Turbulent flow of liquid steel and argon bubbles in slide-gate tundish nozzles: Part II. Effect of operation conditions and nozzle design

A three-dimensional (3-D) finite-volume model, developed and validated in Part I of this two-part article, is employed to study steady-state two-phase turbulent flow of liquid steel and argon bubbles through slide-gate tundish nozzles. Parametric studies are performed to investigate the effects of gas injection, slide-gate orientation, casting speed, gate opening, bubble size, port angle, and port shape on the flow pattern and characteristics of the jet exiting the nozzle port. Argon gas injection bends the jet angle upward, enhances the turbulence level, and reduces the size of the backflow zone. Gas injection becomes less influential with increasing casting speed. The off-center blocking effect of the slide gate generates an asymmetric flow that changes with the gate orientation. The 0-deg gate orientation creates the worst biased flow between the two ports. The 90-deg orientation generates significant swirl and directs the jet slightly toward one of the wide faces. The 45-deg orientation generates both types of asymmetry and, thus, appears undesirable. The horizontal jet angle indicates asymmetric flow in the horizontal plane. It increases with decreasing gate opening and decreasing gas injection rate and ranges from 3 to 5 deg. Most jet characteristics reach their maximum or minimum values near the critical opening of 60 pct (linear).Larger bubbles exert a greater influence on the flow pattern. The vertical jet angle becomes steeper with a steeper port angle and more slender port shape. These results will be useful for nozzle design and for future modeling of flow in the mold.     

Turbulent flow of liquid steel and argon bubbles in slide-gate tundish nozzles: Part I. model development and validation

The quality of continuous-cast steel is greatly affected by the flow pattern in the mold, which depends mainly on the jets flowing from the outlet ports in casting with submerged tundish nozzles. An Eulerian multiphase model using the finite-difference program CFX has been applied to study the three-dimensional (3-D) turbulent flow of liquid steel with argon bubbles in slide-gate tundish nozzles. Part I of this two-part article describes the model formulation, grid refinement, convergence strategies, and validation of this model. Equations to quantify average jet properties at the nozzle exit are presented. Most of the gas exits the upper portion of the nozzle port, while the main downward swirling flow contains very little gas. Particle-image velocimetry (PIV) measurements are performed on a 0.4-scale water model to determine the detailed nature of the swirling velocity profile exiting the nozzle. Predictions with the computational model agree well with the PIV measurements. The computational model is suitable for simulating dispersed bubbly flows, which exist for a wide range of practical gas injection rates. The model is used for extensive parametric studies of the effects of casting operation conditions and nozzle design, which are reported in Part II of this two-part article.     

Three-dimensional analysis of molten steel flow in slab continuous casting mould with rotated ports in submerged entry nozzle

Abstract

The purpose of this study is to develop a three-dimensional (3D) analysis system capable of analysing the flow field of molten steel in the slab continuous casting mould with rotated ports in the submerged entry nozzle. The ultimate goal is to obtain the optimal design for the entry ports of the submerged nozzle, which can introduce favourable flow patterns to remove non-metallic inclusions and avoid entrapment of molten slag and casting powder to produce steel slab of high cleanliness. In this study, a computational fluid dynamics technique, Sola-Surf, is employed to conduct the 3D fluid flow analysis. The technique has the capability of treating fluid flow problems with a free surface that slightly vibrates. The slightly vibrating free surface presents fairly accurately the behaviour of the molten slag–casting powder layer in the continuous casting mould. The developed simulation system is then tested on a slab continuous casting mould to analyse the fluid flow behaviour of molten steel under various nozzle designs. The design conditions include submerged depth of the nozzle, tilted angle of the nozzle port, and rotated angle of the nozzle port. The results of the simulations show that of the various design factors rotation of the nozzle entry ports has the greatest effect on the flow pattern. It can prolong the residence time of the molten steel and stabilise the molten slag–casting powder layer, which is very favourable for obtaining continuous casting slag of high cleanliness.     

Interfacial Fluctuation Behavior of Steel/Slag in Medium-Thin Slab Continuous Casting Mold With Argon Gas Injection

 The flow field of molten steel and the interfacial behaviour between molten steel and liquid slag layer in medium-thin slab continuous casting mold with argon gas injection was studied by numerical simulation, in which the effects of nozzle submergence depth and port angle, casting speed, and argon gas flow rate on the flow and the level fluctuation of molten steel were considered. The results show that the molten steel jet from the submerged entry nozzle (SEN) with three ports into the mold and form three re-circulation zones including one upper re-circulation zone and two lower re-circulation zones. Argon gas injection results in a secondary vortex flow in the upper zone near the nozzle. For a given casting speed and argon gas flow rate, increasing the side port angle and submergence depth of nozzle can effectively restrain the steel/slag interfacial fluctuation. Increasing the casting speed would prick up the level fluctuation. For a fixed casting speed, argon gas flow rate has a critical value, the interfacial fluctuation with argon gas injection are stronger than the case without argon gas injection when the argon gas flow rate is less than the critical value, but when the argon gas flow rate exceeds the critical value, the level fluctuation is calmer than that without argon gas injection.     

