Surface-treated steel sheet

Abstract

Provided is a surface-treated steel sheet comprising a Zn-Al-Mg plated steel sheet having a plating amount of 20-60 g / m2 per one surface and a coating film disposed on at least one main surface of the Zn-Al-Mg plated steel sheet, wherein the coating film comprises a binder resin, doped zinc oxide particles, and a rust preventive, the coating film has an average film thickness of 0.5-3 μm, the doped zinc oxide particles have an average particle diameter of 0.1-2 μm, the content of the doped zinc oxide particles is 20-40 mass% with respect to the coating film, and the content of inorganic substances other than any Zn-containing compounds in the coating film is 0-1 mass% with respect to the coating film.

Description

[Technical Field]

[0001] The present disclosure relates to a surface-treated steel sheet. [Background technology]

[0002] The surface-treated steel sheet is, for example, press-formed and then assembled into a desired shape by spot welding or the like, and then electro-deposition coated, or if electro-deposition coating is omitted, an undercoat is applied, and then the sheet is used as an automobile part. Furthermore, the corrosion resistance of automotive components is often ensured by a chemical conversion coating applied in a chemical conversion treatment process and an electrodeposition coating applied in a subsequent electrodeposition coating process. However, at the joints (sheet joints) of formed materials of surface-treated steel sheets, particularly at the joints of the inner sheets of bag-shaped components and at folded hems, there are sometimes areas where the chemical conversion coating and electrodeposition coating do not wrap around. In such cases, the joints of formed materials are likely to be exposed to a corrosive environment in their bare state. For this reason, there is a demand for surface-treated steel sheets that can ensure corrosion resistance.

[0003] Thus, surface-treated steel sheets must have an electrically conductive coating film to improve weldability so that resistance welding is possible, and must also be provided with corrosion resistance.

[0004] For example, Patent Document 1 describes a surface-treated steel sheet having a coating film on at least one side of the plated steel sheet, the coating film containing a binder resin, V-containing non-oxide ceramic particles (excluding VC particles), and doped zinc oxide particles, the content of the V-containing non-oxide ceramic particles and the doped zinc oxide particles relative to the coating film satisfying the following formula: C Zn ≧10.0 (1) CV ≦0.5 C Zn ···(2) CV ≦70°C Zn ···(3) CV ≥ 0.125 C Zn ···(4) CV ≧2.0 (5) Here, CV means the content (mass%) of the non-oxide ceramic particles containing V, and CZn means the content (mass%) of the doped zinc oxide particles." has been proposed." has been proposed." According to Patent Document 1, it is described that "a surface-treated steel sheet having excellent corrosion resistance and weldability before electrodeposition coating can be provided." [Prior art documents] [Patent documents]

[0005] Patent Document 1: International Publication No. 2018 / 092244 Summary of the Invention [Problem to be solved by the invention]

[0006] Surface-treated steel sheets are widely used for applications such as automobile components, machine components, home appliance components, and building materials, and are required to have improved corrosion resistance and weldability. However, although various research and development efforts have been made up to now, including the one described in Patent Document 1, there is currently a demand for further improvements in the corrosion resistance and weldability of surface-treated steel sheets in response to recent demands.

[0007] Therefore, an object of the present disclosure is to provide a surface-treated steel sheet that is excellent in both corrosion resistance and weldability. [Means for solving the problem]

[0008] Means for solving the above problems include the following aspects. <1> Plating weight per side: 20-60g / m 2 Zn-Al-Mg coated steel sheet and a coating film disposed on at least one main surface of the Zn-Al-Mg-plated steel sheet; and the coating film comprises a binder resin, doped zinc oxide particles, and a rust inhibitor; The average thickness of the coating film is 0.5 to 3 μm, The doped zinc oxide particles have an average particle size of 0.1 to 2 μm, the content of the doped zinc oxide particles is 20 to 40% by mass relative to the coating film; the content of inorganic substances other than the Zn-containing compound in the coating film is 0 to 1% by mass or less relative to the coating film; Surface-treated steel sheet. <2> The rust inhibitor is an organic acid, The content of the rust inhibitor is 1 to 20 mass % relative to the coating film. <1> The surface-treated steel sheet according to claim 1. <3> an intermediate coating film is provided between the Zn-Al-Mg-plated steel sheet and the coating film, the intermediate coating film contains a binder resin, a silane coupling agent, silica fine particles, a phosphoric acid compound, and a fluorine compound; The average thickness of the intermediate coating film is 0.1 to 0.5 μm. <1> or <2> Surface treated steel plate. <4> a chemical composition of the Zn-Al-Mg-based coating layer of the Zn-Al-Mg-based coated steel sheet containing, on average, 4 to 22 mass% Al, 1.0 to 10 mass% Mg, and the remainder including Zn and impurities; <1~ <3> Surface-treated steel sheets according to any one of the above. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a surface-treated steel sheet that is excellent in both corrosion resistance and weldability. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a surface-treated steel sheet according to this embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing an example of a coated member according to this embodiment. [Figure 3] FIG. 3 is a schematic plan view showing a bonded test piece prepared in the corrosion resistance test of the example. [Figure 4]FIG. 4 is a schematic side view showing a bonded test piece prepared in the corrosion resistance test of the example. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment that is an example of the present disclosure will be described. In this specification, when the lower limit of the content of each element in the chemical composition is expressed as "0", this means that the element is an optional component and does not have to be contained. In a numerical range expressed using "to", when the numbers before and after "to" are not followed by "greater than" or "less than", it means a range that includes those numbers as the lower and upper limits. Furthermore, when the numbers before and after "to" are followed by "greater than" or "less than", it means a range that does not include those numbers as the lower or upper limits. In numerical ranges described in stages, the upper limit of a certain numerical range may be replaced by the upper limit of another numerical range described in stages, or may be replaced by a value shown in an example. Also, the lower limit of a certain numerical range may be replaced by the lower limit of another numerical range described in stages, or may be replaced by a value shown in an example. The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0012] <Surface-treated steel sheet> The surface-treated steel sheet according to this embodiment is Plating weight per side: 20-60g / m 2 Zn-Al-Mg coated steel sheet and a coating film disposed on at least one main surface of a Zn-Al-Mg-plated steel sheet; (See Figure 1) The coating film includes a binder resin, doped zinc oxide particles, and a rust inhibitor; The average thickness of the coating film is 0.5 to 3 μm. The doped zinc oxide particles have an average particle size of 0.1 to 2 μm, The content of the doped zinc oxide particles is 20 to 40 mass % of the coating film, The content of inorganic substances other than the Zn-containing compound in the coating film is 0 to 1% by mass or less relative to the coating film. In FIG. 1, 10 denotes a surface-treated steel sheet, 11 denotes a base steel sheet in the Zn-Al-Mg-plated steel sheet (a steel sheet on which the Zn-Al-Mg-plated layer is to be formed), 12 denotes the Zn-Al-Mg-plated layer of the Zn-Al-Mg-plated steel sheet, and 13 denotes a coating film. The main surfaces of the Zn-Al-Mg plated steel sheet refer to two surfaces that face each other in the thickness direction of the Zn-Al-Mg plated steel sheet. Here, one side of the Zn-Al-Mg plated steel sheet means one of a pair of surfaces facing each other in the thickness direction of the Zn-Al-Mg plated steel sheet.

