Weldable colored steel sheet

Abstract: A colored steel sheet having a coating thickness which is sufficiently small to permit welding therethrough, and yet exhibiting an excellent colored and lustrous surface. It is a steel sheet having a surface plated with zinc or zinc alloy, and carrying a chromate film formed on its plated surface, and a colored film formed on the chromate film from a composition comprising specific proportions of a water-soluble or water-dispersible thermosetting resin and a water-soluble dye. The composition may further contain a solid lubricant, or a granular rust-inhibitive pigment, or both. The colored film has a specific range of thickness, and a color other than a black.

Description: FIELD OF THE INVENTION This invention relates to a weldable and colored steel sheet having an excellent appearance which is suitable for use in making electric appliances for domestic use, machines or furniture for office use, copying machines, etc. BACKGROUND OF THE INVENTION The growing demand for a wide variety of products having high quality, particularly in the fields of electric appliances for domestic use, and machines or furniture for office use, has brought about an increasing requirement for weldable and colored steel sheets having an excellent appearance which are made by the continuous and rapid treatment of steel sheets plated with zinc or an alloy thereof. The requirement extends to sheets having a variety of colors. There are known a number of methods which enable the manufacture of weldable black steel sheets in a continuous strip line. It has, however, been difficult to make a weldable steel sheet having any color other than black, since it has been difficult to produce a uniform color in a wide surface area. The following is a brief description of the known methods for forming a colored film on a steel sheet, or on a steel sheet plated with zinc: (a) A resin solution containing an organic or inorganic pigment is applied onto a steel sheet by e.g. spraying or roll coating to form thereon a film having a thickness of several tens of µm. (b) A plating layer existing on a steel sheet is caused to undergo reaction or electrolysis to develop a colored film. This method includes variations as will hereunder be set forth: (b-1) A steel sheet is dipped in a chromating solution containing chromic acid and another acid, and if the bath composition and the reaction temperature are appropriately varied, it is possible to form a lustrous rainbow-colored film, an unlustrous yellowish green film, or a differently colored film on a steel surface plated with pure zinc; (b-2) A chromating solution containing Ag⁺ ions is used to form a black chromate film on a steel surface plated with pure zinc (as described in Japanese Patent Application Laid-Open No. 193376/1983); (b-3) A chromate film is formed on a steel surface plated with pure zinc, and the steel surface is, then, dyed by dipping in a dye solution; (b-4) A steel surface plated with pure zinc is dipped in a solution containing Ni²⁺ or Cu²⁺ ions, so that the metal which the solution contains may be deposited on the steel surface to form a colored film thereon; (b-5) A plating layer of pure zinc is subjected to anodic oxidation in an alkali solution to form a black film; (b-6) An electroplating layer of a Zn-Co, Zn-Ni or Zn-Mo alloy is subjected to anodic treatment to form a black film (Japanese Patent Publication No. 38276/1986); and (b-7) A steel sheet which has been plated with a Zn-Ni alloy is subjected to dipping, spraying, or anodic treatment with a solution containing nitric acid or a nitrate group to develop a black surface (Japanese Patent Publication No. 30262/1987) (c) The surface of a steel sheet, or a steel sheet plated with zinc is subjected to cathode electrolysis treatment to form a colored film (e.g. Japanese Patent Application Laid-Open No. 10292/1987) (d) A solution obtained by adding an organic dye to an aqueous solution of potassium sodium silicate is applied onto a steel surface plated with zinc or an alloy thereof to improve corrosion resistance and adhesion (Japanese Patent Publication No. 30593/1980) All of these methods do, however, have their own drawbacks, as will hereunder be pointed out. (a) This is a common method of coating which employs an organic or inorganic pigment as a coloring agent. The uniform distribution of the pigment in a film is, however, essential to ensure a uniform color tone, and calls for a film thickness of at least about 10 µm. This thickness is too large to permit spot welding. The maximum film thickness that permits welding has been about three µm, and has been too small to ensure the uniform distribution of the pigment. (b) All of the methods as described at (b-2), and (b-5) to (b-7) are useful for forming only a black film. (b-1) This method is likely to form an unevenly colored surface, since its color is greatly affected by even a small difference in thickness of a chromate film. Moreover, it can form only a film having a yellowish or greenish color which is specific to a reactive chromate. (b-2) This method is useful for forming only a black film, as hereinabove stated. Moreover, the solution containing Ag⁺ ions is very expensive. (b-3) The dyeing of the chromate film requires a period of several or more minutes. This period is too long for the continuous coloring treatment of a steel strip, for which only a period not exceeding five seconds is permissible. (b-4) This method requires a treating time of as long as several or more minutes, but can form only a film of poor adhesion. Moreover, a film formed by the deposition of Cu²⁺ ions has the drawback of changing in color as a result of oxidation with the passage of time. (b-5) This method is also unsuitable for the continuous treatment of a strip, since it requires a treating time which is as long as 5 to 20 minutes. (b-6) and (b-7) These methods are both useful for forming only a black film, as hereinabove stated, though they can be carried out rapidly. Moreover, the dissolution of metal ions from a plating layer is not only uneconomical, but also obstructs continuous operation seriously, as the dissolved metal ions deteriorate the solution for the blackening treatment. Furthermore, both of the methods have only a limited scope of application, i.e. (b-6) is applicable only to a steel sheet plated with a Zn-Co, Zn-Ni or Zn-Mo alloy, while (b-7) is applicable only to a sheet plated with a Zn-Ni alloy. (c) This method is mainly employed for forming a black film and can only form a film of poor adhesion. (d) This method is not intended for forming a film having an excellent appearance, nor is it intended for imparting weldability, insofar as the disclosure does not contain any specific reference to the film thickness. Moreover, as the film is basically composed of potassium sodium silicate, the hardened film is unsatisfactory in lubricating property as required during the press forming of the steel sheet, and is, therefore, unsuitable for any steel sheet used for making electric appliances for domestic use, office machines or furniture, etc. Moreover, the disclosure does not specifically define the organic dye to be used. There are a very large number of kinds of organic dyes which are widely different from one another in properties including light fastness, tinting strength and solubility in a solvent. Some dyes are superior to others in e.g. tinting strength, but inferior in light fastness. It is, therefore, needless to say that all of the organic dyes are not equally useful. Under these circumstances, it is an object of this invention to provide a weldable colored steel sheet coated with a film which is sufficiently small in thickness for permitting spot welding, and yet presents an excellently colored and highly lustrous surface. The steel sheet of this invention is an improvement over the product of the known method as described at (a) above, and can therefore overcome all of the drawbacks of the known methods relying upon reaction or electrolysis as described at (b) above. The steel sheet of this invention has a uniformly colored surface having any of a wide variety of colors as desired. It is another object of this invention to provide a colored steel sheet which is not only weldable and excellent in appearance, but also excellent in