Surface-treated steel sheet

Abstract

Provided is a surface-treated steel sheet which does not contain chromium in a surface-treated coating film and which has excellent corrosion resistance and spot weldability after processing. This surface-treated steel sheet is characterized by being provided with: a zinc-plated steel sheet having minute irregularities on the surface thereof; an organic-inorganic composite coating film that does not contain chromium and is formed on the surface of the zinc-plated steel sheet; and a surface-treated coating film that contains a urethane resin and is formed on the organic-inorganic composite coating film. The average value of the total thickness of the organic-inorganic composite coating film and the surface-treated coating film in the protrusions of the zinc-plated steel sheet is 0.10 [mu] m or less, and the average thickness of the surface-treated coating film is 0.4-1.0 [mu] m. The surface-treated steel sheet is characterized in that the film damage rate of the surface-treated film after the surface-treated steel sheet is bent / back-bent with a slider having a tip diameter of 5 mmR is 20% or less.

Description

Technical Field

[0001] The present invention relates to a surface-treated steel sheet that is optimal for applications in automobiles, home appliances, and building materials and that does not contain chromium, which has a high environmental load, in its surface-treated film. Background Art

[0002] Galvanized steel sheets are widely used in the automotive, home appliance, and building material industries. Previously, to improve corrosion resistance, surface-treated steel sheets, obtained by subjecting the surface of galvanized steel sheets to a chromate treatment using a treatment solution primarily composed of chromic acid, dichromic acid, or their salts, were widely used. However, due to recent global environmental issues, there is an increasing demand for galvanized steel sheets with chromate-free surface treatments (chromate-free treated steel sheets), and chromate-free treated steel sheets with improved properties have been proposed.

[0003] On the other hand, when galvanized steel sheets are used in the manufacture of automotive parts, it is also common to not form a surface treatment film. In this case, the galvanized steel sheets are press-formed into formed parts, and multiple formed parts are assembled into components by spot welding, bonding, etc., and then the components are subjected to chemical conversion treatment and subsequent electrodeposition coating to produce automotive parts.

[0004] In this way, the corrosion resistance of automotive parts is ensured by the chemical conversion coating formed by the chemical conversion treatment process and the electrodeposition coating formed by the electrodeposition coating process. However, in the joints of molded parts, etc., the electrodeposition coating sometimes wraps around the parts. Therefore, auxiliary materials such as sealants and wax are used to improve the corrosion resistance of the joints. These auxiliary materials are the main factor in the increase in automobile manufacturing costs, so the demand for reducing auxiliary materials is high. In addition, as a fundamental manufacturing cost reduction plan, there is a tendency to omit the chemical conversion treatment process and the electrodeposition coating process.

[0005] Therefore, research and development of surface-treated steel sheets (e.g., Patent Documents 1 and 2) with the goal of being used in automotive parts, which are formed by forming an organic surface treatment film on the surface of galvanized steel sheets, are gradually being conducted. Such surface-treated steel sheets need to have excellent corrosion resistance after processing and need to be assembled into a predetermined shape, thus requiring spot weldability.

[0006] Patent Document 1 describes a chromium-free alloyed hot-dip galvanized steel sheet having a surface-treated layer on at least one surface of the alloyed hot-dip galvanized steel sheet. The surface-treated layer contains a film-forming component and an inhibitor component. The film-forming component includes an organic silicon compound obtained by reacting multiple silane coupling agents and a cationic polyurethane resin, and the inhibitor component is composed of phosphoric acid and magnesium phosphate.

[0007] Patent document 2 describes a surface-treated steel sheet having a coating on at least one side of a galvanized steel sheet, the coating comprising: a binder resin that is a water-soluble or water-dispersible aqueous resin, conductive particles, a rust-proof pigment, and at least one oxide particle selected from zirconium oxide particles, titanium oxide particles, nickel oxide particles, and tin (IV) oxide particles.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-148109

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2016-194137 Summary of the Invention

[0012] In surface-treated steel sheets formed with an organic surface treatment film on the surface of galvanized steel sheets, generally speaking, the thicker the surface treatment film, the better the corrosion resistance after processing, but the lower the conductivity, and therefore the lower the spot weldability. In other words, corrosion resistance after processing and spot weldability are contradictory properties. Various developments have been made to balance corrosion resistance after processing and spot weldability, but with the recent increase in demand, further improvements are required.

[0013] Therefore, in view of the above-mentioned problems, an object of the present invention is to provide a surface-treated steel sheet that does not contain chromium in the surface-treated film and has excellent corrosion resistance and spot weldability after working.

[0014] To address the above-mentioned issues, the present inventors conducted in-depth research on a method for achieving both high post-processing corrosion resistance and spot weldability in a double-layer surface-treated steel sheet comprising a chromium-free organic-inorganic composite film and a surface-treated film containing a polyurethane resin formed on a galvanized steel sheet, and obtained the following insights. Specifically, the amount of organic-inorganic composite film adhered was minimized, and a specific polyurethane resin with excellent flexibility was used as the polyurethane resin contained in the surface-treated film. This method allows the combined thickness of the organic-inorganic composite film and the surface-treated film to be very thin in the convex portion of the galvanized steel sheet, ensuring spot weldability. Furthermore, damage to the surface-treated film during processing can be suppressed, resulting in excellent post-processing corrosion resistance.

