Transient steel quality under non-isothermal conditions in a multi-strand billet caster tundish: part II. Effect of a flow-control device

A mathematical model was built to simultaneously analyse the effects of non-isothermal conditions and flow-control device on steel quality in a real steelmaking tundish. Liquid steel was used as the operating liquid with a step-input of 23° in a full-scale delta-shaped multi-strand billet caster tundish fitted with a standard impact pad (SIP). The changes in flow pattern and temperature fields of liquid steel in the tundish under isothermal, step-up and step-down conditions were thoroughly studied. Similar to the case of a bare tundish, buoyancy effects were seen to dominate at regions away from the ladle shroud. The presence of SIP modified the flow patterns and resulted into markedly different values of RRI as compared to bare tundish. Finally, the calculated results were qualitatively compared to results obtained from a real steelmaking tundish.     

中间包钢液非等温现象的水模型研究

钢液的非等温现象对中间包内流体流动和传热有着重要的影响。笔者用热态水模型研究了中间包内钢液的流动规律。结果表明：温度变化导致钢液的密度不均匀,使自然对流影响钢液流动;还给出了不同温差、不同控流装置下中间包内钢液温度的分布,并讨论了在不同条件下自然对流对钢液流动的影响。     

Fluid Flow and Mixing in Non-Isothermal Water Model of Continuous Casting Tundish

Fluid flow and mixing of molten steel in a twin-slab-strand continuous casting tundish were investigated using a mixing model under non-isothermal conditions.This model led to a set of ordinary differential equations that were solved with a Runge-Kutta algorithm.Steady state water modeling was carried out under non-isothermal conditions.Experimental data obtained from the water model were used to calibrate the mixing model.Owing to the presence of a mixed convection in the non-isothermal conditions,a channelizing flow would be created in the fluid inside the tundish.A mixing model was designed that was capable of predicting RTD（residence time distribution）curves for different cases in non-isothermal conditions.The relationship between RTD parameters and the Tu（tundish Richardson number）was obtained for various cases under non-isothermal conditions.The results show that the RTD parameters were completely different under isothermal and non-isothermal conditions.The comparison of the RTD curves between the isothermal and non-isothermal conditions presents that the extent of mixing in the tundish in non-isothermal conditions is lower than the mixing extent in isothermal conditions.     

多流中间包内钢液流动规律模拟研究

针对攀钢六流大方坯连铸中间包,采用等温和非等温物理模拟的研究方法,研究了钢水在中间包内的流动规律。实验结果表明自然对流对钢液的流动模式具有重要的影响,因此,对于体积大、流间距长的中间包必须采用非等温的模拟研究方法。实验还证明了在非等温情况下中间包内加控流装置的必要性。     

Numerical Simulation for Effect of Inlet Cooling Rate on Fluid Flow and Temperature Distribution in Tundish

 The fluid flow in tundish is a non-isothermal process and the temperature variation of stream from teeming ladle dominates the fluid flow and thermal distribution in tundish. A numerical model was established to investigate the effect of inlet cooling rate on fluid flow and temperature distribution in tundish based on a FTSC (Flexible Thin Slab Casting) tundish. The inlet cooling rate varies from 0.5 to 0.25 ℃/min. Under the present calculation conditions, the following conclusions were made. When the stream temperature from teeming ladle drops seriously (for inlet cooling rate of 0.5 ℃/min), there is a “backward flow” at the coming end of casting. The horizontal flow along the free surface turns to flow along the bottom of tundish. The bottom flow shortens the fluid flow route in tundish and deteriorates the removal effect of nonmetallic inclusions from molten steel. Nevertheless, when the inlet cooling rate decreases to 0.25 ℃/min, the horizontal flow is sustained during the whole casting period. The present research provides theoretical directions for temperature control in teeming ladle and continuous casting tundish during production of advanced steels.     

Steel Thermo‐Fluid‐Dynamics at Tundish Drainage and Quality Features

In steelmaking and casting, transient operations are very critical for product quality and process regularity. This holds especially for the tundish. Typical drawbacks can occur at ladle change, where, for example, refilling high flow rates induce flow short‐circuits risky for dispersed oxides (“inclusions”) dragging towards the strands. At drainage, vortices formation can affect steel cleanliness via slag entrapment. Such topics were investigated for an industrial tundish with computational fluid dynamics validated tools. The focus was given on a multi‐strand layout more prone to unevenness features. As a matter of fact, the different steel path to reach different strands causes often too high temperature differences and different strand cleanliness levels. Strands closer to the tundish center, are generally hotter and less clean; the others, slightly colder but cleaner. Multiphase models, together with advanced meshing techniques and validated boundary conditions, were used to describe tundish refilling and drainage. Within the operating conditions of concern, a bath height of 300 mm was found as a best compromise between the need of avoiding slag entrapment through vortices and to have maximum yield. Once applied into operating practice, no rejection for cleanliness or customer claims were achieved. As refers to temperature loss from ladle to tundish, a drop at strands of about 2 and of 4°C from tundish inlet to strand, in agreement with plant data over about 700 heats and literature experiments under the same operating conditions, were found.     

