Plated steel sheets and parts containing them

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

[Technical Field]

[0001] This invention relates to plated steel sheets and parts containing the same. [Background technology]

[0002] It is known that improving the corrosion resistance of steel sheets or plated steel sheets is effective in enhancing the chemical conversion treatment properties of the steel sheets or plated steel sheets, thereby uniformly forming a chemical conversion treatment film on these steel sheets.

[0003] In this regard, Patent Document 1 describes a continuous annealing furnace or a cold-rolled steel sheet / hot-dip galvanized steel sheet combined equipment with a continuous annealing furnace in which a cooling method for a cooling zone including 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, radiant cooling, or cooling tube cooling, in which high-strength cold-rolled steel sheets are continuously annealed, the steel sheet surface is exposed to an atmosphere in which iron oxidizes within the steel sheet temperature range, pickled at the exit side of the annealing furnace, and then coated with iron or Ni plating at a rate of 1 to 50 mg / m². 2 A method for manufacturing high-strength cold-rolled steel sheets, characterized by the application of a specific process, is described. Furthermore, Patent Document 1 explains that while oxidation of steel sheets is normally prevented by using an inert atmosphere with an extremely low concentration of oxygen and / or an extremely low dew point gas around the steel sheet, by instead actively exposing it to an oxidizing atmosphere, oxidizing not only Si and Mn but also the iron in the steel sheet, and then pickling it after it leaves the annealing furnace to remove the oxide film of Si, Mn, etc., along with the oxide film of iron in the steel sheet, as well as the oxide film of Si and Mn, etc., by pickling, a high-strength cold-rolled steel sheet with good chemical conversion treatment properties and no transparency can be obtained even with a high content of Si, Mn, etc.

[0004] Patent Document 2 describes a material containing 0.10% to 0.50% by mass of copper (Cu), with a surface residual scale count of 160,000 particles / mm². 2The following describes an automotive steel sheet characterized in that the maximum particle size of copper compound particles exposed on the surface is 2 μm or less. Furthermore, Patent Document 2 teaches that, according to the above configuration, the particle size of copper compound particles exposed on the steel sheet surface, which becomes the cathode point in the chemical conversion treatment, is made 2 μm or less, and the residual scale is made to a predetermined amount or less, thereby providing a steel sheet with excellent chemical conversion treatment properties. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2008-190030 [Patent Document 2] Japanese Patent Publication No. 2020-084238 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Patent Document 2 explains that, in addition to copper (Cu), elements such as nickel (Ni) and tin (Sn) reduce the mechanical properties such as strength and formability required for automotive steel sheets, as well as chemical stability such as corrosion resistance. In particular, copper compounds present on the surface of the steel sheet reduce the ability to perform chemical conversion treatment to improve corrosion resistance. Generally, automotive steel sheets are often processed into the desired part shape by press forming. However, for example, if the plating peels off when the die and the surface of the plated steel sheet slide against each other during press forming, the ability to perform chemical conversion treatment and, consequently, the corrosion resistance of the processed part may decrease.

[0007] Therefore, the present invention aims to provide a plated steel sheet containing Ni, Cu, and Sn that can exhibit improved chemical conversion treatment properties of the processed portion, and a part containing the same. [Means for solving the problem]

[0008] To achieve the above objective, the inventors focused on the elemental distribution on the surface of the steel sheet and conducted investigations. As a result, the inventors found that when the cross-section of the 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 with a predetermined surface coverage ratio, and the size of the uncovered area not covered by at least one of Ni, Cu, and Sn is limited to a predetermined range, thereby significantly improving the chemical conversion treatment properties of the processed area. Based on this, the inventors completed the present invention.

[0009] The present invention, which has achieved the above objectives, is as follows. (1) A plated steel sheet comprising a base steel sheet and a plating disposed on the surface of the base steel sheet, The aforementioned base steel plate is, by mass%, Ni: 0.010~1.000%, Cu: 0.010~1.000%, and It has a chemical composition containing Sn: 0.003 to 1.000%, In the elemental distribution image obtained by measuring the cross-section of the plated steel sheet with EPMA, The surface coverage of the coated region is 25% or more, 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. A plated steel sheet characterized in that the average length of the uncoated region, which is not coated with at least one of Ni, Cu, and Sn, is 10 μm or less. [Ni]+[Cu]+0.6[Sn]≧10...Formula 1 Here, [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 rate is 35% or more. (3) The plated steel sheet according to (2) above, characterized in that the surface coverage rate is 50% or more. (4) The plated steel sheet according to any one of the above items (1) to (3), characterized in that the surface coverage rate is 80% or less. The plated steel sheet according to any one of (1) to (4) above, characterized in that the average length of the non-coated area is 5 μm or less. (6) The chemical composition is in mass %, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000% The plated steel sheet according to any one of (1) to (5) above, characterized by containing the above. (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 component, characterized by containing the plated steel sheet according to any one of (1) to (7) above.

Effect of the Invention

[0010] According to the present invention, there can be provided a plated steel sheet containing Ni, Cu, and Sn, which can exhibit improved chemical conversion treatment properties of the processed part, and a component containing the same.

Brief Description of the Drawings

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

Mode for Carrying Out the Invention

[0012] <Plated Steel Sheet> The plated steel sheet according to an embodiment of the present invention includes a base steel sheet and a plating disposed on the surface of the base steel sheet. 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 in the element distribution image obtained by measuring the cross-section of the plated steel sheet by EPMA, The surface coverage of the coated region is 25% or more, 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. It is characterized by having an average length of 10 μm or less of the uncoated region that is not covered by at least one of Ni, Cu, and Sn. [Ni]+[Cu]+0.6[Sn]≧10...Formula 1 Here, [Ni], [Cu], and [Sn] are the surface concentrations [mass%] of each element.

[0013] Generally, when the chemical conversion treatment properties decrease, areas where the chemical conversion coating, known as "skeleton," has not formed may occur, resulting in reduced corrosion resistance. For example, when elements such as Ni, Cu, and Sn are present in solid solution in a steel sheet, the potential of the steel sheet becomes nobler compared to a state where these elements are not solid-dissolved, which can reduce the etching properties of Fe during chemical conversion treatment. In this case, the chemical conversion treatment properties of the steel sheet decrease. This decrease in chemical conversion treatment properties becomes particularly problematic when the steel sheet contains all three elements: Ni, Cu, and Sn. On the other hand, as mentioned earlier, automotive steel sheets are generally processed into the desired part shape by press forming, and 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 die during press forming. When the plating on a processed area peels off due to press forming or other processes, exposing the base steel sheet, a problem arises, particularly if the base steel sheet contains all three elements Ni, Cu, and Sn simultaneously, as this leads to a decrease in the chemical conversion treatment properties of the exposed base steel sheet in the processed area.

