Steel sheet and component including same

Abstract

Disclosed is a steel sheet containing Cu, Ni, and Sn and having improved coating film adhesion properties. The chemical composition of the steel sheet according to the present disclosure includes, in mass%, 0.010-1.000% of Ni, 0.010-1.000% of Cu, and 0.003-1.000% of Sn. The oxide area ratio in the surface of the steel sheet is 25% or less.

Description

Steel plates and parts containing them

[0001] The present application discloses a steel plate and a component including the same.

[0002] In order to improve the corrosion resistance of a steel sheet, it is effective to adhere a coating film to the surface of the steel sheet. For example, by improving the chemical conversion treatability of the surface of the steel sheet, the adhesion of the coating film to the surface of the steel sheet can be increased.

[0003] Patent Document 1 discloses a method for producing a cold-rolled steel sheet having excellent chemical conversion treatability by continuously annealing the cold-rolled steel sheet in equipment for both cold-rolled steel sheets and hot-dip galvanized steel sheets, exposing the surface of the steel sheet to an atmosphere in which iron oxidizes within a predetermined temperature range, pickling the steel sheet at the outlet side of the annealing furnace, and then plating the steel sheet with a predetermined amount of iron or Ni.

[0004] In addition, Patent Document 2 discloses a method for manufacturing a steel sheet containing copper (Cu) in an amount of 0.10 mass % or more and 0.50 mass % or less, in which the number of residual scales on the surface is 160,000 pieces / mm ２ Patent Document 2 teaches that a steel sheet with excellent chemical conversion treatability can be provided by controlling the particle size of copper compound particles exposed on the steel sheet surface, which serves as a cathode point in chemical conversion treatment, to 2 μm or less and by controlling the amount of residual scale to a predetermined amount or less.

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

[0006] According to new findings by the present inventors, steel sheets containing copper (Cu), nickel (Ni), and tin (Sn) simultaneously tend to have poor paint adhesion. The present application discloses a steel sheet containing Cu, Ni, and Sn and having improved paint adhesion.

[0007] The present application discloses the following aspects as means for solving the above problems. (1) A steel sheet, wherein the chemical composition of the steel sheet includes, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and the area ratio of oxides on the surface of the steel sheet is 25% or less. (2) The steel sheet according to (1), wherein the area ratio of oxides is 15% or less. (3) The steel sheet according to (1), wherein the area ratio of oxides is 5% or less. (4) The steel sheet according to (1), wherein the S strength I １ and S strength I at 1 / 4 of the thickness of the steel plate ２ Relative to I １ / I ２ (5) The steel sheet according to any one of (1) to (3), wherein the S strength I at the surface of the steel sheet is 1.01 or more. １ and S strength I at 1 / 4 of the thickness of the steel plate ２ Relative to I １ / I ２ (6) The steel sheet according to any one of (1) to (3), wherein the S strength I at the surface of the steel sheet is 1.08 or more. １ and S strength I at 1 / 4 of the thickness of the steel plate ２ Relative to I １ / I ２ (7) The steel sheet according to any one of (1) to (6), having a Vickers hardness of 200 Hv or more. (8) The steel sheet according to any one of (1) to (7), having a chemical composition comprising, in mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%. (9) A part, characterized by comprising the steel sheet according to any one of (1) to (8).

[0008] The steel sheet of the present disclosure contains Ni, Cu, and Sn and has improved paint adhesion.

[0009] 1. Background to the Completion of the Steel Sheet of the Present Disclosure Two methods for producing steel sheets are known: a method in which steel sheets are produced in a blast furnace using iron ore, a natural resource, as the main raw material, and a method in which steel sheets are produced in an electric furnace using scrap, a recycled resource, as the main raw material. Electric furnace steel, which uses scrap as the main raw material, contains relatively large amounts of scrap-derived elements (so-called tramp elements), such as Ni, Cu, and Sn. Blast furnace steel may also contain elements such as Ni, Cu, and Sn as additive elements. On the other hand, both steel sheets produced from blast furnace steel and steel sheets produced from electric furnace steel may require high corrosion resistance. In this case, the corrosion resistance of the steel sheet may be improved by adhering a coating film to the surface of the steel sheet. The present inventors have conducted extensive research into the coating adhesion of various steel sheets. As a result, they have found that conventional steel sheets containing Ni, Cu, and Sn have difficulty in ensuring coating adhesion.

