Continuous Casting Tundish Research Articles

Mathematical Modeling of the Tundish of a Single-Strand Slab Caster

The production of quality steel from a casting machine is dependent on a large number of interlinked process parameters of ladle, tundish and casting operations. Continuous casting tundishes provide an important link between the ladle and the caster and the parameters of liquid metal in the tundish get affected by both up-stream and down-stream processes.In this paper, the variation of tundish temperature under the influence of various operating parameters is numerically simulated using a mathematical model. A two-dimensional mathematical model, based on coupled fluid flow and heat transfer, is developed to simulate the tundish operation. The model accounts for turbulent fluid flow and dynamic level changes in the tundish. The model is validated with data published in the literature as well as with data obtained during the plant campaign. Simulation of the tundish operations demonstrates the capability of the model in capturing process dynamics correctly. The effect of a few important process parameters on the tundish outlet temperature is also studied.

Studies by mathematical and physical modelling of fluid flow and inclusion removal phenomena in slab tundish for casting stainless steel using various flow control device designs

A mathematical model, based on the SOLA-SURF scheme and k-ϵ two equation model (where k is the turbulence kinetic energy and ϵ is its dissipation), has been developed to simulate the fluid flow and mass transfer phenomena as well as inclusion removal rate for a single strand stainless steel continuous casting tundish under various designs of flow control device (FCD). A physical model, one-third the scale of the actual slab continuous casting tundish, has also been constructed to observe/measure the fluid flow and mass transfer phenomena directly. The experimental observations/measurements have been used to validate the mathematical model. Satisfactory consistency is noted when the simulated results are compared with the experimental results. The verified mathematical model has been applied to the actual slab continuous casting tundish to evaluate the effects of FCD design on the fluid flow-mass transfer phenomena and inclusion removal rate. It has been found that incorporation of a pouring pad underneath the long nozzle can enhance the opportunity for inclusions to float, and achieves the highest removal rate of inclusions among the four FCD designs evaluated.

Modelling of flow behaviour in continuous casting tundish

One of the steel plants of Steel Authority of India Ltd (SAIL) is considering a proposal to increase the capacity of tundish from the present level of 17 tonne to a targeted level of 25 tonne. As a step towards designing a larger tundish, the flow profile of steel in various alternate tundish configurations considered have been characterized using a 3‐D steady‐state mathematical simulation under assumption of both isothermal and non‐isothermal conditions. Apart from generation of flow profiles of steel inside the tundish, the Residence Time Distribution curves have been predicted for analysis of flow behaviour inside the tundish. Based on the above analysis, the optimum tundish configuration was chosen for further development.

Numerical and Physical Simulation of Tundish Fluid Flow Phenomena

The fluid flow in a continuous casting tundish is numerically and physically simulated by means of water models. Results of residence time distribution (RTD) measurements and laser-optical measurements (Laser Doppler Anemometry - LDA, Digital Particle Image Velocimetry - DPIV) are used to validate the numerical results for water before the numerical simulation is transferred to the steel melt. The investigations are focused on both steady-state and transient casting conditions. To reduce vortexing and turbulence in the tundish different types of turbo-stoppers are installed in the water models and their influence on the spacious flow structure is discussed. The turbo-stopper produces higher turbulence in the inlet region of the tundish, but this region is spatially more limited in relation to the flow without turbo-stopper. Thereby a more homogeneous flow is created at the outlet of the tundish with better conditions for particle separation. Basic design criteria for the geometry of a turbo-stopper are developed. Moreover, the processes of first tundish filling and ladle change are simulated at a downscaled water model and these results are compared with numerical simulations using a Volume of Fluid (VoF) model. This multiphase model is able to reproduce the motion of gas bubbles and waves at the free surface.

Fluid flow analysis in horizontal continuous casting tundish

Water model studies were carried out to analyze fluid flow characteristics in a horizontal continuous casting tundish. The effects of LSN (ladle submerged nozzle) and dams on the flow pattems in a tundish were investigated and casting speeds were also changed as experimental parameters. Dye injection experiments were made to observe qualitative flow patterns and mixing characteristics in a continuous flowing tundish. And quantitative analyses of RTD (residence time distribution) curves using tracer pulse injection were also made to calculate the dead volume fraction in the water model tundish under consideration. From the results it was found that flow modification devices with an open stream have little effect on the flow patterns in the tundish, whether the dam is installed or not. This was confirmed by the flow visualization method showing upward flow near falling stream driven by the buoyancy force caused by air entrapment during liquid falling. However when LSN was installed between a ladle and a tundish there was a great change in flow pattern and a bypassing flow that existed in bare tundish was decreased with the use of dams. Especially, dead volume fraction was decreased with the increase of dam height.

