LOW YIELD RATIO HIGH TENSION STEEL PLATE HAVING SMALL ACOUSTIC ANISTROPY AND EXCELLENT WELDABILITY, AND ITS PRODUCING METHOD

摘要

The invention provides a high tensile steel sheet having a low yield ratio in spite of its tensile strength of =780 MPa, and having low acoustic anisotropy and excellent weldability, and to provide its production method. The steel sheet has a composition comprising, by mass, 0.010 to 0.080% C, 0.02 to 0.50% Si, 1.10 to 3.00% Mn, =1.60% Cu, 0.40 to 2.50% Ni, =0.030% P, =0.010% S, =0.200% Al, =0.0100% N, 0.30 to 2.00% Cr, 0.10 to 1.10% Mo and 0.002 to 0.030% Ti, and the balance Fe with impurities. Further, regarding the following AS and DL, AS=4.00 and DL=2.80 are satisfied, the structure in the part of 1/4 in the sheet thickness is essentially composed of bainitic ferrite including MA by =10 area% (excluding 0%), also, the average value of the major axis/manor axis of old austenite grains is 1.0 to 3.0, and further, the hardness ratio of the MA to the bainitic ferrite is =1.10: AS=[Mn]+[Ni]+2*[Cu], and DL=2.5*[Mo]+30*[Nb]+10*[V]; [X] denotes the content (mass%) of the element X.