Steel plate and its manufacturing method

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

[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 weight reduction, which contributes to improving the fuel efficiency of automobiles. On the other hand, since many automobile parts are manufactured by press forming, high strength and excellent formability are required.

[0003] In this regard, for example, Patent Document 1 describes a high-strength steel sheet characterized by 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%, respectively, as well as V: 0.01~0.05% and / or B: 0.0001~0.005%, with the remainder being iron and unavoidable impurities, and 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 particle size of the prior γ and the standard deviation (σ) of the prior γ particle size distribution to an appropriate range, a high-strength steel sheet with bendability while maintaining a high strength of 980 MPa or more can be realized.

[0004] Patent Document 2 describes a composition in mass%, comprising C: 0.08-0.30%, Si: 3.0% or less, Mn: 1.0-4.0%, P: 0.100% or less, S: 0.02% or less, Al: 1.0% or less, N: 0.008% or less, with the remainder being Fe and unavoidable impurities, and a microstructure in which the total area ratio of polygonal ferrite, fresh martensite, and retained austenite at the 1 / 4 thickness position of the steel plate and at the center of the width of the steel plate is 20% or less of the total area ratio of the entire microstructure. The invention describes a high-strength hot-rolled steel sheet characterized in that the total area ratio of martensite and lower bainite is 65-100% of the entire steel structure, and the standard deviation of the total area ratio of polygonal ferrite, fresh martensite, and retained austenite at the 1 / 4 position of the steel sheet thickness, and at positions 5%, 10%, 15%, 20%, 25%, and 30% of the total width of the steel sheet from the edge toward the center of the width, as well as at the center of the width of the steel sheet, is 7.0% or less. Furthermore, Patent Document 2 teaches that by using the above-mentioned high-strength hot-rolled steel sheet, it is possible to reduce variations in shape during the manufacturing of parts such as automobile parts, thereby obtaining products such as high-strength parts with stable shapes. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-242832 [Patent Document 2] Japanese Patent Publication No. 2021-063253 [Overview of the project] [Problems that the invention aims to solve]

[0006] In automotive parts, improved collision resistance is desired from the perspective of ensuring occupant safety. Therefore, there is a need for high-strength parts that can suppress deformation localization caused by material non-uniformity in response to large deformations that occur during collisions. In connection with this, there is a need for steel materials that can uniformly strengthen the entire part after molding and during paint curing.

[0007] Therefore, an object of the present invention is to provide a steel sheet having a novel structure that can achieve high strength and uniformly strengthen during painting baking.

Means for Solving the Problems

[0008] In order to achieve the above object, the inventors of the present invention focused on the metallographic structure of a steel sheet, particularly a hot-rolled steel sheet, and conducted studies. As a result, first, the inventors found that by forming the metallographic structure of a hot-rolled steel sheet having a predetermined chemical composition mainly of martensite, high strength can be achieved and the uniformity of strengthening during painting baking can be improved. In addition, the inventors restricted the average grain size of the prior austenite grains in the metallographic structure within a predetermined range and reduced the variation in the grain size of the prior austenite grains to make the metallographic structure uniform in the micro region, and further controlled the standard deviation of the dislocation density in the width direction within a predetermined range to make the metallographic structure uniform in the macro region, thereby finding that the uniformity of strengthening during painting baking can be significantly improved, and completing the present invention.

[0009] The present invention that has achieved the above object is as follows. (1) The chemical composition is, in mass%, 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%, ​​​​​​​​​​​​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 The remainder consists of Fe and impurities. The metallic structure, in area percentage, Martensite: 90.0% or more, Contains 3.0% or less of residual austenite. The average particle size of the prior austenite grains is 30.0 μm or less. The standard deviation of the particle size of the prior austenite grains is less than 4.0 μm. The standard deviation of the dislocation density at seven positions, located at 1 / 4 of the plate thickness and extending from the edge towards the center of the width direction at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, is 1.50 × 10⁻⁶. 15 / m 2 A steel plate characterized by the following: (2) The chemical composition is, in mass%, 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% The steel plate according to (1) above, characterized in that it includes at least one of the following. (3) The metal structure is further, in area %, Ferrite: 10.0% or less Baynite: 10.0% or less, Perlite: 10.0% or less The steel plate according to (1) or (2) above, characterized in that it includes at least one of the above. (4) A steel plate as described in any one of the above items (1) to (3), characterized in that the plate thickness is 1.0 to 8.0 mm. (5) A component characterized by including a steel plate as described in any one of the above items (1) to (4). (6) A heating step of heating a slab having the chemical composition described in (1) or (2) above and holding it at a temperature of 1100°C or higher for 6000 seconds or more. A width reduction process comprising pressing the slab with a sizing press device, wherein the width reduction press satisfies the following conditions (a) and (b): (a) The width reduction ratio is 1.0 to 23%, and (b) The conveying speed of the slab is 20 m / min or more. A hot rolling process comprising finishing rolling the slab, wherein the finishing rolling satisfies the following conditions (c) to (e): (c) The reduction ratio in each rolling pass of the stage immediately preceding the final stage and the final stage is 20-50%. (d) The total reduction rate is 90% or more, (e) The final rolling temperature is 960 to 1100°C. A cooling process in which the cooling of the steel sheet is started less than 0.5 seconds after the completion of the hot rolling process, 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 winding process in which cooled steel plates are wound up in a temperature range of 400°C or less. A method for manufacturing steel plates, characterized by including the following: (7) Before the heating step, the average cooling rate at 600-900°C is 10°C / min or more, and the average cooling rate gradient is 40°C / min. 2 The method for manufacturing a steel sheet according to (6) above, further comprising a continuous casting process controlled to be as follows. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a steel sheet that is highly strong and can be uniformly strengthened during paint curing. [Modes for carrying out the invention]

[0011] <Steel plate> The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition in mass%, C: 0.040~0.200%, Si: 0.30~2.00%, Mn: 1.00~4.00%, sol.Al: 0.001~0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001~1.000%, 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 The remainder consists of Fe and impurities. The metallic structure, in area percentage, Martensite: 90.0% or more, Contains 3.0% or less of residual austenite. The average particle size of the prior austenite grains is 30.0 μm or less. The standard deviation of the particle size of the prior austenite grains is less than 4.0 μm. The standard deviation of the dislocation density at seven positions, located at 1 / 4 of the plate thickness and extending from the edge towards the center of the width direction at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, is 1.50 × 10⁻⁶. 15 / m 2 It is characterized by the following:

[0012] As mentioned earlier, automotive parts require high-strength components that can suppress deformation localization caused by material non-uniformity in the face of large deformations that occur during collisions. In connection with this, there is a need for steel materials that can uniformly strengthen (bake harden) the entire part after molding and paint baking. Here, bake hardening is a phenomenon in which interstitial elements (mainly carbon) move and fix to dislocations introduced by press forming (hereinafter also called "pre-strain") during paint baking at 100-200°C, thereby inhibiting their movement and increasing strength. This is also called strain aging.

