Steel sheet and component including same

Abstract

Provided are: a steel sheet characterized by having a chemical composition containing, in mass %, 0.010-1.000% of Ni, 0.010-1.000% of Cu, and 0.003-1.000% of Sn, and by the surface exposure ratio of the concentrated part of at least one of Ni, Cu, and Sn in a cross-section of the steel sheet being 10% or less; and a component including the steel sheet.

Description

Steel plates and parts containing them

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

[0002] It is known that an effective way to improve paint film adhesion on steel sheets is to enhance the chemical conversion treatability of the steel sheet and to form a uniform chemical conversion coating on the steel sheet.

[0003] In relation to this, Patent Document 1 discloses that when a high-strength cold-rolled steel sheet is continuously annealed in a continuous annealing furnace or a cold-rolled steel sheet / hot-dip galvanized steel sheet dual-purpose facility having a continuous annealing furnace, the cooling method of which in a cooling zone including a part or all of the steel sheet temperature range of 600 to 250°C following heating for recrystallization is one or more of gas cooling, diffusion cooling, and cooling pipe cooling, the steel sheet surface is exposed to an atmosphere in which iron oxidizes within the above-mentioned steel sheet temperature range, pickled at the outlet side of the annealing furnace, and then iron or Ni plating is applied to a thickness of 1 to 50 mg / m 2 Furthermore, Patent Document 1 teaches that, although oxidation of the steel sheet is usually prevented by an extremely low concentration of oxygen and / or an inert atmospheric gas with an extremely low dew point around the steel sheet, the steel sheet is instead actively exposed to an oxidizing atmosphere to oxidize not only Si and Mn but also the iron of the steel sheet, and the oxide films of Si, Mn, etc. are removed together with the oxide film of iron on the steel sheet by pickling immediately after leaving the annealing furnace, thereby obtaining a high-strength cold-rolled steel sheet that is free from "hollow-out" and has good chemical conversion treatability even if the contents of Si, Mn, etc. are high.

[0004] In Patent Document 2, a copper (Cu) content of 0.10 mass % or more and 0.50 mass % or less is contained, and the number of residual scales on the surface is 160,000 pieces / mm 2 Patent Document 2 teaches that, according to the above configuration, the particle size of the copper compound particles exposed on the steel sheet surface, which serves as the cathode point in chemical conversion treatment, is 2 μm or less, and the residual scale is reduced to a predetermined amount or less, thereby making it possible to provide a steel sheet with excellent chemical conversion treatability.

[0005] JP 2008-190030 A JP 2020-084238 A

[0006] Patent Document 2 teaches that elements such as nickel (Ni) and tin (Sn) in addition to copper (Cu) reduce the mechanical properties required for automotive steel sheets, such as strength and formability, as well as chemical stability such as corrosion resistance, and that copper compounds present on the surface of the steel sheet in particular reduce the chemical conversion treatability required for improving corrosion resistance. In addition, generally, when chemical conversion treatability is reduced, areas where a chemical conversion coating is not formed, known as "whiteout," may occur, which may result in reduced paint film adhesion.

[0007] Therefore, an object of the present invention is to provide a steel sheet containing Ni, Cu, and Sn that can exhibit improved paint adhesion, and a part including the steel sheet.

[0008] In order to achieve the above object, the present inventors have conducted research focusing on the element distribution on the surface of a steel sheet, and as a result, have found that coating adhesion can be significantly improved by limiting the surface exposure of enriched portions of at least one of Ni, Cu, and Sn in the cross section of the steel sheet to a predetermined range, thereby completing the present invention.

[0009] The present invention has achieved the above-mentioned object as follows: (1) A steel sheet having a chemical composition containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, wherein the surface exposure rate of the Ni, Cu, and Sn-enriched portion in the cross section of the steel sheet is 10% or less. (2) The steel sheet according to (1) above, wherein the surface exposure rate is 5% or less. (3) The steel sheet according to (1) or (2) above, wherein, in grazing incidence X-ray diffraction of the steel sheet surface, the peak half-width of the (211) plane of the ferrite phase is 0.3° or more. (4) The steel sheet according to any one of (1) to (3) above, wherein the chemical composition contains Si, and wherein, in X-ray photoelectron spectroscopy measurement of the steel sheet surface, the thickness of silicon oxide is 10 nm or less. (5) The steel sheet according to any one of (1) to (4) above, characterized in that it has a Vickers hardness of 190 Hv or more. (6) The steel sheet according to any one of (1) to (5) above, characterized in that the chemical composition includes, in mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%. (7) A part, characterized in that it includes the steel sheet according to any one of (1) to (6) above.

[0010] According to the present invention, it is possible to provide a steel sheet containing Ni, Cu, and Sn, which is capable of exhibiting improved paint film adhesion, and a part including the steel sheet.

[0011] 1 is a cross-sectional schematic view of a steel plate according to an embodiment of the present invention, illustrating the surface exposure rate of a portion where at least one of Ni, Cu, and Sn is concentrated.

[0012] <Steel Sheet> A steel sheet according to an embodiment of the present invention 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 is characterized in that the surface exposure rate of enriched portions of at least one of Ni, Cu, and Sn in a cross section of the steel sheet is 10% or less.

[0013] As mentioned above, generally, when chemical conversion treatability is reduced, regions where the chemical conversion coating is not formed, known as "skid zones," may occur, resulting in reduced paint adhesion. For example, when elements such as Ni, Cu, and Sn are present in a steel sheet as a solid solution, the potential of the steel sheet becomes more noble than when these elements are not present in a solid solution, which may reduce the etching ability of Fe during chemical conversion treatment. In this case, the chemical conversion treatability of the steel sheet is reduced, resulting in reduced paint adhesion. Therefore, when a steel sheet simultaneously contains the three elements Ni, Cu, and Sn, this reduced paint adhesion is particularly problematic.

