Steel sheet and component comprising same

Abstract

The present invention provides: a steel sheet which contains Ni, Cu, and Sn, and has excellent coating film adhesion; and a component which comprises the same.　A steel sheet according to the present invention and a component which comprises the same are characterized by having a chemical composition that contains, by mass%, 0.010-1.000% of Ni, 0.010-1.000% of Cu, and 0.003-1.000% of Sn, and are also characterized in that, in the X-ray diffraction pattern of the surface of this steel sheet, the peak half-value width of the (310) plane of the ferrite phase is 0.5° or more.

Description

Steel plates and parts containing them

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

[0002] In order to improve the paint film adhesion to steel sheets, it is effective to enhance the chemical conversion treatability of the steel sheet surface and to form a uniform chemical conversion coating on the steel sheet surface.

[0003] In this regard, for example, 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 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 a steel sheet is usually prevented by an extremely low concentration of oxygen and / or an inert atmosphere gas with an extremely low dew point around the steel sheet, the steel sheet is actively exposed to an oxidizing atmosphere to oxidize not only Si and Mn but also the iron of the steel sheet, and by pickling after leaving an annealing furnace, the oxide films of Si, Mn, etc. are removed together with the oxide film of iron of the steel sheet by pickling, thereby obtaining a high-strength cold-rolled steel sheet that is free from "scales" and has good chemical conversion treatability even if the contents of Si, Mn, etc. are high.

[0004] Patent Document 2 also describes 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 further teaches that the above-described configuration makes it possible to provide a steel sheet with excellent chemical conversion treatability, since 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.

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

[0006] The above-mentioned 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. 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 and having excellent paint film adhesion, and a part including the steel sheet.

[0008] The present invention includes at least the following aspects.

[0009] (Aspect 1) A steel sheet, characterized in that the steel sheet has a chemical composition containing, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and in that, in an X-ray diffraction pattern of the steel sheet surface, the peak half width of the (310) plane of the ferrite phase is 0.5° or more.

[0010] (Embodiment 2) The steel sheet according to the above embodiment 1, wherein the peak half width is 1.0° or more.

[0011] (Embodiment 3) The steel sheet according to the above embodiment 1, wherein the peak half width is 1.3° or more.

[0012] (Aspect 4) The steel sheet according to any one of Aspects 1 to 3, characterized in that in an element distribution image of a cross section of the steel sheet obtained by an electron microprobe analyzer, a specific element-containing portion containing at least one of the Ni, Cu, and Sn in a solid solution state is present in a region from the surface of the steel sheet to a depth of 5 μm.

[0013] (Aspect 5) The steel sheet according to any one of Aspects 1 to 4, wherein the chemical composition further includes Si, and the thickness of silicon oxide is 10 nm or less when measured by X-ray photoelectron spectroscopy on the surface of the steel sheet.

[0014] (Embodiment 6) A component comprising the steel sheet according to any one of embodiments 1 to 5.

[0015] According to the present invention, it is possible to provide a steel sheet containing Ni, Cu, and Sn and having excellent paint film adhesion, and a part including the steel sheet.

[0016] As described 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 solid solution in a steel sheet, 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 such cases, the chemical conversion treatability of the steel sheet may be 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.

[0017] Two commonly known methods for producing steel include, 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. Steel produced by the former method, i.e., blast furnace steel, may contain elements such as Ni, Cu, and Sn as additive elements. Therefore, if these elements are present, the above-mentioned problems must be addressed appropriately. On the other hand, steel produced by the latter method, i.e., electric furnace steel, uses scrap material as the main raw material, as described above. Therefore, the steel contains relatively large amounts of scrap-derived elements such as Ni, Cu, and Sn (so-called tramp elements), and is likely to contain the three elements Ni, Cu, and Sn simultaneously. Therefore, the above-mentioned problems are particularly pronounced in electric furnace steel.

[0018] Therefore, the present inventors conducted research, focusing particularly on the surface condition of the steel sheet, in order to provide a steel sheet having excellent paint adhesion even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, the present inventors discovered that by creating a specific strain state on the surface of a steel sheet containing Ni, Cu, and Sn, the chemical conversion treatability of the steel sheet surface is improved, resulting in excellent paint adhesion. Specifically, the present inventors discovered that by creating a specific strain state on the surface of a steel sheet containing Ni, Cu, and Sn, such that the peak half-width of the (310) plane of the ferrite phase in an X-ray diffraction pattern is 0.5° or more, the chemical conversion treatability of the steel sheet surface is improved and paint adhesion can be significantly improved. Here, in this specification, the term "steel sheet surface" refers to the surface of the steel sheet itself. Therefore, for example, in the case of a plated steel sheet having a plating layer formed on the surface of a base steel sheet, the term "steel sheet surface" refers to the surface of the base steel sheet, not the surface of the plated steel sheet.