[0013] Therefore, the inventors investigated not only the appropriate chemical composition of steel sheets, particularly hot-rolled steel sheets, but also the metallic structure of the hot-rolled steel sheets. First, the inventors found that by configuring the metallic structure of a hot-rolled steel sheet having a predetermined chemical composition with a structure mainly composed of martensite, more specifically, a structure containing 90.0% or more martensite and 3.0% or less retained austenite by area percentage, it is possible to achieve high strength, such as a tensile strength of 980 MPa or more, while also making the strengthening more uniform when the resulting hot-rolled steel sheet is formed by press forming or the like and then subjected to paint baking treatment. Although there is no intention to be bound by any particular theory, it is thought that by making the metallic structure a more uniform structure with 90.0% or more martensite by area percentage, the strength difference within the metallic structure can be reduced compared to cases where other structures softer than martensite, such as ferrite, are relatively abundant, and this reduction in strength difference can improve the uniformity of strengthening during paint baking. Furthermore, retained austenite can act as a fracture initiation point during deformation such as press forming, and dislocations can be introduced non-uniformly into the microstructure during such deformation. Therefore, in addition to controlling martensite to 90.0% or more by area percentage, limiting retained austenite to 3.0% or less by area percentage makes it possible to more significantly reduce strength differences within the metal microstructure.

[0014] Next, the inventors considered that a uniform dislocation distribution within the martensite structure is effective in achieving uniform strengthening, and therefore investigated the appropriate particle size of prior austenite grains in a martensite-based metal structure. More specifically, by refining the prior austenite grains, the density of the prior austenite grain boundaries can be increased. As a result, by refining the prior austenite grains, dislocations are introduced uniformly into the metal structure during press forming, and as a result, it is possible to reduce the strength differences within the metal structure after bake hardening. To explain in more detail, if the particle size of the prior austenite grains is large, the distance between the inside of the grain and the vicinity of the grain boundary increases, making it easy for strain to be introduced unevenly. This is because strain is easily introduced near the grain boundary, while it is not easily introduced inside the grain. 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, and as a result, it is possible to reduce the strength differences within the metal structure after bake hardening.

[0015] However, simply refining the existing austenite grains does not necessarily result in a uniform dislocation distribution within the martensitic structure, and may not adequately reduce the strength differences within the metal structure after sintering. For example, if coarse and fine austenite grains are mixed in the structure before martensitic transformation, the starting temperature of martensitic transformation differs depending on the grain size, resulting in different temperature histories for each grain before cooling is complete. More specifically, austenite grains with larger grain sizes have a higher starting temperature for martensitic transformation compared to austenite grains with smaller grain sizes, and therefore auto-tempering progresses between the completion of transformation and cooling to room temperature. As a result, some of the dislocations introduced during the transformation to martensite annihilate each other through thermal vibrations as they pass through this temperature history. Consequently, the dislocation density within austenite grains that transformed at higher temperatures is relatively lower, while the dislocation density within austenite grains that transformed 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 variability in austenite grain size before martensitic transformation. In other words, by reducing the variability in austenite grain size before martensitic transformation, it is possible to reduce the variability in prior austenite grain size after martensitic transformation, and as a result, the dislocation distribution within the martensitic structure can be homogenized.

[0016] Therefore, in addition to controlling the particle size of the prior austenite grains, the inventors focused on controlling the particle size distribution, more specifically, controlling the variation in particle size, and conducted investigations. As a result, the inventors found that by refining the prior austenite grains within a predetermined range, more specifically by controlling the average particle size of the prior austenite grains to 30.0 μm or less, the density of the prior austenite grain boundaries in the entire steel sheet is increased, while reducing the variation in the particle size of the prior austenite grains, more specifically by controlling the standard deviation of the particle size of the prior austenite grains to less than 4.0 μm, the dislocation distribution within the martensitic structure can be sufficiently homogenized, thereby significantly reducing the strength difference within the metal structure after bake-hardening.

[0017] While controlling the average grain size of prior austenite grains and the standard deviation of the grain size of said prior austenite grains as described above can homogenize the dislocation distribution within the martensitic structure in the microscopic region of the steel sheet, it is not possible to reliably homogenize the dislocation distribution within the martensitic structure in the macroscopic region of the entire steel sheet. On the other hand, as mentioned above, in automotive parts and the like, there is a need for steel materials that can uniformly strengthen the entire part after forming and during paint baking in order to suppress the localization of deformation during collisions. In this regard, the manufacture of steel sheets, including slabs, is generally carried out by rolling, and this rolling is performed using rolling rolls. From the results of numerous tests related to such rolling, it is known that the material and strength of steel sheets are more uniform in the longitudinal direction (rolling direction) than in 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 thickness of the sheet is thinner at the ends and it is also easier to cool, and therefore the strain introduced by rolling is likely to differ in the width direction. Therefore, in order to minimize the difference in strength across the entire steel plate and to achieve a uniform structure, it is important to appropriately control the structure in the width direction.

[0018] Therefore, the inventors further studied to homogenize the dislocation distribution not only in the microscopic region of the steel sheet but also in the macroscopic region of the entire steel sheet within the martensite structure. As a result, the inventors found that 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 of the steel sheet in the direction from the end in the width direction of the steel sheet to the center in the width direction at the position of 1 / 4 of the sheet thickness of the steel sheet was controlled within a specific range, more specifically 1.50×10 15 / m 2 It was found that a uniform metal structure can be realized throughout the steel sheet by controlling it below. In particular, in the width direction of the steel sheet, the thickness reduction at the end is larger than that at the center in the width direction. Therefore, for example, instead of the standard deviation based on the dislocation density measured at equal distances such as 10%, 20%, 30%, 40% and 50% with respect to the total width of the steel sheet in the direction from the end to the center in the width direction, it is considered more effective in realizing a more uniform metal structure to control the standard deviation based on the dislocation density measured relatively more at positions closer to the end as described above. According to the steel sheet according to an embodiment of the present invention, by configuring the metal structure of a steel sheet having a predetermined chemical composition with a structure mainly composed of martensite, high strength is achieved and the uniformity of strengthening during painting baking is improved, and 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 to make the metal structure uniform in the microscopic region, and further the standard deviation of the dislocation density at the above seven positions in the width direction is 1.50×10 15 / m 2 By controlling it below to make the metal structure uniform also in the macroscopic region, it becomes possible to significantly improve the uniformity of strengthening during painting baking. Therefore, by using the steel sheet according to an embodiment of the present invention, it is possible to provide a high-strength component that can significantly suppress or reduce deformation localization in the large deformation generated during a collision. Therefore, the steel sheet according to an embodiment of the present invention is particularly useful for use in the automotive field where excellent collision resistance is required.

