WO2024252887A1 - High strength steel sheet, high strength plated steel sheet, methods for producing same, and member - Google Patents

Abstract

The purpose of the present invention is to provide: a high strength steel sheet and a high strength plated steel sheet which have a strength of 1180 MPa or more, which have excellent bendability and inter-steel-sheet cracking characteristics at spot-welding resistant parts HAZ, and from which it is possible to manufacture components with high dimensional accuracy; methods for producing the high strength steel sheet and the high strength plated steel sheet; and a member.　This high strength steel sheet has a prescribed component composition, has an effective Ti molar fraction (x_{Ti, eff}) of 0.001 or more as determined from x_{Ti, eff} = x_Ti-x_N-x_S (wherein each element symbol represents the molar fraction of a corresponding element in steel), and has a steel structure in which the area percentage of martensite is 80-99% and the total of the area percentage of ferrite and/or the volume percentage retained austenite is more than 0% but not more than 20% at a 1/4-position in the sheet thickness, the Si concentration at a 5-µm positon in the sheet thickness direction from the steel sheet surface is 0.10-0.60%, the number density of MnS present in Mn-segregated portions in the surface of the steel sheet is 5.0 pieces/mm² or less, the standard deviation of Vickers hardness in the surface of the steel sheet is 15 or less, and the hardness fluctuation frequency per 1100 µm in the sheet width direction in the surface of the steel sheet is seven times or less.

Description

High-strength steel sheet, high-strength plated steel sheet, their manufacturing method, and component

This disclosure relates to high-strength steel sheets, high-strength plated steel sheets, their manufacturing methods, and components that are optimal for automotive reinforcing parts and frame structural parts.

In order to achieve both reduction in _CO2 emissions by reducing the weight of vehicles and improvement in crashworthiness by reducing the weight of the vehicle body, efforts are being made to increase the strength of steel sheets for automobiles. In addition, new legal regulations are being introduced one after another. Therefore, in order to increase the strength of the vehicle body, the number of cases in which high-strength steel sheets are applied to the main structural parts and reinforcing parts that form the framework of the automobile cabin (hereinafter also referred to as framework structural parts of automobiles) is increasing. In particular, the number of cases in which high-strength steel sheets with a tensile strength (hereinafter also referred to simply as TS) of 1180 MPa or more is applied is increasing.

High-strength steel sheets used in automotive reinforcing parts and frame structural parts are required to have excellent formability. Furthermore, the formed parts are required to have excellent dimensional accuracy. For example, parts such as crash boxes have punched end faces and bent parts, so from the viewpoint of formability, steel sheets with high stretch flangeability and bendability are suitable. From the viewpoint of part performance, increasing the yield ratio of the steel sheet (YR = yield strength YS / tensile strength TS) increases the impact absorption energy during a collision. Furthermore, from the viewpoint of dimensional accuracy of parts, controlling the yield ratio (YR) of the steel sheet within a certain range makes it possible to suppress springback after steel sheet forming and control the dimensional accuracy of parts. In order to increase the application ratio of high-strength steel sheets to automotive parts, it is required to comprehensively satisfy these characteristics.

Furthermore, among the structural components of automobiles, for example crash boxes have bent sections. Furthermore, in order to increase the application rate of high-strength steel sheets to parts with a relatively small bending radius, there is a demand for high-strength steel sheets with high bendability that does not cause large cracks in bent sections where the value R/t obtained by dividing the bending radius (R) by the plate thickness (t) is less than 5.0.

More recently, it has been confirmed that when high-strength galvanized steel sheets are spot welded, the zinc in the coating layer diffuses into the grain boundaries of the steel sheet surface, causing liquid metal embrittlement (LME). As a result, LME cracks occur in the HAZ of the spot weld. Cracks in the HAZ of the spot weld can occur in high-strength steel sheets that do not have a zinc coating layer, as long as the welding partner is a zinc-coated steel sheet, so this is becoming a problem for all high-strength steel sheets. During mass production, steel sheets of various thicknesses are spot welded, and the hold time required to suppress cracks in the HAZ of the spot weld increases as the sheet thickness increases. However, an increase in the hold time leads to a decrease in productivity, so there is a demand for high-strength steel sheets that can suppress cracks in the HAZ of the spot weld even with a short hold time.

In response to these demands, for example, Patent Document 1 provides a high-strength steel plate with a tensile strength of 980 MPa or more that has excellent bendability and LME resistance properties and can be used to manufacture parts with high dimensional accuracy.

Patent No. 6787535

The high-strength steel plate described in Patent Document 1 satisfies overall requirements for bendability and LME resistance, and makes it possible to manufacture parts with high dimensional accuracy. However, the high-strength steel plate described in Patent Document 1 has a TS of 980 MPa, leaving room for further improvement in strength.

This disclosure was developed in consideration of these circumstances, and aims to obtain a high-strength steel plate of 1,180 MPa or more that has excellent bendability and inter-steel plate cracking resistance in the HAZ of spot welds and can be used to manufacture parts with high dimensional accuracy, as well as to provide an advantageous method for manufacturing the high-strength steel plate.

In this disclosure, being able to manufacture parts with high dimensional accuracy (having high dimensional accuracy during molding) means that the yield ratio (YR) is 65% or more and 90% or less. YR can be calculated by the following formula (2).
YR=YS/TS×100...(2)
Regarding bendability, a bending test is performed by a V-block method with a bending angle of 90 degrees, and five samples are subjected to bending tests at an R where the value R/t obtained by dividing the bending radius (R) by the plate thickness (t) is about 4.5, i.e., 4.3 to 4.7. Next, the length of the crack at the ridgeline of the bend apex of all five samples is evaluated, and if the crack length is 200 μm or less, it is determined that the bendability is excellent.

Regarding the inter-steel-plate cracking resistance of the spot weld HAZ, the cross section of the weld described in the examples was observed with an optical microscope (200x), and the inter-steel-plate cracking resistance of the spot weld HAZ was evaluated according to the following criteria: If it was A or B, it was judged that the spot weld HAZ had excellent inter-steel-plate cracking resistance; if it was C, it was judged that the spot weld HAZ had poor inter-steel-plate cracking resistance.
A: No cracks longer than 0.1 mm were observed with a hold time of 0.16 seconds.
B: A crack having a length of 0.1 mm or more is observed with a hold time of 0.16 seconds, but no crack having a length of 0.1 mm or more is observed with a hold time of 0.20 seconds.
C: A crack having a length of 0.1 mm or more is observed at a hold time of 0.20 seconds.

Here, the hold time refers to the time from when the welding current stops flowing to when the electrode starts to be released.

Means for Solving the Problems The present inventors have conducted extensive research to achieve the above object, and as a result have obtained the following findings.
(1) By making the microstructure mainly comprise martensite (quenched martensite and tempered martensite) and further containing ferrite and/or retained austenite, it is possible to realize a YR, which is an index of dimensional accuracy of a part, of 65% or more and 90% or less.
(2) The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is 0.10% to 0.60%, the number density of MnS present in the Mn segregated portion of the steel sheet surface is 5.0 pieces/mm2 ^or less, and the standard deviation of the Vickers hardness of the steel sheet surface is 15 or less. Furthermore, the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is 7 times or less, thereby realizing good bendability.
(3) By setting the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction to 0.10% or more and 0.60% or less, good resistance to inter-steel sheet cracking in the HAZ of the spot weld can be achieved.

The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows.
[1] In 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, and an effective Ti mole fraction (x _Ti,eff ) calculated by the following formula (1) is contained. a steel structure in which, at a 1/4 position in the sheet thickness direction, an area ratio of martensite is 80% or more and 99% or less, and an area ratio of ferrite and/or a volume ratio of retained austenite is more than 0% and 20% or less in total; a Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is 0.10% or more and 0.60% or less; a number density of MnS present in Mn segregated portions on the steel sheet surface is 5.0 pcs/mm2 or ^less ; a standard deviation of Vickers hardness on the steel sheet surface is 15 or less; and a hardness fluctuation frequency per 1,100 μm in the sheet width direction on the steel sheet surface is 7 or less.
Note x _{Ti, eff} = x _Ti -x _N -x _S ... (1)
In the formula, x _Ti , x _N and x _S represent the content (molar fraction) of each element in the steel sheet.
[2] The high-strength steel plate according to [1], further comprising at least one element selected from the group consisting of, by mass%, 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, REM: 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 plated steel sheet having a plating layer on at least one side of the high-strength steel sheet according to [1] or [2].
[4] A steel slab having the composition according to [1] or [2] is heated at an average heating rate of 25°C/min or less in a temperature range of 900°C to 1150°C, a slab heating temperature of 1150°C or more, and a residence time from 1100°C to the slab heating temperature of 20 min or more. Then, the steel slab is subjected to a finish rolling process with a final pass reduction of 9% to 15%, a pass one pass before the final pass reduction of 15% to 21%, and a pass two passes before the final pass reduction of 20% to 21%. A method for producing a high strength steel sheet, comprising the steps of: hot rolling a steel sheet having a rolling reduction ratio of 1% to 27% to obtain a hot rolled sheet; pickling the hot rolled sheet to obtain a pickled sheet; cold rolling the pickled sheet with a cumulative rolling reduction ratio of 20% to 75% to obtain a cold rolled sheet; and annealing the cold rolled sheet to a heating temperature of 780°C or higher, wherein the average heating rate in a temperature range of 250°C to 700°C is 100°C/s or less and the residence time from 750°C to the heating temperature is 10s or more.
[5] The method for producing a high-strength steel plate according to [4], wherein the oxygen concentration of the atmosphere at a temperature of 750 ° C. or higher and the heating temperature or lower in the annealing step is 0.5 vol.% or higher and 5.0 vol.% or lower, and the dew point of the atmosphere is −35 ° C. or higher.
[6] The method for producing a high-strength steel plate according to [4] or [5], in which a cooling step is further performed following the annealing step, and in which the average cooling rate in the temperature range of 250°C or more and 400°C or less is 1.0°C/s or more.
[7] The method for producing a high-strength steel plate according to [4] or [5], further comprising a cooling step following the annealing step, in which the steel sheet is kept at a heat retention temperature of 100°C or higher and 450°C or lower for 5s or more.
[8] The annealing step is followed by a cooling step, in which the cooling stop temperature is set to 250°C or less, and the cold-rolled sheet is then reheated to a reheating temperature of (the cooling stop temperature + 50°C) or more and 450°C or less, and kept at the reheating temperature for 5s or more. [4] or [5]. A method for producing a high-strength steel sheet.
[9] A method for producing a high-strength plated steel sheet, comprising: performing a plating process on at least one surface of the cold-rolled sheet after the annealing process according to any one of [4] to [8].
[10] A member at least in part using the high-strength steel plate according to [1] or [2].
[11] A member at least partially made of the high-strength plated steel sheet according to [3].

