Plated steel sheet

Abstract

Provided is a plated steel sheet characterized by being a plated steel sheet having a base steel sheet and a plated layer provided on the 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 %, in an elemental distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy, the surface coverage of 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 at least one of Ni, Cu, and Sn is 10 μm or less.

Description

Technical Field

[0001] This invention relates to coated steel sheets. Background Technology

[0002] It is known that in order to improve the adhesion of coatings in steel sheets or clad steel sheets and enhance the chemical conversion treatment properties of such steel sheets or clad steel sheets, it is effective to uniformly form a chemical conversion treatment film on these steel sheets.

[0003] Relatedly, Patent Document 1 describes a method for manufacturing high-strength cold-rolled steel sheet, characterized by continuously annealing the high-strength cold-rolled steel sheet using a continuous annealing furnace or a combined cold-rolled steel sheet / hot-dip galvanized steel sheet equipment equipped with a continuous annealing furnace. In the aforementioned continuous annealing furnace, the cooling method for a portion or all of the cooling zone within the steel sheet temperature range of 600-250°C following the heating for recrystallization is one or more of gas cooling, diffusion cooling, and cooling pipe cooling. During this continuous annealing process, the steel sheet surface is exposed to an atmosphere that causes iron oxidation within the aforementioned steel sheet temperature range. After pickling at the outlet of the annealing furnace, a concentration of 1-50 mg / m³ is applied. 2 Iron or Ni plating. Furthermore, Patent Document 1 teaches that: normally, oxidation of the steel sheet is prevented by using an inactive atmosphere with extremely low concentrations of oxygen and / or extremely low dew points around the steel sheet. In contrast, active exposure to an oxidizing atmosphere not only oxidizes Si and Mn but also oxidizes the iron in the steel sheet. Through pickling when exiting the annealing furnace, the oxide film on the iron and the oxide films on Si, Mn, etc., of the steel sheet are pickled off, thereby obtaining a high-strength cold-rolled steel sheet with good chemical conversion treatment properties, even if the content of Si, Mn, etc. is high, without any "uncovered parts".

[0004] Patent document 2 describes a steel sheet for automobiles, characterized by containing copper (Cu) at a concentration of 0.10% by mass or more and 0.50% by mass or less, and having a residual oxide scale on the surface of 160,000 pieces / mm. 2 The maximum particle size of the copper compound particles exposed on the surface is 2 μm or less. Furthermore, Patent Document 2 teaches that, with the above configuration, the particle size of the copper compound particles exposed on the surface of the steel plate that becomes the cathode point is 2 μm or less during the chemical conversion treatment, and the residual oxide scale is kept below a specified amount, thus providing a steel plate with excellent chemical conversion treatment properties.

[0005] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2008-190030 Patent Document 2: Japanese Patent Application Publication No. 2020-084238 Summary of the Invention

[0006] The problem that the invention aims to solve Patent document 2 teaches that, in addition to copper (Cu), elements such as nickel (Ni) and tin (Sn) not only reduce the mechanical properties required for automotive steel sheets, such as strength and formability, but also reduce chemical stability, such as corrosion resistance. In particular, the chemical conversion treatment capability of copper compounds present on the surface of the steel sheet, used to improve corrosion resistance, is reduced. Furthermore, generally speaking, if the chemical conversion treatment capability is reduced, areas without a chemically converted coating, known as uncovered areas, may sometimes appear, resulting in reduced coating adhesion.

[0007] Therefore, the object of the present invention is to provide a coated steel sheet containing Ni, Cu and Sn that exhibits improved coating adhesion.

[0008] Methods for solving problems To achieve the aforementioned objectives, the inventors conducted research focusing on the elemental distribution on the surface of steel plates. As a result, the inventors discovered that, when measured using Auger electron spectroscopy, coating the surface of the base steel plate with at least one of Ni, Cu, and Sn at a specified surface coverage rate, and with the size of the uncoated areas not covered by these elements limited to a specified range, significantly improves coating adhesion, thus completing the present invention.

[0009] The present invention, which achieves the above objectives, is described below.

[0010] (1) A clad steel sheet, characterized in that it is a clad steel sheet having a base steel sheet and a coating disposed on the surface of the base steel sheet, wherein, The aforementioned base steel plate contains, by mass % Ni: 0.010~1.000% Cu: 0.010~1.000%, and Sn: Chemical composition of 0.003~1.000% In the elemental distribution image obtained by measuring the surface of the above-mentioned coated steel sheet using Auger electron spectroscopy... The surface coating of at least one of Ni, Cu and Sn is 25% or more, and The equivalent circle radius of the uncoated region not covered by at least one of Ni, Cu and Sn is less than 10 μm.

[0011] (2) The plated steel sheet according to (1) above, characterized in that the surface coverage rate is 35% or more.

[0012] (3) The plated steel sheet according to (2) above, characterized in that the surface coverage rate is 50% or more.

[0013] (4) The plated steel sheet according to any one of (1) to (3) above, characterized in that the surface coverage is 80% or less.

[0014] (5) The plated steel sheet according to any one of (1) to (4) above, characterized in that the equivalent circle radius of the non-coated area is 5 μm or less.

[0015] (6) The coated steel sheet according to any one of (1) to (5) above, characterized in that the above chemical composition contains, by mass%, Ni: 0.040~1.000% Cu: 0.040~1.000%, and Sn: 0.004~1.000%.

[0016] (7) The plated steel sheet according to any one of (1) to (6) above is characterized in that it has a Vickers hardness of 200 Hv or more.

[0017] Invention Effects According to the present invention, it is possible to provide a coated steel sheet containing Ni, Cu and Sn that exhibits improved coating adhesion. Detailed Implementation

[0018] <Coated steel sheet> The coated steel sheet according to an embodiment of the present invention is characterized in that it comprises a base steel sheet and a coating disposed on the surface of the base steel sheet. The base steel plate contains, by mass%, a percentage of Ni: 0.010~1.000% Cu: 0.010~1.000%, and Sn: Chemical composition of 0.003~1.000% In the elemental distribution image obtained by measuring the surface of the coated steel sheet using Auger electron spectroscopy... The surface coating of at least one of Ni, Cu and Sn is 25% or more, and The equivalent circle radius of the uncoated region not covered by at least one of Ni, Cu and Sn is less than 10 μm.

