Steel sheet, component including same, and method for manufacturing steel sheet

Abstract

The present invention provides: a steel sheet which has excellent impact resistance characteristics after molding despite having a high strength; a component which includes the steel sheet; and a method for manufacturing the steel sheet. Specifically provided is steel sheet that is characterized by having a predetermined chemical composition and having, at 1 / 2 the sheet thickness from the surface, a metal structure which contains, in area%, a total of 30-85% of acicular bainite, tempered martensite, fresh martensite, and retained austenite, wherein: the grain size of prior austenite grains is 200 µm or less; the standard deviation of Mn concentration is 0.40 mass% or less; and the boundary density at which the crystal misorientation among the acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less is 0.35 / µm or more. Also specifically provided are: a component which includes the steel sheet; and a method for manufacturing the steel sheet.

Description

Steel plate, parts containing the same, and method for manufacturing steel plate

[0001] This invention relates to steel plates, parts containing the same, and methods for manufacturing steel plates.

[0002] In recent years, the automotive industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. To achieve both vehicle weight reduction and collision safety, increasing the strength of the steel plates used is one effective method, and for this reason, the development of high-strength steel plates is progressing.

[0003] On the other hand, many automotive parts are manufactured by press forming. It is generally known that increasing the strength of a steel sheet reduces its formability and toughness.

[0004] In this regard, Patent Document 1 discloses a hot-rolled steel sheet having a predetermined chemical composition, and in the central part of the steel sheet thickness, which is a portion of the steel sheet partitioned at the 3 / 8 thickness position and the 5 / 8 thickness position from the surface of the steel sheet, the average value of the X-ray random intensity ratio of the {100}<011> to {223}<110> orientation group on the sheet surface is 6.5 or less, and the X-ray random intensity ratio of the {332}<113> crystal orientation is 5.0 or less, and the total area ratio of tempered martensite, martensite and lower bainite is more than 85%, and the microstructure has an average grain size of 12.0 μm or less. Patent Document 1 teaches that according to the above configuration, a high-strength steel sheet with excellent stretch flangeability and low-temperature toughness can be provided.

[0005] Patent Document 2 describes a material having a predetermined chemical composition, containing either tempered martensite or lower bainite, or both, in a total volume fraction of 90% or more, and having a dislocation density of 5 × 10⁻⁶ in the martensite and lower bainite. １３ (1 / m ２ ) 1 x 10 １６ (1 / m ２ A high-strength hot-rolled steel sheet having a structure of the following degree and a maximum tensile strength of 980 MPa or more is disclosed. Patent Document 2 teaches that, according to the above configuration, a high-strength steel sheet with a maximum tensile strength of 980 MPa or more and excellent bake-hardenability and low-temperature toughness can be provided.

[0006] Patent Document 3 discloses a hot-rolled steel sheet having a predetermined chemical composition, a structure in which the combined volume fraction of tempered martensite and lower bainite is 90% or more, an average effective grain size of 10 μm or less in the portion from the surface to 1 / 4 of the sheet thickness, and an average effective grain size of 6 μm or less in the portion from the surface to 50 μm. Patent Document 3 teaches that with the above configuration, it is possible to provide a hot-rolled steel sheet with excellent fatigue strength and low-temperature toughness, and a maximum tensile strength of 980 MPa or more.

[0007] Patent Document 4 describes a material having a predetermined chemical composition, with a microstructure comprising, in area percent, ferrite: 1-29%, retained austenite: 5-20%, martensite: less than 10%, pearlite: less than 5%, and the remainder being bainite and / or tempered martensite, where the total length of the interface between ferrite and martensite or retained austenite with an equivalent circular radius of 1 μm or more is 1000 μm. ２ Cold-rolled steel sheets with a thickness of 100 μm or less per unit area are disclosed. Patent Document 4 teaches that, according to the above configuration, it is possible to provide high-strength cold-rolled steel sheets and high-strength hot-dip galvanized cold-rolled steel sheets that are excellent in workability and low-temperature toughness, and in particular excellent in low-temperature toughness after the introduction of plastic strain.

[0008] Patent Document 5 discloses a high-strength hot-rolled steel sheet having a predetermined chemical composition, with a bainite phase comprising 85% or more area, a martensite phase or martensite-austenite mixed phase comprising 15% or less area, and the remainder being a ferrite phase, the average particle size of the second phase being 3.0 μm or less, the average aspect ratio of prior austenite grains being 1.3 or more and 5.0 or less, the area ratio of recrystallized prior austenite grains to unrecrystallized prior austenite grains being 15% or less, and the amount of precipitates with a diameter of less than 20 nm precipitated in the hot-rolled steel sheet being 0.10% or less by mass, and a tensile strength TS of 980 MPa or more. Patent Document 8 teaches that with the above configuration, a high-strength hot-rolled steel sheet can be obtained with a tensile strength TS of 980 MPa or more and excellent punchability and hole-expanding properties.

[0009] Patent Document 6 discloses a steel sheet having a metallic structure characterized by having a predetermined chemical composition and an area fraction of ferrite: 50% to 95%, granular bainite: 5% to 48%, tempered martensite: 2% to 30%, upper bainite, lower bainite, fresh martensite, retained austenite, and pearlite: 5% or less in total, and the product of the area fraction of tempered martensite and the Vickers hardness of tempered martensite: 800 to 10500. Patent Document 9 teaches that with the above configuration, high strength, excellent elongation, and hole-expanding properties can be obtained because granular bainite and the like are included in the metallic structure in an appropriate area fraction.

[0010] International Publication No. 2013 / 103125, International Publication No. 2014 / 132968, International Publication No. 2014 / 188966, International Publication No. 2018 / 179386, International Publication No. 2017 / 017933, International Publication No. 2018 / 142450

[0011] In automotive parts, improved impact resistance is desired from the perspective of ensuring occupant safety. For this reason, steel sheets used as materials for automotive parts, for example, are required to exhibit high impact resistance after being formed by press forming or other methods, so that they are less likely to break even when subjected to impacts such as those caused by car collisions.

[0012] Therefore, the present invention aims to provide a steel sheet that is high in strength and has excellent impact resistance after molding, a part containing the same, and a method for manufacturing the steel sheet.

[0013] To achieve the above objective, the inventors focused on and investigated the microstructure of steel sheets, particularly hot-rolled steel sheets. Specifically, the inventors first found that by configuring the microstructure of a steel sheet having a predetermined chemical composition so that it contains a specific proportion of acicular bainite, tempered martensite, fresh martensite, and retained austenite at a position half the thickness from the surface of the steel sheet, it is possible to improve the impact resistance after forming while achieving the desired high strength. Furthermore, the inventors found that by controlling the particle size of prior austenite grains to 200 μm or less, controlling the standard deviation of the Mn concentration to 0.40 mass% or less, and controlling the boundary density where the crystal orientation difference between acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less to 0.35 / μm or more, it is possible to significantly improve the impact resistance after forming while maintaining high strength, thus completing the present invention.