corrosion resistance, film adhesion and formability. Although the organic solvents have usually been used in a wide variety of coating compositions, the harmfulness of their vapor to the human body has turned out to be a big environmental problem. The latest tendency is, therefore, toward the use of an aqueous coating composition. Under these circumstances, it is still another object of this invention to provide a colored steel sheet coated with a colored film formed from an aqueous composition, and having a color other than black. SUMMARY OF THE INVENTION We, the inventors of this invention, have done an extensive range of research and experimental work to attain the above objects, and particularly, to obtain a uniformly and beautifully colored film having a thickness which is sufficiently small to permit the welding of the underlying steel sheet. As a result, we have found that the above objects can be attained if a colored film having an appropriate thickness is formed from a composition obtained by mixing a base resin and a specific dye in a specific ratio, and that it is possible to produce a weldable colored steel sheet having still higher levels of corrosion resistance, film adhesion and formability if a film having a specific multilayer structure including the above colored film is formed on the steel sheet, or if the composition further contains a specific component as required. Our discovery of these facts forms a basis for this invention. This invention, therefore, resides essentially in a steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface, and a colored film formed in an appropriate thickness on the chromate film from a composition comprising specific proportions of a water-soluble or water-dispersible thermosetting resin and a water-soluble dye, and further containing a specific additive or additives, if required. The following is a more specific summary of this invention: (1) A weldable colored steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m² in terms of metallic chromium, and a colored film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin and 1 to 200 parts by weight of a water-soluble dye as a coloring agent, the colored film having a thickness of 0.3 to 3.0 µm, and a color other than black. (2) A weldable colored steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m² in terms of metallic chromium, and a colored film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin, 1 to 200 parts by weight of a water-soluble dye as a coloring agent and 1 to 100 parts by weight of a solid lubricant, the colored film having a thickness of 0.3 to 3.0 µm, and a color other than black. (3) A weldable colored steel sheet having a surface plated with zinc or alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m² in terms of metallic chromium, and a colored film formed on the chromate film from a composition comprising 100 parts by weight of water-soluble or water-dispersible thermosetting resin as a base resin, 1 to 200 parts by weight of a water-soluble dye as a coloring agent and 1 to 100 parts by weight of a granular rust-inhibitive pigment, the colored film having a thickness of 0.3 to 3.0 µm, and a color other than black. (4) A weldable colored steel sheet having a surface plated with zinc or zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m² in terms of metallic chromium, and a colored film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or water-dispersible thermosetting resin as a base resin, 1 to 200 parts by weight of a water-soluble dye as a coloring agent, 1 to 100 parts by weight of a solid lubricant and 1 to 100 parts by weight of a granular rust-inhibitive pigment, the colored film having a thickness of 0.3 to 3.0 µm, and a color other than black. (5) A weldable steel sheet as set forth at (1), (2), (3) or (4) above, wherein the dye is at least one dye selected from azo, azomethine, quinoline, ketoneimine, fluorone, nitro, xanthene, acenaphthene, quinophthalone, anthraquinone, aminoketone, methine, perylene, coumarin, perinone, triphenyl, triallylmethane, phthalocyanine, isochlorophenol, and azine dyes. (6) A weldable colored steel sheet as set forth at (5) above, wherein the dye is a mixture of a water-soluble azo dye in the form of a complex metal salt and another water-soluble dye. (7) A weldable colored steel sheet as set forth at (1), (2), (3) or (4) above, wherein the dye is an azo dye in the form of a complex metal salt. (8) A weldable colored steel sheet set forth at (2), (4), (5), (6) or (7) above, wherein the solid lubricant is at least material selected from the group consisting of hydrocarbon compounds such as polyolefin wax, fluororesins, fatty acid amides, metallic soaps, metal sulfides such as molybdenum disulfide, graphite, graphite fluoride, boron nitride and polyalkylene glycols. (9) A weldable colored steel sheet as set forth at (3), (4), (5), (6) or (7) above, wherein the granular rust-inhibitive pigment is at least one material selected form the group consisting of sparingly soluble chromium compounds and silica. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic representation of the film structure of a steel sheet embodying this invention; FIGURE 2 is a graphical representation of the ranges defined by this invention for the proportion of a dye used to form a colored film, and for the thickness of the film; and FIGURE 3 is a graph showing the weldability of steel sheet samples in relation to the thickness of the colored film thereon. DETAILED DESCRIPTION OF THE INVENTION This invention is directed to a colored steel sheet other than a black steel sheet carrying a black film (i.e. a film containing as a coloring agent substantially only a blackening agent such as a black dye or pigment). Therefore, the term "colored steel sheet, or film" as therein used does not include any black steel sheet, or film. The colored steel sheet of this invention is defined as one carrying a colored film having a chromatic or achromatic color having a luminosity, or L value in excess of 25, so that may be distinguished from a black steel sheet. It is, however, to be understood that this invention includes also any steel sheet carrying a colored film formed by employing a black dye with a dye having another color, as will hereinafter be described in further detail. It is also to be understood that the scope of this invention does not exclude any colored steel sheet having an achromatic color other than black, though the colored steel sheet of this invention may, in the majority of the cases, have a chromatic color. The colored steel sheet of this invention essentially comprises a steel plated with zinc or zinc alloy, which is starting material, a chromate film formed on its plated surface, and a colored film formed on chromate film from a composition comprising a water-soluble or water-dispersible thermosetting base resin, and a water-soluble dye. Its cross sectional construction is schematically shown in FIGURE 1. The steel sheet used as a starting material is, for example, a steel sheet plated with zinc, an alloy of zinc and iron, an alloy of zinc and nickel, an alloy of zinc and manganese, an alloy of zinc and aluminum, an alloy of zinc, cobalt and chromium, or a similar plating composition further containing one or more elements such as, Ni, Fe, Mn, Mo, Co, Al, and Cr. It is also possible to use as a starting material a steel sheet coated with a disperse plated layer formed from a composition similar to any of those mentioned above, but further containing, for example, a granular resin, silica, or a chromium compound. Moreover, the starting material may be a compositely plated steel sheet carrying a plating film formed by two or more layers of the same or different compositions, for example, a film formed by two or more layers consisting of Zn-Fe alloys having different iron contents, respectively. As far as its appearance is concerned, a similar colored film can be produced on any steel sheet, and even a hot or cold rolled steel sheet which has not been plated can, therefore, be used as the starting material. For the same reason, a stainless steel sheet, or a sheet of an aluminum or titanium alloy can also be used as the starting material. From the standpoint of cost and corrosion resistance, however, it is desirable to use a plated steel