Steel sheet

Abstract

This steel sheet has a composition comprising, by mass, C: 0.07-0.30%, Si: 0.01-0.65%, Mn: 0.80-2.30%, 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 steel sheet has a ferrite area ratio of 0-10.0%, a martensite area ratio of 90.0-100%, and a ferrite average crystal grain size of 15.0 μm or less. The GAM value is 1.0-10.0°, and the number density of crystal grains having a crystal grain size of 20.0 μm or more is 500 / mm2 or less. Also, the steel sheet has a sheet thickness tolerance of ±0.08 mm or less when the sheet width is 400 mm or less, and a maximum warpage in the rolling direction of 10 mm or less when the length in the rolling direction is 1 m.

Description

steel plate

[0001] The present invention relates to a steel sheet.

[0002] Medium-carbon steel sheets and high-carbon steel sheets (hereinafter, abbreviated as "steel sheets") are used as materials for structural components and machine parts in various machines and devices, such as automobiles. These steel sheets are processed into predetermined shapes and then subjected to heat treatments such as quenching and tempering to form various parts. For example, Patent Document 1 proposes a steel sheet for use in such various parts, which has a steel structure in which, by 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.

[0003] International Publication No. 2021 / 085336

[0004] In recent years, due to the increasing need to reduce CO2 emissions, there has been a demand for the elimination of heat treatment after processing steel sheets into a predetermined shape, making it necessary to harden steel sheets. However, hardening steel sheets reduces their workability. Therefore, in order to omit the heat treatment after processing, it is important to harden steel sheets while maintaining their workability. One effective method for hardening steel sheets is to use steel sheets with a martensitic structure. However, martensitic structures tend to lose their shape during quenching, resulting in a decrease in flatness. Furthermore, when steel sheets are used as materials for parts such as chains, the steel sheets themselves are required to have fatigue properties and toughness.

[0005] The steel sheet of Patent Document 1 improves the uniformity of the steel sheet shape by constraining the steel sheet from the front and back sides with two rolls sandwiching the steel sheet so as to satisfy predetermined conditions when the surface temperature of the steel sheet is below (Ms point + 150°C) during water quenching, thereby ensuring the flatness of the steel sheet itself. On the other hand, although the steel sheet of Patent Document 1 is hardened because it is mainly 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 parts, heat treatment must be performed after processing to increase the strength.

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a steel sheet that has high hardness, can omit heat treatment after processing, and is excellent in flatness, fatigue resistance, and toughness.

[0007] As a result of intensive research conducted by the inventors to solve the above-mentioned problems, they discovered that by controlling the composition and metal structure of a steel sheet, it is possible to improve hardness and fatigue resistance while ensuring flatness, and thus completed the present invention.

[0008] That is, 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, wherein the area fraction of ferrite is 0 to 10.0% and the area fraction of martensite is 90.0 to 100%, the average grain size of the ferrite is 15.0 μm or less, the GAM value is 1.0 to 10.0°, and the number density of grains having a grain size of 20.0 μm or more is 500 grains / mm 2 the thickness tolerance is ±0.08 mm or less when the sheet width is 400 mm or less, and the maximum amount of warpage in the rolling direction when the length in the rolling direction is 1 m is 10 mm or less.

[0009] According to the present invention, it is possible to provide a steel sheet that has high hardness, can omit heat treatment after working, and is excellent in flatness, fatigue resistance, and toughness.

[0010] Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, as appropriate, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the present invention, also fall within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0011] <Steel Sheet> The steel sheet according to an embodiment of the present invention has a composition containing 0.07 to 0.30% C, 0.01 to 0.65% Si, 0.80 to 2.30% Mn, 0.100% or less P, 0.100% or less S, 0.100% or less Al, 1.00% or less Cr, and 0.0150% or less N, with the balance being Fe and impurities. Herein, "steel sheet" refers to a plate-shaped (including strip-shaped) material formed from steel. Also, "impurities" refer to components that are mixed in during industrial production of steel sheet due to raw materials such as ore and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities include unavoidable impurities. Examples of impurities include Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca. With regard to the content of each element, including "xx% or less" means that it is xx% or less, but includes an amount exceeding 0% (particularly, exceeding the impurity level). Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0012] 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. Details of the above composition will be described.

