Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a steel sheet, a member using the steel sheet as material, and methods of producing same.
• steel sheets used as material for automotive parts for example, automobile frame parts such as bumpers, are often required, from the viewpoint of press formability, to have high elongation (hereinafter, also referred to as EL).
• Patent Literature (PTL) 1 describes:
• Automotive parts often undergo paint baking.
• 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.
• TS is measured by a tensile test in accordance with JIS Z 2241:2022.
• High EL means an EL of 10 % or more. Further, EL is measured by a tensile test in accordance with JIS Z 2241:2022.
• Excellent toughness after paint baking means that a brittle-ductile transition temperature after aging treatment is -40 °C or lower.
• the aging treatment conditions are a treatment temperature of 170 °C and a treatment time of 20 min.
• the brittle-ductile transition temperature is measured by the Charpy impact test in accordance with JIS Z 2242:2018.
• Excellent crash properties after paint baking means that a YR after aging treatment is 0.85 or more, and a fracture stress ratio after aging treatment is 0.90 or less.
• 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.
• [YR after aging treatment] [YS after aging treatment] / [TS after aging treatment]
• [Fracture stress ratio after aging treatment] [fracture stress after aging treatment] / [TS after aging treatment]
• a steel sheet is obtainable that has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking.
• the 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.
• FIG. 1 is a schematic diagram for explaining definitions of entry side intermesh pressing amount and delivery intermesh pressing amount.
• C is an important basic component of steel.
• C is an important element that affects the area fraction of tempered martensite and the crash properties after paint baking.
• C content is less than 0.030 %, the area fraction of tempered martensite decreases, and achieving a TS of 1180 MPa or more becomes difficult. Further, achieving excellent crash properties after paint baking also becomes difficult.
• the C content exceeds 0.500 %, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, making it difficult to achieve a high EL.
• the C content is therefore 0.030 % or more and 0.500 % or less.
• the C content is preferably 0.050 % or more.
• the C content is more preferably 0.100 % or more.
• the C content is preferably 0.400 % or less.
• the C content is more preferably 0.350 % or less.
• Si is an important basic component of steel.
• 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.
• Si content is less than 0.010 %, achieving a TS of 1180 MPa or more becomes difficult.
• 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 is an important basic component of steel.
• Mn is an important element that affects the area fraction of tempered martensite and toughness after paint baking.
• Mn content is less than 0.10 %, the area fraction of tempered martensite decreases, and achieving a TS of 1180 MPa or more becomes difficult.
• the Mn content exceeds 5.00 %, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, making it difficult to achieve a high EL.
• the Mn content is therefore 0.10 % or more and 5.00 % or less.
• the Mn content is preferably 0.50 % or more.
• the Mn content is more preferably 0.80 % or more.
• the Mn content is preferably 4.50 % or less.
• the Mn content is more preferably 4.00 % or less.
• P 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.
• P is a solid-solution-strengthening element and can increase steel sheet strength. The P content is therefore preferably 0.001 % or more.
• 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 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 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 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Co is an element that improves hardenability. The Co content is therefore preferably 0.001 % or more.
• 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.
• Cu is an element that improves hardenability.
• the Cu content is therefore preferably 0.01 % or more.
• 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.
• 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.
• 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.
• Sb is an element that controls surface layer softening thickness and allows strength adjustment. The Sb content is therefore preferably 0.001 % or more.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Bi is an element that reduces segregation. The Bi content is therefore preferably 0.001 % or more.
• Fe and inevitable impurity examples include Zn, Pb, As, Ge, Sr, and Cs. Such inevitable impurity is allowed to be included as long as a total amount is 0.100 % or less.
• the area fraction of each phase is the area ratio occupied by each phase relative to the entire microstructure.
• the area fraction of tempered martensite is 83 % or more. That is, by making tempered martensite the main phase, in particular by making the area fraction 83 % or more, a TS of 1180 MPa or more is possible to achieve.
• the area fraction of tempered martensite is therefore 83 % or more.
• the area fraction of tempered martensite is preferably 85 % or more.
• the area fraction of tempered martensite is more preferably 87 % or more.
• An upper limit of the area fraction of tempered martensite is not specifically defined.
• the area fraction of tempered martensite is, for example, preferably less than 95 %.
