Steel sheet and method for manufacturing same

Abstract

Disclosed is a steel sheet which has a composition that contains, on a mass basis, 0.35-0.90% of C, 0.01-0.50% of Si, 0.20-1.30% of Mn, 0.100% or less of P, 0.100% or less of S, 0.100% or less of Al, 0.50% or less of Cr, and 0.0150% or less of N, with the balance being made up of Fe and impurities. With respect to this steel sheet, the average crystal grain size of ferrite is 15.0 μm or less, the area ratio of pearlite is 90% or more, the area ratio of crystal grains that have a KAM value of 5° or less and a crystal grain size of 1.0-10.0 μm is 20% or more, and the number density of crystal grains that have a KAM value of 5° or less and a crystal grain size of 20.0 μm or more is 500 / mm2 or less.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel sheet and a method for manufacturing the same.

[0002] Medium-carbon steel sheets and high-carbon steel sheets (hereinafter referred to as "steel sheets") are used as materials for structural and mechanical parts in various machines and devices, such as automobiles. These steel sheets are processed into a predetermined shape and then subjected to heat treatments such as quenching and tempering to become various parts. As steel sheets used for the various parts described above, for example, Patent Document 1 proposes a medium carbon steel cold-rolled steel sheet having a chemical composition consisting of C: 0.2 to 0.8 wt%, Si: 0.35 wt% or less, Mn: 0.90 wt% or less, P: 0.03 wt% or less, S: 0.02 wt% or less, Cr: 0.2 wt% or less, Ca: 0.001 to 0.02 wt%, the balance Fe and unavoidable impurities, and the cleanliness of the steel (JIS G0555) is dA (60 × 400): 0.015% or less, dT (60 × 400): 0.030% or less, and having excellent press workability and a ferrite-pearlite mixed structure.

[0003] Patent No. 4232927

[0004] In recent years, due to the increasing need to reduce CO2 emissions, there has been a demand for steel sheets that can be made without heat treatment after being processed into a predetermined shape, and this has necessitated the hardening of steel sheets. However, hardening steel sheets reduces their workability (e.g., properties such as uniform elongation). Therefore, in order to omit the heat treatment after processing, it is important to harden the steel sheets while maintaining their workability. Furthermore, when steel sheets are used as materials for parts such as chains, the steel sheets themselves are also required to be wear-resistant.

[0005] However, while the steel sheet of Patent Document 1 has good workability and wear resistance, it cannot be said that it is sufficiently hardened. Therefore, in order to ensure the strength required for parts, heat treatment must be performed after processing to increase the strength. 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 has excellent uniform elongation and wear resistance, and a manufacturing method thereof.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that by controlling the composition and metallographic structure of a steel sheet, it is possible to improve hardness and wear resistance while ensuring uniform elongation. The present inventors have also found that a steel sheet having the above-mentioned properties can be obtained by hot rolling and cold rolling a slab having a predetermined composition under predetermined conditions. The present invention was completed against this background.

[0007] That is, the present invention provides a steel sheet having a composition containing, on a mass basis, C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities, wherein the average grain size of ferrite is 15.0 μm or less, the area fraction of pearlite is 90% or more, the area fraction of grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm is 20% or more, and the number density of grains having a KAM value of 5° or less and a grain size of 20.0 μm or more is 500 grains / mm 2 The present invention relates to the following steel plate:

[0008] The present invention also provides a hot rolling process in which a slab having a composition containing, by mass, C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities, is hot rolled under conditions where the rolling temperature in the final stage is 830 to 950°C and the rolling reduction is 10 to 25%, and then cooled to a coiling temperature of 525 to 700°C at an average cooling temperature of 7 to 100°C / second and coiled at the coiling temperature; and a cold rolling process in which the hot-rolled steel sheet obtained in the hot rolling process is cold-rolled at a rolling reduction of 25 to 60%. The present invention relates to a method for producing a steel sheet, which includes the steps of:

[0009] According to the present invention, it is possible to provide a steel sheet having high hardness, which allows for the omission of heat treatment after working, and which has excellent uniform elongation and wear resistance, and a method for manufacturing the same.

