We investigated the effect of B1 compounds on ferrite transformation during continuous cooling in C–Mn steel. TiO, TiN and MgO were employed as B1 compounds of similar lattice parameters to exclude the effect of lattice matching. Single-crystalline grains of the B1 compounds were hot-pressed with steel and embedded in austenite grains by subsequent reheating in austenite region, which were then cooled at 1K/s or 5K/s. Polygonal ferrite was found at all the interface of the three compounds similarly in the case of 1K/s cooling, where the Baker–Nutting (B–N) orientation relationship was well-identified between the ferrite and all the compounds. In the case of 5K/s cooling, ferrite formed from only some parts of the interface of MgO while still from the entire interface of TiO and TiN. The portion of ferrite developed from TiO without the B–N relationship increased, while such trend was not found in ferrite from TiN and MgO. First principle calculation suggests that interfacial energy is the lowest when the B1 compounds and ferrite lie with the B–N relationship, and the energies of the interfaces of TiN and TiO are lower than that of MgO. Increase in interfacial energy is significant for the interface of TiN without the B–N relationship, while negligible for the interface of TiO. Those results are explained by electron bonding behaviors at the interface between ferrite and the B1 compounds, and are well correlated with ferrite formation from the B1 compounds observed.
