Micromechanical study of the influence of the microstructure and composition on the plastic flow and damage properties of Dual-Phase steels

As a consequence of environmental concerns, reducing vehicle weight and fuel consumption is an important challenge for the automotive industry. However, it cannot be reached at the expense of safety. In this context, Dual-Phase steels are presented as key materials for the manufacturing of car structural parts. Indeed, these steel-steel composites combine the high ductility of the ferrite matrix with the high strength of the reinforcement martensite particles. Moreover, the high initial work hardening rate and the continuous yielding of such materials are also of specific interest. If the benefit of such kind of steels is known, the influence of the martensite parameters (carbon content, volume fraction and morphology), on the plastic flow properties and the damage mechanisms is not well understood. Especially, hardly anything has been reported about the fracture toughness. An investigation of the effect of each parameter and on a large range of microstructures is therefore necessary to have a better understanding of their behavior. In this thesis, Dual-Phase steels were generated to investigate separately the influence of such parameters, playing with heat treatments and composition. Their effect on the void nucleation, flow properties and fracture toughness was investigated with tomography, tensile tests and three-point bending, respectively. The critical strength for nucleation was determined by reverse engineering while the evolution of the flow properties as a function of the martensite parameters was predicted with a model combining the per phase response with a Mori-Tanaka homogenization scheme.(FSA 3) -- UCL, 201