Effect of holding time and surface cover in ladles on liquid steel flow in continuous casting tundishes

Mathematical modeling of fluid flow and heat transfer of melt in a typical two-strand slab caster tundish has been done for a complete casting sequence. The complete casting sequence consists of 1 minute of tundish emptying period during the ladle transfer operation followed by 1 minute of tundish filling period by the new ladle and pouring at the normal operating level of the tundish for 46 minutes. The effect of varying ladle stream temperature conditions on the melt flow and heat transfer in the continuous casting tundish has been studied. When the ladle stream temperature decreases appreciably over the casting period, corresponding to heat loss of the melt in the ladle from the top free surface, the incoming melt temperature becomes lower than that of the melt in the bulk of the tundish after about 30 minutes from the start of teeming. This results in melt flow along the bottom of the tundish instead of the normal free surface directed flow. The ladle melt stream temperature shows little variability when the ladle has an insulated top. Corresponding to this situation, the temperature of the incoming melt remains higher than that of the melt in the bulk of the tundish and the normal free surface directed flow is maintained throughout the casting period. The product cast under such condition is expected to have a uniformly low inclusion content. The heat loss condition from the top of the ladle has been shown to be the dominant factor in determining fluid flow and heat-transfer characteristics of the melt in the tundish rather than the holding time of the melt in the ladle. Formerly Graduate Student, Department of Materials Science and Engineering, Ohio State University     

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.     

长水口对连铸中间包钢液保护浇注作用的研究进展

全球钢铁产品很大比例上是通过连铸工艺生产的,而中间包保护浇注是连铸生产高品质洁净钢的关键环节之一。长水口是连接于钢包和中间包之间的耐材质通道,长水口的发明和使用在连铸技术发展过程中起到了重要的作用,并与中间包的保护浇注效果有着紧密的联系,具体包括防止稳态和非稳态浇注过程中的二次氧化和来源于空气/渣/耐材/引流砂等的污染。本文基于中间包钢液污染的来源和形式,引申出了长水口在这些方面可以起到的潜在作用,并回顾了长水口在连铸发展早期的发明、工业实验效果和不断优化的历程。工业实践证实了长水口优良的保护浇注功能,但其实际效果与长水口的结构和操作工艺紧密相关。因此,分析了不同的长水口结构（包括工业化的长水口和一些新的设计理念）对保护浇注的影响,重点评述了喇叭型长水口在改善钢液洁净度和提高生产效率方面的优势。讨论了长水口的浸入深度和偏斜等操作工艺参数与保护浇注之间的关系。结合新时期炼钢?连铸的发展形势,指出了未来长水口结构功能一体化的发展方向,具体表现在长寿化、轻量化、多功能化和绿色化等方面。      

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.     

Transient Fluid Flow in a Continuous Casting Tundish during Ladle Change and Steady‐state Casting

Transient effects occur during both steady‐state casting as well as transient casting, e.g. a ladle change. These effects are caused by transient boundary conditions at the inlet of the tundish. A time‐dependent inlet temperature causes a free convection flow during steady‐state casting. During transient casting, such as during a ladle change, the mass flow at the inlet is time‐dependent and thus a transient flow develops. In general, transient flow is unwanted because transient flow means a change of conditions for the separation of non‐metallic particles. The analysis of the flow in the tundish is carried out by numerical as well as physical simulations. In this case experimental investigations are carried out on a water model. The results of laser optical investigations using Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (DPIV) serve as a validation of the numerical results. The numerical results are then used for the investigation of the thermal melt flow. The effects caused by a changing bath level during transient casting (ladle change) are investigated using the Volume‐of‐Fluid (VoF) model. Beyond this, the interaction between the melt and slag is taken into account, by using the three phase system melt‐slag‐air. In addition to the classical methods a new zonal approach is introduced in this paper. The integral balance localises high turbulence mixing regions as well as the development and intensity of back flows. The levelling of the momentum flux between the inlet and the outlet can also be described.     