[0019] The steel sheets according to embodiments of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0020] [C:0.040~0.200%] Carbon (C) is an effective element for increasing the strength of steel sheets. Furthermore, C forms carbides and / or carbonitrides with Nb in the steel, contributing to microstructure refinement through the pinning effect of the formed precipitates. To fully obtain these effects, the C content should be 0.040% or higher. The C content may also be 0.060% or higher, 0.080% or higher, 0.100% or higher, or 0.120% or higher. On the other hand, excessive C content can lead to the formation of a relatively large amount of retained austenite, resulting in uneven dislocation introduction during deformation such as press forming, and consequently, a large difference in strength within the metal structure after bake-hardening. Therefore, the C content should be 0.200% or lower. The C content may also be 0.180% or lower, 0.160% or lower, 0.150% or lower, or 0.140% or lower.

[0021] [Si: 0.30~2.00%] Si is an effective element for increasing strength as a solid solution strengthening element. To obtain this effect fully, the Si content should be 0.30% or more. The Si content may also be 0.40% or more, over 0.50%, 0.51% or more, 0.52% or more, 0.53% or more, 0.54% or more, 0.55% or more, over 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 treatment properties and formability will decrease, and slab cracking may occur during hot rolling. In addition, a relatively large amount of retained austenite may be generated, and dislocations may be introduced unevenly during deformation such as press forming, resulting in a large difference in strength within the metal structure after bake-hardening. Therefore, the Si content should be 2.00% or less. The Si content may also be 1.80% or less, 1.60% or less, 1.50% or less, or 1.40% or less.

[0022] [Mn: 1.00~4.00%] Mn is an effective element for increasing strength as a hardenability and solid solution strengthening element. To obtain these effects to the fullest, the Mn content should be 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 reduce formability. Therefore, the Mn content should be 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:0.001~0.500%] sol.Al is an element that acts as a deoxidizing agent for molten steel. It also suppresses the precipitation of cementite, which is detrimental to formability. To obtain these effects, the sol.Al content should be 0.001% or higher. The sol.Al content may be 0.010% or higher, 0.020% or higher, 0.030% or higher, 0.050% or higher, or 0.100% or higher. On the other hand, excessive sol.Al content may lead to saturation of the effect and an increase in manufacturing costs. Therefore, the sol.Al content should be 0.500% or lower. The sol.Al content may be 0.400% or lower, 0.300% or lower, or 0.200% or lower. sol.Al refers to acid-soluble Al, specifically solid-solution Al present in the steel.

[0024] [P:0.100% or less] If P is present in excess, moldability may decrease due to grain boundary segregation, etc. Therefore, the P content should be 0.100% or less. The P content may also 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 lead to increased 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 sulfur content can lead to the formation of many sulfides such as MnS, which can reduce moldability. Therefore, the sulfur content should be 0.0300% or less. The sulfur content may also be 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the sulfur content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the sulfur content may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[0026] [N:0.0070% or less] Excessive nitrogen content can form coarse nitrides, reducing moldability. Therefore, the nitrogen content should be 0.0070% or less. The nitrogen content may also be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the nitrogen content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the nitrogen content may be 0.0001% or more, or 0.0005% or more.

[0027] [O:0.0100% or less] O is an element that is introduced during the manufacturing process. Excessive O content can lead to the formation of coarse inclusions, which can reduce the formability of the steel sheet. Therefore, the O content should be 0.0100% or less. The O content may also 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 it to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.

[0028] [Nb:0.001~1.000%] Nb is an element that contributes to the refinement of prior austenite grains and, consequently, to the increased strength of steel sheets by forming carbides, nitrides, and / or carbonitrides in steel through a pinning effect. To fully obtain these effects, the Nb content should be 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, if the Nb content is excessive, coarse carbides and the like may be formed in the steel, which may reduce the formability of the steel sheet. Therefore, the Nb content should be 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 optionally contain at least one of the following elements in place of a portion of the remaining Fe.

[0030] [Cr: 0~0.90%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength and / or corrosion resistance. While the Cr content may be 0%, to obtain these effects, it is preferable that the Cr content be 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Cr content may lead to saturation of the effect and an increase in manufacturing costs. Therefore, it is preferable that the Cr content be 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 :0~0.010%, 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 may be included in the steel sheet as optional elements, or may exist in the steel sheet as trump elements. The content of these elements is as follows: 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%, M The content of each element may be as follows: g: 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%. For the lower limits of these elements, for example, the content of Ti, V, Cu, Mo, Ni, Bi, Zr, Co, Zn, W, Sn, and As may be 0.001%, 0.005%, or 0.008%, respectively. Similarly, the B, Ca, Mg, and REM content 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 remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities are elements that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. minutes be.

[0033] The chemical composition of the steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive 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 embodiment of the present invention includes, by area percent, 90.0% or more of martensite and 3.0% or less of retained austenite. By configuring the metal structure of the steel sheet to include these structures, it is possible to achieve high strength, for example, a tensile strength of 980 MPa or more, while making the strengthening more uniform when the resulting steel sheet is formed by press forming or the like and then subjected to paint baking treatment. More specifically, by controlling the hard martensite to a range of 90.0% or more by area percent to create a more uniform structure, it is possible not only to increase strength but also to reduce strength differences within the metal structure, and as a result of this reduction in strength differences, the uniformity of strengthening during paint baking can be improved. If the area ratio of martensite is less than 90.0%, it may not be possible to achieve the desired strength and / or reduce strength differences within the metal structure, and as a result, it may not be possible to appropriately improve the uniformity of strengthening during paint baking. From the viewpoint of further increasing strength and improving the uniformity of strengthening during paint curing, a higher area ratio of martensite is preferable, for example, it may be 92.0% or more, 94.0% or more, 96.0% or more, or 98.0% or more. The upper limit of the area ratio of martensite is not particularly limited and may be 100.0%, or for example, 99.0% or less. On the other hand, retained austenite can become the starting point of fracture during deformation such as press forming, and furthermore, dislocations may be introduced non-uniformly into the microstructure during such deformation. Therefore, in addition to controlling the martensite to 90.0% or more in area percentage, limiting the retained austenite to 3.0% or less in area percentage makes it possible to more significantly improve the strength difference in the metal structure. If the area ratio of retained austenite exceeds 3.0%, it may become the starting point of fracture during deformation, and / or the introduction of non-uniform dislocations during deformation may cause a large strength difference in the metal structure after curing. Therefore, a lower area percentage of retained austenite is preferable, for example, it may be 2.5% or less, 2.0% or less, 1.5% or less, or 1.0% or less. The lower limit of the area percentage of retained austenite is not particularly limited and may be 0%, or for example, 0.5% or more.

