Steel flow and inclusion separation in continuous casting tundishes

A model for the calculation of flow patterns and inclusion separation in continuous casting tundishes is described. Velocity and turbulence fields for the liquid steel are calculated, assuming three-dimensional, turbulent steady-state flow. A transport equation for particles is solved, which takes into account buoyancy, convection und turbulent dispersion. Particle concentration fields and the percentage of removed particles are calculated as a function of particle rise velocity. The influence of increased tundish width and height and of dams and weirs on the rate of inclusion separation is investigated for a slab caster tundish. Non-dimensional representations and approximation expressions are discussed and used to compare the computed removal rates to measured values from literature.     

Mathematical Modelling of Fluid Flow in a Continuous Casting Tundish with regard to Extraordinary Casting Conditions

The quality of the steel (degree of cleanness in the finished material) can be influenced to a great extent by the flow conditions in the continuous casting tundish and the resulting conditions for the separation of non‐metallic inclusions. Whereas in the literature a lot of numerical simulations about the flow patterns of regular casting tundishes have been published, the effects of disturbances like the absence of a strand or the progressive wear of the flow control devices in the tundish on the flow behaviour will be investigated here. It will be shown that the absence of single strands exerts much less influence on the total flow behaviour in the tundish than variances of the flow control devices.     

Numerical and Physical Simulation of Flow Patterns in a Swirling Flow Tundish

Swirling flow tundish is a new kind of tundish which has shown good effects on flotation of inclusion and reduction of inclusion content. In this paper, studies have been carried out on the flow fields in a one‐strand slab tundish. A full scale model of the flow patterns in the water model tundish was developed using a self‐developed code. RTD curves under different experimental conditions were obtained from both physical and numerical simulations. The effects of the swirling flow chamber geometry and the flowrate on flow patters in the tundish were discussed and compared with results from the numerical simulation. Validation of the self‐developed codes was achieved by comparing the physical and numerical results of the RTD curves and the mean rotational velocities in swirling flow tundish. As a result, significant rotational flow in the swirling flow chamber and asymmetrical flow pattern in the whole tundish were confirmed and the effects of these parameters on dead zone and mean residence time were also obtained. Further and more comprehensive studies are needed to optimize the design and application of such tundishes.     

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).     

Mathematical modelling of the geometry influence of a multiple‐strand tundish on the momentum,heat and mass transfer of steel flow

Simple changes on tundish geometry may lead to significant improvements of transport phenomena of liquid steel in tundishes. In the present case steel flow in a six‐strand billet trough type tundish is mathematically simulated. Numerical results indicate the existence of a high fluid turbulence in the pouring zone and recirculating flows. Steel temperatures in the strands are also different, which from practice it would mean different qualities of billet among the strands. A simple change of design by widening the pouring box improves all the steel flow characteristics. First the turbulence in the pouring box is decreased, the recirculating flows are eliminated and steel temperatures in the six strands become closer to each other. Using a computational technique known as volume of fluid, surface topography of bath including the covering slag was simulated for both types of tundishes. These simulations predicted an open eye of the slag layer for the first tundish while in the second this phenomena was avoided. Thus, it was demonstrated the original hypothesis that small changes in tundish design may lead to a more controlled steel flow.     

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.     

Separation of nonmetallic particles in tundishes

The Separation of nonmetallic particles by buoyancy in tundishes is investigated theoretically. It is shown, that the best possible separation is achieved it the projection of the region through which flow from the inlet to the outlet mainly occurs, covers as large a part of the surface as possible. In contrast, the volume of this region, its distance from the surface and the mean retention time do not influence separation directly. Numerical calculations of the flow fields and particle separation in two different tundish geometries with and without a baffle predict no significant differences in the capability of separating nonmetallic particles. This surprising result may be explained by the principle stated above. In contrast to these findings, observations made during production provide indirect evidence that the insertion of a baffle reduces the concentration of Al₂O₃ at the outlet of the tundish significantly. This would mean that besides buoyancy, another process contributes significantly to the separation of nonmetallics. Such a process is not identified in this paper, but it is shown that turbulent diffusion and inertial impaction are not likely candidates.     

