Considerations on the Solidification and Macroscopic Segregation of Large Steel Ingots and Castings

Abstract

The solidification and macroscopic segregation of steel cast in chilled and sand moulds were investigated. The growth of solidifying steel shell with the lapse of time was calculated under simplified postulations, when steel was cast in moulds with the shape of infinite wide plate or long cylinder and sphere. The longitudinal solidification rate of ingots and castings were measured by bar test method, and compared with the calculated values.
Further, discussion was made on the relation between the rate of vertical or horizontal solidification and the formation of macroscopic segregation, under consideration of inter-dendritic solidification.
The results obtained are as follows;
(1) The calculated results agree well with the measured results by bar test method.
(2) There exists close correlation of macroscopic segregation with condition of solidification and thickness of solid-liquid coexisting zone in dendrite. In other words the macroscopic segregation is induced by the upwards floating of light liquid metal enriched with solute in solidifying dendrite arms.
The easiness of floatation depends upon the direction of macroscopic solidification and also the width of solid-liquid coexisting zone. On the other hand the width of solid-liquid coexisting zone is influenced by the temperature gradient of solidification front and the temperature range of solidus and liquidus of steel. The mutual relations of these factors are made clear.