steel sheet

Abstract

A steel plate has the following composition: by weight, it contains C: 0.07~0.30%, Si: 0.01~0.65%, Mn: 0.80~2.30%, P: less than 0.100%, S: less than 0.100%, Al: less than 0.100%, Cr: less than 1.00%, and N: less than 0.0150%, with the balance being Fe and impurities. The steel plate has a ferrite area fraction of 0~10.0%, a martensite area fraction of 90.0~100%, an average ferrite grain diameter of less than 15.0 μm, a GAM value of 1.0~10.0°, and a grain number density of 500 grains / mm² with a grain diameter of 20.0 μm or more. 2 Furthermore, when the width of the steel plate is 400mm or less, the thickness tolerance is ±0.08mm or less, and when the length in the rolling direction is 1m, the maximum warpage in the rolling direction is 10mm or less.

Description

Technical Field

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

[0002] Medium carbon steel plates and high carbon steel plates (hereinafter referred to as "steel plates") are used as blanks for structural components and mechanical parts used in automobiles and other machinery and devices. After being processed into specified shapes, the steel plates are transformed into various components through heat treatments such as quenching and tempering.

[0003] As for the steel plates used in the various components described above, for example, Patent Document 1 discloses a steel plate having, in terms of area ratio, martensite of 20% or more and 100% or less, ferrite of 0% or more and 80% or less, other metallic phases of 5% or less, and a steel structure in which the dislocation density of the metallic phases on the surface of the steel plate is 30% or more and 80% or less relative to the dislocation density of the metallic phases in the central part of the plate thickness, and the maximum warpage of the steel plate when sheared in the rolling direction with a length of 1 m is 15 mm or less.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: International Publication No. 2021 / 085336 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] In recent years, due to the increasing demand for suppressing CO2 emissions, there is a need to eliminate the need for heat treatment after machining steel sheets into specified shapes, thus requiring the hardening of steel sheets. On the other hand, hardening steel sheets reduces machinability. Therefore, in order to eliminate the need for post-machining heat treatment, it is important to ensure machinability and achieve hardening. As a method for hardening steel sheets, it is effective to produce steel sheets with a martensitic structure, but the shape of the steel sheet is destroyed during quenching due to the martensitic structure, and the flatness is easily reduced.

[0009] In addition, when using steel plates as blanks for components such as chains, the steel plates themselves must also have fatigue characteristics and toughness.

[0010] In the water-quenching process described in Patent Document 1, the steel plate is restrained from both the surface and back sides by two rollers clamping the steel plate in a manner that meets specified conditions in the area where the surface temperature of the steel plate is below (Ms point + 150°C). This improves the uniformity of the steel plate's shape and thus ensures the flatness of the steel plate itself. On the other hand, while the steel plate in Patent Document 1 is hardened due to its predominantly martensitic structure, it cannot be said that post-processing heat treatment can be omitted. Therefore, to ensure the strength of the component, post-processing heat treatment is necessary to improve its strength.

[0011] The present invention was made to solve the above-mentioned problems, and its purpose is to provide a steel plate with high hardness, which can eliminate the need for post-processing heat treatment, and excellent flatness, fatigue resistance and toughness.

[0012] Solution for solving the problem

[0013] In order to solve the above-mentioned problems, the inventors conducted in-depth research and found that by controlling the composition and metallographic structure of the steel plate, flatness can be ensured and hardness and fatigue resistance can be improved, thus completing the present invention.

[0014] That is, the present invention relates to a steel plate having the following composition: by weight, containing C: 0.07~0.30%, Si: 0.01~0.65%, Mn: 0.80~2.30%, P: less than 0.100%, S: less than 0.100%, Al: less than 0.100%, Cr: less than 1.00%, and N: less than 0.0150%, with the balance being Fe and impurities.

[0015] The area fraction of ferrite is 0~10.0%, and the area fraction of martensite is 90.0~100%.

[0016] The average grain diameter of the aforementioned ferrite is less than 15.0 μm.

[0017] GAM value is 1.0~10.0°.

[0018] The number density of grains with a diameter of 20.0 μm or larger is 500 grains / mm. 2 the following,

[0019] For plates with a width of 400mm or less, the thickness tolerance is ±0.08mm or less.

[0020] The maximum warpage in the rolling direction when the rolling length is 1m is less than 10mm.

[0021] The effects of the invention

[0022] According to the present invention, it is possible to provide steel plates with high hardness, which eliminate the need for post-processing heat treatment, and excellent flatness, fatigue resistance and toughness. Detailed Implementation

[0023] The embodiments of the present invention will be described in detail below. It should be understood that the present invention is not limited to the following embodiments. Without departing from the spirit of the present invention, appropriate changes or improvements to the following embodiments based on the common knowledge of those skilled in the art also fall within the scope of the present invention.

