板坯凝固过程夹杂物运动行为

 本文使用离散相模型,利用数值模拟的方法对结晶器中的钢液流动、传热、凝固以及夹杂物的运动进行了耦合计算。通过追踪夹杂物的运动轨迹,并在钢渣界面处对夹杂物进行采样分析,最终计算出夹杂物在结晶器中的上浮率。研究表明,夹杂物在结晶器中的上浮率与其尺寸及拉速的大小均有关系,但受夹杂物密度的影响很小。夹杂物越大、拉速越小,越有利于夹杂物上浮至自由液面。小颗粒夹杂在结晶器中并不能被有效去除。对于粒径为50μm的夹杂物,当拉速为1m/min时其上浮率仅为46%,有37%的夹杂物被凝固坯壳捕捉,主要分布在铸坯表皮下10~25mm处。夹杂物被宽面坯壳捕捉的位置多集中在宽面靠近窄面处,在水口下方被捕捉的夹杂物较少。以往的研究认为只要夹杂物上浮至钢渣界面就能够被保护渣吸收,J.Strandh等的研究表明,夹杂物能否被吸收还取决于保护渣的粘度和润湿性等因素。因此,对于粒径较小的夹杂物,必须在精炼后的软吹氩过程中适当增大钢液的静置时间,尽量减少钢液中小颗粒夹杂的数量。另外,结晶器保护渣的选用对钢液中夹杂物的去除也很重要,不仅要满足其对钢液的保温润滑作用,还要考虑其对夹杂物吸附的影响。     

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.     

210 t RH炉的物理模拟研究

采用物理模拟的方法对210 t 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100～130 m³/h范围内,混匀时间减小幅度最大。当提升气体流量达到190 m³/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160 m³/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28 min内完成,去除最迅速的阶段是前8 min。     

Investigation of inclusion re-entrainment from the steel-slag interface

The mechanism of inclusion elimination from continuous steel casting is investigated at the steelslag interface. Inclusion impact at the interface is considered under the concept of energy balance, with buoyancy forces, fluid dynamic forces, interfacial adhesion, and rebound forces determining whether the particle will pass through the interface or be retained by it. The effects of the inclusion, slag, and steel properties, as well as the effect of inclusion impact velocity, are considered at the interface. The interfacial tension between the slag and the inclusion should be smaller than that between the steel and the inclusion (negative wettability), so that the inclusions can pass into the slag layer and avoid re-entrainment. The inclusion particles that reach an equilibrium state at the steel-slag interface are subject to re-entrainment back into the steel, due to lift forces applied to them by the turbulent boundary layer at the interface. A removal criterion dependent upon the shear stress is introduced, and then the removal rates are calculated from the turbulent burst theory. It is found that the smaller diameter inclusions are trapped at the interface. Of the particles that remain at the interface, it is the larger ones that are more easily removed by the lift forces due to the turbulent shear stress. High slag viscosity is desirable, since it makes inclusion re-entrainment into the casting product more difficult.     

Behaviour of inclusions in RH vacuum degasser

Abstract

Vacuum degassing processes are now an integral part of secondary steelmaking operations and besides the more obvious benefits of steel chemistry control to precise specifications, they also reduce the number of non-metallic inclusions. Care has to be taken, however, that the change in steel composition through loss of C, O, Mn, etc. does not result in a deleterious change in the composition of the remaining inclusions. Attempts in the past to determine the effect of vacuum on inclusion compositions have been through the use of thermodynamic models, following the inclusion engineering approach. The calculated inclusion compositions do not, however, compare well with the inclusions as analysed in samples taken from the liquid steel after the degassing operation. Clearly, it is important to take into account time dependent effects during degassing and this has been achieved by the development of a combined fluid flow-thermodynamic model. Using the computational fluid dynamics model, CFX, to establish the temperature, flow and species contours in a two-dimensional steel ladle under the influence of natural convection, the results are transferred as start conditions in a three-dimensional RH degasser model. A body force is then applied to simulate the argon bubbles that are injected into the up-leg of the degasser and changes in flow, temperature and species concentrations are calculated. Allowing for additions made during the process, the composition of the top slag and any local inclusions within the steel is predicted. The influence on top slag and inclusion chemistry of any glaze on the snorkels of the degasser is also taken into account.     

