Steel sheet and outside sheet member

Abstract

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

Description

Steel plates and outer panel components

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

[0002] The automotive industry is seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. Increasing the strength of the steel sheets used is one effective way to achieve both a lighter vehicle body and collision safety, and against this background, the development of high-strength steel sheets is underway.

[0003] In this regard, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of a steel sheet used as a substrate, characterized in that the substrate contains, by mass%, C: 0.02 to 0.20%, Si: 0.7% or less, Mn: 1.5 to 3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1 to 1.0%, N: 0.010% or less, and Cr: 0.03 to 0.5%, and has a surface oxidation index A during annealing, defined by the mathematical formula A = 400Al / (4Cr + 3Si + 6Mn) in which the contents of Al, Cr, Si, and Mn are specified in the same paragraph, of 2.3 or more, with the balance being Fe and unavoidable impurities, and further the structure of the substrate is composed of ferrite and a second phase, and the second phase is mainly composed of martensite. Furthermore, Patent Document 1 describes that the high-strength hot-dip galvanized steel sheet has excellent surface quality and a tensile strength of 590 MPa or more, making it suitable for use mainly as structural parts of automobiles such as members and rockers.

[0004] Japanese Patent Application Laid-Open No. 2005-220430

[0005] In recent years, in connection with the demand for further improvement in fuel efficiency, there has been an increasing need to reduce the weight of not only structural components such as the members described in Patent Document 1, but also exterior panel components such as roofs, hoods, fenders, and doors. Unlike the structural components described above, these exterior panel components are visible to the public, and therefore, not only are characteristics such as strength important, but also design and surface quality are important. Therefore, excellent appearance after forming is required. Meanwhile, in connection with such demand for weight reduction, even higher strength and thinner thickness are required for the steel sheets used for these exterior panel components. In addition, as the shapes of these exterior panel components become more complex, unevenness tends to occur more easily on the steel sheet surface after forming, and if such unevenness occurs, the appearance quality will be reduced.

[0006] Specifically, for example, in the case of DP steel (Dual Phase steel) consisting of a soft ferrite phase (first phase) and a hard second phase mainly composed of martensite as described in Patent Document 1, the yield strength (hereinafter sometimes referred to as "YS") is low and although it has excellent workability during forming such as press forming, non-uniform deformation is likely to occur during forming, in which the soft phase consisting of ferrite and its surroundings deform preferentially. For this reason, when such DP steel is used for outer panel parts, minute irregularities are generated on the steel sheet surface after forming, which can cause appearance defects called ghost lines or stretcher strain.

[0007] Even if attempts were made to optimize various conditions during steel plate manufacturing in order to improve these poor appearances, there was a risk that the low YS characteristic of DP steel would not be obtained depending on the conditions during steel plate manufacturing, and that workability during forming would be reduced.

[0008] Therefore, an object of the present invention is to provide a high-strength steel sheet having a novel structure that is excellent in appearance after forming and in workability during forming.

[0009] The present invention includes the following aspects.

[0010] (Aspect 1) A steel plate having a chemical composition, in mass%, of 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, wherein the area ratio of the hard phase in a region having a depth from the surface of the steel plate to a 1 / 8 depth position of the plate thickness is 0.20Vm to 0.80Vm, where Vm is the area ratio of the hard phase in a region having a depth from a 3 / 8 depth position to a 5 / 8 depth position of the plate thickness, The metal structure in the region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the plate thickness contains, in area %, 75 to 97% ferrite and 3 to 25% hard phase, and the standard deviation of the area ratio of the hard phase in the region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the plate thickness in a direction perpendicular to the rolling direction and the plate thickness direction is 0.85% or less.

[0011] (Aspect 2) The chemical composition is, in mass%, Cr: 0.01 to 0.80%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.200%, As: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, The steel sheet according to the above-mentioned aspect 1, characterized in that it contains one or more selected from the group consisting of Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%.

[0012] (Aspect 3) The steel sheet according to Aspect 1 or Aspect 2, characterized in that index A represented by the following formula (1) is 0.45% or more: 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 mass%, and are 0% when the corresponding element is not contained.

[0013] (Aspect 4) The steel sheet according to any one of Aspects 1 to 3, characterized in that the following formula (2) is satisfied: (TS-180000 / TS) / Vm≧35 (2), where TS represents tensile strength in MPa, and Vm represents the area ratio of the hard phase in a region having a depth ranging from 3 / 8 to 5 / 8 of the sheet thickness.

[0014] (Aspect 5) The steel sheet according to any one of Aspects 1 to 4, wherein the ferrite has an average grain size of 5.0 to 30.0 μm in a region having a depth from a 3 / 8 depth position to a 5 / 8 depth position of the sheet thickness, and the hard phase has an average grain size of 1.0 to 5.0 μm in a region having a depth from a 3 / 8 depth position to a 5 / 8 depth position of the sheet thickness.

[0015] (Aspect 6) The steel sheet according to any one of Aspects 1 to 5, wherein the hard phase in the region having a depth from a 3 / 8 depth position to a 5 / 8 depth position of the sheet thickness is composed of at least one of martensite, bainite, tempered martensite, and pearlite.

[0016] (Embodiment 7) An outer panel member including the steel plate according to any one of Embodiments 1 to 6.

[0017] According to the present invention, it is possible to provide a high-strength steel sheet that is excellent in appearance after forming and in workability during forming.

[0018] Hereinafter, preferred embodiments of the steel sheet of the present invention will be described in detail. In this specification, various numerical ranges mean ranges including the upper and lower limit values ​​unless otherwise specified.

[0019] For exterior panel components such as roofs and doors, i.e., exterior panel parts, DP steel, which has a relatively low yield strength (YS), is often used to avoid surface defects known as surface distortions that occur during press forming and other processes. However, as described above, DP steel is prone to non-uniform deformation, in which soft phases and their surrounding areas deform preferentially during processes such as press forming. Therefore, when such DP steel is used for exterior panel parts, minute irregularities may appear on the steel sheet surface after forming, resulting in appearance defects known as ghost lines. To explain the occurrence of these ghost lines in more detail, first, during processes such as press forming, the soft phase portion composed of ferrite deforms in a concave manner, while the hard phase portion composed mainly of martensite does not concave or rather deforms in a convex manner. This results in the formation of minute irregularities on the steel sheet surface after forming. These minute irregularities are formed by convex portions extending generally along the rolling direction and concave portions extending generally along the rolling direction, aligned in the sheet width direction perpendicular to the rolling direction. When the surface of the steel sheet after forming is polished, the convex portions of the minute irregularities on the surface of the steel sheet are removed, resulting in the appearance of ghost lines, which are streaky patterns extending in the rolling direction of the steel sheet. The rolling direction can be easily identified based on the extension direction of the crystal grains of the steel sheet. In this specification, the direction orthogonal to the rolling direction and the thickness direction means a direction perpendicular to each of the rolling direction and the thickness direction.

[0020] In order to improve such ghost lines, the present inventors first conducted detailed studies focusing on the morphology of the hard phase in the metal structure of the steel sheet. As a result, the present inventors found that in steel sheets in which a soft phase and a hard phase are mixed, such as DP steel, the degree of ghost lines becomes significant due to the presence of hard phases connected in stripes (striped hard phases) in the metal structure. Furthermore, the present inventors found that by suppressing the formation of such striped hard phases and dispersing the hard phases more uniformly in the metal structure, it is possible to suppress the formation of minute irregularities on the steel sheet surface after forming while sufficiently maintaining the high strength provided by the hard phases, and as a result, the occurrence of ghost lines can be suppressed.

[0021] More specifically, the present inventors have found that reducing Mn segregation during solidification in a slab casting process in which molten steel is solidified to cast a slab is effective in suppressing the formation of banded hard phases. Therefore, the present inventors have conducted further detailed studies on methods for reducing Mn segregation from two perspectives: center segregation and microsegregation.

[0022] The present inventors first considered that suppressing the flow of molten steel during slab casting would be effective in reducing center segregation of Mn, and conducted various studies. More specifically, during solidification, molten steel solidifies from the surface, with the center solidifying last. As the solid phase is expelled from the liquid phase, Mn in the liquid phase becomes concentrated during this stage. If the molten steel flows during solidification, this concentrated Mn tends to eventually gather in the center during the solidification process, resulting in significant center segregation of Mn. Therefore, the present inventors discovered that, when producing steel plate, the center segregation of Mn can be significantly suppressed by appropriately controlling the solidification conditions and suppressing the flow of molten steel.

[0023] Next, the inventors of the present invention considered that promoting Mn diffusion during solidification would be effective in reducing Mn microsegregation, and conducted various studies. To promote Mn diffusion, it is effective to create a structure in which Mn easily diffuses. Therefore, the inventors focused on the δ phase, in which Mn has a fast diffusion rate, and experimentally investigated the influence of each element in steel on Mn microsegregation in order to achieve δ solidification. As a result, the inventors found that when the C and Mn contents are high, δ solidification does not occur during solidification, but the Mn diffusion rate decreases, increasing microsegregation, while when the Si, Al, Cr, and Mo contents are high, Mn diffusion during solidification is promoted, thereby reducing microsegregation.

