Title of Invention

ZINC ELECTRO-PLATED STEEL SHEET HAVING EXCELLENT STAIN RESISTANCE

Abstract

Disclosed is a zinc electro-plated steel sheet comprising a substantially Cr free resin film containing 0.05 to 5% by mass of Na and formed on the zinc electro-plated layer. The zinc electro-plated layer contains at least one element selected from the group consisting of Ni, Fe, Cr, Mo, Sn, Cu, Cd, Ag, Si, Co, In, Ir and W each in an amount expressed in terms of atoms within the following ranges, Ni: 60 to 6000 ppm (ppm means ppm by mass, the same shall apply hereinafter), Fe: 60 to 600 ppm, Cr: 0.5 to 5 ppm, Mo: 30 to 500 ppm, Sn: 0.6 to 20 ppm, Cu: 8 to 3000 ppm, Cd: 0.0001 to 0.02 ppm, Ag: 1.0 to 400 ppm Si: 30 to 2000 ppm, Co: 0.0003 to 0.3 ppm, In: 0.1 to 30 ppm, Ir: 0.01 to 10 ppm, and W: 0.1 to 50 ppm.

Full Text

ZINC ELECTRO-PLATED STEEL SHEET HAVING EXCELLENT STAIN RESISTANCE BACKGROUND OF THE INVENTION Field of the Invention [0001] The present invention relates to an zinc electro-plated (electrogalvanized) steel sheet which has excellent stain resistance (stain resistivity) and a method for producing the same and, more particularly, to a technology for improving stain resistance, capable of effectively suppressing (concealing) unevenness in the appearance of an zinc electroplated steel sheet comprising a resin film containing substantially no Cr due to stain caused by Na in the resin film. The zinc electro-plated (electroplated) steel sheet of the present invention is preferably used in applications mainly for indoor uses, such as chassis and case components for consumer electronics and OA equipments, and steel furnitures. Description of the Related Art [0002] Due to regulation of the use of a toxic substance, an zinc electro-plated steel sheet comprising a chromate-free chemical treatment film containing no hexavalent chromium (nonchromate zinc electro-plated (electrogalvanize) steel sheet) is widely used. Since such a nonchromate zinc electro-plated steel sheet is often used without being coated due to users' requests for omission of a coating, the nonchromate zinc electro-plated steel sheet is sometimes exposed under a high temperature and a humid environment for a long period upon storage of a coil after production, upon working by manufacturers of consumer electronics and OA equipment, and during use by users. [0003] However, the test results of the present inventors revealed that, when the nonchromate zinc electro-plated steel sheet is allowed to stand under a high temperature and a humid environment for such a long period as half a month or more (for example, about 504 hours (21 days)), unevenness in appearance (difference in color tone) occurs in the form of stains on the surface of the steel sheet as shown in Fig. 1C hereinafter. [0004] Such a phenomenon (hereinafter referred sometimes to as "stain") has never been observed in a zinc electro-plated steel sheet subjected to a chromate treatment. The test results of the present inventors also revealed that, unlike corrosion phenomenons which have been reported previously, that is, typically the generation of white rust under a wet environment in the presence of chlorine ions (which is conventionally evaluated 96 hours (4 days) after a salt spray test defined in JIS Z2371) and the generation of blackening under a comparatively mild corrosive environment observed before generation of white rust (initial stage) (which is conventionally evaluated 72 hours (3 days) after a test performed under a constant temperature and constant humidity at 50°C and a relative humidity of 95% or more), when exposing a nonchromate zinc electro-plated steel sheet containing Na in a film under a high temperature and a humid environment, staining is first observed after such a very long period as about half month of more. [0005] However, a technology for improving unevenness in the appearance of a zinc electro-plated steel sheet relates to a method for preventing the white rust or the blackening phenomenon before the generation of the white rust (see, for example, JP 304336B, JP3499544B, JP 3499543B and JP 2004-263252A) and a technology for improving unevenness in appearance, which has an object of preventing staining, has never been provided. SUMMARY OF THE INVENTION [0006] Under these circumstances, the present invention has been made and an object of the present invention is to provide a nonchromate zinc electro-plated steel sheet having not only excellent white rust resistance, but also stain resistance, and a method for producing the same. [0007] The zinc electro-plated steel sheet of the present invention, which can achieve the above object, includes a substantially Cr free resin film containing 0.05 to 5% (% means % by mass, the same shall apply hereinafter) of Na formed on the zinc electro-plated layer, wherein the zinc electro-plated layer contains at least one element selected from the group consisting of Ni, Fe, Cr, Mo, Sn, Cu, Cd, Ag, Si, Co, In, Ir and W each in an amount expressed in terms of atoms within the following ranges, Ni: 60 to 6000 ppm (ppm means ppm by mass, the same shall apply hereinafter), Fe: 60 to 600 ppm, Cr: 0.5 to 5 ppm. Mo: 30 to 500 ppm, Sn: 0.6 to 20 ppm, Cu: 8 to 3000 ppm, Cd: 0.0001 to 0.02 ppm, Ag: 1.0 to 400 ppm Si: 30 to 2000 ppm, Co: 0.0003 to 0.3 ppm, In: 0.1 to 30 ppm, Ir: 0.01 to 10 ppm, and W: 0.1 to 50 ppm. [0008] According to a preferred embodiment, the resin film contains a carboxyl group-containing resin and a Si-based inorganic compound. Typical examples of the Si-based inorganic compound include colloidal silica. [0009] According to a preferred embodiment, the resin film further contains a silane coupling agent. [0010] The method for manufacturing a zinc electro-plated steel sheet of the present invention, which can achieve the above object, includes the steps of: (1) zinc electro-plating using an acidic plating solution containing at least one element selected from the group consisting of Ni: 20 to 2000 ppm, Fe?+: 50 to 5000 ppm, Fe3+: 50 to 5000 ppm, Cr: 5 to 2000 ppm, Mo: 50 to 2000 ppm, Sn: 0.05 to 20 ppm, Cu: 0.05 to 50 ppm, Cd: 0.05 to 5 ppm, Ag: 0.05 to 5ppm, Si: 20 to 2000 ppm, Co: 0.05 to 50 ppm, In: 0.5 to 50 ppm, Ir: 0.05 to 5 ppm and W: 0.5 to 50, and (2) forming a resin film containing 0.05 to 5% by mass of Na. [0011] Since the electro-plated steel sheet of the present invention is constituted as described above, white rust resistance and stain resistance of the nonchromate-treated steel sheet are remarkably improved. BRIEF DESCRIPTION OF THE DRAWINGS [0012] FIG. 1A is a photograph of No. 2 of Example 1. FIG. IB is a photograph of No. 4 of Example 1. FIG. 1C is a photograph of No. 6 of Example 1. DETAILED DESCRIPTION OF THE INVENTION [0013] A "stain (staining)" phenomenon has never been recognized in a conventional chromate-treated steel sheet. Test results of the present inventors revealed that a stain phenomenon is observed by exposing a zinc electro-plated steel sheet containing Na in a chromate-free resin film under a high temperature and a humid environment for a long time (about 504 hours). Na is added with the primary objective of improving abrasion resistance (scratch resistance) by increasing the strength of the chromate-free resin film (details are described hereinafter) , and a nonchroitiate resin film containing Na is widely used for nonchromate-treated steel sheets. [0014] Thus, the present inventors have intensively studied so as to provide a technology capable of preventing (concealing) unevenness in the appearance of stain caused by storing a nonchromate zinc electro-plated steel sheet, which is not subjected to a chromate treatment, under a high temperature and a humid environment for a long period. [0015] As a result, the present inventors have found that stain resistance is improved if the content of a specific element(s) which is described in detail hereinafter (sometimes referred to as a "stain resistance improving element") among impurity elements, which