Steel plates and outer panel components

Abstract

Provided is a steel sheet characterized by: having a prescribed chemical composition; having a metal structure in which, in a sheet thickness cross-section in a direction inclined by 45° with respect to the rolling direction, the area ratio of ferrite is 70-97% and the area ratio of a second phase is 3-30%, the standard deviation of the area ratio of the second phase is at most 0.75%, the integrated intensity of {211}<011> is at least 4.0, and the integrated intensity of {332}<113> is at least 4.0 in a cross-section of Φ2 = 45° of ODF; and having a surface arithmetic average height Sa of 0.10-0.50 μm.

Description

[Technical Field]

[0001] The present invention relates to a steel plate and a shell member. [Background technology]

[0002] Due to the growing need for weight reduction in the context of energy issues, high-strength steel sheets that allow for reduced plate thickness are being widely used in automobile components such as passenger cars and trucks, and this is now even extending to exterior panels such as doors and hoods.

[0003] Panel components of vehicle exterior panels, such as roofs, hoods, fenders, and doors, are visible to the naked eye, and therefore require not only high material strength but also high design and surface quality through forming into complex part shapes. To achieve such high design quality, materials with good formability and high surface quality both before and after processing are required. As one method for increasing strength, dual phase steels (DP steels) have been developed, as disclosed in Patent Document 1, in which hard phases such as martensite and bainite are introduced into a matrix structure consisting of ferrite. However, these hard phases have low ductility, which reduces formability and tends to result in a non-uniform structure, resulting in unevenness on the steel sheet surface after forming. The occurrence of such unevenness poses a problem of reduced appearance, i.e., reduced surface quality.

[0004] Regarding formability, panel parts for the exterior panels of automobile bodies include processing elements such as draw forming at corners (corner edges) or embossed parts of door handles, and a high r value is required.

[0005] In this regard, Patent Document 1 discloses a steel sheet having excellent deep drawability, in which the composition is adjusted and the coefficient of friction between the steel sheet and the rolls in at least the final rolling stand during hot rolling is controlled to increase the area ratio of crystal grains whose {111} plane orientation is parallel to the steel sheet surface from the surface layer to 1 / 4 of the sheet thickness.

[0006] Patent Document 2 discloses a steel sheet having excellent deep drawability, in which the amounts of C and Nb added are adjusted and the cooling conditions after hot rolling and the heating rate during annealing are controlled to develop a {111} recrystallization texture.

[0007] In Patent Document 3, a steel sheet with excellent stretch formability is obtained by controlling the rolling conditions after solidification and the cooling conditions after hot rolling, thereby optimizing the (111) orientation of ferrite. <112> The document discloses a steel sheet having an orientation concentration of 3.0 or more and an orientation concentration of martensite and tempered martensite (252)<2-11> of 5.0 or less.

[0008] Patent Document 4 discloses a steel sheet having an excellent r45 value and ultimate deformability, in which γ-fiber is developed by adjusting the rolling conditions and coiling conditions in the final three stages of finish hot rolling and performing an appropriate cold rolling process. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-108364 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-64193 [Patent Document 3] International Publication No. 2020 / 203159 [Patent Document 4] International Publication No. 2021 / 205943 Summary of the Invention [Problem to be solved by the invention]

[0010] Increasing the strength of a steel sheet generally reduces its formability, such as ductility. Furthermore, as mentioned above, in the case of DP steel, unevenness is likely to occur on the steel sheet surface after forming, and the occurrence of such unevenness poses a problem of poor appearance. Therefore, in high-strength steel sheets, it is generally difficult to improve the appearance after forming while improving the strength and ductility. In addition, properties such as deep drawability are also required for panel-related parts of vehicle body exterior panels. While the above-mentioned Patent Documents 1 to 4 specifically examine mainly improvements in strength and deep drawability, they do not necessarily provide sufficient consideration from the perspective of improving the appearance after forming.

[0011] In view of the above circumstances, an object of the present invention is to provide a steel sheet which has improved tensile strength, ductility and deep drawability, and also has excellent appearance after forming. [Means for solving the problem]

[0012] To achieve the above object, the present inventors conducted research focusing on both the chemical composition and metallographic structure of a steel sheet. First, the present inventors discovered that by configuring the metallographic structure of a steel sheet with a first ferrite phase and a second hard phase in a predetermined ratio, it is possible to achieve a desired high strength while improving ductility. Next, the present inventors discovered that by developing a specific texture in such a metallographic structure mainly composed of ferrite, it is possible to significantly improve deep drawability. Furthermore, the present inventors discovered that, in addition to developing such texture, optimizing the chemical composition of the steel sheet can suppress or reduce Mn microsegregation and reduce the variation in the second phase in a predetermined direction of the metallographic structure, and further controlling the arithmetic mean height Sa of the steel sheet surface within a predetermined range can significantly suppress the occurrence of poor appearance due to minute irregularities on the steel sheet surface, even when strain is imparted by press forming or the like, and thus completed the present invention.

[0013] The present invention, which has achieved the above object, is as follows. (1) Chemical composition, in mass%, C: 0.03 to 0.12%, Si: 0.005 to 1.500%, Mn: 1.0-2.8%, P: 0.100% or less, S: 0.020% or less, N: 0.010% or less, Al: 0.005 to 0.700%, O: 0.010% or less, Cr: 0.10~0.50%, Mo: 0.05 to 0.30%, B: 0.0005~0.005%, Ti: 0.01 to 0.25% Nb: 0.01 to 0.40%, V: 0~0.15%, Ni: 0 to 0.50% Cu: 0-0.50% W: 0~0.15%, Sn: 0 to 0.10% Sb: 0 to 0.10% Ca: 0 to 0.005%, Mg: 0 to 0.0050%, Zr: 0 to 0.015%, Te: 0~0.0100%, REM: 0 to 0.010%, and The balance is Fe and impurities. In the thickness cross section in a direction inclined at 45 degrees to the rolling direction, The area ratio of ferrite is 70 to 97%. The area ratio of the second phase is 3 to 30%; the standard deviation of the area ratio of the second phase is 0.75% or less, In the Φ2=45° cross section of the ODF, {211} <011> The accumulation intensity is 4.0 or more, <113> The metal structure has an accumulation strength of 4.0 or more, A steel sheet characterized in that the arithmetic mean height Sa of the surface is 0.10 to 0.50 μm. (2) The chemical composition is in mass%: V: 0.001~0.15%, Ni: 0.001 to 0.50% Cu: 0.001 to 0.50% W: 0.001 to 0.15%, Sn: 0.001 to 0.10% Sb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Mg: 0.0001 to 0.0050%, Zr: 0.0001 to 0.015%, Te: 0.0001 to 0.0100%, and REM: 0.0001 to 0.010% The steel sheet according to (1) above, characterized in that it contains at least one of the following. (3) The steel sheet according to (1) or (2) above, wherein the ferrite has an average crystal grain size of 5.0 to 30.0 μm, and the second phase has an average crystal grain size of 1.0 to 5.0 μm. (4) The steel sheet according to any one of the above (1) to (3), wherein the second phase is at least one of martensite, bainite, tempered martensite, pearlite, and retained austenite. (5) An outer panel member comprising the steel plate according to any one of (1) to (4) above. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a steel sheet which has improved tensile strength, ductility and deep drawability, and also has excellent appearance after forming. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described. Note that these descriptions are intended to be merely examples of embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0016] <Steel plate> The steel sheet according to the embodiment of the present invention has a chemical composition, in mass%, C: 0.03 to 0.12%, Si: 0.005 to 1.500%, Mn: 1.0-2.8%, P: 0.100% or less, S: 0.020% or less, N: 0.010% or less, Al: 0.005 to 0.700%, O: 0.010% or less, Cr: 0.10~0.50%, Mo: 0.05 to 0.30%, B: 0.0005~0.005%, Ti: 0.01 to 0.25% Nb: 0.01 to 0.40%, V: 0~0.15%, Ni: 0 to 0.50% Cu: 0-0.50% W: 0~0.15%, Sn: 0 to 0.10% Sb: 0 to 0.10% Ca: 0 to 0.005%, Mg: 0 to 0.0050%, Zr: 0 to 0.015%, Te: 0~0.0100%, REM: 0 to 0.010%, and The balance is Fe and impurities. In the thickness cross section in a direction inclined at 45 degrees to the rolling direction, The area ratio of ferrite is 70 to 97%. The area ratio of the second phase is 3 to 30%; the standard deviation of the area ratio of the second phase is 0.75% or less, In the Φ2=45° cross section of the ODF, {211} <011> The accumulation intensity is 4.0 or more, <113> The metal structure has an accumulation strength of 4.0 or more, The arithmetic mean height Sa of the surface is 0.10 to 0.50 μm.

