Aqueous coating composition and method for forming a coating film

An aqueous coating composition comprising:

• an aqueous resin having a hydroxyl group and a carboxyl group (A),
• a water-dispersible blocked polyisocyanate compound (B),
• a hydrophilicized carbodiimide compound (C), and
• an aqueous polyurethane resin (D),
• wherein
• the aqueous resin having a hydroxyl group and a carboxyl group (A) has a hydroxyl value of 80 to 200 mgKOH/g and an acid value of 10 to 40 mgKOH/g in terms of resin solid content,
• the hydrophilicized carbodiimide compound (C) is a compound represented by formula (I), (II), or (III) below,
• the aqueous polyurethane resin (D) has a glass transition point (Tg) of −50° C. or less, and
• a cured film of the aqueous polyurethane resin (D) has an elongation at break of 400% or more at −20° C.,
• wherein X is a bifunctional organic group having at least one carbodiimide group, Y is each same or different structure resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Z is a structure resulting from elimination of a hydroxyl group from a bifunctional polyol having a number-average molecular weight of 200 to 5,000,
• wherein each X is a bifunctional organic group having at least one carbodiimide group, Y is each same or different structure resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether, R0 is hydrogen, a methyl group or an ethyl group, each R1 is an alkylene group having 4 or less carbon atoms, n is 0 or 1, and each m is a number from 0 to 60,
• wherein X is a bifunctional organic group having at least one carbodiimide group, and Y is each same or different structure resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether.