Steel is the most popular structural material. The properties and practical applications of all constructional materials, including steel, are determined mostly by their structure. Non-metallic inclusions are one of the factors that influence the fatigue strength of steel. The physical and chemical reactions that occur in the process of steel melting and solidification produce non-metallic compounds and phases, referred to as inclusions. The quantity of non-metallic inclusions is correlated with the content of dopants in the alloy, while their phase composition and structure, in particular shape, dimensions and dispersion, are determined by the course of metallurgical processes.The experimental material consisted of semi-finished products of high-grade, medium-carbon structural steel. The production process involved two melting technologies: steel melting in a 140-ton basic arc furnace with desulfurization and argon refining variants and in a 100-ton oxygen converter and next subjected to vacuum circulation degassing. Billet samples were collected to analyse the content of non-metallic inclusions with the use of an optical microscope and a video inspection microscope. The application of various heat treatment parameters led to the formation of different microstructures responsible for steel hardness values. Examination was realized on calling out to rotatory curving machine about frequency 6000 periods on minute. The objective of this study was to determine the influence of size proportions and distances between the fine non-metallic inclusions (up to 2 µm in size) on bending fatigue strength of high plasticity constructional steel. The results revealed that fatigue strength is determined by the analysed parameters and tempering temperature.Keywords: steel, high-grade steel, impurities, non-metallic inclusions, fatigue strength, bending fatigue strength
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