MICROSTRUCTURE CONTROL OF HOT ROLLED TRIP STEEL BASED ON DYNAMIC TRANSFORMATION OF UNDERCOOLED AUSTENITE I. Prior Austenite Grain Size

Low-alloy multiphase transformation-induced plasticity (TRIP) steels offer excellent mechanical properties combining high-strength levels with a well ductility. This results from the com- plex synergy between different phases, i.e., ferrite, baintie and retained austenite in them. An in-depth understanding of the novel hot rolled TRIP processing based on dynamic transformation of undercooled austenite (DTUA) is required to study the microstructure evolution under different treatment condi- tions. The parameters determining the stability of the metastable austenite and mechanical property of TRIP steel were revived and investigated experimentally, with a special attention paid to the effect of prior austenite grain size and cooling rate after DTUA on its microstructure and mechanical property. The results show that the kinetics of ferrite during DTUA is accelerated when the prior austenite grain size is small. In addition, reducing the grain size of austenite, the distribution of ferrite, bainite and retained austenite becomes more uniform. Moreover, the size of bainite packets and bainitic ferrite becomes smaller, the volume fraction and carbon content of retained austenite higher, and the finer granular retained austenite vast and distributed uniformly, which result in the investigated steel having