Plated steel sheet

Abstract

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

Description

[Technical Field]

[0001] This invention relates to plated steel sheets. [Background technology]

[0002] It is known that improving the adhesion of coatings to 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 coating 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 project] [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 required for automotive steel sheets, such as strength and formability, as well as chemical stability such as corrosion resistance. In particular, copper compounds present on the surface of steel sheets reduce the chemical conversion treatment properties necessary to improve corrosion resistance. Furthermore, generally, when the chemical conversion treatment properties decrease, areas where the chemical conversion coating, known as "skew," has not formed may occur, which can result in reduced paint adhesion.

[0007] Therefore, the present invention aims to provide a plated steel sheet containing Ni, Cu, and Sn that can exhibit improved coating adhesion. [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 discovered that, when measured by Auger electron spectroscopy, the adhesion of the coating film can be significantly improved by applying plating such that the surface of the base steel sheet is covered with at least one of Ni, Cu, and Sn with a predetermined surface coverage ratio, and the size of the uncovered area not covered by these elements is limited to a predetermined range. Based on this discovery, 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 surface of the plated steel sheet using Auger electron spectroscopy, The surface coverage rate of at least one of Ni, Cu, and Sn is 25% or more, A plated steel sheet characterized in that the equivalent radius of the uncoated region, which is not coated with at least one of Ni, Cu, and Sn, is 10 μm or less. (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. (5) The plated steel sheet according to any one of (1) to (4) above, characterized in that the equivalent radius of the uncoated area is 5 μm or less. (6) The chemical composition is, in mass%, Ni: 0.040~1.000%, Cu: 0.040~1.000%, and Sn: 0.004~1.000% The plated steel sheet according to any one of the above (1) to (5), characterized by containing (7) The plated steel sheet according to any one of the above (1) to (6), characterized by having a Vickers hardness of 200 Hv or more. [Advantages of the Invention]

[0010] According to the present invention, there is provided a plated steel sheet containing Ni, Cu, and Sn, which can exhibit improved coating adhesion. [Embodiments for Carrying Out the Invention]

[0011] [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 contains, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and has a chemical composition containing In the element distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy, The surface coverage rate by at least one of Ni, Cu, and Sn is 25% or more, and The equivalent circle radius of the non-coated region not coated by at least one of Ni, Cu, and Sn is 10 μm or less.

[0012] As described above, generally, when the chemical conversion treatment property deteriorates, there may be regions where a chemical conversion treatment film called "sque" is not formed, and as a result, the coating film adhesion may decrease. For example, when elements such as Ni, Cu, and Sn are dissolved in the steel sheet, the potential of the steel sheet becomes nobler compared to the state where these elements are not dissolved, and the etching property of Fe may decrease during the chemical conversion treatment. In this case, the chemical conversion treatment property of the steel sheet deteriorates, and as a result, the coating film adhesion decreases. Therefore, when the steel sheet contains the three elements of Ni, Cu, and Sn at the same time, such a decrease in the coating film adhesion becomes a particularly problematic issue.

[0013] In addition, as a method for manufacturing a steel sheet, for example, there are generally two methods: a method in which molten iron is obtained in a blast furnace using iron ore, which is a natural resource, as the main raw material, and then molten steel is manufactured through refining in a converter or the like; and a method in which molten steel is manufactured in an electric furnace using scrap materials, which are recycled resources, as the main raw material. Since blast furnace materials can 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 issues. On the other hand, since electric furnace materials use scrap materials as the main raw material as described above, they contain relatively large amounts of elements derived from scrap such as Ni, Cu, and Sn (so-called trump elements), and therefore the above issues become particularly prominent.

[0014] 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 coating adhesion 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 uniformly coat the surface of the base steel sheet containing Ni, Cu, and Sn with at least one of these elements. More specifically, the inventors found that coating adhesion can be significantly improved by applying plating such that, as measured by Auger electron spectroscopy, the surface of the base steel sheet is covered with at least one of Ni, Cu, and Sn with a predetermined surface coverage rate, more specifically 25% or more, and the size of the uncoated area not covered by these elements, i.e., Ni, Cu, and Sn, is within a predetermined range, more specifically, the equivalent circular radius of the uncoated area is limited to 10 μm or less.

