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

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（k;）, inclusion precipitation, and microsegregation by establishing a coupling model of microsegregation and inclusion precipitation, wherein the quantified dependencies of k;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 k;and eventually in microsegregation. k;,k;, and k;were mainly affected by phase composition and k;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’（P;）, kave Pand kave S(k;, the average value of the k;across the whole stages of solidification)decreased monotonically, whereas kave Cincreased linearly. The peritectic reaction impacted on the phase composition and k;, leading to the change in microsegregation. Such effect of the peritectic reaction was more significant at the last stage of solidification. When the P;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 k;, k;and Mn S 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.