[0024] It should be noted that, unless otherwise specified, the "%" in this specification refers to "mass%" when referring to ingredients.

[0025] <Steel Plate>

[0026] The steel plate of the present invention has the following composition: containing C: 0.07~0.30%, Si: 0.01~0.65%, Mn: 0.80~2.30%, P: less than 0.100%, S: less than 0.100%, Al: less than 0.100%, Cr: less than 1.00%, and N: less than 0.0150%, with the balance being Fe and impurities.

[0027] In this specification, "steel plate" refers to a plate-shaped (including strip-shaped) material made of steel.

[0028] Furthermore, the term "impurity" in this specification refers to components that are permissible during the industrial manufacturing of steel plates due to various reasons related to raw materials such as ores and waste, as well as manufacturing processes, and that do not adversely affect the present invention. For example, impurities may also include unavoidable impurities. Examples of impurities include Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca.

[0029] It should be noted that, regarding the content of each element, "below xx%" means the amount contained below xx% but exceeding 0% (especially exceeding the impurity level). Furthermore, the numerical range indicated by "~" in this specification refers to the range including the values ​​before and after "~" as both the lower and upper limits.

[0030] In addition, the steel plate of the embodiments of the present invention may further contain one or more components 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.

[0031] The details of the above components will be explained.

[0032] (C: 0.07~0.30%)

[0033] Carbon (C) is an element required to improve the strength, fatigue resistance, and punching properties of steel sheets. To achieve these effects, the C content is set at 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 will harden and its toughness will decrease. Therefore, the C content is set at 0.30% or less, preferably 0.29% or less, and more preferably 0.28% or less.

[0034] It should be noted that the numerical range of C content mentioned above (the range specifying the upper or lower limit) can be any combination of these numerical ranges. Therefore, for example, the C content can be set to 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 contents of the other elements listed below.

[0035] (Si: 0.01~0.65%)

[0036] Si is an element required for deoxidation. To achieve this effect, the Si content is set at 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 is set at 0.65% or less, preferably 0.60% or less, and more preferably 0.58% or less.

[0037] (Mn: 0.80~2.30%)

[0038] Mn is an element that affects the strength and punching properties of steel sheets. If the Mn content is too low, ferrite is easily formed, thus reducing the strength of the steel sheet and compromising fatigue resistance and punching properties. Therefore, the Mn content is set at 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 hardens and its toughness decreases. Therefore, the Mn content is set at 2.30% or less, preferably 2.25% or less, and more preferably 2.20% or less.

[0039] (P: below 0.100%)

[0040] A lower phosphorus (P) content is preferred, as excessive P content reduces the toughness and other properties of the steel plate. Therefore, 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, a lower P content is preferred, so there is no particular lower limit. However, excessively reducing the P content will lead to increased costs, so the P content can be set to, for example, 0.001% or more.

[0041] (S: below 0.100%)

[0042] Sulfur (S) can form MnS, which can easily become the starting point for damage and reduce 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 limitation on the lower limit. However, excessive reduction of the S content will lead to increased costs, so the S content can be set to, for example, 0.001% or more.

[0043] (Al: below 0.100%)

[0044] 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 can be low, and there is no particular limitation on the lower limit. However, from the viewpoint of obtaining the aforementioned effects brought about by Al, the Al content can be set to, for example, 0.001% or more, 0.003% or more, or 0.005% or more.

[0045] (Cr: less than 1.00%)

[0046] Cr is an effective element for forming the desired metallographic structure. However, if the Cr content is too high, the steel sheet will have high strength, but toughness cannot be guaranteed. 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 can be low, and there is no particular limitation on the lower limit. However, from the viewpoint of obtaining the aforementioned effects brought about by Cr, the Cr content can be set to, for example, 0.01% or more, 0.05% or more, 0.10% or more, 0.20% or more, or 0.30% or more.

[0047] (N: below 0.0150%)

[0048] Nitrogen (N) is an element that forms AlN and suppresses grain coarsening through a pinning effect. However, if the N content is too high, its effect becomes saturated, leading to a decrease in toughness. Therefore, the N content is set to 0.0150% or less, preferably 0.0140% or less, and most preferably 0.0130% or less. On the other hand, the N content can be low, and there is no particular lower limit. However, excessively low N content will lead to increased costs, so the N content can be set to, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0049] (Ni: below 1.000%)

[0050] Ni is an element that increases strength in steel without compromising toughness. However, Ni is an expensive element, and excessive amounts increase 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 can be low, and there is no particular limitation on the lower limit. However, from the viewpoint of obtaining the aforementioned effects brought about by Ni, the Ni content can be set to, for example, 0.001% or more, 0.010% or more, or 0.050% or more.

