EP4675001A1

EP4675001A1 - Steel plate and method for manufacturing same

Info

Publication number: EP4675001A1
Application number: EP24764043.6A
Authority: EP
Inventors: Yusuke TSUNEMI
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-03-01
Filing date: 2024-03-01
Publication date: 2026-01-07
Also published as: JPWO2024181566A1; CN120813716A; MX2025009972A; WO2024181566A1; KR20250140085A; JP7849641B2

Abstract

Provided are a steel sheet and a method for manufacturing the same, the steel sheet having a predetermined chemical composition, in which a microstructure at a 1/4 thickness position that is an area centered on 1/4 position in thickness with a range of 1/8 to 3/8 of a sheet thickness of the steel sheet in a sheet thickness direction of the steel sheet from a surface of the steel sheet, includes, by area ratio, 0% to 60% of ferrite, 0% to 3% of residual austenite, and a remainder containing one or more selected from martensite, bainite, pearlite, and cementite, in a range of up to 2 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is 95% or more, and an in-plane average grain size of the ferrite in an in-plane direction is 2.0 µm or less, in a range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is less than 90%, and a tensile strength of the steel sheet is 950 MPa or more.

Description

TECHNICAL FIELD

The present invention relates to a steel sheet and a method for manufacturing the same.
Priority is claimed on Japanese Patent Application No. 2023-030697, filed March 1, 2023 , the content of which is incorporated herein by reference.

BACKGROUND ART

In order to reduce a weight of a vehicle to improve fuel efficiency and reduce carbon dioxide emissions and to secure safety of an occupant during a collision or the like, a high strength steel sheet is used as a steel sheet for a vehicle. In addition, the high strength steel sheet to be used for a component for a vehicle is required to have not only strength but also properties necessary for forming components, such as bendability.
When the steel sheet is formed into a predetermined member or is subjected to a collision, the steel sheet undergoes bending deformation. In general, the bendability of the steel sheet decreases as the strength increases. Therefore, the present inventors recognized the importance of increasing bendability as formability and bendability as collision performance while maintaining high strength.
For example, as a method for improving bendability of a high strength steel sheet, there are a method of softening a surface layer area of a steel sheet (Patent Document 1) and a method of specifying a microstructure (Patent Document 2). In these methods, a deformation capability is improved by forming the surface layer area of the steel sheet into a ferrite primary phase or a decarburized ferrite phase. However, on the other hand, a hard phase that may serve as a crack origin remains, and thus there is room for improvement in bendability in terms of both the properties described above.

Citation List

Patent Documents

Patent Document 1: Japanese Patent No. 7103509
Patent Document 2: Japanese Patent No. 6536294
Patent Document 3: Japanese Patent No. 6154794

SUMMARY OF INVENTION

Technical Problem

As a method for evaluating bendability of a steel sheet, there are a method in which cracks on a surface after bending are observed (a typical example is a 90° V-bending test) and a method in which cracks are determined by a load change during bending (a typical example is a VDA (German Association of the Automotive Industry) bending test). For example, Patent Document 3 proposes a method for evaluating cracking susceptibility during a collision using a VDA bending test. In the evaluation of the VDA bending test, fracture is determined based on a peak or a decrease in load. However, as a result of an investigation conducted by the present inventors, it has been found that fracture occurs already before the peak of the load. Therefore, in a case where formability or the presence or absence of cracking is important, good bendability is required in the 90° V-bending test, and in a case where collision performance or load transmission characteristics are important, good bendability is required in the VDA bending test.
In view of the above, an object of the present invention is to provide a steel sheet having high strength, excellent bendability as formability, and excellent bendability as collision characteristics and a method for manufacturing the same.

Solution to problem

The gist of the present invention is as follows.

(1) A steel sheet according to one aspect of the present invention includes, as a chemical composition, by mass%: C: 0.070% to 0.15%; Si: 0.10% to 2.00%; Mn: 1.00% to 4.00%; sol. Al: 0.001% to 1.500%; P: 0.0010% to 0.0300%; S: 0.0200% or less; N: 0.0100% or less; O: 0.0100% or less; Ti: 0% to 0.200%; B: 0% to 0.0100%; Cr: 0% to 1.000%; Mo: 0% to 1.000%; Ni: 0% to 1.000%; Cu: 0% to 1.000%; Sn: 0% to 0.500%; Nb: 0% to 0.200%; V: 0% to 0.500%; W: 0% to 0.500%; Ca: 0% to 0.0100%; Mg: 0% to 0.0100%; Bi: 0% to 0.0100%; Sb: 0% to 0.1000%; Zr: 0% to 0.0100%; REM: 0% to 0.1000%; and a remainder: Fe and impurities, in which a microstructure at a 1/4 thickness position that is an area centered on 1/4 position in thickness with a range of 1/8 to 3/8 of a sheet thickness of the steel sheet in a sheet thickness direction of the steel sheet from a surface of the steel sheet, includes, by area ratio, 0% to 60% of ferrite, 0% to 3% of residual austenite, and a remainder containing one or more selected from martensite, bainite, pearlite, and cementite, in a range of up to 2 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is 95% or more, and an in-plane average grain size of the ferrite in an in-plane direction is 2.0 µm or less, in a range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is less than 90%, and a tensile strength of the steel sheet is 950 MPa or more.
(2) In the steel sheet according to (1), the tensile strength may be less than 1,300 MPa.
(3) In the steel sheet according to (1) or (2), a transition region based on a C concentration in the sheet thickness direction of the steel sheet may be 150 µm or less.
(4) In the steel sheet according to any one of (1) to (3), in the range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a fresh martensite fraction may be 10% or less.
(5) In the steel sheet according to any one of (1) to (4), in the range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction may be 50% or more.
(6) A method for manufacturing a steel sheet according to an aspect of the present invention includes: performing cold rolling on a steel sheet including, as a chemical composition, by mass%, C: 0.070% to 0.15%, Si: 0.10% to 2.00%, Mn: 1.00% to 4.00%, sol. Al: 0.001% to 1.500%, P: 0.0010% to 0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0% to 0.200%, B: 0% to 0.0100%, Cr: 0% to 1.000%, Mo: 0% to 1.000%, Ni: 0% to 1.000%, Cu: 0% to 1.000%, Sn: 0% to 0.500%, Nb: 0% to 0.200%, V: 0% to 0.500%, W: 0% to 0.500%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Bi: 0% to 0.0100%, Sb: 0% to 0.1000%, Zr: 0% to 0.0100%, REM: 0% to 0.1000%, and a remainder: Fe and impurities; grinding a surface of the steel sheet subjected to the cold rolling; annealing the steel sheet of which the surface is ground in the grinding; and quenching the steel sheet after the annealing, in which, in the grinding, the surface of the steel sheet subjected to the cold rolling is ground by 0.1 µm or more, in the annealing, a dew point is set to -15°C to 20°C, in the annealing, an annealing temperature is set to 750°C or higher, and in the quenching, quenching is performed to 300°C or lower at an average cooling rate of 0.4 °C/s or faster.
(7) In the method for manufacturing a steel sheet according to (6), in the annealing, the dew point at least in a heating process up to the annealing temperature may be set to -15°C to 20°C.
(8) The method for manufacturing a steel sheet according to (6) or (7) may further include: tempering the steel sheet at 150°C or higher after the quenching.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a steel sheet having high strength, excellent bendability as formability, and excellent bendability as collision characteristics, and a method for manufacturing the same.

DESCRIPTION OF EMBODIMENTS

A steel sheet according to an embodiment of the present invention and a method for manufacturing the same will be described below.
In the description of the following embodiments, a range indicated by "to" includes, in principle, values at both ends thereof as a lower limit and an upper limit of the range. However, numerical values indicated as "more than" or "less than" are not included in the range.
Hereinafter, each configuration of the steel sheet according to the present embodiment will be described.

[Chemical Composition of Steel Sheet]

The steel sheet according to the present embodiment contains the following elements. In the present embodiment, % of an amount of each element means mass%.

C: 0.070% to 0.15%

C (carbon) is an essential element for increasing strength of the steel sheet. When a C content is less than 0.070%, a sufficient tensile strength cannot be obtained. Therefore, the C content is set to 0.070% or more. From the viewpoint of securing ferrite and improving elongation, the C content is preferably 0.08% or more.
On the other hand, when the C content is more than 0.15%, ductility of a material decreases and bendability deteriorates. Therefore, the C content is set to 0.15% or less. From the viewpoint of weldability, the C content is preferably 0.14% or less.

Si: 0.10% to 2.00%

Si (silicon) is a solid solution strengthening element and is an element effective in increasing the strength of the steel sheet. In order to obtain this effect, a Si content is set to 0.10% or more. From the viewpoint of securing a desired ferrite fraction over a wide range of annealing temperatures, the Si content is preferably 0.30% or more.
On the other hand, when Si is excessively contained, chemical convertibility and wettability with hot-dip galvanizing of the steel sheet significantly deteriorate. For this reason, the Si content is set to 2.00% or less. In addition, from the viewpoint of embrittlement of steel components and a decrease in cold formability, the Si content is preferably 1.8% or less.

Mn: 1.00% to 4.00%

Mn (manganese) is a strong austenite stabilizing element and is an effective element for improving hardenability of the steel sheet. In order to obtain this effect, a Mn content is set to 1.00% or more. From the viewpoint of securing the strength, the Mn content is preferably 1.50% or more.
On the other hand, when Mn is excessively contained, the weldability and low temperature toughness deteriorate. Therefore, the Mn content is set to 4.00% or less. From the viewpoint that Mn promotes co-segregation with P or S and causes a significant deterioration in workability, the Mn content is preferably 3.20% or less.

sol. Al: 0.001% to 1.500%

Al (aluminum) is an element having a deoxidizing action on steel. In order to obtain this effect, a sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.005% or more.
On the other hand, even in a case where Al is contained excessively, the effect is saturated, leading to an increase in cost. Additionally, a transformation temperature of the steel rises, leading to an increase in load during hot rolling. Therefore, the sol. Al content is set to 1.500% or less. The sol. Al content is preferably 1.000% or less.

