WO2025169776A1 - Plated steel sheet and component including same - Google Patents

Abstract

Provided are a plated steel sheet and a component including the same, said plated steel sheet comprising a parent material steel sheet and a plating layer that is disposed on the surface of the parent material steel sheet, and being characterized in that the parent material steel sheet has a chemical composition including, by mass %, Ni: 0.010-1.000%, Cu: 0.010-1.000%, and Sn: 0.003-1.000%, and in an element distribution image obtained by measuring a cross-section of the plated steel sheet by EPMA, the thickness of the plating layer in which the total concentration of at least one of Zn and Mn is 10 mass% or more and the concentration of Fe is 10 mass% or more is 5 μm or more.

Description

Plated steel sheets and parts containing them

The present invention relates to plated steel sheets and parts containing the same.

It is known that an effective way to improve the corrosion resistance of steel sheets or plated steel sheets is to increase the chemical conversion treatability of the steel sheets or plated steel sheets and to form a uniform chemical conversion coating on the steel sheets.

In this regard, Patent Document 1 describes a method for producing a high-strength cold-rolled steel sheet, characterized in that when a high-strength cold-rolled steel sheet is continuously annealed in a continuous annealing furnace or in a cold-rolled steel sheet/hot-dip galvanized steel sheet dual-purpose facility having a continuous annealing furnace, the cooling method in a cooling zone including part or all of the steel sheet temperature range of 600 to 250°C following heating for recrystallization is one or more of gas cooling, diffusion cooling, and cooling pipe cooling, the steel sheet surface is exposed to an atmosphere in which iron oxidizes within the above-mentioned steel sheet temperature range, pickled at the outlet of the annealing furnace, and then iron or Ni plating is applied in an amount of 1 to 50 mg/ ^m2 . Furthermore, Patent Document 1 teaches that while oxidation of a steel sheet is normally prevented by an extremely low concentration of oxygen and/or an inert atmospheric gas with an extremely low dew point around the steel sheet, the steel sheet is instead actively exposed to an oxidizing atmosphere to oxidize not only Si and Mn but also the iron in the steel sheet, and by pickling after the steel sheet leaves the annealing furnace, the oxide films of Si, Mn, etc. are removed together with the oxide film on the iron in the steel sheet, thereby obtaining a high-strength cold-rolled steel sheet that is free from "hollow-out" and has good chemical conversion treatability even if the contents of Si, Mn, etc. are high.

Patent Document 2 describes an automotive steel sheet containing 0.10% by mass or more and 0.50% by mass or less of copper (Cu), with the number of residual scales on the surface being 160,000 particles/mm2 or ^less , and the maximum particle size of copper compound particles exposed on the surface being 2 μm or less. Patent Document 2 also teaches that the above configuration makes it possible to provide a steel sheet with excellent chemical conversion treatability, since 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 residual scale is kept to a predetermined amount or less.

Japanese Patent Application Laid-Open No. 2008-190030 Japanese Patent Application Laid-Open No. 2020-084238

Patent Document 2 teaches that elements such as nickel (Ni) and tin (Sn) in addition to copper (Cu) reduce the mechanical properties required of automotive steel sheets, such as strength and formability, as well as chemical stability such as corrosion resistance, and that copper compounds present on the surface of the steel sheet in particular reduce the chemical conversion treatability required to improve corrosion resistance. Furthermore, automotive steel sheets are generally often processed into the desired part shape by press forming. However, for example, if the plating layer peels off when the mold and the surface of the plated steel sheet slide against each other during press forming, the chemical conversion treatability and therefore the corrosion resistance of the processed area may be reduced.

The present invention therefore aims to provide a plated steel sheet containing Ni, Cu, and Sn that exhibits improved chemical conversion treatability in processed areas, and a component that includes the same.

In order to achieve the above objective, the inventors conducted research, focusing particularly on the plating layer. As a result, the inventors discovered that by coating the surface of a base steel sheet containing Ni, Cu, and Sn with a predetermined thickness of an alloy plating layer of Fe and at least one of Zn and Mn, the chemical conversion treatability of the processed area can be significantly improved, leading to the completion of the present invention.

The present invention, which has achieved the above object, is as follows.
(1) A plated steel sheet comprising a base steel sheet and a plating layer disposed on a surface of the base steel sheet,
The base steel plate is, in mass%,
Ni: 0.010 to 1.000%,
A chemical composition including Cu: 0.010 to 1.000% and Sn: 0.003 to 1.000%;
In an element distribution image obtained by measuring a cross section of the plated steel sheet with an EPMA,
A plated steel sheet, characterized in that the plating layer has a total concentration of at least one of Zn and Mn of 10 mass % or more, an Fe concentration of 10 mass % or more, and a thickness of 5 μm or more.
(2) The plated steel sheet according to (1) above, characterized in that the plating layer has a total concentration of at least one of Zn and Mn of 10 mass % or more and an Fe concentration of 10 mass % or more, and has a thickness of 8 μm or more.
(3) The plated steel sheet according to (2) above, characterized in that the plating layer has a total concentration of at least one of Zn and Mn of 10 mass % or more and an Fe concentration of 10 mass % or more, and has a thickness of 10 μm or more.
(4) The chemical composition is, in mass%,
Ni: 0.040-1.000%,
Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%
The plated steel sheet according to any one of (1) to (3) above, comprising:
(5) The plated steel sheet according to any one of (1) to (4) above, characterized in that it has a Vickers hardness of 200 Hv or more.
(6) A part comprising the plated steel sheet according to any one of (1) to (5) above.

