EP4696800A1

EP4696800A1 - High strength steel sheet, high strength plated steel sheet, methods for producing same, and member

Info

Publication number: EP4696800A1
Application number: EP24819120.7A
Authority: EP
Inventors: Hidekazu Minami; Yusuke Wada; Yuki Toji; Katsutoshi Takashima; Mai AOYAMA; Katsuya Hoshino
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-06-09
Filing date: 2024-05-16
Publication date: 2026-02-18
Also published as: JP7673874B1; KR20260003776A; CN121285647A; WO2024252887A1; JPWO2024252887A1; MX2025014478A

Abstract

It is an object to provide a high strength steel sheet of 1180 MPa or more and a high strength coated steel sheet of 1180 MPa or more that have excellent bendability and high resistance to intra-steel sheet cracking in a spot weld HAZ and can be used for the production of parts with high dimensional accuracy, to provide methods for producing the steel sheets, and to provide a member.

A high strength steel sheet has a prescribed chemical composition. An effective Ti mole fraction (x_Ti,eff) determined by x_Ti,eff = x_Ti - x_N - x_S (each element symbol represents the mole fraction of the element in the steel) satisfies 0.001 or more. The high strength steel sheet has, at a position 1/4 of the thickness, a steel microstructure in which the area fraction of martensite is 80% or more and 99% or less and in which the sum of the area fraction of ferrite and/or the volume fraction of retained austenite is more than 0% and 20% or less. At a position 5 µm from the surface of the steel sheet in the thickness direction, the concentration of Si is 0.10% or more and 0.60% or less. The number density of MnS inclusions present in Mn-segregated regions on the surface of the steel sheet is 5.0 pieces/mm² or less. The standard deviation of the Vickers hardness of the surface of the steel sheet is 15 or less, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet is 7 or less.

Description

Technical Field

The present disclosure relates to a high strength steel sheet and a high strength coated steel sheet that are optimal for reinforcing parts and framework structural parts of automobiles, to methods for producing these steel sheets, and to a member.

Background Art

To simultaneously reduce CO₂ emissions by reducing vehicle weight and improve crashworthiness by reducing vehicle body weight, the strength of steel sheets for automobiles has been increased. Moreover, new legal regulations are being introduced continuously. Therefore, high strength steel sheets are being increasingly used for main structural and reinforcing parts that form frameworks of automobile cabins (these parts are hereinafter referred to also as "automotive framework structural parts") for the purpose of increasing vehicle body strength. In particular, high strength steel sheets with a tensile strength (hereinafter referred to simply as "TS") of 1,180 MPa or higher are being increasingly used.
High strength steel sheets used for reinforcing parts and framework structural parts of automobiles are required to have excellent formability. Moreover, the formed parts are required to have high dimensional accuracy. For example, parts such as crash boxes have punched edges and bent portions, so that steel sheets with high stretch flangeability and bendability are suitable from the viewpoint of formability. From the viewpoint of performance of parts, absorbable impact energy at the time of collision can be increased by increasing the yield ratio (YR = yield strength YS/tensile strength TS) of steel sheets. Moreover, from the viewpoint of dimensional accuracy of parts, the yield ratio (YR) of steel sheets is controlled within a certain range. This allows springback of the steel sheets formed into the parts to be reduced, and the dimensional accuracy of the parts can thereby by controlled. To increase the use ratio of high strength steel sheets in automotive parts, it is required that these properties be satisfied in a comprehensive manner.
Among framework structural parts of automobiles, crash boxes, for example, have bent portions. Moreover, to increase the use ratio of high strength steel sheets in parts with relatively small bending radii, there is a demand for high strength steel sheets that have bendability high enough to prevent the occurrence of large cracks in bent portions in which the value of R/t obtained by dividing the bending radius (R) by the sheet thickness (t) is less than 5.0.
Recently, it has been found that, when high strength galvanized steel sheets are spot-welded, zinc in the galvanized layers diffuses into grain boundaries in the surface layers of the steel sheets, causing liquid metal embrittlement (LME). In this case, intra-steel sheet cracking (LME cracking) occurs in the spot weld HAZ. The intra-steel sheet cracking in the spot weld HAZ can occur even in a high strength steel sheet having no galvanized layer when the welding partner is a galvanized steel sheet and is recognized as a problem for both types of high-strength steel sheets. During mass production, steel sheets having different thicknesses are spot-welded. Generally, as the sheet thickness increases, the hold time provided for preventing the occurrence of intra-steel sheet cracking in the spot weld HAZ increases. However, a long hold time causes a reduction in productivity. Therefore, there is a demand for high strength steel sheets in which the occurrence of intra-steel sheet cracking in the spot weld HAZ can be prevented even when the hold time is short.
To meet these demands, Patent Literature 1, for example, provides a high strength steel sheet that has a tensile strength of 980 MPa or more, has excellent bendability and high LME resistance, and can be used for production of parts with high dimensional accuracy.

Citation List

Patent Literature

PTL 1: Japanese Patent No. 6787535

Summary of Invention

Technical Problem

The high strength steel sheet described in Patent Literature 1 satisfies both bendability and LME resistance in a comprehensive manner and can be used for production of parts with high dimensional accuracy. However, the high strength steel sheet described in Patent Literature 1 has a TS grade of 980 MPa, and there is room for further improvement in strength.
The present disclosure has been developed in view of the above circumstances, and it is an object to provide a high strength steel sheet of 1180 MPa or more that has excellent bendability and high resistance to intra-steel sheet cracking in the spot weld HAZ and can be used for production of parts with high dimensional accuracy and to provide an advantageous method for producing the high strength steel sheet.
In the present disclosure, the phrase "parts can be produced with high dimensional accuracy (dimensional accuracy during forming is high)" means that the yield ratio (YR) is 65% or more and 90% or less. The YR is determined using the following formula (2). $YR = YS / TS \times 100$
Regarding the bendability, a bending test is performed using the V-block method with a bending angle of 90 degrees. Specifically, the bending test is performed on five samples, with the bending radius (R) set such that the value of R/t obtained by dividing the bending radius (R) by the sheet thickness (t) is about 4.5, i.e., 4.3 to 4.7. Next, for all the five samples, the length of a crack in a ridge portion at the apex of the bend is evaluated. When the crack length is 200 µm or less, the bendability is considered excellent.
Regarding the resistance to intra-steel sheet cracking in the spot weld HAZ, a cross-section of a weld described in Examples is observed under an optical microscope (200X), and the resistance to intra-steel sheet cracking in the spot weld HAZ is evaluated according to the following criteria. When the rating is A or B, the resistance to intra-steel sheet cracking in the spot weld HAZ is considered excellent. When the rating is C, the resistance to intra-steel sheet cracking in the spot weld HAZ is considered poor.

A: No crack with a length of 0.1 mm or more is found when the hold time is 0.16 seconds.
B: A crack with a length of 0.1 mm or more is found when the hold time is 0.16 seconds, but no crack with a length of 0.1 mm or more is found when the hold time is 0.20 seconds.
C: A crack with a length of 0.1 mm or more is found when the hold time is 0.20 seconds.

Now, the hold time is the period from the end of the application of the welding current to the start of release of the electrodes.

Solution to Problem

To achieve the foregoing object, the inventors have conducted extensive studies and found the following.

(1) With a microstructure composed mainly of martensite (including quenched martensite and tempered martensite) and containing ferrite and/or retained austenite, the YR used as an indicator of the dimensional accuracy of parts can be 65% or more and 90% or less.
(2) Excellent bendability can be achieved when the following conditions are met. The concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is 0.10% or more and 0.60% or less. The number density of MnS inclusions present in Mn-segregated regions on the surface of the steel sheet is 5.0 pieces/mm² or less. The standard deviation of the Vickers hardness of the surface of the steel sheet is 15 or less. A hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet is 7 or less.
(3) High resistance to intra-steel sheet cracking in the spot weld HAZ can be achieved when the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is 0.10% or more and 0.60% or less.

The present disclosure has been made based on the above findings. Specifically, the summary of the present disclosure is as follows.

[1] A high strength steel sheet having a chemical composition containing, in % by mass, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, and Ti: 0.002% or more and 0.200% or less, with the balance being Fe and incidental impurities, wherein an effective Ti mole fraction (x_Ti,eff) determined by formula (1) below is 0.001 or more, wherein the high strength steel sheet has, at a position 1/4 of a thickness, a steel microstructure in which an area fraction of martensite is 80% or more and 99% or less and in which a sum of an area fraction of ferrite and/or a volume fraction of retained austenite is more than 0% and 20% or less, wherein, at a position 5 µm from a surface of the steel sheet in a thickness direction, a concentration of Si is 0.10% or more and 0.60% or less, wherein a number density of MnS inclusions present in Mn-segregated regions on the surface of the steel sheet is 5.0 pieces/mm² or less, wherein a standard deviation of Vickers hardness of the surface of the steel sheet is 15 or less, and wherein a hardness variation frequency per 1100 µm in a sheet width direction on the surface of the steel sheet is 7 or less: $x_{Ti, eff} = x_{Ti} - x_{N} - x_{S},$ where x_Ti, x_N, and x_S are contents (mole fractions) of respective elements in the steel sheet.
[2] The high strength steel sheet according to [1], wherein the chemical composition further contains, in % by mass, at least one element selected from Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REMs: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.
[3] A high strength coated steel sheet including: the high strength steel sheet according to [1] or [2]; and a coated layer on at least one side of the high strength steel sheet.
[4] A method for producing a high strength steel sheet, the method including: heating a steel slab having the chemical composition according to [1] or [2] to a slab heating temperature of 1150°C or higher at an average slab heating rate of 25°C/min or less in a temperature range of 900°C or higher and 1150°C or lower such that a residence time in a range of from 1100°C to the slab heating temperature is 20 minutes or longer; subsequently, hot-rolling the resulting steel slab to obtain a hot rolled sheet, wherein a rolling reduction in a final pass of finish rolling is set to 9% or more and 15% or less, wherein a rolling reduction in a pass immediately preceding the final pass is set to 15% or more and 21% or less, and wherein a rolling reduction in a pass two passes prior to the final pass is set to 21% or more and 27% or less; subsequently, pickling the hot rolled sheet to obtain a pickled sheet; subsequently, cold-rolling the pickled sheet at a cumulative rolling reduction ratio of 20% or more and 75% or less to thereby obtain a cold rolled sheet; and then performing an annealing step of heating the cold rolled sheet to a heating temperature of 780°C or higher such that an average heating rate in a temperature range of 250°C or higher and 700°C or lower is set to 100°C/s or lower and that a residence time in a range of 750°C or higher and the heating temperature or lower is set to 10 seconds or longer.
[5] The method for producing a high strength steel sheet according to [4], wherein, in the annealing step, a concentration of oxygen in an atmosphere in a range of 750°C or higher and the heating temperature or lower is 0.5% by volume or more and 5.0% by volume or less, and wherein the atmosphere has a dew point of -35°C or higher.
[6] The method for producing a high strength steel sheet according to [4] or [5], the method further including, after the annealing step, performing a cooling step such that an average cooling rate in a temperature range of 250°C or higher and 400°C or lower is 1.0°C/s or more.
[7] The method for producing a high strength steel sheet according to [4] or [5], the method further including, after the annealing step, performing a cooling step including heat retention at a heat retention temperature of 100°C or higher and 450°C or lower for 5 seconds or longer.
[8] The method for producing a high strength steel sheet according to [4] or [5], the method further including, after the annealing step: performing a cooling step with a cooling stop temperature set to 250°C or lower; and then reheating the resulting cold rolled sheet to a reheating temperature of (the cooling stop temperature + 50°C) or higher and 450°C or lower to perform heat retention treatment at the reheating temperature for 5 seconds or longer.
[9] A method for producing a high strength coated steel sheet, the method including, after the annealing step according to any one of [4] to [8], performing a coating step of subjecting at least one side of the cold rolled sheet to coating treatment.
[10] A member including the high strength steel sheet according to [1] or [2], the high strength steel sheet being used for at least part of the member.
[11] A member including the high strength coated steel sheet according to [3], the high strength coated steel sheet being used for at least part of the member.

Advantageous Effects of Invention

The present disclosure can provide a high strength steel sheet of 1180 MPa or more that has excellent bendability and high resistance to intra-steel sheet cracking in a spot weld HAZ and can be used for production of a member with high dimensional accuracy and can also provide a member.

