Volgograd State Technical University (Volgograd, Russia)
A. V. Drozdov, Postgraduate Student, Dept. of Materials Technology, drozdov28373@gmail.com
D. V. Rutskiy, Cand. Eng., Associate Prof., Head of the Dept. of Materials Technology, drutskii@vstu.ru
N. A. Zyuban, Dr. Eng., Prof., Dept. of Materials Technology, tecmat49@vstu.ru
Volzhsky Pipe Plant, TMK branch (Volzhsky, Russia)
M. S. Nikitin, Lead Process Engineer, Laboratory of Steelmaking Technology, Chief Steelmaker’s Service, maks.nikitin@tmk-group.com
V. V. Morozov, Head of the Laboratory of Steelmaking Technology, Chief Steelmaker’s Service, vadim.morozov@tmk-group.com
The paper studies the patterns of formation, changes in morphology, sizes and phase composition of non-metallic inclusions (NMI) in liquid metal during the production of bearing steel ShKh15 using the EAF-LF-VD-ССM process flow chart. It has been established that the maximum contamination of metal NMI is observed after smelting in the EAF and amounts to 0.0109%, while the minimum values of the volume fraction and density of inclusions are achieved after desulfurization and alloying in a ladle furnace (LF-2) and amount to 0.0048 % and 2.2 pcs/mm2, respectively. It is shown that during the out-of-furnace treatment, a sequential transformation of oxide inclusions occurs according to the scheme Al2O3 → MgO·Al2O3 → CaO·Al2O3 → 12CaO·7Al2O3 with preservation of the spinel core. It was established that the modification of oxide inclusions with calcium, reduced from highly basic refining slag, is accompanied by the formation of calciumcontaining aluminates, which are liquid or semi-liquid at processing temperatures. This promotes inclusion globularization and increases their proportion to 96.8% at the LF-2 stage. After evacuation, a further redistribution of the phase composition of the inclusions towards calcium-containing aluminates is observed. During tundish pouring, the proportion of inclusions with magnesia spinel cores increases, which may be due to the interaction of the melt with the magnesia lining of the tundish and the transfer of previously formed Mg-containing inclusions. It is shown that the formation of complex oxide inclusions of the Al2O3-CaO-MgO-SiO2 system occurs via a mixed exogenous-endogenous mechanism. The average size of globular inclusions varies from 6.3 to 7.2 μm depending on the process stage, with large inclusions larger than 20 μm present at each stage. Based on the obtained results, process recommendations were proposed to reduce the contamination of ShKh15 steel with globular non-metallic inclusions by optimizing the deoxidation regime, secondary metallurgy parameters, refining slag composition, and argon treatment conditions.
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