50～60kgf/mm²級高張力鋼サブマージアーク溶接金属の組織とじん性におよぼすTi及びNの影響

抄録

Investigations into relationship between Ti content and toughness of 50-60 kgf/mm² tensile strength submerged-arc weld metals containing different amounts of nitrogen to obtain excellent, tough weld metals and also effects of oxygen and Al content on the optimum Ti content have been studied. An addition of 0.04 to 0.08% Ti into weld metals from welding wires gives excellent toughness to the weld metal containing nitrogen of 25 to 80 ppm. The optimum Ti content increases with nitrogen content in weld metal so that improvement of weld metal toughness by Ti addition to weld metal is not considered to be based on only the precipitation of TiN particles as an acicular ferrite nucleation sites during transformation from austenite. The contribution of Ti to improvement of weld metal toughness stems from fine acicular ferrite formation accompanied with reduction in free nitrogen.
Decrease in weld metal oxygen content or addition of Al into weld metal leads to shift the optimum Ti content to lower levels and to decrease precipitated Ti and increase precipitated N. It is therefore considered that with decreasing oxygen in weld metal Ti fixes nitrogen more effectively because the major precipitated Ti consists of oxide and nitride and that Al combines with oxygen in preference to Ti, allowing ready combination of Ti with N, because Al addition produced little AIN precipitation.
As a result, addition of small amounts of Ti to weld metal, linking with decrease in weld metal oxygen and nitrogen content may well be key point of improvement of weld metal toughness.