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.
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°C in two phase coexisting range for 300 sec-10 min. Thereafter, this steel sheet is cooled to 350-500°C at 2-200°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°C for 5 sec-2 min. Thereafter, this galvanized steel sheet is cooled to ≤250°C at ≥5°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.
SUEHIRO MASAYOSHI
MURAKAMI HIDEKUNI
ARAMAKI TAKASHI
