Steel sheet and component including same

Abstract

The present invention provides: a steel sheet which contains Ni, Cu and Sn, and has excellent chemical conversion treatability; and a component which includes said steel sheet.　A steel sheet and a component including the same according to the present invention are characterized by having a chemical composition containing, in mass %, 0.010-1.000% of Ni, 0.010-1.000% of Cu, and 0.003-1.000% of Sn, and by the emission intensities of Ni, Cu, and Sn in the surface layer of the steel sheet as measured by high-frequency glow discharge optical emission spectrometry satisfying the following relationships.　Ni: Surface layer maximum intensity / Bulk intensity ≤ 1.5 Cu: Surface layer maximum intensity / Bulk intensity ≤ 1.3 Sn: Surface layer maximum intensity / Bulk intensity ≤ 15

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 corrosion resistance of 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. In addition, generally, when the chemical conversion treatability is reduced, areas called "whiteout areas" may occur where no chemical conversion coating is formed, which may result in reduced corrosion resistance.

[0007] Therefore, an object of the present invention is to provide a steel sheet containing Ni, Cu, and Sn and having excellent chemical conversion treatability, and a part including the steel sheet.

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

[0009] (Aspect 1) A steel sheet having 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%, wherein the emission intensities of the Ni, Cu, and Sn in the surface layer of the steel sheet measured by high-frequency glow discharge optical emission spectroscopy satisfy the following relationships: Ni: maximum strength of surface layer / bulk strength≦1.5 Cu: maximum strength of surface layer / bulk strength≦1.3 Sn: maximum strength of surface layer / bulk strength≦15

[0010] (Aspect 2) The steel sheet according to Aspect 1, wherein the emission intensities of the Ni, Cu, and Sn in the surface layer of the steel sheet measured by the high-frequency glow discharge optical emission spectroscopy satisfy the following relationships: Ni: maximum strength of surface layer / bulk strength≦1.2 Cu: maximum strength of surface layer / bulk strength≦1.2 Sn: maximum strength of surface layer / bulk strength≦12

[0011] (Aspect 3) The steel sheet according to Aspect 1, wherein the emission intensities of the Ni, Cu, and Sn in the surface layer of the steel sheet measured by the high-frequency glow discharge optical emission spectroscopy satisfy the following relationships: Ni: maximum strength of surface layer / bulk strength≦1.0 Cu: maximum strength of surface layer / bulk strength≦1.0 Sn: maximum strength of surface layer / bulk strength≦10

[0012] (Embodiment 4) The steel sheet according to any one of the above-mentioned embodiments 1 to 3, wherein the chemical composition further contains 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.

[0013] (Embodiment 5) The steel sheet according to any one of the above-mentioned embodiments 1 to 4, characterized in that the steel sheet has a Vickers hardness of 190 Hv or more.

[0014] (Aspect 6) The steel sheet according to any one of Aspects 1 to 5, characterized in that the steel sheet has a chemical composition containing, in mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%.

[0015] (Embodiment 7) A part comprising the steel sheet according to any one of the above embodiments 1 to 6.

[0016] According to the present invention, it is possible to provide a steel sheet containing Ni, Cu, and Sn and having excellent chemical conversion treatability, and a part including the steel sheet.

[0017] FIG. 1 is a schematic diagram of a cross section of a steel plate 1 according to one embodiment of the present invention, cut in the plate thickness direction.

[0018] As described above, generally, when chemical conversion treatability is reduced, regions where a chemical conversion coating is not formed, called "skid zones," may occur, resulting in reduced corrosion resistance. For example, when elements such as Ni, Cu, and Sn are present in a steel sheet as a solid solution, the potential of the steel sheet becomes more noble than the potential when these elements are not present in a solid solution state, which may reduce the etching ability of Fe during chemical conversion treatment. In such cases, it becomes difficult to form a chemical conversion coating, resulting in reduced chemical conversion treatability and, as a result, reduced corrosion resistance. Therefore, when a steel sheet simultaneously contains the three elements Ni, Cu, and Sn, this reduced chemical conversion treatability is particularly problematic.

[0019] 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.