[0015] The gist of the present invention completed based on the above findings is as follows.

[0016] [1] A surface-treated steel plate, characterized by having:

[0017] The galvanized steel sheet has slight bumps on the surface.

[0018] an organic-inorganic composite film containing no chromium formed on the surface of the galvanized steel sheet, and

[0019] A surface treatment film containing a polyurethane resin formed on the above-mentioned organic-inorganic composite film,

[0020] The average value of the total thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet is 0.10 μm or less.

[0021] The average thickness of the surface treatment film is 0.4 to 1.0 μm.

[0022] The surface treatment film damage rate after the surface treated steel sheet was bent / returned using a bead having a tip diameter of 5 mmR was 20% or less.

[0023] [2] The surface-treated steel sheet according to [1] above, wherein the polyurethane resin has an elongation of 400% or more and a maximum tensile stress of 1.0 MPa or more.

[0024] [3] The surface-treated steel sheet according to [1] or [2] above, wherein the polyurethane resin is a solvent-soluble resin.

[0025] [4] The surface-treated steel sheet according to any one of [1] to [3] above, wherein the surface-treated film contains a total of 1 to 50 parts by mass of one or more rust-preventive additives selected from silicon oxide, phosphoric acid compounds, molybdic acid compounds, and vanadium compounds, relative to 100 parts by mass of the polyurethane resin.

[0026] [5] The surface-treated steel sheet according to any one of [1] to [4], wherein the surface-treated film contains 1 to 30 parts by mass of a solid lubricant per 100 parts by mass of the polyurethane resin.

[0027] [6] The surface-treated steel sheet according to any one of [1] to [5], wherein the average thickness of the organic-inorganic composite film is 0.01 to 0.20 μm.

[0028] The surface-treated steel sheet of the present invention does not contain chromium in the surface-treated film and is excellent in both corrosion resistance after working and spot weldability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a diagram schematically showing a cross-sectional structure of a surface-treated steel sheet according to one embodiment of the present invention.

[0030] Figure 2 This is a cross-sectional SEM image of the surface-treated steel plate of Invention Example No. 5.

[0031] Figure 3 This is a diagram showing bending / rebending processes performed when determining the "film damage rate" in one embodiment of the present invention.

[0032] Figure 4 This is a graph showing a combined cycle test conducted in Examples to evaluate post-processing corrosion resistance.

[0033] Figure 5 This is a schematic diagram of a friction coefficient measuring device used to evaluate lubricity in Examples.

[0034] Figure 6 This is a perspective view showing the shape and dimensions of a slider used for evaluating lubricity in Examples. DETAILED DESCRIPTION

[0035] [Surface treated steel sheet]

[0036] Reference Figure 1 A surface-treated steel sheet according to one embodiment of the present invention comprises a galvanized steel sheet, an organic-inorganic composite film formed on the surface of the galvanized steel sheet, and a surface treatment film formed on the organic-inorganic composite film. It should be noted that, in this specification, "the surface of the galvanized steel sheet" refers to the surface of the galvanized film and refers to one or both surfaces of the galvanized steel sheet.

[0037] [[Galvanized steel sheet]]

[0038] The galvanized steel sheet may be any steel sheet having any galvanized coating formed thereon, and examples thereof include galvanized steel sheet, Zn-Fe alloy-plated steel sheet, Zn-Ni alloy-plated steel sheet, Zn-Mn alloy-plated steel sheet, Zn-Co alloy-plated steel sheet, Zn-Cr alloy-plated steel sheet, Zn-Al alloy-plated steel sheet, Zn-Mg alloy-plated steel sheet, or steel sheet having a coating formed thereon containing a plurality of these alloying elements. Furthermore, a galvanized composite steel sheet (e.g., a Zn-SiO2 dispersed galvanized steel sheet) in which a small amount of a dissimilar metal element, metal oxide, polymer, etc. is dispersed in the galvanized coating may also be used as the galvanized steel sheet.

[0039] As the plating method, there is no particular limitation, as long as appropriate preferred methods such as electroplating, chemical plating, hot-dip plating, PVD, CVD, etc. are adopted. In addition, after forming the galvanized film, annealing, temper rolling, surface conditioning, etc. can be appropriately implemented.

[0040] Generally, the surface of a galvanized steel sheet (i.e., the surface of the galvanized coating) has minute irregularities with an arithmetic mean roughness Ra of 0.5 to 1.5 μm. Here, the arithmetic mean roughness Ra is the arithmetic mean roughness obtained from the roughness curve obtained by measuring the surface of the galvanized steel sheet in accordance with JIS B0601 (2001) using a cutoff value λc of 0.8 mm, a cutoff value λs of 2.5 μm, and a measurement distance of 2.5 mm.