P: 0.0010% to 0.0300%

P (phosphorus) is a solid solution strengthening element and is an element effective in increasing the strength of the steel sheet.
In order to obtain this effect, a P content is set to 0.0010% or more. The P content is preferably 0.0050% or more.
On the other hand, when the P content is more than 0.0300%, the steel sheet becomes embrittled due to segregation of P to grain boundaries. In addition, the weldability and toughness deteriorate. Therefore, the P content is set to 0.0300% or less. The P content is preferably 0.0200% or less.

S: 0.0200% or Less

S (sulfur) is an element that causes hot embrittlement and is also an element that inhibits the weldability and corrosion resistance. When a S content is more than 0.0200%, hot workability, weldability, and corrosion resistance significantly decrease. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less.
The S content is preferably low and may be 0%. However, in order to set the S content to less than 0.0001%, a manufacturing cost significantly increases. Therefore, the S content may be set to 0.0001% or more. The S content may be set to 0.0010% or more.

N: 0.0100% or Less

N (nitrogen) is an element that forms coarse nitrides in steel and deteriorates the bendability and hole expansibility. When a N content is more than 0.0100%, the above-described deterioration becomes significant. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0050% or less.
The N content is preferably low and may be 0%. However, reducing the N content excessively increases a denitrification cost. Therefore, the N content may be set to 0.0005% or more from the viewpoint of economic efficiency.

O: 0.0100% or Less

O (oxygen) is an element that forms coarse oxides in steel and deteriorates the bendability and hole expansibility. When an O content is more than 0.0100%, the above-described deterioration becomes significant. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0070% or less.
The O content is preferably small and may be 0%. However, the O content may be set to 0.0001% or more from the viewpoint of the manufacturing cost. The O content may be set to 0.0010% or more.
The steel sheet according to the present embodiment may contain the above elements and a remainder including Fe and impurities. However, for the purpose of improving various properties, one or more elements (optional elements) selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM shown below may be further contained. Since optional elements do not have to be contained, lower limits thereof are 0%.

Ti: 0% to 0.200%

Ti (titanium) is an element that fixes N in steel as TiN, thereby suppressing the formation of BN, which is a factor for reducing hardenability. In addition, Ti is an element that refines an austenite grain size during heating and improves toughness. In a case of obtaining this effect, a Ti content is preferably set to 0.005% or more. The Ti content is more preferably set to 0.010% or more.
On the other hand, when the Ti content is excessive, the ductility of the steel sheet decreases. Therefore, in the case where Ti is contained, the Ti content is set to 0.200% or less. The Ti content is preferably set to 0.050% or less.

B: 0% to 0.0100%

B (boron) is an element that segregates to austenite grain boundaries during welding, thereby strengthening the grain boundaries, and contributing to an improvement in resistance to molten metal embrittlement cracking. In a case of obtaining this effect, a B content is preferably set to 0.0005% or more. The B content is more preferably set to 0.0008% or more.
On the other hand, when the B content is more than 0.0100%, carbides and nitrides are generated, the above-described effects are saturated, and the hot workability decreases. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

Cr: 0% to 1.000%
Mo: 0% to 1.000%
Ni: 0% to 1.000%
Cu: 0% to 1.000%
Sn: 0% to 0.500%

Cr (chromium), Mo (molybdenum), Ni (nickel), Cu (copper), and Sn (tin) are all elements effective in increasing the strength of the steel sheet. In order to obtain the above effect, it is preferable to contain 0.001% or more, more preferably 0.010% or more, and even more preferably 0.050% or more of one or more selected from Cr, Mo, Ni, Cu, and Sn.
On the other hand, when these elements are excessively contained, the effect is saturated and the cost increases. Therefore, in a case where these elements are contained, Cr, Mo, Ni, and Cu contents are each set to 1.000% or less, and a Sn content is set to 0.500% or less. The Cr, Mo, Ni, and Cu contents are each preferably set to 0.600% or less, and the Sn content is preferably set to 0.300% or less.

Nb: 0% to 0.200%
V: 0% to 0.500%
W: 0% to 0.500%

Nb (niobium), V (vanadium), and W (tungsten) are carbide forming elements and are elements effective in increasing the strength of the steel sheet. In order to obtain the above effects, it is preferable to contain 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more of one or more selected from Nb, V, and W.
On the other hand, when these elements are excessively contained, the effect is saturated and the cost increases. Therefore, in a case where these elements are contained, a Nb content is set to 0.200% or less, and V and W contents are each set to 0.500% or less. The Nb content is preferably set to 0.100% or less, and the V and W contents are each preferably set to 0.300% or less.

Ca: 0% to 0.0100%
Mg: 0% to 0.0100%
Bi: 0% to 0.0100%
Sb: 0% to 0.1000%
Zr: 0% to 0.0100%
REM: 0% to 0.1000%

Ca (calcium), Mg (magnesium), Sb (antimony), Zr (zirconium), and REM (rare earth elements) are elements that contribute to fine dispersion of inclusions in steel, and Bi (bismuth) is an element that reduces microsegregation of substitutional alloying elements such as Mn and Si in steel. These elements each contribute to an improvement in bending resistance of the steel sheet. Therefore, V may be contained as necessary.
In order to obtain the above effects, it is preferable to contain 0.0001% or more, and more preferably 0.0010% or more of one or more selected from Ca, Mg, Bi, Sb, Zr, and REM.
On the other hand, when these elements are excessively contained, ductility deteriorates. Therefore, Ca, Mg, Bi, Sb and Zr contents are each set to 0.0100% or less. In addition, a REM content is set to 0.1000% or less. The Ca, Mg, Bi, Sb, and Zr contents are each set to preferably 0.0080% or less, and more preferably 0.0060% or less.
The REM content is preferably 0.0800% or less, more preferably 0.0600% or less, and even more preferably 0.0200% or less.
Here, REM refers to a total of 17 elements including Sc, Y, and lanthanoids, and the REM content means the total amount of these elements. Lanthanoids are industrially added in the form of mischmetal.
A chemical composition of the steel sheet according to the present embodiment can be obtained by the following method.
The chemical composition of the steel sheet described above may be measured by a general chemical composition measurement. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
sol. Al may be measured by ICP-AES using a filtrate obtained by heating and decomposing a sample with an acid.
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-non-dispersive infrared absorption method. In a case where the steel sheet is provided with a plating layer on a surface, the chemical composition may be analyzed after removing the plating layer by mechanical grinding.
As described above, the steel sheet according to the present embodiment contains, as the chemical composition, C, Si, Mn, sol. Al, P, S, O, and N, and a remainder including Fe and impurities, or contains C, Si, Mn, sol. Al, P, S, O, and N and further contains one or more elements selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM, and a remainder including Fe and impurities.

[Microstructure of Steel Sheet]

In the steel sheet according to the present embodiment, a microstructure in a range of 1/8 to 3/8 of a sheet thickness from the surface of the steel sheet in a sheet thickness direction of the steel sheet, with respect to a 1/4 position of the sheet thickness of the steel sheet as a center, includes, by area ratio, 0% to 60% of ferrite, 0% to 3% of residual austenite, and a remainder containing one or more selected from martensite, bainite, pearlite, and cementite. In the present embodiment, this "range of 1/8 to 3/8 of the sheet thickness from the surface of the steel sheet" is referred to as a "1/4 thickness position". The sheet thickness direction of the steel sheet in the present embodiment is a direction perpendicular to the surface of the steel sheet. The area ratio means a proportion of each structure to the entire microstructure in the above range. The remainder of the microstructure may include one or more selected from martensite, bainite, pearlite, and cementite.
The microstructure at the 1/4 thickness position more preferably contains 0% to 30% of ferrite from the viewpoint of securing strength.
The microstructure at the 1/4 thickness position more preferably contains 40% to 100% in total of tempered martensite and fresh martensite from the viewpoint of securing strength.
The microstructure at the 1/4 thickness position more preferably contains 40% to 100% in total of tempered martensite and bainite from the viewpoint of securing bendability.
The microstructure at the 1/4 thickness position preferably contains 0% to 5% in total of pearlite and cementite, and more preferably contains 0% to 3% in total of pearlite and cementite from the viewpoint of securing strength.
The area ratios of ferrite, residual austenite, martensite (including tempered martensite and fresh martensite), and bainite included in the microstructure at the 1/4 thickness position can be measured by the following method.
A sample is collected with a cross section parallel to a rolling direction and the sheet thickness direction of the steel sheet as an observation section, and the observation section is polished and etched with nital. The rolling direction in the present embodiment is parallel to a flat surface of the steel sheet and is a longitudinal direction of the steel sheet when the steel sheet is elongated by rolling. Since highly ductile non-ferrous inclusions such as MnS contained in the steel sheet are also elongated together with the steel sheet by rolling, the rolling direction also coincides with a direction in which the highly ductile non-ferrous inclusions such as MnS are elongated. Therefore, a cutting direction in which an aspect ratio of the highly ductile non-ferrous inclusions such as MnS is the largest by observing a cross section cut on a plane perpendicular to the surface of the steel sheet may be set as the rolling direction. A method of specifying the rolling direction in a case where the rolling direction of the steel sheet is unknown, for example, in a case where the steel sheet is a processed component, will be described later for convenience of explanation.
Next, in a case of observing the structure at the 1/4 thickness position, in a range of 1/8 of the thickness to 3/8 of the thickness (from a 1/8 thickness position of the steel sheet from the surface of the steel sheet to a 3/8 thickness position of the steel sheet from the surface of the steel sheet) with respect to the 1/4 position of the thickness of the steel sheet from the surface of the steel sheet as the center, a total of five visual fields, each visual field set to 250 µm² or more, are observed at a magnification of 5,000-fold using a field-emission scanning electron microscope (FE-SEM). Then, the area ratios of ferrite, residual austenite, martensite, pearlite, cementite, and bainite are measured.
Here, with regard to the identification of each phase, a region having a substructure within grains and having a plurality of long sides of carbides when observed with a scanning electron microscope is determined to be tempered martensite. Here, under similar observation conditions, in a case where brightness within the grains varies finely with respect to grain sizes, the region is determined to have a substructure.
In addition, a region where cementite is precipitated in a lamellar form is determined to be pearlite or cementite. A region with low brightness and no observable substructure is determined to be ferrite. A region with high brightness and no substructure revealed by etching is determined to be fresh martensite or residual austenite. The remainder is determined to be bainite.
The area ratio of each phase is calculated by a point counting method to obtain the area ratio of each structure. In the point counting method, the measurement is performed at 300 or more measurement points per visual field at intervals between the measurement points of 2 µm. Calculated values in each visual field are arithmetically averaged to obtain the area ratio of each structure.
The area ratio of fresh martensite can be obtained by subtracting the area ratio of residual austenite obtained by an EBSD method, which will be described below, from the area ratio of fresh martensite or residual austenite. Then, a sum of this and the area ratio of tempered martensite calculated by the point counting method is taken as the area ratio of martensite. In a case where the area ratio of fresh martensite or residual austenite calculated by the point counting method is smaller than the area ratio of residual austenite obtained by the EBSD method described below, the area ratio of fresh martensite is set to zero.
In the steel sheet according to the present embodiment, the area ratio of residual austenite at the 1/4 thickness position is evaluated by performing high-resolution crystal structure analysis by the EBSD method (electron backscatter diffraction method). Specifically, a sample is collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section, and the observation section is polished to a mirror finish. Furthermore, in order remove a processed layer of a surface layer, electrolytic polishing or mechanical polishing using colloidal silica is performed.
Next, at the 1/4 thickness position of the steel sheet, crystal structure analysis is performed by the EBSD method on five visual fields, each visual field set to a size of 250 µm² or more, at a magnification of 5000-fold. A distance between evaluation points (step) is set to 0.01 to 0.20 µm.
Data obtained by the EBSD method is analyzed using "OIM Analysis 6.0" manufactured by TSL. From the observation results at each position, a region determined to be FCC iron is determined to be residual austenite, and the area ratio of each residual austenite at the 1/4 thickness position is calculated. In a case where the area ratio of fresh martensite or residual austenite calculated by the point counting method is smaller than the area ratio of residual austenite obtained by the EBSD method, the area ratio of fresh martensite or residual austenite calculated by the point counting method is regarded as the area ratio of residual austenite.

