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

High yield ratio high-strength thin steel sheet superior in weldability and ductility characterized by ; being comprised of steel containing, by mass%, C: over 0.030 to less than 0.10%, Si: 0.30 to 0.80%, Mn: 1.7 to 3.2%, P: 0.001 to 0.02%, S: 0.0001 to 0.006%, Al: 0.060% or less, N: 0.0001 to 0.0070%, containing further Ti: 0.01 to 0.055%, Nb: 0.012 to 0.055%, Mo: 0.07 to 0.55%, B: 0.0005 to 0.0040%, and simultaneously statisfying 1.1‰¤14xTi(%)+20xNb(%)+3xMo(%)+300xB(%)‰¤3.7, the balance comprised of iron and unavoidable impurities, and having a yield ratio of 0.64 to less than 0.92, a TSxEl of 3320 or more, an YRxTSxEl 1/2 of 2320 or more, and a maximum tensile strength (TS) of 780 MPa or more.