Steel sheet and method for producing same

Abstract

Provided is a steel sheet which is characterized by having a specific chemical composition, wherein: the metal structure contains, in terms of percentage by area, 90.0% or more of martensite and 3.0% or less of retained austenite; the average grain size of the prior austenite grains is 30.0 μm or less; the standard deviation of the grain size of the prior austenite grains is less than 4.0 μm; and the standard deviation of the dislocation density at seven positions corresponding to 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the entire width of the steel sheet from an end in the width direction to the center in the width direction at a position corresponding to 1 / 4 the sheet thickness is 1.50 × 1015 / m2 or less. Also provided is a method for producing the steel sheet.

Description

Steel plate and its manufacturing method

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

[0002] In recent years, the use of high-strength steel sheets has expanded to reduce the weight of automobiles, contributing to improved fuel efficiency. However, because many automotive parts are manufactured by press forming, they require both high strength and excellent formability.

[0003] In this regard, for example, Patent Document 1 describes a high-strength steel plate containing C: 0.1 to 0.25%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, Cr: 0.1 to 1.5%, Mo: 0.1 to 0.5%, Ti: 0.01 to 0.05%, and Nb: 0.01 to 0.05%, as well as V: 0.01 to 0.05% and / or B: 0.0001 to 0.005%, with the balance being iron and unavoidable impurities, the steel plate having an average grain size of prior austenite of 20 μm or less and a standard deviation (σ) of the prior austenite grain size distribution of 5 μm or less. Furthermore, Patent Document 1 teaches that, as described above, by appropriately adjusting the chemical composition and controlling the average grain size of prior γ and the standard deviation (σ) of the prior γ grain size distribution within appropriate ranges, it is possible to realize a high-strength steel sheet that is bendable while maintaining a high tensile strength of 980 MPa or more.

[0004] Patent Document 2 describes a composition containing, by mass%, C: 0.08 to 0.30%, Si: 3.0% or less, Mn: 1.0 to 4.0%, P: 0.100% or less, S: 0.02% or less, Al: 1.0% or less, N: 0.008% or less, with the balance being Fe and unavoidable impurities, and a steel structure in which the total area ratio of polygonal ferrite, fresh martensite, and retained austenite at a quarter position in the thickness direction of the steel plate and at the center position in the width direction of the steel plate is 20% or less with respect to the entire steel structure. The high-strength hot-rolled steel sheet is characterized in that the total area ratio of martensite and lower bainite is 65 to 100% of the entire steel structure, and the standard deviation of the total area ratio of the polygonal ferrite, the fresh martensite, and the retained austenite is 7.0% or less at a quarter-thickness position of the steel sheet and at positions 5%, 10%, 15%, 20%, 25%, and 30% of the total width of the steel sheet from the edge toward the width center, and at the width center of the steel sheet. Furthermore, Patent Document 2 teaches that the use of the above-mentioned high-strength hot-rolled steel sheet can reduce variation in shape during the production of parts such as automobile parts, thereby enabling the production of products such as high-strength parts with stable shapes.

[0005] JP 2009-242832 A JP 2021-063252 A

[0006] To ensure the safety of passengers, improved crashworthiness is also desired for automotive parts. This requires high-strength parts that can suppress localized deformation caused by uneven material properties when large deformations occur during a collision. In connection with this, there is a demand for steel materials that can strengthen the entire part uniformly during paint baking after forming.

[0007] Therefore, an object of the present invention is to provide a steel sheet having a new structure, which has high strength and can be strengthened uniformly during paint baking.

[0008]

[0006] In order to achieve the above-mentioned object, the present inventors have conducted studies focusing on the metallographic structure of steel sheets, particularly hot-rolled steel sheets. As a result, the present inventors have first found that by configuring the metallographic structure of a hot-rolled steel sheet having a predetermined chemical composition to be mainly composed of martensite, it is possible to achieve high strength and improve the uniformity of strengthening during paint baking. In addition, the present inventors have found that by limiting the average grain size of prior austenite grains in the metallographic structure to within a predetermined range and reducing the variation in the grain size of the prior austenite grains to make the metallographic structure uniform in the microscopic region, and further by controlling the standard deviation of dislocation density in the width direction to within a predetermined range to make the metallographic structure uniform in the macroscopic region, it is possible to significantly improve the uniformity of strengthening during paint baking, and have completed the present invention.

[0009] The present invention has achieved the above object as follows: (1) A steel sheet having a chemical composition, in mass %, of C: 0.040 to 0.200%, Si: 0.30 to 2.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001 to 1.000%, Ti: 0 to 0.200%, V: 0 to 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 to 0.010%, Zn: 0 to 0.010%, W: 0 to 0.100%, Sn: 0 to 0.040%, As: 0 to 0.100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, the metallographic structure contains, in area %, 90.0% or more of martensite and 3.0% or less of retained austenite, the average grain size of prior austenite grains is 30.0 μm or less, the standard deviation of the grain size of the prior austenite grains is less than 4.0 μm, and the standard deviation of the dislocation density at seven positions at 1 / 4 of the sheet thickness and at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width from the end in the width direction toward the width center is 1.50 × 10 15 / m 2(2) A steel sheet characterized in that the chemical composition is, in mass%, Ti: 0.001 to 0.200%, V: 0.001 to 0.300%, Cu: 0.001 to 0.40%, Cr: 0.001 to 0.90%, Mo: 0.001 to 0.12%, Ni: 0.001 to 0.30%, B: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, Bi: 0.001 to 0.010%, Zr: 0.001 to 0.050%, Co: 0.001 to 0.010%, Zn: 0.001 to 0.010%, (1) The steel sheet according to (1), characterized in that it contains at least one of W: 0.001 to 0.100%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, and REM: 0.0001 to 0.0100%. (3) The steel sheet according to (1) or (2), characterized in that the metallographic structure further contains, in area percentages, at least one of ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less. (4) The steel sheet according to any one of (1) to (3), characterized in that the sheet thickness is 1.0 to 8.0 mm. (5) A part, characterized in that it contains the steel sheet according to any one of (1) to (4).(6) A heating step of heating a slab having the chemical composition described in (1) or (2) above and maintaining it in a temperature range of 1100°C or higher for 6000 seconds or more; A width reduction step including width reduction pressing of the slab with a sizing press device, the width reduction pressing satisfying the following conditions (a) and (b): (a) a width reduction ratio of 1.0 to 23%, and (b) a conveying speed of the slab of 20 m / min or higher; A hot rolling step including finish rolling of the slab, the finish rolling satisfying the following conditions (c) to (e): (c) a reduction ratio in each rolling pass of the penultimate stage and the final stage of the rolling is 20 to 50%, (d) a total reduction ratio of 90% or higher, and (e) a final rolling temperature of 960 to 1100°C. A method for manufacturing a steel sheet, comprising: a cooling step of starting cooling of the steel sheet within 0.5 seconds after the completion of the hot rolling step, and then cooling the steel sheet to a temperature of 400°C or less within 20.0 seconds from the start of cooling; and a coiling step of coiling the cooled steel sheet in a temperature range of 400°C or less. (7) Before the heating step, the average cooling rate at 600 to 900°C is 10°C / min or more, and the average cooling rate gradient is 40°C / min. 2 The method for producing a steel sheet according to (6) above, further comprising a continuous casting step of controlling the following:

[0010] According to the present invention, it is possible to provide a steel sheet which has high strength and can be strengthened uniformly during paint baking.