[0051] (Mo: 0.700% or less)

[0052] Mo is an element that improves the strength of steel sheets. However, if the Mo content is too high, the steel sheet becomes hardened, and its 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 can be low, and there is no particular limitation on the lower limit. However, from the viewpoint of obtaining the aforementioned effects brought about by Mo, the Mo content can be set to, for example, 0.001% or more, 0.005% or more, or 0.010% or more.

[0053] (V: below 0.500%, Nb: below 0.500%, Ti: below 0.150%)

[0054] V, Nb, and Ti are all elements that increase the strength of steel plates 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 plate. Therefore, the V content is set to 0.500% or less, preferably 0.480% or less, more preferably 0.460% or less; the Nb content is set to 0.500% or less, preferably 0.480% or less, more preferably 0.460% or less; and the Ti content is set to 0.150% or less, preferably 0.148% or less, more preferably 0.145% or less. On the other hand, the content of these elements can be low, and there is no particular limitation on the lower limit. However, from the viewpoint of obtaining the above-mentioned effects brought about by these elements, the content of V, Nb, and Ti can all be set to, for example, 0.001% or more, 0.003% or more, or 0.005% or more.

[0055] (B: below 0.0100%)

[0056] Boron (B) is an element that segregates at grain boundaries and increases grain boundary strength. However, if the B content is too high, its effect becomes saturated, increasing raw material costs. 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 can be low, and there is no particular limitation on the lower limit. However, from the viewpoint of obtaining the aforementioned effects brought about by B, the B content can be set to, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0057] (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%)

[0058] Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca are impurities, but may not be present in the steel plate. These elements may be present individually or in groups of two or more as impurities.

[0059] The contents of Cu, W and Ta are all 0~0.15%, preferably 0~0.14%.

[0060] The contents of Sn, Sb, Co, As, Mg, Y, Zr, La, Ce and Ca are all 0~0.050%, preferably 0~0.045%.

[0061] Next, the metallographic structure of the steel plate according to the embodiment of the present invention will be described.

[0062] The steel plate of the embodiments of the present invention has a ferrite area fraction of 0~10.0%, a martensite area fraction of 90.0~100%, an average ferrite grain diameter of less than 15.0 μm, a GAM value of 1.0~10.0°, and a grain number density of 500 grains / mm with a grain diameter of more than 20.0 μm. 2 the following.

[0063] (Ferrite area ratio: 0~10.0% and martensite area ratio: 90.0~100%)

[0064] The steel sheet of the present invention is hardened by having a martensitic microstructure, thereby improving fatigue resistance and punching performance. From the viewpoint of achieving this effect, the martensite area ratio is set to 90.0% or more, preferably 91.0% or more. On the other hand, the upper limit of the martensite area ratio is 100% (i.e., it can also be a single-phase martensite microstructure).

[0065] Furthermore, the metallographic structure of the steel sheet according to embodiments of the present invention may contain ferrite. From the viewpoint of ensuring the effects brought about by the 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%.

[0066] It should be noted that the metallographic structure of the steel plate in the embodiments of the present invention may also contain phases other than martensite and ferrite (such as pearlite, bainite, etc.) without hindering the effects brought about by the martensite mentioned above.

[0067] The area ratios of martensite and ferrite are calculated as follows.

[0068] First, the cross-section (L-section) parallel to the rolling direction of the test piece cut from the steel plate was ground and then immersed in a 3% nitric acid-ethanol etching solution to reveal the microstructure. The center of the field of view was chosen at a point representing one-quarter of the steel plate's thickness, and the microstructure was observed at five arbitrary locations using a scanning electron microscope (SEM). The magnification was set to 500–3000x depending on the grain size. In the obtained microstructure images, martensite (white microstructure) and ferrite (black microstructure) were identified, and the area ratios of martensite and ferrite (the proportion of martensite and ferrite area in the overall measurement region) were calculated using image analysis software. The area ratios of martensite and ferrite were the average of the measurements at the five locations. It should be noted that the area ratios of martensite and ferrite can be calculated by taking the remaining area ratio of one (e.g., ferrite) and using it as the area ratio of the other (e.g., martensite).

[0069] (Average grain diameter of ferrite: less than 15.0 μm)

[0070] If the average grain diameter of ferrite is large, it is difficult to obtain a sufficient martensitic structure, resulting in insufficient hardening and reduced fatigue resistance and punching ability. Therefore, the average grain diameter of ferrite is 15.0 μm or less, preferably 13.0 μm or less, and more preferably 11.0 μm or less.

