Mould protection slag plays the following five basic functions in continuous casting production:
It can prevent the secondary oxidation of liquid steel, especially the meniscus. The protective slag is added to the liquid steel surface in the crystallizer, forming a liquid slag layer of a certain thickness after melting, and uniformly covering the liquid steel surface. The liquid slag layer plays a role of isolating the liquid steel from the air, so as to prevent the secondary oxidation of liquid steel.
② Thermal insulation. The radiant heat loss and superheat of molten steel can be reduced by adding protective slag on the surface of high temperature molten steel due to the appearance of three or more layers. Therefore, in the protection slag operation, it is required to have a certain thickness of powder slag layer on the liquid slag layer, that is, black slag operation. The meniscus temperature of crystallizer can be increased by increasing the heat preservation property of protective slag, and the formation or excessive growth of slag ring can be reduced. Especially when casting high carbon steel, it is beneficial to improve the lubrication of billet to improve the thermal insulation performance of protective slag. Increasing the amount of carbon in the slag, changing the type of carbon material, adding heating element or reducing the volume density of the slag can improve the heat preservation of the slag. The particle morphology of protective slag also has a certain effect on thermal insulation. The thermal insulation of powder slag and hollow particle slag is better than that of columnar slag. With the development of high-speed continuous casting slag and ultra-low carbon steel continuous casting slag, hollow particle slag with lower volume density is widely used to avoid the deterioration of heat preservation of the slag.
(3) the ability of protection slag to absorb inclusion. In order to prevent the floating inclusions from being sucked into the solidification shell and causing defects on the surface or under the skin of the billet, the liquid slag layer formed by the melting of the protective slag should be able to absorb and assimilate the floating non-metallic inclusions in the molten steel. Different steel grade floatation of inclusions, the effects on the physical properties of powder is different, such as the inclusion of mild steel, cast steel containing rare earth or containing titanium steel, steel and high melting point of rare earth oxide, titanium oxide and titanium nitride affect the performance of protecting slag, etc., these protective slag inclusions can cause the change of the properties such as viscosity and melting point or alkalinity.
④ The protective slag has the function of uniform heat transfer. Metallurgical workers have realized that the slag film can control the heat transfer from billet to mold. Under the condition that no protective slag is used (such as rapeseed oil as lubricant), the cooling speed of the shell is high at the upper part of the mold due to the contact between the shell and the mold wall. At the lower part of the crystallizer, the shrinkage of the billet shell produces an air gap, resulting in an increase in thermal resistance and a decrease in heat output. Meanwhile, the heat transfer of the air gap is very uneven. If the molten slag can flow evenly into the mold wall and solidified billet shell, a uniform slag film can be formed, which can reduce the upper heat transfer rate and increase the lower heat transfer rate, so as to improve the uniformity of heat transfer and improve the quality of billet.
(5) improve the lubrication of billet. The slag film formed between the mould wall and solidified billet shell can act as lubricant, reduce the resistance of drawing billet and prevent the bond between the billet shell and mould wall. With the continuous improvement of the casting speed, the increase of mold vibration frequency makes it difficult for the protective slag to flow into the channel between the casting slab and the mold, the consumption of protective slag decreases, the friction resistance of the casting slab increases, and when the resistance exceeds the strength of the shell, the steel leakage accident will be caused. In addition, the increase of friction will lead to the increase of longitudinal crack index, and the lubrication of slag film becomes more and more important, which has become the main problem to be solved in continuous casting production. 
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