Modeling of Dynamic Recrystallization and Flow Stress of Nb‐Bearing Steels

The dynamic recrystallization (DRX) and flow stress of Nb‐bearing steels were investigated by means of isothermal single compression testing at temperatures of 850‐105° and at constant strain rate from 0.1 to 20s‐1 using a Gleeble 3800 thermo‐mechanical simulator in order to model the DRX processes and predict the flow stress during plate rolling. On the basis of the measured flow stress, a new model of DRX kinetics was proposed to calculate the volume fraction of dynamically recrystallized grains, which was a function of processing parameters such as deformation temperature, strain, strain rate, the initial austenite grain size and Nb content. The effect of deformation conditions was quantified by the Zener‐Hollomon parameter, in which the activation energy of deformation was expressed as a power function of Nb content. The critical strain was determined by using the method proposed by Jonas and co‐workers. It is shown that the ratio of the critical strain to the peak strain decreases with increasing Nb content, from which an empirical equation was developed. In addition, the influence of Nb content and deformation conditions on the steady state grain size was determined by fitting the experimental results to a linear relationship. Finally, the flow stress of Nb bearing steels was accurately predicted using a one‐internal‐variable evolution equation by taking Nb content as a parameter and including the influence of DRX. The comparison between the experimental and theoretical results confirmed that the modeling had a good accuracy to predict flow stresses during hot deformation.