STEEL SHEET AND MEMBER, AND METHODS OF PRODUCING SAME

Abstract

Provided is a steel sheet that has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking. The steel sheet has a defined chemical composition and a microstructure where an area fraction of tempered martensite is 83% or more, an area fraction of retained austenite is less than 3%, a total area fraction of bainitic ferrite and ferrite 5 % or more and less than 15%, and a 20° or greater grain boundary density in tempered martensite is 1.0 μm/μm2 or more, satisfying the following Expression ⁢ ( 1 )  KAM ⁡ ( S ) / KAM ⁡ ( C ) > 1. ( 1 )

Description

TECHNICAL FIELD

The present disclosure relates to a steel sheet, a member using the steel sheet as material, and methods of producing same.

BACKGROUND

Higher strength steel sheets as material for automotive parts are being developed to both decrease CO₂ emissions by decreasing automobile body weight and improve crashworthiness. Further, new laws and regulations are being introduced one after another. As a result, in the main structural parts of automobiles, the application of steel sheets that have a tensile strength (hereinafter also referred to as TS) of 1180 MPa or more is increasing.

Further, steel sheets used as material for automotive parts, for example, automobile frame parts such as bumpers, are often required, from the viewpoint of press formability, to have high elongation (hereinafter, also referred to as EL).

As a steel sheet used as material for an automotive part, for example, Patent Literature (PTL) 1 describes:

• • “a steel sheet comprising: a chemical composition containing, in mass %,
  • C: 0.12% or more and 0.40% or less,
  • Si: 0.01% or more and 1.5% or less,
  • Mn: more than 1.7% and 3.5% or less,
  • P: 0.05% or less,
  • S: 0.010% or less,
  • sol.Al: 1.00% or less,
  • N: 0.010% or less,
  • B: 0.0002% or more and 0.0050% or less, and
  • one or both of Nb and Ti for a total of 0.010% or more and 0.080% or less, with the balance being Fe and inevitable impurity; and
  • a steel microstructure that has a martensite area fraction of 70% or more, a bainite area fraction of 30% or less, and a total area fraction of ferrite and retained austenite of 10% or less, wherein
  • a number density of carbides that have a major axis length of 0.5 μm or more at a ¼ sheet thickness position of the steel sheet is 60,000/mm² or less,
  • a number density of inclusion particles that have a circle equivalent diameter of 4.0 μm or more in a range from ¼ to ¾ sheet thickness of the steel sheet is 10/mm² or more and 30/mm² or less,
  • a number density of inclusion particles that have a circle equivalent diameter of 4.0 μm or more in a range from a surface to ¼ sheet thickness of the steel sheet is 27/mm² or less, and
  • tensile strength is 1310 MPa or more.”

PTL 2 describes:

• • “a steel sheet comprising: a chemical composition containing, in mass %,
  • C: 0.05% to 0.40%,
  • Si: 0.05% to 3.0%,
  • Mn: 1.5% to 3.5%,
  • Al: 1.5% or less,
  • N: 0.010% or less,
  • P: 0.10% or less,
  • S: 0.005% or less,
  • Cr, Cu, Ni, Sn, and Mo: total of 0.0% to 1.0%,
  • B: 0.000% to 0.005%,
  • Ca: 0.000% to 0.005%,
  • Ce: 0.000% to 0.005%, and
  • La: 0.000% to 0.005%,
  • and further containing one or more selected from the group consisting of
  • Nb: 0.0002% to 0.04%,
  • Ti: 0.0002% to 0.08%, and
  • V and Ta: a total of 0.01% to 0.3%,
  • with the balance being Fe and impurity; and
  • a steel microstructure represented by, in area %,
  • first martensite with two or more iron carbides having a circle equivalent diameter from 2 nm to 500 nm: 20% to 95%,
  • ferrite: 15% or less,
  • retained austenite: 15% or less, and
  • residual microstructure: bainite or second martensite with less than two iron carbides having a circle equivalent diameter from 2 nm to 500 nm, or both of these, wherein
  • a total area fraction of ND//<111> orientation grains and ND//<100> orientation grains is 40% or less,
  • an amount of solute C is 0.44 ppm or more,
  • the ND//<111> orientation grains are crystal grains that have a crystal orientation parallel to the normal direction of the sheet surface, with a deviation of 100 or less from the <111> direction, and
  • the ND//<100> orientation grains are crystal grains that have a crystal orientation parallel to the normal direction of the sheet surface, with a deviation of 10° or less from the <100> direction.”

PTL 3 describes:

• • “a steel sheet comprising: a chemical composition containing, in mass %,
  • C: 0.09% or more and 0.37% or less,
  • Si: more than 0.70% and 2.00% or less,
  • Mn: 2.60% or more and 3.60% or less,
  • P: 0.001% or more and 0.100% or less,
  • S: 0.0200% or less,
  • Al: 0.010% or more and 1.000% or less, and
  • N: 0.0100% or less, with the balance being Fe and inevitable impurity; and
  • a steel microstructure that has an area fraction of martensite with a carbon concentration greater than 0.7×[% C] and less than 1.5×[% C] of 55% or more,
  • an area fraction of tempered martensite with a carbon concentration of 0.7×[% C] or less of 5% or more and 40% or less,
  • a ratio of carbon concentration in retained austenite to a volume fraction of retained austenite of 0.05 or more and 0.40 or less, and
  • an average grain size of the martensite and the tempered martensite of 5.3 μm or less, wherein
  • the steel microstructure further has a surface layer softening thickness of 10 μm or more and 100 μm or less, and
  • tensile strength is 1180 MPa or more,
  • where [% C] represents content in mass % of the component element C in the steel.”

CITATION LIST

Patent Literature

• • PTL 1: JP 7001197 B2
  • PTL 2: JP 6497443 B2
  • PTL 3: JP 6747612 B2

SUMMARY

Technical Problem

Automotive parts often undergo paint baking. Here, toughness and crash properties of a steel sheet may change significantly before and after paint baking. Therefore, in recent years, steel sheets used as material for automotive parts are also required to have excellent toughness and crash properties after paint baking for further improvement of automobile safety.

However, none of the steel sheets described in PTL 1 to PTL 3 take into account toughness and crash properties after paint baking. Accordingly, it is currently desirable to develop a steel sheet that has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking.

In view of the above circumstances, it would be helpful to provide a steel sheet that has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking, along with an advantageous method of producing the steel sheet.

Further, it would be helpful to provide a member using the steel sheet as a material and a method of producing the member.

Here, TS is measured by a tensile test in accordance with JIS Z 2241:2022.

High EL means an EL of 10% or more. Further, EL is measured by a tensile test in accordance with JIS Z 2241:2022.

Excellent toughness after paint baking means that a brittle-ductile transition temperature after aging treatment is −40° C. or lower. Here, the aging treatment conditions are a treatment temperature of 170° C. and a treatment time of 20 min. Further, the brittle-ductile transition temperature is measured by the Charpy impact test in accordance with JIS Z 2242:2018.

Excellent crash properties after paint baking means that a YR after aging treatment is 0.85 or more, and a fracture stress ratio after aging treatment is 0.90 or less. Here, the aging treatment conditions are a treatment temperature of 170° C. and a treatment time of 20 min. The YR after aging treatment and the fracture stress ratio after aging treatment are determined from the TS, yield stress (YS), and fracture stress after aging treatment measured by a tensile test in accordance with JIS Z 2241:2022, determined using the following expressions.

$\left[\mathrm{YR}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]=\left[\mathrm{YS}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]/\left[\mathrm{TS}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]$ $\left[\mathrm{Fracture}\mathrm{stress}\mathrm{ratio}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]=\left[\mathrm{fracture}\mathrm{stress}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]/\left[\mathrm{TS}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]$

Details of the measurement methods are described in the EXAMPLES section below.

Solution to Problem

The inventors conducted intensive studies to achieve the above, and made the following discoveries:

• • (A) To obtain a TS of 1180 MPa or more, it is important that the area fraction of tempered martensite is 83% or more and the total area fraction of ferrite and bainitic ferrite is less than 15%. This allows for obtaining a TS of 1180 MPa or more while securing defined required properties.
  • (B) To obtain a high EL, it is important that the total area fraction of ferrite and bainitic ferrite is 5% or more. This makes it possible to obtain a high EL while securing defined required properties.
  • (C) To obtain excellent toughness after paint baking, it is important to have an area fraction of retained austenite of less than 3%, and a 20° or greater grain boundary density in tempered martensite of 1.0 μm/μm² or more. This allows for obtaining excellent toughness after paint baking while securing the defined required properties.
  • (D) To obtain excellent crash properties after paint baking, it is important to have a 20° or greater grain boundary density in tempered martensite of 1.0 μm/μm² and to satisfy the following Expression (1). This allows for obtaining excellent crash properties after paint baking while securing defined required properties.

$\begin{array}{cc}\mathrm{KAM}\left(S\right)/\mathrm{KAM}\left(C\right)>1.& \left(1\right)\end{array}$

Here,

• • KAM(S) is KAM value at a depth of 100 μm from a surface of the steel sheet, and
  • KAM(C) is KAM value at a mid-thickness position of the steel sheet.

The present disclosure is based on these discoveries and further studies.

Primary features of the present disclosure are as follows.

1. A steel sheet comprising: a chemical composition containing (consisting of), in mass %,

• • C: 0.030% or more and 0.500% or less,
  • Si: 0.010% or more and 2.500% or less,
  • Mn: 0.10% or more and 5.00% or less,
  • P: 0.100% or less,
  • S: 0.0200% or less,
  • N: 0.0100% or less,
  • O: 0.0100% or less, and
  • Al: 1.000% or less,
  • with the balance being Fe and inevitable impurity; and a steel microstructure wherein
  • area fraction of tempered martensite is 83% or more,
  • area fraction of retained austenite is less than 3%,
  • total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%,
  • 20° or greater grain boundary density in the tempered martensite is 1.0 μm/μm² or more, and
  • the following Expression (1) is satisfied,

$\begin{array}{cc}\mathrm{KAM}\left(S\right)/\mathrm{KAM}\left(C\right)>1.& \left(1\right)\end{array}$

• • where
  • KAM(S) is an average KAM value at a depth of 100 μm from a surface of the steel sheet, and
  • KAM(C) is an average KAM value at a mid-thickness position of the steel sheet.

2. The steel sheet according to 1, above, wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of

• • Ti: 0.200% or less,
  • Nb: 0.200% or less,
  • V: 0.200% or less,
  • Ta: 0.10% or less,
  • WV: 0.10 00 or less,
  • B: 0.0100% or less,
  • Cr: 1.00% or less,
  • Mo: 1.00% or less,
  • Ni: 1.00% or less,
  • Co: 0.010% or less,
  • Cu: 1.00% or less,
  • Sn: 0.200% or less,
  • Sb: 0.200% or less,
  • Ca: 0.0100% or less,
  • Mg: 0.0100% or less,
  • REM: 0.0100% or less,
  • Zr: 0.100% or less,
  • Te: 0.100% or less,
  • Hf: 0.10% or less, and
  • Bi: 0.200% or less.
  • 3. The steel sheet according to 1 or 2, above, further comprising a coated or plated layer on a surface.

4. A member formed using the steel sheet according to any one of 1 to 3, above.

5. A method of producing the steel sheet according to any one of 1 to 3, above, the method comprising:

• • a preparation process of preparing a blank sheet having the chemical composition according to 1 or 2, above;
  • a heating process of heating the blank sheet under a set of conditions including an average heating rate in a temperature range from 700° C. to 750° C. of 5.0° C./s or less, and
  • heating to an annealing temperature T1;
  • an annealing process of annealing the blank sheet under a set of conditions including the annealing temperature T1 being 750° C. or more and 850° C. or less, and
  • an annealing time t1 of 10 s or longer and 1000 s or shorter;
  • a bending process of applying bending once or more to the blank sheet using a roller that has a radius of 800 mm or less in a temperature range from the annealing temperature T1 to 700° C.;
  • a first cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 700° C. to 550° C. of 0.5° C./s or more and less than 10.0° C./s,
  • to a first cooling end temperature;
  • a second cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 300° C. to 100° C. of 300° C./s or more, and
  • applying tension to the blank sheet in the temperature range from 300° C. to 100° C. of 5 MPa or more, to a second cooling end temperature;
  • a tempering process of tempering the blank sheet under a set of conditions including a tempering temperature T2 of 100° C. or more and 400° C. or less, and
  • a tempering time t2 of 10 s or longer and 10,000 s or shorter; and
  • a straightening process of applying straightening to the blank sheet by leveling under a set of conditions including a straightening start temperature of 100° C. or less,
  • an entry side intermesh pressing amount of 4.0 mm or more and 10.0 mm or less,
  • a delivery intermesh pressing amount of 1.0 mm or more and 10.0 mm or less,
  • an entry side tension of 20 MPa or more and 500 MPa or less, and
  • a delivery tension of 25 MPa or more and 550 MPa or less.

6. The method of producing a steel sheet according to 5, above, further comprising a coating or plating process of applying a coating or plating treatment to the blank sheet between the first cooling process and the second cooling process, or between the tempering process and the straightening process.

7. A method of producing a member, wherein the steel sheet according to any one of 1 to 3, above, is subjected to at least one of a forming process or a joining process to produce the member.

Advantageous Effect

According to the present disclosure, a steel sheet is obtainable that has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking. By applying the steel sheet of the present disclosure as a material for automotive parts, for example, it is possible to improve fuel efficiency due to an automotive body weight decrease, which can greatly contribute to a decrease in CO₂ emissions. Therefore, the industrial utility value is extremely high.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram for explaining definitions of entry side intermesh pressing amount and delivery intermesh pressing amount.

DETAILED DESCRIPTION

The following describes embodiments of the present disclosure.

[1] Steel Sheet

First, the chemical composition of a steel sheet according to an embodiment of the present disclosure is described. Hereinafter, although the unit in all chemical compositions is “mass %”, this may be indicated simply as “%”, unless otherwise specified.