[0020] Therefore, the present inventors conducted research, focusing particularly on the element distribution in the surface layer of a steel sheet, in order to provide a steel sheet having excellent chemical conversion treatability even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, the present inventors discovered that excellent chemical conversion treatability of a steel sheet surface can be obtained by suppressing the amount of Ni, Cu, and Sn enrichment in the surface layer of the steel sheet containing Ni, Cu, and Sn. Specifically, the present inventors discovered that the chemical conversion treatability of a steel sheet surface containing the three elements Ni, Cu, and Sn can be significantly improved by suppressing the amount of Ni, Cu, and Sn enrichment in the surface layer so that the emission intensities of Ni, Cu, and Sn in the surface layer of the steel sheet measured by high-frequency glow discharge optical emission spectroscopy satisfy the following specific relationships. The amount of Ni, Cu, and Sn enrichment in the surface layer of the steel sheet refers to the amount of Ni, Cu, and Sn in solid solution contained in the surface layer of the steel sheet. Ni: maximum strength of surface layer / bulk strength≦1.5 Cu: maximum strength of surface layer / bulk strength≦1.3 Sn: maximum strength of surface layer / bulk strength≦15 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 in which a plating layer is 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.

[0021] The present invention has been completed based on the above findings, and includes the following embodiments.

[0022] Hereinafter, preferred embodiments of the steel sheet of the present invention will be described in detail.

[0023] <Steel Sheet> A steel sheet according to one embodiment of the present invention has a chemical composition containing, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. The steel sheet according to this embodiment has a characteristic configuration in which the emission intensities of Ni, Cu, and Sn in the surface layer of the steel sheet measured by high-frequency glow discharge optical emission spectroscopy (GDS) satisfy the following relationships: Ni: maximum strength of surface layer / bulk strength ≦ 1.5 Cu: maximum strength of surface layer / bulk strength ≦ 1.3 Sn: maximum strength of surface layer / bulk strength ≦ 15

[0024] In general, chemical conversion treatment involves the generation of electrons by the anodic dissolution (etching) of Fe, and the cathode reaction (2H ＋ +2e - →H ２ , 10H ＋ +NO ３ - +8e - →NH ４ ＋ +3H ２ O) occurs. In relation to this, the pH of the chemical conversion treatment solution near 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. However, as described above, in steel sheets in which Ni, Cu, and Sn are present in a solid solution, the potential of the steel sheet becomes more noble than in steel sheets in which these elements are not present in a solid solution, and the etching ability of Fe during chemical treatment may be reduced. In such cases, the above-mentioned chemical conversion treatment film is less likely to be formed, which may result in reduced chemical treatability. However, in the steel sheet of this embodiment, even if the steel sheet contains the three elements Ni, Cu, and Sn, the concentrations of Ni, Cu, and Sn in the steel sheet surface layer (i.e., the contents of Ni, Cu, and Sn in a solid solution state in the steel sheet surface layer) are suppressed by the method described below so that the emission intensities of Ni, Cu, and Sn in the steel sheet surface layer, measured by high-frequency glow discharge optical emission spectroscopy, respectively, satisfy the above-mentioned specific relationships, thereby enabling the steel sheet to exhibit excellent chemical treatability.

[0025] 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 chemical conversion treatability 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 chemical conversion treatability and / or corrosion resistance are required.

[0026] Hereinafter, each component of the steel sheet of this embodiment will be described in detail.

[0027] [Emission intensities of Ni, Cu, and Sn in the surface layer of steel sheet measured by radio-frequency glow discharge optical emission spectroscopy] [Ni: maximum intensity of surface layer / bulk strength≦1.5] [Cu: maximum intensity of surface layer / bulk strength≦1.3] [Sn: maximum intensity of surface layer / bulk strength≦15] As described above, the steel sheet of this embodiment has a characteristic configuration in which the emission intensities of Ni, Cu, and Sn in the surface layer of the steel sheet measured by radio-frequency glow discharge optical emission spectroscopy (GDS) satisfy the following specific relationships: Ni: maximum intensity of surface layer / bulk strength≦1.5 Cu: maximum intensity of surface layer / bulk strength≦1.3 Sn: maximum intensity of surface layer / bulk strength≦15

