Steel sheet and outer sheet member

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

Steel plates and outer panel components

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

[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 heterogeneous structure, making the steel sheet surface prone to unevenness after forming. The occurrence of such unevenness poses a problem of reduced appearance, i.e., reduced surface quality.

[0004] Regarding formability, panel parts for outer car bodies include processing elements such as corners (corner edges) and door handle embossed portions that require drawing, 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 of the steel sheet to ¼ 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] Patent Document 3 discloses a steel sheet having excellent stretch formability, in which the rolling conditions after solidification and the cooling conditions after hot rolling are controlled to have an accumulation degree of ferrite (111)<112> orientation of 3.0 or more and an accumulation degree of martensite and tempered martensite (252)<2-11> orientation 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.

[0009] JP 2009-108364 A JP 2013-64193 A International Publication No. 2020 / 203159 International Publication No. 2021 / 205943

[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 when such unevenness occurs, there is 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-type parts of vehicle body exterior panels. While the above-mentioned Patent Documents 1 to 4 specifically consider 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.

[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%, is: C: 0.03 to 0.12%, Si: 0.005 to 1.500%, Mn: 1.0 to 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 to 0.50%, Mo: 0.05 to 0.30%, B: 0.0005 to 0.005%, Ti: 0.01 to 0.25%, Nb: 0.01 to 0.40%, V: 0 to 0.15%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, W: 0 to 0.15%, the metal structure comprising 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 to 0.0100%, REM: 0 to 0.010%, and the balance: Fe and impurities, wherein in a thickness cross section in a direction inclined at 45 degrees to the rolling direction, an area fraction of ferrite is 70 to 97%, an area fraction of a second phase is 3 to 30%, and a standard deviation of the area fraction of the second phase is 0.75% or less, and in a Φ2=45° cross section of the ODF, the metal structure has a cluster intensity of {211}<011> of 4.0 or more and a cluster intensity of {332}<113> of 4.0 or more, (2) A steel sheet according to (1), characterized in that the chemical composition includes, in mass %, at least one of 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 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%. (3) The steel sheet according to (1) or (2) above, characterized in that the average grain size of the ferrite is 5.0 to 30.0 μm, and the average grain size of the second phase is 1.0 to 5.0 μm.(4) The steel plate according to any one of (1) to (3) above, characterized in that 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.

[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.

[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 Sheet> A steel sheet according to an embodiment of the present invention has a chemical composition, in mass %, of C: 0.03 to 0.12%, Si: 0.005 to 1.500%, Mn: 1.0 to 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 to 0.50%, Mo: 0.05 to 0.30%, B: 0.0005 to 0.005%, Ti: 0.01 to 0.25%, Nb: 0.01 to 0.40%, V: 0 to 0.15%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, and W. : 0 to 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 to 0.0100%, REM: 0 to 0.010%, and the balance: Fe and impurities, and in a thickness cross section in a direction inclined at 45 degrees to the rolling direction, the steel sheet has a metallographic structure in which the area fraction of ferrite is 70 to 97%, the area fraction of a second phase is 3 to 30%, and the standard deviation of the area fraction of the second phase is 0.75% or less, and in a Φ2=45° cross section of the ODF, the steel sheet has a metallographic structure in which the cluster intensity of {211}<011> is 4.0 or more and the cluster intensity of {332}<113> is 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, as with frame components, there is a demand for higher strength in these exterior panel components. On the other hand, dual phase steel (DP steel), which has a relatively low yield strength, is often used for these exterior panel components in order to avoid surface defects known as surface distortions that occur during press forming and other processes. However, in the case of DP steel, which contains a mixture of a soft phase consisting of ferrite and a hard phase consisting of martensite, etc., non-uniform deformation is likely to occur during processing such as press forming, in which the soft phase and its surroundings deform preferentially. This can lead to minute irregularities on the surface of the steel sheet after forming, resulting in poor appearance. On the other hand, as the strength of steel sheets increases, elements such as Mn are sometimes added in relatively large amounts 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 central segregation and microsegregation, are formed during casting, and these enriched regions are elongated in the rolling direction by hot rolling or cold rolling, resulting in streaky segregation of Mn. This Mn segregation results in the presence of regions with high and low hardenability 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 present inventors conducted research focusing on both the chemical composition and metallographic structure of the steel sheet in order to satisfy these properties. First, the present inventors discovered 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 terms of 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] Furthermore, outer plate components generally have a large rectangular shape. Therefore, from the perspective of ensuring material yield, components 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, since 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. Here, a high r-value generally indicates excellent deep drawability, since a small change in thickness and a large dimensional change in the direction 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, small thickness variation at each location of the component is required, 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. In other words, it is particularly important to reduce the anisotropy of the r-value (hereinafter referred to as the "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. Therefore, the present inventors have investigated improvements in deep drawability and post-forming appearance from this perspective. As a result, the present inventors have found that by developing a specific texture in the above-mentioned metallographic structure mainly composed of ferrite, more specifically, by controlling the clustering intensity of {211}<011> to 4.0 or more and similarly controlling the clustering intensity of {332}<113> to 4.0 or more in the Φ2=45° cross section of the ODF (crystal orientation distribution function) of the metallographic structure, a higher r45 value can be obtained and the Δr value can be sufficiently reduced, thereby significantly improving deep drawability and improving the post-forming appearance.

