HOT-DIP ZINC-ALUMINUM ALLOY COATED STEEL SHEET FOR PREPAINTED STEEL SHEET, PROCESS FOR PRODUCING THE SAME AND PREPAINTED STEEL SHEET

Abstract: A hot-dip zinc-aluminum alloy coated steel sheet produced by coating in a hot dipping bath comprising 0.3 to 3.5 wt.% of Al, 100 ppm or less of Pb and the balance of Zn and unavoidable impurities is provided. The bath may further contain specified amounts of Si as well as Mg, Mn and/or Cu. A process for producing said coated steel sheet is also provided, wherein the temperature of the steel sheet to be dipped into the hot dipping bath is adjusted to be lower than the bath temperature and, if necessary, the amount of the molten zinc-aluminum alloy deposited on the steel sheet is controlled with a special gas wiping type of equipment and, if necessary, the coated steel sheet is reheated at a temperature higher than the melting temperature of the coating layer. A prepainted steel sheet prepared from this steel sheet is also provided.

Description: DESCRIPTION HOT-DIP ZINC-ALUMINUM ALLOY COATED STEEL SHEET FOR PREPAINTED STEEL SHEET, PROCESS FOR PRODUCING THE SAME AND PREPAINTED STEEL SHEET Technical Field: The present invention relates to prepainted steel sheets having excellent properties suitable for use as construction materials such as roof materials and wall materials as well as household electric appliances, hot-dip zinc-aluminum alloy coated steel sheets having properties superior to those of conventional ones and suitable for use as the sheets of the prepainted sheets and a process for producing them. Background Art: Steel sheets coated with zinc ( Zn) or prepainted steel sheets produced therefrom have been used in construction materials or household electric appliances. Further steel sheets coated with zinc-aluminum (Zn-Al) alloy attract attention as materials taking the place of Zn coated steel sheets, since they have corrosion resistance, etc. superior to those of the Zn coated ones. Various processes have been proposed heretofore for producing the Zn-Al alloy coated steel sheets. These processes include one comprising the use of a coating bath comprising 5 to 25 wt.% of Al, 0.1 wt.% or less of Pb and the balance of Zn (see Japanese Patent Publication No. 25220/1976); one comprising the use of a coating bath comprising more than 3.5 wt.% but not more than 10 wt.% of Al, and Mg, Be, Ti and Cu each in a specified range of a small amount relative to the concentration of Pb, Sn or both of them in the coating bath (see Japanese Patent Publication No. 47055/1978); one comprising the use of a coating bath comprising 3 to 15 wt.% of Al, about 85 to 97 wt.% of Zn and small amounts of rare earth elements (see Japanese Patent Publication No. 500475/1982); and one comprising the use of a coating bath comprising 0.05 to 2.0 wt.% of Al, 0.01 to 0.1 wt.% of Mn and the balance of Zn and unavoidable impurities (see Japanese Patent Publication No. 32700/1985). A principal object of these processes is to reduce the weight loss of the coated steel sheets due to corrosion by the addition of Al. The loss due to corrosion of the coated steel sheets in exposure tests in outdoor is usually reduced as the Al concentration in the coating bath is increased and, therefore, the corrosion resistance of the sheets is improved. However, an alloy layer formed on the interface with iron becomes thicker and the adhesion and the workability of the coating layer are seriously reduced as the Al concentration is increased. Methods have been proposed to solve these problems by adding small amounts of various elements. However, they have problems that the use of the limited kinds of elements in limited amounts is troublesome, that the coating pot must be exchanged in order to switch over the coating bath and that the appearance of the coated steel sheet surface is impaired. Under these circumstances, the development of a process for producing coated steel sheets which can satisfy the required qualities with as little as possible elements added is demanded. The following properties are required of Zn-Al alloy coated steel sheets, particularly those to be used as sheets for prepainted steel sheets from the viewpoint of the use of them: (1) the corrosion resistance of the steel sheet surface is improved, (2) a self-sacrificing rust-preventive power of Zn for that part of iron which is exposed when the steel sheet is cut is retained, (3) no crack is formed in a bent part of the steel sheet in the working step, (4) the coating layer has an excellent adhesion which is not reduced with time, and (5) the surface smoothness is excellent. The present applicant proposed a coated steel sheet for prepainted galvanized steel sheets which is produced by coating a steel sheet with hot dipping bath comprising 0.3 to 3.5 wt.% of Al and the balance of Zn and unavoidable impurities (see Japanese Patent Application No. 159469/1983). It is apparent from the phase diagram of Zn-Al alloy that the eutectic point is realized when the alloy comprises 5 wt.% of Al (95 wt.% of Zn). When the Al content is deviated to some extent from 5 wt.%, the texture of the solidified alloy is quite different from that of the 5 wt.% Al alloy unless it is quenched at a very high speed. The Al-Zn alloy having 5 wt.% Al content is eutectic and, therefore, its melting point is low and Al and Zn are dispersed homogeneously irrespective of the cooling rate. However, when the Al content is less than 5 wt.%, for example, 1 wt.%, a primary crystal of Zn containing only very small amount of the Al component is formed in the coating layer and the majority of the Al component remains in the finally solidified grain boundary. Thus no coating layer having homogeneous composition is formed. It has been believed that an Al-Zn alloy having an Al content of 5 wt.% is advantageous for homogeneously dispersing Al and Zn and for giving a stable texture. Disclosure of Invention: After investigations of an Al-Zn alloy having an Al content of 0.3 to 3.5 wt.% which is less than the above-described value, 5 wt.%, the inventors have completed the present invention. According to the present invention, the qualities and properties required of Zn-Al alloy coated steel sheets are satisfied or further improved. Brief Description of Drawings: Figs. 1(a) to (c) are microphotographs of the metal textures on the surfaces of the coating layers formed in Example 5 of the present invention according to Claim 4 and Comparative Example. They are X-ray images of Al on the surface obtained with EPMA. Figs. 1(a), 1(b) and 1(c) are microphotographs of the metal textures of the coating layer surfaces obtained when (steel sheet temperature at dipping time) - (bath temperature) was 20°C, -20°C and -80°C, respectively. Figs. 2(a) and (b) show the concentration distribution of Fe, Zn and Al in the thickness direction of the hot-dip zinc-aluminum alloy coated steel sheets produced in Example 5 of the present invention according to Claim 4 and Comparative Example. They show the concentration distributions obtained when (steel sheet temperature at dipping time) - (bath temperature) was 20°C and -80°C, respectively. Fig. 3 is a schematic drawing of the hot dipping equipment used in Example 6 of the present invention. Figs. 4(a) to (c) are microphotographs of the appearances of the metal textures of the coating layer surface formed in Example 7 of the present invention according to Claim 6 and Comparative Example. Fig. 4(a) is a microphotograph of the metal texture of the coating layer surface formed under conditions comprising (steel sheet temperature at dipping time) - (bath temperature) of -60°C, a nozzle slit clearance of 0.8 mm, an ejecting 2 pressure of gas of 1.0 kg/cm and a distance between front and back nozzles of 50 mm. Fig. 4(b) is one formed under conditions comprising (steel sheet temperature at dipping time) - (bath temperature) of -20°C, a nozzle slit clearance of 0.6 mm, an ejecting pressure of gas 2 of 1.5 kg/cm and a distance between front and back nozzles of 20 mm. Fig. 4(c) is one formed under conditions comprising (temperature of steel sheet at dipping time) - (bath temperature) of -80°C, a nozzle slit clearance of 1.2 mm, an ejecting pressure of gas 2 of 0.1 kg/cm and a distance between front and back nozzles of 20 mm. Figs. 5(a) and (b) show the thickness distributions of the coating layers, wherein Fig. 5(a) shows that of the coating layer having a surface not reheated (Comparative Example) and Fig. 5(b) shows that of the deposit coating layer having a surface reheated at 460°C (Example 8 of the present invention according to Claim 7) . Figs. 6(a) to (c) are microphotographs of the metal textures of the coating layer surfaces obtained in Example 9 according to Claim 8 of the present invention and Comparative Example. They are X-ray images of Al on the surface obtained with EPMA. Figs. 6(a), 6(b) and 6(c) are microphotographs of the metal textures of the coating layer surfaces obtained when the cooling speed was 2°C/sec, 17°C/sec and 47°C/sec, respectively. Best Mode for Carrying Out the Invention: The present invention described in Claims 9 and 10 relates to a prepainted steel sheet produced from the above-described Zn-Al alloy coated steel sheet. The prepainted steel sheets are those previously painted with a paint. They are produced continuously on a large scale by painting galvanized steel sheets or zinc alloy coated steel sheets with a roll coater usually after chemical conversion treatment. The demand for them as starting materials for construction materials, household electric appliances, business machines, etc. is now increasing, since they have excellent, uniform qualities, they can be supplied in large amounts and no painting is necessary after application. The properties required of the prepainted steel sheets are mainly adhesion, corrosion resistance, workability, weather resistance and scratch resistance. However, it is quite difficult to satisfy all o the required properties with only one paint. Thus it was a usual practice to classify paints into two groups in the prior art, i.e. those for under-painting which could impart adhesion and corrosion resistance and paints for toppainting which could impart workability, weather resistance and scratch resistance to further improve the qualities and properties of the prepainted steel sheets. These paints were applied by the so-called two-coat two-bake process wherein the sheets were baked after application of the under-painting paint and baked again after application of the toppainting paint in order to improve the qualities of the prepainted steel sheets. However, recently, durability over a period of as long as, for example, 10 or 20 years is demanded of the construction materials such as roof and wall materials in various cases. The term "durability" as used herein means weather resistance and corrosion resistance. Namely, an excellent durability means that the color tone and gloss are substantially unchanged and no rust is formed in 10 or 20 years. The excellent weather resistance can be given by using a paint having excellent properties against chalking and fading. However, it is difficult to inhibit the rust formation over a long period of time. The formation of red rust is often observed in a portion of roof or wall material worked by roll forming machine only several years after the construction, though the timing of the rust formation varies depending on the environmental conditions. Such rust formation occurs even when the material is exposed to the outdoor for only a short period of time, since cracks are formed in the worked part already prior to the use and in an extreme case, the steel texture is exposed through the cracks formed in the coating layer. To solve this problem, it is necessary to cover the coating layer with a painting film having such an excellent elongation that it can withstand the deformation in the course of working and to use a steel sheet having a coating layer having such an excellent workability that it is not cracked in the course of working. The former requirement was satisfied by producing prepainted steel sheets or metal sheets having excellent corrosion resistance after intensive investigations. The inventors made investigations for the purpose of satisfying the latter requirement. After intensive investigations made for the purpose of satisfying the latter requirement, the inventors have completed the inventions as set forth in Claims 1 to 8 of the present invention. Namely, the inventors have succeeded in the production of Zn-Al alloy coated steel sheets having excellent properties. After further investigations of prepainted steel sheets, the inventors have found that two-coat steel sheets having a performance equivalent to that of three-coat steel sheets can be produced when the Zn-Al alloy coated steel sheets as set forth in Claims 1 to 8 are used. The inventors have completed the inventions as set forth in Claims 9 and 10 on the basis of this finding. An object of the present invention is to solve the above-described problems and the gist thereof resides in: 1. a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, produced by coating the sheet in a hot dipping bath comprising 0.3 to 3.5 wt.% of Al, 100 ppm or less of Pb and the balance of Zn and unavoidable impurities, 2. a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, produced by coating the sheet in a hot dipping bath comprising 0.3 to 3.5 wt.% of Al, 1/100 to 1 part, per part of Al, of Si, 100 ppm or less of Pb and the balance of Zn and unavoidable impurities, 3. a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, produced by coating the sheet in the same hot dipping bath as that of Claim 1 or 2 which further contains 0.01 to 1.5 wt.% of one or more metals selected from the group consisting of Mg, Mn and Cu, 4. a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, wherein the temperature of the steel sheet dipped in the hot dipping bath is lower than that of said bath in the step of coating in the hot dipping bath according to Claim 1, 2 or 3, 5. a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, wherein the steel sheet is taken out of the hot dipping bath and then the amount of the molten zinc-aluminum alloy deposited on the steel sheet is controlled with a gas wiping type of equipment for controlling the amount of molten zinc-aluminum alloy deposit under conditions comprising a slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm and an 2 ejecting pressure of gas of 0.1 to 2.0 kg/cm in the step of coating in the hot dipping bath according to Claim 1 , 2 or 3, 6. a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 4, wherein the steel sheet is taken out of the hot dipping bath and then the amount of the molten zinc-aluminum alloy deposited on the steel sheet is controlled with a gas wiping type of equipment for controlling the amount of molten zinc-aluminum alloy deposit under conditions comprising a slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm and an ejecting pressure of gas 2 of 0.1 to 2.0 kg/cm , 7. a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 5 or 6, wherein hot-dip zinc-aluminum alloy coated steel sheet is reheated to a temperature above the melting temperature of the coating layer, 8. a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 5 or 6 , wherein the hot-dip zinc-aluminum coated steel sheet is cooled to the solidifying point thereof at a rate of at least 10°C/sec, 9. A prepainted steel sheet having excellent workability and corrosion resistance which comprises the hot-dip zinc-aluminum alloy coated steel sheet according to Claim 1 to 3 or the hot-dip zinc-aluminum alloy coated steel sheet produced according to the process of Claims 4 to 8, including a layer formed thereon by chemical conversion treatment and further a surface painting film layer formed thereon, and 10. a prepainted steel sheet according to Claim 9, wherein the chemical conversion layer is one formed by treating the hot-dip zinc-aluminum alloy coated steel sheet with a chromic acid solution containing silica having an average particle diameter of 50 mμ and a 2 specific surface area of 200 m /g in such a manner that the amount of the coating film after drying will be 50 2 to 250 mg/m to impart an excellent scratch resistance. The invention of Claim 1 relates to a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, which is produced by coating in a hot dipping bath comprising 0.3 to 3.5 wt.% of Al, 100 ppm or less of Pb and the balance of Zn and unavoidable impurities. The Al concentration is thus limited because when it exceeds 3.5 wt.%, the self-sacrifying anticorrosive effect of Zn on iron is reduced while when it is insufficient, the effect of improving the corrosion resistance at the surface of the coating layer is unsatisfactory. With at least 0.3 wt.% Al concentration, the workability is also improved and this effect is remarkable when the Al concentration is 0.5 wt.% or higher. The Pb concentration is limited because when it exceeds 100 ppm, the adhesion is reduced with time due to intercrystalline corrosion and consequently the corrosion resistance which is particularly important for the prepainted steel sheet is damaged. With 100 ppm or less of Pb, stable adhesion which is not reduced with time can be obtained. With an Al concentration of 0.3 to 3.5 wt.% and a Pb concentration of 100 ppm or less, a zinc-aluminum alloy coated steel sheet which does not cause reduction in adhesion with time, has an excellent workability and is quite suitable for use as the sheet for a prepainted steel sheet can be obtained. The temperature of the hot dipping bath may be one at which Zn and Al are molten to form a homogeneous melt, such as about 430 to 480°C. The invention as set forth in Claim 2 relates to a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, which is produced by coating in a hot dipping bath comprising 0.3 to 3.5 wt.% of Al, 1/100 to 1 part, per part of Al, of Si, 100 ppm or less of Pb and the balance of Zn and unavoidable impurities. Although a hot-dip zinc-aluminum alloy coated steel sheet having excellent workability and adhesion after aging can be obtained when the Al concentration is in the range of 0.3 to 3.5 wt.