Thermodynamic study on the solute partition coefficients on L/δ and L/δ+γ phase interfaces for 1215 high-sulfur steel solidification by orthogonal design

The solute partition coefficient (ki), characterizing the solute redistribution on the solid-liquid interface, is an important parameter for the study of segregation. Based on thermodynamic equilibrium, the effects of composition fluctuation, phase transition, MnS precipitation, and temperature on the ki for the solidification of 1215 high-sulfur steel were investigated by orthogonal design. The results show that there is only L→δ phase transition for 0.05–0.07wt% C steel solidification, while there are L→δ and L+δ→γ (peritectic reaction) phase transition in turn for the solidification of steel with 0.07–0.09 wt% C. On the L/δ interface, as the temperature decreases, the kSi and kP increase, the kMn decreases, while the kC and kS both decrease first and then increase because of MnS precipitation. In the L+δ+γ coexistent phase, the kiL/δ+γ is comprehensively determined by kiL/δ and kiL/γ. Moreover, the kiL/δ+γ for each solute is closer to the kiL/δ comparing to kiL/γ, because the mass fraction of γ phase (mγ) in the solidified phase (consisting of δ+γ) is relatively small. The peritectic reaction has small effect on the kiL/δ and kiL/γ, but has significant effect on the mγ, leading to the discrepancy in kiL/δ+γ for the steels with different C contents. The orthogonal analysis indicates that the composition fluctuation in 1215 steel has small effect on the ki. The Mn and S contents have the greatest effect, followed by C content. Si and P contents have the least effect. The quantitative ki in terms of temperature, phase composition, and solute content were fitted.