Sn-Zn alloy-plated steel material, battery case, and fuel tank

Abstract

This Sn-Zn alloy-plated steel material has a steel material, a first alloy layer formed on the surface of the steel material and mainly composed of Fe, Cr and Ni, a second alloy layer formed on the first alloy layer and mainly composed of Sn, Fe, Ni and Zn, and an Sn-Zn plating layer formed on the second alloy layer and mainly composed of Sn and Zn, the surface coating rate of the first alloy layer on the surface of the steel material being 70% or more, and the surface coating rate of the second alloy layer on the surface of the steel material being 70% or more. The second alloy layer is an alloy layer mainly composed of at least one of a FeSn2 phase and a Fe3Zn10 phase substituted by a Ni portion, the Sn-Zn plating layer contains 1.0-15.0% by mass of Zn and the balance of Sn and impurities, and the total amount of the first alloy layer, the second alloy layer, and the Sn-Zn plating layer is 5-80 g / m2 per surface.

Description

Technical Field

[0001] This invention relates to Sn-Zn alloy-coated steel, battery casings, and fuel tanks. In particular, it relates to Sn-Zn alloy-coated steel with excellent coating appearance, corrosion resistance, and processability.

[0002] 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

[0003] In recent years, the use of steel sheets for automobiles has been continuously developing towards higher strength, with the aim of improving fuel efficiency through weight reduction. Similarly, in fuel tank steel sheets, due to the need for lighter tanks, more complex vehicle body designs, and considerations for fuel tank storage locations, the shapes of fuel tanks are becoming increasingly complex, demanding excellent formability and high strength.

[0004] In the past, in order to meet the requirements of balancing formability and high strength, high-strength IF steel was developed, which is an interstitial free steel with carbonitride forming elements such as Ti and Nb added to ultra-low carbon steel, and further has solid solution strengthening elements such as P, Si and Mn added to it.

[0005] As mentioned above, to achieve vehicle weight reduction, it is necessary to reduce the thickness of the steel sheets used as raw materials. Consequently, the corrosion resistance of these steel sheets has become more important than ever before. For example, for steel sheets used on the inner surface of the fuel tank, it is required that they do not generate corrosion products that cause filter clogging, and that they do not exhibit perforation corrosion, in response to gasoline, alcohol, or organic acids produced by gasoline deterioration. Furthermore, for steel sheets used on the outer surface of the fuel tank, good corrosion resistance is required for various operating environments, such as areas with scattered de-icing salts and driving in hot and humid conditions. To address these objectives, the application of coatings such as Pb-Sn alloys, Al-Si alloys, and Sn-Zn alloys to the surface of the steel sheets has been proposed and applied in the past.

[0006] However, when the inner and outer surfaces of a fuel tank are exposed to a harsh corrosive environment, corrosion leading to perforation can occasionally occur. Therefore, materials used in fuel tanks require sufficient durability and good corrosion resistance. Furthermore, to cope with such a harsh corrosive environment, various surface-treated steel sheets have been developed to date. Among these, Cr-containing steel sheets or steel sheets coated with Sn-based plating have been disclosed as promising technologies.

[0007] For example, Patent Document 1 discloses a steel plate for a fuel container, which has a Ni, Co, Ni-Co alloy diffusion coating and a Sn, Sn-Zn alloy coating on at least one side of a steel plate containing C: less than 0.02%, Cr: more than 3% to 20%, and acid-soluble Al: 0.005 to 0.10%.

[0008] Furthermore, Patent Document 2 discloses a rust-proof steel plate for fuel tanks, comprising: an alloy layer disposed on steel containing C, Si, Mn, P, Al, and 0.2 ≤ Cr ≤ 6%, and containing one or more of Ni, Fe, Sn, and Zn, with each side having a thickness of 1.5 μm or less; and a tin-zinc alloy plating layer disposed on the alloy layer, comprising 80–99% tin, the remainder being zinc and unavoidable impurities, wherein the zinc crystals containing zinc crystals with a major diameter of 250 μm or more number 20 or less per 0.25 mm. 2 And the thickness of each side is 4 to 50 μm.

[0009] Furthermore, Patent Document 3 discloses the following technology: a hot-dip Zn-Sn coating is formed on the surface of a steel plate, wherein the coating composition, in addition to Zn, contains less than 1-50% Sn, and the steel contains 1-25% Cr. Moreover, Patent Document 3 discloses a hot-dip Zn-Sn based steel plate, characterized in that a pre-coating layer containing Ni, Co, Fe, Cr, Sn, Zn, and Cu is formed at the interface between the coating and the steel plate.

[0010] Furthermore, Patent Document 4 discloses the following technology: a hot-dip galvanized Sn-Zn layer is formed on the surface of a steel plate, wherein the coating composition contains 1 to 50% Zn in addition to Sn, and the ratio of (Zn% of the surface layer composition / Zn% of the total coating) is 0.95 or less. Additionally, Patent Document 4 discloses a hot-dip galvanized Sn-Zn steel plate, characterized in that the steel contains 3 to 25% Cr, and a pre-plating layer containing Ni, Co, Fe, Cr, Sn, Zn, and Cu is formed at the interface between the coating and the steel plate.

[0011] Furthermore, Patent Document 5 discloses an automotive fuel tank or fuel supply pipe that uses an r value with a fracture elongation of 30% or more when processed by uniaxial tension. minA ferritic stainless steel sheet with a value of 1.3 or higher is used as the base material. Specifically, an automotive fuel tank or fuel supply pipe is disclosed, characterized in that the ferritic stainless steel sheet used as the base material contains, by mass percent, C: less than 0.015%, Si: less than 0.5%, Cr: 11.0 to 25.0%, N: less than 0.020%, Ti: 0.05 to 0.50%, Nb: 0.10 to 0.50%, and B: less than 0.0100%. A coating is formed on its surface, consisting of an Al coating, a Zn coating, or an alloy layer of Zn with one or more of Fe, Ni, Co, Mg, Sn, and Al. Furthermore, a zinc-rich coating is formed at the welded portion, and a cationic electrodeposited coating is formed on the entire base material including the welded portion.

[0012] Furthermore, Patent Document 6 discloses a surface-treated stainless steel sheet, characterized in that it comprises: a stainless steel sheet containing, by mass %: C: ≤0.030%, Si: ≤2.00%, Mn: ≤2.00%, P ≤0.050%, S: ≤0.0100%, N: ≤0.030%, Al: 0.010~0.100%, Cr: 10.00%~25.00%, and further containing Ni: 0.10~4.00%, Cu: 0. The coating comprises 0.10–2.00% of one or more of Mo, 0.10–2.00% of V, and 0.10–1.00% of one or two of Ti, 0.01–0.30% of Nb, with the remainder containing unavoidable impurities and Fe, and a Y value defined by a specific formula of -10.4 or less; and an anti-corrosion coating on the surface of a stainless steel plate comprising 0.8–10.0% Zn, with the remainder being Sn and unavoidable impurities.

[0013] Existing technical documents

[0014] Patent documents

[0015] Patent Document 1: Japanese Patent Application Publication No. 61-091390

[0016] Patent Document 2: Japanese Patent Application Publication No. 08-269735

[0017] Patent Document 3: Japanese Patent Application Publication No. 2001-355051

[0018] Patent Document 4: Japanese Patent Application Publication No. 2002-038250

[0019] Patent Document 5: Japanese Patent Application Publication No. 2003-277992

[0020] Patent Document 6: Japanese Patent Application Publication No. 2009-068102

[0021] Patent Document 7: International Publication No. 2019 / 208775 Summary of the Invention

[0022] The problem that the invention aims to solve

[0023] However, the aforementioned prior art is hardly sufficient to adequately address the coating properties such as appearance and adhesion on steel plates, as well as the methods for obtaining them, in order to achieve good corrosion resistance.

[0024] Furthermore, the aforementioned prior art has the following problems.

[0025] To achieve good corrosion resistance in coated steel sheets, a coating with a uniform appearance, i.e., a high coverage rate on the steel sheet, and a certain thickness is required. Hot-dip galvanizing is suitable for achieving this. However, when hot-dip galvanizing steels containing Cr, Si, Mn, etc., the formation of an oxide film on the steel surface may hinder the coating process.

[0026] Furthermore, in the case of hot-dip galvanizing, it is necessary to consider the corrosion resistance caused by the type of alloy layer formed between the base metal and the pre-plating layer, and between the pre-plating layer and the plating bath, due to the heat input during hot-dip galvanizing. However, in the aforementioned prior art, research on overcoming these issues is insufficient. Therefore, unlike small test pieces, when hot-dip galvanizing is performed while continuously passing a wide steel strip through the plate, the elemental distribution on the steel plate surface is not sufficiently formed in the plating methods disclosed in Patent Documents 2-6. Research on the formation of the alloy layer for obtaining a good plating interface is also insufficient in the plating methods disclosed in Patent Documents 2-6. Consequently, uneven surface conditions and alloy layer formation occur during the manufacturing of wide steel plates in industrial production, potentially resulting in areas where plating is not performed ("unplated"). Furthermore, if unplated occurs, it will certainly damage the appearance of the finished product, but it will also lead to a decrease in corrosion resistance and processability.