[0014] Furthermore, two commonly known methods for producing steel sheets are, for example, a method in which molten iron is obtained in a blast furnace using iron ore, a natural resource, as the main raw material, and then molten steel is produced through refining in a converter or the like, and a method in which molten steel is produced in an electric furnace using scrap material, a recycled resource, as the main raw material. Since blast furnace steel may also contain elements such as Ni, Cu, and Sn as additive elements, when these elements are contained, it is necessary to appropriately address the above-mentioned issues. On the other hand, electric furnace steel uses scrap material as the main raw material as described above, and therefore contains relatively large amounts of elements derived from the scrap (so-called tramp elements), such as Ni, Cu, and Sn, and therefore the above-mentioned issues become particularly pronounced.

[0015] Therefore, the present inventors conducted research, focusing particularly on the element distribution on the steel sheet surface, in order to provide a steel sheet that can exhibit excellent paint adhesion even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, the present inventors found that it is effective to limit the surface exposure of at least one of the concentrated Ni, Cu, and Sn portions in the cross section of the steel sheet to a predetermined range. More specifically, the present inventors found that controlling the surface exposure rate of at least one of the concentrated Ni, Cu, and Sn portions in the cross section of the steel sheet to 10% or less can improve the chemical conversion treatability of the steel sheet, thereby significantly improving the paint adhesion of the steel sheet.

[0016] FIG. 1 is a cross-sectional schematic diagram of a steel sheet according to an embodiment of the present invention, illustrating the surface exposure rate of a Ni, Cu, and Sn enriched portion. Referring to FIG. 1, the steel sheet 1 simultaneously contains three elements, Ni, Cu, and Sn, and has an enriched portion 2 in its surface where at least one of Ni, Cu, and Sn is enriched. Here, the "Ni, Cu, and Sn enriched portion" or "Ni, Cu, and Sn enriched portion" refers to a region having a circle equivalent diameter of 0.5 μm or more in an EPMA analysis described below, where the total content of Ni, Cu, and Sn in the region is 0.5 mass% or more and is 1.1 times or more the total content of Ni, Cu, and Sn in the bulk. The "total content of Ni, Cu, and Sn in the bulk" corresponds to the total content of Ni, Cu, and Sn in the "chemical composition of the steel sheet" described below. In FIG. 1, in the cross section of the steel plate 1, the length L of the portion where each enriched portion 2 is exposed on the surface in the direction perpendicular to the plate thickness direction is i The sum of ΣL i (In Figure 1, ΣL i = L1 + L2), and the surface length L0 of the steel plate 1 is ΣL i It can be seen that the relationship / L0 × 100≦10 is satisfied, i.e., the surface exposure rate of at least one of the Ni, Cu, and Sn enriched portions 2 is 10% or less. As mentioned above, Ni, Cu, and Sn may reduce the etching ability of Fe during chemical conversion treatment. However, by controlling the surface exposure rate of at least one of these elements in the enriched portions 2 to 10% or less, it is possible to suppress the reduction in Fe etching ability due to the inclusion of Ni, Cu, and Sn. Therefore, according to the steel sheet according to the embodiment of the present invention, the chemical conversion treatability of the steel sheet can be significantly improved, and as a result, a chemical conversion coating can be formed uniformly over the entire steel sheet, thereby significantly improving paint adhesion. The total content of Ni, Cu, and Sn in the enriched portions may be, for example, 0.5% by mass to 5.0% by mass, or 1.0% by mass to 2.5% by mass.

[0017] The steel sheet according to the embodiment of the present invention encompasses not only electric furnace steel, which inevitably contains Ni, Cu, and Sn as tramp elements, but also blast furnace steel, which contains Ni, Cu, and Sn as essential elements or optional added elements. Furthermore, the steel sheet according to the embodiment of the present invention can achieve superior paint adhesion and, in turn, superior corrosion resistance compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where superior paint adhesion and / or corrosion resistance are required. Each component of the steel sheet according to the embodiment of the present invention will be described in more detail below.

[0018] [Surface Exposure Ratio of Ni, Cu, and Sn Concentrated Portions: 10% or Less] In an embodiment of the present invention, the surface exposure ratio of Ni, Cu, and Sn concentrated portions in the cross section of the steel sheet is controlled to 10% or less. As described above, by controlling the surface exposure ratio of Ni, Cu, and Sn concentrated portions to 10% or less, a chemical conversion coating can be uniformly formed over the entire steel sheet, thereby significantly improving paint adhesion. From the viewpoint of further improving paint adhesion, the lower the surface exposure ratio, the better. For example, the surface exposure ratio of Ni, Cu, and Sn concentrated portions is preferably 8% or less or 6% or less, more preferably 5% or less or 4% or less, and most preferably 3% or less or 2% or more. The lower limit is not particularly limited and may be 0%. For example, the surface exposure ratio of Ni, Cu, and Sn concentrated portions may be 0.5% or more or 1% or more.