This disclosure makes it possible to provide high-strength steel plates and components of 1180 MPa or more that have excellent bendability and inter-steel cracking resistance in the HAZ of spot welds, and that can be used to manufacture parts with high dimensional accuracy.

The following describes embodiments of the present disclosure. Note that the present disclosure is not limited to the following embodiments.

First, we will explain the appropriate range of the composition of the steel plate and the reasons for its limitation. In the following explanation, "%" representing the content of the component elements of the steel plate means "mass %" unless otherwise specified. Furthermore, in this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits.

[C: 0.030% or more and 0.500% or less]
C is one of the important basic components of steel. In particular, in the present disclosure, the area ratio of martensite and ferrite, the volume ratio of retained austenite, the standard deviation of Vickers hardness on the steel sheet surface, and the plate hardness on the steel sheet surface are It is an important element that affects the frequency of hardness fluctuation per 1100 μm in the width direction. If the C content is less than 0.030%, the area ratio of martensite decreases, and the area ratio of ferrite increases, resulting in a hardness fluctuation of 1180 MPa. It becomes difficult to realize the above TS. Also, it becomes difficult to realize the desired YR. On the other hand, when the C content exceeds 0.500%, the hardness distribution of martensite in the sheet width direction becomes As a result, the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the frequency of hardness fluctuations per 1,100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in poor bendability. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably 0.080% or more. The C content is preferably 0.400% or less. The C content is more preferably 0.110% or more. The content 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 steel, and in particular, in the present disclosure, Si is an element that suppresses the formation of carbides during annealing and promotes the formation of retained austenite, thereby affecting the volume fraction of retained austenite. In addition, since Si exhibits a large resistance to tempering softening at temperatures below 400°C, it is an important element that affects the standard deviation of the Vickers hardness of the steel sheet surface and the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface. If the Si content is less than 0.01%, the hardness distribution of martensite in the sheet width direction becomes non-uniform. As a result, the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15. On the other hand, when the Si content exceeds 2.50%, the hardness fluctuation frequency at a position 5 μm from the steel sheet surface in the sheet thickness direction exceeds 7 times, and the bendability is deteriorated. Since the Si concentration in the steel sheet increases, the inter-steel sheet cracking resistance of the spot weld HAZ decreases. Therefore, the Si content is set to 0.01% or more and 2.50% or less. The Si content is preferably set to 0.20% or more. The Si content is preferably set to 2.00% or less. The Si content is more preferably 0.25% or more, and 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 steel. In particular, in the present disclosure, the area ratio of martensite and ferrite, the volume ratio of retained austenite, the standard deviation of Vickers hardness on the steel sheet surface, and the plate hardness on the steel sheet surface are used. It is an important element that affects the frequency of hardness fluctuation per 1100 μm in the width direction. If the Mn content is less than 0.10%, the area ratio of martensite decreases, and the area ratio of ferrite increases, resulting in a hardness of 1180 MPa. On the other hand, if the Mn content exceeds 5.00%, the standard deviation of the Vickers hardness of the steel sheet surface becomes The hardness fluctuation frequency per 1,100 μm in the sheet width direction on the surface of the steel sheet exceeds 7 times, and the bendability is deteriorated. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably 1.00% or more. The Mn content is preferably 4.00% or less, more preferably 2.00% or more, and more preferably 3.50% or less.

[P: 0.100% or less]
If P is excessive, it segregates at the prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, resulting in reduced bendability. Therefore, the P content is set to 0.100% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to set the P content to 0.001% or more. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.001% or more, and more preferably set to 0.070% or less.

[S: 0.0200% or less]
S exists as sulfide and reduces the ultimate deformability of steel, which reduces its bendability. Therefore, the S content must be 0.0200% or less. Although there is no particular lower limit for S, due to limitations in production technology, it is preferable that the S content be 0.0001% or more. Therefore, the S content is set to 0.0200% or less. S Content The content of S is preferably 0.0050% or less.

[Al: 0.100% or less]
If Al is excessive, the _A3 transformation point rises and a large amount of ferrite is included in the microstructure, making it difficult to achieve the desired YR. Therefore, the Al content is set to 0.100%. Although there is no particular lower limit for the Al content, the Al content is set to 0.001% or less because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. Therefore, the Al content is set to 0.100% or less. The Al content is preferably set to 0.001% or more. The Al content is preferably set to 0.050% or less. do.

[N: 0.0100% or less]
N exists as a nitride and reduces the ultimate deformability of the steel sheet, which reduces the bendability. Therefore, the N content must be 0.0100% or less. Although there is no particular lower limit for N, due to limitations in production technology, the N content is preferably 0.0005% or more. Therefore, the N content is set to 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 exists as an oxide and reduces the ultimate deformability of the steel sheet, which reduces the bendability. Therefore, the O content must be 0.0100% or less. Although there is no particular lower limit for O, due to restrictions in production technology, the O content is preferably 0.0001% or more. Therefore, the O content is set to 0.0100% or less. O Content The content of O is preferably 0.0050% or less.

[Ti: 0.002% or more and 0.200% or less]
Ti increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In addition, the addition of Ti increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. The number density of MnS can be reduced. To obtain this effect, the Ti content must be 0.002% or more. On the other hand, when the Ti content exceeds 0.200%, If the Ti content is too high, the amount of carbides, nitrides, or carbonitrides increases, making it difficult to achieve a desired YR. Therefore, the Ti content is set to 0.002% or more and 0.200% or less. The Ti content is preferably 0.006% or more. The Ti content is preferably 0.100% or less. The Ti content is more preferably 0.010% or more. The content is more preferably 0.050% or less.

[The effective Ti mole fraction (x _Ti,eff ) calculated from the formula (1) is 0.001 or more]
Effective Ti mole fraction (x _Ti,eff )=x _Ti −x _N −x _S (1)
In the formula, x _Ti , x _N and x _S represent the content (molar fraction) of each element in the steel sheet.
By setting the effective Ti molar fraction calculated from the above formula (1) to a certain level or more, the number density of MnS present in the Mn segregation portion on the steel sheet surface can be reduced, and good bendability can be achieved. In order to obtain such an effect, the effective Ti molar fraction is set to 0.001% or more. Although the upper limit of the effective Ti molar fraction is not particularly specified, the amount of carbides, nitrides or carbonitrides increases, making it difficult to achieve the desired YR, so it is preferable that the effective Ti molar fraction is set to 0.040 or less. Therefore, the effective Ti molar fraction is set to 0.001% or more. The effective Ti molar fraction is preferably set to 0.002% or more. The effective Ti molar fraction is preferably set to 0.040% or less. The molar fraction is a molar fraction obtained by converting the components contained as mass% into molar fractions.

The high-strength steel plate according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities. Preferably, the high-strength steel plate according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities. Here, examples of unavoidable impurities include Zn, Pb, and As. These impurities are permissible to be contained as long as their total content is 0.100% or less.

In addition to the essential components described above, the composition of the high-strength steel plate disclosed herein may further contain, 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, REM: 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, either alone or in combination.

[Nb: 0.200% or less]
Nb generates a large amount of coarse precipitates and inclusions, which reduces the ultimate deformability of the steel sheet. Therefore, if the Nb content exceeds 0.200%, the bendability is reduced. The Nb content is set to 0.200% or less. Although the lower limit of the Nb content is not particularly specified, by setting the Nb content to 0.001% or more, fine grains can be easily formed during hot rolling or continuous annealing. By forming suitable carbides, nitrides, or carbonitrides, the strength of the steel sheet can be increased and the YR can be controlled within a desired range. For this reason, the Nb content is set to 0.001% or more. Therefore, when Nb is added, the Nb content is set to 0.200% or less. The Nb content is preferably set to 0.001% or more. The Nb content is preferably set to 0.100% or less. Let us assume that.

