Patent ID: 7736449

Claim:
A method for producing a high-strength hot-dip galvanized steel sheet by using an equipment having only a reduction annealing furnace comprising: providing a hot-rolled and cold-rolled steel sheet, with said steel sheet containing, by weight: C: 0.03 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.5 to 2.5%, P: 0.03% or less, S: 0.02% or less, Al: 0.01 to 2.0%, Ni: 0.01 to 0.15%, and balance Fe and unavoidable impurities; with the relationship among Si, Mn and Al satisfying the following expression, Si+Al+Mn≧1.0%; regulating an oxygen concentration of 100 ppm or less in a temperature range of lower than 400° C.; subjecting the steel sheet to a treatment in an atmosphere having an oxygen concentration of 50 ppm or less in a temperature range of 400° C. to 750° C.; annealing said hot-rolled and cold-rolled steel sheet for 10 sec. to 6 min. in a dual phase coexisting temperature range of 750° C. to 900° C.; subjecting said steel sheet to another treatment for 30 sec. or longer in a temperature range of 750° C. or higher in an atmosphere wherein hydrogen concentration, dew point and oxygen concentration in said atmosphere are defined by H (%), D (° C.) and O (ppm), respectively, with H, D and O satisfying the following expressions: 2 ppm≦O≦30 ppm, and 20×exp(0.1 ×D )≦H≦2,000×exp(0.1 ×D ); forming oxides including one or more of Si, Mn and Al and having a length of 3 μm or less on the surface of said steel sheet with an area ratio of 5% to 80%, subsequently cooling said annealed steel sheet to 350° C. to 500° C. at a cooling rate of 2 to 200° C./sec.; subsequently dipping said cooled steel sheet into a hot-dip galvanizing bath after annealing to form a hot-dip galvanizing layer on each surface of said steel sheet; cooling said hot-dip galvanized steel sheet to 250° C. or lower at a cooling rate of 5° C./sec. or more; wherein retained austenite in said steel sheet is 2 to 20% by volume ratio.