Influence of Microstructural Evolution on the Hot Deformation Behavior of an Fe-Mn-Al Duplex Lightweight Steel

The hot deformation behavior of Fe-26Mn-6.2Al-0.05C steel was studied by experimental hot compression tests in the temperature range of 800-1050 degrees C and strain rate range of 0.01-30 s(-1) on a Gleeble-3500 thermal simulation machine. The microstructural evolution during the corresponding thermal process was observed in situ by confocal laser scanning microscopy. Electron backscattered diffraction and transmission electron microscopy analyses were carried out to observe the microstructural morphology before and after the hot deformation. Furthermore, interrupted compression tests were conducted to correlate the microstructural characteristics and softening mechanisms at different deformation stages. The results showed that hot compression tests of this steel were all carried out on a duplex matrix composed of austenite and delta-ferrite. As the deformation temperature increased from 800 to 1050 degrees C, the volume fraction of austenite decreased from 70.9% to 44.0%, while that of delta-ferrite increased from 29.1% to 56.0%. Due to the different stress exponents (n) and apparent activation energies (Q), the generated strain was mostly accommodated by d-ferrite at the commencement of deformation, and then both dynamic recovery and dynamic recrystallization occurred earlier in delta-ferrite than in austenite. This interaction of strain partitioning and unsynchronized softening behavior caused an abnormal hot deformation behavior profile in the Fe-Mn-Al duplex steel, such as yield-like behavior, peculiar work-hardening behavior, and dynamic softening behavior, which are influenced by not only temperature and strain rate but also by microstructural evolution.