inevitably exist attributing to a raw material and so on for Zn plating, is controlled within a predetermined range. Thus, the present invention has been completed. [0016] Although not related to a stain resistance improving technology like the present invention, the aforementioned JP 304336B, JP3499544B, JP 3499543B and JP 2004-263252A describe a method for improving white rust resistance by controlling the content of predetermined elements in an zinc electroplated layer. These patent documents disclose some elements used in the present invention, of which a portion of the concentration overlaps that of an element of the present invention. Specifically, JP 304336B, JP3499544B and JP 3499543B disclose a method for preventing unevenness in appearance such as white rust by adding elements (Ni, In, Cu, Ag, Co), which are more noble than Zn, in a plating bath. JP 2004-263252A discloses a method for the same purpose by adding elements (Fe, Co, Ni, Mn, Mg, Al, Ce, In), which form slightly soluble hydroxides in an alkaline region where Zn dissolves, and elements (Si, Ti, V, Mo, Zr), which are stable in a neutral region and also can stably exist in a corrosive environment, in a plating bath. [0017] As described hereinafter, it is considered that the unevenness in appearance, which is of interest to present invention, differs in cause and also differs in occurrence mechanism from that of these paten documents. [0018] Namely, the unevenness in the appearance of "stain", which is of interest to the present invention, arises only by exposing a nonchromate zinc electro-plated steel sheet under a high temperature and a humid environment for a long period and it is considered that such stain is different in the occurrence mechanism from the unevenness in the appearance of white rust generated under a salt (salt water) atmosphere or blackening generated by exposing under a high temperature and a humid environment for a short time. It is also considered that the "stain" is different from unevenness in the appearance of white rust and blackening in the occurrence mechanism with respect where it is a phenomenon first seen in the case where the nonchromate treatment film contains Na. It is impossible to find any description about the use of a Na-containing nonchromate film even when these patent documents are checked in detail. [0019] As shown in Examples hereinafter, some of the stain resistance improving elements kinds and their contents used in the present invention are different from those of the white rust resistance improving element whose effect has been confirmed in the above patent documents, and it has been found that it is difficult to apply the methods disclosed in the above patent documents to the stain resistance improving technology as they are. [0020] The zinc electro-plated steel sheet of the present invention is described in detail as below. [0021] The zinc electro-plated steel sheet of the present invention is a zinc electro-plated steel sheet which comprises a substantially Cr free resin film containing 0.05 to 5% by mass of Na and formed on a zinc electro-plated layer, wherein the zinc electro-plated layer contains at least one element (stain resistance improving element) selected from the group consisting of Ni, Fe, Cr, Mo, Sn, Cu, Cd, Ag, Si, Co, In, Ir and W each in an amount expressed in terms of atoms within the following ranges, Fe: 60 to 600 ppm, Cr: 0.5 to 5 ppm, Mo: 30 to 500 ppm, Sn: 0.6 to 20 ppm, Cu: 8 to 3000 ppm, Cd: 0.0001 to 0.02 ppm, Ag: 1.0 to 400 ppm, Si: 30 to 2000 ppm, Co: 0.0003 to 0.3 ppm, In: 0.1 to 30 ppm, Ir: 0.01 to 10 ppm, and W: 0.1 to 50 ppm. [0022] A photograph of a zinc electro-plated steel sheet in which staining has occurred is shown in FIG. 1. FIG. 1 is a photograph (5 cm * 5 cm] of a zinc electro-plated steel sheet comprising a Na-containing nonchromate film which was stored under a high temperature and a humid environment at a temperature of 50°C and relative humidity of 95% for 504 hours (21 days), and FIG. 1A to FIG. 1C respectively show Examples No. 2 (evaluation criteria 3, staining occurs), No. 4 (evaluation criteria 2, slight staining occurs) and No. 6 (evaluation criteria 3, no staining occurs) shown in Table 1 of Example 1 described hereinafter. As shown in FIG. 1A, in. the zinc electro-plated steel sheet, the blackish region in which the surface of the film discolored to a blackish dark brown color (the portion where Na is aggregated) and the whitish region in which the surface of the film did not discolor (the portion where Na is not aggregated) coexist, and a stain-like mottled pattern (stain) is observed. The main cause of unevenness in the appearance of stain is considered to be only the portion where Na is aggregated is discolored. [0023] The mechanism capable of effectively preventing stain by the addition of the stain resistance improving element used in the present invention is unclear, but is considered to be as follows. Namely, since formation of a zinc electro- plated layer containing a predetermined amount of the above elements exerts an influence on a crystalline morphology of Zn plating and an oxide of the surface (e.g. a hydroxide layer of Zn, containing the above additional elements formed inevitably on the surface of Zn plating), it becomes possible to scarcely eliminate a difference in color tone between the portion where Na is aggregated and the portion where Na is not aggregated. As a result, unevenness in appearance due to the stain can be eliminated. [0024] All of the above elements have a stain resistance improving action and are roughly classified into (1) elements (Ni, Fe, Sn, Cd, Ir, In, Cu, Ag, Co) which are more noble than Zn, and (2) elements (Cr, Mo, Si, W) which are not more noble than Zn but form an oxide. These elements may be used alone, or two or more kinds of them may be used in combination. [0025] To effectively exert the stain resistance improving action of the above elements, the content of each element in a zinc electro-plated layer is controlled to amounts expressed in terms of atoms within the following range, Ni: 60 ppm or more (preferably 600 ppra or more), Fe: 60 ppm or more (preferably 80 ppm or more), Cr: 0.5 ppm or more (preferably 0.8 ppm or more), Mo: 30 ppm or more (preferably 100 ppm or more), Sn: 0.6 ppm or more (preferably 1.5 ppm or more), Cu: 8.0 ppm or more (preferably 100 ppm or more), Cd: 0.0001 ppm or more (preferably 0.01 ppm or more), Ag: 1.0 ppm or more (preferably 30 ppm or more), Si: 30 ppm or more (preferably 80 ppm or more), Co: 0.0003 ppm or more (preferably 0.001 ppm or more), In: 0.1 ppm or more (preferably 1.0 ppm or more), Ir: 0.01 ppm or more (preferably 0.1 ppm or more), and W: 0.1 ppm or more (preferably 1.0 ppm or more) (see Examples described hereinafter). [0026] When these elements are excessively added, the following problems arise. First, when Fe and Si are excessively added, as shown in Examples described below, the stain resistance improving action becomes lower and also corrosion resistance (particularly, white rust resistance) deteriorates. In contrast, when the stain resistance improving elements except for Fe and Si are excessively added, as shown in Examples described below, white rust resistance deteriorates, although stain resistance is good. To satisfy both characteristics of stain resistance and white rust resistance and to obtain an excellent appearance of the surface, the content of each element in the zinc electroplated layer is controlled to, Ni: 6000 ppm or less, Fe: 600 ppm or less, Cr: 5.0 ppm or less, Mo: 500 ppm or less, Sn: 20 ppm or less, Cu: 3000 ppm or less, Cd: 0.02 ppm or less, Ag: 400 ppm or less, Si: 2000 ppm or less, Co: 0.3 ppm or less, In: 30 ppm or less, Ir: 10 ppm or less, and W: 50 ppm or less. [0027] Of these elements, Ni, Fe, Cr, Mo, Si, Cu, Co, W, In, Cu, and Ag are preferred as an element having the stain resistance improving action, and Ni, Fe, Mo, Cr, and W are more preferred. [0028] The amount of