Lightweight design in the automotive industry is not always combined with the usage of alternative materials like composites. Even high strength steels have high potential for reducing the weight for lightweight design.
For the forging industry a new steel is developed, which enables the TRIP-effect (Transformation Induced Plasticity) for forging parts. This material effect is already well known and used for steel sheet structures. The TRIP-effect is based on the structure of the TRIP-material containing retained austenite, which has the possibility to form residual stresses due to the austenite-martensite transformation under cyclic loading. Beside static properties, the dynamic and cyclic material behaviour has a high importance for parts in the automotive industry. So, for lightweight design, a focus has to be on fatigue behaviour under service loads including overloads for an optimal weight reduction.
The traditional forging steels are the precipitation hardening ferritic-pearlitic steels (PHFP steel) and the martensitic quenched and tempered (Q&T) steels. In comparison to these steels, the new generation of TRIP forging steels has an improved cyclic material behaviour. To utilize this potential, attention has to be paid to the sequence of cooling down the material from the forging temperature because the properties of the material depend on the cooling rate between 500°C and 200°C.
In this paper, the high potential of this new generation of forging steels is shown. First fatigue tests on TRIP-materials demonstrate the good results under service loads, especially under overloads and misusage loads.