真空感应熔炼中夹杂物运动行为数值模拟

为了促进真空感应熔炼GH4169合金过程中夹杂物的去除,建立了电磁-流动-粒子跟踪耦合数学模型,研究了真空感应熔炼过程中熔液流动与夹杂物运动的规律,对比了不同冶炼工艺参数下熔池中流场和夹杂物运动特点,提出了一种促进真空感应熔炼过程中夹杂物去除的工艺优化方法。结果表明,真空感应熔炼过程中,熔池流场中存在2个漩涡,加快漩涡的运动速度可以促进熔池内夹杂物的去除,熔液流动速度随着熔炼的进行先升高后下降,最终趋于稳定。随着电压增加,熔液平均流动速度增加,熔池内夹杂物总去除率升高。随着电流频率升高,熔液平均流动速度降低,熔池内夹杂物总去除率降低。因此可通过增大电压、减小电流频率的方法,增大熔液流动速度,促进真空感应熔炼GH4169合金过程中夹杂物的去除,工艺参数由400 V、3 400 Hz优化为420 V、3 200 Hz后,夹杂物总去除率由91.98%增大到94.87%,提高了2.89%。     

Fluid flow and mass transfer in an inductively stirred four-ton melt of molten steel: A comparison of measurements and predictions

Experimental measurements are reported on melt velocities and on the rate at which immersed carbon rods dissolve in a 4-ton induction furnace, holding a low carbon steel melt. These measurements are compared with theoretical predictions, based on the numerical solution of Maxwell’s equations and the turbulent Navier-Stokes equations. In general, good agreement has been obtained, both regarding the absolute values of the velocities and the mass transfer coefficients and the trends predicted by the theoretical analysis. In addition to providing further proof regarding the applicability of the mathematical modeling technique, the principal contribution of the work is that it provides an improved insight into the behavior of inductively stirred melts. In particular it was found that for an inductively stirred melt both the velocities and the rate of turbulence energy dissipation are relatively uniform spatially, in contrast to bubble stirred systems, where most of the agitation is confined to the jet plume and to the near surface region. It was found, furthermore, that the mass transfer coefficient characterizing the rate of dissolution of immersed carbon rods depends both on the absolute values of the melt velocity and on the local values of the turbulence intensity; thus significant mass transfer will occur in the region of the eye of the circulation, where the absolute value of the mean velocity is small. On leave at Massachusetts Institute of Technology     

The clustering of alumina inclusions

Factors influencing the tendency of alumina particles in steel to agglomerate and form large clusters during deoxidation and solidification have been investigated by deoxidizing melts of Fe-10 pct Ni alloys with 0.1 pct aluminum under an inert atmosphere. Clusters of alumina inclusions are formed when the melt is stirred by either inductive or mechanical means. Nearly all of the inclusions are found in the clusters if the stirring time is long enough or at moderate stirring rates. Inclusions within the clusters increase in size with time while the size of inclusions in nonclustered regions remains essentially constant. The oxygen content of an ingot solidified soon after deoxidation corresponds well with that calculated from the oxygen content of the charged materials. After most inclusions have migrated to the clusters, the oxygen content in nonclustered regions is found to be as low as 2 ppm. Formerly Graduate Student, M.I.T..     

The mechanism of inclusion removal from molten steel by dissolved gas flotation

Dissolved gas flotation (DGF) is used for removing inclusions from molten steel. The supersaturated gas forms bubbles on inclusions and carry them to the melt’s surface. Inclusion removal is modelled visually by applying a water simulation system. A mathematical model is developed to simulate the removal rate of inclusions from steel melts. From the experimental results and mathematical calculation, the mechanism of the inclusion removal by DGF was discussed, and it was concluded that first the relationship between inclusion removal efficiency and dissolved gas content, second the bubble nucleation on suspended inclusion in supersaturated molten, and finally the multi-fine bubbles flotation to the surface.     

Physical refining of steel melts by filtration

The removal of nonmetallic inclusions from steel melts prior to casting has significant merit. Laboratory prepared steel melts containing carefully prepared alumina inclusions have been successfully filtered at 1600 °C. Two distinct types of filters were used: (i) tabular alumina packed bed (0.2 to 0.5 cm nominal diameter) and (ii) extruded monolithic alumina (400 cells per square inch). The kinetics of the filtration process have been modeled, and inclusion removal efficiency of up to 96 pct has been achieved in laboratory melts. The results show that inclusion removal efficiency is a strong function of melt velocity in the range of 0.08 to 0.68 cm per second and is weakly dependent on filter length. The type of filter utilized affects inclusion removal efficiency significantly. The inclusion capture kinetics and the filtration characteristics of the filter media tested are discussed. Formerly with Drexel University, Philadelphia, PA.     

