2003 Volume 44 Issue 11 Pages 2303-2310
A new method for determining flow curves of hot metal under dynamic recrystallization is proposed in this paper. Thermomechanical finite element analysis of a tool and a workpiece under hot compression is used to evaluate the upsetting force obtained by an experiment, as a function of the distributed flow stress of the workpiece. In order to compensate the effect of inhomogeneous distributions of deformation and temperature on the flow curve, inverse analysis for calculating coefficients in a flow stress curve under dynamic recrystallization is coupled with the thermomechanical finite element analysis. The discrepancy in upsetting force between the experiment and thermomechanical finite element analysis is used as an error estimator in the inverse analysis. A new regression formula of the flow curve, which includes four independent parameters, is introduced. It can express the flow curve of hot metal under dynamic recrystallization as well as dynamic recovery and work hardening. Also, four independent parameters included in the proposed formula have clear physical meanings. The proposed method is applied to the hot compression test of plain carbon steel at elevated temperatures. Flow curves are successfully determined for diversified conditions of strain rate and forming temperature. From the determined parameters of the flow curve by the proposed inverse analysis method, it can be concluded that 1) the critical strain for the onset of dynamic recrystallization is dependent on strain rate as well as temperature, 2) the stain rate sensitivity of m=0.13 is acceptable for plain carbon steel under testing conditions, and 3) the temperature sensitivity A is approximately 3000-3600 [K−1]. Through investigation, it has become clearer that an accurate flow curve can be determined by the proposed method. In addition to a quantitative description of the flow curve as a function of strain, strain rate and temperature, metallurgical parameters such as the critical strain for the onset of dynamic recrystallization and steady-state stress could also be estimated directly by the proposed method.