Quantitative effects of phase transition on solute partition coefficient, inclusion precipitation, and microsegregation for high-sulfur steel solidification

Abstract

Segregation and inclusion precipitation are the common behaviours of steel solidification, which are resulted from the redistribution and diffusion of the solute elements at the solid-liquid interface. The effect of the phase transition of high-sulfur free-cutting steel is quantified in the present work for the solute partition coefficient (ki), inclusion precipitation, and microsegregation by establishing a coupling model of microsegregation and inclusion precipitation, wherein the quantified dependencies of ki in terms of temperature, phase and carbon (C) content were applied. Results showed that the solidification temperature range and phase transition of high-sulfur steel that under different solidification paths and C contents were quite different, leading to differences in ki and eventually in microsegregation. kC, kP, and kS were mainly affected by phase composition and kSi was primarily by temperature, while kMn depended on both phase composition and temperature during solidification. Increasing the C content within the interval 0.07-0.48 wt%, the ‘proportion of the δ phase maintained temperature region during solidification’ (Pδ), kPave and kSave (kiave, the average value of the ki across the whole stages of solidification) decreased monotonically, whereas kCave increased linearly. The peritectic reaction impacted on the phase composition and ki, leading to the change in microsegregation. Such effect of the peritectic reaction was more significant at the last stage of solidification. When the Pδ was between 75% and 100% (corresponding to 0.07-0.16 wt% C), the solidification path resulted in a greater effect on the microsegregation of solutes C, P, and S because of the peritectic reaction. The microsegregation of solutes Mn and S were comprehensively influenced by kMn, kS and MnS precipitation as well. The studies would help reveal the solute redistribution at the solid-liquid interface, and improve the segregation of high-sulfur steel by controlling the solidification and precipitation in practice.