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

Technical Field

[0001] This disclosure relates to steel sheets.

Background Art

[0002] In the automotive industry, weight reduction of vehicle bodies is required from the perspective of improving fuel efficiency. To achieve both weight reduction and collision safety of vehicle bodies, increasing the strength of the steel sheets used is one effective method, and based on such background, the development of high-strength steel sheets has been promoted.

[0003] In relation to this, in Patent Document 1, there is described a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of a steel sheet as a substrate, wherein the substrate contains, in 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, Cr: 0.03 to 0.5%, and the annealing surface oxidation index A defined by the mathematical formula: A = 400Al / (4Cr + 3Si + 6Mn) with the contents of Al, Cr, Si, and Mn as the same terms is 2.3 or more, the balance being composed of Fe and inevitable impurities, and further, the structure of the substrate is composed of ferrite and a second phase, and the second phase is mainly martensite. Also, in Patent Document 1, it is described that the high-strength hot-dip galvanized steel sheet has excellent surface quality and a tensile strength of 590 MPa or more, and is suitable mainly for uses as structural parts of automobiles such as members and lockers.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

[0005] In recent years, in line with the demand for further improvements in fuel efficiency, there has been a growing need for weight reduction not only for structural components such as members described in Patent Document 1, but also for exterior components such as roofs, hoods, fenders, and doors. Unlike the structural components mentioned above, these exterior components are visible to the public, so not only are properties such as strength important, but also aesthetics and surface quality, and therefore, they are required to have a superior appearance after molding. On the other hand, in line with this demand for weight reduction, there is a need for even higher strength and thinner steel sheets used in these exterior components. In addition, as the shapes of these exterior components become more complex, the surface of the steel sheet after molding tends to become uneven, and when such unevenness occurs, there is a problem in that the appearance deteriorates.

[0006] More specifically, in the case of DP steel (Dual Phase steel), which consists of a soft ferrite and a hard second phase mainly composed of martensite, as described in Patent Document 1, non-uniform deformation is likely to occur during processing such as press forming, where the soft ferrite phase and its surrounding areas deform preferentially. Therefore, when using composite structure steel composed of such a soft phase and a hard phase, minute irregularities may occur on the surface of the steel sheet after forming, resulting in appearance defects known as ghost lines.

[0007] Regarding such surface irregularities of steel sheets, as a means of suppressing them, 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 of the average Mn concentration in the rolling direction at the 1 / 4 position in the thickness direction by the average Mn concentration is 0.025 or less. Furthermore, Patent Document 4 discloses a steel sheet having a specific chemical composition, in which the metal structure of the surface region consists of ferrite and a second phase with a volume fraction of 0.01 to 5.0%, the metal structure of the internal region consists 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 region is smaller than the volume fraction of the second phase in the internal region, the average grain size of the second phase in the surface region is 0.01 to 4.0 μm, and the intensity ratio of the {001} orientation to the {111} orientation of ferrite is X ODF{001} / {111} A steel sheet containing a texture where the ratio is 0.60 or higher and less than 2.00 is disclosed.

[0008] This disclosure aims to provide a high-strength steel sheet having an improved post-formed appearance through a novel configuration. [Means for solving the problem]

[0009] To achieve the above objective, the Disclosers conducted a detailed study focusing on the morphology of the hard phase in the metal structure. As a result, the Disclosers found that by suppressing the formation of a striped, interconnected hard phase (striped hard phase) and dispersing the hard phase more uniformly in the metal structure, it is possible to maintain high strength based on such a hard phase while improving the appearance defects after molding. Specifically, the Disclosers found that by reducing the central segregation of Mn during solidification, which is a factor in the formation of the striped hard phase, and by reducing the hard phase fraction and its variability, it is possible to significantly improve the appearance defects after molding while maintaining sufficient high strength.

[0010] This disclosure is completed based on these findings and includes the following aspects:

[0011] (Aspect 1) The chemical composition is expressed in mass percent. 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 - 0.50%, B: 0 - 0.0100%, Ti: 0 - 0.100%, Nb: 0 - 0.100%, V: 0 - 0.50%, Ni: 0 - 1.00%, Cu: 0 - 1.00%, W: 0 - 1.00%, Sn: 0 - 1.00%, Sb: 0 - 0.200%, Ca: 0 - 0.0100%, Mg: 0 - 0.0100%, Zr: 0 - 0.0100%, REM: 0 - 0.0100%, and the balance: Fe and impurities, and the index A represented by the following formula (1) is 0.45% or more, the metallographic structure is, in area%, ferrite: 75 - 97% and hard phase: 3 - 25%, and the standard deviation of the hard phase fraction in the rolling right angle direction is 0.75% or less. A steel sheet characterized by this. 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% units, and when the element is not contained, it is 0%.

[0012] (Aspect 2) The above chemical composition is, in mass%, Cr: 0.01 - 0.80%, Mo: 0.01 - 0.50%, B: 0.0001 - 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%, Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100% The steel sheet according to the above aspect 1, characterized by containing one or more selected from the group consisting of.

[0013] (Aspect 3) The steel sheet according to the above aspect 1 or 2, characterized in that the steel sheet satisfies the following formula (2). (TS - 180,000 / TS) / Vm ≥ 35 ···(2) Here, TS is the tensile strength in MPa unit, and Vm is the volume fraction of the hard phase in area% unit.

[0014] (Aspect 4) The steel sheet according to any one of the above aspects 1 to 3, characterized in that the average crystal grain size of the ferrite is 5.0 to 30.0 μm, and the average crystal grain size of the hard phase is 1.0 to 5.0 μm.

