Plated steel sheet and component including same

Abstract

Provided are a plated steel sheet, and a component including same, the plated steel sheet comprising a base material steel sheet and plating disposed on a surface of the base material steel sheet, the plated steel sheet characterized in that: the base material steel sheet has a chemical composition containing, in terms of mass%, 0.010%-1.000% of Ni, 0.010%-1.000% of Cu, and 0.003%-1.000% of Sn; and in an elemental distribution image obtained by measuring a cross-section of the plated steel sheet with EPMA, a surface coverage of a covered region in which a surface concentration of at least one of Ni, Cu, and Sn satisfies [Ni] + [Cu] + 0.6[Sn] ≥ 10, and the surface concentration of Fe is 10 mass% or more is 25% or more, and the average length of a non-covered region not covered with any of Ni, Cu, or Sn is 10 μm or less.

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 corrosion resistance 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 to improve corrosion resistance. Furthermore, automotive steel sheets and the like are generally often processed into the desired part shape by press forming. However, for example, if the plating peels off during sliding between the mold and the surface of the plated steel sheet during press forming, the chemical conversion treatability and therefore the corrosion resistance of the processed part may be reduced.

[0007] Therefore, an object of the present invention is to provide a plated steel sheet containing Ni, Cu, and Sn, which is capable of exhibiting improved chemical conversion treatability in processed portions, and a part including the plated steel sheet.

[0008] In order to achieve the above object, the present inventors have conducted studies focusing on the element distribution on the surface of a steel sheet, and as a result, have found that, when a cross section of a plated steel sheet is measured by EPMA, the surface of the base steel sheet is covered with an alloy plating of at least one of Ni, Cu, and Sn and Fe at a predetermined surface coverage rate, and the size of an uncoated region not covered with at least one of Ni, Cu, and Sn is limited to a predetermined range, thereby significantly improving the chemical conversion treatability of a processed portion, and have completed the present invention.

[0009] The present invention, which has achieved the above-mentioned object, is as follows: (1) A plated steel sheet comprising a base steel sheet and a plating disposed on 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 EPMA, the surface concentration of at least one of Ni, Cu, and Sn satisfies the following formula 1 and the surface coverage rate of a coated region where the surface concentration of Fe is 10 mass % or more is 25% or more, and the average length of an uncoated region not covered with at least one of Ni, Cu, and Sn is 10 μm or less. [Ni] + [Cu] + 0.6[Sn] ≧ 10 ... Formula 1 where [Ni], [Cu], and [Sn] are the surface concentrations [mass %] of each element. (2) The plated steel sheet according to (1) above, characterized in that the surface coverage is 35% or more. (3) The plated steel sheet according to (2) above, characterized in that the surface coverage is 50% or more. (4) The plated steel sheet according to any one of (1) to (3) above, characterized in that the surface coverage is 80% or less. (5) The plated steel sheet according to any one of (1) to (4) above, characterized in that the average length of the uncoated region is 5 μm or less. (6) The plated steel sheet according to any one of (1) to (5) above, characterized in that the chemical composition includes, in mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%. (7) The plated steel sheet according to any one of (1) to (6) above, characterized in that it has a Vickers hardness of 200 Hv or more. (8) A part, characterized in that it includes the plated steel sheet according to any one of (1) to (7) above.

[0010] According to the present invention, it is possible to provide a plated steel sheet containing Ni, Cu, and Sn, which can exhibit improved chemical conversion treatability in processed portions, and a part including the plated steel sheet.

[0011] 1 is a cross-sectional schematic view of a plated steel sheet according to an embodiment of the present invention, illustrating the surface coverage of the plating.

[0012] <Plated Steel Sheet> A plated steel sheet according to an embodiment of the present invention comprises a base steel sheet and a plating disposed on a surface of the base steel sheet, wherein the base steel sheet has a chemical composition containing, in mass%, the following: Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and wherein an element distribution image obtained by measuring a cross section of the plated steel sheet using an electron probe microanalyzer (EPMA) has an element distribution image in which the surface concentration of at least one of Ni, Cu, and Sn satisfies the following formula 1 and the surface concentration of Fe is 10 mass% or more, the surface coverage rate of a coated region where the surface concentration of at least one of Ni, Cu, and Sn is 25% or more, and the average length of an uncoated region not covered with at least one of Ni, Cu, and Sn is 10 μm or less: [Ni] + [Cu] + 0.6[Sn] ≧ 10 (Formula 1), where [Ni], [Cu], and [Sn] are the surface concentrations [mass%] of each element.