• the area fraction of tempered martensite is more preferably 94 % or less.
• the area fraction of tempered martensite is even more preferably 93 % or less.
• the area fraction of retained austenite is less than 3 %. That is, when the area fraction of retained austenite is 3 % or more, achieving excellent toughness after paint baking becomes difficult.
• One of the causes of decreased toughness after paint baking is that retained austenite transforms into deformation-induced martensite during processing, resulting in high-hardness martensite, which becomes an initiation point of a fracture.
• the area fraction of retained austenite is therefore less than 3 %.
• the area fraction of retained austenite is preferably 1 % or less. A lower limit of the area fraction of retained austenite is not specifically defined.
• the area fraction of retained austenite may be 0 %.
• Total area fraction of ferrite and bainitic ferrite 5 % or more and less than 15 %
• the total area fraction of ferrite and bainitic ferrite is more than 5 % and less than 15 %. That is, when the total area fraction of ferrite and bainitic ferrite is 15 % or more, achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the total area fraction of ferrite and bainitic ferrite is less than 5 %, it becomes difficult to achieve a high EL. Therefore, the total area fraction of ferrite and bainitic ferrite is 5 % or more and less than 15 %.
• the total area fraction of ferrite and bainitic ferrite is preferably 6 % or more.
• the total area fraction of ferrite and bainitic ferrite is more preferably 7 % or more.
• the total area fraction of ferrite and bainitic ferrite is preferably 14 % or less.
• the total area fraction of ferrite and bainitic ferrite is more preferably 13 % or less.
• Ferrite and bainitic ferrite may be included individually, or both may be included.
• the area fraction of residual microstructure other than described above is preferably 5 % or less.
• Examples of residual microstructure include pearlite, fresh martensite, and acicular ferrite. These residual microstructures may be included as long as the content is 5 % or less, as they do not affect the properties.
• the area fraction of the residual microstructure may be 0 %.
• the area fraction of tempered martensite, as well as the total area fraction of ferrite and bainitic ferrite, is measured, for example, as follows.
• a sample is cut from the steel sheet such that a thickness cross-section parallel to the rolling direction of the steel sheet (L-section) becomes an observation plane.
• the observation plane of the sample is then polished.
• the observation plane of the sample is then corroded with 3 vol% nital to reveal the microstructure.
• 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.
• tempered martensite has fine irregularities in the microstructure and contains carbides in the microstructure.
• ferrite and bainitic ferrite have a flat microstructure in recessed portions and do not contain carbides.
• the areas occupied by tempered martensite, as well as ferrite and bainitic ferrite are determined.
• the area occupied by tempered martensite, and the area occupied by ferrite and bainitic ferrite are each divided by the total area of the observed field of view and multiplied by 100. Then, the average values of these are taken as the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite, respectively.
• the microstructure of steel sheets is normally approximately vertically symmetrical in the thickness direction. Therefore, any one surface of the steel sheet (front or back) can be set as an initiation point of a thickness position (sheet thickness 0 position), such as the 1/4 sheet thickness position or a depth of 100 ⁇ m from the steel sheet surface.
• the area fraction of retained austenite is measured as follows.
• 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.
• 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 K ⁇ 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.
• 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 %.
• Area fraction of residual microstructure (%)] 100 - [area fraction of tempered martensite (%)] - [total area fraction of ferrite and bainitic ferrite (%)] - [area fraction of retained austenite (%)]
• the 20° or greater grain boundary density of tempered martensite is 1.0 ⁇ m/ ⁇ m 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 ⁇ m/ ⁇ m 2 , achieving excellent crash properties after paint baking becomes difficult.
• the large-angle grain boundary density of tempered martensite is 1.0 ⁇ m/ ⁇ m 2 or more.
• the large-angle grain boundary density of tempered martensite is preferably 1.2 ⁇ m/ ⁇ m 2 or more.
• the large-angle grain boundary density of tempered martensite is more preferably 1.3 ⁇ m/ ⁇ m 2 or more.
• An upper limit of the large-angle grain boundary of tempered martensite is not specifically defined.
• the large-angle grain boundary density of tempered martensite is preferably 3.0 ⁇ m/ ⁇ m 2 or less.
