Steel sheet and component including 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 an elemental distribution image obtained by measuring the surface of the steel sheet by Auger electron spectroscopy, the area ratio of an external oxide containing at least one of Si and Cr being 50% or less; and a component which includes said 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 surface state of a steel sheet, and as a result, have found that chemical conversion treatability can be significantly improved by limiting the proportion of specific outer oxides formed on the steel sheet surface to 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, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, wherein an area ratio of an outer oxide containing at least one of Si and Cr is 50% or less in an element distribution image obtained by measuring the steel sheet surface by Auger electron spectroscopy. (2) The steel sheet according to (1) above, wherein the area ratio of the outer oxide is 20% or less. (3) The steel sheet according to (2) above, wherein the area ratio of the outer oxide is 10% or less. (4) The steel sheet according to any one of (1) to (3) above, wherein the grain size of grains forming an Fe phase on the steel sheet surface is 10 μm or less. (5) The steel sheet according to any one of (1) to (4) above, wherein the steel sheet 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] <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 an element distribution image obtained by measuring the steel sheet surface by Auger electron spectroscopy, the area ratio of outer oxides containing at least one of Si and Cr is 50% or less.

[0012] 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, elements such as Ni, Cu, and Sn, when concentrated on the steel sheet surface, may reduce the chemical conversion treatability of the steel sheet. When the steel sheet simultaneously contains the three elements Ni, Cu, and Sn, such reduced chemical conversion treatability becomes a particular problem.

[0013] 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.

[0014] Therefore, the present inventors conducted studies, focusing particularly on the surface condition 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 it is effective to limit the proportion of a specific outer oxide formed on the steel sheet surface within a predetermined range. More specifically, the present inventors found that, when the steel sheet surface, more specifically the surface of an unplated steel sheet, is measured by Auger electron spectroscopy, the chemical conversion treatability of the steel sheet can be significantly improved by controlling the area ratio of outer oxides containing at least one of Si and Cr to 50% or less. In this specification, the term "steel sheet surface" refers to the surface of the steel sheet itself and therefore does not include, for example, the surface of a plated steel sheet.

[0015] Without intending to be bound by any particular theory, it is believed that when elements such as Ni, Cu, and Sn are concentrated on the surface of a steel sheet, the oxidation of Si, Cr, and other elements contained in the steel is accelerated, resulting in the formation of an outer oxide containing at least one of these elements on the surface of the steel sheet. Outer oxides containing these elements may not be sufficiently dissolved in the chemical conversion treatment solution during chemical conversion treatment. Therefore, if the formation of such outer oxides progresses excessively, the chemical conversion treatability of the steel sheet will be reduced. Therefore, the present inventors conducted research to address the reduction in chemical conversion treatability caused by the surface concentration of Ni, Cu, and Sn in steel sheets containing these three elements simultaneously. As a result, the inventors have found that, as will be described in detail later in connection with the manufacturing method, by subjecting a steel sheet to brush grinding under predetermined conditions at an appropriate stage before the annealing step, it is possible to sufficiently or completely remove the concentrated areas of at least one of Ni, Cu, and Sn on the steel sheet surface, and thereby, when the steel sheet surface is measured by Auger electron spectroscopy, it is possible to control the area ratio of outer oxides containing at least one of Si and Cr to 50% or less. Therefore, in the steel sheet according to the embodiment of the present invention, even though the steel sheet simultaneously contains three elements, Ni, Cu, and Sn, by appropriately controlling the proportion of outer oxides containing at least one of Si and Cr formed on the steel sheet surface within a predetermined range, it is possible to significantly improve the chemical conversion treatability of the steel sheet.

[0016] 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.

