Steel plates and parts containing them

Abstract

The present invention provides a steel sheet containing Ni, Cu, and Sn, which has excellent chemical conversion treatability even after a certain period of time has passed since degreasing, and a part including the steel sheet. The steel sheet and parts including the same of the present invention have a chemical composition of the steel sheet containing, in mass%, Mn: 1.20 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, Sn: 0.003 to 1.000%, and Si: 0.01 to less than 0.75%, and satisfy Mn / (Si+Mn)>0.80, and the number N of Mn oxides per 10 μm exposed on the surface of the steel sheet and having an equivalent circle diameter of 30 nm or more is 30 nm. 30 But, N 30 ≧20.

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 mechanical properties such as strength and formability required of automotive steel sheets, as well as chemical stability such as corrosion resistance, and that copper compounds present on the surface of the steel sheets in particular reduce the ability to be chemically treated to improve corrosion resistance.

[0007] Furthermore, chemical conversion treatment of steel sheets is generally performed by degreasing the steel sheet with a degreasing agent and then immersing it in a chemical conversion treatment solution, but particularly when the steel sheet contains copper (Cu), nickel (Ni), and tin (Sn), the time from degreasing the steel sheet to chemical conversion treatment must be very strictly controlled and kept short. If a certain time (e.g., 10 minutes or more) has passed since degreasing, there is a risk that the chemical conversion treatability will be significantly reduced.

[0008] Therefore, an object of the present invention is to provide a steel sheet containing Ni, Cu, and Sn, which has excellent chemical conversion treatability even after a certain period of time has passed since degreasing, and a part including the steel sheet. [Means for solving the problem]

[0009] The present invention includes at least the following aspects.

[0010] (Aspect 1) A steel plate, The chemical composition of the steel plate is, in mass%, Mn: 1.20 to 3.00% Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, Sn: 0.003 to 1.000%, and Si: 0.01 to less than 0.75% and Mn / (Si+Mn)>0.80 is satisfied, The number of Mn oxide particles per 10 μm with a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet 30 but, N 30 ≧20.

[0011] (Aspect 2) 10 of Mn oxides having a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet Number per μm N 30 but, N 30 3. The steel sheet according to claim 1, wherein the steel sheet satisfies a condition of ≧30.

[0012] (Aspect 3) The Cu concentration measured by high-frequency glow discharge optical emission spectroscopy in the thickness direction from the surface of the steel sheet is 3. The steel sheet according to the above aspect 1 or 2, wherein (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm)≦10.0 is satisfied.

[0013] (Aspect 4) The Cu concentration measured by high-frequency glow discharge optical emission spectroscopy in the thickness direction from the surface of the steel sheet is 3. The steel sheet according to the above aspect 1 or 2, wherein (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm)≦7.0 is satisfied.

[0014] (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:

[0015] (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.

[0016] (Aspect 7) Aspect 7. The steel sheet according to aspect 6, wherein the ratio of hopite in chemical conversion crystals in the chemical conversion coating is 50% or more, as measured by X-ray diffraction.

[0017] (Aspect 8) A part comprising the steel sheet according to any one of the above aspects 1 to 7. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a steel sheet containing Ni, Cu, and Sn, which has excellent chemical conversion treatability even after a certain period of time has passed since degreasing, and a part including the steel sheet. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a schematic diagram for explaining a method for measuring the number N30 of Mn oxides per 10 μm that are exposed on the surface of a steel sheet and have an equivalent circle diameter of 30 nm or more. [Figure 2] FIG. 2 is a schematic diagram for explaining a method for measuring the circle-equivalent diameter R of Mn oxides exposed on the surface of a steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0020] Generally, when the chemical conversion treatability of steel sheet deteriorates, regions where the 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 the steel sheet as solid solutions, the potential of the steel sheet becomes more noble than when these elements are not present in the steel sheet, which can reduce the etching ability of Fe during chemical conversion treatment. In such cases, the formation of a chemical conversion coating becomes difficult, resulting in reduced chemical conversion treatability and, consequently, reduced corrosion resistance. Therefore, this reduced chemical conversion treatability is particularly problematic when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. Furthermore, when elements such as Ni, Cu, and Sn are present in the steel sheet as solid solutions, the steel sheet is prone to forming an oxide coating. Therefore, unless the time between degreasing and chemical conversion treatment is strictly controlled and kept short, the chemical conversion treatability is significantly reduced, resulting in reduced corrosion resistance.

[0021] 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.

[0022] Therefore, the present inventors conducted extensive research to address the decline in phosphatability when a certain time (i.e., 10 minutes) has elapsed between degreasing and phosphatability. As a result, they discovered that Mn outer oxides can dissolve in place of the base material during phosphatability treatment and precipitate phosphatable crystals, thereby improving the phosphatability of steel sheets when a certain time (i.e., 10 minutes) has elapsed between degreasing and phosphatability treatment. On the other hand, it is believed that the concentration of Cu in the surface layer of the steel sheet promotes the oxidation of Si and, if present, Al contained in the steel, leading to the formation of outer oxides of these elements on the surface of the steel sheet. The formation of outer oxides of these elements inhibits the formation of Mn outer oxides, preventing the sufficient improvement in phosphatability achieved by the Mn outer oxides. In this specification, oxides formed on the surface of the steel sheet and exposed from the surface of the steel sheet are referred to as "external oxides," while oxides formed inside the steel sheet and not exposed from the surface of the steel sheet are referred to as "internal oxides."

[0023] The present inventors have discovered 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 1.20 to 3.00% of Mn, 0.010 to 1.000% of Ni, 0.010 to 1.000% of Cu, 0.003 to 1.000% of Sn, and less than 0.01 to 0.75% of Si, and satisfies Mn / (Si+Mn)>0.80, and that the hot-rolled steel sheet is pickled, then brush-ground, and then annealed under specific dew point conditions and heat pattern, thereby reducing the number N of Mn oxides per 10 μm, each of which has an equivalent circle diameter of 30 nm or more and is exposed on the surface of the steel sheet. 30 But, N 30 The inventors have found that by forming such a large amount of Mn oxide on the surface of the steel sheet, it is possible to form a large amount of Mn oxide on the surface of the steel sheet, satisfying the condition of Mn content (Mn content) ≥ 20. The inventors have also found that by forming such a large amount of Mn oxide on the surface of the steel sheet, the Mn oxide dissolves in place of the base material during chemical conversion treatment, allowing a sufficient amount of chemical conversion crystals to precipitate, thereby significantly improving chemical conversion treatability after a certain time (i.e., 10 minutes) has elapsed since degreasing.

