Plated steel sheet and component including same

Abstract

Provided are: a plated steel sheet that is provided with a base steel sheet and a plating layer disposed on the surface of the base steel sheet, and that is characterized in that the base steel sheet has a chemical composition containing, in mass%, 0.010-1.000% of Ni, 0.010-1.000% of Cu, and 0.003-1.000% of Sn, and the thickness of the plating layer in which the total concentration of Zn and / or Mn is at least 60 mass% is not less than 0.10 µm but less than 1.00 µm in an element distribution image obtained by measuring a cross-section of the plated steel sheet by using an EPMA; and a component including the plated steel sheet.

Description

Plated steel sheets and parts containing them

[0001] The present invention relates to a plated steel sheet and a part including the same.

[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 that can exhibit improved paint film adhesion, and a part including the same.

[0008] In order to achieve the above object, the present inventors have conducted research, focusing particularly on the plating layer, and as a result have found that coating the surface of a base steel sheet containing Ni, Cu, and Sn with a plating layer of a predetermined thickness containing primarily at least one of Zn and Mn can significantly improve paint film adhesion, leading to the completion of the present invention.

[0009] The present invention, which has achieved the above-mentioned objects, is as follows: (1) A plated steel sheet comprising a base steel sheet and a plating layer 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, in an element distribution image obtained by measuring a cross section of the plated steel sheet with an electron probe microanalyzer, the plating layer has a concentration of at least one of Zn and Mn in total of 60 mass% or more and a thickness of 0.10 to less than 1.00 μm. (2) The plated steel sheet according to (1) above, wherein the plating layer has a concentration of at least one of Zn and Mn in total of 60 mass% or more and a thickness of 0.30 to less than 1.00 μm. (3) The plated steel sheet according to (2) above, characterized in that the thickness of the plated layer, in which the concentration of at least one of Zn and Mn is 60 mass% or more in total, is 0.50 to less than 1.00 μm. (4) The plated steel sheet according to any one of (1) to (3) 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%. (5) The plated steel sheet according to any one of (1) to (4) above, characterized in that it has a Vickers hardness of 200 Hv or more. (6) A part, characterized in that it includes the plated steel sheet according to any one of (1) to (5) above.

[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, and a part including the same.

[0011] <Plated Steel Sheet> A plated steel sheet according to an embodiment of the present invention is a plated steel sheet including a base steel sheet and a plating layer disposed on a surface of the base steel sheet, wherein the base steel sheet has a chemical composition including, in 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 a cross section of the plated steel sheet with an EPMA, the plating layer has a concentration of at least one of Zn and Mn that is 60 mass% in total and a thickness of 0.10 to less than 1.00 μm.

[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 have conducted research, focusing particularly on the plating layer, 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 have found that it is effective to coat the surface of a base steel sheet containing Ni, Cu, and Sn with a plating layer of a predetermined thickness that mainly contains at least one of Zn and Mn.

[0015] More specifically, Zn and Mn function as anodes during chemical conversion treatment and improve the chemical treatability of steel sheets by dissolving themselves. Therefore, even if a base steel sheet contains Ni, Cu, and Sn, if the manufacturing method can be appropriately controlled to properly coat the surface of the base steel sheet with a plating layer containing primarily at least one of Zn and Mn, the chemical treatability of the plated steel sheet can be significantly improved. Therefore, the inventors first discovered that, by appropriately controlling the manufacturing method as described in detail below, it is possible to suppress the formation of oxides, particularly Mn- and / or Si-based surface oxides, on the surface of a base steel sheet containing Ni, Cu, and Sn, thereby coating the surface of the base steel sheet with a plating layer containing primarily at least one of Zn and Mn. In addition, the present inventors have found that, when a cross section of a plated steel sheet is measured with an EPMA (electron probe microanalyzer), the chemical conversion treatability of the plated steel sheet can be significantly improved by controlling the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 60 mass % or more to 0.10 to less than 1.00 μm. Therefore, in the plated steel sheet according to an embodiment of the present invention, even though the base steel sheet simultaneously contains the three elements Ni, Cu, and Sn, the anodic dissolution (etching) of Zn and / or Mn in the plating layer can be promoted during chemical conversion treatment, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet, and as a result, paint adhesion can be significantly improved.

