Steel plates and parts

Abstract

Disclosed is a steel sheet containing Cu, Ni, and Sn and having improved chemical conversion treatability. The chemical composition of the steel sheet of the present disclosure 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 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 sheet is two 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 three or less per 200 μm length along the surface of the steel sheet. The grain size of grains forming an Fe phase on the surface of the steel sheet is 60 μm or less.

Description

[Technical Field]

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

[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 with excellent chemical conversion treatability by continuously annealing the cold-rolled steel sheet in equipment for both cold-rolled steel sheet and hot-dip galvanized steel sheet, 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 plating the steel sheet with a predetermined amount of iron or Ni.

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

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-190030 [Patent Document 2] Japanese Patent Publication No. 2020-084238 Summary of the Invention [Problem to be solved by the invention]

[0006] 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. [Means for solving the problem]

[0007] The present application discloses the following aspects as means for solving the above problems. (1) A steel plate, The chemical composition of the steel plate is, in mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%, Including, The steel sheet has a concentrated phase of at least one of Ni, Cu, and Sn, 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 sheet is 2 or more per 200 μm length along the surface of the steel sheet, The number of the concentrated 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, The grain size of the grains forming the Fe phase on the surface of the steel sheet is 60 μm or less. steel plate. (2) The steel plate according to (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 (1) or (2), wherein the number of the concentrated phases having a circle equivalent diameter of more than 5.0 μm on the surface of the steel plate is 1 or less per 200 μm length along the surface of the steel plate. (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), which has a Vickers hardness of 200 Hv or more. (7) The chemical composition is in mass%: Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%, The steel sheet according to any one of (1) to (6), comprising: (8) A part comprising the steel sheet according to any one of (1) to (7). [Effects of the Invention]

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

[0009] 1. The process leading up to the completion of the steel plate disclosed herein 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 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, a recycled resource, as the main raw material. Electric furnace steel, which uses scrap as the main raw material, 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. On the other hand, 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 further investigated the chemical conversion treatability of steel sheets containing Ni, Cu, and Sn, and as a result have obtained the following findings. (1) When a concentrated phase of at least one of Ni, Cu, and Sn is finely dispersed on the surface of a steel sheet, the chemical 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 treatability of the steel sheet is thought to be improved. (2) When concentrated phases of at least one of Ni, Cu, and Sn are coarsely dispersed on the surface of a steel sheet, the chemical conversion treatability of the steel sheet deteriorates. It is believed that during chemical conversion treatment, poor etching occurs in the coarse concentrated phases, resulting in poor chemical conversion. (3) Refining the Fe phase on the steel sheet surface improves the chemical treatability of the steel sheet. The grain boundaries of the Fe phase become etching sites during chemical treatment, and it is thought that the finer the Fe phase, the better the chemical treatability. Furthermore, refining the Fe phase makes it easier for the concentrated phase to be finely dispersed, which promotes etching of the base steel during chemical treatment and makes it easier for the chemical treatment film 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. By performing cold rolling under predetermined conditions after pickling, it is possible to reduce the surface irregularities of the steel sheet and impart strain to the surface layer of the steel sheet. By performing annealing under predetermined conditions on the cold-rolled steel sheet with the strain imparted to the surface layer, it is possible to generate a fine Fe phase in the surface layer and nucleate at least one concentrated phase of Ni, Cu, and Sn at the grain boundaries of the grains forming 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 sheet according to one embodiment has the following features. Specifically, 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 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 in the surface of the steel sheet is two 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 in the surface of the steel sheet is three or less per 200 μm length along the surface of the steel sheet. The grain size of grains forming an Fe phase in the surface of the steel sheet is 60 μm or less.

[0013] 2.1 Chemical composition The chemical composition of a steel sheet according to one embodiment includes, by mass, 0.010-1.000% Ni, 0.010-1.000% Cu, and 0.003-1.000% Sn. The chemical composition may also include, by mass, 0.040-1.000% Ni, 0.040-1.000% Cu, and 0.004-1.000% 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 according to 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 according to the present disclosure may include, in addition to Ni, Cu, and Sn, appropriate amounts of any alloying element commonly added in this technical field. The chemical composition of a steel sheet according to one embodiment will be described in detail below. However, the following description is intended to merely exemplify a preferred chemical composition for application to automotive steel sheets and the like. The chemical composition of the steel sheet of the present disclosure is not limited to the specific chemical composition described below. In this application, "%" for components means mass %. Furthermore, in this application, unless otherwise specified, "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: 0~0.0100%, Mo: 0 to 1.000%, Cr: 0 to 1.000%, V: 0~0.150%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050% Zr: 0 to 0.500%, Ca: 0 to 0.050%, REM: 0~0.010%, As: 0~0.100%, Ir: 0 to 1.000%, Zn: 0 to 1.000%, and Remainder: Fe and impurities Each element will be described in more detail below.