[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 0.35 to 0.90% C, 0.01 to 0.50% Si, 0.20 to 1.30% Mn, 0.100% or less P, 0.100% or less S, 0.100% or less Al, 0.50% 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. Furthermore, "impurities" refer to components that are mixed in during industrial production of steel sheet due to various factors, such as raw materials such as ore and scrap, or manufacturing processes, 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 1.000% or less Ni, 0.500% or less Mo, 0.500% or less V, 0.500% or less Nb, 0.150% or less Ti, and 0.0100% or less B. Details of the above composition will be described below.

[0013] (C: 0.35 to 0.90%) C is an element necessary for increasing the strength of the steel sheet and improving its wear resistance and punchability. To fully obtain these effects, the C content is set to 0.35% or more, preferably 0.38% or more, more preferably 0.40% or more, and even more preferably 0.45% or more. On the other hand, if the C content is too high, the steel sheet becomes hard and its toughness and uniform elongation decrease. For this reason, the C content is set to 0.90% or less, preferably 0.88% or less, and more preferably 0.85% 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 0.35 to 0.90%, 0.35 to 0.88%, 0.35 to 0.85%, 0.38 to 0.90%, 0.38 to 0.88%, 0.38 to 0.85%, 0.40 to 0.90%, 0.40 to 0.88%, 0.40 to 0.85%, 0.45 to 0.90%, 0.45 to 0.88%, or 0.45 to 0.85%. The same applies to the numerical ranges of the contents of the other elements below.

[0014] (Si: 0.01 to 0.50%) 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, and more preferably 0.05% or more. On the other hand, if the Si content is too high, ferrite is likely to be formed, which reduces the strength of the steel sheet and makes it impossible to ensure wear resistance and punchability. For this reason, the Si content is set to 0.50% or less, preferably 0.48% or less, and more preferably 0.45% or less.

[0015] (Mn: 0.20 to 1.30%) Mn is an element that affects the strength, toughness, 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 wear resistance and punchability. For this reason, the Mn content is set to 0.20% or more, preferably 0.22% or more, and more preferably 0.25% or more. On the other hand, if the Mn content is too high, the steel sheet becomes hard, reducing toughness and uniform elongation. For this reason, the Mn content is set to 1.30% or less, preferably 1.28% or less, and more preferably 1.25% 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 that forms AlN and suppresses coarsening of crystal grains through a pinning effect. 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: 0.50% or less) Cr is an element effective for forming a predetermined metal structure. However, if the Cr content is too high, the uniform elongation and toughness of the steel sheet will decrease. Therefore, the Cr content is set to 0.50% or less, preferably 0.48% or less, and more preferably 0.47% 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, 0.15% or 0.20%.

[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 more preferably 0.0135% 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: 0.500% 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 0.500% or less, preferably 0.450% or less, and more preferably 0.400% 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 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050%) Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca are impurities and may not be contained in the steel sheet. These elements may be contained alone or in combination of two or more as impurities. The contents of Cu, W, and Ta are each 0 to less than 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 less than 0.050%, preferably 0 to 0.045%.

[0026] Next, the metallographic structure of a steel sheet according to an embodiment of the present invention will be described. The steel sheet according to an embodiment of the present invention has a metallographic structure mainly composed of pearlite. This pearlite has a eutectoid structure in which fine cementite is uniformly dispersed in ferrite. Specifically, the steel sheet according to an embodiment of the present invention has an average grain size of ferrite of 15.0 μm or less, an area ratio of pearlite of 90% or more, an area ratio of grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm of 20% or more, and a number density of grains having a KAM value of 5° or less and a grain size of 20.0 μm or more of 500 grains / mm 2 The following is the result.

[0027] (Average grain size of ferrite: 15.0 μm or less) If the average grain size of ferrite is large, it becomes difficult to obtain sufficient pearlite in a eutectoid structure in which fine cementite is uniformly dispersed in the ferrite, resulting in reduced wear 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. It should be noted that the ferrite defining the average grain size here refers to the ferrite in a mixed structure containing ferrite and pearlite, and not the ferrite that constitutes pearlite. Meanwhile, the smaller the average grain size of ferrite, the better, so the lower limit is not particularly limited. Although fine ferrite grains may be formed immediately after the formation of ferrite grains, the lower limit of the average grain size of ferrite may be 0.1 μm, 0.5 μm, or 0.9 μm. Furthermore, the steel sheet according to the embodiment of the present invention may be composed of a pearlite single-phase structure. In this case, since no ferrite is present, the average grain size of ferrite can be considered to be 0 μm.

