Classifications of defects in continuous casting billet
The defects of continuous casting billet can generally be divided into surface defects, internal defects and shape defects. Surface defects include: surface cracks (including transverse, longitudinal, corner and face cracks), bubbles, slag inclusions, double casting, skin flipping, abnormal vibration marks, leakage, cold splash, scratches, etc. Internal defects include: internal cracks, non-metallic inclusions, center segregation and center looseness, etc. Shape defects include: diamond deformation also known as “de-square”, longitudinal and transverse depressions, etc.
Main causes of defects
The main causes of surface cracks are: too high casting temperature, too fast pulling speed, crystallizer deformation, too fast and uneven primary cooling. The second reason is that the secondary cooling is too intense, the secondary cooling nozzle is not in the right position, and the temperature is too low when the billet is straightened. In addition, subcutaneous blisters will form when the molten steel is not completely deoxidized or contains hydrogen; the molten steel temperature is too low, the pulling speed is too slow, the crystallizer protective slag is poor, the casting is interrupted, and the molten steel splashes, etc., will also cause other surface defects accordingly.
The main reason for internal cracks is that the thermal stress and mechanical stress that the billet bears in the secondary cooling zone are too large; in addition, too fast pulling speed will cause central shrinkage.
The main reasons for de-squareness are mold deformation, too high casting temperature, too fast pulling speed, improper support, etc.
It can be considered that surface defects originate from the crystallizer, and internal defects originate from the secondary cooling zone. Therefore, maintaining the crystallizer, stabilizing various operations in the crystallizer, and providing appropriate molten steel to the crystallizer are necessary conditions to avoid surface defects; and maintaining the secondary cooling equipment and stabilizing the secondary cooling operation are important ways to avoid internal defects.
Factors influencing defects of billet
The factors affecting the defects of continuous casting billets can be summarized into three aspects: molten steel, process, and equipment. Molten steel conditions mainly refer to the deoxidation of molten steel, carbon content, manganese-silicon ratio, and impurity element content; the operating process mainly refers to molten steel temperature, pulling speed, protective casting method, cooling water distribution, argon blowing and stirring of molten steel, wire feeding, etc.; equipment status mainly refers to the operating status of major online equipment such as crystallizers and secondary cooling devices. The continuous casting process is at best a maintenance process, and it is impossible to correct the problems of steelmaking and equipment. Therefore, “steelmaking technology is the foundation.”
The IF+LOD+LRF steelmaking process designed by Shanghai Metallurgy Equipment Group has metallurgical functions such as deoxidation, dephosphorization, desulfurization, decarburization, argon blowing, and composition adjustment, which provides a solid foundation for producing high-quality steel billets to meet the changing market demand.
