Micro-segregation Study of Directional Solidified Fe-Mn-C-Al TWIP Steels

Chemical variation occurring during the solidification process can cause undesirable effects on the material property, especially for alloys with high solute content. The current study investigated the distributions of solute elements in directional solidified Fe-Mn-C-Al TWIP steels and compared the micro-segregation with various pulling rates and C contents using the phase field model and the grid analysis based on the data performed by EPMA, which could provide advice for mitigating micro-segregation in high Mn steel. The results indicated that C and Mn preferentially segregated toward the liquid phase, while Al entered the solid phase. The occurrence of solid-state diffusion was observed, which led to a homogenization effect in the solid phase during solidification. The pulling speed increase made a more severe micro-segregation because the change of pulling speeds had more effect on the SDAS than on the local solidification time; thus, the dimensionless diffusion time decreased with increasing pulling rates, representing a more serious micro-segregation. For a C content increasing from 0.06 to 0.68 wt pct, the micro-segregation level of C and Al reduced first and then increased, which was caused by the change in the solidification mode. The continuous accumulation of solute elements and the slower solute element diffusion in austenite in combination led to the presence of more severe micro-segregation in the case of one-phase solidification. Besides, a C content increase could attract Mn, resulting in more serious Mn micro-segregation.