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

steel plate

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

[0002] In recent years, in response to environmental issues, there has been a demand for lighter automotive parts in order to reduce CO2 gas emissions and improve fuel efficiency. At the same time, societal demands for improved collision safety are also increasing. Increasing the strength of steel materials is an effective way to achieve both weight reduction and improved collision safety. However, increasing the strength of steel materials usually results in a decrease in workability, so steel materials that simultaneously improve both strength and workability are needed.

[0003] In relation to the improvement of strength and workability, for example, Patent Document 1 discloses that the chemical components contain, 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., with P: 0.05% or less, S: 0.010% or less, N: 0.0060% or less, and the balance being Fe and impurities. The hot-rolled steel sheet is characterized in that it is made of pure steel, has a structure containing 5 to 60% ferrite and 30 to 95% bainite by area ratio, and, when boundaries in the structure where the misorientation is 15° or more are defined as grain boundaries and regions surrounded by the grain boundaries and having an equivalent circle diameter of 0.3 μm or more are defined as crystal grains, the proportion of crystal grains having an intragranular misorientation of 5 to 14° by area ratio is 20 to 100%. Patent Document 1 also teaches that by setting the proportion of crystal grains having an intragranular misorientation of 5 to 14° by area ratio to 20% or more, it is possible to improve stretch flangeability (hole expandability) while maintaining the desired steel sheet strength.

[0004] Patent Document 2 describes a high-tensile steel material in which prior γ grains are spherical, characterized in that the steel contains, by mass, 0.06 to 0.19% C, 0.15 to 0.60% Si, 0.60 to 1.80% Mn, 0.05 to 1.20% Cr, 0.05 to 1.00% Mo, and one or more of 0.005 to 0.10% Nb, 0.005 to 0.10% V, and 0.005 to 0.10% Ti, and the steel contains 0.01 to 0.8% by volume of carbonitrides of Nb, Ti, or V having a particle size of 100 nm or less, and the prior γ grains have a grain size number of 7 or more and contain a martensite structure or a mixed structure of martensite and bainite within the prior γ grains. Furthermore, Patent Document 2 teaches that the above-described configuration makes it possible to provide a high-strength steel material that is excellent in toughness, arrestability, and weldability, has a large uniform elongation property exceeding 10%, and is suitable for mass production.

[0005] Patent Document 3 describes a high-carbon steel plate member containing 0.80 mass % to 1.10 mass % C, 0.05 mass % to 0.40 mass % Si, 0.05 mass % to 0.50 mass % Mn, 0.01 mass % to 0.35 mass % Cr, 0.03 mass % or less P, 0.03 mass % or less S, with the balance being Fe and unavoidable impurities, in which the circle-equivalent diameter of martensite blocks defined by an orientation difference of 15° or more is 3.2 μm or less, carbides having a circle-equivalent diameter of 0.2 μm or more are present in an amount of 0.6 to 5.0 area %, the average grain size of the carbides is 0.3 μm or more and 2.0 μm or less, the carbide occupancy rate on prior austenite grain boundaries is 0.35% or less, and an average KAM value measured by EBSD is 0.690 to 0.710. Furthermore, Patent Document 3 teaches that the above-mentioned configuration makes it possible to obtain a high-carbon steel plate member having high levels of both hardness and hardness-toughness balance.

[0006] International Publication No. 2016 / 136672 Japanese Patent Application Laid-Open No. 2002-088440 Japanese Patent Application Laid-Open No. 2018-048375

[0007] As described above, it is known that the workability of steel declines with increasing strength, resulting in a decline in properties such as hole expandability, as described in Patent Document 1. A decline in hole expandability may make it impossible to form the steel into the desired shape, for example, in automobile suspension components. Therefore, in the development of high-strength steel sheets, such as high-strength hot-rolled steel sheets, it is important to achieve high strength while maintaining certain or higher levels of properties appropriate for the application. Furthermore, automotive steel sheets are often processed into the desired part shape by press forming. Because press forming is typically performed in multiple steps, there are many areas where, for example, primary deformation accumulates strain within the steel sheet, leading to additional deformation (e.g., stretch flange deformation). However, because steel sheets undergo work hardening and strength increase when strain is introduced, their workability in subsequent processes generally declines. For this reason, steel sheets are required to exhibit high formability, for example, by possessing excellent work hardening ability (the ability to continue hardening) even when a certain amount of strain is introduced.

[0008] The present invention has been made in view of such circumstances, and its object is to provide a steel sheet having improved hole expandability and work hardening ability despite its high strength due to a novel configuration.

[0009] In order to achieve the above object, the present inventors have conducted research focusing on the metallographic structure of steel sheets, particularly hot-rolled steel sheets. As a result, the present inventors have discovered that by configuring the metallographic structure of a hot-rolled steel sheet having a predetermined chemical composition to be mainly composed of martensite, it is possible to achieve high strength and improved hole expandability, and that by limiting the average grain size of prior austenite grains in the metallographic structure to within a predetermined range and appropriately controlling the proportion of crystal grains having a predetermined intra-granular misorientation in the martensite structure, it is possible to significantly improve work hardening capacity, and have completed the present invention.

[0010] The present invention has achieved the above object as follows: (1) A steel sheet having a chemical composition, in mass %, of C: 0.040 to 0.200%, Si: 0.30 to 2.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001 to 1.000%, Ti: 0 to 0.200%, V: 0 to 0.300%, Cu: 0-0.40%, Cr: 0-0.90%, Mo: 0-0.12%, Ni: 0-0.30%, B: 0-0.0030%, Ca: 0-0.0010%, Mg: 0-0.0010%, Bi: 0-0.010%, Zr: 0-0.050%, Co: 0 to 0.010%, Zn: 0 to 0.010%, 1. A steel sheet comprising: W: 0-0.100%, Sn: 0-0.040%, As: 0-0.100%, REM: 0-0.0100%, and the balance: Fe and impurities; a metal structure containing, in area %, 90.0% or more martensite and 3.0% or less retained austenite; an average grain size of prior austenite grains being 30.0 μm or less; and, when a crystal grain is defined as a region in the martensite surrounded by grain boundaries having an orientation mismatch of 15° or more, the proportion of the crystal grains having an intragranular orientation mismatch of 4° or more is 45.0-70.0% in area %. (2) The chemical composition is, in mass%, Ti: 0.001 to 0.200%, V: 0.001 to 0.300%, Cu: 0.001 to 0.40%, Cr: 0.001 to 0.90%, Mo: 0.001 to 0.12%, Ni: 0.001 to 0.30%, B: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, Bi: 0.001 to 0.010%, Zr: 0.001 to 0.050%, Co: 0.001 to 0.010%, Zn: 0.001 to 0.010%, The steel sheet according to (1) above, characterized in that it contains at least one of W: 0.001 to 0.100%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, and REM: 0.0001 to 0.0100%.(3) The steel sheet according to (1) or (2) above, characterized in that the metallographic structure further includes, in area %, at least one of: ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less. (4) The steel sheet according to any one of (1) to (3) above, characterized in that a ratio, in area %, of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains is 90.0% or more. (5) The steel sheet according to any one of (1) to (4) above, characterized in that a sheet thickness is 1.0 to 7.0 mm. (6) A part, characterized in that it includes the steel sheet according to any one of (1) to (5) above.

