Plated steel sheet

Abstract

Provided is a plated steel sheet comprising a base material steel sheet and plating disposed on a surface of the base material steel sheet, the plated steel sheet characterized in that the base material steel sheet has a chemical composition containing, in terms of mass%, 0.010%-1.000% of Ni, 0.010%-1.000% of Cu, and 0.003%-1.000% of Sn; and in an element distribution image obtained by measuring, with Auger electron spectroscopy, a surface of the plated steel sheet, a surface coverage with at least one of Ni, Cu, and Sn is 25% or more, and the equivalent circle radius of a non-covered region not covered with any of Ni, Cu, or Sn is 10 μm or less.

Description

Plated steel sheet

[0001] The present invention relates to a plated steel sheet.

[0002] It is known that in order to improve the paint film adhesion of steel sheets or plated steel sheets, it is effective to enhance the chemical conversion treatability of the steel sheets or plated steel sheets and to form a uniform chemical conversion coating on these steel sheets.

[0003] In relation to this, Patent Document 1 discloses that when a high-strength cold-rolled steel sheet is continuously annealed in a continuous annealing furnace or a cold-rolled steel sheet / hot-dip galvanized steel sheet dual-purpose facility having a continuous annealing furnace, the cooling method of which in a cooling zone including a part or all of the steel sheet temperature range of 600 to 250°C following heating for recrystallization is one or more of gas cooling, diffusion cooling, and cooling pipe cooling, the steel sheet surface is exposed to an atmosphere in which iron oxidizes within the above-mentioned steel sheet temperature range, pickled at the outlet side of the annealing furnace, and then iron or Ni plating is applied to a thickness of 1 to 50 mg / m 2 Furthermore, Patent Document 1 teaches that, although oxidation of the steel sheet is usually prevented by an extremely low concentration of oxygen and / or an inert atmospheric gas with an extremely low dew point around the steel sheet, the steel sheet is instead actively exposed to an oxidizing atmosphere to oxidize not only Si and Mn but also the iron of the steel sheet, and the oxide films of Si, Mn, etc. are removed together with the oxide film of iron on the steel sheet by pickling immediately after leaving the annealing furnace, thereby obtaining a high-strength cold-rolled steel sheet that is free from "hollow-out" and has good chemical conversion treatability even if the contents of Si, Mn, etc. are high.

[0004] In Patent Document 2, a copper (Cu) content of 0.10 mass % or more and 0.50 mass % or less is contained, and the number of residual scales on the surface is 160,000 pieces / mm 2 Patent Document 2 teaches that, according to the above configuration, the particle size of the copper compound particles exposed on the steel sheet surface, which serves as the cathode point in chemical conversion treatment, is 2 μm or less, and the residual scale is reduced to a predetermined amount or less, thereby making it possible to provide a steel sheet with excellent chemical conversion treatability.

[0005] JP 2008-190030 A JP 2020-084238 A

[0006] Patent Document 2 teaches that elements such as nickel (Ni) and tin (Sn) in addition to copper (Cu) reduce the mechanical properties required for automotive steel sheets, such as strength and formability, as well as chemical stability such as corrosion resistance, and that copper compounds present on the surface of the steel sheet in particular reduce the chemical conversion treatability required for improving corrosion resistance. In addition, generally, when chemical conversion treatability is reduced, areas where a chemical conversion coating is not formed, known as "whiteout," may occur, which may result in reduced paint film adhesion.

[0007] Therefore, an object of the present invention is to provide a plated steel sheet containing Ni, Cu, and Sn, which is capable of exhibiting improved paint film adhesion.

[0008] In order to achieve the above object, the present inventors conducted research focusing on the element distribution on the surface of a steel sheet, and as a result, they found that coating adhesion can be significantly improved by plating the surface of a base steel sheet so that, when measured by Auger electron spectroscopy, the surface is covered with at least one of Ni, Cu, and Sn at a predetermined surface coverage rate and the size of the uncovered region not covered with these elements is limited to a predetermined range, thereby completing the present invention.

[0009] The present invention, which has achieved the above-mentioned object, is as follows: (1) A plated steel sheet comprising a base steel sheet and a plating disposed on the surface of the base steel sheet, wherein the base steel sheet has a chemical composition containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and wherein, in an element distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy, the surface coverage with at least one of Ni, Cu, and Sn is 25% or more, and the circle-equivalent radius of an uncoated region not covered with at least one of Ni, Cu, and Sn is 10 μm or less. (2) The plated steel sheet according to (1) above, wherein the surface coverage is 35% or more. (3) The plated steel sheet according to (2) above, wherein the surface coverage is 50% or more. (4) The plated steel sheet according to any one of (1) to (3) above, characterized in that the surface coverage is 80% or less. (5) The plated steel sheet according to any one of (1) to (4) above, characterized in that the circle-equivalent radius of the uncoated region is 5 μm or less. (6) The plated steel sheet according to any one of (1) to (5) above, characterized in that the chemical composition includes, in mass %, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%. (7) The plated steel sheet according to any one of (1) to (6) above, characterized in that it has a Vickers hardness of 200 Hv or more.

[0010] According to the present invention, it is possible to provide a plated steel sheet containing Ni, Cu, and Sn, which is capable of exhibiting improved paint film adhesion.

[0011] <Plated Steel Sheet> A plated steel sheet according to an embodiment of the present invention comprises a base steel sheet and a plating disposed on a surface of the base steel sheet, wherein the base steel sheet has a chemical composition containing, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and wherein an element distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy shows that the surface coverage with at least one of Ni, Cu, and Sn is 25% or more, and the circle-equivalent radius of an uncoated region that is not covered with at least one of Ni, Cu, and Sn is 10 μm or less.