[0020] The present inventors conducted studies from the perspectives of reducing Mn segregation, reducing the variation in the second phase (hard phase) in the metallographic structure, and improving surface properties in order to further improve the appearance after forming. 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 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 condition, which is flatter than 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, it is believed that 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 that the irregularities present before forming and the minute irregularities that occur on the steel sheet surface after forming cancel each other out, resulting in the 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 a component 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 the 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, if C is contained excessively, the diffusion of Mn during solidification may be hindered, and microsegregation of Mn may not be sufficiently suppressed. 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 for increasing the strength of steel sheet without impairing its ductility. Si is also an element effective for promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully obtain this effect, 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, etc., 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, if Mn is contained excessively, the diffusion of Mn during solidification may be inhibited, and microsegregation of Mn may not be sufficiently suppressed. 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 an element that 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 a decrease in productivity. Therefore, the P content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more. On the other hand, if P is contained excessively, the toughness of the steel plate may decrease. 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 a decrease in 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 is an element that functions as a deoxidizer and is a solid-solution strengthening element that is effective in increasing the strength of steel. Al is also an element that is effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully obtain 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 plate. It is also an element that is effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully obtain 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 form coarse Cr carbides that serve as fracture initiation points. 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 the steel sheet, and is also an element that is effective in promoting the diffusion of Mn 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 has the effect of suppressing recrystallization and coarsening of austenite, promoting flattening, and making it easier to obtain the {223}<252> orientation in the hot-rolled sheet. B is also an element that has the effect of increasing the recrystallization temperature during annealing and suppressing randomization of the texture. To fully obtain these effects, 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, excessive B content may cause the formation of B precipitates, 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 is an element that precipitates as carbides in the hot-rolled sheet structure, reduces solute carbon, facilitates obtaining a {211}<011> orientation in the cold-rolled steel sheet, and contributes to improving the r45 value and Δr. Ti also has the effect of suppressing austenite recrystallization and coarsening, promoting austenite flattening in the hot-rolling process, and facilitating obtaining a {223}<252> orientation in the hot-rolled sheet. Ti is also a precipitation strengthening element that has the effect of refining the structure and improving the strength-formability balance of the steel sheet. To fully obtain these effects, 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 result in the formation of coarse Ti sulfides, Ti nitrides, and / or Ti oxides, which may reduce the formability of the steel sheet. Therefore, the Ti content is 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. Furthermore, Nb promotes the flattening of austenite in the hot rolling process, making it easier to obtain the {223}<252> orientation of the hot-rolled sheet, and suppresses recrystallization and texture randomization during annealing. To fully achieve 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 the formability of the steel sheet. Therefore, 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 to 0.15%) V is an element that contributes to improving the strength of the steel sheet due to strengthening by precipitates, grain refinement strengthening by suppressing ferrite grain growth, and / or dislocation strengthening by suppressing recrystallization. The V content may be 0%, but in order to obtain the above effect, 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 cause a large amount of carbonitrides to precipitate, thereby reducing 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 the steel sheet. The Ni content may be 0%, but in order to obtain the above effect, 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 an element that exists in steel in the form of fine particles and contributes to improving the strength of the steel sheet. The Cu content may be 0%, but in order to obtain the above effect, 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 to 0.15%) W is an element that has the effect of suppressing recrystallization and coarsening of austenite, promoting flattening, and making it easier to obtain the {223}<252> orientation of the hot-rolled sheet. The W content may be 0%, but in order to obtain the above effects, 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 in order 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 coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sb content may be 0%, but in order 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. For this reason, 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 desirably 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 desirably 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 numbers 57 to lutetium (Lu) with atomic numbers 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 to 97%) The metal structure of the steel sheet contains, in area %, 70 to 97% ferrite and 3 to 30% second phase, for example, composed only of 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 strength-ductility balance, the ferrite area ratio is preferably 70 to 90%.