[0024] Although the above-described methods for reducing Mn segregation can improve ghost lines to a certain extent, in order to obtain a more sufficient improvement effect on ghost lines, the present inventors have investigated further methods for improving ghost lines in addition to the above-described methods for reducing Mn segregation. As a result, the present inventors have found that ghost lines can be improved by reducing the hard phase fraction of a steel sheet, even if a certain amount of centerline Mn segregation remains. The present inventors have also found that the solidification structure significantly affects the occurrence of ghost lines, and that even if the centerline Mn segregation is small, if coarse equiaxed crystals are formed in the solidification structure, negative Mn segregation occurs, which increases the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction, making ghost lines more likely to occur. Furthermore, the present inventors have found that a method different from conventional measures against centerline segregation, namely, a method of reducing the equiaxed crystal fraction and controlling the solidification structure to a columnar crystal structure, can suppress negative Mn segregation and improve ghost lines.

[0025] Based on these findings, the inventors have found that ghost lines can be significantly improved while adequately maintaining high strength by controlling the solidification structure during casting to a columnar crystal structure, reducing the central segregation of Mn during solidification, which is a factor in the generation of banded hard phases, and reducing the hard phase fraction and its variation.

[0026] Although the ghost lines have been sufficiently improved by the above-mentioned techniques, it has been found that the appearance defect called stretcher strain can still occur. Furthermore, even if various conditions during steel plate production are controlled within specific ranges as described above in order to improve these appearance defects, it has been found that, depending on the conditions, the low YS characteristic of DP steel cannot be obtained, and the workability during forming may be reduced.

[0027] Therefore, the present inventors have conducted extensive research into a method for achieving both suppression of poor appearance after forming, including not only ghost lines but also stretcher strain, and workability during forming. As a result, the present inventors have found that a high-strength steel sheet that is excellent in both appearance after forming and workability during forming can be obtained by a method of controlling, within specific ranges, the area ratio of the hard phase in a region having a depth from the surface of the steel sheet to a ⅛ depth position of the sheet thickness, i.e., the hard phase fraction in the surface layer of the steel sheet, and the area and variation thereof of the hard phase in a region having a depth from a ⅜ depth position to a ⅙ depth position of the sheet thickness, i.e., the hard phase fraction in the sheet thickness center portion of the steel sheet and its variation.

[0028] The present invention has been completed based on the above findings, and includes the following embodiments.

[0029] Hereinafter, preferred embodiments of the steel sheet of the present invention will be described in detail.

[0030] <Steel Sheet> A steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, of 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. In the steel sheet of this embodiment, the area ratio of the hard phase in a region having a depth from the surface of the steel sheet to a depth position of 1 / 8 of the sheet thickness is 0.20Vm to 0.80Vm, where Vm is the area ratio of the hard phase in a region having a depth from a depth position of 3 / 8 to a depth position of 5 / 8 of the sheet thickness. Furthermore, in the steel plate of this embodiment, the metal structure in a region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the plate thickness contains, in area %, 75 to 97% ferrite and 3 to 25% hard phase, and the standard deviation of the area ratio of the hard phase in the direction perpendicular to the rolling direction and the plate thickness direction in the region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the plate thickness is 0.85% or less.

[0031] As described above, the steel sheet of this embodiment has a specific chemical composition, and the metallographic structure in the region from 3 / 8 to 5 / 8 of the thickness (i.e., the center of the thickness) has a lower hard phase fraction than conventional DP steel, and the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the thickness direction at the center of the thickness (i.e., the variation in the hard phase fraction) is small. Such a specific metallographic structure can be obtained by adopting a specific chemical composition and casting conditions, as described below.

[0032] As described above, the steel sheet of this embodiment has such a unique metal structure, i.e., a metal structure in which the centerline segregation of Mn during solidification is small and the hard phase fraction and its variation are small, and therefore the steel sheet can suppress the generation of minute irregularities on the steel sheet surface after forming while sufficiently maintaining high strength. As a result, the steel sheet of this embodiment can significantly suppress the occurrence of ghost lines and stretcher strain while sufficiently maintaining high strength.

[0033] Furthermore, in the steel sheet of this embodiment, the area ratio of the hard phase in a region having a depth from the surface of the steel sheet to a depth position of 1 / 8 of the sheet thickness, i.e., the hard phase fraction in the surface layer, is 0.20 Vm to 0.80 Vm relative to the area ratio Vm of the hard phase in the center of the sheet thickness. When the hard phase fraction in the surface layer is 0.20 Vm or more, a low yield strength (YS) and a high tensile strength (TS) can be ensured, resulting in a high-strength steel sheet with excellent workability during forming. Furthermore, this further facilitates suppression of stretcher strain that occurs during forming of the steel sheet. Furthermore, when the hard phase fraction in the surface layer of the steel sheet is 0.80 Vm or less, good bendability can be ensured, resulting in a steel sheet with excellent workability during forming.

[0034] In order to control the hard phase fraction in the surface layer of the steel sheet within the range of 0.20 Vm to 0.80 Vm, it is necessary to adopt specific conditions, which will be described later, in the hot rolling process, pickling process, and annealing process during the production of the steel sheet.

[0035] Here, a hard phase fraction of "0.20 Vm" in the surface layer of a steel sheet means that the hard phase fraction Vm in the center portion of the sheet thickness is 0.20 times (0.20 × Vm). The same applies to 0.80 Vm, etc. In this specification, the area fraction (%) of the hard phase and the hard phase fraction (%) are synonymous.

[0036] As described above, the steel sheet of the present embodiment is a high-strength steel sheet that suppresses poor appearance after forming, including not only ghost lines but also stretcher strain, while also achieving good formability during forming, i.e., a high-strength steel sheet that is excellent in both appearance after forming and formability during forming.

[0037] The steel plate of this embodiment will be described in more detail below. In the following description, the unit "%" for the content of each element and the index A means "mass %" unless otherwise specified. In this specification, the term "to" indicating a numerical range means that the numerical values ​​before and after the term are included as the lower and upper limits, respectively, unless otherwise specified. In this specification, the term "x / y depth position in the plate thickness (where x and y are natural numbers satisfying x < y)" refers to a position shifted from the surface of the steel plate in the plate thickness direction by a distance (depth) of x / y in the plate thickness direction toward the center of the steel plate. For example, if the thickness of a steel plate is 2 mm, the term "1 / 8 depth position in the plate thickness" refers to a position 0.25 mm deep from the surface of the steel plate in the plate thickness direction. When a steel sheet has a plating layer on its surface, the "surface of the steel sheet" in the definition of "a depth position that is x / 8 of the sheet thickness from the surface of the steel sheet" means the interface between the steel sheet and the plating layer, and the "sheet thickness" means the sheet thickness of the steel sheet (base material) excluding the plating layer.

[0038] (Chemical Composition) As described above, the steel sheet of this embodiment has the following composition: 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.

[0039] Each element in this chemical composition will be described in detail below.

[0040] [C: 0.030 to 0.100%] C is an element that increases the strength of the steel sheet. To fully obtain this effect, the C content is set to 0.030% or more. The C content may be 0.032% or more, 0.034% or more, or 0.035% or more. On the other hand, if C is contained excessively, the diffusion of Mn during solidification may be hindered, and microsegregation of Mn may not be sufficiently suppressed. Therefore, the C content is set to 0.100% or less. The C content may be 0.095% or less, 0.090% or less, or 0.080% or less.

[0041] [Mn: 1.00 to 2.50%] Mn is an element that improves the hardenability of steel and contributes to improving strength. To fully obtain this effect, the Mn content is set to 1.00% or more. The Mn content may be 1.02% or more, 1.04% or more, or 1.05% or more. On the other hand, if Mn is contained excessively, the diffusion of Mn during solidification may be hindered, and microsegregation of Mn may not be sufficiently suppressed. Therefore, the Mn content is set to 2.50% or less. The Mn content may be 2.40% or less, 2.30% or less, or 2.20% or less.

[0042] [Si: 0.005 to 1.500%] Si is a deoxidizing element for steel and a solid-solution strengthening element effective in increasing the strength of steel sheet without impairing its ductility. Si is also an element 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 may be 0.008% or more, 0.010% or more, or 0.012% or more. On the other hand, excessive Si content may reduce the peelability of scale and cause surface defects. Therefore, the Si content is set to 1.500% or less. The Si content may be 1.200% or less, 1.000% or less, or 0.800% or less.

[0043] [Al: 0.005 to 0.700%] Al functions as a deoxidizer and is a solid-solution strengthening element effective in increasing the strength of steel. Furthermore, Al is also an element 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 may 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 may be 0.600% or less, 0.500% or less, 0.400% or less, or 0.300% or less.

