steel plate

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

[Technical Field]

[0001] This invention relates to steel plates. [Background technology]

[0002] Medium-carbon steel sheets and high-carbon steel sheets (hereinafter referred to as "steel sheets") are used as materials for structural and mechanical components 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 become various components. As a steel sheet used in the various parts described above, for example, Patent Document 1 proposes a steel sheet having a structure in which, by area ratio, martensite: 20% to 100%, ferrite: 0% to 80%, and other metallic phases: 5% or less, and the ratio of the dislocation density of the metallic phase on the surface of the steel sheet to the dislocation density of the metallic phase in the center of the sheet thickness is 30% to 80%, and the maximum warp of the steel sheet when sheared in the rolling direction over a length of 1m is 15mm or less. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2021 / 085336 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In recent years, the increasing need to reduce CO2 emissions has necessitated the omission of heat treatment after processing steel sheets into a predetermined shape, thus requiring the hardening of steel sheets. However, hardening steel sheets reduces their workability. Therefore, in order to omit post-processing heat treatment, it is important to harden the steel sheets while maintaining workability. One effective method for hardening steel sheets is to create steel sheets with a martensitic structure, but martensitic structures tend to lose their shape and flatness during quenching. Furthermore, when steel plates are used as the material for components such as chains, the steel plates themselves are required to have fatigue properties and toughness.

[0005] The steel sheet described in Patent Document 1 improves the uniformity of the steel sheet shape by restraining the steel sheet from the front and back surfaces with two rolls placed on either side of the steel sheet during water quenching, in a region where the surface temperature of the steel sheet is below (Ms point + 150°C), thereby ensuring the flatness of the steel sheet itself. On the other hand, although the steel sheet described in Patent Document 1 is hardened because it is mainly composed of martensite, it cannot be said to be at a level where post-processing heat treatment can be omitted. Therefore, in order to ensure the strength as a component, heat treatment must be performed after processing to increase its strength.

[0006] This invention was made to solve the above-mentioned problems, and aims to provide a steel sheet that has high hardness, eliminates the need for post-processing heat treatment, and is excellent in flatness, fatigue resistance, and toughness. [Means for solving the problem]

[0007] As a result of diligent research to solve the above problems, the inventors of the present invention have discovered that by controlling the composition and microstructure of steel sheets, it is possible to improve hardness and fatigue resistance while ensuring flatness, and have completed the present invention.

[0008] In other words, the present invention has a composition by mass containing 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 remainder being Fe and impurities. The area ratio of ferrite is 0-10.0%, and the area ratio of martensite is 90.0-100%. The average crystal grain size of the ferrite is 15.0 μm or less. The GAM value is between 1.0 and 10.0°. The number density of crystal grains with a grain size of 20.0 μm or larger is 500 grains / mm². 2The following: When the plate width is 400 mm or less, the plate thickness tolerance is ±0.08 mm or less. This relates to a steel plate whose maximum curvature in the rolling direction is 10 mm or less when the length in the rolling direction is 1 m. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a steel sheet that has high hardness, eliminates the need for heat treatment after processing, and exhibits excellent flatness, fatigue resistance, and toughness. [Modes for carrying out the invention]

[0010] The embodiments of the present invention will be described in detail below. The present invention is not limited to the embodiments described below, and it should be understood that modifications, improvements, etc., made to the embodiments described below, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the invention, also fall within the scope of the present invention. In this specification, unless otherwise specified, any "%" indication for ingredients refers to "mass%".

[0011] <Steel plate> The steel sheet according to the embodiment of the present invention has a composition containing 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 remainder being Fe and impurities. Herein, in this specification, "steel plate" means a plate-shaped (including strip-shaped) material formed from steel. Furthermore, in this specification, "impurities" refers to components that are mixed in during the industrial manufacturing of steel sheets due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and which are acceptable as long as they do 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. Furthermore, regarding the content of each element, "xx% or less" means that it is xx% or less, but includes an amount greater than 0% (especially above the impurity level). Also, in this specification, numerical ranges expressed using "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits, respectively.

