Entrapment of Inclusions by Solidified Hooks at the Subsurface of Ultra-Low-Carbon Steel Slab
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TO improve the surface quality of ultra-low carbon steel slabs, which are usually used for skin panels of automobiles and white goods, reducing inclusions[1–3] and argon bubbles[4,5] in slabs is important, especially at the location of 0.5 to 3.5 mm beneath the surface of continuous casting slabs. Subsurface inclusions originate from the entrapment by either the solidification front[6–8] or solidified hooks[9,10] near the meniscus in the mold. The entrapment of non-metallic inclusions and argon bubbles below hooks has been reported in the literature.[11] As a kind of sub-surface solidification-related structure, hooks[11–13] often accompany oscillation marks[14–16] in continuous-casting low-carbon steel slabs and can be revealed by etching longitudinal sections vertical to oscillation marks. The hook forms at the meniscus during mold oscillation and locates within 2 to 3 mm beneath the slab surface.[17] The three-dimensional hook[18] in the mold is a 0.6 to 1-mm-thick curved surface along the slab perimeter like the oscillation XUBIN ZHANG, YING REN, LIFENG ZHANG, and WEN YANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, China. Contact e-mail: [email protected], [email protected] Manuscript submitted February 5, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
mark, which contacts the liquid steel and can entrap inclusions at approximately 0-50 mm below the meniscus according to shell thickness.[19,20] With a high liquidus temperature and thin mushy zone,[13] ultra-low-carbon steels are prone to the formation of bulky hooks. During the past 2 decades, a few studies have investigated the influence of solidified hooks on the entrapment of non-metallic inclusions and argon bubbles. Awajiya et al.[9] measured the hook depth on the wide face of continuous casting slabs, detected inclusions 1 mm beneath the slab surface and calculated the flow velocity of liquid steel near the hook. They concluded that hooks might capture > 200 lm inclusions and high-flow velocity might wash inclusions away from hooks. Deng et al.[10] detected hooks and inclusions in ultra-low- and low-carbon steel slabs and reached similar conclusions. Yamamura and Mizukami[21] performed modeling experiments to reveal the entrapment of bubbles by the solidified shell and discussed the influence of the angle of the solidified shell and the size of bubbles on the entrapment of bubbles. Esaka and Kuroda[5] performed a similar modeling experiment and found that larger shell angles could entrap more bubbles and smaller bubbles were prone to be entrapped. The interfacial morphology and flow velocity of the steel on the solidification front could also make a difference. Miyake et al.[4] studied subsurface bubbles in ultra-low-carbon continuous casting steel slabs and found that solidified hooks could entrap
bubbles. In this period, with the improvement of steel cleanliness, large-sized inclusions have been significantly decreased. However, whether small inclusions are entr
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