steel plate

Abstract

Provided is a steel plate characterized by having a predetermined chemical composition, and characterized in that: the metal structure includes, in area%, 90.0% or more martensite and 3.0% or less retained austenite; the average grain size of former austenite grains is 30.0 μm or less; and when a region surrounded by grain boundaries having an orientation difference of 15° or more in the martensite is defined as crystal grains, the proportion of the crystal grains in which the in-grain orientation difference is 4° or more is, in area%, 45.0-70.0%.

Description

Technical Field

[0001] The present invention relates to a steel sheet.

Background Art

[0002] In recent years, in order to address environmental issues, weight reduction of automotive parts is desired for the purpose of reducing CO2 gas emissions and improving fuel efficiency. On the other hand, social demands for improving collision safety are also increasing. High-strengthening of steel materials is an effective means to achieve both weight reduction and improved collision safety. However, usually, when the steel material is high-strengthened, the workability deteriorates, so there is a need for a steel material that can improve both strength and workability at the same time.

[0003] Regarding the improvement of strength and workability, for example, in Patent Document 1, the chemical components are, in mass %, C: 0.020 to 0.070%, Si: 0.10 to 1.70%, Mn: 0.60 to 2.50%, Al: 0.01 to 1.00%, Ti: 0.015 to 0.170%, Nb: 0.005 to 0.050%, etc., limited to P: 0.05% or less, S: 0.010% or less, N: 0.0060% or less, the balance being composed of Fe and impurities, and the structure contains, in area ratio, 5 to 60% ferrite and 30 to 95% bainite. In the said structure, when a boundary with an orientation difference of 15° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having a circle equivalent diameter of 0.3 μm or more is defined as a crystal grain, the ratio of the crystal grains with an orientation difference of 5 to 14° inside the grains is 20 to 100% in area ratio, and a hot-rolled steel sheet is described. Further, in Patent Document 1, it is taught that by setting the ratio of crystal grains with an orientation difference of 5 to 14° inside the grains to 20% or more in area ratio, it is possible to improve the elongation flange property (hole expansion property) while maintaining the desired steel sheet strength.

[0004] Patent Document 2 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 which has prior γ grains with a particle size number of 7 or higher and which have a martensitic structure or a mixed structure of martensite and bainite within the prior γ grains, characterized in that the prior γ grains are spherical. Furthermore, Patent Document 2 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.

[0005] Patent Document 3 describes a high-carbon steel sheet member comprising C: 0.80 mass% to 1.10 mass%, Si: 0.05 mass% to 0.40 mass%, Mn: 0.05 mass% to 0.50 mass%, Cr: 0.01 mass% to 0.35 mass%, P: 0.03 mass% or less, S: 0.03 mass% or less, with the remainder being Fe and unavoidable impurities, wherein the equivalent circle diameter of the martensite block, defined by an orientation difference of 15° or more, is 3.2 μm or less, carbides with an equivalent circle diameter of 0.2 μm or more are present in an area of ​​0.6 to 5.0%, the average particle size of the carbides is 0.3 μm or more and 2.0 μm or less, the carbide occupancy rate on the prior austenite grain boundaries is 0.35% or less, and the average KAM value measured by EBSD is 0.690 to 0.710. Furthermore, Patent Document 3 teaches that, according to the above configuration, a high-carbon steel sheet member can be obtained in which both hardness and hardness-toughness balance are at a high level. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2016 / 136672 [Patent Document 2] Japanese Patent Publication No. 2002-088440 [Patent Document 3] Japanese Patent Publication No. 2018-048375 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] As described above, it is known that as steel strength increases, its workability decreases, and properties such as hole-expandability, as described in Patent Document 1, decline. When hole-expandability decreases, it may not be possible to process the steel 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 properties appropriate for the application are maintained to a certain extent. In addition, steel sheets for automobiles are often processed into the desired part shape by press forming. Normally, press forming is carried out in multiple steps, so there are relatively many places where, for example, the steel sheet has undergone primary deformation and accumulated strain, 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 decreases. For this reason, steel sheets are required to exhibit high formability even when a certain amount of strain has been introduced, for example, by having 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 that, despite being high in strength, has improved hole-expandability and work-hardening ability through a novel configuration. [Means for solving the problem]

