Submerged Entry Nozzle Design Research Articles

Effects of the asymmetric and oscillating turbulent melt flow on the heat transfer and solidification inside the thin slab continuous casting (TSC) mold under the applied electromagnetic brake (EMBr)

The thin slab casting (TSC) is a breakthrough near-net-shape technique for flat products accompanied by rapid casting and solidification rates. The TSC quality hinges on the turbulence, super-heat flow and growth of the solidified shell. The electromagnetic brake (EMBr) is commonly applied to control the fresh melt flow after feeding through a submerged entry nozzle (SEN). Numerical modelling is a perfect tool to investigate the multiphase phenomena in the continuous casting (CC). The presented study considers the heat transfer through the solid shell and water-cooled copper mold including the averaged thermal resistance of the slag skin and the air gap coupled with the turbulent flow and magnetohydrodynamics (MHD) model using an in-house code developed inside the open-source computational fluid dynamics (CFD) package OpenFOAM®. The model is applied to investigate different undesired asymmetric melt flow issues: (i) with the misaligned or (ii) partially blocked SEN; (iii) caused by the mean flow fluctuations with the natural frequencies; (iv) related to the oscillations of the fresh melt jets for the specific SEN designs and casting regimes. The variation of the flow pattern and superheat distribution is studied and presented for different scenarios both with and without applied EMBr.

Analysis of a New SEN Design with an Inner Flow Divider

To minimize the product imperfections due to slag entrapment and surface defects, the fluid flow pattern inside the mold must be symmetric, commonly named double-roll flow. Thus, the liquid steel must enter into the mold evenly distributed. The submerged entry nozzle (SEN) is crucial in product quality in vertical steel slab continuous casting machines because it distributes the molten steel from the tundish into the mold. This work evaluates the performance of a novel bifurcated nozzle design named “SEN with flow divider”. The symmetry at the outlet ports is obtained by imposing symmetry inside the SEN. The flow divider is a solid barrier attached at the SEN bottom inner wall, the height of which slightly surpasses the upper edges of the outlet ports. The performance analysis is done first using numerical simulations, where the Computational Fluid Dynamics (CFD) technique and the Smoothed Particle Hydrodynamics (SPH) approach are used. Then, experimental tests on a scaled model are also used to evaluate the SEN performance. Numerical and physical simulations showed that the flow divider considerably reduces the SEN outlet jets’ broadness and misalignment, producing compact, aligned, and symmetric jets. Therefore, the SEN design analyzed in this work is a promising alternative to improve process profitability.

Effect of swirling flow tundish submerged entry nozzle outlet design on multiphase flow and heat transfer in mould

ABSTRACTEffect of a swirling flow SEN (submerged entry nozzle) outlet design on the multiphase flow and heat transfer in a mould was investigated by using numerical simulation. It was found that different SEN outlet designs could form different flow patterns and temperature distributions on the upper of the mould. The enlarged outlet SEN design had an effect to decrease the horizontal velocity of liquid steel flowing out the SEN outlet, reducing the steel flow velocity towards the solidification front. Although a higher velocity was found near the slag/steel interface with the enlarged outlet SEN, but the turbulent kinetic energy was lower. The reason was that less circulation flows were formed in the region of the mould top. The weak horizontal flow towards the solidification front with the enlarged outlet SEN induced lower wall shear stresses, at the same time it also formed a lower temperature distribution near the solidified shell.

Effect of Submerged‐Entry Nozzle (SEN) Design on Fluid Flow and Heat Transfer in a Thin‐Slab Steel Caster

Occurrence of breakouts (BO) during casting of narrow slabs is observed to be influenced by the submerged‐entry nozzle (SEN) design in one of ArcelorMittal's thin‐slab casters. To understand the root cause for the BO, three‐dimensional computational fluid dynamics (CFD) models are developed to simulate steady‐state and transient molten steel flow and heat transfer in the SEN and in the CSP caster for three four‐port SEN designs (type‐ A, B, and C), among which exist two major design differences, including the presence of a top insert at SEN tube entrance and height of the flow divider at SEN bottom. CFD models are validated by water model measurements of SEN port opening velocities. Both water model experiments and numerical simulation results suggest that SEN design with a taller bottom flow divider increases both mean and variations of the upper port flow rates and large‐scale asymmetric flows in the CSP mold region. Transient heat transfer simulation results further show that this unstable biased flow in the mold increases “hot spots” near the shell around the funnel‐to‐flat transition region, which could be responsible for local steel re‐melting and potential breakout under certain critical ferrostatic pressure.