[0028] In this specification, the term "surface layer of a steel sheet" refers to a region in the vicinity of a surface 2 of a steel sheet (sometimes referred to as "the outermost surface of a steel sheet") such as a steel sheet 1 shown in Fig. 1. Specifically, the surface layer 3 of a steel sheet is a region located at a depth P ０．１ and a depth position P of 2.0 μm from the steel plate surface 2 ２．０ In this specification, the term "surface layer of the steel sheet" may be simply referred to as "surface layer". In addition, in this specification, "luminescence intensity of Ni, Cu and Sn in the surface layer of the steel sheet measured by high-frequency glow discharge optical emission spectroscopy" means the luminescence intensity of Ni, Cu and Sn in the surface layer of the steel sheet on at least one of the front and back surfaces of the steel sheet measured by high-frequency glow discharge optical emission spectroscopy. On the other hand, in this specification, "bulk" refers to a region other than the surface layer 3 of the steel sheet 1, which is located between the center P of the thickness of the steel sheet 1 and the surface layer 3 of the steel sheet 1. Ｃ In FIG. 1 , for ease of understanding, the steel sheet surface layer 3 and the bulk 4 are shown as a region including the center P of the thickness of the steel sheet 1. Ｃ However, in an actual steel plate, the thickness center P Ｃ Similarly to the upper side, the steel sheet surface layer 3 and the bulk 4 are present on the lower side.

[0029] With regard to the above relationship between the emission intensities of Ni, Cu, and Sn in the surface layer of a steel sheet measured by GDS, the "maximum surface intensity" of an element such as Ni is the maximum value on a graph obtained by smoothing the emission intensity of an element such as Ni in the surface layer of a steel sheet, obtained when GDS measurement is performed from the surface of the steel sheet in the thickness direction at plot intervals of 0.1 μm or less, and means the maximum concentration of the element such as Ni in the surface layer. On the other hand, the "bulk intensity" is the emission intensity of an element such as Ni in the bulk, meaning the bulk concentration of an element such as Ni. Specifically, it is the average value of the emission intensities of elements such as Ni, obtained when GDS measurement is performed in a region from the steel sheet surface to 80 to 100 μm at plot intervals of 0.1 μm or less.

[0030] Therefore, "Ni: maximum strength of surface layer / bulk strength ≦ 1.5" is synonymous with the maximum Ni concentration in the surface layer being 1.5 times or less than the bulk Ni concentration, meaning that the amount of Ni concentration in the surface layer of the steel sheet is suppressed to approximately 1.5 times or less than the Ni concentration in the bulk. Similarly, "Cu: maximum strength of surface layer / bulk strength ≦ 1.3" is synonymous with the maximum Cu concentration in the surface layer being 1.3 times or less than the bulk Cu concentration, meaning that the amount of Cu concentration in the surface layer of the steel sheet is suppressed to approximately 1.3 times or less than the Cu concentration in the bulk. Similarly, "Sn: maximum strength of surface layer / bulk strength ≦ 15" is synonymous with the maximum Sn concentration in the surface layer being 15 times or less than the bulk Sn concentration, meaning that the amount of Sn concentration in the surface layer of the steel sheet is suppressed to approximately 15 times or less than the Sn concentration in the bulk.

[0031] In this way, when the amounts of Ni, Cu, and Sn concentrated in the surface layer of the steel sheet are suppressed so that the luminescence intensities of Ni, Cu, and Sn in the surface layer of the steel sheet measured by GDS respectively satisfy the above-mentioned specific relationships, even if the steel sheet contains the three elements Ni, Cu, and Sn, it can exhibit excellent chemical conversion treatability as described above.

[0032] (Method for Measuring GDS of Ni, Cu, and Sn in the Steel Sheet Surface and Bulk) The GDS of Ni, Cu, and Sn in the steel sheet surface and bulk is measured using a high-frequency glow discharge optical emission spectrometer. Specifically, the steel sheet surface to be measured is placed in an Ar atmosphere, a voltage is applied to generate glow plasma, and the steel sheet surface is analyzed in the depth direction while sputtering. Elements contained in the steel sheet are identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. Depth direction data can be estimated from the sputtering time. Specifically, the relationship between sputtering time and sputtering depth is determined in advance using a standard sample, allowing the sputtering time to be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the steel sheet surface.

[0033] A commercially available analyzer can be used for the GDS measurement. In this embodiment, a high-frequency glow discharge optical emission analyzer "GDS850A" manufactured by LECO Japan LLC is used. The measurement conditions are as follows: Ar gas pressure: 0.3 MPa, anode diameter: 4 mmφ, RF output: 30 W, measurement time: 200 to 1500 seconds.

