High-strength hot-dip galvanized steel sheet and process for producing the same

A process for producing a hot-dip galvanized steel sheet, comprising subjecting a steel material comprising, in mass %,

• C:
• from 0.10 to 0.4%,
• Si:
• from 0.01 to 0.5%,
• Mn:
• from 1.0 to 3.0%,
• O:
• 0.006% or less,
• P:
• 0.04% or less,
• S:
• 0.01% or less,
• Al:
• from 0.1 to 3.0%,
• N:
• 0.01% or less, and
• Si+Al≧0.5%, with the balance being Fe and unavoidable impurities, to a hot rolling treatment at a hot-rolled slab temperature of 1,100° C. or more and a finishing temperature of 850 to 970° C.,
• cooling the steel sheet after the hot rolling to a temperature region of 650° C. or less at 10 to 200° C./sec on average, and taking it up in a temperature range of 650° C. or less,
• cold-rolling the steel sheet at a rolling reduction ratio of 40% or more,
• annealing the steel sheet by setting the maximum temperature during annealing to be from 700 to 900° C., cooling the steel sheet to a temperature region of 350 to 550° C. at an average cooling rate of 0.1 to 200° C./sec, and then holding it in the temperature region for 1 to 1,000 seconds, and
• immersing the steel sheet after holding in the temperature region in a hot-dip galvanizing bath and after the plating treatment, applying an alloying treatment at a temperature of 470 to 580° C.,
• wherein at the time of holding the steel sheet in a temperature region of 350 to 550° C., the steel sheet is repeatedly bent using a roll having a roll diameter of 50 to 2,000 mm to thereby impart a strain to the steel sheet, and
• the longitudinal average stress applied to the steel sheet during the repeated bending is from 2 to 50 MPa.