the stain resistance improving element contained in the zinc electro-plated layer can be measured, for example, using an atomic absorption analysis, inductively coupled plasma emission spectral analysis (ICP) or inductively coupled plasma mass spectrometry (ICP-MS) method. A detailed analysis method is described in the Examples described below. Analysis is preferably carried out after diluting a plated layer with hydrochloric acid etc. so as to eliminate a measurement error caused by a matrix element such as Zn, Na, or S contained in a plating solution. Dilution ratio may be appropriately adjusted to a proper range according to the concentration of the matrix element and the amount of the stain resistance improving element to be measured. In Examples described hereinafter, the content of the element in the plated layer is determined after diluting the plated layer with double-diluted hydrochloric acid. [0029] The coating weight of zinc electro-plating is preferably about 40 g/m2 or less, and more preferably about 30 g/m2 or less, taking account of the crystal size of a Zn single crystal deposited on the surface of the plated layer. The lower limit is not specifically limited from the view point of the crystal size, but is preferably about 3 g/m2, and more preferably about 10 g/mz, taking account of the sacrificial protection action of Zn. [0030] The zinc electro-plated layer may be provided at least on a predetermined surface of a steel sheet as a base material, or may be provided on either or both surfaces of the steel sheet. [0031] The resin film (nonchromate resin film) contains Na in an amount of about 0.05 to 5% (preferably, 0.1% or more and 3% or less, and more preferably 1% or less). Na is usually added in the carboxyl group-containing resin or colloidal silica in order to increase the strength of the nonchromate resin film {preferably containing a carboxyl group-containing resin and a Si-based inorganic compound such as colloidal silica). When the content of Na is less than 0.05%, for example, Na crosslinking between carboxyl groups and Na in the carboxyl group-containing resin is not sufficiently formed thereby decreasing the strength of the film. In contrast, when the content of Na is more than 5%, the amount of soluble Na contained in the film increases and abrasion resistance deteriorates. The amount of Na contained in the resin film is expressed by the sum total of Na in the solid content of the respective components constituting the resin film (the resin component, the Si-based inorganic compound, and the silane coupling agent contained optionally). [0032] The resin film is substantially Cr free. As used herein, the expression "substantially Cr free" means that the amount of Cr to be inevitably included in the process of forming the resin film is allowable. In the present invention, for example, a trace amount of Cr as the stain resistance improving element is sometimes added in the plated layer and the added Cr in the plated layer sometimes contaminates the resin film. In addition, for example, when a trace amount of a Cr compound is eluted from a production vessel and a coating device in the process of preparing and applying a treatment solution used in a nonchromated resin film, Cr may be included in the resin film. In such a case, the amount of Cr contained in the resin film is preferably 0.01% or less. [0033] The resin film preferably contains a resin component of a carboxyl group-containing resin, and a Si-based inorganic compound (typically, colloidal silica). By forming a resin film containing them, corrosion resistance, alkaline degreasing resistance and coatability of the film are improved. [0034] The carboxyl group-containing resin is not specifically limited as long as it has carboxyl groups and includes, for example, a polymer synthesized by polymerizing using a monomer having carboxyl groups such as an unsaturated carboxylic acid as a portion or all of the raw materials, and a resin modified with carboxylic acid utilizing a functional group reaction. [0035] As the carboxyl group-containing resin, commercially available products may be used and the carboxyl group-containing resin includes, for example, Hiteck 53141 (manufactured by TOHO Chemical Industry Co., Ltd.). The resin component may contain an organic resin other than the carboxyl group-containing resin. [0036] The Si-based inorganic compound includes, for example, silicate and/or silica. These compounds may be used alone, or two or more types of them may be used in combination. [0037] Of these compounds, the silicate includes, for example, potassium silicate and lithium silicate. [0038] Typical examples of the silica include colloidal silica and scaly silica. In addition, dry silica such as ground silica, gas phase process silica, silica sol, or fumed silica may be used. [0039] Of these compounds, colloidal silica is used particularly preferably. Consequently, not only the strength of the resin film increased, but also silica is concentrated at the flaw portion of the film under a corrosive environment, and thus corrosion of Zn is suppressed and corrosion resistance is further enhanced. [0040] As the colloidal silica, commercially available products may be used and the colloidal silica includes, for example, SNOWTEX series "ST-40", "ST-XS", "ST-N", "ST-20L", "ST-UP", "ST-ZL", "ST-SS", "ST-O" and "ST-AK" manufactured by Nissan Chemical Industries, Ltd. These products usually contain Na. [0041] The mass ratio of the resin component and the Si-based inorganic compound (typically, colloidal silica), which constitute the resin film, resin component:Si-based inorganic compound, is preferably within a range from about 5 parts to 45 parts:55 parts to 95 parts. When the content of the resin component is small, corrosion resistance, alkaline degreasing resistance and coatability tend to decrease. In contrast, when the content of the resin component is large, abrasion resistance and electrical conductivity may deteriorate. Also, when the content of the Si-based inorganic compound is small, abrasion resistance and electrical conductivity tend Co deteriorate. In contrast, when the content of the Si-based inorganic compound is large, the content of the resin component decreases, and thus film forming properties of the resin film deteriorate and corrosion resistance deteriorates. [0042] The resin film may further contain a silane coupling agent. Since the addition of the silane coupling agent makes the bond between the carboxyl group-containing resin £nd the Si-based inorganic compound strong, elution of Na ions decreases and stain resistance is further improved. [0043] The silane coupling agent preferably has a lower alkoxy group such as an alkyl group having 1 to 5 carbon atoms, an allyl group, or an aryl group. Specific examples thereof include glycidoxy group-containing silane coupling agents such as y-glycidoxypropyltrimethoxysilane, y-glycidoxypropylmethyldimethoxysilane, y-glycidoxypropyltriethoxysilane, and y- glycidoxymethyldimethoxysilane; amino group-containing silane coupling agents such as Y_aminopropyltrimethoxysilane, y-aminopropyltriethoxysilane, W-(/3-aminoethyl?