[0011] According to the present invention, it is possible to provide a steel sheet, particularly a hot-rolled steel sheet, which has improved hole expandability and work hardenability despite having high strength.

[0012] <Steel Sheet> The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition, in mass %, of C: 0.040 to 0.200%, Si: 0.30 to 2.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001 to 1.000%, Ti: 0 to 0.200%, V: 0 to 0.300%, Cu: 0-0.40%, Cr: 0-0.90%, Mo: 0-0.12%, Ni: 0-0.30%, B: 0-0.0030%, Ca: 0-0.0010%, Mg: 0-0.0010%, Bi: 0-0.010%, Zr: 0-0.050%, Co: 0 to 0.010%, Zn: 0 to 0.010%, The alloy is characterized in that it is composed of 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, and the metal structure contains, in area %, 90.0% or more martensite and 3.0% or less retained austenite, the average grain size of prior austenite grains is 30.0 μm or less, and when a crystal grain is defined as a region in the martensite surrounded by grain boundaries with an orientation misalignment of 15° or more, the proportion of the crystal grains with an intragranular orientation misalignment of 4° or more is 45.0 to 70.0% in area %.

[0013] As mentioned above, it is known that properties such as hole expandability decrease with increasing strength of steel. For example, to manufacture automobile suspension components with complex shapes, such as lower arms and trailing arms, a steel sheet is required that has high strength, particularly a tensile strength of 980 MPa or more, which enables weight reduction, while also exhibiting excellent hole expandability. From the perspective of increasing strength, it is preferable for the metal structure of the steel sheet to be composed primarily of martensite. However, martensitic steel has a hierarchical structure that includes substructures such as packets, blocks, and laths within prior austenite grains. Although it has excellent strength, it generally suffers from poor workability. Therefore, in forming operations that are performed in multiple steps, such as press forming, work hardening due to strain introduced in the early stage of deformation generally results in a decrease in workability in the later stage of deformation. Therefore, 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 stage of deformation during press forming.

[0014] Therefore, the present inventors conducted research focusing not only on the chemical composition of a steel sheet, particularly a hot-rolled steel sheet, but also on the metallurgical structure of the hot-rolled steel sheet. First, the present inventors discovered that by forming the metallurgical structure of a hot-rolled steel sheet having a predetermined chemical composition into a structure mainly composed of martensite, more specifically, a structure containing 90.0% or more martensite and 3.0% or less retained austenite, high strength, for example, a tensile strength of 980 MPa or more, can be achieved while significantly improving the hole expandability of the resulting hot-rolled steel sheet. While not intending to be bound by any particular theory, it is believed that by forming the metallurgical structure into a more uniform structure containing 90.0% or more martensite by area, the hardness difference in the metallurgical structure can be reduced compared to when other structures softer than martensite, such as ferrite, are relatively abundant, and that this reduction in hardness difference can improve the hole expandability. Furthermore, since retained austenite can be the starting point for fracture during deformation such as press forming, it is possible to more significantly improve hole expandability by limiting the area percentage of the martensite to 90.0% or more and limiting the area percentage of the retained austenite to 3.0% or less.

[0015] Next, the inventors investigated the improvement of work hardening ability from the perspective of optimizing the grain size of prior austenite grains in a metal structure mainly composed of martensite, because prior austenite grain boundaries act as a resistance force against dislocation motion and are thought to be effective in improving work hardening ability. More specifically, by refining the prior austenite grains, the density of prior austenite grain boundaries can be increased. Therefore, by refining the prior austenite grains, it is possible to increase the hindrance of dislocations and therefore improve work hardening ability. However, simply refining the prior austenite grains may not be enough to exhibit sufficient work hardening ability in the later stages of deformation during a forming operation that is performed in multiple steps, such as press forming. Therefore, the inventors conducted research focusing on the morphology of the martensite structure in addition to controlling the grain size of the prior austenite grains. As a result, the inventors discovered that by refining the prior austenite grains within a predetermined range, more specifically by controlling the average grain size of the prior austenite grains to 30.0 μm or less, the work-hardening ability of the entire hot-rolled steel sheet can be improved while appropriately controlling the proportion of specific crystal grains in the martensite structure, more specifically by defining a crystal grain as an area surrounded by grain boundaries in martensite with an orientation misorientation of 15° or more, and controlling the proportion of crystal grains with an intragranular orientation misorientation of 4° or more within the range of 45.0 to 70.0% by area %, it is possible to achieve a high work-hardening rate even in a state where a certain amount of strain has been introduced, such as in the later stages of deformation in press forming.

[0016] While not intending to be bound by any particular theory, it is believed that by controlling the proportion of crystal grains in martensite with an intragranular misorientation of 4° or more within the range of 45.0 to 70.0% by area, 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 misorientation within crystal grains represents the distribution of dislocations, and the more non-uniformly dispersed the dislocations, the greater the intragranular misorientation generally. Therefore, by including an appropriate proportion of crystal grains with increased intragranular misorientation in martensite to create a certain amount of a structure with non-uniformly dispersed dislocations, non-uniform deformation develops 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 intragranular misorientation of 4° or more are crystal grains with a sufficient misorientation to develop non-uniform deformation, and by controlling such crystal grains within a range of 45.0 to 70.0% by area, it is possible to achieve a desired work-hardening rate. Therefore, even in areas where a steel sheet undergoes primary deformation, such as press forming, and strain accumulates inside the steel sheet before undergoing another deformation (such as stretch flange deformation), the steel sheet according to the present invention maintains high work-hardening capacity, allowing for stable forming. In a metal structure primarily composed of martensite, the fact that the work-hardening capacity of the steel sheet can be improved by controlling the proportion of crystal grains with increased intragranular misorientation within a predetermined range to form a structure in which dislocations are non-uniformly dispersed was not previously known, and has now been revealed for the first time by the present inventors. As a result, the steel sheet according to the present invention can significantly improve hole expandability and work-hardening capacity, despite having a high tensile strength of, for example, 980 MPa or more. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability, and is therefore particularly useful in the automotive field where both properties are required to be achieved.