[0034] There are no particular limitations on the method for suppressing the concentration amounts of Ni, Cu, and Sn in the steel sheet surface layer so that the luminescence intensities of Ni, Cu, and Sn in the steel sheet surface layer measured by GDS each satisfy the above-mentioned specific relationships. For example, the following method can be mentioned.

[0035] First, in the manufacturing process of a steel sheet, a shot blasting treatment is performed on the surface of the steel sheet after the hot rolling process or the cold rolling process, thereby imparting strain to the surface of the steel sheet and removing a specific element-containing portion that contains at least one of Ni, Cu, and Sn in a solid solution state in the surface layer of the steel sheet, which was generated during the hot rolling process. Here, the specific element-containing portion is a region also referred to as an enriched portion, and refers to an enriched region where the concentration of at least one element selected from Ni, Cu, and Sn is 0.3 mass% or more when the distribution state of elements in the surface layer of the steel sheet is analyzed using an electron probe microanalysis (EPMA) element distribution image. For example, when the area analysis is performed using an EPMA element distribution image, a region where the Ni concentration is 0.3 mass% or more, i.e., a Ni-enriched region (also referred to as a Ni-enriched portion), is a specific element-containing portion. Then, by carrying out an annealing process directly or via another process after the above-mentioned shot blasting treatment, recrystallization of Fe in the strained surface layer of the steel sheet is promoted, the grain size of Fe is refined, and the specific element-containing portions (Ni, Cu, and Sn-enriched portions) are trapped and segregated in the large amount of Fe grain boundaries, thereby realizing the suppression of the amount of Ni, Cu, and Sn enrichment in the steel sheet surface layer as described above.

[0036] A specific method for suppressing the concentrations of Ni, Cu, and Sn in the steel sheet surface layer so that the luminescence intensities of Ni, Cu, and Sn in the steel sheet surface layer measured by GDS satisfy the above-mentioned specific relationships will be described later.

[0037] In this embodiment, in order to obtain better chemical conversion treatability, it is more preferable that the luminescence intensities of Ni, Cu, and Sn in the surface layer of the steel sheet measured by GDS satisfy the following relationships: Ni: maximum strength of surface layer / bulk strength≦1.2 Cu: maximum strength of surface layer / bulk strength≦1.2 Sn: maximum strength of surface layer / bulk strength≦12

[0038] Furthermore, in this embodiment, it is particularly preferable that the luminescence intensities of Ni, Cu, and Sn in the surface layer of the steel sheet measured by GDS satisfy the following relationships: Ni: maximum strength of surface layer / bulk strength≦1.0 Cu: maximum strength of surface layer / bulk strength≦1.0 Sn: maximum strength of surface layer / bulk strength≦10

[0039] [Chemical Composition] In this embodiment, the steel sheet has a chemical composition containing, by mass%, Ni: 0.010-1.000%, Cu: 0.010-1.000%, and Sn: 0.003-1.000%. As described above, the present invention aims to provide a steel sheet containing Ni, Cu, and Sn and having excellent chemical conversion treatability. This objective is achieved by suppressing the concentration of Ni, Cu, and Sn in the steel sheet surface layer so that the luminescence intensities of Ni, Cu, and Sn in the steel sheet surface layer, as measured by GDS, respectively, satisfy the specific relationships described above. Therefore, the chemical composition of the steel sheet is not particularly limited except that it contains, by mass%, Ni: 0.010-1.000%, Cu: 0.010-1.000%, and Sn: 0.003-1.000%. It is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the objectives of the present invention.

[0040] Regarding Ni, Cu, and Sn, in terms of the chemical conversion treatability, strength, corrosion resistance, and the like of the steel sheet, the steel sheet of this embodiment preferably has a chemical composition containing, in mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] [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.

[0045] [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.

[0046] [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.

[0047] [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.

[0048] [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.

[0049] [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.

[0050] [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.

[0051] [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.

[0052] [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.

[0053] [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.

[0054] 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.

[0055] [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.

[0056] [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.

[0057] [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.

[0058] [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.

[0059] [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.

[0060] [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.

[0061] [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.

