Innate character and theory model about channel segregation in a sand mold steel ingot

TL;DR

This paper investigates the formation mechanism of channel segregation in steel ingots, revealing the role of MnS inclusions and their movement driven by interfacial tension and buoyancy, supported by experimental observation and theoretical analysis.

Contribution

It introduces a new formation mechanism for channel segregation involving MnS inclusions and their lateral and upward movement in the mushy zone.

Findings

MnS inclusions move laterally and upwards in the mushy zone.

Residual MnS inclusions form chains that promote ferrite chain formation.

The interface tension and buoyancy are key driving forces for MnS movement.

Abstract

The channel segregation is a severe casting defect in steel ingots. The formation of channel segregation is generally attributed to the solute partition and the interdendritic thermosolutal convection in the mushy zone. In this article, the channel segregation in a steel ingot was carefully characterized by detailed experimental observation. A different formation mechanism for the channel segregation was revealed. In the mushy zone of the ingot, large amount of separate MnS inclusions move laterally and upwards. Some MnS inclusions will remain in the moving trace of inclusions. Such residual MnS inclusions appear as large amount of separate MnS inclusion chains. In the subsequent solid phase transition process, promoted by the MnS chains, ferrite prefers to forms form the austenite near the MnS inclusions and shows as large amount of separate ferrite chains. Large amount of ferrite chains align in a strip-like zone, which results in so called channel segregation in macro-etching. 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 act as the drive force for the upwards movement.