[0019] As mentioned above, generally speaking, if the chemical conversion treatment properties decrease, areas where the chemical conversion treatment film has not formed, known as uncovered areas, may occur. As a result, the coating adhesion may sometimes be reduced. For example, when elements such as Ni, Cu, and Sn are present in solid solution in a steel sheet, the potential of the steel sheet increases compared to its undissolved state, and the corrosivity of Fe may sometimes decrease during chemical conversion treatment. In this case, the chemical conversion treatment properties of the steel sheet decrease, resulting in reduced coating adhesion. Therefore, this reduction in coating adhesion is particularly problematic when the steel sheet contains all three elements: Ni, Cu, and Sn.

[0020] Furthermore, as methods for manufacturing steel plates, two methods are generally known: one is to obtain molten iron in a blast furnace using iron ore, a natural resource, as the main raw material, and then refine it in a converter or similar furnace to produce molten steel; the other is to produce molten steel in an electric arc furnace using scrap iron, a recycled resource, as the main raw material. Blast furnace materials may also contain elements such as Ni, Cu, and Sn as additives, and therefore, when these elements are present, the aforementioned issues need to be addressed appropriately. On the other hand, as mentioned above, electric arc furnace materials use scrap iron as the main raw material, and therefore contain a relatively large amount of elements derived from scrap iron, such as Ni, Cu, and Sn (so-called "tramp elements"), making the aforementioned issues particularly significant.

[0021] Therefore, in order to provide a coated steel sheet that exhibits excellent coating adhesion even when the steel sheet contains the three elements Ni, Cu, and Sn simultaneously, the inventors have conducted research, particularly focusing on the elemental distribution on the surface of the steel sheet. As a result, the inventors have 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 inventors have found that, when measured using Auger electron spectroscopy, coating the surface of the base steel sheet with at least one of Ni, Cu, and Sn at a specified surface coverage rate, more specifically at 25% or more, and in a manner where the size of the uncoated area not covered by these elements (Ni, Cu, and Sn) is limited to a specified range, more specifically, the equivalent circular radius of the uncoated area is limited to less than 10 μm, thereby significantly improving coating adhesion.

[0022] While not intending to be bound by any particular theory, it is reasonable to assume that by uniformly coating the surface of the base steel sheet with at least one of Ni, Cu, and Sn, these elements can function appropriately as cathode sites during chemical conversion treatment, thereby promoting the anodic dissolution (corrosion) of Fe present around these elements. To elaborate further, Ni, Cu, and Sn are elements with higher potentials than Fe. Therefore, it is believed that by ensuring these elements are uniformly dispersed on the steel sheet surface rather than in a solid solution state, and thus functioning as effective cathode sites with respect to Fe during chemical conversion treatment as described above, the corrosion of Fe present around them can be promoted, thereby significantly improving the chemical conversion treatability of the steel sheet. As a result, a chemical conversion coating can be uniformly formed throughout the steel sheet, significantly improving coating adhesion. Even when the surface coverage rate of at least one of Ni, Cu, and Sn is very high, for example, 50% or more, large uncoated areas not covered by these elements cannot promote the anodic dissolution of Fe during chemical conversion treatment. As a result, a chemical conversion treatment film cannot be uniformly formed across the entire steel sheet, leading to reduced coating adhesion. Therefore, in the coated steel sheet of the embodiments of the present invention, it is important that at least one of Ni, Cu, and Sn is uniformly coated on the surface of the base steel sheet. Specifically, in the elemental distribution image obtained by measuring the surface of the coated steel sheet using Auger electron spectroscopy, it is important that the surface coverage rate of at least one of Ni, Cu, and Sn is controlled to be 25% or more, and the equivalent circle radius of the uncoated areas not coated with at least one of Ni, Cu, and Sn is controlled to be 10 μm or less. This is because if either of these conditions is not met, a chemical conversion treatment film cannot be uniformly formed across the entire steel sheet during chemical conversion treatment. On the other hand, by satisfying both conditions, a chemical conversion treatment film can be uniformly formed across the entire steel sheet, resulting in significantly improved coating adhesion.

[0023] The clad steel sheet of the embodiments of the present invention not only includes electric furnace material that inevitably contains Ni, Cu, and Sn as impurity elements, but also includes blast furnace material containing Ni, Cu, and Sn as essential elements or optional additive elements. Furthermore, compared with conventional clad steel sheets containing all three elements Ni, Cu, and Sn, the clad steel sheet of the embodiments of the present invention can achieve excellent coating adhesion, and thus excellent corrosion resistance. Therefore, the clad steel sheet of the embodiments of the present invention is particularly useful in applications in the automotive field where excellent coating adhesion and / or corrosion resistance are required. Hereinafter, the constituent elements of the clad steel sheet of the embodiments of the present invention will be described in more detail.

[0024] [Coating] According to embodiments of the present invention, the coating is disposed on the surface of a base steel plate, for example, on at least one side of the base steel plate, preferably on both sides. The coating may contain at least one of Ni, Cu, and Sn, and may also contain other elements such as Zn, Al, and Fe. For example, the coating may substantially contain at least one of Ni, Cu, and Sn, or may contain at least one of Ni, Cu, and Sn, or may be composed of at least one of Ni, Cu, and Sn. The content of Ni, Cu, and Sn in the coating and the coating adhesion amount are not particularly limited, and can be appropriately selected within the range that satisfies the requirements for surface coverage and non-coated areas described in detail later.