[0017] In recent years, there has been an increasing need to reduce the weight of automotive exterior panel components (roofs, hoods, fenders, doors, etc.). Therefore, similar to the case of structural components, there is a demand for higher strength in these exterior panel components. On the other hand, dual phase steels (DP steels), which have relatively low yield strength, are often used for these exterior panel components to avoid surface defects known as surface distortions that occur during press forming and other processes. However, DP steels, which contain a soft phase consisting of ferrite and a hard phase consisting of martensite, are prone to non-uniform deformation during press forming and other processes, in which the soft phase and its surroundings deform preferentially. This can lead to micro-irregularities on the surface of the steel sheet after forming, resulting in poor appearance. Meanwhile, as the strength of steel sheets increases, relatively large amounts of elements such as Mn are sometimes added to improve the hardenability of the steel sheet. Mn is an element that tends to segregate in a streaky manner in steel sheets. More specifically, Mn-enriched regions, such as center segregation and micro-segregation, are formed during casting, and these enriched regions are elongated in the rolling direction during hot rolling and cold rolling, resulting in Mn segregation in a streaky manner. Therefore, due to this Mn segregation, regions with high and low hardenability exist in the steel sheet. As a result, a relatively large number of striped hard phases are formed in the metal structure of the steel sheet after quenching. In this case, the occurrence of poor appearance is particularly noticeable. Furthermore, since formability such as ductility generally decreases as strength increases, it is generally difficult to improve the strength and ductility of high-strength steel sheets while also improving the appearance after forming. In addition, properties such as deep drawability are also required for outer plate members.

[0018] Therefore, the inventors have conducted research focusing on both the chemical composition and metallographic structure of the steel sheet in order to satisfy these properties. First, the inventors have found that by configuring the metallographic structure of the steel sheet with a first phase of ferrite and a second hard phase in a predetermined ratio, more specifically, by configuring the metallographic structure with, in area percentage, 70 to 97% ferrite and 3 to 30% second phase, it is possible to achieve a desired high strength while improving ductility.

[0019] Additionally, outer plate components generally have a large rectangular shape. Therefore, to ensure material yield, they are typically cut from base steel sheets so that the longitudinal direction of the component coincides with the rolling direction or width direction of the base steel sheet. In this regard, because many outer plate components contain deep-drawing processing elements at the corners, increasing the r-value at 45° to the rolling direction (hereinafter referred to as the "r45 value") is particularly important for improving deep drawability. A high r-value generally indicates excellent deep drawability, since a small change in thickness and a large dimensional change perpendicular to the thickness direction result. Furthermore, controlling the r-value is important not only for deep drawability but also for improving the post-formed appearance of the component. For example, to achieve good deep drawability and good post-formed appearance, it is necessary to minimize thickness variation at each location within the component, regardless of the direction of strain introduced during forming. It is particularly important that the r-value, which is an index of resistance to thickness reduction, is independent of the strain direction, i.e., that the anisotropy of the r-value (hereinafter referred to as "Δr value") calculated from the r-values ​​in the rolling direction, the 45° direction relative to the rolling direction, and the direction perpendicular to the rolling direction, is small. Therefore, the present inventors have investigated how to improve deep drawability and the appearance after forming from this perspective. As a result, the present inventors have found that it is possible to develop a specific texture in the above-mentioned metallographic structure mainly composed of ferrite, more specifically, to develop a {211} texture in the Φ2=45° cross section of the ODF (crystal orientation distribution function) of the metallographic structure. <011> The accumulation strength of the <113> It was found that by controlling the integrated strength to 4.0 or more, a higher r45 value can be obtained and the Δr value can be sufficiently reduced, thereby significantly improving deep drawability and improving the appearance after forming.

[0020] The present inventors conducted research to further improve the appearance after forming from the perspectives of reducing Mn segregation, reducing the variation in the second phase (hard phase) in the metallographic structure, and improving surface properties. As a result, the present inventors first found that optimizing the chemical composition of the steel sheet, more specifically, controlling the chemical composition of the steel sheet to contain, by mass%, 0.12% or less of C, 0.005% or more of Si, 2.8% or less of Mn, 0.005% or more of Al, 0.10% or more of Cr, and 0.10% or more of Mo, can significantly suppress or reduce Mn microsegregation. Furthermore, the present inventors found that reducing the variation in the second phase in a predetermined direction in the metallographic structure, more specifically, controlling the standard deviation of the area fraction of the second phase in a direction inclined at 45 degrees to the rolling direction to 0.75% or less, can significantly suppress the generation of microscopic irregularities on the steel sheet surface after forming. In addition, the present inventors have found that by controlling the arithmetic mean height Sa of the steel sheet surface within the range of 0.10 to 0.50 μm, it is possible to suppress or reduce the occurrence of poor appearance due to minute irregularities on the steel sheet surface, even when strain is imparted by press forming or the like. While not intending to be bound by any particular theory, it is believed that, for example, if the value of Sa is too small, the presence of minute irregularities on the steel sheet surface after forming becomes too noticeable relative to the surface texture, which is flat before forming, and the appearance deteriorates. On the other hand, if Sa is a moderate value, i.e., if irregularities that are not noticeable to the naked eye are present, the minute irregularities that occur on the steel sheet surface after forming due to the influence of the irregularities present before forming are not noticeable, or the irregularities present before forming and the minute irregularities that occur on the steel sheet surface after forming cancel each other out, resulting in suppression or reduction of the occurrence of poor appearance.

[0021] Hereinafter, each component of the steel sheet according to the embodiment of the present invention will be described in more detail. First, the reasons for limiting the chemical composition of the steel sheet according to the embodiment of the present invention will be described. Here, "%" for components means mass %. Furthermore, in this specification, unless otherwise specified, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit.

[0022] (C: 0.03 to 0.12%) C is necessary for generating a second phase other than ferrite and is an effective element for increasing the strength of steel sheet. To fully obtain this effect, the C content is set to 0.03% or more. The C content may be 0.04% or more, 0.05% or more, or 0.06% or more. On the other hand, excessive C content may inhibit the diffusion of Mn during solidification, making it impossible to sufficiently suppress Mn microsegregation. For this reason, the C content is set to 0.12% or less. The C content may be 0.10% or less, 0.09% or less, or 0.08% or less.

[0023] (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 effective element in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully achieve 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, 0.100% or more, 0.200% or more, or 0.400% or more. On the other hand, excessive Si content may reduce the peelability of scale and cause surface defects. For this reason, the Si content is set to 1.500% or less. The Si content may be 1.200% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0024] (Mn: 1.0 to 2.8%) Mn is an element necessary for improving the hardenability of steel and obtaining a second phase, which is a hard phase including martensite, and contributes to improving strength. To fully obtain this effect, the Mn content is set to 1.0% or more. The Mn content may be 1.1% or more, 1.3% or more, 1.5% or more, or 1.8% or more. On the other hand, excessive Mn content may inhibit the diffusion of Mn during solidification, and may not sufficiently suppress Mn microsegregation. For this reason, the Mn content is set to 2.8% or less. The Mn content may be 2.6% or less, 2.4% or less, 2.2% or less, or 2.0% or less.

[0025] (P:0.100% or less) P is a solid solution strengthening element and is mixed in during the manufacturing process. The lower limit is not particularly limited and may be 0%. However, reducing the P content requires time for refining, which leads to reduced productivity. Therefore, the P content may be 0.0001% or more, 0.0002% or more, or 0.0005% 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.

[0026] (S:0.020% or less) S is an element that is mixed in during the manufacturing process. The lower limit is not particularly limited and may be 0%. However, reducing the S content requires time for refining, resulting in reduced productivity. Therefore, the S content may be 0.0001% or more, 0.0002% or more, or 0.0005% 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 0.020% or less. The S content may be 0.015% or less, 0.010% or less, 0.006% or less, or 0.003% or less.

[0027] (N:0.010% or less) N is an element that is mixed in during the manufacturing process. The lower limit is not particularly limited and may be 0%. However, reducing the N content requires time for refining, resulting in reduced productivity. Therefore, the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% 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 0.010% or less. The N content may be 0.008% or less, 0.005% or less, or 0.003% or less.

[0028] (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 the diffusion of Mn during solidification and reducing Mn microsegregation. To fully achieve this effect, the Al content is set to 0.005% or more. The Al content may be 0.010% or more, 0.050% or more, 0.100% or more, or 0.200% or more. On the other hand, excessive Al content may deteriorate castability and reduce productivity. For this reason, the Al content is set to 0.700% or less. The Al content may be 0.500% or less, or 0.300% or less.

[0029] (O: 0.010% or less) O is an element that is mixed in during the manufacturing process. The lower limit is not particularly limited and may be 0%. However, reducing the O content requires time for refining, resulting in a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% 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 0.010% or less. The O content may be 0.008% or less, 0.005% or less, or 0.003% or less.

[0030] (Cr: 0.10 to 0.50%) Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel sheets. It is also an element that is effective in promoting Mn diffusion during solidification and reducing Mn microsegregation. To fully achieve these effects, the Cr content is set to 0.10% or more. The Cr content may be 0.12% or more, 0.15% or more, 0.18% or more, 0.20% or more, or 0.25% or more. On the other hand, excessive Cr content may cause the formation of coarse Cr carbides that serve as fracture initiation sites. For this reason, the Cr content is set to 0.50% or less. The Cr content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0031] (Mo: 0.05 to 0.30%) Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. It is also an element that is effective in promoting Mn diffusion during solidification and reducing Mn microsegregation. To fully obtain these effects, the Mo content is set to 0.05% or more. The Mo content may be 0.08% or more or 0.10% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. For this reason, the Mo content is set to 0.30% or less. The Mo content may be 0.25% or less, 0.20% or less, or 0.15% or less.

