Steel sheet and component including same

Abstract

The present invention provides: a steel sheet which is excellent in ultimate deformability isotropy and has high elongation and ultimate deformability in spite of having high strength; and a component including the steel sheet. Provided is a steel sheet having a predetermined chemical composition, the steel sheet being characterized in that the metal structure at a 1 / 4 thickness position from a surface contains, in terms of area%, 70%-95% of bainite and 5%-30% of martensite; the average aspect ratio of prior austenite grains at the 1 / 4 thickness position from the surface is 3.00-5.50; and the standard deviation of hardness in the region between a 3 / 8 thickness position and a 5 / 8 thickness position from the surface is 25 Hv or less.

Description

Steel plates and components containing the same

[0001] The present invention relates to steel plates and components containing the same.

[0002] In recent years, in the automotive industry, weight reduction of vehicle bodies has been demanded from the viewpoint of improving fuel efficiency. In order to achieve both weight reduction of the vehicle body and collision safety, increasing the strength of the steel plates used is one effective method, and thus the development of high-strength steel plates has been promoted under such a background.

[0003] Regarding this, for example, in Patent Document 1, a high-strength steel plate having a predetermined composition, as a microstructure, the total of ferrite and bainite phases being 90% or more in area fraction, and the ratio of the area fraction of the shear texture ({110}<112>, {112}<111>) between the surface layer part (a point at a thickness of 1 / 10t from the surface, where t means the thickness (mm) of the steel material) and the central part (a point exceeding 1 / 10t to 1 / 2t in the thickness direction) being 0.05 to 1.0 is disclosed. In Patent Document 1, it is taught that according to the above configuration, a steel plate having high strength and excellent collision resistance characteristics can be provided.

[0004] On the other hand, many automotive members are made by press-forming sheared blanks. Generally, it is known that the formability of steel plates decreases as the strength increases.

[0005] In relation to this, for example, in Patent Document 2, a high-strength hot-rolled steel plate having a predetermined chemical composition, a microstructure containing one or both of tempered martensite and lower bainite in a total volume fraction of 90% or more, and a standard deviation σ of the Vickers hardness distribution being 15 or less is disclosed. In Patent Document 2, it is taught that according to the above configuration, a high-strength hot-rolled steel plate having a tensile maximum strength of 980 MPa or more and excellent punching hole expansion property and low-temperature toughness can be provided.

[0006] Patent Document 3 discloses a hot-rolled steel sheet having a predetermined chemical composition, characterized in that the metal structure has, in area percent, less than 3.0% retained austenite, less than 15.0% ferrite, less than 5.0% pearlite, an E value indicating the periodicity of the metal structure less than 10.7, an I value indicating the uniformity of the metal structure less than 1.020, a standard deviation of Mn concentration of 0.60 mass% or less, and a tensile strength of 780 MPa or more. Patent Document 3 teaches that, according to the above configuration, a hot-rolled steel sheet with excellent strength and shear workability can be obtained.

[0007] International Publication No. 2020 / 022778, Japanese Patent Publication No. 2015-196891, International Publication No. 2022 / 044494

[0008] As mentioned above, it is generally known that the formability of steel sheets decreases as their strength increases. On the other hand, there is a high demand for high-strength steel sheets with improved properties, such as elongation and ultimate deformability, in order to improve the formability of steel sheets. Here, ultimate deformability is a property that represents how much strain the microstructure can bear during the ductile fracture process, and can be evaluated, for example, by the rate of reduction in sheet thickness.

[0009] Furthermore, if the ultimate deformation capacity of the steel plate is anisotropic, the direction of blank cutting and forming may be restricted, which can lead to a decrease in productivity.

[0010] Therefore, the present invention aims to provide a steel plate and a component containing the same that, despite having high strength, possesses high elongation and ultimate deformability, and also exhibits excellent isotropy of ultimate deformability.

[0011] To achieve the above objective, the inventors focused on and investigated the microstructure of steel sheets. Specifically, they first found that by configuring the microstructure of a steel sheet having a predetermined chemical composition to contain bainite and martensite in specific proportions, they could improve elongation while achieving the desired high strength. In addition, they found that by controlling the average aspect ratio of prior austenite grains to a predetermined range corresponding to a relatively flattened shape, they could more significantly improve elongation while maintaining a high level of ultimate deformability, and furthermore, by controlling the standard deviation of hardness from the surface of the steel sheet to the 3 / 8 to 5 / 8 position of the sheet thickness to a predetermined range, they could significantly improve the isotropy of the ultimate deformability, thus completing the present invention.

[0012] The present invention, which has achieved the above objectives, is as follows. (1) The chemical composition, in mass%, is: C: 0.045-0.120%, Si: 0.01-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, O: 0.0100% or less, Ti: 0.001-0.180%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-0.500%, B: 0-0.0100%, Ca: 0-0.0500%. The composition is Mg: 0-0.050%, REM: 0-0.100%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.050%, Sn: 0-0.050%, and the remainder is Fe and impurities. The microstructure at a position 1 / 4 of the plate thickness from the surface contains, by area percentage, bainite: 70-95% and martensite: 5-30%. The average aspect ratio of prior austenite grains at a position 1 / 4 of the plate thickness from the surface is 3.00-5.50. A steel plate characterized in that the standard deviation of hardness from the surface at a position 3 / 8 to 5 / 8 of the plate thickness is 25 Hv or less.(2) The above chemical composition is as follows, in mass%, Nb: 0.001 to 0.100%, V: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001 to 2.000%, Mo: 0.001 to 3.000%, Ni: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0500%, Mg: 0.0001 to 0.050%, REM: 0.0001 to 0.100%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, Zr: 0.001 to 0.500%, (1) The steel sheet described above, characterized in that it contains at least one of the following: Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%. (3) The steel sheet described above, characterized in that it has a tensile strength of 980 MPa or more. (4) The steel sheet described above, characterized in that it has a plate thickness of 1.0 to 8.0 mm. (5) A component described above, characterized in that it contains the steel sheet described above, according to any one of the following: (1) to (4).

[0013] According to the present invention, it is possible to provide a steel plate and a component containing the same that, despite having high strength, possesses high elongation and ultimate deformability, as well as excellent isotropy of ultimate deformability.

[0014] Figure 1 is a schematic diagram illustrating the measurement of the plate thickness reduction rate, specifically the plate thickness at the fracture point of the test specimen after a tensile test.

[0015] <Steel Sheet> The steel sheet according to the embodiment of the present invention has a chemical composition in mass percent of: C: 0.045 to 0.120%, Si: 0 to 3.00%, Mn: 1.20 to 3.00%, Al: 0.010 to 0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, O: 0.0100% or less, Ti: 0.001 to 0.180%, Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 2.000%, Mo: 0 to 3.000%, Ni: 0 to 0.500%, B: 0 to 0.0100%, Ca: 0 to 0.0500%. The composition is Mg: 0-0.050%, REM: 0-0.100%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.050%, Sn: 0-0.050%, and the remainder is Fe and impurities. The microstructure at a position 1 / 4 of the plate thickness from the surface contains, by area percentage, bainite: 70-95% and martensite: 5-30%. The average aspect ratio of prior austenite grains at a position 1 / 4 of the plate thickness from the surface is 3.00-5.50. A key feature is that the standard deviation of hardness from the surface at a position 3 / 8 to 5 / 8 of the plate thickness is 25 Hv or less.

[0016] As mentioned earlier, it is generally known that the formability of steel sheets decreases with increasing strength. However, there is a high demand for high-strength steel sheets with improved properties, such as ultimate deformability, which is evaluated by the thickness reduction rate, in order to improve the formability of steel sheets. Here, "ultimate deformability" in this specification is considered to be the thickness reduction rate. Therefore, the inventors have investigated not only the appropriate chemical composition of the steel sheet, but also focusing particularly on the metal structure of the steel sheet. To explain in more detail, the inventors have found that by configuring the metal structure of a steel sheet having an optimized chemical composition, particularly a chemical composition containing C: 0.045 to 0.120% and Mn: 1.20 to 3.00% by mass%, to contain bainite and martensite in specific proportions, and more specifically, by configuring it to contain bainite: 70 to 95% and martensite: 5 to 30% by area%, it is possible to improve elongation while achieving the desired high strength, more specifically a tensile strength of 980 MPa or more.