The present invention provides a plated steel sheet containing Ni, Cu, and Sn that exhibits improved chemical conversion treatability in processed areas, as well as a part containing the same.

<Plated steel sheet>
A plated steel sheet according to an embodiment of the present invention is a plated steel sheet including a base steel sheet and a plating layer disposed on a surface of the base steel sheet,
The base steel plate is, in mass%,
Ni: 0.010 to 1.000%,
A chemical composition including Cu: 0.010 to 1.000% and Sn: 0.003 to 1.000%;
In an element distribution image obtained by measuring a cross section of the plated steel sheet with an EPMA,
The plating layer has a total concentration of at least one of Zn and Mn of 10 mass % or more and an Fe concentration of 10 mass % or more, and is characterized by having a thickness of 5 μm or more.

In general, when chemical conversion treatability decreases, areas where the chemical conversion coating is not formed, known as "skid zones," may appear, resulting in decreased corrosion resistance. For example, when elements such as Ni, Cu, and Sn are present in a solid solution in steel sheet, the potential of the steel sheet becomes more noble than when these elements are not present in a solid solution, which can reduce the etching ability of Fe during chemical conversion treatment. In this case, the chemical conversion treatability of the steel sheet decreases. This decrease in chemical conversion treatability is particularly problematic when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. On the other hand, as mentioned above, automotive steel sheets are generally often processed into the desired part shape by press forming. In the case of plated steel sheets, for example, the plating layer may peel off when the surface of the plated steel sheet slides against the mold during press forming. If the plating layer peels off in the processed area due to press forming or other processes, exposing the base steel sheet, this can cause a problem of reduced chemical conversion treatability in the processed area where the base steel sheet is exposed, especially if the base steel sheet simultaneously contains the three elements Ni, Cu, and Sn.

Furthermore, there are two commonly known methods for producing steel plate: one method involves producing molten iron in a blast furnace using iron ore, a natural resource, as the main raw material, and then refining it in a converter or other furnace to produce molten steel; and the other method involves producing molten steel in an electric furnace using scrap material, a recycled resource, as the main raw material. Blast furnace steel can also contain elements such as Ni, Cu, and Sn as additives, so if these elements are present, the above-mentioned issues must be addressed appropriately. On the other hand, electric furnace steel, because it uses scrap material as its main raw material as mentioned above, contains relatively large amounts of elements derived from the scrap, such as Ni, Cu, and Sn (so-called tramp elements), and therefore the above-mentioned issues are particularly pronounced.

Therefore, the inventors conducted research, focusing particularly on the plating layer, to provide a plated steel sheet that can exhibit excellent chemical conversion treatability in processed areas, even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, the inventors discovered that it is effective to coat the surface of a base steel sheet containing Ni, Cu, and Sn with an alloy plating layer of Fe and at least one of Zn and Mn to a predetermined thickness. More specifically, the inventors discovered that, when the cross section of the plated steel sheet is measured using an electron probe microanalyzer (EPMA), by controlling the thickness of the plating layer to 5 μm or more, where the concentration of at least one of Zn and Mn is 10 mass% or more in total and the Fe concentration is 10 mass% or more, the adhesion between the plating layer and the base steel sheet is improved, and peeling of the plating layer can be sufficiently suppressed even during processing, particularly processing that involves sliding such as press working, thereby significantly improving the chemical conversion treatability of processed areas.

More specifically, Zn and Mn function as anodes during chemical conversion treatment and improve the chemical treatability of steel sheets by dissolving themselves. Therefore, even if the base steel sheet contains Ni, Cu, and Sn, if the manufacturing method can be appropriately controlled to properly coat the surface of the base steel sheet with a plating layer containing at least one of Zn and Mn, it is possible to significantly improve the chemical treatability of the plated steel sheet. Therefore, the inventors first discovered that by appropriately controlling the manufacturing method, as will be described in detail later, it is possible to suppress the formation of oxides, particularly Mn- and/or Si-based surface oxides, on the surface of a base steel sheet containing Ni, Cu, and Sn, thereby coating the surface of the base steel sheet with a plating layer containing at least one of Zn and Mn. In addition, the inventors discovered that by alloying at least one of Zn and Mn in the coating layer with Fe in the base steel sheet through a subsequent annealing step, and controlling the thickness of the coating layer to 5 μm or more, where the total concentration of at least one of Zn and Mn is 10 mass% or more and the Fe concentration is 10 mass% or more when the cross section of the coating steel sheet is measured by EPMA, the chemical conversion treatability of the coating steel sheet can be significantly improved. Therefore, in the coating steel sheet according to an embodiment of the present invention, even though the base steel sheet simultaneously contains the three elements Ni, Cu, and Sn, the anodic dissolution (etching) of Zn and/or Mn in the coating layer can be promoted during chemical conversion treatment, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet, resulting in significantly improved corrosion resistance.

In addition, by forming an alloy plating layer of at least one of Zn and Mn with Fe, it is possible to improve the adhesion between the alloy plating layer and the base steel sheet compared to when simply plating at least one of Zn and Mn. Therefore, peeling of the plating layer can be sufficiently suppressed even during processes that involve sliding, such as press working, and this makes it possible to significantly improve the chemical treatability of the processed area.

