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.

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A New Mechanism of Hook Formation during Continuous Casting of Ultra-Low-Carbon Steel Slabs

The restrictions on quality for low carbon continuously cast slab products require that surface defects be kept to a minimum. Currently, the steel industry has developed a wealth of experience on how to apply slabs with oscillation marks to very demanding applications. However, these practices circumvent the problem, rather than solving it. By understanding the formation mechanism of oscillation marks, one can then develop casting practices that can minimize their effect on slab surface quality. The techniques developed in this study allowed a more detailed examination of the mold heat-transfer interactions during continuous casting, such that the variations of heat flux due to irregular solidification near the meniscus could be measured. It is shown that the mechanisms proposed in the literature are not individually sufficient for the formation of an oscillation mark, but that several are necessary and must occur in sequence for an oscillation mark to form. A mechanism is proposed for the formation of oscillation marks that is shown to be in agreement with the trends observed and reported in the literature. Additionally, it is shown that the success of practices used in industry to reduce the severity of oscillation marks can be explained using this proposed hypothesis.

A mold simulator for continuous casting of steel: Part II. The formation of oscillation marks during the continuous casting of low carbon steel

http://ccc.illinois.edu/PDF%20Files/Publications/06_ActaMat_Sengupta_Micrograph_Evidence_Men_Sol_reprint_post.pdf

Micrograph evidence of meniscus solidification and sub-surface microstructure evolution in continuous-cast ultralow-carbon steels

Oscillation marks (OMs) are regular, transverse indentations formed on the surface of continuously cast (CC) steel products. OMs are widely considered defects because these are associated with segregation and transverse cracking. A variety of mechanisms for their formation has been proposed (e.g., overflow, folding, and meniscus freezing), whereas different mark types have also been described (e.g., folded, hooks, and depressions). The current work uses numerical modeling to formulate a unified theory for the onset of OMs. The initial formation mechanism is demonstrated to be caused by fluctuations in the metal and slag flow near the meniscus, which in turn causes thermal fluctuations and successive thickening and thinning of the shell, matching the thermal fluctuations observed experimentally in a mold simulator. This multiphysics modeling of the transient shell growth and explicit prediction of OMs morphology was possible for the first time through a model for heat transfer, fluid flow, and solidification coupled with mold oscillation, including the slag phase. Strategies for reducing OMs in the industrial practice fit with the proposed mechanism. Furthermore, the model provides quantitative results regarding the influence of slag infiltration on shell solidification and OM morphology. Control of the precise moment when infiltration occurs during the cycle could lead to enhanced mold powder consumption and decreased OM depth, thereby reducing the probability for transverse cracking and related casting problems.

A Unified Mechanism for the Formation of Oscillation Marks

A mathematical model of the continuous casting process, which explicitly incorporates the presence of slag, molten steel, heat transfer through the mould walls, and shell solidification, is presented. The model is based on the solution of the Navier–Stokes equations for the multiphase slag–steel–air system under transient conditions, including tracking of the interface between these phases. The use of an extremely fine mesh (100 μm) in the meniscus region allows, for the first time, the direct calculation of liquid slag infiltration into the shell-mould gap. Elsewhere, a coarser mesh is used to capture the influence of the metal flow on the overall solution. Predictions are compared with prior, cold model experiments and high temperature mould simulators. Excellent agreement was found for features such as slag film development and heat flux variations during the oscillation cycle. Furthermore, predictions of shell thicknesses and heat fluxes for a variety of simulated casting speeds are also in good agreement with plant measurements. These findings provide an improved fundamental understanding of the basic principles involved in slag infiltration and solidification inside the mould and how these affect key process parameters, such as powder consumption and shell growth. These parameters have a decisive effect on the formation of oscillations marks and transverse cracks, which are a major source of defects in the casting practice.

Explicit Modelling of Slag Infiltration and Shell Formation during Mould Oscillation in Continuous Casting

Surface defects, such as oscillation marks, ripples, and cracks that can be found on the surface of continuously cast steel, originate in the continuous casting mold. Therefore, a detailed knowledge of initial solidification behavior of steel in a continuous casting mold is necessary because it determines the surface quality of continuously cast slabs. In order to develop an understanding of the initial solidification of continuous cast steels, a “mold simulator” was designed and constructed to investigate heat-transfer phenomena during the initial phase of strand solidification. The mold simulator was used to obtain solidified steel shells of different grades of steel under conditions similar to those found in industrial casting operations. The resulting cast surface morphologies were compared with industrial slabs and were found to be in good agreement, indicating that it is possible to simulate the continuous casting process by a laboratory scale simulator.

C. C. Snyder

Papers

A mold simulator for continuous casting of steel: Part II. The formation of oscillation marks during the continuous casting of low carbon steel

A mold simulator for the continuous casting of steel. Part I. The development of a simulator