[0017] Carbon (C) is effective in increasing the strength of steel sheets because it improves the hardness of martensite. However, excessive C content can reduce elongation and increase hardness variability, thus negatively impacting the hardness standard deviation, which will be explained in more detail later. Similarly, manganese (Mn) is effective in generating the desired amount of martensite to improve the hardenability of steel. However, excessive Mn content can reduce elongation due to excessive martensite formation, and increase hardness variability in the thickness direction due to Mn segregation during casting, which will also negatively impact the hardness standard deviation. Therefore, in order to improve elongation while achieving the desired high strength, and to appropriately control the hardness standard deviation, which will be explained in more detail later, it is extremely important to control the chemical composition of the steel sheet to contain C: 0.045-0.120% and Mn: 1.20-3.00% by mass, in addition to including bainite and martensite in the specific proportions mentioned above.

[0018] Next, the inventors investigated other characteristics of the metal structure in order to further improve elongation and enhance ultimate deformability. As a result, the inventors found that by controlling the average aspect ratio of prior austenite grains to a predetermined range corresponding to a relatively flattened shape, and more specifically by controlling the average aspect ratio of prior austenite grains at a position 1 / 4 of the plate thickness from the surface of the steel plate to 3.00 to 5.50, it is possible to significantly improve elongation while maintaining a high level of ultimate deformability.

[0019] In this study, the inventors found that as the average aspect ratio of prior austenite grains increases, the total elongation of the steel sheet improves, but the ultimate deformability tends to decrease. While we do not intend to be bound by any particular theory, this tendency is thought to be due to a certain correlation between the average aspect ratio of prior austenite grains and the formation of bainite, and furthermore, to a specific combination of these two characteristics. To explain in more detail, in order to control the average aspect ratio of prior austenite grains to 3.00 to 5.50 in the final metal structure, it is necessary to apply a predetermined reduction within a predetermined temperature range during the hot rolling process, and to create a relatively flattened shape for the austenite grains. In this case, austenite grains with a relatively flattened shape can be present in greater numbers in the steel sheet when viewed within the same field of view, for example, compared to more equiaxed austenite grains, and therefore the grain boundary area is larger. In addition, the rolling process applied to form relatively flattened austenite grains introduces a relatively large number of dislocations into the hot-rolled steel sheet. Here, bainite is thought to nucleate from grain boundaries and dislocations. Therefore, by applying appropriate cooling to the hot-rolled steel sheet, which has a large grain boundary area and many introduced dislocations, a relatively large amount of bainite can be generated using these many grain boundaries and dislocations as nucleation sites. In the steel sheet according to the embodiment of the present invention, the average aspect ratio of the prior austenite grains is controlled within the above-mentioned specific range, and by including a large amount of bainite, which is softer than martensite, uniform elongation is improved, and as a result, the total elongation is improved. However, if the prior austenite grains are made excessively flat, the rate of thickness reduction during deformation decreases, leading to a decrease in ultimate deformability. In contrast, in the steel sheet according to the embodiment of the present invention, by controlling the average aspect ratio of the prior austenite grains to 5.50 or less, it is believed that the prior austenite grains are prevented from becoming excessively flat, thereby maintaining a high level of ultimate deformability while enabling a more significant improvement in total elongation.

[0020] On the other hand, in a steel sheet according to an embodiment of the present invention that mainly contains bainite and martensite, the morphology of the prior austenite grains is carried over to the final metal structure. Therefore, if the prior austenite grains are flattened, elongated martensite in the rolling direction tends to remain in the metal structure. When elongated martensite in the rolling direction remains, anisotropy of the ultimate deformability tends to occur in the rolling direction (L direction) and the width direction perpendicular to it (C direction). In such cases, the direction in which blanks can be taken from the steel sheet and formed may be restricted, leading to a decrease in productivity. Therefore, the inventors focused on the hardness distribution of the metal structure and conducted studies to reduce the anisotropy of the ultimate deformability that may occur due to controlling the shape of the prior austenite grains to be relatively flattened. As a result, the inventors have found that by controlling the standard deviation of hardness from the surface of the steel plate at positions 3 / 8 to 5 / 8 of the plate thickness to 25 Hv or less, the anisotropy of the ultimate deformation capacity can be significantly reduced, and therefore a steel plate with excellent isotropy of the ultimate deformation capacity can be provided.

[0021] Generally, when there is a large difference in hardness in a metal structure, stress tends to concentrate in the relatively softer structure. Therefore, in the steel sheet according to the embodiment of the present invention, which mainly contains bainite and martensite, stress tends to concentrate in the bainite, which is a relatively soft structure. In addition, the hardness variation in the thickness direction tends to be large in the center of the steel sheet thickness due to central segregation of Mn during casting, and for this reason, stress concentration in bainite is also likely to occur. In this case, it is thought that the anisotropy of the ultimate deformability becomes more pronounced due to the relatively flattened morphology of prior austenite grains and the stress concentration in bainite. In contrast, according to the steel sheet according to the embodiment of the present invention, it is thought that stress concentration in bainite can be significantly suppressed by controlling the standard deviation of hardness from the surface of the steel sheet at positions 3 / 8 to 5 / 8 of the thickness, which corresponds to the center of the steel sheet thickness where hardness variation is particularly likely to occur, to 25 Hv or less. As a result, according to embodiments of the present invention, the anisotropy of the ultimate deformability can be significantly reduced despite the presence of relatively flattened prior austenite grains, and therefore it is possible to provide a steel sheet with excellent isotropy of the ultimate deformability.

[0022] Therefore, according to the steel sheet according to the embodiment of the present invention, despite having a high strength of 980 MPa or more in tensile strength, it is possible to achieve not only high elongation and ultimate deformability, but also excellent isotropy of the ultimate deformability. Thus, the steel sheet according to the embodiment of the present invention can reliably achieve both high strength and excellent formability, which are conflicting properties, and is particularly useful in the automotive field where these properties are required.

[0023] 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.

[0024] [C: 0.045-0.120%] Carbon (C) is an effective element for increasing the strength of steel sheets by improving the hardness of martensite. In addition, C forms carbides and / or carbonitrides with Ti and / or 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 0.045% 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 elongation will decrease, and the variation in hardness will increase, which may result in a larger standard deviation of the hardness of the final metal structure. Therefore, the C content should be 0.120% or less. The C content may be 0.115% or less, 0.110% or less, 0.105% or less, or 0.100% or less.

[0025] [Si: 0.01-3.00%] Si is an effective element for increasing strength as a solid solution strengthening element. To obtain this effect fully, the Si content should be 0.01% or more. The Si content may be 0.10% or more, 0.30% or more, 0.50% 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 3.00% or less. The Si content may be 2.00% or less, 1.50% or less, or 1.00% or less.

[0026] [Mn: 1.20-3.00%] 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 1.20% or more. The Mn content may be 1.40% or more, 1.60% or more, 1.80% or more, or 2.00% or more. On the other hand, if Mn is included in excess, the elongation will decrease due to the excessive formation of martensite, and the variation in hardness in the thickness direction will increase due to the central segregation of Mn during casting, which may result in a large standard deviation of hardness from the surface of the steel plate to the 3 / 8 to 5 / 8 position of the plate thickness. 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.

[0027] [Al: 0.010-0.400%] Al is an element that acts as a deoxidizing agent for molten steel. Also, if there is insufficient Al, a relatively large amount of inclusions may be formed, which may reduce the ultimate deformability. Therefore, the Al content should be 0.010% or more. The Al content may be 0.020% or more, 0.050% or more, 0.100% or more, 0.120% or more, or 0.140% or more. On the other hand, if there is an excessive amount of Al, coarse oxides may be formed, which may reduce toughness and ductility. Therefore, the Al content should be 0.400% or less. The Al content may be 0.300% or less, 0.250% or less, or 0.200% or less.

[0028] [P: 0.080% or less] Excessive P content may negatively affect weldability and other properties. Therefore, the P content should be 0.080% or less. The P content may also be 0.050% or less, 0.040% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.003% or more.