[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 Layer] According to an embodiment of the present invention, a plating layer 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 layer may contain at least one of Zn and Mn and satisfy the requirements for the Zn and Mn concentrations and plating layer thickness, which will be described in detail later, and therefore may include various plating compositions. Therefore, the plating layer may contain other elements in addition to Zn and Mn, such as Al, Ni, Cu, Sn, Mg, and Fe. More specifically, the plating layer may contain or consist of Zn—Cu and / or Zn—Fe, etc. Naturally, the plating layer may consist essentially of at least one of Zn and Mn, consist of at least one of Zn and Mn, or consist of at least one of Zn and Mn. The plating coverage is not particularly limited and may be appropriately selected within a range that satisfies the requirements for the Zn and Mn concentrations and plating layer thickness, which will be described in detail later.

[0018] [Thickness of plating layer having a total concentration of at least one of Zn and Mn of 60 mass% or more: 0.10 to less than 1.00 μm] In an embodiment of the present invention, in an element distribution image obtained by measuring a cross section of a plated steel sheet using EPMA, the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 60 mass% or more is controlled to 0.10 to less than 1.00 μm. As described above, Zn and Mn function as anodes during chemical conversion treatment and improve the chemical treatability of the steel sheet by dissolving themselves. In an embodiment of the present invention, by controlling the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 60 mass% or more to 0.10 to less than 1.00 μm, it becomes possible for the Zn and Mn in the plating layer to effectively function as anodes during chemical conversion treatment. That is, the anodic dissolution (etching) of Zn and / or Mn in the plating layer during chemical conversion treatment is promoted, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet, resulting in significantly improved paint adhesion. From the viewpoint of further improving paint adhesion, the thickness of the plating layer having a total concentration of at least one of Zn and Mn of 60 mass% or more is preferably as thick as possible, for example, preferably 0.20 μm or more or 0.30 μm or more, more preferably 0.40 μm or more or 0.50 μm or more, and most preferably 0.60 μm or more or 0.70 μm or more. On the other hand, if the thickness of the plating layer having a total concentration of at least one of Zn and Mn of 60 mass% or more is too thick, the workability of the plated steel sheet may be reduced, or the P ratio in the coating obtained by phosphating may be reduced, resulting in reduced paint adhesion. To explain the latter in more detail, the P ratio represents the ratio of hopite (Zn3(PO4)2.4H2O) to phosphophyllite (Zn2Fe(PO4)2.4H2O) in the film obtained by phosphating. More specifically, it is the X-ray diffraction intensity I of the (100) plane of phosphophyllite measured using an X-ray diffractometer. P and X-ray diffraction intensity of the (020) plane of Hopite I H Using I P / (I P +I H) × 100 (%). A higher P ratio means that more Fe-containing phosphophyllite is present, and that phosphophyllite crystals are densely formed on the steel sheet surface. It is also known that a lower P ratio generally leads to poorer paint adhesion. Here, the Fe in phosphophyllite originates from Fe dissolved in the base steel sheet during chemical conversion treatment. Therefore, if Fe does not sufficiently dissolve during chemical conversion treatment, the formation of phosphophyllite is not promoted, resulting in a low P ratio. In an embodiment of the present invention, if the thickness of a coating layer having a total concentration of at least one of Zn and Mn of 60 mass% or more is too thick, the amount of Fe dissolved from the base steel sheet during chemical conversion treatment decreases, resulting in a lower P ratio of the resulting chemical conversion coating and poor paint adhesion. Therefore, the thickness of the coating layer is set to less than 1.00 μm, and may be 0.95 μm or less or 0.90 μm or less.

[0019] [Measurement of the Thickness of a Plating Layer Having a Total Concentration of at Least One of Zn and Mn of 60% by Mass or More] The thickness of a plating layer having a total concentration of at least one of Zn and Mn of 60% by mass or more is measured using an EPMA as follows. First, five samples are taken from the surface of the plated steel sheet so that the cross section of the plated steel sheet can be observed. Next, for each sample, a rectangular area of ​​80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction is defined as one field of view, and five fields of view in total for the five samples are measured using an EPMA (for example, JXA-8500 manufactured by JEOL Ltd.) at an acceleration voltage of 15 kV and a probe current of 5×10 -7 A mapping image is obtained by photographing the specimen at a magnification of 1000 times under the conditions of A. Next, in the obtained element distribution image, a region on the surface of the plated steel sheet where the concentration of at least one of Zn and Mn is 60 mass% or more in total is determined, and the area of ​​the region is divided by the length (100 μm) in the direction perpendicular to the thickness direction of the field of view to determine the value as the thickness of the plating layer for one sample. Finally, the thicknesses obtained for the five samples are averaged, and the calculated value is determined as the thickness of the plating layer where the concentration of at least one of Zn and Mn is 60 mass% or more in total.