[0013] 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 corrosion resistance. 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 state, 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. This reduction in chemical conversion treatability is particularly problematic when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. On the other hand, as mentioned above, automotive steel sheets and the like are generally often processed into the desired part shape by press forming. In the case of plated steel sheets, for example, the plating may peel off when the surface of the plated steel sheet slides against the mold during press forming. When the plating peels off from the processed portion due to press forming or the like, exposing the base steel sheet, a decrease in chemical conversion treatability at the processed portion where the base steel sheet is exposed becomes a problem, particularly when the base steel sheet simultaneously contains the three elements Ni, Cu, and Sn.

[0014] 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.

[0015] Therefore, the present inventors conducted studies, focusing particularly on the element distribution on the steel sheet surface, in order to provide a plated steel sheet that can exhibit excellent chemical conversion treatability in processed portions even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, the present inventors found that it is effective to appropriately coat the surface of a base steel sheet containing Ni, Cu, and Sn with an alloy plating of at least one of these elements and Fe, and to limit the size of the uncoated region that is not coated with at least one of Ni, Cu, and Sn to within a predetermined range. More specifically, the present inventors have found that, when a cross section of a plated steel sheet is measured with an EPMA (electron probe microanalyzer), the surface coverage of coated regions where the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more is 25% or more, and further, by applying plating so that the average length of uncoated regions not coated with at least one of Ni, Cu, and Sn (i.e., uncoated regions not coated with any of Ni, Cu, and Sn) is 10 μm or less, the adhesion between the plating and the base steel sheet can be increased and peeling of the plating can be sufficiently suppressed even in processing, particularly processing that involves sliding such as press processing, and therefore the chemical conversion treatability of the processed portion can be significantly improved.

[0016] Fig. 1 is a cross-sectional schematic diagram of a plated steel sheet according to an embodiment of the present invention, illustrating the surface coverage of the plating. Referring to Fig. 1, the plated steel sheet 1 comprises a base steel sheet 2 and a plating 3 (covered region) disposed on the surface of the base steel sheet 2. The plating 3 contains at least one of Ni, Cu, and Sn and Fe. When the cross section of the plated steel sheet 1 is measured by EPMA, the surface concentration of at least one of Ni, Cu, and Sn in the plating 3 satisfies the above formula 1, and the surface concentration of Fe is 10 mass% or more. Here, in the plated steel sheet 1 of Fig. 1, the length L of each plating 3 is i The sum of ΣL i (In Figure 1, ΣL i = L1 + L2 + L3), and the surface length L0 of the base steel plate 2 is ΣL i / L0 × 100≧25, that is, the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1, and the surface coverage of the coated region where the surface concentration of Fe is 10 mass% or more is 25% or more. In addition, in the plated steel sheet 1 of Figure 1, the spacing between adjacent platings 3 (E1 and E2 in Figure 1) corresponds to the uncoated region that is not coated with at least one of Ni, Cu, and Sn, and the average length of the coated region ((E1 + E2) / 2 in Figure 1) is limited to 10 μm or less.

[0017] Without intending to be bound by any particular theory, it is believed that controlling the surface coverage of such an alloy plating containing at least one of Ni, Cu, and Sn and Fe to 25% or more and limiting the average length of the uncoated region to 10 μm or less, i.e., uniformly covering the surface of the base steel sheet with the alloy plating, allows the Ni, Cu, and Sn in the alloy plating to function appropriately as cathode sites during chemical conversion treatment, thereby promoting the anodic dissolution (etching) of Fe present around the alloy plating. More specifically, Ni, Cu, and Sn are elements that are potentially more noble than Fe. Therefore, by having these elements uniformly dispersed on the steel sheet surface rather than in a solid solution state, these elements function as effective cathode sites in relation to Fe during chemical conversion treatment, as described above, and promote the etching of the surrounding Fe, thereby significantly improving the chemical treatability of the steel sheet. As a result, a chemical conversion coating can be uniformly formed over the entire steel sheet, significantly improving corrosion resistance.