[0012] Furthermore, the steel sheet according to the embodiment of the present invention may further include 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. The details of the above composition will be explained below.

[0013] (C: 0.07~0.30%) Carbon (C) is an essential element for increasing the strength of steel plates and improving their fatigue resistance and punchability. To fully obtain these effects, the C content should be 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 plate will harden and its toughness will decrease. For this reason, the C content should be 0.30% or less, preferably 0.29% or less, and more preferably 0.28% or less. Furthermore, the numerical ranges for the C content (ranges with defined upper or lower limits) mentioned above may be any combination of these ranges. Therefore, for example, the C content can be 0.07~0.30%, 0.08~0.30%, 0.09~0.30%, 0.07~0.29%, 0.08~0.29%, 0.09~0.29%, 0.07~0.28%, 0.08~0.28%, or 0.09~0.28%. The same applies to the numerical ranges for the content of the other elements listed below.

[0014] (Si: 0.01~0.65%) Si is an element necessary for deoxidation. To fully obtain this effect, the Si content should be 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 will decrease. Therefore, the Si content should be 0.65% or less, preferably 0.60% or less, and more preferably 0.58% or less.

[0015] (Mn: 0.80 - 2.30%) Mn is an element that affects properties such as the strength and punching property of the steel plate. If the Mn content is too low, ferrite is likely to form, resulting in a decrease in the strength of the steel plate and the inability to ensure fatigue resistance and punching property. Therefore, the Mn content should be 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 plate will harden and the toughness will decrease. Therefore, the Mn content should be 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 deteriorate. Therefore, the P content should be 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 is, the better, the lower limit is not particularly limited. However, excessive reduction of the P content will lead to an increase in cost, so the P content can be, for example, 0.001% or more.

[0017] (S: 0.100% or less) S forms MnS and is likely to become a starting point for fracture, reducing the workability of the steel plate. Therefore, the S content should be 0.100% or less, preferably 0.095% or less, and more preferably 0.090% or less. On the other hand, since the lower the S content is, the better, the lower limit is not particularly limited. However, excessive reduction of the S content will lead to an increase in cost, so the S content can be, 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, the number of inclusions increases, and the workability of the steel sheet decreases. For this reason, the Al content should be 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 there is no particular lower limit. However, from the viewpoint of obtaining the above-mentioned effects of Al, the Al content can be, for example, 0.001% or more, 0.003% or more, or 0.005% or more.

[0019] (Cr:1.00% or less) Cr is an effective element for forming a specific metallic structure. However, if the Cr content is too high, the steel sheet will become too strong and its toughness will not be ensured. For this reason, the Cr content should be 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 there is no particular lower limit. 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 grain coarsening through a pinning effect. However, if the N content is too high, this effect saturates, leading to a decrease in toughness. For this reason, the N content should be 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, excessive reduction of the N content leads to increased costs, so the N content can be, 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 its strength without impairing its toughness. However, Ni is an expensive element, and excessive amounts increase costs. Therefore, the Ni content should be 1.000% or less, preferably 0.950% or less, and more preferably 0.900% or less. On the other hand, the Ni content can be low, and there is no particular lower limit. However, from the viewpoint of obtaining the above-mentioned effects of Ni, the Ni content can be, 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 steel sheets. However, if the Mo content is too high, the steel sheet hardens, reducing its toughness and uniform elongation. For this reason, the Mo content should be 0.700% or less, preferably 0.650% or less, and more preferably 0.600% or less. On the other hand, the Mo content can be low, and there is no particular lower limit. However, from the viewpoint of obtaining the above effects of Mo, the Mo content can be, 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 steel sheets through carbide precipitation. However, if the content of these elements is too high, a large amount of carbides will be generated, reducing the toughness of the steel sheet. For this reason, the V content should be 0.500% or less, preferably 0.480% or less, more preferably 0.460% or less; the Nb content should be 0.500% or less, preferably 0.480% or less, more preferably 0.460% or less; and the Ti content should be 0.150% or less, preferably 0.148% or less, more preferably 0.145% or less. On the other hand, the content of these elements may be low, and there is no particular lower limit. However, from the viewpoint of obtaining the above effects from these elements, the content of V, Nb, and Ti can all be, 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, its effect saturates, and raw material costs increase. For this reason, the B content should be 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 there is no particular lower limit. However, from the viewpoint of obtaining the above effect of B, the B content can be, 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:0~0.0 50%, 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 have to be present in the steel sheet. These elements may be present as impurities individually or in pairs or combinations. The content of Cu, W, and Ta is all 0-0.15%, preferably 0-0.14%. The content of Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca is all 0-0.050%, preferably 0-0.045%.