[0019] The present invention has been completed based on the above findings, and includes the following embodiments.

[0020] Hereinafter, preferred embodiments of the steel sheet of the present invention will be described in detail.

[0021] <Steel Sheet> A steel sheet according to one embodiment of the present invention 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%. The steel sheet according to this embodiment has a characteristic configuration in which, in an X-ray diffraction pattern of the steel sheet surface, the peak half-width of the (310) plane of the ferrite phase is 0.5° or more. That is, the steel sheet according to this embodiment has a specific strain state in which a certain amount or more of various types of strain are introduced in a dispersed state at the steel sheet surface, such that the peak half-width of the (310) plane of the ferrite phase is 0.5° or more.

[0022] Strain, in other words, is a dislocation or a sub-grain boundary, and is a relatively weak defect called a planar defect or a line defect, so it is susceptible to corrosion. Therefore, if such strain is introduced into the steel sheet surface, it can promote the anodic dissolution (i.e., etching) of Fe in the chemical conversion treatment solution during chemical conversion treatment. In general, in chemical conversion treatment, electrons are generated by the anodic dissolution (etching) of Fe, and on the other hand, the electrons generated by the anodic dissolution of Fe at the cathode site cause a cathodic reaction (2H ＋ +2e - →H ２ , 10H ＋ +NO ３ - +8e - →NH ４ ＋ +3H ２ In connection with this, the pH of the chemical conversion treatment solution in the vicinity of the steel sheet surface increases, and as a result, compounds such as zinc phosphate crystals that constitute the chemical conversion treatment film are precipitated on the steel sheet surface.

[0023] As described above, the steel sheet of this embodiment has a specific strain state on the surface of the steel sheet, in which the peak full width at half maximum of the (310) plane of the ferrite phase is 0.5° or more, and therefore the steel sheet surface can be uniformly and finely etched during chemical conversion treatment, thereby exhibiting good chemical conversion treatability. As a result, the steel sheet of this embodiment can significantly improve paint adhesion.

[0024] The steel sheet of this embodiment includes not only electric furnace steel that inevitably contains Ni, Cu, and Sn as tramp elements, but also blast furnace steel that contains Ni, Cu, and Sn as essential elements or optional added elements. Furthermore, the steel sheet of this embodiment can exhibit 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 of this embodiment is particularly useful in the automotive field, where excellent paint adhesion and / or corrosion resistance are required.

[0025] Hereinafter, each component of the steel sheet of this embodiment will be described in detail.

[0026] [Ferrite Phase (310) Plane Peak Half-Width: 0.5° or More] The steel sheet of this embodiment has a characteristic configuration in which the peak half-width of the ferrite phase (310) plane in the X-ray diffraction pattern of the steel sheet surface is 0.5° or more. The peak half-width of the diffraction pattern obtained by X-ray diffraction (XRD) is one of the evaluation parameters for the crystal lattice defect density in a metal structure, etc., and can quantitatively evaluate the strain state of the steel sheet surface. When the amount of strain is large, the crystal lattice distance expands or contracts from its original length, and this behavior is reflected in the peak half-width obtained by XRD. That is, the greater the amount of strain, the greater the number of crystal lattice distances that are longer or shorter than their original length, resulting in a larger peak half-width. In this embodiment, the peak half-width of the ferrite phase (310) plane in the diffraction pattern obtained by XRD is used to determine the crystal lattice defect density, which serves as an indicator of the strain state for obtaining good chemical conversion treatability. That is, in this embodiment, by limiting the strain state to a specific state in which the peak full width at half maximum of the (310) plane of the ferrite phase is 0.5° or more, the steel sheet surface can be uniformly and finely etched during chemical conversion treatment, and good chemical conversion treatability can be exhibited.

[0027] The peak half width of the (310) plane of the ferrite phase in the diffraction pattern obtained by the above-mentioned XRD may be 0.5° or more on at least one of the front and back surfaces of the steel sheet.

