*3.3. Strain Hardening Behaviors of CR Samples*

The 10% CR and 30% CR samples with good ductility were selected to study the strain hardening behaviors and their strain hardening rates (SHRs; dσT/dεT) were plotted as a function of true strain, presented in Figure 7. The SHR plots of selected samples can be divided into three stages, as shown in Table 4. The SHRs first decreased (Stage A), then increased (Stage B), before finally decreasing again (Stage C) with increasing true strain.

**Figure 7.** Strain hardening rate for 10% CR (A1, B1, C1) and 30% CR samples (A2, B2, C2).

**Table 4.** Values of plastic strain of the samples at each stage. The data are taken from Figure 7.


#### **4. Discussion**

In this paper, the effect of cold deformation on the mechanical properties of 316LN ASS was studied. It is well-known that the microstructural characteristics are a major factor determining the mechanical properties of steels. Therefore, the effect of cold deformation on microstructural evolution and the relationship between the microstructures and mechanical properties of CR 316LN ASS were discussed. The ductility of the ASSs is related to their strain hardening behavior. To obtain high strength 316LN ASS with a reasonable ductility, the strain hardening behaviors of CR samples were discussed.

#### *4.1. The Effect of Cold Deformation on the Microstructures*

The deformation microstructures formed in ASSs during the plastic deformation process depend on their deformation mechanisms, which are determined by their SFEs. The SFE of the 316LN ASS at room temperature is ~18.9 mJ/m<sup>2</sup> (Table 1) and both the strain induced martensite and the mechanical twin could form during plastic deformation [13,20]. When the cold rolling reduction increased, the shear bands comprised of slip bands and mechanical twins formed in austenite grains and then the strain-induced martensite nucleated at the intersection of shear bands [21–23]. The formed ά-martensite replaced the shear bands and the shear bands gradually disappeared as the cold rolling reduction increased. Furthermore, the martensite lath could be broken and mixed with untransformed austenite to form dislocation-cell-type martensite that has a higher density of dislocation (Figure 3f) [24].

Previous studies [25,26] have indicated that heavily cold worked metals could be subdivided by grain boundaries and dislocation boundaries. The cold work inducing the formation of the grain boundary was also found in CR AZ91 alloy [27] and 308L stainless steel [28]. In this 316LN ASS, in addition to dislocation grain boundaries, the boundaries of mechanical twins and strain-induced martensite formed during the CR process can also subdivide untransformed austenite, leading to the decrease of the untransformed austenite structure size. The EBSD results in Figure 4 revealed a similar tendency, where the increased density of grain boundaries led to a decreasing untransformed austenite structure size. Hughes and Hansen's study [29] indicated that the continued subdivision of

grains into crystallites surrounded by dislocation boundaries leads to a large orientation spread based on dislocation accumulation processes. The main grain boundaries formed in the 10% CR sample were LAGB that may originate from the dislocation boundary formed by dislocation accumulated at twins' boundaries or shear bands. The rapidly increasing density of LAGB as the cold reduction increased from 10% to 30% could be attributed to the increased dislocation boundaries caused by the increasing density of dislocation. The difficult dislocation boundaries formed could show an operation of different slip system combinations within the individual crystallites, leading to the possibility of different parts of a grain rotating towards different stable end orientations [29]. If such end orientations are not far apart, large misorientations within the original grain can build up. Considering that the density of the dislocation boundary in the untransformed austenite grains increases with the increasing cold rolling reduction, the probability of large misorientations within the original grains increases with the increasing cold rolling reduction, leading to the increasing density of HAGB. The results in Figures 4 and 5 illustrate this viewpoint very well as the variations of HAGB densities are in good agreement with the variations of the average numbers of misorientations in the original austenite grains.
