In Electric Arc Furnace (EAF) steelmaking, the oxidation period is one of the most critical stages when the oxidation refining route is applied. Its operating quality has a decisive influence on steel cleanliness and composition control, particularly when the charge materials contain relatively high levels of impurities.
When scrap quality is poor or impurity content is high, proper control of the oxidation period becomes essential for ensuring stable downstream refining and achieving the required steel quality.
Main Tasks of the Oxidation Period in EAF Steelmaking
From a metallurgical standpoint, the oxidation period in electric arc furnace steelmaking serves several key purposes:
- Dephosphorization
- Degassing of molten steel
- Removal of non-metallic inclusions
- Uniform heating of molten steel to a temperature above tapping temperature
These objectives are primarily achieved through the boiling action generated by carbon oxidation reactions. Stable and uniform boiling enhances mass transfer between molten steel and slag, which is fundamental to effective phosphorus removal, gas elimination, and inclusion flotation.
General Operating Approach During the Oxidation Period
To ensure successful oxidation refining, operational control typically begins at the end of the melting stage:
- Oxidation reactions are initiated during the late melting stage to create favorable conditions for early dephosphorization;
- Once sufficient molten steel temperature is reached, oxidation is intensified by ore addition and/or oxygen blowing to promote uniform and sustained boiling;
- Based on phosphorus content in the molten steel, part of the slag is allowed to flow out and fresh slag is formed as needed;
- When steel temperature and chemical composition (such as carbon and phosphorus levels) meet the required targets, slag is completely removed, marking the end of the oxidation period and the transition to the reduction stage.
Operating Principles of the Oxidation Period
Although the oxidation period includes multiple refining objectives, they are fundamentally linked to decarburization reactions. However, dephosphorization and decarburization impose different requirements on temperature and slag conditions:
- Dephosphorization requires moderate-to-lower temperatures, a large slag volume, and highly oxidizing slag with good fluidity;
- Decarburization requires higher temperatures and thinner slag conditions.
Based on these characteristics, the overall operating principles of the oxidation period in EAF steelmaking can be summarized as follows:
- Oxidation sequence: phosphorus removal precedes carbon removal;
- Temperature control: gradual temperature increase, slow first and faster later;
- Slag practice: large slag volume for dephosphorization first, controlled slag flushing during phosphorus removal, followed by thin-slag decarburization.
Basic Process Flow of the Oxidation Period
After the furnace charge is fully melted and adequately stirred, molten steel samples are taken according to the composition requirements of the steel grade. Key elements such as carbon, manganese, sulfur, and phosphorus are analyzed.
Subsequently, dephosphorization, decarburization, and temperature raising are carried out in a coordinated manner. When both chemical composition and temperature fall within the specified range, slag is removed to conclude the oxidation period and the process proceeds to the reduction stage.
Throughout oxidation refining, both dephosphorization and decarburization require slag with high oxidizing potential and good fluidity. In practice:
- The optimal slag basicity for dephosphorization is generally 2.5–3.0;
- During decarburization, slag basicity is typically around 2.0.
Therefore, strict control of slag composition, quantity, and fluidity is essential. If slag fluidity deteriorates due to furnace wall collapse or floating refractory materials, prompt slag removal is required.
Oxidation Methods Commonly Used During the Oxidation Period
Several oxidation methods are commonly applied during the oxidation period of electric arc furnace steelmaking, including oxidizing agent addition, oxygen blowing, and combined oxidation methods. Each method exhibits distinct metallurgical and operational characteristics.
Oxidation by Oxidizing Agent Addition
Oxidation using iron ore or similar oxidizing agents has the following features:
- Produces relatively uniform and wide-range boiling, which is beneficial for gas removal and inclusion flotation;
- Is an endothermic process, favorable for dephosphorization but associated with increased electrical energy consumption;
- Oxidizing agents may introduce impurities, potentially affecting steel cleanliness;
- Oxidation reactions occur through slag diffusion, resulting in longer reaction equilibrium times and higher control difficulty.
Oxidation by Oxygen Blowing
In oxygen blowing oxidation, oxygen reacts directly with elements in the molten steel:
- The reaction is exothermic, contributing to rapid temperature rise and shorter refining cycles;
- The slag contains relatively less iron oxide, limiting dephosphorization when oxygen blowing is used alone;
- Dephosphorization efficiency can be enhanced by adjusting blowing practices or combining oxygen blowing with ore addition;
- High oxygen purity and fast reaction kinetics are beneficial for improving steel cleanliness and reducing power consumption.
Operational experience shows that combined oxidation using both oxygen blowing and oxidizing agent addition offers clear advantages in terms of steel quality, operational stability, and overall refining efficiency compared with single oxidation methods.
