Surface-treated steel sheet

Abstract

Provided is a surface-treated steel sheet in which chromium is not contained in a surface-treating film and which is excellent in terms of both post-processing corrosion resistance and spot weldability. The surface-treated steel sheet according to the present invention is characterized by comprising: a zinc-based plated steel sheet having, on a surface thereof, micro recesses and protrusions; an organic-inorganic composite film formed on the surface of the zinc-based plated steel sheet and not containing chromium; and a surface-treating film formed on the organic-inorganic composite film and comprising a polyurethane resin. The surface-treated steel sheet is also characterized in that: in the protruding parts in the zinc-based plated steel sheet, the average of the total thicknesses of the organic-inorganic composite film and the surface-treating film is 0.10 μm or less; the average thickness of the surface-treating film is 0.4 to 1.0 μm; and, after the surface-treated steel sheet is subjected to bending / restoration processing by using a bead having a tip diameter of 5 mmR, the film damage rate of the surface-treating film is 20% or less.

Description

Surface-treated steel sheet

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

[0002] Zinc-based plated steel sheets are widely used in fields such as automobiles, home appliances, and building materials. Conventionally, for the purpose of improving corrosion resistance, surface-treated steel sheets have been widely used, in which the surface of zinc-based plated steel sheets has been subjected to a chromate treatment using a treatment solution containing chromic acid, dichromic acid, or a salt thereof as a main component. However, in light of recent global environmental concerns, there has been an increasing demand for the adoption of zinc-based plated steel sheets subjected to a chromate-free surface treatment (chromate-free treated steel sheets), and chromate-free treated steel sheets with various improved properties have been proposed.

[0003] On the other hand, when zinc-based plated steel sheets are used to manufacture automotive parts, it has been common to not form a surface treatment film. In this case, the zinc-based plated steel sheets are press-formed into formed parts, and multiple formed parts are assembled into parts by spot welding, adhesive bonding, etc., and then the parts are subjected to a chemical conversion treatment and then to electrodeposition coating to manufacture the automotive parts.

[0004] As described above, the corrosion resistance of automotive components is ensured by the chemical conversion coating film formed in the chemical conversion treatment process and the electrodeposition coating film formed in the electrodeposition coating process. However, there are cases where the electrodeposition coating film does not cover enough areas, such as at the joints of molded components. Therefore, secondary materials such as sealers and waxes are used to supplement the corrosion resistance of the joints. Because these secondary materials increase automobile manufacturing costs, there has been a strong demand for reducing the amount of secondary materials used. Furthermore, as a fundamental measure to reduce manufacturing costs, efforts have been made to eliminate 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) in which an organic surface treatment film is formed on the surface of a zinc-based plated steel sheet for application to automotive components has been actively carried out. Such surface-treated steel sheets are required to have excellent corrosion resistance after processing, and also to have good spot weldability because they must be assembled into a predetermined shape.

[0006] Patent Document 1 describes a chromium-free galvannealed steel sheet having, on at least one surface thereof, a surface treatment layer containing a film-forming component including an organosilicon compound obtained by reacting a plurality of silane coupling agents and a cationic polyurethane resin, and an inhibitor component consisting of phosphoric acid and magnesium phosphate.

[0007] Patent Document 2 describes a surface-treated steel sheet having, on at least one surface of a zinc-based plated steel sheet, a coating film containing a binder resin that is a water-soluble or water-dispersible aqueous resin, conductive particles, an anti-corrosion pigment, and at least one type of oxide particles selected from the group consisting of zirconia particles, titania particles, nickel oxide particles, and tin (IV) oxide particles.

[0008] JP 2016-148109 A JP 2016-194137 A

[0009] In surface-treated steel sheets in which an organic surface treatment film is formed on the surface of a zinc-based plated steel sheet, the thicker the surface treatment film, the better the corrosion resistance after processing. However, the thicker the surface treatment film, the lower the electrical conductivity and therefore the lower the spot weldability. In other words, the corrosion resistance after processing and the spot weldability are contradictory properties. Various developments have been made to balance the corrosion resistance after processing and the spot weldability, but further improvements are required in line with recent increasing requirements.

[0010] In view of the above problems, an object of the present invention is to provide a surface-treated steel sheet that does not contain chromium in the surface treatment film and that has excellent corrosion resistance after processing and spot weldability.

[0011] To solve the above-mentioned problems, the present inventors conducted extensive research into a method for achieving both high levels of post-processing corrosion resistance and spot weldability in a two-layer surface-treated steel sheet, in which a chromium-free organic / inorganic composite coating and a surface treatment coating containing a polyurethane resin are formed on a zinc-based plated steel sheet, and have made the following discoveries. Specifically, the inventors minimized the amount of organic / inorganic composite coating applied, while using a specific polyurethane resin with excellent flexibility as the polyurethane resin contained in the surface treatment coating. This enabled the total thickness of the organic / inorganic composite coating and the surface treatment coating to be made very thin at the convex portions of the zinc-based plated steel sheet, ensuring spot weldability. Furthermore, damage to the surface treatment coating during processing could be suppressed, resulting in excellent post-processing corrosion resistance.

