Thin steel sheet with surface coating.

Abstract

Invention details;

Description

Plated steel sheet

[0001] The present invention relates to a plated steel sheet having a plating layer on the surface of a base steel sheet.

[0002] In recent years, the automotive industry has seen a growing need to reduce the weight of not only structural parts such as members, but also exterior panel parts such as roofs, hoods, fenders, doors, etc., in order to improve fuel efficiency, etc. Unlike structural parts, these exterior panel parts are visible to the public, so they are required to have not only strength and other characteristics, but also excellent appearance quality, including design and surface quality.

[0003] In this regard, for example, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet with excellent surface quality. Specifically, the document discloses a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of a steel sheet serving as a substrate, the substrate containing, by mass %, C: 0.02 to 0.20%, Si: 0.7% or less, Mn: 1.5 to 3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1 to 1.0%, N: 0.010% or less, and Cr: 0.03 to 0.5%, the substrate having a surface oxidation index A during annealing defined by the mathematical formula A = 400Al / (4Cr + 3Si + 6Mn) in which the contents of Al, Cr, Si, and Mn are the same as those in the preceding paragraphs, of 2.3 or more, with the balance being Fe and unavoidable impurities, and the structure of the substrate being composed of ferrite and a second phase, the second phase being mainly composed of martensite.

[0004] On the other hand, in relation to the above-mentioned need for weight reduction, steel sheets used in automotive exterior panel parts are required to be even stronger and thinner. Furthermore, as the shapes of exterior panel parts become more complex, unevenness tends to occur more easily on the steel sheet surface after forming. If such unevenness occurs on the steel sheet surface, the appearance quality of the exterior panel part may be reduced. Specifically, for example, dual-phase steel (hereinafter referred to as DP steel) consisting of a soft ferrite (soft phase) and a hard second phase (hard phase) mainly composed of martensite, as disclosed in Patent Document 1, is prone to uneven deformation in which the soft phase and its surroundings deform preferentially during forming, such as press forming. Therefore, when such DP steel is used, minute unevenness may occur on the steel sheet surface after forming, which can cause appearance defects known as ghost lines. Therefore, how to suppress ghost lines in DP steel is an issue.

[0005] Regarding ghost lines, for example, Patent Document 2 discloses a steel sheet that suppresses the occurrence of ghost lines by reducing Mn segregation during steel solidification. Specifically, the steel sheet has a specific chemical composition, a metal structure consisting of 70 to 95% ferrite and 5 to 30% hard phase by area fraction, and a value X1 obtained by dividing the standard deviation in the thickness direction of the average Mn concentration in the rolling direction at a quarter position in the thickness direction by the average Mn concentration at the quarter position in the thickness direction is 0.025 or less. In the steel sheet disclosed in Patent Document 2, large reduction is performed after the steel solidification, thereby reducing Mn segregation, particularly Mn microsegregation at the quarter position in the thickness direction, and reducing the proportion of connected hard phases. As a result, the surface roughness of the steel sheet after forming is said to be improved.

[0006] Patent Document 3 also describes a steel sheet having a specific chemical composition, a metal structure consisting of ferrite with a volume fraction of 70 to 95% and a hard phase with a volume fraction of 5 to 30%, and a Vickers hardness H １／４ The standard deviation of the Vickers hardness H １／４ The value X1 divided by the average value of １／２ The standard deviation of the Vickers hardness H １／２The steel sheet disclosed in Patent Document 3 has a value X2 of 0.030 or less, which is calculated by dividing the average value of X by the average value of X. The steel sheet disclosed in Patent Document 3 is said to be capable of realizing excellent appearance quality in formed products.

[0007] Patent Document 4 describes a panel having a steel plate containing martensite, in which the surface roughness parameter (Sa, where the low-pass filter λs is 0.8 mm and wavelength components of 0.8 mm or less are removed) in the flat part of the center part of the panel is Sa≦0.500 μm, and in the lath of martensite, precipitates with a major axis of 0.05 μm to 1.00 μm and an aspect ratio of 3 or more are formed at a rate of 15 / μm. ２ The yield stress YS measured using a tensile test piece cut out from the flat part １ and the yield stress YS measured on a tensile test specimen cut from the edge of the panel. ２ Relative to YS １ / YS ２ The panel disclosed in Patent Document 4 is said to have an excellent appearance after being molded from a material and to have excellent dent resistance.

[0008] Furthermore, Patent Document 5 discloses a steel sheet having a specific chemical composition, in which the metal structure of a surface region ranging from the surface to a position 20 μm from the surface in the thickness direction is composed of ferrite and a second phase at a volume fraction of 0.01 to 5.0%, and the metal structure of an internal region ranging from a position more than 20 μm from the surface in the thickness direction to a position ¼ of the thickness from the surface in the thickness direction is composed of ferrite and a second phase at a volume fraction of 2.0 to 10.0%, the volume fraction of the second phase in the surface region is smaller than the volume fraction of the second phase in the internal region, the average crystal grain size of the second phase in the surface region is 0.01 to 4.0 μm, and X, which is the intensity ratio between the {001} orientation and the {111} orientation of ferrite, ＯＤＦ｛００１｝／｛１１１｝ Patent Document 5 discloses a steel sheet characterized by including a texture in which the σ is 0.60 or more and less than 2.00. The steel sheet disclosed in Patent Document 5 is said to be capable of suppressing the occurrence of surface irregularities during forming.

[0009] JP 2005-220430 A International Publication No. WO 2022 / 181761 International Publication No. WO 2022 / 254847 International Publication No. WO 2021 / 149810 International Publication No. WO 2020 / 145256

[0010] The techniques disclosed in Patent Documents 2 to 5 are said to be capable of improving the surface roughness and appearance of steel sheets after forming.

[0011] Therefore, an object of the present invention is to provide a plated steel sheet having excellent strength and elongation and an improved appearance after forming, by using a novel structure.

[0012] To achieve the above object, the present inventors conducted extensive research into methods for reducing Mn segregation as well as methods for further improving ghost lines. As a result, they found that even if Mn segregation is reduced, if the segregated portions are not rolled uniformly on the front and back sides of the steel sheet in the hot rolling process, the segregated portions will be unevenly distributed on the front and back sides of the steel sheet, resulting in non-uniform dispersion of the hard phase and the occurrence of ghost lines. Therefore, the present inventors discovered that by using a material that reduces Mn segregation and rolling the steel sheet uniformly on the front and back sides in the hot rolling process to uniformly distribute the hard phase, and further controlling the surface roughness of the coating layer to smooth out unevenness before forming, it is possible to suppress the occurrence of ghost lines while maintaining high strength and elongation, and to significantly improve poor appearance after forming.

[0013] The present invention was completed based on these findings and includes the following aspects.

[0014] (Aspect 1) A plated steel sheet having a base steel sheet and a plating layer provided on a surface of the base steel sheet, wherein the base steel sheet has a chemical composition, in mass%, of C: 0.03 to 0.10%, Si: 0.01 to 1.50%, Mn: 1.0 to 2.5%, Al: 0.005 to 0.700%, Cr: 0.15 to 0.80%, Mo: 0.15 to 0.50%, Ti: 0.03 to 0.10%, P: 0.1000% or less, S: 0.0200% or less, N: 0.015% or less, O: 0.0200% or less, B: 0 to 0.010%, Nb: 0 to 0.10%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.20%, Ca: 0 to 0.010%, Zr: 0 to 0.010%, REM: 0 to 0.010%, and the balance: Fe and impurities, and the metallographic structure of the base steel plate is, in area %, ferrite: 80 to 97%, hard phase: 3 to 20%, the area ratio of a band-shaped hard phase at a position of 3t / 8 to 5t / 8 in the plate thickness t is 0 to 5%, and the absolute value of the difference in hard phase fraction at a position of 1t / 8 to 4t / 8 and a position of 4t / 8 to 7t / 8 in the plate thickness t from the surface of the base steel plate is 0 to 8%, The plated steel sheet has a surface roughness Sa of 0.10 to 0.50 μm.

[0015] (Aspect 2) The plated steel sheet according to Aspect 1, characterized in that the base steel sheet has a chemical composition, in mass%, of one or more elements selected from the group consisting of B: 0.0001 to 0.010%, Nb: 0.001 to 0.10%, V: 0.001 to 0.50%, Ni: 0.001 to 1.00%, Cu: 0.001 to 1.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.20%, Ca: 0.0001 to 0.010%, Zr: 0.0001 to 0.010%, and REM: 0.0001 to 0.010%.

[0016] (Embodiment 3) The plated steel sheet according to embodiment 1 or 2, wherein the hard phase accounts for 5% or more in the metal structure of the base steel sheet.

[0017] (Aspect 4) The plated steel sheet according to any one of Aspects 1 to 3, wherein an absolute value of a difference between the hard phase fractions at a position 1t / 8 to 4t / 8 and a position 4t / 8 to 7t / 8 in a sheet thickness t from the surface of the base steel sheet is 5% or less.

[0018] (Embodiment 5) The plated steel sheet according to any one of the above-mentioned embodiments 1 to 4, wherein the plated steel sheet has a tensile strength of 540 MPa or more.