[0014] Furthermore, there are generally two known methods for manufacturing steel plates: one involves obtaining molten iron in a blast furnace using iron ore, a natural resource, as the main raw material, and then producing molten steel through refining in a converter or the like; and the other involves producing molten steel in an electric arc furnace using scrap material, a recycled resource, as the main raw material. Blast furnace steel can also contain elements such as Ni, Cu, and Sn as additive elements, so when these elements are present, it is necessary to appropriately address the above-mentioned issues. On the other hand, electric arc furnace steel, as mentioned above, uses scrap material as the main raw material, and therefore contains a relatively large amount of scrap-derived elements such as Ni, Cu, and Sn (so-called trump elements), and thus the above-mentioned issues are particularly pronounced.

[0015] Therefore, the inventors conducted research focusing particularly on the elemental distribution on the surface of the steel sheet in order to provide a plated steel sheet that can exhibit excellent chemical conversion treatment properties of the processed area even when the steel sheet contains the three elements Ni, Cu, and Sn simultaneously. As a result, the inventors found that it is effective to appropriately coat the surface of the 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 area not covered by at least one of Ni, Cu, and Sn to a predetermined range. More specifically, the present inventors have found that when measuring the cross-section of a plated steel sheet with an EPMA (electron probe microanalyzer), the surface coverage of the coated 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% by mass or more is 25% or more, and furthermore, the average length of the uncoated region not covered by at least one of Ni, Cu, and Sn (i.e., the uncoated region not covered by any of Ni, Cu, and Sn) is 10 μm or less, thereby improving the adhesion between the plating and the base steel sheet, and sufficiently suppressing peeling of the plating even in processing, especially in processing involving sliding such as press working, and thus significantly improving the chemical conversion treatment properties of the processed part.

[0016] Figure 1 is a schematic cross-sectional view of a plated steel sheet according to an embodiment of the present invention, illustrating the surface coverage of the plating. Referring to Figure 1, the plated steel sheet 1 comprises a base steel sheet 2 and a plating 3 (covering area) 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 with 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% by mass or more. Here, in the plated steel sheet 1 of Figure 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 length L0 of the surface of the base steel plate 2 is ΣL i The condition / L0 × 100 ≥ 25 is satisfied, meaning that the surface coverage of the coated region is 25% or more, 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% by mass or more. Furthermore, in the plated steel sheet 1 in Figure 1, the spacing between adjacent platings 3 (E1 and E2 in Figure 1) corresponds to the uncoated region not covered by at least one of Ni, Cu, and Sn, and the average length of the coated region (in Figure 1, (E1 + E2) / 2) is limited to 10 μm or less.

[0017] While not intended to be bound by any particular theory, it is believed that by controlling the surface coverage rate of such 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 area to 10 μm or less, that is, by uniformly coating the surface of the base steel sheet with the alloy plating, the Ni, Cu, and Sn in the alloy plating can be appropriately made to function as cathode sites during the chemical conversion treatment, and in connection with this, the anodic dissolution (etching) of Fe present around the alloy plating can be promoted. More specifically, Ni, Cu, and Sn are elements that are electrically nobler than Fe. Therefore, by uniformly dispersing these elements on the steel sheet surface rather than in a solid solution state, these elements can be made to function as effective cathode sites in relation to Fe during the chemical conversion treatment, as described above, and the etching of Fe present around them can be promoted, thus significantly improving the chemical conversion treatment properties of the steel sheet. As a result, the chemical conversion treatment film can be uniformly formed over the entire steel sheet, and corrosion resistance can be significantly improved.

[0018] In addition, by using an alloy plating of at least one of Ni, Cu, and Sn with Fe, it is possible to improve the adhesion between the alloy plating and the base steel sheet compared to simply plating with at least one of Ni, Cu, and Sn. Therefore, peeling of the plating can be sufficiently suppressed even in processes involving sliding, such as press working, and thus the chemical conversion treatment properties of the processed area can be significantly improved. Even if the surface coverage rate of the above alloy plating is very high, for example, a very high value of 50% or more, if the uncoated area not covered by at least one of Ni, Cu, and Sn is relatively large, such uncoated areas will not be able to promote the anodic dissolution of Fe during the chemical conversion treatment. As a result, the chemical conversion treatment properties of the entire plated steel sheet, not just the processed area, will be reduced. Therefore, in the plated steel sheet according to the embodiment of the present invention, it is important to uniformly coat the surface of the base steel sheet with an alloy plating of at least one of Ni, Cu, and Sn and Fe. Specifically, when the cross-section of the plated steel sheet is measured by EPMA, it is important to satisfy both the following conditions: the surface coverage rate of the coated area where the surface concentration of at least one of Ni, Cu, and Sn satisfies formula 1 above and the surface concentration of Fe is 10% by mass or more is 25% or more; and the average length of the uncoated area not covered by at least one of Ni, Cu, and Sn is 10 μm or less. This is because if even one of these conditions is not satisfied, it will not be possible to uniformly form the chemical conversion coating over the entire steel sheet during the chemical conversion treatment, and furthermore, it will not be possible to improve the adhesion between the plating and the base steel sheet. On the other hand, by satisfying both of these conditions, it is possible to uniformly form the chemical conversion coating over the entire steel sheet and improve the adhesion between the plating and the base steel sheet, and as a result, it is possible to significantly improve the chemical conversion treatment performance of the processed area.

[0019] The plated steel sheets according to the embodiments of the present invention encompass not only electric furnace materials that inevitably contain Ni, Cu, and Sn as trump elements, but also blast furnace materials that contain Ni, Cu, and Sn as essential or optional additive elements. Furthermore, the plated steel sheets according to the embodiments of the present invention can achieve superior chemical conversion treatment properties of the processed parts compared to conventional plated steel sheets that simultaneously contain all three elements of Ni, Cu, and Sn, and consequently, superior corrosion resistance of the processed parts. Therefore, the plated steel sheets according to the embodiments of the present invention are particularly useful in the automotive sector where superior chemical conversion treatment properties and / or corrosion resistance of the processed parts are required. The components of the plated steel sheets according to the embodiments of the present invention will be described in more detail below.

[0020] [plating] According to embodiments of the present invention, the plating is disposed on the surface of the base steel sheet, for example, on at least one, preferably both, surfaces of the base steel sheet. The plating is sufficient if, when the 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 concentration of Fe is 10% by mass or more. In addition to Ni, Cu, Sn, and Fe, other elements, such as Zn and Al, may also be included. For example, the plating may essentially consist of at least one of Ni, Cu, and Sn and Fe, or consist of at least one of Ni, Cu, and Sn and Fe, or consist of at least one of Ni, Cu, and Sn and Fe. The amount of plating is not particularly limited and should be appropriately selected within a range that satisfies the requirements for surface coverage and uncovered areas, which will be described in detail later. Furthermore, as long as the requirement that the surface coverage rate of the coated area is 25% or more, 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% by mass or more, the plated steel sheet according to the embodiment of the present invention may also include plating that does not satisfy this requirement, for example, 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% by mass.

