Steel plate

Abstract

The present disclosure provides a high-strength steel plate having an improved post-molding appearance.　The steel plate of the present disclosure has a specific chemical composition and is characterized by comprising a metal structure that includes 75-97% of ferrite and 3-25% of a hard phase in terms of area, the standard deviation of the hard phase percentage for the rolling right-angle direction being 0.75% or less.

Description

steel plate

[0001] The present disclosure relates to steel sheets.

[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 describes 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, wherein 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 the substrate 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 Publication No. 2005-220430 International Publication No. 2022 / 181761 International Publication No. 2020 / 145256

[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, so not only characteristics such as strength are important, but also design and surface quality are important, and therefore excellent appearance after forming is required. Meanwhile, in connection with this demand for weight reduction, the steel sheets used for these exterior panel components are also required to be stronger and thinner. In addition, as the shapes of these exterior panel components become more complex, unevenness tends to occur on the steel sheet surface after forming, which poses a problem of degrading the appearance when such unevenness occurs.

[0006] More specifically, for example, in the case of DP steel (Dual Phase steel) composed of soft ferrite and a hard second phase mainly composed of martensite as described in Patent Document 1, non-uniform deformation is likely to occur in which the soft phase composed of ferrite and its surroundings are preferentially deformed during processing such as press forming. Therefore, when such a dual-phase steel composed of a soft phase and a hard phase is used, minute irregularities may occur on the steel sheet surface after forming, resulting in appearance defects called ghost lines.

[0007] As a means for suppressing such unevenness on the steel sheet surface, for example, Patent Document 3 discloses a steel sheet having a specific chemical composition and metal structure, in which the value X1 obtained by dividing the standard deviation in the thickness direction of the average Mn concentration in the rolling direction at a quarter position in the thickness direction by the average Mn concentration is 0.025 or less. Also, Patent Document 4 discloses a steel sheet having a specific chemical composition, in which the metal structure of the surface layer region is composed of ferrite and a second phase with a volume fraction of 0.01 to 5.0%, the metal structure of the inner region is composed of ferrite and a second phase with a volume fraction of 2.0 to 10.0%, the volume fraction of the second phase in the surface layer region is smaller than the volume fraction of the second phase in the inner region, the average crystal grain size of the second phase in the surface layer region is 0.01 to 4.0 μm, and the intensity ratio X between the {001} orientation and the {111} orientation of ferrite is 0.025 or less. ＯＤＦ｛００１｝／｛１１１｝ The present invention discloses a steel sheet having a texture in which the σ is 0.60 or more and less than 2.00.

[0008] The present disclosure aims to provide a high strength steel sheet having an improved appearance after forming through a novel configuration.

[0009] In order to achieve the above object, the present inventors conducted detailed studies focusing on the morphology of hard phases in the metal structure. As a result, the present inventors discovered that by suppressing the formation of hard phases connected in stripes (striped hard phases) and dispersing the hard phases more uniformly in the metal structure, poor appearance after forming can be improved while maintaining high strength due to such hard phases. Specifically, the present inventors discovered that by reducing the center segregation of Mn during solidification, which is a factor in the formation of the striped hard phases, and by reducing the hard phase fraction and minimizing its variation, poor appearance after forming can be significantly improved while sufficiently maintaining high strength.

[0010] The present disclosure has been completed based on these findings and includes the following aspects.

[0011] (Aspect 1) Chemical composition, in mass%, is: C: 0.030 to 0.100%, Mn: 1.00 to 2.50%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, 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%, A steel sheet comprising: 0-1.00% Sn, 0-0.200% Sb, 0-0.0100% Ca, 0-0.0100% Mg, 0-0.0100% Zr, 0-0.0100% REM, and the balance being Fe and impurities, wherein an index A represented by the following formula (1) is 0.45% or more, wherein the metallographic structure is, in area %, 75-97% ferrite and 3-25% hard phase, and wherein the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is 0.75% or less: 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% if the element is not contained.

