Steel plates and auto parts

Abstract

To provide a steel sheet having corrosion resistance and excellent electrocoating property, and an automobile component utilizing the same.SOLUTION: Provided is a steel sheet comprising a plated base steel sheet with a zinc-based plating layer provided on at least one side, and an organic coating provided on the zinc-based plating layer, wherein the plated base steel sheet has an arithmetic mean roughness Ra of 0.5 to 2.3 μm, the number of peaks per inch PPI (peak per inch) is 30 to 150, a thickness of the organic film is 0.3 to 2.1 μm and is smaller than the arithmetic average roughness Ra of the plated base steel sheet, and automobile parts using the same.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and an automobile part. [Background technology]

[0002] For example, the corrosion resistance of automobile parts is ensured by electrocoating the plated base steel material after chemical conversion treatment. Patent Document 1 proposes "a zinc-plated steel sheet having a zinc plating layer formed on the surface of the steel sheet and a zinc phosphate coating formed on the surface of the plating layer and having a crystal size of 3 μm or less, and having surface roughness characteristics of an arithmetic mean roughness Ra of 1.0 μm or more and 1.6 μm or less, as specified in JIS B 0601-1994, a number of peaks per inch PPI of 180 or more, and a maximum peak height Ry of 12 μm or less, as specified in JIS B 0601-1994." According to Patent Document 1, it is possible to provide "a zinc-plated steel sheet with an excellent appearance due to electrodeposition coating, which can improve the coating finish on the surface that has been subjected to sliding contact with a mold during press forming, etc."

[0003] However, in areas where the electrodeposition coating does not adhere well, such as the inner mating parts and bent parts of bag-shaped parts, it may not be possible to ensure a sufficient film thickness of the electrodeposition coating, i.e., corrosion resistance, and it may be necessary to compensate for this corrosion resistance by applying secondary rust-preventive materials such as sealers and waxes.

[0004] In this regard, development is being carried out for the purpose of improving corrosion resistance and eliminating or reducing the need for secondary rust prevention materials by using electrodeposition-coated steel sheets for automotive parts, which are made by electrodeposition-coating a base steel sheet with an organic coating. For example, Patent Document 2 describes a coated steel sheet coated with a composite coating, the composite coating being a composite coating of a composite resin (A) containing at least one resin selected from the group consisting of polyurethane resin particles (A-1) and ethylene-unsaturated carboxylic acid copolymer resin particles (A-2), each having an average particle size of 20 to 100 nm and having a silanol group and / or an alkoxysilyl group, and a vanadium compound (B), and the coating amount is 0.5 to 3 g / m. 2"A coated steel sheet with excellent sweat resistance, characterized by: [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-060020 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-144208 Summary of the Invention [Problem to be solved by the invention]

[0006] However, if the thickness of the organic film is increased in order to improve corrosion resistance, the film resistance increases, the conductivity deteriorates, and the electrodeposition paintability deteriorates.

[0007] Therefore, an object of the present disclosure is to provide a steel sheet having excellent corrosion resistance and excellent electrodeposition paintability, and an automobile part using the same. [Means for solving the problem]

[0008] The means for solving the problem include the following aspects. <1> A steel sheet having a plated base steel sheet having a zinc-based plating layer provided on at least one surface thereof, and an organic coating provided on the zinc-based plating layer, The arithmetic mean roughness Ra of the plated base steel sheet is 0.5 to 2.3 μm, and the number of peaks per inch (PPI) is 30 to 150, The thickness of the organic coating is 0.3 to 2.1 μm, and is smaller than the arithmetic mean roughness Ra of the plated base steel sheet. <2> The organic coating contains at least one resin selected from the group consisting of polyurethane resins and ethylene-unsaturated carboxylic acid copolymer resins having silanol groups and / or alkoxysilyl groups. <1> The steel plate according to claim 1. <3> The organic coating contains at least one additive selected from the group consisting of vanadium compounds, silicon oxide, organic titanium compounds, polyolefin wax, phosphoric acid compounds, thiocarbonyl compounds, niobium oxide, and guanidine compounds. <1> or <2> The steel plate according to claim 1. <4> <1> ~ <3> a formed steel sheet according to any one of the above items, a phosphate coating or a metal salt coating provided on a part or all of the protruding portions of the zinc-based plating layer protruding from the organic coating in the formed steel sheet; an electrodeposition coating film provided on the organic coating film and the phosphate coating film or the metal salt coating film of the steel plate compact; Automotive parts having <5> The phosphate coating is a needle-shaped phosphate coating. <4> The automotive part described in <6> The crystal diameter of the needle-shaped crystal phosphate coating is 0.1 to 5 μm. <5> The automotive part described in <7> The metal salt coating contains a salt of at least one selected from iron, titanium, zirconium, hafnium, indium, tin, bismuth, vanadium, nickel, cerium, molybdenum, and tungsten with an oxide ion, a nitrate ion, a sulfate ion, a fluorine ion, a complex fluorine ion, or a carbonate ion, and is a crystalline or amorphous metal salt coating. <4> The automotive part described in <8> The thickness of the crystalline or amorphous metal salt coating is 0.01 to 3 μm. <7> The automotive part described in [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a steel sheet having excellent corrosion resistance and excellent electrodeposition paintability, and an automobile part using the same. DETAILED DESCRIPTION OF THE INVENTION

[0010] An example of the coated steel sheet of the present disclosure will be described below. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In numerical ranges described in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In a numerical range, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the examples. The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved. A "combination of preferred embodiments" is a more preferred embodiment.

[0011] In this specification, the "average particle size" is the particle size that is the cumulative 50% particle size from the smallest diameter side in the particle size distribution based on the number determined by dynamic light scattering.

[0012] The steel sheet of the present disclosure is A steel sheet having a plated base steel sheet having a zinc-based plating layer provided on at least one surface thereof, and an organic coating provided on the zinc-based plating layer, The base steel sheet has an arithmetic mean roughness Ra of 0.5 to 2.3 μm and a peak per inch (PPI) of 30 to 150, The organic coating has a thickness of 0.3 to 2.1 μm, and is smaller than the arithmetic mean roughness Ra of the base steel sheet.

