Steel sheet and component

Abstract

Disclosed is a steel sheet which contains Cu, Ni and Sn, and has improved chemical conversion treatability. The chemical composition of the steel sheet according to the present disclosure contains, in mass %, 0.010-1.000% of Ni, 0.010-1.000% of Cu, and 0.003-1.000% of Sn. The steel sheet has a concentrated phase of at least one of Ni, Cu, and Sn. The number of the concentrated phases having an equivalent circle diameter of 0.5-5.0 μm on the surface of the steel sheet is two or more per 200 μm of the length along the surface of the steel sheet. The number of the concentrated phases having an equivalent circle diameter of more than 5.0 μm on the surface of the steel sheet is not more than 3 per 200 μm of the length along the surface of the steel sheet. The grain size of the grains forming the Fe phase on the surface of the steel sheet is 60 μm or less.

Description

Steel plates and parts

[0001] The present application discloses steel plates and components.

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

[0003] For example, Patent Document 1 discloses a method for producing a cold-rolled steel sheet having excellent chemical conversion treatability by continuously annealing the cold-rolled steel sheet in equipment for both cold-rolled steel sheets and hot-dip galvanized steel sheets, exposing the surface of the steel sheet to an atmosphere in which iron oxidizes within a predetermined temperature range, pickling the steel sheet at the outlet side of the annealing furnace, and then applying a predetermined amount of iron or Ni plating.

[0004] In addition, Patent Document 2 discloses a method for manufacturing a steel sheet containing copper (Cu) in an amount of 0.10 mass % or more and 0.50 mass % or less, in which the number of residual scales on the surface is 160,000 pieces / mm ２ Patent Document 2 teaches that a steel sheet with excellent chemical conversion treatability can be provided by controlling the particle size of copper compound particles exposed on the steel sheet surface, which serves as a cathode point in chemical conversion treatment, to 2 μm or less and by controlling the amount of residual scale to a predetermined amount or less.

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

[0006] According to new findings by the present inventors, steel sheets containing copper (Cu), nickel (Ni), and tin (Sn) simultaneously tend to have poor chemical conversion treatability. The present application discloses a steel sheet containing Cu, Ni, and Sn and having improved chemical conversion treatability.

[0007] The present application discloses the following multiple aspects as means for solving the above problems: (1) A steel sheet, wherein the chemical composition of the steel sheet includes, in mass %, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, the steel sheet has concentrated phases of at least one of Ni, Cu, and Sn, the number of the concentrated phases having an equivalent circle diameter of 0.5 μm to 5.0 μm on the surface of the steel sheet is 2 or more per 200 μm length along the surface of the steel sheet, the number of the concentrated phases having an equivalent circle diameter of more than 5.0 μm on the surface of the steel sheet is 3 or less per 200 μm length along the surface of the steel sheet, and the grain size of grains forming an Fe phase on the surface of the steel sheet is 60 μm or less. (2) The steel sheet according to (1), wherein the number of the concentrated phases having an equivalent circle diameter of 0.5 μm or more and 5.0 μm or less on the surface of the steel sheet is 4 or more per 200 μm length along the surface of the steel sheet. (3) The steel sheet according to (1) or (2), wherein the number of the concentrated phases having an equivalent circle diameter of more than 5.0 μm on the surface of the steel sheet is 1 or less per 200 μm length along the surface of the steel sheet. (4) The steel sheet according to any one of (1) to (3), wherein the grain size of the grains forming the Fe phase on the surface of the steel sheet is 30 μm or less. (5) The steel sheet according to any one of (1) to (3), wherein the grain size of the grains forming the Fe phase on the surface of the steel sheet is 15 μm or less. (6) The steel sheet according to any one of (1) to (5), wherein the steel sheet has a Vickers hardness of 200 Hv or more. (7) The steel sheet according to any one of (1) to (6), wherein 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%. (8) A part including the steel sheet according to any one of (1) to (7).

[0008] The steel sheet of the present disclosure contains Ni, Cu, and Sn and has improved chemical conversion treatability.

[0009] 1. Background to the Completion of the Steel Sheet of the Present Disclosure Two known methods for producing steel include, for example, a method in which molten iron is obtained in a blast furnace using iron ore, a natural resource, as the primary 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, a recycled resource, as the primary raw material. Electric furnace steel uses scrap as the primary raw material, and therefore contains relatively large amounts of scrap-derived elements (so-called tramp elements), such as Ni, Cu, and Sn. Blast furnace steel may also contain elements such as Ni, Cu, and Sn as additive elements. Meanwhile, both steel sheets produced from blast furnace steel and steel sheets produced from electric furnace steel may require high corrosion resistance. In this case, the corrosion resistance of the steel sheet can be improved by forming a uniform chemical conversion coating on the surface of the steel sheet. In other words, the occurrence of white spots (areas without a chemical conversion coating) on ​​the surface of a steel sheet after chemical conversion treatment tends to reduce the corrosion resistance of the steel sheet. The present inventor has conducted extensive research into the chemical conversion treatability of various steel sheets. As a result, it was found that it is difficult to ensure sufficient chemical conversion treatability with conventional steel sheets containing Ni, Cu, and Sn.

