Patent ID: 8216397

Claim:
A method of producing a dual phase high-strength hot-dip galvanized steel sheet composed of ferrite as a main phase and martensite phase as a secondary phase having high plating adhesion after severe deformation and ductility during heavy working, 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: 0.011 to 1%, and a balance of 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 0.1×(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 a balance of Zn, and satisfying the following equation (3), 100≧( A/ 3 +B/ 6)/( C/ 6)≧0.01 (3), wherein A is the Al content in mass % in the plated layer, B is the Mo content in mass % in the plated layer, and C is the Mo content in mass % in the steel sheet.