R. D. Morales

Mathematical simulation of effects of flow control devices and buoyancy forces on molten steel flow and evolution of output temperatures in tundish

Abstract The effects of flow control devices and buoyancy forces on the melt flow in a large tundish have been mathematically simulated using a k–ϵ turbulence model. Flow control devices included arrangements consisting of a pair of weirs and a pair of dams, a turbulence inhibitor and a pair of dams, and only a turbulence inhibitor. Buoyancy forces were simulated using step inputs of temperature and inputs of varying ladle stream temperature into the tundish. It was found that with inputs of hot steel, flow control devices improve performance by driving the melt upwards through the action of buoyancy forces. A bare tundish is less sensitive to buoyancy forces and shows greater thermal mixing than any other arrangement. Inputs of temperature steps promote higher temperature gradients of liquid steel inside the vessel than inputs of varying ladle stream temperatures. A turbulence inhibitor delays the thermal disturbance compared with a bare tundish or a tundish with a weir–dam arrangement. When using a turbulence inhibitor higher volume fractions of melt obey a plug like flow. The dimensionless quantity Gr/Re2, where Gr and Re are the Grash of and Reynolds number respectively, quantifies buoyancy forces: high values indicate that buoyancy forces have more effect than inertial forces on fluid flow. When Gr/Re2 <5, buoyancy forces have no noticeable influence on fluid flow.

Melt flow optimisation using turbulence inhibitors in large volume tundishes

Abstract A water model of a typical 60 t slab tundish was designed and constructed to determine flow patterns characteristics when using a turbulence inhibiting device and dams; the patterns observed were compared with those predicted by a numerical model. Parameters studies included minimum residence time, volume fractions of piston (plug), perfect mixed, and dead flow, and tracer dispersion. From the quantitative results obtained by water modelling, an optimum flow control system was identified. Further evaluations were carried out to determine the systems performance under different flowrates and during a change of steel grade. The bare tundish and the selected arrangement were mathematically simulated to compare the tracer dispersion of the water model with the flow patterns predicted by the computer model. The results show the importance of using these flow control devices to increase productivity and steel quality.

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

Buoyancy effects on the fluid flow patterns of molten steel in a tundish of a continuous slab caster were mathematically simulated. Thermal responses of step-input temperatures in steel for different flow rates were predicted. The molten steel velocity profiles were determined for these temperature and flow rate variations. The importance of natural flow convection was established through a dimensionless number given by the ratio Gr/Re2. Buoyancy forces proved to be as important as inertial forces, especially in the extremes of the tundish. The simulations indicated that these forces increase the residence time of the molten steel in the tundish, flowing near the slag layer, allowing more time for the non-metallic inclusions to be captured; also, that the turbulence inhibiting device helps to redirect the flow towards the top free surface. It is shown that this device helps to decrease turbulence near the entry zone and has a dumping effect on the temperature step inputs allowing better control of the casting temperature. The traditional flow control devices like weirs and dams were not as effective as the turbulence inhibitor.

Ladle Shroud as a Flow Control Device for Tundish Operations

The performance characteristics of a tundish, such as the flotation of inclusions and slag entrainment, are largely influenced by the fluid-flow phenomena. Physical modeling in water is widely used to understand the fluid flows in a tundish and as a tool to improve, control, and design procedures for high-quality steel processing operations. These approaches were used to study the performance of fluid flow for a new design of ladle shroud. The new design for a dissipative ladle shroud (DLS) was studied, using a one-third scale, delta shaped, four-strand tundish. The results were compared with those achieved with the conventional ladle shroud. Different cases have been analyzed, including a conventional ladle shroud (LS) with a bare tundish and a tundish furnished with an impact pad. Similarly, the new design of the shroud (DLS) was studied under equivalent conditions. The physical experiments included the use of particle image velocimetry (PIV) and conductivity tracer techniques. The PIV measured the instantaneous velocities at the outlet of the DLS and the LS at different flow rates, showing the detailed jetting characteristics of water leaving the two types of ladle shroud. Residence time distribution (RTD) curves were also obtained for the different flow arrangements previously mentioned, and the dispersion of a colored dye tracer was observed at different intervals of time during tundish operation and analyzed using the video visualization technique.

Looking into continuous casting mould

Abstract When you look into the continuous casting mould you can see very little. Consequently, steelmakers have had to rely on plant trials, simulation experiments and physical property measurements on fluxes and steels to gain an understanding of the mechanisms responsible for process problems and product defects. However, in recent years, mathematical modelling has advanced to the stage where they can provide us with great insight into these mechanisms. As a non-mathematical modeller, I was initially sceptical of some of the predictions of the mathematical models. However, I have been completely won over by the ability of these models to simulate accurately the mechanisms responsible for various defects, such as slag entrapment, oscillation mark formation, etc. Mathematical modelling literally allows us to ‘see’ what is happening in the mould. It is a remarkable tool.

