Sn-Zn alloy plated steel, battery cases and fuel tanks

Abstract

This Sn-Zn based-alloy plated steel material is provided with: a steel material; a first alloy layer that is on the surface of the steel material and is mainly composed of Fe, Cr and Ni; a second alloy layer that is on the first alloy layer and is mainly composed of Sn, Fe, Ni and Zn; and an Sn-Zn based plating layer that is on the second alloy layer and is mainly composed of Sn and Zn. The surface coverage of the steel material surface by the first alloy layer is 70% or more. The second alloy layer is mainly composed of at least one of an FeSn2 phase and an Fe3Zn10 phase that is partially substituted by Ni. The Sn-Zn based plating layer is composed of 1.0-15.0% by mass of Zn, with the balance being made up of Sn and impurities. The total coating amount of the first alloy layer, the second alloy layer and the Sn-Zn based plating layer is 5-80 g / m2 per one surface.

Description

[Technical Field]

[0001] The present invention relates to a Sn—Zn-based alloy plated steel material, a battery case and a fuel tank, and more particularly to a Sn—Zn-based alloy plated steel material that has excellent plating appearance, corrosion resistance and workability. This application claims priority based on Japanese Patent Application No. 2023-044958, filed on March 22, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, automobile steel sheets have become stronger in order to reduce vehicle weight and thereby improve fuel efficiency. Steel sheets for fuel tanks are also required to have excellent formability and high strength, as the tanks are becoming lighter, vehicle designs are becoming more complex, and fuel tank shapes are becoming more complex due to the location where the tanks are installed.

[0003] To satisfy the demand for both formability and high strength, high-strength interstitial-free (IF) steels have been developed by adding carbonitride-forming elements such as Ti and Nb to ultra-low carbon steels, and then further adding solid-solution strengthening elements such as P, Si, and Mn.

[0004] As described above, reducing the thickness of steel sheets is required to reduce the weight of vehicle bodies, and as a result, the corrosion resistance of steel sheets has become even more important than before. For example, steel sheets used on the inner surface of fuel tanks are required to have properties that prevent the formation of corrosion products that cause filter clogging and prevent pitting corrosion in response to gasoline, alcohol, or organic acids produced by gasoline degradation. Furthermore, steel sheets used on the outer surface of fuel tanks are required to have good corrosion resistance in various operating environments, such as areas where snow-melting salt is sprayed or where the vehicle is driven in hot and humid regions. To meet these needs, plating of steel sheets with Pb-Sn alloys, Al-Si alloys, Sn-Zn alloys, etc. has been proposed and applied.

[0005] However, when fuel tanks are exposed to severe corrosive environments on the inside and outside, corrosion that can lead to holes may occasionally occur. Therefore, materials used for fuel tanks are increasingly required to have both sufficient durability and good corrosion resistance. To cope with such severe corrosive environments, various surface-treated steel sheets have been developed. Among these, Cr-containing steel sheets and steel sheets plated with Sn-based plating have been disclosed as promising technologies.

[0006] For example, Patent Document 1 discloses a steel sheet for fuel containers having a Ni, Co, Ni-Co alloy diffusion coating layer and a Sn, Sn-Zn alloy plating coating layer on at least one side of the steel sheet containing 0.02% or less of C, more than 3% to 20% of Cr, and 0.005 to 0.10% of acid-soluble Al.

[0007] Patent Document 2 also describes an alloy layer containing one or more of Ni, Fe, Sn, and Zn, and having a thickness of 1.5 μm or less per side, provided on steel containing C, Si, Mn, P, Al, and 0.2≦Cr≦6%, and a steel sheet provided on the alloy layer, which is made of 80 to 99% tin, the remainder being zinc and unavoidable impurities, and in which the zinc crystals contained therein have a major axis of 250 μm or more and the number of the zinc crystals is 20 or less per 0.25 mm. 2 and a tin-zinc alloy plating layer having a thickness of 4 to 50 μm per side.

[0008] Patent Document 3 also discloses a technology in which a hot-dip Zn-Sn coated layer is provided on the surface of a steel sheet, the coated layer composition containing 1 to less than 50% Sn in addition to Zn, and the steel contains 1 to 25% Cr. Patent Document 3 further discloses a hot-dip Zn-Sn coated steel sheet characterized by having a pre-coated layer containing Ni, Co, Fe, Cr, Sn, Zn, and Cu at the interface between the coated layer and the steel sheet.

[0009] Patent Document 4 discloses a technology in which a hot-dip Sn-Zn coated layer is provided on the surface of a steel sheet, the coated layer composition containing 1 to 50% Zn in addition to Sn, and the ratio of the coated layer (Zn% in surface layer composition / Zn% in entire coated layer) is 0.95 or less. Patent Document 4 also discloses a hot-dip Sn-Zn coated steel sheet characterized by containing 3 to 25% Cr in the steel and having a pre-coated layer containing Ni, Co, Fe, Cr, Sn, Zn, and Cu at the interface between the coated layer and the steel sheet.

[0010] In addition, Patent Document 5 describes a substrate having a breaking elongation of 30% or more when processed by uniaxial tension, and a Lankford value (r value) of r min The document discloses an automobile fuel tank or fuel filler pipe using a ferritic stainless steel sheet having a Cr content of 1.3 or more. Specifically, the document discloses an automobile fuel tank or fuel filler pipe characterized in that the ferritic stainless steel sheet substrate contains, by mass, 0.015% or less of C, 0.5% or less of Si, 11.0 to 25.0% of Cr, 0.020% or less of N, 0.05 to 0.50% of Ti, 0.10 to 0.50% of Nb, and 0.0100% or less of B, and that the surface of the ferritic stainless steel sheet is formed with an Al plating layer, a Zn plating layer, or a plating layer consisting of an alloy layer of Zn with one or more of Fe, Ni, Co, Mg, Sn, and Al, and further that a zinc-rich coating film is formed at the welded parts and a cationic electrodeposition coating film is formed on the entire substrate including the welded parts.

[0011] Patent Document 6 also describes a steel sheet containing, in mass %, C:≦0.030%, Si:≦2.00%, Mn:≦2.00%, P≦0.050%, S:≦0.0100%, N:≦0.030%, Al: 0.010 to 0.100%, Cr: 10.00 to 25.00%, and in addition, one or more of Ni: 0.10 to 4.00%, Cu: 0.10 to 2.00%, Mo: 0.10 to 2.00%, and V: 0.10 to 1.00%. The present invention discloses a stainless steel sheet containing one or more of Ti: 0.01-0.30% and one or two of Nb: 0.01-0.30%, with the balance consisting of unavoidable impurities and Fe, in which the Y value defined by a specific formula is -10.4 or less, and a surface-treated stainless steel sheet having on its surface a corrosion-resistant plating layer consisting of Zn: 0.8-10.0%, with the balance consisting of Sn and unavoidable impurities. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 61-091390 [Patent Document 2] Japanese Patent Publication No. 08-269735 [Patent Document 3] Japanese Patent Publication No. 2001-355051 [Patent Document 4] Japanese Patent Publication No. 2002-038250 [Patent Document 5] Japanese Patent Publication No. 2003-277992 [Patent Document 6] Japanese Patent Publication No. 2009-068102 [Patent Document 7] International Publication No. 2019 / 208775 Summary of the Invention [Problem to be solved by the invention]

[0013] However, it is difficult to say that the conventionally disclosed techniques as described above are fully satisfactory in terms of plating properties such as plating adhesion appearance and adhesion on steel sheets to achieve good corrosion resistance, and methods for achieving these.

[0014] Furthermore, the above-mentioned prior art has the following problems. To obtain good corrosion resistance in plated steel sheets, it is necessary to form a uniform coating appearance, i.e., a coating with a high coverage rate on the steel sheet and a certain thickness or more. Hot-dip coating is suitable for achieving this goal. However, when hot-dip coating is applied to steels containing elements such as Cr, Si, and Mn, there is a concern that certain oxide films formed on the steel surface may impair plating properties.

[0015] Furthermore, in the case of hot-dip galvanizing, it is necessary to consider the corrosion resistance of the alloy layer formed between the base steel and the pre-coating and between the pre-coating and the coating bath due to the heat input during hot-dip galvanizing. However, the aforementioned prior art lacks sufficient consideration to overcome this issue. Therefore, when hot-dip galvanizing is performed on an actual wide-width steel strip while it is continuously threaded, unlike small test pieces, the coating methods disclosed in Patent Documents 2 to 6 insufficiently control the element distribution on the steel sheet surface. The coating methods disclosed in Patent Documents 2 to 6 also insufficiently consider the formation of an alloy layer at the interface to achieve good coating properties. For these reasons, variations in the surface condition and alloy layer formation occur during industrial production of wide-width steel sheets, raising concerns about the occurrence of "unplated" areas where the coating is not applied. Furthermore, unplated areas not only impair the product's appearance, but also lead to reduced corrosion resistance and workability.

[0016] Patent Document 7 discloses a Sn-Zn plated steel sheet having an alloy layer containing Ni, Sn, Cr, Fe, and Zn on the surface of the steel sheet containing Cr, Si, Mn, etc., in which the ratio of Fe-Sn-Cr-Zn phase to Sn-Fe-Ni-Zn phase is 1 / 100 to 2 / 1 and the steel sheet surface coverage is 98% or more. However, in the case of Patent Document 7, if an alloy layer containing Zn, which has a low electrochemical potential, is present at the interface between the steel sheet surface and the plating layer, the potential of the alloy layer will be low. Therefore, if a scratch reaches the base steel, corrosion may progress along the alloy layer due to the potential difference between the base steel and the noble metal Sn in the plating.