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

C is an important basic component of steel. In particular, in the steel sheet according to an embodiment of the present disclosure, C is an important element that affects the area fraction of tempered martensite and the crash properties after paint baking. When C content is less than 0.030%, the area fraction of tempered martensite decreases, and achieving a TS of 1180 MPa or more becomes difficult. Further, achieving excellent crash properties after paint baking also becomes difficult. On the other hand, when the C content exceeds 0.500%, the total area fraction of ferrite and bainitic ferrite becomes less than 5%, making it difficult to achieve a high EL. The C content is therefore 0.030% or more and 0.500% or less. The C content is preferably 0.050% or more. The C content is more preferably 0.100% or more. The C content is preferably 0.400% or less. The C content is more preferably 0.350% or less.

[Si: 0.010% or more and 2.500% or less]

Si is an important basic component of steel. In particular, in the steel sheet according to an embodiment of the present disclosure, Si suppresses carbide formation during annealing and promotes formation of retained austenite. That is, Si is an important element that affects the area fraction of retained austenite. When Si content is less than 0.010%, achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the Si content exceeds 2.500%, retained austenite increases excessively, and achieving excellent toughness after paint baking becomes difficult. The Si content is therefore 0.010% or more and 2.500% or less. The Si content is preferably 0.050% or more. The Si content is more preferably 0.100% or more. The Si content is preferably 2.000% or less. The Si content is more preferably 1.200% or less.

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

Mn is an important basic component of steel. In particular, in the steel sheet according to an embodiment of the present disclosure, Mn is an important element that affects the area fraction of tempered martensite and toughness after paint baking. When Mn content is less than 0.10%, the area fraction of tempered martensite decreases, and achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the Mn content exceeds 5.00% the total area fraction of ferrite and bainitic ferrite becomes less than 5%, making it difficult to achieve a high EL. The Mn content is therefore 0.10% or more and 5.00% or less. The Mn content is preferably 0.50% or more. The Mn content is more preferably 0.80% or more. The Mn content is preferably 4.50% or less. The Mn content is more preferably 4.00% or less.

[P: 0.100% or less]

P segregates at prior austenite grain boundaries, embrittling the grain boundaries and decreasing steel sheet ultimate deformability. Therefore, when P content becomes excessive, achieving excellent toughness after paint baking becomes difficult. The P content is therefore 0.100% or less. The P content is preferably 0.070% or less. A lower limit of the P content is not particularly specified. However, P is a solid-solution-strengthening element and can increase steel sheet strength. The P content is therefore preferably 0.001% or more.

[S: 0.0200% or less]

S exists as a sulfide and decreases steel sheet ultimate deformability. Therefore, when S content becomes excessive, achieving excellent toughness after paint baking becomes difficult. The S content is therefore 0.0200% or less. The S content is preferably 0.0050% or less. A lower limit of the S content is not particularly specified. However, in view of production technology constraints, the S content is preferably 0.0001% or more.

[N: 0.0100% or less]

N exists as a nitride and decreases steel sheet ultimate deformability. Therefore, when N content becomes excessive, achieving excellent toughness after paint baking becomes difficult. The N content is therefore 0.0100% or less. The N content is preferably 0.0050% or less. A lower limit of the N content is not particularly specified. However, in view of production technology constraints, the N content is preferably 0.0001% or more.

[O: 0.0100% or less]

O exists as an oxide and decreases steel sheet ultimate deformability. Therefore, when O content becomes excessive, achieving excellent toughness after paint baking becomes difficult. The O content is therefore 0.0100% or less. The O content is preferably 0.0050% or less. A lower limit of the O content is not particularly specified. However, in view of production technology constraints, the O content is preferably 0.0001% or more.

[Al: 1.000% or less]

Al exists as an oxide and decreases steel sheet ultimate deformability. Therefore, when Al content becomes excessive, achieving excellent toughness after paint baking becomes difficult. The Al content is therefore 1.000% or less. The Al content is preferably 0.500% or less. A lower limit of the Al content is not particularly specified. However, in view of production technology constraints, the Al content is preferably 0.001% or more.

Basic chemical composition of the steel sheet according to an embodiment of the present disclosure is described above. The steel sheet according to an embodiment of the present disclosure has a chemical composition including the basic composition above, with the balance being Fe (iron) and inevitable impurity. Here, the steel sheet according to an embodiment of the present disclosure preferably has a chemical composition consisting of the basic composition above, with the balance being Fe and inevitable impurity. In addition to the basic components described above, the steel sheet according to an embodiment of the present disclosure may contain one or more elements selected from the following as optional additive elements, either alone or in combination.

• • Ti: 0.200% or less,
  • Nb: 0.200% or less,
  • V: 0.200% or less,
  • Ta: 0.10% or less,
  • WV: 0.10% or less,
  • B: 0.0100% or less,
  • Cr: 1.00% or less,
  • Mo: 1.00% or less,
  • Ni: 1.00% or less,
  • Co: 0.010% or less,
  • Cu: 1.00% or less,
  • Sn: 0.200% or less,
  • Sb: 0.200% or less,
  • Ca: 0.0100% or less,
  • Mg: 0.0100% or less,
  • REM: 0.0100% or less,
  • Zr: 0.100% or less,
  • Te: 0.100% or less,
  • Hf: 0.10% or less, and
  • Bi: 0.200% or less.

The effects of the present disclosure are obtainable whenever content is equal to or less than the upper limit indicated above, and therefore there is no particular lower limit for the above optional additive elements. Further, when any of the above optional additional elements are included below a preferred lower limit described below, such an element is included as an inevitable impurity.

[Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less]

When each of Ti, Nb, and V are 0.200% or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Ti, Nb, and V are included, the content of each is preferably 0.200% or less. The content of each of Ti, Nb, and V is respectively more preferably 0.100% or less. A lower limit of the content of each of Ti, Nb, and V is not particularly specified. However, Ti, Nb, and V increase the strength of steel sheets by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, the content of each of Ti, Nb, and V is respectively preferably 0.001% or more.

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

When each of Ta and W are 0.10% or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Ta and W are included, the content of each is preferably 0.10% or less. The content of each of Ta and W is respectively more preferably 0.08% or less. A lower limit of the content of each of Ta and W is not particularly specified. However, Ta and W increase the strength of steel sheets by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. Therefore, the content of each of Ta and W is respectively preferably 0.01% or more.

[B: 0.0100% or less]

When B content is 0.0100% or less, this element does not cause cracks inside the steel sheet during casting or hot rolling or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when B is included, the B content is preferably 0.0100% or less. The B content is more preferably 0.0080% or less. A lower limit of the B content is not particularly specified. However, B is an element that segregates at an austenite grain boundary during annealing and improves hardenability. The B content is therefore preferably 0.0003% or more.

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

When each of Cr, Mo, and Ni are 1.00% or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Cr, Mo, and Ni are included, the content of each is preferably 1.00% or less. The content of each of Cr, Mo, and Ni is respectively more preferably 0.80% or less. A lower limit of the content of each of Cr, Mo, and Ni is not particularly specified. However, Cr, Mo, and Ni are elements that improve hardenability. Therefore, the content of each of Cr, Mo, and Ni is respectively preferably 0.01% or more.

[Co: 0.010% or less]

When Co is 0.010% or less, this element does not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Co is included, the Co content is preferably 0.010% or less. The Co content is more preferably 0.008% or less. A lower limit of the Co content is not particularly specified. However, Co is an element that improves hardenability. The Co content is therefore preferably 0.001% or more.

[Cu: 1.00% or less]

When Cu is 1.00% or less, this element does not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Cu is included, the Cu content is preferably 1.00% or less. The Cu content is more preferably 0.80% or less. A lower limit of the Cu content is not particularly specified. However, Cu is an element that improves hardenability. The Cu content is therefore preferably 0.01% or more.

[Sn: 0.200% or less]

When Sn content is 0.200% or less, this element does not cause cracks inside the steel sheet during casting or hot rolling or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Sn is included, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.100% or less. A lower limit of the Sn content is not particularly specified. However, Sn is an element that improves hardenability and is generally also an element that improves corrosion resistance. The Sn content is therefore preferably 0.001% or more.

[Sb: 0.200% or less]

When Sb is 0.200% or less, this element does not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Sb is included, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.100% or less. A lower limit of the Sb content is not particularly specified. However, Sb is an element that controls surface layer softening thickness and allows strength adjustment. The Sb content is therefore preferably 0.001% or more.

[Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less]

When each of Ca, Mg, and REM are 0.0100% or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Ca, Mg, and REM are included, the content of each is preferably 0.0100% or less. The content of each of Ca, Mg, and REM is respectively more preferably 0.0050% or less. A lower limit of the content of each of Ca, Mg, and REM is not particularly specified. However, Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve steel sheet ultimate deformability. Therefore, the content of each of Ca, Mg, and REM is respectively preferably 0.0005% or more.

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

When each of Zr and Te are 0.100% or less, these elements do not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Zr and Te are included, the content of each is preferably 0.100% or less. The content of each of Zr and Te is respectively more preferably 0.080% or less. A lower limit of the content of each of Zr and Te is not particularly specified. However, Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve steel sheet ultimate deformability. Therefore, the content of each of Zr and Te is respectively preferably 0.001% or more.

[Hf: 0.10% or less]

When Hf is 0.10% or less, this element does not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Hf is included, the Hf content is preferably 0.10% or less. The Hf content is more preferably 0.08% or less. A lower limit of the Hf content is not particularly specified. However, Hf is an element that spheroidizes the shape of nitrides and sulfides and improves steel sheet ultimate deformability. The Hf content is therefore preferably 0.01% or more.

[Bi: 0.200% or less]

When Bi is 0.200% or less, this element does not cause large amounts of coarse precipitates or inclusions to form or cause steel sheet ultimate deformability to decrease. Therefore, a decrease in toughness after paint baking does not result. Therefore, when Bi is included, the Bi content is preferably 0.200% or less. The Bi content is more preferably 0.100% or less. A lower limit of the Bi content is not particularly specified. However, Bi is an element that reduces segregation. The Bi content is therefore preferably 0.001% or more.

Elements other than those described above are Fe and inevitable impurity. Examples of inevitable impurity include Zn, Pb, As, Ge, Sr, and Cs. Such inevitable impurity is allowed to be included as long as a total amount is 0.100% or less.

The following describes the microstructure of the steel sheet according to an embodiment of the present disclosure.

The microstructure of the steel sheet according to an embodiment of the present disclosure satisfies the following conditions:

• • area fraction of tempered martensite is 83% or more,
  • area fraction of retained austenite is less than 3%,
  • total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%,
  • 20° or greater grain boundary density in tempered martensite is 1.0 μm/μm² or more, and

Expression (1) is satisfied.

The reasons for each of these limitations are described below. The area fraction of each phase is the area ratio occupied by each phase relative to the entire microstructure.

[Area fraction of tempered martensite: 83% or more]

In the steel sheet according to an embodiment of the present disclosure, it is extremely important that the area fraction of tempered martensite is 83% or more. That is, by making tempered martensite the main phase, in particular by making the area fraction 83% or more, a TS of 1180 MPa or more is possible to achieve. The area fraction of tempered martensite is therefore 83% or more. The area fraction of tempered martensite is preferably 85% or more. The area fraction of tempered martensite is more preferably 87% or more. An upper limit of the area fraction of tempered martensite is not specifically defined. The area fraction of tempered martensite is, for example, preferably less than 95%. The area fraction of tempered martensite is more preferably 94% or less. The area fraction of tempered martensite is even more preferably 93% or less.

[Area fraction of retained austenite: less than 3%]

In the steel sheet according to an embodiment of the present disclosure, it is extremely important that the area fraction of retained austenite is less than 3%. That is, when the area fraction of retained austenite is 3% or more, achieving excellent toughness after paint baking becomes difficult. One of the causes of decreased toughness after paint baking is that retained austenite transforms into deformation-induced martensite during processing, resulting in high-hardness martensite, which becomes an initiation point of a fracture. The area fraction of retained austenite is therefore less than 3%. The area fraction of retained austenite is preferably 1% or less. A lower limit of the area fraction of retained austenite is not specifically defined. The area fraction of retained austenite may be 0%.

[Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%]

In the steel sheet according to an embodiment of the present disclosure, it is extremely important that the total area fraction of ferrite and bainitic ferrite is more than 5% and less than 15%. That is, when the total area fraction of ferrite and bainitic ferrite is 15% or more, achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the total area fraction of ferrite and bainitic ferrite is less than 5%, it becomes difficult to achieve a high EL. Therefore, the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%. The total area fraction of ferrite and bainitic ferrite is preferably 6% or more. The total area fraction of ferrite and bainitic ferrite is more preferably 7% or more. The total area fraction of ferrite and bainitic ferrite is preferably 14% or less. The total area fraction of ferrite and bainitic ferrite is more preferably 13% or less. Ferrite and bainitic ferrite may be included individually, or both may be included.

The area fraction of residual microstructure other than described above is preferably 5% or less. Examples of residual microstructure include pearlite, fresh martensite, and acicular ferrite. These residual microstructures may be included as long as the content is 5% or less, as they do not affect the properties. The area fraction of the residual microstructure may be 0%.

Here, the area fraction of tempered martensite, as well as the total area fraction of ferrite and bainitic ferrite, is measured, for example, as follows.

A sample is cut from the steel sheet such that a thickness cross-section parallel to the rolling direction of the steel sheet (L-section) becomes an observation plane. The observation plane of the sample is then polished. The observation plane of the sample is then corroded with 3 vol % initial to reveal the microstructure. Then, a ¼ sheet thickness position of the steel sheet (a position corresponding to ¼ of the sheet thickness in the depth direction from a steel sheet surface) is observed at 2000× magnification by SEM for ten fields of view. In the observation images, tempered martensite has fine irregularities in the microstructure and contains carbides in the microstructure. Further, ferrite and bainitic ferrite have a flat microstructure in recessed portions and do not contain carbides. Then, for each field of view, the areas occupied by tempered martensite, as well as ferrite and bainitic ferrite, are determined. Next, for each field of view, the area occupied by tempered martensite, and the area occupied by ferrite and bainitic ferrite, are each divided by the total area of the observed field of view and multiplied by 100. Then, the average values of these are taken as the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite, respectively.

