LASER WELDED JOINT WITH EXCELLENT TOUGHNESS OF WELD METAL

摘要

PROBLEM TO BE SOLVED: To provide a welded joint in which the toughness of a weld part is improved by a laser welding, in particular, acicular ferrite of the weld metal structure effective for high CTOD in a manufacturing of a steel structure such as a bridge and a ship building, and structural and piping steel tubes. SOLUTION: Steel having the composition consisting of, by mass, <=0.16% C, <=0.80% Si, <=2.00% Mn, >=0.005% Al, and <=0.025% Ti is used, the oxygen concentration [0]depo of the weld metal satisfies the condition 1: 0.35×(Al+0.57 Ti)<[0]depo<2.0×(Al+0.57Ti), and [0]depo>20 ppm, and the composition of impurities diffused in the weld metal satisfies the condition 2: quantitative analysis of the content elements is performed for the at least ten representative impurities by the energy diffusion type EPMA, and leveling is performed so that the total of Al, Si, Mn and Ti is 100, and the averaged ratio of Si is <=20%.