[0015] (Aspect 5) The steel sheet according to any one of the above aspects 1 to 4, characterized in that the hard phase consists of at least one of martensite, bainite, tempered martensite, and pearlite.

Advantages of the Invention

[0016] According to the present disclosure, it is possible to provide a high-strength steel sheet having an improved appearance after forming.

Modes for Carrying Out the Invention

[0017] Preferred embodiments of the steel sheet of this disclosure will be described in detail below. In this specification, unless otherwise specified, various numerical ranges mean a range including their upper and lower limits.

[0018] <Steel plate> A steel sheet according to one embodiment of this disclosure has a chemical composition in mass%, 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: 0~0.50%, B: 0~0.0100%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.50%, Ni: 0~1.00%, Cu: 0~1.00%, W: 0~1.00%, Sn: 0~1.00%, Sb: 0~0.200%, Ca: 0~0.0100%, Mg: 0~0.0100%, Zr: 0~0.0100%, REM: 0~0.0100%, and The remainder consists of Fe and impurities, with an index A represented by the following formula (1) being 0.45% or more. The metallic structure consists of ferrite: 75-97% and hard phase: 3-25% by area percentage. This material is characterized by having a standard deviation of 0.75% or less of the hard phase fraction in the direction perpendicular to the rolling process. A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb] ···(1) Here, [Si], [P], [Al], [Ti], and [Nb] represent the content of each element in mass percent, and 0% indicates that an element is not present.

[0019] In exterior parts such as roofs and doors, DP steel, which has a relatively low yield strength, is often used to avoid surface defects called surface strain that occur during press forming. However, as mentioned above, in the case of DP steel, which has a mixture of a soft phase consisting of ferrite and a hard phase mainly consisting of martensite, uneven deformation is likely to occur during processing such as press forming, where the soft phase and its surroundings deform preferentially. This uneven deformation can cause minute irregularities on the surface of the steel sheet after forming, resulting in an appearance defect called a ghost line. To explain the occurrence of this ghost line in more detail, first, during processing 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 concave, or rather deforms in a convex manner. As a result, minute irregularities are formed on the surface of the steel sheet after forming. These minute irregularities are formed so that convex parts extending generally along the rolling direction and concave parts extending generally along the rolling direction are aligned in the width direction perpendicular to the rolling direction. Furthermore, when the surface of the steel sheet is polished after forming, the protrusions of the minute irregularities on the surface are removed, making the ghost lines, which are streaky patterns extending in the rolling direction of the steel sheet, visible. The rolling direction can be easily determined based on the stretching direction of the crystal grains of the steel sheet. The direction perpendicular to the rolling direction is the direction perpendicular to both the rolling direction and the thickness direction.

[0020] Therefore, in order to improve such defects in appearance after forming, the Disclosers conducted a detailed study focusing on the morphology of the hard phase in the metal structure. As a result, the Disclosers first discovered that in steel sheets where a soft layer and a hard phase are mixed, such as DP steel, the presence of hard phases connected in a striped pattern in the metal structure makes the degree of ghost lines more pronounced. The Disclosers then found that by suppressing the formation of such striped hard phases and dispersing the hard phase more uniformly in the metal structure, it is possible to suppress the formation of minute irregularities on the surface of the steel sheet after forming while maintaining sufficient high strength, and consequently suppress the occurrence of ghost lines.

[0021] More specifically, the Disclosers have found that reducing Mn segregation during solidification in the slab casting process, in which molten steel is solidified to cast a slab, is effective in suppressing the formation of striped structures associated with the hard phase. In connection with this, the Disclosers have conducted a detailed study on methods for reducing Mn segregation from two perspectives: central segregation and microsegregation.

[0022] First, the Disclosers considered that suppressing the flow of molten steel during slab casting would be effective in reducing Mn central segregation, and conducted various studies. More specifically, molten steel solidifies from the surface inward, with the center solidifying last. At this stage, as the solid phase is discharged from the liquid phase, the Mn in the liquid phase becomes concentrated. If the molten steel flows during solidification, these Mn-enriched areas tend to accumulate in the center during the solidification process, resulting in significant Mn central segregation. Therefore, the Disclosers found that by appropriately controlling the solidification conditions during steel plate manufacturing to suppress such molten steel flow, Mn central segregation can be significantly suppressed.

[0023] On the other hand, the Disclosers considered that promoting the diffusion of Mn during solidification would be effective in reducing Mn microsegregation, and conducted various studies. To promote Mn diffusion, it is effective to create a structure that facilitates Mn diffusion. Therefore, the Disclosers focused on the δ phase, in which Mn has a high diffusion rate, and experimentally investigated the influence of each element in steel on Mn microsegregation in order to achieve δ solidification mode. As a result, the Disclosers found that when the content of C and Mn increases, δ solidification does not occur during solidification, the diffusion rate of Mn decreases and microsegregation increases, but when the content of Si, Al, Cr, and Mo increases, the diffusion of Mn during solidification is promoted, and microsegregation can be reduced.

