Mechanism of hook formation and control in continuously cast slab of ultra-low carbon steel

The mechanism of hook formation was clarified in detail by numerical calculation and experimental observation with optical microscopy, and its distribution characteristics were studied. The results of numerical simulations confirmed that the Freezing-overflow mechanism was the primary cause of hook formation, and revealed that the occurrence of the freezing event was uncertain, while the overflow event occured at the positive strip time (PST). The average pitch of oscillation marks (OMs) on the slab surface was 8.693 mm, while the theoretical pitch was 8.889 mm, with a difference of approximately 2%. The main reason for this discrepancy lied in the varying degrees of overflow, which affected the morphology of the OMs and the position of their deepest points. Notably, this result further confirmed that the freezing and overflow in the meniscus were indeed caused by the periodic oscillation of the mold. A higher superheat hindered hook formation, resulting in a negative correlation between the hook depth distribution around the slab and the temperature distribution within the mold. Therefore, the depth of the corner hook was greater than that of other positions, which was caused by the intensified cooling effect of the corner. Moreover, an analysis was conducted to identify the key factors influencing the development of hooks, drawing insights from the transient fluid-flow and heat transfer characteristics within the mold. Transient fluid-flow and heat transfer contributed to the random and tendency of hook formation. The random aspect manifested in the angles of the hooks, while the tendency aspect was evident in the negative correlation between superheat and hook length. Based on the random and tendency of hook formation and its profile characteristics, a new method of controlling hook depth based on "Sine law" was proposed.