Ladle Furnace (LF) Steelmaking Process
The Ladle Furnace (LF) process is a critical secondary refining stage that significantly enhances steel quality. It involves transferring molten steel from the primary furnace (converter or electric arc furnace) to the LF, where reducing conditions and precise control enable deep refining.
In the LF process, molten steel in its final oxidation stage is transferred from the primary furnace. A key initial step is the removal of 50-90% of the oxidizing slag, followed by the addition of a synthetic reducing slag and deoxidizers to initiate reduction refining. By optimizing process parameters—such as extending stirring time under heating, increasing slag volume, and enhancing stirring power—and achieving near-total slag removal during tapping, the sulfur and oxygen content can be dramatically reduced. Targets of [%S] < 30 ppm and [%O] < 20 ppm are achievable, producing exceptionally clean steel.
The limited solubility of oxygen in solid steel necessitates its effective removal during the liquid stage. Molten steel from the primary furnace is highly oxidized, making deep deoxidation a prerequisite for other refining tasks like desulfurization. The detrimental effects of oxygen include:
Impeding Desulfurization: High oxygen content (or slag oxygen potential) negatively affects the sulfur distribution ratio between slag and steel, hindering deep desulfurization and influencing the nature of residual inclusions.
Causing Carbon Re-oxidation: During solidification, decreasing oxygen solubility leads to its precipitation, potentially reacting with carbon to form CO gas. This can cause defects like blowholes, porosity, and segregation in the final product.
Generating Harmful Inclusions: Precipitated oxygen forms non-metallic inclusions (e.g., oxides of Si, Mn, Al), which degrade mechanical properties (proportional limit, impact energy, elongation) and are a primary cause of defects like hairline cracks.
Exacerbating Sulfur Harm: Oxygen can form low-melting-point eutectics with sulfur (e.g., FeO-FeS), impairing steel plasticity and causing hot shortness during rolling or forging.
The LF primarily employs two deoxidation methods:
(1) Precipitation Deoxidation
This involves directly adding bulk deoxidizers (e.g., aluminum, ferroalloys) to the molten steel after slag removal. The dissolved deoxidizing element (M) reacts with dissolved oxygen ([O]) to form solid or liquid oxide products (MxOy) that float out of the melt. Composite deoxidizers containing aluminum and alkaline earth elements are particularly effective, as they form low-melting-point complex oxides (e.g., calcium aluminate) that coalesce and separate more easily from the steel.
(2) Diffusion Deoxidation
This method adds powdered deoxidizers (e.g., Fe-Si, CaC₂, Al powder) onto the slag surface. The deoxidation reaction occurs at the slag-steel interface, lowering the FeO content in the slag. To re-establish equilibrium, oxygen diffuses from the steel into the slag, progressively reducing the overall oxygen content in the metal.
Sulfur is generally a harmful element, and its removal is a core LF function. Under the reducing, high-basicity slag conditions in the LF, desulfurization proceeds efficiently. The primary reaction is:
[FeS] + (CaO) = (CaS) + (FeO) (1)
The efficiency of this reaction depends on:
Slag Basicity: Higher basicity favors desulfurization, but beyond an optimum point, slag viscosity increases, impairing kinetics.
Slag Redox State: A low (FeO) content is crucial, as indicated by Equation (1). The LF's reducing atmosphere and synthetic slag facilitate this.
Slag Fluidity and Volume: A fluid slag promotes reaction kinetics, while adequate slag volume provides sufficient capacity. However, excessive slag can hinder inclusion flotation and increase cost.
Bottom argon stirring is a vital final step before continuous casting, crucial for homogenizing temperature and composition and promoting inclusion removal. The argon bubbles facilitate inclusion elimination through several mechanisms:
Bubbles are particularly effective at colliding with and adsorbing smaller inclusions. Therefore, optimizing the bottom-stirring system (gas flow rate, porous plug placement, stirring pattern) according to specific plant conditions is essential for maximizing steel cleanliness. This process ensures the final removal of deoxidation products and other non-metallic inclusions, directly impacting the quality of the cast product. 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