[0071] On the other hand, the smaller the average grain diameter of ferrite, the better, so there is no particular lower limit. Additionally, the steel plate in the embodiments of the present invention may also be composed of a single-phase martensitic structure, in which case ferrite is absent, and therefore the average grain diameter of ferrite can be considered as 0 μm.

[0072] The average grain diameter of ferrite was determined according to JIS G0551:2020.

[0073] Specifically, the cross-section (L-section) of the test piece cut from the steel plate, parallel to the rolling direction, was ground and then immersed in a 3% nitric acid-ethanol etching solution to reveal the microstructure. The center of the field of view was chosen at a point representing one-quarter of the steel plate's thickness, and the microstructure was observed at five arbitrary locations using a scanning electron microscope (SEM). The magnification was set to 500–3000x depending on the grain size. The average grain diameter was determined from the obtained microstructure images using the intercept method. The average grain diameter was the average of the measurements from the five locations.

[0074] (GAM value: 1.0~10.0°)

[0075] The GAM (Grain Average Misorientation) value is obtained from crystal orientation analysis data obtained using electron backscatter diffraction (EBSD). It is calculated by setting the distance between measurement points (hereinafter referred to as "step size") to 0.2 μm within a grain separated by large-angle grain boundaries with an orientation difference of more than 15°. The orientation difference of each adjacent measurement point is calculated, and the calculated orientation difference within the same grain is used as the average value.

[0076] When the GAM value is small, the average orientation difference within a grain becomes smaller, resulting in uniform grains with less strain or grains with a continuous orientation gradient. On the other hand, when the GAM value is large, the average orientation difference within a grain becomes larger, resulting in larger local strain within a grain.

[0077] The GAM value is related to the uniform elongation and toughness of the steel plate. By setting the GAM value to 1.0~10.0°, the uniform elongation and toughness of the steel plate can be improved.

[0078] (Number density of grains with a diameter of 20.0 μm or more: 500 grains / mm) 2 the following)

[0079] Large grains with a diameter greater than 20.0 μm reduce toughness. Therefore, the number density of large grains with a diameter greater than 20.0 μm is set to 500 grains / mm. 2 the following.

[0080] The GAM value and the number density of grains with a grain diameter of 20.0 μm or more are calculated as follows.

[0081] First, the section parallel to the rolling direction (L-section) of the test piece cut from the steel plate was ground, and the position at 1 / 4 of the steel plate thickness was used as the center of the field of view. Electron backscattering diffraction (EBSD) analysis was performed within a range of 200 μm in the thickness direction and 200 μm in the long side direction (field of view) at least three fields of view with a spacing of 0.2 μm. Regarding the grain diameter, based on the EBSD data, orientation analysis was performed using OIMAnalysis (manufactured by TSL Solutions KK) as the analysis software. Grain boundaries with an orientation difference of 15° or more from adjacent measurement points were defined as grain boundaries, and the grain diameter was calculated using this software. The GAM value is the average value of the grain diameters within the measurement range.

[0082] In addition, the number density of grains with a grain diameter of 20.0 μm or more is determined by identifying the number of grains with a grain diameter of 20.0 μm or more, calculating the number (in grains), and dividing by the area (mm²) of the measurement region. 2 The number density of the grains is calculated as the average of the measurements from five different locations.

[0083] In embodiments of the present invention, the thickness tolerance of the steel plate is ±0.08 mm or less when the plate width is 400 mm or less, and the maximum warpage in the rolling direction is 10 mm or less when the rolling length is 1 m. If the thickness tolerance and maximum warpage are within these ranges, the steel plate shape can be considered uniform. If the thickness tolerance and maximum warpage are outside the above ranges, the workability (e.g., punching properties) may decrease.

[0084] Here, the thickness tolerance and maximum warpage of the steel plate are determined according to JIS G3141:2021.

[0085] The Vickers hardness of the steel sheet in the embodiments of the present invention is preferably 300-550 Hv, more preferably 310-540 Hv, and even more preferably 315-530 Hv. With a Vickers hardness within this range, it can be said that workability (uniform elongation, toughness) and hardening are ensured. Therefore, post-processing heat treatment of the steel sheet can be omitted.

[0086] Vickers hardness is determined as follows: After grinding a section (L-section) of the test piece cut from the steel plate parallel to the rolling direction, a Vickers hardness test is performed at a point representing 1 / 4 of the steel plate thickness, according to JIS Z2244:2009. The Vickers hardness is then determined. In the Vickers hardness test, the test load is set to 1 kgf (9.807 N), and measurements are taken at any three locations. The average value of these measurements is taken as the result.

