This work presents an analysis of the plate drawing processes carried out in converging dies. The analysis has been made by the upper bound method (UBM), modeling the plastic deformation zone by triangular rigid zones (TRZ), and considering that the processes occur under plane strain and partial friction conditions. The goal is to evaluate how the work-hardening suffered by materials when they are cold-worked affects the energy required to carry out a certain process. In order to achieve this objective, the paper proposes a simplified model for calculating the shear yield stress, k, along the contact surface between die and material when theoretical work-hardening materials are used. The results obtained with this simplified procedure do not differ significantly from those obtained with the complete model, even in the more drastic conditions tried. This result confirms that the simplified model can successfully substitute for the complete model with less calculation.