[0013] Further, the automotive part of the present disclosure is A formed body of the steel plate of the present disclosure; a phosphate coating or a metal salt coating provided on a part or all of the protruding portions of the zinc-based plating layer protruding from the organic coating in the formed steel sheet; an electrodeposition coating film provided on the organic coating film and the phosphate coating film or the metal salt coating film of the steel plate formed body; It has.

[0014] In the steel sheet according to this embodiment, the base steel sheet (i.e., the zinc-based coating layer) has a predetermined surface texture with an arithmetic mean roughness Ra of 0.5 to 2.3 μm and a peaks per inch (PPI) of 30 to 150. An organic coating having a thickness of 0.3 to 2.1 μm and smaller than the arithmetic mean roughness Ra of the sheet is formed on the surface of the base steel sheet having this surface texture. As a result, on the surface of the steel sheet, the protruding portions of the zinc-based plating layer are present so as to protrude from the organic coating, that is, the protruding portions of the zinc-based plating layer are exposed from the organic coating. After the steel sheet is formed, a phosphate film or a metal salt film is formed on a part of or the entire surface of the protruding portions of the zinc-based plating layer, and then an electrodeposition coating film is formed on the organic film and the phosphate film or the metal salt film to produce an automotive part.

[0015] In the steel sheet according to this embodiment, the surface of the zinc-based plating layer other than the protrusions of the zinc-based plating layer is covered with an organic coating, so that the corrosion resistance can be improved while suppressing the film resistance. Since the film resistance of the organic film is reduced, electrodeposition paintability can also be improved. In addition, since a phosphate film or a metal salt film is formed on a part or the entire surface of the protruding portions of the zinc-based plating layer before the electrodeposition coating film is formed, the adhesion of the electrodeposition coating film is improved and the corrosion resistance is also enhanced.

[0016] As will be seen below, the steel sheet and automobile part according to this embodiment have excellent electrodeposition coatability as well as corrosion resistance.

[0017] Hereinafter, the steel sheet and the automobile part according to this embodiment will be described in detail.

[0018] <Steel plate> (plated base steel sheet) -Surface properties of plated base steel sheet- The arithmetic mean roughness Ra of the plated base steel sheet (that is, the zinc-based plated layer) is 0.5 to 2.3 μm, preferably 0.8 to 2.2 μm, and more preferably 1.0 to 2.0 μm.

[0019] The number of peaks per inch (PPI) of the plated base steel sheet (that is, the zinc-based plated layer) is 30 to 150, preferably 50 to 140, and more preferably 70 to 130.

[0020] When the plated base steel sheet (i.e., zinc-based plating layer) has the above-mentioned surface properties, the organic coating imparts sufficient corrosion resistance even when an organic coating having a surface roughness smaller than the arithmetic mean roughness Ra of the plated base steel sheet is formed.

[0021] The above-described surface texture of the plated base steel sheet (i.e., the zinc-based coating layer) can be obtained, for example, by carrying out a skin pass (temper rolling) to impart the surface texture. The surface texture of the present embodiment can be obtained by appropriately adjusting the surface texture and pressure of the roll surface during the skin pass and transferring the texture to the surface of the zinc-based coating layer.

[0022] The arithmetic mean roughness Ra of the coated base steel sheet is a value measured in accordance with JIS B 0601:1994, with the measurement conditions being a cutoff of 0.8 mm and an evaluation length of 8 mm. On the other hand, the number of peaks per inch (PPI) of the plated base steel sheet is measured in accordance with the US SAE standard SAE J911, with an evaluation length of 1 inch (25.4 mm) and a peak count level of ±0.654 μm.

[0023] - Composition of plated base steel sheet - The plated base steel sheet has a zinc-based plating layer formed on at least one side thereof. That is, the plated base steel sheet has a base steel sheet and a zinc-based plating layer formed on at least one side of the base steel sheet.

[0024] The base steel sheet is the steel sheet on which the zinc-based coating layer is to be formed. The base steel sheet is not particularly limited, and may be any type of steel sheet, such as an ultra-low C type (a ferrite-based structure), an Al-k type (a structure containing pearlite in ferrite), a two-phase structure (e.g., a structure containing martensite in ferrite, or a structure containing bainite in ferrite), a deformation-induced transformation type (a structure containing retained austenite in ferrite), or a fine crystal type (a ferrite-based structure).

[0025] The zinc-based plating layer is a plating layer containing zinc. Specific examples of the zinc-based plating layer include well-known zinc-based plating layers such as a zinc plating layer, a zinc-aluminum-magnesium plating layer, a zinc-aluminum-magnesium-silicon plating layer, a zinc-aluminum plating layer, and a zinc-aluminum-silicon plating layer.

[0026] The zinc-based plating layer may also contain small amounts of cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, arsenic, etc. as dissimilar metal elements or impurities.

[0027] In particular, from the viewpoint of corrosion resistance, the zinc-based plating layer is preferably a plating layer containing aluminum in addition to zinc, or a plating layer containing aluminum and magnesium. In other words, using a zinc alloy-plated steel sheet as the plating base steel sheet is preferable because it can provide better corrosion resistance than a zinc-plated steel sheet. Specifically, the zinc-based plating layer is preferably a plating layer containing 0.5% by mass or more and 60% by mass or less of aluminum, with the balance being zinc and impurities, and more preferably a plating layer containing 0.5% by mass or more and 60% by mass or less of aluminum and 0.5% by mass or more and 15% by mass or less of magnesium, with the balance being zinc and impurities. Examples of zinc alloy plating layers containing all of zinc, aluminum, and magnesium include a zinc-aluminum-magnesium plating layer and a zinc-aluminum-magnesium-silicon plating layer. Depending on the ratio of each component, there are various types such as a Zn-6%Al-3%Mg plating layer, a Zn-11%Al-3%Mg-0.2%Si plating layer, a Zn-55%Al-2%Mg-1.6%Si plating layer, and plating layers containing trace amounts of Ni, Cr, Ti, etc. in these plating layers.