[0013] (C: 0.07 to 0.30%) C is an element necessary for increasing the strength of the steel sheet and improving fatigue resistance and punchability. To fully obtain these effects, the C content is set to 0.07% or more, preferably 0.08% or more, and more preferably 0.09% or more. On the other hand, if the C content is too high, the steel sheet becomes hard and its toughness decreases. For this reason, 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 (ranges specifying upper and lower limits) may be any combination of the numerical ranges. Therefore, for example, the C content can 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%. The same applies to the numerical ranges of the contents of the other elements below.

[0014] (Si: 0.01 to 0.65%) Si is an element necessary for deoxidation. To fully obtain this effect, 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. On the other hand, if the Si content is too high, the toughness of the steel plate decreases. For this reason, the Si content is set to 0.65% or less, preferably 0.60% or less, and more preferably 0.58% or less.

[0015] (Mn: 0.80 to 2.30%) Mn is an element that affects the strength, punchability, and other properties of the steel sheet. 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. For this reason, 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. For this reason, the Mn content is set to 2.30% or less, preferably 2.25% or less, and more preferably 2.20% or less.

[0016] (P: 0.100% or less) The lower the P content, the better; if it is too high, properties such as the toughness of the steel plate will be reduced. For this reason, the P content is set to 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less. On the other hand, since the lower the P content, the better, there is no particular lower limit. However, since excessive reduction of the P content leads to an increase in costs, the P content can be set to, for example, 0.001% or more.

[0017] (S: 0.100% or less) 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.

[0018] (Al: 0.100% or less) Al is an element used for deoxidation. However, if the Al content is too high, inclusions increase and the workability of the steel sheet decreases. Therefore, the Al content is set to 0.100% or less, preferably 0.098% or less, and more preferably 0.095% or less. On the other hand, the Al content may be low, and the lower limit is not particularly limited. However, from the viewpoint of obtaining the above-mentioned effects of Al, the Al content can be set to, for example, 0.001% or more, 0.003% or more, or 0.005% or more.

[0019] (Cr: 1.00% or less) Cr is an element effective for forming a predetermined metal structure. However, if the Cr content is too high, the steel plate will have high strength and toughness will not be ensured. Therefore, the Cr content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less. On the other hand, the Cr content may be low, and the lower limit is not particularly limited. However, from the viewpoint of obtaining the above-mentioned effects of Cr, 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.

[0020] (N: 0.0150% or less) N is an element that forms AlN and suppresses coarsening of crystal grains through a pinning effect. However, if the N content is too high, the effect saturates, resulting in a decrease in toughness. Therefore, the N content is set to 0.0150% or less, preferably 0.0140% or less, and preferably 0.0130% or less. On the other hand, the N content may be low, and there is no particular lower limit. However, since excessive reduction of the N content leads to an increase in costs, the N content can be set to, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0021] (Ni: 1.000% or less) Ni is an element that dissolves in steel to improve strength without impairing toughness. However, 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. On the other hand, 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.

[0022] (Mo: 0.700% or less) Mo is an element that improves the strength of a steel sheet. However, if the Mo content is too high, the steel sheet becomes hard, and toughness and uniform elongation decrease. Therefore, the Mo content is set to 0.700% or less, preferably 0.650% or less, and more preferably 0.600% or less. On the other hand, the Mo content may be small, and the lower limit is not particularly limited. However, from the viewpoint of obtaining the above-mentioned effects of Mo, the Mo content can be set to, for example, 0.001% or more, 0.005% or more, or 0.010% or more.

[0023] (V: 0.500% or less, Nb: 0.500% or less, Ti: 0.150% or less) V, Nb, and Ti are all elements that improve the strength of the steel sheet by carbide precipitation. However, if the contents of these elements are too high, a large amount of carbides will be formed, and the toughness of the steel sheet will decrease. 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. On the other hand, the contents of these elements may be small, and there are no particular lower limits. However, from the viewpoint of obtaining the above-described effects of these elements, 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.

[0024] (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.

