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
  • C22C38/04—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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
• • 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
  • 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
• • 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
  • 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")
• • 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
  • 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
• • 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
  • C22C38/08—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
  • C22C38/38—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
  • C21D2211/005—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
• • 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
  • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below

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 are also required to have excellent dimensional accuracy when formed into parts (hereinafter also referred to simply as dimensional accuracy).
• dimensional accuracy For example, in automotive frame parts such as bumpers, it is possible to suppress springback and improve dimensional accuracy by controlling the yield ratio (hereinafter also referred to as YR) of the steel to a certain range.
• YR yield ratio
• Patent Literature (PTL) 1 describes:
• [%S] and [%N] indicate S and N content, in mass%, in the steel, respectively.
• excellent delayed fracture resistance means that a time to delayed fracture when cold press forming involving shearing and punching is:
• the time to delayed fracture is the time from the start of immersion to the beginning of the formation of microcracks when immersed in hydrochloric acid (hydrogen chloride aqueous solution) with a pH of 1 at an aqueous solution temperature of 20 °C.”
• Automotive parts particularly automotive frame parts, have many end faces formed by shearing (hereinafter also referred to as sheared end faces). Therefore, steel sheets used as material for automotive parts are also required to have excellent delayed fracture resistance after shearing. Delayed fracture is a phenomenon that leads to failure as follows. When a part is subjected to high stress due to forming or the like and is placed in a hydrogen entry environment, hydrogen enters the part. Hydrogen that enters into a part causes a decrease in interatomic bonding strength and causes localized deformation. This causes microcracks to form in the part, which eventually leads to failure when the microcracks propagate.
• Delayed fracture resistance is affected by the morphology of sheared end faces. Further, the shape of a sheared end face is affected by a shear angle during shearing (hereinafter also referred to simply as the shear angle). That is, the delayed fracture resistance is affected by the shear angle. For example, even when parts are made from the same steel sheet, when the shear angle is outside an appropriate range, delayed fracture resistance will decrease.
• the shear angle is an angle between upper and lower blades used in shearing (blade angle).
• shear angle range the range of the shear angle at which excellent delayed fracture resistance of the steel sheet after shearing is obtainable
• TS is measured by a tensile test in accordance with JIS Z 2241:2022.
• Excellent dimensional accuracy means that the YR is 65 % or more and 85 % or less.
• YS is yield stress, which, like TS, is measured by a tensile test in accordance with JIS Z 2241:2022.
• Excellent shear angle range means that an appropriate range of the shear angle at which delayed fracture does not occur when load stress is 1000 MPa is 0° to 0.5° or more.
• a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range is obtainable. Further, the steel sheet of the present disclosure can be applied to a wider range of automotive part materials, which can further improve fuel efficiency by decreasing automotive body weight, thereby greatly contributing to decreasing CO 2 emissions. Therefore, the industrial utility value is extremely high.
• FIG. 1 is a schematic diagram for explaining the definition of prior y grain boundary occupancy rate.
• C is an important basic component of steel.
• C is an important element that affects the area fraction of tempered martensite.
• 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.
• the C content exceeds 0.500 %, tempered martensite becomes brittle, and achieving excellent shear angle range 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 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 shear angle range 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 shear angle range.
• 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.
• Mn content exceeds 5.00 %, tempered martensite becomes embrittled, and achieving excellent shear angle range 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 segregates at prior austenite grain boundaries, embrittling grain boundaries and becoming the initiation point of delayed fracture. Therefore, when P content is excessive, achieving an excellent shear angle range 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.
• 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 nitrides and becomes initiation points of delayed fracture. Therefore, when N content is excessive, achieving an excellent shear angle range 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 oxides and becomes initiation points of delayed fracture. Therefore, when O content is excessive, achieving an excellent shear angle range 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 oxides and becomes initiation points of delayed fracture. Therefore, when Al content is excessive, achieving an excellent shear angle range 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 become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. 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 become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. 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 become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. 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.
• 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 become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. 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 become initiation points of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. 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.
• Hf When Hf is 0.10 % or less, this element does not cause large amounts of coarse precipitates or inclusions to form or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Hf is included, the content is 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.
• Bi is 0.200 % or less
• this element does not cause large amounts of coarse precipitates or inclusions to form or become an initiation point of delayed fracture. Therefore, this does not lead to a decrease in shear angle range. Therefore, when Bi is included, the content is 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 shear angle range becomes difficult.
• One of the causes of decreased shear angle range is that retained austenite transforms into deformation-induced martensite during shearing, 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 %, achieving excellent dimensional accuracy becomes difficult. 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 1 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 (%)]
• Prior y grain boundary occupancy rate 20 % or more
• prior y grain boundary occupancy rate 20 % or more in order to realize an excellent shear angle range.