[0015] [C:0.001~0.500%] C is an element that inexpensively increases strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.001% or more. The C content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.040% or more, 0.070% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, excessive C content may result in reduced elongation. For this reason, the C content is preferably 0.500% or less. The C content may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.

[0016] [Si: 0-3.00%] 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: 0.10~3.00%] 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 decrease the 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 improves the quality of the steel. To fully achieve this effect, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0019] [Ni: 0.010~1.000%] [Cu: 0.010~1.000%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. To fully achieve these effects, the 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 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 the formation of oxides on the steel sheet surface, 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.

[0020] [Sn: 0.003~1.000%] 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, particularly Mn- and / or Si-based surface oxides and iron oxides, on the steel sheet surface. 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. The lower the P content, the better, and ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, or may be 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0022] [S:0.100% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, reducing the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content can result in a significant increase in costs. For this reason, the S content may be 0.0001% or more, or may be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content can cause cracks to occur originating from nonmetallic inclusions during cold forming. Therefore, the S content is preferably 0.100% or less. The S content may be 0.050% or less, 0.020% or less, or 0.010% or less.

[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, ideally 0%. However, excessive reduction in N content may result in a significant increase in manufacturing costs. For this reason, 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. For this reason, the O content may be 0.0001% or more, or may be 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 required.

[0026] [Ti: 0~0.150%] [Nb: 0~0.150%] [V:0~0.150%] Ti, Nb, and V form carbonitrides in steel, improving the strength of the steel sheet through precipitation strengthening. The Ti, Nb, and V contents may be 0%, but to achieve 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. However, even if these elements are contained in excess, the effect saturates, and excessive inclusion of these elements in 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: 0~0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0028] [Mo: 0-1.000%] [Cr:0~1.000%] [W:0~1.000%] Mo, Cr, and W are elements that improve the hardenability of steel and contribute to improving its strength. The Mo, Cr, and W contents may be 0%, but to 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. However, even if these elements are contained in excess, the effects saturate, and adding more than necessary to 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 nonmetallic inclusions. The contents of Hf, Mg, Zr, Ca, and REM may be 0%, but to obtain 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. However, 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 contents of Hf, Mg, and Ca 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 general term for 17 elements: scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and the lanthanides lanthanum (La), atomic number 57, through lutetium (Lu), atomic number 71. "REM content" is the total content of these elements.

[0030] [As:0~0.100%] 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 an excessive amount of As is contained, the effect saturates, and adding more As than necessary to the steel sheet increases the manufacturing cost. Therefore, the As content is preferably 0.100% or less. The As content may be 0.080% 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: 0~1.000%] 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 added, the effect saturates, and adding more Ir than necessary to the steel 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, 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 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 steel sheets can be measured by common analytical methods. For example, the chemical composition of steel sheets can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) of chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece is obtained from the 1 / 4 position of the steel sheet thickness, and the composition can be determined by measuring it using a measuring device such as Shimadzu's ICPS-8100 under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using a combustion-infrared absorption method, and N can be measured using an inert gas fusion-thermal conductivity method.

[0035] 2.2 Concentrated phase of at least one of Ni, Cu and Sn A steel sheet according to one embodiment has a phase enriched with at least one of Ni, Cu, and Sn. The term "phase enriched with at least one of Ni, Cu, and Sn" refers to a phase in which, in an EPMA analysis described below, the total content of Ni, Cu, and Sn is 0.5% by mass or more and is 1.1 times or more the total content of Ni, Cu, and Sn in the bulk. 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. Specifically, a 35 mm square test piece is obtained from a quarter-thickness position of the steel sheet, and the total content of Ni, Cu, and Sn in the bulk is determined by measuring the test piece using a measuring instrument such as Shimadzu's ICPS-8100 under conditions based on a previously prepared calibration curve. The total content of Ni, Cu, and Sn in the enriched phase may be, for example, 0.5% by mass to 5.0% by mass, or 1.0% by mass to 2.5% by mass.