[0035] [Remaining tissue] The remaining microstructure other than martensite and retained austenite may be 0% in area percentage, but if the remaining microstructure is present, it may include at least one of the following: 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 will be less than 90.0%, and as a result, the desired strength and uniformity of strengthening during paint curing cannot be achieved. The lower limits for ferrite, bainite, and pearlite may each be 0%, or 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 may be 8.0% or less, 6.0% or less, 5.0% or less, or 4.0% or less, respectively.

[0036] [Identification of metallographic structure and calculation of area ratio] The identification of the metallic structure and calculation of area ratios in steel sheets are performed by optical microscopy observation and X-ray diffraction after etching with Nital reagent or Repera solution. Microscopy observation is performed on the thickness cross section perpendicular to the sheet surface. Preferably, the thickness cross section is parallel to the rolling direction. Specifically, first, a sample is taken from the steel sheet and the observation surface of the sample is etched with Nital. Next, by performing image analysis on a microscopic image obtained at a depth of 1 / 4 of the sheet thickness in a 300 μm × 300 μm field of view using an optical microscope, the total area ratio of martensite and bainite, as well as the area ratios of ferrite and pearlite, are calculated. Next, using a sample whose observation surface has been Repera-etched, the total area ratio of martensite and retained austenite is calculated by performing image analysis on a microscopic image obtained at a depth of 1 / 4 of the sheet thickness in a 300 μm × 300 μm field of view using an optical microscope. Next, using a sample that has been surface-machined to a depth of 1 / 4 of the plate thickness from the direction normal to the rolling surface, 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 total area fraction of martensite and retained austenite calculated earlier. Finally, the area fraction of bainite is calculated by similarly subtracting the obtained area fraction of martensite from the total area fraction of martensite and bainite calculated earlier.

[0037] [Average particle size of old austenite grains: 30.0 μm or less] In the steel sheet according to the embodiment of the present invention, the average particle size of the prior austenite grains is 30.0 μm or less. As mentioned above, the dislocation distribution within the martensitic structure is considered effective in achieving uniform strengthening. In connection with this, 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, and thus it is possible to reduce 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 particle size of the prior austenite grains, the better, and 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. The lower limit is not particularly limited, but the average particle size of the prior austenite grains may be, for example, greater 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 of particle size of former austenite grains: less than 4.0 μm] In embodiments of the present invention, the standard deviation of the particle size of the prior austenite grains is less than 4.0 μm. By limiting the average particle size of the prior austenite grains to 30.0 μm or less, while keeping the standard deviation of the particle size of the prior austenite grains to less than 4.0 μm, that is, by reducing the variation in the particle size of the prior austenite grains, the dislocation distribution within the martensitic structure can be sufficiently homogenized, thereby significantly reducing the strength differences within the metal structure after bake-hardening. From the viewpoint of further enhancing this effect, it is preferable that the standard deviation of the particle size of the prior austenite grains is as small as possible, i.e., the less variation there is, for example, 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. The lower limit is not particularly limited, but the standard deviation of the particle 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 particle size of the old austenite grains and the standard deviation of the particle size of the old austenite grains] The average grain size and standard deviation of the grain size of the prior austenite grains are determined as follows. First, a sample is cut from any position at least 50 mm away from the edge of the steel plate (if a sample cannot be taken from this position, a position avoiding the edge is used) so that a cross-section perpendicular to the plate surface can be observed. The cross-section is preferably parallel to the rolling direction. The size of the sample should be such that approximately 10 mm can be observed in the direction perpendicular to the plate thickness, although this depends on the measuring device. The cross-section of the sample is polished using silicon carbide sandpaper from #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the observation surface is finished by electropolishing. At an arbitrary position in the longitudinal direction of the sample cross-section, at a depth of 1 / 4 of the plate thickness, a region with a length of 50 μm and a thickness of 50 μm is measured by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector can be used. For example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 detector can be used. In this case, the vacuum level inside the EBSD analyzer should be 9.6 × 10⁻⁶. -5The irradiation pressure may be set to Pa or less, the acceleration voltage to 15kV, and the irradiation current level to 13. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between typical prior austenite grains and crystal grains with 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, the average value of the shortest diameter and the longest diameter is calculated, and this average value is taken as the grain size of that prior austenite grain. Excluding prior austenite grains whose entire grain is not included in the imaging field, such as at the edges of the imaging field, the above operation is performed for all prior austenite grains to determine the grain size of all prior austenite grains in the imaging field. 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 of the grain size from the obtained grain sizes of all prior austenite grains.

[0040] [Standard deviation of dislocation density in the width direction: 1.50 × 10] 15 / m 2 below] In an embodiment of the present invention, the standard deviation of the dislocation density at seven positions at 1 / 4 of the plate thickness and at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width from the edge in the width direction toward the center is 1.50 × 10⁻⁶. 15 / m 2 The following applies: As described above, the average particle size of prior austenite grains in the metal structure is limited to 30.0 μm or less, and the standard deviation of the particle size of said 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 seven positions in the width direction of the steel sheet is set to 1.50 × 10⁻⁶. 15 / m 2By controlling the structure as described below to make the metal structure uniform even in the macroscopic region, it is possible to significantly improve the uniformity of strengthening during paint curing. Furthermore, by using such steel sheets in parts manufacturing, it is possible to uniformly strengthen the entire part after forming and during paint curing. Here, "entire part" can mean any position on the steel sheet used in parts manufacturing, but in the embodiment of the present invention, seven positions are selected as measurement positions for dislocation density to support uniform strengthening of the entire part, at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width from the edge in the width direction of the steel sheet towards the center of the width. To further improve the uniformity of strengthening during paint curing, it is preferable that the standard deviation of the above dislocation density is small, for example, 1.40 × 10⁻⁶. 15 / m 2 Below, 1.20 × 10 15 / m 2 Below, 1.00 × 10 15 / m 2 The following or 0.80 × 10 15 / m 2 The following may also apply. The lower limit is not particularly limited, but for example, the standard deviation of the above dislocation density is 0.01 × 10⁻⁶. 15 / m 2 The above is 0.05 × 10 15 / m 2 The above or 0.10 × 10 15 / m 2 That's fine too.