[0028] Although peaks such as the (110) plane can be considered as peaks to be used as indicators of the strain state of the steel sheet surface, the measured peak half-width saturates as the amount of strain increases, and this may not be appropriate as an indicator of the strain state of the steel sheet surface. On the other hand, the (310) plane is appropriate as an indicator of the strain state of the steel sheet surface because the plane index of the peak half-width itself is high, there is little measurement error, and the peak half-width also increases as the amount of strain increases.

[0029] The peak of the (310) plane of the ferrite phase is a peak whose center is located at 2θ = 116.4 ± 0.5. The peak half width of the (310) plane of the ferrite phase means the width at half the peak height of the (310) plane.

[0030] In this embodiment, the peak half width of the (310) plane of the ferrite phase is preferably 1.0° or more, and more preferably 1.3° or more, in order to obtain better chemical conversion treatability.

[0031] The peak half-width of the (310) plane of the ferrite phase in the X-ray diffraction pattern was measured using an X-ray diffractometer (manufactured by Rigaku Corporation, "Ultima III") and a Cu—K α The full width at half maximum of the peak of the (310) plane of the ferrite phase can be determined by measuring the width at half the peak height of the (310) plane of the ferrite phase from the X-ray diffraction pattern obtained by XRD using 1.6 kW X-rays (wavelength λ=1.54 Å) and an X-ray source load power of 40 kV / 40 mA. Specific methods and conditions for measuring the full width at half maximum of the peak of the (310) plane of the ferrite phase will be described later.

[0032] The means for bringing the steel sheet surface into the above-mentioned specific strain state is not particularly limited, but it is preferable to use, for example, a shot blasting treatment. Specific means and conditions for bringing the steel sheet surface into the above-mentioned specific strain state will be described later.

[0033] [Chemical Composition] In this embodiment, 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 and having excellent paint adhesion. This objective is achieved by having the steel sheet surface have the above-mentioned specific strain state, i.e., a specific strain state in which the peak half-width of the (310) plane of the ferrite phase is 0.5° or more. 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%. It is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the objectives of the present invention.

[0034] The chemical composition of the steel plate according to one embodiment of the present invention may include, in addition to Ni, Cu, and Sn, any alloying elements that are commonly added in the technical field of the present invention in appropriate amounts.

[0035] Hereinafter, the chemical compositions that can be employed in the steel sheet of this embodiment will be described in detail. However, 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.

[0036] For example, the steel plate of this embodiment has, 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 : 0 to 0.150%, 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 of these elements will be described in more detail below.

[0037] [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.

[0038] [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.

[0039] [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.

[0040] [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.

[0041] [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 promote the formation of oxides, particularly Si-based surface oxides and iron oxides, on the steel sheet surface. 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.

[0042] [Sn: 0.003 to 1.000%] Sn is an element effective in improving corrosion resistance. To fully obtain this effect, the Sn content is preferably 0.003% or more. The Sn content may be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, excessive Sn content may promote the formation of oxides, particularly Si-based surface oxides and iron oxides, on the steel sheet surface. 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.

[0043] [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.

[0044] [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 to occur 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.

[0045] [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, 0.0050% or less, or 0.0030% or less.

[0046] [O: 0.0100% or less] O is an impurity that is mixed in during the manufacturing process. O is an element that forms coarse inclusions and reduces the workability of steel sheets. Although O can be considered to be included in impurities, the O content will be described in detail below. 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. The O content may be 0.0005% or more or 0.0010% or more. On the other hand, excessive O content may form coarse inclusions, as described above, and reduce 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.

[0047] 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.

[0048] [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 such effects, 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.

[0049] [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 obtain 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.

[0050] [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, excessive inclusion of these elements saturates the effect, and excessive inclusion in 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.

[0051] [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. 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. The REM content is the total content of these elements.

[0052] [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.

[0053] [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.

[0054] [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.

[0055] 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.

[0056] 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.

[0057] [Specific element-containing portion in region from steel sheet surface to a depth of 5 μm] Furthermore, the steel sheet of this embodiment may have a specific element-containing portion containing at least one of Ni, Cu, and Sn in a solid solution state in a region from the steel sheet surface to a depth of 5 μm in an element distribution image of the steel sheet cross section obtained by an electron probe microprobe analyzer (EPMA). Here, containing at least one specific element of Ni, Cu, and Sn in a solid solution state means that the specific element is contained in the steel sheet at a concentration of 0.3 mass% or more when the distribution of elements in the steel sheet cross section is analyzed by area analysis using the element distribution image obtained by EPMA.