[0032] (B: 0.0005 to 0.005%) B suppresses the recrystallization and coarsening of austenite, promotes flattening, and improves the {223} <252> B has the effect of making it easier to obtain the desired orientation. Furthermore, B is an element that has the effect of increasing the recrystallization temperature during annealing and suppressing the randomization of the texture. To fully obtain this effect, the B content is set to 0.0005% or more. The B content may be 0.001% or more, or 0.002% or more. On the other hand, if B is contained in an excessive amount, B precipitates may be formed, which may reduce the strength of the steel sheet. For this reason, the B content is set to 0.005% or less. The B content may be 0.004% or less, or 0.003% or less.

[0033] (Ti: 0.01 to 0.25%) Ti precipitates as carbides in the hot-rolled steel structure, reducing solute carbon and increasing the {211} <011> Ti is an element that makes it easier to obtain austenite orientation and contributes to improving the r45 value and Δr. In addition, Ti suppresses the recrystallization and coarsening of austenite, promotes the flattening of austenite during the hot rolling process, and improves the {223} <252> Ti has the effect of making it easier to obtain orientation. Ti is also a precipitation strengthening element that refines the structure and improves the strength-formability balance of steel sheet. To fully obtain this effect, the Ti content is set to 0.01% or more. The Ti content may be 0.05% or more, 0.08% or more, 0.10% or more, or 0.12% or more. On the other hand, excessive Ti content may form coarse Ti sulfides, Ti nitrides, and / or Ti oxides, which may reduce the formability of the steel sheet. For this reason, the Ti content is set to 0.25% or less. The Ti content may be 0.22% or less, 0.20% or less, 0.18% or less, or 0.15% or less.

[0034] (Nb: 0.01 to 0.40%) Nb precipitates as carbides or nitrides and has the effect of suppressing the recrystallization and coarsening of austenite. Nb also promotes the flattening of austenite during the hot rolling process and reduces the {223} <252> Nb has the effect of making it easier to obtain orientation, suppressing recrystallization during annealing, and suppressing randomization of the texture. To fully obtain these effects, the Nb content is set to 0.01% or more. The Nb content may be 0.02% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive Nb content may increase unrecrystallized ferrite, resulting in a decrease in formability of the steel sheet. For this reason, the Nb content is set to 0.40% or less. The Nb content may be 0.35% or less, 0.30% or less, 0.20% or less, or 0.10% or less.

[0035] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0036] (V:0~0.15%) V is an element that contributes to improving the strength of steel sheets due to strengthening by precipitates, grain refinement strengthening by inhibiting ferrite grain growth, and / or dislocation strengthening by inhibiting recrystallization. The V content may be 0%, but to obtain the above effects, the V content is preferably 0.001% or more. The V content may be 0.005% or more, 0.01% or more, or 0.05% or more. On the other hand, excessive V content may precipitate a large amount of carbonitrides, which may reduce the formability of the steel sheet. Therefore, the V content is preferably 0.15% or less. The V content may be 0.12% or less, 0.10% or less, or 0.08% or less.

[0037] (Ni: 0 to 0.50%) 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 the above effects, the Ni content is preferably 0.001% or more. The Ni content may be 0.005% or more, 0.01% or more, or 0.05% or more. On the other hand, excessive Ni content may reduce the weldability of the steel sheet. For this reason, the Ni content is preferably 0.50% or less. The Ni content may be 0.40% or less, 0.20% or less, or 0.10% or less.

[0038] (Cu: 0 to 0.50%) Cu is present in steel in the form of fine particles and is an element that contributes to improving the strength of steel sheet. The Cu content may be 0%, but to obtain the above effects, the Cu content is preferably 0.001% or more. The Cu content may be 0.005% or more, 0.01% or more, or 0.05% or more. On the other hand, excessive Cu content may reduce the weldability of the steel sheet. For this reason, the Cu content is preferably 0.50% or less. The Cu content may be 0.40% or less, 0.20% or less, or 0.10% or less.

[0039] (W:0~0.15%) W suppresses the recrystallization and coarsening of austenite, promotes flattening, and improves the {223} <252> W is an element that has the effect of making it easier to obtain orientation. The W content may be 0%, but to obtain the above effect, the W content is preferably 0.001% or more. The W content may be 0.005% or more, 0.01% or more, or 0.05% or more. On the other hand, excessive W content may deteriorate hot workability and reduce productivity. For this reason, the W content is preferably 0.15% or less. The W content may be 0.12% or less, 0.10% or less, or 0.08% or less.

[0040] (Sn: 0 to 0.10%) 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 the above effect, the Sn content is preferably 0.001% or more. The Sn content may be 0.005% or more, 0.01% or more, or 0.03% or more. On the other hand, excessive Sn content may generate coarse oxides, which may cause embrittlement of the steel sheet. For this reason, the Sn content is preferably 0.10% or less. The Sn content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[0041] (Sb: 0 to 0.10%) Sb is an element that suppresses the coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sb content may be 0%, but to obtain the above effect, the Sb content is preferably 0.001% or more. The Sn content may be 0.005% or more, 0.01% or more, or 0.03% or more. On the other hand, excessive Sb content may generate coarse oxides, which may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 0.10% or less. The Sb content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[0042] (Ca: 0 to 0.005%) Ca is an element mixed in as a deoxidizer. The Ca content may be 0%, but excessive reduction will increase manufacturing costs. Therefore, the Ca content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive Ca content may generate coarse oxides, which may cause embrittlement of the steel sheet. Therefore, the Ca content is preferably 0.005% or less. The Ca content may be 0.004% or less, 0.003% or less, or 0.002% or less.

[0043] (Mg: 0 to 0.0050%) Mg is an element that can control the morphology of sulfides with trace addition, and is added as needed. The Mg content may be 0%, but excessive reduction will increase manufacturing costs. Therefore, the Mg content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive Mg content may generate coarse inclusions, which may cause embrittlement of the steel sheet. Therefore, the Mg content is preferably 0.0050% or less. The Mg content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.

[0044] (Zr: 0 to 0.015%) Zr is an element mixed in as a deoxidizer. The Zr content may be 0%, but excessive reduction will increase manufacturing costs. Therefore, the Zr content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive Zr content may generate coarse oxides, which may cause embrittlement of the steel sheet. Therefore, the Zr content is preferably 0.015% or less. The Zr content may be 0.010% or less, 0.005% or less, or 0.003% or less.

[0045] (Te: 0 to 0.0100%) Te is an element that can control the morphology of sulfides with trace addition, and is added as needed. The Te content may be 0%, but excessive reduction will increase manufacturing costs. Therefore, the Te content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive Te content may generate coarse inclusions, which may cause embrittlement of the steel sheet. Therefore, the Te content is preferably 0.0100% or less. The Te content may be 0.0090% or less, 0.0080% or less, 0.0070% or less, or 0.0050% or less.

[0046] (REM: 0 to 0.010%) REM is an element mixed in as a deoxidizer. The REM content may be 0%, but excessive reduction leads to increased manufacturing costs. Therefore, the REM content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive REM content may generate coarse oxides, which may cause embrittlement of the steel sheet. Therefore, the REM content is preferably 0.010% or less. The REM content may be 0.008% or less, 0.005% or less, or 0.003% or less. 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.

[0047] The remainder of the steel sheet according to this embodiment, other than the above-described components, is Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the steel sheet according to this embodiment is industrially manufactured.

[0048] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general 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.

[0049] Next, the characteristics of the structure and properties of the steel sheet according to the embodiment of the present invention will be described.

[0050] (Ferrite area ratio: 70-97%) The metal structure of the steel sheet contains, in area %, 70 to 97% ferrite and 3 to 30% second phase, and is composed of, for example, only 70 to 97% ferrite and 3 to 30% hard phase. If the ferrite area ratio is less than 70%, ductility decreases. Therefore, the ferrite area ratio is set to 70% or more. From the viewpoint of improving ductility, a higher ferrite area ratio is preferable, and may be, for example, 75% or more, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if it exceeds 97%, the desired strength may not be obtained. Therefore, the ferrite area ratio is set to 97% or less. The ferrite area ratio may be 96% or less, 93% or less, 90% or less, or 87% or less. From the viewpoint of the balance between strength and ductility, the ferrite area ratio is preferably 70 to 90%.

[0051] (Second phase area ratio: 3~30%) If the area fraction of the second phase, which is a hard phase, is less than 3%, the desired strength cannot be obtained. Therefore, the area fraction of the second phase is set to 3% or more. From the viewpoint of improving strength, a higher area fraction of the second phase is preferable, and may be, for example, 5% or more, 7% or more, 10% or more, or 15% or more. On the other hand, if it exceeds 30%, ductility may decrease. Therefore, the area fraction of the second phase is set to 30% or less. The area fraction of the second phase may be 27% or less, 25% or less, 20% or less, or 18% or less. Here, the second phase refers to a structure consisting of at least one of martensite, bainite, tempered martensite, pearlite, and retained austenite.