[0041] [[Organic-inorganic composite film]]

[0042] The organic-inorganic composite film is a film that does not contain chromium (chromium-free) and is formed on the surface of a galvanized steel sheet. Figure 1 The organic-inorganic composite film preferably has the property of not easily following the unevenness of the surface of the galvanized steel sheet. In other words, the thickness of the organic-inorganic composite film on the convex portion of the galvanized steel sheet is preferably smaller than the thickness of the organic-inorganic composite film on the concave portion of the galvanized steel sheet. As described below, this can substantially reduce the combined thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet, thereby ensuring spot weldability.

[0043] The average thickness of the organic-inorganic composite film is preferably 0.01 to 0.20 μm. If the average thickness of the organic-inorganic composite film is too small, excellent corrosion resistance cannot be obtained. From this viewpoint, the average thickness of the organic-inorganic composite film is preferably 0.01 μm or more, more preferably 0.05 μm or more. On the other hand, if the average thickness of the organic-inorganic composite film is too large, the total thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet cannot be fully reduced, and spot weldability cannot be ensured. From this viewpoint, the average thickness of the organic-inorganic composite film is preferably 0.20 μm or less.

[0044] Here, the "average thickness of the organic-inorganic composite film" is calculated using the following method. The surface layer of a cross-section of the surface-treated steel sheet is observed in three viewing fields using a scanning electron microscope (SEM) at 5000x magnification. The thickness of the organic-inorganic composite film is measured at 20 locations within each viewing field at 1.0 μm intervals. The arithmetic average of the thicknesses at a total of 60 locations across the three viewing fields is used as the "average thickness." The cross-section processing method is not particularly limited; examples include FIB (Focused Ion Beam) processing.

[0045] In order to achieve both post-processing corrosion resistance and spot weldability, the organic-inorganic composite film preferably has excellent electrical conductivity. Specifically, the organic-inorganic composite film is preferably a film obtained by applying a surface treatment solution containing a water-soluble zirconium compound (a), a tetraalkoxysilane (b), a compound having an epoxy group (c), a chelating agent (d), a vanadic acid compound (e), and a metal compound (f) containing at least one selected from Ti, Al, and Zn, and having a pH of 8 to 10, to the surface of a galvanized steel sheet and drying the solution.

[0046] (I) The mass ratio (a / b) of the water-soluble zirconium compound (a) in terms of Zr to the tetraalkoxysilane (b) is 1.0 to 6.0

[0047] (II) The mass ratio (b / c) of the tetraalkoxysilane (b) to the compound (c) having an epoxy group is 0.1 to 1.6

[0048] (III) The mass ratio (b / d) of tetraalkoxysilane (b) to chelating agent (d) is 0.3 to 2.0

[0049] (IV) The mass ratio (e / d) of the V-converted mass of the vanadate compound (e) to the chelating agent (d) is 0.03 to 1.0

[0050] (V) The mass ratio (f / d) of the total metal-converted mass of the metal compound (f) to the chelating agent (d) is 0.05 to 0.8

[0051] Examples of the water-soluble zirconium compound (a) include zirconium nitrate, zirconium oxynitrate, zirconium acetate, zirconium sulfate, zirconium carbonate, ammonium zirconium carbonate, potassium zirconium carbonate, sodium zirconium carbonate, and fluorozirconic acid. One or more of these may be used.

[0052] Examples of the tetraalkoxysilane (b) include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane, and one or more of these can be used.

[0053] Examples of the compound (c) having an epoxy group include silane coupling agents having an epoxy group such as γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, and γ-glycidoxypropyltriethoxysilane; ester compounds having an epoxy group such as diglycidyl adipate, diglycidyl phthalate, and diglycidyl terephthalate; and ether compounds having an epoxy group such as sorbitol polyglycidyl ether, sorbitan polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, glycerol polyglycidyl ether, trimethylpropane polyglycidyl ether, neopentyl glycol diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, and polypropylene glycol diglycidyl ether. More than one of these can be used.

[0054] Examples of the chelating agent (d) include hydroxycarboxylic acids such as tartaric acid and malic acid; polycarboxylic acids such as monocarboxylic acids, dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, citric acid, and adipic acid; and tricarboxylic acids; aminocarboxylic acids such as glycine; phosphonic acids such as 1-hydroxymethane-1,1-diphosphonic acid; and phosphonates, and one or more of these can be used.

[0055] Examples of the vanadic acid compound (e) include ammonium metavanadate and sodium metavanadate, and one or more of these can be used.

[0056] Examples of the metal compound (f) containing at least one selected from Ti, Al and Zn include titanyl sulfate, titanyl nitrate, titanium nitrate, titanyl chloride, titanium chloride, titanium dioxide sol, titanium oxide, potassium oxalate titanate, fluorotitanic acid, ammonium fluorotitanate, titanium lactate, titanium tetraisopropoxide, titanium acetylacetonate, diisopropyl titanium bisacetylacetonate, aluminum oxide, aluminum hydroxide, aluminum sulfate, aluminum nitrate, aluminum phosphate, aluminum chloride, zinc carbonate, zinc oxide, zinc hydroxide, zinc sulfate, zinc nitrate, zinc chloride, zinc phosphate, sodium zincate, potassium zincate, and the like, and one or more thereof can be used.