Description of Embodiments

Embodiments of the present disclosure will next be described. However, the present disclosure is not limited to the following embodiments.
First, appropriate ranges of the chemical composition of a steel sheet and the reasons for the limitations on the ranges of the chemical composition will be described. In the following description, "%" representing the content of a component element of the steel sheet means "% by mass" unless otherwise specified. In the present description, a numerical range represented using "to" means a range including the numerical values before and after the "to" as the lower limit and the upper limit, respectively.

[C: 0.030% or more and 0.500% or less]

C is one of the important basic components of the steel. In particular, in the present disclosure, C is an important element that affects the area fractions of martensite and ferrite, the volume fraction of retained austenite, the standard deviation of the Vickers hardness of the surface of the steel sheet, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet. If the content of C is less than 0.030%, the area fraction of martensite decreases, and the area fraction of ferrite increases, which makes it difficult to achieve a TS of 1180 MPa or more. This also makes it difficult to achieve the desired YR. If the content of C exceeds 0.500%, the hardness distribution of martensite in the sheet width direction becomes nonuniform. In this case, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. Therefore, the content of C is 0.030% or more and 0.500% or less. The content of C is preferably 0.080% or more. The content of C is preferably 0.400% or less. The content of C is more preferably 0.110% or more. The content of C is more preferably 0.350% or less.

[Si: 0.01% or more and 2.50% or less]

Si is one of the important basic components of the steel. In particular, in the present disclosure, Si reduces the formation of carbides during annealing, facilitates the formation of retained austenite, and is an element that affects the volume fraction of retained austenite. Si provides high temper softening resistance at 400°C or lower and is therefore an important element that affects the standard deviation of the Vickers hardness of the surface of the steel sheet and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet. When the content of Si is less than 0.01%, the distribution of the hardness of martensite in the sheet width direction becomes nonuniform. In this case, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. Meanwhile, when the content of Si exceeds 2.50%, the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction increases, so that the resistance to intra-steel sheet cracking in the spot weld HAZ deteriorates. Therefore, the content of Si is 0.01% or more and 2.50% or less. The content of Si is preferably 0.20% or more. The content of Si is preferably 2.00% or less. The content of Si is more preferably 0.25% or more. The content of Si is more preferably 1.50% or less.

[Mn: 0.10% or more and 5.00% or less]

Mn is one of the important basic components of the steel. In particular, in the present disclosure, Mn is an important element that affects the area fractions of martensite and ferrite, the volume fraction of retained austenite, the standard deviation of the Vickers hardness of the surface of the steel sheet, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet. When the content of Mn is less than 0.10%, the area fraction of martensite decreases, and the area fraction of ferrite increase, which makes it difficult to achieve a TS of 1180 MPa or more. This also make it difficult to achieve the desired YR. Meanwhile, when the content of Mn exceeds 5.00%, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. Therefore, the content of Mn is 0.10% or more and 5.00% or less. The content of Mn is preferably 1.00% or more. The content of Mn is preferably 4.00% or less. The content of Mn is more preferably 2.00% or more. The content of Mn is more preferably 3.50% or less.

[P: 0.100% or less]

When the content of P is excessively large, it segregates at prior-austenite grain boundaries and embrittles the grain boundaries, and the ultimate deformation ability of the steel sheet deteriorates, causing deterioration of the bendability. Therefore, the content of P must be 0.100% or less. No particular limitation is imposed on the lower limit of the content of P. However, since P is a solid solution strengthening element and increases the strength of the steel sheet, the content of P is preferably 0.001% or more. Therefore, the content of P is 0.100% or less. The content of P is preferably 0.001% or more and more preferably 0.070% or less.

[S: 0.0200% or less]

S is present as sulfides and causes deterioration of the ultimate deformation ability of the steel, so that the bendability deteriorates. Therefore, the content of S must be 0.0200% or less. No particular limitation is imposed on the lower limit of the content of S. However, in view of the limitations on the production technique, the content of S is preferably 0.0001% or more. Specifically, the content of S is 0.0200% or less. The content of S is preferably 0.0001% or more. The content of S is preferably 0.0050% or less.

[Al: 0.100% or less]

When the content of Al is excessively large, the A₃ transformation temperature increases, and a large amount of ferrite is contained in the microstructure, which makes it difficult to achieve the desired YR. Therefore, the content of Al must be 0.100% or less. No particular limitation is imposed on the lower limit of the content of Al. However, since Al reduces the formation of carbides and facilitates the formation of retained austenite during continuous annealing, the content of Al is preferably 0.001% or more. Specifically, the content of Al is 0.100% or less. The content of Al is preferably 0.001% or more. The content of Al is preferably 0.050% or less.

[N: 0.0100% or less]

N is present as nitrides and causes deterioration of the ultimate deformation ability of the steel sheet, so that the bendability deteriorates. Therefore, the content of N must be 0.0100% or less. No particular limitation is imposed on the lower limit of the content of N. However, in view of the limitations on the production technique, the content of N is preferably 0.0005% or more. Specifically, the content of N is 0.0100% or less. The content of N is preferably 0.0005% or more. The content of N is preferably 0.0050% or less.

[O: 0.0100% or less]

O is present as oxides and causes deterioration of the ultimate deformation ability of the steel sheet, so that the bendability deteriorates. Therefore, the content of O must be 0.0100% or less. No particular limitation is imposed on the lower limit of the content of O. However, in view of the limitations on the production technique, the content of O is preferably 0.0001% or more. Specifically, the content of O is 0.0100% or less. The content of O is preferably 0.0001% or more. The content of O is preferably 0.0050% or less.

[Ti: 0.002% or more and 0.200% or less]

Ti forms a fine carbide, nitride, or carbonitride during hot rolling or annealing and thereby increases the strength of the steel sheet. By adding Ti, the number density of MnS inclusions present in Mn-segregated regions on the surface of the steel sheet can be reduced. To obtain these effects, the content of Ti must be 0.002% or more. When the content of Ti exceeds 0.200%, the amount of the carbide, nitride, or carbonitride increases, and this makes it difficult to achieve the desired YR. Therefore, the content of Ti is 0.002% or more and 0.200% or less. The content of Ti is preferably 0.006% or more. The content of Ti is preferably 0.100% or less. The content of Ti is more preferably 0.010% or more. The content of Ti is more preferably 0.050% or less.

[Effective Ti mole fraction (x_Ti,eff) determined from formula (1) is 0.001 or more]

$Effective Ti mole fraction (x_{Ti, eff}) = x_{Ti} - x_{N} - x_{S}$
In the above formula, x_Ti, x_N, and x_S are contents (mole fractions) of the respective elements in the steel sheet. By adjusting the effective Ti mole fraction determined using formula (1) above to a predetermined value or more, the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet can be reduced, and excellent bendability can be achieved. To obtain this effect, the effective Ti mole fraction is adjusted to 0.001% or more. Now, no particular limitation is imposed on the upper limit of the effective Ti mole fraction. However, the effective Ti mole fraction is preferably 0.040 or less. Otherwise, the amount of the carbide, nitride, or carbonitride increases, and this makes it difficult to achieve the desired YR. Specifically, the effective Ti mole fraction is 0.001% or more. The effective Ti mole fraction is preferably 0.002% or more. The effective Ti mole fraction is preferably 0.040% or less. The mole fraction of a component is determined by converting its mass percentage.
A high strength steel sheet according to an embodiment of the present invention has a chemical composition containing the elements described above and further containing Fe and incidental impurities as the balance. Preferably, the high strength steel sheet according to the present embodiment of the present invention has a chemical composition containing the elements described above, with the balance being Fe and incidental impurities. The incidental impurities include Zn, Pb, and As. The acceptable total content of these impurities is 0.100% or less.
The chemical composition of the high strength steel sheet of the present disclosure may further contain, in addition to the essential components described above, in % by mass, at least one element or a combination of elements selected from Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REMs: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[Nb: 0.200% or less]

Nb forms a large number of coarse precipitates and inclusions, and this causes deterioration of the ultimate deformation ability of the steel sheet. When the content of Nb is more than 0.200%, the bendability deteriorates. Therefore, the content of Nb is 0.200% or less. No particular limitation is imposed on the lower limit of the content of Nb. When the content of Nb is 0.001% or more, a fine carbide, nitride, or carbonitride is formed during hot rolling or continuous annealing, and the strength of the steel sheet thereby increases, so that the YR can be controlled within the desired range. Therefore, the content of Nb is preferably 0.001% or more. When Nb is added, its content is set to 0.200% or less. The content of Nb is preferably 0.001% or more. The content of Nb is preferably 0.100% or less.

[V: 0.200% or less]

V forms a large number of coarse precipitates and inclusions, and this causes deterioration of the ultimate deformation ability of the steel sheet. When the content of V is more than 0.200%, the bendability deteriorates. Therefore, the content of V is set to 0.200% or less. No particular limitation is imposed on the lower limit of the content of V. When the content of V is 0.001% or more, a fine carbide, nitride, or carbonitride is formed during hot rolling or continuous annealing, and the strength of the steel sheet thereby increases, so that the YR can be controlled within the desired range. Therefore, the content of V is preferably 0.001% or more. When V is added, its content is set to 0.200% or less. The content of V is preferably 0.001% or more. The content of V is preferably 0.100% or less.

[Ta: 0.10% or less and W: 0.10% or less]

When the contents of Ta and W are each more than 0.10%, a large number of coarse precipitates and inclusions are formed, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the contents of Ta and W are each set to 0.10% or less. Now, no particular limitation is imposed on the lower limits of the contents of Ta and W. However, Ta and W form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, and this contributes to an increase in the strength of the steel sheet. Therefore, the contents of Ta and W are each preferably 0.01% or more. When Ta and W are added, their contents are each set to 0.10% or less. The contents of Ta and W are each preferably 0.01% or more. The contents of Ta and W are each preferably 0.08% or less.

[B: 0.0100% or less]

When the content of B is 0.0100% or less, no cracking occurs in the steel sheet during casting or hot rolling, and deterioration of the ultimate deformation ability of the steel sheet does not occur, so that the bendability does not deteriorate. Therefore, the content of B is preferably 0.0100% or less. No particular limitation is imposed on the lower limit of the content of B. However, since B is an element that segregates at austenite grain boundaries during annealing to improve the hardenability, the content of B is more preferably 0.0003% or more. When B is contained, its content is set to 0.0100% or less. The content of B is more preferably 0.0003% or more. The content of B is still more preferably 0.0080% or less.

[Cr: 1.00% or less, Mo: 1.00% or less, and Ni: 1.00% or less]

When the contents of Cr, Mo, and Ni are each more than 1.00%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the contents of Cr, Mo, and Ni are each set to 1.00% or less. No particular limitation is imposed on the lower limits of the contents of Cr, Mo, and Ni. However, since these elements improve the hardenability, the contents of Cr, Mo, and Ni are each preferably 0.01% or more. Specifically, when these elements are added, the contents of Cr, Mo, and Ni are each set to 1.00% or less. The contents of Cr, Mo, and Ni are each preferably 0.01% or more. The contents of Cr, Mo, and Ni are each preferably 0.80% or less.

[Co: 0.010% or less]

When the content of Co is more than 0.010%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the content of Co is set to 0.010% or less. No particular limitation is imposed on the lower limit of the content of Co. However, since Co is an element that improves the hardenability, the content of Co is preferably 0.001% or more. Specifically, when Co is added, its content is set to 0.010% or less. The content of Co is preferably 0.001% or more. The content of Co is preferably 0.008% or less.

[Cu: 1.00% or less]

When the content of Cu is more than 1.00%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the content of Cu is set to 1.00% or less. No particular limitation is imposed on the lower limit of the content of Cu. However, since Cu is an element that improves the hardenability, the content of Cu is preferably 0.01% or more. Specifically, when Cu is added, its content is set to 1.00% or less. The content of Cu is preferably 0.01% or more. The content of Cu is preferably 0.80% or less.

[Sn: 0.200% or less]

When the content of Sn is more than 0.200%, cracks are formed in the steel sheet during casting or hot rolling, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the content of Sn is set to 0.200% or less. No particular limitation is imposed on the lower limit of the content of Sn. However, since Sn is an element that improves the hardenability, the content of Sn is preferably 0.001% or more. Specifically, when Sn is added, its content is set to 0.200% or less. The content of Sn is preferably 0.001% or more. The content of Sn is preferably 0.100% or less.

[Sb: 0.200% or less]

When the content of Sb is more than 0.200%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the content of Sb is set to 0.200% or less. No particular limitation is imposed on the lower limit of the content of Sb. However, since Sb is an element that can control the thickness of a softened surface layer to adjust the strength, the content of Sb is preferably 0.001% or more. Specifically, when Sb is added, its content is set to 0.200% or less. The content of Sb is preferably 0.001% or more. The content of Sb is preferably 0.100% or less.

