Classifications

• • 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
  • C21D1/62—Quenching devices
  • C21D1/63—Quenching devices for bath 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
  • 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
  • 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
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
  • C21D9/573—Continuous furnaces for strip or wire with cooling
• • 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
• • 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
• • 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

Definitions

• the present invention relates to steel plates and components that have excellent tensile strength, yield ratio, stretch flangeability, stress corrosion cracking resistance, and material stability, as well as methods for manufacturing the same.
• the steel plates of the present invention can be suitably used as structural components for automobile parts, etc.
• High-strength steel sheets used in automobiles are required to have an excellent yield ratio, excellent stretch-flange formability, and excellent stress corrosion cracking resistance.
• steel sheets with excellent stretch-flange formability are preferred from the perspective of formability.
• excellent yield ratios and excellent stress corrosion cracking resistance are required.
• one issue with using high-strength steel sheets for components is that the increased strength of steel sheets results in increased springback, significantly reducing shape fixability during press forming. Therefore, in the field of press technology, to ensure shape fixability, it is common to predict the amount of shape change after demolding during press forming and design the press die shape accordingly.
• Patent Document 1 discloses a high-strength steel sheet of 1,180 MPa or more that has excellent yield ratio, flatness in the sheet width direction, and work embrittlement resistance, as well as a manufacturing method for the same.
• the technology described in Patent Document 1 does not take into consideration high-strength steel sheets that have excellent stretch flangeability, stress corrosion cracking resistance, and material stability.
• Patent Document 3 discloses a method for manufacturing high-strength steel sheet of 980 MPa or more, which has excellent ductility, stretch flangeability, and stable mechanical properties. However, the technology described in Patent Document 3 does not take into consideration high-strength steel sheet with excellent yield ratio and stress corrosion cracking resistance.
• the present invention was developed in light of these circumstances, and aims to provide steel plates and components with a tensile strength TS of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability, as well as methods for manufacturing the same.
• the tensile strength TS (hereinafter also simply referred to as TS) and the yield ratio YR (hereinafter also simply referred to as YR) can be measured in accordance with JIS Z 2241 (2022).
• Excellent stretch flangeability means that the limiting hole expansion ratio ⁇ (%) determined by a hole expansion test in accordance with JIS Z 2256 (2020) is 30% or more.
• Excellent stress corrosion cracking resistance means that a test specimen obtained from a steel plate is subjected to four-point bending in accordance with ASTM (G39-99), stresses equivalent to YS and TS are applied to the bent vertices of the test specimen, and the test specimen in the stressed state is immersed in 1 mass % sulfuric acid at 25°C for 100 hours, and the test specimen to which a stress equivalent to YS has been applied shows no cracks.
• the present inventors have conducted extensive research to achieve the above-mentioned object and have found the following. (1) By setting the amount of tempered martensite to 95% or more, a TS of 1320 MPa or more can be achieved. (2) By making the total amount of ferrite and bainitic ferrite less than 5%, excellent stretch flangeability can be achieved. (3) By making the amount of retained austenite less than 3%, a YR of 75% or more can be achieved.
• the present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows. [1] In mass%, C: 0.030% or more and 0.450% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; At the 1/4 position of the plate thickness, Area fraction of tempered martensite: 95% or more, Volume fraction of retained austenite: less than 3%; The total area fraction of ferrite and bainitic ferrite is less than 5%; The steel plate has a structure satisfying that the area fraction of the tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m or more and 1.0 ⁇ m or less within grains surrounded by grain boundaries at an angle of 15
• the component composition further comprises, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less,
• the steel sheet according to [1] above containing at least one element selected from the following: [3] The steel sheet according to [1] or [2], having a plating layer on the surface of the steel sheet.
• [4] A member made using the steel plate according to any one of [1] to [3] above.
• [5] A cold-rolled sheet produced by hot rolling, pickling and cold rolling a steel having the component composition according to [1] or [2], Annealing temperature T1: 800 ° C.
• Heating is performed under the condition that the holding time t1 at the annealing temperature T1 is 10 seconds or more, Cooling at an average cooling rate CR1 of 700 to 500 ° C.: 5 ° C./s or more; and cooling by water quenching at an average cooling rate CR2 of 300 ° C./s or more from a quenching start temperature T2 of (Ms - 80 ° C.) or more to 80 ° C., which is equal to or higher than Ms.