sheet, so that the colored steel sheet of this invention may be used for making a final product to which no further painting will be given. Any practically possible method, such as an electrolytic, dip coating, or vapor phase process, can be employed for preparating the starting material. A chromate film is formed on the surface of the starting material by chromate treatment. The combination of the chromate film and a colored film containing specific coloring agent, or dye, as will hereinafter be described, gives a very high level of corrosion resistance to the colored steel sheet of this invention. The chromate film is so formed as to have dry coating weight of 10 to 200 mg/m², preferably 30 to 80 mg/m², in terms of metallic chromium. If its coating weight exceeds 200 mg/m², it is likely to lower the formability and weldability of the steel sheet. If its coating weight is smaller than 10 mg/m², it is likely to lack uniformity and lower the corrosion resistance of the steel sheet. The chromate film preferably contains chromium having a valence of 6, since chromium ions having a valance of 6 have a self-healing action and prevent corrosion occurring from any surface defect on the steel sheet. The chromate treatment for forming the chromate film can be carried out by any known process relying upon reaction, coating or electrolysis. If the colored film is of a color having a high L value, such as yellow, red or blue the chromate film is preferably of a nearly white color, so that the color of the colored film may not be dulled by the color of the underlying chromate film. If a coating type of process is employed for the chromate treatment, it is carried out by employing a coating solution which consists mainly of a partially reduced chromic acid solution and may further contain a water-dispersible or water-soluble organic resin, such as an acrylic resin, and/or silica (colloidal or fumed) having a particle diameter of several to several hundred nanometers (millimicrons). The solution may contain chromium ions having a valence of 3 and ones having a valence of 6 in a ratio of 1:1 to 1:3, and have a pH of 1.5 to 4.0, preferably 2 to 3. The ratio of the chromium ions having a valence of 3 to those having a valence of 6 is adjusted by employing a commonly used organic reducing agent selected from among, for example, saccharides and alcohols, or a commonly used inorganic reducing agent. The coating type of chromate treatment may be carried out by any commonly used method, such as roll coating, dipping, or spraying. The treatment is not followed by water rinsing, but is directly followed by drying to give a chromate film, since rinsing results in the removal of chromium ions having a valence of 6. The film contains chromium ions having valences of 3 and 6 in the ratio in which the solution contains them. A resin film, which will be formed on the chromate film. prevents any excessive flow of chromium ions having a valence of 6 out of the chromate film in a corrosive environment and enables the maintenance of effective passivation and thereby high corrosion resistance for a long period of time. An electrolytic type of chromate treatment includes cathodic treatment in a bath comprising chromic anhydride and one or more kinds of anions selected from among, for example, sulfuric acid, phosphoric fluoride and halogenoxy acids, and is followed by rinsing in water and drying to give a chromate film. The chromate film which is formed by the coating type of process contains a larger amount of chromium having a valence of 6 than that formed by the electrolytic process, and is, therefore, superior in corrosion resistance. The corrosion resistance of the former film becomes still better upon heat treatment, which improves its density and strength, as will hereinafter be described in further detail. The chromate film which is formed by the electrolytic process has the advantages of being high in density and strength even without being given heat treatment, and of being easy to control in coating weight. Description will now be made of the colored film formed on the chromate film and its components. According to this invention, the colored film is principally characterized by comprising a water-soluble,or water-dispersible thermosetting resin as a base resin, and a water-soluble dye as a coloring agent. The film may further contain a solid lubricant which is added to improve its formability, or a rust-inhibitive pigment added to improve its corrosion resistance, or both. It is essential that the coloring agent which is employed to form the colored film be capable of giving a uniformly and beautifully colored surface when the film has a thickness allowing the welding of the underlying steel sheet, i.e. not exceeding three µm. Moreover, it is imperative for the coloring agent not to exert any adverse effect on any other property required of the colored film such as formability or corrosion resistence, when it is mixed with the base resin in a ratio ensuring that the film exhibit a good appearance. There are two major classes of coloring agents: pigments (inorganic and organic), and dyes. The pigments are generally particulate, and are dispersed in a solvent when they are used. They are widely used for coating the surfaces of automobile bodies, and electric appliances for domestic use. Prussian blue and chrome yellow are examples of inorganic pigments and quinacridon and phthalocyanine pigments are examples of organic pigments. The pigments are usually employed to form a coating having a thickness of 10 or more µm which does not permit any welding therethrough. If a pigment is used in a film having a thickness not exceeding three µm to form a weldable steel sheet, the particles of the pigment contact one another and agglomerate, and the film therefore, fails to show a satisfactorily high hiding power. The film is not lustrous, either. The addition of a large amount of pigment to a resin film lowers its formability and adhesive strength. Moreover, the invasion of water through the interstices of the pigment particles is likely to lower the corrosion resistance of the base material. AS is obvious from the foregoing, the use of a pigment as a coloring agent cannot form a satisfactorily thin colored film having a good appearance, and the use of a large amount of pigment results in a film having lower properties. Thus, it has been impossible to form a colored film having a thickness which is sufficiently small to permit welding, and having a good appearance. No combination of two or more inorganic or organic pigments has been found capable of producing any satisfactory result. Referring now to the dyes, it is understood that thousands of dyes are commercially available when counted by tradenames. They are classified by two major methods, i.e. (I) one classifying the dyes by the chemical structure thereof, and (II) one for the practical classification of the dyes based on their properties (see e.g. "Handbook of Organic Chemistry", complied by The Society of Organic Synthetic Chemistry, and published by Gihodo). The method as stated at (I) above classifies the dyes by the chemical group in the molecule which gives rise to color. The dyes as classified by this method include nitroso, nitro, azo (monoazo, disazo, trisazo, tetrakisazo), anthraquinone, indigo, azine, cyanine, phthalocyanine, stilbene, sulfur, triazole, triphenylmethane, acridine, diphenylmethane, and oxazine dyes. The dyes as classified by the method (II) include direct, acid, basic, acid mordant, premetallized, sulfur, vat, azoic, disperse, reactive, oxidation, fluorescent brightener, and oil-soluble (organic solvent-soluble) dyes. It follows from these two ways of classification that, for example, the acid dyes according to the classification by the method (II) include azo (monoazo, disazo, trisazo and tetrakisazo), anthraquinone, triphenylmethane, and azine dyes according to the classification by the method (I). The color of a dye as the most important property thereof is due to the action of a chemical group containing an unsaturated bond, such as -CH=CH-, or -N=N-(chromophore), and a group containing a lone pair of electrons, such as -NH₂ or -OH (auxochrome), whereby, while light having a particular wavelength is absorbed, light having a different wavelength not causing