HIGH TENSILE STEEL SHEET OF LOW ACOUSTICAL ANISOTROPY EXCELLING IN WELDABILITY, AND PROCESS FOR PRODUCING THE SAME

摘要

A high tensile steel sheet of low acoustical anisotropy excelling in weldability, whose tensile strength is on the order of 570 MPa or greater; and a process for producing a steel sheet, based on an accelerated cooling-halfway stopping process realizing high productivity. There is provided a high tensile steel sheet comprising a steel structure of 0.055% = Nb+2Ti = 0.105%, value of A = (Nb+2Ti)×(C+N×12/14) being in the range of 0.0025 to 0.0055, content of bainite being = 30 vol% and content of pearlite plus island shaped martensite being < 3 vol%. In the production thereof, steel is heated to = 1200°C, and preliminarily rolled at = 1020°C. Thereafter, the preliminarily rolled steel is further rolled so that the cumulative draft at temperature of >920°C but <1020°C is =15% while the cumulative draft at temperature of 860° to 920°C falls within the range of 20 to 50%, and an accelerated cooling of 2° to 30°C/sec cooling rate is started from =800°C. The accelerated cooling is stopped at 600° to 700°C, and thereafter cooling is performed at a rate of = 0.4°C/sec.