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MgTiO3 powder was directly added into molten steel at 1873K, and its effect on inclusion characteristics was studied by a scanning electron microscope and an energy dispersive spectrometer. Thermodynamic calculation indicates that MgTiO3 is unstable in molten steel and may decompose into [Ti], [O] and MgO. However, only Ti-bearing inclusions were observed in the treated sample, and Mg-bearing inclusions were absent. This can be explained by the features of wettability and stability for MgO. Compared with a Non-treated sample, both oxide and coarse MnS in treated sample were refined. For the oxide, this originates from the formation of Ti-bearing inclusion. For coarse MnS, this may be due to the fact that Ti can aggravate S segregation. Besides, this aggravation makes coarse MnS less globular. After etching, it was found that in the treated sample, Ti-bearing inclusion can induce the nucleation of intragranular acicular ferrites. This appearance was totally different from that of Non-treated sample, and indicates the effectiveness of external adding method in oxide metallurgy.

The alloys of Ti and Mg are added in molten carbon structural steel in sequence to reveal their influence on features of inclusion and microstructure. Compared with non-treated steel, it is found that they leads to the formation of Ti-Mg-O inclusion and reduction in portion of pure MnS. Experimental results indicate that average inclusion size in treated steel significantly decreases and its number per unit volume increases by 48 %. Moreover, the margin mole ratio of Mg/Ti in all Ti-Mg-O inclusions is larger than 2, which hints they are more complex than single assumption of MgTi2O4. After etching, intragranular acicular ferrite (IAF) microstructure is observed. EDS line analysis suggests that Mn-depletion zone exists in steel matrix, which is proposed to explain the IAF nucleation effect induced by Ti-Mg-O inclusions. Through comparison with only Mg-treated sample, the refinement of inclusion and increase in portion of pure MnS due to Ti adding are revealed.