HIGH TENSILE HOT DIP GALVANIZING STEEL SHEET AND HIGH TENSILE GALVANNEALED STEEL SHEET HAVING EXCELLENT BALANCE IN STRENGTH AND DUCTILITY, PLATING ADHESION, AND CORROSION RESISTANCE

摘要

PROBLEM TO BE SOLVED: To provide a high tensile hot dip galvanizing or galvannealed steel sheet which has excellent adhesion for galvanizing even when a base steel sheet largely containing Si and Mn, has excellent mechanical properties such as press formability and an excellent balance in strength and ductility, and has excellent corrosion resistance. SOLUTION: The surface of a steel sheet having a composition containing, by mass, 0.05 to 0.25% C, 0.50 to 2.00% Si, =3.5% Mn, and 0.01 to 1.0% Al, and in which the mass ratio of Mn/Si in the steel is 0.5 to 2, and having a composite structure consisting of tempered martensite, retained austenite, ferrite, and a low temperature transformed phase, and in which the volume fraction of the tempered martensite is =20%, and the volume fraction of the retained austenite is =2% is provided with a galvanized layer in which the concentration of Al and the concentration of Si in the plated layer satisfy the Mn/Si concentration in the steel, and a specified relational equation, and, further, the content of oxygen in the surface layer of ferrite after the removal of the plated layer is controlled to =0.05 g/m2 .