[0010] The present inventors have conducted further studies on the chemical conversion treatability of steel sheets containing Ni, Cu, and Sn. As a result, they have made the following discoveries. (1) When a concentrated phase of at least one of Ni, Cu, and Sn is finely dispersed on the steel sheet surface, the chemical conversion treatability of the steel sheet is improved. Ni, Cu, and Sn are elements that are potentially more noble than Fe. Therefore, during chemical conversion, the concentrated phases of Ni, Cu, and Sn function as cathode sites, promoting anodic dissolution (etching) of the base steel surrounding the concentrated phases, which is thought to facilitate adhesion of the chemical conversion coating. As a result, the chemical conversion treatability of the steel sheet is thought to be improved. (2) When a concentrated phase of at least one of Ni, Cu, and Sn is coarsely dispersed on the steel sheet surface, the chemical conversion treatability of the steel sheet is deteriorated. It is thought that poor etching occurs in the coarse concentrated phases during chemical conversion, resulting in poor chemical conversion. (3) When the Fe phase is refined on the steel sheet surface, the chemical conversion treatability of the steel sheet is improved. It is believed that the grain boundaries of the grains forming the Fe phase become etching sites during chemical conversion, and that the finer the Fe phase, the better the chemical conversion treatability. Furthermore, it is believed that the refinement of the Fe phase facilitates the fine dispersion of the concentrated phase, which promotes etching of the base steel during chemical conversion, making it easier for the chemical conversion coating to adhere. (4) In the manufacturing process of steel sheet containing Ni, Cu, and Sn, by using a pickling solution containing a certain amount or more of a predetermined accelerator during pickling after hot rolling, it is possible to impart large irregularities to the surface of the hot-rolled steel sheet. After pickling, cold rolling under specified conditions can reduce the surface irregularities of the steel sheet and impart strain to the steel sheet surface layer. By annealing a cold-rolled steel sheet having strain imparted to the surface layer under predetermined conditions, a fine Fe phase can be generated in the surface layer, and at the same time, a concentrated phase of at least one of Ni, Cu, and Sn can be nucleated at the grain boundaries of the grains that form the Fe phase, resulting in the concentrated phase being finely dispersed in the surface layer of the steel sheet.

[0011] The steel sheet of the present disclosure has been completed based on the above findings. The steel sheet of the present disclosure may be manufactured from an electric furnace steel that inevitably contains Ni, Cu, and Sn as tramp elements, or may be manufactured from a blast furnace steel that contains Ni, Cu, and Sn as essential elements or optional added elements. The steel sheet of the present disclosure can ensure superior chemical conversion treatability compared to conventional steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. In this regard, the steel sheet of the present disclosure is particularly useful, for example, in applications in the automotive field where chemical conversion treatability and / or corrosion resistance are required. Hereinafter, embodiments of the steel sheet of the present disclosure will be described, but the steel sheet of the present disclosure is not limited to the following embodiments.

[0012] 2. Steel Plate A steel plate according to one embodiment has the following features. Specifically, the chemical composition of the steel plate includes, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. The steel plate has a concentrated phase of at least one of Ni, Cu, and Sn. The number of the concentrated phases having an equivalent circle diameter of 0.5 μm to 5.0 μm on the surface of the steel plate is two or more per 200 μm length along the surface of the steel plate. The number of the concentrated phases having an equivalent circle diameter of more than 5.0 μm on the surface of the steel plate is three or less per 200 μm length along the surface of the steel plate. The grain size of the grains forming the Fe phase on the surface of the steel plate is 60 μm or less.

[0013] 2.1 Chemical Composition The chemical composition of a steel sheet according to one embodiment includes, by mass, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, and 0.003 to 1.000% Sn. The chemical composition may also include, by mass, 0.040 to 1.000% Ni, 0.040 to 1.000% Cu, and 0.004 to 1.000% Sn. As described above, the technology of the present disclosure ensures improved chemical conversion treatability for a steel sheet simultaneously containing the three elements Ni, Cu, and Sn by controlling the morphology of the enriched phase and the morphology of the Fe phase on the surface. In the chemical composition of the steel sheet of the present disclosure, elements other than Ni, Cu, and Sn are not essential technical features for improving chemical conversion treatability. The chemical composition of the steel sheet of the present disclosure may contain, in addition to Ni, Cu, and Sn, any alloying elements commonly added in this technical field in appropriate amounts. The chemical composition of a steel sheet according to one embodiment will be described in detail below, but the following description is intended to merely exemplify a preferred chemical composition for application to automotive steel sheets, etc. The chemical composition of the steel sheet of the present disclosure is not limited to the specific chemical composition described below. Note that, in this application, "%" for a component means mass %. Furthermore, in this application, unless otherwise specified, the term "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0014] In one embodiment, the chemical composition of the steel sheet is, 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 : 0 to 0.150%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.500%, Ca: 0 to 0.050%, REM: 0 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.

[0015] [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.

[0016] [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.

[0017] [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.

[0018] [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.

[0019] [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 such 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 content of these elements may promote the formation of oxides, particularly Mn- and / or Si-based surface oxides and iron oxides, on the steel sheet surface. From the viewpoint of reducing oxide formation on the steel sheet surface, the Ni and Cu contents are each 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.

[0020] [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 promote the formation of oxides on the steel sheet surface, particularly Mn- and / or Si-based surface oxides and iron oxides. From the viewpoint of reducing the formation of oxides on the steel sheet surface, 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.

[0021] [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.

[0022] [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.

[0023] [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 0.0140% or less, 0.0120% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0024] [O: 0.0100% or less] O is an element that is mixed in during the manufacturing process and forms coarse inclusions, reducing the workability of the steel sheet. Since a lower O content is preferable, the ideal O content 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.