[0027] Here, Patent Document 7 discloses a Sn-Zn coated steel sheet, which has an alloy layer containing Ni, Sn, Cr, Fe, and Zn on the surface of a steel sheet containing Cr or Si, Mn, etc., with a ratio of Fe-Sn-Cr-Zn phase to Sn-Fe-Ni-Zn phase of 1 / 100 to 2 / 1 and a coating coverage of 98% or more on the steel sheet surface. However, in the case of Patent Document 7, if an alloy layer containing Zn with a low electrochemical potential exists at the interface between the steel sheet surface and the coating, the potential of the alloy layer becomes low. Therefore, in the event of defects extending to the base metal, corrosion sometimes progresses along the alloy layer due to the potential difference with the base metal or the noble metal Sn in the coating.

[0028] One embodiment of the present invention aims to solve the problem of providing Sn-Zn alloy-plated steel, battery casing, and fuel tank materials with good plating appearance, high corrosion resistance, and excellent processability.

[0029] Methods for solving problems

[0030] In order to solve the aforementioned problems, the inventors have conducted a thorough investigation into the elemental distribution on the steel surface that affects the composition and coating appearance of steel, as well as the state of the alloy layer between the Sn-Zn coating and the interface. They have also studied in detail the conditions for obtaining a good coating appearance, corrosion resistance, and processability.

[0031] The result was that it was discovered that: through

[0032] (A) To make the steel contain a specified amount of Cr.

[0033] (B) Forming an alloy layer with a specific composition on the steel with a high coating rate, especially an alloy layer mainly composed of Fe, Cr, and Ni (alloy layer A described later).

[0034] (C) A Sn-Zn alloy coating is formed on the alloy layer, thereby reducing the corrosion rate of the steel and exerting the anti-corrosion effect brought about by the formation of the Sn-Zn alloy coating.

[0035] Furthermore, it was discovered that by setting the Cr concentration of the steel to a certain amount and forming an alloy layer with a specific composition at the interface between the steel and the coating, Sn-Zn alloy-coated steel with good corrosion resistance can be obtained.

[0036] This invention is based on these insights, and the main points of the various aspects of this invention are as follows.

[0037] [1] One embodiment of the Sn-Zn alloy-coated steel of the present invention has the following characteristics:

[0038] steel

[0039] A first alloy layer, mainly composed of Fe, Cr, and Ni, is formed on the surface of the aforementioned steel.

[0040] A second alloy layer, mainly composed of Sn, Fe, Ni, and Zn, is formed on the aforementioned first alloy layer.

[0041] A Sn-Zn based coating, mainly composed of Sn and Zn, is formed on the aforementioned second alloy layer.

[0042] The surface coverage of the first alloy layer on the surface of the steel is 70% or more.

[0043] The aforementioned second alloy layer consists of FeSn2 phase and Fe3Zn partially replaced by Ni.10 At least one of the selected alloy layers is the main component.

[0044] The aforementioned Sn-Zn based coating contains, by mass percent, 1.0 to 15.0% Zn, with the remainder being Sn and impurities.

[0045] The total amount of the first alloy layer, the second alloy layer, and the Sn-Zn coating is 5 to 80 g / m² on each side. 2 .

[0046] [2] In the Sn-Zn alloy coated steel described in [1] above, it is also possible that the steel contains, by mass %:

[0047] C: 0.0005~0.030%

[0048] Si: below 0.80%

[0049] Mn: 0.10~2.00%

[0050] P: 0.005~0.040%

[0051] S: below 0.0100%

[0052] Cr: 4.0–18.0%

[0053] Al: 0-0.30%

[0054] Ti: 0~0.300%

[0055] Nb: 0~0.040%

[0056] B: 0~0.0030%

[0057] N: 0~0.030%

[0058] Cu: 0–2.0%

[0059] Ni: 0-3.0%

[0060] Mo: 0–2.00%

[0061] V: 0~2.00%,

[0062] The remainder contains Fe and impurities.

[0063] [3] In the Sn-Zn alloy coated steel described in [2] above, it is also possible that the steel contains REM: 0 to 0.1000% by mass%.

[0064] [4] In the Sn-Zn alloy coated steel described in [1] above, it is also possible that at least one side of the surface of the Sn-Zn coating has a chemical conversion treatment film with a thickness of 0.02 to 2.0 μm.

[0065] [5] In the Sn-Zn alloy coated steel described in [1] or [2] above, it is also possible that at least one side of the surface of the Sn-Zn coating has a coating film with a thickness of 10 to 500 μm.

[0066] [6] In the Sn-Zn alloy coated steel described in [3] above, it is also possible that at least one side of the surface of the Sn-Zn coating has a coating with a thickness of 10 to 500 μm.

[0067] [7] The battery casing of one embodiment of the present invention comprises Sn-Zn alloy coated steel as described in any one of [1] to [6] above.

[0068] [8] One embodiment of the present invention has a fuel tank comprising Sn-Zn alloy coated steel as described in any one of [1] to [6] above.

[0069] Invention Effects

[0070] According to one embodiment of the present invention, it is possible to provide Sn-Zn alloy-plated steel, battery casings, and fuel tanks with good plating appearance, high corrosion resistance, and excellent processability. Attached Figure Description

[0071] Figure 1 This is a schematic diagram showing the cross-sectional structure of the Sn-Zn coated steel according to this embodiment.

[0072] Figure 2 This is a TEM image showing a cross-section of the Sn-Zn based coated steel of this embodiment.

[0073] Figure 3 It means in Figure 2 A graph of points from which qualitative analysis was performed in TEM images.

[0074] Figure 4 It means Figure 3 A graph showing the qualitative analysis results of points (a) to (c).

[0075] Figure 5 This embodiment shows a schematic diagram of the test material used for the corrosion resistance evaluation test.

[0076] Figure 6 This is a perspective view showing the schematic configuration of the lower part of the battery casing in this embodiment.

[0077] Figure 7This is a perspective view showing the general configuration of the fuel tank in this embodiment. Detailed Implementation

[0078] The inventors of this invention have conducted a detailed study on the composition of the steel used as the base material for Sn-Zn based coated steel (hereinafter also referred to as coated steel), the surface condition of the steel affecting the appearance of the coating, and the Sn-Zn based coating and alloy layer. Furthermore, the inventors of this invention have discovered the optimal conditions for obtaining a good coating appearance, good corrosion resistance, and excellent processability.

[0079] The preferred embodiments of the present invention, including the new insights gained, will be described in detail below. Furthermore, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of the invention. In this specification, unless otherwise specified, the "%" in the chemical composition of the coating refers to "mass %". Additionally, the numerical range indicated by "~" refers to the range including the values ​​before and after "~" as lower and upper limits. It should be noted that when the values ​​before and after "~" are marked "more than" or "less than", the numerical range refers to the range excluding these values ​​as lower or upper limits.

[0080] Furthermore, the "external surface corrosion resistance" mentioned in this specification refers to the corrosion resistance exhibited by the coated steel when exposed to the outdoor atmospheric environment. "Internal surface corrosion resistance" refers to the corrosion resistance in environments containing organic acids such as nail acid and acetic acid. For example, it refers to the corrosion resistance exhibited by the coated steel when exposed to an aqueous solution of organic acids generated from water droplets on the coating surface due to condensation, fuel deterioration, etc., resulting in the concentration of these organic acids. It should be noted that when referred to as "corrosion resistance" below, it refers to both "external surface corrosion resistance" and "internal surface corrosion resistance."

[0081] [Sn-Zn alloy coated steel]

[0082] like Figure 1 As shown, the Sn-Zn alloy-coated steel 10 of this embodiment includes: a steel 11 having a specified chemical composition; a first alloy layer 12 formed on at least one surface of the steel 11, mainly composed of Fe, Ni, and Cr; a second alloy layer 13 formed on the first alloy layer 12, mainly composed of Sn, Fe, Zn, and Ni; and a Sn-Zn coating 14 formed on the second alloy layer 13, mainly composed of Sn and Zn.

[0083] Hereinafter, the “first alloy layer 12” will sometimes be referred to as alloy layer A, the “second alloy layer 13” as alloy layer B, and the “Sn-Zn plating layer 14” as “Sn-Zn plating layer C”.

[0084] The first alloy layer (alloy layer A) 12 has a coverage of more than 70% relative to the surface of the steel plate, and the second alloy layer (alloy layer B) 13 is FeSn2 phase and / or Fe3Zn partially replaced by Ni. 10 The alloy layer is predominantly composed of the Sn-Zn phase. Sn-Zn plating layer 14 (Sn-Zn plating layer C) contains 1–15% Zn by mass, with Sn as the remainder and impurities. The total adhesion weight of alloy layer A, alloy layer B, and Sn-Zn plating layer C is 5–80 g / m² per side. 2 .

[0085] <Steel>

[0086] First, let's explain the steel (original sheet) that will be coated.

[0087] The steel used is primarily steel sheet, but there are no particular restrictions on its size. The steel sheet is suitable as long as it is suitable for the standard hot-dip galvanizing process. Specifically, steel sheets suitable for the process of immersing in molten metal and allowing it to solidify are applicable.

[0088] There are no particular restrictions on the type of steel used. For example, ordinary steel, pre-plated steel thinly coated with various metals, low-carbon steel, high-carbon steel, and various high-tensile steel sheets can be used. Furthermore, in the following description, steel sheets are shown as examples of steel (original sheets), but the steel used in this embodiment is not limited to steel sheets.

[0089] The specific composition of steel will be described later.

[0090] <Alloy Layers A, B>

[0091] Next, alloy layers A and B will be explained.

