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
  • 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
• • 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.
• steel sheets and high-carbon steel sheets are used as materials for structural and mechanical parts in various machines and devices, such as automobiles. These steel sheets are processed into a predetermined shape and then subjected to heat treatments such as quenching and tempering to become various parts.
• Patent Document 1 proposes a steel sheet having a steel structure in which, in area ratio, martensite is 20% to 100%, ferrite is 0% to 80%, and other metallic phases are 5% or less, and the ratio of the dislocation density of the metallic phase at the surface of the steel sheet to the dislocation density of the metallic phase at the center of the sheet thickness is 30% to 80%, and the maximum warpage of the steel sheet when sheared at a length of 1 m in the rolling direction is 15 mm or less.
• the steel plate in Patent Document 1 improves the uniformity of the steel plate shape by constraining the steel plate from the front and back sides to meet specified conditions using two rolls placed on either side of the steel plate during water quenching when the surface temperature of the steel plate is below (Ms point + 150°C), thereby ensuring the flatness of the steel plate itself.
• the steel plate in Patent Document 1 is hardened because it is primarily composed of martensite, it cannot be said to be at a level where heat treatment after processing can be omitted. Therefore, in order to ensure the strength required for a part, heat treatment must be performed after processing to increase the strength.
• the present invention was made to solve the above-mentioned problems, and aims to provide a steel sheet that has high hardness, can eliminate heat treatment after processing, and has excellent flatness, fatigue resistance, and toughness.
• the present invention provides a steel sheet having a composition containing, on a mass basis, C: 0.07 to 0.30%, Si: 0.01 to 0.65%, Mn: 0.80 to 2.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 1.00% or less, and N: 0.0150% or less, with the balance being Fe and impurities;
• the area ratio of ferrite is 0 to 10.0%, and the area ratio of martensite is 90.0 to 100%,
• the average grain size of the ferrite is 15.0 ⁇ m or less,
• the GAM value is 1.0 to 10.0°,
• the number density of crystal grains having a crystal grain size of 20.0 ⁇ m or more is 500 grains/mm 2 or less, When the plate width is 400 mm or less, the plate thickness tolerance is ⁇ 0.08 mm or less,
• the present invention relates to a steel sheet having a maximum warpage in the rolling direction of 10 mm or less when the length in the rolling
• the present invention makes it possible to provide steel sheets that are high in hardness, can eliminate post-processing heat treatment, and have excellent flatness, fatigue resistance, and toughness.
• a steel sheet according to an embodiment of the present invention has a composition containing C: 0.07 to 0.30%, Si: 0.01 to 0.65%, Mn: 0.80 to 2.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 1.00% or less, and N: 0.0150% or less, with the balance being Fe and impurities.
• the term "steel plate” refers to a plate-shaped (including strip-shaped) material made of steel.
• impurities refers to components that are mixed in during industrial production of steel sheets due to raw materials such as ores and scraps, or various factors in the manufacturing process, and are acceptable to the extent that they do not adversely affect the present invention.
• impurities include unavoidable impurities.
• impurities include Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca.
• xx% or less means that it is xx% or less, but includes an amount exceeding 0% (particularly, exceeding the impurity level).
• a numerical range expressed using "to” means a range that includes the numerical values before and after "to" as the lower and upper limits.
• the steel sheet according to the embodiment of the present invention may further contain one or more elements selected from the group consisting of Ni: 1.000% or less, Mo: 0.700% or less, V: 0.500% or less, Nb: 0.500% or less, Ti: 0.150% or less, and B: 0.0100% or less.
• C 0.07-0.30%
• C is an element necessary for increasing the strength of the steel sheet and improving its fatigue resistance and punchability.
• the C content is set to 0.07% or more, preferably 0.08% or more, and more preferably 0.09% or more.
• the C content is set to 0.30% or less, preferably 0.29% or less, and more preferably 0.28% or less.
• the above-mentioned numerical ranges of the C content may be any combination of the numerical ranges.
• the C content may be 0.07 to 0.30%, 0.08 to 0.30%, 0.09 to 0.30%, 0.07 to 0.29%, 0.08 to 0.29%, 0.09 to 0.29%, 0.07 to 0.28%, 0.08 to 0.28%, or 0.09 to 0.28%.
