Dynamic response of aluminium containing TRIP steel and its constituent phases

In the automotive industry a lot of effort is put into the development of lightweight car body structures. Therefore, complex multiphase steel grades have been developed with exceptional mechanical properties: they combine high strength values (yield strength, tensile strength, etc.) with an excellent ductility. TRansformation Induced Plasticity steels (TRIP steels) show these properties pre-eminently. To guarantee a controlled dissipation of the energy released during a crash it is essential to characterize and understand the impact-dynamic material properties. In this paper, results are presented of an extensive experimental program to investigate the strain rate dependent mechanical properties of TRIP steel and its constituent phases. These different phases (ferrite, bainite and austenite) were prepared separately to obtain a clear understanding of their individual behaviour within the multiphase steel. A split Hopkinson tensile bar set-up was used for the experiments and microstructural observation techniques such as SEM and XRD revealed the mechanisms governing the observed high strain rate behaviour. The results show clearly that the excellent mechanical properties are not only preserved at higher strain rates, but they are still improved. As in the static case the ferrite phase is responsible for the large deformation in the TRIP steel and bainite causes the high strength levels. The martensite/austenite constituent is responsible for the excellent combination of high stress and strain in the TRIP steel as well as for the important strain hardening.