The Effect of Aluminum on the Divorced Eutectic Transformation of MnS Inclusions

The mechanical properties of steels can be deteriorated by the formation of type II sulfide inclusions. The addition of alloying elements Al, C, and Si is generally considered to promote the transition from type II to type III sulfides. However, the mechanism of how alloying elements, especially Al, affect the transition is still unclear. In this work, the effect of aluminum on the divorced eutectic transformation of MnS inclusions was investigated. The research results show that most sulfides are type II MnS in micro-alloyed YF45MnV steel with Al concentration lower than 0.027 wt pct. The sulfides in the steel with Al concentration higher than 0.038 wt pct are type III MnS. The increase of aluminum content promotes the transition of sulfide morphology from the type II to type III, but increasing cooling rate can have the opposite effect. It is also found that an increase in Al content can significantly affect the equilibrium dissolved oxygen content and surface tension, but has little influence on the activity coefficient of S, precipitation temperature, initial S content in liquid steel, and solid fraction of the precipitation of MnS. High Al content can increase the interface energy between the liquid steel and MnS and promote the divorced eutectic transformation of MnS and form type III MnS.