低炭素低合金鋼溶接熱影響部の限界CTOD特性に及ぼすAl含有量の影響

抄録

Crack Tip Opening Displacement (CTOD) properties in HAZ of 490 N/mm₂ class steels were investigated from viewpoint of Al content in steel. The C-Mn microalloyed steels with Al content of 0.015 and less than 0.001 mass% were selected in this study.
The critical CTOD property in HAZ of low Al content steel was more excellent than that in HAZ of conventional Al content steel. The microstructure in HAZ of low Al content steel mainly consisted of fine ferrite. On the other hand, the microstructure in HAZ of conventional Al content steel consisted of upper bainite with M-A constituent.
Ferrite formation in low Al content steel was easier than that in conventional Al content steel at usual cooling rate during welding, because the region of ferrite formation was shifted to a higher cooling rate at low Al content.
In HAZ of conventional Al content steel, M-A constituent formation is easy due to the suppression of ferrite formation because of the decrease in interfacial energy between γ and α. This is base on condencing the carbon atoms in γ side of γ/α interface due to delaying the diffusion rate of carbon atoms in γ region.
From these results, the effects of low Al content on the improvement of the critical CTOD property in HAZ are affecting the transformation behaviour through promoting the diffusion of carbon atoms, resulting in fine ferrite microstructures without M-A constituent.