Thermomechanical characterization of 22MnB5 steels with special emphasis on stress relaxation and creep behavior
In hot stamping, the deformed austenitic phase is further quenched in the die to produce a fully martensitic microstructure in the component. During this time, the microstructure is refined with a relaxed state of stress. Therefore, it is important to study the time-dependent effects on the material during forming and quenching. This influences the martensite start temperature and further aids to predict the residual stress in the component accurately. In this work, thermo-mechanical characterization of 22MnB5 steels is presented with special emphasis on stress relaxation and creep phenomena. In addition to the monotonous experiments at different strain rates, relaxation and creep tests give a deeper insight into the rate dependent behavior of the material. These tests are conducted for two distinct microstructures (austenite and martensite) using Gleeble 3150. In austenitic phase, relaxation test is conducted at 900 degrees C for different strain levels which lead to a significant drop in the stress level of the material due to recovery. Therefore, creep tests are conducted to determine the real elastic limit of the material at high temperatures. As a result, a significant increase in the creep strain is realized once the stress in the material increases 20 MPa. Martensitic microstructure shows stress relaxation behavior at 200 degrees C (below the martensite finish (Me) temperature) and even at ambient temperature after a total deformation of 3%. These experimental results are the basis for the construction of a viscoplastic constitutive model and the model is able to represent the observed phenomena.