Heterogeneous Strip Originated from Inclusions: Characterization and Physical Mechanism

Abstract

The heterogeneity, such as well-known 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 inclusions is discovered by detailed experimental characterization in a steel ingot. A strip characterized as ferrite chains and inclusions 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 MnS inclusions move laterally and upwards. Some MnS inclusions will remain in the moving trace. Such residual MnS inclusions appear as a large amount of separate 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 MnS movement is further theoretically analyzed. In the mushy zone, the interface tension resultant applied on the MnS inclusions can act as the drive force for the lateral movement of MnS inclusions. And the buoyance applied on the MnS inclusions acts as the drive force for the upwards movement of inclusions.