Patent ID: 8398787
Filing Date: 2013-03-19
Classification: C21D,C22C

Abstract:
1. 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; 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; wherein T is the heating temperature (IC) in the SR treatment and t wherein the steel sheet is obtained by rolling with an exit temperature of not lower than the Ar 3 transformation point.