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

[Plating layer]
According to an embodiment of the present invention, a plating layer is disposed on the surface of a base steel sheet, for example, on at least one, preferably both, surfaces of the base steel sheet. The plating layer may contain at least one of Zn and Mn and Fe, and may have a variety of plating compositions as long as it satisfies the requirements for the concentrations of Zn, Mn, and Fe and the thickness of the plating layer, which will be described in detail later. Therefore, the plating layer may contain other elements in addition to Zn, Mn, and Fe, such as Al, Ni, Cu, Sn, and Mg. More specifically, the plating layer may contain or consist of Zn—Cu—Fe and/or Zn—Ni—Fe, etc. Naturally, the plating layer may consist essentially of at least one of Zn and Mn and Fe, or may consist of at least one of Zn and Mn and Fe, or may consist of at least one of Zn and Mn and Fe. The plating weight is not particularly limited, and may be appropriately selected within a range that satisfies the requirements for the concentrations of Zn, Mn, and Fe and the thickness of the plating layer, which will be described in detail later.

[Thickness of plating layer having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more: 5 μm or more]
In an embodiment of the present invention, in an element distribution image obtained by measuring the cross section of a plated steel sheet using EPMA, the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more is controlled to 5 μm or more. As described above, Zn and Mn function as an anode during chemical conversion treatment and improve the chemical treatability of the steel sheet by dissolving themselves. In an embodiment of the present invention, by controlling the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more to 5 μm or more, it becomes possible for the Zn and Mn in the plating layer to effectively function as an anode during chemical conversion treatment. In other words, the anodic dissolution (etching) of Zn and/or Mn in the plating layer is promoted during chemical conversion treatment, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet, resulting in significantly improved corrosion resistance. From the viewpoint of further improving chemical conversion treatability and thus corrosion resistance, the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more is preferably as thick as possible, for example, 6 μm or more or 8 μm or more, more preferably 10 μm or more, and most preferably 12 μm or more. On the other hand, the upper limit is not particularly limited, and the thickness of the plating layer may be, for example, 30 μm or less, 25 μm or less, 20 μm or less, or 15 μm or less. In an embodiment of the present invention, as long as the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more satisfies the requirement of 5 μm or more, the plating layer may include regions that do not satisfy this requirement, such as regions where the total concentration of at least one of Zn and Mn is less than 10 mass% or regions where the total concentration of at least one of Zn and Mn is 10 mass% or more and the Fe concentration is less than 10 mass%.

[Measurement of thickness of plating layer having a total concentration of at least one of Zn and Mn of 10% by mass or more and an Fe concentration of 10% by mass or more]
The thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10% by mass or more and an Fe concentration of 10% by mass or more is measured using an EPMA as follows: First, point analysis is performed on a cross section of a plated steel sheet from the surface in the depth direction to a depth of 12 μm, with point analysis performed every 1 μm. Similar measurements are performed at five locations in the surface direction of the plated steel sheet, and the average total concentration of at least one of Zn and Mn and the average Fe concentration at each depth are calculated using the five measurements at each depth. Next, it is confirmed whether the average values at each depth from the surface in the depth direction satisfy the requirement of "a total concentration of at least one of Zn and Mn of 10% by mass or more and an Fe concentration of 10% by mass or more." The value obtained by subtracting 2 μm from the depth at which the requirement is no longer satisfied for two consecutive measurements is determined as the "thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10% by mass or more and an Fe concentration of 10% by mass or more."

[Base material steel plate]
In an embodiment of the present invention, the base steel sheet has a chemical composition containing, in mass %, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%. As described above, an object of the present invention is to provide a plated steel sheet containing Ni, Cu, and Sn that can exhibit improved chemical conversion treatability in processed portions, and this object is achieved by controlling the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass % or more and an Fe concentration of 10 mass % or more to 5 μm or more when a cross section of the plated steel sheet is measured by EPMA. Therefore, the chemical composition of the base steel sheet is not particularly limited except that it contains, in mass %, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, and 0.003 to 1.000% Sn, and therefore it is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the object of the present invention. The chemical composition of the base steel sheet may contain, in addition to Ni, Cu, and Sn, appropriate amounts of any alloying elements commonly added in the technical field of the present invention. Hereinafter, the chemical composition of the base steel sheet used in the plated steel sheet according to the embodiment of the present invention will be described in detail, but these descriptions are intended to merely exemplify preferred chemical compositions of base steel sheets for use in automotive steel sheets and the like, and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

In an embodiment of the present invention, for example, the base steel plate contains, 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: 0.0100% 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 0.050%,
Ca: 0-0.010%,
REM: 0-0.010%,
As: 0 to 0.010%,
It is preferable that the chemical composition be Ir: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[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 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.

[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.

[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.

[Al: 0.001-2.000%]
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. Therefore, 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.

[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 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, which may impair plating adhesion in the plating process. Therefore, the Ni and Cu contents are preferably 1.000% or less, and may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.

[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, particularly Mn- and/or Si-based surface oxides and iron oxides, on the steel sheet surface, which may impair plating adhesion in the plating process. Therefore, the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[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. Therefore, 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.

[S: 0.100% or less]
S is an element that generates nonmetallic 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 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.

