Steel sheet and member, and method for producing said steel sheet and method for producing said member

Abstract

Provided is a steel sheet that has a TS of 1320 MPa or more, as well as excellent stretch flangeability, and 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 95 % or more, an area fraction of retained austenite is less than 3 %, a total area fraction of bainitic ferrite and ferrite is less than 5 %, and a 20° or greater grain boundary density in tempered martensite is 1.0 µm / µm2 or more, satisfying the following Expression (1): KAMS / KAMC>1.001

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel sheet, a member using the steel sheet as material, and methods of producing same.BACKGROUND

[0002] Higher strength steel sheets as material for automotive parts are being developed to both decrease CO 2 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 1320 MPa or more is increasing.

[0003] The steel sheets that serve as material for automotive parts are often required to have excellent stretch flangeability. For example, automotive parts such as crash boxes have a punched end face. Therefore, the steel sheets that serve as material for such automotive parts, from the perspective of formability, are required to have excellent stretch flangeability.

[0004] 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 1 / 4 sheet thickness position of the steel sheet is 60,000 / mm 2< or less, a number density of inclusion particles that have a circle equivalent diameter of 4.0 µm or more in a range from 1 / 4 to 3 / 4 sheet thickness of the steel sheet is 10 / mm 2< or more and 30 / mm 2< 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 1 / 4 sheet thickness of the steel sheet is 27 / mm 2< or less, and tensile strength is 1310 MPa or more."

[0005] PTL 2 describes: "a steel sheet comprising: a chemical composition represented by, 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 10° 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."

[0006] PTL 3 describes: "a high strength 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 LISTPatent Literature

[0007] PTL 1: JP 7001197 B2 PTL 2: JP 6497443 B2 PTL 3: JP 6747612 B2 SUMMARY(Technical Problem)

[0008] 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.

[0009] 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 1320 MPa or more, as well as excellent stretch flangeability, and excellent toughness and crash properties after paint baking.

[0010] In view of the above circumstances, it would be helpful to provide a steel sheet that has a TS of 1320 MPa or more, as well as excellent stretch flangeability, and excellent toughness and crash properties after paint baking, along with an advantageous method of producing the steel sheet.

[0011] Further, it would be helpful to provide a member using the steel sheet as a material and a method of producing the member.

[0012] Here, TS is measured by a tensile test in accordance with JIS Z 2241:2022.

[0013] Excellent stretch flangeability means that a maximum hole expansion ratio λ is 30 % or more. The maximum hole expansion ratio λ is measured by a hole expanding test in accordance with JIS Z 2256:2020.

[0014] 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.

[0015] 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.

[0016] Details of the measurement methods are described in the EXAMPLES section below.(Solution to Problem)

[0017] The inventors conducted intensive studies to achieve the above, and made the following discoveries: (A) To achieve a TS of 1320 MPa or more, it is important to have an area fraction of tempered martensite of 95 % or more. This allows for obtaining a TS of 1320 MPa or more while securing defined required properties. (B) To obtain excellent stretch flangeability, it is important to have a total area fraction of ferrite and bainitic ferrite of less than 5 %. This allows for obtaining excellent stretch flangeability 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 / µm 2< 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 / µm 2< and to satisfy the following Expression (1). This allows for obtaining excellent crash properties after paint baking while securing defined required properties. KAM S / KAM C > 1.00

[0018] 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.

[0019] The present disclosure is based on these discoveries and further studies.

[0020] 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 95 % or more, area fraction of retained austenite is less than 3 %, total area fraction of ferrite and bainitic ferrite is less than 5 %, 20° or greater grain boundary density in the tempered martensite is 1.0 µm / µm 2< or more, and the following Expression (1) is satisfied, KAM S / KAM C > 1.00 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, 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 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 800 °C or more, and an annealing time t1 of 10 s or longer; 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 10 °C / s or more, 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)

[0021] According to the present disclosure, a steel sheet is obtainable that has a TS of 1320 MPa or more, as well as excellent stretch flangeability, and 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 CO 2 emissions. Therefore, the industrial utility value is extremely high.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] 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

[0023] The following describes embodiments of the present disclosure.[1] Steel sheet

[0024] 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]

[0025] 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 1320 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 %, tempered martensite becomes brittle, and achieving excellent toughness after paint baking becomes difficult. 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]

[0026] 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 1320 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]

[0027] 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 1320 MPa or more becomes difficult. On the other hand, when the Mn content exceeds 5.00 %, tempered martensite becomes embrittled, and achieving excellent toughness after paint baking becomes difficult. 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]

[0028] 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]

[0029] 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]

[0030] 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]

[0031] 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]

[0032] 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.