[0026] Next, the metal structure of a steel sheet according to an embodiment of the present invention will be described. The steel sheet according to the embodiment of the present invention has a ferrite area ratio of 0 to 10.0%, a martensite area ratio of 90.0 to 100%, an average ferrite grain size 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 / mm². 2 The following applies:

[0027] (Ferrite area ratio: 0-10.0% and martensite area ratio: 90.0-100%) The steel sheet according to the embodiment of the present invention has a metallic structure mainly composed of martensite, which hardens it and improves its fatigue resistance and punchability. From the viewpoint of obtaining this effect, the area ratio of martensite is 90.0% or more, preferably 91.0% or more. On the other hand, the upper limit of the area ratio of martensite is 100% (i.e., it may be a single-phase martensite structure). Furthermore, the metal structure of the steel sheet according to the embodiment of the present invention may include ferrite. From the viewpoint of ensuring the effect of martensite as described above, the area ratio of ferrite is 10.0% or less, preferably 9.0% or less. On the other hand, the lower limit of the area ratio of ferrite is 0%. Furthermore, the metal structure of the steel sheet according to the embodiment of the present invention may include phases other than martensite and ferrite (for example, pearlite, bainite, etc.) to the extent that they do not hinder the effects of martensite described above.

[0028] The area ratios of martensite and ferrite can be determined as follows: First, the cross-section (L-section) parallel to the rolling direction of the test specimen cut from the steel plate is polished, and then immersed in a 3% nital etching solution to reveal the microstructure. Using a scanning electron microscope (SEM), the microstructure is observed at five arbitrary locations on this cross-section, with the center of the field of view being at a position corresponding to 1 / 4 of the steel plate's thickness. The magnification is set to 500-3000x depending on the size of the crystal grains. In the obtained microstructure images, martensite (white microstructure) and ferrite (black microstructure) are identified, and the area ratio of martensite and ferrite (the area percentage of martensite and ferrite in the entire measurement area) is calculated using image analysis software. The area ratio of martensite and ferrite is the average of the measurement results from the five locations. If the metal structure does not contain phases other than martensite and ferrite, The area ratios of martensite and ferrite can be determined by first calculating the area ratio of one of them (for example, the area ratio of ferrite), and then using the remaining area ratio as the area ratio of the other (for example, the area ratio of martensite).

[0029] (Average crystal 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. For this reason, 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, since a smaller average grain size of ferrite is preferable, the lower limit is not particularly limited. Furthermore, the steel sheet according to the embodiment of the present invention may be composed of a single-phase martensite structure, in which case ferrite is not present, and therefore 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-section) parallel to the rolling direction of a test specimen cut from a steel plate is polished, and then immersed in a 3% nital etching solution to reveal the microstructure. Using a scanning electron microscope (SEM), the microstructure is observed at five arbitrary locations on this cross-section, with the center of the field of view being at a point corresponding to 1 / 4 of the steel plate's thickness. The magnification is set to 500-3000x depending on the size of the crystal grains. The average crystal grain size is determined from the obtained microstructure images using the sectioning method. The average crystal grain size is the average of the measurement results from the five locations.

