This paper investigates the effect of de-oxidation inclusions on micro-structure in low carbon (0.07 mass%), high Mn (0.9 mass%) steel. De-oxidation tests were carried out by adding either aluminum (0.05 mass%) or titanium (0.05, 0.03 or 0.015 mass%) to an iron melt in a 400 g-scale vacuum furnace. A Confocal Scanning Laser Microscope (CSLM) was used to evaluate the effect of cooling rate by re-melting and quenching during solidification. Fine secondary de-oxidation particles were obtained in the Ti-killed samples, and the particle density increased with increasing oxygen content, and their size decreased with increasing the cooling rate during solidification.
The secondary Ti de-oxidation particles were found to have an effect on microstructure evolution, such as solidifying microstructure, austenite grain growth and austenite decomposition. The de-oxidation particles were examined through FE-TEM and were identified to be TiO, MnTiO₃ and Mn₂TiO₄, in low oxygen ([O]=7 ppm) and high oxygen ([O]=56, 81 ppm) Ti-killed steels respectively, which were qualitatively same as those predicted by thermodynamic calculations. Stabilities of TiO, MnTiO₃ and Mn₂TiO₄ are influenced by Mn presence. Composition change and decomposition of oxide were estimated through thermodynamic calculations. The effect of the particles on ferrite formation was evaluated through thermo-mechanical treatments. TiO was the most effective for promoting ferrite formation through heterogeneous nucleation. The particles contributed to ferrite formation in the following order, TiO>TiN>MnS> MnTiO₃>Ti₂O₃.
It was found that the secondary Ti de-oxidation particles work are engulfed by the advancing solid phase during solidification based on analysis with PET (Pushing Engulfment Transition) velocity, particle sizes and solidification rates. The particles at dendrite tips and inter-dendritic regions are likely restraining the molten steel flow resulting in a finer solidification microstructure.