[0009] To achieve the above objective, the inventors focused their research on the microstructure of steel sheets, particularly hot-rolled steel sheets. As a result, the inventors discovered that by constructing the microstructure of a hot-rolled steel sheet having a predetermined chemical composition with a martensite-based structure, they could achieve increased strength and improved hole-expanding properties. Furthermore, by limiting the average grain size of prior austenite grains in the microstructure to a predetermined range and appropriately controlling the proportion of crystal grains with a predetermined intra-grain orientation difference in the martensite structure, they could significantly improve work hardening ability, thus completing the present invention.

[0010] The present invention, which has achieved the above objectives, is as follows. (1) The chemical composition is expressed 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, and the metallographic structure contains, by area percentage, 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, when a region surrounded by grain boundaries with an orientation difference of 15° or more in the martensite is defined as a crystal grain, the proportion of the crystal grains with an orientation difference of 4° or more within the grains is 45.0 - 70.0% by area percentage. The steel sheet is characterized by this. (2) The chemical composition contains, by mass percentage, 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 sheet according to (1) above is characterized by containing at least one of them. (3) The metallographic structure further contains, by area percentage, ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 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 sheet according to any one of the above items (1) to (3), characterized in that the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains is 90.0% or more in area percent. (5) A steel plate as described in any one of the above items (1) to (4), characterized in that the plate thickness is 1.0 to 7.0 mm. (6) A component characterized by including a steel plate as described in any one of the above items (1) to (5). [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a steel sheet, particularly a hot-rolled steel sheet, that has 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. In the aforementioned martensite, when a region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a crystal grain, the proportion of such crystal grains with an orientation difference of 4° or more is characterized by being 45.0 to 70.0% in terms of area.

[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 lead to improved 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 grain 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 performed in multiple steps, such as press molding. Therefore, in addition to controlling the grain size of the prior austenite grains, the inventors focused on the morphology of the martensite structure in their investigation. 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 also appropriately controlling the proportion of specific crystal grains in the martensite structure, more specifically, by defining a crystal grain as a region surrounded by grain boundaries with an orientation difference of 15° or more in the martensite, and controlling the proportion of crystal grains with an orientation difference of 4° or more within an area percentage range of 45.0 to 70.0%, 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, it is believed that by controlling the proportion of grains with an intragrain orientation difference of 4° or more within the area percentage range of 45.0-70.0% in martensite, a martensite structure with non-uniformly dispersed dislocations can be formed, and that such a martensite structure contributes to a high work hardening rate in the later stages of deformation such as press forming. More specifically, the intragrain orientation difference represents the distribution state of dislocations, and generally, the more non-uniformly dispersed the dislocations, the larger the intragrain orientation difference becomes. Therefore, by including grains with increased intragrain orientation differences in an appropriate proportion within the martensite, a certain amount of a structure with non-uniformly dispersed dislocations is present, resulting in the development of non-uniform deformation during processing such as press forming. As a result, it is believed that sufficient work hardening ability can be maintained even in the later stages of deformation, and therefore a high work hardening rate can be achieved. In particular, crystal grains with an orientation difference of 4° or more within the grain are crystal grains with an orientation difference sufficient to develop non-uniform deformation, and by controlling such crystal grains to within the range of 45.0 to 70.0% in area percent, it is possible to achieve a desired work hardening rate. Therefore, even in areas where the steel sheet has undergone primary deformation and accumulated strain inside, such as during press forming, and is then subjected to another deformation (such as stretch flange deformation), the steel sheet according to the embodiment of the present invention can be stably formed because it maintains high work hardening ability. The fact that the work hardening ability of a steel sheet can be improved by controlling the proportion of crystal grains with increased orientation differences within the grain to a predetermined range in a martensite-based metal structure to form a structure in which dislocations are non-uniformly dispersed is not previously known and has now been revealed for the first time by the present inventors. As a result, according to the embodiment of the present invention, it is possible to significantly improve hole-expanding properties and work hardening ability, even while having 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 sector 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 be 0.40% or more, 0.50% or more, 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 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% or 0.08%, Ni: 0-0.30% or 0.15%, B: 0-0.0030% or 0.0015%, Ca: 0-0.0010% or 0.0008%, Mg: 0-0.001 The percentages may be 0% 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 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 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, 3.0 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] [Percentage of old austenite grains with an aspect ratio of 2.0 or less out of all old austenite grains: 90.0% or more in area] In the metal structure of the steel sheet according to the embodiment of the present invention, although not particularly limited, the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains may be, for example, 90.0% or more, 92.0% or more, 94.0% or more, or 96.0% or more in area percent. By increasing the proportion of prior austenite grains having such relatively small aspect ratios, the anisotropy of the metal structure can be reduced. Although there is no particular upper limit, for example, the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains may be 100.0%, 99.0% or less, or 98.0% or less. 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 capabilities. This objective 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 appropriately controlling the proportion of crystal grains having a predetermined intra-grain orientation difference in the martensite structure while limiting the average grain size of prior austenite grains in the metal structure to a predetermined range. Therefore, it is clear that the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains is not an essential technical feature for achieving the objective of the present invention.

