## 4 Conclusions  
The tested steel contained 0.23% C on average (Table 1). Therefore, it can be included in the group of low-carbon steels. Taking into account the standard deviation for the carbon content, it can be seen that the carbon content also exceeded the value of 0.25% C, which is an imaginary limit for classifying steels into the low-carbon and medium-carbon groups. Qualitative and quantitative analyses of non-metallic inclusions showed the highest content of the Al2O3 phase, amounting to 39.5% by volume of all inclusions (Tables 4 and 5). The lower relative volumes of 13.4% and 10.4% were occupied by the SiO2 phase and CaO, respectively. The volumes of the subsequent groups of non-metallic inclusions were determined to be at levels below 10%. Taking into account that the Al2O3 phase was present in the microstructure of the steel in the form of fine oval inclusions (Figure 1), it can be concluded that its amount is large. Other marked inclusions had much larger unit \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Temperature**} & \multirow{2}{*}{**Correlation**} & \multicolumn{2}{c}{**Degree of**} & \multirow{2}{*}{\(t_{\text{a}}\) = 0.05 from**} \\  & & **Dissipation, \(k\),** & & **Student’s** \\
**Temperature**\({}^{\circ}\)**C** & **Coefficient**\(r\) & **Around Regression** & & **Distribution for** \\  & & & **Line (7)** & & **\(p\) = (\(n\) - 1)** \\ \hline
200 & 0.8736 & 0.1415 & 4.3972 & \\
300 & 0.8668 & 0.0723 & 4.2579 & \\
400 & 0.8773 & 0.1470 & 4.4775 & 2.4469 \\
500 & 0.9354 & 0.0724 & 6.4800 & \\
600 & 0.9139 & 0.0803 & 5.5146 & \\ all & 0.8101 & 0.1360 & 8.0568 & 2.0452 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Statistical parameters representing mathematical Equation (6), correlation coefficients and dissipation. Figure 7: The bending fatigue strength coefficient, \(k\), of research steel after hardening and tempering for all tempering temperatures depends on the relative content of non-metallic inclusions. volumes (dimensions). For all the tempering temperatures used, an increase in the fatigue strength coefficient was observed with an increase in the relative content of non-metallic inclusions (Figures 2-7). All equations describing the fatigue strength coefficient were statistically significant, which was confirmed by the analysis in Table 6. High reliability was obtained for all tested tempering temperatures. In general, as the tempering temperature increased, i.e., the ductility of the steel increased and its hardness and strength decreased, the correlation coefficient increased. Thus, for microstructures characterized by high plasticity, the presented regression equations are characterized by higher confidence. Such results can be explained by the fact that the brittle incision occurring in the hard matrix during fracture can form a notch favoring the propagation of the microcrack to the incision matrix. Moreover, the initial cracking of hard materials is easier than that of plastic materials [6; 18]. Taking into account the random nature of inclusions in the volume of steel for materials with high hardness, it can be assumed that the formation of cracks initiated by these inclusions is also random. The highest reliability of the tested coefficient was noted for the tempering temperature of 500 \({}^{\circ}\)C.  
Taking into account the results of this study and the studies presented in other papers [16; 28; 30], it can be assumed that, with a high proportion of submicroscopic inclusions, the value of the \(k\) coefficient increases with an increase in the relative content of non-metallic inclusions. This increase is explained by the shape and size of submicroscopic inclusions. The majority of these inclusions are aluminum oxides (Al\({}_{2}\)O\({}_{3}\)). Due to their small volume, they tend to keep the energy as low as possible. For this reason, their shape is similar to nodular. Assuming that they may constitute obstacles in the microstructure affecting the segmental slip bands, the deformation blockade effect by sub-boundaries may occur.