[0012] (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: 0.01 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, The steel sheet according to 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%.

[0013] (Aspect 3) The steel sheet according to Aspect 1 or 2, characterized in that the steel sheet satisfies the following formula (2): (TS-180,000 / TS) / Vm≧35 (2), where TS is the tensile strength in MPa, and Vm is the hard phase fraction in area %.

[0014] (Embodiment 4) The steel sheet according to any one of the above embodiments 1 to 3, wherein the ferrite has an average crystal grain size of 5.0 to 30.0 μm, and the hard phase has an average crystal grain size of 1.0 to 5.0 μm.

[0015] (Embodiment 5) The steel sheet according to any one of the above-mentioned embodiments 1 to 4, wherein the hard phase comprises at least one of martensite, bainite, tempered martensite, and pearlite.

[0016] According to the present disclosure, a high strength steel sheet having an improved appearance after forming can be provided.

[0017] Hereinafter, preferred embodiments of the steel sheet of the present disclosure will be described in detail. In this specification, various numerical ranges mean ranges including the upper and lower limit values ​​unless otherwise specified.

[0018] <Steel Sheet> A steel sheet according to an embodiment of the present disclosure 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%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, 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%, and W. : 0 to 1.00%, Sn: 0 to 1.00%, Sb: 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 index A represented by the following formula (1) is 0.45% or more, the metal structure is, in area %, ferrite: 75 to 97% and hard phase: 3 to 25%, and the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is 0.75% or less. 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 element is not contained.

[0019] For exterior panel components such as roofs and doors, DP steel, which has a relatively low yield strength, is often used to avoid surface defects known as surface distortions that occur during press forming and other processes. However, as mentioned above, DP steel, which contains a mixture of a soft phase consisting of ferrite and a hard phase mainly consisting of martensite, is prone to non-uniform deformation, in which the soft phase and its surroundings deform preferentially during processes such as press forming. This non-uniform deformation can result in minute irregularities on the surface of the steel sheet after forming, which can lead to 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 consisting of ferrite deforms in a concave manner, while the hard phase mainly consisting of martensite does not deform or rather deforms in a convex manner. This results in the formation of minute irregularities on the surface of the steel sheet 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 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. The direction perpendicular to the rolling direction is the direction perpendicular to the rolling direction and the thickness direction.

[0020] Therefore, in order to improve such poor appearance after forming, the present inventors conducted detailed studies focusing on the morphology of the hard phase in the metal structure. As a result, the present inventors first found that in steel sheets in which a soft layer 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 in the metal structure. The present inventors then 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 high strength, 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 structures associated with hard phases. In relation to this, the present inventors have conducted detailed studies on methods for reducing Mn segregation from two perspectives: center segregation and microsegregation.

[0022] First, the present inventors believed 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 Mn-enriched region 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 sheets, suppressing the flow of molten steel by appropriately controlling the solidification conditions can significantly suppress center segregation of Mn.

[0023] On the other hand, the present inventors have conducted various studies, believing that promoting Mn diffusion during solidification is effective in reducing Mn microsegregation. To promote Mn diffusion, it is effective to create a structure in which Mn easily diffuses. Therefore, the present 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 the solidification mode. As a result, the present inventors found that, when the C and Mn contents are high, δ solidification does not occur during solidification, but the Mn diffusion rate decreases and microsegregation increases, whereas when the Si, Al, Cr, and Mo contents are high, Mn diffusion during solidification is promoted and microsegregation can be reduced.