[Ferrite Fraction of Outermost Layer]

In the steel sheet according to the present embodiment, a ferrite fraction in a range of up to 2 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet is 95% or more. In addition, in the steel sheet according to the present embodiment, an in-plane average grain size of ferrite of 2.0 µm or less.
As will be described later, in a method for manufacturing the steel sheet according to the present embodiment, the surface of the steel sheet that has undergone a cold rolling step is ground and annealed, so that crystal grains are refined during annealing due to strain introduced by grinding. Accordingly, ferrite having an in-plane average grain size of 2.0 µm or less is generated in a surface layer area of the steel sheet at a ferrite fraction of 95% or more, and soft ferrite is formed in an outermost layer of the steel sheet. As a result, initiation of microcracks on the surface of the steel sheet, which is particularly observed in a 90° V-bending test, can be suppressed. In addition, propagation of such microcracks can also be suppressed.
The ferrite fraction can be measured using the following method. The ferrite fraction means a proportion of a ferrite structure to the entire microstructure in a range of up to 2 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
The ferrite fraction in a range of up to 2 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet can be measured using the following method.
A sample is collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section, and the observation section is polished and etched with nital. Next, a total of 5 visual fields, each visual field set to 250 µm² or more, are observed using the field-emission scanning electron microscope at a magnification of 5000-fold. Then, the area ratio of each ferrite is measured.
Here, a region with low brightness and no observable substructure is determined as a ferrite structure. The area ratio of the ferrite structure is calculated by the point counting method described above to obtain the area ratio of the ferrite structure.
The range of up to 2 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet means a range within 2 µm along the sheet thickness direction of the steel sheet from the surface of the steel sheet toward an inside of the steel sheet.
The in-plane average grain size of ferrite can be measured using the following method.
A sample with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section and a sample with a cross section parallel to a sheet width direction and the sheet thickness direction of the steel sheet as an observation section are prepared. The observation section of each sample is polished and etched with nital. First, a surface layer of the sample in which the cross section parallel to the rolling direction and the sheet thickness direction is the observation section is photographed at a magnification of 5000-fold, and a connected photograph of a total of five visual fields, each visual field set to a connected photograph of 250 µm² or more obtained by moving a photographing range in the rolling direction, is acquired by the field-emission scanning electron microscope. For the sample with the cross section parallel to the sheet width direction and the sheet thickness direction as the observation section, a surface layer is photographed at a magnification of 5000-fold, and a connected photograph of a total of five visual fields, each visual field set to a connected photograph of 250 µm² or more obtained by moving a photographing range in the sheet width direction, is acquired by the field-emission scanning electron microscope. For the connected photographs of a total of 10 visual fields obtained from the samples, grain sizes of ferrite grains at depth positions of 0.5, 1.0, 1.5, and 2.0 µm from the surface of the steel sheet in the sheet thickness direction are measured. Specifically, at each depth position, a straight line parallel to the surface of the steel sheet is assumed, and for ferrite grains intersected by the straight line, (length of the straight line)/(number of ferrite grains intersected by the straight line - 1) is defined as the grain size of the ferrite grain at the depth position. The length of the straight line, that is, an observation distance is set to 50 µm or more. The in-plane average grain size of ferrite is obtained by arithmetically averaging all the grain sizes of the ferrite grains, that is, the grain sizes obtained from 40 straight lines in 10 visual fields, each visual field including the four depth positions. In a case where the rolling direction of the steel sheet is not known, for example, in a case where the steel sheet is a processed component, any direction in a sheet surface of the component may be set as a reference direction, each of a sample with a cross section parallel to a reference direction and a sheet thickness direction as an observation section and a sample with a cross section parallel to a direction orthogonal to the reference direction in the sheet surface and parallel to the sheet thickness direction as an observation section may be prepared, and the above-described measurement may be performed. By measuring the grain size of ferrite by the above-described method, the average grain size (in-plane average grain size) of ferrite in the measurement range at a portion intersecting the straight line parallel to the surface of the steel sheet, can be evaluated.
When the ferrite grain sizes in a portion parallel to the surface of the steel sheet are fine, bending strain that occurs in a steel surface layer during bending deformation is dispersed into individual fine ferrite grains, and this dispersion suppresses localization of strain within the grains, which is likely to occur in coarse ferrite grains, whereby crack initiation is less likely to occur during bending deformation and crack propagation in the sheet thickness direction is less likely to occur. Therefore, it is important that the ferrite grain sizes on the straight line parallel to the surface of the steel sheet are in a predetermined range (2.0 µm or less) rather than cross-sectional areas of the ferrite grains being small.
The steel sheet according to the present embodiment has such a configuration, and thus it is possible to suppress the initiation and propagation of microcracks on the surface of the steel sheet, particularly in a 90° V-bending test.

[Tensile Strength of Steel Sheet]

The steel sheet according to the present embodiment has a tensile strength of 950 MPa or more. The tensile strength of 950 MPa or more allows preferable use as a steel sheet for a vehicle.
In a case where the contribution to weight reduction of vehicles is taken into consideration, the tensile strength is more preferably 980 MPa or more or 1,050 MPa or more, and even more preferably 1,100 MPa or more.
The steel sheet according to the present embodiment more preferably has a tensile strength of less than 1,300 MPa.
The tensile strength of less than 1,300 MPa has an advantage that elongation can be easily ensured.
On the other hand, a tensile strength of more than 1,400 MPa leads to deterioration of weldability. Therefore, the tensile strength is set to 1,400 MPa or less.
In the steel sheet according to the present embodiment, a transition region based on a C concentration in the sheet thickness direction of the steel sheet is more preferably 150 µm or less. The transition region in the present embodiment is a region where the C concentration is 20% to 90% with respect to a C concentration of a steady state portion, which will be described later. When the transition region based on the C concentration is 150 µm or less, a change in C concentration from a low carbon region to a high carbon region from the surface layer to the inside becomes steeper compared to a case where the transition region based on the C concentration is larger than 150 µm, with the same amount of decarburization. Therefore, the low carbon region having a sufficient thickness can be secured in the surface layer, and the initiation and propagation of microcracks in the surface layer can be suppressed. When the transition region is 100 µm or less, the bendability can be further improved, which is more preferable.
The C concentration is a carbon concentration in the steel sheet. The C concentration can be measured by a method using a GDS (glow discharge optical emission spectrometer) as described below.
Specifically, a surface of a sample is degreased and washed, and the C concentration is continuously measured from the surface of the sample. After the measurement, a reduction in thickness is measured with a micrometer, and assuming that the reduction in thickness has occurred at a constant rate, so that the C concentration at each depth can be obtained. Regarding a measurement time, a measurement time is set such that the measurement depth of the C concentration in the steady state portion is 50 µm or more. After applying noise removal processing using moving average processing with a unit interval of 1.0 µm in a depth direction, a range that is within ±5% of an arithmetic mean of the C concentration of the steady state portion, which is an average C concentration of base metal before a decarburization treatment, is set as the steady state portion.
The steady state portion measured by this measurement method is defined as C concentration 100%. The transition region in the present embodiment is a region in which the C concentration is 20% to 90% in the sheet thickness direction of the steel sheet.
In the steel sheet according to the present embodiment, a fresh martensite fraction is more preferably 10% or less in a range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet. The microstructure in this range is important in terms of suppressing the propagation of microcracks. When the fresh martensite fraction in this range is 10% or less, the effect of suppressing the propagation of microcracks is improved.
The fresh martensite fraction in this range is more preferably 5% or less. The fresh martensite fraction means a proportion of a fresh martensite structure to the entire microstructure in a range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
The fresh martensite fraction is obtained by calculating the proportion of fresh martensite in the microstructure in the above range, based on the area ratio obtained by the above-described measurement method of the area ratio of the microstructure.
The range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet means a range of 5 µm or more and 20 µm or less from the surface of the steel sheet toward the inside of the steel sheet along the sheet thickness direction of the steel sheet from the surface of the steel sheet.
In the steel sheet according to the present embodiment, a ferrite fraction is more preferably 50% or more in the range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
When the ferrite fraction in this range is 50% or more, a hard phase that serves as the origin of fracture is substantially eliminated, and ferrite having good ductility has an effect of suppressing the propagation of cracks.
The ferrite fraction in this range is more preferably 70% or more. The ferrite fraction means a proportion of a ferrite structure to the entire microstructure in the range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
In the steel sheet of the present embodiment, each structure having the above-described properties is appropriately disposed in the sheet thickness direction, so that the initiation and propagation of microcracks are suppressed.
As a method for observing the initiation of microcracks on the surface, a 90° V-bending test can be adopted. The 90° V-bending test is a method capable of observing microcracks on the surface of the steel sheet, that is, initial microcracks, and can evaluate bending performance corresponding to the formability of the steel sheet. Suppression of the initial microcracks means that the properties in the 90° V-bending test are improved, and the formability of the steel sheet is improved.
In addition, as a method for evaluating the propagation of microcracks, a VDA bending test can be adopted. The VDA bending test is a method for determining a crack based on a change in load applied to a measurement target, and can evaluate bending performance corresponding to collision performance. Suppression of the propagation of microcracks means that the properties in the VDA bending test are improved, and the collision performance of the steel sheet as a steel sheet for a vehicle is improved.

