Numerical Simulations of Inclusion Behavior in Gas-Stirred Ladles

A computation fluid dynamics–population balance model (CFD–PBM) coupled model has been proposed to investigate the bubbly plume flow and inclusion behavior including growth, size distribution, and removal in gas-stirred ladles, and some new and important phenomena and mechanisms were presented. For the bubbly plume flow, a modified k-ε model with extra source terms to account for the bubble-induced turbulence was adopted to model the turbulence, and the bubble turbulent dispersion force was taken into account to predict gas volume fraction distribution in the turbulent gas-stirred system. For inclusion behavior, the phenomena of inclusions turbulent random motion, bubbles wake, and slag eye forming on the molten steel surface were considered. In addition, the multiple mechanisms both that promote inclusion growth due to inclusion–inclusion collision caused by turbulent random motion, shear rate in turbulent eddy, and difference inclusion Stokes velocities, and the mechanisms that promote inclusion removal due to bubble-inclusion turbulence random collision, bubble-inclusion turbulent shear collision, bubble-inclusion buoyancy collision, inclusion own floatation near slag–metal interface, bubble wake capture, and wall adhesion were investigated. The importance of different mechanisms and total inclusion removal ratio under different conditions, and the distribution of inclusion number densities in ladle, were discussed and clarified. The results show that at a low gas flow rate, the inclusion growth is mainly attributed to both turbulent shear collision and Stokes collision, which is notably affected by the Stokes collision efficiency, and the inclusion removal is mainly attributed to the bubble-inclusion buoyancy collision and inclusion own floatation near slag–metal interface. At a higher gas flow rate, the inclusions appear as turbulence random motion in bubbly plume zone, and both the inclusion–inclusion and inclusion-bubble turbulent random collisions become important for inclusion growth and removal. With the increase of the gas flow rate, the total removal ratio increases, but when the gas flow rate exceeds 200 NL/min in 150-ton ladle, the total removal ration almost does not change. For the larger size inclusions, the number density in bubbly plume zone is less than that in the sidewall recirculation zones, but for the small size inclusions, the distribution of number density shows the opposite trend.     

Mathematical Modelling of Water Sampler Filling

Steel samples taken from ladles or tundishes during the steel making process can be of significant importance when monitoring the inclusion size and distribution. In order to preserve the original size and distributions of inclusions in the extracted samples, it is important to avoid their collisions and coagulations inside samplers during filling. Thus, it is necessary to investigate the flow during a sampling process to make sure that this is minimized. In addition, it is important to study the turbulence characteristics, since it is known to influence the inclusion growth. This study presents mathematical modelling of sampler filling using water as a media and experimental results for verification. The study focuses on a lollipop‐shaped sampler since it is one of the most common in the industry. The sampler is filled from an inlet pin located at the bottom centre of the main body. In addition, two different turbulence models, the realizable k‐ε model and Wilcox k‐ω model, were used to study the flow pattern in the sampler. The predictions were compared to experimental results obtained by Particle Image Velocimetry (PIV) measurements. It was found that the flow field predictions using the Wilcox k‐ω model agreed best with the flow field obtained by PIV measurements. Furthermore, it was illustrated that the Wilcox k‐ω model can be used for predictions of the different flow regions as well as the positions of the centres of vortexes which are located near the free surface. Thus, it is concluded that the Wilcox k‐ω model can be used in the future to predict the filling of steel samplers.     