[0014] The present invention that achieves the above object is as follows. (1) In mass%: C: 0.040 to 0.180%, Si: 0.20 to 2.00%, Mn: 1.00 to 3.00%, Cr: 0.500 to 1.000%, Ti: 0.020 to 0.150%, B: 0.00003 to 0.00150%, sol.Al: 0.001 to 0.100%, P: 0.1000% or less, S: 0.050% or less, N: 0.0150% or less, O: 0.0060% or less, Mo: 0 to 0.500%, W: 0 to 0.500%, Co: 0 to 3.000%, Nb: 0 to 0.150%, V: 0 to 0.500%, Ta: 0 to 0.100%, Sn: 0 to 0.100%, Sb: 0 to 0.500%, As: 0 to 0.100%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Ca: 0 to 0.0500%, Zr: 0 to 0.500%, Mg: 0 to 0.0500%, REM: 0 to 0.100%, Bi: 0 to 0.100%, and the balance: has a chemical composition consisting of Fe and impurities, at a position 1 / 2 of the plate thickness from the surface, in area%: acicular bainite, tempered martensite, fresh martensite, and retained austenite: 30 to 85% in total, the grain size of prior austenite grains is 200 µm or less, the standard deviation of Mn concentration is 0.40 mass% or less, and the steel sheet has a metal structure in which the boundary density at which the crystal orientation difference in acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less is 0.35 / µm or more.(2) The above chemical composition is as follows, in mass%, Mo: 0.001 to 0.500%, W: 0.001 to 0.500%, Co: 0.001 to 3.000%, Nb: 0.001 to 0.150%, V: 0.001 to 0.500%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.500%, As: 0.001 to 0.100%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Ca: 0.0001 to 0.0500%, Zr: 0.001 to 0.500%, (1) The steel sheet according to (1) above, characterized in that it contains at least one of the following: Mg: 0.0001 to 0.0500%, REM: 0.0001 to 0.100%, and Bi: 0.001 to 0.100%. (3) The steel sheet according to (1) or (2) above, characterized in that, at a position 1 / 4 of the sheet thickness from the surface, it has a metallic structure containing, in area percent, acicular bainite, tempered martensite, fresh martensite, and retained austenite: 30 to 70% in total, and granular bainite: 30 to 50%. (4) The steel sheet according to any one of (1) to (3) above, characterized in that it has a tensile strength of 980 MPa or more. (5) The steel sheet according to any one of (1) to (4) above, characterized in that it has a sheet thickness of 1.0 to 8.0 mm. (6) A part characterized in that it contains the steel sheet according to any one of (1) to (5) above.(7) A hot rolling process that includes heating a slab having the chemical composition described in (1) or (2) above, followed by rough rolling and finish rolling, and satisfying the following conditions (a) to (c): (a) The heating temperature of the slab is 1100 to 1280°C and the heating time of the slab is 10,000 seconds or less; (b) The temperature during rough rolling is 1000 to 1170°C, the rough rolling time is 10 seconds or less, and the rough rolling is performed so that the total reduction ratio during rough rolling is 30% or more; (c) The temperature during finish rolling is 850 to 1000°C and the finish rolling is performed so that the total reduction ratio is 60% or more; A cooling process in which the finish-rolled steel sheet is first cooled from the temperature after the completion of the hot rolling process to 700°C, and then second cooled so that the residence time at 500 to 680°C is 10.0 seconds or less. A method for manufacturing a steel sheet, characterized by including a winding step of winding the second-cooled steel sheet at a winding temperature of 300 to 530°C. (8) The method for manufacturing a steel sheet according to (7) above, characterized in that in the cooling step, the average cooling rate of the primary cooling is 50°C / second or more.

[0015] According to the present invention, it is possible to provide a steel sheet with high strength and excellent impact resistance after molding, a part containing the same, and a method for manufacturing the steel sheet.

[0016] <Steel Sheet> The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has the following composition in mass%, C: 0.040 to 0.180%, Si: 0.20 to 2.00%, Mn: 1.00 to 3.00%, Cr: 0.500 to 1.000%, Ti: 0.020 to 0.150%, B: 0.00003 to 0.00150%, sol. Al: 0.001 to 0.100%, P: 0.1000% or less, S: 0.050% or less, N: 0.0150% or less, O: 0.0060% or less, Mo: 0 to 0.500%, W: 0 to 0.500%, Co: 0 to 3.000%, Nb: 0 to 0.150%, V: 0-0.500%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.500%, As: 0-0.100%, Ni: 0-1.000%, Cu: 0-1.000%, Ca: 0-0.0500%, Zr: 0 to 0.500%, Mg: 0 to 0.0500%, It is characterized by having a metallic structure in which, at a position half the thickness from the surface, it contains, in area percent, 30 to 85% in total acicular bainite, tempered martensite, fresh martensite, and retained austenite, the particle size of the prior austenite grains is 200 μm or less, the standard deviation of the Mn concentration is 0.40 mass% or less, and the boundary density is 0.35 / μm or more, with a crystal orientation difference of acicular bainite, tempered martensite, fresh martensite, and retained austenite being 5° or less.

[0017] As mentioned earlier, in automotive parts, improved impact resistance is desired from the perspective of ensuring occupant safety. For this reason, steel sheets used as materials for automotive parts, for example, are fine on normal road surfaces, but are required to exhibit high impact resistance after forming by press forming or other methods so that they are less likely to break even when subjected to large bumps or impacts such as collisions with automobiles. Therefore, the inventors of the present invention have conducted studies focusing on the metallic structure of the steel sheet, in addition to making the chemical composition of the steel sheet appropriate. To explain in more detail, the inventors first found that by controlling the metallic structure of a steel sheet having an optimized chemical composition, particularly one containing Cr: 0.500 to 1.000%, Ti: 0.020 to 0.150%, and B: 0.00003 to 0.00150% by mass%, so as not to excessively generate hard phases such as acicular bainite, tempered martensite, fresh martensite, and retained austenite at a position half the thickness from the surface of the steel sheet, and more specifically by controlling the area percentage to contain acicular bainite, tempered martensite, fresh martensite, and retained austenite in total of 30 to 85%, it is possible to improve the impact resistance after forming while achieving the desired high strength, more specifically a tensile strength of 980 MPa or more. Here, if hard phases are excessively generated at a position half the thickness from the surface of the steel sheet, toughness decreases, which can lead to a decrease in impact resistance after forming.

[0018] Cr is an element that segregates at grain boundaries during the transformation from austenite (fcc) to bainite (bcc), contributing to the suppression of austenite grain growth. Furthermore, Cr expands the transformation temperature range of granular bainite, and is therefore effective in generating granular bainite as a soft phase, as will be explained in detail later. Consequently, a deficiency of Cr can lead to excessive growth of austenite grains, resulting in larger grain sizes of the prior austenite grains. B is effective in generating tempered martensite and fresh martensite to improve the hardenability of steel. However, a deficiency of B, especially a deficiency of dissolved B, reduces the amount of B segregated at grain boundaries, and therefore can decrease the boundary density in the hard phase where the crystal orientation difference is 5° or less, as will be explained in detail later. Incidentally, Ti is effective in improving toughness and thereby improving impact resistance by forming carbonitrides and refining the microstructure through a pinning effect, and is also effective in improving the strength of steel through precipitation strengthening. Therefore, in order to improve the impact resistance after forming while achieving the desired high strength, it is important to control the formation of hard phases such as acicular bainite, tempered martensite, fresh martensite, and retained austenite at a position half the thickness from the surface of the steel sheet, and to control the chemical composition of the steel sheet to include Cr: 0.500 to 1.000%, Ti: 0.020 to 0.150%, and B: 0.00003 to 0.00150% by mass%.

[0019] Next, the inventors focused on the particle size of prior austenite grains in the metal structure in order to improve impact resistance after molding while maintaining high strength. Specifically, as explained above, they suppressed the excessive growth of prior austenite grains due to elements such as Ti and Cr, and also examined the manufacturing method of the steel sheet in detail, controlling the particle size of prior austenite grains at a position half the thickness from the surface to 200 μm or less. It is generally known that refining the metal structure is an effective means of achieving both high strength and toughness, and by controlling the particle size of prior austenite grains to 200 μm or less, toughness was improved, which in turn slightly improved impact resistance.

[0020] However, it was found that simply controlling the particle size of the prior austenite grains to 200 μm or less was not sufficient to adequately improve the impact resistance after molding. Therefore, the inventors further investigated the metallographic structure in detail. As a result, they found that by controlling the standard deviation of the Mn concentration to 0.40 mass% or less at a position half the thickness from the surface, and controlling the boundary density at which the crystal orientation difference between acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less to 0.35 / μm or more, it is possible to significantly improve the impact resistance after molding while maintaining high strength.

[0021] First, let's explain the standard deviation of Mn concentration. As steel sheets become stronger, relatively large amounts of elements such as Mn are sometimes added to improve their hardenability. However, Mn is an element that tends to segregate in the center of the steel sheet. As a result of this Mn segregation, hard phases such as acicular bainite, tempered martensite, fresh martensite, and retained austenite may be excessively formed in the region from the surface of the steel sheet to half the thickness. Consequently, toughness decreases, and the impact resistance after forming deteriorates. Since the excessive formation of hard phases reduces the impact resistance after forming, it is important to suppress Mn segregation, specifically to control the standard deviation of Mn concentration to 0.40 mass% or less, in order to improve the impact resistance after forming.

[0022] Next, we will explain the boundary density where the crystal orientation difference is 5° or less. Low-angle boundaries with a crystal orientation difference of 5° or less mainly originate from the boundaries of acicular bainite, tempered martensite, and fresh martensite. Although we do not intend to be bound by any particular theory, by setting the boundary density where the crystal orientation difference in the hard phase is 5° or less to 0.35 / μm or higher, the number of such phase boundaries increases, and consequently the grain size of such phases decreases, allowing the impact energy at the center of the steel sheet to be sufficiently absorbed even after forming, thereby significantly improving the impact resistance after forming. Therefore, controlling the boundary density where the crystal orientation difference in acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less to 0.35 / μm or higher is important for significantly improving the impact resistance after forming while maintaining high strength.