Fast and Accurate Prediction of Stratified Steel Temperature During Holding Period of Ladle

Metallurgical and Materials Transactions B - Thermal stratification of liquid steel in a ladle during the holding period and the teeming operation has a direct bearing on the superheat available at...     

Transient flow and heat transfer in a steelmaking ladle during the holding period

Transient, turbulent flow and heat transfer in a ladle during the holding period are numerically investigated. The ladle refractories including the working lining, safety lining, insulation layer, and steel shell have been simultaneously taken into account. No assumptions are made for the heat transfer between the liquid steel and the inside ladle walls. Both the initial ladle heating and the heat loss from the slag surface are changed to examine their effect on thermal stratification in molten steel. A simplified model for the heat loss from the molten steel to the refractory is proposed. Correlations for the history of mean steel temperature, thermal stratification, and heat loss rate are obtained, which can be easily applied for industrial operations. Predictions are compared with experimental data in an industrial ladle and a pilot plant ladle, and those from previous studies.     

低碳铝镇静钢中间包浇注过程夹杂物的行为

 通过对BOF-Ar站-CC炼钢流程生产低碳铝镇静钢的中间包不同浇注时间取样及正常坯的取样,采用氧氮化学分析、光学显微镜以及扫描电镜-能谱(SEM+EDS)等多种方法研究了中间包浇注过程夹杂物特征的变化。结果表明：每炉钢包开浇时与浇注末期,钢中T[O]含量均高于浇注中期的T[O]含量,这是由于换包过程中钢水被二次氧化;中间包钢水及正常坯中的夹杂物,按照其形貌与成分可以分为以下3类：Al2O3基夹杂物,MnS基夹杂物,来自中间包覆盖剂或者钢包下渣所卷入的外来夹杂物。中间包及铸坯中的夹杂物主要以1~4 μm的Al2O3为主,同时在铸坯中发现了大量的MnS夹杂物,使铸坯中夹杂物的数量密度升高。当钢液中硫含量较高时,铸坯中气泡+Al2O3类型的夹杂物增加。在当前的工艺条件下,交换钢包之后的开浇阶段与浇注末期,钢水的二次氧化对铸坯的洁净度产生重要影响,同时应合理控制钢中的硫含量,减少铸坯中气泡+Al2O3类型的夹杂物,避免钢液在凝固过程中析出大量的MnS夹杂物。     

Two-phase sink vortex suction mechanism and penetration dynamic characteristics in ladle teeming process

At the late stage of continuous casting (CC) ladle teeming,sink vortex can suck the liquid slag into tundish,and cause negative influences on the cleanliness of molten steel.To address this issue,a two-phase fluid mechanical modeling method for ladle teeming was proposed.Firstly,a dynamic model for vortex suction process was built,and the profiles of vortex flow field were acquired.Then,based on the level set method (LSM),a two-phase 3D interface coupling model for slag entrapment was built.Finally,in combination with high-order essentially non-oscillatory (ENO) and total variation diminishing (TVD) methods,a LSM-based numerical solution method was proposed to obtain the 3D coupling evolution regularities in vortex suction process.Numerical results show that the vortex with higher kinetic energy can form an expanded sandglass-shape region with larger slag fraction and lower rotating velocity;there is a pressure oscillation phenomenon at the vortex penetration state,which is caused by the energy shock of two-phase vortex penetration coupling.     

湍流控制装置的结构对中间包流体流动特性的影响

通过水力学模型实验，研究了不同结构的湍流控制装置对中间包流体流动特性的影响，结果表明，湍流控制装置的几何结构对中国包流体流动特性有明显的影响，方形无顶缘和圆形无顶缘的湍流控制装置对抑制钢包注流的动能作用不大，而方形带顶缘和圆形带顶缘的湍流控制装置能够较好地改善中间包流体的流动特性，本实验采用长宽比为1．44的矩形湍流控制装置，无论其是否带有顶缘，均难以改善中间包的流动特性。     

热轧卷板表面夹渣缺陷来源分析及控制现状

热轧卷板的表面夹渣缺陷对热轧板的质量及产品性能会产生极其恶劣的影响,会导致产品品级的下降乃至报废等问题,并对产品的服役期限及性能造成一定影响.随着冶炼过程中钢液洁净度的不断提高,夹渣缺陷所造成的质量问题显得尤为严重.而不同生产工艺下表面夹渣缺陷的来源方式略有差异,缺陷的来源主要有精炼过程中钢包渣的卷渣、非稳态浇注时期的...     