[0051] (Second phase area ratio: 3 to 30%) If the area ratio of the second phase, which is a hard phase, is less than 3%, the desired strength cannot be obtained. Therefore, the area ratio of the second phase is set to 3% or more. From the viewpoint of improving strength, a higher area ratio 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 ratio of the second phase is set to 30% or less. The area ratio 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 area ratio of second phase in a direction inclined at 45 degrees relative 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 deformation amount 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. For this reason, 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 the steel sheet is unclear, the following method is used, for example, to identify the rolling direction of the steel sheet. After mirror-polishing the 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 as an axis is observed by the above method. The average length of the major axes of the inclusions in each cross section is calculated, and the cross section with the largest average length of the major axes of the inclusions is identified. The direction parallel to the major axes of the inclusions in that cross section is determined to be the rolling direction.

[0054] (Intensity of {211}<011> and {332}<113> Clusters in the Φ2=45° Cross Section of the ODF: 4.0 or More) In the Φ2=45° cross section of the ODF (crystal orientation distribution function) of the metallographic structure, if either of the intensities of {211}<011> and {332}<113> clusters is less than 4.0, it cannot be said that the texture is developed, and a high r45 value and a low Δr, for example, an r45 value of 1.40 or more and a Δr of 0.40 or less, cannot be achieved. For this reason, the intensities of {211}<011> and {332}<113> clusters are each set to 4.0 or more. From the viewpoint of improving deep drawability and the appearance after forming, the higher the cluster intensities, the more preferable. For example, the intensities of {211}<011> and {332}<113> clusters may be 5.0 or more, 6.0 or more, 7.0 or more, or 8.0 or more, respectively. There is no particular upper limit to the upper limit, but for example, the accumulation intensities of {211}<011> and {332}<113> may be 18.0 or less, 16.0 or less, or 15.0 or less, respectively.

[0055] (Measurement of Cluster Strength by ODF) Cluster strength measurement 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 FE-SEM (field emission scanning electron microscope), for example, the "JSM-7200F (NEO)" manufactured by JEOL Ltd. (accelerating voltage: 15 kV), and as the EBSD detector, for example, the "Velocity" manufactured by TSL Solutions, can be used. Based on the obtained crystal orientation data, OIM ver., a crystal orientation analysis application from EDAX TEXSEM, can be used. Using 7.3, the accumulation intensities of {211}<011> and {332}<113> are calculated from the ODF (crystal orientation distribution function) of the φ2=45° cross section (Bunge formula) based on the ND axis using the spherical harmonic function expansion method of 16th order expansion. The accumulation intensities are measured in three or more regions spaced 100 μm or more apart, and the arithmetic mean value of the measured values ​​for each measurement is used as the measurement result of the accumulation intensities 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 the 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 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 the hard phase, which is at least one of martensite, bainite, tempered martensite, pearlite, and retained austenite, 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 crystal 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, 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 (or 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 out from a position 100 mm or more away from the end face of the steel sheet, and then, using a laser microscope (for example, Keyence Corporation's "VK-X3000"), the unevenness of the surface of the steel sheet (if a plating layer is present on the surface of the steel sheet, the surface of the plating layer) is measured in an 8 mm square area. Depending on the size of the object to be measured, the test piece may be cut out from a position 10 mm or more away from the end face of the steel sheet. In this case, the measurement magnification is 20 times, the resolution in the XY plane is 5 μm, and the resolution in the Z spatial plane is 0.1 nm, and the measurement is performed in a linked manner. Thereafter, the entire measurement area is 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 determined. Sa is the absolute average of z(x, y) in the reference region (A) defined in JIS B0681-2:2018, 4.1.7 "Arithmetic mean height of the scale limited surface." The arithmetic mean height Sa is measured in three or more regions (8 mm square regions) spaced at intervals of 100 μm or more, and the arithmetic mean value of each measurement is obtained.

