Influence of Al content on the characteristics of non-metallic inclusions and precipitation behaviors of AlN inclusions in TWIP steel

The morphology, composition, and number of inclusions in Fe-Mn-C(-Al) twining-induced plasticity (TWIP) steels were investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and an automated program called "INCAFeature". The characteristics of the inclusions in four TWIP steels with different Al contents (0.002% -1.590%) as well as the influence of Al content on the precipitation of AlN inclusions were investigated. In addition, systematic thermodynamics calculations of AlN formed in TWIP steel were carried out using the appropriate thermodynamic data for high-Mn-Al TWIP steel. The results show that AlN would begin to precipitate and locally precipitate around the MnS(Se) -Al₂O₃ inclusions when the Al content in the steel reaches 0.75%. The thermodynamics calculations show that AlN could already form in the liquid TWIP steel at an Al content of 1.07%. Then, AlN would locally precipitate around the MnS(Se) inclusions, thus forming MnS(Se) -AlN aggregates. When the Al content increases to 1.59%, the precipitation temperature of AlN is 42℃ higher than the liquidus temperature of the TWIP steel. Furthermore, precipitated AlN inclusions in the liquid TWIP steel could act as heterogeneous nuclei for MnS(Se) inclusions, thus forming MnS(Se) -AlN inclusions. Moreover, according to the thermodynamics calculation, the lowest N content for AlN formation in the liquid Fe-18.21% Mn-0.64% C-1.59% Al steel is just 0.0043%. Therefore, the N content should be kept as low as possible to avoid the formation of excessive AlN inclusions during melting of Fe-Mn-C(-Al) TWIP steel.