[0036] 2.2.1 Fine thickened phase It is important that the number of enriched phases having a circular equivalent diameter of 0.5 μm to 5.0 μm on the surface of a steel sheet according to one embodiment is two or more per 200 μm along the surface of the steel sheet. According to the inventor's new findings, even if fine enriched phases having a circular equivalent diameter of 2.0 μm to 5.0 μm are present on the surface of the steel sheet, poor chemical conversion is unlikely to occur. In fact, dispersing a large amount of fine enriched phases having a circular equivalent diameter of 2.0 μm to 5.0 μm on the surface of the steel sheet improves the chemical conversion treatability of the steel sheet. 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 around the enriched phases and facilitating the adhesion of the chemical conversion coating. Finely dispersing such enriched phases on the steel sheet surface can improve the chemical conversion treatability of the entire surface of the steel sheet. In one embodiment, the number of enriched 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 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 enriched 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 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 thickened phase 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 makes the enriched phases prone to poor chemical conversion 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 a 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 a 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 a 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 plate is 0 or more, and may be more than 0, per 200 μm length along the surface of the steel plate.

[0038] 2.2.3 Method for measuring the size and number of thickened phases The size and number of the enriched phases on the surface of the steel sheet can be identified by performing local elemental analysis of the cross section of the surface layer of the steel sheet using an electron probe microanalyzer (EPMA). For observation using 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 using 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 × 10 -7A, irradiation time: 50 ms. EPMA analysis is performed on Fe, Si, Mn, Ni, Cu, and Sn. 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. Next, a region is identified from the obtained mapping image, where the total content of Ni, Cu, and Sn is 0.5 mass% or more and 1.1 times the total content of Ni, Cu, and Sn in the bulk. For example, a color bar is displayed in the Ni concentration range of 0.5 to 5.0% in the mapping image to identify a region where the Ni concentration is 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 to Cu and Sn below). Similarly, a color bar is displayed in the Cu concentration range of 0.5 to 5.0% to identify a region where the Cu concentration is 0.5% or more. Similarly, the color bar is displayed with a Sn concentration range of 0.5 to 5.0%, thereby identifying the region where the Sn concentration is 0.5% or more. In the region where two or more of Ni, Cu, and Sn coexist, the color bar is displayed so 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 is 1.1 times the total content of Ni, Cu, and Sn in the bulk). For example, for regions where Ni and Cu coexist, a color bar can be 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).This allows for identification of 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 in a similar manner.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 of ​​each enriched phase is measured using the image processing software "ImageJ." Specifically, the 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, select one of the enriched phases whose area you want to measure on the Clipboard screen, use "Measure" under "Analyze," read the "Area" and "Area fraction" in the "Results" section, and multiply the "Area" and "Area fraction" values ​​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 the areas considered to be enriched phases on the steel sheet surface, the number of enriched phases with a circular equivalent diameter of 0.5 μm to 5.0 μm per 200 μm along the steel sheet surface is calculated, and the number of enriched phases with a circular equivalent diameter of more than 5.0 μm per 200 μm along the steel sheet surface 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 enriched 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" and "the number of enriched 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 grain size of 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 grain size of 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 grain size of the Fe phase on the surface of the steel sheet is not particularly limited, and may be greater 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 identified.

[0041] (1) The "ferrite phase" is distinguished from other phases as follows: After mechanically polishing the cross section of the steel plate 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 steel sheet level, 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 of the cross section revealing the grain boundaries is obtained 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 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 considered to be the "grain size of grains forming the Fe phase." The observation field is 200 μm×200 μm at a magnification of 100 times, and the line length is 200 μm.

[0044] 2.4 Steel Plate Thickness The thickness of the steel sheet of the present disclosure is not particularly limited. The steel sheet 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 according to the present disclosure are not particularly limited. The 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 sheets is measured in accordance with JIS Z 2244-1:2024 as follows. First, a test piece is cut from any position on the steel sheet, excluding the edges, so that a cross section perpendicular to the surface (thickness cross section) can be observed. The thickness cross section of the test piece is polished using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using a diluted solution such as alcohol or a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in pure water. This thickness cross section serves as the measurement surface. Next, the Vickers hardness is measured using a micro Vickers hardness tester at a load of 1 kgf at intervals at least three times the indentation width. Specifically, a total of 20 measurements are taken at random positions along 1 / 4 of the steel sheet's thickness, and the arithmetic average of these measurements is determined as the Vickers hardness of the steel sheet.

