Steel plate and its manufacturing method

Abstract

Provided is a steel sheet that characteristically has: a prescribed chemical composition; a metal structure containing, on an area% basis, at least 90.0% martensite and not more than 3.0% retained austenite; an average grain size for prior austenite grains of not more than 30.0 µm; and a standard deviation on the grain size of the prior austenite grains of at least 4.0 µm. Also provided is a method for producing this steel sheet.

Description

[Technical Field]

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

[0002] In recent years, in response to environmental issues, there has been a growing demand for lighter automotive parts to reduce CO2 emissions and improve fuel efficiency. At the same time, there is an increasing social demand for improved collision safety. To achieve both weight reduction and improved collision safety, increasing the strength of steel materials is an effective means. However, increasing the strength of steel materials usually reduces their workability; therefore, there is a need for steel materials that can improve both strength and workability simultaneously.

[0003] In relation to improving strength and workability, 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, according to the above configuration, it is possible to refine the prior austenite grain size and reduce its variation, thereby realizing a high-strength steel sheet that maintains a high strength of 980 MPa or more while also improving bendability.

[0004] Patent Document 2 describes a high-strength, highly ductile, fine-martensitic steel material containing C: 0.075-0.3 wt%, Mn: 3-10 wt%, Si: 0-2.5 wt%, with the remainder being Fe and unavoidable impurities, having an prior γ grain size of 2.0 μm or less, and possessing a microstructure of equiaxed martensite with single blocks. Patent Document 2 also teaches that this high-strength, highly ductile, fine-martensitic steel material can achieve a tensile strength of 1200 MPa or more and a total elongation of 10% or more.

[0005] Patent Document 3 describes a high-tensile steel material in which, by mass percentage, C: 0.06~0.19%, Si: 0.15~0.60%, Mn: 0.60~1.80%, Cr: 0.05~1.20%, Mo: 0.05~1.00%, and one or more of Nb: 0.005~0.10%, V: 0.005~0.10%, and Ti: 0.005~0.10%, and which contains carbonitrides of Nb, Ti, or V with a particle size of 100 nm or less in volume percentage of 0.01~0.8%, and the prior γ grains have a particle size number of 7 or higher and the prior γ grains have a martensitic structure or a mixed structure of martensite and bainite. Furthermore, Patent Document 3 teaches that, according to the above configuration, it is possible to provide a high-strength steel material that has excellent toughness, arrestability, and weldability, as well as a large uniform elongation characteristic of over 10%, and is suitable for mass production. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2009-242832 [Patent Document 2] Japanese Patent Publication No. 2019-143244 [Patent Document 3] Japanese Patent Publication No. 2002-088440 [Overview of the project] [Problems that the invention aims to solve]

[0007] As described above, as strength increases, the workability of steel materials decreases, and it is known that properties such as hole-expandability also deteriorate, in addition to bending workability, total elongation, and uniform elongation as described in Patent Documents 1 to 3. When hole-expandability deteriorates, it may not be possible to process the material into the desired shape, for example, in automobile suspension parts. For this reason, in the development of high-strength steel sheets such as high-strength hot-rolled steel sheets, it is important to increase strength while ensuring that the properties appropriate for the application are maintained at a certain level or higher. Furthermore, automobile steel sheets are often processed into the desired part shape by press forming. For example, among automobile suspension parts, there are parts with complex shapes such as lower arms and trailing arms. Normally, press forming is carried out in multiple steps, so there are relatively many places where, for example, strain accumulates inside the steel sheet after undergoing primary deformation and then undergoes another deformation (such as stretch flange deformation). However, when strain is introduced into a steel sheet, it works hardens and becomes stronger, so the workability in subsequent processes generally deteriorates. Therefore, steel plates are required to exhibit high formability even when subjected to a certain degree of strain, for example, by possessing excellent work hardening ability (the ability to continue to harden).

[0008] This invention has been made in view of the above circumstances, and its objective is to provide a steel sheet and a method for manufacturing the same that, despite being high in strength, have improved hole-expandability and work-hardening ability through a novel configuration. [Means for solving the problem]

[0009] To achieve the above objectives, the inventors focused on the microstructure of steel sheets, particularly hot-rolled steel sheets, and conducted research. As a result, the inventors discovered that by constructing the microstructure of a hot-rolled steel sheet having a predetermined chemical composition with a structure mainly composed of martensite, it is possible to achieve high strength and improved hole-expanding properties. Furthermore, by limiting the average particle size of prior austenite grains in the microstructure to a predetermined range while increasing the variation in particle size of said prior austenite grains, the work-hardening ability can be significantly improved, thus completing the present invention.

