Effect of Al Content on Formation Mechanism of AlN and MnS Inclusions in Fe-5Mn-xAl-0.15C-0.23Si Medium Mn Steel

The effect of Al content on the formation mechanism of AlN and MnS inclusions in Fe-5Mn-xAl-0.15C-0.23Si (x = 0.018%, 0.95%, 1.93%, 2.97%) medium Mn steel was investigated under as-cast conditions by microstructure observation, thermodynamic analyses, and the Clyne–Kurz microsegregation model. The effect of Al content on the partition coefficient, diffusion coefficient, and secondary dendrite arm spacing was explored first, and then these parameters under different Al contents were used in the Clyne–Kurz model to predict the solute concentration in the residual liquid steel. The results showed that, with increasing Al content, the solute partition coefficients were easily influenced, and the order of the influence was S > C > N > Al > Mn. Among these, the average partition coefficients of S showed a large downward trend (from 0.0107 to 0.0060), while Mn showed little change (approximately 0.71). Based on an electron probe microanalyzer, the partition coefficients of the solutes S, C, N, and Mn were all lower than 1, while solute Al was higher than 1, which were consistent of the results by Thermo-Calc calculations. With the Al content increasing from 0.95% to 2.97%, the precipitation temperature of AlN increased from 1730 K to 1759 K. The local growth time of MnS increased from 0.80 s to 4.96 s.