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NTRODUCTION

HOOKS were the solidification-related structure at the subsurface of the continuous casting slab, and formed at the meniscus with the mold oscillation.[4] The mechanism of hook formation[5–7] through meniscus solidification and subsequent liquid steel overflowing the meniscus was widely agreed. The curved meniscus developed under the interfacial tension[8] between the liquid steel and liquid slag, and then the meniscus solidification[9,10] and the overflow of liquid steel occurred with the cooling of the water in copper plate and the pressure at the meniscus.[11,12] Hooks were usually located near oscillation marks and within 2 to 3 mm from the surface of the continuous casting slab. Curved hooks extended to the inner portion of the mold, and could entrap mold flux,[13,14] argon bubbles,[15,16] and floating non-metallic inclusions[5] moving with the liquid steel in the mold, which damaged the [1–3]

XUBIN ZHANG, YING REN, and LIFENG ZHANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, China. Contact e-mails: [email protected], [email protected] Manuscript submitted February 24, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

surface quality of the slab. For example, sliver defects[17,18] (Figure 1) on the surface of rolled plate was mainly caused by the existence of large-sized inclusions[19] at the subsurface, which was closely related to the entrapment of inclusions by the hook structure at the meniscus during the solidification of steel. This could also explain why most > 300 lm inclusions gathered within 0 to 3 mm from the surface of continuous casting.[20] Since hooks were observed as nail-like shell near oscillation marks by Emi et al.[21] in 1970’s, many studies on hooks at the subsurface of the slab have been conducted. Hook size decreased with the increase of carbon content in steel, which was proposed by Yamamura et al.[2] through dip test and the calculation of the temperature at the meniscus. Awajiya et al.[22] found that most >200 lm inclusions were located near hook lines, and high speed of liquid steel reduced the entrapment of inclusions by hooks. The entrapment of >200 lm inclusions by hooks was also proposed by Deng et al.[23] Esaka et al.[24] found that the entrapment of bubbles by the oblique copper plate was related to the diameter of gas bubbles, the angle of copper plate, and the roughness of the interface through in situ observations. Lee et al.[25] proposed that hook size decreased with the increase in superheat, the increase of casting speed, and the optimization of powder properties in the

Fig. 1—Sliver defects on cold rolling plate.

mold through experiments in the plant. The author[26] also investigated the entrapment of inclusions and bubbles by hooks, and the influence of casting speed, submerged depth of SEN and the application of electromagnetic braking on hook depth. However, few studies have been conducted to comprehensively investigate the influence of casting parameters on hook d