[0025] [Surface coverage of at least one of Ni, Cu, and Sn: ≥25%] In embodiments of the present invention, in the elemental distribution image obtained by measuring the surface of the coated steel sheet using Auger electron spectroscopy, the surface coverage rate of 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 processing. In chemical conversion processing, electrons are generally generated through the anodic dissolution (corrosion) of Fe, while at the cathode site, a cathode reaction (2H+) occurs due to the electrons generated by the anodic dissolution of Fe. + +2e - →H2、10H + +NO3 - +8e - →NH4 + +3H2O). Relatedly, the pH of the chemical conversion treatment solution near the steel plate surface rises, subsequently causing compounds such as zinc phosphate crystals, which constitute the chemical conversion treatment film, to precipitate on the steel plate surface.

[0026] In embodiments of the present invention, by satisfying the requirement for non-coated areas described later, and by controlling the surface coverage rate of the coated steel sheet containing at least one of Ni, Cu, and Sn to 25% or more, the surface of the base steel sheet can be uniformly coated with at least one of Ni, Cu, and Sn, enabling these elements to function effectively as cathode sites. As a result, the aforementioned cathodic reaction can be appropriately carried out on the entire surface of the steel sheet, thus enabling the chemical conversion treatment film to be uniformly formed on the entire steel sheet, thereby significantly improving the coating adhesion. From the viewpoint of further improving coating adhesion, the higher the surface coverage rate, the more preferred. For example, the surface coverage rate of 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. There is no particular upper limit, and the surface coverage rate of at least one of Ni, Cu, and Sn can be, for example, 80% or less, 75% or less, 70% or less, or 65% or less.

[0027] [Equivalent circle radius of uncovered region: less than 10 μm] In embodiments of the present invention, in the elemental distribution image obtained by measuring the surface of the coated steel sheet using Auger electron spectroscopy, the equivalent circle radius of the uncoated regions (i.e., uncoated regions not coated by any of Ni, Cu, and Sn) is controlled to be less than 10 μm. In the presence of relatively large uncoated regions not coated by at least one of Ni, Cu, and Sn, there are naturally no cathode sites in such uncoated regions, thus hindering the anodic dissolution of Fe during chemical conversion treatment. As a result, a uniform chemical conversion film cannot be formed on the entire steel sheet, leading to reduced coating adhesion.

[0028] In embodiments of the present invention, by satisfying the previously described requirement for surface coverage rate, and by ensuring the existence of such uncoated areas, and by controlling these uncoated areas to be 10 μm or less in terms of equivalent circular radius, the surface of the base steel sheet can be uniformly coated with at least one of Ni, Cu, and Sn, enabling these elements to function effectively as cathode sites. As a result, the aforementioned cathodic reaction can be appropriately carried out throughout the surface of the steel sheet, thus enabling the formation of a chemical conversion treatment film uniformly throughout the steel sheet, thereby significantly improving coating adhesion. From the viewpoint of further improving coating adhesion, a smaller equivalent circular radius of the aforementioned uncoated areas is preferred. For example, the equivalent circular radius of the uncoated areas 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. The lower limit is not particularly limited, and the equivalent circular radius of the uncoated areas not coated with at least one of Ni, Cu, and Sn can, for example, be 0.5 μm or more or 1 μm or more.

[0029] [Determination of surface coverage and uncovered areas] The surface coating rate was determined using Auger electron spectroscopy as follows. First, the sample, including the plate surface, was placed in an Auger electron spectroscopy apparatus (e.g., AES PHI-700 type (ULVAC-PHI, FE type)). The sample surface (plate surface) was measured under the following conditions: accelerating voltage: 10 kV, current: 10 nA, and Auger spectroscopy measurement energy range: 40–1690 eV. The measurement area was observed using a SEM at magnification of 1000x or higher, resulting in an area of ​​60 μm × 100 μm or larger. Next, a mapping was created with a lower limit of 80 cps (count / s) to obtain an elemental distribution image. Finally, the obtained elemental distribution image was binarized using the image analysis software "ImageJ" (min=0, max=255), and the coating rate of at least one of Ni, Cu, and Sn relative to the measurement area was calculated. The calculated value was then determined as the surface coating rate of the coated steel plate containing at least one of Ni, Cu, and Sn.

[0030] Regarding the uncoated regions, in the binarized image obtained using the image analysis software "ImageJ" in association with the determination of surface coating rate, the area and number of uncoated regions without at least one of the coatings of Ni, Cu, and Sn are calculated. Next, the average area S of each uncoated region is calculated by dividing the total area of ​​the uncoated regions by the total number of uncoated regions. Finally, the equivalent circle radius r is calculated using the following formula and determined as the equivalent circle radius of the uncoated regions without at least one of the coatings of Ni, Cu, and Sn.

[0031] r = (S / π) 0.5 [Base Material Steel Plate] In embodiments of the present invention, the base steel sheet has a chemical composition comprising, by mass%, 0.010–1.000% Ni, 0.010–1.000% Cu, and 0.003–1.000% Sn. As described above, the object of the present invention is to provide a coated steel sheet containing Ni, Cu, and Sn that exhibits improved coating adhesion. This object is achieved by coating the base steel sheet in a manner that, when measured using Auger electron spectroscopy, the surface of the base steel sheet is covered with at least 25% or more of Ni, Cu, and Sn, and the equivalent circle radius of the uncoated areas not covered by 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 Ni: 0.010~1.000%, Cu: 0.010~1.000%, and Sn: 0.003~1.000% by mass%. Thus, it can be understood that elements other than Ni, Cu, and Sn are not essential technical features for achieving the objectives of this invention. In addition to Ni, Cu, and Sn, the chemical composition of the base steel sheet can contain, in appropriate amounts, any alloying elements commonly added in the technical field of this invention. Hereinafter, the chemical composition of the base steel sheet used in the coated steel sheet according to embodiments of this invention will be described in detail. However, these descriptions are intended merely to illustrate preferred chemical compositions of base steel sheets used in automotive steel sheets, etc., and are not intended to limit the invention to the use of base steel sheets having such specific chemical compositions.