(Plating Layer)

The steel sheet according to the present embodiment may have a plating layer such as a galvanized layer on the surface of the steel sheet as the base metal. The galvanized layer is, for example, a hot-dip galvanized layer. In the present embodiment, the galvanized layer means a plating layer containing 80 mass% or more of Zn. The presence of the hot-dip galvanized layer on the surface improves corrosion resistance.
An adhesion amount of the galvanized layer is not particularly limited. However, from the viewpoint of continuous weldability, the adhesion amount is preferably set to 150 g/m² or less, and more preferably 100 g/m² or less. On the other hand, in terms of improving the corrosion resistance, the adhesion amount is preferably 20 g/m² or more.
A chemical composition of the galvanized layer is not limited, and preferably contains, for example, Al: 0.1% to 2.0%, Fe: 5.0% or less, and a remainder including Zn and impurities.
The adhesion amount and the chemical composition of the galvanized layer are obtained by the following methods. The plating layer is melted using hydrochloric acid containing an inhibitor, and weights before and after melting are compared to each other to obtain the adhesion amount. In addition, a solution obtained by the melting is quantitatively analyzed by ICP to measure the chemical composition of the plating layer.
In a case where the steel sheet has a plating layer on the surface thereof, the position in the sheet thickness direction in the present embodiment is set as a depth from the surface of the base steel sheet (interface between an Fe phase and the plating layer) as a reference.

[Method for Manufacturing Steel Sheet]

Next, a method for manufacturing the steel sheet of the above embodiment will be described.
The method for manufacturing the steel sheet of the present embodiment includes a cold rolling step of performing cold rolling on a steel sheet having a predetermined chemical composition, a grinding step of grinding a surface of the steel sheet subjected to the cold rolling, and an annealing step of annealing the steel sheet of which the surface is ground in the grinding step.

In the cold rolling step, a steel sheet having the following chemical composition is cold-rolled.
The chemical composition includes, by mass%:

C: 0.070% to 0.15%;
Si: 0.10% to 2.00%;
Mn: 1.00% to 4.00%;
sol. Al: 0.001% to 1.500%;
P: 0.0010% to 0.0300%;
S: 0.0200% or less;
N: 0.0100% or less;
O: 0.0100% or less;
Ti: 0% to 0.200%;
B: 0% to 0.0100%;
Cr: 0% to 1.000%;
Mo: 0% to 1.000%;
Ni: 0% to 1.000%;
Cu: 0% to 1.000%;
Sn: 0% to 0.500%;
Nb: 0% to 0.200%;
V: 0% to 0.500%;
W: 0% to 0.500%;
Ca: 0% to 0.0100%;
Mg: 0% to 0.0100%;
Bi: 0% to 0.0100%;
Sb: 0% to 0.1000%;
Zr: 0% to 0.0100%;
REM: 0% to 0.1000%; and
a remainder: Fe and impurities.

Cold rolling conditions are not particularly limited, and the cold-rolled steel sheet can be manufactured by performing cold rolling on the hot-rolled steel sheet under normal conditions.
Manufacturing conditions of the steel sheet to be subjected to the cold rolling step are not limited. For example, molten steel having the above-described chemical composition is cast under normal conditions to obtain a steel piece, and then hot rolling is performed on the steel piece under normal conditions to manufacture a hot-rolled steel sheet. Cold rolling can be performed on the hot-rolled steel sheet.

In the grinding step, the surface of the steel sheet subjected to the cold rolling step is ground in the sheet thickness direction by an average of 0.1 µm or more. By grinding the surface of the steel sheet by 0.1 µm or more, strong strain is introduced into the inside of the steel sheet. Therefore, recrystallization in the steel sheet structure surface layer is promoted during heating in the subsequent annealing step, and fine ferrite crystal grains can be obtained in the outermost layer of the steel sheet.
With regard to the amount of the steel sheet to be ground, from the viewpoint of the amount of strain introduced to the inside of the steel sheet, it is more preferable that the surface of the steel sheet is ground by 0.15 µm or more.
In the grinding step, it is important to introduce large strain to the surface layer of the steel sheet.
The amount (µm) of the steel sheet to be ground is calculated based on a change in the weight of the steel sheet before and after grinding. A weight loss of the steel sheet before and after grinding is divided by the area of the ground steel sheet to obtain a weight loss (g) per 1 m². Then, the amount of grinding (µm) is calculated based on a value (value representing a relationship between the weight loss per 1 m² and the amount of grinding (µm)) from an offline test conducted in advance under the same conditions.
As an example of a grinding method, grinding is performed using a grinding brush at a predetermined rotation speed, reduction, and grinding speed. Although not particularly limited, for example, grinding may be performed using a D-100 grinding brush manufactured by Hotani Co., Ltd, at a rotation speed of 1,000 to 1,500 rpm, a reduction of 2.0 mm, and a grinding speed of about 100 mpm. In addition, the amount of grinding may be adjusted by performing the grinding a plurality of times, for example, two to ten times.
It is preferable to ground the entire sheet surface of the steel sheet.
Only one surface of the steel sheet may be ground, or both surfaces of the steel sheet may be ground. However, it is more preferable to grind both surfaces of the steel sheet in consideration of versatility of the steel sheet as a steel sheet for a vehicle.

The annealing step includes a heating process of heating the steel sheet having the predetermined chemical composition (the same chemical composition as the steel sheet according to the present embodiment to be obtained) to a predetermined annealing temperature (highest heating temperature), a holding process of holding the heated steel sheet at the annealing temperature for a certain period of time, and a cooling process of cooling the steel sheet from the annealing temperature to a predetermined temperature.
From the viewpoint of productivity, it is preferable to perform annealing by passing the steel sheet through a continuous annealing line.

(Heating Process)

In the heating process, the steel sheet is heated to the annealing temperature.
In the heating process up to the annealing temperature in the annealing step, a heating rate is not particularly limited, and it is important to control the following atmosphere.
In the heating process, a dew point is set to -15°C to 20°C. By setting the dew point to -15°C to 20°C, recrystallization is promoted by the strain introduced in the grinding step, and fine crystal grains can be obtained in the outermost layer of the steel sheet. As a result, a decarbonizing reaction is promoted, and the ferrite fraction of the outermost layer of the steel sheet can be set to 95% or more.
When the dew point in a furnace is lower than -15°C, a sufficient ferrite fraction cannot be obtained. Therefore, the dew point is set to -15°C or higher. The dew point is more preferably -10°C or higher from the viewpoint of obtaining a high ferrite fraction.
On the other hand, when the dew point is higher than 20°C, decarburization excessively progresses, the hard phase of the surface layer of the base steel sheet significantly decreases, and coarse oxides are formed in the refined layer, so that plating adhesion and powdering properties decrease. Therefore, the dew point is set to 20°C or lower.

(Holding Process)

After being heated to the annealing temperature under the above conditions, the steel sheet is held at a predetermined highest heating temperature for five seconds or longer. When the holding time is shorter than five seconds, it is not possible to sufficiently secure austenite, which will later become the hard phase.
An upper limit of the holding time is not particularly limited. However, when the holding time is too long, manufacturability of the steel sheet is impaired. Therefore, from the viewpoint of cost, the holding time is preferably shorter than 500 seconds. In addition, from the viewpoint of securing austenite sufficiently at a low cost, the holding time is more preferably about 10 to 120 seconds.
The annealing temperature is set to 750°C or higher in order to sufficiently secure austenite, which will later become the hard phase.
When the annealing temperature is higher than 1,000°C, grain sizes during annealing become coarse, and it becomes difficult to sufficiently obtain fine ferrite grains that contribute to the improvement in bendability of the surface layer. Therefore, the annealing temperature is set to 1,000°C or lower. The annealing temperature is preferably 900°C or lower.
From the heating process, the dew point may be set to -15°C to 20°C in the holding process.
In the above-described annealing step, annealing may be performed without setting the dew point to -15°C to 20°C, and a step of heating the steel sheet at a dew point of -15 °C to 20 °C may be provided separately from the annealing step before or after the annealing step.