[Ca: 0.0100% or less, Mg: 0.0100% or less, and REMs: 0.0100% or less]

When the contents of Ca, Mg, and REMs are each more than 0.0100%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the contents of Ca, Mg, and REMs are each set to 0.0100% or less. Now, no particular limitation is imposed on the lower limits of the contents of Ca, Mg, and REMs. However, since these elements spheroidize nitrides and sulfides and improve the ultimate deformation ability of the steel sheet, the contents of Ca, Mg, and REMs are each preferably 0.0005% or more. Specifically, when Ca, Mg, and REMs are added, their contents are each set to 0.0100% or less. The contents of Ca, Mg, and REMs are each preferably 0.0005% or more. The contents of Ca, Mg, and REMs are each preferably 0.0050% or less. The REMs (rare earth metals) are a generic term for Sc, Y, and 15 elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71, and the content of REMs is the total content of these elements.

[Zr: 0.100% or less and Te: 0.100% or less]

When the contents of Zr and Te are each more than 0.100%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the contents of Zr and Te must be each 0.100% or less. No particular limitation is imposed on the lower limits of the contents of Zr and Te. However, since Zr and Te are elements that spheroidize nitrides and sulfides and improve the ultimate deformation ability of the steel sheet, their contents are each more preferably 0.001% or more. Specifically, when Zr and Te are added, their contents are each set to 0.100% or less. The contents of Zr and Te are each preferably 0.001% or more. The contents of Zr and Te are each preferably 0.080% or less.

[Hf: 0.10% or less]

When the content of Hf is more than 0.10%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the content of Hf is set to 0.10% or less. No particular limitation is imposed on the lower limit of the content of Hf. However, since Hf is an element that spheroidizes nitrides and sulfides and improves the ultimate deformation ability of the steel sheet, the content of Hf is set to preferably 0.01% or more. Specifically, when Hf is added, its content is set to 0.10% or less. The content of Hf is preferably 0.01% or more. The content of Hf is preferably 0.08% or less.

[Bi: 0.200% or less]

When the content of Bi is more than 0.200%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the content of Bi is set to 0.200% or less. Now, no particular limitation is imposed on the lower limit of the content of Bi. However, since Bi is an element that reduces the degree of segregation, its content is preferably 0.001% or more. Specifically, when Bi is added, its content is set to 0.200% or less. The content of Bi is preferably 0.001% or more. The content of Bi is preferably 0.100% or less.
Even when the contents of Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REMs, Zr, Te, Hf, and Bi described above are less than their preferred lower limits, the effects of the invention are not impaired. In this case, they are contained as incidental impurities.
Next, the steel microstructure of the steel sheet will be described.

[Area fraction of martensite: 80% or more and 99% or less]

When the area fraction of martensite is less than 80%, the area fraction of ferrite is large, which makes it difficult to achieve a TS of 1180 MPa or more. This also makes it difficult to achieve the desired YR. Moreover, since the hardness distribution in the sheet width direction becomes nonuniform, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. If the area fraction of martensite exceeds 99%, a sufficient amount of ferrite and/or retained austenite effective in controlling the YR is not present in the steel microstructure, and this makes it difficult to achieve the desired YR. Therefore, the area fraction of martensite is set to 80% or more and 99% or less. The area fraction of martensite is preferably 85% or more. The area fraction of martensite is preferably 98% or less. The area fraction of martensite is more preferably 87% or more. The area fraction of martensite is preferably 97% or less. The term "martensite" as used herein encompasses quenched martensite (fresh martensite), tempered martensite, and bainite. Now, the observation position for the area fraction of martensite is a position 1/4 of the thickness of the steel sheet as described later.

[Sum of area fraction of ferrite and/or volume fraction of retained austenite: more than 0% and 20% or less]

When the sum of the area fraction of ferrite and/or the volume fraction of retained austenite is 0%, the steel microstructure becomes a martensite single-phase microstructure, and this makes it difficult to achieve the desired YR. Meanwhile, when the sum of the area fraction of ferrite and/or the volume fraction of retained austenite exceeds 20%, the area fraction of martensite is small, and this makes it difficult to achieve a TS of 1180 MPa or more. This also makes it difficult to achieve the desired YR. In addition, since the hardness distribution in the sheet width direction becomes nonuniform, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. Therefore, the sum of the area fraction of ferrite and/or the volume fraction of retained austenite is set to more than 0% and 20% or less. The sum of the area fraction of ferrite and/or the volume fraction of retained austenite is preferably 1% or more. The area fraction of martensite is preferably 18% or less. The area fraction of martensite is more preferably 2% or more. The area fraction of martensite is preferably 15% or less. Now, the term "ferrite" as used herein encompasses bainitic ferrite. The observation position for the area fraction of ferrite and the volume fraction of retained austenite is a position 1/4 of the thickness of the steel sheet as described later.
A method for measuring the area fraction of martensite (quenched martensite, tempered martensite, and bainite) and the area fraction of ferrite (bainitic ferrite) is as follows.
A sample is cut from the steel sheet such that a thicknesswise cross section (L cross section) parallel to the rolling direction serves as an observation surface, and the observation surface is mirror-polished using a diamond paste. Then the resulting observation surface is etched with 3 vol.% nital solution to make the microstructure visible. A position 1/4 of the thickness of the steel sheet is used as an observation position, and an SEM (Scanning Electron Microscope) is used to observe three viewing fields each having a viewing area of 17 µm × 23 µm at a magnification of 5000X under the condition of an acceleration voltage of 15 kV. In the obtained microstructure images of the three viewing fields, area fractions of constituent microstructures are calculated by dividing the areas of the constituent microstructures by the measurement area using Adobe Photoshop from Adobe Systems. The constituent microstructures mean ferrite (bainitic ferrite) and martensite (tempered martensite, bainite, and quenched martensite). The obtained values are averaged to obtain the area fractions of the microstructures. In the microstructure images, ferrite (bainitic ferrite) appears as a concave microstructure and is a flat microstructure containing no carbides, and tempered martensite and bainite appear as concave microstructures and are microstructures containing fine carbides. Quenched martensite appears as convex portions and is a microstructure with fine irregularities thereinside. These microstructures can be distinguished from each other. However, it is unnecessary that quenched martensite, tempered martensite, and bainite be distinguishable from each other because their total area fraction is used to determine the area fraction of martensite.
A method for measuring the volume fraction of retained austenite is as follows.
The steel sheet is ground such that a position 1/4 of the thickness from the surface of the steel sheet (a position corresponding to 1/4 of the thickness in the depth direction from the surface of the steel sheet) serves as an observation surface, and the resulting surface is further polished by 0.1 mm by chemical polishing. Then, on the polished surface, the CoKα line from an X-ray diffractometer is used to measure the integrated reflection intensities from (200), (220), and (311) planes of fcc iron (austenite) and (200), (211), and (220) plans of bcc iron. The volume fraction of austenite is determined from the intensity ratios of the integrated reflection intensities from the above planes of fcc iron (austenite) to the integrated reflection intensities from the above planes of bcc iron and is used as the volume fraction of retained austenite.

[Concentration of Si at position 5 µm from surface of steel sheet in thickness direction: 0.10% or more and 0.60% or less]

In the present disclosure, this is an extremely important constituent factor of the invention. To control the resistance to intra-steel sheet cracking in the spot weld HAZ, it is important to control the concentrations of elements at a position 5 µm from the surface of the steel sheet in the thickness direction. In particular, when the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is reduced, the resistance to intra-steel sheet cracking in the spot weld HAZ can be improved. To obtain this effect, it is necessary that the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction be controlled to 0.60% or less. When the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is less than 0.10%, the distribution of the hardness of martensite in the sheet width direction becomes nonuniform. In this case, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. Therefore, the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is set to 0.10% or more and 0.60% or less. The concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is preferably 0.15% or more. The concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is preferably 0.55% or less. The concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is more preferably 0.20% or more. The concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is preferably 0.50% or less. The unit of the concentration of Si is % by mass.
A method for measuring the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction is as follows.
A sample with a rolling direction length of 20 mm and a width direction length of 20 mm is obtained by cutting the steel sheet. The surface of the high strength steel sheet is used as a measurement surface, and measurement is performed by glow discharge optical emission spectrometry (hereinafter referred to as GDS). In this case, the concentration of Si is analyzed in the sheet thickness direction under the conditions of a high-frequency discharge pressure of 300 Pa, a high-frequency output power of 35 W, and a pulse frequency of 100 Hz. The concentration values of Si at positions 5 µm from the surface of the steel sheet are averaged to calculate the concentration of Si at a position 5 µm from the surface of the steel sheet. The measurement data is converted to the concentration of Si by a calibration curve method.

[Number density of MnS inclusions present in Mn-segregated regions on surface of steel sheet: 5.0 pieces/mm² or less]

In the present disclosure, this is an extremely important constituent factor of the invention. To control the bendability, it is important to control the number density of inclusions present in Mn-segregated regions on the surface of the steel sheet, i.e., in regions containing harder martensite than that in surrounding regions. By reducing the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet, the bendability can be improved. To obtain this effect, it is necessary that the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet be set to 5.0 pieces/mm² or less. No particular limitation is imposed on the lower limit of the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet. However, the lower the number density of MnS inclusions, the better. Even when the number density of MnS inclusions is 0.0 pieces/mm², the effects of the disclosure can be obtained. Therefore, the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is set to 5.0 pieces/mm² or less. The number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is preferably 0.0 pieces/mm² or more. The number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is preferably 4.0 pieces/mm² or less.
A method for measuring the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is as follows.
A bending test is performed using the V-block method with a bending angle of 90 degrees. Specifically, the bending test is performed with R set such that the value of R/t is about 4.5, i.e., 4.3 to 4.7. Then a sample with a rolling direction length of 20 mm and a width direction length of 5 mm is obtained by cutting the test piece so as to include a crack in the ridge portion at the apex of the bend. The outer surface of the bend is used as an observation surface, and the observation surface is mirror-polished using a diamond paste. Next, measurement is performed using an electron probe microanalyzer (EPMA) (JXA-8230 manufactured by JEOL Ltd.). In this case, Mn and S are measured in three viewing fields under the conditions of acceleration voltage: 15 kV, measurement region: rolling direction length 1.2 mm × width direction length 1.0 mm, and irradiation current: 1.0 × 10^-7 A. The measurement data is converted to the concentration of C by a calibration curve method. Mn element mapping is performed on the obtained three viewing fields, and regions in which a large amount of Mn is detected are identified as Mn-segregated regions. Then S element mapping is performed to identify S-concentrated portions, i.e., MnS inclusions, in the Mn-segregated regions, and the number of S-concentrated portions is evaluated. The number of MnS inclusions present in the Mn-segregated regions is divided by the measurement surface area 1.2 mm² to calculate the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet.

[Standard deviation of Vickers hardness of surface of steel sheet: 15 or less]

In the present disclosure, this is an extremely important constituent factor of the invention. To control the bendability, it is important to ensure a uniform hardness distribution over the surface of the steel sheet. By reducing the standard deviation of the Vickers hardness of the surface of the steel sheet, the bendability can be improved. To obtain this effect, the standard deviation of the Vickers hardness of the surface of the steel sheet must be 15 or less. No particular limitation is imposed on the lower limit of the standard deviation of the Vickers hardness of the surface of the steel sheet. However, the lower the standard deviation of the Vickers hardness of the surface of the steel sheet, the better. Even when the standard deviation is 0, the effects of the disclosure can be obtained. Therefore, the standard deviation of the Vickers hardness of the surface of the steel sheet is 15 or less. The standard deviation of the Vickers hardness of the surface of the steel sheet is preferably 0 or more. The standard deviation of the Vickers hardness of the surface of the steel sheet is preferably 13 or less.
A method for measuring the standard deviation of the Vickers hardness of the surface of the steel sheet is as follows.
A bending test is performed using the V-block method with a bending angle of 90 degrees. Specifically, the bending test is performed with R set such that the value of R/t is about 4.5, i.e., 4.3 to 4.7. Then a sample with a rolling direction length of 20 mm and a width direction length of 5 mm is obtained by cutting the test piece so as to include a crack in the ridge portion at the apex of the bend. The outer surface of the bend is used as an observation surface, and the observation surface is mirror-polished using a diamond paste. Then the Vickers hardness is measured at 11 points on the mirror-polished observation surface at 100 µm intervals in the sheet width direction using a Vickers hardness meter under the condition of a load of 100 gf. The measurement points are set such that the sixth point is located at a position 500 µm from the edge of the crack in the rolling direction so as to be parallel to the crack. The standard deviation is determined from the obtained results and used as the standard deviation of the Vickers hardness of the surface of the steel sheet.

