Non-metallic inclusion and intragranular nucleation of ferrite in Ti-killed C–Mn steel

The influence of Ti addition on the development of acicular ferrite microstructure during the γ/α transformation in C–Mn steels has been studied. The optical microstructures of the heat-treated specimens with different Ti concentrations were characterized. Transmission electron microscopy analysis with thin foil specimens was carried out to investigate the phase composition of non-metallic inclusions and the local variation of chemical composition around the inclusions. It has been found that an acicular ferrite dominant microstructure could be produced when the Ti concentration is higher than about 50 ppm. The transition from the conventional bainitic microstructure to the interlocking acicular ferrite microstructure occurs in response to the change in the main-component of the non-metallic inclusions from Mn–Si oxide to Ti 2O 3. The Mn depleted zones around Ti 2O 3 particles were detected, which could explain how the intragranular nucleation is facilitated on Ti 2O 3 particles.