Steel sheet and component including the same

Abstract

Provided are: a steel sheet characterized by having a chemical composition containing, in mass %, 0.010-1.000% of Ni, 0.010-1.000% of Cu and 0.003-1.000% of Sn, and by, in GDS measurement from the surface of the steel sheet, the Cu concentration on the surface of the steel sheet being 115% or less of the bulk Cu concentration, and a Cu-deficient region, in which the Cu concentration is 90% or less of the bulk Cu concentration, being present between the surface of the steel sheet and a depth of 5 μm; and a component including the steel sheet.

Description

Steel plates and parts containing them

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

[0002] It is known that in order to improve the corrosion resistance of steel sheets, it is effective to improve the chemical conversion treatability of the steel sheets and to form a uniform chemical conversion coating on the steel sheets.

[0003] In relation to this, Patent Document 1 discloses that when a high-strength cold-rolled steel sheet is continuously annealed in a continuous annealing furnace or a cold-rolled steel sheet / hot-dip galvanized steel sheet dual-purpose facility having a continuous annealing furnace, the cooling method of which in a cooling zone including a part or all of the steel sheet temperature range of 600 to 250°C following heating for recrystallization is one or more of gas cooling, diffusion cooling, and cooling pipe cooling, the steel sheet surface is exposed to an atmosphere in which iron oxidizes within the above-mentioned steel sheet temperature range, pickled at the outlet side of the annealing furnace, and then iron or Ni plating is applied to a thickness of 1 to 50 mg / m 2 Furthermore, Patent Document 1 teaches that, although oxidation of a steel sheet is usually prevented by an extremely low concentration of oxygen and / or an inert atmospheric gas with an extremely low dew point around the steel sheet, a method of actively exposing the steel sheet to an oxidizing atmosphere to oxidize not only Si and Mn but also the iron of the steel sheet, and removing the oxide films of Si, Mn, etc. together with the oxide film of iron of the steel sheet by pickling immediately after leaving an annealing furnace, thereby obtaining a high-strength cold-rolled steel sheet that is free from "scales" and has good chemical conversion treatability even if the contents of Si, Mn, etc. are high.

[0004] In Patent Document 2, a copper (Cu) content of 0.10 mass % or more and 0.50 mass % or less is contained, and the number of residual scales on the surface is 160,000 pieces / mm 2 Patent Document 2 teaches that, according to the above configuration, the particle size of the copper compound particles exposed on the steel sheet surface, which serves as the cathode point in chemical conversion treatment, is 2 μm or less, and the residual scale is reduced to a predetermined amount or less, thereby making it possible to provide a steel sheet with excellent chemical conversion treatability.

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

[0006] Patent Document 2 teaches that elements such as nickel (Ni) and tin (Sn) in addition to copper (Cu) reduce mechanical properties such as strength and formability required of automotive steel sheets, as well as chemical stability such as corrosion resistance, and that copper compounds present on the surface of the steel sheet in particular reduce the ability to perform chemical conversion treatments to improve corrosion resistance.

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

[0008] In order to achieve the above object, the present inventors have conducted research focusing on the element distribution in the surface portion of a steel sheet, and as a result, have found that chemical conversion treatability can be significantly improved by controlling the Cu concentration in the surface portion of a steel sheet within a predetermined range, and have completed the present invention.

[0009] The present invention has achieved the above-mentioned object as follows: (1) A steel sheet having a chemical composition containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, wherein, in GDS measurement from the surface of the steel sheet, the Cu concentration at the steel sheet surface is 115% or less of the Cu concentration in the bulk, and a Cu-depleted region is present within a depth direction of 5 μm from the steel sheet surface, in which the Cu concentration is 90% or less of the Cu concentration in the bulk. (2) The steel sheet according to (1) above, wherein the Cu concentration at the steel sheet surface is 90% or less of the Cu concentration in the bulk. (3) The steel sheet according to (1) or (2) above, wherein, in GDS measurement from the surface of the steel sheet, the Cu concentration at a depth where the Fe concentration is 90% or more is 85% or less of the Cu concentration in the bulk. (4) The steel sheet according to any one of (1) to (3) above, characterized in that the chemical composition includes Si, and that the thickness of silicon oxide is 10 nm or less when measured by X-ray photoelectron spectroscopy on the surface of the steel sheet. (5) The steel sheet according to any one of (1) to (4) above, characterized in that it has a Vickers hardness of 190 Hv or more. (6) The steel sheet according to any one of (1) to (5) above, characterized in that the chemical composition includes, in mass %, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%. (7) A part, characterized in that it includes the steel sheet according to any one of (1) to (6) above.

[0010] According to the present invention, it is possible to provide a steel sheet containing Ni, Cu, and Sn that can exhibit improved chemical conversion treatability, and a part including the steel sheet.

[0011] FIG. 1 is a conceptual diagram showing an example of a GDS measurement result from a steel sheet surface.

[0012] <Steel Sheet> A steel sheet according to an embodiment of the present invention has a chemical composition containing, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, and is characterized in that, in a GDS measurement from the surface of the steel sheet, the Cu concentration at the surface of the steel sheet is 115% or less of the Cu concentration in the bulk, and a Cu-depleted region is present within a depth direction of 5 μm from the steel sheet surface, in which the Cu concentration is 90% or less of the Cu concentration in the bulk.

[0013] Generally, when chemical conversion treatability is reduced, regions where the chemical conversion coating is not formed, called "skid zones," may occur, resulting in reduced corrosion resistance. For example, when elements such as Ni, Cu, and Sn are present in a steel sheet as a solid solution, the potential of the steel sheet becomes more noble than when these elements are not present in a solid solution, which may reduce the etching ability of Fe during chemical conversion treatment. In this case, the chemical conversion treatability of the steel sheet is reduced. Therefore, when a steel sheet simultaneously contains the three elements Ni, Cu, and Sn, such reduced chemical conversion treatability is particularly problematic.

[0014] Furthermore, two commonly known methods for producing steel sheets are, for example, a method in which molten iron is obtained in a blast furnace using iron ore, a natural resource, as the main raw material, and then molten steel is produced through refining in a converter or the like, and a method in which molten steel is produced in an electric furnace using scrap material, a recycled resource, as the main raw material. Since blast furnace steel may also contain elements such as Ni, Cu, and Sn as additive elements, when these elements are contained, it is necessary to appropriately address the above-mentioned issues. On the other hand, electric furnace steel uses scrap material as the main raw material as described above, and therefore contains relatively large amounts of elements derived from the scrap (so-called tramp elements), such as Ni, Cu, and Sn, and therefore the above-mentioned issues become particularly pronounced.

