Steel sheet and component comprising same

Abstract

Provided are: a steel sheet which contains Ni, Cu and Sn, and which has excellent chemical conversion treatability even if a certain period of time has elapsed after degreasing; this steel sheet and a member containing the same are characterized in that the chemical composition of the steel sheet contains, in mass%, 1.20-3.00% of Mn, 0.010-1.000% of Ni, 0.010-1.000% of Cu, 0.003-1.000% of Sn, and 0.01-less than 0.75% of Si, and in that Mn / (Si + Mn) > 0.80 is satisfied, and in that the number N30 per 10 [mu] m of Mn oxides having an equivalent circle diameter of 30 nm or more exposed on the surface of the steel sheet satisfies N30 > = 20.

Description

Technical Field

[0001] This invention relates to steel plates and components comprising the steel plates. Background Technology

[0002] To improve the corrosion resistance of steel plates, it is effective to enhance the chemical conversion treatment properties of the steel plate surface and uniformly form a chemical conversion treatment film on the surface of the steel plate.

[0003] Relatedly, for example, Patent Document 1 discloses a method for manufacturing high-strength cold-rolled steel sheet, characterized in that the high-strength cold-rolled steel sheet is continuously annealed using a continuous annealing furnace or a combined cold-rolled steel sheet / hot-dip galvanized steel sheet equipment having a continuous annealing furnace. In the aforementioned continuous annealing furnace, the cooling method for a portion or all of the cooling zone within the steel sheet temperature range of 600-250°C following heating for recrystallization is one or more of gas cooling, diffusion cooling, and cooling pipe cooling. During such continuous annealing, the surface of the steel sheet is exposed to an atmosphere that causes iron oxidation within the aforementioned steel sheet temperature range. After pickling at the outlet of the annealing furnace, a concentration of 1-50 mg / m³ is applied. 2 Iron or Ni plating. Furthermore, Patent Document 1 teaches that: normally, oxidation of the steel sheet is prevented by using an inactive atmosphere with extremely low concentrations of oxygen and / or extremely low dew points around the steel sheet. In contrast, active exposure to an oxidizing atmosphere not only oxidizes Si and Mn but also oxidizes the iron in the steel sheet. Through pickling when coming out of the annealing furnace, the oxide film of iron and the oxide films of Si, Mn, etc., on the steel sheet are pickled off, thereby obtaining a high-strength cold-rolled steel sheet with good chemical conversion treatment properties, even if the content of Si, Mn, etc. is high, there is no "uncovered part".

[0004] In addition, Patent Document 2 discloses a steel sheet for automobiles, characterized in that it contains copper (Cu) at a concentration of 0.10% by mass or more and 0.50% by mass or less, and the number of residual oxide scales on the surface is 160,000 per mm. 2 The maximum particle size of the copper compound particles exposed on the surface is 2 μm or less. Furthermore, Patent Document 2 teaches that, with the above configuration, the particle size of the copper compound particles exposed on the surface of the steel plate that becomes the cathode point is 2 μm or less during chemical conversion treatment, and the residual oxide scale is kept below a specified amount, thus providing a steel plate with excellent chemical conversion treatment properties.

[0005] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2008-190030 Patent Document 2: Japanese Patent Application Publication No. 2020-084238 Summary of the Invention

[0006] The problem that the invention aims to solve Patent document 2 mentioned above teaches that elements such as copper (Cu), nickel (Ni), and tin (Sn) not only reduce the mechanical properties such as strength and formability required for automotive steel sheets, but also reduce chemical stability such as corrosion resistance. In particular, the chemical conversion treatment of copper compounds present on the surface of the steel sheet to improve corrosion resistance is reduced.

[0007] Furthermore, the chemical conversion treatment of steel plates typically involves degreasing the steel plates with a degreasing agent and then immersing them in a chemical conversion treatment solution. This is especially important when the steel plates contain copper (Cu), nickel (Ni), and tin (Sn), requiring very strict and short control of the time between degreasing and the chemical conversion treatment. If a certain amount of time has passed after degreasing (e.g., more than 10 minutes), the chemical conversion properties may significantly decrease.

[0008] Therefore, the object of the present invention is to provide a steel plate and a component comprising the steel plate, the steel plate being a steel plate containing Ni, Cu and Sn, which exhibits excellent chemical conversion properties even after degreasing and a certain period of time.

[0009] Methods for solving problems The present invention includes at least the following solutions.

[0010] (Option 1) A steel plate, characterized in that, The chemical composition of the above steel plate, expressed as a percentage by mass, includes: Mn: 1.20~3.00% Ni: 0.010~1.000% Cu: 0.010~1.000% Sn: 0.003~1.000%, and Si: 0.01% to less than 0.75%, and The condition Mn / (Si+Mn) > 0.80 is satisfied. The number N of Mn oxides with an equivalent circular diameter of 30 nm or more exposed on the surface of the aforementioned steel plate per 10 μm. 30 Satisfying N 30 ≥20.

[0011] (Option 2) According to the steel plate described in Scheme 1 above, the steel plate is characterized in that the number N of Mn oxides with an equivalent circular diameter of 30 nm or more exposed on the surface of the steel plate is N per 10 μm. 30 Satisfying N 30 ≥30.

[0012] (Option 3) According to the steel plate described in Scheme 1 or 2 above, the Cu concentration measured by high-frequency glow discharge emission spectroscopy along the thickness direction from the surface of the steel plate satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤ 10.0.

[0013] (Option 4) According to the steel plate described in Scheme 1 or 2 above, the Cu concentration measured by high-frequency glow discharge emission spectroscopy along the thickness direction from the surface of the steel plate satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤ 7.0.

[0014] (Option 5) The steel plate according to any one of the above schemes 1 to 4 is characterized in that the chemical composition of the steel plate, in mass % comprises: Ni: 0.040~1.000% Cu: 0.040~1.000%, and Sn: 0.004~1.000%.

[0015] (Option 6) The steel plate according to any one of the above schemes 1 to 5 is characterized in that it has a chemical conversion treatment film on its surface.

[0016] (Option 7) The steel plate according to Scheme 6 above is characterized in that the proportion of hopeite in the chemically transformed crystals of the above-mentioned chemically transformed film, as measured by X-ray diffraction, is more than 50%.

[0017] (Option 8) A component, characterized in that it comprises a steel plate as described in any one of the above-described schemes 1 to 7.

[0018] Invention Effects According to the present invention, it is possible to provide steel plates containing Ni, Cu and Sn, and components comprising the steel plates, which exhibit excellent chemical conversion properties even after a certain period of time following degreasing. Attached Figure Description

[0019] Figure 1 N is used to describe the number of Mn oxide particles (N1) with an equivalent circle diameter greater than 30 nm exposed on the surface of a steel plate per 10 μm. 30 A schematic diagram of the determination method.

[0020] Figure 2This is a schematic diagram illustrating the method for determining the equivalent circle diameter R of Mn oxide exposed on the surface of a steel plate. Detailed Implementation

[0021] Generally, if the chemical conversion treatmentability of a steel sheet decreases, areas where a chemical conversion coating has not formed, known as uncovered areas, may appear, resulting in reduced corrosion resistance. For example, when elements such as Ni, Cu, and Sn are present in solid solution in a steel sheet, the potential of the steel sheet is higher than that of the undissolved state of these elements, sometimes reducing the etching ability of Fe during chemical conversion treatment. In such cases, it is difficult to form a chemical conversion coating, thus reducing chemical conversion treatmentability and consequently reducing corrosion resistance. Therefore, this reduction in chemical conversion treatmentability is particularly problematic when the steel sheet contains Ni, Cu, and Sn simultaneously. Furthermore, if elements such as Ni, Cu, and Sn are present in solid solution in a steel sheet, an oxide film easily forms. If the time from degreasing to chemical conversion treatment is not strictly and briefly controlled, the chemical conversion treatmentability will be significantly reduced, resulting in reduced corrosion resistance.

[0022] There are generally two known methods for manufacturing steel: one is to produce molten iron in a blast furnace using iron ore, a natural resource, as the main raw material, and then refine it in a converter or similar furnace to produce steel; the other is to produce molten steel in an electric arc furnace using scrap iron, a recycled resource, as the main raw material. In steel produced by the former method, i.e., blast furnace feedstock, elements such as Ni, Cu, and Sn may be added, and therefore, the aforementioned problems need to be addressed appropriately when these elements are present. On the other hand, in steel produced by the latter method, i.e., electric arc furnace feedstock, as mentioned above, since scrap iron is used as the main raw material, it not only contains a greater amount of elements from scrap iron (so-called impurity elements, tramp elements), but also tends to contain all three elements simultaneously. Therefore, the aforementioned problems are particularly significant in electric arc furnace feedstock.

[0023] Therefore, the inventors conducted in-depth research to address the decrease in chemical conversion treatability when a certain time (i.e., 10 minutes) has elapsed between degreasing and chemical conversion treatment. The results showed that external oxides of Mn can dissolve instead of the base material during chemical conversion treatment, causing chemical conversion crystallization and improving the chemical conversion treatability of the steel sheet when a certain time (i.e., 10 minutes) has elapsed between degreasing and chemical conversion treatment. On the other hand, it is believed that if Cu is concentrated on the surface of the steel sheet, it will promote the oxidation of Si contained in the steel, and in the case of Al, it will promote the oxidation of Al, forming external oxides of these elements on the surface of the steel sheet. If external oxides of these elements are formed, the formation of external oxides of Mn is suppressed, and the sufficient improvement in chemical conversion treatability brought about by external oxides of Mn cannot be obtained. It should be noted that in this specification, oxides formed on the surface of the steel sheet and exposed from the surface are referred to as "external oxides," and oxides formed inside the steel sheet and not exposed from the surface are referred to as "internal oxides."