[0018] In addition, alloy plating of at least one of Ni, Cu, and Sn with Fe can improve adhesion between the alloy plating and the base steel sheet compared to plating simply with at least one of Ni, Cu, and Sn. Therefore, peeling of the plating can be sufficiently suppressed even during processes involving sliding, such as press working, thereby significantly improving the chemical treatability of the processed area. Even if the surface coverage rate of the alloy plating is very high, for example, 50% or higher, if the uncoated area not covered by at least one of Ni, Cu, and Sn becomes relatively large, this uncoated area will not be able to promote the anodic dissolution of Fe during chemical treatment. As a result, the chemical treatability of not only the processed area but also the entire plated steel sheet will be reduced. Therefore, in the plated steel sheet according to the embodiment of the present invention, it is important that the surface of the base steel sheet is uniformly coated with an alloy plating of at least one of Ni, Cu, and Sn and Fe. That is, when a cross section of the plated steel sheet is measured by EPMA, it is important that the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface coverage of the coated region where the surface concentration of Fe is 10 mass% or more is 25% or more, and that the average length of the uncoated region not covered with at least one of Ni, Cu, and Sn is 10 μm or less. This is because, if either of these conditions is not satisfied, a chemical conversion coating cannot be formed uniformly over the entire steel sheet during chemical conversion treatment, and further, the adhesion between the plating and the base steel sheet cannot be improved. On the other hand, if both of these conditions are satisfied, a chemical conversion coating can be formed uniformly over the entire steel sheet and the adhesion between the plating and the base steel sheet can be improved, resulting in a significant improvement in the chemical treatability of the processed portion.

[0019] 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 chemical conversion treatability in processed portions, and thus superior corrosion resistance in processed portions, 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 chemical conversion treatability and / or corrosion resistance in processed portions are required. Each component of the plated steel sheet according to the embodiment of the present invention will be described in more detail below.

[0020] [Plating] According to an embodiment of the present invention, a plating is disposed on the surface of a base steel sheet, for example, on at least one surface, preferably both surfaces, of the base steel sheet. When a cross section of the plated steel sheet is measured by EPMA, the plating may be such that the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more. In addition to Ni, Cu, Sn, and Fe, other elements such as Zn and Al may also be contained. For example, the plating may essentially consist of at least one of Ni, Cu, and Sn and Fe, or may consist of at least one of Ni, Cu, and Sn and Fe, or may consist of at least one of Ni, Cu, and Sn and Fe. The plating weight is not particularly limited and may be appropriately selected within a range that satisfies the requirements for surface coverage and uncoated areas, which will be described in detail later. Furthermore, as long as the plated steel sheet according to the embodiment of the present invention satisfies the requirements that the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface coverage rate of the coated region where the surface concentration of Fe is 10 mass% or more is 25% or more, it may include plating that does not satisfy these requirements, such as plating or alloy plating in which the surface concentration of at least one of Ni, Cu, and Sn does not satisfy the above formula 1, or plating in which the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is less than 10 mass%.

[0021] [Surface Coverage: 25% or More] In an embodiment of the present invention, in an element distribution image obtained by measuring the cross section of a plated steel sheet with EPMA, the surface coverage of a coating region where the surface concentration of at least one of Ni, Cu, and Sn satisfies the following formula 1 and the surface concentration of Fe is 10% by mass or more is controlled to 25% or more: [Ni] + [Cu] + 0.6[Sn] ≥ 10 ... formula 1 Here, [Ni], [Cu], and [Sn] are the surface concentrations [% by mass] of each element. As mentioned above, Ni, Cu, and Sn in such alloy plating can function as cathode sites during chemical conversion treatment. In chemical conversion treatment, electrons are generally generated by the anodic dissolution (etching) of Fe, and at the cathode sites, the electrons generated by the anodic dissolution of Fe cause a cathodic reaction (2H + +2e - →H2, 10H + +NO3 - +8e - →NH4 + In connection with this, the pH of the chemical conversion treatment solution in the vicinity of the steel sheet surface increases, and as a result, compounds such as zinc phosphate crystals that constitute the chemical conversion treatment film are precipitated on the steel sheet surface.