连铸中间包内钢液流动特性分析模型的研究进展

介绍了中间包内钢液流动特性分析模型的研究进展,针对中间包内钢液的去夹杂流动特性,论述了典型的单、多流中间包去夹杂流动特性分析模型,并通过对比分析找出了较优的单、多流中间包去夹杂流动特性分析模型。对于多流中间包内钢液的流动特性,不仅要考虑非金属夹杂物的去除,还要考虑多流中间包内各流流动特性的一致性,对比分析了典型各流流动特性一致性分析模型的特点并找出了合适的分析模型。所找到的合理的去夹杂及各流流动特性一致性分析模型为中间包内钢液流动特性分析提供了理论依据。     

Mathematical modelling of flows and discrete phase behaviour in a V‐shaped tundish

In steelmaking processes, various efforts are made in order to enhance steel cleanness. Among other approaches, the volume of the tundish has been increased and flow‐controlling devices are used. The extended mean residence time of steel in the tundish leads to an improved inclusion separation. In the present study, the degree of inclusion separation in different tundish configurations is determined through numerical modelling. Flows in the tundish has been calculated under isothermal and non‐isothermal conditions. The numerical model is based on a Euler Lagrange approach. The flow and the temperature field are described through Reynolds averaged transport equations in conjunction with a turbulence model.     

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.     

The effect of tundish wall inclination on the fluid flow and mixing: A modeling study

A mathematical model to represent turbulent fluid flow and mixing in continuous casting tundishes has been developed. The model involves solution of the three-dimensional turbulent Navier-Stokes equation, turbulence being modeled by the so-calledK-ε, two-equation model. Fluid flow parameters and residence time distribution has been predicted in a tundish of rectangular cross section. The model is later extended to predict fluid flow in typical industrial tundishes where walls are not vertical, but rather slightly inclined from the vertical. This results in an interresting variation in fluid flow, which may have important technological implications. The theoretical predictions are compared with measurements obtained in water models. The detailed understanding of the hydrodynamics of the tundish flow can be used to optimize their design for steel cleanliness. YOUDUO HE (on leave from Boutao Institute of Iron and Steel Technology, People's Republic of China) Research Associate     

Optimisation of baffles in six strand round bloom continuous casting tundish: a physical modelling study

Abstract

Physical modelling using water in a one-third scale model was carried out to ascertain the influence of various types of baffles with inclined holes on the liquid flow in a six strand round bloom continuous casting tundish. To characterise the flow in the tundish, residence time distribution (RTD) curves were measured for different types of baffles with inclined holes. Because there is no well known analysis model to characterise the melt flow in multistrand tundishes, a new model was presented to analyse RTD curves and its reasonability was discussed. Furthermore, a new approach for quantifying the similarity among the strands was proposed and the baffle was optimised to improve the inclusion floatation and strand similarity in the tundish.     

连铸中间包内夹杂物去除的水模拟试验与工业实践

通过中间包水模型试验和工业试验,研究了U型挡墙中间包和Y型挡墙中间包对夹杂物去除的影响。结果表明：Y型挡墙中间包能提高夹杂物的去除率。与U型挡墙中间包相比,从中间包到铸坯过程钢中平均总氧去除率显著提高,铸坯中大型夹杂物含量显著减少,而显微夹杂数目变化不大;同时Y型挡墙中间包内各流夹杂物分布较均匀。使用U型挡墙中间包时,中间包到铸坯过程吸氮较少。     

Investigation of Flow Characteristics in a Six‐Strand CC Tundish Combining Plant Measurements,Physical and Mathematical Modeling

The tundish plays a major role in the continuous casting process. The flow in a tundish has a very substantial effect on the quality of the final product and on efficient casting conditions. Efforts are being made worldwide to obtain the most favourable shape of tundish interior by using dams, weirs and gas curtains. The aim of these flow control devices is to reduce the dead zone areas and improve the conditions for the separation of non‐metallic inclusions. Numerous model studies are being carried out to explain the effect of the tundish working space shape and steel flow conditions on the inclusions floating processes. The presented article shows the results of investigations performed to obtain the mass exchange characteristics in the investigated tundish. The measurements were done directly at the steel plant during normal working conditions. By controlling the changing content of manganese in steel, the residence time distribution (RTD) characteristics were acquired. The RTD characteristics are also obtained with a water model of the tundish with dimensional scale of 1:3. Parallel to the water model, numerical simulation based on mathematical modelling of fluid flow, relying on the system of differential equations, is employed in the research work. Numerical simulations were carried out with the finite‐volume commercial code FLUENT using the standard k‐ε turbulence model. The primary purpose of the investigations carried out is to present the characteristics describing the transitory zone in a six‐strand tundish. It is shown that the F‐curve, describing the transitory zone, can be obtained by using different measurement techniques. Tracer concentration characteristics for the model of tundish obtained from both modelling techniques ‐ physical as well as numerical ‐ are very similar.     