absorption is visible to the eye as the color of the dye. Thus, there are various dyes having a variety of colors. These colors are yellow, orange, red, violet, blue, green, brown and black according to the classification by the Color Index. The Color Index (Third Edition, Vols. 1 to 8) published by The Society of Dyers and Colourists describes the commercially available dyes by class, structure, properties, use, etc., and classifies the dyes under "C.I. Generic Name". The Color Index classifies the commercially available dyes in a practically useful way similar to that of classification by the method as stated at (II) above, and gives the classification of the colors which includes the classification of the dyes by the chemical structure in a numerical order starting with 1. If the chemical structural formula of a dye is specifically known, it is shown under "C.I. Constitution Number", and the majority of the the dyes of which the chemical structural formulas are not clear are classified by chemical structure as, for example, azo (monoazo, disazo, etc.), anthraquinone, and azine dyes, as is the case with the classification by the method stated at (I) above. For example, the yellow dyes belonging to the class of acid dyes are grouped under the classification "C.I. Acid Yellow" which includes "C.I. Acid Yellow 1" covering monoazo dyes having a specific chemical structure, "C.I. Acid Yellow 28" covering disazo dyes having a specific chemical structure, and "C.I. Acid Yellow 109" covering anthraquinone dyes having a specific chemical structure, and classifies the dyes by color, properties, use, etc. The "C.I. Generic Name" provides a listing of dyes by the names under which they are commercially available. We have examined the dyes belonging to every class of dyes to see whether each dye satisfies the requirements connected with the manufacture and use of the steel sheet according to this invention. The requirements are: (1) That the dye is soluble or dispersible in a base resin (thermosetting resin) and a solvent (water or an organic solvent), and does not change in color when film containing it is baked for thermal setting,; (2) That the dye enables the formation of a uniformly and vividly colored film having a small thickness allowing the welding of the underlying steel sheet, i.e. not exceeding three µm; and (3) That the color of the dye does not fade even in a film which is likely to be exposed to light from various sources, as one formed on a steel sheet used to make electrical appliances for domestic use, office machines or furniture, etc., i.e. the dye has good fastness to light. It is only the the dyes satisfying all of these three requirements that can be used to make the steel sheet of this invention. We have found that a mixture of a water-soluble dye and a water-soluble or water-dispersible thermosetting resin can form a film having a sufficiently small thickness to allow welding, i.e. not exceeding three µm, and yet exhibiting a uniformly and vividly colored surface. The water-soluble dye generally contains -OH, - COOH, -SO₃H, -NH₂ or another hydrophilic group at the end of its molecule. All the dyes according to the classification as described at (II) above, except the oil-soluble (organic solvent-soluble) dyes are, therefore, water-soluble dyes. This invention does, however, not preclude the use of a water-soluble dye not registered in the Color Index, such as a new dye not yet registered in the Color Index, or a mixture of water-soluble dyes. The mixture may be of dyes having the same color, or of ones having different colors. It is well known that the mixing of yellow and red dyes produces an orange color by the subtractive process. This method can be employed to form films having a wide variety of colors. The water-soluble dyes which can be employed for the purpose of this invention include azo, azomethine, quinoline, ketoneimine, fluorone, nitro, xanthene, acenaphthene, quinophthalone, anthraquinone, aminoketon methine, perylene, coumarin, perinone, triphenyl, triallylmethane, phthalocyanine, isochlorophenol, and azine dyes. It is also possible to use a mixture of two or more dyes. We have found that a water-soluble azo dye composed of a complex metal salt enables the formation of a colored film which is superior in appearance, light fastness and corrosion resistance to any film formed by employing any other dye. The water-soluble azo dye in the form of a complex metal salt can be selected from the eight classes in the Color Index which have hereinabove been mentioned. More particularly, it is a complex compound of an azo dye and a trivalent metal, such as chromium, copper or cobalt. This type of dye is characterized in that (1) it can produce a vivid and uniform color even in a thin film, and (2) It is superior to any other dye in stability (or fastness) to light, apparently because it is a complex compound having a chemical structure formed by two molecules of an azo dye and one atom of a metal (a complex metal salt of the 2:1 type), or by one dye molecule and one melt atom (a complex metal salt of the 1:1 type). Although Cr, Co, Cu, Fe, Al, etc. can be used to form a complex salt with a dye, chromium is used more often than any other metal. The Color Index includes the following azo dyes in the form of complex metal salts: (i) C.I. Acid Yellow: 59, 100, 133, 139, 185, 407 (ii) C.I. Acid Orange: 60, 96, 107, 129, 139 (iii) C.I. Acid Red: 186, 190, 209, 281, 339 (iv) C.I. Acid Violet: 58, 98, 111 (v) C.I. Acid Blue 152, 179, 200, 240, 268 (vi) C.I. Acid Green 35, 49, 52, 60, 92 (vii) C.I. Acid Brown 231, 255, 270, 299, 328 and (viii) C.I. Acid Black 8, 107, 108, 120, 155 It is, of course, possible to use any other azo dye in the form of a complex metal salt that is not registered in the Color Index, such as a new product not yet registered, or a mixture of dyes which contains at least one azo dye in the form of a complex metal salt. If an azo dye in the form of a complex metal salt is used as a coloring agent, it is possible to form a colored film having a thickness which is sufficiently small to permit welding (i.e. does not exceed three µm), and yet presenting a uniform and excellent appearance. As this type of dye is very fast to light, it is possible to produce a colored steel sheet which will not present any problem at all when used for electric appliance for domestic use, office machines or furniture, or other devices or articles that may be exposed to light from a variety of sources. A colored film containing this type of dye has a very high level of corrosion resistance, as hereinabove stated. Its corrosion resistance is by far higher than that of any colored film containing a pigment as a coloring agent. This is apparently due to, among others, the fact that the molecules of the dye are uniformly and densely distributed in the film, and also the fact that, if chromium is used as a complex metal salt composing the water-soluble azo dye, chromium ions having a valence of 3 form an oxcide as a passive film. The corrosion resistance of the steel sheet according to this invention owes itself primarily to the barrier effect produced by the chromate forming a passive film by virtue of chromium ions having a valence of 3 and the barrier effect of the resin film, and is further improved by the barrier effect of the dye itself as hereinabove stated. Other factors contributing to its high corrosion resistance include the self-heating action which is achieved by the conversion of chromium ions having a valence of 6 in the chromate to ones having a valence of 3 and rectifies any defect of the passive film, and the high adhesive strength achieved between the zinc or zinc alloy layer and the chromate film, and between the chromate film and the colored film. The use of an azo dye in the form of a complex chromium salt among others, enables a very high level of corrosion resistance, presumably by virtue of the synergistic effect produced by the trivalent chromium ions in the salt and the underlying chromate, as will be obvious from EXAMPLE 6. Therefore, the most preferable form of the dye for the purpose of this invention is a complex salt containing trivalent chromium ions. A water-soluble dye is easily soluble in water and can, therefore, form a composition for forming a colored film if it is mixed