Abstract
Subsurface hooks formation during initial solidification in the continuous casting mould degrades the quality of steel slabs owing to the associated entrapment of non-metallic inclusions. To understand the mechanism for hook-capturing inclusions, the characteristics of hooks and the inclusion distribution around hooks were investigated by etching experiment and electron microscopic observation. The results reveal that compared with the general shell, hooks had a stronger ability to capture inclusions of 20–300 μm, especially for large inclusions above 250 μm, and the capture area was within their maximum depth. By analysing the forces on inclusions of different sizes at the solidification front, it is found that the Marangoni force has an obvious effect on the entrapment of inclusions. Based on a comparison of the inclusion floating inclination angle, the initial shell and hook inclination angle, the mechanism for the entrapment of inclusions by hooks and shell were proposed. According to this mechanism, the number density of inclusions smaller than 240 μm is greater in the hook zone, while inclusions larger than 240 μm are captured only by hooks. In addition, by studying the characteristics of hooks in slabs produced with different casting parameters, several suggestions are put forward for controlling hooks in industrial production: superheat degree can be set greater than 25 °C and molten steel flow rate can be set greater than 0.35 m3/min, and slab surface cleaning technique should be adopted.
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