[0041] [Method for determining the standard deviation of dislocation density in the width direction] The standard deviation of the dislocation density in the width direction is determined as follows. First, X-ray diffraction samples are taken from seven predetermined positions on the steel plate, i.e., from the edge in the width direction of the steel plate toward the center of the width, at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width of the steel plate. Then, the surface of the collected samples is polished to remove scale, and X-ray diffraction measurements are performed at positions corresponding to 1 / 4 of the thickness of the steel plate. The dislocation density is calculated using a method that uses strain obtained from the full width at half maximum β obtained from the X-ray diffraction measurement. In the diffraction intensity curve obtained by normal X-ray diffraction, two lines, Kα1 and Kα2, with different wavelengths, overlap, so they are separated using the Rachinger method. The Williamsson-Hall method is used to extract the strain. The broadening of the full width at half maximum is influenced 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 × cosθ)) + 2ε × tanθ. Further rearranging this equation, we get βcosθ / λ = 0.9λ / D + 2ε × sinθ / λ. By plotting βcosθ / λ against sinθ / 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 the strain ε to the dislocation density is ρ = 14.4ε 2 / b 2 The following is used: θ represents the peak angle calculated from the θ-2θ method of X-ray diffraction, and λ represents the wavelength of the X-rays used in X-ray diffraction. b is the Burgers vector of Fe(α).

[0042] [plate thickness] The steel plate according to the embodiment of the present invention is not particularly limited, but generally has a thickness of 1.0 to 8.0 mm. 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 is high-strength and allows for uniform strengthening during paint curing, and therefore can significantly suppress or reduce deformation localization in large deformations that occur during collisions. For this reason, the steel sheet according to the embodiment of the present invention is particularly useful in the automotive field where excellent collision resistance is required. In a preferred embodiment, an automotive part, particularly an automotive undercarriage part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automotive undercarriage 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 there are differences in molding depending on the part, they are considered to basically inherit the characteristics and excellent properties of the steel sheet as the material. These automotive parts, particularly automotive undercarriage parts, only need to include the steel sheet according to the embodiment of the present invention in at least a part of these parts, and therefore at least a part of these parts satisfies the above-mentioned chemical composition and structural characteristics.

[0044] [Mechanical properties] [Tensile strength: TS] According to steel sheets having the above chemical composition and metal structure, particularly hot-rolled steel sheets, a high tensile strength, specifically a tensile strength of 980 MPa or higher, can be achieved. The tensile strength is preferably 1000 MPa or higher, 1080 MPa or higher, or 1180 MPa or higher. According to the steel sheets of the embodiment of the present invention, despite having such a very high tensile strength, the uniformity of strengthening during paint baking can be significantly improved by the specific combination of chemical composition and metal 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 lower, 1700 MPa or lower, or 1600 MPa or lower. The tensile strength is measured by taking a JIS No. 5 test piece from a position 50% of the total width from the edge in the width direction of the steel sheet toward the center of the width, with the longitudinal direction of the test piece parallel to the rolling direction of the steel sheet (L direction), and performing a tensile test in accordance with JIS Z 2241:2011.

[0045] <Method of manufacturing steel plates> Next, preferred manufacturing methods for steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below. More specifically, the following describes the manufacturing of hot-rolled steel sheets, but the steel sheets according to embodiments of the present invention include any steel sheets having the chemical composition and metal structure described above, i.e., not only hot-rolled steel sheets, but also cold-rolled steel sheets, plated steel sheets, etc. Therefore, the following description merely describes preferred manufacturing methods when the steel sheet according to embodiments of the present invention is a hot-rolled steel sheet.

[0046] A method for manufacturing steel sheets according to an embodiment of the present invention is: A heating process in which a slab having the chemical composition described above in relation to a steel plate is heated and held at a temperature of 1100°C or higher for 6000 seconds or more, A width reduction process comprising pressing the slab with a sizing press device, wherein the width reduction press satisfies the following conditions (a) and (b): (a) The width reduction ratio is 1.0 to 23%, and (b) The conveying speed of the slab is 20 m / min or more. A hot rolling process comprising finishing rolling the slab, wherein the finishing rolling satisfies the following conditions (c) to (e): (c) The reduction ratio in each rolling pass of the stage immediately preceding the final stage and the final stage is 20-50%. (d) The total reduction rate is 90% or more, (e) The final rolling temperature is 960 to 1100°C. A cooling process in which the cooling of the steel sheet is started less than 0.5 seconds after the completion of the hot rolling process, 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 winding process in which cooled steel plates are wound up in a temperature range of 400°C or less. It is characterized by including the following. In the above manufacturing method, the temperatures described for the slab and steel plate refer to the surface temperature of the slab and the surface temperature of the steel plate, respectively. Each step will be explained in detail below.

[0047] [Casting Process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, followed by casting using methods such as conventional continuous casting or ingot casting. Preferably, a slab having the chemical composition described above in relation to steel plates is cast 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 more, and the average cooling rate gradient is 40°C / min. 2 The following conditions are controlled: an average cooling rate of 10°C / min or higher between 600 and 900°C, and an average cooling rate gradient of 40°C / min. 2 By controlling the continuous casting process as follows, it is possible to ensure that the resulting steel sheet has refined prior austenite grains and a small standard deviation in the fine grains. The average cooling rate at 600-900°C may 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 may be 70°C / min or lower.

[0048] The average cooling rate gradient between 600 and 900°C refers to the average rate of change of the cooling rate per unit time between 600 and 900°C. For example, if 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 when the cooling rate changes from 50°C / min to 10°C / min, the average cooling rate gradient in this manufacturing method remains 40°C / min. 2 The average cooling rate gradient between 600 and 900°C is assumed to be 30°C / min. 2The following may also apply. While there is no particular lower limit, the average cooling rate gradient between 600 and 900°C should be 2°C / min. 2 or above, or 3°C / min 2 That's all.

