The effect of Al content on the characteristics and formation of inclusions in the light-weight Fe–5Mn–xAl steels was investigated in this study. Four synthetic steels with different Al content were produced in the laboratory. The types of observed inclusions were Al₂O₃(pure), Al₂O₃–MnS, AlN(pure), AlN–MnS, AlON–MnS, AlON and MnS. Increasing Al content from 0.5% to 6% increased the total amount of inclusions by 2.5 times. As the Al content increased from 0.5% to 3%, the number of AlN–MnS inclusions increased significantly. Moreover, the AlN(pure) inclusions appeared 6% Al containing steel. Thermodynamic calculations confirmed that AlN inclusions formed during cooling of the steel. It is also observed that AlN can precipitate on Al₂O₃ to form AlN + Al₂O₃ inclusions, classified as multi-phase AlON inclusions in this study. Furthermore, MnS inclusions could co-precipitate with AlN and Al₂O₃ inclusions, but it preferred to co-precipitate with AlN inclusions.

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 Al₂O₃-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 AlON > Al₂O₃. Moreover, it was determined that AlN particles could be easily engulfed at the solidification front relative to Al₂O₃, which resulted in a higher MnS precipitation ratio for Al₂O₃ under slow cooling conditions.

The effect of N content on the characteristics and formation of inclusions in the Fe-5Mn-3Al steels was investigated in this study. Two synthetic steel melts were produced by two different methods—N₂ gas purging and injecting—to introduce nitrogen into the melt. The N content of steel melt varied from 2 to 54 ppm. An increase in the N content to 47 ppm (for 533N-P) and 58 ppm (for 533N-I) increased the total amount of inclusions from 13 to 64 mm⁻² and from 21 to 101 mm⁻², respectively. The observed inclusions were Al₂O_3(pure), Al₂O₃-MnS, AlN_(pure), AlN-MnS, AlON, AlON-MnS, and MnS. When the N content was less than 10 ppm, AlN-MnS inclusions were the primary type of inclusions and they formed as solidification products. With an increase in the N content, AlN_(pure) inclusions became the dominant type of inclusions as AlN was stable in the liquid steel. These findings were confirmed by thermodynamic calculations. The influence of cooling rate on the types of inclusions was studied and a higher number of AlN-MnS inclusions were observed in samples with slow cooling rate.