Thermal deformation model and grain refinement mechanism in δ-ferrite-based lightweight steel

To explore effective strategies for optimising the grain size of experimental steel, a thermal simulation experiment was utilised within a temperature range spanning from 650 °C to 950 °C, at strain rates varying from 0.1 to 10/s, with a true strain of 0.918. Through thermal compression, the grain size could be reduced from 103.4 μm in the forged sample to a minimum of 11.6 μm (0.1 s⁻¹–950 °C). The double-hyperbolic sine constitutive model was developed based on the flow stress curves, with the thermal activation energy for plastic deformation determined to be 514.66 kJmol⁻¹. The critical conditions for recrystallisation were identified as σ c / σ P = 0.98 . Combined with microstructural observations, it was determined that grain refinement primarily occurred through the dynamic recrystallisation mechanism and the deformation induced ferrite transformation mechanism. Additionally, within the deformation temperature range spanning from 850 °C to 950 °C, residual austenite emerged and its distribution was affected by the temperature.