[0023] Therefore, by controlling the particle size of prior austenite grains to 200 μm or less at a position half the thickness from the surface of the steel plate, controlling the standard deviation of the Mn concentration to 0.40 mass% or less, and controlling the boundary density at which the crystal orientation difference between acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less to 0.35 / μm or more, it is possible to significantly improve the impact resistance after molding while maintaining high strength.

[0024] As described above, the steel sheet according to the embodiment of the present invention can achieve high strength with a tensile strength of 980 MPa or more and excellent impact resistance after forming. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve both high strength and excellent impact resistance after forming, which are conflicting properties, and is particularly useful in the automotive field where these properties are required.

[0025] 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 limit and upper limit, respectively, unless otherwise specified.

[0026] [C: 0.040-0.180%] Carbon (C) is an effective element for increasing the strength of steel sheets. In addition, C forms carbides and / or carbonitrides with Ti and Nb in the steel, and contributes to precipitation strengthening based on the precipitates formed. To obtain these effects to the fullest, the C content should be 0.040% or more. The C content may be 0.050% or more, 0.060% or more, 0.070% or more, or 0.080% or more. On the other hand, if the C content is excessive, the impact resistance and elongation after forming may decrease due to an excessive increase in strength. Therefore, the C content should be 0.180% or less. The C content may be 0.160% or less, 0.140% or less, 0.120% or less, or 0.100% or less.

[0027] [Si: 0.20-2.00%] Si is an effective element for increasing strength as a solid solution strengthening element. To obtain this effect sufficiently, the Si content should be 0.20% or more. The Si content may be 0.40% or more, 0.50% or more, 0.60% or more, or 0.70% or more. On the other hand, if the Si content is excessive, a surface quality defect called Si scale may occur. Therefore, the Si content should be 2.00% or less. The Si content may be 1.60% or less, 1.40% or less, 1.20% or less, 1.00% or less, or 0.80% or less.

[0028] [Mn: 1.00-3.00%] Mn is an element effective in increasing strength as a hardenability and solid solution strengthening element. To obtain these effects to the fullest, the Mn content should be 1.00% or more. The Mn content may be 1.20% or more, 1.40% or more, 1.60% or more, or 1.80% or more. On the other hand, if the Mn content is excessive, a large amount of MnS may be generated, which may reduce the impact resistance after molding. Therefore, the Mn content should be 3.00% or less. The Mn content may be 2.80% or less, 2.60% or less, 2.40% or less, or 2.20% or less.

[0029] [Cr: 0.500-1.000%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength. Cr segregates at grain boundaries during the transformation from austenite (fcc) to bainite (bcc), suppressing the growth of austenite grains and contributing to the refinement of prior austenite grains. Furthermore, Cr is effective in generating granular bainite because it broadens the transformation temperature range of granular bainite. To fully obtain these effects, the Cr content should be 0.500% or more. The Cr content may also be 0.550% or more, 0.600% or more, or 0.650% or more. On the other hand, excessive Cr content may lead to saturation of the effect and an increase in manufacturing costs. Therefore, the Cr content should be 1.000% or less. The Cr content may be 0.950% or less, 0.900% or less, 0.800% or less, or 0.700% or less.

[0030] [Ti: 0.020-0.150%] Ti is an element that refines the microstructure through a pinning effect, contributing to improved toughness. Ti also contributes to improving the strength of steel sheets by forming carbonitrides and the like in steel, through precipitation strengthening. To fully obtain these effects, the Ti content should be 0.020% or more. The Ti content may be 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more. On the other hand, excessive Ti content may reduce impact resistance. Therefore, the Ti content should be 0.150% or less. By controlling the Ti content to 0.150% or less, the decrease in impact resistance can be suppressed. The Ti content may be 0.140% or less, 0.130% or less, 0.120% or less, or 0.110% or less.

[0031] [B: 0.00003 to 0.00150%] B is an element that enhances the hardenability of steel and contributes to improving its strength. B is also an element that segregates at grain boundaries and contributes to improving boundary density. To obtain these effects fully, the B content should be 0.00003% or more. The B content may be 0.00010% or more, 0.00050% or more, 0.00070% or more, or 0.00100% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, the B content should be 0.00150% or less. The B content may be 0.00140% or less, 0.00130% or less, 0.00120% or less, or 0.00110% or less.

[0032] [sol. Al: 0.001-0.100%] Sol. Al is an element that acts as a deoxidizing agent for molten steel. To obtain this effect, the sol. Al content should be 0.001% or more. The sol. Al content may be 0.005% or more, or 0.010% or more. On the other hand, if the sol. Al content is excessive, the effect will saturate, and including more than necessary in the steel sheet may lead to an increase in manufacturing costs. Therefore, the sol. Al content should be 0.100% or less. The sol. Al content may be 0.090% or less, 0.070% or less, or 0.050% or less.

[0033] [P: 0.1000% or less] Excessive P content may negatively affect weldability and other properties. Therefore, the P content should be 0.1000% or less. The P content may also be 0.0500% or less, 0.0200% or less, 0.150% or less, 0.0100% or less, or 0.0050% 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, 0.0005% or more, or 0.0010% or more.

[0034] [S: 0.050% or less]  If S is contained excessively, a large amount of MnS is formed, which may reduce toughness. Therefore, the S content is set to 0.050% or less. The S content may be 0.030% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction leads to an increase in cost. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0035] [N: 0.0150% or less]  If N is contained excessively, coarse nitrides are formed, which may reduce toughness. Therefore, the N content is set to 0.0150% or less. The N content may be 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction leads to an increase in cost. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0036] [O: 0.0060% or less]  O is an element mixed in during the manufacturing process. If O is contained excessively, coarse inclusions are formed, which may reduce the toughness of the steel sheet. Therefore, the O content is set to 0.0060% or less. The O content may be 0.0040% or less or 0.0020% or less. The lower limit of the O content is not particularly limited and may be 0%, however, reducing the O content to less than 0.0001% requires a long refining time, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0037] 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 optional elements in place of a part of the balance Fe as required.

[0038] [Mo: 0 to 0.500%] Mo is an element that enhances the hardenability of steel and contributes to the improvement of strength. The Mo content may be 0%, but in order to obtain such effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.005% or more, or 0.010% or more. On the other hand, excessive Mo content may increase deformation resistance during hot working and increase the load on equipment. Therefore, the Mo content is preferably 0.500% or less. The Mo content may be 0.300% or less, 0.250% or less, 0.200% or less, 0.150% or less, or 0.100% or less.

[0039] [W: 0 to 0.500%] W is an element that enhances the hardenability of steel and contributes to the improvement of strength. The W content may be 0%, but in order to obtain such effects, the W content is preferably 0.001% or more. The W content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive W content may reduce weldability. Therefore, the W content is preferably 0.500% or less. The W content may be 0.300% or less, 0.200% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0040] [Co: 0 to 3.000%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but in order to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Co content may reduce hot workability and also leads to an increase in raw material cost. Therefore, the Co content is preferably 3.000% or less. The Co content may be 2.000% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0041] [Nb: 0-0.150%] Nb is an element that forms carbides, nitrides and / or carbonitrides in steel, contributing to the refinement of the microstructure and, consequently, the increased strength of the steel sheet through a pinning effect. The Nb content may be 0%, but to obtain these effects, it is preferable that the Nb content be 0.001% or more. The Nb content may also be 0.005% or more or 0.010% or more. On the other hand, if Nb is included in excess, the effect will saturate, and including more Nb in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, it is preferable that the Nb content be 0.150% or less. The Nb content may also be 0.100% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0042] [V: 0-0.500%] V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but in order to obtain such an effect, it is preferable that the V content be 0.001% or more. The V content may also be 0.005% or more or 0.010% or more. On the other hand, if the V content is excessive, the effect will saturate, and including more V in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, it is preferable that the V content be 0.500% or less. The V content may also be 0.300% or less, 0.200% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0043] [Ta: 0-0.100%] Ta is an element effective in controlling the morphology and increasing the strength of carbides. The Ta content may be 0%, but to obtain these effects, it is preferable that the Ta content be 0.001% or more. The Ta content may be 0.002% or more, 0.050% or more, or 0.010% or more. On the other hand, if the Ta content is excessive, the effect will saturate, and including more Ta in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, it is preferable that the Ta content be 0.100% or less. The Ta content may be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0044] [Sn: 0-0.100%] Sn is an element effective in improving corrosion resistance. The Sn content may be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more, and may be 0.002% or more, or 0.005% or more. On the other hand, if the Sn content is excessive, the effect will saturate, and including more Sn in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, the Sn content is preferably 0.100% or less. The Sn content may be 0.050% or less, 0.030% or less, 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less.