[0019] (Aspect 6) The plated steel sheet according to any one of Aspects 1 to 5, wherein the ferrite has an average crystal grain size of 5.0 to 30.0 μm, and the hard phase has an average crystal grain size of 1.0 to 5.0 μm.

[0020] (Aspect 7) The plated steel sheet according to any one of Aspects 1 to 6, wherein the hard phase comprises at least one of martensite, bainite, tempered martensite, and pearlite.

[0021] According to the present invention, it is possible to provide a high-strength plated steel sheet that is excellent in strength and elongation and has an improved appearance after forming.

[0022] Fig. 1 is a diagram schematically showing a partial cross section of a plated steel sheet 1 according to one embodiment of the present invention. Fig. 2 is a cross-sectional SEM photograph of a general plated steel sheet, which is different from the plated steel sheet 1 of the present invention. In Fig. 2, the portion indicated by the white arrow is a band-shaped hard phase.

[0023] Hereinafter, preferred embodiments of the plated steel sheet 1 of the present invention will be described in detail with reference to the drawings. In this specification, various numerical ranges refer to ranges that include their upper and lower limit values ​​unless otherwise specified. In particular, numerical ranges expressed using "to" refer to ranges that include the numerical values ​​before and after "to" as the lower and upper limit values. However, when the numerical values ​​before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit values.

[0024] <Plated Steel Sheet> As shown in Fig. 1 , a plated steel sheet 1 according to one embodiment of the present invention is a plated steel sheet having a base steel sheet 2 and a plating layer 3 provided on one or both surfaces of the base steel sheet 2. The plated steel sheet 1 of this embodiment has the following characteristics. That is, a plating layer 3 having the characteristics described below is formed on one or both surfaces of the base steel sheet 2. Note that the plating layer 3 may be provided on only one surface of the base steel sheet 2, or on both surfaces.

[0025] In the plated steel sheet 1 of this embodiment, the chemical composition of the base steel sheet 2 is, in mass %, C: 0.03 to 0.10%, Si: 0.01 to 1.50%, Mn: 1.0 to 2.5%, Al: 0.005 to 0.700%, Cr: 0.15 to 0.80%, Mo: 0.15 to 0.50%, Ti: 0.03 to 0.10%, P: 0.1000% or less, S: 0.0200% or less, N: 0.015% or less, O: 0.0200% or less, B: 0 to 0.010%, Nb: 0 to 0.10%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W : 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.20%, Ca: 0 to 0.010%, Zr: 0 to 0.010%, REM: 0 to 0.010%, and the balance: Fe and impurities. The metallographic structure of the base steel sheet 2 is, in area %, ferrite: 80 to 97% and hard phase: 3 to 20%, the area ratio of the band-shaped hard phase at a position 3t / 8 to 5t / 8 of the sheet thickness t is 0 to 5%, and the absolute value of the difference in hard phase fraction between a position 1t / 8 to 4t / 8 and a position 4t / 8 to 7t / 8 of the sheet thickness t from the surface of the base steel sheet 2 is 0 to 8%. Furthermore, the surface roughness Sa of the plated steel sheet 1 of this embodiment is 0.10 to 0.50 μm.

[0026] For exterior panel parts such as roofs, hoods, fenders, and doors, DP steel, which has a relatively low yield strength, is often used to avoid surface defects called surface distortions that occur during press forming and other forming processes. However, DP steel, which contains a mixture of a soft phase consisting of ferrite and a hard phase mainly composed of martensite, is prone to non-uniform deformation, in which the soft phase and its surroundings deform preferentially during press forming and other forming processes, as described above. If such non-uniform deformation causes minute irregularities on the surface of the steel sheet after forming, it can cause appearance defects called ghost lines.

[0027] To explain the occurrence of ghost lines in more detail, first, during forming such as press forming, the soft phase consisting of ferrite deforms in a concave manner, while the hard phase consisting mainly of martensite or the like deforms in a convex manner without concavity. As a result, minute irregularities are formed on the surface of the steel sheet after forming. These minute irregularities are formed in such a way that convex portions extending generally along the rolling direction and concave portions extending generally along the rolling direction are alternately arranged in the width direction perpendicular to the rolling direction. Then, when the surface of the steel sheet after forming is polished, the convex portions of the minute irregularities on the steel sheet surface are scraped off, thereby revealing ghost lines in the form of streaks extending in the rolling direction of the steel sheet.

[0028] In steel sheets such as DP steels that contain a mixture of soft layers and hard phases, the presence of hard phases connected in stripes in the metal structure (hereinafter sometimes referred to as "band-shaped hard phases") makes the degree of ghost lines more pronounced. Therefore, by suppressing the formation of such band-shaped hard phases and dispersing the hard phases more uniformly in the metal structure, it is possible to suppress the formation of minute irregularities on the steel sheet surface after forming, and thus the occurrence of ghost lines.

[0029] The band-shaped hard phase is formed due to central segregation or microsegregation of Mn during solidification of steel. Therefore, in order to suppress the formation of the band-shaped hard phase, it is effective to reduce Mn segregation during solidification in the casting process in which molten steel is solidified to cast a slab.

[0030] However, while these techniques for reducing Mn segregation can achieve a certain degree of ghost line improvement, the improvement is not sufficient. Therefore, the present inventors conducted extensive research into further techniques for improving ghost lines in addition to techniques for reducing Mn segregation. As a result, the present inventors discovered that even if Mn segregation is reduced, if the segregated portions are not uniformly rolled on both sides of the steel sheet during the hot rolling process, the segregated portions will be unevenly distributed on both sides of the steel sheet, resulting in the non-uniform dispersion of hard phases and the occurrence of ghost lines. The present inventors then discovered that by uniformly rolling the steel sheet on both sides during the hot rolling process while reducing Mn segregation to uniformly distribute the hard phases, and further controlling the surface roughness of the coating layer 3 to smooth out pre-forming irregularities, it is possible to suppress the occurrence of ghost lines while maintaining high strength and elongation, and significantly improve poor appearance after forming. The present invention was completed based on these findings.

[0031] As described above, the plated steel sheet 1 according to one embodiment of the present invention is a plated steel sheet 1 having a base steel sheet 2 and a plating layer 3 provided on the surface of the base steel sheet 2, and the base steel sheet 2 has the above-mentioned specific chemical composition. Furthermore, this base steel sheet 2 has a unique metallographic structure that is made up of a lower hard phase fraction than conventional DP steel, has less band-shaped hard phase, and has little bias in the hard phase fraction between the front and back surfaces of the base steel sheet 2. As will be described later, such a metallographic structure can be obtained by employing a specific chemical composition and manufacturing conditions to reduce Mn segregation, and by uniformly rolling the base steel sheet 2 on both surfaces to uniformly disperse the hard phase.

[0032] Furthermore, in the plated steel sheet 1 of this embodiment, as described above, the plating layer 3 provided on the surface of the base steel sheet 2 has a smooth surface. That is, the plated steel sheet 1 of this embodiment has a smooth surface. The plated steel sheet 1 having such a smooth surface can be obtained by selecting various conditions in the annealing step, the plating step, etc., as will be described later.

[0033] In the plated steel sheet 1 of this embodiment, the base steel sheet 2 has the above-described specific metal structure and a smooth surface, and therefore, while maintaining high strength and elongation, the occurrence of ghost lines is suppressed, and poor appearance after forming can be significantly improved. That is, according to this embodiment, it is possible to provide a plated steel sheet 1 that is excellent in strength and elongation and has an even more improved appearance after forming.

[0034] The base steel sheet 2 and the plating layer 3 constituting the plated steel sheet 1 of the present embodiment will be described in more detail below. In the following description, "%", which is the unit of content of each element, means "mass%" unless otherwise specified.

[0035] <Base Steel Plate> In this embodiment, the base steel plate 2 has the following composition as described above: C: 0.03 to 0.10%, Si: 0.01 to 1.50%, Mn: 1.0 to 2.5%, Al: 0.005 to 0.700%, Cr: 0.15 to 0.80%, Mo: 0.15 to 0.50%, Ti: 0.03 to 0.10%, P: 0.1000% or less, S: 0.0200% or less, N: 0.015% or less, O: 0.0200% or less, B: 0 to 0.010%, Nb: 0 to 0.10%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, It has a specific chemical composition consisting of Sn: 0 to 1.00%, Sb: 0 to 0.20%, Ca: 0 to 0.010%, Zr: 0 to 0.010%, REM: 0 to 0.010%, and the balance: Fe and impurities.

[0036] (Chemical Composition) Each element in this chemical composition will be described in detail below.

[0037] [C: 0.03 to 0.10%] C is an element that generates martensite and increases the strength of the base steel plate 2. To fully obtain this effect, the C content is set to 0.03% or more. The C content may be 0.06% or more. On the other hand, from the viewpoint of not inhibiting the diffusion of Mn during solidification and sufficiently suppressing microsegregation of Mn, the C content is set to 0.10% or less. The C content may be 0.09% or less.

[0038] [Si: 0.01 to 1.50%] Si is a deoxidizing element for steel and a solid-solution strengthening element effective for increasing the strength without impairing the ductility of the base steel plate 2. Si is also an element effective for promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully obtain these effects, the Si content is set to 0.01% or more. The Si content may be 0.10% or more. On the other hand, from the viewpoint of preventing surface defects due to reduced scale peelability, the Si content is set to 1.50% or less. The Si content may be 0.50% or less.