[0028] 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.

[0029] (Area fraction of pearlite: 90% or more) The steel sheet according to the embodiment of the present invention has a metal structure mainly composed of pearlite, thereby improving wear resistance and punchability. From the viewpoint of achieving this effect, the area fraction of pearlite is set to 90% or more, preferably 91% or more, more preferably 92% or more, and even more preferably 93% or more. On the other hand, the upper limit of the area fraction of pearlite is not particularly limited, and may be 100% (i.e., the steel sheet may have a pearlite single-phase structure).

[0030] The area ratio of pearlite is determined as follows. First, a cross section (L cross section) parallel to the rolling direction of a test piece cut out 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 of the photograph is set to 500 to 3000 times depending on the size of the crystal grains. In the obtained structural photograph, the region where cementite is uniformly dispersed in ferrite is recognized as pearlite, and the area ratio of pearlite (the area ratio of pearlite to the entire measurement region) is calculated using image analysis software. The area ratio of pearlite is taken as the average value of the measurement results from the five points.

[0031] (Area ratio of crystal grains with a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm: 20% or more) The KAM (Kernel Average Misorientation) value is a parameter that indicates the magnitude of strain, and a larger KAM value means a larger strain. The area ratio of crystal grains with a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm is related to the toughness and uniform elongation of the steel plate. By setting the area ratio of these crystal grains to 20% or more, a sufficient area with small strain is secured, thereby improving the toughness and uniform elongation of the steel plate.

[0032] The area ratio of crystal grains having a KAM value of 5° or less and a crystal grain size of 1.0 to 10.0 μm is determined as follows. First, a cross section (L cross section) parallel to the rolling direction of a test specimen 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 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 ¼ of the thickness of the steel sheet. The crystal grain size of the crystal grains is determined by analyzing the orientation of the crystal grains using the analysis software OIM Analysis (manufactured by TSL Solutions Co., Ltd.) based on the EBSD data. The boundaries with an orientation difference of 5° or more from adjacent measurement points are defined as grain boundaries, and the crystal grains are calculated using the same software. Furthermore, based on the EBSD data, the local orientation difference around each measurement point is mapped using the KAM method, and the KAM value is determined. Here, the KAM method is a method for calculating the local misorientation (KAM value) of each pixel by averaging the misorientations between the six adjacent pixels (first approximation) of a certain regular hexagonal pixel in the measurement data, the 12 pixels outside of that (second approximation), and the 18 pixels outside of that (third approximation). Next, crystal grains with a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm are identified, and their area ratio (the area ratio of the crystal grains to the entire measurement area) is calculated. This area ratio of the crystal grains is the average value of each measurement field.

[0033] (Number density of crystal grains with a KAM value of 5° or less and a grain size of 20.0 μm or more: 500 grains / mm 2 The grains having a KAM value of 5° or less and a grain size of 20.0 μm or more are related to the wear resistance of the steel sheet. 2 If the thickness is less than this, it can be said that the number of coarse crystal grains present in the steel sheet is small, and therefore the wear resistance of the steel sheet can be improved.

[0034] The number density of crystal grains with a KAM value of 5° or less and a grain size of 20.0 μm or more can be determined as follows. First, a backward electron diffraction analysis is performed in the same manner as above to calculate the grain size and KAM value of the crystal grains. Next, the number of crystal grains with a KAM value of 5° or less and a grain size of 20.0 μm or more is determined, and the area (mm 2 The number density of the crystal grains is calculated by dividing the number of the crystal grains by the average value of the number of the crystal grains in each measurement field.

[0035] The steel sheet according to the embodiment of the present invention has the composition and metal structure described above, and therefore can have the following properties.

[0036] (Vickers hardness: 300 to 500 Hv) The steel sheet according to the embodiment of the present invention preferably has a Vickers hardness of 300 to 500 Hv, more preferably 310 to 480 Hv, and even more preferably 315 to 460 Hv. A Vickers hardness within this range ensures workability (uniform elongation and toughness) while being hardened. 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.

[0037] (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. 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.

[0038] 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.