[0062] 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.

[0063] 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.

[0064] [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 chemical conversion treatability.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] On the other hand, the base material is a steel plate that has been ground and / or polished to a depth of 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.

[0069] For each of the evaluation material and base material prepared as described above, the maximum strength of the steel sheet surface is measured in a 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.

[0070] 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

[0071] 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.

[0072] (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.

[0073] (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., Kanefiel, 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 paint film is then 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 carbon 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 carbon concentration of 10% by mass or more, first obtain an element distribution image of carbon using EPMA with a carbon 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%.

[0074] (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%.

[0075] The above-described method for removing a coating film and a chemical conversion coating can be applied not only to XPS measurements but also to various measurements and analyses using samples obtained from automobiles. For example, when using a sample obtained from an automobile to perform the above-described GDS measurement of the steel sheet surface layer or to analyze the chemical composition of the steel sheet, the coating film and the chemical conversion coating can be removed from the sample according to the above-described method for removing a coating film and a chemical conversion coating, and then the GDS measurement and the analysis of the chemical composition can be performed.

[0076] The means for reducing silicon oxide on the steel sheet surface to a certain amount or less is not particularly limited, and 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 in the steel sheet manufacturing process, a means for controlling the coiling temperature as described below, a means for subjecting the annealed steel sheet to skin pass rolling, etc. Among these, the means for subjecting the annealed steel sheet to skin pass rolling is particularly suitable because it can steadily remove silicon oxide on the steel sheet surface by mechanical crushing.

[0077] (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.

[0078] (Mechanical Properties) [Vickers Hardness] In the present embodiment, the strength of the steel plate is not particularly limited, but for example, the steel plate may have a Vickers hardness of 90 Hv or more. The Vickers hardness of the steel plate 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 plate may be 650 Hv or less, 600 Hv or less, 550 Hv or less, or 500 Hv or less.

[0079] 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.

[0080] <Parts> As described above, the steel sheet according to the embodiment of the present invention can achieve superior chemical conversion treatability and, in turn, superior corrosion resistance compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields that require superior chemical conversion treatability 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.

[0081] 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.

[0082] 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 of 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.

[0083] <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.

[0084] The steel sheet of this embodiment 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 slab, a hot rolling step of hot-rolling the steel slab to obtain a hot-rolled steel sheet, a shot blasting step of performing shot blasting on the surface of the hot-rolled steel sheet, a step of cold-rolling the shot-blasted hot-rolled steel sheet to obtain a cold-rolled steel sheet, and an annealing step of annealing the cold-rolled steel sheet.

[0085] Alternatively, the steel sheet of this embodiment can also be produced by a production method including, for example, a casting process in which molten steel having an adjusted chemical composition is cast to form a steel slab, a hot rolling process in which the steel slab is hot-rolled to obtain a hot-rolled steel sheet, a pickling process in which the hot-rolled steel sheet is pickled, a cold rolling process in which the hot-rolled steel sheet is cold-rolled, a shot blasting process in which a surface of the cold-rolled steel sheet is shot blasted, and an annealing process in which the shot-blasted cold-rolled steel sheet is annealed.

[0086] In any case, as described above, the method for manufacturing a steel sheet according to this embodiment involves performing a shot blasting treatment on the surface of the steel sheet after the hot rolling process or the cold rolling process, thereby imparting strain to the surface of the steel sheet and removing specific element-containing portions (Ni, Cu, and Sn-enriched portions) in the surface layer of the steel sheet that contain at least one of Ni, Cu, and Sn in a solid solution state, which were generated in the hot rolling process. Furthermore, by performing an annealing process directly or via another process (e.g., a cold rolling process or a pickling process) after the shot blasting process, the recrystallization of Fe in the strained surface layer of the steel sheet is promoted, the Fe particle size is refined, and the specific element-containing portions (Ni, Cu, and Sn-enriched portions) are trapped and segregated at the large Fe grain boundaries. This allows the amount of Ni, Cu, and Sn enrichment in the surface layer of the steel sheet to be suppressed so that the emission intensities of Ni, Cu, and Sn in the surface layer of the steel sheet measured by GDS as described above satisfy the following specific relationships: Ni: Maximum surface strength / bulk strength ≦ 1.5 Cu: Maximum surface strength / bulk strength ≦ 1.3 Sn: Maximum surface strength / bulk strength ≦ 15

[0087] Preferred conditions for these steps will be described in detail below.