[0057] The pH of the surface treatment liquid is within the range of 8 to 10. If the pH is less than 8, the storage stability, corrosion resistance, and adhesion of the organic-inorganic composite film of the surface treatment liquid decrease, while if the pH exceeds 10, corrosion resistance and conductivity decrease.

[0058] The mass ratio (a / b) of the Zr-equivalent mass of the water-soluble zirconium compound (a) to the tetraalkoxysilane (b) is in the range of 1.0 to 6.0. If the mass ratio (a / b) is less than 1.0, corrosion resistance decreases, and if it exceeds 6.0, conductivity decreases.

[0059] The mass ratio (b / c) of the tetraalkoxysilane (b) to the epoxy group-containing compound (c) is in the range of 0.1 to 1.6. If the mass ratio (b / c) is less than 0.1, corrosion resistance is reduced, and if it exceeds 1.6, the adhesion of the organic-inorganic composite film is reduced.

[0060] The mass ratio (b / d) of tetraalkoxysilane (b) to chelating agent (d) is in the range of 0.3 to 2.0. If the mass ratio (b / d) is less than 0.3 or exceeds 2.0, the corrosion resistance is reduced.

[0061] The mass ratio (e / d) of the V-converted mass of the vanadate compound (e) to the mass ratio (e / d) of the chelating agent (d) is in the range of 0.03 to 1.0. If the mass ratio (e / d) is less than 0.03, the corrosion resistance is reduced, and if it exceeds 1.0, the vanadate compound is difficult to dissolve in the surface treatment solution.

[0062] The mass ratio (f / d) of the total metal-equivalent mass of the metal compound (f) to the chelating agent (d) is in the range of 0.05 to 0.8. If the mass ratio (f / d) is less than 0.05, corrosion resistance is reduced, and if it exceeds 0.8, the metal compound (f) becomes difficult to dissolve in the surface treatment agent.

[0063] [[Surface treatment film]]

[0064] The surface treatment film is formed on the organic-inorganic composite film and is a film containing a polyurethane resin and not containing chromium (a chromium-free) .

[0065] First, regarding both the organic-inorganic composite film and the surface treatment film, refer to Figure 1 In this embodiment, it is important that the average total thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet is 0.10 μm or less. If the average total thickness exceeds 0.10 μm, excellent spot weldability cannot be achieved. If the average total thickness is 0.10 μm or less, thin film portions with small thicknesses of the organic-inorganic composite film and the surface treatment film can be provided, thereby achieving excellent spot weldability.

[0066] In addition, the lower limit of the average of the above-mentioned total thickness is not particularly limited, but from the viewpoint of obtaining excellent corrosion resistance after processing, the average of the above-mentioned total thickness is preferably 0.02 μm or more.

[0067] Here, the "average value of the combined thickness of the organic-inorganic composite film and the surface treatment film on the convex portions of the galvanized steel sheet" is determined by the following method. The surface layer of the surface-treated steel sheet is observed in three viewing fields using a SEM at 10,000x magnification. All convex portions present in the three viewing fields are identified, and the combined thickness of the organic-inorganic composite film and the surface treatment film on these convex portions is calculated. The average value of these sums is used as the "average thickness." The cross-sectional processing method is not particularly limited; examples include FIB processing.

[0068] It is important that the average thickness of the surface treatment film be 0.4 to 1.0 μm. If the average thickness of the surface treatment film is less than 0.4 μm, the film damage rate (described later) cannot be reduced to less than 20%, and excellent post-processing corrosion resistance cannot be achieved. On the other hand, if the average thickness of the surface treatment film exceeds 1.0 μm, the average combined thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet cannot be reduced to less than 0.10 μm, and excellent spot weldability cannot be achieved.

[0069] Here, the "average thickness of the surface treatment film" is determined by the following method. The surface layer of a cross-section of the surface-treated steel plate is observed using an SEM at 5000x magnification in three viewing fields. The thickness of the surface treatment film is measured at 20 locations within each viewing field at 1.0 μm intervals. The arithmetic average of the thicknesses at a total of 60 locations across the three viewing fields is used as the "average thickness." The cross-section processing method is not particularly limited; examples include FIB processing.

[0070] Furthermore, it is important that the film damage rate of the surface treatment film after bending / rebending the surface-treated steel sheet using a slider with a tip diameter of 5 mmR be 20% or less. If this film damage rate exceeds 20%, excellent post-processing corrosion resistance cannot be achieved. A lower film damage rate is preferred, so there is no particular lower limit; the film damage rate can be 0% or greater, and may be 5% or greater.

[0071] Here, the above-mentioned "film damage rate" is obtained by the following method. Figure 3As shown, a 68mm×350mm surface-treated steel plate test piece was pressed with a load of 700kgf using a convex slider and a concave slider, and a bending / return bead test (Drawbead Test) was performed in which the test piece was pulled horizontally at a pulling speed of 1m / min. The drawing was carried out by applying rust-proof cleaning oil (PRETON R352L manufactured by Sugimura Chemical Industry Co., Ltd.), and a 5mmR was used at the tip of the convex slider. Then, the surface of the surface-treated film was observed using an SEM at a magnification of 100 times. In any three fields of view of the reflected electron image, the film damage area was determined by image binarization. The area ratio of the film damage area in each field of view was calculated, and the arithmetic average of the area ratios was used as the "film damage ratio."