[0047] 3. Steel plate manufacturing method Next, a preferred method for manufacturing 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] The steel plate according to one embodiment may be, for example, A casting process in which molten steel with adjusted chemical composition is cast to obtain steel billets. a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet; a pickling step of pickling the hot rolled steel sheet; A cold rolling step of cold-rolling the hot-rolled steel sheet after pickling to obtain a cold-rolled steel sheet; an annealing step of annealing the cold-rolled steel sheet; Each step 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, electric furnace, or the like, various secondary smelting processes are performed, and then the molten steel is cast by a method such as ordinary continuous casting or casting by an ingot method 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 process In the pickling process, the hot-rolled steel sheet obtained in the hot rolling process 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] This manufacturing method has a unique feature 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 the dissolution of the grain boundaries of the Fe phase grains at a concentration of 10 ppm to 1500 ppm. The accelerator promotes the dissolution of the grain boundaries of the Fe phase grains during pickling, thereby creating large irregularities on the surface of the hot-rolled steel sheet, which allows for appropriate strain to be imparted to the steel sheet surface during 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 during cold rolling. As a result, the final steel sheet will not have the appropriate enriched phase or Fe phase. The accelerator contained in the pickling solution may be any accelerator that promotes the dissolution of the grain boundaries of the Fe phase grains. 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] As described above, in the pickling process, 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 process to cold rolling process 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 the hydrogen remains in the steel even during the subsequent annealing. This is thought to facilitate the suppression of coarsening of the enriched phase 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 step, 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 a surface roughness by pickling is cold-rolled to reduce the surface roughness 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, and preferably 2 μm or less. The reduction ratio in 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 in cold rolling may be, for example, 20% or more and 80% or less. After the cold rolling step, the steel sheet may be cooled to room temperature, for example, by air cooling.

[0056] 3.6 Annealing process In the annealing step, 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, and the concentrated phase can be finely dispersed in the surface layer of the steel sheet. The annealing step 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 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., 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-mentioned steel sheet. The steel sheet according to this 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, other structural parts and reinforcing parts that require strength, and exterior panel parts such as roofs, hoods, fenders, and doors that require high designability. At least a portion of these parts may include the steel sheet according to this 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 automobile parts for various measurements and analyses, the following locations and parts (1) to (4) shall be avoided. (1) Welded section: Within 20 mm from the toe of a spot weld, and within 20 mm from the toe of the bead of an arc / laser weld (2) Processed area: A processed area with a curvature radius of less than 15 mm, and an area within 5 mm of the processed area (3) Edge: The edge within 5 mm from the cut edge of the part. (4) Red rust: Areas within 5 mm of areas where red rust is visible

[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 film removal process The paint film was removed from samples cut from automobile bodies under the following conditions to expose the steel sheet. A paint remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) was applied to the surface at room temperature and allowed to stand for approximately 5 minutes. The paint film was then removed by rubbing with a hard sponge or similar (e.g., Kanefiel, manufactured by Aion Co., Ltd.). The sample was then rinsed and dried. The remaining paint film was then confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after rinsing and drying. In the elemental distribution image obtained by EPMA, areas with a carbon concentration of 10% or more by mass were identified. If the area ratio of these areas was 5% or more, the paint film was deemed to have been insufficiently removed. To measure the area ratio of areas with a carbon concentration of 10% or more by mass, an elemental distribution image of carbon was first obtained using an EPMA with a carbon concentration range of 10–30%. The area ratio was then measured by image processing of the elemental distribution image. The image processing software "ImageJ" was used for image processing. After loading the C element distribution image into ImageJ, "Make Binary" under "Process" and "Binary" 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" under "Results," which was used as the area fraction of areas with a C concentration of 10% or more. If the coating was not sufficiently removed, the coating 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, with the paint 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. In the elemental distribution image obtained by EPMA, areas with a P concentration of 5% or more by mass are identified, and if the area ratio of these areas is 5% or more, it is determined that the paint has not been sufficiently removed. To measure the area ratio of areas with a P concentration of 5% or more by mass, first obtain an elemental distribution image of P using an EPMA with a P concentration range of 5-10%. The area ratio is then measured by image processing of the elemental distribution image. The image processing software "ImageJ" was used for image processing. After loading the P element distribution image into ImageJ, "Make Binary" under "Process" was used to binarize the image 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.Other 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. [Example]

[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. Steel plate manufacturing 1.1 Casting process The molten steel was continuously cast to obtain steel billets having the chemical compositions shown in Table 1 below.

[0066] 1.2 Hot rolling process The steel slab was cooled once, then reheated to 1200°C and hot rolled. The hot rolling was performed by rough rolling and finish rolling, with the finish rolling finishing temperature being 900 to 1050°C and the finish rolling reduction being 30%.

