EP4650471A1

EP4650471A1 - Hot-rolled steel sheet

Info

Publication number: EP4650471A1
Application number: EP23916300.9A
Authority: EP
Inventors: Yasuyuki Ogisu; Takeshi Toyoda
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-01-13
Filing date: 2023-12-27
Publication date: 2025-11-19
Also published as: WO2024150687A1; EP4650471A4; JPWO2024150687A1

Abstract

A hot-rolled steel sheet has a predetermined chemical composition, in which at a depth position of 1/4 of a sheet thickness, a microstructure is formed of 30 area% or more of ferrite, 10 area% or more of pearlite, and 0 area% or more and 60 area% or less of a remainder in microstructure, and ferrite grains to be measured by an electron back scattering diffraction method satisfy a predetermined relationship.

Description

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet.
Priority is claimed on Japanese Patent Application No. 2023-003919 filed January 13, 2023 , the content of which is incorporated herein by reference.

BACKGROUND ART

With regard to steel sheets for a vehicle, in order to reduce a weight of a vehicle body and improve fuel efficiency in consideration of the global environment, there is a significantly increasing demand for high strength steel sheets having a small sheet thickness and excellent formability. Among the steel sheets for a vehicle, particularly for high strength cold-rolled steel sheets used for vehicle body frame components, a higher strength is required.
Together with high-strengthening of cold-rolled steel sheets, there is a tendency for hot-rolled steel sheets before cold rolling to harden, and the cold rolling properties of the hot-rolled steel sheets are reduced. Therefore, in order to stably manufacture high strength steel sheets, hot-rolled steel sheets having excellent cold rolling properties are required.
As a hot-rolled steel sheet having excellent cold rolling properties, Patent Document 1 discloses a hot-rolled steel sheet having a sheet thickness of 3 to 20 mm and containing, as a composition, by mass% (hereinafter, the same applies to chemical compositions), C: 0.3% or less (not including 0%), Si: 0.5% or less (not including 0%), Mn: 0.2% to 1%, P: 0.05% or less (not including 0%), S: 0.05% or less (not including 0%), Al: 0.01% to 0.1%, N: 0.008% to 0.025%, and a remainder of iron and unavoidable impurities, in which solid solution N: 0.007% or more, the C content and the N content satisfy a relationship of 10C + N ≤ 3.0, a structure includes, by area ratio relative to an entire structure, pearlite: less than 20% and a remainder: ferrite, and an average grain size of the ferrite is within a range of 3 to 35 µm.

Citation List

Patent Document

Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2014-148739

SUMMARY OF INVENTION

Technical Problem

At the present time, hot-rolled steel sheets having more excellent cold rolling properties than the hot-rolled steel sheet of Patent Document 1 are required.
The present invention has been contrived in view of the above circumstances, and an object thereof is to provide a hot-rolled steel sheet having excellent cold rolling properties.

Solution to problem

The present inventors conducted detailed studies on the chemical composition, microstructure, and manufacturing conditions for cold rolling properties of a hot-rolled steel sheet. The present inventors found that, by increasing a rolling reduction at a stage before hot rolling and performing rolling under light pressure at a final stage, the ferrite fraction of a hot-rolled steel sheet to be obtained increases, the grain size distribution changes, and the cold rolling properties are improved.
The present invention has been contrived in view of the above findings. The gist of the present invention is as described below.

<1> A hot-rolled steel sheet according to one aspect of the present invention including, as a chemical composition, by mass%: C: 0.02% or more and less than 0.20%; Si: 0.010% to 3.000%; Mn: 0.10% to 4.00%; Al: 0.001% to 1.000%; P: 0.0200% or less; S: 0.020% or less; N: 0.020% or less; O: 0.0200% or less; Cr: 0% to 2.000%; B: 0% to 0.010%; Nb: 0% to 0.10%; Mo: 0% to 1.00%; V: 0% to 0.50%; Ti: 0% to 0.4000%; Cu: 0% to 0.500%; W: 0% to 0.100%; Ta: 0% to 0.100%; Ni: 0% to 1.000%; Mg: 0% to 0.050%; Ca: 0% to 0.040%; Y: 0% to 0.050%; Zr: 0% to 0.050%; La: 0% to 0.050%; Co: 0% to 0.500%; Sn: 0% to 0.050%; Sb: 0% to 0.050%; As: 0% to 0.050%; and a remainder of Fe and impurities, in which, at a depth position of 1/4 of a sheet thickness, a microstructure is formed of 30 area% or more of ferrite, 10 area% or more of pearlite, and 0 area% or more and 60 area% or less of a remainder in microstructure, the remainder in microstructure includes at least one of bainite, martensite, and residual austenite, among ferrite grains to be measured by an electron back scattering diffraction method, ferrite grains excluding ferrite grains occupying 5% of a total number of the ferrite grains from a maximum grain size side and ferrite grains occupying 5% of the total number of the ferrite grains from a minimum grain size side of the ferrite grains are treated as evaluation ferrite grains, a minimum value of a grain size of the evaluation ferrite grains is set as a first grain size, a maximum value of the grain size of the evaluation ferrite grains is set as a second grain size, a grain size obtained by adding, to the first grain size, 1/3 of a difference between the second grain size and the first grain size is set as a third grain size, a grain size obtained by adding, to the first grain size, 2/3 of the difference between the second grain size and the first grain size is set as a fourth grain size, and when a range that is equal to or more than the first grain size and equal to or less than the third grain size is set as a first grain size range, a range that is more than the third grain size and equal to or less than the fourth grain size is set as a second grain size range, and a range that is more than the fourth grain size and equal to or less than the second grain size is set as a third grain size range, the number of the evaluation ferrite grains in the first grain size range is 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range is 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range.
<2> In Aspect 2 of the present invention, in the hot-rolled steel sheet according to Aspect 1, an average grain size of the evaluation ferrite grains in the first grain size range may be 1 µm to 10 µm.
<3> In Aspect 3 of the present invention, in the hot-rolled steel sheet according to Aspect 1 or 2, an average grain size of the evaluation ferrite grains in the third grain size range may be more than 35 µm and 80 µm or less.
<4> In Aspect 4 of the present invention, in the hot-rolled steel sheet according to Aspect 1 or 2, an average grain size of the evaluation ferrite grains in the third grain size range may be 40 µm to 80 µm.
<5> In Aspect 4 of the present invention, in the hot-rolled steel sheet according to any one of Aspects 1 to 4, a total elongation may be 40% or more.

Advantageous Effects of Invention

According to the aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent cold rolling properties.

DESCRIPTION OF EMBODIMENTS

The chemical composition and microstructure of a hot-rolled steel sheet (hereinafter, may be simply referred to as the steel sheet according to the present embodiment) according to an embodiment of the present invention, and rolling conditions and the like in a manufacturing method capable of manufacturing the steel sheet will be described in detail below.

First, the chemical composition of the steel sheet according to the present embodiment will be described. The symbol "%" indicating an amount of each element in the chemical composition means mass% unless otherwise specified. In the present specification, the numerical value range represented by using "to" means the range including the numerical values before and after "to" as a lower limit and an upper limit.

[C: 0.02% or more and less than 0.20%]

C is an essential element for high-strengthening of the steel sheet. In a case where the C content is less than 0.02%, a sufficient tensile strength cannot be obtained. Therefore, the C content is set to 0.02% or more. The C content is preferably 0.08% or more.
Meanwhile, in a case where the C content is 0.20% or more, desired cold rolling properties cannot be obtained. Therefore, the C content is set to be less than 0.20%. The C content is preferably 0.15% or less.
In a case where the C content is within the above range, it is possible to secure the tensile properties at a normal level (for example, 540 MPa or more) required for the cold-rolled steel sheet after cold rolling and heat treatment.

[Si: 0.010% to 3.000%]

Si is a solid solution strengthening element, and is an element that is generally contained for high-strengthening of the cold-rolled steel sheet. In order to obtain the cold rolling property improvement effect by structure control, the Si content is set to 0.010% or more. The Si content is preferably 0.100% or more.
Meanwhile, in a case where Si is excessively contained, the steel sheet is embrittled, and it is difficult to sufficiently secure the cold rolling properties even when the structure control is applied. Therefore, the Si content is set to 3.000% or less. The Si content is preferably 2.000% or less.
In a case where the Si content is within the above range, it is possible to secure the tensile properties at a normal level required for the cold-rolled steel sheet after cold rolling and heat treatment.

