Steel sheet and method for manufacturing same

Abstract

Provided are: a steel sheet which has a prescribed chemical composition and has a metallographic structure containing, in area%, 90.0% or more of martensite and 3.0% or less of retained austenite, the average grain diameter of prior austenite grains being 30.0 [mu] m or less, the standard deviation of the grain diameter of the prior austenite grains being less than 4.0 [mu] m, and the average grain diameter of the prior austenite grains being 30.0 [mu] m or less; and the standard deviation of the dislocation density at seven positions of 5%, 10%, 15%, 20%, 25%, 30%, and 50% with respect to the total width at a position of 1 / 4 of the sheet thickness and in a direction from an end portion in the width direction toward a width center portion is 1.50 * 1015 / m2 or less.

Description

Technical Field

[0001] This invention relates to steel plates and methods for manufacturing the same. Background Technology

[0002] In recent years, the application of high-strength steel sheets has been expanding from the perspective of lightweighting, which helps improve the fuel efficiency of automobiles. On the other hand, most automotive parts are manufactured by pressing, thus requiring both high strength and excellent formability.

[0003] Relatedly, for example, Patent Document 1 describes a high-strength steel plate characterized in that, in addition to containing C: 0.1~0.25%, Si: 0.1~0.5%, Mn: 0.5~2.0%, Cr: 0.1~1.5%, Mo: 0.1~0.5%, Ti: 0.01~0.05%, and Nb: 0.01~0.05%, it also contains V: 0.01~0.05% and / or B: 0.0001~0.005%, the remainder being composed of iron and unavoidable impurities, and the average grain size of the original austenite is less than 20 μm, and the standard deviation (σ) of the original austenite grain size distribution is less than 5 μm. Furthermore, as described above, Patent Document 1 teaches that by appropriately adjusting the chemical composition and controlling the average particle size and standard deviation (σ) of the original γ particle size distribution within an appropriate range, it is possible to achieve a high-strength steel plate with a tensile strength of 980 MPa or higher and good bending workability.

[0004] Patent document 2 describes a high-strength hot-rolled steel plate, characterized in that its composition, by mass%, contains C: 0.08~0.30%, Si: less than 3.0%, Mn: 1.0~4.0%, P: less than 0.100%, S: less than 0.02%, Al: less than 1.0%, N: less than 0.008%, with the remainder consisting of Fe and unavoidable impurities. Regarding the steel microstructure, polygonal ferrite, primary martensite, and... are present at the 1 / 4 position of the steel plate thickness and the center of the steel plate width. The total area ratio of retained austenite relative to the overall steel structure is 20% or less, and the total area ratio of martensite and lower bainite relative to the overall steel structure is 65-100%. The standard deviation of the total area ratio of the polygonal ferrite, primary martensite, and retained austenite at positions representing 5%, 10%, 15%, 20%, 25%, and 30% of the total width of the steel plate (located at 1 / 4 of the plate thickness and from the edge towards the center of the width), and at the center of the width of the steel plate, is 7.0% or less. Furthermore, Patent Document 2 teaches that using the aforementioned high-strength hot-rolled steel plate can reduce shape deviations during the manufacturing of components such as automotive parts, thereby enabling the production of high-strength components with stable shapes.

[0005] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2009-242832 Patent Document 2: Japanese Patent Application Publication No. 2021-063252 Summary of the Invention

[0006] The problem that the invention aims to solve In automotive components, improved crash resistance is desired from the perspective of ensuring passenger safety. Therefore, for large deformations during a collision, high-strength components capable of suppressing localized deformation caused by material inhomogeneity are required. Relatedly, steel that can uniformly strengthen the entire component during the painting and baking process after forming is required.

[0007] Therefore, the object of the present invention is to provide a high-strength steel sheet that can be uniformly coated and baked through a new configuration.

[0008] Methods for solving problems To achieve the aforementioned objectives, the inventors conducted research focusing on the microstructure of steel sheets, particularly hot-rolled steel sheets. As a result, firstly, the inventors discovered that by constructing a microstructure of hot-rolled steel sheets with a specified chemical composition primarily composed of martensite, high strength can be achieved, and the uniformity of strengthening during coating baking can be improved. Furthermore, the inventors discovered that by limiting the average grain size of the original austenite grains in the microstructure to a specified range and reducing the deviation in the grain size of these original austenite grains, the microstructure becomes uniform in the microscopic region. Furthermore, by controlling the standard deviation of the dislocation density in the width direction to a specified range, the microstructure also becomes uniform in the macroscopic region. This significantly improves the uniformity of strengthening during coating baking, thus completing the present invention.

[0009] The present invention, which achieves the above objectives, is described below.

[0010] (1) A steel plate, characterized in that its chemical composition, in mass% is: C: 0.040~0.200% Si: 0.30~2.00% Mn: 1.00~4.00% sol.Al: 0.001~0.500% P: Below 0.100% S: Below 0.0300% N: below 0.0070% O: Below 0.0100% Nb: 0.001~1.000% Ti: 0~0.200% V: 0~0.300% Cu: 0~0.40%, Cr: 0~0.90% Mo: 0~0.12%, Ni: 0~0.30% B: 0~0.0030% Ca: 0~0.0010%, Mg: 0~0.0010%, Bi: 0~0.010% Zr: 0~0.050%, Co: 0~0.010%, Zn: 0~0.010%, W: 0~0.100% Sn: 0~0.040% As: 0~0.100% REM: 0~0.0100%, and Remaining components: Fe and impurities. The metallic structure contains, by area %: Martensite: over 90.0%, and Residual austenite: less than 3.0%, The average grain size of the original austenite grains is less than 30.0 μm. The standard deviation of the original austenite grain size is less than 4.0 μm. The standard deviation of dislocation density at the 1 / 4 position of the plate thickness, from the end of the width direction towards the center of the width, relative to the seven positions of 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, is 1.50 × 10⁻⁶. 15 / m 2 the following.

[0011] (2) The steel plate according to (1) above, characterized in that the above chemical composition contains at least one of the following elements in mass percent: Ti: 0.001~0.200% V: 0.001~0.300% Cu: 0.001~0.40% Cr: 0.001~0.90% Mo: 0.001~0.12% Ni: 0.001~0.30% B: 0.0001~0.0030% Ca: 0.0001~0.0010% Mg: 0.0001~0.0010% Bi: 0.001~0.010% Zr: 0.001~0.050% Co: 0.001~0.010%, Zn: 0.001~0.010%, W: 0.001~0.100% Sn: 0.001~0.040% As: 0.001~0.100%, and REM: 0.0001~0.0100%.

[0012] (3) The steel plate according to (1) or (2) above, characterized in that the above-mentioned metal structure further comprises at least one of the following in area percentage: Ferrite: less than 10.0%, Bainite: below 10.0%, and Pearlite: less than 10.0%.

[0013] (4) The steel plate according to any one of (1) to (3) above, characterized in that the plate thickness is 1.0 to 8.0 mm.

[0014] (5) A component, characterized in that it comprises any one of (1) to (4) above.

[0015] (6) A method for manufacturing a steel plate, characterized in that it includes the following steps: Heating process: The slab having the chemical composition described in (1) or (2) above is heated and held at a temperature of 1100°C or above for more than 6000 seconds; Width reduction process: This includes using a finishing pressurizing device to perform width reduction pressing on the above-mentioned slab, wherein the width reduction pressing satisfies the following conditions (a) and (b): (a) Width reduction ratio of 1.0~23%, and (b) The conveying speed of the above-mentioned slab is 20 m / min or more; Hot rolling process: This includes finishing rolling the above-mentioned slab, wherein the finishing rolling satisfies the following conditions (c) to (e): (c) The reduction rate in each rolling pass of the final section and the preceding and final sections is 20-50%. (d) Total reduction rate of 90% or more, and (e) The final rolling temperature is 960~1100℃; Cooling process: Cooling of the steel plate begins within 0.5 seconds after the completion of the hot rolling process described above, and then the steel plate is cooled to a temperature below 400°C within 20.0 seconds from the start of cooling; and Coiling process: The cooled steel plate is coiled in a temperature range below 400℃.

[0016] (7) The method for manufacturing steel plate according to (6) above is characterized in that a continuous casting process is further included before the heating process, wherein the continuous casting process has an average cooling rate of 10°C / min or more at 600-900°C and an average cooling rate gradient of 40°C / min. 2 The following methods can be used for control.