[0015] Therefore, the present inventors conducted studies, focusing particularly on the element distribution in the surface portion of the steel sheet, in order to provide a steel sheet that can exhibit excellent chemical conversion treatability even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, the present inventors found that controlling the Cu concentration in the surface portion of the steel sheet within a predetermined range is effective. More specifically, the present inventors found that controlling the element distribution in the surface portion of the steel sheet so that, in a GDS (glow discharge optical emission spectroscopy) measurement from the surface of the steel sheet, the Cu concentration in the steel sheet surface, more specifically, the surface of an unplated steel sheet, is 115% or less of the bulk Cu concentration, and a Cu-depleted region is present within a depth direction of 5 μm from the steel sheet surface, where the Cu concentration is 90% or less of the bulk Cu concentration. In this specification, the term "steel sheet surface" refers to the surface of the steel sheet itself and does not include, for example, the surface of a plated steel sheet.

[0016] Without intending to be bound by any particular theory, it is believed that Cu tends to concentrate on the surface of a steel sheet containing the three elements Ni, Cu, and Sn simultaneously during the steel sheet manufacturing process. For example, when manufacturing a steel sheet, the steel sheet may be annealed at high temperatures after cold rolling to create a predetermined metallographic structure. In a study conducted by the present inventors, a tendency for Cu to concentrate on the surface of the steel sheet during such annealing was observed. Cu concentration on the surface of the steel sheet is undesirable because it reduces the etching ability of Fe during chemical conversion treatment, which in turn reduces the chemical conversion treatability of the steel sheet. In response to this, the present inventors have discovered that by utilizing the tendency of Cu to concentrate on the surface of the steel sheet, the Cu concentration in the surface portion of the steel sheet can be appropriately controlled, thereby significantly improving the chemical conversion treatability of the steel sheet. More specifically, when bulk Cu diffuses and concentrates on the surface of the steel sheet, a Cu-depleted region is formed directly below the surface due to the surface concentration of Cu. Therefore, in such cases, if only the surface enriched portion of Cu can be at least partially removed or completely removed, the surface portion of the steel sheet after removal can be composed mainly of a Cu-deficient region, which is thought to make it possible to improve the chemical conversion treatability of the steel sheet.

[0017] From this perspective, the inventors conducted further studies focusing on controlling the element distribution in the surface portion of a steel sheet. As a result, the inventors discovered that, as will be described in detail later in connection with the manufacturing method, by concentrating Cu on the surface in an annealing step and then at least partially removing the Cu-enriched portion on the surface by brush grinding, it is possible to control the degree of Cu enrichment on the steel sheet surface within a predetermined range and to control the element distribution in the surface portion of the steel sheet so that a predetermined Cu-depleted region is present directly below the Cu-enriched portion. More specifically, the inventors discovered that by appropriately combining the annealing step and the brush grinding step, it is possible to control the element distribution in the surface portion of the steel sheet so that, in GDS measurement from the surface of the steel sheet, the Cu concentration at the steel sheet surface is 115% or less of the Cu concentration in the bulk, and a Cu-depleted region is present within 5 μm from the steel sheet surface in the depth direction, where the Cu concentration is 90% or less of the Cu concentration in the bulk. In this regard, the present inventors have found that it is possible to suppress the decrease in Fe etching ability caused by Cu concentration on the steel sheet surface, and as a result, it is possible to significantly improve the chemical conversion treatability of the steel sheet.

[0018] The steel sheet according to the embodiment of the present invention encompasses not only electric furnace steel, which inevitably contains Ni, Cu, and Sn as tramp elements, but also blast furnace steel, which contains Ni, Cu, and Sn as essential elements or optional added elements. Furthermore, the steel sheet according to the embodiment of the present invention can achieve superior chemical conversion treatability and, in turn, superior corrosion resistance compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where superior chemical conversion treatability and / or corrosion resistance are required. Each component of the steel sheet according to the embodiment of the present invention will be described in more detail below.

[0019] [Cu-Depleted Region Exists in Which the Cu Concentration at the Steel Sheet Surface is 115% or Less of the Cu Concentration in the Bulk and is 90% or Less of the Cu Concentration in a Depth Direction of 5 μm from the Steel Sheet Surface] In an embodiment of the present invention, a GDS (Glow Discharge Optical Emission Spectroscopy) measurement from the surface of the steel sheet shows that the Cu concentration at the steel sheet surface is 115% or less of the Cu concentration in the bulk and that a Cu-Depleted Region exists in which the Cu concentration is 90% or less of the Cu concentration in the bulk and is 90% or less of the Cu concentration in a depth direction of 5 μm from the steel sheet surface. Here, "bulk" refers to a region of the steel sheet other than the steel sheet surface and including the center of the steel sheet thickness. As described above, by controlling the degree of Cu enrichment at the steel sheet surface to 115% or less of the bulk Cu concentration and creating a Cu-deficient region directly below the surface where the Cu concentration is 90% or less of the bulk Cu concentration, it is possible to suppress the decrease in Fe etching ability due to Cu enrichment at the steel sheet surface, thereby significantly improving the chemical conversion treatability of the steel sheet. From the viewpoint of further improving the chemical conversion treatability, a lower Cu concentration at the steel sheet surface is preferable. Therefore, the Cu concentration at the steel sheet surface is preferably 110% or less or 100% or less of the bulk Cu concentration, more preferably 95% or less or 90% or less of the bulk Cu concentration, and most preferably 88%, 85%, or 82% or less of the bulk Cu concentration. The lower limit is not particularly limited, but the Cu concentration at the steel sheet surface may be, for example, 60% or more, 65% or more, 70% or more, or 75% or more of the bulk Cu concentration.