[0067] 1.3 Pickling The hot-rolled steel sheet was coiled at 100 to 300°C and then subjected to pickling. The pickling solution used 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 process After pickling, the steel was cold-rolled at a 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 shown in Table 2.

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

[0070] [Table 1]

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

[0072] 2.2 Size of the Fe phase on the steel sheet surface The cross section of the steel sheet surface was observed using an SEM to measure the particle size of the particles forming the Fe phase on the steel sheet surface. Details of the SEM measurement method 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 Phosphate treatability 2.4.1 Chemical conversion treatment procedure A 50 mm square sample was cut out 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: Immerse in degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes (2) Washing with water (3) Liquid surface adjustment: Immerse in surface adjuster (Preparen Z) at room temperature for 30 seconds. (4) Chemical conversion treatment: Immersion in zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes (5) Washing with water (6) Drying

[0075] 2.4.2 Evaluation Method The bead portion of the sample that had undergone 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 known as "invisible," was calculated by binarization using the image analysis software "ImageJ." Depending on the area ratio of invisible, the chemical conversion treatability of the processed portion was evaluated using the following evaluation criteria. The invisible 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 -7 In the mapping image taken at 1000x magnification under the conditions of A, the area was defined as the area with an Fe concentration of 70% or more. The area ratio was determined as the average value of five randomly selected fields of view. Rating AAA: Less than 10% of the surface area is clear Rating AA: Clear area ratio 10% or more but less than 15% Rating A: Clear area ratio 15% or more but less than 20% Rating B: 20% or more of the surface area

[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] [Table 2]

[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 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 on the surface of the steel sheet having a circular equivalent diameter of 0.5 μm or more and 5.0 μm or less is 2 or more per 200 μm length along the surface of the steel sheet, the number of enriched phases on the surface of the steel sheet having a circular equivalent diameter of more than 5.0 μm 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 concentrated phases of at least one of Ni, Cu, and Sn are finely dispersed on the steel sheet surface, the concentrated phases of Ni, Cu, and Sn function as cathode sites during chemical conversion, promoting anodic dissolution (etching) of the base steel present around the concentrated phases, making it easier for the chemical conversion coating to adhere to the entire steel sheet surface. Furthermore, it is presumed that reducing the number of coarse concentrated phases of at least one of Ni, Cu, and Sn on the steel sheet surface suppresses poor etching in the coarse concentrated phases. 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. Furthermore, by refining the Fe phase, the concentrated phase described above is more easily dispersed, which promotes etching of the base steel during chemical conversion treatment and makes it easier for the chemical conversion treatment film to adhere.

Claims

1. A steel plate, The chemical composition of the steel plate is, in mass%, C: 0.001 to 0.500%, Si: 0-3.00%, Mn: 0.10-3.00%, Al: 0.001-2.000%, Ni: 0.010 to 1.000%, Cu: 0.010-1.000%, Sn: 0.003-1.000%, P: 0.100% or less, S: 0.100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0-1.000%, Cr: 0-1.000%, V: 0 to 0.150%, W: 0-1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.500%, Ca: 0-0.050%, REM: 0-0.010%, As: 0 to 0.100%, Ir: 0-1.000%, Zn: 0 to 1.000%, and Remainder: Fe and impurities It consists of The steel plate has a concentrated phase of at least one of Ni, Cu, and Sn, 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 sheet is 2 or more per 200 μm length along the surface of the steel sheet, The number of the concentrated 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, The grain size of the grains forming the Fe phase on the surface of the steel plate is 60 μm or less. steel plate.

2. 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 sheet is 4 or more per 200 μm length along the surface of the steel sheet, The steel sheet according to claim 1.

3. The number of the concentrated phases having a circle equivalent 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, The steel sheet according to claim 1.

4. The grain size of the grains forming the Fe phase on the surface of the steel plate is 30 μm or less. The steel sheet according to any one of claims 1 to 3.

5. The grain size of the grains forming the Fe phase on the surface of the steel plate is 15 μm or less. The steel sheet according to any one of claims 1 to 3.

6. Vickers hardness of 200 Hv or more The steel sheet according to any one of claims 1 to 3.

7. The chemical composition is, in mass %, Ni: 0.040-1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004-1.000%, Including, The steel sheet according to any one of claims 1 to 3.

8. The steel sheet according to any one of claims 1 to 3 is included. parts.

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