Patent ID: 8398787

Claim:
A high-strength steel sheet which decreases little in strength after stress relieving annealing and has excellent low temperature joint toughness, the high-strength steel sheet comprising (hereafter % means mass %): C: 0.10 to 0.16%, Si: 0.05 to 0.50%, Mn: 1.3 to 1.9%, Al: 0.01 to 0.05%, Ti: 0.005 to 0.025%, Nb: 0.005 to 0.025%, V: 0.005 to 0.06% and Mo: 0.03 to 0.10%, with the remainder being iron and inevitable impurities, with a CP value defined by equation (1) below being 5.40% or higher, and with a carbon equivalent Ceq defined by equation (2) below being 0.45% or lower; CP value=125[Ti]+111[Nb]+60[V]+15[Mo] (1) wherein [Ti], [Nb], [V] and [Mo] represent the amounts of Ti, Nb, V and Mo contained (mass %), respectively; Ceq =[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15 (2) wherein [C], [Mn], [Cr], [Mo], [V], [Cu] and [Ni] represent the amounts of C, Mn, Cr, Mo, V, Cu and Ni contained (mass %), respectively; wherein the high-strength steel sheet has a change in tensile strength (ΔTS) value lower than 30 MPa and a tensile strength (TS) of 550 MPa or higher after being subjected to long term stress-relief annealing (SR) when the SR treatment is carried out under conditions such that the P value defined by equation (3) below is 18.8 or higher; P value=T(20+log t 0 ) (3) wherein T is the heating temperature (IC) in the SR treatment and t 0 is the heating time (hrs.) in the SR treatment; and wherein the steel sheet is obtained by rolling with an exit temperature of not lower than the Ar 3 transformation point.