• Prior austenite grain hereinafter also referred to as prior y grain
• Prior austenite grain (hereinafter also referred to as prior y grain) boundaries become initiation points of delayed fracture.
• the prior y grain boundaries are occupied by soft ferrite and bainitic ferrite, and in particular that the prior y grain boundary occupancy rate is 20 % or more. This makes it possible to minimize the effect of the shear angle during shearing and suppress the occurrence of delayed fracture, thereby achieving an excellent shear angle range.
• the prior y grain boundary occupancy rate is therefore 20 % or more.
• the prior y grain boundary occupancy rate is preferably 22 % or more.
• the prior y grain boundary occupancy rate is more preferably 24 % or more. There is no particular upper limit to the prior y grain boundary occupancy rate.
• the prior y grain boundary occupancy rate may be 100 %.
• the prior y grain boundary occupancy rate is determined, for example, as follows (see FIG. 1 ).
• One prior y grain confirmed in an observation image in measurement of the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite is hereinafter also referred to as the prior y grain.
• the total circumferential length of the prior y grain (the circumference of the prior y grain, which is the sum of the solid line (prior austenite grain boundary not occupied by ferrite or bainitic ferrite) and the dotted line in FIG. 1 , hereinafter also referred to as L T ) is measured.
• the length of the interface between the prior y grain and the ferrite and bainitic ferrite in contact with the prior y grain (the sum of the prior y grain boundary length of the dotted lines in FIG.
• Prior y grain boundary occupancy rate of the prior y grain (L F /L T ) ⁇ 100
• This measurement is performed on 30 prior y grains in order from the closest to the prior y grain, and the average of the prior y grain boundary occupancy rates measured for each prior y grain is regarded as the prior y grain boundary occupancy rate of the steel sheet being measured.
• L T and L F are 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 1 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. From the obtained microstructure image, L T and L F are measured using an object function of Adobe Illustrator.
• 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 has a TS of 1180 MPa or more, and also has excellent dimensional accuracy and shear angle range. 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 maximum arrival 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 average heating rate in the temperature range of 700 °C to 750 °C affects the prior y grain boundary occupancy rate. That is, by setting the average heating rate to 5.0 °C/s or less, the dissolution of carbides is promoted. This refines the prior y grains, contributing to an increase in the prior y grain boundary occupancy rate. As a result, the shear angle range is improved. 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.
• the maximum arrival temperature T1 When the maximum arrival temperature T1 is less than 800 °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 maximum arrival temperature T1 exceeds 900 °C, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, making achieving excellent dimensional accuracy of parts difficult.
• the maximum arrival temperature T1 is therefore 800 °C or more and 900 °C or less.
• the maximum arrival temperature T1 is preferably 810 °C or more.
• the maximum arrival temperature T1 is preferably 890 °C or less.
• processing may immediately proceed to the cooling process described later, or the maximum arrival temperature T1 may be held for a certain period of time, for example, 1.0 s to 5.0 s, before proceeding to the cooling process.
• the blank sheet is then cooled under a set of conditions including an average cooling rate in a temperature range from the maximum arrival temperature T1 to the intermediate holding temperature T2 of 0.10 °C/s or more and 5.00 °C/s or less.
• first average cooling rate 0.10 °C/s or more and 5.00 °C/s or less
• the first average cooling rate is less than 0.10 °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 0.10 °C/s or more and 5.00 °C/s or less.
• the first average cooling rate is preferably 0.20 °C/s or more.
• the first average cooling rate is preferably 3.00 °C/s or less.
• the first cooling end temperature may be 600 °C or more and 750 °C or less.
• the first cooling end temperature may be the intermediate holding temperature T2.
• the blank sheet is then held under a set of conditions including the intermediate holding temperature T2 being 600 °C or more and 750 °C or less,
• the intermediate holding temperature T2 When the intermediate holding temperature T2 is less than 600 °C, transformation of ferrite and bainitic ferrite from places other than prior y grain boundaries may be promoted. Therefore, it becomes difficult to make the prior y grain boundary occupancy rate 20 % or more, and it also becomes difficult to realize an excellent shear angle range. On the other hand, when the intermediate holding temperature T2 exceeds 750 °C, the total area fraction of ferrite and bainitic ferrite becomes less than 5 %, making it difficult to achieve excellent dimensional accuracy of parts.
• the intermediate holding temperature T2 is therefore 600 °C or more and 750 °C or less.
• the intermediate holding temperature T2 is preferably 610 °C or more.
• the intermediate holding temperature T2 is preferably 740 °C or less.
• the intermediate holding temperature here refers to the holding temperature in the intermediate holding process.
• the intermediate holding temperature may be constant during holding. Further, the intermediate holding temperature is the temperature range of 600 °C or more and 750 °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 intermediate holding time t2 is less than 1.0 s (including a case where no intermediate holding is carried out), the prior y grain boundary occupancy rate becomes less than 20 %, making it impossible to realize an excellent shear angle range.
