3.4 grain boundaries distribution
The grain boundaries distribution maps of X90 bend zone analyzed by EBSD are shown in Fig. 9. In the grain boundaries distribution maps, the black lines represent the high angle grain boundaries (HAGBs) that greater than 15°, and the red lines correspond to low angle grain boundaries (LAGBs) were characterized as boundaries with misorientation of 2° ~ 15°. In the crystallographic theory, these two types of grain boundaries are usually used to represent the main grain boundaries. HAGBs are the grain boundaries of PF and LB. Moreover, some bainite packet boundaries can be regarded as Σ3 (< 111>/ 60°) boundaries according to the typical Kurdjumov-Sachs (K-S) orientation relationship [29, 30]. Some sub-grain boundaries inside the PF are regarded as LAGBs.