[0024] The present invention has been completed based on the above findings, and includes the following embodiments.

[0025] Hereinafter, preferred embodiments of the steel sheet of the present invention will be described in detail.

[0026] <Steel plate> A steel sheet according to one embodiment of the present invention has a specific chemical composition containing, in mass%, 1.20 to 3.00% Mn, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, 0.003 to 1.000% Sn, and less than 0.01 to 0.75% Si, and satisfies Mn / (Si+Mn)>0.80.

[0027] The steel sheet of this embodiment has a number N per 10 μm of Mn oxides having a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet. 30 But, N 30 ≧20.

[0028] 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.

[0029] However, in steel sheets containing Ni, Cu, and Sn in solid solution, the potential of the steel sheet becomes more noble than in steel sheets containing no such elements, which may reduce the etching ability of Fe during chemical conversion treatment. Furthermore, as mentioned above, Cu concentrates on the surface of the steel sheet during the annealing process, promoting the oxidation of Si and, if present, Cr contained in the steel, forming outer oxides of these elements on the surface of the steel sheet. Since the outer oxides of these elements may not dissolve sufficiently in the chemical conversion treatment solution during chemical conversion treatment, the presence of excessive outer oxides of these elements on the surface of the steel sheet inhibits the dissolution of the base material, making it difficult for chemical crystals to precipitate, and reducing the chemical conversion treatability of the steel sheet.

[0030] The steel sheet of this embodiment contains three elements, Ni, Cu, and Sn, but the number N per 10 μm of Mn oxides having a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet is measured by a method described later. 30 But, N 30 Since a large amount of Mn oxides, satisfying the condition (MnO ≥ 20), is formed on the surface of the steel sheet, the Mn oxides dissolve in place of the base material during chemical conversion treatment, allowing a sufficient amount of chemical conversion crystals to precipitate, and excellent chemical conversion treatability can be exhibited even after a certain time (i.e., 10 minutes) has passed since degreasing. The reason why chemical conversion treatability deteriorates after a certain time has passed since degreasing is because Sn, Ni, and Cu in the base material are likely to form an oxide film after degreasing. However, in the steel sheet of this embodiment, by forming an outer oxide of Mn, even if an oxide film is formed, the outer oxide of Mn dissolves instead, ensuring chemical conversion treatability.

[0031] 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 superior chemical conversion treatability and corrosion resistance are required.

[0032] Hereinafter, each component of the steel sheet of this embodiment will be described in detail.

[0033] [Chemical composition] In this embodiment, the steel sheet has a specific chemical composition containing, by mass%, 1.20 to 3.00% Mn, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, 0.003 to 1.000% Sn, and less than 0.01 to 0.75% Si, and satisfying Mn / (Si+Mn)>0.80. As described above, the present invention aims to provide a steel sheet containing Ni, Cu, and Sn, which has excellent chemical conversion treatability even after a certain period of time has passed since degreasing, and the steel sheet has a specific chemical composition containing, by mass%, 1.20 to 3.00% Mn, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, 0.003 to 1.000% Sn, and less than 0.75% Si, and the specific chemical composition satisfies Mn / (Si+Mn)>0.80. 30 But, N 30 The object is achieved by forming a large amount of Mn oxide on the surface of the steel sheet, which satisfies the condition of Mn ≥ 20.

[0034] Therefore, the chemical composition of the steel sheet is not particularly limited except that it contains, in mass%, Mn: 1.20 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, Sn: 0.003 to 1.000%, and Si: 0.01 to less than 0.75%, and satisfies Mn / (Si+Mn)>0.80.

[0035] Regarding 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%.

[0036] 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.

[0037] 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.

[0038] For example, the steel plate of this embodiment has, in mass%, C: 0.001 to 0.500%, Si: 0.01 to less than 0.75% Mn: 1.20~3.00%, 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 Mn / (Si+Mn)>0.80.

[0039] Each of these elements will be described in more detail below.

[0040] [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 reduced 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.

[0041] [Si: 0.01 to less than 0.75%] Si is an effective solid-solution strengthening element for increasing strength. Si also contributes to the formation of outer Mn oxides, which form around Si oxide nuclei. To fully achieve these effects, the Si content is set to 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.15% or more, 0.20% or more, 0.25% or more, or 0.30% 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 less than 0.75%. The Si content may be 0.70% or less, 0.65% or less, 0.60% or less, 0.55% or less, or 0.50% or less. The Si content must satisfy the relationship Mn / (Si + Mn) > 0.80, as described below.

[0042] [Mn: 1.20~3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. Furthermore, Mn is an important element that forms an outer oxide on the surface of steel sheet, contributing to improved phosphatability. To fully achieve this effect, the Mn content is set to 1.20% or more. The Mn content may be 1.40% or more, 1.60% or more, 1.80% or more, or 2.00% or more. On the other hand, excessive Mn content may increase the steel strength but reduce elongation. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less or 2.60% or less. The Mn content must satisfy the relationship Mn / (Si+Mn)>0.80, as described below.

[0043] [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.

[0044] [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.

[0045] [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.

[0046] [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.

[0047] [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.

[0048] [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.

[0049] [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.

[0050] 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.

[0051] [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.

[0052] [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.

[0053] [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.

[0054] [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.

[0055] [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.

[0056] [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.

[0057] 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.

[0058] [Mn / (Si+Mn)>0.80] As described above, the steel sheet of this embodiment is required to have the Si and Mn contents satisfy the relationship Mn / (Si+Mn)>0.80. In this relationship, Mn means the Mn content in mass%. Similarly, Si means the Si content in mass%.