• Si 0.01-0.65%
• Si is an element necessary for deoxidation.
• the Si content is set to 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.
• the Si content is set to 0.65% or less, preferably 0.60% or less, and more preferably 0.58% or less.
• Mn is an element that affects the strength, punchability, and other properties of steel sheets. If the Mn content is too low, ferrite is more likely to form, reducing the strength of the steel sheet and making it impossible to ensure fatigue resistance and punchability. Therefore, the Mn content is set to 0.80% or more, preferably 0.90% or more, and more preferably 1.00% or more. On the other hand, if the Mn content is too high, the steel sheet becomes hard and its toughness decreases. Therefore, the Mn content is set to 2.30% or less, preferably 2.25% or less, and more preferably 2.20% or less.
• the P content is set to 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.
• the P content can be set to, for example, 0.001% or more.
• S forms MnS, which easily becomes the starting point of fracture and reduces the workability of the steel sheet. Therefore, the S content is set to 0.100% or less, preferably 0.095% or less, and more preferably 0.090% or less. On the other hand, the lower the S content, the better, so there is no particular lower limit. However, since excessive reduction of the S content leads to increased costs, the S content can be set to, for example, 0.001% or more.
• Al 0.100% or less
• Al is an element used for deoxidation.
• the Al content is set to 0.100% or less, preferably 0.098% or less, and more preferably 0.095% or less.
• the Al content may be low, and the lower limit is not particularly limited.
• the Al content can be set to, for example, 0.001% or more, 0.003% or more, or 0.005% or more.
• Cr 1.00% or less
• Cr is an element effective for forming a predetermined metal structure.
• the Cr content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less.
• the Cr content may be low, and the lower limit is not particularly limited.
• the Cr content can be, for example, 0.01% or more, 0.05% or more, 0.10% or more, 0.20% or more, or 0.30% or more.
• N is an element that forms AlN and suppresses grain coarsening through a pinning effect.
• the N content is set to 0.0150% or less, preferably 0.0140% or less, and preferably 0.0130% or less.
• the N content may be low, and there is no particular lower limit.
• the N content can be set to, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more.
• Ni is an element that dissolves in steel to improve strength without impairing toughness.
• Ni is an expensive element, and excessive Ni content increases costs. Therefore, the Ni content is set to 1.000% or less, preferably 0.950% or less, and more preferably 0.900% or less.
• the Ni content may be low, and the lower limit is not particularly limited. However, from the viewpoint of obtaining the above-mentioned effects of Ni, the Ni content can be set to, for example, 0.001% or more, 0.010% or more, or 0.050% or more.
• Mo 0.700% or less
• Mo is an element that improves the strength of steel sheet.
• the Mo content is set to 0.700% or less, preferably 0.650% or less, and more preferably 0.600% or less.
• the Mo content may be low, and the lower limit is not particularly limited.
• the Mo content can be set to, for example, 0.001% or more, 0.005% or more, or 0.010% or more.
• V, Nb, and Ti are all elements that improve the strength of steel sheets by carbide precipitation. However, if the contents of these elements are too high, a large amount of carbides are formed, resulting in a decrease in the toughness of the steel sheets. Therefore, the V content is set to 0.500% or less, preferably 0.480% or less, and more preferably 0.460% or less; the Nb content is set to 0.500% or less, preferably 0.480% or less, and more preferably 0.460% or less; and the Ti content is set to 0.150% or less, preferably 0.148% or less, and more preferably 0.145% or less.
• the contents of these elements may be small, and there is no particular lower limit.
• the contents of V, Nb, and Ti may all be set to, for example, 0.001% or more, 0.003% or more, or 0.005% or more.
• B (B: 0.0100% or less) B is an element that segregates at grain boundaries and improves grain boundary strength. However, if the B content is too high, the effect saturates and raw material costs increase. Therefore, the B content is set to 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less. On the other hand, the B content may be low, and the lower limit is not particularly limited. However, from the viewpoint of obtaining the above-mentioned effect of B, the B content can be set to, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more.
• Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca are impurities and may not be contained in the steel sheet. These elements may be contained as impurities either alone or in combination of two or more.
• the content of each of Cu, W, and Ta is 0 to 0.15%, preferably 0 to 0.14%.
• the contents of Sn, Sb, Co, As, Mg, Y, Zr, La, Ce and Ca are all 0 to 0.050%, preferably 0 to 0.045%.
• the area ratio of ferrite is 0 to 10.0%
• the area ratio of martensite is 90.0 to 100%
• the average grain size of ferrite is 15.0 ⁇ m or less
• the GAM value is 1.0 to 10.0°
• the number density of grains having a grain size of 20.0 ⁇ m or more is 500 grains/mm 2 or less.
• the steel sheet according to the embodiment of the present invention has a metallographic structure mainly composed of martensite, which hardens the steel sheet and improves its fatigue resistance and punchability. From the viewpoint of achieving this effect, the area fraction of martensite is set to 90.0% or more, preferably 91.0% or more. On the other hand, the upper limit of the area fraction of martensite is 100% (i.e., the steel sheet may have a martensite single-phase structure).
• the metal structure of the steel sheet according to the embodiment of the present invention may contain ferrite.
• the area fraction of ferrite is 10.0% or less, preferably 9.0% or less.
• the lower limit of the area fraction of ferrite is 0%.
• the metal structure of the steel plate according to the embodiment of the present invention may contain phases other than martensite and ferrite (for example, pearlite, bainite, etc.) to the extent that the effects of martensite described above are not impaired.
• the area ratios of martensite and ferrite are determined as follows. First, a cross section (L cross section) parallel to the rolling direction of a test specimen cut from a steel plate is polished, and then immersed in a 3% nital etchant to reveal the structure. In this cross section, the position corresponding to 1 ⁇ 4 of the thickness of the steel plate is set as the center of the field of view, and the structure is observed at any five locations using a scanning electron microscope (SEM). The magnification is set to 500 to 3000 times depending on the size of the crystal grains.
• SEM scanning electron microscope
• martensite white structure
• ferrite black structure
• the area ratios of martensite and ferrite are taken as the average values of the measurement results at five locations.
• the area ratios of martensite and ferrite may be determined by first determining the area ratio of one of the martensite and ferrite (e.g., the area ratio of ferrite), and then using the remaining area ratio as the area ratio of the other (e.g., the area ratio of martensite).
• the average grain size of ferrite is 15.0 ⁇ m or less, preferably 13.0 ⁇ m or less, and more preferably 11.0 ⁇ m or less.
• the steel sheet according to the embodiment of the present invention may be composed of a martensite single phase structure, in which case ferrite does not exist and the average grain size of ferrite can be considered to be 0 ⁇ m.
• the average grain size of ferrite is determined in accordance with JIS G0551:2020. Specifically, a cross section (L cross section) parallel to the rolling direction of a test piece cut from a steel sheet is polished, and then immersed in a 3% nital etchant to reveal the structure. In this cross section, the position corresponding to 1 ⁇ 4 of the thickness of the steel sheet is set as the center of the field of view, and the structure is observed at any five points using a scanning electron microscope (SEM). The magnification is set to 500 to 3000 times depending on the size of the crystal grains. The average crystal grain size is determined from the obtained microstructure photograph using a cutting method. The average crystal grain size is the average of the measurement results from the five points.
• SEM scanning electron microscope
• the GAM (Grain Average Misorientation) value is a value obtained from crystal orientation analysis data by electron backscattering diffraction (EBSD), and is obtained by measuring within a crystal grain separated by a large-angle grain boundary having an orientation misorientation of 15° or more, setting the distance between measurement points (hereinafter also referred to as the "step size") to 0.2 ⁇ m, calculating the orientation misorientation for each adjacent measurement point, and averaging the calculated orientation misorientation within the same crystal grain.
• EBSD electron backscattering diffraction
• the GAM value is related to the uniform elongation and toughness of the steel sheet, and by adjusting the GAM value to 1.0 to 10.0°, the uniform elongation and toughness of the steel sheet can be improved.
