Electric arc furnace (EAF) steelmaking has certain limitations in producing low-nitrogen steel due to its inherent characteristics. Currently, China’s special steel enterprises include some ordinary steel producers that have adopted the modern EAF-LF/VD-CC process. The challenge lies in how to effectively control nitrogen levels in molten steel during this process, so that the final product achieves a nitrogen content comparable to that of converter steel. This is essential for expanding the range of steel grades produced by EAF and enhancing its market competitiveness. To address this issue, this paper presents field experiments that explore various measures and methods for nitrogen control in modern EAF steelmaking.
Nitrogen content in the molten steel during the EAF process undergoes several changes. The primary sources of nitrogen increase are the arc zone, atmospheric exposure, and raw materials. Denitrification mainly occurs through bubble carrying via the C-O reaction.
In the arc zone, non-deoxidized molten steel does not absorb nitrogen directly because oxygen on the surface hinders it. However, when the electrode heats the molten steel, localized high temperatures occur, increasing nitrogen solubility. Foaming slag is crucial to prevent direct exposure of the molten steel to the arc zone and reduce nitrogen absorption.
The addition of molten iron plays a significant role in reducing nitrogen content. As more molten iron is added, the nitrogen level in the steel decreases linearly. High carbon content in the molten iron promotes denitrification through vigorous C-O reactions, which helps lower nitrogen levels during tapping.
During the tapping process, the nitrogen content in the molten steel is similar whether argon or nitrogen is used for bottom blowing. Field tests showed that nitrogen levels in the electric arc furnace were around 56–69.5×10â»â¶, while argon injection resulted in an average of 66.1×10â»â¶. These values indicate that refining processes need further optimization to achieve lower nitrogen levels.
Deoxidation significantly affects nitrogen absorption. After tapping, the nitrogen content increases rapidly as deoxidizers are introduced. However, once the slag forms and aluminum is added, the nitrogen level stabilizes. The key factors influencing this change include the exposure of the molten steel surface during tapping and the power supply system.
The power supply position also impacts nitrogen levels. Lower positions lead to higher nitrogen absorption, while higher gear positions result in smaller increases. When the furnace is off, the slag covers the molten steel, minimizing nitrogen uptake. Using high-power heating can reduce nitrogen absorption by limiting arc ionization and shortening the time at high temperatures.
During continuous casting, molten steel absorbs nitrogen if exposed to the atmosphere. Covering agents and mold flux have limited effects on nitrogen absorption. Field tests revealed that inclusion levels were higher in the initial castings, with micro-inclusion aggregation observed in certain areas of the slab.
Inclusions in the slab originate from ladle slag, tundish cover, and mold flux. Tracer studies confirmed that BaO and CeOO were present in inclusions, indicating that these sources contribute significantly to foreign inclusions. Implementing process improvements such as optimizing refining steps, using long nozzles, and improving ladle drainage agents reduced inclusions and improved steel cleanliness.
In conclusion, understanding the sources and evolution of inclusions in the BOF-LF-CC process provides a foundation for further improvements. Tracer results confirm that ladle and mold slags are major contributors to inclusions. After implementing improvement measures, both total oxygen content and inclusion numbers were significantly reduced, demonstrating effective progress in achieving cleaner steel production.
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