[N: 0.0100% 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 it 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 0.0100% or less. The N content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

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

[Ti: 0 to 0.150%]
[Nb: 0 to 0.150%]
[V: 0-0.150%]
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 such effects, 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 containing more than necessary 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.

[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 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.

[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. On the other hand, even if these elements are contained in excess, the effects are saturated, and containing more than necessary in 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.

[Hf: 0 to 0.050%]
[Mg: 0 to 0.050%]
[Zr: 0 to 0.050%]
[Ca: 0-0.010%]
[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 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 are saturated, and adding more than necessary to the steel sheet increases production costs. Therefore, the Hf, Mg, and Zr 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 Ca and REM contents are preferably 0.010% or less, and may be 0.005% or less, or 0.003% or less.

[As: 0 to 0.010%]
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 containing more As than necessary in the steel sheet increases the manufacturing cost. Therefore, the As content is preferably 0.010% or less. The As content may be 0.008% or less or 0.005% or less.

[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 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.

In the base steel plate, the remainder, other than the above elements, consists of Fe and impurities. Impurities in the base steel plate are components that are mixed in during the industrial production of the base steel plate due to various factors in the manufacturing process, including raw materials such as ore and scrap.

The chemical composition of the base steel plate can be measured using standard analytical methods. For example, the chemical composition of the base steel plate can be determined by first removing the plating layer by mechanical grinding, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014. Specifically, the composition can be determined by obtaining a 35 mm square test piece from approximately half the plate thickness of the base steel plate and measuring it using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-created calibration curve. C and S, which cannot be measured using ICP-AES, can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

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

As described above, the steel sheet according to the embodiment of the present invention can achieve superior chemical conversion treatability in processed areas, and therefore superior corrosion resistance in processed areas, compared to conventional plated steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the plated steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields that require superior chemical conversion treatability and/or corrosion resistance in processed areas, and is particularly useful for use in parts in the automotive field. In a preferred embodiment, an automobile part is provided that includes the plated steel sheet according to the embodiment of the present invention. 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. It is sufficient for at least a portion of these parts to include the plated steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the characteristics of the plated steel sheet described above. In areas of the steel sheet that do not come into direct contact with a mold during forming, such as press forming, or that come into direct contact with a mold but are relatively lightly processed, the characteristics of the plated steel sheet do not change significantly before and after forming.

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

[Vickers hardness measurement]
Vickers hardness is determined as follows. First, a test piece is cut out from any position of the plated steel sheet, excluding the edge, 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 mirror-finished 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, 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 at intervals of at least three times the indentation. Specifically, a total of 20 points are measured randomly near the half-thickness position of the plated steel sheet, and the arithmetic average of these measurements is determined as the Vickers hardness of the plated steel sheet.

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

Plated steel sheets according to embodiments of the present invention can be manufactured by, for example, performing a casting process in which molten steel with an adjusted chemical composition is cast to form a steel billet; a hot rolling process in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet; a coiling process in which the hot-rolled steel sheet is coiled and then subjected to a primary pickling; a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; a secondary pickling process in which the cold-rolled steel sheet is subjected to a secondary pickling; a plating process in which the secondary pickled cold-rolled steel sheet is plated; and an annealing process in which the resulting plated steel sheet is annealed. While the following specifically describes the manufacture of plated steel sheets obtained by plating cold-rolled steel sheets, plated steel sheets according to embodiments of the present invention encompass not only plated steel sheets obtained by plating cold-rolled steel sheets, but also plated steel sheets obtained by plating hot-rolled steel sheets. Therefore, when manufacturing plated steel sheets obtained by plating hot-rolled steel sheets, for example, the secondary pickling process may be performed after the coiling process without performing the cold-rolling process described below. Each process is described in detail below.

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

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

[Winding process]
The hot-rolled steel sheet obtained in the hot rolling process is coiled in the next coiling process and then subjected to primary pickling. In this manufacturing method, the hot-rolled steel sheet is coiled at a coiling temperature of 520°C or higher. By controlling the coiling temperature to 520°C or higher, an outer oxide layer is formed on the outer (surface) side of the steel sheet, and an inner oxide layer is also formed in the inner (surface layer) side of the steel sheet. This inner oxide layer is mainly composed of Mn- and/or Si-based oxides. Therefore, an Mn—Si-depleted layer is formed directly below the inner oxide layer formed on the surface layer of the steel sheet due to the consumption of Mn and/or Si in the steel caused by the formation of the inner oxide layer. In particular, by controlling the coiling temperature to 520°C or higher, the thickness of the Mn—Si-depleted layer can be controlled to 0.3 μm or higher. Since the outer oxide layer and inner oxide layer are removed by primary pickling after coiling, an Mn—Si-depleted layer having a thickness of 0.3 μm or higher remains on the surface of the hot-rolled steel sheet after primary pickling. By forming the surface of the hot-rolled steel sheet with an Mn-Si depleted layer having a thickness of 0.3 μm or more, it is possible to sufficiently suppress the formation of Mn- and/or Si-based surface oxides on the steel sheet surface during the subsequent annealing process due to the depletion of Mn and Si on the steel sheet surface. Therefore, it is possible to appropriately maintain the plating applied in the plating process before the annealing process, and it is possible to form a plating layer of a desired thickness in the finally obtained plated steel sheet, in which the total concentration of at least one of Zn and Mn is 10 mass % or more and the Fe concentration is 10 mass % or more.