[0039] [Mn: 1.0 to 2.5%] Mn is an element that improves the hardenability of steel, generates martensite, and contributes to improving the strength of the base steel plate 2. To fully obtain these effects, the Mn content is set to 1.0% or more. The Mn content may be 1.1% or more. On the other hand, from the viewpoint of not inhibiting the diffusion of Mn during solidification and sufficiently suppressing microsegregation of Mn, the Mn content is set to 2.5% or less. The Mn content may be 2.0% or less.

[0040] [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 the base steel plate 2. Al is also an element that is effective in promoting the diffusion of Mn during solidification and reducing microsegregation of Mn. In order to fully obtain these effects, the Al content is set to 0.005% or more. The Al content may be 0.010% or more. On the other hand, from the viewpoint of preventing a decrease in productivity due to deterioration of castability, the Al content is set to 0.700% or less. The Al content may be 0.080% or less.

[0041] [Cr: 0.15 to 0.80%] Cr is an element that improves the hardenability of steel and contributes to improving the strength of the base steel plate 2. Cr 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 0.15% or more. The Cr content may be 0.20% or more. On the other hand, from the viewpoint of preventing the formation of coarse Cr carbides that may become the starting point of fracture, the Cr content is set to 0.80% or less. The Cr content may be 0.40% or less.

[0042] [Mo: 0.15 to 0.50%] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the base steel plate 2. Mo is also an element that is effective in promoting the diffusion of Mn during solidification and reducing microsegregation of Mn. In order to fully obtain these effects, the Mo content is set to 0.15% or more. The Mo content may be 0.20% or more. On the other hand, from the viewpoint of preventing a decrease in productivity due to a decrease in hot workability, the Mo content is set to 0.50% or less. The Mo content may be 0.40% or less.

[0043] [Ti: 0.03 to 0.10%] Ti is an element that has the effect of reducing the amounts of S, N, and O, which can generate coarse inclusions that act as fracture initiation points. Ti is also a precipitation strengthening element that has the effect of refining the structure and improving the strength-formability balance of the base steel sheet 2. In order to fully obtain these effects, the Ti content is set to 0.03% or more. The Ti content may be 0.05% or more. On the other hand, from the viewpoint of preventing a decrease in formability of the base steel sheet 2 due to the formation of coarse Ti sulfides, Ti nitrides, and / or Ti oxides, the Ti content is set to 0.10% or less. The Ti content may be 0.08% or less.

[0044] [P: 0.1000% or less] P is an element that is mixed in during the manufacturing process. P is also a solid solution strengthening element. The P content may be 0%. However, reducing the P content to 0% requires time for refining, which leads to a decrease in productivity. Therefore, from the viewpoint of productivity, the P content may be 0.0001% or more or 0.0005% or more. On the other hand, from the viewpoint of preventing a decrease in toughness of the base steel plate 2, the P content is set to 0.1000% or less. The P content may be 0.0150% or less.

[0045] [S: 0.0200% or less] S is an element that is mixed in during the manufacturing process. The S content may be 0%. However, reducing the S content to 0% requires time for refining, which leads to a decrease in productivity. Therefore, from the viewpoint of productivity, the S content may be 0.0001% or more or 0.0005% or more. On the other hand, from the viewpoint of preventing a decrease in formability, such as ductility, hole expandability, stretch flangeability, and / or bendability, of the base steel sheet 2 due to the formation of Mn sulfides, the S content is set to 0.0200% or less. The S content may be 0.0100% or less.

[0046] [N: 0.015% or less] N is an element that is mixed in during the manufacturing process. The N content may be 0%. However, reducing the N content to 0% requires time for refining, resulting in a decrease in productivity. Therefore, from the viewpoint of productivity, the N content may be 0.0001% or more or 0.0005% or more. On the other hand, from the viewpoint of preventing a decrease in formability, such as ductility, hole expandability, stretch flangeability, and / or bendability, of the base steel sheet 2 due to the formation of nitrides, the N content is set to 0.015% or less. The N content may be 0.008% or less.

[0047] [O: 0.0200% or less] O is an element that is mixed in during the manufacturing process. The O content may be 0%. However, reducing the O content to 0% requires time for refining, which leads to a decrease in productivity. Therefore, from the viewpoint of productivity, the O content may be 0.0001% or more or 0.0005% or more. On the other hand, from the viewpoint of preventing a decrease in formability, such as ductility, hole expandability, stretch flangeability, and / or bendability, of the base steel sheet 2 due to the formation of coarse oxides, the O content is set to 0.0200% or less. The O content may be 0.0010% or less.

[0048] The basic chemical composition of the base steel plate 2 of this embodiment is as described above. Furthermore, in this embodiment, the base steel plate 2 may contain one or more of the following optional elements in place of a portion of the remaining Fe, as necessary. These optional elements will be described in detail below.

[0049] [B: 0 to 0.010%] B is an element that generates martensite and contributes to improving the strength of the base steel plate 2. The B content may be 0%, but to fully obtain this effect, the B content may be 0.0001% or more or 0.0005% or more. On the other hand, from the viewpoint of preventing a decrease in strength of the base steel plate 2 due to the generation of B precipitates, the B content is set to 0.010% or less. The B content may be 0.004% or less.

[0050] [Nb: 0 to 0.10%] Nb is a precipitation strengthening element that contributes to improving the strength of the base steel sheet 2 due to strengthening by precipitates, grain refinement strengthening by suppressing ferrite grain growth, and / or dislocation strengthening by suppressing recrystallization. The Nb content may be 0%, but to fully obtain these effects, the Nb content may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of preventing a decrease in formability of the base steel sheet 2 due to an increase in unrecrystallized ferrite, the Nb content is set to 0.10% or less. The Nb content may be 0.08% or less.

[0051] [V: 0 to 0.50%] V is an element that contributes to improving the strength of the base steel sheet 2 due to strengthening by precipitates, grain refinement strengthening by suppressing ferrite grain growth, and / or dislocation strengthening by suppressing recrystallization. The V content may be 0%, but to fully obtain these effects, the V content may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of preventing a decrease in formability of the base steel sheet 2 due to the precipitation of a large amount of carbonitrides, the V content is set to 0.50% or less. The V content may be 0.01% or less.

[0052] [Ni: 0 to 1.00%] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the base steel plate 2. The Ni content may be 0%, but to fully obtain this effect, the Ni content may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of preventing a decrease in the weldability of the base steel plate 2, the Ni content is set to 1.00% or less. The Ni content may be 0.40% or less.

[0053] [Cu: 0 to 1.00%] Cu is an element that exists in steel in the form of fine particles and contributes to improving the strength of the base steel plate 2. The Cu content may be 0%, but to fully obtain this effect, the Cu content may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of preventing a decrease in the weldability of the base steel plate 2, the Cu content is set to 1.00% or less. The Cu content may be 0.40% or less.

[0054] [W: 0 to 1.00%] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the base steel plate 2. The W content may be 0%, but to fully obtain this effect, the W content may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of preventing a decrease in productivity due to a decrease in hot workability, the W content is set to 1.00% or less. The W content may be 0.08% or less.

[0055] [Sn: 0 to 1.00%] Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the base steel plate 2. The Sn content may be 0%, but to fully obtain this effect, the Sn content may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of preventing embrittlement of the base steel plate 2 due to the generation of coarse oxides, the Sn content is set to 1.00% or less. The Sn content may be 0.08% or less.

[0056] [Sb: 0 to 0.20%] Sb is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the base steel plate 2. The Sb content may be 0%, but to fully obtain this effect, the Sb content may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of preventing embrittlement of the base steel plate 2 due to the generation of coarse oxides, the Sb content is set to 0.20% or less. The Sb content may be 0.04% or less.

[0057] [Ca: 0 to 0.010%] Ca is an element mixed in as a deoxidizer. The Ca content may be 0%. However, reducing the Ca content to 0% requires time for refining, which leads to a decrease in productivity. Therefore, from the viewpoint of productivity, the Ca content may be 0.0001% or more or 0.0005% or more. On the other hand, from the viewpoint of preventing embrittlement of the base steel plate 2 due to the generation of coarse oxides, the Ca content is set to 0.010% or less.

[0058] [Zr: 0 to 0.010%] Zr is an element mixed in as a deoxidizer. The Zr content may be 0%. However, reducing the Zr content to 0% requires time for refining, resulting in reduced productivity. Therefore, from the viewpoint of productivity, the Zr content may be 0.0001% or more or 0.0005% or more. On the other hand, from the viewpoint of preventing embrittlement of the base steel plate 2 due to the generation of coarse oxides, the Zr content is set to 0.010% or less.

[0059] [REM: 0 to 0.010%] REM is an element mixed in as a deoxidizer. The REM content may be 0%, but to fully obtain this effect, the REM content may be 0.0001% or more or 0.0005% or more. On the other hand, from the viewpoint of preventing embrittlement of the base steel plate 2 due to the generation of coarse oxides, the REM content is set to 0.010% or less.