[0011] <Steel Sheet> The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition, in mass %, of C: 0.040 to 0.200%, Si: 0.30 to 2.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001 to 1.000%, Ti: 0 to 0.200%, V: 0 to 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 to 0.010%, Zn: 0 to 0.010%, W: 0 to 0.100%, Sn: 0 to 0.040%, As: 0 to 0.100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, the metallographic structure contains, in area %, 90.0% or more of martensite and 3.0% or less of retained austenite, the average grain size of prior austenite grains is 30.0 μm or less, the standard deviation of the grain size of the prior austenite grains is less than 4.0 μm, and the standard deviation of the dislocation density at seven positions at 1 / 4 of the sheet thickness and at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width from the end in the width direction toward the width center is 1.50 × 10 15 / m 2 It is characterized by the following:

[0012] As mentioned above, there is a demand for high-strength automotive parts that can suppress localized deformation due to material inhomogeneity in the face of large deformations that occur during a collision, and in this regard, there is a demand for steel materials that can be uniformly strengthened throughout the part during paint baking after forming (bake hardening). Here, bake hardening is a phenomenon in which interstitial elements (mainly carbon) migrate and adhere to dislocations introduced by press forming (hereinafter also referred to as "pre-strain") through a paint baking process at 100 to 200°C, thereby inhibiting their movement and increasing strength; this phenomenon is also called strain aging.

[0013]

[0005] Therefore, the present inventors conducted studies focusing not only on the chemical composition of a steel sheet, particularly a hot-rolled steel sheet, but also on the metallurgical structure of the hot-rolled steel sheet. First, the present inventors discovered that by forming the metallurgical structure of a hot-rolled steel sheet having a predetermined chemical composition into a structure mainly composed of martensite, more specifically, a structure containing, by area %, 90.0% or more of martensite and 3.0% or less of retained austenite, high strength, for example, a tensile strength of 980 MPa or more, can be achieved, while more uniform strengthening can be achieved when the obtained hot-rolled steel sheet is formed by press forming or the like and then subjected to a paint baking treatment. While not intending to be bound by any particular theory, it is believed that by forming the metallurgical structure into a more uniform structure containing 90.0% or more of martensite by area %, the strength difference in the metallurgical structure can be reduced compared to when other structures softer than martensite, such as ferrite, are contained in a relatively large amount, and this reduction in strength difference can improve the uniformity of strengthening during paint baking. Furthermore, since retained austenite can be the origin of fracture during deformation such as press forming, and furthermore can introduce dislocations non-uniformly into the structure during the deformation, it is possible to more significantly reduce the strength difference in the metal structure by controlling martensite to 90.0% or more by area percent and limiting the retained austenite to 3.0% or less by area percent.

[0014] Next, the inventors conducted research from the perspective of optimizing the grain size of prior austenite grains in a metal structure mainly composed of martensite, because dislocation distribution within the martensite structure is considered effective in achieving uniform strengthening. More specifically, by refining the prior austenite grains, the density of the prior austenite grain boundaries can be increased. Therefore, by refining the prior austenite grains, dislocations are uniformly introduced into the metal structure during press forming, which results in a reduction in strength differences within the metal structure after bake hardening. More specifically, when the grain size of the prior austenite grains is large, the distance between the interior of the grains and the vicinity of the grain boundaries increases, which makes it easy for strain to be introduced non-uniformly. This is because strain is easily introduced near the grain boundaries, but is difficult to introduce into the interior of the grains. Therefore, by refining the prior austenite grains and increasing the density of the prior austenite grain boundaries, dislocations can be introduced more uniformly into the metal structure during press forming, which results in a reduction in strength differences within the metal structure after bake hardening.

[0015] However, simply refining prior austenite grains does not necessarily result in a uniform dislocation distribution within the martensite structure, and strength differences within the metal structure after bake hardening may not be sufficiently reduced. For example, if coarse and fine austenite grains are present in a structure prior to martensite transformation, the onset temperature of martensite transformation varies depending on the grain size of the austenite grains, resulting in different thermal histories for each grain before cooling. Specifically, austenite grains with larger grain sizes have a higher onset temperature of martensite transformation than austenite grains with smaller grain sizes. Therefore, autotempering progresses between the completion of transformation and cooling to room temperature. Therefore, some of the dislocations introduced during the martensite transformation are annihilated by thermal vibration during this thermal history. Therefore, it is believed that the dislocation density in austenite grains transformed at higher temperatures is relatively low, while the dislocation density in austenite grains transformed at lower temperatures is relatively high. As a result, the dislocation distribution within the martensite structure becomes nonuniform. Therefore, in order to homogenize the dislocation distribution in the martensitic structure, it is important to reduce the variation in the austenite grain size before martensitic transformation. In other words, it is thought that by reducing the variation in the austenite grain size before martensitic transformation, it is possible to reduce the variation in the prior austenite grain size after martensitic transformation, and as a result, it is possible to homogenize the dislocation distribution in the martensitic structure.

[0016] Therefore, the present inventors have focused on and investigated not only the control of the grain size of prior austenite grains but also the control of the grain size distribution, more specifically, the control of the grain size variation, and have found that by refining the prior austenite grains within a predetermined range, more specifically, by controlling the average grain size of the prior austenite grains to 30.0 μm or less, the density of prior austenite grain boundaries in the steel sheet as a whole can be increased while reducing the variation in the grain size of the prior austenite grains, more specifically, by controlling the standard deviation of the grain size of the prior austenite grains to less than 4.0 μm, the dislocation distribution in the martensite structure can be sufficiently uniform, and thereby the strength difference in the metal structure after bake hardening can be significantly reduced.

[0017] Although controlling the average grain size of prior austenite grains and the standard deviation of the grain size of the prior austenite grains as described above can homogenize the dislocation distribution in the martensite structure in the microscopic region of the steel sheet, it is not possible to reliably homogenize the dislocation distribution in the martensite structure in the macroscopic region of the entire steel sheet. On the other hand, as described above, in automotive parts and the like, in order to suppress localized deformation during a collision, a steel material that can uniformly strengthen the entire part after forming and paint baking is required. In this regard, steel sheets, including slabs, are generally manufactured by rolling, and the rolling is performed using rolling rolls. Numerous test results related to such rolling have shown that the steel sheet has uniform material properties and strength in the longitudinal direction (rolling direction) compared to the width direction, and therefore the difference in strength in the longitudinal direction is relatively small. On the other hand, in the width direction of the steel sheet, the end portions are thinner and more easily cooled, and therefore the strain introduced by rolling is likely to vary in the width direction. Therefore, in order to reduce the difference in strength throughout the steel sheet and to make the structure uniform, it is important to appropriately control the structure in the width direction.