[0012] As mentioned above, generally, when chemical conversion treatability is reduced, regions where the chemical conversion coating is not formed, known as "skid zones," may occur, resulting in reduced paint adhesion. For example, when elements such as Ni, Cu, and Sn are present in a steel sheet as a solid solution, the potential of the steel sheet becomes more noble than when these elements are not present in a solid solution, which may reduce the etching ability of Fe during chemical conversion treatment. In this case, the chemical conversion treatability of the steel sheet is reduced, resulting in reduced paint adhesion. Therefore, when a steel sheet simultaneously contains the three elements Ni, Cu, and Sn, this reduced paint adhesion is particularly problematic.

[0013] Furthermore, two commonly known methods for producing steel sheets are, for example, a method in which molten iron is obtained in a blast furnace using iron ore, a natural resource, as the main raw material, and then molten steel is produced through refining in a converter or the like, and a method in which molten steel is produced in an electric furnace using scrap material, a recycled resource, as the main raw material. Since blast furnace steel may also contain elements such as Ni, Cu, and Sn as additive elements, when these elements are contained, it is necessary to appropriately address the above-mentioned issues. On the other hand, electric furnace steel uses scrap material as the main raw material as described above, and therefore contains relatively large amounts of elements derived from the scrap (so-called tramp elements), such as Ni, Cu, and Sn, and therefore the above-mentioned issues become particularly pronounced.

[0014] Therefore, the present inventors conducted studies, focusing particularly on the element distribution on the steel sheet surface, in order to provide a plated steel sheet that can exhibit excellent paint adhesion even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, the present inventors found that it is effective to uniformly coat the surface of a base steel sheet containing Ni, Cu, and Sn with at least one of these elements. More specifically, the present inventors found that paint adhesion can be significantly improved by plating the surface of the base steel sheet so that, when measured by Auger electron spectroscopy, the surface of the base steel sheet is covered with at least one of Ni, Cu, and Sn at a predetermined surface coverage, more specifically, a surface coverage of 25% or more, and the size of the uncoated region not covered with these elements, i.e., Ni, Cu, and Sn, is limited within a predetermined range, more specifically, the circle-equivalent radius of the uncoated region is limited to 10 μm or less.

[0015] Without intending to be bound by any particular theory, it is believed that uniformly coating the surface of a base steel sheet with at least one of Ni, Cu, and Sn allows these elements to function appropriately as cathode sites during chemical conversion treatment, thereby promoting the anodic dissolution (etching) of Fe present around these elements. More specifically, Ni, Cu, and Sn are elements that are potentially more noble than Fe. Therefore, by having these elements uniformly dispersed on the steel sheet surface rather than in a solid solution state, these elements function as effective cathode sites in relation to Fe during chemical conversion treatment, as described above, promoting the etching of the surrounding Fe. This is believed to significantly improve the chemical treatability of the steel sheet. As a result, a chemical conversion coating can be formed uniformly over the entire steel sheet, significantly improving paint adhesion. Even if the surface coverage rate of at least one of Ni, Cu, and Sn is very high, for example, 50% or higher, if a relatively large uncoated region not covered by these elements exists, such uncoated region will not be able to promote the anodic dissolution of Fe during chemical conversion treatment. As a result, the chemical conversion coating cannot be formed uniformly over the entire steel sheet, resulting in reduced coating adhesion. Therefore, in the plated steel sheet according to the embodiment of the present invention, it is important to uniformly coat the surface of the base steel sheet with at least one of Ni, Cu, and Sn. In other words, it is important to satisfy both the following requirements in an element distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy: a surface coverage rate of at least one of Ni, Cu, and Sn of 25% or more; and a circle-equivalent radius of the uncoated region not covered by at least one of Ni, Cu, and Sn of 10 μm or less. This is because, if either of these requirements is not satisfied, the chemical conversion coating cannot be formed uniformly over the entire steel sheet during chemical conversion treatment. On the other hand, by satisfying both of these requirements, it is possible to form a chemical conversion coating uniformly over the entire steel sheet, and as a result, it is possible to significantly improve the paint adhesion.

[0016] The plated steel sheet according to the embodiment of the present invention encompasses not only electric furnace steel, which inevitably contains Ni, Cu, and Sn as tramp elements, but also blast furnace steel, which contains Ni, Cu, and Sn as essential elements or optional added elements. Furthermore, the plated steel sheet according to the embodiment of the present invention can achieve superior paint adhesion and, in turn, superior corrosion resistance compared to conventional plated steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the plated steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where superior paint adhesion and / or corrosion resistance are required. Each component of the plated steel sheet according to the embodiment of the present invention will be described in more detail below.

[0017] [Plating] According to an embodiment of the present invention, a plating is disposed on the surface of a base steel sheet, for example, on at least one surface, preferably both surfaces, of the base steel sheet. The plating may contain at least one of Ni, Cu, and Sn, and may also contain other elements, such as Zn, Al, and Fe, in addition to Ni, Cu, and Sn. For example, the plating may consist essentially of at least one of Ni, Cu, and Sn, or may consist of at least one of Ni, Cu, and Sn, or may be composed of at least one of Ni, Cu, and Sn. The contents and coating weights of Ni, Cu, and Sn in the plating are not particularly limited and may be appropriately selected within ranges that satisfy the requirements for surface coverage and uncoated areas, which will be described in detail later.