[0020] [Base Steel Sheet] In an embodiment of the present invention, 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%. 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 controlling the thickness of a plating layer having a concentration of at least one of Zn and Mn in total of 60 mass % or more to 0.10 to less than 1.00 μm when a cross section of the plated steel sheet is measured by EPMA. 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.

[0021] 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.

[0022] [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.

[0023] [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.

[0024] [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.

[0025] [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.

[0026] [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.

[0027] [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, thereby impairing 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.

[0028] [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.

[0029] [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.

[0030] [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.

[0031] 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.

[0032] [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.

[0033] [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.

[0034] [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.

[0035] [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 production 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.

[0036] [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.

[0037] [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.

[0038] 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.

[0039] 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.

[0040] [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.

[0041] As described above, the steel sheet according to the embodiment of the present invention can achieve excellent paint adhesion and, in turn, excellent corrosion resistance compared to conventional plated steel sheets simultaneously containing the three elements Ni, Cu, and Sn. Therefore, the plated steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields requiring excellent paint adhesion and / or corrosion resistance, particularly for use in parts in the automotive field. In a preferred embodiment, an automobile part including the plated steel sheet according to the embodiment of the present invention is provided. Examples of automobile parts include frame parts, bumpers, and other structural and reinforcing parts that require strength, as well as exterior parts such as roofs, hoods, fenders, and doors that require high design quality. It is sufficient for at least a portion of these parts to include the plated steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the characteristics of the plated steel sheet described above. In areas of the steel sheet that do not come into direct contact with a mold during forming, such as press forming, or that come into direct contact with the mold but are relatively lightly processed, the characteristics of the plated steel sheet do not change significantly before and after forming.

[0042] [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.

[0043] [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.

[0044] <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.

[0045] 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.

[0046] [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.

[0047] [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%.

[0048] [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. Because the above-mentioned outer and inner oxide layers are removed by primary pickling after coiling, a Mn—Si-depleted layer having a thickness of 0.3 μm or more remains 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 a plating layer having a desired thickness and a total concentration of at least one of Zn and Mn of 60 mass% or more can be formed on the finally obtained plated steel sheet.

[0049] 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.

[0050] 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.

[0051] 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, it becomes impossible to form a plating layer having a desired thickness in which the concentration of at least one of Zn and Mn is 60 mass % in total in the finally obtained plated steel sheet.

[0052] From the viewpoint of further improving coating adhesion by further increasing the thickness of the coating layer having a total concentration of at least one of Zn and Mn of 60 mass % or more, 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 further thicken the Mn—Si depleted layer. As a result, it is possible to more significantly suppress the formation of Mn- and / or Si-based surface oxides in the annealing step, making it possible to further increase the thickness of the coating layer. There is no particular upper limit to the coiling temperature, but the coiling temperature may be, for example, 600°C or lower.

[0053] [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.

[0054] [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).

[0055] [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.

[0056] 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 formation of a coating layer of a desired thickness having a total concentration of at least one of Zn and Mn of 60 mass% or more 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 plating step is inhibited due to the surface oxides. As a result, the coating cannot be properly adhered, and the coating layer cannot be formed to the desired thickness 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.

[0057] In the secondary pickling process, rinsing after the secondary pickling is also extremely important. For example, if the electrical conductivity of the rinsing solution used in the rinsing 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 rinsing 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 plating layer cannot be formed to the desired thickness on the finally obtained plated steel sheet. In contrast, in the present production method, the rinsing after the secondary pickling is performed with a rinsing solution having an electrical conductivity of 40 mS / m or less, thereby significantly suppressing the formation of iron oxides during the rinsing after the secondary pickling and enabling the plating to be properly adhered in the subsequent plating process.