Design of single element flow control device in twin strand billet tundish for continuous casting of steel using flow simulation model

An optimal single element flow control device (SEFCD) is proposed in the present study for a small capacity twin strand tundish, to produce steel billet with the desired cleanliness, designed by means of numerical experimentation. To achieve this goal, a mathematical model based on the SOLA-SURF technique and the k-ϵ (where k is the kinetic energy of the fluid and ϵ is its dissipation) two equation turbulence model has been developed to simulate the fluid flow phenomena of molten steel, as well as the mass transfer phenomena of tracer during steady state operation in the tundish. The accuracy of the mathematical model has also been verified by water model experiments, which indicate that the mathematical model is sufficiently accurate and suitable as a tool to develop an optimal SEFCD as required in the present study. Several SEFCD designs have then been evaluated using the mathematical model. The simulated results show that a billet tundish with a high, long, trough shaped pouring pad arrangement has the highest residence time, which promotes inclusion removal, and is thus considered the optimal SEFCD design for the billet tundish.

A theoretical and experimental study of continuous-casting tundishes focusing on slag-steel interaction

A model of a tundish has been developed that takes into account the steel, slag, and refractory phases. Predicted temperatures and velocities in the steel and refractory from the model were earlier found to agree well with measured velocities and temperatures. The model was also used to determine the optimal location of flow devices, making the temperature distribution in the steel more even and enhancing the removal of inclusions to the slag. In this study, the focus was on using the model to study the slag/steel interface in the tundish. Predictions showed that slag is dispersed into the steel close to the interface as well as close to the ladle shroud. In order to confirm these predictions, the momentary interfacial solidification sampling (MISS) method was developed. Using this method, a sample of the steel/slag interface could be taken that represented almost an instantaneous picture of the interface. The MISS sampler was used for sampling low-carbon steel in the tundish. Samples were analyzed using ultrasonic testing, optical microscopy, and scanning electron microscopy (SEM). Analysis results confirmed the presence of nonmetallic particles close to the slag/steel interface and close to the ladle shroud, as suggested by the modeling results. The analyses also showed that the slag/steel interface is very irregular, despite the low velocities.

Mechanism of fluid flow in a continuous casting tundish with different turbo‐stoppers

The fluid flow in a continuous casting tundish affects the separation of non‐metallic particles and the cleanliness of the steel. Today, laser‐optical investigations of water models are state of the art and enable detailed information about the effect of baffles, i. e. dams, weirs and turbo‐stoppers, on the flow. In this work 3D‐LDA and 2D‐DPIV‐investigations for different turbo‐stoppers in a water model on a scale of 1:1.7 of a 16 t single strand tundish are presented. Three circular turbo‐stoppers are investigated. Detailed measurements of the mean velocity and turbulence intensity in the tundish with and without turbo‐stopper are shown. With a suitable turbo‐stopper geometry the recirculation area in the tundish centre and short‐circuit flows along the side walls can be avoided and thus more favourable residence time distributions can be obtained. It is shown that the turbo‐stopper produces higher turbulence in the inlet region of the tundish, which is spatially more limited, however, in relation to the flow without turbo‐stopper. Thereby a more homogeneous flow is created at the discharge of the tundish with better conditions for the particle separation. The experimental data yield a good understanding of the flow phenomena in a tundish with turbo‐stopper and are used as validating criterion for numerical simulations (Fluent 5.5) on the basis of the Reynolds equations. The turbulence modelling is based on a two‐equation model (realizable k‐ε model).

VAI CON Temp: continuous temperature monitoring in metallurgical vessels

VOEST-ALPINE Industrieanlagenbau (VAI) has developed the VAI-CON Temp system for the continuous temperature monitoring in metallurgical vessels. The system has been tested for a variety of installations and has reached industrial perfection. It has many potential applications (from the electric arc furnace to the continuous casting tundish) and, thanks to the highly accurate results it provides, it allows a substantially better process control than the conventional, discontinuous measuring systems.

Numerical simulation of fluid flow and disperse phase behaviour incontinuous casting tundishes

A new Euler-Lagrange model of steel melt flows including dispersed secondaryphases in continuous-casting tundishes is presented. The system offundamental equations for the flow field is closed by different turbulencemodels. The calculation of tundish flows and a comparison withexperimental results show that the flow fields of the steel melt and thebehaviour of the dispersed secondary phases in the melt are describedadequately by the model. However, the quality of the results forthe dispersed phase depends essentially on the choice of turbulence model.Criteria for deciding which turbulence model isbest suited for a specific flow situation are suggested. These criteria arededuced from investigations of different tundish flows.

Mathematical Modeling of Iron and Steel Making Processes. Design Optimization of a Single-strand Continuous Caster Tundish Using Residence Time Distribution Data.