[0060] In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0061] Regarding the above optional elements, in this embodiment, the chemical composition of the base steel plate 2 may include, in mass%, one or more elements selected from the group consisting of B: 0.0001 to 0.010%, Nb: 0.001 to 0.10%, V: 0.001 to 0.50%, Ni: 0.001 to 1.00%, Cu: 0.001 to 1.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.20%, Ca: 0.0001 to 0.010%, Zr: 0.0001 to 0.010%, and REM: 0.0001 to 0.010%.

[0062] In this embodiment, the remainder of the base steel plate 2 other than the above elements consists of Fe and impurities. Here, the impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the base steel plate 2. Examples of impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The impurities may be contained in an amount of 0.100% or less in total.

[0063] Here, the chemical composition of the base steel plate 2 can be measured by a general analytical method. For example, the chemical composition of the base steel plate 2 can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0064] (Metallic Structure) [Ferrite: 80 to 97% and Hard Phase: 3 to 20%] In the present embodiment, the metallic structure of the base steel plate 2 is composed of, in area %, 80 to 97% ferrite and 3 to 20% hard phase. By making the metallic structure of the base steel plate 2 such a composite structure, it becomes possible to easily suppress poor appearance after forming while maintaining the strength and ductility (elongation) of the base steel plate 2 within appropriate ranges, specifically, within ranges where the tensile strength and elongation at break measured using a No. 5 test piece of JIS Z 2241:2022, with the longitudinal direction being perpendicular to the rolling direction, are 540 MPa or more and 19% or more, respectively.

[0065] From the viewpoint of further increasing the strength of the base steel plate 2, the area fraction of the hard phase (hereinafter sometimes referred to as the "hard phase fraction") may be 4% or more, 5% or more, or 6% or more. Similarly, the area fraction of ferrite (hereinafter sometimes referred to as the "ferrite fraction") may be 96% or less, 95% or less, or 94% or less. On the other hand, from the viewpoint of further increasing the ductility (elongation) of the base steel plate 2, the hard phase fraction may be 8% or less or 10% or less. Similarly, the ferrite fraction may be 90% or more or 92% or more.

[0066] In this specification, the hard phase of the base steel plate 2 refers to a structure harder than ferrite, and is composed of, for example, at least one of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of improving the strength of the base steel plate 2, the hard phase is preferably composed of at least one of martensite, bainite, and tempered martensite, and more preferably composed of martensite. Note that the metal structure of the base steel plate 2 preferably contains little retained austenite. Specifically, the retained austenite is preferably 3% or less, 1% or less, or 0.5% or less, in area percentage, and more preferably 0%.

[0067] (Identification of Metallographic Structure and Calculation of Area Fraction) The identification of the metallographic structure of the base steel sheet 2 and the calculation of the area fraction are performed as follows. First, a sample (approximately 20 mm in the rolling direction × 50 mm in the width direction × thickness of the base steel sheet) for observing the metallographic structure (microstructure) is collected from a position 100 mm or more away from the end face of the base steel sheet 2 from which the coating layer 3 has been removed. To prepare the sample, the plate thickness cross section perpendicular to the rolling direction is polished as the observation surface and etched by nital corrosion. Next, using a scanning electron microscope (SEM), secondary electron images of the observation surface of the sample are taken at a magnification of 600x and linked. The obtained image data are observed in 10 fields of view in an area of ​​the total plate thickness × 5 mm, and image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe. In the image analysis, ferrite and hard phases are binarized based on differences in brightness, and the area fraction of the hard phase is calculated. The black parts of the image data represent ferrite, and the white parts represent the hard phase. Then, for a total of 10 observation fields, image analysis was performed in the same manner as above to measure the area fraction of the hard phase, and the average value of these area fractions was calculated. This average value was taken as the area fraction of the hard phase, and the remainder was taken as the area fraction of ferrite. The total observation area was the total plate thickness x 50 mm.

[0068] Here, in this specification, the rolling direction of the base steel sheet 2 can be determined, for example, as follows. After mirror-polishing the sheet thickness cross section, the S concentration is measured using an electron probe microanalyzer (EPMA). 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 at positions 3t / 8 to 5t / 8 of the sheet thickness t. At this time, regions with high S concentration and elongation are determined to be inclusions. When observing, observation may be performed from multiple fields of view. Next, using the sheet 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 sheet thickness direction is observed in the same manner as above. For each of the obtained cross sections, the average length of the major axes of multiple inclusions is calculated. Then, the cross section with the largest average value of the major axis length of the inclusions is identified. The direction parallel to the longitudinal axis direction of the inclusions in the cross section is determined as the rolling direction. In the case of a coil (steel strip) or when the rolling direction of the base steel plate 2 can be determined by other means, the rolling direction of the base steel plate 2 does not need to be determined by the above-mentioned determination method.

[0069] When the area fraction of retained austenite needs to be measured in calculating the area fraction of the metallographic structure, the area fraction of retained austenite can be measured by X-ray diffraction of the above-mentioned observation surface. Specifically, using Co-Kα radiation, the integrated intensities of a total of six peaks, namely, α(110), α(200), α(211), γ(111), γ(200), and γ(220), at a quarter position in the sheet thickness direction are obtained, and the volume fraction of retained austenite is calculated using the intensity averaging method. The obtained volume fraction of retained austenite is defined as the area fraction of retained austenite.

[0070] [Area Fraction of Band-Shaped Hard Phase at Positions 3t / 8 to 5t / 8 of Sheet Thickness t is 0 to 5%] In this embodiment, the metallographic structure of the base steel sheet 2 has an area fraction of band-shaped hard phase at positions 3t / 8 to 5t / 8 of sheet thickness t of 0 to 5%. By reducing the amount of band-shaped hard phase contained in the metallographic structure in this way, that is, by dispersing the hard phase more uniformly, it is possible to prevent non-uniform strain during forming and suppress the generation of minute irregularities on the surface of the plated steel sheet 1 after forming. This makes it possible to make ghost lines less likely to occur.

[0071] (Band-like hard phase) In this specification, the term "band-like hard phase" refers to one or more linear hard phases having a thickness of 3 μm or more and a length of 200 μm or more, and extending continuously or intermittently. Here, "thickness" refers to the length in the sheet thickness direction, and "length" refers to the length in the direction perpendicular to the sheet thickness. When multiple linear hard phases are present, the band-like hard phase is one in which the total thickness of the multiple linear hard phases is 3 μm or more. Here, FIG. 2 is a cross-sectional SEM photograph of a general plated steel sheet, which is different from the plated steel sheet 1 of the present invention. In FIG. 2, the portion pointed to by the white arrow is the band-like hard phase.

[0072] (Method for measuring the area fraction of the band-like hard phase at the 3t / 8 to 5t / 8 position of the sheet thickness t) The area fraction of the band-like hard phase at the 3t / 8 to 5t / 8 position of the sheet thickness t can be measured as follows. First, the combined SEM images measured in the above "Identification of metallographic structure and calculation of area fraction", i.e., secondary electron images of the observation surface of the sample are taken at a magnification of 600 times and combined to obtain image data, are observed in 10 fields of view in an area of ​​total sheet thickness × 5 mm (the total field of view is total sheet thickness × 50 mm), and image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe, to calculate the area fraction of one or more linear hard phases having a thickness of 3 μm or more and a length of 200 μm or more and extending continuously or intermittently, i.e., the band-like hard phase, at the 3t / 8 to 5t / 8 position, which is the center of the sheet thickness t.

[0073] The area ratio of the band-shaped hard phase at the 3t / 8 to 5t / 8 position of the plate thickness t is preferably 3% or less, more preferably 2% or less or 1% or less, and particularly preferably 0%, from the viewpoint of further improving the appearance after forming.

[0074] [Absolute Value of Difference in Hard Phase Fraction Between the 1t / 8 to 4t / 8 Position and the 4t / 8 to 7t / 8 Position in the Sheet Thickness t from the Surface of the Base Steel Sheet is 0 to 8%] In the present embodiment, the metallographic structure of the base steel sheet 2 has an absolute value of difference in hard phase fraction between the 1t / 8 to 4t / 8 position and the 4t / 8 to 7t / 8 position in the sheet thickness t from the surface of the base steel sheet 2 of 0 to 8%. Note that "an absolute value of difference in hard phase fraction of 0 to 8%" is synonymous with "a difference in hard phase fraction of -8 to 8%." In the present embodiment, by reducing the bias in hard phase fraction between the front and back surfaces of the base steel sheet 2 in this manner, it is possible to prevent non-uniform strain during forming and suppress the generation of minute irregularities on the surface of the plated steel sheet 1 after forming. This makes it possible to make ghost lines less likely to occur.

[0075] The difference in absolute values ​​of the hard phase fractions at the 1t / 8 to 4t / 8 position and the 4t / 8 to 7t / 8 position in the plate thickness t from the surface of the base steel plate 2 can be calculated from the difference between the area fraction of the hard phase at the 1t / 8 to 4t / 8 position in the plate thickness t from the surface of the base steel plate 2 and the area fraction of the hard phase at the 4t / 8 to 7t / 8 position in the plate thickness t from the surface of the base steel plate 2, which are determined from the linked SEM image measured in the above-mentioned "Identification of metal structure and calculation of area fraction".

[0076] The absolute value of the difference in hard phase fraction between the 1t / 8 to 4t / 8 position and the 4t / 8 to 7t / 8 position in the plate thickness t from the surface of the base steel plate 2 is preferably 6% or less, more preferably 5% or less, even more preferably 4% or less, and particularly preferably 1% or less, from the viewpoint of further improving the appearance after forming.