with a water-soluble or water-dispersible thermosetting resin. A colored film formed from such a composition exhibits a uniformly and excellently colored surface, even if its thickness may be sufficiently small to allow welding, i.e. may not exceed three µm. The colored steel sheet of this invention is manufactured by a process including coating treatment which can be carried out without involving any deterioration of a solution of a film-forming composition, as opposed to any conventional etching type, electrolytic treatment, or any conventional coloring treatment relying upon reaction, such as substitution plating. Moreover, the greatest advantage of the coating type of treatment is that it is applicable to any plated steel sheet, irrespective of the material used for its plating. Reference is now made to the proportion of the water-soluble dye to the water-soluble or water-dispersible thermosetting resin in the colored film on the steel sheet of this invention and its thickness. The colored film on the steel sheet of this invention is formed from a composition comprising 100 parts by weight of a water-soluble or water-dispersible thermosetting resin, and 1 to 200, preferably 5 to 120, parts by weight of a water-soluble dye. The film has a thickness of 0.3 to 3.0, preferably 0.7 to 2.5, µm. If the proportion of the dye is higher, and particularly, in excess of 200 parts by weight, the dye is not fully dissolved in the resin, or solvent, but part thereof settles or forms coarse particles. This is a result which is undesirable not only from an economical standpoint, but also because the undissolved dye has an adverse effect on the color of the film, and because the film is very likely to peel off any bent portion of the steel sheet. The dye fails to produce any satisfactory coloring effect if its proportion is lower than five parts, and particularly, one part, by weight. The film lowers the spot weldability of the steel sheet if its thickness is larger than 2.5 µm, and particularly if its thickness is larger than 3.0 µm. The film fails to show a satisfactorily high hiding power,and allows the steel surface to be seen therethrough, or produces an unevenly colored surface, if its thickness is smaller than 0.5 µm, and particularly if its thickness is smaller than 0.3 µm. FIGURE 2 is a graphical representation of the ranges of the dye proportion and the film thickness as hereinabove set forth. The base resin which is used for the purpose of this invention is a water-soluble or water-dispersible thermosetting resin. These limitations are important for the reasons which will hereinafter be set forth. The resin need be a water-soluble or water-dispersible one, since the dye is a water-soluble one. The resin need be a thermosetting one, since the use of a thermoplastic resin is likely to result in a film which is low in scratch resistance. Specific examples of the applicable thermosetting resins are acrylic copolymer, alkyd, epoxy, polybutadiene, phenolic and polyurethane, resins. It is also possible to use a mixture of two or more such resins, a product of addition condensation formed by amy such a resin and another monomer, or a derivative of any such resin obtained by modification with another resin, Acrylic copolymer alkyd, and epoxy resins are among others preferred. The acrylic copolymer resins are ones which can be synthesized from ordinary unsaturated ethylenic monomers by e.g. solution, emulsion or suspension polymerization. This type of resin is obtained by employing as an essential component a hard monomer such as methacrylate, acrylonitrile, styrene, acrylic acid, acrylamide or vinyltoluene, and adding an appropriate proportion of an unsaturated vinyl monomer to impart hardness, flexibility and crosslinking property to the resulting resin. The resin may be modified by another kind of resin, such as an alkyd, epoxy, or phenolic resin. As regards the alkyd resins, it is possible to use any of the known ones which can be produced by ordinary methods, for example, oil-modified, rosin-modified, phenol-modified, styrenated, silicone-modified, and acryl-modified alkyd resins, and oilfree alkyd (polyester) resins. Examples of the suitable epoxy resins are straight epoxy resins of e.g. the epichlorohidrin or glycidyl ether type, and fatty acid-modified, polybasic acid-modified, acrylic resin-modified, phenol-modified, amine- or polyamine-modified, and urethane-modified epoxy resins. A known curing agent is used with the resin. Examples of the appropriate curing agents are melamine, block isocyanate, and urea. The colored film formed on the steel sheet of this invention as hereinabove described possesses all of the necessary properties. The addition of certain additives as will hereinafter be described, however, makes it possible to form a film having still better properties. Firstly, it is preferable to add a solid lubricant to a film-forming composition to form a colored film having good self-lubricating property. The following is a list of examples of the solid lubricants which are appropriate for the purpose of this invention: (a) Hydrocarbon lubricants, such as natural and synthetic paraffins, microcrystalline wax, polyethylene wax, and chlorinated hydrocarbons; (b) Fluororesins, such as polyfluoroethylene, polyvinyl fluoride, polytetrafluoroethylene, and polyvinylidene fluoride resins; (c) Fatty acid amide lubricants, such as stearic acid amide, palmititic acid amide, methylenebisstearoamide, ethylenebisstearoamide, oleic acid amide, ethyl acid amide, and alkylenebis fatty acid amide; (d) Metallic soaps, such as calcium stearate, lead stearate, calcium laurate, and calcium palmitate; (e) Metal sulfides, such as molybdenum disulfide, and tungsten disulfide; and (f) Other lubricants, such as graphite, graphite fluoride, boron nitride, grease, and alkali metal sulfates. It is important to select a solid lubricant which will not affect the color of the colored film. For example, a lubricant having an achromatic color, such as a hydrocarbon lubricant, or fluororesin, is suitable for use in any film, irrespective of its color, while molybdenum disulfide having a black color is suitable for only a film having a dark color. The solid lubricant is used in the proportion of 1 to 100 parts by weight, preferably 3 to 60 parts by weight, for 100 parts by weight of thermosetting resin. If its proportion is less than 3 parts, and particularly one part, by weight, it does not satisfactorily improve the lubricating property of the film. The use of the lubricant in any proportion in excess of 60 parts, and particularly in excess of 100 parts, by weight, is undesirable, since it lowers the strength of the film as cured, and causes it to adhere partly to a die used for pressing the steel sheet. The colored film formed from a coating composition comprising a water-soluble or water-dispersible base resin and a water-soluble dye has a satisfactorily high level of corrosion resistance by virtue of the synergistic effect produced by the underlying plating layer and the chromate film. It is, however, preferable to add a rust-inhibitive pigment to the composition to form a film which exhibits a still higher level of corrosion resistance particularly when subjected to working, and thereby provides a colored steel sheet having a widened scope of application. It is possible to use as the rust-inhibitive pigment one or more materials selected from among sparingly soluble chromates and silica. The sparingly soluble chromates which can be employed are barium chromate (BaCrO₄), strontium chromate (SrCrO₄), lead chromate (PbCrO₄), zinc chromate (ZnCrO₄·4Zn(OH)₂), calcium chromate (CaCrO₄), potassium zinc chromate (K₂O·4ZnO·4Cro₃·3H₂O), and silver chromate (AgCro₄). The following are examples of the silica used in this invention: (1) Silica produced by pyrogenic process (e.g. AEROSIL 130, AEROSIL 200, AEROSIL 300, OR AEROSIL 380 of Nippon Aerosil Co., Ltd.); (2) Colloidal silica (e.g. Snawtex 20, Snawtex C, Snawtex N, Snawtex O, or Snawtex S of Nissan Chemical Industries, Ltd.); (3) Silica by wet process-precipitated type (e.g. T-32(s), K-41 or F-80 of TOKUYAMA SODA CO., LTD.); and (4) Silica by wet-gel process (e.g. SYLOID 244, SYLOID 150, SYLOID 72, SYLOID 65 OR SHIELDEX of Fuji-Davison Chemical Ltd.). It is possible to use one kind of silica, or