[0013] The surface-treated steel sheet according to this embodiment has the above-described configuration, and is excellent in both corrosion resistance and weldability. The surface-treated steel sheet according to this embodiment was discovered based on the following findings.

[0014] The inventors have investigated the corrosion resistance and weldability of surface-treated steel sheets, and have obtained the following findings. To date, zinc-based plated steel sheets, in which a zinc plating is applied to the steel sheet, have been mainly used as surface-treated steel sheets for applications such as automotive parts in order to ensure corrosion resistance and weldability. In order to further improve corrosion resistance while maintaining weldability, painted steel sheets, in which a coating film containing a binder resin, a rust-preventive pigment, and a rust inhibitor is provided on a zinc-based plated steel sheet, have been studied. On the other hand, at the joints (plate joining parts) of formed materials of surface-treated steel sheets, there are areas where the chemical conversion coating film and electrocoating film do not wrap around, and the joints of the formed materials are exposed to a corrosive environment in a bare state, which can lead to the progression of corrosion. Therefore, by using a Zn-Al-Mg plated steel sheet as the zinc-based plated steel sheet, excellent corrosion resistance is exhibited in the surface-treated steel sheet.

[0015] However, when Zn-Al-Mg plated steel sheet, which has excellent corrosion resistance, is used, Al and Mg, which are components of the plating, adhere to the welding electrode during welding, deteriorating weldability (especially continuous spot welding). Therefore, the coating weight of Zn-Al-Mg coated steel sheet per side is set to 20 to 60 g / m 2 By reducing the amount of Al and Mg to 0.1%, adhesion of Al and Mg to the welding electrode during welding is suppressed. In addition, the coating contains large doped zinc oxide particles with an average particle size of 0.1 to 2 μm as an anti-corrosion pigment, and the content of the doped zinc oxide particles is increased to 20 to 40 mass %, which makes it easier for the welding electrode to come into contact with the doped zinc oxide particles during welding, but makes it less likely for the welding electrode to come into contact with the Zn-Al-Mg-based coating layer of the Zn-Al-Mg-based coated steel sheet. Furthermore, even if a Zn-containing compound such as doped zinc oxide particles comes into contact with a welding electrode during welding and adheres to the electrode, it is unlikely to cause welding defects. This is because, among inorganic substances, a Zn-containing compound adheres to the welding electrode mainly as zinc oxide and has low reactivity with the metal components (e.g., Cu) of the welding electrode, so that even if it adheres to the welding electrode, it is unlikely to interfere with welding. Therefore, the coating film must not contain any inorganic substances other than the Zn-containing compound, or even if it does contain inorganic substances other than the Zn-containing compound, the content of inorganic substances other than the Zn-containing compound must be controlled to 1 mass% or less of the coating film.

[0016] From the above findings, it has been found that the surface-treated steel sheet according to this embodiment is a surface-treated steel sheet that is excellent in both corrosion resistance and weldability.

[0017] Hereinafter, the surface-treated steel sheet according to this embodiment will be described in detail.

[0018] (Zn-Al-Mg plated steel sheet) A Zn-Al-Mg-plated steel sheet is a steel sheet in which a Zn-Al-Mg-plated layer containing at least Zn, Al, and Mg is formed on a base steel sheet. The Zn-Al-Mg-plated layer containing at least Zn, Al, and Mg contains Zn as a main component (the maximum component being, for example, 40 mass% or more of Zn), and is a plating layer with excellent corrosion resistance. The Zn-Al-Mg based plating layer may contain elements such as Si, Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, and C in addition to Zn, Al, and Mg. A specific example of the chemical composition of the Zn-Al-Mg based plating layer is a chemical composition containing, on average, 4 to 22 mass% Al, 1 to 10 mass% Mg, with the remainder containing Zn and impurities. A more preferred specific chemical composition of the Zn-Al-Mg-based plating layer is a chemical composition containing, on average, 4 to 22 mass% Al, 1 to 10 mass% Mg, and 0.0001 to 2 mass% Si, with the remainder containing Zn and impurities. In a Zn-Al-Mg-based coating layer, impurities refer to components contained in the raw materials or components mixed in during the manufacturing process, but not intentionally added. For example, a Zn-Al-Mg-based coating layer may contain trace amounts of components other than Fe as impurities due to mutual atomic diffusion between the base steel sheet and the Zn-Al-Mg-based coating layer.

