Constitutive behaviour under hot stamping conditions

The modelling of the hardening behaviour at high temperatures and a range of strain rates is extensively discussed. The hardening behaviour is characterized using tensile tests done in a Gleeble testing machine. The specimen is heated by an electric current, soaked to get fully austenized, cooled down to its desired testing temperature and then drawn to fracture with a given strain rate without a further drop of the temperature. One of the major challenges of this test is to achieve a uniform temperature distribution over the sample, to ensure homogeneous austenization. A dedicated tensile sample geometry enables a much more uniform temperature distribution than a regular sample geometry. Yielding and hardening behaviour have been characterized with a Kocks-Mecking plot. These characterizations have been used to fit parameters for a physically based hardening model that is applicable in a wide range of strain rates and temperatures. The predicted strains in FEM simulations that use these hardening curves match well with thickness measurements on hot stamped parts.