[0044] [P: 0.100% or less] P is an element that is mixed in during the manufacturing process. P is also a solid-solution strengthening element. The P content may be 0%. However, reducing the P content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the P content may 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 may be 0.080% or less, 0.075% or less, or 0.070% or less.

[0045] [S: 0.0200% or less] S is an element that is mixed in during the manufacturing process. The S content may be 0%. However, reducing the S content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0008% or more. On the other hand, excessive S content may form Mn sulfides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and / or bendability. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0150% or less, 0.0100% or less, or 0.0080% or less.

[0046] [N: 0.0150% or less] N is an element that is mixed in during the manufacturing process. The N content may be 0%. However, reducing the N content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form nitrides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and / or bendability. Therefore, the N content is set to 0.0150% or less. The N content may be 0.0100% or less, 0.0080% or less, or 0.0050% or less.

[0047] [O: 0.0100% or less] O is an element that is mixed in during the manufacturing process. The O content may be 0%. However, reducing the O content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0008% or more. On the other hand, excessive O content may form coarse oxides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and / or bendability. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0070% or less, 0.0050% or less, or 0.0030% or less.

[0048] The basic chemical composition of the steel sheet of this embodiment is as described above. Furthermore, in this embodiment, the steel sheet may contain one or more of the following optional elements in place of a portion of the remaining Fe, as necessary. These optional elements will be described in detail below.

[0049] [Cr: 0 to 0.80%] Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel plate. Cr is also an element that is effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. While the Cr content may be 0%, to obtain these effects, the Cr content is preferably 0.01% or more. The Cr content may be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, excessive Cr content may cause the formation of coarse Cr carbides that serve as fracture initiation points. Therefore, the Cr content is preferably 0.80% or less. The Cr content may be 0.75% or less, 0.70% or less, or 0.65% or less.

[0050] [Mo: 0 to 0.50%] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. Mo is also an element that is effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. The Mo content may be 0%, but to obtain these effects, the Mo content is preferably 0.01% or more. The Mo content may be 0.05% or more or 0.07% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. Therefore, the Mo content is preferably 0.50% or less. The Mo content may be 0.45% or less, 0.40% or less, or 0.35% or less.

[0051] [B: 0 to 0.0100%] B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more, 0.0008% or more, or 0.0010% or more. On the other hand, excessive B content may cause the formation of B precipitates, which may reduce the strength of the steel sheet. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0052] [Ti: 0 to 0.100%] Ti is an element that has the effect of reducing the amounts of S, N, and O, which generate coarse inclusions that act as fracture initiation sites. Ti is also a precipitation strengthening element that has the effect of refining the structure and improving the strength-formability balance of the steel sheet. The Ti content may be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, excessive Ti content may form coarse Ti sulfides, Ti nitrides, and / or Ti oxides, which may reduce the formability of the steel sheet. Therefore, the Ti content is preferably 0.100% or less. The Ti content may be 0.080% or less, 0.075% or less, or 0.070% or less.

[0053] [Nb: 0 to 0.100%] Nb is a precipitation strengthening element that contributes to improving the strength of steel sheets due to strengthening by precipitates, grain refinement strengthening by suppressing ferrite grain growth, and / or dislocation strengthening by suppressing recrystallization. The Nb content may be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.003% or more, 0.004% or more, or 0.005% or more. On the other hand, excessive Nb content may increase unrecrystallized ferrite, thereby reducing the formability of the steel sheet. Therefore, the Nb content is preferably 0.100% or less. The Nb content may be 0.080% or less, 0.070% or less, or 0.060% or less.

[0054] [V: 0 to 0.50%] V is an element that contributes to improving the strength of steel sheet due to strengthening by precipitates, grain refinement strengthening by suppressing ferrite grain growth, and / or dislocation strengthening by suppressing recrystallization. The V content may be 0%, but to obtain these effects, the V content is preferably 0.01% or more. The V content may be 0.02% or more. On the other hand, excessive V content may precipitate a large amount of carbonitrides, reducing the formability of the steel sheet. Therefore, the V content is preferably 0.50% or less. The V content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[0055] [Ni: 0 to 1.00%] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The Ni content may be 0%, but to obtain this effect, the Ni content is preferably 0.01% or more. The Ni content may 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 may be 0.60% or less, 0.50% or less, or 0.40% or less.

[0056] [Cu: 0 to 1.00%] Cu is an element that exists in steel in the form of fine particles and contributes to improving the strength of the steel sheet. The Cu content may be 0%, but to obtain such an effect, the Cu content is preferably 0.01% or more. The Cu content may be 0.03% or more or 0.05% or more. On the other hand, excessive Cu content may reduce the weldability of the steel sheet. Therefore, the Cu content is preferably 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0057] [W: 0 to 1.00%] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The W content may be 0%, but to obtain this effect, the W content is preferably 0.01% or more. The W content may be 0.02% or more. On the other hand, excessive W content may deteriorate hot workability and reduce productivity. Therefore, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0058] [Sn: 0 to 1.00%] Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sn content may be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more. The Sn content may 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 may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0059] [Sb: 0 to 0.200%] Sb is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sb content may be 0%, but to obtain such an effect, the Sb content is preferably 0.001% or more. The Sb content may 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 may be 0.180% or less, 0.170% or less, or 0.160% or less.

[0060] [As: 0 to 0.200%] Although As does not have as significant an effect as Sb and Sn, it is an element that is effective in improving corrosion resistance in an acidic environment. The As content may be 0%, but to obtain such an effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more, 0.004% or more, or 0.006% or more. On the other hand, excessive As content may deteriorate hot workability. Therefore, the As content is preferably 0.200% or less. The As content may be 0.180% or less, 0.170% or less, or 0.160% or less.

[0061] [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 sheet. The Ca, Mg, Zr, and REM contents may be 0%, but to achieve these effects, the Ca, Mg, Zr, and REM contents are preferably 0.0001% or more. The Ca, Mg, Zr, and REM contents may be 0.0005% or more, 0.0010% or more, or 0.0015% or more, respectively. On the other hand, excessive inclusion of these elements may reduce the ductility of the steel sheet. Therefore, the Ca, Mg, Zr, and REM contents are preferably 0.0100% or less. The Ca, Mg, Zr and REM contents may be 0.0080% or less, 0.0070% or less or 0.0060% or less, respectively.

[0062] In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0063] Regarding the above optional elements, in this embodiment, the chemical composition of the steel sheet is: Cr: 0.01 to 0.80%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.200%, As: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, It is preferable that the steel sheet contains one or more elements selected from the group consisting of Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%. When the steel sheet contains such optional elements, it is possible to more reliably maintain high strength, significantly suppress the occurrence of appearance defects after forming, such as ghost lines, and provide a steel sheet with excellent workability during forming.

[0064] In the steel sheet of this embodiment, the balance other than the above elements consists of Fe and impurities. Here, the impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing steel sheets. 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 impurities may be 0.100% or less.

[0065] [Index A: 0.45% or More] In the present embodiment, the chemical composition of the steel sheet preferably has an index A represented by the following formula (1) of 0.45% or more: A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb] (1) Here, [Si], [P], [Al], [Ti], and [Nb] are the contents of each element in mass%, and are 0% when the corresponding element is not contained.

[0066] The index A is determined by the contents of the solid solution strengthening elements Si, P, and Al and the precipitation strengthening elements Ti and Nb, and the larger this value, the higher the strength can be obtained with a smaller hard phase fraction. By setting the index A to 0.45% or more, high strength can be obtained while controlling the hard phase fraction of the steel sheet to a certain level or less.

[0067] The index A may be 0.48% or more, 0.50% or more, or 0.52% or more. The upper limit of the index A is not particularly limited, and the index A may be, for example, 1.50% or less, 1.20% or less, or 1.00% or less.

[0068] The chemical composition of the steel sheet may be measured by a common analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, a 35 mm square test piece is taken from the steel sheet at a depth of approximately 1 / 2 the sheet thickness, and the chemical composition of the steel sheet is determined by measuring the specimen using a measuring device such as the Shimadzu ICPS-8100 under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0069] (Metal Structure) [Area Fraction of Hard Phase in Region from the Surface of Steel Plate to ⅛ Depth of Plate Thickness: 0.20 Vm to 0.80 Vm] In this embodiment, the metal structure of the steel plate has an area fraction of the hard phase in a region from the surface of the steel plate to ⅛ depth of the plate thickness (i.e., hard phase fraction in the surface layer) of 0.20 Vm to 0.80 Vm, where Vm is the area fraction of the hard phase in a region from ⅜ depth to ⅝ depth of the plate thickness (i.e., hard phase fraction in the plate thickness center). When the hard phase fraction in the surface layer of the steel plate is 0.20 Vm or more, low yield strength (YS) and high tensile strength (TS) can be ensured, resulting in a high-strength steel plate with excellent workability during forming. Furthermore, this also makes it easier to suppress stretcher strain that occurs during forming of the steel plate. Furthermore, when the hard phase fraction in the surface layer of the steel sheet is 0.8 Vm or less, good bendability can be ensured, and therefore the steel sheet can have excellent workability during forming. From the viewpoint of further enhancing these effects, the area fraction of the hard phase in the surface layer may be 0.30 Vm or more, 0.35 Vm or more, 0.40 Vm or more, or 0.45 Vm or more. On the other hand, from the viewpoint of further improving workability during forming, the area fraction of the hard phase in the surface layer may be 0.79 Vm or less, 0.78 Vm or less, 0.77 Vm or less, or 0.76 Vm or less.