[0017] Invention Effects According to the present invention, it is possible to provide a high-strength steel sheet that can be uniformly reinforced during coating and baking. Detailed Implementation

[0018] <steel plate> The chemical composition of the steel plate, particularly the hot-rolled steel plate, according to embodiments of the present invention is as follows (in mass%): C: 0.040~0.200% Si: 0.30~2.00% Mn: 1.00~4.00% sol.Al: 0.001~0.500% P: Below 0.100% S: Below 0.0300% N: below 0.0070% O: Below 0.0100% Nb: 0.001~1.000% Ti: 0~0.200% V: 0~0.300% Cu: 0~0.40%, Cr: 0~0.90% Mo: 0~0.12%, Ni: 0~0.30% B: 0~0.0030% Ca: 0~0.0010%, Mg: 0~0.0010%, Bi: 0~0.010% Zr: 0~0.050%, Co: 0~0.010%, Zn: 0~0.010%, W: 0~0.100% Sn: 0~0.040% As: 0~0.100% REM: 0~0.0100%, and Remaining components: Fe and impurities. The metallic structure contains, by area %: Martensite: over 90.0%, and Residual austenite: less than 3.0%, The average grain size of the original austenite grains is less than 30.0 μm. The standard deviation of the original austenite grain size is less than 4.0 μm. The standard deviation of dislocation density at the 1 / 4 position of the plate thickness, from the end of the width direction towards the center of the width, relative to the seven positions of 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, is 1.50 × 10⁻⁶. 15 / m 2 the following.

[0019] As mentioned above, automotive components require high-strength parts capable of suppressing localized deformation caused by material inhomogeneity, which results in large deformations during collisions. Relatedly, steel is required that can be uniformly strengthened (baked hardening) throughout the component after forming and during painting and baking. Here, baked hardening refers to the phenomenon where interstitial elements (mainly carbon) move and become fixed in dislocations generated through pressing (hereinafter also referred to as "pre-straining") at 100-200°C, thus hindering their movement and increasing strength; this is also known as strain aging.

[0020] Therefore, in addition to optimizing the chemical composition of steel sheets, especially hot-rolled steel sheets, the inventors have conducted research specifically focusing on the microstructure of these hot-rolled steel sheets. First, the inventors discovered that by constructing a microstructure of a hot-rolled steel sheet with a prescribed chemical composition consisting primarily of martensite, and more specifically, comprising at least 90.0% martensite and less than 3.0% retained austenite by area percent, high strength, such as a tensile strength of 980 MPa or higher, can be achieved. Furthermore, the strengthening process during the forming of the obtained hot-rolled steel sheet by pressing or similar methods, followed by coating and baking, becomes more uniform. While not intending to be bound by any particular theory, it is believed that by achieving a more uniform microstructure with at least 90.0% martensite by area percent, compared to cases containing more of other microstructures softer than martensite, such as ferrite, the strength difference within the microstructure can be reduced. This reduction in strength difference improves the uniformity of strengthening during coating and baking. In addition, retained austenite may become the starting point of fracture during deformation such as pressing and forming, and dislocations can be introduced unevenly into the microstructure during such deformation. Therefore, in addition to controlling the martensite to be more than 90.0% by area, the retained austenite is also limited to less than 3.0% by area, thereby significantly reducing the strength difference in the metal microstructure.

[0021] Next, the inventors considered that the dislocation distribution within martensite is effective in achieving uniform strengthening, and therefore conducted research from the viewpoint of appropriately optimizing the grain size of the original austenite grains in a martensite-dominated metal structure. More specifically, by refining the original austenite grains, the density of the original austenite grain boundaries can be increased. Therefore, by refining the original austenite grains, dislocations are uniformly introduced into the metal structure during pressing, resulting in a reduction of the strength difference within the baked-hardened metal structure. To explain in more detail, when the grain size of the original austenite grains is large, the distance between the grain and the vicinity of the grain boundaries increases, making strain introduction more uneven. This is because strain is easily introduced near grain boundaries, but difficult to introduce within the grains. Therefore, by refining the original austenite grains and increasing the density of the original austenite grain boundaries, dislocations can be more uniformly introduced into the metal structure during pressing, resulting in a reduction of the strength difference within the baked-hardened metal structure.

[0022] However, simply refining the original austenite grains may not necessarily result in a uniform dislocation distribution within the martensite structure, and sometimes may not sufficiently reduce the strength difference within the baked-hardened metal structure. For example, in the case where coarse and fine austenite grains coexist in the microstructure before martensitic transformation, the onset temperature of the martensitic transformation varies depending on the grain size of the austenite. Therefore, the temperature journey experienced by each grain before cooling is complete differs. Specifically, austenite grains with larger grain sizes have a higher onset temperature for martensitic transformation compared to those with smaller grain sizes, thus undergoing automatic tempering during the period from the completion of the transformation to cooling to room temperature. Consequently, some of the dislocations introduced during the martensitic transformation undergo dipole annihilation due to thermal vibrations during this temperature journey. Therefore, it is believed that the dislocation density within austenite grains undergoing transformation at higher temperatures is relatively lower, while the dislocation density within austenite grains undergoing transformation at lower temperatures is relatively higher. As a result, the dislocation distribution within the martensitic structure becomes non-uniform. Therefore, in order to homogenize the dislocation distribution within the martensitic structure, it is important to reduce the deviation in the austenite grain size before the martensitic transformation. In other words, it is believed that by reducing the deviation in the austenite grain size before the martensitic transformation, the deviation in the original austenite grain size after the martensitic transformation can be reduced, thereby homogenizing the dislocation distribution within the martensitic structure.

[0023] Therefore, in addition to controlling the grain size of the original austenite grains, the inventors have also focused on controlling the grain size distribution, and more specifically, on controlling the deviation of the grain size. As a result, the inventors have discovered that by refining the original austenite grains to a specified range, and more specifically, by controlling the average grain size of the original austenite grains to below 30.0 μm, the density of the original austenite grain boundaries in the steel plate as a whole is increased, and the deviation of the original austenite grain size is reduced. More specifically, by controlling the standard deviation of the original austenite grain size to below 4.0 μm, the dislocation distribution in the martensite structure can be sufficiently homogenized, thereby significantly reducing the strength difference in the metal structure after bake hardening.

[0024] While controlling the average grain size and standard deviation of the original austenite grains as described above can homogenize the dislocation distribution within the martensite structure in the micro-regions of the steel sheet, it cannot reliably homogenize the dislocation distribution within the martensite structure in the macro-regions of the steel sheet as a whole. On the other hand, as mentioned above, in automotive parts and the like, to suppress the localization of deformation during collisions, steel that can uniformly strengthen the entire part after forming and during painting and baking is required. Relatedly, the manufacture of steel sheets, including slabs, is generally carried out by rolling using rolls. According to numerous test results related to such rolling, the steel sheet achieves a more uniform effect in terms of material and strength in the length direction (rolling direction) compared to the width direction, thus the strength difference in the length direction becomes smaller. On the other hand, in the width direction of the steel sheet, the thickness at the ends becomes thinner and it is easier to cool, so the strain introduced by rolling is also prone to differences in the width direction. Therefore, in order to reduce the overall strength difference of the steel plate and make the microstructure more uniform, it is important to properly control the microstructure in the width direction.

[0025] Therefore, the inventors further investigated how to homogenize the dislocation distribution within the martensite structure not only in the microscopic region of the steel plate but also in the macroscopic region of the steel plate as a whole. As a result, the inventors discovered that by controlling the standard deviation of the dislocation density at seven locations—5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width of the steel plate at a position of 1 / 4 of the plate thickness, from the end of the steel plate in the width direction towards the center—relative to 5% of the total width of the steel plate, more specifically, within a specific range, specifically within 1.50 × 10⁻⁶. 15 / m 2 The following allows for the achievement of a uniform metal microstructure throughout the steel sheet. Specifically, in the width direction of the steel sheet, the thickness reduction at the ends is greater than that at the center of the width. Therefore, it is considered effective, for example, in achieving a more uniform metal microstructure, rather than controlling the standard deviation of dislocation density measured at equal distances of 10%, 20%, 30%, 40%, and 50% of the full width of the steel sheet from the ends toward the center of the width. According to the embodiment of the present invention, the steel sheet with a specified chemical composition, consisting of a martensitic microstructure, achieves high strength and improves the uniformity of strengthening during coating and baking. Furthermore, by limiting the average grain size of the original austenite grains in the metal microstructure to below 30.0 μm and controlling the standard deviation of the grain size of these original austenite grains to below 4.0 μm, the metal microstructure becomes uniform in the microscopic region, thereby controlling the standard deviation of the dislocation density at the aforementioned seven locations in the width direction to 1.50 × 10⁻⁶. 15 / m2 This makes the metal structure more uniform in the macroscopic region, thereby significantly improving the uniformity of strengthening during coating baking. Therefore, by using the steel sheet according to embodiments of the present invention, it is possible to provide a high-strength component capable of significantly suppressing or reducing deformation localization in large deformations generated during a collision. Therefore, the steel sheet according to embodiments of the present invention is particularly useful in the automotive field where excellent crashworthiness is required.