[0031] (GAM value: 1.0~10.0°) The GAM (Grain Average Misorientation) value is obtained from crystal orientation analysis data obtained by back-scattering diffraction (EBSD). It is calculated by measuring within crystal grains distinguished by large-angle grain boundaries with orientation differences of 15° or more, with a step size of 0.2 μm between measurement points, calculating the orientation difference for adjacent measurement points, and then averaging the calculated orientation differences within the same crystal grain. When the GAM value is small, the average orientation difference within a single grain is small, resulting in the formation of uniform grains with little strain, or a continuous orientation gradient within the grain. On the other hand, when the GAM value is large, the average orientation difference within a grain is large, resulting in large local strains within a single grain. The GAM value is related to the uniform elongation and toughness of steel sheets, and by setting the GAM value to 1.0 to 10.0°, the uniform elongation and toughness of steel sheets can be improved.

[0032] (Number density of crystal grains with a grain size of 20.0 μm or larger: 500 grains / mm²) 2 below) Coarse crystal grains with a grain size of 20.0 μm or larger reduce toughness. Therefore, the number density of coarse crystal grains with a grain size of 20.0 μm or larger is 500 grains / mm³. 2 The following applies:

[0033] The GAM value and the number density of crystal grains with a grain size of 20.0 μm or larger are determined as follows. First, the cross-section (L-section) parallel to the rolling direction of the test specimen cut from the steel plate is polished. Then, with the center of the field of view being 1 / 4 of the thickness of the steel plate, electron back-scattering diffraction (EBSD) analysis is performed in three or more fields of view, each 200 μm in the thickness direction and 200 μm in the longitudinal direction, at a pitch of 0.2 μm. The grain size of the crystal grains is determined by orientation analysis using the analysis software OIM Analysis (manufactured by TSL Solutions Co., Ltd.) based on the EBSD data. The boundary between adjacent measurement points with an orientation difference of 15° or more is defined as a grain boundary, and the crystal grains are determined using the same software. The GAM value is the average value of each crystal grain size within the measurement range. Furthermore, the number density of crystal grains with a grain size of 20.0 μm or larger is determined by identifying the crystal grains with a grain size of 20.0 μm or larger, calculating the number of grains, and then measuring the area (mm²) of the measurement region. 2 It is calculated by dividing by (). The number density of these crystal grains is the average of the measurement results from five locations.

[0034] In the embodiment of the present invention, the steel sheet has a thickness tolerance of ±0.08 mm or less when the sheet width is 400 mm or less, and a maximum curvature in the rolling direction of 10 mm or less when the length in the rolling direction is 1 m. With such thickness tolerances and maximum curvature, the shape of the steel sheet can be said to be uniform. If the thickness tolerance or maximum curvature is outside the above range, the workability (e.g., punchability) may decrease. Here, the thickness tolerance and maximum warpage of the steel plate 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. With a Vickers hardness within this range, it can be said that hardening is achieved while ensuring workability (uniform elongation and toughness). Therefore, heat treatment after processing the steel sheet can be omitted. Vickers hardness is determined by polishing the cross-section (L-section) parallel to the rolling direction of a test specimen cut from a steel plate, and then performing a Vickers hardness test in accordance with JIS Z2244:2009 at a position that is 1 / 4 of the thickness of the steel plate. In the Vickers hardness test, the measurement load is set to 1 kgf (9.807 N), measurements are taken at any three locations, and the average value of these measurements is taken as the measurement result.

[0036] (Uniform elongation: 1.5% or more) In the embodiment of the present invention, the steel sheet preferably has a uniform elongation of 1.5% or more. A uniform elongation within this range indicates good workability. Uniform elongation is determined by preparing a No. 13B test specimen in accordance with JIS Z2241:2023 and conducting a tensile test with the tensile axis in the rolling direction of the steel sheet. Three No. 13B test specimens are taken from the steel sheet, measurements are taken on these three specimens, and the average value of these measurements is taken as 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 a cold-rolled steel sheet is preferred. The thickness of the steel sheet is not particularly limited, but for example, it can be 10.0 mm or less, 8.0 mm or less, or 6.0 mm or less.