• Tempering temperature T3 100 ° C or more and less than 250 ° C
• An annealing process is performed at a tempering temperature T3 for a holding time t3 of 10 seconds or more and 10,000 seconds or less.
• the time t2 during which the steel sheet is held in a temperature range of 250°C or higher and Ms or lower is set to 1.0 seconds or higher and 10.0 seconds or lower
• a method for producing a steel sheet wherein, during cooling of the water quenching in the annealing step, pressure is applied from the front and back surfaces of the steel sheet with two rolls placed on either side of the steel sheet, and the pressure is applied under the conditions of a roll-to-roll distance of 20 mm or more and 250 mm or less in the steel sheet transport direction of the two rolls and a pressure of 196 N or more.
• a method for manufacturing a component comprising a step of subjecting the steel plate according to any one of [1] to [3] to at least one of forming and joining to form a component.
• the present invention it is possible to obtain steel sheets that have a TS of 1,320 MPa or more, a YR of 75% or more, and that are excellent in stretch flangeability, stress corrosion cracking resistance, and material stability. Furthermore, by applying the steel sheets of the present invention to, for example, automotive structural components, it is possible to reduce the weight of the vehicle body and thereby improve fuel efficiency. Therefore, the industrial value of this steel sheet is extremely great.
• FIG. 1 is a schematic diagram illustrating a method of applying pressure during water cooling in the method of manufacturing a steel sheet according to the present invention.
• the steel sheet of the present invention has a chemical composition containing C: 0.030% or more and 0.450% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities;
• the steel sheet has a structure that satisfies the following conditions at a 1/4 position in the sheet thickness: an area fraction of tempered martensite: 95% or more, a volume fraction of retained austenite: less than 3%, a total area fraction of ferrite and bainitic ferrite: less than 5%, and an area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 ⁇ m or more and 1.0 ⁇ m or less, within grains of tempered martensite surrounded by grain boundaries at an angle of
• C 0.030% or more and 0.450% or less C is one of the important basic components of steel, and in the present invention, it is an important element that affects the area fraction (hereinafter also referred to as fraction) of tempered martensite and stress corrosion cracking resistance. If the C content is less than 0.030%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1320 MPa or more. On the other hand, if the C content exceeds 0.450%, the tempered martensite becomes embrittled, making it difficult to obtain excellent stress corrosion cracking resistance. Therefore, the C content is set to 0.030% or more and 0.450% or less. The C content is preferably set to 0.050% or more. The C content is more preferably set to 0.100% or more. The C content is preferably set to 0.400% or less. The C content is more preferably set to 0.350% or less.
• Si 0.010% or more and 2.500% or less Si is one of the important basic components of steel, and in the present invention, in particular, it is an important element that affects TS and the amount of retained austenite because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. If the Si content is less than 0.010%, it becomes difficult to achieve a TS of 1320 MPa or more. On the other hand, if the Si content exceeds 2.500%, the amount of retained austenite increases excessively, making it difficult to achieve YR ⁇ 75%. Therefore, the Si content is set to 0.010% or more and 2.500% or less. The Si content is preferably set to 0.050% or more.
• Mn 0.10% or more and 5.00% or less
• Mn is one of the important basic components of steel, and in the present invention, it is an important element that affects the fraction of tempered martensite and stress corrosion cracking resistance. If the Mn content is less than 0.10%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1,320 MPa or more. On the other hand, if the Mn content exceeds 5.00%, corrosion of the grain boundaries of the steel sheet is promoted, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the Mn content is set to 0.10% or more and 5.00% or less.
• the Mn content is preferably set to 0.50% or more.
• the Mn content is more preferably set to 0.80% or more.
• the Mn content is preferably set to 4.50% or less.
• the Mn content is more preferably set to 4.00% or less.
• the P content must be 0.100% or less.
• the P content is preferably 0.070% or less.
• the P content is more preferably 0.050% or less, and even more preferably 0.020% or less.
• the P content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.
• S 0.0200% or less S exists as sulfide and becomes the initiation point of stress corrosion cracking. Therefore, if the content exceeds 0.0200%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the S content must be 0.0200% or less.
• the S content is preferably 0.0050% or less.
• the S content is more preferably 0.0030% or less, and even more preferably 0.0020% or less. Although there is no particular lower limit for the S content, due to constraints on production technology, the S content is preferably 0.0001% or more, and more preferably 0.0002% or more.