[0087] (Uniform elongation: ≥1.5%)

[0088] In embodiments 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.

[0089] The uniform elongation was determined by preparing test pieces No. 13B according to JIS Z2241:2023 and conducting tensile tests with the tensile axis as the rolling direction of the steel plate. Three test pieces No. 13B were collected from the steel plate, and the values ​​of these three test pieces were used for measurement. The average value of these values ​​was taken as the measurement result.

[0090] The steel plate used in embodiments of the present invention can be any type of hot-rolled steel plate or cold-rolled steel plate, but is preferably cold-rolled steel plate. The thickness of the steel plate is not particularly limited; for example, it can be 10.0 mm or less, 8.0 mm or less, or 6.0 mm or less.

[0091] <Methods for Manufacturing Steel Plates>

[0092] The method for manufacturing the steel plate according to the embodiments of the present invention is not particularly limited as long as it can manufacture a steel plate having the characteristics described above.

[0093] For example, the steel sheet according to embodiments of the present invention can be manufactured by a method including a hot rolling process and a cold rolling process. In the hot rolling process, after hot rolling a slab having the composition described above, it is cooled in a temperature range from 700°C to 100°C while being coiled, provided that the ratio of sheet thickness to cooling rate (sheet thickness / cooling rate) is 0.005 mm·s / °C or higher. In the cold rolling process, the hot-rolled steel sheet obtained by hot rolling is cold-rolled. It should be noted that, after each process, known processes such as pickling may be further included. These known processes are not particularly limited and can be performed according to known methods.

[0094] The following is a detailed explanation of each process.

[0095] (Hot rolling process)

[0096] There are no particular limitations on the conditions for hot rolling, and it can be carried out according to well-known methods. For example, a slab with the composition described above can be heated to 1100~1350°C and hot rolled at a rolling rate of 5~30%.

[0097] Hot-rolled steel sheets obtained through hot rolling are coiled into coils. At this time, cooling is performed in a temperature range from 700°C to 100°C, provided that the ratio of sheet thickness to cooling rate (sheet thickness / cooling rate) is 0.005 mm·s / °C or higher. Cooling under these conditions ensures the uniformity of the steel sheet shape. From the viewpoint of consistently ensuring this effect, the ratio of sheet thickness to cooling rate is preferably 0.015 mm·s / °C or higher. There is no particular upper limit to the ratio of sheet thickness to cooling rate; for example, it is 0.300 mm·s / °C. On the other hand, if the ratio of sheet thickness to cooling rate is less than 0.005 mm·s / °C, the uniformity of the steel sheet shape cannot be ensured, and the aforementioned sheet thickness tolerance and maximum warpage become larger.

[0098] (Cold rolling process)

[0099] Cold rolling is performed at a rolling rate of 20% to 65%. By performing cold rolling within this rolling rate range, the GAM value can be controlled to be 1.0 to 10.0°, and the number density of grains with a grain diameter of 20.0 μm or larger can be controlled to 500 grains / mm. 2 From the viewpoint of consistently ensuring this effect, the cold rolling rate is preferably 30-60%. On the other hand, if the cold rolling rate exceeds 65%, excessive strength is achieved, while uniform elongation and toughness decrease. Furthermore, if the cold rolling rate is less than 20%, insufficient strength is achieved, and fatigue resistance and punching performance decrease.

[0100] Furthermore, the steel sheet according to embodiments of the present invention can also be manufactured by a method including the following steps: a hot rolling step, in which a slab having the composition described above is hot rolled; a cold rolling step, in which the hot-rolled steel sheet obtained by hot rolling is cold rolled; an annealing step, in which the cold-rolled steel sheet obtained by cold rolling is annealed, and cooled in a temperature range from 700°C to 100°C under the condition that the ratio of plate thickness to cooling rate (plate thickness / cooling rate) is 0.005 mm·s / °C or higher; and a final cold rolling step, in which the cold-rolled annealed steel sheet obtained by the annealing step is cold rolled. It should be noted that, after each step, known steps such as pickling may be further included. These known steps are not particularly limited and can be performed according to known methods.

[0101] The following is a detailed explanation of each process.

[0102] (Hot rolling process)

[0103] There are no particular limitations on the conditions for hot rolling, and it can be carried out according to well-known methods. For example, a slab with the composition described above can be heated to 1100~1350°C and hot rolled at a rolling rate of 5~30%.

[0104] It should be noted that, under this method, there are no particular limitations on the conditions for coiling the hot-rolled steel sheet obtained by hot rolling into a coil.