[0012] The gist of the present invention, which was completed based on the above findings, is as follows: [1] A surface-treated steel sheet comprising: a zinc-based plated steel sheet having a microscopic unevenness on its surface; a chromium-free organic-inorganic composite coating formed on the surface of the zinc-based plated steel sheet; and a surface treatment film containing a polyurethane resin formed on the organic-inorganic composite coating, wherein the average total thickness of the organic-inorganic composite coating and the surface treatment film at the convex portions of the zinc-based plated steel sheet is 0.10 μm or less, the average thickness of the surface treatment film is 0.4 to 1.0 μm, and the film damage rate of the surface treatment film is 20% or less after the surface-treated steel sheet is bent / unbent using a bead with a tip diameter of 5 mmR.

[0013] [2] The surface-treated steel sheet according to the above [1], wherein the polyurethane resin has an elongation of 400% or more and a maximum tensile stress of 1.0 MPa or more.

[0014] [3] The surface-treated steel sheet according to the above [1] or [2], wherein the polyurethane resin is a solvent-soluble resin.

[0015] [4] The surface-treated steel sheet according to any one of the above [1] to [3], wherein the surface treatment film contains 1 to 50 parts by mass in total of one or more rust-preventive additives selected from the group consisting of silicon oxide, phosphate compounds, molybdenum acid compounds, and vanadium compounds per 100 parts by mass of the polyurethane resin.

[0016] [5] The surface-treated steel sheet according to any one of [1] to [4] above, wherein the surface treatment film contains 1 to 30 parts by mass of a solid lubricant per 100 parts by mass of the polyurethane resin.

[0017] [6] The surface-treated steel sheet according to any one of the above [1] to [5], wherein the organic-inorganic composite coating has an average thickness of 0.01 to 0.20 μm.

[0018] The surface-treated steel sheet of the present invention does not contain chromium in the surface treatment film and is excellent in both corrosion resistance after processing and spot weldability.

[0019] FIG. 1 is a diagram schematically illustrating the cross-sectional structure of a surface-treated steel sheet according to one embodiment of the present invention. FIG. 2 is a cross-sectional SEM image of the surface-treated steel sheet of Invention Example No. 5. FIG. 3 is a diagram illustrating a bending / unbending process performed when determining the "coating damage rate" in one embodiment of the present invention. FIG. 4 is a diagram illustrating a combined cycle test performed to evaluate corrosion resistance after processing in the examples. FIG. 5 is a schematic diagram of a friction coefficient measuring device used when evaluating lubricity in the examples. FIG. 6 is a perspective view illustrating the shape and dimensions of a bead used when evaluating lubricity in the examples.

[0020] 1 , a surface-treated steel sheet according to one embodiment of the present invention comprises a zinc-based plated steel sheet, an organic-inorganic composite coating formed on the surface of the zinc-based plated steel sheet, and a surface treatment coating formed on the organic-inorganic composite coating. In this specification, the term "surface of the zinc-based plated steel sheet" refers to the surface of the zinc-based plating coating, and means one or both sides of the zinc-based plated steel sheet.

[0021] [[Zinc-based plated steel sheet]] The zinc-based plated steel sheet may be any steel sheet on which any zinc-based plating film is formed, and examples thereof include zinc-plated steel sheets, Zn—Fe alloy-plated steel sheets, Zn—Ni alloy-plated steel sheets, Zn—Mn alloy-plated steel sheets, Zn—Co alloy-plated steel sheets, Zn—Cr alloy-plated steel sheets, Zn—Al alloy-plated steel sheets, Zn—Mg alloy-plated steel sheets, and steel sheets on which plating films containing a plurality of these alloy elements are formed. Also included are zinc-based composite-plated steel sheets (for example, Zn—SiO ２ Dispersion-plated steel sheets, etc. may also be used as the zinc-based plated steel sheets.

[0022] The plating method is not particularly limited, and any suitable method may be adopted, such as electroplating, electroless plating, hot dip plating, PVD, CVD, etc. After the formation of the zinc-based plating film, annealing treatment, temper rolling treatment, surface conditioning treatment, etc. may be carried out as appropriate.