[Mn: 0.10% to 4.00%]

Mn is an element that acts to improve the hardenability of steel and is generally contained to secure the strength of the cold-rolled steel sheet. In a case where the Mn content is less than 0.10%, it is difficult to obtain the cold rolling property improvement effect by structure control. Accordingly, the Mn content is set to 0.10% or more. The Mn content is preferably 1.00% or more.
Meanwhile, in a case where the Mn content is more than 4.00%, the generation of ferrite in the hot-rolled steel sheet is suppressed, and desired cold rolling properties cannot be obtained. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 1.50% or less.
In a case where the Mn content is within the above range, it is possible to secure the tensile properties at a normal level required for the cold-rolled steel sheet after cold rolling and heat treatment.

[Al: 0.001% to 1.000%]

Al is an element that acts to deoxidize steel. In order to obtain the above-described effect, the Al content is set to 0.001% or more. The Al content is preferably 0.005% or more.
Meanwhile, in a case where Al is excessively contained, the above-described effect saturates to lead to an unnecessary increase in cost. Moreover, the transformation temperature of steel increases and the load during hot rolling increases. Therefore, the Al content is set to be 1.000% or less. The Al content is preferably 0.900% or less. The Al content means a so-called total Al (T-Al) content.

[P: 0.0200% or less]

P is an element contained in steel as an impurity, and is an element that segregates at grain boundaries to embrittle steel and deteriorates the cold rolling properties. Therefore, the P content is set to 0.0200% or less. The P content is preferably 0.0150% or less, and more preferably 0.0100% or less.
The P content is preferably as small as possible, but may be set to be more than 0.0080% in consideration of the time and cost required for removing P.

[S: 0.020% or less]

S is an element contained in steel as an impurity, and is an element that forms sulfide-based inclusions and deteriorates the cold rolling properties. Therefore, the S content is set to 0.020% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.009% or less. The S content is preferably as small as possible and may be 0%. However, the S content may be set to 0.008% or more in consideration of the time and cost required for removing S.

[N: 0.020% or less]

N is an element that forms coarse nitrides in the steel sheet and deteriorates the cold rolling properties of the steel sheet. In a case where the N content is more than 0.020%, the cold rolling properties significantly deteriorate. Therefore, the N content is set to 0.020% or less. The N content may be 0.015% or less, 0.010% or less, or 0.005% or less.
Meanwhile, in a case where the N content is set to be less than 0.001%, the manufacturing cost increases significantly. The N content may be set to 0.001% or more. The N content may be set to 0.002% or more.

[O: 0.0200% or less]

O is an element that forms coarse oxides in steel and deteriorates the cold rolling properties. In a case where the O content is more than 0.0200%, the tendency of deterioration is noticeable. Therefore, the O content is set to 0.0200% or less. The O content may be 0.0150% or less or 0.0100% or less.
The O content is preferably small. However, from the economic perspective, it is not preferable the O content be less than 0.0001% due to an excessive increase in cost. Therefore, the O content may be set to 0.0001% or more. The O content may be set to 0.0010% or more.
The steel sheet according to the present embodiment may contain the above-described elements and a remainder may be Fe and impurities. Here, the impurities are elements that are mixed in from a raw material such as ore or a scrap or due to a variety of factors in manufacturing steps during industrial manufacturing of steel and are allowed to an extent that the properties of the steel sheet according to the present embodiment are not adversely affected. The impurities also include elements that are not intentionally added to the steel sheet according to the present embodiment.
The steel sheet according to the present embodiment may further contain one or more elements (optional elements) among Cr, B, Nb, Mo, V, Ti, Cu, W, Ta, Ni, Mg, Ca, Y, Zr, La, Co, Sn, Sb, and As shown below. Since these elements do not necessarily need to be contained, the lower limits thereof in content are 0%.

[Cr: 0% to 2.000%]

Cr is an element that increases the hardenability and is thus effectively and generally used for high-strengthening of the cold-rolled steel sheet. Therefore, Cr may be contained in steel. In order to obtain the above-described effect, the Cr content is preferably 0.001% or more. Meanwhile, in a case where the Cr content is more than 2.000%, Cr segregates at a center portion of the steel sheet and forms coarse Cr carbides, and the cold rolling properties may be reduced. Therefore, the Cr content is set to 2.000% or less. The Cr content is more preferably 1.000% or less.

[B: 0% to 0.010%]

B is an element that suppresses the generation of ferrite and pearlite during the course of cooling from austenite, promotes the generation of a low-temperature transformation structure such as bainite or martensite, and is beneficial for the high-strengthening of the cold-rolled steel sheet. Therefore, B may be contained in steel. In order to obtain the above-described effects of B, the B content is preferably 0.001% or more. Meanwhile, B leads to the generation of coarse B oxides or borides in steel, which become origins of voids during cold rolling, and thus the cold rolling properties of the steel sheet may deteriorate. Therefore, the B content is set to 0.010% or less. The B content is more preferably 0.009% or less.

[Nb: 0% to 0.10%]

Nb is an effective element for controlling the morphology of carbide. Since the addition of Nb refines the structure, it is also an effective element for improving the toughness of the steel sheet. Therefore, Nb may be contained in steel. In order to obtain the effects of Nb, the Nb content is preferably set to 0.01% or more. Meanwhile, in a case where Nb is excessively added, a large number of coarse Nb carbides are precipitated, which become origins of voids during cold rolling, and thus the cold rolling properties of the steel sheet may deteriorate. Therefore, the Nb content is set to 0.10% or less.

[Mo: 0% to 1.00%]

Mo is an effective element for strengthening of the cold-rolled steel sheet. Therefore, Mo may be contained in steel. For the high-strengthening of the cold-rolled steel sheet by Mo, the Mo content is preferably 0.01% or more. Meanwhile, in a case where Mo is excessively added, the cost increases and coarse Mo carbides are formed, which may reduce the cold rolling properties of the steel sheet. Therefore, the Mo content is set to 1.00% or less. The Mo content is more preferably 0.90% or less.

[V: 0% to 0.50%]

V is an effective element for controlling the morphology of carbide. Since the addition of V refines the structure, it is also an effective element for improving the toughness of the steel sheet. Therefore, V may be contained in steel. In order to obtain the effects of V, the V content is preferably 0.01% or more. Meanwhile, in a case where V is excessively added, a large number of fine V carbides are precipitated, which increases the strength of the steel sheet and significantly deteriorates the ductility, and the cold rolling properties may be reduced. Therefore, the V content is set to 0.50% or less. The V content is more preferably 0.40% or less.

[Ti: 0% to 0.4000%]

Ti is an important element for controlling the morphology of carbide, and is an element that is contained in the steel sheet of the present invention in order to promote an increase in strength of ferrite in the cold-rolled steel sheet by containing a large amount of Ti. Therefore, Ti may be contained in steel. However, in a case where the Ti content is 0.0001% or more, the ferrite strength improvement effect can be obtained. Meanwhile, in a case where Ti is excessively added, the cold rolling properties of the steel sheet may be reduced due to the presence of coarse Ti oxides or Ti carbonitrides in steel. Therefore, the Ti content is set to 0.4000% or less. The Ti content is more preferably 0.3000% or less.

[Cu: 0% to 0.500%]

Cu is an element that contributes to the improvement in strength of the cold-rolled steel sheet, and is contained in the steel sheet of the present invention. Therefore, Cu may be contained in steel. In order to obtain the above-described effect, the Cu content is preferably 0.001% or more. However, in a case where the Cu content is too large, there is a concern that red shortness may be caused and productivity in hot rolling may be degraded. Furthermore, in a case where the Cu content is too large, there is a concern that the cold rolling properties may be reduced due to the formation of coarse inclusions. Therefore, the Cu content is set to 0.500% or less. The Cu content is preferably 0.300% or less.