Velocity Monitoring of Molten Steel in a Continuous Casting Mold Using Three Submerged Entry Nozzle Designs

The horizontal and vertical velocity components of molten steel in a slab continuous casting mold produced by three different two-port Submerged Entry Nozzle (SEN) designs are monitored and compared using Computational Fluid Dynamics (CFD) simulations. These two ports designs correspond to a conventional cylindrical SEN, a plate SEN and an anchor-shaped SEN. Four monitoring points at the molten steel in the centered vertical plane were selected to track the horizontal and the vertical component of the velocity vector. Two of them are located near the free surface and the remaining two are located in the vicinity of the SEN discharge nozzles. Some statistical values of the time series of above the velocity components are analyzed and correlated with the Kelvin-Helmholtz instability and the Karman vortex streets, which cause mold powder entrapment in the molten steel.

The Role of Submerged Entry Nozzle Port Shape on Fluid Flow Turbulence in a Slab Mold

The fluid flow of liquid steel in a wide slab mold (1880 × 230 mm) influenced by two different submerged entry nozzle (SEN) designs (bifurcated nozzles with rectangular, SEN-R, vs square, SEN-S, ports) and immersion depths of 115 and 185 mm was studied using a 1:1 scale water model. To analyze the fluid dynamics, particle image velocimetry and video recording techniques were used. The fluid-flow dynamics indicate that the discharging jets using either SEN design suffer strong wandering and raveling effects that enhance turbulence in the meniscus region. The preceding results show the existence of velocity spikes (defined as velocities with magnitudes that exceed the standard deviation of the average velocity) in the submeniscus region. Using the SEN-R ports yields more velocity spikes per minute with larger magnitudes than using the SEN-S, which could be the main cause of the detrimental quality of steel. The capability of slag entrainment by the flow developed by a nozzle was the criterion employed to evaluate quantitatively the merits of one nozzle over the other. This criterion is based on the capillary number, which gives the ratio between viscous-inertial and surface forces at the metal–slag interface.

Modeling Study of Molten Steel Turbulence Control by SEN Design Improvement in a Conventional Slab Caster

A research study to improve the fluidynamics of a typical slab caster mould by redesigning the submerged entry nozzle (SEN) ports through a complete mathematical analysis was carried out, selected geometries are deeply tested and analyzed by physical modeling. For that a 1:1 scale analogue water model was constructed; PIV techniques, red dye tracer injections, and ultrasonic sensors are employed to study the molten steel flow patterns behavior. A comparison between the original and the improved SEN designs indicate that both geometries show weak double roll flow pattern at shallow position, but the two‐roll flow pattern is well developed at a deeper position. The original design induces vortexes formation at either, shallow or deep immersion. In the other hand, the proposed improved design, where the backflow is eradicated, maintains good free surface stability without any vortex formation. The physical results indicate that the backflow phenomenon induces jet fluctuations, which are reflected in a more turbulent free surface, and its elimination provides more stable flow patterns in the mould.

Review: The “butterfly effect” in continuous casting

The continuous casting (CC) mould may appear very peaceful when viewed from above, but the powder bed hides relentless fluctuations in the following phenomena: metal flow, thermal gradients, chemical reactions and multiple phase transformations. When observed separately, some of these phenomena seem to have a ‘simple behaviour’, which may appear easy to control through the main casting parameters (e.g. casting speed, pouring temperature and powder type) and associated control systems (e.g. mould level control, automatic powder feeding and mould oscillation). However, when combined, these phenomena exhibit periodic fluctuations in behaviour, which is both difficult to predict and control. For instance, the combination of casting speed, submerged entry nozzle design and slab size can cause the metal flow pattern to shift from double roll to single roll and back, which can cause unstable fluctuations in metal level, standing waves, etc. In this respect, the CC process closely resembles a meteorological system where both variations and local fluctuations in temperature, humidity, pressure, etc., can result in effects that are difficult to predict in the long term. This is equivalent to the famous Lorenz premise: ‘Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas?’ In this paper, we give some examples of the ‘butterfly effect’ in CC discussed below by using a mathematical model able to predict the slab solidification inside the mould in which various factors affecting the process stability are analysed and the probable sources of fluctuation are identified.

