Modeling the Effect of Combined Electromagnetic Stirring Modes on Macrosegregation in Continuous Casting Blooms

Metallurgical and Materials Transactions B - Macrosegregation is one of the most frequently observed defects in continuous casting blooms, which causes nonconformity in ultrasonic flaw detection of...     

Multiphase Solidification Modeling of Solidification Structure Evolution and Macrosegregation of Round Bloom Continuous Casting Process with Mold Electromagnetic Stirring and Final Electromagnetic Stirring

Based on Eulerian–Eulerian method, a 3D/2D multiphase solidification model, which takes into consideration the heat transfer and fluid flow with grains nucleation and crystal growth, is developed to predict the macrostructure evolution and macrosegregation in continuously cast round bloom. The results show that the mold electromagnetic stirring (M-EMS) can accelerate superheat dissipation and promote grain nucleation, but the horizontal swirl induced by M-EMS has a strong washing effect on the solidification front and leads to subsurface negative segregation. When the M-EMS current intensity increases from 200 to 300 A, the subsurface negative segregation ratio decreases from 0.935 to 0.875. The final electromagnetic stirring (F-EMS) not only improves the center segregation, but also leads to the formation of negative segregation zone near the round bloom center due to the enhancement of solute washing induced by F-EMS. As the F-EMS current intensity increases from 150 to 300 A, the center segregation ratio decreases from 1.148 to 1.075, and the negative segregation ratio near the strand center decreases from 0.985 to 0.977. Another phenomenon is found that the nucleation of equiaxed grain ahead of columnar tips restrains the solute diffusion and leads to a local macrosegregation in columnar-to-equiaxed transition zone.     

Influence of Electromagnetic Stirring on the Macrosegregation in Arc Remelted Ti and Zr Alloy Ingots

A numerical model of the VAR process has been developed in close collaboration with titanium and zirconium producers.The model is based on the solution of the coupled heat,mass and momentum transport equations in the whole ingot(liquid pool,mushy zone and solidified part)using a finite volume method.It accounts for electromagnetic stirring,buoyancy flows and turbulence effects.Solidification mechanisms implemented in the model include a full coupling between energy and solute transport in the mushy zone.The numerical model has been applied to simulate the remelting of Ti6-4.The influence of the applied magnetic field on the fluid flow and segregation behaviour,for a number of different stirring practices,is presented and discussed.Also,the macrosegregation in Zy4 ingots is investigated.The comparison between the predicted segregation and the experimental results shows the importance of accounting for both the stirring and thermosolutal convection to forecast properly the segregation in remelted ingots.     

Numerical Simulation of Macrosegregation During the Solidification of Alloys under Magnetic Fields

In this paper,a mathematical model considering the electric potential is established for describing the transfer behavior of multicomponent alloy solidification process under steady magnetic fields.By using finite volume method,the corresponding simulation program is self-developed.A validation numerical example is calculated to verify the correctness of the model and solution method,and the model also is applied to analyze steel alloys solidification transportation mechanism and the influence of magnetic parameter.The numerical results indicate that the suitable magnetic field intensity is beneficial to reduce the type and degree of macrosegregation.     

Control of Macrosegregation Behavior by Applying Final Electromagnetic Stirring for Continuously Cast High Carbon Steel Billet

Solidification behavior of continuously cast high carbon steel billets was investigated with an objective of producing high quality billets by determining the optimum final electromagnetic stirring (F-EMS) parameters. Characteristics of centerline segregation were analyzed for lots of billet samples collected from the plant through obtaining the carbon concentrations of drill chips, which were correlated with the operating parameters of the caster and stirrers, but a problem occurred that segregation control results of trial billets with the same casting and stirring parameters often have drastic fluctuations. An attempt was made to find out the induced reasons of this problem by measuring the electromagnetic torque, analyzing the secondary dendrite arm spacing (SDAS) and the corresponding cooling rate of the typical specimens, and observing the longitudinal profile of etched billet samples. Then a simple dynamic secondary cooling model was developed based on the solidified shell thickness control mode, by which the maximum carbon segregation index was reduced effectively, and thus the segregation fluctuation problem was basically solved. Finally, the most favourable stirring parameters were determined as the casting speed of 1.65 m/min, the liquid core thickness of 40 mm, stirring current of 360 A and frequency of 12 Hz.     