[0010] The present inventors have conducted further studies on paint adhesion when steel sheets containing Ni, Cu, and Sn are subjected to chemical conversion treatment and painting. As a result, the following findings have been obtained: (1) Ni, Cu, and Sn may dissolve in the steel sheet and promote the formation of oxides (particularly Mn-based oxides and / or Si-based oxides) on the steel sheet surface. Furthermore, the presence of Ni, Cu, and Sn tends to facilitate the adhesion of oxides (e.g., oxides dissolved during the removal of surface scale and oxides by pickling in the steel sheet manufacturing process) to the steel sheet surface. Furthermore, since Ni, Cu, and Sn are more noble elements than Fe, the presence of Ni, Cu, and Sn tends to promote the oxidation of the steel sheet surface during water washing, etc. Therefore, steel sheets containing Ni, Cu, and Sn may have a large amount of oxides on their surfaces. (2) If a large amount of oxides is present on the surface of a steel sheet, the oxides inhibit the chemical conversion treatment during the chemical conversion treatment of the steel sheet surface, resulting in poor chemical conversion. When a coating film is formed on the surface of a steel sheet with poor chemical conversion, it is difficult to ensure sufficient coating adhesion. In other words, reducing the amount of oxides on the surface of a steel sheet containing Ni, Cu, and Sn can improve coating adhesion on the steel sheet surface. (3) In addition to reducing the amount of oxides on the surface of a steel sheet containing Ni, Cu, and Sn, the presence of S on the steel sheet surface tends to further improve coating adhesion on the steel sheet surface. (4) In the manufacturing process of a steel sheet containing Ni, Cu, and Sn, using a pickling solution containing a predetermined accelerator at a certain concentration or higher during pickling of the steel sheet surface can suppress the adhesion of oxides to the steel sheet surface. S can also be attached to the steel sheet surface. (5) In the manufacturing process of a steel sheet containing Ni, Cu, and Sn, by using water with low electrical conductivity when rinsing the surface of the steel sheet with water, the oxidation-reduction reaction on the surface of the steel sheet can be suppressed, and the generation of oxides on the surface of the steel sheet can be further suppressed.

[0011] The steel sheet of the present disclosure has been completed based on the above findings. The steel sheet of the present disclosure may be manufactured from an electric furnace steel that inevitably contains Ni, Cu, and Sn as tramp elements, or may be manufactured from a blast furnace steel that contains Ni, Cu, and Sn as essential elements or optional added elements. The steel sheet of the present disclosure can ensure excellent paint adhesion compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. In this regard, the steel sheet of the present disclosure is particularly useful, for example, in applications in the automotive field where paint adhesion and / or corrosion resistance are required. Hereinafter, embodiments of the steel sheet of the present disclosure will be described, but the steel sheet of the present disclosure is not limited to the following embodiments.

[0012] 2. Steel Sheet A steel sheet according to one embodiment has the following characteristics. Specifically, the chemical composition of the steel sheet includes, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. The area ratio of oxides on the surface of the steel sheet is 25% or less.

[0013] 2.1 Chemical Composition The chemical composition of a steel sheet according to one embodiment includes, by mass, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, and 0.003 to 1.000% Sn. The chemical composition may also include, by mass, 0.040 to 1.000% Ni, 0.040 to 1.000% Cu, and 0.004 to 1.000% Sn. As described above, the technology disclosed herein ensures improved paint adhesion by controlling oxides on the surface of a steel sheet that simultaneously contains the three elements Ni, Cu, and Sn. In the chemical composition of the steel sheet disclosed herein, elements other than Ni, Cu, and Sn are not essential technical features for improving paint adhesion. The chemical composition of the steel sheet of the present disclosure may contain, in addition to Ni, Cu, and Sn, any alloying elements commonly added in this technical field in appropriate amounts. The chemical composition of a steel sheet according to one embodiment will be described in detail below, but the following description is intended to merely exemplify a preferred chemical composition for application to automotive steel sheets, etc. The chemical composition of the steel sheet of the present disclosure is not limited to the specific chemical composition described below. Note that, in this application, "%" for a component means mass %. Furthermore, in this application, unless otherwise specified, the term "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0014] In one embodiment, the chemical composition of the steel plate is, 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%, V: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0 to 1.000%, The alloy may consist of Cr: 0 to 1.000%, 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.

[0015] [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.

[0016] [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.

[0017] [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.

[0018] [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.

[0019] [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 such effects, the contents of these elements are preferably 0.010% or more, and may be 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, excessive content of these elements may promote the formation of oxides, particularly Mn- and / or Si-based surface oxides and iron oxides, on the steel sheet surface. From the viewpoint of reducing oxide formation on the steel sheet surface, the Ni and Cu contents are each preferably 1.000% or less, and may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.

[0020] [Sn: 0.003 to 1.000%] Sn is an element effective in improving corrosion resistance. To fully obtain this effect, the Sn content is preferably 0.003% or more. The Sn content may be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, excessive Sn content may promote the formation of oxides on the steel sheet surface, particularly Mn- and / or Si-based surface oxides and iron oxides. From the viewpoint of reducing the formation of oxides on the steel sheet surface, 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.

[0021] [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.

[0022] [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.

[0023] [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 0.0140% or less, 0.0120% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0024] [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.

[0025] 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.

[0026] [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.

[0027] [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.

[0028] [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.

[0029] [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 0.300% or less, 0.100% or less, or 0.050% 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 number 57 to lutetium (Lu) with atomic number 71, and the "REM content" is the total content of these elements.

[0030] [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 0.080% or less, 0.050% or less, 0.030% or less, 0.010% or less, 0.008% or less, or 0.005% or less.

[0031] [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.

[0032] [Zn: 0% to 1.000%] Zn is an element that can contribute to controlling the morphology of inclusions. Zn is an optional element, and its content is 0% or more. The Zn content may be 0.001% or more, 0.003% or more, or 0.005% 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.

[0033] In the chemical composition of the steel sheet according to one embodiment, the balance other than the above elements is composed 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.

[0034] The chemical composition of the steel plate can be measured by a common analytical method. For example, the chemical composition of the steel plate can 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 can be obtained from the 1 / 4 position of the steel plate thickness, and the composition can be determined by measuring it under conditions based on a pre-created calibration curve using a measuring device such as Shimadzu ICPS-8100. C and S, which cannot be measured by ICP-AES, can be measured using a combustion-infrared absorption method, and N can be measured using an inert gas fusion-thermal conductivity method.