[0024] The inventors have discovered that even steel plates containing the three elements Ni, Cu, and Sn can achieve a specific chemical composition by including 1.20–3.00% Mn, 0.010–1.000% Ni, 0.010–1.000% Cu, 0.003–1.000% Sn, and 0.01% to less than 0.75% Si, with Mn / (Si+Mn) > 0.80. This is achieved by performing electrobrushing after pickling the hot-rolled steel plate and then annealing it under specific dew point conditions and heating modes. This allows for the formation of N Mn oxide particles with an equivalent circle diameter of 30 nm or more exposed on the steel plate surface, with a density N per 10 μm. 30 Satisfying N 30 A large amount of Mn oxide, ≥20, is produced. Furthermore, the inventors have discovered that by forming a large amount of Mn oxide on the surface of the steel plate in this way, the Mn oxide dissolves instead of the base material during chemical conversion treatment, resulting in the precipitation of a sufficient amount of chemical conversion crystals. This significantly improves the chemical conversion treatment performance after a certain time (i.e., 10 minutes) following degreasing.

[0025] The present invention is based on the above insights and includes the following implementation schemes.

[0026] The preferred embodiments of the steel plate of the present invention will be described in detail below.

[0027] <steel plate> The steel plate of one embodiment of the present invention has the following specific chemical composition: containing, by mass %, Mn: 1.20~3.00%, Ni: 0.010~1.000%, Cu: 0.010~1.000%, Sn: 0.003~1.000% and Si: 0.01~less than 0.75%, and satisfying Mn / (Si+Mn) > 0.80.

[0028] Furthermore, the steel plate of this embodiment has the following characteristic configuration: the number N of Mn oxides with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate is [number] per 10 μm. 30 Satisfying N 30 ≥20.

[0029] Generally, in chemical conversion processes, electrons are generated through the anodic dissolution (etching) of Fe, and on the other hand, a cathodic reaction (2H+) occurs through the electrons generated by the anodic dissolution of Fe. + +2e - →H2、10H + +NO3 - +8e - →NH4 + +3H2O). Relatedly, the pH of the chemical conversion treatment solution near the surface of the steel plate rises, and compounds such as zinc phosphate crystals that constitute the chemical conversion treatment film precipitate on the surface of the steel plate.

[0030] However, for steel sheets where Ni, Cu, and Sn are dissolved in the steel, the potential is higher compared to steel sheets where these elements are not dissolved, sometimes reducing 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 contained in the steel, or, in the case of Cr, promoting the oxidation of Cr, forming external oxides of these elements on the surface of the steel sheet. These external oxides sometimes cannot be fully dissolved in the chemical conversion treatment solution during chemical conversion treatment. Therefore, if these external oxides are excessively present on the surface of the steel sheet, they hinder the dissolution of the base material and make it difficult for chemical conversion crystals to precipitate, reducing the chemical conversion treatability of the steel sheet.

[0031] Even if the steel plate in this embodiment contains the three elements Ni, Cu, and Sn, the method described later forms a number N of Mn oxide particles with an equivalent circular diameter of 30 nm or more exposed on the surface of the steel plate per 10 μm. 30 Satisfying N 30The presence of a large amount of Mn oxide (≥20) allows for the dissolution of the Mn oxide instead of the base material during chemical conversion treatment, resulting in the precipitation of sufficient chemical conversion crystals. Even after a certain period of time (i.e., 10 minutes) following degreasing, excellent chemical conversion treatability is maintained. The degradation of chemical conversion treatability after a certain period of time following degreasing is due to the tendency of Sn, Ni, and Cu in the base material to form oxide films after degreasing. However, in the steel sheet of this embodiment, the formation of external Mn oxides ensures that even if an oxide film forms, the external Mn oxides dissolve, guaranteeing chemical conversion treatability.

[0032] It should be noted that the steel sheet of this embodiment includes not only electric furnace materials that inevitably contain Ni, Cu, and Sn as impurity elements, but also blast furnace materials that contain Ni, Cu, and Sn as essential elements or optional additives. Furthermore, compared to conventional steel sheets containing all three elements (Ni, Cu, and Sn), the steel sheet of this embodiment exhibits superior chemical conversion treatability, and consequently, superior corrosion resistance. Therefore, the steel sheet of this embodiment is particularly useful in the automotive industry, where excellent chemical conversion treatability and corrosion resistance are required.

[0033] The following is a detailed description of each component of the steel plate in this embodiment.

[0034] [Chemical Composition] In this embodiment, the steel plate has the following specific chemical composition: containing, by mass%, Mn: 1.20~3.00%, Ni: 0.010~1.000%, Cu: 0.010~1.000%, Sn: 0.003~1.000%, and Si: 0.01~less than 0.75%, and satisfying Mn / (Si+Mn) > 0.80. As described above, the object of the present invention is to provide a steel plate containing Ni, Cu, and Sn that exhibits excellent chemical conversion properties even after degreasing and a certain period of time, by forming on the surface of the steel plate the number N of Mn oxides with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate per 10 μm. 30 Satisfying N 30 This is achieved by using a large amount of Mn oxides, such as ≥20.

[0035] Therefore, the chemical composition of the steel plate is not particularly limited except that it contains Mn: 1.20~3.00%, Ni: 0.010~1.000%, Cu: 0.010~1.000%, Sn: 0.003~1.000% and Si: 0.01~less than 0.75% by mass, and satisfies Mn / (Si+Mn)>0.80.

[0036] It should be noted that, 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. Furthermore, Ni is preferably 1.000% or less by mass. Similarly, Cu is preferably 0.040% or more by mass. Furthermore, Cu is preferably 1.000% or less by mass. Similarly, Sn is preferably 0.004% or more by mass. Furthermore, Sn is preferably 1.000% or less by mass. It should be noted that preferred contents of these elements will be described later.

[0037] In particular, in this embodiment, the chemical composition of the steel plate preferably contains, by mass%, Ni: 0.040~1.000%, Cu: 0.040~1.000% and Sn: 0.004~1.000%.

[0038] In addition to Ni, Cu, and Sn, the chemical composition of the steel plate of this embodiment can contain any alloying element commonly added in the technical field of this invention in appropriate amounts within a suitable range.

[0039] The following describes in detail the chemical composition that can be used in the steel sheet of this embodiment. The purpose of the following description is simply to illustrate the preferred chemical composition of steel sheets used in automobiles and the like, and it is not intended to limit the present invention to steel sheets having such a specific chemical composition.

[0040] For example, the steel plate of this embodiment may also have a chemical composition consisting of the following: (in mass%), C: 0.001~0.500% Si: 0.01% to below 0.75% Mn: 1.20~3.00% Al: 0.001~2.000% Ni: 0.010~1.000% Cu: 0.010~1.000% Sn: 0.003~1.000% P: Below 0.100% S: Below 0.100% N: below 0.0150% O: Below 0.0100% Ti: 0~0.150%, Nb: 0~0.150%, B: 0~0.0100% Mo: 0~1.000% Cr: 0~1.000% V: 0~0.150%, W: 0~1.000% Hf: 0~0.050%, Mg: 0~0.050%, Zr: 0~0.500%, Ca: 0~0.050%, REM: 0~0.100% As: 0~0.100% Ir: 0~1.000%, and The remainder consists of Fe and impurities. Additionally, the chemical composition satisfies Mn / (Si+Mn) > 0.80.

[0041] The following is a more detailed explanation of each of these elements.

[0042] [C: 0.001~0.500%] Carbon (C) is an inexpensive element that increases strength and is an important element for controlling the strength of steel. To achieve this effect, the C content is preferably set to 0.001% or more. The C content can 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 can sometimes lead to a decrease in elongation. Therefore, the C content is preferably set to 0.500% or less. The C content can be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.

[0043] [Si: 0.01% to less than 0.75%] Si is an element that effectively increases strength as a solid solution strengthening element. Furthermore, Si also contributes to the formation of external oxides of Mn, which form around Si oxide cores. To fully obtain these effects, the Si content is set to 0.01% or more. The Si content can 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 can sometimes lead to an increase in steel strength and a decrease in elongation. Therefore, the Si content is set to less than 0.75%. The Si content can be less than 0.70%, 0.65% or less, 0.60% or less, 0.55% or less, or 0.50% or less. Additionally, as described below, the Si content must satisfy the relationship Mn / (Si+Mn) > 0.80.

[0044] [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 external oxides on the surface of steel sheets, contributing to improved chemical conversion treatment properties. To achieve this full effect, the Mn content is set at 1.20% or more. The Mn content can 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 can sometimes lead to increased steel strength and decreased elongation. Therefore, the Mn content is set at 3.00% or less. The Mn content can be 2.80% or less or 2.60% or less. Additionally, as described below, the Mn content must satisfy the relationship Mn / (Si+Mn) > 0.80.

[0045] [Al: 0.001~2.000%] Al acts as a deoxidizer in steel, contributing to its overall health. To achieve this effect, the Al content is preferably 0.001% or more. The Al content can also 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 can sometimes lead to the formation of coarse Al oxides, resulting in a decrease in the elongation of the steel sheet. Therefore, the Al content is preferably 2.000% or less. The Al content can be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0046] [Ni: 0.010~1.000%] [Cu: 0.010~1.000%] Ni and Cu are elements that contribute to increased strength through precipitation strengthening or solid solution strengthening. To achieve this effect, the Ni and Cu contents are each set at 0.010% or more. Ni and Cu contents can 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 content of these elements can sometimes excessively promote the formation of oxides on the surface of the steel sheet, especially Si-based oxides and iron oxides. Therefore, the Ni and Cu contents are each set at 1.000% or less. Ni and Cu contents can be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less, respectively.

[0047] [Sn: 0.003~1.000%] Sn is an effective element for improving corrosion resistance. To achieve this effect, the Sn content is set at 0.003% or more. Sn content can 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 can sometimes excessively promote the formation of oxides on the surface of the steel plate, especially Si-based oxides and iron oxides. Therefore, the Sn content is set at 1.000% or less. Sn content can be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[0048] [P: below 0.100%] Phosphorus (P) is an element that promotes embrittlement of steel due to grain boundary segregation. Lower P content is preferred, and ideally, it should be 0%. However, excessively low P content can sometimes lead to a significant increase in manufacturing costs. Therefore, the P content can be 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, if P is excessive, as mentioned above, grain boundary segregation can sometimes cause embrittlement of the steel. Therefore, the P content is preferably set to 0.100% or less. The P content can be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0049] [S: Below 0.100%] Sulfur (S) is an element that forms non-metallic inclusions such as MnS in steel, leading to a decrease in the ductility of steel components. Lower S content is preferred, ideally 0%. However, excessively low S content can sometimes result in a significant increase in manufacturing costs. Therefore, the S content can be 0.0001% or more, or 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content can sometimes lead to cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably set to 0.100% or less. The S content can be 0.050% or less, 0.020% or less, or 0.010% or less.