-y-aminopropyltrimethoxysilane, and N-(p-aminoethyl)-y-aminopropylmethyldimethoxysilane; vinyl group-containing silane coupling agents such as vinyltrimethoxysilane, vinyltriethoxysilane, and vinyltris(J3-methoxyethoxy)silane; methacryloxy group-containing silane coupling agents such as y-methacryloxypropyltrimethoxysilane; mereapto group-containing silane coupling agents such as y-mercaptopropyltrimethoxysilane and y- mercaptopropylmethyldimethoxysilane; and halogen group-containing silane coupling agents such as y- chloropropylmethoxysilane and y-chloropropyltrimethoxysilane. These silane coupling agents may be used alone, or two or more kinds of them may be used in combination. [0044] Of these silane coupling agents, glycidoxy group-containing silane coupling agents are preferably used because of particularly high reactivity and excellent corrosion resistance and alkaline resistance. [0045] As the silane coupling agent, commercially available products may be used and the silane coupling agent includes, for example, y-glycidoxypropyltrimethoxysilane "KBM4 03" (manufactured by Shin-Etsu Chemical Co., Ltd.). [0046] The content of the silane coupling agent is preferably about 5 parts by mass or more and 25 parts by mass or less, based on 100 parts by mass of the total amount of the resin component and the Si-based inorganic compound. When the content of the silane coupling agent is small, not only the stain resistance improving action is not effectively exerted, but also reactivity above mentioned between the carboxyl group-containing resin and the Si-based inorganic compound deteriorates and abrasion resistance, coatability and corrosion resistance deteriorate. In contrast, when the content of the silane coupling agent is large, stability of a film forming agent used to form the resin film may deteriorate to cause gelling. Also, the amount of the silane coupling agent, which does not contribute to the reaction, increases, and thus adhesion between the Zn plated layer and the resin film may become lower. [0047] The case where the following resin film was used as a typical nonchromate resin film used in the present invention is described below. This resin film is an improved urethane resin film disclosed by the present applicant and details are as described in JP 2006-43913A (see, for example, paragraphs [0020] to [0071]). While the constitution of the resin film and the method for forming the resin film is briefly described below, it is to be understood that the resin film used in the present invention is not to be limited to these specific examples. [0048] The resin film is obtained from the following aqueous resin solution. The aqueous resin solution contains 5 to 45 parts by mass of an aqueous carboxyl group-containing polyurethane resin solution and an aqueous ethylene-unsaturated carboxylic acid copolymer dispersion as nonvolatile resin components, and 55 to 95 parts by mass of silica particles having an average particle size of 4 to 20 nm in a total amount of 100 parts by mass, and further contains a silane coupling agent in the amount of 5 to 25 parts by mass based on 100 parts by mass of the total amount of the above components. Furthermore, the mixing ratio of the nonvolatile resin component of the aqueous polyurethane resin solution (PU) to that of the aqueous ethylene-unsaturated carboxylic acid copolymer dispersion (EC), PU:EC, is from 9:1 to 2:1 in terms of a mass ratio. [0049] First, the aqueous carboxyl group-containing polyurethane resin solution is described as follows. [0050] As the aqueous carboxyl group-containing polyurethane resin solution, either of an aqueous dispersion in which a carboxyl group-containing polyurethane resin is dispersed in aqueous medium, and an aqueous solution in which the carboxyl group-containing polyurethane resin is dissolved in an aqueous medium can be used. The aqueous medium may contain, in addition to water, a trace amount of a hydrophilic solvent such as an alcohol, N-methyl pyrrolidone or acetone. [0051] The carboxyl group-containing polyurethane resin is preferably obtained by a chain extension reaction of a urethane prepolymer using a chain extender, and the urethane prepolymer is obtained, for example, by reacting a polyisocyanate component and a polyol component which are described hereinafter. [0052] As the polyisocyanate component constituting the urethane prepolymer, for example, at least one kind of a polyisocyanate selected from the group consisting of tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI) and dicyclohexylmethane diisocyanate {hydrogenated MDI) is preferably used. Herein, as the polyol component constituting the urethane prepolymer, all three kinds of polyols such as 1,4-cyclohexane dimethanol, polyetherpolyol, and a polyol having a carboxyl group is used, and preferably all three kinds are diols. Also, the polyetherpolyol is not specifically limited as long as it has at least two hydroxyl groups in a molecular chain and a main skeleton is composed of an alkylene oxide unit, and examples thereof include polyoxyethylene glycol, polyoxypropylene glycol, and polyoxytetramethylene glycol. [0053] Examples of the chain extender used in the chain extension reaction of the urethane prepolymer include, but are not limited to, a polyamine, a low molecular weight polyol, and an alkanolamine, [0054] In the preparation of the aqueous solution of the carboxyl group-containing polyurethane resin, a known method can be employed. Examples thereof include a method in which carboxyl groups of the carboxyl group-containing urethane prepolymer are neutralized with a base and the carboxyl group-containing urethane prepolymer is emulsified and dispersed in an aqueous medium, thereby causing a chain extension reaction, and a method in which the carboxyl group-containing polyurethane resin is emulsified and dispersed by a high shear force in the presence of an emulsifier, thereby causing a chain extension reaction. [0055] The aqueous ethylene-unsaturated carboxylic acid copolymer dispersion is described. [0056] The aqueous ethylene-unsaturated carboxylic acid copolymer dispersion is not specifically limited as long as it is a solution in which an ethylene-unsaturated carboxylic acid copolymer is dispersed in an aqueous medium, and the ethylene-unsaturated carboxylic acid copolymer is a copolymer of ethylene and an ethylenically unsaturated carboxylic acid. Examples of the unsaturated carboxylic acid include (rtieth) acrylic acid, crotonic acid, isocrotonic acid, maleic acid, fumaric acid, and itaconic acid. A copolymer can be obtained by polymerizing at least one kind of these unsaturated carboxylic acids with ethylene using a known high-temperature and high-pressure polymerization method. [0057] The ethylene-unsaturated carboxylic acid copolymer has carboxyl groups and an aqueous dispersion can be prepared by neutralizing the carboxyl groups with an organic base (for example, an amine having a boiling point of 100°C or lower) or monovalent metal ions, such as Na. [0058] The monovalent metal ions are used for neutralisation as described above and are effective to improve solvent resistance and film hardness. A compound of a monovalent metal preferably contains one or more kinds of metals selected from among sodium, potassium, and lithium, and hydroxides, carbonates or oxides of these metals are preferred. Of these compounds, NaOH, KOH, and LiOH are preferred and NaOH is more preferred because of most excellent performances. The present invention improves a stain phenomenon originating in NaOH. [0059] The amount of the compound of the monovalent metal is preferably adjusted within a range from 0.02 to 0.4 mols (2 to 40 mol%) based on 1 mole of carboxyl groups in the ethylene-unsaturated carboxylic acid copolymer. When the amount of the metal compound is less than 0.02 mols, emulsion stability becomes insufficient. In contrast, the case where the amount of the metal compound is more than 0.4 mols is not preferred, because hygroscopicity (particularly to an alkaline solution) of the resulting resin film enhances and corrosion resistance after the degreasing step deteriorates. The lower limit of the metal compound is more preferably 0.03 mols, and still more preferably 0.1 mols, whereas, the upper limit of the metal compound is more preferably 0.5 mols, and still more preferably 0.2 mols. [0060] The case where the total amount {neutralization amount) of the organic base (preferably, an amine having a boiling point of 100CC or lower) and the monovalent metal compound is too large is not preferred because viscosity of an aqueous dispersion rapidly increases to cause solidification, and also an excessive alkaline component can cause deterioration of corrosion resistance and much energy is required so as to volatilize the excessive alkaline