[0072] The polyurethane resin contained in the surface treatment film and the surface treatment liquid used to form the surface treatment film preferably has an elongation of 400% or more and a maximum tensile stress of 1.0 MPa or more. When the elongation is less than 400%, the damage rate of the above-mentioned film exceeds 20%, and excellent corrosion resistance after processing cannot be obtained. In addition, when the maximum tensile stress is less than 1.0 MPa, the strength of the surface treatment film is low, so the damage rate of the above-mentioned film exceeds 20%, and excellent corrosion resistance after processing cannot be obtained. The upper limits of the elongation and the maximum tensile stress are not particularly limited. The elongation of the polyurethane resin can be less than 2000%, and the maximum tensile stress can be less than 300 MPa.

[0073] Here, the "elongation" and "maximum tensile stress" of the polyurethane resin are measured using a tensile testing machine according to a method in accordance with JIS K7161 (2014). The elongation at break of the obtained stress-strain curve is referred to as the "elongation", and the maximum stress is referred to as the "maximum tensile stress".

[0074] The polyurethane resin contained in the surface treatment film and the surface treatment liquid used to form the surface treatment film is preferably a solvent-soluble resin. Polyurethane resins exist in water-dispersible (dispersion) and solvent-dispersible types, but water-dispersible polyurethane resins easily follow the unevenness of the surface of the galvanized steel sheet. That is, there is a trend that the thickness of the surface treatment film on the concave portion of the galvanized steel sheet is equal to the thickness of the surface treatment film on the convex portion of the galvanized steel sheet. In this case, it is impossible to make the average value of the total thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet less than 0.10 μm, and spot weldability cannot be ensured. In contrast, solvent-dispersible polyurethane resins exist in a state dissolved in an organic solvent in the surface treatment liquid, and therefore have difficulty following the unevenness of the surface of the galvanized steel sheet. That is, the thickness of the surface treatment film on the convex portion of the galvanized steel sheet becomes smaller than the thickness of the surface treatment film on the concave portion of the galvanized steel sheet. As a result, the average value of the total thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet can be set to 0.10 μm or less, and the spot weldability can be ensured.

[0075] As the solvent-soluble polyurethane resin having an elongation of 400% or more and a maximum tensile stress of 1.0 MPa or more, known commercially available products such as Burnock 16-416, Burnock 18-472, and Burnock DF-407 manufactured by DIC Corporation can be used.

[0076] The surface treatment film and the surface treatment liquid used to form the surface treatment film preferably contain one or more rust-preventive additives selected from silicon oxide, a phosphate compound, a molybdate compound, and a vanadium compound. This can provide particularly excellent corrosion resistance.

[0077] As silicon oxide, colloidal silica or dry silica as microparticle silicon dioxide can be used. As colloidal silica, for example, Snowtex S, OS, C, NS, XS, NXS (all trade names) manufactured by Nissan Chemical Co., Ltd. can be used. In addition, as dry silica, fumed silica, calcium-exchanged silica, etc. can be used. As fumed silica, AEROSIL130, NX130, 200, RX200, RY200, 300, 300CF, RX300, RY300 (all trade names) manufactured by Japan AEROSIL Co., Ltd. can be used. In addition, as calcium-exchanged silica, SHIELDEX C303 and SHIELDEX AC5 (all trade names) manufactured by WR Grace & Co. can be used. It is known that these silicas contribute to the generation of dense and stable zinc corrosion products under corrosive environments, and inhibit the promotion of corrosion by densely forming these corrosion products on the plating surface.

[0078] As the phosphoric acid compound, for example, phosphate can be used. Phosphate includes all types of salts such as monosalts and double salts, and preferably has poor solubility. In addition, the metal cation constituting the phosphate is not limited, and can be any metal cation of zinc phosphate, magnesium phosphate, calcium phosphate, aluminum phosphate, etc. In addition, there is no limitation on the skeleton, condensation degree, etc. of the phosphate ion, and it can be any of an orthosalt, a dihydrogen salt, a monohydrogen salt, or a phosphite. Furthermore, in addition to orthophosphate, the orthosalt also includes all condensed phosphoric acids such as polyphosphates. With respect to the phosphorus compound, the zinc of the plating metal dissolved by corrosion reacts with the phosphate ions dissociated by hydrolysis to form a complex, thereby forming a dense and poorly soluble protective film to suppress the corrosion reaction.

[0079] As the molybdic acid compound, for example, molybdate can be used. There are no restrictions on the skeleton or degree of condensation of molybdates, and examples thereof include orthomolybdate, paramolybdate, and metamolybdate. Furthermore, all salts, including single and double salts, are encompassed. Examples of double salts include molybdenum phosphate. Molybdic acid compounds exhibit self-healing properties due to their passivation effect. Specifically, in a corrosive environment, they form a dense oxide on the surface of the plated film together with dissolved oxygen, thereby blocking the starting point of corrosion and suppressing the corrosion reaction.

