A four-phase mixed columnar–equiaxed solidification model is introduced to calculate the formation of macrosegregation and shrinkage cavity during solidification of steel ingot. The four phases are the liquid melt, the solidifying solid with columnar morphology, the solidifying solid with equiaxed morphology, and the gas phase (or covering liquid slag). Multiphase/multiphysics transport phenomena (mass, momentum, species and enthalpy) are solved with a volume-average approach. Solidification induced mass and species transfers among metal phases are considered according to the thermodynamics and diffusion-governed crystal growth kinetics. The gas phase (or covering liquid slag) is only required to feed the shrinkage cavity and no mass/species exchange with other metal phases occurs. The following modeling results are obtained: the progress of columnar tip front and growth of columnar tree trunks, the nucleation and growth of equiaxed grains, the melt flow and equiaxed crystal sedimentation, the solute partitioning at the solid/liquid interface, the transport of the solute species and induced macrosegregation, the shrinkage cavity, the interaction or competition between growing columnar and equiaxed phases and the occurrence of columnar to equiaxed transition (CET). Those modeling capacities were verified by the calculation of a 10.5 tons steel ingot. The experimentally determined profile of the shrinkage cavity, Sulfur print and chemical analysis of macrosegregation of the ingot in a vertical section were also available. Satisfactory agreement was obtained between the simulation and experimental result. Finally, a new hypothesis for the initialization of A-segregates is proposed: the motion of equiaxed phase and its interaction with the melt flow in the vicinity of growing columnar tip front lead to formation of an A-shape segregation band starting from the ingot corner just above the columnar-to-equiaxed transition area. This A-segregation band might provide a favored location for the initialization of A-segregates. Further dedicated experiment should be carried out to verify it.