Theoretical and experimental investigations on macrosegregation caused by mixing in the liquid bulk melt

An improved model describing the macrosegregation caused by mixing in the liquid bulk melt is presented. This model includes the mass transfer between the interdendritic liquid and the bulk melt which takes place additionally to the mass transfer via the boundary layer in front of the dendrite tips. Further, laboratory experiments on macrosegregation of iron-carbon alloys are described. The experiments were carried out in a unidirectional solidification device with 1,5 kg melts. During the solidification process the liquid melt was stirred by a ceramic blade stirrer with rotational speeds between 180 and 720 min^?1. The melts were supperheated above the liquidus temperature by amounts between 3 and 80 K. The solidification velocities were 2 to 6 mm/min. The experiments could satisfactorily be described by the model using an exchange factor α for the amount of additional mass transfer. The exchange factor increases with decreasing amounts of superheating and solidification velocity and with increasing stirring speed.     

Numerical Simulation of Nonmetallic Inclusions Behaviour in Gas‐Stirred Ladles

The secondary refining of molten steel in gas‐stirred ladle has played a more and more important role in the production of high quality steel. In the present work, a mathematical model of the fluid flow and inclusions behaviour in a 150t gas‐stirred ladle was presented, and the variations in concentration, size and density for non‐metallic inclusions in the ladle during the refining process were predicted. The results show that during the refining process, the variations in the number density of the inclusions differed depending on size. The inclusions with a diameter less than 25 μm decrease during the whole period, while inclusions with diameter larger than 25 μm increase in the first stage of the treatment and gradually decrease during the later stage. After 15 minutes, all inclusions show a tendency to decrease, but the removal rate for inclusions of smaller size becomes slower. After treatment in the ladle, inclusions with a diameter larger than 50 μm were removed, the number of inclusions with a size between 30‐40 μm was not high, while inclusions that were smaller than 25 μm still remained in the molten steel. Two‐jet bubbling demonstrated an advantage over one‐jet for inclusion removal. The practice of bubbling argon with a higher gas flowrate initially, followed by a lower flowrate in the ladle was found to be beneficial for inclusions removal.     

带过滤器中间包钢水内夹杂物行为的非稳态模拟

通过建立多相流数学模型对带有过滤器中间包钢水内夹杂物的运动轨迹及浇注过程中夹杂物去除率进行模拟研究.在某钢厂提取连铸过程中中间包入口及出口处钢样,对其进行大样电解得到夹杂物在钢水中所占的比例,验证了数学模型正确性,并分析浇注过程对夹杂物去除的影响.结果表明:穿过过滤器的夹杂物速率变小,停留时间增加,去除几率增加,过滤器可以提高夹杂物的去除率;对于粒径在100μm以上的夹杂物,其去除率在稳定后不再发生剧烈波动,但对于粒径小于80μm的夹杂物,其去除率在稳定后依然存在较大波动,浇注中后期20μm以下的夹杂物在出口处甚至出现了增加.夹杂物去除是一个非稳态过程.     

The effect of melt refining upon inclusions in aluminum

A series of aluminum melts has been refined with respect to inclusions by use of ALCOA 469, FILD, or SNIF. The content and size distribution of inclusions in the original-and the refined melts-have been measured by use of neutron activation (oxygen content), gas chromatography (carbide content), sedimentation analysis, and dissolution of metal in hydrochloric acid and subsequent analysis of oxides by means of a Coulter Counter. All the units tested have a beneficial effect and decrease the inclusion content, but the number of analyses are too few to make general conclusions. However, for melts cleaned by use of SNIF, it was found that oxides larger than 50 μm in cross section and borides larger than 20 μm in diameter were removed, while the smaller borides were agglomerated only. The effect of FILD and ALCOA 469 upon the melt tested was removal of borides larger than 5-10 μ m and oxides larger than 15μm in diameter, respectively. Formerly with Central Institute for Industrial Research, Oslo, Norway     