[0028] The method for forming the zinc-based plating layer is not particularly limited, and any of the known methods such as electroplating, hot-dip plating, vapor deposition plating, dispersion plating, and vacuum plating may be used.

[0029] The amount of zinc-based coating layer per side of the steel sheet is not particularly limited, but is preferably 15 g / m 2 More than 140g / m 2 It is preferably 30 g / m or less. 2 More than 90g / m 2 The following is the result. The coating weight of the zinc-based plating layer is 15g / m 2 If the coating weight is less than 140 g / m, the coating weight will be too small, resulting in unplated areas, and the corrosion protection effect of the plating may not be exerted. 2 If it is over 100%, the corrosion resistance is high, but the phenomenon of the plating turning black easily occurs.

[0030] (organic coating) -Thickness of organic film- The thickness of the organic coating is 0.3 to 2.1 μm, which is smaller than the arithmetic mean roughness Ra of the plated base steel sheet. If the organic coating is too thin, corrosion resistance will decrease. If the organic coating is too thick, film resistance will increase and electrodeposition paintability will decrease. Furthermore, if the arithmetic mean roughness Ra of the plated base steel sheet is equal to or greater than the arithmetic mean roughness Ra, the protrusions of the zinc-based plating layer, which form a phosphate coating or metal salt coating to improve the adhesion of the electrodeposition coating film, will not protrude from the organic coating. This will result in decreased corrosion resistance. Therefore, the thickness of the organic coating is set within the above range.

[0031] The thickness of the organic coating is preferably 0.5 to 1.9 μm, more preferably 0.7 to 1.7 μm. The difference between the thickness of the organic coating and the arithmetic mean roughness Ra of the plated base steel sheet (arithmetic mean roughness Ra of the plated base steel sheet - thickness of the organic coating) is preferably 0.2 to 1.8 μm, more preferably 0.5 to 1.4 μm.

[0032] The thickness of the organic coating is measured as follows. A sample with a cross section cut along the thickness direction is cut out from the steel plate (or automotive part) to be measured. The sample is cut out from the center of the steel plate (or automotive part) to be measured. After embedding the test piece in resin, the cut surface is polished to prepare the observation surface. Next, the specimen is observed at 500x magnification using a scanning electron microscope (SEM), and a length of 10 mm is observed in the horizontal direction, and 10 randomly selected continuous recesses in the zinc-based plating layer are selected. Ten selected recesses in the zinc-based plating layer are observed at 3000x magnification, and the maximum film thickness of the organic coating present in the recesses in the zinc-based plating layer is measured, and the average is calculated.

[0033] -resin- The organic coating includes a resin. Examples of the resin include known organic resins such as polyester resin, polyurethane resin, epoxy resin, phenol resin, acrylic resin, polyolefin resin, etc. The resins may be used alone or in combination of two or more.

[0034] Among these, from the viewpoint of improving corrosion resistance, the resin is preferably at least one resin selected from the group consisting of polyurethane resins and ethylene-unsaturated carboxylic acid copolymer resins, each having a silanol group and / or an alkoxysilyl group. These resins contain silanol groups and / or alkoxysilyl groups, which can, for example, react with each other or with other components (e.g., with Si-OH groups in silicon oxide or Ti-OH or Ti-OR groups in organotitanium compounds), forming a composite organic coating and improving corrosion resistance.

[0035] The polyurethane resin having a silanol group and / or an alkoxysilyl group is preferably a polycarbonate polyurethane. Polycarbonate-based polyurethane resins can be obtained as follows. First, a polyurethane prepolymer is produced by reacting an isocyanate group-containing compound, a polycarbonate polyol, a low-molecular-weight polyol, and a compound containing active hydrogen groups and hydrophilic groups. Next, to disperse the polyurethane prepolymer well in water, the hydrophilic groups (carboxyl groups or sulfonic acid groups) of the polyurethane prepolymer are neutralized with a neutralizing agent. The neutralized polyurethane prepolymer is then dispersed in water containing active hydrogen group-containing alkoxysilanes and a polyamine as a chain extender, followed by chain extension. This results in a polycarbonate-based polyurethane resin.

[0036] Examples of isocyanate group-containing compounds include aliphatic diisocyanates (hexamethylene diisocyanate, etc.), alicyclic diisocyanates (1,3-cyclohexane diisocyanate, isophorone diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane, etc.), and aromatic diisocyanates (m-phenylene diisocyanate, p-phenylene diisocyanate, 1,5-naphthalene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4- or 2,6-tolylene diisocyanate or a mixture thereof, 4,4'-toluidine diisocyanate, etc.).

[0037] Examples of polycarbonate polyols include polyols obtained by reacting glycol with dimethyl carbonate, diphenyl carbonate, ethylene carbonate, phosgene, and the like. The glycol may be one or more selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, bisphenol-A, and hydrogenated bisphenol-A.

[0038] Examples of low molecular weight polyols include glycols (ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, etc.), glycerin, trimethylolpropane, and pentaerythritol.

[0039] Examples of compounds containing an active hydrogen group and a hydrophilic group include sulfonic acid-containing compounds (e.g., 2-hydroxyethanesulfonic acid) or derivatives thereof, and carboxyl group-containing compounds (e.g., 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid) or derivatives thereof.

[0040] Examples of the neutralizing agent include ammonia, tertiary amines such as triethylamine and dimethylethanolamine, and alkali metal hydroxides such as sodium hydroxide and potassium hydroxide.

[0041] Examples of active hydrogen group-containing alkoxysilanes include amino group-containing silanes (γ-(2-aminoethyl)aminopropyltrimethoxysilane, γ-(2-aminoethyl)aminopropyltriethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-(2-aminoethyl)aminopropylmethyldiethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, etc.), and mercapto group-containing silanes (γ-mercaptopropyltrimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-mercaptopropylmethyldiethoxysilane, etc.).