[Hardness variation frequency per 1100 µm in sheet width direction on surface of steel sheet: 7 or less]

In the present disclosure, this is an extremely important constituent factor of the invention. By reducing the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet, the desired bendability can be achieved. To obtain this effect, the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet must be 7 or less. No particular limitation is imposed on the lower limit of the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet. However, the lower the hardness variation frequency, the better. Even when the hardness variation frequency is 0, the effects of the disclosure can be obtained. Specifically, the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet is 7 or less. The hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet is preferably 0 or more. The hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet is preferably 6 or less.
The hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet is as follows.
A bending test is performed using the V-block method with a bending angle of 90 degrees. Specifically, the bending test is performed with R set such that the value of R/t is about 4.5, i.e., 4.3 to 4.7. Then a sample with a rolling direction length of 20 mm and a width direction length of 5 mm is obtained by cutting the test piece so as to include a crack in the ridge portion at the apex of the bend. The outer surface of the bend is used as an observation surface, and the observation surface is mirror-polished using a diamond paste. Then the Vickers hardness is measured at 11 points on the mirror-polished observation surface at 100 µm intervals in the sheet width direction using a Vickers hardness meter under the condition of a load of 100 gf. The measurement points are set such that the sixth point is located at a position 500 µm from the edge of the crack so as to be parallel to the crack. The obtained results are used to produce the distribution of the hardness measured on the surface of the steel sheet using the Vickers hardness meter in the sheet width direction. In the hardness distribution, first, the value of {(maximum hardness value Hv_max) - (minimum hardness value Hv_min)} / 2 is calculated. The value of {(maximum hardness value Hv_max) - (minimum hardness value Hv_min)} / 2 is used as a reference variation value. Then, any occurrence in which the absolute value of hardness variation is larger than or equal to the reference variation value is identified. Over the hardness measurement range (length: 1100 µm), the number of occurrences in which the absolute value of hardness variation is larger than or equal to the reference variation value is counted. Specifically, each instance in which the hardness variation value is positive and larger than or equal to the reference variation value is counted as one occurrence, and each instance in which the hardness variation value is negative, and its absolute value is larger than or equal to the reference variation value is counted as one occurrence. Therefore, when one measured hardness variation value is positive and larger than or equal to the reference variation value and another one measured hardness variation value is negative with an absolute value larger than or equal to the reference variation value, the hardness variation frequency is two.
The steel microstructure in the present disclosure may contain remaining microstructures other than the martensite (quenched martensite, tempered martensite, and bainite), ferrite (including bainitic ferrite), and retained austenite described above. Even when pearlite, carbides such as cementite and metastable carbides, and other well-known steel sheet microstructures are contained as remaining microstructures, the effects of the disclosure are not impaired when their area fraction is 5% or less. Examples of the metastable carbides include epsilon (ε) carbide, eta (η) carbide, and chi (χ) carbide.

[High strength steel sheet]

The chemical composition and steel microstructure of the high strength steel sheet are as described above. No particular limitation is imposed on the thickness of the high strength steel sheet. The thickness is generally 0.3 mm or more and 2.8 mm or less.

[High strength coated steel sheet]

The high strength coated steel sheet of the disclosure is a high strength coated steel sheet produced by forming a coated layer on at least one side of the high strength steel sheet of the disclosure. No particular limitation is imposed on the type of coated layer, and the coated layer may be, for example, a hot-dip coated layer or an electroplated layer. The coated layer may be an alloyed coated layer. The coated layer is preferably a galvanized layer. The galvanized layer may contain Al and Mg. The coated layer is also preferably a hot-dip zinc-aluminum-magnesium alloy coated layer (Zn-Al-Mg coated layer). In this case, it is preferable that the content of Al is set to 1% by mass or more and 22% by mass or less and the content of Mg is set to 0.1% by mass or more and 10% by mass or less, with the balance being Zn. The Zn-Al-Mg coated layer may contain, in addition to Zn, Al, and Mg, at least one selected from Si, Ni, Ce, and La in a total amount of 1% by mass or less. No particular limitation is imposed on the coating metal. In addition to the Zn coating, an Al coating etc. may be used.
No particular limitation is imposed on the chemical composition of the coated layer, and any commonly used chemical composition may be used. For example, a hot-dip galvanized layer and a hot-dip galvannealed layer generally contain Fe: 20% by mass or less and Al: 0.001% by mass or more and 1.0% by mass or less. Their chemical compositions further contain one or two or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REMs in a total amount of 0% by mass or more and 3.5% by mass or less, with the balance being Zn and incidental impurities. In the present disclosure, it is preferable to include a hot-dip galvanized layer with a coating weight per side of 20 to 80 g/m² or a hot-dip galvannealed layer obtained by subjecting the hot-dip galvanized layer to galvannealing. When the coated layer is a hot-dip galvanized layer, the content of Fe in the coated layer may be less than 7% by mass. When the coated layer is a hot-dip galvannealed layer, the content of Fe in the coated layer may be 7 to 20% by mass.
Next, a method for producing the high strength steel sheet of the disclosure will be described.
First, a steel slab is produced by melting a steel material having the chemical composition described above. In the present disclosure, no particular limitation is imposed on the method for melting the steel material, and any known melting method using a converter, an electric arc furnace, etc. may be suitably used. Preferably, the steel slab (slab) is produced using a continuous casting method in order to prevent macro segregation. However, the steel slab may be produced using another method such as ingot casting or thin slab casting. After the production of the steel slab, a conventional method may be used to cool the steel slab to room temperature and then reheat the steel slab. Alternatively, an energy saving process such as hot charge rolling or hot direct rolling can be used without any problem. In the hot charge rolling or hot direct rolling, the slab is not cooled. The hot slab is charged into a heating furnace, or the slab is subjected to heat retention treatment for a short time, and then the resulting slab is immediately hot-rolled.
Next, the steel slab is heated to a slab heating temperature of 1150°C or higher at an average slab heating rate of 25°C/min or less in the temperature range of 900°C or higher and 1150°C or lower such that the residence time in a range of from 1100°C to the slab heating temperature is 20 minutes or longer.

[Average slab heating rate in temperature range of 900°C or higher and 1150°C or lower: 25°C/min or less]

In the present disclosure, this is an extremely important constituent factor of the invention. By setting the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower to a low value (25°C/min or less), the detachment of Si from the surface of the steel sheet is facilitated, and the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction can be reduced. Moreover, by setting the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower to a low value, the segregation of Mn formed during casting can be reduced, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet can be reduced. Moreover, the standard deviation of the Vickers hardness of the surface of the steel sheet can be reduced, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet can be reduced. To obtain these effects, the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower is set to 25°C/min or less. No particular limitation is imposed on the lower limit of the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower. However, to effectively prevent an increase in the thickness of a softened surface layer after annealing and to adjust the TS within a more preferred range, the average slab heating rate is set to preferably 5°C/min or more. Therefore, the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower is set to 25°C/min or less. The average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower is preferably 5°C/min or more. The average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower is preferably 15°C/min or less. The slab heating temperature is the temperature of the surface of the steel slab during slab heating.

[Slab heating temperature: 1150°C or higher]

In the present disclosure, this is an extremely important constituent factor of the invention. By setting the slab heating temperature to a high value of 1150°C or higher, the detachment of Si from the surface of the steel sheet is facilitated, and the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction can be reduced. By setting the slab heating temperature to a high value, the segregation of Mn formed during casting is reduced, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is reduced. Moreover, the standard deviation of the Vickers hardness of the surface of the steel sheet can be reduced, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet can be reduced. To obtain these effects, the slab heating temperature is set to 1150°C or higher. No particular limitation is imposed on the upper limit of the slab reheating temperature. However, to effectively prevent an increase in the thickness of the softened surface layer after annealing and to adjust the TS within a more preferred range, the slab heating temperature is set to preferably 1300°C or lower. Therefore, the slab heating temperature is 1150°C or higher. The slab heating temperature is preferably 1180°C or higher. The slab heating temperature is preferably 1300°C or lower. The slab heating temperature is the temperature of the surface of the steel slab during slab heating.

[Residence time in range of from 1100°C to slab heating temperature: 20 minutes or longer]

In the present disclosure, this is an extremely important constituent factor of the invention. By setting the residence time in the range of from 1100°C to the slab heating temperature to a large value of 20 minutes or longer, the detachment of Si from the surface of the steel sheet can be facilitated, and the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction can be reduced. By setting the residence time in the range of from 1100°C to the slab heating temperature to a large value, the segregation of Mn formed during casting is reduced, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is reduced. By setting the residence time in the range of from 1100°C to the slab heating temperature to a large value, the standard deviation of the Vickers hardness of the surface of the steel sheet can be further reduced, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet can be further reduced. To obtain these effects, the residence time in the range of from 1100°C to the slab heating temperature is set to 20 minutes or longer. Now, no particular limitation is imposed on the upper limit of the residence time in the range of from 1100°C to the slab heating temperature. However, to effectively prevent an increase in the thickness of the softened surface layer after annealing and to adjust the TS within a more preferred range, the residence time is set to preferably 100 minutes or shorter. Specifically, the residence time in the range of from 1100°C to the slab heating temperature is set to 20 minutes or longer. The residence time in the range of from 1100°C to the slab heating temperature is preferably 30 minutes or longer. The residence time in the range of from 1100°C to the slab heating temperature is preferably 100 minutes or shorter. The slab heating temperature is the temperature of the surface of the steel slab during slab heating.
The slab is subjected to rough rolling under ordinary conditions to form a sheet bar. When the slab heating temperature is set to be low, it is preferable that the sheet bar is heated before finish rolling using, for example, a bar heater, from the viewpoint of preventing troubles during hot rolling.

[Rolling reduction in final pass of finish rolling: 9% or more and 15% or less]
[Rolling reduction in pass immediately preceding final pass: 15% or more and 21% or less]
[Rolling reduction in pass two passes prior to final pass: 21% or more and 27% or less]

