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 D 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 c 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≦14×Ti(%)+20×Nb(%)+3×Mo(%)+300×B(%)≦3.7, the balance comprised of iron and unavoidable impurities, and having a yield ratio of 0.64 to less than 0.92, a TS×El of 3320 or more, an YR×TS×El^1/2 of 2320 or more, and a maximum tensile strength (TS) of 780 MPa or more.