[0037] [Method for determining the average particle size of the old austenite grains and the proportion of old austenite grains with an aspect ratio of 2.0 or less to the total number of old austenite grains] The average particle size of the prior austenite grains and the proportion of prior austenite grains with an aspect ratio of 2.0 or less to the total number of 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 finish 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. Crystal orientation information is obtained by measuring a region with a length of 50 μm and a thickness of 50 μm at an arbitrary position in the longitudinal direction of the sample cross-section, at a depth of 1 / 4 of the plate thickness, using electron backscatter diffraction at measurement intervals of 0.1 μm. 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 is 9.6 × 10⁻⁶. -5The Pa level may be less than 15kV, the acceleration voltage 15kV, and the irradiation current level 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 of the prior austenite grains is determined by calculating the average grain size from the obtained grain sizes of all prior austenite grains.

[0038] 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. By dividing the total number of prior austenite grains with an aspect ratio of 2.0 or less by the total number of prior austenite grains, the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains is determined.

[0039] [Percentage of crystal grains in martensite with an intragranular orientation difference of 4° or more: 45.0-70.0% by area] In the martensitic structure of the steel sheet according to the embodiment of the present invention, when a region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a crystal grain, the proportion of crystal grains with an orientation difference of 4° or more is controlled to be within the range of 45.0 to 70.0% in area percent. As mentioned above, the more non-uniformly the dislocations are dispersed, the larger the orientation difference within the grain generally becomes, and crystal grains with an orientation difference of 4° or more are crystal grains that have an orientation difference sufficient to form a structure in which dislocations are non-uniformly dispersed and to develop non-uniform deformation. In this regard, in the embodiment of the present invention, by having such crystal grains in the martensite within the range of 45.0 to 70.0% in area percent and having a certain amount of structure in which dislocations are non-uniformly dispersed, non-uniform deformation develops during processing such as press forming, and as a result, sufficient work hardening ability can be maintained even in the later stages of deformation, and therefore a high work hardening rate can be achieved. If the proportion of crystal grains with an orientation difference of 4° or more within the grain is less than 45.0% or more than 70.0% in area percent, non-uniform deformation will not develop easily during processing such as press forming, and therefore the desired work hardening rate will not be achieved. The proportion of crystal grains with an orientation difference of 4° or more within the grain in a martensitic structure may be, for example, 48.0% or more, 50.0% or more, or 55.0% or more in area percent. Similarly, the proportion of crystal grains with an orientation difference of 4° or more within the grain in a martensitic structure may be, for example, 65.0% or less, 62.0% or less, or 60.0% or less in area percent.

