Ti-Deoxidized Products and Formation Mechanism of Intragranular Ferrite in High Grade Pipeline Steels

High grade pipeline steels were prepared using vacuum carbon deoxidization process combined with a final Ti-deoxidation process. The microstructure of the as-cast steels was investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM observation shows that the formation of intragranular ferrite (IGF) structure is induced by fine inclusions. TEM selected area diffraction (SAD) patterns and elemental distribution analysis indicate that these inclusions are mainly Ti2O3 and MnS. It is also found that Ti2O3 may act as nucleus in the formation of MnS during solidification process. Raman spectroscopic analysis demonstrates the presence of another phase, MnTiO3, which could be formed through entrapment of Mn by Ti2O3. It is believed that the formation of Mn-depleted region in the inclusions and thus the formation of MnTiO3 phase will increase the Mn pickup from matrix and promote the formation of IGF during solidification of molten steel.