[0025] 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.

[0026] [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.

[0027] [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.

[0028] [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.

[0029] [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 0.300% or less, 0.100% or less, or 0.050% 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 number 57 to lutetium (Lu) with atomic number 71, and the "REM content" is the total content of these elements.

[0030] [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 0.080% or less, 0.050% or less, 0.030% or less, 0.010% or less, 0.008% or less, or 0.005% or less.

[0031] [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.

[0032] [Zn: 0% to 1.000%] Zn is an element that can contribute to controlling the morphology of inclusions. Zn is an optional element, and its content is 0% or more. The Zn content may be 0.001% or more, 0.003% or more, or 0.005% 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.

[0033] In the chemical composition of the steel sheet according to one embodiment, the balance other than the above elements is composed 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.

[0034] The chemical composition of the steel plate can be measured by a common analytical method. For example, the chemical composition of the steel plate can 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 can be obtained from the 1 / 4 position of the steel plate thickness, and the composition can be determined by measuring it under conditions based on a pre-created calibration curve using a measuring device such as Shimadzu ICPS-8100. C and S, which cannot be measured by ICP-AES, can be measured using a combustion-infrared absorption method, and N can be measured using an inert gas fusion-thermal conductivity method.

[0035] 2.2 Concentrated phase of at least one of Ni, Cu, and Sn The steel sheet according to one embodiment has a concentrated phase of at least one of Ni, Cu, and Sn. "Concentrated phase of at least one of Ni, Cu, and Sn" refers to a phase in which the total content of Ni, Cu, and Sn is 0.5 mass% or more and is 1.1 times or more the total content of Ni, Cu, and Sn in the bulk in the EPMA analysis described below. The "total content of Ni, Cu, and Sn in the bulk" corresponds to the total content of Ni, Cu, and Sn in the "chemical composition of the steel sheet" described above. That is, a 35 mm square test piece is obtained from the 1 / 4 thickness position of the steel sheet, and the total content of Ni, Cu, and Sn in the bulk is determined by measuring it under conditions based on a previously prepared calibration curve using a measuring device such as Shimadzu ICPS-8100. The total content of Ni, Cu, and Sn in the enriched phase may be, for example, 0.5 mass % or more and 5.0 mass % or less, or 1.0 mass % or more and 2.5 mass % or less.

[0036] 2.2.1 Fine Enriched Phases It is important that the number of enriched phases with a circular equivalent diameter of 0.5 μm or more and 5.0 μm or less on the surface of a steel sheet according to one embodiment is two or more per 200 μm length along the surface of the steel sheet. According to the inventor's new findings, even if fine enriched phases with a circular equivalent diameter of 2.0 μm or more and 5.0 μm or less are present on the surface of the steel sheet, poor chemical conversion is unlikely to occur. Rather, by dispersing a large amount of fine enriched phases with a circular equivalent diameter of 2.0 μm or more and 5.0 μm or less on the surface of the steel sheet, the chemical conversion treatability of the steel sheet is improved. Ni, Cu, and Sn are elements that are potentially more noble than Fe. Therefore, during chemical conversion, the enriched phases of Ni, Cu, and Sn function as cathode sites, promoting anodic dissolution (etching) of the base steel present around the enriched phases and facilitating the adhesion of the chemical conversion coating. By finely dispersing such concentrated phases on the steel sheet surface, the chemical conversion treatability of the entire surface of the steel sheet can be improved. In one embodiment, the number of concentrated phases having a circle-equivalent diameter of 0.5 μm or more and 5.0 μm or less on the surface of the steel sheet is preferably 3 or more, more preferably 4 or more, and even more preferably 5 or more per 200 μm length along the surface of the steel sheet. The upper limit of the number of concentrated phases having a circle-equivalent diameter of 0.5 μm or more and 5.0 μm or less on the surface of the steel sheet is not particularly limited. The number may be, for example, 30 or less, 25 or less, 20 or less, or 15 or less per 200 μm length along the surface of the steel sheet.

[0037] 2.2.2 Coarse Enriched Phases It is important that the number of enriched phases having a circle-equivalent diameter of more than 5.0 μm on the surface of a steel sheet according to one embodiment is three or less per 200 μm along the surface of the steel sheet. According to the inventor's new findings, the presence of coarse enriched phases having a circle-equivalent diameter of more than 5.0 μm on the surface of a steel sheet is likely to result in poor chemical conversion in the enriched phases during chemical conversion treatment. This is thought to be because poor etching occurs in the coarse enriched phases during chemical conversion treatment. Reducing the coarse enriched phases on the surface of the steel sheet can improve the chemical conversion treatability of the entire surface of the steel sheet. In one embodiment, the number of enriched phases having a circle-equivalent diameter of more than 5.0 μm on the surface of the steel sheet is preferably two or less, more preferably one or less, per 200 μm along the surface of the steel sheet. The lower limit of the number of enriched phases having a circle-equivalent diameter of more than 5.0 μm on the surface of the steel sheet is not particularly limited. The number of concentrated phases having a circle equivalent diameter of more than 5.0 μm on the surface of the steel sheet is 0 or more, and may be more than 0, per 200 μm length along the surface of the steel sheet.