The thickness of the Mn-Si depletion layer is determined as follows. First, using a high-frequency glow discharge optical emission spectrometer (GDS), the surface of the steel sheet after primary pickling is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The surface of the steel sheet is then sputtered and analyzed in the depth direction. The elements contained in the material are then identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. Depth direction data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted to mass % by creating a calibration curve. When the steel sheet is subjected to GDS measurement after the primary pickling in this manner, the region in the depth direction where the sum of the Mn and Si concentrations is 70% or less of the sum of the Mn and Si concentrations at the half-thickness position is defined as the Mn-Si depleted zone, and its thickness is determined.

The primary pickling is not particularly limited, and may be carried out using a commonly used pickling solution under conditions suitable for removing the outer and inner oxide layers. The primary pickling may be carried out once, or may be carried out multiple times to ensure that the outer and inner oxide layers are completely removed.

In steel sheets containing the three elements Ni, Cu, and Sn, the presence of these elements can promote the formation of Mn- and/or Si-based surface oxides on the steel sheet surface. For this reason, it is extremely difficult to suppress the formation of such surface oxides and properly adhere a coating to steel sheets containing the three elements Ni, Cu, and Sn. However, this manufacturing method significantly suppresses the formation of such surface oxides by combining an Mn-Si depleted layer formed to a predetermined thickness, i.e., 0.3 μm or more, through appropriate control of the coiling temperature in the coiling process with a secondary pickling process, which will be described in detail later. On the other hand, if the coiling temperature in the coiling process is less than 520°C, the formation of an internal oxide layer is insufficient, making it impossible to form an Mn-Si depleted layer with a thickness of 0.3 μm or more. In this case, it becomes impossible to sufficiently suppress the formation of Mn- and/or Si-based surface oxides in the annealing process after the plating process, and it becomes impossible to form a plating layer of the desired thickness in the final plated steel sheet having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more.

From the perspective of further improving chemical conversion treatability by further thickening the plating layer, which has a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more, it is preferable to control the coiling temperature to 550°C or higher. By controlling the coiling temperature to 550°C or higher, the formation of the internal oxide layer can be further promoted, which in turn makes it possible to further thicken the Mn-Si depleted layer. As a result, the formation of Mn- and/or Si-based surface oxides in the annealing process can be more significantly suppressed, making it possible to further increase the thickness of the plating layer. There is no particular upper limit on the coiling temperature, but the coiling temperature may be 600°C or lower, for example.

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

[Secondary pickling process]
The resulting cold-rolled steel sheet is then subjected to secondary pickling in the subsequent secondary pickling step. Specifically, the secondary pickling step involves immersing the cold-rolled steel sheet in an aqueous solution having a hydrochloric acid concentration of 3 to 12% and not containing an inhibitor that inhibits corrosion of the steel sheet at a temperature of 50 to 90°C for 2 to 100 seconds, and then rinsing the cold-rolled steel sheet with a rinse solution having an electrical conductivity of 40 mS/m or less. Even if the outer and inner oxide layers are not sufficiently removed in the previous primary pickling, the secondary pickling using an aqueous hydrochloric acid solution can reliably and completely remove these oxide layers. Furthermore, because the surface condition of the steel sheet changes during cold rolling, and the reason for this is not always clear, the formation of Mn- and/or Si-based surface oxides in the subsequent annealing step may not be sufficiently suppressed due to this change in the surface condition and the presence of Ni, Cu, and Sn in the steel sheet. Therefore, in the present manufacturing method, secondary pickling is performed before the annealing step to condition the surface of the steel sheet, thereby making it possible to sufficiently and reliably suppress the formation of Mn- and/or Si-based surface oxides in the subsequent annealing step. Furthermore, iron oxides may also form on the surface of the steel sheet during cold rolling, which may result in poor plating performance in the subsequent plating step. To reliably remove the iron oxides and ensure plating performance, it is effective to perform pickling using an inhibitor-free aqueous solution having a hydrochloric acid concentration of 3 to 12% after the cold rolling step and before the plating step, followed by rinsing with a wash solution having an electrical conductivity of 40 mS/m or less, as described below. Preferably, the hydrochloric acid concentration of the aqueous hydrochloric acid solution is 4 to 8%, the immersion temperature is 70 to 90°C, and the immersion time is 4 to 50 seconds.

In the secondary pickling process, the water rinse after the secondary pickling is also extremely important. For example, if the electrical conductivity of the water used in the water rinse is relatively high, more specifically, if it is higher than 40 mS/m, iron oxides may form on the surface of the cold-rolled steel sheet during the water rinse after the secondary pickling. The presence of such iron oxides on the surface of the cold-rolled steel sheet inhibits the adhesion of the plating in the subsequent plating process. In this case, the final plated steel sheet will not be able to form a plating layer of the desired thickness having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more. In contrast, in the present manufacturing method, the water rinse after the secondary pickling is performed using a water rinse with an electrical conductivity of 40 mS/m or less, which significantly suppresses the formation of iron oxides during the water rinse after the secondary pickling and enables the plating to adhere properly in the subsequent plating process.

