Modeling Bainite Transformation and Retained Austenite in Hot Rolled TRIP Steel by Instantaneous Carbon Enrichment

Abstract

The rate model inherited from Leblond and Devaux's concept has been applied to multiphase transformations in hot‐rolled TRIP steel. The phase fraction is collected from different laboratory scale process simulations, both multi‐pass rolling for repeated recrystallization and single‐pass cycles. The latter is also investigated by in situ synchrotron X‐ray diffraction (XRD) to acquire instantaneous data for the phase fraction and carbon content. The different cycles allow different austenite conditioning inputs for the model to enlarge the range of thermomechanical parameters. Bainite transformation model is a function of the instantaneous average carbon content in the remaining austenite, which is measured from the shift in the 1‐dimensional diffraction peaks. The modeling can map different process windows for a specific required microstructure of TRIP steel. The bainite formation is remarkably favored by low carbon content. But, the promotion of ferrite formation in the former step increases the carbon content dramatically and yields smaller bainite fraction that consequently larger fraction of retained austenite is allowed. A modified Koistinen–Marburger equation has been established from the in situ XRD data to model the martensite formation during the final cooling to reveal the stability of austenite. Above an average carbon content of 1.475 mass%, the retained austenite becomes stable.