Design of a Submerged Entry Nozzle for Thin Slab Molds Operating at High Casting Speeds

A submerged entry nozzle (SEN) for thin slab casters operating at casting speeds as high as 7.5 m/minute was developed based on fundamental grounds of boundary-layer theory and water modeling experiments. Experimental techniques included tracer injection to observe overall fluid flow patterns, high speed video camera and image analysis to follow dynamic changes of meniscus levels and particle image velocimetry to measure water speeds in the mold. This design was compared with two other SEN designs of nozzles under current commercial use at various thin slab casters. Direct comparisons of mathematical simulations and experimental results among the three SEN's evidenced that this new SEN yields very stable flows which are independent from casting speed and nozzle immersion depth. Fluid flow developed by this SEN consists of a double roll pattern without generation of superficial vortices. The two other SEN's yield instable discharging jets due to and excessive shearing effects with the surrounding fluid inducing severe dissipation of kinetic energy which promotes severe tailing effects inducing strong meniscus oscillations. The proposed design has reported good industrial performances and a longer operating life because the slag protection belt suffers less wear thanks to smaller velocities of the bath in contact with the SEN wall.

The Effects of Electromagnetic Brake on Liquid Steel Flow in Thin Slab Caster

AbstractComputational fluid dynamics (CFD) model with magneto hydro dynamics (MHD) is developed for a thin slab caster to investigate the effects of electromagnetic brake (EMBr) on liquid steel flow in continuous casting mold and to determine the EMBr practices which lead to optimal flow structure. Particle Image Velocimetry (PIV) tests in water model and meniscus flow measurements in real caster are performed to validate the predictions obtained with CFD models. The performance of different submerged entry nozzle designs, SEN 1 and SEN 2, are evaluated. The effects of nozzle submergence in relation to the applied magnetic field on mold flow structure are quantified. There are significant differences between flow structures obtained with SEN 1 and SEN 2, even though both designs have fundamental similarities and contain four ports. EMBr mainly reduces the meniscus velocities for SEN 2 as opposed to the foremost influence of EMBr for SEN 1 that is to significantly slow down the downward jet coming from the bottom ports. In addition, reducing the EMBr strength for shallow nozzle submergence and increasing the EMBr strength for deep nozzle submergence help to maintain similar meniscus activity for all conditions.

Effect of SEN structure on the mold level fluctuation and heat transfer for a medium thin slab caster

It is important to select suitable parameters of a submerged entry nozzle (SEN) for optimizing the flow and temperature patterns in a mold. The effect of SEN design on the mould level stability, meniscus steel flow velocity, and heat transfer of the mold of a medium thin slab caster was studied by means of 1:1 water modeling and industrial testing. The advantages of a 2-port SEN compared with a 3-port SEN are the following: more optimal flow patterns with a lower mold level fluctuation and a lower meniscus steel flow velocity; proper powder consumption without slag bears due to a reasonable liquid powder thickness. The argon flow rate can be reduced and the mold average heat flux and temperature near the edges of the copper plate are reduced. At a casting speed of 2.5 m-min −1, the mold level fluctuation lies within ±5 mm. In addition, soft cooling of the steel shell in the mold is realized, which is suitable for casting crack susceptible steel grades.

Structure of turbulent flow in a slab mold

The structure of the turbulent flow in a slab mold is studied using a water model, various experimental techniques, and mathematical simulations. The meniscus stability depends on the turbulence structure of the flow in the mold; mathematical simulations using the k-e model and the Reynolds-stress model (RSM) indicate that the latter is better at predicting the meniscus profile for a given casting speed. Reynolds stresses and flow vorticity measured through the particle-image velocimetry (PIV) technique are very close to those predicted by the RSM model, and maximum and minimum values across the jet diameter are reported. The backflow in the upper side of the submerged entry nozzle (SEN) port (for a fixed SEN design) depends on the casting speed and disappears, increasing this process parameter. At low casting speeds, the jet does not report enough dissipation of energy, so the upper flow roll is able to reach the SEN port. At high casting speeds, the jet energy is strongly dissipated inside the SEN port, the narrow wall, and in the mold corner, weakening the momentum transfer of the upper flow roll, which is unable to reach the SEN port. At low casting speeds, meniscus instability is observed very close to the SEN, while at high casting speeds, this instability is observed in the mold corner. An optimum casting speed is reported where complete meniscus stability was observed. The flow structure at the free surface indicates a composite structure of islands with large gradients of velocity at high casting speeds. These velocity gradients are responsible for the meniscus instability.

