Steel sheet and sheet member

Abstract

The present invention provides a high-strength steel sheet having a novel configuration and excellent appearance after molding and workability at the time of molding. This steel sheet has a specific chemical composition, and is characterized in that when Vm is the area ratio of a hard phase in a region having a depth from 3 / 8 to 5 / 8 of the sheet thickness, the area ratio of the hard phase in a region having a depth from the surface of the steel sheet to 1 / 8 of the sheet thickness is 0.20-0.80 Vm, and Vm is the area ratio of the hard phase in a region having a depth from 3 / 8 to 5 / 8 of the sheet thickness. The metal structure in the region from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness contains, in area%, 75-97% of ferrite and 3-25% of a hard phase. And the standard deviation of the area ratio of the hard phase in the direction orthogonal to the rolling direction and the plate thickness direction in a region having a depth from a 3 / 8 depth position to a 5 / 8 depth position of the plate thickness is 0.85% or less.

Description

Technical Field

[0001] This invention relates to steel plates. Background Technology

[0002] In the automotive industry, lightweighting of vehicle bodies is a key requirement for improving fuel efficiency. To balance lightweighting and crash safety, using high-strength steel sheets is one effective method, and the development of high-strength steel sheets is being promoted against this backdrop.

[0003] Relatedly, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet, characterized in that it is a hot-dip galvanized steel sheet with a hot-dip galvanized layer on the surface of a steel sheet serving as a substrate. The substrate contains, by mass%, C: 0.02-0.20%, Si: 0.7% or less, Mn: 1.5-3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1-1.0%, N: 0.010% or less, and Cr: 0.03-0.5%. Furthermore, the surface oxidation index A during annealing is 2.3 or more, as defined by the mathematical formula A = 400Al / (4Cr+3Si+6Mn), which uses the contents of Al, Cr, Si, and Mn as terms with the same sign. The remaining portion consists of Fe and unavoidable impurities. Moreover, the microstructure of the substrate consists of ferrite and a second phase, which is mainly martensite. Furthermore, Patent Document 1 describes that the aforementioned high-strength hot-dip galvanized steel sheet has excellent surface quality and tensile strength of 590 MPa or more, making it primarily suitable for use as structural components of automobiles such as beams and rocker arms.

[0004] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2005-220430 Summary of the Invention

[0005] The problem that the invention aims to solve In recent years, in line with the demand for further improvements in fuel efficiency, the need for lightweighting has increased not only for structural components such as beams described in Patent Document 1, but also for exterior panel components such as roofs, hoods, fenders, and doors. Unlike the aforementioned structural components, these exterior panel components are visible to the public, making not only strength and other properties important, but also pattern design and surface quality. Therefore, a superior appearance after forming is required. On the other hand, related to this demand for lightweighting, there is a need for further increases in strength and thinner walls in the steel sheets used in these exterior panel components. Furthermore, with the increasing complexity of the shapes of these exterior panel components, the surface of the formed steel sheet tends to become more prone to unevenness, leading to a decrease in appearance quality.

[0006] Specifically, in the case of DP steel (dual phase steel), as described in Patent Document 1, which consists of a soft ferrite phase (first phase) and a hard second phase dominated by martensite, although it has a low yield strength (hereinafter sometimes referred to as "YS") and excellent machinability during forming such as pressing, it is prone to uneven deformation during processing, where the soft phase formed by ferrite and its surrounding area deform preferentially. Therefore, if such DP steel is used for outer panel components, it is possible to produce minute irregularities on the surface of the formed steel sheet, resulting in appearance defects known as ghost lines or tensile strain.

[0007] Moreover, even if the various conditions during steel plate manufacturing are optimized to improve these appearance defects, it is still possible to fail to obtain the characteristics of DP steel, namely low YS, and reduced processability during forming, depending on the conditions during steel plate manufacturing.

[0008] Therefore, the object of the present invention is to provide a high-strength steel sheet with excellent appearance after forming and excellent processability during forming through a novel configuration.

[0009] Methods for solving problems The present invention includes the following solutions.

[0010] (Option 1) A steel plate, characterized in that, The chemical composition of the above steel plate, expressed as a percentage by mass, is as follows: C: 0.030~0.100% Mn: 1.00~2.50%, Si: 0.005~1.500%, Al: 0.005~0.700% P: Below 0.100% S: Below 0.0200% N: below 0.0150% O: Below 0.0100% Cr: 0–0.80% Mo: 0–0.50%, B: 0~0.0100% Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.50%, Ni: 0~1.00%, Cu: 0~1.00%, W: 0~1.00% Sn: 0~1.00% Sb: 0~0.200%, As: 0~0.200% Ca: 0~0.0100% Mg: 0~0.0100%, Zr: 0~0.0100% REM: 0–0.0100%, and Remaining components: Fe and impurities. When the area fraction of the hard phase in a region having a depth from 3 / 8 to 5 / 8 of the plate thickness is set to Vm, the area fraction of the hard phase in a region having a depth from the surface of the steel plate to 1 / 8 of the plate thickness is 0.20Vm to 0.80Vm. The metal microstructure in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness comprises, in area %: 75-97% ferrite and 3-25% hard phase, and the standard deviation of the area ratio of the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness in a direction orthogonal to the rolling direction and the plate thickness direction is 0.85% or less.

[0011] (Option 2) The steel plate according to Scheme 1 above is characterized in that the above chemical composition, by mass%, contains one or more elements selected from the following: Cr: 0.01~0.80% Mo: 0.01–0.50%, B: 0.0001~0.0100% Ti: 0.001~0.100% Nb: 0.001~0.100% V: 0.01~0.50%, Ni: 0.01~1.00%, Cu: 0.01~1.00%, W: 0.01~1.00% Sn: 0.001~1.00% Sb: 0.001~0.200% As: 0.001~0.200% Ca: 0.0001~0.0100% Mg: 0.0001~0.0100%, Zr: 0.0001~0.0100%, and REM: 0.0001~0.0100%.

[0012] (Option 3) The steel plate according to Scheme 1 or Scheme 2 above is characterized in that the index A represented by the following formula (1) is 0.45% or more.

[0013] A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb] (1) Where [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in terms of mass%, and are 0% when the corresponding element is not present.

[0014] (Option 4) The steel plate according to any one of the above schemes 1 to 3 is characterized in that it satisfies the following formula (2).

[0015] (TS-180000 / TS) / Vm≥35 (2) Where TS represents the tensile strength in MPa, and Vm represents the area fraction of the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness.

[0016] (Option 5) The steel plate according to any one of the above-described embodiments 1 to 4 is characterized in that the average grain size of the ferrite in the region having a depth from 3 / 8 of the plate thickness to 5 / 8 of the plate thickness is 5.0 to 30.0 μm. The average crystal grain size of the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness is 1.0 to 5.0 μm.

[0017] (Option 6) According to any one of the above-described schemes 1 to 5, the steel plate is characterized in that the hard phase in the region having a depth from 3 / 8 of the plate thickness to 5 / 8 of the plate thickness is composed of at least one of martensite, bainite, tempered martensite and pearlite.

[0018] (Option 7) An outer panel component comprising a steel plate as described in any one of the above-described schemes 1 to 6.

[0019] Invention Effects According to the present invention, a high-strength steel sheet with excellent appearance after forming and excellent processability during forming can be provided. Detailed Implementation

[0020] The preferred embodiments of the steel plate of the present invention will be described in detail below. It should be noted that, unless otherwise specified, all numerical ranges in this specification refer to the range including the upper and lower limits.

[0021] In exterior panel components such as roofs and doors, or other outer panel parts, DP steel with a relatively low yield strength (YS) is often used to avoid surface defects, known as surface strain, that occur during pressing and forming processes. However, as mentioned above, DP steel is prone to uneven deformation during pressing and forming processes, where the soft phase and its surrounding area preferentially deform. Therefore, when such DP steel is used in outer panel parts, it is possible to produce a defective appearance known as ghost lines due to the formation of tiny irregularities on the surface of the formed steel sheet. To explain the formation of these ghost lines in more detail, firstly, during pressing and forming processes, the steel sheet deforms in a way that the soft phase portion, composed of ferrite, is recessed, while the hard phase portion, mainly composed of martensite, does not recess or instead deforms in a way that is convex. As a result, tiny irregularities are formed on the surface of the formed steel sheet. These tiny irregularities are formed by convex portions extending approximately along the rolling direction and concave portions extending approximately along the rolling direction, arranged in a width direction orthogonal to the rolling direction. Then, during the grinding of the formed steel sheet surface, the tiny protrusions on the steel sheet surface are removed, thereby making the ghost lines of the striped pattern extending along the rolling direction of the steel sheet more apparent. It should be noted that the rolling direction can be easily determined based on the grain extension direction of the steel sheet. Furthermore, in this specification, the direction orthogonal to both the rolling direction and the thickness direction refers to the direction perpendicular to both the rolling direction and the thickness direction.

[0022] The inventors of this invention first conducted detailed research focusing on the morphology of the hard phase in the metal structure of the steel sheet in order to improve such ghost lines. As a result, the inventors discovered that in steel sheets such as DP steel, where soft and hard phases are mixed, the presence of a hard phase linked in a stripe-like pattern (striped hard phase) in the metal structure significantly increases the degree of ghost lines. Furthermore, the inventors found that by suppressing the formation of such striped hard phases and thus dispersing the hard phase more uniformly in the metal structure, the high strength provided by the hard phase can be adequately maintained, and the formation of minute irregularities on the surface of the formed steel sheet can be suppressed. As a result, the formation of ghost lines can be suppressed.

[0023] In more detail, the inventors of this invention have discovered that reducing Mn segregation during solidification is effective in suppressing the formation of striped hard phases during the slab casting process, where molten steel is solidified to cast slabs. Therefore, the inventors of this invention have conducted more detailed research on methods for reducing Mn segregation from two perspectives: central segregation and microsegregation.

