A New Mechanism of Hook Formation during Continuous Casting of Ultra-Low-Carbon Steel Slabs JOYDEEP SENGUPTA, BRIAN G. THOMAS, HO-JUNG SHIN, GO-GI LEE, and SEON-HYO KIM The initial stages of solidification near the meniscus during continuous casting of steel slabs involve many complex inter-related transient phenomena, which cause periodic oscillation marks (OMs), subsurface hooks, and related surface defects. This article presents a detailed mechanism for the formation of curved hooks and their associated OMs, based on a careful analysis of numerous specially etched samples from ultra-low-carbon steel slabs combined with previous measurements, observations, and theoretical modeling results. It is demonstrated that hooks form by solidification and dendritic growth at the liquid meniscus during the negative strip time. Oscillation marks form when molten steel overflows over the curved hook and solidifies by nucleation of undercooled liquid. The mechanism has been justified by its explanation of several plant observations, including the variability of hook and OM characteristics under different casting conditions, and the relationships with mold powder consumption and negative/positive strip times. I. INTRODUCTION OSCILLATION marks (OMs) [1] and subsurface hooks [2] in continuously cast steel slabs have received much atten- tion from researchers during the past several decades, owing to their association with quality problems. Oscilla- tion marks, such as shown in Figure 1(a), are periodic trans- verse depressions running across the slab surface. The nomenclature derives from their cause, vertical oscillation of the mold, although similar but irregular surface depres- sions 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, var- iations in OM spacing are used to infer liquid level varia- tions in the mold. [4] Typically, OMs are 0.2 to 0.8 mm in depth, depending on steel composition and casting condi- tions. [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 fea- tures 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 ‘‘depres- sion type’’ if they do not. Hooks can also be classified as ‘‘curved’’ if they angle steeply inwards from the surface or ‘‘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. Fur- thermore, 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 pos- itive microsegregation (mainly of Al, [8] Mn, [8] N, [8] and P [16] ) observed adjacent to hooks. In severe cases, the entire slab surface must be ‘‘scarfed’’ or ground to remove OMs, hooks, and their associated defects, leading to an overall loss of yield and productivity. [9,17] To minimize the defects associated with OMs and hooks, industry practice now tends to use higher oscillation fre- quencies and shorter strokes. This reduces the negative strip time (the period during each oscillation cycle when the mold moves downward faster than the casting speed) and produces shallower OMs. [6,10,16] Practices to minimize the depth and severity of subsurface hooks include low density and exothermic mold powder, [11] special submerged entry nozzles (SENs), [12] nonsinusoidal or ‘‘triangular’’ mold oscillation, [9,10] hot top molds, [18] and replacing aluminum deoxidation with a silicon-killing practice. [1] Steel grade greatly affects OM and hook formation. Oscillation marks are most severe in peritectic steels (0.07 to 0.15 pct C), [19] while hooks increase in severity with decreasing carbon content, and especially plague ultra-low-carbon steels. [7,20] This work is part of a larger project to investigate the formation of OMs, hooks, and other phenomena related to initial solidification and surface defects in continuous-cast steel. This article first outlines and critically reviews pre- vious theories of OM and hook formation. Then, a quanti- tative analysis of hooks and OMs is presented, based on numerous specially etched ultra-low-carbon slab samples obtained after controlled casting trials at POSCO Gwan- gyang Works. The remainder of the article presents a detailed 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: bgthomas@uiuc.edu 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 OLUME 37A, MAY 2006—1597