component. However, too small a neutralization amount is not preferred because of poor emulsiflability. Therefore, the total amount of the organic base and the monovalent metal compound is preferably adjusted within a range from 0.3 to 1.0 mols based on 1 mol of carboxyl groups in the ethylene-unsaturated carboxylic acid copolymer. [0061] The aqueous ethylene-unsaturated carboxylic acid copolymer dispersion, in which very fine particles (oil droplets) having an average particle size (particle diameter)of 5 to 50 nm are dispersed in an aqueous medium, is obtained by emulsification using the organic base in combination with the monovalent metal ion. Therefore, it is considered that film forming properties of the resulting resin film, adhesion to a metal sheet and densification of the film are achieved and thus corrosion resistance is improved. The aqueous medium may contain, in addition to water, a hydrophilic solvent such as an alcohol or an ether. The particle size of resin particles of the aqueous dispersion can be measured, for example, by a laser diffraction method using a light scattering photometer (manufactured by Otsuka Electronics Co., Ltd.). [0062] The aqueous ethylene-unsaturated carboxylic acid copolymer dispersion is prepared by charging an ethylene-unsaturated carboxylic acid copolymer in a homogenizer, together with an aqueous medium, optionally heating to a temperature within a range from 70 to 250°C, and adding an organic base such as an amine having a boiling point of 100°C or lower and a compound of a monovalent metal in an appropriate form such as aqueous solution (adding the amine having a boiling point of 100°C lower in advance, or nearly simultaneously adding the amine having a boiling point of 100°C lower and the monovalent metal), followed by stirring through a high shear force. [0063] Next, the aqueous carboxyl group-containing polyurethane resin solution and the aqueous ethylene-unsaturated carboxylic acid copolymer dispersion obtained by the above method are mixed with silica particles and a silane coupling agent, and optionally mixed with a wax and a crosslinking agent to obtain a desired aqueous resin solution. The silica particles, the silane coupling agent, the wax and the crosslinking agent may be added in any stage. However, after adding the crosslinking agent and the silane coupling agent, it is preferred that the mixture is not heated so as not to cause gelling as a result of proceeding of the crosslinking reaction. A typical resin film used in the present invention is described above. [0064] The resin film may contain conventional components (for example, antiskinning agents, leveling agents, defoamers, penetrants, emulsifiers, film-forming auxiliaries, color pigments, lubricants, surfactants, conductive additives for imparting conductivity, thickeners, dispersing agents, desiccants, stabilizers, fungicides, antiseptics, and antifreezing agents) as long as the operation of the present invention is not impaired. [0065] The thickness of the resin film is preferably within a range from about 0.1 to 2 urn, and more preferably from about 0.2 to 1.0 lira. When the thickness of the resin film is less than 0.1 urn, corrosion resistance deteriorates. In contrast, when the thickness of the resin film is more than 2 um, conductivity deteriorates. [0066] On the resin film, a film such as an organic resin film, an organic-inorganic composite film, an inorganic film, or an electrodeposited film may be provided so as to improve corrosion resistance (particularly, white rust resistance) and coatability. [0067] Examples of the organic resin film includes a film formed by using a known resin for coating, for example, a urethane-based resin; an epoxy resin; an acrylic resin; an olefin-based resin such as polyethylene, polypropylene and an ethylene-acrylic acid copolymer; a styrene-based resin such as a polystyrene; a polyester; and a copolymer and a modified product thereof, in combination with colloidal silica, a solid lubricant and a crosslinking agent, if necessary. [0063] Typically, the organic-inorganic composite film includes a film formed by using the organic resin in combination with a soluble glass forming component such as sodium silicate. [0069] Typically, the inorganic film includes a soluble glass film, and a film formed of lithium silicate. [0070] The method for producing a nonchromate zinc electroplated steel sheet of the present invention is described below, [0071] First, a steel sheet substrate (original sheet for plating) serving as a base material is prepared. The steel sheet substrate is not specifically limited as long as it is usually used as a zinc electro-plated steel sheet and, for example, various steel sheets such as a common steel sheet, an Al killed steel sheet, and a high tensile steel sheet. The original sheet for plating is preferably subjected to a pretreatment such as degreasing or acid pickling before zinc electro-plating is conducted. [0072] Next, a zinc electro-plated layer is formed on the steel sheet substrate using a zinc electro-plating method to produce a zinc electro-plated steel sheet. [0073] In an acidic bath used in zinc electro-plating, at least one element selected from the group consisting of Ni:20 to 2000 ppm, Fe2+: 50 to 5000 ppm, Fe3+: 50 to 5000 ppm, Cr: 5 to 2000 ppm, Mo: 50 to 2000 ppm, Sn: 0.05 to 20 ppm, Cu: 0.05 to 50 ppm, Cd: 0.05 to 5 ppm, Ag: 0.05 to 5 ppm, Si: 20 to 2000 ppm, Co: 0.05 to 50 ppm, In: 0.5 to 50 ppm, Ir: 0.05 to 5 ppm, and W: 0.5 to 50 ppm is added in an acidic solution such as sulfuric acid or hydrochloric acid so as to form a desired plated layer. When the amount of elements added is less than the lower limit, stain resistance is not effectively exhibited. In contrast, when the amount of elements added is more than the upper limit, characteristics such as stain resistance and white rust resistance deteriorate (see Examples described hereinafter). f0074] The preferred amount of each element is as follows: Ni: 200 ppm to 2000 ppm, Fe2+: 200 ppm to 2000 ppm, Fe3+: 500 ppm to 2000 ppm, Cr: 50 ppm to 2000 ppm, Mo: 200 ppm to 2000 ppm, Sn; 0.5 ppm to 5 ppm, Cu: 2 ppm to 50 ppm, Cd: 0.5 ppm to 5 ppm, Ag: 0.5 ppm to 5 ppm, Si: 50 ppm to 800 ppm, Co: 0.5 ppm to 5 ppm, In: 2 ppm to 20 ppm, Ir: 0.5 ppm to 5 ppm, and W: 2 ppm to 50 ppm. [0075] The form of the elements to be added in the plating bath is not specifically limited and any form can be adopted as long as the amount of each element to be added in terms of atoms satisfies the above composition range. For example, the elements may be added in the plating solution in a metal state such as a metal powder or a metal foil, or may be added in the form of a metal salt such as a sulfate, a chloride, a phosphate, a carbonate, or an oxide salt. If the elements are added in the form of a metal salt, the valency of the element is not specifically limited and a conventional valency can be adopted. For example, Cr may be trivalent or hexavalent. Mo and W may be either tetravalent and hexavalent. As shown in Examples described below, the elements may be added in the form of a hydrate. [0076] In the plating solution, other conventionally added components may be added, in addition to the above elements. For example, conductive auxiliary agents such as Na2S04, (NH4)2S04, KC1, and NaCl may be added so as to reduce power consumption by enhancing conductivity. [0077] The method of the present invention is characterized by adding a predetermined amount of the above elements having the stain resistance improving action in the plating solution to form a desired zinc electro-plated layer, and other plating conditions are appropriately decided as long as the operation of the present invention is not impaired, but are preferably controlled as follows. [0078] The pH of the plating solution is preferably within a range from 0.5 to 4.0, and more preferably from 1.0 to 2.0, taking account of a relation between current efficiency and a burnt plating phenomenon. [0079] The