In the present disclosure, these are extremely important constituent factors of the invention. The rolling reduction in the pass immediately preceding the final pass is set to be more than or equal to the rolling reduction in the final pass, and the rolling reduction in the pass two passes prior to the final pass is set to be more than or equal to the rolling reduction in the pass immediately preceding the final pass. In this manner, the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction, the standard deviation of the Vickers hardness of the surface of the steel sheet, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet can be controlled appropriately.
When the rolling reduction in the final pass of the finish rolling is less than 9%, the grain size of austenite on the surface of the steel sheet becomes coarse during hot rolling, i.e., the crystal grain size in the annealed sheet becomes coarse, so that the diffusion of Si to the surface of the steel sheet is suppressed. In this case, the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction cannot be reduced. Moreover, precipitation of Ti, i.e., precipitation of sulfides, is reduced, so that the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet cannot be reduced. When the rolling reduction in the final pass exceeds 15%, the segregation of Mn formed during casting cannot be reduced. In this case, the standard deviation of the Vickers hardness of the surface of the steel sheet cannot be reduced, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet cannot be reduced. Therefore, the rolling reduction in the final pass of the finish rolling is set to 9% or more and 15% or less.
When the rolling reduction in the pass immediately preceding the final pass is less than 15%, the grain size of austenite on the surface of the steel sheet becomes coarse during hot rolling, i.e., the crystal grain size in the annealed sheet becomes coarse, so that the diffusion of Si to the surface of the steel sheet is reduced. In this case, the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction cannot be reduced. Moreover, precipitation of Ti, i.e., precipitation of sulfides, is reduced, so that the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet cannot be reduced. Meanwhile, when the rolling reduction in the pass immediately preceding the final pass exceeds 21%, the segregation of Mn formed during casting cannot be reduced. In this case, the standard deviation of the Vickers hardness of the surface of the steel sheet cannot be reduced, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet cannot be reduced. Therefore, the rolling reduction in the pass immediately preceding the final pass in the finish rolling is set to 15% or more and 21% or less.
If the rolling reduction in the pass two passes prior to the final pass is less than 21%, the grain size of austenite on the surface of the steel sheet becomes coarse during hot rolling, i.e., the crystal grain size in the annealed sheet becomes coarse, so that the diffusion of Si to the surface of the steel sheet is reduced. In this case, the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction cannot be reduced. Moreover, precipitation of Ti, i.e., precipitation of sulfides, is reduced, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet cannot be reduced. Meanwhile, when the rolling reduction in the pass two passes prior to the final pass exceeds 27%, the segregation of Mn formed during casting cannot be reduced. In this case, the standard deviation of the Vickers hardness of the surface of the steel sheet cannot be reduced, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet cannot be reduced. Therefore, the rolling reduction in the pass two passes prior to the final pass of the finish rolling is set to 21% or more and 27% or less.
During the finish rolling, the rolling load may increase, and the rolling reduction in a state in which austenite is not recrystallized may increase. In this case, an unexpected microstructure extending in the rolling direction develops, resulting in a deterioration in the workability of the annealed sheet. Therefore, it is preferable to perform the finish rolling at a finish rolling temperature higher than or equal to the Ar₃ transformation temperature. The temperature of coiling performed after hot rolling is preferably 300°C or higher and more preferably 700°C or lower in order to further improve the workability after annealing. The Ar₃ transformation temperature is determined by the following formula. Ar₃ transformation temperature (°C) = 868 - 396 × [%C] + 24.6 × [%Si] - 68.1 × [%Mn] - 36.1 × [%Ni] - 20.7 × [%Cu] - 24.8 × [%Cr]
In the above formula, [% element symbol] represents the content (% by mass) of the element in the chemical composition and is 0 when the element is not contained.
During hot rolling, rough-rolled sheets may be joined together to allow finish rolling to be performed continuously. The rough-rolled sheets may be temporarily coiled. To reduce the rolling load during the hot rolling, part or all of the finish rolling may be performed as lubrication rolling. The lubrication rolling is also effective from the viewpoint of allowing the steel sheet to have a uniform shape and uniform material properties. The coefficient of friction during the lubrication rolling is preferably 0.10 or more and is preferably 0.25 or less.
The thus-produced hot rolled steel sheet is pickled. The pickling allows removal of oxides from the surface of the steel sheet and is important for imparting excellent chemical convertibility to the high strength steel sheet used as the final product and for ensuring excellent coating quality. The pickling may be performed once or repeatedly in a plurality of passes.
Next, the hot rolled sheet subjected to pickling or the hot rolled sheet subjected to pickling and then to optional heat treatment (hot rolled annealed sheet) is cold-rolled to obtain a cold rolled sheet. Preferably, the cold rolling is performed through multi-pass rolling including two or more passes such as tandem-type multi-stand rolling or reverse rolling because uniform strain can be introduced effectively, and a uniform microstructure can be obtained.
To introduce working strain to the surface of the steel sheet to reduce the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction during annealing, it is preferable to perform bending and unbending one or more times before cold rolling. No particular limitation is imposed on the number of repetitions of bending and unbending before cold rolling. However, the number of repetitions is preferably two or more and more preferably three or more. The bending and unbending before cold rolling are generally performed using rolls with a roll diameter of 300 to 1500 mm.

[Cumulative rolling reduction ratio in cold rolling: 20% or more and 75% or less]

By increasing the cumulative rolling reduction ratio in the cold rolling, the area fraction of ferrite can be reduced, i.e., the sum of the area fraction of ferrite and/or the volume fraction of retained austenite can be adjusted to 20% or less. By increasing the cumulative rolling reduction ratio in the cold rolling, many shear bands are introduced to the surface of the steel sheet, and the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction can be reduced. To obtain these effects, the cumulative rolling reduction ratio in the cold rolling is set to 20% or more. If the cumulative rolling reduction ratio in the cold rolling exceeds 75%, the grain size of austenite generated during annealing becomes small, and the amount of retained austenite in the annealed sheet increases. Specifically, since the sum of the area fraction of ferrite and/or the volume fraction of retained austenite increases, the desired YR cannot be achieved. Moreover, the hardness distribution in the sheet width direction becomes nonuniform. In this case, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. Therefore, the cumulative rolling reduction ratio in the cold rolling is set to 20% or more and 75% or less. The cumulative rolling reduction ratio in the cold rolling is preferably 25% or more. The cumulative rolling reduction ratio in the cold rolling is preferably 70% or less. The cumulative rolling reduction ratio in the cold rolling is more preferably 27% or more. The cumulative rolling reduction ratio in the cold rolling is more preferably 60% or less.
The cold rolled sheet obtained as described above is subjected to an annealing step. The annealing conditions are as follows.

[Average heating rate in temperature range of 250°C or higher and 700°C or lower: 100°C/s or lower]

By reducing the average heating rate in the temperature range of 250°C or higher and 700°C or lower, Si diffuses to the surface of the steel sheet, so that the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction can be reduced. To obtain this effect, the average heating rate in the temperature range of 250°C or higher and 700°C or lower must be 100°C/s or lower. No particular limitation is imposed on the lower limit of the average heating rate in the temperature range of 250°C or higher and 700°C or lower. However, from the viewpoint of suppressing the increase in the grain size of austenite during heating and adjusting the YR appropriately, the average heating rate is set to preferably 5°C/s or more and more preferably 10°C/s or more. Specifically, the average heating rate in the temperature range of 250°C or higher and 700°C or lower is set to 100°C/s or lower. The average heating rate in the temperature range of 250°C or higher and 700°C or lower is preferably 5°C/s or more. The average heating rate in the temperature range of 250°C or higher and 700°C or lower is preferably 75°C/s or lower. The average heating rate in the temperature range of 250°C or higher and 700°C or lower is more preferably 10°C/s or more. The average heating rate in the temperature range of 250°C or higher and 700°C or lower is more preferably 50°C/s or lower. The average heating rate is measured based on the surface temperature of the steel sheet.

[Heating temperature: 780°C or higher]

When the heating temperature (annealing temperature) is lower than 780°C, the annealing treatment proceeds in the ferrite-austenite two phase region, and the resulting steel sheet contains a large amount of ferrite after annealing. In this case, the TS of 1180 MPa or more cannot be achieved, and this makes it difficult to achieve the desired YR. Moreover, since the hardness distribution in the sheet width direction is nonuniform, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. Moreover, Si cannot diffuse to the surface of the steel sheet, and the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction cannot be reduced, so that the resistance to intra-steel sheet cracking in the spot weld HAZ cannot be improved. Now, no particular limitation is imposed on the upper limit of the heating temperature. However, when the heating temperature is increased, the thickness of the softened surface layer after annealing increases, and the TS decreases. Moreover, the grain size of prior austenite becomes coarse, and the YR decreases. Therefore, the heating temperature is preferably 1000°C or lower. Specifically, the heating temperature is set to 780°C or higher. The heating temperature is more preferably 820°C or higher. The heating temperature is still more preferably 830°C or higher. The upper limit of the heating temperature is preferably 1000°C or lower. The upper limit of the heating temperature is more preferably 980°C or lower. The heating temperature is measured based on the surface temperature of the steel sheet.

[Residence time in a temperature range of 750°C or higher and heating temperature or lower: 10 seconds or longer]

In the present disclosure, this is an extremely important constituent factor of the invention. By increasing the residence time in the temperature range of 750°C or higher and the heating temperature or lower, the detachment of Si from the surface of the steel sheet can be facilitated, and the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction can be reduced. Moreover, the segregation of Mn formed during casting can be reduced. The standard deviation of the Vickers hardness of the surface of the steel sheet can be reduced, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet can be reduced. To obtain these effects, the residence time in the temperature range of 750°C or higher and the heating temperature or lower is set to 10 seconds or longer. No particular limitation is imposed on the upper limit of the residence time in the temperature range of 750°C or higher and the heating temperature or lower. However, when the residence time in the temperature range of 750°C or higher and the heating temperature or lower is high, the thickness of the softened surface layer after annealing may increase, and the TS may decrease. Moreover, the grain size of prior austenite may increase, and the YR may decrease. Therefore, the residence time is preferably 400 seconds or shorter. Specifically, the residence time in the temperature range of 750°C or higher and the heating temperature or lower is set to 10 seconds or longer. The residence time in the temperature range of 750°C or higher and the heating temperature or lower is preferably 20 seconds or longer. The residence time in the temperature range of 750°C or higher and the heating temperature or lower is preferably 400 seconds or shorter. The residence time in the temperature range of 750°C or higher and the heating temperature or lower is more preferably 25 seconds or longer. The residence time from 750°C or higher to the heating temperature is more preferably 300 seconds or shorter.

[Concentration of oxygen in atmosphere in the temperature range of 750°C or higher and heating temperature or lower: 0.5% by volume or more and 5.0% by volume or less (preferred condition)]

By increasing the concentration of oxygen in the temperature range of 750°C or higher and the heating temperature or lower during annealing, the detachment of Si from the surface of the steel sheet via oxygen in air is facilitated, and the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction can be reduced. Moreover, the detachment of Mn from the surface of the steel sheet is facilitated, and this allows a reduction in the standard deviation of the Vickers hardness of the surface of the steel sheet and a reduction in the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet. To obtain these effects, the concentration of oxygen in the temperature range of 750°C or higher and the heating temperature or lower is preferably 0.5% by volume or more, more preferably 1.0% by volume or more, and still more preferably 1.5% by volume or more. As the concentration of oxygen in the temperature range of 750°C or higher and the heating temperature or lower increases, the thickness of the softened surface layer after annealing increases, and the TS decreases. Therefore, the concentration of oxygen in the temperature range of 750°C or higher and the heating temperature or lower is preferably 5.0% by volume or less, more preferably 4.5% by volume or less, and still more preferably 4.0% by volume or less. The temperature in the range of 750°C or higher and the heating temperature or lower is based on the surface temperature of the steel sheet. Specifically, when the surface temperature of the steel sheet is 750°C or higher and the heating temperature or lower, the concentration of oxygen is adjusted within the above range.

[Dew point of atmosphere in the temperature range of 750°C or higher and heating temperature or lower: -35°C or higher (preferred condition)]

By increasing the dew point of the atmosphere in the temperature range of 750°C or higher and the heating temperature or lower during annealing, the detachment of Si from the surface of the steel sheet via moisture in air can be facilitated, and the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction can be reduced. Moreover, the detachment of Mn from the surface of the steel sheet is facilitated, and this allows a reduction in the standard deviation of the Vickers hardness of the surface of the steel sheet and a reduction in the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet. To obtain these effects, the dew point in the temperature range of 750°C or higher and the heating temperature or lower is preferably -35°C or higher, more preferably -30°C or higher, and still more preferably -25°C or higher. No particular limitation is imposed on the upper limit of the dew point in the temperature range of 750°C or higher and the heating temperature or lower. However, the dew point in the temperature range of 750°C or higher and the heating temperature or lower is preferably 15°C or lower and more preferably 5°C or lower. Otherwise, the thickness of the softened surface layer after annealing increases, and the TS decreases. The temperature of 750°C or higher and the heating temperature or lower is based on the surface temperature of the steel sheet. Specifically, when the surface temperature of the steel sheet is 750°C or higher and the heating temperature or lower, the dew point is adjusted within the above range.

[Cooling step]

After the annealing step, the cold rolled sheet is optionally cooled. No particular limitation is imposed on the cooling rate in the range of the heating temperature or lower and 400°C or higher. However, the cooling rate is preferably 5°C/s or more and 30°C/s or lower. In the temperature range of the heating temperature or lower and 400°C or higher, the high strength steel sheet may be cooled once, and then the temperature of the steel sheet may be again increased.

[Average cooling rate in temperature range of 250°C or higher and 400°C or lower: 1.0°C/s or more (preferred condition)]

When the average cooling rate in the temperature range of 250°C or higher and 400°C or lower is 1.0°C/s or more, the amount of bainitic ferrite included after annealing can be further reduced, and the YR and bendability can be further improved. The average cooling rate in the temperature range of 250°C or higher and 400°C or lower is preferably 1.0°C/s or more, more preferably 2.0°C/s or more, and still more preferably 3.0°C/s or more. No particular limitation is imposed on the upper limit of the average cooling rate in the temperature range of 250°C or higher and 400°C or lower. However, in view of the limitations on the production technique, the average cooling rate is preferably 100.0°C/s or lower and still more preferably 80.0°C/s or lower. When the cooling stop temperature exceeds 250°C, the average cooling rate is the value in the temperature range of the cooling stop temperature or higher and 400°C or lower. The average cooling rate is measured based on the surface temperature of the steel sheet.
A cooling method used in the temperature range of 250°C or higher and 400°C or lower may be gas jet cooling, mist cooling, water cooling, natural cooling, etc.

