The fine-grained mixed microstructure of acicular ferrite (AF) and bainite in YS690MPa steel weld metal contributes to attain high-impact toughness. The morphology and evolutionary mechanism of fine-grained mixed microstructure in this weld metal were investigated. Single or multiple AF grains were nucleated on complex inclusions by forming Mn-depleted zones, where Mn spontaneously diffused into Ti oxide inclusions due to the cation vacancies. It is in good agreement with the theoretical calculation by first principle. The bainite nucleated on austenite grain boundary and then assisted the pre-formed AF to partition the austenite grain into small and separate regions. Furthermore, the later formed ferrite nucleated on the broad surface of pre-formed ferrite plates and grew in those small regions with limited grain size. All of them resulted in the formation of fine-grained mixed microstructure, which provided excellent impact toughness in this weld metal with dimples and quasi-cleavage fracture surface combination.
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