[0018] [Surface Coverage by At Least One of Ni, Cu, and Sn: 25% or More] In an embodiment of the present invention, in an element distribution image obtained by measuring the surface of a plated steel sheet by Auger electron spectroscopy, the surface coverage by at least one of Ni, Cu, and Sn is controlled to be 25% or more. As described above, Ni, Cu, and Sn can function as cathode sites during chemical conversion treatment. In chemical conversion treatment, electrons are generally generated by the anodic dissolution (etching) of Fe, and on the other hand, the electrons generated by the anodic dissolution of Fe at the cathode sites cause a cathodic reaction (2H + +2e - →H2, 10H + +NO3- +8e - →NH4 + In connection with this, the pH of the chemical conversion treatment solution in the vicinity of the steel sheet surface increases, and as a result, compounds such as zinc phosphate crystals that constitute the chemical conversion treatment film are precipitated on the steel sheet surface.

[0019] In an embodiment of the present invention, by controlling the surface coverage of the plated steel sheet with at least one of Ni, Cu, and Sn to 25% or more while satisfying the requirements for the uncoated region described below, the surface of the base steel sheet can be uniformly coated with at least one of Ni, Cu, and Sn, allowing these elements to effectively function as cathode sites. As a result, the above-mentioned cathodic reaction can be appropriately promoted over the entire surface of the steel sheet, allowing a chemical conversion coating to be uniformly formed over the entire steel sheet, thereby significantly improving paint adhesion. From the viewpoint of further improving paint adhesion, the higher the surface coverage, the more preferable it is. For example, the surface coverage with at least one of Ni, Cu, and Sn is preferably 30% or more or 35% or more, more preferably 40% or more or 45% or more, and most preferably 50% or more or 55% or more. While there is no particular upper limit, the surface coverage with at least one of Ni, Cu, and Sn may be, for example, 80% or less, 75% or less, 70% or less, or 65% or less.

[0020] [Circle-equivalent Radius of Uncoated Region: 10 μm or Less] In an embodiment of the present invention, in an element distribution image obtained by measuring the surface of a plated steel sheet by Auger electron spectroscopy, the circle-equivalent radius of an uncoated region not coated with at least one of Ni, Cu, and Sn (i.e., an uncoated region not coated with any of Ni, Cu, and Sn) is controlled to 10 μm or less. If a relatively large uncoated region not coated with at least one of Ni, Cu, and Sn is present, naturally, no cathode sites exist in such an uncoated region, and therefore, the anodic dissolution of Fe cannot be promoted during chemical conversion treatment. As a result, the chemical conversion treatment film cannot be formed uniformly over the entire steel sheet, and coating adhesion is reduced.

[0021] In an embodiment of the present invention, while satisfying the surface coverage requirements described above, by providing such an uncoated region and controlling the uncoated region to a circle-equivalent radius of 10 μm or less, the surface of the base steel sheet can be uniformly coated with at least one of Ni, Cu, and Sn, allowing these elements to effectively function as cathode sites. As a result, the above-mentioned cathodic reaction can be appropriately promoted over the entire surface of the steel sheet, allowing a chemical conversion coating to be uniformly formed over the entire steel sheet, thereby significantly improving paint adhesion. From the viewpoint of further improving paint adhesion, the smaller the circle-equivalent radius of the uncoated region, the more preferable it is. For example, the circle-equivalent radius of the uncoated region not coated with at least one of Ni, Cu, and Sn is preferably 8 μm or less, more preferably 6 μm or less or 5 μm or less, and most preferably 4 μm or less or 3 μm or less. Although there is no particular lower limit, the circle-equivalent radius of the uncoated region not coated with at least one of Ni, Cu, and Sn may be, for example, 0.5 μm or more or 1 μm or more.

[0022] [Measurement of Surface Coverage and Uncovered Region] The surface coverage is measured by Auger electron spectroscopy as follows. First, a sample including a plate surface is placed in an Auger electron spectrometer (e.g., AES PHI-700 (ULVAC-PHI, FE type)), and the sample surface (plate surface) is measured under conditions of an acceleration voltage of 10 kV, a current value of 10 nA, and an Auger spectrum measurement energy range of 40 to 1690 eV. The measurement region is observed at 1000x magnification or more with an SEM, and is set to an area of ​​60 μm × 100 μm or more. Next, a mapping is created with a lower limit of 80 cps (counts / s), and an element distribution image is obtained. Finally, the obtained element distribution image is binarized (min = 0, max = 255) using image analysis software "ImageJ," and the coverage of the measurement area by at least one of Ni, Cu, and Sn is calculated. The calculated value is determined as the surface coverage of the plated steel sheet by at least one of Ni, Cu, and Sn.

