| Summary: | This study focused on the characteristics of complex MnS inclusions in advanced high strength steels. The effect of metal chemistry (Al and N) and the cooling rate of steel were evaluated by analyzing the inclusions present in five laboratory produced steels. The observed complex MnS inclusions contained Al<sub>2</sub>O<sub>3</sub>-MnS, AlN-MnS, and AlON-MnS. An increase in Al content from 0.5% to 6% increased the number of complex MnS inclusions by ~4 times. In comparison, a decrease of ~80% was observed due to the increased N content of steel from <10 ppm to ~50 ppm. MnS precipitation ratio was used to determine the potency of different inclusions forming complex MnS inclusions due to heterogeneous nucleation. It was found that the MnS precipitation ratio of the observed inclusions was related to their misfit with MnS, and it decreased in the order of AlN > AlON > Al<sub>2</sub>O<sub>3</sub>. Moreover, it was determined that AlN particles could be easily engulfed at the solidification front relative to Al<sub>2</sub>O<sub>3</sub>, which resulted in a higher MnS precipitation ratio for Al<sub>2</sub>O<sub>3</sub> under slow cooling conditions.
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