High temperature processed high Nb X80 steel with excellent heat-affected zone toughness

In this letter, the investigation focused on the coarse-grained heat-affected zone (CGHAZ) of a high temperature processed (HTP) high Nb (0.11 wt%) X80 steel. The microstructural examination showed that the microstructure of CGHAZ consisted of granular bainite, baintic ferrite and secondary phases such as martensitic-austenitic (M-A) constituent. The comparatively fine austenite grain size was due to both Ti-Nb precipitates and solute Nb. The Charpy V-notch (CVN) testing revealed that the HTP steel exhibited excellent toughness in CGHAZ, which was indicated by the low ductile-brittle transition temperature (DBTT). The improved toughness was ascribed to the combined contribution from the relatively fine prior austenite grain size, fine sub-microstructures and dispersed fine M-A constituent.