% and the Pb concentration is 100 ppm or less as stated in Claim 1, it is further preferred to add 1/100 to 1 part, per part of Al, of Si. The addition of Si serves to inhibit the formation of the alloy layer at the interface between the steel sheet and the coating layer to thereby make the formation of a thin alloy layer possible. Thus, a zinc-aluminum alloy coated steel sheet having further improved workability and adhesion after aging can be obtained. The Si concentration is limited because when it is as low as about 1/200 of the Al concentration, no improvement in the workability or adhesion can be obtained. Another reason is that the control of the addition of Si in an amount of as small as 1/200 of Al is difficult, since the Al concentration is as low as 0.3 wt.% or less. The lower limit of the Si concentration is thus 1/100 of the Al concentration. The Al concentration is limited for the same reasons as those of Claim 1. The invention stated in Claim 3 relates to a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, which is produced by coating in the same hot dipping bath as that of Claim 1 or 2, which further contains 0.01 to 1.5 wt.% of one or more metals selected from the group consisting of Mg, Mn and Cu. When metallic elements effective in improving the corrosion resistance of the galvanized steel sheet. such as Mg, Mn or Cu, are added to the bath, the effects of the present invention, i.e. satisfactory workability, corrosion resistance and adhesion after aging are further improved. With 0.01 wt.% or more of the metallic elements, the effects can be obtained. The addition of the metallic elements in an amount larger than 1.5 wt.% is not preferred from the viewpoint of the cost or efficiency of working. Mg, Mn and Cu may be used alone or in combination of them. By adjusting the amount of Pb to 100 ppm or less, a stable adhesion after aging can be attained. The invention stated in Claim 4 relates to a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, wherein the temperature of the starting steel sheet dipped in the hot dipping bath is lower than that of the bath in the step of coating in the hot dipping bath according to claim 1, 2 or 3. The feature of the invention stated in Claim 4 resides in that the temperature of the steel sheet dipped in the hot dipping bath at the dipping time is adjusted to a temperature lower than that of the bath. Preferably, the temperature of the steel sheet to be dipped in the hot-dip Zn-Al alloy coating bath is adjusted to a temperature lower than the bath temperature by at most 80°C, particularly by 10 to 60°C. By this process, a hot-dip zinc-aluminum alloy coated steel sheet having an excellent surface corrosion resistance and a high power of protecting the edge of the steel from rust is obtained. Also the corrosion resistance in a bent part is improved. Usually in the production of a hot-dip coated steel sheet in hot dipping equipment provided with a continuous annealing furnace, the temperature of the steel sheet to be dipped in a hot dipping bath is kept higher than a temperature of the bath from the viewpoints of the adhesion and the heating effect of the bath. Since, the steel sheet is thicker than the coating layer and the temperature of the steel sheet is high, the cooling of the coating layer starts with its surface and the interface of the coating layer with the steel sheet is solidified layer. Consequently, the Al concentration is high in the interface and the thickness of the alloy layer is increased to thereby reduce the workability and the self-sacrificing anticorrosive effect on the steel, while the Al concentration on the coating layer surface is low to reduce the corrosion resistance. In the invention stated in Claim 4, the temperature of the steel sheet dipped in the hot dipping bath is kept below the bath temperature contrary to the conventional processes in order to initiate the cooling of the coating metal on the side facing to the steel sheet. By this process, the inventors have succeeded in reducing the Al concentration on the interface side and reducing the amount of the alloy layer formed. In this process, the self-sacrificing anticorrosive power (resistance to the initial red rust formation) of Zn is retained. Further, since the alloy layer formed is thin, no crack is formed in the bent part. On the other hand, the Al concentration in the surface region of the coating layer is increased and, therefore, the corrosion resistance of the surface is improved (namely, the weight loss due to corrosion is reduced) . The feature of the present invention thus resides in that the temperature of the steel sheet to be dipped in the hot-dip Zn-Al alloy coating bath is kept below the temperature of the bath and the temperature of the steel sheet to be pulled out of the bath is kept below the bath temperature as far as possible in order that the solidification of the coating layer be started with the part in contact with the steel sheet and is completed as soon as possible. Such a process has never been known as yet. The hot dipping bath temperature is such that Zn and Al are molten to form a homogeneous melt, for example, about 430 to 480°C. The temperature of the steel sheet to be dipped in the hot dipping bath is preferably kept below the bath temperature by 10 to 80°C. For example, when the bath temperature is 480°C, the temperature of the steel sheet to be dipped in the hot dipping bath is kept in the range of 400 to 470°C. This is because when the temperature of the steel sheet to be dipped in the hot dipping bath is equal to or lower than the bath temperature and also that of the steel sheet to be pulled out of the bath is lower than the bath temperature, the steel sheet will have a thickness larger than that of the coating layer and, since the temperature of the sheet pulled out is lower than the bath temperature, the cooling of the coating layer is started with the inner face and the Al concentration is high in the surface layer, since Al tends to concentrate in a part which is solidified later. However, when the temperature of the steel sheet is lower by more than 80°C, the adhesion of the coating layer is reduced and the lowering in the bath temperature is serious to thereby increase the operation cost. The Al concentration in the grain boundaries where solidification occurs later is higher than that in the grain centers where crystallization occurs in an initial stage. In practice, the Al concentration in the surface layer is uneven and it forms a honey-comb pattern in which parts of a relatively low Al concentration are surrounded by parts of a high Al concentration. However, the area of the parts of the high Al concentration is large enough for improving the corrosion resistance of the whole surface. Although the Al-Zn crystals are formed also in the surface layer upon cooling of the surface layer, also the homogeneous dispersion of Al in the surface layer is accelerated, since the cooling velocity of the whole coating layer is increased by dipping the steel sheet kept at a low temperature. Further the bath is preferably a low temperature bath. The invention stated in Claim 5 relates to a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, wherein the steel sheet is taken out of the hot dipping bath and then the amount of the molten zinc-aluminum alloy deposited on the steel sheet is controlled with a gas wiping type of equipment for controlling the amount of molten zinc-aluminum alloy deposit under conditions comprising a slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm and an 2 ejecting pressure of gas of 0.1 to 2.0 kg/cm in the step of coating in the hot dipping bath according to Claim 1, 2 or 3. The invention stated in Claim 6 relates to a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 4, wherein the steel sheet is taken out of the hot dipping bath and then the amount of the molten zinc-aluminum alloy deposited on the steel sheet is controlled with a gas wiping type of equipment for controlling the amount of molten zinc-aluminum alloy deposit under conditions comprising a slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm and an ejecting pressure of gas 2 of 0.1 to 2.0 kg/cm . As described above, the Pb concentration in the hot dipping bath must be controlled to 100 ppm or less in order to obtain an excellent adhesion after aging. However, when the Pb concentration is 500 ppm or less, the surface of the coating layer has a ripping pattern to impair the appearance thereof unfavorably. After intensive investigations made for the purpose of eliminating such a pattern from the surface, the inventors have succeeded in obtaining an excellent appearance by controlling the amount of the molten zinc-aluminum alloy deposit with a gas wiping type of equipment under conditions comprising a nozzle slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm and an ejecting pressure of gas of 0.1 2 to 2.0 kg/cm . These values are limited for the following reasons: 1. Nozzle slit clearance: When the lower limit is less than 0.6 mm, the secondary pressure variation of the gas is too much to obtain a consistent appearance. When the upper limit exceeds 2.4 mm, the amount of the gas is too much and the energy loss is serious. 