[0050] [N: below 0.0150%] Nitrogen (N) is an element that forms large nitrides in steel sheets, reducing their workability. Lower N content is preferred, ideally 0%. However, excessively low N content can sometimes lead to a significant increase in manufacturing costs. Therefore, the N content can be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if N is excessive, as mentioned above, large nitrides can sometimes form, reducing the workability of the steel sheet. Therefore, the N content is preferably set to 0.0150% or less. The N content can be 0.0080% or less, 0.0050% or less, or 0.0030% or less.

[0051] [O: below 0.0100%] O (O) is an element that can be introduced during the manufacturing process, forming large inclusions and reducing the workability of the steel sheet. Lower O content is preferred, and ideally, it should be 0%. However, excessively low O content can sometimes lead to a significant increase in manufacturing costs. Therefore, the O content can be set to 0.0001% or more, or 0.0005% or more, or 0.0010% or more. On the other hand, if O is present in excess, as mentioned above, large inclusions can sometimes form, reducing the workability of the steel sheet. Therefore, the O content is preferably set to 0.0100% or less. The O content can be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0052] The preferred basic chemical composition of the steel plate of this embodiment is as described above. Furthermore, the steel plate of this embodiment may, as needed, contain at least one of the following elements to replace a portion of the remaining Fe.

[0053] [Ti: 0~0.150%] [Nb: 0~0.150%] [V: 0~0.150%] Ti, Nb, and V have the effect of forming carbonitrides in steel, thereby increasing the strength of the steel sheet through precipitation strengthening. The content of Ti, Nb, and V can also be 0%, but to fully obtain this effect, the content of each element is preferably set to 0.001% or more. The content of Ti, Nb, and V can be 0.002% or more, 0.005% or more, or 0.010% or more, respectively. On the other hand, even if these elements are excessively present, the effect saturates, and their presence in the steel sheet beyond what is necessary leads to an increase in manufacturing costs. Therefore, the content of each element is preferably set to 0.150% or less. The content of each element can 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, respectively.

[0054] [B: 0~0.0100%] Boron (B) enhances grain boundary strength through grain boundary segregation, thereby improving low-temperature toughness. While the B content can be 0%, it is preferable to set it to 0.0001% or more to fully achieve this effect. The B content can be 0.0002%, 0.0005%, or 0.0010% or more. On the other hand, even with excessive B content, the effect saturates, potentially leading to increased manufacturing costs. Therefore, the B content is preferably set to 0.0100% or less. The B content can be 0.0050%, 0.0030%, 0.0020%, or 0.0015% or less.

[0055] [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 increased strength. While the content of Mo, Cr, and W can be 0%, it is preferable to set the content of each element to 0.001% or more to achieve the desired effect. The content of Mo, Cr, and W can be 0.010% or more, 0.020% or more, or 0.030% or more, respectively. On the other hand, even with excessive content of these elements, the effect becomes saturated, and their presence in the steel sheet beyond what is necessary leads to increased manufacturing costs. Therefore, the content of each element is preferably 1.000% or less. The content of each element can be 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less, respectively.

[0056] [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 non-metallic inclusions. The content of Hf, Mg, Zr, Ca, and REM can be 0%, but to achieve the desired effect, it is preferable that the content of each of Hf, Mg, Zr, Ca, and REM is 0.0001% or more. The content of Hf, Mg, Zr, Ca, and REM can be 0.0005% or more or 0.001% or more. On the other hand, even if these elements are excessively present, the effect will saturate, and their presence in the steel sheet beyond what is necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the content of Hf and Mg is 0.050% or less, and the content of Zr is 0.500% or less. The content of Hf, Mg, and Zr can be 0.010% or less, 0.005% or less, or 0.003% or less, respectively. Similarly, it is preferable that the content of Ca is 0.050% or less, and the content of REM is 0.100% or less. The contents of Ca and REM can be less than 0.010%, less than 0.005%, or less than 0.003%, respectively.

[0057] In addition, REM refers to the collective name of 17 elements, namely scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanide elements. REM content is the total content of these elements.

[0058] [As: 0~0.100%] As is an effective element for improving corrosion resistance. The As content can be 0%, but to achieve the desired effect, it is preferable to set the As content to 0.001% or more. The As content can be 0.002% or more, or 0.003% or more. On the other hand, even with excessive As content, the effect saturates, and exceeding the necessary amount in the steel sheet leads to increased manufacturing costs. Therefore, the As content is preferably set to 0.100% or less. The As content can be 0.008% or less, or 0.005% or less.

[0059] [Ir: 0~1.000%] Ir is an element that increases the strength of grain boundaries due to segregation at the original austenite grain boundaries. The Ir content can be 0%, but to fully achieve this effect, the Ir content is preferably set to 0.001% or more. The Ir content can be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even with excessive Ir content, the effect saturates, and its presence in the steel sheet beyond what is necessary leads to increased manufacturing costs. Therefore, the Ir content is preferably set to 1.000% or less. The Ir content can be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0060] In steel plates, the remaining components besides the aforementioned elements consist of Fe and impurities. These impurities are components introduced during the industrial manufacturing of steel plates, typically from raw materials such as ores or scrap iron, due to various reasons during the manufacturing process.

[0061] [Mn / (Si+Mn)>0.80] As described above, the Si and Mn contents of the steel plate in this embodiment need to satisfy the relationship Mn / (Si+Mn) > 0.80. Furthermore, in this formula, Mn refers to the Mn content in mass% units. Similarly, Si refers to the Si content in mass% units.

[0062] It is known that Si and Mn in steel plates form oxides during annealing. However, when the Si content in the steel plate is high (e.g., 1.00% by mass or more) and the Mn content is low, Mn oxides are difficult to form on the surface of the steel plate, and therefore it is possible that a sufficient amount of Mn oxides may not be formed on the surface of the steel plate.

[0063] Therefore, in the steel plate of this embodiment, by sufficiently increasing the Mn content to a level where Mn / (Si+Mn) exceeds 0.80, the formation of external oxides of Mn is promoted, thereby achieving the formation of a sufficient amount of Mn oxides on the surface of the steel plate.

[0064] It should be noted that Mn / (Si+Mn) can be 0.81 or higher, 0.82 or higher, 0.83 or higher, 0.84 or higher, or 0.85 or higher. Furthermore, the upper limit of Mn / (Si+Mn) can be 1.00, 0.99, 0.98, 0.97, 0.96 or 0.95.

[0065] The chemical composition of the steel plate can be determined using general analytical methods. For example, the chemical composition of the steel plate can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) according to JIS G 1201:2022. Specifically, for example, a 35 mm square test piece can be obtained from approximately one-quarter of the thickness of the steel plate and measured using a Shimadzu ICPS-8100 (measuring device) under conditions based on a pre-prepared standard curve. C and S, which cannot be determined by ICP-AES, can be determined using the combustion-infrared absorption method, and N can be determined using the inactive gas melting-thermal conductivity method.

[0066] The number N of Mn oxides with an equivalent circular diameter of 30 nm or more exposed on the surface of the steel plate per 10 μm. 30 ] [N30 ≥20] As described above, the steel plate of this embodiment has the following characteristic configuration: the number N of Mn oxides with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate is [number missing] per 10 μm. 30 Satisfying N 30 ≥20.

[0067] In this specification, "surface of steel sheet" refers to the outermost surface of the steel sheet, and in the case of a chemical conversion treatment film on the steel sheet, it refers to the interface between the chemical conversion treatment film and the base metal. It should be noted that, in this specification, "surface layer of steel sheet" refers to the area near the surface of the steel sheet, specifically the area between a depth of 0.1 μm in the thickness direction from the surface of the steel sheet and a depth of 2.0 μm in the thickness direction from the surface of the steel sheet. It should also be noted that, in this specification, "surface layer of steel sheet" is sometimes simply referred to as "surface layer".

[0068] Furthermore, the Mn oxides exposed on the surface of the steel plate with an equivalent circle diameter of 30 nm or more refer to the external oxides of Mn with an equivalent circle diameter R of 30 nm or more as measured by the method described later. These Mn oxides include not only Mn oxides (MnO), but also composite oxides of Mn and Si (specifically, Mn2SiO4, MnSiO3, and MnCr2O4).

[0069] Such exposed Mn oxide on the surface of the steel plate can be achieved by the following specific method: the resulting steel plate has a chemical composition containing 1.20~3.00% Mn, 0.010~1.000% Ni, 0.010~1.000% Cu, 0.003~1.000% Sn, and 0.01~less than 0.75% Si, and satisfies Mn / (Si+Mn) > 0.80. After pickling the hot-rolled steel plate, it is subjected to brush grinding, followed by an annealing process under specific dew point and temperature conditions. It should be noted that the specific manufacturing method of the steel plate according to this embodiment will be described later.

[0070] As described above, if Mn oxides with an equivalent circular diameter of 30 nm or more are formed on the surface of the steel plate, the number N per 10 μm is... 30 Satisfying N 30 When a large amount of Mn oxide (≥20) is present, the Mn oxide dissolves instead of the parent material during chemical conversion treatment, resulting in the precipitation of sufficient chemical conversion crystals. Even after a certain period of time following degreasing, it can still exhibit excellent chemical conversion treatment properties.

[0071] In this embodiment, from the viewpoint of obtaining better chemical conversion treatability, the number N of Mn oxides with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate is defined as follows: 30 Preferred to satisfy N 30 ≥30. It should be noted that N refers to the number of Mn oxide particles N with an equivalent circle diameter greater than 30 nm exposed on the surface of the steel plate per 10 μm. 30 It can be 32 or more, 34 or more, 36 or more, 38 or more, or 40 or more. Additionally, the number N of Mn oxide particles with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate is defined as follows: 30 It can be less than 60 or less than 50.