[0024] First, the inventors of this invention believed that suppressing the flow of molten steel during slab casting was effective in reducing center segregation of Mn, and therefore conducted various studies. To explain in more detail, molten steel solidifies gradually from the surface, finally solidifying at the center. During this process, as the solid phase gradually displaces from the liquid phase, Mn in the liquid phase gradually becomes concentrated. If the molten steel flows during solidification, this concentrated portion of Mn tends to concentrate at the center during solidification, resulting in significant center segregation of Mn. Therefore, the inventors of this invention discovered that by appropriately controlling the solidification conditions during steel sheet manufacturing to suppress such molten steel flow, center segregation of Mn can be significantly suppressed.

[0025] Secondly, the inventors of this invention believe that promoting Mn diffusion during solidification is effective in reducing Mn microsegregation, and have therefore conducted various studies. To promote Mn diffusion, creating a microstructure that facilitates Mn diffusion is effective. Therefore, the inventors of this invention focused on the δ phase, where Mn diffuses rapidly, and investigated the influence of various elements in the steel on Mn microsegregation through experiments to achieve δ solidification. As a result, the inventors of this invention found that if the contents of C and Mn increase, δ solidification will not occur during solidification, the diffusion rate of Mn will decrease, and microsegregation will increase. On the other hand, if the contents of Si, Al, Cr, and Mo increase, Mn diffusion during solidification will be promoted, and microsegregation can be reduced.

[0026] While the methods described above for reducing Mn segregation can improve ghost lines to some extent, in order to achieve a more complete improvement in ghost lines, the inventors of this invention not only studied the methods for reducing Mn segregation but also further studied methods for improving ghost lines. As a result, the inventors of this invention have realized that by reducing the hard phase fraction of the steel plate, ghost lines can be improved even if some degree of central Mn segregation remains.

[0027] Furthermore, the inventors of this invention have realized that the solidification structure has a significant impact on the formation of ghost lines. Even if the center segregation of Mn is small, if coarse equiaxed crystals are formed in the solidification structure, negative segregation of Mn occurs, and the unevenness of the hard phase fraction in the direction orthogonal to the rolling direction and the thickness direction increases, making it easier for ghost lines to form. Moreover, the inventors of this invention have realized that by using a method different from conventional center segregation countermeasures—namely, reducing the equiaxed crystal fraction and controlling the solidification structure to a columnar crystal structure—negative segregation of Mn can be suppressed, thereby improving ghost lines.

[0028] Based on these understandings, the inventors of this invention have discovered that high strength can be adequately maintained and ghost lines can be significantly improved by controlling the solidification structure during casting to a columnar crystal structure and reducing the central segregation of Mn during solidification, which is the cause of the formation of the striped hard phase, while reducing the hard phase fraction and its inhomogeneity.

[0029] It was determined that although the ghost lines were significantly improved using the methods described above, appearance defects known as tensile strain still occurred. Furthermore, it was determined that even if the various conditions during steel sheet manufacturing were controlled within specific ranges as described above to improve these appearance defects, the characteristic of DP steel—low YS—may not be obtained, and the processability during forming may be reduced, depending on the conditions.

[0030] Therefore, the inventors of this invention have conducted in-depth research on methods that simultaneously suppress post-forming appearance defects, including not only ghost lines but also tensile strain, and improve processability during forming. As a result, the inventors of this invention have discovered that by controlling the area ratio of the hard phase in a region extending from the surface of the steel sheet to a depth of 1 / 8 of the sheet thickness (i.e., the hard phase fraction in the surface layer of the steel sheet), and the area and unevenness of the hard phase in a region extending from a depth of 3 / 8 to 5 / 8 of the sheet thickness (i.e., the hard phase fraction and unevenness in the center of the steel sheet thickness) within specific ranges, high-strength steel sheets with excellent post-forming appearance and processability during forming can be obtained.

[0031] The present invention is based on the above understanding and includes the following implementation scheme.

[0032] The preferred embodiments of the steel plate of the present invention will be described in detail below.

[0033] <steel plate> The chemical composition of the steel plate according to one embodiment of the present invention, in mass percent, is as follows: C: 0.030~0.100% Mn: 1.00~2.50%, Si: 0.005~1.500%, Al: 0.005~0.700% P: Below 0.100% S: Below 0.0200% N: below 0.0150% O: Below 0.0100% Cr: 0–0.80% Mo: 0–0.50%, B: 0~0.0100% Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.50%, Ni: 0~1.00%, Cu: 0~1.00%, W: 0~1.00% Sn: 0~1.00% Sb: 0~0.200%, As: 0~0.200% Ca: 0~0.0100% Mg: 0~0.0100%, Zr: 0~0.0100% REM: 0–0.0100%, and Remaining components: Fe and impurities.

[0034] Furthermore, in this embodiment, when the area ratio of the hard phase in the region having a depth from 3 / 8 of the plate thickness to 5 / 8 of the plate thickness is set to Vm, the area ratio of the hard phase in the region having a depth from the surface of the steel plate to 1 / 8 of the plate thickness is 0.20Vm to 0.80Vm.

[0035] Furthermore, in the steel plate of this embodiment, the metal microstructure in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness comprises, in area percent, 75-97% ferrite and 3-25% hard phase, and the standard deviation of the area ratio of the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness in a direction orthogonal to the rolling direction and the plate thickness direction is 0.85% or less.

[0036] The steel plate of this embodiment has a specific chemical composition as described above, and a unique metallic structure as follows: the metallic structure in the region from 3 / 8 to 5 / 8 of the plate thickness (i.e., the center of the plate thickness) has a lower hard phase fraction than conventional DP steel, and the standard deviation (i.e., unevenness of hard phase fraction) of the hard phase fraction in the center of the plate thickness in directions orthogonal to the rolling direction and the plate thickness direction is small. Such a unique metallic structure can be obtained by employing specific chemical compositions and casting conditions, as described later.

[0037] As described above, the steel sheet of this embodiment possesses a unique metallic structure, namely, a small central segregation of Mn during solidification and a small fraction and unevenness of the hard phase. Therefore, it can sufficiently maintain high strength and suppress the formation of minute irregularities on the surface of the formed steel sheet. Consequently, the steel sheet of this embodiment can sufficiently maintain high strength and significantly suppress the generation of ghost lines and tensile strain.

[0038] Furthermore, in the steel sheet of this embodiment, the area fraction of the hard phase in the region extending from the surface of the steel sheet to a depth of 1 / 8 of the sheet thickness, i.e., the hard phase fraction in the surface layer relative to the area fraction Vm of the hard phase in the center of the sheet thickness, is 0.20Vm to 0.80Vm. If the hard phase fraction in the surface layer is 0.20Vm or more, low yield strength (YS) and high tensile strength (TS) can be ensured, thus enabling the production of a high-strength steel sheet with excellent processability during forming. Furthermore, this also makes it easier to further suppress tensile strain generated during the forming of the steel sheet. In addition, if the hard phase fraction in the surface layer of the steel sheet is 0.80Vm or less, good bending properties can be ensured, thus enabling the production of a steel sheet with excellent processability during forming.

[0039] It should be noted that in order to control the hard phase fraction on the surface of the steel plate within the range of 0.20Vm to 0.80Vm, specific conditions, as described later, need to be adopted in the hot rolling, pickling, and annealing processes during steel plate manufacturing.

[0040] The hard phase fraction on the surface of the steel plate, "0.20Vm", refers to 0.20 times (0.20 × Vm) the hard phase fraction Vm at the center of the plate thickness. Similarly, 0.80Vm and others are expressed in the same way. It should be noted that in this specification, the area fraction (%) of the hard phase and the hard phase fraction (%) have the same meaning.

[0041] As described above, the steel sheet of this embodiment is a high-strength steel sheet that combines the suppression of poor appearance after forming, including not only ghost lines but also tensile strain, with the processability during forming; that is, a high-strength steel sheet with excellent appearance after forming and processability during forming.

[0042] The steel plate of this embodiment will now be described in more detail. It should be noted that, in the following description, the unit "%" for the content of each element and the index A refers to "mass %" unless otherwise specified. Furthermore, in this specification, the "~" indicating a numerical range is used to mean the lower and upper limits of the values ​​described before and after it, unless otherwise specified.

[0043] In addition, in this specification, the "x / y depth position of the plate thickness (where x and y are set as natural numbers satisfying x < y)" refers to the position where the distance (depth) of only x / y of the plate thickness is moved from the surface in the plate thickness direction of the steel plate toward the center portion of the steel plate. For example, when the plate thickness of the steel plate is 2 mm, the "1 / 8 depth position of the plate thickness" refers to the position where the depth is 0.25 mm in the plate thickness direction from the surface of the steel plate. It should be noted that when the steel plate has a coating on the surface, the "surface of the steel plate" in the definition of "from the surface of the steel plate to the x / 8 depth position of the plate thickness" refers to the interface between the steel plate and the coating, and the "plate thickness" refers to the plate thickness of the steel plate (base material) excluding the coating.

[0044] (Chemical composition) The steel plate of this embodiment is as described above: C: 0.030 to 0.100%, Mn: 1.00 to 2.50%, Si: 0.005 to 1.500%, Al: 0.005 to 0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 0.80%, Mo: 0 to 0.50%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, As: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and The balance: Fe and impurities.

[0045] Hereinafter, each element in this chemical composition will be described in detail.

[0046] [C: 0.030~0.100%] Carbon (C) is an element that increases the strength of steel plates. To achieve this effect, the C content is set at 0.030% or more. The C content can also be 0.032% or more, 0.034% or more, or 0.035% or more. On the other hand, if the C content is excessive, it may hinder the diffusion of manganese (Mn) during solidification, failing to adequately suppress Mn microsegregation. Therefore, the C content is set at 0.100% or less. The C content can also be 0.095% or less, 0.090% or less, or 0.080% or less.

[0047] [Mn: 1.00~2.50%] Manganese (Mn) is an element that improves the hardenability of steel and contributes to increased strength. To achieve this effect, the Mn content is set at 1.00% or more. The Mn content can also be 1.02% or more, 1.04% or more, or 1.05% or more. On the other hand, if the Mn content is excessive, it may hinder Mn diffusion during solidification and fail to adequately suppress Mn microsegregation. Therefore, the Mn content is set at 2.50% or less. The Mn content can also be 2.40% or less, 2.30% or less, or 2.20% or less.