[0017] The problem to be solved by one embodiment of the present invention is to provide Sn-Zn alloy plated steel, battery case and fuel tank materials that have a good plating appearance, high corrosion resistance and excellent workability. [Means for solving the problem]

[0018] In order to solve the above-mentioned problems, the inventors conducted extensive research into the element distribution on the steel surface that affects the composition of the steel and the appearance of the coating, as well as the state of the alloy layer at the interface with the Sn-Zn coating, and conducted detailed studies into the conditions for obtaining good coating appearance, corrosion resistance, and workability.

[0019] the result, (A) The steel contains a predetermined amount of Cr. (B) Forming an alloy layer having a specific composition, particularly an alloy layer mainly composed of Fe, Cr, and Ni (alloy layer A described later), on the steel material with a high coverage rate; (C) Forming a Sn-Zn alloy plating layer on the alloy layer. It was discovered that this process reduces the corrosion rate of steel materials and also provides corrosion protection by forming an alloy layer with the Sn-Zn plating.

[0020] Furthermore, they discovered that by setting the Cr concentration in the steel to a certain level and forming an alloy layer with a specific composition at the interface between the steel and the plating layer, it is possible to obtain a Sn-Zn alloy-plated steel material with good corrosion resistance. The present invention has been completed based on these findings, and the gist of each aspect of the present invention is as follows.

[0021] [1] A Sn-Zn alloy plated steel material according to one embodiment of the present invention is Steel and a first alloy layer mainly composed of Fe, Cr, and Ni on the surface of the steel material; a second alloy layer mainly composed of Sn, Fe, Ni, and Zn on the first alloy layer; A Sn-Zn based plating layer mainly composed of Sn and Zn is formed on the second alloy layer. and a surface coverage rate of the first alloy layer on the surface of the steel material is 70% or more; The second alloy layer is composed of FeSn2 phase and Fe3Zn partially substituted with Ni. 10 an alloy layer mainly composed of at least one of the phases, the Sn-Zn-based plating layer is composed of, by mass%, 1.0 to 15.0% Zn, the remainder being Sn and impurities; The total amount of the first alloy layer, the second alloy layer, and the Sn-Zn-based plating layer is 5 to 80 g / m per side. 2 is. [2] The Sn-Zn alloy plated steel material according to the above item [1], wherein the steel material comprises, in mass %, C: 0.0005~0.030%, Si: 0.80% or less, Mn: 0.10 to 2.00%, P: 0.005~0.040%, S: 0.0100% or less, Cr: 4.0 to 18.0%, Al: 0-0.30% Ti: 0 to 0.300% Nb: 0 to 0.040%, B: 0~0.0030%, N: 0~0.030%, Cu: 0-2.0% Ni: 0 to 3.0% Mo: 0-2.00%, V: 0~2.00%, and the remainder may be Fe and impurities. [3] The Sn-Zn alloy plated steel material described in [2] above is The steel material may further contain, by mass %, REM: 0 to 0.1000%. [4] The Sn—Zn alloy plated steel material described in [1] above may have a chemical conversion coating having a thickness of 0.02 to 2.0 μm on at least one surface of the Sn—Zn alloy plated layer. [5] The Sn—Zn alloy plated steel material according to the above [1] or [2] may have a coating film having a thickness of 10 to 500 μm on at least one surface of the Sn—Zn alloy plated layer. [6] The Sn—Zn alloy plated steel material described in [3] above may have a coating film having a thickness of 10 to 500 μm on at least one surface of the Sn—Zn alloy plated layer. [7] A battery case according to one aspect of the present invention includes the Sn—Zn-based alloy plated steel material according to any one of the above [1] to [6]. [8] A fuel tank according to one aspect of the present invention includes the Sn—Zn-based alloy plated steel material according to any one of the above [1] to [6]. [Effects of the Invention]

[0022] According to one embodiment of the present invention, it is possible to provide a Sn—Zn alloy plated steel material, a battery case and a fuel tank that have a good plating appearance, high corrosion resistance and excellent workability. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a schematic diagram showing a cross-sectional structure of a Sn—Zn-plated steel material according to an embodiment of the present invention. [Figure 2] 1 is a TEM image showing a cross section of a Sn—Zn-plated steel material according to the present embodiment. [Figure 3] FIG. 3 shows points where qualitative analysis was performed in the TEM image of FIG. 2. [Figure 4] FIG. 4 is a diagram showing the results of qualitative analysis of each of the points (a) to (c) shown in FIG. [Figure 5]FIG. 2 is a schematic diagram of a test material used in an evaluation test of corrosion resistance in this example. [Figure 6] FIG. 2 is a perspective view showing a schematic configuration of a lower part of the battery case of the present embodiment. [Figure 7] 1 is a perspective view showing a schematic configuration of a fuel tank according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0024] The present inventors have conducted extensive research into the composition of the steel material that serves as the base material for Sn-Zn plated steel (hereinafter also referred to as plated steel), the surface condition of the steel material that affects the coating appearance, and the Sn-Zn coating layer and alloy layer, and have found the optimal conditions for obtaining good coating appearance, good corrosion resistance, and excellent workability.

[0025] Below, preferred embodiments of the present invention will be described in detail, including the newly discovered findings. The present invention is not limited to the configurations disclosed in the present embodiments, and various modifications are possible without departing from the spirit of the present invention. In this specification, the "%" used to indicate the content of each element in the chemical composition of the plating layer means "mass %" unless otherwise specified. A numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. When the numerical values ​​before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit.

[0026] In addition, "external corrosion resistance" as used herein refers to the corrosion resistance exhibited when a plated steel material is exposed to an outdoor atmospheric environment. "Internal corrosion resistance" refers to the corrosion resistance in an organic acid environment such as formic acid or acetic acid, and refers to the corrosion resistance exhibited when a plated steel material is exposed to an organic acid aqueous solution produced when organic acids produced by fuel degradation or the like are concentrated in water droplets formed on the surface of the plated layer due to condensation or the like. Note that, hereinafter, when simply referring to "corrosion resistance," it refers to both "external corrosion resistance" and "internal corrosion resistance."

[0027] [Sn-Zn alloy plated steel] As shown in FIG. 1, the Sn-Zn-based alloy plated steel material 10 of this embodiment comprises a steel material 11 having a predetermined chemical composition, a first alloy layer 12 mainly composed of Fe, Ni, and Cr formed on at least one surface of the steel material 11, a second alloy layer 13 mainly composed of Sn, Fe, Zn, and Ni formed on the first alloy layer 12, and an Sn-Zn-based plating layer 14 mainly composed of Sn and Zn formed on the second alloy layer 13. Hereinafter, the "first alloy layer 12" may be referred to as alloy layer A, the "second alloy layer 13" as alloy layer B, and the "Sn-Zn-based plating layer 14" as "Sn-Zn-based plating layer C."

[0028] The coverage of the first alloy layer (alloy layer A) 12 to the steel sheet surface is 70% or more, and the second alloy layer (alloy layer B) 13 is FeSn2 phase and / or Fe3Zn partially substituted with Ni. 10 The Sn-Zn-based plating layer 14 (Sn-Zn-based plating layer C) is composed of 1 to 15% by mass of Zn, with the remainder being Sn and impurities. The total coating weight of alloy layer A, alloy layer B, and Sn-Zn-based plating layer C is 5 to 80 g / m per side. 2 is.

[0029] <Steel> First, the steel material (original sheet) to be plated will be described. The steel material is, for example, mainly a steel plate, but its size is not particularly limited. The steel plate may be any steel plate that can be applied to a normal hot-dip galvanizing process. Specifically, this applies to steel plates that can be applied to a process in which the steel plate is immersed in molten metal and solidified.

[0030] The material of the steel material is not particularly limited. Various steel sheets can be used as the steel material, such as general steel, pre-plated steel thinly plated with various metals, low-carbon steel, high-carbon steel, and various high-tensile steels. In the following description, a steel sheet will be used as an example of the steel material (original sheet), but the steel material of this embodiment is not limited to a steel sheet. The specific composition of the steel material will be described later.

[0031] <Alloy layer A, B> Next, the alloy layers A and B will be described. The present inventors have conducted extensive research into the effects of the surface condition of a steel sheet and an alloy layer on the coating appearance, and have found that by forming an alloy layer having a specific composition when applying a hot-dip Sn-Zn coating, the resulting coated steel sheet can have high corrosion resistance and excellent formability.

[0032] The alloy layers formed between the steel sheet and the Sn-Zn plating layer C are a layer mainly composed of Fe-Cr-Ni, which contains α-Fe and Cr and Ni (hereinafter referred to as alloy layer A), and a layer mainly composed of Sn-Fe-Ni-Zn (hereinafter referred to as alloy layer B). Specifically, alloy layer B is composed of FeSn2 phase and Fe3Zn, which are partially substituted with Ni. 10 It is composed of equal phases, for example, Fe x Ni (3-x) Zn 10 The specific configuration of the alloy layer B will be described in detail later.