[0058] The specific element-containing portion is a region also referred to as an enriched portion, and refers to an enriched region in which the concentration of at least one element selected from Ni, Cu, and Sn is 0.3 mass% or more when the distribution of elements in the region from the surface of the steel sheet to a depth of 5 μm is analyzed using an EPMA element distribution image. For example, when the region is analyzed using an EPMA element distribution image, the Ni concentration is 0.3 mass% or more, i.e., the Ni-enriched region (also referred to as a Ni-enriched portion) is the specific element-containing portion.

[0059] (Method for measuring the concentration of specific elements in a region from the steel sheet surface to a depth of 5 μm using EPMA) When performing area analysis of an element distribution image of a steel sheet cross section using EPMA, first, a region from the steel sheet surface to a depth of 5 μm in the steel sheet cross section obtained by cutting the steel sheet in the sheet thickness direction is photographed using an EPMA (for example, JXA-8500 manufactured by JEOL Ltd.) at a magnification of 1000 times under conditions of an acceleration voltage of 15 kV and an irradiation current of 5 × 10 A to obtain a mapping image. Next, the obtained element distribution image is binarized (min = 0, max = 255) using image analysis software "ImageJ" and area analysis is performed with pixels of 0.25 μm × 0.25 μm, and quantitative analysis of Ni, Cu, and Sn is performed to determine the concentrations of these specific elements in the above region.

[0060] As described above, when Ni, Cu, and Sn are present in the form of solid solution in a steel sheet, the potential of the steel sheet becomes more noble than when these elements are not present in the steel sheet, which reduces the etching ability of Fe during chemical conversion treatment and, in turn, may result in a decrease in the chemical conversion treatability of the steel sheet.

[0061] However, the steel sheet of this embodiment has a specific strain state on the surface of the steel sheet in which the peak half width of the (310) plane of the ferrite phase is 0.5° or more. Therefore, even if the steel sheet has a specific element-containing portion containing at least one of Ni, Cu, and Sn in a solid solution state in a region up to a depth of 5 μm from the surface of the steel sheet, the steel sheet can exhibit good chemical conversion treatability and, as a result, excellent paint film adhesion.

[0062] [Silicon oxide (SiO ２ ) thickness: 10 nm or less] In addition, the steel sheet of this embodiment may contain Si in the chemical composition. In this case, the steel sheet of this embodiment may contain silicon oxide (SiO ２) is preferably 10 nm or less. When the chemical composition of the steel sheet contains Si, silicon oxide may be formed on the steel sheet surface. Silicon oxide is a surface oxide that particularly reduces chemical conversion treatability, so by reducing this silicon oxide to a certain amount or less, good chemical conversion treatability can be more reliably obtained. As a result, the steel sheet of this embodiment can more reliably exhibit excellent paint film adhesion.

[0063] The thickness of silicon oxide measured on the steel sheet surface by XPS is preferably 9 nm or less, 8 nm or less, or 7 nm or less, in order to more reliably obtain good chemical conversion treatability. The lower limit of the thickness of silicon oxide measured on the steel sheet surface by XPS is not particularly limited, and may be 0 nm, i.e., no silicon oxide is present, or may be more than 0 nm, or may be 1 nm or more.

[0064] The XPS measurement is carried out as follows: First, an evaluation material for specifying the thickness of silicon oxide, which is a surface oxide, and a reference base material are prepared.

[0065] Here, the evaluation material is a steel plate for which the thickness of silicon oxide is to be specified, cut out from a target product or the like. The evaluation material is one from which oil and dirt have been removed from the surface without changing the thickness of silicon oxide. Specifically, if oil has been applied to the surface of the steel plate to be evaluated, the evaluation material is obtained after removing the oil by an appropriate method that does not cause surface oxidation of the steel plate. Note that an example of an appropriate method that does not cause surface oxidation of the steel plate is a method of removing the oil using a solvent.

[0066] On the other hand, the base material is a steel plate that has been ground and / or polished to a depth of approximately 100 to 500 μm from the surface of the steel plate, and the arithmetic mean roughness Ra of the surface has been adjusted 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 using a wet method using distilled water or ethanol.

[0067] For each of the evaluation material and base material prepared as described above, the maximum strength of the steel sheet surface is measured in the bond energy range of 532.9±0.4 eV. If the value of the maximum strength of the evaluation 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 evaluation material. Next, the evaluation material is subjected to XPS measurement at 1 nm intervals in the thickness direction by sputtering. The thickness at which the value of the maximum strength of the evaluation material / the maximum strength of the base material becomes less than 1.2 is determined to be the thickness of silicon oxide.