[0042] Examples of polyamines as chain extenders include polyamines (diamines such as ethylenediamine, 1,2-propanediamine, 1,6-hexamethylenediamine, and piperazine, diethylenetriamine, dipropylenetriamine, and triethylenetetramine), and hydrazines.

[0043] Here, the reaction to obtain a polyurethane prepolymer from an active hydrogen compound such as a polyol and an isocyanate group-containing compound is preferably carried out at a reaction temperature of 30 to 100°C in the presence or absence of an organic solvent.

[0044] When an organic solvent is used, it is preferable to use one that has a relatively high solubility in water. Specific examples of the organic solvent include acetone, methyl ethyl ketone, acetonitrile, and N-methylpyrrolidone.

[0045] The polyurethane prepolymer can be dispersed in water using, for example, a homogenizer, a mixer, etc. The temperature at this time is preferably from room temperature to about 70°C.

[0046] When the reaction is carried out in a solvent, the solvent can be removed by distillation under reduced pressure, if necessary.

[0047] Examples of ethylene-unsaturated carboxylic acid copolymer resins having silanol groups and / or alkoxysilyl groups include resins obtained by reacting an epoxy group-containing alkoxysilane with an aqueous dispersion resin liquid obtained by neutralizing an ethylene-methacrylic acid copolymer resin with at least one of an alkali metal hydroxide, ammonia, and an amine and dispersing it in water.

[0048] Examples of epoxy group-containing alkoxysilanes include γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

[0049] The epoxy group-containing alkoxysilane is reacted preferably in a proportion of 0.1 to 20% by mass, more preferably 1 to 10% by mass, based on the solid content of the aqueous dispersion resin.

[0050] In the reaction with the epoxy group-containing alkoxysilane, a polyfunctional epoxy compound may be used in combination. Examples of the polyfunctional epoxy compound include sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, propylene glycol diglycidyl ether, triglycidyl tris(2-hydroxyethyl)isocyanurate, bisphenol A diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether.

[0051] The reaction between the epoxy group-containing alkoxysilane and polyfunctional epoxy compound and the aqueous dispersion resin is preferably carried out at 50 to 100° C. for 0.5 to 12 hours.

[0052] Either the polyurethane resin or the ethylene-unsaturated carboxylic acid copolymer resin can be used alone, but from the viewpoint of improving corrosion resistance, it is preferable to use them in combination in a mass ratio (polyurethane resin / ethylene-unsaturated carboxylic acid copolymer resin) of 20 / 80 to 90 / 10.

[0053] The polyurethane resin and ethylene-unsaturated carboxylic acid copolymer resin are preferably used as resin particles. The average particle size of the resin particles is preferably 20 to 100 nm. When the average particle size of the resin particles is within the above range, the formability of the organic coating and the adhesion to the zinc-based plating layer are improved, thereby improving corrosion resistance.

[0054] The content of the resin is preferably 20 to 100% by mass, more preferably 40 to 80% by mass, based on the organic coating.

[0055] -Rust prevention ingredients- In addition to the resin, the organic coating may contain at least one additive selected from the group consisting of vanadium compounds, silicon oxide, organic titanium compounds, polyolefin wax, phosphoric acid compounds, thiocarbonyl compounds, niobium oxide, and guanidine compounds. Vanadium compounds, silicon oxide, organic titanium compounds, phosphate compounds, thiocarbonyl compounds, niobium oxide, and guanidine compounds are rust-preventing components that contribute to improving corrosion resistance, and are preferably contained in the organic coating. Furthermore, polyolefin wax is a lubricating component and is preferably contained in the organic film because it prevents damage to the organic film.

[0056] The vanadium compound exists in the organic film as water-soluble vanadium ions and prevents damage to the organic film, thereby improving corrosion resistance.

[0057] Examples of vanadium compounds include ammonium vanadate, sodium vanadate, potassium vanadate, vanadium oxyacetylacetonate, and vanadium acetylacetonate.

[0058] The content of the vanadium compound is preferably 0.05 to 10% by mass, more preferably 0.1 to 5% by mass, based on the resin.

[0059] When silicon oxide is used as a resin in the form of a polyurethane resin or an ethylene-unsaturated carboxylic acid copolymer resin containing silanol groups and / or alkoxysilyl groups, the Si-OH groups of the silicon oxide react with the silanol groups and / or alkoxysilyl groups, forming a composite organic coating and improving corrosion resistance.

[0060] Examples of silicon oxide include well-known silicon oxides such as combustion silica, deflagration silica, precipitation silica, gel silica, colloidal silica, sol-gel silica, etc. Commercially available silicon oxide products include ST-YL, ST-ZL, MP-1040 (manufactured by Nissan Chemical Industries, Ltd.), PL-7 (manufactured by Fuso Chemical Co., Ltd.), and SI-80P (manufactured by Catalysts and Chemical Industries Co., Ltd.).

[0061] The average particle size of silicon oxide is preferably 5 to 200 nm. When the average particle size of silicon oxide is within the above range, corrosion resistance is improved.

[0062] The content of silicon oxide is preferably 5 to 100% by mass, more preferably 20 to 50% by mass, based on the resin.

[0063] When an organotitanium compound is used as the resin, a polyurethane resin or an ethylene-unsaturated carboxylic acid copolymer resin having a silanol group and / or an alkoxysilyl group reacts with the Ti-OH or Ti-OR' group of the organotitanium compound and the silanol group and / or the alkoxysilyl group, resulting in a composite organic coating and improved corrosion resistance.

[0064] Examples of organic titanium compounds include dipropoxybis(triethanolaminato)titanium, dipropoxybis(diethanolaminato)titanium, dibutoxybis(triethanolaminato)titanium, dibutoxybis(diethanolaminato)titanium, dipropoxybis(acetylacetonato)titanium, dibutoxybis(acetylacetonato)titanium, dihydroxybis(lactato)titanium monoammonium salt, dihydroxybis(lactato)titanium diammonium salt, propanedioxytitanium bis(ethylacetoacetate), and oxotitanium bis(monoammonium oxalate).

[0065] The content of the organic titanium compound is preferably 0.05 to 3% by mass, more preferably 0.1 to 2% by mass, based on the resin.