[0048] [Si: 0.005~1.500%] Si is a deoxidizing element in steel and an effective solid solution strengthening element for increasing strength without compromising the ductility of the steel sheet. Furthermore, Si is also effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully obtain these effects, the Si content is set to 0.005% or more. The Si content can also be 0.008% or more, 0.010% or more, or 0.012% or more. On the other hand, if the Si content is excessive, the peelability of the oxide scale may decrease, resulting in surface defects. Therefore, the Si content is set to 1.500% or less. The Si content can also be 1.200% or less, 1.000% or less, or 0.800% or less.

[0049] [Al: 0.005~0.700%] Al acts as a deoxidizer and is an effective solid solution strengthening element for improving the strength of steel. Furthermore, Al is also effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully obtain these effects, the Al content is set to 0.005% or more. The Al content can also be 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, excessive Al content may deteriorate castability and reduce productivity. Therefore, the Al content is set to 0.700% or less. The Al content can also be 0.600% or less, 0.500% or less, 0.400% or less, or 0.300% or less.

[0050] [P: below 0.100%] Phosphorus (P) is an element introduced during the manufacturing process. Furthermore, P is also a solid solution strengthening element. The P content can be 0%. However, refining to reduce the P content to below 0.0001% requires time, leading to reduced productivity. Therefore, the P content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive P content may reduce the toughness of the steel plate. Therefore, the P content is set to 0.100% or less. The P content can also be 0.080% or less, 0.075% or less, or 0.070% or less.

[0051] [S: Below 0.0200%] Sulfur (S) is an element introduced during the manufacturing process. The S content can also be 0%. However, refining to reduce the S content to below 0.0001% requires time, leading to reduced productivity. Therefore, the S content can also be 0.0001% or more, 0.0005% or more, or 0.0008% or more. On the other hand, excessive S content can potentially form Mn sulfides, reducing the formability of the steel sheet, such as ductility, hole expansion, tensile flange properties, and / or bending. Therefore, the S content is set to 0.0200% or less. The S content can also be 0.0150% or less, 0.0100% or less, or 0.0080% or less.

[0052] [N: below 0.0150%] Nitrogen (N) is an element introduced during the manufacturing process. The N content can also be 0%. However, refining to reduce the N content to below 0.0001% requires time, leading to reduced productivity. Therefore, the N content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content can lead to the formation of nitrides, reducing the formability of the steel sheet, such as ductility, hole expansion, tensile flange properties, and / or bending. Therefore, the N content is set to 0.0150% or less. The N content can also be 0.0100% or less, 0.0080% or less, or 0.0050% or less.

[0053] [O: below 0.0100%] O is an element introduced during the manufacturing process. The O content can also be 0%. However, refining to reduce the O content to below 0.0001% requires time, leading to reduced productivity. Therefore, the O content can also be 0.0001% or more, 0.0005% or more, or 0.0008% or more. On the other hand, excessive O content can lead to the formation of coarse oxides, reducing the formability of the steel sheet, such as ductility, hole expansion, tensile flange properties, and / or bending. Therefore, the O content is set to 0.0100% or less. The O content can also be 0.0070% or less, 0.0050% or less, or 0.0030% or less.

[0054] The basic chemical composition of the steel plate in this embodiment is as described above. Furthermore, in this embodiment, the steel plate may, as needed, contain one or more of the following optional elements to replace a portion of the remaining Fe. These optional elements will be described in detail below.

[0055] [Cr: 0~0.80%] Cr is an element that improves the hardenability of steel and contributes to the increase of steel plate strength. Furthermore, Cr is also effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. The Cr content can be 0%, but to achieve these effects, the Cr content is preferably 0.01% or more. The Cr content can also be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Cr content is excessive, coarse Cr carbides may form, becoming the starting point for damage. Therefore, the Cr content is preferably 0.80% or less. The Cr content can also be 0.75% or less, 0.70% or less, or 0.65% or less.

[0056] [Mo: 0~0.50%] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. Furthermore, Mo is also effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. The Mo content can be 0%, but to achieve these effects, the Mo content is preferably 0.01% or more. The Mo content can also be 0.05% or more, or 0.07% or more. On the other hand, excessive Mo content may reduce hot workability and thus productivity. Therefore, the Mo content is preferably 0.50% or less. The Mo content can also be 0.45% or less, 0.40% or less, or 0.35% or less.

[0057] [B: 0~0.0100%] Boron (B) is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The B content can be 0%, but to achieve this effect, a B content of 0.0001% or more is preferred. The B content can also be 0.0005% or more, 0.0008% or more, or 0.0010% or more. On the other hand, if the B content is excessive, B precipitates may form, thereby reducing the strength of the steel sheet. Therefore, the B content is preferably 0.0100% or less. The B content can also be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0058] [Ti: 0~0.100%] Ti is an element that reduces the amount of S, N, and O that contribute to the formation of coarse inclusions that act as the starting point for damage. Furthermore, Ti is also a precipitation strengthening element that refines the microstructure and improves the strength-formability balance of the steel sheet. The Ti content can be 0%, but to achieve these effects, the Ti content is preferably 0.001% or more. The Ti content can also be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if Ti is excessively present, coarse Ti sulfides, Ti nitrides, and / or Ti oxides may form, thereby reducing the formability of the steel sheet. Therefore, the Ti content is preferably 0.100% or less. The Ti content can also be 0.080% or less, 0.075% or less, or 0.070% or less.

[0059] [Nb: 0~0.100%] Nitrogen (Nb) is a precipitation strengthening element that contributes to increased steel sheet strength through strengthening by precipitates, grain refinement strengthening by inhibiting ferrite grain growth, and / or dislocation strengthening by inhibiting recrystallization. The Nb content can be 0%, but to achieve these effects, the Nb content is preferably 0.001% or more. The Nb content can also be 0.003% or more, 0.004% or more, or 0.005% or more. On the other hand, if the Nb content is excessive, it may increase the amount of unrecrystallized ferrite, thereby reducing the formability of the steel sheet. Therefore, the Nb content is preferably 0.100% or less. The Nb content can also be 0.080% or less, 0.070% or less, or 0.060% or less.

[0060] [V: 0~0.50%] V is an element that contributes to improving the strength of steel sheets through strengthening by precipitates, grain refinement strengthening by inhibiting ferrite grain growth, and / or dislocation strengthening by inhibiting recrystallization. The V content can be 0%, but to achieve these effects, the V content is preferably 0.01% or more. The V content can also be 0.02% or more. On the other hand, if the V content is excessive, a large amount of carbonitrides may precipitate, thereby reducing the formability of the steel sheet. Therefore, the V content is preferably 0.50% or less. The V content can also be 0.40% or less, 0.30% or less, or 0.20% or less.

[0061] [Ni: 0~1.00%] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The Ni content can be 0%, but to achieve this effect, a Ni content of 0.01% or more is preferred. The Ni content can also be 0.03% or more, or 0.05% or more. On the other hand, excessive Ni content may reduce the weldability of the steel sheet. Therefore, the Ni content is preferably 1.00% or less. The Ni content can also be 0.60% or less, 0.50% or less, or 0.40% or less.

[0062] [Cu: 0~1.00%] Cu is an element that exists in steel in the form of fine particles and contributes to the strength of the steel sheet. The Cu content can be 0%, but to achieve this effect, a Cu content of 0.01% or more is preferred. The Cu content can also be 0.03% or more, or 0.05% or more. On the other hand, if the Cu content is excessive, the weldability of the steel sheet may decrease. Therefore, the Cu content is preferably 1.00% or less. The Cu content can also be 0.80% or less, 0.60% or less, or 0.40% or less.

[0063] [W: 0~1.00%] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel plates. The W content can be 0%, but to achieve this effect, a W content of 0.01% or more is preferred. The W content can also be 0.02% or more. On the other hand, excessive W content may reduce hot workability and thus productivity. Therefore, the W content is preferably 1.00% or less. The W content can also be 0.80% or less, 0.60% or less, or 0.40% or less.

[0064] [Sn: 0~1.00%] Sn is an element that inhibits grain coarsening and contributes to improving the strength of steel sheets. The Sn content can be 0%, but to achieve this effect, the Sn content is preferably 0.001% or more. The Sn content can also be 0.004% or more. On the other hand, excessive Sn content may cause embrittlement of the steel sheet. Therefore, the Sn content is preferably 1.00% or less. The Sn content can also be 0.80% or less, 0.60% or less, or 0.40% or less.

[0065] [Sb: 0~0.200%] Sb is an element that inhibits grain coarsening and contributes to improving the strength of steel sheets. The Sb content can be 0%, but to achieve this effect, an Sb content of 0.001% or more is preferred. The Sb content can also be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, excessive Sb content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 0.200% or less. The Sb content can also be 0.180% or less, 0.170% or less, or 0.160% or less.

[0066] [As: 0~0.200%] While As does not have the same significant effect as Sb and Sn, it is an effective element for improving corrosion resistance in acidic environments. The As content can be 0%, but to achieve this effect, an As content of 0.001% or more is preferred. The As content can also be 0.002%, 0.004%, or 0.006% or more. On the other hand, excessive As content may reduce hot workability. Therefore, the As content is preferably 0.200% or less. The As content can also be 0.180%, 0.170%, or 0.160% or less.

[0067] [Ca: 0–0.0100%], [Mg: 0–0.0100%], [Zr: 0–0.0100%], and [REM: 0–0.0100%] Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets. While the content of Ca, Mg, Zr, and REM can be 0%, to achieve this effect, the content of each is preferably 0.0001% or more. The content of Ca, Mg, Zr, and REM can also be 0.0005% or more, 0.0010% or more, or 0.0015% or more, respectively. On the other hand, excessive content of these elements may reduce the ductility of the steel sheet. Therefore, the content of each is preferably 0.0100% or less. The content of each is also preferably 0.0080% or less, 0.0070% or less, or 0.0060% or less, respectively.

[0068] It should be noted that in this specification, REM refers to the collective term for 17 elements, namely scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides, namely lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. REM content is the total content of these elements.

