Nitrogen Control in Production of N-Alloyed Stainless Steels in AOD Converter: Application of Sieverts’ Law

In numerous stainless steels, nitrogen is used as an alloying element to improve mechanical and corrosion properties. The typical nitrogen content is up to 0.5 pct. A common method for alloying is to blow nitrogen gas into an Argon Oxygen Decarburization (AOD) converter. Depending on the steel composition, operating practice, and process conditions, the hitting grade of the N content can be a problem. According to Sieverts’ law, nitrogen solubility depends on the nitrogen partial pressure. In this study, the applicability of Sieverts’ law to control the nitrogen content in stainless steels was investigated by test trials in an industrial AOD converter using N ₂–Ar mixtures. The validity was established in a wide range of nitrogen content (0.1–0.4 pct) and large variety of alloying (Cr, Ni, Mo, Mn) with considerable influence on N solubility. The validity was well established by approaching the equilibrium N content both from above and below. As a conclusion, the nitrogen content can be controlled by regulating the partial pressure of nitrogen in the blowing gas mixture. For practical purposes, the final nitrogen content can be predicted in different situations and guide graphs were drawn to determine the required N ₂–Ar ratio. These tools can be utilized when designing blowing practices for the AOD converter for nitrogen-alloyed stainless steels.