[0092] The inventors conducted various studies on the influence of steel sheet surface condition and alloy layer on the appearance of the coating. The results showed that by forming an alloy layer with a specific composition during hot-dip Sn-Zn plating, high corrosion resistance and excellent processability can be obtained in the resulting coated steel sheet.

[0093] The alloy layers formed between the steel plate and the Sn-Zn coating C consist of a Fe-Cr-Ni layer (hereinafter referred to as alloy layer A) mainly composed of Cr and Ni in α-Fe, and a Sn-Fe-Ni-Zn layer (hereinafter referred to as alloy layer B). Specifically, alloy layer B consists of FeSn2 phase or Fe3Zn partially replaced by Ni. 10 Equal composition, for example, containing Fe x Ni (3-x) Zn 10 The specific composition of alloy layer B will be detailed later.

[0094] Alloy layers A and B are composed of elements that diffuse from the steel plate, the Ni-based pre-plating layer, and the hot-dip galvanized Sn-Zn alloy layer C, respectively, when forming the alloy layer between the steel plate and the Sn-Zn plating layer C during the hot-dip galvanizing process.

[0095] As described above, alloy layer A is primarily a Fe-Cr-Ni layer containing Cr and Ni in α-Fe. Here, "primarily a Fe-Cr-Ni layer" means that alloy layer A is occupied by at least 90% by volume of a Fe-Cr-Ni alloy. In other words, alloy layer A may also contain alloys other than Fe-Cr-Ni alloys, which may be present in less than 10% by volume. For example, alloys other than Fe-Cr-Ni alloys are Fe-Ni-Cr-Zn layers (alloy layer D) that contribute to deterioration of corrosion resistance. If the volume fraction of other alloys such as alloy layer D in alloy layer A is less than 10%, it will not adversely affect the effects of the present invention. From the viewpoint of improving corrosion resistance, alloy layer A is preferably formed of a Fe-Cr-Ni alloy. Fe-Cr-Ni alloys refer to ternary alloys containing Fe, Cr, and Ni. Furthermore, in addition to Fe, Cr, and Ni, alloy layer A may also contain small amounts of other elements (less than 1% by mass) as long as it does not adversely affect the characteristics of this embodiment.

[0096] In this embodiment, the corrosion resistance of the coated steel sheet can be improved by including Cr and Ni in alloy layer A. On the other hand, if alloy layer A contains a large amount of elements other than Fe, Cr, and Ni, the corrosion resistance may sometimes deteriorate. For example, if Zn from the plating bath penetrates into alloy layer A, alloy layer A becomes an Fe-Cr-Ni-Zn alloy, resulting in a low potential and thus sometimes reduced corrosion resistance. Therefore, alloy layer A, which is mainly composed of Fe-Cr-Ni alloy, is relatively effective in terms of corrosion resistance.

[0097] The coverage of alloy layer A relative to the steel plate surface (surface coverage) is 70% or more. Alloy layer A is a layer that effectively contributes to the corrosion resistance of the coated steel plate. Therefore, a higher coverage of alloy layer A relative to the steel plate surface is preferred. Preferably, the surface coverage of alloy layer A is 90% or more. More preferably, it is desirable for one or both sides of the steel plate to be completely covered by alloy layer A. That is, the surface coverage of alloy layer A can be 100%. The method for determining the surface coverage will be described later.

[0098] Alloy layer B is primarily composed of Sn-Fe-Ni-Zn. Specifically, alloy layer B consists of FeSn2 phase and Fe3Zn partially replaced by Ni. 10 The layer is dominated by at least one of the phases. Alloy layer B can be composed of FeSn2 phase and Fe3Zn partially replaced by Ni. 10Phase composition. Alloy layer B, for example, contains ZnNi phase, FeSn2 phase, Fe3Zn phase. 10 Phases: FeSn2 phase partially replaced by Ni, Fe3Zn phase partially replaced by Ni 10 Phase (e.g., Fe) x Ni (3-x) Zn 10 In this context, "a layer primarily composed of Sn-Fe-Ni-Zn" refers to a state where over 90% by volume of alloy layer B is occupied by a Sn-Fe-Ni-Zn alloy. In other words, alloy layer B may also contain less than 10% by volume of alloys other than Fe-Cr-Ni alloys. As described above, alloys other than Fe-Cr-Ni alloys are, for example, Fe-Ni-Cr-Zn layers (alloy layer D) that contribute to deterioration of corrosion resistance. If the volume fraction of other alloys, such as alloy layer D, in alloy layer B is less than 10%, it will not adversely affect the effectiveness of the present invention.

[0099] From the perspective 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. It should be noted that, in addition to these elements, alloy layer B may also contain small amounts of other elements (less than 1% by mass) as long as they do not adversely affect its properties.

[0100] In addition, the Sn-Fe-Ni-Zn-based layer of alloy layer B is formed on alloy layer A in a hot-dip Sn-Zn plating bath, and consists of FeSn2 phase and / or FeSn2 phase and Fe3Zn. 10 Phase and / or Fe3Zn partially replaced by Ni 10 The layers consist of phases. That is, alloy layer B is electrochemically inferior, second only to the electrochemically inferior Sn-Zn system coating (hereinafter referred to as Sn-Zn system coating C) in terms of the Zn monolayer. Specifically, for example, in the event of defects reaching the base metal, when the coated steel sheet is exposed to a corrosive environment, firstly, after the sacrificial dissolution of Zn in Sn-Zn system coating C, the exposed Fe3Zn in alloy layer B, partially replaced by Ni, will... 10 Zn dissolves from the substrate. However, after the Zn dissolves from the surface of alloy layer B, the expensive alloy layer A is exposed, thus leading to sacrificial dissolution of Sn in the next stage of the Sn-Zn plating layer C. However, due to the low cathodic reaction amount in the exposed base metal, the anodic dissolution rate of Sn is also slow, and long-term corrosion resistance can be expected.

[0101] There is no particular limitation 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 can be set to 0.1 μm or more or 0.3 μm or more. Alternatively, the upper limit of the average value of the total thickness can be set to 3.0 μm or less or 2.0 μm or less.

[0102] In addition, such as Figure 1 As shown, alloy layer B has a layered region (layered region) 13A and a needle-like region (needle-like region) 13B. The thickness of alloy layer B is defined as the distance from the bottom of layered region 13A (the interface between alloy layer A and layered region 13A) to the top of needle-like region 13B. That is, the thickness of alloy layer B is set as the sum of the thickness of layered region 13A and the height of needle-like region 13B. Here, the "height of needle-like region 13B" is set as the average of 10 needle-like crystal heights in the TEM image used for the coverage measurement described later. When the average total thickness of alloy layer A and alloy layer B is less than 0.1 μm, the corrosion resistance of the outer surface of the coated steel sheet is insufficient. By making the average total thickness 0.1 μm or more, the coverage of alloy layer A and alloy layer B can be further improved, improving the corrosion resistance of the outer surface, including the welded portion, and the coating appearance. When the average thickness of the alloy layer exceeds 3.0 μm, the hard and brittle layer increases, and the processability deteriorates. By setting the average combined thickness of alloy layer A and alloy layer B to below 3.0 μm, processability can be better maintained.

[0103] The constituent elements and surface coverage of alloy layers A and B can be determined by focused ion beam (FIB), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS).

[0104] First, a test specimen for measurement is cut and processed from the coated steel sheet of this embodiment using FIB, and the cross-section of the specimen is observed by TEM. A Cu mesh is used to hold the specimen during TEM observation. An example of cross-sectional observation is shown below. Figure 2 Then, in Figure 2 TEM images Figure 3 At the points shown, electron beam diffraction and EDS were performed to obtain the following results: Figure 4 The electron beam diffraction pattern and composition analysis diagram are shown. Furthermore, in Figure 3In the case of the example shown, point (a) corresponds to alloy layer A (Fe-Cr-Ni alloy), point (b) corresponds to Cr-containing α-Fe as the steel sheet, and point (c) corresponds to alloy layer B (Fe-Ni-Sn-Zn alloy). Thus, the constituent elements of alloy layers A and B can be determined based on Figure 3 the electron diffraction pattern and qualitative composition analysis diagram shown.

[0105] In addition, the coverage rate of alloy layer A on the steel sheet surface is the value obtained by dividing the length of alloy layer A covering the steel sheet surface by the length along the steel sheet surface. That is, for example, in Figure 3 the cross-sectional image along the thickness direction shown, the value obtained by dividing the covering length of alloy layer A (corresponding to (a)) by the length of the surface of the steel sheet (corresponding to (b)) is taken as the coverage rate. It should be noted that when calculating the coverage rate, that is, when obtaining the covering length of alloy layer A, the measurement reference length is set to 5 μm.

[0106] Here, the present inventors investigated the detailed conditions of each plating condition in the hot dip plating process and the alloy layer formed at the interface of the steel sheet / Sn-Zn coating C, and as a result, it was found that when the heat load during the hot dip plating process is too high (for example, the plating bath temperature is high, the plating bath immersion time is long, etc.), alloy layer A disappears and a layer mainly composed of Fe-Cr-Ni-Zn (alloy layer D) is formed. Since alloy layer D contains Zn, alloy layer D shows a potential that is nobler than that of steel or Sn in the plating electrochemically. Therefore, it was found that, for example, in an organic acid environment generated due to oxidative deterioration of fuel inside a fuel tank, in a snow-melting salt spreading area outside the fuel tank or a battery case, or in a salt damage corrosion environment along the coastline, Zn at the interface of alloy layer D and the upper alloy layer B (Fe-Ni-Sn-Zn) dissolves out mutually, and corrosion progresses along the interface of alloy layer D / alloy layer B. Moreover, it was found that the higher the ratio of alloy layer D (layer mainly composed of Fe-Cr-Ni-Zn), the more deteriorated the corrosion resistance of the plated steel sheet.