[0038] 2.2.3 Method for measuring the size and number of enriched phases The size and number of the enriched phases on the surface of a steel sheet can be identified by performing local elemental analysis of a cross section of the steel sheet surface layer using an electron probe microanalyzer (EPMA). For observation with EPMA, a sample is taken so that the cross section parallel to the thickness direction of the steel sheet becomes the observation surface. Furthermore, before analysis with EPMA, the sample is embedded in resin and the observation surface is mirror-polished. For EPMA analysis, for example, a JXA-8500 manufactured by JEOL Ltd. is used, with an acceleration voltage of 15 kV and a probe current of 5 x 10 －７A, irradiation time: 50 ms. EPMA analysis is performed under the following conditions. The EPMA analysis targets Fe, Si, Mn, Ni, Cu, and Sn, and a mapping image is obtained for a region of the cross section of the steel sheet, extending from the surface of the steel sheet to a depth of 10 μm and extending 200 μm along the surface of the steel sheet. Next, from the obtained mapping image, a region is identified in which the total content of Ni, Cu, and Sn in the region is 0.5 mass% or more and is 1.1 times or more the total content of Ni, Cu, and Sn in the bulk. For example, in the mapping image, a color bar is displayed with a Ni concentration range of 0.5 to 5.0%, thereby identifying a region with a Ni concentration of 0.5% or more (however, if the Ni content in the bulk is 0.5 mass% or more, the lower limit of the Ni concentration is 1.1 times the Ni content in the bulk. The same applies hereinafter to Cu and Sn). Similarly, by displaying the color bar with a Cu concentration range of 0.5 to 5.0%, the region where the Cu concentration is 0.5% or more is identified. Similarly, by displaying the color bar with a Sn concentration range of 0.5 to 5.0%, the region where the Sn concentration is 0.5% or more is identified. For regions where two or more of Ni, Cu, and Sn coexist, the color bar display should be such that the total concentration of Ni, Cu, and Sn is 0.5% or more (however, if the total content of Ni, Cu, and Sn in the bulk is 0.5% or more by mass, the lower limit of the total concentration of Ni, Cu, and Sn should be 1.1 times the total content of Ni, Cu, and Sn in the bulk). As an example, for a region where Ni and Cu coexist, a color bar is displayed with a Ni concentration range of 0.2 to 5.0% (however, if the Ni content in the bulk is 0.2% by mass or more, the lower limit of the Ni concentration is 1.1 times the Ni content in the bulk) and a Cu concentration range of 0.3 to 5.0% (however, if the Cu content in the bulk is 0.3% by mass or more, the lower limit of the Cu concentration is 1.1 times the Cu content in the bulk), thereby making it possible to identify regions where the total concentration of Ni and Cu is 0.5% or more and 1.1 times the total content of Ni and Cu in the bulk. Regions where Ni and Sn coexist, regions where Cu and Sn coexist, and regions where Ni, Cu, and Sn coexist can also be identified by similar means.Of the regions thus deemed to be enriched phases, those present on the surface of the steel sheet (exposed on the surface) are deemed to be "enriched phases on the surface of the steel sheet." The area of ​​the enriched phase is calculated, and the area-equivalent circle diameter is calculated based on the area. This area-equivalent circle diameter is deemed to be the "circle-equivalent diameter of the enriched phase on the surface of the steel sheet." Here, the area per enriched phase is measured using the image processing software "ImageJ." Specifically, a mapping image is imported into ImageJ, and "Set scale" under "Analyze" is used to calibrate the scale bar in the mapping image and the scale on ImageJ. Next, "Make Binary" under "Binary" under "Process" is used to binarize the enriched phase so that it is displayed in black and the regions other than the enriched phase are displayed in white. After binarization, one of the enriched phases whose area is to be measured is selected on the "Clipboard" screen, and then "Measure" under "Analyze" is used to read the "Area" and "Area fraction" in the "Results" section. The values ​​of "Area" and "Area fraction" are multiplied to determine the area of ​​each enriched phase. Note that enriched phases that are partially cut off due to being caught on the outer edge of the observation area are excluded from the area measurement. Furthermore, for regions deemed to be enriched phases on the surface of the steel sheet, the number of enriched phases with a circular equivalent diameter of 0.5 μm to 5.0 μm per 200 μm along the surface of the steel sheet is calculated, and the number of enriched phases with a circular equivalent diameter of more than 5.0 μm per 200 μm along the surface of the steel sheet is calculated. In the present application, the average value for any five locations on the cross section of the steel sheet is used as "the number of concentrated phases on the surface of the steel sheet having a circular equivalent diameter of 0.5 μm or more and 5.0 μm or less per 200 μm length along the surface of the steel sheet" or "the number of concentrated phases on the surface of the steel sheet having a circular equivalent diameter of more than 5.0 μm per 200 μm length along the surface of the steel sheet."

[0039] 2.3 Fe Phase It is important that the particle size of the particles forming the Fe phase on the surface of the steel sheet according to one embodiment is 60 μm or less. By refining the Fe phase on the steel sheet surface, the chemical conversion treatability of the steel sheet is improved. It is believed that the grain boundaries of the Fe phase become etching sites during chemical conversion treatment, and the finer the Fe phase, the more etching sites there are, making it easier for the chemical conversion coating to adhere. Furthermore, it is believed that refining the Fe phase makes it easier for the concentrated phase to be finely dispersed, which promotes etching of the base steel around the concentrated phase during chemical conversion treatment and makes it easier for the chemical conversion coating to adhere. The "Fe phase" refers to a ferrite phase or a prior austenite phase. The particle size of the particles forming the Fe phase on the surface of the steel sheet is preferably 40 μm or less, more preferably 30 μm or less, even more preferably 20 μm or less, and particularly preferably 15 μm or less. The lower limit of the particle size of the particles forming the Fe phase on the surface of the steel sheet is not particularly limited, and may be more than 0 μm, 1 μm or more, 3 μm or more, 5 μm or more, or 7 μm or more.

