###### Abstract  
The fatigue strength of steel is an important parameter determining the use of the alloy. Conducting material durability tests depending on the working conditions of the material requires a lot of work. Therefore, the industry knows methods to estimate the fatigue life of steel on the basis of other parameters or measurements of other mechanical properties. One of such parameters is the fatigue strength coefficient, which allows one to link the fatigue strength with the hardness results of a specific steel grade. Alloys produced in industrial conditions contain impurities that can affect the properties of steel, including fatigue strength. Impurities in steel depend mainly on the technology of its production. One of the technologies that allows one to obtain high-purity steel is by subjecting it to secondary metallurgy treatment consisting of desulfurization and refining with argon. The fatigue strength of steel depends, among other things, on the morphology of impurities. In the work, the influence of the distance between small non-metallic inclusions with a diameter of less than 2 \(\upmu\)m on the fatigue strength of steel, expressed by the fatigue resistance factor, was assessed. The research was carried out in industrial conditions on seven independent melts of low-carbon steel capable of forming a martensite microstructure. Several dozen fatigue strength tests were carried out for each of the casts. The volume fraction, size, and distribution of pollutants were examined. It was found that the main impurity is Al\({}_{2}\)O\({}_{3}\), with a diameter of about 1.8 \(\upmu\)m occurring at a distance of about 12 \(\upmu\)m. The distance between small non-metallic inclusions affects the fatigue resistance factor, and small non-metallic inclusions with a diameter of less than 2 \(\upmu\)m hinder the destruction of high-ductility steel. The paper presents an example of the structure of non-metallic inclusions for heat, the relative volume of inclusions, the average impurity diameter and impurity spacing for impurity dimensional ranges, the impurity spacing \(\Lambda\) for the total volume of impurities, and the bending fatigue strength coefficient tested in steel after hardening and tempering at different tempering temperatures. steel; carbon steel; fatigue strength; inclusions; non-metallic inclusion Article  
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particles introduced into the steel through the use of contaminated recycled feedstock. This group also includes particles from technological equipment used in production, e.g., refractory materials constituting the lining of the furnace, launders, etc. [24; 25; 26]. The vast majority of works describing the impact of inclusions on steel durability analyze hard steels, most often bearing and tool steels (this is the same group of steels) [27; 28; 29; 30; 31; 32]. In these steels, almost every brittle inclusion is a structural micro-notch, which is the focus of the crack. Inclusions with a low tendency to brittle fracture are attributed to a decrease in the durability of steel due to the impact on the boundary of the inclusion and its matrix. The authors report that the size of inclusions at 5 \(\upmu\)m significantly reduces the fatigue strength of steel. As a rule, hard steels tend to develop a discontinuity that causes a fracture when a critical discontinuity is formed, which is a natural notch [33; 34; 35; 36]. A different group of materials is steel, which has high plasticity. These steels are prone to slippage of atomic planes and plastic deformations, both in micro-areas and on a macroscopic scale. In these steels, the impact of non-metallic inclusions is not as negative as in hard steels. The complexity of the interaction is likely the reason why researchers are less interested in these materials. There are also studies indicating the influence of inclusions of small dimensions on increasing the durability of steel. The results of these works were also confirmed by the author's research.