[0024] Although ghost lines can be improved by the above-described methods for reducing Mn segregation, in order to fully achieve the improvement effect over the entire length and width of the coil, the present inventors investigated further methods for improving ghost lines in addition to the above-described methods for reducing Mn segregation. As a result, the present inventors first discovered that ghost lines can be improved by reducing the hard phase fraction of the steel sheet, even if a certain amount of center segregation of Mn remains. Furthermore, the present inventors discovered that the solidification structure also has a significant effect on the occurrence of ghost lines, and that even if the center 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 worsens the poor appearance after forming. Therefore, unlike conventional countermeasures against center segregation (Note: It is common knowledge among those skilled in the art that an increase in the equiaxed crystal fraction is necessary to improve center segregation. For example, Takaho Kawawa et al., "Tetsu to Hagane," Vol. 60 (1974) No. 5, pp. 486-500; Hiroshi Kumai et al., "Tetsu to Hagane," Vol. 60 (1974) No. 7, pp. 894-914), the present inventors have discovered that by reducing the equiaxed crystal fraction and controlling the solidification structure to a columnar crystal structure, negative segregation of Mn can be suppressed and ghost lines can be improved. Based on this knowledge, the present inventors have discovered that by controlling the solidification structure during casting to a columnar crystal structure, reducing the center segregation of Mn during solidification, which is a cause of the formation of striped hard phases, and reducing the hard phase fraction and minimizing its variation, it is possible to significantly improve poor appearance after forming while sufficiently maintaining high strength.

[0025] In one embodiment of the present disclosure, the steel sheet has a specific chemical composition as described above, and a unique metallographic structure that has a lower hard phase fraction than conventional DP steel and has a small variation in the hard phase fraction in the direction perpendicular to the rolling direction. Such a metallographic structure can be obtained by adopting a specific chemical composition and casting conditions, as described below, and controlling the solidification structure during casting to be columnar.

[0026] The steel sheet of this embodiment has such a unique metal structure, i.e., a metal structure with small centerline segregation of Mn during solidification and a small hard phase fraction and its variation, and therefore 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 appearance defects after forming, such as ghost lines, while sufficiently maintaining high strength. In other words, according to this embodiment, a high-strength steel sheet with improved appearance after forming can be provided.

[0027] The steel sheet 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.

[0028] (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%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, 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%, It has a specific chemical composition of Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, and the balance: Fe and impurities, with an index A represented by the following formula (1) being 0.45% or greater: 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% if the element is not contained.

[0029] Each element in this chemical composition will be described in detail below.

[0030] [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 0.035% or more, 0.040% or more, or 0.050% 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.

[0031] [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 1.20% or more, 1.30% or more, or 1.40% 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 2.25% or less, 2.00% or less, or 1.85% or less.

[0032] [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 0.010% or more, 0.050% or more, or 0.100% 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 1.000% or less, 0.500% or less, or 0.300% or less.

[0033] [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 0.001% 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 0.060% or less, 0.040% or less, or 0.020% or less.

[0034] [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 0.0010% 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 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0035] [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 effective in promoting Mn diffusion 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, 0.020% or more, or 0.025% 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, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less.

[0036] [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.

[0037] [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 0.0010% 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, 0.0040% or less, or 0.0020% or less.

[0038] 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.

[0039] [Cr: 0 to 0.80%] Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel sheets. 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 0.001% or more, and more preferably 0.01% or more. The Cr content may be 0.10% or more, 0.20% or more, or 0.30% 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 0.70% or less, 0.60% or less, or 0.50% or less.

[0040] [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 0.001% or more, more 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 0.40% or less, 0.30% or less, or 0.20% or less.

[0041] [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, 0.0010% or more, or 0.0015% 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 0.0030% or less.

[0042] [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 refines the structure and improves the strength-formability balance of the steel sheet. While the Ti content may be 0%, 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 result in the formation of 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, 0.060% or less, or 0.030% or less.

[0043] [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 0.005% or more, 0.007% or more, or 0.010% 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 0.060% or less, 0.040% or less, or 0.030% or less.

[0044] [V: 0 to 0.50%] V is an 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 V content may be 0%, but to obtain these effects, the V content is preferably 0.001% or more, more preferably 0.005% or more, and even more 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, 0.20% or less, or 0.10% or less.

[0045] [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 such an effect, the Ni content is preferably 0.001% or more, and more 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, 0.40% or less, or 0.20% or less.