[0059] It is known that Si and Mn in steel sheets form oxides during annealing. However, if the Si content in a steel sheet is high (for example, 1.00 mass% or more) and the Mn content is low, Mn oxides are less likely to form on the surface of the steel sheet, and a sufficient amount of Mn oxide may not form on the surface of the steel sheet.

[0060] Therefore, in the steel sheet of this embodiment, the Mn content is set sufficiently high so that Mn / (Si+Mn) exceeds 0.80, thereby promoting the formation of outer oxides of Mn and realizing the formation of a sufficient amount of Mn oxide on the surface of the steel sheet.

[0061] The ratio Mn / (Si+Mn) may be 0.81 or more, 0.82 or more, 0.83 or more, 0.84 or more, or 0.85 or more. The upper limit of the ratio Mn / (Si+Mn) may be 1.00, 0.99, 0.98, 0.97, 0.96, or 0.95.

[0062] 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 composition can be determined by measuring it using a measuring device such as Shimadzu's ICPS-8100 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.

[0063] [Number of Mn oxide particles per 10 μm with a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet N 30 ] [N 30 ≥ 20] As described above, the steel sheet of this embodiment has a number N per 10 μm of Mn oxides having a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet. 30 But, N 30 ≧20.

[0064] 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".

[0065] The manganese oxides exposed on the surface of the steel sheet and having an equivalent circle diameter of 30 nm or more are outer oxides of manganese having an equivalent circle diameter R of 30 nm or more as measured by the measurement method described below. These manganese oxides include oxides of manganese only (MnO) as well as composite oxides of manganese and silicon (specifically, Mn2SiO4, MnSiO3, and MnCr2O4).

[0066] Such Mn oxides exposed on the surface of a steel sheet can be achieved by a specific method in which the chemical composition of the resulting steel sheet contains 1.20 to 3.00% Mn, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, 0.003 to 1.000% Sn, and less than 0.01 to 0.75% Si, and satisfies Mn / (Si + Mn) > 0.80, and the hot-rolled steel sheet is pickled, brush-ground, and then annealed under specific dew point and temperature conditions. A specific method for producing the steel sheet of this embodiment will be described later.

[0067] As mentioned above, the number of Mn oxide particles per 10 μm with a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet, N 30 But, N 30 When a large amount of Mn oxides is formed on the surface of a steel sheet, satisfying the condition of

[0045] ≥ 20, the Mn oxides dissolve in place of the base material during chemical conversion treatment, allowing a sufficient amount of chemical conversion crystals to precipitate, and excellent chemical conversion treatability can be exhibited even after a certain period of time has passed since degreasing.

[0068] In this embodiment, in order to obtain better chemical conversion treatment properties, the number N of Mn oxide particles per 10 μm having a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet is set to 1. 30 But, N 30 It is preferable that the number N of Mn oxide particles having an equivalent circle diameter of 30 nm or more per 10 μm exposed on the surface of the steel sheet is satisfied. 30 The number N of Mn oxides per 10 μm having an equivalent circle diameter of 30 nm or more exposed on the surface of the steel sheet may be 32 or more, 34 or more, 36 or more, 38 or more, or 40 or more. 30 may be 60 or less or 50 or less.

[0069] The number of Mn oxide particles per 10 μm with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel sheet, N 30 can be measured by energy dispersive X-ray spectroscopy (TEM / EDS) using a transmission electron microscope as follows. Figure 1 shows the number N of Mn oxides per 10 μm with a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet. 30 2 is a schematic diagram for explaining a method for measuring the size R of the circle-equivalent diameter of Mn oxides exposed on the surface of a steel sheet.

[0070] (The number of Mn oxide particles per 10 μm with a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet) 30 (Method of measurement) In TEM / EDS analysis, first, a sample is taken so that the observation surface is a cross section parallel to the L direction (rolling direction) and thickness direction (plate thickness direction) of the steel sheet to be measured. If the rolling direction is unknown, cross sections cut in the thickness direction at angles of 0°, 45°, 90°, and 135° to an arbitrary direction are observed, and the cross section with the highest aspect ratio of precipitates is taken as the cross section parallel to the rolling direction and thickness direction.

[0071] Next, as a pretreatment for TEM / EDS analysis, a carbon protective film is formed on the surface of the sampling area using FIB. Subsequently, a thin film sample is prepared using the FIB-cross-sectional μ-sampling method. A Hitachi High-Tech NB5000 FIB-SEM, for example, is used, with an acceleration voltage of 5 to 40 kV during FIB processing and a Mo mesh.

[0072] Then, a TEM, such as a JEM-2100F manufactured by JEOL Ltd., is used as a TEM, and a JED-300T manufactured by JEOL Ltd. is used as an EDS analyzer, such as a JED-300T manufactured by JEOL Ltd., is used to perform TEM / EDS analysis under conditions of an acceleration voltage of 200 kV. Observation is performed using BF-STEM images at a magnification of, for example, 500,000x. By creating a series of photographs as shown in Figure 1, a BF-STEM image and an EDS mapping image are obtained over a range of 10 µm or more. Note that Figure 1 schematically shows the state of multiple Mn oxides 2 exposed on the surface S1 of the steel sheet 1 in a series of photographs with a measurement field of view of 1 µm x 1 µm.

[0073] From the BF-STEM images and EDS mapping images obtained in the range of 10 μm or more, the number N of Mn oxides per 10 μm with a circle equivalent diameter R of 30 nm or more exposed on the surface of the steel sheet was determined. 30 Count.

[0074] Here, the Mn oxide exposed on the surface of the steel sheet is, for example, as shown in FIG. 2, a Mn oxide layer on a line L along the surface S1 of the steel sheet in a cross section of the steel sheet 1 cut in the thickness direction. s This refers to Mn oxide 2 that intersects with

[0075] The size R (nm) of the Mn oxide exposed on the surface of the steel sheet was calculated as the area S (nm) of the oxide part, as shown in Figure 2. The obtained Mn mapping image was binarized (min = 0, max = 255) using the image analysis software "ImageJ" to separate the Mn oxide part from the steel part. 2 ) and calculate the following equation: R = (S / π) 0.5 It is calculated as follows.