[W: 0% to 0.100%]

W is an element that forms carbides, is an effective element for high-strengthening of the cold-rolled steel sheet, and is contained in the steel sheet of the present invention. Therefore, W may be contained in steel. In order to obtain the above-described effects, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more.
Meanwhile, even in a case where W is excessively contained, the effects saturate and the cost increases. Therefore, in a case where W is contained, the W content is set to 0.100% or less. The W content is more preferably 0.050% or less.

[Ta: 0% to 0.100%]

Ta is an effective element for controlling the morphology of carbide and improving the strength of the cold-rolled steel sheet, and is contained in the steel sheet of the present invention. Therefore, Ta may be contained in steel. In order to obtain the above-described effects, the Ta content is preferably 0.001% or more. Meanwhile, in a case where the Ta content is too large, there is a concern that a large number of fine Ta carbides may be precipitated to lead to a reduction in ductility of the steel sheet, and the cold rolling properties of the steel sheet may be reduced. Therefore, the Ta content is set to 0.100% or less.
The Ta content is more preferably 0.090% or less. The Ta content is still more preferably 0.080% or less.

[Ni: 0% to 1.000%]

Ni is an effective element for improving the strength of the cold-rolled steel sheet, and is contained in the steel sheet of the present invention. Therefore, Ni may be contained in steel. In order to obtain the above-described effect, the Ni content is preferably 0.001% or more. The Ni content is more preferably 0.100% or more. Meanwhile, in a case where the Ni content is too large, there is a concern that the ductility of the steel sheet may be reduced to lead to a reduction in cold rolling properties. Therefore, the Ni content is set to 1.000% or less. The Ni content is preferably 0.500% or less.

[Mg: 0% to 0.050%]

Mg is an element that controls the morphology of sulfide and oxide and contributes to the improvement in bendability of the steel sheet, and is contained in the steel sheet of the present invention. Therefore, Mg may be contained in steel. In order to obtain the above-described effects, the Mg content is preferably 0.001 % or more. However, in a case where the Mg content is too large, there is a concern that the cold rolling properties may be reduced due to the formation of coarse inclusions. Therefore, the Mg content is set to 0.050% or less. The Mg content is preferably 0.030% or less.

[Ca: 0% to 0.040%]

Ca is an element that can control the morphology of sulfide with a small amount. Therefore, Ca may be contained in steel. In order to obtain the above-described effect, the Ca content is preferably 0.001% or more. However, in a case where the Ca content is too large, coarse Ca oxides may be generated, and there is a concern that the Ca oxides may become origins of cracks during cold rolling, and as a result, the cold rolling properties may deteriorate. Therefore, the Ca content is set to 0.040% or less. The Ca content is preferably 0.020% or less.

[Y: 0% to 0.050%]

Y is an element that effectively acts to control the morphology of sulfide even in a case where the amount thereof is small. Therefore, Y may be contained in the steel sheet. In order to obtain the above-described effect, the Y content is preferably 0.001 % or more. However, in a case where the Y content is too large, there is a concern that coarse Y oxides may be generated, and the cold rolling properties and the fracture resistance may thus be reduced. Therefore, the Y content is set to 0.050% or less. The Y content is more preferably 0.020% or less.

[Zr: 0% to 0.050%]

Zr is an element that can control the morphology of sulfide with a small amount. Therefore, Zr may be contained in steel. In order to obtain the above-described effect, the Zr content is preferably 0.001% or more. However, in a case where the Zr content is too large, there is a concern that coarse Zr oxides may be generated, and the cold rolling properties may thus be reduced. Therefore, the Zr content is set to 0.050% or less. The Zr content is preferably 0.020% or less.

[La: 0% to 0.050%]

La is an element that effectively acts to control the morphology of sulfide even in a case where the amount thereof is small. Therefore, La may be contained in the steel sheet. In order to obtain the above-described effect, the La content is preferably 0.001% or more. However, in a case where the La content is too large, there is a concern that coarse La oxides may be generated, and the cold rolling properties and the fracture resistance may thus be reduced. Therefore, the La content is set to 0.050% or less. The La content is preferably 0.020% or less.

[Co: 0% to 0.500%]

Co is an effective element for improving the strength of the cold-rolled steel sheet. Therefore, Co may be contained in steel. In order to obtain the above-described effect, the Co content is preferably 0.001% or more. However, in a case where the Co content is too large, there is a concern that the cold rolling properties may be reduced. Therefore, the Co content is set to 0.500% or less. The Co content is preferably 0.200% or less.

[Sn: 0% to 0.050%]

Sn is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet. In addition, there is a concern that Sn may cause a reduction in cold rolling properties of the steel sheet attributed to the embrittlement of ferrite. Therefore, the Sn content is preferably as small as possible. The Sn content is set to 0.050% or less. The Sn content is preferably 0.020% or less. However, reducing the Sn content to less than 0.001% leads to an excessive increase in refining costs, and thus the Sn content may be set to 0.001% or more.

[Sb: 0% to 0.050%]

Similar to Sn, Sb is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet. There is a concern that Sb may strongly segregate at grain boundaries to lead to the embrittlement of the grain boundaries, a reduction in ductility, and a reduction in cold rolling properties. Therefore, the Sb content is preferably as small as possible. The Sb content is set to 0.050% or less. The Sb content is preferably 0.020% or less.
Reducing the Sb content to less than 0.001% leads to an excessive increase in the refining costs, and thus the Sb content may be set to 0.001% or more.

[As: 0% to 0.050%]

Similar to Sn and Sb, As is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet. There is a concern that As may strongly segregate at grain boundaries to lead to a decrease in cold rolling properties. Therefore, the As content is preferably as small as possible. The As content is set to 0.050% or less, and is preferably 0.020% or less. Reducing the As content to less than 0.001% leads to an excessive increase in the refining costs, and thus the As content may be set to 0.001% or more.
The chemical composition of the steel sheet according to the present embodiment can be obtained by the following method.
The above-described chemical composition of the steel sheet may be measured by a general chemical composition. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). In addition, C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-nondispersive infrared absorption method.

Next, the microstructure at a depth position of 1/4 of the sheet thickness of the steel sheet according to the present embodiment (a position that is away from the surface by 1/4 of the sheet thickness along the sheet thickness direction) will be described.
In the description of the microstructure of the steel sheet according to the present embodiment, microstructural fractions are indicated by area ratios. Therefore, unless otherwise specified, "%" in the description of the microstructure represents "area%".
In the steel sheet according to the present embodiment, the microstructure at a depth position of 1/4 of the sheet thickness (a position that is away from the surface by 1/4 of the sheet thickness along the sheet thickness direction) is formed of 30 area% or more of ferrite, 10 area% or more of pearlite, and 0 area% or more and 60 area% or less of the remainder in microstructure.

(Ferrite: 30 Area% or More)

Ferrite is a relatively soft phase in the steel sheet, and improves the ductility of the steel sheet and the cold rolling properties. Therefore, ferrite is 30 area% or more at a depth position of 1/4 of the sheet thickness. Ferrite is preferably 35 area% or more. The upper limit of the ferrite is not particularly limited, but is, for example, 90 area% or less in the steel sheet according to the present invention.

(Pearlite: 10 Area% or More)

Pearlite is a structure having a large amount of cementite in the structure. Pearlite consumes C (carbon) in steel, which contributes to an increase in strength, and causes softening. Therefore, pearlite is 10 area% or more at a depth position of 1/4 of the sheet thickness. Pearlite is more preferably 15 area% or more.
The upper limit of the pearlite is not particularly limited, but is, for example, 70 area% or less.