[0029] [S: 0.0100% or less] Excessive sulfur content can lead to the formation of large amounts of MnS, which can reduce the ultimate deformability. Therefore, the sulfur content should be 0.0100% or less. The sulfur content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the sulfur content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the sulfur content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0030] [N: 0.0050% or less] Excessive N content can form coarse nitrides, reducing toughness. Therefore, the N content should be 0.0050% or less. The N content may also be 0.0040% or less, 0.0030% or less, or 0.0020% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0031] [O: 0.0100% or less] O is an element that is introduced during the manufacturing process. Excessive O content can lead to the formation of coarse oxides, which can reduce the ultimate deformability. Therefore, the O content should be 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing it to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0032] [Ti: 0.001-0.180%] Ti precipitates as TiC in steel, improving its strength through precipitation strengthening. Ti also contributes to improved toughness by refining the microstructure through its pinning effect. To fully obtain these effects, the Ti content should be 0.001% or more. The Ti content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if Ti is included in excess, the TiC becomes coarse, and in connection with this, the aspect ratio of the prior austenite grains becomes excessively large, the ultimate deformability decreases, and its anisotropy may become pronounced. Therefore, the Ti content should be 0.180% or less. The Ti content may be 0.170% or less, 0.160% or less, 0.150% or less, 0.140% or less, or 0.130% or less.

[0033] [Nb: 0-0.100%] 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 be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if Nb is present in excess, coarse carbides and the like are formed in the steel, which can lead to an excessively large aspect ratio of the prior austenite grains, reducing the ultimate deformability and making the anisotropy more pronounced. Therefore, it is preferable that the Nb content be 0.100% or less. The Nb content may be 0.080% or less, 0.060% or less, 0.040% or less, or 0.030% or less.

[0034] [V: 0-1.000%] V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but to obtain such an effect, it is preferable that the V content be 0.001% or more. The V content may be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if the V content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the V content be 1.000% or less. The V content may be 0.500% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0035] [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 in order to obtain such an effect, the Cu content is preferably 0.001% or more, and may be 0.010% or more, 0.050% or more, 0.100% or more, or 0.150% or more. On the other hand, if the Cu content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, the Cu content is preferably 1.000% or less, and may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less.

[0036] [Cr: 0-2.000%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength. The Cr content may be 0%, but to obtain such effects, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if the Cr content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the Cr content be 2.000% or less. The Cr content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, or 0.200% or less.

[0037] [Mo: 0-3.000%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. The Mo content may be 0%, but to obtain such effects, it is preferable that the Mo content be 0.001% or more. The Mo content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if the Mo content is excessive, the deformation resistance during hot working may increase, and the equipment load may increase. Therefore, it is preferable that the Mo content be 3.000% or less. The Mo content may be 1.000% or less, 0.500% or less, 0.300% or less, or 0.150% or less.

[0038] [Ni: 0-0.500%] Ni is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.001% or more, and may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if the Ni content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, the Ni content is preferably 0.500% or less, and may be 0.300% or less, 0.100% or less, 0.075% or less, or 0.050% or less.

[0039] [B: 0-0.0100%] B improves low-temperature toughness by segregating at grain boundaries and increasing grain boundary strength. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% 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 is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, or 0.0010% or less.

[0040] [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, 0.0010% or more, 0.0020% or more, or 0.0030% 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 will lead to an increase in manufacturing costs. Therefore, the Ca content is preferably 0.0500% or less, and may be 0.0300% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0041] [Mg: 0-0.050%] Mg is an element that can control the morphology of nonmetallic inclusions. The Mg content may be 0%, but to obtain such an effect, the Mg content is preferably 0.0001% or more, and may be 0.001% or more, or 0.002% or more. On the other hand, if the Mg content is excessive, the effect will saturate, and including more Mg than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, the Mg content is preferably 0.050% or less, and may be 0.030% or less, 0.010% or less, 0.008% or less, 0.006% or less, or 0.004% or less.

[0042] [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 0.001% or more, 0.002% or more, or 0.004% or more. On the other hand, if REM is included in excess, the effect will saturate, and including more REM in the steel sheet than necessary will 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, 0.050% or less, 0.030% 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.

[0043] [Bi: 0-0.100%] Bi is an element effective in improving corrosion resistance. The Bi content may be 0%, but to obtain such an effect, it is preferable that the Bi content be 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 plate than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the Bi content be 0.100% or less. The Bi content may also be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

[0044] [Ta: 0 to 0.100%] Ta is an element effective for controlling the morphology of carbides and increasing strength. The Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, if Ta is contained excessively, a large number of fine Ta carbides precipitate, leading to an excessive increase in the strength of the steel material and, as a result, possibly reducing ductility. Therefore, the Ta content is preferably 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.

[0045] [Zr: 0 to 0.500%] Zr is an element that can control the morphology of non-metallic inclusions. The Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.001% or more. The Zr content may be 0 / 010% or more, 0.050% or more, 0 / 100% or more, 0.200% or more, or 0.250% or more. On the other hand, even if Zr is contained excessively, the effect saturates, and including it in the steel sheet more than necessary leads to an increase in production cost. Therefore, the Zr content is preferably 0.500% or less. The Zr content may be 0.400% or less, 0.350% or less, 0.330% or less, or 0.310% or less.

[0046] [Co: 0 to 3.000%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be O%, but in order to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.010% or more, 0.050% or more, <0.100% or more, or 0.150% or more. On the other hand, if Co is contained excessively, the hot workability may decrease, which also leads to an increase in raw material cost. Therefore, the Co content is preferably 3.000% or less. The Co content may be 1.000% or less, 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less.

[0047] [Zn: 0 to 0.200%] Zn is an element that can be contained in the steel sheet when using scrap or the like as the steel raw material. Therefore, the Zn content is preferably 0.200% or less, and may be 0.180% or less, 0.160% or less, or 0.140% or less. The Zn content may be 0%, but reducing it to less than 0.001% requires time for refining and causes a decrease in productivity. Therefore, the Zn content may be 0.001% or more, 0.010% or more, 0.050% or more, 0.080% or more, or 0.100% or more.

[0048] [W: 0 to

[0048] 0.200%] W is an element that enhances the hardenability of steel and contributes to an improvement in strength. The W content may be 0%, but in order to obtain such an effect, the W content is preferably 0.

[0048] 001% or more. The W content may be 0.010% or more, 0.050% or more, 0.100% or more, 0.130% or more, 0.150% or more, or 0.170% or more. On the other hand, if the W content is excessively high, the weldability may decrease. Therefore, the W content is preferably 0.200% or less. The W content may be 0.190% or less.

[0049] [Sb: 0 to 0.500%] Sb is an element effective for improving corrosion resistance. The Sb content may be 0%, but in order to obtain such an effect, the Sb content is preferably 0.001% or more, and may be 0.005% or more, 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if the Sb content is excessively high, the toughness may decrease. Therefore, the Sb content is preferably 0.500% or less, and may be 0.300% or less, 0.200% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0050] [As: 0-0.050%] 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 be 0.002% or more, 0.003% or more, or 0.004% 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 will lead to an increase in manufacturing costs. Therefore, it is preferable that the As content be 0.050% or less. The As content may be 0.030% or less, 0.020% or less, 0.010% or less, or 0.008% or less.

[0051] [Sn: 0-0.050%] Sn is an element effective in improving corrosion resistance. The Sn content may be 0%, but to obtain this effect, the Sn content is preferably 0.001% or more, and may be 0.005% or more, 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, excessive Sn content may lead to a decrease in toughness. Therefore, the Sn content is preferably 0.050% or less. The Sn content may also be 0.045% or less.

[0052] In the steel sheet according to the embodiment of the present invention, the remainder other than the above-mentioned elements may include Fe and impurities. Impurities are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap, and components that are included in a range that does not affect the effect of the present invention. If necessary, the total content of impurities may be 1.00% or less, 0.50% or less, 0.10% or less, or 0.05% or less.

[0053] 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.

