Formation Mechanism of Inclusions in Steel by Ti-Al Deoxidation and Their Physicochemical Properties

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Thermodynamic properties of the Al_2O_3-SiO_2-TiOx oxide system, which are important for the morphology control of steel using non-metallic inclusions formed during steelmaking processes, have been investigated.Firstly, phase relations for the Al_2O_3-SiO_2-TiOx system at 1873 K were clarified. Solubilities of Al_2O_3 and TiOx were found to be 28 mass% and 50 mass%, respectively, and the ratio of Ti^<4+>/ Ti^<3+> increased with increase in SiO_2 content of the oxide, which indicated that both Ti^<4+> and Ti^<3+> behaved as network formers also from the results of Raman spectroscopy of the quenched samples.Secondly, the activity of each constituent was measured. Oxide melts were equilibrated with Fe-C-Si alloys and the activity of SiO_2 was determined from the final Si content and activity coefficient of Si in the alloy. On the other hand, Knudsen cell effusion method was employed for the activity of Al_2O_3 From the vapor pressures of Al, Si, AlO and SiO for the molten oxide equilibrated with Al-Si-Ti alloy, the activities of Al_2O_3 and SiO_2 were determined. The obtained value of SiOs agreed well with that by chemical equilibration technique, and that of TiOx was also derived by Toop's method. These values demonstrated that the affinity of Si0_2 for Al_2O_3 was almost the same as that of TiOx.The findings drawn in the present research quantitatively provide the driving force necessary in discussing the mechanism of inclusion formation. Also, the composition of steel equilibrated with various compositions of oxide inclusions can be assessed from the results, which leads to an optimization in deoxidation process control.