[0020] [Measurement of Cu Concentration on Steel Sheet Surface and Bulk by GDS] The Cu concentration on the steel sheet surface and bulk by GDS is measured as follows. First, using a high-frequency glow discharge optical emission spectrometer (e.g., Model "GDS850A" manufactured by LECO Japan LLC), the steel sheet surface is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The steel sheet surface is then sputtered and analyzed in the depth direction. The elements contained in the material are identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. Depth data can be estimated from the sputtering time. Specifically, the relationship between sputtering time and sputtering depth can be calculated in advance using a standard sample, allowing the sputtering time to be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted to mass % by creating a calibration curve. The measurement conditions are as follows. The calibration curve is corrected using a standard sample (Brammer, BSH-1B). Ar gas pressure: 0.3 MPa, anode diameter: 4 mmφ, RF output: 30 W, measurement time: 200 to 1500 seconds

[0021] FIG. 1 is a conceptual diagram showing an example of GDS measurement results from the steel sheet surface. Referring to FIG. 1 , the average Cu concentration measured as described above in the region from the steel sheet surface to a depth of 0.1 μm (region A in FIG. 1 ) is determined as the "Cu concentration at the steel sheet surface." In the GDS measurement, peaks presumably associated with contamination on the steel sheet surface are ignored when calculating the Cu concentration at the steel sheet surface. Meanwhile, the average value of the Cu emission intensity is calculated within a depth range where the Cu emission intensity is sufficiently stable. For example, the average value of the Cu emission intensity in a region from the surface of the steel sheet to a depth of 80 to 100 μm (region B in FIG. 1 ) is calculated, and this is determined as the "bulk Cu concentration." More specifically, when measuring the Cu concentration at the steel sheet surface, the region from the surface of the steel sheet to a depth of 5 μm is measured at plot intervals of 0.01 μm or less, and the average value of the Cu emission intensity in a region from the surface of the steel sheet to a depth of 0.1 μm is calculated. When measuring the bulk Cu concentration, measurements are made at plot intervals of 0.1 μm or less, and the average value of Cu emission intensity in a region from the steel sheet surface to 80 to 100 μm is calculated. Based on the thus obtained "Cu concentration at the steel sheet surface" and "Cu concentration in the bulk," the ratio of the Cu concentration at the steel sheet surface to the Cu concentration in the bulk is determined.

[0022] [Presence or absence of Cu depleted region] In the GDS measurement, the Cu concentration is calculated at each measurement point in the region from the surface of the steel sheet to a depth of 5 μm (region C in FIG. 1 ), and if any of these Cu concentrations is 90% or less of the bulk Cu concentration determined above, it is determined that a Cu depleted region exists. The Cu depleted region may exist in a region from the surface to a depth of 5 μm on at least one of the front and back surfaces of the steel sheet.

[0023] [Chemical Composition of Steel Sheet] In an embodiment of the present invention, the steel sheet has a chemical composition containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. As described above, an object of the present invention is to provide a steel sheet containing Ni, Cu, and Sn that can exhibit improved chemical conversion treatability, and the object is achieved by controlling the element distribution in the surface portion of the steel sheet so that, in GDS measurement from the surface of the steel sheet, the Cu concentration at the steel sheet surface is 115% or less of the Cu concentration in the bulk, and a Cu-depleted region is present within 5 μm from the steel sheet surface in the depth direction, in which the Cu concentration is 90% or less of the Cu concentration in the bulk. Therefore, the chemical composition of the steel sheet is not particularly limited except that it contains, in mass %, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, and 0.003 to 1.000% Sn. Therefore, it is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the object of the present invention. The chemical composition of the steel sheet can include, in addition to Ni, Cu, and Sn, appropriate amounts of any alloying elements commonly added in the technical field of the present invention. The chemical composition of the steel sheet according to the embodiment of the present invention will be described in detail below, but these descriptions are intended to merely exemplify preferred chemical compositions of steel sheets for application in automotive steel sheets and the like, and are not intended to limit the present invention to steel sheets having such specific chemical compositions.

[0024] In an embodiment of the present invention, for example, the steel plate contains, in mass %, C: 0.001 to 0.500%, Si: 0 to 3.00%, Mn: 0.10 to 3.00%, Al: 0.001 to 2.000%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, Sn: 0.003 to 1.000%, P: 0.100% or less, S: 0.100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0 to 1.000%, Cr: 0 to 1.000%, V It is preferable that the alloy has a chemical composition consisting of: W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.500%, Ca: 0 to 0.050%, REM: 0 to 0.010%, As: 0 to 0.100%, Ir: 0 to 1.000%, Zn: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0025] [C: 0.001 to 0.500%] C is an element that inexpensively increases strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.001% or more. The C content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.040% or more, 0.070% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.500% or less. The C content may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.

[0026] [Si: 0 to 3.00%] Si is an element that is effective in increasing strength as a solid solution strengthening element. The Si content may be 0%, but to obtain this effect, the Si content is preferably 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.30% or more, 0.50% or more, 0.80% or more, or 1.00% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.00% or less. The Si content may be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.

[0027] [Mn: 0.10 to 3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.50% or more, 1.00% or more, 1.30% or more, 1.50% or more, or 1.80% or more. On the other hand, excessive Mn content may increase the steel strength but reduce elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[0028] [Al: 0.001 to 2.000%] Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel. To fully obtain this effect, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0029] [Ni: 0.010 to 1.000%] [Cu: 0.010 to 1.000%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. To fully obtain these effects, the contents of these elements are preferably 0.010% or more, and may be 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, excessive inclusion of these elements may reduce the etching ability of Fe during chemical conversion treatment, resulting in reduced chemical conversion treatability of the steel sheet. Therefore, the Ni and Cu contents are preferably 1.000% or less, and may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.

[0030] [Sn: 0.003 to 1.000%] Sn is an element effective in improving corrosion resistance. To fully obtain this effect, the Sn content is preferably 0.003% or more. The Sn content may be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, excessive Sn content may reduce the etching ability of Fe during chemical conversion treatment, resulting in reduced chemical conversion treatability of the steel sheet. Therefore, the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[0031] [P: 0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. Since a lower P content is preferable, ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0032] [S: 0.100% or less] S is an element that generates non-metallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content may result in a significant increase in costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content may cause cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably 0.100% or less. The S content may be 0.050% or less, 0.020% or less, or 0.010% or less.

