Continuous casting mold slag is a key auxiliary material that coats the surface of the molten steel in the mold during the continuous steel casting process. It is known as the “blood of continuous casting.” Its core functions are to stabilize the molten steel in the mold, improve the quality of the ingot, and ensure smooth continuous casting. It can be divided into the following five core functions, each of which directly impacts continuous casting efficiency and ingot quality:
Isolating the steel from air to prevent secondary oxidation
Molten steel is at high temperatures (above approximately 1500°C) in the mold. When in contact with air, it is oxidized by gases such as O₂ and N₂, forming non-metallic inclusions such as Al₂O₃ and SiO₂. These inclusions remain in the ingot, causing cracks and inclusion defects, and reducing the mechanical properties of the steel. When mold slag coats the surface of the molten steel, it forms a dense film or solid slag layer, completely isolating the molten steel from the atmosphere. Furthermore, the deoxidizing components (such as CaO and Al₂O₃) it contains absorb some of the formed inclusions in the molten steel, purifying the steel and improving the purity of the ingot.
Absorbing Inclusions and Purifying Molten Steel
During the continuous casting process, the molten steel inevitably contains small amounts of inclusions that have fallen from the charge and refractory materials, as well as oxide inclusions generated by deoxidation. The molten slag formed by melting the mold slag has excellent “adsorptive properties.” Its viscosity and surface tension are precisely controlled, allowing it to act like a “sponge,” absorbing and dissolving inclusions on and near the surface of the molten steel. It is then discharged from the mold as the slag layer is renewed (either by skimming or by removing a small amount adhering to the surface during casting), preventing inclusions from remaining within the ingot and causing defects.
Insulation to maintain a stable molten steel temperature
The mold is a water-cooled copper jacket. Molten steel dissipates heat rapidly within the mold. If the temperature drops too quickly, the molten steel’s viscosity increases, fluidity deteriorates, and a “shell” may form on the upper portion of the mold, hindering steel replenishment and causing problems such as leaks and nodules.
The solid and molten layers of mold slag have low thermal conductivity, effectively preventing the molten steel from dissipating heat to the mold walls, reducing temperature loss, maintaining temperature uniformity within the mold, and ensuring good fluidity, creating conditions for stable solidification of the ingot.
Lubricate the ingot and mold walls to reduce friction
During continuous casting, the ingot is pulled downward from the mold at a constant speed (usually 0.5-5 m/min). This creates intense friction between the ingot shell and the mold copper walls. Excessive friction can cause the shell to be “stretched” (forming longitudinal or transverse cracks), or even tearing the shell, leading to leaks. Mold slag forms a uniform film between the molten steel and the mold wall. This film fills the tiny gaps between the billet shell and the copper wall, reducing mechanical friction. Furthermore, the flow of the film dissipates frictional heat, preventing localized high temperatures from causing the billet shell to stick. Furthermore, some mold slag penetrates the tiny pores of the billet shell, further enhancing lubrication and ensuring smooth billet drawing.
Controlling the heat transfer rate of the molten steel in the mold to improve billet solidification quality
The quality of billet solidification is directly dependent on the uniformity of heat transfer within the mold. If the molten steel transfers heat to the mold wall too quickly, the billet shell will solidify quickly but with uneven thickness, which can easily lead to cracks. If the heat transfer is too slow, the billet shell will be too thin, potentially causing leaks.
By regulating the thickness and thermal conductivity of the mold slag film, mold slag precisely controls the heat transfer rate of the molten steel to the mold, ensuring uniform and slow solidification of the billet shell from the surface to the interior, forming a dense, defect-free solidification structure and reducing defects such as longitudinal cracks, corner cracks, and subcutaneous bubbles.
Additional Information: Functional Emphasis of Different Types of Mold Flux
Depending on the type of continuous casting steel (e.g., mild steel, alloy steel, stainless steel) and the cross-section of the ingot (slab, billet, round billet), the mold flux composition (CaO/SiO₂ ratio, Al₂O₃, MgO content, etc.) and physical properties (melting point, viscosity, melting rate) are tailored to each type, with specific functional emphases:
Bill casting mold flux: Prioritizes lubricity and rapid melting to prevent corner cracks;
Slab casting mold flux: Prioritizes heat transfer uniformity and inclusion absorption to reduce surface defects;
Alloy steel mold flux: Requires increased viscosity and melting point to accommodate the poor fluidity of the molten steel, while also enhancing deoxidation and inclusion absorption.
In summary, the various functions of continuous casting mold flux work in synergistic fashion. It serves as a “protective layer” and “purifier” for the molten steel, a “lubricant” between the ingot and the mold, and a “heat transfer regulator.” It is one of the core materials for ensuring a stable continuous casting process and producing high-quality ingots.
