What is the "three-layer structure" for protecting slag?
Release time:
2021-09-02
In continuous casting production, except for small-section billets, most use protective slag (with submerged entry nozzle). Practice has proved that this process is one of the important conditions to ensure the quality of slabs and normal operation. In the early 1970s, this technology was successfully applied to slab continuous casting in China, solving the problem of longitudinal cracks in slabs, and then gradually promoted. In recent years, a series of protective slag have been introduced, and some special steel continuous casting protective slag have been developed, creating the necessary conditions for the further development of continuous casting in China.
In order to fully exert the five functions of mold powder, it is necessary to make the powder added to the mold form a "three-layer structure". The key to forming the "three-layer structure" is to control protective slag the melting speed of the powder, that is, the slag powder added to the steel surface should not melt into liquid at once, but should melt gradually. For this reason, carbon particles are usually added to protective slag as a regulator of melting rate.
The speed of controlling the melting speed of carbon particles depends on the type and quantity of carbon particles added. Carbon is a high-temperature resistant material. Very fine carbon powder is adsorbed around the slag particles, separating the slag particles from each other, hindering the contact and fusion between the slag materials, and slowing down the melting speed. If the amount of carbon powder added is insufficient, the slag layer temperature has not yet reached the sintering temperature of the slag, the carbon particles are burned out, the sintering layer is developed, the melting speed is too fast, and the liquid slag layer is too thick. If too much carbon powder is added, some carbon particles will still exist after the slag is completely melted, which will cause the sintering layer to shrink and make the sintering layer too thin. When the amount of carbon powder is moderate, some carbon particles are burned out in the sintering layer, and the remaining molten slag is effectively controlled by the carbon particles, so that a sintering layer with an appropriate thickness of liquid molten slag layer can be obtained.
Carbon materials include graphite and carbon black. Graphite particles are coarser, with a particle size of 60~80μm, and have poor separability and retardation, but their initial oxidation temperature is higher (about 560℃), and the oxidation rate is slower, and their ability to control the melting rate in the high-temperature zone is stronger. Carbon black has an amorphous structure, small particles (0.06~0.10μm), strong separation and retardation, low initial oxidation temperature (500℃), and fast oxidation rate. Therefore, carbon black has a stronger ability to control the melting rate in the low-temperature zone of the slag layer, but its control efficiency is lower in the high-temperature zone. Even if the dosage is increased, the improvement effect is limited.
Generally, the amount of carbon powder added is 4~7%.
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