[0068] 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

[0069] When the above-mentioned XPS measurement is performed using a sample obtained from an automobile, the coating film and chemical conversion coating film are removed from the sample according to the following method for removing the coating film and chemical conversion coating film before the XPS measurement is performed.

[0070] (Method for Removing Paint Film and Chemical Conversion Coating) In order to remove the paint film and chemical conversion coating from the sample obtained from the automobile, the following (1) paint film removing step and (2) chemical conversion coating removing step are carried out in this order.

[0071] (1) Paint Removal Process: A paint remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface of a sample cut from an automobile body at room temperature and allowed to stand for 5 minutes. The paint remover-coated surface of the sample is then rubbed with a hard sponge (e.g., "Kanefeel," manufactured by AION Co., Ltd.) to remove the paint from the sample surface. The sample surface after the paint removal is then washed with water and dried. The remaining state of the paint film is confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after washing and drying. In the element distribution image obtained by EPMA, regions with a C concentration of 10% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the paint film has not been sufficiently removed. To measure the area ratio of regions with a C concentration of 10% by mass or more, first obtain an element distribution image of C using EPMA with a C concentration range of 10 to 30%. Next, the obtained C element distribution image is subjected to image processing to measure the area fraction. Image analysis software "ImageJ" is used for image processing. Specifically, the C element distribution image is loaded into ImageJ, and then binarized using "Make Binary" in "Binary" under "Process" so that regions where the C concentration is 10% by mass or more are displayed as black, and regions where the C concentration is less than 10% by mass are displayed as white. After binarization, "Measure" under "Analyze" is used to read the value of "Area fraction" in "Results." This read value is determined as the area fraction of the region where the C concentration is 10% by mass or more. If the coating film is not sufficiently peeled off, the coating film is repeatedly removed until the area fraction of the region where the C concentration is 10% by mass or more becomes less than 5%.

[0072] (2) Chemical Conversion Coating Removal Process: A sample cut from an automobile body and having the coating removed is subjected to a method in accordance with JIS K 3151:1996 to remove the chemical conversion coating from the sample surface. Specifically, the sample after coating removal is immersed in a 5% chromic acid solution heated to 75°C for 15 minutes to remove the chemical conversion coating from the sample surface. Next, the surface of the sample after chemical conversion coating removal is washed with water and dried. At this time, the state of remaining chemical conversion crystals is confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after washing and drying. In the element distribution image obtained by EPMA, regions with a P concentration of 5% by mass or more are identified, and if the area ratio of such regions is 5% or more, it is determined that the chemical conversion coating has not been sufficiently removed. To measure the area ratio of regions with a P concentration of 5% by mass or more, first obtain an element distribution image of P using EPMA with a P concentration range of 5 to 10%. Next, the obtained P element distribution image is subjected to image processing to measure the area fraction. Image analysis software "ImageJ" is used for image processing. Specifically, the P element distribution image is loaded into ImageJ, and then binarized using "Make Binary" in "Binary" under "Process" so that regions with a P concentration of 5% by mass or more are displayed as black and regions with a P concentration of less than 5% by mass are displayed as white. After binarization, "Measure" under "Analyze" is used to read the value of "Area fraction" in "Results." This read value is determined as the area fraction of the region with a P concentration of 5% by mass or more. If the chemical conversion coating is not sufficiently removed, removal of the chemical conversion coating is repeated until the area fraction of the region with a P concentration of 5% by mass or more becomes less than 5%.

[0073] The above-described method for removing coatings and chemical conversion coatings is not limited to XPS measurements, and can also be applied when various measurements and analyses are performed using samples obtained from automobiles. For example, when performing the above-described XRD or EPMA measurements or analyzing the chemical composition of a steel sheet using a sample obtained from an automobile, the coatings and chemical conversion coatings can be removed from the sample according to the above-described method for removing coatings and chemical conversion coatings, and then the XRD or EPMA measurement or chemical composition analysis can be performed.

[0074] The means for reducing the amount of silicon oxide on the steel sheet surface to a certain level or less is not particularly limited, but examples thereof include a means for rinsing the steel sheet with a rinsing solution having low electrical conductivity when rinsing the steel sheet after shot blasting the surface of the steel sheet in the manufacturing process of the steel sheet, and a means for controlling the coiling temperature, which will be described later.

[0075] (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.