[0038] <Method of manufacturing steel plates> The method for manufacturing steel sheets according to the embodiment of the present invention is not particularly limited as long as it can produce steel sheets having the above-described characteristics. For example, a steel sheet according to an embodiment of the present invention can be manufactured by a method that includes a hot-rolling step in which a slab having the composition described above is hot-rolled, and then, when wound into a coil, is cooled in a temperature range of 700 to 100°C under conditions where the ratio of plate thickness to cooling rate (plate thickness / cooling rate) is 0.005 mm·sec / °C or more, and a cold-rolling step in which the hot-rolled steel sheet obtained from the hot-rolling is cold-rolled. After each step, known steps such as pickling steps may be further included. These known steps are not particularly limited and can be carried out in accordance with known methods. The following provides a detailed explanation of each step.

[0039] (Hot rolling process) The conditions for hot rolling are not particularly limited and can be carried out in accordance with known methods. For example, a slab having the composition described above can be heated to 1100-1350°C and hot-rolled at a rolling rate of 5-30%. Hot-rolled steel sheets obtained by hot rolling are wound into coils. At this time, they are 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 higher. Cooling under these conditions ensures uniformity of the steel sheet shape. From the viewpoint of stably ensuring this effect, it is preferable that the ratio of sheet thickness to cooling rate is 0.015 mm·sec / °C or higher. There is no particular upper limit to the ratio of sheet thickness to cooling rate, but for example, it is 0.300 mm·sec / °C. On the other hand, if the ratio of sheet thickness to 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 described above become large.

[0040] (Cold rolling process) Cold rolling is performed with a rolling ratio of 20-65%. By performing cold rolling within this rolling ratio range, a GAM value of 1.0-10.0° and a number density of 500 grains / mm² for grains with a grain size of 20.0 μm or larger can be achieved. 2 Each of the following can be controlled. From the viewpoint of stably ensuring this effect, the rolling ratio of cold rolling is preferably 30-60%. On the other hand, if the rolling ratio of cold rolling exceeds 65%, the strength becomes too high, and uniform elongation and toughness decrease. Also, if the rolling ratio of cold rolling is less than 20%, the strength becomes insufficient, and fatigue resistance and punchability decrease.

[0041] Furthermore, the steel sheet according to the embodiment of the present invention can also be manufactured by a method comprising: a hot rolling step of hot rolling a slab having the composition described above; a cold rolling step of cold rolling the hot-rolled steel sheet obtained from hot rolling; an annealing step of annealing the cold-rolled steel sheet obtained from cold rolling and cooling it in the temperature range of 700 to 100°C under conditions where the ratio of plate thickness to cooling rate (plate thickness / cooling rate) is 0.005 mm·sec / °C or more; and a final cold rolling step of cold rolling the cold-rolled and annealed steel sheet obtained from the annealing step. After each step, known steps such as pickling steps may be further included. These known steps are not particularly limited and can be carried out in accordance with known methods. The following provides a detailed explanation of each step.

[0042] (Hot rolling process) The conditions for hot rolling are not particularly limited and can be carried out in accordance with known methods. For example, a slab having the composition described above can be heated to 1100-1350°C and hot-rolled at a rolling rate of 5-30%. In this method, the conditions for winding the hot-rolled steel sheet obtained by hot rolling into a coil are not particularly limited.

[0043] (Cold rolling process) The conditions for the cold rolling process are not particularly limited and can be carried out in accordance with known methods. For example, a hot-rolled steel sheet obtained by hot rolling can be cold-rolled at a rolling ratio of 30-65%.

