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.     

Inclusion behavior and heat-transfer phenomena in steelmaking tundish operations: part III. applications—computational approach to tundish design

The attributes of various tundish designs and flow modifiers have been studied based on the computational and physical modeling efforts developed in parts I and II. The importance of thermal natural convection effects in such vessels has been demonstrated. Flow patterns and residual ratios (RR) for various tundish designs of equal volume and metal throughput showed that a conventional biflow, twin-strand, trough-type tundish fitted with flow modifiers allows for the greatest removal of inclusions, while a one-way flow, twin-strand, wedge-shaped tundish performs best in the absence of flow modifiers. More detailed analyses of the conventional trough-type tundish were performed to assess the effects of double-weir-double-dam and single-weir-double-dam arrangements. The effectiveness of steeply sloping sidewalls for enhanced removal of inclusions was demonstrated; however, this measure was associated with penalties in the form of greater metal temperature losses. Formerly Doctoral Candidate. Formerly Postdoctoral Fellow, Department of Mining and Metallurgical Engineering, McGill Metals Processing Centre, McGill University. Macdonald Professor of Metallurgy.     

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.     

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.     

板坯连铸中间包结构优化的研究

结合某公司双冲击点板坯中间包生产现状,利用水模实验对中间包内钢水流动特性进行了研究.结果表明:原中间包结构存在明显的短路流,钢水停留时间偏短,死区体积分率偏高,不利于夹杂物上浮去除.同时,由于在高度较大的两湍流控制器间钢水不能流出,影响金属收得率.这种具有双冲击点的中间包,应按照对称的两个单流板坯中间包进行结构优化.采...     

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.     

Fluid Flow and Inclusion Motion in a Multi-Heat Teeming Tundish for Heavy Steel Ingot

 Besides the function of molten steel supply in the casting process, tundish has a capability of decreasing the formation of macro-scale inclusions and promoting the flotation of the remaining inclusions. Both the round shape tundish and the elliptical shape tundish were studied for the multi-heat teeming ingot. The transportation of molten steel and the motion of inclusions were compared using numerical simulation and physical modeling. The results showed that the short molten steel residence time, the large dead volume, and the low efficiency in inclusion removal for the round shape tundish were obtained even when a flow control device was installed. After comparing the two kinds of tundishes with the same capacity, it indicated that in the elliptical shape tundish, the molten steel residence time was prolonged by 1.6 times and the mean residence time increased by 6%; the dead volume was decreased from 18% to 13%, the ratio of the plug volume to the dead volume increased by 68%, and the inclusion flotation efficiency increased by 17%. The trial results indicated that, after using the elliptical tundish, the equivalent diameter of single defect was decreased from 2.5 to 1.6 mm, and no intensity defect was observed. The total oxygen content was 21×10-6, which was decreased by 30%.     

Modelling performance of four‐strand, 12 t,delta shaped continuous casting tundish fitted with different flow modifying arrangements for better steel quality

Abstract

The performance of a 12?t, delta shaped, four‐strand, billet casting tundish was investigated using a full scale (1∶1) isothermal model using water as the simulating fluid for steel. Three different modelling experiments were carried out. Quantifiable parameters like ‘number of slag beads’ in transient physical modelling of slag entrainment, or ‘residual ratio of inclusions’ (RRI %), in steady state mathematical modelling of inclusion removal and ‘mean residence time’, ‘dead volume fraction’, etc., in steady state mathematical computation of residence time distributions, were used as the performance indicators. Results for three different flow modifying dam arrangements were considered and compared with those of a bare tundish. Computational fluid dynamic analysis showed that different flow modifying dam arrangements significantly alters the flow pattern within the tundish. On the basis of these performance indicators, the best arrangement was identified. The assumption in this paper that similar conclusions can be drawn from experiments carried out either in transient or in steady state conditions was verified. It was shown that both transient physical modelling experiments and the steady state mathematical predictions, point to the same conclusion.     

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 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.     

鞍钢板坯连铸机中间包挡墙设置优化与改进

采用数学模型方法研究了连铸中间包过程的钢水流动，混合以及停留状况，并以提高钢水平均停留时间为依据优化了鞍钢第三炼钢厂板坯连铸中间包的挡墙设置，通过对中间包过程钢水夹杂物运动行为的数学模化和仿真计算探讨了夹杂物的碰撞长大。上浮排除以及被耐火材料墙壁粘结吸附等现象。在实际生产中对有关研究结果进行了验证分析。     

Large‐Eddy Simulations and Particle‐Image Velocimetry Measurements of Tundish Flow