[0040] [Method for determining the proportion of grains in martensite with an intragranular orientation difference of 4° or more] The proportion of crystal grains in martensite with an intra-grain orientation difference of 4° or more is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from a steel plate so that the cross-section perpendicular to the plate surface is the observation surface. Next, at a depth of 1 / 4 of the plate thickness from the surface of the steel plate, an EBSD analysis is performed at a measurement interval of 0.2 μm in a region of 200 μm perpendicular to the plate thickness direction and 100 μm in the plate thickness direction to obtain crystal orientation information. Here, the EBSD analysis is performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points / second. At this time, the vacuum level inside the EBSD analyzer is 9.6 × 10⁻⁶. -5The pressure may be less than Pa, the acceleration voltage may be 15kV, and the irradiation current level may be 13. Next, the martensite structure is identified from the obtained crystal orientation information using the "Phase Map" function installed in the "OIM Analysis®" software attached to the EBSD analyzer. Then, in the identified martensite structure, the region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a crystal grain, the average orientation difference within the crystal grain is calculated, and the percentage of crystal grains with an orientation difference of 4° or more is determined. The crystal grains and the average orientation difference within the grain defined as above can similarly be calculated using the "OIM Analysis®" software attached to the EBSD analyzer. In this invention, "intragrain orientation difference" refers to "Grain Orientation Spread (GOS)," which is the orientation dispersion within the crystal grain. As described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD and X-ray Diffraction Methods," Hidehiko Kimura et al., Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 71, No. 712, 2005, pp. 1722-1728, the value of the orientation difference within a grain is obtained as the average value of the misorientation between a reference crystal orientation and all measurement points within the same crystal grain. In the embodiment of the present invention, the reference crystal orientation is the orientation averaged from all measurement points within the same crystal grain. The GOS value can be calculated using the software "OIM Analysis® Version 7.0.1" attached to the EBSD analyzer.

[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 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 the average value should be used. 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, particularly steel sheets having preferred properties, 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 an example of a preferred manufacturing method 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 hot rolling process comprising finishing rolling the slab, wherein the finishing rolling satisfies the following conditions (a) to (c): (a) The reduction ratio in the rolling pass one stage before the final stage is 30-50%, and the reduction ratio in the final rolling pass is 20-50%. (b) The total reduction rate is 90% or more, (c) The rolling temperature in the rolling pass immediately preceding the final stage is 970-1100°C, and the final rolling temperature is 960-1050°C. Within 0.5 to 10.0 seconds after the completion of the hot rolling process, the cooling of the finish-rolled steel sheet is started, and then within 20.0 seconds from the start of cooling, the steel sheet is cooled to a temperature of 400°C or lower in a cooling process. A winding process in which a cooled steel plate is wound up in a temperature range of 400°C or less, and A strain application process is performed on the obtained steel plate, in which a strain of 0.16 to 1.40% in absolute value is applied to the steel plate at a position where the thickness of the steel plate is 1 / 4 of the way through, by repeatedly applying positive and negative strains three or more times. 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] [Heating process] First, a slab having the chemical composition described above in relation to the steel plate is heated and held at a temperature of 1100°C or higher for 6000 seconds or more. From the viewpoint of productivity, it is preferable to use a slab obtained by continuous casting, but a slab obtained by casting and splitting may also be used, and if necessary, slabs that have been hot-worked or cold-worked may also be used. 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 the initiation points of cracks. 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 process described later, starting from these carbides, resulting in a martensite area ratio of less than 90.0%, and consequently, 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.

[0048] [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.