[0019] Here, the base steel sheet for the Zn-Al-Mg-plated steel sheet (the steel sheet on which the Zn-Al-Mg-plated layer is to be formed) is not particularly limited. The base steel sheet may be a hot-rolled steel sheet obtained by hot-rolling a slab having an appropriate chemical composition for steel, or a cold-rolled steel sheet obtained by cold-rolling such a hot-rolled steel sheet. The strength of the base steel sheet may be, for example, 270 to 1470 MPa in tensile strength class. The thickness of the base steel plate is not particularly limited, but is preferably 0.4 to 3.2 mm, more preferably 0.5 to 2.4 mm. The thickness of the Zn-Al-Mg plated steel sheet is measured using a microgauge (CPM-MX manufactured by Mitutoyo Corporation).

[0020] Zn-Al-Mg coated steel sheet with a coating weight of 20 to 60 g / m 2 is. If the coating weight is too low, the corrosion resistance will decrease, whereas if the coating weight is too high, Al and Mg will easily adhere to the welding electrode during welding, resulting in decreased weldability. Therefore, the plating weight is set within the above range. Plating weight is 25-50g / m 2 is preferable, and 30 to 40 g / m 2 is more preferred. Here, the plating weight was measured as follows. The coating weight of the plating layer can be measured by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet (pickling), and then measuring the change in weight before and after pickling. If a coating film is present, it can be measured by removing the coating film with a chemical (remover, etc.) or resin shot before performing the above pickling.

[0021] The method for forming the Zn-Al-Mg-based coating layer of the Zn-Al-Mg-based coated steel sheet is not particularly limited. For example, the Zn-Al-Mg-based coating layer can be formed by hot-dip galvanization or the like. The Zn-Al-Mg-based coating layer can be formed by either a continuous process or a batch process. Furthermore, after the Zn-Al-Mg-based coating layer is formed, it may be subjected to treatments such as zero-spangle treatment for uniform appearance, annealing treatment for modifying the Zn-Al-Mg-based coating layer, and temper rolling for adjusting the surface condition or material properties.

[0022] (paint film) The coating film contains a binder resin, doped zinc oxide particles, and a rust inhibitor. If necessary, the coating film may contain other components.

[0023] -Binder resin- The binder resin may be either a water-soluble or water-dispersible aqueous resin that dissolves or disperses in water, or a solvent-based resin that dissolves or disperses in an organic solvent, but from the standpoints of production cost and environmental friendliness, a water-based resin is preferred.

[0024] Examples of the water-based resin include water-soluble or water-dispersible resins such as polyester resin, urethane resin, polyolefin resin, acrylic resin, epoxy resin, phenol resin, and mixed resins of two or more of these resins. When a polyester resin is used, the molecular weight is preferably 10,000 to 30,000. A molecular weight of 10,000 or more ensures sufficient processability. On the other hand, a molecular weight of 30,000 or less increases the number of bonding sites of the resin itself, ensuring excellent adhesion to the electrodeposition coating film. Furthermore, when crosslinking is performed using a curing agent such as melamine, the crosslinking reaction is carried out sufficiently, ensuring the performance of the coating film. Here, the molecular weight of the polyester resin is the weight average molecular weight. When a urethane resin is used, the urethane resin is preferably in the form of an emulsion with an emulsion particle size of 10 to 100 nm (preferably 20 to 60 nm). When the emulsion particle size is 10 nm or more, costs can be kept low. On the other hand, when the emulsion particle size is 100 nm or less, the gaps between the emulsion particles are small when the coating film is formed, ensuring the barrier properties of the coating film. Examples of urethane resin types include ether-based, polycarbonate-based, ester-based, and acrylic graphite-based types. These may be used alone or in combination.

[0025] Examples of solvent-based resins include polyester resins, urethane resins, epoxy resins, acrylic resins, and mixed resins of two or more of these resins.

[0026] Here, the binder resin may be a crosslinked resin having a crosslinked structure or a non-crosslinked resin not having a crosslinked structure, but a non-crosslinked resin is preferable from the viewpoint of low-temperature film formation of the coating film. As a crosslinking agent (curing agent) that imparts a crosslinked structure to the binder resin, a water-soluble crosslinking agent is preferable. Specific examples of the crosslinking agent include melamine, isocyanate, silane compounds, zirconium compounds, and titanium compounds.

[0027] The amount of crosslinking agent added is preferably 5 to 30 parts by mass per 100 parts by mass of resin solids. When the amount of crosslinking agent added is 5 parts by mass or more, the crosslinking reaction with the binder resin is ensured, resulting in sufficient performance as a coating film. On the other hand, when the amount of crosslinking agent added is 30 parts by mass or less, the crosslinking reaction proceeds moderately, preventing the coating film from becoming excessively hard. This ensures processability. In addition, the paint stability of silane compounds, zirconium compounds, and titanium compounds can be ensured.

[0028] The binder resin content is preferably 10.0 to 90.0% by mass relative to the coating film (total solids content of the coating film). When the binder resin content is 10.0% by mass or more, the binder function is easily exerted and the cohesive strength of the coating film is improved. As a result, internal failure of the coating film (cohesive failure of the coating film) is less likely to occur during adhesion tests and molding processing. On the other hand, when the binder resin content is 90.0% by mass or less, the proportion of pigment components contained in the coating film is reduced, making it easier to achieve both weldability, corrosion resistance, and adhesion to the electrodeposition coating film. The content of the binder resin is more preferably 15.0 to 80.0 mass % of the coating film (total solid content of the coating film) in order to exhibit the binder function and simultaneously achieve weldability, corrosion resistance, and adhesion to the electrodeposition coating film.

