A coupled numerical model was developed to predict macrosegregation in continuously cast steel billets. Continuum formulation was adopted to investigate macroscopic transport behavior of momentum, heat and species in the continuous casting process. In order to model fluid flow damping in the mushy zone solidification was considered to occur in two successive stages. In the first stage with the solid fraction below a certain critical value, the relative viscosity concept was employed to treat fluid flow in the dilute mushy zone. In the second stage with the solid fraction above critical value, the permeability concept was used to treat the concentrated mushy zone as a porous medium. Turbulent fluid flow during continuous casting was treated by the standard k-ε model. Thermal and solutal convections were also considered in the simulation of solute redistribution in the mushy zone. The effect of interdendritic flow on surface temperature, metallurgical length and centerline segregation were investigated. It is suggested that the present model can be successfully applied to simulate the macrosegregation in continuous casting of steel billets.