The micromechanics of TRIP-assisted multiphase steels

The industrial interest for TRIP-assisted multiphase steels is now a fact. Indeed, major requirements of the automotive industry since a few years, concern weight reduction of vehicles in order to lower fuel consumption and limit the rejection of greenhouse gas. Moreover, constraining safety criteria are imposed to automotive body. Projects like the Ultra Light Steel Auto Body (ULSAB) project, demonstrate the possibility of reducing the weight of car bodies by 25% by using high performance steels as well as new forming and assembly techniques. The aim of this thesis is to characterise and to model the different mechanisms of plastic straining occurring in TRIP-assisted multiphase steels. This approach allows to understand and to predict the forming properties exhibited by these steels. By coupling different techniques such as nano-indentation, neutron diffraction and digital image correlation, it was possible to establish the in situ flow behaviour of the constitutive phases of several TRIP-assisted multiphase steels. Moreover, the dependence of the kinetics of the martensitic transformation on various imposed stress states has been ascertained. This micromechanical characterisation demonstrates that, more than any other materials, TRIP-assisted multiphase steels must be designed not only as a function of the desired application but also as a function of the peculiarities of the forming process necessary to achieve this application. Finally, all the experimental results constituted the input data necessary for the constitutive model predicting the mechanical behaviour of TRIP-assisted steels that takes into account the transformation of metastable austenite. Thanks to this model, it has been possible to assess the most important features of the complex mechanical behaviour of the TRIP-aided steels.