• the GAM value and the number density of crystal grains having a grain size of 20.0 ⁇ m or more can be determined as follows. First, a cross section (L cross section) parallel to the rolling direction of a test piece cut from a steel sheet is polished, and then electron backscattering diffraction (EBSD) analysis is performed at a 0.2 ⁇ m pitch over three or more fields of view, with a range of 200 ⁇ m in the thickness direction and 200 ⁇ m in the longitudinal direction, with the center of the field of view being a position corresponding to 1/4 of the thickness of the steel sheet. The grain size of the crystal grains is analyzed using the analysis software OIM Analysis (manufactured by TSL Solutions Co., Ltd.) based on the EBSD data.
• OIM Analysis manufactured by TSL Solutions Co., Ltd.
• the grain size is determined by defining the boundary with an orientation difference of 15° or more from adjacent measurement points as the grain boundary, and the crystal grain is calculated using the same software.
• the GAM value is the average value of each crystal grain size within the measurement range.
• the number density of crystal grains having a grain size of 20.0 ⁇ m or more is calculated by identifying the number of crystal grains having a grain size of 20.0 ⁇ m or more and dividing the number by the area ( mm2 ) of the measurement region.
• the number density of crystal grains is the average of the measurement results at five locations.
• the thickness tolerance is ⁇ 0.08 mm or less when the sheet width is 400 mm or less, and the maximum warpage in the rolling direction is 10 mm or less when the length in the rolling direction is 1 m. With such thickness tolerance and maximum warpage, it can be said that the steel sheet shape is uniform. If the thickness tolerance or maximum warpage is outside the above ranges, the workability (e.g., punchability) may be reduced.
• the thickness tolerance and maximum warpage of the steel plate are determined in accordance with JIS G3141:2021.
• the steel sheet according to the embodiment of the present invention preferably has a Vickers hardness of 300 to 550 Hv, more preferably 310 to 540 Hv, and even more preferably 315 to 530 Hv.
• a Vickers hardness within this range can be said to ensure workability (uniform elongation and toughness) while being hardened. Therefore, heat treatment after working the steel sheet can be omitted.
• the Vickers hardness is determined by polishing a cross section (L cross section) parallel to the rolling direction of a test piece cut out from a steel sheet, and then performing a Vickers hardness test at a position corresponding to 1 ⁇ 4 of the thickness of the steel sheet in accordance with JIS Z2244: 2009. In the Vickers hardness test, a measurement load of 1 kgf (9.807 N) is used, and measurements are taken at three arbitrary locations, with the average value being the measurement result.
• the steel sheet according to the embodiment of the present invention preferably has a uniform elongation of 1.5% or more.
• a uniform elongation in this range can be said to have good workability.
• the uniform elongation is determined by preparing a 13B test piece in accordance with JIS Z2241:2023 and conducting a tensile test with the tensile axis in the rolling direction of the steel sheet. Three 13B test pieces are taken from the steel sheet, and measurements are performed on these three 13B test pieces, with the average value being the measurement result.
• the steel sheet according to the embodiment of the present invention may be either a hot-rolled steel sheet or a cold-rolled steel sheet, but is preferably a cold-rolled steel sheet.
• the thickness of the steel sheet is not particularly limited, but can be, for example, 10.0 mm or less, 8.0 mm or less, or 6.0 mm or less.
• the method for manufacturing a steel sheet according to an embodiment of the present invention is not particularly limited as long as it can manufacture a steel sheet having the above-described characteristics.
• the steel sheet according to the embodiment of the present invention can be produced by a method including a hot rolling step in which a slab having the composition described above is hot rolled, and then cooled in a temperature range of 700 to 100°C under conditions such that the ratio of the thickness to the cooling rate (thickness/cooling rate) is 0.005 mm ⁇ sec/°C or more when coiled into a coil, and a cold rolling step in which the hot-rolled steel sheet obtained by hot rolling is cold rolled.
• a known step such as a pickling step may be further included. These known steps are not particularly limited and can be carried out in accordance with known methods. Each step will be described in detail below.
• the hot rolling conditions are not particularly limited, and the hot rolling can be carried out according to a known method.
• a slab having the composition described above may be heated to 1100 to 1350°C and hot rolled at a rolling reduction of 5 to 30%.
