Microstructures and stability of retained austenite in TRIP steels

Transformation induced plasticity (TRIP) steels exhibit a combination of high strength and ductility due to their multi-phase microstructure, including ferrite, bainite and retained austenite which transforms to martensite under the external stress. The characterization of microstructures is necessary for understanding the relationship between microstructure and property. In the present work, an effort to determine the volume fraction of various phases was made for the conventional TRIP steel containing silicon. The microstructures in the TRIP steel were characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, especially, an effective method was developed to identify multi-phase microstructures by atomic force microscopy based on the height difference. Furthermore, the stability of retained austenite determining TRIP effect was evaluated by electrical resistance tests and tensile tests. The results show that retained austenite does not generate martensitic transformation at −80 °C and exhibits a good thermodynamic stability, and the transition temperature from stress-induced martensitic transformation to strain-induced martensitic transformation is determined as about −5 °C, and thus strain-induced martensitic transformation over −5 °C (somewhat lower than room temperature) is favorable for the application of TRIP steels in the automobile industry.