[0017] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0018] [C: 0.040 to 0.200%] C is an element effective in increasing the strength of steel sheet. Furthermore, C forms carbides and / or carbonitrides with Nb in steel, and contributes to refining the structure due to the pinning effect of the formed precipitates. To fully obtain these effects, the C content is set to 0.040% or more. The C content may be 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, excessive C content may reduce workability. Therefore, the C content is set to 0.200% or less. The C content may be 0.180% or less, 0.160% or less, 0.150% or less, or 0.140% or less.

[0019] [Si: 0.30 to 2.00%] Si is an element that is effective in increasing strength as a solid solution strengthening element. To fully obtain this effect, the Si content is set to 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, excessive Si content may deteriorate chemical conversion treatability and workability, and may cause slab cracking during hot rolling. Therefore, the Si content is set to 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, 1.50% or less, or 1.40% or less.

[0020] [Mn: 1.00 to 4.00%] Mn is an element that is effective in increasing strength as an element for hardenability and solid solution strengthening. To fully obtain these effects, the Mn content is set to 1.00% or more. The Mn content may be 1.20% or more, 1.50% or more, 1.80% or more, 2.00% or more, or 2.20% or more. On the other hand, excessive Mn content may decrease workability. Therefore, the Mn content is set to 4.00% or less. The Mn content may be 3.80% or less, 3.50% or less, 3.20% or less, 3.00% or less, or 2.80% or less.

[0021] [Sol. Al: 0.001 to 0.500%] Sol. Al is an element that acts as a deoxidizer for molten steel. Sol. Al also suppresses the precipitation of cementite, which is harmful to hole expandability. To achieve these effects, the sol. Al content is set to 0.001% or more. The sol. Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive sol. Al content may saturate the effect and increase manufacturing costs. Therefore, the sol. Al content is set to 0.500% or less. The sol. Al content may be 0.400% or less, 0.300% or less, or 0.200% or less. Sol. Al refers to acid-soluble Al, which refers to solute Al present in steel in a solid solution state.

[0022] [P: 0.100% or less] Excessive P content may result in reduced workability due to grain boundary segregation, etc. Therefore, the P content is set to 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0023] [S: 0.0300% or less] Excessive S content may result in the formation of large amounts of sulfides such as MnS, which may reduce workability. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the S content may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[0024] [N: 0.0070% or less] Excessive N content may form coarse nitrides and reduce workability. Therefore, the N content is set to 0.0070% or less. The N content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[0025] [O: 0.0100% or less] O is an element that is mixed in during the manufacturing process. Excessive O content may form coarse inclusions, reducing the workability of the steel sheet. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long refining time, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more or 0.0005% or more.

[0026] [Nb: 0.001 to 1.000%] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, contributing to the refinement of prior austenite grains through a pinning effect and, ultimately, to the increased strength of the steel sheet. To fully obtain these effects, the Nb content is set to 0.001% or more. The Nb content may be 0.005% or more, 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, or 0.300% or more. On the other hand, excessive Nb content may generate coarse carbides and the like in the steel, reducing the workability of the steel sheet. Therefore, the Nb content is set to 1.000% or less. The Nb content may be 0.800% or less, 0.600% or less, or 0.500% or less.

[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 contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0028] [Cr: 0 to 0.90%] Cr is an element that improves the hardenability of steel and contributes to improving strength and / or corrosion resistance. The Cr content may be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if Cr is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Cr content is preferably 0.90% or less, and may be 0.70% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[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 to 0.010%, Zr: 0 to 0.050%, Co: 0 to 0.010%, Zn: 0 to 0.010%, W: 0 to 0.100%, Sn: 0 to 0.040%, As: 0 to 0.100%, and REM: 0 to 0.0100%] Ti, V, Cu, Mo, Ni, B, Ca, Mg, Bi, Zr, Co, Zn, W, Sn, As, and REM may be contained in the steel sheet as optional elements or may be present in the steel sheet as tramp elements. The contents of these elements are as follows: Ti: 0 to 0.200% or 0.100%, V: 0 to 0.300% or 0.200%, Cu: 0 to 0.40% or 0.20%, Mo: 0 to 0.12% or 0.08%, Ni: 0 to 0.30% or 0.15%, B: 0 to 0.0030% or 0.0015%, Ca: 0 to 0.0010% or 0.0008%, Mg: 0 to 0.001%. The lower limits of these elements may be, for example, 0.001% or more, 0.005% or more, 0.008% or more, 0.001% or more, 0.005% or more, 0.005% or more, 0.008% or more, 0.001% or more, 0.005% or more, 0.008% or more, 0.005% or more, 0.008% or more, or 0.008% or more, respectively. Similarly, the B, Ca, Mg and REM contents 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 balance other than the above elements consists of Fe and impurities. The 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 industrially manufacturing the steel sheet. Impurities are also allowed to be contained within a range that does not affect the effects of the present invention.

[0031] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[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 present invention includes, by area percentage, 90.0% or more martensite and 3.0% or less retained austenite. By configuring the metal structure of the steel sheet to include these structures, high strength, for example, a tensile strength of 980 MPa or more, can be achieved while significantly improving the hole expandability of the resulting steel sheet. More specifically, controlling the hard martensite to a range of 90.0% or more by area percentage to create a more uniform structure not only contributes to increased strength, but also reduces hardness differences in the metal structure, thereby improving hole expandability due to such reduced hardness differences. If the area fraction 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 martensite area fraction is preferable, and may be, for example, 92.0% or more, 94.0% or more, 96.0% or more, or 98.0% or more. The upper limit of the martensite area fraction is not particularly limited and may be 100.0%, for example, 99.0% or less. On the other hand, since retained austenite can become the origin of fracture during deformation such as press forming, by controlling the martensite area fraction to 90.0% or more and limiting the retained austenite area fraction to 3.0% or less, hole expandability can be more significantly improved. If the area fraction of retained austenite exceeds 3.0%, it can become the origin of fracture during deformation, resulting in a decrease in hole expandability. From the viewpoint of further improving hole expandability, a lower retained austenite area fraction is preferable, and may be, for example, 2.5% or less, 2.0% or less, 1.5% or less, or 1.0% or less. The lower limit of the area ratio of retained austenite is not particularly limited and may be 0%, or may be, for example, 0.5% or more.

