## 3 Results and Discussion
### Mechanical Properties  
Figure 11 shows the hardness graph of the studied samples in the standard state after the addition of silicon and obtaining the CFB structure and the microalloyed CFB samples. The Brinell hardness value in the standard sample (without silicon) was 380 HB, which  
Figure 10: EBSD images of the PAG distribution of (**a**) 1.6511 Si2, (**b**) 1.6511 Si2V, and (c) 1.6511 Si2M. By adding vanadium to the chemical composition the PAG size increased and by adding a rare earth element it decreased. increased to 410 HB with the addition of silicon due to the solid solution strengthening and microstructure change to CFB. Comparing the hardness of other microalloyed samples with this sample, it seems that the hardness value depends on the amount of retained austenite in the matrix. In sample 1.6511 Si2V, with the higher amount of retained austenite, a lower hardness value was measured (405 HB), and in sample 1.6511 Si2M, with lower retained austenite, a higher hardness value (415 HB) was observed. Figure 12 demonstrates the engineering stress-strain curve and Table 2 shows the 0.2% offset yield and tensile strength of the studied samples. The yield strength of all specimens mainly followed the Hall Patch relationship [40]. The 1.6511 Si2M sample with an average PAG size of 7 \(\upmu\)m showed an average yield strength of 878 MPa, and the 1.6511 Si2V sample with an average PAG size of 30 \(\upmu\)m showed a yield strength of 787 MPa. It should be noted that the standard tensile strength is also affected by the matrix structure and solid solution strengthening. The lower yield stress of the standard samples, with an average grain size of 12 \(\upmu\)m compared to other samples, especially 1.6511 Si2 with a similar grain size (average 15 \(\upmu\)m), is mainly due to the lack of silicon in the matrix of the base steel. Figure 11: Variation in the hardness of the samples based on their identification codes. Figure 12: Typical engineering stress–strain curve of samples. Studies by [PERSON] et al. [41] on duplex steels show that the elastic modulus of ferrite is higher than that of austenite. However, the austenite in these steels has a higher work hardening coefficient and tensile strength. Due to the higher percentage of retained austenite (about 20 vol%) in the microstructure of the 1.6511 Si2V sample, the tensile strength and relative elongation are higher than the other samples (tensile strength in this sample is 1063 MPa, and the relative elongation is about 10%). Reducing the percentage of retained austenite in the 1.6511 Si2M sample compared to the 1.6511 Si2V sample from 20 to 12 vol% led to a 7% reduction in tensile strength to 980 MPa and a 3.5% reduction in elongation. It should be considered that despite the strong relationship between the tensile strength and the PAG size and the percentage of retained austenite in the matrix phase, other factors such as the grain boundary characteristics, the morphology of inclusions in the structure, and many other metallurgical variables can also affect the final properties of the steel. The behavior of the 1.6511 Si2V sample in the tensile test indicates that despite a high amount of retained austenite in the matrix, they are not bulky and unstable, so they do not turn into untampered martensite due to applied stresses. The conversion of retained austenite to untampered martensite can cause a reduction in the ultimate tensile strength (UTS) from 1063 \(\pm\) 5 MPa for the 1.6511 Si2V sample to 965 \(\pm\) 4 MPa for the 1.6511 Si2 sample, as presented in Figure 12. On the other hand, stable retained austenite can increase the tensile strength in two ways: crack blinding and TRIP [37]. The SEM image shows the fracture surfaces of 1.6511 Si2 after impact testing, as shown in Figure 13. It is worth noting that both phenomena are likely to occur together where the presence of dimples (Figure 13a) is related to the existence of a soft phase, while the crack closure is associated with the TRIP phenomenon (Figure 13b).