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 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.     

Modeling Study of Turbulent Flow Effect on Inclusion Removal in a Tundish with Swirling Ladle Shroud

The present study is focused on the assessment of a new concept of ladle shroud capable to control the turbulence promoted by the steel entry jet in a continuous casting tundish; the new proposal is a Swirling Ladle Shroud (SLS). It presumed that the SLS decreases the impact velocities in the tundish bottom close to 1/3 of that provided by a conventional shroud. In this mathematical study an analysis of turbulence control and particle removal is made by comparing the SLS with two different conventional tundish arrangements. Particle sizes included 1, 5, 20, 40, 60, 80, 100, 120, 140 and 160 microns. Simulations also included the effects of the mass flow rate on the removal efficiency of non‐metallic inclusions, considering 3.8 and 7.6 ton/min mass flow rates. It was found that the SLS is capable to handle different mass flow rates, opposite to the conventional arrangements where at any increase of mass flow rate, these devices become inefficient to control turbulence, reducing considerably the inclusion removal efficiency. These results illustrate that using a SLS, the turbulent flow control and the particle removal may be better with this new proposal.     

Modelling the effects of off-centered ladle streams on fluid flow of liquid steel in a slab tundish

The effects of off-centered ladle streams on fluid flow of liquid steel in a two-strand slab tundish were studied using water modelling techniques and particle image velocimetry (PIV) measurements. Fluid flow in an off-centered ladle stream was compared with a centered ladle stream. In a centered position the proposed arrangement (turbulence inhibitor plus dams) yields higher fractions of plug flow than the current arrangement (baffles). This effect is stronger at the high casting rate. An off-centered position increases dead volumes at a low casting rate of 1.7 t/min. At the high casting rate of 3.8 t/min, effects of the off-centered position are decreased although the TI-D arrangement always renders better flow characteristics. Velocity fields determined with PIV measurements, demonstrated that the TI-D arrangements provide the fluid with a plug flow characteristic.     

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.     

Physical and Mathematical Modeling of Inert Gas-Shrouded Ladle Nozzles and Their Role on Slag Behavior and Fluid Flow Patterns in a Delta-Shaped, Four-Strand Tundish

Inert gas shrouding practices were simulated using a full-scale, four-strand water model of a 12-tone, delta-shaped tundish. Compressed air was aspirated into the ladle shroud to model volumetric flow rates that range between 2 and 10 pct of steel entry flows. Bubble trajectories, slag layer movements, and flow fields, were visualized. Flow fields were visualized using particle image velocimetry (PIV). A numerical model also was developed using discrete phase modeling (DPM) along with the standard k-ε turbulence model with two-way turbulence coupling. Predicted flow fields and bubble trajectories corresponded with the water model experiments.     

Modelling of air ingress and pressure distribution in ladle shroud system for continuous casting of steel

The pressure distribution and fluid flow profiles whithin the slide gate and shroud nozzle for the continuous casting of steel have been investigated using a full scale water model and a CFD (computational fluid dynamics) model. The water modelling has shown that a large quantity of air can be drawn into the liquid stream if there is any breakdown of the seals in the vicinity of the slide gate. The 3-dimensional numerical solution for highly turbulent flow has predicted the pressure distribution and velocity profile within the slide gate and shroud. Based on the experimental and numerical modelling, it has been shown that cavitation can occur near the slide gate during ladle teeming. This can be a source for erosion of the refractories. Improvements to the design of the ladle shroud system are recommended.     

侧孔长水口两流非对称中间包流场优化

 中间包流体流动直接影响连铸坯的质量,为了缩小非对称中间包两流之间的流动差异,利用水模型和数值模拟方法,研究了侧孔长水口中间包流体流动特性。研究结果表明：直通型长水口、圆形湍流控制器和单挡墙结构中间包两出口流体差异较大,侧孔长水口和多孔挡墙结构中间包两流之间平均停留时间差异是直通型长水口中间包的3/4,且近长水口侧出口的平均停留时间延长7.7%。     

长水口吹氩生成微小气泡工业实验研究

在连铸生产中采用大流量长水口吹氩,并采用“冷钢片沾钢法”沾取中间包钢液试样,成功沾取了中间包钢液中微小氩气泡。冷钢片沾样表面气泡为中间包上部钢/渣界面和炉渣中氩气泡,尺寸主要位于1.0~3.0 mm,但该尺寸不能反映中间包钢液内部长水口吹氩生成气泡,冷钢片沾样内部气泡为钢液内部长水口吹氩生成的气泡。结合扫描电镜和共聚焦显微镜对沾取试样内部气泡形貌、尺寸和数量进行了分析,结果表明大部分气泡为独立圆形气泡,偶见少量粘连和聚合气泡;钢液内部氩气泡尺寸主要位于100~1000 μm,平均尺寸为500 μm左右;气泡在长水口出口及其下方较为弥散,气泡数量可达15.2 cm?2。采用扫描电镜结合能谱分析,发现部分气泡内粘附有夹杂物,有些气泡粘附多个夹杂物;气泡粘附Al₂O₃夹杂物的几率高于粘附CaO(?MgO)?Al₂O₃?SiO₂复合夹杂物的几率。      

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...     

连铸保护浇注工艺改进

通过改进长水口机械手结构,采用一种新型浸入式开浇长水口,开浇前进行包盖吹氩、浇注过程中对长水 口、浸入式水口板间进行吹氩保护,避免空气进入钢包注流、中间包冲击区、开浇初期钢水、中间包注流等区域,防 止钢水二次氧化,减少了浇注过程中钢水铝损和钢中［N］质量分数,提高了连铸坯质量。     

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.     

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     

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.     

单侧孔长水口优化异型四流中间包流场

采用数值模拟方法研究了不同形式长水口下异型四流中间包的钢液流场和停留时间分布特征.数值结果表明:原型中间包存在较大的死区,各流流动情况存在着巨大的差异,而把直通型长水口替换成单侧孔型长水口之后,中间包死区比例减少,各流流动差异性基本上被消除.在挡墙的左侧产生了旋转流场,有利于夹杂物的碰撞聚合和去除.     

Numerical and Physical Simulation of Tundish Fluid Flow Phenomena

The fluid flow in a continuous casting tundish is numerically and physically simulated by means of water models. Results of residence time distribution (RTD) measurements and laser‐optical measurements (Laser Doppler Anemometry – LDA, Digital Particle Image Velocimetry‐DPIV) are used to validate the numerical results for water before the numerical simulation is transferred to the steel melt. The investigations are focused on both steady‐state and transient casting conditions. To reduce vortexing and turbulence in the tundish different types of turbo‐stoppers are installed in the water models and their influence on the spacious flow structure is discussed. The turbo‐stopper produces higher turbulence in the inlet region of the tundish, but this region is spatially more limited in relation to the flow without turbo‐stopper. Thereby a more homogeneous flow is created at the outlet of the tundish with better conditions for particle separation. Basic design criteria for the geometry of a turbo‐stopper are developed. Moreover, the processes of first tundish filling and ladle change are simulated at a downscaled water model and these results are compared with numerical simulations using a Volume of Fluid (VoF) model. This multiphase model is able to reproduce the motion of gas bubbles and waves at the free surface.     

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。     

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.     

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...     

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.