[0029] -Doped zinc oxide particles- The doped zinc oxide particles are conductive zinc oxide particles. When the coating film contains the conductive doped zinc oxide particles, the weldability is improved.

[0030] Doped zinc oxide particles include, for example, particles that exhibit electrical conductivity by doping zinc oxide particles with at least one element (hereinafter also referred to as "dope element") selected from the group consisting of elements in Group 13 of the periodic table and elements in Group 15 of the periodic table.

[0031] Examples of Group 13 elements of the periodic table include B, Al, Ga, and In. Examples of Group 15 elements of the periodic table include P and As. Among these, from the viewpoint of improving electrical conductivity, the doping element is preferably Al or Ga. Furthermore, from the viewpoint of cost, the doping element is more preferably Al.

[0032] From the viewpoint of improving the electrical conductivity, the content of the doping element is preferably 0.05 to 5 atom %, more preferably 0.1 to 5 atom %, based on the undoped zinc oxide particles.

[0033] The doped zinc oxide particles have an average particle size of 0.1 to 2 μm. If the average particle size is too small, the welding electrode tends to come into contact with the Zn-Al-Mg-based coating layer of the Zn-Al-Mg-coated steel sheet during welding, resulting in poor weldability.On the other hand, if the average particle size is too large, the doped zinc oxide particles are less likely to come into uniform contact with the welding electrode (e.g., Cu electrode) during welding, resulting in the welding electrode tending to come into contact with the Zn-Al-Mg-based coating layer of the Zn-Al-Mg-coated steel sheet and poor weldability. Therefore, the average particle size of the doped zinc oxide particles is set to the above range. The average particle size of the doped zinc oxide particles is preferably 0.2 to 1.5 μm, more preferably 0.4 to 1.0 μm.

[0034] The "average particle size" of the doped zinc oxide particles refers to the average primary particle size when the doped zinc oxide particles exist alone in the coating film, and refers to the average secondary particle size representing the particle size of the oxide particles in the aggregated state when the doped zinc oxide particles exist in the aggregated state.

[0035] The average particle size of the doped zinc oxide particles is determined by the following measurement method. First, the surface-treated substrate on which the coating film was formed was cut along the thickness direction to expose the cross section. After embedding with resin, the exposed cross section was polished using a polishing machine. After polishing, the cross section was vapor-deposited with gold to create a sample for cross-sectional observation. The resulting cross section was observed using a scanning electron microscope to obtain an observation image of the cross section in the coating film. Twenty-five doped zinc oxide particles present in the field of view of the observation image were randomly selected, and the long and short side lengths of each doped zinc oxide particle were measured. Finally, the average long and short side lengths were calculated, and the average particle size was calculated by arithmetically averaging the arithmetic mean long and short side lengths. The doped zinc oxide particles present in the field of view of the observation image were identified as particles located in the region where Al was detected within the region where Zn was detected in EPMA analysis of the field of view. The observation conditions for the scanning electron microscope are as follows: Measurement equipment: JEOL Ltd. JSM-7200F ·Magnification: 5000x Acceleration voltage: 10 keV Image analysis software: Image J Ver.1.54f

[0036] The content of the doped zinc oxide particles is 20 to 40 mass % relative to the coating film (total solid content of the coating film). If the content of doped zinc oxide particles is low, the welding electrode is more likely to come into contact with the Zn-Al-Mg-based coating layer of the Zn-Al-Mg-based coated steel sheet during welding, resulting in poor weldability.On the other hand, if the content of doped zinc oxide particles is high, the proportion of conductive pigment in the coating film decreases, resulting in insufficient performance such as weldability and corrosion resistance. Therefore, the content of the doped zinc oxide particles is set to the above range. The content of the doped zinc oxide particles is preferably 25 to 35 mass %, more preferably 27 to 33 mass %.

[0037] -Rust inhibitor- As the rust inhibitor, organic rust inhibitors such as organic acids, organic acid salts, amine salts, and esters are used to impart high weldability. This is because when organic rust inhibitors are used, they burn and carbonize due to the heat during welding, making them less likely to adhere to the welding electrode. Among these, organic acids are preferred because they can impart high corrosion resistance. Examples of organic acids include carboxylic acids (tartaric acid, tannic acid, oleic acid, dimer acid, naphthalene acid, etc.). Examples of organic acid salts include metal carboxylic acid soaps (such as lanolin Ca, naphthenate Zn, oxidized wax Ca, and Ba salts), and sulfonates (such as sodium sulfonate, calcium sulfonate, and barium sulfonate). Examples of esters include glycerin esters of higher fatty acids, sorbitan monoisostearate, and sorbitan norate. The content of the rust inhibitor is preferably 1 to 20 mass% of the coating film (total solid content of the coating film). When the content of the rust inhibitor is 1 mass% or more, corrosion resistance is sufficiently improved. On the other hand, when the content of the rust inhibitor is 20 mass% or less, a decrease in the liquid stability of the coating material is suppressed. From the viewpoint of corrosion resistance and paint stability, the content of the rust inhibitor is more preferably 1 to 3 mass % relative to the coating film (total solid content of the coating film).