• Al 1.000% or less Since Al exists as an oxide and serves as the initiation point for stress corrosion cracking, if the Al content exceeds 1.000%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the Al content must be 1.000% or less.
• the Al content is preferably 0.500% or less.
• the Al content is more preferably 0.200% or less, and even more preferably 0.100% or less.
• the lower limit of the Al content is not particularly specified, it is preferably 0.001% or more due to constraints on production technology.
• the Al content is more preferably 0.002% or more.
• the Al content is more preferably 0.005% or more, and even more preferably 0.010% or more.
• N 0.0100% or less N exists as nitrides and can be the starting point for stress corrosion cracking. Therefore, if the N content exceeds 0.0100%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the N content must be 0.0100% or less.
• the N content is preferably 0.0050% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more, more preferably 0.0002% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.
• O 0.0100% or less
• O exists as an oxide and can be the starting point for stress corrosion cracking. Therefore, if the O content exceeds 0.0100%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the O content must be 0.0100% or less.
• the O content is preferably 0.0050% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more.
• a steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities.
• the balance consists of Fe and unavoidable impurities.
• unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs, and the total content of these impurities is permitted to be 0.100% or less.
• the steel sheet of the present invention further contains, by mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, and Cu: 1.00% or less.
• It may contain at least one element selected from Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination.
• Ti 0.200% or less, Nb: 0.200% or less, V: 0.200% or less If Ti, Nb, and V are each 0.200% or less, large amounts of coarse precipitates and inclusions are not formed, and they do not become the starting point of stress corrosion cracking, so stress corrosion cracking resistance is not reduced. Therefore, when at least one of Ti, Nb, and V is contained, the contents of Ti, Nb, and V are each 0.200% or less. Preferably, each of these contents is 0.100% or less.
• the contents of Ti, Nb, and V are each preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more, because they form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet.
• Ta 0.10% or less
• W 0.10% or less
• the contents of Ta and W are each 0.10% or less.
• each of these contents is 0.08% or less.
• the Ta and W contents are preferably 0.01% or more, respectively, because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing.
• the content of each of these elements is more preferably 0.02% or more, and further preferably 0.03% or more.
• B 0.0100% or less If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and stress corrosion cracking resistance will not deteriorate. Therefore, when B is contained, the B content is set to 0.0100% or less.
• the B content is preferably set to 0.0080% or less. Although there is no particular lower limit for the B content, since B is an element that segregates at austenite grain boundaries during annealing and improves hardenability, the B content is preferably 0.0003% or more.
• Cr 1.00% or less
• Mo 1.00% or less
• Ni 1.00% or less
• Cr, Mo, and Ni contents are each 1.00% or less.
• each of these contents is 0.90% or less.
• Each of these contents is preferably 0.80% or less.
• the Cr, Mo, and Ni contents are each preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.
• Co 0.010% or less If Co is 0.010% or less, coarse precipitates and inclusions do not increase and do not become the starting point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, when Co is contained, the Co content is preferably 0.010% or less. The Co content is preferably 0.008% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, the Co content is preferably 0.001% or more, and more preferably 0.002% or more.
• Cu 1.00% or less If Cu is 1.00% or less, coarse precipitates and inclusions do not increase and do not become the starting point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, when Cu is contained, the Cu content is set to 1.00% or less.
• the Cu content is preferably set to 0.80% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is preferably 0.01% or more.
• Sn 0.200% or less If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the Sn content will not become the starting point for stress corrosion cracking, so stress corrosion cracking resistance will not be reduced. Therefore, if Sn is contained, the Sn content is set to 0.200% or less. The Sn content is preferably set to 0.190% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.
• Sb 0.200% or less If Sb is 0.200% or less, coarse precipitates and inclusions do not increase and do not become the starting point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, if Sb is contained, the Sb content is set to 0.200% or less. The Sb content is preferably set to 0.190% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is more preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more.
• Ca 0.0100% or less
• Mg 0.0100% or less
• REM 0.0100% or less
• the contents of Ca, Mg, and REM are set to 0.0100% or less.
• each of these contents is set to 0.0070% or less. More preferably, each of these contents is set to 0.0050% or less.