[0107] From the above, in the plated steel sheet of the present embodiment, alloy layer A is a layer mainly composed of Fe-Cr-Ni, alloy layer B is a layer mainly composed of Sn-Fe-Ni-Zn, and the coverage rate of alloy layer A on the steel sheet surface is 70% or more, which is effective for showing excellent corrosion resistance.

[0108] <Sn-Zn coating>

[0109] An Sn-Zn alloy coating C is provided on alloy layer B. The average chemical composition of the Sn-Zn alloy coating C contains 1.0 to 15.0% of Zn by mass, and the balance Sn and impurities.

[0110] When the Zn content in the Sn-Zn alloy coating C is less than 1%, the protective function of zinc against corrosion and the protective effect of zinc corrosion products on the steel plate cannot be fully expected, and thus the corrosion resistance sometimes deteriorates. Therefore, the lower limit of the Zn content in the Sn-Zn alloy coating C is 1.0% or more. Preferably, it is 1.2% or more, more preferably 2.0% or more, and even more preferably 3.0% or more.

[0111] If the Zn content in the Sn-Zn alloy coating C exceeds 16.0%, the barrier properties of Sn cannot be fully utilized, and thus the corrosion resistance sometimes deteriorates. Therefore, the upper limit of the Zn content in the Sn-Zn alloy coating C is 15.0% or less. Preferably, it is 12.0% or less, more preferably 10.0% or less, and even more preferably 8.8% or less.

[0112] Good corrosion resistance can be obtained by setting the Zn content in the Sn-Zn alloy coating C to a range of 1.0% to 15%. Furthermore, if the Zn content in the Sn-Zn alloy coating C is 8.8% or less, primary Sn crystals precipitate and Zn is finely dispersed, thus obtaining even better corrosion resistance.

[0113] In the average chemical composition of the Sn-Zn alloy coating C, the remainder other than Zn is Sn and impurities. Additionally, to improve corrosion resistance, it may also contain one or more of Mg, Al, Mo, and W, totaling less than 1%.

[0114] Impurities in Sn-Zn alloy coating C refer to components contained in the raw materials or mixed in during the manufacturing process, and are not intentionally present. For example, in Sn-Zn alloy coating C, due to the mutual atomic diffusion between the steel plate (base metal) and the plating bath, trace amounts of components other than Fe are sometimes mixed in as impurities.

[0115] The average chemical composition of the coating can be determined by the following methods.

[0116] First, in the following mixed aqueous solution (stripping solution) at 25°C with the pH adjusted to 7-8, 25A / dm 2 The coating was stripped using anodic electrolysis. The endpoint was set at the point where the potential began to change. Nitric acid was added to the aqueous solution after the coating was stripped, and the solution was analyzed by ICP-MS until it became clear, thereby determining the average chemical composition of the coating.

[0117] <Stripping solution>: Triethanolamine 100mL / L, glycolic acid 60mL / L, sodium bromide 20g / L, ammonium nitrate 50g / L.

[0118] The adhesion amount of Sn-Zn alloy coating C is 5-80 g / m² per side. 2 More preferably, it is 15–60 g / m2 .

[0119] Adhesion amount less than 5g / m on one side 2 However, good corrosion resistance cannot be guaranteed at this stage. Therefore, the lower limit for the adhesion amount of C in the Sn-Zn alloy coating is set at 5 g / m. 2 The above. Alternatively, it can be preferably set to 12g / m³. 2 The above, preferably 15g / m 2 The above is further preferred to be 20g / m 2 That's all. Additionally, if the adhesion amount exceeds 80g / m²... 2 This would increase material costs. Furthermore, excessive coating thickness would result in uneven coating thickness, pattern defects, or reduced weldability of the coated steel sheet. Therefore, the upper limit for the adhesion amount of Sn-Zn alloy coating C is set at 80 g / m². 2 The following can also be preferred, set to 70g / m³. 2 The following is more preferably 60g / m 2 The following is a further preferred value: 50g / m 2 the following.

[0120] The adhesion amount of the Sn-Zn alloy coating C can be determined by fluorescence X-ray diffraction (standard curve method). It should be noted that the adhesion amount is evaluated on each side of the coated steel sheet. Therefore, when the Sn-Zn alloy coating C is applied to both sides of the steel sheet, the adhesion amount is preferably determined by fluorescence X-ray diffraction. It should be noted that the adhesion amount of the Sn-Zn alloy coating C on at least one side of the steel sheet is 5–80 g / m². 2 Hot-dip galvanized Sn-Zn alloy steel sheets were deemed to meet the above requirements regarding the amount of adhesion.

[0121] <Painting>

[0122] The Sn-Zn alloy-coated steel of this embodiment can be used without coating, but by implementing coatings appropriate to the purpose, its corrosion resistance, formability, and designability can be further improved.

[0123] When the Sn-Zn alloy-coated steel of this embodiment is applied, for example, to fuel tanks for automobiles, the Sn-Zn coating may sometimes be damaged during the manufacturing process of the fuel tank due to welding or brazing. In such cases, applying an anti-corrosion coating to the Sn-Zn alloy-coated steel can achieve higher rust resistance.

[0124] The thickness of the anti-corrosion coating can be set to, for example, 10–500 μm.

[0125] When the anti-corrosion coating is a black coating based on spraying or electrodeposition, if the coating thickness is less than 10 μm, the barrier properties of the coating become insufficient, and corrosion under the coating can easily proceed. Therefore, the coating thickness is preferably 10 μm or more.

[0126] Furthermore, to prevent coating damage and corrosion caused by cracking at the bottom of the fuel tank and to further improve rust resistance, a crack-resistant coating can be applied to the Sn-Zn alloy-plated steel of this embodiment. In this case, the thickness of the crack-resistant coating can be set to 100 μm or more. However, if the coating thickness exceeds 500 μm, cracking may occur due to shrinkage during drying, leading to corrosion saturation. Additionally, increasing the coating thickness requires additional coating and drying processes to prevent cracking, making thicker coatings uneconomical. Therefore, a coating thickness of 500 μm or less is preferable.

[0127] Therefore, the coating thickness of the anti-corrosion coating is preferably set to 10–500 μm. It should be noted that in addition to spraying and spray coating, electrodeposition coating and other methods can also be used to apply the coating.

[0128] <Chemical Conversion Treatment of Coatings>

[0129] By performing a chemical conversion treatment on the Sn-Zn alloy-plated steel of this embodiment and setting a chemical conversion treatment film, corrosion resistance can be improved. Furthermore, by setting the chemical conversion treatment film as a substrate for the black coating, corrosion resistance can be further improved. Moreover, by setting the chemical conversion treatment film as a substrate for the coating film, the adhesion of the coating film can be further improved. As the chemical conversion treatment method, known techniques such as chromate coatings containing trivalent chromium but not hexavalent chromium, and chromate-free coatings containing no chromium, can be used.

[0130] When the thickness of the chemically converted coating is less than 0.02 μm, the corrosion resistance of the outer surface is insufficient. Therefore, the effective coating thickness for achieving corrosion resistance is preferably set to 0.02 μm or more. Furthermore, when the thickness of the chemically converted coating exceeds 2.0 μm, it hinders resistance welding. Therefore, it is preferable to set it to 2.0 μm or less without hindering resistance welding.

[0131] Lubricating film

[0132] Furthermore, to ensure formability during cold working processes such as pressing, an organic lubricating film can be formed directly above the Sn-Zn coating C of the Sn-Zn alloy-coated steel in this embodiment or on the chemically converted film. In this case, the coefficient of friction of the lubricating film is preferably 0.15 or less. Considering weldability, the thickness of the lubricating film is preferably 2.0 μm or less.

[0133] <Surface Roughness of Sn-Zn Plated Steel>

[0134] The surface roughness Ra of Sn-Zn plated steel can be 0.05 to 1.0 μm. By subjecting the Sn-Zn plated steel after plating solidification or after the formation of a chemical conversion treatment film to temper rolling, the surface roughness Ra of the Sn-Zn plated steel can be controlled to 0.05 to 1.0 μm. Thereby, the workability of the plated steel sheet can be maintained and good weldability can be obtained.

[0135] When the surface roughness Ra of Sn-Zn plated steel is less than 0.05 μm, when forming and processing the plated steel as a raw material, the oil film of the lubricating oil breaks, and the workability decreases. Therefore, the surface roughness Ra of Sn-Zn plated steel is preferably 0.05 μm or more, more preferably 0.1 μm or more.

[0136] On the other hand, when the surface roughness Ra of Sn-Zn plated steel exceeds 1.0 μm, the metal adhesion between the plated steel and the mold increases, and the workability decreases. Therefore, the surface roughness Ra of Sn-Zn plated steel is preferably 1.0 μm or less, more preferably 0.5 μm or less.

[0137] <Antirust Oil, Lubricating Oil>

[0138] Furthermore, in order to improve the antirust property and formability, an antirust oil or a lubricating oil can be coated on the surface of the Sn-Zn coating C of the Sn-Zn alloy plated steel or on the surface of the chemical conversion treatment film.

