Surface-treated steel sheet

Abstract

A surface-treated steel sheet comprising: a Zn-Al-Mg plated steel sheet in which the amount of plating layer deposited per metal surface is 20-60 g / m2; and a coating film disposed on at least one main surface of the Zn-Al-Mg-based plated steel sheet, the coating film containing a binder resin, doped zinc oxide particles, and an anti-rust agent, the average film thickness of the coating film being 0.5-3 [mu] m, the average particle diameter of the doped zinc oxide particles being 0.1-2 [mu] m, the content of the doped zinc oxide particles being 20-40 mass% with respect to the coating film, and the average particle diameter of the doped zinc oxide particles being 0.1-2 [mu] m. The content of inorganic substances other than a Zn-containing compound in the coating film is 0-1% by mass or less with respect to the coating film.

Description

Technical Field

[0001] This disclosure relates to a surface-treated steel sheet. Background Technology

[0002] Surface-treated steel sheets, for example, are assembled into the desired shape by pressure forming, spot welding, etc., and then electrophoretic coating is applied. If electrophoretic coating is omitted, intermediate coating is applied, so that they can be used as automotive components.

[0003] Furthermore, the corrosion resistance of automotive components is mostly ensured through a chemical conversion coating based on a chemical conversion treatment process and an electrophoretic coating based on a subsequent electrophoretic coating process. However, at the joints of the formed materials of surface-treated steel sheets (plate overlaps), especially at the plate joints on the inner surface of bag-shaped components and at bent edges, areas are sometimes not completely covered by the chemical conversion coating and electrophoretic coating. In this case, the possibility of the joints of the formed materials being exposed to a corrosive environment in an exposed state increases. Therefore, surface-treated steel sheets that can ensure corrosion resistance are required.

[0004] Therefore, the surface-treated steel sheet needs to be made conductive by making the coating conductive in a way that allows for resistance welding to improve weldability and impart corrosion resistance.

[0005] For example, Patent Document 1 proposes "a surface-treated steel sheet, which is a surface-treated steel sheet having a coating on at least one side of a plated steel sheet, said coating comprising an adhesive resin, non-oxide ceramic particles containing V (excluding VC particles), and doped zinc oxide particles, wherein the content of the non-oxide ceramic particles containing V and the doped zinc oxide particles relative to the coating satisfies 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) Wherein, CV refers to the content (mass%) of the non-oxide ceramic particles containing V, and CZn refers to the content (mass%) of the doped zinc oxide particles.

[0006] According to Patent Document 1, it is described as "a surface-treated steel plate that can provide excellent corrosion resistance and weldability before electrophoretic coating".

[0007] Existing technical documents Patent documents Patent Document 1: International Publication No. 2018 / 092244 Summary of the Invention

[0008] The problem that the invention aims to solve Surface-treated steel sheets, including those used in automotive components, mechanical components, home appliance components, and building materials, require improved corrosion resistance and weldability.

[0009] However, various research and developments have been carried out, including the aforementioned Patent Document 1, but due to recent requirements, the current situation is that further improvements in the corrosion resistance and weldability of surface-treated steel sheets are desired.

[0010] Therefore, the subject of this disclosure is to provide surface-treated steel sheets with excellent corrosion resistance and weldability.

[0011] Methods for solving problems The means to solve the above problems include the following methods.

[0012] <1> A surface-treated steel sheet, wherein it has: Zn-Al-Mg coated steel sheets have a coating thickness of 20–60 g / m² on each side. 2 ;and A coating is disposed on at least one main surface of the Zn-Al-Mg based coated steel sheet; The coating film comprises an adhesive resin, doped zinc oxide particles, and a rust inhibitor. The average thickness of the coating is 0.5–3 μm. The average particle size of the doped zinc oxide particles is 0.1–2 μm. The content of the doped zinc oxide particles is 20-40% by mass relative to the coating film. The content of inorganic substances in the coating, excluding Zn compounds, is less than 0-1% by mass relative to the coating.

[0013] <2> According to the surface-treated steel plate described in <1> above, wherein, The rust inhibitor is an organic acid. The content of the rust inhibitor is 1 to 20% by mass relative to the coating.

[0014] <3> The surface-treated steel plate according to <1> or <2> above, wherein, An intermediate coating is provided between the Zn-Al-Mg based coated steel sheet and the coating film. The intermediate coating film comprises an adhesive resin, a silane coupling agent, silica microparticles, a phosphoric acid compound, and a fluorine compound. The average thickness of the intermediate coating is 0.1–0.5 μm.

[0015] <4> The surface-treated steel sheet according to any one of <1> to <3> above, wherein the chemical composition of the Zn-Al-Mg coating of the Zn-Al-Mg coated steel sheet, on an average basis, contains Al: 4-22% by mass, Mg: 1.0-10% by mass, and the remainder includes Zn and impurities.

[0016] Invention Effects According to this disclosure, surface-treated steel sheets with excellent corrosion resistance and weldability can be provided. Attached Figure Description

[0017] Figure 1 This is a schematic cross-sectional view showing an example of a surface-treated steel sheet according to this embodiment.

[0018] Figure 2 This is a cross-sectional view showing an example of a coating component according to this embodiment.

[0019] Figure 3 This is a schematic top view showing the bonded test piece produced in the corrosion resistance test of the embodiment.

[0020] Figure 4 This is a schematic side view showing the bonded test piece produced in the corrosion resistance test of the embodiment. Detailed Implementation

[0021] The following describes one example of an implementation method of this disclosure.

[0022] Furthermore, in this specification, when the lower limit of the content of each element in the chemical composition is marked as "0", it means that the element is an optional component and may not be present.