[0039] <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 manufacture a steel sheet having the above-described characteristics. For example, the steel sheet according to an embodiment of the present invention can be manufactured by a method including: a hot rolling step in which a slab having the composition described above is hot-rolled under conditions where the rolling temperature in the final stage is 830 to 950°C and the rolling reduction is 10 to 25%, followed by cooling to a coiling temperature of 525 to 700°C at an average cooling rate of 7 to 100°C / second and coiling at that coiling temperature; and a cold rolling step in which the hot-rolled steel sheet obtained in the hot rolling step is cold-rolled at a rolling reduction of 25 to 60%, but which does not include an annealing step after the cold rolling step. Each step will be described in detail below.

[0040] (Hot Rolling Process) In hot rolling, the rolling of a slab is performed in multiple stages. Usually, the final stage of rolling is called finish rolling, and the other stages of rolling are called rough rolling. It is preferable to heat the slab to 1100°C or higher to sufficiently redissolve Ti carbonitrides and the like. Rough rolling is performed on the slab to adjust the plate thickness, etc. Rough rolling is performed as long as the desired dimensions are secured, and the conditions are not particularly limited. Finish rolling (final stage of rolling) is performed under conditions where the rolling temperature is 830 to 950°C and the rolling reduction is 10 to 25%. If the rolling temperature is less than 830°C and the rolling reduction is more than 25%, a great burden is placed on the rolling mill, which may cause equipment trouble. On the other hand, if the rolling temperature exceeds 950°C and the rolling ratio is less than 10%, the crystal grains become coarse, and the number density of crystal grains having a KAM value of 5° or less and a crystal grain size of 20.0 μm or more is reduced to 500 grains / mm 2 From the viewpoint of stably avoiding these problems, the rolling temperature is preferably 840 to 940°C, and the rolling ratio is preferably 11 to 24%.

[0041] After hot rolling, the steel sheet is cooled to a coiling temperature of 525 to 700°C at an average cooling rate of 7 to 100°C / s, and then coiled at the coiling temperature. If the coiling temperature is less than 525°C, pearlite is not generated, and a pearlite-based structure cannot be obtained. If the coiling temperature exceeds 700°C, pickling properties deteriorate. If the average cooling rate is less than 7°C / s, ferrite precipitates in large amounts, making it impossible to obtain a pearlite-based structure. If the rate exceeds 100°C / s, it becomes difficult to obtain a flat steel sheet.

[0042] (Cold Rolling Step) The hot-rolled steel sheet obtained in the hot rolling step is cold-rolled at a rolling reduction of 25 to 60%. If the rolling reduction is less than 25%, the cementite is not sufficiently refined, and the number density of crystal grains having a KAM value of 5° or less and a crystal grain size of 20.0 μm or more is reduced to 500 grains / mm 2 It becomes difficult to control the area ratio of crystal grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm to 20% or more. In addition to the above-described steps, known steps may be performed. For example, a pickling step may be performed between the hot rolling step and the cold rolling step. The conditions for these steps are not particularly limited, and can be performed in accordance with the conditions of known steel sheet manufacturing methods.

[0043] 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.

[0044] A 250 mm thick slab having the composition shown in Table 1 (the balance being impurities other than Fe and the elements shown in Table 1) was produced by a continuous casting method. Next, the slab was heated to 1100 to 1250°C and held for 1 hour, after which a hot rolling step was carried out. Hot rolling was carried out under the conditions of the rolling temperature and rolling reduction in the final stage shown in Table 2. Thereafter, the slab was cooled to the coiling temperature shown in Table 2 at the average cooling temperature shown in Table 2, and coiled at the coiling temperature. Next, the coiled hot-rolled steel sheet was pickled to remove oxide scale, and then cold-rolled at the rolling reduction shown in Table 2 to obtain a 2 mm thick cold-rolled steel sheet. Furthermore, for Comparative Examples 14 to 16, annealing was carried out at 710°C for 40 hours after cold rolling.

[0045]

[0046]

[0047] The cold-rolled steel sheets and cold-rolled annealed steel sheets obtained in the above examples were evaluated as follows.

[0048] (Average grain size of ferrite and area ratio of pearlite) The average grain size of ferrite was measured 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 mm in thickness.