[0058] 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.

[0059] [Plating Step] Next, in the plating step, plating is applied to at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). The plating step can be carried out by any appropriate plating process that is effective for achieving a desired thickness of a plating layer in which the concentration of at least one of Zn and Mn is 60 mass % in total, 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 Zn and Mn and optional additive elements at predetermined concentrations, at a current density of 5 to 20 A / dm 2 The current density is preferably 8 to 15 A / dm and the current application time is 2.0 to 20.0 seconds. 2 The current application time is 3.0 to 10.0 seconds.

[0060] 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, 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, it is possible to form a plating layer of the desired thickness having a total concentration of at least one of Zn and Mn of 60 mass% or more. 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 secondary pickling step.

[0061] According to this manufacturing method, in steel sheets where 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 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. In this regard, by carrying out an appropriate subsequent plating process, it is possible to produce plated steel sheets in which the thickness of the plating layer, measured by EPMA, is controlled to 0.10 to less than 1.00 μm and in which the concentration of at least one of Zn and Mn is 60 mass% or more in total. As mentioned above, Zn and Mn function as anodes during chemical conversion treatment and improve the chemical treatability of the steel sheet by dissolving themselves. Therefore, with a plated steel sheet produced according to this production method, the anodic dissolution (etching) of Zn and / or Mn in the plating layer can be promoted during chemical conversion treatment, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet, resulting in significantly improved paint adhesion. Therefore, plated steel sheets produced by this 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 used in automobiles and building materials.

[0062] The plated steel sheet according to the embodiment of the present invention can be used as the various automotive parts described above, for example, after a chemical conversion coating or paint film is optionally formed on the surface. Whether an automotive part having a paint film or chemical conversion coating includes the plated steel sheet according to the embodiment of the present invention can be determined by removing the paint film or chemical conversion coating from a sample taken from the automotive part. In this case, the location from which the sample is taken, the paint film removal step, and the chemical conversion coating removal step are as follows.

[0063] [Sample collection locations] Samples will be collected from automotive parts, avoiding the following locations (i) to (iv): (i) Locations within 20 mm from the toe of spot welds and locations within 20 mm from the toe of the bead of arc / laser welds (ii) Processed areas with a curvature radius of less than 15 mm, and locations within 5 mm from such processed areas (iii) Edges within 5 mm from the cut end surface of the part (iv) Locations within 5 mm from locations where red rust is visible

[0064] [Paint Removal Process] The paint film is removed from a sample cut from an automobile body under the following conditions to expose the steel sheet. A paint remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface at room temperature and allowed to stand for approximately 5 minutes. The paint film is then removed by rubbing with a hard sponge or similar (e.g., Kanefiel, manufactured by Aion Co., Ltd.). The sample is then rinsed with water and dried. The remaining paint film is then confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after rinsing and drying. In the element distribution image obtained by EPMA, regions with a carbon concentration of 10% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the paint film has not been sufficiently removed. To measure the area ratio of regions with a carbon concentration of 10% by mass or more, first obtain an element distribution image of carbon using an EPMA with a carbon concentration range of 10 to 30%. Specific measurement conditions for EPMA are as follows: Apparatus: JEOL Ltd. JXA-8230 Electron Probe Microanalyzer Acceleration voltage: 15 kV Irradiation current: 0.05 μA Area analysis: WDS Analysis interval: 300 μm or more Area fraction: Average value of 5 fields of view Next, the area fraction is measured by image processing of the obtained element distribution image. Image analysis software "ImageJ" is used for image processing, and the C element distribution image is read into ImageJ, and then binarized in "Make Binary" under "Binary" in "Process" so that areas with a C concentration of 10% by mass or more are displayed as black and areas with a C concentration of less than 10% by mass are displayed as white. After binarization, "Measure" under "Analyze" is used to read the value of "Area fraction" under "Results," and this value is determined as the area fraction of the area where the C concentration is 10% by mass or more. If the coating film is not sufficiently removed, the removal of the coating film is repeated until the area ratio of the region having a C concentration of 10% by mass or more becomes less than 5%.