[0023] The uncoated regions are calculated by calculating the total area and number of uncoated regions that are not coated with at least one of Ni, Cu, and Sn in a binarized image obtained using the image analysis software "ImageJ" in connection with the measurement of the surface coverage. Next, the average area S per uncoated region is calculated by dividing the total area of ​​the uncoated regions by the total number of uncoated regions. Finally, the circle-equivalent radius r is calculated using the following formula, and this is determined as the circle-equivalent radius of the uncoated regions that are not coated with at least one of Ni, Cu, and Sn. r = (S / π) 0.5

[0024] [Base Steel Sheet] In an embodiment of the present invention, the base steel sheet has a chemical composition containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. As described above, an object of the present invention is to provide a plated steel sheet containing Ni, Cu, and Sn that can exhibit improved paint adhesion, and this object is achieved by plating the base steel sheet so that, when measured by Auger electron spectroscopy, the surface of the base steel sheet is covered with at least one of Ni, Cu, and Sn at a surface coverage rate of 25% or more, and the circle-equivalent radius of the uncoated region not covered with these elements is limited to 10 μm or less. Therefore, the chemical composition of the base steel sheet is not particularly limited except that it contains, in mass %, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, and 0.003 to 1.000% Sn, and therefore it is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the object of the present invention. The chemical composition of the base steel sheet may contain, in addition to Ni, Cu, and Sn, appropriate amounts of any alloying elements commonly added in the technical field of the present invention. Hereinafter, the chemical composition of the base steel sheet used in the plated steel sheet according to the embodiment of the present invention will be described in detail, but these descriptions are intended to merely exemplify preferred chemical compositions of base steel sheets for use in automotive steel sheets and the like, and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0025] In an embodiment of the present invention, for example, the base steel plate contains, in mass %, C: 0.001 to 0.500%, Si: 0 to 3.00%, Mn: 0.10 to 3.00%, Al: 0.001 to 2.000%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, Sn: 0.003 to 1.000%, P: 0.100% or less, S: 0.100% or less, N: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0 to 1.000%, Cr: 0 to 1.000%, V: 0 to 0.150%, W It is preferable that the alloy has a chemical composition consisting of: Cr: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.010%, REM: 0 to 0.010%, As: 0 to 0.010%, Ir: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0026] [C: 0.001 to 0.500%] C is an element that inexpensively increases strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.001% or more. The C content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.040% or more, 0.070% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.500% or less. The C content may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.

[0027] [Si: 0 to 3.00%] Si is an element that is effective in increasing strength as a solid solution strengthening element. The Si content may be 0%, but to obtain this effect, the Si content is preferably 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.30% or more, 0.50% or more, 0.80% or more, or 1.00% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.00% or less. The Si content may be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.

[0028] [Mn: 0.10 to 3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.50% or more, 1.00% or more, 1.30% or more, 1.50% or more, or 1.80% or more. On the other hand, excessive Mn content may increase the steel strength but reduce elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[0029] [Al: 0.001 to 2.000%] Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel. To fully obtain this effect, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0030] [Ni: 0.010 to 1.000%] [Cu: 0.010 to 1.000%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. To fully achieve these effects, the contents of these elements are preferably 0.010% or more, and may be 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, excessive content of these elements may promote the formation of oxides, particularly Mn- and / or Si-based surface oxides and iron oxides, on the steel sheet surface, which may impair plating adhesion in the plating process. Therefore, the Ni and Cu contents are each preferably 1.000% or less, and may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.

[0031] [Sn: 0.003 to 1.000%] Sn is an element effective in improving corrosion resistance. To fully obtain this effect, the Sn content is preferably 0.003% or more. The Sn content may be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, excessive Sn content may promote the formation of oxides, particularly Mn- and / or Si-based surface oxides and iron oxides, on the steel sheet surface, which may impair plating adhesion in the plating process. Therefore, the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[0032] [P: 0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. Since a lower P content is preferable, ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0033] [S: 0.100% or less] S is an element that generates non-metallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content may result in a significant increase in costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content may cause cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably 0.100% or less. The S content may be 0.050% or less, 0.020% or less, or 0.010% or less.

[0034] [N: 0.0100% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, the ideal N content is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0100% or less. The N content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0035] The base steel sheet preferably has the basic chemical composition described above. Furthermore, the base steel sheet may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0036] [Ti: 0 to 0.150%] [Nb: 0 to 0.150%] [V: 0 to 0.150%] Ti, Nb, and V form carbonitrides in steel and have the effect of improving the strength of the steel sheet through precipitation strengthening. The Ti, Nb, and V contents may be 0%, but to obtain this effect, the Ti, Nb, and V contents are preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, even if these elements are contained in excess, the effect saturates, and adding more than necessary to the steel increases manufacturing costs. Therefore, the Ti, Nb, and V contents are preferably 0.150% or less, and may be 0.120% or less, 0.100% or less, 0.080% or less, 0.050% or less, 0.020% or less, or 0.015% or less.

[0037] [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but to achieve this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0038] [Mo: 0 to 1.000%] [Cr: 0 to 1.000%] [W: 0 to 1.000%] Mo, Cr, and W are elements that improve the hardenability of steel and contribute to improving its strength. The Mo, Cr, and W contents may be 0%, but to achieve these effects, the Mo, Cr, and W contents are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel increases manufacturing costs. Therefore, the Mo, Cr, and W contents are preferably 1.000% or less, and may be 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0039] [Hf: 0-0.050%] [Mg: 0-0.050%] [Zr: 0-0.050%] [Ca: 0-0.010%] [REM: 0-0.010%] Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of non-metallic inclusions. The Hf, Mg, Zr, Ca, and REM contents may be 0%, but to achieve these effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel sheet increases manufacturing costs. Therefore, the Hf, Mg, and Zr contents are preferably 0.050% or less, and may be 0.010% or less, 0.005% or less, or 0.003% or less. Similarly, the Ca and REM contents are each preferably 0.010% or less, and may be 0.005% or less or 0.003% or less.

[0040] [As: 0 to 0.010%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more or 0.003% or more. On the other hand, even if an excessive amount of As is contained, the effect saturates, and containing more As than necessary in the steel sheet increases the manufacturing cost. Therefore, the As content is preferably 0.010% or less. The As content may be 0.008% or less or 0.005% or less.