[0033] [N: 0.0150% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, the ideal N content is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0150% or less. The N content may be 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0034] [O: 0.0100% or less] O is an impurity introduced during the manufacturing process, forming coarse inclusions and reducing the workability of the steel sheet. O can be considered to be included in the impurities. To specifically explain the O content, the lower the O content, the better; ideally, it is 0%. However, excessive reduction in the O content may result in a significant increase in manufacturing costs. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive O content may form coarse inclusions, as described above, reducing the workability of the steel sheet. Therefore, the O content is preferably 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0035] The preferred basic chemical composition of the steel sheet is as described above. Furthermore, the steel sheet may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0036] [Ti: 0 to 0.150%] [Nb: 0 to 0.150%] [V: 0 to 0.150%] Ti, Nb, and V form carbonitrides in steel and have the effect of improving the strength of the steel sheet through precipitation strengthening. The Ti, Nb, and V contents may be 0%, but to obtain this effect, the Ti, Nb, and V contents are preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, even if these elements are contained in excess, the effect saturates, and adding more than necessary to the steel increases manufacturing costs. Therefore, the Ti, Nb, and V contents are preferably 0.150% or less, and may be 0.120% or less, 0.100% or less, 0.080% or less, 0.050% or less, 0.020% or less, or 0.015% or less.

[0037] [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but to achieve this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0038] [Mo: 0 to 1.000%] [Cr: 0 to 1.000%] [W: 0 to 1.000%] Mo, Cr, and W are elements that improve the hardenability of steel and contribute to improving its strength. The Mo, Cr, and W contents may be 0%, but to achieve these effects, the Mo, Cr, and W contents are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel increases manufacturing costs. Therefore, the Mo, Cr, and W contents are preferably 1.000% or less, and may be 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0039] [Hf: 0-0.050%] [Mg: 0-0.050%] [Zr: 0-0.500%] [Ca: 0-0.050%] [REM: 0-0.010%] Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of non-metallic inclusions. The Hf, Mg, Zr, Ca, and REM contents may be 0%, but to achieve these effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel sheet increases production costs. Therefore, the Hf, Mg, and Ca contents are preferably 0.050% or less, and may be 0.010% or less, 0.005% or less, or 0.003% or less. Similarly, the Zr content is preferably 0.500% or less, and may be 0.100% or less, 0.050% or less, or 0.010% or less. Similarly, the REM content is preferably 0.010% or less, and may be 0.005% or less, or 0.003% or less. REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic numbers 57 to lutetium (Lu) with atomic numbers 71, and the REM content is the total content of these elements.

[0040] [As: 0 to 0.100%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more or 0.003% or more. On the other hand, even if excessive As is contained, the effect saturates, and containing more As than necessary in the steel sheet increases manufacturing costs. Therefore, the As content is preferably 0.100% or less. The As content may be 0.050% or less, 0.010% or less, 0.008% or less, or 0.005% or less.

[0041] [Ir: 0 to 1.000%] Ir is an element that segregates at prior austenite grain boundaries to increase the strength of the grain boundaries. The Ir content may be 0%, but to obtain this effect, the Ir content is preferably 0.001% or more. The Ir content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even if an excessive amount of Ir is contained, the effect saturates, and adding more Ir than necessary to the steel material increases the manufacturing cost. Therefore, the Ir content is preferably 1.000% or less. The Ir content may be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0042] [Zn: 0 to 1.000%] Zn is an element effective in controlling the shape of inclusions. The Zn content may be 0%, but to obtain this effect, the Zn content is preferably 0.001% or more. The Zn content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even if Zn is contained in an excessive amount, the effect saturates, and adding more Zn than necessary to the steel material increases the manufacturing cost. Therefore, the Zn content is preferably 1.000% or less. The Zn content may be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0043] The remainder of the steel sheet other than the above elements consists of Fe and impurities. The impurities in the steel sheet are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the steel sheet is industrially manufactured.

[0044] The chemical composition of the steel plate may be measured by a general analytical method. For example, the chemical composition of the steel plate may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece is obtained from the 1 / 4 position of the steel plate thickness, and the test piece is measured using a Shimadzu ICPS-8100 (measuring device) or the like under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

[0045] [Cu Concentration at a Depth Where the Fe Concentration is 90% or More is 85% or Less of the Bulk Cu Concentration] In a preferred embodiment of the present invention, in a GDS measurement from the surface of a steel sheet, the Cu concentration at a depth where the Fe concentration is 90% or more is controlled to 85% or less of the bulk Cu concentration. When GDS measurement is performed from the surface of a steel sheet, a state in which the Cu concentration at a depth near the surface where the Fe concentration first becomes 90% or more is controlled to 85% or less of the bulk Cu concentration means that surface segregation of Cu is sufficiently suppressed. Therefore, by controlling the Cu concentration in GDS measurement within this range, it is possible to further suppress the decrease in Fe etching ability caused by surface segregation of Cu, and as a result, it is possible to more significantly improve the chemical conversion treatability of the steel sheet.

[0046] From the viewpoint of further improving the chemical conversion treatability of the steel sheet, the Cu concentration at the depth where the Fe concentration is 90% or more is preferably as low as possible, more preferably 80% or less or 75% or less of the bulk Cu concentration, and most preferably 70% or less of the bulk Cu concentration. The lower limit is not particularly limited, and for example, the Cu concentration at the depth where the Fe concentration is 90% or more may be 50% or more, 55% or more, or 60% or more of the bulk Cu concentration. The Cu concentration at the depth where the Fe concentration is 90% or more may be determined by the same method as described in [Measurement of Cu Concentration on the Steel Sheet Surface and in the Bulk by GDS]. Specifically, GDS measurement is first performed from the surface of the steel sheet in the depth direction, and the Cu concentration at the depth where the Fe concentration first becomes 90% or more is calculated, and then the ratio of this Cu concentration to the bulk Cu concentration is determined.

[0047] [Silicon Oxide Thickness in X-ray Photoelectron Spectroscopy Measurement: 10 nm or Less] In a preferred embodiment of the present invention, when the chemical composition of a steel sheet contains Si, for example, when the chemical composition of the steel sheet contains 0.01 to 3.00% Si by mass, the thickness of silicon oxide (SiO) is controlled to 10 nm or less in X-ray photoelectron spectroscopy (XPS) measurement of the steel sheet surface. When the chemical composition of the steel sheet contains Si, silicon oxide may be formed on the steel sheet surface. If this silicon oxide becomes thick, it may not be sufficiently dissolved in the chemical conversion treatment solution during chemical conversion treatment. Therefore, to further improve the chemical conversion treatability of the steel sheet, the smaller the thickness of silicon oxide on the steel sheet surface, the more preferable it is. In this regard, the inventors have discovered that controlling the silicon oxide thickness to 10 nm or less in X-ray photoelectron spectroscopy measurement of the steel sheet surface, in combination with the characteristics of the Cu concentration in the surface portion of the steel sheet, can more reliably and significantly improve the chemical conversion treatability of the steel sheet.

