HIGH-YIELD-RATIO HIGH-STRENGTH THIN STEEL SHEET AND HIGH-YIELD-RATIO HIGH-STRENGTH HOT-DIP GALVANIZED THIN STEEL SHEET EXCELLING IN WELDABILITY AND DUCTILITY AS WELL AS HIGH-YIELD-RATIO HIGH-STRENGTH ALLOYED HOT-DIP GALVANIZED THIN STEEL SHEET AND PROCESS FOR PRODUCING THE SAME

摘要

A high-yield-ratio high-strength thin steel sheet excelling in weldability and ductility, constituted of a steel comprising, by mass, more than 0.030 to less than 0.10% of C, 0.30 to 0.80% of Si, 1.7 to 3.2% of Mn, 0.001 to 0.02% of P, 0.0001 to 0.006% of S, 0.060% or less of Al and 0.0001 to 0.0070% of N and further comprising 0.01 to 0.055% of Ti, 0. 012 to 0.055% of Nb, 0.07 to 0.55% of Mo and 0.0005 to 0.0040% of B, these satisfying the relationship 1.1 <= 14 x Ti(%) + 20 x Nb(%) + 3 x Mo(%) + 300 x B(%) <= 3.7, with the balance composed of iron and unavoidable impurities, characterized in that the steel sheet exhibits a yield ratio of 0.64 to below 0.92, TS x El of 3320 or greater, YR x TS x El1/2 of >=2320 and maximum tensile strength (TS) of 780 MPa or greater.