[0032] In embodiments of the present invention, for example, the base steel plate preferably has a chemical composition comprising: in mass %, C: 0.001~0.500% Si: 0~3.00% Mn: 0.10~3.00% Al: 0.001~2.000% Ni: 0.010~1.000% Cu: 0.010~1.000% Sn: 0.003~1.000% P: Below 0.100% S: Below 0.100% N: below 0.0100% Ti: 0~0.150%, Nb: 0~0.150%, B: 0~0.0100% Mo: 0~1.000% Cr: 0~1.000% V: 0~0.150%, W: 0~1.000% Hf: 0~0.050%, Mg: 0~0.050%, Zr: 0~0.050%, Ca: 0~0.010%, REM: 0~0.010%, As: 0~0.010% Ir: 0~1.000%, and The remainder consists of Fe and impurities. The following provides a more detailed explanation of each element.

[0033] [C: 0.001~0.500%] Carbon (C) is an inexpensive element that increases strength and is an important element for controlling the strength of steel. To achieve this effect, the C content is preferably 0.001% or more. The C content can 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 can sometimes lead to a decrease in elongation. Therefore, the C content is preferably set to 0.500% or less. The C content can be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.

[0034] [Si: 0~3.00%] Si is an element that effectively increases strength as a solid solution strengthening element. The Si content can be 0%, but to achieve this effect, the Si content is preferably set to 0.01% or more. The Si content can 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 can sometimes lead to an increase in steel strength and a decrease in elongation. Therefore, the Si content is preferably set to 3.00% or less. The Si content can be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.

[0035] [Mn: 0.10~3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully achieve this effect, the Mn content is preferably set at 0.10% or more. The Mn content can 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 can sometimes lead to an increase in steel strength and a decrease in elongation. Therefore, the Mn content is preferably set at 3.00% or less. The Mn content can be 2.80% or less, 2.50% or less, or 2.00% or less.

[0036] [Al: 0.001~2.000%] Al acts as a deoxidizer in steel, contributing to its overall health. To achieve this effect, the Al content is preferably 0.001% or more. The Al content can also 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 leads to the formation of coarse Al oxides, reducing the elongation of the steel sheet. Therefore, the Al content is preferably 2.000% or less. The Al content can be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0037] [Ni: 0.010~1.000%] [Cu: 0.010~1.000%] Ni and Cu are elements that contribute to increased strength through precipitation strengthening or solid solution strengthening. To achieve this effect, the content of these elements is preferably 0.010% or more, but may also 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, if these elements are excessively present, they may sometimes promote the formation of oxides on the surface of the steel sheet, especially surface oxides of Mn and / or Si-based oxides and iron oxides. In this case, the adhesion of the coating in the plating process is hindered. Therefore, the content of Ni and Cu is preferably 1.000% or less, but may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.

[0038] [Sn: 0.003~1.000%] Sn is an effective element for improving corrosion resistance. To achieve this effect, the Sn content is preferably 0.003% or more. The Sn content can 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, if Sn is excessively present, it can sometimes promote the formation of oxides on the steel plate surface, especially Mn and / or Si-based surface oxides and iron oxides. In this case, the adhesion of the coating during the plating process is hindered. Therefore, the Sn content is preferably 1.000% or less. The Sn content can be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[0039] [P: below 0.100%] Phosphorus (P) is an element that promotes embrittlement of steel due to grain boundary segregation. Lower P content is preferred, and ideally, it should be 0%. However, excessively low P content can sometimes lead to a significant increase in cost. Therefore, the P content can be set to 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, if P is excessive, as mentioned above, grain boundary segregation can sometimes cause embrittlement of the steel. Therefore, the P content is preferably set to 0.100% or less. The P content can be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0040] [S: Below 0.100%] Sulfur (S) is an element that reduces the ductility of steel components by forming non-metallic inclusions such as MnS. Lower S content is preferred, ideally 0%. However, excessively low S content can sometimes lead to a significant increase in cost. Therefore, the S content can be 0.0001% or more, or 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content can sometimes lead to cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably set to 0.100% or less. The S content can be 0.050% or less, 0.020% or less, or 0.010% or less.

[0041] [N: below 0.0100%] Nitrogen (N) is an element that forms large nitrides in steel sheets, reducing their workability. Lower N content is preferred, ideally 0%. However, excessively low N content can sometimes lead to a significant increase in manufacturing costs. Therefore, the N content can be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if N is excessive, as mentioned above, large nitrides can sometimes form, reducing the workability of the steel sheet. Therefore, the N content is preferably set to 0.0100% or less. The N content can be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0042] The preferred basic chemical composition of the base steel plate is as described above. Furthermore, the base steel plate may, as needed, contain at least one of the following elements to replace a portion of the remaining Fe.

[0043] [Ti: 0~0.150%] [Nb: 0~0.150%] [V: 0~0.150%] Ti, Nb, and V have the effect of forming carbonitrides in steel, thereby increasing the strength of the steel sheet through precipitation strengthening. The content of Ti, Nb, and V can be 0%, but to achieve this effect, the content of Ti, Nb, and V is preferably 0.001% or more, and can also be 0.002% or more, 0.005% or more, or 0.010% or more, respectively. On the other hand, even if these elements are excessively present, the effect saturates, and their presence in the steel sheet beyond what is necessary leads to an increase in manufacturing costs. Therefore, the content of Ti, Nb, and V is preferably 0.150% or less, and can 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, respectively.

[0044] [B: 0~0.0100%] Boron (B) improves low-temperature toughness by increasing grain boundary strength through grain boundary segregation. The B content can be 0%, but to achieve this effect, a B content of 0.0001% or more is preferred. The B content can be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, even with excessive B content, the effect saturates, which may lead to increased manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content can also be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0045] [Mo: 0~1.000%] [Cr: 0~1.000%] [W: 0~1.000%] Mo, Cr, and W are elements that improve the hardenability of steel and contribute to increased strength. The content of Mo, Cr, and W can be 0%, but to achieve this effect, the content of Mo, Cr, and W is preferably 0.001% or more, and can also be 0.010% or more, 0.020% or more, or 0.030% or more, respectively. On the other hand, even if these elements are excessively present, their effects saturate, and their presence in the steel sheet beyond what is necessary leads to increased manufacturing costs. Therefore, the content of Mo, Cr, and W is preferably 1.000% or less, and can be 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less, respectively.