[0041] [Ir: 0 to 1.000%] Ir is an element that segregates at prior austenite grain boundaries to increase the strength of the grain boundaries. The Ir content may be 0%, but to obtain this effect, the Ir content is preferably 0.001% or more. The Ir content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even if an excessive amount of Ir is contained, the effect saturates, and adding more Ir than necessary to the steel material increases the manufacturing cost. Therefore, the Ir content is preferably 1.000% or less. The Ir content may be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0042] The remainder of the base steel plate other than the above elements consists of Fe and impurities. The impurities in the base steel plate are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel plate is industrially manufactured.

[0043] The chemical composition of the base steel sheet may be measured by a common analytical method. For example, the chemical composition of the base steel sheet may be measured by first removing the plating layer by mechanical grinding, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece may be obtained from the base steel sheet at approximately half the thickness, and the test piece may be measured using a Shimadzu ICPS-8100 (measuring device) or the like under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

[0044] [Thickness of Base Steel Plate] The thickness of the base steel plate is not particularly limited, but is generally 0.2 to 8.0 mm. For example, the thickness may be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the thickness of the base steel plate may be, for example, 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0045] [Mechanical Properties] The plated steel sheet according to the embodiment of the present invention may have, for example, a Vickers hardness of 90 Hv or more, without being particularly limited thereto. The Vickers hardness may be 150 Hv or more, 200 Hv or more, 250 Hv or more, 300 Hv or more, 350 Hv or more, 400 Hv or more, or 450 Hv or more. The upper limit is not particularly limited, but the Vickers hardness may be, for example, 650 HV or less, 600 HV or less, 550 HV or less, or 500 HV or less.

[0046] [Measurement of Vickers Hardness] Vickers hardness is determined as follows. First, a test piece is cut out from any position of the plated steel sheet, excluding the edge, so that a cross section (thickness cross section) perpendicular to the surface can be observed. The thickness cross section of the test piece is polished using #600 to #1500 silicon carbide paper, and then mirror-finished using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water, and this thickness cross section is used as the measurement surface. Next, the Vickers hardness is measured using a micro Vickers hardness tester at a load of 1 kgf at intervals of at least three times the indentation. Specifically, a total of 20 points are measured randomly near the half-thickness position of the plated steel sheet, and the arithmetic average of these measurements is determined as the Vickers hardness of the plated steel sheet.

[0047] <Method for manufacturing plated steel sheet> Next, a preferred method for manufacturing a plated 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 plated steel sheet according to an embodiment of the present invention, but is not intended to limit the plated steel sheet to one manufactured by the manufacturing method described below.

[0048] The plated steel sheet according to the embodiment of the present invention can be manufactured by, for example, a casting process in which molten steel having an adjusted chemical composition is cast to form a slab, a hot rolling process in which the slab is hot-rolled to obtain a hot-rolled steel sheet, a coiling process in which the hot-rolled steel sheet is coiled and then subjected to a primary pickling, a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, an annealing process in which the cold-rolled steel sheet is annealed, a secondary pickling process in which the annealed cold-rolled steel sheet is secondarily pickled, and a plating process in which the obtained base steel sheet is plated. The following specifically describes the manufacture of a plated steel sheet obtained by plating a cold-rolled steel sheet, but the plated steel sheet according to the embodiment of the present invention encompasses not only a plated steel sheet obtained by plating a cold-rolled steel sheet, but also a plated steel sheet obtained by plating a hot-rolled steel sheet. Therefore, when manufacturing a plated steel sheet obtained by plating a hot-rolled steel sheet, for example, a secondary pickling process may be performed after the coiling process without performing the cold-rolling process and annealing process described below. Each process will be described in detail below.

[0049] [Casting Step] The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0050] [Hot Rolling Process] A hot-rolled steel plate can be obtained by hot-rolling a cast steel slab. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0051] [Coiling Process] The hot-rolled steel sheet obtained in the hot rolling process is coiled in the next coiling process and then subjected to primary pickling. In this manufacturing method, the hot-rolled steel sheet is coiled at a coiling temperature of 520°C or higher. By controlling the coiling temperature to 520°C or higher, an outer oxide layer is formed on the outer surface (surface) of the steel sheet, and an inner oxide layer is also formed in the inner surface (surface layer) of the steel sheet. This inner oxide layer is mainly composed of Mn- and / or Si-based oxides. Therefore, an Mn—Si-depleted layer is formed directly below the inner oxide layer formed in the surface layer of the steel sheet due to the consumption of Mn and / or Si in the steel by the formation of the inner oxide layer. In particular, by controlling the coiling temperature to 520°C or higher, the thickness of the Mn—Si-depleted layer can be controlled to 0.3 μm or higher. The above-mentioned outer and inner oxide layers are removed by primary pickling after coiling, leaving a Mn—Si-depleted layer having a thickness of 0.3 μm or more on the surface of the hot-rolled steel sheet after the primary pickling. By forming a Mn—Si-depleted layer having a thickness of 0.3 μm or more on the surface of the hot-rolled steel sheet, the steel sheet surface is depleted in Mn and Si, which makes it possible to sufficiently suppress the formation of Mn- and / or Si-based surface oxides on the steel sheet surface in the subsequent annealing process. Therefore, plating can be appropriately performed in the subsequent plating process, and the desired surface coverage can be achieved in the finally obtained plated steel sheet.