• the large-angle grain boundary density of tempered martensite is determined, for example, as follows.
• a test piece for microstructure observation is collected from the steel sheet.
• the collected test piece is then polished by colloidal silica vibrational polishing so that the cross-section in the rolling direction (L-section) becomes the observation plane.
• the observation plane is mirror-finished.
• EBSD electron backscatter diffraction
• the step size is set to 0.10 ⁇ m, and the measurement area is 50 ⁇ m square (50 ⁇ m ⁇ 50 ⁇ m). 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) exceeds 1.00.
• KAM(S) is the KAM value at a depth of 100 ⁇ m from the steel sheet surface
• KAM(C) is the KAM value at the mid-thickness position of the steel sheet.
• 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.
• KAM(S)/KAM(C) is preferably 1.110 or less.
• KAM(S) and KAM(C) can be determined, for example, as follows.
• 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 ⁇ m from the steel sheet surface and at the mid-thickness position of the steel sheet, and average values are used.
• 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.
• 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.
• a galvanized layer with Zn as the main component Zn content of 50.0 mass% or more
• 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.
• 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.
• GI hot-dip galvanized steel sheet
• GA galvannealed steel sheet
• EG electrogalvanized steel sheet
• Coated or plated layers other than galvanized layers may include aluminum coated or plated layers and alloy coated or plated layers.
• alloy coated or plated layers examples include hot-dip zinc-aluminum-magnesium alloy coated layers and Zn-Ni electroplated alloy layers.
• 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.
• 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.
• the member according to an embodiment of the present disclosure is a member formed using the steel sheet described above as a material.
• the material, the steel sheet is subjected to at least one of a forming process or a joining process to make the member.
• the steel sheet described above has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking. Therefore, the member according to an embodiment of the present disclosure is particularly suitable for application as a material for automotive parts. This allows for improved fuel efficiency due to an automotive body weight decrease, which can greatly contribute to a decrease in CO 2 emissions.
• the following describes a method of producing a steel sheet according to an embodiment of the present disclosure.
• 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.
• a blank sheet having the chemical composition described above is prepared.
• 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.
• a method of smelting the steel slab 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.
• hot rolling examples 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.
• 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.
• Ar 3 transformation temperature (°C) 868 - 396 ⁇ [%C] + 24.6 ⁇ [%Si] - 68.1 ⁇ [%Mn] - 36.1 ⁇ [%Ni] - 20.7 ⁇ [%Cu] - 24.8 ⁇ [%Cr]
• [%element symbol] in the above expression represents the content in mass% of the element in the chemical composition described above.
• Pickling is capable of removing oxides from the surface of the hot-rolled steel sheet, and is preferably carried out to secure good chemical convertibility and coating quality in the final steel sheet product. Pickling may be carried out in one or more batches. Further, the hot-rolled steel sheet after pickling may be subjected to heat treatment.
• the total rolling reduction in the cold rolling is preferably 30 % or more.
• the total rolling reduction in the cold rolling is preferably 80 % or less. The defined effect can be obtained without limiting the number of rolling passes or the rolling reduction for each pass.
• 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.
• the average heating rate in the temperature range of 700 °C to 750 °C 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.
• the average heating rate is preferably 0.1 °C/s or more.
• the blank sheet is annealed under a set of conditions including the annealing temperature T1 being 750 °C or more and 850 °C or less, and annealing time t1 being 10 s or longer and 1000 s or shorter.
• the annealing temperature T1 When the annealing temperature T1 is less than 750 °C, the total area fraction of ferrite and bainitic ferrite becomes 15 % or more, and achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the annealing temperature T1 exceeds 850 °C, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, making it difficult to achieve a high EL.
• the annealing temperature T1 is therefore 750 °C or more and 850 °C or less.
• the annealing temperature T1 is preferably 760 °C or more.
• the annealing temperature T1 is more preferably 770 °C or more.
• the annealing temperature T1 is preferably 840 °C or less.
• the annealing temperature T1 is more preferably 830 °C or less.
• the annealing temperature referred to here is the holding temperature during the annealing process. Further, the annealing temperature may remain constant during holding. Further, the annealing temperature is the temperature range of 750 °C or more and 850 °C or less, and when temperature fluctuation is within ⁇ 10 °C of the set temperature, the annealing temperature does not have to be constant during holding.