In steel sheets containing the three elements Ni, Cu, and Sn simultaneously, the presence of these elements promotes the formation of Mn- and/or Si-based surface oxides during the annealing process, as well as the formation of iron oxides during water rinsing after secondary pickling. For this reason, it is extremely difficult to suppress the formation of these oxides in steel sheets containing the three elements Ni, Cu, and Sn simultaneously and to form a coating layer of the desired thickness in the final plated steel sheet in which the total concentration of at least one of Zn and Mn is 10 mass% or more and the Fe concentration is 10 mass% or more. Therefore, it is highly unexpected and surprising that the formation of these oxides can be significantly suppressed by combining a Mn-Si depleted layer formed to a predetermined thickness, i.e., 0.3 μm or more, due to appropriate control of the coiling temperature in the coiling process with specific secondary pickling and water rinsing in the secondary pickling process. To further suppress the formation of iron oxides, the lower the electrical conductivity of the washing solution, the better; specifically, it is preferably 25 mS/m or less, and more preferably 15 mS/m or less.

[Plating process]
Next, in the plating step, plating is applied to at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). The plating step can be carried out by any appropriate plating process, such as electroplating, vapor deposition plating, thermal spraying, or cold spraying, that is effective in achieving the desired thickness of a plating layer in which the total concentration of at least one of Zn and Mn is 10% by mass or more and the Fe concentration is 10% by mass or more. Preferably, the plating step is carried out by electroplating. Electroplating can be carried out using a bath containing at least one of Zn and Mn and optional additive elements at predetermined concentrations, under conditions of a current density of ⁵ to 20 A/dm2 and a current application time of 2.0 to 20.0 seconds. Preferably, the current density is 8 to 15 A/ ^dm2 and the current application time is 3.0 to 10.0 seconds.

In this manufacturing method, in order to ensure proper plating adhesion, it is important to perform the plating step after sufficiently or completely removing Mn- and/or Si-based surface oxides and iron oxides from the surface of the base steel sheet in the secondary pickling step. In other words, it is important to perform the plating step after the secondary pickling step. Conversely, as long as the plating step is performed after the secondary pickling step, performing the plating step before the secondary pickling step is not necessarily excluded. For example, by dividing the plating step into two steps, first performing the first plating treatment before the secondary pickling step and then performing the second plating treatment after the secondary pickling step, it is possible to form a plating layer of the desired thickness having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more. Alternatively, it is possible to perform another plating treatment before the secondary pickling step and then perform the plating step according to this manufacturing method after the secondary pickling step.

[Annealing process]
Finally, the obtained plated steel sheet is annealed. The annealing step involves heating the cold-rolled steel sheet to a temperature of 700 to 950°C in an atmosphere with a dew point of -40 to 20°C and holding the temperature for 0 to 300 seconds. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere containing 1 to 10% hydrogen (e.g., 4% hydrogen and the balance nitrogen). The annealing step allows at least one of Zn and Mn in the plated layer applied in the plated step to be alloyed with Fe in the base steel sheet, making it possible to form a plated layer of a desired thickness having a total concentration of at least one of Zn and Mn of 10% by mass or more and an Fe concentration of 10% by mass or more.

This manufacturing method, particularly for steel sheets for which improving chemical treatability is difficult due to the simultaneous inclusion of the three elements Ni, Cu, and Sn, enables sufficient or complete removal of Mn- and/or Si-based surface oxides and iron oxides from the surface of the base steel sheet by combining a Mn-Si depleted layer formed to a predetermined thickness, i.e., 0.3 μm or greater, through appropriate control of the coiling temperature in the coiling process with specific secondary pickling and water rinsing in the secondary pickling process. In this regard, subsequent appropriate plating and annealing processes can be used to produce plated steel sheets with a controlled thickness of 5 μm or greater, in which the total concentration of at least one of Zn and Mn is 10 mass% or greater and the Fe concentration is 10 mass% or greater, as measured on the cross section by EPMA. As previously mentioned, Zn and Mn function as anodes during chemical treatment and improve the chemical treatability of the steel sheet by dissolving themselves. Therefore, with plated steel sheets manufactured according to this manufacturing method, the anodic dissolution (etching) of Zn and/or Mn in the plating layer can be promoted during chemical conversion treatment, allowing for the formation of a uniform chemical conversion coating over the entire steel sheet, resulting in significantly improved corrosion resistance. Additionally, by alloying at least one of Zn and Mn with Fe through the annealing process to form an alloy plating layer, the adhesion between the alloy plating layer and the base steel sheet can be improved compared to when at least one of Zn and Mn is simply plated. As a result, peeling of the plating layer can be sufficiently suppressed even during processes involving sliding, such as press working, thereby significantly improving the chemical treatability of the processed area. Therefore, plated steel sheets manufactured according to this manufacturing method can achieve superior corrosion resistance compared to conventional plated steel sheets containing the three elements Ni, Cu, and Sn simultaneously. This can contribute to industrial development by extending the life of plated steel sheets for automobiles and building materials.

Plated steel sheets according to embodiments of the present invention can be used as the various automotive parts described above, for example, after a chemical conversion coating or paint film is optionally formed on the surface. Whether an automotive part having a paint film or chemical conversion coating includes a plated steel sheet according to embodiments of the present invention can be determined by removing the paint film or chemical conversion coating from a sample taken from the automotive part. In this case, the location from which the sample is taken, the paint film removal process, and the chemical conversion coating removal process are as follows.

