Steel plates and parts containing them

Abstract

An object of the present invention is to provide a steel sheet that has excellent corrosion resistance after painting, even if the steel sheet contains Ni, Cu, and Sn, and a part including the steel sheet. The steel sheet and the part including the same of the present invention have a chemical composition of the steel sheet containing, in mass %, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and satisfies Si / (Si+Mn)≧0.20, and the intensity of Sn measured by X-ray photoelectron spectroscopy in the thickness direction from the surface of the steel sheet is ≦100% (I on the outermost surface) 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 and (I of depth 50 nm) 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.7.

Description

[Technical Field]

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

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

[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 surface of the steel sheet is exposed to an atmosphere in which iron oxidizes within the above-mentioned steel sheet temperature range, pickled at the outlet side of the annealing furnace, and then iron or Ni plating is applied to the steel sheet at 1 to 50 mg / m 2 Furthermore, Patent Document 1 teaches that, while oxidation of a steel sheet is usually prevented by an extremely low concentration of oxygen and / or an inert gas atmosphere 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 in the steel sheet, and that by pickling after leaving an annealing furnace, the oxide films of Si, Mn, etc. are removed together with the oxide film on the iron in the steel sheet, thereby obtaining a high-strength cold-rolled steel sheet that is free from "bleaching" and has good chemical 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 2Patent Document 2 further teaches that the above-mentioned 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 surface of the steel sheet, 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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-190030 [Patent Document 2] Japanese Patent Publication No. 2020-084238 Summary of the Invention [Problem to be solved by the invention]

[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. Furthermore, generally, when chemical conversion treatability is reduced, areas where a chemical conversion coating is not formed, known as "white spots," may occur, which may result in reduced corrosion resistance after painting.

[0007] Therefore, an object of the present invention is to provide a steel sheet containing Ni, Cu, and Sn and having excellent corrosion resistance after painting, and a part including the steel sheet. [Means for solving the problem]

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

[0009] (Aspect 1) A steel plate, The chemical composition of the steel plate is, in mass%, Si: 0.30 to 3.00% Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and Si / (Si+Mn)≧0.20 is satisfied, The intensity of Sn measured by X-ray photoelectron spectroscopy from the surface of the steel sheet in the thickness direction is (Top surface I 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5, and, (50 nm deep I 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.7.

[0010] (Aspect 2) The intensity of Sn measured by X-ray photoelectron spectroscopy from the surface of the steel sheet in the thickness direction is (Top surface I 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.3.

[0011] (Aspect 3) The intensity of Sn measured by X-ray photoelectron spectroscopy from the surface of the steel sheet in the thickness direction is (50 nm deep I 484.9 ) / (I at a depth of 50 nm 486.5 3. The steel sheet according to claim 1, wherein the relationship satisfies the following: )≦0.5.

[0012] (Aspect 4) The chemical composition of the steel plate is, in mass%, The steel sheet according to any one of the above aspects 1 to 3, characterized in that it contains 0.50 to 3.00% of Si.

[0013] (Aspect 5) The chemical composition of the steel plate is, in mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000% 5. The steel sheet according to any one of the above aspects 1 to 4, comprising:

[0014] (Aspect 6) 6. The steel sheet according to any one of the above aspects 1 to 5, wherein the steel sheet has a chemical conversion coating on a surface thereof.

[0015] (Aspect 7) A part comprising the steel sheet according to any one of the first to sixth aspects. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a steel sheet containing Ni, Cu, and Sn and having excellent corrosion resistance after painting, and a part including the steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0017] As mentioned above, generally, when the chemical conversion treatability of a steel sheet deteriorates, regions where a chemical conversion coating is not formed, known as "skid zones," may appear, resulting in reduced corrosion resistance. For example, when elements such as Ni, Cu, and Sn are present in a solid solution in the steel sheet, the potential of the steel sheet becomes more noble than when these elements are not present in the solid solution, which can reduce the etching ability of Fe during chemical conversion treatment. In particular, when a steel sheet simultaneously contains the three elements Ni, Cu, and Sn, Ni and Cu tend to concentrate Sn on the steel sheet surface during the annealing process. The concentrated Sn on the steel sheet surface inhibits the chemical conversion treatment, causing poor etching of the steel sheet surface during chemical conversion treatment and making it difficult to form a chemical conversion coating. Therefore, in such cases, the chemical conversion treatability deteriorates, resulting in reduced corrosion resistance after painting. Therefore, when a steel sheet simultaneously contains the three elements Ni, Cu, and Sn, this reduced corrosion resistance after painting is particularly problematic.

[0018] Two commonly known methods for producing steel are: one in which molten iron is obtained in a blast furnace using iron ore, a natural resource, as the primary raw material, and then refined in a converter or other furnace to produce molten steel; and another in which molten steel is produced in an electric furnace using scrap material, a recycled resource, as the primary 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. 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 primary 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 prone to simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the above-mentioned problems are particularly pronounced in electric furnace steel.

[0019] 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 that has excellent corrosion resistance after painting, even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, it was found that, among the three elements Ni, Cu, and Sn, Sn in particular is likely to concentrate on the surface of the steel sheet during the annealing process due to the presence of Ni and Cu, and the content of metallic Sn in a solid solution state (hereinafter sometimes referred to as "solid solution Sn") on the surface of the steel sheet is likely to increase. The solid solution Sn present on the surface of the steel sheet inhibits chemical conversion treatment, causing poor etching of the steel sheet surface during chemical conversion treatment and resulting in deterioration of corrosion resistance after painting.

[0020] The present inventors have found that by oxidizing Sn in the surface layer of a steel sheet to form tin oxide (hereinafter sometimes referred to as "Sn oxide"), the content of dissolved Sn on the surface of the steel sheet can be reduced, and further, by removing the Sn oxide on the surface of the steel sheet, the chemical conversion treatability of the surface of the steel sheet can be significantly improved, resulting in excellent corrosion resistance after painting. Specifically, the present inventors have found that even in the case of a steel sheet containing three elements, Ni, Cu, and Sn, the chemical composition of the resulting steel sheet contains 0.30 to 3.00% Si and satisfies Si / (Si+Mn)≧0.20, the hot-rolled steel sheet is brush-ground after pickling, is annealed at a high dew point, the residence time between 600 and 700°C in the annealing step is 12.5 seconds or less, and further, the annealing step is followed by pickling, and the intensity of Sn measured by X-ray photoelectron spectroscopy in the thickness direction from the surface of the steel sheet is 100% or less (the outermost surface I 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 and (I 484.9 ) / (I at a depth of 50 nm 486.5 It has been found that the content of dissolved Sn and oxidized Sn on the surface of a steel sheet can be reduced so as to satisfy the condition (R)≦0.7. Furthermore, it has been found that this significantly improves the chemical conversion treatability of the surface of the steel sheet, resulting in excellent corrosion resistance after painting.

[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 plate> A steel sheet according to one embodiment of the present invention has a specific chemical composition that contains, in mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and satisfies Si / (Si+Mn)≧0.20.