[0023] Generally, a zinc-based plated steel sheet has microscopic irregularities on its surface (i.e., the surface of the zinc-based plated film) with an arithmetic mean roughness Ra of 0.5 μm to 1.5 μm. Here, the arithmetic mean roughness Ra is the arithmetic mean roughness obtained from a roughness curve obtained by measuring the surface of a zinc-based plated steel sheet under the conditions of a cutoff value λc of 0.8 mm, a cutoff value λs of 2.5 μm, and a measurement distance of 2.5 mm in accordance with JIS B0601 (2001).

[0024] [Organic-inorganic composite coating] The organic-inorganic composite coating is a chromium-free coating formed on the surface of a zinc-based plated steel sheet. Referring to FIG. 1 , the organic-inorganic composite coating preferably has the property of being less likely to conform to the irregularities on the surface of the zinc-based plated steel sheet. That is, it is preferable that the thickness of the organic-inorganic composite coating in the convex portions of the zinc-based plated steel sheet is smaller than the thickness of the organic-inorganic composite coating in the concave portions of the zinc-based plated steel sheet. This allows the total thickness of the organic-inorganic composite coating and the surface treatment coating in the convex portions of the zinc-based plated steel sheet to be sufficiently small, as described below, and ensures spot weldability.

[0025] The average thickness of the organic-inorganic composite coating is preferably 0.01 to 0.20 μm. If the average thickness of the organic-inorganic composite coating is too small, excellent corrosion resistance cannot be obtained. From this perspective, the average thickness of the organic-inorganic composite coating is preferably 0.01 μm or more, and more preferably 0.05 μm or more. On the other hand, if the average thickness of the organic-inorganic composite coating is too large, the total thickness of the organic-inorganic composite coating and the surface treatment coating at the convex portions of the zinc-based plated steel sheet cannot be made sufficiently small, and spot weldability cannot be ensured. From this perspective, the average thickness of the organic-inorganic composite coating is preferably 0.20 μm or less.

[0026] Here, the "average thickness of the organic-inorganic composite coating" is determined by the following method: The cross-sectional surface layer portion of the surface-treated steel sheet is observed in three fields of view at a magnification of 5000 times using a scanning electron microscope (SEM), and the thickness of the organic-inorganic composite coating is measured at a total of 20 locations per field of view at 1.0 μm intervals, and the arithmetic mean value of the thicknesses at a total of 60 locations across the three fields of view is defined as the "average thickness." The method for processing the cross section is not particularly limited, but examples include FIB (Focused Ion Beam) processing.

[0027] The organic-inorganic composite coating preferably has excellent electrical conductivity in order to achieve both corrosion resistance after processing and spot weldability. Specifically, the organic-inorganic composite coating is preferably obtained by applying to the surface of a zinc-plated steel sheet 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 vanadate compound (e), and a metal compound (f) containing at least one element selected from the group consisting of Ti, Al, and Zn in amounts that satisfy the following conditions (I) to (V), and having a pH of 8 to 10, and then drying the liquid. (I) The mass ratio (a / b) of the water-soluble zirconium compound (a) converted into Zr to the tetraalkoxysilane (b) is 1.0 to 6.0. (II) The mass ratio (b / c) of the tetraalkoxysilane (b) to the compound (c) having an epoxy group is 0.1 to 1.6. (III) The mass ratio (b / d) of the tetraalkoxysilane (b) to the chelating agent (d) is 0.3 to 2.0. (IV) The mass ratio (e / d) of the vanadate compound (e) converted into V to the chelating agent (d) is 0.03 to 1.0. (V) The mass ratio (f / d) of the metal compound (f) converted into the total metal to the chelating agent (d) is 0.05 to 0.8.

[0028] Examples of the water-soluble zirconium compound (a) include zirconium nitrate, zirconium oxynitrate, zirconyl acetate, zirconyl sulfate, zirconium carbonate, ammonium zirconium carbonate, potassium zirconium carbonate, sodium zirconium carbonate, and hydrofluoric zirconate, and one or more of these can be used.

[0029] Examples of the tetraalkoxysilane (b) include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane, and one or more of these can be used.

[0030] 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 adipic acid diglycidyl ester, phthalic acid diglycidyl ester, and terephthalic acid diglycidyl ester; 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. One or more of these can be used.

[0031] Examples of the chelating agent (d) include hydroxycarboxylic acids such as tartaric acid and malic acid, monocarboxylic acids, polycarboxylic acids such as dicarboxylic acids or tricarboxylic acids such as oxalic acid, malonic acid, succinic acid, citric acid and adipic acid, 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.

[0032] Examples of the vanadic acid compound (e) include ammonium metavanadate and sodium metavanadate, and one or more of these can be used.

[0033] Examples of the metal compound (f) containing at least one selected from the group consisting of Ti, Al, and Zn include titanyl sulfate, titanyl nitrate, titanium nitrate, titanyl chloride, titanium chloride, titania sol, titanium oxide, potassium oxalate titanate, titanium hydrofluoric acid, ammonium titanium fluoride, titanium lactate, titanium tetraisopropoxide, titanium acetylacetonate, diisopropyltitanium bisacetylacetone, 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, and potassium zincate, and one or more of these can be used.

