Alloying and Refining of High Nitrogen Steels in Ceramic Crucible Plasma Furnace at Pressure Higher than Atmospheric
Metallurgy and Engineering Metrials Technical University of Czestochowa, Poland
Faculty of Metallurgy and Materials Engineering, Technical University of Czestochowa Al. Armii Krajowej 19, 42-200 Czestochowa, Poland
Technical University of Czestochowa, Poland
Faculty of Metallurgy and Engineering Metrials Technical University of Czestochowa, Poland
The one stage process of melting Cr-Mn steels in a hermetic plasma furnace, described in this work, is based on the removing of harmful impurities as oxygen, sulphur and phosphorus from these steels and simultaneously saturating the liquid metal by nitrogen from a gas phase. Investigations have been conducted in the hermetic plasma furnace which is fitted with a 100 kW DC plasma torch. The 24 kg of steels have been melted in the spinel crucible (MgO · A1203) and under a pressure inside the furnace of up to 0.5 MPa. Synthetic slag (65 wt. % CaO + 35 wt. % CaF2) and metallic calcium or calcium carriers (CaF2-Ca and CaH2) were used as refining compounds. For the austenitic steels, the sum of the [%0]tot+[%S] ≤ 150 ppm can be attained recurrently under these conditions.
In the presence of CaO - CaF2 slag which was liquid under the experimental conditions, the final contents of nitrogen in Cr - Mn steels were about 20 ÷ 40% higher than the equilibrium contents (without slag) under the actual partial pressure of nitrogen in the gas phase. The value of 0.93% was obtained as a maximium final concentration of nitrogen in H18AG18 austenitic steel. This value is 2.4 times higher than the standard value of nitrogen solubility for this steel under normal pressure. A layout of the installation for production of high nitrogen steels is proposed.