a mixture of two or more kinds of silica. Any silica as hereinabove listed remains stable when dispersed in a water-soluble or water-dispersible base resin. It is, however, important to select a rust-inhibitive pigment which will not have any substantial effect on the color of the colored film. Therefore, it is unsuitable to use, for example, a yellow sparingly soluble chromate in any film having a different color, such as red or blue. The rust-inhibitive pigment (or pigments) is incorporated as one of the constituents of the film-forming composition. The rust-inhibitive pigment is used in the proportion of 1 to 100 parts, preferably 3 to 60 parts, by weight for 100 parts by weight of thermosetting resin. If its proportion is less than one part, or even three parts, by weight, it exhibits no or hardly any rust inhibiting effect. The use of the pigment in any proportion exceeding 100 or even 60 parts by weight should be avoided, as it is likely not only to lower the lubricating property of the film, but also to produce a film surface having a lower degree of luster, or a greatly altered color. The addition of both the solid lubricant and the granular rust-inhibitive pigment enables the formation of a colored film which is outstandingly good in both formability and corrosion resistance on any formed steel sheet portion. In this case, they are each used in the proportion of 1 to 100, preferably 3 to 60 parts by weight. Moreover, another color pigment (inorganic or organic), or dye can be added to form a colored film having its color tone and luster controlled exquisitely as desired. The addition of an inorganic or organic pigment enables the formation of a film having an improved hiding power and thereby a deeper color, or an appropriately controlled degree of luster. The addition of a water-soluble dye having a different color, or chemical structure enables the formation of a film having a color differing from the colors of the individual dyes, as hereinabove suggested. The colored film as hereinabove described can be formed if the composition which has been diluted with a solvent as required is applied in an appropriate film thickness onto a steel sheet by e.g. roll squeezing or coating, or air-knife coating, and is baked by heating the sheet to a temperature of 80°C to 300°C, preferably 120°C to 250°C. Any ordinary method can be employed without any limitation in particular for the application and baking of the composition. It is, however, a great advantage of the steel sheet according to this invention that it can be manufactured by any coating equipment that iron and steel manufacturers usually have for producing high corrosion resistance surface treated steel sheets. As is obvious from the foregoing, this invention provides a colored steel sheet which is weldable, since its colored film has a thickness not exceeding three µm, which is smaller than the film on any conventionally available steel sheet, and yet which has a uniformly colored and lustrous surface presenting an excellent appearance. The colored steel sheet of this invention is a product which is excellent in both various properties and productivity. In addition to its excellent appearance and weldability, it is outstandingly characterized by the high adhesive strength, formability, corrosion resistance and light fastness of its colored film. As it can be manufactured by any existing coating and baking equipment including a roll coater, its manufacture can be accomplished by a process of greatly improve productivity which is free from any problem as has hitherto been caused by the deterioration of the coating solution by the dissolution of the plating layer on any conventional colored steel sheet made by reactive etching. As the colored film on the steel sheet of this invention is formed from an aqueous composition, it is unlikely to exert any such adverse effect on the human body as may be produced by any film formed from a composition containing an organic solvent. The colored steel sheet exhibits particularly high levels of corrosion resistance and light fastness if its colored film contains an azo dye in the form of a complex metal salt. EXAMPLE The invention will now be described more specifically with reference to EXAMPLE 1 to 7 each directed to a steel sheet used for making an electrical appliance for domestic use, or office machine or furniture, as well as COMPARATIVE EXAMPLES 1 to 7 corresponding to EXAMPLES 1 to 7, respectively. Each steel sheet had a surface which had been electroplated with a Zn-Ni alloy having a nickel content of 12%, or a Zn-Fe alloy having an iron content of 25%. After it had been degreased with an alkali, rinsed with water, and dried, it was coated with a chromate solution by a roll coater, or was electrolyzed in an electrolytic bath for chromate treatment, whereby a chromate film was formed on the plated surface of the steel sheet. After it had been dried, the film was coated with a resin solution by a roll coater. The resin film was dried, heated, and air cooled. More specifically, the coating type of chromate treatment and the electrolytic chromate treatment were carried out, as will be described below: Coating type of chromate treatment: The chromating solution contained chromium ions having a valence of 3 and ones having a valence of 6 in the ratio of 2:3, and had a pH of 2.5 (after adjustment by KOH), and a solid content of 20 g per liter. The solution was applied by the roll coater onto the steel surface at ordinary room temperature, and dried. Electrolytic chromate treatment: The bath contained 50 g of CrO₃ and 0.5 g of H₂SO₄ per liter, and had a temperature of 50°C. The film was formed by cathodic treatment using a current density of 4.9 A/dm² and an electrolyzing time of 20 seconds, followed by rinsing with water and drying. TABLES 1 to 4 show the details of the base resins, coloring agents, solid lubricants, and granular rust-inhibitive pigments used for preparing the film compositions as employed in the EXAMPLES of this invention and COMPARATIVE EXAMPLES. TABLES 5 to 30 show the plated steel sheets employed, the chromate films formed thereon, the compositions used for forming colored films thereon, and the results of the tests conducted on the colored steel sheets obtained. The compositions were prepared by mixing the constituents thereof as shown in TABLES 1 to 4 in the proportions as shown in the relevant TABLES, and were diluted with a solvent if required. The following is a description of the tests conducted for the evaluation of each colored steel sheet: (1) Appearance: The colored film on each steel sheet was visually evaluated for its appearance, and particularly for its color uniformity. The symbols used to show the results of evaluation in the relevant tables have the following meanings, respectively: O ○: Uniform and beautiful; ○ : Substantially uniform and beautiful; ▵-X: Rough and lacking luster, or uneven in color. (2) Luster: The luster of the colored film was measured at incident and reflecting angles of 60° by an instrument of Suga Test Instruments Co., Ltd. A greater value means a higher degree of luster. (3) Weldability: Spot welding was conducted on each steel sheet and the number of continuously formed weld spots was counted as a measure of its weldability. Spot Welding Conditions: Electrode: Cr-Cu, D type: Electrode: dia.: 6 mm; Welding current: 10 kA; Welding pressure: 200 kg; Welding time: 12 cycles/60 Hz. The symbols used to show the results have the following meanings: O ○: 1000 or more spots; ○ : 700 or more spots; ▵: Less than 700 spots. (4) Corrosion resistance of flat and formed portions: A salt spray test conforming to the requirements of JIS Z-2371 was conducted for a maximum of 480 hours on a flat portion of each steel sheet and also on a cross-cut and Erichsen-extruded portion having a height of 7 mm. The corrosion resistance of the flat portion was judged by the length of time which had passed before white rust formed in 5% by area of its surface, and the corrosion resistance of the formed portion by the length of time which had passed before white rust began to flow down along the tested portion. The symbols used to show the results have the following meanings: O ○: No white rust occurred; + ○: Over 240 hours, but within 480 hours; ○ : Over 120 hours, but within 