[0034] The pH of the surface treatment solution is set to be within the range of 8 to 10. If the pH is less than 8, the storage stability of the surface treatment solution, corrosion resistance, and adhesion of the organic-inorganic composite coating decrease, while if the pH is more than 10, corrosion resistance and electrical conductivity decrease.

[0035] The mass ratio (a / b) of the water-soluble zirconium compound (a) converted to Zr and the tetraalkoxysilane (b) is set to be in the range of 1.0 to 6.0. If the mass ratio (a / b) is less than 1.0, the corrosion resistance decreases, and if it exceeds 6.0, the electrical conductivity decreases.

[0036] The mass ratio (b / c) of the tetraalkoxysilane (b) to the compound having an epoxy group (c) is set to be within the range of 0.1 to 1.6. If the mass ratio (b / c) is less than 0.1, the corrosion resistance decreases, and if it exceeds 1.6, the adhesion of the organic-inorganic composite coating decreases.

[0037] The mass ratio (b / d) of the tetraalkoxysilane (b) to the chelating agent (d) is set to a range of 0.3 to 2.0. When the mass ratio (b / d) is less than 0.3 or exceeds 2.0, the corrosion resistance decreases.

[0038] The mass ratio (e / d) of the vanadate compound (e) converted into V to the chelating agent (d) is set to be within the range of 0.03 to 1.0. If the mass ratio (e / d) is less than 0.03, the corrosion resistance decreases, and if it exceeds 1.0, it becomes difficult to dissolve the vanadate compound in the surface treatment solution.

[0039] The mass ratio (f / d) of the total metal-equivalent mass of the metal compound (f) to the chelating agent (d) is set to be in the range of 0.05 to 0.8. If the mass ratio (f / d) is less than 0.05, the corrosion resistance decreases, and if it exceeds 0.8, it becomes difficult for the metal compound (f) to dissolve in the surface treatment agent.

[0040] [Surface Treatment Film] The surface treatment film is formed on the organic-inorganic composite film, contains a polyurethane resin, and is a chromium-free film.

[0041] First, with regard to both the organic-inorganic composite coating and the surface treatment coating, referring to Figure 1, in this embodiment, it is important that the average total thickness of the organic-inorganic composite coating and the surface treatment coating at the convex portion of the zinc-based plated steel sheet is 0.10 µm or less. If the average total thickness exceeds 0.10 µm, excellent spot weldability cannot be obtained. If the average total thickness is 0.10 µm or less, thin film portions with small thicknesses of the organic-inorganic composite coating and the surface treatment coating can be provided, thereby obtaining excellent spot weldability.

[0042] Although there is no particular restriction on the lower limit of the average total thickness, from the viewpoint of obtaining excellent corrosion resistance after processing, the average total thickness is preferably 0.02 μm or more.

[0043] Here, the "average total thickness of the organic-inorganic composite coating and the surface treatment coating at the convex portions of the zinc-based plated steel sheet" is determined by the following method: The cross-sectional surface layer portion of the surface-treated steel sheet is observed in three fields of view at a magnification of 10,000 times using an SEM, all convex portions present in the three fields of view are identified, the total thickness of the organic-inorganic composite coating and the surface treatment coating at all the convex portions is determined, and the arithmetic mean value thereof is defined as the "average thickness." The method for processing the cross section is not particularly limited, but examples include FIB processing.

[0044] It is important that the average thickness of the surface treatment film is 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 below) cannot be reduced to 20% or less, and excellent corrosion resistance after processing cannot be obtained. On the other hand, if the average thickness of the surface treatment film exceeds 1.0 μm, the average total thickness of the organic-inorganic composite film and the surface treatment film at the convex portions of the zinc-based plated steel sheet cannot be reduced to 0.10 μm or less, and excellent spot weldability cannot be obtained.

[0045] Here, the "average thickness of the surface treatment film" is determined by the following method: The cross-sectional surface layer portion of the surface-treated steel sheet is observed in three fields of view at a magnification of 5000 times using an SEM, and the thickness of the surface treatment film is measured at a total of 20 points at 1.0 μm intervals per field of view, and the arithmetic mean value of the thicknesses at a total of 60 points in the three fields of view is taken as the "average thickness." The method of cross-sectional processing is not particularly limited, but examples include FIB processing.

[0046] It is also important that the damage rate of the surface treatment film after bending / unbending the surface-treated steel sheet with a bead having a tip diameter of 5 mmR is 20% or less. If the damage rate exceeds 20%, excellent corrosion resistance after bending cannot be obtained. Since the smaller the damage rate, the better, there is no particular lower limit; the damage rate may be 0% or more, and may be 5% or more.

