Introduction to the Electric Arc Furnace Steelmaking Process and Key Operations

Introduction to the Electric Arc Furnace Steelmaking Process and Key Operations

1. Charging

Charging is the initial operation in the electric arc furnace (EAF) steelmaking process, involving the loading of raw materials, primarily scrap steel and sometimes hot metal (molten iron), into the furnace.

2. Slag Formation

This operation involves adjusting the composition, basicity, viscosity, and reactivity of the slag. For instance, during oxygen blowing, the goal is to generate a slag with sufficient fluidity and basicity to effectively transfer oxygen to the metal surface. This facilitates the reduction of sulfur and phosphorus to levels below the specified limits for the target steel grade, while also minimizing splashing and slag spillage.

3. Slag Removal

Depending on the specific smelting conditions and objectives, slag removal or replacement is performed at various stages. For example:

   When using a single-slag practice, the oxidizing slag must be removed ("skimmed") at the end of the oxidation period.

   When using a double-slag practice to create a reducing slag, the initial oxidizing slag must be completely removed to prevent phosphorus reversion (the return of phosphorus from the slag to the molten steel).

4. Bath Stirring

Energy is supplied to the molten bath to induce motion in the molten steel and slag, thereby improving the kinetics of metallurgical reactions. Stirring can be achieved through several methods, including gas injection (e.g., inert gases like Ar or N₂), mechanical means, or electromagnetic induction.

5. Dephosphorization

This is the chemical reaction aimed at reducing the phosphorus content in molten steel. Phosphorus is a detrimental impurity, as high levels can cause brittleness in steel at low temperatures, known as "cold shortness." The embrittling effect worsens with increasing carbon content. Standard specifications typically limit phosphorus to a maximum of 0.045% for common grades, with even stricter limits for high-quality steels.

6. EAF Bottom Stirring

Gases such as N₂, Ar, CO₂, CO, CH₄, or O₂ are injected into the molten bath through tuyeres (nozzles) installed in the furnace bottom. This serves to accelerate melting and enhance metallurgical reactions. The benefits of bottom stirring include:

   Reduced tap-to-tap time and lower power consumption.

   More efficient dephosphorization and desulfurization.

   Increased recovery of alloying elements like manganese.

   Improved homogenization of steel composition and temperature, leading to better quality, lower costs, and higher productivity.

7. Melting Period

Specifically for EAF operations, the melting period spans from initial power-on until the solid charge is completely liquefied. The primary objectives are to melt the charge rapidly, raise its temperature, and form a preliminary slag.

8. Oxidation Period and Decarburization

In conventional EAF practice, the oxidation period typically begins after charge meltdown and extends to the removal of the oxidizing slag. A key task is decarburization, where oxygen reacts with carbon to form CO gas. Sufficient decarburization (often >0.2%) is crucial for refining steel purity by removing gases and inclusions. The oxidation period also handles phosphorus removal and bath homogenization. With the rise of secondary metallurgy, much of this oxidation refining is now often shifted to ladle or separate refining units.

9. Refining Period

This general term refers to process stages where specific harmful elements or compounds are removed from the molten steel. This is achieved through chemical reactions that transfer them into the gas phase or float them into a slag layer for removal.

10. Reduction Period

In conventional EAF practice, the reduction period follows the oxidation period and precedes tapping. Its main purpose is to create a reducing slag atmosphere for final deoxidation, desulfurization, precise composition adjustment, and temperature homogenization. This period is largely eliminated in modern high-power and ultra-high-power EAF operations, which focus on fast melting and transfer refining tasks to external units.

11. Secondary Metallurgy (Ladle Refining)

This involves transferring the primary molten steel from the EAF (or other primary steelmaking vessel) to a separate vessel for further refining. The steelmaking process is thus divided into two steps:

   Primary Smelting: Melting, initial dephosphorization, decarburization, and rough alloying in an oxidizing atmosphere.

   Refining: Final degassing, deoxidation, desulfurization, inclusion removal, and fine-tuning of composition, often under vacuum, inert gas, or reducing atmospheres.

This division enhances steel quality, shortens primary furnace time, and optimizes cost and process flexibility. Ladle refining methods vary widely, classified by pressure (atmospheric or vacuum) and treatment approach (ladle processing vs. dedicated ladle furnace refining).

12. Ladle Stirring

Stirring molten steel during secondary metallurgy is critical. It homogenizes temperature and composition and accelerates refining reactions by improving mass transfer at phase interfaces. Reactions that might take 30-60 minutes statically (e.g., desulfurization) can be completed in 3-5 minutes with stirring. Stirring also dramatically increases the rate at which non-metallic inclusions float out of the steel bath.

13. Ladle Wire Feeding

This technique involves feeding cored or solid wires (e.g., containing Ca-Si alloy powder, aluminum, or carbon) into the ladle of molten steel using a wire feeder. It enables deep desulfurization, calcium treatment (for inclusion shape control), and precise micro-alloying or composition trimming. It also contributes to final deoxidation and improves steel cleanliness.

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