In the method for manufacturing the steel sheet according to the present embodiment, a quenching step and a tempering step may be performed after the holding process in the annealing step. Accordingly, the fresh martensite fraction can be reduced, and the propagation of microcracks can be further suppressed. In particular, from the viewpoint of reducing the fresh martensite fraction in a range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, it is preferable to perform the tempering step in addition to the quenching step.
In the quenching step, the steel sheet that has been heated and held in the annealing step is cooled and quenched so that the temperature of the steel sheet reaches 300°C or lower. By performing the quenching so that the temperature of the steel sheet reaches 300°C or lower, it is possible to reduce fresh martensite that serves as an origin of fracture. From the viewpoint of further reducing fresh martensite that serves as the origin of fracture, it is more preferable to perform quenching to 250°C or lower.
An average cooling rate during quenching is set to 0.4 °C/s or faster in order to obtain a hard layer. There is no particular upper limit to the average cooling rate during quenching, but it is difficult to cool the steel sheet at 500 °C/s or faster from the viewpoint of cost. By performing quenching at an average cooling rate of 0.4 °C/s or faster, the ferrite fraction of the surface layer (5 to 20 µm) can be set to less than 90%. Excessive softening of the surface layer not only leads to a reduction in bending strength but also hinders securing the strength of the steel sheet.
In the tempering step, tempering is performed on the steel sheet cooled and held in the quenching step so that the temperature of the steel sheet reaches 150°C or higher. As conditions for the tempering, a holding time for the tempering is set to 2 seconds or longer. Although no particular upper limit is defined, it is preferable to set the holding time for the tempering to 500 seconds or shorter, since the effect is saturated. In order to obtain tempered martensite that is sufficiently soft, it is preferable to perform the tempering so that the temperature of the steel sheet reaches 200°C or higher. In addition, in order to secure the strength, it is more preferable to perform the tempering so that the temperature of the steel sheet reaches 500°C or higher.
In the above description, the quenching step is performed after the annealing step. However, after the holding process of the annealing step, the steel sheet may be once cooled to a predetermined temperature, and then heated again to perform the quenching step.

(Plating Step)

In the method for manufacturing the steel sheet according to the present embodiment, a plating step may be performed between the annealing step and the quenching step, between the quenching step and the tempering step, or after the tempering step. The plating step may be performed as a part of the quenching step.
In addition, the plating step may be performed after the tempering step. In a case where the plating step is performed after the tempering step, the above-described plating layer may be formed by electroplating.

Examples

Slabs having the chemical compositions shown in Tables 1A and 1B were hot-rolled and coiled under the hot rolling conditions shown in Tables 2A and 2B to obtain various hot-rolled steel sheets having the sheet thicknesses shown in Tables 2A and 2B. The hot-rolled steel sheets were cold-rolled under the cold rolling conditions shown in Tables 2A and 2B to obtain various cold-rolled steel sheets having the sheet thicknesses shown in Tables 2A and 2B (cold rolling step).

In the following tables, the underlines indicate numerical values or the like outside the ranges of the present invention.

[Table 1A]

Kind of steel	Chemical composition (mass%, remainder: Fe and impurities)
Kind of steel	C	Si	Mn	P	S	Al	O	N	Ti	B	Cr	Mo
A	0.123	1.24	2.64	0.014	0.0031	0.028	0.002	0.001	0.021	0.0025
B	0.082	0.84	2.84	0.005	0.0016	0.038	0.003	0.002	0.038	0.0038	0.12
C	0.138	0.05	3.02	0.016	0.0015	0.018	0.001	0.001	0.038	0.0031		0.07
D	0.092	0.29	1.57	0.029	0.0027	0.462	0.003	0.002
E	0.146	0.77	2.25	0.005	0.0015	0.027	0.003	0.003	0.010	0.0010
F	0.136	0.34	2.18	0.021	0.0022	0.040	0.002	0.002	0.042	0.0028		0.27
G	0.081	1.88	3.68	0.009	0.0019	0.039	0.001	0.002
H	0.075	1.86	2.07	0.021	0.0023	0.025	0.003	0.004	0.033	0.0031		0.24
I	0.102	0.47	1.60	0.023	0.0025	0.247	0.003	0.002	0.019	0.0013	0.37
J	0.095	0.88	2.30	0.020	0.0014	0.035	0.004	0.003
K	0.124	0.19	2.17	0.008	0.0033	0.017	0.003	0.003	0.039	0.0013
L	0.133	1.61	2.70	0.029	0.0013	0.015	0.001	0.003
M	0.120	1.86	3.11	0.008	0.0019	0.020	0.003	0.002
N	0.085	1.24	1.83	0.012	0.0032	0.039	0.003	0.003	0.013	0.0010		0.42
O	0.097	0.27	1.34	0.022	0.0019	0.022	0.003	0.003	0.027	0.0037
P	0.117	0.92	1.78	0.021	0.0020	0.012	0.003	0.003	0.018	0.0022	0.33
Q	0.130	0.62	2.04	0.004	0.0032	0.032	0.003	0.003	0.026	0.0011
R	0.110	0.54	1.54	0.010	0.0018	0.032	0.004	0.003	0.017	0.0023
S	0.118	1.62	1.62	0.010	0.0034	0.012	0.002	0.002	0.044	0.0027
T	0.097	1.39	2.67	0.004	0.0019	0.039	0.003	0.003	0.046	0.0025		0.2
U	0.127	0.25	3.10	0.006	0.0037	0.039	0.004	0.004	0.011	0.0018
V	0.139	0.82	2.73	0.016	0.0011	0.021	0.004	0.002	0.039	0.0038	0.28
w	0.111	0.40	2.51	0.006	0.0021	0.023	0.003	0.001	0.021	0.0018
X	0.112	1.37	1.62	0.015	0.0013	0.017	0.001	0.002	0.034	0.0037	0.21
Z	0.067	0.80	1.84	0.020	0.0024	0.033	0.003	0.001	0.041	0.0038		0.13
AA	0.163	0.11	2.97	0.018	0.0028	0.026	0.001	0.002	0.048	0.0014
AB	0.088	2.82	3.04	0.008	0.0033	0.011	0.002	0.001			0.32
AC	0.126	0.92	4.67	0.023	0.0036	0.030	0.003	0.001	0.046	0.0031
AD	0.086	0.55	3.11	0.013	0.0011	1.648	0.004	0.003	0.030	0.0032
AE	0.114	1.96	0.89	0.005	0.0033	0.022	0.003	0.002	0.012	0.0037
AF	0.122	0.81	1.54	0.046	0.0036	0.023	0.001	0.003	0.030	0.0033
AG	0.129	1.04	2.10	0.010	0.0257	0.034	0.001	0.003	0.018	0.0039		0.35

[Table 1B]

Kind of steel	Chemical composition (mass%, remainder: Fe and impurities)
Kind of steel	Ni	Cu	Sn	Nb	V	W	Ca	Mg	Bi	Sb	Zr	REM
A				0.01
B						0.28
C							0.009
D	0.07	0.14									0.002
E			0.18									0.007
F					0.29				0.006
G			0.07			0.16
H							0.003
I								0.005
J	0.17			0.02
K		0.22			0.11
L
M
N				0.02
O							0.004
p	0.54
Q				0.03				0.007
R		0.74								0.002
S				0.14
T					0.43
U	0.27								0.002
V
W						0.08						0.003
X											0.005
z
AA
AB
AC						0.16
AD
AE		0.19
AF	0.26
AG

[Table 2A]

Experiment No.	Component	Hot rolling conditions						Cold rolling conditions
Experiment No.	Component	Heating temperature [°C]	Rolling end temperature [°C]	Time from end of rolling to start of cooling [SEC]	Cooling rate [°C/SEC]	Coiling temperature [°C]	Sheet thickness after rolling [mm]	Cold rolling reduction [%]	Sheet thickness after cold rolling [mm]
1	A	1250	950	3.4	30	550	3.2	50	1.6
2	A	1250	950	3.4	30	550	3.2	50	1.6
3	A	1250	950	3.4	30	550	3.2	50	1.6
4	A	1250	950	3.4	30	550	3.2	50	1.6
5	A	1250	950	3.4	30	550	3.2	50	1.6
6	A	1250	950	3.4	30	550	3.2	50	1.6
7	A	1250	950	3.4	30	550	3.2	50	1.6
8	A	1250	950	3.4	30	550	3.2	50	1.6
9	A	1250	950	3.4	30	550	3.2	50	1.6
10	A	1250	950	3.4	30	550	3.2	50	1.6
11	A	1250	950	3.4	30	550	3.2	50	1.6
12	A	1250	950	3.4	30	550	3.2	50	1.6
13	A	1250	950	3.4	30	550	3.2	50	1.6
14	A	1250	950	3.4	30	550	3.2	50	1.6
15	A	1250	950	3.4	30	550	3.2	50	1.6
16	A	1250	950	3.4	30	550	4.5	64	1.6
17	A	1250	950	3.4	30	550	2.5	36	1.6
18	A	1250	950	3.4	30	550	3.2	50	1.6
19	B	1250	950	3.4	30	550	3.2	50	1.6
20	B	1250	950	3.4	30	550	3.2	50	1.6
21	B	1250	950	3.4	30	550	3.2	50	1.6
22	B	1250	950	3.4	30	550	3.2	50	1.6
23	C	1250	950	3.4	30	550	3.2	50	1.6
24	D	1250	950	3.4	30	550	3.2	50	1.6
25	E	1250	950	3.4	30	550	3.2	50	1.6
26	G	1250	950	3.4	30	550	3.2	50	1.6
27	H	1250	950	3.4	30	600	3.2	50	1.6