temperature of the plating solution is preferably adjusted within a range from 50 to 70°C. [0080] The relative flow rate of the plating solution is preferably within a range from about 0.3 to 5 m/sec. As used herein, relative flow rate means the difference between a rate in a flow direction of the plating solution and a rate in a passing direction of a steel sheet as an original sheet for plating. [0081] The type of electrode (anode) used in electro-plating is not specifically limited as long as it is conventionally used. The electrode includes, for example, lead-based electrodes such as a Pb-Sn electrode, a Pb-In electrode, a Pb-Ag electrode, and a Pb-In-Ag electrode; iridium oxide electrodes; and zinc electrodes. [0082] As a plating cell, either of vertical and horizontal type cells can be used. The zinc electro-plating (electrogalvanizing) is not specifically limited and includes, for example, a constant current plating method and a pulse plating method. [0083] After forming the plated layer as described above, a resin film (nonchromate film) is formed in the following manner. Before forming the resin film, the surface of the plated layer may be subjected to a known pretreatment using ,for example, Co, Ni, Mo, V, a phosphate, or an amine such as a nitrate for the purpose of improving film adhesion, improving corrosion resistance and controlling appearance. [0084] Specifically, first, a chromate-free chemical treatment solution, which contains a predetermined amount of a resin component such as a carboxyl group-containing resin and a Si-based inorganic compound and also preferably contains a predetermined amount of a silane coupling agent (hereinafter simply referred to sometimes as "treatment solution"), is prepared. The treatment solution is prepared by dissolving or dispersing the following components in an aqueous solvent capable of completely dissolving the components (for example, hydrochloric acid, nitric acid solution, etc.). [0085] The mass ratio of the resin component and the Si-based inorganic compound contained in the treatment solution, resin component: Si-based inorganic compound, is preferably within a range from 5 parts to 45 parts:55 parts to 95 parts. When the amount of the resin component such as a carboxyl group-containing resin is small, corrosion resistance, alkaline degreasing resistance and coatability tend to deteriorate. In contrast, when the amount of the resin component is large, abrasion resistance and conductivity may deteriorate. Also, when the amount of colloidal silica is small, abrasion. resistance and conductivity tend to deteriorate. In contrast, when the amount of colloidal silica is large, since the amount of the resin component decreases, film forming properties of the resin film deteriorate and corrosion resistance may deteriorate. [0086] The treatment solution may further contain a silane coupling agent. The content of the siiane coupling agent in the treatment solution is preferably within a range from about 5 to 25 parts by mass, based on 100 parts by mass of the total of the resin component and the Si-based inorganic compound, as shown in Examples described hereinafter. When the content of the silane coupling agent is small, not only the stain resistance improving action is not effectively exerted, but also reactivity between, the carboxyl group-containing resin and the Si-based inorganic compound becomes lower and abrasion resistance, coatability and corrosion resistance deteriorate. In contrast, when the content of the silane coupling agent is large, stability of a film forming agent used to form the resin film may deteriorate to cause gelling. Also, the amount of the silane coupling agent, which does not contribute to the reaction, increases, and thus adhesion between the Zn plated layer and the resin film may become lower. [0087] To the treatment solution, wax and a crosslinking agent may be optionally added, in addition to the above components. Furthermore, the treatment solution may contain conventional components (for example, antiskinning agents, leveling agents, defoamers, penetrants, emulsifiers, film-forming auxiliaries, coloration pigments, lubricants, surfactants, conductive additives for imparting conductivity, thickeners, dispersing agents, desiccants, stabilizers, mildewproofing agents, antiseptics, and antifreezing agents) as long as the operation of the present invention is not impaired. [0088] When the treatment solution containing the above components is applied to either or both surfaces of a metal sheet using a known method, for example, a roll coating method, a spray coating method, a curtain flow coating method, a knife coating method, a bar coating method, a dip coating method, or a brush coating method, heated and then dried, a zinc electro-plated steel sheet comprising a desired resin film can be obtained. [0089] The heating and drying temperature is preferably a temperature at which a crosslinking reaction between the carboxyl group-containing resin and Si-based inorganic compound sufficiently proceeds (for example, a sheet temperature of about 90 to 100°C). Also, when a spherical polyethylene wax is used as the lubricant/ processability in the processing step is improved by maintaining a spherical shape, and therefore drying is preferably conducted at a temperature within a range from about 70 to 130°C. Ex amp1e s [0090] The present invention is further illustrated by the following examples. It is to be understood that the present invention is not limited to the examples, and various changes and modifications can be made from the aspects described hereinbefore and hereinafter. Such changes and modifications are intended to be within the scope of the invention and the appended claims. [0091] Example 1 In this Example 1, the influence of the kind and the content of elements contained in a plated layer on stain resistance and further on white rust resistance was studied. Herein, a resin film is produced in the same manner as in Example 1 of aforementioned JP 2006-43913A (Japanese Patent Application No. 2004-224454). [0092] fl) Preparation of Aqueous Resin Solution A resin film was formed using an aqueous resin solution, which contains an aqueous carboxyl group-containing polyurethane resin solution, an aqueous ethylene-unsaturated carboxylic acid copolymer dispersion, silica particles and a silane coupling agent. The specific method is as follows. [0093] (1-1) Preparation of Aqueous Carboxyl Group-Containing Polyurethane Resin Solution In a synthesizing apparatus (inner volume: 0.8 L) equipped with a stirrer, a thermometer and a temperature controller, 60 g of polytetramethylene ether glycol (average molecular weight: 1000) manufactured by Hodogaya Chemical Co., Ltd. as a polyol component, 14 g of 1,4-cyclohexane dimethanol and 20 g of dimethylolpropionic acid were charged, and then 30.0 g of N-methyl pyrrolidone as a reaction solvent was added, 104 g of tolylene diisocyanate (hereinafter abbreviated sometimes to "TDI") as an isocyanate component was charged and the mixture was reacted by heating between 80 °C and 85°C for 5 hours. The NCO content of the resulting prepolymer was 8.9%. Furthermore, the prepolymer was neutralized by adding 16 g of triethylamine and emulsified at 50°C for 4 hours by adding an aqueous mixed solution of 16 g of ethylenediamine and 480 g of water, followed by a chain extension reaction to obtain an aqueous polyurethane resin dispersion (nonvolatile resin component: 29.1%, acid value: 41.4) . [0094] (1-2) Preparation of Aqueous Ethylene-Unsaturated Carboxylic Acid Copolymer Dispersion In an autoclave of an emulsification device (inner volume: 0,8 L) equipped with a stirrer, a thermometer and a temperature controller, 626 parts by mass of water and 160 parts by mass of an ethylene-acrylic acid copolymer (acrylic acid: 20% by mass, melt index (MI): 300) were added. To 1 mole of carboxyl groups of the ethylene-acrylic acid copolymer, 40 mol% of triethylamine and 15 