[0052] (Standard deviation of the area ratio of the second phase in a direction inclined at 45 degrees to the rolling direction: 0.75% or less) By setting the standard deviation of the area ratio of the second phase in a direction inclined at 45 degrees relative to the rolling direction to 0.75% or less, it is possible to further reduce the variation in the amount of deformation of the steel sheet in the 45-degree direction during forming, such as press forming, and as a result, it is possible to achieve a good post-forming appearance. Therefore, the standard deviation of the area ratio of the second phase is set to 0.75% or less. From the viewpoint of improving the post-forming appearance, the lower the standard deviation of the area ratio of the second phase, the more preferable it is, and it may be, for example, 0.70% or less, 0.65% or less, 0.60% or less, or 0.55% or less. The lower limit is not particularly limited and may be 0%. For example, the standard deviation of the area ratio of the second phase may be 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more.

[0053] When the rolling direction of a steel sheet is unclear, the following method, for example, is employed to identify the rolling direction of the steel sheet. After mirror-polishing a thickness cross-section of the steel sheet, the S concentration is measured using an electron probe microanalyzer (EPMA) (for example, the "JXA-8230" manufactured by JEOL Ltd.). The measurement conditions are an acceleration voltage of 15 kV and a measurement pitch of 1 μm, and a distribution image is measured over a 500 μm square range in the center of the sheet thickness. At this time, an elongated region with a high S concentration is determined to be an inclusion such as MnS. When observing, observation is made in three or more fields spaced 100 μm or more apart. Next, using the thickness cross-section initially observed by the above method as a reference, a plane parallel to a plane rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed by the above method. The average value of the long axis length of the multiple inclusions in each obtained cross-section is calculated for each cross-section, and the cross-section with the largest average long axis length of the inclusions is identified. The direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.

[0054] (ODF Φ2=45° section {211} <011> and {332} <113> (Integration strength: 4.0 or more) In the Φ2=45° section of the ODF (crystal orientation distribution function) of the metal structure, {211} <011> and {332} <113> If either of the accumulation intensities is less than 4.0, it cannot be said that the texture is well developed, and a high r45 value and a low Δr, for example, an r45 value of 1.40 or more and a Δr value of 0.40 or less, cannot be achieved. <011> and {332} <113> The aggregate strength of each of these is 4.0 or more. From the viewpoint of deep drawability and improving the appearance after forming, the higher the aggregate strength, the better. For example, {211} <011> and {332} <113> The accumulation intensity of each of these may be 5.0 or more, 6.0 to 7.0 or more, or 8.0 or more. <011> and {332} <113> may have an accumulation intensity of 18.0 or less, 16.0 or less, or 15.0 or less, respectively.

[0055] (Measurement of accumulation strength by ODF) Measurement of the integrated strength by ODF is performed as follows. First, a cross section parallel to the rolling direction and thickness direction of the steel sheet is cut out, and the surface is prepared by mechanical grinding and electrolytic polishing. If the rolling direction of the steel sheet is unclear, the rolling direction can be identified by the method using inclusions described above. The same applies to other measurements. Next, the crystal orientation is measured at 1.00 μm intervals in a 100 μm × 100 μm region at a depth position half the sheet thickness using the SEM-EBSD method. As the SEM, a field emission scanning electron microscope (FE-SEM), for example, the "JSM-7200F (NEO)" (accelerating voltage: 15 kV) manufactured by JEOL Ltd., and as the EBSD detector, for example, the "Velocity" manufactured by TSL Solutions, Inc. can be used. Based on the obtained crystal orientation data, OIM ver. 7.3, a crystal orientation analysis application from EDAX TEXSEM, was used to obtain the ODF (crystal orientation distribution function) of the φ2 = 45° cross section (Bunge method) based on the ND axis using the 16th-order spherical harmonic function expansion method. <011> and {332} <113> The accumulation strength is measured in three or more areas spaced at least 100 μm apart, and the arithmetic mean value of the measured values ​​is used as the measurement result of the accumulation strength by ODF for that cross section.

[0056] (Average grain size of ferrite: 5.0 to 30.0 μm) The average grain size of ferrite in the steel sheet is the average grain size of ferrite generated during annealing and subsequent cooling. The density and grain size of the hard phase change depending on this average grain size of ferrite. In this embodiment, the average grain size of ferrite is preferably 5.0 to 30.0 μm. Controlling the average grain size of ferrite within such a fine range increases the uniformity of the structure and further improves the appearance after forming. Specifically, when the average grain size of ferrite is 5.0 μm or more, aggregation of the hard phase after ferrite generation is less likely to occur, preventing uneven strain during forming and further improving the appearance after forming. On the other hand, when the average grain size of ferrite is 30.0 μm or less, variation in the grain size of ferrite is reduced, preventing uneven strain during forming and further improving the appearance after forming. The average grain size of ferrite may be 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. Similarly, the average grain size of the ferrite may be 28.0 μm or less, 25.0 μm or less, 20.0 μm or less, 16.0 μm or less, 14.0 μm or less, or 12.0 μm or less.

[0057] (Average grain size of the second phase: 1.0 to 5.0 μm) The average grain size of the hard phase in the steel sheet is the average grain size of at least one of martensite, bainite, tempered martensite, pearlite, and retained austenite that is generated during annealing and subsequent cooling. In this embodiment, the average grain size of the hard phase is preferably 1.0 to 5.0 μm. Controlling the average grain size of the hard phase within such a fine range increases the uniformity of the structure and further improves the appearance after forming. Specifically, when the average grain size of the hard phase is 1.0 μm or more, aggregation of the hard phase is less likely to occur, preventing uneven strain during forming and further improving the appearance after forming. On the other hand, when the average grain size of the hard phase is 5.0 μm or less, variation in the grain size of the hard phase is reduced, preventing uneven strain during forming and further improving the appearance 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 crystal grain size of the hard phase may be 4.8 μm or less, or 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.8 μm or less, 3.6 μm or less, or 3.4 μm or less.

[0058] (Sa: 0.10 to 0.50 μm) In this embodiment, the arithmetic mean height Sa of the steel sheet surface (the surface of the plating layer, if a plating layer is present on the surface of the steel sheet) is 0.10 to 0.50 μm. By controlling the surface roughness of the steel sheet in this manner and smoothing out the irregularities on the steel sheet surface before forming, it is possible to make the surface irregularities after forming less noticeable. This makes it possible to more reliably improve poor appearance after forming. In this embodiment, from the viewpoint of further improving the appearance after forming, the arithmetic mean height Sa is preferably 0.45 μm or less, more preferably 0.43 μm or less, and most preferably 0.40 μm or less or 0.35 μm or less. Furthermore, the arithmetic mean height Sa may be 0.15 μm or more, 0.20 μm or more, or 0.25 μm or more. A preferred range of Sa to further improve the appearance after forming is, for example, 0.20 to 0.45 μm.

[0059] (Method for measuring Sa) Sa is measured as follows. First, a test piece is cut from a position at least 100 mm away from the edge of the steel sheet. Next, a laser microscope (e.g., Keyence's VK-X3000) is used to measure the unevenness of the surface of the steel sheet (or the surface of the plating layer, if present) in an 8 mm square area. Depending on the size of the object to be measured, the test piece may be cut from a position at least 10 mm away from the edge of the steel sheet. The measurement magnification is 20x, the resolution in the XY plane is 5 μm, and the resolution in the Z plane is 0.1 nm. The measurement is then performed in a linked manner. The entire measurement area is then subjected to low-pass filtering with a cutoff value of 0.25 mm using a Gaussian filter specified in JIS B0681-2:2018, and the arithmetic mean height Sa is calculated. Sa is the absolute average of z(x, y) in the reference area (A) defined in JIS B0681-2:2018, 4.1.7 "Arithmetical mean height of the scale limited surface." The arithmetic mean height Sa is measured in three or more areas (8 mm square areas) spaced at least 100 μm apart, and the arithmetic mean value of each measurement is calculated.

[0060] (plate 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. A steel plate having such a thickness is suitable for use as a material 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 becomes significant. The thickness of the steel plate is measured using a micrometer.

[0061] (plating) The steel sheet according to the embodiment of the present invention may be a cold-rolled sheet (cold-rolled steel sheet), but may further include a plating layer on its surface for the purpose of improving corrosion resistance, etc. The plating layer may be either a hot-dip plating layer or an electroplated layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplated layer on its surface. Examples of the hot-dip plating layer 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 the electroplated layer include an electrogalvanized layer (EG), an electrolytic Zn-Ni alloy plating layer, etc. Preferably, the plating layer is a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer. The coating weight of the coating layer is not particularly limited and may be a general coating weight.

[0062] Next, the method for observing and measuring the above-specified structure will be described.