[0088] [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.

[0089] [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%.

[0090] The hot-rolled steel sheet after finish rolling is coiled at a predetermined coiling temperature and then subjected to the subsequent shot blasting process or cold rolling 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.

[0091] 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 chemical conversion treatability can be exhibited.

[0092] From the viewpoint of further improving chemical conversion treatability, 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-depleted 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.

[0093] [Shot blasting process] In the steel sheet manufacturing method of this embodiment, the shot blasting process is a process of imparting strain to the steel sheet surface of the hot-rolled steel sheet or the cold-rolled steel sheet and performing shot blasting to remove specific element-containing portions (i.e., Ni-, Cu-, and Sn-enriched portions) containing at least one of Ni, Cu, and Sn in a solid solution state in the steel sheet surface layer generated in the hot rolling process. Note that brush grinding cannot impart a sufficient amount of strain to the steel sheet surface.

[0094] The treatment conditions in the shot blasting process are not particularly limited as long as they can impart strain to the surface of the steel sheet and remove specific element-containing portions (enriched portions) in the surface layer of the steel sheet that have been formed in the hot rolling process. For example, the shot blasting process can be carried out 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 Shot blasting may be performed with a projection 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 projection material is not particularly limited, but may be, for example, 10 to 150 m / sec.

[0095] 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. Furthermore, in this case, it is preferable to rinse the steel sheet surface with water having an electrical conductivity of 80 mS / m or less. If such 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 water used for rinsing the steel sheet surface is 60 mS / m or less.

[0096] [Annealing step] The annealing step is a step of annealing the steel sheet after the shot blasting step. By performing the annealing step directly after the shot blasting step or via another step (for example, a cold rolling step, a pickling step, etc.), the recrystallization of Fe in the strained steel sheet surface layer is promoted, the Fe grain size is refined, and the specific element-containing portion (Ni, Cu, and Sn-enriched portion) is trapped and segregated in the large amount of Fe grain boundaries, so that the Ni, Cu, and Sn enrichment amounts in the steel sheet surface layer can be suppressed so that the Ni, Cu, and Sn emission intensities in the steel sheet surface layer measured by GDS respectively satisfy the specific relationships.

[0097] The annealing step involves heating the steel sheet to a temperature of 700 to 950°C in an atmosphere with a dew point of -40 to 20°C and holding the temperature for 0 to 300 seconds. 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 containing 1 to 10% hydrogen (e.g., 3% hydrogen and the balance nitrogen). The temperature in the annealing step is preferably 750 to 950°C, more preferably 780 to 900°C. The holding time in the annealing step is preferably 30 to 200 seconds, more preferably 50 to 150 seconds.

[0098] The basic steps of the manufacturing method of the steel sheet of this embodiment are as described above. In the steel sheet of this embodiment manufactured by the manufacturing method including the steps described above, the concentrations of Ni, Cu, and Sn in the steel sheet surface layer are suppressed so that the emission intensities of Ni, Cu, and Sn in the steel sheet surface layer measured by GDS satisfy the specific relationships described above. Therefore, even if the steel sheet contains the three elements Ni, Cu, and Sn, surface oxides are unlikely to be generated as described above, and excellent chemical conversion treatability can be exhibited.

[0099] In addition to the above-described steps, the method for producing a steel sheet according to this embodiment may further include any processing steps that are generally performed in steel sheet manufacturing methods. Specifically, the method for producing a steel sheet according to this embodiment may optionally include a cold rolling step of cold-rolling a hot-rolled steel sheet before or after a shot blasting step to obtain a cold-rolled steel sheet, a pickling step of pickling the steel sheet before or after the shot blasting step, a cooling step of cooling the annealed steel sheet, and a skin-pass rolling step of skin-pass rolling the annealed steel sheet.

[0100] (Cold Rolling Step) The cold rolling step is a step in which the hot rolled steel sheet before or after the shot blasting step 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, sheet thickness, etc., and may be, for example, 20 to 80%. After the cold rolling step, the steel sheet may be cooled to room temperature, for example, by air cooling.