[0054] [Metal structure] [Bainite: 70-95%, and martensite: 5-30%] The metal structure of the steel sheet according to the embodiment of the present invention contains, by area percentage, bainite: 70-95% and martensite: 5-30%. By constructing the metal structure of the steel sheet with a structure mainly composed of bainite in this way, while including a predetermined amount of martensite, it is possible to increase the strength while increasing the elongation of the steel sheet. From the viewpoint of improving elongation, a higher area ratio of bainite is preferable, for example, it may be 75% or more, 78% or more, or 80% or more. On the other hand, if the area ratio of bainite becomes too high, the area ratio of martensite becomes low, and as a result, it may not be possible to achieve the desired strength, for example, a tensile strength of 980 MPa or more. Therefore, the area ratio of bainite is set to 95% or less, for example, 90% or less, 88% or less, or 85% or less.

[0055] From the viewpoint of improving strength, a higher area ratio of martensite is preferable, for example, it may be 7% or more, 10% or more, or 15% or more. On the other hand, from the viewpoint of improving elongation, a lower area ratio of martensite is preferable, for example, it may be 25% or less, 22% or less, or 20% or less.

[0056] [Residual Structure] The metallic structure of the steel sheet according to the embodiment of the present invention includes bainite and martensite as described above, and may also include other residual structures, but the area ratio of the residual structure is preferably small, and may be 0%. The area ratio of the residual structure is not particularly limited, but may be, for example, 0 to 10%, 0 to 8%, 0 to 5%, 0 to 4%, or 0 to 3%. In other words, the total area ratio of bainite and martensite may be, for example, 90 to 100%, 92 to 100%, 95 to 100%, 96 to 100%, or 97 to 100%. The lower limit of the residual structure may be 1% or 2%. If residual structure is present, it may include at least one of ferrite, pearlite, and retained austenite. From the viewpoint of improving ultimate deformability, it is preferable that the residual structure does not contain ferrite (i.e., the area ratio of ferrite is 0%).

[0057] [Identification of Metallic Structure and Calculation of Area Ratio] The area ratios of bainite, martensite, and the remaining structure are measured by the following method. Note that in addition to carbides such as cementite, various other precipitates may be present in each of the aforementioned structures. These precipitates will be treated as the area ratio of each structure (e.g., bainite, martensite, ferrite, or pearlite) in which they are present. A test piece is taken from the steel plate so that the metallic structure at the 1 / 4 thickness position (from the surface in the thickness direction, in the range from the 1 / 8 thickness position to the 3 / 8 thickness position) can be observed. The thickness cross section of the test piece is finished with mirror polishing, and after LePera etching, a 200 μm (thickness direction) × 600 μm (perpendicular to the thickness direction) region at the 1 / 4 thickness position is observed using an FE-SEM (thermal field emission scanning electron microscope, JEOL JSM-7001F), and image analysis is performed.

[0058] In repera corrosion, martensite and retained austenite are not corroded. Therefore, by calculating the area percentage of the uncorroded region, the sum of the area percentages of martensite and retained austenite can be obtained.

[0059] The area fraction of retained austenite is obtained by X-ray diffraction. For a test piece taken from a steel plate, the plate surface is ground down to a position 1 / 4 of the plate thickness (from the surface in the thickness direction from a position 1 / 8 of the plate thickness to a position 3 / 8 of the plate thickness), and the exposed surface is used as the observation surface. After mirror polishing this observation surface, it is finished by electropolishing. Using a Rigaku RINT-2500, Mo-Kα, the integrated intensities of a total of five peaks, α(200), α(211), γ(200), γ(220), and γ(311), are determined on the observation surface, and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0060] The total area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained by X-ray diffraction from the sum of the area ratios of martensite and retained austenite obtained by observation using the FE-SEM described above. If the calculated total area ratio of martensite is a negative value, the total area ratio of martensite is set to 0%.

[0061] The area ratio of pearlite is obtained by the following method: In the same region (200 μm × 600 μm) used to determine the area ratios of martensite and retained austenite, only the eroded layer is removed by polishing and the surface is mirror-finished. Then, the region is etched with Nital solution, observed using FE-SEM, and image analysis is performed. Regions in which cementite and ferrite are arranged in a lamellar pattern are identified as pearlite, and the area ratio of that region is calculated to obtain the area ratio of pearlite.

[0062] The area fraction of ferrite is obtained by the following method. Furthermore, the following procedure is performed on areas other than those identified as pearlite by the above method. Colloidal polishing or electrolytic polishing is performed on the same area (200 μm × 600 μm) used when determining the area fractions of martensite and retained austenite, and then crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.2 μm. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity® ultrafast EBSD detector) is used. At this time, the vacuum level inside the apparatus is 9.6 × 10⁻⁶. －５ The pressure should be below Pa, the acceleration voltage 25kV, and the irradiation current level 16.

[0063] Using the obtained crystal orientation information, the following analysis is performed using version 7 or later of OIM Analysis (registered trademark) from EDAX / TSL solution. Measurement points with a crystal orientation difference of 15° or more are considered grain boundaries, and the region enclosed by these grain 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 considered ferrite, and their area fraction is calculated to obtain the ferrite area fraction.

[0064] The area ratio of bainite is obtained by subtracting the area ratios of martensite, retained austenite, pearlite, and ferrite obtained above from 100%. If the calculated area ratio of bainite is a negative value, the area ratio of bainite is set to 0%. In this embodiment, since the area ratio of the metal structure is calculated by image analysis using FE-SEM, X-ray diffraction, and EBSD analysis, the sum of each structure may not equal 100%. In that case, the area ratio of each structure is corrected so that the sum equals 100%. For example, if the sum of the area ratios of each structure is 103%, the area ratio of each structure is corrected by multiplying it by "100 / 103".

[0065] The observation conditions for the FE-SEM are as follows: Electron gun type: Thermal emission type Current irradiation number: 9 Working distance (WD): 10 mm Acceleration voltage: 20 kV Objective aperture number: 4 Pixel count: 5120 x 3840

[0066] [Average aspect ratio of prior austenite grains at a position 1 / 4 of the plate thickness from the surface: 3.00 to 5.50] In the metal structure of the steel plate according to the embodiment of the present invention, the average aspect ratio of prior austenite grains at a position 1 / 4 of the plate thickness from the surface is 3.00 to 5.50. By controlling the average aspect ratio of prior austenite grains to 3.00 to 5.50, it is possible to maintain a high level of ultimate deformability while significantly improving elongation due to the combination with the bainite area ratio described above. From the viewpoint of improving elongation, a higher average aspect ratio of prior austenite grains is preferable, for example, it may be 3.20 or higher, 3.50 or higher, 3.80 or higher, or 4.00 or higher. On the other hand, from the viewpoint of improving ultimate deformability, a lower average aspect ratio of prior austenite grains is preferable, for example, it may be 5.00 or lower, 4.80 or lower, 4.60 or lower, 4.40 or lower, or 4.20 or lower. The aspect ratio of the old austenite grain is the value obtained by dividing the long axis of the old austenite grain by its short axis, and it takes a value of 1.00 or greater.

[0067] [Measurement of the average aspect ratio of prior austenite grains at a position 1 / 4 of the plate thickness from the surface] The average aspect ratio of prior austenite grains is measured by the following method. A sample is taken at an arbitrary position at a distance greater than or equal to the plate thickness from the end face, preferably at a position 1 / 4 of the plate width from the end face, so that the metallographic structure of the cross section (plate thickness direction × rolling direction cross section) with the plate width direction as the normal direction can be observed. The rolling direction of the steel plate is determined by the following method. A test piece is taken so that the plate thickness cross section of the steel plate can be observed. The test piece is taken with the direction perpendicular to the plate surface as the Z direction, and a total of six pieces are taken by rotating the Z direction every 30° around this axis. The plate thickness cross section of the taken test piece is polished, and the prior austenite grain boundaries are exposed using the etching solution described above, and the average value of the aspect ratio of the prior austenite grains is calculated by the cutting method. The test piece with the largest average aspect ratio of prior austenite grains is identified, and the direction in which that test piece was taken is determined to be the rolling direction of the steel plate. In other words, the direction parallel to the thickness cross-section of the test specimen and perpendicular to the thickness direction is determined to be the rolling direction of the steel plate. The size of the sample depends on the measuring device, but for example, a rectangular parallelepiped with the total thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction may be used. Next, after mirror polishing the observation surface, it is etched using a picric acid saturated aqueous solution according to the Bechet-Beaujard method, which uses a picric acid saturated aqueous solution as specified in Annex JA. 2 of JIS G 0551:2020. The grains that appear black due to the etching are identified as prior austenite grains. The observation surface in which the prior austenite grains are revealed is observed with an optical microscope, and at a magnification of 1000x or more, eight or more fields of view with a field of view of 200 μm in the thickness direction and 600 μm or more in the rolling direction are photographed for the internal region (the region from the surface to a depth of 1 / 8 of the thickness to a depth of 3 / 8 of the thickness) with a field of view of 200 μm in the thickness direction and 600 μm or more in the rolling direction are photographed. From the captured tissue images, the ratio of the long axis to the short axis obtained for each prior austenite grain is calculated, and the average aspect ratio of the prior austenite grains is calculated by weighting it by the area of ​​each prior austenite grain according to the following formula. Here, di is the aspect ratio (major axis / minor axis) of the i-th ancient austenite grain, Ai is the area of ​​the i-th ancient austenite grain, and n is the total number of ancient austenite grains measured. If the above method fails to sufficiently reveal the prior austenite grains, the prior austenite grains are identified and their average aspect ratio is determined by the reconstruction method described in "Development of a Reconstruction Method of Prior Austenite Microstructure Using EBSD Data of Martensite" (Kengo HATA, Masayuki WAKITA, Kazuki FUJIWARA, Kaori KAWANO, NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 114 (2017), pp. 26-31).