[0026] The steel plate according to embodiments of the present invention will now be described in more detail. In the following description, unless otherwise specified, "%" as the unit for the content of each element refers to "mass%". Furthermore, in this specification, unless otherwise specified, the "~" indicating a numerical range is used to mean the lower and upper limits of the values ​​described before and after it.

[0027] [C: 0.040~0.200%] C is an effective element for improving the strength of steel sheets. Furthermore, C forms carbides and / or carbonitrides with Nb in steel, which also contributes to the microstructure refinement resulting from the pinning effect of the formed precipitates. To fully obtain these effects, the C content is set to 0.040% or more. The C content can be 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, if there is an excessive amount of C, more retained austenite is formed, which unevenly introduces dislocations during deformation such as pressing, resulting in a larger strength difference within the baked-hardened metal microstructure. Therefore, the C content is set to 0.200% or less. The C content can be 0.180% or less, 0.160% or less, 0.150% or less, or 0.140% or less.

[0028] [Si: 0.30~2.00%] Si is an effective element for enhancing strength as a solid solution strengthening element. To fully achieve this effect, the Si content is set to 0.30% or more. The Si content can be 0.40% or more, more than 0.50%, 0.51% or more, 0.52% or more, 0.53% or more, 0.54% or more, 0.55% or more, more than 0.55%, 0.60% or more, 0.70% or more, 0.85% or more, 1.00% or more, or 1.20% or more. On the other hand, if the Si content is excessive, the chemical conversion processability and formability decrease, and sometimes slab cracking occurs during hot rolling. In addition, more retained austenite is formed, and dislocations are unevenly introduced during deformation such as pressing, resulting in a larger strength difference within the metal structure after bake hardening. Therefore, the Si content is set to 2.00% or less. The Si content can be 1.80% or less, 1.60% or less, 1.50% or less, or 1.40% or less.

[0029] [Mn: 1.00~4.00%] Mn is an effective element for hardenability and for enhancing strength as a solid solution strengthening element. To fully obtain these effects, the Mn content is set to 1.00% or more. The Mn content can be 1.20% or more, 1.50% or more, 1.80% or more, 2.00% or more, or 2.20% or more. On the other hand, if the Mn content is excessive, the formability may decrease. Therefore, the Mn content is set to 4.00% or less. The Mn content can be 3.80% or less, 3.50% or less, 3.20% or less, 3.00% or less, or 2.80% or less.

[0030] [sol.Al: 0.001~0.500%] Sol.Al acts as a deoxidizer in molten steel. Additionally, sol.Al inhibits the precipitation of cementite, which is detrimental to formability. To achieve these effects, the sol.Al content is set at 0.001% or more. The sol.Al content can be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, even with excessive sol.Al content, the effect saturates, potentially leading to increased manufacturing costs. Therefore, the sol.Al content is set at 0.500% or less. The sol.Al content can be 0.400% or less, 0.300% or less, or 0.200% or less. Sol.Al refers to acid-soluble Al, indicating solid-solution Al present in the steel.

[0031] [P: below 0.100%] Excessive phosphorus (P) content can sometimes reduce formability due to grain boundary segregation. Therefore, the P content is set to 0.100% or less. The P content can be 0.050%, 0.030%, 0.020%, or 0.015% or less. There is no specific lower limit for the P content; it can be 0%, but excessive reduction will lead to increased costs. Therefore, the P content can be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0032] [S: Below 0.0300%] Excessive sulfur (S) content can sometimes lead to the formation of large amounts of sulfides such as MnS, reducing formability. Therefore, the S content is set to below 0.0300%. The S content can be below 0.0200%, 0.0100%, or 0.0050%. There is no specific lower limit for the S content; it can be 0%, but excessive reduction will increase costs. Therefore, the S content can be above 0.0001%, 0.0010%, or 0.0030%.

[0033] [N: below 0.0070%] Excessive nitrogen (N) content can sometimes form coarse nitrides, reducing formability. Therefore, the N content is set to 0.0070% or less. The N content can be 0.0050%, 0.0040%, or 0.0030% or less. There is no specific lower limit for the N content; it can be 0%, but excessive reduction will lead to increased costs. Therefore, the N content can be 0.0001% or more, or 0.0005% or more.

[0034] [O: below 0.0100%] O (O) is an element introduced during the manufacturing process. Excessive O content can sometimes form large inclusions, reducing the formability of the steel sheet. Therefore, the O content is set to be below 0.0100%. The O content can be below 0.0080%, 0.0060%, or 0.0040%. There is no specific lower limit for the O content; it can be 0%, but refining to below 0.0001% requires time, leading to reduced productivity. Therefore, the O content can be above 0.0001% or above 0.0005%.

[0035] [Nb: 0.001~1.000%] Nitrogen (Nb) is an element that forms carbides, nitrides, and / or carbonitrides in steel, contributing to the refinement of the original austenite grains through a pinning effect, thereby increasing the strength of the steel sheet. To fully achieve these effects, the Nb content is set to 0.001% or more. The Nb content can be 0.005% or more, 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, or 0.300% or more. On the other hand, if the Nb content is excessive, coarse carbides may sometimes form in the steel, reducing the formability of the steel sheet. Therefore, the Nb content is set to 1.000% or less. The Nb content can be 0.800% or less, 0.600% or less, or 0.500% or less.

[0036] The basic chemical composition of the steel plate according to embodiments of the present invention is as described above. Furthermore, the steel plate may, as needed, contain at least one of the following elements to replace a portion of the remaining Fe.

[0037] [Cr: 0~0.90%] Cr is an element that improves the hardenability of steel and contributes to increased strength and / or corrosion resistance. The Cr content can be 0%, but to achieve these effects, the Cr content is preferably 0.001% or more, specifically 0.01%, 0.05%, or 0.10% or more. On the other hand, even with excessive Cr content, the effect saturates, which may lead to increased manufacturing costs. Therefore, the Cr content is preferably 0.90% or less, specifically 0.70%, 0.50%, 0.40%, or 0.30% or less.

[0038] [Ti: 0~0.200%, V: 0~0.300%, Cu: 0~0.40%, Mo: 0~0.12%, Ni: 0~0.30%, B: 0~0.0030%, Ca: 0~0.0010%, Mg: 0~0.0010%, B i: 0~0.010%, Zr: 0~0.050%, Co: 0~0.010%, Zn: 0~0.010%, W: 0~0.100%, Sn: 0~0.040%, As: 0~0.100% and REM: 0~0.0100%] Ti, V, Cu, Mo, Ni, B, Ca, Mg, Bi, Zr, Co, Zn, W, Sn, As, and REM can be included in the steel plate as any selected element, or sometimes as impurity elements. The content of these elements can be: Ti: 0~0.200% or 0.100%, V: 0~0.300% or 0.200%, Cu: 0~0.40% or 0.20%, Mo: 0~0.12%, 0.09%, 0.08%, 0.06% or 0.04%, Ni: 0~0.30% or 0.15%, B: 0~0.0030% or 0.0015%, Ca: 0~0.0010% or 0.0008%. %, Mg: 0~0.0010% or 0.0008%, Bi: 0~0.010%, Zr: 0~0.050% or 0.030%, Co: 0~0.010%, Zn: 0~0.010%, W: 0~0.100% or 0.050%, Sn: 0~0.040% or 0.020%, As: 0~0.100% or 0.050%, and REM: 0~0.0100% or 0.0050%. Regarding the lower limits for these elements, the contents of Ti, V, Cu, Mo, Ni, Bi, Zr, Co, Zn, W, Sn, and As can be above 0.001%, above 0.005%, or above 0.008%, respectively. Similarly, the contents of B, Ca, Mg and REM can be above 0.0001%, above 0.0002%, or above 0.0005%.