[0049] (Area ratio of crystal grains with a KAM value of 5° or less and a crystal grain size of 1.0 to 10.0 μm, and number density of crystal grains with a KAM value of 5° or less and a crystal grain size of 20.0 μm or more) The area ratio of a predetermined crystal grain and the number density of a predetermined crystal grain were determined according to the method described above. The test specimen 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, the area ratio of crystal grains with a KAM value of 5° or less and a crystal grain size of 1.0 to 10.0 μm is 20% or more, which is defined as condition A, and those that satisfy this condition are represented as OK, and those that do not satisfy this condition are represented as NG. Furthermore, if the number density of crystal grains with a KAM value of 5° or less and a crystal grain size of 20.0 μm or more is 500 grains / mm 2 The following is set as condition B, and what satisfies this condition is expressed as OK, and what does not satisfy this condition is expressed as NG.

[0050] (Vickers Hardness) Vickers hardness was measured according to the method described above.

[0051] (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).

[0052] (Wear Resistance) Using abrasive paper (SiC with a grain size of #800) as a disk, a pin-on-disk test was performed under dry conditions at room temperature to measure the amount of wear. The cold-rolled steel sheet and the cold-rolled annealed steel sheet were processed into pin test pieces with a diameter of 5 mm. The pin-on-disk test was performed under a load of 20 N, a friction speed (rotational speed) of 0.66 m / s, and a friction time of 2.5 minutes (friction distance of 100 m), and the amount of wear was measured at a friction distance of 50 m to 100 m. In this evaluation, the amount of wear was 35 × 10 -5 mm 3 / Nm or less is OK (good wear resistance), 35 x 10 -5 mm 3 A value exceeding this is expressed as NG (insufficient wear resistance).

[0053] (Toughness) V-notch test pieces were taken from the cold-rolled steel sheets and cold-rolled annealed steel sheets, and subjected to a Charpy impact test at 150°C. This test was performed in accordance with JIS Z2242:2023, and the test pieces were V-notched, had a plate thickness of 2.0 mm (dimensional tolerance ±0.05 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).

[0054] (Punching Property) A hole with a diameter of 10 mm was punched from a cold-rolled steel sheet and a cold-rolled annealed 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 punching property), and a sample with a sagging amount of more than 100 μm was evaluated as NG (insufficient punching property).

[0055]

[0056] As shown in Table 3, the cold-rolled steel sheets of Examples 1 to 35 had appropriate steel sheet compositions and metallographic structures, and therefore achieved good results in the evaluation of each characteristic. In contrast, the cold-rolled steel sheet of Comparative Example 1 had an excessively high C content, resulting in high strength but insufficient uniform elongation and toughness. The cold-rolled steel sheet of Comparative Example 2 had an excessively low C content, resulting in insufficient strength and a small pearlite area fraction, resulting in insufficient wear resistance and punchability. The cold-rolled steel sheet of Comparative Example 3 had an excessively high Si content, resulting in a small pearlite area fraction and insufficient wear resistance and punchability. The cold-rolled steel sheet of Comparative Example 4 had an excessively low Mn content, resulting in a small pearlite area fraction and insufficient wear resistance and punchability. The cold-rolled steel sheet of Comparative Example 5 had an excessively high Mn content, resulting in high strength but insufficient uniform elongation and toughness. The cold-rolled steel sheet of Comparative Example 6 had an excessively high Cr content, resulting in high strength but insufficient uniform elongation and toughness.

[0057] In the cold-rolled steel sheet of Comparative Example 7, the rolling ratio during cold rolling was too high, so the area ratio of crystal grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm could not be made 20% or more. As a result, the uniform elongation and toughness were insufficient. In the cold-rolled steel sheet of Comparative Example 8, the rolling ratio during cold rolling was too low, so the number density of crystal grains having a KAM value of 5° or less and a grain size of 20.0 μm or more could not be made 500 grains / mm 2 In the cold-rolled steel sheet of Comparative Example 9, the rolling reduction ratio in the final stage of hot rolling was too low, and therefore the number density of crystal grains having a KAM value of 5° or less and a grain size of 20.0 μm or more could not be reduced to 500 grains / mm 2 For the cold-rolled steel sheet of Comparative Example 10, the rolling temperature in the final stage of hot rolling was too high, and therefore the number density of crystal grains having a KAM value of 5° or less and a grain size of 20.0 μm or more could not be reduced to 500 grains / mm 2or less. As a result, the wear resistance was insufficient. In the cold-rolled steel sheet of Comparative Example 11, the rolling reduction ratio in the final stage of hot rolling was too low and the rolling temperature in the final stage of hot rolling was too high, so the ferrite coarsened and the area ratio of pearlite was reduced. As a result, the wear resistance and punchability were insufficient. In the cold-rolled steel sheet of Comparative Example 12, the average cooling rate during cooling after hot rolling was too slow, so the ferrite coarsened and the area ratio of pearlite was reduced. As a result, the wear resistance and punchability were insufficient. In the cold-rolled steel sheet of Comparative Example 13, the ferrite coarsened because it did not contain Al and N. As a result, the wear resistance and punchability were insufficient. In Comparative Examples 14 to 16, the cold-rolled annealed steel sheets were annealed after cold rolling, so the number density of crystal grains with a KAM value of 5° or less and a grain size of 20.0 μm or more was set to 500 grains / mm 2 Furthermore, in Comparative Example 16, it was not possible to make the area ratio of crystal grains having a KAM value of 5° or less and a crystal grain size of 1.0 to 10.0 μm 20% or more. For this reason, these cold-rolled annealed steel sheets were insufficient in strength, and also insufficient in wear resistance and punchability.

