Role of Early Slag Formation During the Melting Stage
Early slag formation during the melting stage plays a key role in arc stabilization, heat transfer, bath protection, and the creation of favorable oxidizing conditions for refining reactions.
Arc Stabilization and Thermal Efficiency
A slag layer covering the molten steel stabilizes the electric arc and reduces radiative heat loss. Increased slag blackness improves heat absorption and overall heat transfer efficiency.
Protection of the Molten Bath and Inclusion Absorption
Early slag formation isolates molten steel from furnace atmosphere, reducing gas absorption while capturing inclusions introduced with scrap.
Control of Element Oxidation and Evaporation
Slag coverage limits the volatilization of elements and provides suitable conditions for the oxidation and removal of phosphorus, silicon, and manganese.
For arc stabilization and bath coverage alone, a slag amount of approximately 1.0–1.5% of scrap weight is sufficient. Excessive slag increases non-productive energy consumption.
Early Dephosphorization Practice During the Melting Stage
To achieve dephosphorization in the melting stage, the slag must possess adequate oxidizing potential, basicity, and volume. Modern EAF practice often shifts dephosphorization from the oxidation stage to the melting stage, allowing phosphorus levels to enter the controlled range once melting is completed.
At typical melting-stage temperatures of 1500–1540°C, low-temperature, basic, fluid, and oxidizing slags are thermodynamically favorable for phosphorus removal. Slag materials are therefore charged in advance, raising total slag volume to 3–5% of scrap weight.
During oxygen-assisted melting, relatively low bath temperature and weak carbon–oxygen reactions allow slag FeO content to exceed 20%. With slag basicity controlled at 1.8–2.0, approximately 50–70% of phosphorus in the charge can be removed. Timely slag removal after melting significantly shortens the oxidation stage. This practice is now widely applied in carbon and low-alloy steel production.
Oxygen-Assisted Melting Operation in the EAF Melting Stage
Oxygen-assisted melting begins once scrap near the furnace door reaches red heat and molten steel becomes visible upon furnace tilting. Some plants initiate early oxygen-assisted melting by igniting scrap near the furnace door using auxiliary carbonaceous materials.
Operational principles include:
- Cutting and pushing scrap simultaneously;
- Avoiding concentrated oxygen blowing at a single point;
- Immediately cutting through bridged scrap to allow immersion into the molten bath;
- Opening the furnace door channel before full oxygen lancing.
Main Oxygen Lancing Methods During Melting
Cutting Method for Bridged Scrap
Used to break scrap bridges and allow gradual immersion into the bath. Oxygen utilization is relatively low and melting speed is slower.
Oxygen Blowing on the Slag Surface
Applied when scrap bridging is absent and carbon content is high. This method offers high oxygen efficiency and fast heating, but produces large flames and harsh operating conditions.
Establishing and Expanding the Central Molten Pool
Oxygen is first blown into the center to connect the three electrode melt zones, forming a molten pool that is gradually expanded. Combined with high power input and foamy slag formation, this accelerates melting.
Oxygen Lancing Strategy for Cold Zones
Lancing strategy should be adjusted according to furnace campaign stage. In early furnace life, electrode-zone scrap can be cleared first. In later furnace life, scrap near furnace walls should be cleared sequentially toward the center.
Practical experience shows that proper carbon addition during the melting stage, combined with decarburization under power input and bath agitation from carbon–oxygen reactions, provides an efficient oxygen-assisted melting mechanism.
Safety and Abnormal Conditions During the Melting Stage
Oxygen lances must not approach the arc zone, as electric current may be transmitted through the lance, posing a serious risk of electric shock.
Poor electrical conductivity during melting is typically caused by:
- Non-conductive materials mixed in the scrap;
- Excessive voids between scrap pieces;
- Mechanical failure of the electrode lifting system.
Corrective measures include adding conductive materials beneath the electrode, such as pig iron or small electrode pieces, while avoiding excessive carbon pickup.
Scrap Bridging Phenomenon in the Melting Stage
Scrap bridging results from improper scrap charging and localized melting. Bridging can cause sudden scrap collapse, electrode breakage, or violent bath boiling. Prevention relies on proper scrap distribution and controlled oxygen-assisted melting.
Power Control at the End of the Melting Stage
When more than three-quarters of the scrap has melted, the arc is no longer shielded. Continued operation at maximum power can damage the furnace roof and walls, making power reduction necessary.
Measures to Shorten Melting Stage Time in EAF Steelmaking
Effective measures include:
- Fast furnace turnaround and rational scrap charging;
- Oxygen-assisted melting;
- Fuel–oxygen burners;
- Scrap preheating;
- Hot metal charging;
- Retained steel and slag practice;
- Rational power supply strategy.