This paper contains the results of a design optimization study performed on the steady-casting operation of the Columbus Stainless single-strand stainless steel caster tundish. Residence Time Distribution (RTD) data such as minimum residence time (or plug flow volume fraction) and dead volume are used as objective functions in the mathematical optimization process. Water is used in the first two case studies as modeling fluid to allow for comparison with water model results. Liquid steel is used in the last case study to investigate the effect of temperature and buoyancy on the resulting flow patterns and the optimum design. Two separate tundish configurations are considered. The first has one dam and one weir, while the second comprises a baffle with angled holes and an impact pad. Significant improvements of up to 34% in minimum residence time are obtained for the second configuration.

Mathematical simulation of the influence of buoyancy forces on the moltensteel flow in a continuous casting tundish

Buoyancy effects on the fluid flow patterns of molten steel in a tundish of acontinuous slab caster were mathematically simulated. Thermal responses ofstep-input temperatures in steel for different flow rates were predicted. Themolten steel velocity profiles were determined for these temperature and flowrate variations. The importance of natural flow convection was establishedthrough a dimensionless number given by the ratio Gr/Re2. Buoyancy forcesproved to be as important as inertial forces, especially in the extremes of thetundish. The simulations indicated that these forces increase the residencetime of the molten steel in the tundish, flowing near the slag layer, allowingmore time for the non-metallic inclusions to be captured; also, that theturbulence inhibiting device helps to redirect the flow towards the top freesurface. It is shown that this device helps to decrease turbulence near theentry zone and has a dumping effect on the temperature step inputs allowingbetter control of the casting temperature. The traditional flow controldevices like weirs and dams were not as effective as the turbulence inhibitor.

Two-fluid simulation on the mixed convection flow pattern in a nonisothermal water model of continuous casting tundish

A transient two-fluid model is applied to simulate fluid flow and heat transfer in a nonisothermal water model of continuous casting (CC) tundish. The original liquid in the bath is defined as the first fluid, and the inlet stream, with the temperature variation, is defined as the second fluid. The flow patterns and heat transfer are predicted by solving the three-dimensional (3-D) transient transport equations for each fluid. The results predicted by the two-fluid model make the effect of natural convection more clear compared with the generally used single fluid model k-ɛ turbulence model.

Physical and mathematical modelling of tundish flows using digital particle image velocimetry and CFD‐methods

The flow in continuous casting tundishes is considerably responsible for the flotation of non‐metallic inclusions and the quality of steel. Because tests in liquid steel can hardly be performed, water model testing considering the geometric and dynamic similarity laws is often used. Here, quantitative investigations with time and cost expensive laser measurement techniques are possible. The digital particle image velocimetry (DPIV) can be seen as a completion and is an effective method to obtain reliable results.

Fluid flow and inclusion removal in continuous casting tundish

Three-dimensional fluid flow in continuous casting tundishes with and without flow control devices is first studied. The results indicate that flow control devices are effective to control the strong stirring energy within the inlet zone, and other zones are with much uniform streamline. By dividing tundish into two zones with different inclusion removal mechanisms the inclusion removal is calculated. Three modes of inclusion removal from molten steel in the tundish, i.e., flotation to the free surface, collision and coalescence of inclusions to form larger ones, and adhesion to the lining solid surfaces, are taken into account. The Brownian collision, Stokes collision, and turbulent collision are examined and discussed. The suitable coagulation coefficient is discussed, and a value of 0.18 is derived. Calculation results indicate that, besides flotation, collision of inclusion and adhesion to the lining solid surfaces are also important ways for inclusion removal from molten steel in tundish especially for the smaller inclusions. The flotation removal holds 49.5 pct, and the adhesion removal holds 29.5 pct for the tundish with flow control devices; the collision effect is reflected in improving flotation and adhesion. Finally, industrial experiment data are used to verify the inclusion removal model.

Fundamentals of inclusion removal from liquid steel by bubble flotation

Attempting to remove the maximum number of inclusions from liquid steel by bubble flotation is an important means of improving steel cleanliness. There has, however, been no complete theoretical fundamental study on this issue, and the present review represents a tentative step towards this objective. An attempt is made to apply results from the field of mineral processing to the liquid steel–inclusion–bubble system for the case of silica inclusions and argon bubbling. However, it should be noted that this approach has limitations: the size of the bubbles and particles involved and the degree of turbulence, the surface tension of the liquid, and the contact angle between inclusion and bubble are quite different in the two systems. The mechanism of bubble–inclusion interaction, bubble size and rising velocity, liquid film formation and rupture between bubble and inclusion, collision time, film drainage time, sliding time, and collision and adhesion probabilities are discussed. A simple mathematical model of inclusion removal by bubble flotation is described, and the effects of gas flowrate, bubble size, and time on inclusion removal are discussed. Based on the results of the model, it is clear that the optimum bubble diameter for inclusion removal is 1–5 mm. It is proposed that the shroud protecting transfer of liquid metal from ladle to continuous casting tundish is a good place to inject gas to remove inclusions by bubble flotation. The effect of turbulence on the removal of inclusions by bubble flotation is briefly discussed.