RH去除Al2O3夹杂物的数值模拟及工艺分析

 RH已经成为炉外精炼的重要组成部分,以180 t RH设备参数为基础建立三维模型,通过工艺试验和数值模拟的方法分析夹杂物分布及去除情况。试验结果表明,RH净循环时间为300 s夹杂物去除效率最高,但是夹杂物去除净循环时间不超过850 s。数值模拟比较夹杂物直径60、40、20 μm去除行为影响因素情况,通过数值模拟与金相试验的方法相比较,两者质量分数相差15.8%,因此说明数值模拟的方法可以应用到RH炉外精炼工艺过程中指导生产实践的预测。     

吹氩精炼钢包内非金属夹杂物去除机理

 通过物理模拟试验,研究分析了底吹氩精炼钢包内夹杂物去除机理以及吹氩量对其的影响规律。结果表明：钢包中夹杂物的上浮主要是通过上升的钢液流携带,底吹氩量对夹杂物在钢包表面的钢-渣界面去除行为存在重要影响。吹氩量较小时,钢-渣界面稳定,夹杂物在浮力、毛细作用力等共同作用下穿过平坦的钢-渣界面而被吸收;吹氩量较大时,钢-渣界面波动大,渣眼周围发生卷渣,夹杂物被卷入的液滴吸收,随液滴进入渣层;吹氩量大,渣眼周围形成渣泡,夹杂物被渣泡吸收,随渣泡进入渣层。吹氩量达到一定时,夹杂物被钢-渣界面的吸收成为其被去除的限制性环节,且吹氩量较大时夹杂物去除效果最差,为实际吹氩精炼过程吹气量的控制提供了指导。     

Mathematical Modelling of Molten Steel Flow Process in a Whole RH Degasser during the Vacuum Circulation Refining Process: Application of the Model and Results

A three‐dimensional mathematical model for the molten steel flow during the RH refining process has been applied to the circulatory flow processes in both a practical RH degasser and its water model unit. The model was presented earlier [1] and one of its characteristics is that ladle, snorkels and vacuum vessel are regarded as a whole. Using this model, the fluid flow field and the gas holdups of liquid phases and others have been computed respectively for a 90 t RH degasser and its water model unit with a 1/5 linear scale. The results show that the mathematical model can properly describe the flow pattern of molten steel during the refining process in an RH degasser. Except in the area close to the liquid's free surface and in the zone between the two snorkels in the ladle, a strong mixing of the molten steel occurs, especially in the vacuum vessel. However, there is a boundary layer between the descending liquid stream from the down‐snorkel and its surrounding liquid, which is a typical liquid‐liquid two‐phase flow, and the molten steel in the ladle is not in a perfect mixing state. The lifting gas blown is ascending mostly near the up‐snorkel wall, which is more obvious under the conditions of a practical RH degasser, and the flow pattern of the bubbles and molten steel in the up‐snorkel is closer to an annular flow. The calculated circulation rates for the water model unit at different lifting gas rates are in good agreement with experimentally determined values.     

RH装置内夹杂物聚合与去除的三维数值模拟

 摘 要：基于RH精炼装置内流场,建立了一个研究夹杂物聚合与去除行为的三维数学模型。数值结果表明,夹杂物在钢包内上升管左侧存在一个侧“V”分布,上升管下方及下降管右侧均存在大的环状分布;夹杂物浓度和数量密度具有相似的空间分布;钢包内夹杂物的平均数量密度及浓度大于真空室内的平均值;夹杂物在气液两相区浓度较低;整个过程前400秒夹杂物去除最快,900秒后可去除大部分夹杂物,在处理后期,夹杂物平均特征尺寸有减小的趋势。     

RH精炼钢中全氧量变化与操作改进

为了使钢中全氧量控制在一个适当的水平,在武钢炼钢总厂RH真空脱气装置对低碳、超低碳钢进行了脱氧净化试验.结果表明,影响全氧去除的因素按作用高低依次是出钢溶解氧水平,溶解氧与钢包渣的交互作用,钢包渣,和真空处理净化时间.通过改进工艺生产了全氧量 ≤ 10×10^-6,非金属夹杂物尺寸<10μm的清洁钢水,建立了一个钢包内钢水全氧浓度随时间变化的新方程,该方程考虑了全氧的表观平衡含量及环流、扩散传质对去除氧化物夹杂速率的影响.     

六流T型中间包夹杂物去除行为的数值模拟

以41 t圆坯连铸中间包为研究对象,运用ANSYS软件,结合流动模型、拉格朗日离散相模型模拟钢液流动和夹杂物行为。通过用户自定义函数为中间包钢渣界面的边界条件设置了一种新的判定标准,即根据夹杂物的受力情况判定夹杂物上浮去除或进入钢液。通过数值模拟计算中间包出口处夹杂物数密度,并与工业试验结果对比,发现新的判定标准较传统的捕捉(trap)边界条件具有更高的计算精度。基于上述模型计算了稳态浇铸过程中间包内不同粒径氧化铝夹杂物的上浮时间、上浮位置及去除率。上述结果为更好地研究中间包内夹杂物行为提供了支撑。     