[0069] Regarding the optional elements mentioned above, in this embodiment, the chemical composition of the steel plate preferably contains one or more elements selected from the following: Cr: 0.01~0.80% Mo: 0.01–0.50%, B: 0.0001~0.0100% Ti: 0.001~0.100% Nb: 0.001~0.100% V: 0.01~0.50%, Ni: 0.01~1.00%, Cu: 0.01~1.00%, W: 0.01~1.00% Sn: 0.001~1.00% Sb: 0.001~0.200% As: 0.001~0.200% Ca: 0.0001~0.0100% Mg: 0.0001~0.0100%, Zr: 0.0001~0.0100%, and REM: 0.0001~0.0100%.

[0070] If the steel sheet contains optional elements, it is possible to more reliably produce a steel sheet that can maintain high strength, significantly suppress the occurrence of post-forming appearance defects such as ghost lines, and has excellent machinability during forming.

[0071] In the steel sheet of this embodiment, the remaining portion besides the aforementioned elements consists of Fe and impurities. Impurities refer to components introduced during the industrial manufacturing of steel sheets due to various factors in the manufacturing process, such as the use of raw materials like ores and waste. Examples of impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The total amount of impurities may be 0.100% or less.

[0072] [Index A: Above 0.45%] In this embodiment, the chemical composition of the steel plate preferably has an index A of 0.45% or more as represented by the formula (1) below.

[0073] A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb] (1) Where [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in terms of mass%, and are 0% when the corresponding element is not present.

[0074] The index A is determined by the content of Si, P, and Al as solid solution strengthening elements and Ti and Nb as precipitation strengthening elements. The larger the value, the higher the strength can be obtained with a lower hard phase fraction. By setting the index A to 0.45% or higher, the hard phase fraction of the steel plate can be controlled below a certain level while obtaining high strength.

[0075] Index A can also be above 0.48%, above 0.50%, or above 0.52%. It should be noted that there is no specific upper limit for Index A, but for example, Index A can also be below 1.50%, below 1.20%, or below 1.00%.

[0076] The chemical composition of steel plates can be determined using general analytical methods. For example, the chemical composition of steel plates can be determined using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Specifically, a 35mm square test piece is collected from approximately half the depth of the steel plate, and the chemical composition of the steel plate can be determined using a measuring device such as the Shimadzu ICPS-8100, based on pre-prepared standard lines. C and S, which cannot be determined by ICP-AES, can be determined using the combustion-infrared absorption method; N can be determined using the inert gas melting-thermal conductivity method; and O can be determined using the inert gas melting-non-dispersive infrared absorption method.

[0077] (Metallic structure) [Area fraction of the hard phase in the region extending from the surface of the steel plate to a depth of 1 / 8 of the plate thickness: 0.20Vm~0.80Vm] In this embodiment, regarding the microstructure of the steel sheet, when the area fraction of the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the sheet thickness (i.e., the hard phase fraction at the center of the sheet thickness) is set to Vm, the area fraction of the hard phase in the region having a depth from the surface of the steel sheet to 1 / 8 of the sheet thickness (i.e., the hard phase fraction of the surface layer) is 0.20Vm to 0.80Vm. If the hard phase fraction of the surface layer of the steel sheet is 0.20Vm or more, low yield strength (YS) and high tensile strength (TS) can be ensured, thus enabling the production of a high-strength steel sheet with excellent processability during forming. Furthermore, tensile strain generated during the forming of the steel sheet can be easily suppressed. In addition, if the hard phase fraction of the surface layer of the steel sheet is 0.8Vm or less, good bending properties can be ensured, thus enabling the production of a steel sheet with excellent processability during forming. From the viewpoint of further improving these effects, the area fraction of the hard phase in the surface layer can also be 0.30Vm or more, 0.35Vm or more, 0.40Vm or more, or 0.45Vm or more. On the other hand, from the viewpoint of further improving processability during forming, the area fraction of the hard phase in the surface layer can also be 0.79Vm or less, 0.78Vm or less, 0.77Vm or less, or 0.76Vm or less.

[0078] [Metallic structure in a region extending from 3 / 8 to 5 / 8 of the plate thickness: ferrite: 75–97% and hard phase: 3–25%] In this embodiment, the microstructure of the steel sheet in the region having a depth from 3 / 8 to 5 / 8 of the sheet thickness (i.e., the center of the sheet thickness) comprises 75-97% ferrite and 3-25% hard phase by area. By setting the microstructure of the steel sheet in the center of the sheet thickness to such a specific composite microstructure, the strength of the steel sheet can be maintained within a suitable range, more specifically achieving a tensile strength of 500 MPa or more, and poor appearance after forming can be suppressed. From the viewpoint of further improving the strength of the steel sheet, the area fraction of the hard phase in the center of the sheet thickness can also be 4% or more, 5% or more, 6% or more, 7% or more, or 8% or more. Similarly, the area fraction of ferrite in the center of the sheet thickness can also be 96% or less, 95% or less, or 94% or less. On the other hand, from the viewpoint of further improving the appearance after forming, the area fraction of the hard phase in the center of the sheet thickness can also be 22% or less, 20% or less, or 18% or less. Similarly, the area fraction of ferrite in the center of the plate thickness can also be 78% or more, 80% or more, 82% or more, 85% or more, 90% or more, or 92% or more.

[0079] In the steel sheet of this embodiment, the hard phase refers to a structure harder than ferrite, such as being composed of at least one of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of improving the strength of the steel sheet, the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the sheet thickness is preferably composed of at least one of martensite, bainite, and tempered martensite, and more preferably composed of martensite. In the metallic microstructure of the steel sheet, retained austenite is preferably minimal; specifically, retained austenite is preferably less than 1% or less than 0.5% by area, and more preferably 0%.

[0080] (Identification of metallic structures and calculation of area fraction) The identification of the metal structure and the calculation of the area fraction are performed as follows. First, determine the rolling direction of the obtained steel sheet. The rolling direction of the steel sheet can be determined as follows.

[0081] Test pieces were collected by means of an observable cross-section parallel to the surface of the steel plate. The cross-sections of the test pieces collected along the thickness direction were then mirror-polished and observed using an optical microscope. The observation surface was set as a plane parallel to the surface of the steel plate at any depth position within the range of 1 / 4 to 1 / 2 of the plate thickness along the thickness direction. Within this observation surface, the direction parallel to the grain elongation direction was identified as the rolling direction.

[0082] The direction orthogonal to the rolling direction and thickness direction of the steel plate is defined as the plate width direction, and the length of the steel plate in the plate width direction is defined as the plate width W. A sample for observing the metallic structure (microstructure) is collected at a position of W / 4 or 3W / 4 of the plate width W (i.e., a position W / 4 of the plate width direction from any end of the steel plate at any point in the plate width direction). The sample dimensions are, for example, set to 20 mm in the rolling direction × 20 mm in the plate width direction × the thickness of the steel plate.

[0083] Next, the metallic microstructure was observed using a FE-SEM (field emission scanning electron microscope, such as JEOL JSM-7200F, accelerating voltage: 15kV, light source: FE, device resolution: 1.2nm). At this time, regarding the field of view, a 100μm × 100μm area in the thickness direction of the steel plate corresponding to the location of the object being observed was used as the field of view. That is, when observing a metallic microstructure having a depth from the surface of the steel plate to 1 / 8 of its thickness, a 50μm × 100μm area (50μm in the thickness direction and 100μm in the field of view perpendicular to the thickness direction) was observed in the thickness direction of the steel plate. On the other hand, when observing the microstructure of the steel plate in a region with a depth from 3 / 8 to 5 / 8 of the plate thickness (i.e., the center of the plate thickness), a 100 μm × 100 μm region from 3 / 8 to 5 / 8 of the plate thickness was observed along the thickness direction of the steel plate. It should be noted that the resolution was set to 1280 × 960 pixels in any observation of any region. As a sample adjustment, the plate thickness section in the width direction was used as the observation surface for grinding, and etching was performed using nitric acid ethanol etching. Then, the "microstructure" was classified based on SEM images at 500 or 1000x magnification. Ferrite and hard phases could be distinguished by differences in brightness.

[0084] For steel plates etched using nitric acid-ethanol etching, the aforementioned 100μm × 100μm region was observed at 500x or 1000x magnification. Image analysis software Image J (Ver. 1.54f) was used for image analysis to determine the area fraction of the hard phase. Specifically, the secondary electron image of the metal structure was binarized according to different brightness levels. The black portion of the image data was taken as ferrite, and the unetched white portion as the hard phase, and the area fraction of the hard phase was calculated. For any metal structure, three measurement sites were set. Image analysis was performed at these sites in the same manner to determine the area fraction of the hard phase, and the arithmetic mean of these area fractions was calculated. This average value was taken as the area fraction of the hard phase, and the remaining portion was taken as the area fraction of ferrite. It should be noted that the observation of the microstructure in the region of the steel plate extending from the surface to 1 / 8 of the plate thickness, and the observation of the microstructure in the region of the steel plate extending from 3 / 8 to 5 / 8 of the plate thickness, are different only in the field of view along the thickness direction of the steel plate, and the area fraction of the hard phase can be determined in the same way.

[0085] Furthermore, when it is necessary to determine the area fraction of retained austenite, the area fraction of retained austenite can be determined by X-ray diffraction performed on the aforementioned observation surface. Specifically, using Co-Kα rays, the integrated intensities of the six peaks α (110), α (200), α (211), γ (111), γ (200), and γ (220) at a depth of 1 / 2 the plate thickness are calculated, and the volume fraction of retained austenite is calculated using the intensity averaging method. The obtained volume fraction of retained austenite is then used as the area fraction of retained austenite.

[0086] [Standard deviation of the area fraction of the hard phase in a region with a depth from 3 / 8 to 5 / 8 of the plate thickness, in a direction orthogonal to the rolling direction and the plate thickness direction: less than 0.85%] In this embodiment, the standard deviation of the hard phase fraction in the metal structure of the steel sheet in the region having a depth from 3 / 8 to 5 / 8 of the sheet thickness (i.e., the center of the sheet thickness) in directions orthogonal to the rolling and thickness directions is 0.85% or less. It should be noted that the standard deviation of the hard phase fraction refers to the standard deviation of the area fraction of the hard phase itself. As described above, for defects in the appearance after forming, such as ghost lines, not only is Mn segregation significant, but the solidification structure also plays a crucial role. For example, even when the center segregation of Mn is small, if coarse equiaxed crystals are formed in the solidification structure, negative Mn segregation may occur, increasing the unevenness of the hard phase fraction in directions orthogonal to the rolling and thickness directions, thus worsening the appearance defects after forming. However, in the steel sheet of this embodiment, since the standard deviation of the hard phase fraction in the region having a depth from 3 / 8 of the sheet thickness to 5 / 8 of the sheet thickness in the direction orthogonal to the rolling direction and the sheet thickness direction is 0.85% or less, that is, the non-uniformity of the hard phase fraction in the direction orthogonal to the rolling direction and the sheet thickness direction is constant or less, it is possible to significantly suppress appearance defects after forming.

