The final annealed sheets were tensile tested by 15% strain along the rolling direction to evaluate the surface ridging behavior. Figure 1 shows the macro-morphologies of the tensile specimens. The specimen of No. 1 shows the corrugations on the surface after tension. Obviously, the specimen of No. 2 shows better surface quality after tension, indicating that this specimen indicates higher resistance against surface ridging. The average roughness (\(R_{\text{a}}\)) and maximum peak (\(R_{\text{p}}\)) were measured and are listed in Table 3. In comparison with the specimen of No. 1, the \(R_{\text{a}}\) and \(R_{\text{p}}\) for the specimen of No. 2 are decreased by 31% and 29%, respectively. This indicates that the hot rolling technology has a big influence on the surface ridging behavior of the final sheets, and decreasing FET properly can enhance the surface ridging resistance of the final sheets.

Fig. 1

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	Fig. 1 Surface morphologies of the final sheets after tensile test by a strain of 15%

Table 3

Table 3

Table 3 Surface ridging data detected after tensile straining by 15% Average roughness, \(R_{\text{a}}\) (μ m) Maximum peak, \(R_{\text{p}}\) (μ m) No. 1 1.94 6.00 No. 2 1.33 4.24

Table 3 Surface ridging data detected after tensile straining by 15%

Since the occurrence of the surface ridging of FSS is proposed to relate with the distribution of micro-texture, the micro-texture evolution during manufacture process was monitored. Figure 2 shows the orientation maps of the hot rolled and annealed sheets of the two specimens in longitudinal sections. After hot rolling and annealing, the specimens are mainly composed of equiaxed recrystallized grains. The grains in the surface layer are much finer than those in the center layer, which is caused by the shear deformation on the surface during hot rolling. For the specimen of No. 1, there still exist a few of elongated unrecrystallized grains with {001}//RD orientation (in red color) as indicated by the arrows in Fig. 2a. Besides, some abnormal growth grains are observed at subsurface layer. For the specimen of No. 2, however, the grains are recrystallized completely. There are more fraction of {111}//ND orientations (in blue color) in the center layer. And the grain size in the specimen of No. 2 is much finer than that in the specimen of No. 1.

Fig. 2

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	Fig. 2 Orientation maps of the hot rolled and annealed sheets: a No. 1, b No. 2

Figure 3 shows the orientation maps and the distributions of specific orientations of the cold rolled and annealed sheet at the center layer of No. 1. To investigate the distribution of micro-texture, a large-scale area (3 mm × 2 mm) was analyzed by EBSD with a step of 5 μ m. From the orientation map, it can be seen that the grain colonies in which the grains with similar orientations get together formed and distributed along the rolling direction (as indicated by arrows in Fig. 3a). The orientation analysis indicates that the grain colonies have mainly orientation with certain deviation from the {001}//RD-α fiber textures. Also, the recrystallization texture with {111} < 112 > and {111} < 110 > orientations tends to be arranged along the rolling direction (Fig. 3b).

Fig. 3

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	Fig. 3 Micro-texture of the cold rolled and annealed sheets at center layer of No. 1 specimen: a orientation map, b distribution of specific orientations

Figure 4 shows the orientation map and the distribution of specific orientations of the cold rolled and annealed sheet at the center layer of No. 2. It can be seen that much more {111}//ND-γ fiber textures including {111} < 112 > and {111} < 110 > orientations are achieved for the specimen of No. 2. And these orientations seem to be distributed homogeneously in the center layer. Although some grain colonies can be observed in the sample as indicated by arrows, the distribution of the grain colonies in the specimens of No. 2 reduced markedly as compared to the specimen of No. 1.

Fig. 4

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	Fig. 4 Micro-texture of the cold rolled and annealed sheets at center layer of No. 2 specimen: a orientation map, b distribution of specific orientations

Figure 5 shows the orientation maps for the sheets of No. 2 after hot rolling and partially annealing at 950 ° C for 90 s. Two types of recrystallization nucleation sites can be observed, high-angle grain boundaries and highly misoriented regions within shear bands. Besides, the hot rolled shear bands occurred preferentially in the {111}//ND-oriented grains, and most of the recrystallized grains preserved {111} < 110 > and {111} < 112 > orientations. Doherty et al. [21] have pointed out that the shear bands are resulted from highly localized deformation and preserve high stored energies. The micro-texture analysis in this work indicates that these areas can serve as nuclei sites for recrystallization. However, no in-grain shear bands were observed for the specimen of No. 1 with higher FET.

