Cracks in Continuous Casting Billet
Stress cracks are one of the more common defects that occur in continuous casting billet. These cracks can form during the molten steel’s transfer from the ladle to the billet mold and can continue to develop in subsequent cooling and handling of the casting billet. The cause of these stress cracks is generally attributed to thermal cycling and stresses that are formed when the steel passes through different temperature zones along the casting train. In order to understand the causes and potential solutions of these stress cracks it is important to understand the different sources that can cause these issues.
The thermal cycling of the cast billet is the primary cause of stress cracks in the billet. When the steel is transferred from the ladle it passes through the different temperature zones in the casting train, first entering the tundish, then the mold, and finally the chill zone. During this process the steel is subjected to multiple thermal cycles as it cools down from the temperatures in the ladle to the temperatures of the chill zone. This thermal cycling can create tension between the surface and the core of the billet as the surface cools down at a faster rate than the internal temperature of the steel. This tension can cause the surface of the billet to expand, creating tension in the steel that can eventually lead to stress cracks.
In addition to the thermal cycling during the casting process, the subsequent rolling process can cause additional cracks in the billet. During the rolling process, the cooling rate of the steel is increased and the temperature gradients between the surface and the core can cause tensile stresses that can eventually lead to stress cracks. This is most likely to occur to billets that have had longer exposure to high temperatures during the casting process.
In order to reduce and prevent the formation of stress cracks in continuous casting billet there are several things that must be done. Firstly, it is important to ensure that the steel is heated in a uniform manner so that the internal and external temperatures of the steel remain close to each other. It is also important to ensure that the cooling rates of the steel remain consistent during the casting process so that the thermal cycling of the steel is minimized. Lastly, it is important to ensure that the rolling process is performed in a manner that minimizes the cooling rate of the steel on the surface so that there is no significant difference between the internal and external temperatures of the billet. By following these steps, it is possible to reduce the risk of stress cracks in continuous casting billet.
