Microstructure evolution and room temperature mechanical properties of a thermomechanically processed ferrite-based low density steel

Abstract

The high temperature deformation behavior of a duplex ferrite based low density steel was investigated and the governing restoration mechanisms were determined. Toward this end, the isothermal hot compression tests in the temperature range of 800–1000 °C under the strain rate of 0.001–0.1 s−1 were executed. Interestingly, an appreciable ferrite grain refinement was achieved through thermomechanical processing and the ferrite phase was dynamically recrystallized through continuous mechanism at various temperature and strain rates. The ferrite grain size decreases from ∼900 μm (initial microstructure) down to ∼35 and 50 μm for the microstructure deformed at 800 °C/0.001s−1 and 1000 °C/0.001s−1, respectively. The occurrence of grain refinement within the austenite was also observed which increased the stability of austenite against thermally induced martensitic transformation during quenching. The deformation induced transformation of the austenite to ferrite was also characterized as one of the involved refinement mechanisms which was intensified under the higher strain rates at lower temperature of 800 °C. Considering the portion of the fine ferrite grain resulted from strain induced transformation, the grain size under the lower strain rate of 0.001s−1 decreases down to the ∼14 μm. Finally, the room temperature mechanical properties of the thermomechanically processed specimens was assessed through miniaturized shear punch testing method. It was found that the workability of deformed specimens was significantly improved owning to the simultaneous occurrence of dynamic recrystallization and deformation induced transformation.