High-strength hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet having fatigue resistance, corrosion resistance, ductility and plating adhesion, after severe deformation, and a method of producing the same

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, which hot-dip galvanized steel sheet being excellent in appearance and workability, 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;
• hot rolling the cast slab including finishing the hot-rolling at a temperature of 880 to 1,100° C. to provide a hot-rolling steel sheet;
• coiling the hot-rolled steel sheet;
• then pickling and cold-rolling the coiled 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×(Ac3−Ac1)+Ac1 (° C.) to not more than Ac3+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 −50° C. to the plating bath temperature +50° C. at a cooling rate of 0.1 to 100° C./sec.;
• then dipping the steel sheet in the plating bath;
• keeping the steel sheet in the temperature range from the plating bath temperature −50° C. to the plating bath temperature +50° C. for 2 to 200 seconds including the dipping time;
• and,
• thereafter, 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.

A method of producing a dual phase high-strength high-ductility hot-dip galvanized steel sheet composed of ferrite as a main phase and martensite phase as a secondary phase, the hot-dip galvanized steel sheet being excellent in corrosion resistance, 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;
• then rough-rolling the cast slab at the total reduction rate of 60 to 99% and at a temperature of 1,000 to 1,150° C.;
• followed by finishing rolling to provide a hot-rolled steel sheet;
• coiling the hot-rolled steel sheet;
• then pickling and cold-rolling the coiled hot-rolled steel sheet;
• thereafter, annealing the cold-rolled steel sheet for 10 seconds to 30 minutes in the temperature range from not less than 0.12×(Ac3−Ac1)+Ac1 (° C.) to not more than Ac3+50 (° C.);
• then, after the annealing, cooling the steel sheet, when the highest attained temperature during annealing is defined as Tmax (° C.), to the temperature range from Tmax −200° C. to Tmax −100° C. at a cooling rate of Tmax/1,000 to Tmax/10° C./sec.;
• thereafter, cooling the steel sheet to the temperature range from the plating bath temperature −30° C. to the plating bath temperature +50° C. at a cooling rate of 0.1 to 100° C./sec.;
• then dipping the steel sheet in the plating bath;
• keeping the steel sheet in the temperature range from the plating bath temperature −30° C. to the plating bath temperature +50° C. for 2 to 200 seconds including the dipping time;
• and,
• thereafter, 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.