HIGH-STRENGTH STEEL SHEET OF 450 MPa OR HIGHER YIELD STRESS AND 570 MPa OR HIGHER TENSILE STRENGTH HAVING LOW ACOUSTIC ANISOTROPY AND HIGH WELDABILITY AND PROCESS FOR PRODUCING THE SAME

摘要

A high-tensile steel plate of low acoustic anisotropy and high weldability having yield stress of 450MPa or greater and tensile strength of 570 MPa or greater and a process for producing the steel plate are provided. The steel has an Si content of 0.10% or less, thereby achieving a volume ratio of island martensite of 3% or less, contains Nb ≥ 0.025% and Ti ≥ 0.005% so as to satisfy 0.045% ≤ [Nb] + 2 × [Ti] ≤ 0.105%, contains Nb, Ti, C and N in ranges such that the value of A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14) is 0.0022 to 0.0055, and has a steel structure wherein bainite volume ratio is 30% or more and pearlite volume ratio is less than 5%.