[0068] If the sample contains prior austenite grains with an equivalent diameter of less than 2 μm, these grains are excluded before performing the above measurements. This is because prior austenite grains with an equivalent diameter of less than 2 μm do not adversely affect the properties of the steel sheet according to this embodiment.

[0069] [Standard deviation of hardness at positions 3 / 8 to 5 / 8 of the plate thickness from the surface: 25 Hv or less] In the metal structure of the steel plate according to the embodiment of the present invention, the standard deviation of hardness at positions 3 / 8 to 5 / 8 of the plate thickness from the surface is 25 Hv or less. As mentioned above, the morphology of the prior austenite grains is carried over to the final metal structure, so if the prior austenite grains are flattened, elongated martensite in the rolling direction is likely to remain in the metal structure. When elongated martensite in the rolling direction remains, anisotropy of the ultimate deformability is likely to occur in the rolling direction (L direction) and the width direction perpendicular to it (C direction). For this reason, when the average aspect ratio of the prior austenite grains is controlled to 3.00 to 5.50, anisotropy of the ultimate deformability is naturally likely to occur. However, in the steel sheet according to the embodiment of the present invention, by controlling the standard deviation of hardness from the steel sheet surface at positions 3 / 8 to 5 / 8 of the sheet thickness, which corresponds to the center of the sheet thickness where hardness variations tend to occur due to central segregation of Mn, etc., to 25 Hv or less, it becomes possible to significantly suppress stress concentration in bainite, which is a relatively soft structure among the metall structure mainly composed of bainite and martensite. As a result, despite containing relatively flattened prior austenite grain morphology, the anisotropy of the ultimate deformability can be significantly reduced, and therefore it becomes possible to provide a steel sheet with excellent isotropy of the ultimate deformability. From the viewpoint of improving the isotropy of the ultimate deformability, a lower standard deviation of hardness is preferable, and may be, for example, 22 Hv or less, 20 Hv or less, 15 Hv or less, or 10 Hv or less. The lower limit is not particularly limited, but the standard deviation of hardness may be, for example, 1 Hv or more or 5 Hv or more.

[0070] [Measurement of standard deviation of hardness at positions 3 / 8 to 5 / 8 of the plate thickness from the surface] The hardness at positions 3 / 8 to 5 / 8 of the plate thickness from the surface is measured using a micro-Vickers test in accordance with JIS Z 2244-1:2020. Samples for the micro-Vickers test can be prepared as follows. First, a sample is cut from any position at least 50 mm away from the end face (if a sample cannot be taken from this position, a position avoiding the end face is used) so that a cross-section perpendicular to the plate surface and parallel to the rolling direction can be observed. It is preferable that the cross-section is parallel to the rolling direction. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the direction perpendicular to the plate thickness. After polishing the cross-section of the above sample using silicon carbide paper from #600 to #1500, it is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the observation surface is finished by electropolishing. The micro-Vickers test measures hardness with a test force of 0.9807 N (load 100 gf) along a straight line oblique to the thickness direction, from the 3 / 8 to 5 / 8 position from the surface, such that the spacing between indentations in the thickness direction is 10 μm and the minimum distance between adjacent indentations is at least three times the average diagonal length d of the indentations. However, if the minimum distance between adjacent indentations is less than three times the average diagonal length d of the indentations, the measurement position is shifted in a direction perpendicular to the thickness direction. In this case, there will be many straight lines oblique to the thickness direction. For example, if you measure the hardness at five points at 82 μm intervals along a straight line with a 7° slope perpendicular to the thickness direction, and then repeat the process of measuring the hardness at five points at 82 μm intervals along the same 7° slope (whereas the distance in the thickness direction between the first measurement point on an adjacent straight line and the last measurement point on the previous straight line is 10 μm; in this case, the distance in the thickness direction between adjacent straight lines with a 7° slope is 50 μm), you can measure the hardness at 10 μm intervals in the thickness direction within an area of ​​approximately 0.4 mm width (width perpendicular to the thickness direction). Then, by calculating the standard deviation based on the hardness obtained from all measurement points, you can obtain the standard deviation of hardness from the surface at positions 3 / 8 to 5 / 8 of the thickness.

[0071] [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.

[0072] As described above, the steel sheet according to the embodiment of the present invention achieves high strength, high elongation and ultimate deformability, as well as excellent isotropy of the ultimate deformability. Therefore, it is possible to reliably achieve a high level of balance between the conflicting properties of high strength and excellent formability. Accordingly, 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, which 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 metallic 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 metallic structure characteristics do not change particularly before and after forming.

[0073] [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, a specific combination of the chemical composition and metal structure described above can achieve high elongation and ultimate deformability, as well as excellent isotropy of the ultimate deformability. 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 plate cannot be determined, a JIS No. 5 test specimen may be taken from any direction on the surface of the steel plate.

[0074] [Total Elongation (EL)] According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, elongation can also be improved, and more specifically, a total elongation of 10.0% or more can be achieved. The total elongation is preferably 12.0% or more. There is no particular upper limit, but for example, the total elongation may be 30.0% or less, 25.0% or less, or 20.0% 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 surface of the steel sheet. If it is difficult to obtain a JIS No. 5 test specimen, a small test specimen may be used. In this case, a known conversion formula (for example, the Iron and Steel Institute of Japan Standardization Committee Data Sheet Subcommittee: p. 1136, No. 8, vol. 56 (1970) "Dimensional effect of test specimen on elongation value") can be used to convert it to the total elongation of a JIS No. 5 test specimen.

[0075] [Ultimate Deformability: Thickness Reduction Rate After Tensile Test in the L and C Directions] According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the ultimate deformability can also be improved. Specifically, the thickness reduction rate after tensile testing in the L and C directions (hereinafter also simply referred to as the thickness reduction rate), which is an indicator of ultimate deformability, can be improved, and more specifically, a thickness reduction rate of 60.0% or more in the L and C directions can be achieved. The thickness reduction rate in the L and C directions is preferably 65.0% or more. There is no particular upper limit, but for example, the thickness reduction rate in the L and C directions may be 80.0% or less or 75.0% or less. The thickness reduction rate in the L direction is determined by taking a JIS No. 5 test piece from the direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel sheet (L direction) and performing a tensile test in accordance with JIS Z2241:2022. Specifically, first, the thickness of the specimen is measured at the center in the longitudinal and width directions before the tensile test, and then the fractured portion is embedded in resin with the ends butted together after the tensile test. After that, the specimen is cut at the center of its width, and the thickness of the fractured portion is measured by cross-sectional observation. The "thickness of the fractured portion after the tensile test" will be explained in more detail using Figure 1. Figure 1 shows a cross-section 10a at the center in the width direction of the specimen 10 after the tensile test, and has a fractured portion 10b because it has fractured due to the tensile test. The thickness 11 of the fractured portion is defined as the "thickness of the fractured portion after the tensile test". Using the measured thicknesses, the thickness reduction rate is calculated from (1 - thickness of the fractured portion after the tensile test / thickness of the center in the longitudinal and width directions before the tensile test) × 100. Furthermore, the plate thickness reduction rate in the C direction is calculated by taking a JIS No. 5 test specimen 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 (C direction), and performing a tensile test in the same manner as for the plate thickness reduction rate in the L direction.