• the contents of Ca, Mg, and REM are each preferably 0.0005% or more, more preferably 0.0006% or more, and even more preferably 0.0007% or more.
• the Zr and Te contents are preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.
• Hf 0.10% or less If the Hf content is 0.10% or less, coarse precipitates and inclusions do not increase and do not become the starting point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, if Hf is contained, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.08% or less. Although the lower limit of the Hf content is not particularly specified, the Hf content is preferably 0.001% or more, and more preferably 0.010% or more, because Hf is an element that makes the shapes of nitrides and sulfides spherical and reduces the number of sites that become the starting points of stress corrosion cracking.
• Bi 0.200% or less If Bi is 0.200% or less, coarse precipitates and inclusions do not increase and do not become the starting point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, if Bi is contained, the Bi content is set to 0.200% or less. The Bi content is preferably set to 0.100% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.
• Tempered martensite area fraction of 95% or more This is one of the important constituent elements of the present invention.
• the area fraction of tempered martensite must be 95% or more. Therefore, the area fraction of tempered martensite is 95% or more.
• the area fraction of tempered martensite is preferably 96% or more.
• the area fraction of tempered martensite is more preferably 97% or more. There is no particular upper limit to the area fraction of tempered martensite, but it may be 100%.
• the area fraction of tempered martensite is measured as follows: After polishing the L-section of a steel sheet, it is corroded with 1 vol. % nital, and a portion of the sheet that is 1/4 of the sheet thickness (a position corresponding to 1/4 of the sheet thickness in the depth direction from the surface of the steel sheet) is observed using an SEM at a magnification of 2000 times and a field of view of 30 ⁇ m ⁇ 30 ⁇ m in 10 fields of view.
• the tempered martensite has fine irregularities inside and contains carbides.
• the area fraction of the tempered martensite can be calculated from the average value of these values.
• Retained austenite volume fraction less than 3% This is one of the important constituent elements of the present invention. If the volume fraction of retained austenite is 3% or more, it becomes difficult to achieve YR ⁇ 75%. The reason why it becomes difficult to achieve YR ⁇ 75% is that the retained austenite transforms into martensite during the tensile test, resulting in a decrease in YS. Therefore, the retained austenite is set to less than 3%. Preferably, the volume fraction of retained austenite is set to 1% or less. There is no particular lower limit for the retained austenite. The volume fraction of retained austenite may be 0%.
• the volume fraction of retained austenite is measured as follows: After polishing the steel plate to 1/4 of the plate thickness, the surface is further polished by 0.1 mm using chemical polishing.
• the retained austenite volume fraction is determined by measuring the integrated intensity ratios of the diffraction peaks of the ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ planes of fcc iron and the ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ planes of bcc iron using CoK ⁇ radiation in an X-ray diffractometer, and then averaging the nine integrated intensity ratios obtained.
• Total area fraction of ferrite and bainitic ferrite less than 5% This is one of the important constituent elements of the present invention. If the total area fraction of ferrite and bainitic ferrite is 5% or more, it becomes difficult to achieve excellent stretch flangeability. Therefore, the total area fraction of ferrite and bainitic ferrite is less than 5%. This total area fraction is preferably 3% or less. This total area fraction is more preferably 2% or less. There is no particular lower limit for the total area fraction of ferrite and bainitic ferrite. The total area fraction of ferrite and bainitic ferrite may even be 0%.
• the method for measuring the total area fraction of ferrite and bainitic ferrite is as follows: After polishing the L-section of the steel plate, it is etched with 1 vol. % nital, and a portion of 1/4 of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed using an SEM at 2000x magnification with a field of view of 30 ⁇ m x 30 ⁇ m, using 10 fields of view. Note that in the above structural image, the ferrite and bainitic ferrite are recessed and the interior of the structure is flat, and does not contain carbides. The total area fraction of ferrite and bainitic ferrite can be calculated from the average of these values.
• the total area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m is calculated.
• the total area fraction of all tempered martensite is also calculated.
• the total area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 ⁇ m or more and 1.0 ⁇ m or less within the grains is divided by the total area fraction of all tempered martensite to calculate the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 ⁇ m or more and 1.0 ⁇ m or less.
• the time t2 during which the steel sheet is held in a temperature range of 250°C or higher and Ms or lower is set to 1.0 seconds or higher and 10.0 seconds or lower.