[0022] In an embodiment of the present invention, by controlling the surface coverage of the plated steel sheet by alloy plating of at least one of Ni, Cu, and Sn with Fe to 25% or more while satisfying the requirements of the uncoated region described below, the surface of the base steel sheet can be uniformly coated with the alloy plating, allowing Ni, Cu, and Sn to effectively function as cathode sites. As a result, the above-mentioned cathodic reaction can be appropriately promoted over the entire surface of the steel sheet, thereby forming a chemical conversion coating uniformly over the entire steel sheet and improving adhesion between the alloy plating and the base steel sheet compared to when the steel sheet is simply plated with at least one of Ni, Cu, and Sn. As a result, peeling of the plating can be sufficiently suppressed even during processing involving sliding, such as press working, thereby significantly improving the chemical conversion treatability of the processed portion. From the perspective of further improving chemical conversion treatability, a higher surface coverage is preferable. For example, the surface coverage of the coating region in which the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more 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, but the surface coverage may be, for example, 80% or less, 75% or less, 70% or less, or 65% or less.

[0023] [Average Length of Uncoated Regions: 10 μm or Less] In an embodiment of the present invention, in an element distribution image obtained by measuring the cross section of a plated steel sheet using EPMA, the average length of uncoated regions not coated with at least one of Ni, Cu, and Sn (i.e., uncoated regions not coated with any of Ni, Cu, and Sn) is controlled to 10 μm or less. If the uncoated regions not coated with at least one of Ni, Cu, and Sn become relatively large, naturally, no cathode sites exist in such uncoated regions, making it impossible to promote the anodic dissolution of Fe during chemical conversion treatment. As a result, the chemical conversion coating cannot be formed uniformly over the entire steel sheet, and the chemical conversion treatability of not only the processed portions but the entire plated steel sheet is reduced.

[0024] In an embodiment of the present invention, by satisfying the above-described surface coverage requirements while allowing such uncoated regions to exist and controlling the average length of the uncoated regions to 10 μm or less, the surface of the base steel sheet can be uniformly coated with an alloy plating of at least one of Ni, Cu, and Sn and Fe, allowing Ni, Cu, and Sn to effectively function as cathode sites. As a result, the above-described cathodic reaction can be appropriately promoted over the entire surface of the steel sheet, allowing a chemical conversion coating to be uniformly formed over the entire steel sheet, thereby significantly improving corrosion resistance. From the viewpoint of further improving chemical conversion treatability and, in turn, corrosion resistance, the smaller the average length of the uncoated regions, the more preferable. For example, the average length of the uncoated regions not covered with at least one of Ni, Cu, and Sn is preferably 8 μm or less, more preferably 6 μm or less or 5 μm or less, and most preferably 4 μm or less or 3 μm or less. Although there is no particular lower limit, the average length of the uncoated region that is not coated with at least one of Ni, Cu, and Sn may be, for example, 0.5 μm or more, or 1 μm or more.

[0025] [Measurement of Surface Coverage and Uncovered Region] The surface coverage is measured using an EPMA as follows. First, five samples are taken from the plated steel sheet so that a cross section including the surface 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 an irradiation current of 5×10 -7 The element distribution image is then binarized (min = 0, max = 255) using image analysis software "ImageJ" to determine the length L of the coating region in the surface portion of the plated steel sheet where the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass % or more. i Finally, the sum of the L obtained for the five samples is calculated. i Calculate the average of the total ΣLi and the surface length L0 of the corresponding base steel plate (length of the long side in the field of view: 100 μm) is calculated as ΣL i / L0 × 100 is calculated, and the calculated value is determined as the surface coverage of the coating region where the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass % or more.

[0026] The number of uncoated regions not coated with at least one of Ni, Cu, and Sn and the length of each uncoated region are calculated in the binarized image obtained using the image analysis software "ImageJ" in connection with the measurement of the surface coverage. Next, the average length per uncoated region is calculated by dividing the total length of the uncoated regions by the total number of uncoated regions ((E1 + E2) / 2 in Figure 1). Finally, the average value of the average lengths per uncoated region obtained for the five samples is calculated, and the calculated average value is determined as the average length of the uncoated regions not coated with at least one of Ni, Cu, and Sn.