[0046] [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 0.001% or more, and more 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 0.60% or less, 0.40% or less, or 0.20% or less.

[0047] [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 0.001% or more, and more preferably 0.01% or more. The W content may be 0.02% or more or 0.10% 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, 0.50% or less, or 0.20% or less.

[0048] [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, and more preferably 0.01% or more. The Sn content may be 0.05% or more or 0.08% 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, 0.50% or less, or 0.20% or less.

[0049] [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 this effect, the Sb content is preferably 0.001% or more. The Sb content may be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, excessive Sn content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 1.00% or less, and more preferably 0.20% or less. The Sb content may be 0.10% or less, 0.05% or less, or 0.01% or less.

[0050] [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, 0.0060% or less or 0.0030% or less, respectively.

[0051] 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.

[0052] 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: 0.01 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, It is preferable that the steel sheet contains one or more elements selected from the group consisting of 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 and significantly suppress the occurrence of appearance defects after forming, such as ghost lines.

[0053] 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.

[0054] [Index A: 0.45% or More] In this embodiment, the chemical composition of the steel sheet 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) where [Si], [P], [Al], [Ti], and [Nb] are the contents of each element in mass%, and are 0% when the element is not contained.

[0055] 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.

[0056] 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 1.50% or less, 1.20% or less, or 1.00% or less, for example.

[0057] Here, 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). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0058] (Metal Structure) [Ferrite: 75 to 97% and Hard Phase: 3 to 25%] In the present embodiment, the metal structure of the steel sheet is composed of, in area percentage, 75 to 97% ferrite and 3 to 25% hard phase. By making the metal structure of the steel sheet such a composite structure, the strength of the steel sheet can be maintained within an appropriate range, more specifically, a tensile strength of 500 MPa or more can be achieved, while suppressing poor appearance after forming. From the viewpoint of further increasing the strength of the steel sheet, the area fraction of the hard phase may be 7% or more, 10% or more, 12% or more, or 15% or more. Similarly, the area fraction of ferrite may be 93% or less, 90% or less, 88% or less, or 85% or less. On the other hand, from the viewpoint of further improving the appearance after forming, the area fraction of the hard phase may be 24% or less, 22% or less, or 20% or less. Similarly, the area fraction of ferrite may be 76% or more, 78% or more, or 80% or more.

[0059] 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 is preferably composed of at least one of martensite, bainite, and tempered martensite, and more preferably composed of martensite. In this embodiment, 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 area percentage.

[0060] (Identification of Metallic Structure and Calculation of Area Fraction) Identification of the metallic structure and calculation of the area fraction are performed as follows. First, a sample (having a size of, for example, 20 mm in the rolling direction × 20 mm in the width direction × thickness of the steel plate) for observing the metallic structure (microstructure) is taken from the W / 4 position or 3W / 4 position of the sheet width W of the obtained steel sheet (i.e., a position W / 4 in the width direction from either end of the steel sheet). Next, the metallic structure (microstructure) is observed using a scanning electron microscope (SEM). The observation fields were: (i) one field centered in the thickness direction at a position 125 μm from the steel sheet surface (the surface excluding the plating layer, if present) (hereinafter referred to as the "125 μm thickness position"); (ii) one field centered in the thickness direction at a position 1 / 2 the thickness from the steel sheet surface (hereinafter referred to as the "1 / 2 thickness position"); and (iii) eight fields between (i) and (ii) divided into nine equal sections by dividing the distance from the 125 μm thickness position to the 1 / 2 thickness position. To prepare the sample, the cross section of the thickness perpendicular to the rolling direction was polished as the observation surface and etched by nital etching. Next, the "microstructure" was classified from SEM photographs at a magnification of 500 or 1000 times. Ferrite and hard phases could be distinguished by differences in brightness.