[0033] Alloy layers A and B are composed of elements diffused from the steel sheet, the Ni-based pre-plated layer, and the hot-dip Sn-Zn alloy plated layer C when the alloy layer is formed between the steel sheet and the Sn-Zn plated layer C during the hot-dip plating process.

[0034] As described above, the alloy layer A is a layer mainly composed of Fe—Cr—Ni, which contains α-Fe and Cr and Ni. Here, "a layer mainly composed of Fe—Cr—Ni" refers to a state in which 90 volume % or more of the alloy layer A is occupied by an Fe—Cr—Ni alloy. In other words, the alloy layer A may contain less than 10 volume % of an alloy other than the Fe—Cr—Ni alloy. The alloy other than the Fe—Cr—Ni alloy here refers, for example, to an Fe—Ni—Cr—Zn layer (alloy layer D) that contributes to a deterioration in corrosion resistance. If the volume fraction of the other alloy, such as alloy layer D, in the alloy layer A is less than 10%, the effect of the present invention is not adversely affected. From the viewpoint of improving corrosion resistance, the alloy layer A is preferably composed of an Fe—Cr—Ni alloy. The Fe—Cr—Ni alloy refers to a ternary alloy containing Fe, Cr, and Ni. The alloy layer A may contain a small amount of elements (1 mass % or less in element ratio) other than Fe, Cr, and Ni, as long as the elements do not adversely affect the characteristics of this embodiment.

[0035] In this embodiment, the inclusion of Cr and Ni in alloy layer A can improve the corrosion resistance of the plated steel sheet. On the other hand, if alloy layer A contains a large amount of elements other than Fe, Cr, and Ni, the corrosion resistance may deteriorate. For example, if Zn from the plating bath penetrates into alloy layer A and alloy layer A becomes an Fe-Cr-Ni-Zn alloy, the potential becomes less noble, which may reduce the corrosion resistance. Therefore, it is effective for corrosion resistance that alloy layer A has an Fe-Cr-Ni alloy as the main phase.

[0036] The coverage rate of the alloy layer A on the steel sheet surface (surface coverage rate) is 70% or more. The alloy layer A is a layer that effectively improves the corrosion resistance of the plated steel sheet. Therefore, the higher the coverage rate of the alloy layer A on the steel sheet surface, the better. Preferably, the surface coverage rate of the alloy layer A is 90% or more. More preferably, one or both entire surfaces of the steel sheet are covered with the alloy layer A. In other words, the surface coverage rate of the alloy layer A may be 100%. How to determine the surface coverage rate will be described later.

[0037] The alloy layer B is a layer mainly composed of Sn-Fe-Ni-Zn. Specifically, the alloy layer B is composed of an FeSn2 phase and an Fe3Zn phase partially substituted with Ni. 10 The alloy layer B is a layer mainly composed of at least one of the FeSn2 phase and the Fe3Zn phase partially substituted with Ni. 10 The alloy layer B may be composed of, for example, a ZnNi phase, an FeSn2 phase, an Fe3Zn 10 phase, FeSn2 phase partially substituted with Ni, Fe3Zn phase partially substituted with Ni 10 phases (e.g., Fe x Ni (3-x) Zn 10 ) and the like. Here, "a layer mainly composed of Sn-Fe-Ni-Zn" refers to a state in which 90% by volume or more of alloy layer B is occupied by an Sn-Fe-Ni-Zn alloy. In other words, alloy layer B may contain less than 10% by volume of an alloy other than an Fe-Cr-Ni alloy. As described above, an alloy other than an Fe-Cr-Ni alloy is, for example, an Fe-Ni-Cr-Zn layer (alloy layer D) that contributes to deterioration of corrosion resistance. In alloy layer B, if the volume fraction of other alloys such as alloy layer D is less than 10%, it does not adversely affect the effects of the present invention.

[0038] From the viewpoint of improving corrosion resistance, alloy layer B is preferably composed of a Sn-Fe-Ni-Zn alloy. A Sn-Fe-Ni-Zn alloy refers to a quaternary alloy containing Sn, Fe, Ni, and Zn. In addition to these elements, alloy layer B may contain small amounts of elements (1 mass% or less) within a range that does not adversely affect its properties.

[0039] The alloy layer B, which is mainly composed of Sn-Fe-Ni-Zn, is a layer formed on the alloy layer A in a hot-dip Sn-Zn plating bath, and is composed of FeSn2 phase and / or FeSn2 phase, Fe3Zn 10 phase and / or partially Ni-substituted Fe3Zn 10In other words, alloy layer B is electrochemically second only to the Zn single layer in the Sn-Zn based plating layer (hereinafter referred to as Sn-Zn based plating layer C), which is electrochemically the most base. In other words, for example, if a scratch that reaches the base steel occurs and the plated steel sheet is exposed to a corrosive environment, After sacrificial dissolution of Zn in Sn-Zn based plating layer C, Fe3Zn was exposed in alloy layer B, with Ni partially substituted. 10 However, after the Zn in the surface layer of alloy layer B dissolves, the noble alloy layer A is exposed, and in the next stage, the Sn in the Sn-Zn-based plating layer C undergoes sacrificial dissolution. However, because the amount of cathodic reaction in the exposed steel substrate is small, the rate of anodic dissolution of Sn is also slow, and long-term corrosion resistance can be expected.

[0040] There are no particular limitations on the total thickness of alloy layer A and alloy layer B. For example, the lower limit of the average value of the total thickness of alloy layer A and alloy layer B may be 0.1 μm or more, or 0.3 μm or more. Furthermore, the upper limit of the average value of the total thickness may be 3.0 μm or less, or 2.0 μm or less.

[0041] As shown in FIG. 1 , alloy layer B has a lamellar region (layer region) 13A and an acicular region (acicular region) 13B. The thickness of alloy layer B is defined as the distance from the bottom of lamellar region 13A, i.e., the interface between alloy layer A and lamellar region 13A, to the top end of acicular region 13B. That is, the thickness of alloy layer B is the sum of the thickness of lamellar region 13A and the height of acicular region 13B. Here, the "height of acicular region 13B" is the average height of 10 acicular crystals in a TEM image used to measure the coverage rate, as described below. If the average total thickness of alloy layer A and alloy layer B is less than 0.1 μm, the outer surface corrosion resistance of the plated steel sheet is insufficient. By setting the average total thickness to 0.1 μm or more, the coverage rate of alloy layer A and alloy layer B can be further improved, resulting in improved outer surface corrosion resistance, including welds, and improved plating appearance. If the average thickness of the alloy layer exceeds 3.0 μm, the amount of hard and brittle layers will increase, resulting in poor workability. By keeping the average total thickness of alloy layer A and alloy layer B at 3.0 μm or less, workability can be maintained even more favorably.

[0042] The constituent elements and surface coverage of each of the alloy layers A and B can be measured using a focused ion beam (FIB), a transmission electron microscope (TEM), and energy dispersive X-ray spectroscopy (EDS).

[0043] First, a measurement sample is cut out from the plated steel sheet of this embodiment using FIB, and the cross section of the sample is observed using TEM. A Cu mesh is used as the sample holding mesh during TEM observation. An example of a cross section is shown in FIG. 2. Electron diffraction and EDS are then performed at the points shown in FIG. 3 in the TEM image of FIG. 2, and an electron diffraction pattern and elemental analysis chart as shown in FIG. 4 are obtained. In the example shown in FIG. 3, point (a) corresponds to alloy layer A (Fe-Cr-Ni alloy), point (b) corresponds to the Cr-containing α-Fe steel sheet, and point (c) corresponds to alloy layer B (Fe-Ni-Sn-Zn alloy). In this way, the constituent elements of each of alloy layers A and B can be determined from the electron diffraction pattern and qualitative elemental analysis chart as shown in FIG. 3.

[0044] The coverage of the alloy layer A on the steel sheet surface is the length of the alloy layer A covering the steel sheet surface divided by the length along the steel sheet surface. That is, for example, in a cross-sectional image taken along the thickness direction as shown in Fig. 3, the coverage is the value obtained by dividing the coverage length of the alloy layer A (corresponding to (a)) by the length along the surface of the steel sheet (corresponding to (b)). When calculating the coverage, i.e., when determining the coverage length of the alloy layer A, the measurement reference length is 5 µm.

[0045] Here, when the present inventors investigated each plating condition in the molten plating process and the details of the alloy layer formed at the interface between the steel sheet / Sn-Zn plating layer C, it was found that when the heat load during the molten plating process was too high (for example, the plating bath temperature was high, the immersion time in the plating bath was long, etc.), the alloy layer A disappeared and a layer mainly composed of Fe-Cr-Ni-Zn (alloy layer D) was formed. Since the alloy layer D contains Zn, the alloy layer D shows a lower potential electrochemically than steel or Sn in the plating. Therefore, for example, in an organic acid environment generated by the oxidative degradation of fuel inside the fuel tank, or in a salt damage corrosion environment such as a snow melting salt spraying area outside the fuel tank or battery box or along the coastline, the Zn at the interface between the alloy layer D and the upper alloy layer B (Fe-Ni-Sn-Zn) elutes with each other, and it was found that corrosion progresses along the interface between the alloy layer D / alloy layer B. And it was found that the corrosion resistance of the plated steel sheet deteriorates as the ratio of the alloy layer D (layer mainly composed of Fe-Cr-Ni-Zn) increases. From the above, in the plated steel sheet of the present embodiment, it is effective for showing excellent corrosion resistance that the alloy layer A is a layer mainly composed of Fe-Cr-Ni, the alloy layer B is a layer mainly composed of Sn-Fe-Ni-Zn, and the coverage rate of the alloy layer A on the steel sheet surface is 70% or more.

