High-strength steel sheet with a tensile strength of 980 MPa or more and excellent low-temperature toughness of multilayer prime joints

摘要

PROBLEM TO BE SOLVED: To provide a high strength steel sheet which is excellent in low temperature toughness at the HAZ of a joint portion even while having the tensile strength of 980 MPa or more.SOLUTION: The steel sheet contains 0.12-0.16% (meaning mass%, and the same hereinafter) of C, 0.05-0.5% of Si, 0.8-1.2% of Mn, 0.01-0.07% of Al, 0.5-0.8% of Mo, 0.025-0.08% of Nb, 0.0004-0.002% of B, and 0.8% or less of Cr and/or 0.06% or less of V, respectively, and the balance comprising iron and inevitable impurities. An MP value specified by equation (1) is 0.550 or larger, and a carbon equivalent Ceq specified by equation (2) is 0.63% or less, wherein equation (1) : MP value=(12/[C])×(([Mo]/96)+([Nb]/93)) and equation (2): Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15.