[0080] As the vanadium compound, for example, a pentavalent vanadium compound or a tetravalent vanadium compound can be used. In particular, a tetravalent vanadium compound is preferred from the viewpoint of corrosion resistance.

[0081] The content of the rust-preventive additive is preferably within a range of 1 to 50 parts by mass relative to 100 parts by mass of the polyurethane resin in the surface treatment film and 100 parts by mass of the polyurethane resin solids content in the surface treatment liquid. A content of 1 part by mass or greater can fully enhance post-processing corrosion resistance. A content of 50 parts by mass or less can maintain the proportion of polyurethane resin in the surface treatment film, thereby preventing degradation of post-processing corrosion resistance.

[0082] The surface treatment film and the surface treatment liquid for forming the surface treatment film preferably contain a solid lubricant for the purpose of improving workability as needed.

[0083] Examples of the solid lubricant include the following solid lubricants, and one or more of these may be used.

[0084] (1) Polyolefin wax, paraffin wax: such as polyethylene wax, synthetic paraffin wax, natural paraffin wax, microcrystalline wax, etc.

[0085] (2) Fluororesin particles: such as polyvinyl fluoride resin, polyvinyl fluoride resin, polyvinylidene fluoride resin, etc.

[0086] In addition to these, one or more of fatty acid amide compounds (for example, stearic acid amide, palmitic acid amide, methylene bisstearamide, ethylene bisstearamide, oleic acid amide, ethanesulfonic acid amide, alkylene bisfatty acid amide, etc.), metal soaps (for example, calcium stearate, lead stearate, calcium laurate, calcium palmitate, etc.), metal sulfides (for example, molybdenum disulfide, tungsten disulfide, etc.), graphite, graphite fluoride, boron nitride, polyalkylene glycol, alkali metal sulfate, etc. can also be used.

[0087] The solid lubricant content is preferably in the range of 1 to 30 parts by mass per 100 parts by mass of the polyurethane resin in the surface treatment film and 100 parts by mass of the polyurethane resin solids content in the surface treatment liquid. A content of 1 part by mass or greater achieves a sufficient improvement in lubricity. A content of 30 parts by mass or less maintains the proportion of polyurethane resin in the surface treatment film, preventing degradation of post-processing corrosion resistance.

[0088] The surface treatment film and the surface treatment liquid for forming the surface treatment film may contain one or more of other oxide particles (for example, aluminum oxide, zirconium oxide, titanium oxide, cerium oxide, etc.), organic inhibitors (for example, hydrazine and its derivatives, thiol compounds, thiocarbamates, etc.) as corrosion inhibitors as needed. The content of the corrosion inhibitor is preferably in the range of 1 to 10 parts by mass relative to 100 parts by mass of the polyurethane resin for the surface treatment film and 100 parts by mass of the solid content of the polyurethane resin for the surface treatment liquid. If it is 1 part by mass or more, the effect of corrosion inhibition can be fully obtained. In addition, if it is 10 parts by mass or less, the proportion of the polyurethane resin in the surface treatment film will not be reduced, and the corrosion resistance after processing will not be impaired.

[0089] The surface treatment film and the surface treatment liquid for forming the surface treatment film may contain one or more of organic coloring pigments (e.g., condensed polycyclic organic pigments, phthalocyanine organic pigments, etc.), coloring dyes, inorganic pigments, chelating agents (e.g., thiols, etc.), coupling agents (e.g., silane coupling agents, titanium coupling agents, etc.) as other additives as needed. The content of these additives is preferably in the range of 1 to 5 parts by mass relative to 100 parts by mass of the polyurethane resin for the surface treatment film and 100 parts by mass of the solid content of the polyurethane resin for the surface treatment liquid. If it is 1 part by mass or more, the effect of the additive can be fully obtained. In addition, if it is 5 parts by mass or less, the proportion of the polyurethane resin in the surface treatment film will not be reduced, and the corrosion resistance after processing will not be impaired.

[0090] As described above, the surface treatment film contains a polyurethane resin and optionally contains a rust preventive additive, a solid lubricant, a corrosion inhibitor, and other additives, and is preferably composed of these.

[0091] [Method for manufacturing surface-treated steel sheet]

[0092] Next, a method for producing a surface-treated steel sheet according to an embodiment of the present invention will be described.

[0093] First, a surface treatment liquid containing a water-soluble zirconium compound (a), a tetraalkoxysilane (b), a compound having an epoxy group (c), a chelating agent (d), a vanadic acid compound (e), and a metal compound (f) containing at least one selected from Ti, Al, and Zn in the range satisfying the conditions of (I) to (V) above, and having a pH of 8 to 10, is applied to the surface of a galvanized steel sheet and dried to form an organic-inorganic composite film. The surface treatment liquid can be applied by any one of a coating method (rod coating, roller coating, etc.), a spray method, and a dipping method (and roller drawing). As a heating and drying device, a dryer, a hot air furnace, a high-frequency induction heating furnace, an infrared furnace, etc. can be used. Heating and drying is preferably carried out in a range of 60 to 200° C. in terms of the plate temperature.

