Abstract
A continuum model for the transport phenomena in solidification systems is used to investigate the formation mechanism of macrosegregation in a 3.3 t steel ingot. Numerical scheme with explicit time stepping in solidification problems is developed for solving coupled temperature and concentration fields, and equations of momentum. Experimental measurements and numerical results in the literature are used as indications of the validity of present prediction. Influences of the mold shape and the melt superheat upon macrosegregation are investigated. Results show that for ingots with the same weight, reducing the size of hot top favors a pronounced positive-negative-positive concentration distribution along the centerline. A-segregates and positive-negative-positive concentration distribution are not found in the ingot with a modified hot top that has a more uniform section area. Higher superheat reduces the height of bottom negative segregation cone. For cases with superheat larger than zero, positively segregated patches are observed at the ingot bottom.

Key words
macrosegregation, steel ingot, mold shape, melt superheat

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