Generation mechanisms of non-metallic inclusions in high-cleanliness steel

There have been many research reports regarding the deoxidation and oxygen reduction in steel since around 1970, some reports have discussed the genesis, growth and separation mechanisms of individual oxide inclusions in steel during refining. However, most of the investigation of deoxidation dealt with the steel with high oxygen content, where the sum of such base-metal oxides as Fe _xO, MnO and Cr₂O₃, in the secondary refining slag amounted to 5 to 10 mass%, or higher. In this research, the behavior of the inclusions in clean steel practice was investigated, where the base metal oxides in slag do not exceed 1 mass%. It has been found that there are two important mechanisms of oxide inclusion generation other than "Deoxidation reaction" and "slag trapping" which are often assumed to be the origins of oxides during secondary refining. One is "Desulfurization reaction"; the amount of alumina formed by desulfurization after deoxidation is greater than that by the deoxidation reaction. The other is, namely, "Intrinsic reaction"; lime-bearing inclusions are formed by deoxidation/oxidation reactions of CaO and Al.