[0070] [Metal Structure in a Region Having a Depth from the 3 / 8 Depth Position to the 5 / 8 Depth Position of the Plate Thickness: Ferrite: 75-97% and Hard Phase: 3-25%] In this embodiment, the metal structure in the region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the plate thickness of the steel plate (i.e., the plate thickness center portion) contains, in area %, 75-97% ferrite and 3-25% hard phase. By forming the metal structure in the plate thickness center portion of the steel plate into such a specific composite structure, it is possible to maintain the strength of the steel plate within an appropriate range, more specifically, to achieve a tensile strength of 500 MPa or more, while suppressing poor appearance after forming. From the viewpoint of further increasing the strength of the steel plate, the area fraction of the hard phase in the plate thickness center portion may 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 plate thickness center portion may 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 portion of the sheet thickness may be 22% or less, 20% or less, or 18% or less. Similarly, the area fraction of ferrite in the center portion of the sheet thickness may be 78% or more, 80% or more, 82% or more, 85% or more, 90% or more, or 92% or more.

[0071] In the steel sheet of this embodiment, the hard phase refers to a structure harder than ferrite, and is composed of, for example, 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 the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness is preferably composed of at least one of martensite, bainite, and tempered martensite, and more preferably composed of martensite. It is preferable that the metal structure of the steel sheet contains little retained austenite. Specifically, the retained austenite is preferably less than 1% or less than 0.5%, and more preferably 0%, in terms of area percentage.

[0072] (Identification of Metallographic Structure and Calculation of Area Fraction) Identification of the metallographic structure and calculation of the area fraction are performed as follows. First, the rolling direction of the obtained steel plate is identified. The rolling direction of the steel plate can be determined as follows. A test piece is taken so that a cross section parallel to the surface of the steel plate can be observed. The cross section of the test piece taken in the plate thickness direction is mirror-polished and then observed using an optical microscope. The observation surface is a plane parallel to the surface of the steel plate at an arbitrary depth position in the plate thickness direction between a ¼ depth position and a ½ depth position, and the direction parallel to the extension direction of the crystal grains in the observation surface is determined to be the rolling direction. The direction perpendicular 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 metallographic structure (microstructure) is taken from the W / 4 position or 3W / 4 position of the plate width W (i.e., a position W / 4 in the plate width direction from either end of the steel plate in the plate width direction). The size of the sample is, for example, 20 mm in the rolling direction × 20 mm in the plate width direction × thickness of the steel plate.

[0073] Next, the metallographic structure is observed using an FE-SEM (field emission scanning electron microscope, for example, JEOL's "JSM-7200F", acceleration voltage: 15 kV, light source: FE, instrument resolution: 1.2 nm). At this time, the observation field is a 100 μm × 100 μm region corresponding to the observation target position in the thickness direction of the steel plate. That is, when observing the metallographic structure of a region having a depth from the surface of the steel plate to a depth position of 1 / 8 of the plate thickness, a 50 μm × 100 μm region (50 μm in the plate thickness direction, and a 100 μm region in the observation field in the direction perpendicular to the plate thickness direction) is observed from the surface of the steel plate to a depth position of 1 / 8 of the plate thickness in the plate thickness direction. On the other hand, when observing the metal structure of a region having a depth from 3 / 8 to 5 / 8 of the thickness of the steel plate (i.e., the center of the plate thickness), a 100 μm × 100 μm region between the 3 / 8 to 5 / 8 depth positions of the thickness of the steel plate in the plate thickness direction is observed. Note that the resolution for observing any region is 1280 × 960 pixels. To prepare the sample, the plate thickness cross section in the plate width direction is polished as the observation surface and etched by nital corrosion. Next, the "microstructure" is classified from SEM photographs at magnifications of 500 or 1000 times. Ferrite and hard phases can be distinguished by differences in brightness.

[0074] For the steel sheet etched by nital corrosion, the 100 μm × 100 μm region is observed at 500x or 1000x magnification, and image analysis is performed using image analysis software Image J (Ver. 1.54f) to determine the area fraction of the hard phase. Specifically, the secondary electron image of the metallographic structure is binarized based on differences in brightness, and the black parts of the image data are considered to be ferrite, and the uncorroded white parts are considered to be the hard phase, and the area fraction of the hard phase is calculated. For each metallographic structure, three measurement points are used, and image analysis is performed at these measurement points in the same manner as above to measure the area fraction of the hard phase. These area fractions are then arithmetically averaged to calculate the average value. This average value is considered to be the area fraction of the hard phase, and the remainder is considered to be the area fraction of ferrite. In addition, when observing the metal structure of a region having a depth from the surface of the steel plate to a depth position of 1 / 8 of the plate thickness, and when observing the metal structure of a region having a depth from a depth position of 3 / 8 to a depth position of 5 / 8 of the plate thickness of the steel plate, the only difference is the observation field in the plate thickness direction of the steel plate, and the area fraction of the hard phase can be measured in the same way.

[0075] Furthermore, when it is necessary to measure the area fraction of retained austenite, the area fraction of retained austenite can be measured by X-ray diffraction of the above observation surface. Specifically, using Co-Kα radiation, the integrated intensities of a total of six peaks, namely, α(110), α(200), α(211), γ(111), γ(200), and γ(220), at a depth of ½ of the plate thickness are obtained, and the volume fraction of retained austenite is calculated using the intensity averaging method. The obtained volume fraction of retained austenite is defined as the area fraction of retained austenite.

[0076] [Standard deviation of the area fraction of the hard phase in the direction perpendicular to the rolling direction and the thickness direction in a region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness: 0.85% or less] In the present embodiment, the metallographic structure of the steel sheet has a standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the thickness direction in a region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness (i.e., the sheet thickness center portion) of 0.85% or less. Note that the standard deviation of the hard phase fraction means the standard deviation of the area fraction of the hard phase itself. As described above, appearance defects after forming, such as ghost lines, are significantly affected not only by Mn segregation but also by the influence of the solidification structure. For example, even if the centerline segregation of Mn is small, if coarse equiaxed crystals are formed in the solidification structure, negative segregation of Mn occurs, which increases the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the thickness direction, and may worsen the appearance defects after forming. However, in the steel sheet of this embodiment, the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction in the region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness is 0.85% or less, that is, the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction is a certain level or less, so that it is possible to significantly suppress poor appearance after forming.

[0077] From the viewpoint of more reliably suppressing poor appearance after forming, the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction in a region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness may be 0.80% or less, 0.75% or less, 0.70% or less, or 0.65% or less. Note that the lower limit of the standard deviation of the hard phase fraction is not particularly limited, and the standard deviation of the hard phase fraction may be, for example, 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.

[0078] Regarding the hard phase fraction, it is preferable that the ratio of the average value of the area fraction of the hard phase to its standard deviation be 0.10 or less (i.e., standard deviation of hard phase fraction / average value of area fraction of hard phase≦0.10). The ratio is preferably 0.09 or less, 0.08 or less, or 0.07 or less. The lower limit of the ratio is 0, but may be set to 0.01 if necessary.

[0079] The standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the thickness direction of the metallographic structure at the center of the sheet thickness can be determined as follows. First, a 100 μm × 100 μm region between a 3 / 8 depth position and a 5 / 8 depth position of the sheet thickness in a cross section parallel to the direction perpendicular to the rolling direction and the thickness direction of the steel sheet and perpendicular to the surface of the steel sheet is observed at a magnification of 500x or 1000x using an FE-SEM (field emission scanning electron microscope, for example, JEOL JSM-7200F, acceleration voltage: 15 kV, light source: FE, instrument resolution: 1.2 nm) to obtain a secondary electron image. The resolution is 1280 × 960 pixels. Similar to the hard phase area fraction in the above-mentioned "Identification of Metallic Structure and Calculation of Area Fraction," image analysis is performed on this secondary electron image using image analysis software Image J (Ver. 1.54f), and the hard phase area fraction is measured every 100 μm within an 8 mm range in the direction perpendicular to the rolling direction and thickness direction of the steel plate (plate width direction) from the W / 4 position to the 3W / 4 position of the plate width W, and the standard deviation is calculated. Note that the observation range in the direction perpendicular to the rolling direction and thickness direction (plate width direction) may be less than 8 mm or may exceed 8 mm. However, the lower limit of the observation range for the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and thickness direction (plate width direction) is 4 mm, and the upper limit is 12 mm.