Mathematical Modelling of Non‐equilibrium Decarburization Process during Vacuum Circulation Refining of Molten Steel: Application of the Model and Results (I) ‐ Decarburization Process and Influences of Some Technological Factors

A novel three‐dimensional mathematical model proposed and developed for the non‐equilibrium decarburization process during the vacuum circulation (RH) refining of molten steel has been applied to the refining process of molten steel in a 90‐t multifunction RH degasser. The decarburization processes of molten steel in the degasser under the conditions of RH and RH‐KTB operations have been modelled and analysed, respectively, using the model. The results demonstrate that the changes in the carbon and oxygen contents of liquid steel with the treatment time during the RH and RH‐KTB refining processes can be precisely modelled and predicted by use of the model. The distribution patterns of the carbon and oxygen concentrations in the steel are governed by the flow characteristics of molten steel in the whole degasser. When the initial carbon concentration in the steel is higher than 400 · 10⁻⁴ mass%, the top oxygen blowing (KTB) operation can supply the oxygen lacking for the decarburization process, and accelerate the carbon removal, thus reaching a specified carbon level in a shorter time. Moreover, a lower oxygen content is attained at the decarburization endpoint. The average contributions at the up‐snorkel zone, the bath bulk and the free surface with the droplets in the vacuum vessel in the refining process are about 11, 46 and 42% of the overall amount of decarburization, respectively. The decarburization roles at the gas bubble‐molten steel interface in the up‐snorkel and the droplets in the vacuum vessel should not be ignored for the RH and RH‐KTB refining processes. For the refining process in the 90‐t RH degasser, a better efficiency of decarburization can be obtained using an argon blow rate of 417 I(STP)/min, and a further increase in the argon blowing rate cannot obviously improve the effectiveness in the RH refining process of molten steel under the conditions of the present work.     

Cleanliness of GCr15 bearing steel refined by RH and VD vacuum processes

The cleanliness of GCr15 bearing steels produced by RH and VD vacuum refining processes was compared. Evolutions of total oxygen (TO), total nitrogen (TN), total sulfur (TS), and inclusions were investigated. Bearing steel has high requirements for cleanliness. RH refining has advantages in reducing TO and TN content, removing solid inclusion, and circulating efficiency. After RH refining, the TO content in molten steel decreased by 61%, the TN content decreased by 15%, and the number density of inclusions decreased by 75%. The stirring of slag and steel was strong during the VD refining, which was beneficial to the desulfurization, the TS content in liquid steel decreased by 50%. The circulation rate of the liquid steel in the VD refining was much lower than that in RH refining. The stronger stirring of slag and steel during VD refining resulted in the slag entrainment. The contact angle between inclusions with different liquid phase fractions and liquid steel is different. Inclusions with liquid phase fraction less than 27% are not wetted with liquid steel and are easy to collide, grow and float up for removal, while the adhesion work of liquid inclusions is greater and difficult to remove from liquid steel.     

Effect of initial large-sized inclusion content on inclusion removal during electroslag remelting of H13 die steel

The current paper focuses on the influence of initial large-sized inclusion content in the consumable electrode on inclusion removal during electroslag remelting (ESR) of H13 die steel. Considering the relationship between the inclusion size and the interfacial energy change of the slag/inclusion/steel system during the inclusion transfer across the steel/slag interface, the thermodynamic conditions for inclusion removal from steel to the slag were put forward. The results showed that the content of large-sized inclusions in final ESR ingot was decreased by approximately 13.66% with the increase in large-sized inclusion content in the consumable electrode from 11.36?mg/10?kg to 16.50?mg/10?kg. The interfacial energy change of the slag/inclusion/steel system decreases with the increase in inclusion radius during the absorption process of inclusions by slag, which is beneficial for inclusion removal.     

六流T型中间包夹杂物去除行为的数值模拟

以41 t圆坯连铸中间包为研究对象,运用ANSYS软件,结合流动模型、拉格朗日离散相模型模拟钢液流动和夹杂物行为。通过用户自定义函数为中间包钢渣界面的边界条件设置了一种新的判定标准,即根据夹杂物的受力情况判定夹杂物上浮去除或进入钢液。通过数值模拟计算中间包出口处夹杂物数密度,并与工业试验结果对比,发现新的判定标准较传统的捕捉(trap)边界条件具有更高的计算精度。基于上述模型计算了稳态浇铸过程中间包内不同粒径氧化铝夹杂物的上浮时间、上浮位置及去除率。上述结果为更好地研究中间包内夹杂物行为提供了支撑。     

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.     