The microstructure of steel sheets is normally approximately vertically symmetrical in the thickness direction. Therefore, any one surface of the steel sheet (front or back) can be set as an initiation point of a thickness position (sheet thickness 0 position), such as the ¼ sheet thickness position or a depth of 100 μm from the steel sheet surface.

Further, the area fraction of retained austenite is measured as follows.

That is, the steel sheet is mechanically ground to a depth of ¼-0.1 mm so that the ¼ sheet thickness position of the steel sheet becomes the observation position, and then further polished by 0.1 mm by chemical polishing. Using the polished surface as the observation plane, an integrated intensity of the diffraction peaks of bcc iron {200}, {211}, and {220} is compared to that of fcc iron (austenite) {200}, {220}, and {311} using Co Kα radiation with an X-ray diffractometer. A volume fraction of retained austenite is then calculated from the ratio of the integrated intensity of each plane. Then, assuming that the retained austenite is uniform in three dimensions, the volume fraction of the retained austenite is taken as the area fraction of retained austenite.

Further, the area fraction of the residual microstructure is determined by subtracting the area fraction of tempered martensite, the total area fraction of ferrite and bainitic ferrite, and the area fraction of retained austenite from 100%.

$\left[\mathrm{Area}\mathrm{fraction}\mathrm{of}\mathrm{residual}\mathrm{microstructure}\left(\%\right)\right]=100-\left[\mathrm{area}\mathrm{fraction}\mathrm{of}\mathrm{tempered}\mathrm{martensite}\left(\%\right)\right]-\left[\mathrm{total}\mathrm{area}\mathrm{fraction}\mathrm{of}\mathrm{ferrite}\mathrm{and}\mathrm{bainitic}\mathrm{ferrite}\left(\%\right)\right]-\left[\mathrm{area}\mathrm{fraction}\mathrm{of}\mathrm{retained}\mathrm{austenite}\left(\%\right)\right]$

[20° or greater grain boundary density in tempered martensite: 1.0 μm/μm² or more]

In the steel sheet according to an embodiment of the present disclosure, it is extremely important that the 20° or greater grain boundary density of tempered martensite is 1.0 μm/μm² or more. Large-angle grain boundaries in tempered martensite, particularly 20° or greater grain boundaries in tempered martensite, become sites of carbon segregation during paint baking, helping prevent steel sheet fracture. As a result, fracture stress during tensile deformation decreases. Therefore, when 200 or greater grain boundary density in tempered martensite (hereinafter also referred to as large-angle grain boundary density of tempered martensite) is less than 1.0 μm/μm², achieving excellent crash properties after paint baking becomes difficult. Therefore, the large-angle grain boundary density of tempered martensite is 1.0 μm/p m² or more. The large-angle grain boundary density of tempered martensite is preferably 1.2 μm/μm² or more. The large-angle grain boundary density of tempered martensite is more preferably 1.3 μm/μm² or more. An upper limit of the large-angle grain boundary of tempered martensite is not specifically defined. For example, the large-angle grain boundary density of tempered martensite is preferably 3.0 μm/μm² or less.

Here, the large-angle grain boundary density of tempered martensite is determined, for example, as follows.

A test piece for microstructure observation is collected from the steel sheet. The collected test piece is then polished by colloidal silica vibrational polishing so that the cross-section in the rolling direction (L-section) becomes the observation plane. The observation plane is mirror-finished. At the ¼ sheet thickness position of the steel sheet (the position corresponding to ¼ of the thickness in the depth direction from the surface of the steel sheet), electron backscatter diffraction (EBSD) measurement is performed to obtain local crystal orientation data. In the EBSD measurement, the step size is set to 0.10 μm, and the measurement area is 50 μm square (50 μm×50 μm). Then, using the analysis software OIM Analysis 7, the obtained local crystal orientation data is analyzed. The analysis of the local crystal orientation data is executed for each of 10 fields of view at the ¼ sheet thickness position of the steel sheet, and an average value is used. Further, prior to the analysis of the local crystal orientation data, a cleanup process is executed once using a grain dilatation function of the analysis software (grain tolerance angle: 5, minimum grain size: 2, single iteration: ON). Next, the 20° or greater grain boundaries in tempered martensite are displayed, and a total length of the 20° or greater grain boundaries in tempered martensite is determined. Then, by dividing the total length of the 200 or greater grain boundaries in tempered martensite by the area of the measurement region, the large-angle grain boundary density of tempered martensite is determined.

$\left[\mathrm{KAM}\left(S\right)/\mathrm{KAM}\left(C\right)>1.\right]$

In the steel sheet according to an embodiment of the present disclosure, it is extremely important that KAM(S)/KAM(C) exceeds 1.00. Here, KAM(S) is the KAM value at a depth of 100 μm from the steel sheet surface, and KAM(C) is the KAM value at the mid-thickness position of the steel sheet. As a result of various studies, the inventors discovered that changing the dislocation distribution state from the surface layer of the steel sheet to the interior is effective for improving crash properties after paint baking. In particular, by making KAM(S)/KAM(C) exceed 1.00, excellent crash properties after paint baking are obtainable. KAM(S)/KAM(C) is therefore more than 1.00. KAM(S)/KAM(C) is preferably 1.03 or more. An upper limit of KAM(S)/KAM(C) is not particularly defined. For example, KAM(S)/KAM(C) is preferably 1.110 or less.

Here, KAM(S) and KAM(C) can be determined, for example, as follows. In the same manner as the measurement of the large-angle grain boundary density of tempered martensite, EBSD measurement is carried out, and the obtained local crystal orientation data is analyzed. The analysis of the local crystal orientation data is executed for 10 fields of view at a depth of 100 μm from the steel sheet surface and at the mid-thickness position of the steel sheet, and average values are used. Then, from the analysis results of the local crystal orientation data at the depth of 100 μm from the steel sheet surface and at the mid-thickness position of the steel sheet, a chart of the KAM values of the bcc phase for each position is created, and average values are used as KAM(S) and KAM(C), respectively.

Mechanical properties of the steel sheet according to an embodiment of the present disclosure are as described above.

Further, the steel sheet according to an embodiment of the present disclosure may include a coated or plated layer on a surface. The coated or plated layer may be on only one surface of the steel sheet or may be on both surfaces. The coated or plated layer is not particularly limited. As a coated or plated layer, a galvanized layer with Zn as the main component (Zn content of 50.0 mass % or more) is an example. Further, examples of galvanized layers include hot-dip galvanized layers, galvannealed layers, and electrogalvanized layers. A steel sheet that has a galvanized layer may also be referred to as a galvanized steel sheet. Further, a steel sheet that has a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer may also be referred to as a hot-dip galvanized steel sheet (GI), a galvannealed steel sheet (GA), or an electrogalvanized steel sheet (EG), respectively.

Coated or plated layers other than galvanized layers may include aluminum coated or plated layers and alloy coated or plated layers. As alloy coated or plated layers, examples include hot-dip zinc-aluminum-magnesium alloy coated layers and Zn—Ni electroplated alloy layers.

Further, coating weight per side of the coated or plated layer is not particularly limited. The coating weight per side of the coated or plated layer is preferably 20 g/m² or more. The coating weight per side is preferably 80 g/m² or less.

The thickness of the steel sheet according to an embodiment of the present disclosure is not particularly limited. The thickness of the steel sheet is preferably 0.50 mm or more. The thickness of the steel sheet is preferably 2.50 mm or less.

[2] Member

A member according to an embodiment of the present disclosure is described below.

The member according to an embodiment of the present disclosure is a member formed using the steel sheet described above as a material. For example, the material, the steel sheet, is subjected to at least one of a forming process or a joining process to make the member.

Here, the steel sheet described above has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking. Therefore, the member according to an embodiment of the present disclosure is particularly suitable for application as a material for automotive parts. This allows for improved fuel efficiency due to an automotive body weight decrease, which can greatly contribute to a decrease in CO₂ emissions.

[3] Method of Producing Steel Sheet

The following describes a method of producing a steel sheet according to an embodiment of the present disclosure.

The method of producing a steel sheet according to an embodiment of the present disclosure includes:

• • a preparation process of preparing a blank sheet having the chemical composition described above;
  • a heating process of heating the blank sheet under a set of conditions including an average heating rate in a temperature range from 700° C. to 750° C. of 5.0° C./s or less, and
  • heating to an annealing temperature T1;
  • an annealing process of annealing the blank sheet under a set of conditions including the annealing temperature T1 being 750° C. or more and 850° C. or less, and
  • an annealing time t1 of 10 s or longer and 1000 s or shorter;
  • a bending process of applying bending once or more to the blank sheet using a roller that has a radius of 800 mm or less in a temperature range from the annealing temperature T1 to 700° C.;
  • a first cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 700° C. to 550° C. of 0.5° C./s or more and less than 10.0° C./s,
  • to a first cooling end temperature;
  • a second cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 300° C. to 100° C. of 300° C./s or more, and
  • applying tension to the blank sheet in the temperature range from 300° C. to 100° C. of 5 MPa or more,
  • to a second cooling end temperature;
  • a tempering process of tempering the blank sheet under a set of conditions including a tempering temperature T2 of 100° C. or more and 400° C. or less, and
  • a tempering time t2 of 10 s or longer and 10,000 s or shorter; and
  • a straightening process of applying straightening to the blank sheet by leveling under a set of conditions including a straightening start temperature of 100° C. or less,
  • an entry side intermesh pressing amount of 4.0 mm or more and 10.0 mm or less,
  • a delivery intermesh pressing amount of 1.0 mm or more and 10.0 mm or less,
  • an entry side tension of 20 MPa or more and 500 MPa or less, and
  • a delivery tension of 25 MPa or more and 550 MPa or less.

Unless otherwise specified, each of the temperatures above refers to a surface temperature of the steel sheet. Further, the average heating rate and the average cooling rate are based on the surface temperature of the steel sheet, unless otherwise specified.

Preparation Process

First, a blank sheet having the chemical composition described above is prepared. For example, a blank sheet can be prepared by hot rolling a steel slab into a hot-rolled steel sheet, then subjecting the hot-rolled steel sheet to optional pickling and heat treatment, and then cold rolling to obtain a cold-rolled steel sheet. The conditions of these processes are not particularly limited and may follow a conventional method.

For example, a method of smelting the steel slab (steel material) may be any known method, such as by use of a converter, an electric furnace, or the like. The steel slab is preferably smelted by continuous casting to help prevent macro-segregation.

Examples of hot rolling include methods such as rolling the steel slab after heating, direct rolling the steel slab after continuous casting without heating, and rolling the steel slab after applying a short heating treatment following continuous casting. Further, slab heating temperature, slab soaking duration, and coiling temperature in hot rolling are not particularly limited. The slab heating temperature is preferably 1100° C. or more. The slab heating temperature is preferably 1300° C. or less. The slab soaking duration is preferably 30 min or more. The slab soaking duration is preferably 250 min or less. The rolling finish temperature is preferably the Ar₃ transformation temperature or more. The coiling temperature is preferably 350° C. or more. The coiling temperature is preferably 650° C. or less. The Ar₃ transformation temperature is determined by the following expression.

${\mathrm{Ar}}_3\mathrm{transformation}\mathrm{temperature}\left(°C.\right)=868-396\times \left[\%C\right]+24.6\times \left[\%\mathrm{Si}\right]-68.1\times \left[\%\mathrm{Mn}\right]-36.1\times \left[\%\mathrm{Ni}\right]-20.7\times \left[\%\mathrm{Cu}\right]-24.8\times \left[\%\mathrm{Cr}\right]$

Here, [% element symbol] in the above expression represents the content in mass % of the element in the chemical composition described above.

Pickling is capable of removing oxides from the surface of the hot-rolled steel sheet, and is preferably carried out to secure good chemical convertibility and coating quality in the final steel sheet product. Pickling may be carried out in one or more batches. Further, the hot-rolled steel sheet after pickling may be subjected to heat treatment.

The total rolling reduction in the cold rolling is preferably 30% or more. The total rolling reduction in the cold rolling is preferably 80% or less. The defined effect can be obtained without limiting the number of rolling passes or the rolling reduction for each pass.

Heating Process

Next, the blank sheet prepared in the preparation process is heated to the annealing temperature Ti under a set of conditions including an average heating rate of 5.0° C./s or less in the temperature range of 700° C. to 750° C. The annealing temperature T1 is explained in the annealing process described later.

[Average heating rate in temperature range of 700° C. to 750° C.: 5.0° C./s or less]

The inventors have carried out intensive studies and found that the average heating rate in the temperature range of 700° C. to 750° C. (hereinafter also referred to simply as average heating rate) affects the density of large-angle grain boundaries of tempered martensite. That is, by setting the average heating rate to 5.0° C./s or less, the dissolution of carbides is promoted. As a result, prior austenite grain boundaries are refined, and the number of 200 or greater grain boundaries after martensitic transformation increases. Therefore, the density of large-angle grain boundaries of tempered martensite in the final steel sheet product also increases, improving crash properties after paint baking. Accordingly, the average heating rate is 5.0° C./s or less. The average heating rate is preferably 3.0° C./s or less. A lower limit of the average heating rate is not specifically defined. For example, the average heating rate is preferably 0.1° C./s or more.

Annealing Process

Next, the blank sheet is annealed under a set of conditions including the annealing temperature T1 being 750° C. or more and 850° C. or less, and annealing time t1 being 10 s or longer and 1000 s or shorter.

[Annealing temperature T1: 750° C. or more and 850° C. or less]

When the annealing temperature T1 is less than 750° C., the total area fraction of ferrite and bainitic ferrite becomes 15% or more, and achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the annealing temperature T1 exceeds 850° C., the total area fraction of ferrite and bainitic ferrite becomes less than 5%, making it difficult to achieve a high EL. The annealing temperature T1 is therefore 750° C. or more and 850° C. or less. The annealing temperature T1 is preferably 760° C. or more. The annealing temperature T1 is more preferably 770° C. or more. The annealing temperature T1 is preferably 840° C. or less. The annealing temperature T1 is more preferably 830° C. or less. The annealing temperature referred to here is the holding temperature during the annealing process. Further, the annealing temperature may remain constant during holding. Further, the annealing temperature is the temperature range of 750° C. or more and 850° C. or less, and when temperature fluctuation is within ±10° C. of the set temperature, the annealing temperature does not have to be constant during holding.