[0018] Therefore, the inventors have further studied to homogenize the dislocation distribution in the martensite structure not only in the micro region of the steel sheet but also in the macro region of the entire steel sheet. As a result, the inventors have found that the standard deviation of the dislocation density at seven positions, namely, 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width of the steel sheet from the end portion in the width direction of the steel sheet toward the width center portion, is within a specific range, more specifically, 1.50 × 10 15 / m 2 It has been found that a uniform metallographic structure can be achieved throughout the steel sheet by controlling the standard deviation as described below. In particular, in the width direction of the steel sheet, the thickness reduction is greater at the end portions than at the width center portion. For this reason, it is considered effective to control the standard deviation based on dislocation densities measured relatively frequently at locations closer to the end portions as described above, rather than the standard deviation based on dislocation densities measured at equal distances, for example, 10%, 20%, 30%, 40%, and 50% of the entire width of the steel sheet from the end portions toward the width center portion. According to the steel sheet according to the embodiment of the present invention, the metallographic structure of the steel sheet having a predetermined chemical composition is formed from a martensite-based structure, thereby achieving high strength and improving the uniformity of strengthening during paint baking. In addition, the average grain size of prior austenite grains in the metallographic structure is limited to 30.0 μm or less, and the standard deviation of the grain size of the prior austenite grains is controlled to less than 4.0 μm, thereby making the metallographic structure uniform in the microscopic region. Furthermore, the standard deviation of the dislocation densities at the above seven positions in the width direction is controlled to 1.50 × 10 15 / m 2 By controlling the following to make the metal structure uniform even in the macroscopic region, it becomes possible to significantly improve the uniformity of strengthening during paint baking. Therefore, by using the steel sheet according to the embodiment of the present invention, it becomes possible to provide high-strength parts that can significantly suppress or reduce localized deformation in large deformation that occurs during a collision. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in the automotive field, where excellent crash resistance is required.

[0019] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0020] [C: 0.040 to 0.200%] C is an element effective in increasing the strength of steel sheet. Furthermore, C forms carbides and / or carbonitrides with Nb in steel, contributing to refinement of the structure through 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 may be 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, excessive C content may generate a relatively large amount of retained austenite, which may introduce non-uniform dislocations during deformation such as press forming, resulting in increased strength differences within the metal structure after bake hardening. Therefore, the C content is set to 0.200% or less. The C content may be 0.180% or less, 0.160% or less, 0.150% or less, or 0.140% or less.

[0021] [Si: 0.30 to 2.00%] Si is an effective solid-solution strengthening element for increasing strength. To fully achieve this effect, the Si content is set to 0.30% or more. The Si content may 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, excessive Si content may deteriorate chemical conversion treatability and formability and may cause slab cracking during hot rolling. In addition, relatively large amounts of retained austenite may be generated, which may lead to non-uniform dislocations during deformation such as press forming, resulting in increased strength differences within the metal structure after bake hardening. Therefore, the Si content is set to 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, 1.50% or less, or 1.40% or less.

[0022] [Mn: 1.00 to 4.00%] Mn is an element that is effective in increasing hardenability and 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 may 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, excessive Mn content may decrease formability. Therefore, the Mn content is set to 4.00% or less. The Mn content may be 3.80% or less, 3.50% or less, 3.20% or less, 3.00% or less, or 2.80% or less.

[0023] [Sol. Al: 0.001 to 0.500%] Sol. Al is an element that acts as a deoxidizer for molten steel. Sol. Al also suppresses the precipitation of cementite, which is harmful to formability. To achieve these effects, the sol. Al content is set to 0.001% or more. The sol. Al content may 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, excessive sol. Al content may saturate the effect and increase manufacturing costs. Therefore, the sol. Al content is set to 0.500% or less. The sol. Al content may be 0.400% or less, 0.300% or less, or 0.200% or less. Sol. Al refers to acid-soluble Al, which refers to solute Al present in steel in a solid solution state.

[0024] [P: 0.100% or less] Excessive P content may result in reduced formability due to grain boundary segregation, etc. Therefore, the P content is set to 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0025] [S: 0.0300% or less] Excessive S content may result in the formation of large amounts of sulfides such as MnS, which may reduce formability. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the S content may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[0026] [N: 0.0070% or less] Excessive N content may form coarse nitrides and reduce formability. Therefore, the N content is set to 0.0070% or less. The N content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[0027] [O: 0.0100% or less] O is an element that is mixed in during the manufacturing process. Excessive O content may cause the formation of coarse inclusions, which may reduce the formability of the steel sheet. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long refining time, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more or 0.0005% or more.

[0028] [Nb: 0.001 to 1.000%] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, thereby contributing to the refinement of prior austenite grains through a pinning effect, thereby increasing the strength of the steel sheet. To fully obtain these effects, the Nb content is set to 0.001% or more. The Nb content may 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, excessive Nb content may generate coarse carbides and the like in the steel, which may reduce the formability of the steel sheet. Therefore, the Nb content is set to 1.000% or less. The Nb content may be 0.800% or less, 0.600% or less, or 0.500% or less.

[0029] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0030] [Cr: 0 to 0.90%] Cr is an element that improves the hardenability of steel and contributes to improving strength and / or corrosion resistance. The Cr content may be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if Cr is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Cr content is preferably 0.90% or less, and may be 0.70% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0031] [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%, Bi : 0 to 0.010%, Zr: 0 to 0.050%, Co: 0 to 0.010%, Zn: 0 to 0.010%, W: 0 to 0.100%, Sn: 0 to 0.040%, As: 0 to 0.100%, and REM: 0 to 0.0100%] Ti, V, Cu, Mo, Ni, B, Ca, Mg, Bi, Zr, Co, Zn, W, Sn, As, and REM may be contained in the steel sheet as optional elements or may be present in the steel sheet as tramp elements. The contents of these elements are as follows: Ti: 0 to 0.200% or 0.100%, V: 0 to 0.300% or 0.200%, Cu: 0 to 0.40% or 0.20%, Mo: 0 to 0.12%, 0.09%, 0.08%, 0.06%, or 0.04%, Ni: 0 to 0.30% or 0.15%, B: 0 to 0.0030% or 0.0015%, Ca: 0 to 0.0010% or 0.0008%, M The lower limits of these elements may be, for example, Ti, V, Cu, Mo, Ni, Bi, Zr, Co, Zn, W, Sn, and As, each of which may be 0.001% or more, 0.005% or more, or 0.008% or more. Similarly, the B, Ca, Mg and REM contents may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0032] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the steel sheet is industrially manufactured, and components that are contained to an extent that does not affect the effects of the present invention.

[0033] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0034] [Metal Structure] [Martensite: 90.0% or More and Retained Austenite: 3.0% or Less] The metal structure of the steel sheet according to the present invention includes, by area percentage, 90.0% or more martensite and 3.0% or less retained austenite. By configuring the metal structure of the steel sheet to include these structures, high strength, for example, a tensile strength of 980 MPa or more, can be achieved, while more uniform strengthening can be achieved when the resulting steel sheet is formed by press forming or the like and then subjected to a paint baking process. More specifically, controlling the hard martensite content to 90.0% or more by area percentage to create a more uniform structure not only contributes to increased strength, but also reduces strength differences within the metal structure, thereby improving the uniformity of strengthening during paint baking. If the area percentage of martensite is less than 90.0%, the desired strength cannot be achieved and / or the strength differences within the metal structure cannot be reduced, resulting in insufficient improvement of the uniformity of strengthening during paint baking. From the viewpoint of further increasing strength and improving the uniformity of strengthening during paint baking, a higher martensite area fraction is preferable, and may be, for example, 92.0% or more, 94.0% or more, 96.0% or more, or 98.0% or more. The upper limit of the martensite area fraction is not particularly limited and may be 100.0%, for example, 99.0% or less. On the other hand, since retained austenite can serve as fracture initiation points during deformation such as press forming and can introduce non-uniform dislocations into the structure during such deformation, controlling the martensite area fraction to 90.0% or more and limiting the retained austenite area fraction to 3.0% or less can more significantly improve the strength difference within the metal structure. If the area fraction of retained austenite exceeds 3.0%, the retained austenite may serve as fracture initiation points during deformation and / or the strength difference within the metal structure after bake hardening will be large due to the introduction of non-uniform dislocations during deformation. Therefore, the area ratio of the retained austenite is preferably as low as possible, and may be, for example, 2.5% or less, 2.0% or less, 1.5% or less, or 1.0% or less. The lower limit of the area ratio of the retained austenite is not particularly limited and may be 0%, or may be, for example, 0.5% or more.