[0040] In the present application, in accordance with JIS G 0551:2020, the cross section of the steel sheet is mechanically polished to a mirror finish, and then the etching solution is changed to reveal the grain boundaries of the "ferrite phase" and the "prior austenite phase," and then the "grain size of the grains forming the Fe phase" is specified.

[0041] (1) The "ferrite phase" is distinguished from other phases as follows: After mechanically polishing the cross section of the steel sheet to a mirror finish, nital is used as an etching solution to reveal the grain boundaries. Regions that have a substructure within the grains (lath boundaries, block boundaries) 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.

[0042] (2) The "prior austenite phase" is identified as follows: 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. The etching conditions are 50°C and 4 minutes.

[0043] The "grain size of grains forming the Fe phase" is measured as follows. First, two samples are prepared for each level of steel sheet, and the above (1) and (2) are performed, respectively, to reveal the grain boundaries of grains forming the ferrite phase in the cross section of one sample, and the grain boundaries of grains forming the prior austenite phase in the cross section of the other sample. A secondary electron image is obtained for the cross section where the grain boundaries are revealed using a field emission scanning electron microscope. In the secondary electron image, for each of the ferrite phase and prior austenite phase present (exposed on the surface) of the steel sheet, the grain size is measured by the line segment method in accordance with JIS G 0551:2020, by calculating the average line segment length per crystal grain from the number of intersections between the length of the drawn line segment and the grain boundary, and the arithmetic mean value D is determined. Similar measurements are performed for five visual fields, and the average value D of the five visual fields is regarded as the "grain size of grains forming the Fe phase." The observation field is set to 200 μm×200 μm at a magnification of 100 times, and the length of the line segment is set to 200 μm.

[0044] 2.4 Thickness of Steel Plate The thickness of the steel plate of the present disclosure is not particularly limited. The steel plate according to one embodiment may have a thickness of, for example, 0.2 mm or more and 8.0 mm or less. 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, or 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0045] 2.5 Mechanical Properties The mechanical properties of the steel sheet of the present disclosure are not particularly limited. A steel sheet according to one embodiment may have a Vickers hardness of, for example, 80 Hv or more. The Vickers hardness may be 90 Hv or more, 100 Hv or more, 150 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, and the Vickers hardness may be, for example, 650 Hv or less, 600 Hv or less, 550 Hv or less, or 500 Hv or less.

[0046] The Vickers hardness of steel plate is measured in accordance with JIS Z 2244-1:2024 as follows. First, a test piece is cut out from any position except the end of the steel plate so that a cross section (thickness cross section) perpendicular to the surface can be observed. 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, the Vickers hardness is measured using a micro Vickers hardness tester at a load of 1 kgf and 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's thickness, and the arithmetic average of these measurements is determined as the Vickers hardness of the steel plate.

[0047] 3. Manufacturing Method of Steel Sheet Next, a preferred manufacturing method of the steel sheet according to the embodiment will be described. The following description is intended to exemplify a characteristic method for manufacturing the steel sheet according to the embodiment, and is not intended to limit the steel sheet of the present disclosure to one manufactured by the manufacturing method described below.

[0048] A steel sheet according to one embodiment can be manufactured through, for example, a casting process in which molten steel having an adjusted chemical composition is cast to obtain a steel slab, a hot rolling process in which the steel slab is hot-rolled to obtain a hot-rolled steel sheet, a pickling process in which the hot-rolled steel sheet is pickled, a cold-rolling process in which the pickled 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. Each process will be described in detail below.

[0049] 3.1 Casting Process The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes are performed, and then the molten steel is cast by a method such as ordinary continuous casting or ingot casting to obtain a steel slab.

[0050] 3.2 Hot Rolling Process In the hot rolling process, the cast steel slab is hot rolled to obtain a hot-rolled steel sheet. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, and then hot rolling it. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100°C or higher and 1250°C or lower. 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°C or higher and 1050°C or lower, and the reduction ratio of finish rolling may be 10% or higher and 50% or lower.

[0051] 3.3 Pickling Step In the pickling step, the hot-rolled steel sheet obtained in the hot rolling step is optionally coiled and then pickled. There are no particular restrictions on the coiling temperature of the hot-rolled steel sheet, but it is preferably 520°C or higher from the viewpoint of internal oxidation.

[0052] In this manufacturing method, one feature resides in the pickling solution used during the pickling of the hot-rolled steel sheet. Specifically, it is important that the pickling solution contains an accelerator that promotes dissolution of the grain boundaries of the particles that form the Fe phase, at a concentration of 10 ppm to 1500 ppm. During pickling, the accelerator promotes dissolution of the grain boundaries of the particles that form the Fe phase, thereby imparting large irregularities to the surface of the hot-rolled steel sheet, thereby enabling appropriate strain to be imparted to the steel sheet surface layer in the cold-rolling process described below. If the accelerator concentration in the pickling solution is too low, the surface irregularities of the hot-rolled steel sheet will be insufficient, and appropriate strain will not be imparted to the steel sheet surface layer during cold rolling. As a result, the final steel sheet will not have appropriate enriched phases or Fe phases. The accelerator contained in the pickling solution may be any accelerator that promotes dissolution of the grain boundaries of the particles that form the Fe phase. For example, an S-containing organic compound such as thioglycolic acid, thiosulfuric acid, thiocyanic acid, thiocarboxylic acid, thiourea, sodium thiosulfate, or ammonium thiosulfate is preferred. The concentration of the accelerator contained in the pickling solution is preferably 50 ppm or more and 1500 ppm or less, more preferably 100 ppm or more and 1500 ppm or less.

