Evolution of oxide non-metallic inclusions in production of pipe steel

Under the conditions of pipe rolling mill, during the production of 12 heats of medium-carbon aluminum-killed steel, 10 samples were taken from each heat at different stages of production – from the beginning of ladle treatment to pipe rolling. The samples were examined by high-temperature extraction in a carrier gas to determine the total oxygen content and by scanning electron microscopy to determine the composition and size distribution of non-metallic inclusions. The dynamics of change in the content of inclusions was established according to the following criteria: total oxygen content, volume fraction, density and average diameter of inclusions. The composition of non-metallic inclusions was plotted on ternary diagrams: the oxide component on the diagrams CaO – Al₂O₃ – SiO₂ and CaO – Al₂O₃ – MgO, the sulfide – on Ca – Mn – S. The trajectory of change in the chemical composition of inclusions in the steelmaking process was established. The paper shows the role of steel treatment with calcium and importance of observing thermodynamic conditions. A relationship is established between deviations in calcium treatment parameters, composition of non-metallic inclusions, steel castability and sorting of pipe products by surface and internal defects. Decreasing total oxygen content, practically does not occur at the main stage of ladle treatment. The highest intensity of steel refining from inclusions is observed at the stage between the samples before and after steel treatment with calcium, this time interval is less than 10 min. Comparison of the obtained data on the chemical composition of inclusions with the calculated data on the boundary of liquid phase region of the CaO – Al₂O₃ – SiO₂ – MgO system shows that at this stage, the chemical and phase composition of non-metallic inclusions is transformed. After the addition of calcium, the composition of oxide non-metallic inclusions shifted to the liquid region of the CaO – Al₂O₃ – SiO₂ system at MgO content of 10 wt. % and 1600 °C.

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