[0035] [Remaining Structure] The remaining structure other than martensite and retained austenite may be 0% by area. However, if a remaining structure is present, the remaining structure may include at least one of ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less. If the total area fraction of at least one of ferrite, bainite, and pearlite exceeds 10.0%, the area fraction of martensite will be less than 90.0%, resulting in failure to achieve the desired strength and uniformity of strengthening during paint baking. The lower limits of ferrite, bainite, and pearlite may each be 0%, or may be, 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 of ferrite, bainite, and pearlite may each be 8.0% or less, 6.0% or less, 5.0% or less, or 4.0% or less, respectively.

[0036] [Identification of Metallic Structure and Calculation of Area Ratio] Identification of the metallic structure of a steel sheet and calculation of its area ratio are performed by optical microscope observation and X-ray diffraction method after corrosion using a Nital reagent or a Repeller solution. The microstructure observation using an optical microscope is performed on a plate thickness cross section perpendicular to the plate surface. The plate thickness cross section is preferably parallel to the rolling direction. Specifically, a sample is first taken from the steel sheet, and the observation surface of the sample is etched with Nital. Next, image analysis is performed on a microstructure photograph obtained using an optical microscope with a 300 μm × 300 μm field at a depth of 1 / 4 of the plate thickness to calculate the total area ratio of martensite and bainite, as well as the area ratios of ferrite and pearlite. Next, using a sample whose observation surface has been subjected to Repeller corrosion, image analysis is similarly performed on a microstructure photograph obtained using an optical microscope with a 300 μm × 300 μm field at a depth of 1 / 4 of the plate thickness to calculate the total area ratio of martensite and bainite. Next, using a sample that has been face-milled from the normal direction of the rolled surface to a depth of 1 / 4 of the plate thickness, the volume fraction of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of retained austenite is equivalent to the area fraction, this is taken as the area fraction of retained austenite. The area fraction of martensite is calculated by subtracting the obtained area fraction of retained austenite from the previously calculated total area fraction of martensite and retained austenite. Finally, the area fraction of bainite is calculated by subtracting the obtained area fraction of martensite from the previously calculated total area fraction of martensite and bainite.

[0037] [Average grain size of prior austenite grains: 30.0 μm or less] In the steel sheet according to the embodiment of the present invention, the average grain size of the prior austenite grains is 30.0 μm or less. As described above, dislocation distribution within the martensite structure is considered effective in achieving uniform strengthening. In this regard, by refining the prior austenite grains, the density of the prior austenite grain boundaries can be increased. Therefore, by refining the prior austenite grains to 30.0 μm or less, dislocations can be uniformly introduced into the metal structure during press forming, thereby reducing the strength difference within the metal structure after bake hardening. From the viewpoint of further reducing the strength difference within the metal structure after bake hardening, the smaller the average grain size of the prior austenite grains, the more preferable it is. For example, it may 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. Although the lower limit is not particularly limited, the average grain size of the prior austenite grains may be, for example, more than 3.0 μm, 3.2 μm or more, 3.5 μm or more, 3.7 μm or more, 4.0 μm or more, 4.2 μm or more, 4.5 μm or more, 4.7 μm or more, 5.0 μm or more, 6.0 μm or more, 8.0 μm or more, 10.0 μm or more, or 12.0 μm or more.

[0038] [Standard Deviation in Grain Size of Prior Austenite Grains: Less than 4.0 μm] In an embodiment of the present invention, the standard deviation in the grain size of the prior austenite grains is less than 4.0 μm. By limiting the average grain size of the prior austenite grains to 30.0 μm or less while setting the standard deviation in the grain size of the prior austenite grains to less than 4.0 μm, i.e., by reducing the variation in the grain size of the prior austenite grains, it is possible to sufficiently homogenize the dislocation distribution in the martensite structure, and thereby significantly reduce the strength difference in the metal structure after bake hardening. From the viewpoint of further enhancing this effect, the smaller the standard deviation in the grain size of the prior austenite grains, i.e., the smaller the variation, the more preferable. For example, the standard deviation may 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. Although the lower limit is not particularly limited, the standard deviation in the grain size of the prior austenite grains may be, for example, 0.5 μm or more, 0.8 μm or more, 1.0 μm or more, 1.2 μm or more, or 1.5 μm or more.

[0039] [Method for Determining the Average Grain Size and Standard Deviation of Prior Austenite Grain Size] The average grain size and standard deviation of the grain size of prior austenite grains are determined as follows. First, a sample is cut from any position 50 mm or more away from the end face of the steel sheet (if a sample cannot be taken from this position, a position avoiding the end) so that a thickness cross section perpendicular to the sheet surface can be observed. The thickness cross section is preferably parallel to the rolling direction. The size of the sample depends on the measuring device, but it should be large enough to observe approximately 10 mm in a direction perpendicular to the thickness direction. The cross section of the sample is polished using #600 to #1500 silicon carbide paper, and then finished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. Next, the observation surface is finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, at a depth of 1 / 4 of the plate thickness, a region of 50 μm in length and 50 μm in the plate thickness direction is measured at measurement intervals of 0.1 μm by electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used, for example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. In this case, the degree of vacuum inside the EBSD analyzer is 9.6 × 10 -5The pressure may be 1 Pa or less, the acceleration voltage may be 15 kV, and the probe current level may be 13. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and crystal grains having a body-centered structure after transformation. The following method is used to calculate the crystal orientation of the prior austenite grains. First, a crystal orientation map of the prior austenite grains is created using the method described in Acta Materialia, 58 (2010), 6393-6403. For one of the prior austenite grains included in the observation field of view, the average value of the shortest diameter and the longest diameter is calculated, and this average value is used as the grain size of the prior austenite grain. The above operation is performed for all prior austenite grains, excluding prior austenite grains whose entirety is not included in the observation field of view, such as those at the ends of the observation field of view, to determine the grain sizes of all prior austenite grains in the observation field of view. The average grain size and standard deviation of the grain size of the prior austenite grains are determined by calculating the average grain size and standard deviation from the grain sizes of all the prior austenite grains obtained.

[0040] [Standard deviation of dislocation density in the width direction: 1.50 × 10 15 / m 2 In the embodiment of the present invention, the standard deviation of dislocation density at seven positions at 1 / 4 of the sheet thickness and at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width from the end in the width direction toward the center of the width is 1.50 × 10 15 / m 2 As described above, the average grain size of the prior austenite grains in the metal structure is limited to 30.0 μm or less, and the standard deviation in the grain size of the prior austenite grains is controlled to less than 4.0 μm, thereby making the metal structure uniform in the microscopic region. In addition, the standard deviation of the dislocation density at the above seven positions in the width direction of the steel sheet is set to 1.50 × 10 15 / m 2By controlling the metal structure as described below to make it uniform even in the macroscopic region, it becomes possible to significantly improve the uniformity of strengthening during paint baking. Furthermore, by using such a steel sheet in the manufacture of a part, it becomes possible to uniformly strengthen the entire part during paint baking after forming. Here, the entire part can mean any position on the steel sheet used to manufacture the part, but in an embodiment of the present invention, seven positions are selected as dislocation density measurement positions to support uniform strengthening throughout the part: 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the entire width from the end portion in the width direction of the steel sheet toward the center of the width. In order to further improve the uniformity of strengthening during paint baking, it is preferable that the standard deviation of the dislocation density is as small as possible, and for example, 1.40 × 10 15 / m 2 Below, 1.20 x 10 15 / m 2 Below, 1.00 x 10 15 / m 2 or less or 0.80 x 10 15 / m 2 Although there is no particular lower limit, for example, the standard deviation of the dislocation density may be 0.01 × 10 15 / m 2 More than 0.05 x 10 15 / m 2 or more or 0.10 x 10 15 / m 2 It may be more than that.

