Key Takeaways & Executive Findings
- •• The freezing–overflow mechanism is confirmed as the primary cause of hook formation in ultra-low carbon steel slabs, with freezing occurring unpredictably and overflow during positive strip time. • The average pitch of oscillation marks (8.693 mm) closely matches the theoretical value (8.889 mm), with a 2% difference attributed to varying overflow degrees, validating the role of mold oscillation. • Higher superheat reduces hook depth, and corner hooks are deeper due to intensified cooling, indicating a negative correlation between hook depth and mold temperature distribution. • A novel 'sine law' method is proposed for controlling hook depth, leveraging the randomness and tendency of hook formation influenced by transient fluid flow and heat transfer.
Abstract
The hook formation mechanism in continuously cast slabs of ultra-low carbon steel was analyzed in detail through numerical calculations and experimental observations using optical microscopy, and its distribution characteristics were determined. Numerical simulations confirmed that the freezing–overflow mechanism is the primary cause of hook formation. They also revealed that the freezing event occurs unpredictably, while the overflow event takes place during the positive strip time. 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%. This discrepancy primarily results from varying degrees of overflow, which affects the morphology of the OMs and the positions 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. Higher superheat hindered hook formation, leading to 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, key factors influencing hook development were analyzed, providing insights into transient fluid flow and heat transfer characteristics within the mold. Transient fluid flow and heat transfer contributed to the randomness and tendency of hook formation. This randomness was reflected in the varying angles of the hooks, whereas the tendency was evident in the negative correlation between superheat and hook length. Based on the randomness and tendency of hook formation and its profile characteristics, a new method for controlling hook depth based on “sine law” is proposed.
1. Introduction
Subsurface hooks, which significantly affect the quality of low-carbon steel slabs, have gained widespread attention in recent years. These unique microstructural features often accompany deep oscillation marks (OMs) beneath the surface (0.5–3 mm) of low-carbon (C < 0.1wt%) slabs [1–3]. During the initial solidification of molten steel within the meniscus region, hooks form and grow from the shell surface towards the interior of the molten steel. This phenomenon causes bubbles, inclusions, and flux to be easily entrapped in hooks, resulting in the formation of defects [4–5]. These defects may lead to surface imperfections, such as slivers and blisters, during subsequent rolling processes. In severe cases, the removal of these defects through grinding or scarfing [6–7] increases production costs and reduces the overall yield of finished products.
To minimize defects associated with the formation of hooks, several previous studies have focused on understanding the mechanisms behind hook formation and the effect of process parameters on these hooks. Emi et al. [8] first discovered the presence of hooks while studying the solidification structure of OMs. Subsequently, extensive research on the mechanism of hook formation was conducted. Three main hypotheses for this mechanism have been proposed: 1) freezing and stripping, 2) shell tip bending and overflow, and 3) meniscus freezing and overflow (freezing–overflow mechanism). The freezing and stripping mechanism results from the damage behavior of the mold oscillation to the shell tip. Savage and Pritchard [9] showed that, during the positive strip time (PST), the upward movement of the mold causes the primary shell to adhere to the mold wall to be torn owing to its high temperature and low ductility. Sato [10] suggested that during the PST, molten steel flows towards the shell tip, freezes against the hot mold face, and forms gaps. During the negative strip time (NST), two solid shells weld together, forming the OM and hooks, and the gaps disappear. However, this mechanism applies only to steel casting using oil lubrication. In the mold with mold powder as a lubricant, no direct contact exists between the molten steel and mold wall. Furthermore, no evidence of hot tearing or welding is observed in the microstruct...
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Wenjie Tong, Sen Luo, Xiaohua Wang, Chunxin Wei, Weiling Wang, Miaoyong Zhu (2025). Hook formation and control mechanisms in continuously cast slabs of ultra-low carbon steel. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3112-z
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Frequently Asked Questions
What is the primary mechanism of hook formation in ultra-low carbon steel slabs?
The primary mechanism is the freezing–overflow mechanism, where freezing occurs unpredictably and overflow happens during the positive strip time, as confirmed by numerical simulations.
How does superheat affect hook formation?
Higher superheat hinders hook formation, leading to a negative correlation between hook depth and superheat. This means that lower superheat results in deeper hooks.
Why are corner hooks deeper than other positions?
Corner hooks are deeper due to intensified cooling at the corners of the mold, which causes a lower temperature and thus promotes more hook growth.
What is the 'sine law' method for controlling hook depth?
The 'sine law' method is a new control strategy proposed based on the randomness and tendency of hook formation, which uses a sinusoidal pattern to modulate process parameters to achieve desired hook depths.
What is the significance of the oscillation mark pitch difference?
The average pitch of oscillation marks (8.693 mm) is about 2% less than the theoretical pitch (8.889 mm), which is attributed to varying overflow degrees, confirming the role of mold oscillation in hook formation.
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