[0076] [Isotropy of Ultimate Deformability: Difference in Thickness Reduction Rate After Tensile Tests in the L and C Directions] According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, excellent isotropy of ultimate deformability can also be achieved. Specifically, the difference in thickness reduction rate after tensile tests in the L and C directions (hereinafter also simply referred to as the LC difference of thickness reduction rate) as an indicator of the isotropy of ultimate deformability can be improved, and more specifically, an LC difference of thickness reduction rate of 8.0% or less can be achieved. The LC difference of thickness reduction rate is preferably 4.0% or less. The lower limit is not particularly limited, but for example, the LC difference of thickness reduction rate may be 0.1% or more or 0.3% or more. The LC difference of thickness reduction rate is determined by calculating the difference in thickness reduction rates in the L and C directions based on the thickness reduction rates in the L and C directions measured by the method described above.

[0077] The steel sheets according to the embodiments of the present invention include any steel sheets having the chemical composition and metal structure described above, namely hot-rolled steel sheets, cold-rolled steel sheets, and steel sheets having a plating layer on at least one surface thereof. If necessary, the invention may be limited to hot-rolled steel sheets (including steel sheets having a plating layer on at least one surface thereof).

[0078] <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 as mentioned above, steel sheets according to embodiments of the present invention include not only any steel sheets having the chemical composition and metal structure described above, i.e., hot-rolled steel sheets, but also cold-rolled steel sheets, plated steel sheets, etc. Therefore, the following description merely describes preferred manufacturing methods when the steel sheet according to embodiments of the present invention is a hot-rolled steel sheet.

[0079] A method for manufacturing a steel sheet according to an embodiment of the present invention includes a hot rolling step that includes heating a slab having the chemical composition described above in relation to a steel sheet, followed by rough rolling and finish rolling, and satisfies the following conditions (a) to (c): (a) the slab heating temperature is 1100 to 1300°C; (b) in rough rolling, rolling is performed at a temperature of 1160 to 1210°C and the reduction ratio is 35% or more, two or more times; (c) in finish rolling, the total reduction ratio in the temperature range of less than 1050°C is 40 to 60%; and (d) The exit temperature of the final pass in finish rolling is 895°C or lower. The method is characterized by including a slow cooling step in which the finish-rolled steel sheet is cooled to a slow cooling start temperature of 500 to 680°C at an average cooling rate of 30°C / second or more, and then slow-cooled at an average cooling rate of 10°C / second or less for 2.0 seconds or more, and an accelerated cooling step in which the slow-cooled steel sheet is accelerated-cooled to 200°C at an average cooling rate of 30 to 120°C / second, and then wound at a winding temperature of 100°C or lower.

[0080] [Hot Rolling Process] [(a) Slab Heating Temperature: 1100-1300°C] 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 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 is preferably 1100°C or higher, and more preferably 1230°C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of the capacity of the heating equipment and productivity, it is preferably 1300°C or lower, and more preferably 1270°C or lower. Furthermore, from the viewpoint of solid-solving the alloying elements in the slab, it is preferable to set the holding time in the temperature range of 1100 to 1300°C to 1000 seconds or more. There is no particular upper limit to the holding time, but from the viewpoint of productivity, it is preferable to set it to 4000 seconds or less.

[0081] [Rough Rolling] [(b) Rolling at a temperature of 1160-1210°C and a reduction ratio of 35% or more: 2 or more times] In this manufacturing method, rough rolling is extremely important for obtaining the desired standard deviation of hardness. This is because by performing rolling at a temperature of 1160-1210°C and a reduction ratio of 35% or more two or more times during rough rolling, recrystallization is promoted up to the center of the plate thickness, thereby reducing the variation in austenite grain size across the entire plate thickness. If there is variation in the grain size of austenite grains, variations in hardness are likely to occur in the hard structure generated during the accelerated cooling process, especially in the martensite. Therefore, in order to control the standard deviation of hardness at the 3 / 8 to 5 / 8 position from the surface of the steel plate to 25 Hv or less in the final obtained metal structure, it is extremely important to perform rough rolling at a temperature of 1160-1210°C and a reduction ratio of 35% or more two or more times.

[0082] If the temperature is lower than 1160°C, the reduction ratio is lower than 35%, and / or the number of rough rolling cycles that satisfy the temperature and reduction ratio conditions is one or less, recrystallization cannot be sufficiently promoted across the entire thickness of the sheet. As a result, it may not be possible to obtain the desired standard deviation of hardness. The upper limits of the temperature and reduction ratio in rough rolling are not particularly limited and should be determined from the viewpoint of cracking during slab heating, rough rolling load, and ensuring the temperature for finish rolling. Although not particularly limited, the reduction ratio in rough rolling may be 50% or less. Furthermore, there is no particular upper limit to the number of rough rolling cycles at a temperature of 1160 to 1210°C and a reduction ratio of 35% or more, and the number of such rough rolling cycles may be, for example, five or fewer or three or fewer.

[0083] [(c) Total reduction ratio in the temperature range below 1050°C during finish rolling: 40-60%] In this manufacturing method, finish rolling is also very important for obtaining the microstructure of the steel sheet, in particular the desired average aspect ratio of prior austenite and bainite area ratio. By performing finish rolling in the non-recrystallization temperature range below 1050°C, the austenite grains can be made into a relatively flattened shape, making it possible to form prior austenite grains with an average aspect ratio of 3.00 to 5.50 in the final microstructure. In addition, the rolling applied to form relatively flattened austenite grains introduces a relatively large number of dislocations into the steel sheet after hot rolling. Here, since bainite is nucleated from grain boundaries and dislocations, by applying appropriate cooling to the steel sheet after hot rolling, which has a large grain boundary area and many introduced dislocations, it is possible to generate a relatively large amount of bainite using these many grain boundaries and dislocations as nucleation sites, making it possible to achieve the desired bainite area ratio in the final microstructure. When finishing rolling is performed at temperatures above 1050°C, recrystallization occurs, making it impossible to obtain the desired average aspect ratio of prior austenite grains in the final resulting metal structure. Similarly, recrystallization reduces the number of grain boundaries and dislocations that function as bainite nucleation sites, which may result in the inability to obtain the desired bainite area ratio.

[0084] Furthermore, if the total reduction ratio in the temperature range below 1050°C is less than 40%, the strain introduced will be small, and therefore the number of dislocations introduced will also be small, which may result in the average aspect ratio of the prior austenite grains being less than 3.00 and / or the bainite area ratio being less than 70%. Therefore, the total reduction ratio in the temperature range below 1050°C should be 40% or more, preferably 43% or more. On the other hand, if the total reduction ratio in the temperature range below 1050°C is more than 60%, the strain introduced will be large, and therefore the number of dislocations introduced will also be large, which may result in the average aspect ratio of the prior austenite grains exceeding 5.50 and / or the bainite area ratio exceeding 95%. Therefore, the total reduction ratio should be 60% or less, preferably 57% or less. Here, "total reduction ratio in the temperature range below 1050°C (%)" is the reduction ratio defined by the following formula, where t1 is the plate thickness when the temperature drops below 1050°C and t2 is the final plate thickness. Total reduction ratio (%) in the temperature range below 1050°C = 100 × {1 - (t2 / t1)}

[0085] [Exit temperature of the final pass in finish rolling: 895°C or less] In this manufacturing method, the exit temperature of the final pass in finish rolling is controlled to 895°C or less. If the exit temperature of the final pass in finish rolling exceeds 895°C, the dislocations introduced into the austenite grains by hot rolling recover and decrease unevenly, causing variations in the transformation rate and potentially increasing the standard deviation of the hardness of the final steel sheet. Therefore, the exit temperature of the final pass in finish rolling is set to 895°C or less. On the other hand, there is no particular lower limit to the exit temperature of the final pass in finish rolling, but it may be, for example, 860°C or higher.