(Remainder in Microstructure)

The remainder in microstructure is 0 area% or more and 60 area% or less. The remainder in microstructure refers to a structure remaining after pearlite and ferrite are removed from the microstructure. The remainder in microstructure includes at least one of bainite, martensite, and residual austenite. The remainder in microstructure is preferably a structure primarily including at least one of bainite and martensite. Here, the "structure primarily including at least one of bainite and martensite" means that a total of bainite and martensite is 60 area% or more relative to the total area of the remainder in microstructure. The total of bainite and martensite is more preferably 80 area% or more relative to the total area of the remainder in microstructure. The upper limit of the total of bainite, martensite, and residual austenite relative to the total area of the remainder in microstructure may be 100 area%.
Bainite is softer than martensite. Therefore, in the steel sheet according to the present embodiment that emphasizes ductility rather than strength, the structure is preferably controlled so that bainite is formed instead of martensite. Here, martensite refers to fresh martensite and tempered martensite, but in the steel sheet according to the present embodiment that emphasizes ductility rather than strength, the structure is preferably controlled so that tempered martensite is formed instead of fresh martensite. For example, bainite is set to 10 area% or more. Preferably, bainite is 15 area% or more. Martensite is, for example, set to 10 area% or less. Preferably, martensite is 5 area% or less. Residual austenite is, for example, set to 5 area% or less.
Next, methods for identification of each microstructure at a depth position of 1/4 of the sheet thickness and for area ratio calculation will be described.
The identification of each microstructure and the calculation of the area and the area ratio can be performed by observing a cross section of the steel sheet parallel to the rolling direction and perpendicular to the sheet surface at 1,000 to 50,000-fold magnification using electron back scattering diffraction (EBSD), X-ray measurement, corrosion using a Nital reagent or a LePera liquid, and a scanning electron microscope. In the measurement of the area ratio of any structure, the measurement is performed at three points, and an average value of the measured values is calculated. In this case, in the present invention, for example, software "OIM Data Collection TM (ver. 7)" manufactured by TSL Solutions, Inc. is used as software for acquiring crystal orientation data.
The area and the area ratio of ferrite can be measured by the following method. That is, an observation surface is finished by colloidal silica polishing or electrolytic polishing, and the measurement is performed at intervals (pitch) of 0.2 µm in a square region ranging from 1/8 to 3/8 of the thickness, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface of the steel sheet (a square having a side whose length ranges from 1/8 to 3/8 along the sheet thickness direction), by EBSD attached to a scanning electron microscope. Conditions for sample preparation and the like are set within the ranges of the conditions recommended in the "Standards for crystal orientation difference measurement for material evaluation by electron back scattering diffraction (EBSD) method" of the JSMS Standards. The value of grain average misorientation (GAM) is calculated from the measurement data. In addition, a region in which the value of grain average misorientation is less than 0.5° is treated as ferrite, and the area and the area ratio thereof are measured. Here, a boundary having a crystal orientation difference of 15° or more is determined as a grain boundary, and a region surrounded by the grain boundary is defined as a crystal grain. The grain average misorientation is a value obtained by calculating an orientation difference between adjacent measurement points and averaging the orientation differences among all measurement points in the crystal grains.
For the area and the area ratio of bainite, a sample is collected so that a cross section thereof in the sheet thickness direction that is parallel to the rolling direction of the steel sheet serves as an observation surface, the observation surface is polished and etched with a Nital liquid, a square region ranging from 1/8 to 3/8 of the thickness, centered at a depth of 1/4 of the sheet thickness, is observed by a field emission scanning electron microscope (FE-SEM), and the calculation is performed using known image analysis software. Using, for example, "ImageJ" as the image analysis software, the area ratio can be calculated. Here, "ImageJ" is open source and public domain image processing software, and is widely used by those skilled in the art.
In the observation by the FE-SEM, for example, structures in the square observation region are distinguished as follows. Bainite is an aggregation of lath-shaped crystal grains, and is a region containing no iron-based carbides having a major axis of 20 nm or more in the lath structure thereof. In a case where iron-based carbides having a major axis of 20 nm or more in the lath structure thereof are contained, bainite is a region in which the carbides belong to a single variant, that is, an iron-based carbide group elongated in the same direction. Here, the iron-based carbide group elongated in the same direction means a group in which a difference in elongation direction of the iron-based carbide group is within 5°.
Similar to bainite, for the area ratio of martensite, a sample is collected so that a cross section in the sheet thickness direction that is parallel to the rolling direction of the steel sheet serves as an observation surface, the observation surface is polished and etched with a Nital liquid, a square region ranging from 1/8 to 3/8 of the thickness, centered at a depth of 1/4 of the sheet thickness, is observed by a field emission scanning electron microscope (FE-SEM), and the calculation is performed using known image analysis software. Using, for example, "ImageJ" as the image analysis software, the area ratio can be calculated. Here, "ImageJ" is open source and public domain image processing software, and is widely used by those skilled in the art. Since martensite has a high dislocation density and has a substructure such as a block or a packet in the grain, martensite is distinguished from other microstructures in an electron channeling contrast image for which a scanning electron microscope is used.
The area and the area ratio of residual austenite are measured by the following method. That is, an observation surface is finished by colloidal silica polishing or electrolytic polishing, diffracted electrons are measured at intervals (grid-like arrangement) of 0.2 µm in the sheet thickness direction and the rolling direction in a square region ranging from 1/8 to 3/8 of the thickness, centered at a depth of 1/4 of the sheet thickness from the surface of the steel sheet, by EBSD attached to a scanning electron microscope, and the obtained pseudo-Kikuchi pattern is analyzed to identify the crystal orientation and the crystal system. Conditions for sample preparation and the like are set within the ranges of the conditions recommended in the "Standards for crystal orientation difference measurement for material evaluation by electron back scattering diffraction (EBSD) method" of the JSMS Standards. A region detected as an FCC phase from the measurement data is determined as residual austenite, and the area and the area ratio of the region are measured.
The area ratio of pearlite can be obtained as follows. A sample is collected so that a cross section in the sheet thickness direction that is parallel to the rolling direction of the steel sheet serves as an observation surface, the observation surface is polished and corroded with a Nital reagent, and a square region ranging from 1/8 to 3/8 of the thickness, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface of the steel sheet, is observed using a secondary electron image obtained by a scanning electron microscope. A region in which bright and dark contrasts in the secondary electron image are lamellar is defined as pearlite, and the area ratio thereof is calculated using the above-described image analysis software "ImageJ". The determination of pearlite by the contrasts of the secondary electron image is generally performed by those skilled in the art, and the pearlite can be easily determined by those skilled in the art. In a case where the total area ratio of the structures obtained by the above evaluation method is different from 100%, a value obtained by multiplying the area ratio of each structure by 100/(total area ratio of structures) is defined as the area ratio of the structures.

(Evaluation Ferrite Grains)

