Effects of carbon on the CG HAZ toughness and transformation of X80 pipeline steel

X80 pipeline steel produced by TMCP has high strength and high toughness with ultrafine grain microstructure. The microstructure coarsens and the toughness worsens at the coarse grained (CG) HAZ apparently after the weld simulation. The experimental results indicate that the bainitic ferrite and the second phases formed at cooling are different with the variation of carbon content in base metal. In low carbon steels, the bainitic ferrite laths are long and narrow, and the second phases are complex, including residual austenite, martensite, the M-A constituent and the Fe3C carbide. The formation of Fe3C carbide is the main reason of the poor toughness in the CG HAZ. The ultra-low carbon content in base metal, however, can improve the CG HAZ toughness through restraining the formation of carbides, decreasing the M-A constituent and increasing the residual austenite content, which are beneficial to the CG HAZ toughness.