Automotive outer panels are required to have shape fixability when press forming is performed and depression resistance after press forming has been performed. Therefore, a material which is used for manufacturing automotive outer panels is required to have a low yield strength when subjected to forming and a hardenability when subjected to paint baking, that is, a so-called paint-bake hardenability. Patent Literature 1 through Patent Literature 3 disclose methods for manufacturing bake-hardening steel sheets which can be manufactured by using a continuous annealing furnace.
In addition, a bake-hardening steel sheet is required to have press workability. Patent Literature 4 discloses a method for improving such properties by controlling the volume ratio of carbides in grains with respect to the total carbon precipitated during continuous annealing and an over-ageing treatment to be 80% or more.
In addition, Patent Literature 5 discloses a method for achieving a paint-bake hardenability at a low temperature of 150° C. or lower by controlling the number density of carbides in grains to be 4×104 pieces/mm2 or more.
Moreover, a steel sheet for automotive outer panels is required to have excellent corrosion resistance. Patent Literature 6 discloses a method for manufacturing a galvanized steel sheet which is subjected to a galvanizing treatment in order to increase corrosion resistance.
Regarding a galvanizing technique, in the case of a galvanized steel sheet having a coating weight of 40 g/m2 or more, hydrogen which penetrates into a steel sheet from a hydrogen-containing reducing atmospheric gas during continuous annealing is enclosed behind the coating layer. Subsequently, after the steel sheet has been cooled to room temperature, over-saturated hydrogen is gasified at the interface between the surface of the steel sheet and the coating layer, which causes a problem of a blister defect in which the coating layer becomes swollen. The blister defect tends to become more evident in the case where the coating weight is large and where an alloying treatment is not performed. As an example of a method for preventing such a blister defect, Patent Literature 7 discloses a method for controlling the size and amount of Ti-based precipitates in ultralow-carbon steel having a C content of less than 0.005%.