[0019] [Measurement of Surface Exposure Rate of At Least One of Ni, Cu, and Sn-Enriched Portions] The surface exposure rate of at least one of Ni, Cu, and Sn-enriched portions is measured using an electron probe microanalyzer (EPMA) as follows. First, five samples are taken from the surface of the steel sheet so that the cross section of the steel sheet can be observed. Next, for each sample, a rectangular area of ​​80 μm in the sheet thickness direction and 1500 μm in the direction perpendicular to the sheet thickness direction is defined as one field of view, and a total of five fields of view for the five samples are measured using an EPMA (for example, JXA-8500 manufactured by JEOL Ltd.) at an acceleration voltage of 15 kV and a probe current of 5 × 10 -7A mapping image is obtained by photographing the specimen at 1000x magnification under conditions of A, irradiation time: 50 ms, and beam diameter: 0.2 μm. Next, from the obtained mapping image, a region where the total content of Ni, Cu, and Sn is 0.5 mass% or more and is 1.1 times or more the total content of Ni, Cu, and Sn in the bulk is identified, and this region is determined to be a region where at least one of Ni, Cu, and Sn is concentrated. For example, in the mapping image, a color bar is displayed with a Ni concentration range of 0.5 to 5.0%, thereby identifying a region where the Ni concentration is 0.5% or more (however, if the Ni content in the bulk is 0.5 mass% or more, the lower limit of the Ni concentration is 1.1 times the Ni content in the bulk. The same applies hereinafter to Cu and Sn). Similarly, a color bar is displayed with a Cu concentration range of 0.5 to 5.0%, thereby identifying a region where the Cu concentration is 0.5% or more. Similarly, the region where the Sn concentration is 0.5% or more is identified by displaying the color bar in the Sn concentration range of 0.5 to 5.0%. In the region where two or more of Ni, Cu, and Sn coexist, the color bar display is adjusted so that the total concentration of Ni, Cu, and Sn is 0.5% or more (however, if the total content of Ni, Cu, and Sn in the bulk is 0.5% or more by mass, the lower limit of the total concentration of Ni, Cu, and Sn is set to 1.1 times the total content of Ni, Cu, and Sn in the bulk). As an example, for a region where Ni and Cu coexist, the color bar is displayed in a Ni concentration range of 0.2 to 5.0% (however, if the Ni content in the bulk is 0.2 mass% or more, the lower limit of the Ni concentration is 1.1 times the Ni content in the bulk), and a Cu concentration range of 0.3 to 5.0% (however, if the Cu content in the bulk is 0.3 mass% or more, the lower limit of the Cu concentration is 1.1 times the Cu content in the bulk). This allows for the identification of a region where the total concentration of Ni and Cu is 0.5% or more and 1.1 times the total content of Ni and Cu in the bulk. Regions where Ni and Sn coexist, regions where Cu and Sn coexist, and regions where Ni, Cu, and Sn coexist can also be identified by similar means. Next, the length L in the direction perpendicular to the thickness direction of the portion where each enriched portion is exposed on the surface is calculated. iand calculate the sum of them (L1 + L2 in FIG. 1). Finally, the L obtained for the five samples i Calculate the average of the total ΣL i and the length L0 of the surface of the corresponding steel plate (length of the long side in the field of view: 1500 μm) is calculated as ΣL i / L0 × 100 is calculated, and the calculated value is determined as the surface exposure rate of the portion where at least one of Ni, Cu, and Sn is concentrated.

[0020] [Chemical Composition of Steel Sheet] In an embodiment of the present invention, the steel sheet has a chemical composition containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. As described above, the present invention aims to provide a steel sheet containing Ni, Cu, and Sn that can exhibit improved paint adhesion. This objective is achieved by controlling the surface exposure rate of enriched portions of at least one of Ni, Cu, and Sn in the steel sheet cross section to 10% or less. Therefore, the chemical composition of the steel sheet is not particularly limited other than containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. Therefore, it is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the objectives of the present invention. The chemical composition of the steel sheet may contain, in addition to Ni, Cu, and Sn, any alloying elements that are generally added in appropriate amounts in the technical field of the present invention. The chemical composition of the steel sheet according to the embodiment of the present invention will be described in detail below, but these descriptions are intended to merely exemplify preferred chemical compositions of steel sheets for application in automotive steel sheets and the like, and are not intended to limit the present invention to steel sheets having such specific chemical compositions.

[0021] In an embodiment of the present invention, for example, the steel plate contains, in mass %, C: 0.001 to 0.500%, Si: 0 to 3.00%, Mn: 0.10 to 3.00%, Al: 0.001 to 2.000%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, Sn: 0.003 to 1.000%, P: 0.100% or less, S: 0.100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0 to 1.000%, Cr: 0 to 1.000%, V It is preferable that the alloy has a chemical composition consisting of: W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.500%, Ca: 0 to 0.050%, REM: 0 to 0.010%, As: 0 to 0.100%, Ir: 0 to 1.000%, Zn: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0022] [C: 0.001 to 0.500%] C is an element that inexpensively increases strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.001% or more. The C content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.040% or more, 0.070% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.500% or less. The C content may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.

[0023] [Si: 0 to 3.00%] Si is an element that is effective in increasing strength as a solid solution strengthening element. The Si content may be 0%, but to obtain this effect, the Si content is preferably 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.30% or more, 0.50% or more, 0.80% or more, or 1.00% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.00% or less. The Si content may be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.

[0024] [Mn: 0.10 to 3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.50% or more, 1.00% or more, 1.30% or more, 1.50% or more, or 1.80% or more. On the other hand, excessive Mn content may increase the steel strength but reduce elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[0025] [Al: 0.001 to 2.000%] Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel. To fully obtain this effect, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0026] [Ni: 0.010 to 1.000%] [Cu: 0.010 to 1.000%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. To fully obtain these effects, the contents of these elements are preferably 0.010% or more, and may be 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, excessive inclusion of these elements may reduce the etching ability of Fe during chemical conversion treatment, resulting in reduced chemical conversion treatability of the steel sheet. Therefore, the Ni and Cu contents are preferably 1.000% or less, and may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.

[0027] [Sn: 0.003 to 1.000%] Sn is an element effective in improving corrosion resistance. To fully obtain this effect, the Sn content is preferably 0.003% or more. The Sn content may be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, excessive Sn content may reduce the etching ability of Fe during chemical conversion treatment, resulting in reduced chemical conversion treatability of the steel sheet. Therefore, the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[0028] [P: 0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. Since a lower P content is preferable, ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0029] [S: 0.100% or less] S is an element that generates non-metallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content may result in a significant increase in costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content may cause cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably 0.100% or less. The S content may be 0.050% or less, 0.020% or less, or 0.010% or less.

[0030] [N: 0.0150% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, the ideal N content is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0150% or less. The N content may be 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0031] [O: 0.0100% or less] O is an element that is mixed in during the manufacturing process and forms coarse inclusions, reducing the workability of the steel sheet. Since a lower O content is preferable, the ideal O content is 0%. However, excessive reduction in the O content may result in a significant increase in manufacturing costs. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive O content may form coarse inclusions, as described above, reducing the workability of the steel sheet. Therefore, the O content is preferably 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0032] The preferred basic chemical composition of the steel sheet is as described above. Furthermore, the steel sheet may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0033] [Ti: 0 to 0.150%] [Nb: 0 to 0.150%] [V: 0 to 0.150%] Ti, Nb, and V form carbonitrides in steel and have the effect of improving the strength of the steel sheet through precipitation strengthening. The Ti, Nb, and V contents may be 0%, but to obtain this effect, the Ti, Nb, and V contents are preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, even if these elements are contained in excess, the effect saturates, and adding more than necessary to the steel increases manufacturing costs. Therefore, the Ti, Nb, and V contents are preferably 0.150% or less, and may be 0.120% or less, 0.100% or less, 0.080% or less, 0.050% or less, 0.020% or less, or 0.015% or less.

