What is The Process of Ladle Refining Furnace?

The ladle refining furnace, or LF furnace for short, is one of the core refining equipment in the steelmaking process. Its typically located after the primary refining furnace (such as a converter or electric arc furnace) and before the continuous casting machine, hence it is also called a "ladle furnaces" or "secondary refining equipment."

Components of an LF Furnace

• Furnace body (the ladle itself): A ladle lined with refractory material, with permeable bricks at the bottom for blowing inert gas (usually argon) for stirring.

• Electrode heating system: The LRF typically uses three graphite electrodes, generating a powerful electric arc through a transformer to heat the molten steel, similar to the principle of an electric arc furnace.

• Alloy and slag charging system: Used to add alloys, deoxidizers, and slag-forming materials into the ladle.

• Water-cooled furnace lid: Covered during refining, serving to seal, collect dust, and improve the working environment.

• Dust removal system: Collects the fumes generated during refining.

• Automatic control system: Controls electrode raising and lowering, power, charging, and argon blowing operations.

Steel Ladle Refining Furnace Process Flow

I. Steel Preparation and Acceptance

1. Ladle Preparation: Inspect the integrity of the ladle's refractory lining and bottom permeable bricks to ensure they are in good condition.

2. Accepting Primary Refining Steel: Pour or hoist the molten steel from the converter or electric arc furnace into the ladle at the ladle furnaces station. At this stage, the molten steel typically contains a high amount of oxygen and is at a high temperature, but its composition and temperature are uneven.

II. Initial Stirring and Evaluation

1. Argon Stirring: After the ladle is in place, immediately blow argon gas through the bottom permeable bricks. The rising argon gas bubbles generate a strong stirring effect on the molten steel, rapidly homogenizing its composition and temperature.

2. Sampling & Temperature Measurement: Under stirring conditions, the operator measures the molten steel temperature with a probe and takes a steel sample with a sampler, sending it to the rapid analysis laboratory for chemical composition analysis.

III. Deoxidation & Alloying

1. Oxygen Determination: Determine the oxygen content in the molten steel based on the steel grade requirements and the results of rapid analysis.

2. Deoxidation: Deoxidizers (such as aluminum blocks, ferrosilicon, silicon carbide, etc.) are added to the ladle to react with oxygen in the molten steel to generate oxides, creating conditions for subsequent reduction slag formation.

3. Preliminary Alloying: Based on the target composition, a large quantity of alloying materials (such as FeMn, FeSi, FeCr, etc.) are calculated and added to roughly adjust the composition of the molten steel.

IV. Slag Formation & Refining

This is the most crucial refining stage of the ladle furnaces, usually carried out simultaneously with heating.

1. Reduction Slag Formation: Slag-forming materials, typically composed of lime (CaO), fluorite (CaF₂), etc., are added to the surface of the molten steel. Under arc heating and a strong reducing atmosphere, a high-basicity (high CaO/SiO₂) and low-iron oxide (FeO) "white slag" or "carbide slag" is formed.

2. Deep Desulfurization: The high-basicity reduction slag has a strong desulfurization capacity. Sulfur is transferred from the molten steel to the slag, forming stable CaS. Continuous argon blowing and stirring increases the steel-slag contact interface, accelerating the desulfurization reaction. [S] + (CaO) + C = (CaS) + CO

3. Arc Heating: Lowering the graphite electrode generates an electric arc to heat the molten steel. The heating power and time are precisely controlled according to the target outlet temperature. The LRF  heating process also promotes the melting of slag and the refining reaction.

V: Composition Fine-tuning & Temperature Control

1. Re-sampling & Temperature Measurement: After refining in the ladle furnaces for a period of time, samples are taken again for analysis and temperature measurement.

2. Fine-tuning: Based on the new analysis results, a small amount of alloy is precisely calculated and added to ensure that the steel composition accurately meets the internal control standard. Simultaneously, the need for continued heating is determined based on the temperature.

VI: Final Adjustment & Purification

Once the composition and temperature meet the standards, the final purification stage begins.

1. Soft Argon Blowing: The argon blowing intensity is reduced to a level that only causes slight undulations on the surface of the molten steel. Strong stirring can trap slag, while weak stirring (soft blowing) promotes the upward movement of tiny inclusions, allowing them to be adsorbed by the slag, without trapping slag inside the molten steel. This is a crucial step in improving the cleanliness of the molten steel.

2. Wire Feeding: Wire feeding may be performed as needed. Metal wire containing elements such as CaSi, pure calcium, or rare earth elements is injected at high speed into the depths of the molten steel using a wire feeder.

Purpose: To modify inclusions (transforming them from harmful Al₂O₃ to low-melting-point calcium aluminates, improving steel properties) and further desulfurize and deoxidize.

VII: Leaving the Station

1. Final Inspection: A final temperature measurement and sampling are conducted to confirm that all indicators are within acceptable limits.

2. Ladle Lifting: Argon blowing is stopped, the electrodes are raised, and the ladle furnace cover is removed. The molten steel refined in the LF refining furnaces is lifted to the continuous casting machine for casting.

The ladle refining furnace is an indispensable link in the modern steel production process, whether it's a "long process" or a "short process." By heating and holding the molten steel, precisely alloying it, deeply desulfurizing it, and purifying it to remove inclusions, it greatly improves the quality stability, purity, and mechanical properties of the steel. It can be said that without secondary refining technologies such as the LF furnace, it would be impossible to economically and stably produce the high-end steel needed by today's society.