[0035] 2.2 Surface Oxide Area Ratio It is important that the oxide area ratio on the surface of the steel sheet according to this embodiment is 25% or less. In this embodiment, the term "surface oxide" refers to oxides present on the surface of the steel sheet. This oxide is not limited to Fe oxides, but also includes oxides of metals contained in the steel sheet, such as Si, Mn, Al, Ni, and Cu, as well as oxides of various components contained in the steel sheet, such as P and S. Specific examples of surface oxides referred to herein include oxides generated during water washing. According to the inventor's new findings, controlling the oxide area ratio on the surface of the steel sheet to 25% or less makes it difficult for the chemical conversion treatment of the steel sheet surface to be inhibited, improving the chemical conversion treatability of the steel sheet surface and, as a result, facilitating improved adhesion of a coating film to the steel sheet surface. The surface oxide area ratio is preferably 20% or less, more preferably 15% or less, even more preferably 10% or less, and particularly preferably 5% or less.

[0036] The area ratio of the oxide on the surface of the steel sheet can be determined by performing elemental analysis of the steel sheet surface using an electron probe microanalyzer (EPMA). For the EPMA analysis, an evaluation sample for determining the surface oxide area ratio and a reference base sample are prepared. The evaluation sample is cut from the steel sheet for which the surface oxide area ratio is to be determined, with surface oil and dirt removed without changing the surface oxide area ratio. Specifically, if oil is applied to the surface of the steel sheet to be evaluated, the oil is removed using an appropriate method that does not cause surface oxidation of the steel sheet (e.g., the same method as the degreasing method optionally performed before chemical conversion treatment) before obtaining the evaluation sample. The base sample is prepared by grinding and / or polishing the steel sheet to a depth of approximately 100 to 500 μm from the surface of the steel sheet to remove surface oxide and adjust the arithmetic mean roughness (Ra) of the surface to 0.8 μm or less. The grinding and polishing method for the base sample is not particularly limited, but care must be taken to prevent surface oxidation during grinding and polishing. In other words, grinding and polishing that would cause the steel sheet to reach a high temperature of 50°C or higher should be avoided. Furthermore, for example, wet polishing using distilled water or ethanol is preferable for finish polishing. EPMA analysis is performed using, for example, a JXA-8500 manufactured by JEOL Ltd., with an acceleration voltage of 15 kV and a probe current of 5 x 10 －７A, irradiation time: 50 ms. First, EPMA analysis is performed on the surfaces of both the evaluation material and the base material under the above conditions in a measurement field of 1500 μm × 1500 μm, and the oxygen (O) intensity is identified for each measurement point in the field of view. First, the average O intensity is determined in the measurement field of the base material. Next, in the measurement field of the evaluation material, areas where the O intensity is 5 times or more the average O intensity in the base material are mapped, and areas of the evaluation material where the O intensity is 5 times or more the average O intensity in the base material are considered to be "areas where surface oxides exist." In the field of view of the evaluation material, areas where surface oxides are considered to exist as described above are colored white (or black), and other areas are colored black (or white) using image processing software, and the area ratio of the areas where surface oxides exist in the entire measurement field of the evaluation material is measured. Specifically, by setting the lower limit of the O mapping color bar value to five times the average O intensity of the base material, a mapping image in which only the areas where surface oxide is present are brightly displayed is obtained. The area fraction of the surface oxide in the mapping image is then measured using the image processing software "ImageJ." First, using "Make Binary" in "Binary" under "Process," the image is binarized so that areas where surface oxide is present are displayed as black and areas where no surface oxide is present are displayed as white. After binarization, using "Measure" under "Analyze," the value of "Area fraction" in "Results" is read to determine the area fraction. For any five fields of view on the surface of the evaluation material, the "area fraction of the area where surface oxide is present in the entire measurement field" is calculated, and the average value is calculated. The calculated average value is considered to be the "area fraction of the oxide on the surface of the steel sheet."

[0037] 2.3 S Intensity According to the findings of the present inventors, in a steel sheet containing Ni, Cu, and Sn, the amount of oxides on the surface of the steel sheet is reduced, and in addition, S adheres to the surface of the steel sheet, improving the chemical conversion treatability of the surface of the steel sheet and facilitating further improvement in the adhesion of a coating film on the surface of the steel sheet. For example, in a steel sheet according to one embodiment, the S intensity I １ and S strength I at 1 / 4 of the plate thickness of the steel plate２ Relative to I １ / I ２ is preferably 1.01 or more, 1.02 or more, 1.03 or more, 1.04 or more, 1.05 or more, 1.06 or more, 1.07 or more, 1.08 or more, 1.09 or more, 1.10 or more, 1.11 or more, 1.12 or more, 1.13 or more, 1.14 or more, or 1.15 or more. The reason why adhesion of S to the surface of a steel sheet improves the chemical treatability of the surface of the steel sheet is presumed to be as follows. That is, in chemical conversion treatment, a surface conditioning treatment is sometimes performed as a pretreatment. In the surface conditioning treatment, for example, Ti colloid is adsorbed onto the surface of the steel sheet. By adsorbing Ti colloid onto the surface of the steel sheet, the Ti colloid becomes a nucleation site during chemical conversion treatment, thereby improving chemical treatability. It is presumed that adhesion of S to the surface of the steel sheet makes it easier for this Ti colloid to be adsorbed, which is thought to result in further improvement in chemical treatability and further improvement in paint adhesion. Alternatively, a surface conditioning treatment may be performed using a phosphate-based (zinc compound-based) surface conditioner as a pretreatment for chemical conversion treatment. Even when a phosphate-based (zinc compound-based) surface conditioning is performed, good paint film adhesion is more likely to be obtained if S is attached to the surface of the steel sheet.