[0066] Polyolefin wax increases the lubricity of the organic film and prevents damage to the organic film caused by contact with the mold during processing such as press molding. If the organic film is damaged, its corrosion resistance decreases, so by preventing damage, corrosion resistance is maximized. Examples of polyolefin waxes include hydrocarbon waxes (paraffin, microcrystalline, polyethylene, etc.) and derivatives thereof, etc. Examples of derivatives include carboxylated polyolefins, chlorinated polyolefins, etc.

[0067] The softening point of the polyolefin wax is preferably 80 to 130° C. When the softening point of the polyolefin wax is within the above range, sufficient lubricity can be imparted to the organic coating. The softening point is measured in accordance with ASTM D3954 "Standard Test Method for Dropping Point of Wax."

[0068] The average particle size of the polyolefin wax is preferably 0.5 to 4 μm. When the average particle size of the polyolefin wax is within the above range and the softening point of the polyolefin wax is within the above range, sufficient lubricity can be imparted to the organic coating.

[0069] The content of the polyolefin wax is preferably 0.1 to 20% by mass, more preferably 0.5 to 10% by mass, based on the resin.

[0070] The phosphate ions of the phosphate compound form a phosphate layer on the surface of the zinc-based plating layer, thereby passivating the layer and improving corrosion resistance. Examples of the phosphoric acid compound include phosphoric acids such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, and tetraphosphoric acid, and phosphates such as triammonium phosphate, diammonium hydrogen phosphate, trisodium phosphate, and disodium hydrogen phosphate.

[0071] The content of the phosphoric acid compound is preferably 0.01 to 5% by mass, more preferably 0.05 to 3% by mass, based on the resin.

[0072] The thiocarbonyl compound prevents white rust and improves corrosion resistance. The thiocarbonyl compound includes a thiocarbonyl compound represented by the following formula (1). Formula (1):XC(=S)-Y In the formula, X and Y are the same or different and represent H, OH, SH, or NH2, or a hydrocarbon group having 1 to 15 carbon atoms which may have OH, SH, or NH2 as a substituent and may contain -O-, -NH-, -S-, -CO-, or -CS-, and X and Y may be bonded to form a ring.

[0073] The thiocarbonyl compound represented by general formula (1) refers to a compound having a thiocarbonyl group represented by the following formula (I), and among these, a thiocarbonyl group having a nitrogen atom or an oxygen atom represented by the following formula (II) is preferred. Formula (I):-C(=S)- Formula (II): NC(=S)- or OC(=S)-

[0074] As the thiocarbonyl compound, a compound capable of forming a thiocarbonyl group-containing compound in an aqueous solution or in the presence of an acid or alkali can also be used.

[0075] Specific examples of the thiocarbonyl compound include thiourea represented by the formula: NC(S)—N and derivatives thereof (for example, methylthiourea, dimethylthiourea, trimethylthiourea, ethylthiourea, diethylthiourea, 1,3-dibutylthiourea, phenylthiourea, diphenylthiourea, 1,3-bis(dimethylaminopropyl)-2-thiourea, ethylenethiourea, and propylenethiourea). Specific examples of the thiocarbonyl compound include carbothioic acids represented by the formula: RC(═S)—OH or the formula: RC(═S)—SH and salts thereof (for example, thioacetic acid, thiobenzoic acid, dithioacetic acid, sodium methyldithiocarbamate, sodium dimethyldithiocarbamate, dimethyldithiocarbamic acid triethylamine salt, sodium diethyldithiocarbamate, pentamethylenedithiocarbamic acid piperidine salt, pipecolyldithiocarbamic acid pipecoline salt, potassium o-ethylxanthogenate, etc.).

[0076] The content of the thiocarbonyl compound is preferably 0.1 to 10% by mass, more preferably 0.2 to 5% by mass, based on the resin.

[0077] Niobium oxide prevents white rust and improves corrosion resistance. Niobium oxide sol produced by a known method can be used. The niobium oxide sol is not particularly limited, and examples thereof include those produced by known methods described in JP-A-6-321543, JP-A-8-143314, JP-A-8-325018, etc. Niobium oxide sol commercially available from Taki Chemical Co., Ltd. may also be used.

[0078] The average particle size of the niobium oxide is preferably 100 nm or less, more preferably 2 to 50 nm, and even more preferably 2 to 20 nm.

[0079] The content of niobium oxide is preferably 0.1 to 5 mass %, more preferably 0.2 to 3 mass %, calculated as Nb2O5 relative to the resin.

[0080] The guanidine compound prevents white rust and improves corrosion resistance. The guanidine compound includes a guanidine compound represented by the following formula (2). Formula (2): X'-NH-C(=NH)-NH-Y' In the formula, X' and Y' are the same or different and represent H, NH2, a phenyl group, or a methylphenyl group (tolyl group), or a group which may have H, NH2, a phenyl group, or a methylphenyl group (tolyl group) as a substituent and which contains -C(=NH)-, -CO-, or -CS-.

[0081] Specific examples of the guanidine compound include guanidine, aminoguanidine, guanylthiourea, 1,3-di-o-tolylguanidine, 1-o-tolylbiguanide, and 1,3-diphenylguanidine.

[0082] The content of the guanidine compound is preferably 0.1 to 5% by mass, more preferably 0.2 to 3% by mass, based on the resin.

[0083] -Other ingredients- The organic coating may also contain other components as is well known. Other components include, for example, inorganic pigments (titanium oxide (TiO2), zinc oxide (ZnO), calcium carbonate (CaCO3), barium sulfate (BaSO4), alumina (Al2O3), kaolin clay, carbon black, iron oxide (Fe2O3, Fe3O4), organic pigments, etc.

[0084] -Formation of organic film- The organic coating is formed using a paint containing the above components as well as an organic solvent, and the paint may contain an antifoaming agent, a leveling agent, etc. The organic coating is formed by applying a coating material by a commonly used method such as bar coating, roll coating, air spraying, airless spraying, or dipping, and then heating the coating. The heating temperature is preferably 50 to 250°C, and more preferably 70 to 220°C.