[0077] [Average grain size of ferrite: 5.0 to 30.0 μm] The average grain size of ferrite in the base steel sheet 2 is the average grain size of ferrite generated during annealing. The density and grain size of the hard phase change depending on this average grain size of ferrite. In this embodiment, the average grain size of ferrite is preferably 5.0 to 30.0 μm. Controlling the average grain size of ferrite within such a fine range increases the uniformity of the structure and further improves the appearance after forming. Specifically, when the average grain size of ferrite is 5.0 μm or more, aggregation of the hard phase after ferrite generation is less likely to occur, preventing uneven strain during forming and further improving the appearance after forming. On the other hand, when the average grain size of ferrite is 30.0 μm or less, variation in the grain size of ferrite is reduced, preventing uneven strain during forming and further improving the appearance after forming. The average grain size of ferrite may be 8.0 μm or more, 10.0 μm or more, or 12.0 μm or more. Similarly, the average grain size of the ferrite may be 28.0 μm or less, 25.0 μm or less, or 21.0 μm or less.

[0078] The average grain size of ferrite in the base steel sheet 2 can be measured as follows. First, the combined SEM images measured in the above "Identification of Metallic Structure and Calculation of Area Fraction" section, i.e., secondary electron images of the observation surface of the sample taken at 600x magnification, are combined to obtain image data. Ten fields of view are observed in an area of ​​total sheet thickness × 5 mm (total field of view is total sheet thickness × 50 mm). Image analysis is performed using image analysis software such as Adobe Photoshop (registered trademark) CS5, and the number of ferrite particles in each of the 10 fields of view is calculated. The average area ratio per ferrite particle is calculated by dividing the total area ratio of ferrite (i.e., the total area ratio of ferrite in the 10 fields of view) by the total number of ferrite particles in the 10 fields of view. The equivalent circle diameter is calculated from this average area ratio, and the obtained equivalent circle diameter is used as the average grain size of ferrite.

[0079] [Average grain size of hard phase: 1.0 to 5.0 μm] The average grain size of the hard phase in the base steel sheet 2 is the average grain size of hard phases such as martensite, pearlite, bainite, and retained austenite generated during annealing. In this embodiment, the average grain size of the hard phase is preferably 1.0 to 5.0 μm. Controlling the average grain size of the hard phase within such a fine range increases the uniformity of the structure and further improves the appearance after forming. Specifically, when the average grain size of the hard phase is 1.0 μm or more, aggregation of the hard phase is less likely to occur, preventing uneven strain during forming and further improving the appearance after forming. On the other hand, when the average grain size of the hard phase is 5.0 μm or less, variation in the grain size of the hard phase is reduced, preventing uneven strain during forming and further improving the appearance after forming. The average grain size of the hard phase may be 1.5 μm or more. Similarly, the average grain size of the hard phase may be 4.8 μm or less, or 4.5 μm or less.

[0080] The average grain size of the hard phase can be measured as follows. First, the combined SEM images measured in the above "Identification of Metallic Structure and Calculation of Area Fraction" section, i.e., secondary electron images of the observation surface of the sample taken at 600x magnification, are combined to obtain image data. Ten fields of view are observed in an area of ​​total plate thickness × 5 mm (total field of view is total plate thickness × 50 mm). Image analysis is performed using Adobe Photoshop (registered trademark) CS5 image analysis software, and the number of hard phase particles in each of the 10 fields of view is calculated. The average area ratio per hard phase particle is calculated by dividing the area ratio of the entire hard phase (i.e., the total area ratio of the hard phase in the 10 fields of view) by the total number of hard phase particles in the 10 fields of view. The equivalent circle diameter is calculated from this average area ratio, and the resulting equivalent circle diameter is used as the average grain size of the hard phase.

[0081] (Sheet Thickness) In the present embodiment, the thickness of the plated steel sheet 1 is not particularly limited. For example, the plated steel sheet 1 may have a thickness of 0.1 to 2.0 mm. A plated steel sheet 1 having such a thickness is suitable for use as a material for covering members such as doors and hoods. The plated steel sheet 1 may have a thickness of 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the plated steel sheet 1 may have a thickness of 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 plated steel sheet 1 0.2 mm or more, it becomes easier to maintain the shape of a molded product flat, and additional effects such as improved dimensional accuracy and shape accuracy can be obtained. On the other hand, by making the plated steel sheet 1 1.0 mm or less, the weight reduction effect of the member is significant. The plated steel sheet 1 thickness is measured, for example, using a micrometer.

[0082] <Plated Layer> In the present embodiment, the plated layer 3 formed on the surface of the base steel sheet 2 may be either a hot-dip plated layer or an electroplated layer. Examples of the hot-dip plated layer include a hot-dip galvanized layer (GI), a galvannealed layer (GA), a hot-dip aluminum plated layer, a hot-dip Zn—Al alloy plated layer, a hot-dip Zn—Al—Mg alloy plated layer, and a hot-dip Zn—Al—Mg—Si alloy plated layer. Examples of the electroplated layer include an electrogalvanized layer (EG) and an electrolytic Zn—Ni alloy plated layer. Among these, the plated layer 3 is preferably a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer.

[0083] [Surface roughness Sa of plated steel sheet is 0.10 to 0.50 μm] In this embodiment, the surface roughness Sa of the plated steel sheet 1 (surface roughness of the plating layer 3 provided on the surface of the base steel sheet 2) is 0.10 to 0.50 μm. By controlling the surface roughness of the plated steel sheet 1 in this manner and smoothing out the irregularities on the surface of the plated steel sheet 1 before forming, it is possible to make the surface irregularities less noticeable after forming. This makes it possible to more reliably improve poor appearance after forming.

[0084] In this embodiment, from the viewpoint of further improving the appearance after forming, the surface roughness Sa of the plated steel sheet 1 is preferably 0.45 μm or less, more preferably 0.43 μm or less, even more preferably 0.35 μm or less, and particularly preferably 0.33 μm or less. Furthermore, the surface roughness Sa of the plated steel sheet 1 may be 0.20 μm or more or 0.25 μm or more.

[0085] The surface roughness Sa of the plated steel sheet 1 can be measured as follows. First, a test piece is taken from the plated steel sheet 1 to be measured. The test piece is taken from a position 100 mm or more away from the end face of the plated steel sheet 1. Next, a laser microscope is used to measure the unevenness of the test piece surface in an 8 mm × 8 mm area. The measurement conditions are a measurement magnification of 20 times, a resolution of 5 μm in the XY plane, and a resolution of 0.1 nm in the Z spatial plane, and the measurement is performed in a linked manner. After that, the entire measurement area is subjected to a filtering process to remove unevenness with a period of 0.25 mm or less (i.e., a low-pass filter λs of 0.25 mm), and the arithmetic mean height Sa is determined in accordance with JIS B0681-2:2018, 4.1.7, "Arithmetical mean height of the scale limited surface."

[0086] The arithmetic mean height Sa thus obtained is defined as the surface roughness Sa of the plated steel sheet 1. The smaller the low-pass filter λs, the larger the surface roughness Sa. For example, in the case of automotive steel sheets for which appearance is important, Sa (λs = 0.25 mm) when the low-pass filter λs is 0.25 mm is approximately three times larger than Sa (λs = 0.8 mm) when the low-pass filter λs is 0.8 mm. Therefore, a surface roughness of 0.10 to 0.50 μm at Sa (λs = 0.25 mm) means that the surface has very few irregularities and is a smooth surface.

[0087] In this embodiment, the coating weight of the plating layer 3 is set to 20 g / m as the coating weight per one side of the base steel sheet 2 from the viewpoint of adjusting the surface roughness Sa of the plated steel sheet 1 to 0.10 to 0.50 μm. ２The coating weight of the plating layer 3 is 20 g / m ２ When the thickness is more than this, the plating layer 3 can be formed more uniformly on the surface of the base steel sheet 2, and the appearance after forming can be improved. In addition, the coating weight of the plating layer 3 is set to 120 g / m from the viewpoint of adjusting the surface roughness Sa of the plated steel sheet 1 to 0.10 to 0.50 μm. ２ The coating weight of the plating layer 3 is 120 g / m or less. ２ When the coating weight of the plating layer 3 is 25 g / m or less, the adhesion of the plating layer 3 is further improved. ２ or more than 30 g / m ２ Similarly, the coating weight of the plating layer 3 may be 110 g / m or more. ２ or less than 100 g / m ２ It may be the following:

[0088] (Mechanical Properties) The plated steel sheet 1 of this embodiment, which is composed of the base steel sheet 2 having the above-mentioned specific chemical composition and metallographic structure and the plating layer 3 provided on the surface thereof and has the above-mentioned specific surface roughness, can achieve high strength, specifically a tensile strength of 540 MPa or more, and excellent ductility (elongation), specifically a breaking elongation of 19% or more. Here, the tensile strength and breaking elongation are the tensile strength and breaking elongation measured using a No. 5 test piece of JIS Z 2241:2022 cut out from the plated steel sheet 1 and with the longitudinal direction perpendicular to the rolling direction.

