Air leak through snorkel of RH degasser and its effect on nitrogen removal in molten steel

Experimental and theoretical studies were carried out to understand the effects of air leak through the snorkel of a RH degasser on desorption and absorption of nitrogen in molten steel during vacuum treatment. The pressure and gas composition in steel pipes inserted into the refractory of up-and down-legs were examined. Snorkels were sealed with argon gas and change of nitrogen content in steel were compared with conventional treatment without argon gas seal. The outer side pressure of the down-leg is higher than the inner side one, whereas the pressure distribution of the up-leg is to the contrary. The main gas composition in the down-leg is nitrogen and that of the up-leg is argon. These mean that the air leak spot exists in the down-leg. Desorption and absorption rates of nitrogen are balanced at 20 ppm when snorkels are sealed with argon gas, whereas balance content is 27 ppm for conventional treatment without argon gas seal. The kinetic analysis on nitrogen behavior made clear that the amount of nitrogen gas leak when argon gas was used for sealing snorkels was reduced on the average by one half and standard deviation of that value by one fifth compared with the conventional treatment. The relation between the rate constant for nitrogen removal and nitrogen absorption rate by air leak was shown in order to obtain the aimed final nitrogen content at the fixed treatment time.