[0046] <Sn-Zn plating layer> An Sn-Zn alloy plating layer C is provided on the alloy layer B. The average chemical composition of the Sn-Zn alloy plating layer C consists of 1.0 to 15.0% of Zn in mass%, and the balance of Sn and impurities.

[0047] When the amount of Zn in the Sn-Zn alloy plating layer C is less than 1%, the sacrificial corrosion prevention function of Zn for the steel sheet and the protective action of the zinc corrosion product cannot be sufficiently expected, so the corrosion resistance may deteriorate. Therefore, the lower limit value of the amount of Zn in the Sn-Zn alloy plating layer C is set to 1.0% or more. Preferably it is 1.2% or more, more preferably 2.0% or more, and still more preferably is 3.0% or more. If the Zn content in the Sn-Zn alloy plating layer C exceeds 16.0%, the barrier properties of Sn may not be fully exhibited, resulting in a deterioration in corrosion resistance. Therefore, the upper limit of the Zn content in the Sn-Zn alloy plating layer C is set to 15.0% or less, preferably 12.0% or less, more preferably 10.0% or less, and even more preferably 8.8% or less.

[0048] Good corrosion resistance can be obtained by setting the Zn content in the Sn-Zn alloy plating layer C in the range of 1.0 to 15%. Furthermore, when the Zn content in the Sn-Zn alloy plating layer C is 8.8% or less, primary crystals of Sn precipitate and Zn is finely dispersed, resulting in more preferable corrosion resistance.

[0049] The balance other than the Zn in the average chemical composition of the Sn-Zn alloy plating layer C is Sn and impurities. To improve corrosion resistance, the Sn-Zn alloy plating layer C may further contain one or more of Mg, Al, Mo, and W in a total amount of 1% or less. Impurities in the Sn-Zn alloy plating layer C refer to components contained in the raw materials or components mixed in during the manufacturing process, but not intentionally added. For example, trace amounts of components other than Fe may be mixed into the Sn-Zn alloy plating layer C as impurities due to mutual atomic diffusion between the steel sheet (base steel) and the plating bath.

[0050] The average chemical composition of the plating layer can be identified by the following method. First, the following mixed aqueous solution (stripping solution) was adjusted to pH 7-8 at 25°C, and the 2 The plating layer is stripped using anodic electrolysis. The end point is when the potential begins to change. After the plating layer has been stripped, nitric acid is added to the aqueous solution, and the solution is dissolved until it becomes transparent. The solution is then analyzed using ICP-MS analysis to identify the average chemical composition of the plating layer.

[0051] <Stripping solution>: Triethanolamine 100mL / L, glycolic acid 60mL / L, sodium bromate 20g / L, ammonium nitrate 50g / L.

[0052] The coating weight of the Sn-Zn alloy plating layer C is 5 to 80 g / m per side. 2 and more preferably 15 to 60 g / m 2 is.

[0053] Adhesion amount is 5g / m on one side 2 If the coating weight is less than 5g / m, good corrosion resistance cannot be ensured. 2 More than 12 g / m is preferable. 2 More preferably, 15 g / m 2 More preferably, 20 g / m 2 The coating amount may be 80 g / m or more. 2 If the coating weight exceeds 1000g / m², the material cost will increase. Furthermore, if the coating weight is excessively high, the coating thickness will vary, resulting in pattern defects and reduced weldability of the plated steel sheet. Therefore, the upper limit of the coating weight of the Sn-Zn alloy coating layer C is set at 80g / m². 2 The thickness should be less than 70 g / m. 2 Less than 60 g / m 2 More preferably 50 g / m or less 2 The following may also be used.

[0054] The coating weight of the Sn-Zn alloy plating layer C can be measured by X-ray fluorescence analysis (calibration curve method). The coating weight is evaluated for each side of the plated steel sheet. Therefore, when the Sn-Zn alloy plating layer C is provided on both sides of the steel sheet, it is preferable to measure the coating weight by X-ray fluorescence analysis. It is also preferable that the coating weight of the Sn-Zn alloy plating layer C is 5 to 80 g / m on at least one side of the steel sheet. 2 It is judged that the hot-dip Sn—Zn alloy plated steel sheet having the above coating mass satisfies the above requirements.

[0055] <Painting> The Sn-Zn alloy plated steel material of this embodiment can be used without painting, but by applying a coating according to the purpose, the corrosion resistance, formability, and design properties can be further improved.

[0056] When the Sn-Zn alloy-plated steel material of this embodiment is applied to, for example, a fuel tank for an automobile, the Sn-Zn plating layer may be damaged by welding or brazing during the manufacture of the fuel tank. In such cases, even higher rust resistance can be achieved by applying a corrosion-resistant coating to the Sn-Zn alloy-plated steel material.

[0057] The thickness of the anticorrosive coating can be, for example, 10 to 500 μm. When the anticorrosion coating is a black shower coat or electrodeposition coating, if the coating thickness is less than 10 μm, the coating barrier properties will be insufficient, and corrosion under the coating will easily progress. For this reason, a coating thickness of 10 μm or more is preferable.

[0058] Furthermore, to prevent paint film damage and corrosion due to chipping at the bottom of the fuel tank and further enhance rust resistance, a chipping-resistant coating can be applied to the Sn—Zn alloy-plated steel material of this embodiment. In this case, the thickness of the chipping-resistant coating can be 100 μm or more. However, if the coating film thickness exceeds 500 μm, the coating film may crack due to shrinkage during drying, resulting in saturation of corrosion resistance. Furthermore, if the coating film thickness is increased, the number of coating and drying processes must be increased to prevent cracking, which is not economical. For this reason, a coating film thickness of 500 μm or less is preferred.

[0059] Therefore, it is preferable that the thickness of the anticorrosion coating is 10 to 500 μm. As the coating method, in addition to the spray method and shower coating, electrodeposition coating method and the like can also be applied.

[0060] <Chemical conversion coating> By subjecting the Sn-Zn alloy-plated steel material of the present embodiment to chemical conversion treatment to provide a chemical conversion coating film, the corrosion resistance can be improved. Further, by providing a chemical conversion coating film as a base for black painting, the corrosion resistance can be further improved. Furthermore, by providing a chemical conversion coating film as a base for the coating film, the adhesion of the coating film can be further improved. As the chemical conversion treatment method, known techniques such as a chromate film of trivalent chromium containing no hexavalent chromium or a chromium-free chromate film can be used.

[0061] When the film thickness of the chemical conversion coating film is less than 0.02 μm, the outer surface corrosion resistance is insufficient. Therefore, it is preferable that the effective film thickness for exhibiting the corrosion resistance effect is 0.02 μm or more. Also, when the film thickness of the chemical conversion coating film exceeds 2.0 μm, the resistance welding property is inhibited. Therefore, it is preferable that the film thickness is 2.0 μm or less so as not to inhibit the resistance welding property.

[0062] <Lubricating coating film> Furthermore, in order to ensure the press formability during cold working such as press forming, an organic lubricating coating film may be formed directly above the Sn-Zn plating layer C of the Sn-Zn alloy-plated steel material of the present embodiment or on the chemical conversion coating film. In this case, the friction coefficient value of the lubricating coating film is preferably 0.15 or less. Considering the weldability, the film thickness of the lubricating coating film is preferably 2.0 μm or less.

[0063] <Surface roughness of Sn-Zn alloy-plated steel material> The surface roughness Ra of the Sn-Zn alloy-plated steel material may be 0.05 to 1.0 μm. By subjecting the Sn-Zn alloy-plated steel material after plating solidification or after forming the chemical conversion coating film to skin pass rolling, the surface roughness Ra of the Sn-Zn alloy-plated steel material can be controlled to be 0.05 to 1.0 μm. Thereby, good weldability can be obtained while maintaining the workability of the plated steel sheet. When the surface roughness Ra of the Sn-Zn alloy-plated steel material is less than 0.05 μm, the oil film of the lubricating oil breaks during the forming process using the plated steel material as a raw material, and the workability deteriorates. Therefore, the surface roughness Ra of the Sn-Zn alloy-plated steel material is preferably 0.05 μm or more, more preferably 0.1 μm or more. On the other hand, if the surface roughness Ra of the Sn-Zn plated steel material exceeds 1.0 μm, metal adhesion between the plated steel material and the mold increases, and workability deteriorates. Therefore, the surface roughness Ra of the Sn-Zn plated steel material is preferably 1.0 μm or less, and more preferably 0.5 μm or less.

[0064] <Rust prevention oil, lubricating oil> Furthermore, the surface of the Sn-Zn-based plating layer C of the Sn-Zn-based alloy-plated steel material or the surface of the chemical conversion coating may be coated with an anti-rust oil or a lubricating oil to improve rust resistance and formability.

[0065] <Chemical composition of steel (steel plate)> The chemical components of the steel sheet that is the base material of the Sn—Zn-plated steel sheet according to this embodiment will be described below. In the following description, the units of chemical components are expressed in "mass %."