[0053] In the pickling process, as described above, it is important to impart large irregularities to the surface of the hot-rolled steel sheet. The arithmetic mean roughness Ra of the surface of the hot-rolled steel sheet after the pickling process is, for example, 4 μm or more, and may be 6 μm or more and 10 μm or less. The temperature and time in the pickling process are not particularly limited as long as appropriate irregularities can be imparted to the surface of the hot-rolled steel sheet. According to the findings of the present inventors, when pickling is performed using the above-mentioned pickling solution, appropriate irregularities can be imparted to the surface of the hot-rolled steel sheet when the temperature of the pickling solution is 40° C. or more and 90° C. or less and the pickling time is 40 seconds or more and 300 seconds or less. Pickling may be performed only once or may be performed in multiple steps.

[0054] 3.4 Time from Pickling to Cold Rolling The time from the end of pickling to the start of cold rolling is preferably within 24 hours. By keeping this time within 24 hours, the cold rolling is performed while the hydrogen that penetrated into the steel during pickling remains, and the hydrogen in the steel is converted to non-diffusible hydrogen during cold rolling, so that hydrogen remains in the steel even during subsequent annealing. This is thought to make it easier to suppress coarsening of concentrated phases of at least one of Ni, Cu, and Sn in the surface layer of the finally obtained steel sheet. The time from the end of pickling to the start of cold rolling is more preferably within 10 hours, even more preferably within 3 hours, and particularly preferably within 0.5 hours.

[0055] 3.5 Cold Rolling Process In the cold rolling process, the hot-rolled steel sheet after pickling is cold-rolled to obtain a cold-rolled steel sheet. In this manufacturing method, the hot-rolled steel sheet that has been given surface irregularities by pickling is subjected to cold rolling to reduce the surface irregularities of the steel sheet (flatten the protrusions) and impart strain to the surface layer of the steel sheet. The arithmetic mean roughness Ra of the surface of the cold-rolled steel sheet after cold rolling may be, for example, 3 μm or less, preferably 2 μm or less. The reduction ratio of the cold rolling may be any reduction ratio that can impart appropriate strain to the surface layer of the steel sheet, and can be appropriately determined depending on the desired metal structure and sheet thickness. The reduction ratio of the cold rolling may be, for example, 20% or more and 80% or less. After the cold rolling process, the steel sheet may be cooled to room temperature, for example, by air cooling.

[0056] 3.6 Annealing Process In the annealing process, the cold-rolled steel sheet is annealed. As described above, strain is imparted to the surface layer of the cold-rolled steel sheet. By annealing the cold-rolled steel sheet with strain imparted to the surface layer, the strain generates a fine Fe phase, and at the grain boundaries of the grains forming the Fe phase, a concentrated phase of at least one of Ni, Cu, and Sn is nucleated, thereby allowing the concentrated phase to be finely dispersed in the steel sheet surface layer. The annealing process may include, for example, heating the cold-rolled steel sheet to a temperature of 700°C to 950°C in an atmosphere with a dew point of -40°C to 20°C and holding the temperature for 0 seconds to 300 seconds. The atmosphere in the annealing process 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., 4% hydrogen and the balance nitrogen).

[0057] As described above, when producing a steel sheet containing the three elements Ni, Cu, and Sn simultaneously, by devising the pickling step after hot rolling, the cold rolling step after pickling, and the annealing step after cold rolling as described above, the morphology of the enriched phase and Fe phase on the surface of the finally obtained steel sheet can be appropriately controlled, and the chemical conversion treatability of the steel sheet can be significantly improved.

[0058] 4. Parts The technology of the present disclosure also has an aspect of a part including the above-described steel sheet. The steel sheet according to the present embodiment is useful for use in parts in technical fields requiring excellent chemical conversion treatability, and is particularly useful for use in parts in the automotive field. That is, the part may be an automobile part. 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 design quality. At least a portion of these parts may include the steel sheet according to the present embodiment. In other words, at least a portion of these parts satisfies the characteristics of the steel sheet described above.

[0059] When taking samples from automotive parts for various measurements and analyses, the following locations and areas (1) to (4) shall be avoided: (1) Welded parts: 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 (2) Processed parts: Processed parts with a curvature radius of less than 15 mm and locations within 5 mm from the processed parts (3) Edges: Edges within 5 mm from the cut end surface of the part (4) Red rust: Locations within 5 mm from locations where red rust is visible to the naked eye

[0060] The steel sheet of the present disclosure may be used as the above-mentioned automotive part, for example, after a chemical conversion coating or paint film is optionally formed on the surface thereof. Whether or not an automotive part having a paint film or chemical conversion coating includes the steel sheet of the present disclosure can be determined by removing the paint film or chemical conversion coating from the automotive part. In this case, the paint film removal step and the chemical conversion coating removal step are as follows.