[0034] [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but to achieve this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0035] [Mo: 0 to 1.000%] [Cr: 0 to 1.000%] [W: 0 to 1.000%] Mo, Cr, and W are elements that improve the hardenability of steel and contribute to improving its strength. The Mo, Cr, and W contents may be 0%, but to achieve these effects, the Mo, Cr, and W contents are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel increases manufacturing costs. Therefore, the Mo, Cr, and W contents are preferably 1.000% or less, and may be 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0036] [Hf: 0-0.050%] [Mg: 0-0.050%] [Zr: 0-0.500%] [Ca: 0-0.050%] [REM: 0-0.010%] Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of non-metallic inclusions. The Hf, Mg, Zr, Ca, and REM contents may be 0%, but to achieve these effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel sheet increases production costs. Therefore, the Hf, Mg, and Ca contents are preferably 0.050% or less, and may be 0.010% or less, 0.005% or less, or 0.003% or less. Similarly, the Zr content is preferably 0.500% or less, and may be 0.100% or less, 0.050% or less, or 0.010% or less. Similarly, the REM content is preferably 0.010% or less, and may be 0.005% or less, or 0.003% or less. REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic numbers 57 to lutetium (Lu) with atomic numbers 71, and the REM content is the total content of these elements.

[0037] [As: 0 to 0.100%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more or 0.003% or more. On the other hand, even if excessive As is contained, the effect saturates, and containing more As than necessary in the steel sheet increases manufacturing costs. Therefore, the As content is preferably 0.100% or less. The As content may be 0.050% or less, 0.010% or less, 0.008% or less, or 0.005% or less.

[0038] [Ir: 0 to 1.000%] Ir is an element that segregates at prior austenite grain boundaries to increase the strength of the grain boundaries. The Ir content may be 0%, but to obtain this effect, the Ir content is preferably 0.001% or more. The Ir content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even if an excessive amount of Ir is contained, the effect saturates, and adding more Ir than necessary to the steel material increases the manufacturing cost. Therefore, the Ir content is preferably 1.000% or less. The Ir content may be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0039] [Zn: 0 to 1.000%] Zn is an element effective in controlling the shape of inclusions. The Zn content may be 0%, but to obtain this effect, the Zn content is preferably 0.001% or more. The Zn content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even if Zn is contained in an excessive amount, the effect saturates, and adding more Zn than necessary to the steel material increases the manufacturing cost. Therefore, the Zn content is preferably 1.000% or less. The Zn content may be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0040] The remainder of the steel sheet other than the above elements consists of Fe and impurities. The impurities in the steel sheet are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the steel sheet is industrially manufactured.

[0041] The chemical composition of the steel plate may be measured by a general analytical method. For example, the chemical composition of the steel plate may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece is obtained from the 1 / 4 position of the steel plate thickness, and the test piece is measured using a Shimadzu ICPS-8100 (measuring device) or the like under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

[0042] [Peak half width of the (211) plane of the ferrite phase in grazing incidence X-ray diffraction: 0.3° or more] In a preferred embodiment of the present invention, the peak half width of the (211) plane of the ferrite phase in grazing incidence X-ray diffraction of the steel sheet surface is controlled to 0.3° or more. By controlling the peak half width of the (211) plane of the ferrite phase to 0.3° or more, the chemical conversion treatability of the steel sheet can be further improved, and as a result, the paint adhesion of the steel sheet can be more significantly improved.

[0043] More specifically, the inventors discovered that the peak half-width of the (211) plane of the ferrite phase in grazing incidence X-ray diffraction of the steel sheet surface correlates with the strain state of the steel sheet surface, and that the greater the strain introduced into the steel sheet surface, the greater the peak half-width of the (211) plane of the ferrite phase. Here, as the strain introduced into the steel sheet surface increases, naturally, many dislocations are introduced into the steel sheet surface, and subgrain boundaries may be formed due to dislocation rearrangement. While not intending to be bound by any particular theory, it is believed that corrosion generally occurs easily due to linear defects such as dislocations and planar defects such as grain boundaries. Therefore, introducing a large amount of strain into the steel sheet surface to introduce dislocations or form subgrain boundaries can promote the anodic dissolution (etching) of Fe during chemical conversion treatment. From this perspective, the inventors conducted further research focusing on the introduction of strain into the steel sheet surface. As a result, the inventors have found that, as will be described in detail later in connection with the manufacturing method, by introducing appropriate strain into the steel sheet surface by brush grinding, and thereby controlling the peak half-width of the (211) plane of the ferrite phase in the grazing incidence X-ray diffraction of the steel sheet surface to 0.3° or more, it is possible to further improve the chemical conversion treatability of the steel sheet, and as a result, to more significantly improve the paint film adhesion of the steel sheet.

[0044] From the viewpoint of further improving the paint adhesion of the steel sheet, the larger the peak half width of the (211) plane of the ferrite phase, the more preferable, and it may be, for example, 0.4° or more. There is no particular upper limit, but the peak half width of the (211) plane of the ferrite phase may be, for example, 1.2° or less, 1.0° or less, 0.8° or less, 0.6° or less, or 0.5° or less.

[0045] [Measurement of Peak Half Width of the (211) Plane of the Ferrite Phase in Grazing Incidence X-ray Diffraction] The peak half width of the (211) plane of the ferrite phase in grazing incidence X-ray diffraction is calculated by obtaining an X-ray diffraction pattern using an X-ray diffractometer (for example, "EMPYREAN" manufactured by Spectris, a division of Malvern Panalytical Co., Ltd.) under the following conditions: X-ray source: Cu-Kα ray (wavelength λ = 1.54 Å), Kβ removal method: Ni filter (absorption edge wavelength: λ = 1.49 Å), incident angle: 0.5°, X-ray source load power: 1.6 kW (tube voltage / tube current = 45 kV / 40 mA), divergence slit: 0.18°, goniometer radius: 240 mm, measurement mode: continuous, and step: 0.01°, and then measuring the width at 1 / 2 the peak height position of the (211) plane of the ferrite phase from the obtained X-ray diffraction pattern.

