Effect of nitrogen-containing alloy and N 2 injection on nitrogen absorption and cleanliness of molten steel in RH vacuum

The industrial trials of N₂ injection and nitrogen-containing alloy addition for increasing the nitrogen content in molten steel were carried out on a 300 t ladle Ruhrstahl–Heraeus (RH) treatment. The effects of the order of N₂ injection and alloy addition, as well as vacuum chamber pressure on the nitrogen dissolution rate of the molten steel were investigated. A nitrogen dissolution model for N₂ injection of RH was established and the predicted result was in good agreement with the measured. The equilibrium [N] contents corresponding to pressure of 7.5–8.5 kPa, 3.0–4.0 kPa and <0.1 kPa were 160–170 ppm, 110–130 ppm and 90–100 ppm, respectively. The [N] content increase rate was slow with only N₂ injection, and the [N] content was not further increased after reaching the equilibrium. The [N] content increase rate was fast with nitrogen-containing alloy addition, while the [N] content rapidly rised to a supersaturation state if the amount of nitrogen brought in by the alloy exceeded the nitrogen equilibrium. Thereafter, the [N] content decreased to near the equilibrium value. The nitrogen-containing alloy addition led to a deterioration of the cleanliness of molten steel, and the earlier addition of alloys had more time to remove the inclusions and to reduce the T.O content.