Mechanical properties, damage behaviour and retained austenite stability in low-alloyed TRIP steels

Multiphase steels are in great demand for automotive applications due to their excellent mechanical properties which are reflected in high strength values and good formability. In comparison to the often used conventional dual phase steels, TRIP steels are characterized by higher forming capabilities as a reason for the phase transformation of meta-stable retained austenite to the much harder phase martensite. But these types of steels demonstrate a failure behaviour which is very complex and currently still improperly understood. The damage evolution is not only influenced by the TRIP effect, the stress state and rate of deformation but also by the microstructural constitution of the single phases or the existence of microstructural heterogeneities, respectively. This work deals with the phase transformation, mechanical properties and damage behavior from microstructural aspects under industrial boundary conditions. In order to create a connection between microstructural constitution and global mechanical properties, the microstructure was resolved locally. Investigations of the local phase transformation, microstructure morphology, distribution of alloying elements and the constitution of the fracture surface after different types of forming tests were conducted. The development of the TRIP effect, which is very crucial for this type of steels, was analysed on the macroscopical and microscopical scale.