[0105] (Cold rolling process)

[0106] There are no particular limitations on the conditions for the cold rolling process; it can be carried out using well-known methods. For example, hot-rolled steel sheets obtained through hot rolling can be cold-rolled at a rolling rate of 30-65%.

[0107] (Annealing process)

[0108] There are no particular restrictions on the annealing conditions; it can be carried out according to well-known methods. For example, the cold-rolled steel sheet obtained in the cold rolling process can be held at 750~900℃ for 1~5 minutes.

[0109] After annealing, the sheet is cooled in a temperature range from 700°C to 100°C, provided that the ratio of sheet thickness to cooling rate (sheet thickness / cooling rate) is 0.005 mm·s / °C or higher. Cooling under these conditions ensures the uniformity of the steel sheet shape. From the viewpoint of consistently ensuring this effect, the ratio of sheet thickness to cooling rate is preferably 0.015 mm·s / °C or higher. There is no particular upper limit to the ratio of sheet thickness to cooling rate; for example, it is 0.300 mm·s / °C. On the other hand, if the ratio of sheet thickness to cooling rate is less than 0.005 mm·s / °C, the uniformity of the steel sheet shape cannot be ensured, and the aforementioned sheet thickness tolerances and maximum warpage become larger.

[0110] (Final cold rolling process)

[0111] The final cold rolling is performed at a rolling rate of 1.0% or more but less than 30%. By performing the final cold rolling within this rolling rate range, the GAM value can be controlled to be 1.0~10.0°, and the number density of grains with a grain diameter of 20.0 μm or more can be controlled to 500 grains / mm. 2 From the viewpoint of consistently ensuring this effect, the cold rolling rate is preferably 1.5% to 25%. On the other hand, if the cold rolling rate is 30% or more, the strength is excessively increased, and the uniform elongation and toughness decrease. In addition, if the cold rolling rate is less than 1.0%, the strength is insufficient, and the fatigue resistance and punching ability decrease.

[0112] Example

[0113] The following examples illustrate the content of the present invention in detail, but the present invention is not limited to these examples.

[0114] Steel plates can be manufactured using the following two methods.

[0115] <Method A>

[0116] A 250 mm thick slab with the composition shown in Table 1 (balance being Fe and impurities other than those shown in Table 1) is manufactured using continuous casting. The slab is then heated to 1300°C, hot-rolled at a final pass rate of 15%, and then coiled and cooled in a temperature range from 700°C to 100°C at a thickness-to-cooling-rate ratio (thickness / cooling-rate) as shown in Table 2. Finally, the hot-rolled steel sheet is cold-rolled at the rolling rate shown in Table 2 to obtain a cold-rolled steel sheet.

[0117] It should be noted that the width of the steel sheet during cold rolling is set to 400mm or less. Additionally, in Method A of Table 2, the sheet thickness refers to the thickness of the hot-rolled sheet, and the cooling rate refers to the cooling rate when the sheet is coiled.

[0118] <Method B>

[0119] A 250 mm thick slab with the composition shown in Table 1 (balance: Fe and impurities other than those shown in Table 1) is manufactured using continuous casting. The slab is then heated to 1250 °C and hot-rolled at a final pass rate of 15%, coiled into a coil. Next, the hot-rolled steel sheet is cold-rolled at the rolling rate shown in Table 2 to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is then annealed at 850 °C for 3 minutes, followed by cooling in a temperature range from 700 °C to 100 °C at a thickness-to-cooling-rate ratio (thickness / cooling-rate) shown in Table 2. Finally, the annealed cold-rolled sheet is cold-rolled again at the rolling rate shown in Table 2 to obtain a cold-rolled steel sheet.

[0120] It should be noted that the width of the steel sheet during cold rolling is set to be 400mm or less. Additionally, in Method B of Table 2, the sheet 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.

[0121] [Table 1]

[0122]

[0123] [Table 2]

[0124]

[0125] The cold-rolled steel sheets obtained in the above embodiments were evaluated as follows.

[0126] (Area ratio of ferrite and martensite, average grain diameter of ferrite)

[0127] The area ratio of ferrite was calculated using the method described above, and the remaining area ratio was used as the area ratio of martensite. Additionally, the average grain diameter of ferrite was calculated using the same method. It should be noted that the test piece was 15 mm in the rolling direction × 10 mm in the width direction × 2.5 mm in thickness.

[0128] (GAM value and number density of grains with a diameter of 20.0 μm or more)

[0129] The GAM value and the specified grain number density were determined using the method described above. It should be noted that the test piece measures 15mm in the rolling direction, 10mm in the width direction, and 2mm in thickness.