In the steel sheet of the present embodiment, a distribution state of ferrite grains in the sheet thickness direction is set within a predetermined range to secure cold rolling properties. Specifically, fine ferrite grains are relatively increased, and coarse ferrite grains are relatively increased. The reason why the cold rolling properties can be secured by setting the distribution state of the ferrite grains in the sheet thickness direction within a predetermined range is not clear, but the inventors assume the reason as follows. The coexistence of fine grains and coarse grains results in the production of a distribution of strain applied to the crystal grains. It is assumed that, in a case where the distribution is present, the strain can be released from grains having high strain to grains having low strain (strain dispersion) during rolling, and thus larger cold rolling properties can be secured than in a case where a normal distribution is present. In order to more accurately evaluate the distribution state of the grain sizes of the ferrite grains, among all the ferrite grains, ferrite grains excluding the smallest 5% and the largest 5% in grain size are defined as ferrite grains for evaluation to evaluate the distribution state of the grain sizes.
Specifically, in the ferrite grains to be measured at a depth position of 1/4 of the sheet thickness by the electron back scattering diffraction (EBSD) method described above, ferrite grains excluding ferrite grains occupying 5% of the total number of the ferrite grains from the maximum grain size side and ferrite grains occupying 5% of the total number of the ferrite grains from the minimum grain size side of the ferrite grains are defined as evaluation ferrite grains. In this case, in a case where a number n of grains from the maximum grain size side or the minimum grain size side is less than 5% of the total number and a number n + 1 of grains is more than 5% of the total number, n + 1 grains are excluded. Here, a minimum value of the grain size of the evaluation ferrite grains is set as a first grain size, a maximum value of the grain size of the evaluation ferrite grains is set as a second grain size, a grain size obtained by adding, to the first grain size, 1/3 of a difference between the second grain size and the first grain size is set as a third grain size, and a grain size obtained by adding, to the first grain size, 2/3 of the difference between the second grain size and the first grain size is set as a fourth grain size. In addition, a range that is equal to or more than the first grain size and equal to or less than the third grain size is set as a first grain size range, a range that is more than the third grain size and equal to or less than the fourth grain size is set as a second grain size range, and a range that is more than the fourth grain size and equal to or less than the second grain size is set as a third grain size range.
The number of evaluation ferrite grains in the first grain size range of the steel sheet according to the present embodiment is 2.5 times or more and 3.0 times or less the number of evaluation ferrite grains in the second grain size range. In addition, the number of evaluation ferrite grains in the third grain size range of the steel sheet according to the present embodiment is 2.0 times or more and 2.5 times or less the number of evaluation ferrite grains in the second grain size range. By adjusting the number of the evaluation ferrite grains in the first grain size range to 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range and setting the number of the evaluation ferrite grains in the third grain size range to 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range, the evaluation ferrite grains in each grain size range have a clear distribution, and thus the strain dispersion is noticeable. Therefore, the cold rolling properties of the steel sheet can be improved. The number of the evaluation ferrite grains in the first grain size range of the steel sheet according to the present embodiment is preferably 2.6 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range. The number of the evaluation ferrite grains in the third grain size range of the steel sheet according to the present embodiment is preferably 2.2 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range.
The number of the evaluation ferrite grains in the first grain size range of the steel sheet according to the present embodiment is preferably 1.1 times or more and 1.4 times or less the number of the evaluation ferrite grains in the third grain size range.
The number of the evaluation ferrite grains in the first grain size range of the steel sheet according to the present embodiment is more preferably 1.2 times or more and 1.4 times or less the number of the evaluation ferrite grains in the third grain size range.
In a case where the number of the evaluation ferrite grains in the first grain size range of the steel sheet according to the present embodiment is 1.1 times or more and 1.4 times or less the number of the evaluation ferrite grains in the third grain size range, the cold rolling properties of the steel sheet can be further improved.
The average grain size of the evaluation ferrite grains in the first grain size range is preferably 1 µm to 10 µm. The average grain size of the evaluation ferrite grains in the first grain size range is more preferably 1 µm to 6 µm. In a case where the average grain size of the evaluation ferrite grains in the first grain size range is 1 µm to 10 µm, a large amount of strain can be accumulated in fine crystal grains, and the strain can be efficiently dispersed to larger crystal grains. Therefore, the cold rolling properties of the steel sheet can be further improved.
The average grain size of the evaluation ferrite grains in the second grain size range is preferably more than 10 µm and 35 µm or less. The average grain size of the evaluation ferrite grains in the second grain size range is more preferably 15 µm to 35 µm, and still more preferably 20 µm to 30 µm. In a case where the average grain size of the evaluation ferrite grains in the second grain size range is more than 10 µm and 35 µm or less, the cold rolling properties of the steel sheet can be further improved.
The average grain size of the evaluation ferrite grains in the third grain size range is preferably more than 35 µm and 80 µm or less. The average grain size of the evaluation ferrite grains in the third grain size range is more preferably 40 µm to 80 µm, and still more preferably 60 µm to 80 µm. In a case where the average grain size of the evaluation ferrite grains in the third grain size range is more than 35 µm and 80 µm or less, the cold rolling properties of the steel sheet can be further improved.
Next, a method of measuring ferrite grains at a depth position of 1/4 of the sheet thickness will be described.
The identification of ferrite and the calculation of the grain size can be performed by observing a cross section of the steel sheet parallel to the rolling direction and perpendicular to the sheet surface at 1,000 to 50,000-fold magnification using electron back scattering diffraction (EBSD) and a scanning electron microscope. In this case, in the present invention, for example, software "OIM Data Collection TM (ver. 7)" manufactured by TSL Solutions, Inc. is used as software for acquiring crystal orientation data.
An observation surface is finished by colloidal silica polishing or electrolytic polishing, and the measurement is performed at intervals (pitch) of 0.2 µm in a square region ranging from 1/8 to 3/8 of the thickness, centered at a position that is at a depth of 1/4 of the sheet thickness from the surface of the steel sheet (a square having a side whose length ranges from 1/8 to 3/8 along the sheet thickness direction), by EBSD attached to a scanning electron microscope. Conditions for sample preparation and the like are set within the ranges of the conditions recommended in the "Standards for crystal orientation difference measurement for material evaluation by electron back scattering diffraction (EBSD) method" of the JSMS Standards. The value of grain average misorientation (GAM) is calculated from the measurement data. In addition, a region in which the value of grain average misorientation is less than 0.5° is defined as a ferrite grain. Here, a boundary having a crystal orientation difference of 15° or more is determined as a grain boundary, and a region surrounded by the grain boundary is defined as a crystal grain. The grain average misorientation is a value obtained by calculating an orientation difference between adjacent measurement points and averaging the orientation differences among all measurement points in the crystal grains. Grain sizes of the obtained ferrite grains are measured. The grain size is an equivalent circle diameter. Here, the equivalent circle diameter of a crystal grain means a diameter of a circle having an area equal to the area of the crystal grain.

[Total Elongation is 40% or More]

The total elongation of the steel sheet according to the present embodiment is preferably 40% or more from the viewpoint of cold rolling properties. The total elongation is more preferably 50% or more.
The total elongation can be obtained by performing a tensile test according to JIS Z 2241: 2011 with a JIS No. 5 tensile test piece collected from the steel sheet in a direction perpendicular to the rolling direction and the sheet thickness direction.

The sheet thickness of the steel sheet according to the present embodiment is not limited. The sheet thickness of the steel sheet according to the present embodiment may be 1.5 mm to 5.0 mm, for example.

The steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (IV):

(I) a heating step of heating a slab having the above-described chemical composition to 1,100°C or higher and lower than 1,350°C;
(II) a finish rolling step of continuously passing the slab after the heating step through a plurality of rolling stands to perform rolling so that σ determined by Expression (1) satisfies 50 ≤ σ ≤ 80 in each of the initial four rolling stands and satisfies 3 ≤ σ ≤ 10 in the final stand;
(III) a cooling step of cooling the slab to a temperature range of 550°C to 650°C at an average cooling rate of 30 °C/s or higher by starting the cooling within 0.2 seconds to 2.0 seconds after the end of the finish rolling step; and (IV) a coiling step of performing coiling so that a coiling temperature range is 550°C to 650°C after the cooling step.

Hereinafter, the steps will be described. Hereinbelow, each temperature is a surface temperature of a slab or a steel sheet.

[Heating Step]

In the heating step, a slab having the same chemical composition as the above-described steel sheet according to the present embodiment is preferably heated to 1,100°C or higher and lower than 1,350°C. In a case where the heating temperature is lower than 1,100°C, homogenization of materials may become insufficient. In addition, in a case where the heating temperature is set to 1,350°C or higher, it is difficult for the number of the evaluation ferrite grains in the first grain size range, the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range to satisfy a predetermined relationship.