[0039] In the steel plate of the embodiments of the present invention, the remaining portion other than the aforementioned elements consists of Fe and impurities. Impurities refer to components that are mixed in during the industrial manufacturing of steel plates due to various reasons in the manufacturing process, such as raw materials like ores and scrap iron, as well as components that can be included within a range that does not affect the effects of the present invention.

[0040] The chemical composition of the steel plate according to the embodiments of the present invention can be determined by conventional analytical methods. For example, the chemical composition of the steel plate can be determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S can be determined using combustion-infrared absorption method, N can be determined using inactive gas melting-thermal conductivity method, and O can be determined using inactive gas melting-non-dispersive infrared absorption method.

[0041] [Metal Structure] [Martensite: ≥90.0% and retained austenite: ≤3.0%] The metal microstructure of the steel sheet according to embodiments of the present invention contains 90.0% or more martensite and 3.0% or less retained austenite by area percentage. By constructing the metal microstructure of the steel sheet in a manner that includes these components, high strength, such as a tensile strength of 980 MPa or more, can be achieved, and the strengthening process during the forming of the steel sheet obtained by pressing or the like, followed by coating and baking, can be made more uniform. More specifically, by controlling the hard martensite to a range of 90.0% or more by area percentage to form a more uniform microstructure, not only is high strength achieved, but the strength difference in the metal microstructure can also be reduced. Due to this reduction in strength difference, the uniformity of strengthening during coating and baking can be improved. When the area percentage of martensite is less than 90.0%, the desired strength cannot be achieved and / or the strength difference in the metal microstructure cannot be reduced. As a result, the uniformity of strengthening during coating and baking is sometimes not adequately improved. From the viewpoint of further increasing strength and improving the uniformity of strengthening during coating baking, a higher martensite area ratio is preferred, for example, 92.0% or more, 94.0% or more, 96.0% or more, or 98.0% or more. There is no particular upper limit to the martensite area ratio; it can be 100.0%, for example, 99.0% or less. On the other hand, retained austenite may become the initiation point of fracture during deformation such as pressing, and may then introduce dislocations unevenly into the microstructure during this deformation. Therefore, in addition to controlling the martensite to be 90.0% or more in terms of area percentage, limiting the retained austenite to 3.0% or less in terms of area percentage can more significantly improve the strength difference in the metal microstructure. If the area ratio of retained austenite exceeds 3.0%, the strength difference within the bake-hardened metal microstructure increases due to their role as the initiation point of fracture during deformation and / or the introduction of dislocations unevenly during deformation. Therefore, a lower area ratio of retained austenite is preferred, for example, it can be below 2.5%, below 2.0%, below 1.5%, or below 1.0%. There is no particular limitation on the lower limit of the area ratio of retained austenite; it can be 0%, or for example, above 0.5%.

[0042] [Remaining Organization] The remaining microstructure other than martensite and retained austenite can be 0% by area percentage. However, if the remaining microstructure is present, it may include at least one of ferrite (10.0% or less), bainite (10.0% or less), and pearlite (10.0% or less). If the combined area percentage of at least one of ferrite, bainite, and pearlite exceeds 10.0%, the area percentage of martensite is less than 90.0%, and therefore, the desired strength and uniform strengthening during coating baking cannot be achieved. The lower limits for ferrite, bainite, and pearlite can be 0%, for example, 0.1% or more, 0.5% or more, 1.0% or more, 2.0% or more, or 3.0% or more, respectively. Similarly, the upper limits for ferrite, bainite, and pearlite can be 8.0% or less, 6.0% or less, 5.0% or less, or 4.0% or less, respectively.

[0043] [Identification of metallic structures and calculation of area ratio] The identification of the microstructure and the calculation of the area ratio in the steel plate were performed using optical microscopy after etching with nitric acid-ethanol reagent or LePera solution, and X-ray diffraction. Microstructure observation was performed on a section of the plate perpendicular to the plate surface using optical microscopy. Preferably, the section of the plate thickness was parallel to the rolling direction. Specifically, firstly, a sample was collected from the steel plate, and the observation surface of the sample was etched with nitric acid-ethanol. Next, an image analysis was performed on a microstructure photograph obtained at a depth of 1 / 4 of the plate thickness with a field of view of 300 μm × 300 μm using an optical microscope, from which the total area ratio of martensite and bainite, and the area ratios of ferrite and pearlite were calculated. Then, using a sample with the observation surface etched with LePera, an image analysis was performed on a microstructure photograph obtained at a depth of 1 / 4 of the plate thickness with a field of view of 300 μm × 300 μm using an optical microscope, from which the total area ratio of martensite and retained austenite was calculated. Next, using a sample cut to a depth of 1 / 4 of the plate thickness from the end face normal to the rolling surface, the volume fraction of retained austenite was calculated by X-ray diffraction. Since the volume fraction of retained austenite is equal to its area fraction, it was used as the area fraction of retained austenite. The area fraction of martensite was calculated by subtracting the area fraction of retained austenite from the previously calculated total area fraction of martensite and retained austenite. Finally, the area fraction of bainite was calculated by similarly subtracting the area fraction of martensite from the previously calculated total area fraction of martensite and bainite.

[0044] [Average grain size of the original austenite: below 30.0 μm] In the steel sheet of the embodiment of the present invention, the average grain size of the original austenite grains is 30.0 μm or less. As described above, the dislocation distribution within the martensite structure is considered effective in achieving uniform strengthening. Relatedly, by refining the original austenite grains, the density of the original austenite grain boundaries can be increased. Therefore, by refining the original austenite grains to 30.0 μm or less, dislocations can be uniformly introduced into the metal structure during pressing, thereby reducing the strength difference within the baked-hardened metal structure. From the viewpoint of further reducing the strength difference within the baked-hardened metal structure, a smaller average grain size of the original austenite grains is preferred, for example, it can be 28.0 μm or less, 25.0 μm or less, 22.0 μm or less, 20.0 μm or less, 18.0 μm or less, or 15.0 μm or less. There is no specific lower limit, and the average grain size of the original austenite grains can be, for example, greater than 3.0 μm, greater than 3.2 μm, greater than 3.5 μm, greater than 3.7 μm, greater than 4.0 μm, greater than 4.2 μm, greater than 4.5 μm, greater than 4.7 μm, greater than 5.0 μm, greater than 6.0 μm, greater than 8.0 μm, greater than 10.0 μm, or greater than 12.0 μm.

[0045] [Standard deviation of the original austenite grain size: less than 4.0 μm] In embodiments of the present invention, the standard deviation of the original austenite grain size is less than 4.0 μm. By limiting the average grain size of the original austenite grains to 30.0 μm or less, and on the other hand, by making the standard deviation of the original austenite grain size less than 4.0 μm, i.e., reducing the deviation of the original austenite grain size, the dislocation distribution within the martensite structure can be sufficiently homogenized, thereby significantly reducing the strength difference within the baked-hardened metal structure. From the viewpoint of further improving such an effect, the smaller the standard deviation of the original austenite grain size, the more preferred, i.e., the smaller the deviation, the more preferred. For example, it can be 3.8 μm or less, 3.6 μm or less, 3.4 μm or less, 3.2 μm or less, 3.0 μm or less, 2.8 μm or less, 2.6 μm or less, 2.4 μm or less, 2.2 μm or less, or 2.0 μm or less. There is no specific lower limit. The standard deviation of the original austenite grain size can be, for example, above 0.5 μm, above 0.8 μm, above 1.0 μm, above 1.2 μm, or above 1.5 μm.