[Heat retention temperature in cooling step: 100°C or higher and 450°C or lower (preferred condition)]

Preferably, in the cooling step, heat retention treatment is performed in the temperature range of 100°C or higher and 450°C or lower for 5 seconds or longer. By subjecting the high strength steel sheet to the heat retention treatment in the above range, the YR and bendability can be adjusted within more preferred ranges. Moreover, the area fraction of bainitic ferrite can be further reduced, and the TS can be further improved. The heat retention temperature in the cooling step is more preferably 150°C or higher and still more preferably 200°C or higher. The heat retention temperature in the cooling step is more preferably 400°C or lower and still more preferably 350°C or lower. The temperature in the cooling step is based on the surface temperature of the steel sheet.

[Heat retention time in cooling step: 5 seconds or longer (preferred condition)]

By heat retention at the above-described heat retention temperature in the cooling step, the YR and bendability can be adjusted within more preferred ranges. To obtain this effect, the heat retention time at the heat retention temperature in the cooling step is preferably 5 seconds or longer, more preferably 10 seconds or longer, and still more preferably 15 seconds or longer. No particular limitation is imposed on the upper limit of the heat retention time at the heat retention temperature in the cooling step. To adjust the TS within a more preferred range, the heat retention time at the heat retention temperature in the cooling step is preferably 500 seconds or shorter and more preferably 250 seconds or shorter.

[Cooling stop temperature: 250°C or lower (preferred condition)]

In the cooling step described above, the cooling stop temperature is preferably 250°C or lower and more preferably 200°C or lower. When the cooling stop temperature is 250°C or lower, the formation of a large amount of retained austenite after annealing can be prevented, and the YR and bendability can be further improved. No particular limitation is imposed on the lower limit of the cooling stop temperature. However, from the viewpoint of productivity, the cooling stop temperature is preferably higher than or equal to room temperature. The cooling stop temperature is measured based on the surface temperature of the steel sheet.
No particular limitation is imposed on the average cooling rate to 250°C or lower. However, to further improve the TS, the average cooling rate to 250°C or lower is preferably 1°C/s or more and more preferably 2°C/s or more. In view of the limitations on the production technique, the average cooling rate to 250°C or lower is preferably 1000°C/s or lower and more preferably 150°C/s or lower.
The cold rolled sheet may be further cooled from the cooling stop temperature to room temperature. No particular limitation is imposed on the average cooling rate from the cooling stop temperature to room temperature, and any method may be used to cool the cold rolled sheet to room temperature. Examples of the cooling method that can be used include gas jet cooling, mist cooling, water cooling, and natural cooling.
The cold rolled sheet subjected to the annealing described above and cooled to the cooling stop temperature may be rolled. The elongation rate in the rolling is preferably 0.05% or more and more preferably 0.10% or more. By setting the elongation rate in the rolling performed after the cooling to the cooling stop temperature to 0.05% or more, the YR can be controlled within the desired range. The elongation rate in the rolling is preferably 2.00% or less and more preferably 1.00% or less. By setting the elongation rate in the rolling after the cooling to the cooling stop temperature to 2.00% or less, the volume fraction of retained austenite can be adjusted within a more preferred range. Moreover, the bendability and the degree of damage to sheared end faces in a corrosive environment can be adjusted within more preferred ranges.
The rolling performed after cooling to the cooling stop temperature may be performed in an apparatus connected to the continuous annealing apparatus described above (on-line) or may be performed in an apparatus separated from the continuous annealing apparatus (off-line). The target elongation rate may be achieved by a single rolling pass, or a plurality of rolling passes may be performed to obtain a total elongation rate of 0.05% or more and 2.00% or less. The rolling described above is generally temper rolling. However, any working method such as repeated bending using a tension leveler, or rolls may be used so long as an elongation rate comparable to that obtained by temper rolling can be obtained.

[Reheating temperature: (cooling stop temperature + 50°C) or higher and 450°C or lower (preferred condition)]

After the cooling to the cooling stop temperature or after the rolling performed after the cooling to the cooling stop temperature, the high strength steel sheet may be reheated (reheating step). By reheating the high strength steel sheet, the YR and bendability can be adjusted within more preferred ranges. To obtain this effect, the reheating temperature is preferably (the cooling stop temperature + 50°C) or higher, more preferably (the cooling stop temperature + 100°C) or higher, and still more preferably (the cooling stop temperature + 150°C) or higher. Meanwhile, as the reheating temperature increases, martensite is further tempered, and the TS decreases. Therefore, the reheating temperature is preferably 450°C or lower, more preferably 400°C or lower, and still more preferably 380°C or lower. The reheating temperature is based on the surface temperature of the steel sheet.

[Heat retention time at reheating temperature: 5 seconds or longer (preferred condition)]

By performing heat retention treatment at the reheating temperature, the YR and bendability can be adjusted within more preferred ranges. To obtain this effect, the heat retention time at the reheating temperature is preferably 5 seconds or longer, more preferably 10 seconds or longer, and still more preferably 15 seconds or longer. No particular limitation is imposed on the upper limit of the heat retention time at the reheating temperature. However, to adjust the TS within a more preferred range, the heat retention time at the reheating temperature is preferably 500 seconds or shorter and more preferably 250 seconds or shorter.
The steel sheet may be cooled from the reheating temperature to room temperature. No particular limitation is imposed on the cooling rate from the reheating temperature to room temperature, and any method may be used for the cooling to room temperature. Examples of the cooling method that can be used include gas jet cooling, mist cooling, water cooling, and natural cooling.
When the high strength steel sheet is used as a trade product, the high strength steel sheet is generally cooled and then used as the trade product.

[Method for producing high strength coated steel sheet]

A high strength coated steel sheet can be obtained by subjecting at least one side of the high strength steel sheet produced as described above to coating treatment. Examples of the coating treatment include hot-dip galvanizing treatment and galvannealing treatment performed after the hot-dip galvanizing treatment. The annealing and galvanization may be performed continuously in a single line. Moreover, the coated layer may be formed by electroplating such as Zn-Ni alloy electroplating, or hot-dip zinc-aluminum-magnesium alloy plating may be performed. The galvanization has mainly been described above. However, no particular limitation is imposed on the type of coating metal, and the coating may be Zn coating, Al coating, etc.
When hot-dip galvanizing treatment is performed, it is preferable that the high strength steel sheet is immersed in a galvanizing bath at 440°C or higher and 500°C or lower to perform the hot-dip galvanizing treatment and then the coating weight is controlled by, for example, gas wiping. Preferably, the hot-dip galvanization is performed using a galvanizing bath with an Al content of 0.10% by mass or more and 0.23% by mass or less. The temperature range when galvannealing treatment is performed after the hot-dip galvanization is preferably 470°C or higher and 600°C or lower and more preferably 470°C or higher and 560°C or lower. By performing the galvannealing at 470°C or higher, a more preferred Zn-Fe alloying rate is achieved, and more preferred productivity is achieved. By performing the galvannealing at 600°C or lower, transformation of untransformed austenite to pearlite is prevented, and a more preferred TS is achieved. Electrogalvanizing treatment may be performed. The coating weight per side is preferably 20 to 80 g/m² (double-sided coating). Regarding the hot-dip galvannealed steel sheet (GA), it is preferable that galvannealing described later is performed to adjust the concentration of Fe in the coated layer to 7 to 15% by mass.
Regarding the coating treatment, the high strength steel sheet subjected to the annealing step may be subjected to the coating treatment in the temperature range of the heating temperature or lower and 400°C or higher without cooling the high strength steel sheet, or the cold rolled steel sheet may be cooled to lower than 400°C, reheated such that the temperature of the steel sheet is increased to 400°C or higher, and then subjected to the coating treatment.
The high strength coated steel sheet subjected to the coating treatment may be rolled. The elongation rate in the rolling is preferably 0.05% or more and more preferably 0.10% or more. By setting the elongation rate in the rolling performed after the coating treatment to 0.05% or more, the YR can be controlled within the desired range. The elongation rate in the rolling is preferably 2.00% or less and more preferably 1.00% or less. By setting the elongation rate in the rolling in the coating treatment to 2.00% or less, the volume fraction of retained austenite can be adjusted within a more preferred range, and the bendability and the degree of damage to sheared end faces in a corrosive environment can be adjusted within more preferred ranges.
The rolling performed after the coating treatment may be performed in an apparatus connected to the continuous annealing apparatus described above (on-line) or may be performed in an apparatus separated from the continuous annealing apparatus (off-line). The target elongation rate may be achieved by a single rolling pass, or a plurality of rolling passes may be performed to obtain a total elongation rate of 0.05% or more and 2.00% or less. The rolling described above is generally temper rolling. However, any working method such as repeated bending using a tension leveler, or rolls may be used so long as an elongation rate comparable to that obtained by temper rolling can be obtained. Reheating treatment may be performed after the rolling performed after the coating treatment.
No particular limitation is imposed on the other production conditions. However, from the viewpoint of productivity, it is preferable that the series of treatments including the annealing, hot-dip galvanization, galvannealing treatment, etc. described above is performed in a hot-dip galvanization line known as a CGL (Continuous Galvanizing Line). After the hot-dip galvanization, wiping may be performed in order to adjust the coating weight. The coating conditions other than the conditions described above may be set according to a conventional procedure for hot-dip galvanization.
When the high strength coated steel sheet is used as a trade product, the high strength coated steel sheet is generally cooled to room temperature and then used as the trade product.
Production conditions other than the conditions described above may be set according to a conventional procedure.

[Member]

A member according to an embodiment of the invention will be described.
The member according to the present embodiment of the invention is a member prepared using the high strength steel sheet or high strength coated steel sheet according to the preceding embodiment of the invention. The member according to the present embodiment of the invention is prepared, for example, by forming the high strength steel sheet or high strength coated steel sheet according to the preceding embodiment of the invention into an intended shape by, for example, cold pressing. Therefore, the member has the steel microstructure and properties of the high strength steel sheet, or the high strength coated steel sheet even after forming. The member according to the present embodiment of the invention is preferably used for framework structural parts of automobiles and reinforcing parts of automobiles.
The high strength steel sheet according to the preceding embodiment of the invention is a high strength steel sheet of 1180 MPa or more that has excellent bendability and high resistance to intra-steel sheet cracking in the spot weld HAZ and is usable for the production of parts with high dimensional accuracy. Therefore, the member according to the present embodiment of the invention can contribute to a reduction in vehicle body weight and can be particularly preferably used for framework structural parts of automobiles or variety of members for reinforcing parts of automobiles.

EXAMPLES

Steels having chemical compositions shown in Table 1 with the balance being Fe and unavoidable impurities were produced using a converter and then subjected to continuous casting to obtain slabs. Each of the slabs obtained was cooled, then heated, hot-rolled, subjected to pickling treatment, and then cold-rolled. Table 2 shows the cooling rate after casting, the hot-rolling conditions, the pickling conditions, and the cold-rolling conditions. Unannealed cold rolled steel sheets with a thickness of 1.4 mm were thereby produced. In some Examples, cold rolled steel sheets with different thicknesses were also produced.