[V: 0.200% or less]
V generates a large amount of coarse precipitates and inclusions, which reduces the ultimate deformability of the steel sheet. Therefore, if the V content exceeds 0.200%, the bendability is reduced. The V content is set to 0.200% or less. Although the lower limit of the V content is not particularly specified, by setting the V content to 0.001% or more, fine grains can be easily formed during hot rolling or continuous annealing. By forming suitable carbides, nitrides, or carbonitrides, the strength of the steel sheet can be increased and the YR can be controlled within a desired range. For this reason, the V content is set to 0.001% or more. Therefore, when V is added, the V content is set to 0.200% or less. The V content is preferably set to 0.001% or more. The V content is preferably set to 0.100% or less. Let us assume that.

[Ta: 0.10% or less, W: 0.10% or less]
If the Ta and W contents exceed 0.10%, large amounts of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel sheet, and therefore the bendability is reduced. The contents of Ta and W are each set to 0.10% or less. The lower limits of the contents of Ta and W are not particularly specified, but fine carbides, nitrides or carbonitrides may be generated during hot rolling or continuous annealing. Therefore, the Ta and W contents are preferably 0.01% or more. The Ta and W contents are each preferably 0.01% or more, and 0.08% or less.

[B: 0.0100% or less]
If the content of B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not decrease, so that the bendability will not decrease. The amount of B is preferably 0.0100% or less. Although there is no particular lower limit for the B content, since B is an element that segregates at the austenite grain boundaries during annealing and improves hardenability, the amount of B is preferably 0.0100% or less. The content of B is more preferably 0.0003% or more. Therefore, when B is contained, its content is 0.0100% or less. The content of B is more preferably 0.0003% or more. The B content is more preferably 0.0080% or less.

[Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less]
If the content of Cr, Mo and Ni exceeds 1.00%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, thereby reducing the bendability. The Ni content is set to 1.00% or less. The lower limits of the Cr, Mo and Ni contents are not particularly specified, but since these elements improve hardenability, the Cr, Mo and Ni contents are set to 1.00% or less. The content of each of is preferably 0.01% or more. Therefore, when added, the content of each of Cr, Mo and Ni is 1.00% or less. The contents of Cr, Mo and Ni are as follows: The content of Cr, Mo and Ni is preferably 0.01% or more. The content of Cr, Mo and Ni is preferably 0.80% or less.

[Co: 0.010% or less]
If the Co content exceeds 0.010%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, thereby reducing the bendability. The lower limit of the Co content is not particularly specified, but since Co is an element that improves hardenability, the Co content is preferably 0.001% or more. Therefore, when Co is added, the Co content is set to 0.010% or less, preferably 0.001% or more, and preferably 0.008% or less. .

[Cu: 1.00% or less]
If the Cu content exceeds 1.00%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is preferably 0.01% or more. Therefore, when added, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.01% or more. The Cu content is preferably set to 0.80% or less. .

[Sn: 0.200% or less]
If the Sn content exceeds 0.200%, cracks are generated inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet is reduced, resulting in reduced bendability. The Sn content is set to 0.200% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability, the Sn content is set to 0.001% or more. Therefore, when Sn is added, the Sn content is set to 0.200% or less. The Sn content is preferably set to 0.001% or more. The Sn content is preferably set to 0. The percentage shall be 100% or less.

[Sb: 0.200% or less]
If the Sb content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. The lower limit of the Sb content is not particularly specified, but since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is set to 0.001 % or more. Therefore, when Sb is added, the Sb content is 0.200% or less. The Sb content is preferably 0.001% or more. The Sb content is preferably shall be 0.100% or less.

[Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less]
If the content of Ca, Mg and REM exceeds 0.0100%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. The contents of Ca, Mg and REM are each set to 0.0100% or less. Although there is no particular lower limit for the contents of Ca, Mg and REM, these elements are effective in spheroidizing the shape of nitrides and sulfides and in preventing the formation of the steel sheet. Since these elements improve the ultimate deformability of steel, the contents of Ca, Mg and REM are preferably 0.0005% or more. Therefore, when added, the contents of Ca, Mg and REM are The content of Ca, Mg and REM is preferably 0.0005% or more. The content of Ca, Mg and REM is preferably 0.0050% or less. In addition, REM (rare earth elements) is a collective term for 15 elements ranging from Sc, Y, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content here refers to the total amount of these elements. The total content of the elements.

[Zr: 0.100% or less, Te: 0.100% or less]
If the Zr and Te contents are each more than 0.100%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, thereby reducing the bendability. The lower limits of the Zr and Tellurium contents are not particularly specified, but they are elements that make the shape of nitrides and sulfides spheroidal and improve the ultimate deformability of the steel sheet. Therefore, the content of Zr and the content of Te are preferably 0.001% or more. Therefore, when added, the content of Zr and the content of Te are 0.100% or less. The content of Zr and the content of Te are each preferably 0.080% or less.

[Hf: 0.10% or less]
If the Hf content exceeds 0.10%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. The lower limit of the Hf content is not specified, but since Hf is an element that makes the shape of nitrides and sulfides spheroidal and improves the ultimate deformability of the steel sheet, the Hf content is set to 10% or less. The content of Hf is preferably 0.01% or more. Therefore, when Hf is added, the content of Hf is 0.10% or less. The content of Hf is preferably 0.01% or more. The amount is preferably 0.08% or less.

[Bi: 0.200% or less]
If the Bi content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, thereby reducing the bendability. The Bi content is not particularly limited, but since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more. When Bi is added, the Bi content is set to 0.200% or less, preferably 0.001% or more, and preferably 0.100% or less.

Note that the above-mentioned Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi do not impair the effects of the present invention when their respective contents are below the preferred lower limit values. Therefore, they are considered to be included as unavoidable impurities.

Next, we will explain the steel structure of steel plates.

[Area ratio of martensite: 80% or more and 99% or less]
If the area ratio of martensite is less than 80%, the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve a desired YR. Furthermore, the hardness distribution in the sheet width direction is made non-uniform, so that the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in a decrease in bendability. On the other hand, if the area ratio of martensite exceeds 99%, ferrite and/or retained austenite effective for controlling the YR does not exist in the steel structure, making it difficult to achieve a desired YR. Therefore, the area ratio of martensite is set to 80% or more and 99% or less. The area ratio of martensite is preferably set to 85% or more. The area ratio of martensite is preferably set to 98% or less. The area ratio of martensite is more preferably set to 87% or more. The area ratio of martensite is preferably set to 97% or less. The martensite referred to here includes tempered martensite and bainite in addition to quenched martensite (fresh martensite). The observation position for the area ratio of martensite is a quarter position in the sheet thickness of the steel sheet, as described later.

[Total of area ratio of ferrite and/or volume ratio of retained austenite: more than 0% and not more than 20%]
When the total of the area ratio of ferrite and/or the volume ratio of retained austenite is 0%, the steel structure becomes a martensite single phase structure, so that it is difficult to realize the desired YR. On the other hand, when the total of the area ratio of ferrite and/or the volume ratio of retained austenite exceeds 20%, the area ratio of martensite decreases, so that it is difficult to realize a TS of 1180 MPa or more. It is also difficult to realize the desired YR. Furthermore, since the hardness distribution in the sheet width direction is made non-uniform, the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and furthermore, the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, so that the bendability is reduced. Therefore, the total of the area ratio of ferrite and/or the volume ratio of retained austenite is more than 0% and 20% or less. The total of the area ratio of ferrite and/or the volume ratio of retained austenite is preferably 1% or more. The area ratio of martensite is preferably 18% or less. The area ratio of martensite is more preferably 2% or more. The area ratio of martensite is preferably 15% or less. Note that the ferrite referred to here includes bainitic ferrite. Note that the observation position for the area ratio of ferrite and the volume ratio of retained austenite is a quarter position of the sheet thickness of the steel sheet, as described later.

The method for measuring the area ratio of martensite (quenched martensite, tempered martensite, and bainite) and ferrite (bainitic ferrite) is as follows.

After cutting out the sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate is the observation surface, the observation surface is mirror-polished using diamond paste, and then etched with 3 vol. % nital to reveal the structure. Using a scanning electron microscope (SEM) with an acceleration voltage of 15 kV, the observation position is set to 1/4 of the plate thickness of the steel plate, and three fields of view are observed at a magnification of 5000 times and a field of view of 17 μm x 23 μm. The area ratio of each constituent structure divided by the measured area is calculated for the three fields of view using Adobe Photoshop from Adobe Systems Inc. Here, each constituent structure means ferrite (bainitic ferrite), martensite (tempered martensite, bainite, and quenched martensite). These values are averaged to determine the area ratio of each structure. In the above structure image, ferrite (bainitic ferrite) is a flat structure that does not contain carbides in the recessed portions, while tempered martensite and bainite are structures that contain fine carbides in the recessed portions. Hardened martensite is a structure that is a convex portion and has fine irregularities inside the structure, and they are distinguishable from each other. Note that tempered martensite, tempered martensite, and bainite do not need to be distinguishable from each other, since the total area ratio is calculated as the area ratio of martensite.

The method for measuring the volume fraction of retained austenite is as follows:

After grinding, the steel plate is polished by chemical polishing for a further 0.1 mm so that the observation surface is located 1/4 of the plate thickness from the surface (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface). For this surface, an X-ray diffraction device is used with a Co Kα source to measure the integrated reflection intensity of the (200), (220), and (311) surfaces of fcc iron (austenite) and the (200), (211), and (220) surfaces of bcc iron. The volume fraction of austenite is calculated from the intensity ratio of the integrated reflection intensity from each surface of fcc iron (austenite) to the integrated reflection intensity from each surface of bcc iron, and this is taken as the volume fraction of retained austenite.

[Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction: 0.10% or more and 0.60% or less]
In the present disclosure, this is an extremely important invention constituent element. In order to control the inter-steel plate crack resistance property of the spot welded HAZ, it is important to control the concentration of elements at a position 5 μm from the steel plate surface in the plate thickness direction. In particular, by reducing the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction, the inter-steel plate crack resistance property of the spot welded HAZ can be improved. In order to obtain such an effect, it is necessary to make the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction 0.60% or less. On the other hand, if the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction is less than 0.10%, the hardness distribution of martensite in the plate width direction becomes uneven. Therefore, the standard deviation of the Vickers hardness of the steel plate surface exceeds 15, and further, the frequency of hardness fluctuation per 1100 μm in the plate width direction on the steel plate surface exceeds 7 times, and the bendability is reduced. Therefore, the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction is set to 0.10% or more and 0.60% or less. The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is preferably 0.15% or more. The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is preferably 0.55% or less. The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is more preferably 0.20% or more. The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is preferably 0.50% or less. The unit of this Si concentration is mass%.

The method for measuring the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction is as follows.

A sample measuring 20 mm in the rolling direction and 20 mm in the width direction is cut from the steel plate. The surface of the high-strength steel plate is used as the measurement surface, and measurements are made using glow discharge optical emission spectrometry (GDS). The Si concentration is analyzed along the plate thickness direction under conditions of a high-frequency discharge pressure of 300 Pa, a high-frequency output of 35 W, and a pulse frequency of 100 Hz. The Si concentration at a position 5 μm from the steel plate surface is averaged and calculated as the Si concentration at a position 5 μm from the plate thickness surface. The measurement data is converted to Si concentration using the calibration curve method.

[Number density of MnS present in Mn segregation part on steel sheet surface: 5.0 pieces/ ^mm2 or less]
In the present disclosure, this is an extremely important invention constituent element. In order to control the bendability, it is important to control the number density of inclusions present in the Mn segregation part of the steel sheet surface, that is, the area containing martensite that is harder than the surroundings. By reducing the number density of MnS present in the Mn segregation part of the steel sheet surface, the bendability can be improved. In order to obtain such an effect, it is necessary to make the number density of MnS present in the Mn segregation part of the steel sheet surface 5.0 pieces/ ^mm2 or less. In addition, the lower limit of the number density of MnS present in the Mn segregation part of the steel sheet surface is not particularly limited, but the lower the number density of MnS, the more preferable it is, and even if it is 0.0 pieces/ ^mm2 , the effect of the present disclosure can be obtained. Therefore, the number density of MnS present in the Mn segregation part of the steel sheet surface is 5.0 pieces/ ^mm2 or less. The number density of MnS present in the Mn segregation part of the steel sheet surface is preferably 0.0 pieces/ ^mm2 or more. The number density of MnS present in the Mn segregated portion on the steel sheet surface is preferably 4.0 pieces/mm2 or ^less .

The method for measuring the number density of MnS present in the Mn segregated area on the steel sheet surface is as follows.

A bending test is performed by the V-block method with a bending angle of 90 degrees, and a bending test is performed at R where R/t is about 4.5, i.e., 4.3 to 4.7. A sample of 20 mm in the rolling direction and 5 mm in the width direction is cut to include the crack at the ridge of the bend apex. The surface on the outside of the bend is used as the observation surface, and the observation surface is mirror-polished using diamond paste. Then, the measurement is performed using an electron probe micro analyzer (EPMA; Electron Probe Micro Analyzer) (JXA-8230: manufactured by JEOL Ltd.). At this time, Mn and S are measured in three fields of view under the conditions of acceleration voltage: 15 kV, measurement area: rolling direction 1.2 mm x width direction 1.0 mm, and irradiation current: 1.0 x 10 ^-7 A. The measurement data is converted to C concentration by a calibration curve method. Then, when the obtained three visual fields are mapped with Mn element, the places where Mn is detected in large amounts are identified as Mn segregation parts. Furthermore, by mapping with S element, S concentrated parts in the Mn segregation parts, that is, MnS, are identified and their number is evaluated. The number of MnS present in the obtained Mn segregation parts is divided by the measurement area of 1.2 ^mm2 to calculate the number density of MnS present in the Mn segregation parts on the steel sheet surface.

[Standard deviation of Vickers hardness of steel plate surface: 15 or less]
This is an extremely important invention constituent element in the present disclosure. In order to control the bendability, it is important to make the hardness distribution of the steel sheet surface uniform, and by reducing the standard deviation of the Vickers hardness of the steel sheet surface, the bendability can be improved. In order to obtain such an effect, it is necessary to make the standard deviation of the Vickers hardness of the steel sheet surface 15 or less. Although the lower limit of the standard deviation of the Vickers hardness of the steel sheet surface is not particularly limited, the lower the standard deviation of the Vickers hardness of the steel sheet surface, the more preferable it is, and even if it is 0, the effect of the present disclosure can be obtained. Therefore, the standard deviation of the Vickers hardness of the steel sheet surface is 15 or less. The standard deviation of the Vickers hardness of the steel sheet surface is preferably 0 or more. The standard deviation of the Vickers hardness of the steel sheet surface is preferably 13 or less.

The method for measuring the standard deviation of the Vickers hardness of the steel plate surface is as follows:

A bending test is performed using the V-block method with a bending angle of 90 degrees, with an R/t of approximately 4.5, i.e., 4.3 to 4.7. A sample of 20 mm in the rolling direction and 5 mm in the width direction is cut to include the crack at the ridge of the bend apex. The outer surface of the bend is used as the observation surface, and the observation surface is mirror-polished using diamond paste. Next, the Vickers hardness of the observation surface after mirror polishing is measured at 11 points along the plate width direction at 100 μm intervals under a load of 100 gf using a Vickers hardness tester. The measurement position is 500 μm away from the end of the crack in the rolling direction, with the sixth point being parallel to the crack. The standard deviation is calculated from the obtained results to calculate the standard deviation of the Vickers hardness of the steel plate surface.

[Hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface: 7 times or less]
This is an extremely important invention constituent element in the present disclosure. By reducing the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface, the desired bendability can be achieved. In order to obtain such an effect, it is necessary to make the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface 7 times or less. The lower limit of the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is not particularly limited, but the lower the hardness fluctuation frequency, the more preferable it is, and even if it is 0 times, the effect of the present disclosure can be obtained. Therefore, the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is 7 times or less. The hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is preferably 0 times or more. The hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is preferably 6 times or less.

Here, the frequency of hardness variation per 1,100 μm in the width direction on the steel plate surface is as follows:

A bending test is performed by the V-block method with a bending angle of 90 degrees, and a bending test is performed at an R where R/t is about 4.5, i.e., 4.3 to 4.7. A sample of 20 mm in the rolling direction and 5 mm in the width direction is cut to include the crack at the ridge of the bend apex. The surface on the outside of the bend is used as the observation surface, and the observation surface is mirror-polished using diamond paste. Next, the Vickers hardness of 11 points is measured along the plate width direction at 100 μm intervals with a Vickers hardness tester under a load of 100 gf for the observation surface after mirror polishing. The measurement position is a position 500 μm away from the end of the crack in the rolling direction, and the sixth point is measured parallel to the crack. From the obtained results, a hardness distribution is created by measuring the surface of the steel plate in the plate width direction with a Vickers hardness tester. In the hardness distribution, the value of {(maximum hardness Hv _max )-(minimum hardness Hv _min )}/2 is first calculated. The value of {(maximum hardness Hv _max )-(minimum hardness Hv _min )}/2 is used as the standard amount of fluctuation, and each time the hardness fluctuates above or below the standard amount of fluctuation, it is counted as one occurrence, and the number of times the hardness fluctuates in the area where the hardness was measured (length 1100 μm) is measured. In other words, when the hardness fluctuates above or below the standard amount of fluctuation, it is counted as one occurrence, and when the hardness fluctuates below the standard amount of fluctuation, it is counted as one occurrence. Therefore, when the hardness fluctuates above and below once each, the hardness fluctuation frequency is two occurrences.

Furthermore, the steel structure according to the present disclosure may contain residual structures other than the above-mentioned martensite (quenched martensite, tempered martensite, bainite), ferrite (including bainitic ferrite), and retained austenite. The effect of the present disclosure is not impaired even if the residual structures include pearlite, cementite, metastable carbides, and other carbides known as steel plate structures, as long as the area ratio is within a range of 5% or less. Here, metastable carbides include, for example, epsilon (ε) carbides, eta (η) carbides, chi (χ) carbides, etc.

[High-strength steel plate]
The chemical composition and steel structure of the high strength steel plate are as described above. The plate thickness of the high strength steel plate is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less.

[High-strength plated steel sheet]
The high-strength plated steel sheet of the present disclosure is a high-strength plated steel sheet having a plating layer on at least one side of the high-strength steel sheet of the present disclosure. The type of plating layer is not particularly limited, and may be, for example, either a hot-dip plating layer or an electroplating layer. The plating layer may also be an alloyed plating layer. The plating layer is preferably a zinc plating layer. The zinc plating layer may contain Al and Mg. Hot-dip zinc-aluminum-magnesium alloy plating (Zn-Al-Mg plating layer) is also preferred. In this case, it is preferred that the Al content is 1 mass% or more and 22 mass% or less, the Mg content is 0.1 mass% or more and 10 mass% or less, and the balance is Zn. In addition, in the case of the Zn-Al-Mg plating layer, in addition to Zn, Al, and Mg, one or more selected from Si, Ni, Ce, and La may be contained in a total of 1 mass% or less. Note that the plating metal is not particularly limited, and may be Al plating or the like in addition to the above-mentioned Zn plating.