[Sample collection location]
Sampling from automobile parts will be carried out avoiding the following points (i) to (iv).
(i) Within 20 mm from the toe of a spot weld, within 20 mm from the toe of the bead of an arc/laser weld; (ii) A processed part with a radius of curvature of less than 15 mm, and within 5 mm from such a processed part; (iii) An edge within 5 mm from the cut end surface of a part; (iv) A part within 5 mm from a part where red rust is visible to the naked eye.

[Paint film 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, areas with a carbon concentration of 10% by mass or more are identified, and if the area ratio of these areas is 5% or more, it is determined that the paint film has not been sufficiently removed. To measure the area ratio of areas with a carbon concentration of 10% by mass or more, first obtain an element distribution image of carbon using an EPMA with a carbon concentration range of 10-30%. Specific measurement conditions for EPMA are as follows:
Apparatus: JXA-8230 electron probe microanalyzer manufactured by JEOL Ltd. Acceleration voltage: 15 kV
Irradiation current: 0.05μA
Surface analysis: WDS
Analysis interval: 300 μm or more Area ratio: average value of 5 fields of view Next, the area ratio is measured by image processing of the obtained element distribution image. Image processing is performed using image analysis software "ImageJ". After loading the C element distribution image into ImageJ, it is binarized using "Make Binary" in "Binary" under "Process" so that areas with a C concentration of 10% by mass or more are displayed as black and areas with a C concentration of less than 10% by mass are displayed as white. After binarization, "Measure" under "Analyze" is used to read the value of "Area fraction" in "Results", and this value is determined as the area ratio of areas with a C concentration of 10% by mass or more. If peeling of the coating film is insufficient, removal of the coating film is repeated until the area ratio of areas with a C concentration of 10% by mass or more becomes less than 5%.

[Chemical conversion coating removal process]
For a sample cut from an automobile body and having the paint removed, for example, if the chemical conversion coating is a zinc phosphate coating, the chemical conversion coating is removed in accordance with JIS K 3151:1996. Specifically, the sample is immersed in a 5% chromic acid aqueous solution heated to 75°C for 15 minutes to remove the chemical conversion coating. The sample is 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 element distribution image obtained by EPMA, regions with a P concentration of 5% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the chemical conversion coating has not been sufficiently removed. To measure the area ratio of regions with a P concentration of 5% by mass or more, first obtain an element distribution image of P using EPMA with a P concentration range of 5 to 10%. The obtained element distribution image is then image-processed to measure the area ratio. Image analysis software "ImageJ" was used for image processing. The P element distribution image was then loaded into ImageJ, and binarized using "Make Binary" in "Binary" under "Process" so that areas with a P concentration of 5% by mass or more were displayed as black, and areas with a P concentration of less than 5% by mass were displayed as white. After binarization, "Measure" under "Analyze" was used to read the value for "Area fraction" in "Results," and this value was determined as the area fraction of areas with a P concentration of 5% by mass or more. If peeling of the chemical conversion coating was insufficient, removal of the chemical conversion coating was repeated until the area fraction of areas with a P concentration of 5% by mass or more became less than 5%.

The present invention will be explained in more detail below using examples, but the following examples are merely examples of the present invention and the present invention is in no way 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, plated steel sheets according to embodiments of the present invention were manufactured under various conditions, and the properties of the manufactured plated steel sheets were investigated.

First, molten steel was cast using a continuous casting method to form a steel billet having the chemical composition shown in Table 1. After cooling, the steel billet was reheated to 1200°C and hot rolled, and then coiled at the coiling temperature shown in Table 2. Hot rolling was carried out by rough rolling and finish rolling, with the finishing temperature for finish rolling being 900-1050°C and the reduction in finish rolling being 30%. Next, the resulting hot-rolled steel sheet was subjected to primary pickling and then cold-rolled at a reduction of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

Next, the obtained cold-rolled steel sheet was subjected to secondary pickling. Specifically, the secondary pickling was performed by immersing the cold-rolled steel sheet in an inhibitor-free aqueous solution having a 5% hydrochloric acid concentration at a temperature of 80°C for 4.5 seconds, and then rinsing the cold-rolled steel sheet with a rinse solution having an electrical conductivity shown in Table 2. Next, the cold-rolled steel sheet (base steel sheet) that had been subjected to secondary pickling was electroplated using a bath containing metal species such as Zn and Mn at predetermined concentrations under conditions of a current density of 10 A/ ^dm2 and a current application time of 5.0 seconds, thereby obtaining a plated steel sheet having a plating layer attached to both sides of the base steel sheet. Finally, the obtained plated steel sheet was subjected to an annealing step in which the plated steel sheet was heated to a temperature of 800°C in an atmosphere with a dew point of 0°C and 4% hydrogen (nitrogen balance) in a furnace with an oxygen concentration of 20 ppm or less, and maintained at that temperature for 100 seconds, thereby obtaining a plated steel sheet in which at least one of Zn and Mn was alloyed with Fe and, optionally, Cu. The "thickness of the plating layer" in Table 2 indicates the thickness of the plating layer in which the concentration of at least one of Zn and Mn is 10 mass % or more in total and the Fe concentration is 10 mass % or more, when the cross section of the plated steel sheet is measured by EPMA. As shown in Table 2, the plating layer may contain Cu in addition to Zn, Mn, and Fe.