[0089] (Tensile strength) The tensile strength of the plated steel sheet 1 is preferably 540 MPa or more. The tensile strength of the plated steel sheet 1 is more preferably 550 MPa or more or 600 MPa or more. There is no particular upper limit to the tensile strength of the plated steel sheet 1, but the tensile strength may be, for example, 980 MPa or less or 850 MPa or less. Setting the tensile strength to 850 MPa or less has the advantage of making it easier to ensure formability when the plated steel sheet 1 is press-formed.

[0090] (Fracture elongation) From the viewpoint of formability, the fracture elongation of the plated steel sheet 1 is preferably 19% or more. The fracture elongation of the plated steel sheet 1 is more preferably 20% or more or 21% or more. There is no particular upper limit to the fracture elongation of the plated steel sheet 1, but from the viewpoint of productivity, the fracture elongation may be, for example, 35% or less or 33% or less.

[0091] The tensile strength (TS) and elongation at break can be measured as follows: First, a No. 5 test piece according to JIS Z 2241:2022 is taken from the width center of the plated steel sheet 1 to be measured, with the longitudinal direction being perpendicular to the rolling direction. Next, a tensile test according to JIS Z 2241:2022 is performed using this test piece, whereby the tensile strength TS (MPa) and elongation at break EL (%) can be measured.

[0092] (Surface Properties After Forming) Furthermore, the plated steel sheet 1 of this embodiment has excellent surface properties even after forming, with a post-forming Sa of 0.10 to 0.50 μm and a post-forming Str of 0.30 to 1.00. Therefore, the plated steel sheet 1 of this embodiment can provide an excellent post-forming appearance.

[0093] (Post-forming Sa is 0.10 to 0.50 μm) As described above, the plated steel sheet 1 of this embodiment can have surface characteristics such that post-forming Sa is 0.10 to 0.50 μm. Post-forming Sa is the average value of the height differences (absolute values) at each point relative to the average plane of the surface after strain is imparted during forming. If post-forming Sa is 0.50 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.

[0094] The post-forming Sa can be measured as follows. First, a No. 5 test piece according to JIS Z 2241:2022 is taken from the plated steel sheet 1 to be measured, with the longitudinal direction perpendicular to the rolling direction. The test piece is taken from a position at least 100 mm away from the end face of the plated steel sheet 1. 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, the surface roughness of the test piece after the tensile test is measured in an 8 mm × 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 a filtering process (i.e., a low-pass filter λs of 0.25 mm) to remove irregularities with a period of 0.25 mm or less, and the arithmetic mean height Sa is determined in the same manner as for the surface roughness Sa of the plated steel sheet 1 described above. The arithmetic mean height Sa thus obtained is defined as the post-molding height Sa.

[0095] (Post-forming Str is 0.30 to 1.00) As described above, the plated steel sheet 1 of this embodiment can have surface characteristics such that the post-forming Str is 0.30 to 1.00. The post-forming Str is an index that takes a value in the range of 0 to 1 and represents the anisotropy of the surface unevenness after strain is imparted during forming. Str, which is the aspect ratio of the surface texture, is one of the spatial parameters of the surface texture defined in JIS B0681-2:2018, Section 4.2.2, "Texture aspect ratio," and is known to indicate the strength of the surface anisotropy and take a value in the range of 0 to 1. A Str close to 0 indicates a highly anisotropic surface texture, such as a streak pattern, while a Str close to 1 indicates an isotropic surface texture that is not direction-dependent. If the post-molding Str is 0.30 or more, even if ghost lines occur during molding, they are difficult to see.

[0096] The post-forming Str can be measured as follows. First, a No. 5 test piece according to JIS Z 2241:2022 is taken from the plated steel sheet 1 to be measured, with the longitudinal direction being perpendicular to the rolling direction. The test piece is taken from a position at least 100 mm away from the end face of the plated steel sheet 1. Next, a tensile strain of 5% is applied to the test piece in the longitudinal direction by a tensile test in accordance with JIS Z 2241:2022. Then, using a laser microscope, the unevenness of the test piece surface 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 spatial plane, and the measurements are performed in a linked manner. The entire measurement area is then subjected to a filtering process (i.e., a low-pass filter λs of 0.25 mm) to remove unevenness with a period of 0.25 mm or less, and the post-forming Str is calculated.

[0097] As described above, the plated steel sheet 1 of this embodiment has high strength and elongation, and can maintain an excellent appearance even after forming, such as press forming. Therefore, the plated steel sheet 1 of this embodiment is very useful for use as exterior panel parts, such as roofs, hoods, fenders, and doors, which require high design quality in automobiles.

[0098] <Method for manufacturing plated steel sheet> Next, an example of a method for manufacturing the plated steel sheet 1 according to one embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing the plated steel sheet 1 according to one embodiment of the present invention, and is not intended to limit the plated steel sheet 1 to one manufactured by the manufacturing method described below.

[0099] The method for producing the plated steel sheet 1 of this embodiment includes a casting step of casting a slab having the above-mentioned specific chemical composition, a hot rolling step of hot-rolling the cast slab, a cold rolling step of cold-rolling the hot-rolled steel sheet, an annealing step of holding the cold-rolled steel sheet in a predetermined atmosphere at a predetermined temperature range, a cooling step of cooling the annealed cold-rolled steel sheet, a plating step of forming a plating layer 3 on the surface of the cooled cold-rolled steel sheet, and a skin-pass rolling step of skin-pass rolling the steel sheet after the plating step. Preferred conditions for these steps will be described below.

[0100] (Casting Step) In the method for producing the plated steel sheet 1 of this embodiment, the casting step is a step of casting a slab having the above-mentioned specific chemical composition. The casting step includes performing soft reduction using a continuous casting machine equipped with a plurality of reduction rolls adjacent to each other in the conveying direction of the slab, the roll pitch between the adjacent reduction rolls being 290 mm or less. In this specification, soft reduction refers to a reduction gradient of 0.6 mm or more per meter in the casting direction.

[0101] As described above, in the plated steel sheet 1 of this embodiment, it is essential that the base steel sheet 2 has a unique metallographic structure that is composed of a lower hard phase fraction and less band-shaped hard phase than conventional DP steel. To obtain such a metallographic structure, it is important to control the solidification structure during casting to be columnar. Specifically, in the casting process, by setting the superheat ΔT (i.e., the difference between the molten steel temperature and the solidification temperature of the molten steel) of the molten steel having the specific chemical composition to 25°C or higher and further setting the segment pressing force to 450 tons or higher, the solidification structure is controlled to a columnar crystal structure with an equiaxed crystal fraction of 15% or less. This method, which differs from conventional methods for preventing center segregation, can also suppress center segregation. The superheat ΔT is more preferably 30°C or higher. Furthermore, the superheat ΔT is preferably 40°C or lower. The molten steel temperature is the temperature of the molten steel in the tundish and can be determined by actual measurement. The solidification temperature can be determined from the chemical composition of the molten steel using a known solidification temperature estimation formula.

[0102] Conventional measures to improve centerline segregation involve minimizing the superheat ΔT (at least below 25°C) and increasing the equiaxed crystal fraction (at least to more than 15%), but such conventional measures do not provide sufficient improvement. In this embodiment, casting conditions that are completely different from the conventional measures described above, namely, unique casting conditions of setting the superheat ΔT to 25°C or higher and setting the segment pressing force to 450 tons or higher, are adopted to control the solidification structure to a columnar crystal structure, thereby suppressing negative Mn segregation. As a result, Mn microsegregation is reduced, and ghost lines can be sufficiently improved.

[0103] The equiaxed crystal ratio (%) can be calculated by taking an etched print of a cross section of the slab in the thickness direction in the width direction, visually determining the boundary between the columnar crystal structure and the equiaxed crystal structure, measuring the thickness (mm) of the equiaxed crystal structure of the slab and the thickness (mm) of the slab, and dividing the thickness of the equiaxed crystal structure by the thickness of the slab and multiplying the result by 100.

[0104] Furthermore, in the casting process, soft reduction is performed using a continuous casting machine in which the roll pitch of adjacent reduction rolls is 290 mm or less, thereby suppressing the flow of molten steel during solidification and reducing the concentration of Mn in the center. This makes it possible to suppress center segregation of Mn. It is more preferable that the roll pitch of adjacent reduction rolls is 280 mm or less.

[0105] (Hot rolling step) In the method for producing the plated steel sheet 1 of this embodiment, the hot rolling step is a step of hot rolling a cast slab. In the hot rolling step, it is preferable to heat the slab to 1200°C or higher prior to hot rolling. By setting the heating temperature to 1200°C or higher, the rolling reaction force does not become excessively large in the hot rolling, making it easier to obtain the target thickness. There is no particular upper limit to the heating temperature, but from an economical viewpoint, it is preferable that the heating temperature be 1300°C or lower.

[0106] In the hot rolling process, the heated slab is subjected to rough rolling and finish rolling. Here, as described above, it is essential for the plated steel sheet 1 of this embodiment to have a unique metallographic structure in which the base steel sheet 2 has a lower hard phase fraction than conventional DP steel, has less band-shaped hard phase, and has little bias in the hard phase fraction between the front and back surfaces of the base steel sheet 2. Such a metallographic structure can be obtained by selecting various rough rolling conditions in this hot rolling process as follows, and uniformly rolling the base steel sheet 2 on both surfaces to uniformly distribute the hard phase.