[0023] When using "~" to represent a numerical range, if the values ​​before and after "~" are not marked with "exceeding" or "below", the range includes those values ​​as lower and upper limits. Conversely, if the values ​​before and after "~" are accompanied by "exceeding" or "below", the range does not include those values ​​as lower or upper limits.

[0024] Within the numerical range recorded in stages, the upper limit of a certain stage's numerical range can be replaced by the upper limit of other stage's numerical ranges, or it can be replaced by the value shown in the embodiment. Similarly, the lower limit of a certain stage's numerical range can be replaced by the lower limit of other stage's numerical ranges, or it can be replaced by the value shown in the embodiment.

[0025] The term "process" is not only used to describe a single process, but also to include any process that achieves its intended purpose, even if it cannot be clearly distinguished from other processes.

[0026] <Surface-treated steel sheet> The surface-treated steel plate of this embodiment has the following characteristics: Zn-Al-Mg coated steel sheets have a coating thickness of 20–60 g / m² on each side. 2 ;and A coating is applied to at least one main surface of a Zn-Al-Mg based coated steel sheet (refer to...). Figure 1 ); The coating consists of binder resin, doped zinc oxide particles, and rust inhibitor. The average film thickness of the coating is 0.5–3 μm. The average particle size of the doped zinc oxide particles is 0.1–2 μm. The content of doped zinc oxide particles is 20-40% by mass relative to the coating film. The content of inorganic substances in the coating, excluding Zn compounds, is less than 0-1% by mass relative to the coating.

[0027] Here, in Figure 1 In the text, 10 represents a surface-treated steel plate, 11 represents the base steel plate (the steel plate of the object to which a Zn-Al-Mg coating is formed) in Zn-Al-Mg coated steel plates, 12 represents the Zn-Al-Mg coating of the Zn-Al-Mg coated steel plate, and 13 represents the coating film.

[0028] Furthermore, the main surfaces of Zn-Al-Mg coated steel sheets refer to the two opposite surfaces in the thickness direction of the Zn-Al-Mg coated steel sheets.

[0029] Furthermore, "single-sided" in Zn-Al-Mg coated steel sheet refers to one of two opposite sides of a pair of Zn-Al-Mg coated steel sheets in the thickness direction.

[0030] The surface-treated steel sheet of this embodiment, through the above-described configuration, becomes a surface-treated steel sheet with excellent corrosion resistance and weldability. The surface-treated steel sheet of this embodiment was discovered through the following insights.

[0031] The inventors conducted research on the corrosion resistance and weldability of surface-treated steel sheets. The results yielded the following insights.

[0032] To date, zinc-coated steel sheets, which are galvanized, have been primarily used in surface-treated steel sheets for applications such as automotive parts to ensure corrosion resistance and weldability. To further improve corrosion resistance while maintaining weldability, coated steel sheets with a coating containing binder resin, rust-inhibiting pigments, and rust inhibitors have been investigated.

[0033] On the other hand, in the joints (plate overlaps) of the forming materials of surface-treated steel sheets, the chemical conversion treatment film and the electrophoretic coating film do not completely cover the parts, and the joints of the forming materials are sometimes exposed to the corrosive environment in an exposed state, thus undergoing corrosion.

[0034] Therefore, by using Zn-Al-Mg coated steel sheets as zinc-coated steel sheets, the surface-treated steel sheets exhibit excellent corrosion resistance.

[0035] However, if Zn-Al-Mg coated steel sheets with excellent corrosion resistance are used, the Al and Mg that are components of the coating will adhere to the welding electrode during welding, thereby deteriorating the weldability (especially the continuous spot welding).

[0036] Therefore, by reducing the coating adhesion amount on each single side of the Zn-Al-Mg coated steel sheet to 20-60 g / m², 2 It can inhibit the adhesion of Al and Mg to the welding electrode during welding.

[0037] In addition, as an anti-rust pigment contained in the coating, large-diameter doped zinc oxide particles with an average particle size of 0.1 to 2 μm are used, and the content of doped zinc oxide particles is as high as 20 to 40% by mass. As a result, during welding, the welding electrode can easily come into contact with the doped zinc oxide particles, but it is difficult for it to come into contact with the Zn-Al-Mg coating of the Zn-Al-Mg coated steel sheet.

[0038] Furthermore, when doped zinc oxide particles and other Zn-containing compounds come into contact with the welding electrode during welding, poor welding is unlikely to occur even if Zn-containing compounds are attached. This is because, in inorganic materials, when Zn-containing compounds adhere to the welding electrode, they mainly adhere to the electrode in the form of zinc oxide, and their reactivity with the metal components of the welding electrode (such as Cu) is low. Therefore, even if they adhere to the welding electrode, they are unlikely to hinder welding.

[0039] Therefore, even if the coating does not contain inorganic substances other than Zn compounds, or if it does contain inorganic substances other than Zn compounds, the content of inorganic substances other than Zn compounds should be controlled to be less than 1% by mass relative to the coating.

[0040] Based on the above insights, it has been found that the surface-treated steel sheet of this embodiment is a surface-treated steel sheet with excellent corrosion resistance and weldability.

[0041] The details of the surface-treated steel plate of this embodiment will now be described.

[0042] (Zn-Al-Mg coated steel sheet) Zn-Al-Mg coated steel sheet is a steel sheet with a Zn-Al-Mg coating containing at least Zn, Al, and Mg formed on a base steel sheet. A Zn-Al-Mg coating containing at least Zn, Al, and Mg is a coating that contains Zn as the main component (for example, containing more than 40% by mass of Zn as the maximum component) and has excellent corrosion resistance.

[0043] In Zn-Al-Mg coatings, in addition to Zn, Al and Mg, 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 can also be included.

[0044] The specific chemical composition of Zn-Al-Mg coatings, for example, based on average composition, can be listed as containing Al: 4-22% by mass, Mg: 1-10% by mass, with the remainder including Zn and impurities.