[0046] [Silicon Oxide Thickness in X-ray Photoelectron Spectroscopy Measurement: 10 nm or Less] In a preferred embodiment of the present invention, when the chemical composition of a steel sheet contains Si, for example, when the chemical composition of the steel sheet contains 0.01 to 3.00% Si by mass, the thickness of silicon oxide (SiO) on the steel sheet surface is controlled to 10 nm or less in X-ray photoelectron spectroscopy (XPS) measurement. When the chemical composition of a steel sheet contains Si, silicon oxide may be formed on the steel sheet surface, and if this silicon oxide becomes thick, it may not be sufficiently dissolved in the chemical conversion treatment solution during chemical conversion treatment. Therefore, in order to further improve the chemical conversion treatability of the steel sheet and thereby more significantly improve paint adhesion, it is preferable that the thickness of silicon oxide on the steel sheet surface is as small as possible. In this regard, the present inventors have found that by controlling the thickness of silicon oxide to 10 nm or less in X-ray photoelectron spectroscopy measurements of the steel sheet surface, in combination with controlling the surface exposure rate of the concentrated portion of at least one of Ni, Cu, and Sn, it is possible to more reliably and significantly improve the paint adhesion of the steel sheet.

[0047] From the viewpoint of further improving the coating adhesion of the steel sheet, as described above, the smaller the thickness of the silicon oxide, the more preferable, and it may be, for example, 8 nm or less, 6 nm or less, 5 nm or less, 4 nm or less, 3 nm or less, or 2 nm or less. The lower limit is not particularly limited, and may be 0 nm. For example, the thickness of the silicon oxide may be 0.5 nm or more, or 1 nm or more.

[0048] [Measurement of Silicon Oxide Thickness by X-ray Photoelectron Spectroscopy] The thickness of silicon oxide is measured by X-ray photoelectron spectroscopy (XPS) as follows. First, an evaluation material for determining the thickness of the silicon oxide (surface oxide) and a reference base material are prepared. The evaluation material is a steel plate for which the silicon oxide thickness is to be determined, cut out from a target product or the like, and the surface oil and dirt have been removed without changing the silicon oxide thickness. Specifically, if oil is applied to the surface of the steel plate to be evaluated, the oil is removed by an appropriate method that does not cause surface oxidation of the steel plate (e.g., a method of removing it with a solvent, etc.) before obtaining the evaluation material. The base material is prepared by grinding and / or polishing a steel plate to a depth of approximately 100 to 500 μm from the surface of the steel plate, and adjusting the arithmetic mean roughness Ra of the surface to 0.8 μm or less. The grinding and polishing method for the base material is not particularly limited, but care must be taken to prevent surface oxidation during grinding and / or polishing. In other words, grinding and polishing methods that result in high temperatures must be avoided. Furthermore, when finish polishing is performed, it is preferable to perform the finish polishing by wet polishing using distilled water or ethanol. For each of the test material and base material prepared as described above, the maximum strength is measured on the steel sheet surface in a binding energy range of 532.9±0.4 eV. If the value of the maximum strength of the test material / the maximum strength of the base material is 1.2 or greater, it is determined that silicon oxide, a surface oxide, is present on the surface of the test material. Next, the test material is subjected to XPS measurement at 1 nm intervals in the thickness direction by sputtering, and the thickness at which the value of the maximum strength of the test material / the maximum strength of the base material becomes less than 1.2 is determined as the thickness of silicon oxide.

[0049] The XPS measurement conditions for determining the thickness of silicon oxide are as follows: X-ray source: mono-Al Kα (1486.6 eV) X-ray diameter: 50 to 200 μm Measurement area: 100 to 700 μm × 100 to 700 μm Degree of vacuum: 1×10 -10 ~1 x 10 -11 torr (1 torr = 133.32 Pa) Acceleration voltage: 1 to 10 kV

[0050] [Thickness of Steel Plate] The thickness of the steel plate is not particularly limited, but is generally 0.2 to 8.0 mm. For example, the thickness may be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the thickness of the steel plate may be, for example, 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0051] As described above, the steel sheet according to the embodiment of the present invention can achieve excellent paint adhesion and, in turn, excellent corrosion resistance compared to conventional steel sheets simultaneously containing the three elements Ni, Cu, and Sn. Therefore, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields requiring excellent paint adhesion and / or corrosion resistance, and is particularly useful for use in parts in the automotive field. In a preferred embodiment, an automobile part including the steel sheet according to the embodiment of the present invention is provided. Examples of automobile parts include frame parts, bumpers, and other structural and reinforcing parts that require strength, as well as exterior panel parts such as roofs, hoods, fenders, and doors that require high designability. It is sufficient for at least a portion of these parts to include the steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the characteristics of the steel sheet described above. In a forming process such as press forming, in a region of the steel sheet that does not come into direct contact with a mold or that comes into direct contact with the mold but is relatively lightly processed, the characteristics of the steel sheet do not change significantly before and after forming.

[0052] [Mechanical Properties] The steel sheet according to the embodiment of the present invention may have a Vickers hardness of, for example, 90 Hv or more, but is not particularly limited thereto. The Vickers hardness may be 150 Hv or more, 190 Hv or more, 200 Hv or more, 250 Hv or more, 300 Hv or more, 350 Hv or more, 400 Hv or more, or 450 Hv or more. The upper limit is not particularly limited, but the Vickers hardness may be, for example, 650 Hv or less, 600 Hv or less, 550 Hv or less, or 500 Hv or less.

[0053] [Measurement of Vickers Hardness] Vickers hardness is determined in accordance with JIS Z 2244-1:2024 as follows. First, a test piece is cut out so that a cross section (thickness cross section) perpendicular to the surface can be observed from any position except the end of the steel plate. The thickness cross section of the test piece is polished using silicon carbide paper #600 to #1500, and then mirror-finished using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water, and this thickness cross section is used as the measurement surface. Next, using a micro Vickers hardness tester, the Vickers hardness is measured at a load of 1 kgf at intervals of at least three times the indentation. Specifically, a total of 20 points are measured randomly at 1 / 4 of the steel plate thickness, and the arithmetic average of these measurements is determined as the Vickers hardness of the steel plate.