[0080] [Relationship between tensile strength and hard phase fraction of steel sheet: (TS-180000 / TS) / Vm≧35] In this embodiment, the steel sheet preferably satisfies the following formula (2): (TS-180000 / TS) / Vm≧35 (2) Here, TS represents the tensile strength (MPa) of the steel sheet, and Vm represents the area fraction (area %) of the hard phase in a region having a depth ranging from 3 / 8 to 5 / 8 of the sheet thickness.

[0081] When a steel sheet satisfies such formula (2), even if center segregation of Mn remains to some extent, it is easy to suppress the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction, and as a result, it is possible to more easily prevent poor appearance after forming of the steel sheet.

[0082] The tensile strength (TS) of a steel plate can be measured by taking a No. 5 tensile test piece of JIS Z2241:2011, with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction, from the steel plate and conducting a tensile test in accordance with JIS Z2241:2011. Note that if the No. 5 tensile test piece cannot be taken from the steel plate to be measured (for example, if the size of the measurement sample is small), a tensile test piece of any size, with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction, can be used instead of the No. 5 tensile test piece.

[0083] [Average grain size of ferrite in a region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness: 5.0 to 30.0 μm] In the steel sheet of this embodiment, the average grain size of ferrite in the metal structure in a region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness of the steel sheet (i.e., the center portion of the sheet thickness) is preferably 5.0 to 30.0 μm. Controlling the average grain size of ferrite within such a fine range can further improve the appearance of the steel sheet after forming. The average grain size of ferrite may be 6.0 μm or more or 7.0 μm or more. Similarly, the average grain size of ferrite may be 27.0 μm or less, 21.0 μm or less, 15.0 μm or less, or 11.0 μm or less.

[0084] The average grain size of ferrite in the center of the sheet thickness is determined as follows: First, the KAM value of ferrite is calculated by KAM (Kernel Average Misorientation) analysis and GAIQ (Grain Average Image Quality) analysis in an electron probe microanalyzer (EPMA) measurement, which is a crystal analysis method using an SEM.

[0085] A field emission scanning electron microscope (e.g., JEOL's "JSM-7001F") is used as the SEM observation device, and EBSD analysis can be performed using, for example, TSL's "OIM Analysis 7." In EBSD analysis, a 50 μm × 50 μm region between a 3 / 8 depth position and a 5 / 8 depth position of the plate thickness of a steel plate whose plate cross section has been mirror-polished is analyzed at intervals (pitch) of 0.05 μm. KAM analysis is an analysis in which the average value of the misorientation (°) between a "certain pixel" that is the measurement point and all adjacent pixels is used as the KAM value of the "certain pixel," and a KAM map based on local crystal misorientation can be created. By such KAM analysis, the KAM value in ferrite is analyzed. The region where ferrite exists in the EBSD measurement results is determined by the GAIQ analysis described below.

[0086] GAIQ (Grain Average Image Quality) analysis is performed on the EBSD measurement results obtained under the same measurement conditions as the KAM value to obtain the GAIQ value. Based on this GAIQ value, the region is divided into a region with a GAIQ value of 50,000 or more and a region with a GAIQ value of less than 50,000, and the region (crystal grain) with a GAIQ value of 50,000 or more is determined to be a region where ferrite exists. On the other hand, the region (crystal grain) with a GAIQ value of less than 50,000 is determined to be a region where a hard phase exists. Note that the GAIQ analysis is an analysis in which the average value within one crystal grain of the IQ value, which represents the clarity of the Kikuchi pattern at a "certain pixel" that is the measurement point, is used as the GAIQ value of that crystal grain.

[0087] In the EBSD measurement results, the circle equivalent diameters are calculated for all crystal grains (ferrite grains) located in the region corresponding to the ferrite determined by the GAIQ analysis. Here, the crystal grains are defined as the region surrounded by the grain boundary, which is the boundary between regions where the crystal orientation differs by 15° or more. The value obtained by arithmetically averaging these values ​​is then defined as the average crystal grain size of the ferrite. The above-mentioned average crystal grain size of the ferrite is measured at five locations, and the arithmetic average value is determined as the average crystal grain size of the ferrite.

[0088] [Average grain size of the hard phase in the region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness: 1.0 to 5.0 μm] In the steel sheet of this embodiment, the average grain size of the hard phase in the metal structure in the region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness of the steel sheet (i.e., the center portion 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 can further improve the appearance of the steel sheet after forming. The average grain size of the hard phase may 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 may 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.

[0089] The average grain size of the hard phase in the center of the sheet thickness is determined as follows. First, a sample having a steel sheet cross section perpendicular to the sheet surface is collected, and the steel sheet cross section is etched with a Nital reagent to obtain the observation surface. A 100 μm x 100 μm region within the 3 / 8 to 5 / 8 sheet thickness position of this observation surface is used as the observation region, and the hard phase is identified using an FE-SEM (e.g., a JEOL JSM-7200F, measured at an acceleration voltage of 15 kV). Specifically, the metal structure is binarized based on differences in brightness using image analysis software Image J (Ver. 1.54f), and the hard phase is identified. Specifically, when a Nital solution is used, the black portions of the image data represent ferrite, and the uncorroded white portions represent the hard phase. Next, the circle-equivalent diameters of all identified hard phases are calculated. This operation is carried out in three observation regions, and the circle equivalent diameters of all the hard phases obtained are arithmetically averaged, and the obtained value is determined as the average crystal grain size of the hard phase.

[0090] (Thickness) In this embodiment, the thickness of the steel plate is not particularly limited and can be determined appropriately depending on the type of final product, etc. For example, the steel plate may have a thickness of 0.1 to 2.0 mm. Steel plates having such thicknesses are suitable for use as materials for covering members such as doors and hoods. The thickness of the steel plate may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the thickness of the steel plate 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 making the thickness of the steel plate 0.2 mm or more, it becomes easier to maintain the shape of the molded product flat, and additional effects such as improved dimensional accuracy and shape accuracy can be obtained. On the other hand, by making the thickness 1.0 mm or less, the weight reduction effect of the member is significant. The thickness of the steel plate is measured using a micrometer.

[0091] (Plated Layer) The steel sheet of this embodiment may further have a plated layer on its surface for the purpose of improving corrosion resistance, etc. When the steel sheet has a plated layer, the plated layer may be provided on one or both sides of the steel sheet. The plated layer may be either a hot-dip plated layer or an electroplated layer. Examples of the hot-dip plated layer include a hot-dip galvanized layer (GI), a galvannealed layer (GA), a hot-dip aluminum plated layer, a hot-dip Zn—Al alloy plated layer, a hot-dip Zn—Al—Mg alloy plated layer, and a hot-dip Zn—Al—Mg—Si alloy plated layer. Examples of the electroplated layer include an electrogalvanized layer (EG) and an electrolytic Zn—Ni alloy plated layer. Of these, the plated layer is preferably a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer. The coating weight of the plated layer is not particularly limited, and a general coating weight may be used.

[0092] (Mechanical Properties) According to the steel sheet of this embodiment having the above-mentioned specific chemical composition and metallographic structure, high tensile strength, specifically a tensile strength of 500 MPa or more, can be achieved. The tensile strength of the steel sheet is preferably 540 MPa or more, more preferably 600 MPa or more. The upper limit of the tensile strength is not particularly limited, but the tensile strength may be, for example, 980 MPa or less or 850 MPa or less. By setting the tensile strength to 850 MPa or less, it is easier to ensure workability when press-forming the steel sheet.

[0093] The steel sheet of the present embodiment has high strength, specifically a tensile strength of 500 MPa or more, yet is excellent in workability in press forming and the like, and can maintain an excellent appearance even after forming. In a preferred embodiment, an exterior panel member, particularly an automobile exterior panel member, is provided that includes the steel sheet of the present embodiment. Examples of automobile exterior panel members include roofs, hoods, fenders, doors, and the like, which require high designability. These exterior panel members, particularly automobile exterior panel members, only need to include the steel sheet of the present embodiment in at least a portion of the exterior panel member, and therefore at least a portion of these exterior panel members will satisfy the chemical composition and metallographic characteristics described above. In portions of the steel sheet that are relatively lightly processed in forming, such as press forming, the metallographic characteristics do not change particularly before and after forming.

[0094] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to one embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to one embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0095] The method for producing a steel sheet according to this embodiment includes at least 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 an oxide film (scale) formed during the hot rolling, a cold rolling step of cold rolling the hot-rolled steel sheet after pickling, and an annealing step of holding the cold-rolled steel sheet in a predetermined atmosphere at a predetermined temperature range.

[0096] Furthermore, in addition to the above-mentioned steps, the method for manufacturing a steel sheet according to this embodiment may optionally include other steps such as a cooling step for cooling the cold-rolled steel sheet after annealing, a plating step for forming a plating layer on the surface of the cold-rolled steel sheet after cooling, and a skin-pass rolling step for skin-pass rolling the steel sheet after the plating step.

[0097] Preferred conditions for these steps will be described below.