• the hot-rolled steel sheet obtained by hot rolling is wound into a coil.
• the steel sheet is cooled in a temperature range of 700 to 100°C under conditions such that the ratio of the sheet thickness to the cooling rate (sheet thickness/cooling rate) is 0.005 mm ⁇ sec/°C or more. By cooling under such conditions, uniformity of the steel sheet shape can be ensured.
• the ratio of the sheet thickness to the cooling rate is preferably 0.015 mm ⁇ sec/°C or more.
• the upper limit of the ratio of the sheet thickness to the cooling rate is not particularly limited, but is, for example, 0.300 mm ⁇ sec/°C.
• the ratio of the sheet thickness to the cooling rate is less than 0.005 mm ⁇ sec/°C, uniformity of the steel sheet shape cannot be ensured, and the sheet thickness tolerance and maximum warpage amount described above will increase.
• Cold rolling is performed at a reduction ratio of 20 to 65%.
• the reduction ratio of cold rolling is preferably 30 to 60%.
• the reduction ratio of cold rolling exceeds 65%, the strength becomes too high, and uniform elongation and toughness decrease.
• the reduction ratio of cold rolling is less than 20%, the strength is insufficient, and fatigue resistance and punchability decrease.
• the steel sheet according to the embodiment of the present invention can also be produced by a method including a hot rolling process in which a slab having the composition described above is hot rolled, a cold rolling process in which the hot rolled steel sheet obtained by hot rolling is cold rolled, an annealing process in which the cold rolled steel sheet obtained by cold rolling is annealed and cooled in a temperature range of 700 to 100 ° C under conditions where the ratio of sheet thickness to cooling rate (sheet thickness/cooling rate) is 0.005 mm sec/° C. or more, and a final cold rolling process in which the cold-rolled annealed steel sheet obtained in the annealing process is cold rolled.
• a known process such as a pickling process may be further included. These known processes are not particularly limited and can be carried out according to known methods. Each step will be described in detail below.
• the hot rolling conditions are not particularly limited, and the hot rolling can be carried out according to a known method.
• a slab having the composition described above may be heated to 1100 to 1350°C and hot rolled at a rolling reduction of 5 to 30%.
• the conditions for winding the hot-rolled steel sheet obtained by hot rolling into a coil are not particularly limited.
• the conditions for the cold rolling step are not particularly limited, and the cold rolling step can be carried out in accordance with a known method.
• the hot-rolled steel sheet obtained by hot rolling may be cold-rolled at a rolling reduction ratio of 30 to 65%.
• the annealing conditions are not particularly limited, and can be performed according to a known method.
• the cold-rolled steel sheet obtained by cold rolling may be held at 750 to 900°C for 1 to 5 minutes.
• cooling is performed in a temperature range of 700 to 100°C under conditions such that the ratio of the sheet thickness to the cooling rate (sheet thickness/cooling rate) is 0.005 mm ⁇ sec/°C or more. Cooling under such conditions can ensure uniformity of the steel sheet shape. From the viewpoint of stably ensuring this effect, the ratio of the sheet thickness to the cooling rate is preferably 0.015 mm ⁇ sec/°C or more.
• the upper limit of the ratio of the sheet thickness to the cooling rate is not particularly limited, but is, for example, 0.300 mm ⁇ sec/°C. On the other hand, if the ratio of the sheet thickness to the cooling rate is less than 0.005 mm ⁇ sec/°C, uniformity of the steel sheet shape cannot be ensured, and the sheet thickness tolerance and maximum warpage amount described above will increase.
• the final cold rolling is performed at a rolling ratio of 1.0% or more but less than 30%.
• the final cold rolling within this rolling ratio range, it is possible to control the GAM value to 1.0 to 10.0° and the number density of crystal grains with a grain size of 20.0 ⁇ m or more to 500 grains/ mm2 or less.
• the cold rolling reduction ratio is preferably 1.5 to 25%.
• the cold rolling reduction ratio is 30% or more, the strength becomes too high, resulting in a decrease in uniform elongation and toughness.
• the cold rolling reduction ratio is less than 1.0%, the strength is insufficient, resulting in a decrease in fatigue resistance and punchability.