[0046] [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 nonmetallic inclusions. The content of Hf, Mg, Zr, Ca, and REM can be 0%, but to achieve this effect, the content of these elements is preferably 0.0001% or more, or 0.0005% or more, or 0.001% or more, respectively. On the other hand, even if these elements are excessively present, the effect saturates, and their presence in the steel sheet beyond what is necessary leads to increased manufacturing costs. Therefore, the content of Hf, Mg, and Zr is preferably 0.050% or less, or 0.010%, 0.005%, or 0.003% or less, respectively. Similarly, the content of Ca and REM is preferably 0.010% or less, or 0.005% or less, or 0.003% or less, respectively.

[0047] [As: 0~0.010%] As is an effective element for improving corrosion resistance. The As content can be 0%, but to achieve this effect, the As content is preferably 0.001% or more. The As content can be 0.002% or more, or 0.003% or more. On the other hand, even with excessive As content, the effect saturates, and exceeding the necessary amount in the steel sheet leads to increased manufacturing costs. Therefore, the As content is preferably 0.010% or less. The As content can be 0.008% or less, or 0.005% or less.

[0048] [Ir: 0~1.000%] Ir is an element that increases the strength of grain boundaries due to segregation at the original austenite grain boundaries. The Ir content can be 0%, but to achieve this effect, the Ir content is preferably 0.001% or more. The Ir content can be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even with excessive Ir content, the effect saturates, and its presence in steel beyond what is necessary leads to increased manufacturing costs. Therefore, the Ir content is preferably 1.000% or less. The Ir content can be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0049] In the base steel sheet, the remaining portion besides the aforementioned elements consists of Fe and impurities. Impurities in the base steel sheet refer to components that are mixed in during the industrial manufacturing of the base steel sheet due to various reasons in the manufacturing process, such as raw materials like ores and scrap iron.

[0050] The chemical composition of the base steel sheet can be determined using general analytical methods. For example, the coating is first removed by mechanical grinding, and then measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on the cut material according to JIS G 1201:2014. Specifically, for example, a 35 mm square test piece can be obtained from approximately half the thickness of the base steel sheet and measured using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-prepared standard curve. C and S, which cannot be measured by ICP-AES, are determined using the combustion-infrared absorption method, and N is determined using the inactive gas melting-thermal conductivity method.

[0051] [Thickness of the base steel plate] There is no particular limitation on the thickness of the base steel plate, which generally ranges from 0.2 to 8.0 mm. For example, the thickness can 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 can be, for example, less than 7.0 mm, less than 6.0 mm, less than 5.0 mm, or less than 4.0 mm.

[0052] [Mechanical Properties] The coated steel sheet used in embodiments of the present invention is not particularly limited, but may, for example, have a Vickers hardness of 90 Hv or higher. The Vickers hardness may be 150 Hv or higher, 200 Hv or higher, 250 Hv or higher, 300 Hv or higher, 350 Hv or higher, 400 Hv or higher, or 450 Hv or higher. There is no particular upper limit; for example, the Vickers hardness may be below 650 HV, below 600 HV, below 550 HV, or below 500 HV.

[0053] [Determination of Vickers Hardness] The Vickers hardness is determined as follows. First, a test piece is cut out in a manner that allows observation from any position on the coated steel sheet, except at the ends, of a section perpendicular to the surface (thickness section). The thickness section of the test piece is ground using silicon carbide paper of grit #600 to #1500, and then polished to a mirror finish using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. This thickness section is used as the measurement surface. Next, the Vickers hardness is measured using a micro Vickers hardness tester at intervals of at least three times the indentation length under a load of 1 kgf. Specifically, a total of 20 points are randomly measured near the 1 / 2 mark of the thickness of the coated steel sheet, and their arithmetic mean is determined as the Vickers hardness of the coated steel sheet.

[0054] <Manufacturing Method of Coated Steel Sheet> Next, a preferred manufacturing method for the coated steel sheet according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the coated steel sheet according to an embodiment of the present invention, and is not intended to limit the coated steel sheet to a steel sheet manufactured by the manufacturing method described below.

[0055] The coated steel sheet according to embodiments of the present invention can be manufactured, for example, by performing the following steps: a casting process in which molten steel with adjusted chemical composition is cast to form a billet; a hot rolling process in which the billet 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 first 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 second pickling process in which the annealed cold-rolled steel sheet is subjected to a second pickling; and a coating process in which the obtained base steel sheet is coated. Hereinafter, the manufacture of coated steel sheet in which cold-rolled steel sheet is coated is specifically shown, but the coated steel sheet according to embodiments of the present invention includes not only coated steel sheet in which cold-rolled steel sheet is coated, but also coated steel sheet in which hot-rolled steel sheet is coated. Therefore, when manufacturing coated steel sheet in which hot-rolled steel sheet is coated, for example, the cold rolling and annealing processes described below may be omitted, and a second pickling process may be performed after the coiling process. The following is a detailed description of each process.

[0056] [Casting Process] There are no particular restrictions on the conditions for the casting process. For example, after smelting in a blast furnace or electric furnace, various secondary smelting processes can be carried out, followed by casting using conventional continuous casting or ingot casting methods.

[0057] [Hot rolling process] Hot-rolled steel sheets can be obtained by hot rolling cast steel billets. The hot rolling process is carried out by reheating the cast steel billet after direct or temporary cooling. In the case of reheating, the heating temperature of the steel billet can be, for example, 1100~1250℃. In the hot rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction rate of each rolling process can be appropriately determined according to the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling can be 900~1050℃, and the reduction rate of finish rolling can be 10~50%.

