Study on Process Technology of Gas Bubbling Curtain Substituting Dam in the Tundish of Slab Casting

This paper presents experimental results from physical and mathematical simulation of liquid steel flow in the tundish adopting gas bubbling curtain substituting dam and industrial trials have been investigated based the experimental results. Physical and mathematical simulation results show the gas bubbling curtain can improve pattern of liquid flow in the tundish , accelerate inclusions flotation,and simultaneity metallurgy effect has been further improved when combining weir and turbulence inhibitor. The industrial trial results confirm that content of oxygen , nitrogen and inclusions in continuous casting billets have been reduced when gas bubbling curtain combining weir and turbulence inhibitor.     

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.     

中间包上水口环形吹氩下气泡大小和迁移行为研究

摘要：通过水模型实验研究了上水口环形吹氩工艺下中间包和结晶器内气泡形貌,并结合数值模拟分析了透气砖位置、拉坯速度和吹氩量对中间包和结晶器内气泡尺寸、气泡迁移和中间包近液面钢液流动的影响。结果表明：上水口环形吹氩形成以塞棒为中心的圆台状气泡羽流,气泡浓度沿径向向外逐渐减少;附壁效应使得气泡羽流偏向塞棒壁面流动,增大气泡的碰撞聚并概率和近塞棒壁面的羽流上升速度,对中间包液面产生较大冲击作用;同时,部分细小气泡会随钢液进入水口及结晶器内部;增大吹氩量,中间包内环形气泡羽流中气泡数目明显增多,中间包近液面钢液上升速度增大;增大拉坯速度,环形气泡羽流的宽度和气泡数量逐渐减小,近液面速度减小;增大透气环距水口中心距离,中间包内气泡弥散度增大,环形气泡羽流宽度也随之增大,气泡羽流对中间包液面冲击作用减弱;增大吹氩量和拉坯速度、减小透气环距水口中心距离,进入结晶器的气量和气泡尺寸逐渐增大。实验条件下,透气环内外径为110mm/140mm、拉坯速度为1.2m/min时,吹氩量为4L/min较为合适。     

Optimization of Two‐Strand Industrial Tundish Work with Use of Turbulence Inhibitors: Physical and Numerical Modeling

The paper shows the results of the research obtained by physical and mathematical modeling of steel flow and mixing in the tundish. Two‐strand continuous casting tundish was under consideration. It has been working in one of polish steel plants. The change of concast slab assortment was caused by the changeable market terms. So, the tundish with the new system of steel flow controller was needed. Up to now baffles with the notch have played this role. Their placement cause the excessive consumption of the lining of the tundish front line. As a consequence the turbulence inhibitor (TI) was applied. Four different types of this inhibitor were designed. As a result of the experimental measurements and numerical simulations, the RTD curves of F‐type were obtained. Basing on these curves time constants for examined types were determined. Additionally, the research results were complemented by the E‐type curves. The percentage participations of dead volume flow, dispersed plug flow, and well‐mixed volume flow were calculated. The research gives possibility to estimate the designed TIs and their influence on the tundish work.     

Fluid Flow,Heat Transfer and Inclusion Motion in a Four‐Strand Billet Continuous Casting Tundish

Inclusions in the steel in a four‐strand continuous casting tundish, billet and wire products are firstly investigated with industrial trials, and the fraction of inclusions removed in terms of total oxygen in the tundish is measured. Then the 3‐dimenional fluid flow, heat transfer and inclusion motion in the tundish are numerically simulated. The κ‐? two‐equation model is used to model turbulence. Inclusion motion and trajectories are calculated by considering drag force and buoyancy force, coupling the effect of turbulent fluctuation (Random Walk Model). The effect of strands‐blocking on the fluid flow, heat transfer and inclusion removal is studied. A new design of tundish is proposed focusing on removing more inclusions from the molten steel.     

两流板坯连铸58t中间包过滤器的水模型试验和应用

通过几何相似比0.29:1的水模型试验了湍流抑制器+挡墙+挡坝和湍流抑制器+挡墙+过滤器两种控流装置的钢液流动,研究了通道式过滤器对58t中间包钢液流场的影响。结果表明,原中间包（湍流抑制器+挡墙+挡坝）活塞区体积小,死区体积高达29.53%,优化中间包加入过滤器后（湍流抑制器+挡墙+过滤器）短路流基本消失,钢液的实际平均停留时间延长,死区体积由29.53%减小至13.52%。50t中间包,230mm×1100mm连铸板坯,拉速1.25~1.30m/min工业生产结果表明,使用过滤器后,中间包浇注区的夹杂物尺寸明显小于冲击区,中间包浇注区T[O]由原86×10^-6降至30×10^-6,连铸坯大多数夹杂物尺寸≤10μm,没有发现≥30μm夹杂物。     

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).     