2. Distance between front and back nozzles: The lower limit is 10 mm, because when it is less than 10 mm, the vibrating strip is apt to be brought into contact with the nozzle to cause troubles. The upper limit is 40 mm, because a shorter distance gives a better result and the appearance is impaired when it exceeds 40 mm. 3. Ejecting pressure of gas: The lower limit is 0.1 kg/cm 2 because when 2 it is below 0.1 kg/cm , the amount of the deposition cannot be controlled. 2 The upper limit is 2.0 kg/cm because when it exceeds 2.0 kg/cm 2, the energy loss is large and a more consistent appearance can be obtained with a lower pressure. The control of the amount of the deposition is necessary in order to conform to Z 27 specified in JIS G 3302 or G 90 specified in ASTM A 525. The invention stated in Claim 7 relates to a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 5 or 6, wherein the hot-dip zinc-aluminum alloy coated steel sheet is reheated to a temperature above melting temperature of the coating layer. The reheating temperature is preferably 420°C or higher, because the appearance of the sheet can be improved at such a high temperature. Although the reheating temperature is preferably 420°C or above, a high temperature is not preferred from the viewpoints of both energy and equipment cost. Thus, a temperature in the range of 420"C to 560°C is more preferred. In this invention, the effect similar to that of Claim 4 can be obtained even when the temperature of the steel sheet dipped in the hot dipping bath is higher than that of the hot dipping bath, because the temperature of the steel sheet is lower than that of the coating layer in the step of melting the coating layer surface again and, therefore, the recrystallization of the molten coating metal starts with the side in contact with the steel sheet. The alloy layer formed between the steel sheet and the coating layer is not molten again, because it has a high melting point and, therefore, the Al distribution in the alloy layer is kept unchanged and the adhesion of the coating layer is kept high. The invention stated in Claim 8 relates to a process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 5 or 6, wherein the hot-dip zinc-aluminum alloy coated steel sheet is cooled to the solidifying point thereof at a rate of at least 10°C/sec. The lower limit of the cooling rate is 10 c C/sec, because stable corrosion resistance can be obtained at a cooling rate of at least 10°C/sec. Although the higher limit thereof is not limited, a cooling rate of 150°C/sec or less is desirable from the viewpoints of the energy cost and equipment. The invention stated in Claim 9 relates to a prepainted steel sheet having excellent workability and corrosion resistance which comprises the hot-dip zinc-aluminum alloy coated steel sheet according to Claim 1 to 3 or the hot-dip zinc-aluminum alloy coated steel sheet produced according to the process of Claims 4 to 8, a layer formed thereon by chemical conversion treatment and further a surface painting film layer formed thereon. The surface painting film layer comprises those of one-coat, two coat, three-coat, four-coat type, etc. Usually, a two-coat layer is used. The present invention will be described in detail with reference to a two-coat surface painting film layer. First, an under-painting paint is applied to the sheet and baked. The sheet used is the hot-dip zinc-aluminum alloy coated steel sheet according to Claims 1 to 8. The sheet may have a layer formed by chemical conversion treatment having a thickness of about 0.1 to 5 μ. The chemical conversion treatment is conducted in order to improve the corrosion resistance of the sheet and the adhesion of the paint to the steel sheet. The chemical conversion treatments include, for example, a treatment with a phosphate such as zinc phosphate, iron phosphate, manganese phosphate or cobalt phosphate, and a treatment with a chromate such as electrolytic chromate treatment and applied chromate treatment. As the under-painting paints, those ordinarily used for the production of prepainted steel sheets can be used. They include, for example, paints prepared by mixing coloring pigment, rustproof pigment, body, etc. in a resin solution mainly comprising a resin such as epoxy, oil-free polyester, acrylic or urethane resin. Among them, an under-painting paint mainly comprising the epoxy resin which has excellent adhesion and corrosion resistance or the oil-free polyester resin which has also a good workability is preferred. The thickness of the under-paint is 1 to 15 μ, preferably 2 to 12 μ, because the corrosion resistance and scratch resistance are further improved with a thickness of at least 2 μ and the workability is further improved with that of 12 μ or less. When the thickness exceeds 12 μ, the cost is increased. The rustproof pigment may contain 5 to 35% of at least one of strontium chromate, zinc chromate, red lead, zinc plumbate, calcium plumbate, lead cyanamide, basic lead chromate, basic lead silicochromate, basic zinc molybdate and calcium zinc molybdate depending on the use and environments. With at least 5% of this pigment, the rust formation in an early stage can be completely inhibited and no blister is formed with 35% or less thereof. After formation of the under-paint, a top-paint paint is applied thereto and baked to form a topcoat. The top-painting paint comprises preferably acrylic resin, oil-free polyester resin, silicone polyester resin, silicone acrylic resin, alkyd resin, polyurethane resin, polyimide resin, polyamide resin, fluororesin or the like. The thickness of the top-paint is 8 to 50 μ, preferably 10 to 45 μ, because the scratch resistance, workability and weather resistance are improved with the thickness of at least 10 μ but the cost is increased when the thickness is beyond 45 μ. When the Pb concentration exceeds 100 ppm, the adhesion after aging is reduced due to intercrystalline corrosion and, as a result, the corrosion resistance of a worked part which is important particularly in the prepainted steel sheet is also reduced. The concentrations of Al and Pb must be thus limited in order to produce the prepainted steel sheet having excellent properties. The invention stated in Claim 10 relates to a prepainted steel sheet according to Claim 9, wherein the chemical conversion layer is one formed by treating the sheet with a chromic acid solution containing silica having an average particle diameter of 50 mμ and specific 2 surface area of 200 m /g in such a manner that the amount 2 of the coating film after drying will be 50 to 250 mg/m to impart an excellent scratch resistance. In the invention stated in Claim 10, the chromic acid solution containing silica having an average particle diameter of 50 mμ is used. The smaller the average particle diameter of silica, the better the workability and adhesion, though the scratch resistance is not improved when the average particle diameter is too small. When the average particle diameter is too large, the particles are liable to be precipitated disadvantageously. In this invention, the specific surface area 2 of silica in the chromic acid solution is 200 m /g. Although it varies depending on the average particle diameter, no scratch resistance can be obtained when the specific surface area is excessively small. The chemically converted layer is formed with the above-described chromic acid solution in such a manner that the amount of the coating film after drying will be 50 to 250 mg/m 2. With at least 50 mg/m2 of the film after drying, both the scratch and corrosion resistances 2 are improved and with 250 mg/m or less thereof, the workability, adhesion and scratch resistance are improved. (Examples) The following Examples will further illustrate the present invention. The materials to be coated were low-carbon aluminum killed steel sheets (0.8 mm x 914 mm x coil) in all the cases. The sheets were hot-dip coated with Zn-Al alloy with Sendzimir continuous zinc coating equipment. The invention stated in Claim 1 of the present application will be described with reference to Examples. Example 1 Hot-dip zinc-aluminum alloy coated steel sheets were produced with hot dipping baths having various Al concentrations and the workability thereof was examined in order to confirm the effects obtained by the addition of Al according to the present invention. The conditions were as follows: sheet thickness: 0.8 mm bath temperature: 460°C dipping time in the bath: 4 sec 2 amount of deposition: 120 to 260 g/m Pb concentration: 50 ppm (1) Test method: The 0T and 2T bending test methods according to JIS G 3312 were employed. The term "0T bend" and "2T bend" refer to the bends realized when a steel sheet having a thickness of the base metal of T is bend with a hand vise or other