• the annealing time t1 When the annealing time t1 is less than 10 s, the total area fraction of ferrite and bainitic ferrite becomes 15 % or more, and achieving a TS of 1180 MPa or more becomes difficult. On the other hand, when the annealing time t1 exceeds 1000 s, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, making it difficult to achieve a high EL.
• the annealing time t1 is therefore 10 s or longer and 1000 s or shorter.
• the annealing time t1 is preferably 30 s or longer.
• the annealing time t1 is preferably 800 s or shorter.
• the annealing time t1 referred to here is the holding time at the annealing temperature T1.
• 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.
• the inventors found that carrying out bending in the temperature range from the annealing temperature T1 to 700 °C affects the large-angle grain boundary density of tempered martensite.
• carrying out bending using a roller that has a radius of 800 mm or less in the bending temperature range promotes martensitic transformation nucleation.
• 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.
• the number of times bending using a roller that has a radius of 800 mm or less in the bending temperature range (hereinafter also referred to as bending count) is set to be once or more.
• the radius of the roller used for bending is preferably 600 mm or less.
• a lower limit of the radius of the roller used for bending is not particularly restricted.
• the radius of the roller used for bending is preferably 100 mm or more.
• 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.
• 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).
• 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.
• the blank sheet is cooled to a first cooling end temperature under a set of conditions including an average cooling rate of 0.5 °C/s or more and less than 10.0 °C/s in a temperature range from 700 °C to 550 °C.
• Average cooling rate in temperature range from 700 °C to 550 °C: 0.5 °C/s or more and less than 10.0 °C/s]
• first average cooling rate When the average cooling rate in the temperature range from 700 °C to 550 °C (hereinafter also referred to as first average cooling rate) is 10.0 °C/s or more, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, and achieving a high EL becomes difficult. On the other hand, when the first average cooling rate is less than 0.5 °C/s, the total area fraction of ferrite and bainitic ferrite becomes 15 % or more, and achieving a TS of 1180 MPa or more becomes difficult.
• the first average cooling rate is therefore 0.5 °C/s or more and less than 10.0 °C/s.
• the first average cooling rate is preferably 1.0 °C/s or more.
• the first average cooling rate is preferably 8.0 °C/s or less.
• the first cooling end temperature may be, for example, from 550 °C to 300 °C.
• 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.
• 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.
• 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.
• the second average cooling rate is preferably 2000 °C/s or less.
• 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.
• 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.
• 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.
• 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.
• the applied tension is preferably 100 MPa or less.
• the second cooling end temperature may be, for example, less than 100 °C.
• the second cooling end temperature may be, for example, around room temperature.
• the bending in the bending process described above increases the number of nucleation sites, which are initiation points of martensitic transformation.
• the application of tension in the second cooling process promotes the martensitic transformation itself. That is, the effects obtained from both are different.
• 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.
• Tempered martensite is formed by tempering treatment, where martensite is tempered.
• 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.
• the tempering temperature T2 exceeds 400 °C, tempering of martensite progresses excessively, and achieving a TS of 1180 MPa or more becomes difficult.
• the tempering temperature T2 is therefore 100 °C or more and 400 °C or less.
• the tempering temperature T2 is preferably 150 °C or more.
• the tempering temperature T2 is preferably 350 °C or less.
• the tempering temperature referred to here is the holding temperature during the tempering process.
• the tempering temperature may be constant during holding. Further, the tempering temperature is in the range from 100 °C or more to 400 °C or less, and when temperature fluctuation is within ⁇ 10 °C of the set temperature, the tempering temperature does not have to be constant during holding.
• tempered martensite is formed by tempering treatment, where martensite is tempered.
• 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.
• the tempering time t2 exceeds 10,000 s, tempering of martensite progresses excessively, and achieving a TS of 1180 MPa or more becomes difficult.
• 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.
• the tempering time t2 refers to the holding time at the tempering temperature T2.
• the cooling after tempering is not specifically defined. For example, it is sufficient to cool by any method according to a conventional method.
• the cooling end temperature after tempering may be, for example, around room temperature.
• 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.
• 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.