[Annealing time t1: 10 s or longer and 1000 s or shorter]

When the annealing time t1 is less than 10 s, the total area fraction of ferrite and bainitic ferrite becomes 15% or more, and achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the annealing time t1 exceeds 1000 s, the total area fraction of ferrite and bainitic ferrite becomes less than 5%, making it difficult to achieve a high EL. The annealing time t1 is therefore 10 s or longer and 1000 s or shorter. The annealing time t1 is preferably 30 s or longer. The annealing time t1 is preferably 800 s or shorter. The annealing time t1 referred to here is the holding time at the annealing temperature Ti.

Bending Process

Next, the blank sheet is subjected to bending once or more using a roller that has a radius of 800 mm or less in the temperature range from the annealing temperature T1 to 700° C.

[Number of times bending using a roller that has a radius of 800 mm or less in the temperature range from the annealing temperature Ti to 700° C.: once or more]

As a result of intensive studies, the inventors found that carrying out bending in the temperature range from the annealing temperature T1 to 700° C. (hereinafter also referred to as bending temperature range) affects the large-angle grain boundary density of tempered martensite. In particular, carrying out bending using a roller that has a radius of 800 mm or less in the bending temperature range promotes martensitic transformation nucleation. As a result, martensite is refined, and 200 or greater grain boundaries after martensitic transformation increase. Therefore, the density of large-angle grain boundaries of tempered martensite in the final steel sheet product also increases, improving crash properties after paint baking. Accordingly, the number of times bending using a roller that has a radius of 800 mm or less in the bending temperature range (hereinafter also referred to as bending count) is set to be once or more.

The radius of the roller used for bending is preferably 600 mm or less. A lower limit of the radius of the roller used for bending is not particularly restricted. For example, the radius of the roller used for bending is preferably 100 mm or more.

Further, the number of bends may be one or more. The number of bends is preferably two or more. An upper limit of the number of bends is not specifically defined. For example, the number of bends is preferably 15 or fewer. The bending may be carried out by bending in one direction with a roller and then bending back the same amount in the opposite direction. In such a case, the number of bends is counted as two (one for bending and one for bending back). Further, the bending angle is preferably 800 or more. The bending angle is preferably 1100 or less. This results in a greater effect of promoting martensitic transformation nucleation. The bending angle is the angle (acute angle) formed between the sheet passing direction of the steel sheet on the roller entry side and the sheet passing direction of the steel sheet on the roller delivery side.

When bending using a roller that has a radius of 800 mm or less is carried out at least once in the bending temperature range, additional bending that does not satisfy the described conditions may be carried out.

First Cooling Process

Next, the blank sheet is cooled to a first cooling end temperature under a set of conditions including an average cooling rate of 0.5° C./s or more and less than 10.0° C./s in a temperature range from 700° C. to 550° C. [0081][Average cooling rate in temperature range from 700° C. to 550° C.: 0.5° C./s or more and less than 10.0° C./s]

When the average cooling rate in the temperature range from 700° C. to 550° C. (hereinafter also referred to as first average cooling rate) is 10.0° C./s or more, the total area fraction of ferrite and bainitic ferrite becomes less than 5%, and achieving a high EL becomes difficult. On the other hand, when the first average cooling rate is less than 0.5° C./s, the total area fraction of ferrite and bainitic ferrite becomes 15% or more, and achieving a TS of 1180 MPa or more becomes difficult. The first average cooling rate is therefore 0.5° C./s or more and less than 10.0° C./s. The first average cooling rate is preferably 1.0° C./s or more. The first average cooling rate is preferably 8.0° C./s or less.

The first cooling end temperature may be, for example, from 550° C. to 300° C. Further, a coating or plating treatment may be applied to the blank sheet between the first cooling process and the second cooling process described later. Details about the coating or plating treatment are described later.

Second Cooling Process

Next, the blank sheet is cooled to the second cooling end temperature under a set of conditions including an average cooling rate of 300° C./s or more in a temperature range from 300° C. to 100° C., and a tension of 5 MPa or more applied to the blank sheet in the temperature range from 300° C. to 100° C.

[Average cooling rate in temperature range from 300° C. to 100° C.: 300° C./s or more]

When the average cooling rate in the temperature range from 300° C. to 100° C. (hereinafter also referred to as second average cooling rate) is less than 300° C./s, the area fraction of retained austenite becomes 3% or more, and achieving excellent toughness after paint baking becomes difficult. The second average cooling rate is therefore 300° C./s or more. The second average cooling rate is preferably 800° C./s or more. An upper limit of the second average cooling rate is not specifically defined. For example, the second average cooling rate is preferably 2000° C./s or less.

[Tension applied to blank sheet in temperature range from 300° C. to 100° C.: 5 MPa or more]

As a result of intensive studies, the inventors found that applied tension to the blank sheet during cooling in the temperature range from 300° C. to 100° C. affects the large-angle grain boundary density of tempered martensite. In particular, when the tension applied to the blank sheet in the temperature range from 300° C. to 100° C. (hereinafter also referred to simply as applied tension) is 5 MPa or more, martensitic transformation is promoted. As a result, martensite is refined, and 200 or greater grain boundaries after martensitic transformation increase. Therefore, the density of large-angle grain boundaries of tempered martensite in the final steel sheet product also increases, improving crash properties after paint baking. Accordingly, the applied tension is 5 MPa or more. The applied tension is preferably 10 MPa or more. An upper limit of the applied tension is not specifically defined. For example, the applied tension is preferably 100 MPa or less.

Further, the second cooling end temperature may be, for example, less than 100° C. The second cooling end temperature may be, for example, around room temperature.

The bending in the bending process described above increases the number of nucleation sites, which are initiation points of martensitic transformation. On the other hand, the application of tension in the second cooling process promotes the martensitic transformation itself. That is, the effects obtained from both are different.

Tempering Process

Next, the blank sheet is tempered under a set of conditions including the tempering temperature T2 being 100° C. or more and 400° C. or less, and tempering time t2 being 10 s or longer and 10,000 s or shorter.

[Tempering temperature T2: 100° C. or more and 400° C. or less]

Tempered martensite is formed by tempering treatment, where martensite is tempered. Here, when the tempering temperature T2 is less than 100° C., martensite is not sufficiently tempered, resulting in a microstructure mainly composed of quenched martensite. In such a microstructure mainly composed of quenched martensite, excellent toughness after paint baking cannot be obtained. On the other hand, when the tempering temperature T2 exceeds 400° C., tempering of martensite progresses excessively, and achieving a TS of 1180 MPa or more becomes difficult. The tempering temperature T2 is therefore 100° C. or more and 400° C. or less. The tempering temperature T2 is preferably 150° C. or more. The tempering temperature T2 is preferably 350° C. or less. The tempering temperature referred to here is the holding temperature during the tempering process. The tempering temperature may be constant during holding. Further, the tempering temperature is in the range from 100° C. or more to 400° C. or less, and when temperature fluctuation is within ±10° C. of the set temperature, the tempering temperature does not have to be constant during holding.

[Tempering time t2: 10 s or longer and 10,000 s or shorter]

As mentioned above, tempered martensite is formed by tempering treatment, where martensite is tempered. Here, when the tempering time t2 is shorter than 10 s, martensite is not sufficiently tempered, resulting in a microstructure mainly composed of quenched martensite. In such a microstructure mainly composed of quenched martensite, excellent toughness after paint baking cannot be obtained. On the other hand, when the tempering time t2 exceeds 10,000 s, tempering of martensite progresses excessively, and achieving a TS of 1180 MPa or more becomes difficult. Accordingly, the tempering time t2 is 10 s or longer and 10,000 s or shorter. The tempering time t2 is preferably 50 s or longer. The tempering time t2 is preferably 5000 s or shorter. Here, the tempering time t2 refers to the holding time at the tempering temperature T2.

The cooling after tempering is not specifically defined. For example, it is sufficient to cool by any method according to a conventional method. The cooling end temperature after tempering may be, for example, around room temperature. Further, a coating or plating treatment may be applied to the blank sheet between the tempering process and the straightening process described below. Details about the coating or plating treatment are described later.

Straightening Process

Next, the blank sheet is subjected to straightening by leveling (using a leveler). At this time, satisfying the following conditions is extremely important in the method of producing the steel sheet according to an embodiment of the present disclosure.

• • Straightening start temperature of 100° C. or less,
  • an entry side intermesh pressing amount of 4.0 mm or more and 10.0 mm or less,
  • a delivery intermesh pressing amount of 1.0 mm or more and 10.0 mm or less,
  • an entry side tension of 20 MPa or more and 500 MPa or less, and
  • a delivery tension of 25 MPa or more and 550 MPa or less.

[Straightening start temperature: 100° C. or less]

When the straightening start temperature exceeds 100° C., the steel sheet becomes soft. As a result, the amount of strain introduced into the surface layer and the center of the steel sheet by leveling changes, and KAM(S)/KAM(C) becomes 1.00 or less. Therefore, crash properties after paint baking are decreased. The straightening start temperature is therefore 100° C. or less. The straightening start temperature is preferably 80° C. or less. A lower limit of the straightening start temperature is not specifically defined. For example, the straightening start temperature is preferably −10° C. or more.

[Entry side intermesh pressing amount: 4.0 mm or more and 10.0 mm or less]

When the entry side intermesh pressing amount is less than 4.0 mm, the amount of processing is insufficient. As a result, KAM(S)/KAM(C) becomes 1.00 or less, and the crash properties after paint baking decrease. An upper limit of the entry side intermesh pressing amount is 10.0 mm or less in view of production technology constraints. The entry side intermesh pressing amount is therefore 4.0 mm or more and 10.0 mm or less. The entry side intermesh pressing amount is preferably 5.0 mm or more.

[Delivery intermesh pressing amount: 1.0 mm or more and 10.0 mm or less]

When the delivery intermesh pressing amount is less than 1.0 mm, the amount of processing is insufficient. As a result, KAM(S)/KAM(C) becomes 1.00 or less, and the crash properties after paint baking decrease. An upper limit of the delivery intermesh pressing amount is 10.0 mm or less in view of production technology constraints. The delivery intermesh pressing amount is therefore 1.0 mm or more and 10.0 mm or less.

Here, the entry side intermesh pressing amount, as indicated in FIG. 1, refers to the pressing amount of the second roller from the entry side (roller 2 in FIG. 1) against the steel sheet plane (the surface of the steel sheet where roller 2 is disposed) in leveling. Further, the delivery intermesh pressing amount refers to the pressing amount of the second roller from the delivery side (roller 8 in FIG. 1) against the steel sheet plane (the surface of the steel sheet where roller 8 is disposed) in leveling. The number of rollers in the leveling (leveler) is not particularly limited. For example, five or more rollers is preferred.

[Entry side tension: 20 MPa or more and 500 MPa or less]

When the entry side tension is less than 20 MPa, the amount of processing is insufficient. Therefore, KAM(S)/KAM(C) becomes 1.00 or less, and the crash properties after paint baking decrease. The upper limit of the entry side tension is 500 MPa in view of production technology constraints. Accordingly, the entry side tension is 20 MPa or more and 500 MPa or less. The entry side tension is preferably 100 MPa or more.

[Delivery tension: 25 MPa or more and 550 MPa or less]

Due to leveler apparatus constraints, the delivery tension is higher than the entry side tension. Here, when the delivery intermesh pressing amount is less than 25 MPa, the amount of processing is insufficient. Therefore, KAM(S)/KAM(C) becomes 1.00 or less, and the crash properties after paint baking decrease. The upper limit of the delivery tension is 550 MPa or less in view of production technology constraints. Accordingly, the delivery tension is 25 MPa or more and 550 MPa or less. The delivery tension is preferably 100 MPa or more.

Coating or Plating Process

Further, optionally, the steel sheet may be subjected to coating or plating treatment. Coating or plating treatment is not particularly limited. Examples of coating or plating treatment include galvanizing treatment such as hot-dip galvanizing treatment, galvannealing treatment, and electrogalvanization treatment. Other than galvanizing treatment, examples of coating or plating treatment include aluminum coating or plating treatment and alloy coating or plating treatment. Examples of alloy coating or plating treatment include hot-dip zinc-aluminum-magnesium alloy coating treatment and Zn—Ni electro-alloy plating treatment. Treatment conditions may follow conventional methods. As mentioned above, the coating or plating treatment is preferably carried out between the first cooling process and the second cooling process, or between the tempering process and the straightening process. For example, hot-dip galvanizing treatment or galvannealing treatment is preferably carried out between the first cooling process and the second cooling process. Further, electrogalvanization treatment or Zn—Ni electro-alloy plating treatment is preferably carried out between the tempering process and the straightening process.

In the case of hot-dip galvanizing treatment and galvannealing treatment, from the perspective of productivity, the series of treatments including the heating process, the annealing process, and the coating or plating treatment process is preferably carried out on a continuous galvanizing line (CGL). After the hot-dip galvanizing, wiping may be carried out for adjusting the coating amount.

Conditions other than those described above are not particularly limited, and a conventional method may be used. According to the method of producing the steel sheet according to an embodiment of the present disclosure described above, a steel sheet is obtainable that has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking. The obtained steel sheet may be suitably used as a material for automotive parts, for example.

[4] Method of Producing Member

The following describes a method of producing a member according to an embodiment of the present disclosure.

The method of producing a member according to an embodiment of the present disclosure includes process of at least one of forming or joining the steel sheet described above to make the member.

Here, a forming method is not particularly limited, and a typical processing method such as press forming may be used, for example. Further, a joining method is also not particularly limited, and for example, typical welding such as spot welding, laser welding, arc welding, and the like, rivet joining, swaging joining, and the like may be used. Forming and joining conditions are not particularly limited and may follow a conventional method.

