Based on its function in the core process of continuous casting, mold flux can be divided into two stages: melting and solidification.
The first stage is marked by the addition of mold flux into the mold, where it gradually melts under heat to form liquid slag. This stage mainly involves the transformation of mold flux from a solid phase to a liquid phase, hence it is called the melting stage. During the melting stage, the mold flux forms a layered structure of powder-sintered-liquid slag: the powder layer has low density and slow heat conduction, mainly providing thermal insulation; the liquid slag isolates the air, preventing secondary oxidation of the molten steel.
The second stage is marked by the seepage of liquid slag from the meniscus into the gap between the mold and the billet shell, until the slag film exits the mold. This stage mainly involves the solidification of liquid mold flux under a high temperature gradient, hence it is called the solidification stage. The solidification stage protects the normal growth of the billet shell by forming a solid-liquid composite slag film: the good lubricating properties of the liquid slag film ensure the smooth exit of the cast billet from the mold; the solid slag film controls the heat transfer rate, protecting the uniform growth of the billet shell.
Both melting and solidification protect the steel flow, so melting and solidification constitute a complete process of protecting the steel flow.
As the temperature of the mold flux increases, direct reactions first occur between the solid phases of the powder slag. The reaction temperature is far below the melting point of the reactants or their eutectic point. Because some fluxes exist in the mold flux, such as alkali metal carbonates, oxides, fluorides, and glass, the temperature at which they begin to form a liquid phase is far below the eutectic temperature of the main components. These small amounts of liquid phase play a significant role in sintering. The liquid phase wets the surface of the solid particles, and surface tension causes the powder slag particles to approach each other, tighten, and solidify, forming a sintered phase. As the temperature further increases, near the melting point of the mold flux, the sintered phase gradually melts into small liquid droplets. These small droplets come into contact with each other and aggregate into larger droplets, finally forming a liquid slag layer.
The melting rate of the mold flux in the mold is mainly affected by the following factors:
Longitudinal heat flux density. It is affected by various casting parameters, such as pulling speed, superheating degree, and vibration;
Free carbon content in the protective slag. Carbon particles isolate mineral particles and slow down the coalescence of molten slag droplets. Therefore, the higher the carbon content, the longer the coalescence time and the slower the melting speed;
Type of carbon and size of carbon particles. The larger the carbon particle size, the faster the melting speed;
Carbonate components in the protective slag. The decomposition of carbonates can stir the slag layer, thereby increasing the thermal conductivity of the slag layer and accelerating the melting speed. The order of influence of several common carbonates on the melting speed is: MgCO3 > Li2CO3 > CaCO3 > Na2CO3 > BaCO3;
Particle size of the protective slag. The smaller the particle size, the faster the melting speed;
Melting range of the protective slag.
FAQ
Why must carbonaceous materials (such as graphite or carbon black) be added to the protective slag to control the melting process?
A: Carbonaceous materials act as a “skeleton” in the protective slag. Because carbon has an extremely high melting point, it coats the surface of low-melting-point slag particles, preventing them from agglomerating into large droplets too quickly. By adjusting the type and content of carbon, the melting rate of the protective slag can be precisely controlled. Without carbon, the protective slag would melt instantly, resulting in an excessively thick and rapidly consumed liquid slag layer, making it impossible to maintain a stable structural layer.
What is the ideal “liquid slag layer thickness”? What are the consequences of abnormal thickness?
A: The ideal liquid slag layer thickness is typically maintained between 8mm and 15mm.
Too thin (<5mm): This leads to insufficient lubrication, increases friction between the billet shell and the copper plate, and easily induces sticking and leakage.
Excessive thickness (>20mm): While providing good lubrication, it may lead to uneven slag flow to the side of the billet shell. Furthermore, increased hydraulic pressure may cause fluctuations in the liquid level in the copper mould tube, resulting in slag entrapment defects.