suitable means to give an inner diameter of the bend of 0T and 2T, respectively. The cracks of the coating layer in the bend part were examined and the results were classified into five groups. The standard is shown in Table 1 and the results are shown in Table 2. Table 1 (Standard of evaluating degree of cracking) Table 2 The numerals in -Table 2 are the averages of the results. Example 2 Hot-dip zinc-aluminum alloy coated steel sheets were produced with hot dipping baths having various Pb concentrations and the prolonged adhesion thereof was examined in order to confirm the effects of Pb added according to the present invention. The conditions were as follows : sheet thickness: 0.8 mm bath temperature: 460°C dipping time in the bath: 4 sec 2 amount of deposition: 120 to 260 g/m (2) Test method (adhesion after aging or adhesion after working: A paint was applied to the test pieces in a thickness of about 5 μ and then baked in order to prevent formation of white rust. The test pieces were immersed in hot water kept at 80°C for 3 days and taken out. The painting film was removed with a stripping agent. A semi-spherical steel mass weighing 5 kg and having a radius of 3/4 inch was dropped from a height of 500 mm. An adhesive tape was applied to the projecting coated surface to forcedly strip it to thus determine the adhesion of the coating layer. The standard is shown in Table 3 and the results are shown in Table 4. Table 3 (Standard of adhesion after aging or adhesion after- working) Table 4 * The hot dipping bath comprised Al, Pb and the balance of Zn. The numerals in Table 4 are the averages of the results The following Example 3 will illustrate the invention stated in Claim 2 of the present application. Example 3 Hot-dip zinc-aluminum alloy coated steel sheets were produced with hot dipping baths containing various amounts of Si and the workability and adhesion thereof were examined in order to confirm the effects obtained by the addition of Si according to the present invention. (Test method) A semi-spherical steel mass weighing 5 kg and having a radius of 3/4 inch was dropped from a height of 500 mm and an adhesive tape was applied to the projecting coated surface to forcedly strip it to thus determine the adhesion of the coating layer. The results are shown in Table 5. The standard is the same as that shown in Table 3. The conditions were as follows: sheet thickness: 0.8 mm bath temperature: 460°C dipping time in the bath: 4 sec 2 amount of deposition: 120 to 260 g/m Pb concentration: 50 ppm In Table 5, the results of 0T crack formation according to the bending test are shown in the upper row and those of 0T tape tests on adhesion after working are shown in the lower row. When the Al concentration is in the range of 0.3 to 3.5 wt.% and the Pb concentration is not higher than 100 ppm, hot-dip zinc-aluminum alloy coated steel sheets having excellent workability and adhesion after aging can be obtained. It is preferred, however, to add 1/100 to 1 part of Si per part of Al to control the formation of the alloy layer and, therefore, to form only a thin alloy layer. By this process, a hot-dip zinc-aluminum coated steel sheet further improved in adhesion after aging can be obtained. When only 1/200 part of Si is added per part of Al, no improvement is observed. It was thus confirmed that when the Al concentration was 0.3 to 3.5 wt.%, Pb concentration was 100 ppm or less and Si concentration was 1/100 to 1 part per part of Al, an intended hot-dip zinc-aluminum alloy coated steel sheet which has excellent workability and adhesion can be obtained. The following Example 4 will illustrate the invention stated in Claim 3 of the present application. Example 4 Hot-dip zinc-aluminum alloy coated steel sheets were produced with hot dipping baths containing various amounts of Mg, Mn or Cu and the corrosion resistance and adhesion after aging thereof were examined in order to confirm the effects obtained by the addition of it. The conditions were as follows: sheet thickness: 0.8 mm bath temperature: 460°C dipping time in the bath: 4 sec 2 amount of deposition: 120 to 260 g/m Pb concentration: 50 ppm (Test method) Test pieces prepared under the conditions shown in Table 7 were subjected to the chromate treatment. An epoxy resin under-paint having a thickness of 5 μ and then a silicone polyester resin top-paint having a thickness of 15 μ were formed. After baking, they were subjected to a salt spray test according to JIS Z 2371 and the corrosion resistance in the 0T part of each of the prepainted steel sheets was examined. The results are shown in Table 7. The standard is shown in Table 6. Table 6 (Standard of evaluation of corrosion resistance) The adhesion after aging was tested in the same manner as that of Example 2 and the results were evaluated according to the standard shown in Table 3. When a metallic element such as Mg, Mn or Cu capable of improving the corrosion resistance of the galvanized steel sheet was added to the hot dipping bath, the effect of the present invention could be further improved. The effective concentration of the metallic element was 0.01 wt.% or higher. A combination of Si with Mg, Mn or Cu is also usable. By controlling the Pb concentration below 100 ppm, a stable adhesion after aging could be obtained. Table 7 * Amount of Si was 1 part per part of Al. • The numerals of the evaluation results in Table 7 are the averages of the results. The following Example 5 will illustrate the invention stated in Claim 4 of the present application. Example 5 Hot-dip zinc-aluminum alloy coated steel sheets were produced at various steel sheet temperatures and hot dipping bath temperatures as shown in Table 10 and the workability and corrosion resistance thereof were examined in order to confirm the effects of the present invention. The results are shown in Table 10. In this example, the temperature difference between the steel sheet and the bath was controlled to be 0 to 80°C to reduce the energy required to maintain the bath temperature. (Test methods) Surface corrosion resistance test The four edges of each of the test pieces having a size of 60 mm x 60 mm were sealed by coating. Further the whole surface of the test piece other than the surface to be tested was also sealed by painting and then dried. The painting was conducted in such a manner that the area of the exposed surface of the test piece would be 50 mm x 50 mm. Then the test piece was thrown into a salt spray testing instrument and tested according to JIS Z 2371. After the completion of the test conducted for 100 Hr. , the test piece was taken out, corrosion products were removed from the exposed surface thereof and the test piece was weighed. The loss due to corrosion 2 (g/m ) was determined by dividing the difference in weight between that before the test and that after the test with the area of the test surface. The standard is shown in Table 8. Table 8 (Standard of evaluation of surface corrosion resistance) Initial red rust test on edge The test piece was placed in a salt spray testing instrument for 160 Hr. and red rust formed on the edges thereof was examined. The salt spray test was conducted according to JIS Z 2371. The standard of the examination of the red rust is shown in Table 9 Table 9 (Standard of examination of initial red rust formed on edge) The lower is the temperature of the steel sheet at the time of dipping into the bath, as compared with the bath temperature, the better is the corrosion resistance. The temperature difference between the sheet and the bath is preferably within 80°C to reduce the energy required to maintain the bath temperature. The Al distribution on the coating layer surface at various steel sheet temperatus at dipping time is shown in Figs. 1. The samples used for the determination of this distribution were prepared under the following conditions: bath composition: 1 wt.% of Al, 0.005 wt.% of Pb, 0.02 wt.% of Si and the balance of Zn sheet thickness: 0.8 mm bath temperature 460°C dipping time in the bath 4 sec Figs. 1(a), 1(b) and 1(c) are microphotographs of the metal textures obtained when (steel plate temperature at dipping time) - (bath temperature) was 20°C, -20°C and -80°C, respectively. The Al distribution was determined with EPMA (EMX-SM 7; a product of Shimadzu Seisakusho Ltd.). Fig. 2(b) shows the distributions of Fe, Zn and Al in the cross section of the hot-dip Zn-Al alloy coated steel sheet. This figure substantiates the fact that Al in the coating layer was distributed on the surface layer thereof to improve the corrosion resistance. Fig. 2(a) shows a cross section of the coating layer of a hot-dip Zn-Al alloy coated steel sheet produced by a conventional process. It is apparent that Al is distributed densely in the alloy layer. It is thus apparent from the above figures that Al is distributed mostly in the surface layer of the coating layer in the products of the present invention unlike the products produced by a conventional process. As described above, the present invention provides a hot-dip Zn-Al alloy coated steel sheet which has an excellent corrosion resistance of the coating layer, which is prevented from the initial red rust formation on the edges of the sheet and which has an excellent workability of the coating layer. Table 10 he numerals of the evaluation results in Table 10 are the aver'ageβ of the results. The following Example 6 will illustrate the invention stated in Claim 5 of the present application. Example 6 The steel sheet was pulled out of the hot dipping bath and the surface smoothness of the sheet was improved by means of a gas wiping type of equipment for controlling the amount of zinc deposit under conditions comprising a slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm 2 and an ejecting of 0.1 to 2.0 kg/cm . The appearance (smoothness of the coating layer surface) of the produced hot-dip zinc-aluminum alloy coated steel sheet was examined. The conditions were as follows : line speed: 100 m/min sheet thickness: 0.8 mm bath composition: 1 wt.