[0085] <Automotive parts> (Steel plate molding) The formed steel sheet is a formed steel sheet according to the present disclosure. The steel sheet can be formed by a known forming method such as press forming or bending.

[0086] (phosphate coating) Examples of the phosphate that constitutes the phosphate coating include ammonium salts of phosphoric acid, alkali metal salts of phosphoric acid, and alkaline earth metal salts of phosphoric acid. From the viewpoint of improving the adhesion of the electrodeposition coating film and enhancing corrosion resistance, the phosphate coating is preferably a phosphate coating with needle-like crystals, and the crystal diameter of the phosphate coating with needle-like crystals is preferably 0.1 to 5 μm, more preferably 0.5 to 3 μm.

[0087] Here, the crystal diameter of the needle-shaped crystals in the phosphate coating is measured as follows: A sample having a cross section cut along the thickness direction is cut out from the steel plate (or automobile part) to be measured. The sample is cut out from the center of the steel plate (or automobile part) to be measured. After embedding the test piece in resin, the cut surface is polished to serve as the observation surface. Next, the sample is observed at 500x magnification using a scanning electron microscope (SEM), and the particle sizes of the phosphate crystals at any 10 locations are measured and then averaged.

[0088] The phosphate film is formed on some or all of the protrusions of the zinc-based plating layer that protrude from the organic coating. The phosphate film is formed when the zinc-based plating layer dissolves in the treatment solution, and the pH at the interface between the treatment solution and the zinc-based plating layer increases due to the effect of hydrogen gas generated by the dissolution, causing the dissolved zinc-based plating layer components to react with the phosphate components in the treatment solution, resulting in precipitation as crystals.

[0089] (metal salt coating) Examples of metal species that may form the metal salt coating include iron, titanium, zirconium, hafnium, indium, tin, bismuth, vanadium, nickel, cerium, molybdenum, and tungsten. Examples of salt species constituting the metal salt coating include oxide ions, nitrate ions, sulfate ions, fluorine ions, complex fluorine ions, and carbonate ions. Specific examples include zirconium oxide, hafnium oxide, indium oxide, tin oxide, bismuth oxide, vanadium oxide, nickel oxide, cerium oxide, molybdenum oxide, tungsten oxide, zirconium nitrate, iron sulfide, zirconium fluoride, titanium fluoride, hafnium fluoride, and indium fluoride. The complex fluorine ion is an anion in which two or more fluorine atoms are bonded to a central atom. Suitable central atoms include boron, silicon, titanium, zirconium, and hafnium. Specific examples of the complex fluorine ion include a hexafluorosilicate ion, a hexafluorotitanate ion, a hexafluorozirconate ion, and a hexafluorohafnium salt ion.

[0090] From the viewpoint of improving the adhesion of the electrodeposition coating film and enhancing corrosion resistance, the metal salt coating preferably contains a salt of at least one element selected from iron, titanium, zirconium, hafnium, indium, tin, bismuth, vanadium, nickel, cerium, molybdenum, and tungsten with an oxide ion, a nitrate ion, a sulfate ion, a fluoride ion, a complex fluoride ion, or a carbonate ion, and is a crystalline or amorphous metal salt coating. The thickness of the crystalline or amorphous metal salt coating is preferably 0.01 to 3 μm, and more preferably 0.05 to 1 μm.

[0091] Among these, the metal salt coatings are particularly preferably coatings of zirconium oxide, titanium oxide, and hafnium oxide.

[0092] Here, the thickness of the metal salt film is measured as follows. A sample with a cross section cut along the thickness direction is cut out from the steel plate (or automotive part) to be measured. The sample is cut out from the center of the steel plate (or automotive part) to be measured. After embedding the test piece in resin, the cut surface is polished to prepare the observation surface. Next, the sample is observed under a scanning electron microscope (SEM) at 500x magnification, and the thickness of the metal salt film is measured at any 10 locations, and the average is calculated.

[0093] The metal salt film is formed on some or all of the protrusions of the zinc-based plating layer that protrude from the organic film. The metal salt film is formed when the zinc-based plating layer dissolves in the treatment solution, and the pH at the interface between the treatment solution and the zinc-based plating layer rises due to the effect of hydrogen gas generated by the dissolution, causing the dissolved zinc-based plating layer components to react with the metal salt components in the treatment solution, resulting in insolubilization.

[0094] (Coverage area ratio of phosphate film or metal salt film) The phosphate film or metal salt film is formed on part or all of the protruding portions of the zinc-based plating layer that protrude from the organic film. The coverage of the phosphate film or metal salt film is preferably 30 to 100% of the protruding portions of the zinc-based plating layer protruding from the organic film, which improves the adhesion of the electrodeposition coating film and also improves corrosion resistance.

[0095] The coverage of the phosphate film or metal salt film is measured as follows. A sample is cut out from the automobile part to be measured. The sample is cut out from the center of the automobile part to be measured. After embedding the test piece in resin, the cut surface is polished to prepare the observation surface. Next, the specimen is observed at 500x magnification using a scanning electron microscope (SEM) over a 10 mm horizontal length, and the total length of the protruding parts of the zinc-based plating layer protruding from the organic coating and the covering length of the phosphate coating or metal salt coating are calculated. The coverage rate of the phosphate film or metal salt film is then calculated using the formula: coverage rate of the phosphate film or metal salt film = coverage length of the phosphate film or metal salt film / (total length of the protruding portion of the zinc-based plating layer protruding from the organic film and coverage length of the phosphate film or metal salt film) x 100. The above operation is carried out at any three locations on the automobile part to be measured, and the arithmetic average is calculated. The length of the protrusions of the zinc-based plating layer refers to the length of the outline of the protrusions of the zinc-based plating layer protruding from the organic coating. The coating length of the phosphate film or metal salt film indicates the boundary length where the phosphate film or metal salt film is in contact with the protrusions of the zinc-based plating layer.