EXAMPLES

Steel having the chemical compositions listed in Table 1 (the balance being Fe and inevitable impurity) was melted in a converter and made into steel slabs by a continuous casting method. The steel slabs were then heated. The steel slabs were then hot rolled to produce hot-rolled steel sheets. Pickling treatment was then carried out on the hot-rolled steel sheets. The hot-rolled steel sheets were then subjected to cold rolling to obtain cold-rolled steel sheets. In this way, blank sheets were prepared. The prepared blank sheets then underwent the heating process, the annealing process, the bending process, the first cooling process, the second cooling process, the tempering process, and the straightening process under conditions including the conditions listed in Table 2 to obtain final product steel sheets (thickness: 0.6 mm to 2.2 mm). The first cooling end temperature in each case was set to 550° C. to 300° C. Both the second cooling end temperature and the cooling end temperature after tempering were set to room temperature. The bending angle in the bending was set to 800 to 110°. Further, some of the steel sheets (those listed as GI, GA, and EG in Table 2) were subjected to coating or plating treatment. Among these, for those listed as GI and GA in Table 2, coating treatment was carried out between the first cooling process and the second cooling process. For those listed as EG in Table 2, plating treatment was carried out between the tempering process and the straightening process. Conditions not specified were followed according to conventional methods.

Using the obtained steel sheets, the area fraction of tempered martensite, the area fraction of retained austenite, the total area fraction of ferrite and bainitic ferrite, the large-angle grain boundary density of tempered martensite, and KAM(S)/KAM(C) were determined in the manner described above. Results are listed in Table 3.

Further, each evaluation was carried out according to the following procedure. The evaluation results are listed in Table 3.

(Evaluation of TS)

From each obtained steel sheet, a JIS No. 5 test piece (gauge length: 50 mm, parallel portion width: 25 mm) was taken so that the direction perpendicular to the rolling direction of the steel sheet was the longitudinal direction of the test piece. Next, a tensile test was conducted according to JIS Z 2241:2022 using the test piece, and TS and EL were measured. The crosshead speed was set to 1.67×10⁻¹ mm/s. Evaluation was based on the following criteria.

• • Good (pass, very good): TS was 1180 MPa or more
  • Poor (fail): TS was less than 1180 MPa

(Evaluation of EL)

The EL measured in the above tensile tests was evaluated according to the following criteria.

• • Good (pass, very good): EL was 10% or more
  • Poor (fail): EL was less than 10%
    (Evaluation of Toughness after Paint Baking)

For each obtained steel sheet, a plurality of sheets were stacked and fastened with bolts. Next, after confirming that there were no gaps between the steel sheets, a V-notch having a depth of 2 mm was applied to the stacked steel sheets, and a stacked Charpy test piece (hereinafter also referred to simply as the test piece) was prepared. The number of stacked steel sheets was set to the number that most closely approaches a thickness of 10 mm for the test piece (when there were two numbers that were closest to 10 mm, the smaller number was chosen). For example, when the thickness of the steel sheet was 1.2 mm, eight sheets of the steel sheet were stacked together. That is, the thickness of the test piece was 9.6 mm. Further, the test piece was prepared so that the sheet transverse direction of the steel sheet (direction perpendicular to the rolling direction) was the longitudinal direction of the test piece. The prepared test piece was then subjected to aging treatment at a treatment temperature of 170° C. for a treatment time of 20 min. Using the test piece that was subjected to the aging treatment, a Charpy impact test was carried out in a test temperature range of −120° C. to +120° C. From the obtained percent brittle fracture, a transition curve was determined, and the temperature at which the percent brittle fracture reached 50% was defined as the brittle-ductile transition temperature. Based on the following criteria, the toughness after paint baking was evaluated. Other than the above conditions, JIS Z 2242:2018 was followed.

Excellent (pass, particularly good): the brittle-ductile transition temperature after aging treatment was −60° C. or lower.

Good (pass, very good): the brittle-ductile transition temperature after aging treatment was −40° C. or lower (excluding Excellent results).

Poor (fail): the brittle-ductile transition temperature after aging treatment was higher than −40° C.

(Crash Properties after Paint Baking)

The obtained steel sheets were each subjected to aging treatment at a treatment temperature of 170° C. for a treatment time of 20 min. From each steel sheet subjected to aging treatment, a JIS No. 5 test piece (gauge length: 50 mm, parallel portion width: 25 mm) was taken so that the direction perpendicular to the rolling direction of the steel sheet was the longitudinal direction of the test piece. Then, using the test piece, a tensile test was carried out according to JIS Z 2241:2022 in the same manner as in the TS evaluation described above, measuring the TS, yield stress (YS), and fracture stress after aging treatment. Based on the following criteria, the crash properties after paint baking were then evaluated.

Good (pass, very good): the YR after aging treatment was 0.85 or more, and the fracture stress ratio after aging treatment was 0.90 or less.

Poor (fail): at least one of the YR after aging treatment being 0.85 or more and the fracture stress ratio after aging treatment being 0.90 or less was not satisfied.

The YR and fracture stress ratio after aging treatment are determined by the following expressions, respectively.

$\left[\mathrm{YR}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]=\left[\mathrm{YS}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]/\left[\mathrm{TS}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]$ $\left[\mathrm{Fracture}\mathrm{stress}\mathrm{ratio}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]=\left[\mathrm{fracture}\mathrm{stress}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]/\left[\mathrm{TS}\mathrm{after}\mathrm{aging}\mathrm{treatment}\right]$

Further, the fracture stress is the stress at the fracture point in the tensile test (the stress applied when the test piece fractures).

TABLE 1
Steel
sample	Chemical composition (mass %)
ID	C	Si	Mn	P	S	N	O	Al	Ti	Nb	B	Cu	Other	Remarks
A	0.245	0.251	1.25	0.008	0.0010	0.0042	0.0049	0.049						Conforming steel
B	0.232	0.296	1.28	0.008	0.0008	0.0057	0.0041	0.042						Conforming steel
C	0.212	0.235	1.16	0.006	0.0010	0.0051	0.0068	0.022						Conforming steel
D	0.201	0.289	1.89	0.008	0.0009	0.0018	0.0042	0.056						Conforming steel
E	0.217	0.252	1.53	0.010	0.0009	0.0034	0.0052	0.032						Conforming steel
F	0.037	0.162	1.52	0.010	0.0010	0.0063	0.0025	0.045						Conforming steel
G	0.024	0.299	1.01	0.012	0.0007	0.0010	0.0014	0.045						Comparative steel
H	0.479	0.136	1.26	0.012	0.0006	0.0067	0.0057	0.010						Conforming steel
I	0.505	0.220	1.19	0.013	0.0010	0.0052	0.0033	0.030						Comparative steel
J	0.234	0.011	1.23	0.009	0.0009	0.0018	0.0019	0.037						Conforming steel
K	0.200	0.005	1.38	0.009	0.0010	0.0049	0.0070	0.036						Comparative steel
L	0.217	2.376	1.24	0.010	0.0009	0.0022	0.0065	0.020						Conforming steel
M	0.240	2.580	1.64	0.010	0.0012	0.0067	0.0051	0.059						Comparative steel
N	0.203	0.218	0.15	0.007	0.0011	0.0032	0.0016	0.030						Conforming steel
O	0.208	0.243	0.07	0.008	0.0013	0.0070	0.0015	0.024						Comparative steel
P	0.242	0.233	4.92	0.006	0.0012	0.0055	0.0019	0.019						Conforming steel
Q	0.203	0.122	5.16	0.013	0.0007	0.0040	0.0042	0.048						Comparative steel
R	0.221	0.207	1.62	0.097	0.0013	0.0036	0.0044	0.028						Conforming steel
S	0.240	0.289	1.92	0.109	0.0007	0.0019	0.0021	0.036						Comparative steel
T	0.249	0.171	1.39	0.013	0.0192	0.0039	0.0047	0.029						Conforming steel
U	0.220	0.164	1.94	0.011	0.0203	0.0016	0.0018	0.048						Comparative steel
V	0.223	0.223	1.50	0.013	0.0008	0.0038	0.0061	0.924						Conforming steel
W	0.220	0.166	1.17	0.009	0.0011	0.0022	0.0031	1.049						Comparative steel
X	0.206	0.192	1.03	0.007	0.0015	0.0094	0.0054	0.021						Conforming steel
Y	0.247	0.168	1.63	0.010	0.0009	0.0111	0.0058	0.050						Comparative steel
Z	0.212	0.101	1.81	0.011	0.0011	0.0045	0.0088	0.047						Conforming steel
AA	0.236	0.221	1.11	0.012	0.0010	0.0041	0.0112	0.045						Comparative steel
AB	0.207	0.201	1.40	0.010	0.0012	0.0019	0.0048	0.025						Conforming steel
AC	0.224	0.209	1.72	0.011	0.0013	0.0037	0.0014	0.039	0.001					Conforming steel
AD	0.246	0.278	1.09	0.009	0.0005	0.0044	0.0063	0.019	0.199					Conforming steel
AE	0.234	0.256	1.75	0.010	0.0007	0.0029	0.0052	0.047	0.214					Comparative steel
AF	0.249	0.241	1.21	0.013	0.0014	0.0027	0.0024	0.018			0.0004			Conforming steel
AG	0.221	0.112	1.95	0.007	0.0009	0.0041	0.0013	0.031			0.0085			Conforming steel
AH	0.237	0.210	1.09	0.008	0.0014	0.0059	0.0041	0.054			0.0120			Comparative steel
AI	0.201	0.154	1.35	0.009	0.0011	0.0044	0.0030	0.056		0.002				Conforming steel
AJ	0.214	0.249	1.06	0.009	0.0013	0.0042	0.0022	0.012		0.196				Conforming steel
AK	0.242	0.201	1.62	0.012	0.0012	0.0065	0.0037	0.022		0.209				Comparative steel
AL	0.236	0.155	1.48	0.008	0.0010	0.0052	0.0058	0.060				0.01		Conforming steel
AM	0.240	0.161	1.48	0.012	0.0014	0.0029	0.0039	0.011				0.99		Conforming steel
AN	0.235	0.103	1.95	0.009	0.0011	0.0027	0.0035	0.053				1.15		Comparative steel
AO	0.225	0.210	1.32	0.012	0.0009	0.0033	0.0056	0.013					V: 0.049	Conforming steel
AP	0.227	0.236	1.88	0.006	0.0010	0.0033	0.0021	0.037					Ta: 0.06	Conforming steel
AQ	0.203	0.290	1.80	0.013	0.0011	0.0026	0.0060	0.014					W: 0.05	Conforming steel
AR	0.219	0.173	1.41	0.006	0.0010	0.0066	0.0023	0.021					Cr: 0.32	Conforming steel
AS	0.223	0.262	1.87	0.012	0.0012	0.0056	0.0068	0.052					Mo: 0.85	Conforming steel
AT	0.244	0.246	1.19	0.009	0.0007	0.0066	0.0036	0.056					Co: 0.007	Conforming steel
AU	0.202	0.141	1.33	0.010	0.0010	0.0029	0.0057	0.054					Ni: 0.02	Conforming steel
AV	0.225	0.184	1.39	0.012	0.0010	0.0036	0.0018	0.033					Sn: 0.012	Conforming steel
AW	0.249	0.233	1.13	0.014	0.0013	0.0046	0.0014	0.034					Sb: 0.058	Conforming steel
AX	0.235	0.157	1.23	0.010	0.0009	0.0012	0.0034	0.017					Ca: 0.0088	Conforming steel
AY	0.231	0.258	1.70	0.014	0.0012	0.0020	0.0039	0.044					Mg: 0.0016	Conforming steel
AZ	0.209	0.252	1.82	0.015	0.0009	0.0058	0.0061	0.038					Zr: 0.067	Conforming steel
BA	0.220	0.211	1.45	0.006	0.0009	0.0040	0.0049	0.056					Te: 0.024	Conforming steel
BB	0.237	0.192	1.76	0.013	0.0013	0.0035	0.0035	0.026					Hf: 0.07	Conforming steel
BC	0.235	0.113	1.82	0.006	0.0009	0.0035	0.0022	0.029					REM: 0.0077	Conforming steel
BD	0.237	0.214	1.30	0.008	0.0008	0.0021	0.0043	0.034					Bi: 0.149	Conforming steel
BE	0.231	0.236	1.19	0.014	0.0011	0.0022	0.0014	0.057					Zn: 0.090	Conforming steel
BF	0.216	0.134	1.62	0.007	0.0007	0.0031	0.0052	0.013					Pb: 0.052	Conforming steel
BG	0.208	0.220	1.37	0.012	0.0013	0.0032	0.0060	0.053					As: 0.028	Conforming steel
BH	0.214	0.185	1.47	0.010	0.0012	0.0068	0.0012	0.038					Ge: 0.033	Conforming steel
BI	0.209	0.268	1.89	0.006	0.0013	0.0052	0.0010	0.028					Sr: 0.011	Conforming steel
BJ	0.226	0.189	1.38	0.014	0.0008	0.0020	0.0053	0.044					Cs: 0.025	Conforming steel
BK	0.239	0.295	1.82	0.006	0.0011	0.0059	0.0046	0.052						Conforming steel
BL	0.248	0.276	1.58	0.014	0.0007	0.0035	0.0055	0.014						Conforming steel
BM	0.227	0.259	1.94	0.008	0.0008	0.0041	0.0012	0.030						Conforming steel
BN	0.234	0.142	1.20	0.007	0.0015	0.0039	0.0048	0.053						Conforming steel
BO	0.243	0.291	1.41	0.012	0.0010	0.0025	0.0070	0.046						Conforming steel
Underlined: indicates value outside scope of present disclosure.

