Electric Arc Furnace Systems – Stable Electrical Arc Furnace Designs

Electric Arc Furnace Systems – Stable Electrical Arc Furnace Designs

The Electric Arc Furnace (EAF) steelmaking process is systematically divided into five key stages: Raw Material Collection, Pre-Smelting Preparation, Melting Period, Oxidation Period, and Reduction Period.

1. Raw Material Collection

Scrap steel is the primary feedstock. Its quality directly impacts final steel quality, production cost, and furnace productivity. Key requirements are:

   Cleanliness: Surfaces should be clean with minimal rust. Impurities reduce charge conductivity, prolong melting, hinder dephosphorization, and erode the refractory lining. Heavy rust or oil contamination lowers steel and alloy element yield.

   Contaminant Control: Scrap must be free of non-ferrous metals (Pb, Sn, As, Zn, Cu). Lead (dense, low-melting) sinks and damages the hearth; tin, arsenic, and copper cause hot brittleness. Sealed containers, explosives, and toxic materials are prohibited for safety.

   Known Composition: The chemical composition must be known, with sulfur and phosphorus content kept low.

   Size Specification: Scrap dimensions must be appropriate (cross-section ≤150x150 mm, max length ≤350 mm) to ensure efficient melting and charging. Pig iron may be added (≤30% of charge) to adjust carbon content.

2. Pre-Smelting Preparation (Batching)

Proper batching is essential for smooth operation and meeting technical specifications. Key principles:

   Accurate Calculation & Weighing: Precise calculation and weighing of charge materials are mandatory.

   Size Distribution: Charge materials should have a mix of sizes (small: 15-20%, medium: 40-50%, large: 40%) for dense packing, rapid melting, and good electrical contact.

   Composition Targets: The charge composition is tailored to the target steel grade and process:

       Carbon: Initial carbon should be 0.3-0.4% above the specification lower limit to enable sufficient oxidation (decarburization). Excess carbon prolongs oxidation and causes overheating.

       Silicon & Manganese: Typically limited to ≤0.8% and ≤0.3%, respectively, to avoid delaying the boil.

       Phosphorus & Sulfur: Levels should be as low as possible, with phosphorus ideally <0.05%.

   Charging Practice: About 1.5% (of charge weight) lime is placed on the furnace bottom to form an early slag, aiding dephosphorization. Charging follows a specific pattern: small scrap at the bottom and center, large scrap placed strategically among smaller pieces, with medium scrap on top. The goal is a dense, centered burden (higher in the middle, lower near the door) that melts quickly and evenly.

3. Melting Period

This stage spans from power-on to complete liquefaction, consuming about half the total heat time and two-thirds of the electrical energy.

   Objective: Melt the charge rapidly with minimal energy while protecting the furnace lining, forming a suitable slag, stabilizing the arc, and initiating early dephosphorization.

   Sub-stages & Power Strategy:

1.  Arc Strike Phase: With a full, cold charge, intermediate voltage and ~2/3 of rated power are used to protect the roof from arc radiation.
2.  Bore-in Phase: Once electrodes are surrounded by scrap, maximum power is applied to melt a "well" around each electrode (~1/4 of melt time).
3.  Rise/Melt-down Phase: As a molten pool forms, maximum power continues until most scrap is melted (~1/2 of melt time).
4.  Final Melt Phase: When >75% melted and arcs are exposed, power is reduced to protect sidewalls and roof.
5. Oxidation Period

   Primary Tasks:

1.  Further reduce phosphorus to ≤0.010-0.015%.
2.  Remove gases (H₂, N₂) and non-metallic inclusions.
3.  Heat and homogenize the bath to a temperature 10-20°C above the target tapping temperature.

   Ore Oxidation Method: Iron ore (80-90% FeₓOᵧ) is added. It decomposes to FeO, which oxidizes carbon ([C] + (FeO) → [Fe] + {CO}↑) and phosphorus. The CO gas causes a vigorous "carbon boil," essential for agitation, heating, and gas/inclusion removal.

   Operational Points:

       Begin after temperature exceeds 1550°C with good slag fluidity.

       Add ore in small batches (1-2% of charge weight per batch, >5 min intervals).

       Maintain a basic (R=CaO/SiO₂ ≈ 2-3), oxidizing (FeO ≈ 12-20%), and fluid slag.

       Achieve a decarburization rate (Vc) ≥ 0.06% C/min.

       Control endpoint carbon and phosphorus via intermediate sampling.

       Conclude with a "clean boil" (5-10 min without ore) to allow gases and inclusions to float out.

       Remove Oxidation Slag: Before reduction, the phosphorus-rich, oxidizing slag must be completely removed ("skimmed") to prevent phosphorus reversion.

5. Reduction Period

This final refining stage occurs under a reducing slag after the oxidative slag is removed.

   Primary Tasks: Deoxidation, desulfurization, final chemistry adjustment, and temperature fine-tuning.

   Typical Operation Sequence:

1.  Slag-Off & New Slag Formation: Quickly remove the old slag and add a lime-fluorspar mixture (e.g., 3:1 ratio) to form a thin, basic, reducing "white slag" to cover the steel.
2.  Pre-deoxidation & Carbide Slag Formation: Add a strong deoxidizer (e.g., Fe-Si, Al) to the slag surface. Then add a carbonaceous material (e.g., coke breeze, ~2.5-4 kg/t) and increase power. Under the high-temperature arc, this forms calcium carbide (CaC₂) in the slag, creating a highly reducing "carbide slag."
3.  Refining Under Reducing Slag: Maintain the white, reducing slag for 20-30 minutes to achieve deep deoxidation and desulfurization. Monitor for carburization from the slag.
4.  Final Adjustments & Tapping: Make final alloy additions for chemistry, ensure temperature is precisely at the tapping point, then tap the furnace.
5.  We are a professional electric furnace manufacturer. For further inquiries, or if you require submerged arc furnaces, electric arc furnaces, ladle refining furnaces, or other melting equipment, please do not hesitate to contact us at  susie@aeaxa.com