Morphological evolution of HAZ microstructures in low carbon steel during simulated welding thermal cycle

•Bainite formation can be found in a wide range of cooling rate for experimental steel.•Lath structure of ferrite is first formed within the range of bainite transformation.•Granular bainite derived from the lath structure occurs in the final stage of transformation.•The complex inclusions do not effectively assist the nucleation of acicular ferrite. It is necessary to reveal the derivation of bainite microstructures with different appearance because these complex appearances are closely related to the transformation nature and their final mechanical properties. In this work, the morphological evolution of HAZ microstructures during simulated welding thermal cycle was investigated in a low carbon steel that was intentionally treated with combined addition of Ti and Mg elements. The inclusions present in the matrix have a complex core/shell composite structure composed of Al-Ca-Mg-Ti-O and MnS. However, only very few inclusions with large size (about 2∼6μm) can stimulate the intragranular nucleation of ferritic laths. Thus, this behavior did not show an obvious refinement effect on HAZ microstructure. Partial bainite microstructure revealed the ferritic lath structures always first form at the beginning of the transformation irrespective of the final morphologies of microstructure (granular bainite or bainitic ferrite), implying that the granular bainite is actually derived from the ferritic lath structures although these lath structures will become gradually obscure at the later stages of the transformation.