[0054] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0055] The steel sheet according to the embodiment of the present invention can be manufactured by, for example, performing a casting process in which molten steel having an adjusted chemical composition is cast to form a slab, a hot rolling process in which the slab is hot-rolled to obtain a hot-rolled steel sheet, a coiling process in which the hot-rolled steel sheet is coiled and then pickled, a brush grinding process in which the obtained hot-rolled steel sheet is brush-ground, and a water-rinsing process in which the brush-ground steel sheet is water-rinsed. While the manufacturing of a hot-rolled steel sheet will be specifically described below, the steel sheet according to the embodiment of the present invention encompasses not only a hot-rolled steel sheet but also a cold-rolled steel sheet. Therefore, when manufacturing a cold-rolled steel sheet, for example, a cold-rolling process may be performed after the water-rinsing process. Each process will be described in detail below.

[0056] [Casting Step] The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0057] [Hot Rolling Process] A hot-rolled steel plate can be obtained by hot-rolling a cast steel slab. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0058] [Coiling Process] The hot-rolled steel sheet obtained in the hot rolling process is coiled in the next coiling process and then pickled. In this manufacturing method, the hot-rolled steel sheet may be coiled at room temperature. However, from the viewpoint of suppressing the formation of silicon oxide (SiO) on the surface of the finally obtained steel sheet, it is preferable to coil the hot-rolled steel sheet at a coiling temperature of 520°C or higher. More specifically, by controlling the coiling temperature to 520°C or higher, an outer oxide layer is first formed on the outer surface (surface) of the steel sheet, and an inner oxide layer is also formed in the inner surface (surface layer) of the steel sheet. In particular, in the case of a steel sheet containing Si, this inner oxide layer is mainly composed of Si-based oxides. Therefore, a Si-depleted layer is formed immediately below the inner oxide layer formed on the surface layer of the steel sheet due to the consumption of Si in the steel due to the formation of the inner oxide layer. In particular, by controlling the coiling temperature to 520°C or higher, a Si-depleted layer with a sufficient thickness can be formed. Since the outer oxide layer and the inner oxide layer are removed by pickling after coiling, a Si-depleted layer remains on the surface of the hot-rolled steel sheet after pickling. By forming the surface of the hot-rolled steel sheet with a Si-depleted layer, it becomes possible to suppress the formation of silicon oxide (SiO2) on the surface of the steel sheet in a subsequent water-rinsing process or the like due to the lack of Si on the steel sheet surface. The upper limit of the coiling temperature is not particularly limited, but the coiling temperature may be, for example, 600°C or lower.

[0059] The pickling is not particularly limited, and may be carried out using a commonly used pickling solution, such as a hydrochloric acid solution containing an inhibitor that suppresses corrosion of the steel sheet, under conditions suitable for removing the outer and inner oxide layers. The pickling may be carried out once, or may be carried out multiple times to ensure that the outer and inner oxide layers are completely removed.

[0060] [Brush grinding step] The obtained hot-rolled steel sheet is subjected to the next brush grinding step with a grinding amount of 3.0 g / m 2Brush grinding is performed under the above conditions. By performing brush grinding under such conditions, it is possible to sufficiently or completely remove at least one of the concentrated portions of Ni, Cu, and Sn on the steel sheet surface. As a result, it is possible to achieve a desired surface exposure rate of at least one of the concentrated portions of Ni, Cu, and Sn on the cross section of the steel sheet in the finally obtained steel sheet, more specifically, a surface exposure rate of 10% or less. The grinding amount by brush grinding is 3.0 g / m 2 If the brush grinding amount is less than 6.0 g / m, it is not possible to sufficiently remove the concentrated portions of at least one of Ni, Cu, and Sn on the steel sheet surface, and as a result, it is not possible to achieve a desired surface exposure rate of the concentrated portions. From the viewpoint of further reducing the surface exposure rate, the greater the amount of grinding by brush grinding, the more preferable it is, and for example, 6.0 g / m 2 It is preferable that the content is 8.0 g / m or more. 2 More preferably, it is equal to or greater than this.

[0061] The amount of grinding by brush grinding was 8.0 g / m 2 By controlling the amount as described above, not only can the concentrated portions of at least one of Ni, Cu, and Sn on the steel sheet surface be removed, but also more strain can be introduced into the steel sheet surface. By introducing more strain into the steel sheet surface, it is possible to control the peak half-width of the (211) plane of the ferrite phase in the grazing incidence X-ray diffraction of the steel sheet surface to 0.3° or more. As mentioned above, the introduction of appropriate strain corresponding to such peak half-width can appropriately introduce defects such as dislocations into the steel sheet surface. Therefore, the presence of such defects can promote the anodic dissolution (etching) of Fe during chemical conversion treatment, thereby further improving the chemical conversion treatability of the steel sheet, and as a result, it is possible to more significantly improve the paint adhesion of the steel sheet. The upper limit of the grinding amount is not particularly limited, but may be, for example, 20.0 g / m 2 or less than 15.0 g / m 2The amount of grinding by the abrasive brush can be adjusted by any appropriate method known to those skilled in the art. Although not particularly limited, the amount of grinding by the abrasive brush can be adjusted by appropriately selecting, for example, the type of abrasive brush (e.g., H115 manufactured by Hotani Co., Ltd.), the number of abrasive brushes, the rotation speed, the brush pressure, the coating liquid used, and the like.