[0044] (Annealing process) The annealing conditions are not particularly limited and can be carried out according to known methods. For example, a cold-rolled steel sheet obtained by cold rolling can be held at 750-900°C for 1-5 minutes. After annealing, the steel sheet is cooled in the temperature range of 700 to 100°C under conditions where the ratio of thickness to cooling rate (thickness / cooling rate) is 0.005 mm·sec / °C or higher. Cooling under these conditions ensures uniformity of the steel sheet shape. From the viewpoint of stably ensuring this effect, it is preferable that the ratio of thickness to cooling rate is 0.015 mm·sec / °C or higher. There is no particular upper limit to the ratio of thickness to cooling rate, but for example, it is 0.300 mm·sec / °C. On the other hand, if the ratio of thickness to cooling rate is less than 0.005 mm·sec / °C, uniformity of the steel sheet shape cannot be ensured, and the thickness tolerance and maximum warpage described above will increase.

[0045] (Final cold rolling process) The final cold rolling is performed with a rolling ratio of 1.0% to less than 30%. By performing the final cold rolling within this rolling ratio range, the GAM value is 1.0 to 10.0°, and the number density of crystal grains with a grain size of 20.0 μm or larger is 500 grains / mm². 2 Each of the following can be controlled. From the viewpoint of stably ensuring this effect, the rolling ratio of cold rolling is preferably 1.5 to 25%. On the other hand, if the rolling ratio of cold rolling is 30% or more, the strength becomes too high, and uniform elongation and toughness decrease. Also, if the rolling ratio of cold rolling is less than 1.0%, the strength becomes insufficient, and fatigue resistance and punchability decrease. [Examples]

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

[0047] The steel plates were manufactured using the following two methods. <Method A> A 250 mm thick slab with the composition shown in Table 1 (the remainder being Fe and impurities other than the elements shown in Table 1) was manufactured by continuous casting. Next, the slab was heated to 1300°C and hot-rolled with a final rolling ratio of 15%. Then, when winding it into a coil, it was cooled in the temperature range of 700 to 100°C at a ratio of plate thickness to cooling rate (plate thickness / cooling rate) shown in Table 2. After that, the hot-rolled steel sheet was (final) cold-rolled at the rolling ratio shown in Table 2 to obtain a cold-rolled steel sheet. Furthermore, the width of the steel sheet during cold rolling was limited to 400 mm or less. Also, in Method A in Table 2, the sheet thickness refers to the sheet thickness during hot rolling, and the cooling rate refers to the cooling rate when winding into a coil.

[0048] <Method B> A 250 mm thick slab with the composition shown in Table 1 (the remainder being Fe and impurities other than the elements shown in Table 1) was manufactured by continuous casting. Next, the slab was heated to 1250°C and hot-rolled at a final rolling rate of 15%, and wound into a coil. Next, the hot-rolled steel sheet was cold-rolled at the rolling rate 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 the temperature range of 700 to 100°C at the ratio of sheet thickness to cooling rate (sheet thickness / cooling rate) shown in Table 2. After that, the cold-rolled and annealed sheet was cold-rolled at the rolling rate shown in Table 2 to obtain a cold-rolled steel sheet. Furthermore, the width of the steel sheet during cold rolling was limited to 400 mm or less. In addition, in Method B in Table 2, the sheet thickness refers to the sheet 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] [Table 1]

[0050] [Table 2]

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

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

[0053] (GAM value, and number density of crystal grains with a crystal 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 piece had dimensions of 15 mm in the rolling direction × 10 mm in the width direction × 2 mm in thickness. Also, in Table 3, those with a GAM value of 1.0 to 10.0° are indicated as OK, and those with a GAM value not within 1.0 to 10.0° are indicated as NG. Furthermore, the number density of crystal grains with a crystal grain size of 20.0 μm or more is abbreviated as "number density", and when the number density is 500 grains / mm 2 or less, it is indicated as OK, and when the number density exceeds 500 grains / mm 2 it is indicated as NG.

[0054] (Plate thickness tolerance) The plate thickness tolerance of the cold-rolled steel sheet was measured in accordance with JIS G3141:2021. Specifically, the plate thickness was measured at three arbitrary locations at least 15 mm inward from the ends in the width direction of the cold-rolled steel sheet (with a plate width of 400 mm or less). In this measurement, those with a deviation (tolerance) from the set plate thickness of ±0.08 mm or less were indicated as OK (small plate thickness tolerance), and those with a deviation (tolerance) from the set plate thickness exceeding ±0.08 mm were indicated as NG (large plate thickness tolerance).

