连铸结晶器内湍流大涡模拟

采用大涡模拟研究连铸钢水非定常湍流特性,比较了雷诺平均数学模型与大涡模拟数学模型对预测结晶器内钢水湍流运动的影响.模型采用一种低温液态金属模型的超声波多普勒速度仪（UDV）的测量结果进行验证,表明大涡模拟比雷诺平均模拟与实验测量结果更加吻合.瞬态湍流研究表明,大涡模拟优于雷诺平均数学模型,能够捕捉到水口附近高频率的湍流脉动现象.水口出流钢水区域内小尺度湍流结构起支配作用,射流以阶梯状上下摆动向结晶器内扩散.受湍流大尺度作用,结晶器内流场偏流现象始终存在,并且呈周期性变化,随着对流场时间平均的增加,结晶器内的不对称现象逐渐消除.     

连铸中间包上水口及浸入式水口结瘤物分析

水口结瘤一直是困扰连铸生产的问题,对连铸生产中发生结瘤的中间包上水口和浸入式水口从上到下做了全面的解剖分析,通过SEM、EDS、X-RD等手段对水口的结瘤物进行分析研究。结果表明,中间包上水口结瘤严重,钢水完全凝结;在浸入式水口渣线以上,发生了严重的结瘤,厚度约为16 mm,渣线下的结瘤较轻,结瘤物主要是Al2O3夹杂着钢水凝结而成,由于钢水中的酸溶铝含量较高,水口处Al2O3的富集使钢水凝结,堵塞水口。根据研究得出,中间包上水口和浸入式水口结瘤堵塞的原因为钢水酸溶铝含量较高,钢水在水口处温降过大、拉速过低等。     

Numerical calculation of fluid flow in a continuous casting tundish

The deposition of deoxidation and reoxidation products in continuous casting tundish nozzles results in poor surface quality of the cast product and in extreme cases, disruption of the operating schedule. Since the flow of steel in the tundish is central to the behavior of nonmetallic inclusions in the tundish, the details of the flow field are determined numerically by appropriate solutions of the equations of change. The effects of the changes of the internal configuration of the tundish and various casting parameters on the computed velocity fields, particle trajectories, and particle residence times are investigated. The conclusions drawn from the computed results are compared with the results of various independent water model studies. Formerly with The Pennsylvania State University, is now with the United States Navy assigned to the Newport News Shipbuilding and Drydock Company     

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.     

Coupled turbulent flow,heat, and solute transport in continuous casting processes

A fully coupled fluid flow, heat, and solute transport model was developed to analyze turbulent flow, solidification, and evolution of macrosegregation in a continuous billet caster. Transport equations of total mass, momentum, energy, and species for a binary iron-carbon alloy system were solved using a continuum model, wherein the equations are valid for the solid, liquid, and mushy zones in the casting. A modified version of the low-Reynolds numberk-ε model was adopted to incorporate turbulence effects on transport processes in the system. A control-volume-based finite-difference procedure was employed to solve the conservation equations associated with appropriate boundary conditions. Because of high nonlinearity in the system of equations, a number of techniques were used to accelerate the convergence process. The effects of the parameters such as casting speed, steel grade, nozzle configuration on flow pattern, solidification profile, and carbon segregation were investigated. From the computed flow pattern, the trajectory of inclusion particles, as well as the density distribution of the particles, was calculated. Some of the computed results were compared with available experimental measurements, and reasonable agreements were obtained.     

双流连铸中间包流场优化方案的数值模拟

利用商业软件对武钢双流板坯连铸中间包原方案和优化方案进行了数值模拟,通过流场和停留时间分布曲线(RTD曲线)分析了各方案的中包流场特性,经水力学模型试验验证,其结果与数值模拟结果一致。研究表明,优化控流方案死区体积比Vd降至5.33%,比原方案降低了77.4%,平均停留时间Ta达到364.5s,比原方案增加了70.5s,优化后的中包流场更加合理,有利于夹杂物去除,该优化方案是适合双流板坯连铸中包的控流方案。     

板坯连铸机结晶器内钢液流场的数值模拟

基于流体力学的基本理论,利用商业软件fluent的,κ-ε湍流模型,实现了对结晶器内钢液流场的三维数学模拟.重点分析了浸入式水口的形状、插入深度、水口侧孔倾角以及拉速等工艺参数对结晶器钢液流场的影响.结果表明,对于断面为1280 mm × 180 mm的板坯结晶器,水口插入深度为150 mm,水口倾角为向下15°,拉坯速度为1 m/min时,结晶器内的流场较好.     

宽板坯连铸结晶器内钢水流动的数值模拟

以150mm×1600～3250mm宽板坯连铸结晶器为研究对象,利用大型商业软件ANSYS CFX10.0建立了1个三维有限体积模型,对结晶器内钢液的流动进行数值模拟.研究了拉速、浸入深度、水口倾角、断面宽度等工艺参数对结晶器内流场和窄面冲击压力的影响.结果表明:随着拉速的增大,表面流速和钢液对窄面的冲击压力都显著增加,采用较大的水口倾角和浸入深度,可以抑制液面波动,减少卷渣.     

