Control of nitrogen in non-oriented silicon steel

Cold Rolled Non-Oriented (CRNO) silicon steels are normally specified by a low core loss value. Essential prerequisites to achieve such low core loss include minimisation of nitrogen, sulphur etc., the fine precipitates of which inhibit grain growth. A number of measures are undertaken to achieve low level of nitrogen in steel. These measures include use of low - nitrogen hot metal, improvement in purity of oxygen used for BOF blowing, lowering of tapping temperature, open tap practice, use of sacrificial deoxidiser or limestone for generating CO2 gas to provide protective layer on liquid steel, improving the nitride capacity of top slag during ladle refining, using dry ice to avoid pick up of nitrogen during teeming etc. Most of these measures were adopted at Rourkela Steel Plant during heat making for CRNO grade of steel and it was possible to bring down the level of nitrogen to less than 40 ppm consistently.