[0025] (Cu: 0-0.15%, W: 0-0.15%, Ta: 0-0.15%, Sn: 0-0.050%, Sb: 0-0.050%, Co: 0-0.050%, As: 0-0.050%, Mg: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, and Ca: 0-0.050%) Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca are impurities and do not necessarily need to be contained in the steel sheet. These elements may be contained as impurities either alone or in combination of two or more. The contents of Cu, W, and Ta are each 0 to 0.15%, preferably 0 to 0.14%.The contents of Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca are each 0 to 0.050%, preferably 0 to 0.045%.

[0026] Next, the metal structure of the steel sheet according to the embodiment of the present invention will be described. The steel sheet according to the embodiment of the present invention has an area ratio of ferrite of 0 to 10.0%, an area ratio of martensite of 90.0 to 100%, an average grain size of ferrite of 15.0 μm or less, a GAM value of 1.0 to 10.0°, and a number density of grains with a grain size of 20.0 μm or more of 500 grains / mm 2 The following is the result.

[0027] (Area fraction of ferrite: 0 to 10.0% and area fraction of martensite: 90.0 to 100%) 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 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., a martensite single-phase structure is also possible). Furthermore, the metallographic structure of the steel sheet according to the embodiment of the present invention may contain ferrite. From the viewpoint of ensuring the effects of martensite described above, the area fraction of ferrite is 10.0% or less, preferably 9.0% or less. On the other hand, the lower limit of the area fraction of ferrite is 0%. Note that the metallographic structure of the steel sheet according to the embodiment of the present invention may contain phases other than martensite and ferrite (e.g., pearlite, bainite, etc.) within a range that does not impair the effects of martensite described above.

[0028] 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 ¼ 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. In the obtained structure photograph, martensite (white structure) and ferrite (black structure) are identified, and the area ratios of martensite and ferrite (the area ratios of martensite and ferrite to the entire measurement area) are calculated using image analysis software. 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).

[0029] (Average grain size of ferrite: 15.0 μm or less) If the average grain size of ferrite is large, it becomes difficult to obtain a sufficient martensite structure, resulting in insufficient hardening and reduced fatigue resistance and punchability. Therefore, the average grain size of ferrite is 15.0 μm or less, preferably 13.0 μm or less, and more preferably 11.0 μm or less. On the other hand, the smaller the average grain size of ferrite, the better, so there is no particular lower limit. Furthermore, the steel sheet according to the embodiment of the present invention may be composed of a martensite single-phase structure. In this case, since no ferrite is present, the average grain size of ferrite can be considered to be 0 μm.

[0030] 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 specimen cut from a steel sheet is polished, and then immersed in a 3% nital etching solution to reveal the structure. In this cross section, the position corresponding to ¼ 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 grains. The average grain size is determined from the obtained microstructure photograph using a cutting method. The average grain size is the average of the measurement results from five points.

[0031] (GAM value: 1.0 to 10.0°) The GAM (Grain Average Misorientation) value is a value obtained from crystal orientation analysis data by electron backscattering diffraction (EBSD). It is a value obtained by measuring the distance between measurement points (hereinafter also referred to as "step size") of 0.2 μm within a crystal grain separated by a large-angle grain boundary having a misorientation of 15° or more, calculating the misorientation between adjacent measurement points, and averaging the calculated misorientation within the same crystal grain. When the GAM value is small, the average misorientation within a single crystal grain is small, resulting in uniform crystal grains with little strain, or a continuous orientation gradient within the crystal grain. On the other hand, when the GAM value is large, the average misorientation within the crystal grain is large, resulting in large local strain within a single crystal grain. 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.

[0032] (Number density of crystal grains with a grain size of 20.0 μm or more: 500 grains / mm 2 Coarse crystal grains with a grain size of 20.0 μm or more reduce toughness. Therefore, the number density of coarse crystal grains with a grain size of 20.0 μm or more is 500 grains / mm 2 The following applies.

[0033] The GAM value and the number density of crystal grains with a grain size of 20.0 μm or more are 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 μm pitch over three or more fields of view (fields of view) of 200 μm in the thickness direction and 200 μm 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. 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 calculation is performed 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 μm or more was determined by identifying the crystal grains having a grain size of 20.0 μm or more, determining the number (pieces), and calculating the area (mm 2 The number density of crystal grains is calculated by dividing the number density by the average value of the measurement results at five locations.