[0033] 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, 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.

[0034] 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]

[0035] 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]

[0036] 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]

[0037] 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]

[0038] 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]

[0039] 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]

[0040] 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]

[0041] 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]

[0042] 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]

[0043] 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]

[0044] 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]

[0045] 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]

[0046] 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.

[0047] 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.

[0048] The following describes the microstructure of the steel sheet according to an embodiment of the present disclosure.

[0049] The microstructure of the steel sheet according to an embodiment of the present disclosure satisfies the following conditions: area fraction of tempered martensite is 95 % or more, area fraction of retained austenite is less than 3 %, total area fraction of ferrite and bainitic ferrite is less than 5 %, 20° or greater grain boundary density in tempered martensite is 1.0 µm / µm 2< or more, and Expression (1) is satisfied.

[0050] 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: 95 % or more]

[0051] In the steel sheet according to an embodiment of the present disclosure, it is extremely important that the area fraction of tempered martensite is 95 % or more. That is, by making tempered martensite the main phase, in particular by making the area fraction 95 % or more, a TS of 1320 MPa or more is possible to achieve. The area fraction of tempered martensite is therefore 95 % or more. The area fraction of tempered martensite is preferably 96 % or more. The area fraction of tempered martensite is more preferably 97 % or more. An upper limit of the area fraction of tempered martensite is not specifically defined. The area fraction of tempered martensite may be 100 %.[Area fraction of retained austenite: less than 3 %]

[0052] 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: less than 5 %]

[0053] 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 less than 5 %. That is, when the total area fraction of ferrite and bainitic ferrite is 5 % or more, achieving a TS of 1320 MPa or more becomes difficult. Further, achieving excellent stretch flangeability becomes difficult. Therefore, the total area fraction of ferrite and bainitic ferrite is less than 5 %. The total area fraction of ferrite and bainitic ferrite is preferably 3 % or less. The total area fraction of ferrite and bainitic ferrite is more preferably 2 % or less. A lower limit of the total area fraction of ferrite and bainitic ferrite is not specifically defined. The total area fraction of ferrite and bainitic ferrite may be 0 %. Ferrite and bainitic ferrite may be included individually, or both may be included.

[0054] 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 %.

[0055] 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.

[0056] 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% nital to reveal the microstructure. Then, a 1 / 4 sheet thickness position of the steel sheet (a position corresponding to 1 / 4 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.

[0057] 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 1 / 4 sheet thickness position or a depth of 100 µm from the steel sheet surface.

[0058] Further, the area fraction of retained austenite is measured as follows.

[0059] That is, the steel sheet is mechanically ground to a depth of 1 / 4 - 0.1 mm so that the 1 / 4 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.

[0060] 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 %. [20° or greater grain boundary density in tempered martensite: 1.0 µm / µm 2< or more]

[0061] 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 / µm 2< 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 20° 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 / µm 2< , achieving excellent crash properties after paint baking becomes difficult. Therefore, the large-angle grain boundary density of tempered martensite is 1.0 µm / µm 2< or more. The large-angle grain boundary density of tempered martensite is preferably 1.2 µm / µm 2< or more. The large-angle grain boundary density of tempered martensite is more preferably 1.3 µm / µm 2< 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 / µm 2< or less.

[0062] Here, the large-angle grain boundary density of tempered martensite is determined, for example, as follows.

[0063] 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 1 / 4 sheet thickness position of the steel sheet (the position corresponding to 1 / 4 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 1 / 4 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 20° 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.[KAM(S) / KAM(C) > 1.00]

[0064] 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.

[0065] Here, KAM(S) and KAM(C) can be determined, for example, as follows.

[0066] 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.

[0067] Mechanical properties of the steel sheet according to an embodiment of the present disclosure are as described above.