[0072] The number N of Mn oxides with an equivalent circular diameter of 30 nm or more exposed on the surface of the steel plate per 10 μm. 30 The determination can be performed using energy-dispersive X-ray spectroscopy (TEM / EDS) based on transmission electron microscopy, as described below. Figure 1 N is used to describe the number of Mn oxide particles (N1) with an equivalent circle diameter greater than 30 nm exposed on the surface of a steel plate per 10 μm. 30 A schematic diagram of the determination method. Figure 2 This is a schematic diagram illustrating the method for determining the equivalent circle diameter R of Mn oxide exposed on the surface of a steel plate.

[0073] (Number N of Mn oxides with an equivalent circle diameter greater than 30 nm exposed on the surface of the steel plate per 10 μm) 30 (Determination method) When performing TEM / EDS analysis, samples are first collected with a cross-section parallel to the L-direction (rolling direction) and thickness direction (plate thickness direction) of the steel plate being measured as the observation plane. It should be noted that when the rolling direction is unclear, cross-sections cut along the thickness direction at 0°, 45°, 90°, and 135° relative to any direction are observed, and the cross-section with the highest aspect ratio of precipitates is taken as the cross-section parallel to both the rolling and thickness directions.

[0074] Next, as a pretreatment for TEM / EDS analysis, a carbon protective film is fabricated on the surface of the sampling site using FIB. Then, a thin film sample is prepared using the FIB-cross-section μ-sampling method (also known as cross-sectional micro-sampling). For FIB-SEM, for example, a Hitachi High-Tech NB5000 is used, with the accelerating voltage during FIB processing set to 5~40kV, and a Mo mesh is employed.

[0075] Then, using, for example, a JEM-2100F manufactured by NEC Corporation as a TEM and a JED-300T manufactured by NEC Corporation as an EDS analyzer, TEM / EDS analysis was performed at an accelerating voltage of 200 kV. Observations were performed on BF-STEM images at a magnification of, for example, 500,000x. This was achieved by fabricating... Figure 1 The sequential photographs shown yielded BF-STEM images and EDS-mapped images covering a range exceeding 10 μm. It should be noted that... Figure 1 The morphology of multiple Mn oxides 2 exposed on the surface S1 of steel plate 1 is schematically shown in a series of photographs with a measurement field of 1 μm × 1 μm.

[0076] Then, based on the BF-STEM and EDS mapping images of the range of 10 μm or more obtained as described above, the number N of Mn oxides with an equivalent circle diameter R of 30 nm or more exposed on the surface of the steel plate was counted per 10 μm. 30 .

[0077] Here, the Mn oxides exposed on the surface of the steel plate refer to, for example, ... Figure 2 As shown, in the section where the steel plate 1 is cut along the thickness direction, the line segment L along the surface S1 of the steel plate... s Crossed Mn oxides 2.

[0078] For the size R (nm) of the Mn oxide exposed on the surface of the steel plate, such as Figure 2 As shown, in order to divide the obtained Mn mapping image into Mn oxide and steel parts, binarization was performed using the image analysis software "ImageJ" (min=0, max=255), and the area S (nm) of the oxide part was measured. 2 ), through the following formula R=(S / π) 0.5 Calculate it.

[0079] It should be noted that, regarding Mn oxides, in the part identified as Mn oxides by binarization based on "ImageJ" of the Mn mapping image, the part identified as Mn oxides (MnO, MnCr2O4, Mn2SiO4 and MnSiO3) by the analysis of diffraction images in TEM / EDS analysis is defined as Mn oxides.

[0080] For the aforementioned Mn oxides with an equivalent circle diameter R greater than 30 nm exposed on the surface of the steel plate, the number N per 10 μm is... 30 The measurements were performed at five randomly selected sites, and the arithmetic mean was used.

[0081] In addition, when the steel plate being measured is a chemically converted steel plate, for the oxides exposed on the surface of the steel plate, the unattached part of the chemically converted crystals is defined as the oxides exposed on the surface of the steel plate, and the attached part of the chemically converted crystals is defined as the oxides existing at the interface between the chemically converted crystals and the base metal.

[0082] [Cu concentration measured by high-frequency glow discharge emission spectroscopy along the thickness direction from the surface of the steel plate] [(Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤ 10.0] Furthermore, in this embodiment, the Cu concentration measured by high-frequency glow discharge emission spectroscopy (GDS) along the thickness direction from the surface of the steel plate preferably satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤ 10.0. It should be noted that "Cu concentration at a depth of 5 nm" refers to the Cu concentration at a depth of 5 nm along the thickness direction from the surface of the steel plate. Similarly, "Cu concentration at a depth of 10 μm" refers to the Cu concentration at a depth of 10 μm along the thickness direction from the surface of the steel plate.

[0083] As described above, Cu concentrates on the surface of the steel sheet during the annealing process, which promotes the oxidation of Si contained in the steel, or, in the case of Cr, promotes the oxidation of Cr, forming external oxides of these elements on the surface of the steel sheet. Therefore, it is difficult for external oxides of Mn to form on the surface of the steel sheet. Therefore, in the steel sheet of this embodiment, by reducing the portion of Cu concentrated on the surface of the steel sheet (hereinafter, sometimes referred to as "Cu concentration area"), more Mn oxides can be formed on the surface of the steel sheet, resulting in better chemical conversion treatment properties. It should be noted that the Cu concentration area formed on the surface of the steel sheet is a portion where Cu is concentrated not only compared to the center portion in the thickness direction of the steel sheet, but also compared to other portions of the surface of the steel sheet.

[0084] The means to reduce the Cu concentration on the surface of the steel plate will be described in detail in the manufacturing method described later. It can be achieved by brush grinding the surface of the steel plate after pickling the hot-rolled steel plate.

[0085] From the viewpoint of further improving chemical conversion processability, the Cu concentration measured by GDS along the thickness direction from the surface of the steel plate more preferably satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤ 7.0. Furthermore, (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) can be 6.5 or less, 6.0 or less, or 5.5 or less. Moreover, (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) can be 0 or more, 1.0 or more, 1.5 or more, 2.0 or more, or 2.5 or more.

[0086] The determination of Cu concentration based on GDS can be carried out according to the following determination method.

[0087] (Method for determining Cu concentration in the thickness direction from the surface of the steel plate based on GDS) The GDS determination of Cu concentration was performed using a high-frequency glow discharge emission spectrometer. Specifically, the method was as follows: the surface of the steel plate to be measured was exposed to an Ar atmosphere, and while sputtering the surface of the steel plate to generate glow plasma under an applied voltage, analysis was performed in the depth direction. Then, based on the emission spectral wavelengths of Cu emitted by atoms excited in the glow plasma, the Cu elements contained in the material were identified, and the emission intensity of the identified Cu elements was estimated.

[0088] Data in the depth direction can be estimated based on sputtering time. Specifically, by pre-determining the relationship between sputtering time and sputtering depth using standard samples, sputtering time can be converted into sputtering depth. Therefore, the sputtering depth converted from sputtering time can be defined as the depth in the thickness direction from the surface of the steel plate.

[0089] It should be noted that when the steel plate being measured is a chemically converted steel plate, the surface of the steel plate is set as the interface between the base metal and the chemically converted coating.

[0090] GDS measurements can be performed using commercially available analytical apparatus. In this embodiment, a high-frequency glow discharge emission spectrometer "GDS850A" manufactured by LECO Japan Co., Ltd. is used. The measurement conditions are described below.

[0091] Ar gas pressure: 0.3 MPa Anode diameter: 4mm RF output: 30W Measurement time: 200~1500 seconds [Chemical conversion treatment of the film] The steel plate of this embodiment can have a chemical conversion treatment film on its surface. The steel plate of this embodiment has a large amount of Mn oxide formed on its surface, resulting in excellent chemical conversion treatment properties. Therefore, a dense chemical conversion treatment film with high structural uniformity can be formed on the surface of the steel plate, resulting in excellent corrosion resistance.

[0092] It should be noted that, for example, well-known zinc phosphate-based chemical conversion treatment solutions and zirconium-based chemical conversion treatment solutions can be used to form the chemical conversion treatment film.

[0093] Furthermore, in this embodiment, the steel plate preferably contains 50% or more of the zinc phosphate rock (Zn3(PO4)2•4H2O) in the chemically converted crystals of the chemically converted film, as measured by X-ray diffraction (XRD).

[0094] When chemical conversion treatment is performed using a zinc phosphate-based chemical conversion treatment solution, chemical conversion crystals are formed starting from the Mn oxides formed on the surface of the steel plate, thereby increasing the proportion of zinc phosphate ore. If the proportion of zinc phosphate ore in the chemical conversion crystals of the chemical conversion treatment film is 50% or more, a large amount of Mn oxides can be reliably formed on the surface of the steel plate, thus achieving the aforementioned excellent chemical conversion treatment properties more reliably.

[0095] Here, the ratio of zinc phosphate rock in the chemically transformed crystals of the chemically transformed film is calculated by the following formula, with H: the integral intensity of the zinc phosphate rock peak and P: the integral intensity of the phosphophyllite peak.

[0096] The percentage of zinc phosphate rock (%) = H / (H+P) × 100 It should be noted that the integrated intensity of each peak was calculated as follows: XRD analysis was performed using a Cr vacuum tube, and the peaks of phosphoproteolith (2θ=14.88°) and zinc phosphate rock (2θ=14.55°) were separated, and the values ​​were calculated from the integrated values ​​of their respective peak intensities.

[0097] XRD analysis can be performed using an X-ray diffraction device such as "EMPAYREAN" (registered trademark) manufactured by Malvern Panalytical, under the following conditions.