[0046] [Methods for determining the average grain size and standard deviation of the original austenite grain size] The average grain size and standard deviation of the original austenite grain size were determined as follows. First, a sample was cut from any position at least 50 mm from the end face of the steel plate (avoiding positions where sample collection is not possible at that position), in a way that allows observation of a section perpendicular to the plate surface. The section is preferably parallel to the rolling direction. The sample size also depends on the measuring device, and is set to approximately 10 mm in size that can be observed in the direction perpendicular to the plate thickness. After grinding the section of the sample using #600 to #1500 silicon carbide paper, a mirror finish was achieved using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. Next, the observation surface was finished by electrolytic polishing. At any position along the length of the sample section and at a depth of 1 / 4 of the plate thickness, a region 50 μm in length and 50 μm in the plate thickness direction was measured by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. For the measurement, any EBSD analysis apparatus consisting of a thermal field emission scanning electron microscope and an EBSD detector is sufficient. For example, an EBSD analysis apparatus using a JEOL JSM-7001F detector and a TSL DVC5 detector is suitable. In this case, the vacuum level within the EBSD analysis apparatus can be set to 9.6 × 10⁻⁶. -5 The acceleration voltage was set to 15 kV and the irradiation current level to 13 Pa. Using the obtained crystal orientation information, the crystal orientation of the original austenite grains was calculated based on the general crystal orientation relationship between the original austenite grains and the transformed body-centered structure grains. The method for calculating the crystal orientation of the original austenite grains was as follows: First, a crystal orientation map of the original austenite grains was prepared using the method described in Acta Materialia, 58 (2010), 6393-6403. For one original austenite grain contained in the field of view, the average of the shortest diameter and the longest diameter was calculated, and this average was taken as the grain size of the original austenite grain. Excluding the original austenite grains at the ends of the field of view, which cannot be contained in the entire field of view, the above operation was performed on all the original austenite grains, and the grain size of all the original austenite grains in the field of view was determined. The average grain size and standard deviation of the original austenite grains are determined by calculating the average grain size and standard deviation from the grain size of all the original austenite grains.

[0047] Standard deviation of dislocation density in the width direction: 1.50 × 10⁻⁶ 15 / m 2 the following] In an embodiment of the invention, the standard deviation of the dislocation density at a position of 1 / 4 of the plate thickness, in the direction from the end in the width direction toward the center of the width, relative to the seven positions of 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, is 1.50 × 10⁻⁶.15 / m 2 Below, as described above, by limiting the average grain size of the original austenite grains in the metal microstructure to 30.0 μm or less and controlling the standard deviation of the grain size of the original austenite grains to less than 4.0 μm, the metal microstructure is made uniform in the microscopic region. Furthermore, by controlling the standard deviation of the dislocation density at the aforementioned seven locations in the width direction of the steel plate to 1.50 × 10⁻⁶, 15 / m 2 This makes the metal structure uniform even in the macroscopic region, thereby significantly improving the uniformity of strengthening during coating and baking. Furthermore, by using such a steel sheet in component manufacturing, the entire component can be uniformly strengthened after forming and during coating and baking. Here, "entire component" can refer to all locations on the steel sheet used in component manufacturing, but in this embodiment of the invention, the locations for measuring the dislocation density to support uniform strengthening of the entire component are selected as seven locations: 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, from the end of the steel sheet in the width direction towards the center of the width. To further improve the uniformity of strengthening during coating and baking, a smaller standard deviation of the dislocation density is preferred, for example, 1.40 × 10⁻⁶. 15 / m 2 Below, 1.20×10 15 / m 2 Below, 1.00×10 15 / m 2 Below or 0.80×10 15 / m 2 The following is an example. No specific lower limit is specified; for instance, the standard deviation of the dislocation density mentioned above could be 0.01 × 10⁻⁶. 15 / m 2 Above, 0.05×10 15 / m 2 Above or 0.10×10 15 / m 2 above.

[0048] [Method for determining the standard deviation of dislocation density in the width direction] The standard deviation of the dislocation density in the width direction was determined as follows. First, X-ray diffraction samples were collected at specified locations on the steel plate, namely, seven positions relative to the full width of the steel plate: 5%, 10%, 15%, 20%, 25%, 30%, and 50%, from the end of the steel plate towards the center of the width. The surface of the collected samples was then ground to remove oxide scale, and X-ray diffraction measurements were performed at positions corresponding to 1 / 4 of the steel plate thickness. The dislocation density was calculated using a method that converts the strain from the half-maximum amplitude β determined by X-ray diffraction. In the diffraction intensity curves obtained by conventional X-ray diffraction, Kα1 and Kα2 rays of different wavelengths overlap, and therefore are separated using the Rachinger method. The Williamson-Hall method was used for strain extraction. The broadening of the half-maximum amplitude is affected by the crystallite size D and the strain ε, and can be calculated as the sum of these two factors using the following formula: β = β1 + β2 = (0.9λ / (D×cosθ)) + 2ε×tanθ. Further transforming this equation, we get βcosθ / λ = 0.9λ / D + 2ε × sinθ / λ. The strain ε is calculated based on the slope of the straight line by plotting βcosθ / λ relative to sinθ / λ. The diffraction lines used for the calculation are set to (110), (200), (211), (220), (310), and (222). The dislocation density is calculated from the strain ε using ρ = 14.4ε. 2 / b 2 θ refers to the peak angle calculated using the θ-2θ method of X-ray diffraction, and λ refers to the wavelength of the X-rays used in X-ray diffraction. b is the Burgers vector of Fe(α).

[0049] [Plate thickness] The steel plate used in the embodiments of the present invention is not particularly limited, and typically has a plate thickness of 1.0 to 8.0 mm. For example, the plate thickness can be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or can be 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

[0050] The steel sheet of the embodiments of the present invention is of high strength and can be uniformly strengthened during coating and baking, thus significantly suppressing or reducing deformation localization in large deformations during collisions. Therefore, the steel sheet of the embodiments of the present invention is particularly useful in the automotive field where excellent crashworthiness is required. In a preferred embodiment, automotive parts, particularly automotive running gear, incorporating the steel sheet of the embodiments of the present invention are provided. Examples of automotive running gear include lower arms, trailing arms, etc. These parts are manufactured by pressing the steel sheet of the embodiments of the present invention. Although the forming varies depending on the part, they are considered to essentially inherit the characteristics and excellent properties of the steel sheet as the raw material. These automotive parts, particularly automotive running gear, only need to contain the steel sheet of the embodiments of the present invention in at least a portion of these parts, thus satisfying the above-described characteristics of chemical composition and microstructure in at least a portion of these parts.

[0051] [Mechanical Properties] [Tensile Strength: TS] According to the steel sheet, particularly hot-rolled steel sheet, which has the above-described chemical composition and metallic structure, high tensile strength, specifically 980 MPa or more, can be achieved. The tensile strength is preferably 1000 MPa or more, 1080 MPa or more, or 1180 MPa or more. Despite having such very high tensile strength, the steel sheet according to the embodiments of the present invention, through the specific combination of the chemical composition and metallic structure described above, can significantly improve the uniformity of strengthening during coating baking. There is no particular upper limit to the tensile strength; for example, the tensile strength of the steel sheet can be 1780 MPa or less, 1700 MPa or less, or 1600 MPa or less. The tensile strength is determined as follows: A JIS 5 test piece is collected from the end of the steel sheet in the width direction toward the center of the width, at a position relative to 50% of the total width, in a direction parallel to the rolling direction of the steel sheet (L direction), and a tensile test is performed according to JIS Z 2241:2011.

[0052] <Methods for manufacturing steel plates> Next, a preferred manufacturing method for the steel sheet according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the steel sheet according to an embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the method described below. More specifically, the manufacture of hot-rolled steel sheets is specifically shown below, but the steel sheet according to an embodiment of the present invention includes not only any steel sheet having the chemical composition and metallic structure described above, i.e., hot-rolled steel sheets, but also cold-rolled steel sheets, galvanized steel sheets, etc. Therefore, the following description is merely an explanation of a preferred manufacturing method when the steel sheet according to an embodiment of the present invention is a hot-rolled steel sheet.

[0053] The method for manufacturing the steel plate according to embodiments of the present invention includes the following steps: Heating process: The slab having the chemical composition described above in relation to the steel plate is heated and held at a temperature of 1100°C or higher for more than 6000 seconds; Width reduction process: The slab is subjected to width reduction pressing using a finishing pressurizing device, wherein the width reduction pressing satisfies the following conditions (a) and (b): (a) Width reduction ratio of 1.0~23%, and (b) The conveying speed of the above-mentioned slab is 20 m / min or more; Hot rolling process: This includes finishing rolling the above-mentioned slab, wherein the finishing rolling satisfies the following conditions (c) to (e): (c) The reduction rate in each rolling pass of the final section and the preceding and final sections is 20-50%. (d) Total reduction rate of 90% or more, and (e) The final rolling temperature is 960~1100℃; Cooling process: Cooling of the steel plate begins within 0.5 seconds after the completion of the hot rolling process, and then the steel plate is cooled to a temperature below 400°C within 20.0 seconds from the start of cooling; and Coiling process: The cooled steel sheet is coiled at a temperature below 400°C. In the above manufacturing method, the temperatures recorded for the slab and the steel sheet refer to the surface temperatures of the slab and the steel sheet, respectively. The following is a detailed description of each process.