[0098] (Casting Process) In the method for producing a steel sheet according to this embodiment, the casting process is a process of casting a slab having the specific chemical composition. The casting process includes soft reduction using a continuous casting machine having a plurality of reduction rolls adjacent to each other in the slab transport direction, the roll pitch between the adjacent reduction rolls being 290 mm or less. In this specification, soft reduction refers to a reduction gradient of 0.6 mm or more per meter in the casting direction.

[0099] As described above, the steel sheet of this embodiment is essential to have a unique metal structure with a lower hard phase fraction than conventional DP steel and small variations in the hard phase fraction in the rolling direction and the direction perpendicular to the thickness direction at the center of the plate thickness. To obtain such a unique metal structure, it is important to control the solidification structure during casting to be columnar. Specifically, in the casting process, by setting the superheat ΔT (the difference between the molten steel temperature and the solidification temperature of the molten steel) of the molten steel having the specific chemical composition to 25°C or higher and further setting the segment pressing force to 450 tons or higher, the solidification structure is controlled to a columnar crystal structure with an equiaxed crystal fraction of 15% or less. This method, which differs from conventional center segregation countermeasures, can also suppress center segregation. The superheat ΔT is preferably 30°C or higher. Furthermore, the superheat ΔT is preferably 40°C or lower. The molten steel temperature is the temperature of the molten steel in the tundish and can be determined by actual measurement. The solidification temperature can be determined from the chemical composition of the molten steel using a known solidification temperature estimation formula.

[0100] Conventional countermeasures for improving centerline segregation involve minimizing the superheat ΔT (at least below 25°C) and increasing the equiaxed crystal fraction (at least to more than 15%), but such conventional countermeasures do not provide sufficient improvement. In the present embodiment, casting conditions completely different from conventional countermeasures are adopted, namely, unique casting conditions of setting the superheat ΔT to 25°C or higher and setting the segment pressing force to 450 tons or higher, thereby controlling the solidification structure to a columnar crystal structure, thereby suppressing negative Mn segregation. As a result, Mn microsegregation is reduced, and post-forming appearance such as ghost lines can be sufficiently improved.

[0101] The equiaxed crystal ratio (%) can be calculated by taking an etched print of a cross section of the thickness of the slab in the width direction (the direction perpendicular to the conveying direction and the thickness direction), visually determining the boundary between the columnar crystal structure and the equiaxed crystal structure, measuring the thickness (mm) of the equiaxed crystal structure at the center of the thickness of the slab and the thickness (mm) of the slab, and dividing the thickness of the equiaxed crystal structure by the thickness of the slab and multiplying the result by 100.

[0102] Furthermore, in the casting process, soft reduction is performed using a continuous casting machine in which the roll pitch of adjacent reduction rolls is 290 mm or less, thereby suppressing the flow of molten steel during solidification and reducing the concentration of Mn in the center. This makes it possible to suppress center segregation of Mn. Note that the roll pitch of adjacent reduction rolls is preferably 280 mm or less.

[0103] (Hot rolling process) In the steel sheet manufacturing method of this embodiment, the hot rolling process is a process of hot rolling a cast slab. In this process, it is preferable to heat the slab, whose temperature has dropped to less than 1100°C after casting, to 1100°C or higher prior to hot rolling. By setting the heating temperature to 1100°C or higher, the rolling reaction force does not become excessively large in hot rolling, making it easier to obtain the desired product thickness. There is no particular upper limit to the heating temperature, but from an economical viewpoint, it is preferable that the heating temperature be less than 1300°C. In the hot rolling process, the heated slab is subjected to rough rolling and finish rolling.

[0104] The finish rolling end temperature is preferably 950° C. or less. By setting the finish rolling end temperature to 950° C. or less, the average crystal grain size of the hot-rolled steel sheet and the final product can be reduced, and sufficient yield strength and high surface quality after forming can be ensured.

[0105] Furthermore, in order to control the hard phase fraction in the surface layer of the steel sheet within the range of 0.20 Vm to 0.80 Vm, the air-cooling time t (seconds) after finish rolling is set to a time that satisfies the following formula (3): 0.067t≦ln(1300 / (FT+273)) (3) Here, in formula (3), t represents the air-cooling time (seconds) after finish rolling, and FT represents the surface temperature (°C) of the steel sheet after finish rolling.

[0106] If the air-cooling time after finish rolling satisfies the above formula (3), the internal oxide layer is easily removed in the subsequent pickling step, and as a result, the internal oxide layer can be minimized.

[0107] The hot-rolled steel sheet obtained in this manner is coiled at a predetermined coiling temperature (CT). In order to control the hard phase fraction in the surface layer of the steel sheet to within a range of 0.20 Vm to 0.80 Vm, the coiling temperature is set to a temperature of 580°C or less. By setting the coiling temperature to a temperature of 580°C or less, it is possible to make it difficult for an internal oxide layer to form. Note that the coiling temperature may be, for example, a temperature of 450°C or more.

[0108] (Pickling process) In the steel sheet manufacturing method of this embodiment, the pickling process is a process for removing scale formed during hot rolling. In the pickling process, the hot-rolled steel sheet that is continuously transported is immersed in a pickling tank containing an acidic cleaning solution, thereby removing scale formed on the surface of the hot-rolled steel sheet. As the acidic cleaning solution, for example, hydrochloric acid, sulfuric acid, or the like can be used.

[0109] In the pickling process, the pickling time and temperature are set according to the chemical components (Si and Mn) of the steel sheet and the coiling temperature (CT). However, in order to control the hard phase fraction in the surface layer of the steel sheet within the range of 0.20 Vm to 0.80 Vm, the pickling time and temperature are set to satisfy the following formula (4): 0.4 × (T - 15) × (t Ａ−20)≧(5[Si]+0.3[Mn]−0.6)×(CT+273) (4) where T in formula (4) represents the pickling temperature (°C), t Ａ represents the pickling time (seconds), [Si] represents the Si content (mass%), [Mn] represents the Mn content (mass%), and CT represents the coiling temperature (°C) after hot rolling.

[0110] Pickling time t Ａ When the temperature T of the pickling process satisfies the above formula (4), the internal oxide layer is more easily removed, and as a result, the internal oxide layer can be reduced as much as possible.

[0111] The pickling time t Ａ can be arbitrarily changed by adjusting the conveying speed of the hot-rolled steel sheet.

[0112] (Cold Rolling Step) In the steel sheet manufacturing method of this embodiment, the cold rolling step is a step of cold rolling the hot-rolled steel sheet after pickling. In the cold rolling step, it is preferable to cold-roll the hot-rolled steel sheet so that the cumulative reduction is, for example, 50 to 90%. By controlling the cumulative reduction within this range, it is possible to ensure the desired sheet thickness and further ensure sufficient uniformity of the material in the sheet width direction.

[0113] (Annealing step) In the steel sheet manufacturing method of this embodiment, the annealing step is a step of holding the cold-rolled steel sheet in a predetermined atmosphere and at a predetermined temperature range. In the annealing step, it is preferable to perform an annealing treatment in which the cold-rolled steel sheet is heated to a soaking temperature of 750 to 900°C and held at that temperature. By setting the soaking temperature to 750°C or higher, it is possible to sufficiently promote the recrystallization of ferrite and the reverse transformation from ferrite to austenite, thereby obtaining a desired metal structure in the final product. On the other hand, by setting the soaking temperature to 900°C or lower, it is possible to densify the crystal grains and obtain sufficient strength.

[0114] Furthermore, in this annealing process, in order to control the hard phase fraction in the surface layer of the steel sheet within the range of 0.20 Vm to 0.80 Vm, conditions such as oxygen potential are controlled so as to satisfy the following three conditions (i) to (iii) during humid annealing, i.e., high dew point annealing: (i) Oxygen potential in heating zone: -0.9≦log(pH２ O / pH ２ ) ≦-0.3 (ii) Isothermal zone oxygen potential: -2.4 ≦ log(pH ２ O / pH ２ ) ≦-0.8 (iii) Temperature of the heating zone outlet and soaking zone: Ac1+20≦T≦Ac3-30, where pH ２ O is the water vapor partial pressure (Pa) of the atmosphere in the heating zone and the soaking zone, and pH ２ is the hydrogen partial pressure (Pa) in the heating zone atmosphere and the soaking zone atmosphere, and T is the temperature (°C) at the outlet of the heating zone and in the soaking zone. The Ac1 point (°C) and Ac3 point (°C) are calculated according to the chemical composition of the steel sheet using the following formulas: Ac1 = 751 - 27C + 18Si - 12Mn - 23Cu - 23Ni + 24Cr + 23Mo - 40V - 6Ti + 230Nb - 169Al Ac3 = 911 - 436C + 30Si - 25Mn - 5Cr + 15Mo + 136Ti - 19Nb + 101Al

[0115] In the annealing step, by controlling conditions such as oxygen potential so as to satisfy the above three conditions in high dew-point annealing, it is possible to easily control the hard phase fraction in the surface layer and the center portion of the steel sheet within the above specific range.