[0047] Here, the "coating damage rate" is determined by the following method. As shown in Figure 3, a 68 mm x 350 mm test piece of surface-treated steel sheet is subjected to a bending / bending-back drawbead test in which the test piece is pulled horizontally at a pulling rate of 1 m / min while being held down with a load of 700 kgf using convex and concave beads. The drawing is performed after applying a rust-preventive cleaning oil (Pleton R352L, manufactured by Sugimura Chemical Industry Co., Ltd.), and the tip of the convex bead has a radius of 5 mm. Thereafter, the surface of the surface treatment film is observed with an SEM at a magnification of 100x. Damaged areas of the coating are identified by image binarization in any three fields of view of the backscattered electron image. The area ratio of the damaged areas of the coating in each field of view is determined, and the average value thereof is used as the "coating damage rate."

[0048] The polyurethane resin contained in the surface treatment film and the surface treatment solution for forming the same preferably has an elongation of 400% or more and a maximum tensile stress of 1.0 MPa or more. If the elongation is less than 400%, the film damage rate exceeds 20%, making it impossible to obtain excellent corrosion resistance after processing. Furthermore, if the maximum tensile stress is less than 1.0 MPa, the strength of the surface treatment film is low, making it impossible to obtain excellent corrosion resistance after processing. Although there are no particular limitations on the upper limits of the elongation and maximum tensile stress, the elongation of the polyurethane resin may be 2000% or less, and the maximum tensile stress may be 300 MPa or less.

[0049] 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 K 7161 (2014), and the elongation at break in the obtained stress-strain curve is defined as the "elongation" and the maximum stress as the "maximum tensile stress."

[0050] The polyurethane resin contained in the surface treatment film and the surface treatment solution for forming it is preferably a solvent-soluble resin. Polyurethane resins are available in water-dispersed (dispersion) and solvent-dispersed types. Water-dispersed polyurethane resins are more likely to conform to the surface irregularities of a zinc-based plated steel sheet. That is, the thickness of the surface treatment film in the recesses of the zinc-based plated steel sheet tends to be equivalent to the thickness of the surface treatment film in the protrusions of the zinc-based plated steel sheet. In this case, the average total thickness of the organic-inorganic composite film and the surface treatment film in the protrusions of the zinc-based plated steel sheet cannot be reduced to 0.10 μm or less, and spot weldability cannot be ensured. In contrast, solvent-dispersed polyurethane resins exist in a state dissolved in an organic solvent in the surface treatment solution, and therefore are less likely to conform to the surface irregularities of the zinc-based plated steel sheet. That is, the thickness of the surface treatment film in the protrusions of the zinc-based plated steel sheet is smaller than the thickness of the surface treatment film in the recesses of the zinc-based plated steel sheet. As a result, the average total thickness of the organic-inorganic composite coating and the surface treatment coating on the convex portions of the zinc-based plated steel sheet can be reduced to 0.10 μm or less, ensuring spot weldability.

[0051] As a 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.

[0052] The surface treatment film and the surface treatment solution for forming the same preferably contain one or more rust-preventive additives selected from the group consisting of silicon oxide, phosphate compounds, molybdenum compounds, and vanadium compounds, thereby achieving particularly excellent corrosion resistance.

[0053] As silicon oxide, colloidal silica or dry silica, which is fine silica particles, can be used. As colloidal silica, for example, Snowtex S, OS, C, NS, XS, and NXS (all trade names) manufactured by Nissan Chemical Industries, Ltd. can be used. Furthermore, as dry silica, fumed silica or calcium-exchanged silica can be used. As fumed silica, AEROSIL 130, NX130, 200, RX200, RY200, 300, 300CF, RX300, and RY300 (all trade names) manufactured by Nippon Aerosil Co., Ltd. can be used. Furthermore, as calcium-exchanged silica, SHIELDEX C303 and SHIELDEX AC5 (all trade names) manufactured by W. R. Grace & Co. can be used. These silicas are known to contribute to the formation of dense and stable zinc corrosion products in a corrosive environment, and the dense formation of these corrosion products on the plating surface inhibits the acceleration of corrosion.

[0054] The phosphoric acid compound may be, for example, a phosphate. Phosphates include all types of salts, including simple and double salts, and are preferably sparingly soluble. Furthermore, there are no limitations on the metal cations constituting the phosphate, and any metal cation, such as zinc phosphate, magnesium phosphate, calcium phosphate, or aluminum phosphate, may be used. Furthermore, there are no limitations on the skeleton or degree of condensation of the phosphate ion, and any of normal salts, dihydrogen salts, monohydrogen salts, or phosphites may be used. Furthermore, normal salts include all condensed phosphoric acids, such as orthophosphates and polyphosphates. This phosphorus compound inhibits corrosion by forming a dense, sparingly soluble protective film through a complexation reaction between zinc from the plating metal eluted by corrosion and phosphate ions dissociated by hydrolysis.