[0060] (Thickness) In this embodiment, the thickness of the steel plate is not particularly limited. For example, the steel plate may have a thickness of 0.1 to 2.0 mm. Steel plates having such thicknesses are suitable for use as materials for covering members such as doors and hoods. The thickness of the steel plate may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the thickness of the steel plate may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. For example, by making the thickness of the steel plate 0.2 mm or more, it becomes easier to maintain the shape of the molded product flat, and additional effects such as improved dimensional accuracy and shape accuracy can be obtained. On the other hand, by making the thickness 1.0 mm or less, the weight reduction effect of the member is significant. The thickness of the steel plate is measured using a micrometer.

[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 hot-dip 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 sample is taken with a thickness cross section at 45° to the rolling direction of the steel sheet as the observation surface, and then the observation surface is polished and etched with nital. Next, secondary electron images of the steel sheet structure are taken and linked using an FE-SEM (e.g., a JSM-7200F (NEO) manufactured by JEOL Ltd., accelerating voltage: 15 kV) at a magnification of 600x and a resolution of 1280 x 960 pixels. The white areas (relatively bright contrast areas) of the obtained image data represent the structure of the second phase (martensite, bainite, tempered martensite, pearlite, and retained austenite), and the black areas (relatively dark contrast areas) represent ferrite. The image is then analyzed in a field of view of the total sheet thickness x 5 mm to determine the area ratios of ferrite and second phase. The white and black areas can be determined, for example, using image analysis software ImageJ. For each pixel of an SEM secondary electron image, the binarization threshold, which indicates black if the structure is ferrite and white if it is not, is determined using a method that employs the average brightness value as the threshold, as described in "Glasbey, CA (1993), "Analysis of histogram-based thresholding algorithms", CVGIP: Graphical Models and Image Processing 55:532-537." This algorithm is implemented in ImageJ, and binarization is performed automatically by using the Auto threshold function and setting the threshold determination method to Method = Mean. That is, the binarization threshold is automatically determined from a histogram after smoothing by replacing each pixel value with the average of pixel values ​​within a radius of 15 pixels from the pixel of interest center in ImageJ with Method = Mean and radius = 15. Image analysis is performed in three or more visual fields spaced 100 μm or more apart, and the arithmetic mean of the measured values ​​in each visual field is taken as the area ratio of ferrite and the second phase in the observation surface.

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

[0066] (Method for measuring the average crystal grain size of the second phase) From the combined SEM image measured above, image analysis is performed in a field of view of the total plate thickness × 5 mm to calculate the number of second phase (hard phase) particles. 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 in three or more fields, each spaced 100 μm apart, and the arithmetic mean of the values ​​measured in each field 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 the steel sheet having the above-mentioned specific chemical composition and metallographic 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 press-forming the steel sheet.

[0069] (Total elongation) From the viewpoint of improving formability such as ductility, the total elongation of the steel sheet is preferably 16.0% or more, and more preferably 18.0% or more. 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 being perpendicular to the rolling direction. Next, a tensile test in accordance with JIS Z 2241:2022 is performed using this test piece to measure 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 Properties After Forming) 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. Therefore, the steel sheet of this embodiment can obtain an excellent post-forming appearance.

[0072] (Post-forming Sa: 0.10 to 0.55 μm) As described above, the steel sheet of this embodiment can have surface characteristics such that post-forming Sa is 0.10 to 0.55 μm. Post-forming Sa is the average value of the height difference (absolute value) of each point relative to the average surface 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 JIS B0681-2:2018, 4.1.7 "Arithmetic mean height of the scale limited surface." If post-forming Sa is 0.55 μm or less, the post-forming appearance will be excellent. Note that 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 No. 5 tensile test piece according to JIS Z 2241:2022 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 Z 2241:2022. Then, using a laser microscope (e.g., Keyence Corporation'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 space 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 intervals of 100 μm or more, and the arithmetic mean value of each measurement is obtained. The arithmetic mean height Sa thus obtained is referred to as 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 of measuring r45 value and Δr) The r45 value and Δr, which are plastic strain ratios, were measured in accordance with the provisions of JIS Z 2254: 2008. Test specimens were taken from three or more locations spaced at intervals of 100 μm or more, and the arithmetic mean values ​​of the measured values ​​for each test specimen were used as the measurement results for the r45 value and Δr of the steel plate to be 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 an 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 where these properties are required. 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, only need to include the steel sheet according to the embodiment of the present invention in at least a portion of the exterior panel member, and therefore, at least a portion of these exterior panel members will satisfy the chemical composition and metallographic characteristics described above. In portions of the steel sheet that are relatively lightly processed in forming, such as press forming, the metallographic characteristics do not change significantly before and after forming.