Effects of EMBr position, mould curvature and slide gate on fluid flow of steel in slab mould

Abstract Influences of mould curvature, slide gate, electromagnetic brake (EMBr) position and magnetic forces on steel flow in a slab mould were studied using a mathematical model. Positions of the EMBr include magnets at the same level as the discharging ports of the submerged entry nozzle (SEN) and magnets below the SEN tip. The slide gate induces a biased flow. Regarding the EMBr in the first position, it was found that increasing the magnetic flux density leads to rises of the discharging jets and the elimination of the two upper roll flows with lower roll flows with smaller velocities. In the case of the second position, the EMBr eliminates the effects of the biased flow in the meniscus velocity profile and induces a downward uniform flow eliminating the lower roll flow. In either case, the magnetic flux density does not affect the velocity profile in the discharging ports in the SEN.

Concept of dynamic foaming index and its application to control of slag foaming in electric arc furnace steelmaking

Abstract To sustain a foam in steelmaking processes, two basic requirements should be fulfilled, i.e. appropriate physical properties of the slag such as high viscosity, low density, and low superficial tension, and the generation of sufficient reaction gas. To date, foaming indexes have been focused on the physical properties of refining slags. In the present paper a dynamic foaming index (DFI) that involves both above requirements is proposed, using a kinetic model of the electric arc furnace process to calculate the generation rate of reaction gas, mainly CO. When the arc distortion, as affected by electrode submergence in the foam, is compared with the DFI, calculated via the kinetic model, it is observed that both parameters follow very similar trends. This finding indicates the feasibility of knowing the foaming conditions of a heat in advance, or of using the kinetic model online to control the foaming phenomena. Furthermore, experimental results relating to dynamic behaviour of the slag chemistry are well simulated using the kinetic model. To take into account the effect of size distribution of carbon particles injected into the slag to reduce FeO, a Monte Carlo simulation has been integrated into the process simulator, allowing a more realistic prediction of the current steelmaking process.

Multiphase Flow Modeling of Slag Entrainment During Ladle Change-Over Operation

Steel transfer from the ladle to a single-strand tundish using a conventional ladle shroud (CLS), and a dissipative ladle shroud (DLS) is studied during the transient period of ladle change-over operation. Fluid velocities and fluid flow turbulence statistics during this unsteady operation were recorded by an ultrasound velocimetry probe in a 1/3 scale water–oil–air analog model (to emulate steel-slag-air system). Reynolds stress model and volume of fluid model allow the tracking of water–oil, water–air, and oil-air interfaces during this operation. Velocity measurements indicate a very high turbulence with the formation of a water–air bubbles-oil emulsion. Flow turbulence and the intensity of the emulsification decrease considerably due to an efficient dissipation of the turbulent kinetic energy employing the DLS instead of the CLS. The modeling results indicate that DLS is widely recommended to substitute flow control devices to improve the fluid dynamics of liquid steel during this transient operation.

Improvement of steel billet cleanliness and thermal responses of multiple strands through fluid flow control

Abstract Fluid flow of liquid steel in a six-strand billet tundish using a simple squared advanced pouring box (APB) promotes the formation of heavy skulls, particularly in the corners of the vessel, and heterogeneous product cleanliness and thermal responses in the strands. To improve the flow patterns, two other APBs were designed and tested through water model tracer injection techniques, ultrasound velocimetry and mathematical simulations. The first APB is the actual design. The second APB maintains the same geometry but is equipped with two circular orifices with zero angle respect to the horizontal at each lateral side placed toward the tundish back wall. The third APB geometry is also squared with elliptic orifices with double upward angles also oriented to the tundish back wall. The first two devices induce long shearing–circulating flows, leaving stagnant regions inside the fluid bulk and particularly in the four upper corners of the tundish, which explains the existence of skulls in the actual tundish. The third device provides a long recirculating flow that eliminates all stagnant regions. Mathematical simulations have indicated that this flow carries energy, through convection mechanisms, toward the upper bath surface, indicating the elimination of skulls. Trials at the caster proved that all skulling problems were eliminated, and thermal and metallurgical evenness was reached using this simple flow control device.

Simulation of the aluminum alloy A356 solidification cast in cylindrical permanent molds

A mathematical model based on the control volume method with fixed mesh was selected in order to simulate the solidification of cylindrical castings poured in permanent steel mold. The latent heat was incorporated using the effective specific heat. The application of the model allowed us to obtain the solidification front and the temperature fields at any time from the pouring. The mold was made of the SAE 1010 steel. Two mold temperatures were evaluated: 25°C and 300°C. The mathematical model showed sensitivity to changes in mold temperatures. For the casting poured with an initial mold temperature of 300°C, the solidification time was greater than that of the casting poured in the mold at 25°C. When the perfect contact condition between the mold and the metal was considered, the theoretical solidification times were shorter than the experimental results. When the imperfect contact supposition was assumed, this resulted in longer times of solidification very close to the experimental data. A reasonable fitting was reached when the heat transfer coefficient between mold and casting surfaces in the range of 100 to 500 W/m 2 °K was used for the experiments with the mold at 25°C.