Mechanism and Model of Nitrogen Absorption of Molten Steel During N₂ Injection Process in RH Vacuum

Herein, the mechanism of nitrogen absorption of the molten steel by injecting N-2 was studied during the RH vacuum refining process. The dissolution behaviour of N-2 in the molten steel was analyzed through the thermodynamic and kinetic calculations. For the N-2 bubbles in the upward flow, the phenomenon of the nitrogen absorption, equilibrium, and denitrification occurred simultaneously in the up snorkel. The process is divided into three stages: (1) the larger static pressure of the molten steel makes the equilibrium value higher than the [N] content when the N(2 )bubbles begin to rise at the N-2 inlet. Then, the nitrogen diffuses from N-2 bubbles to the molten steel and the [N] content increases; (2) the [N] content in the molten steel reaches the equilibrium value; (3) the smaller static pressure of the molten steel makes the equilibrium value lower than the [N] content when N2 bubbles are close to the surface of the molten steel in the vacuum chamber. Then, the nitrogen diffuses from the molten steel to N-2 bubbles and the [N] content decreases. In addition, a nitrogen dissolution model was established which took into account the static pressure both the molten steel and the vacuum chamber. The nitrogen dissolution model was verified by the industrial trials in a 300 t ladle RH treatment. The predicted results by the optimized nitrogen dissolution model agreed well with the measured results. Results showed that the nitrogen dissolution rate increased significantly with the increasing of the vacuum chamber pressure and the decreasing of the initial [N] content in the molten steel, respectively, while the temperature of molten steel had less effect on the nitrogen dissolution rate.