[0086] [Slow Cooling Process] In the next slow cooling process, the finish-rolled steel sheet is cooled to a slow cooling start temperature of 500-680°C at an average cooling rate of 30°C / second or more, particularly on a runout table (ROT), and then slowly cooled, for example, by air cooling, at an average cooling rate of 10°C / second or less for 2.0 seconds or more. Performing such slow cooling allows the bainite transformation to proceed sufficiently, making it possible to achieve the desired bainite area ratio in the final resulting metal structure. If the slow cooling start temperature is above 680°C, excessive ferrite will be generated, and therefore the desired bainite and / or martensite area ratio cannot be obtained in the final resulting metal structure. Therefore, the slow cooling start temperature should be 680°C or lower, preferably 670°C or lower. On the other hand, if the slow cooling start temperature is below 500°C, there will be insufficient bainite, and therefore the desired metal structure cannot be obtained in the final resulting steel sheet. Therefore, the slow cooling start temperature should be 500°C or higher, preferably 510°C or higher.

[0087] On the other hand, if the average cooling rate of slow cooling exceeds 10°C / second, martensitic transformation occurs, and therefore a relatively large amount of martensite is generated in the final metal structure, making it impossible to achieve the desired elongation. Therefore, the average cooling rate of slow cooling should be 10°C / second or less, preferably 8°C / second or less. The lower limit is not particularly limited, and the average cooling rate of slow cooling may be, for example, 1°C / second or more. Also, if the slow cooling time is less than 2.0 seconds, bainite will not be sufficiently generated, and therefore the desired bainite area ratio cannot be obtained in the final metal structure. On the other hand, the upper limit is not particularly limited, and from the viewpoint of productivity, the slow cooling time may be, for example, 10.0 seconds or less, 8.0 seconds or less, or 6.0 seconds or less.

[0088] [Accelerated Cooling Process] The slowly cooled steel sheet is then accelerated to 100°C at an average cooling rate of 30 to 120°C / second in the next accelerated cooling process, and then wound up at a winding temperature of 100°C or lower. By accelerating the cooling at a relatively fast average cooling rate after slow cooling, martensite can be appropriately precipitated, making it possible to form a metal structure in the final steel sheet that contains bainite and martensite in specific proportions. On the other hand, if the average cooling rate of the accelerated cooling is less than 30°C / second, sufficient martensite will not be generated, and therefore the desired martensite area ratio cannot be obtained in the final metal structure. In such cases, the desired tensile strength cannot be achieved. Therefore, the average cooling rate should be 30°C / second or higher, preferably 34°C / second or higher. On the other hand, if the average cooling rate of the accelerated cooling exceeds 120°C / second, sufficient bainite will not be generated, and / or excessive martensite will be generated, and similarly, the desired bainite area ratio and / or martensite area ratio cannot be obtained in the final metal structure. Therefore, the average cooling rate should be 120°C / second or less, preferably 100°C / second or less.

[0089] If the winding temperature exceeds 100°C, the cooling rate in the temperature range above 100°C, specifically, for example, 100 to 300°C, decreases, which can lead to tempering and decomposition of MA (Martensite-Austenite constant; a composite phase of martensite and austenite). Since tempering and MA decomposition vary depending on local differences in elemental concentration, microstructure size, and morphology, variations in hardness can occur in the center of the sheet thickness where these differences are large, and the standard deviation of the hardness of the final steel sheet may increase. Therefore, the winding temperature should be 100°C or lower. The lower limit is not particularly limited, but for example, the winding temperature may be 15°C or higher or 20°C or higher.

[0090] 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 containing C: 0.045 to 0.120% and Mn: 1.20 to 3.00% by mass%, to contain bainite: 70 to 95% and martensite: 5 to 30% by area%, it is possible to improve elongation while achieving high strength, for example, a tensile strength of 980 MPa or more. Furthermore, by controlling the average aspect ratio of prior austenite grains at a position 1 / 4 of the sheet thickness from the surface to within the range of 3.00 to 5.50, it is possible to more significantly improve elongation while maintaining a high level of ultimate deformability. In addition, by controlling the standard deviation of hardness at positions 3 / 8 to 5 / 8 of the sheet thickness from the surface to 25 Hv or less, the anisotropy of the ultimate deformability can be significantly reduced, and therefore it is possible to provide a steel sheet with excellent isotropy in ultimate deformability. Therefore, steel sheets manufactured by the above manufacturing method can achieve both high strength and excellent workability, which are conflicting properties, making them particularly useful in the automotive sector where both of these properties are required.

[0091] 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.

[0092] In the following embodiments, steel sheets according to the present invention, particularly hot-rolled steel sheets, were manufactured under various conditions, and the tensile strength, total elongation, thickness reduction rate, and LC difference of the thickness reduction rate of the obtained steel sheets were investigated.

[0093] 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 under the conditions shown in Table 3 and held for 1000 to 2000 seconds, after which hot rolling was performed. Hot rolling was carried out by performing rough rolling and finish rolling under the conditions shown in Table 3. In Table 3, the notation "G" in rough rolling indicates that rolling was performed two or three times at a temperature of 1160 to 1210°C with a reduction ratio of 35% or more, and the notation "NG" indicates that rolling was performed less than two times at a temperature of 1160 to 1210°C with a reduction ratio of 35% or more. Next, the finish-rolled steel sheets were water-cooled at an average cooling rate of 30°C / second or more to the slow cooling start temperature shown in Table 3, and then slowly cooled (air-cooled) at an average cooling rate of 10°C / second or less under the conditions shown in Table 3. The slowly cooled steel sheets were then processed under manufacturing No. For samples 1-43, 45, and 46, the steel sheets were accelerated to at least 100°C at the average cooling rate shown in Table 3, wound at the winding temperature shown in Table 3, and obtained steel sheets with a thickness of 2.9 mm. For production No. 44, the steel sheets were accelerated to 301°C at the average cooling rate shown in Table 3, wound at the winding temperature shown in Table 3, and obtained steel sheets with a thickness of 2.9 mm.

[0094]

[0095]

[0096]

[0097] The properties of the obtained steel plates were measured and evaluated by the following method.

[0098] [Tensile Strength (TS) and Total Elongation (EL)] 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.

[0099] [Thickness Reduction Rate] The thickness reduction rate was calculated by measuring the thickness reduction rate in the L direction and the thickness reduction rate in the C direction, and taking the smaller of the two values ​​as the thickness reduction rate of the steel plate. The thickness reduction rate in the L direction was calculated by taking a JIS No. 5 test piece from the direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate (L direction), and performing a tensile test in accordance with JIS Z2241:2022. For the test piece, the thickness of the plate at the center in the longitudinal and width directions before the tensile test and the thickness of the plate at the fracture point after the tensile test were measured, and the thickness was calculated from (1 - thickness of the plate at the fracture point after the tensile test / thickness of the plate at the center in the longitudinal and width directions before the tensile test) × 100. Furthermore, the plate thickness reduction rate in the C direction was calculated by taking a JIS No. 5 test specimen 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 (C direction), performing a tensile test in the same manner as for the plate thickness reduction rate in the L direction, and calculating the plate thickness at the fracture site before and after the tensile test.

[0100] [LC difference of plate thickness reduction rate] Based on the plate thickness reduction rates in the L direction and C direction measured by the method described above, the LC difference of the plate thickness reduction rate was determined by calculating the difference between the plate thickness reduction rates in the L direction and C direction.

[0101] Steel sheets with a tensile strength (TS) of 980 MPa or higher, a total elongation (EL) of 10.0% or higher, a plate thickness reduction rate (an indicator of ultimate deformation capacity) of 60.0% or higher, and an LC difference in plate thickness reduction rate (an indicator of isotropy of ultimate deformation capacity) of 8.0% or lower were evaluated as having high strength, high elongation and ultimate deformation capacity, and excellent isotropy of ultimate deformation capacity. The results are shown in Table 4. The measurement results of the area ratio of the metal structure in Table 4 were rounded to the first decimal place and then proportionally apportioned so that the sum equals 100%.