[0063] (Method for measuring the area ratio of ferrite and second phase) First, a specimen is taken from a cross-section of the steel sheet at a 45° angle to the rolling direction, and the observation surface is then polished and etched with nital. Secondary electron images of the steel sheet are then taken and linked using a FE-SEM (e.g., a JEOL "JSM-7200F (NEO)" microscope at 600x magnification and a resolution of 1280 x 960 pixels) (e.g., a JEOL "JSM-7200F (NEO)" microscope, accelerating voltage: 15 kV). The white areas (relatively bright areas) in the resulting image data represent the structure of the second phase (martensite, bainite, tempered martensite, pearlite, and retained austenite), while the black areas (relatively dark areas) represent ferrite. The image is then analyzed over a field of view measuring the total sheet thickness x 5 mm to determine the area ratios of ferrite and the second phase. The white and black areas can be determined, for example, using the image analysis software ImageJ. The threshold for binarization of each pixel in an SEM secondary electron image, which represents ferrite as black and non-ferrite as white, is determined using the method described in Glasbey, CA (1993), "An analysis of histogram-based thresholding algorithms," CVGIP: Graphical Models and Image Processing 55:532-537, which uses the average brightness value as the threshold. This algorithm is implemented in ImageJ, and binarization is performed automatically by using the Auto threshold function and setting the threshold method to Method = Mean. That is, the binarization threshold is automatically determined from the smoothed histogram by replacing each pixel value with the average pixel value within a 15-pixel radius of the pixel of interest in ImageJ, setting Method = Mean and radius = 15. Image analysis is performed on at least three fields of view, each spaced at least 100 μm apart, and the arithmetic mean of the values ​​measured in each field of view is used as the area fraction of ferrite and the second phase in the observation surface.

[0064] (Method for measuring the standard deviation of the area ratio of the second phase in a direction inclined at 45 degrees to the rolling direction) The standard deviation of the area ratio of the second phase is calculated from the combined SEM images measured above. First, the steel sheet is divided into 100 μm (0.1 mm) sections in a 45° direction relative to the rolling direction, and the area ratio of the second phase across the entire sheet thickness is calculated for each divided section. The standard deviation of the area ratio of the second phase is calculated based on the area ratio of the second phase calculated from each of the 500 divided images in total. Image analysis is performed on three or more fields of view, each spaced 100 μm or more apart, and the arithmetic mean of the measured values ​​in each field of view is taken as the standard deviation of the area ratio of the second phase on that observation surface.

[0065] (Method for measuring the average grain size of ferrite) From the combined SEM image measured above, image analysis is performed in a field of view of the total plate thickness x 5 mm to calculate the number of ferrite particles. The average area ratio per ferrite particle is calculated by dividing the ferrite area ratio by the number of ferrite particles. The circle equivalent diameter is calculated from this average area ratio, and the obtained circle equivalent diameter is used as the average ferrite crystal grain size. Image analysis is performed in three or more fields, each spaced 100 μm apart, and the arithmetic mean of the measurements in each field is used as the average ferrite crystal grain size in that observation surface.

[0066] (Method for measuring the average crystal grain size of the second phase) The number of second phase (hard phase) particles is calculated by performing image analysis on the combined SEM image measured above in a field of view of the total plate thickness x 5 mm. The average area ratio per second phase particle is calculated by dividing the second phase area ratio by the number of second phase particles. The circle equivalent diameter is calculated from this average area ratio, and the obtained circle equivalent diameter is taken as the average crystal grain size of the hard phase second phase. Image analysis is performed on three or more fields of view, each spaced 100 μm or more apart, and the arithmetic mean of the measurements in each field of view is taken as the average crystal grain size of the second phase in that observation surface.

[0067] (mechanical properties) The steel sheet of this embodiment, which is composed of a steel sheet having the above-mentioned specific chemical composition and metal structure and the above-mentioned specific arithmetic mean height Sa formed on the surface thereof, can achieve high strength and ductility (elongation), specifically, a tensile strength of 540 MPa or more and a total elongation of 15.0% or more.

[0068] (tensile strength) The tensile strength of the steel sheet is preferably 550 MPa or more, more preferably 600 MPa or more or 650 MPa or more. There is no particular upper limit to the tensile strength of the steel sheet, but the tensile strength may be, for example, 980 MPa or less or 950 MPa or less. Setting the tensile strength to 950 MPa or less has the advantage of making it easier to ensure formability when the steel sheet is press-formed.

[0069] (total elongation) The total elongation of the steel sheet is preferably 16.0% or more, more preferably 18.0% or more, from the viewpoint of improving formability such as ductility. There is no particular upper limit to the total elongation of the steel sheet, but from the viewpoint of productivity, the total elongation may be, for example, 30.0% or less or 25.0% or less.

[0070] The tensile strength (TS) and total elongation (EL) can be measured as follows. First, a No. 5 tensile test piece according to JIS Z 2241:2022 is cut out from the steel sheet to be measured, with the longitudinal direction perpendicular to the rolling direction. Next, a tensile test in accordance with JIS Z 2241:2022 is performed using this test piece, thereby measuring the tensile strength TS (MPa) and butt elongation (total elongation) EL (%). Tensile test pieces are taken from three or more locations spaced 100 μm or more apart, and the arithmetic mean values ​​of the measured values ​​for each tensile test piece are used as the measurement results for the tensile strength (TS) and total elongation (EL) of the steel sheet to be measured.

[0071] (Surface characteristics after molding) Furthermore, the steel sheet of this embodiment has a post-forming Sa controlled to 0.10 to 0.55 μm, and therefore has excellent surface properties even after forming, making it possible to obtain an excellent post-forming appearance with the steel sheet of this embodiment.

[0072] (Sa after molding: 0.10~0.55μm) As described above, the steel sheet of this embodiment can have surface characteristics such that the post-forming Sa is 0.10 to 0.55 μm. The post-forming Sa is the average value of the height difference (absolute value) at each point relative to the average plane of the surface after strain is imparted during forming, and more specifically, it is the average absolute value of z(x, y) in the reference region (A) defined in 4.1.7 "Arithmetical mean height of the scale limited surface" of JIS B0681-2:2018. When the post-forming Sa is 0.55 μm or less, the post-forming appearance is excellent. Note that the post-forming Sa may be 0.10 μm or more from the viewpoint of productivity.

[0073] The post-forming Sa can be measured as follows. First, a JIS Z2241:2022 No. 5 tensile test piece is cut from the steel sheet to be measured, with the longitudinal direction perpendicular to the rolling direction. The test piece is cut from a position at least 100 mm away from the end face of the steel sheet. Depending on the size of the object to be measured, the test piece may be cut from a position at least 10 mm away from the end face of the steel sheet. Next, a tensile strain of 5% is applied to the test piece in the longitudinal direction using a tensile test in accordance with JIS Z2241:2022. Then, using a laser microscope (e.g., Keyence's "VK-X3000"), the unevenness of the test piece surface (or the surface of the plating layer, if present on the surface of the test piece) after the tensile test is measured in an 8 mm x 8 mm area. The measurement conditions are a measurement magnification of 20x, a resolution of 5 μm in the XY plane, and a resolution of 0.1 nm in the Z plane, and the measurements are performed in a linked manner. The entire measurement area is then subjected to low-pass filtering with a cutoff value of 0.25 mm using a Gaussian filter specified in JIS B0681-2:2018 to determine the arithmetic mean height Sa. The arithmetic mean height Sa is measured in three or more areas spaced at least 100 μm apart, and the arithmetic mean value of each measurement is calculated. The arithmetic mean height Sa obtained in this way is the post-molding Sa.

[0074] (r45 value: 1.40 or more and Δr: 0.40 or less) The r45 value of the steel sheet is preferably 1.40 or more, more preferably 1.50 or more. The upper limit of the r45 value of the steel sheet is not particularly limited, but may be 3.00 or less, 2.50 or less, or 2.00 or less. The Δr of the steel sheet is preferably 0.40 or less, more preferably 0.35 or less, or 0.30 or less.

[0075] (Method for measuring r45 value and Δr) The r45 value and Δr, which are the plastic strain ratios, were measured in accordance with the provisions of JIS Z 2254:2008. Test pieces were taken from three or more locations spaced at least 100 μm apart, and the arithmetic mean values ​​of the measured values ​​for each test piece were used as the measurement results for the r45 value and Δr of the steel plate being measured.

[0076] The steel sheet according to the embodiment of the present invention can achieve high strength, for example, a tensile strength of 540 MPa or more, improved ductility and deep drawability, and even excellent appearance after forming. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in parts in technical fields requiring these properties. In a preferred embodiment, an exterior panel member, particularly an automobile exterior panel member, is provided, comprising the steel sheet according to the embodiment of the present invention. Examples of automobile exterior panel members include roofs, hoods, fenders, doors, and the like, which require high design quality. These exterior panel members, particularly automobile exterior panel members, may comprise the steel sheet according to the embodiment of the present invention in at least a portion thereof, and therefore, at least a portion of these exterior panel members satisfies the above-described chemical composition and metallographic characteristics. In portions of the steel sheet that are relatively lightly processed in forming, such as press forming, the metallographic characteristics do not change significantly before and after forming.

[0077] <Steel sheet manufacturing method> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0078] The method for producing a steel sheet according to this embodiment includes a casting process for casting a steel slab having the specific chemical composition described above, a hot rolling process for hot-rolling the cast steel slab, a cold rolling process for cold-rolling the hot-rolled sheet, a primary annealing process for holding the cold-rolled sheet at a predetermined temperature range, a secondary annealing process for holding the cold-rolled sheet at a predetermined temperature range under a predetermined atmosphere, a cooling process for cooling the annealed cold-rolled sheet, an optional plating process for forming a plating layer on the surface of the cooled cold-rolled sheet, and a skin-pass rolling process. These processes are not particularly limited, and may be carried out under any suitable conditions selected appropriately to obtain the specific metal structure described above. Preferred conditions for these processes are described below.