[0038] S strength I on the surface of the steel plate １ , and S strength I at 1 / 4 of the thickness of the steel plate ２ and , respectively, refer to the S intensity measured by fluorescent X-rays. Specifically, first, the surface of the steel sheet is irradiated with fluorescent X-rays to measure the intensity I due to the presence of S element. １ On the other hand, the surface at 1 / 4 of the plate thickness of the steel plate is exposed by grinding or the like, and fluorescent X-rays are irradiated onto this surface in the same way to identify the intensity I due to the presence of S element. ２ The intensity I １ and intensity I ２ From this, the intensity ratio I １ / I ２ The S intensity is measured by fluorescent X-rays at five arbitrary points on the surface of the steel plate and on the 1 / 4 surface of the plate thickness, and the intensity ratio I is calculated based on the average value of the five points. １ / I ２The fluorescent X-ray measurement is performed using a Simultix14 device manufactured by Rigaku Corporation, with an acceleration voltage of 30 kV, until the maximum intensity in the obtained spectrum reaches 10,000 counts. The maximum peak intensity at an energy of 2.3±0.3 keV is taken as the intensity of S.

[0039] 2.4 Thickness of Steel Plate The thickness of the steel plate of the present disclosure is not particularly limited. The steel plate according to one embodiment may have a thickness of, for example, 0.2 mm or more and 8.0 mm or less. 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, or 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0040] 2.5 Mechanical Properties The mechanical properties of the steel sheet according to the present disclosure are not particularly limited. The steel sheet according to one embodiment may have a Vickers hardness of, for example, 90 Hv or more. The Vickers hardness may be 150 Hv or more, 200 Hv or more, 250 Hv or more, 300 Hv or more, 350 Hv or more, 400 Hv or more, or 450 Hv or more. The upper limit is not particularly limited, and the Vickers hardness may be, for example, 650 Hv or less, 600 Hv or less, 550 Hv or less, or 500 Hv or less.

[0041] The Vickers hardness of steel plate is measured in accordance with JIS Z 2244-1:2024 as follows. First, a test piece is cut out from any position except the end of the steel plate so that a cross section (thickness cross section) perpendicular to the surface can be observed. The thickness cross section of the test piece is polished using 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, the Vickers hardness is measured using a micro Vickers hardness tester at a load of 1 kgf and 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's thickness, and the arithmetic average of these measurements is determined as the Vickers hardness of the steel plate.

[0042] 3. Manufacturing Method of Steel Sheet Next, a preferred manufacturing method of the steel sheet according to the embodiment will be described. The following description is intended to exemplify a characteristic method for manufacturing the steel sheet according to the embodiment, and is not intended to limit the steel sheet of the present disclosure to one manufactured by the manufacturing method described below.

[0043] 3.1 First embodiment A method for producing a steel sheet according to a first embodiment may include, for example, a casting step of casting molten steel having an adjusted chemical composition to obtain a steel slab, a hot rolling step of hot-rolling the steel slab to obtain a hot-rolled steel sheet, a pickling step of pickling the hot-rolled steel sheet, and a water-rinsing step of rinsing the hot-rolled steel sheet after the pickling, and preferably further includes a drying step of drying the steel sheet after the water-rinsing. In this case, after the surface properties of the steel sheet are appropriately adjusted by pickling and water-rinsing (and drying), a chemical conversion treatment is applied to the surface of the steel sheet, and then a coating film is adhered to the surface of the steel sheet. Each step will be described in detail below.

[0044] 3.1.1 Casting Process The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes are performed, and then the molten steel is cast by a method such as ordinary continuous casting or ingot casting to obtain a steel slab.

[0045] 3.1.2 Hot Rolling Process In the hot rolling process, the cast steel slab is hot-rolled to obtain a hot-rolled steel sheet. The hot rolling process is performed by reheating the cast steel slab directly or after cooling it once, and then hot-rolling it. When reheating is performed, the heating temperature of the steel slab may be, for example, 1100°C or higher and 1250°C or lower. 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 depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900°C or higher and 1050°C or lower, and the reduction ratio of finish rolling may be 10% or higher and 50% or lower.

[0046] 3.1.3 Pickling process In the pickling process, the hot-rolled steel sheet obtained in the hot rolling process is optionally coiled and then pickled. There are no particular restrictions on the coiling temperature of the hot-rolled steel sheet, but it is preferably 520°C or higher from the viewpoint of internal oxidation.