[0021] [Surface coverage rate: 25% or more] In an embodiment of the present invention, in the element distribution image obtained by measuring the cross-section of the plated steel sheet by EPMA, the surface coverage rate of the 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 be 25% or more. [Ni] + [Cu] + 0.6[Sn] ≥ 10 ··· Formula (1) Here, [Ni], [Cu], and [Sn] are the surface concentrations [mass%] of the respective elements. As described above, Ni, Cu, and Sn in such alloy plating can function as cathode sites during the chemical conversion treatment. In the chemical conversion treatment, generally, anodic dissolution (etching) of Fe occurs to generate electrons. On the other hand, at the cathode site, a cathode reaction (2H + + 2e - → H2, 10H + + NO3 - + 8e - → NH4 + + 3H2O) occurs. In relation to this, the pH of the chemical conversion treatment solution near the steel sheet surface rises, and accordingly, compounds such as zinc phosphate crystals constituting the chemical conversion coating are deposited on the steel sheet surface.

[0022] In embodiments of the present invention, by controlling the surface coverage rate of the plated steel sheet with an alloy plating of at least one of Ni, Cu, and Sn and Fe to 25% or more while satisfying the requirements for the uncoated area described later, the surface of the base steel sheet can be uniformly coated with the alloy plating, and Ni, Cu, and Sn can be effectively utilized as cathode sites. As a result, the above-mentioned cathode reaction can be appropriately carried out across the entire surface of the steel sheet, so that a chemical conversion treatment film is uniformly formed over the entire steel sheet, and the adhesion between the alloy plating and the base steel sheet can be improved compared to the case where at least one of Ni, Cu, and Sn is simply plated. As a result, peeling of the plating can be sufficiently suppressed even in processing involving sliding such as press working, and therefore the chemical conversion treatment performance of the processed area can be significantly improved. From the viewpoint of further improving the chemical conversion treatment performance, a higher surface coverage rate is preferable. For example, the surface coverage of a coated region where the surface concentration of at least one of Ni, Cu, and Sn satisfies formula 1 above and the surface concentration of Fe is 10% by mass or more is preferably 30% or 35%, more preferably 40% or 45%, and most preferably 50% or 55%. 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 uncovered area: 10 μm or less] In the embodiment of the present invention, in the elemental distribution image obtained by measuring the cross-section of the plated steel sheet with EPMA, the average length of the uncoated region not covered by at least one of Ni, Cu, and Sn (i.e., the uncoated region not covered by any of Ni, Cu, and Sn) is controlled to 10 μm or less. If the uncoated region not covered by at least one of Ni, Cu, and Sn becomes relatively large, then naturally, since there are no cathode sites in such an uncoated region, it becomes impossible to promote the anodic dissolution of Fe during the chemical conversion treatment. As a result, it becomes impossible to form a chemical conversion treatment film uniformly over the entire steel sheet, and the chemical conversion treatment performance of the entire plated steel sheet, not just the processed area, deteriorates.

[0024] In embodiments of the present invention, while satisfying the surface coverage requirements described above, by allowing such uncovered regions to exist and controlling the average length of these uncovered 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, and Ni, Cu, and Sn can be effectively utilized as cathode sites. As a result, the above-mentioned cathode reaction can be appropriately carried out across the entire surface of the steel sheet, so that the chemical conversion coating can be uniformly formed over the entire steel sheet, and therefore corrosion resistance can be significantly improved. From the viewpoint of further improving chemical conversion treatment properties and thus corrosion resistance, it is preferable that the average length of the uncovered regions be as small as possible. For example, the average length of the uncovered regions not covered by at least one of Ni, Cu, and Sn is preferably 8 μm or less, more preferably 6 μm or less or 5 μm or less, and most preferably 4 μm or less or 3 μm or less. The lower limit is not particularly limited, but the average length of the uncoated region not covered by 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 areas] The surface coverage is measured using EPMA as follows: First, five samples are taken from the plated steel sheet so that the 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. An EPMA (e.g., JXA-8500 manufactured by JEOL Ltd.) is used to measure the total of five fields of view from the five samples, with an acceleration voltage of 15 kV and an irradiation current of 5 × 10⁻¹⁰ -7 Under conditions A, a map image is obtained by taking a picture at 1000x magnification. Next, the obtained elemental distribution image is binarized using the image analysis software "ImageJ" (min=0, max=255), and the length L of the coated region where the surface concentration of at least one of Ni, Cu, and Sn on the surface portion of the plated steel sheet satisfies the above formula 1 and the surface concentration of Fe is 10% by mass or more is determined. iThe sum of (L1 + L2 + L3 in Figure 1) is calculated. Finally, the L obtained for the five samples is calculated. i Calculate the average of the sums and ΣL i Let L0 be the length of the surface of the corresponding base steel plate (length of the longest side in the field of view: 100 μm), and ΣL i / L0 × 100 is calculated, and the calculated value is determined as the surface coverage rate of the coated area 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% by mass or more.

[0026] In relation to the measurement of surface coverage, the number of uncoated regions and the length of each uncoated region are calculated from the binarized image obtained using the image analysis software "ImageJ". 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 (in Figure 1, (E1 + E2) / 2). Finally, the average value of the average length 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 that are not covered by at least one of Ni, Cu, and Sn.

[0027] [Base material steel plate] In embodiments of the present invention, the base steel sheet has a chemical composition comprising, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. The present invention aims to provide a plated steel sheet containing Ni, Cu, and Sn, as described above, that can exhibit improved chemical conversion treatment properties of the processed portion. This objective is achieved by plating the plated steel sheet such that, when the cross-section of the plated steel sheet is measured by EPMA, the surface coverage of the coated region where the surface concentration of at least one of Ni, Cu, and Sn satisfies formula 1 above and the surface concentration of Fe is 10% by mass or more is 25% or more, and furthermore, the average length of the uncoated region not covered by 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 Ni: 0.010~1.000%, Cu: 0.010~1.000%, and Sn: 0.003~1.000% by mass%, and thus it is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the objectives of the present invention. In addition to Ni, Cu, and Sn, the chemical composition of the base steel sheet may contain appropriate amounts of any alloying elements that are commonly added in the art of the present invention. The chemical composition of the base steel sheet used in plated steel sheets according to embodiments of the present invention will be described in detail below, but these descriptions are intended merely as examples of preferred chemical compositions of base steel sheets for application 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 embodiments of the present invention, for example, the base steel sheet is, by mass%, C: 0.001~0.500%, Si: 0~3.00%, Mn: 0.10~3.00%, Al: 0.001~2.000%, Ni: 0.010~1.000%, Cu: 0.010~1.000%, Sn: 0.003~1.000%, P: 0.100% or less, S: 0.100% or less, N: 0.0100% or less, Ti: 0~0.150%, Nb: 0~0.150%, B: 0~0.0100%, Mo: 0~1.000%, Cr: 0~1.000%, V: 0~0.150%, W: 0~1.000%, Hf: 0~0.050%, Mg: 0~0.050%, Zr: 0~0.050%, Ca: 0~0.010%, REM: 0~0.010%, As: 0~0.010%, Ir: 0~1.000%, and Remainder: Fe and impurities It is preferable to have a chemical composition consisting of the following. Each element will be described in more detail below.