[0087] From the viewpoint of more reliably suppressing defects in the appearance after forming, the standard deviation of the hard phase fraction in a region having a depth from 3 / 8 to 5 / 8 of the plate thickness, in a direction orthogonal to the rolling direction and the plate thickness direction, can also be 0.80% or less, 0.75% or less, 0.70% or less, or 0.65% or less. It should be noted that the lower limit of the standard deviation of the hard phase fraction is not particularly limited, but for example, the standard deviation of the hard phase fraction can also be 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.

[0088] It should be noted that, for the hard phase fraction, the ratio of the average area fraction of the hard phase to its standard deviation is preferably 0.10 or less (i.e., the standard deviation of the hard phase fraction / the average area fraction of the hard phase ≤ 0.10). This ratio is preferably 0.09 or less, 0.08 or less, or 0.07 or less. The lower limit of this ratio is 0, but it can also be set to 0.01 as needed.

[0089] The standard deviation of the hard phase fraction in the metal structure at the center of the plate thickness, in a direction orthogonal to the rolling and thickness directions, can be determined as follows: First, for a 100 μm × 100 μm region of a section parallel to the direction orthogonal to the rolling and thickness directions and perpendicular to the surface of the plate, from 3 / 8 to 5 / 8 of the plate thickness depth, observe the region using a FE-SEM (field emission scanning electron microscope, such as JEOL JSM-7200F, accelerating voltage: 15 kV, light source: FE, device resolution: 1.2 nm) at 500x or 1000x magnification to obtain a secondary electron image. It should be noted that the resolution is set to 1280 × 960 pixels. For this secondary electron image, similar to the area fraction of the hard phase in the "Identification of Metallic Structure and Calculation of Area Fraction" section above, image analysis software ImageJ (Ver. 1.54f) was used for image analysis. The area fraction of the hard phase per 100 μm was measured within an 8 mm range in the direction orthogonal to the rolling and thickness directions of the steel plate (plate width direction) from the W / 4 to 3W / 4 position of the plate width W, and its standard deviation was calculated. It should be noted that the observation range in the direction orthogonal to the rolling and thickness directions (plate width direction) can be less than 8 mm or more than 8 mm. However, the lower limit of the observation range for the standard deviation of the hard phase fraction in the direction orthogonal to the rolling and thickness directions (plate width direction) is set to 4 mm, and the upper limit is set to 12 mm.

[0090] [The relationship between the tensile strength of steel plate and the hard phase fraction is: (TS-180000 / TS) / Vm≥35] In this embodiment, the steel plate preferably satisfies the following formula (2).

[0091] (TS-180000 / TS) / Vm≥35 (2) Where TS represents the tensile strength of the steel plate (MPa), and Vm represents the area fraction (area %) of the hard phase in the region with a depth from 3 / 8 to 5 / 8 of the plate thickness.

[0092] If the steel plate satisfies equation (2), then even if the center segregation of Mn remains to a certain extent, the uneven distribution of hard phase in the direction orthogonal to the rolling direction and the plate thickness direction can be easily suppressed, and thus it is easier to avoid producing poor appearance of the steel plate after forming.

[0093] The tensile strength (TS) of steel plate can be determined as follows: A No. 5 tensile test specimen according to JIS Z2241:2011, with its length direction orthogonal to the rolling direction and the thickness direction, is collected from the steel plate, and a tensile test is performed according to JIS Z2241:2011. It should be noted that if the aforementioned No. 5 tensile test specimen cannot be collected from the steel plate from which the test specimen is to be taken (e.g., when the size of the test specimen is small), a tensile test specimen of any size with its length direction orthogonal to the rolling direction and the thickness direction can be used instead of the No. 5 tensile test specimen.

[0094] [The average grain size of ferrite in the region with a depth from 3 / 8 to 5 / 8 of the plate thickness is 5.0–30.0 μm.] In the steel sheet of this embodiment, the average grain size of ferrite in the microstructure of the steel sheet in the region having a depth from 3 / 8 to 5 / 8 of the sheet thickness (i.e., the center of the sheet thickness) is preferably 5.0 to 30.0 μm. Controlling the average grain size of ferrite within such a fine range further improves the appearance of the formed steel sheet. The average grain size of ferrite can also be 6.0 μm or more, or 7.0 μm or more. Similarly, the average grain size of ferrite can also be 27.0 μm or less, 21.0 μm or less, 15.0 μm or less, or 11.0 μm or less.

[0095] The average grain size of ferrite in the center of the plate thickness is determined as follows.

[0096] First, the KAM value of ferrite was calculated by using SEM crystal analysis methods, namely KAM (Kernel Average Misorientation) analysis and GAIQ (Grain Average Image Quality) analysis measured by an electron probe microanalyzer (EPMA).

[0097] As the SEM observation device, a field emission scanning electron microscope (e.g., JEOL's "JSM-7001F") is used, and for example, TSL's "OIM Analysis 7" can be used for EBSD analysis. In EBSD analysis, a 50μm × 50μm region from 3 / 8 to 5 / 8 of the thickness of the steel sheet after mirror polishing is analyzed at 0.05μm intervals. KAM analysis is the analysis of the KAM value of a "pixel" relative to the measurement point, using the average of the orientation differences (°) with all adjacent pixels. A KAM map can be created based on local crystal orientation differences. The KAM value in ferrite is analyzed using this KAM analysis. The determination of the region containing ferrite in the EBSD measurement results is performed using the GAIQ analysis described below.

[0098] For EBSD measurement results obtained under the same measurement conditions as KAM values, GAIQ (Grain Average Image Quality) analysis was performed to obtain GAIQ values. Based on these GAIQ values, regions with GAIQ values ​​of 50,000 or higher and regions with GAIQ values ​​lower than 50,000 were divided into regions where GAIQ values ​​are 50,000 or higher. Regions (grains) with GAIQ values ​​of 50,000 or higher were identified as regions containing ferrite. On the other hand, regions (grains) with GAIQ values ​​lower than 50,000 were identified as regions containing hard phases. It should be noted that GAIQ analysis uses the average value within a grain of IQ value, which represents the sharpness of the Kikuchi pattern as a "one pixel" at the measurement point, as the GAIQ value of that grain.

[0099] In the aforementioned EBSD measurements, the equivalent circle diameter was calculated for all grains (ferrite grains) located in the region corresponding to the ferrite determined by GAIQ analysis. Here, a grain is defined as the region enclosed by the boundaries of regions with a crystal orientation phase difference of 15° or more, i.e., grain boundaries. Then, the value obtained by arithmetic averaging these values ​​was taken as the average grain size of the ferrite.

[0100] The average crystal grain size of ferrite was measured at more than 5 locations, and their arithmetic mean was determined as the average crystal grain size of ferrite.

[0101] [The average grain size of the hard phase in the region with a depth from 3 / 8 to 5 / 8 of the plate thickness is 1.0–5.0 μm.] In the steel sheet of this embodiment, the average grain size of the hard phase in the metal microstructure of the steel sheet in the region having a depth from 3 / 8 to 5 / 8 of the sheet thickness (i.e., the center of the sheet thickness) is preferably 1.0 to 5.0 μm. Controlling the average grain size of the hard phase within such a fine range further improves the appearance of the formed steel sheet. The average grain size of the hard phase can also be 1.2 μm or more, 1.5 μm or more, 1.7 μm or more, or 2.0 μm or more. Similarly, the average grain size of the hard phase can also be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.5 μm or less, or 3.0 μm or less.

[0102] The average crystal grain size of the hard phase in the center of the plate thickness is determined as follows.

[0103] First, a sample with a steel plate cross-section perpendicular to the plate surface was collected. The cross-section after etching with nitric acid-ethanol reagent was used as the observation surface. A 100μm × 100μm area within the 3 / 8 to 5 / 8 thickness region of this observation surface was selected as the observation area, and the hard phase was identified using FE-SEM (e.g., JEOL JSM-7200F, measured at an accelerating voltage of 15kV). Specifically, Image J (Ver. 1.54f) image analysis software was used to binarize the metal structure according to different brightness levels to identify the hard phase. Specifically, with the use of nitric acid-ethanol solution, the black areas in the image data represent ferrite, and the unetched white areas represent the hard phase. Next, the equivalent circle diameter of all identified hard phases was calculated. This operation was performed in three observation areas, and the arithmetic mean of the obtained equivalent circle diameters of all hard phases was determined as the average grain size of the hard phase.

[0104] (plate thickness) In this embodiment, the thickness of the steel sheet is not particularly limited and can be appropriately determined according to the type of the final product. For example, the steel sheet may have a thickness of 0.1 to 2.0 mm. Steel sheets with such thicknesses are suitable for use as raw materials for cover components such as car doors and engine hoods. It should be noted that the thickness of the steel sheet may also be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the thickness of the steel sheet may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. For example, by setting the thickness of the steel sheet to 0.2 mm or more, it becomes easier to maintain the flatness of the formed product, and additional effects such as improved dimensional and shape accuracy can be obtained. On the other hand, by setting the thickness to 1.0 mm or less, the lightweight effect of the component becomes significant. The thickness of the steel sheet is measured using a micrometer.

[0105] (Coating) The steel sheet of this embodiment may also have a coating on its surface for the purpose of improving corrosion resistance, etc. When the steel sheet has a coating, the coating may be applied to one or both sides of the steel sheet. Furthermore, the coating can be either a hot-dip galvanized layer or an electroplated layer. Examples of hot-dip galvanized layers include hot-dip zinc plating (GI), alloyed hot-dip zinc plating (GA), hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. Examples of electroplated layers include electroplated zinc plating (EG) and electroplated Zn-Ni alloy plating. The coating is preferably a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electroplated zinc plating layer. The amount of coating applied is not particularly limited and can be a general amount.