[0038] (Other ingredients) Examples of other components include well-known additives such as lubricants, silane coupling agents, and leveling agents. However, if inorganic substances other than Zn-containing compounds adhere to the welding electrode during welding, the weldability will decrease. Therefore, the content of inorganic substances other than the Zn-containing compound in the coating film is set to 0 to 1% by mass or less relative to the coating film. That is, the coating film does not contain any inorganic substances other than the Zn-containing compound, or if it does contain any inorganic substances other than the Zn-containing compound, the content of the inorganic substances other than the Zn-containing compound is controlled to 1 mass % or less of the coating film. Here, the Zn-containing compound includes the doped zinc oxide particles described above. Examples of the Zn-containing compound including the doped zinc oxide particles include Al-doped zinc oxide, Sn-doped zinc oxide, zinc oxide, zinc hydroxide, and inorganic zinc salts. Examples of inorganic zinc include zinc sulfate, zinc nitrate, and zinc phosphide. Inorganic substances other than Zn-containing compounds including doped zinc oxide particles do not include metal salts of organic substances (such as metal salts of organic acids).

[0039] The solid lubricant can impart excellent lubricity to the coating film and can also improve the powdering resistance of the coating film. Examples of the solid lubricant include the following solid lubricants (1) and (2).

[0040] (1) Polyolefin wax, paraffin wax: for example, polyethylene wax, synthetic paraffin, natural paraffin, microcrystalline wax, chlorinated hydrocarbons, etc. (2) Fluorine resin wax: for example, polyfluoroethylene resin (polytetrafluoroethylene resin, etc.), polyvinyl fluoride resin, polyvinylidene fluoride resin, etc.

[0041] When polyethylene wax is used as the solid lubricant, the average particle size is preferably 0.5 to 10 μm. When the average particle size of the polyethylene wax is 0.5 μm or more, surface thickening of the polyethylene wax is suppressed, and a decrease in adhesion between the coating film and the electrodeposition coating film can be suppressed. On the other hand, when the average particle size of the solid lubricant is 10 μm or less, peeling of the polyethylene wax from the coating film is suppressed, and lubricity and corrosion resistance can be ensured. The average particle size of the solid lubricant is more preferably 1 to 5 μm in order to obtain excellent adhesion between the coating film and the electrodeposition coating film, corrosion resistance, lubricity, and powdering resistance.

[0042] The softening point of the solid lubricant is preferably 100 to 135° C., more preferably 110 to 130° C. When the softening point of the solid lubricant is 100 to 135° C., the lubricity and powdering resistance of the coating film are further improved.

[0043] The content of the solid lubricant is preferably 0.1 to 10 mass% of the coating film (total solid content of the coating film). When the content of the solid lubricant is 0.1 mass% or more, sufficient lubricity is obtained. On the other hand, when the content of the solid lubricant is 10 mass% or less, deterioration in adhesion between the coating film and the electrodeposition coating film and corrosion resistance can be suppressed. The content of the solid lubricant is more preferably 0.5 to 5 mass % of the coating film (total solid content of the coating film), and even more preferably 0.5 to 2.5 mass %, from the viewpoints of adhesion between the coating film and the electrodeposition coating film, lubricity, and corrosion resistance.

[0044] (average coating thickness) The average thickness of the coating film is 0.5 to 3 μm. If the average thickness of the coating film is too thin, sufficient corrosion resistance cannot be obtained. In addition, sufficient adhesion between the coating film and the electrodeposition coating film cannot be obtained. On the other hand, if the average thickness of the coating film is too thick, the cohesive strength of the coating film decreases, resulting in insufficient weldability. Therefore, the average thickness of the coating film is set within the above range. The average thickness of the coating film is preferably 0.8 to 2.2 μm, more preferably 1.2 to 1.8 μm.

[0045] The average coating thickness is measured as follows. First, the surface-treated substrate with the coating film is cut along the thickness direction to expose the cross section, which is then embedded in resin and polished using a polishing machine. After polishing, Au is vapor-deposited onto the cross section to prepare a sample for cross-sectional observation. The cross section thus obtained is observed with a scanning electron microscope to obtain an observation image of the cross section of the coating film. In the observation image, the film thickness of the coating film is measured at 10 points, and the arithmetic mean value is calculated. When measuring the film thickness of the coating film, the film thickness is measured at 10 points equally spaced at 40 μm intervals in the direction in which the coating film extends on the observation image. The observation conditions for the scanning electron microscope are as follows: Measurement equipment: JEOL Ltd. JSM-7200F ·Magnification: 3000x Acceleration voltage: 10 keV

[0046] (Formation of coating film) The method for forming a coating film is not particularly limited, and well-known methods can be used. For example, a coating film-forming composition (paint) is obtained by mixing a binder resin, doped zinc oxide particles, a rust inhibitor, and, if necessary, other components in a solvent. The solvent may be water or an organic solvent, but water is preferred from the standpoints of production cost and environmental friendliness. In other words, the coating film-forming composition is preferably an aqueous composition. The coating film is then formed by applying the coating film-forming composition to at least one surface of a Zn-Al-Mg-plated steel sheet and drying, or by drying and heating. Note that the surface of the Zn-Al-Mg-plated steel sheet is preferably degreased and washed with water before applying the coating film-forming composition. The coating film-forming composition is applied by a coating method such as bar coating, roll coating, blade coating, or curtain coating. The drying temperature of the coating film is preferably 50 to 120°C, more preferably 60 to 100°C, in terms of the maximum temperature that can be reached on the surface of the Zn-Al-Mg plated steel sheet. The heating temperature of the coating film is preferably 100 to 240°C, more preferably 120 to 210°C, in terms of the maximum temperature that can be reached on the surface of the Zn-Al-Mg plated steel sheet.