Fig. 5

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	Fig. 5 Orientation maps for the sheets of No. 2 after hot rolling and partial annealing at 950 ° C for 90 s

To investigate the recrystallization nucleation mechanisms, the hot rolled slab of No. 2 was annealed at 950 ° C for different time to observe the recrystallization nucleation process by TEM as shown in Fig. 6. After annealing at 950 ° C for 10 s, the deformed grains have little changes, and the in-grain shear bands can be still observed in the grains. This is in the recovery stage (Fig. 6a). As the annealing time increased, the dislocations rearranged through sliding and climbing, and a more equiaxed subgrain structure in the deformed bands formed (as indicated by arrows in Fig. 6b). Moreover, the misorientation angles between the subgrains were calculated through the Kikuchi lines obtained by TEM. The misorientations at the subgrain boundaries were higher than those within the deformation bands. For the larger diameter and high orientation gradient from adjacent matrix [21], the subgrain served as a nucleus for recrystallization and grown into large grain at expense of neighbors (Fig. 6c, d). It is concluded that the formation of recrystallized grains near the in-grain shear bands is to be the coalescence of subgrains.

Fig. 6

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	Fig. 6 TEM images showing crystallization in the hot rolled sample (No. 2) annealed at 950 ° C for different time: a 10 s, b 60 s, c 75 s, d 90 s

Figure 7 shows the TEM images of the hot rolled specimen of No. 1 annealed at 950 ° C for different time. Only nucleation at the grain boundaries can be observed. Since the high recovery rate preserved in the specimen with higher FET, the dislocation density in deformed grains in No. 1 seems to be lower than that in No. 2 (Fig. 7a). With the annealing time increasing, some dislocations rearrange. But no recrystallized nucleation in grain interiors is observed for the specimen annealed for 60 s (Fig. 7b). With increasing annealing time to 105 s, big recrystallization grains nucleate at the grain boundaries (Fig. 7c). That is, for the 21%Cr FSS, when it is hot rolled at relatively higher temperatures, nucleation at the grain boundaries is the main recrystallization mechanism during hot rolling and annealing.

Fig. 7

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	Fig. 7 TEM images showing crystallization in the hot rolled sample (No. 1) annealed at 950 ° C for different time: a 30 s, b 60 s, c 105 s

Tsuji et al. [22, 23, 24] have investigated the effect of initial orientation of solidified columnar crystals on the cold rolling behavior and recrystallization behavior of a 19%Cr FSS. Their works suggested that the formation of the grain colonies is supposed to come from the columnar grains in cast slab. For the high stacking fault energy (SFE) preserved in FSSs [25], dynamic recovery is the main softening behavior during hot rolling and no dynamic recrystallization occurs for the tested material, and also the occurrence of phase transformation between austenite and ferrite [20]. The columnar grains tend to transfer to the {001} < 110 > and {112} < 110 > orientations during hot rolling [26]. These grains show a lower recrystallization rate than other grains during annealing after hot rolling (Fig. 2a). And these grains also tend to be stable during cold rolling and finally evolve to the grain colonies during the final annealing process.

For the specimen of No. 1, with relative higher FET, there are no shear bands that were observed in the hot deformed grains. This is because FSS possesses high SFE, and the dislocation can cross-slip and climb easily at high temperatures, resulting in a high dynamic recovery rate. This high dynamic recovery rate avoids the occurrence of heavy plastic deformation in localization. When the temperature decreases, the dynamic recovery rate becomes slow. The slow rate of dynamic recovery and high deformation resistance at low FET would cause more deformation energy to be preserved in the sample, causing more slip systems to be activated and therefore increasing the formation possibility of shear bands.

The formation of nucleation sites in shear bands would refine the microstructure of the hot rolled and annealed sheet. This is useful to break up the columnar grains. As a result, the formation of grain colonies in the final sheets is reduced by decreasing the FET, resulting in improving the surface ridging resistance of FSS. Therefore, it is recommended that the surface ridging resistance of 21%Cr ultra-purified FSS can be improved by optimizing the hot rolling conditions.