[0049] [Heating process] 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 cases where the slab temperature is held at a constant temperature of 1100°C or higher, but also cases where the slab temperature fluctuates within a temperature range of 1100°C or higher. By holding the slab at a temperature of 1100°C or higher for 6000 seconds or more, coarse carbides present in the structure can be completely dissolved, eliminating crack initiation points. If the holding temperature is below 1100°C or the holding time is less than 6000 seconds, the coarse carbides will not be completely dissolved. If the coarse carbides are not completely dissolved, ferrite and bainite transformations will occur in the cooling step described later, starting from these carbides, resulting in a martensite area ratio of less than 90.0%, and as a result, the desired strength and / or uniformity of strengthening during paint curing cannot be achieved. The upper limit of the heating temperature of the slab is preferably 1300°C or lower, or 1200°C or lower. Similarly, the upper limit of the holding time in the temperature range of 1100°C or higher is preferably 10,000 seconds or lower.

[0050] [Width reduction process] [(a) Width reduction ratio: 1.0~23%] In this manufacturing method, the slab is subjected to width reduction pressing using a sizing press. When manufacturing steel plates, the plate thickness is thinner at the edges in the width direction, and it is also more susceptible to cooling. Therefore, the strain introduced by rolling tends to vary in the width direction. For this reason, in order to minimize the difference in strength throughout the steel plate and to make the structure uniform, it is important to appropriately control the structure in the width direction. By controlling the width reduction rate of the width reduction pressing using the sizing press within the range of 1.0 to 23%, the anisotropic solidification structure (columnar crystals) at the lateral edges of the slab is destroyed, homogenizing the structure and reliably suppressing the introduction of heterogeneous strain into the slab. As a result, it is possible to reduce the deviation in strength and structure in the width direction of the steel plate. On the other hand, if the width reduction rate is less than 1.0%, the above solidification structure cannot be reliably destroyed and the structure cannot be homogenized, and the resulting metal structure will have a large variation in dislocation density in the width direction. Furthermore, if the width reduction ratio exceeds 23%, the deviation of strain in the width direction becomes too large, and similarly, the variation in dislocation density in the width direction in the final metal structure becomes large. Preferably, the width reduction ratio of the width reduction press using a sizing press device is controlled within the range of 5 to 20%.

[0051] [(b) Slab transport speed: 20 m / min or more] In the width reduction process, the slab transport speed is controlled to 20 m / min or higher. During the width reduction process, the slab edges lose heat when they come into contact with the sizing rolls, causing a decrease in the temperature of the slab edges. Therefore, it is important to minimize the widthwise deviation caused by this temperature drop. By increasing the slab transport speed, specifically by controlling it to 20 m / min or higher, the contact time between the slab edges and the sizing rolls can be shortened, thereby sufficiently suppressing heat loss by the sizing rolls. Consequently, in combination with the control of the width reduction rate described above, the widthwise deviation of the steel plate can be reliably reduced. As a result, the standard deviation of the dislocation density in the widthwise direction can be controlled within a predetermined range, significantly improving the uniformity of strengthening during paint curing. On the other hand, if the slab transport speed is less than 20 m / min, the amount of heat lost from the sizing rolls increases, resulting in a large temperature difference in the widthwise direction of the slab. In this case, the variation in the widthwise dislocation density in the final metal structure becomes large. Preferably, the slab transport speed is 30 m / min or higher. There is no particular upper limit, but for example, the slab transport speed may be 50 m / min or less.

[0052] [Hot rolling process] [Rough rolling] In this manufacturing method, for example, a heated slab may be subjected to rough rolling before finish rolling to adjust the plate thickness. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions are ensured.

[0053] [(c) Reduction ratio in each rolling pass of the second-to-last stage and the final stage: 20-50%] The heated slab, or a slab that has been roughly rolled as needed, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of multiple rolling stands, for example, five or more rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, the reduction ratio in the last two rolling passes, i.e., the second-to-last and last rolling passes, is controlled to 20-50%. Performing rolling at such relatively high reduction ratios in the second-to-last and last rolling passes promotes recrystallization and refines the microstructure. If the reduction ratio in the second-to-last and / or last rolling passes is less than 20%, recrystallization may not be completed or sufficiently promoted, and the desired average grain size and / or standard deviation of grain size of prior austenite grains may not be achieved in the microstructure of the final steel sheet. If the desired average grain size and / or standard deviation of grain size of prior austenite grains cannot be achieved, the strength differences within the microstructure after bake-hardening cannot be sufficiently reduced. On the other hand, if the reduction ratio in each rolling pass of the stage before the final stage and / or the final stage is too high, the rolling load will be excessive, and the load on equipment such as the rolling mill will increase. For this reason, the reduction ratio in each rolling pass of the stage before the final stage and the final stage should be 50% or less. Preferably, the reduction ratio in each rolling pass of the stage before the final stage and the final stage should be 45% or less.

[0054] [(d) Total reduction rate: 90% or more] In this manufacturing method, the total reduction ratio in finish rolling is controlled to 90% or higher. Since Mn contained in steel is an element that reduces the fracture energy at grain boundaries, if there are regions where Mn is locally concentrated, crack generation during plastic deformation such as press forming may be promoted. Therefore, from the viewpoint of further improving formability, it is effective to suppress or reduce the localized concentration of Mn. By controlling the total reduction ratio in finish rolling to 90% or higher, Mn can be diffused into the steel, and in connection with this, it is possible to suppress or reduce the variation in Mn concentration in the steel, that is, to suppress or reduce the localized concentration of Mn. If the total reduction ratio in finish rolling is less than 90%, the variation in Mn concentration will be relatively high, and it may not be possible to sufficiently suppress the development of regions where Mn is locally concentrated and the fracture energy is reduced. The upper limit of the total reduction ratio in finish rolling may be, for example, 99% or less or 98% or less. Here, the total reduction ratio in finish rolling is calculated by the following formula. Total reduction ratio (%) = (Thickness of sheet before finish rolling - Thickness of sheet after finish rolling) / Thickness of sheet before finish rolling × 100

[0055] [(e) Final rolling temperature: 960~1100℃] In this manufacturing method, in addition to controlling the reduction ratio in each of the two rolling passes after the finish rolling stage, the final rolling temperature (the end temperature of the finish rolling stage) is also extremely important for controlling the microstructure of the steel sheet. If the final rolling temperature is below 960°C, recrystallization may not be completed or may not be sufficiently promoted, and the desired average grain size and / or standard deviation of grain size of the prior austenite grains may not be achieved in the microstructure of the final steel sheet. If the desired average grain size and / or standard deviation of grain size of the prior austenite grains cannot be achieved, the strength differences within the microstructure after bake hardening cannot be sufficiently reduced. On the other hand, if the final rolling temperature exceeds 1100°C, the prior austenite grains become generally coarser, and the desired average grain size and / or standard deviation of grain size of the prior austenite grains may not be achieved. In this case as well, it naturally becomes impossible to sufficiently reduce the strength differences within the microstructure after bake hardening.