[0024] In the steel sheet of this embodiment, the intensity of Sn measured by X-ray photoelectron spectroscopy in the thickness direction from the surface of the steel sheet is481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 and (I of depth 50 nm) 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.7.

[0025] Generally, in chemical conversion treatment, electrons are generated by the anodic dissolution (etching) of Fe, and the electrons generated by the anodic dissolution of Fe cause a cathodic reaction (2H + +2e - →H2, 10H + +NO3 - +8e - →NH4 + As a result, the pH of the chemical conversion solution near the surface of the steel sheet rises, and as a result, compounds such as zinc phosphate crystals that make up the chemical conversion coating are precipitated on the surface of the steel sheet.

[0026] However, in steel sheets containing dissolved Ni, Cu, and Sn, the potential of the steel sheet becomes more noble than in steel sheets containing no dissolved Ni, Cu, or Sn, which can reduce the etching ability of Fe during chemical conversion treatment. Among the three elements, Ni, Cu, and Sn, Sn in particular tends to concentrate on the surface of the steel sheet during the annealing process due to the presence of Ni and Cu, which increases the concentration of dissolved Sn on the surface of the steel sheet. As described above, dissolved Sn on the surface of the steel sheet inhibits the chemical conversion treatment of the steel sheet surface, causing poor etching during chemical conversion treatment and resulting in reduced corrosion resistance after painting.

[0027] Even if the steel sheet of this embodiment contains three elements, Ni, Cu, and Sn, the intensity of Sn measured by X-ray photoelectron spectroscopy (XPS) from the surface of the steel sheet in the thickness direction by the method described below is less than 100% (I on the outermost surface). 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 and (I 484.9 ) / (I at a depth of 50 nm 486.5The contents of dissolved Sn and oxidized Sn on the surface of the steel sheet are reduced so as to satisfy the condition of .gtoreq.0.05%.)≦0.7. As a result, the steel sheet of this embodiment can significantly improve the chemical conversion treatability of the surface of the steel sheet, and as a result, can obtain excellent corrosion resistance after painting.

[0028] 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 compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn, and can therefore exhibit superior corrosion resistance after painting. Therefore, the steel sheet of this embodiment is particularly useful in the automotive field, where excellent corrosion resistance after painting is required.

[0029] Hereinafter, each component of the steel sheet of this embodiment will be described in detail.

[0030] [Chemical composition] In this embodiment, the steel sheet has a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and satisfying Si / (Si + Mn) ≧ 0.20. As described above, an object of the present invention is to provide a steel sheet that has excellent corrosion resistance after painting, even if it contains Ni, Cu, and Sn, and this object is achieved by reducing the content of dissolved Sn and Sn oxide on the surface of the steel sheet so that the Sn intensity measured by XPS from the surface to the thickness direction of the steel sheet satisfies the above specific relationship.

[0031] Therefore, the chemical composition of the steel sheet is not particularly limited except that it contains, in mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and satisfies Si / (Si+Mn)≧0.20.

[0032] With regard to Ni, Cu, and Sn, from the viewpoint of the chemical conversion treatability, strength, and corrosion resistance of the steel sheet, Ni is preferably 0.040% or more by mass. Ni is preferably 1.000% or less by mass. Similarly, Cu is preferably 0.040% or more by mass. Cu is preferably 1.000% or less by mass. Similarly, Sn is preferably 0.004% or more by mass. Sn is preferably 1.000% or less by mass. The preferred contents of these elements will be described later. In particular, in this embodiment, the chemical composition of the steel sheet preferably contains, in mass %, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%.

[0033] The chemical composition of the steel plate of this embodiment may contain, in addition to Ni, Cu, and Sn, any alloying element that is generally added in the technical field of the present invention, in an amount within an appropriate range.

[0034] The chemical compositions that can be employed in the steel sheet of this embodiment will be described in detail below. The following description is intended to merely exemplify preferred chemical compositions of steel sheets for use in automobiles and the like, and is not intended to limit the present invention to steel sheets having such specific chemical compositions.

[0035] For example, the steel plate of this embodiment has, in mass%, C: 0.001 to 0.500%, Si: 0.30 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: 0~0.0100%, Mo: 0 to 1.000%, Cr: 0 to 1.000%, V: 0~0.150%, 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: 0~0.100%, As: 0~0.100%, Ir: 0 to 1.000%, and Remainder: Fe and impurities The chemical composition may be such that Si / (Si+Mn)≧0.20.

[0036] Each of these elements will be described in more detail below.

[0037] [C: 0.001~0.500%] 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: 0.30~3.00%] Si is an effective solid-solution strengthening element for increasing strength. Si also contributes to the formation of Mn oxides, which are formed around Si oxide nuclei. To fully obtain these effects, the Si content is set to 0.30% or more. The Si content may be 0.35% or more, 0.40% or more, 0.45% or more, 0.50% or more, 0.55% or more, or 0.60% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. Therefore, the Si content is set to 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. Among these, the Si content is preferably 0.50 to 3.00%. The Si content must satisfy the relationship Si / (Si+Mn)≧0.20, as described below.

[0039] [Mn: 0.10~3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. The Mn content may be 0%, but 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. Furthermore, excessive Mn content may result in the formation of large amounts of Mn oxide on the steel surface. In such cases, a large amount of oxygen is consumed in the formation of Mn oxide on the steel surface, which may result in insufficient formation of the above-mentioned Sn oxide and insufficient reduction of solute Sn. Therefore, 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. The Mn content must satisfy the relationship Si / (Si + Mn) ≥ 0.20, as described below.

[0040] [Al: 0.001 to 2.000%] Al acts as a deoxidizer for steel and improves the quality of the steel. To fully achieve 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: 0.010~1.000%] [Cu: 0.010~1.000%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. To fully achieve these effects, the Ni and Cu contents are each set to 0.010% or more. The Ni and Cu contents 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, respectively. On the other hand, excessive inclusion of these elements may excessively promote the formation of oxides, particularly Si-based surface oxides and iron oxides, on the surface of the steel sheet. Therefore, the Ni and Cu contents are each set to 1.000% or less. The Ni and Cu contents may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less, respectively.

[0042] [Sn: 0.003~1.000%] Sn is an element effective in improving corrosion resistance. To fully obtain this effect, the Sn content is set to 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 excessively promote the formation of oxides, particularly Si-based surface oxides and iron oxides, on the surface of the steel sheet. Therefore, the Sn content is set to 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. The lower the P content, the better, and ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in manufacturing costs. For this reason, the P content may be 0.0001% or more, or may be 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 nonmetallic inclusions such as MnS in steel, reducing the ductility of steel parts. The lower the S content, the better, and ideally it is 0%. However, excessive reduction in the S content can significantly increase manufacturing costs. Therefore, the S content may be 0.0001% or more, or may be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content can cause cracks to occur originating from nonmetallic 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. The lower the N content, the better, and ideally it 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 0.0080% or less, 0.0050% or less, or 0.0030% or less.

