## 2 Materials and Methods  
All samples in this study were prepared by the casting and austempering process. Melts preparation was conducted using a carbon steel scrap, alloying elements and ferroalloys in an Inductotherm network frequency induction furnace with a capacity of 800 kg. The optimal content of 0.1 wt% for vanadium was selected based on the research conducted by [PERSON] et al. [23], [PERSON] et al. [24], and [PERSON] et al. [25]. However, optimal content of 0.03 wt.% for rare earth was selected according to the studies of [PERSON] et al. [26] and [PERSON] et al. [27]. The chemical composition of the studied samples is listed in Table 1. The chemical composition of the samples was measured by a FOUNDRY-MASTER Pro2 optical emission spectrometer (OES). The casting procedure for all 1.6511, 1.6511 Si2, 1.6511 Si2M, and 1.6511 Si2V specimens was conducted in sand molds prepared according to ASTM A781-05 style keel blocks (Figure 1a). A CFB matrix in the as-casted sample was achieved by using isothermal heat treatment in the lower baintite temperature range and adding 2 wt.% silicon to the matrix. The austempering process was carried out at 300 \({}^{\circ}\)C for 5 h in a KNO\({}_{3}\) salt bath and NaNO\({}_{3}\) with 50% of each salt. The heat treatment process cycle is shown in Figure 1b. For further studies, samples were machined from the tensile zone of the austempered keel blocks based on ASTM A781-05. To investigate the effect of alloying elements on the transformation kinetics, isothermal dilatometry was conducted on the as-cast samples using BAHR 805A/D dilatometry in cycles similar to Figure 1b. Continued cooling dilatometry was also performed to check the effect of alloying elements on the critical temperatures of all samples. An Olympus PME3 light microscope and TESCAN-LMU 30 kV electron microscope equipped with energy-dispersive X-ray spectroscopy (EDS) were used to study the microstructure, and working distances were 4-5 mm. The ASTM E3-11(2017) standard was used for the preparation of metallographic specimens. Sample preparation was started by a mechanically wet grounding with 80-2500 SiC grit paper; subsequently, a 0.05 microns diamond polishing paste was employed for final preparation. The specimens were etched in 4% Nital solution (4% nitric acid + 96% ethanol) for 5 s. Phase composition of the matrix characterization was performed using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). A \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Sample Description** & **Identification Code** & **C** & **Si** & **Mn** & **V** & **RE** & **Cr** & **Mo** & **Ni** \\ \hline DIN 36 CrNiMo4 & 1.6511 & 0.4 & 0.4 & 0.8 & 0 & 0 & 0.9 & 0.4 & 0.9 \\ DIN 36 CrNiMo4-2 Si & 1.6511 Si2 & 0.4 & 2 & 0.8 & 0 & 0.9 & 0.4 & 0.9 \\ DIN 36 CrNiMo4-2 Si-0.03 RE & 1.6511 Si2M & 0.4 & 2 & 0.8 & 0 & 0.03 & 0.9 & 0.4 & 0.9 \\ DIN 36 CrNiMo4-2 Si-0.1V & 1.6511 Si2V & 0.4 & 2 & 0.8 & 0.1 & 0 & 0.9 & 0.4 & 0.9 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the studied samples and their identification code. Figure 1: (**a**) Schematic of the keel blocks; (**b**) heat-treatment process cycle; and (**c**) CTOD samples. Philips PW 1710 diffractometer, with its tube anode being Cu with K\(\alpha\) = 1.54242 A, was used for the XRD analysis. The angle of the goniometer was selected from 20 to 120 degrees to include all the main steel peaks. XRD results were also analyzed by Xpert high score software. The EBSD was performed with an EDAX EBSD detector. Hardness tests were performed by Koppa Brinell (Universal), the tensile tests by GOTECH The AI-7000-LAU TESTING MACHINES at a 2 mm/min elongation rate at ambient temperature, and the Charpy impact at ambient temperature, according to the ASTM E23-2a standard, to evaluate the mechanical properties of the samples. Each test was repeated three times to examine the reproducibility of the results. CTOD sample preparation was performed according to the BS 7448-1 standard with a square cross-section.