[0065] [Chemical Conversion Coating Removal Process] For a sample cut from an automobile body and having the coating removed, for example, if the chemical conversion coating is a zinc phosphate coating, the chemical conversion coating is removed in accordance with JIS K 3151:1996. Specifically, the sample is immersed in a 5% chromic acid aqueous solution heated to 75°C for 15 minutes to remove the chemical conversion coating. The sample is then washed with water and dried. The remaining state of chemical conversion crystals is confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after washing and drying. In the element distribution image obtained by EPMA, regions with a P concentration of 5% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the chemical conversion coating has not been sufficiently removed. To measure the area ratio of regions with a P concentration of 5% by mass or more, first obtain an element distribution image of P using EPMA with a P concentration range of 5 to 10%. The obtained element distribution image is then image-processed to measure the area ratio. Image analysis software "ImageJ" was used for image processing. The P element distribution image was then loaded into ImageJ, and binarized using "Make Binary" in "Binary" under "Process" so that areas with a P concentration of 5% by mass or more were displayed as black, and areas with a P concentration of less than 5% by mass were displayed as white. After binarization, "Measure" under "Analyze" was used to read the value for "Area fraction" in "Results," and this value was determined as the area fraction of areas with a P concentration of 5% by mass or more. If peeling of the chemical conversion coating was insufficient, removal of the chemical conversion coating was repeated until the area fraction of areas with a P concentration of 5% by mass or more became less than 5%.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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 hydrochloric acid concentration of 5% 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 metal species such as Zn and Mn at predetermined concentrations at a current density of 10 A / dm 2 and current application time of 5.0 seconds to obtain plated steel sheets having plating layers containing the metal species shown in Table 2 attached to both surfaces of the base steel sheet. The "thickness of the plating layer" in Table 2 indicates the thickness of the plating layer in which the concentration of at least one of Zn and Mn in total is 60 mass % or more when the cross section of the plated steel sheet is measured by EPMA.

[0070]

[0071]

[0072] The properties of the obtained plated steel sheets were measured and evaluated by the following methods.

[0073] [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.

[0074] 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%.

[0075] 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.

[0076] 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 a Mn—Si depleted layer having a thickness of 0.3 μm or more. As a result, the thickness of the coating layer having a total concentration of at least one of Zn and Mn of 60 mass% or more was less than 0.10 μm, 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 thickness of the coating layer was less than 0.10 μm, and coating adhesion was reduced. In Comparative Examples 24 and 26, 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. As a result, the thickness of the plating layer was less than 0.10 μm, and the paint adhesion was reduced. In Comparative Example 27, the thickness of the plating layer, in which the concentration of at least one of Zn and Mn was 60 mass % or more in total, was too thick, which is thought to have reduced the amount of Fe dissolved from the base steel sheet during the chemical conversion treatment, resulting in a lower P ratio in the chemical conversion treatment film. As a result, the paint adhesion was reduced.

[0077] In contrast, in the plated steel sheets according to all Examples, the coating adhesion of the plated steel sheets was significantly improved by controlling the thickness of the coating layer, in which the total concentration of at least one of Zn and Mn was 60 mass% or more as measured by EPMA, to 0.10 to less than 1.00 μm. In particular, in Examples 2, 3, 8, 9, 14, 15, and 20, in which the coating layer thickness was 0.30 μm or more, the coating adhesion was evaluated as AA, indicating further improvement. In Examples 4 to 6, 10 to 12, 16 to 18, 21, and 22, in which the coating layer thickness was 0.50 μm or more, the coating adhesion was evaluated as AAA, indicating further improvement.

Claims

1. A plated steel sheet comprising a base steel sheet and a plating layer disposed 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 a cross section of the plated steel sheet with an EPMA, A plated steel sheet, characterized in that the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 60 mass % or more is 0.10 to less than 1.00 μm.

2. 2. The plated steel sheet according to claim 1, wherein the thickness of the plating layer having a total concentration of at least one of Zn and Mn of 60 mass % or more is 0.30 to less than 1.00 μm.

3. The plated steel sheet according to claim 2, wherein the thickness of the plating layer having a concentration of at least one of Zn and Mn in total of 60 mass % or more is 0.50 to less than 1.00 μm.

4. 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:

5. 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.

6. A part comprising the plated steel sheet according to any one of claims 1 to 3.