[0024] Although ghost lines can be improved by the methods for reducing Mn segregation described above, in order to obtain a sufficient improvement effect across the entire length and width of the coil, the Disclosers investigated further methods for improving ghost lines in addition to the methods for reducing Mn segregation described above. As a result, the Disclosers first found that by reducing the hard phase fraction of the steel sheet, ghost lines can be improved even if some degree of central Mn segregation remains. Furthermore, the Disclosers found that the solidification structure also has a significant influence on the occurrence of ghost lines, and even if the central Mn segregation is small, if coarse equiaxed crystals are formed in the solidification structure, negative segregation of Mn occurs, the variation in the hard phase fraction in the direction perpendicular to rolling increases, and the appearance defects after forming worsen. Therefore, unlike conventional measures to address central segregation (Note: It is common knowledge among those skilled in the art that increasing the equiaxed fraction is necessary to improve central segregation. For example, Takahiro Kawawa et al.: "Iron and Steel", Vol. 60 (1974) No. 5, pp. 486-500; Hiroshi Kumai et al.: "Iron and Steel", Vol. 60 (1974) No. 7, pp. 894-914), we have found that by reducing the equiaxed 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 these findings, we have found that by controlling the solidification structure during casting to a columnar crystal structure, reducing central segregation of Mn during solidification which is a factor in the formation of striped hard phases, and reducing the hard phase fraction and its variation, it is possible to significantly improve the appearance defects after molding while maintaining high strength.

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

[0026] The steel sheet of this embodiment has a unique metallic structure, namely, a metallic structure in which central segregation of Mn during solidification is small, and the hard phase fraction and its variation are small. Therefore, it is possible to suppress the generation of minute irregularities on the surface of the steel sheet after forming while maintaining sufficient high strength. As a result, the steel sheet of this embodiment can significantly suppress the occurrence of post-forming appearance defects such as ghost lines while maintaining sufficient high strength. In other words, according to this embodiment, it is possible to provide a high-strength steel sheet with an improved post-forming appearance.

[0027] The steel plate of this embodiment will be described in more detail below. In the following description, unless otherwise specified, the "%" in the content of each element and the unit of index A means "mass%".

[0028] (chemical composition) The steel plate of this embodiment is as described above. 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: 0~0.50%, B: 0~0.0100%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.50%, Ni: 0~1.00%, Cu: 0~1.00%, W: 0~1.00%, Sn: 0~1.00%, Sb: 0~0.200%, Ca: 0~0.0100%, Mg: 0~0.0100%, Zr: 0~0.0100%, REM: 0~0.0100%, and The remainder consists of Fe and impurities, and has a specific chemical composition in which the exponent A represented by the following formula (1) is 0.45% or more. A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb] ···(1) Here, [Si], [P], [Al], [Ti], and [Nb] represent the content of each element in mass percent, and 0% indicates that an element is not present.

[0029] The following provides a detailed explanation of each element in this chemical composition.

[0030] [C:0.030~0.100%] Carbon (C) is an element that increases the strength of steel plates. To obtain this effect sufficiently, the C content should be 0.030% or more. The C content may also be 0.035% or more, 0.040% or more, or 0.050% or more. On the other hand, if the C content is excessive, the diffusion of manganese (Mn) during solidification may be inhibited, and the microsegregation of manganese may not be sufficiently suppressed. Therefore, the C content should be 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, or 0.080% or less.

[0031] [Mn: 1.00~2.50%] Mn is an element that enhances the hardenability of steel and contributes to improving its strength. To fully obtain this effect, the Mn content should be 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 the Mn content is excessive, the diffusion of Mn during solidification may be inhibited, and it may not be possible to sufficiently suppress the microsegregation of Mn. Therefore, the Mn content should be 2.50% or less. The Mn content may be 2.25% or less, 2.00% or less, or 1.85% or less.

[0032] [Si: 0.005~1.500%] Si is a deoxidizing element for steel and an effective solid solution strengthening element that increases the strength of steel sheets without impairing their ductility. Si is also an effective element that promotes the diffusion of Mn during solidification, thereby reducing Mn microsegregation. To fully obtain these effects, the Si content should be 0.005% or higher. The Si content may also be 0.010% or higher, 0.050% or higher, or 0.100% or higher. On the other hand, excessive Si content can reduce scale detachability and cause surface defects. Therefore, the Si content should be 1.500% or lower. The Si content may also be 1.000% or lower, 0.500% or lower, or 0.300% or lower.

[0033] [P:0.100% or less] P is an element that is introduced during the manufacturing process. Furthermore, 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 more time for refining, leading to decreased 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 sheet. Therefore, the P content should be 0.100% or less. The P content may also 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 introduced during the manufacturing process. The S content may be 0%. However, reducing the S content to less than 0.0001% requires more time for refining, leading to a decrease in 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 can form Mn sulfides, which can reduce the formability of the steel sheet, such as ductility, hole-expandability, stretch flangeability, and / or bendability. Therefore, the S content should be 0.0200% or less. The S content may also be 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0035] [Al:0.005~0.700%] Al is an element that functions as a deoxidizing agent and is an effective solid solution strengthening element that increases the strength of steel. Al is also an effective element that promotes the diffusion of Mn during solidification, thereby reducing Mn microsegregation. To fully obtain these effects, the Al content should be 0.005% or higher. The Al content may also be 0.010% or higher, 0.020% or higher, or 0.025% or higher. On the other hand, excessive Al content can worsen castability and reduce productivity. Therefore, the Al content should be 0.700% or lower. The Al content may also be 0.600% or lower, 0.400% or lower, 0.300% or lower, 0.200% or lower, or 0.100% or lower.