[0045] [Sb: 0-0.500%] Sb is an element effective in improving corrosion resistance. The Sb content may be 0%, but to obtain such an effect, the Sb content is preferably 0.001% or more, and may be 0.002% or more, or 0.005% or more. On the other hand, if Sn is included in excess, the effect will saturate, and including more than necessary in the steel sheet may lead to an increase in manufacturing costs. Therefore, the Sb content is preferably 0.500% or less, and may be 0.200% or less, 0.100% or less, 0.050% or less, 0.030% or less, 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less.

[0046] [As: 0-0.100%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain such an effect, it is preferable that the As content be 0.001% or more. The As content may also be 0.002% or more, or 0.005% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel plate may lead to an increase in manufacturing costs. Therefore, it is preferable that the As content be 0.100% or less. The As content may also be 0.050% or less, 0.030% or less, 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less.

[0047] [Ni: 0-1.000%] Ni is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but to obtain such an effect, it is preferable that the Ni content be 0.001% or more. The Ni content may also be 0.005% or more or 0.010% or more. On the other hand, if Ni is included in excess, the effect will saturate, and including more Ni in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, it is preferable that the Ni content be 1.000% or less. The Ni content may also be 0.500% or less, 0.200% or less, 0.100% or less or 0.050% or less.

[0048] [Cu: 0-1.000%] Cu is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Cu content may be 0%, but to obtain such an effect, it is preferable that the Cu content be 0.001% or more. The Cu content may also be 0.005% or more or 0.010% or more. On the other hand, if Cu is included in excess, the effect will saturate, and including more Cu in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, it is preferable that the Cu content be 1.000% or less. The Cu content may also be 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0049] [Ca: 0-0.0500%] Ca is an element that can control the morphology of nonmetallic inclusions. The Ca content may be 0%, but to obtain such an effect, the Ca content is preferably 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, if Ca is included in excess, the effect will saturate, and including more Ca in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, the Ca content is preferably 0.0500% or less. The Ca content may be 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.

[0050] [Zr: 0-0.500%] Zr is an element that can control the morphology of nonmetallic inclusions. The Zr content may be 0%, but to obtain such an effect, it is preferable that the Zr content be 0.001% or more. The Zr content may also be 0.002% or more, or 0.005% or more. On the other hand, if the Zr content is excessive, the effect will saturate, and including more Zr in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, it is preferable that the Zr content be 0.500% or less. The Zr content may also be 0.300% or less, 0.100% or less, 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0051] [Mg: 0-0.0500%] Mg is an element that can control the morphology of nonmetallic inclusions. The Mg content may be 0%, but to obtain such an effect, it is preferable that the Mg content be 0.0001% or more. The Mg content may be 0.0005% or more or 0.0010% or more. On the other hand, if the Mg content is excessive, the effect will saturate, and including more Mg in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, it is preferable that the Mg content be 0.0500% or less. The Mg content may be 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less or 0.0050% or less.

[0052] [REM: 0-0.100%] REM is an element that can control the morphology of nonmetallic inclusions. The REM content may be 0%, but to obtain such an effect, it is preferable that the REM content be 0.0001% or more. The REM content may be 0.0005% or more or 0.001% or more. On the other hand, if the REM content is excessive, the effect will saturate, and including more REM in the steel sheet than necessary may lead to an increase in manufacturing costs. Therefore, it is preferable that the REM content be 0.100% or less. The REM content may be 0.080% or less, 0.060% or less, 0.040% or less, 0.020% or less, or 0.010% or less. In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0053] [Bi: 0-0.100%] Bi has the effect of improving the ductility of steel sheets by refining the solidification structure. The Bi content may be 0%, but in order to obtain such an effect, the Bi content is preferably 0.001% or more. The Bi content may also be 0.002% or more. On the other hand, if Bi is included in excess, the effect will saturate, and including more Bi in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Bi content is preferably 0.100% or less. The Bi content may also be 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, or 0.005% or less.

[0054] In the steel sheet according to the embodiment of the present invention, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial manufacture of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap, and components that are included in a range that does not affect the effects of the present invention.

[0055] 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 spectroscopy (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.

[0056] [Metal structure] [Acicular bainite, tempered martensite, fresh martensite, and retained austenite: 30-85% in total] The metal structure of the steel sheet according to the embodiment of the present invention, more specifically the metal structure in the region from the surface of the steel sheet to half the thickness, includes, in area percent, acicular bainite, tempered martensite, fresh martensite, and retained austenite: 30-85% in total. By appropriately controlling the hard phases such as acicular bainite, tempered martensite, fresh martensite, and retained austenite in the region from the surface of the steel sheet to half the thickness, high strength, for example, a tensile strength of 980 MPa or more, can be achieved, and the impact resistance after forming can be improved. From the viewpoint of further increasing strength, it is preferable that the total area ratio of acicular bainite, tempered martensite, fresh martensite, and retained austenite is higher, for example, 50% or more, 55%, 60% or more, 65% or more, or 70% or more. On the other hand, if the total area ratio of acicular bainite, tempered martensite, fresh martensite, and retained austenite becomes too high, the impact resistance after molding may decrease. Therefore, the total area ratio of acicular bainite, tempered martensite, fresh martensite, and retained austenite should be 85% or less, and may be, for example, 83% or less, 80% or less, or 75% or less.

[0057] [Remaining Structure] In the steel sheet according to the embodiment of the present invention, more specifically in the region from the surface of the steel sheet to a position half the thickness of the sheet, the remaining structure other than acicular bainite, tempered martensite, fresh martensite, and retained austenite may be at least one of granular bainite and ferrite. The area ratio of the remaining structure is not particularly limited, but may be, for example, 15% or more and 70% or less. In this case, more specifically, the total area ratio of at least one of granular bainite and ferrite may be 15% or more and 70% or less. From the viewpoint of improving the strength of the steel sheet, it is preferable to have less soft phase such as granular bainite and ferrite. Therefore, the area ratio of the remaining structure may be 60% or less, 50% or less, or 40% or less. On the other hand, from the viewpoint of improving the impact resistance after forming, it is preferable to include a large amount of these soft phases. Therefore, the area ratio of the remaining structure may be 15% or more, 18% or more, or 20% or more.

[0058] [Calculation of Area Ratios of Ferrite, Granular Bainite, Acicular Bainite, Tempered Martensite, Fresh Martensite, and Retained Austenite] The area ratios of ferrite, granular bainite, acicular bainite, tempered martensite, fresh martensite, and retained austenite are calculated by electron backscatter diffraction (EBSD) as follows. Specifically, first, a sample is taken from the steel plate so that the thickness cross section perpendicular to the plate surface becomes the observation surface. Although it is preferable that the thickness cross section is parallel to the rolling direction, it is not necessary for the thickness cross section to be parallel to the rolling direction, for example, when the rolling direction of the steel plate cannot be determined. Next, EBSD analysis is performed at measurement intervals of 0.2 μm on a rectangular region centered at the 1 / 2 thickness position from the steel plate surface, with a length of 200 μm in the thickness direction and a length of 600 μm perpendicular to the thickness direction, to obtain crystal orientation information for this rectangular region. EBSD analysis was performed using a system consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001200F) and an EBSD detector (TSL EDAX Velocity ultrafast EBSD detector DVC5 type detector), with a resolution of 9.6 × 10⁻⁶. －５The procedure is performed under the conditions of a vacuum of Pa or less, an acceleration voltage of 125 kV, and an irradiation current level of 16. In the obtained crystal orientation information, measurement points with a crystal orientation difference of 15° or more are considered crystal boundaries, and the region enclosed by these crystal boundaries is considered a crystal grain. Next, the difference in crystal orientation between all measurement points within the crystal grain is calculated, and the average value of this difference is calculated to obtain the GAM value (Grain Average Misorientation value) of that crystal grain. Crystal grains with a GAM value of 0.5° or less are identified as ferrite, and their area fraction is calculated. Subsequently, for crystal grains with a GAM value greater than 0.5°, crystal grains with a boundary density of less than 0.40 / μm and a crystal orientation difference of 5° or more are identified as granular bainite, and their area fraction is calculated. The crystal grains in regions other than ferrite and granular bainite are identified as acicular bainite, tempered martensite, fresh martensite, and retained austenite, and their total area fraction is calculated. Here, the boundary density for crystal orientation differences of 5° or less is calculated by dividing the total length of the crystal boundary by the measured area for boundaries within the crystal grain where the crystal orientation difference is 5° or less.

