*3.1. The Microstructural Evolution with Increasing Cold Rolling Reduction*

Optical micrographs of 316LN commercial ASS and 10%, 30%, 50%, 70%, and 90% CR samples are presented in Figure 1. As shown in Figure 1a, the coarse-grained austenite existed in this commercial steel. The content of strain-induced martensite increased as the cold rolling reduction increased, as shown in Figure 1b–f. Many shear bands were discovered in the austenite phase of the 10% CR sample (Figure 1b). When the cold reduction increased to 30%, some strain-induced martensite formed at shear bands. When the cold reduction increased to 50%, the untransformed austenite was elongated along the rolling direction (Figure 1d). The content of martensite further increased and the formed martensite segmented the untransformed austenite when the cold reduction was increased to 70%, as shown in Figure 1e. When the cold reduction increased to 90%, the large block of untransformed austenite still existed. Meanwhile, the formed martensite seemed to be divided into fine martensite and mixed with fine austenite structure (white circle in Figure 1f). The shear bands were basically replaced by martensite and became difficult to distinguish, as shown in Figure 1f.

**Figure 1.** *Cont.*

**Figure 1.** Optical micrographs of microstructures for (**a**) original; (**b**) 10% CR; (**c**) 30% CR; (**d**) 50% CR; (**e**) 70% CR; and (**f**) 90% CR 316LN ASSs. The cold rolling direction (RD) is shown in (**f**). The straw color phase is the strain-induced martensite and the white phase is the austenite matrix.

The XRD patterns of 316LN commercial ASS, 10%, 30%, 50%, 70%, and 90% CR samples are presented in Figure 2a. According to the XRD patterns, only ά-martensite formed during cold deformation and the increased cold reduction led to the development of ά-martensite peaks. The measurement of volume fractions of ά-martensite in CR ASSs based on XRD has been reported in many studies [17–19]. The measured results based on the equation of Dickson [17] are presented in Figure 2b. It can be observed that the volume fraction of ά-martensite increased with increasing cold reduction and about 46% ά-martensite was achieved in the 90% CR sample.

**Figure 2.** (**a**) X-ray diffraction patterns of 316LN ASS, 10%, 30%, 50%, 70%, and 90% CR samples and (**b**) the volume fractions of ά-martensite in CR samples as a function of cold reduction.

Figure 3 illustrates that the microstructures evolve with the increasing cold rolling reduction. As shown in Figure 3a,b, shear bands and mechanical twins existed in the 10% CR sample and the dislocations gathered at shear bands and twins' boundaries. When the cold reduction increased to 30%, the strain-induced martensite was observed and the dislocation movement was suppressed by the martensite boundary, which resulted in the gathering of dislocations around the martensite boundary (Figure 3c). Meanwhile, dislocation boundaries (DB) were formed to segment the untransformed austenite structure (Figure 3d). When the cold reduction increased to 50%, many martensite laths formed (Figure 3e). Furthermore, the dislocation-cell-type martensite was observed in samples when the cold reduction was increased to 90% (Figure 3f).

**Figure 3.** Representative bright field transmission electron micrographs of deformation microstructures in (**a**,**b**) 10% CR; (**c**,**d**) 30% CR; (**e**) 50% CR; and (**f**) 90% CR samples. The diffraction pattern confirmed (**b**) deformation twin, (**c**) martensite lath, and (**f**) dislocation-cell-type martensite. DB in (**b**) means dislocations boundary.

*Metals* **2018**, *8*, 522

The original and 10–40% CR samples were characterized by EBSD and are presented in Figure 4. The main structure of the original sample was coarse austenite grains with many annealing twins (Figure 4a). A small amount of strain-induced martensite and grain boundaries formed in austenite grains after 10% cold rolling. The formed grain boundary was mainly a low angle grain boundary (0◦ < LAGB < 15◦). The density of the grain boundary and volume fraction of martensite increased with the increasing cold rolling reduction. Figure 4f indicates that the densities of both LAGB and the high angle grain boundary (HAGB > 15◦) increased with the increasing cold rolling reduction. The difference between LAGB and HAGB is that the density of LAGB first increased rapidly and then increased slowly when the cold reduction increased from 10% to 40%, while for HAGB, the increase rate change is the opposite. Sizes of austenite structure, percentage fractions, and densities of the austenite grain boundary with the different misorientation ranges based on Figure 4 are listed in Table 2. As shown in Table 2, the sizes of the austenite structure decreased with the increasing cold reduction. Figure 5a–d show the IPF map and Figure 5e–h show the corresponding misorientation variation along the black lines in untransformed austenite grains of 10% CR, 20% CR, 30% CR, and 40% CR samples, respectively. The results indicate that the main misorientation of point to point in untransformed austenite grains of the 10% CR sample was below 15◦. When the cold reduction increased to 30% and 40%, the densities of high misorientation (>15◦) increased obviously. The average numbers of misorientation in untransformed austenite grains of CR samples increased rapidly as the cold reduction increased from 10% to 40%, as shown in Figure 5e–h.

**Figure 4.** *Cont.*

**Figure 4.** Electron backscattered diffraction (EBSD) micrographs grain boundary reconstruction maps for austenite phase of (**a**) original; (**b**) 10% CR; (**c**) 20% CR; (**d**) 30% CR; and (**e**) 40% CR samples, respectively; (**f**) The variation density of low angle grain boundary (LAGB) and high angle grain boundary (HAGB) as a function of cold rolling reduction. The white area is the austenite structure and the black area is martensite.

**Figure 5.** *Cont.*

**Figure 5.** (**a**–**d**) IPF images for (**a**) 10% CR; (**b**) 20% CR; (**c**) 30% CR; and (**d**) 40% CR samples, respectively. (**e**–**h**) Misorientation variations along the black lines (black lines in figures) in untransformed austenite grain for (**e**) 10% CR; (**f**) 20% CR; (**g**) 30% CR; and (**h**) 40% CR samples, respectively.


**Table 2.** Austenite structure sizes, percentage fractions, and densities of the austenitic grain boundary with different misorientation ranges of samples with varying cold reductions.