[0077] <Method for manufacturing steel sheet> 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, 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 so as to obtain the specific metal structure described above. Preferred conditions for these processes are described below.

[0079] (Casting Process) (Superheat ΔT: 25°C or More, and Segment Pressing Force: 450 Tons or More) In the casting process, from the viewpoint of productivity, it is preferable to cast a slab by a continuous casting method. 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 specific chemical composition to 25°C or more and further setting the segment pressing force to 450 tons or more, the solidification structure can be controlled to a columnar crystal structure with an equiaxed crystal fraction of 15% or less and the central segregation of Mn can also be suppressed. A columnar crystal structure with an equiaxed crystal fraction of 15% or less is effective in suppressing localized Mn enrichment. 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 to 1300°C) 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. Furthermore, if the temperature exceeds 1300°C, the life of the heating furnace may be shortened. For this reason, the heating temperature is set to 1300°C or lower.

[0081] (Rough rolling) In the present 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 of 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 to 100%) The heated slab, or the slab that has been subjected to rough rolling as needed, is then subjected to finish rolling. Finish rolling is performed using a tandem rolling mill consisting of multiple rolling stands, for example, three or more, preferably four to six, rolling stands. The total reduction in the final three stages of finish rolling is set to 40% or more in order to develop the {223}<252> orientation in the hot-rolled sheet and thereby increase the integrated strength of {211}<011> and {332}<113> in the final product. The {223}<252> orientation in the hot-rolled sheet is the crystal orientation that becomes the nucleus of {211}<011> during the subsequent introduction of shear bands by cold rolling and recrystallization by annealing. Therefore, by developing the {223}<252> orientation in the hot-rolled sheet, it is possible to increase the concentration of {211}<011> and other orientations in the final product after cold rolling and annealing. 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 performed in a temperature range from the finish rolling end temperature to the finish rolling end temperature + 100°C.

[0083] (Effective rolling index in the final three stages of finish rolling: 1.2 or more) If austenite recrystallization progresses excessively between passes during hot rolling, the texture integration weakens and the crystal grains become equiaxed, making it impossible to obtain a hot-rolled sheet with the desired texture. That is, in order to develop the {223}<252> orientation in the hot-rolled sheet, it is preferable to develop the austenite texture. Therefore, in the present manufacturing method, as described 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. In order to develop the {223}<252> texture in the hot-rolled sheet, 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 effective rolling index allows for better development of the 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 ferrite after cooling.As a result of further investigation, it was found that the combination for flattening the crystal grains while enhancing the texture in the final three stages of finish rolling can be determined by the following method.

[0085] First, among the final three rolling stands, attention is focused on the rolling stand (F1) that performs rolling first. From the relationship between the Ti content (mass%) contained in the steel, the rolling strain at F1, and the finish rolling end temperature FT (°C), dT is determined, which is a value related to the time it takes for 50% of austenite to recrystallize after F1 rolling. eff-p Ask for. Here, t ini is the plate thickness at the F1 entry side (mm), and t F1 is the plate thickness (mm) after F1 rolling. Ti is the Ti content (mass%) contained in the steel.

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

[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.

[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-aWhen the value 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 from the following formula (vi). Here, t F3 is the plate thickness (mm) after F3 rolling.

[0089] On the other hand, dT eff-s-a When the rolling strain of F3 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 from the following formula (vii).

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

[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 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). Here, W Mo , W V , W W , W Zr and W Nb are the contents (mass%) of Mo, V, W, Zr, and Nb contained in the steel, respectively, and are substituted with 0 if no Mo, V, W, Zr, or Nb is contained.

[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 completion of finish rolling: 0.5 to 2.0 seconds) After the completion of finish rolling, cooling must be performed before the recrystallization of austenite is completed. 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 are generated in the hot-rolled sheet structure, which increases the structural non-uniformity of the final product and deteriorates the appearance after forming. For this reason, the cooling start time after completion of finish rolling is set to 0.5 seconds or more. On the other hand, if it exceeds 2.0 seconds, cooling will be performed after the recrystallization of austenite is completed after the completion of finish rolling. For this reason, in order to ensure that cooling is performed before the recrystallization of austenite is completed, the cooling start time after completion of finish rolling is set to 2.0 seconds or less.