[0049] [(a) Reduction ratio in the rolling pass immediately preceding the final stage: 30-50%, and reduction ratio in the final rolling pass: 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, it is necessary to appropriately control the reduction ratio in each of the two subsequent rolling passes. Specifically, the reduction ratio in the rolling pass before the final stage is controlled to 30-50%, and the reduction ratio in the final rolling pass is controlled to 20-50%. By performing rolling at such relatively high reduction ratios in the rolling passes before the final stage and in the final stage, recrystallization can be promoted and the metal structure can be refined, and in addition, the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains can be increased. If the reduction ratio in the rolling pass immediately preceding the final stage is less than 30%, and / or the reduction ratio in the final rolling pass is less than 20%, recrystallization may not be completed or 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 proportion of prior austenite grains with an aspect ratio of 2.0 or less may be relatively small. 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 immediately preceding 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 immediately preceding the final stage and the final stage should be 50% or less. Preferably, the reduction ratio in each rolling pass immediately preceding the final stage and the final stage is 45% or less.

[0050] [(b) Total reduction rate: 90% or more] In this manufacturing method, the total reduction ratio during finish rolling is controlled to 90% or more. Since manganese (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 initiation 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 during finish rolling to 90% or more, 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 during finish rolling is less than 90%, the variation in Mn concentration becomes relatively high, and it may not be possible to sufficiently suppress the development of regions where Mn is locally concentrated and the fracture energy is reduced. In addition, if the total reduction ratio in finish rolling is less than 90%, the accumulated strain during rolling will be insufficient, which may result in recrystallization not being completed or not being sufficiently promoted, and the desired average grain size of prior austenite grains may not be achieved in the final steel sheet's microstructure, and / or the proportion of prior austenite grains with an aspect ratio of 2.0 or less may be relatively small. 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

[0051] [(c) Rolling temperature in the rolling pass one stage before the final stage: 970~1100℃, final rolling temperature: 960~1050℃] In this manufacturing method, in addition to controlling the reduction ratio in each of the two rolling passes after the finish rolling stage, the rolling temperature in the rolling pass immediately preceding the final stage (the entry temperature of the rolling pass immediately preceding the final stage) and the final rolling temperature (the end temperature of the finish rolling) are also extremely important for controlling the microstructure of the steel sheet. If the rolling temperature in the rolling pass immediately preceding the final stage is less than 970°C, and / or the final rolling temperature is less than 960°C, recrystallization may not be completed or 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 proportion of prior austenite grains with an aspect ratio of 2.0 or less may be relatively small. 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 rolling temperature of the stage immediately preceding the final stage exceeds 1100°C, and / or the final rolling temperature exceeds 1050°C, the prior austenite grains become coarse, and it may not be possible to achieve the desired average grain size of the prior austenite grains. In this case, as expected, sufficient work hardening ability cannot be obtained.

[0052] [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 the desired proportion of prior austenite grains with an aspect ratio of 2.0 or less in the microstructure of the final steel sheet.

[0053] If the time from the completion of the hot rolling process to the start of cooling is less than 0.5 seconds, recrystallization may not be completed or 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 desired proportion of prior austenite grains with an aspect ratio of 2.0 or less may not be obtained. Also, 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, and the desired average grain size of prior austenite grains will not be obtained. As a result, in either case, sufficient work hardening ability cannot be achieved 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, ferrite, bainite, and / or pearlite will be formed during cooling, and the area ratio of martensite will be less than 90.0%, resulting in the inability to obtain the desired strength and / or hole-expanding properties.

[0054] [Winding process] Next, the cooled steel sheet is wound up at a temperature of 400°C or lower. If the winding temperature exceeds 400°C, as in the cooling process, ferrite, bainite, and / or pearlite will form during winding, resulting in a martensite area ratio of less than 90.0%, and consequently, the desired strength and / or hole-expanding properties cannot be obtained.