• Steel sheets were prepared by the following two methods. ⁇ Method A> A 250 mm thick slab having the composition shown in Table 1 (the balance being impurities other than Fe and the elements shown in Table 1) was produced by a continuous casting method. Next, the slab was heated to 1300°C and hot rolled at a final stage rolling reduction of 15%, and then, when wound into a coil, cooled in a temperature range of 700 to 100°C with the ratio of the plate thickness to the cooling rate (plate thickness/cooling rate) shown in Table 2. Thereafter, the hot-rolled steel sheet was (finally) cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet. The width of the steel sheet during cold rolling was set to 400 mm or less. In Method A in Table 2, the sheet thickness means the sheet thickness after hot rolling, and the cooling rate means the cooling rate when coiled.
• ⁇ Method B> A 250 mm thick slab having the composition shown in Table 1 (the balance being impurities other than Fe and the elements shown in Table 1) was produced by a continuous casting method. Next, the slab was heated to 1250°C and hot rolled at a final stage rolling reduction of 15%, and then wound into a coil. Next, the hot-rolled steel sheet was cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet. Next, the cold-rolled steel sheet was annealed by holding it at 850°C for 3 minutes, and then cooled in a temperature range from 700 to 100°C with the ratio of the sheet thickness to the cooling rate (sheet thickness/cooling rate) shown in Table 2.
• the cold-rolled annealed sheet was finally cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet.
• the width of the steel sheet during cold rolling was set to 400 mm or less.
• the sheet thickness means the sheet thickness of the cold-rolled steel sheet obtained by cold rolling before annealing
• the cooling rate means the cooling rate after annealing.
• the area ratio of ferrite was determined according to the above-mentioned method, and the remaining area ratio was taken as the area ratio of martensite.
• the average grain size of ferrite was also determined according to the above-mentioned method.
• the test specimens had dimensions of 15 mm in the rolling direction, 10 mm in the width direction, and 2.5 mm in thickness.
• the GAM value and the number density of the predetermined crystal grains were determined according to the above-mentioned method.
• the test specimen had a size of 15 mm in the rolling direction, 10 mm in the width direction, and 2 mm in thickness.
• OK those with a GAM value of 1.0 to 10.0° are indicated as OK, and those with a GAM value other than 1.0 to 10.0° are indicated as NG.
• number density of crystal grains with a grain size of 20.0 ⁇ m or more is abbreviated as "number density,” with those with a number density of 500 grains/ mm2 or less being OK and those with a number density exceeding 500 grains/ mm2 being NG.
• the thickness tolerance of the cold-rolled steel sheet was measured in accordance with JIS G3141:2021. Specifically, the thickness was measured at three arbitrary locations 15 mm or more inside from the widthwise end of the cold-rolled steel sheet (sheet width 400 mm or less). In this measurement, a deviation (tolerance) from the set thickness of ⁇ 0.08 mm or less was expressed as OK (small thickness tolerance), and a deviation (tolerance) from the set thickness of more than ⁇ 0.08 mm was expressed as NG (large thickness tolerance).
• the maximum warpage in the rolling direction of the cold-rolled steel sheet was measured in accordance with JIS G3141:2021. Specifically, the cold-rolled steel sheet was sheared to a length of 1 m in the rolling direction, and the sheared cold-rolled steel sheet was placed on a surface plate (horizontal table). The value of the warpage (also referred to as flatness) was calculated by subtracting the thickness of the cold-rolled steel sheet from the maximum vertical distance from the upper surface of the surface plate to the surface of the cold-rolled steel sheet. Furthermore, the warpage was measured with one side of the cold-rolled steel sheet facing up, and then the warpage was measured with the other side facing up. The maximum value of the measured warpage was taken as the maximum warpage. A maximum warpage of 10 mm or less was designated OK (high flatness), and a maximum warpage of more than 10 mm was designated NG (low flatness).
• Vickers hardness Vickers hardness was measured according to the method described above.
• the uniform elongation was measured according to the method described above. In this evaluation, a uniform elongation of 1.5% or more was evaluated as OK (good uniform elongation), and a uniform elongation of less than 1.5% was evaluated as NG (insufficient uniform elongation).