[0056] [Cooling process] [Time from completion of hot rolling process to start of cooling: less than 0.5 seconds] [Time from the start of cooling until the temperature drops below 400°C: 20.0 seconds or less] The finish-rolled steel sheet is cooled in the next cooling process less than 0.5 seconds after the completion of the hot rolling process, and then cooled to a temperature of 400°C or lower within 20.0 seconds of the start of cooling. By performing this cooling control, it is possible to achieve the desired average grain size and standard deviation of prior austenite grains in the microstructure of the final steel sheet.

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

[0058] [Winding process] Finally, the cooled steel sheet is wound up at a temperature of 400°C or lower to produce the steel sheet. If the winding temperature exceeds 400°C, the martensite area ratio will be less than 90.0%, as in the cooling process, and as a result, the desired strength and / or uniformity of strengthening during paint curing cannot be achieved.

[0059] According to the steel sheet manufactured by the above manufacturing method, the metal structure is composed of 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, while also making the strengthening more uniform when the resulting steel sheet is formed by press forming or other methods and then subjected to paint baking treatment. Furthermore, by limiting the average particle size of prior austenite grains in the metal structure to 30.0 μm or less and controlling the standard deviation of the particle size of said prior austenite grains to less than 4.0 μm, the metal structure is made uniform in the microscopic region, and the standard deviation of the dislocation density in the width direction is 1.50 × 10⁻⁶. 15 / m 2 By controlling the process as described below to achieve a uniform metallic structure even in the macroscopic domain, it becomes possible to significantly improve the uniformity of strengthening during paint curing. Therefore, steel sheets manufactured by the above manufacturing method can provide high-strength parts that can significantly suppress or reduce deformation localization in large deformations that occur during collisions. Thus, these steel sheets are particularly useful for use in the automotive sector where excellent collision 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 in any way to these examples. [Examples]

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

[0062] First, the molten steel is cast using a continuous casting method, with an average cooling rate of 12°C / min at 600-900°C and an average cooling rate gradient of 30°C / min. 2Slabs with various chemical compositions shown in Table 1 were formed by casting under the specified conditions. These slabs were heated to a temperature of 1100-1200°C and held for the time shown in Table 2, followed by width reduction and hot rolling. Width reduction was performed using a sizing press at the width reduction ratio and slab transport speed shown in Table 2. Hot rolling was performed by rough rolling and finish rolling. More specifically, rough rolling was performed under the same conditions for all examples and comparative examples, and finish rolling was performed 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 wound under the conditions shown in Table 2 to obtain steel sheets with a thickness of 1.6-3.2 mm.

[0063] [Table 1]

[0064] [Table 2]

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

[0066] [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 taken from seven positions in the width direction of the steel plate, from the edge towards the center, at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width of the steel plate. Then, the surface of the collected samples was polished to remove 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 derived the strain obtained from the full width at half maximum (FWHM) β obtained from the X-ray diffraction measurement. In the diffraction intensity curve obtained by normal X-ray diffraction, two lines with different wavelengths, Kα1 and Kα2, overlap, so they were separated using the Rachinger method. The Williamsson-Hall method was used to extract the strain. The broadening of the FWHM is influenced by the crystallite size D and the strain ε, and can be calculated as the sum of both factors by the following equation: β = β1 + β2 = (0.9λ / (D × cosθ)) + 2ε × tanθ. Further rearranging this equation, we obtain βcosθ / λ = 0.9λ / D + 2ε × sinθ / λ. By plotting βcosθ / λ against sinθ / 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 dislocation density from the strain ε is ρ = 14.4ε 2 / b 2 The following was used: θ represents the peak angle calculated from the θ-2θ method of X-ray diffraction, and λ represents 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 variation in TS in the width direction] First, tensile test specimens of type 5 according to JIS Z2241:2011 were taken at seven positions in the width direction of the steel plate, from the edge towards the center of the width, at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, with the test direction parallel to the rolling direction. Next, tensile tests were performed on these specimens in accordance with JIS Z2241:2011 to obtain seven tensile strength (TS) values, and finally, the variation in TS in the width direction was determined by calculating the difference between the maximum and minimum values. Furthermore, the tensile strength at the 50% position was determined as the tensile strength (TS) of the steel plate.

[0068] [Standard deviation of curing 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, seven tensile test specimens of JIS Z2241:2011 No. 5 were taken at seven positions in the width direction of the steel plate, from the edge toward the center of the width, at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, with the test direction parallel to the rolling direction. Next, a 1% pre-strain was applied to each specimen, followed by heat treatment at 170°C for 20 minutes. Then, the bake-hardening amount of each specimen was determined by subtracting the stress at the time of 1% pre-strain application from the stress obtained when the specimen was tensed again. Finally, the standard deviation of the bake-hardening amount in the width direction was determined based on the bake-hardening amounts of the seven specimens obtained.

[0069] Steel sheets with a tensile strength (TS) of 980 MPa or higher, a variation in TS in the width direction of 70 MPa or less, and a standard deviation of bake hardening amount in the width direction of 20 MPa or less were evaluated as high-strength steel sheets capable of uniform strengthening during paint baking. The results are shown in Table 3.

[0070] [Table 3]

[0071] Referring to Tables 1-3, it is thought that in Comparative Example 24, the width reduction ratio in the width reduction process was too low, which prevented sufficient destruction of the anisotropic solidification structure at the lateral end of the slab. As a result, the variation in dislocation density in the width direction was large in the final metal structure, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were large. In Comparative Example 25, it is thought that the deviation in strain in the width direction was too large because the width reduction ratio in the width reduction process was too high. As a result, similarly, the variation in dislocation density in the width direction was large in the final metal structure, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were large. In Comparative Example 26, it is thought that the contact time between the slab end and the sizing roll was long due to the slow slab transport speed in the width reduction process, resulting in a large amount of heat removal from the sizing roll and a large temperature difference in the width direction of the slab. As a result, the variation in dislocation density in the width direction was large in the final metal structure, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were large. In Comparative Example 27, the holding time in the temperature range above 1100°C during the heating process was short, resulting in incomplete solid solution of coarse carbides. It is believed that these carbides initiated ferrite and bainite transformations during the subsequent cooling process. As a result, the area ratio 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, it is believed that recrystallization was not completed or sufficiently promoted due to low reduction ratios in the rolling passes of the second-to-last and final stages, respectively, during the finish rolling process. As a result, the average grain size and standard deviation 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, it is believed that recrystallization was not completed or sufficiently promoted due to a low final rolling temperature during the finish rolling process. As a result, the average grain size and standard deviation of the prior austenite grains in the final resulting metal structure were large, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were also large. In Comparative Example 31, it is believed that the prior austenite grains became generally coarser due to a high final rolling temperature during the finish rolling process. As a result, the average grain size of the prior austenite grains in the final resulting metal structure was large, and the variation in TS in the width direction and the standard deviation of the bake hardening amount were also large. In Comparative Example 32, it is believed that grain growth progressed because the time from the completion of the hot rolling process to the start of the cooling process was 0.5 seconds or more. As a result, the desired standard deviation of the prior austenite grain size 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, it is believed that grain growth progressed too much overall because the time from the completion of the hot rolling process to the start of the cooling process exceeded 10.0 seconds. 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 bake hardening amount were large. In Comparative Example 34, the time from the start of cooling to below 400°C in the cooling process was long, resulting in a martensite area ratio of 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 Example 35, the winding temperature was high, so similarly the martensite area ratio 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.