[0130] Furthermore, in Table 3, GAM values ​​of 1.0–10.0° are represented as OK, while GAM values ​​outside this range are represented as NG. Moreover, the number density of grains with a diameter of 20.0 μm or larger is simply referred to as "number density," and a number density of 500 grains / mm is defined as... 2 The following situation is considered OK: the density exceeds 500 particles / mm. 2 The case is represented as NG.

[0131] (Plate thickness tolerance)

[0132] The thickness tolerance of cold-rolled steel sheets is determined according to JIS G3141:2021. Specifically, the thickness is measured at any three locations within 15 mm of the end of the cold-rolled steel sheet (width 400 mm or less) in the width direction. In this measurement, a deviation (tolerance) from the set thickness of ±0.08 mm or less is indicated as OK (small thickness tolerance), and a deviation (tolerance) exceeding ±0.08 mm is indicated as NG (large thickness tolerance).

[0133] (Maximum warpage)

[0134] The maximum warpage in the rolling direction of cold-rolled steel sheet is measured according to JIS G3141:2021. Specifically, the cold-rolled steel sheet is cut with a length of 1m in the rolling direction, and the cut sheet is placed on a platform (horizontal table). The warpage (also known as flatness) is calculated by subtracting the thickness of the cold-rolled steel sheet from the maximum value of the vertical distance from the upper surface of the platform to the surface of the cold-rolled steel sheet. Furthermore, the warpage is measured with one side of the cold-rolled steel sheet as the top, and then with the other side as the top. The maximum value of these measured warpages is taken as the maximum warpage. A maximum warpage of 10mm or less is indicated as OK (high flatness), and a maximum warpage exceeding 10mm is indicated as NG (low flatness).

[0135] (Vickers hardness)

[0136] The Vickers hardness was determined using the method described above.

[0137] (Uniform elongation)

[0138] The uniform elongation was determined according to the method described above. In this evaluation, a uniform elongation of 1.5% or more was indicated as OK (good uniform elongation), and a uniform elongation of less than 1.5% was indicated as NG (insufficient uniform elongation).

[0139] (Fatigue resistance)

[0140] Fatigue resistance was assessed using a plane bending fatigue test according to JIS Z2275:1978 to determine the fatigue limit. Specifically, test pieces (b = 15 mm, R = 30 mm) specified in JIS Z2275:1978 were cut from cold-rolled steel sheets and subjected to a plane bending fatigue test using a plane bending testing machine under stress ratio R = -1 and frequency of 25 Hz. The stress cycles to fracture were measured at various stress amplitudes, and the stress-to-fatigue ratio (SN) curve was calculated to determine the fatigue strength (fatigue limit) after 10,000,000 cycles. In this evaluation, a fatigue limit of 0.8 × TS (tensile strength) or higher was designated as OK (good fatigue resistance), while a fatigue limit less than 0.8 × TS was designated as NG (inadequate fatigue resistance).

[0141] It should be noted that the tensile strength was determined using the method specified in JIS Z2241:2023 for JIS No. 5 tensile test specimens. The crosshead displacement speed for the tensile test was set to 30 mm / min.

[0142] (toughness)

[0143] V-notch test pieces were collected from cold-rolled steel sheets and subjected to Charpy impact tests at 150°C. The tests were conducted in accordance with JIS Z2242:2023. The test pieces were collected with a V-notch and a sheet thickness of 2.0 mm, with the length direction parallel to the rolling direction.

[0144] In this evaluation, a brittle fracture surface area ratio of less than 70% is considered OK (good toughness), while a brittle fracture surface area ratio exceeding 70% is considered NG (insufficient toughness).

[0145] (Cut-off)

[0146] A 10mm diameter hole is punched from a cold-rolled steel sheet with a 3% punching clearance. The end face of the punched sample is observed, and the size of the burr (burr amount) is measured. The burr amount is set as the difference in height between the center of the punched sample and the end face.

[0147] In this evaluation, cases with a collapse amount of less than 100 μm are indicated as OK (good punching performance), while cases with a collapse amount of more than 100 μm are indicated as NG (insufficient punching performance).

[0148] The evaluation results are shown in Table 3.

[0149] [Table 3]

[0150]

[0151] As shown in Table 3, the cold-rolled steel sheets of Examples 1 to 35 have good results in Vickers hardness, uniform elongation, fatigue resistance, toughness and punching properties due to the appropriate composition and metallographic structure of the steel sheets. In addition, the results in thickness tolerance and maximum warpage are also good.

