Investigation of characteristic and evolution of fine-grained bainitic microstructure in the coarse-grained heat-affected zone of super-high strength steel for offshore structure

E690 steel is a novel developed super-high strength steel for offshore structure. The microstructure-property relationship in the coarse-grained heat-affected zone of E690 steel subjected to high heat input welding with rapid cooling are studied, and the evolution of fine-grained bainitic microstructure is in-situly and crystallographically characterized. The results reveal that the fine-grained bainitic microstructure with abundant high angle grain boundaries is obtained in the coarse-grained heat-affected zone, attributing to rapid cooling. The formation of fine-grained bainite is attributed to extensive impingements and intersections of bainite sheaves forming via both primary nucleation and sympathetic nucleation. Abundant of high angle grain boundaries exist between the sheaves of bainite. The fine-grained bainitic microstructure with abundant of high angle grain boundaries in the coarse-grained heat-affected zone of this super-high strength steel contributes to excellent low temperature impact toughness.