[Finish Rolling Step]

In the finish rolling step, the slab after the heating step is subjected to rough rolling as necessary, and then rolled by being continuously passed through a plurality of rolling stands. In the manufacturing method of the steel sheet according to the present embodiment, the slab after the heating step is preferably rolled so that σ (unit: kgf/mm²) determined by Expression (1) satisfies 50 ≤ σ ≤ 80 in each of the initial four rolling stands and satisfies 3 ≤ σ ≤ 10 in the final rolling stand. $σ = \exp (0.753 + 3000 / T) \times ε^{0.21} \times ε^{' 0.13}$
Here, T is a temperature (K) immediately before entering the stand, ε is an equivalent plastic strain, and ε' is a strain rate (/s).
By adjusting σ determined by Expression (1) to 50 or more in each of the initial four rolling stands, the amount of the strain accumulated in the steel sheet during and immediately after processing can be increased, and the crystal grains formed by recrystallization during and after processing can be appropriately refined. In addition, by controlling rolling conditions of the final rolling stand to be described later, the number of the evaluation ferrite grains in the first grain size range, the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range after the austenite to ferrite transformation are likely to satisfy a predetermined relationship.
It is difficult to perform the rolling so that σ determined by Expression (1) is more than 80 in each of the initial four rolling stands. In addition to this, in a case where the rolling is performed so that σ is more than 80, an excessively fine structure is made, and thus even in a case where the rolling conditions of the final rolling stand to be described below are controlled, it is difficult for the number of the evaluation ferrite grains in the first grain size range, the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range after the austenite to ferrite transformation to satisfy a predetermined relationship.
In the finish rolling step, in the final rolling stand, the rolling is preferably performed to satisfy 3 ≤ σ ≤ 10. In a case where the rolling is performed so that σ determined by Expression (1) satisfies 50 ≤ σ ≤ 80 in each of the initial four rolling stands and satisfies 3 ≤ σ ≤ 10 in the final rolling stand, the amount of strain that is introduced varies between the crystal grains. Therefore, the grain growth selectively occurs in the coiling step, and a duplex grain structure of the ferrite grains, which is characteristic in the present invention, is formed after the austenite to ferrite transformation.
The finish rolling start temperature is preferably 1,000°C or higher. Therefore, appropriate strain is likely to be accumulated in each of the initial four rolling stands. In a case where the finish rolling start temperature is 1,000°C or higher, strain appropriate for austenite grains during and immediately after processing is likely to be accumulated in each of the initial four rolling stands.
The interpass time between the rolling stands in the finish rolling step is preferably 10.0 seconds or shorter. Therefore, strain appropriate for austenite grains during and immediately after processing is likely to be accumulated. The lower limit does not need to be particularly limited and may be adjusted to be as short as possible. However, the lower limit is set to be about 0.1 seconds in consideration of a practical equipment configuration. Furthermore, appropriate strain is likely to be accumulated by adjusting the interpass time between the rolling stands in the finish rolling step to 0.2 seconds or longer and 3.0 seconds or shorter.
The temperature on the outlet side of the final rolling stand in the finish rolling step is preferably adjusted to 850°C or higher and 1,000°C or lower. In a case where the temperature on the outlet side of the final rolling stand is adjusted to 850°C or higher and 1,000°C or lower, strain appropriate for austenite grains during and immediately after processing is likely to be accumulated in the final rolling stand.
The cumulative rolling reduction in the finish rolling step is preferably adjusted to 60% or more. In a case where the cumulative rolling reduction is adjusted to 60% or more, strain appropriate for austenite grains during and immediately after processing is likely to be accumulated.

"Cooling Step"

In the cooling step, the cooling is preferably started between 0.2 seconds and 2.0 seconds after the end of the finish rolling step. Cooling at 30 °C/sec or faster between 0.2 seconds to 2.0 seconds can stop the growth of austenite grains. The average cooling rate is preferably 30 °C/sec or faster. The upper limit of the average cooling rate is not particularly limited, but is 180°C/sec or slower, for example. Therefore, after the austenite to ferrite transformation, the number of grains of the evaluation ferrite crystals in the first grain size range is likely to be 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range is likely to be 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range. The average cooling rate can be obtained by dividing a difference between the temperature at the start of cooling and the temperature at the end of cooling by a time from the start of cooling to the end of cooling.
In the cooling step, cooling to a temperature range of 550°C to 650°C (cooling stop temperature range) is preferable. Due to the cooling to a temperature range of 550°C to 650°C, the formation of martensite, bainite, and the like is suppressed, and the formation of pearlite and ferrite can be promoted.

"Coiling Step"

In the coiling step, the coiling is performed so that a coiling temperature is within a temperature range of 550°C to 650°C after the cooling step. Due to the coiling within a temperature range of 550°C to 650°C, the formation of martensite, bainite, and the like is suppressed, and the formation of pearlite and ferrite can be promoted. Examples
The present invention will be described in greater detail with reference to examples.

Slabs having the chemical compositions shown in Tables 1 and 2 were cast. The slabs after casting were heated under the conditions shown in Table 3 and subjected to finish rolling. After the finish rolling, cooling and coiling were performed under the conditions shown in Table 4. The average interpass time between rolling stands in Table 3 is an average value of the interpass times between the rolling stands. The interpass time between the rolling stands in each example was 0.2 seconds or longer and 3.0 seconds or shorter.

[Table 1]

Component	Remainder: Fe and Impurities (mass%)
Component	C	Si	Mn	Al	P	S	N	O	Cr	B	Nb	Mo	V
a	0.02	0.020	0.90	0.002	0.0012	0.008	0.004	0.0190
b	0.19	2.700	0.10	0.621	0.0090	0.019	0.010	0.0017	0.500	0.009		0.90
C	0.06	1.700	2.20	0.057	0.0100	0.014	0.007	0.0007			0.09		0.47
d	0.17	1.300	3.90	0.998	0.0200	0.018	0.003	0.0010		0.010
e	0.07	2.900	1.10	0.027	0.0046	0.010	0.010	0.0022	1.900
f	0.24	0.120	1.30	0.790	0.0097	0.010	0.001	0.0001
g	0.01	0.190	1.40	0.035	0.0008	0.015	0.003	0.0018				0.10
h	0.12	3.800	3.40	0.286	0.0099	0.019	0.010	0.0019		0.004
i	0.13	0.005	3.50	0.014	0.0057	0.011	0.010	0.0002	0.100
j	0.16	1.200	4.60	0.057	0.0069	0.012	0.003	0.0018			0.10
k	0.03	0.800	0.05	0.878	0.0025	0.018	0.002	0.0010					0.10
1	0.18	1.900	0.60	1.500	0.0088	0.016	0.002	0.0015
m	0.11	1.700	2.90	0.039	0.0080	0.017	0.090	0.0009
n	0.13	1.200	2.90	0.471	0.0400	0.060	0.004	0.0018
o	0.15	1.100	1.90	0.572	0.0093	0.012	0.019	0.0017
p	0.03	1.500	3.00	0.792	0.0073	0.017	0.008	0.0520

[Table 2]

Component	Remainder: Fe and Impurities (mass%)
Component	Ti	Cu	w	Ta	Ni	Mg	Ca	Y	Zr	La	Co	Sn	Sb	As
a
b	0.2800 0.2800										0.400
c								0.030					0.03
d				0.100	0.300		0.002		0.002		0.002
e		0.400	0.100			0.002		0.001		0.001		0.001	0.001	0.001
f	0.3900
g
h
i
j
k
l						0.002
m	0.2000							0.020
n							0.003
o				0.020
P										0.020

[Table 3A]

Test No.	Component	Heating Step	Finish Rolling Step									Sheet Thickness	Remarks
Test No.	Component	Heating Temperature (°C)	σ in First Rolling Stand (kgf/mm²)	σ in Second Rolling Stand from First (kgf/mm²)	σ in Third Rolling Stand from First (kgf/mm²)	σ in Fourth Rolling Stand from First (kgf/mm²)	σ in Final Rolling Stand (kgf/mm²)	Finish Rolling Start Temperature (°C)	Average Interpass Time Between Rolling Stands (sec)	Temperature on Outlet Side of Final Rolling Stand (°C)	Cumulative Rolling Reduction (%)	Sheet Thickness	Remarks
1	a	1241	73	74	68	58	7	1171	0.5	919	92	2.6	Invention Example
2	a	1120	73	52	75	58	8	1042	2.9	862	85	1.5	Invention Example
3	a	1208	50	53	72	58	3	1032	0.3	853	82	2.8	Invention Example
4	a	1206	60	50	79	57	2	969	0.8	907	81	2.5	Comparative Example
5	a	1251	50	78	51	58	13	1178	0.2	824	81	4.9	Comparative Example
6	b	1207	54	58	67	66	9	1147	2.6	904	85	4.4	Invention Example
7	b	1221	68	55	76	54	10	1054	2.0	942	87	3.9	Invention Example
8	b	1390	52	66	77	79	9	1074	1.0	888	81	2.9	Comparative Example
9	b	1080	72	64	63	38	4	1004	3.0	853	78	2.9	Comparative Example
10	c	1109	64	58	72	68	9	1157	2.6	852	90	1.5	Invention Example
11	c	1148	74	54	67	55	7	1118	0.7	905	80	1.5	Invention Example
12	c	1244	53	68	67	87	5	1185	1.0	888	96	2.9	Comparative Example
13	c	1177	54	66	58	61	9	1033	2.1	888	97	1.7	Comparative Example
14	d	1333	52	56	64	59	10	1009	0.9	920	81	1.7	Invention Example
15	d	1226	51	63	50	53	9	1065	2.0	917	96	1.9	Invention Example
16	d	1101	75	62	74	61	4	1054	1.5	998	95	4.6	Comparative Example
17	d	1264	69	63	55	73	9	1048	1.8	929	80	1.5	Comparative Example