[0076] The Mn oxides were defined as those parts that were identified as Mn oxides (MnO, MnCr2O4, Mn2SiO4, and MnSiO3) by analyzing the diffraction image in TEM / EDS analysis, among the parts that were recognized as Mn oxides when the Mn mapping image was binarized using "ImageJ."

[0077] The number of Mn oxide particles per 10 μm with a circle equivalent diameter R of 30 nm or more exposed on the surface of the above steel sheet, N 30 Measurements are made at five randomly selected locations, and the arithmetic mean value is used.

[0078] Furthermore, when the steel sheet to be measured is a chemically treated steel sheet, the oxides exposed on the surface of the steel sheet are those exposed on the surface of the steel sheet in areas where chemical crystals are not attached, and those present at the interface between the chemical crystals and the base steel in areas where chemical crystals are attached.

[0079] [Cu concentration measured from the surface of the steel sheet through the thickness direction using radio frequency glow discharge optical emission spectroscopy] [(Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm)≦10.0] Furthermore, in the steel sheet of this embodiment, the Cu concentration measured by high-frequency glow discharge optical emission spectroscopy (GDS) from the surface of the steel sheet in the thickness direction preferably satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm)≦10.0. Note that "Cu concentration at a depth of 5 nm" refers to the Cu concentration at a position 5 nm deep from the surface of the steel sheet in the thickness direction. Similarly, "Cu concentration at a depth of 10 μm" refers to the Cu concentration at a position 10 μm deep from the surface of the steel sheet in the thickness direction.

[0080] As described above, Cu concentrates on the surface of the steel sheet during the annealing process, promoting the oxidation of Si and, if present, Cr contained in the steel, causing the formation of outer oxides of these elements on the surface of the steel sheet. Therefore, the steel sheet of this embodiment reduces the number of Cu-enriched areas on the surface of the steel sheet (hereinafter, sometimes referred to as "Cu-enriched areas"). This allows more Mn oxide to form on the surface of the steel sheet, thereby achieving better chemical conversion treatability. The Cu-enriched areas formed on the surface of the steel sheet are not only Cu-enriched compared to the center of the steel sheet in the thickness direction, but also Cu-enriched compared to other areas on the surface of the steel sheet.

[0081] A means for reducing Cu-enriched portions on the surface of the steel sheet will be described in detail later in connection with the manufacturing method, but this can be achieved by brush grinding the surface of the hot-rolled steel sheet after pickling.

[0082] To further improve chemical conversion treatability, it is more preferable that the Cu concentration measured by GDS from the surface of the steel sheet in the thickness direction satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm)≦7.0. The Cu concentration at a depth of 5 nm / (Cu concentration at a depth of 10 μm) may be 6.5 or less, 6.0 or less, or 5.5 or less. Furthermore, the Cu concentration at a depth of 5 nm / (Cu concentration at a depth of 10 μm) may be 0 or more, 1.0 or more, 1.5 or more, 2.0 or more, or 2.5 or more.

[0083] The Cu concentration can be measured by GDS measurement according to the following measurement method.

[0084] (Method for measuring Cu concentration by GDS measurement from the surface of steel sheet to the thickness direction) GDS measurements of Cu concentration are performed using a high-frequency glow discharge optical emission spectrometer. Specifically, the surface of the steel plate to be measured is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The surface of the steel plate is then sputtered and analyzed in the depth direction. The Cu element contained in the steel plate is then identified from the emission spectrum wavelength specific to Cu, which is emitted when atoms are excited in the glow plasma, and the emission intensity of the identified Cu element is estimated.

[0085] Depth data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, sputtering time can be converted into sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the steel plate in the thickness direction.

[0086] When the steel sheet to be measured is a chemically treated steel sheet, the surface of the steel sheet is the interface between the base steel and the chemically treated coating.

[0087] 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: 4mmφ RF output: 30W Measurement time: 200 to 1500 seconds

[0088] [Chemical conversion coating] The steel sheet of this embodiment may have a chemical conversion coating on its surface. The steel sheet of this embodiment has a certain amount or more of Mn oxides formed 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, the steel sheet can exhibit excellent corrosion resistance.

[0089] 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.

[0090] Furthermore, in the steel sheet of this embodiment, the ratio of hopite (Zn3(PO4)2.4H2O) in the chemical conversion crystals in the chemical conversion coating, as measured by X-ray diffraction (XRD), is preferably 50% or more.

[0091] In the case of chemical conversion treatment using a zinc phosphate-based chemical conversion treatment solution, chemical crystals are formed starting from the manganese oxides formed on the surface of the steel sheet, resulting in a high ratio of hopeite. If the ratio of hopeite in the chemical crystals in the chemical conversion treatment film is 50% or more, a certain amount or more of manganese oxides will be reliably formed on the surface of the steel sheet, making it possible to more reliably achieve the excellent chemical conversion properties described above.

[0092] Here, the ratio of hopite in the chemical conversion crystals in the chemical conversion coating is a ratio calculated by the following formula, where H is the integrated intensity of the hopite peak and P is the integrated intensity of the phosphophyllite peak. Hopite ratio (%) = H / (H+P) x 100 The integrated intensity of each peak is calculated by performing XRD analysis using a Cr tube, separating the phosphophyllite peak (2θ = 14.88°) and the hopite peak (2θ = 14.55°), and then calculating the integrated value of each peak intensity.

[0093] The XRD analysis may be carried out using, for example, an X-ray diffractometer such as "EMPYREAN" (registered trademark) manufactured by Malvern Panalytical under the following conditions. Tube: Cr Detector: 1Der Output: 45kV, 40mA Measurement range: 2θ=10~130°

[0094] (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.