[0068] 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.

[0069] 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.

[0070] 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 / m 2< or more. The coating weight per side is preferably 80 g / m 2< or less.

[0071] 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

[0072] A member according to an embodiment of the present disclosure is described below.

[0073] 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.

[0074] Here, the steel sheet described above has a TS of 1320 MPa or more, and has excellent stretch flangeability, as well as 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 CO 2 emissions.[3] Method of producing steel sheet

[0075] The following describes a method of producing a steel sheet according to an embodiment of the present disclosure.

[0076] 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 800 °C or more, and an annealing time t1 of 10 s or longer; 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 10 °C / s or more, 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.

[0077] 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

[0078] 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.

[0079] 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.

[0080] 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 Ar 3 transformation temperature or more. The coiling temperature is preferably 350 °C or more. The coiling temperature is preferably 650 °C or less. The Ar 3 transformation temperature is determined by the following expression.

[0081] Here, [%element symbol] in the above expression represents the content in mass% of the element in the chemical composition described above.

[0082] 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.

[0083] 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

[0084] Next, the blank sheet prepared in the preparation process is heated to the annealing temperature T1 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.

[0085] [Average heating rate in temperature range of 700 °C to 750 °C: 5.0 °C / s or less]

[0086] 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 20° 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

[0087] Next, the blank sheet is annealed under a set of conditions including the annealing temperature T1 being 800 °C or more and the annealing time t1 being 10 s or longer.[Annealing temperature T1: 800 °C or more]

[0088] When the annealing temperature T1 is less than 800 °C, the total area fraction of ferrite and bainitic ferrite becomes 5 % or more, and achieving a TS of 1320 MPa or more becomes difficult. Further, achieving excellent stretch flangeability becomes difficult. The annealing temperature T1 is therefore 800 °C or more. The annealing temperature T1 is preferably 820 °C or more. An upper limit of the annealing temperature T1 is not specifically defined. For example, the annealing temperature T1 is preferably 1000 °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 a temperature range of 800 °C or more, 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]

[0089] When the annealing time t1 is less than 10 s, the total area fraction of ferrite and bainitic ferrite becomes 5 % or more, and achieving a TS of 1320 MPa or more becomes difficult. Further, achieving excellent stretch flangeability becomes difficult. The annealing time t1 is therefore 10 s or longer. The annealing time t1 is preferably 30 s or longer. An upper limit of the annealing time t1 is not specifically defined. For example, the annealing time t1 is preferably 1000 s or shorter. The annealing time t1 referred to here is the holding time at the annealing temperature T1.• Bending process

[0090] 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 T1 to 700 °C: once or more]

[0091] 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 20° 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.

[0092] 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.

[0093] 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 80° or more. The bending angle is preferably 110° 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.

[0094] 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

[0095] Next, the blank sheet is cooled to a first cooling end temperature under a set of conditions including an average cooling rate of 10 °C / s or more in a temperature range from 700 °C to 550 °C.[Average cooling rate in temperature range from 700 °C to 550 °C: 10 °C / s or more]

[0096] 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 less than 10 °C / s, the total area fraction of ferrite and bainitic ferrite becomes 5 % or more, and achieving a TS of 1320 MPa or more becomes difficult. Further, achieving excellent stretch flangeability becomes difficult. The first average cooling rate is therefore 10 °C / s or more. The first average cooling rate is preferably 30 °C / s or more. An upper limit of the first average cooling rate is not specifically defined. For example, the first average cooling rate is preferably 2000 °C / s or less.

[0097] 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

[0098] 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]

[0099] 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]

[0100] 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 20° 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.

[0101] 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.

[0102] 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

[0103] 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]

[0104] 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 1320 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]

[0105] 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 1320 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.

[0106] 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

[0107] 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: 100 °C or less. Entry side intermesh pressing amount: 4.0 mm or more and 10.0 mm or less. Delivery intermesh pressing amount: 1.0 mm or more and 10.0 mm or less. Entry side tension: 20 MPa or more and 500 MPa or less. Delivery tension: 25 MPa or more and 550 MPa or less. [Straightening start temperature: 100 °C or less]

[0108] 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]

[0109] 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]

[0110] 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.

