DEPHOSPHORIZATION OF HIGH-CHROMIUM STEELS USING RARE EARTH OXIDES

High phosphorus content in chromium steels causes reducing of the service characteristics and increasing of steel liability to brittle fracture. It is known that the process of dephosphoration in reducing conditions by treating the metal with calcium or REM depends on the possibility of retention of these elements in the melt, where the loss of chromium by this process do not exist. This work presents results of the study of dephosphorization of chromium steel (13 % Cr) by slag mixtures containing rare earth metals. The proposed thermodynamic model confi rmed the possibility of dephosphoration of high-alloyed steels with low contents of carbon, when other ways to get it are not possible. The experiments were performed in a resistance furnace with a graphite heater, argon was used as a protective atmosphere at a temperature of 1600 °C. After melting of the metal the mixture of the slag was added for 5 minutes. Moreover, the main infl uence on this process has an oxidation potential of the system “metal – slag”, which in turn is determined by the aluminum content in the steel. The volume of removed phosphorus increases with increasing content of rare earth metals oxides in the slag mixture. When this content reaches 40 % of rare earth metals oxides in the slag mixture (slag ratio 0.03) the volume of removed phosphorus amounts to 0.007 %, which corresponds to the dephosphoration degree of 20 %. When slag-metal ratio is 0.1 and 20 % of rare earth oxides in the slag, the phosphorus concentration is reduced to 0.014 %, i.e. by 36 %. The experimental data on dephosphoration on chromium-containing metal are consistent with the calculated data under the assumption of phosphorus transfer into the slag in the form of cerium phosphide. After the replacement of rare earth oxides (REO) to CaO the reduction ratio (% REO)/(% CaO) the degree of dephosphoration in the beginning is not changed, then, since (% REO)/(% CaO) = 0,67 it is rapidly declining, despite the fact that the concentration of aluminum in the metal does not change. It was noticed, that increasing of the REO content in the slag higher than 25 % is not economically attractive, as the degree of dephosphorization is constant.

steel refining, dephosphorization, rare earth oxides, synthetic slag, chromium steel, dephosphorization degree

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