[Table 2B]

Experiment No.	Component	Hot rolling conditions						Cold rolling conditions
Experiment No.	Component	Heating temperature [°C]	Rolling end temperature [°C]	Time from end of rolling to start of cooling [SEC]	Cooling rate [°C/SEC]	Coiling temperature [°C]	Sheet thickness after rolling [mm]	Cold rolling reduction [%]	Sheet thickness after cold rolling [mm]
28	I	1250	950	3.4	30	600	3.2	50	1.6
29	K	1250	950	3.4	30	550	3.2	50	1.6
30	L	1250	950	3.4	30	550	3.2	50	1.6
31	M	1250	950	3.4	30	550	3.2	50	1.6
32	N	1250	950	3.4	30	550	3.2	50	1.6
33	O	1250	950	3.4	30	550	3.2	50	1.6
34	P	1250	950	3.4	30	550	3.2	50	1.6
35	Q	1250	950	3.4	30	550	3.2	50	1.6
36	R	1250	950	3.4	30	550	3.2	50	1.6
37	S	1250	950	3.4	30	550	3.2	50	1.6
38	T	1250	950	3.4	30	550	3.2	50	1.6
39	U	1250	950	3.4	30	550	3.2	50	1.6
40	v	1250	950	3.4	30	550	3.2	50	1.6
41	w	1250	950	3.4	30	550	3.2	50	1.6
42	X	1250	950	3.4	30	550	3.2	50	1.6
43	Y	1250	950	3.4	30	550	3.2	50	1.6
44	Z	1250	950	3.4	30	550	3.2	50	1.6
45	AA	1250	950	3.4	30	550	3.2	50	1.6
46	AB	1250	950	3.4	30	550	3.2	50	1.6
47	AC	1250	950	3.4	30	550	3.2	50	1.6
48	AD	1250	950	Cracking occurred hot rolling
49	AE	1250	950	3.4	30	550	3.2	18.75	2.6
50	AF	1250	950	3.4	30	550	3.2	-12.5	3.6
51	AG	1250	950	Cracking occurred during hot rolling
52	A	1250	950	3.4	30	550	3.2	50	1.6
53	A	1250	950	3.4	30	550	3.2	50	1.6
54	A	1250	950	3.4	30	550	3.2	50	1.6

The surfaces of these cold-rolled steel sheets were ground (grinding step). The amount of grinding on the surface of the steel sheet is shown in Tables 3A and 3B.
In addition, the steel sheets were heated and held to the annealing temperatures at dew points shown in Tables 3A and 3B (annealing step).
Conditions controlled during the annealing are shown in Tables 3A and 3B.
After the holding, quenching was performed at the average cooling rate and the cooling stop temperature shown in Tables 3A and 3B, and tempering was performed at the heat treatment temperature and the heat treatment time shown in Tables 3A and 3B.

In addition, in some of the examples, hot-dip plating was performed under the conditions shown in Tables 3A and 3B (Kind of plating (GA: hot-dip galvannealing, GI: hot-dip galvanizing), steel sheet temperature before plating, and alloying temperature), to form a plating layer on the cold-rolled steel sheet after annealing.

[Table 3A]

Experiment No.	Kind of steel	Grinding step	Annealing step			Quenching step and tempering step		Heat treatment step		Plating step
		Grinding step	Heating process	Soaking process		Quenching step and tempering step		Heat treatment step		Plating step
		Amount of grinding [µm]	Dew point [°C]	Annealing temperature [°C]	Soaking time [SEC]	Average cooling rate from annealing temperature to cooling stop temperature [°C/SEC]	Cooling stop temperature [°C]	Heat treatment temperature [°C]	Heat treatment time [SEC]	Kind of plating	Temperature before plating [°C]	Alloying temperature [°C]
1	A	0.37	0.7	814	84	1.1	51	246	7	GA	460	521
2	A	0.05	10.2	867	140	2.0	41	133	19	GA	460	512
3	A	0.02	6.8	830	142	3.9	53	229	26	GA	460	560
4	A	0.21	-14.1	840	124	4.3	104	163	31	GA	460	541
5	A	0.08	-19.7	812	61	0.5	42	177	495	GA	460	543
6	A	0.39	-1.8	746	162	4.8	48	101	29	GA	460	548
7	A	0.19	-4.8	765	58	3.0	213	262	156	GA	460	511
8	A	0.30	9.6	854	542	0.4	76	262	134	GA	460	529
9	A	0.46	14.5	836	186	241.5	196	284	87	-	-	-
10	A	0.29	-7.3	859	32	167.4	234	298	480	-	-	-
11	A	0.37	19.0	839	37	0.3	114	259	219	GA	460	496
12	A	0.21	17.7	812	178	0.9	321	332	312	GA	460	507
13	A	0.11	13.2	863	9	4.4	170	-	-	GA	460	535
14	A	0.46	15.0	860	84	2.5	227	-	-	GA	460	578
15	A	0.27	11.3	809	12	3.9	246	268	27	GI	460	-
16	A	0.37	-7.3	825	174	1.5	83	246	11	GA	460	580
17	A	0.19	15.5	815	66	2.2	41	176	29	GA	460	551
18	A	0.25	24.5	814	21	1.4	52	203	21	GA	460	530
19	B	0.26	-3.5	786	14	2.6	165	312	29	GA	460	506
20	B	0.00	-7.1	809	63	2.3	61	287	31	GA	460	488
21	B	0.03	2.9	857	156	2.8	69	331	211	GA	460	528
22	B	0.12	-1.3	821	194	3.0	175	330	21	GA	460	541
23	C	0.12	12.1	832	27	4.9	148	233	227	GA	460	528
24	D	0.13	-3.1	792	54	2.5	41	291	417	GI	460	-
25	E	0.28	-5.4	814	18	2.8	156	202	23	GI	460	-
26	G	0.27	4.1	831	171	0.5	240	308	161	GA	460	497
27	H	0.42	7.0	799	71	3.3	35	154	46	GA	460	497

[Table 3B]

Experiment No.	Kind of steel	Grinding step	Annealing step			Quenching step and tempering step		Heat treatment step		Plating step
		Grinding step	Heating process	Soaking process		Quenching step and tempering step		Heat treatment step		Plating step
		Amount of grinding [µm]	Dew point [°C]	Annealing temperature [°C]	Soaking time [SEC]	Average cooling rate from annealing temperature to cooling stop temperature [°C/SEC]	Cooling stop temperature [°C]	Heat treatment temperature [°C]	Heat treatment time [SEC]	Kind of plating	Temperature before plating [°C]	Alloying temperature [°C]
28	I	0.31	4.3	841	56	2.3	221	265	33	GA	460	515
29	K	0.14	-2.8	858	79	3.2	36	268	26	GA	460	577
30	L	0.47	18.2	848	186	1.3	230	295	21	GI	460	-
31	M	0.37	17.6	820	159	1.2	160	211	406	GA	460	521
32	N	0.12	6.1	825	115	3.2	163	265	12	GA	460	554
33	O	0.46	6.8	841	22	3.2	153	173	323	-	-	-
34	P	0.33	19.3	795	91	1.2	94	149	33	GA	460	563
35	Q	0.24	12.2	867	6	1.4	150	248	420	GA	460	563
36	R	0.29	-2.9	804	190	1.9	235	295	365	GA	460	572
37	S	0.43	-7.8	807	34	2.4	220	282	73	GA	460	491
38	T	0.19	-4.7	806	37	1.4	44	290	33	GA	460	492
39	U	0.42	-7.6	800	155	4.6	153	336	193	GA	460	501
40	v	0.16	8.3	783	150	1.4	59	323	36	GA	460	511
41	w	0.24	17.9	856	83	1.0	178	285	9	GA	460	505
42	X	0.28	0.4	794	114	0.7	155	343	134	GA	460	503
43	Y	0.39	-1.9	828	137	4.8	243	302	302	GA	460	568
44	Z	0.48	11.2	834	115	0.7	179	197	53	GA	460	559
45	AA	0.34	-0.4	867	149	2.2	186	306	14	GA	460	493
46	AB	0.30	8.6	813	111	2.8	98	198	54	GA	460	501
47	AC	0.39	15.2	845	60	0.6	178	279	244	GA	460	567
48
49	AE	0.22	19.2	861	72	3.3	79	247	19	GA	461	515
50	AF	0.11	8.4	780	72	3.1	194	267	37	GA	462	539
51
52	A	0.32	3.3	803	112	4.2	453	227	207	GA	460	521
53	A	0.11	-0.8	819	84	4.4	141	261	12	GA	460	501
54	A	0.18	-9.1	821	103	2.1	178	153	134	GA	460	499

For the obtained steel sheets, the ferrite fraction and the in-plane average grain size of ferrite in a range of up to 2 µm from the surface of the steel sheet were measured by the following method.

(Ferrite Fraction (Vα))

A sample was collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section. The observation section was polished and then etched with nital, and a total of five visual fields, each visual field set to 250 µm² or more, were observed at a magnification of 5,000-fold using the field-emission scanning electron microscope. Then, the area ratio of ferrite was measured for each of the samples.
Here, a region with low brightness and no observable substructure was determined to be a ferrite structure. The area ratio of the ferrite structure was calculated by a point counting method in which measurement was performed at 300 or more measurement points per visual field. Calculated values in each visual field were arithmetically averaged to obtain the ferrite fraction (Vα).