[0098] Vacuum tube: Cr Detector: 1Der Output: 45kV, 40mA Measurement range: 2θ = 10~130° (Thickness of the steel plate) In this embodiment, the thickness of the steel plate can be a general thickness. Examples of such thicknesses include 0.2 to 8.0 mm. Furthermore, the thickness of the steel plate can 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 can be 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0099] It should be noted that when the steel plate has a chemical conversion treatment coating, the total thickness of the steel plate is the sum of the thickness of the chemical conversion treatment coating and the thickness of the coating itself. The thickness of the chemical conversion treatment coating can be, for example, 1 μm or more, 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. Alternatively, the thickness of the chemical conversion treatment coating can be, for example, less than 60 μm, less than 55 μm, less than 50 μm, less than 45 μm, or less than 40 μm.

[0100] (Mechanical properties) Vickers hardness For the steel plate of this embodiment, as a measure of strength, it can, for example, have a Vickers hardness of 90 HV or higher. The Vickers hardness of the steel plate can be 150 HV or higher, 190 HV or higher, 200 HV or higher, 250 HV or higher, 300 HV or higher, 350 HV or higher, 400 HV or higher, or 450 HV or higher. Alternatively, the Vickers hardness of the steel plate can be 650 HV or lower, 600 HV or lower, 550 HV or lower, or 500 HV or lower.

[0101] It should be noted that Vickers hardness is determined according to JIS Z 2244-1:2024 as follows.

[0102] First, test pieces are cut out in a manner that allows observation of a section perpendicular to the surface (thickness section) from any position on the steel plate, except at the ends. The thickness section of the cut test pieces is ground using silicon carbide paper of #600 to #1500.

[0103] Next, using a liquid obtained by dispersing diamond powder with a particle size of 1~6μm in a diluent such as alcohol or pure water, the cross-section of the test piece is precision machined into a mirror surface, and this cross-section is used as the measurement surface.

[0104] Next, using a micro Vickers hardness tester, the Vickers hardness of the test piece was measured at intervals more than three times the indentation length under a load of 1 kgf. Specifically, a total of 20 points were randomly measured near 1 / 4 of the thickness of the test piece, and their arithmetic mean was determined as the Vickers hardness of the steel plate.

[0105] <Components> As described above, compared to conventional steel sheets containing all three elements Ni, Cu, and Sn, the steel sheet of this embodiment exhibits superior chemical conversion properties even after degreasing and a certain period of time, thereby achieving excellent corrosion resistance. Therefore, the steel sheet of this embodiment is useful for use in components and other components in technical fields requiring excellent chemical conversion properties and corrosion resistance. In particular, the steel sheet of this embodiment is useful for use in automotive components and other similar applications.

[0106] In a preferred embodiment, an automotive component comprising a steel sheet according to an embodiment of the present invention is provided. Examples of automotive components include frame components, bumpers, and other structural and reinforcing components requiring strength. Furthermore, other examples of automotive components include exterior panel components such as roofs, hoods, fenders, and doors, which require high design flexibility. These components only need to include a steel sheet according to an embodiment of the present invention in at least a portion of them. Therefore, at least a portion of these components is a portion that satisfies the characteristics of the steel sheet according to the above-described embodiment. In parts of the steel sheet that do not directly contact the mold during forming processes such as stamping, or where the degree of processing is relatively low even if direct contact with the mold is present, the characteristics of the steel sheet do not change significantly before and after forming. For example, in a component comprising a steel sheet according to an embodiment of the present invention, the portion of the sample collection area described later (i.e., the portion avoiding the portions described later (i) to (iv)) can be identified as a non-processed portion, which retains the characteristics of the steel sheet according to the above-described embodiment before and after forming as a component.

[0107] The steel sheet of the embodiments of the present invention can be used as various automotive parts as described above after a chemical conversion treatment film or coating is arbitrarily formed on its surface. Whether an automotive part having a coating or chemical conversion treatment film contains the steel sheet of the embodiments of the present invention can be determined by removing the coating or chemical conversion treatment film from a sample taken from that automotive part. The sample collection site, coating removal process, and chemical conversion treatment film removal process are described below.

[0108] (Sampling site) When performing various measurements and analyses on automotive parts, samples should be collected in a manner that avoids the locations described in (i) to (iv) below.

[0109] (i) Welded parts: the area within 20mm of the seam edge of the spot weld and the area within 20mm of the seam edge of the arc / laser weld.

[0110] (ii) Machining section: Machining section with a radius of curvature of less than 15 mm, and the part within 5 mm of the above-mentioned machining section.

[0111] (iii) End: The end within 5 mm of the cut end face of the component.

[0112] (iv) Red rust: The area within 5 mm from the point where red rust appears, as seen visually.

[0113] (Coating removal process) For the surface of a sample cut from an automotive part or sheet metal, apply a coating release agent (NEOREVER (registered trademark) #160, manufactured by SANSAIKAKO Co., Ltd.) at room temperature and let it stand for 5 minutes. Then, use a stiff sponge (e.g., "Kanefeel" (registered trademark), manufactured by AION Co., Ltd.) to wipe the surface of the sample coated with the coating release agent, thereby peeling the coating off the surface of the sample.

[0114] Next, the surface of the sample after the coating was peeled off was washed with water and dried. At this point, the residual state of the coating was confirmed by SEM-EPMA measurement of the surface of the washed and dried sample (100 μm square, 5 fields of view).

[0115] Based on the elemental distribution image of EPMA, regions with a C concentration of 10% or more by mass are identified. If the area ratio of such regions is 5% or more, it is determined that the coating has not been sufficiently peeled off.

[0116] The determination of the area ratio of regions with a C concentration of 10% by mass or higher first involves obtaining an elemental distribution image of C in EPMA with a C concentration range of 10% to 30%. The specific measurement conditions for EPMA are described below.

[0117] Apparatus: JXA-8230 electron probe microanalyzer manufactured by NEC Corporation Accelerating voltage: 15kV Irradiation current: 0.05μA Surface Analysis: WDS Analysis interval: 300 μm or more Area ratio: the average of 5 fields of view Next, the area fraction was determined by image processing of the obtained C elemental distribution image. Image processing was performed using the image analysis software "ImageJ". Specifically, after importing the C elemental distribution image into ImageJ, the "Make Binary" function in "Process" was used to binarize the area, displaying regions with a C concentration of 10% or more by mass as black and regions with a C concentration less than 10% by mass as white. After binarization, the "Measure" function in "Analyze" was used to read the "Area fraction" value in "Results". This read value was then determined as the area fraction of regions with a C concentration of 10% or more by mass.

[0118] If the coating is not sufficiently peeled off, the coating removal process is repeated until the area ratio of the region with a C concentration of 10% or more is less than 5%.

[0119] (Chemical conversion treatment for film removal) For samples obtained by cutting from automotive parts or steel sheets and removing the coating, if it is necessary to remove the chemical conversion treatment film for various measurements, the chemical conversion treatment film is removed using an appropriate method. For example, in the case of a zinc phosphate coating, the chemical conversion treatment film is removed from the sample surface by following the method according to JIS K 3151:1996. Specifically, the sample after coating removal is immersed in a 5% chromic acid aqueous solution heated to 75°C for 15 minutes, thereby removing the chemical conversion treatment film from the sample surface.

[0120] Next, the surface of the sample after removing the chemical conversion film was washed with water and dried. At this point, the residual state of the chemical conversion crystals was confirmed by SEM-EPMA measurement of the surface of the washed and dried sample (100 μm square, 5 fields of view).

[0121] Based on the elemental distribution image of EPMA, regions with a P concentration of 5% or higher were identified. When the area ratio of such regions was 5% or higher, it was determined that the chemical conversion treatment of the film was not sufficiently peeled off.

[0122] In determining the area fraction of regions with a P concentration of 5% by mass or higher, an elemental distribution image of P in EPMA with a P concentration range of 5-10% was first obtained. Next, the area fraction was determined by image processing of the obtained P elemental distribution image. Image processing was performed using the image analysis software "ImageJ". Specifically, after importing the aforementioned P elemental distribution image into ImageJ, binarization was performed using the "Make Binary" function in "Process" > "Binary", displaying regions with a P concentration of 5% by mass or higher as black and regions with a concentration less than 5% by mass as white. After binarization, the "Area fraction" value in "Results" was read using the "Measure" function in "Analyze". This read value was then determined as the area fraction of regions with a P concentration of 5% by mass or higher.

[0123] If the chemical conversion treatment of the film is insufficient, the chemical conversion treatment to remove the film is repeated until the area ratio of the region with a P concentration of 5% or more is less than 5%.

[0124] <Methods for manufacturing steel plates> Next, a preferred method for manufacturing a steel plate according to one embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a steel plate according to one embodiment of the present invention, and is not intended to limit the steel plate to a steel plate manufactured by the method described below.

[0125] The steel plate of this embodiment can be manufactured, for example, by a manufacturing method including the following steps: a casting step in which molten steel with the chemical composition of the final steel plate being the specific chemical composition described above is cast to form a steel billet; a hot rolling step in which the steel billet is hot rolled to obtain a hot-rolled steel plate; a pickling step in which the hot-rolled steel plate is pickled; an electric brush grinding step in which the surface of the pickled steel plate is ground with an electric brush; a cold rolling step in which the hot-rolled steel plate after electric brush grinding is cold rolled to obtain a cold-rolled steel plate; an annealing step in which the cold-rolled steel plate is annealed under specific conditions where the dew point at 600°C is -10°C or higher, the dew point at the highest temperature is 5°C or higher than the dew point at 600°C, and the residence time between 600°C and 700°C is set to 40 seconds or more; and a chemical conversion treatment step in which the steel plate is immersed in a chemical conversion treatment solution to form a chemical conversion treatment film on the surface of the steel plate.

[0126] In the steel plate manufacturing method of this embodiment, firstly, the chemical composition of the final steel plate is adjusted to include, by mass%, Mn: 1.20~3.00%, Ni: 0.010~1.000%, Cu: 0.010~1.000%, Sn: 0.003~1.000%, and Si: 0.01~less than 0.75%, and the Mn content is greater than Mn / (Si+Mn)>0.80. This makes it easier to form a large amount of Mn oxide on the surface of the steel plate.