高拉速板坯连铸结晶器浸入式水口的水模型研究

利用染料示踪法,采用波高传感器和旋桨式流速仪在全比例水模型中研究了四种浸入式水口(A型:凹型,15°(上角度)-15°(下角度);B型:凸型,15°-15°;C型:凹型,40°-15°,D型:凸型,40°-15°)下板坯连铸结晶器内的流场和液面特征.发现采用凹型水口时结晶器液面的波动与表面流速均小于凸型水口.凹型水口F的表面流速变化的功率(频率为0.03~0.1Hz)比凸型水口小约50%,所以凹型水口更有利于减少结晶器内卷渣的发生.在高拉速条件下(拉速为1.8m·min-1,较大的水口出口上角度有利于抑制水口出口流股的漩涡流,进而减少剪切卷渣的发生.四种水口中C型水口条件下结晶器液面的表面流速最小,约为0.27m·s-1,为提高拉速留有较大余地,所以适合高拉速连铸的最佳浸入式水口为C型.      

宽板坯连铸结晶器内氩气分布的水模型

 针对塞棒、上水口吹氩时,吹入的氩气有多少进入到结晶器钢水内和氩气在结晶器断面上如何分布的问题,采用1：1水模型,重点研究了吹气量、吹气方式、拉速、浸入式水口倾角等工艺参数对不同宽度结晶器内气体分布状况的影响。结果表明,吹气量和吹气方式对结晶器内气体分布几乎没有影响,而拉速和水口倾角基本上就决定了宽度方向上的气体分布状况。小的结晶器断面宽度、小的水口倾角、高拉速时窄面附近含气比例更高,氩气泡更易被凝固坯壳所捕捉而使铸坯产生气泡缺陷。     

Interracial Fluctuation Behavior of Steel/Slag in Medium-Thin Slab Continuous Casting Mold With Argon Gas Injection

The flow field of molten steel and the interfacial behaviour between molten steel and liquid slag layer in medium-thin slab continuous casting mold with argon gas injection were studied by numerical simulation, in which the effects of nozzle submergence depth and port angle, casting speed, and argon gas flow rate on the flow and the level fluctuation of molten steel were considered. The results show that the molten steel is jetted from the submerged entry nozzle (SEN) with three ports into the mold and forms three recirculation zones including one upper recirculation zone and two lower recirculation zones. Argon gas injection results in a secondary vortex flow in the upper zone near the nozzle. For a given casting speed and argon gas flow rate, increasing the side port angle and submergence depth of nozzle can effectively restrain the steel/slag interfacial fluctuation. Increasing the casting speed would prick up the level fluctuation. For a fixed casting speed, argon gas flow rate has a critical value, the interfacial fluctuation with argon gas injection are stronger than the case without argon gas injection when the argon gas flow rate is less than the critical value, but when the argon gas flow rate exceeds the critical value, the level fluctuation is calmer than that without argon gas injection.     

浸入式水口倾角对宽幅连铸结晶器内流动行为的影响

浸入式水口出口角度影响射流的动能损失,进而影响结晶器内的流动行为.建立了比例为1 ∶ 4的水模型,在模型拉速为0.425 m/min、水口浸入深度为40 mm的条件下,借助粒子图像测速技术研究了浸入式水口出口倾角为0°和+5°对宽幅连铸结晶器内流动行为的影响.结果表明,0°和+5°水口条件下,射流在模型内的流动方式有很...     

Water modelling study of the jet characteristics in a continuous casting mould

The jet characteristics and the fluid flow pattern in a continuous slab caster have been studied using a water model. The fluid jet is studied under free fall and submerged discharge conditions. In the latter case, the jet was followed by dye-injection technique and image analyser was used to find out the effect of nozzle parameters on jet-spread angle, jet-discharge angle and the volume entrainment by the jet. All free-fall jets with nozzle port angle zero and upward are found to be spinning. Some of the free-fall jets with downward nozzle-port angle are found to be spinning and rest are smooth. The spinning direction of the jets are found to change with time. The well depth, port diameter and the inner diameter of the nozzle have a clear effect on the free-fall jets with downward port angle. The jet-spread angle is found to be about 17° for smooth jets. The spread angle for spinning jet increases as the nozzle-port angle is increased from downward 25 to upward 15°. The jet-discharge angle is always downward even when the nozzle-discharge ports are angled upward. The extent of volume entrainment by the spinning jet is higher and it increases as the nozzle-port angle is increased from 25 downward to 15° upward.     