[0015] While not intended to be bound by any particular theory, it is believed that by uniformly coating the surface of the base steel sheet with at least one of Ni, Cu, and Sn, these elements can be appropriately utilized as cathode sites during the chemical conversion treatment, thereby promoting the anodic dissolution (etching) of Fe present around these elements. More specifically, Ni, Cu, and Sn are elements that are electrically nobler than Fe. Therefore, by uniformly dispersing these elements on the steel sheet surface rather than in a solid solution state, they can be made to function as effective cathode sites in relation to Fe during the chemical conversion treatment, as described above, thereby promoting the etching of Fe present around them. This is believed to significantly improve the chemical conversion treatment properties of the steel sheet. As a result, a chemical conversion coating can be uniformly formed over the entire steel sheet, significantly improving coating adhesion. Even if the surface coverage rate by at least one of Ni, Cu, and Sn is very high, for example, 50% or more, if there are relatively large uncovered areas that are not covered by these elements, such uncovered areas will not be able to promote the anodic dissolution of Fe during the chemical conversion treatment. As a result, the chemical conversion treatment film cannot be formed uniformly over the entire steel sheet, and the adhesion of the coating film will decrease. Therefore, in the plated steel sheet according to the embodiment of the present invention, it is important to uniformly cover the surface of the base steel sheet with at least one of Ni, Cu, and Sn. That is, it is important to satisfy both the control of the surface coverage rate by at least one of Ni, Cu, and Sn to 25% or more in the elemental distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy, and the control of the equivalent radius of the uncovered area that is not covered by at least one of Ni, Cu, and Sn to 10 μm or less. This is because if either of these conditions is not satisfied, it will not be possible to uniformly form the chemical conversion treatment film over the entire steel sheet during the chemical conversion treatment. On the other hand, by satisfying both of these conditions, the chemical conversion coating can be formed uniformly across the entire steel sheet, resulting in a significant improvement in coating adhesion.

[0016] 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 coating adhesion and, consequently, superior corrosion resistance compared to conventional plated steel sheets that simultaneously contain all three elements: Ni, Cu, and Sn. Therefore, the plated steel sheets according to the embodiments of the present invention are particularly useful in the automotive sector where excellent coating adhesion and / or corrosion resistance are required. The components of the plated steel sheets according to the embodiments of the present invention will be described in more detail below.

[0017] [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 may contain at least one of Ni, Cu, and Sn, and may also contain other elements in addition to Ni, Cu, and Sn, such as Zn, Al, and Fe. For example, the plating may essentially consist of at least one of Ni, Cu, and Sn, consist of at least one of Ni, Cu, and Sn, or be composed of at least one of Ni, Cu, and Sn. The content of Ni, Cu, and Sn in the plating and the amount of plating deposited are 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.

[0018] [Surface coverage ratio of at least one of Ni, Cu, and Sn: 25% or more] In embodiments of the present invention, the surface coverage rate of at least one of Ni, Cu, and Sn is controlled to 25% or more in the elemental distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy. As mentioned above, Ni, Cu, and Sn can function as cathode sites during chemical conversion treatment. In chemical conversion treatment, electrons are generally generated by the anodic dissolution (etching) of Fe, while at the cathode sites, a cathode reaction (2H) occurs due to the electrons generated by the anodic dissolution of Fe.+ +2e - →H2, 10H + +NO3 - +8e - →NH4 + A reaction occurs where +3H2O occurs. In connection with this, the pH of the chemical conversion treatment solution near the surface of the steel sheet rises, and consequently, compounds such as zinc phosphate crystals that constitute the chemical conversion treatment film precipitate on the surface of the steel sheet.

[0019] In embodiments of the present invention, by controlling the surface coverage rate of the plated steel sheet with at least one of Ni, Cu, and Sn to 25% or more while satisfying the requirements for the uncovered areas described later, the surface of the base steel sheet can be uniformly covered with at least one of Ni, Cu, and Sn, and these elements 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 the adhesion of the coating can be significantly improved. From the viewpoint of further improving the adhesion of the coating, a higher surface coverage rate is preferable. For example, the surface coverage rate with at least one of Ni, Cu, and Sn is preferably 30% or more or 35% or more, more preferably 40% or more or 45% or more, and most preferably 50% or more or 55% or more. There is no particular upper limit, but the surface coverage rate with at least one of Ni, Cu, and Sn may be, for example, 80% or less, 75% or less, 70% or less, or 65% or less.