[0116] (Cooling Step) In the steel sheet manufacturing method of this embodiment, the cooling step is a step of cooling the cold-rolled steel sheet after annealing. In the cooling step, the cold-rolled steel sheet is preferably cooled so that the average cooling rate from the soaking temperature is 5 to 50°C / s. By setting the average cooling rate to 5°C / s or more, excessive transformation to ferrite is suppressed and the amount of hard phases such as martensite produced is increased, thereby achieving the desired strength. Furthermore, by setting the average cooling rate to 50°C / s or less, the steel sheet can be cooled more uniformly in the sheet width direction (the direction perpendicular to the rolling direction and the sheet thickness direction).

[0117] (Plating Step) In the steel sheet manufacturing method of this embodiment, the plating step is a step of forming a plating layer on the surface of the steel sheet by performing a plating treatment on the surface of the cooled cold-rolled steel sheet. Examples of plating treatments include hot-dip plating, alloying hot-dip plating, and electroplating. For example, the plating treatment may involve performing a hot-dip galvanizing treatment on the surface of the steel sheet, or the hot-dip galvanizing treatment may be followed by an alloying treatment. The specific conditions for the plating treatment and the alloying treatment are not particularly limited, and any appropriate conditions known to those skilled in the art can be adopted. For example, the alloying temperature may be 450 to 600°C.

[0118] In the steel sheet manufacturing method of this embodiment, the plating step is a step performed for the purpose of improving corrosion resistance, etc., and is not an essential step. Therefore, if the steel sheet does not include a plating layer, there is no need to perform such a plating step.

[0119] The present invention is not limited to the above-described embodiments or the following examples, and appropriate combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention.

[0120] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0121] In the following examples, steel sheets according to one embodiment of the present invention were produced under various conditions, and the tensile strength, workability, and appearance after forming of the resulting steel sheets were examined.

[0122] First, a slab having the chemical composition shown in Table 1 was cast by continuous casting using a continuous casting machine equipped with multiple reduction rolls arranged at a roll pitch of 290 mm or less, with a soft reduction gradient of 0.6 mm or more per meter in the casting direction. The remainder other than the components shown in Table 1 is Fe and impurities. Various conditions in this casting process are shown in Table 2. In Table 2, the casting condition "ΔT" is the condition that superheat ΔT ≥ 25°C, and the casting condition "pressing force" is the condition that segment pressing force ≥ 450 tons. In each example, cases where these conditions are met are marked "OK," and cases where they are not met are marked "NG," and the results are shown in Table 2.

[0123] Next, the obtained slab was subjected to a hot rolling process. Various conditions in the hot rolling process are shown in Table 2. In Table 2, the "air cooling time" hot rolling condition for each steel plate is a condition that the air cooling time after finish rolling satisfies the following formula (3), and the "coiling temperature" hot rolling condition is a condition that the coiling temperature (CT) ≦ 580°C: 0.067t ≦ ln(1300 / (FT + 273)) (3) In formula (3), t represents the air cooling time (seconds) after finish rolling, and FT represents the surface temperature (°C) of the steel plate after finish rolling. In each example, cases where these conditions are satisfied are indicated as "OK", and cases where they are not satisfied are indicated as "NG", and are respectively shown in Table 2. Steel Plate No. For No. 1, a slab having the chemical composition shown in Table 1 was heated and held at 1230°C for 30 minutes or more, and then hot-rolled steel sheets having a thickness of 2.3 mm were produced at a finish rolling temperature of 900°C, an air-cooling time after finish rolling of 1 second, and a coiling temperature of 550°C.

[0124] Furthermore, the hot-rolled steel sheet after the hot rolling process was subjected to a pickling process. The various conditions in the pickling process are shown in Table 2. In Table 2, the "time and temperature" of the pickling conditions are conditions in which the pickling time and the pickling temperature satisfy the following formula (4): 0.4 × (T - 15) × (t Ａ −20)≧(5[Si]+0.3[Mn]−0.6)×(CT+273) (4) where T in formula (4) represents the pickling temperature (°C), t Ａrepresents the pickling time (seconds), [Si] represents the Si content (mass%), [Mn] represents the Mn content (mass%), and CT represents the coiling temperature (°C) after hot rolling. In each example, cases where this condition was met are indicated as "OK", and cases where this condition was not met are indicated as "NG", and these are shown in Table 2. For steel sheet No. 1, the hot-rolled steel sheet produced as described above was pickled in hydrochloric acid with a concentration of 10%, at a temperature of 70°C, and for 100 seconds.

[0125] The pickled hot-rolled steel sheets were then subjected to a cold-rolling process (cumulative reduction of 80%), an annealing process (soaking temperature of 800°C), and a cooling process (average cooling rate of 10°C / sec) to produce steel sheets. The surfaces of the obtained steel sheets were appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrogalvanized layer (EG) consisting of the plating types shown in Table 2. For Steel Sheet No. 1, the pickled steel sheet was cold-rolled to a thickness of 0.45 mm. The obtained cold-rolled steel sheets were then subjected to a hot-dip galvannealed process in a CGL (continuous hot-dip galvanizing line). In CGL, the steel sheets were heated under conditions where the temperature of the heating zone was 780°C, the dew point was -2°C, and the hydrogen concentration was 2.6% by volume, and the temperature of the soaking zone was 780°C, the dew point was -40°C, and the hydrogen concentration was 2.7% by volume, and then the steel sheets were subjected to a hot-dip galvanizing treatment and then to an alloying treatment at 560°C. In Table 2, steel sheets No. 8 and 33 are examples where the surface of the steel sheet was not subjected to a plating treatment.

[0126] The various conditions in the annealing process are shown in Table 2. In Table 2, the annealing conditions "oxygen potential in the heating zone," "oxygen potential in the soaking zone," and "temperatures at the outlet of the heating zone and in the soaking zone" must satisfy the following three conditions (i) to (iii). (i) Oxygen potential in the heating zone: -0.9≦log(pH ２ O / pH ２ ) ≦-0.3 (ii) Isothermal zone oxygen potential: -2.4 ≦ log(pH ２ O / pH ２) ≦−0.8 (iii) Temperature of the heating zone outlet and soaking zone: Ac1+20≦T≦Ac3−30 In each example, if these conditions are met, it is marked as “OK”, and if they are not met, it is marked as “NG”, and each is shown in Table 2.

[0127] Samples were taken from the steel plate produced as described above, and their chemical compositions were analyzed. It was confirmed that there was no change from the chemical composition of the slab shown in Table 1. Note that underlines next to chemical compositions in Table 1 indicate that the composition is outside the scope of the present invention. Also, underlines next to various numerical values ​​in Table 2 indicate that the composition is outside the scope of the present invention, that the production conditions do not allow the steel plate of the present invention to be obtained, or that the properties of the steel plate are unfavorable. Note that "Ratio of hard phase fraction at 3 / 8 to 5 / 8 depth positions of plate thickness; Vs / Vm" in Table 2 means the ratio Vs / Vm of the hard phase fraction Vs in a region having a depth from the surface of the steel plate to 1 / 8 depth position of the plate thickness, to the hard phase fraction Vm in a region having a depth from 3 / 8 to 5 / 8 depth position of the plate thickness. Therefore, this ratio Vs / Vm being 0.20 to 0.80 is synonymous with "the area ratio of the hard phase in the region having a depth from the surface of the steel plate to a depth position of 1 / 8 of the plate thickness being 0.20Vm to 0.80Vm."

[0128]

[0129]

[0130] The metallographic structure and various properties of the surface layer and the center portion of the sheet thickness of the obtained steel sheet were measured and evaluated by the above-mentioned or following methods.

[0131] (Tensile Strength and Yield Ratio) JIS Z2241:2011 No. 5 tensile test specimens, with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction, were taken from the steel plates, and tensile tests were performed in accordance with JIS Z2241:2011 to measure the tensile strength (MPa) and yield stress (0.2% proof stress) of the steel plates. Furthermore, the yield ratio YR (%), defined as (yield stress) / (tensile strength), was calculated from these measurement results. Then, from these results, steel plates with a tensile strength of 500 MPa or more were evaluated as high-strength steel plates, and steel plates with a yield ratio of 0.70 or less were evaluated as steel plates with excellent workability.

[0132] (Workability) A close contact bending test was performed using the push bending method of the bending test method for metallic materials in JIS Z2248:2022 to evaluate workability. A rectangular test piece having a width of 30 mm and a length of 100 mm was taken from the steel plate so that the axial direction of the bending test was parallel to the rolling direction of the steel plate. After the close contact bending test, the test piece was visually evaluated for the presence or absence of cracks at the ridge line of the bend apex. A test piece without cracks was deemed to have passed, and is shown as "OK" in Table 2. A test piece with cracks observed in the visual evaluation was deemed to have failed, and is shown as "NG" in Table 2.