[0062] [Water-Rinsing Step] Brush grinding causes powdery deposits to adhere to the steel sheet surface. Therefore, the brush-ground steel sheet must be rinsed in the subsequent water-rinsing step. Preferably, the water-rinsing step is performed by rinsing the steel sheet with a rinsing solution having an electrical conductivity of 40 mS / m or less, e.g., rinsing water. For example, if the rinsing solution used in the water-rinsing step has a relatively high electrical conductivity, an oxidation-reduction reaction may occur on the steel sheet surface during rinsing, resulting in the formation of silicon oxide on the steel sheet surface. If the silicon oxide becomes thick, it may not be sufficiently dissolved in the chemical conversion treatment solution during chemical conversion treatment. Therefore, to further improve the chemical conversion treatability of the steel sheet and thereby significantly improve the paint film adhesion, it is preferable that the thickness of the silicon oxide formed on the steel sheet surface is as small as possible. In this manufacturing method, by performing the water-rinsing step after brush grinding with a rinsing solution having an electrical conductivity of 40 mS / m or less, the oxidation-reduction reaction on the steel sheet surface during rinsing after brush grinding can be sufficiently or completely suppressed. This significantly suppresses the formation of silicon oxide. In particular, when a large amount of strain is introduced into the steel sheet surface by brush grinding, the introduction of strain promotes the formation of silicon oxide on the steel sheet surface. Therefore, rinsing with a rinsing solution having a lower electrical conductivity, particularly rinsing water having a lower electrical conductivity, is very effective in suppressing the formation of silicon oxide.

[0063] Furthermore, in the present production method, it is preferable not only to use a washing solution having a lower electrical conductivity in the washing step, specifically an electrical conductivity of 40 mS / m or less, but also to combine such washing with a coiling temperature of 520°C or more in the previous coiling step. By combining a Si-deficient layer formed on the steel sheet surface due to a coiling temperature of 520°C or more with washing with a washing solution having an electrical conductivity of 40 mS / m or less, it becomes possible to reliably and sufficiently suppress the formation of silicon oxide in the washing step, even when a large amount of strain is introduced into the steel sheet surface by brush grinding. From the viewpoint of further suppressing the formation of silicon oxide, it is preferable that the electrical conductivity of the washing solution is as low as possible, specifically, 30 mS / m or less is preferred, and 20 mS / m or less is more preferred.

[0064] According to this manufacturing method, in steel sheets where improving paint film adhesion is difficult due to the simultaneous inclusion of three elements, Ni, Cu, and Sn, it is possible to achieve a surface exposure rate of 10% or less of the enriched areas of at least one of Ni, Cu, and Sn in the cross section of the steel sheet by appropriately controlling the grinding amount, particularly in the brush grinding process. As mentioned above, when Ni, Cu, and Sn are present in solid solution in the steel sheet, the potential of the steel sheet becomes more noble than when these elements are not present in solid solution, which reduces the etching ability of Fe during chemical conversion treatment and, therefore, may reduce the chemical conversion treatability of the steel sheet. However, with steel sheets manufactured according to this manufacturing method, as described above, the surface exposure rate of the enriched areas of at least one of Ni, Cu, and Sn in the cross section of the steel sheet is controlled to 10% or less, allowing the chemical conversion treatment film to be formed uniformly over the entire steel sheet during chemical conversion treatment, resulting in significantly improved paint film adhesion. In addition, the introduction of strain into the steel sheet surface by brush grinding promotes the anodic dissolution of Fe during chemical conversion treatment. Furthermore, by combining the formation of a Si-depleted layer due to appropriate control of the coiling temperature in the coiling process with a specific water rinse in the water rinsing process after brush grinding, the formation of silicon oxide on the steel sheet surface can be suppressed, thereby making it possible to more significantly improve the paint adhesion of the steel sheet. Therefore, steel sheets manufactured by this manufacturing method can achieve superior corrosion resistance compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. This can contribute to industrial development by extending the service life of steel sheets used in automobiles and building materials.

[0065] The steel sheet according to the embodiment of the present invention may be used as the various automotive parts described above, for example, after a chemical conversion coating or paint film is optionally formed on the surface thereof. Whether or not an automotive part having a paint film or chemical conversion coating includes the steel sheet according to the embodiment of the present invention can be determined by removing the paint film or chemical conversion coating from a sample taken from the automotive part. In this case, the location from which the sample is taken, the paint film removal step, and the chemical conversion coating removal step are as follows.

[0066] [Sample collection locations] Samples will be collected from automotive parts, avoiding the following locations (i) to (iv): (i) Locations within 20 mm from the toe of spot welds and locations within 20 mm from the toe of the bead of arc / laser welds (ii) Processed areas with a curvature radius of less than 15 mm, and locations within 5 mm from such processed areas (iii) Edges within 5 mm from the cut end surface of the part (iv) Locations within 5 mm from locations where red rust is visible

[0067] [Paint Removal Process] The paint film is removed from a sample cut from an automobile body under the following conditions to expose the steel sheet. A paint remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface at room temperature and allowed to stand for approximately 5 minutes. The paint film is then removed by rubbing with a hard sponge or similar (e.g., Kanefiel, manufactured by AION Co., Ltd.). The sample is then rinsed with water and dried. The remaining paint film is then confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after rinsing and drying. In the element distribution image obtained by EPMA, regions with a carbon concentration of 10% by mass or more are identified, and if the area ratio of such regions is 5% or more, it is determined that the paint film has not been sufficiently removed. To measure the area ratio of regions with a carbon concentration of 10% by mass or more, first obtain an element distribution image of carbon using an EPMA with a carbon concentration range of 10 to 30%. The obtained element distribution image is then subjected to image processing to measure the area ratio. Image analysis software "ImageJ" was used for image processing. The C element distribution image was then loaded into ImageJ, and binarized using "Make Binary" in "Binary" under "Process" so that areas with a C concentration of 10% by mass or more were displayed as black and areas with a C concentration of less than 10% by mass were displayed as white. After binarization, "Measure" under "Analyze" was used to read the value for "Area fraction" in "Results," and this value was determined as the area fraction of areas with a C concentration of 10% by mass or more. If peeling of the coating film was insufficient, removal of the coating film was repeated until the area fraction of areas with a C concentration of 10% by mass or more became less than 5%.