[0059] [Particle size of prior austenite grains: 200 μm or less] In the metal structure of the steel sheet according to the embodiment of the present invention, the particle size of prior austenite grains, more specifically, the particle size of prior austenite grains in the region from the surface of the steel sheet to 1 / 2 of the sheet thickness, is 200 μm or less. From the viewpoint of improving the impact properties after forming, the smaller the particle size of prior austenite grains, the better, and for example, it may be 180 μm or less, 160 μm or less, 140 μm or less, or 120 μm or less. The lower limit is not particularly limited, but the particle size of prior austenite grains may be 10 μm or more, 20 μm or more, or 50 μm or more.

[0060] [Measurement of the grain size of the old austenite grains] The grain size of the old austenite grains is measured by optical microscopy. First, a sample is taken from the thickness cross section of the steel plate parallel to the rolling direction, with the thickness cross section at the center in the width direction serving as the observation surface. While it is preferable that the thickness cross section be parallel to the rolling direction, it is not necessary to be parallel to the rolling direction if the rolling direction of the steel plate cannot be determined. Next, after mirror polishing the thickness cross section, the old austenite grain boundaries are exposed using an etching solution (an aqueous solution containing picric acid, a surfactant, and oxalic acid), and an optical microscope is used to observe a rectangular area from the steel plate surface, centered at the 1 / 2 thickness position, with an area of ​​200 μm in the thickness direction and 600 μm in the direction perpendicular to the thickness direction. Finally, the old austenite grains are approximated as ellipses, and the average value of the top 5 circle equivalent diameters of the old austenite grains in the measurement area is calculated to obtain the grain size of the old austenite grains.

[0061] [Standard deviation of Mn concentration: 0.40 mass% or less] In the metallographic structure of the steel sheet according to the embodiment of the present invention, the standard deviation of the Mn concentration, more specifically the standard deviation of the Mn concentration in the region from the surface of the steel sheet to 1 / 2 of the sheet thickness, is 0.40 mass% or less. Such a low standard deviation of Mn concentration is associated with the suppression of Mn segregation. Therefore, due to the suppression of Mn segregation, the excessive formation of a hard phase consisting of at least one of acicular bainite, tempered martensite, fresh martensite, and retained austenite can be suppressed, the decrease in toughness can be suppressed, and thereby the impact resistance after forming can be improved. From the viewpoint of improving the impact resistance after forming, a lower standard deviation of Mn concentration is preferable, and may be, for example, 0.35 mass% or less, 0.30 mass% or less, or 0.25 mass% or less. A smaller lower limit of the standard deviation of Mn concentration is desirable. However, due to constraints in the manufacturing process, the practical lower limit of the standard deviation of the Mn concentration is 0.05 mass%, and the standard deviation of the Mn concentration may be, for example, 0.10 mass% or more, 0.15 mass% or more, or 0.20 mass% or more.

[0062] [Measurement of Standard Deviation of Mn Concentration] The standard deviation of Mn concentration is measured as follows. First, a sample is taken from the surface of the steel plate so that the metal structure in the cross-sectional area at the 1 / 2 thickness position can be observed. Although it is preferable that the cross-sectional area at the thickness is parallel to the rolling direction, it is not necessary for the cross-sectional area at the thickness to be parallel to the rolling direction if the rolling direction of the steel plate cannot be determined. Depending on the measuring device, the sample should be large enough to be observed at a distance of about 10 mm perpendicular to the thickness direction. Next, after the sample is mirror-polished, the standard deviation of Mn concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions are an acceleration voltage of 15 kV and a magnification of 5000x, and distribution images are measured at more than 40,000 locations at measurement intervals of 0.1 μm in areas of 20 μm perpendicular to the thickness direction of the sample and 20 μm in the thickness direction of the sample. Then, the standard deviation of Mn concentration is obtained by calculating the standard deviation based on the Mn concentrations obtained from all measurement points.

[0063] [Boundary density where the crystal orientation difference in acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less: 0.35 / μm or more] In the metal structure of the steel sheet according to the embodiment of the present invention, the boundary density where the crystal orientation difference in acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less, more specifically the boundary density in the region from the surface of the steel sheet to 1 / 2 of the sheet thickness is 0.35 / μm or more. From the viewpoint of improving the impact resistance after forming, a higher boundary density is preferable, and may be, for example, 0.40 / μm or more or 0.45 / μm or more. The upper limit of the boundary density is not particularly limited, but may be 1.00 / μm or less, 0.90 / μm or less, or 0.80 / μm or less.

[0064] [Measurement of Boundary Density] Using the same method as described above for [Calculation of Area Ratio of Acicular Bainite, Granular Bainite, and Ferrite], EBSD analysis is performed to obtain crystal orientation information for this rectangular region. In the obtained crystal orientation information, measurement points with a crystal orientation difference of 15° or more are considered crystal boundaries, and the region enclosed by these crystal boundaries is considered a crystal grain. Next, the crystal orientation difference between all measurement points within the crystal grain is calculated, and the average value of this difference is calculated to obtain the GAM value (Grain Average Misorientation value) for that crystal grain. Then, for crystal grains with a GAM value greater than 0.5°, the boundary density where the crystal orientation difference is 5° or less is calculated. Here, the boundary density where the crystal orientation difference is 5° or less is calculated by dividing the total length of the boundary by the measurement area for boundaries within the crystal grain where the crystal orientation difference is 5° or less.

[0065] [Acicular bainite, tempered martensite, fresh martensite, and retained austenite at a position 1 / 4 of the thickness from the surface: 30-70% in total] The microstructure of a steel sheet according to a preferred embodiment of the present invention, more specifically the microstructure in the region of the steel sheet at a position 1 / 4 of the thickness from the surface, contains, in area percent, acicular bainite, tempered martensite, fresh martensite, and retained austenite: 30-70% in total. Hard structures such as acicular bainite, tempered martensite, fresh martensite, and retained austenite have the effect of increasing strength. To obtain such an effect, the total area ratio of acicular bainite, tempered martensite, fresh martensite, and retained austenite at a position 1 / 4 of the thickness from the surface is preferably 30% or more, and may be 35% or more, 40% or more, 45% or more, 50% or more, or 55% or more. On the other hand, if these hard phases are included in excess, the desired formability, specifically the desired elongation and hole-expanding properties, may not be obtained. Therefore, the total area ratio of acicular bainite, tempered martensite, fresh martensite, and retained austenite at a position 1 / 4 of the plate thickness from the surface is preferably 70% or less, and may be 68% or less, 66% or less, 64% or less, 62% or less, or 60% or less.

[0066] [Granular bainite at 1 / 4 thickness from the surface: 30-50%] The metallographic structure of a steel sheet according to a preferred embodiment of the present invention, more specifically the metallographic structure in the region at 1 / 4 thickness from the surface of the steel sheet, contains 30-50% granular bainite by area percentage. Since granular bainite has the effect of improving the strength-ductility balance, by containing 30-50% granular bainite by area percentage in the metallographic structure in the region at 1 / 4 thickness, high formability, specifically total elongation of 10% or more and hole expansion ratio of 50% or more, can be achieved. In order to fully obtain such effects, the area percentage of granular bainite at 1 / 4 thickness from the surface is preferably 30% or more, and may be 35% or more or 40% or more. On the other hand, if granular bainite is included in excess, the desired strength may not be obtained. Therefore, the area percentage of granular bainite at 1 / 4 thickness from the surface is preferably 50% or less, and may be 45% or less.

[0067] [Remaining structure at a position 1 / 4 of the plate thickness from the surface] In a preferred embodiment of the present invention, the steel sheet, more specifically in the region at a position 1 / 4 of the plate thickness from the surface of the steel sheet, may contain remaining structures other than granular bainite, acicular bainite, tempered martensite, fresh martensite, and retained austenite, but the area ratio of the remaining structure may be 0%. The remaining structure may be ferrite. The area ratio of the remaining structure is not particularly limited, but may be less than 20%, 15% or less, 10% or less, 5% or less, or 1% or less. In other words, the total area ratio of granular bainite, acicular bainite, tempered martensite, fresh martensite, and retained austenite may be greater than 80% to 100%, 85% to 100%, 90% to 100%, 95% to 100%, or 99% to 100%. The lower limit of the remaining structure may be 0.1%, 0.5%, or 1%.