[0094] Next, a surface treatment liquid containing a polyurethane resin and optionally containing a rust preventive additive, a solid lubricant, a corrosion inhibitor, and other additives, preferably consisting of these, is applied to the surface of the organic-inorganic composite film and dried to form a surface-treated film. The surface treatment liquid can be applied by any one of a coating method (rod coating, roller coating, etc.), a spraying method, and an immersion method (and roller drawing). After applying the surface treatment liquid, it is heated and dried without washing with water. As a heating and drying device, a dryer, a hot air furnace, a high-frequency induction heating furnace, an infrared furnace, etc. can be used. Heating and drying is preferably carried out in a range of 60 to 200° C. based on the plate temperature.

[0095] Example

[0096] As the base material, a galvanized steel sheet described in Table 1 was used, which had a cold-rolled steel sheet having a thickness of 0.8 mm as a base.

[0097] The surface treatment solution shown below was applied to a galvanized steel sheet using a bar coater and heated with an induction heater to a sheet temperature of 140° C. to form an organic-inorganic composite film. The average thickness of the organic-inorganic composite film determined by the above method is shown in Table 5.

[0098] [Surface treatment liquid]

[0099] Ingredient (a): 100 parts by mass of sodium zirconium carbonate

[0100] Component (b): 60 parts by mass of tetraethoxysilane

[0101] Component (c): 138 parts by mass of polyethylene glycol diglycidyl ether

[0102] Component (d): 1-hydroxymethane-1,1-diphosphonic acid + 49 parts by mass of tartaric acid

[0103] Ingredient (e): 17 parts by mass of sodium metavanadate

[0104] Component (f): 3 parts by mass of ammonium fluorotitanate

[0105] The surface treatment liquid had a pH of 8.4, a mass ratio (a / b) of 1.7, a mass ratio (b / c) of 0.4, a mass ratio (b / d) of 1.2, a mass ratio (e / d) of 0.3, and a mass ratio (f / d) of 0.06.

[0106] Next, the rust preventive additives listed in Table 3 and the solid lubricants listed in Table 4 were added to the polyurethane resins listed in Table 2 in the amounts listed in Table 5 to prepare a surface treatment solution. This surface treatment solution was applied to the surface of the organic-inorganic composite film using a bar coater and heated with an induction heater until the plate temperature reached 140°C to form a surface treatment film, thereby producing a surface-treated steel sheet. Of the polyurethane resin products listed in Table 2, solvent-soluble products A1 to A3, A6, and A7 used methyl ethyl ketone as the dilution solvent, dissolving the polyurethane resin in methyl ethyl ketone. Water-dispersible products A4 and A5 used water as the dilution solvent, dispersing the polyurethane resin in water. The amounts of rust preventive additives and solid lubricants listed in Table 5 are based on 100 parts by mass of the solid content of the polyurethane resin in the surface treatment solution, which is equivalent to the amount per 100 parts by mass of the polyurethane resin in the surface treatment film. The average thickness of the surface treatment film, determined by the above method, is shown in Table 5.

[0107] In addition, the "average value of the total thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet" and the "film damage rate" were calculated by the above method and are shown in Table 5. In addition, a cross-sectional SEM image of the surface-treated steel sheet of Invention Example No. 5 is shown as a representative invention example. Figure 2 .

[0108] The obtained surface-treated steel sheets were subjected to the following performance evaluations, and the results are shown in Table 5.

[0109] [Corrosion resistance after processing]

[0110] After the drawbead test for determining the film damage rate, the samples were cut into 70 mm × 150 mm pieces and subjected to chemical conversion treatment using a degreasing agent: FC-E6403 (40°C, spraying for 120 seconds), a surface conditioner: PL-X (room temperature, 20 seconds), and a chemical conversion treatment agent: Palbond PB-L3065 (35°C, 120 seconds) manufactured by Japan Parkerizing Co., Ltd., and then a cationic electrodeposition coating (V-50 manufactured by Nippon Paint Co., Ltd.) was formed to a film thickness of 20 μm. Figure 4 The combined cycle test (CCT) according to SAE-J2334 was performed as shown, and the white rust area ratio after 150 cycles was determined. The corrosion resistance after processing was evaluated according to the following criteria.

[0111] ◎: No white rust

[0112] ○: White rust area rate is less than 20%

[0113] ×: White rust occurrence area ratio is 20% or more

[0114] [Spot weldability]

[0115] A continuous spot welding test was conducted on each sample using a CF type Cr-Cu electrode, a pressing force of 150 kgf, a current application time of 10 cycles / 60 Hz, and a welding current of 8 kA. The spot weldability was evaluated according to the following criteria.