[0033] [Remaining Structure] The remaining structure other than martensite and retained austenite may be 0% by area. However, if a remaining structure is present, the remaining structure may include at least one of ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less. If the total area fraction of at least one of ferrite, bainite, and pearlite exceeds 10.0%, the area fraction of martensite will be less than 90.0%, resulting in failure to achieve the desired strength and hole expandability. The lower limits of ferrite, bainite, and pearlite may each be 0%, or may be, for example, 0.1% or more, 0.5% or more, 1.0% or more, 2.0% or more, or 3.0% or more, respectively. Similarly, the upper limits of ferrite, bainite, and pearlite may each be 8.0% or less, 6.0% or less, 5.0% or less, or 4.0% or less, respectively.

[0034] [Identification of Metallic Structure and Calculation of Area Ratio] Identification of the metallic structure of a steel sheet and calculation of its area ratio are performed by optical microscope observation and X-ray diffraction method after corrosion using a Nital reagent or a Repeller solution. The microstructure observation using an optical microscope is performed on a plate thickness cross section perpendicular to the plate surface. The plate thickness cross section is preferably parallel to the rolling direction. Specifically, first, a sample is taken from the steel sheet, and the observation surface of the sample is etched with Nital. Next, image analysis is performed on a microstructure photograph obtained using an optical microscope with a 300 μm × 300 μm field at a depth of 1 / 4 of the plate thickness to calculate the total area ratio of martensite and bainite, as well as the area ratios of ferrite and pearlite. Next, using a sample whose observation surface has been subjected to Repeller corrosion, image analysis is similarly performed on a microstructure photograph obtained using an optical microscope with a 300 μm × 300 μm field at a depth of 1 / 4 of the plate thickness to calculate the total area ratio of martensite and retained austenite. Next, using a sample that has been face-milled from the normal direction of the rolled surface to a depth of 1 / 4 of the plate thickness, the volume fraction of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of retained austenite is equivalent to the area fraction, this is taken as the area fraction of retained austenite. The area fraction of martensite is calculated by subtracting the obtained area fraction of retained austenite from the previously calculated total area fraction of martensite and retained austenite. Finally, the area fraction of bainite is calculated by subtracting the obtained area fraction of martensite from the previously calculated total area fraction of martensite and bainite.

[0035] [Average grain size of prior austenite grains: 30.0 μm or less] In the steel sheet according to the embodiment of the present invention, the average grain size of the prior austenite grains is 30.0 μm or less. As described above, the prior austenite grain boundaries act as a resistance force against dislocation motion and are considered to be effective in improving work hardening capacity. In this regard, by refining the prior austenite grains, the density of the prior austenite grain boundaries can be increased. Therefore, by refining the prior austenite grains to 30.0 μm or less, it is possible to increase the obstacles to dislocations, and therefore it is possible to improve the work hardening capacity of the resulting steel sheet. From the viewpoint of further improving the work hardening capacity of the steel sheet, the smaller the average grain size of the prior austenite grains, the more preferable it is, and it may be, 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. Although there is no particular lower limit, the average grain 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] [Proportion of Prior Austenite Grains with Aspect Ratios of 2.0 or Less to All Prior Austenite Grains: 90.0% or More in Area %] In the metallographic structure of the steel plate according to the embodiment of the present invention, the proportion of prior austenite grains with aspect ratios of 2.0 or less to all prior austenite grains is not particularly limited, but may be, for example, 90.0% or more, 92.0% or more, 94.0% or more, or 96.0% or more in area %. Increasing the proportion of such prior austenite grains with a relatively small aspect ratio can reduce the anisotropy of the metallographic structure. While the upper limit is not particularly limited, for example, the proportion of prior austenite grains with aspect ratios 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 has high strength but has improved hole expandability and work hardenability. The objective is achieved by forming the metallographic structure of a steel sheet having a predetermined chemical composition from a structure mainly composed of martensite, and by limiting the average grain size of prior austenite grains in the metallographic structure to within a predetermined range while appropriately controlling the proportion of crystal grains having a predetermined intragranular misorientation in the martensite structure. 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 Grain Size of Prior Austenite Grains and the Proportion of Prior Austenite Grains with an Aspect Ratio of 2.0 or Less to All Prior Austenite Grains] The average grain size of prior austenite grains and the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains are determined as follows. First, a sample is cut from any position 50 mm or more away from the end face of the steel sheet (if a sample cannot be taken from this position, a position avoiding the end) so that a thickness cross section perpendicular to the sheet surface can be observed. The thickness cross section is preferably parallel to the rolling direction. The size of the sample depends on the measuring device, but it should be large enough to observe approximately 10 mm in a direction perpendicular to the thickness direction. The cross section of the sample is polished using #600 to #1500 silicon carbide paper, and then finished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. Next, the observation surface is finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, at a depth of 1 / 4 of the plate thickness, a region of 50 μm in length and 50 μm in the plate thickness direction is measured at measurement intervals of 0.1 μm by electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used, for example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. In this case, the degree of vacuum inside the EBSD analyzer is 9.6 × 10 -5The pressure may be 1 Pa or less, the acceleration voltage may be 15 kV, and the probe current level may be 13. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and crystal grains having a body-centered structure after transformation. The following method is used to calculate the crystal orientation of the prior austenite grains. First, a crystal orientation map of the prior austenite grains is created using the method described in Acta Materialia, 58 (2010), 6393-6403. For one of the prior austenite grains included in the observation field of view, the average value of the shortest diameter and the longest diameter is calculated, and this average value is used as the grain size of the prior austenite grain. The above operation is performed for all prior austenite grains, excluding prior austenite grains whose entirety is not included in the observation field of view, such as those at the ends of the observation field of view, to determine the grain sizes of all prior austenite grains in the observation field of view. The average grain size of the prior austenite grains is determined by calculating the average grain size from the grain sizes of all the obtained prior austenite grains.

