Abstract:The effect of prior-austenite grain size (PAGS) on bainitic ferrite (BF) morphology and effective grain size in a pipeline steel under an industrial cooling conditions was investigated. With PAGSs of 22.3 m and 37.0 m, BF laths with a parallel morphology could be seen, while as the PAGS is increased to 52.4 m or 62.8 m, more interlocking BF microstructures were observed. Detailed crystallographic analysis identified that intense BF variant selection occurs for the small-grained austenite, but due to the large austenite grain quantity and thus the large total austenite grain boundary area per unit volume of the small-grained austenite, with the reduction of PAGS from 62.8 m to 22.3 m, the effective grain size of the transformed microstructure gradually decreases from 9.5 m to 5.6 m. In the meanwhile, the small-grained austenite transformation was dominated by one close-packed plane (CP) group, but for the larger austenite grain, different CP groups were observed to be adjacent to each other, producing the interlocking structure. A mechanism based on the carbon distribution near the BF/austenite interface is proposed to account for this difference.