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.     

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.     

气幕挡墙中间包仿真优化及其冶金效果

根据水钢炼钢厂150 mm×150 mm六流连铸32 t中间包的结构操作工艺参数,采用数模仿真法研究了气幕挡墙技术对中间包内钢液流动特性及夹杂物去除的影响,并对HPB235和65钢进行了工业试验。结果表明,气幕挡墙可以有效改善钢液的流动状态,均衡各出口停留时间,有效延长钢液的平均停留时间,降低死区体积,提高夹杂物去除率。该技术适应多流中间包纯净钢冶炼的需求。     

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.     

中间包底吹氩水模型试验及冶金效果

通过水模型试验和工业试验,研究了中间包底吹氩参数对中间包流场的影响。水模型试验结果表明:中间包底吹气可以改善中间包流场,较佳的吹气参数是透气砖距离冲击区765 mm,吹气量在0.06~0.10 m3/h。工业试验结果表明:中间包采用底吹氩后,T[O]降低了26.67%;大型夹杂物总含量由0.430 mg/kg降至0.079 mg/kg,降低了81.82%。     

鞍钢宽厚板坯中间包气幕挡墙研究

以相似原理为基础,通过实验室水模拟实验,对鞍钢单流宽厚板坯连铸中间包应用气幕挡墙进行了优化研究。水模实验主要研究了原型中间包和气幕挡墙、耐火挡墙不同组合方案钢液的流动特征,将实验确定的最佳方案在大生产上进行了工业试验。结果表明,工业试验和水模实验的结果具有一致性。中间包应用气幕挡墙提高了钢的洁净度,显微夹杂物降低了23.73%,大型夹杂物降低了28.15%。     

连铸中间包气幕挡墙水模拟实验研究

通过水模拟实验,研究了气幕挡墙位置、气体流量及采用双透气砖对中间包流场的影响;认为采用气幕挡墙可有效延长钢水在中包的平均停留时间,降低死区,混匀钢水;并提出工业性试验的可行性建议。     

六流小方坯中间包气幕挡墙的数学模拟

根据6流200 mm×200 mm方坯30 t中间包的结构操作工艺参数,采用欧拉两流体模型、多孔介质模型、欧拉-拉格朗日随机轨道模型及Monte-Carlo法,并引入气泡吸附模型,用数学模拟法对比研究了采用气幕挡墙技术对6流中间包内钢液流动特性及夹杂物去除的影响。结果表明,采用气幕挡墙技术优化后,可以有效改善钢液的流动状态,均衡各出口停留时间,有效延长钢液的平均停留时间,降低死区体积,提高夹杂物去除率,适应多流中间包超纯净钢冶炼的需求。     

四流中间包气幕挡墙水模拟研究

采用不同参数的多孔透气砖进行了25 t四流中间包的底部吹气水模拟实验,结合RTD曲线和标准差分析,研究透气砖面积(210～105 mm×56～28 mm) 、4种吹气位置和吹气量(60～120 L/h)对中间包液体流动特性的影响,并与不吹气的情况进行了对比。结果表明,中间包吹气能改善液体的流动特性,均衡其在不同水口间的停留时间,并能延长峰值时间和降低死区体积。透气砖靠近挡墙,采用小气量和小面积更有利于改善现有中间包流场。     

气幕挡墙中间包钢水流动的数值模拟

根据二流连铸1500mm×250mm板坯时中间包的操作工艺参数,采用欧拉两流体模型,多孔介 质模型和拉格朗日随机轨道模型,模拟计算了采用湍流控制器和气幕挡墙技术的中间包内钢液流动特性和 夹杂物的运动轨迹,并用Monte-Carlo法统计了夹杂物的总去除率。模拟结果表明,采用中间包气幕挡墙技术 可以有效改善钢液的流动特性,延长钢液停留时间,减小死区体积;当吹气量为0.90m³/h时,夹杂物去除率比 不吹气工艺增加15.6%     

不同位置气幕挡墙方案对两流连铸中间包内流场的影响

在相似理论的基础上,通过水力学模拟对两流板坯连铸中间包3种不同气幕挡墙形式下的包内流场进行研究。实验结果表明：在同一吹气量（28L/h）时,气幕挡墙置于中间包端部（方案一）,在气幕的两侧形成两个方向相反的回流区域,延长了钢液的平均停留时间,采用该方案时中间包内死区比例为18.5%;气幕挡墙置于中间包墙坝之间（方案二）,也形成了两个较大的回流区,增加了钢液间的混合和夹杂物的去除,中间包内死区最小为17.4%,为三种方案之最优;气幕挡墙置于中间包挡墙之前（方案三）,气幕挡墙并未形成有效的气幕,中间包内部流体未得到充分混匀,出现27.5%的较大的死区。     

Hydraulics and Mathematics Simulation on the Weir and Gas Curtain in Tundish of Ultrathick Slab Continuous Casting

The molten steel flow pattern in continuous casting tundish could be improved if the flow control devices were properly used. With reasonable application of weir and air curtain, the disturbance at injection zone of the tundish was reduced. The flow path of the molten steel was changed, and the short-circuit flow was eliminated. Therefore, the residence time was lengthened. An air curtain with bubbles floating could promote the surface flow and improve inclusions removal. The application of weir and air curtain in an ultrathick slab continuous casting tundish was investigated with hydraulics and mathematics simulation. The residence time and dead volume fraction were studied through orthogonal experiments with different positions of the flow control devices. The efficiency of three factors was analyzed, and the optimum positions of the weir and air curtain were obtained. Besides, the discrete phase model was suitable for simulation of the interaction between gas bubbles and molten steel, and the mathematics results shown the optimum one got larger inclusion flotation rate.     