[0047] The manufacturing method according to the first embodiment has one feature in the pickling solution used during pickling of the hot-rolled steel sheet. Specifically, it is important that the pickling solution contains an accelerator that promotes dissolution of the grain boundaries of the Fe phase at a concentration of 10 ppm to 1500 ppm. By using a pickling solution containing an accelerator at a certain concentration or higher during pickling, adhesion of oxides to the surface of the steel sheet during pickling can be suppressed. As a result, the paint adhesion on the surface of the steel sheet can be improved. Furthermore, by using a pickling solution containing an accelerator at a certain concentration or higher during pickling, S derived from the accelerator can remain on the surface of the steel sheet after pickling. This reduces the S intensity I on the surface of the steel sheet. １ The amount of the accelerator increases, improving chemical conversion treatability through the estimated mechanism described above, which can contribute to improving paint film adhesion on the steel sheet surface. The accelerator contained in the pickling solution may be any accelerator as long as it suppresses oxide adhesion, and examples thereof include organic compounds containing S, such as thioglycolic acid, thiosulfuric acid, thiocyanic acid, thiocarboxylic acid, and thiourea, and salts thereof. The concentration of the accelerator contained in the pickling solution is preferably 50 ppm or more and 1500 ppm or less, more preferably 100 ppm or more and 1500 ppm or less.

[0048] In the manufacturing method according to the first embodiment, the temperature and time in the pickling step are not particularly limited as long as the surface properties of the hot-rolled steel sheet can be made appropriate as described above. According to the findings of the present inventors, when pickling is performed using the above-mentioned pickling solution, the surface properties of the hot-rolled steel sheet can be appropriately adjusted when the temperature of the pickling solution is 40°C or higher and 90°C or lower and the pickling time (contact time between the hot-rolled steel sheet and the pickling solution) is 25 seconds or higher and 300 seconds or lower, preferably 40 seconds or higher and 300 seconds or lower. Pickling may be performed only once or may be performed multiple times.

[0049] In addition, in conventionally known pickling processes, even if the detergency of the pickling itself is strengthened by, for example, increasing the acid concentration of the pickling solution or raising the pickling temperature, the adhesion of the oxides described above is not necessarily suppressed, and further, since the surface of the steel sheet after pickling is active and in a state in which oxides are likely to form, the area ratio of the surface oxide of the steel sheet finally produced is not necessarily reduced. In order to reduce the area ratio of the surface oxide of the steel sheet finally produced, as described above, it is considered effective to use a pickling solution containing a specific accelerator at a predetermined concentration or higher in the pickling process.

[0050] 3.1.4 Rinsing Step In the rinsing step, the hot-rolled steel sheet after pickling is rinsed with water. According to the findings of the present inventors, if water with high electrical conductivity is used in the rinsing step, an oxidation-reduction reaction occurs on the surface of the steel sheet, and oxides are likely to form on the surface of the steel sheet. In this regard, in the manufacturing method according to the first embodiment, it is preferable to rinse the steel sheet with water having an electrical conductivity of 200 mS / m or less in the rinsing step, thereby suppressing the formation of oxides on the surface of the steel sheet. The electrical conductivity of the water used in the rinsing step is more preferably 100 mS / m or less, even more preferably 50 mS / m or less, and particularly preferably 20 mS / m or less.

[0051] 3.1.5 Drying Step In the drying step, the steel sheet is dried after water rinsing. There are no particular restrictions on the atmosphere during drying, but it is particularly preferable to carry out drying using nitrogen with a dew point of -40°C or less. By carrying out drying using a gas with a low dew point (gas with little moisture), surface oxidation of the steel sheet is suppressed, and the area ratio of oxides on the surface of the steel sheet can be further reduced.

[0052] By undergoing the steps according to the first embodiment described above, the area ratio of oxides on the surface of the steel sheet containing Ni, Cu, and Sn becomes 25% or less. Thus, when chemical conversion treatment is performed on a steel sheet with reduced surface oxides, a chemical conversion coating can be more appropriately formed on the surface of the steel sheet compared to when chemical conversion treatment is performed on a steel sheet with a large amount of surface oxides. Details of the chemical conversion treatment are known. The chemical conversion treatment may be, for example, a zinc phosphate treatment. Furthermore, the chemical conversion treatment may include, for example, a surface conditioning treatment using a powder surface conditioning such as Ti colloid or a zinc compound-based liquid surface conditioning as a pretreatment. Thus, when a steel sheet with reduced surface oxides by predetermined pickling and water washing is subjected to chemical conversion treatment, and then a coating film is formed on the surface of the steel sheet by electrodeposition coating or the like, the adhesion of the coating film on the surface of the steel sheet can be significantly improved compared to when the same treatment is performed on a steel sheet with a large amount of surface oxides.

[0053] 3.2 Second Form A method for producing a steel sheet according to a second form may include, for example, a casting step of casting molten steel having an adjusted chemical composition to obtain a steel slab, a hot rolling step of hot-rolling the steel slab to obtain a hot-rolled steel sheet, a first pickling step of pickling the hot-rolled steel sheet, a cold-rolling step of cold-rolling the hot-rolled steel sheet after the first pickling to obtain a cold-rolled steel sheet, an annealing step of annealing the cold-rolled steel sheet, a second pickling step of pickling the annealed steel sheet, and a water-rinsing step of rinsing the steel sheet after the second pickling, and preferably further includes a drying step of drying the steel sheet after the water-rinsing. In this case, after the annealing step, the steel sheet may be optionally shaped and degreased, and then subjected to the second pickling step and the water-rinsing step (and drying step) to appropriately adjust the surface properties of the steel sheet, followed by subjecting the surface of the steel sheet to a chemical conversion treatment, and then adhering a coating film to the surface of the steel sheet.

[0054] 3.2.1 Casting Process and Hot Rolling Process The casting process and hot rolling process are the same as those in the first embodiment.