[0068] [Chemical Conversion Coating Removal Process] The chemical conversion coating is removed from a sample cut from an automobile body, with the coating removed, in accordance with JIS K 3151:1996. Specifically, the sample is immersed in a 5% chromic acid solution heated to 75°C for 15 minutes to remove the chemical conversion coating. The sample is then rinsed with water and dried. The remaining state of chemical conversion crystals is confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after rinsing and drying. In the element distribution image obtained by EPMA, regions with a P concentration of 5% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the chemical conversion coating has not been sufficiently removed. To measure the area ratio of regions with a P concentration of 5% by mass or more, first obtain an element distribution image of P using EPMA, with the P concentration range set to 5-10%. The obtained element distribution image is then image-processed to measure the area ratio. Image analysis software "ImageJ" was used for image processing. The P element distribution image was then loaded into ImageJ, and binarized using "Make Binary" in "Binary" under "Process" so that areas with a P concentration of 5% by mass or more were displayed as black, and areas with a P concentration of less than 5% by mass were displayed as white. After binarization, "Measure" under "Analyze" was used to read the value for "Area fraction" in "Results," and this value was determined as the area fraction of areas with a P concentration of 5% by mass or more. If peeling of the chemical conversion coating was insufficient, removal of the chemical conversion coating was repeated until the area fraction of areas with a P concentration of 5% by mass or more became less than 5%.

[0069] The present invention will be described in more detail below with reference to examples, but the following examples are merely illustrative of the present invention and the present invention is not limited to these examples. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention.

[0070] In the following examples, steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the produced steel sheets were investigated.

[0071] First, molten steel was cast by a continuous casting method to form a steel billet having the chemical composition shown in Table 1. The steel billet was once cooled, reheated to 1200°C, hot rolled, and then coiled at a coiling temperature of 520°C. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at a temperature of 900 to 1050°C and a finish rolling reduction of 30%. Next, the obtained hot-rolled steel sheet was pickled and then brush-ground (H115, manufactured by Hotani Co., Ltd.) with the grinding amount shown in Table 2. Finally, the steel sheet was washed with wash water having the electrical conductivity shown in Table 2 to obtain a steel sheet having a thickness of 3.0 mm.

[0072]

[0073]

[0074] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0075] [Evaluation of Paint Adhesion] Paint adhesion was evaluated as follows. First, a 50 mm x 50 mm sample of the steel plate produced above was subjected to a zinc phosphate treatment (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.) as a chemical conversion treatment under the following conditions: Degreasing: Immersion in a degreaser (Fine Cleaner E2032A / B) at 40°C for 2 minutes, followed by rinsing with water. Surface conditioning: Immersion in a surface conditioner (Preparen Z) at room temperature for 30 seconds. Chemical conversion treatment: Immersion in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, followed by rinsing with water and drying.

[0076] Chemically treated steel sheet samples were electrocoated (Powernics Excel 1200, manufactured by Nippon Paint Industrial Coating Co., Ltd.) at an electrodeposition temperature of 30°C to a film thickness of 16 μm, followed by a baking treatment at 170°C for 30 minutes. The electrocoated samples were then subjected to a saltwater immersion test (SDT). Specifically, the electrocoated samples were immersed in a 5% NaCl aqueous solution at 55°C for 1000 hours. After the SDT test, the removed samples were dried, and then a tape peeling test was performed on one side of the sample. The peeled tape was scanned, and the peeled area ratio of the coating was calculated by binarization using the image analysis software "ImageJ." The coating adhesion was evaluated as follows: AAA: Peeled area ratio less than 5%; AA: Peeled area ratio 5-10%; A: Peeled area ratio 10-15%; B: Peeled area ratio more than 15%.

[0077] Steel sheets that were rated AAA, AA, or A for paint adhesion were evaluated as containing Ni, Cu, and Sn and capable of exhibiting improved paint adhesion. The results are shown in Table 2.

[0078] Referring to Table 2, in Comparative Examples 31 to 35, the grinding amount by brush grinding was 3.0 g / m 2 As a result, the surface exposure rate of the concentrated portions of at least one of Ni, Cu, and Sn in the cross section of the steel sheet exceeded 10%, and the paint film adhesion was reduced.

[0079] In contrast, in the steel plates according to all the examples, the amount of grinding by brush grinding was 3.0 g / m 2By controlling the above, it was possible to sufficiently remove at least one of the concentrated areas of Ni, Cu, and Sn on the steel sheet surface, and as a result, the surface exposure rate of at least one of the concentrated areas of Ni, Cu, and Sn on the cross section of the steel sheet was controlled to 10% or less, and the paint adhesion of the steel sheet was significantly improved. In particular, in Examples 3, 4, 9, 10, 15, 16, 21, 22, 27, and 28, where the surface exposure rate was 5% or less, the paint adhesion was evaluated as AA, and paint adhesion was further improved. Furthermore, in Examples 5, 6, 11, 12, 17, 18, 23, 24, 29 and 30, in which the surface exposure rate was 5% or less, the peak half-width of the (211) plane of the ferrite phase was 0.3° or more in the grazing incidence X-ray diffraction of the steel sheet surface, and the silicon oxide thickness was 10 nm or less in the X-ray photoelectron spectroscopy measurement of the steel sheet surface, the coating adhesion was evaluated as AAA, which indicated that the coating adhesion was further improved.

[0080] 1 Steel plate 2 Part where at least one of Ni, Cu and Sn is concentrated

Claims

1. A steel sheet having a chemical composition containing, by mass%, Ni: 0.010-1.000%, Cu: 0.010-1.000%, and Sn: 0.003-1.000%, wherein the surface exposure rate of enriched portions of at least one of Ni, Cu, and Sn in the cross section of the steel sheet is 10% or less.

2. The steel sheet according to claim 1, characterized in that the surface exposure rate is 5% or less.

3. The steel sheet according to claim 1 or 2, wherein the peak half-width of the (211) plane of the ferrite phase in the grazing incidence X-ray diffraction of the steel sheet surface is 0.3° or more.

4. The steel sheet according to any one of claims 1 to 3, characterized in that the chemical composition contains Si, and the thickness of silicon oxide is 10 nm or less when measured on the surface of the steel sheet by X-ray photoelectron spectroscopy.

5. The steel sheet according to any one of claims 1 to 4, characterized in that it has a Vickers hardness of 190 Hv or more.

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

7. A part, characterized in that it comprises a steel sheet according to any one of claims 1 to 6.

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