[0047] <Other aspects of surface-treated steel sheets> The surface-treated steel sheet according to this embodiment may have an intermediate coating film (functional coating film such as a chemical conversion coating film) between the Zn-Al-Mg-plated steel sheet and the coating film. The intermediate coating film preferably contains a binder resin, a silane coupling agent, silica fine particles, a phosphate compound, and a fluorine compound, and has an average film thickness of 0.1 to 0.5 μm. By having an intermediate coating film with the above composition and average film thickness, it is possible to improve corrosion resistance without compromising weldability. This is because a relatively thin intermediate coating film has little effect on weldability and improves the adhesion between the plated steel sheet and the coating film, thereby improving corrosion resistance. Therefore, the surface-treated steel sheet according to this embodiment preferably has an intermediate coating film (functional coating film) with the above composition and average film thickness between the Zn-Al-Mg-plated steel sheet and the coating film.

[0048] The average thickness of the intermediate coating is measured in the same manner as the average thickness of the coating.

[0049] <Painting materials> The painted member of this embodiment comprises a formed material obtained by forming the surface-treated steel sheet of this embodiment, and an electrocoating film formed on the coating film of the formed material (i.e., the coating film of the surface-treated steel sheet) (see Figure 2). In Fig. 2, 100 denotes a coated member, 10A denotes a formed member, 11A denotes a base steel sheet in the formed member (the steel sheet on which the Zn-Al-Mg-based plating layer is to be formed), 12A denotes the Zn-Al-Mg-based plating layer of the formed member, 13A denotes a coating film on the formed member, and 14 denotes an electrodeposition coating film. Fig. 2 shows an example of a coated member having a hat-shaped cross section, but the overall shape and cross-sectional shape of the coated member are not particularly limited and may be any shape.

[0050] The coated member according to this embodiment is manufactured, for example, as follows: First, a surface-treated steel sheet is formed into a formed product of the desired shape using, for example, well-known forming techniques such as cutting and press forming. If necessary, the formed product may be assembled into the desired shape by welding (spot welding, etc.).

[0051] Next, the coating film of the molding material is subjected to an electrodeposition coating treatment. As a result, an electrodeposition coating film is formed on the coating film. The electrodeposition coating treatment may be either anionic electrodeposition coating or cationic electrodeposition coating, but cationic electrodeposition coating is preferred from the viewpoint of corrosion resistance.

[0052] In particular, when an electrodeposition coating film is formed by cationic electrodeposition coating using an aqueous paint containing a resin [for example, an aqueous resin (such as an acrylic resin, polyester resin, alkyd resin, epoxy resin, or polyurethane resin) having a hydrophilic group such as a carboxyl group, a hydroxyl group, a methylol group, an amino group, a sulfonic acid group, or a polyoxyethylene bond, and a functional group such as a hydroxyl group that reacts with a curing agent, and a curing agent (such as a melamine resin or a blocked polyisocyanate)], and other additives (such as a coloring pigment, an optical interference pigment, an extender pigment, a dispersant, an anti-settling agent, a reaction accelerator, an antifoaming agent, a thickener, an anti-rust agent, an ultraviolet absorber, or a surface conditioner), the adhesion between the coating film and the electrodeposition coating film is likely to be improved.

[0053] Thereafter, other coating films such as an intermediate coating film and a top coating film may be formed on the electrodeposition coating film of the molding material, if necessary.

[0054] Through these steps, the coated member according to this embodiment is manufactured.

[0055] Before electrodeposition coating, the molded material on which the coating film has been formed may be degreased, the surface adjusted, and then chemically treated (for example, phosphate treatment, Zr treatment, etc.). By performing chemical conversion treatment, a chemical conversion treatment film is less likely to form on the coating film, but a chemical conversion treatment film is formed in required areas other than the coating film. This improves the adhesion of the electrodeposition coating film to the entire molded material (coated member).

[0056] The coated members according to this embodiment are widely used in applications such as automobile members (automobile bodies, suspension members, etc.), machine members (casings, etc.), home appliance members (casings, etc.), and building materials (roofs, walls, etc.). [Example]

[0057] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to the following examples. The present disclosure allows various conditions to be adopted as long as they do not deviate from the gist of the disclosure and the object of the disclosure is achieved.

[0058] <Example> 1. Manufacturing of surface-treated steel sheets 1.1 Preparation of galvanized steel sheet The following five types of zinc-based plated steel sheets were prepared, and the surfaces were degreased by immersion in an aqueous solution (2.5 mass%, 40°C) of an aqueous alkaline degreasing agent (FC-301 manufactured by Nippon Parkerizing Co., Ltd.) for 2 minutes, followed by rinsing with water and drying to obtain zinc-based plated steel sheets for surface treatment. However, cold-rolled steel sheets were also prepared as steel sheets for surface treatment.

[0059] ZA: Zn-Al-Mg ternary hot-dip galvanized steel sheet (Zn-11%Al-3%Mg-0.2%Si) (sheet thickness 0.8mm) ZB: Zn-Al-Mg ternary hot-dip galvanized steel sheet (Zn-2%Al-2%Mg) (sheet thickness 0.8 mm) ZC: Hot-dip galvanized steel sheet (thickness 0.8 mm) ZD: Galvannealed steel sheet (thickness 0.8 mm, 10% Fe by mass) C: Cold-rolled steel sheet (thickness 0.8 mm, unplated)

[0060] The coating weights are shown in Table 2. The coating weights were controlled by adjusting the plating pull-up speed and wiping amount during plating preparation.

[0061] 1.2 Formation of intermediate coating (chemical conversion coating) Next, the following chemical conversion treatment solution S was prepared and applied to the above-mentioned zinc-based plated steel sheet or cold-rolled steel sheet while changing the bar coat count so as to obtain the average film thickness of the intermediate coating film shown in Table 2. Thereafter, the zinc-based plated steel sheet or cold-rolled steel sheet was heated and dried in a hot air furnace so that the temperature reached on the surface of the zinc-based plated steel sheet or cold-rolled steel sheet was 70°C, and then air-dried, thereby forming an intermediate coating film on the surface of the zinc-based plated steel sheet or cold-rolled steel sheet. However, in some cases, no intermediate coating was formed.