[Table 3B]

Test No.	Component	Heating Step	Finish Rolling Step									Sheet Thickness	Remarks
Test No.	Component	Heating Temperature (°C)	σ in First Rolling Stand (kgf/mm²)	σ in Second Rolling Stand from First (kgf/mm²)	σ in Third Rolling Stand from First (kgf/mm²)	σ in Fourth Rolling Stand from First (kgf/mm²)	o in Final Rolling Stand (kgf/mm²)	Finish Rolling Start Temperature (°C)	Average Interpass Time Between Rolling Stands (sec)	Temperature on Outlet Side of Final Rolling Stand (°C)	Cumulative Rolling Reduction (%)	Sheet Thickness	Remarks
18	e	1147	77	78	60	65	7	1137	1.4	897	81	2.3	Invention Example
19	e	1336	52	56	69	76	8	1056	3.0	885	92	1.5	Invention Example
20	e	1340	62	59	72	82	18	1037	1.4	805	95	2.4	Comparative Example
21	f	1274	72	56	62	51	5	1020	2.8	920	88	3.5	Comparative Example
22	g	1312	78	71	60	64	6	1111	1.5	855	83	4.1	Comparative Example
23	h	1205	71	61	63	67	8	1001	1.7	840	84	2.3	Comparative Example
24	i	1235	66	77	76	54	7	1028	1.1	998	88	4.1	Comparative Example
25	j	1114	66	50	64	64	6	1134	1.3	897	98	1.9	Comparative Example
26	k	1185	66	58	72	43	7	1073	2.4	832	90	2.2	Comparative Example
27	l	1200	71	77	74	74	8	1053	1.5	834	93	3.4	Comparative Example
28	m	1266	73	75	62	70	9	1127	2.7	912	84	2.5	Comparative Example
29	n	1173	76	51	71	79	4	1013	3.0	908	86	1.5	Comparative Example
30	o	1214	57	69	62	61	9	1140	1.8	804	89	3.7	Comparative Example
31	p	1159	51	74	66	56	8	1078	2.7	809	91	3.7	Comparative Example
32	a	1261	65	64	58	52	7	1151	1.5	919	92	2.8	Invention Example
33	b	1150	68	52	75	58	8	1040	2.9	862	85	5.0	Invention Example

[Table 4]

Test No.	Cooling Step			Coiling Step	Remarks
Test No.	Time Until Cooling is Started (sec)	Cooling Rate (°C/sec)	Cooling Stop Temperature (°C)	Coiling Temperature (°C)	Remarks
1	1.4	177	592	607	Invention Example
2	0.3	77	578	627	Invention Example
3	0.2	127	581	646	Invention Example
4	0.4	154	629	614	Comparative Example
5	0.2	62	639	609	Comparative Example
6	0.8	117	567	585	Invention Example
7	0.6	91	619	634	Invention Example
8	0.7	75	618	570	Comparative Example
9	0.6	87	578	610	Comparative Example
10	0.7	84	645	622	Invention Example
11	0.8	67	629	611	Invention Example
12	0.7	123	560	585	Comparative Example
13	3.0	69	629	645	Comparative Example
14	0.5	166	638	630	Invention Example
15	0.8	177	565	649	Invention Example
16	0.2	149	680	678	Comparative Example
17	0.2	121	472	464	Comparative Example
18	0.7	32	643	642	Invention Example
19	0.8	141	584	582	Invention Example
20	0.3	76	568	617	Comparative Example
21	0.6	70	605	630	Comparative Example
22	0.7	120	585	650	Comparative Example
23	0.1	31	570	564	Comparative Example
24	0.4	111	603	645	Comparative Example
25	0.4	167	565	601	Comparative Example
26	0.6	110	583	613	Comparative Example
27	1.0	71	627	588	Comparative Example
28	0.7	35	553	620	Comparative Example
29	0.4	121	645	648	Comparative Example
30	0.6	33	602	632	Comparative Example
31	0.6	83	642	571	Comparative Example
32	0.8	97	587	585	Invention Example
33	0.6	81	609	634	Invention Example

A test piece for SEM observation was collected from the obtained hot-rolled steel sheet as described above, and a cross section thereof in a sheet thickness direction that was parallel to a rolling direction was polished. Then, a microstructure at a depth position of 1/4 of a sheet thickness was observed by the above-described method to obtain area ratios of ferrite, pearlite, bainite, martensite, and residual austenite at the depth position of 1/4 of the sheet thickness. The obtained results are shown in Table 5. Similarly, the grain sizes of ferrite grains were measured by the above-described method, and a number N1 and an average grain size of evaluation ferrite grains in a first grain size range, a number N2 and an average grain size of evaluation ferrite grains in a second grain size range, and a number N3 and an average grain size of evaluation ferrite grains in a third grain size range were evaluated. The obtained results are shown in Table 5. A ratio (N1/N2) of the number N1 of the evaluation ferrite grains in the first grain size range to the number N2 of the evaluation ferrite grains in the second grain size range, a ratio (N3/N2) of the number N3 of the evaluation ferrite grains in the third grain size range to the number N2 of the evaluation ferrite grains in the second grain size range, and a ratio (N1/N3) of the number N1 of the evaluation ferrite grains in the first grain size range to the number N2 of the evaluation ferrite grains in the third grain size range are shown in Table 6.
The total elongation was obtained by performing a tensile test according to JIS Z 2241: 2011 with a JIS No. 5 tensile test piece collected from the hot-rolled steel sheet in a direction perpendicular to the rolling direction. The obtained results are shown in Table 6.
Since strain is applied in a complicated manner during cold rolling, elongation tests that can evaluate only simple strain are insufficient. Therefore, 90°-bending workability was evaluated as the cold rolling properties of the hot-rolled steel sheet by a 90°-V-block test. When the sheet thickness of a steel sheet was denoted by t and the minimum inside bend radius of a punch was denoted by R, the test piece was pushed into a 90-degree die using a 90-degree punch having a curvature at which a ratio R/t was 1, and then the test piece was taken out to visually observe the outside of the bend. The results of visual observation were represented by "X" in a case where cracks occurred and by "O" in a case where no abnormality was shown. A maximum width of a gap of 1 mm or more was defined as "crack". The obtained results are shown in Table 6.

As can be seen from Table 6, all of the steels of the present invention were excellent in cold rolling properties.