[0038] Next, for one of the prior austenite grains included in the above 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 the prior austenite grain. When the rolling direction is unknown, cross sections are observed at angles of 0°, 45°, 90°, and 135° relative to an arbitrary direction, and the cross section with the highest aspect ratio among them 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. The above operation is performed for all prior austenite grains, excluding prior austenite grains whose entire crystal grains are not included in the photographed field, such as the edges of the photographed field, to determine the aspect ratios of all prior austenite grains in the photographed field. The proportion of prior austenite grains whose aspect ratios are 2.0 or less to all prior austenite grains is determined by dividing the total number of prior austenite grains whose aspect ratios are 2.0 or less by the number of all prior austenite grains.

[0039] [Proportion of Crystal Grains in Martensite Having an Intragranular Misorientation of 4° or More: 45.0 to 70.0% by Area] In the martensite structure of the steel sheet according to the present invention, when a crystal grain is defined as a region surrounded by grain boundaries having a misorientation of 15° or more, the proportion of crystal grains having an intragranular misorientation of 4° or more is controlled within a range of 45.0 to 70.0% by area. As described above, the more non-uniformly dispersed dislocations are, the larger the intragranular misorientation generally becomes. Crystal grains having an intragranular misorientation of 4° or more are crystal grains with a sufficient misorientation to form a structure in which dislocations are non-uniformly dispersed and develop non-uniform deformation. In this regard, in an embodiment of the present invention, by having such crystal grains present in the martensite within a range of 45.0 to 70.0% by area, a certain amount of a structure in which dislocations are non-uniformly dispersed is present, thereby developing 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, and therefore a high work hardening rate can be achieved. If the proportion of crystal grains with an intragranular misorientation of 4° or more is less than 45.0% or more than 70.0% by area, non-uniform deformation is unlikely to occur during processing such as press forming, making it impossible to achieve the desired work-hardening rate. The proportion of crystal grains with an intragranular misorientation of 4° or more in the martensitic structure may be, for example, 48.0% or more, 50.0% or more, or 55.0% or more by area. Similarly, the proportion of crystal grains with an intragranular misorientation of 4° or more in the martensitic structure may be, for example, 65.0% or less, 62.0% or less, or 60.0% or less by area.

[0040] [Method for determining the proportion of crystal grains in martensite having an intragranular misorientation of 4° or more] The proportion of crystal grains in martensite having an intragranular misorientation of 4° or more is measured by electron backscattered diffraction (EBSD). More specifically, a sample is first taken from the steel sheet so that the observation surface is the thickness cross section perpendicular to the sheet surface. Next, at a depth of ¼ of the sheet thickness from the steel sheet surface, a region of 200 μm in the direction perpendicular to the sheet thickness direction and 100 μm in the sheet thickness direction is analyzed by EBSD at a measurement interval of 0.2 μm to obtain crystal orientation information. Here, the EBSD analysis is performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) at an analysis speed of 50 to 300 points per second. At this time, the degree of vacuum in the EBSD analyzer is 9.6 × 10 -5The EBSD analysis may be performed at a pressure of 100 Pa or less, an acceleration voltage of 15 kV, and a probe current level of 13. Next, the obtained crystal orientation information is used to identify the martensite structure using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. Next, in the identified martensite structure, regions surrounded by grain boundaries with a misorientation of 15° or more are defined as crystal grains, the average misorientation within the crystal grains is calculated, and the percentage of crystal grains with a misorientation within the grains of 4° or more is determined. The crystal grains and the average misorientation within the grains defined above can also be calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. In the present invention, the "misorientation within the grain" refers to the "grain orientation spread (GOS)," which is the orientation dispersion within the crystal grains. The value of the misorientation within a grain is determined as the average value of the misorientation between a reference crystal orientation and all measurement points within the same crystal grain, as described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD and X-ray Diffraction Methods," Kimura Hidehiko et al., Transactions of the Japan Society of Mechanical Engineers (Part A), Vol. 71, No. 712, 2005, pp. 1722-1728. In an embodiment of the present invention, the reference crystal orientation is the average orientation of all measurement points within the same crystal grain. The GOS value can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" provided with the EBSD analyzer.

[0041] [Thickness] The steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 8.0 mm, although not particularly limited thereto. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

[0042] The steel sheet according to the embodiment of the present invention can reliably achieve the contradictory properties of high strength and excellent formability, and is useful for use in parts in technical fields where both properties are required, particularly in parts in the automotive field. Therefore, in a preferred embodiment, an automotive part, particularly an automotive suspension part, including the steel sheet according to the embodiment of the present invention is provided. Examples of automotive suspension parts include lower arms and trailing arms. These automotive parts, particularly automotive suspension parts, may contain the steel sheet according to the embodiment of the present invention in at least a portion thereof, and therefore, at least a portion of these parts satisfy the above-described chemical composition and structural characteristics. In a portion of a steel sheet that has been processed relatively lightly in a forming process such as press forming, the characteristics of the steel sheet do not change significantly before and after forming. A portion of a steel sheet that has been processed relatively lightly is determined by characteristics such as a smooth shape that has not been subjected to deformation such as bending, a small rate of change in sheet thickness, etc.

[0043] [Mechanical Properties] [Tensile Strength: TS] Steel sheets having the above-described chemical composition and metallographic structure, particularly hot-rolled steel sheets, can achieve high tensile strength, specifically tensile strength of 980 MPa or more. The tensile strength is preferably 1000 MPa or more, 1080 MPa or more, or 1180 MPa or more. Despite having such extremely high tensile strength, steel sheets according to embodiments of the present invention can achieve excellent hole expandability and work hardening capacity due to the specific combination of chemical composition and metallographic structure described above. The upper limit of tensile strength is not particularly limited, but the tensile strength of the steel sheet may be, for example, 1780 MPa or less, 1700 MPa or less, or 1600 MPa or less. Tensile strength is measured by taking a JIS No. 5 test piece in a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel sheet (C direction) and conducting a tensile test in accordance with JIS Z 2241:2011. For example, if it is difficult to obtain 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, the lower limit is set to 0.5 mm to ensure appropriate evaluation. For example, if it is difficult to obtain 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 can be performed in accordance with JIS 2244-1:2020, and the hardness (HV) converted into tensile strength can be used. Samples for the micro-Vickers test can be prepared in the same manner as samples for evaluating the average grain size and aspect ratio of prior austenite grains. The micro-Vickers test can be performed by measuring 30 points at 1 / 4 the plate thickness with a load of 500 gf 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] Steel sheets having the above chemical composition and metallographic structure can achieve high hole expandability, specifically, a hole expansion ratio of 45% or more. The hole expansion ratio may be preferably 50% or more, more preferably 60% or more or 70% or more. The upper limit of the hole expansion ratio is not particularly limited, but may be, for example, 150% or less, 120% or less, or 100% or less. The hole expansion ratio is determined as follows. First, a test piece 100 mm wide x 100 mm long 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 using a conical punch with an apex angle of 60° until a crack penetrating the plate thickness occurs. The hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) of each test piece is calculated using the following formula: This hole expansion test is carried out three times, and the average value is determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