The composition of the plating layer is not particularly limited, and may be any common one. For example, in the case of a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, it generally contains Fe: 20 mass% or less, and Al: 0.001 mass% or more and 1.0 mass% or less. Furthermore, it contains one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% or more and 3.5 mass% or less, with the balance being Zn and unavoidable impurities. In the present disclosure, it is preferable to have a hot-dip galvanized layer with a plating coating weight of 20 to 80 g/m ² per side, and an alloyed hot-dip galvanized layer further alloyed therewith. In addition, when the plating layer is a hot-dip galvanized layer, the Fe content in the plating layer is less than 7 mass%, and when the plating layer is an alloyed hot-dip galvanized layer, the Fe content in the plating layer may be 7 to 20 mass%.

Next, we will explain the manufacturing method for the high-strength steel plate disclosed herein.

First, a steel material having the above-mentioned composition is melted to produce a steel slab. In the present disclosure, the method of melting the steel material is not particularly limited, and any known melting method such as a converter or electric furnace is suitable. In addition, the steel slab is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method or a thin slab casting method. In addition to the conventional method of cooling the steel slab to room temperature after production and then reheating it, energy-saving processes such as direct rolling and direct rolling, in which the slab is loaded into a heating furnace without being cooled, or is rolled immediately after a short period of heat retention, can also be applied without any problems.

Then, the steel slab is heated at an average heating rate of 25°C/min or less in the temperature range of 900°C to 1150°C, a slab heating temperature of 1150°C or more, and a residence time from 1100°C to the slab heating temperature of 20 minutes or more.

[Average heating rate of slab in temperature range of 900° C. to 1150° C.: 25° C./min or less]
This is an extremely important invention constituent element in the present disclosure. By slowing down the average heating rate of the slab in the temperature range of 900°C to 1150°C to 25°C/min or less, it is possible to promote the desorption of Si from the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, by slowing down the average heating rate of the slab in the temperature range of 900°C to 1150°C, it is possible to reduce the Mn segregation formed during casting and reduce the number density of MnS present in the Mn segregation part on the steel sheet surface. Furthermore, it is possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the average heating rate of the slab in the temperature range of 900°C to 1150°C is set to 25°C/min or less. Although the lower limit of the average heating rate of the slab in the temperature range of 900°C to 1150°C is not particularly specified, it is preferably 5°C/min or more in order to suitably prevent an increase in the softened thickness of the surface layer after annealing and to bring TS within a more suitable range. Therefore, the average heating rate of the slab in the temperature range of 900°C to 1150°C is 25°C/min or less. The average heating rate of the slab in the temperature range of 900°C to 1150°C is preferably 5°C/min or more. The average heating rate of the slab in the temperature range of 900°C to 1150°C is preferably 15°C/min or less. The slab heating temperature is the surface temperature of the steel slab during slab heating.

[Slab heating temperature: 1150°C or higher]
In the present disclosure, this is an extremely important invention constituent element. By increasing the slab heating temperature to 1150°C or more, it is possible to promote the desorption of Si from the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, by increasing the slab heating temperature, the Mn segregation formed during casting is reduced, and the number density of MnS present in the Mn segregation part on the steel sheet surface is reduced. In addition, it is possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the slab heating temperature is set to 1150°C or more. Note that the upper limit of the slab reheating temperature is not particularly specified, but it is preferable to set it to 1300°C or less in order to suitably prevent an increase in the surface layer softening thickness after annealing and to set TS within a more suitable range. Therefore, the slab heating temperature is set to 1150°C or more. The slab heating temperature is preferably set to 1180°C or more. The slab heating temperature is preferably set to 1300°C or less. The slab heating temperature refers to the temperature of the surface of the steel slab during heating.

[Dwell time from 1100 ° C. to the slab heating temperature: 20 min or more]
This is an extremely important invention constituent element in the present disclosure. By lengthening the residence time from 1100 ° C. to the slab heating temperature to 20 min or more, it is possible to promote the desorption of Si from the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, by lengthening the residence time from 1100 ° C. to the slab heating temperature, it is possible to reduce the Mn segregation formed during casting and reduce the number density of MnS present in the Mn segregation part on the steel sheet surface. By lengthening the residence time from 1100 ° C. to the slab heating temperature, it is possible to further reduce the standard deviation of the Vickers hardness of the steel sheet surface and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the residence time from 1100 ° C. to the slab heating temperature is set to 20 min or more. Although there is no particular upper limit for the residence time from 1100°C to the slab heating temperature, it is preferable to set it to 100 min or less in order to preferably prevent an increase in the softened thickness of the surface layer after annealing and to set TS within a more preferable range. Therefore, the residence time from 1100°C to the slab heating temperature is 20 min or more. The residence time from 1100°C to the slab heating temperature is preferably 30 min or more. The residence time from 1100°C to the slab heating temperature is preferably 100 min or less. The slab heating temperature is the surface temperature of the steel slab during slab heating.

The slab is made into a sheet bar by rough rolling under normal conditions, but if the slab heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[Reduction rate of the final pass of finish rolling: 9% to 15%]
[Reduction rate of the pass before the final pass: 15% to 21%]
[Reduction rate of the pass two passes before the final pass: 21% or more and 27% or less]
This is an extremely important invention constituent element in the present disclosure. The reduction ratio of the pass one before the final pass is set to be equal to or greater than the reduction ratio of the final pass, and the reduction ratio of the pass two passes before the final pass is set to be equal to or greater than the reduction ratio of the pass one before the final pass. This makes it possible to appropriately control the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction, the standard deviation of the Vickers hardness of the steel sheet surface, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface.

If the reduction rate of the final pass of the finish rolling is less than 9%, the austenite grain size on the steel sheet surface during hot rolling becomes coarse, i.e., the crystal grain size in the annealed sheet becomes coarse, and the diffusion of Si to the steel sheet surface is suppressed. As a result, it is not possible to reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, since Ti precipitation, i.e., sulfide precipitation, is suppressed, it is not possible to reduce the number density of MnS present in the Mn segregated part of the steel sheet surface. On the other hand, if the reduction rate of the final pass exceeds 15%, it is not possible to reduce the Mn segregation formed during casting. Therefore, it is not possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface or the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction rate of the final pass of the finish rolling is set to 9% or more and 15% or less.

If the reduction rate before the final pass is less than 15%, the austenite grain size on the steel sheet surface during hot rolling becomes coarse, i.e., the crystal grain size in the annealed sheet becomes coarse, and the diffusion of Si to the steel sheet surface is suppressed. As a result, it is not possible to reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, since Ti precipitation, i.e., sulfide precipitation, is suppressed, it is not possible to reduce the number density of MnS present in the Mn segregated part on the steel sheet surface. On the other hand, if the reduction rate before the final pass exceeds 21%, it is not possible to reduce the Mn segregation formed during casting. Therefore, it is not possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface or the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction rate before the final pass of finish rolling is set to 15% to 21%.

If the reduction rate of the second pass before the final pass is less than 21%, the austenite grain size on the steel sheet surface during hot rolling becomes coarse, i.e., the crystal grain size in the annealed sheet becomes coarse, and the diffusion of Si to the steel sheet surface is suppressed. As a result, it is not possible to reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, since Ti precipitation, i.e., sulfide precipitation, is suppressed, it is not possible to reduce the number density of MnS present in the Mn segregated part of the steel sheet surface. On the other hand, if the reduction rate of the second pass before the final pass exceeds 27%, it is not possible to reduce the Mn segregation formed during casting. Therefore, it is not possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface or the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction rate of the second pass before the final pass of finish rolling is set to 21% to 27%.

Finish rolling is preferably performed at a finish rolling temperature of the Ar3 transformation point or higher, since the rolling load increases, the reduction rate in the unrecrystallized state of _austenite increases, and abnormal structures elongated in the rolling direction develop, which may result in a decrease in the workability of the annealed sheet. In addition, the coiling temperature after hot rolling is preferably 300°C or higher and 700°C or lower in order to improve the workability after annealing. The _Ar3 transformation point temperature is calculated by the following formula.
Ar ₃ transformation point (°C) = 868-396×[%C]+24.6×[%Si]-68.1×[%Mn]-36.1×[%Ni]-20.7×[%Cu]-24.8×[%Cr]
In the above formula, the symbol [% element] represents the content (mass %) of the corresponding element in the above composition, and is set to 0 when the corresponding element is not contained.

The rough rolled sheets may be joined together during hot rolling and continuously finished rolling may be performed. The rough rolled sheets may also be wound up once. To reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniformity of the steel sheet shape and material quality. The friction coefficient during lubricated rolling is preferably 0.10 or more and 0.25 or less.

The hot-rolled steel sheet produced in this manner is then pickled. Pickling is capable of removing oxides from the steel sheet surface, and is therefore important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet product. Pickling may be performed once or multiple times.

Then, the hot-rolled sheet after pickling, or the hot-rolled sheet (hot-rolled annealed sheet) that has been optionally heat-treated after pickling, is cold-rolled to produce a cold-rolled sheet. Since strain is introduced uniformly and efficiently and a uniform structure is obtained, it is preferable to perform cold rolling by multi-pass rolling that requires two or more passes, such as tandem multi-stand rolling or reverse rolling.

In addition, it is preferable to perform bending and bending back at least once each before cold rolling in order to introduce processing strain into the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction during annealing. There are no particular restrictions on the number of times each of bending and bending back is performed before cold rolling, but it is preferable to perform it at least twice, and more preferably three times or more. In addition, bending and bending back before cold rolling is generally performed using rolls with a roll diameter of 300 to 1500 mm.