[0020] [Equivalent radius of uncovered area: 10 μm or less] In the embodiment of the present invention, in the elemental distribution image obtained by measuring the surface of the plated steel sheet by Auger electron spectroscopy, the equivalent radius 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 there is a relatively large uncoated region not covered by at least one of Ni, Cu, and Sn, 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, the chemical conversion treatment film cannot be formed uniformly over the entire steel sheet, and the adhesion of the coating film is reduced.

[0021] In embodiments of the present invention, while satisfying the surface coverage requirements described above, by allowing such uncovered regions to exist and controlling the equivalent radius of these uncovered regions to 10 μm or less, the surface of the base steel sheet can be uniformly coated with at least one of Ni, Cu, and Sn, and these elements 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 the adhesion of the coating can be significantly improved. From the viewpoint of further improving the adhesion of the coating, it is preferable that the equivalent radius of the uncovered region be as small as possible. For example, the equivalent radius of the uncovered region 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 equivalent radius of the uncovered 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.

[0022] [Measurement of surface coverage and uncovered areas] The measurement of the surface coverage rate is carried out by Auger electron spectroscopy as follows. First, a sample including the plate surface is placed in an Auger electron spectrometer (for example, AES PHI-700 type (manufactured by ULVAC-PHI, FE type)), and the measurement of the sample surface (plate surface) is performed under the conditions of an acceleration voltage of 10 kV, a current value of 10 nA, and a measurement energy range of the Auger spectrum of 40 to 1690 eV. The measurement area is observed at 1000 times or more with SEM so as to be an area of 60 μm × 100 μm or more. Next, mapping with a lower limit of 80 cps (count / s) is created to obtain an elemental distribution image. Finally, the obtained elemental distribution image is binarized (min = 0, max = 255) by image analysis software “ImageJ”, the coverage rate by at least one of Ni, Cu, and Sn with respect to the measurement area is calculated, and the calculated value is determined as the surface coverage rate of the plated steel sheet by at least one of Ni, Cu, and Sn.

[0023] For the non-coated area, in the binarized image obtained by the image analysis software “ImageJ” in relation to the measurement of the surface coverage rate, the area and the number of the entire non-coated area not coated by at least one of Ni, Cu, and Sn are calculated. Next, the average area S per non-coated area is calculated by dividing the total area of the non-coated area by the number of the entire non-coated area. Finally, the equivalent circle radius r is calculated by the following formula, and this is determined as the equivalent circle radius of the non-coated area not coated by at least one of Ni, Cu, and Sn. r = (S / π) 0.5

[0024] [Base metal steel sheet] 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 coating adhesion. This objective is achieved by plating the base steel sheet such that, when measured by Auger electron spectroscopy, the surface of the base steel sheet is covered with at least one of Ni, Cu, and Sn with a surface coverage rate of 25% or more, and the equivalent radius of the uncovered area not covered by these elements is limited to 10 μm or less. Therefore, the chemical composition of the base steel sheet is not particularly limited except that it contains Ni: 0.010~1.000%, Cu: 0.010~1.000%, and Sn: 0.003~1.000% by mass%, 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.

[0025] 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.

[0026] [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.

[0027] [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.

[0028] [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.

[0029] [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.

[0030] [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.

[0031] [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.

[0032] [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.

[0033] [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.

[0034] [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.

[0035] 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.

[0036] [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.

[0037] [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.

[0038] [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.

[0039] [Hf: 0~0.050%] [Mg: 0~0.050%] [Zr:0~0.050%] [Ca: 0~0.010%] [REM: 0~0.010%] Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of 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.

[0040] [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.

[0041] [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.

[0042] 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.

[0043] 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.

[0044] [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.

[0045] [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.

[0046] [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.

[0047] <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.

[0048] 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, an annealing step of annealing the cold-rolled steel sheet, a secondary pickling step of secondary pickling the annealed cold-rolled steel sheet, and a plating step of applying plating to the obtained base 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 and annealing step described below. Each step will be described in detail below.

[0049] [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.

[0050] [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%.

[0051] [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. Consequently, the plating process can be carried out appropriately, and the desired surface coverage can be achieved in the final plated steel sheet.