[0062] S1: A chemical conversion treatment solution with a solids concentration of 10% consisting of silane coupling agents, silica particles, urethane resins, phosphoric acid compounds, and fluorine compounds. S2: A chemical conversion treatment solution with a solids concentration of 10% consisting of Zr compounds, silane coupling agents, phosphate compounds, and vanadium compounds.

[0063] 1.3 Coating formation Next, a paint for forming a coating film was prepared by mixing the components to a solids concentration of 20 mass % to form a coating film having the composition ratio (mass %) shown in Table 1. The paint was applied to a zinc-plated steel sheet or cold-rolled steel sheet, or to a chemical conversion coating, using a bar coater, while changing the bar coat count and dilution ratio so as to obtain the average film thickness shown in Table 2, and then dried in an oven under conditions that resulted in the maximum temperature reached (PMT) shown in Table 2, or by drying and heating, to form a coating film. The components contained in the paint are listed below.

[0064] (binder resin, etc.) J1: Epoxy resin (ADEKA ADEKA RESIN EM-0461N) J2: Polyester resin (Vyloner MD1480 manufactured by Byron) J3: Urethane resin (Superflex 150 manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) J4: Epoxy resin (ADEKA ADEKA RESIN EP-4100) J5: Polyester resin (Byron 200 manufactured by Byron) J6: Melamine resin (Allnex Cymel 325) J7: Water-soluble isocyanate (Dai-ichi Kogyo Seiyaku Co., Ltd. Elastron BN-77)

[0065] (Doped zinc oxide particles) Z1: Doped zinc oxide particles (23-Kt, manufactured by Hakusui Tech Co., Ltd., average particle size 0.5 μm) Z2: Doped zinc oxide particles (23-K manufactured by Hakusui Tech Co., Ltd., average particle size 0.2 μm) Z3: Doped zinc oxide particles (Pazet CK, manufactured by Hakusui Tech Co., Ltd., average particle size 0.03 μm) Z4: Zinc oxide particles (general reagent, average particle size 0.5 μm) Z5: Titanium nitride particles (general reagent, average particle size 2.0 μm)

[0066] (rust inhibitor) B1: A mixture of tannic acid (general reagent) and zinc naphthenate (general reagent) in a mass ratio of 1:1 B2: A mixture of tartaric acid (general reagent) and zinc naphthenate (general reagent) in a mass ratio of 1:1 B3: A mixture of tannic acid (general reagent) and trizinc phosphate (general reagent) in a mass ratio of 1:1 B4: A mixture of tartaric acid (general reagent) and trizinc phosphate (general reagent) in a mass ratio of 1:1 B5: Colloidal silica (particle size 20 nm) B6: Aluminum dihydrogen triphosphate (particle size 2 μm) B7: Calcium ion-exchanged silica (Ca exchange rate 9%) (particle size 2 μm) B8: Silica (particle size 2 μm)

[0067] (solid lubricant) W1: Polyethylene wax (CHEMBASE S-394 MG) W2: Polytetrafluoroethylene (PTFE) particles (CHEMBASE SST-1MG-RC)

[0068] 2. Performance evaluation test The surface-treated steel sheets of each example were subjected to the following performance evaluation tests.

[0069] 2.1 Corrosion resistance test (Preparation 1) Test piece A was cut to a size of 70 mm x 150 mm, and test piece B was cut to a size of 50 mm x 100 mm, from the surface-treated steel sheet of each example. Next, as shown in Figures 3 and 4, test piece A and test piece B were overlapped so that their centers overlapped and their longitudinal directions were parallel in a plan view, and then test piece A and test piece B were joined at their respective center positions by spot welding to produce joined test piece C. The spot welding was performed at one location in the center of test piece A and test piece B, and the spot welding conditions were as follows: electrode used: tip diameter 5 mm, R40 CF type Cr-Cu electrode, pressure: 1.96 kN, welding current: 8 kA, current application time: 12 cycles / 50 Hz. In addition, when spot welding, a 0.1 mm gauge was sandwiched between test piece A and test piece B, and a gap (clearance) was provided between test piece A and test piece B.

[0070] (Preparation 2) For the bonded test specimens C prepared from the surface-treated steel sheets of each example, surface conditioning was performed at room temperature for 20 seconds using a surface conditioning treatment agent, Preparen X (trade name), manufactured by Nihon Parkerizing Co., Ltd. Furthermore, a chemical conversion treatment (phosphate treatment) was performed using a chemical conversion treatment solution (zinc phosphate treatment solution), Palbond 3020 (trade name), manufactured by Nihon Parkerizing Co., Ltd. The temperature of the chemical conversion treatment solution was set to 43°C, and the bonded test specimens C were immersed in the chemical conversion treatment solution for 120 seconds, then rinsed with water and dried. After the chemical conversion treatment (phosphate treatment), the bonded test specimens C were electrodeposited with a cationic electrodeposition paint manufactured by Nippon Paint Co., Ltd. using a ramp current of 160V, and further baked at a baking temperature of 170°C for 20 minutes. The average thickness of the electrodeposition coating film was 10 μm for all bonded test specimens C.