[0055] [Strain application process] Finally, the resulting steel sheet is subjected to a strain of 0.16 to 1.40% in absolute value at a position 1 / 4 of the sheet thickness, repeated three or more times with alternating positive and negative values, thereby manufacturing the steel sheet. By repeatedly applying such strain to the steel sheet, dislocations can be introduced non-uniformly into the grains of the martensitic structure, and as a result, the proportion of grains with an orientation difference of 4° or more can be controlled to be within the range of 45.0% to 70.0% in area percent. If the absolute value of the strain applied is less than 0.16% or the number of times the strain is applied is less than three, the non-uniform introduction of dislocations into the grains will be insufficient, resulting in the proportion of grains with an orientation difference of 4° or more being less than 45.0% in area percent, and the desired work hardening rate cannot be achieved. On the other hand, if the absolute value of the strain applied exceeds 1.40%, excessive dislocation accumulation in the crystal grains occurs, resulting in the proportion of crystal grains with an orientation difference of 4° or more exceeding 70.0% in area percentage, making it impossible to achieve the desired work hardening rate. There is no particular upper limit to the number of times strain is applied, but from the viewpoint of manufacturability, it is preferable to have 10 or fewer, 7 or fewer, or 5 or fewer times.

[0056] Since the martensitic structure inherits dislocations within the austenite grains before transformation, generally, martensitic grains produced from unrecrystallized austenite grains have high intragranular orientation differences, while martensitic grains produced from recrystallized austenite grains have low intragranular orientation differences. In this manufacturing method, as described above, the structure is refined by promoting recrystallization during the hot rolling process, thereby controlling the average grain size of the prior austenite grains to 30.0 μm or less. However, in this case, since martensite is generated from recrystallized austenite grains, the intragranular orientation differences of the martensitic grains become low, making it impossible to achieve an area percentage of 45.0-70.0% for grains with an intragranular orientation difference of 4° or more. Therefore, in this manufacturing method, by appropriately controlling the conditions of the hot rolling process to create a prior austenite structure in which recrystallization is almost or completely completed, the structure is refined and the work hardening ability of the steel sheet as a whole is improved. At the same time, by introducing dislocations non-uniformly into some of the grains of the martensitic structure during the strain application process to appropriately create orientation differences, it becomes possible to achieve a high work hardening rate even in a state where some strain has been introduced, such as in the later stages of deformation in press forming.

[0057] The strain application process can be carried out by any suitable method known to those skilled in the art. While not particularly limited, such methods include, for example, bending and unbending deformation using a tension leveler. In this case, the absolute value of the applied strain can be easily changed by varying the size of the rolls in the tension leveler, the relative positional relationship between the rolls, and the thickness of the steel plate. Therefore, by appropriately controlling these parameters, it is possible to easily apply a desired strain at the 1 / 4 thickness position of the steel plate.