[0048] From the viewpoint of further improving the chemical conversion treatability of the steel sheet, as described above, the smaller the thickness of the silicon oxide, the more preferable, and it may be, for example, 9 μm or less, 8 μm or less, 7 μm or less, or 6 μm or less. The lower limit is not particularly limited, and it may be 0 μm. For example, the thickness of the silicon oxide may be 1 μm or more, 3 μm or more, or 5 μm or more.

[0049] [Measurement of Silicon Oxide Thickness by X-ray Photoelectron Spectroscopy] The thickness of silicon oxide is measured by X-ray photoelectron spectroscopy (XPS) as follows. First, an evaluation material for determining the thickness of the silicon oxide (surface oxide) and a reference base material are prepared. The evaluation material is a steel plate for which the silicon oxide thickness is to be determined, cut out from a target product or the like, and the surface oil and dirt have been removed without changing the silicon oxide thickness. Specifically, if oil is applied to the surface of the steel plate to be evaluated, the oil is removed by an appropriate method that does not cause surface oxidation of the steel plate (e.g., a method of removing it with a solvent, etc.) before obtaining the evaluation material. The base material is prepared by grinding and / or polishing a steel plate to a depth of approximately 100 to 500 μm from the surface of the steel plate, and adjusting the arithmetic mean roughness Ra of the surface to 0.8 μm or less. The grinding and polishing method for the base material is not particularly limited, but care must be taken to prevent surface oxidation during grinding and / or polishing. In other words, grinding and polishing methods that result in high temperatures must be avoided. Furthermore, when finish polishing is performed, it is preferable to perform the finish polishing by wet polishing using distilled water or ethanol. For each of the test material and base material prepared as described above, the maximum strength is measured on the steel sheet surface in a binding energy range of 532.9±0.4 eV. If the value of the maximum strength of the test material / the maximum strength of the base material is 1.2 or greater, it is determined that silicon oxide, a surface oxide, is present on the surface of the test material. Next, the test material is subjected to XPS measurement at 1 nm intervals in the thickness direction by sputtering, and the thickness at which the value of the maximum strength of the test material / the maximum strength of the base material becomes less than 1.2 is determined as the thickness of silicon oxide.

[0050] The XPS measurement conditions for determining the thickness of silicon oxide are as follows: X-ray source: mono-Al Kα (1486.6 eV) X-ray diameter: 50 to 200 μm Measurement area: 100 to 700 μm × 100 to 700 μm Degree of vacuum: 1×10 -10 ~1 x 10 -11 torr (1 torr = 133.32 Pa) Acceleration voltage: 1 to 10 kV

[0051] [Thickness of Steel Plate] The thickness of the steel plate is not particularly limited, but is generally 0.2 to 8.0 mm. For example, the thickness may be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the thickness of the steel plate may be, for example, 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0052] As described above, the steel sheet according to the embodiment of the present invention can achieve superior chemical conversion treatability and, therefore, superior corrosion resistance compared to conventional steel sheets simultaneously containing the three elements Ni, Cu, and Sn. Therefore, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields requiring superior chemical conversion treatability and / or corrosion resistance, and is particularly useful for use in parts in the automotive field. In a preferred embodiment, an automobile part including the steel sheet according to the embodiment of the present invention is provided. Examples of automobile parts include frame parts, bumpers, and other structural and reinforcing parts that require strength, as well as exterior panel parts such as roofs, hoods, fenders, and doors that require high designability. It is sufficient for at least a portion of these parts to include the steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the characteristics of the steel sheet described above. In a forming process such as press forming, in a region of the steel sheet that does not come into direct contact with a mold or that comes into direct contact with the mold but is relatively lightly processed, the characteristics of the steel sheet do not change significantly before and after forming.

[0053] [Mechanical Properties] The steel sheet according to the embodiment of the present invention may have a Vickers hardness of, for example, 90 Hv or more, but is not particularly limited thereto. The Vickers hardness may be 150 Hv or more, 190 Hv or more, 200 Hv or more, 250 Hv or more, 300 Hv or more, 350 Hv or more, 400 Hv or more, or 450 Hv or more. The upper limit is not particularly limited, but the Vickers hardness may be, for example, 650 Hv or less, 600 Hv or less, 550 Hv or less, or 500 Hv or less.

[0054] [Measurement of Vickers Hardness] Vickers hardness is determined in accordance with JIS Z 2244-1:2024 as follows. First, a test piece is cut out so that a cross section (thickness cross section) perpendicular to the surface can be observed from any position except the end of the steel plate. The thickness cross section of the test piece is polished using silicon carbide paper #600 to #1500, and then mirror-finished using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water, and this thickness cross section is used as the measurement surface. Next, using a micro Vickers hardness tester, the Vickers hardness is measured at a load of 1 kgf at intervals of at least three times the indentation. Specifically, a total of 20 points are measured randomly at 1 / 4 of the steel plate thickness, and the arithmetic average of these measurements is determined as the Vickers hardness of the steel plate.

[0055] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0056] The steel sheet according to the embodiment of the present invention can be manufactured by, for example, carrying out a casting process in which molten steel having an adjusted chemical composition is cast to form a steel billet, a hot rolling process in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet, a coiling process in which the hot-rolled steel sheet is coiled and then pickled, a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, an annealing process in which the cold-rolled steel sheet is annealed, a brush grinding process in which the annealed cold-rolled steel sheet is brush-ground, and a water-rinsing process in which the brush-ground steel sheet is water-rinsed. Each process will be described in detail below.

