Formation of Nonmetallic Inclusions during Ladle Treatment of Pipe Steels

The technology of melting and ladle treatment of pipe steels K56, 09G2S, and KEI55 is analyzed to determine the factors affecting the metal quality and the formation and removal of oxide nonmetallic inclusions (NIs). To determine the causes of increasing the phosphorus content in the metal in an ladle–furnace unit (LFU), we develop a mathematical model to calculate the slag mass entering the metal on tapping from an electric arc furnace (EAF) and remaining on the steel ladle walls from the previous heat. This model is based on a system of material balance equations. Fractional gas analysis and scanning electron microscopy with electron-probe microanalysis are used to study the metal samples taken along the entire technological scheme of melting and ladle treatment of K56, 09G2S, and KEI55 pipe steels. The total content of oxygen and nitrogen in the selected metal samples, the characteristic types of oxide NIs, and their number are determined at each stage of ladle treatment and casting. The steel samples are shown to contain the most unfavorable for pipe steels undeformable NIs, namely, aluminates, aluminosilicates, and calcium aluminosilicates. The increase in the aluminum–magnesium spinel NI content in the metal samples is assumed to be due to the influence of deoxidizers and the inert gas flow rate on the destruction of the lining. The introduction of ferroniobium with an increase in the argon blowing intensity is found to decrease the nitrogen content in the metal due to the removal of formed titanium nitrides into a slag. The increase in the oxygen and nitrogen content in the samples taken from a continuous casting machine for the heats of 09G2S and KEI55 steels is assumed to indicate secondary oxidation of the metal on casting. A software package is developed using the C++ programming language and the Visual Studio environment in order to predict the number of various types of NIs having formed during ladle treatment depending on its conditions. The convergence of the calculated and experimental values is shown to be satisfactory.