[0095] (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. The Vickers hardness is determined in accordance with JIS Z 2244-1:2024 as follows.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] <Parts> As described above, the steel sheet of this embodiment can achieve excellent chemical conversion treatability and therefore excellent corrosion resistance, even after a certain period of time has passed since degreasing, 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 excellent chemical conversion treatability and corrosion resistance. In particular, the steel sheet of this embodiment is useful for use in parts and the like in the automotive field.

[0100] 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.

[0101] 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.

[0102] (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.

[0103] (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.

[0104] 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).

[0105] 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.

[0106] 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.

[0107] 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%.

[0108] (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 to perform various measurements, 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.

[0109] 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).

[0110] 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.

[0111] 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.

[0112] 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%.

[0113] <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.

[0114] 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, 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, a brush grinding process in which the surface of the steel sheet after pickling is ground with a brush, and 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 specific conditions in which the dew point when 600 ° C. is reached is -10 ° C. or higher, the dew point at the highest temperature reached is 5 ° C. or higher than the dew point of 600 ° C., and the residence time between 600 and 700 ° C. is 40 seconds or more, and 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.

[0115] In the method for producing a steel sheet of this embodiment, first, the chemical composition of the molten steel is adjusted so that the chemical composition of the steel sheet finally obtained contains, in mass%, 1.20 to 3.00% Mn, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, 0.003 to 1.000% Sn, and less than 0.01 to 0.75% Si, and satisfies Mn / (Si+Mn)>0.80, i.e., so that the Mn content is high, thereby facilitating the formation of a large amount of Mn oxide on the surface of the steel sheet.

[0116] 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, pickled, and then the surface of the hot-rolled steel sheet is brush-ground to remove Cu-enriched areas and Mn-depleted layers that inhibit the formation of outer Mn oxides. Note that the Mn-depleted layer is formed as a Mn-depleted layer with a low Mn content in the surface layer of the steel sheet due to the formation of an internal Mn oxide layer during coiling after hot rolling. The presence of such a Mn-depleted layer in the surface layer of the steel sheet makes it difficult for Mn oxides to form on the surface of the steel sheet.

[0117] In the method for producing a steel sheet according to the present embodiment, the cold-rolled steel sheet obtained by cold-rolling the brush-ground steel sheet is annealed under specific conditions in which the dew point at 600°C is -10°C or higher, the dew point at the highest temperature reached is 5°C or higher than the dew point at 600°C, and the residence time between 600 and 700°C is 40 seconds or longer, thereby forming Si oxides inside the steel sheet and increasing the number N per 10 μm of Mn oxides having a circle-equivalent diameter of 30 nm or more. 30 But, N 30 In this case, a certain amount or more of Mn oxide is formed on the surface of the steel sheet, satisfying the condition (1) above.

[0118] To explain the specific conditions of this annealing step in more detail, first, the cold-rolled steel sheet is annealed at a dew point of -10°C or higher, which facilitates the formation of Mn oxides. Furthermore, by making the dew point at the maximum temperature 5°C or higher than the dew point at 600°C, the formation of an outer Mn oxide prevails over the formation of an inner complex oxide of Mn and Si. Then, by setting the residence time between 600 and 700°C to 40 seconds or longer, Si within the steel sheet is consumed to form an inner Si oxide, which suppresses the formation of an inner complex oxide of Mn and Si at 700°C or higher, resulting in the dominant formation of an outer Mn oxide.

[0119] According to the method for producing a steel sheet of this embodiment, even if the steel sheet contains three elements, Ni, Cu, and Sn, the number N per 10 μm of Mn oxides having a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet can be reduced. 30 But, N 30 ≧20, a certain amount of Mn oxide can be formed on the surface of the steel sheet, and excellent chemical conversion treatability can be obtained even if a certain amount of time has passed since degreasing.

[0120] Hereinafter, preferred conditions for each step in the method for producing a steel sheet according to this embodiment will be described in detail.

[0121] [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%, 1.20 to 3.00% Mn, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, 0.003 to 1.000% Sn, and less than 0.01 to 0.75% Si, and satisfies Mn / (Si+Mn)>0.80, i.e., a specific chemical composition with a high Mn content.

[0122] 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.

[0123] [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.

[0124] 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%.

[0125] 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.

[0126] [Pickling process] In the method for producing a steel sheet according to this embodiment, the pickling step after hot rolling is a step of pickling the coiled hot-rolled steel sheet with 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 Cu-enriched portion and Mn-depleted layer of the hot-rolled steel sheet.

[0127] For example, the pickling process may be carried out by immersing the hot-rolled steel sheet in a hydrochloric acid solution of a predetermined concentration for a predetermined time after hot rolling. The pickling solution used in the pickling process is, for example, a hydrochloric acid solution of a concentration of 3 to 12%. The temperature at which the hot-rolled steel sheet is immersed in the pickling solution is, for example, 50 to 90°C. The time for immersing the hot-rolled steel sheet in the pickling solution is, for example, 1 to 30 seconds.

[0128] The pickling may be carried out in one step, or may be carried out in several steps in order to completely remove the Cu-enriched portions and the Mn-depleted layers.

[0129] [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 the brush grinding process under the above conditions. By performing brush grinding under these conditions, it is possible to sufficiently remove the Cu-enriched areas and Mn-depleted layers that inhibit the formation of Mn outer oxides.

[0130] In the brush grinding step, not only the Cu-enriched portions and Mn-depleted layers on the surface of the steel sheet but also the Ni- and / or Sn-enriched portions can be removed.

[0131] The amount of grinding by brush grinding is preferably as large as possible in order to more reliably remove the Cu-enriched portion and the Mn-depleted layer. For example, 4.0 g / m 2 It 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:

[0132] 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.

[0133] [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.