[0036] [N:0.0150% or less] N is an element that is introduced during the manufacturing process. The N content may be 0%. However, reducing the N content to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the N content is excessive, nitrides may form, which may reduce the formability of the steel sheet, such as ductility, hole-expandability, stretch flangeability, and / or bendability. Therefore, the N content should be 0.0150% or less. The N content may also 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 introduced during the manufacturing process. The O content may be 0%. However, reducing the O content to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the O content is excessive, coarse oxides may form, which may reduce the formability of the steel sheet, such as ductility, hole-expandability, stretch flangeability, and / or bendability. Therefore, the O content should be 0.0100% or less. The O content may also be 0.0070% or less, 0.0040% or less, or 0.0020% or less.

[0038] The basic chemical composition of the steel sheet in 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 needed. These optional elements will be described in detail below.

[0039] [Cr: 0~0.80%] Cr is an element that enhances the hardenability of steel and contributes to improving the strength of steel sheets. Furthermore, Cr is an effective element in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. While the Cr content may be 0%, to obtain these effects, a Cr content of 0.001% or more is preferable, and 0.01% or more is more preferable. 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 lead to the formation of coarse Cr carbides, which can become the starting point for fracture. Therefore, a Cr content of 0.80% or less is preferable. The Cr content may also be 0.70% or less, 0.60% or less, or 0.50% or less.

[0040] [Mo: 0~0.50%] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. Mo is also an effective element in promoting the diffusion of Mn during solidification and reducing microsegregation of Mn. While the Mo content may be 0%, to obtain these effects, the Mo content is preferably 0.001% or more, and more preferably 0.01% or more. The Mo content may also be 0.05% or more, or 0.07% or more. On the other hand, excessive Mo content can reduce hot workability and decrease productivity. Therefore, the Mo content is preferably 0.50% or less. The Mo content may also be 0.40% or less, 0.30% or less, or 0.20% or less.

[0041] [B: 0~0.0100%] B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the B content is excessive, B precipitates may form, which may reduce the strength of the steel sheet. Therefore, it is preferable that the B content be 0.0100% or less. The B content may also be 0.0080% or less, 0.0060% or less, or 0.0030% or less.

[0042] [Ti: 0~0.100%] Ti is an element that reduces the amount of S, N, and O, which generate coarse inclusions that act as fracture initiation points. Ti is also a precipitation strengthening element that refines the microstructure and improves the strength-formability balance of the steel sheet. While the Ti content may be 0%, it is preferable that the Ti content be 0.001% or higher to obtain these effects. The Ti content may also be 0.005% or higher, 0.007% or higher, or 0.010% or higher. On the other hand, excessive Ti content can lead to the formation of coarse Ti sulfides, Ti nitrides, and / or Ti oxides, reducing the formability of the steel sheet. Therefore, the Ti content is preferably 0.100% or lower. The Ti content may also be 0.080% or lower, 0.060% or lower, or 0.030% or lower.

[0043] [Nb: 0~0.100%] Nb is a precipitation strengthening element that contributes to improving the strength of steel sheets through precipitation strengthening, grain refinement strengthening by suppressing the growth of ferrite crystal grains, 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, if the Nb content is excessive, the amount of unrecrystallized ferrite may increase, which may reduce 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 ~ 0.50%] V is an element that contributes to improving the strength of steel sheets through strengthening by precipitates, strengthening by refining due to the suppression of ferrite grain growth, and / or dislocation strengthening due to the suppression of recrystallization. The V content may be 0%, but in order 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 also be 0.02% or more. On the other hand, if the V content is excessive, a large amount of carbonitride may precipitate, which may reduce the formability of the steel sheet. Therefore, the V content is preferably 0.50% or less. The V content may also be 0.40% or less, 0.20% or less, or 0.10% or less.

[0045] [Ni: 0~1.00%] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The Ni content may be 0%, but to obtain this 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, if the Ni content is excessive, the weldability of the steel sheet may decrease. 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: 0~1.00%] Cu is an element that exists in steel in the form of fine particles and contributes to improving the strength of steel sheets. 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, if the Cu content is excessive, the weldability of the steel sheet may decrease. Therefore, the Cu content is preferably 1.00% or less. The Cu content may be 0.60% or less, 0.40% or less, or 0.20% or less.

[0047] [W: 0~1.00%] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. 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, if the W content is excessive, the hot workability may decrease and productivity may decrease. 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: 0~1.00%] Sn is an element that suppresses grain coarsening and contributes to improving the strength of steel sheets. The Sn content may be 0%, but to obtain this 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: 0~0.200%] Sb is an element that suppresses grain coarsening and contributes to improving the strength of steel sheets. The Sb content may be 0%, but to obtain this effect, it is preferable that the Sb content be 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 sheets. The content of Ca, Mg, Zr, and REM may be 0%, but to obtain such an effect, it is preferable that the content of Ca, Mg, Zr, and REM be 0.0001% or more, 0.0010% or more, or 0.0015% or more, respectively. On the other hand, if these elements are included in excess, the ductility of the steel sheet may decrease. Therefore, it is preferable that the content of Ca, Mg, Zr, and REM be 0.0100% or less, respectively. The content of Ca, Mg, Zr, and REM may be 0.0080% or less, 0.0060% or less, or 0.0030% or less, respectively.

