Heat resistant precoated steel sheet and process for the production thereof

A precoated steel sheet suitable for use in the manufacture of a muffler of an automobile exhaust gas system comprising:

• a hot dip aluminized steel sheet,
• a chromate pretreatment layer formed on each surface of said hot dip aluminized steel sheet and having a chromium pick-up of 10 to 50 mg/m.sup.2,
• a primer coat formed on said pretreatment layer and comprising a cured polyamide imide resin and 8 to 52 parts by weight, based on 100 parts by weight of said resin, of strontium chromate intimately admixed with said resin,
• a top coat formed on one of the primer coats and comprising a cured silicone-modified polyester resin with a modification degree of from 30 to 50% and 8 to 32 parts by weight, based on 100 parts by weight of said resin, of aluminum scale intimately admixed with said resin, and
• a black top coat formed on the other primer coat and comprising a cured silicone-modified polyester resin with a modification degree of from 30 to 50% having intimately admixed therewith 1.5 to 5 parts by weight of aluminum scale, 3 to 5 parts by weight of a non calcined black pigment and 4 to 6 parts by weight of silica powder, based on 100 parts by weight of said resin.

A process for the production of a precoated steel sheet suitable for use in the manufacture of a muffler of an automobile exhaust gas system comprising the steps of:

• treating a hot dip aluminized steel sheet, the surfaces of which have been cleaned, with a chromic acid solution to form a chromate pretreatment layer having a chromium pick-up of 10 to 50 mg/m.sup.2 on each surface of said hot dip aluminized steel sheet,
• applying onto said pretreatment layer a primer coat paint comprising a polyamide imide resin having a molecular weight of from about 2000 to 7000 dissolved in a solvent and 8 to 52 parts by weight, based on 100 parts by weight of said resin, of strontium chromate intimately admixed with said resin, and curing the paint at a temperature of from 260.degree. to 320.degree. C. to form a primer coat, and
• applying onto one of said primer coats a top coat paint comprising a silicone-modified polyester resin with a modification degree of from 30 to 50% dissolved in a solvent, a curing agent for said resin and 8 to 32 parts by weight, based on 100 parts by weight of said resin, of aluminum scale intimately admixed with said resin, while applying onto the other of said primer coats a black top coat paint comprising a silicone-modified polyester resin with a modification degree of from 30 to 50% dissolved in a solvent and having intimately admixed therewith 1.5 to 5 parts by weight of aluminum scale, 3 to 5 parts by weight of a non- calcined black pigment and 4 to 6 parts by weight of silica powder, based on 100 parts by weight of said resin, and a curing agent for said resin, and curing the paints at a temperature of from 220.degree. to 260.degree. C. to form top coats.