[0046] [O:0.0100% or less] O is an element that is mixed in during the manufacturing process and forms coarse inclusions, reducing the workability of the steel sheet. The lower the O content, the better, and ideally it is 0%. However, excessive reduction in the O content may result in a significant increase in manufacturing costs. For this reason, the O content may be 0.0001% or more, or 0.0005% or more, or 0.0010% or more. On the other hand, excessive O content may form coarse inclusions, as described above, reducing the workability of the steel sheet. Therefore, the O content is preferably 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0047] The preferred basic chemical composition of the steel sheet of this embodiment is as described above. Furthermore, the steel sheet of this embodiment may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0048] [Ti: 0~0.150%] [Nb: 0~0.150%] [V: 0~0.150%] Ti, Nb, and V form carbonitrides in steel, improving the strength of the steel sheet through precipitation strengthening. The Ti, Nb, and V contents may be 0%, but to fully obtain these effects, the Ti, Nb, and V contents are preferably 0.001% or more. The Ti, Nb, and V contents may be 0.002% or more, 0.005% or more, or 0.010% or more. However, excessive inclusion of these elements saturates the effect, and excessive inclusion of these elements in steel increases manufacturing costs. Therefore, the Ti, Nb, and V contents are preferably 0.150% or less. The Ti, Nb, and V contents 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: 0~0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but to fully 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: 0-1.000%] [Cr:0~1.000%] [W: 0~1.000%] 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 fully obtain these effects, the Mo, Cr, and W contents are preferably 0.001% or more. The Mo, Cr, and W contents may be 0.010% or more, 0.020% or more, or 0.030% or more. However, even if these elements are contained in excess, the effects are saturated, and excessive inclusion of these elements in steel increases manufacturing costs. Therefore, the Mo, Cr, and W contents are preferably 1.000% or less. The Mo, Cr, and W contents may be 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less, respectively.

[0051] [Hf:0~0.050%] [Mg: 0~0.050%] [Zr: 0~0.500%] [Ca: 0-0.050%] [REM:0~0.100%] Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of nonmetallic inclusions. The Hf, Mg, Zr, Ca, and REM contents may be 0%, but to fully obtain these effects, the Hf, Mg, Zr, Ca, and REM contents are preferably 0.0001% or more. The Hf, Mg, Zr, Ca, and REM contents may be 0.0005% or more or 0.001% or more. However, excessive inclusion of these elements saturates the effect, and excessive inclusion of these elements in the steel sheet increases production costs. Therefore, the Hf and Mg contents are preferably 0.050% or less, and the Zr content is preferably 0.500% or less. The Hf, Mg, and Zr contents may be 0.010% or less, 0.005% or less, or 0.003% or less, respectively. Similarly, the Ca content is preferably 0.050% or less, and the REM content is preferably 0.100% or less. The Ca and REM contents may be 0.010% or less, 0.005% or less, or 0.003% or less, respectively. REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0052] [As:0~0.100%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to fully 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 an excessive amount of As is contained, the effect saturates, and adding more than necessary to the steel sheet increases the manufacturing cost. Therefore, the As content is preferably 0.100% or less. The As content may be 0.008% or less, or 0.005% or less.

[0053] [Ir: 0~1.000%] 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 fully 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 added, the effect saturates, and adding more Ir than necessary to a steel material increases manufacturing costs. 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] The remainder of the steel sheet other than the above elements consists of Fe and impurities. Impurities in 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 steel sheet is industrially produced.

[0055] [Si / (Si+Mn)≧0.20] As described above, the steel sheet of this embodiment is required to have the Si and Mn contents satisfy the relationship Si / (Si+Mn)≧0.20. In this relationship, Si means the Si content in mass%. Similarly, Mn means the Mn content in mass%.

[0056] It is known that Si and Mn in steel sheets form oxides during annealing. Furthermore, during high-temperature annealing, the internal oxide tends to form Mn oxide around the Si oxide as a nucleus. When the Si content is low, a large amount of Mn oxide is formed on the surface of the steel sheet. In this case, a large amount of oxygen is consumed to form Mn oxide on the surface of the steel sheet, which may result in insufficient formation of the above-mentioned Sn oxide and insufficient reduction of the remaining metallic Sn content.

[0057] Therefore, in the steel sheet of this embodiment, by increasing the Si content sufficiently to the extent that Si / (Si+Mn) is 0.20 or more, it is possible to suppress the formation of Mn oxides on the surface of the steel sheet and to sufficiently form the above-mentioned Sn oxides, i.e., it is possible to sufficiently reduce the content of remaining metallic Sn.

[0058] The ratio Si / (Si+Mn) may be 0.21 or more, 0.22 or more, 0.23 or more, 0.24 or more, or 0.25 or more. The upper limit of the ratio Si / (Si+Mn) may be 1.00, 0.80, 0.60, or 0.50.

[0059] The chemical composition of steel sheets can be measured by a common analytical method. For example, the chemical composition of steel sheets can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) of chips in accordance with JIS G 1201:2022. Specifically, for example, a 35 mm square test piece is obtained from the steel sheet at approximately 1 / 4 of the thickness, and the test piece is measured using a Shimadzu ICPS-8100 (measuring device) or the like under conditions based on a pre-prepared calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using a combustion-infrared absorption method, and N can be measured using an inert gas fusion-thermal conductivity method.

[0060] [Sn intensity measured from the surface of the steel sheet in the thickness direction using X-ray photoelectron spectroscopy] [(I on the top surface 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5] [(50 nm depth I 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.7] As described above, in the steel sheet of this embodiment, the intensity of Sn measured by X-ray photoelectron spectroscopy (XPS) from the surface of the steel sheet in the thickness direction is 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 and (I of depth 50 nm) 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.7.

[0061] Here, in this specification, "surface of steel sheet" means the outermost surface of the steel sheet, and in the case where the steel sheet has a chemical conversion coating, means the interface between the chemical conversion coating and the base steel. In addition, in this specification, "surface layer of steel sheet" means a region near the surface of the steel sheet, and specifically means a region between a position at a depth of 0.1 μm from the surface of the steel sheet in the thickness direction and a position at a depth of 2.0 μm from the surface of the steel sheet in the thickness direction. In this specification, "surface layer of steel sheet" may be simply referred to as "surface layer".

[0062] Regarding the Sn intensity measured by XPS from the surface of the steel sheet in the thickness direction, 481-490 ) / (I at a depth of 50 nm 481-490 "Meeting the condition )≦0.5" means that the Sn content at the surface of the steel sheet, specifically the total content of solute Sn and oxidized Sn, is 0.5 times or less the Sn content at a depth of 50 nm from the surface of the steel sheet in the thickness direction (i.e., the total content of solute Sn and oxidized Sn). In other words, this means that the Sn content at the surface of the steel sheet is so small that it is half or less of the Sn content inside the steel sheet (surface layer of the steel sheet).