[0051] In this specification, REM refers to the collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0052] Regarding the above optional elements, in this embodiment, the chemical composition of the steel sheet is as follows: Cr: 0.01~0.80%, Mo: 0.01~0.50%, B: 0.0001~0.0100%, Ti: 0.001~0.100%, Nb: 0.001~0.100%, V: 0.01~0.50%, Ni: 0.01~1.00%, Cu: 0.01~1.00%, W: 0.01~1.00%, Sn: 0.01~1.00%, Sb: 0.001~0.200%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Zr: 0.0001~0.0100%, and REM: 0.0001~0.0100% It is preferable that the steel sheet contains one or more elements selected from the group consisting of the following. When the steel sheet contains such arbitrarily selected elements, it is possible to more reliably maintain high strength while significantly suppressing the occurrence of post-forming appearance defects such as ghost lines.

[0053] In the steel sheet of this embodiment, the remainder of the elements other than those mentioned above consists of Fe and impurities. Here, impurities are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. Examples of impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The total amount of impurities may be 0.100% or less.

[0054] [Index A: 0.45% or more] In this embodiment, the chemical composition of the steel sheet is such 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) Here, [Si], [P], [Al], [Ti], and [Nb] represent the content of each element in mass percent, and 0% indicates that an element is not present.

[0055] Index A is determined by the content of solid solution strengthening elements Si, P, and Al, and precipitation strengthening elements Ti and Nb. The larger this value, the higher the strength that can be obtained with a smaller hard phase fraction. By setting Index A to 0.45% or higher, high strength can be obtained while controlling the hard phase fraction of the steel sheet to below a certain level.

[0056] Furthermore, Index A may be 0.48% or higher, 0.50% or higher, or 0.52% or higher. There is no particular upper limit to Index A, but for example, Index A may be 1.50% or lower, 1.20% or lower, or 1.00% or lower.

[0057] Here, the chemical composition of the steel sheet can be measured using general analytical methods. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-nondispersive infrared absorption method.

[0058] (Metal structure) [Ferrite: 75-97% and hard phase: 3-25%] In this embodiment, the metallographic structure of the steel sheet consists of 75-97% ferrite and 3-25% hard phase by area percentage. By making the metallographic structure of the steel sheet such a composite structure, it is possible to suppress defects in appearance after forming while maintaining the strength of the steel sheet within an appropriate range, more specifically achieving a tensile strength of 500 MPa or more. 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 consists of at least one of, for example, martensite, bainite, tempered martensite, and pearlite. From the viewpoint of improving the strength of the steel sheet, the hard phase preferably consists of at least one of martensite, bainite, and tempered martensite, and more preferably consists of martensite. In this embodiment, it is preferable that the metallic structure of the steel sheet contains little retained austenite, specifically, the retained austenite is preferably less than 1% or less than 0.5% by area, and more preferably 0%.

[0060] (Identification of metallographic structure and calculation of area fraction) The identification of the metal structure and calculation of the area fraction are performed as follows: First, a sample for observing the metal structure (microstructure) is taken from the W / 4 position or 3W / 4 position of the obtained steel sheet width W (i.e., a position W / 4 in the width direction from either end of the steel sheet in the width direction) (for example, the size is 20 mm in the rolling direction × 20 mm in the width direction × the thickness of the steel sheet). Next, the metal structure (microstructure) is observed using a scanning electron microscope (SEM). At this time, the observation field is set up in the thickness direction of the steel plate and consists of a total of 10 fields of view: (i) one field of view with the center in the thickness direction set at a position 125 μm from the surface of the steel plate (excluding the plating layer if plating is present) (hereinafter referred to as the "125 μm thickness position"), (ii) one field of view with the center in the thickness direction set at a position half the thickness from the surface of the steel plate (hereinafter referred to as the "half thickness position"), and (iii) eight fields of view between (i) and (ii) above, divided at intervals of nine equal parts from the distance between the 125 μm thickness position to the half thickness position. For sample preparation, the thickness cross section perpendicular to the rolling direction is polished as the observation surface and etched by Nital corrosion. Next, the "microstructure" is classified from SEM images at a magnification of 500 or 1000x. Ferrite and hard phases can be distinguished by the difference in brightness.

[0061] For steel plates etched with Nital corrosion, the 10 observation fields described above are observed at 500x or 1000x magnification, and image analysis is performed using Adobe Photoshop CS5 to determine the area fraction of the hard phase. Ferrite and the hard phase are binarized based on the difference in brightness, and the area fraction of the hard phase is calculated. For a total of 10 observation fields, the area fraction of the hard phase is measured by performing image analysis in the same manner as above, and these area fractions are averaged to calculate the average value. This average value is taken as the area fraction of the hard phase, and the remainder is taken as the area fraction of ferrite. The observation area of ​​each field is 150 μm in the thickness direction and 250 μm in the rolling direction (in this case, the observation area is 150 × 250 = 37500 μm). 2 ) Furthermore, if it is necessary to measure the area fraction of retained austenite, it can be measured by X-ray diffraction on the observation surface described above. Specifically, using Co-Kα radiation, the integrated intensities of a total of six peaks—α(110), α(200), α(211), γ(111), γ(200), and γ(220)—at a position 1 / 4 of the plate thickness direction are determined, and the volume fraction of retained austenite is calculated using the intensity averaging method. The obtained volume fraction of retained austenite is then taken as the area fraction of retained austenite.