[0061] For the steel sheet etched by nital corrosion, the above 10 observation fields are observed at a magnification of 500x or 1000x, and image analysis is performed using image analysis software such as Adobe Photoshop (registered trademark) CS5 to determine the area fraction of the hard phase. Ferrite and hard phase are binarized based on the difference in brightness, and the area fraction of the hard phase is calculated. Image analysis is performed in the same manner as above for a total of 10 observation fields to measure the area fraction of the hard phase, and these area fractions are averaged to calculate an average value. This average value is the area fraction of the hard phase, and the remainder is the area fraction of ferrite. The observation area of ​​each field is 150 μm in the sheet thickness direction and 250 μm in the rolling direction (in this case, the observation area is 150 × 250 = 37,500 μm). ２) 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 on 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 1 / 4 position in the sheet thickness direction are obtained, and the volume fraction of retained austenite is calculated using the intensity averaging method. The obtained volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0062] [Standard Deviation of Hard Phase Fraction in the Direction Transverse to the Rolling Direction: 0.75% or Less] In this embodiment, the metallographic structure of the steel sheet has a standard deviation of the hard phase fraction in the direction transverse to the rolling direction of 0.75% or less. The standard deviation of the hard phase fraction refers to the standard deviation of the area fraction of the hard phase itself. As described above, post-forming appearance defects such as ghost lines are significantly affected not only by Mn segregation but also by 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 may occur, increasing the variation in the hard phase fraction in the direction transverse to the rolling direction and worsening the post-forming appearance defects. However, in the steel sheet of this embodiment, the standard deviation of the hard phase fraction in the direction transverse to the rolling direction is 0.75% or less, i.e., the variation in the hard phase fraction in the direction transverse to the rolling direction is below a certain level, thereby significantly suppressing post-forming appearance defects. 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.

[0063] The standard deviation of the hard phase fraction in the direction transverse to the rolling direction of the metallographic structure can be determined as follows. First, a cross section of the steel sheet parallel to the direction transverse to the rolling direction and perpendicular to the surface of the steel sheet is observed at a magnification of 500x or 1000x using a scanning electron microscope (SEM) to obtain an SEM photograph. This SEM photograph is then analyzed using image analysis software, similar to the above-mentioned hard phase area fraction. The hard phase area fraction is measured every 100 µm within an 8 mm range in the direction transverse to the rolling direction of the steel sheet, and its standard deviation is calculated. The observation range in the direction transverse to the rolling direction may be less than 8 mm or more than 8 mm. However, the lower limit of the observation range for the standard deviation of the hard phase fraction in the direction transverse to the rolling direction is 4 mm, and the upper limit is 12 mm.

[0064] From the viewpoint of further improving the appearance after forming, the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction may be 0.65% or less, 0.55% or less, or 0.45% or less. The lower limit of the standard deviation 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.

[0065] [Relationship between tensile strength and hard phase fraction of steel sheet: (TS-180,000 / TS) / Vm≧35] In this embodiment, the steel sheet preferably satisfies the following formula (2): (TS-180,000 / TS) / Vm≧35 (2) where TS represents tensile strength (MPa) and Vm represents hard phase fraction (area %).

[0066] 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 as a result, it is possible to more easily prevent poor appearance of the steel sheet after forming.

[0067] The tensile strength (TS) of a steel plate can be measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the steel plate, with the longitudinal direction being perpendicular to the rolling direction, and conducting a tensile test in accordance with JIS Z2241:2011.

[0068] [Average grain size of ferrite: 5.0 to 30.0 μm] In the steel sheet of this embodiment, the average grain size of ferrite in the metal structure 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 7.0 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. Similarly, the average grain size of ferrite may be 27.0 μm or less, 25.0 μm or less, 20.0 μm or less, or 16.0 μm or less.