The properties of the resulting plated steel sheets were measured and evaluated using the following methods.

[Evaluation of chemical conversion treatment properties of processed parts]
The chemical conversion treatability of the processed portion was evaluated by evaluating the chemical conversion treatability of the bead portion after a drawbead test. Specifically, first, a 200 mm × 30 mm sample of the plated steel sheet produced above was coated with NOX-RUST550NH and then pressed against a mold with an R = 4 mm at a pressing load of 3 kN. Next, using a tensile testing machine UST-10T manufactured by Oriental Co., Ltd., the sample was pulled out at a speed of 100 mm/min so that the sliding distance was 100 mm. Next, the sample that had been subjected to the drawbead test was subjected to zinc phosphate treatment as a chemical conversion treatment under the following conditions.
Degreasing: Immersed in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes, then rinsed with water. Surface conditioning: Immersed in a surface conditioning agent (Preparen Z) at room temperature for 30 seconds. Chemical conversion treatment: Immersed in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, then rinsed with water and dried.

The bead portions of the chemically treated samples were observed using SEM-EPMA, and the area ratio of the portion where the chemical conversion coating was not formed, commonly known as the "clear" area, was calculated by binarization using the image analysis software "ImageJ." The chemical conversion treatability of the processed portion was evaluated according to the area ratio of the clear-cut area using the following evaluation criteria. The clear-cut area was defined as an area with an Fe concentration of 70% or more in a mapping image obtained by photographing an area of 80 μm x 60 μm or more at 1000x magnification using an EPMA (JXA-8500 manufactured by JEOL Ltd.) under conditions of an acceleration voltage of 15 kV and an irradiation current of 5 x ^10-7 A. The area ratio of the clear-cut area was determined as the average value of five randomly selected fields of view.
AAA: Transparent area ratio less than 20% AA: Transparent area ratio 20-25% A: Transparent area ratio 25-35%
B: Skewer area ratio over 35%

Steel sheets with a rating of AAA, AA, or A for chemical conversion treatability in the processed area were evaluated as containing Ni, Cu, and Sn and capable of exhibiting improved chemical conversion treatability in the processed area. The results are shown in Table 2.

Referring to Table 2, in Comparative Example 23, the low coiling temperature resulted in insufficient formation of an internal oxide layer, making it impossible to form an Mn-Si depleted layer with a thickness of 0.3 μm or more. As a result, the thickness of the plating layer, which had a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more, was less than 5 μm, and the chemical conversion treatability of the processed part was reduced. In Comparative Example 25, in addition to the low coiling temperature, the high electrical conductivity of the water used for rinsing after the secondary pickling meant that the formation of Mn- and/or Si-based surface oxides during the annealing process could not be sufficiently suppressed, and furthermore, the formation of iron oxides during rinsing after the secondary pickling could not be sufficiently suppressed. As a result, the thickness of the plating layer was less than 5 μm, and the chemical conversion treatability of the processed part was reduced. In Comparative Examples 24, 26, and 27, the high electrical conductivity of the water used for rinsing after the secondary pickling presumably prevented the formation of iron oxides during rinsing after the secondary pickling. As a result, the thickness of the plating layer was less than 5 μm, and the chemical conversion treatability of the processed area was reduced.

In contrast, in the plated steel sheets of all Examples, when the cross section was measured by EPMA, the plating was performed so that the thickness of the plating layer, in which the total concentration of at least one of Zn and Mn was 10 mass% or more and the Fe concentration was 10 mass% or more, was limited to 5 μm or more. This significantly improved the chemical treatability of the processed portions of the plated steel sheets. In particular, in Examples 2, 3, 8, 9, 14, 15, and 20, in which the plating layer thickness was 8 μm or more, the chemical treatability of the processed portions was evaluated as AA, further improving the chemical treatability of the processed portions. In Examples 4 to 6, 10 to 12, 16 to 18, 21, and 22, in which the plating layer thickness was 10 μm or more, the chemical treatability of the processed portions was evaluated as AAA, further improving the chemical treatability of the processed portions.

Claims (6)

A plated steel sheet comprising a base steel sheet and a plating layer disposed on a surface of the base steel sheet,
The base steel plate is, in mass%,
Ni: 0.010 to 1.000%,
A chemical composition including Cu: 0.010 to 1.000% and Sn: 0.003 to 1.000%;
In an element distribution image obtained by measuring a cross section of the plated steel sheet with an EPMA,
A plated steel sheet, characterized in that the plating layer has a total concentration of at least one of Zn and Mn of 10 mass % or more, an Fe concentration of 10 mass % or more, and a thickness of 5 μm or more. The plated steel sheet according to claim 1, characterized in that the thickness of the plating layer, in which the total concentration of at least one of Zn and Mn is 10 mass% or more and the Fe concentration is 10 mass% or more, is 8 μm or more. The plated steel sheet according to claim 2, characterized in that the plating layer has a total concentration of at least one of Zn and Mn of 10 mass% or more, an Fe concentration of 10 mass% or more, and a thickness of 10 μm or more. The chemical composition is, in mass %,
Ni: 0.040-1.000%,
Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%
The plated steel sheet according to any one of claims 1 to 3, comprising: The plated steel sheet according to any one of claims 1 to 4, characterized in that it has a Vickers hardness of 200 Hv or more. A part comprising the plated steel sheet according to any one of claims 1 to 5.