[0055] 3.2.2 First Pickling Step There are no particular restrictions on the pickling conditions in the first pickling step. The first pickling step may be the same as the pickling step according to the first embodiment described above, or may be the same as a conventionally known general pickling step. The hot-rolled steel sheet after the first pickling may be subjected to water rinsing and drying. There are no particular restrictions on the conditions for water rinsing and drying, and the first pickling step may be the same as the water rinsing step and drying step according to the first embodiment described above, or may be the same as a conventionally known general water rinsing step and drying step.

[0056] 3.2.3 Cold Rolling Step In the cold rolling step, the hot-rolled steel sheet after the first pickling is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness. The reduction ratio of the cold rolling may be, for example, 20% or more and 80% or less. After the cold rolling step, the steel sheet may be cooled to room temperature, for example, by air cooling.

[0057] 3.2.4 Annealing Step In the annealing step, the cold-rolled steel sheet is annealed. The annealing step may include, for example, heating the cold-rolled steel sheet to a temperature of 700°C or higher and 950°C or lower in an atmosphere with a dew point of -40°C or higher and 20°C or lower, and holding the temperature for 0 seconds or higher and 300 seconds or lower. The atmosphere in the annealing step may be a reducing atmosphere, more specifically, a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (e.g., 4% hydrogen and the balance nitrogen).

[0058] 3.2.5 Second Pickling Step In the second pickling step, the annealed steel sheet is pickled. In the second pickling step, conditions are adopted that reduce the area ratio of oxides on the surface of the steel sheet. That is, in the second pickling step, as in the pickling step according to the first embodiment, a pickling solution containing a specific accelerator at a predetermined concentration or higher is preferably used. Details are as described above. Note that, before the second pickling step, the steel sheet may be optionally formed, degreased, or the like. Known methods may be used for forming the steel sheet and degreasing it.

[0059] 3.2.6 Water-washing step and drying step The water-washing step and drying step according to the second embodiment are preferably the same as those according to the first embodiment. That is, it is preferable to perform water-washing using water with low electrical conductivity, and then perform drying under a low dew point. Details are as described above.

[0060] By going through the steps according to the second embodiment, the area ratio of oxides on the surface of the steel sheet containing Ni, Cu, and Sn is reduced to 25% or less. By performing a chemical conversion treatment on the steel sheet with reduced surface oxides and then forming a coating film on the steel sheet surface, the adhesion of the coating film can be significantly improved.

[0061] 3.3 Supplementary Note: As described above, both the first and second manufacturing methods are characterized by appropriately controlling the surface oxides of the steel sheet before chemical conversion treatment. In other words, various processes can be adopted as a manufacturing process for the steel sheet as long as the surface oxides of the steel sheet can be appropriately controlled before chemical conversion treatment. For example, after obtaining a hot-rolled steel sheet or a cold-rolled steel sheet containing Ni, Cu, and Sn, the above-mentioned specific pickling process and the above-mentioned specific water-washing process can be performed before chemical conversion treatment of the hot-rolled steel sheet or the cold-rolled steel sheet, preferably by performing the above-mentioned specific pickling process, the above-mentioned specific water-washing process, and the above-mentioned specific drying process. This makes it possible to appropriately control the oxides on the surface of the steel sheet before chemical conversion treatment.

[0062] 4. Parts (e.g., automobile parts) The parts of the present disclosure are characterized by including the steel sheet according to the above-described embodiment. The steel sheet according to the embodiment can be used as an automobile part, for example, after a chemical conversion coating or paint film is optionally formed on its surface. That is, the automobile part according to the embodiment is characterized by including the steel sheet according to the present disclosure. Whether an automobile part having a paint film or a chemical conversion coating includes the steel sheet according to the present disclosure can be determined by removing the paint film or the chemical conversion coating from the automobile part. In this case, the paint film removal step and the chemical conversion coating removal step are as follows.

[0063] 4.1 Paint Removal Process The paint film is removed from a sample cut from an automobile body under the following conditions to expose the steel sheet. A paint remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface at room temperature and allowed to stand for approximately 5 minutes. The paint film is then removed by rubbing with a hard sponge or similar (e.g., Kanefiel, manufactured by AION Co., Ltd.). The sample is then rinsed with water and dried. The remaining paint film is then confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after rinsing and drying. In the element distribution image obtained by EPMA, regions with a carbon concentration of 10% by mass or more are identified, and if the area ratio of these regions is 5% or more, the paint film is deemed to have been insufficiently removed. To measure the area ratio of regions with a carbon concentration of 10% by mass or more, an element distribution image of carbon is first obtained using EPMA with a carbon concentration range of 10 to 30%. The area ratio is then measured by image processing of the element distribution image. Image processing was performed using the image processing software "ImageJ." After loading the C element distribution image into ImageJ, "Make Binary" in "Binary" under "Process" was used to binarize the image so that areas with a C concentration of 10% or more were displayed in black and areas with a C concentration of less than 10% were displayed in white. After binarization, "Measure" under "Analyze" was used to read the value for "Area fraction" in "Results," which was taken as the area fraction of areas with a C concentration of 10% or more. If the coating film was not sufficiently removed, the coating film was repeatedly removed until the area fraction of areas with a C concentration of 10% by mass or more was less than 5%.

