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

In this article, an electromagnetic field simulation and a flow analysis model are performed to describe the three-dimensional electromagnetic field distribution and the electromagnetically driven flow characteristics in a round-bloom mold with a low-frequency in-mold rotary electromagnetic stirrer. The interaction between the induced flow and the inertial impinging jet from a straight-through submerged entry nozzle (SEN) of the caster is considered. The effects of stirrer current and frequency on the electromagnetic field and the flow in the round-bloom mold are investigated, and a strategy to optimize the stirring parameters is proposed. The results show that the distributions of magnetic flux density and electromagnetic force magnitude are nonuniform in a three-dimensional electromagnetic stirring (EMS) configuration. There exists a significant axial induced component of electromagnetic force. The flow in the in-mold EMS system is characterized by a dominant swirling movement at the transverse sections, coupled with the recirculating flows in the axial direction. An upper recirculation zone and a lower recirculation zone with the reverse melt flowing are found near the strand wall at the axial location close to the middle of the stirrer, and another recirculation zone is formed due to the interference of the induced flow with the jet from SEN. The meniscus surface has a swirl flow, and the meniscus level rises near the bloom strand wall and sinks around the SEN wall. All of these flow features are closely associated with metallurgical performances of the in-mold rotary stirrer. With the increase of stirring current and the decrease of frequency, the magnetic flux magnitude increases. There is an optimum frequency to obtain a peak of electromagnetic force magnitude and maximum tangential velocity. For a mold rotary EMS system, to determine the optimum stirring intensity, it is necessary to make a compromise between a larger tangential velocity and a relatively quiescent meniscus surface.

Numerical Simulation of Fluid Flow in a Round Bloom Mold with In-Mold Rotary Electromagnetic Stirring

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

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.

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

Finite element analysis of electromagnetic and fluid flow phenomena in rotary electromagnetic stirring of steel

The magnetohydrodynamic (MHD) flow around a suspended particle in a liquid metal subjected to electric and magnetic fields can affect the force exerted by the applied electromagnetic field on the particle. In this article, a novel approach to the computational simulation of three-dimensional nonlinear MHD flow in two-phase systems is proposed. The electromagnetic field in the conducting fluid, including the particle, is represented using the current-vector potential (T) and reduced magnetic scalar potential (Ψ) to avoid the discontinuity of the electric field at the fluid-particle interface. To avoid the solution of the electromagnetic field in free space and to account exactly for the electromagnetic field interactions with the fluid and the particle, the electric and magnetic fields are specified at the boundary of the fluid-flow domain using Ampere’s law. This formulation permits the numerical solution of the coupled electromagnetic and fluid-flow equations on a common mesh. The discretized equations are derived using a finite-element formulation, and an iterative procedure is described for the efficient solution of these equations. This method is used to investigate the electromagnetic and fluid-flow phenomena in electromagnetic separation of a nonconducting spherical particle in crossed uniform electric and magnetic fields at intermediate Hartmann numbers. The computed results show that the magnetic field has no effect on either the velocity field or the net force on the particle when the Hartmann number is less than 1. Beyond this threshold value of the Hartmann number, the velocity decreases almost linearly with increasing magnetic-field strength. The damping of the flow by the magnetic field manifests itself in a reduction of the separation force, even though it is relatively small for this system.

A new method for three-dimensional numerical simulation of electromagnetic and fluid-flow phenomena in electromagnetic separation of inclusions from liquid metal

T. T. Natarajan

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

Finite element analysis of electromagnetic and fluid flow phenomena in rotary electromagnetic stirring of steel

A new method for three-dimensional numerical simulation of electromagnetic and fluid-flow phenomena in electromagnetic separation of inclusions from liquid metal

Detection of mold events using thermocouple measurements