Study of electromagnetic separation of nometallic inclusions from aluminum melt

Several methods using the concept of electromagnetic separation of nonmetallic inclusions from an aluminum melt were analytically studied and compared in this article. The migration rate of inclusion particles for each method by exerting different types of electromagnetic field, viz. DC electric field with steady magnetic field, AC magnetic field, and AC electric field, was calculated and compared with the gravity settling velocity. Effects of various operating parameters, such as the imposing time of electromagnetic field, the radius of circular pipe, and the magnitude of electromagnetic force, on inclusion removal efficiency were analyzed and illustrated for each case. It was found that the removal efficiency increases with the increase of imposing time and decreases dramatically with the increase of pipe radius, especially for the AC electric field method. As far as the removal of small inclusions is concerned, the AC magnetic field method is the most favorable one because it has the least dependence of removal efficiency upon the particle size. However, high frequency is needed in order to get the optimum results, and the best values of a/δ are in the range of 2 to 3.     

铝及其合金熔体净化技术的实践

本文分析了铝熔体中氢与Al2O3夹杂物的来源及相互作用．提出了铝熔体净化必须“除气除渣同时进行，除渣是除气的前提”的原则．介绍了气体 精炼剂-气体-过滤综台净化技术．及提高铝熔体净化效果的新技术途径。     

钢包底吹氩中非金属夹杂物分布的水力学模拟

 为了研究夹杂物在不同工艺参数条件下以及其本身性质对夹杂物去除的影响,确定最优的底吹参数和夹杂物分布规律,通过水力学模拟试验,利用高速摄像仪和专业的图像处理软件（Image Pro Plus）研究了某钢厂60 t精炼炉吹气量、时间、粒径大小对夹杂物的去除影响以及在不同高度上夹杂物的空间分布规律。结果表明,钢包底吹过程中,吹气时间对夹杂物的去除影响规律相同,14 min时夹杂物基本去除,流量为0.08 m3/h,去除率最高;200～355 μm的夹杂物比105～150 μm的微型夹杂物更容易被去除;夹杂物在气液两相区的数量分布比其他区域少,且特征尺寸较大,在距离透气砖较远的底部存在一个弱流区,此区域夹杂物的密度较大,不易去除。     

Ejection of steel and slag droplets from gas stirred steel melts

Abstract

Liquid ejections from gas stirred melts can be classified into small film and jet droplets caused by bubble bursting and larger splashes resulting from gas channels formed at higher exiting gas velocities. In view of the conditions in ladle metallurgy, experimental investigations were carried out at moderate to low gas flowrates in an arc heated, bottom stirred 150 kg steel melting furnace and an 80 L water tank. Droplets were collected at different heights above the melt level, while gas flowrates, viscosities, surface tensions, and slag layer thickness were varied. The number of steel droplets collected decreased greatly with height (in the range 30-110 mm) and with size (in the range 0·1-1·8 mm). Calculations showed that the entrainment of droplets is strongly influenced by the velocity of upward flowing gases. While at low flowrates typical for secondary metallurgy (0·1 m s ^-1), only droplets <50 μm will be entrained, BOF (basic oxygen furnace) typical flowrates (20-50 m s ^-1) will cause particles up to 500 μm to be carried into the dust removal systems. Higher surface tensions resulted in increased droplet ejection, while higher viscosities led to a decreasing quantity of ejected melt. Slag layers led to a decrease in the ejection of steel droplets and to an increase in ejected slag but they did not completely stop steel ejection, because gas bubbles appear to entrain steel drops when they rise through the slag layer. Bubble bursting in a pure slag system caused large but few slag droplets due to the high viscosity of the slag as compared to the steel melt.     

Temperature dependences of the ultrasound velocity in liquid bismuth and lead and their alloys

The temperature dependences of ultrasound velocity a in Pb-Bi melts are measured by a pulsedphase method. The measurements were performed in the temperature range between the liquidus temperature and 1400 K, as a rule, in cooling of a sample. The experimentally measured temperature dependences of the ultrasound velocity are used to calculate the polytherms of the adiabatic compressibilities of the alloys and to plot ultrasound velocity and adiabatic compressibility isotherms. The nonuniformity of the ultrasound velocity in the melts along the vertical coordinate has received much attention. This heterogeneity is explained in terms of a metastable melt heterogeneity.     