Effect of Actual Cooling Rate of Ladle Stream on Persistent Metallurgical Performance of a Given Tundish

To evaluate the effect of actual cooling rate of liquid steel in the ladle on the metallurgical performances of a tundish, a transient and coupled computational model was developed to reveal the flow fields, temperature fields, residence time distribution of the molten steel and the inclusion removal efficiency in a typical single-strand tundish with given geometry and process parameters. The results showed that, with the decrease of the ladle stream cooling rate, the temperature difference of bulk flow at the outlet of tundish over a normal casting period decreased from 11.3 to 2.6 K, and the dead volume fraction of the tundish decreased from 17.58% to 14. 35%, while the inclusion removal efficiency was increased especially for the inclusions with the diameter less than 50 μm, whose removal ratio could be increased by 20.62%. When the cooling rate was less than 0.3 K · min-1 , however, the variation rates of the three evaluation criterions above declined significantly, which suggested that a critical value existed for the effect of the cooling rate of ladle stream on the tundish performances. The establishment of the critical ladle stream cooling rate should be very important to achieve persistent metallurgical properties of tundish over the whole casting stage, together with the reasonable ladle insulation design.     

Experimental investigation of flow control devices effect on inclusion separation in a four strand tundish

In continuous casting, molten steel often contains non-metallic inclusions that have a very high melting temperature and thus remain suspended in the molten flow. Given the lower density of these substances compared to the melt, they can be filtered in the form of top slag by proper use of buoyancy forces. Thus, providing a desirable flow pattern, reduced flow turbulence, and adequate residence time for inclusions in the tundish will improve the inclusion separation performance and therefore the product quality. In this study, a reduced-scale water model of a tundish was used to evaluate the effect of melt level in the tundish on the flow pattern and inclusion separation performance. Then, the flow control devices are added to this simple tundish model to examine the effect of them on the flow behaviour and inclusion separation. Experimental investigation showed that raising the water level in the model tundish has desirable effects on inclusion separation and the use of the dam at the inlet with tall dam improves the flow pattern and inclusion separation almost 20% in the form of top slag.     

连铸钢包浇注过程中旋涡的形成机理

连铸钢包浇注末期出现的旋涡卷渣现象会对连铸钢水质量产生重要的影响,为了有效控制生产中的旋涡卷渣现象,对钢包浇注末期旋涡的产生机理开展详细研究.依据相似原理,构建了相似比为1∶6的钢包物理模型,对钢包浇注过程中旋涡产生及卷渣过程进行研究,分析了初始液面高度、出水口尺寸与布置位置、出水口结构等因素对旋涡形成规律的影响.结果表明：旋涡形成过程与初始液面高度无关;钢包出水口尺寸和布置位置对于旋涡形成过程影响显著.随着出水口直径的增大,旋涡临界高度逐渐增加;随着偏心率的增加,旋涡临界高度逐渐降低,最佳偏心率为3/4.提出三片挡块式出水口结构,可以显著降低钢包浇注末期的旋涡临界高度,降低高度达50％左右.     

A Model for Predicting the Melt Temperature in the Ladle and in the Tundish as a Function of Operating Parameters during Continuous Casting