DYANMICS OF UNSTEADY ONE-PHASE TURBULENT FLOWS IN A CONTINUOUS CASTING MOULD

The fluid flow dynamics of one-phase flows of liquid steel in a continuous casting mould was physically modelled using digital particle image velocimetry technology and mathematically simulated using a large eddy simulation approach. Both analyses showed that the flow of liquid through the mould fluctuates with time yielding asymmetrical and biased flows through both submerged entry nozzle (SEN) ports. Reynolds stresses close to the SEN ports increase approximately tenfold when the casting speed is increased from 0.8 m/min to 1.8 m/min. The highest Reynolds stresses are located in the boundary between the entering jet and the liquid bulk inside the mould producing high shear rates. Vorticity maps observe a random behaviour over time of magnitude and sign in the flow field at low casting speeds (0.8 m/min) and a layered structure with a more regular behaviour at high casting speeds (1.2 and 1.8 m/min). Vorticity and velocity/vector maps should be useful for SEN design aimed at higher casting speeds than the conventional ones with good control of fluid flow turbulence.On a modélisé physiquement la dynamique d’écoulement du fluide d’écoulements monophasés d’acier liquide dans un moule pour la coulée continue, en utilisant la technique de vélocimétrie d’image digitale de particule, et on l’a simulé mathématiquement en utilisant une approche de simulation d’un grand tourbillon. Les deux analyses ont montré que l’écoulement de liquide dans le moule fluctue avec le temps, produisant des écoulements asymétriques et biaisés à travers les deux orifices de la buse d’entrée submergée (SEN). Les contraintes de Reynolds près des orifices SEN augmentent approximativement dix fois lorsque l’on augmente la vitesse de coulée de 0.8 m/min à 1.8 m/min. Les contraintes de Reynolds les plus élevées se situent à la limite entre le jet d’entrée et la masse de liquide à l’intérieur du moule, produisant des taux élevés de cisaillement. Des représentations graphiques de vorticité observent un comportement aléatoire par rapport au temps de la grandeur et du signe du champ d’écoulement aux basses vitesses de coulée (0.8 m/min) et une structure en couches avec un comportement plus régulier aux vitesses élevées de coulée (1.2 et 1.8 m/min). Les représentations graphiques de vorticité et de vecteur/vélocité devraient être utiles pour la conception de SEN ayant pour objectif des vitesses de coulage plus élevées que les vitesses conventionnelles avec un bon contrôle de la turbulence de l’écoulement du fluide.

Improving oxide cleanness on basis of MIDAS method

The MIDAS method (Mannesmann inclusion detection by analysing surfboards) was established in 1987 and enabled steelmakers to detect less frequent macroscopic inclusion particles in large volumes of steel. The basic idea of MIDAS is the flat cross-rolling of a strand sample perpendicular to the casting direction. Improvements in secondary metallurgy and casting technology have been achieved, since it became possible to define macrocleanness in exact numbers. In this paper, MIDAS results are presented for various strand sections: billets, blooms, jumbo blooms, round strands, conventional slabs and thin slabs. The variation of cleanness during sequence casting, at the front portion and end portion of heats, at ladle change and during steady state casting is investigated. The effect of teeming disturbances and accidental casting events on macrocleanness is studied. Selection of clean material can be optimised by adjusting the cutting length and downgrading critical casting portions. Detected inclusion particles in ultrasonic tested 'surfboard' samples are subsequently identified by metallography and by SEM or EPMA. The exogenous sources of contaminations and the formation mechanisms of inclusion particles are elucidated. Fundamental aspects, such as flotation due to buoyancy forces, coagulation of liquid particles, agglomeration of solid particles due to turbulent fluid flow, and the emulsification of slag droplets from ladle, tundish or mould, are now more deeply understood and enable progressive steps towards clean steel practice. MIDAS results correlate with material behaviour and properties at manufacturing and end-use of long and flat steel products. Future objectives should stress the stabilisation of optimised steady state conditions to realise a higher consistency and reproducibility of properties. Revamping of older continuous casting machines with curved moulds and the introduction of vertical sections are most essential for clean steelmaking. To avoid subsurface inclusions at the midwidth of the strand, submerged entry nozzle design and fluid flow pattern in the mould and liquid pool of continuous cast slabs should be reconsidered.