板坯连铸结晶器内非对称流动现象的数值模拟

基于流体力学基本原理,采用Fluent软件建立板坯连铸结晶器及浸入式水口的三维有限元体积模型,模拟研究了水口不对中和水口单孔结瘤条件下结晶器内流体的流动特征和温度场分布状况.结果表明:水口对中不良时,结晶器两侧回流明显不对称,液面会产生旋涡,水口偏向侧温度高于水口偏离侧;水口出口单孔结瘤时,未结瘤侧流股增强,液面流速增大,并且液面有涡流产生,结瘤侧新鲜钢液减少,温度偏低.     

Mechanism of alumina buildup in tundish nozzles during continuous casting of aluminum-killed steels

During continuous casting of aluminum-killed steels, the pouring rate through the tundish nozzle often diminishes, posing serious operating problems. This happens because a buildup of microscopic alumina particles at the nozzle orifice effectively decreases the nozzle orifice diameter and causes constriction of the liquid-steel pouring stream. There are two major aspects of the constriction problem: (1) the source of the depositing particles, and (2) the mechanism by which the particles deposit at the nozzle orifice. Detailed microscopic examination of the buildup of alumina particles in nozzles from various casting trials and petrographic examination of the nozzle refractory indicated that the particles depositing at the nozzle orifice were already present in the melt because of deoxidation and reoxidation processes. A model proposed herein explains why and how the alumina particles deposit and stay at the nozzle orifice. The model considers a microscopically thin boundary layer at the nozzle bore where the velocity of the liquid steel approaches zero. Particles passing close to the refractory surface and in the slow-moving boundary layer attach to the wall and to each other. The presence of eddy currents in and close to the turbulent boundary layer increase the particle to particle collision. Such collisions of the particles thrown from outer region of boundary layers with the already attached particles keep on dumping the particles toward the refractory surface. The high interfacial energy of alumina inclusions in steel is a driving force for particles to attach to the refractory wall and to each other, and high-température sintering then occurs to form a network of alumina particles.     

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.     

Collision-controlled growth of composites in casting nozzles

Like the open-stream nozzles and submerged entry nozzles on c.c. tundishes, ladle nozzles can also become blocked. Accumulations of Al₂O₃, CaO · 2Al₂O₃, CaO · Al₂O₃, MgO · Al₂O₃ and Ti₃O₅ particles were observed on five sleeves removed from rotary ladle nozzles. The type of particles suspended in the steel depends on metallurgy. Growth takes place in bushy ramification in a coral-shaped structure and is controlled by flow separation, interfacial turbulence and gradient collision. Heat transfer on the nozzle wall is impaired. Steel which has infiltrated freezes if a heat sink is present. A stable composite of steel and oxide is formed. CaS depositions in c.c. tundish submerged entry nozzles are also examined. These CaS deposits become coated with viscous aluminates of the (C₁₂A₇ + CA) type, prior, here too, to impregnation with steel. The overall result is a reduction in interfacial energy.     

Numerical simulation of deformation-induced segregation in continuous casting of steel

The deformation-induced macrosegregation in continuous casting of steel has been simulated using a finite-volume scheme. For that purpose, a two-dimensional heat-flow computation was first performed in a Eulerian reference frame attached to the mold, assuming a unique solidification path, i.e., a unique relationship between temperature and enthalpy. This gave the stationary enthalpy field in the longitudinal section of the slab. On the other hand, bulging of the slab between two rolls was calculated in the same section, assuming plane-strain deformation and using the ABAQUS code. The Lagrangian reference frame was attached to the slab, and the rolls were moved at the surface until a stationary, bulging deformation profile was reached. The bulging of the surface was then used as an input condition for the calculation of the velocity and pressure fields in the interdendritic liquid. Using a fairly simple hypothesis for the deformation of the solid skeleton, the mass conservation and Darcy equations were solved in a Eulerian reference frame. This calculation was performed in an iterative loop, within which the solute conservation equation was also solved. At convergence and using the enthalpy field, this calculation allowed to obtain the temperature, the volume fraction of solid, and the average concentration fields, in addition to the fluid velocity and pressure. It is shown that the positive centerline segregation of carbon in the slab is well reproduced with this model. The effects of shrinkage and soft reduction were also investigated.     

Three-dimensional FEM study of fluid flow in mould for beam blank continuous casting: influence of straight through conduit type SEN

Abstract

In order to increase the beam blank cleanliness, the aim of this work is to analyse the flow field in the mould of beam blank continuous casting, to find the factors influencing the strand cleanliness and then to optimise the process parameters. A three-dimensional steady finite element model was developed to simulate and analyse the turbulent flow field in the mould. The volume of fluid model was used to track the free surface evolution at the meniscus. The influences of processing parameters, such as casting speed and nozzle parameters, on the molten steel flow in the strand (such as vortex location, liquid steel impact depth, velocity and fluctuation of the liquid steel at free surface) were analysed and the optimum processing parameters determined based on mass calculation. The results of this research project have been applied in actual production, and it has been shown that they are very useful and efficient for improving the steel cleanliness and controlling the surface cracks on the beam blank web.     

连铸板坯凝固传热过程的计算机模拟

建立了板坯连铸的凝固传热数学模型,充分考虑了弧形铸坯的几何特点,采用有限单元法求解控制方程。通过变参数φ来考虑铸坯内钢水和两相糊状区的导热系数,计算了参数φ、结晶热通量、拉速和二冷强度对铸坯表面温度和坯壳厚度的影响。结果表明,模型的准确性得到提高,拉速对固液界面有明显影响,二冷强度对铸坯表面温度影响非常大。     

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.     

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.