• 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.
• the straightening start temperature is preferably -10 °C or more.
• the entry side intermesh pressing amount 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.
• 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.
• the entry side intermesh pressing amount 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.
• 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.
• the entry side tension 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.
• the delivery tension is higher than the entry side tension.
• the delivery intermesh pressing amount is less than 25 MPa, the amount of processing is insufficient. Therefore, KAM(S)/KAM(C) becomes 1.00 or less, and the crash properties after paint baking decrease.
• the upper limit of the delivery tension is 550 MPa or less in view of production technology constraints. Accordingly, the delivery tension is 25 MPa or more and 550 MPa or less.
• the delivery tension is preferably 100 MPa or more.
• the steel sheet may be subjected to coating or plating treatment.
• Coating or plating treatment is not particularly limited.
• coating or plating treatment include galvanizing treatment such as hot-dip galvanizing treatment, galvannealing treatment, and electrogalvanization treatment.
• examples of coating or plating treatment include aluminum coating or plating treatment and alloy coating or plating treatment.
• 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.
• 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.
• hot-dip galvanizing treatment or galvannealing treatment is preferably carried out between the first cooling process and the second cooling process.
• electrogalvanization treatment or Zn-Ni electro-alloy plating treatment is preferably carried out between the tempering process and the straightening process.
• 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).
• wiping may be carried out for adjusting the coating amount.
• Conditions other than those described above are not particularly limited, and a conventional method may be used. According to the method of producing the steel sheet according to an embodiment of the present disclosure described above, a steel sheet is obtainable that has a TS of 1180 MPa or more, high EL, and also excellent toughness and crash properties after paint baking. The obtained steel sheet may be suitably used as a material for automotive parts, for example.
• 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.
• a forming method is not particularly limited, and a typical processing method such as press forming may be used, for example.
• 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.
• 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°.
• 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.
• 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.
• a tensile test was conducted according to JIS Z 2241:2022 using the test piece, and TS and EL were measured.
• the crosshead speed was set to 1.67 ⁇ 10 -1 mm/s. Evaluation was based on the following criteria.
• the EL measured in the above tensile tests was evaluated according to the following criteria.
• a plurality of sheets were stacked and fastened with bolts.
• 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.
• 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.
• 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.
• the obtained steel sheets were each subjected to aging treatment at a treatment temperature of 170 °C for a treatment time of 20 min.
• 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.
• 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.
• the YR and fracture stress ratio after aging treatment are determined by the following expressions, respectively.
• [YR after aging treatment] [YS after aging treatment] / [TS after aging treatment]
• [Fracture stress ratio after aging treatment] [fracture stress after aging treatment] / [TS after aging treatment]
• the fracture stress is the stress at the fracture point in the tensile test (the stress applied when the test piece fractures).
• Table 2 No. Steel sample ID Heating process Annealing process Bending process First cooling process Second cooling process Tempering process Straightening process Type* Remarks Average 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.
• Tempered M tempered martensite, retained ⁇ : retained austenite, F: ferrite, B: bainitic ferrite No. Steel sample ID Microstructure * Evaluation result Remarks Area fraction Large-angle grain boundary density of tempered M ( ⁇ d/ ⁇ m 2 ) KAM(S) /KAM(C) TS EL Toughness after paint baking Crash properties after paint baking Tempered M (%) Retained ⁇ (%) F + B (%) TS (MPa) Evaluation EL (%) Evaluation Brittle-ductile transition temp.
• Tempered M tempered martensite, retained ⁇ : retained austenite, F: ferrite, B: bainitic ferrite No. Steel sample ID Microstructure* Evaluation result Remarks Area fraction Large-angle grain boundary density of tempered M ( ⁇ m/ ⁇ m 2 ) KAM(S) /KAM(C) TS EL Toughness after paint baking Crash properties after paint baking Tempered M (%) Retained ⁇ (%) F + B (%) TS (MPa) Evaluation EL (%) Evaluation Brittle-ductile transition temp.
• TS, EL as erll as toughness and crash properties after paint baking, all reached passing criteria.
• members obtained by forming and members obtained by joining all had a target shape without crack occurrence, and TS, EL, as well as toughness and crash properties after paint baking, all reached passing criteria.

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