[0134] [Annealing process] In the steel sheet manufacturing method of this embodiment, the annealing step is a step of annealing the cold-rolled steel sheet after the cold-rolling step under specific conditions in which the dew point when 600°C is reached is -10°C or higher, the dew point at the highest temperature reached is 5°C or higher than the dew point of 600°C, and the residence time between 600 and 700°C is 40 seconds or longer. By annealing the cold-rolled steel sheet under these specific conditions, Si oxides are formed inside the steel sheet, and the number N of Mn oxides per 10 μm having the above-mentioned circle equivalent diameter of 30 nm or more is increased. 30 But, N 30 ≧20, and as a result, excellent chemical conversion treatability can be obtained even after a certain period of time has passed since degreasing.

[0135] In the annealing step, the dew point when the temperature reaches 600°C needs to be -10°C or higher in order to facilitate the formation of Mn oxides. The dew point may be -9°C or higher or -8°C or higher. The dew point may also be +10°C or lower, +9°C or lower, or +8°C or lower.

[0136] Furthermore, in the annealing process, in order to prioritize the formation of Mn outer oxides over the formation of Mn-Si inner complex oxides, it is necessary to increase the dew point at the maximum temperature by 5°C or more relative to the dew point at 600°C. Furthermore, it is preferable that the dew point at the maximum temperature be increased by 10°C or more relative to the dew point at 600°C. Furthermore, it is preferable that the dew point at the maximum temperature be between -5°C and +20°C. By controlling the dew point in the annealing process in this manner, Si oxides are formed inside the steel sheet, and Mn oxides are formed on the surface of the steel sheet, and the Si content in the oxides on the surface of the steel sheet can be reduced. Furthermore, by forming inner Si oxides, Cu can be trapped and its concentration in the steel sheet surface can be suppressed.

[0137] In the annealing process, the residence time between 600 and 700°C needs to be 40 seconds or longer in order to consume Si inside the steel sheet to form an internal oxide of Si, suppress the formation of an internal composite oxide of Mn and Si at 700°C or higher, and preferentially form an external oxide of Mn. If the time at 600 to 700°C is less than 40 seconds, a composite oxide of Mn and Si will form inside the steel sheet, making it difficult for Mn oxide to form on the surface of the steel sheet.

[0138] The residence time between 600 and 700°C in the annealing step is preferably 60 seconds or more. The residence time may be 65 seconds or more or 70 seconds or more. The residence time may be 100 seconds or less or 90 seconds or less.

[0139] The annealing step may be carried out under conditions other than the above-mentioned specific conditions that are appropriate for annealing the cold-rolled steel sheet while forming Mn oxides on the surface of the steel sheet. For example, the annealing step includes heating to an annealing temperature of 700 to 950°C in an atmosphere in which the dew point when the temperature reaches 600°C is -10 to +10°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 40 seconds or more.

[0140] 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.

[0141] 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).

[0142] [Chemical conversion treatment process] In the method for producing a steel sheet according to this embodiment, the chemical conversion treatment step 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 coating on the surface of the steel sheet. The chemical conversion treatment step may be carried out under conditions suitable for forming a chemical conversion coating on the surface of the steel sheet. For example, in the chemical conversion treatment step, before immersing the steel sheet after pickling 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.

[0143] 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.

[0144] 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.

[0145] 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]

[0146] 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.

[0147] 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%.

[0148] 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 ...

[0149] Next, the brush-ground hot-rolled steel sheet was cold-rolled at a rolling reduction of 50% to obtain a cold-rolled steel sheet.

[0150] 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.

[0151] In this manner, various steel plates having a thickness of 1.6 mm were obtained as examples or comparative examples. When the chemical compositions of the steel plates thus obtained were analyzed, they were found to be the same as the chemical compositions of the steel slabs before hot rolling.

[0152] [Table 1]

[0153] [Table 2]

[0154] For each of the steel sheets obtained as described above, the number N of Mn oxide particles per 10 μm with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel sheet was measured according to the following methods. 30Various measurements were carried out, including Cu concentration measured by GDS from the surface of the steel sheets in the thickness direction, Vickers hardness, and the ratio of hopite in the chemical conversion crystals in the chemical conversion coating. Furthermore, the chemical conversion treatability of each steel sheet was evaluated according to the following evaluation methods. The results of these measurements and evaluations are shown in Table 2. Note that underlines next to various values ​​in Tables 1 and 2 indicate values ​​that are outside the scope of the present invention or represent unfavorable manufacturing conditions.

[0155] (The number of Mn oxide particles per 10 μm with a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet) 30 (Method of measurement) First, a sample is taken so that the cross section parallel to the L direction (rolling direction) and thickness direction (plate thickness direction) of the steel plate to be measured serves as the observation surface. If the rolling direction is unknown, cross sections cut in the thickness direction at angles of 0°, 45°, 90°, and 135° to an arbitrary direction are observed, and the cross section with the highest aspect ratio of precipitates is taken as the cross section parallel to the rolling direction and thickness direction.

[0156] Next, as a pretreatment for TEM / EDS analysis, a carbon protective film is formed on the surface of the sampling area using FIB. Subsequently, a thin film sample is prepared using the FIB-cross-sectional μ-sampling method. A Hitachi High-Tech NB5000 FIB-SEM, for example, is used, with an acceleration voltage of 5 to 40 kV during FIB processing and a Mo mesh.

[0157] Then, a TEM, such as a JEM-2100F manufactured by JEOL Ltd., is used as a TEM, and a JED-300T manufactured by JEOL Ltd. is used as an EDS analyzer, and TEM / EDS analysis is performed under conditions of an acceleration voltage of 200 kV. Observation is performed using BF-STEM images at a magnification of, for example, 500,000x. By creating a series of photographs as shown in Figure 1, BF-STEM images and EDS mapping images of a range of 10 μm or more are obtained.

[0158] From the BF-STEM images and EDS mapping images obtained in the range of 10 μm or more, the number N of Mn oxides per 10 μm with a circle equivalent diameter R of 30 nm or more exposed on the surface of the steel sheet was determined. 30 Count.

[0159] The size R (nm) of the Mn oxide exposed on the surface of the steel sheet was calculated as the area S (nm) of the oxide part, as shown in Figure 2. The obtained Mn mapping image was binarized (min = 0, max = 255) using the image analysis software "ImageJ" to separate the Mn oxide part from the steel part. 2 ) and calculate the following equation: R = (S / π) 0.5 It is calculated as follows.