[0045] More preferably, the chemical composition of a specific Zn-Al-Mg coating, on an average basis, can be listed as containing Al: 4 to 22% by mass, Mg: 1 to 10% by mass, Si: 0.0001 to 2% by mass, with the remainder being Zn and impurities.

[0046] Furthermore, in Zn-Al-Mg coatings, impurities refer to components contained in the raw materials or components introduced during the manufacturing process that are not intentionally present. For example, in Zn-Al-Mg coatings, due to the mutual atomic diffusion between the base steel sheet and the Zn-Al-Mg coating, trace amounts of components other than Fe are sometimes introduced as impurities.

[0047] Here, there are no particular limitations on the base steel sheet (the steel sheet to which the Zn-Al-Mg coating is formed) used as the base steel sheet. The base steel sheet can be a hot-rolled steel sheet obtained by hot rolling a slab having a suitable 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, for example, can be set to 270–1470 MPa in the tensile strength grade.

[0048] There are no particular restrictions on the thickness of the base steel plate, but it is preferably 0.4 to 3.2 mm, and more preferably 0.5 to 2.4 mm.

[0049] The thickness of Zn-Al-Mg coated steel sheets was measured using a micrometer (Mitutoyo CPM-MX, manufactured by Mitutoyo Corporation).

[0050] The coating thickness on each single side of the Zn-Al-Mg coated steel sheet is 20–60 g / m². 2 .

[0051] If the coating thickness is too low, the corrosion resistance will decrease. On the other hand, if the coating thickness is too high, Al and Mg will easily adhere to the welding electrode during welding, thereby reducing weldability.

[0052] Therefore, the coating adhesion amount is set to the range described above.

[0053] The preferred coating adhesion amount is 25-50 g / m². 2 More preferably 30-40 g / m 2 .

[0054] The determination of coating adhesion amount is described below.

[0055] The amount of coating adhesion can be determined by dissolving (pickling) the coating in an acidic solution containing a corrosion inhibitor that inhibits corrosion of the base steel sheet, and measuring the weight change before and after pickling. Alternatively, in the presence of a coating film, the amount can be determined by performing the aforementioned pickling after removing the coating film using chemicals (removing agents, etc.) or resin shot peening.

[0056] There are no particular limitations on the method for forming the Zn-Al-Mg coating on Zn-Al-Mg coated steel sheets. For example, the Zn-Al-Mg coating can be formed using hot-dip galvanizing. The formation of the Zn-Al-Mg coating can be either continuous or intermittent. In addition, after the Zn-Al-Mg coating is formed, treatments such as zero-spangle treatment for uniform appearance, annealing treatment for modification of the Zn-Al-Mg coating, and tempering rolling for adjusting surface condition or material properties can also be performed.

[0057] (Coating) The coating contains a binder resin, doped zinc oxide particles, and a rust inhibitor. Other components may also be included as needed.

[0058] - Adhesive resin - The binder resin can be any of the following: a water-soluble or water-dispersible aqueous resin dissolved or dispersed in water, or a solvent-based resin dissolved or dispersed in an organic solvent. From the perspective of manufacturing cost and environmental adaptability, aqueous resins are preferred.

[0059] As water-based resins, examples include polyester resins, polyurethane resins, polyolefin resins, acrylic resins, epoxy resins, phenolic resins, and mixtures of two or more of these resins, which are water-soluble or water-dispersible resins.

[0060] When using polyester resin, the molecular weight is preferably between 10,000 and 30,000. A molecular weight of 10,000 or higher ensures sufficient processability. Conversely, a molecular weight of 30,000 or lower increases the number of bonding sites within the resin itself, ensuring excellent adhesion to the electrophoretic coating film. Furthermore, when using a curing agent such as melamine for crosslinking, the crosslinking reaction proceeds sufficiently, ensuring the performance of the coating film. Here, the molecular weight of the polyester resin is the weight-average molecular weight.

[0061] When using polyurethane resin, the preferred form of the polyurethane resin is an emulsion with a particle size of 10 to 100 nm (preferably 20 to 60 nm). If the emulsion particle size is 10 nm or larger, cost increases can be suppressed. On the other hand, if the emulsion particle size is 100 nm or smaller, the gaps between the emulsions are smaller during coating, ensuring the barrier properties of the coating. Types of polyurethane resins include ether-based, polycarbonate-based, ester-based, and acrylic-grafted types. They can be used alone or in combination.

[0062] Solvent-based resins include polyester resins, polyurethane resins, epoxy resins, acrylic resins, and mixtures of two or more of these resins.

[0063] Here, the adhesive resin can be either a cross-linked resin with a cross-linking structure or a non-cross-linked resin without a cross-linking structure, but from the viewpoint of low-temperature film formation, a non-cross-linked resin is preferred. As the cross-linking agent (curing agent) that imparts a cross-linking structure to the adhesive resin, a water-soluble cross-linking agent is preferred. Specifically, melamine, isocyanate, silane compounds, zirconium compounds, titanium compounds, etc., are preferred as cross-linking agents.

[0064] The amount of crosslinking agent added is preferably 5 to 30 parts by weight relative to 100 parts by weight of the resin solids. If the amount of crosslinking agent added is 5 parts by weight or more, the crosslinking reaction with the binder resin can be ensured, resulting in sufficient coating performance. On the other hand, if the amount of crosslinking agent added is 30 parts by weight or less, the crosslinking reaction proceeds moderately, and excessive hardening of the coating film can be suppressed. This ensures processability. Furthermore, the stability of coatings containing silane compounds, zirconium compounds, and titanium compounds can also be ensured.