[0111] 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]

[0112] 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]

[0113] 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 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

[0114] 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.

[0115] 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.

[0116] 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 1320 MPa or more, as well as excellent stretch flangeability, and 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

[0117] The following describes a method of producing a member according to an embodiment of the present disclosure.

[0118] 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.

[0119] 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

[0120] 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 80° 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.

[0121] 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.

[0122] Further, each evaluation was carried out according to the following procedure. The evaluation results are listed in Table 3.(Evaluation of TS)

[0123] 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 was 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 1320 MPa or more Poor (fail): TS was less than 1320 MPa (Evaluation of stretch flangeability)

[0124] The evaluation of stretch flangeability was carried out by a hole expanding test in accordance with JIS Z 2256:2020. That is, each obtained steel sheet was sheared to a size of 100 mm × 100 mm, and a test piece was taken. A hole having a diameter of 10 mm was then punched through the test piece with a clearance of 12.5 %. The test piece was then held down by a blank holding force of 9 tonnes (88.26 kN) using a die with an inner diameter of 75 mm. Then, in that state, a conical punch having a vertex angle of 60° was pressed into the hole of the test piece, and the diameter of the hole of the test piece at the crack initiation limit (when cracking occurs) was measured. The maximum hole expansion ratio λ was obtained by the following expression. λ % = D f − D 0 / D 0 × 100

[0125] Here, D f is diameter in mm of the hole in the test piece at the crack initiation limit (when cracking occurs), and D 0 is diameter in mm of the hole in the test piece at start. The stretch flangeability was evaluated based on the following criteria. Good (pass, very good): λ was 30 % or more Poor (fail): λ was less than 30 % (Evaluation of toughness after paint baking)

[0126] 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)

[0127] 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.

[0128] 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.

[0129] 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.

[0130] The YR and fracture stress ratio after aging treatment are determined by the following expressions, respectively.

[0131] 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]

