Formation of core-shell structured carbides via interphase precipitations in V-Nb microalloyed steels

It is one of the most effective methods to manipulate the atomic-scale structures of carbides formed by interphase precipitation to enhance the strengths of the microalloyed steel. In this study, conventional TEM, STEM-EDS, and HR-STEM were carried out to understand the origin of fine carbide precipitation in V-Nb multiple microalloyed ferritic steels. TEM characterizations showed that the multiple additions of V and Nb resulted in B1 structured alloy carbides with (V,Nb)C-cores and VC-shells, resulting in denser carbide precipitation compared to single additions of these elements. Thermo-Calc calculations and TEM results suggest that the precipitation kinetics of the (V,Nb)C-cores depends on the balance between its driving force and the misfit strain with the surrounding matrix, while that of the VC-shells is achieved by aging precipitation.