[0055] (Maximum camber) The maximum warpage in the rolling direction of cold-rolled steel sheets was measured in accordance with JIS G3141:2021. Specifically, a cold-rolled steel sheet was sheared to a length of 1 m in the rolling direction, and the sheared sheet was placed on a surface plate (horizontal stand). The value of warpage (also called flatness) was calculated by subtracting the thickness of the cold-rolled steel sheet from the maximum vertical distance from the top surface of the surface plate to the surface of the cold-rolled steel sheet. In addition, the warpage was measured with one side of the cold-rolled steel sheet facing upwards, and then measured with the other side facing upwards. The maximum value among the measured warpages was defined as the maximum warpage. A maximum warpage of 10 mm or less was indicated as OK (high flatness), and a maximum warpage exceeding 10 mm was indicated as NG (low flatness).

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

[0057] (Uniform stretching) The uniform elongation was measured according to the method described above. In this evaluation, a uniform elongation of 1.5% or more is indicated as OK (good uniform elongation), and a uniform elongation of less than 1.5% is indicated as NG (insufficient uniform elongation).

[0058] (Fatigue resistance) Fatigue resistance was determined by conducting a plane bending fatigue test in accordance with JIS Z2275:1978, and the fatigue limit was determined. Specifically, a No. 1 test specimen (b = 15 mm, R = 30 mm) as specified in JIS Z2275:1978 was cut from a cold-rolled steel sheet, and a plane bending fatigue test was performed using a plane bending testing machine under the conditions of a stress ratio R = -1 and a frequency of 25 Hz. Here, the stress cycles until fracture were measured at each stress amplitude, the SN curve was determined, and the fatigue strength (fatigue limit) at 10,000,000 cycles was determined. In this evaluation, if the fatigue limit was 0.8 × TS (tensile strength) or higher, it was expressed as OK (good fatigue resistance), and if the fatigue limit was less than 0.8 × TS, it was expressed as NG (insufficient fatigue resistance). The tensile strength was determined using the method specified in JIS Z2241:2023, employing a JIS No. 5 tensile test specimen. The crosshead displacement rate for the tensile test was set to 30 mm / min.

[0059] (Dynamic properties) V-notch test specimens were taken from cold-rolled steel sheets and subjected to Charpy impact tests at 150°C. The tests were conducted in accordance with JIS Z2242:2023, and the test specimens were V-notched with a thickness of 2.0 mm, and taken so that the length direction was parallel to the rolling direction. In this evaluation, materials with a brittle fracture surface ratio of 70% or less are classified as OK (good toughness), while those with a brittle fracture surface ratio exceeding 70% are classified as NG (insufficient toughness).

[0060] (Punching properties) A 10mm diameter hole was punched out of a cold-rolled steel sheet with a punching clearance of 3%. The edge of the punched sample was observed, and the amount of sag was measured. The amount of sag was defined as the height difference between the center of the punched sample and the edge. In this evaluation, a sag amount of 100 μm or less is indicated as OK (good punching performance), and a sag amount exceeding 100 μm is indicated as NG (insufficient punching performance).

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

[0062] [Table 3]

[0063] As shown in Table 3, the cold-rolled steel sheets of Examples 1 to 35 had appropriate steel sheet composition and metal structure, resulting in good Vickers hardness, uniform elongation, fatigue resistance, toughness, and punchability, as well as good results for sheet thickness tolerance and maximum warpage. In contrast, the cold-rolled steel sheet of Comparative Example 1 had too much carbon content, resulting in high strength but insufficient toughness. The cold-rolled steel sheet in Comparative Example 2 had insufficient strength due to its low carbon content, resulting in poor fatigue resistance and punchability. The cold-rolled steel sheet in Comparative Example 3 had insufficient toughness due to its excessively high Si content. The cold-rolled steel sheet in Comparative Example 4 had too little Mn content, resulting in excessive ferrite precipitation and insufficient strength, fatigue resistance, and punchability. The cold-rolled steel sheet in Comparative Example 5 had too much Mn content, resulting in high strength but insufficient toughness. The cold-rolled steel sheet in Comparative Example 6 had excessively high Mn and Cr content, resulting in high strength but insufficient toughness.