[Table 2]

No.	Steel type	Hot rolling step							Cold rolling step		Remarks
		Average slab heating rate from 900 to 1150°C	Slab heating temperature	Residence time from 1100°C to slab heating temperature	Rolling reduction in pass two passes prior to final pass of finish rolling	Rolling reduction in pass immediately preceding final pass of finish rolling	Rolling reduction in final pass of finish rolling	Thickness	Cumulative rolling reduction ratio	Thickness
		(°C/min)	(°C)	(min)	(%)	(%)	(%)	(mm)	(%)	(mm)
1	A	5	1200	40	23	19	11	2.8	50	1.4	Inventive Example
2	A	5	1200	40	23	19	11	2.8	50	1.4	Inventive Example
3	A	5	1200	40	23	19	11	2.8	50	1.4	Inventive Example
4	A	25	1200	40	23	19	11	4.3	35	2.8	Inventive Example
5	A	5	1200	20	23	19	11	4.3	35	2.8	Inventive Example
6	A	5	1200	40	27	19	11	4.3	35	2.8	Inventive Example
7	A	5	1200	40	23	19	11	4.3	35	2.8	Inventive Example
8	A	5	1200	40	23	19	11	4.3	35	2.8	Inventive Example
9	B	8	1300	50	26	18	12	2.3	40	1.4	Inventive Example
10	C	7	1250	50	21	20	11	2.5	45	1.4	Inventive Example
11	C	7	1250	50	21	20	11	2.5	45	1.4	Inventive Example
12	C	7	1250	50	21	20	11	2.5	45	1.4	Inventive Example
13	C	35	1250	50	21	20	11	2.5	45	1.4	Comparative Example
14	C	7	1050	50	21	20	11	2.5	45	1.4	Comparative Example
15	C	7	1250	5	21	20	11	2.5	45	1.4	Comparative Example
16	C	7	1250	50	30	20	11	2.5	45	1.4	Comparative Example
17	C	7	1250	50	30	20	11	5.1	45	2.8	Comparative Example
18	C	7	1250	50	21	20	7	2.5	45	1.4	Comparative Example
19	C	7	1250	50	21	20	11	1.6	10	1.4	Comparative Example
20	C	7	1250	50	21	20	11	9.3	85	1.4	Comparative Example
21	C	7	1250	50	21	20	11	18.7	85	2.8	Comparative Example
22	C	7	1250	50	21	20	11	2.5	45	1.4	Comparative Example
23	C	7	1250	50	21	20	11	2.5	45	1.4	Comparative Example
24	C	7	1250	50	21	20	11	2.5	45	1.4	Comparative Example
25	C	7	1200	50	21	20	11	2.5	45	1.4	Inventive Example
26	C	7	1200	50	21	20	11	2.5	45	1.4	Inventive Example
27	C	7	1200	50	21	20	11	2.5	45	1.4	Inventive Example
28	D	15	1200	150	25	19	12	3.1	55	1.4	Inventive Example
29	E	10	1250	40	25	19	13	3.1	55	1.4	Comparative Example
30	F	10	1250	40	25	19	13	3.1	55	1.4	Comparative Example
31	G	10	1250	40	22	19	13	3.1	55	1.4	Comparative Example
32	H	10	1250	40	22	19	13	3.1	55	1.4	Comparative Example
33	I	10	1250	40	22	19	13	3.1	55	1.4	Comparative Example
34	J	10	1250	40	22	19	13	3.1	55	1.4	Comparative Example
35	K	10	1250	40	22	19	13	3.1	55	1.4	Comparative Example
36	L	25	1250	100	24	18	12	2.8	50	1.4	Inventive Example
37	M	20	1150	100	21	20	11	2.8	50	1.4	Inventive Example
38	N	2	1200	20	23	19	10	2.8	50	1.4	Inventive Example
39	O	15	1200	40	27	17	13	2.8	50	1.4	Inventive Example
40	P	15	1200	40	24	21	9	2.8	50	1.4	Inventive Example
41	Q	10	1200	100	25	18	13	1.9	25	1.4	Inventive Example
42	R	10	1200	20	25	16	11	4.7	70	1.4	Inventive Example
43	S	8	1200	40	21	21	11	1.8	20	1.4	Inventive Example
44	T	8	1200	100	21	16	13	3.1	55	1.4	Inventive Example
45	U	8	1200	50	25	17	11	3.1	55	1.4	Inventive Example
46	V	3	1200	50	23	18	12	2.8	50	1.4	Inventive Example
47	W	5	1250	40	24	17	12	3.1	55	1.4	Inventive Example
48	X	5	1250	50	22	19	11	3.1	55	1.4	Inventive Example
49	Y	3	1250	50	21	18	14	3.1	55	1.4	Inventive Example
50	Z	5	1250	100	23	18	13	3.1	55	1.4	Inventive Example

Underline: Outside the corresponding range.

[Table 3]

No.	Steel type	Annealing step					Cooling step				Reheating step		Coating treatment condition	Remarks
		Average heating rate from 250 to 700°C	Residence time from 750°C to heating temperature	Heating temperature	Oxygen concentration from 750°C to heating temperature	Dew point from 750°C to heating temperature	Average cooling rate from 250 to 400°C	Heat retention temperature	Heat retention time	Cooling stop temperature	Reheating temperature	Heat retention time at reheating temperature	Type
		(°C/s)	(s)	(°C)	(% by volume)	(°C)	(°C/s)	(°C)	(s)	(°C)	(°C)	(s)
1	A	20	30	950	2.0	-17	14.7	-	-	50	350	50	CR	Inventive Example
2	A	20	30	950	2.0	-17	14.7	300	70	-	-	-	CR	Inventive Example
3	A	20	30	950	2.0	-17	14.7	150	120	-	-	-	CR	Inventive Example
4	A	20	30	950	2.0	-17	14.7	150	120	-	-	-	CR	Inventive Example
5	A	20	30	950	2.0	-17	14.7	150	120	-	-	-	CR	Inventive Example
6	A	20	30	950	2.0	-17	14.7	150	120	-	-	-	CR	Inventive Example
7	A	20	30	950	6.0	-17	14.7	150	120	-	-	-	CR	Inventive Example
8	A	20	30	950	2.0	+5	14.7	150	120	-	-	-	CR	Inventive Example
9	B	40	100	850	2.5	-11	9.8	-	-	80	250	100	GA	Inventive Example
10	C	20	60	890	3.0	-5	5.0	-	-	Room temperature	300	60	GA	Inventive Example
11	C	20	60	890	3.0	-5	5.0	350	60	-	-	-	GA	Inventive Example
12	C	20	60	890	3.0	-5	5.0	200	100	-	-	-	GA	Inventive Example
13	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
14	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
15	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
16	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
17	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
18	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
19	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
20	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
21	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
22	C	120	60	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
23	C	20	5	890	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
24	C	20	60	750	2.0	-5	5.0	-	-	Room temperature	300	60	GA	Comparative Example
25	C	20	60	890	2.0	-5	0.5	-	-	Room temperature	300	60	GA	Inventive Example
26	C	20	60	890	2.0	-5	5.0	-	-	300	300	60	GA	Inventive Example
27	C	20	60	890	2.0	-5	5.0	-	-	Room temperature	500	60	GA	Inventive Example
28	D	18	30	850	1.5	-10	9.8	-	-	Room temperature	300	60	GA	Inventive Example
29	E	18	60	880	3.0	-20	3.0	-	-	5.0	200	90	GA	Comparative Example
30	F	18	60	880	3.0	-20	3.0	-	-	5.0	200	90	GA	Comparative Example
31	G	18	60	880	3.0	-20	3.0	-	-	50	200	90	GA	Comparative Example
32	H	18	60	880	3.0	-20	3.0	-	-	5.0	200	90	GA	Comparative Example
33	I	18	60	880	3.0	-20	3.0	-	-	5.0	200	90	GA	Comparative Example
34	J	18	60	880	3.0	-20	3.0	-	-	5.0	200	90	GA	Comparative Example
35	K	18	60	880	2.0	-20	3.0	-	-	Room temperature	200	90	GA	Comparative Example
36	L	16	100	850	1.5	-25	10.1	-	-	5.0	250	5.0	EG	Inventive Example
37	M	23	100	825	2.0	-16	15.1	-	-	Room temperature	300	100	GA	Inventive Example
38	N	5	30	830	3.0	-16	10.3	-	-	80	250	100	GA	Inventive Example
39	O	24	300	840	2.5	-16	29.8	-	-	Room temperature	250	100	GA	Inventive Example
40	P	15	30	850	4.0	-30	14.5	-	-	80	300	50	GA	Inventive Example
41	Q	14	200	830	3.0	-25	15.2	-	-	100	300	100	GA	Inventive Example
42	R	14	60	870	2.0	-24	30.3	-	-	50	250	100	GA	Inventive Example
43	S	80	60	990	3.5	-13	9.9	-	-	50	250	100	GA	Inventive Example
44	T	10	15	900	3.0	-15	7.9	-	-	50	350	30	CR	Inventive Example
45	U	50	100	800	2.5	-20	8.4	-	-	50	250	100	CR	Inventive Example
46	V	50	60	850	6.0	-15	3.1	-	-	150	250	100	GA	Inventive Example
47	W	50	60	920	0.2	-16	10.1	-	-	50	450	100	GA	Inventive Example
48	X	50	60	880	2.5	-38	15.1	-	-	50	450	5	GI	Inventive Example
49	Y	45	150	880	2.5	+5	15.1	-	-	50	450	5	GI	Inventive Example
50	Z	30	150	850	2.0	-10	10.3	-	-	50	250	100	GA	Inventive Example

Underline: Outside the corresponding range.

Next, annealing, cooling, and reheating were performed under the conditions shown in Tables 2 and 3 to thereby obtain high strength cold rolled steel sheets (CRs). Some of the steel sheets were subjected to coating treatment to thereby obtain hot-dip galvanized steel sheets (GIs), hot-dip galvannealed steel sheets (GAs), and an electrogalvanized steel sheet (EG). The hot-dip galvanizing bath used for the GIs was a zinc bath containing Al: 0.14 to 0.19% by mass, and the hot-dip galvanizing bath used for the GAs was a zinc bath containing Al: 0.14% by mass. The temperature of the baths was 470°C. The coating weight per side for the GIs was about 45 to 72 g/m² (double-sided coating), and the coating weight per side for the GAs was about 45 g/m² (double-sided coating). In the GAs, the concentration of Fe in the coated layer was 9% by mass or more and 12% by mass or less. In the EG in which the coated layer was a Zn-Ni coated layer, the content of Ni in the coated layer was 9% by mass or more and 25% by mass or less.

The thus-obtained high strength cold rolled steel sheets and high strength coated steel sheets were used as steel samples, and the tensile properties, bendability, resistance to intra-steel sheet cracking in the spot weld HAZ were evaluated using test methods described later. The results are shown in Table 4.

[Table 4]

No.	Steel type	Area fraction of M	Sum of area fraction of α and/or volume fraction of retained γ	Remaining microstructure	Concentration of Si at position 5 µm from surface of steel sheet in thickness direction	Number density of MnS inclusions present in Mn segregated regions on surface of steel sheet	Standard deviation of Vickers hardness of surface of steel sheet	Hardness variation frequency per 1100 µm in sheet width direction on surface of steel sheet	YS	TS	YR	Bendability	Resistance to intra-steel sheet cracking in spot weld HAZ	Remarks
No.	Steel type	(%)	(%)	Remaining microstructure	(%)	(pieces/mm²)		(frequency)	(MPa)	(MPa)	(%)	(µm)	Resistance to intra-steel sheet cracking in spot weld HAZ	Remarks
1	A	92	7	θ	0.44	2.5	8	4	1287	1570	82	158	A	Inventive Example
2	A	93	6	θ	0.44	2.5	9	4	1192	1509	79	142	A	Inventive Example
3	A	93	6	θ	0.44	2.5	10	4	1148	1594	72	177	A	Inventive Example
4	A	92	6	θ	0.33	4.1	13	6	1317	1587	83	192	B	Inventive Example
5	A	92	4	θ	0.56	4.3	15	7	1339	1575	85	181	B	Inventive Example
6	A	93	7	θ	0.57	3.2	14	6	1325	1577	84	196	B	Inventive Example
7	A	93	4	θ	0.59	2.6	11	2	1221	1471	83	84	A	Inventive Example
8	A	91	9	θ	0.43	2.6	9	5	1105	1473	75	128	A	Inventive Example
9	B	95	4	θ	0.55	3.3	12	5	1465	1744	84	76	B	Inventive Example
10	C	88	10	θ	0.57	0.8	3	5	1027	1181	87	102	B	Inventive Example
11	C	88	11	θ	0.57	0.8	6	5	930	1240	75	108	B	Inventive Example
12	C	89	7	θ	0.57	0.8	6	5	881	1224	72	149	B	Inventive Example
13	C	89	7	θ	0.62	5.8	18	9	1000	1205	83	272	C	Comparative Example
14	C	89	8	θ	0.63	7.5	20	8	1041	1269	82	330	C	Comparative Example
15	C	89	8	θ	0.62	6.7	17	12	1060	1277	83	265	C	Comparative Example
16	C	88	7	θ	0.57	0.8	18	9	1006	1257	80	271	B	Comparative Example
17	C	89	8	θ	0.55	0.8	19	10	1012	1249	81	239	C	Comparative Example
18	C	89	7	θ	0.63	6.7	5	5	1069	1243	86	209	C	Comparative Example
19	C	70	27	θ	0.62	0.8	5	5	700	1147	61	57	C	Comparative Example
20	C	75	23	θ	0.57	0.8	3	5	707	1219	58	200	B	Comparative Example
21	C	77	22	θ	0.55	0.8	4	5	746	1204	62	199	C	Comparative Example
22	C	80	19	θ	0.63	0.8	6	5	768	1181	65	194	C	Comparative Example
23	C	89	7	θ	0.62	0.8	19	10	1124	1263	89	299	C	Comparative Example
24	C	66	33	θ	0.63	0.8	21	8	716	1155	62	258	C	Comparative Example
25	C	81	19	θ	0.57	0.8	6	5	781	1184	66	192	B	Inventive Example
26	C	84	15	θ	0.57	0.8	4	5	788	1212	65	187	B	Inventive Example
27	C	88	8	θ	0.57	0.8	4	5	1066	1184	90	100	B	Inventive Example
28	D	96	2	θ	0.60	3.3	12	5	1631	1989	82	95	B	Inventive Example
29	E	20	76	θ	0.47	0.8	6	5	504	813	62	175	B	Comparative Example
30	F	82	17	θ	2.01	0.8	5	2	898	1213	74	196	C	Comparative Example
31	G	18	80	θ	0.53	0.8	5	6	338	614	55	164	B	Comparative Example
32	H	87	9	θ	0.48	0.8	19	9	966	1255	77	261	B	Comparative Example
33	I	88	9	θ	0.30	5.8	6	5	801	1272	63	278	A	Comparative Example
34	J	88	8	θ	0.63	0.8	5	6	1158	1259	92	71	C	Comparative Example
35	K	88	10	θ	0.79	7.5	6	5	751	1252	60	275	C	Comparative Example
36	L	92	8	θ	0.32	4.2	14	7	1315	1547	85	192	A	Inventive Example
37	M	91	4	θ	0.51	5.0	15	6	1222	1472	83	193	B	Inventive Example
38	N	90	9	θ	0.57	4.2	14	6	1214	1445	84	181	B	Inventive Example
39	O	94	4	θ	0.56	3.3	13	6	1458	1757	83	196	B	Inventive Example
40	P	93	5	θ	0.59	4.2	10	6	1304	1590	82	195	B	Inventive Example
41	Q	81	17	θ	0.49	2.5	8	4	958	1474	65	101	B	Inventive Example
42	R	82	17	θ	0.55	3.3	12	4	1321	2002	66	184	B	Inventive Example
43	S	84	15	θ	0.54	3.3	11	5	1104	1672	66	192	B	Inventive Example
44	T	92	3	θ	0.11	2.5	13	7	1351	1518	89	200	A	Inventive Example
45	U	84	15	θ	0.44	2.5	13	6	971	1494	65	186	A	Inventive Example
46	V	91	5	θ	0.60	2.5	10	2	1220	1470	83	84	A	Inventive Example
47	W	91	7	θ	0.59	0.0	14	7	1414	1589	89	194	B	Inventive Example
48	X	92	7	θ	0.34	3.3	15	7	1420	1595	89	185	B	Inventive Example
49	Y	91	8	θ	0.44	2.5	8	4	1133	1471	77	128	A	Inventive Example
50	Z	94	5	θ	0.18	3.3	12	6	1430	1663	86	163	A	Inventive Example