[0139] <Chemical Composition of Steel (Steel Sheet)>

[0140] The chemical composition of the steel sheet which is the base material of the Sn-Zn plated steel sheet of the present embodiment will be described. In addition, in the following description, the unit of the chemical composition represents "mass%".

[0141] C: 0.0005 to 0.030%

[0142] Carbon (C) is an element that contributes to the strength of steel sheets. However, since it reduces the ductility of the steel sheet and hinders its formability, a low C content is preferable. Furthermore, in steels containing Cr, C is also a cause of intergranular corrosion in welded and brazed sections. Therefore, C content is preferably limited to below a specified level. In this embodiment, the upper limit of C content is set to 0.030% or less. The upper limit of 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, when the C content is below 0.0005%, it is difficult to ensure the strength of the steel sheet, and the cost of smelting increases. Therefore, the lower limit of C content is set to 0.0005% or more. The lower limit of C content is preferably 0.0008% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0143] Si: below 0.80%

[0144] Si is an element that acts as a solid solution strengthening element, contributing to the strength of steel sheets. However, excessive Si content can sometimes reduce the ductility of the steel sheet and negatively impact its hot-dip galvanization properties. Therefore, the Si content is preferably set below a certain level. Thus, 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. It should be noted that the lower limit of the Si content is not specifically defined and can be 0%. Considering refining costs, the lower limit of the Si content can be set to 0.001%, or it can be set to 0.01% or more. More preferably, it is 0.05% or more, and even more preferably 0.10% or more.

[0145] Mn: 0.10~2.00%

[0146] Like Si, manganese (Mn) is an element that contributes to the strength of steel sheets through solid solution strengthening. However, excessive Mn content can sometimes reduce the ductility of the steel sheet and negatively impact hot-dip galvanizing. Therefore, the Mn content is preferably limited to a certain amount. When the Mn content is below 0.10%, the aforementioned effects are sometimes not achieved. On the other hand, if the Mn content exceeds 2.00%, the formability of the sheet is impaired, and Mn oxides are formed on the surface of the base steel sheet, sometimes resulting in impaired plating properties. Therefore, the Mn content is set between 0.10% and 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.

[0147] P: 0.005~0.040%

[0148] Polymer (P) is an element that acts as a solid solution strengthening element, contributing to the strength of steel plates. Furthermore, P is effective in improving corrosion resistance in certain saline environments. On the other hand, P reduces ductility and also deteriorates resistance to secondary processing due to grain boundary segregation. Therefore, the upper limit of P content is set to 0.040% or less. The upper limit of 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, when the P content is below 0.005%, the effect on improving the strength and corrosion resistance of the base steel plate is insufficient. Therefore, the lower limit of P content is set to 0.005% or more. The lower limit of P content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.018% or more.

[0149] S: below 0.0100%

[0150] S is an impurity element introduced during steel refining. Furthermore, S combines with Mn and Ti to form precipitates, deteriorating processability. 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. Additionally, the base steel sheet of the Sn-Zn alloy-coated steel sheet of this embodiment does not need to contain S. Therefore, the S content can be 0%. However, reducing the S content to less than 0.0005% increases manufacturing 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.

[0151] Cr: 4.0–18.0%

[0152] Cr is an important element for ensuring the corrosion resistance of the base steel plate. The higher the Cr content, the more effective it is in improving corrosion resistance. Therefore, the lower limit of Cr content is set to 4.0% or more. When the Cr content is below 4.0%, even if the Sn-Zn alloy coating of this embodiment is implemented, sufficient salt corrosion resistance may not be obtained, especially in welded areas and cut ends. The lower limit of Cr content is preferably 5.0% or more, more preferably 6.0% or more, and even more preferably 7.0% or more. In addition, the lower limit of Cr content can also be 8.0% or more, 10.5% or more, or 12.0% or more.

[0153] On the other hand, Cr is also an element that reduces the ductility of steel sheets, so the Cr content is preferably kept below a certain level. Specifically, if the Cr content exceeds 18.0%, cold workability, such as pressing and forming, decreases, and raw material costs increase. Therefore, the Cr content is set to 18.0% or less. The upper limit of the Cr content is preferably 15.0% or less, more preferably 13.0% or less, and even more preferably 11.0% or less. In addition, especially when pressing the base steel sheet into complex shapes such as saddle-shaped cans, it is necessary to ensure higher cold workability. In such cases, it is preferable to keep the upper limit of the Cr content less than 10.5%.

[0154] In the Sn-Zn alloy-coated steel sheet of this embodiment, the above-mentioned composition of the steel sheet may further contain one or more of the following: Al: 0.01-0.30%, Ti: 0.010-0.300%, Nb: 0.001-0.040%, B: 0.0002-0.0030%, and N: 0.0010-0.0300%. However, even if the steel sheet does not contain these components, the hot-dip Sn-Zn alloy-coated steel sheet of this embodiment can solve this problem, therefore the lower limit value of these elements is 0%.

[0155] Al: 0–0.30%

[0156] Al is an element used as a deoxidizer in steel refining. A deoxidizing effect is achieved when the Al content is above 0.01%. However, if the Al content exceeds 0.30%, it can lead to a decrease in the toughness and machinability of the weld. Therefore, the upper limit for Al content is set at 0.30%.

[0157] Ti: 0~0.300%

[0158] Ti has a strong affinity for C and N, forming carbonitrides and thus inhibiting intergranular corrosion. Furthermore, Ti reduces dissolved C and N in steel, improving the workability of the steel sheet. This effect is further enhanced when the Ti content is 0.010% or higher. On the other hand, if the Ti content exceeds 0.300%, the ductility of the steel sheet may decrease, and the strength and toughness of the welded joints may also decrease. Therefore, the upper limit for Ti content is set at 0.300%.

[0159] Nb: 0~0.040%

[0160] Like Ti, Nb has a strong affinity for both C and N, forming carbonitrides and thus inhibiting intergranular corrosion. Furthermore, Nb reduces the amount of C and N dissolved in steel, improving the workability of the steel sheet. This effect is further enhanced when the Nb content is 0.001% or higher. On the other hand, if the Nb content exceeds 0.040%, the ductility of the steel sheet may decrease, and the strength and toughness of the welded joints may also be reduced. Therefore, the upper limit for Nb content is set at 0.040%.

[0161] B: 0~0.0030%

[0162] Boron (B) is an element that improves grain boundary strength and reduces brittleness during secondary processing by segregating at grain boundaries. This effect is achieved when the B content is 0.0002% or higher, therefore the lower limit of the B content can be set to 0.0002% or higher. More preferably, the lower limit of the B content is 0.0003% or higher. On the other hand, if the B content exceeds 0.0030%, the ductility of the steel plate may decrease, and the strength and toughness of the welded parts may also decrease. Furthermore, when the B content is excessive, corrosion resistance may decrease due to the formation of borides. Therefore, the upper limit of the B content is set to 0.0030% or lower. More preferably, the upper limit of the B content is 0.0020% or lower.

[0163] N: 0~0.0300%

[0164] Nitrogen (N) is an impurity element introduced during steel refining. Furthermore, N forms nitrides of Ti, Al, and Nb, which also affect processability. Therefore, in the presence of N, it is preferable to keep the N content below a certain level to avoid reducing processability. 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 can be set to 0.0010% or more.

[0165] The steel sheet used as the base material for the Sn-Zn alloy-coated steel sheet of this embodiment can achieve even better corrosion resistance by further containing at least one of Cu: 0.01–2.0%, Ni: 0.01–3.0%, Mo: 0.01–2.00%, and V: 0.01–2.00%. However, even if the base steel sheet does not contain these components, the hot-dip Sn-Zn alloy-coated steel sheet of this embodiment can still achieve excellent corrosion resistance, therefore the lower limit value of these elements is 0%.

[0166] Cu: 0–2.0%

[0167] Cu is an effective element for improving the corrosion resistance of steel sheets. This effect is observed when the Cu content is 0.01% or higher, therefore the lower limit of the Cu content can be set to 0.01% or higher. More preferably, the lower limit of the Cu content is 0.03% or higher. However, excessive Cu content can sometimes adversely affect the brittleness during hot rolling. Therefore, the upper limit of the Cu content is set to 2.0% or lower. More preferably, the upper limit of the Cu content is 1.5% or lower.

[0168] Ni: 0–3.0%

[0169] Ni is an effective element for improving the corrosion resistance of steel plates. This effect is observed when the Ni content is 0.01% or higher, therefore the lower limit of Ni content can be set to 0.01% or higher. More preferably, the lower limit of Ni content is 0.03% or higher. However, excessive Ni content can sometimes adversely affect ductility and weld toughness. Therefore, the upper limit of Ni content is set to 3.0% or lower. More preferably, the upper limit of Ni content is 2.0% or lower.

[0170] Mo: 0–2.00%

[0171] Mo is an effective element for improving the corrosion resistance of steel plates. This effect is observed when the Mo content is 0.01% or higher, therefore the lower limit of the Mo content can be set at 0.01% or higher. However, if the Mo content is too high, ductility may decrease, therefore the upper limit of the Mo content is set at 2.00% or lower.