[0079] (Casting process) (Superheat ΔT: 25°C or higher, and segment pressing force: 450 tons or higher) In the casting process, it is preferable to cast the slab by continuous casting from the viewpoint of productivity. It is also important to control the solidification structure during casting so that it becomes 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 above-mentioned specific chemical composition to 25°C or more and further setting the segment pressing force to 450 tons or more, it is possible to control the solidification structure to a columnar crystal structure with an equiaxed crystal fraction of 15% or less and suppress central segregation of Mn. A columnar crystal structure with an equiaxed crystal fraction of 15% or less is effective in suppressing local Mn enrichment, and from the viewpoint of further promoting the formation of such a structure, it is more preferable that the superheat ΔT be 30°C or more. The upper limit of the superheat is not particularly limited, but it may be, for example, 50°C or less.

[0080] (Hot rolling process) (Heating temperature: 1050~1300℃) In the hot rolling process, the slab is heated prior to hot rolling. This heating temperature is set to 1050°C or higher because it is necessary to dissolve Ti in the steel. However, temperatures above 1300°C may shorten the life of the heating furnace. For this reason, the heating temperature is set to 1300°C or lower.

[0081] (rough rolling) In this manufacturing method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0082] (Total reduction in the final three stages of finish rolling: 40-100%) The heated slab or the slab that has been subjected to rough rolling as required is then subjected to finish rolling. Finish rolling is carried out using a tandem rolling mill consisting of multiple rolling stands, for example, three or more stands, preferably four to six stands. The total reduction in the final three stages of finish rolling is <252> This develops the orientation and ultimately the final product. <011> and {332} <113> The {223} content of the hot-rolled sheet is set to 40% or more to increase the aggregate strength. <252> The orientation was changed to {211} by the subsequent introduction of shear bands by cold rolling and recrystallization by annealing. <011> Therefore, in hot-rolled sheets, the {223} <252> By developing the orientation, the final product after cold rolling and annealing has a {211} <011> It is possible to increase the concentration of the above. The total reduction in the final three stages (the sum of the reductions in each stage) is set to 100% or less. At least the final three stages of rolling, including the final rolling stand, are carried out in a temperature range from the finish rolling end temperature to the finish rolling end temperature + 100°C.

[0083] (Effective rolling index of the final three stages of finishing rolling: 1.2 or more) If the recrystallization of austenite between passes during hot rolling progresses excessively, the accumulation of texture weakens and the crystal grains become equiaxed, making it impossible to obtain a hot-rolled sheet with the desired texture. <252> In order to develop the orientation, it is preferable to develop the austenite texture. Therefore, in the present manufacturing method, as will be explained below, the finish rolling is appropriately evaluated and controlled using an effective rolling index that calculates the progress of recrystallization in the final three stages of rolling, thereby enabling sufficient development of the austenite texture. <252> To develop the texture, the effective rolling index is set to 1.2 or more. There is no particular upper limit for the effective rolling index, but a larger index allows for the development of austenite texture.

[0084] For example, if the reduction in a single stage is large, or if the accumulated strain during multi-stage rolling is sufficient to cause recrystallization, the austenite becomes equiaxed and the texture weakens, resulting in a weakened texture of the ferrite after cooling.As a result of further investigation, we found that the following method can be used to determine the combination of rolling conditions for flattening the grains while enhancing the texture in the final three stages of finish rolling.

[0085] First, out of the final three rolling stands, we focused on the rolling stand (F1) that performs rolling first. From the relationship between the Ti content (mass%) in the steel, the rolling strain in F1, and the finish rolling temperature FT (°C), we calculated dT, which is a value related to the time it takes for 50% of the austenite to recrystallize after rolling in F1. eff-p Ask for.

number

number

number

[0086] dT eff-p When is greater than 2.0, it is determined that no recrystallization occurred in F1, and the cumulative rolling strain up to F2 is used to determine dT, which is the value related to the time it takes for 50% of the austenite to recrystallize after rolling F2. eff-s-a is calculated using the following formula (iv).

number

[0087] On the other hand, dT eff-p When dT is 2.0 or less, it is determined that recrystallization has occurred in F1.eff-s-a The value corresponding to the rolling strain of F2 alone is calculated using the following formula (v) to obtain dT eff-s-b Let's say.

number

[0088] The dT calculated above eff-s-a or dT eff-s-b Whether recrystallization occurs after F2 is determined based on the value of dT eff-s-a When is greater than 2.0, it is determined that no recrystallization occurred in either F1 or F2, and the effective rolling index C eff-t-a is calculated using the following equation (vi).

number

[0089] On the other hand, dT eff-s-a When the value is 2.0 or less, it is determined that recrystallization has occurred between F2 and F3 due to the accumulated strain up to F2, and the effective rolling index C eff-t-b is calculated using the following equation (vii).

number

[0090] Furthermore, dT eff-s-b When is greater than 2.0, the effective rolling index C is calculated from the cumulative rolling strain of F2 and F3. eff-t-c is calculated using the following formula (viii).

number

[0091] On the other hand, dT eff-s-b When the value is 2.0 or less, it is determined that recrystallization has occurred after F2 rolling, and the effective rolling index C is calculated by the above formula (vii). eff-t-b Calculate.

[0092] When the steel contains one or more elements selected from Ti, Nb, V, Mo, W, and Zr, the K value is determined by the following formula (ix) instead of the above formula (ii).

number

[0093] By controlling the hot rolling so that the effective rolling index obtained by the above procedure is 1.2 or more, austenite grains that are flat in the rolling direction and have a well-developed texture can be effectively obtained. In this embodiment, the effective rolling index is preferably 1.5 or more from the viewpoint of further developing the texture.

[0094] (Cooling start time after finishing rolling: 0.5 to 2.0 seconds) After the completion of finish rolling, cooling must be carried out before the recrystallization of austenite is complete. However, if the time from the completion of finish rolling to the start of cooling is less than 0.5 seconds, coarse ferrite and pearlite will form in the hot-rolled sheet structure, resulting in increased structural heterogeneity in the final product and a poor appearance after forming. For this reason, the cooling start time after the completion of finish rolling is set to 0.5 seconds or more. On the other hand, if the time exceeds 2.0 seconds, cooling will be carried out after the recrystallization of austenite is complete after the completion of finish rolling. For this reason, to ensure that cooling is carried out before the recrystallization of austenite is complete, the cooling start time after the completion of finish rolling is set to 2.0 seconds or less.

[0095] (Average cooling rate after finishing rolling: 15 to 200°C / sec) If the average cooling rate after the finish rolling is less than 15°C / sec, the {223} <252> The texture cannot be fully developed, and this leads to the {211} <011> and / or {332} <113> If the average cooling rate exceeds 200°C / s, the strength of the hot-rolled sheet will become excessively high, which will result in an excessive load on the cold rolling machine in the subsequent cold rolling process. Therefore, the average cooling rate is set to 200°C / s or less.

[0096] (Finishing temperature: 800-950℃) In the hot rolling process, the end temperature of finish rolling is preferably 800°C or higher. When the end temperature of finish rolling is 800°C or higher, a more uniform metal structure is formed, which allows the desired standard deviation of the area ratio of the second phase to be achieved in the final product, and as a result, a higher quality post-forming appearance can be obtained. On the other hand, although there is no particular upper limit for the end temperature of finish rolling, from an economical viewpoint, the end temperature of finish rolling is preferably 950°C or lower.

[0097] (Hot rolled coil winding temperature: 300-600℃) If the coiling temperature of the hot-rolled coil is less than 300°C, the strength of the hot-rolled sheet will be too high, and the load during cold rolling after pickling will be high. For this reason, the coiling temperature is set to 300°C or higher. On the other hand, if the coiling temperature exceeds 600°C, the {223} <252> The texture cannot be fully developed, and this leads to the {211} <011> and / or {332} <113> If the temperature is too high, the integrated strength of the hot rolled steel sheet will not be increased sufficiently. In addition, coarse ferrite and pearlite will be generated in the hot rolled steel sheet structure, which will result in a large amount of non-uniformity in the structure of the final product and a poor appearance after forming. For this reason, the coiling temperature is set to 600°C or less.

[0098] (Cold rolling process) (cold rolling ratio) The hot-rolled sheet is then subjected to an appropriate pickling treatment to remove scale, and then to a cold-rolling process. By setting the cold-rolling reduction to 65% or more, cold-rolling strain is accumulated, which is then refined in the subsequent annealing process, improving the uniformity of the structure and ensuring the appearance after forming while also achieving {332} <113> The texture can be developed and the r45 value can be increased to the desired value. On the other hand, if the cold rolling reduction is too high, discontinuous recrystallization becomes vigorous, causing randomization of the texture due to shear bands, which deteriorates the r45 value, Δr, and / or the appearance after forming. For this reason, the cold rolling reduction is set to 90% or less.