[0066] C: 0.0005 to 0.030% C is an element that affects the strength of steel sheets. However, because C reduces the ductility of steel sheets and impairs press formability, it is preferable to keep the C content as low as possible. Furthermore, in Cr-containing steels, C is also an element that causes intergranular corrosion at welds and brazed joints. Therefore, it is preferable to limit the C content to a predetermined level. In this embodiment, the upper limit of the C content is set to 0.030% or less. The upper limit of the C content is preferably 0.020% or less, more preferably 0.010% or less, and even more preferably 0.008% or less. On the other hand, if the C content is less than 0.0005%, it becomes difficult to ensure the strength of the steel sheet and the cost of smelting increases. Therefore, the lower limit of the C content is set to 0.0005% or more. The lower limit of the C content is preferably 0.0008% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0067] Si:0.80% or less Si is a solid-solution strengthening element that acts on the strength of steel sheet. On the other hand, an excessively high Si content may reduce the ductility of the steel sheet and adversely affect hot-dip galvanizability. Therefore, it is preferable to limit the Si content to a certain amount or less. Therefore, the upper limit of the Si content is set to 0.80% or less. The upper limit of the Si content is preferably 0.60% or less, more preferably 0.50% or less, and even more preferably 0.40% or less. The lower limit of the Si content is not particularly specified and may be 0%. In consideration of refining costs, the lower limit of the Si content may be set to 0.001% or 0.01% or more. It is more preferably 0.05% or more, and even more preferably 0.10% or more.

[0068] Mn: 0.10 to 2.00% Mn, like Si, is an element that acts on the strength of steel sheet through solid solution strengthening. On the other hand, an excessively high Mn content may reduce the ductility of the steel sheet and adversely affect hot-dip galvanization. Therefore, it is preferable to limit the Mn content to a certain amount or less. If the Mn content is less than 0.10%, the above effects may not be obtained. On the other hand, if the Mn content exceeds 2.00%, press formability may be impaired and Mn oxides may be formed on the surface of the base steel sheet, impairing galvanization. Therefore, the Mn content is set to 0.10% to 2.00%. The upper limit of the Mn content is preferably 1.50% or less, more preferably 1.00% or less, and even more preferably 0.80% or less. The lower limit of the Mn content is preferably 0.15% or more, more preferably 0.20% or more, and even more preferably 0.40% or more.

[0069] P: 0.005 to 0.040% P is a solid solution strengthening element that acts on the strength of steel sheet. It is also effective in improving corrosion resistance in some salt damage environments. On the other hand, P is an element that reduces ductility and also an element that segregates to grain boundaries and deteriorates secondary work embrittlement resistance. Therefore, the upper limit of the P content is set to 0.040% or less. The upper limit of the P content is preferably 0.030% or less, more preferably 0.025% or less, and even more preferably 0.020% or less. On the other hand, if the P content is less than 0.005%, the effect of improving the strength and corrosion resistance of the base steel sheet is poor. Therefore, the lower limit of the P content is set to 0.005% or more. The lower limit of the P content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.018% or more.

[0070] S: 0.0100% or less S is an impurity element that is mixed into steel during refining. S also combines with Mn and Ti to form precipitates, degrading workability. Therefore, the S content is set to 0.0100% or less. The upper limit of the S content is preferably 0.0080% or less, more preferably 0.0060% or less, and even more preferably 0.0040% or less. The base steel sheet of the Sn-Zn alloy-plated steel sheet according to this embodiment does not need to contain S. Therefore, the S content may be 0%. However, reducing the S content to less than 0.0005% increases production costs. Therefore, the S content is preferably set to 0.0005% or more. The lower limit of the S content is more preferably 0.0007% or more, even more preferably 0.0008% or more, and even more preferably 0.0010% or more.

[0071] Cr: 4.0 to 18.0% Cr is an important element for ensuring the corrosion resistance of the base steel sheet. The higher the Cr content, the more effective it is in improving corrosion resistance. Therefore, the lower limit of the Cr content is set to 4.0% or more. If the Cr content is less than 4.0%, there is a concern that sufficient salt corrosion resistance may not be obtained, particularly at welds and cut end surfaces, even if the Sn-Zn-based alloy plating according to this embodiment is applied. The lower limit of the Cr content is preferably 5.0% or more, more preferably 6.0% or more, and even more preferably 7.0% or more. The lower limit of the Cr content may also be set to 8.0% or more, 10.5% or more, or 12.0% or more. On the other hand, Cr is also an element that reduces the ductility of steel sheets, so it is preferable to limit the Cr content to a certain amount or less. Specifically, if the Cr content exceeds 18.0%, cold workability in press forming and the like decreases and material costs increase. Therefore, the Cr content is set to 18.0% or less. The upper limit of the Cr content may be preferably 15.0% or less, more preferably 13.0% or less, and even more preferably 11.0% or less. Furthermore, when press-forming the base steel sheet into a complex shape, such as a saddle-shaped tank, it is necessary to ensure higher cold workability. In such cases, it is preferable to set the upper limit of the Cr content to less than 10.5%.

[0072] In the Sn-Zn alloy plated steel sheet according to this embodiment, the above-mentioned components of the steel sheet may further contain one or more of Al: 0.01 to 0.30%, Ti: 0.010 to 0.300%, Nb: 0.001 to 0.040%, B: 0.0002 to 0.0030%, and N: 0.0010 to 0.0300%. However, since the hot-dip Sn-Zn alloy plated steel sheet according to this embodiment can solve the problem even if the steel sheet does not contain these components, the lower limit of these elements is 0%.

[0073] Al: 0 to 0.30% Al is an element used as a deoxidizer during steel refining. When the Al content is 0.01% or more, a deoxidizing effect is obtained. However, when the Al content exceeds 0.30%, it may lead to a decrease in the toughness of the weld and a decrease in workability. Therefore, the upper limit of the Al content is set to 0.30%.

[0074] Ti: 0 to 0.300% Ti has a strong affinity with C and N, and forms carbonitrides to inhibit intergranular corrosion. Furthermore, Ti reduces the amount of dissolved C and N in steel, improving the workability of the steel sheet. This effect is enhanced when the Ti content is 0.010% or more. On the other hand, when the Ti content exceeds 0.300%, the ductility of the steel sheet decreases, and the strength and toughness of the weld may also decrease. Therefore, the upper limit of the Ti content is set to 0.300%.

[0075] Nb: 0 to 0.040% Like Ti, Nb has a strong affinity with C and N, and forms carbonitrides to inhibit intergranular corrosion. Furthermore, Nb reduces the amount of dissolved C and N in steel, thereby improving the workability of the steel sheet. This effect is enhanced when the Nb content is 0.001% or more. On the other hand, if the Nb content exceeds 0.040%, the ductility of the steel sheet decreases, and the strength and toughness of the weld may also decrease. Therefore, the upper limit of the Nb content is set to 0.040%.

[0076] B: 0 to 0.0030% B is an element that segregates at grain boundaries to increase grain boundary strength and improve secondary work embrittlement resistance. When the B content is 0.0002% or more, this effect is obtained, so the lower limit of the B content may be set to 0.0002% or more. More preferably, the lower limit of the B content is 0.0003% or more. On the other hand, when the B content exceeds 0.0030%, the ductility of the steel sheet decreases, and the strength and toughness of the weld may decrease. Furthermore, when the B content is excessive, boride formation may decrease the corrosion resistance. Therefore, the upper limit of the B content is set to 0.0030% or less. The upper limit of the B content is more preferably 0.0020% or less.

[0077] N: 0 to 0.0300% N is an impurity element that is mixed into steel during refining. N also forms nitrides of Ti, Al, and Nb, and affects workability. Therefore, when N is added, it is preferable to limit the N content to a certain amount or less so as not to reduce workability. Specifically, the N content is set to 0.0300% or less. On the other hand, reducing the N content to less than 0.0010% increases manufacturing costs. Therefore, the lower limit of the N content may be set to 0.0010% or more.

[0078] More preferable corrosion resistance can be obtained by further containing at least one of Cu: 0.01 to 2.0%, Ni: 0.01 to 3.0%, Mo: 0.01 to 2.00%, and V: 0.01 to 2.00% in the steel sheet serving as the base material for the Sn-Zn alloy-plated steel sheet according to this embodiment. However, since the hot-dip Sn-Zn alloy-plated steel sheet according to this embodiment can obtain excellent corrosion resistance even if the base steel sheet does not contain these elements, the lower limit of these elements is 0%.

[0079] Cu: 0 to 2.0% Cu is an element effective in improving the corrosion resistance of steel sheets. This effect is exhibited when the Cu content is 0.01% or more, so the lower limit of the Cu content may be set to 0.01% or more. The lower limit of the Cu content is more preferably 0.03% or more. Furthermore, if the Cu content is too high, it may have an adverse effect on brittleness during hot rolling. Therefore, the upper limit of the Cu content is set to 2.0% or less. The upper limit of the Cu content is more preferably 1.5% or less.

[0080] Ni: 0 to 3.0% Ni is an element effective in improving the corrosion resistance of steel sheets. This effect is exhibited when the Ni content is 0.01% or more, so the lower limit of the Ni content may be set to 0.01% or more. The lower limit of the Ni content is more preferably 0.03% or more. Furthermore, if the Ni content is too high, it may have an adverse effect on ductility and weld toughness. Therefore, the upper limit of the Ni content is set to 3.0% or less. The upper limit of the Ni content is more preferably 2.0% or less.

