The electric arc furnace (EAF) steelmaking process has certain limitations when it comes to producing low-nitrogen steel due to its inherent characteristics. In China, many special steel enterprises, including some traditional steelmakers, have adopted the modern EAF-LF/VD-CC production process. The challenge lies in implementing effective measures to control nitrogen levels in molten steel during this process, aiming to bring the nitrogen content of EAF products up to the level seen in converter steelmaking. This is crucial for expanding the range of steel grades produced by EAF and enhancing their market competitiveness. To address this issue, this paper presents field experiments that investigate various methods for controlling nitrogen in modern EAF steelmaking.
During the EAF process, nitrogen in molten steel can increase through several mechanisms: nitrogen absorption in the arc zone, atmospheric exposure, and impurities from raw materials. Denitrification primarily occurs via bubble carrying during the C-O reaction. Understanding these factors is essential for managing nitrogen levels effectively.
One key area of study is the nitrogen addition in the arc zone. Non-deoxidized molten steel tends to resist nitrogen absorption due to the surface activity of oxygen. However, when the electrode heats the molten steel, localized high temperatures can lead to increased nitrogen solubility. Therefore, forming a foamy slag layer is critical to prevent direct exposure of the molten steel to the arc zone and reduce nitrogen pickup.
The impact of adding molten iron on nitrogen levels was also examined. Increasing the amount of molten iron led to a significant reduction in nitrogen content, with a nearly linear decrease observed. High carbon content in the molten iron enhances denitrification by promoting more vigorous CO reactions, which help lower nitrogen levels during tapping.
In the tapping process, the nitrogen content is similar whether using bottom-blowing argon or other methods. Field tests showed that nitrogen levels varied slightly depending on the refining stage. It was found that deoxidation processes, such as aluminum feeding, significantly affect nitrogen absorption. After aluminum is added, the dissolved oxygen decreases, leading to increased nitrogen uptake. However, once the slag forms, nitrogen levels stabilize.
The power supply system also plays a role in nitrogen content. Lower power positions tend to result in higher nitrogen increases, while higher gear settings reduce this effect. When the power is off, the slag provides protection, minimizing nitrogen absorption. Using high-power heating for short durations can limit arc ionization and reduce nitrogen entry into the molten steel.
During continuous casting, nitrogen absorption occurs if the molten steel is exposed to the atmosphere. While tundish cover agents and mold flux have limited impact, proper slag management and nozzle design are essential to minimize inclusion formation. Inclusion sources were identified as ladle slag, tundish refractories, and mold flux.
After implementing improvements such as optimized refining processes and better slag management, the number of inclusions and total oxygen content in the slab were significantly reduced. These changes improved the cleanliness and quality of the final product.
In conclusion, understanding the behavior of inclusions and their sources is vital for improving steel cleanliness. By applying the findings from this study, significant progress has been made in reducing nitrogen and oxygen content, ultimately enhancing the performance of EAF-produced steel.
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