Patent ID: 7520943

Claim:
A hot-rolled steel plate containing C of about 0.03 to about 0.1%, Si of about 0.01 to about 0.5%, Mn of about 1.2 to about 2.5% and Al of about 0.08% or less by mass, wherein a metal structure is a substantially three-phase structure of ferrite, bainite, and island martensite, and an area fraction of the island martensite is about 3 to about 20%, in addition, the steel plate comprises a complex carbide precipitated in the ferrite phase and further comprises either: (1) Mo of about 0.05 to about 0.4% and Ti of about 0.005 to about 0.04%, wherein the remainder is substantially Fe, and C/(Mo+Ti) which is a ratio of C amount to total amount of Mo and Ti in percent by atom is 1.2 to 3; (2) Mo of about 0.05 to about 0.4% and Ti of about 0.005 to about 0.04%, in addition, contains Nb of about 0.005 to about 0.07% and/or V of about 0.005 to about 0.1%, wherein the remainder is substantially Fe, and C/(Mo+Ti+Nb+V) which is a ratio of the C amount to total amount of Mo, Ti, Nb and V in percent by atom is 1.2 to 3; and, (3) at least two selected from Ti of about 0.005 to about 0.04%, Nb of about 0.005 to about 0.07% and V of about 0.005 to about 0.1%, wherein the remainder is substantially Fe, and C/(Ti+Nb+V) which is a ratio of the C amount to total amount of Ti, Nb and V in percent by atom is 1.2 to 3.