• pressure is applied to the front and back surfaces of the steel sheet using two rolls placed on either side of the steel sheet, and the pressure is applied under the conditions of a distance between the two rolls in the steel sheet conveying direction of 20 mm or more and 250 mm or less, and a pressure of 196 N or more.
• Holding time t1 at annealing temperature T1 10 seconds or more If the holding time t1 at annealing temperature T1 is less than 10 seconds, the total area fraction of ferrite and bainitic ferrite will be 5% or more, making it difficult to achieve a TS of 1320 MPa or more and difficult to achieve excellent stretch flangeability. Therefore, the holding time t1 at annealing temperature T1 is set to 10 seconds or more.
• the holding time t1 at annealing temperature T1 is preferably 30 seconds or more. There is no particular need to limit the upper limit, but the holding time t1 at annealing temperature T1 is preferably 1000 seconds or less.
• Average cooling rate CR1 from 700 to 500 ° C 5 ° C/s or more If the average cooling rate CR1 from 700 to 500 ° C is less than 5 ° C/s, the total area fraction of ferrite and bainitic ferrite will be 5% or more, making it difficult to achieve a TS of 1320 MPa or more and achieving excellent stretch flangeability. Therefore, the average cooling rate CR1 from 700 to 500 ° C is set to 5 ° C/s or more.
• the average cooling rate CR1 is preferably 10 ° C/s or more. There is no particular need to limit the upper limit, but the average cooling rate CR1 is preferably 50 ° C/s or less.
• the average cooling rate CR1 is calculated by (cooling start temperature (700° C.) ⁇ cooling stop temperature (500° C.))/cooling time (s) from the cooling start temperature (700° C.) to the cooling stop temperature.
• Specific examples of cooling at the average cooling rate CR1 include water cooling and mist cooling.
• Quenching start temperature T2 (Ms - 80°C) or higher and Ms or lower This is one of the important constituent elements of the present invention.
• the quenching start temperature T2 By setting the quenching start temperature T2 to (Ms - 80°C) or higher and Ms or lower, it is possible to obtain a structure in which the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m in grains surrounded by grain boundaries at an angle of 15° or more is 5% to 50%.
• the quenching start temperature T2 is lower than (Ms - 80°C)
• the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m in grain size exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance.
• the quenching start temperature T2 exceeds Ms
• the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m becomes less than 5%, making it difficult to achieve excellent material stability. Therefore, the quenching start temperature T2 is set to (Ms - 80°C) or more and Ms or less.
• the quenching start temperature T2 is preferably (Ms - 40°C) or more.
• the quenching start temperature T2 is preferably (Ms - 10°C) or less.
• Ms is the martensitic transformation start temperature (°C)
• Average cooling rate CR2 from quenching start temperature T2 to 80°C 300°C/s or more If the average cooling rate CR2 from quenching start temperature T2 to 80°C is less than 300°C/s, the volume fraction of retained austenite will be 3% or more, making it difficult to achieve a YR of 75% or more. Therefore, the average cooling rate CR2 from quenching start temperature T2 to 80°C is set to 300°C/s or more.
• the average cooling rate CR2 is preferably 800°C/s or more. There is no particular need to limit the upper limit, but the average cooling rate CR2 is preferably 3000°C/s or less.
• the time t2 during which the steel sheet is held in the temperature range of 250°C to Ms is 1.0 second to 10.0 seconds. This is one of the important constituent features of the present invention.
• the time during which the steel sheet is held in the temperature range of 250°C to Ms in the cooling treatment is 1.0 second to 10.0 seconds.
• the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m inclusive in grains surrounded by grain boundaries at an angle of 15° or more exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance.
• the cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for less than 1.0 second, the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m within grains surrounded by grain boundaries at an angle of 15° or more will be less than 5%, making it difficult to achieve excellent material stability. Therefore, the cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for 1.0 second to 10.0 seconds. The cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for preferably 1.5 seconds or more. The cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for preferably 5.0 seconds or less.
• Tempering temperature T3 100°C or higher but lower than 250°C This is one of the important constituent elements of the present invention.
• tempered martensite refers to a structure in which martensite at 80°C or lower is subjected to a heat treatment at a tempering temperature of 100°C or higher for a holding time of 10 seconds or longer. Therefore, if the tempering temperature T3 is lower than 100°C, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, which deteriorates the stress corrosion cracking resistance of the as-quenched martensite.