[0041] [Method for Determining Standard Deviation of Dislocation Density in the Width Direction] The standard deviation of dislocation density in the width direction is determined as follows. First, samples for X-ray diffraction are taken from seven predetermined positions on the steel sheet, i.e., from the end of the steel sheet in the width direction toward the center of the width, at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width of the steel sheet. The surfaces of the taken samples are then polished to remove scale, and X-ray diffraction measurements are performed at positions corresponding to 1 / 4 of the steel sheet thickness. The dislocation density is calculated using a method of converting the strain obtained from the half-width β of the X-ray diffraction measurement. In the diffraction intensity curve obtained by ordinary X-ray diffraction, the Kα1 and Kα2 rays, which have different wavelengths, overlap, so they are separated using the Rachinger method. The Williamson-Hall method is used to extract the strain. The broadening of the half-width is affected by the crystallite size D and the strain ε, and can be calculated as the sum of both factors using the following formula. β = β1 + β2 = (0.9λ / (D × cos θ)) + 2ε × tan θ. Further modifying this equation, β cos θ / λ = 0.9λ / D + 2ε × sin θ / λ. By plotting β cos θ / λ against sin θ / λ, the strain ε can be calculated from the slope of the line. The diffraction lines used for the calculation are (110), (200), (211), (220), (310), and (222). The conversion of strain ε to dislocation density is ρ = 14.4ε 2 / b 2 is used. θ means the peak angle calculated by the θ-2θ method of X-ray diffraction, λ means the wavelength of the X-rays used in X-ray diffraction, and b is the Burgers vector of Fe(α).

[0042] [Thickness] The steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 8.0 mm, although not particularly limited thereto. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 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.

[0043] The steel sheet according to the embodiment of the present invention has high strength and can be uniformly strengthened during paint baking, thereby significantly suppressing or reducing localized deformation during large deformations that occur during a collision. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where excellent crashworthiness is required. In a preferred embodiment, an automotive part, particularly an automotive suspension part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automotive suspension parts include lower arms and trailing arms. These parts are manufactured by press-forming the steel sheet according to the embodiment of the present invention, and although the forming process differs depending on the part, it is believed that they basically inherit the characteristics and excellent properties of the steel sheet that is the raw material. It is sufficient for at least a portion of these automotive parts, particularly automotive suspension parts, to include the steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the above-mentioned chemical composition and structural characteristics.

[0044] [Mechanical Properties] [Tensile Strength: TS] A steel sheet having the above-described chemical composition and metallographic structure, particularly a hot-rolled steel sheet, can achieve high tensile strength, specifically a tensile strength of 980 MPa or more. The tensile strength is preferably 1000 MPa or more, 1080 MPa or more, or 1180 MPa or more. Despite having such extremely high tensile strength, the steel sheet according to the present invention can significantly improve the uniformity of reinforcement during paint baking due to the specific combination of the chemical composition and metallographic structure described above. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel sheet may be 1780 MPa or less, 1700 MPa or less, or 1600 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from a position 50% of the total width of the steel sheet from the end of the width direction toward the width center, with the longitudinal direction of the test piece parallel to the rolling direction of the steel sheet (L direction), and conducting a tensile test in accordance with JIS Z 2241:2011.

[0045] <Method for Manufacturing Steel Sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, although the following specifically describes the manufacture of a hot-rolled steel sheet, the steel sheet according to an embodiment of the present invention encompasses any steel sheet having the chemical composition and metallographic structure described above, i.e., not only a hot-rolled steel sheet, but also a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description merely describes a preferred method for manufacturing a steel sheet according to an embodiment of the present invention when the steel sheet is a hot-rolled steel sheet.

[0046] A method for producing a steel plate according to an embodiment of the present invention includes: a heating step of heating a slab having the chemical composition described above in relation to the steel plate and holding the slab in a temperature range of 1100°C or higher for 6000 seconds or longer; a width reduction step including width-reducing the slab with a sizing press, the width reduction pressing satisfying the following conditions (a) and (b): (a) a width reduction ratio of 1.0 to 23%, and (b) a conveying speed of the slab of 20 m / min or higher; a hot rolling step including finish-rolling the slab, the finish rolling satisfying the following conditions (c) to (e): (c) a reduction ratio in each rolling pass of the penultimate stage and the final stage of the rolling is 20 to 50%, (d) a total reduction ratio of 90% or higher, and (e) a final rolling temperature of 960 to 1100°C. The method is characterized by comprising: a cooling step in which cooling of the steel sheet is started within 0.5 seconds after the completion of the hot rolling step, and then the steel sheet is cooled to a temperature of 400°C or less within 20.0 seconds from the start of cooling; and a coiling step in which the cooled steel sheet is coiled in a temperature range of 400°C or less. In the above manufacturing method, the temperatures described for the slab and the steel sheet refer to the surface temperatures of the slab and the steel sheet, respectively. Each step will be described in detail below.

[0047] [Casting Process] The conditions of the casting process are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a method such as ordinary continuous casting or casting by an ingot method. Preferably, a slab having the chemical composition described above in relation to the steel plate is cast in the continuous casting process. In a preferred embodiment, the temperature history during solidification is appropriately controlled in the continuous casting process, and more specifically, the average cooling rate at 600 to 900°C is 10°C / min or more, and the average cooling rate gradient is 40°C / min or more. 2 The average cooling rate between 600 and 900°C is 10°C / min or more, and the average cooling rate gradient is 40°C / min. 2 By controlling the continuous casting process so as to satisfy the following conditions, it is possible to ensure that prior austenite grains are refined and that a small standard deviation in the fine grains is achieved in the metallographic structure of the finally obtained steel sheet. The average cooling rate at 600 to 900°C may be 12°C / min or more, 15°C / min or more, 18°C / min or more, or 20°C / min or more. Similarly, the average cooling rate at 600 to 900°C may be 70°C / min or less.

[0048] The average cooling rate gradient in the range of 600 to 900°C refers to the average rate of change of the cooling rate per unit time in the range of 600 to 900°C. 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, even if the cooling rate is changed from 50°C / min to 10°C / min, the average cooling rate gradient in this manufacturing method is 40°C / min. 2 The average cooling rate gradient from 600 to 900°C is 30°C / min. 2 Although the lower limit is not particularly limited, the average cooling rate gradient in the range of 600 to 900°C is 2°C / min. 2 or more or 3°C / min 2 It may be more than that.