• a fatigue limit of 0.8 ⁇ TS (tensile strength) or more was evaluated as OK (good fatigue resistance), and a fatigue limit of less than 0.8 ⁇ TS was evaluated as NG (insufficient fatigue resistance).
• the tensile strength was determined by the method specified in JIS Z2241:2023 using a JIS No. 5 tensile test piece. The crosshead displacement speed in the tensile test was 30 mm/min.
• V-notch test pieces were taken from the cold-rolled steel sheets and subjected to a Charpy impact test at 150° C. The test was performed in accordance with JIS Z2242:2023, and the test pieces were V-notched, had a thickness of 2.0 mm, and were taken so that the length direction was parallel to the rolling direction. In this evaluation, a specimen having a brittle fracture rate of 70% or less was designated as OK (good toughness), and a specimen having a brittle fracture rate of more than 70% was designated as NG (insufficient toughness).
• the cold-rolled steel sheets of Examples 1 to 35 had appropriate steel sheet compositions and metal structures, and therefore had good results in Vickers hardness, uniform elongation, fatigue resistance, toughness, and punchability. The results were also good for the plate thickness tolerance and maximum warpage.
• the cold-rolled steel sheet of Comparative Example 1 had an excessively high C content, resulting in high strength and insufficient toughness.
• the cold-rolled steel sheet of Comparative Example 2 had an insufficient strength due to an excessively low C content, and the fatigue resistance and punchability were insufficient.
• the cold-rolled steel sheet of Comparative Example 3 had insufficient toughness because the Si content was too high.
• the cold-rolled steel sheet of Comparative Example 4 had an excessively low Mn content, which resulted in an increased amount of ferrite precipitation, resulting in insufficient strength, fatigue resistance, and punchability.
• the cold-rolled steel sheet of Comparative Example 5 had an excessively high Mn content, and therefore had high strength but insufficient toughness.
• the cold-rolled steel sheet of Comparative Example 6 had an excessively high Mn content and Cr content, and therefore had high strength but insufficient toughness.
• the cold-rolled steel sheet of Comparative Example 7 had an excessively high rolling reduction ratio during (final) cold rolling, and therefore could not achieve a GAM value of 1.0 to 10.0°, resulting in insufficient uniform elongation and toughness.
• the rolling reduction ratio during final cold rolling was too low, so that the GAM value could not be set to 1.0 to 10.0° and the number density of crystal grains having a grain size of 20.0 ⁇ m or more could not be set to 500 grains/ mm2 or less, and the sheet thickness tolerance and maximum warpage also increased. As a result, the fatigue resistance and punchability were insufficient.
• the cold-rolled steel sheet of Comparative Example 9 had an increased thickness tolerance and maximum warpage because the cooling conditions after annealing were inappropriate.
• the present invention makes it possible to provide steel sheets that have high hardness, can eliminate heat treatment after processing, and have excellent flatness, fatigue resistance, and toughness.
• the present invention can provide a steel sheet that has high hardness, can eliminate heat treatment after processing, and has excellent flatness, fatigue resistance, and toughness.
• the area ratio of ferrite is 0 to 10.0%, and the area ratio of martensite is 90.0 to 100%,
• the average grain size of the ferrite is 15.0 ⁇ m or less,
• the GAM value is 1.0 to 10.0°,
• the number density of crystal grains having a crystal grain size of 20.0 ⁇ m or more is 500 grains/mm 2 or less, When the plate width is 400 mm or less, the plate thickness tolerance is ⁇ 0.08 mm or less, A steel plate having a maximum warpage in the rolling direction of 10 mm or less when the length in the rolling direction is 1 m.
• [3] The steel sheet according to [1] or [2], wherein the impurities include, on a mass basis, one or more selected from the group consisting of Cu: 0 to 0.15%, W: 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, and Ca: 0 to 0.050%.

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• 2025
  • 2025-01-24 JP JP2025572254A patent/JPWO2025164543A1/ja active Pending
  • 2025-01-24 WO PCT/JP2025/002292 patent/WO2025164543A1/ja active Pending

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