气幕挡墙中间包夹杂物去除的水模型研究

以连铸1500mm×250mm板坯的中间包为原型,几何相似比1/3,采用自制多孔透气砖进行中间包底吹气的水模型试验。结果表明,气幕挡墙中间包中,夹杂物通过斯托克斯机理上浮,气泡吸附和钢液上扬流动而上浮去除;当吹气量≥0.08m~3/h,距入口中心533～800mm处形成有效的气幕挡墙,显著改善钢液流动性,夹杂物去除率由不吹气的44.86%增加至72.86%,并改善小颗粒夹杂物的去除。工业试验表明,采用气幕挡墙,使T[O]降低12%以上。     

薄板坯连铸中间包吹氩行为的物理模拟

中间包吹氩是适应洁净钢冶炼的新技术、新方法。依据相似原理,在实验室建立了一定相似比的薄板坯连铸中间包物理模型。比较了传统堰坝组合,吹氩气幕挡墙分别代替堰、坝对中间包流体流动特征的影响,并尝试在中间包内采用双气幕挡墙的控流方式。结果表明,气幕挡墙分别代替堰、坝,中间包停留时间和死区体积与堰 坝组合十分相近;双气幕挡墙代替堰 坝组合,除死区体积较大外,停留时间接近堰 坝组合。上述结论为气幕挡墙技术的进一步应用提供了参考依据。     

Analysis of the mathematical model of the gas bubbling curtain injection on the bottom and the walls of a continuous casting tundish

In the present work, the behaviour of a gas bubbling curtain (GBC) in the continuous casting tundish of industry is studied by using a physical model in the scale of 1:3. This study was made with the following configurations of gas injection inside the tundish: no gas injection, gas injection on the bottom and combined gas injection (CGI) on the bottom and the walls. Mathematical simulations were performed using the Eulerian approach. Mathematical modelling was also used to determine the residence time distribution curves (RTD curves). For the validation of the mathematical model, experiments were made using the physical models to obtain the homogenisation time of salt solutions. All of the results were validated by the experimental results of the RTD curves. The plug flow fraction remained almost constant when gas injection was used on the walls, however the mean residence time and the dead volume fraction, increased and decreased, respectively. The CGI on the bottom and walls allowed the entire cross-section to be covered with the GBC which acted as a barrier to the passage of inclusions.     

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.     

板坯连铸中间包气幕挡墙夹杂物去除的研究

分析了中间包不加气幕挡墙和加气幕挡墙吹氩时夹杂物上浮速度与气泡直径、夹杂物直径和密度的关系。气泡尺寸对夹杂物的上浮速度影响比较明显,夹杂物的密度对上浮速度的影响不大。进行了中间包气幕挡墙的工业试验,试验表明:中间包底吹气对钢液中小夹杂物的去除作用不明显,而对大型的夹杂物去除效果显著。     

气幕挡墙中间包内渣钢界面卷混现象

 中间包内采用气幕挡墙是有效去除钢水中细小夹杂的新技术。采用物理模拟方法,研究了连铸中间包采用气幕挡墙时其内流动行为和渣钢界面卷混现象,考察了吹气流量、气幕位置以及与湍流控制器组合对中间包内的渣钢界面的影响规律。结果表明,气幕挡墙的位置和吹气量是影响中间包卷渣的主要因素。     

Modelling Study of Inclusions Removal by Bubble Flotation in the Tundish

Non‐metallic inclusion removal from liquid steel in a tundish is studied using two‐phase flow modelling by Particle Image Velocimetry (PIV) techniques and mathematical simulation. The removal efficiency is studied as a function of Flow Control Devices (FCD) arrangements inside the tundish, gas bubbling and mass flow rate. The mathematical two‐phase model includes an Eulerian‐Eulerian approach for the gas‐liquid system and a Lagrange approach for the solid particles trajectories. The validation of this model was acceptably proved through PIV measurements and colour tracer experiments in a two‐phase water model. The removal efficiency of the tundish in the cases of gas bubbling becomes independent of particle size and FCD arrangements. An increase of mass flow rate decreases the particle mean residence time in the tundish and therefore the removal efficiency. Under the same conditions coupling‐uncoupling phenomena of solid particles from the liquid flow depends strongly on their response time. Where this phenomena occurs, it is determined that the particle response time in the model goes from 10^?5 to 10^?3 seconds for particle size ranging from 20 to 160 μm, respectively; this transition is dependent on particles size and mass flow rate.     

中间包吹氩去除夹杂物的模拟研究

通过水模型模拟实验,选用苯胺模拟夹杂物,有机硅油模拟中间包渣,研究了中间包吹氩去除夹杂物的效果;通过刺激—响应法测定中间包吹氩RTD曲线,探讨了中间包吹氩去除夹杂物的过程和机理。研究结果表明:在中间包吹氩条件下,活塞区体积分数最大的吹氩位置不一定会得到最大的夹杂物去除效果,在注流区附近吹氩,不但可以提高气泡捕捉夹杂物颗粒的概率,而且促进了夹杂物颗粒相互碰撞长大,去除夹杂物效果更为明显。