[0069] The average grain size of ferrite in a steel sheet is determined as follows. First, in the region of a steel sheet etched with a Nital reagent from the surface to the half-thickness position in the sheet thickness direction, 10 fields of view in the same sheet thickness direction as the measurement of the area fraction of ferrite and hard phases described above are observed at 500x magnification. Image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe, and the area fraction of ferrite and the number of ferrite particles in each field of view are calculated. Next, the area fraction of ferrite and the number of ferrite particles in the 10 fields of view are summed, and the total area fraction of ferrite is divided by the total number of ferrite particles to calculate the average area fraction per ferrite particle. From this average area fraction and the number of particles, the equivalent circle diameter is calculated, and the obtained equivalent circle diameter is determined as the average grain size of ferrite. The observation area of ​​each field of view was 150 μm in the plate thickness direction and 250 μm in the rolling direction (in this case, the observation area was 150 × 250 = 37,500 μm). ２ )

[0070] [Average grain size of hard phase: 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 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, or 4.0 μm or less.

[0071] The average grain size of the hard phase is determined as follows. First, in the region of a steel sheet etched with a Nital reagent from the surface to the half-thickness position in the thickness direction, 10 fields of view in the same thickness direction as the measurement of the area fraction of ferrite and hard phase described above are observed at a magnification of 500x. Image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe, and the area fraction of the hard phase and the number of hard phase particles in each field of view are calculated. Next, the area fraction of the hard phase and the number of ferrite particles in the 10 fields of view are each summed, and the total area fraction of the hard phase is divided by the total number of hard phase particles to calculate the average area fraction per hard phase particle. From this average area fraction and the number of particles, the equivalent circle diameter is calculated, and the obtained equivalent circle diameter is determined as the average grain size of the hard phase. The observation area of ​​each field of view was 150 μm in the plate thickness direction and 250 μm in the rolling direction (in this case, the observation area was 150 × 250 = 37,500 μm). ２ )

[0072] (Thickness) In this embodiment, the thickness of the steel plate is not particularly limited. 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.

[0073] (Plating) The steel sheet of this embodiment may further include a plating layer on the surface for the purpose of improving corrosion resistance, etc. The plating layer may be either a hot-dip plating layer or an electroplated layer. Examples of hot-dip plating layers include a hot-dip galvanized layer (GI), a galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn—Al alloy plating layer, a hot-dip Zn—Al—Mg alloy plating layer, and a hot-dip Zn—Al—Mg—Si alloy plating layer. Examples of electroplated layers include an electrogalvanized layer (EG) and an electrogalvanized Zn—Ni alloy plating layer. Among these, the plating layer is preferably a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer. The coating weight of the plating layer is not particularly limited and may be a general coating weight.

[0074] (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. Setting the tensile strength to 850 MPa or less has the advantage of easily ensuring formability when the steel sheet is press-formed.

[0075] The steel sheet of this embodiment has high strength, specifically a tensile strength of 500 MPa or more, yet can maintain an excellent appearance even after forming such as press working, etc. Therefore, the steel sheet of this embodiment is very useful for use as exterior panel parts such as roofs, hoods, fenders, and doors of automobiles, which require high designability.

[0076] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present disclosure will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present disclosure, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0077] The method for producing a steel sheet according to this embodiment includes a casting step of casting a slab having the specific chemical composition. The casting step 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.

[0078] (Casting Process) As described above, the steel sheet of this embodiment must have a unique metallographic structure that has a lower hard phase fraction than conventional DP steel and a small variation in the hard phase fraction in the direction perpendicular to the rolling direction. To obtain such a metallographic 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 more 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.

[0079] 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 this embodiment, casting conditions that are completely different from conventional countermeasures, 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, are adopted to control the solidification structure to a columnar crystal structure, thereby suppressing negative Mn segregation. As a result, Mn microsegregation is reduced, and ghost lines can be sufficiently improved.

[0080] The equiaxed crystal ratio (%) can be calculated by taking an etched print of a cross section of the thickness direction of the slab in the width 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.

[0081] 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. It is more preferable that the roll pitch of adjacent reduction rolls is 280 mm or less.