[0062] [Standard deviation of hard phase fraction in the direction perpendicular to rolling: 0.75% or less] In this embodiment, the metal structure of the steel sheet has a standard deviation of 0.75% or less for the hard phase fraction in the direction perpendicular to the rolling direction. Note that the standard deviation of the hard phase fraction refers to the standard deviation of the area fraction of the hard phase itself. As described above, the solidification structure, as well as Mn segregation, greatly influences the appearance defects after forming, such as ghost lines. For example, even if the central segregation of Mn is small, if coarse equiaxed crystals are formed in the solidification structure, negative segregation of Mn occurs, which can increase the variation in the hard phase fraction in the direction perpendicular to the rolling direction and 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 is 0.75% or less, that is, the variation in the hard phase fraction in the direction perpendicular to the rolling direction is below a certain level, so that appearance defects after forming can be significantly suppressed. Furthermore, regarding the hard phase fraction, it is preferable that the ratio of the average area fraction of the hard phase to its standard deviation is 0.10 or less (i.e., standard deviation of hard phase fraction / average area fraction of hard phase ≤ 0.10). The above ratio is preferably 0.09 or less, 0.08 or less, or 0.07 or less. The lower limit of the above ratio is 0, but if necessary, the lower limit may be set to 0.01.

[0063] The standard deviation of the hard phase fraction in the microstructure perpendicular to the rolling direction can be determined as follows. First, a scanning electron microscope (SEM) is used to observe the area between a position 50 μm from one surface of the steel sheet and a position 50 μm from the other surface of the steel sheet, in a cross section parallel to the rolling direction and perpendicular to the steel sheet surface, at a magnification of 500x or 1000x, and an SEM image is obtained. Image analysis is performed on this SEM image using image analysis software, similar to the area fraction of the hard phase described above, to measure the area fraction of the hard phase at 100 μm intervals within an 8 mm range perpendicular to the rolling direction of the steel sheet, and its standard deviation is calculated. The observation range perpendicular to the rolling direction may be less than 8 mm or greater than 8 mm. However, the lower limit of the observation range for the standard deviation of the hard phase fraction perpendicular to the rolling direction is set to 4 mm, and the upper limit is set to 12 mm.

[0064] 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, from the viewpoint of further improving the appearance after molding. The lower limit of such standard deviation is not particularly limited, but for example, the standard deviation of the hard phase fraction may be 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 plate: (TS - 180,000 / TS) / Vm ≥ 35] In this embodiment, it is preferable that the steel plate satisfies the following formula (2). (TS-180,000 / TS) / Vm≧35 (2) Here, TS represents the tensile strength (MPa), and Vm represents the hard phase fraction (area %).

[0066] When a steel sheet satisfies equation (2) in this way, even if some degree of central segregation of Mn remains, it is easier to suppress variations in the hard phase fraction in the direction perpendicular to the rolling process, and as a result, defects in the appearance of the steel sheet after forming can be made less likely to occur.

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

[0068] [Average crystal grain size of ferrite: 5.0~30.0 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. By controlling the average grain size of ferrite within such a fine range, the appearance of the steel sheet after forming can be further improved. 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, 10 fields of view in the thickness direction of a steel sheet etched with Nital reagent are observed at 500x magnification in the region from the surface to the 1 / 2 position in the thickness direction, the same as the measurement of the area fraction of ferrite and hard phase described above. Image analysis is performed using Adobe's "Photoshop® CS5" image analysis software to calculate the area fraction of ferrite and the number of ferrite particles in each field of view. Next, the area fraction of ferrite and the number of ferrite particles in the 10 fields of view are summed, and the average area fraction per ferrite particle is calculated by dividing the total area fraction of ferrite by the total number of ferrite particles. From this average area fraction and the number of particles, the equivalent diameter of a circle is calculated, and the obtained equivalent diameter of a circle is determined as the average grain size of ferrite. The observation area of ​​each field of view is 150 μm in the thickness direction and 250 μm in the rolling direction (in this case, the observation area is 150 × 250 = 37500 μm). 2 )

[0070] [Average grain size of hard phase: 1.0~5.0 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. By controlling the average grain size of the hard phase within such a fine range, the appearance of the steel sheet after forming can be further improved. 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, 10 fields of view in the same thickness direction as the measurement of the ferrite and hard phase area fractions described above are observed at a magnification of 500x in the region from the surface of the steel plate etched with Nital reagent to the position at 1 / 2 of the plate thickness in the thickness direction. Image analysis is performed using Adobe's "Photoshop® CS5" image analysis software to calculate the area fraction of the hard phase and the number of hard phase particles in each field of view. Next, the area fraction of the hard phase and the number of ferrite particles in the 10 fields of view are summed, and the average area fraction per hard phase particle is calculated by dividing the total area fraction of the hard phase by the total number of hard phase particles. The equivalent diameter of a circle is calculated from this average area fraction and the number of particles, and the obtained equivalent diameter of a circle is determined as the average grain size of the hard phase. The observation area for each field of view is 150 μm in the thickness direction and 250 μm in the rolling direction (in this case, the observation area is 150 × 250 = 37,500 μm). 2 )

[0072] (plate thickness) In this embodiment, the thickness of the steel plate is not particularly limited, but for example, the steel plate may have a thickness of 0.1 to 2.0 mm. Steel plates with such thicknesses are suitable for use as materials for lid components 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 setting the thickness of the steel plate to 0.2 mm or more, it becomes easier to maintain the flatness of the molded product shape, and additional effects such as improved dimensional accuracy and shape accuracy can be obtained. On the other hand, by setting the thickness to 1.0 mm or less, the effect of reducing the weight of the component becomes significant. The thickness of the steel plate is measured with a micrometer.

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

[0074] (Mechanical properties) According to the steel sheet of this embodiment having the specific chemical composition and metal structure described above, a 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. There is no particular upper limit to the tensile strength, but for example, the tensile strength may be 980 MPa or less or 850 MPa or less. Setting the tensile strength to 850 MPa or less has the advantage of making it easier to ensure formability when pressing the steel sheet.