[0127] Furthermore, in the steel plate manufacturing method of this embodiment, the steel billet with the specific chemical composition described above is hot-rolled and pickled. Then, the surface of the hot-rolled steel plate is subjected to electrobrushing, thereby removing the Cu-concentrated portion and the Mn-deficient layer that hinder the formation of external oxides of Mn. It should be noted that the Mn-deficient layer is formed on the surface of the steel plate by the formation of an internal oxide layer of Mn during coiling after hot rolling, resulting in a low Mn content. If such an Mn-deficient layer exists on the surface of the steel plate, it is difficult for Mn oxides to form on the surface of the steel plate.

[0128] Furthermore, in the steel plate manufacturing method of this embodiment, the cold-rolled steel plate obtained by cold rolling the steel plate after brush grinding is annealed under specific conditions: the dew point at 600°C is -10°C or higher, the dew point at the highest temperature is 5°C or higher than the dew point at 600°C, and the residence time between 600°C and 700°C is set to 40 seconds or more. This results in the formation of Si oxide inside the steel plate and the formation of Mn oxide with an equivalent circle diameter of 30 nm or more per 10 μm on the surface of the steel plate. 30 Satisfying N 30 Mn oxides in amounts of ≥20.

[0129] To explain the specific conditions of this annealing process in more detail, firstly, by annealing the cold-rolled steel sheet at a dew point above -10°C, Mn oxides are easily formed. Furthermore, by setting the dew point at the highest reaching temperature to be at least 5°C higher than the dew point at 600°C, the formation of external Mn oxides is more dominant than the formation of internal composite oxides of Mn and Si. Moreover, by maintaining a residence time of 40 seconds or more between 600 and 700°C, the internal Si in the steel sheet is consumed, leading to the formation of internal Si oxides, which suppresses the formation of internal composite oxides of Mn and Si above 700°C, preferentially forming external Mn oxides.

[0130] According to the steel plate manufacturing method of this embodiment, even steel plates containing the three elements Ni, Cu, and Sn can form on the surface of the steel plate a number N of Mn oxides with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate per 10 μm. 30 Satisfying N 30 Mn oxides of ≥20, even after degreasing and a certain period of time, can still exhibit excellent chemical conversion properties.

[0131] Hereinafter, the preferred conditions for each step in the steel plate manufacturing method of this embodiment will be described in detail.

[0132] [Casting Process] In the steel plate manufacturing method of this embodiment, the casting process is a process of casting molten steel with adjusted chemical composition to form a steel billet. The adjustment of the chemical composition needs to be carried out in a specific manner so that the final steel plate contains, by mass percent, Mn: 1.20~3.00%, Ni: 0.010~1.000%, Cu: 0.010~1.000%, Sn: 0.003~1.000%, and Si: 0.01~less than 0.75%, and has a high Mn content satisfying Mn / (Si+Mn) > 0.80.

[0133] For casting processes other than chemical composition, the conditions are those commonly known in the art. For example, for casting processes, after smelting in a blast furnace, electric furnace, or the like, various secondary smelting processes are carried out, followed by casting using conventional continuous casting, ingot casting, or other methods.

[0134] [Hot rolling process] In the steel plate manufacturing method of this embodiment, the hot rolling process is a process of hot rolling a steel billet to obtain a hot-rolled steel plate. The hot rolling process is performed by hot rolling the cast steel billet directly or after temporary cooling and then reheating it. When reheating, the heating temperature of the steel billet is set to, for example, 1100~1250°C.

[0135] In the hot rolling process, roughing and finishing rolling are typically performed. The temperature and reduction rate of each rolling step can be appropriately determined based on the desired metal microstructure and plate thickness. For example, the finishing rolling end temperature is 900~1050℃, and the finishing rolling reduction rate is 10~50%.

[0136] The hot-rolled steel sheet, after finishing, is coiled at a specified coiling temperature for the subsequent pickling process. In this embodiment, the coiling of the hot-rolled steel sheet is performed at a coiling temperature of 520°C or higher. The coiling temperature can be 550°C or higher. Alternatively, the coiling temperature can be 600°C or lower.

[0137] [Pickling process] In the steel sheet manufacturing method of this embodiment, the pickling process after hot rolling is a process of pickling the coiled hot-rolled steel sheet using a pickling solution. The pickling process uses a commonly used pickling solution, such as a hydrochloric acid solution with a specified concentration containing an inhibitor that inhibits corrosion of the steel sheet, and is carried out under conditions suitable for removing the aforementioned Cu-concentrated portion and Mn-deficient layer of the hot-rolled steel sheet.

[0138] For example, the pickling process can be carried out by immersing the hot-rolled steel sheet in a hydrochloric acid solution of a specified concentration for a specified time. The pickling solution used in the pickling process is, for example, a hydrochloric acid solution with a concentration of 3-12%. Furthermore, the temperature at which the hot-rolled steel sheet is immersed in the pickling solution is, for example, 50-90°C. Moreover, the immersion time of the hot-rolled steel sheet in the pickling solution is, for example, 1-30 seconds.

[0139] It should be noted that pickling can be performed in one go, or in multiple stages to reliably remove the Cu-concentrated portion and the Mn-deficient layer.

[0140] [Electrical Brush Grinding Process] In the steel plate manufacturing method of this embodiment, the brush grinding process is carried out at a grinding amount of 3.0 g / m. 2 The above conditions are conditions for performing electrobrushing on pickled hot-rolled steel sheets. By performing electrobrushing under these conditions, Cu-concentrated areas and Mn-deficient layers that hinder the formation of external oxides of Mn can be effectively removed.

[0141] It should be noted that in the brush grinding process, not only can the Cu-concentrated portion and Mn-deficient layer on the surface of the steel plate be removed, but also the concentrated portion of at least one of Ni and Sn can be removed.

[0142] From the viewpoint of more reliably removing Cu-concentrated areas and Mn-deficient layers, a higher grinding depth in brush grinding is preferred; for example, 4.0 g / m is preferred. 2 The above, more preferably 5.0 g / m 2 The above. The upper limit for the grinding depth in brush grinding is, for example, 20.0 g / m.2 The following can be 15.0 g / m 2 the following.

[0143] The amount of material removed by brush grinding can be adjusted by any suitable method known to those skilled in the art. For example, the amount of material removed by brush grinding can be adjusted by appropriately selecting the type of brush (e.g., Hotani H115), wire material, nap length, rotational speed, density, brush pressure, and the coating liquid used.

[0144] [Cold rolling process] In the steel sheet manufacturing method of this embodiment, the cold rolling process is a process of cold rolling a hot-rolled steel sheet that has been brush-ground to obtain a cold-rolled steel sheet. The reduction rate of cold rolling can be appropriately determined according to the desired metal structure, sheet thickness, etc. The reduction rate of cold rolling is, for example, 20% to 80%. After the cold rolling process, the sheet can be cooled to room temperature by air cooling, for example.

[0145] [Annealing process] In the steel sheet manufacturing method of this embodiment, the annealing process is performed under specific conditions: the dew point of the cold-rolled steel sheet after the cold rolling process is -10°C or higher when reaching 600°C, the dew point at the highest reaching temperature is 5°C or higher than the dew point at 600°C, and the residence time between 600°C and 700°C is set to 40 seconds or more. By annealing the cold-rolled steel sheet under these specific conditions, Si oxide can be formed inside the steel sheet, and Mn oxide with an equivalent circle diameter of 30 nm or more can be formed on the surface of the steel sheet at a number N per 10 μm. 30 Satisfying N 30 A certain amount of Mn oxide, ≥20, is present. Moreover, as a result, excellent chemical conversion treatment properties are achieved even after a certain period of time following degreasing.

[0146] During the annealing process, from the viewpoint that Mn oxides are easily formed, it is necessary to ensure that the dew point at 600°C is above -10°C. The dew point can be above -9°C or above -8°C. Alternatively, the dew point can be below +10°C, below +9°C, or below +8°C.

[0147] Furthermore, in the annealing process, from the viewpoint that the formation of external oxides of Mn is more dominant than the formation of internal composite oxides of Mn and Si, it is necessary to make the dew point at the highest reaching temperature at least 5°C higher than the dew point at 600°C. Moreover, the dew point at the highest reaching temperature is preferably at least 10°C higher than the dew point at 600°C. Additionally, the dew point at the highest reaching temperature is preferably set to -5°C or higher and +20°C or lower. By controlling the dew point in the annealing process in this way, Si oxides can be formed inside the steel sheet, and Mn oxides can be formed on the surface of the steel sheet, thereby reducing the Si content in the oxides on the surface of the steel sheet. Furthermore, by forming internal oxides of Si, Cu can be trapped, suppressing the concentration of Cu in the surface layer of the steel sheet.

[0148] Furthermore, in the annealing process, from the perspective of consuming the Si inside the steel plate to form internal Si oxides, suppressing the formation of internal composite oxides of Mn and Si above 700°C, and preferentially forming external oxides of Mn, the residence time between 600 and 700°C needs to be more than 40 seconds. It should be noted that when the residence time at 600-700°C is less than 40 seconds, composite oxides of Mn and Si form inside the steel plate, making it difficult for Mn oxides to form on the surface of the steel plate.

[0149] The dwell time at 600-700°C during the annealing process is preferably 60 seconds or more. This dwell time can be 65 seconds or more, or 70 seconds or more. Alternatively, the dwell time can be 100 seconds or less, or 90 seconds or less.

[0150] In the annealing process, conditions other than the specific conditions mentioned above can be implemented as long as they are suitable for annealing the cold-rolled steel sheet while simultaneously forming Mn oxide on the surface of the steel sheet. For example, the annealing process includes heating to an annealing temperature of 700 to 950°C in an atmosphere with a dew point of -10°C to +10°C at 600°C, and holding at this temperature for 0 to 300 seconds. However, the annealing temperature of 700 to 950°C requires a dwell time of at least 40 seconds between 600 and 700°C.

[0151] Furthermore, the annealing temperature is preferably 750°C or higher, more preferably 780°C or higher. The annealing temperature is also 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. Additionally, the holding time is preferably 200 seconds or lower, more preferably 150 seconds or lower.