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

[0046] A method for producing a steel plate according to an embodiment of the present invention comprises: a heating step of heating a slab having the chemical composition described above in relation to the steel plate and holding the slab in a temperature range of 1100°C or higher for 6000 seconds or longer; a hot rolling step including finish rolling the slab, wherein the finish rolling satisfies the following conditions (a) to (c): (a) the rolling reduction in the rolling pass one stage before the final stage is 30 to 50%, and the rolling reduction in the final rolling pass is 20 to 50%, (b) the total rolling reduction is 90% or more, and (c) the rolling temperature in the rolling pass one stage before the final stage is 970 to 1100°C, and the final rolling temperature is 960 to 1050°C; a cooling step of starting cooling of the finish-rolled steel plate within 0.5 to 10.0 seconds after completion of the hot rolling step, and then cooling the steel plate to a temperature of 400°C or lower within 20.0 seconds from the start of cooling; The method is characterized by including: a winding step in which the cooled steel sheet is wound in a temperature range of 400°C or less; and a strain imparting step in which the obtained steel sheet is imparted with a strain of 0.16 to 1.40% in absolute value at a quarter-thickness position of the steel sheet by alternating positive and negative strains three or more times. In the above manufacturing method, the temperatures described for the slab and steel sheet refer to the surface temperatures of the slab and steel sheet, respectively. Each step will be described 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. However, a slab obtained by casting and blooming can also be used, and if necessary, a slab obtained by hot working or cold working may be used. In this manufacturing method, holding at a temperature of 1100°C or higher includes not only holding the slab at a constant temperature of 1100°C or higher, but also holding the slab at a fluctuating temperature within the temperature range of 1100°C or higher. Holding the slab at a temperature of 1100°C or higher for 6000 seconds or more allows the coarse carbides present in the structure to be completely dissolved, thereby eliminating the initiation points of cracks. If the holding temperature is lower than 1100°C or the holding time is shorter than 6000 seconds, the coarse carbides will not be completely dissolved. If the solid solution of coarse carbides is incomplete, ferrite or bainite transformation originating from such carbides occurs during the cooling process described below, resulting in an area ratio of martensite of less than 90.0%, making it impossible to obtain the desired strength and / or hole expandability. The upper limit of the heating temperature of the slab is preferably 1300°C or less or 1200°C or less. Similarly, the upper limit of the holding time in the temperature range of 1100°C or more is preferably 10,000 seconds or less.

[0048] [Hot Rolling Step] [Rough Rolling] In the present manufacturing method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0049] [(a) Reduction in the rolling pass one stage before the final stage: 30 to 50%, and reduction in the rolling pass at the final stage: 20 to 50%] The heated slab, or the slab that has been subjected to rough rolling as needed, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a 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 in each of the rolling passes at the last two stages. Specifically, the reduction in the rolling pass one stage before the final stage is controlled to 30 to 50%, and the reduction in the rolling pass at the final stage is controlled to 20 to 50%. By performing rolling with such a relatively high reduction rate in each rolling pass at the stage before the final stage and at the final stage, recrystallization can be promoted to refine the metallographic structure, and the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains can also be increased. If the reduction rate in the rolling pass at the stage before the final stage is less than 30% and / or the reduction rate in the rolling pass at the final stage is less than 20%, 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 metallographic structure of the steel sheet obtained in the end, 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 rate in each rolling pass at the stage before the final stage and / or at the final stage is too high, the rolling load becomes excessive, increasing the load on equipment such as a rolling mill. For this reason, the rolling reduction in each rolling pass of the next to last stage and the final stage is set to 50% or less, and preferably, the rolling reduction in each rolling pass of the next to last stage and the final stage is set to 45% or less.

[0050] [(b) Total Reduction: 90% or More] In this manufacturing method, the total reduction in finish rolling is controlled to 90% or more. Since Mn contained in steel is an element that reduces the fracture energy of grain boundaries, the presence of regions where Mn is locally concentrated may promote crack generation during plastic deformation in press forming, etc. Therefore, from the perspective of further improving hole expandability, it is effective to suppress or reduce the local concentration of Mn. By controlling the total reduction in finish rolling to 90% or more, Mn can be diffused into the steel, and in this regard, the variation in Mn concentration in the steel can be suppressed or reduced, that is, the local concentration of Mn can be suppressed or reduced. If the total reduction in finish rolling is less than 90%, the variation in Mn concentration becomes relatively high, and it may not be possible to sufficiently suppress the development of regions where Mn is locally concentrated and the fracture energy is reduced. In addition, if the total reduction in finish rolling is less than 90%, the accumulated strain during rolling will be insufficient, and recrystallization will not be completed or will not be sufficiently promoted, which may result in the failure to achieve the desired average grain size of prior austenite grains in the metal structure of the finally obtained steel sheet, and / or the proportion of prior austenite grains having an aspect ratio of 2.0 or less may become relatively small. The upper limit of the total reduction in finish rolling may be, for example, 99% or less or 98% or less. Here, the total reduction in finish rolling is calculated by the following formula: Total reduction (%) = (sheet thickness before finish rolling - sheet thickness after finish rolling) / sheet thickness before finish rolling × 100

[0051] [(c) Rolling temperature in the rolling pass one stage before the last: 970 to 1100°C, final rolling temperature: 960 to 1050°C] In this manufacturing method, in addition to controlling the reduction ratio in each of the last two rolling passes of the finish rolling, the rolling temperature in the rolling pass one stage before the last (the entry temperature of the rolling pass one stage before the last) and the final rolling temperature (the end temperature of finish rolling) are also very important in controlling the metallographic structure of the steel sheet. If the rolling temperature in the rolling pass one stage before the last 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 metallographic structure of the steel sheet obtained in the end, and / or the proportion of prior austenite grains having 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 may not be obtained. On the other hand, if the rolling temperature in the penultimate rolling 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, it is also naturally impossible to obtain sufficient work hardening ability.

