LASER WELDED JOINT AND LASER WELDING METHOD OF STEEL MATERIAL EXCELLENT IN TOUGHNESS OF WELD METAL PART

摘要

PROBLEM TO BE SOLVED: To provide a laser welded joint formed of a steel material being suitably used for a welded structure such as a line pipe, having a tensile strength of ≥780 MPa, being excellent in toughness of a weld metal part, and also to provide a laser welding method.SOLUTION: In the laser welded joint, the weld metal has a component composition that contains, in mass%, ≤0.14% C and ≤0.02% O and satisfies a Ceq value of 0.35-0.65%, and has a microstructure comprised of bainite or a mixed structure of bainite and martensite where an acicular M-A structure (island martensite) with an aspect ratio of ≥4 is ≤5% in an area ratio. The Ceq value is expressed by a formula: Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14, wherein Ceq is carbon equivalent (mass%), and each of C, Mn, Si, Ni, Cr, Mo, and V is the content of each alloy element (mass%). An inert gas where an oxygen-supplying gas is ≤10% in a volume ratio is used as a shield gas.