[0106] (Mechanical properties) The steel sheet of this embodiment, having the specific chemical composition and metallic structure described above, can achieve a high tensile strength, specifically 500 MPa or more. The tensile strength of the steel sheet is preferably 540 MPa or more, and more preferably 600 MPa or more. There is no particular limitation on the upper limit of the tensile strength, but for example, it can be 980 MPa or less, or 850 MPa or less. By setting the tensile strength to 850 MPa or less, it becomes easier to ensure the processability of the steel sheet during pressing and forming.

[0107] Although the steel sheet of this embodiment has high strength, specifically a tensile strength of 500 MPa or more, it exhibits excellent processability, such as pressing and forming, and maintains an excellent appearance even after forming. In a preferred embodiment, outer panel components incorporating the steel sheet of the embodiments of the present invention can be provided, particularly automotive outer panel components. Examples of automotive outer panel components include roofs, hoods, fenders, and doors, which require highly patterned designs. These outer panel components, particularly automotive outer panel components, only need to incorporate the steel sheet of the embodiments of the present invention in at least a portion of these outer panel components, thus satisfying the previously described characteristics of chemical composition and metal structure in at least a portion of these outer panel components. For portions of the steel sheet where the processing in forming, such as pressing, is relatively low, the characteristics of the metal structure do not change significantly before and after forming.

[0108] <Methods for manufacturing steel plates> Next, a preferred method for manufacturing a steel plate according to one embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a steel plate according to one embodiment of the present invention, and is not intended to limit the steel plate to a steel plate manufactured by the method described below.

[0109] The steel plate manufacturing method of this embodiment includes at least the following steps: a casting step of casting a slab having the above-mentioned specific chemical composition; a hot rolling step of hot rolling the cast slab; a pickling step of removing the oxide film (oxide scale) generated during hot rolling; a cold rolling step of cold rolling the pickled hot-rolled steel plate; and an annealing step of holding the cold-rolled steel plate in a specified atmosphere and in a specified temperature range.

[0110] Furthermore, in addition to the processes described above, the steel plate manufacturing method of this embodiment may optionally include other processes such as the following: a cooling process for cooling the annealed cold-rolled steel plate; a plating process for forming a coating on the surface of the cooled cold-rolled steel plate; and a surface finishing process for performing surface finishing on the steel plate after the plating process.

[0111] The following explains the preferred conditions for these processes.

[0112] (Casting process) In the steel plate manufacturing method of this embodiment, the casting process is a process of casting a slab having the specific chemical composition described above. The casting process includes performing light reduction using a continuous casting machine having a plurality of adjacent pressure rolls in the slab transport direction, and the distance between adjacent pressure rolls being 290 mm or less. It should be noted that, in this specification, light reduction refers to a reduction gradient of 0.6 mm or more per 1 m of casting forward direction.

[0113] As described above, the steel plate of this embodiment possesses a unique metallic structure characterized by a lower hard phase fraction than conventional DP steel, and minimal non-uniformity of the hard phase fraction in the center of the plate thickness in directions orthogonal to the rolling and thickness directions. To obtain this unique metallic structure, it is crucial to control the solidification structure during casting to achieve columnar crystal formation. Specifically, in the casting process, by setting the superheat ΔT (the difference between the molten steel temperature and its solidification temperature) of the molten steel with the aforementioned specific chemical composition to 25°C or higher, and further setting the pressure of the sector section to 450 tons or higher, center segregation can be suppressed even when employing a method different from conventional center segregation countermeasures, such as controlling the solidification structure to a columnar crystal structure with an equiaxed crystal ratio of 15% or less. The superheat ΔT is preferably 30°C or higher. Furthermore, the superheat ΔT is preferably 40°C or lower. It should be noted that the molten steel temperature refers to the temperature of the molten steel within the tundish, which can be determined through actual measurement. The solidification temperature can be determined from the chemical composition of the molten steel using a well-known formula for estimating the solidification temperature.

[0114] It should be noted that conventional measures to improve center segregation involve maximizing the reduction of overheating ΔT (at least set below 25°C) and increasing the equiaxed crystal ratio (at least to more than 15%), but these conventional measures do not provide sufficient improvement. In this embodiment, by employing casting conditions completely different from conventional measures—specifically setting the overheating ΔT to 25°C or higher and setting the pressure on the fan-shaped segment to 450 tons or higher—the solidification structure is controlled to be columnar, thereby suppressing negative segregation of Mn. As a result, microsegregation of Mn is reduced, and the appearance of the formed product, such as ghost lines, is significantly improved.

[0115] The equiaxed crystal ratio (%) can be calculated as follows: Take an etched positive print of the thickness section of the slab in the width direction (orthogonal to the transport direction and the thickness direction), determine the boundary between the columnar crystal structure and the equiaxed crystal structure by visual observation, measure the thickness (mm) of the equiaxed crystal structure in the center of the slab thickness and the thickness (mm) of the slab, divide the thickness of the equiaxed crystal structure by the thickness of the slab and multiply by 100.

[0116] Furthermore, in the casting process, by using a continuous casting machine with a roll spacing of 290 mm or less between adjacent rolls to perform light reduction, the flow of molten steel during solidification can be suppressed, reducing the concentration of Mn towards the center. This suppresses center segregation of Mn. It should be noted that the roll spacing between adjacent rolls is preferably 280 mm or less.

[0117] (Hot rolling process) In the steel plate manufacturing method of this embodiment, the hot rolling process is a process of hot rolling the cast slab. In this process, it is preferable to heat the slab, whose temperature has dropped to below 1100°C after casting, to 1100°C or higher before hot rolling. By setting the heating temperature to 1100°C or higher, the rolling reaction force during hot rolling does not become excessive, and the target product thickness can be easily obtained. There is no particular upper limit to the heating temperature, but from an economic point of view, the heating temperature is preferably set to below 1300°C. In the hot rolling process, the heated slab is subjected to rough rolling and finish rolling.

[0118] It should be noted that the finishing rolling temperature is preferably set below 950°C. By setting the finishing rolling temperature below 950°C, the average grain size of the hot-rolled steel sheet and the final product can be reduced, thereby ensuring sufficient yield strength and a high surface finish after forming.

[0119] In addition, in order to control the hard phase fraction of the surface layer of the steel plate to be within the range of 0.20Vm to 0.80Vm, the air cooling time t (seconds) after finishing rolling is set to satisfy the following formula (3).

[0120] 0.067t≤ln(1300 / (FT+273)) (3) In equation (3), t represents the air cooling time (seconds) after finishing rolling, and FT represents the surface temperature (°C) of the steel plate after finishing rolling.

[0121] If the air cooling time after finishing rolling satisfies the above formula (3), the internal oxide layer becomes easier to remove by the subsequent pickling process. As a result, the internal oxide layer can be suppressed to a minimum.

[0122] The hot-rolled steel sheet obtained through this process is coiled at a specified coiling temperature (CT). To control the hard phase fraction on the surface of the steel sheet within the range of 0.20Vm to 0.80Vm, the coiling temperature is set to below 580°C. By setting the coiling temperature below 580°C, the formation of an internal oxide layer is less likely. It should be noted that the coiling temperature can also be, for example, above 450°C.

[0123] (Pickling process) In the steel sheet manufacturing method of this embodiment, the pickling process is a process for removing the oxide scale generated during hot rolling. In the pickling process, the oxide scale formed on the surface of the hot-rolled steel sheet is removed by immersing the continuously transported hot-rolled steel sheet in a pickling tank containing an acidic washing solution. Examples of acidic washing solutions include hydrochloric acid and sulfuric acid.

[0124] In the pickling process, the pickling time and pickling temperature are set according to the chemical composition (Si and Mn) and coiling temperature (CT) of the steel plate. However, in order to control the hard phase fraction of the surface layer of the steel plate to within the range of 0.20Vm to 0.80Vm, the pickling time and pickling temperature are set in accordance with the following formula (4).

[0125] 0.4×(T-15)×(t) A -20)≥(5[Si]+0.3[Mn]-0.6)×(CT+273) (4) In equation (4), T represents the pickling temperature (°C), and t A The pickling time (seconds) is indicated by [Si], the Si content (mass%) by [Mn], the Mn content (mass%) by [Mn], and the coiling temperature after hot rolling (°C).

[0126] If the pickling time t A If the pickling temperature T satisfies the above equation (4), the internal oxide layer becomes easier to remove, and as a result, the internal oxide layer can be reduced to a great extent.

[0127] It should be noted that the pickling time t AThe speed of hot-rolled steel sheet handling can be changed arbitrarily by adjusting the conveying speed.

[0128] (Cold rolling process) In the steel sheet manufacturing method of this embodiment, the cold rolling process is a process of cold rolling a pickled hot-rolled steel sheet. In the cold rolling process, it is preferable, for example, to cold roll the hot-rolled steel sheet with a cumulative reduction rate of 50% to 90%. By controlling the cumulative reduction rate within such a range, the desired sheet thickness can be ensured, and further, the uniformity of the material in the width direction can be sufficiently ensured.

[0129] (Annealing process) In the steel sheet manufacturing method of this embodiment, the annealing process is a process of holding the cold-rolled steel sheet in a specified atmosphere and within a specified temperature range. In the annealing process, it is preferable to perform an annealing treatment in which the cold-rolled steel sheet is heated to a soaking temperature of 750–900°C and held there. By setting the soaking temperature to 750°C or higher, sufficient recrystallization of ferrite and the reverse phase transformation from ferrite to austenite can be achieved, resulting in a desired metallic structure in the final product. On the other hand, by setting the soaking temperature to 900°C or lower, grain densification can be achieved, resulting in sufficient strength.

[0130] Furthermore, in this annealing process, in order to control the hard phase fraction of the surface layer of the steel plate to a range of 0.20Vm to 0.80Vm, in humidified annealing, i.e. high dew point annealing, the oxygen potential and other conditions are controlled in a manner that satisfies the following three conditions (i) to (iii).