Mathematical Modeling of Fluid Flow Phenomena during Tundish Filling and Subsequent Initial Casting Operation in Steel Continuous Casting Process.

The purpose of this study is to develop a mathematical model to analyze the fluid flow phenomena of molten steel in the tundish during its filling stage and subsequent initial casting operation in the continuous casting process of steel. The ultimate goal is to assure smooth initial casting operation without nozzle clogging by avoiding high deposition rate of inclusion on any of the tundish outlets during tundish filling and subsequent initial casting operation. The mathematical model is developed based on a computational fluid dynamics technique, named SOLA-MAC, and the k-e two-equation turbulence model. SOLA-MAC technique has the ability to handle the flow problem encountered in tundish filling, which is a transient flow problem with highly distorted free surfaces and the locations of the free surfaces are to be determined by theory. A fluid particle method is also employed in this study to analyze the distribution of inclusions in the molten steel and the extent of inclusion contamination in various strands of the continuous casting tundish. A water model that is one-fourth the scale of an actual billet continuous caster is also constructed in this study. Water model experiments are conducted to verify the accuracy and reliability of the mathematical model. The developed model is first tested on the water model to calculate the flow pattern of water in the tundish during the very early stage of filling operation. The simulated filling patterns are compared to the water model experiments. Good consistency is observed. The model is then tested on an actual billet continuous caster with four strands to simulate the fluid flow phenomena of molten steel in the tundish during the filling and subsequent initial casting operations. Inclusion distribution and the extents of inclusion contamination among the outlets of the various strands in the tundish are also analyzed. The simulated results show that for the left half of tundish, inclusion contamination in #2 strand is significantly more severe then that in #1 strand. This is confirmed by the actual experience on the shop floor of that particular billet caster that #2 strand experiences more difficulty in clogging problem during the initial casting operation than #1 strand does.

Investigation of transient fluid flow and heat transfer in a continuous casting tundish by numerical analysis verified with nonisothermal water model experiments

The tundish process is complicated due to the periodic nature of its operation during the changing of ladles, which makes it both a transient and nonisothermal process. A nonisothermal water model, with temperature variations in the inlet stream, is described in this article. A three-dimensional (3-D) transient mathematical model to simulate the fluid flow and heat transfer in this water model is also introduced. From the experimental and numerical results, it is shown that there is a significant thermal buoyancy force contributing to the flow in this nonisothermal water model.

Modeling of inclusion removal in a tundish

Mathematical models have been developed to predict the removal of alumina inclusions from molten steel in a continuous casting tundish, including the effects of turbulent collisions, reoxidation, flotation, and removal on the inclusion size distribution. The trajectories of inclusion particles are tracked through the three-dimensional (3-D) flow distribution, which was calculated with the K-ɛ turbulence model and includes thermal buoyancy forces based on the coupled temperature distribution. The predicted distributions are most consistent with measurements if reoxidation is assumed to increase the number of small inclusions, collision agglomeration is accounted for, and inclusion removal rates are based on particle trajectories tracked through a nonisothermal 3-D flow pattern, including Stokes flotation based on a cluster density of 5000 kg/m3 and random motion due to turbulence. Steel samples should be taken from as deep as possible in the tundish near the outlet and at several residence times after the ladle is opened, in order to best measure the Al2O3 concentration entering the submerged entry nozzle to the mold. Inclusion removal rates vary greatly with size and with the presence of a protective slag cover to prevent reoxidation. The random motion of inclusions due to turbulence improves the relatively slow flotation of small inclusions to the top surface flux layer. However, it also promotes collisions, which slow down the relatively fast net removal rates of large inclusions. For the conditions modeled, the flow pattern reaches steady state soon after a new ladle opens, but the temperature and inclusion distributions continue to evolve even after 1.3 residence times. The removal of inclusions does not appear to depend on the tundish aspect ratio for the conditions and assumptions modeled. It is hoped that this work will inspire future measurements and the development of more comprehensive models of inclusion removal. These validated models should serve as powerful quantitative tools to predict and optimize inclusion removal during molten steel processing, leading to higher quality steel.

Optimization of steel flow in the continuous casting tundish by water model simulation

In continuous casting, the distributing tundish is now considered as a metallurgical reactor with the task of improving the cleanliness of steel. Its ability to perform metallurgical functions such as dissolution, homogenization, inclusion flotation mainly depends on the flow conditions of the liquid steel passing through it.