Nature Commun. 5, 5572 (2014)

Credit: NATURE PUBLISHING GROUP

Segregation during the solidification of steel is the process by which chemical gradients are frozen into a casting. Channel segregates — linear regions of chemical inhomogeneity — are particularly undesirable as they present substantial challenges during subsequent processing of an ingot. Now, Dianzhong Li and colleagues identify oxide-based inclusions, which experience a buoyancy force in the melt, as being key to controlling channel segregate formation. By casting steel under conditions to control oxygen content, they find that channel segregates are supressed in ingots with lower oxygen concentration, reducing the size and number of Al2O3 inclusions. Simulations reveal that the inclusions experience a buoyancy force in the melt, the extent of which is dependent on their size; inclusions with a diameter of ∼5–30 μm alter melt flow around dendrite arms, destabilizing the mushy zone and promoting the formation of channel segregates. To validate their findings, the authors cast 100-tonne steel ingots in low oxygen conditions, resulting in the almost complete elimination of channel segregates.