[0052] 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.

[0053] 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.

[0054] 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, with 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, and it will be difficult to properly adhere the plating in the subsequent plating process. As a result, the desired surface coverage rate cannot be achieved in the final plated steel sheet.

[0055] From the viewpoint of further increasing the surface coverage and improving coating adhesion, 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 depleted 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.

[0056] [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.

[0057] [Annealing process] Next, the obtained cold-rolled steel sheet is annealed. The annealing process involves heating the cold-rolled steel sheet to a temperature of 700 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).

[0058] [Secondary pickling process] The annealed cold-rolled steel sheet is 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. This secondary pickling using an aqueous hydrochloric acid solution can sufficiently or completely remove the Mn and / or Si-based surface oxides formed on the surface of the cold-rolled steel sheet during the annealing process. More specifically, by forming a predetermined Mn-Si deficient layer in the winding process, the formation of Mn and / or Si-based surface oxides in the annealing process can be sufficiently suppressed compared to the case where such a Mn-Si deficient layer is not present. However, since the formation of these surface oxides is not completely suppressed in the annealing process, it is important to perform a proper secondary pickling after the annealing process in order to properly adhere the plating in the subsequent plating process.

[0059] Therefore, the combination of a winding temperature of 520°C or higher in the winding process and the secondary pickling described above in the secondary pickling process is important. Such a specific combination allows for sufficient or complete removal of Mn and / or Si-based surface oxides formed on the surface of the cold-rolled steel sheet during the annealing process, and as a result, the desired surface coverage and the equivalent radius of the uncovered area can be achieved in the final plated steel sheet. For example, if the hydrochloric acid aqueous solution contains an inhibitor, or if the hydrochloric acid concentration of the hydrochloric acid aqueous solution is less than 3%, or if the immersion temperature is less than 50°C, and / or the immersion time is less than 2 seconds, the Mn and / or Si-based surface oxides formed on the surface of the cold-rolled steel sheet during the annealing process cannot be sufficiently removed, and the adhesion of the plating is inhibited in the subsequent plating process due to these surface oxides. As a result, the plating cannot be properly adhered, and the desired surface coverage and / or the equivalent radius of the uncovered area cannot be achieved in the final plated steel sheet. Preferably, the hydrochloric acid concentration of the hydrochloric acid aqueous solution is 4-8%, the immersion temperature is 70-90°C, and the immersion time is 4-50 seconds.

[0060] 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. If such iron oxides are present on the surface of the cold-rolled steel sheet, the adhesion of the plating in the subsequent plating process will be inhibited, similar to the case of Mn and / or Si-based surface oxides. In this case, it will be impossible to achieve the desired surface coverage and the equivalent radius 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.

[0061] 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 in steel sheets containing the three elements Ni, Cu, and Sn simultaneously, and to properly adhere the plating in the subsequent plating process. 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.

[0062] [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 the equivalent radius 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.

[0063] 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 the equivalent radius 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.

[0064] According to this manufacturing method, in steel sheets where coating adhesion is 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 an appropriate subsequent plating process, it is possible to manufacture a plated steel sheet having a plating in which, when measured by Auger electron spectroscopy, the surface of the base steel sheet is covered with at least one of Ni, Cu, and Sn with a surface coverage rate of 25% or more, and the equivalent radius of the uncovered area not covered by these elements 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 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 steel sheet surface rather than being dissolved. This allows for the uniform formation of a chemical conversion coating across the entire steel sheet during chemical conversion treatment, resulting in a significant improvement in coating adhesion. Therefore, plated steel sheets manufactured by this method achieve superior corrosion resistance compared to conventional plated steel sheets containing all three elements (Ni, Cu, and Sn). As a result, they can contribute to industrial development by extending the lifespan of plated steel sheets used in automobiles and building materials.

[0065] 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]

[0066] 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.

[0067] 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 at the winding temperatures shown in Table 2. Hot rolling was carried out by rough rolling and finish rolling, with the finish rolling ending at 900-1050°C and a reduction ratio of 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. Next, the obtained cold-rolled 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.

[0068] Next, the annealed cold-rolled steel sheets underwent 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 did 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. Finally, the resulting base steel sheets were subjected to current density 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 an energizing time of 1.0 second to obtain a plated steel sheet in which the plating was attached to both sides of the base steel sheet.

