## 3 Results and Discussion
### Microstructure of Ferrite and Martensite
#### 3.2.2 Constituent Phase, Substructure and Recrystallization  
The martensite and ferrite have a similarity in crystal structure. Due to the lower carbon content in ferrite, their crystallography is affected by the carbon content. The BC of ferrite are relatively clear and sharp, thus the BC of ferrite shows as grey-white in the BC diagram. Therefore, the BC distribution of the Kikuchi is an effective technique to analyze these two phases [23]. The area fractions of ferrite and martensite phase at three locations (Figure 6j) was calculated by the BC distribution of the Kikuchi (Figure 6). Figure 6a,d,g shows that the small amount of ferrite phases (grey-white area) exists at the core location, while more ferrites are at the top and bottom surfaces. The calculated results show that the area fractions of ferrite phase at the top and bottom surfaces and core are about 20%, 13%, and 9%, respectively (Figure 6j). This result illustrates that the microstructure distribution shows an inhomogeneity state even in such a thin specification plate, and during rapid cooling condition, it will have an influence on the mechanical behavior of the steel. These distribution patterns of ferrite grains at three typical locations correspond to the distribution pattern of ferrite grains by microstructure examination (Figure 4d-f) and the grain boundary type diagram (Figure 5b,e,h). These indicate that the smaller the amounts of ferrites, the lower the proportions of high-angle grain boundaries. Both SEM and BC investigation confirms that the distribution inhomogeneity of martensite and ferrite phases formed by multi-stage cooling is more obvious, and the content of ferrite on the top surface is even found to be two times higher than that of ferrite at the core location and more than that of ferrite at the bottom surface. It is considered that the surfaces of the steel have more rapid cooling rates to reach the initial transformation temperature of ferrite and suffer more severe strain, of which both factors can improve nucleation and growth of ferrite [24; 25; 26; 27; 28; 29]. Meanwhile, the microstructure difference existing between two surfaces can be attributed to cooling rate differences. This microstructure difference through the thickness should not be neglected in the application of the steel, the effect of microstructure inhomogeneity on the mechanical properties must be considered in the subsequent processing of the materials. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Direction\textbackslash{}Position** & **Top Surface** & **Core Position** & **Bottom Surface** \\ \hline Grain sizes in horizontal direction/\(\upmu\)m & 1.4805 & 1.4856 & 1.3048 \\ Grain sizes in vertical direction/\(\upmu\)m & 1.2356 & 1.3744 & 1.1638 \\ Grain size ratios & 1.1982 & 1.0809 & 1.1212 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Statistics of average effective grain size at typical locations. The KAM diagram can qualitatively describe the level of accumulated dislocation density within the grains by calculating the average misorientation between neighboring sites, the magnitude of the KAM value in the KAM diagram are represented by coloring, the smaller the KAM values, the lower the level of accumulated dislocation density. It can be seen in Figure 6b,e,h that the martensite area has green color with higher KAM values. A comparative analysis between the BC diagram (Figure 6a,d,g) and the KAM diagram (Figure 6b,e,h) shows that dislocation density within the ferrite grains is lower because the  
Figure 6: BC, KAM, and recrystallization diagrams at typical locations on the RD-ND plane: (**a,e,i**) top surface; (**b,f,j**) core location; (**c,g,k**) bottom surface; (**a–c**) band contrast diagram; (**e–g**) KAM diagram; (**i–k**) recrystallization diagram; (**d,h,l**) quantitative statistics.