## 3 Results and Discussion
### Microstructure of Ferrite and Martensite  
#### 3.2.1 Grain Boundary Feature and Effective Grain Size  
Generally, due to consisting of packets, blocks, and laths [22], lath martensite grains have many high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs). The ferrite grains otherwise only have HAGBs because they form at elevated temperature. By means of EBSD techniques, types and propositions of grain boundary in two phases will be accurately detected, and quantities and distribution of both phases can be obtained. Furthermore, effective grain size can be calculated according to the distribution of HAGBs. The grain misorientation and distribution of grain boundaries are shown in Figure 5. The red lines represent the LAGBs with misorientation lying between 2 and 15\({}^{\circ}\), and the  
Figure 4: Microstructure morphology of typical locations: (**a**,**d**) top surface; (**b**,**e**) core location; (**c**,**f**) bottom surface; (**a**–**c**) transverse cross-section (TD-ND plane); (**d**–**f**) transverse longitudinal section (RD-ND plane). HAGBs with misorientation larger than 15\({}^{\circ}\) are described by black lines. Grain orientation distribution as shown in Figure 5a,d,g proves that the morphologies of martensite and ferrite do show obvious directional features. From Figure 5c,f,i, the distribution pattern of the misorientation angles between neighboring grains have a bimodal-type. Distribution is found with the peak appearing at low angles (smaller than 20 deg) and high angles (from 50 to 61 deg). This distribution contributed to the packet and block characteristics of lath martensite. The results reveal that the distribution pattern of misorientation angles at three typical locations have the same features. Comparing with the LAGBs, the density of HAGBs only take a few proportions. The density of the HAGBs at three typical locations are calculated in Figure 5c,f,i, and it is found that the smallest density of the HAGBs exist at the core location while higher density of the HAGBs appear at the top and bottom surfaces. It is difficult to figure out martensite and ferrite phases in Figure 5b,e,h, but because of the absence of LAGBs in the ferrite grains, the ferrite grains appear white in Figure 5b,e,h. In the figures, it is evidence that the amounts of ferrite grains at the top and bottom  
Figure 5: Distribution of grain orientation and grain boundary types at typical locations on the RD-ND plane: (**a,d,g**) grain orientation distribution; (**b,e,h**) grain boundary type distribution; (**c,f,i**) grain boundary content statistics; (**a–c**) top surface; (**d–f**) core position; (**g–i**) bottom surface. location are more than that of the ferrite grains at the core location. It is concluded that the distribution of ferrite and martensite grains is non-uniform through the thickness. The grains of the prior austenite are broken severely during the rolling. Grain boundaries were not revealed completely through the traditional chemical erosion method, and the statistics for grain size of the prior austenite by this means are not accurate. In martensite grains, many HAGBs of packets and blocks in the martensite will affect the accuracy of the test. Thus, the effective grain size is measured by using EBSD. In addition, in order to quantitatively understand the variation of effective grain size in each area, the average effective grain size in the horizontal and vertical directions at three typical locations was calculated based on the distribution data of HAGBs, as shown in Table 2. The calculated values show that the ratios of grain size at three typical locations are greater than 1, which further proves that primary austenite has characteristics of deformation. Meanwhile, the grain size ratios of two surfaces are found to be larger than that of the core location. The reasons for these may be attributed to the effect of deformation degree and cooling rate.