mol% of sodium hydroxide were added, followed by high speed stirring under an atmosphere of 150°C and 5 Pa and further cooling to 40°C to obtain an aqueous dispersion of the ethylene-acrylic acid copolymer. Subsequently, to the aqueous dispersion, 4,4'-bis(ethyleneiminocarbonylamino)diphenylmethane (CHEMITITE(registered trademark) DZ-22E, manufactured by NIPPON SHOKUBAI CO., LTD.,) as a crosslinking agent was added in the proportion of 5 parts by mass based on 100 parts by mass of the nonvolatile resin component of the ethylene-acrylic acid copolymer. [0095] (1-3) Preparation of Aqueous Resin Solution The aqueous carboxyl group-containing polyurethane resin solution, the aqueous ethylene-acrylic acid copolymer dispersion and colloidal silica ("ST-XS", manufactured by Nissan Chemical Industries, Ltd. average particle size 4 to 6 nm) were mixed in a mixing ratio of 5 parts by mass:25 parts by mass:70 parts by mass, respectively(a total amount of 100 parts by mass expressed in terms of the nonvolatile component). To the 100 parts by mass of the mixture, 10 parts by mass of Y~glycidoxypropyltrimethoxysilane ("KBM403" manufactured by Shin-Etsu Chemical Co., Ltd.) as a silane coupling agent was further added to prepare an aqueous resin solution. [0096] (2) Manufacturing of Zinc Electro-Plated Steel Sheet As a original sheet for plating, an Al killed cold-rolled steel sheet produced by a conventional method was used. This Al killed cold-rolled steel sheet was degreased, acid-pickled and then electroplated by a circulatory type plating apparatus having a plating area measuring 180 mm * 300 mm under the following conditions using a sulfate bath to obtain a zinc electro-plated steel sheet. [0097] (Composition of Plating Solution) Plating solutions, which contain the following components and also contain various elements shown in Table 1 to Table 3 in the amounts within the ranges shown in the tables added therein, were used. As shown in the tables, all of Ni, Fe, Cr, Sn, Cu, Cd, Co and W were added in the forms of sulfate, Mo was added in the form of sodium molybdate, Si was added in the form of colloidal silica, Ag was added in the form of silver nitrate, In was added in the form of a hydroxide, and Ir was added in the form of a bromide, respectively. For comparison, a plating solution, which does not contain any of these elements, was also prepared. 2nSCWH::0 350 g/L Na;SO^ 70 g/L H2S04 20g/L [0098] Other electroplating conditions are as follows. Current density: 100 A/dm2 Plating bath temperature: 60±5aC Plating solution flow rate: 1.3 m/sec Electrode (anode): IrOx electrode Coating weight: 20 g/mz [0099] (3) Manufacturing of Zinc Electro-Plated Steel Sheet Comprising Resin Film The aqueous resin solution obtained in (1) was applied on the Zn plated layer obtained in (2) by a roll reduction method (coated on one surface) and then dried with heating in an experimental furnace at a furnace temperature of 220DC, and a sheet temperature of 95°C to obtain a zinc electroplated steel sheet comprising a resin film (nonchromate film) having a thickness of 0.4 um. [0100] The resin film thus obtained contains the resin component, the colloidal silica and the silane coupling agent in a mass ratio of resin component:colloidal silica:silane coupling agent = about 30 parts:70 parts:10 parts. [0101] Also, the content of Na in the resin film was measured by an atomic absorption spectrophotometry (apparatus: used SOLARA-M6 manufactured by Jarrel-Ash Company). As a result, it was 1.2% by mass. Specifically, the content of Na in the resin component constituting the resin film was 0.55% by mass, and the content of" Na in the colloidal silica was 1.71 by mass. [0102] (4) Analysis of Stain Resistance Improving Element in Plated Layer A stain resistance improving element contained in the plated layer thus obtained was analyzed by the following method. [0103] First, a sample for analysis was prepared by cutting the zinc electro-plated steel sheet thus obtained into pieces measuring 50 mm * 50 mm. The resulting sample was put in a double-diluted hydrochloric acid solution and dipped until the dissolution reaction of Zn was completed to obtain a dipping solution (1). In this example 1, in order to avoid a measurement error caused by substitution deposition of a stain resistance improving element once dissolved in the solution on the surface of a steel sheet as a base material, immediately after the completion of the reaction, the sample was pulled up and then dipped again in a hydrochloric acid solution (double-diluted solution) separately prepared for 30 seconds to obtain a dipping solution (2). After quantitatively determine the dipping solutions (1) and (2) thus obtained, the amount of the stain resistance improving element (except for Cu) was determined using an ICP-MS analyzer (Model PLASMAQUAD manufactured by VGI Corp.). With respect to Cu, analysis was conducted using an ICP analyzer (ICPV-1000 manufactured by Shimadzu Corporation). [0104] (5) Evaluation of Stain Resistance Each of the zinc electro-plated steel sheets thus obtained was placed in a constant temperature constant humidity testing apparatus maintained at a temperature of 50°C and relative humidity of 95% or more and stored for 504 hours, and then the appearance of the surface was visually (macroscopically) observed and stain resistance was evaluated according to the following criteria. In this example, samples with the evaluation criteria of "1" or "2" were rated "Pass" (examples of the present invention). 1: No stain occurs 2: Slight stain occurs 3: Stain occurs [0105] (6) Evaluation of White Rust Resistance With respect to the respective zinc electro-plated steel sheets thus obtained, a salt spray test defined in JIS Z2371 was carried out and an area ratio of occurrence of white rust after the lapse of 96 hours was determined and white rust resistance was evaluated according to the following criteria. In this example, samples with the evaluation criteria of "A" or "B" were rated "Pass" (examples of the present invention). A: less than 5% B: 5% or more and less than 10% C: 10% or more and less than 50% D: 501 or more [0106] These results are shown in Table 1 to Table 3. For reference, in Table 1 to Table 3, the results of an example (No. 1) in which no element is added in a plated layer are shown. [0107] In Table 1 to Table 3, "concentration in a plating solution" means the content (mg) of an object substance in 1 liter (L) of the plating solution, and "content in a plated layer" means the content by mass of an object substance in the plated layer. [0108] In Table 3, " [01123 As is apparent from the results shown in Table 1 to Table 3, the following consideration can be made. [0113] All of Nos. 3 to 7 (examples containing Ni) , Nos. 11 to 14 (examples containing Fe2+) , Nos. 18 to 20 (examples containing Fe3+) , Nos. 23 to 28 (examples containing Cr) , Nos. 32 to 35 (examples containing Na), Nos. 38 to 42 (examples containing Si), Nos. 46 to 49 (examples containing Sn), Nos. 52 to 56 (examples containing Cu), Nos.59 to 62 (examples containing Co), Nos. 66 to 69 (examples containing W), Nos. 72 to 75 (examples containing In), Nos. 78 to 81 (examples containing Cd), Nos. 84 to 87 (examples containing Ag) and Nos. 90 to 93 (examples containing Ir) are examples of the present invention in which the amount of the stain resistance improving element in the plating solution and the plated layer satisfies the scope of the present invention, and nonchromate zinc electro-plated steel sheets having excellent stain resistance and white rust resistance compared to No. 1 (containing no element added therein) were obtained. [0114] In contrast, No. 2 is a sample in which the content of Ni in the plating solution and the plated layer is small, Nos. 9 and 10 are samples in which the content of Fe2+ is small, Nos. 16 and 17 are samples in which the content of FeJ+ is small, No. 22 is a sample in which the content of Cr is small, Nos. 30 and 31 are samples