[0058] [Winding process] The hot-rolled steel sheet obtained in the hot-rolling process is coiled in the subsequent coiling process and then subjected to a pickling. In this manufacturing method, the coiling of the hot-rolled steel sheet is carried out at a coiling temperature of 520°C or higher. By controlling the coiling temperature to 520°C or higher, an external oxide layer is formed on the outer surface of the steel sheet, and an internal oxide layer is also formed on the inner surface of the steel sheet. This internal oxide layer is mainly composed of Mn and / or Si-based oxides. Therefore, directly below the internal oxide layer formed on the surface of the steel sheet, Mn and / or Si in the steel are consumed due to the formation of this internal oxide layer, thereby forming an Mn-Si deficient layer. In particular, by controlling the coiling temperature to 520°C or higher, the thickness of the Mn-Si deficient layer can be controlled to be 0.3 μm or higher. The aforementioned external oxide layer and internal oxide layer are removed by a pickling after coiling, so that an Mn-Si deficient layer with a thickness of 0.3 μm or higher remains on the surface of the hot-rolled steel sheet after the first pickling. By constructing the surface of the hot-rolled steel sheet with a Mn-Si-deficient layer having a thickness of 0.3 μm or more, Mn and Si are absent from the steel sheet surface, thus effectively suppressing the formation of Mn and / or Si-based surface oxides on the steel sheet surface during subsequent annealing processes. Therefore, plating can be appropriately implemented in subsequent plating processes, achieving the desired surface coverage in the final plating steel sheet.

[0059] The thickness of the Mn-Si deficient layer was determined as follows. First, using a high-frequency glow discharge emission spectroscopy (GDS) device, the surface of a steel sheet after a single pickling was exposed to an Ar atmosphere. While generating glow plasma by applying voltage, the surface of the steel sheet was sputtered, and analysis was performed in the depth direction. Then, the elements contained in the material were identified by the emission spectral wavelengths specific to each element emitted by atoms excited in the glow plasma, and the luminescence intensity of the identified elements was estimated. The depth direction data could be estimated based on the sputtering time. Specifically, by pre-determining the relationship between sputtering time and sputtering depth using standard samples, the sputtering time could be converted to sputtering depth. Therefore, the sputtering depth converted from sputtering time could be defined as the depth from the surface of the material. The obtained luminescence intensity was converted to mass percent by constructing a standard curve. In the case of GDS measurement of a steel sheet after a single pickling, the region where the combined Mn and Si concentrations in the depth direction are less than 70% relative to the half-thickness position of the sheet was defined as the Mn-Si deficient layer, and its thickness was determined.

[0060] There are no particular limitations on the number of pickling operations; any pickling solution suitable for removing the aforementioned external and internal oxide layers can be used. A single pickling operation can be performed, or multiple operations can be performed to reliably remove the external and internal oxide layers.

[0061] In steel sheets containing Ni, Cu, and Sn, the presence of these elements can sometimes 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 achieve proper coating adhesion in steel sheets containing Ni, Cu, and Sn. However, according to this manufacturing method, by combining a Mn-Si deficient layer formed to a specified thickness of 0.3 μm or more due to proper control of the winding temperature in the winding process with the secondary pickling process described in detail later, the formation of such surface oxides can be significantly suppressed. On the other hand, if the winding temperature in the winding process is less than 520°C, the formation of the internal oxide layer becomes insufficient, thus preventing the formation of an Mn-Si deficient layer with a thickness of 0.3 μm or more. 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 achieve proper coating adhesion in subsequent plating processes. As a result, the desired surface coverage cannot be achieved in the final coated steel sheet.

[0062] From the viewpoint of further improving surface coverage and thus coating adhesion, the winding temperature is preferably controlled at 550°C or higher. By controlling the winding temperature to 550°C or higher, the formation of the internal oxide layer can be further promoted, thereby making the Mn-Si deficient layer thicker. As a result, the formation of Mn and / or Si-based surface oxides during the annealing process can be more significantly suppressed, further improving the surface coverage. There is no particular upper limit to the winding temperature; for example, the winding temperature can be below 600°C.

[0063] [Cold rolling process] Cold-rolled steel sheets can be obtained by cold rolling after hot-rolled steel sheets have undergone pickling or similar processes. The reduction rate during cold rolling can be appropriately determined based on the desired metal structure and sheet thickness, for example, it can be 20% to 80%. After the cold rolling process, the sheets can be cooled to room temperature, for example, by air cooling.

[0064] [Annealing process] Next, the obtained cold-rolled steel sheet is annealed. The annealing process involves heating the cold-rolled steel sheet to a temperature of 700-950°C in an atmosphere with a dew point of -40 to 20°C and holding it for 0 to 300 seconds. The atmosphere in the annealing process can be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere with 1-10% hydrogen (e.g., 4% hydrogen and nitrogen balance).

[0065] [Secondary pickling process] After annealing, the cold-rolled steel sheet undergoes a secondary pickling process. Specifically, this secondary pickling process involves immersing the cold-rolled steel sheet in an aqueous solution containing 3-12% hydrochloric acid (without an inhibitor to suppress corrosion) at a temperature of 50-90°C for 2-100 seconds, followed by rinsing with a water wash solution having a conductivity of less than 40 mS / m. This secondary pickling with the hydrochloric acid solution effectively removes 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 deficiency layer during the coiling process, the formation of Mn and / or Si-based surface oxides during the annealing process can be sufficiently suppressed compared to the absence of such a layer. However, since the formation of these surface oxides is not completely suppressed during the annealing process, it is important to perform a proper secondary pickling after the annealing process to ensure proper adhesion of the coating in the subsequent plating process.

[0066] Therefore, the combination of a coiling temperature of 520°C or higher in the coiling process and the aforementioned secondary pickling in the secondary pickling process is important. This specific combination allows for the sufficient or complete removal of Mn and / or Si-based surface oxides formed on the surface of the cold-rolled steel sheet during the annealing process. As a result, the desired surface coverage rate and equivalent circular radius of the uncoated area can be achieved in the final coated steel sheet. For example, if the hydrochloric acid aqueous solution contains inhibitors, or the hydrochloric acid concentration is less than 3%, or the immersion temperature is less than 50°C, and / or 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 process cannot be sufficiently removed. In subsequent coating processes, these surface oxides hinder coating adhesion. Consequently, the coating cannot adhere properly, and the desired surface coverage rate and / or equivalent circular radius of the uncoated area cannot be achieved in the final coated steel sheet. The preferred hydrochloric acid aqueous solution has a hydrochloric acid concentration of 4-8%, an immersion temperature of 70-90℃, and an immersion time of 4-50 seconds.