240 hours; - ○: Over 72 hours, but within 120 hours; ▵: Over 24 hours, but within 72 hours; x: Within 24 hours. (5) Adhesive strength of colored film: One hundred squares defining a checkered pattern and having a distance of 1 mm from one another were cut in the colored film on each steel sheet, and an adhesive tape was bonded to, and removed from, the checkered surface to see how the film would peel off the steel sheet. The symbols used to show the results have the following meanings: O ○: No peeling occurred; ○: Peeling occurred to less than 10% by area of the film; ▵: Peeling occurred to from 10%, inclusive, to less than 20%, by area; x: Peeling occurred to 20% or more by area. (6) Press Formability A specimen of each colored steel sheet was subjected to hat-drawing, by 10 mm-extrusion through a 50 mm⊘- die with a 120 mm⊘ blank, and an adhesive tape was bonded to, and removed from, the drawn portion of the film to determine how the film would peel off the steel sheet and adhere to the tape, and change in appearance. The symbols used to show the results have the following meanings: O ○: No powdery peeling occurred; + ○: Some powdery peeling occurred only locally, and the film remained substantially unchanged in appearance; ○: The powdery peeling of the film colored the tape very slightly, but the film remained substantially unchanged in appearance; -O ○: The powdery peeling of the film colored the tape slightly, and the film was slightly whitened; ▵: The powdery peeling of the film colored the tape, and the film was heavily whitened; x: The tape was extremely colored as a result of the complete peeling of the film. (7) Light fastness: The colored film on each steel sheet was exposed to light applied by a fadeometer in accordance with the Second Light Exposure Method as specified by JIS L-0842, and its fade resistance was ranked in accordance with the Blue Scale, as follows: O ○: Grade 7 or 8 of the Blue Scale; ○: Grade 5 or 6; ▵: Grade 3 or 4; x: Grade 1 or 2. EXAMPLE 1 AND COMPARATIVE EXAMPLE 1 Colored steel sheets embodying this invention and having colored films formed by using different kinds of coloring agents were each evaluated for appearance, luster, weldability, formability, film adhesion, corrosion resistance and light fastness. Comparative colored steel sheets were likewise evaluated. Further information on the steel sheets and the results of their evaluation are shown in TABLES 5 and 6. All of the colored films contained 70 parts by weight of coloring agent against 100 parts by weight of base resin, and had a thickness of 1.5 µm. As is obvious from TABLE 6, the steel sheets of this invention which had been prepared by using water-soluble dyes as the coloring agents exhibited the intended appearance and luster, while having colored film thickness allowing welding. On the other hand, the comparative steel sheets, on which pigments had been used, were poor in appearance and luster, apparently because of too small a film thickness, and were also inferior in press formability, corrosion resistance and film adhesion. Steel sheets 8 to 11 of this invention, on which azo dyes in the form of complex metal salts had been used, showed a particularly high level of light fastness, and a particularly high level of corrosion resistance owing to the corrosion inhibiting action of the dyes themselves. EXAMPLE 2 AND COMPARATIVE EXAMPLE 2 Water-soluble monoazo and phthalocyanine dyes, and water-soluble azo dyes in the form of complex metal salts were used to form colored films having different thicknesses, and analysis was made of the effects which the different film thicknesses might have on the properties of the films. Further information on the steel sheets and the results of their evaluation are shown in TABLE 7 and 10. FIGURE 3 shows the results of weldability tests in relation to film thickness. As is obvious therefrom. the steel sheets began to lower their weldability when the colored film had a thickness in excess of 2.5 µm, and the steel sheets having a film thickness in excess of 3.0 µm could not be properly welded. EXAMPLE 3 AND COMPARATIVE EXAMPLE 3 Water-soluble monoazo dyes, and water-soluble azo dyes in the form of complex metal salts were used to form colored films containing different proportions of dyes, and analysis was made of the effects which the different dye proportions might have on the properties of the films. Further information on the steel sheets and the results of their evaluation are shown in TABLE 11 and 14. EXAMPLE 4 AND COMPARATIVE EXAMPLE 4 Water-soluble monoazo, disazo and phthalocyanine dyes, and water-soluble azo dyes in the form of complex metal salts were used to form colored films on differently plated steel surfaces, of which the majority carried a chromate film thereon. The method for chromating treatment, the coating weight of chromium, the base resin and the baking temperature were also varied, and analysis was made of the effects which the type of plating, the presence of a chromate film, the method for chromating treatment, the coating weight of chromium, the base resin and the baking temperature might have on the properties of the colored films. Further information on the steel sheets and the results of their evaluation are shown in TABLE 15 to 20. EXAMPLE 5 AND COMPARATIVE EXAMPLE 5 Colored films were formed on steel sheets by using water-soluble monoazo, phthalocyanine and disazo dyes, and water-soluble azo dyes in the form of complex metal salts, and also a solid lubricant and a granular rust-inhibitive pigment. They were evaluated for various properties. Analysis was made of the effects which the proportions of the solid lubricant and the granular rust-inhibitive pigment might have on the properties of the films. Further information on the steel sheets and their results of their evaluation are shown in TABLE 21 to 28. EXAMPLE 6 AND COMPARATIVE EXAMPLE 6 Evaluation for corrosion resistance, etc. was made of a steel sheet embodying this invention and having a colored film containing water-soluble azo dye in the form of a complex chromium salt, and of a comparative steel sheet having a clear film not containing any such dye. Further information of the steel sheets and the results of their evaluation are shown in TABLES 29 and 30. TABLES 29 and 30 show also Sample No. 7 of COMPARATIVE EXAMPLE 4 having a colored film formed on the steel surface on which no chromate film had been formed. As is obvious from TABLES 29 and 30, the steel sheet embodying this invention (comprising a zinc-plated steel sheet, a chromate film formed thereon and having a coating weight of 50 mg/m², and a colored film formed thereon and having a thickness of 1.5 µm) were superior in corrosion resistance to the comparative steel sheet comprising a zinc-plated steel sheet, a chromate film formed thereon and having a coating weight of 50 mg/m², and a clear film formed thereon. The superiority in corrosion resistance of the steel sheet embodying this invention is not only due to the barrier effect produced by the chromate film and the resin film, but also due to the fact that the dye itself forms a passive film, as the molecules of the dye mixed with the resin are uniformly and densely distributed in the colored film. Moreover, it is apparent that the corrosion resistance of the steel sheet embodying this invention does not only owe itself to the colored film, but is further enhanced by the interaction of the colored film with the underlying chromate film. This is obvious from its comparison with Sample No. 7 of COMPARATIVE EXAMPLE 4 having no chromate film formed thereon. This steel sheet of COMPARATIVE EXAMPLE 4 having a colored film formed directly on a zinc plated steel surface was by far inferior in corrosion resistance to the steel sheet embodying this invention. It is evident that the outstandingly high corrosion resistance of the steel sheet embodying this invention was not only due to the barrier effect added to by the chromate film, but also due to the synergistic effect produced by the chromate film and the colored film formed thereon. EXAMPLE 7 AND COMPARATIVE EXAMPLE 7 A chromate film having a coating weight of 50 mg/m² in terms of chromium was formed by a continuous roll coater on the degreased surface of a steel sheet plated with a Zn-Ni alloy containing 12% Ni. A composition