[0052] The thickness of the Mn—Si depleted zone is determined as follows. First, using a high-frequency glow discharge optical emission spectrometer (GDS), the surface of the steel sheet after primary pickling is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The surface of the steel sheet is then sputtered and analyzed in the depth direction. The elements contained in the material are identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. The depth direction data can be estimated from the sputtering time. Specifically, the relationship between sputtering time and sputtering depth can be calculated in advance using a standard sample, allowing the sputtering time to be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted to mass % by creating a calibration curve. When the steel sheet after primary pickling is measured using GDS in this way, the region in the depth direction where the sum of the Mn and Si concentrations is 70% or less of the sum of the Mn and Si concentrations at the half-thickness position is defined as the Mn—Si depleted zone, and its thickness is determined.

[0053] The primary pickling may be carried out using a commonly used pickling solution under conditions suitable for removing the outer and inner oxide layers, without any particular limitation. The primary pickling may be carried out once, or may be carried out multiple times to ensure complete removal of the outer and inner oxide layers.

[0054] In steel sheets containing the three elements Ni, Cu, and Sn simultaneously, the presence of these elements may promote the formation of Mn- and / or Si-based surface oxides on the steel sheet surface. Therefore, it is extremely difficult to suppress the formation of such surface oxides and properly adhere a coating to steel sheets containing the three elements Ni, Cu, and Sn simultaneously. However, according to the present manufacturing method, by combining a Mn—Si-depleted layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the coiling temperature in the coiling process with a secondary pickling process described in detail below, it becomes possible to significantly suppress the formation of such surface oxides. On the other hand, if the coiling temperature in the coiling process is less than 520°C, the formation of an internal oxide layer is insufficient, and as a result, a Mn—Si-depleted layer having a thickness of 0.3 μm or more cannot be formed. In this case, the formation of Mn- and / or Si-based surface oxides cannot be sufficiently suppressed in the annealing process, making it difficult to properly adhere a coating to the steel sheet in the subsequent plating process. As a result, the desired surface coverage cannot be achieved in the final plated steel sheet.

[0055] From the viewpoint of further increasing the surface coverage and further improving the coating adhesion, it is preferable to control the coiling temperature to 550°C or higher. By controlling the coiling temperature to 550°C or higher, it is possible to further promote the formation of an internal oxide layer, which in turn makes it possible to make the Mn-Si depleted layer thicker. As a result, it is possible to more significantly suppress the formation of Mn- and / or Si-based surface oxides in the annealing step, and it is possible to further increase the surface coverage. There is no particular upper limit to the coiling temperature, but the coiling temperature may be, for example, 600°C or lower.

[0056] [Cold Rolling Step] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling step, the sheet may be cooled to room temperature, for example, by air cooling.

[0057] [Annealing Step] Next, the obtained cold-rolled steel sheet is annealed. The annealing step involves heating the cold-rolled steel sheet to a temperature of 700 to 950°C in an atmosphere with a dew point of -40 to 20°C and holding the temperature for 0 to 300 seconds. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (e.g., 4% hydrogen and the balance nitrogen).

[0058] [Secondary Pickling Process] The annealed cold-rolled steel sheet is then subjected to secondary pickling in the subsequent secondary pickling process. Specifically, the secondary pickling process is carried out by immersing the cold-rolled steel sheet in an aqueous solution having a hydrochloric acid concentration of 3 to 12% that does not contain an inhibitor for suppressing corrosion of the steel sheet at a temperature of 50 to 90°C for 2 to 100 seconds, and then rinsing the cold-rolled steel sheet with a rinse solution having an electrical conductivity of 40 mS / m or less. The secondary pickling process using an aqueous hydrochloric acid solution can sufficiently or completely remove Mn- and / or Si-based surface oxides formed on the surface of the cold-rolled steel sheet during the annealing process. More specifically, by forming a predetermined Mn—Si-depleted layer in the coiling process, the formation of Mn- and / or Si-based surface oxides in the annealing process can be sufficiently suppressed compared to when such an Mn—Si-depleted layer is not present. However, since the formation of the surface oxide is not completely suppressed in the annealing step, it is important to properly perform secondary pickling even after the annealing step in order to ensure proper plating adhesion in the subsequent plating step.

[0059] Therefore, the combination of a coiling temperature of 520°C or higher in the coiling step and the secondary pickling in the secondary pickling step is important. Such a specific combination can sufficiently or completely remove Mn- and / or Si-based surface oxides formed on the surface of the cold-rolled steel sheet during the annealing step, thereby enabling the desired surface coverage and equivalent circle radius of the uncoated region to be achieved in the finally obtained plated steel sheet. For example, if the hydrochloric acid aqueous solution contains an inhibitor, if the hydrochloric acid concentration of the hydrochloric acid aqueous solution is less than 3%, if the immersion temperature is less than 50°C, and / or if the immersion time is less than 2 seconds, the Mn- and / or Si-based surface oxides formed on the surface of the cold-rolled steel sheet during the annealing step cannot be sufficiently removed, and the adhesion of the coating in the subsequent coating step is inhibited due to the surface oxides. As a result, the coating cannot be properly adhered, and the desired surface coverage and / or equivalent circle radius of the uncoated region cannot be achieved in the finally obtained plated steel sheet. Preferably, the hydrochloric acid concentration of the aqueous hydrochloric acid solution is 4 to 8%, the immersion temperature is 70 to 90° C., and the immersion time is 4 to 50 seconds.