[0017] [Area Ratio of Outer Oxide Containing at Least One of Si and Cr: 50% or Less] In an embodiment of the present invention, in an element distribution image obtained by measuring the steel sheet surface by Auger electron spectroscopy, the area ratio of the outer oxide containing at least one of Si and Cr is controlled to 50% or less. As described above, controlling the area ratio of the outer oxide containing at least one of Si and Cr to 50% or less can significantly improve the chemical conversion treatability of the steel sheet. From the viewpoint of further improving the chemical conversion treatability, the lower the area ratio of the outer oxide, the better. For example, the area ratio of the outer oxide containing at least one of Si and Cr is preferably 40% or less or 30% or less, more preferably 20% or less, and most preferably 10% or less or 8% or less. The lower limit is not particularly limited and may be 0%. For example, the area ratio of the outer oxide containing at least one of Si and Cr may be 1% or more, 2% or more, or 3% or more.

[0018] [Measurement of the Area Fraction of Outer Oxides Containing at Least One of Si and Cr] The area fraction of outer oxides containing at least one of Si and Cr is measured by Auger electron spectroscopy as follows. First, an evaluation material for identifying the outer oxides formed on the steel sheet surface and a reference base material are prepared. The evaluation material is a steel sheet whose surface condition is maintained. The base material is ground to a depth of 200 μm or more from the steel sheet surface (if grinding to 200 μm or more is difficult due to the thin sheet thickness, grinding to a depth of 25% or more of the sheet thickness) and less than half the sheet thickness position to remove the outer oxides and adjust the arithmetic mean roughness Ra to 2 μm or less. More specifically, the base material is first roughly ground by mechanical polishing to a depth of 200 μm or more from the surface and less than half the sheet thickness position. Emery paper #80 is used for rough grinding. The rough grinding is also wet polishing (distilled water). Next, using emery paper #2000 or the like, finish polishing is performed until the arithmetic mean roughness Ra is 2 μm or less. Finish polishing is performed with ethanol (95% or more) to prevent oxidation of the steel sheet surface during polishing. Measurements are performed by Auger electron spectroscopy for each sample of the evaluation material and base material according to the following procedure. First, the sample is placed in an Auger electron spectroscopy device (e.g., AES PHI-700 (ULVAC-PHI, FE type)), and the sample surface is measured under conditions of an acceleration voltage of 10 kV, a current value of 10 nA, and an Auger spectrum measurement energy range of 40 to 1690 eV. The measurement area is observed at 1000x magnification or more using a SEM, with a size of 60 μm x 100 μm or more. Next, a mapping is created with a lower limit of 80 cps (counts / s), and an element distribution image is obtained. A region in the evaluation material where both O and at least one of Si and Cr are detected and where the emission intensities are 1.15 times or more the emission intensities of Si, Cr, and O in the base material is considered to be a region where "an outer oxide containing at least one of Si and Cr" is present. The emission intensities of Si, Cr, and O in the base material refer to the average values ​​of the emission intensities of each element when an area of ​​60 μm × 100 μm or more is measured.Image analysis software "ImageJ" was used for image processing. The obtained element distribution image was loaded into ImageJ, and then binarized using "Make Binary" in "Binary" under "Process" so that regions where an outer oxide containing at least one of Si and Cr is present are displayed as black, and regions where the outer oxide is not present are displayed as white (min = 0, max = 255). After binarization, "Measure" under "Analyze" was used to read the value of "Area fraction" in "Results." More specifically, the area fraction of the black regions was read, and this value was determined as the area fraction of the outer oxide containing at least one of Si and Cr.

[0019] [Chemical Composition of Steel Sheet] In an embodiment of the present invention, the steel sheet has a chemical composition containing, by mass%, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, and 0.003 to 1.000% Sn. As described above, the present invention aims to provide a steel sheet containing Ni, Cu, and Sn that can exhibit improved chemical conversion treatability. This objective is achieved by controlling the area ratio of outer oxides containing at least one of Si and Cr to 50% or less when the steel sheet surface is measured by Auger electron spectroscopy. Therefore, the chemical composition of the steel sheet is not particularly limited except that it contains, by 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 objective of the present invention. The chemical composition of the steel sheet may contain, in addition to Ni, Cu, and Sn, any alloying elements that are generally added in appropriate amounts 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.