[0057] [Casting Step] The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0058] [Hot Rolling Process] A hot-rolled steel plate can be obtained by hot-rolling a cast steel slab. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0059] [Coiling Process] The hot-rolled steel sheet obtained in the hot rolling process is coiled in the next coiling process and then pickled. In this manufacturing method, the hot-rolled steel sheet may be coiled at room temperature. However, from the viewpoint of suppressing the formation of silicon oxide (SiO) on the surface of the finally obtained steel sheet, it is preferable to coil the hot-rolled steel sheet at a coiling temperature of 520°C or higher. More specifically, by controlling the coiling temperature to 520°C or higher, an outer oxide layer is first formed on the outer surface (surface) of the steel sheet, and an inner oxide layer is also formed in the inner surface (surface layer) of the steel sheet. In particular, in the case of a steel sheet containing Si, this inner oxide layer is mainly composed of Si-based oxides. Therefore, a Si-depleted layer is formed immediately below the inner oxide layer formed on the surface layer of the steel sheet due to the consumption of Si in the steel due to the formation of the inner oxide layer. In particular, by controlling the coiling temperature to 520°C or higher, a Si-depleted layer with a sufficient thickness can be formed. Since the outer oxide layer and the inner oxide layer are removed by pickling after coiling, a Si-depleted layer remains on the surface of the hot-rolled steel sheet after pickling. By forming the surface of the hot-rolled steel sheet with a Si-depleted layer, it becomes possible to suppress the formation of silicon oxide (SiO2) on the surface of the steel sheet in a subsequent water-rinsing process or the like due to the lack of Si on the steel sheet surface. The upper limit of the coiling temperature is not particularly limited, but the coiling temperature may be, for example, 600°C or lower.

[0060] The pickling is not particularly limited, and may be carried out using a commonly used pickling solution, such as a hydrochloric acid solution containing an inhibitor that suppresses corrosion of the steel sheet, under conditions suitable for removing the outer and inner oxide layers. The pickling may be carried out once, or may be carried out multiple times to ensure that the outer and inner oxide layers are completely removed.

[0061] [Cold Rolling Step] After pickling the hot-rolled steel sheet, the hot-rolled steel sheet can be cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling step, the steel sheet may be cooled to room temperature, for example, by air cooling.

[0062] [Annealing Step] Next, the obtained cold-rolled steel sheet is annealed. The annealing step involves heating the cold-rolled steel sheet to an annealing temperature of 700 to 900°C in an atmosphere with a dew point of -20 to 20°C and holding the temperature for 10 to 300 seconds. By performing the annealing step under these conditions, Cu can be enriched on the surface of the cold-rolled steel sheet, and a Cu-depleted region can be formed directly below the surface due to the surface enrichment of Cu. Therefore, in the subsequent brush grinding step, by appropriately removing the Cu-enriched portion enriched on the surface by brush grinding, it is possible to obtain a steel sheet in which, in a GDS measurement from the surface of the steel sheet, the Cu concentration on the steel sheet surface is 115% or less of the bulk Cu concentration and a Cu-depleted region exists within a depth direction of 5 μm from the steel sheet surface, in which the Cu concentration is 90% or less of the bulk Cu concentration.

[0063] If the dew point is below -20°C, the annealing temperature is below 700°C, and / or the holding time is less than 10 seconds, Cu cannot be sufficiently concentrated on the steel sheet surface, and a Cu-depleted region cannot be properly formed directly thereunder. On the other hand, if the dew point exceeds 20°C, the annealing temperature exceeds 900°C, and / or the holding time exceeds 300 seconds, Cu is excessively concentrated from the bulk to the surface portion of the steel sheet, and similarly, a Cu-depleted region cannot be properly formed on the surface portion of the steel sheet. From the viewpoint of properly promoting Cu enrichment on the steel sheet surface to more reliably form a Cu-depleted region and, in relation to this, further reducing the Cu concentration in the surface portion after brush grinding, it is preferable to perform the annealing process under conditions of a moderate high temperature and high dew point. For example, by performing the annealing process under conditions of an annealing temperature of 800 to 900°C, a dew point of 0 to 20°C, and a holding time of 80 to 300 seconds, Cu enrichment on the steel sheet surface can be properly promoted. As a result, in the finally obtained steel sheet, the Cu concentration at the depth where the Fe concentration is 90% or more can be significantly reduced, specifically, the Cu concentration at the depth where the Fe concentration is 90% or more can be reduced to 70% or less of the bulk Cu concentration, thereby making it possible to further improve the chemical conversion treatability of the steel sheet. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (e.g., 3% hydrogen and the balance nitrogen).

[0064] [Brush grinding step] The annealed cold-rolled steel sheet is subjected to the next brush grinding step with a grinding amount of 3.0 to 20.0 g / m 2 Brush grinding is performed under the above conditions. By performing brush grinding under these conditions, it is possible to properly or sufficiently remove the Cu-enriched portions on the steel sheet surface. As a result, it is possible to obtain a steel sheet in which the Cu concentration on the steel sheet surface is 115% or less of the Cu concentration in the bulk, and in which a Cu-depleted region exists within a depth of 5 μm from the steel sheet surface, in which the Cu concentration is 90% or less of the Cu concentration in the bulk. The grinding amount by brush grinding is 3.0 g / m 2If the brush grinding amount is less than 20.0 g / m, the Cu-enriched portions on the steel sheet surface cannot be sufficiently removed, and as a result, the Cu concentration on the steel sheet surface cannot be controlled to 115% or less of the bulk Cu concentration. 2 If the brush grinding rate exceeds 1000 kJ / s, not only the Cu-enriched portions on the steel sheet surface but also most or all of the Cu-depleted regions immediately below them may be removed, making it impossible to achieve the desired chemical conversion treatability. In order to further reduce the ratio of the Cu concentration on the steel sheet surface to the Cu concentration in the bulk without removing or excessively removing the Cu-depleted regions, it is preferable to make the grinding amount by brush grinding relatively large, for example, 6.0 g / m 2 It is preferable that this is equal to or greater than this.

[0065] The amount of grinding by the abrasive brush can be adjusted by any appropriate method known to those skilled in the art, and is not particularly limited, but can be adjusted by appropriately selecting, for example, the type of abrasive brush (e.g., H115 manufactured by Hotani Co., Ltd.), the number of abrasive brushes, the rotation speed, the brush pressure, and the coating liquid used.