[0060] In the secondary pickling process, the water rinse after the secondary pickling is also extremely important. For example, if the electrical conductivity of the water used in the water rinse is relatively high, more specifically, if it is higher than 40 mS / m, iron oxides may form on the surface of the cold-rolled steel sheet during the water rinse after the secondary pickling. The presence of such iron oxides on the surface of the cold-rolled steel sheet inhibits the adhesion of the plating in the subsequent plating process, as in the case of Mn- and / or Si-based surface oxides. In this case, the desired surface coverage and the circle-equivalent radius of the uncoated region cannot be achieved in the final plated steel sheet. In contrast, in the present production method, the water rinse after the secondary pickling is performed using a water rinse having an electrical conductivity of 40 mS / m or less, thereby significantly suppressing the formation of iron oxides during the water rinse after the secondary pickling and enabling appropriate adhesion of the plating in the subsequent plating process.

[0061] In steel sheets containing the three elements Ni, Cu, and Sn simultaneously, the presence of these elements promotes the formation of Mn- and / or Si-based surface oxides during the annealing process and the formation of iron oxides during water rinsing after secondary pickling. For this reason, it is extremely difficult to suppress the formation of these oxides in steel sheets containing the three elements Ni, Cu, and Sn simultaneously and ensure proper plating adhesion in the subsequent plating process. Therefore, it is quite unexpected and surprising that the formation of these oxides can be significantly suppressed by combining a Mn-Si depleted layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the coiling temperature in the coiling process with specific secondary pickling and water rinsing in the secondary pickling process. From the viewpoint of further suppressing the formation of iron oxides, the lower the electrical conductivity of the rinsing solution, the more preferable it is. Specifically, it is preferably 25 mS / m or less, and more preferably 15 mS / m or less.

[0062] [Plating Step] Next, in the plating step, at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet) are plated. The plating step can be carried out by any suitable plating process that is effective for achieving the desired surface coverage and the circle-equivalent radius of the uncoated region, such as electroplating, vapor deposition plating, thermal spraying, or cold spraying. Preferably, the plating step is carried out by electroplating. Electroplating is carried out using a bath containing at least one of Ni, Cu, and Sn at a predetermined concentration, at a current density of 0.1 to 5.0 A / dm 2 The current density is preferably 0.3 to 2.0 A / dm and the current application time is 0.1 to 10.0 seconds. 2 The current application time is 0.5 to 5.0 seconds.

[0063] In this manufacturing method, in order to ensure proper adhesion of the plating, it is important to perform the plating step after sufficiently or completely removing Mn- and / or Si-based surface oxides and iron oxides from the surface of the base steel sheet in the secondary pickling step. That is, it is important to perform the plating step after the secondary pickling step. Conversely, as long as the plating step is performed after the secondary pickling step, performing the plating step before the secondary pickling step is not necessarily excluded. For example, it is possible to achieve the desired surface coverage and the circle-equivalent radius of the uncoated region by dividing the plating step into two steps, first performing the first plating treatment before the secondary pickling step and then performing the second plating treatment after the secondary pickling step. Alternatively, it is also possible to perform another plating treatment before the secondary pickling step and then perform the plating step according to this manufacturing method after the second pickling step.

[0064] According to this manufacturing method, in a steel sheet for which improving paint adhesion is difficult due to the simultaneous inclusion of the three elements Ni, Cu, and Sn, it is possible to sufficiently or completely remove Mn- and / or Si-based surface oxides and iron oxides from the surface of the base steel sheet by combining, in particular, an Mn-Si depleted layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the coiling temperature in the coiling step with specific secondary pickling and water rinsing in the secondary pickling step. In this regard, by carrying out a subsequent appropriate plating step, it is possible to produce a plated steel sheet having a coating in which the surface of the base steel sheet is covered with at least one of Ni, Cu, and Sn at a surface coverage rate of 25% or more, and the circle-equivalent radius of the uncoated region not covered with these elements is limited to 10 μm or less, as measured by Auger electron spectroscopy. As mentioned above, when Ni, Cu, and Sn are present in a steel sheet as solid solutions, the potential of the base steel sheet becomes more noble than when these elements are not present in solid solution, which can reduce the etching ability of Fe during chemical conversion treatment and, as a result, reduce the chemical treatability of the steel sheet. However, with plated steel sheets produced according to the present production method, at least one of Ni, Cu, and Sn is present on the steel sheet surface in a uniformly dispersed state rather than in a solid solution state, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet during chemical conversion treatment, resulting in significantly improved paint adhesion. Therefore, plated steel sheets produced according to the present production method can achieve superior corrosion resistance compared to conventional plated steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. This can contribute to industrial development by extending the life of plated steel sheets for automobiles and building materials.

[0065] The present invention will be described in more detail below with reference to examples, but the following examples are merely illustrative of the present invention and are not intended to limit the present invention in any way. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention.

[0066] In the following examples, plated steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the produced plated steel sheets were investigated.

[0067] First, molten steel was cast by a continuous casting method to form a steel billet having the chemical composition shown in Table 1. The steel billet was once cooled, reheated to 1200°C, hot-rolled, and then coiled at the coiling temperature shown in Table 2. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at a temperature of 900 to 1050°C and a reduction ratio of 30%. Next, the obtained hot-rolled steel sheet was subjected to primary pickling and then cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet having a thickness of 1.6 mm. Next, the obtained cold-rolled steel sheet was subjected to an annealing process in which it was heated to a temperature of 800°C in a furnace with an oxygen concentration of 20 ppm or less and an atmosphere with a dew point of 0°C and 4% hydrogen (nitrogen balance), and held there for 100 seconds.