[0034] In the steel sheet according to the embodiment of the present invention, 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 rolling direction length is 1 m. With such thickness tolerance and maximum warpage, the steel sheet shape can be said to be uniform. If the thickness tolerance or maximum warpage is outside the above range, workability (e.g., punchability) may be reduced. Here, the thickness tolerance and maximum warpage of the steel sheet are determined in accordance with JIS G3141:2021.

[0035] 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 ensures hardening while maintaining workability (uniform elongation and toughness). Therefore, heat treatment after processing the steel sheet can be omitted. Vickers hardness is determined by polishing a cross section (L cross section) parallel to the rolling direction of a test piece cut from the steel sheet, and then performing a Vickers hardness test in accordance with JIS Z2244:2009 at a position corresponding to 1 / 4 of the thickness of the steel sheet. In the Vickers hardness test, a measurement load of 1 kgf (9.807 N) is applied, and measurements are taken at three random locations, with the average value being the measurement result.

[0036] (Uniform elongation: 1.5% or more) 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.

[0037] 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 may be, for example, 10.0 mm or less, 8.0 mm or less, or 6.0 mm or less.

[0038] <Method for Manufacturing Steel Sheet> The method for manufacturing a steel sheet according to an embodiment of the present invention is not particularly limited as long as it can produce a steel sheet having the above-described characteristics. For example, the steel sheet according to an 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 sheet thickness to the cooling rate (sheet 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. After each step, a known step such as a pickling step may be further included. These known steps are not particularly limited and can be performed according to known methods. Each step will be described in detail below.

[0039] (Hot Rolling Process) The hot rolling conditions are not particularly limited and can be performed according to known methods. For example, 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. At this time, the steel sheet is cooled in the 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. By cooling under such conditions, uniformity of the steel sheet shape can be ensured. From the viewpoint of stably ensuring this effect, the ratio of the thickness to the cooling rate is preferably 0.015 mm·sec / °C or more. The upper limit of the ratio of the 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., the uniformity of the steel sheet shape cannot be ensured, and the sheet thickness tolerance and maximum amount of warpage described above increase.

[0040] (Cold Rolling Step) Cold rolling is performed at a rolling ratio of 20 to 65%. By performing cold rolling within this rolling ratio range, it is possible to achieve a GAM value of 1.0 to 10.0° and a number density of crystal grains with a grain size of 20.0 μm or more of 500 grains / mm 2Each of these can be controlled as follows. From the viewpoint of stably ensuring this effect, the cold rolling reduction ratio is preferably 30 to 60%. On the other hand, if the cold rolling reduction ratio exceeds 65%, the strength becomes too high, and the uniform elongation and toughness decrease. Furthermore, if the cold rolling reduction ratio is less than 20%, the strength is insufficient, and the fatigue resistance and punchability decrease.

[0041] Furthermore, 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 and annealed steel sheet obtained in the annealing process is cold rolled. After each process, 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 process will be described in detail below.

[0042] (Hot rolling step) The conditions for hot rolling are not particularly limited, and hot rolling can be performed in accordance with a known method. For example, 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%. In this method, the conditions for coiling the hot-rolled steel sheet obtained by hot rolling are not particularly limited.

[0043] (Cold Rolling Step) The conditions for the cold rolling step are not particularly limited, and the cold rolling step can be performed in accordance with a known method. For example, the hot-rolled steel sheet obtained by hot rolling may be cold-rolled at a rolling ratio of 30 to 65%.

[0044] (Annealing Process) The annealing conditions are not particularly limited and can be performed according to known methods. For example, the cold-rolled steel sheet obtained by cold rolling may be held at 750 to 900°C for 1 to 5 minutes. After annealing, the steel sheet is cooled in the 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. 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.

[0045] (Final cold rolling step) The final cold rolling is performed at a rolling reduction ratio of 1.0% or more and less than 30%. By performing the final cold rolling within this rolling reduction range, the GAM value is set to 1.0 to 10.0° and the number density of crystal grains with a grain size of 20.0 μm or more is set to 500 grains / mm 2 Each of these can be controlled as follows. From the viewpoint of stably ensuring this effect, the cold rolling reduction ratio is preferably 1.5 to 25%. On the other hand, if the cold rolling reduction ratio is 30% or more, the strength becomes too high, and uniform elongation and toughness decrease. Furthermore, if the cold rolling reduction ratio is less than 1.0%, the strength is insufficient, and fatigue resistance and punchability decrease.