[0061] 4.1 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 these regions is 5% or more, the paint film is deemed to have been insufficiently removed. To measure the area ratio of regions with a carbon concentration of 10% by mass or more, an element distribution image of carbon is first obtained using EPMA with a carbon concentration range of 10 to 30%. The area ratio is then measured by image processing of the element distribution image. Image processing was performed using the image processing software "ImageJ." After loading the C element distribution image into ImageJ, "Make Binary" in "Binary" under "Process" was used to binarize the image so that areas with a C concentration of 10% or more were displayed in black and areas with a C concentration of less than 10% were displayed in white. After binarization, "Measure" under "Analyze" was used to read the value for "Area fraction" in "Results," which was taken as the area fraction of areas with a C concentration of 10% or more. If the coating film was not sufficiently removed, the coating film was repeatedly removed until the area fraction of areas with a C concentration of 10% by mass or more was less than 5%.

[0062] 4.2 Chemical Conversion Coating Removal Process The chemical conversion coating is removed from samples cut from automobile bodies and the coating removed, according to JIS K3151. Specifically, the samples are immersed in a 5% chromic acid solution heated to 75°C for 15 minutes to remove the chemical conversion coating. The samples are then rinsed 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. Regions with a P concentration of 5% or more by mass are identified in the EPMA element distribution image, and if the area ratio of these 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 P concentration of 5% or more by mass, first obtain an element distribution image of P using an EPMA with a P concentration range of 5 to 10%. The element distribution image is then image-processed to measure the area ratio. Image processing was performed using the image processing software "ImageJ." After loading the P element distribution image into ImageJ, the image was binarized using "Make Binary" under "Binary" in "Process" so that areas with a P concentration of 5% or more were displayed in black and areas with a P concentration of less than 5% were displayed in white. After binarization, "Measure" under "Analyze" was used, and the value for "Area fraction" in "Results" was taken as the area fraction of areas with a P concentration of 5% by mass or more. If removal 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 was less than 5%.

[0063] 5. Others The steel sheet of the present disclosure has excellent chemical conversion treatability and can have a chemical conversion coating formed uniformly on its surface. In this regard, the technology of the present disclosure also has an aspect of a surface-treated steel sheet having the above-mentioned steel sheet and a chemical conversion coating formed on at least a portion of the surface of the steel sheet. A known coating may be used as the chemical conversion coating.

[0064] The present invention will be described in more detail below with reference to examples. However, 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. In the following examples, steel sheets according to one embodiment were manufactured under various conditions, and the properties of the manufactured steel sheets were investigated.

[0065] 1. Production of Steel Plates 1.1 Casting Process Molten steel was continuously cast to obtain steel billets having the chemical compositions shown in Table 1 below.

[0066] 1.2 Hot Rolling Step After cooling the slab, it was reheated to 1200°C and hot rolled. The hot rolling was performed by rough rolling and finish rolling, and the finish rolling temperature was 900 to 1050°C, and the finish rolling reduction was 30%.

[0067] 1.3 Pickling The hot-rolled steel sheets were coiled at 100 to 300°C and then subjected to pickling. In the pickling, a pickling solution was used that contained 10% hydrochloric acid as the acid, 0.04% Ivit 710K manufactured by Asahi Chemical Industry Co., Ltd. as an inhibitor to prevent excessive pickling of the base steel, and glycol thiosulfate as an accelerator at the concentrations shown in Table 2. The temperature of the pickling solution during pickling was 85°C, and the pickling time was 75 seconds.

[0068] 1.4 Cold Rolling Step After pickling, the steel sheet was cold rolled at a rolling reduction of 50% to obtain a cold rolled steel sheet having a thickness of 1.6 mm. The time from the end of pickling to the start of cold rolling is as shown in Table 2.

[0069] 1.5 Annealing Process A cold-rolled steel sheet was annealed in a furnace with an oxygen concentration of 20 ppm or less under any of the following conditions A to C to obtain a steel sheet containing Ni, Cu, and Sn simultaneously. The chemical composition of the obtained steel sheet was the same as that of the steel slab before hot rolling. A: The cold-rolled steel sheet was heated to 800°C in an atmosphere with a dew point of -40°C and 4% hydrogen (nitrogen balance) and held for 100 seconds. B: The cold-rolled steel sheet was heated to 820°C in an atmosphere with a dew point of -10°C and 4% hydrogen (nitrogen balance) and held for 80 seconds. C: The cold-rolled steel sheet was heated to 850°C in an atmosphere with a dew point of 10°C and 4% hydrogen (nitrogen balance) and held for 100 seconds.

[0070]

[0071] 2. Evaluation of Steel Sheets 2.1 Size and Number of Enriched Phases on the Steel Sheet Surface Local elemental analysis was performed on the cross section of the steel sheet surface layer using EPMA to measure the "number of enriched phases with a circle equivalent diameter of 0.5 μm to 5.0 μm on the surface of the steel sheet per 200 μm along the surface of the steel sheet" and the "number of enriched phases with a circle equivalent diameter of more than 5.0 μm on the surface of the steel sheet per 200 μm along the surface of the steel sheet." Details of the measurement method using EPMA are as described above. The results are shown in Table 2 below.

[0072] 2.2 Size of Fe phase on steel sheet surface The cross section of the steel sheet surface was observed using an SEM to measure the "grain size of the particles forming the Fe phase on the steel sheet surface." Details of the measurement method using an SEM are as described above. The results are shown in Table 2 below.

[0073] 2.3 Vickers Hardness The Vickers hardness of the steel sheets was measured. Details of the Vickers hardness measurement method are as described above. The results are shown in Table 2 below.