方坯连铸结晶器电磁搅拌的数值模拟

根据麦克斯韦电磁理论,利用有限元分析软件ANSYS对240 mm×280 mm方坯结晶器电磁搅拌的磁场进行数值模拟。分析电流强度、电流频率及结晶器铜管等对搅拌器内磁场分布的影响,得到了磁场特性与电磁搅拌参数的关系。研究结果表明:旋转磁场在结晶器搅拌区域内产生旋转电磁力,使钢液在水平方向形成旋转流动;磁感应强度随电流强度的增大呈线性递增;在低频搅拌条件下频率对电磁搅拌强度的影响较小,而铜管厚度对磁场强度影响较大;模拟结果与实测结果基本吻合。     

Mathematical Modeling of the Solidification Structure Evolution in the Presence of Ultrasonic Stirring

Ultrasonic treatment (UST) was studied in this work to improve the quality of the cast ingots as well as to control the solidification structure evolution. Ultrasonically induced cavitation consists of the formation of small cavities (bubbles) in the molten metal followed by their growth, pulsation, and collapse. These cavities are created by the tensile stresses that are produced by acoustic waves in the rarefaction phase. The pressure for nucleation of the bubbles (e.g., cavitation threshold pressure) may decrease with increasing the amount of dissolved gases and especially with the amount of inclusions in the melt. Modeling and simulation of casting solidification of alloys with UST requires complex multiscale computations, from computational fluid dynamics (CFD) macroscopic modeling through mesoscopic to microscopic modeling, as well as strategies to link various length-scales emerged in modeling of microstructural evolution. The developed UST modeling approach is based on the numerical solution of the Lilley model (that is founded on Lighthills’s acoustic analogy), fluid flow, heat transfer equations, and mesoscopic modeling of the grain structure. The CFD analysis tool is capable of modeling acoustic streaming and ultrasonic cavitation. It is used in this work to study ingot solidification under the presence of ultrasound. The UST model was applied to low-temperature alloys including Al- and Mg-based alloys. Although the predicted ultrasonic cavitation region is relatively small, the acoustic streaming is strong and, thus, the created/survived bubbles/nuclei are transported into the bulk liquid quickly. The predicted grain size under UST condition is at least one order of magnitude lower than that without UST.     

电磁搅拌下连铸圆坯凝固传热及应力的数值模拟

 针对电磁搅拌作用下碳钢在连铸结晶器内的凝固过程,考虑铸坯和结晶器内的接触状态,建立了完全热力耦合的三维稳态有限元模型。结果显示,电磁搅拌加快了钢液热量的释放,使气隙生长延迟,与无外加磁场的情况相比,坯壳内的等效米塞斯应力加大,凝固前沿附近超过了材料能承受的抗拉强度,且处于脆性温度范围区间,可能诱发裂纹形成。     

Effect of Electromagnetic Stirring on the Distribution of Nb Phase in Cu-5wt%Nb Alloys

The effect of electromagnetic stirring(EMS)on the distribution of Nb particles in solidified microstructure of a Cu-5wt%Nb alloy were studied.In this paper,we have Cu-5wt%Nb alloy remelted and solidified with or without electromagnetic stirring,to study the effect of EMS on the distribution of Nb phase and the hardness of ingots.The300A/16Hz EMS has refined the mircostrucrure of Cu-5%Nb alloy,with smaller particles sizes and larger amounts,and is benefit to the homogenous distribution of particles.The fitted lognormal distribution of particles diameter that in case without or with EMS are compared,and the EMS case has the highest frequency percentage in the range of smaller size.EMS considerably increased the hardness of Cu-5%Nb alloy ingot.     

DC-EAF熔池电磁搅拌与传热的数值计算

本研究应用Spalding数值计算方法,移植顶吹转炉成功的数模与计算软件,加入电磁场洛仑兹力,在理论分析的基础上研制了30t底电极直流电弧炉熔池电磁流场温度场数学模型,开发了计算软件,计算了流场、温度场。     

方坯连铸二冷区电磁旋转搅拌数值模拟

运用电磁流体力学基本理论及磁场边界更新法,给出了方坯连铸二冷区电磁旋转搅拌的数学模型,并利用CFX软件进行了钢液流场和磁场的数值模拟,结果表明,外加水平波磁场可以在搅拌区域内产生电磁力,使钢液在水平方向形成旋转流动,而在垂直方向形成不均匀流动,电源频率和电流强度对旋转搅拌强度有影响。     

Numerical Simulation of Electromagnetic Stirring in Secondary Cooling Zone of CSP Casting

A there-dimensional unsteady coupled mathematical model has been used to analyze the turbulent flow,temperature fields,magnetic field and macroscopic solidification of molten steel in compact strip production(CSP)thin slab continuous casting with electromagnetic stirring(EMS)on secondary cooling zone,in which induced current caused both by flow movement and external magnetic field and end effect of stirrer has been considered.Lorenz force and Joule heat contributed by induction current also have been considered.EMS in secondary cooling zone results in the obverse flow changes in molten steel flow pattern,and leads to horizontal recirculation flow within stirrer zone.Degree of superheat is reduced or eliminated rapidly when molten steel is stirred by EMS.The process of EMS makes the temperature in molten steel homogeneous.After a relatively shot period of time when using EMS,the flow pattern and temperature field will reach to a newquasi-static-state.Molten steel could be uniformly stirred by a one-sided stirrer concerned.     