A mathematical model for predicting the melt temperatures in the ladle and in the tundish during continuous casting has been developed. First of all, a chain of models was created for the following stages of the ladle cycle; the preheating of the empty ladle, filling of the ladle, period in the ladle furnace, waiting period prior to casting, the casting period, and, finally, the free cooling period of the empty ladle. Models, written in CFD code, were used in sequence so that each simulation continued from the results of the simulation of the previous stage. An intermediate model was constructed to estimate the outlet temperature of melt drained from the ladle. Then the work was continued by performing simulations in the tundish, using as input the temperature of the simulated melt feed from the ladle and, as an initial condition, the temperature field of the remaining melt in the tundish. The final model “TEMPARV3” was created and tested by means of measured tundish data received from a steel plant. By means of statistical analysis the coefficients of correlation between the test data and model data at the start, in the middle period, and at the end of casting were calculated to be 0.9, 0.92 and 0.87, respectively. So, the most effective predictive power of the model in the tundish by means of a sequential casting schedule is realized during the middle period of the casting process. The model is applied interactively by a user interface, which expresses the predicted melt temperatures numerically and with graphical curves. The predictive model can be used off‐line as a tool for scheduling the stage operations in advance. The program may be utilized on‐line to estimate the superheat needed and to control periods of the operation. In extreme cases, when the model alerts the operator about the danger of superheat loss having a critical effect on casting, the operator has a chance to take adjustment measures. In addition to production work, the model could be of benefit for studying changes in operating parameters, for training operators, and for use as a “low‐cost computational pilot plant” in process development in general.     

连铸铸余渣用于中高碳钢水精炼技术研究

连铸过程中浇完钢水后,钢水罐中会残存钢水精炼后生成的具有较低氧势的钢渣,即铸余渣。这类钢渣在炼钢温度下具有良好的熔化性能,可以利用渣罐、废弃中包、钢水罐收集,加工后再用于钢厂精炼生产中。通过对其成分的研究及工艺试验,找出其最佳工艺途径以降低成本,该工艺在生产实践中取得了成功。采用DIN50602标准检验出钢中硫化物评级指数由0.91降低至0.18,氧化物评级指数由0.56提高至0.91。     

Q195热轧带钢大型夹杂物来源分析及控制

大型夹杂物对热轧带钢表面质量有着重要的影响。采用示踪剂试验、大样电解的方法,对某厂Q195热轧带钢中大型夹杂物来源、成分、含量进行研究,同时对生产过程中钢包、中间包顶渣进行分析。研究结果表明,研究中大型夹杂物主要为Al₂O₃-SiO₂-MnO、SiO₂-MnO复合夹杂和SiO₂夹杂物,粒度为50~2 000 μm,含量为(40 ~145) mg/10 kg,尺寸较大且含量较高,主要源于钢包、中间包顶渣、引流剂卷入。同时发现钢包、中间包顶渣氧含量较高,碱度较低。应对钢包、中间包覆盖剂进行优化,提升碱度,以达到顶渣吸附去除夹杂物的目的。     

A Physical Model Study of Two-Phase Gas–Liquid Flows in a Ladle Shroud

Metallurgical and Materials Transactions B - Argon-steel flows inside a ladle shroud during teeming from a ladle to a tundish have been modelled physically. To this end, full-scale Perspex models...     

Study of fluid flow in tundishes due to different types of inlet streams

Flow pattern and residence time distribution induced by different methods of pouring of molten steel into a tundish are studied by means of a water model with and without flow modifiers (FM). It was found that the submerged stream without FMs produces short circuiting and argon shrouded stream leads to excessive surface directed flow of liquid in a tundish. Open stream pouring generates surface turbulence and does not produce short circuiting. The effect of FM is found to be different for different methods of pouring, e.g. dam eliminates short circuiting and produces surface directed flow for submerged stream, whereas weir limits the surface turbulence within the inlet region for open stream pouring. Dam is found to reduce the dead volume for argon shrouded pouring.     

大罐顶渣向钢液增氧因素分析

 依据实验数据计算结果,对钢液增氧的因素及其采取的相应措施进行了分析与探讨。研究结果表明：在氧化性顶渣条件下,钢液中w（［Als］）的变化率≥20％,而在还原性顶渣条件下,钢液中w（［Als］）的变化率≤10％;氧化性顶渣对钢液供氧的贡献率大于钢包与中间包衔接的水口处吸气对钢液供氧的贡献率;减少转炉下渣量及对顶渣进行改质可减轻炉渣向钢液供氧程度;大罐下渣量不会使中间包成为新的增氧源。