[0067] In the secondary pickling process, the subsequent water rinse is also extremely important. For example, when the conductivity of the water used in the water rinse is high, more specifically above 40 mS / m, iron oxides sometimes form on the surface of the cold-rolled steel sheet during the water rinse after the secondary pickling. If such iron oxides are present on the surface of the cold-rolled steel sheet, they, like Mn and / or Si-based surface oxides, hinder the adhesion of the coating in the subsequent plating process. In this case, the desired surface coverage and equivalent circle radius of the uncoated area cannot be achieved in the final plated steel sheet. In contrast, in this manufacturing method, by using a water rinse with a conductivity of less than 40 mS / m for the water rinse after the secondary pickling, the formation of iron oxides during the water rinse after the secondary pickling can be significantly suppressed, and the coating can adhere properly in the subsequent plating process.

[0068] In steel sheets containing Ni, Cu, and Sn, the presence of these elements promotes not only the formation of Mn and / or Si-based surface oxides during the annealing process but also the formation of iron oxides during the water wash following the secondary pickling. Therefore, in steel sheets containing Ni, Cu, and Sn, it is extremely difficult to suppress the formation of these oxides and ensure proper adhesion of the coating in subsequent plating processes. Surprisingly, the formation of these oxides can be significantly suppressed by combining a Mn-Si-deficient layer, formed to a specified thickness of 0.3 μm or more due to appropriate control of the coiling temperature during the coiling process, with a specific combination of secondary pickling and water washing in the secondary pickling process. From the viewpoint of further suppressing the formation of iron oxides, a lower conductivity of the water wash solution is preferred; specifically, it is preferably 25 mS / m or less, and more preferably 15 mS / m or less.

[0069] [Plating Process] Next, in the plating process, plating is performed on at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). The plating process can be carried out by any suitable plating treatment that effectively achieves the desired surface coverage rate and the equivalent circular radius of the uncoated area, such as electroplating, vapor deposition, spraying, or cold spraying. The plating process is preferably carried out by electroplating. Electroplating can be performed using a bath containing at least one of Ni, Cu, and Sn at a specified concentration, at a current density of 0.1~5.0 A / dm³. 2 The process is carried out under conditions where the energizing time is 0.1 to 10.0 seconds. A current density of 0.3 to 2.0 A / dm² is preferred. 2 The power-on time is 0.5 to 5.0 seconds.

[0070] In this manufacturing method, for proper coating adhesion, it is important to perform the coating process after the secondary pickling step has sufficiently or completely removed the surface oxides of Mn and / or Si-based materials, as well as iron oxides, from the surface of the base steel sheet. In other words, it is crucial that the coating process be performed after the secondary pickling step. Conversely, as long as the coating process is performed after the secondary pickling step, it is not necessary to perform the coating process before the secondary pickling step. For example, the coating process can be divided into two steps: first, a first coating treatment is performed before the secondary pickling step, and then a second coating treatment is performed after the secondary pickling step, thereby achieving the desired surface coverage rate and the equivalent circle radius of the uncoated area. Alternatively, other coating treatments can be performed before the secondary pickling step, and the coating process of this manufacturing method can be performed after the secondary pickling step.

[0071] According to this manufacturing method, in steel sheets where the adhesion of the coating is difficult to improve due to the simultaneous presence of Ni, Cu, and Sn, Mn-Si deficient layers, formed to a specified thickness of 0.3 μm or more by appropriate control of the winding temperature in the winding process, combined with specific secondary pickling and water washing in the secondary pickling process, can be sufficiently or completely removed from the surface of the base steel sheet, including surface oxides of Mn and / or Si-based materials and iron oxides. Relatedly, by implementing appropriate subsequent plating processes, a coated steel sheet can be manufactured, wherein, as measured by Auger electron spectroscopy, the surface of the base steel sheet is covered by at least one of Ni, Cu, and Sn with a surface coverage rate of 25% or more, and the equivalent circle radius of the uncoated areas not covered by these elements is limited to 10 μm or less. As mentioned above, if Ni, Cu, and Sn are present in solid solution in the steel sheet, the potential of the base steel sheet is higher compared to the state where these elements are not in solid solution. This reduces the corrosiveness of Fe during chemical conversion treatment, and consequently, sometimes reduces the chemical conversion treatability of the steel sheet. However, in the galvanized steel sheet manufactured according to this method, at least one of Ni, Cu, and Sn is uniformly dispersed on the steel sheet surface, not in solid solution. Therefore, during chemical conversion treatment, a uniform chemical conversion film can be formed throughout the steel sheet, resulting in significantly improved coating adhesion. Thus, the galvanized steel sheet manufactured according to this method achieves superior corrosion resistance compared to conventional galvanized steel sheets containing all three elements (Ni, Cu, and Sn). Therefore, its extended service life contributes to industrial development in the use of galvanized steel sheets for automotive and construction materials.

[0072] The present invention will now be described in more detail through embodiments, but these embodiments are merely examples of the present invention, and the present invention is not limited to these embodiments in any way. The present invention can, of course, be modified in any way without departing from its spirit.

[0073] Example In the following embodiments, coated steel sheets according to embodiments of the present invention were manufactured under various conditions, and the characteristics of the manufactured coated steel sheets were investigated.

[0074] First, molten steel is cast using a continuous casting method to form a steel billet with the chemical composition shown in Table 1. This billet is temporarily cooled and then reheated to 1200°C for hot rolling, followed by coiling at the coiling temperatures shown in Table 2. Hot rolling is performed through roughing and finishing rolling, with the finishing rolling end temperature at 900–1050°C and a reduction rate of 30%. Next, the resulting hot-rolled steel sheet is subjected to a single pickling process, followed by cold rolling with a reduction rate of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm. Then, the resulting cold-rolled steel sheet is subjected to an annealing process in a furnace with an oxygen concentration below 20 ppm, in an atmosphere with a dew point of 0°C and 4% hydrogen (nitrogen balance), heating to 800°C and holding for 100 seconds.