	TABLE 2
	First
						cooling	Second cooling
		Heating			Bending	process	process
	process		process	First	Second
	Average	Annealing process	Number	average	average		Tempering process
	Steel	heating	Annealing	Annealing	of	cooling	cooling	Applied	Tempering	Tempering
	sample	rate	temp. T1	time t1	bends	rate	rate	tension	temp. T2	time t2
No.	ID	(° C./s)	(° C.)	(s)	(times)	(° C./s)	(° C./s)	(MPa)	(° C.)	(s)
1	A	0.8	791	189	3	4.8	906	11	190	732
2	B	0.6	793	193	3	3.4	919	14	185	688
3	B	0.1	800	165	4	1.6	964	14	182	572
4	B	0.7	799	61	3	1.1	807	16	178	676
5	B	4.5	802	198	2	2.6	966	13	199	871
6	B	5.2	797	151	3	4.9	854	17	185	502
7	B	0.4	758	66	4	2.4	889	14	207	789
8	B	0.4	744	94	4	2.7	813	13	209	765
9	B	0.4	846	111	4	2.7	994	11	183	947
10	B	0.9	859	67	2	4.0	808	16	192	822
11	B	0.8	801	15	3	2.0	848	13	181	803
12	B	0.2	798	2	2	3.4	948	19	172	566
13	B	0.5	795	984	4	1.4	998	12	210	860
14	B	0.9	810	1054	3	2.0	928	12	199	590
15	B	0.3	805	199	1	3.0	883	20	210	988
16	B	0.9	797	117	0	4.4	820	18	193	831
17	B	0.8	805	110	0	3.8	954	18	190	992
18	B	0.3	790	133	9	2.5	977	10	208	939
19	B	0.7	808	103	4	0.6	807	18	202	669
20	B	0.4	794	70	5	0.3	867	16	199	681
21	B	0.4	791	132	2	8.4	899	11	175	804
22	B	0.9	793	69	4	10.5	934	11	181	986
23	B	0.5	790	139	4	3.8	305	19	185	692
24	B	0.3	810	164	3	3.2	254	12	195	507
25	B	0.6	794	179	5	3.0	1992	12	189	968
26	B	0.3	791	78	3	2.3	978	17	178	523
27	B	0.6	794	87	3	3.4	837	6	176	812
28	B	0.8	796	70	4	2.7	964	2	176	777
29	B	0.9	806	71	4	1.8	984	44	189	736
30	B	0.9	809	180	3	3.4	896	59	188	510
31	B	0.4	796	77	2	1.8	943	15	105	663
32	B	0.4	801	83	3	1.2	854	13	105	979
33	B	0.4	799	106	3	1.8	933	16	368	866
34	B	0.6	793	159	4	3.7	903	12	384	768
35	B	0.6	794	76	3	2.2	822	15	199	12
36	B	0.9	809	70	2	3.2	863	11	195	34
37	B	1.0	795	169	3	4.5	855	18	177	9542
38	B	0.3	794	151	4	2.9	935	16	178	9684
39	B	0.6	803	150	2	1.9	944	10	209	710
40	B	0.8	803	148	3	3.4	965	12	196	557
41	B	0.2	791	59	2	3.0	972	13	189	889
42	B	0.9	809	188	3	2.6	805	12	184	523
43	B	1.0	794	71	4	3.0	939	19	180	641
44	B	0.9	795	71	3	2.7	891	20	191	876
45	B	1.0	808	151	4	2.0	817	20	194	806
46	B	0.4	796	160	4	1.3	994	13	196	837
47	B	0.7	802	181	5	3.6	958	15	181	728
48	B	0.5	800	162	3	3.3	943	18	208	775
49	B	0.3	799	101	3	2.9	874	12	184	751
50	B	0.4	801	129	4	3.4	840	11	199	580
51	B	0.6	799	195	4	1.8	981	14	185	960
52	B	0.3	807	115	5	3.4	995	14	180	530
53	B	0.6	795	182	3	4.2	921	18	203	843
54	B	0.6	809	83	4	2.2	940	14	171	650
55	B	0.9	793	72	2	2.9	950	19	171	526
56	B	0.6	796	166	5	3.0	877	11	202	631
57	B	0.9	804	103	4	2.7	838	14	173	796
58	B	0.9	800	185	5	3.6	975	11	193	733
59	B	0.5	793	123	3	1.8	850	19	170	578
60	B	0.7	808	112	3	4.3	867	17	178	583
61	C	0.4	806	76	2	2.1	833	12	184	651
62	D	0.4	794	171	4	3.0	950	17	193	800
63	E	0.3	798	144	5	1.3	838	16	209	792
64	F	0.7	806	98	4	4.8	958	10	185	699
65	G	0.3	791	84	4	1.6	869	12	195	808
66	H	0.8	797	87	4	1.9	880	18	180	638
67	I	0.3	804	111	3	3.6	800	19	188	914
68	J	0.3	806	139	3	1.3	901	12	180	946
69	K	0.7	804	96	5	4.0	912	13	191	511
70	L	0.9	803	121	2	1.4	896	13	180	712
71	M	0.5	800	153	3	1.9	911	17	184	595
72	N	0.4	797	101	4	1.1	957	14	200	777
73	O	0.4	807	132	3	3.2	940	17	186	650
74	P	0.5	808	92	4	4.1	903	18	188	790
75	Q	0.7	800	198	4	1.0	873	15	188	662
76	R	0.7	796	103	3	2.1	888	13	171	769
77	S	0.5	793	92	3	2.2	961	16	207	539
78	T	0.5	800	59	2	3.3	918	10	187	658
79	U	0.9	795	165	5	2.6	933	12	179	839
80	V	0.8	794	153	4	1.3	979	15	198	797
81	W	0.2	797	81	2	2.7	917	10	185	848
82	X	0.3	805	163	3	2.2	853	13	177	717
83	Y	1.0	808	103	4	3.0	912	15	178	773
84	Z	0.8	803	63	3	1.1	889	19	178	671
85	AA	0.5	810	170	3	1.6	999	11	198	557
86	AB	0.3	797	170	3	4.9	921	14	171	791
87	AC	0.6	792	194	5	4.5	948	11	205	758
88	AD	0.3	798	132	3	4.4	873	18	187	574
89	AE	0.5	791	63	3	3.2	923	20	192	714
90	AF	0.6	803	82	2	3.3	810	19	173	922
91	AG	0.5	790	60	4	4.2	977	17	203	526
92	AH	0.3	801	56	3	1.4	808	12	202	524
93	AI	0.6	793	126	2	1.8	869	18	179	868
94	AJ	0.8	800	179	2	3.0	910	15	172	585
95	AK	0.7	796	141	3	5.0	891	13	210	901
96	AL	0.7	791	109	4	3.2	873	16	195	759
97	AM	0.2	810	81	4	4.6	941	17	171	672
98	AN	0.3	806	194	4	2.9	846	18	174	555
99	AO	0.1	802	188	2	4.2	955	18	208	967
100	AP	4.7	804	79	3	4.0	997	10	180	853
101	AQ	0.3	757	176	3	1.4	818	15	182	529
102	AR	1.0	844	178	4	1.7	883	13	191	551
103	AS	0.6	801	15	5	4.6	842	18	171	793
104	AT	0.4	799	958	2	2.1	849	20	209	562
105	AU	0.6	807	102	1	4.2	849	13	208	892
106	AV	0.4	802	160	10	4.2	889	18	194	715
107	AW	0.7	800	62	5	0.7	893	16	189	742
108	AX	0.2	792	116	3	9.4	856	13	179	641
109	AY	0.8	798	103	4	2.5	322	19	180	532
110	AZ	0.5	795	73	3	2.1	1891	15	172	677
111	BA	0.4	796	180	2	4.2	811	6	196	980
112	BB	0.3	802	147	5	2.3	852	46	182	725
113	BC	0.8	801	109	5	4.7	824	11	111	517
114	BD	0.8	795	51	4	3.5	883	14	368	653
115	BE	0.3	795	174	5	3.6	992	13	199	11
116	BF	0.4	798	90	5	2.1	968	19	187	9658
117	BG	0.3	808	168	3	2.8	976	11	194	908
118	BH	1.0	796	122	4	3.0	888	19	196	910
119	BI	0.6	797	60	4	1.6	870	19	204	834
120	BJ	0.9	791	182	4	1.6	840	13	202	733
121	BK	0.7	800	59	3	4.2	840	14	196	929
122	BL	0.5	795	94	3	2.4	949	12	207	630
123	BM	1.0	806	189	3	2.1	972	19	181	984
124	BN	0.7	807	93	2	1.3	852	15	183	857
125	BO	0.2	807	58	2	4.5	973	16	186	682
	Straightening process
				Entry
				side	Delivery
				intermesh	intermesh	Entry
		Steel	Straightening	pressing	pressing	side	Delivery
		sample	start temp.	amount	amount	tension	tension
	No.	ID	(° C.)	(mm)	(mm)	(MPa)	(MPa)	Type*	Remarks
	1	A	34	6.7	1.5	168	177	CR	Example
	2	B	40	6.9	1.4	201	209	CR	Example
	3	B	26	7.0	1.7	224	232	CR	Example
	4	B	29	6.1	1.4	201	209	CR	Example
	5	B	32	6.1	1.3	228	237	CR	Example
	6	B	32	6.9	1.7	177	184	CR	Comparative
									Example
	7	B	29	6.3	1.8	233	240	CR	Example
	8	B	20	6.5	2.0	245	253	CR	Comparative
									Example
	9	B	40	7.1	1.4	174	181	CR	Example
	10	B	32	7.7	1.7	257	264	CR	Comparative
									Example
	11	B	29	7.9	1.7	184	192	CR	Example
	12	B	39	7.7	1.7	271	280	CR	Comparative
									Example
	13	B	29	7.2	2.0	167	173	CR	Example
	14	B	33	6.4	1.7	244	252	CR	Comparative
									Example
	15	B	37	6.3	1.2	282	292	CR	Example
	16	B	32	7.3	2.0	272	279	CR	Comparative
									Example
	17	B	31	7.7	1.5	284	290	CR	Comparative
									Example
	18	B	40	7.9	1.8	236	245	CR	Example
	19	B	32	6.9	1.5	251	260	CR	Example
	20	B	21	6.2	1.1	252	258	CR	Comparative
									Example
	21	B	38	7.1	1.1	263	273	CR	Example
	22	B	24	6.5	1.9	249	259	CR	Comparative
									Example
	23	B	24	6.5	1.4	197	205	CR	Example
	24	B	26	6.0	1.8	160	167	CR	Comparative
									Example
	25	B	22	6.1	1.8	269	277	CR	Example
	26	B	29	6.0	1.5	209	215	CR	Example
	27	B	39	6.2	1.8	159	165	CR	Example
	28	B	21	6.5	1.9	183	192	CR	Comparative
									Example
	29	B	21	6.9	1.6	252	257	CR	Example
	30	B	34	7.2	1.6	225	234	CR	Example
	31	B	29	7.0	1.9	235	244	CR	Example
	32	B	25	7.7	1.2	253	262	CR	Example
	33	B	34	6.7	1.6	216	225	CR	Example
	34	B	25	6.1	1.8	283	290	CR	Example
	35	B	30	8.0	1.2	189	197	CR	Example
	36	B	23	7.4	1.6	249	258	CR	Example
	37	B	25	7.0	1.3	187	193	CR	Example
	38	B	40	7.1	1.9	285	294	CR	Example
	39	B	3	7.2	1.1	172	179	CR	Example
	40	B	38	6.0	1.2	213	218	CR	Example
	41	B	88	6.2	1.4	253	258	CR	Example
	42	B	105	6.5	1.2	180	189	CR	Comparative
									Example
	43	B	39	4.3	2.0	196	202	CR	Example
	44	B	34	2.5	1.2	258	265	CR	Comparative
									Example
	45	B	25	9.4	1.5	232	239	CR	Example
	46	B	30	9.6	1.0	279	288	CR	Example
	47	B	31	8.0	1.0	299	304	CR	Example
	48	B	37	7.4	1.0	204	211	CR	Example
	49	B	20	9.4	8.4	254	262	CR	Example
	50	B	35	9.6	8.6	257	264	CR	Example
	51	B	20	7.9	1.3	24	31	CR	Example
	52	B	23	6.7	1.1	15	26	CR	Comparative
									Example
	53	B	32	6.1	1.6	415	423	CR	Example
	54	B	28	7.1	1.9	487	493	CR	Example
	55	B	24	7.4	1.3	26	34	CR	Example
	56	B	40	7.1	1.6	14	27	CR	Comparative
									Example
	57	B	26	7.5	1.7	487	540	CR	Example
	58	B	40	7.6	1.7	497	545	CR	Example
	59	B	26	6.1	1.9	293	303	GA	Example
	60	B	24	6.2	1.8	276	284	GA	Example
	61	C	34	7.7	1.2	160	167	GA	Example
	62	D	27	7.6	1.7	187	193	GA	Example
	63	E	25	6.5	1.6	242	248	GA	Example
	64	F	39	6.3	1.7	281	286	CR	Example
	65	G	39	8.0	1.2	176	185	CR	Comparative
									Example
	66	H	36	7.6	1.9	195	204	GA	Example
	67	I	26	6.1	1.8	188	194	GA	Comparative
									Example
	68	J	24	6.2	1.1	157	164	GI	Example
	69	K	20	7.9	1.1	174	181	GA	Comparative
									Example
	70	L	34	8.0	1.3	278	286	GA	Example
	71	M	27	7.0	1.9	192	198	GA	Comparative
									Example
	72	N	39	7.2	1.7	274	280	GA	Example
	73	O	32	6.3	1.3	261	269	GI	Comparative
									Example
	74	P	22	6.6	1.1	265	271	GA	Example
	75	Q	24	7.4	1.0	275	282	GA	Comparative
									Example
	76	R	39	6.0	1.7	223	230	GA	Example
	77	S	35	6.9	1.1	160	165	GA	Comparative
									Example
	78	T	39	6.8	1.8	249	255	GA	Example
	79	U	29	6.5	1.3	242	252	GI	Comparative
									Example
	80	V	23	6.4	1.1	176	183	GA	Example
	81	W	29	7.4	2.0	161	170	GA	Comparative
									Example
	82	X	27	6.0	1.2	152	158	GA	Example
	83	Y	40	6.8	1.5	185	193	GA	Comparative
									Example
	84	Z	24	7.3	1.8	247	252	CR	Example
	85	AA	25	7.3	1.2	182	187	CR	Comparative
									Example
	86	AB	29	7.5	1.6	184	190	GA	Example
	87	AC	30	7.1	1.8	280	285	GA	Example
	88	AD	25	6.1	1.9	197	206	GA	Example
	89	AE	37	7.6	2.0	210	217	GA	Comparative
									Example
	90	AF	22	6.3	1.6	224	233	GA	Example
	91	AG	25	6.8	1.6	277	284	GA	Example
	92	AH	21	7.6	1.4	226	232	GA	Comparative
									Example
	93	AI	35	6.2	1.1	229	237	CR	Example
	94	AJ	29	6.6	1.2	157	166	CR	Example
	95	AK	39	7.1	1.6	198	206	CR	Comparative
									Example
	96	AL	33	7.9	1.7	283	290	CR	Example
	97	AM	33	7.6	1.7	244	250	CR	Example
	98	AN	35	7.7	2.0	207	217	CR	Comparative
									Example
	99	AO	34	7.6	1.3	240	246	CR	Example
	100	AP	29	7.8	2.0	160	169	CR	Example
	101	AQ	21	7.7	1.1	286	294	CR	Example
	102	AR	34	7.6	1.1	217	225	CR	Example
	103	AS	39	6.1	1.7	269	279	CR	Example
	104	AT	35	7.9	1.3	183	190	GA	Example
	105	AU	37	7.1	1.2	219	228	GA	Example
	106	AV	28	6.8	1.4	220	225	CR	Example
	107	AW	22	6.6	1.3	161	166	CR	Example
	108	AX	22	7.2	1.2	283	288	CR	Example
	109	AY	23	6.3	1.2	275	281	EG	Example
	110	AZ	36	7.1	1.4	184	190	GI	Example
	111	BA	39	6.6	1.2	290	300	EG	Example
	112	BB	37	7.0	1.3	297	305	GI	Example
	113	BC	37	7.6	1.1	160	167	CR	Example
	114	BD	28	7.7	1.1	191	201	CR	Example
	115	BE	29	7.9	1.2	176	185	CR	Example
	116	BF	29	7.0	1.1	268	274	CR	Example
	117	BG	6	6.3	1.7	265	272	CR	Example
	118	BH	94	7.8	1.0	295	302	CR	Example
	119	BI	34	4.1	1.8	193	200	CR	Example
	120	BJ	23	9.8	1.3	162	169	CR	Example
	121	BK	25	7.1	1.2	24	32	EG	Example
	122	BL	33	7.7	1.3	487	544	GI	Example
	123	BM	34	6.4	1.5	249	255	EG	Example
	124	BN	25	6.8	1.4	256	265	GI	Example
	125	BO	30	7.9	1.1	191	200	GA	Example
	Underlined: indicates value outside scope of present disclosure.
	*CR: cold-rolled steel sheet (uncoated), GI: hot-dip galvanized steel sheet (without alloying treatment), GA: galvannealed steel sheet, EG: electrogalvanized steel sheet