[0071] (Corrosion resistance test for joints) After the electrodeposition coating, the end face of test piece A of bonded test piece C was sealed, and a corrosion cycle test was conducted on bonded test piece C with salt water sprayed on the side of test piece B of bonded test piece C. The corrosion cycle consisted of 2 hours of salt water spray (SST: Salt Spray Test, 5% NaCl, 35°C atmosphere), 2 hours of dry (60°C), and 4 hours of wet (50°C, 98% RH) conditions, and was conducted for 360 cycles. The corrosion cycle test was conducted in accordance with JASO M609-91. After the corrosion test, the spot welded portion of the joined test piece C was drilled out with a drill or the like to separate test pieces A and B. The corrosion condition of the overlapping surface of test piece A with test piece B was observed and the following ratings were assigned. Furthermore, test piece A was immersed in an ammonium citrate aqueous solution to remove the corrosion products, and then the corrosion depth of test piece A was measured. Specifically, test piece A after the corrosion products were removed was cut into eight equal pieces (in FIG. 3, two left and right sections and four top and bottom sections, for a total of eight equal sections), and the corrosion depth (mm) of each small piece was measured. The largest corrosion depth (mm) of the eight small pieces was defined as the maximum corrosion depth (mm), and was evaluated as the result of the corrosion resistance test of the plate joint of this example. In the corrosion resistance test, a sample was judged to have excellent corrosion resistance if it received a rating of "3," "4," or "5." The results are shown in Table 2. The corrosion depth was measured using a micrometer (CPM-MX manufactured by Mitutoyo Corporation). 1: Red rust occurs on the entire mating surface of test piece A with test piece B, or the maximum corrosion depth is 0.4 mm or more. 2: Red rust occurs on 20% or more of the mating surface of test piece A with test piece B, or the maximum corrosion depth is 0.2 to less than 0.4 mm. 3: Red rust occurs on 5% or more but less than 20% of the mating surface of test piece A with test piece B, or the maximum corrosion depth is 0.1 mm or more but less than 0.2 mm 4: Red rust occurred on less than 5% of the mating surface of test piece A with test piece B, or the maximum corrosion depth was more than 0 mm and less than 0.1 mm. 5: No red rust occurred on the mating surface of test piece A with test piece B, and the maximum corrosion depth was 0 mm (none)

[0072] 2.2 Spot weldability Each surface-treated steel sheet was spot-welded 500 times using a CF-type Cr-Cu electrode with a tip diameter of 5 mm and R40, with a pressure of 1.96 kN, a welding time of 12 cycles / 50 Hz, and varying the welding current to achieve a nugget diameter of 4√t (t = thickness of the surface-treated steel sheet). The nugget diameter and electrode condition after 500 welding points were observed and the following ratings were assigned. Note that spot welding points where no current was applied were judged as those where no nugget was formed. The presence or absence of nugget formation and nugget diameter were measured by observing a cross section of the sheet thickness including the center of the welding point. In the weldability test, a specimen was judged to have excellent weldability when it was rated as "3," "4," or "5." The results are shown in Table 2. 1:500 RBI not possible 2: 500 dots can be performed, but 10 dots must be non-energized (including non-energized). After 500 dots are completed, the diameter of the electrode contact surface using pressure-sensitive paper must be 1.7 times or more the diameter of the electrode contact surface at the first dot. 3: 500 dots are performed, no power is turned off, and the diameter of the electrode contact surface using pressure-sensitive paper at the end of 500 dots is 1.4 times or more but less than 1.7 times the diameter of the electrode contact surface at the first dot. 4: 500 dots are performed, no power is turned off, and the diameter of the electrode contact surface using pressure-sensitive paper at the end of 500 dots is 1.1 times or more but less than 1.4 times the diameter of the electrode contact surface at the first dot. 5: 500 dots are performed, there is no non-current flow, no electrode wear, or the diameter of the electrode contact surface using pressure-sensitive paper at the end of 500 dots is less than 1.1 times the diameter of the electrode contact surface at the first dot 3 and 4, 20A indicates test piece A, 20B indicates test piece B, 20C indicates joined test piece C, and 22 indicates a spot welded portion.

[0073] [Table 1]

[0074] [Table 2-1]

[0075] [Table 2-2]

[0076] From the above results, it can be seen that the surface-treated steel sheets of the present invention are superior in both corrosion resistance and weldability to the surface-treated steel sheets of the comparative examples. In Table 1, the notation "inorganic substance*1" indicates the composition ratio of "inorganic substances other than Zn-containing compounds including doped zinc oxide particles" to the coating film.

[0077] The disclosure of Japanese Patent Application No. 2023-056126 is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. Plating weight per side: 20 to 60 g / m ２ Zn-Al-Mg-plated steel sheet; a coating film disposed on at least one main surface of the Zn—Al—Mg-plated steel sheet; and the coating film comprises a binder resin, doped zinc oxide particles, and a rust inhibitor; The average thickness of the coating film is 0.5 to 3 μm, the doped zinc oxide particles are conductive zinc oxide particles, the doped zinc oxide particles have an average particle size of 0.1 to 2 μm; the content of the doped zinc oxide particles is 20 to 40% by mass relative to the coating film; the content of inorganic substances other than the Zn-containing compound in the coating film is 0 to 1% by mass or less relative to the coating film; The Zn-containing compounds are doped zinc oxide, zinc oxide, zinc hydroxide, and inorganic zinc salts. Surface-treated steel sheet.

2. The rust inhibitor is an organic acid, The content of the rust inhibitor is 1 to 20 mass% relative to the coating film. The surface-treated steel sheet according to claim 1.

3. an intermediate coating film is provided between the Zn-Al-Mg-plated steel sheet and the coating film, the intermediate coating film contains a binder resin, a silane coupling agent, silica fine particles, a phosphoric acid compound, and a fluorine compound; The average film thickness of the intermediate coating film is 0.1 to 0.5 μm. The surface-treated steel sheet according to claim 1.

4. a chemical composition of the Zn-Al-Mg-based plating layer of the Zn-Al-Mg-based plated steel sheet containing, on average, 4 to 22 mass% Al, 1.0 to 10 mass% Mg, and the remainder including Zn and impurities; The surface-treated steel sheet according to claim 1.

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