[0020] 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.

[0021] [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.

[0022] [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.

[0023] [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.

[0024] [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.

[0025] [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 achieve 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 result in the formation of a relatively large amount of outer oxide containing at least one of Si and Cr, which may result in a decrease in the 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.

[0026] [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 result in the formation of a relatively large amount of outer oxide containing at least one of Si and Cr, which may result in a decrease in the 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.

[0027] [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.

[0028] [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.

[0029] [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.

[0030] [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.

[0031] 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.

[0032] [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.

[0033] [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.

[0034] [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.

[0035] [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.

[0036] [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.

[0037] [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.

[0038] [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.

[0039] 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.

[0040] 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.

[0041] [Particle size of grains forming the Fe phase on the steel sheet surface: 10 μm or less] In a preferred embodiment of the present invention, the particle size of grains forming the Fe phase on the steel sheet surface (hereinafter also simply referred to as "particle size of the Fe phase") is controlled to 10 μm or less. Here, "Fe phase" refers to the ferrite phase and the prior austenite phase. By controlling the particle size of the Fe phase on the steel sheet surface to 10 μm or less, the chemical conversion treatability of the steel sheet can be further significantly improved.

[0042] More specifically, the inventors have discovered that the particle size of the Fe phase at the steel sheet surface is correlated with the enrichment of Ni, Cu, and Sn at the steel sheet surface, and that the smaller the particle size of the Fe phase at the steel sheet surface, the more the enrichment of Ni, Cu, and Sn at the steel sheet surface can be suppressed. While not intending to be bound by any particular theory, it is believed that elements such as Ni, Cu, and Sn tend to segregate at the grain boundaries of the Fe phase, and therefore, by refining the Fe phase and increasing the number of grain boundaries, these elements can be dispersed and segregated at many grain boundaries. As a result, it is believed that it is possible to significantly suppress the local enrichment of Ni, Cu, and Sn at the steel sheet surface. As will be described in detail later in connection with the manufacturing method, by introducing appropriate strain into the steel sheet surface by brush grinding and then annealing the steel sheet, it is possible to promote recrystallization of the metal structure at the steel sheet surface, thereby refining the Fe phase at the steel sheet surface. As a result, compared to simply mechanically removing the enriched portions of at least one of Ni, Cu, and Sn on the steel sheet surface by relatively weak brush grinding, it is possible to more reliably suppress the formation of outer oxides containing at least one of Si and Cr. In addition, it is generally known that corrosion is likely to occur at linear defects such as dislocations and planar defects such as grain boundaries. Therefore, by refining the Fe phase on the steel sheet surface and increasing the number of grain boundaries of the Fe phase, it is believed that these grain boundaries can effectively function as etching sites during chemical conversion treatment, thereby promoting the anodic dissolution (etching) of Fe. From this perspective, the inventors conducted further studies focusing on the introduction of strain into the steel sheet surface. As a result, the inventors have found that, as will be described in detail later in connection with the manufacturing method, by introducing appropriate strain into the steel sheet surface by brush grinding and then annealing the steel sheet, recrystallization of the metal structure on the steel sheet surface can be promoted, thereby making it possible to control the particle size of the Fe phase on the steel sheet surface to 10 μm or less, and that such refinement of the Fe phase can more significantly improve the chemical conversion treatability of the steel sheet.

[0043] From the viewpoint of further improving the chemical conversion treatability of the steel sheet, the smaller the particle size of the Fe phase on the steel sheet surface, the more preferable, and it may be, for example, 8 μm or less, 6 μm or less, or 5 μm or less. The lower limit is not particularly limited, but the particle size of the Fe phase on the steel sheet surface may be, for example, 0.5 μm or more, 1 μm or more, 2 μm or more, or 3 μm or more.

[0044] [Measurement of particle size of grains forming the Fe phase on the steel sheet surface] The particle size of grains forming the Fe phase on the steel sheet surface is determined using a field emission scanning electron microscope in accordance with JIS G 0551:2020 by mechanically polishing the cross section of the steel sheet to a mirror finish, and then changing the etching solution to reveal the grain boundaries of the "ferrite phase" and the "prior austenite phase". This will be explained in more detail below.