• the intermediate holding time t2 exceeds 2000.0 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 intermediate holding time t2 is therefore 1.0 s or longer and 2000.0 s or shorter.
• the intermediate holding time t2 is preferably 10.0 s or longer.
• the intermediate holding time t2 is preferably 1500.0 s or shorter.
• the intermediate holding time t2 is a holding time at the intermediate holding temperature T2.
• the inventors have found that applying tension to the blank sheet during intermediate holding affects the prior y grain boundary occupancy rate.
• a tension to the blank sheet hereinafter also referred to simply as applied tension
• 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, for example, preferably 100 MPa or less.
• a coating or plating treatment may be applied to the blank sheet between the intermediate holding process and the second cooling process described below. Details about the coating or plating treatment are described later.
• the blank sheet is cooled 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, to the second cooling end temperature.
• the second average cooling rate is less than 300 °C/s, the area fraction of retained austenite becomes 3 % or more, making it difficult to achieve an excellent shear angle range.
• 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.
• the second cooling end temperature may be, for example, less than 100 °C. Further, the second cooling end temperature may be, for example, around room temperature.
• the blank sheet is tempered under a set of conditions including the tempering temperature T3 being 100 °C or more and 400 °C or less, and the tempering time t3 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 T3 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 shear angle range cannot be obtained.
• the tempering temperature T3 exceeds 400 °C, tempering of martensite progresses excessively, and achieving a TS of 1180 MPa or more becomes difficult.
• the tempering temperature T3 is therefore 100 °C or more and 400 °C or less.
• the tempering temperature T3 is preferably 150 °C or more.
• the tempering temperature T3 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 t3 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 shear angle range cannot be obtained.
• the tempering time t3 exceeds 10,000 s, tempering of martensite progresses excessively, and achieving a TS of 1180 MPa or more becomes difficult. Accordingly, the tempering time t3 is 10 s or longer and 10,000 s or shorter.
• the tempering time t3 is preferably 50 s or longer.
• the tempering time t3 is preferably 5000 s or shorter.
• the tempering time t3 refers to the holding time at the tempering temperature T3.
• 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.
• the blank sheet may be worked under conditions that result in an equivalent plastic strain of 0.10 % or more and 5.00 % or less. Further, after the working, the blank sheet may be reheated to a temperature of 100 °C or more and 400 °C or less.
• the blank sheet may be subjected to a coating or plating treatment. Details about the coating or plating treatment are described below.
• the blank 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 intermediate holding process and the second cooling process, or after the tempering process.
• hot-dip galvanizing treatment or galvannealing treatment is preferably carried out between the intermediate holding process and the second cooling process.
• electrogalvanization treatment or Zn-Ni electro-alloy plating treatment is preferably carried out after the tempering process.
• the series of treatments including the heating 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.
• the blank sheet may be worked under conditions that result in an equivalent plastic strain of 0.10 % or more and 5.00 % or less. Further, after the working, the blank sheet (coated or plated steel sheet) may be reheated to a temperature of 100 °C or more and 400 °C or less.
• Conditions other than those described above are not particularly limited, and a conventional method may be used.
• a steel sheet is obtainable that has a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range.
• the obtained steel sheet may be suitably used as a material for automotive parts, for example.
• the steel sheet is typically cooled to room temperature before being traded.
• 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.
• 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 intermediate holding process and the second cooling process. For those listed as EG in Table 2, plating treatment was carried out after the tempering 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 YS were measured.
• the crosshead speed was set to 1.67 ⁇ 10 -1 mm/s.
• the TS was evaluated according to the following criteria:
• YR 100 ⁇ YS / TS
• the dimensional accuracy was evaluated according to the following criteria:
• the obtained steel sheets were each sheared into a size of 16 mm ⁇ 75 mm with the longitudinal direction perpendicular to the rolling direction to prepare test pieces.
• the clearance during shearing was set to 15 % in each case.
• the shear angle during shearing was changed in increments of 0.25° within a range of 0° to 2.0°.
• a four-point bending test was carried out in accordance with ASTM (G39-99), and a stress of 1000 MPa was applied to the tip of the bend test piece.
• the test piece was immersed in hydrochloric acid of pH 3 at 25 °C for 100 h. After immersion, each test piece was visually inspected for the presence or absence of cracks.
• the shear angle range was evaluated according to the following criteria.
• the appropriate range of shear angle versus delayed fracture was the range of shear angles at which no cracks were observed in the test pieces in the test described above. For example, when no cracks were observed in any of the test pieces prepared with shear angles of 0° to 0.75° during shearing, but cracks were observed in the test pieces prepared with a shear angle of 1.00° or more during shearing, then the appropriate range of shear angles versus delayed fracture was "0° to 0.75°", and the test piece was evaluated as "Good (pass, very good)".

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