重钢5流方坯连铸中间包控流装置的数理研究

对重钢24t中间包控流装置进行水力学模型试验和数值模拟,通过测定停留时间分布(RTD)曲线和夹杂物的排除量,研究了不同尺寸湍流控制器对中间包流体流动特性的影响。研究结果表明,使用合适的湍流控制器和现有挡墙设置组合,可以延长水口响应时间及平均停留时间,提高活塞流区体积分数30%及降低死区体积分数50%,中间包内流体流动特性得到改善。     

Particle Distribution and Separation in Continuous Casting Tundish

The tundish as a part of a continuous casting machine combines the discontinuous ladle metallurgy with the continuous solidification of slabs in the mould. The tundish plays a major role in the challenging task of “clean steel” production. That means the smallest number of inclusions and high cleanliness in all steel grades after changing the conditions at the inlet of the tundish. Inclusions hinder the metal forming process and lead often to fatigue. The cleanliness of steels is important to fulfil the customer's requirements. In the present study inclusion removal was simulated in a 1:3 scaled water model of a single‐strand tundish for the production of stainless steels with a particle counter. The particle counter is capable of counting a large number of particles with a wide range of diameters. The separation rate for particle diameters from d_P = 1 ‐ 250 μm was determined with a counter for the water model tundish. With similarity conditions for the particles this deposition rate can be transformed to the melt flow in a steel tundish. The separation rate was measured for different flow rates in the water model tundish. A larger flow rate decreased the separation rate. Additionally, the separation rate for the tundish fitted with an impact pad was measured and showed a significant increase of separation for particles with a smaller diameter. Furthermore, the particle distribution in the tundish for different size groups of particles was investigated with and without an impact pad. Numerical simulations were carried out with the finite‐volume commercial code FLUENT using the realizable k‐ε turbulence model. A special boundary condition for the separation of particles at the surface was implemented.     

基于PXI技术的中间包流场优化的水力学试验

试验利用PXI连铸综合水力学试验平台,采用1∶2的中间包水模型研究坝堰和多孔挡墙对某钢厂180mm×240mm小方坯六流连铸中间包中流动的影响,通过分析不同方案中间包的相关区域的流场特性和停留时间分布曲线（RTD曲线）来得出相关试验结论。试验结果表明,无坝堰和多孔挡墙的中间包内注流区的流体的流动均匀性不好,同时响应时间(7s)和峰值时间(224s)均比较短;优化后的中间包内的流体的流动特性得到较大程度改善,响应时间和峰值时间分别延长99和159s,死区体积减小了45.71%,活塞流体积从12.03%增大到25.47%。     

Liquid steel flow in continuous casting machine: modelling and measurement

Abstract

Single phase (liquid steel) and two-phase (liquid steel and argon bubbles) three-dimensional computational fluid dynamic and heat transfer models were developed for the continuous casting machines of ArcelorMittal. The computational domains include tundishes, slide gates, submerged entry nozzles and moulds. The effects of buoyancy, tundish design, tundish practices, nozzle design and caster practices on flow structure were investigated. Mathematical modelling is discussed in detail. In addition, submeniscus velocity measurements in the slab caster mould are performed with the method of torque measurement. A consumable probe is inserted into the liquid steel meniscus from the top of the mould through mould powder and slag layer. The liquid steel flow applies a drag force to the probe, which then generates a torque. This torque value is measured and then converted back to velocity. The concept and challenges of the technique are discussed, and the effects of casting parameters on mould flow structure are investigated. Product quality in relation to real time meniscus velocity measurements is also discussed.     

Numerical Investigation of the Fluid Flow in Continuous Casting Tundish Using Analysis of RTD Curves

 A detailed mathematical procedure of the optimization of the fluid flow in a tundish water model with and without flow control devices (weir and dam) was carried out using the commercial CFD code FLUENT 60. The (k ε) two equation model was used to model turbulence. The residence time distribution (RTD) curves were used to analyze the behavior of the flow in tundish. The location of flow control devices in the tundish was studied. The results show that the flow modifiers play an important role in promoting the floatation of nonmetallic inclusions in steel. Comparing the three geometric configurations that are considered (bare tundish, weir, weir+dam), the tundish equipped with the arrangement (weir+dam) is a best and optimal geometric configuration of tundish.     

Inclusion behavior and heat-transfer phenomena in steelmaking tundish operations: Part I. Aqueous modeling

An in-house computer code, METFLO 3D, which can model three-dimensional (3-D) turbulent flow, heat transfer, and inclusion flotation, has been developed for steelmaking tundishes. Also, sensor equipment has been developed for the continuous detection of particles suspended in aqueous systems. The probe, based on the resistive pulse principle, withdraws a continuous sample of the feed water to be monitored. A full-scale (isothermal) water model tundish (Stelco Research Centre, Burlington, Canada) was used to test experimental data on particle separation within the tundish. These experimental data were compared with predictions calculated by the mathematical model developed for tundish flows, and satisfactory agreement was achieved. Formerly Doctoral Candidate, Department of Mining and Metallurgical Engineering, McGill Metals Processing Centre, McGill University. Macdonald Professor of Metallurgy