[0065] The content of the binder resin relative to the coating film (total solids content of the coating film) is preferably 10.0 to 90.0% by mass. If the content of the binder resin is 10.0% by mass or more, it is easier to exhibit its function as a binder, and the cohesiveness of the coating film is improved. As a result, it is difficult for internal damage to the coating film (coagulation failure of the coating film) to occur during adhesion tests and molding processes. On the other hand, if the content of the binder resin is 90.0% by mass or less, the proportion of pigment components in the coating film is reduced, making it easier to balance weldability, corrosion resistance, and adhesion to the electrophoretic coating film.

[0066] From the viewpoint of exhibiting adhesive function while also considering weldability, corrosion resistance, and adhesion to the electrophoretic coating film, the content of adhesive resin relative to the coating film (total solids content of the coating film) is more preferably 15.0 to 80.0% by mass.

[0067] -Doped zinc oxide particles- The doped zinc oxide particles are conductive zinc oxide particles. If the coating contains conductive doped zinc oxide particles, the solderability is improved.

[0068] Examples of doped zinc oxide particles include those that exhibit conductivity by doping zinc oxide particles with at least one element selected from Group 13 and Group 15 of the periodic table (hereinafter also referred to as "doping element").

[0069] Elements in Group 13 of the periodic table include B, Al, Ga, and In. Elements in Group 15 of the periodic table include P and As. Among these, from the viewpoint of improving conductivity, Al or Ga is preferred as the doping element. Furthermore, from the viewpoint of cost, Al is more preferred as the doping element.

[0070] From the viewpoint of improving conductivity, the content of doped elements relative to undoped zinc oxide particles is preferably 0.05 to 5 atom%, more preferably 0.1 to 5 atom%.

[0071] The average particle size of the doped zinc oxide particles is 0.1–2 μm.

[0072] If the average particle size is too small, the welding electrode will easily come into contact with the Zn-Al-Mg coating of the Zn-Al-Mg coated steel sheet during welding, thus reducing weldability. On the other hand, if the average particle size is too large, the doped zinc oxide particles will have difficulty making uniform contact with the welding electrode (e.g., a Cu electrode) during welding, resulting in the welding electrode easily coming into contact with the Zn-Al-Mg coating of the Zn-Al-Mg coated steel sheet, thus reducing weldability.

[0073] Therefore, the average particle size of the doped zinc oxide particles is set to the range described above.

[0074] 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.

[0075] The "average particle size" of doped zinc oxide particles refers to the average first-order particle size when the doped zinc oxide particles exist alone in the coating film, and to the average second-order particle size when the doped zinc oxide particles are aggregated together.

[0076] The average particle size of the doped zinc oxide particles was determined by the following measurement method.

[0077] First, a surface-treated substrate with a coating was cut along its thickness to expose the cross-section. Resin was then embedded, and the exposed cross-section was ground using a grinder. After grinding, Au was deposited onto the cross-section to create a sample for cross-section observation. The obtained cross-section was observed using a scanning electron microscope to obtain an image of the cross-section within the coating. Twenty-five doped zinc oxide particles were randomly selected from the field of view of this image, and the long and short side lengths of each particle were measured. Finally, the average long side length and the arithmetic mean of the short side lengths were calculated, and then the arithmetic mean of the long and short side lengths was taken as the average particle size. Furthermore, the doped zinc oxide particles present in the field of view of the image were identified as particles located in regions where both Zn and Al were detected, based on EPMA analysis of that field of view.

[0078] The observation conditions for scanning electron microscopy are described below.

[0079] • Measuring device: JSM-7200F manufactured by Nippon Electronics Co., Ltd. Magnification: 5000x Accelerating voltage: 10keV Image analysis software: Image J Ver. 1.54f The content of doped zinc oxide particles is 20-40% by mass relative to the coating film (total solids content of the coating film).

[0080] If the content of doped zinc oxide particles is low, the welding electrode will easily come into contact with the Zn-Al-Mg coating of the Zn-Al-Mg coated steel sheet during welding, thus reducing weldability. On the other hand, if the content of doped zinc oxide particles is high, the proportion of conductive pigment in the coating film will decrease, resulting in insufficient weldability, corrosion resistance, and other properties.

[0081] Therefore, the content of doped zinc oxide particles is set within the above range.

[0082] The content of doped zinc oxide particles is preferably 25-35% by mass, more preferably 27%-33% by mass.

[0083] Rust Inhibitor As rust inhibitors, organic rust inhibitors such as organic acids, organic acid salts, amine salts, and esters are used to impart high weldability. This is because if organic rust inhibitors are used, they will burn and carbonize due to the heat during welding, making it difficult for them to adhere to the welding electrode. Among them, organic acids, which can impart high corrosion resistance, are preferred.

[0084] Examples of organic acids include carboxylic acids (tartaric acid, tannic acid, oleic acid, dimer acid, naphthalene acid, etc.).

[0085] Examples of organic acid salts include carboxylic acid metal soaps (calcium lanolinate, zinc naphthenate, calcium oxalate, barium salts, etc.) and sulfonates (sodium sulfonate, calcium sulfonate, barium sulfonate).

[0086] Examples of esters include glycerides of higher fatty acids, sorbitan monoisostearate, and sorbitan oleate.

[0087] The content of the rust inhibitor relative to the coating film (total solids content of the coating film) is preferably 1 to 20% by mass. If the content of the rust inhibitor is 1% by mass or more, the corrosion resistance is sufficiently improved. On the other hand, if the content of the rust inhibitor is 20% by mass or less, the decrease in the liquid stability of the coating can be suppressed.

[0088] From the viewpoint of corrosion resistance and coating stability, the content of rust inhibitor is more preferably 1 to 3 by mass relative to the coating film (total solids content of the coating film).

[0089] (Other ingredients) Other components include, for example, well-known additives such as lubricants, silane coupling agents, and leveling agents.