[0075] The steel sheet of this embodiment has high strength, specifically a tensile strength of 500 MPa or more, yet maintains an excellent appearance even after forming such as press working. For this reason, the steel sheet of this embodiment is extremely useful for use as exterior parts such as roofs, hoods, fenders, and doors in automobiles, where high aesthetic appeal is required.

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

[0077] The steel plate manufacturing method of this embodiment includes a casting step of casting a slab having the above-mentioned specific chemical composition. This casting step includes performing light reduction using a continuous casting machine equipped with a plurality of adjacent reduction rolls in the slab transport direction, wherein the roll pitch between adjacent reduction rolls is 290 mm or less. In this specification, light reduction refers to reduction having a reduction gradient of 0.6 mm or more per meter in the casting direction.

[0078] (Casting process) In the steel sheet of this embodiment, as mentioned above, it is essential that it has a unique metallic structure with a lower hard phase fraction than conventional DP steel and small variation in the hard phase fraction in the direction perpendicular to rolling. To obtain such a metallic structure, it is important to control the solidification structure during casting so that it becomes columnar. Specifically, in the casting process, the superheat ΔT (difference between the molten steel temperature and the solidification temperature of the molten steel) of molten steel having the above-mentioned specific chemical composition is set to 25°C or higher, and the segment pressing force is set to 450 tons or higher. This allows for the control of the solidification structure to a columnar crystal structure with an equiaxed crystal ratio of 15% or less, using a method different from conventional measures to prevent central segregation, while also suppressing central segregation. A superheat ΔT of 30°C or higher is more preferable. Furthermore, a superheat ΔT of 40°C or lower is preferable. 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 measures to improve central segregation involve minimizing the superheat ΔT (at least to less than 25°C) and increasing the equiaxed crystallinity (at least to more than 15%). However, such conventional measures do not provide sufficient improvement. In this embodiment, casting conditions that are completely different from conventional measures, namely, a superheat ΔT of 25°C or higher and a segment pressing force of 450 tons or more, are adopted to control the solidification structure to a columnar crystal structure, thereby suppressing negative segregation of Mn. As a result, microsegregation of Mn is reduced, and ghost lines can be sufficiently improved.

[0080] The equiaxed crystallinity (%) can be calculated by taking an etch print of the slab's thickness cross-section in the width direction, visually observing the boundary between the columnar and equiaxed crystal structures, measuring the thickness of the equiaxed crystal structure (mm) at the center of the slab's thickness and the slab's thickness (mm), and then dividing the equiaxed crystal structure thickness by the slab's thickness and multiplying by 100.

[0081] Furthermore, in the casting process, performing light reduction using a continuous casting machine in which the roll pitch of adjacent reduction rolls is 290 mm or less can suppress the flow of molten steel during solidification and reduce the concentration of Mn in the center. This can suppress central segregation of Mn. It is more preferable that the roll pitch of adjacent reduction rolls be 280 mm or less.

[0082] The steel sheet manufacturing method of this embodiment may include other steps in addition to the casting step described above, such as a hot rolling step, a cold rolling step, an annealing step, and a cooling step. Furthermore, this manufacturing method may optionally include a plating step. These steps are not particularly limited, and any appropriate conditions can be appropriately selected and carried out so that a metallic structure containing the ferrite and hard phase described above in a predetermined area fraction is obtained in relation to the steel sheet. Preferred conditions for these steps will be briefly described below.

[0083] (Hot rolling process) Prior to hot rolling, it is preferable to heat the slab to 1100°C or higher. By heating 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 economic standpoint, it is preferable to heat to less than 1300°C. In the hot rolling process, rough rolling and finish rolling are performed on the heated slab. The hot-rolled steel sheet obtained in this way is wound at a winding temperature of, for example, 450 to 650°C.

[0084] Furthermore, it is preferable that the finish rolling completion temperature be 950°C or lower. By setting the finish rolling completion temperature to 950°C or lower, the average grain size of the hot-rolled steel sheet and the final product can be reduced, making it possible to ensure sufficient yield strength and high surface quality after forming. In addition, by setting the winding temperature to 450-650°C, the average 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 appropriate pickling treatment to remove scale, and then subjected to the cold-rolling process. In the cold-rolling process, it is preferable to cold-roll the hot-rolled steel sheet so that, for example, the cumulative reduction ratio is 50 to 90%. By controlling the cumulative reduction ratio within this range, it is possible to secure the desired sheet thickness, ensure sufficient uniformity of the material in the width direction of the sheet, and prevent the rolling load from becoming excessive and making rolling difficult.

[0086] (Annealing process) In the annealing process, it is preferable to heat and hold the cold-rolled steel sheet to a soaking temperature of 750-900°C. By setting the soaking temperature to 750°C or higher, the recrystallization of ferrite and the reverse transformation from ferrite to austenite are sufficiently advanced, making it possible to obtain the desired metallic structure in the final product. On the other hand, by setting the soaking temperature to 900°C or lower, the crystal grains are densified, and sufficient strength can be obtained.

[0087] (cooling process) In the cooling process, the cold-rolled steel sheet is cooled after the annealing process. In the cooling process, it is preferable to cool the cold-rolled steel sheet so that the average cooling rate from the soaking temperature is 5 to 50°C / second. By setting the average cooling rate to 5°C / second or higher, excessive transformation to ferrite is suppressed, and the amount of hard phase such as martensite is increased, allowing the desired strength to be obtained. Furthermore, by setting the average cooling rate to 50°C / second or lower, the steel sheet can be cooled more uniformly in the width direction.

