A large-deformation constitutive model applicable to the calculation of roll force and torque in heavy-reduction rolling has been presented. The concept of the volume fraction of dynamically recrystallized grains, which depicts the flow stress softening correctly with the level of strain, strain rate and temperature has been newly introduced in the proposed model. The material constants required in the proposed model have been obtained by a series of hot-torsion tests.A laboratory-scale hot-plate rolling experiment, together with three-dimensional finite element analysis coupled with the proposed model, has been performed to investigate the accuracy of the proposed constitutive model. The soundness of the proposed model has been demonstrated through a series of finite element simulations with temperature and reduction changed.The finite element predictions of roll force based on the proposed model and the experimental results was shown to be in fair agreement whereas those based on the Misaka-Yoshimoto model, in which dynamic recrystallization was not considered, failed to predict the roll force precisely at heavy reduction. The results also revealed that, for a typical reduction, the flow stress softening effect was not observed during deformation, whereas the effect was considerable when the material underwent heavy reduction.

This paper presents a constitutive model for two interstitial-free (IF) steels that can be used for calculating the roll force during hot rolling. The constitutive models assume that the deformation behaviour of material can be described with reference to strain, strain rate, and temperature together with the volume fraction of dynamically recrystallized grains. The two IF steels are Nb-Ti-added IF steel containing 0.001 wt% B and 0.1 wt% P, and IF steel with no B and P. To verify the proposed constitutive model, a laboratory-scale hot-plate rolling experiment, together with three-dimensional finite element analysis coupled with the proposed model, has been performed. The capability of the proposed model was demonstrated through comparing predicted roll forces with measured values. A series of finite element simulations has then been carried out to study the variation in the distribution of the dynamically recrystallized volume fraction in the deformed workpiece by changing the rolling temperature and reduction ratio. Good agreement was obtained between the predicted roll forces and experimental measurements for two IF steels. The roll force of IF steel with P and B components was 11.6 per cent larger than that of IF steel without them since dynamic recrystallization was delayed owing to the alloying elements. It has been found that, as the reduction ratio increases, the effect of those alloys on the volume fraction of dynamic recrystallization becomes more dominant and leads to considerable differences between the roll force of the steels.