[0064] In Comparative Example 7, the cold-rolled steel sheet had an excessively high rolling ratio during the (final) cold rolling process, making it impossible to achieve a GAM value of 1.0 to 10.0°. As a result, uniform elongation and toughness were insufficient. In Comparative Example 8, the cold-rolled steel sheet had too low a rolling ratio during the final cold rolling process, resulting in a GAM value of 1.0 to 10.0° and a grain density of 500 grains / mm² for grains with a size of 20.0 μm or larger. 2 The following could not be achieved, and the plate thickness tolerance and maximum warpage also increased. As a result, fatigue resistance and punching performance became insufficient. In Comparative Example 9, the cold-rolled steel sheet exhibited increased thickness tolerance and maximum warpage due to inappropriate cooling conditions after annealing.

[0065] As can be seen from the above results, the present invention provides a steel sheet that has high hardness, eliminates the need for post-processing heat treatment, and exhibits excellent flatness, fatigue resistance, and toughness.

[0066] Therefore, by adopting the following embodiments [1] to [6], the present invention can provide a steel sheet that has high hardness, eliminates the need for post-processing heat treatment, and exhibits excellent flatness, fatigue resistance, and toughness.

[0067] [1] A composition having the following 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 remainder being Fe and impurities. The area ratio of ferrite is 0-10.0%, and the area ratio of martensite is 90.0-100%. The average crystal grain size of the ferrite is 15.0 μm or less. The GAM value is between 1.0 and 10.0°. The number density of crystal grains with a grain size of 20.0 μm or larger is 500 grains / mm². 2 The following: When the plate width is 400 mm or less, the plate thickness tolerance is ±0.08 mm or less. A steel plate in which the maximum curvature in the rolling direction is 10 mm or less when the length in the rolling direction is 1 m. [2] The steel sheet according to [1], further comprising 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, on a mass basis. [3] The steel sheet according to [1] or [2], wherein the impurities include one or more selected by mass from the group consisting of 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] Cold-rolled steel sheet, the steel sheet described in any one of [1] to [3]. [5] A steel plate described in any one of [1] to [4], having a Vickers hardness of 300 to 550 Hv. [6] A steel sheet described in any one of [1] to [5], having a uniform elongation of 1.5% or more.

Claims

1. The composition, by mass, contains C: 0.07-0.30%, Si: 0.01-0.65%, Mn: 0.80-2.30%, P: 0.001-0.100%, S: 0.001-0.100%, Al: 0.001-0.100%, Cr: 0.01-1.00%, and N: 0.0001-0.0150%, with the remainder being Fe and impurities. The area ratio of ferrite is 0-10.0%, and the area ratio of martensite is 90.0-100%. The average crystal grain size of the ferrite is 15.0 μm or less. The GAM value is between 1.0 and 10.0°. The number density of crystal grains with a grain size of 20.0 μm or larger is 500 grains / mm². 2 The following: When the plate width is 400 mm or less, the plate thickness tolerance is ±0.08 mm or less. A steel plate in which the maximum curvature in the rolling direction is 10 mm or less when the length in the rolling direction is 1 m.

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

3. The steel sheet according to claim 1 or 2, wherein the impurities include one or more selected from the group consisting of 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%, on a mass basis.

4. The steel sheet according to claim 1 or 2, which is a cold-rolled steel sheet.

5. The steel plate according to claim 1 or 2, wherein the Vickers hardness is 300 to 550 Hv.

6. The steel sheet according to claim 1 or 2, wherein the uniform elongation is 1.5% or more.

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