Heterogeneous strip originated from the separate enriched melt: Innate character and physical mechanism

Abstract

The chemical heterogeneity, well-known as macrosegregation, is a major problem in the casting of steel ingots. Most heterogeneity generally originated from the solute partition and solute distribution in the solidification process with interdendritic convection. In this article, a new heterogeneous phenomenon originated from the movement of separate enriched melt is discovered in a steel ingot. A strip characterized as ferrite chains and MnS chains is revealed by macro-etching. The formation mechanism for the heterogeneous strip is proposed. In the mushy zone of the ingot, a large amount of separate S-enriched melts move laterally and upwards. Some S-enriched melts will remain in the moving trace. Such residual S-enriched melts produce a large amount of MnS inclusion chains. In the subsequent solid phase transition process, promoted by the MnS chains, ferrite prefers to be transited from the austenite near the MnS inclusions and shows as a large amount of separate ferrite chains. A large amount of ferrite chains align in a strip-like zone, which results in the heterogeneous strip phenomenon in macro-etching process. The physical model about the driving force for the movement of separate S-enriched melts is further theoretically analyzed. In the mushy zone, the interface tension resultant applied on the separate S-enriched melts can act as the drive force for the lateral movement of separate S-enriched melts. And the buoyance applied on the separate S-enriched melts acts as the drive force for the upwards movement. The interaction between the impurity movement and the solute segregation is also discussed.