[0068] [Calculation of the area ratio of the metal structure at a position 1 / 4 of the plate thickness from the surface] The metal structure at a position 1 / 4 of the plate thickness from the surface is calculated in the same way as [Calculation of the area ratio of fresh martensite and retained austenite], [Calculation of the area ratio of tempered martensite and pearlite], and [Calculation of the area ratio of acicular bainite, granular bainite, and ferrite], except that the measurement area is a rectangular region of 200 μm in the thickness direction and 600 μm in the direction perpendicular to the thickness direction, centered at a position 1 / 4 of the plate thickness from the surface of the steel plate.

[0069] [Plate Thickness] The steel plate according to the embodiment of the present invention is not particularly limited, but generally has a plate thickness of 1.0 to 8.0 mm. For example, the plate 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, 5.0 mm or less, or 4.0 mm or less.

[0070] As described above, the steel sheet according to the embodiment of the present invention can achieve high strength and excellent impact resistance even after forming, thus reliably achieving a high level of balance between the conflicting properties of high strength and excellent impact resistance after forming. Furthermore, in addition to the above, by appropriately controlling the metal structure at the 1 / 4 position of the sheet thickness, excellent formability can be achieved in addition to high strength and excellent impact resistance after forming. The steel sheet according to the embodiment of the present invention is particularly useful for use in parts in technical fields where the balance of these properties is required. In a preferred embodiment, an automobile part, particularly an automobile undercarriage part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automobile undercarriage parts include lower arms and trailing arms. These automobile parts, particularly automobile undercarriage parts, only need to include the steel sheet according to the embodiment of the present invention in at least a portion of these parts, and therefore at least a portion of these parts satisfy the chemical composition and metal structure characteristics described above. In parts of the steel sheet that do not come into direct contact with the mold during forming such as press forming, and where the degree of processing is relatively low, the characteristics of the metal structure do not change particularly before and after forming.

[0071] [Mechanical Properties] [Tensile Strength (TS)] According to the steel sheet having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 980 MPa or more, can be achieved. The tensile strength is preferably 1050 MPa or more, and more preferably 1100 MPa or more. According to the steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, excellent impact resistance after forming can be achieved by a specific combination of the chemical composition and metal structure described above. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel sheet may be 1500 MPa or less, 1400 MPa or less, or 1300 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from a direction (C direction) where the longitudinal direction of the test piece is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be determined, a JIS No. 5 test piece may be taken from any direction on the surface of the steel sheet. If it is difficult to obtain a JIS No. 5 test specimen from the sample to be measured, a tensile test can be performed on a micro-test specimen, and the tensile strength can be calculated by converting it to the equivalent value for a JIS No. 5 test specimen. For example, an ASTM E8 Sub-size (6 mm wide) can be used as a micro-test specimen. Needless to say, the test specimen should be taken from a part that is not affected (or is less affected) by work hardening or heat and strain during cutting.

[0072] [Impact resistance after forming: Charpy impact value after pre-straining] According to the steel sheet having the above chemical composition and metal structure, excellent impact resistance is achieved even after forming, specifically, the Charpy impact value after applying pre-strain simulating the forming of the part is 30 J / cm². ２ The above can be achieved. There is no particular upper limit, but for example, the Charpy impact value after pre-strain is 150 J / cm. ２The following is acceptable. The Charpy impact value after pre-straining is determined as follows: A JIS No. 5B tensile test specimen is prepared with the width direction (C direction) of the steel plate aligned with the longitudinal direction of the test specimen. After applying a compressive strain of 10% in the longitudinal direction of the test specimen to the steel, a V-notch test specimen is prepared. Then, it is measured and calculated by a test in accordance with the provisions of JIS Z 2242:2018. Specifically, three Charpy impact values ​​are measured at -40°C using an impact blade with a radius of 2 mm, and the value is calculated by averaging them. If a sub-sized test specimen is used, it is converted to a full-size Charpy impact value according to the thickness of the test specimen. If the width direction (C direction) of the steel plate cannot be determined, a JIS No. 5B tensile test specimen is prepared with an arbitrary direction (0° direction) aligned with the longitudinal direction of the test specimen. Similarly, JIS No. 5B tensile test specimens are prepared at 45° intervals with respect to any direction (0° direction), with the 45°, 90°, or 135° direction being the longitudinal direction of the test specimen, and the Charpy impact value is obtained using the method described above. Among the test specimens obtained from the four directions, the one with the lowest Charpy impact value is taken as the Charpy impact value obtained from the steel plate after pre-straining.

[0073] [Total Elongation (tEl)] A steel sheet according to a preferred embodiment of the present invention, specifically having the above-mentioned chemical composition and metal structure, and having a predetermined metal structure at a position 1 / 4 of the sheet thickness from the surface, can have high total elongation in addition to high tensile strength and excellent impact resistance after forming, and more specifically, can achieve a total elongation of 10% or more. There is no particular upper limit, but for example, the total elongation may be 30% or less, 25% or less, or 20% or less. The total elongation is measured by taking a JIS No. 5 test specimen from a direction (C direction) where the longitudinal direction of the test specimen is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the sheet surface of the steel sheet.

[0074] [Hole Expansion Ratio (λ)] A steel sheet according to a preferred embodiment of the present invention, specifically having the above-mentioned chemical composition and metal structure, and having a predetermined metal structure at a position 1 / 4 of the plate thickness from the surface, can have high tensile strength and excellent impact resistance after forming, as well as excellent hole expansion properties, and more specifically, can achieve a hole expansion ratio of 50% or more. The hole expansion ratio may preferably be 55% or more. There is no particular upper limit to the hole expansion ratio, but for example, the hole expansion ratio may be 100% or less or 80% or less. The hole expansion ratio is determined as follows. First, a test piece with a width of 100 mm and a length of 100 mm 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 widened using a conical punch with a 60° apex angle until a crack penetrates the plate thickness. 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}

[0075] <Method for Manufacturing Steel Sheets> Next, preferred methods for manufacturing steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below. More specifically, the following describes the manufacturing of hot-rolled steel sheets, but the steel sheets according to embodiments of the present invention include any steel sheets having the chemical composition and metal structure described above, i.e., not only hot-rolled steel sheets, but also cold-rolled steel sheets, plated steel sheets, etc. Therefore, the following description merely describes preferred manufacturing methods when the steel sheets according to embodiments of the present invention are hot-rolled steel sheets.

[0076] A method for manufacturing a steel sheet according to an embodiment of the present invention comprises a hot rolling step that includes heating a slab having the chemical composition described above in relation to a steel sheet, and then rough rolling and finish rolling, satisfying the following conditions (a) to (c): (a) the heating temperature of the slab is 1100 to 1280°C and the heating time of the slab is 10,000 seconds or less; (b) the temperature during rough rolling is 1000 to 1170°C, the rough rolling time is 10.0 seconds or less, and the rough rolling is performed so that the total reduction ratio during rough rolling is 30% or more; (c) the temperature during finish rolling is 850 to 1000°C and the finish rolling is performed so that the total reduction ratio is 60% or more; a cooling step in which the finish-rolled steel sheet is first cooled from the temperature after the completion of the hot rolling step to 700°C, and then second cooled so that the residence time at 500 to 680°C is 10.0 seconds or less. The process is characterized by including a winding step in which the secondary-cooled steel sheet is wound at a winding temperature of 300 to 530°C.

[0077] [Hot Rolling Process] [(a) Slab heating temperature: 1100-1280°C, slab heating time: 10,000 seconds or less] First, a slab having the chemical composition described above in relation to the steel sheet is heated. From the viewpoint of productivity, the slab to be used is preferably cast by the continuous casting method, but it may also be manufactured by the ingot casting method or the thin slab casting method. The slab to be used contains a relatively large amount of alloying elements in order to obtain a high-strength steel sheet. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to solid dissolve the alloying elements in the slab. If the heating temperature is too low, the alloying elements will not sufficiently solid dissolve in the slab, leaving coarse alloy carbides, which may cause brittle cracking during hot rolling. For this reason, the heating temperature should be 1100°C or higher. On the other hand, if the heating temperature is excessively high, the austenite grain size will coarseen, and the grain size of the prior austenite grains in the final steel sheet may exceed 200 μm. For this reason, the heating temperature should be 1280°C or lower. The heating temperature is preferably 1250°C or lower. Furthermore, by heating for, for example, 500 seconds or more, preferably 1000 seconds or more, in the temperature range of 1100°C to 1280°C, the alloying elements can be sufficiently dissolved in the slab. On the other hand, if the heating time is excessively long, the austenite grain size will coarseen, and the grain size of the prior austenite grains in the final steel sheet may exceed 200 μm. Therefore, the heating time should be 10,000 seconds or less.