comprising 100 parts by weight of a water-soluble epoxy ester resin (No. 1 in TABLE 1) and 70 parts by weight of Aizen Spilon Yellow-1W-Liq. (Hodogaya Chemical Co., Ltd.; TABLE 2, No. 10) was applied onto the chromate film by the continuous roll coater, and cured by heating at 210°C to form a colored film having a thickness of 1.5 µm, whereby a colored steel sheet embodying this invention was prepared. For the sake of comparison, a steel sheet plated with a Zn-12%Ni alloy and having a degreased surface was dipped for five seconds in a bath of an aqueous solution of nitric acid having a concentration of 5% by weight and temperature of 25°C, was rinsed with water, and was dried, whereby a black film was formed on the steel sheet. The comparative steel sheet, however, began to lower its blackness when about 0.03 m² of its surface per liter of the solution had been blackened, and when about 0.04 m² of its surface had been treated, the L value as a measure of its blackness exceeded 20, and no further continuation of blackening treatment was possible. On the other hand, continuous treatment was possible for the manufacture of the steel sheet embodying this invention as long as the supply of the film-forming composition could be continued. After the samples had been prepared, a part of the remainder of the solution which had been used for forming the colored film on the steel sheet embodying this invention was collected from the tray in the roll coater, and a part of the aqueous solution of nitric acid used for blackening the comparative steel sheet was likewise collected. The zinc content of each solution was determined by an apparatus for atomic absorption spectroscopy (Model Z-8100 of Hitachi Limited) as a measure of the amount of zinc eluted from the plating layer. Only a trace, or even a smaller amount of zinc was detected from the solution employed for the purpose of this invention, but the solution used for the comparative purpose was found to contain zinc in the amount indicating the elution of about 5 g of Zn-12%Ni alloy per m² of the treated surface. The salt spray test as hereinbefore described was conducted for evaluating each sample for corrosion resistance. The flat tested portion of the steel sheet embodying this invention was free of any white rust when test lasting for 480 hours was over, but the whole surface of the comparative steel sheet was covered with white rust in about one to two hours. A chromate film having a coating weight of 50 mg/m² in terms of chromium was formed by coating on the blackened surface of a comparative steel sheet, and a clear film having a thickness of 1.5 µm was formed on the chromate film from the resin shown at No. 1 in TABLE 1. Then, the salt spray test as hereinbefore described was conducted again on the comparative steel sheet. White rust was found of about 5% by area of the tested surface when the test lasting for 240 hours was over. Thus, it was improvement in corrosion resistance over the steel sheet having no chromate or resin film formed on its surface. The comparative black steel sheet may also be satisfactory in corrosion resistance as a material for making an electrical appliance for domestic use, but can only be manufactured by a process comprising the steps of forming a black film by reactive blackening treatment on a steel sheet plated with a Zn-Ni alloy, rinsing with water, drying, forming a chromate film, and forming a clear resin film, while the colored steel sheet of this invention can be manufactured by a process comprising the steps of forming a chromate film on a plated steel sheet, and forming a colored film thereon. It is, therefore, another great advantage of the colored steel sheet of this invention that it can be manufactured by a simplified process not including any of the steps of reactive coloring treatment, rinsing and drying. The following symbols, which will appear in many of the tables, have the following meanings: *1: See TABLE 1; *2 See TABLE 2; *3 Parts by weight of the coloring agent for 100 parts by weight of base resin; *4 See TABLE 3; *5 Parts by weight of solid lubricant for 100 parts by weight of base resin; *6 See TABLE 4; *7 Parts by weight of granular rust-inhibitive pigment for 100 parts by weight of base resin.

Claims: A weldable colored steel sheet comprising a steel sheet having a surface plated with zinc or zinc alloy, and carrying a chromate film formed on said plated surface and having a coating weight of 10 to 200 mg/m² in terms of metallic chromium, and a colored film formed on said chromate film from a composition comprising 100 parts by weight of a water-soluble or water-dispersible thermosetting resin as a base resin and 1 to 200 parts by weight of water-soluble dye as a coloring agent, said colored film having a thickness of 0.3 to 3.0 µm, and a color other than black. A weldable colored steel sheet comprising a steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on said plated surface and having a coating weight of 10 to 200 mg/m² in terms of metallic chromium, and a colored film formed on said chromate film from a composition comprising 100 parts by weight of a water-soluble or water-dispersible thermosetting resin as a base resin, 1 to 200 parts by weight of a water-soluble dye as a coloring agent and 1 to 100 parts by weight of a solid lubricant, said colored film having a thickness of 0.3 to 3.0 µm, and a color other than black. A weldable colored steel sheet comprising a steel sheet having a surface plated with zinc or zinc alloy, and carrying a chromate film formed on said plated surface and having a coating weight of 10 to 200 mg/m² in terms of metallic chromium, and a colored film formed on said chromate film from a composition comprising 100 parts by weight of a water-soluble or water-dispersible thermosetting resin as a base resin, 1 to 200 parts by weight of a water-soluble dye as a coloring agent and 1 to 100 parts by weight of a granular rust-inhibitive pigment, said colored film having a thickness of 0.3 to 3.0 µm, and a color other than black. A weldable colored steel sheet comprising a steel sheet having a chromate film formed on said plated surface and having a coating weight of 10 to 200 mg/m² in terms of metallic chromium, and a colored film formed on said chromate film from a composition comprising 100 parts by weight of a water-soluble or water-dispersible thermosetting resin as a base resin, 1 to 200 parts by weight of a water-soluble dye as a coloring agent, 1 to 100 parts by weight of a solid lubricant and 1 to 100 parts by weight of granular rust-inhibitive pigment, said colored film having a thickness of 0.3 to 3.0 µm, and a color other than black. A weldable colored steel sheet as set forth in any of claims 1 to 4, wherein said dye is at least one dye selected from the group consisting of azo, azomethine, quinoline, ketoneimine, fluorone, nitro, xanthene, acenaphthene, quinophthalone, anthraquinone, aminoketone, methine, perylene, coumarin, perinone, triphenyl, triallylmethane, phthalocyanine, isochlorophenol, and azine dyes. A weldable colored steel sheet as set forth in claim 5, wherein said dye is a mixture of a water-soluble azo dye in the form of a complex metal salt, and at lease one different water-soluble dye. A weldable colored steel sheet as set forth in any of claims 1 to 4, wherein said dye is an azo dye in the form of a complex metal salt. A weldable colored steel sheet as set forth in claim 2, 4, 5, 6 or 7, wherein said solid lubricant is at least one material selected from the group consisting of polyolefin wax and other hydrocarbon compounds, fluororesins, fatty acid amides, metallic soaps, molybdenum disulfide and other metal sulfides, graphite, graphite fluoride, boron nitride and polyalkylene glycols. A weldable colored steel sheet as set forth in claim 3, 4, 5, 6 or 7, wherein said rust-inhibitive pigment is at least one material selected from the group consisting of sparingly soluble chromium compounds and silica.

Inventors: WATANABE TOYOFUMI NKK CORPORAT, YAMASHITA MASAAKI NKK CORPORAT, YOSHIMI NAOTO NKK CORPORATION, WATANABE, TOYOFUMI, NKK CORPORATION, YAMASHITA, MASAAKI, NKK CORPORATION, YOSHIMI, NAOTO, NKK CORPORATION
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1992
Main classification: C23C22
IPC codes: B05D5, B23K35, B05D7, C23C22, C09D7, B32B15