[0152] The atmosphere in the annealing process can be a reducing atmosphere, or more specifically a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere with a hydrogen concentration of 1 to 10% (e.g., 3% hydrogen and nitrogen balance).

[0153] [Chemical conversion treatment process] In the steel sheet manufacturing method of this embodiment, the chemical conversion treatment step involves immersing the steel sheet after annealing and pickling in a chemical conversion treatment solution to form a chemical conversion treatment film on the surface of the steel sheet. The chemical conversion treatment step can be carried out under conditions suitable for forming a chemical conversion treatment film on the surface of the steel sheet. For example, before immersing the pickled steel sheet in the chemical conversion treatment solution, the steel sheet can be degreased with a degreasing agent, washed with water, and then its surface treated with a surface conditioner.

[0154] Examples of chemical conversion solutions used in chemical conversion processes include well-known zinc phosphate-based chemical conversion solutions and zirconium-based chemical conversion solutions.

[0155] Furthermore, the present invention is not limited to the above-described embodiments and the following examples. Without departing from the purpose and spirit of the present invention, appropriate combinations, substitutions, and modifications can be made.

[0156] The present invention will now be described in more detail by way of examples, but the following examples are merely illustrative of the invention and the invention is not limited to these examples in any way.

[0157] Example In the following embodiments, steel plates according to embodiments of the present invention were manufactured under various conditions, and the properties of the resulting steel plates were investigated.

[0158] First, molten steel is cast using a continuous casting method to form a steel billet with the chemical composition shown in Table 1. Then, the billet is temporarily cooled and reheated to 1200°C for hot rolling, followed by coiling at a coiling temperature of 520°C or higher. Hot rolling is carried out by roughing and finishing rolling, with the finishing rolling ending at 900–1050°C and a reduction rate of 30%.

[0159] Next, the obtained hot-rolled steel sheet was pickled and then ground using a grinding brush (Hotani H115) at a speed of 6.0 g / m. 2 The surface of the hot-rolled steel plate is subjected to electric brush grinding with a certain grinding amount.

[0160] Next, the hot-rolled steel sheet after brush grinding is cold-rolled with a reduction rate of 50% to obtain a cold-rolled steel sheet.

[0161] Then, for this cold-rolled steel sheet, annealing was carried out in a furnace with an oxygen concentration of less than 20 ppm, in an atmosphere with a dew point and 3% hydrogen (nitrogen balance) as shown in Table 2, at an annealing temperature of 800°C and held for 100 seconds. It should be noted that the residence time between 600 and 700°C when heating to the annealing temperature is shown in Table 2.

[0162] As described above, various steel plates with a thickness of 1.6 mm were obtained as examples or comparative examples. Furthermore, the chemical composition of the steel plates thus obtained was analyzed, and the results showed that it was the same as that of the steel billet before hot rolling.

[0163] For the various steel plates obtained above, the number N of Mn oxides with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate is determined according to the following methods, per 10 μm. 30 Various measurements were performed on the Cu concentration, Vickers hardness, and the ratio of zinc phosphate minerals in the chemically converted crystals of the chemically converted coating, obtained from the thickness direction of the steel sheet using GDS measurements. Furthermore, the chemical conversion treatability of various steel sheets was evaluated according to the following evaluation method. These measurement and evaluation results are shown in Table 2. It should be noted that the underlines under the various values ​​in Tables 1 and 2 indicate manufacturing conditions outside the scope of this invention or unpreferred conditions.

[0164] (Number N of Mn oxides with an equivalent circle diameter greater than 30 nm exposed on the surface of the steel plate per 10 μm) 30 (Determination method) First, samples are collected using a cross-section parallel to the L-direction (rolling direction) and thickness direction (plate thickness direction) of the steel plate being measured as the observation surface. It should be noted that when the rolling direction is unclear, cross-sections cut along the thickness direction at 0°, 45°, 90°, and 135° relative to any direction are observed, and the cross-section with the highest aspect ratio of precipitates is taken as the cross-section parallel to both the rolling and thickness directions.

[0165] Next, as a pretreatment for TEM / EDS analysis, a carbon protective film is fabricated on the surface of the sampling site using FIB. Then, a thin film sample is prepared using the FIB-section μ-sampling method. For FIB-SEM, for example, a Hitachi High-Tech NB5000 is used, with the accelerating voltage during FIB processing set to 5~40kV, and a Mo mesh is employed.

[0166] Then, using, for example, a JEM-2100F manufactured by NEC Corporation as a TEM and a JED-300T manufactured by NEC Corporation as an EDS analyzer, TEM / EDS analysis was performed at an accelerating voltage of 200 kV. Observations were performed on BF-STEM images at a magnification of, for example, 500,000x. This was achieved by fabricating... Figure 1 The continuous photographs shown yielded BF-STEM images and EDS-mapped images in the range of 10 μm and above.

[0167] Then, based on the BF-STEM and EDS mapping images of the range of 10 μm or more obtained as described above, the number N of Mn oxides with an equivalent circle diameter R of 30 nm or more exposed on the surface of the steel plate was counted per 10 μm. 30 .

[0168] For the size R (nm) of the Mn oxide exposed on the surface of the steel plate, such as Figure 2 As shown, in order to divide the obtained Mn mapping image into Mn oxide and steel parts, binarization was performed using the image analysis software "ImageJ" (min=0, max=255), and the area S (nm) of the oxide part was measured. 2 ), through the following formula R=(S / π) 0.5 Calculate it.

[0169] It should be noted that, regarding Mn oxides, in the part identified as Mn oxides by binarization based on "ImageJ" of the Mn mapping image, the part identified as Mn oxides (MnO, MnCr2O4, Mn2SiO4 and MnSiO3) by the analysis of diffraction images in TEM / EDS analysis is defined as Mn oxides.

[0170] For the aforementioned Mn oxides with an equivalent circle diameter R greater than 30 nm exposed on the surface of the steel plate, the number N per 10 μm is... 30 The measurements were performed at five randomly selected sites, and the arithmetic mean was used.

[0171] In addition, when the steel plate being measured is a chemically converted steel plate, for the oxides exposed on the surface of the steel plate, the unattached part of the chemically converted crystals is defined as the oxides exposed on the surface of the steel plate, and the attached part of the chemically converted crystals is defined as the oxides existing at the interface between the chemically converted crystals and the base metal.

[0172] (Method for determining Cu concentration in the thickness direction from the surface of the steel plate based on GDS) The GDS determination of Cu concentration was performed using a high-frequency glow discharge emission spectrometer. Specifically, the method was as follows: the surface of the steel plate to be measured was exposed to an Ar atmosphere, and while sputtering the surface of the steel plate to generate glow plasma under an applied voltage, analysis was performed in the depth direction. Then, based on the emission spectral wavelengths of Cu emitted by atoms excited in the glow plasma, the Cu elements contained in the material were identified, and the emission intensity of the identified Cu elements was estimated.

[0173] Data in the depth direction can be estimated based on sputtering time. Specifically, by pre-determining the relationship between sputtering time and sputtering depth using standard samples, sputtering time can be converted into sputtering depth. Therefore, the sputtering depth converted from sputtering time can be defined as the depth in the thickness direction from the surface of the steel plate.

[0174] It should be noted that when the steel plate being measured is a chemically converted steel plate, the surface of the steel plate is set as the interface between the base metal and the chemically converted coating.

[0175] GDS measurements can be performed using commercially available analytical apparatus. In this embodiment, a high-frequency glow discharge emission spectrometer "GDS850A" manufactured by LECO Japan Co., Ltd. is used. The measurement conditions are described below.

[0176] Ar gas pressure: 0.3 MPa Anode diameter: 4mm RF output: 30W Measurement time: 200~1500 seconds (Method for determining Vickers hardness) First, test pieces are cut out in a manner that allows observation of a section perpendicular to the surface (thickness section) from any position on the steel plate, except at the ends. The thickness section of the cut test pieces is then ground using silicon carbide paper of grit #600 to #1500.

[0177] Next, using a liquid obtained by dispersing diamond powder with a particle size of 1~6μm in a diluent such as alcohol or pure water, the cross-section of the test piece is precision machined into a mirror surface, and this cross-section is used as the measurement surface.

[0178] Next, using a micro Vickers hardness tester, the Vickers hardness of the test piece was measured at intervals more than three times the indentation length under a load of 1 kgf. Specifically, a total of 20 points were randomly measured near 1 / 4 of the thickness of the test piece, and their arithmetic mean was determined as the Vickers hardness of the steel plate.

[0179] (Method for determining the ratio of zinc phosphate rock in chemically transformed crystals in chemically transformed films) The proportion of zinc phosphate rock (Zn3(PO4)2•4H2O) in the chemically transformed crystals of the chemically transformed film was determined by X-ray diffraction (XRD).

[0180] Here, the ratio of zinc phosphate rock in the chemically transformed crystals of the chemically transformed film is calculated by the following formula, with H: the integral intensity of the zinc phosphate rock peak and P: the integral intensity of the phosphophyllite peak.

[0181] The percentage of zinc phosphate rock (%) = H / (H+P) × 100 It should be noted that the integrated intensity of each peak was calculated as follows: XRD analysis was performed using a Cr vacuum tube, and the peaks of phosphoproteolith (2θ=14.88°) and zinc phosphate rock (2θ=14.55°) were separated, and the values ​​were calculated from the integrated values ​​of their respective peak intensities.

[0182] XRD analysis was performed using, for example, the “EMPAYREAN” (registered trademark) manufactured by Malvern Panalytical, as an X-ray diffraction device under the following conditions.

[0183] Vacuum tube: Cr Detector: 1Der Output: 45kV, 40mA Measurement range: 2θ = 10~130° [Evaluation of chemical transformation treatability] The chemical conversion treatability of the steel plate is evaluated as follows.

[0184] First, a 50mm × 50mm sample of the steel plate to be evaluated was degreased under the following conditions.