[0052] [Cooling Step] [Time from completion of hot rolling step to start of cooling: 0.5 to 10.0 seconds] [Time from start of cooling to reach 400°C or less: 20.0 seconds or less] Cooling of the finish-rolled steel sheet is started in the next cooling step within 0.5 to 10.0 seconds after completion of the hot rolling step, and then cooled to a temperature of 400°C or less within 20.0 seconds from the start of cooling. By controlling cooling in this way, it is possible to achieve a desired average grain size of prior austenite grains and a desired proportion of prior austenite grains having an aspect ratio of 2.0 or less in the metallographic structure of the finally obtained 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 may not be sufficiently promoted, resulting in the failure to achieve the desired average grain size of prior austenite grains in the metallographic structure of the final steel sheet, and / or the failure to obtain the desired proportion of prior austenite grains with an aspect ratio of 2.0 or less. Furthermore, if the time from the completion of the hot rolling process to the start of cooling exceeds 10.0 seconds, grain growth proceeds too much, making it impossible to obtain the desired average grain size of prior austenite grains. As a result, in either case, the steel sheet cannot achieve sufficient work hardening capacity. On the other hand, if the cooling time from the start of cooling to 400°C or below exceeds 20.0 seconds or the cooling stop temperature exceeds 400°C, ferrite, bainite, and / or pearlite are formed during cooling, resulting in an area fraction of martensite of less than 90.0%, and as a result, the desired strength and / or hole expandability cannot be obtained.

[0054] [Coiling process] Next, the cooled steel sheet is coiled in a temperature range of not more than 400° C. If the coiling temperature exceeds 400° C., as in the cooling process, ferrite, bainite and / or pearlite are generated during coiling, and the area ratio of martensite becomes less than 90.0%, and as a result, the desired strength and / or hole expandability cannot be obtained.

[0055] [Straining Process] Finally, the resulting steel sheet is subjected to a strain of 0.16 to 1.40% absolute value at a quarter-thickness position of the steel sheet, alternating positive and negative strains three or more times to produce the steel sheet. Repeated application of strain to the steel sheet allows dislocations to be introduced non-uniformly into the martensitic structure grains, thereby controlling the proportion of grains with an intragranular misorientation of 4° or more within a range of 45.0 to 70.0% by area. If the absolute value of the applied strain is less than 0.16% or the number of times strain is applied is less than three, the non-uniform introduction of dislocations into the grains is insufficient, resulting in a proportion of grains with an intragranular misorientation of 4° or more being less than 45.0% by area, making it impossible to achieve the desired work hardening rate. On the other hand, if the absolute value of the applied strain exceeds 1.40%, dislocations will accumulate excessively in the crystal grains, and the proportion of crystal grains with an intragranular misorientation of 4° or more will exceed 70.0% in terms of area %, making it impossible to achieve the desired work-hardening rate. There is no particular upper limit on the number of times strain is applied, but from the viewpoint of manufacturability, it is preferably 10 times or less, 7 times or less, or 5 times or less.

[0056] Because martensite structures inherit dislocations within austenite grains before transformation, generally, the crystal grains of a martensite structure formed from unrecrystallized austenite grains have a high intragranular misorientation value, while the crystal grains of a martensite structure formed from recrystallized austenite grains have a low intragranular misorientation value. In the present manufacturing method, as described above, the structure is refined by promoting recrystallization in 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, because martensite is formed from recrystallized austenite grains, the crystal grains of the martensite structure have a low intragranular misorientation value, and it becomes impossible to achieve an area percentage of 45.0 to 70.0%. Therefore, in the present manufacturing method, the conditions of the hot rolling process are appropriately controlled to produce a prior austenite structure in which recrystallization is almost or completely completed, thereby refining the structure and improving the work hardening capacity of the steel sheet as a whole, and by introducing dislocations unevenly into some of the crystal grains of the martensite structure in the strain imparting process to appropriately generate misorientation, it becomes possible to achieve a high work hardening rate even in a state in which a certain degree of strain has been introduced, such as in the later deformation stage of press forming.

[0057] The strain imparting step can be performed by any appropriate method known to those skilled in the art. Examples of such methods include, but are not limited to, bending and unbending deformation using a tension leveler or the like. In this case, the absolute value of the strain to be imparted can be easily changed by changing the size of the rolls in the tension leveler, the relative positional relationship between the rolls, and the thickness of the steel sheet. Therefore, by appropriately controlling these parameters, it is possible to easily impart a desired strain at the 1 / 4 position of the steel sheet thickness.

[0058] Steel sheets manufactured by the above manufacturing method have a more uniform metal structure containing, by area percentage, 90.0% or more martensite and 3.0% or less retained austenite, thereby achieving high strength, for example, a tensile strength of 980 MPa or more, while significantly improving hole expandability due to reduced hardness differences. Furthermore, by controlling the average grain size of prior austenite grains in the metal structure to 30.0 μm or less, the work-hardening capacity of the steel sheet as a whole is improved, and by controlling the proportion of martensite grains with an intragranular misorientation of 4° or more 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 a state where a certain degree of strain is introduced, such as in the later stages of deformation in press forming. Therefore, steel sheets manufactured by the above manufacturing method can reliably achieve the contradictory properties of high strength and excellent workability, making them particularly useful in the automotive field, where both of these 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 to these examples in any way.

[0060] In the following examples, steel sheets according to the embodiments of the present invention, particularly hot-rolled steel sheets, were produced under various conditions, and the tensile strength (TS), hole expansion ratio (λ) and work hardening rate (WHR) of the obtained steel sheets were investigated.

[0061] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Tables 1 and 2. These slabs were heated to a temperature of 1100 to 1200°C and held for the times shown in Table 3, and then hot-rolled. Hot-rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling conditions were the same for all Examples and Comparative Examples, and finish rolling was performed under the conditions shown in Table 3 using a tandem rolling mill consisting of five rolling stands. Next, the finish-rolled steel sheet was cooled and coiled under the conditions shown in Table 3. Finally, the absolute value of the strain shown in Table 3 was repeatedly changed between positive and negative at a 1 / 4 position of the steel sheet thickness a predetermined number of times, thereby obtaining a steel sheet having a thickness shown in Table 4.

