Modeling the viscoplastic behavior and grain size in a hot worked Nb-bearing high-Mn steel

In this study, hot compression tests were performed on a Nb-containing high-Mn steel at temperatures in range of 850–1150 °C and strain rates of 0.001–1 s⁻¹. The stacking fault energy of the material at different conditions, i.e. without Nb, with Nb and C in solution and with Nb and C as NbC, was calculated by JMat Pro software as 10.9, 10.15 and 10 mJ/m². The software predicted that the stacking fault energy reaches 200–250 mJ/m² as temperature rises to the deformation range. A new model with interactive effects of temperature, strain and strain rate was constructed on the basis of Johnson-Cook equation to predict the experimental flow curves satisfactorily. Optical microscopy and SEM-EBSD observations showed that strain-induced boundary migration (SIBM) is the major mechanism of dynamic recrystallization giving rise to a necklace structure at the early stages of recrystallization. It was found that very fine layers of lath martensite form within the large austenitic grains during post-deformation quenching. However, the martensite layers were less discernible in the small DRX grains. The dynamically recrystallized grain size decreased with increase in strain rate or decrease in temperature. The model developed based on Bate and Hutchinton approach showed that the recrystallized grain size has an indirect relationship with the flow stress level.