[0132] Table 1Steel sample IDChemical composition (mass%)RemarksCSiMnPSNOAlTiNbBCuOtherA0.2200.2271.530.0140.00150.00600.00250.017Conforming steelB0.2100.2231.870.0140.00050.00650.00300.045Conforming steelC0.2120.1241.980.0120.00130.00350.00450.048Conforming steelD0.2140.2931.280.0140.00060.00430.00380.052Conforming steelE0.2280.1831.120.0090.00090.00260.00440.044Conforming steelF0.0370.1451.600.0130.00130.00460.00160.034Conforming steelG0.0240.2091.550.0100.00110.00520.00260.016Comparative steelH0.4960.2081.030.0130.00110.00370.00630.038Conforming steelI0.5050.1491.210.0120.00100.00110.00490.012Comparative steelJ0.2390.0221.680.0080.00050.00540.00230.050Conforming steelK0.2250.0051.250.0110.00130.00630.00210.046Comparative steelL0.2062.3341.720.0060.00080.00610.00240.053Conforming steelM0.2142.5801.080.0070.00130.00200.00160.054Comparative steelN0.2420.2520.420.0080.00050.00210.00410.018Conforming steelO0.2370.2430.070.0080.00130.00330.00470.035Comparative steelP0.2280.2004.640.0060.00130.00210.00330.023Conforming steelQ0.2030.1625.160.0110.00070.00690.00110.023Comparative steelR0.2010.1541.360.0970.00120.00550.00610.037Conforming steelS0.2360.1631.640.1090.00140.00110.00130.045Comparative steelT0.2490.2241.720.0150.01920.00220.00400.036Conforming steelU0.2190.2151.900.0050.02030.00320.00690.060Comparative steelV0.2390.2181.320.0130.00060.00180.00370.924Conforming steelW0.2110.1351.270.0100.00130.00570.00491.049Comparative steelX0.2120.1521.600.0090.00120.00940.00120.046Conforming steelY0.2120.2871.910.0060.00090.01110.00250.038Comparative steelZ0.2070.1321.650.0070.00060.00320.00880.030Conforming steelAA0.2150.2641.180.0050.00080.00280.01120.035Comparative steelAB0.2380.1071.830.0090.00060.00520.00280.014Conforming steelAC0.2260.2341.590.0050.00070.00610.00280.0320.002Conforming steelAD0.2450.1271.320.0100.00110.00270.00140.0560.183Conforming steelAE0.2200.1211.360.0060.00070.00430.00680.0260.214Comparative steelAF0.2130.2731.570.0070.00060.00210.00450.0420.0001Conforming steelAG0.2080.1721.230.0090.00150.00350.00510.0240.0098Conforming steelAH0.2240.2191.800.0050.00080.00580.00250.0190.0120Comparative steelAI0.2500.1001.290.0130.00050.00340.00410.0310.002Conforming steelAJ0.2370.1871.610.0100.00130.00170.00440.0530.190Conforming steelAK0.2440.1441.710.0060.00060.00660.00190.0210.209Comparative steelAL0.2270.2341.190.0130.00060.00490.00480.0360.03Conforming steelAM0.2050.1541.470.0100.00140.00500.00510.0130.96Conforming steelAN0.2290.2581.880.0070.00130.00110.00390.0401.15Comparative steelAO0.2050.1731.660.0120.00140.00530.00400.058V:0.071Conforming steelAP0.2000.2681.120.0140.00080.00400.00670.022Ta:0.08Conforming steelAQ0.2030.1371.440.0110.00050.00240.00250.018W:0.07Conforming steelAR0.2070.2551.390.0060.00120.00690.00700.059Cr:0.61Conforming steelAS0.2310.2231.020.0110.00140.00340.00350.034Mo:0.37Conforming steelAT0.2260.2751.700.0130.00130.00570.00510.055Co:0.008Conforming steelAU0.2460.2831.360.0140.00100.00700.00490.045Ni:0.78Conforming steelAV0.2100.1071.040.0120.00080.00320.00370.021Sn:0.112Conforming steelAW0.2200.2411.700.0120.00070.00630.00330.054Sb:0.099Conforming steelAX0.2430.2271.500.0130.00060.00610.00490.014Ca:0.0023Conforming steelAY0.2140.1771.950.0090.00100.00170.00240.060Mg:0.0031Conforming steelAZ0.2010.1611.570.0090.00140.00600.00680.028Zr:0.067Conforming steelBA0.2310.2111.380.0120.00070.00130.00450.036Te:0.030Conforming steelBB0.2190.2001.540.0110.00080.00570.00450.043Hf:0.09Conforming steelBC0.2410.1291.780.0090.00110.00170.00600.048REM:0.0048Conforming steelBD0.2100.2621.860.0100.00130.00460.00590.051Bi:0.062Conforming steelBE0.2280.1561.350.0100.00060.00610.00340.030Zn:0.091Conforming steelBF0.2430.1151.810.0130.00100.00410.00150.030Pb:0.076Conforming steelBG0.2460.2201.500.0060.00070.00400.00400.050As:0.094Conforming steelBH0.2230.2051.860.0100.00060.00570.00110.015Ge:0.052Conforming steelBI0.2240.1501.180.0110.00070.00580.00560.052Sr:0.050Conforming steelBJ0.2220.2601.630.0130.00070.00410.00530.041Cs:0.025Conforming steelBK0.2270.1851.320.0090.00050.00540.00520.041Conforming steelBL0.2350.1091.580.0140.00070.00650.00650.036Conforming steelBM0.2410.2251.550.0060.00130.00390.00150.035Conforming steelBN0.2430.2841.180.0060.00050.00650.00280.054Conforming steelBO0.2410.2861.810.0120.00100.00380.00350.036Conforming steelUnderlined: indicates value outside scope of present disclosure. [Table 2]