[0029] [C:0.001~0.500%] Carbon (C) is an element that increases strength inexpensively and is an important element for controlling the strength of steel. To obtain this effect fully, it is preferable that the C content be 0.001% or more. The C content may also 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 lead to a decrease in elongation. For this reason, it is preferable that the C content be 0.500% or less. The C content may also be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.

[0030] [Si: 0~3.00%] Si is an effective element for increasing strength as a solid solution strengthening element. The Si content may be 0%, but to obtain such an effect, it is preferable that the Si content be 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, if the Si content is excessive, it may lead to a decrease in elongation along with an increase in steel strength. For this reason, it is preferable that the Si content be 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: 0.10~3.00%] Mn is an element that enhances the hardenability of steel and is effective in increasing its strength. To fully obtain these effects, it is preferable that the Mn content be 0.10% or more. The Mn content may also 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, if the Mn content is excessive, it may lead to a decrease in elongation along with an increase in steel strength. For this reason, it is preferable that the Mn content be 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, or 2.00% or less.

[0032] [Al:0.001~2.000%] Al acts as a deoxidizing agent for steel and is an element that has the effect of sounding down steel. To obtain this effect sufficiently, it is preferable that the Al content be 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, if the Al content is excessive, coarse Al oxide may be generated, which may reduce the elongation of the steel sheet. For this reason, it is preferable that the Al content be 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: 0.010~1.000%] [Cu: 0.010~1.000%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. To fully obtain such effects, the content of each of these elements is 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, if these elements are present in excess, it may promote the formation of oxides on the surface of the steel sheet, particularly Mn and / or Si-based surface oxides and iron oxides, in which case the adhesion of the plating in the plating process will be inhibited. Therefore, the content of Ni and Cu is 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: 0.003~1.000%] Sn is an effective element for improving corrosion resistance. To obtain this effect fully, 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, if the Sn content is excessive, it may promote the formation of oxides on the surface of the steel sheet, particularly Mn and / or Si-based surface oxides and iron oxides, in which case the adhesion of the plating in the plating process will be inhibited. 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 steel embrittlement. A lower P content is preferable, and ideally it should be 0%. However, excessive reduction of the P content can lead to a significant increase in cost. For this reason, the P content may be 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, excessive P content can lead to steel embrittlement due to grain boundary segregation, as described above. Therefore, it is preferable to have a P content of 0.100% or less. The P content may also 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 nonmetallic inclusions such as MnS in steel, leading to a decrease in the ductility of steel parts. A lower S content is preferable, and ideally it should be 0%. However, excessive reduction of the S content can lead to a significant increase in cost. For this reason, 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 can lead to cracking during cold forming, originating from nonmetallic inclusions. Therefore, it is preferable to have an S content of 0.100% or less. The S content may also 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, reducing their workability. A lower N content is preferable, ideally 0%. However, excessive reduction of the N content can lead to a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or higher, or 0.0005% or higher, or 0.0010% or higher. On the other hand, excessive N content can lead to the formation of coarse nitrides, as described above, reducing the workability of the steel sheet. Therefore, the N content is preferably 0.0100% or less. The N content may also be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0038] The preferred basic chemical composition of the base steel sheet is as described above. Furthermore, the base steel sheet may, if necessary, contain at least one of the following elements in place of a portion of the remaining Fe.

[0039] [Ti: 0~0.150%] [Nb: 0~0.150%] [V: 0~0.150%] Ti, Nb, and V have the effect of improving the strength of steel sheets by forming carbonitrides in the steel and strengthening through precipitation. The Ti, Nb, and V content may be 0%, but in order to obtain such an effect, the Ti, Nb, and V content is 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, if these elements are included in excess, the effect will saturate, and including them in the steel more than necessary will lead to an increase in manufacturing costs. Therefore, the Ti, Nb, and V content is 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: 0~0.0100%] B improves low-temperature toughness by segregating at grain boundaries and increasing grain boundary strength. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the B content be 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0041] [Mo: 0~1.000%] [Cr: 0~1.000%] [W: 0~1.000%] Mo, Cr, and W are elements that enhance the hardenability of steel and contribute to improving its strength. While the content of Mo, Cr, and W may be 0%, to obtain such effects, it is preferable that the content of Mo, Cr, and W be 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 included in excess, the effect will saturate, and including them in the steel more than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the content of Mo, Cr, and W be 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 nonmetallic inclusions. The content of Hf, Mg, Zr, Ca, and REM may be 0%, but to obtain such an effect, the content of each of these elements is preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, if these elements are included in excess, the effect will saturate, and including them in the steel sheet more than necessary will lead to an increase in manufacturing costs. Therefore, the content of Hf, Mg, and Zr is preferably 0.050% or less, and may be 0.010% or less, 0.005% or less, or 0.003% or less, respectively. Similarly, the content of Ca and REM is preferably 0.010% or less, and may be 0.005% or less, or 0.003% or less, respectively.

[0043] [As: 0~0.010%] As is an effective element for improving corrosion resistance. While the As content may be 0%, it is preferable that the As content be 0.001% or more to obtain this effect. The As content may also be 0.002% or more, or 0.003% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the As content be 0.010% or less. The As content may also be 0.008% or less, or 0.005% or less.

[0044] [Ir: 0~1.000%] Ir is an element that segregates at prior austenite grain boundaries, increasing their strength. While the Ir content may be 0%, it is preferable that the Ir content be 0.001% or higher to obtain this effect. The Ir content may also be 0.003% or higher, 0.005% or higher, or 0.010% or higher. On the other hand, excessive Ir content leads to saturation of the effect, and including more Ir than necessary in the steel increases manufacturing costs. Therefore, it is preferable that the Ir content be 1.000% or lower. The Ir content may also be 0.500% or lower, 0.100% or lower, 0.030% or lower, or 0.015% or lower.

[0045] In the base steel sheet, the remainder other than the elements mentioned above consists of Fe and impurities. Impurities in the base steel sheet refer to components that are mixed in during the industrial production of the base steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap.