[0131] (i) Heating potential with oxygen: -0.9 ≤ log(pH2O / pH2) ≤ -0.3 (ii) Mesotropical oxygen potential: -2.4 ≤ log(pH2O / pH2) ≤ -0.8 (iii) Temperature of the heating zone / heating zone: Ac1+20≤T≤Ac3-30 Where pH2O is the partial pressure of water vapor in the heating zone atmosphere and the homogenous zone atmosphere (Pa), pH2 is the partial pressure of hydrogen in the heating zone atmosphere and the homogenous zone atmosphere (Pa), and T is the temperature (°C) of the heating zone outlet side and the homogenous zone. In addition, points Ac1 (°C) and Ac3 (°C) are determined based on the chemical composition of the steel plate using the following formula.

[0132] Ac1=751-27C+18Si-12Mn-23Cu-23Ni+24Cr+23Mo-40V-6Ti+230Nb-169Al Ac3=911-436C+30Si-25Mn-5Cr+15Mo+136Ti-19Nb+101Al In the annealing process, by controlling conditions such as oxygen potential in high dew point annealing in accordance with the above three conditions, it is easy to control the hard phase fraction of the surface layer and the center of the plate thickness of the steel plate within the specific range mentioned above.

[0133] (Cooling process) In the steel sheet manufacturing method of this embodiment, the cooling process is a process of cooling the annealed cold-rolled steel sheet. In the cooling process, it is preferable to cool the cold-rolled steel sheet at an average cooling rate of 5 to 50°C / second from the homogenization temperature. By setting the average cooling rate to 5°C / second or more, excessive phase transformation to ferrite can be suppressed, and the formation of hard phases such as martensite can be increased to obtain the desired strength. Furthermore, by setting the average cooling rate to 50°C / second or less, the steel sheet can be cooled more uniformly in the width direction (the direction orthogonal to the rolling direction and the thickness direction).

[0134] (Plating process) In the steel sheet manufacturing method of this embodiment, the plating process is a process of forming a coating on the surface of a cooled cold-rolled steel sheet by performing a plating treatment. Examples of plating treatments include hot-dip galvanizing, alloyed hot-dip galvanizing, and electroplating. For example, as a plating treatment, the surface of the steel sheet can be subjected to hot-dip galvanizing, or alloying can be performed after hot-dip galvanizing. The specific conditions for the plating and alloying treatments are not particularly limited, and any suitable conditions known to those skilled in the art can be used. For example, the alloying temperature can be 450–600°C.

[0135] It should be noted that in the steel plate manufacturing method of this embodiment, the plating process is performed for the purpose of improving corrosion resistance, etc. Since it is not a necessary process, it is not necessary to perform such a plating process when the steel plate does not have a coating.

[0136] It should be noted that the present invention is not limited to the above-described embodiments or the following examples. Appropriate combinations, substitutions, and modifications can be made without departing from the purpose and spirit of the present invention.

[0137] The present invention will be described in more detail below with reference to embodiments, but the present invention is not limited to these embodiments.

[0138] Example In the following embodiments, a steel sheet according to one embodiment of the present invention was manufactured under various conditions, and the tensile strength or workability, the appearance characteristics after forming, etc. of the obtained steel sheet were investigated.

[0139] First, using a continuous casting machine equipped with multiple pressure rolls arranged with a roll spacing of 290 mm or less, a continuous casting method with a light pressure gradient of 0.6 mm or more per 1 m casting advance direction is implemented to cast a slab with the chemical composition shown in Table 1. The remainder other than the composition shown in Table 1 consists of Fe and impurities. Various conditions in this casting process are shown in Table 2. In Table 2, the casting condition "ΔT" is a condition such as superheating ΔT ≥ 25°C, and the casting condition "pressing pressure" is a condition such as sector pressing pressure ≥ 450 tons. In each example, the condition that is met is recorded as "OK", and the condition that is not met is recorded as "NG", as shown in Table 2.

[0140] Next, the obtained slab is subjected to a hot rolling process. The various conditions for the hot rolling process are shown in Table 2.

[0141] In Table 2, the "air cooling time" of the hot rolling conditions of each steel plate is the air cooling time after finishing rolling that satisfies the following equation (3), and the "coiling temperature" of the hot rolling conditions is the coiling temperature (CT) ≤ 580℃.

[0142] 0.067t≤ln(1300 / (FT+273)) (3) In equation (3), t represents the air cooling time (seconds) after finishing rolling, and FT represents the surface temperature (°C) of the steel plate after finishing rolling.

[0143] In each case, the cases that meet these conditions are recorded as "OK" and the cases that do not meet them are recorded as "NG", as shown in Table 2.

[0144] It should be noted that for steel plate No.1, the slab with the chemical composition shown in Table 1 is heated at 1230°C and held for more than 30 minutes, and then hot-rolled steel plate with a thickness of 2.3 mm is produced by finishing at a finishing temperature of 900°C, air cooling time after finishing at 1 second, and coiling temperature of 550°C.

[0145] Then, a pickling process is performed on the hot-rolled steel sheet after the hot rolling process. The various conditions in the pickling process are shown in Table 2. In Table 2, the "time / temperature" of the pickling conditions is that the pickling time and pickling temperature satisfy the following equation (4).

[0146] 0.4×(T-15)×(t) A -20)≥(5[Si]+0.3[Mn]-0.6)×(CT+273) (4) In equation (4), T represents the pickling temperature (°C), and t AThe pickling time (seconds) is indicated by [Si], the Si content (mass%) by [Mn], the Mn content (mass%) by [Mn], and the coiling temperature after hot rolling (°C).

[0147] In each case, the condition is recorded as "OK" if it is met, and "NG" if it is not met, as shown in Table 2.

[0148] It should be noted that for steel plate No.1, the hot-rolled steel plate manufactured as described above will be pickled with hydrochloric acid at a concentration of 10%, a temperature of 70°C, and a time of 100 seconds.

[0149] Then, for the pickled hot-rolled steel sheets, a cold rolling process (cumulative reduction rate of 80%), an annealing process (soaking temperature of 800°C), and a cooling process (average cooling rate of 10°C / second) are performed to manufacture steel sheets. The surface of each steel sheet is then subjected to a suitable plating treatment to form a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), or an electro-galvanized layer (EG) made of the plating types shown in Table 2.

[0150] It should be noted that for steel plate No. 1, the pickled steel plate is cold-rolled until the thickness is 0.45 mm. Then, the resulting cold-rolled steel plate undergoes alloying hot-dip galvanizing treatment via CGL (continuous hot-dip galvanizing production line). In CGL, the steel plate is heated under the following conditions: heating zone temperature of 780°C, dew point of -2°C, hydrogen concentration of 2.6% by volume, soaking zone temperature of 780°C, dew point of -40°C, and hydrogen concentration of 2.7% by volume. After hot-dip galvanizing, alloying treatment is performed at 560°C.

[0151] It should be noted that in Table 2, steel plates No. 8 and 33 are examples of steel plates for which no surface coating treatment has been applied.

[0152] The various conditions in the annealing process are shown in Table 2. In Table 2, the "oxygen potential of the heating zone", "oxygen potential of the homogenizing zone" and "temperature of the heating zone exit side / homogenizing zone" of the annealing conditions are respectively three conditions that satisfy the following (i) to (iii).

[0153] (i) Heating potential with oxygen: -0.9 ≤ log(pH2O / pH2) ≤ -0.3 (ii) Mesotropical oxygen potential: -2.4 ≤ log(pH2O / pH2) ≤ -0.8 (iii) Temperature of the heating zone / heating zone: Ac1+20≤T≤Ac3-30 In each case, the cases that meet these conditions are recorded as "OK" and the cases that do not meet them are recorded as "NG", as shown in Table 2.

[0154] Samples were collected from the steel sheet manufactured as described above, and its chemical composition was analyzed. The results confirmed that the chemical composition was unchanged from that of the slab shown in Table 1. It should be noted that the underlines indicating chemical compositions in Table 1 indicate that the invention is outside the scope of this invention. Furthermore, the underlines indicating various values ​​in Table 2 indicate that the invention is outside the scope of this invention, or that the manufacturing conditions for the steel sheet that do not meet the requirements of this invention, or that various properties of the steel sheet are not preferred. It should be noted that the "ratio of hard phase fraction to hard phase fraction at depths of 3 / 8 to 5 / 8 of the plate thickness; Vs / Vm" in Table 2 refers to the ratio Vs of the hard phase fraction in the region extending from the surface of the steel sheet to 1 / 8 of the plate thickness to the hard phase fraction Vm in the region extending from 3 / 8 to 5 / 8 of the plate thickness. Therefore, the ratio Vs / Vm of 0.20 to 0.80 has the same meaning as "the area fraction of the hard phase in the region having a depth from the surface of the steel plate to a depth of 1 / 8 of the plate thickness is 0.20Vm to 0.80Vm".

[0155] The microstructure and various properties of the obtained steel plate in the surface layer and the center of the plate thickness are measured and evaluated using the methods described above or below.

[0156] (Tensile strength and yield ratio) Tensile test specimens (No. 5) according to JIS Z2241:2011, taken from steel plates with their length direction orthogonal to the rolling and thickness directions, were subjected to tensile tests to determine the tensile strength (MPa) and yield stress (0.2% of yield strength). The yield ratio YR (%), defined as (yield stress) / (tensile strength), was then calculated from these results. Based on these results, steel plates with a tensile strength of 500 MPa or higher were evaluated as high-strength steel plates, and steel plates with a yield ratio of 0.70 or lower were evaluated as steel plates with excellent workability.

[0157] (Processability) The compressive bending method of JIS Z2248:2022 (Metallic Materials Bending Test Method) was used to evaluate workability. Long strips, 30 mm wide and 100 mm long, were collected from the steel plate with the bending test axis parallel to the rolling direction. After the compressive bending test, the presence or absence of cracks at the bend apex was visually evaluated. The absence of cracks was considered acceptable and marked "OK" in Table 2. The presence of cracks during the visual evaluation was considered unacceptable and marked "NG" in Table 2.

