In EAF steelmaking, controlling the phosphorus content in molten steel is essential for ensuring product quality. Excess phosphorus reduces toughness and ductility, making effective dephosphorization a critical step in the process. The dephosphorization reaction mainly occurs through the interaction between phosphorus in steel and the oxidizing components of slag. Therefore, creating a slag system with strong oxidizing ability, proper basicity, and good fluidity is the key.
Favorable Conditions for Dephosphorization in EAF Steelmaking
To achieve efficient dephosphorization, both thermodynamic and kinetic conditions must be satisfied. The main factors include:
- Lower Reaction Temperature
A lower temperature favors phosphorus removal. Around 1300 °C is considered the most effective range for the dephosphorization reaction. - Optimized Slag Composition
Strongly oxidizing slag with FeO content between 12%–20%, proper basicity (R=2–3), and good fluidity provides ideal conditions for phosphorus transfer from steel to slag. - Increased Slag Amount and Slag Replacement
Enhancing phosphorus activity in steel and applying slag changing or flowing slag operations can significantly improve dephosphorization efficiency. - Intensified Stirring
Strong stirring of the molten pool improves steel–slag interfacial reactions. During the melting stage, correct operations can remove 50%–70% of phosphorus. The remaining phosphorus can be further removed in the oxidation stage through new slag formation, automatic slagging, or powder injection. - Proper Slag System Selection
Choosing a reasonable slag system ensures favorable thermodynamic conditions for phosphorus removal.
Phosphorus Reversion in EAF Steelmaking and Its Prevention
- If not properly controlled, phosphorus that has already been removed may re-enter the molten steel—a phenomenon known as phosphorus reversion. This typically occurs in the following cases:
- Incomplete slag removal in traditional EAFs, allowing phosphorus-rich slag to enter the ladle.
- Slag carryover during tapping in modern EAF steelmaking.
- In the refining furnace, unstable calcium phosphate compounds in slag decompose under reducing conditions, releasing phosphorus back into the steel.
Main Measures to Prevent Phosphorus Reversion
- Minimize Slag Carryover: Tilt the furnace earlier and reduce the tapping beam to minimize slag entering the ladle.
- Increase Lime Addition: When slag carryover is unavoidable due to large tap holes, adding extra lime helps stabilize the slag.
- Timely Slag Removal: After tapping, ladles should be promptly deslagged to reduce phosphorus reversion.
- Improve Slag Basicity: When deslagging is difficult, increase lime input during refining. Higher basicity helps suppress phosphorus reversion, and under reducing conditions, slag can still remove small amounts of phosphorus.
- Deep Dephosphorization in EAF: If the phosphorus content at the end of the EAF process is far from the target, deeper dephosphorization should be carried out to minimize the negative impact of phosphorus reversion on final steel composition control.
