Patent ID: 7615126

Claim:
A high strength hot rolled steel plate having excellent bore expandability and ductility, comprising a steel comprising, by mass, 0.01 to 0.08% of carbon; 0.90 to 1.50% of silicon; 0.50 to 2.50% of manganese; phosphorus ≦0.03%; sulfur ≦0.005%; 0.05 to 0.20% of titanium; and 0.01 to 0.04% of niobium; and the balance consisting of iron and unavoidable impurities, wherein the contents of carbon (C), silicon (Si), manganese (Mn), titanium (Ti), and niobium (Nb) satisfy a requirement represented by formula 115≦(917−480[C %]+100[Si %]−100[Mn %])−(790×([Ti %]+[Nb %]/2) 0.05 )≦235, and wherein not less than 80% of all grains are accounted for by grains having a ratio (ds/dl) of minor axis (ds) to major axis (dl) of not less than 0.1, said steel plate consisting of a ferrite-bainite duplex steel structure consisting of not less than 80% of ferrite and the balance bainite, said steel plate having a strength of not less than 770 N/mm 2 .