[0073] Comparative Examples 36 and 38 had low C and Si content, respectively, which resulted in a decrease in TS. On the other hand, Comparative Examples 37 and 39 had high C and Si content, respectively, which resulted in the formation of a relatively large amount of retained austenite. As a result, dislocations were introduced unevenly when 1% pre-strain was applied, and the standard deviation of the bake hardening amount increased. Comparative Example 40 had a low Mn content, which reduced hardenability, resulting in a low martensite area ratio, a decrease in TS, and a large variation in TS in the width direction and a large standard deviation of the bake hardening amount. Comparative Examples 41 and 44 had high Mn and Nb content, respectively, which reduced formability, and consequently, the standard deviation of the bake hardening amount increased. Comparative Example 42 had a low sol.Al content, which similarly reduced formability and a large standard deviation of the bake hardening amount. Comparative Example 43 had a low Nb content, which is thought to have prevented sufficient promotion of the refinement of prior austenite grains by the pinning effect. As a result, the average grain size of 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, it is thought that the deviation of strain in the width direction increased because the width reduction ratio in the width reduction process was large. As a result, the variation in dislocation density in the width direction increased in the final metal structure, and the variation in TS in the width direction and the standard deviation of the bake hardening amount increased. In Comparative Example 46, it is thought that grain growth progressed because the time from the completion of the hot rolling process to the start of the cooling process was 0.5 seconds or more. As a result, the desired standard deviation of the 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 bake hardening amount increased.

[0074] In contrast, in all the steel sheets relating to the invention, by having a predetermined chemical composition and appropriately controlling each condition in the manufacturing method, the metal structure contains, by area %, 90.0% or more of martensite and 3.0% or less of retained austenite, the average particle size of prior austenite grains is 30.0 μm or less, the standard deviation of the particle size of prior austenite grains is less than 4.0 μm, and the standard deviation of the dislocation density in the width direction is 1.50 × 10⁻⁶. 15 / m 2 We were able to obtain steel sheets that meet the following criteria. As a result, we were able to achieve high strength with a tensile strength of 980 MPa or higher, and significantly improve the uniformity of strengthening during paint curing. In addition, in all of the steel sheets according to the invention examples, the remaining microstructure other than martensite and retained austenite (retained γ) consisted of at least one of ferrite, bainite, and pearlite.

Claims

1. The chemical composition is expressed in mass percent. C: 0.040-0.200%, Si: 0.30-2.00%, Mn: 1.00-4.00%, Sol. Al: 0.001–0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001-1.000%, Ti: 0-0.200%, V: 0-0.300%, Cu: 0 to 0.40%, Cr: 0-0.90%, Mo: 0 to 0.12%, Ni: 0 to 0.30%, B: 0 to 0.0030%, Ca: 0-0.0010%, Mg: 0 to 0.0010%, Bi: 0 to 0.010%, Zr: 0 to 0.050%, Co: 0 to 0.010%, Zn: 0 to 0.010%, W: 0-0.100%, Sn: 0 to 0.040%, As: 0 to 0.100%, REM: 0-0.0100%, and The remainder consists of Fe and impurities. The metallic structure, in area percentage, Martensite: 90.0% or more, Contains 3.0% or less of residual austenite. The average particle size of the prior austenite grains is 30.0 μm or less. The standard deviation of the particle size of the prior austenite grains is less than 4.0 μm. The standard deviation of the dislocation density at seven positions, located at 1 / 4 of the plate thickness and extending from the edge in the width direction towards the center of the width at 5%, 10%, 15%, 20%, 25%, 30%, and 50% of the total width, is 1.50 × 10⁻⁶. 15 / m 2 A hot-rolled steel sheet characterized by the following:

2. The aforementioned chemical composition is, in mass%, Ti: 0.001 to 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 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: 0.001-0.010%, W: 0.001-0.100%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, and REM: 0.0001~0.0100% The hot-rolled steel sheet according to claim 1, characterized in that it includes at least one of the following.

3. The aforementioned metal structure is further, in area %, Ferrite: 10.0% or less Bainite: 10.0% or less, Perlite: 10.0% or less The hot-rolled steel sheet according to claim 1 or 2, characterized in that it includes at least one of the following.

4. A hot-rolled steel sheet according to claim 1 or 2, characterized in that the sheet thickness is 1.0 to 8.0 mm.

5. A component characterized by comprising the hot-rolled steel sheet described in claim 1 or 2.

6. A heating step comprising heating a slab having the chemical composition described in claim 1 or 2 and holding it at a temperature of 1100°C or higher for 6000 seconds or more. A width reduction process comprising pressing the slab with a sizing press device, wherein the width reduction press satisfies the following conditions (a) and (b): (a) The width reduction ratio is 1.0 to 23%, and (b) The transport speed of the slab is 20 m / min or more. A hot rolling process comprising finishing rolling the slab, wherein the finishing rolling satisfies the following conditions (c) to (e): (c) The reduction ratio in each rolling pass of the second-to-last stage and the final stage is 20-50%. (d) The total reduction rate is 90% or more, (e) The final rolling temperature is between 960 and 1100°C. A cooling process in which the cooling of the steel sheet is started less than 0.5 seconds after the completion of the hot rolling process, 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 winding process in which cooled steel plates are wound up in a temperature range of 400°C or less. A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, characterized by including the following:

7. Prior to the heating step, the average cooling rate at 600-900°C is 10°C / min or more, and the average cooling rate gradient is 40°C / min. 2 The method for manufacturing a hot-rolled steel sheet according to claim 6, further comprising a continuous casting process controlled to be as follows.

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