Modeling of inclusion removal in a tundish

Mathematical models have been developed to predict the removal of alumina inclusions from molten steel in a continuous casting tundish, including the effects of turbulent collisions, reoxidation, flotation, and removal on the inclusion size distribution. The trajectories of inclusion particles are tracked through the three-dimensional (3-D) flow distribution, which was calculated with the K-ɛ turbulence model and includes thermal buoyancy forces based on the coupled temperature distribution. The predicted distributions are most consistent with measurements if reoxidation is assumed to increase the number of small inclusions, collision agglomeration is accounted for, and inclusion removal rates are based on particle trajectories tracked through a nonisothermal 3-D flow pattern, including Stokes flotation based on a cluster density of 5000 kg/m³ and random motion due to turbulence. Steel samples should be taken from as deep as possible in the tundish near the outlet and at several residence times after the ladle is opened, in order to best measure the Al₂O₃ concentration entering the submerged entry nozzle to the mold. Inclusion removal rates vary greatly with size and with the presence of a protective slag cover to prevent reoxidation. The random motion of inclusions due to turbulence improves the relatively slow flotation of small inclusions to the top surface flux layer. However, it also promotes collisions, which slow down the relatively fast net removal rates of large inclusions. For the conditions modeled, the flow pattern reaches steady state soon after a new ladle opens, but the temperature and inclusion distributions continue to evolve even after 1.3 residence times. The removal of inclusions does not appear to depend on the tundish aspect ratio for the conditions and assumptions modeled. It is hoped that this work will inspire future measurements and the development of more comprehensive models of inclusion removal. These validated models should serve as powerful quantitative tools to predict and optimize inclusion removal during molten steel processing, leading to higher quality steel.     

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.     

底吹氩过程LF炉盖内气体流动的数值模拟

基于流体模型和湍流修正模型,借助流体工程模拟软件Fluent 6.3.26对吹氩过程中210 t LF精炼炉盖内气体的流动、混合和质量、动量传输进行了计算模拟,分析了其流动行为和分布状态。结果表明,随钢包净空高度增加,液面上部氩气回旋区扩大,"死区"减小;当氩气流量增至500 L/min时,吹氩孔位于0.68 R的盖内流动效果优于0.3 R的效果;正常工作状态下的合理抽气压力为-150 Pa;在合理的抽气压力和吹氩孔位置的情况下,300 L/min的氩气流量基本可以满足要求,强搅拌时可增至500 L/min。     

Research on molten flow simulation of 30t VOD ladle smelting stainless steel

30tVOD ladle smelting stainless steel was used as the process background, and the numerical and physical simulations were applied to investigate the flow and mixing characteristics of molten steel during the 30tVOD refining process. The cold experiment was employed to investigate the mixing behaviors of molten steel under the bottom and combined blowing systems. The flow features of molten steel were analyzed by numerical simulation with different blowing conditions. The results show that when the plug position of the ladle is located on 1/4R(R is the radius of ladle bottom), whether top- bottom blowing or bottom blowing, the molten steel is mixed well, and mixing time compared to the central position is reduced by 46% and 14% respectively. In addition, the comparison of flow field and turbulent kinetic energy of the molten steel show that when the bottom blowing position is 1/4R, the flow direction of the molten steel caused by the top blowing and the bottom blowing is better near the bath surface, and active volume ratio of molten steel is higher than that of other blowing positions. Current results were validated in industrial VOD furnace, which indicates that 1/4R is regarded as the optimum injection position for combined stirring.     

Prediction of temperature in secondary steelmaking: mathematical modelling of fluid flow and heat transfer in gas purged ladle

As a first step towards prediction of temperatures in secondary steelmaking, mathematical modelling of fluid flow and heat transfer in ladle furnace was undertaken. A two‐dimensional quasi‐single phase model has been developed for turbulent recirculating flow by solving Reynolds averaged Navier‐Stokes equations along with a two‐equation k‐? model. The model was then extended to include thermal transport in a conjugate domain (i.e., molten steel + refractory shell + steel shell). The flow model was validated with water model data reported in literature by different researchers. Good agreement for velocity field and satisfactory agreement for turbulent kinetic energy field were obtained. The thermal model showed good agreement with results predicted in literature. The paper also presents findings of tests for sensitivity of flow on modelling and process parameters. By comparison with water model experiments, it has been demonstrated that the flow field in a ladle with a porous plug can be represented using a gas voidage fraction in the plume obtained from experiments with nozzles for axial gas injection from the bottom. Flow and thermal fields were insensitive to initial turbulence level at nozzle. Maximum temperature inhomogeneity in the melt was 2 °C after 1.5 min and negligible after 3 min of onset of gas purging.     