[0064] 4.2 Chemical Conversion Coating Removal Process The chemical conversion coating is removed from a sample cut from an automobile body and the coating removed, according to JIS K3151. Specifically, the sample is immersed in a 5% chromic acid aqueous solution heated to 75°C for 15 minutes to remove the chemical conversion coating. The sample is then rinsed and dried. It is preferable to rinse with water having an electrical conductivity of 200 mS / m or less. The electrical conductivity of the water used for rinsing is more preferably 100 mS / m or less, even more preferably 50 mS / m or less, and particularly preferably 20 mS / s or less. The remaining state of chemical conversion crystals after rinsing and drying 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, areas with a P concentration of 5% or more by mass are identified, and if the area ratio of these areas is 5% or more, it is determined that the coating has not been sufficiently removed. To measure the area fraction of regions with a P concentration of 5% by mass or greater, first obtain an elemental distribution image of P using an EPMA with a P concentration range of 5-10%. The elemental distribution image is then processed to measure the area fraction. The image processing software "ImageJ" was used for image processing. After loading the P elemental distribution image into ImageJ, the image was binarized using "Make Binary" under "Process" and "Binary" so that regions with a P concentration of 5% or greater are displayed in black and regions with a P concentration of less than 5% are displayed in white. After binarization, "Measure" under "Analyze" was used, and the numerical value for "Area fraction" in "Results" was taken as the area fraction of regions with a P concentration of 5% by mass or greater. If removal of the chemical conversion coating was insufficient, removal of the chemical conversion coating was repeated until the area fraction of regions with a P concentration of 5% by mass or greater was reduced to less than 5%.

[0065] When taking samples from automotive parts for various measurements and analyses, the following locations and areas (1) to (4) shall be avoided: (1) Welded parts: 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 (2) Processed parts: Processed parts with a curvature radius of less than 15 mm and locations within 5 mm from the processed parts (3) Edges: Edges within 5 mm from the cut end surface of the part (4) Red rust: Locations within 5 mm from locations where red rust is visible to the naked eye

[0066] 5. Other The steel sheet of the present disclosure has excellent paint adhesion and allows a paint film to be formed uniformly on its surface. In this regard, the technology of the present disclosure also has an aspect of a surface-treated steel sheet having the above-mentioned steel sheet and a chemical conversion coating and a coating formed on at least a portion of the surface of the steel sheet. The steel sheet may not have a plating layer. Known chemical conversion coatings and known coatings may be used as the chemical conversion coating and the coating. Furthermore, the technology of the present disclosure is highly effective even when the Cr content of the steel sheet is low. The Cr content of the steel sheet may be 0% or more, 0.001% or more, 0.010% or more, 0.020% or more, or 0.030% or more, or may be 1.000% or less, 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0067] The present invention will be described in more detail below with reference to examples. However, 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. In the following examples, steel sheets according to one embodiment were manufactured under various conditions, and the properties of the manufactured steel sheets were investigated.

[0068] 1. Production of Steel Plates 1.1 Casting Process Molten steel having the chemical composition shown in Table 1 below was continuously cast to obtain steel billets.

[0069] 1.2 Hot Rolling Process After cooling the slab, it was reheated to 1200°C and hot rolled. The hot rolling was carried out by rough rolling and finish rolling, the finish rolling temperature was 900 to 1050°C, and the finish rolling reduction was 30%.

[0070] 1.3 Pickling The hot-rolled steel sheet was coiled at 580°C and then subjected to pickling. In the pickling, a pickling solution containing 10% hydrochloric acid as the acid and thioglycolic acid as an accelerator at the concentration shown in Table 2 was used. The temperature of the pickling solution during pickling was 90°C, and the pickling time was 80 seconds.

[0071] 1.4 Water Rinsing After pickling, the hot-rolled steel sheets were rinsed using rinse water having the electrical conductivity shown in Table 2. "Rinse water" refers to water used to rinse the hot-rolled steel sheets. The electrical conductivity of the rinse water was adjusted by a known method, such as by mixing ion-exchanged water. The temperature of the rinse water used to rinse the hot-rolled steel sheets was 18°C, and the rinsing time was 30 seconds. After rinsing, the steel sheets were dried using a blower. Here, only in Example 48 below, drying was performed by blowing nitrogen with a dew point of -40°C or lower. The dried steel sheets were subjected to the chemical conversion treatment described below, and the paint film adhesion on the steel sheet surface was evaluated.

[0072]

[0073] 2. Evaluation of Steel Sheets 2.1 Area Ratio of Oxide on Steel Sheet Surface The "area ratio of oxide on the steel sheet surface" was measured by performing elemental analysis on the steel sheet surface using EPMA. Details of the measurement method using EPMA are as described above. The results are shown in Table 2 below.

[0074] 2.2 Amount of S on the steel sheet surface The S intensity I on the surface of the steel sheet was measured by fluorescent X-rays. １ and S strength I on the surface at 1 / 4 of the plate thickness of the steel plate ２ and the intensity ratio I １ / I ２ The details of the measurement method using fluorescent X-rays are as described above. The results are shown in Table 2 below.

[0075] 2.3 Vickers Hardness The Vickers hardness of the steel sheets was measured. Details of the Vickers hardness measurement method are as described above. The results are shown in Table 2 below.