[0102]

[0103] Referring to Tables 1-4, in Comparative Example 1, the high carbon content resulted in the formation of a large amount of martensite, leading to a decrease in EL (electroluminescence). Furthermore, the hardness of the martensite increased, resulting in a larger hardness difference within the metal structure, and consequently, a larger standard deviation of hardness in the metal structure. As a result, the LC (coefficient of reduction) difference in the plate thickness reduction rate increased. On the other hand, in Comparative Example 2, the low carbon content resulted in insufficient martensite hardness. As a result, TS (total strength) decreased.

[0104] In Comparative Example 3, the high Mn content resulted in the formation of a large amount of martensite, and the central segregation of Mn during casting likely led to a larger standard deviation in the hardness of the final metal structure. As a result, the EL decreased, and the LC difference in the plate thickness reduction rate increased. On the other hand, in Comparative Example 4, the low Mn content likely resulted in insufficient martensite formation. As a result, the area ratio of martensite decreased, and the TS decreased.

[0105] In Comparative Example 5, the high Ti content likely led to the formation of coarse carbides and other materials, resulting in an excessively large aspect ratio of the prior austenite grains in the final metal structure. As a result, the plate thickness reduction rate decreased, and the LC difference of the plate thickness reduction rate increased. In Comparative Example 6, the high Nb content likely led to the formation of coarse carbides and other materials, resulting in an excessively large aspect ratio of the prior austenite grains in the final metal structure. As a result, the plate thickness reduction rate decreased, and the LC difference of the plate thickness reduction rate increased. In Comparative Example 7, the low Al content likely led to the formation of a relatively large number of inclusions in the steel plate. As a result, the plate thickness reduction rate decreased. In Comparative Example 8, the high S content likely led to the formation of a large amount of MnS in the steel plate. As a result, the plate thickness reduction rate decreased. In Comparative Example 9, the high O content likely led to the formation of coarse oxides. As a result, the plate thickness reduction rate decreased.

[0106] In Comparative Example 37, the number of rolling cycles at temperatures of 1160-1210°C and reduction ratios of 35% or more was insufficient during rough rolling. As a result, recrystallization could not be promoted across the entire thickness of the sheet, leading to variations in the austenite grain size during the rough rolling stage. Consequently, the grain sizes of the bainite and martensite obtained in the final product also varied, resulting in a larger standard deviation of hardness. Consequently, the LC difference in the sheet thickness reduction rate increased.

[0107] In Comparative Example 38, the total reduction ratio in the temperature range below 1050°C during finish rolling was high, which is thought to have resulted in a large amount of strain being introduced and a high aspect ratio of the prior austenite grains. As a result, the plate thickness reduction rate decreased. On the other hand, in Comparative Example 39, the total reduction ratio in the temperature range below 1050°C during finish rolling was low, which is thought to have resulted in insufficient strain being introduced and a low aspect ratio of the prior austenite grains. Furthermore, since bainite nucleates from grain boundaries and dislocations, it is thought that the grain boundary area was insufficient and / or dislocation introduction was insufficient, resulting in a low bainite area ratio. As a result, the martensite area ratio increased and the EL decreased.

[0108] In Comparative Example 40, it is thought that excessive ferrite was formed due to a high slow cooling start temperature. As a result, the area ratio of martensite decreased, and the total stress (TS) decreased. On the other hand, in Comparative Example 41, it is thought that insufficient bainite was formed due to a low slow cooling start temperature. As a result, the electroluminescence (EL) decreased.

[0109] In Comparative Example 42, it is thought that bainite did not form sufficiently due to the short slow cooling time. As a result, the area ratio of bainite was low, and the EL decreased. In Comparative Example 43, it is thought that martensite did not form sufficiently because the average cooling rate of the rapid cooling was low. As a result, the area ratio of martensite was low, and the TS decreased.

[0110] In Comparative Example 44, accelerated cooling to 301°C and subsequent high coiling temperature resulted in a low cooling rate in the 100-300°C temperature range, which is thought to have caused tempering and decomposition of MA (Martensite-Austenite constant; a composite phase of martensite and austenite). Since tempering and MA decomposition vary depending on local elemental concentrations, microstructure size, and morphology, variations in hardness occurred in the center of the sheet thickness where these differences were large, resulting in a large standard deviation in the hardness of the final steel sheet. Consequently, the LC difference in the sheet thickness reduction rate increased.

[0111] In Comparative Example 45, the exit temperature of the final pass during finish rolling was high, which is thought to have caused the dislocations introduced into the austenite grains by hot rolling to recover and decrease unevenly, resulting in variations in the transformation rate and a larger standard deviation in the hardness of the final steel sheet. As a result, the LC difference in the thickness reduction rate increased.

[0112] In the steel sheet according to the embodiment of the present invention, the standard deviation of hardness at the 3 / 8 to 5 / 8 position from the surface is a characteristic of the variation in hardness at the center of the sheet thickness. By controlling the standard deviation of hardness at the 3 / 8 to 5 / 8 position from the surface to 25 Hv or less, it is possible to provide a steel sheet with excellent isotropy in ultimate deformation capacity, as described above. Here, it is important to control the standard deviation of hardness at the 3 / 8 to 5 / 8 position from the surface, and the above effect may not be obtained if the standard deviation of hardness is controlled in a range that includes positions other than the 3 / 8 to 5 / 8 position from the surface. Specifically, for example, in Comparative Examples 37, 44, and 45, although the standard deviation of hardness at the total thickness and the standard deviation of hardness at the position 1 / 8 to 3 / 8 of the plate thickness from the surface were 25 Hv or less, the standard deviation of hardness at the position 3 / 8 to 5 / 8 of the plate thickness from the surface exceeded 25 Hv. As a result, the LC difference of the plate thickness reduction rate increased, that is, the isotropy of the ultimate deformation capacity deteriorated. Therefore, it is important to control the standard deviation of hardness at the position 3 / 8 to 5 / 8 of the plate thickness from the surface. The standard deviation of hardness at the total thickness and the standard deviation of hardness at the position 1 / 8 to 3 / 8 of the plate thickness from the surface are measured by the method described in paragraph 0061, except for changing the measurement position.

[0113] 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 steel sheets in which the metal structure, at a position 1 / 4 of the thickness from the surface, contained bainite: 70-95% and martensite: 5-30% by area%, the average aspect ratio of prior austenite grains at a position 1 / 4 of the thickness from the surface was 3.0-5.5, and the standard deviation of hardness at a position 3 / 8 to 5 / 8 of the thickness from the surface was 25 Hv or less. As a result, despite having a high strength of tensile strength of 980 MPa or more, it also had high elongation and ultimate deformability, and excellent isotropy of ultimate deformability.

[0114] 10 Test specimen 10a Cross-section of the center in the width direction 10b Fracture 11 Thickness of the fracture

Claims

1. The chemical composition, in mass%, is as follows: C: 0.045-0.120%, Si: 0.01-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, O: 0.0100% or less, Ti: 0.001-0.180%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-0.500%, B: 0-0.0100%, Ca: 0-0.0500%. The composition is Mg: 0-0.050%, REM: 0-0.100%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.050%, Sn: 0-0.050%, and the remainder is Fe and impurities. The microstructure at a position 1 / 4 of the plate thickness from the surface contains, by area percentage, bainite: 70-95% and martensite: 5-30%. The average aspect ratio of prior austenite grains at a position 1 / 4 of the plate thickness from the surface is 3.00-5.

50. A steel plate characterized in that the standard deviation of hardness from the surface at a position 3 / 8 to 5 / 8 of the plate thickness is 25 Hv or less.

2. The chemical composition is as follows, in mass%,: Nb: 0.001-0.100%, V: 0.001-1.000%, Cu: 0.001-1.000%, Cr: 0.001-2.000%, Mo: 0.001-3.000%, Ni: 0.001-0.500%, B: 0.0001-0.0100%, Ca: 0.0001-0.0500%, Mg: 0.0001-0.050%, REM: 0.0001-0.100%, Bi: 0.001-0.100%, Ta: 0.001-0.100%, Zr: 0.001-0.500%, The steel sheet according to claim 1, characterized by containing at least one of the following: Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%.

3. The steel plate according to claim 1 or 2, characterized in that its tensile strength is 980 MPa or more.

4. A steel plate according to any one of claims 1 to 3, characterized in that the plate thickness is 1.0 to 8.0 mm.

5. A component characterized by comprising a steel plate as described in any one of claims 1 to 4.

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