Induction furnace steelmaking has become a preferred method for many modern steel plants and foundries due to its high efficiency, flexible operation, and stable output. To ensure high-quality molten steel, long furnace lining life, and safe production, mastering the full induction furnace steelmaking process is essential.
This guide provides a complete overview—from raw material preparation to furnace maintenance and the handling of bridging during melting.
1. Raw Materials: The Foundation of High-Quality Steelmaking
Raw materials directly determine melting efficiency and steel quality. Among them, scrap steel is the primary input for induction furnaces.
1.1 Scrap Steel Requirements
To maintain stable furnace operation and clean molten steel, scrap must meet the following:
- Clean surface with minimal rust
- Must not contain non-ferrous metals such as aluminum, tin, zinc, or copper
- No sealed containers, flammables, explosives, toxic items, or radioactive materials
- Known chemical composition, with low sulfur (S) and phosphorus (P)
- Reasonable sizing to fit the furnace crucible
1.2 Scrap Steel Management
- Classify by source, chemical composition, and size
- Remove sealed containers, mud, and hazardous objects
- Cut oversized scrap before use
- Briquetted scrap should be stored separately
1.3 Alloying Materials
Common alloy additions include:
- Ferro-silicon (FeSi):alloying and deoxidation
- Silicon-manganese (Si-Mn):alloying and deoxidation
- Mill scale:used mainly for decarburization
2. Furnace Building and Drying: Key to Furnace Lining Life
A properly built and dried furnace ensures safe operation and extended service life.
2.1 Furnace Building
2.1.1 Crucible Refractory Material & Equipment Check
- Confirm the accuracy of refractory material specifications
- Before lining, perform water pressure tests, trial runs, and tilting mechanism checks
2.1.2 Base Layer Installation
- Lay one layer of asbestos cloth, keeping the surface smooth and flat
2.1.3 Placing the Crucible Mold
- Mold deviation ≤ 5 mm
- Vent holes 3–6 mm, spaced 150–200 mm
- Keep crucible wall thickness uniform and aligned with the induction coil
- Fix mold using wooden wedges
2.1.4 Building the Crucible Wall
- Loosen each layer by 5–10 mm before adding material
- Remove all debris to avoid affecting lining quality
- Vibrate each layer with a vibration fork and side hammer in a cross-pattern
- Remove the wedges when reaching one-third height
- The lining must be built in one continuous operation to prevent moisture absorption
2.2 Furnace Drying
Standard induction furnace drying procedure:
- Heat at 100°C/h to 900°C and hold for 2 hours
- Increase power to fully melt the lining to a full-furnace state; remove slag at ~1430°C
- Raise temperature to 1580°C and soak for 1 hour
- Begin charging when ~30% of the initial material is melted
- Do not allow bridging; ensure each batch melts before the next addition
3. Batching and Charging: Improving Melting Efficiency
3.1 Batching Principles
- Accurate calculation and weighing of furnace charge
- Proper ratio of large, medium, and small scrap
- Match charge materials with steel quality requirements
- Ensure the overall chemical composition meets the process targets
3.2 Charging Method
- Use a charging trolley with manual assistance
- Material distribution ratio:
- Small pieces: 15–20%
- Medium pieces: 40–50%
- Briquettes: 30–55%
- Principle: dense at the bottom, loose at the top
4. Melting and Tapping: The Core of Steelmaking
4.1 Melting Stage
- Start melting only after confirming equipment condition
- Melting involves volatilization and oxidation of elements
- Si and Al oxidize easily
- Gas absorption increases with temperature; early slag formation helps reduce gas absorption
4.2 Methods to Shorten Melting Time
- Charge quickly after tapping to reduce heat loss
- Arrange scrap properly to accelerate melting
4.3 Alloying the Molten Steel
- When 80% of the charge is melted, take samples for analysis
- Add alloy materials based on results
- Si-Mn recovery rate typically 88–95%
4.4 Tapping Requirements
- Chemical composition must meet specification
- Temperature must satisfy continuous casting or pouring requirements
5. Induction Furnace Maintenance: Ensuring Safe and Stable Operation
Proper maintenance significantly extends the service life of the crucible and prevents furnace leakage accidents.
5.1 Operation in the Early Stages of a New Lining
- During the first 48 hours, limit power to 60–80% to avoid excessive electromagnetic stirring
5.2 Cooling System Requirements
- Circulating water must run for ~12 hours after shutdown
- Furnace temperature must drop below 200°C to protect the lining and coil
5.3 Charging Precautions
- Never use mechanical charging when the furnace is cold
- Avoid hitting or damaging the crucible during charging
5.4 Furnace Lining Cooling Methods
- Natural cooling
- Forced air cooling
- Avoid excessive temperature differences to prevent cracking
5.5 Daily Furnace Lining Inspection
- Inspect after every heat
- Pay close attention to the spout area and known weak points
- Cracks ≥ 2 mm must be repaired immediately
5.6 Criteria for Lining Removal
- Lining thickness < 50 mm
- Excessively high alarm current
- Direct current rising while voltage drops → indicates possible leakage
5.7 Restarting a Cold Furnace
- Fill with cold charge
- Heat at 300°C/h to 1050°C and hold
- No molten metal allowed during heating
- Add low-melting materials first, ferroalloys last
- Galvanized or wet materials must be added on top to avoid splashing
6. Handling Bridging During Melting
Bridging is a common but dangerous condition in induction furnace steelmaking.
Proper Bridging Treatment
- Monitor furnace conditions during charging
- High-temperature molten steel under bridging may damage the lining and cause explosions
- Reduce power to 25% of holding power
- Tilt the furnace to allow molten steel to melt a hole through the bridge
- Return to upright position and add charge through the hole to eliminate the bridge
- Avoid overheating the molten steel throughout the process
