## 3 Results and Discussion
### Relationship between Microstructure and Mechanical Property
Theoretically, in addition to the content of martensite, the plasticity may be related to the grain size, grain orientation, and texture. Firstly, the grain size data in Table 2 show that the grain size in the core area is the largest, thus the grain size is not the main reason for its higher elongation value. Secondly, the plasticity level highly coincides with the variation regulation of the grain size ratio at three typical locations (Table 2), where the smaller the grain size ratio, the better the plasticity, and the larger the grain size ratio, the lower the plasticity. The results mean that grain size ratio has a dominate role in affecting mechanical properties, while the grain size ratio is closely related to the deformation degree of grains, it is reasonable to consider that these deformed martensite and ferrite grains inherit the deformation characteristics of prior austenite. These results also indicate that the orientation and textures caused by deformation have great influence on the mechanical properties. As shown in Table 3, the proportions of \(\alpha\)-fiber texture ({[223]<110>, [113]<110>, and {112}<110>} and near \(\gamma\)-fiber texture {332}<113> components, have higher values at three typical positions; they are the main factors affecting the mechanical properties of the steel. The content of austenitic recrystallized phase transition textures {001}<110> and {110}<110> at three typical positions remain nearly the same and have relatively low values, respectively, this infers that it is not the main factor affecting the plasticity. According to the IPFs (Figure 8d-f), hard orientation [111] along the TD direction is stronger than that along the RD direction at three positions, all \(\upmu\) values are higher in the regions of [100] to [110], which is called soft orientation, this results in higher strength and lower plasticity along the TD direction and lower strength and higher plasticity along the RD direction [31]. The soft orientation of the RD increases gradually in sequence of top surface, core position, and bottom surface, as shown in Figure 8d-f, while the plasticity of the top surfaces in the RD direction is lower than that of the bottom surface and core position (Table 4). This indicates that the soft orientation of RD has a certain influence on the plasticity. It is reported that \(\gamma\) texture orientation offers advantages in improving the plasticity [32, 33]. It can be seen in Table 3 that the contents of the {332}<113> near \(\gamma\) texture have higher levels, thus, although the soft orientation at the core is weaker than that of the bottom surface, the higher contents of the near \(\gamma\) texture orientation at the core will improve the elongation of the steel. Meanwhile, the texture of {223}<110>, {113}<110>, and {112}<110> leads to an increase in the strength in the TD direction, which results in the strength of TD being higher than that of the RD direction [33]. The analysis of the above results shows that the distribution inhomogeneity of the varieties of texture types exist in the thickness direction of steel plate. The distribution inhomogeneity of the varieties of texture types exists through the thickness of the steel plate. The hard orientation [111] along the TD direction is stronger than that along the RD direction resulting in higher strength and lower plasticity along the TD direction and lower strength and higher plasticity along the RD direction. The higher contents of \(\alpha\)-fiber texture and near \(\gamma\)-fiber texture play a great role in the mechanical properties. It is interpreted that the surface layer of the steel plate sustains a more severe shear stress from the friction between the rolls and the steel plate during the hot rolling process, with the increase in the detected position away from the surface layer, the core area will be involved in a plane strain state, and the level of shear force decreases, this leads to the differences in texture strength between surface and the core area [34].