[0090] However, if inorganic substances other than Zn compounds adhere to the welding electrode during welding, the weldability is reduced.

[0091] Therefore, the content of inorganic substances in the coating film, excluding Zn-containing compounds, is set to be less than 0 to 1% by mass relative to the coating film.

[0092] In other words, the coating film does not contain any inorganic substances other than Zn compounds, or even if it does contain them, the content of inorganic substances other than Zn compounds is controlled to be less than 1% by mass relative to the coating film.

[0093] Here, Zn-containing compounds include the aforementioned doped zinc oxide particles. Examples of Zn-containing compounds that include 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.

[0094] Furthermore, inorganic substances other than Zn-containing compounds including doped zinc oxide particles are not applicable to metal salts of organic substances (metal salts of organic acids, etc.).

[0095] Solid lubricants can impart excellent lubricity to coatings and improve their resistance to chalking. Examples of solid lubricants include those listed below (1) to (2).

[0096] (1) Polyolefin waxes and paraffin waxes: such as polyethylene wax, synthetic paraffin wax, natural paraffin wax, microcrystalline wax, chlorinated hydrocarbons, etc. (2) Fluoropolymer waxes: such as polyvinyl fluoride resin (polytetrafluoroethylene resin, etc.), polyvinyl fluoride resin, polyvinylidene fluoride resin, etc. When using polyethylene wax as a solid lubricant, the average particle size is preferably 0.5 to 10 μm. If the average particle size of the polyethylene wax is 0.5 μm or more, surface enrichment of the polyethylene wax is suppressed, which can prevent a decrease in the adhesion between the coating and the electrophoretic coating film. On the other hand, if the average particle size of the solid lubricant is 10 μm or less, the polyethylene wax can be prevented from falling off the coating film, thereby ensuring lubricity and corrosion resistance. From the perspective of obtaining excellent adhesion, corrosion resistance, lubricity, and chalking resistance of the coating and the electrophoretic coating film, the average particle size of the solid lubricant is more preferably 1 to 5 μm.

[0097] The softening point of the solid lubricant is preferably 100℃~135℃, more preferably 110~130℃. If the softening point of the solid lubricant is 100℃~135℃, the lubricity and powdering resistance of the coating film are further improved.

[0098] The content of solid lubricant relative to the coating film (total solid content of the coating film) is preferably 0.1 to 10% by mass. If the content of solid lubricant is 0.1% by mass or more, sufficient lubricity can be obtained. On the other hand, if the content of solid lubricant is 10% by mass or less, the reduction in the adhesion and corrosion resistance between the coating film and the electrophoretic coating film can be suppressed.

[0099] From the viewpoint of the adhesion, lubricity, and corrosion resistance of the coating and the electrophoretic coating film, the content of solid lubricant relative to the coating film (total solid content of the coating film) is more preferably 0.5 to 5% by mass, and even more preferably 0.5 to 2.5% by mass.

[0100] (Average film thickness) The average film thickness of the coating is 0.5–3 μm.

[0101] If the average film thickness of the coating is too thin, sufficient corrosion resistance cannot be achieved. Furthermore, the adhesion between the coating and the electrophoretic coating film cannot be adequately obtained. On the other hand, if the average film thickness of the coating is too thick, the coating's cohesion decreases, thereby compromising weldability.

[0102] Therefore, the average film thickness of the coating is set to the range described above.

[0103] The average film thickness of the coating is preferably 0.8 to 2.2 μm, more preferably 1.2 to 1.8 μm.

[0104] The average film thickness of the coating is measured as follows.

[0105] First, a surface-treated substrate with a coating is cut along its thickness to expose the cross-section. After resin embedding, the exposed cross-section is ground using a grinder. Following grinding, Au is vapor-deposited onto the cross-section to create a sample for cross-section observation.

[0106] The obtained cross-section was observed using a scanning electron microscope to obtain an image of the cross-section in the coating. The coating thickness was measured at 10 locations in this image, and the arithmetic mean was calculated. Furthermore, when measuring the coating thickness, the coating thickness was measured at 10 points spaced 40 μm apart along the direction of coating extension in the image.

[0107] The observation conditions for scanning electron microscopy are described below.

[0108] • Measuring device: JSM-7200F manufactured by Nippon Electronics Co., Ltd. Magnification: 3000x Accelerating voltage: 10keV (Coating formation) There are no particular limitations on the method for forming the coating film, and known methods can be used. For example, a coating film forming composition (paint) can be obtained by mixing a binder resin, doped zinc oxide particles, and a rust inhibitor with other components added as needed in a solvent. The solvent can be water or an organic solvent, but water is preferred from the viewpoint of manufacturing cost and environmental adaptability. That is, the coating film forming composition is preferably an aqueous composition. Then, the coating film forming composition is applied to at least one side of a Zn-Al-Mg based coated steel sheet and dried, or dried and heated, thereby forming a coating film. Furthermore, it is preferable to degrease and wash the surface of the Zn-Al-Mg based coated steel sheet before applying the coating film forming composition.

[0109] Here, the coating of the composition for film formation is performed using coating methods such as bar coating, roller coating, doctor blade coating, and curtain coating.

[0110] The drying temperature of the coating film, measured by the highest temperature reached on the surface of the Zn-Al-Mg coated steel sheet, is preferably 50-120°C, more preferably 60-100°C.

[0111] The heating temperature of the coating film is preferably 100-240°C, more preferably 120-210°C, based on the highest temperature reached on the surface of the Zn-Al-Mg coated steel sheet.

[0112] <Other methods for surface treatment of steel sheets> The surface-treated steel sheet of this embodiment may also have an intermediate coating (functional coating such as chemical conversion treatment film) between the Zn-Al-Mg system plated steel sheet and the coating film.

