## 3 Results and Discussion  
### Microstructural Evolution and Transformation Kinetics  
Figure 2 shows the SEM metallography of 1.6511 steel in two-state, optimized states by adding 2 wt% silicon and the standard composition. The optimized silicon sample has a carbide-free bainite structure. XRD results (Figure 3) show that in the pattern of the standard WN 1.6511 sample a cementie peak presents, but due to the small dimensions of the carbides, the peak intensity is low and wide (indicated by the circle in Figure 3). In contrast, this peak is removed in the optimized sample. Matching these patterns by the SEM images (Figure 2), it can be concluded that the structure of the optimized sample consists only of bainitic ferrite and austenite, or in other words, carbide-free bainite. In the following, the effects of adding alloying elements to this  
Figure 2: Microstructure of 1.6511: (**a**) standard composition and (**b**) optimized states by adding 2 wt% silicon. Note: The microstructure has changed from martensitic bainitic to CFB with the addition of silicon. Figure 4 presents the dilatometric test results of samples with different chemical compositions, including the silicon-modified alloy (1.6511 Si2) and silicon-modified alloy after adding 0.1 wt% of vanadium (1.6511 Si2V), as well as the silicon-modified alloy after adding 0.03 wt% of rare earth alloy (1.6511 Si2M) at a continuous cooling rate of 4 \({}^{\circ}\)C/s. In the 1.6511 Si2 sample, three transformations peaks are observed at temperatures of 556.55, 437.9, and 282.4 \({}^{\circ}\)C, which are related to the transformation of austenite to perlite (or ferrite), bainite, and martensite, respectively, according to the temperature range. After adding vanadium to the chemical composition, the transformation of austenite to perlite (or ferrite) did not occur, and the temperatures of the bainite and martensitic transformations were transferred to lower temperatures. A similar behavior is observed after the addition of cerium to the chemical composition. The peak associated with the perlite (or ferrite) transformation was removed; in other words, due to the addition of both microalloy elements, the critical cooling rate for the formation of diffusionless transformations was reduced. However, unlike vanadium, cerium and lanthanum have increased the initiation temperature of the martensitic transformation (Ms in the 1.6511 Si2 sample is 282.4 \({}^{\circ}\)C, in the 1.6511 Si2V sample it is 230 \({}^{\circ}\)C, and in the sample containing the rare earth element it is 290 \({}^{\circ}\)C). In other words, with the addition of rare earth elements (cerium and lanthanum), the process length for the bainite formation becomes shorter. Microscopic studies (Figure 5) show that the microstructure after adding vanadium and rare earth elements is in the form of granular bainite. However, in samples containing vanadium, this granular bainite is carbide-free; however, in the sample containing rare earth elements, the formation of carbide phases in the microstructure was not prevented. No perlite formation in the structure of samples containing microalloying elements led to carbon retained in the chemical composition of the parent phase (austenite), and it could be more thermodynamic stable at lower temperatures. The shifting of the bainite transformation temperature to lower temperatures in both samples can be due to this issue, along with increasing the strength of the matrix phase due to the solid solution strengthening effect. However, because of the formation of carbide in the sample containing cerium and lanthanum in  
Figure 3: XRD patterns of (**a**) the 1.6511 sample and (**b**) the silicon-modified (1.6511 Si2) sample austempeered at 300 \({}^{\circ}\)C. Note: The cementite peak vanishes when adding silicone. the bainite transformation, the carbon content of the parent phase has decreased, and as a result, its thermodynamic stability has decreased, and martensitic transformation occurred at higher temperatures than the vanadium-containing sample.