[0172] V: 0~2.00%

[0173] Like Mo, V is an effective element for improving the corrosion resistance of steel plates. This effect is observed when the V content is 0.01% or higher, therefore the lower limit for V content can be set at 0.01% or higher. However, excessive V content can sometimes negatively impact ductility. Therefore, the upper limit for V content is set at 2.00% or lower.

[0174] It should be noted that the chemical composition of the base steel sheet constituting the Sn-Zn alloy-coated steel sheet of this embodiment includes Fe and impurities in addition to the elements mentioned above. Impurities refer to elements that originate from steel raw materials and / or are mixed in during the steelmaking process, and are not intentionally present in the base steel sheet. Furthermore, impurities are elements that are permissible within a range that do not impair the characteristics of the Sn-Zn alloy-coated steel sheet of this embodiment.

[0175] [Manufacturing method of Sn-Zn alloy coated steel sheet]

[0176] Next, the manufacturing method of the Sn-Zn alloy-coated steel sheet of this embodiment will be described. The Sn-Zn alloy-coated steel sheet of this embodiment can be manufactured by either an immersion hot-dip coating method (intermittent hot-dip coating method) or a continuous hot-dip coating method.

[0177] The method for manufacturing Sn-Zn alloy coated steel sheet according to this embodiment includes: an acid pickling step of electrolytically pickling a base steel sheet having the above-mentioned chemical composition to produce an acid pickled steel sheet; a pre-plating step of forming a Ni coating, a Ni-Fe coating, or a Fe-Ni coating on at least one side of the acid pickled steel sheet to produce a pre-plated steel sheet; and a plating step of performing Sn-Zn alloy coating treatment on the pre-plated steel sheet to produce a Sn-Zn alloy coated steel sheet.

[0178] It should be noted that the base steel sheet used in the pickling process can be, for example, a base steel sheet manufactured as follows.

[0179] First, a slab with the aforementioned chemical composition is cast. Specifically, a slab is formed by casting molten steel with the chemical composition described above. It should be noted that the casting method for the slab is not particularly limited. Then, the obtained slab is hot-rolled to produce a hot-rolled sheet. Alternatively, the hot-rolled sheet may be hot-rolled and annealed. Next, the hot-rolled sheet or the hot-rolled and annealed sheet is pickled and then cold-rolled to produce a cold-rolled steel sheet of a specified thickness. To prevent cracking of the steel sheet during cold rolling, intermediate annealing may be performed during cold rolling. Alternatively, the obtained cold-rolled steel sheet may be annealed. Thus, a base steel sheet is prepared for the manufacturing method of the Sn-Zn alloy coated steel sheet of this embodiment.

[0180] The following is a detailed explanation of each process.

[0181] <Pickling Process>

[0182] In the pickling process, cold-rolled steel sheets or cold-rolled annealed sheets are subjected to electrolytic pickling.

[0183] For example, cold-rolled steel sheets or cold-rolled annealed sheets are immersed in an aqueous sulfuric acid solution containing nitrates and / or sulfates and fluorosilicates and / or fluoroborates, i.e., a pickling solution, to dissolve the oxide film on the surface of the steel sheet. Then, the residues generated by the dissolution are removed, for example, by brushing or spraying, followed by washing with water and drying to adjust the surface condition of the steel sheet.

[0184] The sulfuric acid used as the main agent in the pickling solution can be set at a concentration of 50–300 g / L. When the sulfuric acid concentration is below 50 g / L, the pickling efficiency may decrease. Furthermore, when the pickling concentration exceeds 300 g / L, it becomes over-pickling, which can sometimes negatively impact plating properties.

[0185] Sodium nitrate, potassium nitrate, ammonium nitrate, etc., can be used as nitrates, and the concentration of nitrates can be set from 50 to 200 g / L. At concentrations below 50 g / L, no effect is obtained; conversely, at concentrations above 200 g / L, the effect becomes saturated.

[0186] Sodium sulfate can be used as a sulfate, and the concentration can be set between 50 and 200 g / L. When the sulfate concentration is below 50 g / L, no effect is obtained; conversely, when the sulfate concentration exceeds 200 g / L, the effect becomes saturated.

[0187] When using both nitrates and sulfates, the combined concentration of nitrates and sulfates can be set to 100–200 g / L.

[0188] As fluorosilicates and fluoroborates, sodium fluorosilicate, potassium fluorosilicate, sodium fluoroborate, ammonium fluoroborate, and other fluorosilicates can be used. The pickling solution may contain any one or more of fluorosilicates and fluoroborates in a total of 5 to 100 g / L. If the total content of any one or more of fluorosilicates and fluoroborates is less than 5 g / L, it will not help to improve the removal rate of the oxide film; on the other hand, if the total content exceeds 100 g / L, the effect will saturate. The total content of any one or more of fluorosilicates and fluoroborates is preferably 10 g / L or more.

[0189] It should be noted that for steel plates supplied for the pickling process, surface finishing can be carried out using shot peening, grinding brushes, etc. before pickling.

[0190] <Pre-plating process>

[0191] Next, the pre-plating process will be explained.

[0192] A metallic coating (pre-coating) is formed on the pickled steel plate (pickled steel plate) with Ni coating, Ni-Fe coating or Fe-Ni coating as the main body.

[0193] An electrolytic bath (Watt's bath) primarily composed of Ni sulfate, Ni chloride, and boric acid is used as a base. After adjusting the pH of the bath with sulfuric acid, a pickled steel sheet is immersed in the bath, thereby forming a Ni-based metallic coating (Ni plating). Alternatively, a Ni-Fe metallic coating (Ni-Fe plating) can be formed by adding ferrous sulfate to the aforementioned Watt's bath. The proportion of Fe in the Ni-Fe plating is preferably set to less than 10-50% by mass.

[0194] From the perspective of coating adhesion, adjusting the pH of the electrolytic bath to below 2.5 is effective. When the pH of the electrolytic bath exceeds 2.5, the corrosive power of the steel surface decreases, and oxides or hydroxides form at the interface between the pre-coating and the base steel, potentially deteriorating the coating adhesion. Therefore, the preferred pH of the electrolytic bath is below 1.5.

[0195] The adhesion amount of the Ni coating or Ni-Fe coating on each side is preferably set to 0.1–3.0 g / m² based on metal conversion. 2 The adhesion amount is less than 0.1g / m. 2 At this time, the coating is insufficient, thus a uniform Sn-Fe-Ni-Zn alloy layer (alloy layer B) cannot be formed, resulting in insufficient corrosion resistance. On the other hand, the adhesion amount of the Ni coating or Ni-Fe coating exceeds 3.0 g / m. 2 In such cases, the effect of improving corrosion resistance becomes saturated, and an excessively thick alloy layer A and / or alloy layer B is formed at the interface between the Sn-Zn alloy coating C and the steel plate. Sometimes, the coating adhesion is reduced when forming Sn-Zn alloy coated steel plates as raw materials.

[0196] In addition to the Ni coating and Ni-Fe coating described above, the pre-plating layer in this embodiment can also be an Fe-Ni coating. Specifically, before the plating process (Sn-Zn plating) described later, an Fe-Ni based metal coating layer (Fe-Ni coating) with Fe as the main component is formed on the pickled steel sheet. This promotes the formation of the Sn-Fe-Cr layer (alloy layer A) and the Sn-Fe-Ni-Zn layer (alloy layer B) during the plating process, further improving the appearance of the Sn-Zn based coating layer C, and refining the primary Sn crystals to improve corrosion resistance. The Fe-based metal coating layer (Fe-Ni coating) can be formed using a bath mainly composed of ferrous sulfate, Ni sulfate, Ni chloride, and boric acid.

[0197] <Plating Process>

[0198] The steel sheet (pre-plated steel sheet) having the aforementioned metallic coating (pre-coating) is subjected to Sn-Zn based hot-dip galvanizing treatment to form a Sn-Zn based coating C. The Sn-Zn based coating C is formed by hot-dip galvanizing. Furthermore, in the formation of the Sn-Zn based coating C, either the flux method or the Sendzimir strip nitriding zinc plating method can be preferably used.

[0199] The bath temperature for Sn-Zn based plating is set below 260°C. At bath temperatures above 260°C, excessive alloy layer growth occurs, forming a Fe-Ni-Cr-Zn layer (alloy layer D) that contributes to the deterioration of corrosion resistance. Preferably, the bath temperature is set below 255°C. Furthermore, the lower limit of the bath temperature varies depending on the Zn content in the plating. For example, the bath temperature is preferably set above the melting point of the plating layer plus 25°C.

[0200] The composition of the Sn-Zn plating bath can be appropriately adjusted to obtain the chemical composition of the Sn-Zn plating layer C described above. For example, the Zn concentration in the bath composition can be set to 1% to 15%. When the Zn concentration in the bath composition is less than 1%, the rust prevention effect provided by Zn is insufficient, and the protective effect from the formation of Zn corrosion products is also insufficient, potentially leading to a deterioration in corrosion resistance. Furthermore, when the Zn concentration in the bath composition exceeds 15%, the plating layer becomes porous (porous) during Zn corrosion in a corrosive environment, potentially resulting in a deterioration in corrosion resistance.

[0201] The temperature of the steel sheet before immersion in the Sn-Zn based plating bath (immersion temperature) is preferably set to 50°C or higher. When the immersion temperature is below 50°C, insufficient reaction with the flux results in inadequate removal of surface oxides, which may lead to uncoated areas and deterioration of corrosion resistance. The preferred immersion temperature is 60°C or higher.