[Table 5A]

Test No.	Microstructure at Depth Position of 1/4 of Sheet Thickness (area%)						Evaluation Ferrite Grains at Depth Position of 1/4 of Sheet Thickness						Remarks
	Ferrite	Pearlite	Remainder in Microstructure				First Grain Size Range		Second Grain Size Range		Third Grain Size Range
	Ferrite	Pearlite	Bainite	Martensite	Residual Austenite-	Total Remainder in Microstructure	Number of Grains (number)	Average Grain Size (µm)	Number of Grains (number)	Average Grain Size (µm)	Number of Grains (number)	Average Grain Size (µm)
1	69	17	4	5	5	14	135	10	50	23	102	62	Invention Example
2	60	18	16	3	3	22	117	7	41	31	95	65	Invention Example
3	38	20	31	7	4	42	130	9	52	28	114	54	Invention Example
4	21	18	2	58	1	61	32	1	125	30	256	50	Comparative Example
5	24	16	5	55	0	60	55	8	146	24	365	66	Comparative Example
6	38	19	35	4	4	43	105	10	36	31	74	54	Invention Example
7	47	12	41	0	0	41	126	4	49	20	102	51	Invention Example
8	42	16	34	4	4	42	103	8	179	24	410	69	Comparative Example
9	12	6	8	72	2	82	123	3	124	34	306	63	Comparative Example
10	48	49	3	0	0	3	113	1	42	18	91	51	Invention Example
11	77	10	8	4	1	13	123	4	43	33	93	60	Invention Example
12	17	20	55	4	4	63	53	6	138	17	305	40	Comparative Example
13	82	10	1	0	7	8	126	8	145	32	309	42	Comparative Example
14	61	15	6	15	3	24	130	1	47	22	106	40	Invention Example
15	37	12	7	43	1	51	100	1	35	29	73	73	Invention Example
16	33	4	61	1	1	63	107	9	123	20	290	51	Comparative Example
17	55	11	6	14	14	34	109	1	146	19	356	71	Comparative Example

[Table 5B]

Test No.	Microstructure at Depth Position of 1/4 of Sheet Thickness (area%)						Evaluation Ferrite Grains at Depth Position of 1/4 of Sheet Thickness						Remarks
	Ferrite	Pearlite	Remainder in Microstructure				First Grain Size Range		Second Grain Size Range		Third Grain Size Range
	Ferrite	Pearlite	Bainite	Martensite	Residual Austenite	Total Remainder in Microstructure	Number of Grains (number)	Average Grain Size (µm)	Number of Grains (number)	Average Grain Size (µm)	Number of Grains (number)	Average Grain Size (µm)
18	34	16	40	5	5	50	123	1	44	20	103	(µm) 49	Invention Example
19	33	11	6	48	2	56	101	2	34	23	77	79	Invention Example
20	72	11	7	5	5	17	110	10	101	29	237	47	Comparative Example
21	32	18	42	4	4	50	105	2	99	33	247	66	Comparative Example
22	35	13	52	0	0	52	107	7	138	27	302	77	Comparative Example
23	33	13	44	5	5	54	120	7	100	19	217	47	Comparative Example
24	76	19	5	0	0	5	100	1	93	19	227	66	Comparative Example
25	52	13	35	0	0	35	101	5	144	25	340	69	Comparative Example
26	71	12	9	8	0	17	111	4	139	22	286	80	Comparative Example
27	61	18	8	8	5	21	130	7	144	21	331	42	Comparative Example
28	61	13	10	4	12	26	115	5	104	29	253	50	Comparative Example
29	36	18	12	32	2	46	112	7	122	26	265	48	Comparative Example
30	36	12	9	39	4	52	115	6	149	18	337	56	Comparative Example
31	67	11	20	1	1	22	114	2	108	18	244	68	Comparative Example
32	48	19	25	4	4	33	130	1	47	12	106	65	Invention Example
33	57	12	31	0	0	31	100	1	35	17	73	37	Invention Example

[Table 6]

Test No.	N1/N2	N3/N2	N1/N3	Total Elongation (%)	Cold Rolling Properties	Remarks
1	2.7	2.0	1.3	51	O	Invention Example
2	2.9	2.3	1.2	41	O	Invention Example
3	2.5	2.2	1.1	50	O	Invention Example
4	0.3	2.1	0.1	36	X	Comparative Example
5	0.4	2.5	0.2	33	X	Comparative Example
6	2.9	2.1	1.4	56	O	Invention Example
7	2.6	2.1	1.2	55	O	Invention Example
8	0.6	2.3	0.3	27	X	Comparative Example
9	1.0	2.5	0.4	24	X	Comparative Example
10	2.7	2.2	1.2	57	O	Invention Example
11	2.9	2.2	1.3	47	O	Invention Example
12	0.4	2.2	0.2	24	X	Comparative Example
13	0.9	2.1	0.4	22	X	Comparative Example
14	2.8	2.3	1.2	47	O	Invention Example
15	2.9	2.1	1.4	43	O	Invention Example
16	0.9	2.4	0.4	31	X	Comparative Example
17	0.7	2.4	0.3	24	X	Comparative Example
18	2.8	2.3	1.2	46	O	Invention Example
19	2.9	2.3	1.3	58	O	Invention Example
20	1.1	2.4	0.5	30	X	Comparative Example
21	1.1	2.5	0.4	30	X	Comparative Example
22	0.8	2.2	0.4	31	X	Comparative Example
23	1.2	2.2	0.6	24	X	Comparative Example
24	1.1	2.4	0.4	34	X	Comparative Example
25	0.7	2.4	0.3	26	X	Comparative Example
26	0.8	2.1	0.4	26	X	Comparative Example
27	0.9	2.3	0.4	26	X	Comparative Example
28	1.1	2.4	0.5	20	X	Comparative Example
29	0.9	2.2	0.4	33	X	Comparative Example
30	0.8	2.3	0.3	36	X	Comparative Example
31	1.1	2.3	0.5	25	X	Comparative Example
32	2.8	2.3	1.2	14	O	Invention Example
33	2.9	2.1	1.4	3	O	Invention Example

INDUSTRIAL APPLICABILITY

A hot-rolled steel sheet of the present disclosure has excellent cold rolling properties, and thus has high industrial applicability.

Claims

A hot-rolled steel sheet comprising, as a chemical composition, by mass%:
C: 0.02% or more and less than 0.20%;

Si: 0.010% to 3.000%;

Mn: 0.10% to 4.00%;

Al: 0.001% to 1.000%;

P: 0.0200% or less;

S: 0.020% or less;

N: 0.020% or less;

O: 0.0200% or less;

Cr: 0% to 2.000%;

B: 0% to 0.010%;

Nb: 0% to 0.10%;

Mo: 0% to 1.00%;

V: 0% to 0.50%;

Ti: 0% to 0.4000%;

Cu: 0% to 0.500%;

W: 0% to 0.100%;

Ta: 0% to 0.100%;

Ni: 0% to 1.000%;

Mg: 0% to 0.050%;

Ca: 0% to 0.040%;

Y: 0% to 0.050%;

Zr: 0% to 0.050%;

La: 0% to 0.050%;

Co: 0% to 0.500%;

Sn: 0% to 0.050%;

Sb: 0% to 0.050%;

As: 0% to 0.050%; and

a remainder of Fe and impurities,

wherein, at a depth position of 1/4 of a sheet thickness, a microstructure is formed of

30 area% or more of ferrite,

10 area% or more of pearlite, and
0 area% or more and 60 area% or less of a remainder in microstructure,

the remainder in microstructure includes at least one of bainite, martensite, and residual austenite,

among ferrite grains to be measured by an electron back scattering diffraction method, ferrite grains excluding ferrite grains occupying 5% of a total number of the ferrite grains from a maximum grain size side and ferrite grains occupying 5% of the total number of the ferrite grains from a minimum grain size side of the ferrite grains are treated as evaluation ferrite grains,

a minimum value of a grain size of the evaluation ferrite grains is set as a first grain size,

a maximum value of the grain size of the evaluation ferrite grains is set as a second grain size,

a grain size obtained by adding, to the first grain size, 1/3 of a difference between the second grain size and the first grain size is set as a third grain size,

a grain size obtained by adding, to the first grain size, 2/3 of the difference between the second grain size and the first grain size is set as a fourth grain size, and

when a range that is equal to or more than the first grain size and equal to or less than the third grain size is set as a first grain size range,

a range that is more than the third grain size and equal to or less than the fourth grain size is set as a second grain size range, and

a range that is more than the fourth grain size and equal to or less than the second grain size is set as a third grain size range,

the number of the evaluation ferrite grains in the first grain size range is 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range, and

the number of the evaluation ferrite grains in the third grain size range is 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range.
The hot-rolled steel sheet according to Claim 1,
wherein an average grain size of the evaluation ferrite grains in the first grain size range is 1 µm to 10 µm.
The hot-rolled steel sheet according to Claim 1,
wherein an average grain size of the evaluation ferrite grains in the third grain size range is more than 35 µm and 80 µm or less.
The hot-rolled steel sheet according to Claim 1,
wherein an average grain size of the evaluation ferrite grains in the third grain size range is 40 µm to 80 µm.
The hot-rolled steel sheet according to any one of Claims 1 to 4,
wherein a total elongation is 40% or more.