[0046] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0047] Steel sheets were produced 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 continuous casting. Next, the slab was heated to 1300°C and hot-rolled at a final stage reduction of 15%, and then cooled in a temperature range of 700 to 100°C at a ratio of the sheet thickness to the cooling rate (sheet thickness / cooling rate) shown in Table 2 when coiled into a coil. The hot-rolled steel sheet was then (finally) cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet. Note that the width of the steel sheet during cold rolling was 400 mm or less. In Method A in Table 2, the sheet thickness refers to the sheet thickness after hot rolling, and the cooling rate refers to the cooling rate when coiled into a coil.

[0048] <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 continuous casting. Next, the slab was heated to 1250°C and hot-rolled at a final stage reduction of 15%, and then wound into a coil. Next, the hot-rolled steel sheet was cold-rolled at the 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 at a ratio of the thickness to the cooling rate (thickness / cooling rate) shown in Table 2. Thereafter, the cold-rolled annealed sheet was finally cold-rolled at the reduction shown in Table 2 to obtain a cold-rolled steel sheet. Note that the width of the steel sheet during cold rolling was 400 mm or less. In Method B in Table 2, the thickness refers to the thickness of the cold-rolled steel sheet obtained by cold rolling before annealing, and the cooling rate refers to the cooling rate after annealing.

[0049]

[0050]

[0051] The cold-rolled steel sheets obtained in the above examples were evaluated as follows.

[0052] (Area ratio of ferrite and martensite, average grain size of ferrite) The area ratio of ferrite was determined according to the method described above, 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 method described above. The test specimens had dimensions of 15 mm in the rolling direction, 10 mm in the width direction, and 2.5 mm in thickness.

[0053] (GAM value and number density of crystal grains with a grain size of 20.0 μm or more) The GAM value and the number density of predetermined crystal grains were determined according to the method described above. The test specimens had a size of 15 mm in the rolling direction x 10 mm in the width direction x 2 mm in thickness. In Table 3, a GAM value of 1.0 to 10.0° is indicated as OK, and a GAM value outside the range of 1.0 to 10.0° is indicated as NG. Furthermore, the number density of crystal grains with a grain size of 20.0 μm or more is abbreviated as "number density," and a number density of 500 grains / mm 2 The number density is 500 pieces / mm or less. 2 Anything above this is marked as NG.

[0054] (Thickness Tolerance) 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 ±0.08 mm or more was expressed as NG (large thickness tolerance).

[0055] (Maximum Warpage) 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 of the cold-rolled steel sheet 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).

[0056] (Vickers Hardness) Vickers hardness was measured according to the method described above.

[0057] (Uniform elongation) 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).

[0058] (Fatigue Resistance) Fatigue resistance was measured by a plane bending fatigue test in accordance with JIS Z2275:1978 to determine the fatigue limit. Specifically, a No. 1 test piece (b: 15 mm, R: 30 mm) specified in JIS Z2275:1978 was cut out from the cold-rolled steel sheet, and a plane bending fatigue test was performed using a plane bending tester under the conditions of a stress ratio R = -1 and a frequency of 25 Hz. Here, the stress cycle to fracture at each stress amplitude was measured, an S-N curve was obtained, and the fatigue strength (fatigue limit) at 10,000,000 cycles was determined. In this evaluation, a fatigue limit of 0.8 × TS (tensile strength) or more was expressed as OK (good fatigue resistance), and a fatigue limit of less than 0.8 × TS was expressed as NG (insufficient fatigue resistance). The tensile strength was determined using a JIS No. 5 tensile test piece according to the method specified in JIS Z2241:2023. The crosshead displacement rate in the tensile test was 30 mm / min.

[0059] (Toughness) 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 plate thickness of 2.0 mm, and were taken so that the length direction was parallel to the rolling direction. In this evaluation, a brittle fracture rate of 70% or less was expressed as OK (good toughness), and a brittle fracture rate of more than 70% was expressed as NG (insufficient toughness).

