Three-Dimensional Distribution of Hooks in Al-Killed Low-Carbon Continuous Casting Steel Slabs
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AS a subsurface solidification-related structure, hooks[1–3] often occur near oscillation marks[4–10] in low-carbon steel and can be revealed through etching the longitudinal section vertical to oscillation marks. Hooks form at meniscus during mold oscillation and usually locate within 2 to 3 mm below the surface of the continuous casting (CC) slab. With high liquidus temperature and thin mushy zone, low-carbon steel grades (< 0.1 pct) are prone to the formation of hooks. Based on the deflection angle of the hook to the inner slab, hooks are classified as curved type and straight type.[1,3,11–13] Hooks, especially the curved hooks, can capture the mold flux, argon bubbles, and floating inclusions[14] near the meniscus in steel,[15,16] which may
XUBIN ZHANG, WEN YANG, SHENGDONG WANG, and LIFENG ZHANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, China. Contact emails: yangwenfl[email protected]; [email protected] QIANGQIANG WANG is with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. Manuscript submitted February 5, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
deteriorate the surface quality of the slab. Burning the surface of the slab is conducted in the plant to remove the defects from hooks, resulting in high productivity loss.[11] During the past several decades, many studies on the influence of operating parameters on the formation of hooks have been conducted. Measures of the adoption of mold powder with higher surface tension, higher viscosity, and lower solidification temperature,[17] the employment of triangular or nonsinusoidal mold oscillation, the application of mold material with low thermal conductivity,[1] and electromagnetic fields for initial solidification control[18] have been presented to decrease the defects in the slab caused by severe oscillation marks and hooks. Besides, the delivery of molten steel with much more superheat tends to make hooks become thinner and shallower, as does the increase of casting speed.[19] The formation mechanisms of hooks proposed so far can be classified into three main groups: (1) discontinued shell growth-based mechanism,[20–22] (2) shell bending and overflow-based mechanism,[4,8,23] and (3) meniscus-solidified and overflow-based mechanism.[2,6,16,24–26] The third mechanism is proposed that hooks form through periodic meniscus solidification and subsequent liquid steel overflow.[27] In the current study, the third mechanism is used to explain the formation and transformation of hooks. The reported studies on the hooks are summarized in Table I.[2,4,6,12,13,19,27–39]
Table I. Author
Steel Grade
Emi et al.
low-carbon steel
Takeuchi and Brimacombe
low-carbon and middle-carbon steel
Kubota et al.
ultra-low-carbon steel ultra-low-carbon, low-carbon, and middle-carbon steel extra-low-carbon steel
Yamamura et al.
Genzano et al. Shin et al.
ultra-low-carbon steel
Awajiya et al.
ultra-low-carbon steel ultra-low-carbon steel
Sengup
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