[0046] The chemical composition of the base steel sheet can be measured using general analytical methods. For example, the chemical composition of the base steel sheet can be determined 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 can be obtained from around the 1 / 2 thickness point of the base steel sheet, and the composition can be determined by measuring it using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

[0047] [Thickness of the base steel plate] The thickness of the base steel sheet is not particularly limited, but generally it is between 0.2 and 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 sheet 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 treatment properties of the processed part, and consequently superior corrosion resistance of the processed part, 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 useful for use in parts in technical fields where superior chemical conversion treatment properties and / or corrosion resistance of the processed part are required, and is particularly useful for use in parts in the automotive field. In a preferred embodiment, an automotive part including the plated steel sheet according to the embodiment of the present invention is provided. Examples of automotive parts include frame parts, bumpers, other structural parts and reinforcing parts that require strength, and exterior parts such as roofs, hoods, fenders, and doors that require high design quality. These parts only need to include the plated steel sheet according to the embodiment of the present invention in at least a part thereof, and therefore at least a part of these parts will satisfy the characteristics of the plated steel sheet described above. In forming processes such as press forming, the characteristics of the plated steel sheet do not change particularly before and after forming in parts of the steel sheet that do not come into direct contact with the mold or, even if they come into direct contact with the mold, are processed to a relatively low degree.

[0049] [Mechanical properties] The plated steel sheet according to the embodiment of the present invention is not particularly limited, but may have a Vickers hardness of, for example, 90 Hv or higher. The Vickers hardness may be 150 Hv or higher, 200 Hv or higher, 250 Hv or higher, 300 Hv or higher, 350 Hv or higher, 400 Hv or higher, or 450 Hv or higher. The upper limit is not particularly limited, but for example, the Vickers hardness may be 650 HV or lower, 600 HV or lower, 550 HV or lower, or 500 HV or lower.

[0050] [Measurement of Vickers hardness] Vickers hardness is determined as follows: First, a test piece is cut from any position on the plated steel sheet, excluding the edges, so that a cross-section perpendicular to the surface (thickness cross-section) can be observed. The thickness cross-section of the test piece is polished using #600 to #1500 silicon carbide sandpaper, and then polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. This thickness cross-section is used as the measurement surface. Next, the Vickers hardness is measured using a micro Vickers hardness tester with a load of 1 kgf at intervals of at least three times the indentation length. Specifically, a total of 20 points are randomly measured around the 1 / 2 position of the plated steel sheet thickness, and the arithmetic mean of these measurements is determined as the Vickers hardness of the plated steel sheet.

[0051] <Method for manufacturing plated steel sheets> Next, preferred manufacturing methods for plated steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing plated steel sheets according to embodiments of the present invention, and is not intended to limit the plated steel sheets to those manufactured by the manufacturing methods described below.

[0052] A plated steel sheet according to an embodiment of the present invention can be manufactured by, for example, a casting step of casting molten steel with an adjusted chemical composition to form a steel billet, a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet, a winding step of winding the hot-rolled steel sheet and then performing primary pickling, a cold rolling step of cold rolling the winded hot-rolled steel sheet to obtain a cold-rolled steel sheet, a secondary pickling step of secondary pickling the cold-rolled steel sheet, a plating step of applying plating to the secondary-pickled cold-rolled steel sheet, and an annealing step of annealing the obtained plated steel sheet. The following describes in detail the manufacturing of a plated steel sheet obtained by plating a cold-rolled steel sheet, but the plated steel sheet according to an embodiment of the present invention includes not only plated steel sheets obtained by plating a cold-rolled steel sheet, but also plated steel sheets 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, the secondary pickling step may be performed after the winding step without performing the cold rolling step described below. Each step will be described in detail below.

[0053] [Casting Process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, followed by casting using methods such as conventional continuous casting or ingot casting.

[0054] [Hot rolling process] Hot-rolled steel sheets can be obtained by hot-rolling cast steel billets. The hot-rolling process is carried out by hot-rolling the cast steel billet either directly or after it has been cooled and then reheated. When reheating is performed, the heating temperature of the steel billet may be, for example, 1100 to 1250°C. In the hot-rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction ratio of each rolling step can be appropriately determined according to the desired metal structure and sheet thickness. For example, the finishing temperature of the finish rolling may be 900 to 1050°C, and the reduction ratio of the finish rolling may be 10 to 50%.

[0055] [Winding process] The hot-rolled steel sheet obtained in the hot-rolling process is wound in the next winding process and then subjected to primary pickling. In this manufacturing method, the winding of the hot-rolled steel sheet is carried out at a winding temperature of 520°C or higher. By controlling the winding temperature to 520°C or higher, an external oxide layer is formed on the outside (surface) of the steel sheet, and an internal oxide layer is also formed inside (surface) of the steel sheet. This internal oxide layer is mainly composed of Mn and / or Si-based oxides. Therefore, directly beneath the internal oxide layer formed on the surface of the steel sheet, a Mn-Si depleted layer is formed due to the consumption of Mn and / or Si in the steel by the formation of the internal oxide layer. In particular, by controlling the winding temperature to 520°C or higher, the thickness of the Mn-Si depleted layer can be controlled to 0.3 μm or more. Since the above-mentioned external and internal oxide layers are removed by primary pickling after winding, a Mn-Si depleted layer with a thickness of 0.3 μm or more remains on the surface of the hot-rolled steel sheet after primary pickling. By forming the surface of the hot-rolled steel sheet with a Mn-Si deficient layer having a thickness of 0.3 μm or more, the deficient Mn and Si on the steel sheet surface effectively suppresses the formation of Mn and / or Si-based surface oxides on the steel sheet surface during the subsequent annealing process. Therefore, the plating applied in the plating process before the annealing process can be properly maintained, and the desired surface coverage can be achieved in the final plated steel sheet.

[0056] The thickness of the Mn-Si depletion layer is determined as follows. First, using a radiofrequency glow discharge emission spectrometer (GDS), the surface of the steel sheet after primary pickling is subjected to an Ar atmosphere, and a voltage is applied to generate a 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 emission spectral wavelengths of the elements emitted when atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated. The data in the depth direction can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth using a standard sample beforehand, the sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from 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. In this way, when the steel sheet after primary pickling is measured by GDS, the region in the depth direction where the sum of the Mn concentration and Si concentration is 70% or less of the sum of the Mn concentration and Si concentration at the 1 / 2 position of the sheet thickness is defined as the Mn-Si depletion layer, and its thickness is determined.

[0057] The primary pickling is not particularly limited and should be carried out using a commonly used pickling solution under conditions suitable for removing the external and internal oxide layers. The primary pickling may be performed once or in multiple steps to ensure the complete removal of the external and internal 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 plating to steel sheets containing the three elements Ni, Cu, and Sn simultaneously. However, according to this manufacturing method, by combining a Mn-Si deficient layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the winding temperature in the winding process, with a secondary pickling process which will be explained in detail later, it is possible to significantly suppress the formation of such surface oxides. On the other hand, if the winding temperature in the winding process is less than 520°C, the formation of the internal oxide layer will be insufficient, and as a result, it will not be possible to form a Mn-Si deficient layer with a thickness of 0.3 μm or more. In this case, it will not be possible to sufficiently suppress the formation of Mn and / or Si-based surface oxides in the annealing process after the plating process, and the desired surface coverage rate will not be achieved in the final plated steel sheet.