[0133] Table 2No.Steel sample IDHeating processAnnealing processBending processFirst cooling processSecond cooling processTempering processStraightening processType*RemarksAverage heating rate (°C / s)Annealing temp. T1 (°C)Annealing time t1 (s)Number of bends (times)First average cooling rate (°C / s)Second average cooling rate (°C / s)Applied tension (MPa)Tempering temp. T2 (°C)Tempering time t2 (s)Straightening start temp. (°C)Entry side intermesh pressing amount (mm)Delivery intermesh pressing amount (mm)Entry side tension (MPa)Delivery tension (MPa)1A0.285492395485116177622217.01.6262271CRExample2B0.883499489093319201621377.51.5246253CRExample3B0.1834173494389118206513297.71.9225230CRExample4B0.2848164495294116184699317.61.6154164CRExample5B4.5838134498481413192579207.01.8194203CRExample6B5.2831178499483712197819397.81.5196205CRComparative Example7B0.6804179384589219179820387.52.0160167CRExample8B0.3794106382690411202770377.31.6197203CRComparative Example9B0.7941118380796616182904226.11.3167176CRExample10B0.984381491295815199739326.51.7271277CRExample11B0.283415484789319181601227.71.6162167CRExample12B0.98492582881818178563257.32.0298306CRComparative Example13B0.9836984491283415190822277.31.2275282CRExample14B0.6856186496395418198560357.41.1185190CRExample15B0.783354186988219196612246.41.1219226CRExample16B0.285553080286014210547298.01.2200208CRComparative Example17B0.583751088792816173694236.31.3194202CRComparative Example18B0.2856187988499918184624386.61.2290299CRExample19B0.885810031396612192662256.02.0257263CRExample20B0.88401502892819210887206.71.3218226CRComparative Example21B0.58281265185986819181902206.71.2233241CRExample22B0.4853148286986318193518246.21.2270278CRExample23B0.383483582330519178835297.82.0287296CRExample24B0.5859151390225420187888306.71.9254260CRComparative Example25B0.98231183954199216197956387.71.1245252CRExample26B0.482192398298915202584366.01.0246252CRExample27B0.283719139248676185995326.11.2278286CRExample28B0.885610238989232205945256.51.0227237CRComparative Example29B1.0833109389897844200962327.11.7204210CRExample30B0.9840152384491559202904266.71.8257263CRExample31B0.7860108399582311105717346.51.8251259CRExample32B0.3849200384488816105871408.01.0287292CRExample33B0.6860137381486012368930246.31.1281289CRExample34B0.2836166290682017384549306.81.0250259CRExample35B0.38326249429311219112276.11.9262269CRExample36B0.883112858448151620934357.01.6260266CRExample37B0.28331433877951161969542396.41.1180187CRExample38B0.88381674881924152089684357.21.6214223CRExample39B0.782617939229201819163337.91.8296302CRExample40B0.4846171392296914199988206.91.1214224CRExample41B0.9843147499799211209544887.41.7234241CRExample42B0.48361374947970191888631056.21.8150158CRComparative Example43B0.6839176386489415206775334.31.2234243CRExample44B0.9850154283097716182526392.51.3264272CRComparative Example45B0.8831135499795613182596289.41.6210216CRExample46B0.6836134385094818208718229.61.6279288CRExample47B0.2847178585392616176676367.91.0249258CRExample48B0.685052296096112178811376.61.0222231CRExample49B0.4854121586993914188952389.48.4190196CRExample50B0.982262580198116207599339.68.6243250CRExample51B0.784191399386016206883357.61.22431CRExample52B0.9853182582380113203891397.51.71526CRComparative Example53B0.3832174284397917189627337.21.1415424CRExample54B0.7845189397693115187852406.61.4487495CRExample55B0.7834158485998518192760407.51.62634CRExample56B0.3860142293493412178771327.91.21427CRComparative