Tailoring the mechanical properties of high-manganese TRIP/TWIP steels by reversion annealing
Frederike Berrenberg*,Christian Haase, Luis A. Barrales-Mora, Dmitri A. Molodov
Institute of Physical Metallurgy and Metal Physics, RWTH Aachen University, Aachen, 52074 Germany
Due to the outstanding mechanical properties, the commercial and scientific interest in high-manganese Transformation- and Twinning-Induced Plasticity (TRIP/TWIP) steels has constantly increased during the last 25 years. A major shortcoming of these steels that still limits their practical application is the relatively low yield strength in the recrystallized condition. It has previously been shown that a combination of cold rolling and recovery annealing serves as a promising approach to overcome this problem. However, recovery-annealed TWIP steels are characterized by a low work-hardening rate due to the high fraction of deformation twins that are already present in the microstructure from the previous rolling process.
The aim of the present study was to combine the beneficial influences of both the TRIP and the TWIP effect on the mechanical behavior and to adjust the contribution of both effects by suitable thermo-mechanical processing. Different high-manganese TRIP/TWIP steels with varying volume fractions of ε-martensite (5–24%) after pre-deformation were studied. In order to determine the influence of the initial microstructure, four specific conditions were studied, i.e. pre-deformed, reversion-annealed, recovery-annealed and fully recrystallized. Reversion annealing resulted in the transformation of the ε-martensite to austenite at low temperature and time (350°C/2min). During recovery annealing at 550°C for 5 min, annihilation of dislocations was initiated in addition to the ε-martensite transformation. Both heat treatments resulted in a bimodal microstructure comprising strong austenite with high fractions of deformation twins and dislocations that retained the high yield strength and soft reverted austenite that facilitated an improved work-hardening capacity. Variation of the initial ε-martensite fraction and annealing conditions allowed tailoring of the combination of yield strength, ductility and work-hardening capacity. The possibility of precisely setting a specific microstructure provides a promising new approach to the design of crash relevant structural components for the automotive industry made of TWIP/TRIP steels.