Metallic materials typified by iron are commonly used in a state of being coated for the purpose of protection from thermal and chemical damage factors, thereby improving durability and imparting an attractive appearance. These coated metals likely require various properties such as corrosion resistance, contamination resistance, and heat resistance. Applications utilizing heat resistance include, for example, exhaust components for vehicles and motorcycles, cooking utensils, and air conditioners and heating units. These applications generally require resistance to temperatures of about 300 to 400° C. and, in particular, exhaust components for vehicles require resistance to temperatures of 500° C. or more.
For these applications, since a postcoating method of coating after forming into a predetermined shape leads to high cost and is not suitable for mass production, a precoating method of working after previously coating has been reviewed. According to the precoating method, however, working is performed after forming a film, and thus extreme workability is generally required for the coating film. For example, a thin steel sheet likely requires the workability sufficient to cause neither cracking nor peeling of the film in a 2T to 4T bending test, such that a 180° bending is performed in the state where two to four steel sheets each having the same thickness as that of a test material are laid one upon another, and the adhesion between the film capable of enduring the above test and an underlaying metal.
A silicone resin is known as a material of a film for heat-resistant precoated steel sheet. The heat resistance of the coating film made of the silicone resin remarkably varies depending on the kind and content of the organic group to be introduced into the silicone resin. In general, as the content of the organic group increases, the resultant film is more flexible and is excellent in workability and adhesion on working. On the other hand, the content of the organic substance should be decreased so as to enhance the heat resistance of the film. Thus, the above-mentioned workability and adhesion on working of the film are deteriorated.
Japanese Patent Application, First Publication Nos. S63-172640, H02-265742 and H08-10701 describe, as coated steel sheets which satisfy both heat resistance and workability using a silicone resin, precoated steel sheets produced by using a coating composition containing, as a main component, a silicone resin having an alkyl group, an alkenyl group, and a phenyl group introduced therein. These coating films made of the silicone resin have a workability that is typically required of the precoated steel sheet because of comparatively high content of the organic group, but have a low resistance to temperatures of 400° C. or less. Japanese Patent Application, First Publication No. H08-245922 describes a coating film made of a monomethylsilicone resin having a comparatively low content of the organic group, and the resin coating film is generally insufficient in workability.
To solve such a problem, Japanese Patent Application, First Publication No. 2002-234109 describes a method of using two kinds of resins having different heat resistances in combination. Japanese Patent Application, First Publication No. 2002-307606 describes a resin coating film containing a methylsilicone resin and a linear methylphenylsilicone resin. To improve the workability of the above-mentioned coating film described in Japanese Patent Application, First Publication No. H08-245922, Japanese Patent Application, First Publication No. 2002-80974 describes a method of using a coating composition containing a monomethylsilicone resin as a main component, an isocyanate, and an epoxy resin. As described above, however, satisfactory heat resistance and workability imparted by introduction of an organic component conflict with each other, and there have never been obtained those which generally satisfy both properties at a high level.
The heat treatment temperature should preferably be set to a high temperature so as to form a film having high heat resistance, and the heat-resisting temperature of the film tends to decrease so as to decrease the heat treatment temperature. It was difficult to obtain a precoated steel sheet having high heat resistance because of this problem.