A viscoplastic constitutive model to represent the material behaviour during hot rolling of steel

Abstract
A viscoplastic constitutive model, coded as a User Material subroutine (UMAT) for use with the ABAQUS/Standard Finite Element (FE) software, is applied in order to characterise the rapid changes of strain rate and microstructure occurring in the roll bite during hot rolling of steel products. The model uses internal (state) variables to account for work hardening due to the accumulation of dislocations and work softening associated with dynamic recrystallisation. The material constants in the model are determined by a simple and effective semi-empirical method using flow stress data obtained from compression tests on a typical S275 grade steel carried out on a Gleeble 1500 thermomechanical simulator at temperatures between 800 and 1200°C and at strain rates in the range : 0.1 - 10.0 s^-1. It is shown that the constitutive model can be used to predict satisfactorily the history dependence of the flow stress and the work softening associated with dynamic recrystallisation. The application of the constitutive model to a 3D bar rolling problem is also illustrated and discussed.