[0062]

[0063]

[0064]

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

[0066] [Tensile Strength (TS)] The tensile strength (TS) was measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel plate (C direction) 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 test piece having a width of 100 mm and a length of 100 mm was taken from the steel plate, and a punching tool having a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%) was used to create a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the burr was placed on the die side, and the initial hole was expanded with a conical punch having an apex angle of 60 ° until a crack penetrating the plate thickness occurred. The hole diameter d1 mm at the time of crack occurrence was measured, and the hole expansion ratio λ (%) of 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 Capacity] Work hardening capacity was evaluated by determining the work hardening rate (WHR) of the steel sheet from a tensile test. Specifically, the region where the strain during tensile deformation (true strain) during the same tensile test as in the TS measurement was 0.04 or more was simulated as the state in the later stage of deformation during press forming, and the maximum work hardening rate (WHR) in this region was calculated using the following formula: WHR (MPa) = dσ / dε, where σ is the true stress and ε is the true strain.

[0069] When the tensile strength (TS) of the steel sheet is 980 MPa or more, the hole expansion ratio (λ) is 45% or more, and the maximum work hardening rate (WHR) in the region where the true strain is 0.04 or more is 1000 MPa or more, the steel sheet was evaluated as having improved hole expandability and work hardening ability despite its high strength. The results are shown in Table 4.

[0070]

[0071] With reference to Tables 1 to 4, in Comparative Example 2, the holding time in the temperature range of 1100°C or higher in the heating step was short, which presumably resulted in incomplete solid solution of coarse carbides, and ferrite and bainite transformations, etc., originating from these carbides during the subsequent cooling step. As a result, the area ratio of martensite was less than 90.0%, resulting in reduced TS and λ. In Comparative Examples 3 and 6, the reduction rates in the rolling passes one stage before the final stage and the final stage in the finish rolling were low, respectively, which presumably resulted in incomplete or insufficient promotion of recrystallization. As a result, the average grain size of prior austenite grains in the final metallographic structure increased, resulting in reduced work hardening capacity of the steel sheet. In Comparative Examples 4 and 7, the rolling temperatures in the rolling passes one stage before the final stage and the final stage in the finish rolling were low, resulting in incomplete or insufficient promotion of recrystallization. As a result, the average grain size of prior austenite grains in the final metallographic structure increased, resulting in reduced work hardening capacity of the steel sheet. In Comparative Examples 5 and 8, the rolling temperatures in the rolling pass one stage before the final stage in the finish rolling and the final rolling temperature were high, respectively, which is thought to have caused the prior austenite grains to become coarse overall. As a result, the average grain size of the prior austenite grains in the final metallographic structure became large, and the work-hardening ability of the steel sheet decreased. In Comparative Example 9, the time from the completion of the hot rolling process to the start of the cooling process was short, which is thought to have prevented recrystallization from being completed or sufficiently promoted. As a result, the average grain size of the prior austenite grains in the final metallographic structure became large, and the work-hardening ability of the steel sheet decreased. In Comparative Example 10, the time from the completion of the hot rolling process to the start of the cooling process was long, which is thought to have caused grain growth to progress excessively 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 400°C or below in the cooling process was long, which is thought to have caused the martensite area ratio to be less than 90.0%, resulting in decreased TS and λ. In Comparative Example 12, the coiling temperature was high, so the area ratio of martensite was also less than 90.0%, and TS and λ decreased.In Comparative Example 13, the absolute value of the strain imparted in the strain imparting step 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 in martensite with an intragranular misorientation of 4° or more was reduced, and the work-hardening rate of the steel sheet was reduced. In Comparative Example 14, the absolute value of the strain imparted in the strain imparting step 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 in martensite with an intragranular misorientation of 4° or more was increased, and the work-hardening rate of the steel sheet was reduced. In Comparative Example 15, the number of times strain was imparted in the strain imparting step 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 in martensite with an intragranular misorientation of 4° or more was reduced, and the work-hardening rate of the steel sheet was reduced. In Comparative Example 16, the strain imparting step was omitted, 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 in martensite with an intragranular misorientation of 4° or more decreased, and the work hardening rate of the steel sheet decreased.

[0072] In Comparative Examples 37 and 39, TS decreased due to low C and Si contents, respectively. On the other hand, in Comparative Examples 38 and 40, C and Si contents were high, respectively, resulting in the formation of relatively large amounts of retained austenite, and λ decreased. In Comparative Example 41, hardenability decreased due to the low Mn content, which resulted in a low area fraction of martensite and a decrease in TS. In Comparative Example 42, λ decreased due to the high Mn content. In Comparative Example 43, it is believed that the low sol. Al content did not sufficiently suppress cementite precipitation. As a result, λ decreased. In Comparative Example 44, it is believed that the low Nb content did not sufficiently promote the 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, and the work hardening ability of the steel sheet decreased. In Comparative Example 45, it is believed that the high Nb content caused the formation of coarse carbides and the like in the steel. As a result, λ decreased.

[0073] In contrast, all of the steel sheets according to the invention have a predetermined chemical composition. By appropriately controlling the manufacturing conditions, the steel sheets have a metal structure containing, by area %, 90.0% or more martensite and 3.0% or less retained austenite, with an average grain size of 30.0 μm or less and a martensite grain ratio of 45.0 to 70.0% by area. As a result, despite the high tensile strength of 980 MPa or more, the hole expandability and work hardening capacity were significantly improved.

Claims

1. The chemical composition, in mass%, is C: 0.040-0.200%, Si: 0.30-2.00%, Mn: 1.00-4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001-1.000%, 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 to 0.0100%, and The balance is Fe and impurities. The metal structure is, in 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, When a region surrounded by grain boundaries in the martensite having an orientation mismatch of 15° or more is defined as a crystal grain, the proportion of the crystal grains having an intragranular orientation mismatch of 4° or more is 45.0 to 70.0% in terms of area %.

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

3. The metal structure further comprises, in area %, Ferrite: 10.0% or less, Bainite: 10.0% or less, and Perlite: 10.0% or less The steel sheet according to claim 1 or 2, characterized in that it contains at least one of the following:

4. 3. The steel sheet according to claim 1, wherein the proportion of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains is 90.0% or more in terms of area %.

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 in that it comprises a steel sheet according to claim 1 or 2.