[0063] In this specification, I 481-490 is the integral value of the emitted photoelectron intensity in the binding energy range of 481 to 490 eV, and means the integral value of the emitted photoelectron intensity derived from solid-solution Sn and oxidized Sn. Similarly, I 484.9 means the intensity of emitted photoelectrons that appears at a binding energy of 484.9 eV and originates from solid-solution Sn. Furthermore, I 486.5 is the intensity of emitted photoelectrons appearing at a binding energy of 486.5 eV, and Sn 2+ , that is, the intensity of emitted photoelectrons derived from Sn oxide. The measurement method for these will be described later.

[0064] Furthermore, regarding the Sn intensity measured by XPS from the surface of the steel sheet in the thickness direction, 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.7” means that the content of solute Sn at a depth of 50 nm from the surface of the steel sheet in the thickness direction is 2+ That is, it means that the content of solute Sn in the surface layer of the steel sheet is relatively small because a large amount of Sn oxide is formed in the surface layer of the steel sheet.

[0065] These relationships regarding the strength of Sn in the steel sheet surface layer can be realized by the following specific techniques: the chemical composition of the resulting steel sheet contains 0.30 to 3.00% Si and satisfies Si / (Si + Mn) ≧ 0.20, the hot-rolled steel sheet is brush-ground after pickling, annealed at a high dew point, the residence time between 600 and 700°C in the annealing step is 12.5 seconds or less, and pickling is further performed after the annealing step. Note that a specific method for producing the steel sheet of this embodiment will be described later.

[0066] As mentioned above, the intensity of Sn measured by XPS from the surface of the steel sheet in the thickness direction is 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 and (I 484.9 ) / (I at a depth of 50 nm 486.5 When the content of dissolved Sn and oxidized Sn on the surface of the steel sheet is reduced so as to satisfy the condition (Ratio of Sn content to total dissolved Sn and oxidized Sn)≦0.7, the chemical conversion treatability of the surface of the steel sheet can be significantly improved, and as a result, excellent corrosion resistance after painting can be obtained.

[0067] In this embodiment, in order to obtain better corrosion resistance after painting, the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet is 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.3. 481-490 ) / (I at a depth of 50 nm 481-490 ) may be 0.2 or less.

[0068] In addition, in this embodiment, in order to obtain better corrosion resistance after painting, the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet is 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.5. 484.9 ) / (I at a depth of 50 nm 486.5 ) may be 0.4 or less.

[0069] (XPS measurement method) The XPS measurement may be carried out as follows.

[0070] First, a sample for measuring the Sn strength is prepared. Here, the sample is a steel sheet to be measured cut out from a target product. The sample has had oil and dirt removed from its surface using a solvent or the like. If the steel sheet has a paint film or a chemical conversion coating, or if the steel sheet is a steel sheet included in an automobile part, the paint film or the chemical conversion coating is removed by a paint film removal step or a chemical conversion coating removal step described below.

[0071] The intensity of the sample prepared as described above at a predetermined binding energy (BE) is measured from the surface of the sample to a depth of 50 nm in the thickness direction using, for example, a photoelectron spectrometer JPS-9200 manufactured by JEOL Ltd. Specifically, the measurement is performed according to the following procedure. First, the sample holder of the device is evacuated. The maximum pressure inside the holder during measurement is 3.0 × 10 -6 The pressure inside the holder during sputtering is 1.0×10 Pa at most. -5 It is Pa. Before measuring the sample, a 20 nm thick SiO2 wafer is used to measure the time required to remove 20 nm by sputtering. After measuring the outermost surface, the sample is sputtered for 2.5 times the time obtained above, and the strength is measured. -2 The measurement is performed in Ar gas at 100 Pa. The measurement value thus obtained is the measurement value at a depth of 50 nm.

[0072] The Sn intensity is determined by smoothing the BE values ​​at 0.1 eV intervals and using the BE value at 484.9 eV on the resulting curve.

[0073] And, I in XPS measurement 481-490 is the integral value of the emitted photoelectron intensity in the range of 481 eV≦BE≦490 eV.

[0074] In addition, I 484.9 is the emitted photoelectron intensity at BE=484.9 eV. Similarly, I486.5 is the emitted photoelectron intensity at BE=486.5 eV.

[0075] The specific measurement conditions for XPS are as follows: X-ray source: Mg source X-ray diameter: Φ1mm X-ray output: 12kV, 25mA Accelerating voltage: 3 kV

[0076] [Chemical conversion coating] As described above, the steel sheet of this embodiment may have a chemical conversion coating on its surface. The steel sheet of this embodiment has a reduced content of dissolved Sn and Sn oxide on the surface of the steel sheet and has excellent chemical treatability, so that a dense chemical conversion coating with a highly uniform structure can be formed on the surface of the steel sheet, and as a result, excellent corrosion resistance after painting can be exhibited.

[0077] The chemical conversion treatment liquid used to form the chemical conversion coating may be, for example, a known zinc phosphate-based chemical conversion treatment liquid or a zirconium-based chemical conversion treatment liquid.

[0078] (Thickness of steel plate) In this embodiment, the thickness of the steel plate may be a general thickness. Examples of such thicknesses include a thickness of 0.2 to 8.0 mm. Furthermore, the thickness of the steel plate 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 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less. When the steel sheet has a chemical conversion coating, the total thickness of the steel sheet is the above-mentioned thickness plus the thickness of the chemical conversion coating. The thickness of the chemical conversion coating may be, for example, 1 μm or more, 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The thickness of the chemical conversion coating may be, for example, 60 μm or less, 55 μm or less, 50 μm or less, 45 μm or less, or 40 μm or less.

[0079] (mechanical properties) [Vickers hardness] The steel sheet of this embodiment may have a Vickers hardness of 90 HV or more as a strength. 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 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.

[0080] 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 of the steel plate, excluding the edge, so that a cross section perpendicular to the surface (thickness cross section) can be observed. The thickness cross section of the cut test piece is polished using silicon carbide paper of #600 to #1500.

[0081] Next, a liquid in which diamond powder having a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water is used to polish the thickness cross section of the test piece to a mirror finish, and this thickness cross section is used as the measurement surface.

[0082] Next, the Vickers hardness of the test piece is measured at intervals of at least three times the indentation using a micro Vickers hardness tester under a load of 1 kgf. Specifically, measurements are taken at 20 random points in total near the 1 / 4 position of the plate thickness of the test piece, and the arithmetic average of these measurements is determined as the Vickers hardness of the steel plate.

[0083] <Parts> As described above, the steel sheet of this embodiment can achieve superior chemical conversion treatability and, in turn, superior corrosion resistance after painting compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the steel sheet of this embodiment is useful for use in parts and the like in technical fields that require superior corrosion resistance after painting. In particular, the steel sheet of this embodiment is useful for use in parts and the like in the automotive field.

[0084] 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 design quality. These parts may at least partially include the steel sheet according to an embodiment of the present invention. Therefore, these parts at least partially satisfy the characteristics of the steel sheet according to the above-described embodiment. In a part of a steel sheet that does not come into direct contact with a mold during press forming or other forming, or that comes into direct contact with a mold but is relatively lightly processed, the characteristics of the steel sheet do not change significantly before and after forming. For example, in a part including a steel sheet according to an embodiment of the present invention, the portion from which samples are taken (i.e., the portion avoiding the portions (i) to (iv) described below) can be recognized as a non-processed portion, and this portion retains the characteristics of the steel sheet according to the above-described embodiment before and after forming into a part.

[0085] The steel sheet according to the embodiment of the present invention may be used as the various automotive parts described above, for example, after a chemical conversion coating or paint film is optionally formed on the surface thereof. Whether or not an automotive part having a paint film or chemical conversion coating includes the steel sheet according to the embodiment of the present invention can be determined by removing the paint film or chemical conversion coating from a sample taken from the automotive part. In this case, the location from which the sample is taken, the paint film removal step, and the chemical conversion coating removal step are as follows.

[0086] (Sample collection location) When taking samples from automobile parts for various measurements and analyses, the following points (i) to (iv) shall be avoided. (i) Welds: Within 20 mm of the toe of spot welds and within 20 mm of the toe of arc / laser welds. (ii) Processed portion: A processed portion with a curvature radius of less than 15 mm, and a location within 5 mm of the processed portion. (iii) Edge: The edge within 5 mm of the cut end face of the part. (iv) Red rust: Areas within 5 mm of areas where red rust is visible.

[0087] (Paint film removal process) A paint remover (Neo River (registered trademark) #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface of a sample cut out from an automobile part or steel plate at room temperature and allowed to stand for 5 minutes. The surface of the sample to which the paint remover has been applied is then rubbed with a hard sponge (for example, Kanefiel (registered trademark), manufactured by Aion Co., Ltd.) to remove the paint from the surface of the sample.

[0088] Next, the surface of the sample after the coating film removal is washed with water and dried. At this time, the remaining state of the coating film is confirmed by SEM-EPMA measurement of the surface of the sample after washing and drying (100 μm square, 5 fields of view).

[0089] In the element distribution image obtained by EPMA, a region where the C concentration is 10 mass % or more is identified, and if the area ratio of this region is 5% or more, it is determined that the coating film has not been sufficiently peeled off.

[0090] To measure the area ratio of the region where the C concentration is 10 mass % or more, first, an element distribution image of C is obtained using an EPMA with the C concentration range set to 10 to 30%. The specific measurement conditions for the EPMA are as follows: Apparatus: JEOL Ltd. JXA-8230 electron probe microanalyzer Accelerating voltage: 15 kV Irradiation current: 0.05 μA Surface analysis:WDS Analysis interval: 300 μm or more Area ratio: Average value of 5 fields of view Next, the obtained C element distribution image is processed to measure the area fraction. The image analysis software "ImageJ" is used for image processing. Specifically, the above C element distribution image is loaded into ImageJ, and then binarized using "Make Binary" in "Binary" under "Process" so that areas with a C concentration of 10 mass% or more are displayed in black and areas with a C concentration of less than 10 mass% are displayed in white. After binarization, "Measure" under "Analyze" is used to read the value for "Area fraction" in "Results." This read value is determined as the area fraction of areas with a C concentration of 10 mass% or more.

[0091] If the coating film is not sufficiently removed, the removal of the coating film is repeated until the area ratio of the region where the C concentration is 10 mass % or more becomes less than 5%.

[0092] (chemical conversion coating removal process) When the chemical conversion coating needs to be removed from a sample cut from an automobile part or steel plate and the coating removed in order to perform various measurements such as XPS, the chemical conversion coating is removed using an appropriate method. For example, if the chemical conversion coating is a zinc phosphate coating, the chemical conversion coating is removed from the sample surface using a method in accordance with JIS K 3151:1996. Specifically, the chemical conversion coating is removed from the sample surface by immersing the removed sample in a 5% aqueous chromic acid solution heated to 75°C for 15 minutes.

[0093] Next, the surface of the sample after removing the chemical conversion coating is washed with water and dried. At this time, the state of remaining chemical crystals is confirmed by SEM-EPMA measurement of the surface of the sample after washing and drying (100 μm square, 5 fields of view).

[0094] In the element distribution image obtained by EPMA, a region where the P concentration is 5 mass % or more is identified, and if the area ratio of this region is 5% or more, it is determined that the chemical conversion coating has not been sufficiently peeled off.

[0095] To measure the area fraction of regions with a P concentration of 5% by mass or greater, first obtain an elemental distribution image of P using an EPMA with a P concentration range of 5-10%. Next, the obtained elemental distribution image of P is processed to measure the area fraction. The image analysis software "ImageJ" is used for image processing. Specifically, the above elemental distribution image of P 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 greater are displayed as black and regions with a P concentration of less than 5% by mass are displayed as white. After binarization, use "Measure" under "Analyze" to read the value of "Area fraction" in "Results." This value is determined as the area fraction of regions with a P concentration of 5% by mass or greater.

[0096] If the chemical conversion coating is not sufficiently removed, removal of the chemical conversion coating is repeated until the area ratio of the region where the P concentration is 5 mass % or more becomes less than 5%.

[0097] <Steel sheet manufacturing method> Next, a preferred method for manufacturing a steel plate according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel plate according to an embodiment of the present invention, but is not intended to limit the steel plate to one manufactured by the manufacturing method described below.

[0098] The steel sheet of this embodiment may be, for example, a casting process in which molten steel having the above-mentioned specific chemical composition is cast to form a steel billet, and a hot rolling process in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet. A pickling process in which the hot-rolled steel sheet is pickled, and a brush grinding process in which the surface of the pickled steel sheet is ground with a brush. A cold rolling process in which the brush-ground hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, and an annealing process in which the cold-rolled steel sheet is annealed under conditions of a high dew point of -10 ° C. or more and 20 ° C. or less and a residence time between 600 and 700 ° C. of 12.5 seconds or less, and a post-annealing pickling process in which the annealed cold-rolled steel sheet is pickled, and optionally, a chemical conversion treatment process in which the steel sheet is immersed in a chemical conversion treatment solution to form a chemical conversion treatment film on the surface of the steel sheet.

[0099] In the method for producing a steel sheet according to this embodiment, first, the chemical composition of molten steel is adjusted so that the final steel sheet has a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and satisfying Si / (Si+Mn)≧0.20. As a result, as described above, Mn oxides are formed inside the steel sheet using Si oxides as nuclei, suppressing the formation of Mn oxides on the surface of the steel sheet and facilitating the oxidation of Sn. In other words, the content of remaining metallic Sn is easily reduced.

[0100] Furthermore, in the method for producing a steel sheet according to this embodiment, a steel slab having the above-described specific chemical composition is hot-rolled and pickled, and then the surface of the hot-rolled steel sheet is brush-ground to remove at least a portion of the Sn remaining after pickling, which is unlikely to oxidize during annealing.

