The hot deformation behavior of a newly designed Fe–5Mn–3Al–0.1C (wt.%) medium manganese steel was investigated using hot compression tests in the temperature range of 900 to 1150 °C, at constant strain rates of 0.1, 1, 2.5, 5, 10, and 20 s−1. A detailed analysis of the hot deformation parameters, focusing on the flow behavior, hot processing map, dynamic recrystallization (DRX) critical stress, and nucleation mechanism, was undertaken to understand the hot rolling process of the newly designed steel. The flow behavior is sensitive to deformation parameters, and the Zener–Hollomon parameter was coupled with the temperature and strain rate. Three-dimensional processing maps were developed considering the effect of strain and were used to determine safe and unsafe deformation conditions in association with the microstructural evolution. In the deformation condition, the microstructure of the steel consisted of δ-ferrite and austenite; in addition, there was a formation of DRX grains within the δ-ferrite grains and austenite grains during the hot compression test. The microstructure evolution and two types of DRX nucleation mechanisms were identified; it was observed that discontinuous dynamic recrystallization (DDRX) is the primary nucleation mechanism of austenite, while continuous dynamic recrystallization (CDRX) is the primary nucleation mechanism of δ-ferrite. The steel possesses unfavorable toughness at the deformation temperature of 900 °C, which is mainly due to the presence of coarse κ-carbides along grain boundaries, as well as the lower strengthening effect of grain boundaries. This study identified a relatively ideal hot processing region for the steel. Further exploration of hot roll tests will follow in the future.
In this contribution, a series of isothermal compression tests for the 825 nickel-based alloy were performed using a Gleeble-3800 computer-controlled thermomechanical simulator at the compression temperature range of 850 °C to 1150 °C and the strain rate range of 0.14 s−1 to 2.72 s−1. The hot deformation equation of the alloy is derived from the piecewise model based on the theory of work hardening-dynamic recovery and dynamic recrystallization (DRX), respectively. Comparisons between the predicted and experimental data indicate that the proposed constitutive model had a highly accurate prediction. The deformation rate and temperature effect were associated with microstructural change, and the evolution of the microstructure was analyzed through electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The dislocation densities of the alloy at the deformation of 850 °C and 2.72 s−1 is higher than at the other deformation, the higher dislocation density is the higher stored energy and the higher degree of DRX. As well, two types of DRX nucleation mechanisms have been identified: discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX). Changes in grain boundary have significant effect on the DRX nucleation of the alloy, twin boundaries act as potential barriers limiting dislocation slip and motion and eventually leading to the accumulation of dislocation during plastic deformation. This study identified that the major contribution which results in the growth of new twins in DRX grains is the new boundary of Σ3 twins.
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