% of Al, 0.005 wt.% of Pb, 0.02 wt.% of Si and the balance of Zn bath temperature: 460°C dipping time in the bath: 4 sec Fig. 3 is a schematic drawing of the hot dipping equipment having a gas wiping type of means for controlling the amount of zinc deposit used in this Example. (Test method) The appearance (smoothness) of the coating layer surface was evaluated on the basis of the standard shown in Table 11. The results are shown in Tables 12(1) and 12(2). Table 11 (Standard of evaluation of appearance smoothness of coating layer surface) Although the Pb concentration in the bath must be controlled to be 100 ppm or less to obtain an excellent adhesion after aging as described above, a Pb concentration of 500 ppm or less is not preferred for obtaining a good appearance, because a rough ripply pattern is formed on the surface with a Pb concentration of 500 ppm or.less. It is apparent from Tables 12(1) and 12(2) that a good appearance (smoothness) can be obtained by the invention stated in Claim 5 of the present application. The control of the amount of the deposition is necessary in order to conform to Z 27 of JIS G 3302 or G90 of AST A525. The control thereof in the range 2 of 50 to 400 g/m is easy in Example 6. Table 12(1) Upper row: appearance (smoothness -of the coating layer surface) Lower row: Possibility of control of the amount of deposition The numerals of the evaluation results in Table 12(1) are the averages of the results. Table 12(2) The numerals of the evaluation, results in Table 12(2) are the averages of the results. The following Example 7 will illustrate the invention stated in Claim 6 of the present application. Example 7 The steel sheet was pulled out of the hot dipping bath and treated with a gas wiping type of equipment for controlling the amount of zinc deposit under conditions comprising a slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm and a gas ejecting pressure of 0.1 to 2.0 kg/cm 2 to give a hot-dip zmc-alummum alloy coated steel sheet. The appearance (surface smoothness) of the sheet was examined. The conditions were as follows: sheet thickness: 0.8 mm bath temperature: 460°C dipping time in the bath: 4 sec 2 amount of deposition: 250 g/m bath composition: 0.5 wt.% of Al, 0.005 wt.% of Pb, 0.01 wt.% of Si and the balance of Zn (Test method) The appearance (smoothness) of the coating layer surface was evaluated on the basis of the standard shown in Table 11. The results are shown in Table 13. The numerals of the smoothness in Table 13 are the averages of the results. Figs. 4(a) to (c) show the appearances of the sheets produced in this Example and Comparative Example. Fig. 4(a) is microphotograph of a sheet produced under conditions comprising (steel sheet temperature at dipping time) - (bath temperature) of -60°C, a nozzle slit clearance of 0.8 mm, a gas ejecting pressure of 2 1.0 kg/cm and a distance between front and back nozzles of 50 mm (the result of the evaluation of the appearance: 1). Fig. 4(b) is that produced under conditions comprising (steel sheet temperature at dipping time) - (bath temperature) of -20°C, a nozzle slit 2 clearance of 0.6 mm, a gas ejecting pressure of 1.5 kg/cm and distance between front and back nozzles of 20 mm (the result of the evaluation of the appearance: 3). Fig. 4(c) is that produced under conditions comprising (steel sheet temperature at dipping time) - (bath temperature) of -80°C, a nozzle slit clearance of 1.2 mm, 2 a gas ejecting pressure of 0.1 kg/cm and a distance between front and back nozzles of 20 mm (the result of the evaluation of the appearance: 5). By the process stated in Claim 6, a hot-dip zinc-aluminum alloy coated steel sheet having excellent appearance and corrosion resistance and being suitable for use as a material for prepainted steel sheet could be obtained. The following Example 8 will illustrate the invention stated in Claim 7 of the present application. Example 8 The hot-dip zinc-aluminum alloy coated steel sheet produced by the process of Claim 5 or 6 was reheated to various temperatures shown in Table 14 which were above the melting temperature of the coating layer in order to make its surface smooth. The appearance (smoothness), thickness of the coating layer and Al distribution in the obtained hot-dip zinc-aluminum alloy coated steel sheet suitable for use as a material for a prepainted steel sheet were examined. The conditions were as follows: sheet thickness: 0.8 mm bath temperature: 460°C dipping time in the bath: 4 sec 2 amount of deposition: 250 g/m bath composition: 0.5 wt.% of Al, 0.005 wt.% of Pb, -0.01 wt.% of Si and the balance of Zn The standard of the evaluation of the appearance (smoothness) of the coating layer surface is shown in Table 11 and the results thereof are shown in Table 14. Table 14 The numerals of the evaluation results are the averages of the results. The surface smoothness was remarkably improved at a reheating temperature of 420°C or above. The results of the determination of the thicknesses of the coating layers obtained without any reheating and with the reheating at 460°C are shown in Figs. 5. It is apparent from Figs. 5 that the scattering in the thickness of the coating layer was narrow when the reheating was conducted according to the present invention. The scattering of the thickness of the coating layer was closely related to the appearance. Namely, the narrower the scattering, the less the formation of the ripples. The thickness of the coating layer was determined with a micro-fluorescence X-ray device (SPT-157 SLS; a product of Seiko Denshi Co., Ltd.) with a beam diameter of 0.1 mm. The distribution of Al atoms, etc. was examined with ESCA. The distribution of Fe, Zn and Al in the cross section of the coating layer on the hot-dip An-Al alloy coated steel sheet is shown in Table 15. The results support the fact that Al is distributed in the surface layer of the coating layer and the coated steel sheet of the present invention has an excellent corrosion resistance. The conditions were as follows: sheet thickness: 0.8 mm bath temperature: 460°C dipping time in the bath: 4 sec 2 amount of deposition: 250 g/m bath composition: 1 wt.% of Al, 0.003 wt.% of Pb, 0.01 wt.% of Si and the balance of Zn The ESCA instrument used was JPS-90 SX of JEOL, Ltd. The acceleration voltage (V) was 500 and the etching o rate was 250 A/min (in terms of Si0 2 ) (the etching rate of Zn is about 4 times as high as that of SiO_). T able 15 (Atomic distribution change with depth from surface layer) The following Example 9 will illustrate the invention stated in Claim 8 of the present application. Example 9 Hot-dip zinc-aluminum alloy coated steel sheets were produced at various cooling speeds as shown in Table 16 in order to obtain a stable corrosion resistance, The workability and corrosion resistance of the sheets were examined. The conditions were as follows: sheet thickness: 0.8 mm bath temperature: 460°C dipping time in the bath: 4 sec 2 amount of deposition: 120 to 260 g/m bath composition: 1 wt.% of Al, 0.005 wt.% of Pb, 0.01 wt.