[0152] In contrast, the cold-rolled steel sheet of Comparative Example 1 has high strength but insufficient toughness due to its excessive carbon content.

[0153] The cold-rolled steel sheet of Comparative Example 2 has insufficient strength, fatigue resistance, and punching properties due to its low C content.

[0154] The cold-rolled steel sheet in Comparative Example 3 has insufficient toughness due to excessive Si content.

[0155] The cold-rolled steel sheet of Comparative Example 4 has insufficient Mn content, resulting in excessive ferrite precipitation, insufficient strength, and inadequate fatigue resistance and punching properties.

[0156] The cold-rolled steel sheet in Comparative Example 5 has high strength but insufficient toughness due to its excessive Mn content.

[0157] The cold-rolled steel sheet of Comparative Example 6 has high strength but insufficient toughness due to excessive Mn and Cr content.

[0158] The cold-rolled steel sheet of Comparative Example 7 could not achieve a GAM value of 1.0 to 10.0° due to an excessively high rolling rate during (final) cold rolling. As a result, the uniform elongation and toughness became insufficient.

[0159] The cold-rolled steel sheet of Comparative Example 8 could not achieve a GAM value of 1.0~10.0° and a grain density of 500 grains / mm with a grain diameter of 20.0 μm or larger due to the low rolling rate during final cold rolling. 2 Furthermore, the sheet thickness tolerance and maximum warpage also increase. Therefore, fatigue resistance and punching performance become insufficient.

[0160] The cold-rolled steel sheet in Comparative Example 9 had increased thickness tolerance and maximum warpage due to inappropriate cooling conditions after annealing.

[0161] As can be seen from the above results, according to the present invention, it is possible to provide steel plates with high hardness, which can eliminate the need for post-processing heat treatment, and excellent flatness, fatigue resistance and toughness.

[0162] Therefore, by employing the following methods [1] to [6], the present invention can provide a steel plate with high hardness, which can eliminate the need for post-processing heat treatment, and excellent flatness, fatigue resistance and toughness.

[0163] [1] A steel plate having the following composition: on a mass basis, containing C: 0.07~0.30%, Si: 0.01~0.65%, Mn: 0.80~2.30%, P: less than 0.100%, S: less than 0.100%, Al: less than 0.100%, Cr: less than 1.00%, and N: less than 0.0150%, with the balance being Fe and impurities.

[0164] The area fraction of ferrite is 0~10.0%, and the area fraction of martensite is 90.0~100%.

[0165] The average grain diameter of the aforementioned ferrite is less than 15.0 μm.

[0166] GAM value is 1.0~10.0°.

[0167] The number density of grains with a diameter of 20.0 μm or larger is 500 grains / mm. 2 the following,

[0168] For plates with a width of 400mm or less, the thickness tolerance is ±0.08mm or less.

[0169] The maximum warpage in the rolling direction when the rolling length is 1m is less than 10mm.

[0170] [2] The steel plate according to [1] further contains, by weight, one or more of the following: 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.

[0171] [3] The steel plate according to [1] or [2], wherein the above-mentioned impurities, on a mass basis, contain 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%.

[0172] [4] The steel plate according to any one of [1] to [3] is a cold-rolled steel plate.

[0173] [5] The steel plate according to any one of [1] to [4] has a Vickers hardness of 300 to 550 Hv.

[0174] [6] The steel plate according to any one of [1] to [5] has a uniform elongation of 1.5% or more.

Claims

1. A steel plate having the following composition, based on mass: C: 0.07~0.30%, Si: 0.01~0.65%, Mn: 0.80~2.30%, P: less than 0.100%, S: less than 0.100%, Al: less than 0.100%, Cr: less than 1.00%, and N: less than 0.0150%, with the balance being Fe and impurities. The area fraction of ferrite is 0~10.0%, and the area fraction of martensite is 90.0~100%. The average grain diameter of the ferrite is less than 15.0 μm. GAM value is 1.0~10.0°. The number density of grains with a diameter of 20.0 μm or larger is 500 grains / mm. 2 the following, For plates with a width of 400mm or less, the thickness tolerance is ±0.08mm or less. The maximum warpage in the rolling direction when the rolling length is 1m is less than 10mm.

2. The steel plate according to claim 1, further comprising, by weight, one or more components 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 plate according to claim 1 or 2, wherein, The impurities, on a mass basis, contain 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%.

4. The steel plate according to any one of claims 1 to 3, wherein it is a cold-rolled steel plate.

5. The steel plate according to any one of claims 1 to 4, wherein the Vickers hardness is 300 to 550 Hv.

6. The steel plate according to any one of claims 1 to 5, wherein the uniform elongation is 1.5% or more.

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