Correct prediction of composition heterogeneities and grain structure across a steel ingot is critical in optimizing the industrial processing parameters for enhanced performance. The columnar to equiaxed transtion (CET) is a microstructural transition which is strictly controlled as it affects the mechanical properties of the final product along with the macrosegregation patterns. Larger equiaxed regions are preferred for most industrial applications. CET is significantly affected by the number density of equiaxed grains and by the nucleation undercooling. 8 kg 42CrMo4 alloy steel ingots (240 mm x 60 mm x 60 mm) were cast. The cast structure was characterized by ASCOMETAL. The experiments were simulated with a process-scale model of solidification that incorporates a multiscale description of the microstructure formation. The goal of the present study is to show the capabilities of such a process-scale solidification model to explain the observed structure distributions (extent of the columnar and equiaxed zones, equiaxed-to-columnar transition).