| 摘要 |
<p>PROBLEM TO BE SOLVED: To manufacture a high strength galvanized steel sheet excellent in pres-formability and coating stickiness useful to members for motor car, building, electric machinery and apparatus, etc. SOLUTION: The steel sheet composed, by mass %, of 0.04-0.2% C, 0.2-1.8% Si, 0.5-2.5% Mn and 0.01-1.0% Al and 0.4%<=Si+0.8Al<=1.8% in the relation of Si and Al and the balance Fe with the inevitable impurities, is annealed at 650-900 deg.C in two phase coexisting range for 300 sec-10 min. Thereafter, this steel sheet is cooled to 350-500 deg.C at 2-200 deg.C/sec cooling rate and held to this temperature range for 5 sec-20 min and successively, the galvanizing treatment is applied and the galvanized steel sheet is held at the temperature range of 470-600 deg.C for 5 sec-2 min. Thereafter, this galvanized steel sheet is cooled to <=250 deg.C at >=5 deg.C/sec cooling rate and the retained austenite in the steel sheet structure of 2-20% volume ratio is secured and also, the Zn alloy coated layer containing 8-15% Fe, <=1% Al with inevitable impurities on the steel sheet surface, is formed.</p> |