[0055] As the molybdic acid compound, for example, molybdate can be used. The molybdate is not limited in terms of its skeleton or degree of condensation, and examples thereof include orthomolybdate, paramolybdate, and metamolybdate. This includes all salts, including single and double salts, and examples of double salts include phosphomolybdate. Molybdic acid compounds exhibit self-repairing properties due to their passivation effect. In other words, in a corrosive environment, they form a dense oxide on the plating film surface together with dissolved oxygen, thereby blocking corrosion initiation sites and inhibiting the corrosion reaction.

[0056] As the vanadium compound, for example, a pentavalent vanadium compound or a tetravalent vanadium compound can be used, with a tetravalent vanadium compound being particularly preferred from the viewpoint of corrosion resistance.

[0057] The content of the rust-preventive additive is preferably within a range of 1 to 50 parts by mass in total per 100 parts by mass of polyurethane resin in the surface treatment film, and per 100 parts by mass of the solid content of polyurethane resin in the surface treatment solution. If the content is 1 part by mass or more, the effect of improving corrosion resistance after processing can be sufficiently obtained. Furthermore, if the content is 50 parts by mass or less, the proportion of polyurethane resin in the surface treatment film does not decrease, and corrosion resistance after processing is not impaired.

[0058] The surface treatment film and the surface treatment solution for forming the same preferably contain a solid lubricant as needed to improve processability.

[0059] Examples of solid lubricants include the following, and one or more of these can be used: (1) polyolefin wax, paraffin wax: for example, polyethylene wax, synthetic paraffin, natural paraffin, microcrystalline wax, etc. (2) fluororesin fine particles: for example, polyfluoroethylene resin, polyvinyl fluoride resin, polyvinylidene fluoride resin, etc. In addition to these, one or more of the following may be used: fatty acid amide compounds (for example, stearic acid amide, palmitic acid amide, methylene bisstearamide, ethylene bisstearamide, oleic acid amide, esylic 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 sulfates, etc.

[0060] The content of the solid lubricant is preferably within the range of 1 to 30 parts by mass per 100 parts by mass of polyurethane resin in the surface treatment film, and 100 parts by mass of the solid content of polyurethane resin in the surface treatment solution. An amount of 1 part by mass or more can sufficiently improve lubricity. Furthermore, an amount of 30 parts by mass or less does not decrease the proportion of polyurethane resin in the surface treatment film, and does not impair corrosion resistance after processing.

[0061] The surface treatment film and the surface treatment solution for forming the same may optionally contain one or more corrosion inhibitors, such as other oxide fine particles (e.g., aluminum oxide, zirconium oxide, titanium oxide, cerium oxide, etc.), organic inhibitors (e.g., hydrazine and its derivatives, thiol compounds, thiocarbamates, etc.). The content of the corrosion inhibitor is preferably within the range of 1 to 10 parts by mass per 100 parts by mass of the polyurethane resin in the surface treatment film, and 100 parts by mass of the solids content of the polyurethane resin in the surface treatment solution. If the content is 1 part by mass or more, a sufficient corrosion inhibition effect can be obtained. Furthermore, if the content is 10 parts by mass or less, the proportion of the polyurethane resin in the surface treatment film is not reduced, and corrosion resistance after processing is not impaired.

[0062] The surface treatment film and the surface treatment liquid for forming the same may contain, as necessary, one or more of other additives, such as organic coloring pigments (e.g., condensed polycyclic organic pigments, phthalocyanine organic pigments, etc.), coloring dyes, inorganic pigments, chelating agents (e.g., thiols), and coupling agents (e.g., silane coupling agents, titanium coupling agents, etc.). The content of these additives is preferably within the range of 1 to 5 parts by mass per 100 parts by mass of polyurethane resin in the surface treatment film, and 100 parts by mass of polyurethane resin solids in the surface treatment liquid. If the content is 1 part by mass or more, the effect of the additive can be fully obtained. Furthermore, if the content is 5 parts by mass or less, the proportion of polyurethane resin in the surface treatment film is not reduced, and corrosion resistance after processing is not impaired.

[0063] As described above, the surface treatment film contains a polyurethane resin, and optionally contains anti-rust additives, solid lubricants, corrosion inhibitors, and other additives, and preferably consists of these.

[0064] [Method for producing surface-treated steel sheet] Next, a method for producing a surface-treated steel sheet according to one embodiment of the present invention will be described.