[0075] Next, the annealed cold-rolled steel sheet undergoes a secondary pickling process. Specifically, the secondary pickling is performed by immersing the cold-rolled steel sheet in an aqueous solution with a 5% hydrochloric acid concentration and no inhibitor at 80°C for 4.5 seconds, followed by rinsing the cold-rolled steel sheet with a water washing solution having the conductivity shown in Table 2. Finally, for the obtained base steel sheet, a bath containing one of the metals (Ni, Cu, and Sn) shown in Table 2 at a specified concentration is used at a current density of 0.5 A / dm³. 2 Electroplating was performed under the condition of 1.0 second of energization time to obtain a coated steel plate with coatings attached to both sides of the base steel plate.

[0076] The properties of the obtained coated steel sheet were measured and evaluated using the following methods.

[0077] [Evaluation of coating adhesion] The coating adhesion was evaluated as follows. First, a 50mm × 50mm sample of the coated steel sheet manufactured above was subjected to zinc phosphate treatment as a chemical conversion treatment under the following conditions.

[0078] Degreasing: Soak in a degreasing agent (FINE CLEANER E2083) at 40°C for 2 minutes, then wash with water. Surface conditioning: Immerse for 30 seconds at room temperature using surface conditioner (Prepalene Z). Chemical conversion treatment: Immerse in zinc phosphate treatment agent (PALBOND L3020) at 40°C for 2 minutes, followed by washing and drying. For samples of coated steel sheets that underwent chemical conversion treatment, electrodeposition coating (POWERNICS EXCEL 1200: manufactured by Nippon Paint Industrial Coatings) was performed at an electrodeposition temperature of 30°C and a film thickness of 18 μm, followed by baking at 170°C for 30 minutes. Next, a salt water immersion test (SDT) was conducted on the electrodeposited samples; specifically, the electrodeposited samples were immersed in a 5% NaCl aqueous solution at 50°C for 1000 hours. After the SDT test, the samples were dried, and then a tape peel test was performed on one side of the sample. The peeled tape was scanned, and the area ratio of the peeled coating was calculated using image analysis software "ImageJ" to evaluate the coating adhesion.

[0079] AAA: Peeling area ratio less than 5% AA: Stripping area rate of 5% or more but less than 10% A: Peeling area ratio 10~15% B: The stripping area exceeds 15%. The coating adhesion ratings of AAA, AA, and A were used to evaluate coated steel sheets containing Ni, Cu, and Sn that showed improved coating adhesion. The results are shown in Table 2.

[0080] Referring to Table 2, in Comparative Example 23, due to the low winding temperature, the formation of the internal oxide layer was insufficient, resulting in the inability to form a Mn-Si deficiency layer with a thickness of 0.3 μm or more. As a result, the surface coverage of Ni was less than 25%, and the coating adhesion was reduced. In Comparative Example 25, it was believed that in addition to the low winding temperature, the conductivity of the washing solution used in the water wash after the second pickling was also high, thus failing to sufficiently suppress the formation of Mn and / or Si-based surface oxides during the annealing process, and consequently failing to sufficiently suppress the formation of iron oxides during the water wash after the second pickling. As a result, the surface coverage of Ni was less than 25%, and the equivalent circle radius of the non-coated area exceeded 10 μm, resulting in reduced coating adhesion. In Comparative Examples 24, 26, and 27, it was believed that the conductivity of the washing solution used in the water wash after the second pickling was high, thus failing to sufficiently suppress the formation of iron oxides during the water wash after the second pickling. As a result, the surface coverage of at least one of Ni, Cu and Sn is less than 25%, and the equivalent circle radius of the uncovered area also exceeds 10 μm, resulting in reduced coating adhesion.

[0081] In contrast, in all the examples of coated steel sheets, when measured using Auger electron spectroscopy, the surface of the base steel sheet was covered by at least one of Ni, Cu, and Sn with a surface coverage rate of 25% or more, and the equivalent circle radius of the uncoated areas not covered by these elements was limited to 10 μm or less, thereby significantly improving the coating adhesion of the coated steel sheet. Particularly in Examples 2, 3, 8, 9, 14, 15, and 20, with a surface coverage rate of 35% or more, the coating adhesion was evaluated as AA, indicating further improvement in coating adhesion. In Examples 4-6, 10-12, 16-18, 21, and 22, with a surface coverage rate of 50% or more, the coating adhesion was evaluated as AAA, indicating further improvement in coating adhesion.

Claims

1. A galvanized steel sheet, characterized in that, It is a coated steel sheet having a base steel plate and a coating disposed on the surface of the base steel plate, wherein, The base steel plate contains, by mass%, a percentage of Ni: 0.010~1.000% Cu: 0.010~1.000%, and Sn: Chemical composition of 0.003~1.000% In the elemental distribution image obtained by measuring the surface of the coated steel sheet using Auger electron spectroscopy... The surface coating of at least one of Ni, Cu and Sn is 25% or more, and The equivalent circle radius of the uncoated region not covered by at least one of Ni, Cu and Sn is less than 10 μm.

2. The plated steel sheet according to claim 1, characterized in that, The surface coverage rate is 35% or more.

3. The plated steel sheet according to claim 2, characterized in that, The surface coverage rate is 50% or more.

4. The galvanized steel sheet according to any one of claims 1 to 3, characterized in that, The surface coverage rate is below 80%.

5. The galvanized steel sheet according to any one of claims 1 to 4, characterized in that, The equivalent circle radius of the uncovered region is less than 5 μm.

6. The plated steel sheet according to any one of claims 1 to 5, characterized in that, The chemical composition contains, by mass%, Ni: 0.040~1.000% Cu: 0.040~1.000%, and Sn: 0.004~1.000%.

7. The galvanized steel sheet according to any one of claims 1 to 6, characterized in that, It has a Vickers hardness of over 200 Hv.

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