[0202] As described above, by appropriately controlling the bath temperature and immersion temperature, a desired Sn-Fe-Cr layer (alloy layer A) and Sn-Fe-Ni-Zn phase (alloy layer B) can be formed between the base steel plate and the Sn-Zn coating C.

[0203] It should be noted that when forming the Sn-Zn coating C by flux method, the temperature of the steel plate before immersion in the Sn-Zn plating bath can be controlled by adjusting the temperature of the flux applied to the pre-plated steel plate.

[0204] In the flux method, a flux solution is applied to a pre-plated steel sheet having the aforementioned metallic coating, and then the sheet is immersed in a plating bath for hot-dip plating. Specifically, an aqueous flux solution of 2–45% by mass (based on halogen conversion) is applied to the pre-plated steel sheet, and the sheet is then immersed in a plating bath. In the flux method, a predetermined alloy layer A and B can be formed after effectively removing the oxide film from the surface of the pre-plated layer, thus providing a strong effect in suppressing coating repulsion. Fluxes containing chlorides such as ZnCl2, NH4Cl, and HCl, and bromides such as ZnBr2 and NH4Br are effective as fluxes used.

[0205] In addition, before applying the flux solution, a solution of 1-10% dilute hydrochloric acid is pre-applied to the pre-plated steel plate, thereby improving the appearance of the plating.

[0206] The temperature of the flux solution is set to 50°C or higher. By maintaining the flux solution temperature above 50°C, the temperature of the steel sheet after flux coating can be increased, thereby promoting the removal of oxide film and alloying reaction on the steel sheet surface. There is no specific upper limit set for the flux solution temperature; for example, it can be set below 90°C. If the flux solution temperature is higher than 90°C, the amount of flux evaporation increases, making flux solution processing more difficult, and therefore this is not preferred.

[0207] In addition, when forming a Sn-Zn based coating C by the Sendzimir hot-dip galvanizing method (Sendzimir method), the temperature of the steel plate before immersion in the galvanizing bath can be controlled by adjusting the temperature conditions inside the nozzle.

[0208] In the Sendzimir process, conventional annealing and plating equipment, including heating zone, soaking zone, cooling zone, and plating process, can be used.

[0209] The temperature of the isothermal zone can be set to 700–870°C to obtain a recrystallized structure, and the dew point can be set to below -20°C. Furthermore, the atmosphere in the isothermal zone is an N2-H2 atmosphere, and the H2 concentration can be adjusted to be above 1% and below 100%.

[0210] In the Sendzimir process, before immersing the pre-plated steel sheet in the plating bath, the immersion temperature of the steel sheet is adjusted to the same temperature as the bath temperature in a cooling zone, and then it is immersed in the plating bath. In the Sendzimir process, the bath temperature only needs to be below 260°C.

[0211] After the plating process, the coating adhesion is adjusted to 5-80 g / m² per side by means of air wiping and other methods. 2 .

[0212] Through the above processes, a layer with a surface coverage of more than 70% and mainly composed of Ni-Fe-Cr (alloy layer A) and a layer mainly composed of Sn-Fe-Ni-Zn formed on alloy layer A (alloy layer B) can be formed.

[0213] The manufacturing method of this embodiment, as described above, enables the production of Sn-Zn based coated steel sheets that possess a good coating appearance, high corrosion resistance, and excellent processability, making them suitable for automotive applications, particularly fuel tanks. Furthermore, the manufacturing method of this embodiment allows for a stable supply of the Sn-Zn based coated steel sheets of this embodiment.

[0214] [Battery casing]

[0215] The Sn-Zn coated steel sheet of this embodiment is suitable, for example, for use in battery housings mounted in automobiles. Figure 6 This is a perspective view showing an example of the lower part 100 of the battery casing.

[0216] Figure 6 The lower part 100 shown is merely one example, and its shape, size, etc., are not limited to this. Furthermore, even when the Sn-Zn based coated steel sheet of this embodiment is processed into… Figure 6 Even with the shape of the battery casing shown, the components constituting the Sn-Zn-based coated steel sheet of this embodiment will not be affected. Furthermore, when obtaining the battery casing, welding and painting processes can be performed on the upper part after the base steel sheet having the above chemical composition has been processed into the desired lower shape. Additionally, although not shown, the Sn-Zn-based coated steel sheet of this embodiment can be appropriately applied to the upper part of the battery casing, similarly to the lower part 100.

[0217] The lower part 100 has a Sn-Zn coated steel sheet of this embodiment, which has excellent corrosion resistance, especially the corrosion resistance of the outer surface.

[0218] [Fuel Tank]

[0219] The Sn-Zn coated steel sheet of this embodiment is also suitable for use in fuel tanks in automobiles. Figure 7 This is a perspective view showing an example of fuel tank 200.

[0220] The fuel tank 200, for example, has an upper tank body 201, a lower tank body 202, a partition 203, and a secondary tank body 204. The Sn-Zn coated steel sheet of this embodiment is suitable for the tank body 201, the lower tank body 202, the partition 203, and the secondary tank body 204.

[0221] Figure 7 The fuel tank 200 shown is merely one example, and its shape, size, etc., are not limited to this. Furthermore, even when the Sn-Zn based coated steel sheet of this embodiment is processed into… Figure 7 Even with the shape of the fuel tank shown, the components constituting the Sn-Zn based coated steel sheet described in this embodiment will not be affected. It should be noted that when obtaining the fuel tank 200, the pre-plating process and plating process described above can be performed after the base steel sheet having the above chemical composition has been processed into the shapes of the components constituting the fuel tank 200.

[0222] The fuel tank 200 has a Sn-Zn coated steel sheet as described in this embodiment, and therefore has excellent corrosion resistance, especially on the inner surface.

[0223] Example

[0224] The following describes in more detail an embodiment of the Sn-Zn-based coated steel sheet and its preferred manufacturing method according to an example of the present invention. It should be noted that the examples shown below are merely one example of the Sn-Zn-based coated steel sheet of this embodiment, and the Sn-Zn-based coated steel sheet of this embodiment is not limited to the examples shown below. Various conditions can be employed to achieve the purpose of the present invention without departing from its spirit.

[0225] <Example 1>

[0226] The steel with the composition shown in Table 1 below is smelted, and then hot-rolled, pickled, and cold-rolled to produce a cold-rolled steel sheet with a thickness of 0.8 mm. After annealing, the cold-rolled steel sheet is subjected to a pre-plating process and a Sn-Zn based plating process under the conditions shown in Table 2.

[0227] (Pickling process)

[0228] In the following electrolytic pickling bath, the steel plate side is used as the cathode, and the current is 10 A / dm³. 2 Perform an electrolytic pickling process for 5 seconds. Then, wash and dry the surface of the steel plate to produce a pickled steel plate.

[0229] Electrolytic pickling bath: sulfuric acid (120g / L) + sodium nitrate (100g / L) + sodium sulfate (120g / L) + hexafluorosilicic acid (15g / L), bath temperature 50℃.

[0230] (Pre-plating process)

[0231] Apply 1g / m to pickled steel plates 2 After Fe-Ni plating or Ni plating, the following Sn-Zn plating process is performed.

[0232] Regarding Fe-Ni plating, specifically, after adjusting the pH of the Fe-Ni plating bath to 1.5 using sulfuric acid, the steel plate side is used as the cathode in the bath at 10 A / dm². 2 After a 0.12-second electrolytic treatment, the steel plate surface is washed with water and dried. In the Fe-Ni coating, Ni accounts for 25% by mass, with the remainder being Fe and impurities.

[0233] •Fe-Ni plating bath: 110g / L ferrous sulfate, 75g / L nickel sulfate, 140g / L nickel chloride, 30g / L boric acid, bath temperature 45℃.

[0234] Regarding Ni plating, after adjusting the Ni plating bath to pH 4.0 or 1.5 using sulfuric acid, the steel plate side is used as the cathode in the bath at 10 A / dm². 2 After an electrolytic treatment of 0.11 seconds, the steel plate surface is washed with water and dried. The Ni coating is composed of 100% Ni by mass.

[0235] · Ni plating bath: nickel sulfate 75 g / L, nickel chloride 140 g / L, boric acid 30 g / L, bath temperature 45 °C.

[0236] (Sn-Zn plating process)

[0237] After the pre-plating process, the Sn-Zn plating process is carried out by the flux method or the Sendzimir method. As the flux aqueous solution, a ZnCl2-NH4Cl aqueous solution is roll-coated. The composition of Zn in the plating bath is adjusted as shown in Table 2. The bath temperature and the plate temperature when the steel plate is immersed in the bath are adjusted as shown in Table 2. In the case of the flux method, the steel plate is immersed for 8 seconds, and in the case of the Sendzimir method, after the steel plate is immersed for 3 seconds, the coating adhesion amount is adjusted by wiping with N2 gas.

[0238] The compositions and adhesion amounts of alloy layer A, alloy layer B, and coating layer C in the obtained plated steel plate are shown in Table 3. In addition, the coverage rate of alloy layer A is the ratio of alloy layer A to the surface coverage of the steel material, and the part other than alloy layer A is alloy layer D.

[0239] Next, the corrosion resistance and workability shown below are evaluated respectively.

[0240] <Corrosion resistance evaluation>

[0241] The corrosion resistance is evaluated by the following composite cycle test.