[0107] In the hot rolling process, the starting temperature of rough rolling is preferably 1150°C or lower. When the starting temperature of rough rolling is 1150°C or lower, the influence of heat removal by the rolling rolls is reduced, and the base steel sheet 2 can be rolled uniformly on both sides. On the other hand, the starting temperature of rough rolling is preferably 1050°C or higher. When the starting temperature of rough rolling is 1050°C or higher, it is possible to control the rolling reaction force so that it does not become excessively large.

[0108] In the hot rolling process, the reduction ratio of the first pass of rough rolling is preferably 45% or less. When the reduction ratio of the first pass of rough rolling is 45% or less, the influence of heat extraction by the rolling rolls is reduced, and the base steel sheet 2 can be rolled uniformly on both sides.

[0109] In the hot rolling process, rough rolling is carried out under conditions that satisfy the following formula (1) in order to control the absolute value of the difference in hard phase fraction between the 1t / 8 to 4t / 8 position and the 4t / 8 to 7t / 8 position in the sheet thickness t from the surface of the base steel sheet 2 to 0 to 8%: 200 (°C% / mm)≦(start temperature of rough rolling×reduction rate of first pass) / diameter of rolling roll≦420 (°C% / mm) (1)

[0110] When rough rolling is performed under conditions that satisfy the above formula (1), it is possible to reduce the influence of heat removal by the rolling rolls and uniformly roll both sides of the base steel sheet 2. In the above formula (1), it is more preferable that "(start temperature of rough rolling × reduction rate of first pass) / diameter of rolling roll (work roll)" is 350°C% / mm or more and 420°C% / mm or less.

[0111] In the hot rolling step, the end temperature of finish rolling is preferably 800°C or higher. When the end temperature of finish rolling is 800°C or higher, the average crystal grain size of the hot-rolled steel sheet and the final product can be reduced, thereby ensuring sufficient yield strength and achieving a higher-quality post-forming appearance. On the other hand, although there is no particular upper limit for the end temperature of finish rolling, from an economical viewpoint, the end temperature of finish rolling is preferably 980°C or lower.

[0112] Furthermore, in order to satisfy formula (1), the diameter of the rolling roll (work roll) used in rough rolling in the hot rolling step is preferably 100 mm or more. When the diameter of the rolling roll is 100 mm or more, strain is less likely to concentrate on the surface in contact with the rolling roll, and the base steel sheet 2 can be rolled uniformly on both sides. On the other hand, there is no particular upper limit to the diameter of the rolling roll, but from an economical viewpoint, it is preferably 700 mm or less. The rolling roll used in the hot rolling step may be heated in advance. When the rolling roll is heated in advance, heat removal from the base steel sheet 2 by the rolling roll is suppressed, and unevenness in heat removal from the base steel sheet 2 by the rolling roll can be reduced.

[0113] The hot-rolled steel sheet obtained in the above hot rolling process is coiled at a coiling temperature of, for example, 450 to 700°C. By setting the coiling temperature to 450°C or higher, the strength of the hot-rolled steel sheet does not become excessively high, and the load during cold rolling after pickling can be reduced. On the other hand, by setting the coiling temperature to 700°C or lower, coarse ferrite and pearlite are less likely to be generated in the structure of the hot-rolled steel sheet, and the uniformity of the structure after annealing can be improved, resulting in a higher-quality post-forming appearance.

[0114] (Cold Rolling Step) The hot-rolled steel sheet obtained in the hot rolling step is subjected to an appropriate pickling treatment to remove scale, and then is subjected to a cold rolling step.

[0115] In the cold rolling process, it is preferable to cold roll the hot-rolled steel sheet so that the cumulative reduction (i.e., cold rolling reduction) is, for example, 65 to 90%. Controlling the cumulative reduction to 65% or more ensures the desired sheet thickness, and also increases the uniformity of the structure by accumulating cold-rolling strain and refining the grains in the subsequent annealing process, resulting in a higher-quality post-forming appearance. On the other hand, controlling the cumulative reduction to 90% or less can prevent the rolling load from becoming excessively large, making rolling difficult.

[0116] (Annealing Step) The cold-rolled steel sheet obtained in the cold rolling step is subjected to an annealing step in which it is held in a predetermined atmosphere in a predetermined temperature range.

[0117] In the annealing process, the cold-rolled steel sheet is preferably held in a reducing atmosphere, for example, at a holding temperature in the temperature range of 750 to 900°C, for a predetermined time. Here, the term "reducing atmosphere" refers to an atmosphere mainly composed of a reducing gas composed of hydrogen and an inert gas such as nitrogen or argon. The reducing gas used is a mixed gas of hydrogen and nitrogen with a concentration of 2 to 15% in order to adjust the surface roughness Sa of the plated steel sheet 1 to 0.10 to 0.50 μm. A reducing gas of this concentration easily reduces the surface of the base steel sheet 2 and can improve wettability to the plating, resulting in a high-quality post-forming appearance.

[0118] Furthermore, the dew point of the reducing atmosphere in the annealing step is set to -5°C to 10°C in order to adjust the surface roughness Sa of the plated steel sheet 1 to 0.10 to 0.50 μm. If the dew point is -5°C or higher, the surface of the base steel sheet 2 is easily reduced and wettability to the plating can be improved, resulting in a high-quality post-forming appearance. On the other hand, if the dew point is 10°C or lower, condensation is less likely to occur in the manufacturing equipment, and there is little risk of disrupting the operation of the manufacturing equipment.

[0119] Furthermore, in the annealing step, if the hydrogen concentration of the reducing atmosphere is outside the range of 2 to 15% or the dew point is outside the range of -5°C to 10°C, decarbonization and demanganization occur from the surface of the steel sheet, which may cause uneven plating adhesion, and the surface layer of the steel sheet becomes soft, which may result in a decrease in the tensile strength of the base steel sheet 2.

[0120] As described above, the holding temperature in the annealing step is preferably 750°C or higher. When the holding temperature in the annealing step is 750°C or higher, the recrystallization of ferrite and the reverse transformation from ferrite to austenite can be sufficiently promoted, making it easier to obtain the desired metal structure in the final product. In addition, the holding temperature in the sintering step is preferably 900°C or lower. When the holding temperature in the annealing step is 900°C or lower, the desired microstructure fraction can be obtained, and the crystal grains can be densified to obtain sufficient strength.

[0121] In the annealing step, the time for maintaining the holding temperature, i.e., the holding time, is preferably 20 seconds or more. If the holding time in the annealing step is 20 seconds or more, the recrystallization of ferrite and the reverse transformation from ferrite to austenite can be sufficiently promoted, making it easier to obtain the desired metal structure in the final product. Furthermore, the holding time in the annealing step is preferably 300 seconds or less. If the holding time in the annealing step is 300 seconds or less, the desired microstructure fraction can be obtained, and the crystal grains can be densified to obtain sufficient strength.

[0122] (Cooling Step) The cold-rolled steel sheet after the annealing step is subjected to the cooling step, which is a step of cooling the cold-rolled steel sheet heated in the annealing step.

[0123] In the cooling step, the cooling rate when cooling the cold-rolled steel sheet is preferably 5°C / sec or more. When the cooling rate is 5°C / sec or more, excessive transformation to ferrite can be suppressed, and the amount of hard phases such as martensite produced can be increased, making it easier to obtain the desired strength. Furthermore, the cooling rate is preferably 50°C / sec or less. When the cooling rate is 50°C / sec or less, the base steel sheet 2 can be cooled more uniformly in the width direction.

[0124] In the cooling step, the cooling stop temperature is preferably 450°C or higher. When the cooling stop temperature is 450°C or higher, reheating of the plating bath or alloying treatment is not required in the subsequent plating step, and manufacturing costs can be reduced. Furthermore, the cooling stop temperature is preferably 650°C or lower. When the cooling stop temperature is 650°C or lower, the amount of hard phases such as martensite produced is increased, making it easier to obtain the desired strength.

[0125] (Plating step) The cooled cold-rolled steel sheet is subjected to a plating step in order to form a plating layer 3 on its surface. In the plating step, a plating treatment is performed on the surface of the cold-rolled steel sheet, thereby forming a predetermined plating layer 3 on the surface of the base steel sheet 2.

[0126] As the plating treatment, known treatments such as hot dip plating, alloying hot dip plating, electroplating, etc. can be used. For example, as the plating treatment, a hot dip galvanizing treatment may be performed on the surface of the base steel sheet 2, or an alloying treatment may be performed after the hot dip galvanizing treatment. Specific conditions for the plating treatment and the alloying treatment are not particularly limited, and any appropriate conditions known to those skilled in the art can be used.

[0127] The coating weight of the plating layer 3 formed by the plating treatment is set to 20 g / m as the coating weight per one side of the base steel sheet 2 from the viewpoint of adjusting the surface roughness Sa of the plated steel sheet 1 to 0.10 to 0.50 μm as described above. ２ or more and 120 g / m ２ The deposition amount is as follows.

[0128] When alloying treatment is performed in the plating step, the alloying treatment temperature is preferably 480°C or higher. When the alloying treatment temperature is 480°C or higher, carbides are less likely to be generated, making it easier to ensure the desired ductility. Furthermore, the alloying treatment temperature is preferably 600°C or lower. When the alloying treatment temperature is 600°C or lower, alloying proceeds quickly, allowing productivity to be improved.