[0081] Mo: 0 to 2.00% Mo is an element effective in improving the corrosion resistance of steel sheets. This effect is exhibited when the Mo content is 0.01% or more, so the lower limit of the Mo content may be set to 0.01% or more. However, since an excessively high Mo content may reduce ductility, the upper limit of the Mo content is set to 2.00% or less.

[0082] V: 0 to 2.00% Like Mo, V is an element effective in improving the corrosion resistance of steel sheets. This effect is exhibited when the V content is 0.01% or more, so the lower limit of the V content may be set to 0.01% or more. Furthermore, too much V may adversely affect ductility. Therefore, the upper limit of the V content is set to 2.00% or less.

[0083] The chemical composition of the base steel sheet constituting the Sn-Zn alloy-plated steel sheet of this embodiment, excluding the above elements, includes Fe and impurities. Impurities refer to elements derived from steel raw materials and / or mixed in during the steelmaking process, and are not intentionally contained in the base steel sheet. Furthermore, impurities are elements that are permissible within a range that does not impair the properties of the Sn-Zn alloy-plated steel sheet of this embodiment.

[0084] [Manufacturing method for Sn-Zn alloy plated steel sheets] Next, a method for producing the Sn—Zn alloy plated steel sheet of this embodiment will be described. The Sn—Zn alloy plated steel sheet of this embodiment can be produced by either an immersion hot-dip galvanizing method (batch hot-dip galvanizing method) or a continuous hot-dip galvanizing method.

[0085] The method for producing an Sn-Zn-based alloy-plated steel sheet of this embodiment includes a pickling step of electrolytically pickling a base steel sheet having the above-described chemical composition to obtain a pickled steel sheet, a pre-plating step of forming a Ni plating, a Ni-Fe plating or an Fe-Ni plating on at least one side of the pickled steel sheet to obtain a pre-plated steel sheet, and a plating step of subjecting the pre-plated steel sheet to an Sn-Zn-based plating treatment to obtain an Sn-Zn-based plated steel sheet.

[0086] The base steel sheet to be subjected to the pickling step may be manufactured as follows, for example. First, a slab having the above-described chemical composition is cast. Specifically, the slab is formed by casting molten steel adjusted to have the above-described chemical composition. The slab casting method is not particularly limited. The obtained slab is then hot-rolled to form a hot-rolled sheet. The hot-rolled sheet may be subjected to hot-rolled sheet annealing. Next, the hot-rolled sheet or the hot-rolled annealed sheet is pickled and then cold-rolled to form a cold-rolled steel sheet of a predetermined thickness. In order to prevent cracks and the like in the steel sheet during cold rolling, intermediate annealing may be performed between cold rolling steps. Furthermore, the obtained cold-rolled steel sheet may be annealed. In this manner, a base steel sheet to be used in the manufacturing method of the Sn—Zn-based alloy plated steel sheet of this embodiment is prepared. Each step will be described in detail below.

[0087] <Acid washing process> In the pickling process, the cold-rolled steel sheet or the cold-rolled annealed steel sheet is subjected to electrolytic pickling. For example, a cold-rolled steel sheet or a cold-rolled annealed steel sheet is immersed in a pickling solution that is a sulfuric acid aqueous solution containing nitrates and / or sulfates and also fluorosilicates and / or fluoroborates to dissolve the oxide film on the surface of the steel sheet. Thereafter, any residue deposits produced by the dissolution are removed, for example, with a brush or spray, and the steel sheet is then rinsed with water and dried to prepare the surface condition.

[0088] The sulfuric acid used as the main agent in the pickling solution should have a concentration of 50 to 300 g / L. If the sulfuric acid concentration is less than 50 g / L, the pickling efficiency may decrease. If the pickling concentration is more than 300 g / L, the pickling may become excessively pickled, which may have a negative effect on plating properties.

[0089] Nitrates that can be used include sodium nitrate, potassium nitrate, and ammonium nitrate, and the concentration of the nitrate should be 50 to 200 g / L. If the concentration is less than 50 g / L, no effect can be obtained, while if the concentration is more than 200 g / L, the effect becomes saturated.

[0090] Sodium sulfate or the like can be used as the sulfate, and the sulfate concentration is preferably 50 to 200 g / L. If the sulfate concentration is less than 50 g / L, no effect can be obtained, while if the sulfate concentration is more than 200 g / L, the effect becomes saturated. When both nitrate and sulfate are used, the total concentration of nitrate and sulfate should be 100 to 200 g / L.

[0091] Examples of fluorosilicates and fluoroborates that can be used include fluorosilicates such as sodium fluorosilicate and potassium fluorosilicate, and fluoroborates such as sodium fluoroborate and ammonium fluoroborate. The pickling solution preferably contains 5 to 100 g / L of one or more fluorosilicates and fluoroborates in total. If the total content of one or more fluorosilicates and fluoroborates is less than 5 g / L, they will not contribute to improving the oxide film removal rate, while if the total content exceeds 100 g / L, the effect will be saturated. The total content of one or more fluorosilicates and fluoroborates is preferably 10 g / L or more.

[0092] The steel sheet to be subjected to the pickling step may be subjected to surface finishing using shot blasting, abrasive brushes, or the like before pickling.

[0093] <Pre-plating process> Next, the pre-plating step will be described. A metal coating layer (pre-plated layer) mainly made of Ni plating, Ni-Fe plating or Fe-Ni plating is formed on the steel sheet after pickling (pickled steel sheet).

[0094] A metal coating layer mainly composed of Ni (Ni plating) can be formed by immersing a pickled steel sheet in an electrolytic bath (Watts bath) containing Ni sulfate, Ni chloride, and boric acid as its main components, after adjusting the pH of the bath with sulfuric acid. Alternatively, a Ni-Fe-based metal coating layer (Ni-Fe plating) can be formed by using a bath in which ferrous sulfate is added to the Watts bath. The proportion of Fe in the Ni-Fe plating is preferably 10 to less than 50% by mass.

[0095] From the viewpoint of plating adhesion, it is effective to adjust the pH of the electrolytic bath to 2.5 or less. If the pH of the electrolytic bath exceeds 2.5, the etching power on the steel sheet surface decreases, and oxides or hydroxides are formed at the interface between the pre-plating and the base steel sheet, which may result in deterioration of plating adhesion. The pH of the electrolytic bath is preferably 1.5 or less.

[0096] The amount of Ni plating or Ni-Fe plating per side is 0.1 to 3.0 g / m2 in terms of metal. 2 The coating amount is preferably 0.1 g / m 2 If the coating weight is less than 3.0 g / m, the coating ability is insufficient, and a uniform Sn-Fe-Ni-Zn alloy layer (alloy layer B) is not formed, resulting in insufficient corrosion resistance. 2 If the thickness exceeds 1000 nm, the effect of improving corrosion resistance will be saturated and excessively thick alloy layer A and / or alloy layer B will be formed at the interface between the Sn-Zn alloy plated layer C and the steel sheet, which may result in reduced plating adhesion during forming using the Sn-Zn alloy plated steel sheet as a material.

[0097] In addition to the Ni plating and Ni-Fe plating, the pre-plating layer of this embodiment may also be an Fe-Ni plating. Specifically, before the plating process (Sn-Zn plating) described below, an Fe-Ni-based metal coating layer (Fe-Ni plating) containing Fe as the main component is formed on the pickled steel sheet. This promotes the formation of an Sn-Fe-Cr layer (alloy layer A) and an Sn-Fe-Ni-Zn layer (alloy layer B) in the plating process, further improving the appearance of the Sn-Zn-based plating layer C and refining the primary Sn to improve corrosion resistance. The Fe-based metal coating layer (Fe-Ni plating) can be formed using a bath containing ferrous sulfate, nickel sulfate, nickel chloride, and boric acid as the main components.

[0098] <Plating process> The steel sheet (pre-plated steel sheet) on which the above-described metal coating layer (pre-plating) has been formed is subjected to an Sn—Zn-based hot-dip plating process to form an Sn—Zn-based plating layer C. The Sn—Zn-based plating layer C is formed by a hot-dip plating method. Either a flux method or a Sendzimir method can be suitably used to form the Sn—Zn-based plating layer C.

[0099] The bath temperature of the Sn-Zn-based plating bath should be less than 260°C. If the bath temperature is 260°C or higher, the alloy layer will grow excessively, resulting in the formation of an Fe-Ni-Cr-Zn layer (alloy layer D) that contributes to a deterioration of corrosion resistance. Preferably, the bath temperature is 255°C or lower. The lower limit of the bath temperature varies depending on the Zn content in the plating. For example, the bath temperature is preferably 25°C or higher than the melting point of the plating layer.

[0100] The bath composition of the Sn-Zn-based plating bath may be adjusted appropriately to obtain the chemical composition of the Sn-Zn-based plating layer C described above. For example, the Zn concentration in the bath composition may be 1% to 15%. If the Zn concentration in the bath composition is less than 1%, the sacrificial rust prevention effect of Zn may be insufficient, and the protective effect due to the formation of Zn corrosion products may be insufficient, resulting in a deterioration of corrosion resistance. Furthermore, if the Zn concentration in the bath composition is more than 15%, the plating layer may become porous when corroded in a corrosive environment, resulting in a deterioration of corrosion resistance.