[0060] (Punchability) A hole with a diameter of 10 mm was punched from a cold-rolled steel sheet with a punching clearance of 3%. The end surface of the punched sample was observed, and the size of sagging (amount of sagging) was measured. The amount of sagging was defined as the difference in height between the center and the end surface of the punched sample. In this evaluation, a sample with a sagging amount of 100 μm or less was evaluated as OK (good punchability), and a sample with a sagging amount of more than 100 μm was evaluated as NG (insufficient punchability).

[0061] The results of the above evaluations are shown in Table 3.

[0062]

[0063] As shown in Table 3, the cold-rolled steel sheets of Examples 1 to 35 had appropriate steel sheet compositions and metal structures, and therefore exhibited good results in Vickers hardness, uniform elongation, fatigue resistance, toughness, and punchability, as well as good results in sheet thickness tolerance and maximum warpage. In contrast, the cold-rolled steel sheet of Comparative Example 1 had too much C content, resulting in high strength but insufficient toughness. The cold-rolled steel sheet of Comparative Example 2 had too little C content, resulting in insufficient strength and insufficient fatigue resistance and punchability. The cold-rolled steel sheet of Comparative Example 3 had too much Si content, resulting in insufficient toughness. The cold-rolled steel sheet of Comparative Example 4 had too little Mn content, resulting in a large amount of ferrite precipitation, resulting in insufficient strength and insufficient fatigue resistance and punchability. The cold-rolled steel sheet of Comparative Example 5 had too much Mn content, resulting in 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.

[0064] The cold-rolled steel sheet of Comparative Example 7 had a too high rolling ratio during (final) cold rolling, so it was not possible to achieve a GAM value of 1.0 to 10.0°. As a result, the uniform elongation and toughness were insufficient. The cold-rolled steel sheet of Comparative Example 8 had a too low rolling ratio during final cold rolling, so it was not possible to achieve a GAM value of 1.0 to 10.0° and a number density of crystal grains with a grain size of 20.0 μm or more below 500 grains / mm 2 The cold-rolled steel sheet of Comparative Example 9 could not be cooled to or below the above range, and the thickness tolerance and the 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.

[0065] As can be seen from the above results, the present invention can provide a steel sheet that has high hardness, can omit heat treatment after working, and is excellent in flatness, fatigue resistance, and toughness.

[0066] Therefore, by adopting the following aspects [1] to [6], the present invention can provide a steel sheet that has high hardness, can omit heat treatment after processing, and is excellent in flatness, fatigue resistance, and toughness.

[0067] [1] A steel sheet having a composition containing, by mass, 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, wherein 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 the ferrite is 15.0 μm or less, the GAM value is 1.0 to 10.0°, and the number density of grains having a grain size of 20.0 μm or more is 500 grains / mm 2 [2] The steel sheet according to [1], further comprising, on a mass basis, one or more 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, wherein 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 when the length in the rolling direction is 1 m is 10 mm or less. [3] The steel sheet according to [1] or [2], wherein the impurities include, by mass, 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%. [4] The steel sheet according to any one of [1] to [3], which is a cold-rolled steel sheet. [5] The steel sheet according to any one of [1] to [4], which has a Vickers hardness of 300 to 550 Hv. [6] The steel sheet according to any one of [1] to [5], having a uniform elongation of 1.5% or more.

Claims

1. A steel sheet having a composition containing, by mass, 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, wherein 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 the ferrite is 15.0 μm or less, the GAM value is 1.0 to 10.0°, and the number density of grains having a grain size of 20.0 μm or more is 500 grains / mm 2 a thickness tolerance of ±0.08 mm or less when the plate width is 400 mm or less, and a maximum warpage in the rolling direction of 10 mm or less when the length in the rolling direction is 1 m.

2. The steel plate according to claim 1, further comprising, on a mass basis, 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.

3. The steel sheet according to claim 1 or 2, wherein the impurities include one or more selected from the group consisting of, by mass, Cu: 0-0.15%, W: 0-0.15%, Ta: 0-0.15%, Sn: 0-0.050%, Sb: 0-0.050%, Co: 0-0.050%, As: 0-0.050%, Mg: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, and Ca: 0-0.050%.

4. The steel sheet according to any one of claims 1 to 3, which is a cold-rolled steel sheet.

5. The steel sheet according to any one of claims 1 to 4, having a Vickers hardness of 300 to 550 Hv.

6. The steel sheet according to any one of claims 1 to 5, having a uniform elongation of 1.5% or more.

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