REDA与RH精炼过程钢液流动规律比较

以300 t REDA和RH精炼装置为研究对象,借助计算流体力学软件模拟REDA与RH两种精炼工艺下钢液流动行为,从精炼过程流场形态、循环流量、氩气行程及熔池表面湍动能等方面进行分析,研究结果表明:RH对钢包底部熔池的搅拌作用强于REDA,REDA的单浸渍管结构有利于延长浸渍管寿命及提高钢液循环流量,REDA只需RH提升气体流量的30%便能达到相同的循环流量。     

GCr15轴承钢RH和VD真空精炼过程钢洁净度对比

摘要：对比了RH和VD真空精炼工艺生产的GCr15轴承钢精炼过程的洁净度水平,研究了钢液中总氧（TO）、总氮（TN）、总硫（TS）以及夹杂物变化规律。轴承钢对洁净度要求较高,RH精炼工艺在降低钢中TO、TN含量,固相夹杂物去除,循环效率上均具有优势。经过RH精炼后,钢液中TO含量下降了61%,TN含量下降了15%,夹杂物数密度降低了75%;VD精炼过程中,钢渣反应剧烈,脱硫效果优异,VD精炼后钢液中TS含量下降了50%,但钢液循环速率远落后于RH精炼,且更容易发生卷渣。不同液相分数的夹杂物与钢液的接触角不同,液相分数小于27%的夹杂物与钢液不润湿,容易碰撞长大和上浮去除,而液态夹杂物黏附功更大,难以从钢液中去除。     

Mathematical modelling of molten steel flow in a whole degasser during RH refining process

Abstract

A three-dimensional mathematical model for molten steel flow in a whole degasser during the RH (Ruhrstahl–Heraeus) refining process is proposed. The model has been developed considering the physical characteristics of the process, particularly the behaviour of gas–liquid two phase flow in the up snorkel and the momentum exchange between the two phases. The fluid flow fields and gas holdups of liquid phases, among other parameters, in a 90 t RH degasser and a water model unit of one-fifth linear scale have been computed using this mathematical model. The results show that the flow pattern of molten steel in a whole RH degasser can be well represented by the mathematical model. Apart from the area close to the free surface and the zone between the two snorkels in the ladle, the molten steel in an RH degasser, especially in the vacuum vessel, is reasonably fully mixed during the refining process. However, there is a boundary layer between the descending liquid stream from the down snorkel and the surrounding liquid, which is typical liquid–liquid two phase flow, and the molten steel in the ladle is not perfectly mixed. The blown lifting gas ascends mostly near the up snorkel wall, which is more obvious under the conditions of an actual RH degasser, and the flow pattern of bubbles and molten steel in the up snorkel is closer to annular flow. Calculated circulation rates for the water model unit at various lifting gas rates are in good agreement with values determined by means of water modelling experiments.     

轴承钢二次精炼过程夹杂物演变规律

 研究了轴承钢LF精炼和RH真空处理过程各类夹杂物的成分、种类和数量变化,并结合热力学模拟计算了夹杂物与钢液的界面参数,并对试验结果进行分析讨论。夹杂物分析结果表明,精炼25 min后,脱氧产物Al₂O₃消失,钢中夹杂物以纯尖晶石、含少量CaO的尖晶石、CaO·2Al₂O₃和CaO·Al₂O₃为主。继续精炼65 min至LF精炼结束,钢中夹杂物仍以纯尖晶石、含少量CaO的尖晶石、CaO·2Al₂O₃和CaO·Al₂O₃为主。RH真空处理25 min后,钢中夹杂物总数量较LF精炼结束降低75%,其中,纯尖晶石和含少量CaO的尖晶石去除率分别为99.5%和93.2%,CaO·2Al₂O₃去除率为67%。RH破空后钢中夹杂物以液态钙铝酸盐CaO·Al₂O₃和12CaO·7Al₂O₃为主。精炼过程尖晶石类夹杂物尺寸集中在10 μm以下,尺寸大于20 μm夹杂物主要为处于液相区的钙铝酸盐,这些钙铝酸盐在LF精炼前期就已经存在。与钢水接触角大于90°的固态夹杂物纯尖晶石、含少量CaO的尖晶石和CaO·2Al₂O₃在RH真空处理过程容易去除,与钢水接触角小于90°的液态夹杂物CaO·Al₂O₃和12CaO·7Al₂O₃不易去除。因此,将LF精炼结束的夹杂物控制为固态夹杂物有利于RH真空处理过程夹杂物的高效去除。热力学计算结果表明,当钢中w(T[O])为0.001 0%、w([Mg])大于0.000 18%时,脱氧产物Al₂O₃热力学上就不能稳定存在。铝脱氧、高碱度渣精炼条件下很难稳定地获得固态Al₂O₃夹杂物。为获得完全固态尖晶石或高熔点钙铝酸盐夹杂物,钢中w([Ca])需控制在0.000 1%以内。钢中w([Ca])大于0.000 2%,就具备生成液态夹杂物的热力学条件。