Nitrogen control of ultra-low-carbon Al-killed steel in a smelting process

Based on the fact of high nitrogen pick-up, large nitrogen fluctuation and poor control in some steel works, the main aspects of nitrogen removal and nitrogen pick-up in an ultra-low-carbon Al-killed steel smelting process were summarized by analyzing the process data and sampling. The main sections for nitrogen removal are BOF decarburization period and vacuum treatment. High decarburization amount can lower the nitrogen content at the BOF endpoint. There is just a limited effect of switching between nitrogen and argon on the final nitrogen content at the BOF endpoint before the point of 70% oxygen blowing. Free oxygen is favorable for deni-trification by promoting the reaction of carbon and oxygen under the RH mode, in contrast under the VD mode. When free oxygen in liquid steel is controlled above 200×10^-6, the nitrogen absorption during tapping can be controlled within 5×10^-6. The nitrogen introduction of furnace charge is the important factor of nitrogen pick-up during the vacuum refining process, and it is up to 11×10^-6. The minimum of nitrogen pick-up can reduce to 1×10^-6 by adopting gasket sealing and argon blowing protection simultaneously.