GALVANNEALED STEEL SHEET AND ITS PRODUCTION

摘要

PURPOSE:To develop a galvannealed steel sheet having excellent workability of a plating layer in the stage of heating and galvannealing a galvanized steel plate by galvannealing the steel sheet under specific heat treating conditions. CONSTITUTION:Production of a glavannealed steel sheet by heating a steel sheet having a galvanized layer to form an Fe-Zn alloy layer is accomplished by heating quickly the galvanized steel plate once to a temp. region of 675-780 deg.C at a heating rate of >=25 deg.C/sec and forming an Fe-Zn alloy layer by Zn of the plating layer and Fe of the steel sheet. The steel sheet is cooled at a cooling rate of >=20 deg.C/sec and the galvannealed steel sheet having the Fe-Zn alloy layer consisting of the GAMMA phase of the Fe-Zn alloy contg. 20-28% Fe and the Fe-Zn alloy phase of <6% Fe and having 15-23% concn. Fe as a whole and having excellent bending property and pressing property is obtd.