[0049] [Heating Step] The cast slab is heated in the next heating step and held at a temperature of 1100°C or higher for 6000 seconds or more. In this manufacturing method, holding at a temperature of 1100°C or higher includes not only holding the slab at a constant temperature of 1100°C or higher, but also holding the slab at a temperature of 1100°C or higher while fluctuating within the temperature range. Holding the slab at a temperature of 1100°C or higher for 6000 seconds or more allows the coarse carbides present in the structure to be completely dissolved, eliminating crack initiation points. If the holding temperature is lower than 1100°C or the holding time is shorter than 6000 seconds, the coarse carbides will not be completely dissolved. If the coarse carbides are not completely dissolved, ferrite or bainite transformation will occur originating from these carbides during the cooling step described below, resulting in an area ratio of martensite of less than 90.0%, which results in the failure to achieve the desired strength and / or uniformity of strengthening during paint baking. The upper limit of the heating temperature of the slab is preferably 1300° C. or less or 1200° C. or less. Similarly, the upper limit of the holding time in the temperature range of 1100° C. or more is preferably 10,000 seconds or less.

[0050] [Width Reduction Process] [(a) Width Reduction Ratio: 1.0 to 23%] In this manufacturing method, the slab is subjected to width reduction pressing using a sizing press. When manufacturing a steel plate, the plate thickness is thin at the widthwise edges and is easily cooled, which means that the strain introduced by rolling is likely to vary across the width. Therefore, in order to minimize the strength difference throughout the steel plate and achieve a uniform structure, it is important to appropriately control the structure in the width direction. By controlling the width reduction ratio of the width reduction press using a sizing press within the range of 1.0 to 23%, the anisotropic solidification structure (columnar crystals) at the lateral edges of the slab can be destroyed to homogenize the structure, and the introduction of non-uniform strain within the slab can be reliably suppressed. As a result, it is possible to reduce the deviation in strength and structure of the steel plate across the width. On the other hand, if the width reduction ratio is less than 1.0%, the solidification structure cannot be reliably destroyed to homogenize the structure, resulting in large variations in dislocation density across the width in the final metal structure. On the other hand, if the width reduction exceeds 23%, the deviation in strain in the width direction becomes too large, and similarly, the variation in dislocation density in the width direction in the finally obtained metal structure becomes large. Preferably, the width reduction in the width reduction press using a sizing press is controlled within the range of 5 to 20%.

[0051] [(b) Slab Conveying Speed: 20 m / min or More] In the width reduction process, the slab conveying speed is controlled to 20 m / min or more. During the width reduction process, heat is removed from the slab edge when it comes into contact with the sizing roll, resulting in a temperature drop at the slab edge. Therefore, it is important to minimize the widthwise deviation caused by this temperature drop. By increasing the slab conveying speed, specifically controlling it to 20 m / min or more, the contact time between the slab edge and the sizing roll can be shortened, thereby sufficiently suppressing heat removal by the sizing roll. Therefore, in combination with the above-described width reduction rate control, the widthwise deviation of the steel sheet can be reliably reduced. As a result, the standard deviation of dislocation density in the widthwise direction can be controlled within a predetermined range, significantly improving the uniformity of strengthening during paint baking. On the other hand, if the slab conveying speed is less than 20 m / min, the amount of heat removed from the slab sizing roll increases, resulting in a large temperature difference in the widthwise direction of the slab. In this case, the final metallographic structure will have a large variation in dislocation density in the widthwise direction. Preferably, the slab conveying speed is 30 m / min or more. There is no particular upper limit, but the slab conveying speed may be, for example, 50 m / min or less.

[0052] [Hot Rolling Step] [Rough Rolling] In the present manufacturing method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0053] [(c) Reduction in each rolling pass of the next to last stage and the last stage: 20 to 50%] The heated slab, or the slab that has been subjected to rough rolling as needed, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a plurality of rolling stands, for example, a tandem rolling mill consisting of five or more rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, the reduction in each rolling pass of the last two stages, i.e., the next to last stage and the last stage, is controlled to 20 to 50%. By performing rolling with such a relatively high reduction in each rolling pass of the next to last stage and the last stage, recrystallization can be promoted and the metal structure can be refined. If the reduction in each rolling pass of the next to last stage and / or the last stage is less than 20%, recrystallization may not be completed or sufficiently promoted, and the desired average grain size and / or standard deviation in grain size of prior austenite grains may not be achieved in the metal structure of the steel sheet finally obtained. If the desired average grain size and / or standard deviation in grain size of prior austenite grains cannot be achieved, it becomes impossible to sufficiently reduce the strength difference in the metal structure after bake hardening. On the other hand, if the reduction ratio in the rolling passes one stage before the final stage and / or in the final stage is too high, the rolling load becomes excessive, increasing the load on equipment such as a rolling mill. For this reason, the reduction ratio in each rolling pass one stage before the final stage and in the final stage is set to 50% or less. Preferably, the reduction ratio in each rolling pass one stage before the final stage and in the final stage is 45% or less.

[0054] [(d) Total Reduction: 90% or More] In this manufacturing method, the total reduction in finish rolling is controlled to 90% or more. Since Mn contained in steel is an element that reduces the fracture energy of grain boundaries, the presence of locally enriched Mn regions may promote crack generation during plastic deformation, such as press forming. Therefore, from the perspective of further improving formability, it is effective to suppress or reduce local Mn enrichment. By controlling the total reduction in finish rolling to 90% or more, Mn can be diffused into the steel, which in turn suppresses or reduces the variation in Mn concentration in the steel, i.e., the local Mn enrichment. If the total reduction in finish rolling is less than 90%, the variation in Mn concentration becomes relatively high, and it may not be possible to sufficiently suppress the development of regions where Mn is locally enriched and the fracture energy is reduced. The upper limit of the total reduction in finish rolling may be, for example, 99% or less or 98% or less. Here, the total reduction in finish rolling is calculated by the following formula: Total reduction (%) = (plate thickness before finish rolling - plate thickness after finish rolling) / plate thickness before finish rolling x 100

[0055] [(e) Final Rolling Temperature: 960 to 1100°C] In this manufacturing method, in addition to controlling the reduction ratio in each rolling pass of the latter two stages of finish rolling, the final rolling temperature (the end temperature of finish rolling) is also very important in controlling the metallographic structure of the steel sheet. If the final rolling temperature is less than 960°C, recrystallization may not be completed or sufficiently promoted, and the desired average grain size and / or standard deviation in grain size of prior austenite grains may not be achieved in the metallographic structure of the finally obtained steel sheet. If the desired average grain size and / or standard deviation in grain size of prior austenite grains cannot be achieved, the strength difference in the metallographic structure after bake hardening may not be sufficiently reduced. On the other hand, if the final rolling temperature is more than 1100°C, the prior austenite grains may become coarse overall, and the desired average grain size and / or standard deviation in grain size of prior austenite grains may not be achieved. In this case, it goes without saying that the strength difference in the metallographic structure after bake hardening may not be sufficiently reduced.

[0056] [Cooling step] [Time from completion of hot rolling step to start of cooling: less than 0.5 seconds] [Time from start of cooling to reach 400°C or less: 20.0 seconds or less] Cooling of the finish-rolled steel sheet is started in the next cooling step less than 0.5 seconds after completion of the hot rolling step, and then cooled to a temperature of 400°C or less within 20.0 seconds from the start of cooling. By controlling cooling in this way, it becomes possible to achieve the desired average grain size and standard deviation of the grain size of prior austenite grains in the metallographic structure of the finally obtained steel sheet.

