Study on the thermodynamic stability and evolution of inclusions in Al-Ti deoxidized steel

To investigate the thermodynamic stability and the evolution process of inclusions in the Al-Ti deoxidized steels, both laboratory experiments and thermodynamic calculations were conducted in the present work. Scanning electron microscope-energy dispersive spectrometer (SEM-EDS) and Al2O3-Ti2O3-TiO2 phase diagrams were used to investigate the composition of oxide inclusions after adding various contents of Al and Ti. The results show that the TiO x content of inclusions increases with the increase in titanium addition in steel ranging from 0.31 to 1.88%, and the typical inclusions are transferred from pure Al2O3 to multi-phase Al-Ti complex inclusions and Al2O3-TiO x complex inclusions with uniform composition. The TiO x content in the inclusions first increased and then decreased with the extension of deoxidation time in the steels with deoxidants of [% Al] = 0.055 and [% Al] = 0.71, and the content of TiO x in the inclusions is highest at 360 s of deoxidation. There is a small amount of pure titanium oxide and liquid Al2O3-TiO x composite inclusions in the steel with deoxidants of [% Al] = 0.13 and [% Ti] = 1.88 at 120 s of deoxidation, and then this part of inclusions gradually turns into the Al2O3-rich phase. The predominance diagrams of the Al-Ti-O-Fe system were obtained based on the classical thermodynamic calculation and FactSage calculation with different databases and products. The calculated results were compared with the experimental data, and the discrepancies on the stable region of oxides in the predominance diagrams were discussed. The evolution and transformation of inclusions during the solidification process were analyzed based on FactSage calculation.