[0045] (1) The "ferrite phase" is distinguished from other phases as follows: First, the cross section of the steel sheet is mechanically polished to a mirror finish, and then nital is used as an etching solution to reveal the grain boundaries. Regions that have a substructure (lath boundaries, block boundaries) within the grains and where carbides are precipitated with multiple variants are judged to be tempered martensite. Regions where cementite is precipitated in a lamellar form are judged to be pearlite. Regions that are relatively low in brightness and where no substructure is visible are judged to be ferrite. Regions that are relatively high in brightness and where the substructure is not revealed by etching are judged to be fresh martensite or retained austenite. Regions that do not fall into any of the above categories are judged to be bainite.

[0046] (2) The "prior austenite phase" is identified as follows: First, the cross section of the steel sheet is mechanically polished to a mirror finish, and then the grain boundaries of the prior austenite phase are revealed using AGS etchant manufactured by Yamamoto Scientific Tool Research Co., Ltd. as an etching solution. The etching conditions are 50°C and 4 minutes.

[0047] The grain size of the grains forming the Fe phase on the steel sheet surface is measured as follows. Two samples are prepared for each level of steel sheet, and the above operations (1) and (2) are performed. A secondary electron image of the observation surface is obtained using a field emission scanning electron microscope. The average line segment length per crystal grain is calculated using the line segment method in accordance with JIS G 0551:2020 from the number of intersections between the length of the drawn line segment and the grain boundary, thereby measuring the grain size of the ferrite grains forming the ferrite phase and the grain size of the grains forming the prior austenite phase, and the number average value D is determined. Similar measurements are performed for five visual fields, and the average value D of the five visual fields is determined as the grain size of the grains forming the Fe phase on the steel sheet surface. The observation field is 40 μm × 40 μm at a magnification of 2000 times, and the line segment length is 40 μm.

[0048] [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.

[0049] 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.

[0050] [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.

[0051] [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.

[0052] <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.

[0053]

[0013] A steel sheet according to an embodiment of the present invention can be manufactured by, for example, 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 brush grinding process in which the obtained hot-rolled steel sheet is brush-ground, a cold rolling process in which the brush-ground hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, and an annealing process in which the cold-rolled steel sheet is annealed. Below, a case in which the cold-rolling process is performed after the brush grinding process will be specifically described, but the order of the brush grinding process and the cold-rolling process is not necessarily limited to this; the brush grinding process may be performed after the cold-rolling process, and then the annealing process may be performed thereafter. Each process will be described in detail below.

[0054] [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.

[0055] [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%.

[0056] [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 is coiled 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 directly below the inner oxide layer formed in the surface layer of the steel sheet due to the consumption of Si in the steel caused by the formation of the inner oxide layer. In particular, by controlling the coiling temperature to 520°C or higher, a Si-depleted layer with sufficient thickness can be formed. The outer oxide layer and inner oxide layer are removed by pickling after coiling, so 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-deficient layer, it is possible to suppress the formation of outer oxides containing Si and further Cr on the steel sheet surface in the subsequent annealing step due to the Si deficiency on the steel sheet surface. The upper limit of the coiling temperature is not particularly limited, but the coiling temperature may be 600°C or lower, for example.

[0057] 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.

