Formation Mechanism of AlN Inclusions in High‐Al Medium‐Mn Steel

Herein, the morphology of AlN inclusions in high‐Al medium‐Mn steel was observed, and their formation mechanism was discussed. The results indicate that AlN inclusions exhibit the following two types: single particle polyhedral type and multiparticle aggregation type. AlN inclusions can also form composite inclusions with MnS, TiN, and Al 2 O 3 inclusions. Density functional theory (DFT) calculations elucidate that the formation of AlN particles involves a transformation process from solute [Al] [N] to (AlN) n clusters, and subsequently to AlN crystals. As the value of n increases, the stability difference between (AlN) n clusters and AlN crystals decreases, and (AlN) n clusters become more stable. During the solidification process, (AlN) n clusters with n > 30 can be transformed into AlN crystals, and a decrease in temperature is beneficial for nucleation and further reduces the n value. During the growth process of AlN particles, [Al] [N] first forms an (AlN) n layer on the surface of the particles, and the (AlN) n layer on the surface of the particles undergoes a phase transition, promoting the aggregation of two particles. At last, MnS and TiN inclusions precipitate later than AlN, and the two‐dimensional (2D) mismatch theory confirms that AlN inclusions can provide heterogeneous nucleation sites for MnS and TiN inclusions.