[0076] (Mechanical Properties) In the present embodiment, the strength of the steel sheet is not particularly limited, but for example, the steel sheet may have a Vickers hardness of 90 HV or more. The Vickers hardness of the steel sheet 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 of the Vickers hardness is not particularly limited, but for example, the Vickers hardness of the steel sheet may be 650 HV or less, 600 HV or less, 550 HV or less, or 500 HV or less.

[0077] The Vickers hardness is determined 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 cut-out thickness cross section of the test piece is polished using #600 to #1500 silicon carbide paper. Next, 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, the thickness cross section of the test piece is mirror-finished, and this thickness cross section is used as the measurement surface. Next, using a micro Vickers hardness tester, the Vickers hardness of the test piece is measured 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 thickness of the test piece, and the arithmetic average of these measurements is determined as the Vickers hardness of the steel plate.

[0078] <Parts> 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 that simultaneously contain 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 that require excellent paint adhesion and / or corrosion resistance. In particular, the steel sheet according to the embodiment of the present invention is useful for use in parts in the automotive field.

[0079] In a preferred embodiment, an automobile part including a steel sheet according to an embodiment of the present invention is provided. Examples of automobile parts include frame parts, bumpers, and other structural and reinforcing parts that require strength. Further, other examples of automobile parts include exterior panel parts such as roofs, hoods, fenders, and doors that require high designability. At least a portion of these parts may include a steel sheet according to an embodiment of the present invention. Therefore, at least a portion of these parts satisfies the characteristics of the steel sheet according to the embodiment described above. In a portion of the steel sheet that does not come into direct contact with a mold during forming such as press forming, or that comes into direct contact with the mold but is processed to a relatively low degree, the characteristics of the steel sheet do not change particularly before and after forming.

[0080] When taking samples from automotive parts for various measurements and analyses, the following locations (i) to (iv) shall be avoided. (i) Welded parts: Locations within 20 mm from the toes of spot welds and arc / laser welds. (ii) Processed parts: Processed parts with a curvature radius of less than 15 mm and locations within 5 mm from the processed parts. (iii) Edges: Edges within 5 mm from the cut end surface of the part. (iv) Red rust: Locations within 5 mm from locations where red rust is visible to the naked eye.

[0081] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to one 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 one embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0082] A steel sheet according to one embodiment of the present invention can be manufactured by a manufacturing method including, for example, a casting step of casting molten steel having an adjusted chemical composition to form a steel billet, a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet, a shot blasting step of performing shot blasting to impart strain to the steel sheet surface of the hot-rolled steel sheet, and a pickling step of pickling the hot-rolled steel sheet.

[0083] Preferred conditions for these steps will be described in detail below.

[0084] [Casting Step] In the method for producing a steel sheet according to this embodiment, the casting step is a step of casting molten steel having an adjusted chemical composition to form a steel slab. The conditions for the casting step are not particularly limited. For example, the casting step may involve melting in a blast furnace, an electric furnace, or the like, followed by various secondary smelting processes, and then casting by a method such as ordinary continuous casting or ingot casting.

[0085] [Hot Rolling Process] In the steel sheet manufacturing method of this embodiment, the hot rolling process is a process in which a steel slab is hot-rolled to obtain a hot-rolled steel sheet. The hot rolling process is carried out by hot-rolling a cast steel slab, either directly or after cooling, followed by reheating. 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%.

[0086] The hot-rolled steel sheet after finish rolling is coiled at a predetermined coiling temperature and subjected to the subsequent shot blasting treatment process. In the steel sheet manufacturing method of this embodiment, the hot-rolled steel sheet is coiled at a coiling temperature of 520°C or higher. By controlling the coiling temperature to 520°C or higher, a surface oxide is formed on the outside (surface) of the steel sheet, and an internal oxide is also formed in the inside (surface layer) of the steel sheet. In general, in the case of a steel sheet containing Si, the internal oxide is mainly composed of Si-based oxides. Therefore, a Si-depleted layer is formed directly below the internal oxide formed in the surface layer of the steel sheet, due to the consumption of Si in the steel due to the formation of the internal oxide.

[0087] In particular, by controlling the coiling temperature to 520°C or higher, the thickness of the Si-depleted layer can be controlled to 0.3 μm or higher. The surface oxides and internal oxides described above are removed in the shot blasting process and pickling process after coiling, and therefore, a Si-depleted layer having a thickness of 0.3 μm or higher remains on the surface of the hot-rolled steel sheet after these processes. By forming the surface of the hot-rolled steel sheet with a Si-depleted layer having a thickness of 0.3 μm or higher, the steel sheet surface is depleted in Si, and therefore the formation of Si-based surface oxides on the steel sheet surface can be sufficiently suppressed. As a result, better coating adhesion can be achieved.