	TABLE 3
	Microstructure*
	Large-angle
					grain
					boundary
	Area fraction	density of		Evaluation result
	Steel	Tempered	Retained		tempered		TS	EL
	sample	M	γ	F + B	M		TS		EL
No.	ID	(%)	(%)	(%)	(μm/μm2)	KAM(S)/KAM(C)	(MPa)	Evaluation	(%)	Evaluation
1	A	88	1	11	1.3	1.05	1341	Good	14	Good
2	B	91	0	9	1.3	1.07	1354	Good	13	Good
3	B	89	1	10	2.5	1.05	1311	Good	12	Good
4	B	89	1	10	1.1	1.07	1356	Good	13	Good
5	B	90	0	10	1.2	1.04	1330	Good	13	Good
6	B	89	0	11	0.9	1.07	1294	Good	14	Good
7	B	85	1	14	2.0	1.07	1206	Good	15	Good
8	B	84	1	15	2.0	1.06	1125	Poor	16	Good
9	B	94	0	6	1.5	1.08	1529	Good	10	Good
10	B	96	1	3	1.8	1.06	1548	Good	8	Poor
11	B	86	0	14	1.3	1.04	1226	Good	12	Good
12	B	78	1	21	1.7	1.08	984	Poor	18	Good
13	B	94	0	6	1.2	1.04	1541	Good	10	Good
14	B	99	0	1	1.4	1.08	1541	Good	9	Poor
15	B	87	1	12	1.1	1.06	1265	Good	13	Good
16	B	93	0	7	0.7	1.07	1384	Good	11	Good
17	B	91	1	8	0.9	1.05	1429	Good	12	Good
18	B	89	0	11	2.8	1.05	1317	Good	13	Good
19	B	86	0	14	1.4	1.03	1223	Good	14	Good
20	B	85	0	15	1.2	1.05	1036	Poor	17	Good
21	B	94	1	5	2.0	1.06	1542	Good	10	Good
22	B	97	0	3	2.0	1.07	1509	Good	8	Poor
23	B	89	2	9	1.3	1.06	1388	Good	12	Good
24	B	85	5	10	1.8	1.03	1350	Good	13	Good
25	B	90	0	10	1.4	1.04	1330	Good	13	Good
26	B	89	1	10	1.4	1.06	1311	Good	12	Good
27	B	87	1	12	1.0	1.03	1289	Good	12	Good
28	B	87	1	12	0.8	1.05	1248	Good	14	Good
29	B	89	1	10	1.1	1.05	1315	Good	14	Good
30	B	90	1	9	1.1	1.07	1340	Good	11	Good
31	B	92	1	7	1.3	1.05	1532	Good	11	Good
32	B	91	1	8	1.8	1.07	1537	Good	10	Good
33	B	90	1	9	1.7	1.05	1200	Good	13	Good
34	B	91	1	8	1.0	1.08	1207	Good	14	Good
35	B	90	1	9	1.1	1.08	1522	Good	10	Good
36	B	90	1	9	1.8	1.06	1508	Good	10	Good
37	B	91	0	9	1.5	1.05	1202	Good	13	Good
38	B	89	0	11	1.1	1.06	1203	Good	13	Good
39	B	91	0	9	1.2	1.05	1374	Good	12	Good
40	B	87	1	12	1.5	1.04	1257	Good	14	Good
41	B	89	1	10	1.4	1.02	1306	Good	13	Good
42	B	90	1	9	1.7	0.97	1376	Good	13	Good
43	B	93	0	7	1.6	1.01	1394	Good	11	Good
44	B	90	0	10	1.5	0.92	1359	Good	11	Good
45	B	91	0	9	1.4	1.10	1382	Good	12	Good
46	B	88	1	11	1.7	1.14	1274	Good	14	Good
47	B	89	1	10	1.9	1.04	1362	Good	11	Good
48	B	89	1	10	1.7	1.08	1362	Good	12	Good
49	B	88	1	11	1.2	1.15	1311	Good	12	Good
50	B	89	1	10	1.8	1.19	1303	Good	14	Good
51	B	89	0	11	1.1	1.01	1274	Good	13	Good
52	B	89	0	11	1.6	0.99	1280	Good	12	Good
53	B	88	1	11	1.6	1.14	1306	Good	14	Good
54	B	91	1	8	1.9	1.19	1412	Good	11	Good
55	B	91	1	8	1.2	1.02	1400	Good	12	Good
56	B	90	1	9	1.7	0.95	1389	Good	12	Good
57	B	90	0	10	1.5	1.14	1339	Good	11	Good
58	B	88	1	11	1.4	1.18	1281	Good	13	Good
59	B	89	1	10	1.7	1.06	1303	Good	13	Good
60	B	89	1	10	1.6	1.05	1355	Good	13	Good
61	C	90	0	10	1.4	1.07	1328	Good	11	Good
62	D	88	1	11	1.3	1.05	1268	Good	13	Good
63	E	92	1	7	1.6	1.05	1448	Good	10	Good
64	F	85	1	14	1.0	1.05	1196	Good	14	Good
65	G	81	0	19	2.0	1.05	1147	Poor	15	Good
66	H	95	0	5	1.0	1.05	1526	Good	11	Good
67	I	98	0	2	1.4	1.06	1701	Good	8	Poor
68	J	92	1	7	1.0	1.04	1227	Good	14	Good
69	K	88	1	11	1.3	1.06	1070	Poor	16	Good
70	L	91	2	7	2.0	1.06	1568	Good	11	Good
71	M	85	4	11	1.8	1.06	1433	Good	12	Good
72	N	85	1	14	1.9	1.03	1222	Good	15	Good
73	O	78	1	21	1.6	1.05	1089	Poor	16	Good
74	P	94	1	5	2.0	1.05	1521	Good	10	Good
75	Q	96	1	3	1.4	1.05	1763	Good	9	Poor
76	R	93	0	7	1.7	1.07	1379	Good	11	Good
77	S	92	0	8	1.6	1.04	1429	Good	11	Good
78	T	89	1	10	1.3	1.04	1471	Good	12	Good
79	U	93	0	7	1.6	1.06	1476	Good	12	Good
80	V	89	0	11	1.0	1.05	1343	Good	13	Good
81	W	91	1	8	2.0	1.06	1390	Good	13	Good
82	X	88	0	12	1.6	1.05	1281	Good	14	Good
83	Y	91	0	9	1.5	1.05	1400	Good	12	Good
84	Z	89	1	10	1.1	1.07	1282	Good	14	Good
85	AA	92	1	7	1.7	1.04	1363	Good	11	Good
86	AB	91	1	8	1.9	1.07	1439	Good	12	Good
87	AC	89	0	11	1.2	1.07	1312	Good	13	Good
88	AD	91	1	8	1.8	1.05	1534	Good	11	Good
89	AE	88	1	11	1.1	1.07	1702	Good	10	Good
90	AF	91	1	8	1.5	1.06	1384	Good	12	Good
91	AG	91	0	9	1.7	1.04	1522	Good	11	Good
92	AH	89	0	11	1.7	1.06	1752	Good	11	Good
93	AI	91	1	8	1.3	1.04	1460	Good	11	Good
94	AJ	91	1	8	1.1	1.06	1549	Good	11	Good
95	AK	91	1	8	1.9	1.06	1787	Good	11	Good
96	AL	90	0	10	1.8	1.07	1350	Good	12	Good
97	AM	89	1	10	1.5	1.08	1522	Good	11	Good
98	AN	91	0	9	1.8	1.07	1739	Good	10	Good
99	AO	88	1	11	2.6	1.04	1278	Good	12	Good
100	AP	89	0	11	1.1	1.07	1274	Good	14	Good
101	AQ	86	0	14	1.4	1.07	1192	Good	13	Good
102	AR	94	0	6	1.3	1.03	1522	Good	11	Good
103	AS	85	1	14	1.1	1.06	1192	Good	15	Good
104	AT	93	1	6	1.7	1.05	1509	Good	12	Good
105	AU	93	0	7	1.2	1.05	1467	Good	11	Good
106	AV	88	0	12	2.9	1.05	1185	Good	16	Good
107	AW	86	0	14	1.2	1.05	1209	Good	14	Good
108	AX	93	1	6	1.9	1.07	1505	Good	11	Good
109	AY	89	2	9	1.5	1.05	1414	Good	12	Good
110	AZ	89	1	10	1.5	1.06	1275	Good	13	Good
111	BA	90	0	10	1.2	1.04	1325	Good	11	Good
112	BB	87	1	12	2.0	1.07	1276	Good	12	Good
113	BC	91	0	9	1.8	1.05	1510	Good	11	Good
114	BD	93	0	7	1.9	1.05	1196	Good	10	Good
115	BE	90	1	9	1.2	1.04	1548	Good	10	Good
116	BF	88	1	11	1.3	1.06	1218	Good	11	Good
117	BG	91	0	9	1.1	1.03	1425	Good	13	Good
118	BH	92	1	7	1.4	1.02	1481	Good	12	Good
119	BI	91	0	9	1.3	1.02	1393	Good	11	Good
120	BJ	88	1	11	1.6	1.12	1336	Good	11	Good
121	BK	91	1	8	1.3	1.02	1454	Good	11	Good
122	BL	91	1	8	1.1	1.14	1436	Good	11	Good
123	BM	92	0	8	1.1	1.07	1482	Good	12	Good
124	BN	89	1	10	1.2	1.07	1408	Good	11	Good
125	BO	91	0	9	1.3	1.07	1495	Good	11	Good
	Evaluation result
			Toughness after	Crash properties
			paint baking	after paint baking
			Brittle-			Fracture
			ductile			stress
			transition			ratio
		Steel	temp. after aging		YR after	after
		sample	treatment		aging	aging
	No.	ID	(° C.)	Evaluation	treatment	treatment	Evaluation	Remarks
	1	A	−94	Excellent	0.93	0.85	Good	Example
	2	B	−94	Excellent	0.94	0.84	Good	Example
	3	B	−107	Excellent	0.91	0.82	Good	Example
	4	B	−107	Excellent	0.92	0.84	Good	Example
	5	B	−49	Good	0.91	0.88	Good	Example
	6	B	−15	Poor	0.91	0.95	Poor	Comparative
								Example
	7	B	−105	Excellent	0.91	0.85	Good	Example
	8	B	−102	Excellent	0.90	0.84	Good	Comparative
								Example
	9	B	−94	Excellent	0.91	0.82	Good	Example
	10	B	−97	Excellent	0.92	0.83	Good	Comparative
								Example
	11	B	−109	Excellent	0.94	0.84	Good	Example
	12	B	−94	Excellent	0.91	0.83	Good	Comparative
								Example
	13	B	−100	Excellent	0.92	0.82	Good	Example
	14	B	−100	Excellent	0.92	0.81	Good	Comparative
								Example
	15	B	−55	Good	0.92	0.87	Good	Example
	16	B	−25	Poor	0.91	0.93	Poor	Comparative
								Example
	17	B	−17	Poor	0.91	0.91	Poor	Comparative
								Example
	18	B	−105	Excellent	0.94	0.84	Good	Example
	19	B	−105	Excellent	0.92	0.83	Good	Example
	20	B	−108	Excellent	0.94	0.84	Good	Comparative
								Example
	21	B	−99	Excellent	0.92	0.83	Good	Example
	22	B	−104	Excellent	0.91	0.85	Good	Comparative
								Example
	23	B	−48	Good	0.92	0.84	Good	Example
	24	B	−4	Poor	0.90	0.84	Good	Comparative
								Example
	25	B	−104	Excellent	0.91	0.85	Good	Example
	26	B	−94	Excellent	0.94	0.84	Good	Example
	27	B	−45	Good	0.91	0.90	Good	Example
	28	B	−25	Poor	0.92	0.91	Poor	Comparative
								Example
	29	B	−109	Excellent	0.92	0.83	Good	Example
	30	B	−100	Excellent	0.92	0.82	Good	Example
	31	B	−50	Good	0.93	0.82	Good	Example
	32	B	−58	Good	0.92	0.83	Good	Example
	33	B	−94	Excellent	0.91	0.81	Good	Example
	34	B	−97	Excellent	0.90	0.80	Good	Example
	35	B	−49	Good	0.91	0.82	Good	Example
	36	B	−48	Good	0.92	0.82	Good	Example
	37	B	−91	Excellent	0.92	0.84	Good	Example
	38	B	−101	Excellent	0.91	0.81	Good	Example
	39	B	−110	Excellent	0.94	0.80	Good	Example
	40	B	−93	Excellent	0.90	0.84	Good	Example
	41	B	−92	Excellent	0.87	0.81	Good	Example
	42	B	−91	Excellent	0.81	0.83	Poor	Comparative
								Example
	43	B	−103	Excellent	0.86	0.81	Good	Example
	44	B	−92	Excellent	0.82	0.81	Poor	Comparative
								Example
	45	B	−98	Excellent	0.96	0.84	Good	Example
	46	B	−93	Excellent	0.96	0.85	Good	Example
	47	B	−90	Excellent	0.87	0.81	Good	Example
	48	B	−106	Excellent	0.87	0.82	Good	Example
	49	B	−104	Excellent	0.95	0.81	Good	Example
	50	B	−101	Excellent	0.96	0.84	Good	Example
	51	B	−96	Excellent	0.86	0.84	Good	Example
	52	B	−108	Excellent	0.83	0.83	Poor	Comparative
								Example
	53	B	−107	Excellent	0.97	0.85	Good	Example
	54	B	−95	Excellent	0.97	0.83	Good	Example
	55	B	−105	Excellent	0.86	0.85	Good	Example
	56	B	−93	Excellent	0.84	0.85	Poor	Comparative
								Example
	57	B	−109	Excellent	0.96	0.84	Good	Example
	58	B	−94	Excellent	0.97	0.83	Good	Example
	59	B	−110	Excellent	0.92	0.82	Good	Example
	60	B	−96	Excellent	0.93	0.82	Good	Example
	61	C	−100	Excellent	0.93	0.83	Good	Example
	62	D	−98	Excellent	0.94	0.85	Good	Example
	63	E	−110	Excellent	0.91	0.80	Good	Example
	64	F	−96	Excellent	0.87	0.80	Good	Example
	65	G	−91	Excellent	0.84	0.83	Poor	Comparative
								Example
	66	H	−97	Excellent	0.93	0.84	Good	Example
	67	I	−96	Excellent	0.93	0.82	Good	Comparative
								Example
	68	J	−109	Excellent	0.94	0.80	Good	Example
	69	K	−101	Excellent	0.93	0.81	Good	Comparative
								Example
	70	L	−44	Good	0.93	0.81	Good	Example
	71	M	−17	Poor	0.92	0.84	Good	Comparative
								Example
	72	N	−99	Excellent	0.90	0.84	Good	Example
	73	O	−104	Excellent	0.93	0.85	Good	Comparative
								Example
	74	P	−102	Excellent	0.92	0.81	Good	Example
	75	Q	−97	Excellent	0.92	0.82	Good	Comparative
								Example
	76	R	−56	Good	0.90	0.82	Good	Example
	77	S	0	Poor	0.91	0.82	Good	Comparative
								Example
	78	T	−45	Good	0.93	0.82	Good	Example
	79	U	−7	Poor	0.90	0.83	Good	Comparative
								Example
	80	V	−57	Good	0.91	0.85	Good	Example
	81	W	−11	Poor	0.93	0.80	Good	Comparative
								Example
	82	X	−45	Good	0.92	0.84	Good	Example
	83	Y	−25	Poor	0.91	0.83	Good	Comparative
								Example
	84	Z	−48	Good	0.93	0.84	Good	Example
	85	AA	−4	Poor	0.90	0.81	Good	Comparative
								Example
	86	AB	−102	Excellent	0.93	0.80	Good	Example
	87	AC	−103	Excellent	0.93	0.82	Good	Example
	88	AD	−49	Good	0.94	0.84	Good	Example
	89	AE	−5	Poor	0.91	0.81	Good	Comparative
								Example
	90	AF	−92	Excellent	0.93	0.81	Good	Example
	91	AG	−55	Good	0.91	0.81	Good	Example
	92	AH	−24	Poor	0.94	0.81	Good	Comparative
								Example
	93	AI	−99	Excellent	0.92	0.84	Good	Example
	94	AJ	−46	Good	0.92	0.85	Good	Example
	95	AK	−29	Poor	0.92	0.85	Good	Comparative
								Example
	96	AL	−105	Excellent	0.93	0.84	Good	Example
	97	AM	−57	Good	0.92	0.82	Good	Example
	98	AN	−21	Poor	0.93	0.81	Good	Comparative
								Example
	99	AO	−110	Excellent	0.92	0.81	Good	Example
	100	AP	−51	Good	0.93	0.90	Good	Example
	101	AQ	−97	Excellent	0.91	0.83	Good	Example
	102	AR	−110	Excellent	0.94	0.84	Good	Example
	103	AS	−92	Excellent	0.91	0.82	Good	Example
	104	AT	−101	Excellent	0.92	0.81	Good	Example
	105	AU	−52	Good	0.93	0.86	Good	Example
	106	AV	−102	Excellent	0.91	0.81	Good	Example
	107	AW	−102	Excellent	0.91	0.83	Good	Example
	108	AX	−91	Excellent	0.93	0.85	Good	Example
	109	AY	−49	Good	0.91	0.82	Good	Example
	110	AZ	−96	Excellent	0.91	0.80	Good	Example
	111	BA	−51	Good	0.91	0.88	Good	Example
	112	BB	−92	Excellent	0.92	0.80	Good	Example
	113	BC	−96	Excellent	0.93	0.84	Good	Example
	114	BD	−108	Excellent	0.92	0.82	Good	Example
	115	BE	−51	Good	0.90	0.80	Good	Example
	116	BF	−106	Excellent	0.91	0.82	Good	Example
	117	BG	−91	Excellent	0.94	0.81	Good	Example
	118	BH	−106	Excellent	0.86	0.84	Good	Example
	119	BI	−101	Excellent	0.87	0.83	Good	Example
	120	BJ	−92	Excellent	0.91	0.80	Good	Example
	121	BK	−102	Excellent	0.85	0.83	Good	Example
	122	BL	−103	Excellent	0.92	0.84	Good	Example
	123	BM	−107	Excellent	0.90	0.82	Good	Example
	124	BN	−91	Excellent	0.93	0.82	Good	Example
	125	BO	−104	Excellent	0.94	0.82	Good	Example
	Underlined: indicates value outside scope of present disclosure.
	*Tempered M: tempered martensite, retained γ: retained austenite, F: ferrite, B: bainitic ferrite

