In electric arc furnace (EAF) steelmaking, the oxidation stage plays a vital role in determining steel cleanliness, refining efficiency, and overall metallurgical quality. During this stage, decarburization, dephosphorization, and desulfurization are carried out through controlled oxidation reactions, slag formation, and bath agitation, enabling effective gas removal, inclusion flotation, and chemical composition adjustment.
Proper control of the oxidation stage is essential not only for achieving the required steel quality, but also for limiting melting time, reducing refractory wear, and avoiding unnecessary energy consumption.
How Should the Decarburization Amount Be Determined During the Oxidation Stage?
The required decarburization amount during the EAF oxidation stage depends on several factors, including steel grade requirements, refining practice, and scrap quality. In general, poorer raw material quality or stricter steel quality requirements call for a higher decarburization degree.
Production practice shows that insufficient decarburization fails to achieve effective gas removal and inclusion elimination. Conversely, excessive decarburization does not significantly improve steel quality, but instead prolongs refining time, increases refractory erosion, and leads to unnecessary consumption of energy and labor. Therefore, excessive decarburization is neither economical nor technically justified.
For oxidation refining practice, the decarburization amount during the oxidation stage is generally considered appropriate within the range of 0.20%–0.50%.
How Should the Decarburization Rate Be Controlled?
The decarburization rate directly affects bath behavior and refining effectiveness. If the rate is too slow, bath boiling is weak, resulting in insufficient degassing and poor inclusion removal. If the rate is too fast, decarburization may be completed in a short time, causing violent bath agitation, steel exposure, severe reoxidation, and intensified refractory erosion. In extreme cases, splashing or metal loss accidents may occur.
Therefore, EAF decarburization requires a controlled and moderate reaction rate. An appropriate decarburization rate should ensure that the degassing rate of molten steel exceeds the gas absorption rate, while allowing inclusions to float up and be removed efficiently. Under normal oxygen-blowing conditions, the decarburization rate is commonly controlled within 0.005%–0.05% per minute.
Similarities and Differences Between Dephosphorization and Desulfurization Conditions
Dephosphorization and desulfurization during the oxidation stage share several common slag requirements:
- Sufficient slag volume
- High slag basicity
- Good slag fluidity
However, their optimal reaction conditions differ significantly:
- Dephosphorization favors lower temperatures, whereas desulfurization requires higher temperatures
- Dephosphorization requires a relatively high FeO content in slag (approximately 14%–20%), while desulfurization requires low slag oxidizing potential
As a result, simultaneous control of decarburization, dephosphorization, and desulfurization during the oxidation stage requires careful balancing of temperature, slag composition, and slag oxidizing ability.
How Can Decarburization and Dephosphorization Efficiency Be Improved?
Practical measures to improve decarburization and dephosphorization efficiency during the oxidation stage include:
- When temperature conditions permit, adding an appropriate amount of oxidizing agents (such as iron ore or mill scale) to promote dephosphorization at an early stage
- Combining oxygen blowing with oxidizer addition to intensify oxidation reactions
- Maintaining a relatively high and stable slag basicity, typically within 0–3.0
- Supplying power reasonably to keep the molten bath at an active reaction temperature
Typical Operating Scenarios During the Oxidation Stage
Low Carbon and Low Temperature
When both carbon content and temperature are low during the oxidation stage, a small oxygen flow can be applied to the slag surface while carbon powder is added into the furnace to promote slag foaming. At the same time, maximum electrical power should be supplied to raise the bath temperature. If necessary, discarded ferrosilicon powder or similar materials may be added to utilize chemical heat for temperature recovery.
High Carbon and High Phosphorus
This condition typically occurs at the early oxidation stage. The appropriate operation involves adding lime and blowing slag to promote dephosphorization while supplying power to increase temperature, maintaining slag basicity above 2.0. As temperature rises and bath boiling improves, lime addition should continue together with an appropriate amount of iron ore or mill scale, allowing decarburization and dephosphorization to proceed simultaneously.
When slag foaming becomes stable and automatic slag flowing is possible, part of the slag may be drained. Based on chemical analysis, lime and oxidizers can then be replenished to further enhance decarburization and dephosphorization, preparing for the slag-off stage. Compared with the traditional “dephosphorization first, decarburization later” approach, this combined operation helps shorten refining time.
High Carbon and Low Phosphorus
In this case, bath temperature should be controlled within 1550–1580°C, and slag basicity should be maintained between 2.0 and 2.8. Excessively high basicity increases slag viscosity and hinders decarburization, while insufficient basicity weakens the reaction and increases the risk of violent boiling.
During decarburization, if bath temperature becomes too high, iron ore or mill scale may be added together with oxygen blowing to achieve temperature control and decarburization, while simultaneously removing residual phosphorus.
Low Carbon and High Phosphorus, and Transition to the Reduction Stage
When carbon content is low but phosphorus remains high, slag oxidizing ability is usually strong and slag fluidity is good. In this situation, lime mixed with carbon powder should be added to maintain slag basicity above 2.0, while intermittently supplementing lime with carbon powder to reduce FeO content and promote slag foaming. This enhances arc coverage and strengthens dephosphorization at the steel–slag interface.
Bath stirring should be applied to improve heat transfer. If slag fluidity and temperature permit, small-flow oxygen blowing may be used for slag flowing. After dephosphorization is completed and temperature is appropriate, slag should be removed. Carbonaceous recarburizers can then be added to the exposed molten steel surface, followed by slag formation to enter the reduction stage.
If decarburization during the oxidation stage is insufficient, steel quality may deteriorate. In such cases, argon stirring in the ladle after tapping is often required to improve steel cleanliness. It is critical to ensure adequate temperature during slag-off; otherwise, phosphorus reversion may occur due to melting of cold-zone scrap.
Note on Process Scope and Applicability
The discussion above is focused on the oxidation stage of traditional three-stage electric arc furnace steelmaking practice.
It should be noted that in civil and commercial steel production, modern EAFs are increasingly characterized by large capacity, high efficiency, and intelligent control, and have become the mainstream technology route. However, traditional three-stage EAF practice remains irreplaceable for certain special steel grades and under specific regional production conditions.
Moreover, despite differences in equipment scale and automation level, the fundamental metallurgical principles governing decarburization, dephosphorization, desulfurization, slag control, and molten bath behavior are largely shared between traditional and modern EAF steelmaking. For this reason, understanding oxidation-stage operation in traditional EAF practice continues to hold significant engineering and practical value.