[0113] As an intermediate coating film, it is preferable to have an intermediate coating film containing an adhesive resin, a silane coupling agent, silica microparticles, a phosphoric acid compound, and a fluorine compound, and having an average film thickness of 0.1 to 0.5 μm.

[0114] By using an intermediate coating with the above-described composition and average film thickness, corrosion resistance can be improved with relatively little impairment to weldability. This is because if the intermediate coating is relatively thin, the impact on weldability is smaller, the adhesion between the coated steel sheet and the coating is improved, thereby enhancing corrosion resistance.

[0115] Therefore, the surface-treated steel sheet of this embodiment preferably has an intermediate coating (functional coating) with the above-mentioned composition and the above-mentioned average film thickness between the Zn-Al-Mg based coated steel sheet and the coating film.

[0116] The average film thickness of the intermediate coating was measured in the same manner as the average film thickness of the coating.

[0117] <Painted Components> The coating component of this embodiment includes: a forming material formed from the surface-treated steel sheet of this embodiment, and an electrophoretic coating film formed on the coating film of the forming material (i.e., the coating film of the surface-treated steel sheet). Figure 2 ).

[0118] Here, Figure 2 In this context, 100 represents the coated component, 10A represents the forming material, 11A represents the base steel plate in the forming material (the steel plate of the object to which a Zn-Al-Mg coating is formed), 12A represents the Zn-Al-Mg coating of the forming material, 13A represents the coating film of the forming material, and 14 represents the electrophoretic coating film. Figure 2The example shown is a painted component with a defined cross-section in the shape of a hat with a brim, but there are no particular restrictions on the overall shape or cross-sectional shape of the painted component, and it can have any shape.

[0119] The coated component of this embodiment is manufactured as follows. First, a surface-treated steel sheet is formed using known forming techniques such as cutting and pressure forming to obtain a formed material of the desired shape. The formed material can also be assembled into a desired shape by welding (spot welding, etc.) as needed.

[0120] Next, an electrophoretic coating treatment is performed on the coating film of the formed material. This forms an electrophoretic coating film on the coating film. The electrophoretic coating treatment can be either anionic electrophoretic coating or cationic electrophoretic coating, but from the perspective of corrosion resistance, cationic electrophoretic coating is preferred.

[0121] In particular, if an electrophoretic coating film is formed by using a water-based coating containing a resin [e.g., a water-based resin (acrylic resin, polyester resin, alkyd resin, epoxy resin, polyurethane resin, etc., which have hydrophilic groups such as carboxyl, hydroxyl, hydroxymethyl, amino, sulfonic acid, polyoxyethylene bonds and functional groups such as hydroxyl groups that react with the curing agent)], a curing agent (melamine resin, end-capped polyisocyanate, etc.) and other additives (coloring pigments, light interference pigments, extender pigments, dispersants, anti-settling agents, reaction promoters, defoamers, thickeners, rust inhibitors, ultraviolet absorbers, surface conditioners, etc.)], the adhesion between the coating film and the electrophoretic coating film is easily improved.

[0122] Then, other coating films such as intermediate coating films and top coating films can also be formed on the electrophoretic coating film of the shaped material as needed.

[0123] These processes are used to manufacture the coated component of this embodiment.

[0124] Furthermore, before electrophoretic coating, the molded material with the coating film can be degreased and surface-adjusted, followed by chemical conversion treatment (such as phosphate treatment, Zr treatment, etc.). Through chemical conversion treatment, it is difficult to form a chemical conversion film on the coating itself, but a chemical conversion film can be formed in necessary areas outside the coating. Therefore, as a whole, the adhesion of the electrophoretic coating film can be improved.

[0125] The painted components of this embodiment are widely used in automotive components (car bodies, running gear, etc.), mechanical components (frames, etc.), home appliance components (frames, etc.), building materials (roofs, walls, etc.).

[0126] Example The following embodiments are shown and the present disclosure is further described, but the present disclosure is not limited to the following embodiments. Various conditions can be adopted as long as they do not depart from the spirit of the present disclosure and can achieve the purpose of the present disclosure.

[0127] Example 1. Manufacturing of surface-treated steel plates 1.1 Preparation of Zinc-Coated Steel Sheets Prepare the following 5 types of zinc-plated steel sheets, immerse them in an aqueous solution (2.5% by mass, 40°C) of water-based alkaline degreasing agent (Nippon Parkerizing Co., Ltd. FC-301) for 2 minutes to degrease the surface, then wash and dry them to produce zinc-plated steel sheets for surface treatment.

[0128] However, cold-rolled steel sheets are also prepared as steel sheets for surface treatment.

[0129] ZA: Zn-Al-Mg ternary hot-dip galvanized steel sheet (Zn-11%Al-3%Mg-0.2%Si) (plate thickness 0.8mm) ZB: Zn-Al-Mg ternary hot-dip galvanized steel sheet (Zn-2%Al-2%Mg) (plate thickness 0.8mm) ZC: Hot-dip galvanized steel sheet (0.8mm thick) ZD: Alloyed hot-dip galvanized steel sheet (0.8mm thickness, 10% Fe by weight) C: Cold-rolled steel sheet (0.8mm thick, uncoated) The coating adhesion amount is shown in Table 2. The adhesion amount is controlled by adjusting the coating pull-out speed and friction contact amount during coating production.

[0130] 1.2 Formation of the intermediate coating (chemical conversion treatment film) Next, the following chemical conversion treatment solution S is prepared, and the bar coating number (mesh size) is changed to achieve the average film thickness of the intermediate coating shown in Table 2. The solution is then applied to the zinc-plated steel sheet or cold-rolled steel sheet. The sheet is then heated and dried in a hot air furnace at a surface temperature of 70°C, and then air-dried, thereby forming an intermediate coating on the surface of the zinc-plated steel sheet or cold-rolled steel sheet.

