The defects of billet continuous casting mainly include surface defect reconnection, inclusion and scar, scratch and vibration mark. The internal defects of billet mainly include segregation, subcutaneous bubbles, uneven composition, loose internal inclusion center and transgranular. The reason for the reconnection is that the pouring is interrupted due to the operation failure, and the reconnection defect is formed on the surface of the slab. Slow drawing causes deep vibration marks on the surface of the slab and forms reconnection.
The methods to avoid inclusions and scabs are to ensure the stability of the liquid level of the mold, improve the performance of the protective slag, increase the thickness of the slag layer, stabilize the casting speed, and improve the corrosion resistance of the tundish nozzle and plug.
The reason for the scratch is that there is scrap on the foot roll, the tension straightening roll is uneven or the secondary cooling is uneven, resulting in the deviation of the slab, and the slab is in contact with the tension straightening frame to scratch the slab surface.
The defects of billet continuous casting mainly include internal defects, such as segregation and subcutaneous gas
Bubble, uneven composition, internal slag inclusion, loose center and transgranular.
Causes of segregation: the internal segregation of cast steel mainly includes composition segregation and cooling segregation. The degree of segregation of impurity elements in the billet is determined by chemical composition and cooling rate. When the cooling rate is appropriate, the higher the carbon content of steel, the more serious the segregation is.
Methods to avoid segregation include: slow cooling for steel with high carbon content. For the steel with high segregation control requirements, electromagnetic stirring or vibration can be used.
The reason for the formation of subcutaneous bubbles: hydrogen immersion in molten steel during pouring.
Methods to avoid subcutaneous bubbles: deoxidize the molten steel, lay protective slag during pouring, avoid molten steel pollution, and bake the molten steel ladle and the middle thoroughly to ensure complete drying. Prevent the protective slag from being affected by moisture, avoid water seepage and water leakage of the mold, and replace it in time in case of leakage.
Causes of uneven composition: insufficient mixing of molten steel due to short argon blowing time, insufficient insertion depth of argon blowing gun during top argon blowing and insufficient bottom blowing pressure during bottom argon blowing, resulting in uneven composition of molten steel.
MMethods to avoid uneven composition: argon blowing treatment of molten steel shall be carried out in strict accordance with the process requirements.
Causes of internal slag inclusion: the molten steel is impure, and there are many impurities and non-metallic oxides in the molten steel. The protection of molten steel during pouring is not good, resulting in serious secondary oxidation. The liquid level of the mold fluctuates seriously, and impurities are involved in the billet.
Internal slag inclusion avoidance method: argon blowing treatment shall be carried out on the molten steel in strict accordance with the process requirements to promote the full floating of inclusions. Strengthen the protection of molten steel, such as ladle covering agent and steel flow protection pouring, so as to prevent the secondary oxidation of molten steel.
Reasons for the formation of central porosity and transgranular: determined by the solidification characteristics of steel, the higher the carbon content, the easier it is to have porosity and transgranular at the center. Secondary cooling rate. The greater the secondary cooling rate of the billet, the easier it is to have porosity and transgranular at the center. Pull too fast. The faster the drawing speed, the longer the liquid core, the worse the feeding capacity of molten steel, and the more prone to porosity and transgranular.
Methods to avoid central porosity and transgranular: select appropriate cooling speed and blank drawing speed, and adopt low-temperature rapid casting process.