[0242] (x1) Outer surface corrosion resistance:

[0243] As shown in the schematic diagram (plan view) of the corrosion resistance test specimen material Figure 5 , through JASO (Automobile Standards of the Society of Automotive Engineers) M610-92 "Automobile Component Appearance Corrosion Test Method", the test materials of a flat material of 70×150 mm and a plate of 35×100 mm spot-welded at three points on a plate of 70×150 mm are evaluated. Here, in the flat material of 70×150 mm, its end face and back face are sealed. In Figure 5 , the three-point spot welding is denoted as the spot welding part Y, and the sealing of the end face and back face is denoted as the sealing part Z. In addition, no sealing part is provided on the plate of 35×100 mm. The evaluation is carried out based on the rust generation area ratio of the flat part and the end face part of the 35×100 mm plate. The specimens evaluated as A and B for the end face part are set as qualified. In addition, for the samples evaluated as C, since they can be used well as long as they are painted, they are judged as qualified.

[0244] [Evaluation conditions]

[0245] Test period: 360 cycles (120 days)

[0246] [Evaluation Criteria] (Evaluated by the area ratio of rust generation)

[0247] A: Red rust generation is less than 0.1%

[0248] B: Red rust generation is 0.1% or more and less than 1% or white rust is generated (white rust is less than 20%)

[0249] C: Red rust generation is 1% or more and less than 5% or obvious white rust (white rust is 20% or more and less than 70%)

[0250] X: Red rust generation is 5% or more or significant white rust (white rust exceeds 70%)

[0251] (x2) Corrosion resistance of the inner surface:

[0252] Cut out a test piece with a size of 110 mm × 110 mm from the obtained plated steel sheet. Using this test piece, a flanged cup is made by cylindrical deep drawing using a φ50 mm punch. A 50 ml aqueous solution of gasoline at 10% by mass containing 500 ppm of formic acid, 1000 ppm of acetic acid, and 165 ppm of NaCl is sealed in the cup and placed in a constant temperature bath at 45°C for 1000 hours. After the test, visually observe the sample to confirm whether red rust is generated from the bottom surface of the cup. Samples evaluated as A and B are considered qualified.

[0253] [Evaluation Criteria]

[0254] A: No rust

[0255] B: White rust is generated

[0256] X: Red rust is generated

[0257] [Workability Evaluation]

[0258] Workability is evaluated by a cylindrical deep drawing test.

[0259] (y1) Cylindrical deep drawing test:

[0260] Using a flat-bottom cylindrical die with a punch diameter of φ50 mm, cylindrical deep drawing is carried out. Use Noxrust530-F40 (manufactured by Nihon Parkerizing) as the lubricant and perform it with an anti-wrinkle pressure of 700 kgf. Evaluate using the maximum drawing ratio (blank diameter ÷ punch diameter) that can be deep drawn at this time and the plating appearance of the processed part. Samples evaluated as A, B, and C are considered qualified.

[0261] [Evaluation Criteria]

[0262] A: Can be formed, no defects in the coating, drawing ratio of 2.3 or more

[0263] B: Can be formed without coating defects, with a draw ratio of 2.2 or higher.

[0264] C: Can be formed without coating defects, with a draw ratio of 2.0 or higher.

[0265] X: Although it can be formed, the draw ratio is less than 2.0, or adhesion occurs in the coating.

[0266] Table 1

[0267]

[0268] Table 2

[0269]

[0270] The evaluation results are shown in Table 3. It should be noted that the underlines in Tables 2-3 indicate manufacturing conditions that are outside the scope of this embodiment, deviate from the preferred conditions, or have unpreferred characteristic values.

[0271] As shown in Table 3, the performance of Invention Examples C1 to C15 is good. On the other hand, Comparative Examples C1 to C8 deviate from the preferred manufacturing conditions of the present invention, and at least one of the following—the surface coverage of alloy layer A (first alloy layer), the amount of Zn in plating layer C (Sn-Zn based plating layer), and the amount of adhesion on each side of plating layer C—deviates from the range of this embodiment. As a result, in Comparative Examples C1 to C8, at least one of corrosion resistance and processability deteriorates.

[0272] <Example 2>

[0273] Next, using steel No. A1 as shown in Table 1, a Sn-Zn plating process was performed according to preparation method No. B3 as shown in Table 2. Then, a chemical conversion treatment film was formed as shown in Table 4. As chemical conversion treatment films, Nos. D1 to D6 all formed chromate-free films on plating layer C. Specifically, trivalent chromium, silicon dioxide (SiO2), and phosphate ions (PO42-) were used. 3+ A reagent with chromate as the main component is applied to the coating, and then baked at 80°C in a hot air drying oven to form a chromate-free film.

[0274] Furthermore, for No. D4 to D6, a coating was further formed on the chemically converted film. In No. D4 to D5, "Amilac 1000 (manufactured by KANSAIPAINT Co., Ltd.)" was used as the coating material, and the film was formed by baking in an oven at 130°C for 20 minutes. In No. D6, "High-Resolution E-Super (manufactured by Nippon Paint Co., Ltd.)" was used as the coating material, and a coating was formed by recoating.

[0275] The composition and adhesion amount of alloy layer A, alloy layer B, and coating layer C in the obtained coated steel sheet are shown in Table 4.

[0276] Next, corrosion resistance and processability were evaluated in the same manner as described above. The evaluation results are shown in Table 4. Additionally, underlines in Table 4 indicate manufacturing conditions or characteristic values ​​that deviate from the preferred specifications outside the scope of this embodiment.

[0277] As shown in Table 4, the performance of invention examples D1 to D6 is good.

[0278]

[0279] <Example 3>

[0280] Next, the invention examples No. C2 and C14 shown in Table 3 are processed into Figure 6 and Figure 7 After showing the shape of the lower part and fuel tank, test pieces are cut out from the center of the flat parts of the bottom and top surfaces. The composition and adhesion of alloy layer A, alloy layer B, and coating layer C in the coated steel plate, as well as corrosion resistance and processability, are evaluated in the same manner as above.

[0281] As a result, the performance of the battery casing and fuel tank, including all components of the coated steel sheet, is good within the scope of this embodiment.

[0282] Industrial availability

[0283] According to the above-described scheme of the present invention, hot-dip galvanized Sn-Zn alloy steel sheets with excellent coating appearance, corrosion resistance, and processability can be obtained. Therefore, the obtained hot-dip galvanized Sn-Zn alloy steel sheets are applicable to the automotive field, especially to automobiles used for fuel tanks and battery housings (particularly automobile fuel tanks and battery housings), home appliances, building materials, and other fields, thus exhibiting high industrial applicability.

[0284] Explanation of reference numerals in the attached figures

[0285] 10: Sn-Zn series alloy coated steel (coated steel)

[0286] 11: Steel

[0287] 12: First alloy layer (alloy layer A)

[0288] 13: Second alloy layer (alloy layer B)

[0289] 13A: Layered region

[0290] 13B: Needle-like region

[0291] 14: Sn-Zn based coatings (Sn-Zn based coating C, coating C)

[0292] 100: lower part

[0293] 200: Fuel tank

[0294] 201: Upper part of the box

[0295] 202: Lower part of the box

[0296] 203: Partition

[0297] 204: Sub-box

Claims

1. A Sn-Zn alloy-coated steel, characterized in that, It has the following characteristics: steel A first alloy layer, mainly composed of Fe, Cr, and Ni, is formed on the surface of the steel. A second alloy layer mainly composed of Sn, Fe, Ni, and Zn is formed on the first alloy layer, and A Sn-Zn based coating, mainly composed of Sn and Zn, is formed on the second alloy layer. The first alloy layer has a surface coverage of more than 70% on the surface of the steel. The second alloy layer is composed of FeSn2 phase and Fe3Zn partially replaced by Ni. 10 At least one of the selected alloy layers is the main component. The Sn-Zn based coating contains 1.0 to 15.0% Zn by mass, with the remainder being Sn and impurities. The total amount of the first alloy layer, the second alloy layer, and the Sn-Zn coating is 5-80 g / m² on each side. 2 .

2. The Sn-Zn alloy-coated steel according to claim 1, characterized in that, The steel contains, by mass percent: C：0.0005～0.030％、 Si: below 0.80% Mn: 0.10~2.00% P：0.005～0.040％、 S: below 0.0100% Cr：4.0～18.0％、 Al：0～0.30％、 Ti: 0~0.300% Nb: 0~0.040% B：0～0.0030％、 N：0～0.030％、 Cu: 0–2.0% Ni: 0-3.0% Mo: 0–2.00% V：0～2.00％， The remainder contains Fe and impurities.

3. The Sn-Zn alloy-coated steel according to claim 2, characterized in that, The steel also contains REM at a rate of 0-1.000% by mass.

4. The Sn-Zn alloy-coated steel according to claim 1 or 2, characterized in that, At least one side of the Sn-Zn coating has a chemically converted film with a thickness of 0.02 to 2.0 μm.

5. The Sn-Zn alloy-coated steel according to claim 1 or 2, characterized in that, At least one side of the Sn-Zn-based coating has a coating film with a thickness of 10 to 500 μm.

6. The Sn-Zn alloy-coated steel according to claim 3, characterized in that, At least one side of the Sn-Zn-based coating has a coating film with a thickness of 10 to 500 μm.

7. A battery casing comprising the Sn-Zn alloy-coated steel as described in claim 1.

8. A fuel tank comprising the Sn-Zn alloy coated steel as described in claim 1.

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