[0057] If the time from the end of the hot rolling process to the start of cooling is 0.5 seconds or longer, grain growth proceeds, making it impossible to obtain the desired standard deviation in the grain size of the prior austenite grains. Furthermore, if the time from the end of the hot rolling process to the start of cooling exceeds 10.0 seconds, grain growth proceeds excessively overall, making it impossible to obtain the desired average grain size and / or standard deviation in the grain size of the prior austenite grains. As a result, in either case, it becomes impossible to sufficiently reduce the strength difference within the metal structure after bake hardening. On the other hand, if the cooling time from the start of cooling to 400°C or below exceeds 20.0 seconds or the cooling stop temperature exceeds 400°C, the area fraction of martensite becomes less than 90.0%, making it impossible to achieve the desired strength and / or uniformity of strengthening during paint baking.

[0058] [Coiling process] Finally, the cooled steel sheet is coiled in a temperature range of 400° C. or less to produce a steel sheet. If the coiling temperature exceeds 400° C., the area ratio of martensite will be less than 90.0%, as in the cooling process, and as a result, it will be impossible to achieve the desired strength and / or uniformity of strengthening during paint baking.

[0059] According to the steel sheet manufactured by the above manufacturing method, the metallographic structure is constituted by a more uniform structure containing, by area %, 90.0% or more of martensite and 3.0% or less of retained austenite, thereby achieving high strength, for example, a tensile strength of 980 MPa or more, and at the same time, strengthening can be made more uniform when the obtained steel sheet is formed by press forming or the like and then subjected to a paint baking treatment. Furthermore, by limiting the average grain size of prior austenite grains in the metallographic structure to 30.0 μm or less and controlling the standard deviation of the grain size of the prior austenite grains to less than 4.0 μm, the metallographic structure is made uniform in the microscopic region, and further, the standard deviation of the dislocation density in the width direction is reduced to 1.50 × 10 15 / m 2 By controlling the metal structure as described below to make it uniform even in the macroscopic region, it becomes possible to significantly improve the uniformity of strengthening during paint baking. Therefore, the steel sheet manufactured by the above manufacturing method can provide high-strength parts that can significantly suppress or reduce localized deformation in large deformations that occur during collisions. Therefore, the steel sheet is particularly useful for use in the automotive field, where excellent crash resistance is required.

[0060] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0061] In the following examples, steel sheets according to the embodiments of the present invention, particularly hot-rolled steel sheets, were produced under various conditions, and the tensile strength (TS), variation in TS in the width direction of the steel sheet, and variation (standard deviation) in the amount of bake hardening in the width direction of the steel sheet were investigated for the obtained steel sheets.

[0062] First, molten steel is continuously cast at a temperature of 600 to 900°C with an average cooling rate of 12°C / min and an average cooling rate gradient of 30°C / min. 2The steel sheets were cast under the conditions of [Table 1] to form slabs having various chemical compositions shown in Table 1, and these slabs were heated to a temperature of 1100 to 1200°C and held for the times shown in Table 2, followed by width reduction and hot rolling. Width reduction was carried out using a sizing press at the width reduction rate and slab transport speed shown in Table 2. Hot rolling was carried out by performing rough rolling and finish rolling. More specifically, rough rolling was carried out under the same conditions in all Examples and Comparative Examples, and finish rolling was carried out using a tandem rolling mill consisting of five rolling stands under the conditions shown in Table 2. Finally, the finish-rolled steel sheets were cooled and coiled under the conditions shown in Table 2 to obtain steel sheets having thicknesses of 1.6 to 3.2 mm.

[0063]

[0064]

[0065] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0066] [Standard deviation of dislocation density in the width direction] The standard deviation of dislocation density in the width direction was determined as follows. First, samples for X-ray diffraction were taken from seven positions of 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width of the steel plate from the end of the width direction to the center of the width of the steel plate. Next, the surface of the taken sample was polished to remove scale, and X-ray diffraction measurement was performed at a position corresponding to 1 / 4 of the plate thickness of the steel plate. The dislocation density was calculated using a method of converting the strain obtained from the half-width β of the X-ray diffraction measurement. In the diffraction intensity curve obtained by ordinary X-ray diffraction, two rays with different wavelengths, Kα1 and Kα2, overlap, so they were separated using the Rachinger method. The Williamson-Hall method was used to extract the strain. The spread of the half-width is affected by the crystallite size D and the strain ε, and can be calculated as the sum of both factors using the following formula. β = β1 + β2 = (0.9λ / (D × cos θ)) + 2ε × tan θ. Further modifying this equation, β cos θ / λ = 0.9λ / D + 2ε × sin θ / λ. By plotting β cos θ / λ against sin θ / λ, the strain ε was calculated from the slope of the line. The diffraction lines used for the calculation were (110), (200), (211), (220), (310), and (222). The conversion of the strain ε to the dislocation density is ρ = 14.4ε 2 / b 2 was used. θ means the peak angle calculated by the θ-2θ method of X-ray diffraction, and λ means the wavelength of the X-rays used in X-ray diffraction. b is the Burgers vector of Fe(α), which was set to 0.25 nm in this example.

[0067] [Tensile strength (TS) and width direction TS variation] First, No. 5 tensile test specimens according to JIS Z2241:2011 were taken from seven positions of the steel sheet in the width direction, namely, 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width from the end to the width center. The test direction was parallel to the rolling direction. Next, these tensile test specimens were subjected to a tensile test according to JIS Z2241:2011 to obtain seven tensile strength (TS) values. Finally, the difference between the maximum and minimum values ​​was calculated to determine the width direction TS variation. The tensile strength at the 50% position was determined as the tensile strength (TS) of the steel sheet.

[0068] [Standard Deviation of Bake Hardening Amount in the Width Direction] The standard deviation of the bake hardening amount in the width direction was determined as follows. First, as in the case of TS measurement, No. 5 tensile test specimens according to JIS Z2241:2011 were taken from seven positions of the steel sheet, namely, 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width from the end portion in the width direction toward the center of the width direction. The test direction was parallel to the rolling direction. Next, each test specimen was prestrained by 1% and then heat-treated at 170°C for 20 minutes. The amount of bake hardening for each test specimen was determined by subtracting the stress at the time of 1% prestrain from the stress when the test specimen was re-tensile. 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 specimens obtained.

[0069] Steel sheets with a tensile strength (TS) of 980 MPa or more, a widthwise TS variation of 70 MPa or less, and a widthwise bake hardening standard deviation of 20 MPa or less were evaluated as having high strength and capable of being uniformly strengthened during paint baking. The results are shown in Table 3.

[0070]

[0071] With reference to Tables 1 to 3, in Comparative Example 24, the width reduction rate in the width reduction step was low, which is thought to have prevented the anisotropic solidification structure at the lateral edges of the slab from being sufficiently destroyed. As a result, the final metallographic structure had a large variation in dislocation density in the width direction, and the standard deviation of the width-direction TS fluctuation and the bake hardening amount increased. In Comparative Example 25, the width reduction rate in the width reduction step was too large, which is thought to have increased the deviation in strain in the width direction. As a result, the final metallographic structure had a large variation in dislocation density in the width direction, and the standard deviation of the width-direction TS fluctuation and the bake hardening amount increased. In Comparative Example 26, the slab conveying speed in the width reduction step was slow, which is thought to have prolonged the contact time between the slab edges and the sizing rolls, increasing the amount of heat removed from the sizing rolls and resulting in a large temperature difference in the width direction of the slab. As a result, the final metallographic structure had a large variation in dislocation density in the width direction, and the standard deviation of the width-direction TS fluctuation and the bake hardening amount increased. In Comparative Example 27, the holding time in the temperature range of 1100°C or higher in the heating step was short, which is thought to have resulted in incomplete solid solution of coarse carbides, and ferrite and bainite transformation, etc., occurring from these carbides during the subsequent cooling step. As a result, the area fraction of martensite was less than 90.0%, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were large. In Comparative Examples 28 and 29, the reduction rates in the penultimate and final rolling passes in the finish rolling were low, which is thought to have resulted in incomplete or insufficient promotion of recrystallization. As a result, the average grain size and standard deviation of the grain size of prior austenite grains in the final metal structure were large, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were large.