[0160] The Mn oxides were defined as those parts that were identified as Mn oxides (MnO, MnCr2O4, Mn2SiO4, and MnSiO3) by analyzing the diffraction image in TEM / EDS analysis, among the parts that were recognized as Mn oxides when the Mn mapping image was binarized using "ImageJ."

[0161] The number of Mn oxide particles per 10 μm with a circle equivalent diameter R of 30 nm or more exposed on the surface of the above steel sheet, N 30 Measurements are made at five randomly selected locations, and the arithmetic mean value is used.

[0162] Furthermore, when the steel sheet to be measured is a chemically treated steel sheet, the oxides exposed on the surface of the steel sheet are those exposed on the surface of the steel sheet in areas where chemical crystals are not attached, and those present at the interface between the chemical crystals and the base steel in areas where chemical crystals are attached.

[0163] (Method for measuring Cu concentration by GDS measurement from the surface of steel sheet to the thickness direction) GDS measurements of Cu concentration are performed using a high-frequency glow discharge optical emission spectrometer. Specifically, the surface of the steel plate to be measured is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The surface of the steel plate is then sputtered and analyzed in the depth direction. The Cu element contained in the steel plate is then identified from the emission spectrum wavelength specific to Cu, which is emitted when atoms are excited in the glow plasma, and the emission intensity of the identified Cu element is estimated.

[0164] Depth data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, sputtering time can be converted into sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the steel plate in the thickness direction.

[0165] When the steel sheet to be measured is a chemically treated steel sheet, the surface of the steel sheet is the interface between the base steel and the chemically treated coating.

[0166] 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: 4mmφ RF output: 30W Measurement time: 200 to 1500 seconds

[0167] (Vickers hardness measurement method) 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.

[0168] 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.

[0169] 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.

[0170] (Method for measuring the ratio of hopite in chemical crystals in chemical conversion coatings) The ratio of hopite (Zn3(PO4)2·4H2O) in the conversion crystals in the conversion coating is measured using X-ray diffraction (XRD).

[0171] Here, the ratio of hopite in the chemical conversion crystals in the chemical conversion coating is a ratio calculated by the following formula, where H is the integrated intensity of the hopite peak and P is the integrated intensity of the phosphophyllite peak. Hopite ratio (%) = H / (H+P) x 100 The integrated intensity of each peak is calculated by performing XRD analysis using a Cr tube, separating the phosphophyllite peak (2θ = 14.88°) and the hopite peak (2θ = 14.55°), and then calculating the integrated value of each peak intensity.

[0172] The XRD analysis is carried out using an X-ray diffractometer, such as "EMPYREAN" (registered trademark) manufactured by Malvern Panalytical, under the following conditions. Tube: Cr Detector: 1Der Output: 45kV, 40mA Measurement range: 2θ=10~130°

[0173] [Evaluation of chemical conversion treatment properties] The chemical conversion treatability of the steel sheets 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 steel sheet samples were immersed in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes, and then rinsed with water. The steel sheet samples were then dried and allowed to stand in the atmosphere for 10 minutes (in Table 2, samples that were left to stand at atmospheric pressure for 10 minutes are marked with an "O"). For comparison, chemical conversion treatability was also evaluated under conditions in which the samples were not left to stand in the atmosphere for 10 minutes after degreasing, but were immediately subjected to the next chemical conversion treatment after degreasing (in Table 2, samples that were not left to stand at atmospheric pressure for 10 minutes are marked with an "X").

[0174] 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.

[0175] Next, the surfaces of the steel sheet samples that had been subjected to chemical conversion treatment (100 μm square, 5 fields of view) were measured by SEM-EPMA, and regions where the P concentration was 0.5 mass% or more or the Zr concentration was 0.2 mass% or more were identified in the element distribution image obtained by EPMA, and the area ratios of those regions were then measured. The area ratio of the region where the P concentration is 0.5 mass% or more or the Zr concentration is 0.2 mass% or more was measured by first obtaining an element distribution image of P or Zr using an EPMA set to a P concentration range of 0.5 mass% or more or a Zr concentration range of 0.2 mass% or more, and then performing image processing on the obtained element distribution image of P or Zr. Image analysis software "ImageJ" was used for image processing. Specifically, the P or Zr element distribution image was loaded into ImageJ, and then binarized using "Make Binary" under "Process" and "Binary" so that areas with a P concentration of 0.5 mass% or more or a Zr concentration of 0.2 mass% or more were displayed in black, and areas with a P concentration of less than 0.5 mass% or a Zr concentration of less than 0.2 mass% were displayed in white. After binarization, the "Area fraction" value in "Results" was read using "Measure" under "Analyze." This value was determined to be the area fraction of areas with a P concentration of 0.5 mass% or more or a Zr concentration of 0.2 mass% or more.

[0176] Areas where the P concentration was 0.5% by mass or more or the Zr concentration was 0.2% by mass or more were determined to be "areas where a chemical conversion coating was formed," and areas other than areas where the P concentration was 0.5% by mass or more or the Zr concentration was 0.2% by mass or more were determined to be "areas where a chemical conversion coating was not formed." The area ratio of areas where the P concentration was 0.5% by mass or more or the Zr concentration was 0.2% by mass or more (areas where a chemical conversion coating was formed) was subtracted from the area of ​​the surface to be measured to calculate the area ratio of areas where a chemical conversion coating was not formed, so-called "clear-surface areas" (hereinafter, the area ratio of these areas will be referred to as the "clear-surface area ratio").

[0177] Then, the chemical conversion treatability of the steel sheet was evaluated according to the void area ratio using the following evaluation criteria. AAA: Less than 10% of the surface area is clear AA: Clear area ratio 10% or more but less than 15% A: Clear area ratio 15% to 20% B: Over 20% of the surface area

[0178] Steel sheets with a phosphatability rating of AAA, AA, or A were evaluated as having excellent phosphatability, and steel sheets with a phosphatability rating of B were evaluated as having poor phosphatability. Steel sheets with a rating of "Good" in the item "Leaved in air for 10 minutes after degreasing" in Table 2 and a rating of AAA, AA, or A for phosphatability were evaluated as having excellent phosphatability even after a certain time (i.e., 10 minutes) had passed since degreasing. The results are shown in Table 2.