[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, thereby achieving high strength, such as a tensile strength of 980 MPa or more, while significantly improving hole-expandability due to the reduction of 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 proportion of crystal grains with an intra-grain orientation difference of 4° or more in the martensite to within the range of 45.0% to 70.0% by area percentage, it is possible to achieve a high work hardening rate 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 using a continuous casting method to form slabs with various chemical compositions as 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. Next, the finish-rolled steel sheets were cooled and wound under the conditions shown in Table 3, and finally, a predetermined number of strains were applied at the 1 / 4 thickness position of the steel sheet, while repeatedly changing the absolute value of the strain shown in Table 3 to obtain a steel sheet 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)] 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 2, 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 thought 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 TS and λ decreased. In Comparative Examples 3 and 6, it is thought that recrystallization was not completed or sufficiently promoted because the reduction ratios in the rolling pass one stage before the final stage and the final stage of the finish rolling process were low, respectively. 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 Examples 4 and 7, it is thought that recrystallization was not completed or sufficiently promoted because the rolling temperature in the rolling pass one stage before the final stage and the final rolling temperature of the finish rolling process were low, respectively. 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 Examples 5 and 8, the rolling temperature of the rolling pass immediately preceding the final stage and the final rolling temperature were high, respectively, which is thought to have resulted in the prior austenite grains becoming coarser overall. As a result, the average grain size of the prior austenite grains in the final metal structure increased, and the work hardening ability of the steel sheet decreased. In Comparative Example 9, the time between the completion of the hot rolling process and the start of the cooling process was short, which is thought to have prevented or prevented sufficient recrystallization. As a result, the average grain size of the prior austenite grains in the final metal structure increased, and the work hardening ability of the steel sheet decreased. In Comparative Example 10, the time between the completion of the hot rolling process and the start of the cooling process was long, which is thought to have caused excessive grain growth overall. As a result, the desired average grain size of the prior austenite grains could not be obtained, and the work hardening ability of the steel sheet decreased. In Comparative Example 11, 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 a decrease in TS and λ. In Comparative Example 12, due to the high winding temperature, the martensite area ratio similarly fell below 90.0%, resulting in a decrease in TS and λ.In Comparative Example 13, the absolute value of the strain applied during the strain application process was small, which is thought to have resulted in insufficient non-uniform introduction of dislocations within the crystal grains. As a result, the proportion of crystal grains with an orientation difference of 4° or more within the martensite decreased, and the work hardening rate of the steel sheet decreased. In Comparative Example 14, the absolute value of the strain applied during the strain application process was large, which is thought to have resulted in excessive accumulation of dislocations in the crystal grains. As a result, the proportion of crystal grains with an orientation difference of 4° or more within the martensite increased, and the work hardening rate of the steel sheet decreased. In Comparative Example 15, the number of times strain was applied during the strain application process was small, which is thought to have resulted in insufficient non-uniform introduction of dislocations within the crystal grains. As a result, the proportion of crystal grains with an orientation difference of 4° or more within the martensite decreased, and the work hardening rate of the steel sheet decreased. In Comparative Example 16, the strain application process was omitted, so non-uniform introduction of dislocations within the crystal grains was not achieved. As a result, the proportion of grains with an intra-grain orientation difference of 4° or more in the martensite decreased, and the work hardening rate of the steel sheet decreased.

[0072] Comparative Examples 37 and 39 had low C and Si content, respectively, which resulted in a decrease in TS. On the other hand, Comparative Examples 38 and 40 had high C and Si content, respectively, which led to the formation of a relatively large amount of retained austenite and a decrease in λ. Comparative Example 41 had low Mn content, which resulted in reduced hardenability, a lower martensite area ratio, and a decrease in TS. Comparative Example 42 had high Mn content, which resulted in a decrease in λ. Comparative Example 43 had low sol.Al content, which is thought to have prevented sufficient suppression of cementite precipitation, resulting in a decrease in λ. Comparative Example 44 had low Nb content, which is thought to have prevented sufficient promotion of refinement of prior austenite grains due to the pinning effect, resulting in a larger average grain size of prior austenite grains in the final metal structure and a decrease in the work hardening ability of the steel sheet. Comparative Example 45 had high Nb content, which is thought to have 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 proportion of crystal grains with an intra-grain orientation difference of 4° or more in the martensite was 45.0-70.0% by area%. As a result, despite having a high strength of tensile strength of 980 MPa or more, 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. A steel sheet characterized in that, when a region surrounded by grain boundaries with an orientation difference of 15° or more in the martensite is defined as a crystal grain, the proportion of crystal grains with an orientation difference of 4° or more within the grain is 45.0% to 70.0% in area percent.

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: 0.001-0.010%, Zr: 0.001 to 0.050%, Co: 0.001 to 0.010%, Zn: 0.001 to 0.010%, W: 0.001-0.100%, Sn: 0.001-0.040%, 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 the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains is 90.0% or more in area percent.

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

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

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