A new mechanism of hook formation during continuous casting of ultra-low-carbon steel slabs
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ION marks (OMs) and subsurface hooks in continuously cast steel slabs have received much attention from researchers during the past several decades, owing to their association with quality problems. Oscillation marks, such as shown in Figure 1(a), are periodic transverse depressions running across the slab surface. The nomenclature derives from their cause, vertical oscillation of the mold, although similar but irregular surface depressions occur even with a stationary mold.[3] Oscillation marks form during the brief initial stage of solidification close (within ;15 mm) to the liquid steel level, where the solidifying shell tip meets the liquid meniscus. Indeed, variations in OM spacing are used to infer liquid level variations in the mold.[4] Typically, OMs are 0.2 to 0.8 mm in depth, depending on steel composition and casting conditions.[5] The spacing between OMs, called ‘‘pitch,’’ shown in Figure 1(a), ranges from 3 to 15 mm.[5,6,7] Periodic oscillation of the mold facilitates uniform infiltration of the mold flux into the gap between the mold wall and steel shell, and is needed to prevent sticking and breakouts. Subsurface ‘‘hooks’’ are distinctive microstructural features that accompany some OMs and can be identified by etching transverse sections through the slab surface.[1,2,8] Figure 1(b) shows the 3-D shape of a typical hook beneath the root of an OM, which does not vary much with distance along the slab perimeter. Oscillation marks can be classified as ‘‘hook type’’ if they have a subsurface hook or ‘‘depression type’’ if they do not. Hooks can also be classified as ‘‘curved’’ if they angle steeply inwards from the surface or [1]
[2]
JOYDEEP SENGUPTA, formerly NSERC Canada Postdoctoral Fellow, is currently Research Associate with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana–Champaign, IL. BRIAN G. THOMAS, Wilkins Professor of Mechanical Engineering, is with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana–Champaign, IL. Contact e-mail: [email protected] HO-JUNG SHIN and GO-GI LEE, Graduate Students, and SEON-HYO KIM, Professor, are with the Department of Materials Science and Engineering, Pohang, University of Science and Technology (POSTECH), South Korea. Manuscript submitted July 24, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
‘‘straight’’ if they are shallow and lie just beneath and parallel to the surface.[9,10,11] Examples of each type of hook are shown in Figures 1(c) and (d). Oscillation marks associated with curved hooks are generally deeper and wider than those without any hooks or with straight hooks.[12] Hooks often entrap mold flux, floating inclusions, and bubbles that ultimately form surface defects, such as slivers and blisters,[13,14] after rolling and annealing. Furthermore, transverse cracks[8,15,16] (included in Figure 1(a)) often initiate near the roots of OMs. This is due to the hotter, weaker shell and the associated coarser subsurface grain structure, and embrittling precipitates, caused by positive m
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