[0054] [Casting Process] The conditions for the casting process are not particularly limited. For example, after smelting in a blast furnace or electric furnace, various secondary smelting processes can be performed, followed by casting using conventional continuous casting or ingot casting methods. It is preferable to cast slabs with the chemical composition described above, which is related to the steel plate, in the continuous casting process. In a preferred embodiment, the temperature history during solidification is appropriately controlled in the continuous casting process; more specifically, the average cooling rate at 600–900°C is 10°C / min or higher, and the average cooling rate gradient is 40°C / min. 2 The following control method is used: to ensure an average cooling rate of 10°C / min or higher at 600~900°C, and an average cooling rate gradient of 40°C / min. 2The following methods control the continuous casting process, reliably achieving the refinement of the original austenite grains and small standard deviations within the fine grains in the final steel sheet's microstructure. The average cooling rate at 600–900°C can be 12°C / min or higher, 15°C / min or higher, 18°C / min or higher, or 20°C / min or higher. Similarly, the average cooling rate at 600–900°C can be 70°C / min or lower.

[0055] The average cooling rate gradient within the 600–900°C range refers to the average rate of change of the cooling rate per unit time within this range. For example, when the cooling rate changes from 10°C / min to 50°C / min, the average cooling rate gradient in this manufacturing method is 40°C / min. 2 Conversely, when the cooling rate changes from 50°C / min to 10°C / min, the average cooling rate gradient in this manufacturing method is also 40°C / min. 2 The average cooling rate gradient between 600 and 900°C can be 30°C / min. 2 The following is not specifically defined as a lower limit; the average cooling rate gradient between 600 and 900°C can be 2°C / min. 2 Above or 3℃ / min 2 above.

[0056] [Heating Process] The cast slab is heated in the subsequent heating process and held at a temperature above 1100°C for at least 6000 seconds. In this manufacturing method, holding at a temperature above 1100°C includes not only holding at a constant temperature above 1100°C, but also holding at varying temperatures above 1100°C. By holding the slab at a temperature above 1100°C for at least 6000 seconds, coarse carbides present in the microstructure can be completely dissolved, eliminating the initiation point of cracking. If the holding temperature is below 1100°C or the holding time is less than 6000 seconds, the dissolution of coarse carbides becomes incomplete. If the coarse carbides are not completely dissolved, ferrite-bainite phase transformations originating from such carbides occur during the cooling process described later, resulting in a martensite area fraction of less than 90.0%, which fails to achieve the desired strength and / or uniform strengthening during coating baking. The upper limit of the heating temperature of the slab is preferably below 1300°C or 1200°C. Similarly, the upper limit of the holding time in the temperature range above 1100°C is preferably below 10000 seconds.

[0057] [Width pressing process] [(a) Width reduction ratio: 1.0~23%] In this manufacturing method, a finishing pressurizing device is used to perform width reduction pressing on the slab. During steel sheet manufacturing, the sheet thickness decreases at the ends in the width direction, and cooling is easier there; therefore, the strain introduced by rolling is prone to vary in the width direction. Thus, it is important to appropriately control the microstructure in the width direction to reduce the overall strength difference of the steel sheet and to achieve a homogeneous microstructure. By controlling the width reduction rate of the width reduction pressing using the finishing pressurizing device within the range of 1.0% to 23%, the anisotropic solidified structure (columnar crystals) in the transverse ends of the slab can be destroyed, resulting in a homogenized microstructure, and the non-uniform introduction of strain into the slab can be reliably suppressed. As a result, the strength and microstructure deviations in the width direction of the steel sheet can be reduced. On the other hand, if the width reduction rate is less than 1.0%, the aforementioned solidified structure cannot be reliably destroyed to achieve microstructure homogenization, and the deviation in dislocation density in the width direction in the final metal microstructure becomes larger. Furthermore, if the width reduction exceeds 23%, the strain deviation in the width direction becomes excessive, and similarly, the deviation in dislocation density in the width direction increases in the final metal structure. Preferably, the width reduction during width reduction pressing using a finishing pressurizing device is controlled within the range of 5% to 20%.

[0058] [(b) Slab conveying speed: 20 m / min or higher] In the width reduction process, the slab conveying speed is controlled to be 20 m / min or higher. During the width reduction process, the slab end dissipates heat upon contact with the sizing rolls, causing a temperature drop at the slab end. Therefore, minimizing the width-direction deviation caused by this temperature drop becomes important. By increasing the slab conveying speed, specifically controlling it to 20 m / min or higher, the contact time between the slab end and the sizing rolls can be shortened, thus effectively suppressing heat dissipation from the sizing rolls. Therefore, by combining this with the aforementioned width reduction rate control, the width-direction deviation of the steel sheet can be reliably reduced. As a result, the standard deviation of the dislocation density in the width direction can be controlled within a specified range, significantly improving the uniformity of strengthening during coating baking. On the other hand, if the slab conveying speed is less than 20 m / min, the heat dissipation from the sizing rolls increases, and the temperature difference in the width direction of the slab becomes larger. In this case, the deviation of the dislocation density in the width direction in the final metal structure becomes larger. Preferably, the slab conveying speed is 30 m / min or higher. There is no specific upper limit; for example, the conveying speed of the slab can be below 50 m / min.

[0059] [Hot rolling process] [Rough rolling] In this method, for example, for heated slabs, rough rolling can be performed before finish rolling to adjust the slab thickness. Rough rolling is not particularly limited as long as the desired slab size can be ensured.

[0060] [(c) Reduction rate in each rolling pass before and after the final section: 20-50%] The heated slab, or slabs that have undergone rough rolling as needed, are then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a continuous rolling mill consisting of multiple rolling stands, for example, five or more rolling mills. In this manufacturing method, during the finish rolling of the heated slab, the reduction rate in each rolling pass of the last two sections (the section before the final section and the final section) is controlled to be 20-50%. By performing rolling at such a high reduction rate in the section before the final section and each rolling pass of the final section, recrystallization is promoted, resulting in a finer metal microstructure. If the reduction rate in the section before the final section and / or each rolling pass of the final section is less than 20%, recrystallization is not completed or sufficiently promoted, and sometimes the desired average grain size and / or standard deviation of the original austenite grains cannot be achieved in the metal microstructure of the final steel sheet. When the desired average grain size and / or standard deviation of the original austenite grains cannot be achieved, the strength difference within the bake-hardened metal microstructure cannot be sufficiently reduced. On the other hand, if the reduction rate in the preceding section and / or each rolling pass of the final section is too high, the rolling load becomes excessive, and the load on equipment such as the rolling mill increases. Therefore, the reduction rate in the preceding section and each rolling pass of the final section is set to 50% or less. Preferably, the reduction rate in the preceding section and each rolling pass of the final section is 45% or less.

[0061] [(d) Total reduction rate: 90% or more] In this manufacturing method, the total reduction rate during finishing rolling is controlled to be 90% or higher. Mn, contained in steel, is an element that reduces the grain boundary breaking energy. Therefore, if there are regions of localized Mn concentration, it can sometimes promote cracking during plastic deformation such as pressing. Therefore, from the viewpoint of further improving formability, suppressing or reducing localized Mn concentration is effective. By controlling the total reduction rate during finishing rolling to 90% or higher, Mn can diffuse within the steel, and consequently, deviations in the Mn concentration in the steel can be suppressed or reduced, i.e., localized Mn concentration can be suppressed or reduced. If the total reduction rate during finishing rolling is lower than 90%, the deviation in Mn concentration becomes relatively high, and sometimes it is not possible to sufficiently suppress the development of regions where Mn localized concentration and breaking energy can be reduced. The upper limit of the total reduction rate during finishing rolling can be, for example, 99% or lower or 98% or lower. Here, the total reduction rate during finishing rolling is calculated using the following formula.

[0062] Total reduction rate (%) = (thickness of plate before finishing rolling - thickness of plate after finishing rolling) / thickness of plate before finishing rolling × 100 [(e) Final rolling temperature: 960~1100℃] In this manufacturing method, in addition to controlling the reduction rate in each rolling pass of the last two stages of finishing rolling, the final rolling temperature (the end temperature of finishing rolling) is also crucial in controlling the microstructure of the steel sheet. If the final rolling temperature is below 960°C, recrystallization is incomplete or insufficiently promoted, and sometimes the desired average grain size and / or standard deviation of the original austenite grains cannot be achieved in the final microstructure of the steel sheet. When the desired average grain size and / or standard deviation of the original austenite grains cannot be achieved, the strength difference within the bake-hardened microstructure cannot be sufficiently reduced. On the other hand, if the final rolling temperature exceeds 1100°C, the original austenite grains become coarse overall, and sometimes the desired average grain size and / or standard deviation of the original austenite grains cannot be achieved. In this case, the strength difference within the bake-hardened microstructure also cannot be sufficiently reduced.