[0101] (Pickling process) The pickling process is a process of pickling a steel sheet before or after the shot blasting process. The pickling process is a process of pickling a steel sheet before or after the shot blasting process to remove surface oxides and internal oxides. The conditions for the pickling process are not particularly limited, and the pickling process may be carried out using a commonly used pickling solution under conditions appropriate for removing the surface oxides and internal oxides. Pickling may be carried out once, or may be carried out multiple times to ensure that the surface oxides and internal oxides are removed.

[0102] (Cooling Step) The cooling step is a step of cooling the steel sheet after annealing. The cooling conditions are not particularly limited, and for example, the cold-rolled steel sheet may be cooled so that the average cooling rate from the soaking temperature is 5 to 50°C / second. By setting the average cooling rate to 5°C / second or more, excessive transformation to ferrite is suppressed and the amount of hard phases such as martensite produced is increased, thereby achieving the desired strength. Furthermore, by setting the average cooling rate to 50°C / second or less, the steel sheet can be cooled more uniformly in the sheet width direction (the direction perpendicular to the rolling direction and the sheet thickness direction).

[0103] (Skin-pass rolling process) The skin-pass rolling process is a process of subjecting the annealed steel sheet to skin-pass rolling. The skin-pass rolling process is a process of subjecting the annealed steel sheet to skin-pass rolling to crush and remove surface oxides. The reduction rate in the skin-pass rolling process is not particularly limited, but may be, for example, 2.0% or less, 1.5% or less, or 1.0% or less, from the viewpoint of facilitating the removal of surface oxides. Furthermore, the reduction rate may be 0.2% or more, 0.3% or more, or 0.4% or more.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] First, molten steel was cast by a continuous casting method to form a billet having the chemical composition shown in Table 1. After the billet was cooled once, it was reheated to 1200°C and hot rolled, and then coiled at a coiling temperature of 520°C or higher. The hot rolling was carried out by performing rough rolling and finish rolling, and the finish rolling end temperature was 900 to 1050°C, and the finish rolling reduction was 30%. Next, the obtained hot-rolled steel sheets were subjected to shot blasting treatment using "TSH30" manufactured by IKK Shot Co., Ltd. as a shot material at a blasting amount shown in Table 2. The steel sheets after the shot blasting treatment were rinsed with water and then cold-rolled at a reduction of 50% to obtain cold-rolled steel sheets having a thickness of 1.6 mm. Furthermore, the cold-rolled steel sheets were annealed in a furnace having an oxygen concentration of 20 ppm or less under annealing conditions of heating to a temperature of 800°C in an atmosphere with a dew point of -40°C and 3% hydrogen (nitrogen balance) and holding for 100 seconds (annealing condition A shown in Table 2), or under annealing conditions of heating to a temperature of 820°C in an atmosphere with a dew point of -10°C and 3% hydrogen (nitrogen balance) and holding for 80 seconds (annealing condition B shown in Table 2), to obtain various steel sheets serving as examples or comparative examples. Note that some of the steel sheets were finally subjected to skin-pass rolling at the reduction shown in Table 2.

[0108]

[0109]

[0110] The various steel sheets obtained as described above were subjected to various measurements such as Vickers hardness, GDS of Ni, Cu, and Sn in the steel sheet surface and bulk, and XPS. Furthermore, the chemical conversion treatability 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. Note that the underlines next to various values ​​in Table 2 indicate values ​​outside the range of the present invention. In addition, the SiO ２ The "-" in the thickness indicates SiO ２ indicates that no

[0111] [Evaluation of Chemical Conversion Treatability] The chemical conversion treatability of the steel sheet was evaluated as follows. First, a 200 mm x 30 mm steel sheet sample was coated with NOX-RUST550NH and then pressed against a die with an R of 4 mm with a pressing load of 3 kN. Next, using a tensile testing machine UST-10T manufactured by Oriental Co., Ltd., the sample was pulled out at a speed of 100 mm / min so that the sliding distance was 100 mm. Next, the sample was subjected to a zinc phosphate treatment as a chemical conversion treatment under the following conditions. Degreasing: The sample was immersed in a degreaser (Fine Cleaner E2083) at 40°C for 2 minutes, followed by rinsing with water. Surface conditioning: The sample was immersed in a surface conditioner (Preparen Z) at room temperature for 30 seconds. Chemical conversion treatment: The sample was immersed in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, followed by rinsing with water and drying.