The Use of the LSHR Sampler to Determine Total Oxygen Contents in the Ladle and Tundish for Different Steel Grades

The LSHR sampler is a new development for providing a sample volume large enough to study semi‐macro and macro inclusions in clean steel. During the development of the LSHR sampler it was found that the pin where the steel enters the sampler can be used for the determination of the total oxygen content. Therefore, samples were taken at several different steel plants both in the ladle and the tundish as well as for the following five different steel grades: bearing steel, stainless steel 304L and 316L, DWI steel and ULC steel. The general conclusion regarding the sampling is that it was successful for all conditions and steel grades studied. The sample pins were examined for total oxygen content using both the inert‐gas‐fusion method and the fractional‐gas‐analysis method. Thereafter, the standard deviations with respect to the total oxygen data were used as a measure of the samples reliability. The general conclusion regarding the total oxygen determinations is that they were accurate for the five steel grades studied and the conditions under which the sampling took place.     

钢包精炼炉(LF)钢液流动的数学物理模拟与优化

应用相似原理和流体力学理论,利用电导率法和PHOENICS平台对钢包精炼炉(上、下口直径分别为2 310 mm和1 984 mm,内腔高2 620 mm,透气砖直径104 mm)钢液流动状态进行了水模拟和数学模拟,由实验和计算结果得出,离钢包底中心520 mm的2^#底吹位置比离中心354 mm的1^#位置获得的流场和均匀时间更合理,在≤300 L/min时,随吹气量增大,混匀时间减少,但>300 L/min时,继续增大吹气量,混匀时间不再变化。40 t BOF-LF-CC生产结果表明,采用优化工艺后,钢中全氧量降低2×10^-6,达到15.2×10^-6,钢中夹杂物也显著减少。     

Mathematical Model for Growth and Removal of Inclusions in Molten Steel under Gas‐stirring Conditions

A three‐dimensional mathematical model has been developed to predict growth and removal of inclusions during gas stirring through eccentric tuyeres in a ladle. In the model, the efficiency of inclusion removal is investigated under three different collision mechanisms: Brownian, turbulent and Stokes collision. The Importance of the three approaches of wall adhesion, Stokes flotation and bubble adhesion on inclusion removal is analysed and the efficiency of inclusion removal through three types of tuyeres in central, eccentric and multi‐tuyere form is studied. The results indicate that inclusion growth resulting from turbulent collision is most important and the effect of Stokes collision is remarkable with increased inclusion size, while inclusion growth resulting from Brownian collision is negligible. Removal by Stokes flotation is the main mechanism for large inclusions, while inclusion removal by wall adhesion is negligible. The smaller the bubbles are, the higher the efficiency of inclusion removal is. The type of tuyere arrangement has a great effect on inclusion removal. Inclusion removal in a 135t ladle with one eccentric tuyere is more efficient than in a ladle with central tuyere or multi‐tuyere design.     

210t RH炉的物理模拟研究

摘要：采用物理模拟的方法对210t RH炉的混匀时间、循环流量和去除夹杂物效果进行了研究,并结合实验结果制定了RH炉合理的工艺参数。结果表明,随着提升气量的增大,RH炉的钢液混匀时间缩短,特别是提升气量在100～130m3/h范围内,混匀时间减小幅度最大。当提升气体流量达到190m3/h后,混匀时间达到最小。RH炉钢液循环流量随提升气量的增加而增大,提升气量大于160m3/h后,循环流量开始变化比较平缓。夹杂物去除过程基本上是在前28min内完成,去除最迅速的阶段是前8min。     

120tLF炉钢包底吹工艺优化

针对天钢120LF精炼炉的实际情况并结合水模试验数据,对不同钢包底吹流量的钢液混匀时间、夹杂物去除效果、钢包卷渣的临界流量等进行了研究,制定了钢包底吹流量参数。最终通过引进新的钢包底吹设备使得工艺优化得以顺利完成。取得了良好的效果。     

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.     

旋流钢包长水口对夹杂物碰撞聚合影响的数值模拟

与传统钢包长水口进行对比,通过改变钢包长水口的结构以实现钢液的旋转,并研究钢液旋转对夹杂物碰撞聚合的影响.运用ANSYS软件并结合所述的模拟条件,利用PBM模型模拟了旋流钢包长水口内钢液旋转时夹杂物的碰撞聚合情况,通过正交试验得出夹杂物碰撞聚合的最佳参数组合方案.结果表明:在钢包长水口内安装螺旋型旋流引导装置使钢液发生...     