[0096] (electrodeposition coating film) The electrodeposition coating film may be a well-known electrodeposition coating film, and may be either an anionic electrodeposition coating film or a cationic electrodeposition coating film, but from the viewpoint of corrosion resistance, a cationic electrodeposition coating film is preferred. The electrodeposition coating film may be, for example, an electrodeposition coating film formed by electrodeposition coating treatment using a water-based paint containing a resin, a curing agent, and other additives. Examples of the resin include aqueous resins (known aqueous resins such as acrylic resins, polyester resins, alkyd resins, epoxy resins, and polyurethane resins) having hydrophilic groups such as carboxyl groups, hydroxyl groups, methylol groups, amino groups, sulfonic acid groups, and polyoxyethylene bonds, and functional groups such as hydroxyl groups that react with curing agents. Examples of the curing agent include melamine resin and blocked polyisocyanate. Other additives include known additives such as color pigments, optical interference pigments, extender pigments, dispersants, anti-settling agents, reaction accelerators, antifoaming agents, thickeners, rust inhibitors, ultraviolet absorbers, and surface conditioners.

[0097] The thickness of the electrodeposition coating film is preferably 5 to 50 μm, more preferably 10 to 40 μm.

[0098] If necessary, other coating films such as an intermediate coating film and a top coating film may be formed on the electrodeposition coating film.

[0099] (Automotive parts applications) The automotive parts of the present disclosure can be applied to, for example, center pillar outers, door outers, roof rail outers, side panels, fenders, side sills, side members, rear members, floor cloths, etc. [Example]

[0100] The present disclosure will be described in more detail below with reference to examples, although the present disclosure is not limited to these examples.

[0101] (Preparation of plated steel sheets) The following coated base steel sheets were prepared. However, to impart surface texture, the surface texture or pressure of the roll surface was adjusted and skin pass was performed, so that the arithmetic mean roughness Ra and peaks per inch (PPI) of the coated base steel sheets (i.e., zinc-based coating layer) were as shown in Table 7. GA: Galvannealed steel sheet (thickness: 0.8 mm, coating composition: Fe: 10 mass%, balance: Zn, coating weight: 45 g / m 2 ) GI: Hot-dip galvanized steel sheet (sheet thickness = 0.8 mm, coating weight = 60 g / m 2 ) Zn-11Al-3Mg-0.2Si: Zn-Al-Mg-Si based coated steel sheet (sheet thickness 0.8 mm, coating composition = Al: 11 mass%, Mg: 3 mass%, Si: 0.2 mass%, balance: Zn, coating weight = 60 g / m 2 ) Zn-6Al-3Mg: Zn-Al-Mg-based coated steel sheet (sheet thickness 0.8 mm, coating composition = Al: 6 mass%, Mg: 3 mass%, balance: Zn, coating weight = 60 g / m 2 )

[0102] (organic coating resin) -Production of aqueous dispersion of polyurethane resin particles- --Production Example 1-- A reaction vessel was charged with 4,4'-methylenebis(cyclohexyl isocyanate), a polycarbonate diol with a molecular weight of 2000, neopentyl glycol, dimethylolpropionic acid, and N-methylpyrrolidone as a solvent. The mixture was stirred at 80°C for 6 hours and then neutralized with dimethylethanolamine to obtain a polyurethane prepolymer solution. The polyurethane prepolymer solution obtained by the reaction was then dispersed in water containing hydrazine and γ-(2-aminoethyl)aminopropyltriethoxysilane using a homodisper to obtain an aqueous dispersion of polyurethane resin particles containing silanol groups and / or ethoxysilyl groups. The solids concentration was 30% by mass, and the average particle size measured by dynamic light scattering was 39 nm.

[0103] --Production Example 2-- The polyurethane prepolymer obtained in the same manner as in Production Example 1 was dispersed in an aqueous hydrazine solution using a Homodisper to obtain an aqueous dispersion of polyurethane resin particles. The solid content was 30% by mass and the average particle size was 36 nm.

[0104] -Production of aqueous dispersion of ethylene-unsaturated carboxylic acid copolymer resin particles- --Production Example 3-- Ethylene-methacrylic acid copolymer resin (20% by mass of methacrylic acid), sodium hydroxide (5.6% by mass relative to the resin), and deionized water were added to a reaction vessel and stirred at 95°C for 6 hours to obtain a water-based resin dispersion with a solids content of 20%. 0.8% by mass of γ-glycidoxypropyltrimethoxysilane and 0.8% by mass of glycerol polyglycidyl ether were further added to this water-based resin dispersion and reacted at 85°C for 2 hours to obtain a water-based dispersion of ethylene-methacrylic acid copolymer resin particles having silanol groups and / or methoxysilyl groups. The solids concentration was 21% by mass, and the average particle size was 76 nm.

[0105] --Production Example 4-- Ethylene-methacrylic acid copolymer resin (20% by weight methacrylic acid), 3.7% by weight sodium hydroxide, 6.3% by weight aqueous ammonia (25% by weight), and deionized water were added to a reaction vessel and stirred at 95°C for 6 hours to obtain an aqueous resin dispersion with a solids content of 20%. To this aqueous resin dispersion, 1.2% by weight of γ-glycidoxypropyltriethoxysilane and 0.6% by weight of pentaerythritol polyglycidyl ether were further added and reacted at 85°C for 2 hours to obtain an aqueous dispersion of ethylene-methacrylic acid copolymer resin particles having silanol groups and / or methoxysilyl groups. The solids concentration was 21% by weight, and the average particle size was 84 nm.

[0106] (Examples 1 to 38, Comparative Examples 1 to 8) Plated base steel sheets of the steel types shown in Table 7 were degreased by spraying with a 2% aqueous solution of alkaline degreasing agent (Surf Cleaner 155, manufactured by Nippon Paint Co., Ltd.) at 60°C for 30 seconds. A water-based paint was prepared using the resin particles obtained in the above Production Example and the substances shown in Tables 1 to 6 according to the formulation shown in Table 7. The resulting water-based paint was baked using a bar coater in a hot air drying oven with an atmospheric temperature of 500°C until the plate temperature reached 150°C, forming an organic coating having the thickness shown in Table 7. However, the comparative example 5 did not form an organic film.