Underline: Outside the corresponding range.
M: martensite, α: ferrite and/or bainitic ferrite, retained γ: retained austenite, θ: cementite and/or metastable carbides

[Tensile test]

The tensile test was performed according to JIS Z 2241:2022. A JIS No. 5 test specimen was cut from one of the obtained steel sheets such that the test specimen was orthogonal to the rolling direction of the steel sheet. The tensile test was performed under the condition of a crosshead speed of 1.67×10^-1 mm/s to measure the YS and TS. In the present invention, when the yield ratio (YR) was 65% or more and 90% or less, the dimensional accuracy was considered high. The YR was calculated using the calculation method described in formula (2) above.

[Bending test]

The bending test was performed according to JIS Z 2248:2022. A strip-shaped test specimen having a width of 30 mm and a length of 100 mm was cut from one of the obtained steel sheets such that a direction parallel to the rolling direction of the steel sheet was aligned with the axial direction of the bending test. Then a 90° V-bending test was performed under the conditions of a pressing load of 100 kN and a press-holding time of 5 seconds. In the present disclosure, the bending test was performed on five samples, with the bending radius (R) set such that the value of R/t obtained by dividing the bending radius (R) by the sheet thickness (t) was about 4.5, i.e., 4.3 to 4.7. Next, for all the five samples, the length of a crack in a ridge portion at the apex of the bend was evaluated. When the crack length was 200 µm or less, the bendability was considered excellent. The crack length was evaluated by performing the measurement on the ridge portion at the apex of the bend using a digital microscope (RH-2000 manufactured by HIROX CO., LTD.) at a magnification of 40 to 160X.

[Test for intra-steel sheet cracking in spot weld HAZ]

A test specimen with a thickness of 1.4 mm, a length of 30 mm, and a width of 100 mm was cut out such that its lengthwise direction coincided with the rolling direction and placed on a hot-dip galvanized steel sheet for the test having the same size as the cut test specimen and including a hot-dip galvanized layer with a coating weight per side of 50 g/m² to thereby prepare a sheet set. Next, a servo motor pressing-type single-phase AC (50 Hz) resistance welder and electrodes with a tip radius of 6 mm were used to perform resistance welding while the sheet set was inclined 5° with respect to the electrodes of the resistance welder and a clearance of 1.5 mm was provided between the lower electrode and the lower steel sheet. Specifically, the sheet set was resistance-welded under the welding conditions of a welding pressure of 3.5 kN and a holding time of 0.16 seconds or 0.20 seconds while the welding current and the welding time were adjusted such that the nugget had a diameter of 5.9 mm, and the sheet set with a weld was thereby obtained. Next, the sheet set with the weld was cut into halves such that the weld was included in the cross sections, and one of the cross sections of the weld was observed under an optical microscope (200X). The resistance to intra-steel sheet cracking in the spot weld HAZ was evaluated according to the criteria described above. The same evaluation was also performed on samples with thicknesses of 0.8 mm and 2.3 mm.
The area fraction of martensite, the area fraction of ferrite, the volume fraction of retained austenite, the concentration of Si at a position 5 µm from the surface of the steel sheet in the thickness direction, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet were determined by the methods described above. Moreover, the standard deviation of the Vickers hardness of the surface of the steel sheet and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet were determined. As for the remaining microstructures, observation was performed using the following method. A sample was cut from one of the steel sheets such that a thicknesswise cross section (L cross section) parallel to the rolling direction of the steel sheet served as the observation surface, and the observation surface was mirror-polished using a diamond paste. Then the resulting observation surface was etched with 3 vol.% nital solution to make the microstructure visible. A position 1/4 of the thickness of the steel sheet was used as an observation position, and an SEM was used to observe three viewing fields each having a viewing area of 17 µm × 23 µm at a magnification of 5000X under the condition of an acceleration voltage of 15 kV. Carbides in the obtained microstructure images were identified as remaining microstructures.
As shown in Table 4, in Inventive Examples, the TS, YR, bendability, and resistance to intra-steel sheet cracking in the spot weld HAZ were excellent. In Comparative Examples, at least one of the TS, YR, bendability, and resistance to intra-steel sheet cracking in the spot weld HAZ was poor.
The embodiments of the invention have been described. However, the invention is not limited to the description of the embodiments that forms part of the disclosure of the invention. Specifically, other embodiments, Examples, operating techniques, etc. made based on the embodiments by those skilled in the art are all within the scope of the invention. For example, in the series of heat treatments in the production method described above, no particular limitation is imposed on the facilities etc. used to heat-treat the steel sheet, so long as the thermal history conditions are satisfied.

Industrial Applicability

According to the present invention, a high strength steel sheet of 1180 MPa or more can be obtained, which has excellent bendability and high resistance to intra-steel sheet cracking in the spot weld HAZ and can be used to produce parts with high dimensional accuracy.
In particular, the high strength steel sheet of the invention has high resistance to intra-steel sheet cracking in the spot weld HAZ and can therefore be used for automobile framework structural parts with different sizes and shapes while high component strength is ensured. This allows a reduction in vehicle body weight, and the fuel economy can thereby be improved, so that the high strength steel sheet is extremely valuable for industrial applications.

Claims

A high strength steel sheet having a chemical composition containing, in % by mass,
C: 0.030% or more and 0.500% or less,

Si: 0.01% or more and 2.50% or less,

Mn: 0.10% or more and 5.00% or less,

P: 0.100% or less,

S: 0.0200% or less,

Al: 0.100% or less,

N: 0.0100% or less,

O: 0.0100% or less, and

Ti: 0.002% or more and 0.200% or less,

with the balance being Fe and incidental impurities,

wherein an effective Ti mole fraction (x_Ti,eff) determined by formula (1) below is 0.001 or more,

wherein the high strength steel sheet has, at a position 1/4 of a thickness, a steel microstructure

in which an area fraction of martensite is 80% or more and 99% or less and

in which a sum of an area fraction of ferrite and/or a volume fraction of retained austenite is more than 0% and 20% or less,

wherein, at a position 5 µm from a surface of the steel sheet in a thickness direction, a concentration of Si is 0.10% or more and 0.60% or less,

wherein a number density of MnS present in Mn-segregated regions on the surface of the steel sheet is 5.0 pieces/mm² or less,

wherein a standard deviation of Vickers hardness of the surface of the steel sheet is 15 or less, and

wherein a hardness variation frequency per 1100 µm in a sheet width direction on the surface of the steel sheet is 7 or less: $x_{Ti, eff} = x_{Ti} - x_{N} - x_{S},$

where x_Ti, x_N, and x_S are contents (mole fractions) of respective elements in the steel sheet.
The high strength steel sheet according to claim 1, wherein the chemical composition further contains, in % by mass, at least one element selected from
Nb: 0.200% or less,

V: 0.200% or less,

Ta: 0.10% or less,

W: 0.10% or less,

B: 0.0100% or less,

Cr: 1.00% or less,

Mo: 1.00% or less,

Ni: 1.00% or less,

Co: 0.010% or less,

Cu: 1.00% or less,

Sn: 0.200% or less,

Sb: 0.200% or less,

Ca: 0.0100% or less,

Mg: 0.0100% or less,

REMs: 0.0100% or less,

Zr: 0.100% or less,

Te: 0.100% or less,

Hf: 0.10% or less, and

Bi: 0.200% or less.
A high strength coated steel sheet comprising: the high strength steel sheet according to claim 1 or 2; and a coated layer on at least one side of the high strength steel sheet.
A method for producing a high strength steel sheet, the method comprising:
heating a steel slab having the chemical composition according to claim 1 or 2 to a slab heating temperature of 1150°C or higher at an average slab heating rate of 25°C/min or less in a temperature range of 900°C or higher and 1150°C or lower such that a residence time in a range of from 1100°C to the slab heating temperature is 20 minutes or longer;

subsequently, hot-rolling the resulting steel slab to obtain a hot rolled sheet, wherein a rolling reduction in a final pass of finish rolling is set to 9% or more and 15% or less, wherein a rolling reduction in a pass immediately preceding the final pass is set to 15% or more and 21% or less, and wherein a rolling reduction in a pass two passes prior to the final pass is set to 21% or more and 27% or less;

subsequently, pickling the hot rolled sheet to obtain a pickled sheet;

subsequently, cold-rolling the pickled sheet at a cumulative rolling reduction ratio of 20% or more and 75% or less to thereby obtain a cold rolled sheet; and

then performing an annealing step of heating the cold rolled sheet to a heating temperature of 780°C or higher such that an average heating rate in a temperature range of 250°C or higher and 700°C or lower is set to 100°C/s or lower and that a residence time in the temperature range of 750°C or higher and the heating temperature or lower is set to 10 seconds or longer.
The method for producing a high strength steel sheet according to claim 4, wherein, in the annealing step, a concentration of oxygen in an atmosphere in the temperature range of 750°C or higher and the heating temperature or lower is 0.5% by volume or more and 5.0% by volume or less, and wherein the atmosphere has a dew point of -35°C or higher.
The method for producing a high strength steel sheet according to claim 4 or 5, the method further comprising, after the annealing step, performing a cooling step such that an average cooling rate in a temperature range of 250°C or higher and 400°C or lower is 1.0°C/s or more.
The method for producing a high strength steel sheet according to claim 4 or 5, the method further comprising, after the annealing step, performing a cooling step including heat retention at a heat retention temperature of 100°C or higher and 450°C or lower for 5 seconds or longer.
The method for producing a high strength steel sheet according to claim 4 or 5, the method further comprising, after the annealing step: performing a cooling step with a cooling stop temperature set to 250°C or lower; and then reheating the resulting cold rolled sheet to a reheating temperature of (the cooling stop temperature + 50°C) or higher and 450°C or lower to perform heat retention at the reheating temperature for 5 seconds or longer.
A method for producing a high strength coated steel sheet, the method comprising, after the annealing step according to any one of claims 4 to 8, performing a coating step of subjecting at least one side of the cold rolled sheet to coating treatment.
A member comprising the high strength steel sheet according to claim 1 or 2, the high strength steel sheet being used for at least part of the member.
A member comprising the high strength coated steel sheet according to claim 3, the high strength coated steel sheet being used for at least part of the member.