Strain-Associated Alpha-Ferrite Phase Transformation in a Micro-Laminated Low-Density Steel

The alpha-ferrite phase transformation behavior during and after hot working in the ferrite and austenite dual-phase region of a Fe-4Al-2.5Mn-2Ni-0.18C (wt%) low-density steel is investigated by means of an optical microscope, scanning electron microscope, transmission Kikuchi diffraction, transmission electron microscopy, and energy dispersive spectroscopy. The phase diagram of the experimental steel is calculated by Thermal-Calc software, and the results show that delta-ferrite always exists after solidification, which means that the hot working is always in the dual-phase region. The alpha-ferrite is transformed from the austenite, and there are two ways, one is direct growth on delta-ferrite, and the other is strain-associated nucleation at the prior austenite grain boundaries. Measuring alloy composition is the most direct way to distinguish alpha-ferrite and delta-ferrite. The strain-associated nucleation alpha-ferrite appears at the deformation temperature below 1000 degrees C. The reduction of deformation temperature and the increase of strain promote alpha-ferrite nucleation. The growth of strain-associated alpha-ferrite at the prior austenite grain boundary is not apparent. As the strain-associated alpha-ferrite is distributed along the prior austenite grain boundaries, the martensite lamellae are divided effectively, resulting in an increase in elongation.