[0101] The temperature of the steel sheet before immersion in the Sn-Zn-based plating bath (immersion temperature) is preferably 50°C or higher. If the immersion temperature is less than 50°C, the reaction with the flux will be insufficient, resulting in insufficient removal of surface oxides, which may result in unplated areas and reduced corrosion resistance. Preferably, the immersion temperature is 60°C or higher. As described above, by appropriately controlling the bath temperature and immersion temperature, it is possible to form the desired Sn-Fe-Cr layer (alloy layer A) and Sn-Fe-Ni-Zn phase (alloy layer B) between the base steel sheet and the Sn-Zn-based plating layer C.

[0102] When the Sn-Zn-based plating layer C is formed by the flux method, the temperature of the steel sheet before immersion in the plating bath can be controlled by adjusting the temperature of the flux applied to the pre-plated steel sheet before immersion in the Sn-Zn plating bath.

[0103] In the flux method, a pre-plated steel sheet with the above-mentioned metal coating layer is coated with a flux solution and then immersed in a plating bath to perform hot-dip plating. Specifically, a 2-45 mass% flux solution (equivalent to halogen) is applied to the pre-plated steel sheet, and the steel sheet is then immersed in the plating bath. The flux method effectively removes the oxide film on the pre-plated surface before forming the specified alloy layers A and B, which is highly effective in suppressing plating repelling. Fluxes containing chlorides such as ZnCl2, NH4Cl, and HCl, or bromides such as ZnBr2 and NH4Br, are effective. Furthermore, by applying a solution of 1 to 10% diluted hydrochloric acid to the pre-plated steel sheet before applying the flux solution, the plating appearance can be further improved.

[0104] The temperature of the flux solution is set to 50°C or higher. By setting the temperature of the flux solution to 50°C or higher, it is possible to increase the temperature of the steel sheet after applying the flux solution, thereby accelerating the removal of the oxide film on the steel sheet surface and the alloying reaction. There is no particular upper limit to the temperature of the flux solution, but it may be set to 90°C or lower, for example. If the temperature of the flux solution is higher than 90°C, the amount of evaporation of the flux increases, making the flux solution difficult to handle, which is not preferable.

[0105] Furthermore, when the Sn—Zn-based coating layer C is formed by the Sendzimir hot-dip coating method (Sendzimir method), the temperature of the steel sheet before immersion in the coating bath can be controlled by adjusting the temperature conditions in the snout.

[0106] In the Sendzimir process, a general annealing and plating facility consisting of a heating zone, a soaking zone, a cooling zone, a plating process, etc. may be used. The temperature of the soaking zone is preferably 700 to 870°C, where a recrystallized structure can be obtained, with a dew point of -20°C or less. The atmosphere in the soaking zone is preferably an N2-H2 atmosphere, with the H2 concentration adjusted to between 1% and 100%.

[0107] In the Sendzimir process, before immersing the pre-plated steel sheet in the plating bath, the immersion temperature of the steel sheet is adjusted in a cooling zone to the same temperature as the bath temperature, and then the steel sheet is immersed in the plating bath. In the Sendzimir process, it is advisable to request that the bath temperature of the plating bath be less than 260°C.

[0108] After the plating process, the plating coating weight is reduced to 5 to 80 g / m per side by air wiping or other means. 2 Adjust to.

[0109] By going through the above steps, a layer (alloy layer A) mainly composed of Ni-Fe-Cr with a surface coverage rate of 70% or more and a layer (alloy layer B) mainly composed of Sn-Fe-Ni-Zn can be formed on alloy layer A.

[0110] The manufacturing method of this embodiment as described above makes it possible to manufacture a Sn—Zn-plated steel sheet that has a good plating appearance as a product and has high corrosion resistance and excellent formability suitable for the automotive field, particularly for fuel tank applications. Furthermore, the manufacturing method of this embodiment makes it possible to stably supply the Sn—Zn-plated steel sheet of this embodiment.

[0111] [Battery case] The Sn—Zn plated steel sheet of this embodiment is suitable for use in, for example, battery cases to be mounted in automobiles. Fig. 6 is a perspective view showing an example of a lower part 100 of a battery case.

[0112] The lower part 100 shown in FIG. 6 is merely an example, and the shape, dimensions, and the like are not limited thereto. Furthermore, even if the Sn-Zn-plated steel sheet of this embodiment is processed into the shape of the battery case shown in FIG. 6 , this does not affect the various requirements constituting the Sn-Zn-plated steel sheet of this embodiment described above. When obtaining a battery case, a base steel sheet having the above-described chemical composition may be processed into the desired lower shape, and then a welding process to the upper side and a painting process may be performed. Furthermore, although not shown, the Sn-Zn-plated steel sheet of this embodiment can also be suitably applied to the upper parts of a battery case, similar to the lower part 100.

[0113] The lower member 100 has excellent corrosion resistance, particularly outer surface corrosion resistance, because it includes the Sn—Zn plated steel sheet of this embodiment.

[0114] [Fuel Tank] The Sn—Zn plated steel sheet of this embodiment can also be suitably used for, for example, fuel tanks to be mounted on automobiles. Fig. 7 is a perspective view showing an example of a fuel tank 200.

[0115] The fuel tank 200 includes, for example, an upper tank part 201, a lower tank part 202, a separator 203, and a sub-tank 204. The Sn—Zn-plated steel sheet of this embodiment is suitable for use in the upper tank part 201, the lower tank part 202, the separator 203, and the sub-tank 204.

[0116] The fuel tank 200 shown in Fig. 7 is merely an example, and the shape, dimensions, etc. are not limited thereto. Furthermore, even if the Sn-Zn-plated steel sheet of this embodiment is processed into the shape of the fuel tank shown in Fig. 7, the above-described requirements for constituting the Sn-Zn-plated steel sheet of this embodiment are not affected. Note that, when obtaining the fuel tank 200, the above-described pre-plating step, plating step, etc. may be carried out after a base steel sheet having the above-described chemical composition is processed into the shapes of the components constituting the fuel tank 200.

[0117] The fuel tank 200 has an Sn—Zn plated steel sheet according to this embodiment, and therefore has excellent corrosion resistance, particularly corrosion resistance on the inner surface. [Example]

[0118] The Sn-Zn-plated steel sheet according to one embodiment of the present invention and a suitable method for producing the same will be described in more detail below with reference to examples. Note that the examples shown below are merely examples of the Sn-Zn-plated steel sheet according to this embodiment, and the Sn-Zn-plated steel sheet according to this embodiment is not limited to the examples shown below. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0119] Example 1 Steels having the compositions shown in Table 1 below were melted and hot-rolled, pickled, and cold-rolled to produce cold-rolled steel sheets with a thickness of 0.8 mm. After annealing, these cold-rolled steel sheets were subjected to a pre-plating process and a Sn-Zn plating process under the conditions shown in Table 2.

[0120] (pickling process) In the electrolytic pickling bath shown below, the steel sheet side is set as the cathode and the current is 10A / dm 2 The steel sheet surface was then rinsed with water and dried to prepare a pickled steel sheet.

[0121] Electrolytic pickling treatment bath: sulfuric acid (120 g / L) + sodium nitrate (100 g / L) + sodium sulfate (120 g / L) + hexafluorosilicic acid (15 g / L), bath temperature 50°C.

[0122] (Pre-plating process) Pickled steel sheet is plated with Fe-Ni or Ni at 1g / m 2 After that, the following Sn-Zn plating step was carried out.

[0123] For Fe-Ni plating, the following Fe-Ni plating bath was adjusted to pH 1.5 with sulfuric acid, and then the steel sheet was placed in the bath with a current of 10 A / dm 2 After electrolysis at 4000 kJ / min for 0.12 seconds, the steel sheet surface was rinsed with water and dried. The composition of the Fe-Ni plating was 25 mass % Ni, with the remainder being Fe and impurities.

[0124] Fe-Ni plating bath: ferrous sulfate 110g / L, nickel sulfate 75g / L, nickel chloride 140g / L, boric acid 30g / L, bath temperature 45℃.

[0125] For Ni plating, the following Ni plating bath is adjusted to pH 4.0 or 1.5 with sulfuric acid, and then the steel sheet is placed in the bath with a current of 10 A / dm 2 After electrolytic treatment at 100°C for 0.11 seconds, the steel sheet surface was washed with water and dried. The composition of the Ni plating was 100% by mass of Ni.

[0126] Ni plating bath: nickel sulfate 75g / L, nickel chloride 140g / L, boric acid 30g / L, bath temperature 45℃.

[0127] (Sn-Zn plating process) After the pre-plating process, the Sn-Zn plating process was carried out using either the flux method or the Sendzimir method. A ZnCl2-NH4Cl aqueous solution was roll-applied as the flux solution. The Zn composition of the plating bath was adjusted as shown in Table 2. The bath temperature and the sheet temperature when immersing the steel sheet in the bath were adjusted as shown in Table 2. The steel sheet was immersed for 8 seconds in the flux method and for 3 seconds in the Sendzimir method, and then the coating weight was adjusted by N2 gas wiping.

[0128] The structures and coating weights of alloy layer A, alloy layer B, and plating layer C in the obtained plated steel sheet are shown in Table 3. The coverage of alloy layer A is the ratio of the surface of the steel material covered by alloy layer A, and the area other than alloy layer A was alloy layer D.