[0099] (First annealing process) The cold-rolled sheet is then heated to a maximum temperature of 600 to Ac1°C at an average heating rate of 0.01 to 5.0°C / s in the subsequent primary annealing step, and is then held at this temperature for 5 to 60 minutes. This treatment allows the precipitation of fine carbides with a low Mn concentration and therefore relatively easy solubility after recrystallization, resulting in the formation of {211} <011> This allows the texture to be developed and the standard deviation of the area ratio of the second phase (hard phase) after secondary annealing to be reduced.

[0100] (Average heating rate for primary annealing: 0.01 to 5.0°C / sec) From the viewpoint of productivity, the average heating rate of the primary annealing is set to 0.01°C / s or more. On the other hand, if the average heating rate exceeds 5.0°C / s, carbides are formed first, which causes the recrystallization structure to become random, resulting in the {211} <011> The integrated strength of the materials, etc., decreases, and the r45 value and / or Δr deteriorate. For this reason, the average heating rate should be 5.0°C / sec or less.

[0101] (Maximum heating temperature for primary annealing: 600~Ac1℃) If the maximum heating temperature of the first annealing is less than 600°C, recrystallization does not proceed sufficiently, and the {211} <011> The formation of texture is inhibited. For this reason, the maximum heating temperature is set to 600°C or higher. On the other hand, if the maximum heating temperature exceeds the Ac1 transformation point and becomes too high, carbides dissolve and austenite transformation occurs, which causes the texture to become more random, resulting in a deterioration in the r45 value and / or Δr. For this reason, the maximum heating temperature is set to Ac1°C or lower. The Ac1 point can be calculated from the chemical composition of the steel sheet using the following formula (1) described in Leslie, "Leslie Steel Materials Science," translated and edited by Shigeyasu Koda, Maruzen Co., Ltd., 1985, p. 273: Ac1(℃)=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr...(1) Here, the element symbols in the above formula represent the content (mass %) of each element.

[0102] (Primary annealing time: 5 to 60 minutes) If the holding time of the first annealing is less than 5 minutes, fine carbides with low Mn concentration will precipitate after recrystallization, resulting in a {211} <011> It may not be possible to develop a texture, and as a result, Δr and other parameters may not be reduced sufficiently. For this reason, the holding time is set to 5 minutes or more. On the other hand, if the holding time exceeds 60 minutes, Mn will concentrate in the carbides, and the carbides will not be able to dissolve during annealing, making it impossible to uniformly generate the second phase in the subsequent secondary annealing process. As a result, it will not be possible to sufficiently reduce the standard deviation of the area ratio of the second phase in the final product. For this reason, the holding time is set to 60 minutes or less.

[0103] (Secondary annealing process) In the subsequent secondary annealing step, the cold-rolled sheet that has undergone primary annealing is heated to a maximum heating temperature of Ac1 to 900°C at an average heating rate of 2 to 30°C / s in an atmosphere with a dew point of -30 to 20°C and a hydrogen concentration of 2 to 20% (nitrogen balance), and is then held at this maximum heating temperature for 30 to 500 seconds. By carrying out such treatment, it is possible to appropriately promote ferrite recrystallization and reverse transformation from ferrite to austenite, thereby densifying the crystal grains and sufficiently reducing the steel sheet surface, thereby achieving the desired metallographic structure fraction and texture integration strength in the final product, as well as an excellent appearance.

[0104] (Average heating rate for secondary annealing: 2-30°C / sec) If the average heating rate for the secondary annealing is less than 2°C / s, the austenite produced by reverse transformation will become coarse, the texture will become random, and the final product will have a large degree of structural inhomogeneity, resulting in a deterioration in the r45 value and the appearance after forming. For this reason, the average heating rate should be 2°C / s or more. On the other hand, if the average heating rate exceeds 30°C / s, condensation will form on the equipment, hindering its operation. For this reason, the average heating rate should be 30°C / s or less.

[0105] (Hydrogen concentration in secondary annealing: 2 to 20%) If the hydrogen concentration (nitrogen balance) in the atmosphere for secondary annealing is less than 2%, the surface of the base steel sheet will not be sufficiently reduced, the plating wettability of the base steel sheet will deteriorate, and the appearance after forming will deteriorate. For this reason, the hydrogen concentration in the atmosphere for secondary annealing is set to 2% or more. On the other hand, if the hydrogen concentration exceeds 20%, it will not be possible to maintain the dew point at 20°C or below. For this reason, the hydrogen concentration is set to 20% or below.

[0106] (Dew point for secondary annealing: -30 to 20°C) If the dew point of the secondary annealing is below -30°C, the surface will not be sufficiently reduced, the plating wettability of the base steel sheet will deteriorate, and the appearance after forming will deteriorate. For this reason, the dew point of the secondary annealing is set to -30°C or higher. On the other hand, if the dew point is above 20°C, condensation will occur in the equipment, hindering its operation. For this reason, the dew point is set to 20°C or lower.

[0107] (Maximum heating temperature for secondary annealing: Ac1-900℃) If the maximum heating temperature of the secondary annealing is less than Ac1°C, the recrystallization of ferrite and the reverse transformation from ferrite to austenite will be insufficient. Therefore, the maximum heating temperature is set to Ac1°C or higher. On the other hand, if the maximum heating temperature exceeds 900°C, the crystal grains will not be densified, and as a result, the desired metal structure fraction and, ultimately, the desired mechanical properties will not be obtained in the final product. Therefore, the maximum heating temperature is set to 900°C or lower.

[0108] (Secondary annealing holding time: 30 to 500 seconds) If the holding time for the secondary annealing is less than 30 seconds, the recrystallization of ferrite and the reverse transformation from ferrite to austenite will be insufficient. Therefore, the holding time is set to 30 seconds or more. On the other hand, if the holding time exceeds 500 seconds, the crystal grains will not be densified, and as a result, the desired metal structure fraction and sufficient strength will not be obtained in the final product. Therefore, the holding time is set to 500 seconds or less.

[0109] (cooling process) The cold-rolled sheet after the second annealing is then cooled to a cooling stop temperature of 450 to 650°C at an average cooling rate of 3 to 30°C / s in the next cooling step. <011> and {332} <113> It is possible to increase the integrated strength of the alloy and achieve a desired metal structure fraction.

[0110] (Average cooling rate: 3~30℃ / sec) If the average cooling rate in the cooling process is too low, austenite will transform into pearlite, resulting in a loss of strength. Therefore, the average cooling rate should be 3°C / s or higher. On the other hand, if the average cooling rate exceeds 30°C / s, a transformation accompanied by new nucleation will occur, resulting in the formation of {211} <011> and {332} <113> Therefore, the average cooling rate is set to 30°C / sec or less.

[0111] (Cooling stop temperature: 450~650℃) If the cooling stop temperature is less than 450°C, reheating to the plating bath and alloying treatment is required, which increases manufacturing costs. Therefore, the cooling stop temperature is set to 450°C or higher. On the other hand, if the cooling stop temperature is higher than 650°C, the amount of ferrite becomes excessive, the area ratio of the second phase decreases, and the strength decreases. Therefore, the cooling stop temperature is set to 650°C or lower.

[0112] (Plating process) (Alloying temperature: 450-600°C) The cold-rolled sheet after the cooling step is optionally subjected to alloying treatment in the next plating step. When the plating step is performed, if the alloying treatment temperature is less than 450°C, interdiffusion of elements between the steel material and the molten metal is difficult to occur, and alloying does not proceed sufficiently. For this reason, the alloying treatment temperature is set to 450°C or higher. On the other hand, if the alloying treatment temperature is higher than 600°C, alloying proceeds excessively, and plating peeling may occur due to embrittlement of the plating layer. For this reason, the alloying treatment temperature is set to 600°C or lower. The alloying treatment temperature is preferably 460 to 550°C.

[0113] (Alloying treatment time: 10 to 1000 seconds) If the alloying treatment time in the plating step is less than 10 seconds, the alloying treatment cannot be stably achieved. Therefore, the alloying treatment time is set to 10 seconds or more. On the other hand, if the alloying treatment time exceeds 1000 seconds, productivity decreases. Therefore, the alloying treatment time is set to 1000 seconds or less.

[0114] (Skin pass rolling process) (Skin pass rolling ratio: 0.5 to 3.0%) The cold-rolled sheet after the cooling step or the plated steel sheet after the optional plating step is finally subjected to skin-pass rolling in the skin-pass step. If the skin-pass rolling ratio is less than 0.5%, the surface of the cold-rolled sheet after the cooling step or the plated steel sheet after alloying by plating cannot be sufficiently smoothed. As a result, the desired Sa cannot be achieved in the final product, and the appearance after forming deteriorates. For this reason, the skin-pass rolling ratio is set to 0.5% or more. On the other hand, if the skin-pass rolling ratio exceeds 3.0%, the ductility may decrease. For this reason, the skin-pass rolling ratio is set to 3.0% or less. The skin-pass rolling ratio is preferably 0.6 to 1.5%.

[0115] According to the above manufacturing method, the steel sheet according to the embodiment of the present invention can be obtained.

[0116] The present invention is not limited to the above-described embodiments or the examples described below, and appropriate combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention.

[0117] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0118] In the following examples, a steel sheet according to one embodiment of the present invention (i.e., a steel sheet of an invention example) and a steel sheet to be compared therewith (i.e., a steel sheet of a comparative example) were produced under various conditions, and the tensile strength, total elongation, deep drawability, and appearance after forming of each of the obtained steel sheets were evaluated.