Example57B0.4827120393998216191623336.81.8487540CRExample58B0.384882398992020206797307.61.1497545CRExample59B0.9843123481882615207811337.01.4231237GAExample60B0.5833149383792418181507236.41.9170176GAExample61C0.3854181495785517202895296.31.5274280GAExample62D0.7850150495495017196577307.11.5201210GAExample63E0.684476499785019181784397.41.8190200GAExample64F0.5836110591596918190573216.91.3283292CRExample65G0.485554484496613208778237.41.3277285CRComparative Example66H0.8858164583291012175941266.41.9193201GAExample67I0.4820115499094020206669236.01.0170175GAComparative Example68J0.28511684100082115196832386.41.1293302GIExample69K0.4854101495990416185753377.31.4269278GAComparative Example70L0.483381598297413184911406.41.0284293GAExample71M0.683269482684316187964336.31.6245250GAComparative Example72N0.6854147387188815197990297.51.3187193GAExample73O0.8828150487392216188807266.11.4289299GIComparative Example74P0.682988398290317199721276.01.4201208GAExample75Q0.7841196482898910185679356.41.1208213GAComparative Example76R0.3836180592887211180528226.41.2214223GAExample77S0.482669593986019204706337.51.1271277GAComparative Example78T0.7858200283886811176685316.01.8225233GAExample79U0.4835122288093118184886287.41.9172177GIComparative Example80V0.983056383280710199905397.81.0188197GAExample81W0.682269283792115171738366.21.4228237GAComparative Example82X0.8836119582193918180617216.11.3191200GAExample83Y0.6853108486994416182793357.11.8158166GAComparative Example84Z1.0839167392986413204987216.21.7295303CRExample85AA0.5853126389696918207523276.81.3177182CRComparative Example86AB0.9822106397686415208980267.91.1229237GAExample87AC0.4843128398497217204785267.31.7277286GAExample88AD0.483155392387912200630256.21.4150159GAExample89AE0.3832171498888815180812287.41.8269277GAComparative Example90AF0.5840192489287313194579257.61.6199206GAExample91AG0.6822128494995020205503387.91.8169177GAExample92AH0.8850100294185611203864236.91.2291298GAComparative Example93AI1.0838126489997119198950317.51.9262272CRExample94AJ0.984952492281719188634266.81.2175183CRExample95AK0.582771385988818207911296.21.2195203CRComparative Example96AL0.9840184385293913190577386.61.6177186CRExample97AM0.5825163396291511203985296.81.7292300CRExample98AN0.382270290289313182731307.21.6186192CRComparative Example99AO0.1820163486182514190760257.41.8288294CRExample100AP4.784468292599215171671238.01.6173181CRExample101AQ0.3811119488797612205678267.41.5182188CRExample102AR0.494756388695912181979337.71.8237244CRExample103AS0.583915393680116176967366.11.5221231CRExample104AT0.9828958584789620200568296.41.5244252GAExample105AU1.0843138191487311207568256.91.5164171GAExample106AV0.48271431084298515202802277.11.2202208CRExample107AW0.88315331181817172891267.91.7199205CRExample108AX0.3851775189884212173529297.21.7290298CRExample109AY0.582753386032218178692317.61.2199208EGExample110AZ0.2821724880189119199702327.11.8269277GIExample111BA0.383615338869206209628386.01.5258264EGExample112BB0.5830100388899746195614247.41.3288295GIExample113BC0.2828197296681411111874316.91.6178186CRExample114BD1.0847197283093620368931376.11.6223228CRExample115BE0.484516639299051020511237.11.4197207CRExample116BF0.8837944996857121879658387.41.0180189CRExample117BG0.986019529118061120361967.31.3213221CRExample118BH0.582792482290014181900946.31.8282291CRExample119BI0.7822173394097212172804244.11.9294300CRExample120BJ0.9847189485191712187733359.81.6198203CRExample121BK0.9837192481189415171801237.41.32432EGExample122BL0.382291498185014198635217.31.8487544GIExample123BM0.983465488796411187558227.61.1248254EGExample124BN0.9829184395683314185766247.41.2297307GIExample125BO0.6846127486795018176797236.11.6274280GAExampleUnderlined: 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]