[0101] In addition, in the steel sheet manufacturing method of this embodiment, the cold-rolled steel sheet obtained by cold-rolling the brush-ground steel sheet is annealed under conditions of a high dew point of -10°C or higher and 20°C or lower, and a residence time between 600°C and 700°C of 12.5 seconds or less, thereby sufficiently oxidizing Sn and sufficiently reducing the amount of metallic Sn remaining on the surface of the steel sheet, i.e., solute Sn. Note that the temperature range of 600°C to 700°C is a temperature range in which Sn can diffuse to the surface in the steel, but Sn does not oxidize, so it is necessary to reduce the residence time of the steel sheet in this temperature range. If the residence time in this temperature range exceeds 12.5 seconds, even if Sn oxidizes, the amount of solute Sn on the surface of the steel sheet cannot be sufficiently reduced.

[0102] In the method for producing a steel sheet according to this embodiment, the cold-rolled steel sheet after annealing is pickled to remove Sn oxides and Si-based oxides from the surface of the steel sheet.

[0103] According to the manufacturing method of the steel sheet of this embodiment, even if the steel sheet contains three elements, Ni, Cu, and Sn, the intensity of Sn measured by X-ray photoelectron spectroscopy (XPS) in the thickness direction from the surface of the steel sheet is481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 and (I 484.9 ) / (I at a depth of 50 nm 486.5 Since the content of dissolved Sn and oxidized Sn on the surface of the steel sheet can be reduced so as to satisfy the condition (R)≦0.7, the chemical conversion treatability of the surface of the steel sheet can be significantly improved, and as a result, excellent corrosion resistance after painting can be obtained.

[0104] Hereinafter, preferred conditions for each step in the method for producing a steel sheet according to this embodiment will be described in detail.

[0105] [Casting process] In the method for producing a steel sheet according to this embodiment, the casting step is a step of casting molten steel with an adjusted chemical composition to form a steel slab. The chemical composition needs to be adjusted so that the chemical composition of the steel sheet finally obtained contains, in mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and satisfies Si / (Si+Mn)≧0.20.

[0106] The conditions for the casting process other than the chemical composition may be ordinary conditions known in the art. For example, the casting process may involve melting in a blast furnace, an electric furnace, or the like, followed by various secondary smelting processes, and then casting by ordinary continuous casting, ingot casting, or the like.

[0107] [Hot rolling process] In the method for producing a steel sheet according to this embodiment, the hot rolling step is a step of hot rolling a steel slab to obtain a hot-rolled steel sheet. The hot rolling step is performed by hot rolling a cast steel slab directly or after cooling it once and then reheating it. When reheating is performed, the heating temperature of the steel slab is, for example, 1100 to 1250°C.

[0108] In the hot rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling is 900 to 1050°C, and the reduction of finish rolling is 10 to 50%.

[0109] The hot-rolled steel sheet after finish rolling is coiled at a predetermined coiling temperature and then subjected to the subsequent pickling process. In this embodiment, the hot-rolled steel sheet is coiled at a coiling temperature of 520°C or higher. The coiling temperature may be 550°C or higher. The coiling temperature may also be 600°C or lower.

[0110] [Pickling process] In the method for producing a steel sheet according to this embodiment, the pickling step is a step of pickling the coiled hot-rolled steel sheet using a pickling solution. The pickling step may be carried out using a commonly used pickling solution, for example, a hydrochloric acid solution of a predetermined concentration containing an inhibitor that suppresses corrosion of the steel sheet, under conditions suitable for removing the outer and inner oxide layers of the hot-rolled steel sheet. The pickling may be carried out in one step, or may be carried out in multiple steps to ensure that the outer and inner oxide layers are completely removed.

[0111] [Brush grinding process] In the steel sheet manufacturing method of this embodiment, the brush grinding step is performed by grinding the hot-rolled steel sheet after pickling with a grinding amount of 3.0 g / m 2 This is a brush grinding process under the above conditions. By performing brush grinding under these conditions, it is possible to sufficiently remove the dissolved Sn on the surface of the steel sheet remaining after pickling, which is resistant to oxidation during the subsequent annealing process. As a result, it is possible to sufficiently reduce the amount of dissolved Sn on the surface of the steel sheet.

[0112] In the brush grinding step, not only Sn but also at least one of Ni and Cu can be removed from the surface of the steel sheet.

[0113] The amount of grinding by brush grinding is preferably as large as possible in order to more reliably remove dissolved Sn from the surface of the steel sheet. For example, 4.0 g / m 2It is preferable that the content is 5.0 g / m or more. 2 The upper limit of the amount of grinding by brush grinding is, for example, 20.0 g / m 2 less than 15.0 g / m 2 It may be the following:

[0114] The amount of grinding by brush grinding can be adjusted by any appropriate method known to those skilled in the art. For example, the amount of grinding by brush grinding can be adjusted by appropriately selecting the type of brush (e.g., H115 manufactured by Hotani Co., Ltd.), wire material, bristle length, rotation speed, density, brush pressure, and coating liquid to be used.

[0115] [Cold rolling process] In the steel sheet manufacturing method of this embodiment, the cold rolling step is a step of cold rolling the brush-ground hot-rolled steel sheet 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. The reduction ratio of the cold rolling is, for example, 20 to 80%. After the cold rolling step, the steel sheet may be cooled to room temperature by, for example, air cooling.

[0116] [Annealing process] In the method for producing a steel sheet according to this embodiment, the annealing step is a step in which the cold-rolled steel sheet after the cold-rolling step is annealed under conditions of a high dew point of −10° C. or more and 20° C. or less, and a residence time of 12.5 seconds or less between 600° C. and 700° C. By annealing the cold-rolled steel sheet under these specific conditions, Sn can be sufficiently oxidized, and the amount of solute Sn remaining on the surface of the steel sheet can be sufficiently reduced.

[0117] The dew point in the annealing step is preferably 0° C. or higher, and more preferably 10° C. or higher. The residence time between 600 and 700° C. in the annealing step is preferably 11.0 seconds or less, and more preferably 10.0 seconds or less. The residence time between 600 and 700° C. may be, for example, 5.0 seconds or more or 6.0 seconds or more.

[0118] In the annealing step, conditions other than the above-mentioned specific conditions may be those appropriate for annealing the cold-rolled steel sheet while sufficiently oxidizing Sn. For example, the annealing step includes heating to a temperature of 700 to 950°C in an atmosphere with a dew point of -10 to 20°C, and then holding the temperature for 0 to 300 seconds. However, when heating to the annealing temperature of 700 to 950°C, the residence time between 600 and 700°C must be 12.5 seconds or less.

[0119] The annealing temperature is preferably 750°C or higher, more preferably 780°C or higher. The annealing temperature is preferably 950°C or lower, more preferably 900°C or lower. The holding time at the annealing temperature is preferably 30 seconds or higher, more preferably 50 seconds or higher. The holding time is preferably 200 seconds or lower, more preferably 150 seconds or lower.

[0120] 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 with a hydrogen concentration of 1 to 10% (for example, 3% hydrogen and the balance nitrogen).