[0179] As shown in Table 2, the Mn content is high, Mn / (Si+Mn) is more than 0.80, and the number N of Mn oxides per 10 μm exposed on the surface of the steel sheet has an equivalent circle diameter of 30 nm or more. 30 But, N 30 It was found that all of the steel sheets of Examples Nos. 1 to 25, which satisfied the condition of ≧20, were steel sheets with excellent chemical conversion treatability.

[0180] In particular, the steel sheets of Examples Nos. 2, 3, 5, 6, 9, 12, 15, 16, 18, and 19, which were annealed under conditions in which the dew point at the maximum temperature reached in the annealing process was 10°C or more higher than the dew point at 600°C, or the residence time between 600 and 700°C was 60 seconds or more, had a low number N per 10µm of Mn oxides with a circle equivalent diameter of 30nm or more exposed on the surface of the steel sheet. 30 But, N 30 ≧ 30, or the Cu concentration measured by GDS met (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≦ 7.0, resulting in an AA rating, indicating superior chemical conversion treatability.

[0181] Furthermore, the steel sheets of Examples Nos. 4, 7, 10, 13, and 20, which were annealed under conditions in which the dew point at the maximum temperature reached in the annealing process was 10°C or more higher than the dew point at 600°C, and the residence time between 600 and 700°C was 60 seconds or more, had a low number N per 10µm of Mn oxides with a circle equivalent diameter of 30nm or more exposed on the surface of the steel sheet. 30 But, N 30≧30, and the Cu concentration measured by GDS met (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm)≦7.0, earning an evaluation of AAA, demonstrating that the coating has extremely excellent chemical conversion treatability.

[0182] On the other hand, the Mn content is low or Mn / (Si+Mn) is 0.80 or less, and the number N of Mn oxides per 10 μm that are exposed on the surface of the steel sheet and have an equivalent circle diameter of 30 nm or more is 30 But, N 30 The steel sheets of Comparative Examples Nos. 26, 28, 30, 31, 33 and 35, which did not satisfy the condition of ≧20, were all rated B and were found to be steel sheets with poor chemical conversion treatability.

[0183] Specifically, it is presumed that the low Mn content of steel sheet No. 26 resulted in poor phosphatability because a sufficient amount of Mn oxide was not formed on the surface of the steel sheet. Furthermore, the evaluation results of the phosphatability of steel sheet No. 27, which was manufactured under the same conditions, showed that good phosphatability could be obtained by performing phosphatability treatment promptly after degreasing.

[0184] It is presumed that the high Si content of steel sheet No. 28 caused internal oxidation, which prevented sufficient Mn oxide from forming on the surface of the steel sheet, resulting in poor phosphatability.In addition, the evaluation results for the phosphatability of steel sheet No. 29, which was manufactured under the same conditions, showed that good phosphatability could be achieved by performing phosphatability treatment promptly after degreasing.

[0185] Steel sheet No. 30 was not brush-ground after pickling, and Cu-enriched areas and Mn-depleted layers remained, which is presumably why Mn oxides were not sufficiently formed on the surface of the steel sheet.

[0186] It is presumed that steel sheet No. 31 had a low dew point when it reached 600°C, so internal Si oxidation did not occur, and instead a composite oxide of Mn and Si formed internally at 700°C or above, which is why Mn oxide did not form sufficiently on the surface of the steel sheet.In addition, the evaluation results for the phosphatability of steel sheet No. 32, which was manufactured under the same conditions, showed that good phosphatability could be achieved by performing phosphatability immediately after degreasing.

[0187] For steel sheet No. 33, the difference between the dew point when it reached 600°C and the dew point when it reached the maximum temperature was small, and it is presumed that internal oxidation of Mn, with Si oxides forming inside as nuclei, was dominant, which is why Mn oxides were not sufficiently formed on the surface of the steel sheet.In addition, the evaluation results for the phosphatability of steel sheet No. 34, which was manufactured under the same conditions, showed that good phosphatability could be achieved by performing phosphatability immediately after degreasing.

[0188] It is presumed that steel sheet No. 35 had a short residence time between 600 and 700°C, which prevented the formation of internal Si oxides and instead consumed Mn in the internal oxidation of the Mn-Si composite oxide, resulting in insufficient formation of Mn oxides on the surface of the steel sheet. Furthermore, the evaluation results for the phosphatability of steel sheet No. 36, manufactured under the same conditions, showed that good phosphatability could be achieved by performing phosphatability treatment promptly after degreasing.

Claims

1. A steel plate, The chemical composition of the steel plate is, in mass%, C: 0.001 to 0.500%, Si: 0.01 to less than 0.75% Mn: 1.20-3.00%, 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 Mn / (Si+Mn)>0.80 is satisfied; The number N of Mn oxide particles having a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet per 10 μm ３０ but, N ３０ ≧20.

2. The number N of Mn oxide particles having a circle equivalent diameter of 30 nm or more exposed on the surface of the steel sheet per 10 μm ３０ but, N ３０ 2. The steel sheet according to claim 1, wherein the steel sheet satisfies a tensile strength of ≥ 30.

3. The Cu concentration measured by high-frequency glow discharge optical emission spectroscopy in the thickness direction from the surface of the steel sheet is The steel sheet according to claim 1 or 2, wherein (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm)≦10.0 is satisfied.

4. The Cu concentration measured by high-frequency glow discharge optical emission spectroscopy in the thickness direction from the surface of the steel sheet is The steel sheet according to claim 1 or 2, wherein (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm)≦7.0 is satisfied.

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. 7. The steel sheet according to claim 6, wherein the ratio of hopite in the chemical conversion crystals in the chemical conversion coating is 50% or more as measured by X-ray diffraction.

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

Images