[0065] First, 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 vanadate compound (e), and a metal compound (f) containing at least one element selected from the group consisting of Ti, Al, and Zn, in amounts satisfying the above conditions (I) to (V), and having a pH of 8 to 10, is applied to the surface of a zinc-plated steel sheet and then dried to form an organic-inorganic composite coating. The surface treatment solution may be applied by a coating method (such as bar coating or roll coating), a spray method, or an immersion method (and roll squeezing). Heat-drying methods that can be used include dryers, hot air ovens, high-frequency induction heating furnaces, and infrared ovens. Heat-drying is preferably performed at a sheet temperature of 60 to 200°C.

[0066] Next, a surface treatment liquid containing a polyurethane resin and, optionally, a rust prevention 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 treatment film. The surface treatment liquid can be applied by any of coating methods (bar coating, roll coating, etc.), spraying, and immersion (and roll squeezing). After coating with the surface treatment liquid, the film is heated and dried without rinsing with water. Examples of heating and drying methods that can be used include a dryer, a hot air oven, a high-frequency induction heating oven, and an infrared oven. Heat drying is preferably performed at a sheet temperature of 60 to 200°C.

[0067] As the base material, a zinc-based plated steel sheet shown in Table 1 was used, with a cold-rolled steel sheet having a thickness of 0.8 mm as the base.

[0068] The surface treatment solution shown below was applied to a zinc-based plated steel sheet using a bar coater, and the sheet was heated with an induction heater so that the ultimate sheet temperature was 140° C. to form an organic-inorganic composite coating. The average thickness of the organic-inorganic composite coating determined by the method described above is shown in Table 5. [Surface treatment liquid] Component (a): Sodium zirconium carbonate 100 parts by mass Component (b): Tetraethoxysilane 60 parts by mass Component (c): Polyethylene glycol diglycidyl ether 138 parts by mass Component (d): 1-hydroxymethane-1,1-diphosphonic acid + tartaric acid 49 parts by mass Component (e): Sodium metavanadate 17 parts by mass Component (f): Ammonium titanium fluoride 3 parts by mass 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.

[0069] Next, the rust-preventive additives shown in Table 3 and the solid lubricants shown in Table 4 were added to the polyurethane resin shown in Table 2 in the amounts shown in Table 5 to prepare a surface treatment solution. This surface treatment solution was applied to the surface of the organic-inorganic composite coating using a bar coater and heated using an induction heater to reach a sheet temperature of 140°C, forming a surface treatment film and obtaining a surface-treated steel sheet. Among the polyurethane resin products shown in Table 2, solvent-soluble types A1 to A3, A6, and A7 were diluted with methyl ethyl ketone, in which the polyurethane resin was dissolved, while water-dispersible types A4 and A5 were diluted with water, in which the polyurethane resin was dispersed. The amounts of rust-preventive additives and solid lubricants shown in Table 5 are the amounts added per 100 parts by mass of the polyurethane resin solids in the surface treatment solution, which is equivalent to the content per 100 parts by mass of the polyurethane resin in the surface treatment film. The average thickness of the surface treatment film determined using the method described above is also shown in Table 5.

[0070] Furthermore, the "average total thickness of the organic-inorganic composite coating and the surface treatment coating at the convex portions of the zinc-based plated steel sheet" and the "coating damage rate" were determined using the methods described above, and are shown in Table 5. Furthermore, as a representative example of the invention, a cross-sectional SEM image of the surface-treated steel sheet of invention example No. 5 is shown in Figure 2.

[0071] The obtained surface-treated steel sheets were subjected to the following performance evaluations, and the results are shown in Table 5.

[0072] [Corrosion Resistance After Processing] After a drawbead test to determine the coating damage rate, samples were cut to 70 mm x 150 mm and subjected to chemical conversion treatment using a degreaser manufactured by Nihon Parkerizing Co., Ltd.: FC-E6403 (sprayed at 40°C for 120 seconds), a surface conditioner: PL-X (room temperature for 20 seconds), and a chemical conversion treatment agent: Palbond PB-L3065 (35°C for 120 seconds). A cationic electrodeposition paint (V-50 manufactured by Nippon Paint Co., Ltd.) was then applied to a film thickness of 20 μm. Each sample was then subjected to a combined cycle test (CCT) based on SAE-J2334 shown in FIG. 4, and the area ratio of white rust occurrence after 150 cycles was determined. The corrosion resistance after processing was evaluated according to the following criteria: ◎: No white rust occurrence ○: White rust occurrence area ratio less than 20% ×: White rust occurrence area ratio 20% or more

[0073] [Spot Weldability] A continuous spot welding test was performed on each sample under the following conditions: electrode used: CF type Cr-Cu electrode, pressure: 150 kgf, current application time: 10 cycles / 60 Hz, welding current: 8 kA, and the spot weldability was evaluated according to the following criteria: ○: Number of continuous spots: 2000 or more ×: Number of continuous spots: less than 2000