[0121] [Pickling process after annealing] In the steel sheet manufacturing method of this embodiment, the post-annealing pickling step is a step of pickling the annealed cold-rolled steel sheet to remove Sn oxides and Si-based oxides from the surface of the steel sheet. The post-annealing pickling step may be performed under conditions suitable for removing Sn oxides and Si-based oxides from the surface of the steel sheet using a commonly used pickling solution, for example, a hydrochloric acid solution of a predetermined concentration containing an inhibitor that suppresses corrosion of the steel sheet. For example, the post-annealing pickling step may be performed by immersing the annealed cold-rolled steel sheet in a hydrochloric acid solution of a predetermined concentration for a predetermined time.

[0122] The pickling solution used in the post-annealing pickling step is, for example, a hydrochloric acid solution with a concentration of 3 to 12%. In the post-annealing pickling step, the temperature at which the cold-rolled steel sheet is immersed in the pickling solution is, for example, 50 to 90° C. The time for which the cold-rolled steel sheet is immersed in the pickling solution is, for example, 1 to 30 seconds.

[0123] The pickling may be carried out in one step, or may be carried out in several steps in order to completely remove the Sn oxide and Si-based oxides.

[0124] [Chemical conversion treatment process] In the steel sheet manufacturing method of this embodiment, the chemical conversion treatment step is an optional step performed when manufacturing a steel sheet having a chemical conversion treatment film on its surface, and is a step of immersing the steel sheet after the annealing and pickling steps in a chemical conversion treatment solution to form a chemical conversion treatment film on the surface of the steel sheet. The chemical conversion treatment step may be performed under conditions appropriate for forming a chemical conversion treatment film on the surface of the steel sheet. For example, in the chemical conversion treatment step, before immersing the pickled steel sheet in the chemical conversion treatment solution, the steel sheet may be degreased with a degreasing agent, washed with water, and then the surface of the steel sheet may be treated with a surface conditioner.

[0125] Examples of the chemical conversion treatment liquid used in the chemical conversion treatment step include known zinc phosphate-based chemical conversion treatment liquids and zirconium-based chemical conversion treatment liquids.

[0126] 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.

[0127] 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. [Example]

[0128] 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.

[0129] First, molten steel was cast by continuous casting to form a billet having the chemical composition shown in Table 1. The billet was then cooled once, reheated to 1200°C, hot rolled, and coiled at a coiling temperature of 520°C or higher. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at a temperature of 900 to 1050°C and a reduction ratio of 30%.

[0130] Next, the obtained hot-rolled steel sheet was subjected to pickling, and then the surface of the hot-rolled steel sheet was polished with abrasive powder of 6.0 g / m using a grinding brush (H115 manufactured by Hotani Co., Ltd.). 2 The brush grinding was performed with a grinding amount of 10 ...

[0131] Next, the brush-ground hot-rolled steel sheet was cold-rolled at a rolling reduction of 50% to obtain a cold-rolled steel sheet.

[0132] The cold-rolled steel sheets were then annealed in a furnace with an oxygen concentration of 20 ppm or less, in an atmosphere with a dew point and hydrogen of 3% (nitrogen balance) as shown in Table 2, under the annealing conditions of heating to a temperature of 800°C and holding for 100 seconds. When heating to the annealing temperature, the residence time between 600 and 700°C was as shown in Table 2.

[0133] Furthermore, the annealed steel sheets were pickled by immersing them in a 5% hydrochloric acid solution at a temperature of 80°C for 5.0 seconds to obtain various steel sheets having a thickness of 1.6 mm as examples or comparative examples. When the chemical compositions of the steel sheets obtained in this manner were analyzed, they were found to be the same as those of the steel slabs before hot rolling.

[0134] [Table 1]

[0135] [Table 2]

[0136] For the various steel sheets obtained as described above, various XPS and Vickers hardness measurements were carried out according to the following measurement methods. Furthermore, the corrosion resistance of the various steel sheets after painting was evaluated according to the following evaluation method. The results of these measurements and evaluations are shown in Table 2. Note that the underlines next to various values ​​in Tables 1 and 2 indicate that the values ​​are outside the range of the present invention or are unfavorable manufacturing conditions.

[0137] (XPS measurement method) For the steel plate sample to be measured, the intensity at a predetermined binding energy (BE) is measured from the surface of the sample to a depth of 50 nm in the thickness direction using a photoelectron spectrometer JPS-9200 manufactured by JEOL Ltd. Specifically, the measurement is performed according to the following procedure. First, the sample holder of the device is evacuated. The maximum pressure inside the holder during measurement is 3.0 × 10 -6 The pressure inside the holder during sputtering is 1.0×10 Pa at most. -5 It is Pa. Before measuring the sample, a 20 nm thick SiO2 wafer is used to measure the time required to remove 20 nm by sputtering. After measuring the outermost surface, the sample is sputtered for 2.5 times the time obtained above, and the strength is measured. -2 The measurement is performed in Ar gas at 100 Pa. The measurement value thus obtained is the measurement value at a depth of 50 nm.

[0138] The Sn intensity is determined by smoothing the BE values ​​at 0.1 eV intervals and using the BE value at 484.9 eV on the resulting curve.

[0139] The integral of the emitted photoelectron intensity in the range of 481 eV ≤ BE ≤ 490 eV is I 481-490 Furthermore, the emitted photoelectron intensity at BE=484.9 eV is I 484.9 is the emitted photoelectron intensity at BE=486.5 eV, 486.5 respectively.

[0140] The specific measurement conditions for XPS are as follows: X-ray source: Mg source X-ray diameter: Φ1mm X-ray output: 12kV, 25mA Accelerating voltage: 3 kV

[0141] (Vickers hardness measurement method) The Vickers hardness is determined in accordance with JIS Z 2244-1:2024 as follows. First, a test piece is cut from any position on the steel plate, excluding the edges, so that a cross section perpendicular to the surface (thickness cross section) can be observed. The cut-out thickness cross section of the test piece is polished using #600 to #1500 silicon carbide paper. Next, the thickness cross section of the test piece is polished to a mirror finish using a diluted solution such as alcohol or a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in pure water, and this thickness cross section is used as the measurement surface.

[0142] Next, the Vickers hardness of the test piece is measured at intervals of at least three times the indentation using a micro Vickers hardness tester under a load of 1 kgf. Specifically, measurements are taken at 20 random points in total near the 1 / 4 position of the plate thickness of the test piece, and the arithmetic average of these measurements is determined as the Vickers hardness of the steel plate.

[0143] [Evaluation of corrosion resistance after painting] The corrosion resistance of the steel sheets after painting was evaluated as follows. First, a 50 mm x 50 mm sample of the manufactured steel plate to be evaluated was subjected to a degreasing treatment under the following conditions. Degreasing treatment: The sample was immersed in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes, and then rinsed with water.