[0119] First, a slab having the chemical composition shown in Table 1 below was cast by a continuous casting method under conditions of a segment pressing force of 450 tons or more so that the superheat ΔT of the molten steel was the value shown in Table 1. Next, the obtained slab was subjected to a hot rolling process under the conditions shown in Table 2 below. The hot rolling was performed by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all Examples and Comparative Examples, and finish rolling was performed using a tandem rolling mill consisting of six rolling stands. In all Examples and Comparative Examples, the final three stages of finish rolling were performed in a temperature range from the finish rolling end temperature to the finish rolling end temperature + 100°C. Next, the obtained hot-rolled sheet was subjected to a cold rolling process, a first annealing process, a second annealing process, a cooling process, an optional coating process, and a skin-pass rolling process under the conditions shown in Table 2 below, to obtain a cold-rolled sheet or a plated steel sheet having the coating type shown in Table 2 below. In the first annealing step, the steel sheet was heated to the maximum heating temperature shown in Table 2, and then held at a temperature between 600 and Ac1°C for the holding time also shown in Table 2. On the other hand, in the second annealing step, the steel sheet was heated to the maximum heating temperature shown in Table 2, and then held at that heating temperature for the holding time also shown in Table 2.

[0120] [Table 1]

[0121] [Table 2]

[0122] (Steel plate evaluation) The obtained steel sheets were measured for their metal structure, surface properties before forming (i.e., arithmetic mean height Sa before forming), mechanical properties (i.e., tensile strength and total elongation), deep drawability (i.e., r45 value and Δr), and surface properties after forming (i.e., Sa after forming), and each property and appearance after forming were evaluated.

[0123] Steel sheets with a tensile strength of 540 MPa or more, a total elongation of 15.0% or more, an r45 value of 1.40 or more, a Δr of 0.40 or less, and a post-forming Sa of 0.10 to 0.55 μm were evaluated as having improved tensile strength, ductility, and deep drawability, as well as excellent appearance after forming. The results are shown in Table 3 below.

[0124] [Table 3]

[0125] With reference to Tables 1 to 3, in Comparative Example 2, the finish rolling end temperature was low, so a uniform metal structure could not be formed, and the standard deviation of the area ratio of the second phase in the final product exceeded 0.75%. As a result, the appearance after forming was poor. In Comparative Example 3, the coiling temperature was high, so the {223} <252> It is thought that the texture could not be fully developed, and coarse ferrite and other particles were generated in the hot-rolled sheet structure, resulting in a large heterogeneity in the structure of the final product. <011> The accumulated strength of the second phase could not be sufficiently increased, and the standard deviation of the area ratio of the second phase exceeded 0.75%, resulting in poor deep drawability and poor appearance after forming. In Comparative Example 4, the cold rolling strain could not be sufficiently accumulated due to the low cold rolling reduction, and it is believed that the uniformity of the structure could not be improved by grain refinement in the subsequent annealing process. As a result, the {332} <113> The accumulated strength of the Mn alloy could not be sufficiently increased, resulting in poor deep drawability and poor appearance after forming. In Comparative Example 6, the superheat ΔT in the casting process was low, which is thought to be why the solidification structure during casting did not become columnar, and local Mn enrichment could not be sufficiently suppressed. As a result, the standard deviation of the area ratio of the second phase exceeded 0.75%, resulting in poor appearance after forming. In Comparative Example 8, the high cold rolling reduction is thought to have caused discontinuous recrystallization, resulting in randomized texture. As a result, the {211} <011> The accumulated strength of the steel sheet could not be sufficiently increased, and the standard deviation of the area ratio of the second phase exceeded 0.75%, resulting in poor deep drawability and poor appearance after forming. In Comparative Example 10, the dew point in the second annealing step was low, which presumably led to insufficient reduction of the steel sheet surface and poor plating wettability of the steel sheet. As a result, the desired arithmetic mean height Sa was not obtained in the final product, resulting in poor appearance after forming. In Comparative Example 11, the holding time in the first annealing step was long, which presumably led to Mn concentration in the carbides, preventing the carbides from dissolving during annealing and preventing the second phase from being uniformly formed in the subsequent second annealing step. As a result, the standard deviation of the area ratio of the second phase exceeded 0.75% in the final product, resulting in poor appearance after forming.

[0126] In Comparative Example 12, the total reduction in the final three stages of finish rolling was low, so the {223} <252> It is thought that the texture could not be fully developed. <011> In Comparative Example 14, the effective rolling index in the hot rolling process was low, so the {223} <252> It is thought that the texture could not be fully developed. <011> In Comparative Example 15, the average cooling rate in the hot rolling process was slow, so the {223} <252> It is thought that the texture could not be fully developed. <011> In Comparative Example 17, the average heating rate in the first annealing step was too fast, which is thought to have caused the carbides to form first, resulting in randomized recrystallization. As a result, the {211} <011> and {332} <113> In Comparative Example 18, the maximum heating temperature in the first annealing step was low, which is thought to be why recrystallization did not proceed sufficiently. As a result, the {211} <011> In Comparative Example 19, the maximum heating temperature in the first annealing step was high, which is thought to have led to the dissolution of carbides and the occurrence of austenite transformation, leading to the randomization of the texture. As a result, the {211} <011> and {332} <113> In Comparative Example 20, the holding time in the first annealing step was too short, which is thought to be why fine carbides with a low Mn concentration could not be precipitated after recrystallization. As a result, the {211} <011> The integrated strength of the steel sheet decreased, and the deep drawability also decreased.

[0127] In Comparative Example 23, the average cooling rate in the cooling process was fast, so the {211} <011> and {332} <113> In Comparative Example 24, the desired arithmetic mean height Sa was not obtained in the final product due to the low skin-pass rolling ratio, and the appearance after forming was poor. In Comparative Example 25, it is believed that the maximum heating temperature in the secondary annealing step was high, making it impossible to densify the crystal grains. As a result, the area ratio of the second phase in the final product increased, and ductility was poor. In Comparative Examples 26 and 28, it is believed that the high C and Mn contents, respectively, inhibited the diffusion of Mn during solidification, making it impossible to sufficiently suppress the microsegregation of Mn. As a result, the appearance after forming was poor. In Comparative Examples 27 and 29, it is believed that the low Si and Al contents, respectively, inhibited the diffusion of Mn during solidification, making it impossible to sufficiently suppress the microsegregation of Mn. As a result, the appearance after forming was also poor. In Comparative Examples 30 to 32, the low B, Ti, and Nb contents, respectively, made it impossible to sufficiently suppress the {223} <252> It is thought that the texture could not be fully developed. <011> and / or {332} <113> The integrated strength of the sheet could not be increased sufficiently, and the deep drawability was reduced.

[0128] In contrast, in all of the steel sheets according to the examples of the present invention, by having a predetermined chemical composition and metal structure and further by appropriately controlling the arithmetic surface height Sa, it was possible to achieve a tensile strength of 540 MPa or more and a total elongation of 15.0% or more, and to provide steel sheets that not only had improved tensile strength, ductility, and deep drawability, but also had excellent appearance after forming.

Claims

1. The chemical composition, in mass%, is C: 0.03-0.12%, Si: 0.005-1.500%, Mn: 1.0-2.8%, P: 0.100% or less, S: 0.020% or less, N: 0.010% or less, Al: 0.005-0.700%, O: 0.010% or less, Cr: 0.10-0.50%, Mo: 0.05-0.30%, B: 0.0005-0.005%, Ti: 0.01 to 0.25%, Nb: 0.01-0.40%, V: 0 to 0.15%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, W: 0 to 0.15%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Ca: 0-0.005%, Mg: 0 to 0.0050%, Zr: 0 to 0.015%, Te: 0 to 0.0100%, REM: 0 to 0.010%, and The balance is Fe and impurities. In the thickness cross section in a direction inclined at 45 degrees to the rolling direction, The area ratio of ferrite is 70 to 97%, The area ratio of the second phase is 3 to 30%, the standard deviation of the area ratio of the second phase is 0.75% or less, In the Φ2=45° cross section of the ODF, the metal structure has an accumulation intensity of {211}<011> of 4.0 or more and an accumulation intensity of {332}<113> of 4.0 or more, A steel sheet characterized in that the arithmetic mean height Sa of the surface is 0.10 to 0.50 μm.

2. The chemical composition is, in mass %, V: 0.001 to 0.15%, Ni: 0.001 to 0.50%, Cu: 0.001 to 0.50%, W: 0.001 to 0.15%, Sn: 0.001 to 0.10%, Sb: 0.001 to 0.10%, Ca: 0.0001-0.005%, Mg: 0.0001 to 0.0050%, Zr: 0.0001 to 0.015%, Te: 0.0001 to 0.0100%, and REM: 0.0001-0.010% The steel sheet according to claim 1, comprising at least one of the following:

3. 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 second phase has an average grain size of 1.0 to 5.0 μm.

4. The steel plate according to claim 1 or 2, wherein the second phase is at least one of martensite, bainite, tempered martensite, pearlite, and retained austenite.

5. An outer panel member comprising the steel plate according to claim 1 or 2.

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