Studies on the effect of Nb on microstructure refinement and impact toughness improvement in coarse-grained heat-affected zone of X80 pipeline steels

To investigate the effect of niobium (Nb) on the microstructure and properties of the coarse-grained heat-affected zone (CGHAZ) in X80 pipeline steel, this study examined X80 pipeline steels with three different Nb contents: Nb58 (0.058 wt% Nb), Nb76 (0.076 wt% Nb), and Nb91 (0.091 wt% Nb). The microstructure and lowtemperature impact toughness of the CGHAZ were thoroughly analyzed under identical welding conditions. The results indicate that, among the three pipeline steels, the impact toughness at - 40 degrees C increases with the Nb content, ranging from 66 J for Nb58 to 195 J for Nb91. The microstructure within the CGHAZ of Nb91 is the finest, characterized by smaller prior austenite grains (PAGs) and a higher density of high-angle grain boundaries (HAGBs). Moreover, in the CGHAZ of the experimental samples, relatively large NbC particles were observed, while no small-sized NbC particles capable of exerting a pinning effect were detected. These findings suggest that the grain refinement effect of Nb in the CGHAZ is primarily governed by the solute drag effect of Nb, rather than the pinning effect of NbC. Building on this insight, a kinetic grain growth model considering the drag effect of solute Nb was modified to accurately predict the size of PAGs within the CGHAZ during welding. Validation of this model showed a strong correlation with experimental observations in the CGHAZ of welded joints. The grain growth prediction model is significant in guiding the regulation of microstructure refinement and improving impact toughness.