[0076] 2.4 Chemical conversion treatability 2.4.1 Chemical conversion treatment procedure A 50 mm square sample was cut from the above steel plate, and the sample was subjected to a zinc phosphate treatment (Valbond L3020, manufactured by Nihon Parkerizing Co., Ltd.) as a chemical conversion treatment by carrying out the following steps (1) to (6) in order: (1) Degreasing: Immersion in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes (2) Water rinsing (3) Liquid surface conditioning: Immersion in a surface conditioning agent (Preparen Z) at room temperature for 30 seconds (4) Chemical conversion treatment: Immersion in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes (5) Water rinsing (6) Drying

[0077] 2.4.2 Electrodeposition Coating After the chemical conversion treatment, the surface of a 50 mm square sample was subjected to electrodeposition coating under the following conditions to form a coating film: Electrodeposition liquid: Powernics Excel 1200 (manufactured by Nippon Paint Industrial Coatings Co., Ltd.) Electrodeposition temperature: 30°C Coating thickness: 16 μm Baking temperature: 170°C Baking time: 30 minutes

[0078] 2.4.3 Evaluation method A warm saltwater immersion test was carried out on the electrodeposition coated samples under the following conditions: Immersion liquid: 5% NaCl aqueous solution Immersion time: 1000 hours Immersion temperature: 55°C

[0079] After immersion, the coating film was peeled off from one side of the sample using tape, and the peeled tape was scanned to calculate the peeled area ratio of the coating film by binarization using image analysis software "ImageJ." Depending on the calculated peeled area ratio, the coating adhesion was evaluated according to the following evaluation criteria: Evaluation AAAA: Peeled area less than 1% Evaluation AAA: Peeled area ratio 1 to less than 5% Evaluation AA: Peeled area ratio 5 to less than 10% Evaluation A: Peeled area ratio 10 to less than 15% Evaluation B: Peeled area ratio 15% or more

[0080] In this example, steel sheets with coating adhesion ratings of AAAA, AAA, AA, and A were evaluated as "steel sheets containing Ni, Cu, and Sn and having improved coating adhesion." The results are shown in Table 2 below.

[0081]

[0082] The results shown in Tables 1 and 2 reveal the following.

[0083] As shown in Comparative Examples 41 to 45, when pickling is performed using a pickling solution containing a low concentration of an accelerator, or when the pickling time is short as shown in Comparative Example 50, the area ratio of oxides on the surface of the steel sheet becomes excessive, which deteriorates the subsequent chemical conversion treatability and the paint film adhesion on the surface of the steel sheet.

[0084] As shown in Examples 1 to 40 and 46 to 49, when the area ratio of oxides on the surface of the steel sheet is 25% or less, the subsequent chemical conversion treatability is good and the adhesion of the paint film on the surface of the steel sheet is improved. Oxides on the surface of the steel sheet are thought to hinder chemical conversion treatment, and reducing the area ratio of oxides on the surface of the steel sheet to 25% or less is thought to improve the chemical conversion treatability and, as a result, the paint film adhesion.

[0085] The results of Examples 1 to 40 and 46 to 49 show that the effect of improving paint adhesion on steel sheets containing Ni, Cu, and Sn simultaneously is achieved regardless of the composition of elements other than Ni, Cu, and Sn.

[0086] As shown in Examples 1 to 40 and 46 to 49, the Si intensity I on the surface of the steel plate containing Ni, Cu and Sn simultaneously １ Increase the intensity ratio I １ / I ２ is controlled to 1.01 or more, preferably 1.08 or more, more preferably 1.15 or more), thereby １ In comparison with the case where the temperature is low, the adhesion of the coating film to the steel sheet surface tends to be further improved.

[0087] In particular, the area ratio of oxides on the surface of a steel sheet containing Ni, Cu, and Sn simultaneously is controlled to 25% or less, and the Si intensity ratio I １ / I ２ It can be said that by controlling the value of the tensile strength to 1.15 or more, the adhesion of the coating film to the steel sheet surface can be further significantly improved.

[0088] In addition, in Example 48, the drying after pickling of Example 47 was carried out with nitrogen at -40°C, but compared to Example 47, the area ratio of the surface oxide was reduced and the coating adhesion was further improved.

Claims

1. A steel plate having a chemical composition, in mass%, of Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and an area ratio of oxides on the surface of the steel plate is 25% or less.

2. The steel sheet according to claim 1, wherein the area ratio of the oxide is 15% or less.

3. The steel sheet according to claim 1, wherein the area ratio of the oxide is 5% or less.

4. S strength I on the surface of the steel plate １ and S strength I at 1 / 4 of the thickness of the steel plate ２ Relative to I １ / I ２ The steel sheet according to any one of claims 1 to 3, wherein is 1.01 or more.

5. S strength I on the surface of the steel plate １ and S strength I at 1 / 4 of the thickness of the steel plate ２ Relative to I １ / I ２ The steel sheet according to any one of claims 1 to 3, wherein is 1.08 or more.

6. S strength I on the surface of the steel plate １ and S strength I at 1 / 4 of the thickness of the steel plate ２ Relative to I １ / I ２ The steel sheet according to any one of claims 1 to 3, wherein is 1.15 or more.

7. The steel sheet according to any one of claims 1 to 6, having a Vickers hardness of 200 Hv or more.

8. The steel sheet according to any one of claims 1 to 7, wherein the chemical composition comprises, in mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%.

9. A part, characterized in that it comprises a steel sheet according to any one of claims 1 to 8.

Images