[0066] [Water-Rinsing Step] Brush grinding causes powdery deposits to adhere to the steel sheet surface. Therefore, the brush-ground steel sheet must be rinsed in the subsequent water-rinsing step. Preferably, the water-rinsing step is performed by rinsing the steel sheet with a rinsing solution having an electrical conductivity of 80 mS / m or less, e.g., rinsing water. For example, if the rinsing solution used in the water-rinsing step has a relatively high electrical conductivity, an oxidation-reduction reaction may occur on the steel sheet surface during rinsing, resulting in the formation of silicon oxide on the steel sheet surface. If the silicon oxide becomes thick, it may not dissolve sufficiently in the chemical conversion treatment solution during chemical conversion treatment. Therefore, to further improve the chemical conversion treatability of the steel sheet, it is preferable that the thickness of the silicon oxide formed on the steel sheet surface is as small as possible. In this manufacturing method, by performing the water-rinsing after brush grinding with a rinsing solution having an electrical conductivity of 80 mS / m or less, the oxidation-reduction reaction on the steel sheet surface during water-rinsing after brush grinding can be sufficiently or completely suppressed. This significantly suppresses the formation of silicon oxide. In particular, when a large amount of strain is introduced into the steel sheet surface by brush grinding, the introduction of strain promotes the formation of silicon oxide on the steel sheet surface. Therefore, rinsing with a rinsing solution having a lower electrical conductivity, particularly rinsing water having a lower electrical conductivity, is very effective in suppressing the formation of silicon oxide.

[0067] Furthermore, in the present production method, it is preferable not only to use a wash solution having a lower electrical conductivity in the wash step, specifically an electrical conductivity of 80 mS / m or less, but also to combine such wash with a coiling temperature of 520°C or higher in the previous coiling step. By combining a Si-deficient layer formed on the steel sheet surface due to a coiling temperature of 520°C or higher with wash using a wash solution having an electrical conductivity of 80 mS / m or lower, it becomes possible to reliably and sufficiently suppress the formation of silicon oxide in the wash step, even when a large amount of strain is introduced into the steel sheet surface by brush grinding. From the viewpoint of further suppressing the formation of silicon oxide, it is preferable that the wash solution have a lower electrical conductivity, specifically, 60 mS / m or less.

[0068] According to this manufacturing method, in a steel sheet for which improving chemical conversion treatability is difficult due to the simultaneous inclusion of the three elements Ni, Cu, and Sn, it is possible to control the Cu concentration in the surface portion of the steel sheet within a predetermined range by appropriately combining the annealing step and the brush grinding step, thereby significantly improving chemical conversion treatability. More specifically, as described above, by first enriching Cu at the surface and forming a Cu-depleted region immediately below the enriched Cu region in the annealing step, and then appropriately controlling the grinding amount of the Cu-enriched region at the surface in the brush grinding step, it is possible to obtain a steel sheet in which the Cu concentration at the steel sheet surface is 115% or less of the Cu concentration in the bulk and a Cu-depleted region exists within a depth of 5 μm from the steel sheet surface, in which the Cu concentration is 90% or less of the Cu concentration in the bulk. As mentioned above, when Ni, Cu, and Sn are present in a steel sheet as solid solutions, the potential of the steel sheet becomes more noble than when these elements are not present in a solid solution state. This reduces the Fe etching ability during chemical conversion treatment, which may result in a decrease in the chemical treatability of the steel sheet. However, with a steel sheet manufactured according to the present manufacturing method, the degree of Cu enrichment at the steel sheet surface can be controlled to 115% or less of the bulk Cu concentration while creating a predetermined Cu-depleted region directly below the Cu enrichment. In addition, by performing the annealing process under conditions of a moderate high temperature and high dew point, the Cu concentration at a depth where the Fe concentration is 90% or more can be significantly reduced. Furthermore, by combining a Si-depleted layer formed by appropriately controlling the coiling temperature in the coiling process with a specific water rinse in the water rinse process after brush grinding, the formation of silicon oxide on the steel sheet surface can be suppressed, thereby further significantly improving the chemical treatability of the steel sheet. Therefore, the steel sheet produced by this production method can achieve superior corrosion resistance compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn, and can contribute to industrial development by extending the service life of steel sheets used in automobiles and building materials.

[0069] The steel sheet according to the embodiment of the present invention can be used as the various automotive parts described above, for example, after a chemical conversion coating or paint film is optionally formed on its surface, more specifically, on the surface of the steel sheet itself that has not been plated. Whether or not an automotive part having a paint film or chemical conversion coating includes the steel sheet according to the embodiment of the present invention can be determined by removing the paint film or chemical conversion coating from a sample taken from the automotive part. In this case, the location from which the sample is taken, the paint film removal step, and the chemical conversion coating removal step are as follows.

[0070] [Sample collection locations] Samples will be collected from automotive parts, avoiding the following locations (i) to (iv): (i) Locations within 20 mm from the toe of spot welds and locations within 20 mm from the toe of the bead of arc / laser welds (ii) Processed areas with a curvature radius of less than 15 mm, and locations within 5 mm from such processed areas (iii) Edges within 5 mm from the cut end surface of the part (iv) Locations within 5 mm from locations where red rust is visible

[0071] [Paint Removal Process] The paint film is removed from a sample cut from an automobile body under the following conditions to expose the steel sheet. A paint remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface at room temperature and allowed to stand for approximately 5 minutes. The paint film is then removed by rubbing with a hard sponge or similar (e.g., Kanefiel, manufactured by AION Co., Ltd.). The sample is then rinsed with water and dried. The remaining paint film is then confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after rinsing and drying. In the element distribution image obtained by EPMA, regions with a carbon concentration of 10% by mass or more are identified, and if the area ratio of such regions is 5% or more, it is determined that the paint film has not been sufficiently removed. To measure the area ratio of regions with a carbon concentration of 10% by mass or more, first obtain an element distribution image of carbon using an EPMA with a carbon concentration range of 10 to 30%. The obtained element distribution image is then subjected to image processing to measure the area ratio. Image analysis software "ImageJ" was used for image processing. The C element distribution image was then loaded into ImageJ, and binarized using "Make Binary" in "Binary" under "Process" so that areas with a C concentration of 10% by mass or more were displayed as black and areas with a C concentration of less than 10% by mass were displayed as white. After binarization, "Measure" under "Analyze" was used to read the value for "Area fraction" in "Results," and this value was determined as the area fraction of areas with a C concentration of 10% by mass or more. If peeling of the coating film was insufficient, removal of the coating film was repeated until the area fraction of areas with a C concentration of 10% by mass or more became less than 5%.

