Surface defects in continuous-casting slabs
Surface defects in continuous-casting slabs are a major problem that affects product quality and operational safety, resulting in increased costs, reduced output and a lack of product consistency. In order to reduce these problems and increase the production yield, it is important to have a good understanding of the causes and effects of the surface defects.
The most common surface defects on continuous-casting slabs are seam defects, surface cracking and pocks, also known as surface pores. Seam defects are caused by poor lubrication of the slab, resulting in surface cracking which occurs due to thermal stresses and micro-defects from the formation of the semi-solidified surface. Pocks are a result of vapour bubbles trapped in the slab, which form a porous surface.
One of the main causes of surface defects in continuous slabs is the quality of the mould lubricant. Poorly installed or poorly maintained lubrication systems can lead to insufficient lubrication of the surface of the slabs, resulting in improper or insufficient cooling of the surface and forming the seam defect.
In addition to insufficient lubrication, the composition and distillation of the lubricant can lead to surface defects. Low-quality lubricants that are contaminated with, for example, carbon particles, are more likely to form surface defects. Furthermore, when the lubricant has been distilled for a longer period of time, the cooling effect on the surface becomes weakened, leading to a reduced production yield and surface defects.
The temperature of the slab at the time of casting can also contribute to surface defects. If the temperature of the slab is too high, the cooling rate of the surface will be insufficient, causing the seam defects to form. Additionally, the temperature of the mould and the laminar flow of the lubricant can also cause surface defects. If there are large temperature differences between the mould and the lubricant, it can affect the effective lubricant coverage, leading to poorer surface protection.
Finally, high casting speeds can lead to surface defects, as the cooling rate of the surface is decreased due to the higher casting speed. This can cause thermal stresses and pock formation on the surface as the surface is unable to cool properly.
In order to reduce the occurrence of surface defects in continuous slabs, it is important to maintain the lubrication systems and keep the lubricant clean. It is also essential to preheat the slabs before the casting process and to adjust the casting speed and mould temperature according to the slab temperature in order to ensure an efficient cooling rate for the surface of the slabs.
By following the above mentioned preventive measures, one can reduce the occurrence of surface defects in continuous-cast slabs. Quality control techniques such as ultrasonic testing and X-ray testing can also be used to detect defects early and prevent them from becoming more severe.