[0095] (Average Cooling Rate After Finish Rolling: 15 to 200°C / s) If the average cooling rate after finish rolling is less than 15°C / s, the {223}<252> texture of the hot-rolled sheet cannot be sufficiently developed, and as a result, the integrated strength of {211}<011> and / or {332}<113> in the final product cannot be sufficiently increased. Therefore, the average cooling rate is set to 15°C / s or more. On the other hand, if the average cooling rate exceeds 200°C / s, the strength of the hot-rolled sheet becomes excessively high, which results in 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 of finish rolling: 800 to 950°C) In the hot rolling process, the finishing temperature of finish rolling is preferably 800°C or higher. When the finishing 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 in this regard, a higher quality post-forming appearance can be obtained. On the other hand, although there is no particular upper limit for the finishing temperature of finish rolling, from an economical viewpoint, the finishing temperature of finish rolling is preferably 950°C or lower.

[0097] (Coiling temperature of hot-rolled coil: 300 to 600°C) If the coiling temperature of the hot-rolled coil is less than 300°C, the strength of the hot-rolled sheet becomes too high, resulting in a high load during cold rolling after pickling. Therefore, the coiling temperature is set to 300°C or higher. Furthermore, if the coiling temperature exceeds 600°C, the {223}<252> texture of the hot-rolled sheet cannot be sufficiently developed, and as a result, the integrated strength of {211}<011> and / or {332}<113> in the final product cannot be sufficiently increased. Furthermore, coarse ferrite and pearlite are generated in the hot-rolled sheet structure, which increases the structural heterogeneity of the final product and deteriorates the appearance after forming. Therefore, the coiling temperature is set to 600°C or lower.

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

[0099] (Primary Annealing Step) In the subsequent primary annealing step, the obtained cold-rolled sheet is heated to a maximum heating temperature of 600 to Ac1°C at an average heating rate of 0.01 to 5.0°C / s and held at 600 to Ac1°C for 5 to 60 minutes. By carrying out such a treatment, it is possible to precipitate fine carbides with a low Mn concentration and therefore relatively easily dissolve after recrystallization, thereby developing a {211}<011> texture and reducing the standard deviation of the area fraction of the second phase (hard phase) after secondary annealing.

[0100] (Average heating rate in primary annealing: 0.01 to 5.0°C / second) From the viewpoint of productivity, the average heating rate in primary annealing is set to 0.01°C / second or more. On the other hand, if the average heating rate exceeds 5.0°C / second, carbides are generated first, which randomizes the recrystallized structure, reduces the integration strength of {211}<011> and the like, and deteriorates the r45 value and / or Δr. For this reason, the average heating rate is set to 5.0°C / second or less.

[0101] (Maximum heating temperature of primary annealing: 600 to Ac1°C) If the maximum heating temperature of primary annealing is less than 600°C, recrystallization does not proceed sufficiently, and the formation of the {211}<011> texture of ferrite, which is effective in reducing Δr, 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, austenite transformation occurs, and the texture becomes randomized, resulting in a deterioration of 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 (° C.)=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] (Holding time of primary annealing: 5 to 60 minutes) If the holding time of primary annealing is less than 5 minutes, fine carbides with a low Mn concentration will not be precipitated after recrystallization, and the {211}<011> texture will not be developed, and as a result, Δr and other parameters may not be sufficiently reduced. 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 be concentrated in the carbides, and the carbides will not be able to dissolve during annealing, making it impossible to uniformly generate a second phase in the subsequent secondary annealing step. As a result, it will be impossible 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 Step) In the subsequent secondary annealing step, the cold-rolled sheet that has been primarily annealed is heated to a maximum heating temperature of Ac1 to 900°C at an average heating rate of 2 to 30°C / second in an atmosphere with a dew point of -30 to 20°C and a hydrogen concentration of 2 to 20% (nitrogen balance), and is held at this maximum heating temperature for 30 to 500 seconds. By performing such treatment, it is possible to appropriately promote the recrystallization of ferrite and the reverse transformation from ferrite to austenite, thereby densifying the crystal grains and sufficiently reducing the steel sheet surface, thereby making it possible to obtain the desired metal structure fraction and texture integration strength in the final product, as well as achieving an excellent appearance.