[0116] ○: The number of consecutive points is more than 2000

[0117] ×: The number of continuous points is less than 2000

[0118] [Lubricity]

[0119] In order to evaluate press formability, the friction coefficient of each sample was measured as follows. Figure 5 This is an explanatory diagram showing a schematic diagram of the friction coefficient measuring device used. In this device, a sample 1 is fixed to a sample table 2. The sample table 2 is fixed to the upper surface of a horizontally movable slide table 3. A slide table support 5 that can move up and down and has a roller 4 in contact with it is provided on the lower surface of the slide table 3. By pushing up the support 5, a pressing load N is generated by the slider 6 to the sample 1. The first load cell 7 for measuring the pressing load N is installed on the slide table support 5. The slide table 3 is moved in the horizontal direction while the pressing force is applied. The second load cell 8 for measuring the sliding resistance F generated at this time is installed at one end of the slide table 3 and above the guide rail 9. It should be noted that the test was performed by applying "PRETON R352L", a rust-proof cleaning oil manufactured by Sugimura Chemical Industry Co., Ltd., as a lubricating oil, to the surface of the sample 1.

[0120] Figure 6 A perspective view showing the shape and dimensions of the slider used. Figure 5 The lower surface of the slider 6 shown slides while being pressed against the surface of the sample 1 . Figure 6 The shape of the slider 6 shown is 10 mm wide and 59 mm long in the sliding direction. The lower parts of both ends in the sliding direction are composed of curved surfaces with a curvature of 4.5 mmR. The lower surface of the slider pressing the sample has a flat surface with a width of 10 mm and a length of 50 mm in the sliding direction.

[0121] The friction coefficient was measured under the following conditions. Figure 6 The slide shown in the figure was subjected to a pressing load N of 400 kgf, and the sample extraction speed (horizontal movement speed of the slide table 3) was 20 cm / min. The friction coefficient μ between the sample and the slide was calculated using the formula: μ = F / N. The lower the friction coefficient μ, the higher the lubricity, indicating excellent press formability. The evaluation criteria are as follows.

[0122] ◎: Friction coefficient μ less than 0.22

[0123] ○: Friction coefficient μ is 0.22 or more and less than 0.28

[0124] ×: Friction coefficient μ is 0.28 or more

[0125] [Table 1]

[0126]

[0127] [Table 2]

[0128]

[0129] [Table 3]

[0130] type Rust-proof additives Product Name Manufacturer 1 Calcium ion exchange silica SHIELDEX C303 WR Race & Co. 2 Fumed silica AEROSIL 200 Japan AEROSIL Co., Ltd. 3 Aluminum dihydrogen tripolyphosphate K-WHITE G105 TEIKA Co., Ltd. 4 Disodium molybdate Disodium molybdate Fujifilm Wako Pure Chemical Industries, Ltd. 5 Vanadyl sulfate Vanadyl sulfate Shinko Chemical Co., Ltd.

[0131] [Table 4]

[0132] type solid lubricants Product Name Manufacturer 1 polyethylene Ceridust3620 Clariant Chemicals 2 polyethylene ChemipearlW900 Mitsui Chemicals Co., Ltd. 3 Tetrafluoroethylene KD-100CS Kitamura Co., Ltd.

[0133] [Table 5]

[0134]

[0135] Industrial applicability

[0136] The surface-treated steel sheet of the present invention does not contain chromium in the surface-treated film and is excellent in both corrosion resistance and spot weldability after working, and is therefore most suitable for applications in automobiles, home appliances, and building materials.

[0137] Explanation of symbols

[0138] 1 sample

[0139] 2. Sample table

[0140] 3 Sliding table

[0141] 4 rollers

[0142] 5 Sliding table support

[0143] 6 Sliders

[0144] 7. 1st load cell

[0145] 8 Second load cell

[0146] 9 rails

[0147] N Compression load

[0148] F Sliding resistance

Claims

1. A surface-treated steel plate, characterized in that: have: The galvanized steel sheet has slight irregularities on the surface. a chromium-free organic-inorganic composite film formed on the surface of the galvanized steel sheet, and a surface treatment film containing a polyurethane resin formed on the organic-inorganic composite film; The average value of the total thickness of the organic-inorganic composite film and the surface treatment film on the convex portion of the galvanized steel sheet is 0.10 μm or less. The average thickness of the surface treatment film is 0.4 to 1.0 μm. The film damage rate of the surface treatment film after the surface treated steel sheet was bent / returned using a slider having a tip diameter of 5 mmR was 20% or less.

2. The surface-treated steel sheet according to claim 1, wherein The polyurethane resin has an elongation of 400% or more and a maximum tensile stress of 1.0 MPa or more.

3. The surface-treated steel sheet according to claim 1 or 2, wherein: The polyurethane resin is a solvent-soluble resin.

4. The surface-treated steel sheet according to any one of claims 1 to 3, wherein The surface treatment film contains 1 to 50 parts by mass of a total of one or more rust-preventive additives selected from silicon oxide, a phosphoric acid compound, a molybdic acid compound, and a vanadium compound, based on 100 parts by mass of the polyurethane resin.

5. The surface-treated steel sheet according to any one of claims 1 to 4, wherein The surface treatment film contains 1 to 30 parts by mass of a solid lubricant based on 100 parts by mass of the polyurethane resin.

6. The surface-treated steel sheet according to any one of claims 1 to 5, wherein The average thickness of the organic-inorganic composite film is 0.01 to 0.20 μm.

Images