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

[0073] The present invention will be described in more detail below with reference to examples, but the following examples are merely illustrative of the present invention and the present invention is not limited to these examples. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention.

[0074] In the following examples, steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the produced steel sheets were investigated.

[0075] First, molten steel was cast by a continuous casting method to form a steel billet having the chemical composition shown in Table 1. The steel billet was once cooled, reheated to 1200°C, hot-rolled, and then coiled at a coiling temperature of 520°C. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at a temperature of 900 to 1050°C and a reduction ratio of 30%. Next, the obtained hot-rolled steel sheet was pickled and then cold-rolled at a reduction ratio of 50%. Next, the obtained cold-rolled steel sheet was annealed in a furnace with an oxygen concentration of 20 ppm or less in an atmosphere of 3% hydrogen (balanced by nitrogen) under the annealing conditions shown in Table 2. Next, the annealed cold-rolled steel sheet was brush-ground (H115 manufactured by Hotani Co., Ltd.) to the grinding amount shown in Table 2, and finally, washed with wash water having the electrical conductivity shown in Table 2 to obtain a steel sheet having a thickness of 1.6 mm.

[0076]

[0077]

[0078] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0079] [Evaluation of Chemical Conversion Treatability] Chemical conversion treatability was evaluated as follows. First, a 50 mm x 50 mm sample of the steel sheet produced above was subjected to a zinc phosphate treatment (SD5350 system: standard manufactured by Nippon Paint Industrial Coatings Co., Ltd.) as a chemical conversion treatment under the following conditions: Degreasing: Immersion in a degreaser (Fine Cleaner E2032A / B) at 40°C for 2 minutes, followed by rinsing with water. Surface conditioning: Immersion in a surface conditioner (Preparen X) at room temperature for 30 seconds. Chemical conversion treatment: Immersion in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, followed by rinsing with water and drying.

[0080] For steel sheet samples that had been subjected to chemical conversion treatment, the sample surface was observed using secondary electron images from an SEM, and the area ratio of the portion where the chemical conversion coating was not formed, commonly called "clear," was calculated by binarization using the image analysis software "ImageJ." The chemical conversion treatability of the steel sheet was evaluated according to the area ratio of the clear-cut portion using the following evaluation criteria: AAA: Clear-cut area ratio less than 10% AA: Clear-cut area ratio 10 to less than 15% A: Clear-cut area ratio 15 to 20% B: Clear-cut area ratio more than 20%

[0081] Steel sheets with phosphatability ratings of AAA, AA, and A were evaluated as containing Ni, Cu, and Sn and capable of exhibiting improved phosphatability. The results are shown in Table 2.

[0082] Referring to Table 2, in Comparative Example 31, the annealing temperature was low, so Cu could not be sufficiently concentrated on the steel sheet surface, and a Cu-deficient region could not be properly formed immediately thereunder. As a result, the phosphatability was reduced. In Comparative Example 32, the holding time in the annealing step was short, so Cu could not be sufficiently concentrated on the steel sheet surface, and a Cu-deficient region could not be properly formed immediately thereunder. As a result, the phosphatability was reduced. In Comparative Example 33, the dew point in the annealing step was low, so Cu could not be sufficiently concentrated on the steel sheet surface, and a Cu-deficient region could not be properly formed immediately thereunder. As a result, the phosphatability was reduced. In Comparative Example 34, it is believed that the dew point in the annealing step was high, so Cu was excessively concentrated from the bulk to the surface portion of the steel sheet. As a result, a Cu-deficient region could not be properly formed in the surface portion of the steel sheet, and the phosphatability was also reduced. In Comparative Example 35, the grinding amount by brush grinding was 3.0 g / m 2 As a result, the Cu concentration at the steel sheet surface exceeded 115% of the Cu concentration in the bulk, resulting in a deterioration in chemical conversion treatability.

[0083] In contrast, in the steel sheets according to all the examples, the annealing process was carried out under conditions of a dew point of −20 to 20° C., an annealing temperature of 700 to 900° C., and a holding time of 10 to 300 seconds, and the grinding amount by the subsequent brush grinding was 3.0 to 20.0 g / m 2 By controlling as described above, it was possible to appropriately or sufficiently remove the Cu-enriched portions on the steel sheet surface and to create Cu-deficient regions directly below them, resulting in a significant improvement in the chemical conversion treatability of the steel sheet. In particular, in Examples 2 to 4, 8 to 10, 14 to 16, 20 to 22, and 26 to 28, in which the Cu concentration at the steel sheet surface was 115% or less of the bulk Cu concentration and the Cu concentration at a depth from the steel sheet surface where the Fe concentration was 90% or more in the GDS measurement was 85% or less of the bulk Cu concentration, the chemical conversion treatability was evaluated as AA, and the chemical conversion treatability was further improved. Furthermore, in Examples 5, 6, 11, 12, 17, 18, 23, 24, 29 and 30 in which the Cu concentration at the steel sheet surface was 115% or less of the bulk Cu concentration, the Cu concentration at a depth where the Fe concentration was 90% or more in the GDS measurement from the steel sheet surface was 70% or less of the bulk Cu concentration, and the thickness of silicon oxide was 10 nm or less in the X-ray photoelectron spectroscopy measurement of the steel sheet surface, the chemical conversion treatability was evaluated as AAA, and further improvement in chemical conversion treatability was achieved.

Claims

1. A steel sheet having a chemical composition containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, wherein GDS measurement from the steel sheet surface shows that the Cu concentration at the steel sheet surface is 115% or less of the Cu concentration in the bulk, and that a Cu-deficient region is present within a depth direction of 5 μm from the steel sheet surface, in which the Cu concentration is 90% or less of the Cu concentration in the bulk.

2. The steel sheet according to claim 1, wherein the Cu concentration at the surface of the steel sheet is 90% or less of the Cu concentration in the bulk.

3. A steel sheet according to claim 1 or 2, characterized in that, in GDS measurement from the steel sheet surface, the Cu concentration at a depth where the Fe concentration is 90% or more is 85% or less of the bulk Cu concentration.

4. The steel sheet according to any one of claims 1 to 3, characterized in that the chemical composition contains Si, and the thickness of silicon oxide is 10 nm or less when measured on the surface of the steel sheet by X-ray photoelectron spectroscopy.

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

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

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

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