Patent ID: 7824509

Claim:
A method of producing a high-strength hot-dip galvanized steel sheet composed of ferrite as a main phase, 3 to 50 volume % of austenite as a secondary phase and 2 to 47 volume % of bainite as a third phase and said ferrite and bainite have 50 to 97 volume % in total, having high plating adhesion after severe deformation and ductility during heavy working, and excellent in corrosion resistance and workability in an environment containing chloride ion, comprising: casting a steel consisting essentially of, in mass, C: 0.0001 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.01 to 3%, Al: 0.31 to 4%, Mo: 0.001 to 1% and the balance being Fe and unavoidable impurities to provide a cast slab; thereafter, hot-rolling the cast slab into a hot-rolled steel sheet and coiling it, and then pickling and cold-rolling the hot-rolled steel sheet to provide a cold-rolled steel sheet; thereafter, annealing the cold-rolled steel sheet for 10 seconds to 30 minutes in the temperature range from not less than 0.1x(Ac 3 −Ac 1 )+Ac 1 (° C.) to not more than Ac 3 +50 (° C.); then cooling the steel sheet to the temperature range from 650 to 700° C. at a cooling rate of 0.1 to 10° C/sec.; thereafter, cooling the steel sheet to the temperature range from the plating bath temperature to the plating bath temperature +100° C. at a cooling rate of 1 to 100° C/sec.; keeping the steel sheet in the temperature range from the zinc plating bath temperature to the zinc plating bath temperature +100° C. for 1 to 3,000 seconds including the subsequent dipping time; dipping the steel sheet in the zinc plating bath at a temperature of 460 to 470° C.; and, after that, cooling the steel sheet to room temperature; so as to control a concentration of Al and Mo in the plated layer, containing, in mass, Al: 0.001 to 4%, Mo: 0.0001 to 0.1%, and the balance being Zn, and satisfying the following equation (3), 100≧(A/3+B/6)/(C/6)≧0.01 (3) wherein A as Al content (in mass %) and B as Mo content (in mass %) in the plated layer, and C as Mo content (in mass %) in the steel sheet.