(In-Plane Average Grain Size of Ferrite)

A sample with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet an observation section and a sample with a cross section parallel to the sheet width direction and the sheet thickness direction of the steel sheet as an observation section were prepared. The observation sections of the samples were polished and etched with nital. In the surface layer of each sample, a total of five visual fields, each visual field set to 250 µm² or more, were observed at a magnification of 5,000-fold using the field-emission scanning electron microscope.¥ For each sample, the grain sizes in the rolling direction and in the sheet width direction of ferrite grains present at positions 0.5, 1.0, 1.5, and 2.0 µm away from the surface of the steel sheet in the sheet thickness direction were measured. All of the grain sizes of these ferrite grains were arithmetically averaged to obtain the in-plane average grain size of ferrite.

(Structure at 1/4 Thickness Position)

The structure at the 1/4 thickness position was observed by the following method.
A sample is collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section, and the observation section is polished and etched with nital. Next, in a range of 1/8 of the thickness to 3/8 of the thickness (from a 1/8 thickness position of the steel sheet from the surface of the steel sheet to a 3/8 thickness position of the steel sheet from the surface of the steel sheet) with respect to the 1/4 position of the thickness of the steel sheet from the surface of the steel sheet as the center, a total of five visual fields, each visual field set to 250 µm² or more, were observed at a magnification of 5,000-fold using the field-emission scanning electron microscope. In addition, the area ratios of ferrite, residual austenite, martensite, bainite, pearlite, and cementite were each measured.
The area ratio of ferrite was denoted by (Vα), the area ratio of residual austenite was denoted by (Vγ), the area ratio of bainite was denoted by (VB), the area ratio of fresh martensite was denoted by (VfM), and the area ratio of tempered martensite was denoted by (VtM). Pearlite and cementite are included as other structures, and total values thereof are shown.
The identification of each phase was performed as follows. A region having a substructure within grains and having a plurality of long sides of carbides when observed with a scanning electron microscope was determined to be tempered martensite. In addition, a region where cementite is precipitated in a lamellar form was determined to be pearlite or cementite. A region with low brightness and no observable substructure was determined to be ferrite. A region with high brightness and no substructure revealed by etching was determined to be fresh martensite or residual austenite. The remainder was determined to be bainite.
The area ratio of each phase was calculated by a point counting method to obtain the area ratio of each structure. In the point counting method, measurement was performed at 300 or more measurement points per visual field. Calculated values in each visual field are arithmetically averaged to obtain the area ratio of each structure.
The area ratio of fresh martensite was obtained by subtracting the area ratio of residual austenite obtained by the above-described EBSD method from the area ratio of fresh martensite or residual austenite. A sum of this and the area ratio of tempered martensite calculated by the point counting method was taken as the area ratio of martensite.
The area ratio of residual austenite at the 1/4 thickness position was evaluated by performing high-resolution crystal structure analysis by the EBSD method. A sample was collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section, the observation section is polished to a mirror finish, and in order remove a processed layer of a surface layer, electrolytic polishing or mechanical polishing using colloidal silica was performed.
Next, at the 1/4 thickness position of the steel sheet, crystal structure analysis was performed by the EBSD method on five visual fields, each visual field set to a size of 250 µm² or more, at a magnification of 5000-fold. In addition, the distance between evaluation points (step) was set to 0.01 to 0.20 µm.
Data obtained by the EBSD method was analyzed using "OIM Analysis 6.0" manufactured by TSL. From the observation results at each position, a region determined to be FCC iron was determined to be residual austenite, and the area ratio of each residual austenite at the 1/4 thickness position was calculated.
In addition, the ferrite fraction (Vα) and fresh martensite fraction (VfM) in a range of 5 to 20 µm from the surface of the steel sheet were measured by the same method as described above.

The results are shown in Tables 4A and 4B.

[Table 4A]

Experiment No.	Surface to 2 µm		5 to 20 µm		1/4 position
Experiment No.	Vα (%)	In-plane average grain size of ferrite [µm]	Vα (%)	VfM (%)	Va (%)	VB (%)	VfM (%)	VtM (%)	Vγ (%)	Others (%)
1	96	1.12	37	1	31	19	4	47	0	0
2	85	2.87	16	9	0	1	4	93	3	0
3	63	3.44	22	7	21	1	2	76	1	0
4	99	1.58	58	8	13	0	4	81	1	0
5	35	1.42	43	5	31	44	0	22	2	0
6	98	1.09	78	7	76	1	1	22	0	0
7	97	0.84	88	6	56	8	0	35	0	0
8	100	1.38	14	6	6	66	0	26	2	0
9	96	1.29	49	6	16	2	2	79	1	0
10	95	1.10	70	5	2	3	0	94	1	0
11	99	1.37	95	2	14	68	0	17	0	0
12	96	1.13	51	8	21	42	4	32	1	0
13	100	1.59	68	21	17	0	17	66	0	0
14	99	1.17	36	25	15	8	15	59	3	0
15	95	1.59	78	6	35	0	6	59	1	0
16	97	0.76	79	5	24	0	6	68	3	0
17	97	1.36	63	5	31	3	1	64	1	0
18	98	2.57	73	3	29	5	10	56	0	0
19	95	1.27	78	5	47	2	2	46	3	0
20	46	5.65	70	0	37	0	1	59	3	0
21	72	2.43	12	4	7	2	0	91	0	0
22	97	1.83	66	2	28	3	4	64	0	0
23	96	1.94	82	3	0	1	1	98	0	0
24	95	1.46	86	1	53	1	0	45	2	0
25	97	1.38	63	8	25	2	6	65	2	0
26	98	1.06	86	7	27	44	0	21	3	5
27	100	0.98	89	4	44	1	1	52	1	0

[Table 4B]

Experiment No.	Surface to 2 µm		5 to 20 µm		1/4 position
Experiment No.	Va (%)	In-plane average grain size of ferrite [µm]	Vα (%)	VfM (%)	Va (%)	VB (%)	VfM (%)	VtM (%)	Vγ (%)	Others (%)
28	98	1.27	50	2	38	1	4	57	1	0
29	99	1.05	21	0	0	0	1	98	1	0
30	96	1.01	37	7	0	35	6	59	0	0
31	100	1.44	83	5	21	11	0	67	1	0
32	98	0.89	78	6	26	0	10	61	3	0
33	95	0.88	34	7	33	1	0	63	3	0
34	99	1.38	72	5	41	35	1	23	0	0
35	99	1.56	83	5	0	13	0	84	3	0
36	96	1.15	75	2	37	0	0	60	3	0
37	98	0.71	45	3	30	4	2	63	2	0
38	98	1.40	82	4	49	15	0	34	2	0
39	96	0.93	72	6	57	4	0	37	2	0
40	96	1.90	74	3	22	5	1	71	1	0
41	98	1.41	72	8	0	23	14	62	0	0
42	99	1.33	79	8	32	40	0	28	1	0
43	95	0.96	51	6	35	1	0	62	2	0
44	98	1.15	44	3	28	46	1	25	0	0
45	99	0.88	21	4	0	0	12	87	2	0
46	97	0.97	54	2	48	3	1	47	0	0
47	99	1.13	78	1	0	33	1	65	1	0
48
49	99	1.08	54	1	33	3	2	59	2	0
50	98	1.98	50	3	45	4	2	47	2	0
51
52	97	1.21	41	11	30	29	6	34	1	0
53	95	1.17	52	7	25	40	3	32	0	0
54	95	1.27	68	3	34	23	7	36	0	0

(Transition Region Based on C Concentration)

The C concentration of the steel sheet was measured using a glow discharge optical emission spectrometer (GD-Profiler 2 manufactured by HORIBA). The surface of the sample was degreased and washed, and the C concentration is continuously measured from the surface of the sample. After the measurement, a reduction in thickness was measured with a micrometer, and assuming that the reduction in thickness had occurred at a constant rate, the C concentration at each depth was obtained. Regarding a measurement time, a measurement time was set such that a steady state portion having a C concentration of 50 µm or more could be obtained. The steady state portion was defined as a region with a fluctuation range of ±5% after noise removal.
Based on the results obtained by the above measurement, a region having a C concentration of 20% to 90% in a range in the sheet thickness direction of the steel sheet was defined as the transition region.
In addition, the tensile strength of the steel sheet was measured by the following method.

(Yield Stress (YS))

A JIS No. 5 test piece was collected from a direction perpendicular to the rolling direction of the steel sheet, and the yield stress was measured in accordance with JIS Z 2241:2011.

(Tensile Strength (TS))

A JIS No. 5 tensile test piece was collected from a direction (width direction) perpendicular to the rolling direction and the thickness direction of the steel sheet, and a tensile test was conducted in accordance with JIS Z 2241:2011 to measure the tensile strength (TS).

(Elongation (EL))

A JIS No. 5 test piece was collected from the direction perpendicular to the rolling direction of the steel sheet, and the elongation of the steel sheet was measured in accordance with JIS Z 2241:2011.
The obtained steel sheets were subjected to the following two bending tests, and the performance in each bending test was evaluated.

A bending test was conducted in accordance with VDA238-100, and bendability performance was evaluated by giving an evaluation point according to the bending angle of bending in the VDA standard as follows. The test piece was collected in a direction in which the bending ridge was parallel to the rolling direction.

0 points: Bending angle < 120 - TS × 0.05
1 point: Bending angle ≥ 120 - TS × 0.05
2 points: Bending angle ≥ 120 - TS × 0.04

<90° V-Bending Test>

A 90° V-bending test was conducted in accordance with JIS Z 2248. A test piece had a strip shape of 30 mm × 150 mm.
As a result of the test, bending performance was evaluated by giving an evaluation point according to the limit r/t obtained in 90° V-bending, as follows. Here, t at the limit r/t is the sheet thickness, and r is the minimum bending radius at which no crack occurs.

0 points: Limit r/t > 2.0 × (TS/1000)
1 point: Limit r/t ≤ 2.0 × (TS/1000)
2 points: Limit r/t ≤ 1.5 × (TS/1000)

The test pieces in which the sum of the evaluation point from the VDA bending test and the evaluation point of the 90° V-bending test was 3 points or more were determined to have good (OK) bendability, and other test pieces were determined to have poor (NG) bendability.

