One of the quality criteria for Interstitial Fee (IF) steels is the metal purity with respect to non-metallic inclusions (NMI), which are harmful for the plastic properties of the material. Furthermore, they cause a formation of surface defects in flat rolled products and reduce the rate of steel casting due to nozzle clogging. This article presents the results of a study of the content, composition, size and morphology of non-metallic inclusions and clusters in steel samples taken during ladle treatment, casting as well as from slabs and steel sheets after rolling of IF steel. The characteristics of NMI and clusters were determined by using conventional two-dimensional quantitative metallographic investigations of polished sections of steel samples (2D method), electrolytic extraction (EE method) of samples followed by investigations of inclusions and clusters by using scanning electron microscopy and energy dispersive spectroscopy and fractional gas analysis (FGA method). By using EE method, different types of inclusions and clusters, their formation, growth and behavior during different stages of IF steel production were studied. The results obtained by the EE method agreed well with the results of the quantitative determination of oxide NMI by using the FGA method. The method of fractional gas analysis shows the dynamics of changes in the content of various types of oxide non-metallic inclusions during ladle treatment and casting of steel. The obtained results can be used to analyze the causes of the formation of harmful NMI in the metal and to optimize ladle treatment of IF steel grades.
Modern trends in the automotive industry are aimed at reducing vehicle weight and increasing its safety. The high ductility of IF-BH steel in combination with increased strength allows the use of thinner rolled products in the car body structure. Achievement of high plastic properties in IF-BH steel is ensured by the low content of nitrogen and carbon (C <40 ppm, N <40 ppm) and controlled carbon content in the solid solution. The majority of sheet surface defects are associated with non-metallic inclusions. The presence of non-metallic inclusions in the finished product violates its homogeneity, deteriorates the surface properties of steel, fatigue strength and plastic characteristics of the metal. As a result of the work performed by the methods of fractional gas and micro-X-ray spectral analysis on a scanning electron microscope, metal samples of three IF-BH steel melts were analyzed, taken along the entire process chain of production. The main types of oxide non-metallic inclusions in the steel were quantitatively determined, as well as the total oxygen and nitrogen content in the metal. It is shown that the main types of oxide non-metallic inclusions in the investigated metal samples are aluminates, silicates and spinel. An increase in nitrogen content in the metal after breakdown of the vacuum on the VD was established, which indicates secondary oxidation of the metal. In metal of the samples in the section between the tundish and the crystallizer, nitrogen content in the metal increases which indicates the secondary metal oxidation. At the same time, the content of inclusions of aluminates in the metal increased and the number of inclusions of aluminum-magnesium spinel decreased. A mathematical model and software have been developed that describe the formation and removal of oxide nonmetallic inclusions. Adequacy of the program was confirmed by good convergence between the calculated and laboratory data on the total content [O] in various types of oxides for the first sample at the ladle-furnace unit and in the slab.
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