[0129] Next, the corrosion resistance and workability were evaluated as follows.

[0130] <Corrosion resistance evaluation> The corrosion resistance was evaluated by the following combined cycle test.

[0131] (x1) External corrosion resistance: As shown in the schematic diagram (plan view) of the corrosion resistance test specimens in Figure 5, 70 × 150 mm flat plates and specimens in which a 35 × 100 mm plate was three-point spot-welded to a 70 × 150 mm plate were evaluated according to JASO (Society of Automotive Engineers of Japan) M610-92 "External Corrosion Test Method for Automotive Parts." The 70 × 150 mm flat plates were sealed at their edges and backsides. In Figure 5, the three-point spot weld is shown as spot weld Y, and the seals on the edges and backside are shown as seal Z. The 35 × 100 mm plate had no seals. Evaluation was based on the rust area ratio of the flat plate and the edge of the 35 × 100 mm plate. Samples with edge grades A and B were deemed acceptable. Samples with a grade C were also deemed acceptable because they could be used well if painted.

[0132] [Evaluation criteria] Test period: 360 cycles (120 days)

[0133] [Evaluation criteria] (Evaluated by rust area ratio) A: Red rust occurrence less than 0.1% B: Red rust occurs at 0.1% to less than 1% or white rust occurs (white rust less than 20%) C: Red rust occurs at 1% or more but less than 5% or white rust is noticeable (white rust occurs at 20% or more but less than 70%) X: Red rust occurs on 5% or more or white rust is noticeable (white rust occurs on over 70%)

[0134] (x2) Internal corrosion resistance: Test pieces measuring 110 mm x 110 mm were cut from the resulting plated steel sheets, and flanged cups were fabricated using these test pieces by cylindrical deep drawing with a φ50 mm punch. A total of 50 ml of a 10% by mass aqueous solution of gasoline containing 500 ppm formic acid, 1000 ppm acetic acid, and 165 ppm NaCl was sealed in the cup and left in a constant temperature bath at 45°C for 1000 hours. After the test, the samples were visually inspected to determine whether or not red rust had formed on the bottom of the cup. Samples rated A and B were deemed to have passed.

[0135] [Evaluation criteria] A: No rust B: White rust occurs X: Red rust occurs

[0136] <Processability evaluation> The workability was evaluated by a cylindrical deep drawing test. (y1) Cylindrical deep drawing test: Cylindrical deep drawing was performed using a flat-bottom cylindrical die with a punch diameter of 50 mm. Noxrust 530-F40 (manufactured by Nihon Parkerizing) was used as the lubricant, and the blank holding pressure was 700 kgf. The maximum possible drawing ratio (blank diameter ÷ punch diameter) and the plating appearance of the processed area were evaluated. Samples with ratings of A, B, and C were deemed to have passed.

[0137] [Evaluation criteria] A: Formable, no defects in the plating layer, drawing ratio of 2.3 or more B: Formable, no defects in the plating layer, drawing ratio of 2.2 or more C: Formable, no defects in the plating layer, drawing ratio of 2.0 or more X: Forming is possible, but the drawing ratio is less than 2.0 or galling occurs in the plating layer.

[0138] [Table 1]

[0139] [Table 2]

[0140] [Table 3]

[0141] The evaluation results are shown in Table 3. Note that the underlines in Tables 2 and 3 indicate that the product is outside the range of this embodiment, that the production conditions are not within the preferred range, or that the characteristic value is not preferred.

[0142] As shown in Table 3, the performance of all of the invention examples C1 to C15 was good. On the other hand, the comparative examples c1 to c8 were outside the preferred manufacturing conditions of the present invention, and at least one of the surface coverage of the alloy layer A (first alloy layer), the Zn content of the plating layer C (Sn—Zn-based plating layer), and the coating weight per side of the plating layer C was outside the range of this embodiment. As a result, at least one of the corrosion resistance and the workability was deteriorated in the comparative examples c1 to c8.

[0143] <Example 2> Next, using steel No. A1 shown in Table 1, a Sn-Zn based plating process was carried out by manufacturing method No. B3 shown in Table 2. After that, a chemical conversion coating was formed as shown in Table 4. For all of Nos. D1 to D6, a chromate-free coating was formed on the plating layer C as the chemical conversion coating. Specifically, trivalent chromium, silica (SiO2), phosphate ions (PO4 3+ A chromate-free coating was formed by applying a chemical agent mainly containing ) onto the plating layer and baking it at 80°C in a hot air drying oven. Furthermore, for Nos. D4 to D6, an additional coating was formed on top of the chemical conversion coating. For Nos. D4 to D5, "Amilac 1000 (manufactured by Kansai Paint Co., Ltd.)" was used as the coating material, and the coating was formed by baking for 20 minutes in an oven at 130°C. For No. D6, "Hi-Rubber E Super (manufactured by Nippon Paint Co., Ltd.)" was used as the coating material, and the coating was formed by applying multiple coats.

[0144] Table 4 shows the composition and coating weight of each of the alloy layer A, alloy layer B, and plating layer C in the obtained plated steel sheet.

[0145] Next, the corrosion resistance and workability were evaluated in the same manner as above. The evaluation results are shown in Table 4. Note that the underlined values ​​in Table 4 indicate that the values ​​are outside the range of this embodiment, that the manufacturing conditions are not preferable, or that the characteristic values ​​are not preferable.

[0146] As shown in Table 4, the performance of all of the invention examples D1 to D6 was good.

[0147] [Table 4]

[0148] Example 3 Next, Invention Examples C2 and C14 shown in Table 3 were processed into the shapes of the lower section and fuel tank shown in Figures 6 and 7, and then test specimens were cut out from the center of the flat portion on the bottom or top surface. In the same manner as above, the configurations and coating weights of alloy layer A, alloy layer B, and plating layer C in the plated steel sheets, as well as the corrosion resistance and formability, were evaluated. As a result, the battery cases and fuel tanks in which the configurations of the plated steel sheets were within the range of this embodiment performed well. [Industrial Applicability]

[0149] According to the above-described aspect of the present invention, a hot-dip Sn-Zn alloy-plated steel sheet having excellent coating appearance, corrosion resistance, and workability can be obtained. Therefore, the obtained hot-dip Sn-Zn alloy-plated steel sheet is suitable for use in the automotive field, particularly in automobiles suitable for fuel tanks and battery cases (particularly automobile fuel tanks and battery cases), home appliances, building materials, and the like, and therefore has high industrial applicability. [Explanation of symbols]

[0150] 10...Sn-Zn alloy plated steel (plated steel) 11...Steel material 12...First alloy layer (alloy layer A) 13...Second alloy layer (alloy layer B) 13A...Layered region 13B…Acicular region 14...Sn-Zn based plating layer (Sn-Zn based plating layer C, plating layer C) 100...Roa 200…Fuel tank 201...Tank upper 202...Tank lower 203...Separator 204...Subtank

Claims

1. Steel and a first alloy layer mainly composed of Fe, Cr, and Ni on a surface of the steel material; a second alloy layer mainly composed of Sn, Fe, Ni, and Zn on the first alloy layer; A Sn-Zn based plating layer mainly composed of Sn and Zn is formed on the second alloy layer. and a surface coverage rate of the first alloy layer on the surface of the steel material is 70% or more; The second alloy layer is FeSn ２ phase, and Fe partially substituted with Ni ３ Zn １０ an alloy layer mainly composed of at least one of the phases, The Sn—Zn-based plating layer is composed of, by mass%, 1.0 to 15.0% Zn, the remainder being Sn and impurities; The total amount of the first alloy layer, the second alloy layer, and the Sn—Zn-based plating layer is 5 to 80 g / m per side. ２ is The Sn-Zn alloy plated steel material is characterized by the above.

2. The steel material comprises, in mass%, C: 0.0005-0.030%, Si: 0.80% or less, Mn: 0.10-2.00%, P: 0.005-0.040%, S: 0.0100% or less, Cr: 4.0-18.0%, Al: 0-0.30%, Ti: 0-0.300%, Nb: 0 to 0.040%, B: 0 to 0.0030%, N: 0 to 0.030%, Cu: 0-2.0%, Ni: 0-3.0%, Mo: 0-2.00%, V: 0 to 2.00%, The Sn—Zn based alloy plated steel material according to claim 1, characterized in that it contains the above and the balance is Fe and impurities.

3. The steel material further comprises, in mass%, REM: 0-1.000%, The Sn—Zn based alloy plated steel material according to claim 2, characterized in that it contains

4. 3. The Sn—Zn alloy plated steel material according to claim 1, wherein a chemical conversion coating having a thickness of 0.02 to 2.0 μm is provided on at least one surface of the Sn—Zn alloy plated layer.

5. 3. The Sn—Zn alloy plated steel material according to claim 1, wherein a coating film having a thickness of 10 to 500 μm is provided on at least one surface of the Sn—Zn alloy plated layer.

6. 4. The Sn—Zn alloy plated steel material according to claim 3, wherein a coating film having a thickness of 10 to 500 μm is provided on at least one surface of the Sn—Zn alloy plated layer.

7. A battery case comprising the Sn—Zn-based alloy plated steel material according to claim 1.

8. A fuel tank comprising the Sn—Zn-based alloy plated steel material according to claim 1.

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