Inertial and buoyancy driven water flows under gas bubbling and thermal stratification conditions in a tundish model

Steel flow dominated by inertial and buoyancy flows under gas bubbling and thermal stratification conditions, in a one-strand tundish, was studied using a 2/5 scale water model. The use of a turbulence inhibitor yields plug flow volume fractions well above 40 pct for a casting rate of 3.12 tons/min under isothermal conditions. Small flow rates of gas injection (246 cm³/min), through a gas curtain, improved the fluid flow by enhancing the plug flow volume fraction. Higher flow rates originated an increase of back-mixing flow, thus forming recirculating flows in both sides of this curtain. Step inputs of hot water drove streams of this fluid toward the bath surface due to buoyancy forces. A rise in gas flow rate led to a thermal homogenization in two separated cells of flow located at each side of the gas curtain. Step inputs of cold water drove streams of fluid along the tundish bottom. Use of the gas curtain homogenized the lower part of the tundish as well as the upper part of the bath at the left side of the curtain. However, temperature at the top corner of the tundish, in the outlet box, remained very different than the rest of the temperatures inside this tundish. High gas flow rates (912 cm³/min) were required to homogenize the bath after times as long as twice the mean residence time of the fluid. Particle image velocimetry (PIV) measurements corroborated the formation of recirculating flows at both sides of the gas curtain.     

Numerical and Physical Modeling of Steel Flow in a Two-Strand Tundish for Different Casting Conditions

This article presents computational and water model studies of the three-dimensional turbulent fluid flow in a two-strand tundish for steady-state and transient casting conditions. First, it presents the flow field measurements obtained at a 1:3-scale water model of the tundish with the particle-image velocimetry (PIV) method during steady-state casting. The PIV measurements were performed using the Reynolds-similarity criterion. Thereafter, numerical simulation is carried out with the computational fluid dynamic software, FLUENT, using the realizable k-ε turbulence model. The numerical model is validated using the measurement results obtained with the water model. The results of the numerical calculations are in good agreement with the PIV measurements. On this basis, the validated numerical model is adapted to simulate the 1:1-scale steel flow with boundary conditions that are derived from the real casting process. The nonisothermal, unsteady numerical calculations concerning the cooling process of steel melt inside the tundish are done for a 1:1-scale industrial facility—a 69-t two-strand tundish with a 380-t ladle. The influence of transient boundary conditions at the outlet of the tundish (one blocked strand) on the flow structure and mixing process of fluid during the casting process are investigated. The evaluation of the flow structure is performed using a zonal method, which relates the fluid flow with the mixing processes.     

通道式感应加热7流中间包流场的物理模拟

 为解决通道式感应加热7流中间包各流一致性差、第2和第6流钢水停留时间短的问题,通过水模拟试验对该中间包流场进行优化。等温试验结果表明,通过改变通道结构以及添加双挡坝可改善流体的流动状况。与原型结构相比,优化后中间包总体平均停留时间延长了277.8 s,死区比例降低了30.16%,各流一致性得到较大改善。非等温试验表明,感应加热引起的包内自然对流不可忽略,流体经加热从通道流出后会形成明显上升流。通道内外温差越大,中间包各流的一致性越好。针对原型方案,通道内外温差为5 ℃时,中间包内流体停留时间即较无温差时明显延长,死区基本消除。     

中间包温度分布的模拟研究

利用湍流理论,建立中间包温度场的数学模型.通过建立中间包温度场的有限元模型,分析了有限元法在模拟中间包温度场方面的应用.基于有限元法对中间包温度场及其温度分布进行了分析研究,结果表明:中间包内钢水的流动在许多情况下表现为一种非等温状态下的流动,不同结构中间包内钢液温度分布有差异.优化后的中间包钢液的温度分布均匀,有利于提高连铸坯的质量.     

Dissipation of Turbulent Kinetic Energy in a Tundish by Inhibitors with Different Designs

Turbulent flow of liquid steel and its control is studied using different geometries of turbulence inhibitors. Four designs of turbulence inhibitors were characterized through experiments of tracer injection in a water model and mathematical simulations using the Reynolds Stress Model (RSM) of turbulence. Inhibitor geometries included octagonal‐regular, octagonal‐irregular, pentagonal and squared. A layer of silicon oil was used to model the behaviour of tundish flux during steel flow. Fluid flows in a tundish using these geometries were compared with that in a bare tundish. Experimental and simulation results indicate that the flow in a bare tundish and a tundish using turbulence inhibitors open large areas of oil close to the ladle shroud due to strong shear stresses at the water‐oil interface with the exception of the squared inhibitor. Oil layer opening phenomena are explained by the high gradient of the dissipation rate of turbulent kinetic energy. Using the squared inhibitor the kinetic energy reports a high gradient from the tundish floor to the free bath surface as compared with other geometries.     