Study on dimension and migration behavior of bubble under annular argon blowing at tundish upper nozzle

The water model experiments were carried out to study the bubble morphology in the tundish and mold with the process of annular argon blowing at tundish upper nozzle. The effects of the position of gas permeable brick, the casting speed and the argon flow rate on the bubble size distribution, the bubble migration behavior and the flow behavior of liquid steel near the liquid level in tundish were further investigated, coupled with the numerical simulation. The results show that with the process of annular argon blowing at tundish upper nozzle, a frustum cone shaped bubble plume can be formed around the stopper rod. The concentration of argon bubbles gradually decreases outward along the radial direction of the stopper rod. Owing to the wall attached effect, the bubble plumes float upward along the stopper rod, which can increase the collision probability between bubbles and the velocity of bubble plumes, causing a larger impact strength on the liquid level in tundish. In addition, a part of small bubbles are wrapped into the nozzle and the mold due to the drag force of liquid steel. With increasing argon flow rate, the number of bubbles in annular bubble plumes and the vertical velocity of liquid steel near the liquid level in tundish increase significantly. With increasing casting speed, the width and the bubble number of annular bubble plumes gradually decrease, leading to a decrease of the vertical velocity of liquid steel near the liquid level in tundish. Increasing the distance between the annular gas permeable brick and the center of tundish upper nozzle, the dispersion of bubbles and the width of bubble plumes increase, and the impact strength of bubbles acting on the liquid level in tundish becomes weaker. As the argon flow rate and the casting speed increase, and the distance between the gas permeable brick and the center of tundish upper nozzle decreases, the gas volume and bubble size in the mold increase. Under the experimental conditions, when the inner and outer diameters of the annular gas permeable brick are 110mm and 140mm, respectively, and the casting speed is 1.2m/min, the appropriate argon flow rate is 4L/min.     

底吹气中间包内钢液流动及夹杂物去除模拟研究

通过40 t中间包的1:3水模型实验,对小方坯连铸6流中间包内钢液的流场进行了测定,研究了5种不同底吹气方案对中间包内钢液流动特性和夹杂物去除的影响。结果表明,在注流区吹气能延长钢水在中间包内的停留时间,减弱湍流强度,同时注流区内吹气可将气泡击碎成弥散小气泡促进了夹杂物上浮,夹杂物去除效果最佳。在近流与中流水口之间吹气,可改善各流的流动特性,中间包内的流动状况最佳,但近流处夹杂物去除效果略差。在中流和远流水口之间吹气,流场改善效果不理想。     

ASF方坯八流中间包内钢液流场的模拟研究

以某钢厂的八流连铸中间包为研究对象,采用数值模拟方法对不同控流结构中间包内钢液流场进行了研究,并通过水模型实验和工业生产进行验证。模拟结果表明:ASF中间包内的挡墙和湍流器能够明显改善钢液的流动状态和温度分布,加上双坝后效果更佳,钢液的流动存在4个环流区,不仅增加了钢液的混合程度,而且中间包内钢液的温度分布更均匀且低温区较少。水模试验表明在空包中加入湍流器和挡墙能明显改善各流流动特性的一致性,且在其基础上加入双坝能进一步改善钢液的流动特性,与数值模拟结果一致,此外,工业生产也完全达到预期效果。     

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 behaviour in slab continuous casting tundish with different configurations of gas bubbling curtain

Abstract

Fluid flow characteristics in a two strand slab continuous casting tundish with different configurations of argon gas bubbling curtain (GBC) were investigated in physical modelling experiments. It was found from this research that the GBC with a small flow rate acted as a gas dam and could greatly improve the flow characteristics in the tundish. It increased dramatically the peak concentration time and plug flow volume, decreased greatly the dead volume, created surface directed flow and eliminated short circuiting. Therefore, the fluid flow characteristics in a tundish with GBC were favourable to the floatation and separation of inclusions from molten steel. The flow characteristics with low gas flow rate and short distance of the bubbling curtain from the tundish outlet were better than those with high gas flow rate and large distance of the curtain to the outlet. The optimal configuration for the improvement in fluid flow characteristics was turbulence inhibitor (TI)–weir–dam–GBC (TI–W–D–GBC), followed by TI–channel weir (CW)–GBC, TI–W–GBC and TI–GBC.     