[0010] The present invention that can achieve the above object is as follows. (1) The chemical composition is, by 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%, Ti: 0 to 0.200%, V: 0 to 0.300%, Cu: 0 to 0.40%, Cr: 0 to 0.90%, Mo: 0 to 0.12%, Ni: 0 to 0.30%, B: 0 to 0.0030%, Ca: 0 to 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 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 metal structure is, by area%, martensite: 90.0% or more, and retained austenite: 3.0% or less, the average grain size of prior austenite grains is 30.0 μm or less, A steel sheet characterized in that the standard deviation of the grain size of prior austenite grains is 4.0 μm or more. (2) The chemical composition is, by 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%, 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% The steel sheet according to (1) above, characterized by containing at least one of them. (3) The above-mentioned metal structure further has, in area percentage, Ferrite: 10.0% or less, Bainite: 10.0% or less, and Pearlite: 10.0% or less The steel sheet according to (1) or (2) above, characterized by containing at least one of them. (4) The steel sheet according to any one of (1) to (3) above, characterized in that the plate thickness is 1.0 to 7.0 mm. (5) A component, characterized by containing the steel sheet according to any one of (1) to (4) above. (6) A continuous casting process for casting a slab having the chemical composition according to (1) or (2) above, wherein the average cooling rate at 600 to 900 °C is 10 to 50 °C / min and the average cooling rate gradient is 40 °C / min 2 A continuous casting process controlled to be as follows, A heating process for heating the cast slab and holding it for 6000 seconds or more in a temperature range of 1100 °C or higher, A hot rolling process comprising finishing rolling the slab, wherein the finishing rolling satisfies the following conditions (a) to (c): (a) The reduction ratio in each rolling pass of the second-to-last stage and the final stage is 20-50%. (b) The total reduction rate is 90% or more, (c) The final rolling temperature is 960 to 1100°C. A cooling process in which the cooling of the finish-rolled steel sheet is started within 0.5 to 10.0 seconds after the completion of the hot rolling process, and then the steel sheet is cooled to a temperature of 400°C or lower 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: [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a steel sheet, particularly a hot-rolled steel sheet, and a method for manufacturing the same, which have high strength while also having improved hole-expanding properties and work-hardening capabilities. [Modes for carrying out the invention]

[0012] <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. It is characterized by a standard deviation of 4.0 μm or more in the particle size of the prior austenite grains.

[0013] As mentioned earlier, it is known that properties such as hole-expandability decrease as the strength of steel increases. For example, in order to manufacture parts with complex shapes such as lower arms and trailing arms in the suspension of automobiles, a steel sheet is required that has high strength, especially a tensile strength of 980 MPa or more that enables weight reduction, while also having excellent hole-expandability. From the viewpoint of increasing strength, it is preferable that the metal structure of the steel sheet be composed mainly of martensite. However, martensitic steel has a hierarchical structure that includes substructures such as packets, blocks, and laths within the prior austenite grains, and although it has excellent strength, it generally has the problem of poor workability. Therefore, in forming operations that are carried out in multiple steps, such as press forming, workability generally decreases in the later stages of deformation due to work hardening caused by strain introduced in the early stages of deformation. Accordingly, there is a need for a steel sheet that can achieve both high strength and workability by exhibiting high work hardening ability even in the later stages of deformation in press forming.

[0014] 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 constructing 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 significantly improving the hole-expanding properties of the resulting hot-rolled steel sheet. Although not intended to be bound by any particular theory, it is believed that by making the metallic structure more uniform with 90.0% or more martensite by area percentage, the hardness difference in 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 hardness difference can improve the hole-expanding properties. Furthermore, since retained austenite can act as a fracture initiation point during deformation such as press forming, limiting the retained austenite to 3.0% or less in area, in addition to controlling the martensite to 90.0% or more in area, makes it possible to significantly improve hole expansion properties.

[0015] Next, the inventors considered that prior austenite grain boundaries act as resistance to dislocation movement and are effective in improving work hardening ability. Therefore, they investigated how to improve work hardening ability from the perspective of optimizing the particle size of prior austenite grains in a martensite-based metal structure. More specifically, by refining the prior austenite grains, the density of prior austenite grain boundaries can be increased. As a result, refining the prior austenite grains can increase the obstacles to dislocations, and thus improve work hardening ability. However, simply refining the prior austenite grains may not be sufficient to achieve adequate work hardening ability in the later stages of deformation in molding operations that are divided into multiple processes, such as press molding. Therefore, in addition to controlling the particle size of prior austenite grains, the inventors focused on controlling the particle size distribution, more specifically, controlling the variation in particle size, and investigated this. As a result, the inventors have found that by refining the prior austenite grains to a predetermined range, more specifically by controlling the average particle size of the prior austenite grains to 30.0 μm or less, the work hardening ability of the entire hot-rolled steel sheet can be improved, while by increasing 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 4.0 μm or more, a high work hardening rate can be achieved even in conditions where some strain has been introduced, such as in the later stages of deformation during press forming.

