*7.1. Method to Eradicate Center Cavity Defects*

As discussed above, in the HSBC process, employing a double impingement feeding system, the molten metal streams towards the center. Keeping in mind that at lower metal heads (<50 mm), the quantity of the molten metal delivered onto the moving belt is also low, as per Equation (1). As observed experimentally, under the rapid heat extraction rate to the moving belt (i.e., 500 K/sec) [3], the molten metal passing over the moving belt, tends to solidify almost instantaneously. Since the molten metal does not have enough time to level off before the completion of solidification, this results in a strip with a thicker edge, and a comparatively thinner center.

The opposite is true for high metal heads inside the tundish (>50 mm). In this case, the velocity of the molten metal exiting the refractory nozzle slot outlet is high enough to trigger a strong net inward flow. Under these conditions, the molten metal will have enough time to fill the center empty region and to evenly spread throughout the thickness of the strip before the completion of solidification. This helps to eliminate any center cavity defect and to achieve a uniform thickness of the strip across its width.

$$\mathbf{V} = \mathbf{C}\_{\text{D}} \sqrt{2 \mathbf{g} \mathbf{h}} \tag{1}$$

where V is the velocity, h is the molten metal head inside the tundish, and CD is the coeffecient of discharge.