[0027] [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 chemical conversion treatability in processed portions. This object is achieved by applying plating so that, when a cross section of the plated steel sheet is measured by EPMA, the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface coverage rate of a coated region where the surface concentration of Fe is 10 mass % or more is 25% or more, and further the average length of an uncoated region that is not covered with at least one of Ni, Cu, and Sn is 10 μm or less. Therefore, the chemical composition of the base steel sheet is not particularly limited except that it contains, in mass %, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, and 0.003 to 1.000% Sn, and therefore it is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the object of the present invention. The chemical composition of the base steel sheet may contain, in addition to Ni, Cu, and Sn, appropriate amounts of any alloying elements commonly added in the technical field of the present invention. Hereinafter, the chemical composition of the base steel sheet used in the plated steel sheet according to the embodiment of the present invention will be described in detail, but these descriptions are intended to merely exemplify preferred chemical compositions of base steel sheets for use in automotive steel sheets and the like, and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0028] 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.

[0029] [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.

[0030] [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.

[0031] [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.

[0032] [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.

[0033] [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.

[0034] [Sn: 0.003 to 1.000%] Sn is an element effective in improving corrosion resistance. To fully obtain this effect, the Sn content is preferably 0.003% or more. The Sn content may be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, excessive Sn content may promote the formation of oxides, particularly Mn- and / or Si-based surface oxides and iron oxides, on the steel sheet surface, which may impair plating adhesion in the plating process. Therefore, the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[0035] [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.

[0036] [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.

[0037] [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.

[0038] 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.

[0039] [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.

[0040] [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.

[0041] [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.

[0042] [Hf: 0-0.050%] [Mg: 0-0.050%] [Zr: 0-0.050%] [Ca: 0-0.010%] [REM: 0-0.010%] Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of non-metallic inclusions. The Hf, Mg, Zr, Ca, and REM contents may be 0%, but to achieve these effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel sheet increases manufacturing costs. Therefore, the Hf, Mg, and Zr contents are preferably 0.050% or less, and may be 0.010% or less, 0.005% or less, or 0.003% or less. Similarly, the Ca and REM contents are each preferably 0.010% or less, and may be 0.005% or less or 0.003% or less.

[0043] [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.

[0044] [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.

[0045] 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.

[0046] 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.

[0047] [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.

[0048] As described above, the steel sheet according to the embodiment of the present invention can achieve superior chemical conversion treatability in processed portions, and thus superior corrosion resistance in processed portions, 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 superior chemical conversion treatability and / or corrosion resistance in processed portions, and is particularly useful for use in parts in the automotive field. In a preferred embodiment, an automobile part is provided that includes the plated steel sheet according to the embodiment of the present invention. Examples of automobile parts include frame parts, bumpers, and other structural and reinforcing parts that require strength, as well as exterior plate parts such as roofs, hoods, fenders, and doors that require high designability. 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.

[0049] [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.

[0050] [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.

[0051] <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.

[0052] The plated steel sheet according to the embodiment of the present invention can be manufactured by, for example, performing 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, a secondary pickling process in which the cold-rolled steel sheet is secondarily pickled, a plating process in which the secondarily pickled cold-rolled steel sheet is plated, and an annealing process in which the obtained plated steel sheet is annealed. 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 described below. Each process will be described in detail below.

[0053] [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.

[0054] [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%.

[0055] [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 the surface of the hot-rolled steel sheet with a Mn—Si-depleted layer having a thickness of 0.3 μm or more, 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, the plating applied in the plating process before the annealing process can be appropriately maintained, and the desired surface coverage can be achieved in the finally obtained plated steel sheet.

[0056] 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.

[0057] 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.

[0058] In steel sheets containing the three elements Ni, Cu, and Sn simultaneously, the presence of these elements in the steel sheet 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 an Mn—Si-depleted layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the coiling temperature in the coiling process with a secondary pickling process described in detail below, it is 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, which makes it impossible to form an Mn—Si-depleted layer having 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 after the plating process, and the desired surface coverage cannot be achieved in the finally obtained plated steel sheet.

[0059] From the viewpoint of further increasing the surface coverage and further improving the chemical conversion treatability, 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 the internal oxide layer, which in turn makes it possible to make the Mn-Si depleted layer thicker. As a result, it is possible to more significantly suppress the formation of Mn- and / or Si-based surface oxides in the annealing step, and it is possible to further increase the surface coverage. There is no particular limitation on the upper limit of the coiling temperature, but the coiling temperature may be, for example, 600°C or lower.

[0060] [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.

