Response of retained austenite to quenching temperature in a novel low density Fe-Mn-Al-C steel processed by hot rolling-air cooling followed by non-isothermal partitioning

The quenching temperatures below Ms are difficult to uniformly control during large-scale processing of quenched and partitioned (Q&P) steels, which results in significant variation in tensile properties for the entire coil. The present study is aimed at obtaining a stable elongation for a wide range of quenching temperatures in hot rolled Q&P steels through microstructural design. Instead of cold rolling and annealing processes, a novel process involving hot rolling and air cooling followed by cooling in furnace was applied to a newly designed 0.25C-3Mn-2Al (wt.%) steel. The results indicated that a stable amount of retained austenite (RA) ∼ 19–23% was obtained during air-cooling to finish temperatures in the range of ∼280–360 °C, resulting in no loss in elongation at low quenching temperatures. The RA that was promoted by the transfer of carbon from ferrite/bainite reached up to 1.346 μm, whereas the RA embedded in martensitic laths was ∼40 nm. The two types of RA provided continuous TRIP effect during tensile deformation and the ultra-fine martensite lath of ∼100 nm ensured tensile strength greater than 1091 MPa. Finally, a stable total elongation of ∼19.2–20.5% in combination with tensile strength of ∼1091–1196 MPa was obtained over a wide quenching temperature range of 80 °C, which broadened the processing window for the production of hot rolled Q&P steels. In addition, the isothermal transformation during the initial partitioning stage was the underlying reason to obtain stable amount of RA. The CCE (Constrained Carbon Equilibrium) model combined with isothermal transformation was used to quantitatively calculate the volume fraction of RA.