[0078] [(b) Temperature in rough rolling: 1000 to 1170°C] [Rough rolling time: within 10 seconds] [Total reduction ratio in rough rolling: 30% or more] By performing rough rolling in which the temperature in rough rolling is 1000 to 1170°C, the rough rolling time is within 10 seconds, and the total reduction ratio in rough rolling is 30% or more, it is possible to promote the diffusion of Mn atoms due to recovery and recrystallization of austenite grains, and reduce the standard deviation of Mn concentration from the surface to the 1 / 2 plate thickness position. When the temperature in rough rolling exceeds 1170°C, or the rough rolling time exceeds 10 seconds, in addition to the growth of austenite grains, the decrease in grain boundary density prevents grain boundary diffusion of Mn from acting sufficiently, and the standard deviation of Mn concentration at the 1 / 2 plate thickness position from the surface of the finally obtained steel sheet may exceed 0.40 mass%. In addition, if the total reduction ratio of rough rolling in the temperature range of 1000 to 1170°C is less than 30%, the promotion of atomic diffusion of Mn via recovery and recrystallization of austenite grains does not proceed sufficiently, and the standard deviation of Mn concentration at the 1 / 2 plate thickness position from the surface of the finally obtained metal structure may exceed 0.40 mass%. Therefore, in order to control the standard deviation of Mn concentration to 0.40 mass% or less at the 1 / 2 plate thickness position from the surface of the finally obtained metal structure, it is important to perform rough rolling in which the temperature in rough rolling is 1000 to 1170°C and the total reduction ratio reaches 30% or more within 10 seconds from the start of rough rolling.

[0079] The total reduction ratio (%) in the temperature range of 1000 to 1170°C refers to the inlet plate thickness in rough rolling within this temperature range as t １０ (mm), and the outlet plate thickness t in rough rolling within this temperature range １１ (mm), which can be expressed as 100×{1-(t １１ / t １０ )}.

[0080] [(c) Temperature during finish rolling: 850 to 1000°C, and total reduction ratio: 60% or more] The temperature during finish rolling is 850°C to 1000°C, and the total reduction ratio is 60% or more. If the total reduction ratio during finish rolling is less than 60%, the recrystallized austenite grains will not be sufficiently refined, and the presence of coarse austenite grains may result in the boundary density in the final steel sheet where the crystal orientation difference between acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less being less than 0.35 / μm. Therefore, the total reduction ratio during finish rolling should be 60% or more.

[0081] The total reduction ratio (%) in the temperature range of 850°C to 1100°C refers to the initial inlet thickness before rolling in this temperature range. ２0 Let t be the exit plate thickness after the final stage of rolling in this temperature range. ２1 When this is done, 100 × {1 - (t ２1 / t ２0 It can be expressed as ).

[0082] [Cooling Process] [Primary Cooling from the Temperature After Hot Rolling to 700°C] The finish-rolled steel sheet is primary cooled from the temperature after hot rolling to 700°C. In the steel sheet manufacturing method in a preferred embodiment of the present invention, the average cooling rate in primary cooling may be 50°C / second or more. By setting the average cooling rate to 50°C / second or more, the microstructure fraction at a position 1 / 4 of the sheet thickness from the surface of the final obtained metal structure can be controlled to a desired microstructure fraction. The upper limit of the primary cooling rate is not particularly limited, but may be, for example, 200°C / second or less, and preferably 150°C / second or less.

[0083] [Residence time at 500-680°C: 10.0 seconds or less] The cooled steel sheet is then subjected to secondary cooling so that it remains in the temperature range of 500-680°C for a residence time of 10.0 seconds or less. If the residence time exceeds 10.0 seconds, excessive granular bainite is generated, and the total area ratio of acicular bainite, tempered martensite, fresh martensite, and retained austenite at the 1 / 2 thickness position of the final steel sheet may be less than 30.0%. Therefore, the residence time at 500-680°C should be 10.0 seconds or less.

[0084] [Winding Process] [Winding Temperature: 300-530°C] The cooled steel sheet is wound up in a temperature range of 300-530°C. For steel types with a large amount of heat generated due to transformation, the winding temperature may be higher than the temperature at which it was cooled in the cooling process. That is, even a steel sheet cooled to 500°C in the cooling process may generate heat due to transformation, and may reach over 500°C in the winding process. In such cases, this manufacturing method controls the winding temperature to ensure that the sheet is wound up in a temperature range of 300-530°C. If the winding temperature is below 300°C, excessive amounts of acicular bainite, tempered martensite, fresh martensite, and retained austenite may be generated, and the total area ratio of these in the final steel sheet may exceed 85.0%. Therefore, the winding temperature should be 300°C or higher. On the other hand, if the winding temperature exceeds 530°C, excessive granular bainite may be generated, and the total area ratio of acicular bainite, tempered martensite, fresh martensite, and retained austenite in the final steel sheet may be less than 30.0%. Therefore, the winding temperature should be 530°C or lower.

[0085] According to the steel sheet manufactured by the above manufacturing method, by configuring the metallography of the steel sheet having an optimized chemical composition, particularly one containing Ti: 0.020 to 0.150% by mass, B: 0.00003 to 0.00150%, and Cr: 0.500 to 1.000%, such that at a position half the thickness from the surface of the steel sheet, the total area percentage of acicular bainite, tempered martensite, fresh martensite, and retained austenite is 30 to 85%, it is possible to achieve high strength, more specifically a tensile strength of 980 MPa or more, while improving the impact resistance after forming. By controlling the particle size of prior austenite grains to 200 μm or less at a position half the thickness from the surface of the steel sheet, controlling the standard deviation of the Mn concentration to 0.40 mass% or less, and controlling the boundary density where the crystal orientation difference between acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less to 0.35 / μm or more, it is possible to significantly improve the impact resistance after forming while maintaining high strength. Therefore, steel sheets manufactured by the above manufacturing method can achieve both high strength and excellent impact resistance after forming, which are conflicting properties, making them particularly useful in the automotive sector where both properties are required.

[0086] 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.

[0087] 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), Charpy impact value after pre-straining, total elongation (tEl), and hole expansion ratio (λ) of the obtained steel sheets were investigated.

[0088] 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 at 1100 to 1250°C under the holding times shown in Table 3, and then hot-rolled. Hot rolling was carried out by rough rolling at 1000 to 1170°C and finish rolling at 850 to 1000°C under the conditions shown in Table 3. Next, the finish-rolled steel sheets were primary-cooled from the temperature after the completion of the hot-rolling process to 700°C at the average cooling rate shown in Table 3, and then secondary-cooled to a temperature range of 500 to 680°C for the residence time shown in Table 3. Finally, the secondary-cooled steel sheets were wound at the winding temperature shown in Table 3 to obtain steel sheets with a thickness of 2.2 to 4.0 mm.

[0089]

[0090]

[0091]

[0092] The properties of the obtained steel plates were measured and evaluated by the following method.

[0093] [Tensile Strength (TS), Total Elongation (tEl)] Tensile strength (TS) and total elongation (El) were measured by taking a JIS No. 5 test specimen with a length of 200 mm and a thickness of 2.5 mm from the direction in which 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 2241:2022. More specifically, the test was performed at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test specimen, allowing strain to be introduced until fracture occurred.

[0094] [Impact Resistance after Forming; Charpy Impact Value after Pre-straining] First, a JIS No. 5B tensile test specimen was taken with the width direction (C direction) of the steel plate aligned with the longitudinal direction of the test specimen. Next, a 10% compressive strain was applied to the steel material in the longitudinal direction of the test specimen, and then a V-notch test specimen was prepared. The Charpy impact value was calculated by measuring three Charpy impact values ​​at -40°C using an impact blade with a radius of 2 mm, in accordance with the provisions of JIS Z 2242:2018, and averaging them. When a sub-sized test specimen was used, the value was converted to the full-size Charpy impact value according to the thickness of the test specimen. The impact resistance after forming was evaluated as follows by measuring the Charpy impact value on pre-strained specimens that simulated forming into parts: Good: Charpy impact value after pre-straining is 30 J / cm ２ The above is NG: The Charpy impact value after pre-straining is 30 J / cm. ２ less than

[0095] [Hole Expansion Ratio (λ)] The hole expansion ratio (λ) was determined as follows. First, a test piece measuring 100 mm in width and 100 mm in length was taken from the steel plate, and a punched hole (initial hole: hole diameter d0 = 10 mm) was created 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 was expanded using a conical punch with a 60° apex angle until a crack penetrating the plate thickness occurred, and the hole diameter d1 mm at the time of crack occurrence was measured. 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}

[0096] Steel plates with a tensile strength (TS) of 980 MPa or higher and good impact resistance after forming were evaluated as high-strength steel plates with excellent impact resistance after forming. The results are shown in Tables 4 and 5.