[0185] Degreasing treatment: The samples were immersed in a degreasing agent (FINE CLEANER E2083) at 40°C for 2 minutes, followed by rinsing with water. After drying, the steel plate samples were allowed to stand at atmospheric pressure for 10 minutes (in Table 2, samples that stood at atmospheric pressure for 10 minutes are marked with "〇"). Additionally, for comparison, the chemical conversion treatability was evaluated under conditions where the samples were not allowed to stand at atmospheric pressure for 10 minutes after degreasing, but were immediately subjected to subsequent chemical conversion treatment (in Table 2, samples that were not allowed to stand at atmospheric pressure for 10 minutes are marked with "×").

[0186] Next, the degreased steel plate samples were subjected to either (i) zinc phosphate (Zn) treatment or (ii) zirconium (Zr) treatment as chemical conversion treatment.

[0187] (i) Zn phosphate treatment Surface conditioning treatment: Immerse in surface conditioner (Prepalene Z) for 30 seconds at room temperature.

[0188] Chemical conversion treatment: Immerse in zinc phosphate treatment agent (Palbond L3020 bath material) at 40°C for 2 minutes, then wash with water and dry.

[0189] (ii) Zr treatment Chemical conversion treatment: Immerse in Zr-based treatment agent (PLC-2010) at 45°C for 2 minutes, then wash with water and dry.

[0190] Next, for the steel plate samples that have undergone chemical conversion treatment, SEM-EPMA measurements were performed on the surface of the samples (100 μm square, 5 fields of view). In the elemental distribution image based on EPMA, the area fraction of the region with a P concentration of 0.5% or more or a Zr concentration of 0.2% or more was determined.

[0191] The area ratio of regions with a P concentration of 0.5% by mass or above or a Zr concentration of 0.2% by mass or above was determined as follows: First, the elemental distribution images of P or Zr in EPMA with the P concentration range set to 0.5% by mass or above or the Zr concentration range set to 0.2% by mass or above were obtained, and then the obtained P or Zr elemental distribution images were processed.

[0192] Image processing was performed using the image analysis software "ImageJ". Specifically, after importing the aforementioned P or Zr elemental distribution image into ImageJ, binarization was performed using the "Make Binary" function under "Binary" in the "Process" menu. This ensured that areas with a P concentration of 0.5% or higher (by mass) or a Zr concentration of 0.2% or higher were displayed as black, while areas with a P concentration less than 0.5% or a Zr concentration less than 0.2% (by mass) were displayed as white. After binarization, the "Measure" function under "Analyze" was used to read the "Area fraction" value in the "Results" section. This read value was then determined as the area fraction of the regions with a P concentration of 0.5% or higher (by mass) or a Zr concentration of 0.2% or higher (by mass).

[0193] Then, the areas with a P concentration of 0.5% by mass or more or a Zr concentration of 0.2% by mass or more are defined as "parts with a chemically converted film," and the areas other than those with a P concentration of 0.5% by mass or more or a Zr concentration of 0.2% by mass or more are defined as "parts without a chemically converted film." By subtracting the area ratio of the areas with a P concentration of 0.5% by mass or more or a Zr concentration of 0.2% by mass or more (parts with a chemically converted film) from the surface area of ​​the test object, the area ratio of the part without a chemically converted film, referred to as the so-called "uncovered part" (hereinafter, the area ratio of this part is referred to as the "uncovered part area ratio"), is calculated.

[0194] Then, based on the area ratio of the uncovered portion, the chemical conversion treatability of the steel sheet is evaluated according to the following evaluation criteria.

[0195] AAA: The uncovered area ratio is less than 10%. AA: The uncovered area is more than 10% but less than 15% A: The uncovered area is between 15% and 20%. B: The uncovered area exceeds 20%. Steel plates rated AAA, AA, and A for chemical conversion treatability were considered to have excellent chemical conversion treatability, while those rated B were considered to have poor chemical conversion treatability. Then, steel plates with a "0" rating for "10 minutes of standing at atmosphere after degreasing" and a chemical conversion treatability rating of AAA, AA, and A were considered to exhibit excellent chemical conversion treatability even after a certain time (i.e., 10 minutes) following degreasing. The results are shown in Table 2.

[0196] As shown in Table 2, the number N of Mn oxides per 10 μm with high Mn content, Mn / (Si+Mn) ratio exceeding 0.80, and equivalent circle diameters of 30 nm or more exposed on the surface of the steel plate is [not specified]. 30 Satisfying N 30 The steel plates of Examples No.1 to 25 with a thickness of ≥20 are all steel plates with excellent chemical conversion properties.

[0197] In particular, it is known that for the steel sheets of Examples No. 2, 3, 5, 6, 9, 12, 15, 16, 18, and 19, which were annealed under conditions where the dew point at the highest temperature reached during the annealing process was more than 10°C higher than the dew point at 600°C, or where the residence time between 600 and 700°C was more than 60 seconds, the number N of Mn oxides with an equivalent circle diameter of more than 30 nm exposed on the surface of the steel sheet was... 30 Satisfying N 30 ≥30 / or Cu concentration based on GDS measurement that meets (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤7.0, has superior chemical conversion treatability rated as AA.

[0198] Furthermore, it can be seen that for the steel sheets of Examples No. 4, 7, 10, 13, and 20, which were annealed under the conditions that the dew point at the highest reached temperature was more than 10°C higher than the dew point at 600°C and the residence time between 600°C and 700°C was more than 60 seconds, the number N of Mn oxides with an equivalent circle diameter of more than 30 nm exposed on the surface of the steel sheet is... 30 Satisfying N 30 ≥30, and the Cu concentration measured based on GDS satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤7.0, indicating extremely excellent chemical conversion treatability rated AAA.

[0199] In contrast, it can be seen that for Mn oxides with low Mn content, or Mn / (Si+Mn) ratio below 0.80, and an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate, the number N per 10 μm is... 30 Not satisfied with N 30 The steel plates of comparative examples No. 26, 28, 30, 31, 33, and 35 with a strength of ≥20 were all steel plates with poor chemical conversion treatment properties, rated as B.

[0200] Specifically, for steel sheet No. 26, due to its low Mn content, a sufficient amount of Mn oxide was not formed on the surface of the steel sheet, which is presumably a result of poor chemical conversion treatability. Furthermore, the evaluation results of the chemical conversion treatability of steel sheet No. 27, manufactured under the same conditions, indicate that good chemical conversion treatability can be obtained by rapidly performing chemical conversion treatment after degreasing.

[0201] The No. 28 steel plate has a high Si content, with internal oxidation being the dominant process. Mn oxides were not sufficiently formed on the surface, which is presumably the result of poor chemical conversion treatability. Furthermore, the evaluation results of the chemical conversion treatability of the No. 29 steel plate manufactured under the same conditions show that good chemical conversion treatability can be obtained by rapidly performing chemical conversion treatment after degreasing.

[0202] For steel plate No. 30, no electrobrushing was performed after pickling, and Cu-concentrated areas and Mn-deficient layers remained. Therefore, it is speculated that Mn oxide was not fully formed on the surface of the steel plate.

[0203] For steel sheet No. 31, the dew point is low at 600°C, and no internal Si oxidation occurs. Above 700°C, a composite oxide of Mn and Si forms internally, suggesting that Mn oxide is not sufficiently formed on the surface of the steel sheet. Furthermore, the evaluation results of the chemical conversion treatability of steel sheet No. 32 manufactured under the same conditions indicate that good chemical conversion treatability can be obtained by rapidly performing chemical conversion treatment after degreasing.

[0204] For steel sheet No. 33, the difference between the dew point at 600°C and the dew point at the highest temperature is small, indicating that internal oxidation of Mn with Si oxide cores is dominant. Therefore, it is speculated that Mn oxides were not sufficiently formed on the surface of the steel sheet. Furthermore, the evaluation results of the chemical conversion treatability of steel sheet No. 34 manufactured under the same conditions show that good chemical conversion treatability can be obtained by rapidly performing chemical conversion treatment after degreasing.

[0205] For steel sheet No. 35, the short residence time between 600 and 700°C prevented the formation of internal oxides of Si. The internal oxidation of Mn and the Si composite oxide consumed Mn, suggesting that Mn oxides were not sufficiently formed on the steel sheet surface. Furthermore, the evaluation results of the chemical conversion treatability of steel sheet No. 36 manufactured under the same conditions indicate that good chemical conversion treatability can be obtained by rapidly performing chemical conversion treatment after degreasing.

Claims

1. A steel plate, characterized in that, The chemical composition of the steel plate, expressed as a percentage by mass, includes: Mn: 1.20~3.00% Ni: 0.010~1.000% Cu: 0.010~1.000% Sn: 0.003~1.000%, and Si: 0.01% to less than 0.75%, and The condition Mn / (Si+Mn) > 0.80 is satisfied. The number N of Mn oxides with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate per 10 μm. 30 Satisfying N 30 ≥20.

2. The steel plate according to claim 1, characterized in that, The number N of Mn oxides with an equivalent circle diameter of 30 nm or more exposed on the surface of the steel plate per 10 μm. 30 Satisfying N 30 ≥30.

3. The steel plate according to claim 1 or 2, characterized in that, The Cu concentration measured by high-frequency glow discharge emission spectroscopy along the thickness direction from the surface of the steel plate satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤ 10.

0.

4. The steel plate according to claim 1 or 2, characterized in that, The Cu concentration measured by high-frequency glow discharge emission spectroscopy along the thickness direction from the surface of the steel plate satisfies (Cu concentration at a depth of 5 nm) / (Cu concentration at a depth of 10 μm) ≤ 7.

0.

5. The steel plate according to any one of claims 1 to 4, characterized in that, The chemical composition of the steel plate, expressed as a percentage by mass, includes: Ni: 0.040~1.000% Cu: 0.040~1.000%, and Sn: 0.004~1.000%.

6. The steel plate according to any one of claims 1 to 5, characterized in that, The surface of the steel plate has a chemically converted coating.

7. The steel plate according to claim 6, characterized in that, The proportion of zinc phosphate rock in the chemically transformed crystals of the chemically transformed film, as determined by X-ray diffraction, is over 50%.

8. A component, characterized in that, The steel plate comprising any one of claims 1 to 7.

Images