[0063] [Cooling Process] [Time from the end of the hot rolling process to the start of cooling: less than 0.5 seconds] [Time from the start of cooling to reaching below 400°C: less than 20.0 seconds] After the hot rolling process, the finished steel sheet begins cooling in the subsequent cooling process within a period of less than 0.5 seconds, and then cools to a temperature below 400°C within 20.0 seconds from the start of cooling. By performing such cooling control, the desired average grain size and standard deviation of the original austenite grains can be achieved in the metal structure of the final steel sheet.

[0064] If the time from the end of the hot rolling process to the start of cooling is more than 0.5 seconds, grain growth occurs, and the desired standard deviation of the original austenite grain size cannot be obtained. Furthermore, if the time from the end of the hot rolling process to the start of cooling exceeds 10.0 seconds, excessive grain growth occurs overall, and the desired average grain size and / or standard deviation of the original austenite grain size cannot be obtained. As a result, in either case, the strength difference within the baked-hardened metal structure cannot be sufficiently reduced. On the other hand, when the cooling time from the start of cooling to below 400°C exceeds 20.0 seconds or the cooling stop temperature exceeds 400°C, the martensite area ratio is less than 90.0%, resulting in the inability to achieve the desired strength and / or uniform strengthening during coating baking.

[0065] [Winding process] Finally, the cooled steel sheet is rolled in a temperature range below 400°C to manufacture a steel sheet. If the rolling temperature exceeds 400°C, similar to the cooling process, the martensite area ratio is less than 90.0%, resulting in the inability to achieve the desired strength and / or uniform strengthening during coating baking.

[0066] The steel sheet manufactured by the above-described manufacturing method achieves high strength, such as a tensile strength of 980 MPa or higher, by forming a more uniform microstructure comprising martensite of 90.0% or more and retained austenite of 3.0% or less per square meter of area. Furthermore, the steel sheet undergoes more uniform strengthening during forming processes such as pressing, followed by coating and baking treatments. Moreover, by limiting the average grain size of the original austenite grains in the microstructure to 30.0 μm or less and controlling the standard deviation of the grain size to less than 4.0 μm, the microstructure becomes uniform in the microscopic region, and the standard deviation of the dislocation density in the width direction is controlled to 1.50 × 10⁻⁶. 15 / m 2 This process also homogenizes the metal structure in the macroscopic region, thereby significantly improving the uniformity of strengthening during paint baking. Therefore, the steel sheet manufactured by the above method can provide a high-strength component capable of significantly suppressing or reducing deformation localization in large deformations generated during a collision. Consequently, this steel sheet is particularly useful in the automotive industry, where excellent crashworthiness is required.

[0067] The present invention will now be described in more detail by way of examples, but the present invention is not limited to these examples in any way.

[0068] Example In the following embodiments, steel plates, particularly hot-rolled steel plates, of embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), variation of TS in the width direction, and deviation (standard deviation) of bake hardening in the width direction of the obtained steel plates were investigated.

[0069] First, using the continuous casting method, the average cooling rate at 600~900℃ is 12℃ / min, and the average cooling rate gradient is 30℃ / min. 2 Molten steel was cast under specific conditions to form slabs with the various chemical compositions shown in Table 1. These slabs were heated to a temperature of 1100–1200°C and held for the times shown in Table 2, followed by width reduction and hot rolling. Width reduction was performed using a finishing pressurizing device at the width reduction rates and slab conveying speeds shown in Table 2. Hot rolling was performed by roughing and finishing rolling. More specifically, roughing was performed under the same conditions in all examples and comparative examples, while finishing was performed using a continuous rolling mill consisting of 5 rolling mills under the conditions shown in Table 2. Finally, the finished steel sheets were cooled and coiled under the conditions shown in Table 2 to obtain steel sheets with a thickness of 1.6–3.2 mm.

[0070] The properties of the obtained steel plate were measured and evaluated using the following methods.

[0071] Standard deviation of dislocation density in the width direction The standard deviation of the dislocation density in the width direction was determined as follows. First, X-ray diffraction samples were collected at seven locations relative to the full width of the steel plate: 5%, 10%, 15%, 20%, 25%, 30%, and 50%, from the end of the plate towards the center of the width. The surface of the collected samples was then ground to remove oxide scale, and X-ray diffraction measurements were performed at locations corresponding to 1 / 4 of the plate thickness. The dislocation density was calculated using strain derived from the half-maximum amplitude β determined by X-ray diffraction. In the diffraction intensity curves obtained through conventional X-ray diffraction, Kα1 and Kα2 rays of different wavelengths overlap, and therefore were separated using the Rachinger method. Strain extraction was performed using the Williamson-Hall method. The broadening of the half-maximum amplitude is affected by the crystallite size D and the strain ε, and can be calculated as the sum of these two factors using the following formula: β = β1 + β2 = (0.9λ / (D×cosθ)) + 2ε×tanθ. Further transforming this equation, we get βcosθ / λ = 0.9λ / D + 2ε × sinθ / λ. The strain ε is calculated from the slope of the straight line by plotting βcosθ / λ relative to sinθ / λ. The diffraction lines used for the calculation are set to (110), (200), (211), (220), (310), and (222). The dislocation density is calculated from the strain ε using ρ = 14.4ε. 2 / b 2 θ refers to the peak angle calculated using the θ-2θ method of X-ray diffraction, and λ refers to the wavelength of the X-rays used in X-ray diffraction. b is the Burgers vector of Fe(α), which is set to 0.25 nm in this embodiment.

[0072] [Variation of tensile strength (TS) and TS in the width direction] First, tensile test specimens (No. 5) according to JIS Z2241:2011 were collected at seven locations relative to the width of the steel plate: 5%, 10%, 15%, 20%, 25%, 30%, and 50%, with the test direction parallel to the rolling direction. Next, tensile tests were performed on these specimens according to JIS Z2241:2011, yielding seven tensile strength (TS) values. The difference between the maximum and minimum values ​​was then calculated to determine the variation of TS in the width direction. Furthermore, the tensile strength at the aforementioned 50% location was determined as the tensile strength (TS) of the steel plate.

[0073] Standard deviation of bake hardening amount in the width direction The standard deviation of bake hardening in the width direction was determined as follows. First, similar to the TS determination, tensile test pieces (JIS Z2241:2011 No. 5) were collected at seven locations relative to the full width of the steel plate: 5%, 10%, 15%, 20%, 25%, 30%, and 50%, with the test direction parallel to the rolling direction. Next, each test piece was pre-strained with 1% and then heat-treated at 170°C for 20 minutes. The bake hardening amount of each test piece was then determined by subtracting the stress at which the 1% pre-strain was applied from the stress at which the test piece was re-stretched. Finally, the standard deviation of the bake hardening amount in the width direction was determined based on the bake hardening amounts of the seven test pieces.

[0074] Steel sheets with a tensile strength (TS) of 980 MPa or higher, a TS variation of 70 MPa or less in the width direction, and a standard deviation of bake hardening in the width direction of 20 MPa or less are evaluated as high-strength steel sheets that can be uniformly strengthened during coating and baking. The results are shown in Table 3.

[0075] Referring to Tables 1-3, it is believed that Comparative Example 24, due to its low width reduction rate in the width reduction process, failed to sufficiently destroy the anisotropic solidified structure in the transverse end of the slab. As a result, the final metal structure exhibited a larger deviation in dislocation density in the width direction, a larger variation in strain tolerance (TS) in the width direction, and a larger standard deviation in bake hardening. Comparative Example 25, due to its excessive width reduction rate in the width reduction process, exhibited a larger deviation in strain in the width direction. Similarly, the final metal structure also showed a larger deviation in dislocation density in the width direction, a larger variation in TS in the width direction, and a larger standard deviation in bake hardening. Comparative Example 26, due to its slow slab conveying speed in the width reduction process, resulted in a longer contact time between the slab end and the sizing roller, leading to increased heat dissipation from the sizing roller and a larger temperature difference in the width direction of the slab. Consequently, the final metal structure also showed a larger deviation in dislocation density in the width direction, a larger variation in TS in the width direction, and a larger standard deviation in bake hardening. It is believed that in Comparative Example 27, due to the short holding time in the temperature region above 1100°C during the heating process, the coarse carbides were not completely dissolved. In the subsequent cooling process, ferrite and bainite phase transformations occurred starting from these carbides. As a result, the martensite area ratio was less than 90.0%, and the variation in span direction (TS) and the standard deviation of bake hardening increased. It is believed that in Comparative Examples 28 and 29, due to the low reduction rates of the rolling passes in the preceding and final stages of the finishing rolling, recrystallization was incomplete or not sufficiently promoted. As a result, the average grain size and standard deviation of the original austenite grains in the final metal microstructure increased, as did the variation in span direction (TS) and the standard deviation of bake hardening.