[0016] While not intended to be bound by any particular theory, controlling the standard deviation of the particle size of prior austenite grains to 4.0 μm or more allows for the formation of a mixed grain structure containing both coarse and fine grains. This mixed grain structure is thought to contribute to a high work hardening rate in the later stages of deformation, such as press forming. More specifically, forming a mixed grain structure containing both coarse and fine grains induces non-uniform deformation during processing such as press forming. As a result, sufficient work hardening ability can be maintained even in the later stages of deformation, thus enabling the achievement of a high work hardening rate. Consequently, even in areas where the steel sheet has undergone primary deformation and accumulated strain within it during press forming, and is then subjected to further deformation (such as stretch flange deformation), the steel sheet according to the embodiment of the present invention can be stably formed because it maintains a high work hardening ability. The fact that, in a martensite-based metal structure, the work hardening ability of steel sheets can be improved by increasing the variation in the particle size of prior austenite grains, thereby forming a mixed-grain structure in which coarse and fine grains are mixed, was previously unknown and has now been revealed for the first time by the present inventors. As a result, according to the steel sheet according to the embodiment of the present invention, it is possible to significantly improve hole-expanding properties and work hardening ability, even while achieving high strength, for example, a tensile strength of 980 MPa or more. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve both high strength and excellent workability, which are conflicting properties, and is therefore particularly useful in the automotive field where both of these properties are required.

[0017] 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.

[0018] [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 may reduce workability. 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.

[0019] [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 treatment properties and workability will decrease, and slab cracking may occur during hot rolling. 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.

[0020] [Mn: 1.00~4.00%] Mn is an effective element for increasing strength as a hardenability and solid solution strengthening element. To fully obtain these effects, 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 workability. 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.

[0021] [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 hole expansion. 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.

[0022] [P:0.100% or less] If phosphorus (P) is present in excessive amounts, processability 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.

[0023] [S:0.0300% or less] Excessive sulfur content can lead to the formation of many sulfides such as MnS, which can reduce processability. 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.

[0024] [N:0.0070% or less] Excessive nitrogen (N) content can form coarse nitrides, reducing processability. Therefore, the N content should be 0.0070% or less. The N content may also 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 lead to increased costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[0025] [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 workability 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.

[0026] [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 workability 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.

[0027] 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.

[0028] [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.

[0029] [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.

[0030] 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, for example, components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when manufacturing steel sheets industrially. 。

[0031] 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.

[0032] [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 significantly improving the hole-expandability of the resulting steel sheet. More specifically, by controlling the hard martensite to within the 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 the hardness difference in the metal structure, and as a result of this reduction in hardness difference, the hole-expandability can be improved. If the area ratio of martensite is less than 90.0%, the desired strength and hole-expandability cannot be achieved. From the viewpoint of further increasing strength and improving hole-expandability, a higher area ratio of martensite is preferable, for example, 92.0% or more, 94.0% or more, 96.0% or more, or 98.0% or more. The upper limit of the area percentage of martensite is not particularly limited and may be 100.0%, or for example, 99.0% or less. On the other hand, since retained austenite can be the starting point of fracture during deformation such as press forming, in addition to controlling the martensite to 90.0% or more in area percentage, it is possible to more significantly improve hole expansion properties by limiting the retained austenite to 3.0% or less in area percentage. If the area percentage of retained austenite exceeds 3.0%, it becomes the starting point of fracture during deformation, and hole expansion properties decrease. From the viewpoint of further improving hole expansion properties, it is preferable that the area percentage of retained austenite be as low as possible, 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 percentage of retained austenite is not particularly limited and may be 0%, or for example, 0.5% or more.

[0033] [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 contain 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 hole-expanding properties 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.

[0034] [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.

[0035] [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, prior austenite grain boundaries act as a resistance force against dislocation movement and are considered effective in improving work hardening ability. In connection with this, by refining the prior austenite grains, the density of prior austenite grain boundaries can be increased. Therefore, by refining the prior austenite grains to 30.0 μm or less, the obstacles to dislocations can be increased, and thus the work hardening ability of the resulting steel sheet can be improved. From the viewpoint of further improving the work hardening ability of the steel sheet, the smaller the average particle size of the prior austenite grains, the more preferable it is, for example, 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, 4.0 μm or more, 4.1 μm or more, 4.2 μm or more, 4.5 μm or more, 4.7 μm or more, 5.0 μm or more, 8.0 μm or more, 10.0 μm or more, or 12.0 μm or more.