[0058] As can be seen from the above results, the present invention can provide a steel sheet that has high hardness, allows for the omission of heat treatment after processing, and is excellent in uniform elongation and wear resistance, and a manufacturing method thereof.

[0059] Therefore, by adopting the following aspects [1] to [9], the present invention can provide a steel sheet having high hardness, which can omit heat treatment after processing, and which has excellent uniform elongation and wear resistance, and a manufacturing method thereof.

[0060] [1] On a mass basis, it contains C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities, and the average grain size of ferrite is 15.0 μm or less, the area ratio of pearlite is 90% or more, the area ratio of grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm is 20% or more, and the number density of grains having a KAM value of 5° or less and 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: 0.500% 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 [1] or [2], wherein the impurities include, by mass, one or more selected from the group consisting of Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 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], having a Vickers hardness of 300 to 500 Hv. [6] The steel sheet according to any one of [1] to [5], having a uniform elongation of 1.5% or more.

[0061] [7] A hot rolling process in which a slab having a composition containing, by mass, C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities, is hot rolled under conditions where the rolling temperature in the final stage is 830 to 950°C and the rolling reduction is 10 to 25%, and then cooled to a coiling temperature of 525 to 700°C at an average cooling temperature of 7 to 100°C / second and coiled at the coiling temperature; and a cold rolling process in which the hot-rolled steel sheet obtained in the hot rolling process is cold-rolled at a rolling reduction of 25 to 60%. [8] The method for producing a steel sheet according to [7], wherein the slab further contains, by mass, one or more elements selected from the group consisting of Ni: 1.000% or less, Mo: 0.500% or less, V: 0.500% or less, Nb: 0.500% or less, Ti: 0.150% or less, and B: 0.0100% or less. [9] The method for producing a steel sheet according to [7] or [8], wherein the impurities include, on a mass basis, one or more selected from the group consisting of Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050%.

Claims

1. A composition containing, by mass, C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities, wherein the average grain size of ferrite is 15.0 μm or less, the area ratio of pearlite is 90% or more, the area ratio of grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 μm is 20% or more, and the number density of grains having a KAM value of 5° or less and a grain size of 20.0 μm or more is 500 grains / mm 2 The following is a steel plate.

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: 0.500% 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 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 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 500 Hv.

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

7. A hot rolling process in which a slab having a composition containing, by mass, C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities, is hot rolled under conditions where the rolling temperature in the final stage is 830 to 950°C and the rolling reduction is 10 to 25%, and then cooled at an average cooling temperature of 7 to 100°C / second to a coiling temperature of 525 to 700°C and coiled at the coiling temperature; and a cold rolling process in which the hot-rolled steel sheet obtained in the hot rolling process is cold-rolled at a rolling reduction of 25 to 60%. and wherein the method for producing a steel sheet does not include an annealing step after the cold rolling step.

8. The method for producing steel plate according to claim 7, wherein the slab further contains, on a mass basis, one or more elements selected from the group consisting of Ni: 1.000% or less, Mo: 0.500% or less, V: 0.500% or less, Nb: 0.500% or less, Ti: 0.150% or less, and B: 0.0100% or less.

9. The method for producing a steel sheet according to claim 7 or 8, wherein the impurities include one or more selected from the group consisting of, by mass, Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050%.

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