In the early stage of continuous casting process, the casting method was open casting, and rapeseed oil was used for protection casting. However, with the increase of continuous casting billet steel types and the increase of drawing speed, higher requirements were placed on the quality of the billet and the service life of the crystallizer. Rapeseed oil could no longer meet the development needs of continuous casting. In order to adapt to the development of continuous casting technology, continuous casting protection slag was developed in the 1960s.
Continuous casting protection slag plays an important role in the continuous casting process. In the process of exerting its function, there are many critical stages. As shown in Figure 1, in each stage, the continuous casting protection slag must perform well, otherwise it will reduce the surface and internal quality of the continuous casting billet. Selecting and applying appropriate continuous casting protection slag can prevent quality defects such as longitudinal cracks on the surface of the continuous casting billet, and can prevent accidents such as bonding and steel leakage during the continuous casting production process.
Figure 1 The stage in which the mold protection slag plays its role and its influence on the quality of the casting and the adhesion leakage.
During the continuous casting process, after the protection slag is added to the crystallizer, it is heated on the surface of the molten steel to sinter and melt, forming a liquid slag layer. The protection slag near the liquid slag layer has been sintered into a transition layer before it reaches the melting temperature. Above the transition layer is the original (solid) slag layer. This is what we usually call the three-layer structure of the protection slag – liquid slag layer, sintering layer, and original (solid) slag layer, as shown in Figure 2. The above-mentioned liquid slag and the molten steel form a meniscus near the wall of the crystallizer. Under the action of crystallizer vibration, gravity and capillary adsorption, the liquid slag flows through the meniscus into the gap between the crystallizer wall and the billet shell to form a slag film. The liquid slag pool on the surface of the molten steel and the liquid slag are consumed between the crystallizer wall and the billet shell to form a dynamic balance.
Figure 2 Three-layer structure of protective slag in the crystallizer
When protective slag was first used, people had a relatively simple understanding of the role of protective slag. After the joint efforts of metallurgical workers, they have a deeper understanding of the function and principle of protective slag. Protective slag has the following five major functions in the continuous casting crystallizer:
Insulation and heat preservation to reduce heat loss of molten steel
The continuous casting protective slag must have good insulation and heat preservation properties, which can inhibit the formation of bridges and crusts in the crystallizer during continuous casting, and at the same time increase the meniscus temperature, so that the shell growth at the meniscus of the ingot is stable, and at the same time, the meniscus can have a larger curvature radius.With the increase of the curvature radius of the meniscus, the bubbles and oxide inclusions floating from the molten steel at the bottom of the crystallizer are not easily captured by the meniscus, and are more likely to float to the liquid slag layer, thereby making the molten steel cleaner.
Isolate air to prevent secondary oxidation
Under the high temperature of the molten steel, the protective slag will quickly form a liquid slag layer, which can hinder the diffusion of air to the molten steel surface. The three-layer structure of the protective slag is used to isolate oxygen and nitrogen, so that the molten steel is free from secondary oxidation and absorption of nitrogen. It not only prevents the secondary oxidation of the molten steel and avoids the oxidation of alloy elements in the molten steel, but also prevents the molten steel from absorbing nitrogen from the air, which is beneficial to improve the cleanliness of the molten steel.
Lubricate the ingot to reduce the adhesion of the ingot
In order to reduce the occurrence of cracks and steel leakage accidents and increase the service life of the crystallizer, the copper plate of the crystallizer and the ingot must be well lubricated to reduce the mutual friction. At the meniscus around the crystallizer, due to the vibration of the crystallizer and the capillary action of the gap between the ingot shell and the copper plate, the liquid slag formed by the melting of the protective slag is sucked in and fills the gap between the copper plate and the ingot shell, forming a slag film, which plays a lubricating role.
Improve the heat transfer of the crystallizer
In the crystallizer, air gaps are generated due to the contraction of the ingot shell, which increases the thermal resistance. Adding protective slag to fill the air gap with a uniform slag film can reduce the thermal resistance of the air gap, significantly improve the heat transfer of the crystallizer, and make the ingot shell grow evenly. According to actual measurements, the thermal conductivity coefficients are: air gap about 0.09W/(m2·K), slag film about 1.2W/(m2·K), pure copper about 389W/(m2·K), that is, the slag film is about 1/325 of pure copper, 13 times larger than the air gap.
Absorption of non-metallic inclusions
Research shows that adding alkali metal compounds such as Na+, K+, Ba2+ with relatively large ionic radius to continuous casting mold slag can destroy and break the structure of large network bodies, thereby reducing the viscosity of the mold slag and improving the ability of liquid slag to absorb inclusions. Adding F- with a radius similar to O2- to the mold slag can also improve the absorption capacity of continuous casting mold slag for inclusions.