[0061] [Secondary Pickling Step] The resulting cold-rolled steel sheet is subjected to secondary pickling in the subsequent secondary pickling step. Specifically, the secondary pickling step involves 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. Even if the outer and inner oxide layers are not sufficiently removed in the previous primary pickling, the above-described secondary pickling using an aqueous hydrochloric acid solution can reliably and completely remove these oxide layers. Furthermore, because the surface condition of the steel sheet changes during cold rolling, and the reason for this is not necessarily clear, the formation of Mn- and / or Si-based surface oxides in the subsequent annealing step may not be sufficiently suppressed due to this change in the surface condition and the presence of Ni, Cu, and Sn in the steel sheet. Therefore, in the present manufacturing method, secondary pickling is performed before the annealing step to condition the surface of the steel sheet, thereby making it possible to sufficiently and reliably suppress the formation of Mn- and / or Si-based surface oxides in the subsequent annealing step. Furthermore, iron oxides may also form on the surface of the steel sheet during cold rolling, which may result in poor plating performance in the subsequent plating step. To reliably remove the iron oxides and ensure plating performance, it is effective to perform pickling using an inhibitor-free aqueous solution having a hydrochloric acid concentration of 3 to 12% after the cold rolling step and before the plating step, followed by rinsing with a wash solution having an electrical conductivity of 40 mS / m or less, as described below. 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.

[0062] In the secondary pickling process, the water rinse after the secondary pickling is also extremely important. For example, if the electrical conductivity of the water used in the water rinse is relatively high, more specifically, if it is higher than 40 mS / m, iron oxides may form on the surface of the cold-rolled steel sheet during the water rinse after the secondary pickling. The presence of such iron oxides on the surface of the cold-rolled steel sheet inhibits the adhesion of the plating in the subsequent plating process. In this case, the desired surface coverage and average length of the uncoated region cannot be achieved in the final plated steel sheet. In contrast, in the present production method, the water rinse after the secondary pickling is performed using a water rinse having an electrical conductivity of 40 mS / m or less, thereby significantly suppressing the formation of iron oxides during the water rinse after the secondary pickling and enabling the plating to be properly adhered in the subsequent plating process.

[0063] 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 and achieve a desired surface coverage in steel sheets containing the three elements Ni, Cu, and Sn simultaneously. 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.

[0064] [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 suitable plating process effective for achieving the desired surface coverage and average length of the uncoated region, such as electroplating, vapor deposition plating, thermal spraying, or cold spraying. Preferably, the plating step is carried out by electroplating. Electroplating is carried out using a bath containing at least one of Ni, Cu, and Sn at a predetermined concentration, at a current density of 0.1 to 5.0 A / dm 2 The current density is preferably 0.3 to 2.0 A / dm and the current application time is 0.1 to 10.0 seconds. 2 The current application time is 0.5 to 5.0 seconds.

[0065] In this manufacturing method, in order to ensure proper adhesion of the plating, it is important to perform the plating step after sufficiently or completely removing Mn- and / or Si-based surface oxides and iron oxides from the surface of the base steel sheet in the secondary pickling step. That is, it is important to perform the plating step after the secondary pickling step. Conversely, as long as the plating step is performed after the secondary pickling step, performing the plating step before the secondary pickling step is not necessarily excluded. For example, it is possible to achieve the desired surface coverage and average length of the uncoated region by dividing the plating step into two steps, first performing the first plating treatment before the secondary pickling step and then performing the second plating treatment after the secondary pickling step. Alternatively, it is also possible to perform another plating treatment before the secondary pickling step and then perform the plating step according to this manufacturing method after the secondary pickling step.

[0066] [Annealing Step] Finally, the obtained plated 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). The annealing step allows at least one of Ni, Cu, and Sn in the plating applied in the plating step to be alloyed with Fe in the base steel sheet, thereby forming a coating region in which the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more.

