Study on the positive segregation in columnar-to-equiaxed transition zone

Abstract

Laboratory experiments of steel ingots under three cooling modes and industrial trials of continuous casting (CC) blooms and billets were conducted, respectively. Corresponding results of the macrosegregation and the solidification structure were investigated. The mechanism of the positive segregation in the columnar-to-equiaxed transition (CET) zone was proposed. For laboratory experiments of ingots under three cooling modes, including water cooling, water cooling for 15 s followed by air cooling, and air cooling, obvious positive segregation was generated in the CET zone, and the degree of segregation was 1.024, 1.025, and 1.015, respectively. For industrial trials of three kinds of steel, CC bloom, round bloom, and billet, positive segregation was formed in the CET zone, and the degree of segregation was 1.06, 1.04, and 1.06, respectively. With the growth of columnar dendrites, solute elements were rejected on the liquid–solid interface. The downward flow near the columnar tip region and the upward flow in the center melt can carry the solute-rich melt to the molten pool. When the CET occurred, equiaxed grains accumulated around the columnar dendrite tips, and the solid network became less permeable. The liquid flow in the network was significantly suppressed, and thereby the solute-enriched liquid was ‘arrested’ locally. Solute-enrich liquid would be further sucked into the interdendritic region owing to the solidification shrinkage. As the sample was fully solidified, the obvious positive segregation was found at the CET zone.