% of Si and the balance of Zn The workability test was conducted in the same manner as that of Example 1 and the results were evaluated on the basis of the standard shown in Table 1. In the corrosion resistance test, the initial red rust formation in the edges was examined in the same manner as that of Example 5 and the results were evaluated on the basis of the standard shown in Table 9. Table 16 The numerals of the evaluation results are the averages of the results. It is apparent that the corrosion resistance was improved when the cooling was conducted at a speed of at least 10°C/sec after the hot dipping. Figs. 6(a) to (c) are microphotographs showing the metal textures of the coating layer surfaces. They are X-ray images of Al on the surfaces formed with EPMA. Figs. 6(a), 6(b) and 6(c) are microphotographs of the metal textures of the coating layer surfaces obtained when the cooling speed was 2°C/sec, 17°C/sec and 47°C/sec, respectively. It is apparent from Figs. 6(a) to (c) that as the cooling speed was increased, the Al distribution on the coating layer surface became homogeneous and Al concentrated in the surface layer. The following Examples 11 to 19 will illustrate the invention stated in Claim 9 of the present I application. Example 11 Hot-dip zinc-aluminum alloy coated steel sheets were produced with baths containing various amounts of Al and Pb in order to confirm the effects of Al and Pb added to the bath according to the present invention (Example 1). Each of them was used as the material sheet. It was treated with a chromate. An epoxy resin paint containing 15% of a rustproof pigment was applied thereto. After baking at 210°C for 35 sec, an under-paint having a dry thickness of 3 y was given. Then an oil-free polyester resin paint as the top-painting paint was applied thereto and baked at 220°C for 45 sec to form a top-paint having a dry thickness of 11 μ. The properties of the prepainted steel sheets thus prepared were examined. The results are shown in Table 17. The workability was determined by the same 2T bending test method as that of Example 1 and the results were evaluated on the basis of the standard shown in Table 1. The adhesion was determined also in the same manner as that of Example 1. Namely, after the 2T bending test, an adhesive tape was applied to the 2T part of the prepainted steel sheet and the forced stripping test was conducted. The results were evaluated on the basis of the standard shown in Table 3. The corrosion resistance test was conducted according to JIS Z 2371 in the same manner as that of Example 4. The flat part and 2T part were subjected to the salt spray test (SST, 1000 Hr. ) . The results were evaluated on the basis of the standard shown in Table 6. The workability, adhesion and corrosion resistance were determined immediately after the preparation of the prepainted steel sheets and after six months. When the Al concentration was in the range of 0.3 to 3.5 wt.%, the prepainted steel sheets having excellent properties could be obtained. When the Pb concentration exceeded 100 ppm, the workability, adhesion and corrosion resistance were reduced with the elapse of time. Table 17 The numerals of the evaluation results are the averages of the results. Example 12 Hot-dip zinc-aluminum alloy coated steel sheets were prepared with hot dipping baths containing various amounts of Si in order to confirm the effect of Si added to the bath according to the present invention (Example 3). Each of them was used as the material sheet. It was treated with a chromate in the same manner as that of Example 11. An under-paint and then a top-paint were formed thereon to prepare a prepainted steel sheet in the same manner as that of Example 11. The workability, adhesion and corrosion resistance of the prepainted steel sheet were determined in the same manner as that of Example 11. The conditions and the results are shown in Table 18. Table 18 The numerals of the evaluation results are the averages of the results. Example 13 Hot-dip zinc-aluminum alloy coated steel sheets were prepared with hot dipping baths containing various amounts of Mg, Mn or Cu in order to confirm the effect of them added to the bath according to the present invention (Example 4). Each of them was used as the material sheet. It was treated with a chromate. An epoxy resin paint containing 15% of a rustproof pigment was applied thereto. After baking at 210 C C for 35 sec, an under-paint having a dry thickness of 3 μ was given. Then an oil-free polyester resin paint as the top-painting paint was applied thereto and taked at 220°C for 45 sec to form a top-paint having a dry thickness of 11 μ. The workability, adhesion and corrosion resistance of the prepainted steel sheets were determined. The conditions and the results are shown in Table 19. The workability was determined by the 2T bending test in the same manner as that of Example 1 and the results were evaluated on the basis of the standard shown in Table 1. The adhesion in the 2T part of the prepainted steel sheet was determined also in the same manner as that of Example 11 and the results were evaluated o . the basis of the standard shown in Table 3. The corrosion resistance test was conducted according to JIS Z 2371 in the same manner as that of Example 4. The flat part and 2T part were subjected to the salt spray test (SST, 1000 Hr. ) . The results were evaluated on the basis of the standard shown in Table 6. It is apparent from Table 19 that when 0.01 to 1.5 wt.% of one or more of Mg, Mn and Cu was(were) added to the bath, the workability, adhesion and corrosion resistance of the prepainted steel sheet were further improved. Table 19 The numerals of the evaluation results are the averages of the results. Example 14 Hot-dip zinc-aluminum alloy coated steel sheets were prepared by the process stated in Claim 5 in which the slit clearance and the distance between front and back nozzles of the gas wiping type of deposit control equipment were varied. The sheets were painted and baked in the same manner as that of Example 12 to form prepainted steel sheets. The surface smoothness of these sheets was determined on the basis of the standard shown in Table 11 in the same manner as that of Example 6. The conditions and the results are shown in Table 20. The conditions were as follows: bath composition: Al concentration 1.0 wt.% Pb concentration 0.005 wt.% Si concentration 0.02 wt.% Zn concentration the balance According to the present invention, the prepainted steel sheet having a good surface smoothness could be obtained. Table 20 T he numerals of the smoothness test results are the averages. Example 15 Hot-dip zinc-aluminum alloy coated steel sheets were prepared by varying the bath temperature and the temperature of the steel sheet in order to confirm the effect of the temperature of the steel sheets to be dipped in the hot dipping bath according to the present invention (Example 5). Each of the sheets was used as the material sheet. It was treated with a chromate. An epoxy resin paint containing 15% of a rustproof paint was applied thereto. After baking at 210°C for 35 sec, an under-paint having a dry thickness of 3 μ was given. Then an oil-free polyester resin paint as the top-painting paint was applied thereto and baked at 220°C for 45 sec to form a top-paint having a dry thickness of 11 μ. The workability, adhesion and corrosion resistance of the prepainted steel sheets were determined. The conditions and the results are shown in Table 21. The workability was determined by the 2T bending test in the same manner as that of Example 1 and the results were evaluated on the basis of the standard shown in Table 1. The adhesion in the 2T part of the prepainted steel sheet was determined also in the same manner as that of Example 11 and the results were evaluated on the basis of the standard shown in Table 3. The corrosion resistance test was conducted according to JIS Z 2371 in the same manner as that of Example 4. The flat part and 2T part were subjected to the salt spray test (SST, 1000 Hr. ) . The results were evaluated on the basis of the standard shown in Table 6. It is apparent from Table 21 that when the temperature of the steel sheet to be dipped in the hot dipping bath is lower than the temperature of the bath, well-balanced, quite excellent workability, adhesion and corrosion resistance were obtained. Table 21 0 The numerals of the evaluation results are the averages of the results. Example 16 Hot-dip zinc-aluminum alloy coated steel sheets were prepared by the process stated in Claim 6 in which the temperatures of the bath and the sheet to be dipped in the bath, and the slit clearance, gas ejecting pressure and the distance between front and back nozzles of the gas wiping type of deposit control equipment were varied. The sheets were treated with a chromate and then painted and baked by the two-coat/two-bake process in the same manner as that of Example 11 to form prepainted steel sheets. The surface smoothness of each of the prepainted steel sheets was determined on the basis of the standard shown in Table 11 in the same manner as that of Example 6. The conditions and the results are shown in Table 22. The bath composition was as follows: Al concentration 0.5 wt.% Pb concentration 0.005 wt.% Si concentration 0.01 wt.% Zn concentration the balance It is apparent from Table 22 that the surface smoothness of the prepainted steel sheets could be remarkably improved according to the present invention. Table 22 The numerals of the smoothness test results are the averages of the results. Example 17 Hot-dip zinc-aluminum alloy coated steel sheets were prepared by the process stated in Claim 7 in which the reheating temperature was varied. The sheets were treated with a chromate and then painted and baked by the two-coat/two-bake process in the same manner as that of Example 11 to form prepainted steel sheets. The surface smoothness of the prepainted steel sheets was determined on the basis of the standard shown in Table 11 in the same manner as that of Example 6. The smoothness test results and the reheating temperatures are shown in Table 23. The bath composition was as follows: Al concentration 0.5 wt.