Hydrodynamic effects created by argon stirring liquid steel in a one-strand tundish

Continuous casting of steel ensures the flexible production of different grades of slabs intended for the Rolling Mill departments. Therefore, it seems justifiable to search new solutions for tundish metallurgy. This study puts forward a new gas-permeable barrier’s design solution with two circular porous plugs, whose purpose is to punctually agitate the liquid steel in tundish. Two argon injection system locations were tested. The results for the hydrodynamic pattern of liquid steel and bubbles’ behaviour in the tundish were obtained from laboratory experiments with water glass model and computer simulations. Within the framework of the investigation, three argon flow rates, i.e. 5, 10 and 15?NL?min^?1, were considered. The designed gas-permeable barriers effectively stimulate active flow in the tundish for isothermal and non-isothermal conditions, while reducing the stagnant flow share and the transient zone extent.     

Effect of Thermal Buoyancy on Fluid Flow and Inclusion Motion in Tundish without Flow Control Devices--Part Ⅰ: Fluid Flow

The κ-ε two-equation model is used to simulate the fluid flow in the continuous casting tundish coupling with the effect of thermal buoyancy. The natural convection induced by the thermal buoyancy generates an upward flow pattern especially at the outlet zone, and has little effect on the fluid flow in the inlet zone. The maximum viscosity is 700 times larger than the laminar viscosity, which indicates the strong turbulent flow in the tundish. The maximum temperature difference in the whole tundish is 8.2 K. The temperature near the stopper rod and the short wall is obviously lower than that in the inlet zone. The existence of the stopper rod has a big effect on the fluid flow entering the SEN and the mold. All the characteristics of the tundish geometry should be considered to accurately simulate the fluid flow in the tundish.     

气幕挡墙对中间包内钢液流场影响的数值模拟

根据实际中间包的操作工艺参数,采用欧拉-欧拉两流体模型,用数值模拟法模拟计算了中间包底吹对钢液流动的影响.结果表明,底吹改变了中间包内钢液的流动状态,在气幕挡墙的两侧分别形成了方向相反的回流区.底吹气体流量对钢液流动状态、气泡分布影响显著,底吹气体流量太大或太小都不利于改善钢液的流动状态.     

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.     

中间包连浇过程非稳态流场与温度场的数值模拟研究

通过计算得出在浇注过程中连铸中间包包壁瞬态热量损失作为边界条件的基础上,建立了连铸中间包内钢液流动与传热耦合数学模型,对连浇过程中中间包内非稳态的温度场和流场进行了数值模拟,考察了中间包连浇5个包次过程中钢液热量损失、温度分布以及流场情况,为现场操作和工艺优化提供依据和指导。     

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.     

Fluid Dynamics of Vortex Formation in Tundish Operations: Physical Modelling

The present study focuses on the fluid dynamics of vortex formation in tundish operations using a physical water analogue model and Particle Image Velocimetry (PIV) measurements. The aim is to get fundamental knowledge of the nature of vortex formation that clearly represents this flow phenomenon. The results indicate that vortex formation is directly related to the highly turbulent flows present in the tundish. Great changes of velocity fields during short times and large distances between the upper edge of the dam and the free surface of the bath enhance the formation of vortex flows. Therefore, increasing tundish capacity does not guarantee steel cleanliness through long residence times if the dam height is not adjusted to the new level of liquid in the tundish.     

连铸中间包技术改造与多孔控流挡墙的优化设计

介绍了连铸中间包由对称四流向非对称五流的技术改造，并采用数值模拟和水力学模拟的方法优化得出控流挡墙的设计方案。投产实践证明，控流挡墙能够明显改善中间包内钢液的流动特性，同时增加了产量、提高了钢坯质量。     

非对称控流结构中间包流场一致性的物理模拟

国内某钢厂使用的两流板坯连铸中间包因受固定位置排渣口的限制,包内控流装置采用左右不对称布置。生产实践发现,排渣口侧的水口对应铸坯大型夹杂物含量高、热轧卷探伤合格率低,疑与中间包流场的一致性有关。为此,采用1∶3.5的水模型对中间包流场进行了模拟研究,并基于流体动力学原理对其控流效果进行了优化。结果表明,原型中间包两个水口的滞止时间差高达36 s,钢液在排渣口侧的1号水口形成短路流,因而导致两流铸坯洁净度的差异。经水模优化后,方案F1下两流平均停留时间标准差和滞止时间标准差分别可降到0.12和0.35 s,明显改善了中间包内两流浇铸流动特性的一致性,且死区比例较原型降低8.87%、平均停留时间延长了30 s。