## 3 Results and Discussion
### Microstructural Evolution and Transformation Kinetics
Some of the observed changes in microstructure by the addition of these alloying elements could be due to changes in the activity of carbon by incorporating them, so the effect of these elements on carbon was investigated. The linear EDS method was employed to examine the effects of the addition of alloying elements on the carbon content of the austenite interface, as presented in Figure 9. The EDS analysis is not accurate enough to detect the carbon content, and thus only qualitatively employed it. Figure 8: EBSD phase map of the (**a**) 1.6511 Si2, (**b**) 1.6511 Si2V, and (**c**) 1.6511 Si2M samples. The red phase shows the retained austenite. Note: Retained austenite increased with vanadium and decreased with the rare earth elements. Figure 7: Optical and electron metallography of 1.6511 Si2V (**a**,**b**) and 1.6511 Si2M (**c**,**d**) after austenite at 300 \({}^{\circ}\)C. Note: All samples have CFB matrix. It is observed that by adding vanadium, carbon has a higher tendency to remain in austenite, and the average carbon content of austenite slightly increases by the addition of vanadium. Unlike vanadium, the presence of cerium and lanthanum in the CFB matrix increases the carbon content in ferrite and decreases it in austenite. If the EDS results are correct, it is possible to investigate the reason for the earlier or later cessation of the bainite transformation by adding alloying elements. Bainite transformation takes place as long as the thermodynamic conditions are possible to convert austenite into bainite. By decreasing the stability of austenite, bainite transformation needs less activation energy. So, in equal temperature in the alloy, its matrix austenite stability is less; hence, the bainite transformation progresses more. One of the important items that austenite stability depends on is carbon content. Decreasing carbon content resulted in austenite becoming unstable, so it can be concluded that bainite transformation can be assisted by an alloying element that reduces the carbon content of austenite. In this regard, rare earth elements assist bainite transformation, while vanadium postpone this transformation. In other words, vanadium assisted carbon diffusion, but rare earth elements act as a barrier to diffusion. Figure 10 shows the prior austenite grain size measured using EBSD analysis. Comparing Figure 10a-c shows that the addition of vanadium into the CFB steels leads to the escalation of the grain size of the PAG from an average of 15 \(\upmu\)m to 30 \(\upmu\)m, while the average grain size of PAG was reduced to about 7 \(\upmu\)m by the addition of rare earth elements. Grain growth with the addition of vanadium contradicts the results of many scientific studies and the reason for the use of vanadium in tool steels [34; 35]. Grain growth due to the addition of vanadium is observed in only a few studies [11; 30]. Therefore, the lack of information in this regard is quite noticeable. Based on the points presented in [11; 30], it seems that encapsulation of vanadium into the CFB steel prevented the formation of carbide phases; it also disrupted the formation of other nitrites and oxides. Lack of oxides and carbides that act as barriers to grain growth will lead to grain growth. The rare earth elements usually prevent grain growth due to segregation in the interfaces of austenite and ferrite [36]. These elements can form oxidation and nitride phases with very high stability during solidification, and these oxides and nitrides act as heterogeneous nucleation sites during solidification [37]. During solidification, rare earth elements increase the volume fraction of the equiaxed zone by changing the surface tension of the solidification phases and reducing the distance dendrite arm spacing (DAS) [38]. All of the above could have occurred due to the steel under study, which resulted in fine-grained and microstructure modification.