[0074] 2.4 Chemical conversion treatability 2.4.1 Chemical conversion treatment procedure A 50 mm square sample was cut from the above steel plate, and the sample was subjected to zinc phosphate treatment as a chemical conversion treatment by carrying out the following steps (1) to (6) in order: (1) Degreasing: Immersion in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes (2) Water rinsing (3) Liquid surface conditioning: Immersion in a surface conditioning agent (Preparen Z) at room temperature for 30 seconds (4) Chemical conversion treatment: Immersion in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes (5) Water rinsing (6) Drying

[0075] 2.4.2 Evaluation method The bead portion of the sample that had been subjected to chemical conversion treatment was observed using SEM-EPMA, and the area ratio of the portion where the chemical conversion coating film was not formed, commonly called "clear," was calculated by binarization using the image analysis software "ImageJ." Depending on the area ratio of the clear area, the chemical conversion treatability of the processed portion was evaluated according to the following evaluation criteria. Clear area was measured using an EPMA (JXA-8500 manufactured by JEOL Ltd.) at an acceleration voltage of 15 kV and a probe current of 5 x 10 -7In a mapping image obtained by photographing at 1000x magnification under the conditions of A, it was defined as a region where the Fe concentration was 70% or more. The void area ratio was determined as the average value of five randomly selected fields of view. Evaluation AAA: void area ratio less than 10% Evaluation AA: void area ratio 10% or more and less than 15% Evaluation A: void area ratio 15% or more and less than 20% Evaluation B: void area ratio 20% or more

[0076] In this example, steel sheets with phosphatability ratings of AAA, AA, and A were evaluated as "steel sheets containing Ni, Cu, and Sn and having improved phosphatability." The results are shown in Table 2 below.

[0077]

[0078] The results shown in Tables 1 and 2 reveal the following.

[0079] As shown in Comparative Examples 51 to 55, when pickling is performed using a pickling solution containing a low concentration of an accelerator, the finally obtained steel sheet has a small number of concentrated phases with a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less on the surface of the steel sheet, and / or a large number of concentrated phases with a circle equivalent diameter of more than 5.0 μm on the surface of the steel sheet, resulting in a deterioration in the chemical conversion treatability of the steel sheet.

[0080] As shown in Comparative Example 56, in the hot-rolled steel sheet disclosed in Patent Document 2, the Fe phase on the surface becomes coarse, and the chemical conversion treatability deteriorates.

[0081] As shown in Examples 1 to 50, when the number of enriched phases having a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less on the surface of the steel sheet is 2 or more per 200 μm length along the surface of the steel sheet, the number of enriched phases having a circle equivalent diameter of more than 5.0 μm on the surface of the steel sheet is 3 or less per 200 μm length along the surface of the steel sheet, and the particle size of the particles forming the Fe phase on the surface of the steel sheet is 60 μm or less, the chemical conversion treatability of the steel sheet is significantly improved compared to Comparative Examples 51 to 56.

[0082] The results of Examples 1 to 50 show that the effect of improving the chemical conversion treatability of steel sheets containing Ni, Cu, and Sn simultaneously is achieved regardless of the composition of elements other than Ni, Cu, and Sn.

[0083] The reasons for the improved chemical conversion treatability of steel sheets in Examples 1 to 50 compared to Comparative Examples 51 to 56 are presumed to be as follows. First, when a concentrated phase of at least one of Ni, Cu, and Sn is finely dispersed on the steel sheet surface, the concentrated phase of Ni, Cu, and Sn functions as a cathode site during chemical conversion, promoting anodic dissolution (etching) of the base steel present around the concentrated phase, making it easier for the chemical conversion coating to adhere to the entire steel sheet surface. Furthermore, by reducing the coarse concentrated phase of at least one of Ni, Cu, and Sn on the steel sheet surface, poor etching in the coarse concentrated phase is presumed to be suppressed. Furthermore, by refining the Fe phase on the steel sheet surface, the grain boundaries of the grains that form the Fe phase become etching sites during chemical conversion treatment, and by refining the Fe phase, the concentrated phase is more easily dispersed finely, which is thought to promote etching of the base steel during chemical conversion treatment and make it easier for the chemical conversion treatment film to adhere.

Claims

1. A steel plate having a chemical composition, in mass%, of Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, the steel plate having a concentrated phase of at least one of Ni, Cu, and Sn, the number of the concentrated phases having an equivalent circle diameter of 0.5 μm to 5.0 μm on the surface of the steel plate being 2 or more per 200 μm length along the surface of the steel plate, the number of the concentrated phases having an equivalent circle diameter of more than 5.0 μm on the surface of the steel plate being 3 or less per 200 μm length along the surface of the steel plate, and the grain size of grains forming an Fe phase on the surface of the steel plate being 60 μm or less.

2. The steel plate according to claim 1, wherein the number of the concentrated phases having a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less on the surface of the steel plate is 4 or more per 200 μm length along the surface of the steel plate.

3. The steel plate according to claim 1 or 2, wherein the number of the concentrated phases having a circular equivalent diameter of more than 5.0 μm on the surface of the steel plate is one or less per 200 μm length along the surface of the steel plate.

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 30 μm or less.

5. 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 15 μm or less.

6. The steel sheet according to any one of claims 1 to 5, having a Vickers hardness of 200 Hv or more.

7. The steel sheet according to any one of claims 1 to 6, wherein the chemical composition comprises, in mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%.

8. A part comprising the steel sheet according to any one of claims 1 to 7.

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