Patent ID: 8182740

Claim:
High-strength steel sheet excellent in hole-expandability and ductility, consisting of, in mass %, C: not less than 0.01% and not more than 0.20%, Si: not more than 1.5%, Al: from 0.45% to 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), and having the structure primarily comprising of ferrite, and martensite, [Mg%]≧([O%]/16×0.8)×24 (1) [S%]≦([Mg%]/24−[O%]/16×0.8+0.00012)×32 (2) [S%]≦0.0075/[Mn%] (3).