% Pb concentration 0.005 wt.% Si concentration 0.01 wt.% Zn concentration the balance The prepainted steel sheets having excellent smoothness could be thus prepared by the present invention. Table 23 The numerals of the smoothness test results are the averages of the results. Example 18 The hot-dip zinc-aluminum alloy coated steel sheets prepared at various cooling speeds by the process stated in Claim 8 were used. Each of the sheets was treated with a chromate and then painted and baked by the two-coat/two-bake process in the same manner as that of Example 11 to form prepainted steel sheets. The workability, adhesion and corrosion resistance of the prepainted steel sheets were determined. The results are shown in Table 24 together with the cooling speeds. The workability was determined by the 2T bending test in the same manner as that of Example 1 and the results were evaluated on the basis of the standard shown in Table 1. The adhesion in the 2T part of the prepainted steel sheet was determined also in the same manner as that of Example 11 and the results were evaluated on the basis of the standard shown in Table 3. The corrosion resistance test was conducted according to JIS Z 2371 in the same manner as that of Example 4. The flat part and 2T part were subjected to the salt spray test (SST, 1000 h). The results were evaluated on the basis of the standard shown in Table 6. The bath composition was as follows: Al concentration 1.0 wt.% Pb concentration 0.005 wt.% Si concentration 0.01 wt.% Zn concentration the balance The prepainted steel plates having excellent workability, adhesion and corrosion resistance could be prepared by the present invention. Table 24 The numerals of the evaluation results are the averages of the results. The following Example 20 will illustrates the invention stated in Claim 10 of the present application. Example 20 Prepainted steel sheets of the present invention were prepared and the properties of them were determined as follows to confirm the effect of the present invention. Hot-dip zinc-aluminum alloy coated steel sheets were prepared in a hot dipping bath having the following composition (Example 1) and they were used as the materials: The bath composition: Al concentration 1.0 wt.% Pb concentration 0.005 wt.% Si concentration 0.01 wt.% Zn concentration the balance Then the sheets were subjected to chemical conversion treatment to form a coating film in various amounts on the materials (hot-dip zinc-aluminum alloy coated steel sheets) with a chromate solution (type A) having a Cr to Si ratio of 55:45 which comprised a mixture of a solution containing silica having an average particle 2 diameter of 10 mμ (specific surface area: 200 m /g) and a solution containing silica having an average particle 2 diameter of 50 mμ (specific surface area: 50 m /g) in a ratio of 1:1; a chromate solution (type B) having a Cr to Si ratio of 55:45 comprising only the solution containing silica having an average particle diameter 2 of 50 mμ (specific surface area: 200 m /g) or a phosphate solution . Then an epoxy resin paint containing 15% of a rustproof pigment was applied to each of the treated zinc-aluminum alloy coated steel sheet. After baking at 210°C for 35 sec, an under-paint having a dry thickness of 3 μ was given. Then an oil-free polyester resin paint as the top-painting paint was applied thereto and baked at 220°C for 45 sec to form a top-paint having a dry thickness of 11 μ. The scratch resistance, workability, adhesion and corrosion resistance of the prepared prepainted steel sheets were determined. The results are shown in Table 27 together with the conditions. The scratch resistance was determined by applying a copper coin to the painted surface at an angle of 45° and moved under a load of 3 kg. The results were evaluated on the basis of the standard shown in Table 26. The workability was determined by the 2T bend test in the same manner as that of Example 1 and the results were evaluated on the basis of the standard shown in Table 1. The adhesion in the 2T part of the prepainted steel sheet was determined also in the same manner as that of Example 11 and the results were evaluated on the basis of the standard shown in Table 3. The corrosion resistance test was conducted according to JIS Z 2371 in the same manner as that of Example 4. The flat part and 2T part of the prepainted steel sheet were subjected to the salt spray test (SST, 1000 Hr. ) . The results were evaluated on the basis of the standard shown in Table 6. The prepainted steel sheets having not only excellent workability, adhesion and corrosion resistance but also an excellent scratch resistance could be prepared by the present invention. Table 26 (Standard of evaluation of scratch resistance) Table 27 The chromate solution of type A had a Cr to Si ratio of 55:45 and comprised a mixture of a solution containing silica having an average particle diameter 2 of 10 mμ (specific surface area: 200 /g) and that having an average particle diameter of 50 mμ (specific surface 2 area: 50 m /g) in a ratio of 1:1. The chromate solution of type B had a Cr to Si ratio of 55:45 and comprised only a solution containing silica having an average particle diameter of 2 50 mμ (specific surface area: 200 m /g). The numerals of the evaluation results in the above Table are the averages of the results.

Claims: CLAIMS 1. A hot—dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, produced by coating the sheet in a hot dipping bath comprising 0.3 to 3.5 wt.% of Al, 100 pm or less of Pb and the balance of Zn and unavoidable impurities. 2. A hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, produced by coating the sheet in a hot dipping bath comprising 0.3 to 3.5 wt.% of Al, 1/100 to 1 part, per part of Al, of Si, 100 ppm or less of Pb and the balance of Zn and unavoidable impurities. 3. A hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, produced by coating the sheet in the same hot dipping bath as that of Claim 1 or 2, which further contains 0.01 to 1.5 wt.% of one or more metals selected from the group consisting of Mg, Mn and Cu. 4. A process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, wherein the temperature of the steel sheet dipped in the hot dipping bath is lower than that of said bath in the step of coating in the hot dipping bath according to Claim 1, 2 or 3. 5. A process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet, wherein the steel sheet is taken out of the hot dipping bath and then the amount of the molten zinc-aluminum alloy deposited on the steel sheet is controlled with a gas wiping type of equipment for controlling the amount of molten zinc-aluminum alloy deposit under conditions comprising a slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm and an 2 ejecting pressure of gas of 0.1 to 2.0 kg/cm in the step of coating the sheet in the hot dipping bath according to Claim 1, 2 or 3. 6. A process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 4, wherein the steel sheet is taken out of the hot dipping bath and then the amount of the molten zinc-aluminum alloy deposited on the steel sheet is controlled with a gas wiping type of equipment for controlling the amount of molten zinc-aluminum alloy deposit under conditions comprising a slit clearance of 0.6 to 2.4 mm, a distance between front and back nozzles of 10 to 40 mm and an ejecting pressure of gas 2 of 0.1 to 2.0 kg/cm . 7. A process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 5 or 6, wherein the hot-dip zinc-aluminum alloy coated steel sheet is reheated to a temperature above the melting temperature of the coating layer. 8. A process for producing a hot-dip zinc-aluminum alloy coated steel sheet for a prepainted steel sheet according to Claim 5 or 6, wherein the hot-dip zinc-aluminum alloy coated steel sheet is cooled to the solidifying point thereof at a rate of at least 10°C/sec. 9. A prepainted steel sheet having excellent workability and corrosion resistance which comprises the hot-dip zinc-aluminum alloy coated steel sheet according to Claims 1 to 3 or the hot-dip zinc-aluminum alloy coated steel sheet produced according to the process of Claims 4 to 8, including a layer formed thereon by chemical conversion treatment and further a surface painting film layer formed thereon. 10. A prepainted steel sheet according to Claim 9, wherein the chemical conversion layer is one formed by treating the sheet with a chromic acid solution containing silica having an average particle diameter of 50 mμ and a specific surface area of 200 m 2/g in such a manner that the amount of the coating film after drying will 2 be 50 to 250 mg/m to impart an excellent scratch resistance.

Inventors: KAMEYA TORU, OHTA KOJI, OKUZAKI YUUJI, SHIMIZU HISANORI, TAKAGI MASAAKI, KAMEYA, TORU, OHTA, KOJI, OKUZAKI, YUUJI, SHIMIZU, HISANORI, TAKAGI, MASAAKI
Applicants: TAIYO STEEL, TAIYO STEEL CO., LTD.
Application year: 1989
Main classification: C23C2
IPC codes: C23C2, C23C22