[0076] Comparative Example 30 was considered to have insufficient or incomplete recrystallization due to the low final rolling temperature during finishing. As a result, the average grain size and standard deviation of the original austenite grains in the final microstructure increased, as did the variation in TS in the width direction and the standard deviation of bake hardening. Comparative Example 31 was considered to have coarse-grained original austenite grains due to the high final rolling temperature during finishing. As a result, the average grain size of the original austenite grains in the final microstructure increased, as did the variation in TS in the width direction and the standard deviation of bake hardening. Comparative Example 32 was considered to have grain growth due to the time exceeding 0.5 seconds from the end of the hot rolling process to the start of the cooling process. As a result, the desired standard deviation of the original austenite grain size could not be obtained, and the variation in TS in the width direction and the standard deviation of bake hardening increased. Comparative Example 33 was considered to have excessive grain growth due to the time exceeding 10.0 seconds from the end of the hot rolling process to the start of the cooling process. As a result, the desired average grain size of the original austenite grains could not be obtained, and the variation in TS in the width direction and the standard deviation of bake hardening amount increased. In Comparative Example 34, the cooling process took a long time from the start of cooling to reaching below 400°C, therefore the martensite area ratio was less than 90.0%, and the variation in TS in the width direction and the standard deviation of bake hardening amount increased. In Comparative Example 35, due to the high winding temperature, the martensite area ratio was also less than 90.0%, and the variation in TS in the width direction and the standard deviation of bake hardening amount increased.

[0077] Comparative Examples 36 and 38, due to their low C and Si contents, respectively, exhibited reduced TS (strain tolerance). On the other hand, Comparative Examples 37 and 39, due to their high C and Si contents, respectively, resulted in the formation of more retained austenite. As a result, dislocations were introduced non-uniformly when a 1% pre-strain was applied, leading to a larger standard deviation of bake hardening. Comparative Example 40, due to its low Mn content, exhibited reduced hardenability, resulting in a lower martensite area ratio, reduced TS, and a larger variation in TS along the width direction and a larger standard deviation of bake hardening. Comparative Examples 41 and 44, due to their high Mn and Nb contents, respectively, exhibited reduced formability, and consequently, a larger standard deviation of bake hardening. Comparative Example 42, due to its low sol.Al content, also exhibited reduced formability and a larger standard deviation of bake hardening. It is believed that Comparative Example 43, due to its low Nb content, could not sufficiently promote the refinement of the original austenite grains caused by the pinning effect. As a result, the average grain size of the original austenite grains in the final metal microstructure increased, and the variation in strain tolerance (TS) in the width direction and the standard deviation of bake hardening increased. Comparative Example 45 was considered to have a large width reduction rate in the width reduction process, resulting in a larger deviation in strain tolerance in the width direction. Consequently, in the final metal microstructure, the deviation in dislocation density in the width direction increased, and the variation in TS and the standard deviation of bake hardening increased. Comparative Example 46 was considered to have undergone grain growth because the time from the end of the hot rolling process to the start of the cooling process was more than 0.5 seconds. As a result, the desired standard deviation in the grain size of the original austenite grains could not be obtained, and the variation in TS and the standard deviation of bake hardening increased.

[0078] In contrast, in all the steel plates of the invention examples, by having a specified chemical composition and appropriately controlling the conditions in the manufacturing method, it is possible to obtain a metal microstructure containing, by area percent, more than 90.0% martensite and less than 3.0% retained austenite, with an average grain size of less than 30.0 μm for the original austenite grains, a standard deviation of the grain size of the original austenite grains of less than 4.0 μm, and a standard deviation of the dislocation density in the width direction of 1.50 × 10⁻⁶. 15 / m 2 The following steel sheet. Furthermore, as a result, it is possible to achieve a high strength of 980 MPa or more, and the uniformity of strengthening during coating and baking is significantly improved. Additionally, in all the steel sheets of the invention examples, the remaining microstructure other than martensite and retained austenite (retained γ) is at least one of ferrite, bainite, and pearlite.

Claims

1. A steel plate, characterized in that, Its chemical composition, expressed as a percentage by mass, is as follows: C：0.040~0.200%、 Si: 0.30~2.00% Mn: 1.00~4.00% sol.Al: 0.001~0.500% P: Below 0.100% S: Below 0.0300% N: below 0.0070% O: Below 0.0100% Nb: 0.001~1.000% Ti: 0~0.200% V：0~0.300%、 Cu: 0~0.40%, Cr：0~0.90%、 Mo: 0~0.12%, Ni: 0~0.30% B：0~0.0030%、 Ca: 0~0.0010%, Mg: 0~0.0010%, Bi: 0~0.010% Zr：0~0.050%、 Co: 0~0.010%, Zn: 0~0.010%, W：0~0.100%、 Sn: 0~0.040% As: 0~0.100% REM: 0~0.0100%, and Remaining components: Fe and impurities. The metallic structure contains, by area %: Martensite: over 90.0%, and Residual austenite: less than 3.0%, The average grain size of the original austenite grains is less than 30.0 μm. The standard deviation of the original austenite grain size is less than 4.0 μm. The standard deviation of dislocation density at the 1 / 4 position of the plate thickness, from the end of the width direction towards the center of the width, relative to the seven positions of 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, is 1.50 × 10⁻⁶. 15 / m 2 the following.

2. The steel plate according to claim 1, characterized in that, The chemical composition, expressed as a percentage by mass, contains at least one of the following elements: Ti: 0.001~0.200% V：0.001~0.300%、 Cu: 0.001~0.40% Cr：0.001~0.90%、 Mo: 0.001~0.12% Ni: 0.001~0.30% B：0.0001~0.0030%、 Ca: 0.0001~0.0010% Mg: 0.0001~0.0010% Bi: 0.001~0.010% Zr：0.001~0.050%、 Co: 0.001~0.010%, Zn: 0.001~0.010%, W：0.001~0.100%、 Sn: 0.001~0.040% As: 0.001~0.100%, and REM: 0.0001~0.0100%.

3. The steel plate according to claim 1 or 2, characterized in that, The metal microstructure further comprises at least one of the following, in area percentage: Ferrite: less than 10.0% Bainite: less than 10.0%, and Pearlite: less than 10.0%.

4. The steel plate according to claim 1 or 2, characterized in that, The plate thickness is 1.0~8.0mm.

5. A component, characterized in that, It comprises the steel plate as described in claim 1 or 2.

6. A method for manufacturing a steel plate, characterized in that, It includes the following processes: Heating process: Heating a slab having the chemical composition described in claim 1 or 2 and holding it at a temperature above 1100°C for more than 6000 seconds; Width reduction process: This includes using a finishing pressurizing device to perform width reduction pressing on the slab, wherein the width reduction pressing satisfies the following conditions (a) and (b): (a) Width reduction ratio of 1.0~23%, and (b) The conveying speed of the slab is 20 m / min or higher; Hot rolling process: This includes finishing rolling the slab, wherein the finishing rolling satisfies the following conditions (c) to (e): (c) The reduction rate in each rolling pass of the final section and the preceding and final sections is 20-50%. (d) Total reduction rate of 90% or more, and (e) The final rolling temperature is 960~1100℃; Cooling process: Cooling of the steel plate begins within 0.5 seconds after the completion of the hot rolling process, and then the steel plate is cooled to a temperature below 400°C within 20.0 seconds from the start of cooling; and Coiling process: The cooled steel plate is coiled in a temperature range below 400℃.

7. The method for manufacturing a steel plate according to claim 6, characterized in that, The heating process further includes a continuous casting process, in which an average cooling rate of 10°C / min or more and an average cooling rate gradient of 40°C / min are provided at 600-900°C. 2 The following methods can be used for control.

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