In this work, we develop a thermo-viscoplasticity model for body-centered cubic (BCC) metals based on a two-temperature theory of nonequilibrium thermodynamics. Modeling the plastic deformation here involves two subsystems, viz., a configurational subsystem related to grain growth, dislocation motion, and a kinetic vibrational subsystem describing the vibration of atoms. Due to a separation of the time scales, the two subsystems are described by two different temperatures. In this study, we introduce a grain boundary density, in addition to the mobile and forest dislocation densities, as an internal variable. The focus in this paper is on how large plastic deformation is affected by the evolving grain boundaries. In order to check the predictive quality of the model, numerical simulations are conducted and validated against available experimental evidence wherever possible.