[0067] According to this production method, in a steel sheet for which improvement of chemical conversion treatability is difficult due to the simultaneous inclusion of the three elements Ni, Cu, and Sn, it is possible to sufficiently or completely remove Mn- and / or Si-based surface oxides and iron oxides from the surface of the base steel sheet by combining, in particular, an Mn-Si depleted layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the coiling temperature in the coiling step with specific secondary pickling and water rinsing in the secondary pickling step. In this regard, by subsequently performing appropriate plating and annealing steps, it is possible to produce a plated steel sheet having a coating in which, when a cross section is measured by EPMA, the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and is covered by a coated region having a surface concentration of Fe of 10 mass% or more at a surface coverage rate of 25% or more, and the average length of the uncoated region not covered by at least one of Ni, Cu, and Sn is limited to 10 μm or less. As mentioned above, when Ni, Cu, and Sn are present in a steel sheet as a solid solution, the potential of the base steel sheet becomes more noble than when these elements are not present in a solid solution state. This reduces the etching ability of Fe during chemical conversion treatment, which in turn may result in a decrease in the chemical treatability of the steel sheet. However, with plated steel sheets manufactured according to the present manufacturing method, at least one of Ni, Cu, and Sn is present on the steel sheet surface not in a solid solution state but uniformly dispersed, allowing a chemical conversion coating to be uniformly formed over the entire steel sheet during chemical conversion treatment. In addition, alloying at least one of Ni, Cu, and Sn with Fe through the annealing process improves the adhesion between the alloy plating and the base steel sheet compared to when at least one of Ni, Cu, and Sn is simply plated. As a result, peeling of the plating can be sufficiently suppressed even during processes involving sliding, such as press working, thereby significantly improving the chemical treatability of the processed area. Therefore, the plated steel sheet 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, and can contribute to industrial development by extending the life of plated steel sheets used in automobiles and building materials.

[0068] 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.

[0069] [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

[0070] [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%.

[0071] [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%.

[0072] 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.

[0073] 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.

[0074] First, molten steel was cast by a continuous casting method to form a steel billet having the chemical composition shown in Table 1. After the steel billet was cooled once, it was reheated to 1200°C and hot rolled, and then coiled at the coiling temperature shown in Table 2. The hot rolling was carried out by performing rough rolling and finish rolling, and the finishing temperature of the finish rolling was 900 to 1050°C, and the reduction ratio of the finish rolling was 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.

[0075] Next, the obtained 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. Next, the cold-rolled steel sheet (base steel sheet) that had been subjected to secondary pickling was immersed in a bath containing a metal species (one of Ni, Cu, and Sn) shown in Table 2 at a predetermined concentration at a current density of 0.5 A / dm 2 Finally, the obtained plated steel sheet was subjected to an annealing step in which the plated steel sheet was heated to a temperature of 800°C in an atmosphere having a dew point of 0°C and hydrogen of 4% (nitrogen balance) in a furnace with an oxygen concentration of 20 ppm or less, and maintained at that temperature for 100 seconds, thereby obtaining a plated steel sheet in which at least one of Ni, Cu, and Sn was alloyed with Fe.

[0076]

[0077]

[0078] The properties of the obtained plated steel sheets were measured and evaluated by the following methods.

[0079] [Evaluation of Chemical Conversion Treatability of Processed Part] The chemical conversion treatability of the processed part was evaluated by evaluating the chemical conversion treatability of the bead part after a drawbead test. Specifically, first, a 200 mm x 30 mm sample of the plated steel sheet produced above was coated with NOX-RUST550NH and then pressed against a die with an R = 4 mm at a pressing load of 3 kN. Next, using a tensile testing machine UST-10T manufactured by Oriental Co., Ltd., the sample was pulled out at a speed of 100 mm / min so that the sliding distance was 100 mm. Next, the sample that had undergone the drawbead test was subjected to a zinc phosphate treatment as a chemical conversion treatment under the following conditions. Degreasing: Immersion in a degreaser (Fine Cleaner E2083) at 40°C for 2 minutes, followed by rinsing with water. Surface conditioning: Immersion in a surface conditioner (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.