[0036] [Standard deviation of particle size of former austenite grains: 4.0 μm or greater] In embodiments of the present invention, the standard deviation of the particle size of the prior austenite grains is 4.0 μm or more. By limiting the average particle size of the prior austenite grains to 30.0 μm or less, while increasing the standard deviation of the particle size of the prior austenite grains to 4.0 μm or more, that is, by increasing the variability in the particle size of the prior austenite grains, a mixed grain structure in which coarse and fine grains are mixed can be formed. It is believed that by forming such a mixed grain structure, non-uniform deformation is induced during processing such as press molding, and as a result, sufficient work hardening ability can be maintained even in a state in which a certain amount of strain has been introduced, such as in the later stages of deformation in press molding, and therefore a high work hardening rate can be achieved. From the viewpoint of further improving the work hardening ability of the steel sheet, a larger standard deviation in the particle size of the prior austenite grains is preferable, i.e., a greater variation is preferable. For example, it may be 4.5 μm or more, 5.0 μm or more, greater than 5.0 μm, 5.1 μm or more, 5.2 μm or more, 5.3 μm or more, 5.4 μm or more, 5.5 μm or more, 6.0 μm or more, 8.0 μm or more, or 10.0 μm or more. There is no particular upper limit, but since the average particle size of the prior austenite grains is 30.0 μm or less, the upper limit of the standard deviation is naturally limited and it cannot take any value. There is no particular upper limit, but the standard deviation in the particle size of the prior austenite grains may be, for example, 20.0 μm or less, 15.0 μm or less, 12.0 μm or less, 10.0 μm or less, or 8.0 μm or less.

[0037] In order to achieve the desired work hardening ability in the present invention, it is extremely important to limit the average particle size of prior austenite grains in the metal structure to 30.0 μm or less, while controlling the standard deviation of the particle size of said prior austenite grains to 4.0 μm or more. This is because if either of these characteristics is not satisfied, at least one of the effects of improving work hardening ability due to the refinement of prior austenite grains and the effect of improving work hardening ability due to a mixed grain structure of coarse and fine grains will be insufficient. In particular, in metal structures mainly composed of martensite, the particle size of prior austenite grains is generally relatively uniform, meaning that the standard deviation of the particle size is relatively small. For this reason, in metal structures where martensite accounts for 90.0% or more by area, it is not common to deliberately increase the variation in the particle size of said prior austenite grains while limiting the average particle size of said prior austenite grains to within the range of 30.0 μm or less, and therefore it is extremely difficult to create such a metal structure, and no such method has been known to date. In this invention, the present inventors have discovered, for the first time, that by carrying out the continuous casting process, hot rolling process, and cooling process of the slab under appropriate conditions, it is possible to form a metal structure in which coarse and fine grains are mixed while refining the prior austenite grains, and furthermore, that such a metal structure improves work hardening ability. Therefore, according to the embodiment of the present invention, it is possible to achieve high strength and improved hole-expanding properties, while significantly improving work hardening ability, through a metal structure that is mainly composed of martensite, in which the prior austenite grains are refined and coarse and fine grains are mixed.

[0038] [Average aspect ratio of old austenite grains: 3.0 or less] The average aspect ratio of the prior austenite grains is not particularly limited, but may be, for example, 3.0 or less, 2.5 or less, 2.0 or less, 1.8 or less, 1.6 or less, or 1.4 or less. Reducing the average aspect ratio of the prior austenite grains can reduce the anisotropy of the metal structure. The lower limit is not particularly limited, but for example, the average aspect ratio of the prior austenite grains may be 0.6 or more, 0.7 or more, or 0.8 or more. As described above, the present invention aims to provide a steel sheet that is high in strength yet has improved hole-expanding properties and work-hardening ability. This is achieved by constructing the metal structure of a steel sheet having a predetermined chemical composition with a structure mainly composed of martensite, and by increasing the variation in the grain size of the prior austenite grains while limiting the average grain size of the prior austenite grains in the metal structure to a predetermined range. Therefore, it is clear that the average aspect ratio of the prior austenite grains is not an essential technical feature for achieving the objectives of the present invention.