[0134] Table 3No.Steel sample IDMicrostructureEvaluation resultRemarksArea fractionLarge-angle grain boundary density of tempered M (µn / µm 2< )KAM(S) / KAM(C)TSStretch flangeabilityToughness after paint bakingCrash properties after paint bakingTempered M (%)Retained γ (%)F+B (%)TS (MPa)Evaluationλ (%)EvaluationBrittle-ductile transition temp. after aging treatment (°C)EvaluationYR after aging treatmentFracture stress ratio after aging treatmentEvaluation1A99101.71.071569Good57Good-102Excellent0.940.84GoodExample2B98111.21.061489Good49Good-108Excellent0.910.82GoodExample3B99102.11.061502Good53Good-97Excellent0.920.81GoodExample4B99011.01.051520Good57Good-98Excellent0.930.84GoodExample5B99011.11.041509Good58Good-45Good0.910.90GoodExample6B100000.81.051522Good57Good-6Poor0.910.93PoorComparative Example7B96041.51.041363Good32Good-107Excellent0.930.82GoodExample8B91181.61.081231Poor18Poor-92Excellent0.900.84GoodComparative Example9B97121.21.051497Good47Good-109Excellent0.900.83GoodExample10B99101.21.081524Good50Good-95Excellent0.930.84GoodExample11B96132.01.061337Good34Good-100Excellent0.920.85GoodExample12B95051.41.041351Good26Poor-103Excellent0.900.81GoodComparative Example13B100001.71.071541Good56Good-90Excellent0.910.83GoodExample14B98111.61.041491Good58Good-93Excellent0.930.84GoodExample15B98021.21.041470Good51Good-47Good0.930.87GoodExample16B99010.81.031466Good52Good-18Poor0.920.93PoorComparative Example17B97120.71.041517Good58Good-6Poor0.940.94PoorComparative Example18B99102.51.041547Good59Good-98Excellent0.910.85GoodExample19B96041.41.061357Good34Good-106Excellent0.910.84GoodExample20B95051.31.031361Good18Poor-100Excellent0.910.82GoodComparative Example21B100001.21.071563Good52Good-100Excellent0.940.84GoodExample22B98111.51.061498Good58Good-98Excellent0.930.80GoodExample23B97211.11.041537Good60Good-53Good0.910.82GoodExample24B96401.71.081341Good49Good-19Poor0.910.81GoodComparative Example25B98021.81.041465Good49Good-106Excellent0.910.81GoodExample26B100001.31.041522Good52Good-95Excellent0.920.80GoodExample27B98021.21.071492Good45Good-45Good0.920.86GoodExample28B97120.81.081449Good46Good-26Poor0.910.94PoorComparative Example29B100001.31.051524Good59Good-104Excellent0.900.84GoodExample30B99011.11.041483Good58Good-102Excellent0.940.80GoodExample31B98021.61.061654Good52Good-57Good0.920.81GoodExample32B97121.71.041652Good58Good-52Good0.910.81GoodExample33B98111.61.041358Good58Good-97Excellent0.900.82GoodExample34B98021.31.061367Good45Good-109Excellent0.920.81GoodExample35B98111.81.061633Good46Good-48Good0.900.82GoodExample36B99011.41.041648Good54Good-52Good0.930.82GoodExample37B97122.01.051337Good54Good-97Excellent0.920.85GoodExample38B100001.21.031351Good60Good-101Excellent0.900.82GoodExample39B98111.71.051507Good47Good-98Excellent0.930.83GoodExample40B98111.81.061491Good55Good-104Excellent0.940.82GoodExample41B97121.51.021447Good59Good-95Excellent0.860.84GoodExample42B98111.70.951517Good54Good-96Excellent0.810.81PoorComparative Example43B99011.91.021472Good48Good-105Excellent0.850.82GoodExample44B99011.90.931529Good54Good-105Excellent0.810.84PoorComparative 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indicates value outside scope of present disclosure. (*) Tempered M: tempered martensite, retained γ: retained austenite, F: ferrite, B: bainitic ferrite

[0135] As indicated in Table 3, for the Examples according to the present disclosure, TS, stretch flangeability, as well as toughness and crash properties after painting 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, stretch flangeability, as well as toughness and crash properties after paint baking, all reached passing criteria.

[0136] On the other hand, for the Comparative Examples, at least one criterion was failed among TS, stretch flangeability, and toughness and crash properties after paint baking.

Claims

1. A steel sheet comprising: a chemical composition containing, 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 95 % or more, area fraction of retained austenite is less than 3 %, total area fraction of ferrite and bainitic ferrite is less than 5 %, 20° or greater grain boundary density in the tempered martensite is 1.0 µm / µm2 or more, and the following Expression (1) is satisfied, KAM S / KAM C > 1.00 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 any one of claims 1 to 4.

6. A method of producing the steel sheet according to any one of claims 1 to 4, the method comprising: a preparation process of preparing a blank sheet having the chemical composition according to claim 1 or 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 800 °C or more, and an annealing time t1 of 10 s or longer; 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 10 °C / s or more, 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 any one of claims 1 to 4 is subjected to at least one of a forming process or a joining process to produce the member.

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