[0058] [Brush grinding step] The obtained hot-rolled steel sheet is subjected to the next brush grinding step with a grinding amount of 3.0 g / m 2 Brush grinding is performed under the above conditions. By performing brush grinding under these conditions, it is possible to sufficiently or completely remove the concentrated areas of at least one of Ni, Cu, and Sn on the steel sheet surface. As a result, when the steel sheet surface is measured by Auger electron spectroscopy in the finally obtained steel sheet, it is possible to control the area ratio of outer oxides containing at least one of Si and Cr to 50% or less. The grinding amount by brush grinding is 3.0 g / m 2 If the amount is less than 6.0 g / m, it is not possible to sufficiently remove the concentrated portions of at least one of Ni, Cu, and Sn on the steel sheet surface, and as a result, it is not possible to achieve a desired area ratio of the outer oxide. From the viewpoint of further reducing the area ratio of the outer oxide, the greater the amount of grinding by brush grinding, the more preferable it is, and for example, 6.0 g / m 2 It is preferable that the content is 8.0 g / m or more. 2 More preferably, it is equal to or greater than this.

[0059] The amount of grinding by brush grinding was 8.0 g / m 2 By controlling the above, not only can the concentrated portions of at least one of Ni, Cu, and Sn on the steel sheet surface be simply removed, but also more strain can be introduced into the steel sheet surface. Introducing more strain into the steel sheet surface can promote recrystallization of the metal structure on the steel sheet surface in the subsequent annealing process, thereby refining the Fe phase on the steel sheet surface. As mentioned above, the refinement of the Fe phase due to the introduction of such moderate strain, more specifically, the refinement of the Fe phase to a particle size of 10 μm or less, can increase the number of grain boundaries of the Fe phase, allowing these grain boundaries to effectively function as etching sites during chemical conversion treatment. As a result, the anodic dissolution (etching) of Fe can be promoted during chemical conversion treatment, thereby further improving the chemical conversion treatability of the steel sheet. The upper limit of the grinding amount is not particularly limited, but may be, for example, 20.0 g / m 2 or less than 15.0 g / m 2The amount of grinding by the abrasive brush can be adjusted by any appropriate method known to those skilled in the art. Although not particularly limited, the amount of grinding by the abrasive brush 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, the coating liquid used, and the like.

[0060] [Cold Rolling Step] After brush grinding the hot rolled steel sheet, the hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet. The reduction ratio of the cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling step, the sheet may be cooled to room temperature, for example, by air cooling.

[0061] [Annealing Step] Finally, the obtained cold-rolled steel sheet is annealed. The annealing step involves heating the cold-rolled steel sheet to a temperature of 700 to 950°C in an atmosphere with a dew point of -40 to 20°C and holding the temperature for 0 to 300 seconds. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (e.g., 3% hydrogen and the balance nitrogen). In particular, the grinding amount in the brush grinding step is 6.0 g / m 2 or more or 8.0 g / m 2 By controlling the above and carrying out the annealing step under high temperature and high dew point conditions, preferably an annealing temperature of 820°C or higher, a dew point of -10°C or higher, and a holding time of 80 seconds or longer, and more preferably an annealing temperature of 850°C or higher, a dew point of 10°C or higher, and a holding time of 100 seconds or longer, it is possible to promote recrystallization of the metal structure on the steel sheet surface. This makes it possible to refine the Fe phase on the steel sheet surface to a particle size of 10 μm or smaller. In this case, as described above, it is possible to make the grain boundaries of the Fe phase effectively function as etching sites during chemical conversion treatment, and as a result, it is possible to promote anodic dissolution (etching) of Fe during chemical conversion treatment, thereby further improving the chemical conversion treatability of the steel sheet.

[0062] According to the present manufacturing method, it is possible to achieve a desired area ratio of outer oxides containing at least one of Si and Cr in the cross section of a steel sheet, specifically an area ratio of 50% or less, by appropriately controlling the grinding amount, particularly in the brush grinding step, even in steel sheets where the steel sheet is difficult to improve in chemical treatability due to the simultaneous inclusion of three elements: Ni, Cu, and Sn. As mentioned above, outer oxides containing these elements may not be sufficiently dissolved in the chemical conversion treatment solution during chemical conversion treatment. Therefore, excessive formation of such outer oxides would result in a decrease in the chemical treatability of the steel sheet. However, with the steel sheet manufactured according to the present manufacturing method, the area ratio of outer oxides containing at least one of Si and Cr in the cross section of the steel sheet is reliably controlled to 50% or less, as described above, thereby significantly improving the chemical treatability despite the simultaneous inclusion of three elements: Ni, Cu, and Sn. In addition, by combining the introduction of appropriate strain into the steel sheet surface by brush grinding with annealing under appropriate conditions, it is possible to promote recrystallization of the metal structure on the steel sheet surface. In this regard, the Fe phase on the surface of the steel sheet can be refined, and as a result, the chemical conversion treatability of the steel sheet can be more significantly improved. Therefore, the steel sheet manufactured by this manufacturing method can achieve superior corrosion resistance compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the steel sheet can contribute to industrial development by extending the life of steel sheets used in automobiles and building materials.