[0074] [Lubricity] To evaluate press formability, the coefficient of friction of each sample was measured as follows. FIG. 5 is an explanatory diagram showing an outline of the friction coefficient measurement device used. In this device, sample 1 is fixed to sample stage 2. Sample stage 2 is fixed to the upper surface of horizontally movable slide table 3. A vertically movable slide table support base 5 having a roller 4 in contact with the slide table 3 is provided on the underside of slide table 3. By pushing up the support base 5, a pressing load N is generated on sample 1 by bead 6. A first load cell 7 for measuring this pressing load N is attached to slide table support base 5. With this pressing force applied, slide table 3 is moved horizontally. A second load cell 8 for measuring the sliding resistance force F generated at this time is attached above rail 9 at one end of slide table 3. The test was conducted using a rust-preventive cleaning oil "Pleton R352L" manufactured by Sugimura Chemical Industry Co., Ltd. as a lubricant applied to the surface of sample 1.

[0075] Figure 6 is a perspective view showing the shape and dimensions of the bead used. The bead 6 shown in Figure 5 slides while its underside is pressed against the surface of the sample 1. The shape of the bead 6 shown in Figure 6 is 10 mm wide, 59 mm long in the sliding direction, and the lower ends of both ends in the sliding direction are curved surfaces with a curvature of 4.5 mmR, and the underside of the bead against which the sample is pressed has a flat surface with a width of 10 mm and a length of 50 mm in the sliding direction.

[0076] The friction coefficient was measured under the following conditions. Using the bead shown in FIG. 6, the pressing load N was 400 kgf, and the sample withdrawal speed (horizontal movement speed of the slide table 3) was 20 cm / min. The friction coefficient μ between the sample and the bead was calculated using the formula μ = F / N. The smaller this friction coefficient μ, the higher the lubricity and the better the press formability can be evaluated. The evaluation criteria are as follows: ◎: Friction coefficient μ is less than 0.22 ○: Friction coefficient μ is 0.22 or more but less than 0.28 ×: Friction coefficient μ is 0.28 or more

[0077]

[0078]

[0079]

[0080]

[0081]

[0082] The surface-treated steel sheet of the present invention does not contain chromium in the surface treatment film and has excellent corrosion resistance and spot weldability after processing, making it ideal for use in automobiles, home appliances, and building materials.

[0083] REFERENCE SIGNS LIST 1 Sample 2 Sample stage 3 Slide table 4 Roller 5 Slide table support 6 Bead 7 First load cell 8 Second load cell 9 Rail N Pressing load F Sliding resistance force

Claims

1. A zinc-based plated steel sheet having a microscopic unevenness on its surface; a chromium-free organic-inorganic composite coating formed on the surface of the zinc-based plated steel sheet; a surface treatment film containing a polyurethane resin formed on the organic-inorganic composite film; A surface-treated steel sheet having the average total thickness of the organic-inorganic composite coating and the surface treatment coating at the protruding portions of the zinc-based plated steel sheet is 0.10 μm or less; The average thickness of the surface treatment film is 0.4 to 1.0 μm, the damage rate of the surface treatment film after bending / unbending the surface-treated steel sheet with a bead having a tip diameter of 5 mmR is 20% or less; A surface-treated steel sheet characterized by:

2. The coated 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 , wherein the polyurethane resin is a solvent-soluble resin.

4. A surface-treated steel sheet as described in claim 2, wherein the polyurethane resin is a solvent-soluble resin.

5. The surface-treated steel sheet according to any one of claims 1 to 4, wherein the surface treatment film contains 1 to 50 parts by mass in total of one or more rust-preventive additives selected from the group consisting of silicon oxide, phosphate compounds, molybdenum acid compounds, and vanadium compounds, per 100 parts by mass of the polyurethane resin.

6. 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 per 100 parts by mass of the polyurethane resin.

7. The surface-treated steel sheet according to claim 5, wherein the surface treatment film contains 1 to 30 parts by mass of a solid lubricant per 100 parts by mass of the polyurethane resin.

8. The surface-treated steel sheet according to any one of claims 1 to 4, wherein the organic-inorganic composite coating has an average thickness of 0.01 to 0.20 µm.

9. The surface-treated steel sheet according to claim 5, wherein the average thickness of the organic-inorganic composite coating is 0.01 to 0.20 μm.

10. The surface-treated steel sheet according to claim 6, wherein the organic-inorganic composite coating has an average thickness of 0.01 to 0.20 μm.

11. The surface-treated steel sheet according to claim 7, wherein the average thickness of the organic-inorganic composite coating is 0.01 to 0.20 μm.