[0080] The bead portion of the sample that had been subjected to chemical conversion treatment was observed using SEM-EPMA, and the area ratio of the portion where the chemical conversion coating film was not formed, commonly called "clear," was calculated by binarization using the image analysis software "ImageJ." Depending on the area ratio of the clear portion, the chemical conversion treatability of the processed portion was evaluated according to the following evaluation criteria. Clear portion was evaluated using an EPMA (JXA-8500 manufactured by JEOL Ltd.) at an acceleration voltage of 15 kV and an irradiation current of 5 × 10 -7 In a mapping image obtained by photographing an area of ​​80 μm x 60 μm or more at 1000x magnification under condition A, the area was defined as an area with an Fe concentration of 70% or more. The void area ratio was determined as the average value of five randomly selected fields of view. AAA: void area ratio less than 20% AA: void area ratio 20-25% A: void area ratio 25-35% B: void area ratio more than 35%

[0081] Plated steel sheets that were evaluated as AAA, AA, or A for the chemical conversion treatability of the processed portion were evaluated as plated steel sheets containing Ni, Cu, and Sn and capable of exhibiting improved chemical conversion treatability of the processed portion. The results are shown in Table 2. The "surface coverage rate of the coated region" in Table 2 indicates the surface coverage rate of the coated region where the surface concentrations of the metal species shown in Table 2 satisfy the above formula 1 and the surface concentration of Fe is 10 mass% or more, when the cross section of the plated steel sheet is measured by EPMA.

[0082] Referring to Table 2, in Comparative Example 23, the low coiling temperature resulted in insufficient formation of an internal oxide layer, and as a result, it was not possible to form an Mn—Si depleted layer having a thickness of 0.3 μm or more. As a result, the surface coverage by the Ni and Fe alloy plating was less than 25%, and the chemical conversion treatability of the processed part 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 was not sufficiently suppressed. As a result, the surface coverage by the Ni and Fe alloy plating was less than 25%, and the average length of the uncoated region exceeded 10 μm, thereby reducing the chemical conversion treatability of the processed part. In Comparative Examples 24, 26, and 27, the high electrical conductivity of the washing solution used for rinsing after the secondary pickling presumably prevented the formation of iron oxides during rinsing after the secondary pickling presumably prevented the formation of iron oxides during rinsing after the secondary pickling. As a result, the surface coverage rate of the alloy plating of Fe with at least one of Ni, Cu, and Sn was less than 25%, and the average length of the uncoated area exceeded 10 μm, resulting in a decrease in the chemical conversion treatability of the processed area.

[0083] In contrast, in the plated steel sheets according to all Examples, when the cross section was measured by EPMA, the plated steel sheets were covered with a surface coverage of 25% or more by a coated region in which the surface concentration of at least one of Ni, Cu, and Sn satisfied the above formula 1 and the surface concentration of Fe was 10 mass% or more, and the average length of the uncoated region not covered with at least one of Ni, Cu, and Sn was limited to 10 μm or less. This significantly improved the phosphatability of the processed portions of the plated steel sheets. In particular, in Examples 2, 3, 8, 9, 14, 15, and 20, in which the surface coverage was 35% or more, the phosphatability of the processed portions was evaluated as AA, further improving the phosphatability of the processed portions. In Examples 4 to 6, 10 to 12, 16 to 18, 21, and 22, in which the surface coverage was 50% or more, the phosphatability of the processed portions was evaluated as AAA, further improving the phosphatability of the processed portions.

[0084] 1 Plated steel sheet 2 Base steel sheet 3 Coating L1, L2 and L3 Length of each coating L0 Surface length of base steel sheet E1 and E2 Uncoated area

Claims

1. A plated steel sheet comprising a base steel sheet and a plating disposed on a surface of the base steel sheet, wherein the base steel sheet has a chemical composition containing, in mass%, the following: Ni: 0.010-1.000%, Cu: 0.010-1.000%, and Sn: 0.003-1.000%, and an element distribution image obtained by measuring a cross section of the plated steel sheet with an electron probe microanalyzer (EPMA) has an element distribution image in which the surface concentration of at least one of Ni, Cu, and Sn satisfies the following formula 1 and the surface coverage of the coated region where the surface concentration of Fe is 10 mass% or more is 25% or more, and the average length of the uncoated region not covered with at least one of Ni, Cu, and Sn is 10 μm or less: [Ni] + [Cu] + 0.6[Sn] ≧ 10 ... formula 1, where [Ni], [Cu], and [Sn] are the surface concentrations [mass%] of each element.

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

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

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

5. The plated steel sheet according to any one of claims 1 to 4, characterized in that the average length of the uncoated region is 5 μm or less.

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

7. The plated steel sheet according to any one of claims 1 to 6, characterized in that it has a Vickers hardness of 200 Hv or more.

8. A part comprising the plated steel sheet according to any one of claims 1 to 7.

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