[0069] [Table 1]

[0070] [Table 2]

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

[0072] [Evaluation of coating adhesion] The adhesion of the coating film was evaluated as follows. First, a 50mm x 50mm sample of the plated steel sheet manufactured above was subjected to zinc phosphate treatment as a chemical conversion 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.

[0073] For samples of plated steel sheets that had undergone chemical conversion treatment, electrodeposition coating (Powernix Excel 1200: manufactured by Nippon Paint Industrial Coating Co., Ltd.) was performed at an electrodeposition temperature of 30°C and a film thickness of 18 μm, followed by a baking treatment at 170°C for 30 minutes. Next, a salt water immersion test (SDT) was performed on the electrodeposited samples. Specifically, the electrodeposited samples were immersed in a 5% NaCl aqueous solution at 50°C for 1000 hours. After the SDT test, the removed samples were dried, and then a tape peel test was performed on one side of the sample. The peeled tape was scanned, and the area percentage of the coating that had peeled off was calculated by binarization using the image analysis software "ImageJ". The coating adhesion was evaluated as follows. AAA: Peeling area ratio less than 5% AA: Peeling area ratio less than 5-10% A: Peeling area ratio 10-15% B: Peeling area rate over 15%

[0074] Plated steel sheets containing Ni, Cu, and Sn were evaluated as having improved coating adhesion when the coating adhesion rating was AAA, AA, or A. The results are shown in Table 2.

[0075] 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 Ni was less than 25%, leading to reduced coating adhesion. 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 further prevented sufficient suppression of the formation of iron oxides during rinsing after secondary pickling. As a result, the surface coverage by Ni was less than 25%, and the equivalent radius of the uncoated area exceeded 10 μm, resulting in reduced coating adhesion. 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 by at least one of Ni, Cu, and Sn was less than 25%, and the equivalent radius of the uncovered area also exceeded 10 μm, resulting in reduced coating adhesion.

[0076] In contrast, in all the examples, when measured by Auger electron spectroscopy, the surface of the base steel sheet was covered with at least one of Ni, Cu, and Sn with a surface coverage rate of 25% or more, and the equivalent radius of the uncovered area not covered by these elements was limited to 10 μm or less. By applying the plating in this manner, the adhesion of the coating to the plated steel sheet was significantly improved. In particular, in Examples 2, 3, 8, 9, 14, 15, and 20, where the surface coverage rate was 35% or more, the coating adhesion was evaluated as AA, indicating further improvement in coating adhesion. In Examples 4-6, 10-12, 16-18, 21, and 22, where the surface coverage rate was 50% or more, the coating adhesion was evaluated as AAA, indicating further improvement in coating adhesion.

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 surface of the plated steel sheet using Auger electron spectroscopy, The surface coating rate of at least one of Ni, Cu, and Sn is 25% or more, and The equivalent radius of the uncoated region, which is not covered by at least one of Ni, Cu, and Sn, is 10 μm or less. The aforementioned surface coverage rate is determined by placing the sample, including the plate surface, in an Auger electron spectrometer, measuring the sample surface (plate surface) under the conditions of an acceleration voltage of 10 kV, a current of 10 nA, and an Auger spectrum measurement energy range of 40 to 1690 eV. The measurement area is observed at 1000x or higher using a SEM, ensuring it is at least 60 μm × 100 μm in size. Then, a mapping with a lower limit of 80 cps is created to obtain an elemental distribution image. The obtained elemental distribution image is binarized, and the coverage rate of at least one of Ni, Cu, and Sn in the measurement area is calculated. A plated steel sheet characterized in that the equivalent radius of the uncoated area is determined by calculating the total area and number of uncoated areas that are not covered by at least one of Ni, Cu, and Sn in the binarized image obtained in connection with the measurement of the surface coverage rate, then calculating the average area S per uncoated area by dividing the total area of ​​the uncoated areas by the total number of uncoated areas, and then calculating the equivalent radius r using the formula r = (S / π) 0.

5.

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 any one of claims 1 to 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 3, characterized in that the equivalent radius of the uncoated area 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 3, characterized by including the following:

7. A plated steel sheet according to any one of claims 1 to 3, characterized by having a Vickers hardness of 200 Hv or more.

Images