[0072] In Comparative Example 30, the final rolling temperature in the finish rolling was low, which is thought to have caused recrystallization to be incomplete or insufficiently promoted. As a result, the average grain size and standard deviation of the grain size of the prior austenite grains in the final metallographic structure were large, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were large. In Comparative Example 31, the final rolling temperature in the finish rolling was high, which is thought to have caused the prior austenite grains to become coarse overall. As a result, the average grain size of the prior austenite grains in the final metallographic structure was large, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were large. In Comparative Example 32, the time from the completion of the hot rolling process to the start of the cooling process was 0.5 seconds or more, which is thought to have caused grain growth to proceed. As a result, the desired standard deviation of the grain size of the prior austenite grains could not be obtained, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were large. In Comparative Example 33, the time from the completion of the hot rolling process to the start of the cooling process exceeded 10.0 seconds, which is thought to have caused grain growth to proceed excessively overall. As a result, the desired average grain size of prior austenite grains could not be obtained, and the variation in TS in the width direction and the standard deviation of the amount of bake hardening increased. In Comparative Example 34, the time from the start of cooling in the cooling step until the temperature reached 400°C or less was long, so the area fraction of martensite was less than 90.0%, and the variation in TS in the width direction and the standard deviation of the amount of bake hardening increased. In Comparative Example 35, the coiling temperature was high, so the area fraction of martensite was also less than 90.0%, and the variation in TS in the width direction and the standard deviation of the amount of bake hardening increased.

[0073] In Comparative Examples 36 and 38, the TS decreased due to the low C and Si contents, respectively. On the other hand, in Comparative Examples 37 and 39, the C and Si contents were high, respectively, resulting in the formation of relatively large amounts of retained austenite. As a result, dislocations were introduced non-uniformly when 1% prestrain was applied, and the standard deviation of the bake hardening amount increased. In Comparative Example 40, the hardenability decreased due to the low Mn content, which resulted in a low area fraction of martensite, a decrease in TS, and an increase in the variation in TS in the width direction and the standard deviation of the bake hardening amount. In Comparative Examples 41 and 44, the formability decreased due to the high Mn and Nb contents, respectively, and this resulted in an increase in the standard deviation of the bake hardening amount. In Comparative Example 42, the sol. Al content was low, which similarly resulted in a decrease in formability and an increase in the standard deviation of the bake hardening amount. In Comparative Example 43, the Nb content was low, which is thought to have prevented the prior austenite grain refinement due to the pinning effect from being sufficiently promoted. As a result, the average grain size of the prior austenite grains in the final metal structure increased, and the variation in TS in the width direction and the standard deviation of the bake hardening amount increased. In Comparative Example 45, the large width reduction rate in the width reduction step is thought to have increased the deviation in strain in the width direction. As a result, the variation in dislocation density in the final metal structure increased, and the variation in TS in the width direction and the standard deviation of the bake hardening amount increased. In Comparative Example 46, the time from the completion of the hot rolling step to the start of the cooling step was 0.5 seconds or more, which is thought to have caused grain growth to progress. As a result, the desired standard deviation in the grain size of the prior austenite grains could not be obtained, and the variation in TS in the width direction and the standard deviation of the bake hardening amount increased.

[0074] In contrast, in all of the steel sheets according to the examples of the invention, by having a predetermined chemical composition and further appropriately controlling the conditions in the manufacturing method, the metal structure contains, in area %, 90.0% or more of martensite and 3.0% or less of retained austenite, the average grain size of prior austenite grains is 30.0 μm or less, the standard deviation of the grain size of the prior austenite grains is less than 4.0 μm, and the standard deviation of the dislocation density in the width direction is 1.50 × 10 15 / m2 It was possible to obtain steel sheets having the following properties. As a result, it was possible to obtain high strength steel sheets having a tensile strength of 980 MPa or more and to significantly improve the uniformity of strengthening during paint baking. In addition, in all of the steel sheets according to the invention, the remaining structure other than martensite and retained austenite (retained γ) was at least one of ferrite, bainite, and pearlite.

Claims

1. The chemical composition, in mass%, is: C: 0.040-0.200%, Si: 0.30-2.00%, Mn: 1.00-4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001 to 1.000%, Ti: 0 to 0.200%, V: 0 to 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 to 0.010%, Zn: 0 to 0.010%, W: 0-0.100%, Sn: 0-0.040%, As: 0-0.100%, REM: 0-0.0100%, and the balance: Fe and impurities; the metal structure contains, in area %, 90.0% or more of martensite and 3.0% or less of retained austenite; the average grain size of prior austenite grains is 30.0 μm or less; the standard deviation of the grain size of the prior austenite grains is less than 4.0 μm; and the standard deviation of dislocation density at seven positions at 1 / 4 of the sheet thickness and at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width from the end in the width direction toward the center of the width is 1.50×10 15 / m 2 A steel plate characterized in that:

2. The chemical composition is, in mass%, Ti: 0.001 to 0.200%, V: 0.001 to 0.300%, Cu: 0.001 to 0.40%, Cr: 0.001 to 0.90%, Mo: 0.001 to 0.12%, Ni: 0.001 to 0.30%, B: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, Bi: 0.001 to 0.010%, Zr: 0.001 to 0.050%, Co: 0.001 to 0.010%, Zn: 0.001 to 0.010%, The steel plate according to claim 1, comprising at least one of W: 0.001 to 0.100%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, and REM: 0.0001 to 0.0100%.

3. The steel plate according to claim 1 or 2, characterized in that the metal structure further contains, by area percentage, at least one of the following: ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less.

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

5. A part, characterized in that it comprises the steel sheet according to claim 1 or 2.

6. A heating process for heating a slab having the chemical composition according to claim 1 or 2, and maintaining the slab at a temperature of 1100°C or higher for 6000 seconds or more; A width reduction process including width-reducing the slab with a sizing press, the width reduction press satisfying the following conditions (a) and (b); (a) a width reduction ratio of 1.0 to 23%, and (b) a conveying speed of the slab of 20 m / min or more; A hot rolling process including finish-rolling the slab, the finish rolling satisfying the following conditions (c) to (e); (c) a reduction ratio in each rolling pass of the rolling pass one stage before the final stage and the final stage of the slab of 20 to 50%, (d) a total reduction ratio of 90% or more, and (e) a final rolling temperature of 960 to 1100°C. a cooling step of starting cooling of the steel plate within less than 0.5 seconds after completion of the hot rolling step, and then cooling the steel plate to a temperature of 400°C or less within 20.0 seconds from the start of cooling; and a coiling step of coiling the cooled steel plate in a temperature range of 400°C or less.

7. Before the heating step, the average cooling rate at 600 to 900°C is 10°C / min or more, and the average cooling rate gradient is 40°C / min. 2 The method for producing a steel plate according to claim 6, further comprising a continuous casting step of controlling the following:

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