[0129] (Skin-pass rolling process) In the method for producing the plated steel sheet 1 of this embodiment, the steel sheet after the plating process, i.e., the plated steel sheet 1, is subjected to skin-pass rolling. That is, the steel sheet after the plating process is subjected to the skin-pass rolling process. In this skin-pass rolling process, the skin-pass rolling ratio is 0.8% to 2.1% from the viewpoint of adjusting the surface roughness Sa of the plated steel sheet 1 to 0.10 to 0.50 μm. When the skin-pass rolling ratio is within this range, dislocations are accumulated while the surface of the plating layer 3 is smoothed, and the yield point elongation is easily eliminated.

[0130] The plated steel sheet 1 of the above embodiment can be manufactured by the manufacturing method described above.

[0131] 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.

[0132] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0133] In the following examples, plated steel sheets according to one embodiment of the present invention (i.e., plated steel sheets of invention examples) and plated steel sheets for comparison therewith (i.e., plated steel sheets of comparative examples) were produced under various conditions, and the tensile strength, breaking elongation, appearance after forming, and other properties of each of the obtained plated steel sheets were evaluated.

[0134] (Production of plated steel sheet) First, a continuous casting machine equipped with a plurality of reduction rolls arranged at a roll pitch of 290 mm or less was used to cast slabs having the chemical compositions shown in Table 1 below by a continuous casting method in which soft reduction was performed with a reduction gradient of 0.6 mm or more in the casting direction. The segment pressing force was 450 tons or more. The balance other than the components shown in Table 1 consisted of Fe and impurities. The superheat conditions for each example are shown in Table 2 below. The underlined chemical compositions in Table 1 indicate that the compositions are outside the scope of the present invention.

[0135] Next, the obtained slabs were subjected to a hot rolling process, a cold rolling process, an annealing process, and a cooling process under the conditions shown in Table 2 below, thereby obtaining cold-rolled steel sheets. Furthermore, both surfaces of the obtained cold-rolled steel sheets were plated to form galvannealed layers (GA), thereby obtaining plated steel sheets Nos. 1 to 26. Regarding the notation of superheat ΔT in Table 2, a superheat ΔT of 25°C or more was represented as "OK," and a superheat ΔT of less than 25°C was represented as "NG." Slabs with a superheat ΔT of 25°C or more had an equiaxed crystal fraction of 15% or more, and slabs with a superheat ΔT of less than 25°C had an equiaxed crystal fraction of less than 15%.

[0136] Furthermore, when the chemical composition of the samples taken from the above cold-rolled steel sheets was analyzed, it was confirmed that there was no change from the chemical composition of the slab shown in Table 1.

[0137] (Evaluation of Plated Steel Sheets) For the obtained plated steel sheets Nos. 1 to 26, the metallographic structure of the base steel sheet, the surface characteristics before forming (i.e., surface roughness Sa of the plated steel sheet), the mechanical strength (i.e., tensile strength and elongation at break), and the surface characteristics after forming (i.e., Sa and Str after forming) were measured, and the respective strengths, elongations, and appearances after forming were evaluated. Note that, because the rolling direction of the plated steel sheets was known, the above-mentioned method for determining the rolling direction was not used.

[0138] In the evaluation, a plated steel sheet was evaluated as having excellent strength and elongation and an improved post-forming appearance if it satisfied the following criteria: a tensile strength of 540 MPa or more, an elongation at break of 19% or more, a post-forming Sa of 0.10 to 0.50 μm, and a post-forming Str of 0.30 to 1.00. The results are shown in Table 2 below. Note that the "value of (Equation 1)" in Table 2 refers to the calculated value of "(start temperature of rough rolling × reduction rate of first pass) / diameter of rolling roll." In addition, underlines next to various numerical values ​​in Table 2 indicate that the value is outside the range of the present invention, that the plated steel sheet of the present invention cannot be obtained under certain manufacturing conditions, or that the mechanical properties or surface properties after forming do not satisfy the above criteria.

[0139]

[0140]

[0141] As shown in Table 2, the inventive examples of steel sheets Nos. 1, 6, 8, 11, 12, 14, 15 and 18, which had chemical compositions, metal structures and surface roughnesses Sa of the plated steel sheets within the ranges of the present invention, all had tensile strengths of 540 or more, elongations at break of 19% or more, post-forming Sa in the range of 0.10 to 0.50 μm and post-forming Str in the range of 0.30 to 1.00. In other words, it was found that the plated steel sheets of the inventive examples all had excellent strength and elongation and further improved post-forming appearance.

[0142] On the other hand, in Comparative Examples 2, 3, 9, and 13, in which the absolute value of the difference in hard phase fraction between the 1t / 8 to 4t / 8 position and the 4t / 8 to 7t / 8 position was outside the range of the present invention, the post-forming Sa and Str were all outside the appropriate ranges, and good post-forming appearance was not obtained. In Comparative Examples 4, 5, 10, 17, and 19, in which the hard phase fraction was outside the range of the present invention, the tensile strength and breaking elongation were all outside the appropriate ranges, and good strength and elongation were not obtained. In Comparative Examples 4 and 5, the post-forming Sa was also outside the appropriate range, and good post-forming appearance was not obtained. Furthermore, in Comparative Examples 7, 20, and 21, in which the surface roughness Sa of the plated steel sheet was outside the range of the present invention, the post-forming Sa and Str were all outside the appropriate ranges, and good post-forming appearance was not obtained. Finally, in Comparative Examples 7, 20, and 21, in which the area ratio of the band-shaped hard phase, etc., were .... In the comparative examples 16 and 22 to 26, the post-molding Sa and Str were all outside the appropriate range, and a good post-molding appearance was not obtained.

[0143] 1 Plated steel sheet 2 Base steel sheet 3 Plated layer

Claims

DEPCT681. Surface-coated thin steel sheets consisting of a base thin steel sheet and a plating layer arranged on the surface of the base thin steel sheet, in which the chemical composition of the base thin steel sheet is additionally included, in percentage by mass: C: 0.03 to 0.10 percent, Si: 0.01 to 1.50 percent, Mn: 1.0 to 2.5 percent, Al: 0.005 to 0.700 percent, Cr: 0.15 to 0.80 percent, Mo: 0.15 to 0.50 percent, Ti:

0. O: 0.3 to 0.10 percent, P: 0.1000 percent or less, S: 0.0200 percent or less, N: 0.015 percent or less, O: 0.0200 percent or less, B: 0 to 0.010 percent, Nb: 0 to 0.10 percent, V: 0 to 0.50 percent, Ni: 0 to 1.00 percent, Cu: 0 to 1.00 percent, W: 0 to 1.00 percent, Sn: 0 to 1.00 percent, Sb: 0 to 0.20 percent, Ca: 0 to 0.010 percent, Zr: 0 to 0.010 percent, REM: 0 to 0.0-10 percent, and the remainder: Fe and impurities, the microstructure of the base thin steel sheet includes, in percentage by area, ferrite: 80 to 97 percent, solid phase: 3 to 20 percent, the area ratio of the strip solid phase at the position of 3t / 8 to 5t / 8 of the sheet thickness “t” is 0 to 5 percent, the absolute value of the difference of the solid phase fraction at the positions of 1t / 8 to 4t / 8 and 4t / 8 to 7t / 8 of the sheet thickness “t” from the surface of the base thin steel sheet is 0 to 8 percent, and the surface roughness Sa of the plated thin steel sheet is 0.10 to 0.50 microns.

2. Thin steel sheets coated according to claim 1, whereby the chemical composition of the base thin steel sheet includes, in percentage by mass, one or more elements selected from the group consisting of B: 0.0001 to 0.010 percent, Nb: 0.001 to 0.10 percent, V: 0.001 to 0.50 percent, Ni: 0.001 to 1.00 percent, Cu: 0.001 to 1.00 percent, W: 0.001 to 1.00 percent, Sn: 0.001 to 1.00 percent, Sb: 0.001 to 0.20 percent, Ca: 0.0001 to 0.010 percent, Zr: 0.0001 to 0.010 percent, and REM: 0.0001 to 0.010 percent.010 percent 3. Surface-treated thin steel sheet according to Requisition 1 or 2 where in the microstructure of the base thin steel sheet, the solid phase is 5 percent or more.

4. Surface-treated thin steel sheet according to any of Requisitions 1 through 3 where the absolute value of the difference in the solid phase fraction at positions 1t / 8 to 4t / 8 and 4t / 8 to 7t / 8 of the sheet thickness “t” from the surface of the base thin steel sheet is 5 percent or less.

5. Surface-treated thin steel sheet according to any of Requisitions 1 through 3. 1 to 4 where the tensile strength of the surface-treated thin steel sheet is 540 MPa or more; 6. Surface-treated thin steel sheet according to any of the claims 1 to 5 where the average crystalline grain size of ferrite is 5.0 to 30.0 micrometers and the average crystalline grain size of the solid phase is 1.0 to 5.0 micrometers; 7. Surface-treated thin steel sheet according to any of the claims 1 to 6 where the solid phase consists of at least one structure which is martensite, bainite, temper martensite, and perlite;

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