## 2 Materials and Methods  
The tests were carried out on steel smelted in an electric arc furnace with a capacity of 140 tons in industrial conditions. The steel was smelted with the addition of about 25% steel scrap. Seven independent melts were carried out. The metal was tapped into a ladle and then desulfurized with a \"desulfex\" mixture. After the desulfurization process, the metal was refined with argon by blowing through a porous mold. The duration of the procedure oscillated in the range of 8-10 min. After refining, billets with a square cross-section of 100 \(\times\) 100 mm were rolled from the cast steel using the classical method. 95 sections were taken for testing from individual billets. Five were used to determine the chemical composition of steel and the impurities present in it, while the remaining ones were intended for metallographic and mechanical tests. The following tests were carried out: chemical composition, metallographic, relative volume of non-metallic inclusions, dimensional structure of impurities, phase composition of oxide inclusions, hardness, and fatigue properties. The chemical composition was tested using the ARL FICA Quantometerand classical chemistry methods. Oxygen content was determined with LECO devices. The total volume of non-metallic inclusions was determined by the extraction method. Determination of the dimensional structure of non-metallic inclusions was carried out on an automatic station for dimensional computer analysis using the Quantimet 720 video microscope. For better identification of non-metallic inclusions, a 400\(\times\) magnification was used. The observation area suitable for observation at 100\(\times\) magnification has been preserved. Automatic counting of inclusions was carried out for their diameters of 2 \(\upmu\)m, 5 \(\upmu\)m, and 10 \(\upmu\)m. The share of inclusions below 2 \(\upmu\)m was calculated by subtracting from the total volume of inclusions the volume determined for a diameter greater than or equal to 2 \(\upmu\)m. The analysis was based on the fact that the area of inclusions determined in the elementary section is proportional to the volume of inclusions in the elementary volume. The tests were carried out on cylindrical samples with a diameter of 10 mm and a working section length of 50 mm (PN-76/H-04327 [46]). Considering that steel works in various applications, and in order to diversify its properties, it was decided to subject the samples to hardening and tempering in the widest possible temperature range. Hardening was carried out after austenitizing at a temperature of 880 \({}^{\circ}\)C and cooling in water. Tempering was carried out for 120 min in the range from 200 \({}^{\circ}\)C to 600 \({}^{\circ}\)C, changing it every 100 \({}^{\circ}\)C. Cooling was carried out in air. Fatigue strength tests were carried out on the MUJ 6000 machine VEB Werkstoffprurmaschinenkombinat, Leipzig, Germany. Rotational bending was carried out at a frequency of 6000 rpm. The value of 10\({}^{7}\) cycles was adopted as the basis for determining the conventional limit of fatigue strength. During the tests, the stresses were gradually changed, maintaining the intervals between the levels of 40 MPa (which allowed for results in the field of limited fatigue strength). The stress values were selected in such a way that the number of cycles characterizing the limited fatigue strength oscillated within the range of 10\({}^{4}\)-10\({}^{6}\). The ranges of chemical composition of the analyzed steel from seven heats are presented in Table 1. The standard deviation of the chemical composition of steel was determined using the Statistica program (for the sample, i.e., the square root of the variance) for 7 heats and three repetitions of measurements for each heat (21 measurements).