GALVANNEALED STEEL SHEET HAVING ULTRA HIGH STRENGTH AND MANUFACTURING METHOD OF THE SAME

摘要

PURPOSE: A super high strength alloyed and melted galvanized steel sheet and a manufacturing method thereof are provided to improve tensile strength by controlling residual austenite fraction and alloying heating temperature according to components and the temperature of a fast cooling section. CONSTITUTION: A steel sheet is annealed. The annealed steel sheet is cooled. The cooled steel sheet is dipped in a zinc plating bath, zinc plating is processed. The zinc plated steel sheet is inductively heated and is heat-treated for alloying. The temperature(T) of a cooling section is determined in order to satisfying 30-70 for a value of a below first relative formula in a cooling step. A sheet speed(V) is over 70 in the value of a below second relative formula in an alloying treatment step.