Large‐eddy simulations (LES) and digital particle‐image velocimetry (DPIV) of a tundish flow are performed to investigate the turbulent flow structures and vortex dynamics. The LES is carried out using an implicit approach. In implicitly filtered LES, the computational grid and the discretization operators are considered as the filtering tools of the governing equations. The numerical computations are performed by solving the viscous conservation equations for compressible fluids. An implicit dual‐time stepping scheme combined with low Mach number pre‐conditioning and a multigrid accelerating technique is implemented for LES computations. The impact of jet spreading, jet impingement on the wall, and wall jets on the flow field and steel quality is investigated. The characteristics of the flow field in a one‐strand tundish such as the time‐dependent turbulent flow structure and vortex dynamics are analysed and compared with experimental results. To validate the numerical results, DPIV measurements are performed in a reduced 1:1.7 scaled water model. The investigations focus on steady‐state casting conditions for the flow in a tundish. The results evidence a good agreement between the LES and experimental data. The LES solutions provide an extremely detailed insight into the highly intricate turbulent flow structure. Even phenomena like funnel‐shaped vortices downward the shroud jet are well captured.     

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).     

流钢孔对中间包流体行为影响的数值模拟

以某钢厂双流中间包为基础,建立了相似比为1∶3的物理模型和三维稳态数学模型,研究挡坝控流装置上流钢孔的孔径、数量和倾斜角度等变化因素对中间包内流场的影响。结果表明,孔径大小和流钢孔的倾斜角度对中间包内钢液的流动特性至关重要。在中间包挡坝上开设的流钢孔孔径为120 mm、倾斜角度为15°时,可改变钢液流向,延长钢液流动轨迹,增加夹杂物去除概率,与原型相比,中间包内钢液平均停留时间增加近28 s,死区比例降低46.5%,理论残钢量降低79%。     

四流圆坯中间包的结构优化研究

为净化钢液,提高夹杂物在中间包内的上浮率,通过水模型实验,对4流中间包不同挡墙形式下包内流场及夹杂物去除情况进行了模拟研究.结果表明:现用挡墙控流下的中间包同一侧两流之间的流体流动特性存在很大差异,与内侧相比,外侧流的最短停留时间、峰值时间长、死区体积大,造成中间包内钢液温度不均匀,夹杂物不能有效地上浮去除.改进后的挡墙能很好地解决各流间的同步性,死区比例比原型降低6.65 %,对夹杂物的去除效果增强.工业试验表明,改进后的挡墙在降低总氧和夹杂物方面均优于原挡墙.     

Modelling steel flow in a three‐strand billet tundish using a turbulence inhibitor

A three‐strand tundish belonging to a billet caster was water modelled and plant trials were performed to compare the performance of a pouring box and a turbulence inhibitor in terms of melt flow parameters and steel cleanliness. A tailor made turbulence inhibitor for this tundish is useful to accomplish with flow control of fluid turbulence and even melt redirection to all strands. The turbulence inhibitor helps to decrease nitrogen pickup during ladle changes and to float out inclusions towards the covering slag. As a consequence, rod operations to take of alumina deposits from nozzle walls are considerably decreased using a turbulence inhibitor.     

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。     

薄板坯连铸中间包内抑湍器的模拟研究

控制中间包内钢液的合理流动对夹杂物的排除有重要影响,为此建立了模拟薄板坯连铸中间包流动情况的水模型。通过测定停留时间分布(RTD)曲线,研究了不同组合控流装置对中间包流体流动特性的影响。结果表明,结构及尺寸合理的抑湍器能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及降低死区体积分数;抑湍器与单坝组合的控流装置在控制流体流动方面效果极佳,而抑湍器与单墙单坝、单墙双坝组合的控流装置的控流效果不太理想。     

两流非对称中间包结构优化与应用研究

采用水模型实验、数值模拟相结合的方法分析了原型中间包和优化中间包在钢流流场、中间包流动特性方面的差异.结果表明:优化中间包2#水口平均停留时间延长了8.0%,两水口流体平均停留时间之差下降了36.3%.流体在优化中间包内流动轨迹更加复杂,延长了流体在中间包内停留时间.通过工业实验证实了优化方案的可行性.工业试验表明:采用圆形湍流控制器加单挡墙组成控流装置的原型中间包,两水口钢液平均温差为5℃,浇注得到的钢坯试样中,140~300μm夹杂物数量为0.7 mg;而采用非对称长方形湍流控制器加多孔挡墙组成控流装置的中间包,两水口钢液平均温差为3℃或2℃,约为原型中间包两水口钢液平均温差的1/2;浇注得到的钢坯试样中,140~300μm夹杂物数量为0.2 mg,约为原型中间包的1/3.说明采用非对称长方形湍流控制器加多孔挡墙组成控流装置的中间包对两水口温度的均一性起到了显著作用,且更能有效地去除钢液中的夹杂物.