[0082] The method for producing a steel sheet according to this embodiment may include other steps such as a hot rolling step, a cold rolling step, an annealing step, and a cooling step in addition to the casting step. Furthermore, the present production method may optionally include a plating step. These steps are not particularly limited, and may be carried out under any appropriate conditions appropriately selected so as to obtain a metal structure containing the ferrite and hard phases described above in relation to the steel sheet at predetermined area fractions. Preferred conditions for these steps will be briefly described below.

[0083] (Hot rolling process) Prior to hot rolling, the slab is preferably heated to 1100°C or higher. By setting the heating temperature to 1100°C or higher, the rolling reaction force during hot rolling does not become excessively large, making it easier to obtain the desired product thickness. There is no particular upper limit to the heating temperature, but from an economical viewpoint, the heating temperature is preferably less than 1300°C. In the hot rolling process, the heated slab is subjected to rough rolling and finish rolling. The hot-rolled steel sheet obtained in this manner is coiled at a coiling temperature of, for example, 450 to 650°C.

[0084] The finish rolling end temperature is preferably 950°C or lower. By setting the finish rolling end temperature to 950°C or lower, the average crystal grain size of the hot-rolled steel sheet and the final product can be reduced, ensuring sufficient yield strength and high surface quality after forming. Furthermore, by setting the coiling temperature to 450 to 650°C, the average crystal grain size can be reduced and scale growth can be suppressed.

[0085] (Cold Rolling Process) The hot-rolled steel sheet obtained by the hot-rolling process is subjected to an appropriate pickling treatment to remove scale, and then subjected to a cold-rolling process. In the cold-rolling process, 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, while preventing the rolling load from becoming excessive and making rolling difficult.

[0086] (Annealing step) 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 maintained 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.

[0087] (Cooling Step) In the cooling step, the cold-rolled steel sheet after the annealing step is cooled. 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 width direction.

[0088] (Plating Step) For the purpose of improving corrosion resistance, etc., the surface of the obtained cold-rolled steel sheet may be subjected to a plating treatment. Examples of plating treatments include hot-dip plating, alloying hot-dip plating, and electroplating. For example, the steel sheet surface may be subjected to hot-dip galvanizing treatment as the plating treatment, or 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 may be employed. For example, the alloying temperature may be 450 to 600°C.

[0089] The present disclosure 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 purpose and intent of the present disclosure.

[0090] The present disclosure will be explained in more detail below by way of examples, but the present disclosure is not limited to these examples.

[0091] In the following examples, steel sheets according to an embodiment of the present disclosure were produced under various conditions, and the tensile strength, post-forming appearance, and other properties of the resulting steel sheets were examined.

[0092] First, a continuous casting machine equipped with a plurality of reduction rolls arranged at a roll pitch of 290 mm or less was used to cast a slab having the chemical composition shown in Table 1 and a thickness of 200 to 300 mm by a continuous casting method in which soft reduction was performed with a reduction gradient of 0.6 mm or more per meter in the casting direction. The balance other than the components shown in Table 1 was Fe and impurities. In each example, casting condition (I) was a condition in which superheat ΔT was 25°C or higher, and casting condition (II) was a condition in which the segment pressing force was 450 tons or higher. In each example, cases in which these conditions were met (denoted "OK") and cases in which they were not met (denoted "NG") are shown in Table 2.

[0093] Next, the obtained slab was subjected to a hot rolling process (heating temperature 1200°C, finish rolling end temperature 900°C, and coiling temperature 550°C), a cold rolling process (cumulative reduction 80%), an annealing process (soaking temperature 800°C), and a cooling process (average cooling rate 10°C / sec) to produce a cold-rolled steel sheet with a thickness of 0.4 mm. The surface of the obtained cold-rolled steel sheet was appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrogalvanized layer (EG). In addition, when the chemical composition of a sample taken from the produced cold-rolled steel sheet was analyzed, it was found to be unchanged from the chemical composition of the slab shown in Table 1.

[0094]

[0095]

[0096] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0097] (Tensile Strength) The tensile strength of the steel plate was measured by taking a No. 5 tensile test piece according to JIS Z2241:2011 from the steel plate, with the longitudinal direction being perpendicular to the rolling direction, and conducting a tensile test in accordance with JIS Z2241:2011.