[Cumulative reduction rate of cold rolling: 20% or more and 75% or less]
By increasing the cumulative reduction rate of cold rolling, the area ratio of ferrite can be reduced, that is, the total of the area ratio of ferrite and/or the volume ratio of retained austenite can be set to 20% or less. In addition, by increasing the cumulative reduction rate of cold rolling, a large amount of shear bands can be introduced into the steel sheet surface, and the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction can be reduced. In order to obtain such an effect, the cumulative reduction rate of cold rolling is set to 20% or more. On the other hand, if the cumulative reduction rate of cold rolling exceeds 75%, the grain size of austenite generated during annealing becomes fine, and the amount of retained austenite in the annealed sheet increases. In other words, the total area ratio of ferrite and/or the volume ratio of retained austenite increases, so that the desired YR cannot be realized. In addition, the hardness distribution in the sheet width direction is made non-uniform, so that the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in a decrease in bendability. Therefore, the cumulative reduction ratio of the cold rolling is set to 20% or more and 75% or less. The cumulative reduction ratio of the cold rolling is preferably set to 25% or more. The cumulative reduction ratio of the cold rolling is preferably set to 70% or less. The cumulative reduction ratio of the cold rolling is more preferably set to 27% or more. The cumulative reduction ratio of the cold rolling is more preferably set to 60% or less.

The cold-rolled sheet obtained as described above is then subjected to an annealing process. The annealing conditions are as follows:

[Average heating rate in the temperature range of 250° C. to 700° C.: 100° C./s or less]
By lowering the average heating rate in the temperature range of 250°C to 700°C, Si diffuses to the steel sheet surface, and the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction can be reduced. In order to obtain such an effect, it is necessary to set the average heating rate in the temperature range of 250°C to 700°C to 100°C/s or less. The lower limit of the average heating rate in the temperature range of 250°C to 700°C is not particularly specified, but from the viewpoint of suppressing coarsening of the austenite grain size during heating and optimizing the YR, it is preferably 5°C/s or more, and more preferably 10°C/s or more. Therefore, the average heating rate in the temperature range of 250°C to 700°C is 100°C/s or less. The average heating rate in the temperature range of 250°C to 700°C is preferably 5°C/s or more. The average heating rate in the temperature range of 250°C to 700°C is preferably 75°C/s or less. The average heating rate in the temperature range of 250° C. or more and 700° C. or less is more preferably 10° C./s or more. The average heating rate in the temperature range of 250° C. or more and 700° C. or less is more preferably 50° C./s or less. The average heating rate is measured based on the temperature of the steel sheet surface.

[Heating temperature: 780℃ or higher]
If the heating temperature (annealing temperature) is less than 780°C, the annealing process will be performed in the two-phase region of ferrite and austenite, and since a large amount of ferrite will be contained after annealing, it will be impossible to achieve a TS of 1180 MPa or more, and the desired YR will not be achieved. In addition, the hardness distribution in the sheet width direction becomes non-uniform, so that the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the hardness per 1100 μm in the sheet width direction on the steel sheet surface is The frequency of fluctuation exceeds 7 times, and the bendability is deteriorated. Furthermore, since Si cannot diffuse to the steel sheet surface, the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction cannot be reduced, and the spot weld The resistance to inter-plate cracking in the HAZ cannot be improved. Although there is no particular upper limit for the heating temperature, if the heating temperature is increased, the thickness of the softened surface layer after annealing increases, TS decreases, and the old Since the austenite grain size becomes coarse and the YR decreases, the heating temperature is preferably 1000° C. or less. Therefore, the heating temperature is set to 780° C. or more. The heating temperature is more preferably 820° C. or higher. The heating temperature is further 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. The heating temperature is measured based on the temperature of the steel sheet surface.

[Residence time between 750 ° C. and the heating temperature: 10 s or more]
In the present disclosure, this is an extremely important invention constituent element. By increasing the residence time between 750 ° C. and the heating temperature, it is possible to promote the desorption of Si from the plate thickness surface and reduce the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction. In addition, it is possible to reduce Mn segregation formed during casting, reduce the standard deviation of the Vickers hardness of the steel plate surface, and reduce the hardness fluctuation frequency per 1100 μm in the plate width direction on the steel plate surface. In order to obtain such effects, the residence time between 750 ° C. and the heating temperature is set to 10 s or more. Note that the upper limit of the residence time between 750 ° C. and the heating temperature is not particularly specified. However, if the residence time between 750 ° C. and the heating temperature is increased, the surface softening thickness after annealing may increase, TS may decrease, or the prior austenite grain size may become coarse, decreasing YR, so 400 s or less is preferable. Therefore, the residence time between 750 ° C. and the heating temperature is set to 10 s or more. The residence time between 750 ° C. and the heating temperature is preferably set to 20 s or more. The residence time from 750° C. to the heating temperature is preferably 400 s or less. The residence time from 750° C. to the heating temperature is more preferably 25 s or more. The residence time from 750° C. to the heating temperature is more preferably 300 s or less.

[Oxygen concentration in the atmosphere at a heating temperature of 750° C. or higher: 0.5% by volume or higher and 5.0% by volume or lower (preferred conditions)]
During annealing, by increasing the oxygen concentration at 750 ° C. or higher and the heating temperature or lower, it is possible to promote Si desorption from the sheet thickness surface via oxygen in the air, and reduce the Si concentration at a position of 5 μm from the steel sheet surface in the sheet thickness direction. In addition, it is possible to promote Mn desorption from the steel sheet surface, reduce the standard deviation of the Vickers hardness of the steel sheet surface, and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the oxygen concentration at 750 ° C. or higher and the heating temperature or lower is preferably 0.5 vol.% or higher, more preferably 1.0 vol.% or higher, and even more preferably 1.5 vol.% or higher. On the other hand, with an increase in the oxygen concentration at 750 ° C. or higher and the heating temperature or lower, the surface softening thickness after annealing increases and TS decreases. Therefore, the oxygen concentration at 750 ° C. or higher and the heating temperature or lower is preferably 5.0 vol.% or lower, more preferably 4.5 vol.% or lower, and even more preferably 4.0 vol.% or lower. The temperature of 750° C. or higher and the heating temperature or lower is based on the surface temperature of the steel sheet. That is, when the surface temperature of the steel sheet is 750° C. or higher and the heating temperature or lower, the oxygen concentration is adjusted to be within the above range.

[Dew point of the atmosphere at a heating temperature of 750° C. or higher: −35° C. or higher (preferred conditions)]
During annealing, by increasing the dew point of the atmosphere at 750°C or higher and the heating temperature or lower, it is possible to promote Si desorption from the sheet thickness surface through moisture in the air, and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, it is possible to promote Mn desorption from the steel sheet surface, reduce the standard deviation of the Vickers hardness of the steel sheet surface, and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the dew point at 750°C or higher and the heating temperature or lower is preferably -35°C or higher, more preferably -30°C or higher, and even more preferably -25°C or higher. Note that the upper limit of the dew point at 750°C or higher and the heating temperature or lower is not particularly specified. However, since the surface layer softening thickness after annealing increases and TS decreases, the dew point at 750°C or higher and the heating temperature or lower is preferably 15°C or lower, more preferably 5°C or lower. Note that the temperature at 750°C or higher and the heating temperature or lower is based on the steel sheet surface temperature. That is, when the surface temperature of the steel sheet is 750° C. or higher and the heating temperature or lower, the dew point is adjusted to fall within the above range.

[Cooling process]
After the annealing step, the cold-rolled sheet is optionally cooled. The average cooling rate at a temperature range of 400°C or more below the heating temperature is not particularly limited, but is preferably 5°C/s or more and 30°C/s or less. In addition, in the temperature range of 400°C or more below the heating temperature, the high-strength steel sheet may be cooled once and the steel sheet temperature may be increased again.

[Average cooling rate in the temperature range of 250° C. to 400° C.: 1.0° C./s or more (preferred conditions)]
If the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C/s or more, the amount of bainitic ferrite contained 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 to 400°C is preferably 1.0°C/s or more, more preferably 2.0°C/s or more, and even more preferably 3.0°C/s or more. The upper limit of the average cooling rate in the temperature range of 250°C to 400°C is not particularly specified, but due to constraints on production technology, it is preferably 100.0°C/s or less, and more preferably 80.0°C/s or less. When the cooling stop temperature exceeds 250°C, the average cooling rate is the value in the temperature range of the cooling stop temperature to 400°C. The average cooling rate is measured based on the temperature of the steel sheet surface.

In addition, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be used as cooling methods in the temperature range of 250°C to 400°C.

[Heat retention temperature in cooling process: 100° C. or higher and 450° C. or lower (preferred conditions)]
In the cooling step, it is preferable to keep the heat at a temperature range of 100°C or more and 450°C or less for 5s or more. By keeping the temperature within the above range, the high-strength steel plate can be kept in a more suitable range for YR and bendability. In addition, the area ratio of bainitic ferrite can be further reduced, and TS can be further improved. The heat retention temperature in the cooling step is more preferably 150°C or more, and more preferably 200°C or more. In addition, the heat retention temperature in the cooling step is more preferably 400°C or less, and more preferably 350°C or less. The temperature in the cooling step is based on the surface temperature of the steel plate.