• the tempering temperature T3 is 250°C or higher, the tempering of martensite proceeds excessively, and the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m inclusive in grains surrounded by grain boundaries at an angle of 15° or more exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the tempering temperature T3 is set to 100°C or higher but lower than 250°C.
• the tempering temperature T3 is preferably set to 150°C or higher.
• the tempering temperature T3 is preferably set to 220°C or lower.
• tempered martensite refers to a structure in which martensite at 80°C or less is subjected to heat treatment at a tempering temperature of 100°C or more for a holding time of 10 seconds or more. Therefore, if the holding time t3 at tempering temperature T3 is less than 10 seconds, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, which deteriorates the stress corrosion cracking resistance of the as-quenched martensite.
• the holding time t3 at the tempering temperature T3 is set to 10 seconds or more and 10,000 seconds or less.
• the holding time t3 at the tempering temperature T3 is preferably set to 50 seconds or more.
• the holding time t3 at the tempering temperature T3 is preferably set to 5,000 seconds or less.
• cooling after tempering there is no particular requirement for cooling after tempering, and any method may be used to cool to the desired temperature. It is desirable for the desired temperature to be around room temperature.
• the steel sheet may be subjected to a plating process.
• the plating process is not particularly limited.
• Examples of plating processes include galvanizing processes such as hot-dip galvanizing, galvannealed hot-dip galvanizing, and electrogalvanizing.
• Examples of plating processes other than galvanizing include aluminum plating and alloy plating.
• Examples of alloy plating processes include hot-dip zinc-aluminum-magnesium alloy plating and Zn-Ni electroalloy plating. Conventional treatment conditions may be used for all of these processes.
• the plating process is preferably performed during cooling from the annealing temperature T1 to the quenching start temperature T2 or after the tempering process.
• Forming can be performed using common processing methods such as press working without any restrictions.
• joining can be performed using common welding methods such as spot welding and arc welding, as well as rivet joining and crimping without any restrictions.
• the steel sheets (high-strength cold-rolled steel sheets) obtained in this manner were used as test steels to evaluate tensile properties, stretch flangeability, stress corrosion cracking resistance, and material stability according to the following test methods.
• the amount of tempered martensite (area fraction), the amount of retained austenite (volume fraction), the total amount of ferrite and bainitic ferrite (area fraction), and the fraction (area fraction) of tempered martensite containing five or more carbides with a particle size of 0.1 ⁇ m or more and 1.0 ⁇ m or less were determined.
• the hole expansion test was performed in accordance with JIS Z 2256 (2020). After shearing the obtained steel plate to 100 mm x 100 mm, a hole having a diameter of 10 mm was punched with a clearance of 12.5%, and then a conical punch having an apex angle of 60 ° was pressed into the hole while holding it down with a blank holding force of 9 ton (88.26 kN) using a die having an inner diameter of 75 mm. The hole diameter at the crack initiation limit was measured, and the limit hole expansion ratio: ⁇ (%) was calculated from the following formula, and the hole expandability was evaluated from the value of this limit hole expansion ratio.
• the stretch flangeability was determined to be good when the hole expanding ratio ( ⁇ ), which is an index of stretch flangeability, was 30% or more, regardless of the strength of the steel sheet.
• the obtained steel plate was sheared to a size of 20 mm x 75 mm with the direction perpendicular to the rolling direction as the longitudinal direction, and 2 mm was removed from both sides of the longitudinal end face by mechanical grinding to process into 16 mm x 75 mm test specimens.
• the obtained test specimens were subjected to four-point bending in accordance with ASTM (G39-99), and stresses equivalent to YS and TS were applied to the bend vertices of the test specimens.
• the test specimens in the stressed state were immersed in 1 mass% sulfuric acid at 25 °C for 100 hours. After the test, the presence or absence of cracks was confirmed for each test specimen by visual inspection.
• Table 3 (Table 3-1, Table 3-2) had a tensile strength TS of 1,320 MPa or more, a yield ratio YR of 75% or more, and were excellent in stretch flangeability, stress corrosion cracking resistance, and material stability, while the comparative examples were inferior in at least one of these characteristics.
• the components obtained by forming and joining the steel plates of the present invention have a tensile strength TS of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability, similar to the steel plates of the present invention, and have a tensile strength of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability.

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