[0131] However, in some cases, the formation of the intermediate coating was not implemented.

[0132] S1: A chemical conversion treatment solution with a solid content of 10%, consisting of silane coupling agent, silica microparticles, polyurethane resin, phosphoric acid compound, and fluorine compound. S2: A chemical conversion treatment solution with a solid content of 10%, consisting of Zr compounds, silane coupling agents, phosphoric acid compounds, and vanadium compounds. 1.3 Formation of the coating film Next, in order to form a coating film with the composition ratio (mass%) shown in Table 1, the components are mixed to a solid content concentration of 20% by mass to prepare a coating for film formation. The coating is applied to zinc-plated steel sheet, cold-rolled steel sheet, or chemically converted film using a bar coater with varying bar coating number (mesh size) and dilution rate to achieve the average film thickness shown in Table 2. The coating is then dried in an oven at the maximum reach temperature (PMT) recorded in Table 2, or subjected to drying and heating, thereby forming a coating film.

[0133] In addition, the components contained in the coating are as follows.

[0134] (Adhesive resins, etc.) J1: Epoxy resin (ADEKA RESIN EM-0461N manufactured by ADEKA Corporation) J2: Polyester resin (Vylonal MD1480 manufactured by Vylon Corporation) J3: Polyurethane resin (Superflux 150 manufactured by Daiichi Kogyo Pharmaceutical Co., Ltd.) J4: Epoxy resin (ADEKA RESIN EP-4100 manufactured by ADEKA Corporation) J5: Polyester resin (Vylon 200 manufactured by Vylon Corporation) J6: Melamine resin (Cymel 325 manufactured by Allnex) J7: Water-soluble isocyanate (ELASTRON BN-77 manufactured by Daiichi Kogyo Pharmaceutical Co., Ltd.) (Doped zinc oxide particles) (0065) Z1: Doped zinc oxide particles (produced by HAKUSUI TECH Co., Ltd., 23-Kt, average particle size 0.5μm) Z2: Doped zinc oxide particles (produced by HAKUSUI TECH Co., Ltd., 23-K, average particle size 0.2μm) Z3: Doped zinc oxide particles (manufactured by HAKUSUI TECH Co., Ltd., Pazet CK, average particle size 0.03μm) Z4: Zinc oxide particles (general reagent, average particle size 0.5μm) Z5: Titanium nitride particles (general reagent, average particle size 2.0 μm) (Rust inhibitor) B1: A mixture of tannic acid (a common reagent) and zinc naphthenate (a common reagent) in a 1:1 mass ratio. B2: A mixture of tartaric acid (a common reagent) and zinc naphthenate (a common reagent) in a 1:1 mass ratio. B3: A mixture of tannic acid (a common reagent) and zinc triphosphate (a common reagent) in a 1:1 mass ratio. B4: A mixture of tartaric acid (a common reagent) and zinc triphosphate (a common reagent) in a 1:1 mass ratio. B5: Colloidal silica (particle size 20nm) B6: Aluminum dihydrogen phosphate (particle size 2μm) B7: Calcium ion-exchange silica (Ca exchange rate 9%) (particle size 2μm) B8: Silica (particle size 2μm) (Solid lubricant) W1: Polyethylene wax (Chembese S-394 MG) W2: Polytetrafluoroethylene (PTFE) particles (manufactured by CHEMBASE, SST-1MG-RC) 2. Performance evaluation test The following performance evaluation tests were conducted on the surface-treated steel sheets of each example.

[0135] 2.1 Corrosion Resistance Test (Preparation 1) Test piece A, cut to a size of 70mm × 150mm, and test piece B, cut to a size of 50mm × 100mm, were taken from the surface-treated steel plates of each example.

[0136] Next, as Figures 3-4 As shown in the top view, with test pieces A and B overlapping each other at their respective centers and parallel in their length directions, test pieces A and B are joined together by spot welding at their respective center positions to create the joined test piece C.

[0137] Furthermore, the spot welding location was set at the center of test pieces A and B. The spot welding conditions were set as follows: electrode: CF type Cr-Cu electrode with a tip diameter of 5mm and an R40 rating; applied pressure: 1.96kN; welding current: 8kA; energizing time: 12 cycles / 50Hz. Additionally, during spot welding, a 0.1mm gauge was inserted between test pieces A and B, and a gap (allowance) was maintained between them.

[0138] (Preparation 2) For the bonding test piece C made from the surface-treated steel sheet in each example, surface conditioning was performed for 20 seconds at room temperature using PREPARENE X (trade name), a surface conditioning agent manufactured by Nippon Parkerizing Co., Ltd. Furthermore, chemical conversion treatment (phosphate treatment) was performed using PALBOND 3020 (trade name), a chemical conversion treatment solution (zinc phosphate treatment solution) manufactured by Nippon Parkerizing Co., Ltd. The temperature of the chemical conversion treatment solution was set to 43°C. After immersing the bonding test piece C in the chemical conversion treatment solution for 120 seconds, it was washed with water and dried. After the above chemical conversion treatment (phosphate treatment), a cationic electrophoretic coating manufactured by NIPPON PAINT Co., Ltd. was applied using a 160V ramp current, followed by baking at a baking temperature of 170°C for 20 minutes. The average film thickness of the electrophoretic coating was 10 μm in each bonding test piece C.

[0139] (Corrosion resistance test of plate overlap) After the electrophoretic coating process described above, the end face of test piece A of the bonding test piece C was sealed. A corrosion cycle test was then performed on the bonding test piece C while spraying salt water onto the test piece B side of the bonding test piece C. Each corrosion cycle consisted of 2 hours of salt water spray (SST, 5% NaCl, 35°C atmosphere), 2 hours of drying (60°C), and 4 hours of wetting (50°C, 98%RH), for a total of 360 cycles. The corrosion cycle test was conducted according to JASO M609-91.