方坯连铸结晶器电磁搅拌磁场的数值模拟

对方坯连铸结晶器电磁搅拌磁场进行了数值模拟,分析了不同电流强度、电流频率、结晶器铜管以及结晶器内水套对电磁搅拌器内磁场分布的影响,得到了磁场特性与电磁搅拌参数的关系,模拟结果与实测结果一致;并得出采用合适的电磁搅拌参数可以在结晶器内获得合理有效地电磁搅拌强度,从而可以更有效地改善铸坯的质量.     

板坯连铸的电磁搅拌

回顾了电磁搅拌应用于扁平产品连铸生产中的各种形式,并按液态钢水流动形态和冶金效果给予分类。多模式电磁搅拌属于第三代结晶器搅拌技术,它将电磁搅拌的3种功能通过不同操作模式集成在一起,显著改善了产品质量和连铸机的性能。     

Studies on Transport Phenomena during Continuous Casting of an Al-Alloy in Presence of Electromagnetic Stirring

In the present work, a numerical study is performed to predict the transport phenomena during continuous casting of an aluminum alloy (A356) in presence of weak stirring. A set of volume averaged single phase conservation equations (mass, momentum, energy and species) is used to represent the casting process. The electromagnetic forces are incorporated in the momentum equations. The governing equations are solved based on the pressure-based finite volume method according to the SIMPLER algorithm using TDMA solver along with an enthalpy update scheme. The simulation predicts the temperature, solid fraction and species in the computational domain. A parametric study is also performed.     

Numerical Research on Magnetic Field,Temperature Field and Flow Field During Melting and Directionally Solidifying TiAl Alloys by Electromagnetic Cold Crucible

Metallurgical and Materials Transactions B - The electromagnetic cold crucible (EMCC) technique is an effective method to melt and directionally solidify reactive and high-temperature materials...     

Numerical Simulation of Linear Electromagnetic Stirring in Secondary Cooling Region of Slab Caster

SymbolList　　B———Magneticfluxdensity ,T ;　　B ———ConjugatedcomplexnumberofB ,T ;　　Bc———Magneticfluxdensityinboundaryproducedbycoilcurrent,T ;　　Bi———Magneticfluxdensityinboundaryproducedbyin nerinductioncurrent ,T ;　　F———Electromagneticforce ,N·m- 3;　　g———Gravitationalacceleration ,m·s- 2 ;　　I———Coilcurrent ,A ;　　j———Symbolusedcommonlyinelectricalnotation(=- 1) ;　　J———Inductioncurrentdensity ,A·m- 2 ;　　k———Turbulencekineticenergy ,m2·…     

Numerical Simulation of Magnetic Field Distribution in Aluminum Melting Furnace With Electromagnetic Stirring

In metallurgical processing,effective and reliable electromagnetic stirring of the melt is one of the prerequisites for higher productivity and improved process performance.Reasonable structure and electrical parameters of the stirrer are greatly significant on improving and enhancing the stirring quality.In this paper,ANSYS software is used to research the effect of stirrer parameters on magnetic field distribution in aluminum melting furnace.The results show that magnetic flux density distributes as two humps at the direction(X)of metal length.Magnetic flux density distributes as a hump at the direction(Z)of metal width,reaches its maximum at the pool center and decays gradually toward the edge.It is also demonstrated that magnetic flux density increases by 2.65mT,as kilo-ampere-turns increase by 2.The center distance between two coils changes the distribution of magnetic flux density rather than its magnitude,while the distance from coils to the bottom of molten aluminum changes the magnitude of magnetic flux density but no change of distribution.     

Experimental and Numerical Modeling of Segregation in Metallic Alloys

Electromagnetic levitation (EML) has been used as an experimental technique for investigating the effect of the nucleation and cooling rate on segregation and structure formation in metallic alloys. The technique has been applied to aluminum-copper alloys. For all samples, the primary phase nucleation has been triggered by the contact of the levitated droplet with an alumina plate at a given undercooling. Based on the recorded temperature curves, the heat extraction rate and the nucleation undercooling for the primary dendritic and the secondary eutectic structures have been determined. Metallurgical characterizations have consisted of composition measurements using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometry and the analysis of SEM images. The distribution maps drawn for the composition, the volume fraction of the eutectic structure, and the dendrite arm spacing (DAS) reveal strong correlations. Analysis of the measurements with the help of a cellular-automaton (CA)–finite-element (FE) model is also proposed. The model involves a new coupling scheme between the CA and FE methods and a segregation model accounting for diffusion in the solid and liquid phases. Extensive validation of the model has been carried out on a typical equiaxed grain configuration, i.e., considering the free growth of a mushy zone in an undercooled melt. It demonstrates its capability of dealing with mass exchange inside and outside the envelope of a growing primary dendritic structure. The model has been applied to predict the temperature curve, the segregation, and the eutectic volume fraction obtained upon single-grain nucleation and growth from the south pole of a spherical domain with and without triggering of the nucleation of the primary solid phase, thus simulating the solidification of a levitated droplet. Predictions permit a direct interpretation of the measurements.