Studies of the deformation process of a continuously cast billet with a defect of shape “rhomboidity” have been developed in relatively recent times and were caused on the one hand by the active introduction of capacities for the production of continuously cast billet, and on the other hand, by the expansion of the use of caliber-free rolling technology in the stand of roughing groups of continuous rolling mills. The results of a comprehensive study on the evaluation of changes in contact conditions in the box gauge of the roughing stand of a rolling mill during the deformation of an continuously cast billet with a defect of shape “rhomboidity” are presented. Processing of experimental data using the proposed Кскр criterion, which, unlike the existing ones, takes into account the ratio of individual elements of the contact area, showed that an increase in the rhomboidity of the billet in the studied range of values leads to a redistribution of contact areas along the walls of the box pass. The change in the degree of pinching of the billet in the box pass а = Bз/Bк contributes to the overall stabilization of the rolling process. The mechanism of stabilization of the rolling process is based on the fact that as the degree of pinching increases on the second side face, an increase in the contact area is observed, and as a result, a compensating torque occurs. This circumstance allows us to recommend large values of the degree of metal pinching in the box caliber for the rolling conditions of billet with a defect of shape “rhomboidity”.