[0112] The chemically treated samples were observed using SEM-EPMA, and the area ratio of the area where the chemical conversion coating was not formed, commonly called "clear," was calculated by binarization using the image analysis software "ImageJ." The chemical conversion treatability of the steel sheet was evaluated according to the area ratio of the clear-cut area using the following evaluation criteria. The clear-cut area was measured using an EPMA (JXA-8500 manufactured by JEOL Ltd.) at an acceleration voltage of 15 kV and a probe current of 5 × 10 -7 In a mapping image obtained by photographing at 1000x magnification under condition A, it was defined as a region where the Fe concentration was 70% or more. The void area ratio was determined as the average value of five randomly selected fields of view. AAA: void area ratio less than 20% AA: void area ratio 20-25% A: void area ratio 25-35% B: void area ratio more than 35%

[0113] Steel sheets with phosphatability ratings of AAA, AA and A were evaluated as containing Ni, Cu and Sn and having excellent phosphatability.

[0114] As shown in Table 2, the steel sheets of Comparative Examples 29 to 33, in which the shot blasting amount was small and the relationship between the bulk strength and the maximum surface strength of Ni, Cu, and Sn in the steel sheet surface layer measured by GDS did not satisfy the above-mentioned specific relationship, did not suppress the amount of Ni, Cu, and Sn concentration in the steel sheet surface layer, and therefore had poor chemical conversion treatability. On the other hand, the steel sheets of Examples 1 to 28 of the present invention, in which the shot blasting amount was large and the relationship between the bulk strength and the maximum surface strength of Ni, Cu, and Sn in the steel sheet surface layer measured by GDS satisfied the above-mentioned specific relationship, had excellent chemical conversion treatability because the amount of Ni, Cu, and Sn concentration in the steel sheet surface layer was suppressed.

[0115] Furthermore, as shown in Table 2, the comparison results of the steel sheets of Examples 1 to 28 of the present invention reveal that as the shot blasting amount increases, the concentration of Ni, Cu, and Sn in the surface layer of the steel sheet is further suppressed, and even more excellent chemical conversion treatability can be exhibited. Furthermore, by performing skin pass rolling, silicon oxide (SiO ２ ) can be reduced to 10 nm or less, and in particular, from the comparison results between Examples 13 and 14, even when the distribution state of Ni, etc. is the same, it is possible to reduce the thickness of SiO ２ It was found that by making the thickness of the coating 10 nm or less, even better chemical conversion treatability can be obtained.

[0116] 1 Steel plate 2 Steel plate surface 3 Steel plate surface layer 4 Bulk P ０．１ Depth position 0.1 μm from the steel plate surface P ２．０ Depth position 2.0 μm from the steel plate surface P Ｃ Steel plate thickness center

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%, wherein the emission intensities of the Ni, Cu, and Sn in the surface layer of the steel sheet measured by high-frequency glow discharge optical emission spectroscopy satisfy the following relationships: Ni: Maximum strength of surface layer / Bulk strength ≦ 1.5 Cu: Maximum strength of surface layer / Bulk strength ≦ 1.3 Sn: Maximum strength of surface layer / Bulk strength ≦ 15 2. The steel sheet according to claim 1, wherein the emission intensities of the Ni, Cu, and Sn in the surface layer of the steel sheet measured by the high-frequency glow discharge optical emission spectroscopy satisfy the following relationships: Ni: maximum strength of surface layer / bulk strength≦1.2 Cu: maximum strength of surface layer / bulk strength≦1.2 Sn: maximum strength of surface layer / bulk strength≦12 3. The steel sheet according to claim 1, wherein the emission intensities of the Ni, Cu, and Sn in the surface layer of the steel sheet measured by the high-frequency glow discharge optical emission spectroscopy satisfy the following relationships: Ni: maximum strength of surface layer / bulk strength≦1.0 Cu: maximum strength of surface layer / bulk strength≦1.0 Sn: maximum strength of surface layer / bulk strength≦10 4. The steel sheet according to any one of claims 1 to 3, characterized in that the chemical composition further contains Si, and the thickness of silicon oxide is 10 nm or less when measured on the steel sheet surface by X-ray photoelectron spectroscopy.

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

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

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

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