大方坯径向水口旋流效应及其对凝固的影响

通过改变水口侧孔钢水流动方向可以控制结晶器内钢水流动与换热。采用流体动力学与凝固模拟方法对比研究了浸入式四分径向水口不同出流方向对大方坯连铸结晶器内流动、传热和凝固行为的影响。结果表明,侧孔方向对浇注过程结晶器内钢水的流动与凝固行为有显著影响。当水口侧孔水平旋转角度为30°时,结晶器内形成较好的水平旋流,可以有效降低侧孔出流钢水对坯壳的冲刷作用,并有利于结晶器内自由液面过热度的提高。比较不同侧孔出流角度发现,利用普通径向四分水口在一定安装角度下的旋流效应不仅对于初生坯壳的均匀生长以及自由液面的冶金效果产生有利影响,还可能在不改变水口结构条件下获得类似结晶器电磁搅拌的旋流效应。     

水口结构对连铸结晶器内钢水流动的影响

采用雷诺平均（RANS）数学模拟方法,研究波浪形和山形水口底部结构对结晶器内钢水湍流现象及表面流速的影响。通过1∶1结晶器模型水模拟进行了验证,表明采用OA光纤测速仪对结晶器表面流速测量的结果和数值模拟结果吻合较好。研究表明波浪形水口可以抑制水口流出钢水的射流,改善结晶器内钢水流场,降低表面流速稳定液面,进而改善铸坯表面质量。     

Transient Turbulent Flow in a Liquid-Metal Model of Continuous Casting,Including Comparison of Six Different Methods

Computational modeling is an important tool to understand and stabilize transient turbulent fluid flow in the continuous casting of steel to minimize defects. The current work combines the predictions of two steady Reynolds-averaged Navier–Stokes (RANS) models, a “filtered” unsteady RANS model, and two large eddy simulation (LES) models with ultrasonic Doppler velocimetry (UDV) measurements in a small-scale liquid GaInSn model of the continuous casting mold region fed by a bifurcated well-bottom nozzle with horizontal ports. Both mean and transient features of the turbulent flow are investigated. LES outperformed all models while matching the measurements, except in locations where measurement problems are suspected. The LES model also captured high-frequency fluctuations, which the measurements could not detect. Steady RANS models were the least accurate methods. Turbulent velocity variation frequencies and energies decreased with distance from the nozzle port regions. Proper orthogonal decomposition analysis, instantaneous velocity patterns, and Reynolds stresses reveal that velocity fluctuations and flow structures associated with the alternating-direction swirl in the nozzle bottom lead to a wobbling jet exiting the ports into the mold. These turbulent flow structures are responsible for patterns observed in both the time average flow and the statistics of their fluctuations.     

Effects of clogging, argon injection, and continuous casting conditions on flow and air aspiration in submerged entry nozzles

The inter-related effects of nozzle clogging, argon injection, tundish bath depth, slide-gate opening position, and nozzle-bore diameter on the steel flow rate and pressure in continuous-casting slide-gate nozzles are quantified using computational models of three-dimensional (3-D) multiphase turbulent flow. The results are validated with measurements on operating steel continuous slab-casting machines and are presented for practical conditions with the aid of an inverse model. Predictions show that initial clogging may enhance the steel flow rate due to a potential streamlining effect before it becomes great enough to restrict the flow channel. The clogging condition can be detected by comparing the measured steel flow rate to the expected flow rate for those conditions, based on the predictions of the inverse model presented here. Increasing argon injection may help to reduce air aspiration by increasing the minimum pressure, which is found just below the slide gate. More argon is needed to avoid a partial-vacuum effect at intermediate casting speeds and in deeper tundishes. Argon flow should be reduced during shallow tundish and low casting speed conditions (such as those encountered during a ladle transition) in order to avoid detrimental effects on flow pattern. Argon should also be reduced at high casting speed, when the slide gate is open wider and the potential for air aspiration is less. The optimal argon flow rate depends on the casting speed, tundish level, and nozzle-bore diameter and is quantified in this work for a typical nozzle and range of bore diameters and operating conditions.     