[0158] (Appearance after molding) The appearance of the steel sheet after forming is evaluated by the degree of ghost lines and the presence or absence of tensile strain on the surface of the pressed door. The surface after pressing is ground with a grinding wheel. Ghost lines are defined as stripes of several millimeters in spacing that extend along the rolling direction. Based on the following criteria corresponding to the degree of stripe formation, a score of 1 to 5 is assigned. On the ground surface of the pressed steel sheet, any 100mm × 100mm area is visually inspected. A case where no stripes extending along the rolling direction are observed is designated as "1"; a case where the maximum length of the stripes is less than 20mm is designated as "2"; a case where the maximum length of the stripes exceeds 20mm but is less than 50mm is designated as "3"; a case where the maximum length of the stripes exceeds 50mm but is less than 70mm is designated as "4"; and a case where the maximum length of the stripes exceeds 70mm is designated as "5". Cases rated "3" or below are considered to have excellent appearance after molding and are thus deemed acceptable. On the other hand, cases rated "4" or above are considered to have poor appearance after molding and are thus deemed unacceptable.

[0159] Furthermore, when visually inspecting the surface after pressing, the condition where surface defects, known as tensile strain, are formed by localized linear or dendritic uneven patterns, is defined as "SS" and is judged as unqualified due to poor appearance after pressing.

[0160] Referring to Table 2, for the comparative example of steel plate No. 3, which underwent the casting process under the condition of ΔT=15℃, the following results were obtained: the standard deviation of the hard phase fraction of the metal structure in the center of the plate thickness was 0.89%, ghost lines were produced, and the appearance after forming was poor.

[0161] In the case of the comparative example No. 4 steel plate, which underwent annealing under the condition that the oxygen potential was -1.5, the following results were obtained: the ratio of the hard phase fraction Vs of the surface metal structure to the hard phase fraction Vm of the metal structure in the center of the plate thickness, Vs / Vm, was 0.83 (the hard phase fraction Vs of the surface metal structure was 0.83Vm), resulting in poor machinability.

[0162] In the case of steel plate No. 5, which underwent annealing under the condition that the oxygen potential of the isothermal zone was -0.6, the following results were obtained: the ratio of the hard phase fraction Vs of the surface metal structure to the hard phase fraction Vm of the metal structure in the center of the plate thickness, Vs / Vm, was 0.00 (the hard phase fraction Vs of the surface metal structure was 0.00Vm), the yield ratio was high, and tensile strain was generated, resulting in a poor appearance after forming.

[0163] In the case of steel plate No. 6, which underwent annealing at a heating temperature of 860°C, the following results were obtained: the ratio of the hard phase fraction Vs of the surface metal structure to the hard phase fraction Vm of the metal structure in the center of the plate thickness, Vs / Vm, was 0.17 (the hard phase fraction Vs of the surface metal structure was 0.17Vm), resulting in a high yield ratio and tensile strain, leading to a poor appearance after forming.

[0164] Taking steel plate No. 14, which was cast under a pressure of 350 tons, as a comparative example, the following results were obtained: the standard deviation of the hard phase fraction of the metal structure in the center of the plate thickness was 0.88%, ghost lines were produced, and the appearance after forming was poor.

[0165] For the comparative example of steel plate No. 15, which was hot-rolled under the conditions of a finishing rolling temperature of 930°C and an air cooling time of 1.7 seconds, i.e., not satisfying the above formula (3), the following results were obtained: the ratio of the hard phase fraction Vs of the surface metal structure to the hard phase fraction Vm of the metal structure in the center of the plate thickness, Vs / Vm, was 0.17 (the hard phase fraction Vs of the surface metal structure was 0.17Vm), the yield ratio was high, and tensile strain was generated, resulting in a poor appearance after forming.

[0166] Taking the steel plate No. 16, which underwent a hot rolling process at a coiling temperature of 670°C, as a comparative example, the following results were obtained: the ratio of the hard phase fraction Vs of the surface metal structure to the hard phase fraction Vm of the metal structure in the center of the plate thickness, Vs / Vm, was 0.14 (the hard phase fraction Vs of the surface metal structure was 0.14Vm), resulting in a high yield ratio and the generation of tensile strain, leading to a poor appearance after forming.

[0167] For the comparative example of steel plate No. 17, which was subjected to pickling under the conditions of a coiling temperature of 570°C, a pickling temperature of 60°C, and a pickling time of 100 seconds (i.e., the conditions of the above formula (4) were not met), the following results were obtained: the ratio of the hard phase fraction Vs of the surface metal structure to the hard phase fraction Vm of the metal structure in the center of the plate thickness, Vs / Vm, was 0.14 (the hard phase fraction Vs of the surface metal structure was 0.14Vm), the yield ratio was high, and tensile strain was generated, resulting in a poor appearance after forming.

[0168] For the comparative example of steel plate No. 21, which was hot rolled at a coiling temperature of 690°C, pickled at a pickling temperature of 65°C and a pickling time of 80 seconds (i.e., not meeting the conditions of the above formula (4)), and annealed with an oxygen potential of -0.5 at the homogenization temperature, the following results were obtained: the ratio of the hard phase fraction Vs of the surface metal structure to the hard phase fraction Vm of the metal structure in the center of the plate thickness, Vs / Vm, was 0.00 (the hard phase fraction Vs of the surface metal structure was 0.00Vm), the yield ratio was high, and tensile strain was generated, resulting in a poor appearance after forming.

[0169] For the comparative example of steel plate No. 22, which was hot-rolled under the conditions of a finishing rolling temperature of 920°C and an air cooling time of 1.5 seconds (i.e., not satisfying the above formula (3)), and pickled under the conditions of a coiling temperature of 570°C, a pickling temperature of 55°C, and a pickling time of 90 seconds (i.e., not satisfying the above formula (4)), the following results were obtained: the ratio of the hard phase fraction Vs of the surface metal structure to the hard phase fraction Vm of the metal structure in the center of the plate thickness, Vs / Vm, was 0.13 (the hard phase fraction Vs of the surface metal structure was 0.13Vm), the yield ratio was high, and tensile strain was generated, resulting in a poor appearance after forming.

[0170] For the comparative example No. 29, which uses comparative steel S with a C content of 0.118%, the following results were obtained: the standard deviation of the hard phase fraction of the metal structure in the center of the plate thickness was 0.87%, resulting in ghost lines and poor appearance after forming.

[0171] For the comparative example of steel plate No. 30 using comparative steel T with a Mn content of 2.71%, the following results were obtained: the ferrite fraction and hard phase fraction of the metal structure in the center of the plate thickness were 73% and 27%, respectively, and the standard deviation of the hard phase fraction of the metal structure in the center of the plate thickness was 0.91%, resulting in poor machinability, ghost lines, and poor appearance after forming.

[0172] For the comparative example No. 31, which uses comparative steel U with a Si content of 1.620%, the following results were obtained: tensile strain occurred, resulting in poor appearance after forming.

[0173] For the comparative example No. 32, which uses comparative steel V with a C content of 0.023%, the following results were obtained: the ferrite fraction and hard phase fraction of the metal structure in the center of the plate thickness were 98% and 2%, respectively, resulting in reduced tensile strength and poor strength.

[0174] For the comparative example No. 33, which uses comparative steel W with a Mn content of 0.85%, the following results were obtained: the tensile strength decreased, the strength was poor, and the yield ratio increased.

[0175] On the other hand, in the present invention examples of steel plates No. 1, 2, 7 to 13, 18 to 20, and 23 to 28, a high strength of 500 MPa or more can be maintained, and the processability such as pressing and forming is excellent. Even when strain is imparted by pressing and forming, the generation of ghost lines and tensile strain on the surface of the steel plate can be significantly suppressed.

Claims

1. A steel plate, characterized in that, The chemical composition of the steel plate, expressed as a percentage by mass, is: C：0.030～0.100%、 Mn: 1.00~2.50%, Si: 0.005~1.500%, Al：0.005～0.700%、 P: Below 0.100% S: Below 0.0200% N: below 0.0150% O: Below 0.0100% Cr：0～0.80%、 Mo: 0–0.50%, B：0～0.0100%、 Ti: 0~0.100%, Nb: 0~0.100%, V：0～0.50%、 Ni: 0~1.00%, Cu: 0~1.00%, W：0～1.00%、 Sn: 0~1.00% Sb: 0~0.200%, As: 0~0.200% Ca: 0~0.0100% Mg: 0~0.0100%, Zr：0～0.0100%、 REM: 0–0.0100%, and Remaining components: Fe and impurities. When the area fraction of the hard phase in a region having a depth from 3 / 8 to 5 / 8 of the plate thickness is set to Vm, the area fraction of the hard phase in a region having a depth from the surface of the steel plate to 1 / 8 of the plate thickness is 0.20Vm to 0.80Vm. The microstructure in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness comprises, by area %: 75-97% ferrite and 3-25% hard phase, and the standard deviation of the area fraction of the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness in a direction orthogonal to the rolling direction and the plate thickness direction is less than 0.85%.

2. The steel plate according to claim 1, characterized in that, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following: Cr：0.01～0.80%、 Mo: 0.01–0.50%, B：0.0001～0.0100%、 Ti: 0.001~0.100% Nb: 0.001~0.100% V：0.01～0.50%、 Ni: 0.01~1.00%, Cu: 0.01~1.00%, W：0.01～1.00%、 Sn: 0.001~1.00% Sb: 0.001~0.200% As: 0.001~0.200% Ca: 0.0001~0.0100% Mg: 0.0001~0.0100%, Zr: 0.0001~0.0100%, and REM: 0.0001~0.0100%.

3. The steel plate according to claim 1 or 2, characterized in that, The index A represented by the following formula (1) is 0.45% or higher. A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb] (1) Where [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in terms of mass%, and are 0% when the corresponding element is not present.

4. The steel plate according to any one of claims 1 to 3, characterized in that, It satisfies the following equation (2). (TS-180000 / TS) / Vm≥35 (2) Wherein, TS represents the tensile strength of the steel plate in MPa, and Vm represents the area fraction of the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness.

5. The steel plate according to any one of claims 1 to 4, characterized in that, The average grain size of the ferrite in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness is 5.0–30.0 μm. The average crystal grain size of the hard phase in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness is 1.0 to 5.0 μm.

6. The steel plate according to any one of claims 1 to 5, characterized in that, The hard phase in the region having a depth from 3 / 8 to 5 / 8 of the plate thickness is composed of at least one of martensite, bainite, tempered martensite, and pearlite.

7. An outer panel component comprising the steel plate according to any one of claims 1 to 6.

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