*4.3. Effect of CR Microstructures on Strain Hardening Behavior*

The 10% CR and 30% CR samples presenting three-stage strain hardening curves and high SHRs reflect the high strain hardening ability. However, there are subtle differences between the samples in the SHRs, which are caused by their different microstructures. The strain range at stage A in 10% CR samples is larger than that in 30% CR samples. The variation of SHRs at an early stage is usually affected by the nucleation and slip of dislocation in ASSs [33–35]. Compared with the 10% CR sample, the 30% CR sample has more dislocations and a higher density grain boundary, which are not conducive to dislocation movement during the tensile process, resulting in its smaller strain range at stage A. The strain range at stages B and C was considered beneficial to plasticity in ASSs that was attributed to the transformation induced plasticity (TRIP) or twinning induced plasticity (TWIP) effect [36]. The tensile induced martensite or mechanical twin will nucleate and grow at stages B and C [37]. When the cold reduction increased from 10% to 30%, the austenite structure size decreased from 7.3 μm to 1.3 μm (Table 2). Grain refinement can increase the stability of austenite and inhibit the tensile induced martensite formation during plastic deformation [38], making the strain range at stages B and C in the 30% CR sample smaller than that of the 10% CR sample. With the increase of cold reduction, the finer austenite structure in samples is not conducive to the nucleation and movement

of dislocation. Furthermore, the high content of martensite in the samples reduces the capacity to accommodate dislocations. These factors lead to the rapid decrease of the strain hardening ability of the samples when the cold rolling reduction increased.
