STEEL FOR WELDING STRUCTURE HAVING WELDED JOINT WITH SUPERIOR CTOD PROPERTIES IN WELD HEAT AFFECTED ZONE

摘要

There is provided welding structural steel including, by weight, 0.01 to 0.2% C, 0.1 to 0.5% Si, 1.0 to 3.0% Mn, 0.01 to 0.1% Ti, 0.5 to 3.0% Ni, 0.0010 to 0.01% B, 0.003 to 0.006% N, 0.030% or less P, 0.005 to 0.05% Al, 0.030% or less S, 0.05% or less O, unavoidable impurities, and the balance of Fe. The Ti, O, N and B satisfy relations of 0.2≤Ti/O≤0.5, 2≤Ti/N≤5, 5≤O/B≤10 and 0.7≤(Ti+4B)/O≤1.5. A fine structure of a welded joint of the welding structural steel includes acicular ferrite with a structure fraction of 85% or more. In the welded joint, TiO oxides are distributed uniformly in a structure at a spacing of 0.5μm or less, and have a grain size ranging from 0.01 to 0.1 μm. The number of TiO oxides is 1.0x107/mm3 or more. The welding structural steel has high strength characteristics in high heat input welding and employs TiO oxides and soluble B, so that the transformation into acicular ferrite is accelerated in a welded metal zone, thereby ensuring superior CTOD properties at the welded joint.