This paper describes the thermomechanical and thermochemical erosion mechanisms of continuous casting nozzle slides.
Keywords:slidenozzle; corrosion mechanism; thermomechanical corrosion; thermochemical corrosion
Due to their different structures, uses, and conditions of use, refractory materials for skateboards show different damage forms. For example, two-layer skateboards and three-layer skateboards are different. The three-layer skateboards are fixed by the upper and lower slides. Both sides of the middle slide are sliding surfaces. Therefore, the surface is prone to fluffing. Once the crack expands, the casting hole will peel off more seriously in the sliding direction; while the upper and lower plates of the two-layer skateboard are fixed on the upper and lower nozzles through snap-in buckles, and there is only one sliding surface, so under the same conditions, the casting hole peeling off in the sliding direction will be better.
The difference between the damage of straight-forward reciprocating and rotary skateboards lies in the difference in the direction of cracking, but the basic damage forms are not much different. The service conditions of the ladle and the tundish are different, so the refractory material for the slide plate also has different damage forms. First, the slide plate of the tundish hardly interacts with the slag; and the refractory material of the sliding nozzle device for the tundish is pre-heated to about 800°C, and the temperature difference at the beginning of steel casting is from 700-800°C at the beginning to the casting temperature (1520-1560°C); When in use, the temperature difference in one cycle is from 100-400°C to 1600-1670°C.
The tundish slide is less affected by thermal shock, and the main reason for its damage is the wear caused by the steel flow or the blockage caused by the opening and closing of the fixed throttle. These factors will cause the difference in the form and degree of corrosion of the ladle slide and the tundish slide. In addition, due to the different types of cast steel and the different casting methods (die casting or continuous casting), the corrosion conditions and degrees of corrosion are also different. According to many analyzes of the slide plate damage process, there are three main reasons, namely, thermomechanical erosion, thermochemical erosion and the influence of operational factors.
Thermomechanical damage
During the use of the slide plate, thermomechanical corrosion is first produced. The temperature of the ladle slide plate before work is very low. During casting, the inner hole of the slide plate suddenly comes into contact with high-temperature molten steel (1600°C) and suffers a strong thermal shock (the temperature change is about 1400°C). Therefore, a tensile stress exceeding the strength of the slide plate will be generated outside the casting hole, resulting in the formation of radial cracks centered on the casting hole.
The appearance of cracks is conducive to the diffusion, accumulation and penetration of foreign impurities, and accelerates chemical erosion. At the same time, the chemical corrosion reaction promotes the formation and expansion of cracks, and this cycle gradually expands and destroys the casting holes of the slide plate. Moreover, the scouring of high-temperature molten steel will damage the refractory material close to the friction part with molten steel, causing the material to peel off.
Thermomechanical damage mainly includes theories of thermal shock fracture theory and thermal shock damage theory. The thermal shock fracture theory mainly focuses on the crack nucleation problem, and the thermal shock damage theory mainly focuses on the crack growth problem.
Through a lot of practical research, it is found that the thermal shock resistance of commonly used sliding plate refractories is compared, and the materials are Al2O3-C, ZrO2, spinel-C and MgO-C.
Thermochemical attack
Thermal and chemical erosion is another major cause of slide material damage. The refractory materials used for slide plates are exposed to high-temperature molten steel and slag during use, and a series of chemical reactions occur to cause chemical erosion. According to the different chemical damage mechanisms of different steel types to the slide and the different conditions of use, selecting the slide of the corresponding material can increase the service life of the slide and reduce the cost of refractory materials.
For example, slide plates made of carbon composite refractories: MgO-C, MgO Al2O3-C, and ZrO2 slides have poorer thermal shock stability than Al2O3-ZrO2-C, and are suitable for use as tundish slides under special steel conditions. Since Al2O3 and MgO·Al2O3 can react with Ca to form low-melting substances, MgO-C or ZrO2 sliding plates should be used for calcium-treated steels; since Al2O3 and ZrO2 can react with FeO to form low-melting substances, for high-oxygen steels, MgO-C or MgO·Al2O3-C sliding plates should be selected. According to the practice of Baosteel, Al2O3-ZrO2-C sliding plate can be used as a tundish sliding plate to realize multi-furnace continuous casting when casting killed steel, which is beyond the reach of other material sliding plates.
Regardless of the material, its thermochemical erosion generally includes the following forms:
(1) Oxidation erosion of carbon-containing slides;
(2) [Ca], [Mn], [Fe] in molten steel corrode refractory materials;
(3) Chemical changes in the substances contained in the refractory itself.