降低大方坯中间包临界液面高度工艺研究

中间包底部水口附近结构和流场形态影响中包临界液面高度。通过在中间包底部水口附近加抑流件的水模型优化实验及工业试验,确认了加抑流件可以降低中包临界液面高度,在工业生产中抑流件使用是安全的,中间包合理停浇液面对尾坯纯净度无影响。     

连铸中间包内钢液流动特性及控流技术

中间包内钢液的流动状态对钢中夹杂物的去除效率影响较大。增加中间包容量可使铸坯内弧—侧Al2O3总量显著降低，在中间包使用挡墙和坝可减少内、外侧钢流温度差和拉漏发生率，去除钢中夹杂物。过滤器、湍流缓冲器和旋涡控制装置等相关技术的应用均提高了铸坯的清洁度。     

Water Modelling Experiments of Argon Bubbling Curtain in a Slab Continuous Casting Tundish

Fluid flow characteristics in a two‐strand slab tundish with Ar bubbling curtain were studied in water modelling experiments. It was found that the Ar bubbling curtain can greatly improve the flow characteristics in the tundish with a weir, a dam and a turbulence inhibitor. It dramatically increased the peak concentration time and plug volume and greatly decreased the dead volume, but hardly influenced the minimum residence time. Therefore, the fluid flow characteristics in a tundish with Ar bubbling curtain were favourable to the flotation and separation of inclusions from molten steel. The flow characteristics with low gas flow rate and short distance of the Ar bubbling curtain from the tundish outlet were better than those with high gas flow rate and large distance of the curtain from the outlet.     

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.     

Modelling the effects of off-centered ladle streams on fluid flow of liquid steel in a slab tundish

The effects of off-centered ladle streams on fluid flow of liquid steel in a two-strand slab tundish were studied using water modelling techniques and particle image velocimetry (PIV) measurements. Fluid flow in an off-centered ladle stream was compared with a centered ladle stream. In a centered position the proposed arrangement (turbulence inhibitor plus dams) yields higher fractions of plug flow than the current arrangement (baffles). This effect is stronger at the high casting rate. An off-centered position increases dead volumes at a low casting rate of 1.7 t/min. At the high casting rate of 3.8 t/min, effects of the off-centered position are decreased although the TI-D arrangement always renders better flow characteristics. Velocity fields determined with PIV measurements, demonstrated that the TI-D arrangements provide the fluid with a plug flow characteristic.     

连铸中间罐采用新型控流装置的模拟试验

结合本钢炼钢厂板坯中间罐的控流装置,通过实验室对比试验研究,开发了新型控流装置——湍流控制器和微细泡气幕挡墙。试验结果表明,二者大大地改善了中间罐流体流动特征参数和流动轨迹。并且还具有防止注流开浇产生喷溅和减轻注流卷吸气体的作用。     

Theoretical modelling of an isokinetical steel feeding system for near-net-shape strip casting

During near-net-shape casting as considered here, liquid steel in thicknesses between 5 and 15 mm has to be put onto a cooled belt recirculating with speeds of 30 to 60 m/min. On the belt the liquid steel solidifies, before it can be subjected to one or two in-line hot deformation steps. The requirement that the surface and the shape of the strips should already have the best possible structure upon being cast means that high demands are placed above all on the liquid steel feeding system. Especially the liquid metal should ideally be cast with the lowest possible turbulence and at a casting speed which is equal to the speed of the conveyor belt or cooled conveyor belt. In other words, the relative speed between the cast liquid and the cooled conveyor should become “zero” in order that the cast liquid can be more or less “laid” onto the cooled conveyor (known as isokinetic feeding). Different kinds of feeding liquid metals onto a circulating belt are reported with main emphasis on the theoretical modelling of the so-called underpressure feeding systems. It is shown that, on the one hand, the casting speed can be regulated independently of the input mass flow rate into the tundish and, on the other hand, the liquid can be accumulated in the tundish or discharged from it at constant casting speed.     

Effects of tundish size,tundish design and casting flow rate on fluid flow phenomena of liquid steel

Fluid dynamics of liquid steel in continuous casting tundishes is closely related to tundish volume and geometry, existence of flow control devices and steel flow rate. To study this complex interaction physical and mathematical models were used in the present work. The first one was based in a 1/3 scale water model with injection of tracers and the second one on the solution to the steady state-turbulent Navier-Stokes equations using the K-ε [1] approximation for the turbulent viscosity. When the tundish size is increased from 30 t to 50 t the tracer indicates a strong bypassing in the second case. The mathematical predictions indicate very high fluid velocities along the tundish bottom in agreement with the experimental findings. The employment of turbulence inhibitors promotes a counter-flow that surrounds the incoming stream jet of liquid from the inlet nozzle with steel displacing itself, after leaving this zone, along the upper free surface of the liquid. The addition of well designed baffles complements the action of the turbulence inhibitor to obtain a higher volume fraction under a plug flow pattern giving a softer flow of liquid steel. Besides, the positioning of baffles inside a tundish should be performed according to the steel flow rate.