[0063] 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.

[0064] [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

[0065] [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%.

[0066] [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%.

[0067] 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.

[0068] 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.

[0069] 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 temperature being 900 to 1050°C and the finish rolling reduction being 30%. Next, the obtained hot-rolled steel sheet was pickled and then brush ground (H115 manufactured by Hotani Co., Ltd.) with the grinding amount shown in Table 2. Next, the brush-ground hot-rolled steel sheet was cold rolled at a rolling reduction of 50%. Finally, the obtained cold-rolled steel sheet was subjected to an annealing process in an atmosphere of 3% hydrogen (nitrogen balance) in a furnace with an oxygen concentration of 20 ppm or less under annealing conditions A (dew point -40°C, annealing temperature 800°C, and holding time 100 seconds), B (dew point -10°C, annealing temperature 820°C, and holding time 80 seconds), or C (dew point 10°C, annealing temperature 850°C, and holding time 100 seconds), to obtain a steel sheet having a thickness of 1.6 mm.

[0070]

[0071]

[0072] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0073] [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.

[0074] 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%

[0075] 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.

[0076] Referring to Table 2, in Comparative Examples 29 to 33, the grinding amount by brush grinding was 3.0 g / m 2 It is believed that because the concentration of at least one of Ni, Cu, and Sn was less than 100%, it was not possible to sufficiently remove the concentrated areas of at least one of Ni, Cu, and Sn on the steel sheet surface. As a result, when the steel sheet surface was measured by Auger electron spectroscopy, the area ratio of outer oxides containing at least one of Si and Cr exceeded 50%, and chemical conversion treatability was reduced.

[0077] In contrast, in the steel plates according to all the examples, the amount of grinding by brush grinding was 3.0 g / m 2 By controlling the above, it was possible to sufficiently remove concentrated areas of at least one of Ni, Cu, and Sn on the steel sheet surface. As a result, the area ratio of the outer oxide containing at least one of Si and Cr in the steel sheet cross section was controlled to 50% or less, thereby significantly improving the phosphatability of the steel sheet. In particular, in Examples 2 to 5, 7 to 10, 12, 13, 18, and 24, in which the area ratio of the outer oxide was 20% or less and / or the particle size of the Fe phase on the steel sheet surface was 10 μm or less, the phosphatability was evaluated as AA, thereby further improving the phosphatability. In Examples 14 to 16, 20 to 22, and 26 to 28, in which the area ratio of the outer oxide was 10% or less and the particle size of the Fe phase on the steel sheet surface was 10 μm or less, the phosphatability was evaluated as AAA, thereby further improving the phosphatability.

Claims

1. A steel sheet having a chemical composition containing, by mass%, Ni: 0.010-1.000%, Cu: 0.010-1.000%, and Sn: 0.003-1.000%, wherein in an element distribution image obtained by measuring the steel sheet surface by Auger electron spectroscopy, the area ratio of outer oxides containing at least one of Si and Cr is 50% or less.

2. The steel sheet according to claim 1, characterized in that the area ratio of the outer oxide is 20% or less.

3. The steel sheet according to claim 2, wherein the area ratio of the outer oxide is 10% or less.

4. The steel sheet according to any one of claims 1 to 3, wherein the grain size of the grains forming the Fe phase on the surface of the steel sheet is 10 μm or less.

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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