[0068] Next, the annealed cold-rolled steel sheet was subjected to secondary pickling. Specifically, the secondary pickling was performed by immersing the cold-rolled steel sheet in an inhibitor-free aqueous solution having a 5% hydrochloric acid concentration at a temperature of 80°C for 4.5 seconds, and then rinsing the cold-rolled steel sheet with a rinsing solution having an electrical conductivity shown in Table 2. Finally, the obtained base steel sheet was immersed in a bath containing a metal species (one of Ni, Cu, and Sn) shown in Table 2 at a predetermined concentration at a current density of 0.5 A / dm 2 and current application time of 1.0 second, to obtain a plated steel sheet having a plating applied to both sides of the base steel sheet.

[0069]

[0070]

[0071] The properties of the obtained plated steel sheets were measured and evaluated by the following methods.

[0072] [Evaluation of Coating Adhesion] Coating adhesion was evaluated as follows. First, a 50 mm x 50 mm sample of the plated steel sheet produced above was subjected to a zinc phosphate treatment as a chemical conversion treatment under the following conditions: Degreasing: Immersion in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes, followed by rinsing with water. Surface conditioning: Immersion in a surface conditioning agent (Preparen Z) at room temperature for 30 seconds. Chemical conversion treatment: Immersion in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, followed by rinsing with water and drying.

[0073] Chemically treated plated steel sheet samples were electrocoated (Powernics Excel 1200, manufactured by Nippon Paint Industrial Coating Co., Ltd.) at an electrodeposition temperature of 30°C to a film thickness of 18 μm, followed by a baking treatment at 170°C for 30 minutes. The electrocoated samples were then subjected to a saltwater immersion test (SDT). Specifically, the electrocoated samples were immersed in a 5% NaCl aqueous solution at 50°C for 1000 hours. After the SDT test, the removed samples were dried, and then a tape peel test was performed on one side of the sample. The peeled tape was scanned, and the peeled area ratio of the coating was calculated by binarization using the image analysis software "ImageJ." The coating adhesion was evaluated as follows: AAA: Peeled area ratio less than 5%; AA: Peeled area ratio 5-10%; A: Peeled area ratio 10-15%; B: Peeled area ratio more than 15%.

[0074] Steel sheets that were rated AAA, AA, or A for paint adhesion were evaluated as containing Ni, Cu, and Sn and capable of exhibiting improved paint adhesion. The results are shown in Table 2.

[0075] Referring to Table 2, in Comparative Example 23, the low coiling temperature resulted in insufficient formation of an internal oxide layer, and as a result, it was not possible to form an Mn—Si depleted layer having a thickness of 0.3 μm or more. As a result, the Ni surface coverage was less than 25%, and coating adhesion was reduced. In Comparative Example 25, in addition to the low coiling temperature, the high electrical conductivity of the washing solution used for rinsing after the secondary pickling presumably prevented the formation of Mn- and / or Si-based surface oxides in the annealing step, and furthermore, the formation of iron oxides during rinsing after the secondary pickling presumably also failed to be sufficiently suppressed. As a result, the Ni surface coverage was less than 25%, and the circle-equivalent radius of the uncoated region exceeded 10 μm, resulting in reduced coating adhesion. In Comparative Examples 24, 26, and 27, the high electrical conductivity of the washing solution used for rinsing after the secondary pickling presumably prevented the formation of iron oxides during rinsing after the secondary pickling presumably failed to be sufficiently suppressed. As a result, the surface coverage rate with at least one of Ni, Cu and Sn was less than 25%, and the circle-equivalent radius of the uncoated area exceeded 10 μm, resulting in a decrease in coating adhesion.

[0076] In contrast, in the plated steel sheets according to all Examples, when measured by Auger electron spectroscopy, the surface of the base steel sheet was covered with at least one of Ni, Cu, and Sn at a surface coverage of 25% or more, and the circle-equivalent radius of the uncoated area not covered with these elements was limited to 10 μm or less, thereby significantly improving the paint adhesion of the plated steel sheet. In particular, in Examples 2, 3, 8, 9, 14, 15, and 20, which had a surface coverage of 35% or more, the paint adhesion was evaluated as AA, indicating further improvement in paint adhesion. In Examples 4 to 6, 10 to 12, 16 to 18, 21, and 22, which had a surface coverage of 50% or more, the paint adhesion was evaluated as AAA, indicating further improvement in paint adhesion.

Claims

1. A plated steel sheet comprising a base steel sheet and a plating arranged on a surface of the base steel sheet, The base steel plate is, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%; In an element distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy, The surface coverage by at least one of Ni, Cu, and Sn is 25% or more, and A plated steel sheet, characterized in that an uncoated region that is not coated with at least one of Ni, Cu, and Sn has a circle-equivalent radius of 10 μm or less.

2. The plated steel sheet according to claim 1 , wherein the surface coverage is 35% or more.

3. The plated steel sheet according to claim 2, wherein the surface coverage is 50% or more.

4. The plated steel sheet according to any one of claims 1 to 3, wherein the surface coverage is 80% or less.

5. The plated steel sheet according to any one of claims 1 to 3, wherein the uncoated region has an equivalent circle radius of 5 µm or less.

6. The chemical composition is, in mass %, Ni: 0.040-1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004-1.000% The plated steel sheet according to any one of claims 1 to 3, comprising:

7. The plated steel sheet according to any one of claims 1 to 3, characterized in that it has a Vickers hardness of 200 Hv or more.