[0107] Next, a phosphate film or metal salt film was formed on some or all of the protruding portions of the zinc-based plating layer that protruded from the organic film, as follows: In Comparative Example 4, however, the phosphate film or metal salt film was formed directly on the zinc-based plating layer; and in Comparative Examples 7 and 8, the organic film was thick and the zinc-based plating layer did not protrude, so although phosphate treatment was performed, no phosphate film was formed. To form a phosphate coating, the specimen was immersed in a surface conditioning treatment agent (Preparen X, manufactured by Nihon Parkerizing) at room temperature for 20 seconds. The specimen was then further immersed in a phosphating treatment agent (Palbond 3020, manufactured by Nihon Parkerizing) at 43°C to form a phosphate coating. The immersion time was adjusted to form a coating of zinc phosphate with needle-shaped crystals having the crystal diameters shown in Table 7. To form a metal salt coating, the specimen was immersed in a surface conditioning treatment agent (Preparen X, manufactured by Nihon Parkerizing) at room temperature for 20 seconds. The specimen was then further immersed in a metal salt treatment agent at 40°C to form a metal salt coating. The immersion time was adjusted to form a metal salt coating having the thickness shown in Table 7.

[0108] Details of each metal salt shown in Table 7 are as follows. Ti salt = amorphous titanium oxide film (hexafluorotitanic acid, metal salt treatment agent prepared to provide 0.2 Ti equivalent / kg) Zr salt = amorphous zirconium oxide coating (Palcido 1500, manufactured by Nihon Parkerizing) Hf salt = amorphous hafnium oxide film (hexafluorohafnic acid, metal salt treatment agent prepared to give 0.2 Hf / kg equivalent) V salt = amorphous vanadium oxide coating (ammonium metavanadate, metal salt treatment agent prepared to provide a V equivalent of 0.2 / kg)

[0109] Next, a cationic electrocoating paint manufactured by Nippon Paint was electrocoated onto the organic coating and the phosphate or metal salt coating at a voltage of 160 V and baked at a baking temperature of 170°C for 20 minutes to form an electrocoated film having a thickness of 10 μm. However, in some cases, electrodeposition coating was not possible.

[0110] Test steel plates were prepared by the above procedures.

[0111] (evaluation) -Electrodeposition paintability- The electrodeposition paintability was evaluated based on the adhesion of the electrodeposition coating film to the test steel plate of each example. The electrodeposition paintability was evaluated by creating 100 1mm squares on the electrodeposition coating film with a cutter knife and applying polyester tape. The tape was then peeled off and the number of peeled squares was counted to determine the peeling rate (%) of the electrodeposition coating film. Evaluation was then conducted according to the following criteria. 5: Peeling rate less than 1% 4: Peeling rate more than 1% but less than 5% 3: Peeling rate more than 5% but less than 10% 2: Peeling rate over 10% 1: Electrodeposition coating is not possible

[0112] -Corrosion resistance- The test steel plate of each example was subjected to 90 cycles of corrosion in accordance with JASO-M609, and then the area ratio (%) of red rust formation was evaluated according to the following criteria. 5: No red rust 4: Red rust occurrence area rate: over 0% and up to 5% 3: Red rust occurrence area rate is over 5% and 20% or less 2: Red rust occurrence area rate is over 20% and 30% or less 1: Red rust occurrence area rate over 30%

[0113] [Table 1]

[0114] [Table 2]

[0115] [Table 3]

[0116] [Table 4]

[0117] [Table 5]

[0118] [Table 6]

[0119] [Table 7-1]

[0120] [Table 7-2]

[0121] From the above results, it can be seen that the present example has excellent electrodeposition coating properties as well as corrosion resistance compared to the comparative example.

Claims

1. A steel sheet comprising a plated base steel sheet having a zinc-based plating layer provided on at least one surface thereof, and an organic coating provided on the zinc-based plating layer in direct contact with the zinc-based plating layer, the arithmetic mean roughness Ra of the plated base steel sheet is 0.5 to 2.3 μm and the number of peaks per inch (PPI) is 30 to 150; the organic coating has a thickness of 0.3 to 2.1 μm and is smaller than the arithmetic mean roughness Ra of the plated base steel sheet; The steel sheet has protruding portions of the zinc-based plating layer that protrude from the organic coating.

2. The organic coating is a polyurethane having a silanol group and / or an alkoxysilyl group.

2. The steel sheet according to claim 1, which contains at least one resin selected from the group consisting of resins and ethylene-unsaturated carboxylic acid copolymer resins.

3. 3. The steel sheet according to claim 1, wherein the organic coating contains at least one additive selected from the group consisting of vanadium compounds, silicon oxides, organic titanium compounds, polyolefin waxes, phosphate compounds, thiocarbonyl compounds, niobium oxides, and guanidine compounds.

4. A formed body of the steel plate according to any one of claims 1 to 3; a phosphate coating or a metal salt coating provided on a part or all of the protruding portions of the zinc-based plating layer protruding from the organic coating in the formed steel sheet; an electrodeposition coating film provided on the organic coating film and the phosphate coating film or the metal salt coating film of the steel plate compact; Automotive parts having

5. 5. The automobile part according to claim 4, wherein the phosphate coating is a phosphate coating having needle-shaped crystals.

6. 6. The automobile part according to claim 5, wherein the crystal diameter of the needle-shaped crystals of the phosphate coating is 0.1 to 5 μm.

7. 5. The automotive component according to claim 4, wherein the metal salt coating contains a salt of at least one element selected from the group consisting of iron, titanium, zirconium, hafnium, indium, tin, bismuth, vanadium, nickel, cerium, molybdenum, and tungsten with an oxide ion, a nitrate ion, a sulfate ion, a fluoride ion, a complex fluoride ion, or a carbonate ion, and is a crystalline or amorphous metal salt coating.

8. 8. The automobile part according to claim 7, wherein the thickness of the crystalline or amorphous metal salt coating is 0.01 to 3 μm.

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