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.     

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%     

Numerical Evaluation of Two k-ε Turbulence Model for Predicting Flow and Solidification in Continuous Casting Slab

ListofSymbol　　cp———Specificheat ,J·kg-1·℃ -1;　　C1,C2 ,Cμ———Turbulentmodelconstants ;　　Dnoz———EquivalentdynamicdiameterofSEN ,m ;　　fs,f1———Solidfractionandliquidfraction ;　　f1,f2 ,fμ———Turbulentmodelterms ;　　g———Gravitationalacceleration ,m·s-2 ;　　k———Turbulencekineticenergy ,m2 ·s-2 ;　　k1———Laminarthermalconductivity ,W·m-1·℃-1;　　ks———Thermalconductivityofsolidsteel,W·m-1·℃ -1;　　p———Pressure ,Pa ;　　Prt———TurbulentPran…     

Effect of Turbulence Modeling on the Melt Flow and Inclusions Transport in a Steel Filtration Experiment

The current article deals with the effect of turbulence modeling on inclusions transport and melt flow in an induction crucible furnace (ICF), which was employed in order to investigate the efficiency and the performance of a ceramic filter. Furthermore, the influence of the discrete random-walk dispersion model on the behavior of inclusions in the ICF was investigated. Different turbulence models were employed in order to predict the turbulent melt flow. The numerical results show that the flow field is affected by the turbulence modeling method. Moreover, the distribution of the turbulent kinetic energy depends considerably on the choice of the turbulence model. In addition, the turbulence model and dispersion model also affect the inclusion transport in the melt. The filtration rate is also affected by the choice of the turbulence model.     

Influence of ferromanganese additions on microalloyed engineering steel

Abstract

Plant trials with the addition of four different low carbon and medium carbon ferromanganese alloys to microalloyed engineering steels have been carried out at Ovako Steel, Hofors, Sweden. The process route for this type of steel at Ovako is electric arc furnace (EAF), ladle arc furnace (LAF), ASEA-SKF vacuum degasser, LAF and finally uphill teeming. For each ferromanganese alloy type, five experiments were accomplished by adding alloy at the ladle furnace station, i.e. 20 charges in total. During the experiments, liquid steel rapid solidification samples were taken to study the behaviour of inclusion formation and removal during ladle treatment. Also, samples for LSHR testing (liquid sampling hot rolling) were acquired for one heat of each of the ferromanganese containing alloys. Microinclusion assessments were carried out according to the method described in Swedish standard SS 111116. Macroinclusion assessments were carried out using immersed ultrasonic testing. Element analysis and mappings were done using a scanning electron microscope to establish the effect of ferromanganese addition on the composition of inclusions present in the steel. In samples taken 3 min after the ferromanganese addition to the LAF, about 30% of the analysed inclusions contain Al₂O₃, MnO and MnS. The Al₂O₃ content is concluded to originate from the deoxidation practice, while MnO and MnS contents are likely to originate from the addition of ferromanganese. The results also show that the addition of ferromanganese alloys leads to an increased amount of macroinclusions in the LAF up to 9 min after alloy addition. Steel samples taken after degassing but just before casting indicate that all the tested ferromanganese alloys give the same result with respect to the studied microinclusion characteristics at the end of ladle treatment. Last, the inclusion composition in the final steel samples is unrelated to the type of ferromanganese alloy used. In this case, the main compositions are Al₂O₃-MgO and MnS.     

110t不锈钢钢包炉底吹氩水模拟

依据相似原理建立钢包的物理模拟体系,采用水模型对110t LF钢包底吹氩过程进行研究,分析了吹气量、吹氩位置、钢包覆盖渣和钢包液面高度对钢包混匀的影响,并进行了相应的试验验证。研究结果表明：水模型试验结果和大工业应用具有较好的一致性,验证了水模型的可行性;钢包液面高度越高,混匀时间越长;吹气量越大,混匀时间越短;相同的液面高度和吹气量下,底吹氩最佳位置为0.33r 附近;钢包覆盖渣较黏时会使钢液流动显著减慢,增大吹气量容易产生卷渣现象。