[0088] From the viewpoint of further improving the coating adhesion, it is preferable to control the coiling temperature to 550°C or higher. By controlling the coiling temperature to 550°C or higher, it is possible to further promote the formation of internal oxides, which in turn makes it possible to make the Si-deficient layer thicker. As a result, it is possible to more significantly suppress the formation of Si-based surface oxides. There is no particular upper limit to the coiling temperature, but the coiling temperature may be, for example, 600°C or lower.

[0089] [Shot Blasting Step] In the steel sheet manufacturing method of this embodiment, the shot blasting step is a step of performing shot blasting to impart strain to the surface of the hot-rolled steel sheet. The treatment conditions in the shot blasting step are not particularly limited as long as they can bring the steel sheet surface into the above-mentioned specific strain state, i.e., the specific strain state in which the peak half-width of the (310) plane of the ferrite phase in the X-ray diffraction pattern is 0.5° or more. For example, the shot blasting may be performed using a shot material with an average particle size of 0.1 to 5.0 mm (for example, "TSH30" manufactured by IKK Shot Co., Ltd.) at a pressure of 5 kg / m ２ The shot blasting treatment may be carried out with a shot amount of 50 kg / m or more. ２ It is preferable that the saturation is 100 kg / m or more. ２ More preferably, it is 200 kg / m or more. ２ The upper limit of the projection amount is not particularly limited, but it is, for example, 800 kg / m ２ The projection speed of the blast material is not particularly limited, and may be, for example, 10 to 150 m / sec. Note that the projection amount required for removing scale (for example, the projection amount in the comparative examples of the present invention in the examples described later) is not enough to put the steel sheet surface into the above-mentioned specific strain state.

[0090] Furthermore, in this embodiment, a water-rinsing step may be carried out after the shot blasting treatment, in which the steel sheet surface is rinsed with water. For example, in this embodiment, a pickling step may be carried out after the shot blasting treatment, and then a water-rinsing step may be carried out. In the water-rinsing step, the steel sheet surface is preferably rinsed with wash water having an electrical conductivity of 80 mS / m or less. If such wash water having an electrical conductivity of 80 mS / m or less is used when rinsing the steel sheet surface, oxidation-reduction reactions are less likely to occur on the steel sheet surface during rinsing, and the generation of surface oxides that cause a decrease in chemical conversion treatability can be significantly suppressed. It is more preferable that the electrical conductivity of the wash water used for rinsing the steel sheet surface is 60 mS / m or less.

[0091] [Pickling Step] In the steel sheet manufacturing method of this embodiment, the pickling step is a step of pickling the hot-rolled steel sheet before or after the shot blasting step to remove surface oxides and internal oxides. The conditions for the pickling step are not particularly limited, and the pickling step may be performed using a commonly used pickling solution under conditions appropriate for removing the surface oxides and internal oxides. The pickling may be performed once, or may be performed multiple times to ensure that the surface oxides and internal oxides are removed.

[0092] The basic steps of the manufacturing method of the steel sheet of this embodiment are as described above. The steel sheet of this embodiment manufactured by the manufacturing method including the above steps has a steel sheet surface in the above-mentioned specific strain state, particularly due to the above-mentioned shot blasting step, i.e., a specific strain state in which the peak half-width of the (310) plane of the ferrite phase in the X-ray diffraction pattern is 0.5° or more. Therefore, during chemical conversion treatment, the steel sheet surface can be uniformly and finely etched, and good chemical conversion treatability can be exhibited. As a result, the steel sheet of this embodiment can exhibit significantly improved paint adhesion.

[0093] The method for manufacturing a steel sheet according to this embodiment may further include, in addition to the above-described steps, any processing steps that are generally carried out in methods for manufacturing a steel sheet.

[0094] The present invention is not limited to the above-described embodiments or the following examples, and appropriate combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention.

[0095] The present invention will be described in more detail below with reference to examples. However, the following examples are merely examples of the present invention, and the present invention is not limited to these examples in any way.

[0096] In the following examples, steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the resulting steel sheets were investigated.

