Oxidation Barrier (Liquid Slag Layer)
The liquid slag layer formed on the molten steel surface acts as a physical seal that isolates molten steel from air. This prevents secondary oxidation and nitrogen pick-up, and avoids direct contact between liquid steel and the powder/sinter layer.
For low-carbon and ultra-low-carbon grades, controlling carbon pick-up is critical. A stable liquid slag layer together with low-FeO slag chemistry reduces surface carburization and improves slab cleanliness. In practice, “black slag operation” with a continuous powder/sinter cover is used to keep the meniscus protected.
Thermal Insulation (Powder / Sinter / Solid Slag Layer)
The granular and sintered layers on top of the liquid slag reduce radiative heat loss from the molten pool. Good insulation keeps the meniscus hot enough to avoid premature freezing, “cold steel” formation, slag rim growth, and uneven shell initiation.
Maintaining meniscus temperature also improves initial shell plasticity and helps prevent surface defects such as pins, scars, and uneven oscillation marks. Insulation performance depends on powder thickness, bulk density, particle structure, and carbon addition rather than only the liquid slag layer.
Inclusion Absorption and Steel Cleanliness (Liquid Slag Layer)
The liquid slag absorbs deoxidation products, Al₂O₃ clusters, refractory erosion particles, and metallic inclusions arriving at the steel–slag interface. Effective assimilation depends on slag viscosity, basicity, melting behavior, and interfacial properties.
If the slag becomes saturated or its viscosity is poorly matched to casting speed, absorption capacity drops and re-entrainment or slag entrapment may occur. Thus slag design must balance fluidity with stability during continuous consumption.
Lubrication and Sticking Prevention
Liquid slag infiltrates the gap between the mould wall and the solidifying shell, forming a thin liquid slag film near the shell and a solid slag film near the copper plate. The liquid film provides lubrication, lowers friction, and prevents shell sticking or breakout.
Lubrication quality is governed by:
- liquid slag layer thickness
- slag viscosity
- casting speed
- mould oscillation
- slag consumption
A common practical indicator is the viscosity–casting-speed match; poor matching leads to high friction, sticker breakouts, star cracks, or irregular slag films.
Heat-Transfer Control
The solid slag film controls horizontal heat extraction from shell to mould. Heat transfer is influenced by solid slag film thickness, crystallinity, roughness, gas gaps, and thermal contact resistance.
For peritectic, medium-carbon, or crack-sensitive steels, excessive heat extraction at the meniscus intensifies the peritectic reaction and increases longitudinal surface-crack risk. Higher basicity, controlled crystallization, and thicker solid slag film can reduce heat flux.
However, stronger heat suppression often thins the liquid slag film and worsens lubrication. Therefore the key optimization target is not “maximum insulation” or “maximum lubrication” alone, but the balanced slag film structure for a given steel grade and casting speed.