[0039] [Method for determining the average particle size of the old austenite grains, the standard deviation of the particle size of the old austenite grains, and the average aspect ratio of the old austenite grains] The average grain size of the prior austenite grains, the standard deviation of the grain size of the prior austenite grains, and the average aspect ratio of the prior austenite grains are determined as follows. First, a sample is cut from an arbitrary 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. It is preferable that the cross-section is 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. After polishing the cross-section of the sample using silicon carbide sandpaper from #600 to #1500, it is 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] Next, for one of the prior austenite grains included in the observation field, the ratio of the diameter in the thickness direction to the diameter in the rolling direction (rolling direction diameter / thickness direction diameter) is calculated, and this value is taken as the aspect ratio of that prior austenite grain. If the rolling direction is unknown, the cross section is observed at 0°, 45°, 90°, and 135° to any direction, and the cross section with the highest aspect ratio is taken as the cross section parallel to the rolling direction, and the ratio of the diameter in the thickness direction to the diameter in the rolling direction (rolling direction diameter / thickness direction diameter) is calculated. 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 aspect ratio of all prior austenite grains in that imaging field. The average aspect ratio of the prior austenite grains is determined by taking the arithmetic mean of the obtained aspect ratios of all prior austenite grains.

[0041] [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.

[0042] The steel sheet according to the embodiment of the present invention can reliably achieve both high strength and excellent workability, which are conflicting properties, and is useful for use in parts in technical fields where both of these properties are required, and is particularly useful for use in parts in the automotive field. For this reason, 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 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 microstructure characteristics. In parts of the steel sheet that have undergone relatively little processing during forming, such as press forming, the characteristics of the steel sheet do not change particularly before and after forming. Parts of the steel sheet that have undergone relatively little processing are determined by characteristics such as having a smooth shape without deformation such as bending, and having a small rate of increase or decrease in plate thickness.

[0043] [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 embodiment of the present invention, despite having such a very high tensile strength, excellent hole-expanding properties and work-hardening capabilities can be achieved through specific combinations of the chemical composition and metal structure described above. The upper limit of the tensile strength is not particularly limited; 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 specimen from the direction (C direction) where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011. For example, if it is difficult to take a JIS No. 5 test specimen due to dimensional constraints, other test specimens described in JIS Z 2241:2011 can be used. However, if the plate thickness is less than 0.5 mm, 0.5 mm shall be set as the lower limit for proper evaluation. For example, if it is difficult to take a JIS No. 5 test specimen due to dimensional constraints and it is also difficult to use other test specimens described in JIS Z 2241:2011, a micro-Vickers test in accordance with JIS Z 2244-1:2020 may be performed, and the hardness (HV) may be converted to tensile strength and used. The sample to be used for the micro-Vickers test can be prepared in the same way as the sample used to evaluate the average particle size and aspect ratio of prior austenite grains. The micro-Vickers test should be performed by taking 30 measurements at a load of 500 gf at the 1 / 4 position of the plate thickness, and using the average value. The conversion can be performed using the following formula. Tensile strength [MPa] = 3.12 × Vickers hardness [HV] + 16

[0044] [Hole expansion ratio: λ] According to the steel sheet having the above chemical composition and metal structure, high hole expansion properties, specifically a hole expansion ratio of 45% or more, can be achieved. The hole expansion ratio is preferably 50% or more, more preferably 60% or more, or 70% or more. There is no particular upper limit to the hole expansion ratio, but for example, the hole expansion ratio may be 150% or less, 120% or less, or 100% or less. The hole expansion ratio is determined as follows. First, a test piece measuring 100 mm in width and 100 mm in length is taken from the steel sheet, and a punched hole (initial hole: hole diameter d0 = 10 mm) is made using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole is expanded with a conical punch with a vertex angle of 60° until a crack that penetrates the thickness of the sheet occurs, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) for each test piece is calculated using the following formula. This hole expansion test is performed three times, and the average value is determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[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 continuous casting process for casting a slab having the chemical composition described above in relation to a steel plate, wherein the average cooling rate at 600-900°C is 10-50°C / min and the average cooling rate gradient is 40°C / min. 2The continuous casting process is controlled as follows: A heating process in which the cast slab is heated and held at a temperature of 1100°C or higher for 6000 seconds or more. A hot rolling process comprising finishing rolling the slab, wherein the finishing rolling satisfies the following conditions (a) to (c): (a) The reduction ratio in each rolling pass of the second-to-last stage and the final stage is 20-50%. (b) The total reduction rate is 90% or more, (c) The final rolling temperature is 960 to 1100°C. A cooling process in which the cooling of the finish-rolled steel sheet is started within 0.5 to 10.0 seconds after the completion of the hot rolling process, and then the steel sheet is cooled to a temperature of 400°C or lower 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] [Continuous casting process] First, a slab having the chemical composition described above in relation to the steel plate is cast in a continuous casting process, and the temperature history during solidification is appropriately controlled, more specifically, the average cooling rate at 600-900°C is 10-50°C / min, and the average cooling rate gradient is 40°C / min. 2 The following conditions are controlled: an average cooling rate of 10-50°C / min at 600-900°C, and an average cooling rate gradient of 40°C / min. 2 By controlling the continuous casting process as described below, it becomes possible to achieve the desired average grain size and standard deviation of prior austenite grains in the microstructure of the final steel sheet.