[0097] 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 cooled once, reheated to 1200°C, and hot-rolled. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling end temperature being 900 to 1050°C, the coiling temperature being 520°C or higher, and the finish rolling reduction being 30%. Next, the obtained hot-rolled steel sheet was subjected to shot blasting treatment using TSH30 manufactured by IKK Shot Co., Ltd. as the shot material at the blast amount shown in Table 2. The steel sheet after shot blasting was pickled and then washed with wash water having the electrical conductivity shown in Table 2, thereby obtaining various steel sheets serving as examples or comparative examples.

[0098]

[0099]

[0100]

[0101] The various steel sheets obtained as described above were subjected to various measurements including Vickers hardness, XRD, EPMA, and XPS. The XRD measurements were carried out under the conditions shown in Table 3. Furthermore, the paint adhesion of the various steel sheets was evaluated according to the following evaluation method. The results of these measurements and evaluations are shown in Tables 1 and 2. The underlines next to the various values ​​in Table 2 indicate values ​​outside the range of the present invention. Furthermore, the SiO ２ The "-" in the thickness indicates SiO ２ indicates that no

[0102] [Evaluation of Paint Adhesion] Paint adhesion of steel sheets was evaluated as follows. First, a 50 mm x 50 mm sample of the steel sheet produced as described above was subjected to a zinc phosphate treatment as a chemical conversion treatment under the following conditions. Degreasing: The steel sheet was immersed in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes, followed by rinsing with water. Surface conditioning: The steel sheet was immersed in a surface conditioner (Preparen Z) at room temperature for 30 seconds. Chemical conversion treatment: The steel sheet was immersed in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, followed by rinsing with water and drying.

[0103] Chemically treated steel sheet samples were subjected to electrodeposition coating (Powernics Excel 1200, manufactured by Nippon Paint Industrial Coating Co., Ltd.) at an electrodeposition temperature of 30°C and a film thickness of 16 μm, followed by a baking treatment at 170°C for 30 minutes. Next, the electrodeposition-coated samples were subjected to a saltwater immersion test (SDT). Specifically, the electrodeposition-coated 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 a tape peel test was performed on one side of the sample. The peeled tape was scanned, and the area ratio of the coating peeled was calculated by binarization using the image analysis software "ImageJ." The coating adhesion was evaluated as follows. AAAA: Peeled area ratio less than 3% AAA: Peeled area ratio 3 to less than 5% AA: Peeled area ratio 5 to less than 10% A: Peeled area ratio 10 to 15% B: Peeled area ratio more than 15%

[0104] Steel sheets with paint film adhesion ratings of AAA, AA and A were evaluated as steel sheets containing Ni, Cu and Sn and having excellent paint film adhesion.

[0105] As shown in Table 2, the steel sheets of Comparative Examples 30 to 34, in which the shot blasting amount was small and the peak half width of the (310) plane of the ferrite phase in the X-ray diffraction pattern was less than 0.5°, did not have the above-mentioned specific strain state and therefore had poor paint adhesion. On the other hand, the steel sheets of Examples 1 to 29 of the present invention, in which the shot blasting amount was large and the peak half width of the (310) plane of the ferrite phase in the X-ray diffraction pattern was 0.5° or more, had the above-mentioned specific strain state and therefore had excellent paint adhesion.

[0106] Furthermore, as shown in Table 2, the comparison results of the steel sheets of Examples 1 to 29 of the present invention reveal that as the shot blasting amount increases, the half width of the peak of the (310) plane of the ferrite phase in the X-ray diffraction pattern also increases, and even better paint film adhesion can be exhibited.

[0107] In addition, in Examples 16 and 22, in which the thickness of the silicon oxide was very thin, no peeling of the coating occurred, indicating that even better coating adhesion can be achieved by suppressing the amount of silicon oxide generated.

Claims

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%, and wherein in an X-ray diffraction pattern of the steel sheet surface, the peak half-width of the (310) plane of the ferrite phase is 0.5° or more.

2. The steel sheet according to claim 1, wherein the peak half width is 1.0° or more.

3. The steel sheet according to claim 1, characterized in that the peak half width is 1.3° or more.

4. A steel sheet according to any one of claims 1 to 3, characterized in that in an element distribution image of a cross section of the steel sheet taken by an electron microprobe analyzer, a specific element-containing portion containing at least one of the Ni, Cu, and Sn in a solid solution state is present in a region from the surface of the steel sheet to a depth of 5 μm.

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

6. A part, characterized in that it comprises a steel sheet according to any one of claims 1 to 3.

7. A part, characterized in that it comprises the steel sheet according to claim 4.

8. A part, characterized in that it comprises the steel sheet according to claim 5.

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