## 3 Results and Discussion
### Microstructure of Ferrite and Martensite
#### 3.2.2 Constituent Phase, Substructure and Recrystallization
KAM values corresponding to the ferrite grains in the BC diagram are less than 1, while the KAM values in the martensite region are mainly at the range of 1 to 2, indicating that dislocation density in the martensite region is higher than that inside the ferrite grains. According to the KAM diagram (Figure 6b,e,h), the dislocation density distribution at three typical locations is statistically listed in Figure 6k. The results reveal that the fraction of areas with high KAM values at two surfaces (Figure 6b,h) are fewer than that of high KAM values at the core location, this indicates that relatively fewer areas of martensite and a higher ferrite content are present at the two surfaces while higher percentage of areas with KAM values greater than 1 exist at the core location. The results from the KAM plots (Figure 6k) also confirm that the variation of proportions of the ferrite phase and martensite phase at three typical locations is consistent with the statistical results by high-angle grain boundary distribution (Figure 5c,f,i) and BC (Figure 6j). Figure 6c,f,i is the recrystallization diagram at three locations on the RD-ND plane. Blue, yellow, and red areas represent recrystallization, substructure, and non-recrystallization regions, respectively. By comparing the recrystallization diagram (Figure 6c,f,i) with the BC diagram (Figure 6a,d,g), it is discovered that the martensitic phase area mainly corresponds to the incompletely recrystallized region and the ferrite area mainly belongs to the substructure region. It is also found that recrystallization mainly exists within the ferrite grains while only a few scattered, small-sized recrystallized, and substructured regions are dispersed in the martensitic phase, no large recrystallized and substructured regions were found within the martensite grains. Meanwhile, by careful observation, only a few parts of ferrite grains were found to be subjected to the recrystallization process, more recrystallization regions within the ferrite grains appeared at the top and bottom surfaces, and the least recrystallized areas are at the core position. It is discovered that the formed martensite do not recrystallize due to their formation at rapid cooling rate and lower temperature, and ferrite grains can be recrystallized because they are formed at elevated temperature. The more ferrite, the greater the recrystallization region. This is the reason why the recrystallization region is only found within the ferrite grains. It is interesting that some of the formed ferrites still do not become recrystallized after air cooling. This result infers that there is stress concentration within ferrite grains. These findings also correspond closely to analysis results of accumulated dislocation density in the KAM diagrams (Figure 6b,e,h). It is concluded from the above EBSD analysis that the austenite grains at two surfaces undergo more deformation than that in the core position during the rolling process, leading to the higher deformation storage energy within the austenite grains at the surfaces, meanwhile, the cooling rate of two surfaces is faster than that of the core area, both factors cause the austenite grains to start the phase transformation earlier and form more ferrite phases and thereafter induce recrystallization of some ferrite grains after the air-cooling stage [24; 30]. Usually, the cooling rate of the top surface is faster than that of the bottom surface during the rolling process of steel plate, so the top surface has higher percentage of ferrite grains area and higher recrystallization area in the ferrite grains.