[0098] Furthermore, the measured tensile strength of the steel sheet was used to calculate the value of the relational expression between the tensile strength (TS) of the steel sheet and the hard phase fraction (Vm): (TS-180,000 / TS) / Vm.

[0099] (Post-Forming Appearance) The post-forming appearance of the steel sheet was evaluated based on the degree of ghost lines that appeared on the surface of the door outer after forming. The press-formed surface was grinded, and stripes that appeared on the surface at intervals of several millimeters were judged to be ghost lines. Based on the following criteria, the degree of occurrence of the stripes was scored from 1 to 5. An arbitrary 100 mm x 100 mm area was visually inspected, and a score of "1" was assigned if no stripes were observed; a score of "2" was assigned if the maximum length of the stripes was 20 mm or less; a score of "3" was assigned if the maximum length of the stripes was more than 20 mm but not more than 50 mm; a score of "4" was assigned if the maximum length of the stripes was more than 50 mm but not more than 70 mm; and a score of "5" was assigned if the maximum length of the stripes was more than 70 mm. A score of "3" or less was deemed to be excellent in post-forming appearance and thus passed. On the other hand, a score of "4" or more was deemed to be poor in post-forming appearance and thus failed.

[0100] A steel sheet having a tensile strength of 500 MPa or more and an appearance after forming of 3 or less was evaluated as a high strength steel sheet having an improved appearance after forming. The results are shown in Table 2.

[0101] Referring to Table 2, the comparative steel sheets Nos. 3, 4, 12, 16, 17, and 23 to 27, whose chemical compositions and standard deviations of the hard phase fraction in the metallographic structure were outside the ranges of the present invention, all had poor appearance after forming. Furthermore, the comparative steel sheet No. 28, whose chemical composition and the ferrite fraction and hard phase fraction in the metallographic structure were outside the ranges of the present invention, had a good appearance after forming, but the hard phase fraction was low due to the low C content, and sufficient strength was not obtained.

[0102] On the other hand, in the steel sheet Nos. 1, 2, 5 to 11, 13 to 15, and 18 to 22, which are examples of the present invention, a high tensile strength of 500 MPa or more was maintained, and even when strain was imparted by press forming, the generation of minute irregularities on the steel sheet surface was suppressed, thereby significantly suppressing the occurrence of ghost lines.

Claims

1. The chemical composition, in mass%, is C: 0.030-0.100%, Mn: 1.00-2.50%, Si: 0.005-1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005-0.700%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-0.80%, 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: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and The balance is Fe and impurities, and the index A represented by the following formula (1) is 0.45% or more. The metal structure is, in terms of area percentage, ferrite: 75 to 97% and hard phase: 3 to 25%; A steel sheet, characterized in that the standard deviation of the hard phase fraction in a direction perpendicular to the rolling direction is 0.75% or less. 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 when no element is contained, it is 0%.

2. The chemical composition, in mass%, Cr: 0.01-0.80%, Mo: 0.01-0.50%, 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: 0.01-1.00%, Cu: 0.01 to 1.00%, W: 0.01-1.00%, Sn: 0.01-1.00%, Sb: 0.001-0.200%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Zr: 0.0001 to 0.0100%, and REM: 0.0001~0.0100% The steel sheet according to claim 1, further comprising at least one selected from the group consisting of:

3. The steel plate according to claim 1 or 2, characterized in that the steel plate satisfies the following formula (2): (TS-180,000 / TS) / Vm≧35...(2) where TS is the tensile strength in MPa and Vm is the hard phase fraction in area %.

4. 3. The steel sheet according to claim 1, wherein the ferrite has an average grain size of 5.0 to 30.0 μm, and the hard phase has an average grain size of 1.0 to 5.0 μm.

5. 3. The steel sheet according to claim 1, wherein the hard phase comprises at least one of martensite, bainite, tempered martensite, and pearlite.