An Investigation on Nitrogen Desorption from Molten Steel by Reaction with Methane (CH4) Gas

Abstract

Typical nitrogen level in molten steel before tapping in Electric Arc Furnace (EAF) is approximately 50-100 ppm. Currently existing process based on vacuum degassing after the tapping can remove only up to 20 ~ 30% of the nitrogen in the steel. The needs for clean steel with low impurities and non-metallic inclusions have been increasing due to stringent quality requirements. Nitrogen level in the EAF steelmaking process must be controlled below 40 ppm to produce such as API, S50C, 590DP and electrical steel. Therefore, it is necessary to remove nitrogen in the steel during the steelmaking process. In this regard, a fundamental study has been made on the removal of nitrogen from molten steel and methane (CH4) gas injection was tested to accelerate desorption of the nitrogen. Methane (CH4) gas accelerated the rate of nitrogen desorption from liquid steel compared to the case when Ar gas was blown. The nitrogen desorption rate under present experimental conditions exhibited a second order dependence on [N], regardless of gas types blown onto the steel in the range of 0.011~ 0.029 wt% [O] and over 0.008 wt% [N]. However, a first order reaction was exhibited under 0.004 wt% [N] for both Ar and Ar+CH4(5%) gas blowing regardless of wt% [O], it is thought that rarefied nitrogen in the melt might cause the liquid mass transfer rate as rate controlling step.Methane gas decreased [O], surface active element, remarkably and increased the chemical reaction rate constant. The rate constant kr of Ar + CH4 gas was maximum 3 ~ 4 times greater than that of Ar gas.The rate constant kr was 2 ~ 3 times increased due to the increased θν as oxygen was removed by CH4 gas .The rate constant kr was 1.1 ~ 1.3 times increased due to the increased the activity coefficient of nitrogen as CH4 gas supplied the carbon into the melt.