In electric arc furnace (EAF) steelmaking, the decarburization rate plays a decisive role in determining melting time, refining efficiency, and overall productivity. For high-quality steel grades, increasing carbon input is often necessary to meet chemical composition requirements. Under such conditions, accelerating decarburization becomes the key to shortening the steelmaking cycle and stabilizing operations.
This article systematically analyzes the main methods to increase the decarburization rate in EAF steelmaking, as well as the fundamental metallurgical factors that control the decarburization reaction.
Key Process Measures to Increase Decarburization Rate in EAF Steelmaking
1. Proper Control of Heel Steel and Slag Retention
After tapping, the remaining molten steel and slag in the furnace generally contain relatively high oxygen levels. Increasing the amount of retained steel and slag helps oxidize elements in the scrap that inhibit decarburization during the early stage of melting. This practice also improves oxygen blowing efficiency and accelerates the decarburization reaction.
2. Optimization of Charge Mix and Scrap Composition
More than half of the melting and refining progress in an EAF is influenced by charge composition. For heats with high carbon input, reducing the proportion of scrap with high silicon and manganese content is essential. This minimizes competitive oxidation reactions and simplifies the decarburization process.
3. Application of Staged Decarburization
In all-scrap heats, most carbon-bearing materials or pig iron should be charged in the first basket, typically accounting for more than 60% of the total carbon addition. When hot metal is used, it can be added entirely in the first charge. This ensures a high initial carbon content in the molten bath, allowing oxygen blowing to focus on decarburization after silicon and manganese are largely oxidized. As a result, approximately 20%–50% of total carbon removal can be achieved during the melting stage, reducing the burden on the oxidation period.
4. Addition of Iron-Bearing Materials Containing FeO
Charging materials such as mill scale or direct reduced iron increases FeO content in the slag, promoting slag formation and enhancing decarburization kinetics. The addition amount should be matched to the hot metal ratio. Excessive additions can hinder bath heating and decarburization efficiency. In practice, the recommended range is approximately 5%–50% of the hot metal weight.
5. Rational Power Input Strategy
High power input during the early stage of melting rapidly raises bath temperature into the range favorable for decarburization. Once decarburization begins, power input can be adjusted or temporarily reduced, relying on the exothermic nature of the reaction to maintain temperature. If decarburization is incomplete near tapping temperature, low-power operation can be applied to enhance bath agitation through arc stirring, thereby promoting carbon removal. This effect is particularly evident in DC electric arc furnaces.
6. Adjustment of Slag Basicity and Foamy Slag Condition
Foamy slag with a binary basicity in the range of approximately 2.0–3.0 is especially favorable for decarburization. Such slag improves reaction conditions at the steel–slag interface, enhances oxygen utilization, and supports slag coating on consumable oxygen lances, reducing lance wear and stabilizing blowing operations.
7. Use of Combined Blowing Techniques
Each oxygen blowing method has inherent advantages and limitations. For example, supersonic coherent jet lances exhibit strong decarburization capability within the molten steel but are relatively weaker at the steel–slag interface. Combined blowing techniques can balance internal bath decarburization and interfacial reactions, thereby increasing the overall decarburization rate.
8. Maintenance of Proper Furnace Profile and Hearth Condition
A reasonable furnace geometry promotes molten steel circulation and enhances the ability of oxygen jets to penetrate the slag layer. Excessively deep hearths or severe hearth wear in later furnace campaigns can significantly impair decarburization. Timely hearth repair helps restore favorable reaction conditions and improve decarburization efficiency.
9. Controlled Furnace Tilting During Refining
Appropriate and continuous furnace tilting facilitates carbon diffusion toward the reaction zone and improves oxygen blowing effectiveness. During early melting, tilting toward the slag door side is beneficial. In the middle and later stages, tilting toward the tapping side and alternating forward–backward tilting after decarburization weakens can help sustain the reaction rate.
10. Increasing Oxygen Supply Intensity
Within a reasonable operating range, higher oxygen pressure and flow rate improve oxygen utilization efficiency, directly accelerating the decarburization reaction.
Key Kinetic Steps Governing the Decarburization Reaction
The decarburization process in EAF steelmaking is primarily controlled by three fundamental steps:
- Diffusion of carbon and oxygen within the molten steel bath
- Formation of CO gas bubbles
- Conditions allowing CO bubbles to escape effectively from the molten steel
Any factor that restricts these processes will reduce the decarburization rate.
Influence of Molten Steel Composition on Decarburization Rate
Once a molten bath is formed in the EAF, decarburization can occur. However, elements such as silicon, manganese, and phosphorus—being more readily oxidized than carbon—compete for available oxygen. Their presence suppresses decarburization until they are largely oxidized.
Due to selective oxidation behavior, significant decarburization typically begins only after silicon and manganese contents are substantially reduced. Thermodynamically, carbon oxidation becomes noticeable at approximately 1368 °C, while vigorous decarburization generally occurs when bath temperature exceeds about 1480 °C.
Effect of Temperature on Decarburization Kinetics
Higher temperature increases the internal energy of molten steel and accelerates decarburization reactions. In practice, the onset of decarburization is often judged by bath boiling behavior or the appearance of carbon flame in electrode holes or off-gas ducts.
When the average bath temperature exceeds approximately 1540 °C, slag viscosity decreases, promoting FeO diffusion toward the reaction zone. Under such conditions, decarburization reactions tend to proceed preferentially compared with other oxidation reactions.
Influence of Slag Properties on Decarburization
The physical and chemical characteristics of slag play a decisive role in EAF decarburization:
- Slag basicity: Basic slag facilitates the removal of elements that inhibit carbon–oxygen reactions and improves gas escape conditions at the steel–slag interface. Low basicity slag may form a dense structure, hinder gas release, and even trigger severe boiling.
- Slag viscosity: Excessively high viscosity limits FeO diffusion, while excessively low viscosity increases splashing and reduces slag stability—both unfavorable for decarburization.
- Slag quantity: Excessive slag dilutes FeO concentration and weakens oxygen transfer, while insufficient slag destabilizes reactions.
- FeO content in slag: FeO acts as both a solvent for slag formation and a key oxidizing agent for decarburization. Excessive FeO increases the risk of violent boiling, whereas insufficient FeO reduces decarburization driving force. FeO content must therefore be carefully controlled through oxygen blowing, carbon injection, and blowing practices.
Effect of Oxygen Blowing Methods on Decarburization Mechanism
In ultra-high-power electric arc furnaces, decarburization is mainly achieved through two mechanisms: reactions at the steel–slag interface and direct oxygen jet penetration into the molten steel. Consumable lances primarily promote interfacial reactions, while supersonic lances combine both mechanisms. Overall, however, decarburization at the steel–slag interface remains the dominant pathway.
Selecting and optimizing oxygen blowing methods is therefore essential for improving decarburization efficiency in EAF steelmaking.