[0048] When the average cooling rate at 600 to 900 °C is less than 10 °C / min, the cooling rate is slow, and the crystal grains formed by transformation into a body-centered cubic structure (bcc structure) during solidification become coarse, and finally the average grain size of the prior austenite grains in the obtained metal structure becomes larger than 30.0 μm. In this case, sufficient work hardening ability cannot be achieved in the obtained steel sheet. On the other hand, when the average cooling rate at 600 to 900 °C exceeds 50 °C / min, the cooling rate is too fast, and in the transformation process of the solidification structure, the crystal grains become fine and uniform. Although the average grain size of the prior austenite grains in the finally obtained metal structure becomes smaller, the variation in the grain size cannot be increased. That is, the standard deviation of the grain size of the prior austenite grains becomes smaller than 4.0 μm, and similarly, sufficient work hardening ability cannot be achieved.

[0049] In this manufacturing method, the average cooling rate gradient at 600 to 900 °C refers to the average of the change rate of the cooling rate per unit time at 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 and conversely, even when the cooling rate changes from 50 °C / min to 10 °C / min, the average cooling rate gradient in this manufacturing method is 40 °C / min 2 shall be. When the average cooling rate gradient at 600 to 900 °C exceeds 40 °C / min 2 , non-uniform cooling occurs because the fluctuation of the cooling rate is too large. In such a case, a phenomenon occurs in which only specific crystal grains grow abnormally in the transformation process of the solidification structure, and the desired average grain size and / or standard deviation of the grain size of the prior austenite grains cannot be obtained. As a result, sufficient work hardening ability cannot be achieved in the finally obtained steel sheet. The average cooling rate gradient at 600 to 900 °C is preferably 30 °C / min 2 or less. The lower limit is not particularly limited, but the average cooling rate gradient at 600 to 900 °C is 2 °C / min 2 or more or 3 °C / min 2That's all.

[0050] [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 solid solution of coarse carbides will be incomplete. If the solid solution of coarse carbides is incomplete, 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 hole-expanding properties cannot be obtained. 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 higher is preferably 10,000 seconds or less.

[0051] [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.

[0052] [(a) 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 second-to-last and final rolling passes is controlled to 20-50%. Performing rolling at such relatively high reduction ratios in the second-to-last and final rolling passes promotes recrystallization and refines the microstructure, and also reduces the average aspect ratio of the prior austenite grains. If the reduction ratio in the second-to-last and / or final rolling passes is less than 20%, recrystallization may not be completed or sufficiently promoted, and the desired average grain size of the prior austenite grains may not be achieved in the microstructure of the final steel sheet, and / or the average aspect ratio of the prior austenite grains may be relatively large. If the desired average grain size of prior austenite grains cannot be achieved, sufficient work hardening ability cannot be obtained. On the other hand, if the reduction ratio in each rolling pass of the second-to-last 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 second-to-last stage and the final stage should be 50% or less. Preferably, the reduction ratio in each rolling pass of the second-to-last stage and the final stage should be 45% or less.

[0053] [(b) 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 hole expansion properties, 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

[0054] [(c) 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 of prior austenite grains may not be achieved in the microstructure of the final steel sheet, and / or the average aspect ratio of the prior austenite grains may become relatively large. If the desired average grain size of prior austenite grains cannot be achieved, sufficient work hardening ability cannot be obtained. On the other hand, if the final rolling temperature exceeds 1100°C, the prior austenite grains become generally coarse, and the desired average grain size of prior austenite grains and / or the standard deviation of the grain size of prior austenite grains may not be achieved. In this case as well, sufficient work hardening ability cannot be obtained.

[0055] [Cooling process] [Time from completion of hot rolling process to start of cooling: 0.5~10.0 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 within 0.5 to 10.0 seconds after the completion of the hot rolling process, and then cooled to a temperature of 400°C or lower within 20.0 seconds from 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.