[0133] (Appearance after forming) The appearance of the steel sheet after forming was evaluated based on the degree of ghost lines appearing on the surface of the door outer after press forming and the presence or absence of stretcher strain. The surface after press forming was grinded, and stripes appearing on the surface at intervals of the order of several millimeters and extending in the rolling direction were judged to be ghost lines. The appearance was rated on a scale of 1 to 5 based on the following criteria according to the degree of appearance of the stripes. On the grinded surface of the steel sheet after press forming, an arbitrary 100 mm x 100 mm area was visually inspected, and a rating of "1" was given if no stripes extending in the rolling direction were observed; a rating of "2" was given if the maximum length of the stripes was 20 mm or less; a rating of "3" was given if the maximum length of the stripes was more than 20 mm but not more than 50 mm; a rating of "4" was given if the maximum length of the stripes was more than 50 mm but not more than 70 mm; and a rating of "5" was given if the maximum length of the stripes was more than 70 mm. A rating of "3" or less was deemed to be excellent in appearance after molding and was judged to be pass. On the other hand, a rating of "4" or more was deemed to be poor in appearance after molding and was judged to be fail. In addition, when the surface after press molding was visually observed, if surface defects consisting of localized linear or dendritic uneven patterns called stretcher strain were confirmed, the rating was "SS," and the product was judged to be failing due to poor appearance after molding.

[0134] Referring to Table 2, in the comparative example of steel sheet No. 3, which was cast under the condition of ΔT = 15°C, the standard deviation of the hard phase fraction in the metallographic structure at the center of the sheet thickness was 0.89%, resulting in the occurrence of ghost lines and poor appearance after forming. In the comparative example of steel sheet No. 4, which was annealed under the condition of a heating zone oxygen potential value of -1.5, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure at the surface layer to the hard phase fraction Vm of the metallographic structure at the center of the sheet thickness was 0.83 (the hard phase fraction Vs of the metallographic structure at the surface layer was 0.83Vm), resulting in poor workability. In the comparative example of steel sheet No. 4, which was annealed under the condition of a soaking zone oxygen potential value of -0.6, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure at the surface layer to the hard phase fraction Vm of the metallographic structure at the center of the sheet thickness was 0.83 (the hard phase fraction Vs of the metallographic structure at the surface layer was 0.83Vm), resulting in poor workability. In Comparative Example No. 5, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure of the sheet thickness center was 0.00 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.00 Vm), resulting in a high yield ratio and stretcher strain, and poor appearance after forming. In Comparative Example No. 6, in which the annealing process was performed under conditions in which the soaking zone temperature was 860°C, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure of the sheet thickness center was 0.17 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.17 Vm), resulting in a high yield ratio and stretcher strain, resulting in poor appearance after forming. In Comparative Example No. 7, in which the casting process was performed under conditions in which the pressing force was 350 tons, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure of the sheet thickness center was 0.17 Vm. In Comparative Example No. 14, the standard deviation of the hard phase fraction in the metallographic structure at the center of the sheet thickness was 0.88%, and ghost lines occurred, resulting in poor appearance after forming. In Comparative Example No. 15, which was hot-rolled under conditions where the finish rolling temperature was 930°C and the air-cooling time was 1.7 seconds, i.e., conditions that did not satisfy the above formula (3), the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure at the surface layer to the hard phase fraction Vm of the metallographic structure at the center of the sheet thickness was 0.17 (the hard phase fraction Vs of the metallographic structure at the surface layer was 0.17Vm), and the yield ratio was high and stretcher strain occurred, resulting in poor appearance after forming.In the comparative example of steel sheet No. 16, which was subjected to a hot rolling process under conditions where the coiling temperature was 670°C, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure in the sheet thickness center portion was 0.14 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.14Vm), and the yield ratio was high and stretcher strain occurred, resulting in a poor appearance after forming. Steel sheet No. 16, which was subjected to a pickling process under conditions where the coiling temperature was 570°C, the pickling temperature was 60°C, and the pickling time was 100 seconds, i.e., conditions which do not satisfy the above formula (4), was subjected to a pickling process under conditions where the coiling temperature was 570°C, the pickling temperature was 60°C, and the pickling time was 100 seconds, In Comparative Example No. 17, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure in the sheet thickness center was 0.14 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.14Vm), the yield ratio was high, and stretcher strain occurred, resulting in a poor appearance after forming. Steel sheet No. 17 was subjected to a hot rolling process under conditions of a coiling temperature of 690°C, a pickling process under conditions of a pickling temperature of 65°C and a pickling time of 80 seconds, i.e., conditions that do not satisfy the above formula (4), and an annealing process under conditions of an oxygen potential in the soaking zone of -0.5. In Comparative Example No. 21, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure in the thickness center portion was 0.00 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.00Vm), the yield ratio was high, and stretcher strain occurred, resulting in a poor appearance after forming. Steel sheet No. 21 was subjected to a hot rolling process under conditions of a finish rolling temperature of 920°C and an air cooling time of 1.5 seconds, i.e., conditions that do not satisfy the above formula (3), and a pickling process under conditions of a coiling temperature of 570°C, a pickling temperature of 55°C, and a pickling time of 90 seconds, i.e., conditions that do not satisfy the above formula (4). In Comparative Example No. 22, the ratio Vs / Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure in the thickness center portion was 0.13 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.13Vm), resulting in a high yield ratio and the occurrence of stretcher strain, resulting in poor appearance after forming. In Comparative Example No. 29, which used Comparative Steel S with a C content of 0.118%, the standard deviation of the hard phase fraction of the metallographic structure in the thickness center portion was 0.87%, resulting in the occurrence of ghost lines, resulting in poor appearance after forming.In the comparative example of steel sheet No. 30 using comparative steel T with a Mn content of 2.71%, the ferrite and hard phase fractions in the metallographic structure at the center of the sheet thickness were 73% and 27%, respectively, and the standard deviation of the hard phase fraction in the metallographic structure at the center of the sheet thickness was 0.91%, resulting in poor workability, ghost lines, and poor appearance after forming. In the comparative example of steel sheet No. 31 using comparative steel U with a Si content of 1.620%, stretcher strain occurred, resulting in poor appearance after forming. In the comparative example of steel sheet No. 32 using comparative steel V with a C content of 0.023%, the ferrite and hard phase fractions in the metallographic structure at the center of the sheet thickness were 98% and 2%, respectively, resulting in low tensile strength and poor strength. In the comparative example of steel sheet No. 32 using comparative steel W with a Mn content of 0.85%, the ferrite and hard phase fractions in the metallographic structure at the center of the sheet thickness were 98% and 2%, respectively, resulting in poor tensile strength and poor strength. In the comparative example 33, the tensile strength was reduced, resulting in poor strength and a high yield ratio.

[0135] On the other hand, the steel sheets Nos. 1, 2, 7 to 13, 18 to 20, and 23 to 28, which are examples of the present invention, maintained a high strength of 500 MPa or more, while exhibiting excellent workability such as press forming, and were able to significantly suppress the occurrence of ghost lines and stretcher strain on the steel sheet surface even when strain was imparted by press forming.

Claims

1. A steel plate having a chemical composition, in mass%, of C: 0.030-0.100%, Mn: 1.00-2.50%, Si: 0.005-1.500%, Al: 0.005-0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, 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: 0.0200% or less, and : 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, wherein an area ratio of a hard phase in a region having a depth from a surface of the steel plate to a 1 / 8 depth position of the plate thickness is 0.20Vm to 0.80Vm, where Vm is an area ratio of a hard phase in a region having a depth from a 3 / 8 depth position to a 5 / 8 depth position of the plate thickness, The steel sheet is characterized in that the metal structure in a region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness contains, in area percentage, ferrite: 75 to 97% and hard phase: 3 to 25%, and the standard deviation of the area ratio of the hard phase in a direction perpendicular to the rolling direction and the sheet thickness direction in the region having a depth from the 3 / 8 depth position to the 5 / 8 depth position of the sheet thickness is 0.85% or less.

2. The chemical composition is, in mass%, Cr: 0.01 to 0.80%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.200%, As: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, The steel sheet according to claim 1, further comprising one or more selected from the group consisting of Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%.

3. The steel sheet according to claim 1 or 2, characterized in that the index A represented by the following formula (1) is 0.45% or more: 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 mass%, and are 0% when the corresponding element is not contained.

4. The steel plate according to any one of claims 1 to 3, characterized in that the following formula (2) is satisfied: (TS-180000 / TS) / Vm≧35 (2), where TS represents the tensile strength of the steel plate in MPa, and Vm represents the area ratio of the hard phase in a region having a depth ranging from 3 / 8 to 5 / 8 of the plate thickness.

5. A steel sheet according to any one of claims 1 to 4, characterized in that the average grain size of the ferrite in a region having a depth ranging from 3 / 8 to 5 / 8 of the sheet thickness is 5.0 to 30.0 μm, and the average grain size of the hard phase in a region having a depth ranging from 3 / 8 to 5 / 8 of the sheet 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 a region having a depth ranging 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 member comprising the steel plate according to any one of claims 1 to 6.

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