Patent ID: 8192683

Claim:
High-strength steel sheet excellent in hole-expandability and ductility, consisting essentially of, in mass %, C: not less than 0.01% and not more than 0.20%, Si: not more than 1.5%, Al: not less than 0.18% to not more than 1.5%, Mn: not less than 0.5% and not more than 3.5%, P: not more than 0.2%, S: not less than 0.0005% and not more than 0.009%, N: not more than 0.009%, Mg: not less than 0.0006% and not more than 0.01%, O: not more than 0.005% and Ti: not less than 0.01% and not more than 0.20% and/or Nb: not less than 0.01% and not more than 0.10%, with the balance being iron and unavoidable impurities, having the Mn %, Mg %, S % and O % satisfying equations (1) to (3), allowing precipitation of Mg-sulfides while impeding the precipitation of Mn-sulfides, the Al % and Si % satisfying equation (4), and the Ti %, C %, Mn % and Nb % satisfying equations (5) to (7), and containing not less than 5.0×10 2 per square millimeter and not more than 1.0×10 7 per square millimeter of composite precipitates of MgO, MgS and (Nb, Ti)N of not smaller than 0.05 μm and not larger than 3.0 μm, having a structure primarily comprising bainite, and [Mg %]≧([O %]/16×0.8)×24 (1) [S %]≦([Mg %]/24−[O %]/16×0.8+0.00012)×32 (2) [S %]≦0075/[Mn %] (3) [Si %]+2.2×[Al %]≧0.35 (4) 0.9≦48/12×[C %]/[Ti %]<1.7 (5) 50227×[C %]−4479×[Mn %]>−9860 (6) 811×[C %]+135×[Mn %]+602×[Ti %]+794×[Nb %]>465 (7).