Refining of Aluminum and Steel Melts by the Use of Multi-Cellular Extruded Ceramic Filters

Abstract

Laboratory prepared melts of steel containing Al₂O₃ inclusions and aluminum containing TiB₂ inclusions have been successfully filtered using multicellular extruded ceramic filters. Relatively high inclusion removal efficiencies have been achieved in both low temperature and high temperature melt systems —68 % inclusion removal efficiency in the Al-TiB₂ system (1020 K) using cordierite multicellular filters and 96 % inclusion removal efficiency in the steel–Al₂O₃ system (1873 K). The results have been analysed using Apelian and Mutharasan's kinetic model for filtration of the metallic melts [1]. The inclusion capture kinetics and filtration characteristics of the porous media used in this investigation are discussed.     

Redistribution of second phase particles in solidifying alloy melts—entrapment and microsegregation within dendritic networks

Particles disperesed in a melt get redistributed during solidifacation due to buoyant motion and interactactions between the particles and the solidification front. In alloy melts the distortion of the concentration field by the particle leads to entrapment of the particle within the dendritic network. The particles are forced to be confined to the interdenritic eutectic thereby causing a “microsegregation” of the particles in addition to the “macrosegregation” brought about by buoyant motion. In this paper a computer model which simulates the movement and segregation of particles in solidifying alloy melts, is described. The effect of particle size, particle volume fraction and heat extraction rate on the segregation is demonstrated for SiC particles in solidifying Al−4.5% Cu melts. The model can be used for predicting the particle distribution in cast metal matrix composites.     

Influence of Melt Feeding Scheme and Casting Parameters During Direct-Chill Casting on Microstructure of an AA7050 Billet

Direct-chill (DC) casting billets of an AA7050 alloy produced with different melt feeding schemes and casting speeds were examined in order to reveal the effect of these factors on the evolution of microstructure. Experimental results show that grain size is strongly influenced by the casting speed. In addition, the distribution of grain sizes across the billet diameter is mostly determined by melt feeding scheme. Grains tend to coarsen towards the center of a billet cast with the semi-horizontal melt feeding, while upon vertical melt feeding the minimum grain size was observed in the center of the billet. Computer simulations were preformed to reveal sump profiles and flow patterns during casting under different melt feeding schemes and casting speeds. The results show that solidification front and velocity distribution of the melt in the liquid and slurry zones are very different under different melt feeding scheme. The final grain structure and the grain size distribution in a DC casting billet is a result of a combination of fragmentation effects in the slurry zone and the cooling rate in the solidification range.     

Redistribution of particles during casting of composite melts: Effects of buoyancy and particle pushing

When a melt containing a dispersion of second phase particles is solidified, the initial distribution of the particles can change due to three phenomena, namely, buoyant motion of the particles, pushing of the particles by the moving solidification front, and by convection currents in the melt. This paper presents a computer simulation model using which, the net redistribution due to the combined effect of the first two phenomena can be predicted. the existing theory for calculating the critical velocity for particle pushing is extended to include the effect of the buoyancy force and a numerical correlation is developed for easy calculation of the critical velocity. This correlation is incorporated into a computer programme which tracks the position, velocity and direction of the solidification front as well as the position of each particle in the melt as a function of time. The final positions of the particles describe the distribution of the particles in the solidified material. Predicted distributions for various heat extraction rates and particle sizes are presented for a system of silicon carbide particles in a pure aluminium melt solidifying unidirectionally as well as multidirectionally in cylindrical moulds. It is shown that for any heat extraction rate there is an optimum particle size which gives the maximum uniformity of distribution in the solidified material.     

Inclusion Growth and Removal in Gas‐Stirred Ladles

A static modelling approach was used to study the growth and removal of inclusions during gas stirring in a ladle. A mathematical model of a gas‐stirred ladle was used to predict the data necessary to calculate growth and removal of inclusions. Results indicated that inclusion growth resulting from laminar shear collisions is negligible in comparison with growth from turbulent and Stokes collisions. Furthermore, the need for a model describing inclusion flotation by spherical‐cap bubbles was identified. Since the existing models presented in the literature are only valid for spherical bubbles, a model for the removal of inclusions by spherical‐cap bubbles was developed. Inclusion removal to the slag, refractory and by bubble flotation was compared. The mechanism determined to be responsible for the removal of the majority of inclusions larger than 25 μm was Stokes flotation and for the majority of the smaller inclusions, bubble flotation by spherical‐cap bubbles (assuming plane contact between the inclusion and the bubble).