[0059] From the viewpoint of further increasing the surface coverage and improving the chemical conversion treatment properties, it is preferable to control the winding temperature to 550°C or higher. By controlling the winding temperature to 550°C or higher, the formation of the internal oxide layer can be further promoted, which in turn makes it possible to make the Mn-Si deficient layer thicker. As a result, the formation of Mn and / or Si-based surface oxides in the annealing process can be suppressed even more significantly, making it possible to further increase the surface coverage. The upper limit of the winding temperature is not particularly limited, but for example, the winding temperature may be 600°C or lower.

[0060] [Cold rolling process] After pickling or performing other processes on hot-rolled steel sheets, cold-rolled steel sheets can be obtained by cold-rolling them. The reduction ratio during cold rolling can be appropriately determined according to the desired metal structure and sheet thickness, and may be, for example, 20-80%. After the cold-rolling process, the sheet may be cooled to room temperature by air cooling, for example.

[0061] [Secondary pickling process] The obtained cold-rolled steel sheet is then subjected to a secondary pickling process. Specifically, the secondary pickling process involves immersing the cold-rolled steel sheet in an aqueous solution with a hydrochloric acid concentration of 3-12% that does not contain inhibitors to suppress corrosion of the steel sheet, at a temperature of 50-90°C for 2-100 seconds, and then washing the cold-rolled steel sheet with a water washing solution having an electrical conductivity of 40 mS / m or less. Even if the external and internal oxide layers were not sufficiently removed in the preceding primary pickling, these oxide layers can be reliably and completely removed by the above secondary pickling using an aqueous hydrochloric acid solution. Furthermore, because the surface condition of the steel sheet changes due to cold rolling, although the reason is not always clear, it is sometimes not possible to sufficiently suppress the formation of Mn and / or Si-based surface oxides in the subsequent annealing process due to such changes in the surface condition and the Ni, Cu, and Sn present in the steel sheet. Therefore, in this manufacturing method, by performing secondary pickling before the annealing process to prepare the surface condition of the steel sheet, it is possible to sufficiently and reliably suppress the formation of Mn and / or Si-based surface oxides in the subsequent annealing process. In addition, iron oxides may form on the surface of the steel sheet during cold rolling, and in such cases, there is a risk of plating defects in the subsequent plating process. In order to reliably remove such iron oxides and ensure plating quality, it is effective to pickle with an aqueous solution having a hydrochloric acid concentration of 3-12% that does not contain inhibitors after the cold rolling process and before the plating process, and then rinse with water using a washing solution having an electrical conductivity of 40 mS / m or less, as will be explained later. Preferably, the hydrochloric acid concentration of the aqueous hydrochloric acid solution is 4-8%, the immersion temperature is 70-90°C, and the immersion time is 4-50 seconds.

[0062] 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 rinsing is relatively high, more specifically, higher than 40 mS / m, iron oxides may form on the surface of the cold-rolled steel sheet during rinsing after 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, it becomes impossible to achieve the desired surface coverage and average length of the uncoated area in the final plated steel sheet. In contrast, in this manufacturing method, by performing the rinsing after secondary pickling with a rinsing solution having an electrical conductivity of 40 mS / m or less, the formation of iron oxides during rinsing after secondary pickling can be significantly suppressed, making it possible to properly adhere the plating in the subsequent plating process.

[0063] In steel sheets containing the three elements Ni, Cu, and Sn simultaneously, the presence of these elements in the steel sheet promotes the formation of iron oxides during rinsing after secondary pickling, in addition to the formation of Mn and / or Si-based surface oxides during the annealing process. Therefore, it is extremely difficult to suppress the formation of these oxides and achieve the desired surface coverage in the final plated steel sheet in steel sheets containing the three elements Ni, Cu, and Sn simultaneously. Consequently, the fact that the formation of these oxides can be significantly suppressed by combining a Mn-Si deficient layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the winding temperature in the winding process, with specific secondary pickling and rinsing in the secondary pickling process, is quite unexpected and surprising. From the viewpoint of further suppressing the formation of iron oxides, it is preferable that the electrical conductivity of the rinsing solution be as low as possible, specifically preferably 25 mS / m or less, and more preferably 15 mS / m or less.

[0064] [Plating process] Next, in the plating process, plating is applied to at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). The plating process can be carried out by any suitable plating treatment effective in achieving the desired surface coverage and average length of the uncoated area, such as electroplating, vapor deposition, thermal spraying, or cold spraying. Preferably, the plating process is carried out by electroplating. Electroplating is performed using a bath containing at least one of Ni, Cu, and Sn at a predetermined concentration, with a current density of 0.1 to 5.0 A / dm². 2 This can be carried out under conditions of energizing time of 0.1 to 10.0 seconds. Preferably, the current density is 0.3 to 2.0 A / dm 2 The energizing time is 0.5 to 5.0 seconds.

[0065] In this manufacturing method, in order to properly adhere the plating, it is important to carry out the plating process after thoroughly 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 process; in other words, it is important to carry out the plating process after the secondary pickling process. Conversely, as long as the plating process is carried out after the secondary pickling process, carrying out the plating process before the secondary pickling process is not necessarily excluded. For example, it is possible to achieve the desired surface coverage rate and average length of the uncoated area by dividing the plating process into two steps: first, performing the first plating treatment before the secondary pickling process, and then performing the second plating treatment after the secondary pickling process. Alternatively, it is also possible to perform another plating treatment before the secondary pickling process and then carry out the plating process according to this manufacturing method after the secondary pickling process.

[0066] [Annealing process] Finally, the resulting plated steel sheet is annealed. The annealing process 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 it for 0 to 300 seconds. The atmosphere in the annealing process may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere with 1 to 10% hydrogen (e.g., a balance of 4% hydrogen and nitrogen). Through the above annealing process, at least one of the Ni, Cu, and Sn in the plating applied in the plating process alloys with Fe in the base steel sheet, making it possible to form a coated region in which the surface concentration of at least one of the Ni, Cu, and Sn satisfies formula 1 above and the surface concentration of Fe is 10% by mass or more.