[0104] (Average heating rate in secondary annealing: 2 to 30°C / sec) If the average heating rate in secondary annealing is less than 2°C / sec, the austenite produced by reverse transformation will become coarse, the texture will become random, and the structural non-uniformity of the final product will increase, deteriorating the r45 value and the appearance after forming. For this reason, the average heating rate is set to 2°C / sec or more. On the other hand, if the average heating rate exceeds 30°C / sec, condensation will occur in the equipment, hindering its operation. For this reason, the average heating rate is set to 30°C / sec 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 less. For this reason, the hydrogen concentration is set to 20% or less.

[0106] (Dew point of secondary annealing: -30 to 20°C) If the dew point of secondary annealing is less than -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 secondary annealing is set to -30°C or higher. On the other hand, if the dew point is higher than 20°C, condensation will occur on the equipment, hindering its operation. For this reason, the dew point is set to 20°C or lower.

[0107] (Maximum heating temperature of secondary annealing: Ac1 to 900°C) If the maximum heating temperature of secondary annealing is less than Ac1°C, the recrystallization of ferrite and the reverse transformation from ferrite to austenite will be insufficient. For this reason, the maximum heating temperature is set to Ac1°C or higher. On the other hand, if the maximum heating temperature exceeds 900°C, it will be impossible to densify the crystal grains, and as a result, it will be impossible to obtain the desired metal structure fraction and, ultimately, the desired mechanical properties in the final product. For this reason, the maximum heating temperature is set to 900°C or lower.

[0108] (Secondary annealing holding time: 30 to 500 seconds) If the secondary annealing holding time 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 step) The cold-rolled sheet that has been subjected to the second annealing is then cooled at an average cooling rate of 3 to 30° C. / sec to a cooling stop temperature of 450 to 650° C. By performing cooling under these conditions, it becomes possible to increase the integration strength of {211}<011> and {332}<113> and to achieve a desired metal structure fraction.

[0110] (Average cooling rate: 3 to 30°C / sec) If the average cooling rate in the cooling step is too low, austenite will transform into pearlite, resulting in a loss of strength. For this reason, the average cooling rate is set to 3°C / sec or more. On the other hand, if the average cooling rate exceeds 30°C / sec, a transformation accompanied by new nucleation will occur, resulting in insufficient accumulation of {211}<011> and {332}<113>. For this reason, the average cooling rate is set to 30°C / sec or less.

[0111] (Cooling stop temperature: 450 to 650°C) If the cooling stop temperature is less than 450°C, reheating to the plating bath and alloying treatment will be necessary, increasing production costs. For this reason, 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 will be excessive, the area ratio of the second phase will decrease, and the strength will decrease. For this reason, the cooling stop temperature is set to 650°C or lower.

[0112] (Plating step) (Alloying treatment temperature: 450 to 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 unlikely 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 may proceed 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 process or the plated steel sheet after the optional plating process is finally subjected to skin-pass rolling in the skin-pass process. If the skin-pass rolling ratio is less than 0.5%, the surface of the cold-rolled sheet after the cooling process or the plated steel sheet after plating alloying 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 was performed under the same conditions 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 a coating of 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]

[0121]

[0122] (Evaluation of Steel Sheets) For the obtained steel sheets, the 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) of the steel sheets were measured, and each property and the appearance after forming were evaluated.

[0123] Steel sheets having 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]

[0125] Referring to Tables 1 to 3, in Comparative Example 2, the low finish rolling temperature prevented the formation of a uniform metal structure, and the standard deviation of the second phase area ratio in the final product exceeded 0.75%. As a result, the post-forming appearance was poor. In Comparative Example 3, the high coiling temperature prevented the {223}<252> texture from being fully developed in the hot-rolled sheet, and coarse ferrite and other grains were formed in the hot-rolled sheet structure, resulting in increased structural heterogeneity in the final product. In relation to this, the {211}<011> integration strength in the final product could not be sufficiently increased, and the standard deviation of the second phase area ratio exceeded 0.75%, resulting in poor deep drawability and post-forming appearance. In Comparative Example 4, the low cold-rolling reduction ratio prevented the accumulation of cold-rolling strain, and the subsequent annealing process failed to improve structural uniformity through grain refinement. As a result, the integrated strength of {332}<113> in the final product could not be sufficiently increased, and the deep drawability and post-forming appearance were deteriorated. In Comparative Example 6, the superheat ΔT in the casting process was low, so 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 fraction of the second phase exceeded 0.75%, and the post-forming appearance was deteriorated. In Comparative Example 8, the high cold-rolling reduction rate is thought to have caused discontinuous recrystallization, resulting in randomized texture. As a result, the integrated strength of {211}<011> in the final product could not be sufficiently increased, and the standard deviation of the area fraction of the second phase exceeded 0.75%, and the deep drawability and post-forming appearance were deteriorated. In Comparative Example 10, the dew point in the secondary annealing process was low, so the steel sheet surface was not sufficiently reduced, and the plating wettability of the steel sheet was deteriorated. As a result, the desired arithmetic mean height Sa was not obtained in the final product, and the post-forming appearance was deteriorated. In Comparative Example 11, the holding time in the first annealing step was long, which is thought to have caused Mn to concentrate in the carbides, making it impossible to dissolve the carbides during annealing, and therefore preventing the uniform formation of the second phase in the subsequent second annealing step. As a result, the standard deviation of the area ratio of the second phase in the final product exceeded 0.75%, resulting in a poor appearance after forming.