[0097]

[0098]

[0099] In Table 4, "NB" means acicular bainite, "tM" means tempered martensite, "fM" means fresh martensite, "retained γ" means retained austenite, and "GB" means granular bainite. "Prior γ grains" means prior austenite grains. "Boundary density" refers to boundary density where the crystal orientation difference is 5° or less.

[0100] In Comparative Example 3, the long holding time at 1100-1250°C caused the austenite grain size to coarseen, resulting in the final steel sheet having a prior austenite grain size exceeding 200 μm. As a result, the impact resistance after forming decreased. In Comparative Example 4, the low total reduction rate at 1000-1170°C prevented sufficient recovery of austenite grains and promotion of Mn atomic diffusion through recrystallization, resulting in the final steel sheet having a standard deviation of Mn concentration exceeding 0.40 mass%. As a result, the impact resistance after forming decreased. In Comparative Example 5, the long residence time at 1000-1170°C prevented sufficient Mn grain boundary diffusion due to a decrease in grain boundary density in addition to austenite grain growth, resulting in the final steel sheet having a standard deviation of Mn concentration exceeding 0.40 mass%. As a result, the impact resistance after forming decreased. In Comparative Example 6, the total reduction ratio at 850-1000°C was low, resulting in insufficient refinement of the recrystallized austenite grains. The presence of coarse austenite grains likely led to a boundary density of less than 0.35 / μm in the final steel sheet, where the crystal orientation difference between acicular bainite, tempered martensite, fresh martensite, and retained austenite was 5° or less. As a result, the impact resistance after forming was reduced.

[0101] In Comparative Example 9, the high winding temperature resulted in excessive granular bainite formation, and it is believed that the total area ratio of acicular bainite, tempered martensite, fresh martensite, and retained austenite in the final steel sheet was less than 30.0%. As a result, the impact resistance properties after forming and the TS (transfer strength) were reduced. In Comparative Example 10, the low winding temperature resulted in excessive acicular bainite, tempered martensite, fresh martensite, and retained austenite formation, and it is believed that the total area ratio of these materials in the final steel sheet exceeded 85.0%. As a result, the impact resistance properties after forming were reduced.

[0102] In Comparative Example 24, the low carbon content likely resulted in the final steel sheet having a total area ratio of less than 30.0% for acicular bainite, tempered martensite, fresh martensite, and retained austenite. As a result, the impact resistance properties after forming and the TS (Total Strength) were reduced. In Comparative Example 25, the high carbon content likely led to an excessive increase in strength, resulting in reduced impact resistance after forming.

[0103] In Comparative Example 26, the low Mn content resulted in reduced strength in the final steel sheet. In Comparative Example 27, the high Mn content likely led to excessive MnS formation, resulting in reduced impact resistance after forming.

[0104] In Comparative Example 28, the high Cr content likely caused excessive growth of austenite grains, resulting in the prior austenite grain size exceeding 200 μm in the final steel sheet. Consequently, the impact resistance after forming was reduced. In Comparative Example 29, the high Ti content likely contributed to the reduced impact resistance after forming.

[0105] In Comparative Example 30, the high B content resulted in excessively high hardenability, increased strength of the hard phase, and consequently, reduced impact resistance after molding.

[0106] In contrast, in all the examples of the invention, the steel sheets had a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, it was possible to obtain a steel sheet having a metal structure in which, by area percent, acicular bainite, tempered martensite, fresh martensite, and retained austenite totaled 30.0 to 85.0%, the particle size of prior austenite grains at a position half the thickness from the surface of the steel sheet was 200 μm or less, the standard deviation of the Mn concentration was 0.40 mass% or less, and the boundary density was 0.35 / μm or more, with a crystal orientation difference of acicular bainite, tempered martensite, fresh martensite, and retained austenite of 5° or less. As a result, the steel sheets had high strength with a tensile strength of 980 MPa or more and excellent impact resistance after forming.

[0107] In particular, Examples 1, 2, and 11-23 controlled the average cooling rate of the primary cooling from the temperature after the hot rolling process to 700°C to 50°C / second or higher. As a result, it was possible to control the composition so that, at a position 1 / 4 of the plate thickness from the surface, the total area percentage contained acicular bainite, tempered martensite, fresh martensite, and retained austenite: 30.0-70.0%, and granular bainite: 30.0-50.0%. As a result, the material exhibited high strength, excellent impact resistance after molding, a total elongation (tEL) of 10% or more, and porosity (λ) of 50% or more, as well as excellent formability.

[0108] In all of the examples of the invention, the remaining microstructure other than acicular bainite, tempered martensite, fresh martensite, and retained austenite at a position 1 / 2 of the plate thickness from the surface was at least one of granular bainite and ferrite, and the remaining microstructure other than granular bainite, tempered martensite, fresh martensite, and retained austenite at a position 1 / 4 of the plate thickness from the surface was ferrite.

Claims

1. In mass percent, C: 0.040–0.180%, Si: 0.20–2.00%, Mn: 1.00–3.00%, Cr: 0.500–1.000%, Ti: 0.020–0.150%, B: 0.00003–0.00150%, sol. Al: 0.001 to 0.100%, P: 0.1000% or less, S: 0.050% or less, N: 0.0150% or less, O: 0.0060% or less, Mo: 0 to 0.500%, W: 0 to 0.500%, Co: 0 to 3.000%, Nb: 0 to 0.150%, V: 0-0.500%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.500%, As: 0-0.100%, Ni: 0-1.000%, Cu: 0-1.000%, Ca: 0-0.0500%, Zr: 0 to 0.500%, Mg: 0 to 0.0500%, A steel sheet characterized by having a metallic structure having a chemical composition consisting of REM: 0-0.100%, Bi: 0-0.100%, and the remainder: Fe and impurities, containing, in area percent, 30-85% in total of acicular bainite, tempered martensite, fresh martensite, and retained austenite at a position half the thickness from the surface, with a prior austenite grain size of 200 μm or less, a standard deviation of Mn concentration of 0.40 mass% or less, and a boundary density of 0.35 / μm or more where the crystal orientation difference between acicular bainite, tempered martensite, fresh martensite, and retained austenite is 5° or less.

2. The chemical composition is as follows, in mass%, Mo: 0.001-0.500%, W: 0.001-0.500%, Co: 0.001-3.000%, Nb: 0.001-0.150%, V: 0.001-0.500%, Ta: 0.001-0.100%, Sn: 0.001-0.100%, Sb: 0.001-0.500%, As: 0.001-0.100%, Ni: 0.001-1.000%, Cu: 0.001-1.000%, Ca: 0.0001-0.0500%, Zr: 0.001-0.500%, The steel sheet according to claim 1, characterized in that it contains at least one of the following: Mg: 0.0001 to 0.0500%, REM: 0.0001 to 0.100%, and Bi: 0.001 to 0.100%.

3. The steel sheet according to claim 1 or 2, characterized in that, at a position 1 / 4 of the sheet thickness from the surface, it has a metallic structure comprising, by area percentage, acicular bainite, tempered martensite, fresh martensite, and retained austenite: 30 to 70% in total, and granular bainite: 30 to 50%.

4. A steel plate according to any one of claims 1 to 3, characterized in that it has a tensile strength of 980 MPa or more.

5. A steel plate according to any one of claims 1 to 4, characterized in that it has a plate thickness of 1.0 to 8.0 mm.

6. A component characterized by comprising a steel plate as described in any one of claims 1 to 5.

7. A hot rolling step comprising heating a slab having the chemical composition described in claim 1 or 2, then rough rolling and finish rolling, satisfying the following conditions (a) to (c): (a) the heating temperature of the slab is 1100 to 1280°C and the heating time of the slab is 10,000 seconds or less; (b) the temperature during rough rolling is 1000 to 1170°C, the rough rolling time is 10 seconds or less, and the rough rolling is performed such that the total reduction ratio during rough rolling is 30% or more; (c) the temperature during finish rolling is 850 to 1000°C and the finish rolling is performed such that the total reduction ratio is 60% or more; a cooling step of primary cooling the finish-rolled steel sheet from the temperature after the completion of the hot rolling step to 700°C, and then secondary cooling such that the residence time at 500 to 680°C is 10.0 seconds or less. A method for manufacturing a steel sheet, characterized by including a winding step in which a secondary cooled steel sheet is wound at a winding temperature of 300 to 530°C.

8. The method for manufacturing a steel sheet according to claim 7, characterized in that, in the cooling step, the average cooling rate of the primary cooling is 50°C / second or more.

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