PURPOSE: To swiftly apply galvannealing to a high strength steel sheet used for a high strength galvannealed steel sheet, more specifically, e.g. as an automobile steel sheet at a low cost.
CONSTITUTION: The surface of a steel sheet is coated with an inorganic zinc compound by 0.05 to 500g/m2 expressed in terms of Zn content, which is subjected to final annealing in a reducing or nonoxidizing atmosphere, is thereafter passed through hot-dip zinc and is alloyed under heating. It is coated with any of hydroxide, oxide, sulfide, nitrate and sulfate or a mixture thereof as an inorganic zinc compound. Or, it is coated with a zinc ion-contg. acidic aq. soln., is thereafter brought into contact with an alkaline aq. soln. and is coated with zinc hydroxide, or it is coated with a zinc ion-contg. acidic aq. soln., is thereafter brought into contact with sulfide-ion contg. alkaline aq. soln. and is coated with zinc sulfide. The high strength galvannealed steel sheet can be produced with good productivity at a low cost, and the limitation to the compsn. of the steel sheet is remarkably reduced.
SUZUKI SHINICHI
MIYASAKA AKIHIRO