[0144] Next, the degreased steel sheet samples were subjected to the following chemical conversion treatment: (i) zinc phosphate (Zn phosphate) treatment or (ii) zirconium (Zr) treatment. (i) Zn phosphate treatment Surface conditioning treatment: Immersed in a surface conditioning agent (Preparen Z) at room temperature for 30 seconds. Chemical conversion treatment: The plate was immersed in a zinc phosphate treatment agent (Palbond L3020 building material) at 40°C for 2 minutes, then washed with water and dried. (ii) Zr treatment Chemical conversion treatment: The plate was immersed in a Zr-based treatment agent (PLC-2010) at 45°C for 2 minutes, then washed with water and dried.

[0145] Next, an area of ​​50 mm in the L direction × 100 mm in the C direction of the steel plate sample that had been subjected to the chemical conversion treatment was subjected to electrodeposition coating under the following conditions. Electrodeposition liquid: Powernics Excel 1200 (manufactured by Nippon Paint Industrial Coating Co., Ltd.) Electrodeposition temperature: 30℃ Film thickness: 18 μm Electrodeposition baking: 30 minutes at 170°C

[0146] Furthermore, a cut was made in an area of ​​50 mm in the L direction and 100 mm in the C direction on the electrodeposition coated sample, and the JASO corrosion test "JASO M 609:1991" was performed for 30 cycles.

[0147] After the JASO corrosion test, the removed sample was dried and the maximum width of the coating blister was measured. The corrosion resistance of the sample after painting was evaluated based on the maximum width of the coating blister measured according to the following criteria. AAA: Maximum paint blister width is less than 0.9 mm AA: Maximum paint blister width is 0.9mm or more and less than 1.2mm A: Maximum paint blister width is 1.2mm or more and less than 1.5mm B: Maximum paint blister width is 1.5mm or more

[0148] Steel sheets that were rated AAA, AA, or A for corrosion resistance after painting were evaluated as having excellent corrosion resistance after painting, while steel sheets that were rated B for corrosion resistance after painting were evaluated as having poor corrosion resistance after painting.

[0149] As shown in Table 2, the Si content is high, the Si / (Si+Mn) ratio is 0.20 or more, and the Sn intensity measured by XPS in the thickness direction from the surface of the steel sheet is 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 and (I of depth 50 nm) 484.9 ) / (I at a depth of 50 nm 486.5 It was found that all of the steel sheets of Examples Nos. 1 to 29, which satisfied the condition of 0.7≦0.7, were steel sheets with excellent corrosion resistance after painting.

[0150] In particular, the steel sheets of Examples Nos. 2, 3, 5, 6, 9, 12, 15, 16, 18, 19, 22, and 23, which were annealed under conditions where the dew point was 0°C or higher or the residence time between 600 and 700°C was 10.0 seconds or less in the annealing process, further showed a good adhesion to the outermost surface of the steel sheets. 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.3, or (I 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.5, and it was found that the coating had better corrosion resistance after painting.

[0151] Furthermore, the steel sheets of Examples Nos. 4, 7, 10, 13, 20, and 24, which were annealed under the conditions of a dew point of 0°C or higher and a residence time of 10.0 seconds or less between 600 and 700°C in the annealing process, showed a good annealing effect (I 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.3 and (I 484.9 ) / (I at a depth of 50 nm 486.5 )≦0.5, and it was found that the coating had extremely excellent corrosion resistance.

[0152] On the other hand, if the Si content is low, Si / (Si+Mn) is less than 0.20, or if the I 481-490 ) / (I at a depth of 50 nm 481-490 )≦0.5 or (I 484.9 ) / (I at a depth of 50 nm 486.5 It was found that the steel sheets of Comparative Examples Nos. 30 to 37, which did not satisfy the condition 0.7≦0.7, were all steel sheets with poor corrosion resistance after painting.

[0153] Specifically, it is presumed that because steel sheet No. 30 has a low Si / (Si+Mn) ratio, Mn oxides are mainly formed on the surface of the steel sheet, suppressing the oxidation of Sn.

[0154] It is presumed that the steel sheet No. 31 has a low Si content, which reduces the starting points for nucleation of internal oxidation of Sn, thereby suppressing Sn oxidation.

[0155] It is presumed that the oxidation of Sn was suppressed in steel sheet No. 32 because the Si content was low and the Si / (Si+Mn) ratio was low.

[0156] Since steel sheet No. 33 was not brush ground, it is presumed that the Sn-enriched areas formed during pickling were not completely oxidized during annealing and remained in a metallic state.

[0157] It is presumed that the No. 34 steel sheet was not pickled after annealing, and therefore the Sn oxide on the surface was not removed.

[0158] It is presumed that the dew point of steel sheet No. 35 during annealing was low, and therefore Sn did not oxidize sufficiently.

[0159] It is presumed that steel sheet No. 36 had a long residence time between 600 and 700°C, at which Sn does not oxidize, and the Sn was not sufficiently oxidized, resulting in it concentrating in the metallic state on the surface of the steel sheet.

[0160] It is presumed that the dew point during annealing of steel sheet No. 37 was high, and Fe oxide was mainly formed on the surface of the steel sheet, suppressing the oxidation of Sn.

Claims

1. A steel plate, The chemical composition of the steel plate is, in mass%, C: 0.001 to 0.500%, Si: 0.30-3.00%, Mn: 0.10-3.00%, Al: 0.001-2.000%, Ni: 0.010 to 1.000%, Cu: 0.010-1.000%, Sn: 0.003-1.000%, 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: 0-1.000%, Cr: 0-1.000%, V: 0 to 0.150%, W: 0-1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.500%, Ca: 0-0.050%, REM: 0-0.100%, As: 0 to 0.100%, Ir: 0 to 1.000%, and Remainder: Fe and impurities consists of, and Si / (Si+Mn)≧0.20 is satisfied, The intensity of Sn measured by X-ray photoelectron spectroscopy in the thickness direction from the surface of the steel plate is (I on the top surface ４８１－４９０ ) / (I at a depth of 50 nm ４８１－４９０ )≦0.5, and, (I at a depth of 50 nm ４８４．９ ) / (I at a depth of 50 nm ４８６．５ ) ≦0.

7.

2. The intensity of Sn measured by X-ray photoelectron spectroscopy in the thickness direction from the surface of the steel plate is (I on the top surface ４８１－４９０ ) / (I at a depth of 50 nm ４８１－４９０ 2. The steel sheet according to claim 1, wherein the following condition is satisfied: )≦0.

3.

3. The intensity of Sn measured by X-ray photoelectron spectroscopy in the thickness direction from the surface of the steel plate is (I at a depth of 50 nm ４８４．９ ) / (I at a depth of 50 nm ４８６．５ 3. The steel sheet according to claim 1, wherein the following condition is satisfied: )≦0.

5.

4. The chemical composition of the steel plate is, in mass%, The steel plate according to claim 1 or 2, characterized in that it contains Si: 0.50 to 3.00%.

5. The chemical composition of the steel plate is, in mass%, Ni: 0.040-1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004-1.000% The steel sheet according to claim 1 or 2, characterized in that it comprises:

6. The steel sheet according to claim 1 or 2, wherein the surface of the steel sheet has a chemical conversion coating.

7. A component, characterized in that it comprises a steel sheet according to claim 1 or 2.