[Heat retention time in cooling process: 5 seconds or more (optimal conditions)]
By maintaining the heat retention temperature in the cooling step, the YR and bendability can be kept within a more suitable range. In order to obtain such an effect, the heat retention time at the heat retention temperature in the cooling step is preferably 5 seconds or more, more preferably 10 seconds or more, and even more preferably 15 seconds or more. Although there is no particular upper limit to the heat retention time at the heat retention temperature in the cooling step, in order to keep the TS within a more suitable range, the heat retention time at the heat retention temperature in the cooling step is preferably 500 seconds or less, and more preferably 250 seconds or less.

[Cooling stop temperature: 250°C or less (preferred conditions)]
In the above-mentioned cooling step, the cooling stop temperature is preferably 250°C or less, more preferably 200°C or less. If the cooling stop temperature is 250°C or less, it is possible to prevent a large amount of retained austenite from being generated after annealing, and to further improve the YR and bendability. Although the lower limit of the cooling stop temperature is not particularly specified, it is preferably room temperature or higher from the viewpoint of productivity. The cooling stop speed is measured based on the temperature of the steel sheet surface.

The average cooling rate to 250°C or less is not particularly specified, but in order to further improve TS, the average cooling rate to 250°C or less is preferably 1°C/s or more, and more preferably 2°C/s or more. On the other hand, due to constraints in production technology, the average cooling rate to 250°C or less is preferably 1000°C/s or less, and more preferably 150°C/s or less.

The cold-rolled sheet may be further cooled from the cooling stop temperature to room temperature. The average cooling rate from the cooling stop temperature to room temperature is not particularly limited, and any method may be used to cool to room temperature. Cooling methods that may be used include gas jet cooling, mist cooling, water cooling, and air cooling.

The cold-rolled sheet annealed as described above may be cooled to the cooling stop temperature and then rolled. The elongation rate of rolling is preferably 0.05% or more, and more preferably 0.10% or more. By setting the elongation rate of rolling performed after cooling to the cooling stop temperature to 0.05% or more, the YR can be controlled to a desired range. Furthermore, the elongation rate of rolling is preferably 2.00% or less, and more preferably 1.00% or less. By setting the elongation rate of rolling after cooling to the cooling stop temperature to 2.00% or less, the volume fraction of retained austenite can be set within a more suitable range. Furthermore, the bendability and the degree of damage to the sheared end surface in a corrosive environment can be set within a more suitable range.

The rolling after cooling to the cooling stop temperature may be performed on an apparatus continuous with the above-mentioned continuous annealing apparatus (online), or on an apparatus not continuous with the above-mentioned continuous annealing apparatus (offline). The desired elongation may be achieved in a single rolling pass, or multiple rolling passes may be performed to achieve a total elongation of 0.05% to 2.00%. The rolling described here generally refers to temper rolling, but as long as it can impart an elongation equivalent to that of temper rolling, it may also be a processing method using repeated bending with a tension leveler or rolls.

[Reheating temperature: (cooling stop temperature + 50°C) or more and 450°C or less (preferred conditions)]
After cooling to the cooling stop temperature, or after further rolling after 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 set within a more suitable range. In order to obtain such an effect, the reheating temperature is preferably (cooling stop temperature + 50 ° C) or more, more preferably (cooling stop temperature + 100 ° C) or more, and even more preferably (cooling stop temperature + 150 ° C) or more. On the other hand, as the reheating temperature increases, the tempering of martensite progresses and the TS decreases, so the reheating temperature is preferably 450 ° C or less, more preferably 400 ° C or less, and even more preferably 380 ° C or less. The reheating temperature is based on the surface temperature of the steel sheet.

[Heat retention time at reheating temperature: 5 seconds or more (preferred conditions)]
By maintaining the heat at the reheating temperature, the YR and bendability can be kept within a more suitable range. In order to obtain such an effect, the heat-maintaining time at the reheating temperature is preferably 5 seconds or more, more preferably 10 seconds or more, and even more preferably 15 seconds or more. Although there is no particular upper limit to the heat-maintaining time at the reheating temperature, in order to keep the TS within a more suitable range, the heat-maintaining time at the reheating temperature is preferably 500 seconds or less, and more preferably 250 seconds or less.

It should be noted that the material may be cooled from the reheating temperature to room temperature, but the cooling rate from the reheating temperature to room temperature is not particularly limited, and any method can be used to cool the material to room temperature. Cooling methods that can be used include gas jet cooling, mist cooling, water cooling, and air cooling.

When high-strength steel plates are traded, they are usually cooled to room temperature before being traded.

[Method of manufacturing high-strength plated steel sheet]
A high-strength plated steel sheet can be obtained by subjecting at least one side of the high-strength steel sheet produced as described above to a plating process. For example, examples of the plating process include a hot-dip galvanizing process and a process of alloying after hot-dip galvanizing. Annealing and galvanizing may be performed continuously in one line. Alternatively, a plating layer may be formed by electroplating such as Zn-Ni electric alloy plating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied. Note that, although the above description has focused on the case of galvanizing, the type of plating metal such as Zn plating or Al plating is not particularly limited.

In addition, when hot-dip galvanizing is performed, it is preferable to adjust the coating weight by gas wiping or the like after immersing the high-strength steel sheet in a galvanizing bath of 440 ° C. or more and 500 ° C. or less to perform hot-dip galvanizing. It is preferable to use a galvanizing bath having an Al content of 0.10 mass % or more and 0.23 mass % or less for hot-dip galvanizing. In addition, the temperature range when performing alloying treatment of galvanizing after hot-dip galvanizing is preferably 470 ° C. or more and 600 ° C. or less, more preferably 470 ° C. or more and 560 ° C. or less. By performing alloying treatment at 470 ° C. or more, the Zn-Fe alloying rate is more suitable and the productivity is more suitable. In addition, by performing alloying treatment at 600 ° C. or less, it is possible to prevent untransformed austenite from transforming into pearlite, and TS is more suitable. In addition, electrogalvanizing treatment may be performed. The coating weight is preferably 20 to 80 g/m ² per side (double-sided coating), and the galvannealed steel sheet (GA) is preferably subjected to the following alloying treatment to adjust the Fe concentration in the coating layer to 7 to 15 mass %.

As for the plating process, after the annealing process, the high-strength steel sheet may be plated at a temperature range of 400°C or more below the heating temperature without being cooled, or the cold-rolled steel sheet may be cooled to below 400°C, and then the steel sheet temperature may be raised again to 400°C or more before plating is performed.

The high-strength plated steel sheet that has been subjected to the above-mentioned plating process may be subjected to rolling. The elongation rate of rolling is preferably 0.05% or more, and more preferably 0.10% or more. By setting the elongation rate of rolling performed after plating to 0.05% or more, it is possible to control the YR within a desired range. Furthermore, the elongation rate of rolling is preferably 2.00% or less, and more preferably 1.00% or less. By setting the elongation rate of rolling in plating to 2.00% or less, it is possible to set the volume fraction of retained austenite within a more suitable range, and to set the bendability and the degree of damage to the sheared end surface in a corrosive environment within more suitable ranges.

The rolling after plating may be performed on equipment connected to the above-mentioned continuous annealing equipment (online), or on equipment not connected to the above-mentioned continuous annealing equipment (offline). The desired elongation may be achieved in one rolling operation, or multiple rolling operations may be performed to achieve a total elongation of 0.05% to 2.00%. The rolling described here generally refers to temper rolling, but any processing method involving repeated bending using a tension leveler or rolls may be used as long as it can impart an elongation equivalent to that of temper rolling. Reheating may also be performed after rolling after plating.

Other manufacturing method conditions are not particularly limited. However, from the viewpoint of productivity, it is preferable to carry out the above-mentioned series of processes such as annealing, hot-dip galvanizing, and alloying treatment of zinc plating in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping is possible to adjust the coating weight. Note that plating conditions other than those mentioned above can be based on standard hot-dip galvanizing methods.

When high-strength plated steel sheets are traded, they are usually cooled to room temperature before being traded.

　Production conditions other than those mentioned above can be carried out according to conventional methods.

[Materials]
Next, a member according to one embodiment of the present invention will be described.

The member according to one embodiment of the present invention is a member made using the high-strength steel sheet or high-strength plated steel sheet according to one embodiment of the present invention described above. The member according to one embodiment of the present invention is, for example, a high-strength steel sheet or high-strength plated steel sheet according to one embodiment of the present invention described above that is formed into a desired shape by cold press working or the like. Therefore, even after being formed into a member, it has the steel structure and various properties of the high-strength steel sheet and the high-strength plated steel sheet. The member according to one embodiment of the present invention is preferably used for automobile frame structural parts or automobile reinforcing parts.

The high-strength steel plate according to one embodiment of the present invention is a high-strength steel plate of 1180 MPa or more that has excellent bendability and inter-steel crack resistance in the HAZ of spot welds, and can be used to manufacture parts with high dimensional accuracy. Therefore, a member according to one embodiment of the present invention can contribute to reducing the weight of the vehicle body, and can be particularly suitably used for automobile frame structural parts or automobile reinforcing parts in general.

Steel having the composition shown in Table 1, with the remainder consisting of Fe and unavoidable impurities, was melted in a converter and formed into slabs by continuous casting. After cooling, the resulting slabs were heated and hot rolled, followed by pickling and then cold rolling. Table 2 shows the cooling rate after casting, the hot rolling conditions, the pickling conditions, and the cold rolling conditions. Pre-annealed cold-rolled steel sheets with a thickness of 1.4 mm were produced. In some examples, sheets with different thicknesses were also produced.

