Prediction and control of subsurface hooks in continuous cast ultra-low-carbon steel slabs

Abstract

Subsurface hook formation during initial solidification in the continuous casting mould degrades the quality of steel slabs owing to the associated entrapment of argon bubbles and non-metallic inclusions. To minimise hook depth and to improve slab quality, extensive plant experiments were performed and analysed to quantify the effect of casting parameters on hook characteristics using the no. 2-1 caster at POSCO Gwangyang Works, Korea. The results reveal that meniscus heat flux plays an important role in controlling hook characteristics. Hook depth correlates with oscillation mark depth, hook shell thickness, and hook length. Based on regression analysis, this paper proposes an equation to predict hook depth in ultra-low-carbon steels as a function of casting speed, superheat, oscillation frequency, surface level fluctuations, and mould flux properties. Use of this quantitative equation enables improved control of subsurface quality in the continuous casting of steel slabs.

A Unified Mechanism for the Formation of Oscillation Marks

“…Moreover, increased superheat would be expected to result in solidification further down the mold and a thicker liquid slag pool, which would effectively increase the rim-to-shell distance and thereby result in shallower marks. This is in agreement with the results reported by Da¨cker and Sohlgren, [30] Lee et al, [31] and Itoyama. [34] It has been proposed that d OM can be reduced by decreasing the length of the solid meniscus [32] ; this can be achieved by reducing the vertical heat transfer (i.e., improving the thermal insulation of the bed) or by controlling the metal flow in the meniscus (through metal-flow velocity reductions, i.e., electromagnetic braking, or promoting a certain flow pattern redirection, i.e., electromagnetic stirring).…”

“…In-plant observations agree with the latter finding. [31] This conflict can be explained by the fact that a tide change is not instantaneous and there is a period of confusion before a stable flow is achieved (for instance a downward flow for infiltration at t n start ). These periods of confused flow reduce the overall time for slag infiltration and become more important as the oscillation cycle shortens (which occurs with increased frequency despite the increase in the number of cycles per second).…”

“…[1]), which also results in deeper OMs and is consistent with plant observations. [31] Additionally, the effect of casting speed on d OM can be predicted correctly through t n (Eq. [1] and Figure 12) because any increase in v c would lead to a shorter t n and reduced d OM .…”

“…These findings are in agreement with a variety of plant measurements. [9,30,31] Because the effects of oscillation characteristics on d OM can be predicted successfully through their effect on t n , it may be possible to represent the effects of the confused flow transition period through the negative strip time. Equation [1] also shows that these transition periods will become relatively more important as v c increases.…”

Hook evolution and inclusion entrapment during initial solidification process of continuous casting slab

“…Metallographic examination of ultra-low carbon steel slabs [8,[25][26] showed that the hooks were located 2-4 mm beneath the slab surface. The hooks were the most prevalent and thickest at the corner of the slab and then on the narrow surface and broad surface of the slab.…”