[0088] (Plating process) For the purpose of improving corrosion resistance, etc., the surface of the obtained cold-rolled steel sheet may be plated. Examples of plating treatments include hot-dip galvanizing, alloying hot-dip galvanizing, and electroplating. For example, the steel sheet surface may be plated with hot-dip galvanizing, or an alloying treatment may be performed after hot-dip galvanizing. The specific conditions for the plating treatment and 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.

[0089] This disclosure is not limited to the embodiments described above or the following examples, and can be combined, substituted, or modified as appropriate without departing from the purpose and spirit of this disclosure.

[0090] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples. [Examples]

[0091] In the following embodiments, a steel sheet according to one embodiment of the present disclosure was manufactured under various conditions, and the tensile strength and post-forming appearance characteristics of the obtained steel sheet were investigated.

[0092] First, using a continuous casting machine equipped with multiple reduction rolls arranged with a roll pitch of 290 mm or less, slabs with a thickness of 200-300 mm and a light reduction with a reduction gradient of 0.6 mm or more per meter in the casting direction were cast. The remainder other than the components shown in Table 1 consists of Fe and impurities. In each example, casting condition (I) is a superheat ΔT of 25°C or higher, and casting condition (II) is a segment pressing force of 450 tons or higher. In each example, Table 2 shows the cases where these conditions are met (indicated as "OK") and cases where they are not met (indicated as "NG").

[0093] Next, the obtained slab was subjected to a hot rolling process (heating temperature 1200°C, finish rolling completion temperature 900°C, and winding temperature 550°C), a cold rolling process (cumulative reduction ratio 80%), an annealing process (soaking temperature 800°C), and a cooling process (average cooling rate 10°C / second) 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), an alloyed hot-dip galvanized layer (GA), or an electro-galvanized layer (EG). Furthermore, when the chemical composition of samples taken from the manufactured cold-rolled steel sheet was analyzed, there was no change from the chemical composition of the slab shown in Table 1.

[0094] [Table 1]

[0095] [Table 2]

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

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

[0098] Furthermore, using the measured tensile strength of the steel plate, the value of the relationship between the tensile strength (TS) of the steel plate and the hard phase fraction (Vm) was calculated: (TS - 180,000 / TS) / Vm.

[0099] (Appearance after molding) The appearance of the steel plate after forming was evaluated based on the degree of ghost lines that appeared on the surface of the door outer after forming. The surface after press forming was ground with a grinding wheel, and the resulting stripe patterns at intervals of several millimeters were identified as ghost lines. A score from 1 to 5 was assigned based on the following criteria according to the degree of streak formation: A random area of ​​100 mm x 100 mm was visually inspected, and if no streak patterns were observed, it was assigned a score of "1". If the maximum length of the streak pattern was 20 mm or less, it was assigned a score of "2". If the maximum length of the streak pattern was greater than 20 mm but 50 mm or less, it was assigned a score of "3". If the maximum length of the streak pattern was greater than 50 mm but 70 mm or less, it was assigned a score of "4". If the maximum length of the streak pattern was greater than 70 mm, it was assigned a score of "5". A score of "3" or less was judged as a pass, indicating excellent post-forming appearance. On the other hand, a score of "4" or higher was judged as a fail, indicating poor post-forming appearance.

[0100] Steel plates with a tensile strength of 500 MPa or higher and a post-forming appearance rating of 3 or lower were evaluated as high-strength steel plates with improved post-forming appearance. The results are shown in Table 2.

[0101] Referring to Table 2, in the comparative examples of steel sheets No. 3, 4, 12, 16, 17, and 23-27, where the standard deviation of the hard phase fraction in the chemical composition and metal structure was outside the range of the present invention, the appearance after forming deteriorated in all cases. Furthermore, in the comparative example of steel sheet No. 28, where the chemical composition and the ferrite fraction and hard phase fraction in the metal structure were outside the range of the present invention, although the appearance after forming was good, the low carbon content resulted in a low hard phase fraction, and sufficient strength could not be obtained.

[0102] On the other hand, in the present invention examples of steel plates No. 1, 2, 5-11, 13-15, and 18-22, while maintaining high strength of tensile strength of 500 MPa or more, it was possible to suppress the generation of minute irregularities on the surface of the steel plate and significantly suppress the occurrence of ghost lines even when strain was applied by press forming.

Claims

1. The chemical composition is expressed in mass percent. 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: 0-0.0100%, and The remainder consists of Fe and impurities, with an index A represented by the following formula (1) being 0.45% or more. The metallic structure consists of ferrite: 75-97% and hard phase: 3-25% by area percentage. The hard phase consists of at least one of martensite, bainite, tempered martensite, and pearlite. A steel sheet characterized in that 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] represent the content of each element in mass percent, and 0% indicates that an element is not present.

2. The aforementioned chemical composition is, 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: 0.01-0.50%, 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 plate according to claim 1, characterized in that it contains one or more selected from the group consisting of the following.

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) Here, TS is the tensile strength in MPa, and Vm is the hard phase fraction in area percentage.

4. The steel sheet according to claim 1 or 2, characterized in that the average grain size of the ferrite is 5.0 to 30.0 μm, and the average grain size of the hard phase is 1.0 to 5.0 μm.

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