in which the content of Na is small, No. 37 is a sample in which the content of Si is small, Nos. 4 4 and 4 5 are samples in which the content of Sn is small, No. 51 is a sample in which the content of Cu is small, No. 58 is a sample in which the content of Co is small, No. 65 is a sample in which the content of W is small, No. 71 is a sample in which the content of In is small, No. 77 is a sample in which the content of Cd is small, No. 82 is a sample in which the content of Ag is small and No. 89 is a sample in which the content of Ir is small, and stain resistance deteriorated as compared with No. 1 (containing no element added therein) in all examples. [0115] In contrast, No. 8 is a sample in which the content of Ni in the plating solution and the plated layer is large, No. 29 is a sample in which the content of Cr is large, No. 36 is a sample in which the content of Mo is large, No. 43 is a sample in which the content of Si is large, No. 50 is a sample in which the content of Sn is large, No. 64 is a sample in which the content of Co is large, No. 70 is a sample in which the content of W is large, No. 7 6 is a sample in which the content of In is large, No. 82 is a sample in which the content of Cd is large and No. 94 is a sample in which the content of Ir is large, and stain resistance was excellent but white rust resistance deteriorated as compared with No. 1 (containing no element added therein) in all examples. [0116] No. 15 is a sample in which the content of Fe2+ is large, No, 21 is a sample in which the content of Fe3"1" is large and No. 43 is a sample in which the content of Si is large, and stain resistance deteriorated as compared with No. 1 (containing no element added therein) in all examples. Also, in Nos. 15 and 21, white rust resistance deteriorated. [0117] Example 2 In this example 2, a stain resistance improving action due to the addition of a silane coupling agent was studied. As described below, nonchromate zinc electro-plated steel sheets comprising three kinds of resin films, each having a different mixing ratio of a resin component and colloidal silica, were prepared and the influence of the silane coupling agent on each nonchromate zinc electro-plated steel sheet was examined. [0118] (Nos. 95 to 98) In the same manner as in Example 1, except that the silane coupling agent was further added in an amount within the ranges of 0 parts by mass, 10 parts by mass, 20 parts by mass and 30 parts by mass, as shown in Table 4, based on 100 parts by mass of the total amount of the carboxyl group-containing polyure"hane aqueous resin solution, the ethylene-acrylic acid copolymer aqueous dispersion and colloidal silica (mixing ratio = 5 parts by mass:25 parts by mass:70 parts by mass, respectively) in "(1-3) Preparation of Aqueous Resin Solution" in Example 1, resin films were formed. [0119] In the same manner as in Example 1, except that a plating solution containing all elements shown in Table 5 was used in "(2) Production of Zinc Electro-Plated Steel Sheet" in Example 1, zinc electro-plated steel sheets were produced. The respective elements shown in Table 5 were added in the form shown in Example 1. [0120] Next, nonchromate zinc electro-plated steel sheets were produced in the same manner as in Example 1, and then stain resistance and white rust resistance were evaluated. [0121] (Noa. 99 to 102) In the same manner as in Example 1, except that sodium hydroxide was not added in "(1-2) Preparation of Aqueous Ethylene-Unsaturated Carboxylic Acid Copolymer Dispersion" in Example 1 and "ST-AK" manufactured by Nissan Chemical Industries, Ltd. was used as the colloidal silica in " (1-3) Preparation of Aqueous Resin Solution" in Example 1, and that the mixing ratio of the aqueous carboxyl group-containing polyurethane resin solution, the aqueous ethylene-acrylic acid copolymer dispersion (containing no Na) and the colloidal silica was replaced by 5 parts by mass:30 parts by mass:65 parts by mass, respectively and the silane coupling agent was further added in an amount within the ranges of 0 parts by mass, 10 parts by mass, 20 parts by mass and 30 parts by mass, as shown in Table 4, based on 100 parts by mass of the total amount of these components in Example 1, resin films were formed. [0122] In the same manner as in Example 1, except that a plating solution containing all elements shown in Table 5 was used in "(2) Production of Zinc Electro-Plated Steel Sheet" in Example 1, zinc electro-plated steel sheets were produced. The respective elements shown in Table 5 were added in the form shown in Example 1. [0123] Next, nonchromate zinc electro-plated steel sheets were produced in the same manner as in Example 1, and then stain resistance and white rust resistance were evaluated. [0124] (Nos. 103 to 106) In the same manner as in Example 1, except that colloidal silica obtained by adding 5.1% by mass of NaOH in "ST-XS" manufactured by Nissan Chemical Industries, Ltd. so as to increase the strength was used as the colloidal silica in "(1-3) Preparation of Aqueous Resin Solution" in Example 1, and that the mixing ratio of the aqueous carboxyl group-containing polyurethane resin solution, the aqueous ethylene-acrylic acid copolymer dispersion and the colloidal silica was replaced by 6 parts by mass:34 parts by mass:60 parts by mass, respectively and the silane coupling agent was further added in an amount within the ranges of 0 parts by mass, 10 parts by mass, 20 parts by mass and 30 parts by mass, as shown in Table 4, based on 100 parts by mass of the total amount of these components in Example 1, resin films were formed. [0125] In the same manner as in Example 1, except that a plating solution containing all elements shown in Table 5 was used in "(2) Production of Zinc Electro-Plated Steel Sheet" in Example 1, zinc electro-plated steel sheets were produced. The respective elements shown in Table 5 were added in the form shown in Example 1. [0126] Next, nonchromate zinc electro-plated steel sheets were produced in the same manner as in Example 1, and then stain resistance and white rust resistance were evaluated. What is claimed is: 1. A zinc electro-plated steel sheet having excellent stain resistance which sheet comprises a substantially Cr free resin film containing 0.05 to 5% by mass of Na and formed, on a zinc electro-plated layer, wherein the zinc electro-plated layer contains at least one element selected from the group consisting of Ni, Fe, Cr, Mo, Sn, Cu, Cd, Ag, Si, Co, In, Ir and W each in an amount expressed in terms of atoms within the following ranges Ni: 60 to 6000 ppm by mass, Fe: 60 to 600 ppm by mass, Cr: 0.5 to 5 ppm by mass, Mo: 30 to 500 ppm by mass, Sn: 0.6 to 20 ppm by mass, Cu: 8 to 3000 ppm by mass, Cd: 0.0001 to 0.02 ppm by mass, Ag: 1.0 to 400 ppm by mass, Si: 30 to 2000 ppm by mass, Co: 0.0003 to 0.3 ppm by mass, In: 0.1 to 30 ppm by mass, Ir: 0.01 to 10 ppm by mass, and W: 0.1 to 50 ppm by mass. 2. The zinc electro-plated steel sheet according to claim 1, wherein the resin film contains a carboxyl group- containing resin and an Si-based inorganic compound. 3. The zinc electro-plated steel sheet according to claim 2, wherein the resin film further contains a silane coupling agent. 4. A method for producing a zinc electro-plated steel sheet having excellent stain resistance which method comprises the steps of: (1) Zinc electro-plating using an acidic plating solution containing at least one element selected from the group consisting of Ni: 20 to 2000 ppm by mass, Fe2+: 50 to 5000 ppm by mass, Fe3+: 50 to 5000 ppm by mass, Cr: 5 to 2000 ppm by mass, Mo: 50 to 2000 ppm by mass, Sn: 0.05 to 20 ppm by mass, Cu: 0.05 to 50 ppm by mass, Cd: 0.05 to 5 ppm. by mass, Ag: 0.05 to 5 ppm by mass, Si: 20 to 2000 ppm by mass, Co: 0.05 to 50 ppm by mass, In: 0.5 to 50 ppm by mass, ir: 0.05 to 5 ppm by mass, and W: 0.5 to 50 ppm by mass, and (2) forming a resin film containing 0.05 to 5% by mass of Na.

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