[0056] If the time from the completion of the hot rolling process to the start of cooling is less than 0.5 seconds, grain growth will not proceed sufficiently, and it will be impossible to obtain the desired standard deviation of the grain size of the prior austenite grains. On the other hand, 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, and it will be impossible to obtain the desired average grain size of the prior austenite grains and / or the standard deviation of the grain size of the prior austenite grains. As a result, in either case, it will be impossible to achieve sufficient work hardening ability in the steel sheet. 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 area ratio of martensite will be less than 90.0%, and as a result, it will be impossible to obtain the desired strength and / or hole-expanding properties.

[0057] [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 hole-expanding properties cannot be obtained.

[0058] According to the steel sheet manufactured by the above manufacturing method, the metal structure is composed of a more uniform structure containing 90.0% or more martensite and 3.0% or less retained austenite by area percentage. This achieves high strength, such as a tensile strength of 980 MPa or more, while significantly improving hole-expandability due to a reduction in hardness differences. Furthermore, by controlling the average particle size of prior austenite grains in the metal structure to 30.0 μm or less, the work hardening ability of the steel sheet as a whole is improved, and by controlling the standard deviation of the particle size of the prior austenite grains to 4.0 μm or more, a high work hardening rate can be achieved even in conditions where some strain has been introduced, such as in the later stages of deformation during press forming. Therefore, the steel sheet manufactured by the above manufacturing method can reliably achieve both high strength and excellent workability, which are conflicting properties, making it particularly useful in the automotive sector where both properties are required.

[0059] 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]

[0060] 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), hole expansion ratio (λ), and work hardening rate (WHR) of the obtained steel sheets were investigated.

[0061] First, molten steel was cast by continuous casting under the conditions shown in Table 3 to form slabs with various chemical compositions shown in Tables 1 and 2. These slabs were heated to a temperature of 1100-1200°C and held for the time shown in Table 3, and then hot-rolled. Hot rolling was carried out 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 carried out using a tandem rolling mill consisting of five rolling stands under the conditions shown in Table 3. Finally, the finish-rolled steel sheets were cooled and wound under the conditions shown in Table 3 to obtain steel sheets with the thickness shown in Table 4.

[0062] [Table 1]

[0063] [Table 2]

[0064] [Table 3]

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

[0066] [Tensile Strength (TS)] The tensile strength (TS) was measured by taking a JIS No. 5 test specimen from the orientation where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2011.

[0067] [Hole expansion ratio (λ)] The hole expansion ratio was determined as follows. First, a 100mm wide x 100mm long test piece was taken from the steel plate, and a punched hole (initial hole: hole diameter d0 = 10mm) was created using a punching tool with a punch diameter of 10mm and a die diameter of 10.25~11.5mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole was expanded using a conical punch with a 60° apex angle until a crack penetrating the plate thickness occurred. The hole diameter d1mm at the time of crack occurrence was measured, and the hole expansion ratio λ (%) for each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0068] [Work hardening ability] Work hardening ability was evaluated by determining the work hardening rate (WHR) of the steel sheet from tensile tests. Specifically, the region in which the strain during tensile deformation (true strain) was 0.04 or higher during a tensile test, the same as in TS measurement, was simulated as the late-stage deformation state in press forming, and the maximum value of the work hardening rate (WHR) in this region was determined using the following formula. WHR(MPa) = dσ / dε In the formula, σ is the true stress and ε is the true strain.

[0069] Steel sheets with a tensile strength (TS) of 980 MPa or higher, a hole expansion ratio (λ) of 45% or higher, and a maximum work hardening rate (WHR) of 1000 MPa or higher in the region where the true strain is 0.04 or higher were evaluated as steel sheets with improved hole expansion properties and work hardening capabilities despite their high strength. The results are shown in Table 4.

[0070] [Table 4]