[0126] In Comparative Example 12, the total reduction amount in the final three stages of finish rolling was low, which is thought to have prevented the {223}<252> texture from being sufficiently developed in the hot-rolled sheet. Associated with this, the {211}<011> cluster strength was not sufficiently increased in the final product, resulting in poor deep drawability. In Comparative Example 14, the effective rolling index in the hot-rolling process was low, which is thought to have prevented the {223}<252> texture from being sufficiently developed in the hot-rolled sheet. Associated with this, the {211}<011> cluster strength was not sufficiently increased in the final product, resulting in poor deep drawability. In Comparative Example 15, the average cooling rate in the hot-rolling process was slow, which is thought to have prevented the {223}<252> texture from being sufficiently developed in the hot-rolled sheet. Associated with this, the {211}<011> cluster strength was not sufficiently increased in the final product, resulting in poor deep drawability. In Comparative Example 17, the average heating rate in the first annealing step was high, which is thought to have caused carbides to form first, resulting in randomized recrystallized texture. As a result, the integrated strength of {211}<011> and {332}<113> was reduced in the final product, and deep drawability was impaired. In Comparative Example 18, the maximum heating temperature in the first annealing step was low, which is thought to have caused recrystallization not to progress sufficiently. As a result, the integrated strength of {211}<011> was reduced in the final product, and deep drawability was impaired. In Comparative Example 19, the maximum heating temperature in the first annealing step was high, which is thought to have caused carbides to dissolve and austenite transformation to occur, leading to increased randomization of the texture. As a result, the integrated strength of {211}<011> and {332}<113> was reduced in the final product, and deep drawability was impaired. In Comparative Example 20, it is considered that the holding time in the first annealing step was too short to allow fine carbides with a low Mn concentration to precipitate after recrystallization, resulting in a decrease in the accumulated strength of {211}<011> in the final product and a decrease in deep drawability.

[0127] In Comparative Example 23, the average cooling rate in the cooling step was too fast, resulting in insufficient accumulation of {211}<011> and {332}<113> in the final product, and thus poor deep drawability. In Comparative Example 24, the skin-pass rolling reduction was too low, preventing the desired arithmetic mean height Sa in the final product and resulting in poor appearance after forming. In Comparative Example 25, the maximum heating temperature in the secondary annealing step was too high, presumably preventing grain densification. As a result, the area ratio of the second phase in the final product increased, resulting in poor ductility. In Comparative Examples 26 and 28, the high C and Mn contents, respectively, presumably inhibited Mn diffusion during solidification, thereby preventing sufficient suppression of Mn microsegregation. As a result, poor appearance after forming. In Comparative Examples 27 and 29, the low Si and Al contents, respectively, presumably inhibited Mn diffusion during solidification, preventing sufficient suppression of Mn microsegregation. As a result, similar poor appearance after forming. In Comparative Examples 30 to 32, the low B, Ti, and Nb contents presumably prevented the {223}<252> texture from being fully developed in the hot-rolled steel sheets. As a result, the accumulated strength of {211}<011> and / or {332}<113> in the final products was not sufficiently increased, resulting in poor deep drawability.

[0128] In contrast to this, 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 appropriately controlling the arithmetic surface height Sa, it was possible to provide steel sheets that achieved a tensile strength of 540 MPa or more and a total elongation of 15.0% or more, and that had improved tensile strength, ductility, and deep drawability, as well as 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.