[0067] According to this manufacturing method, in steel sheets where the chemical conversion treatment properties are difficult to improve due to the simultaneous presence of three elements, Ni, Cu, and Sn, it becomes 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 deficient layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the winding temperature in the winding process, with specific secondary pickling and water washing in the secondary pickling process. In connection with this, by carrying out appropriate subsequent plating and annealing processes, it is possible to manufacture a plated steel sheet with a plating that, when the cross-section is measured by EPMA, is covered with a surface coverage rate of 25% or more by a covered 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, and the average length of the uncovered region not covered by at least one of Ni, Cu, and Sn is limited to 10 μm or less. As mentioned earlier, when Ni, Cu, and Sn are dissolved in the steel sheet, the potential of the base steel sheet becomes nobler compared to when these elements are not dissolved. This can reduce the etching properties of Fe during the chemical conversion treatment, and consequently, reduce the chemical conversion treatment properties of the steel sheet. However, with plated steel sheets manufactured according to this manufacturing method, at least one of Ni, Cu, and Sn is uniformly dispersed on the surface of the steel sheet rather than being dissolved, allowing for the uniform formation of a chemical conversion treatment film across the entire steel sheet during the chemical conversion treatment. In addition, by alloying at least one of Ni, Cu, and Sn with Fe through the annealing process to create an alloy plating, it is possible to improve the adhesion between the alloy plating and the base steel sheet compared to simply plating with at least one of Ni, Cu, and Sn. As a result, peeling of the plating can be sufficiently suppressed even in processes involving sliding, such as press working, and therefore the chemical conversion treatment properties of the processed area can be significantly improved. Therefore, plated steel sheets manufactured by this method can achieve superior corrosion resistance compared to conventional plated steel sheets containing the three elements Ni, Cu, and Sn simultaneously. This allows for extended lifespan in applications such as automotive and building materials, contributing to industrial development.

[0068] The plated steel sheet according to the embodiment of the present invention can be used as various automobile parts as described above, for example, after a chemical conversion coating or paint film is optionally formed on its surface. Whether or not an automobile 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 automobile part. The sample collection location, paint film removal process, and chemical conversion coating removal process in this case are as follows.

[0069] [Sample collection location] When collecting samples from automotive parts, avoid the following locations (i) to (iv). (i) Within 20 mm of the toe of a spot weld, and within 20 mm of the toe of the bead of an arc / laser weld. (ii) Machining area with a radius of curvature of less than 15 mm, and areas within 5 mm of said machining area (iii) Ends within 5 mm from the cut end face of the part (iv) Areas within 5 mm of the area where red rust is visible to the naked eye

[0070] [Paint film removal process] For a sample cut from the automobile body, the paint film is removed under the following conditions to expose the steel plate. A paint remover (Neoriver #160, manufactured by Sansai Chemical Co., Ltd.) is applied to the surface at room temperature and left to stand for about 5 minutes. Then, the paint film is removed by rubbing with a hard sponge or the like (e.g., Kanefeel, manufactured by AION Co., Ltd.). After that, it is washed with water and dried. At this time, the remaining state of the paint film is confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after washing with water and drying. In the elemental distribution image obtained by EPMA, regions with a C concentration of 10 mass% or more are identified, and if the area ratio of these regions is 5% or more, it is judged that the paint film has not been removed sufficiently. To measure the area ratio of regions with a C concentration of 10 mass% or more, first, an elemental distribution image of C is obtained in EPMA with the C concentration range set to 10-30%. The specific measurement conditions for EPMA are as follows. Equipment: JEOL Ltd. JXA-8230 Electron Probe Microanalyzer Acceleration voltage: 15kV Irradiation current: 0.05μA Surface analysis:WDS Analysis interval: 300 μm or longer Area ratio: Average value of 5 fields of view Next, the area fraction is measured by image processing of the obtained elemental distribution image. The image analysis software "ImageJ" is used for image processing. After loading the elemental distribution image of C into ImageJ, the image is binarized using "Make Binary" in "Process" > "Binary" so that areas with a C concentration of 10 mass% or more are displayed in black and areas with a C concentration of less than 10 mass% are displayed in white. After binarization, the value of "Area fraction" in "Results" is read using "Measure" in "Analyze" and this value is determined as the area fraction of the areas with a C concentration of 10 mass% or more. If the paint film is not sufficiently removed, the removal of the paint film is repeated until the area fraction of the areas with a C concentration of 10 mass% or more is less than 5%.

[0071] [Removal of chemical conversion coating] For samples cut from the automobile body and with the paint film removed, if the chemical conversion coating is, for example, a zinc phosphate coating, the chemical conversion coating is removed in accordance with JIS K 3151:1996. Specifically, the chemical conversion coating is removed by immersion in a 5% chromic acid aqueous solution heated to 75°C for 15 minutes. After that, it is washed with water and dried. At this time, the remaining state of the chemical conversion crystals is confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after washing with water and drying. In the elemental distribution image obtained by EPMA, regions where the P concentration is 5% by mass or more are identified, and if the area ratio of the region is 5% or more, it is judged that the chemical conversion coating has not been removed sufficiently. To measure the area ratio of regions where the P concentration is 5% by mass or more, first, an elemental distribution image of P is obtained in EPMA with the P concentration range set to 5-10%. Then, the area ratio is measured by image processing of the obtained elemental distribution image. Image processing is performed using the image analysis software "ImageJ". After loading the elemental distribution image of P into ImageJ, the image is binarized using "Make Binary" in "Process" > "Binary" so that areas with a P concentration of 5 mass% or more are displayed in black and areas with a P concentration of less than 5 mass% are displayed in white. After binarization, the value of "Area fraction" in "Results" is read using "Measure" in "Analyze" and this value is determined as the area percentage of areas with a P concentration of 5 mass% or more. If the chemical conversion coating is not sufficiently removed, the removal of the chemical conversion coating is repeated until the area percentage of areas with a P concentration of 5 mass% or more is less than 5%.

[0072] The present invention will be described in more detail below with reference to examples, but these examples are merely examples of the present invention, and the present invention is not limited in any way to these examples. Needless to say, the present invention can be modified as desired without departing from the spirit of the invention. [Examples]

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

[0074] First, molten steel was cast using a continuous casting method to form steel billets having the chemical composition shown in Table 1. After the steel billets were cooled, they were reheated to 1200°C and hot-rolled, and then wound up at the winding temperatures shown in Table 2. Hot rolling was carried out by rough rolling and finish rolling, with the finishing temperature being 900-1050°C and the reduction ratio of the finish rolling being 30%. Next, the obtained hot-rolled steel sheets were subjected to primary pickling, and then cold-rolled at a reduction ratio of 50% to obtain cold-rolled steel sheets with a thickness of 1.6 mm.

[0075] Next, the obtained cold-rolled steel sheets were subjected to secondary pickling. Specifically, the secondary pickling was carried out by immersing the cold-rolled steel sheets in an aqueous solution with a 5% hydrochloric acid concentration that does not contain inhibitors at a temperature of 80°C for 4.5 seconds, and then washing the cold-rolled steel sheets with a washing solution having the electrical conductivity shown in Table 2. Subsequently, the secondary-pickled cold-rolled steel sheets (base steel sheets) were subjected to a current density of 0.5 A / dm² using a bath containing one of the metal species (Ni, Cu, and Sn) shown in Table 2 at a predetermined concentration. 2 Furthermore, electroplating was performed under conditions of energization time of 1.0 second to obtain plated steel sheets in which plating was attached to both sides of the base steel sheet. Finally, the obtained plated steel sheets were subjected to an annealing process in which they were heated to a temperature of 800°C in an atmosphere with an oxygen concentration of 20 ppm or less, a dew point of 0°C, and 4% hydrogen (nitrogen balance), and held for 100 seconds to obtain plated steel sheets in which at least one of Ni, Cu, and Sn was alloyed with Fe.

