To understand the mechanism behind the transformation during double-hit scenario, a schematic diagram is proposed as shown in Fig. 7. It is known that the austenite to ferrite transformation occurs due to energy supplied by the applied stress as reported in Ref [19]. Once the energy is above the total energy obstacles, which consist of the free energy difference between the phases, and the works of shear accommodation and lattice dilatation, transformation can take place. However, upon isothermal holding, re-transformation process can take place. This reverse transformation normally requires more than 60s of holding time to fully transform the DT ferrite back into austenite by diffusional mechanism. However, when the holding time to allow for re-transformation is short, e.g., 5 s in the present case, a partial re-transformation occurs that leads to trace amounts of re-transformed austenite to be formed along with DT ferrite. Therefore, during the second hit deformation, the austenite transforms into ferrite in a similar manner with the first deformation, which involves a driving force supplied by the applied stress. During this process, the dynamic recrystallization of ferrite which was formed during the previous deformation hit can also occur. Thus, the overall effect leads to lower critical stress for the onset of DT during the second hit deformation.