连铸结晶器内钢液流场的水模型研究

采用1:1水模型研究了260 mm×300 mm坯连铸结晶器中钢液在拉速0.5～0.8 m/min、浸入深度90～120 mm和5种水口结构时的流场。试验结果表明,合理的水口结构有利于降低冲击深度,现有的直筒型水口过大,优化后的水口可使冲击深度降低30%～40%;随拉速增大,冲击深度增加,而浸入深度对冲击深度影响不大。     

Effect of Nozzle Base Material on the Rate of Clogging during the Continuous Casting of Aluminum‐Killed Steels

One approach to solving the problem of nozzle clogging during the continuous casting of aluminum‐killed steels is through careful selection of nozzle materials. In this study, the rate of clogging was measured while casting steel through simulated nozzles produced from alumina, zirconia, magnesia, zirconia‐graphite, and alumina‐graphite ‐ three common base materials and two common carbon‐containing nozzle materials. Spent nozzles were characterized using optical and cathodoluminescence microscopy. Interactions between the nozzles and steel were not observed in the alumina, zirconia, and zirconia‐graphite nozzles. Slight interactions were observed in the magnesia nozzles as alumina inclusions within the steel interacted with the nozzle to produce alumina‐magnesia spinel. Greater amounts of interaction were observed with alumina‐graphite nozzles. No statistical differences in the mean rate of clogging were observed between all of the pure oxide nozzles and zirconia‐graphite nozzles. However, the alumina‐graphite nozzles clogged at a much higher rate than the other nozzles. The higher rate of clogging is thought to be due to refractory‐steel interactions.     

Simulation of fluid flow inside a continuous slab-casting machine

A finite element model has been developed and applied to compute the fluid flow distribution inside the shell in the mold region of a continuous, steel slab-casting machine. The model was produced with the commercial program FIDAP, which allows this nonlinear, highly turbulent problem to be simulated using the K- ε turbulence model. It consists of separate two-dimensional (2-D) models of the nozzle and a section through the mold, facing the broad face. The predicted flow patterns and velocity fields show reasonable agreement with experimental observations and measurements conducted using a transparent plastic water model. The effects of nozzle angle, casting speed, mold width, and turbulence simulation parameters on the flow pattern have been investigated. The overall flow field is relatively insensitive to process parameters. Formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Illinois-Urbana     

Advantageof anticlogging nozzles over conventional aluminographite nozzles in continuous casting of steel slabs

Abstract

The goal of this work was to establish the advantages of the application of anticlogging nozzles compared with conventional aluminographite nozzles in the continuous casting of steel slabs. Anticlogging nozzles are used in many steelmaking plants to inhibit scab formation at the internal side of the nozzle wall. The formation of aluminate-corundum scab decreases internal nozzle diameter, leading to the reduction of steel flow and eventual blocking of the nozzle. The results of monitoring the behaviour of a large number of conventional aluminographite and anticlogging nozzles in a campaign lasting two months are presented. The data reveal lower average values of internal erosion caused by the steel and reduced thicknesses of scab formed at the internal side of the anticlogging nozzle wall relative to conventional nozzles. At the same time, the average values of external erosion, caused by the action of ungranulated casting powders, were considerably increased relative to external erosion resulting from the action of granulated casting powder.     

薄板坯连铸结晶器钢液流动数值模拟

以唐钢第一炼钢厂薄板坯连铸机为背景,利用商业模拟软件FLUENT,根据热轧薄板厂目前连铸操作的工艺参数,采用湍流模型建立数学模型,模拟了薄板坯连铸结晶器内钢液流场分布情况,系统地研究了水口结构、水口上出口倾角在指定拉速、浸入深度下对结晶器内流场的影响,为水口优化提供理论依据。在固定拉速和指定的浸入深度条件下,通过对比分析几种不同水口结构对结晶器内流场及温度场影响规律,最终确定采用五孔水口上出口倾角取15°为最佳水口结构方案。     

提高410S不锈钢连浇炉数的工艺实践

邢钢一步法(脱磷站+60 t AOD+LF)生产410S不锈钢过程中,由于AOD的冶炼周期远大于连铸机浇钢和脱磷站的处理时间和连铸中间包水口下部侵蚀严重无法实现多炉连浇,严重影响连铸机作业率和整体钢铁料消耗。提高单中间包连浇炉数有利于减少中间包的使用数量、提高连铸机的作业率、降低钢铁料消耗、降低连铸机辅材及能源介质消耗。通过合理提高入炉冷态返回废钢比例(3.5t/炉),选择合适合金硅含量(3.5%)来缩短410S不锈钢AOD的冶炼周期至71 min,连铸机采用中间包分体水口快换,使连浇炉数由6炉提高到12炉。