Surface-treated steel sheet with excellent corrosion resistance and method for manufacturing same

A method for manufacturing a surface-treated steel sheet with excellent corrosion resistance comprising the steps of:

• (a) applying onto a surface of a zinc-based plated steel or an aluminum-based plated steel a surface treatment coating composition which contains the following ingredients (α) to (δ):
• (α) a water-epoxy resin dispersion which is prepared by dispersing in water a (Z) modified epoxy resin obtained by a reaction of a (W) epoxy group-containing resin, an (X) primary amine compound and/or secondary amine compound, and an active hydrogen-containing compound in which a part or entire of the compound is structured by a (Y) hydrazine derivative having active hydrogen;
• (β) a water dispersed polyurethane resin having a mass ratio of resin solid matter in the water-epoxy resin dispersion to resin solid matter in the water dispersed polyurethane resin in the surface treatment coating composition, expressed as (water-epoxy resin dispersion)/(water dispersed polyurethane resin), being 95/5 to 5/95;
• (γ) a silane coupling agent in an amount of 1 to 300 parts by mass of solid matter therein to 100 parts by mass of the resin solid matter in the water-epoxy resin dispersion in the surface treatment coating composition;
• and
• (δ) phosphoric acid and/or a hexafluorometal acid in an amount of 0.1 to 80 parts by mass of solid matter therein to 100 parts by mass of the resin solid matter in the water-epoxy resin dispersion in the surface treatment coating composition;
• (b) drying the applied surface treatment coating composition at an ultimate sheet temperature of 30° to 150° C., thus forming a surface-treatment coating having a coating thickness of 0.01 to 1 μm;
• (c) forming a top coating having a coating thickness of 0.3 to 2 μm on the surface treatment coating by applying a coating composition for the top coating containing an (E) high molecular weight epoxy group-containing resin having a number average molecular weight of 6,000 to 20,000, wherein said high molecular weight epoxy group-containing resin is a hydrazine derivative-modified epoxy resin;
• and
• (d) drying the applied coating composition for a top coating at an ultimate sheet temperature of 30° to 150° C.