[0076] [Table 1]

[0077] [Table 2]

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

[0079] [Evaluation of chemical treatment properties of the processed area] The chemical treatment properties of the processed area were evaluated by assessing the chemical treatment properties of the bead area after the draw bead test. Specifically, first, a 200mm x 30mm sample of the plated steel sheet manufactured above was coated with NOX-RUST550NH, and then pressed against a mold with R=4mm under a pressing load of 3kN. Next, it was drawn using an Oriental Corporation UST-10T tensile testing machine at a speed of 100mm / min to achieve a sliding distance of 100mm. Then, the sample that underwent the draw bead test was treated with zinc phosphate as a chemical treatment under the following conditions. Degreasing: Immerse in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes, then rinse with water. Surface preparation: Immerse in a surface preparation agent (Preparen Z) at room temperature for 30 seconds. Chemical treatment: Immerse in zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, then rinse with water and dry.

[0080] The bead portion of the chemically treated sample was observed using SEM-EPMA, and the area ratio of the portion where the chemically treated film was not formed, commonly known as "skew," was calculated by binarization using the image analysis software "ImageJ." Based on the area ratio of skew, the chemical treatment performance of the processed portion was evaluated according to the following evaluation criteria. Skew was measured using EPMA (JXA-8500, manufactured by JEOL Ltd.) with an acceleration voltage of 15kV and an irradiation current of 5×10⁻¹⁰ -7 Under condition A, a mapping image was obtained by capturing an area of ​​80 μm × 60 μm or larger at 1000x magnification, and the area with an Fe concentration of 70% or higher was defined as such. The scale area ratio was determined as the average value of 5 randomly selected fields of view. AAA: Skeleton area ratio less than 20% AA: Skeleton area ratio less than 20-25% A: Skeleton area ratio 25-35% B: Over 35% of the surface area

[0081] Plated steel sheets containing Ni, Cu, and Sn were evaluated as having improved chemical conversion treatment properties in the processed area if the chemical conversion treatment properties of the processed area were rated as AAA, AA, and A. The results are shown in Table 2. The "Surface coverage of the coated area" in Table 2 indicates the surface coverage of the coated area when the cross-section of the plated steel sheet is measured by EPMA, and the surface concentrations of the metal species shown in Table 2 satisfy formula 1 above and the surface concentration of Fe is 10% by mass or more.

[0082] Referring to Table 2, in Comparative Example 23, the low winding temperature resulted in insufficient formation of the internal oxide layer, which prevented the formation of a Mn-Si deficient layer with a thickness of 0.3 μm or more. As a result, the surface coverage by the Ni-Fe alloy plating was less than 25%, and the chemical conversion treatment performance of the processed area was reduced. In Comparative Example 25, in addition to the low winding temperature, the high electrical conductivity of the rinsing solution used for rinsing after secondary pickling prevented sufficient suppression of the formation of Mn and / or Si-based surface oxides during the annealing process, and furthermore, the formation of iron oxides during rinsing after secondary pickling was also not sufficiently suppressed. As a result, the surface coverage by the Ni-Fe alloy plating was less than 25%, and the average length of the uncoated area exceeded 10 μm, further reducing the chemical conversion treatment performance of the processed area. In Comparative Examples 24, 26, and 27, the high electrical conductivity of the rinsing solution used for rinsing after secondary pickling prevented sufficient suppression of the formation of iron oxides during rinsing after secondary pickling. As a result, the surface coverage rate of the alloy plating of at least one of Ni, Cu, and Sn with Fe was less than 25%, and the average length of the uncoated area also exceeded 10 μm, resulting in a decrease in the chemical conversion treatment properties of the processed area.

[0083] In contrast, in all the examples, when the cross-section of the plated steel sheet was measured by EPMA, the surface coverage rate was 25% or more, with the plated area having a surface concentration of at least one of Ni, Cu, and Sn satisfying formula 1 and a surface concentration of Fe of 10% by mass or more, and the average length of the uncoated area not covered by at least one of Ni, Cu, and Sn was limited to 10 μm or less. By applying the plating in this manner, the chemical conversion treatmentability of the processed portion of the plated steel sheet could be significantly improved. In particular, in Examples 2, 3, 8, 9, 14, 15, and 20, where the surface coverage rate was 35% or more, the chemical conversion treatmentability of the processed portion was evaluated as AA, indicating further improvement in the chemical conversion treatmentability of the processed portion. In Examples 4-6, 10-12, 16-18, 21, and 22, where the surface coverage rate was 50% or more, the chemical conversion treatmentability of the processed portion was evaluated as AAA, indicating further improvement in the chemical conversion treatmentability of the processed portion. [Explanation of Symbols]

[0084] 1. Plated steel sheet 2 Base steel plate 3 Plating L1, L2, and L3 are the lengths of the plating. L0 Surface length of the base steel plate E1 and E2 uncovered areas

Claims

1. A plated steel sheet comprising a base steel sheet and a plating disposed on the surface of the base steel sheet, The aforementioned base steel plate is, by mass%, Ni: 0.010-1.000%, Cu: 0.010 to 1.000%, and It has a chemical composition containing Sn: 0.003 to 1.000%, In the elemental distribution image obtained by measuring the cross-section of the plated steel sheet with EPMA, The surface coverage of the coated region is 25% or more, 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. A plated steel sheet characterized in that the average length of the uncoated region, which is not coated with at least one of Ni, Cu, and Sn, is 10 μm or less. [Ni]+[Cu]+0.6[Sn]≧10...Formula 1 Here, [Ni], [Cu], and [Sn] are the surface concentrations [mass%] of each element. The surface coverage ratio is determined by taking five samples from a plated steel sheet so that the cross-section including the surface of the plated steel sheet can be observed, defining one field of view as a rectangular area of ​​80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction, and imaging five fields of view in total for the five samples using an EPMA under conditions of acceleration voltage: 15 kV and irradiation current: 5 × 10⁻⁷ A at a magnification of 1000x, binarizing the obtained elemental distribution images, calculating the total length Li of the covered area where the surface concentration of at least one of Ni, Cu, and Sn on the surface portion of the plated steel sheet satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more, calculating the average of the total Li obtained for the five samples as ΣLi, and calculating ΣLi / L₀ × 100 from the length L₀ 100 μm of the surface of the corresponding base steel sheet.

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

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

4. The plated steel sheet according to claim 3, characterized in that the surface coverage rate 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 aforementioned chemical composition is, in mass%, Ni: 0.040-1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004-1.000% A plated steel sheet according to any one of claims 1 to 4, characterized by including the following:

7. A plated steel sheet according to any one of claims 1 to 4, characterized by having a Vickers hardness of 200 Hv or more.

8. A component characterized by comprising a plated steel sheet according to any one of claims 1 to 4.

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