Next, annealing, cooling, and reheating were performed under the conditions shown in Tables 2 and 3 to obtain high-strength cold-rolled steel sheets (CR). Furthermore, some of the thin steel sheets were plated to obtain hot-dip galvanized steel sheets (GI), galvannealed hot-dip galvanized steel sheets (GA), and electrogalvanized steel sheets (EG). For the hot-dip galvanizing bath, a zinc bath containing 0.14 to 0.19 mass% Al was used for GI, and a zinc bath containing 0.14 mass% Al was used for GA, with the bath temperature set to 470°C. For the coating weight, for GI, it was about 45 to 72 g/m ² per side (double-sided plating), and for GA, it was about 45 g/m ² per side (double-sided plating). For GA, the Fe concentration in the plating layer was set to 9 mass% or more and 12 mass% or less. For EG, in which the plating layer is a Zn-Ni plating layer, the Ni content in the plating layer was set to 9 mass% or more and 25 mass% or less.

The high-strength cold-rolled steel sheets and high-strength plated steel sheets obtained in the above manner were used as test steels to evaluate the tensile properties, bendability, and inter-sheet cracking properties of the spot weld HAZ according to the following test methods. The results are shown in Table 4.

[Tensile test]
The tensile test was performed in accordance with JIS Z 2241:2022. From the obtained steel plate, a JIS No. 5 test piece was taken perpendicular to the rolling direction of the steel plate, and a tensile test was performed under the condition of a crosshead speed of 1.67×10 ⁻¹ mm/s to measure YS and TS. In the present invention, a yield ratio (YR) of 65% or more and 90% or less was determined to have high dimensional accuracy. YR was calculated by the calculation method described in the above formula (2).

[Bending test]
The bending test was performed in accordance with JIS Z 2248:2022. From the obtained steel plate, a rectangular test piece with a width of 30 mm and a length of 100 mm was taken so that the axial direction of the bending test was parallel to the rolling direction of the steel plate. Then, a 90° V bending test was performed under the conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds. In this disclosure, bending tests were performed on five samples at R, where the value R/t obtained by dividing the bending radius (R) by the plate thickness (t) is about 4.5, that is, 4.3 to 4.7. Next, the crack length at the ridgeline of the bending apex of all five samples was evaluated, and it was determined that the bending property was excellent when the crack length was 200 μm or less. Here, the crack length was evaluated by measuring the ridgeline of the bending apex at a magnification of 40 to 160 times using a digital microscope (RH-2000: manufactured by Hirox Co., Ltd.).

[Spot weld HAZ inter-plate cracking test]
The test pieces were cut out to a thickness of 1.4 mm, length of 30 mm, and width of 100 mm with the rolling direction as the longitudinal direction, and were stacked with test hot-dip galvanized steel sheets cut out to the same size and having a coating weight of 50 g/ ^m2 per side of the hot-dip galvanized layer to form a plate assembly. Next, resistance welding was performed using a servo motor pressure type single-phase AC (50 Hz) resistance welding machine and an electrode with a tip diameter of 6 mm, with the plate assembly tilted by 5° with respect to the electrode of the resistance welding machine and with a clearance of 1.5 mm between the lower electrode and the lower steel sheet. Specifically, resistance welding was performed on the plate assembly under the conditions of a pressure of 3.5 kN, a hold time of 0.16 seconds or 0.20 seconds, and a welding current and welding time that resulted in a nugget diameter of 5.9 mm, to form a plate assembly with a welded portion. Next, the plate assembly with the weld was cut in half to include the weld, and the cross section of the weld was observed with an optical microscope (200x) to evaluate the inter-steel plate cracking resistance of the spot weld HAZ according to the above criteria. In addition, for some samples, the same evaluation was performed with plate thicknesses of 0.8 mm and 2.3 mm.

According to the above-mentioned method, the area ratio of martensite and ferrite, the volume ratio of retained austenite, the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction, and the number density of MnS present in the Mn segregated part of the steel plate surface were obtained. Furthermore, the standard deviation of the Vickers hardness of the steel plate surface and the frequency of hardness fluctuation per 1100 μm in the plate width direction on the steel plate surface were obtained. In addition, the remaining structure was observed by the method described below. After cutting out the sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate was the observation surface, the observation surface was mirror-polished using diamond paste, and then etched with 3 vol. % nital to reveal the structure. Under the condition of an acceleration voltage of 15 kV, the observation position was set to 1/4 of the plate thickness of the steel plate, and three fields of view were observed at a magnification of 5000 times and a field of view of 17 μm x 23 μm. Carbide was identified as the remaining structure from the obtained structure image.

As shown in Table 4, the examples of the present invention are excellent in TS, YR, bendability, and inter-steel cracking resistance in the HAZ of the spot welds. On the other hand, the comparative examples are inferior in one or more of TS, YR, bendability, and inter-steel cracking resistance in the HAZ of the spot welds.

The above describes an embodiment of the present invention, but the present invention is not limited to the description of this embodiment, which is a part of the disclosure of the present invention. In other words, other embodiments, examples, and operational techniques made by those skilled in the art based on this embodiment are all included in the scope of the present invention. For example, in the series of heat treatments in the above-mentioned manufacturing method, there are no particular limitations on the equipment for subjecting the steel plate to heat treatment, as long as the thermal history conditions are satisfied.

The present invention provides high-strength steel plates of 1180 MPa or more that have excellent bendability and inter-steel-sheet cracking resistance in the HAZ of spot welds, and can be used to manufacture parts with high dimensional accuracy.

In particular, the high-strength steel plate of the present invention has excellent resistance to inter-steel plate cracking in the HAZ of spot welds, making it possible to apply it to automotive structural components of various sizes and shapes while still achieving high component strength. This makes it possible to improve fuel efficiency by reducing the weight of the vehicle body, making it extremely valuable in industry.

Claims (11)

In mass percent,
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;
a component composition in which an effective Ti mole fraction (x _Ti,eff ) calculated from the following formula (1) is 0.001 or more, with the remainder being Fe and unavoidable impurities;
At the 1/4 plate thickness position,
The area ratio of martensite is 80% or more and 99% or less,
A steel structure having an area ratio of ferrite and/or a volume ratio of retained austenite in total of more than 0% and not more than 20%;
The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is 0.10% or more and 0.60% or less;
The number density of MnS present in the Mn segregation portion on the steel sheet surface is 5.0 pieces/ ^mm2 or less;
The standard deviation of the Vickers hardness of the steel plate surface is 15 or less.
The hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is 7 times or less;
High strength steel plate.
Note x _{Ti, eff} = x _Ti -x _N -x _S ... (1)
In the formula, x _Ti , x _N and x _S represent the content (molar fraction) of each element in the steel sheet. The composition further includes, in mass%,
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,
REM: 0.0100% or less,
Zr: 0.100% or less,
Te: 0.100% or less,
Hf: 0.10% or less,
Bi: 0.200% or less,
The high strength steel plate according to claim 1, further comprising at least one element selected from the group consisting of: A high-strength plated steel sheet having a plating layer on at least one side of the high-strength steel sheet according to claim 1 or 2. A steel slab having the composition according to claim 1 or 2,
The slab is heated at an average heating rate of 25° C./min or less in a temperature range of 900° C. or more and 1150° C. or less, a slab heating temperature of 1150° C. or more, and a residence time from 1100° C. to the slab heating temperature of 20 min or more;
Next, the steel slab is subjected to hot rolling in which the reduction rate of the final pass of the finish rolling is 9% or more and 15% or less, the reduction rate of the pass before the final pass is 15% or more and 21% or less, and the reduction rate of the pass two passes before the final pass is 21% or more and 27% or less to obtain a hot-rolled sheet,
Next, the hot-rolled sheet is subjected to pickling to obtain a pickled sheet.
Next, the pickled sheet is subjected to cold rolling with a cumulative rolling reduction of 20% to 75% to obtain a cold-rolled sheet,
Next, an annealing step is performed in which the cold-rolled sheet is heated to a heating temperature of 780°C or more, and in this case, the average heating rate in the temperature range of 250°C to 700°C is 100°C/s or less, and the residence time from 750°C to the heating temperature is 10s or more. A method for producing a high-strength steel sheet. The method for manufacturing high-strength steel plate according to claim 4, wherein the oxygen concentration of the atmosphere at a temperature of 750°C or higher and lower than the heating temperature during the annealing process is 0.5% by volume or higher and 5.0% by volume or lower, and the dew point of the atmosphere is -35°C or higher. The method for manufacturing high-strength steel plate according to claim 4 or 5, further comprising a cooling step following the annealing step, in which the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C/s or more. The method for manufacturing high-strength steel plate according to claim 4 or 5, further comprising a cooling step following the annealing step, in which the plate is kept at a temperature of 100°C or higher and 450°C or lower for 5 seconds or more. The method for manufacturing high-strength steel sheet according to claim 4 or 5, further comprising a cooling step following the annealing step, in which the cooling stop temperature is set to 250°C or less, and then reheating the cold-rolled sheet to a reheating temperature of (the cooling stop temperature + 50°C) or more and 450°C or less, and maintaining the temperature at the reheating temperature for 5 seconds or more. A method for producing a high-strength plated steel sheet, comprising the steps of: After the annealing step described in any one of claims 4 to 8, a plating step is performed to plate at least one side of the cold-rolled sheet. A member at least partially made of the high-strength steel plate according to claim 1 or 2. A member at least partially made of the high-strength plated steel sheet described in claim 3.