[0071] Referring to Tables 1-4, in Comparative Example 4, the average cooling rate between 600 and 900°C in the continuous casting process was slow, which is thought to have resulted in coarse grains. As a result, the average grain size of prior austenite grains in the final metal structure became large, and the work hardening ability of the steel sheet decreased. In Comparative Example 5, the average cooling rate between 600 and 900°C in the continuous casting process was fast, which is thought to have resulted in fine and uniform grains during the transformation process of the solidification structure. As a result, the standard deviation of the grain size of prior austenite grains in the final metal structure became small, and the work hardening ability of the steel sheet decreased. In Comparative Example 6, the average cooling rate gradient between 600 and 900°C in the continuous casting process was large, which is thought to have resulted in excessive fluctuations in the cooling rate and uneven cooling. As a result, the desired average grain size and standard deviation of grain size of prior austenite grains could not be obtained, and the work hardening ability of the steel sheet decreased. In Comparative Example 7, 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 martensite area ratio was less than 90.0%, and λ decreased. In Comparative Examples 8 and 9, the reduction ratios in the rolling passes one stage before the final stage and the final stage of the finish rolling process were low, respectively, which is believed to have prevented or prevented sufficient recrystallization. As a result, the average grain size of prior austenite grains in the final metal structure increased, and the work hardening ability of the steel sheet decreased. In Comparative Example 10, the final rolling temperature during the finish rolling process was low, which is believed to have prevented or prevented sufficient recrystallization. As a result, the average grain size of prior austenite grains in the final metal structure increased, and the work hardening ability of the steel sheet decreased. In Comparative Example 11, the final rolling temperature during the finish rolling process was high, which is believed to have resulted in the prior austenite grains becoming coarser overall. As a result, the average grain size and standard deviation of prior austenite grains in the final metal structure increased, leading to a decrease in the work hardening ability of the steel sheet.In Comparative Example 12, it is believed that grain growth did not progress sufficiently because the time between the completion of the hot rolling process and the start of the cooling process was too short. As a result, the desired standard deviation of the prior austenite grain size could not be obtained, and the work hardening ability of the steel sheet decreased. In Comparative Example 13, it is believed that grain growth progressed too much overall because the time between the completion of the hot rolling process and the start of the cooling process was too long. As a result, the desired average grain size and standard deviation of the prior austenite grain size could not be obtained, and the work hardening ability of the steel sheet decreased. In Comparative Example 14, the time from the start of cooling to below 400°C in the cooling process was too long, resulting in a martensite area ratio of less than 90.0% and a decrease in λ. In Comparative Example 15, the coiling temperature was high, similarly resulting in a martensite area ratio of less than 90.0% and a decrease in λ.

[0072] 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 and a decrease in λ. Comparative Example 40 had low Mn content, which resulted in reduced hardenability, and consequently a lower martensite area ratio, leading to a decrease in both TS and λ. Comparative Example 41 had high Mn content, which resulted in a decrease in λ. In Comparative Example 42, it is thought that the low sol.Al content prevented sufficient suppression of cementite precipitation, resulting in a decrease in λ. In Comparative Example 43, it is thought that the low Nb content prevented sufficient promotion of refinement of prior austenite grains due to the pinning effect. As a result, the average grain size of prior austenite grains in the final metal structure increased, reducing the work hardening ability of the steel sheet. In Comparative Example 44, it is thought that the high Nb content resulted in the formation of coarse carbides and other materials in the steel, resulting in a decrease in λ.

[0073] In contrast, in all the examples of the invention, by having a predetermined chemical composition and appropriately controlling each condition in the manufacturing method, it was possible to obtain steel sheets in which the metal structure contained, by area%, 90.0% or more martensite and 3.0% or less retained austenite, the average particle size of prior austenite grains was 30.0 μm or less, and the standard deviation of the particle size of prior austenite grains was 4.0 μm or more. As a result, despite having a high strength of 980 MPa or more in tensile strength, hole-expanding properties and work-hardening ability were significantly improved.

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 4.0 μm or more. A steel plate characterized by having a thickness of 1.0 to 7.0 mm.

2. The aforementioned chemical composition is, in mass%, Ti: 0.001 to 0.200%, V: 0.001-0.300%, Cu: 0.001 to 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 steel plate 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 steel plate according to claim 1 or 2, characterized in that it includes at least one of the following.

4. The steel sheet according to claim 1 or 2, characterized in that it has a Si content of 0.51% or more.

5. The steel plate according to claim 1 or 2, characterized in that the Mo content is 0.09% or less.

6. The steel sheet according to claim 1 or 2, characterized in that the standard deviation of the particle size of the prior austenite grains is greater than 5.0 μm.

7. A component characterized by comprising the steel plate described in claim 1 or 2.

8. A continuous casting process for casting a slab having the chemical composition described in claim 1 or 2, wherein the average cooling rate at 600 to 900°C is 10 to 50°C / min, and the average cooling rate gradient is 40°C / min 2 The continuous casting process is controlled to be as follows: A heating process in which the cast slab is heated and held at a temperature of 1100°C or higher for 6000 seconds or more. A hot rolling process comprising finishing rolling the slab, wherein the finishing rolling satisfies the following conditions (a) to (c): (a) The reduction ratio in each rolling pass of the second-to-last stage and the final stage is 20-50%. (b) The total reduction rate is 90% or more, (c) The final rolling temperature is 960 to 1100°C. A cooling process in which the cooling of the finish-rolled steel sheet is started within 0.5 to 10.0 seconds after the completion of the hot rolling process, and then the steel sheet is cooled to a temperature of 400°C or lower 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 steel sheet according to claim 1 or 2, characterized by including the following:

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