[0140] Then, after the corrosion test, the spot weld of the joint test piece C was hollowed out with a drill bit or the like, and test pieces A and B were separated. The corrosion status of the overlapping surface of test piece A and test piece B was observed and the following scores were given.

[0141] In addition, the corrosion depth of test piece A was measured after removing corrosion products by immersing it in an ammonium citrate aqueous solution. Specifically, test piece A, after removing corrosion products, was divided into 8 equal parts ( Figure 3 The plate was divided into eight equal parts (two parts to the left and right, and four parts to the top and bottom). After being made into small pieces, the corrosion depth (mm) of each piece was measured. The largest corrosion depth (mm) among the eight pieces was set as the maximum corrosion depth (mm) and used as the result of the corrosion resistance test of the plate overlap in this embodiment.

[0142] In such corrosion resistance tests, a rating of "3", "4" or "5" indicates excellent corrosion resistance. The results are shown in Table 2.

[0143] In addition, the corrosion depth was measured using a micrometer (CPM-MX manufactured by Mitutoyo Co., Ltd.).

[0144] 1: The mating surface between test piece A and test piece B shows overall red rust or a maximum corrosion depth of 0.4 mm or more. 2: More than 20% of the interface between test piece A and test piece B shows red rust or a maximum corrosion depth of 0.2 mm or more but less than 0.4 mm. 3: More than 5% but less than 20% of the interface between test piece A and test piece B exhibits red rust or a maximum corrosion depth of 0.1 mm or more but less than 0.2 mm. 4: Less than 5% red rust or a maximum corrosion depth exceeding 0 mm but less than 0.1 mm is observed at the interface between test piece A and test piece B. 5: No red rust was observed at the interface between test piece A and test piece B, and the maximum corrosion depth was 0 mm (none). 2.2 Spot weldability For each surface-treated steel plate, using a CF-type Cr-Cu electrode with a top diameter of 5mm and an R40 rating, under a pressure of 1.96kN and an energizing time of 12 cycles / 50Hz, the welding current was varied, and 500 spot welds were performed with a weld nugget diameter of 4√t (t = thickness of the surface-treated steel plate). The weld nugget diameter and electrode condition were observed during the 500 spot welds, and the following scores were assigned. Furthermore, spot welds without energization were judged based on whether a weld nugget was formed. Additionally, the presence or absence of a weld nugget and its diameter were determined by observing the plate thickness section including the center of the spot weld.

[0145] In such weldability tests, a score of "3", "4", or "5" indicates excellent weldability. The results are shown in Table 2.

[0146] 1. 500 points cannot be implemented. 2: It can perform 500 dots, but there are cases where there is no power supply (including no power supply) in 10 dots. When the 500 dots are finished, the diameter of the electrode contact surface of the pressure-sensitive paper is more than 1.7 times the diameter of the electrode contact surface of the first dot.

[0147] 3. After performing 500 dot markings without any power failures, the diameter of the electrode contact surface using the pressure-sensitive paper at the end of the 500 dot markings should be at least 1.4 times and less than 1.7 times the diameter of the electrode contact surface during the first dot marking. 4. After performing 500 dot markings without any power failures, the diameter of the electrode contact surface using the pressure-sensitive paper at the end of the 500 dot markings should be at least 1.1 times and less than 1.4 times the diameter of the electrode contact surface during the first dot marking. 5. Perform 500 dot markings without any power failures, resulting in no electrode wear, or at the end of the 500 dot markings, the diameter of the electrode contact surface using the pressure-sensitive paper is less than 1.1 times the diameter of the electrode contact surface during the first dot marking. Here, in Figures 3-4 In the diagram, 20A represents test piece A, 20B represents test piece B, 20C represents joint test piece C, and 22 represents the spot weld section.

[0148] As can be seen from the above results, the surface-treated steel plate of this embodiment has better corrosion resistance and weldability compared with the surface-treated steel plate of the comparative example.

[0149] In addition, in Table 1, "inorganic substances" The expression “1” indicates the composition ratio of “inorganic substances other than Zn-containing compounds including doped zinc oxide particles” relative to the coating film.

[0150] Furthermore, the entire contents disclosed in Japanese Patent Application No. 2023-056126 are incorporated herein by reference.

[0151] All documents, patent applications and technical standards described in this specification are incorporated herein by reference, and the content of each document, patent application and technical standard incorporated herein by reference is identical to the specific and separately described herein.

Claims

1. A surface-treated steel plate, wherein, have: Zn-Al-Mg coated steel sheets have a coating thickness of 20–60 g / m² on each side. 2 ;and A coating is disposed on at least one main surface of the Zn-Al-Mg based coated steel sheet; The coating film comprises an adhesive resin, doped zinc oxide particles, and a rust inhibitor. The average thickness of the coating is 0.5–3 μm. The average particle size of the doped zinc oxide particles is 0.1–2 μm. The content of the doped zinc oxide particles is 20-40% by mass relative to the coating film. The content of inorganic substances in the coating, excluding Zn compounds, is less than 0-1% by mass relative to the coating.

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

3. The surface-treated steel plate according to claim 1, wherein, An intermediate coating is provided between the Zn-Al-Mg based coated steel sheet and the coating film. The intermediate coating film comprises an adhesive resin, a silane coupling agent, silica microparticles, a phosphoric acid compound, and a fluorine compound. The average thickness of the intermediate coating is 0.1–0.5 μm.

4. The surface-treated steel plate according to claim 1, wherein, The chemical composition of the Zn-Al-Mg coating on the Zn-Al-Mg coated steel sheet, on average, contains Al: 4-22% by mass, Mg: 1.0-10% by mass, and the remainder includes Zn and impurities.

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