As indicated in Table 3, for the Examples according to the present disclosure, TS, EL, as well as toughness and crash properties after paint baking, all reached passing criteria. Further, using the steel sheets of the Examples, members obtained by forming and members obtained by joining all had a target shape without crack occurrence, and TS, EL, as well as toughness and crash properties after paint baking, all reached passing criteria.

On the other hand, for the Comparative Examples, at least one criterion was failed among TS, EL, and toughness and crash properties after paint baking.

Claims

1. A steel sheet comprising: a chemical composition containing, in mass %, KAM ⁡ ( S ) / KAM ⁡ ( C ) > 1. ( 1 )

C: 0.030% or more and 0.500% or less,

Si: 0.010% or more and 2.500% or less,

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

P: 0.100% or less,

S: 0.0200% or less,

N: 0.0100% or less,

O: 0.0100% or less, and

Al: 1.000% or less,

with the balance being Fe and inevitable impurity; and a steel microstructure wherein

area fraction of tempered martensite is 83% or more,

area fraction of retained austenite is less than 3%,

total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%,

20° or greater grain boundary density in the tempered martensite is 1.0 μm/μm2 or more, and

the following Expression (1) is satisfied,

where

KAM(S) is an average KAM value at a depth of 100 μm from a surface of the steel sheet, and

KAM(C) is an average KAM value at a mid-thickness position of the steel sheet.

2. The steel sheet according to claim 1, wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of

Ti: 0.200% or less,

Nb: 0.200% or less,

V: 0.200% or less,

Ta: 0.10% or less,

W: 0.10% or less,

B: 0.0100% or less,

Cr: 1.00% or less,

Mo: 1.00% or less,

Ni: 1.00% or less,

Co: 0.010% or less,

Cu: 1.00% or less,

Sn: 0.200% or less,

Sb: 0.200% or less,

Ca: 0.0100% or less,

Mg: 0.0100% or less,

REM: 0.0100% or less,

Zr: 0.100% or less,

Te: 0.100% or less,

Hf: 0.10% or less, and

Bi: 0.200% or less.

3. The steel sheet according to claim 1, further comprising a coated or plated layer on a surface.

4. The steel sheet according to claim 2, further comprising a coated or plated layer on a surface.

5. A member made using the steel sheet according to claim 1.

6. A method of producing the steel sheet according to claim 1, the method comprising:

a preparation process of preparing a blank sheet having the chemical composition according to claim 1;

a heating process of heating the blank sheet under a set of conditions including an average heating rate in a temperature range from 700° C. to 750° C. of 5.0° C./s or less, and

heating to an annealing temperature T1;

an annealing process of annealing the blank sheet under a set of conditions including the annealing temperature T1 being 750° C. or more and 850° C. or less, and

an annealing time t1 of 10 s or longer and 1000 s or shorter;

a bending process of applying bending once or more to the blank sheet using a roller that has a radius of 800 mm or less in a temperature range from the annealing temperature T1 to 700° C.;

a first cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 700° C. to 550° C. of 0.5° C./s or more and less than 10.0° C./s,

to a first cooling end temperature;

a second cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 300° C. to 100° C. of 300° C./s or more, and

applying tension to the blank sheet in the temperature range from 300° C. to 100° C. of 5 MPa or more, to a second cooling end temperature;

a tempering process of tempering the blank sheet under a set of conditions including a tempering temperature T2 of 100° C. or more and 400° C. or less, and

a tempering time t2 of 10 s or longer and 10,000 s or shorter; and

a straightening process of applying straightening to the blank sheet by leveling under a set of conditions including a straightening start temperature of 100° C. or less,

an entry side intermesh pressing amount of 4.0 mm or more and 10.0 mm or less,

a delivery intermesh pressing amount of 1.0 mm or more and 10.0 mm or less,

an entry side tension of 20 MPa or more and 500 MPa or less, and

a delivery tension of 25 MPa or more and 550 MPa or less.

7. The method of producing a steel sheet according to claim 6, further comprising a coating or plating process of applying a coating or plating treatment to the blank sheet between the first cooling process and the second cooling process, or between the tempering process and the straightening process.

8. A method of producing a member, wherein the steel sheet according to claim 1 is subjected to at least one of a forming process or a joining process to produce the member.

9. A member made using the steel sheet according to claim 2.

10. A member made using the steel sheet according to claim 3.

11. A member made using the steel sheet according to claim 4.

12. A method of producing the steel sheet according to claim 2, the method comprising:

a preparation process of preparing a blank sheet having the chemical composition according to claim 2;

a heating process of heating the blank sheet under a set of conditions including an average heating rate in a temperature range from 700° C. to 750° C. of 5.0° C./s or less, and

heating to an annealing temperature T1;

an annealing process of annealing the blank sheet under a set of conditions including the annealing temperature T1 being 750° C. or more and 850° C. or less, and

an annealing time t1 of 10 s or longer and 1000 s or shorter;

a bending process of applying bending once or more to the blank sheet using a roller that has a radius of 800 mm or less in a temperature range from the annealing temperature T1 to 700° C.;

a first cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 700° C. to 550° C. of 0.5° C./s or more and less than 10.0° C./s,

to a first cooling end temperature;

a second cooling process of cooling the blank sheet under a set of conditions including an average cooling rate in a temperature range from 300° C. to 100° C. of 300° C./s or more, and

applying tension to the blank sheet in the temperature range from 300° C. to 100° C. of 5 MPa or more, to a second cooling end temperature;

a tempering process of tempering the blank sheet under a set of conditions including a tempering temperature T2 of 100° C. or more and 400° C. or less, and

a tempering time t2 of 10 s or longer and 10,000 s or shorter; and

a straightening process of applying straightening to the blank sheet by leveling under a set of conditions including a straightening start temperature of 100° C. or less,

an entry side intermesh pressing amount of 4.0 mm or more and 10.0 mm or less,

a delivery intermesh pressing amount of 1.0 mm or more and 10.0 mm or less,

an entry side tension of 20 MPa or more and 500 MPa or less, and

a delivery tension of 25 MPa or more and 550 MPa or less.

13. The method of producing a steel sheet according to claim 12, further comprising a coating or plating process of applying a coating or plating treatment to the blank sheet between the first cooling process and the second cooling process, or between the tempering process and the straightening process.

14. A method of producing a member, wherein the steel sheet according to claim 2 is subjected to at least one of a forming process or a joining process to produce the member.

15. A method of producing a member, wherein the steel sheet according to claim 3 is subjected to at least one of a forming process or a joining process to produce the member.

16. A method of producing a member, wherein the steel sheet according to claim 4 is subjected to at least one of a forming process or a joining process to produce the member.