Methods of improving the deep drawing properties of austenitic stainless steels

In extensive tests the mechanical properties of the austenitic stainless steels 1.4301, 1.4318 and 1.4376 were investigated. Compared to the conventional ferritic deep-drawing steels, DC04 and TRIP700 and the duplex stainless steel 1.4462, the austenitic steels seem to offer better properties. In quasistatic and dynamic tensile tests the expected material behaviour could be observed, whereas technological test procedures such as forming limit diagrams, cupping tests and hole expansion tests, partly reveal more unfavourable properties. The different austenite stability, which in dependency on the temperature and the stress state, leads to a strain-induced martensite transformation, as well as the existence of δ-ferrite, accounts for this behaviour. Based on the data of the material testing, the martensitic transformation and the temperature evolution during forming of the austenitic steels, a constitutive model with a coupled thermo-mechanical analysis method was implemented into an implicit FE-code. By using the multitude of experimental data which were complemented by industrial scaled tests, a suitable model could be established which employed mainly Hänsel's works. This model allowed the martensitic transformation to be accurately modelled as a function of temperature and stress state which was verified by experiments again. The simulations performed evidenced the influence of the amount of formed martensite in the formation of austenitic steels, and showed that great care has to be taken in choosing the heat transmission parameters affecting the thermal analysis and simulation in industrial processes.