The investigation of complex material behaviour of steel like transformation-induced plasticity (TRIP) and stress-dependent phase transformation (SDPT) is a large field of current research. The simulation of the material behaviour of work-pieces in complex situations requires a knowledge as deep as possible about such phenomena. In addition, there are effects in the case of non-constant stress which cannot be explained by the widely used Leblond model for TRIP. Therefore, we consider a TRIP model taking into account back stress due to TRIP itself. Based on experimental data for the isothermal pearlitic transformation of the steel 100Cr6 (SAE52100) under step-wise loads we calculate material parameters for the extended TRIP model. Regardless of the preliminary character of the performed tests, all experiments show a back-stress effect with a decrease of the TRIP strain after unloading.