The results are shown in Tables 5A and 5B.

[Table 5A]

Experiment No.	Width of transition region [µm]	YS (Mpa)	TS (Mpa)	EL (%)	Bending test of VDA standard	90° V-bending test	Bending evaluation point
Experiment No.	Width of transition region [µm]	YS (Mpa)	TS (Mpa)	EL (%)	Evaluation point based on bending angle	Evaluation point based on limit τ/t	Bending evaluation point
1	49	739	1172	11	2	2	OK	Invention Example
2	176	699	1028	12	0	1	NG	Comparative Example
3	128	782	1041	13	0	1	NG	Comparative Example
4	137	729	1107	12	1	2	OK	Invention Example
5	110	637	1057	11	1	0	NG	Comparative Example
6	35	675	935	13	2	2	OK	Comparative Example
7	42	816	996	14	2	2	OK	Invention Example
8	40	648	1024	11	2	2	OK	Invention Example
9	39	829	1087	13	2	2	OK	Invention Example
10	29	848	1244	12	2	2	OK	Invention Example
11	48	630	933	12	2	2	OK	Comparative Example
12	47	743	924	14	2	2	OK	Comparative Example
13	38	788	1238	12	1	2	OK	Invention Example
14	43	772	1221	11	1	2	OK	Invention Example
15	58	814	1222	12	2	2	OK	Invention Example
16	27	791	1211	12	2	2	OK	Invention Example
17	46	731	1074	12	2	2	OK	Invention Example
18	198	674	1025	13	1	1	NG	Comparative Example
19	33	739	1202	11	2	2	OK	Invention Example
20	172	722	1158	11	0	0	NG	Comparative Example
21	162	757	1054	13	0	0	NG	Comparative Example
22	49	743	1179	11	2	2	OK	Invention Example
23	62	741	1179	11	2	2	OK	Invention Example
24	88	643	1058	11	2	2	OK	Invention Example
25	38	733	1136	12	2	2	OK	Invention Example
26	44	582	964	12	2	2	OK	Invention Example
27	38	771	1293	11	2	2	OK	Invention Example

[Table 5B]

Experiment No.	Width of transition region [µm]	YS (Mpa)	TS (Mpa)	EL (%)	Bending test of VDA standard	90° V-bending test	Bending evaluation point
Experiment No.	Width of transition region [µm]	YS (Mpa)	TS (Mpa)	EL (%)	Evaluation point based on bending angle	Evaluation point based on limit r/t	Bending evaluation point
28	54	743	1243	11	2	2	OK	Invention Example
29	48	716	1063	12	2	2	OK	Invention Example
30	49	641	985	12	2	2	OK	Invention Example
31	45	795	1061	13	2	2	OK	Invention Example
32	78	877	1111	14	2	2	OK	Invention Example
33	37	649	968	12	2	2	OK	Invention Example
34	49	484	987	10	2	2	OK	Invention Example
35	56	697	1019	12	2	2	OK	Invention Example
36	52	783	1018	14	2	2	OK	Invention Example
37	43	688	1102	11	2	2	OK	Invention Example
38	71	740	979	13	2	2	OK	Invention Example
39	44	830	1078	14	2	2	OK	Invention Example
40	46	807	1172	12	2	2	OK	Invention Example
41	46	763	1170	12	2	2	OK	Invention Example
42	60	625	1020	13	2	2	OK	Invention Example
43	40	791	1129	13	2	2	OK	Invention Example
44	38	542	912	11	1	1	NG	Comparative Example
45	41	853	1188	13	1	1	NG	Comparative Example
46	39	850	1315	12	2	0	NG	Comparative Example
47	51	830	1182	13	1	0	NG	Comparative Example
48								Comparative Example
49	42	745	893	15	1	1	NG	Comparative Example
50	148	723	1059	12	1	1	NG	Comparative Example
51								Comparative Example
52	61	811	1197	12	0	1	NG	Comparative Example
53	118	763	1043	13	1	2	OK	Invention Example
54	51	806	1182	12	2	2	OK	Invention Example

As can be seen from the above results, in Experiments Nos. 44 to 47, 49, and 50 in which the chemical composition was outside the ranges of the present invention, the results were poor in the bending evaluation point. In Experiments Nos. 48 and 51, cracks occurred during hot rolling, and subsequent tests could not be performed.
In addition, even though the chemical composition was within the ranges of the present invention, in Experiments Nos. 2, 3, 20, and 21 in which the amount of grinding in the grinding step was small, the ferrite fraction and the in-plane average grain size of ferrite in a range of up to 2 µm from the surface of the steel sheet were outside the ranges of the present invention, and the results were poor in the bending evaluation point.
In addition, even though the chemical composition was within the ranges of the present invention, in Experiments Nos. 5 and 18 in which the dew point in the heating process of the annealing step was outside the ranges of the present invention, the ferrite fraction and the in-plane average grain size of ferrite in a range of up to 2 µm from the surface of the steel sheet were outside the ranges of the present invention, and the results were poor in the bending evaluation point.
In addition, even though the chemical composition was within the ranges of the present invention, in Experiment No. 6 in which the annealing temperature in the annealing step was outside the range of the present invention and in Experiments Nos. 11 and 12 in which the conditions in the quenching step were outside the ranges of the present invention, a desired tensile strength was not obtained.
In addition, even though the chemical composition was within the ranges of the present invention, in Experiment No. 52 in which the cooling stop temperature during quenching was outside the ranges of the present invention, the results were poor in the bending evaluation point
In the experiment examples other than the above, the chemical composition and the conditions in the manufacturing method were within the ranges of the present invention, and good results were obtained in the bend evaluation point.

INDUSTRIAL APPLICABILITY

The steel sheet of the present disclosure has high strength, excellent bendability as formability, and excellent bendability as collision characteristics, and thus has high industrial applicability.

Claims

A steel sheet comprising, as a chemical composition, by mass%:
C: 0.070% to 0.15%;

Si: 0.10% to 2.00%;

Mn: 1.00% to 4.00%;

sol. Al: 0.001% to 1.500%;

P: 0.0010% to 0.0300%;

S: 0.0200% or less;

N: 0.0100% or less;

O: 0.0100% or less;

Ti: 0% to 0.200%;

B: 0% to 0.0100%;

Cr: 0% to 1.000%;

Mo: 0% to 1.000%;

Ni: 0% to 1.000%;

Cu: 0% to 1.000%;

Sn: 0% to 0.500%;

Nb: 0% to 0.200%;

V: 0% to 0.500%;

W: 0% to 0.500%;

Ca: 0% to 0.0100%;

Mg: 0% to 0.0100%;

Bi: 0% to 0.0100%;

Sb: 0% to 0.1000%;

Zr: 0% to 0.0100%;

REM: 0% to 0.1000%; and

a remainder: Fe and impurities,

wherein a microstructure at a 1/4 thickness position that is an area centered on 1/4 position in thickness with a range of 1/8 to 3/8 of a sheet thickness of the steel sheet in a sheet thickness direction of the steel sheet from a surface of the steel sheet, includes, by area ratio, 0% to 60% of ferrite, 0% to 3% of residual austenite, and a remainder containing one or more selected from martensite, bainite, pearlite, and cementite,

in a range of up to 2 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is 95% or more, and an in-plane average grain size of the ferrite in an in-plane direction is 2.0 µm or less,

in a range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is less than 90%, and

a tensile strength of the steel sheet is 950 MPa or more.
The steel sheet according to Claim 1,
wherein the tensile strength is less than 1,300 MPa.
The steel sheet according to Claim 1 or 2,
wherein a transition region based on a C concentration in the sheet thickness direction of the steel sheet is 150 µm or less.
The steel sheet according to Claim 1 or 2,
wherein in the range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a fresh martensite fraction is 10% or less.
The steel sheet according to Claim 1 or 2,
wherein in the range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is 50% or more.
A method for manufacturing a steel sheet, the method comprising:
performing cold rolling on a steel sheet including, as a chemical composition, by mass%,

C: 0.070% to 0.15%,

Si: 0.10% to 2.00%,

Mn: 1.00% to 4.00%,

sol. Al: 0.001% to 1.500%,

P: 0.0010% to 0.0300%,

S: 0.0200% or less,

N: 0.0100% or less,

O: 0.0100% or less,

Ti: 0% to 0.200%,

B: 0% to 0.0100%,

Cr: 0% to 1.000%,

Mo: 0% to 1.000%,

Ni: 0% to 1.000%,

Cu: 0% to 1.000%,

Sn: 0% to 0.500%,

Nb: 0% to 0.200%,

V: 0% to 0.500%,

W: 0% to 0.500%,

Ca: 0% to 0.0100%,

Mg: 0% to 0.0100%,

Bi: 0% to 0.0100%,

Sb: 0% to 0.1000%,

Zr: 0% to 0.0100%,

REM: 0% to 0.1000%, and

a remainder: Fe and impurities;

grinding a surface of the steel sheet subjected to the cold rolling;

annealing the steel sheet of which the surface is ground in the grinding; and

quenching the steel sheet after the annealing,

wherein, in the grinding, the surface of the steel sheet subjected to the cold rolling is ground by 0.1 µm or more,

in the annealing, a dew point is set to -15°C to 20°C,

in the annealing, an annealing temperature is set to 750°C or higher, and

in the quenching, quenching is performed to 300°C or lower at an average cooling rate of 0.4 °C/s or faster.
The method for manufacturing a steel sheet according to Claim 6,
wherein in the annealing, the dew point at least in a heating process up to the annealing temperature is set to -15°C to 20°C.
The method for manufacturing a steel sheet according to Claim 6 or 7, further comprising:
tempering the steel sheet at 150°C or higher after the quenching.