Abstract In the paper the multicsale model of segregation process is presented. The course of solidification is considered in a macro scale under the assumption that the temperature field in the casting domain is equalized and only time-dependent. Such model can be a good approximation of the conditions of volumetric solidification. The final form of the differential equation results from the energy balance at the same time Mehl–Johnson–Avrami–Kolmogoroff approach to the solidification modelling is taken into account. The temporary solidification rate determines the course of microsegregation proceeding in the control volume (‘envelope’) corresponding to the final grain radius. The distribution of alloy component in the control volume domain is estimated by a broken line. The courses of the segments creating the broken line result from the mass balance, the thickness of boundary layer near the solidification front and the condition given on the moving boundary. The changes of concentration determine the new values of solidification point.