effects of electromagnetic stirring on fluid flow and ... · parameters of m-ems by combining...

Metall. Res. Technol. 115, 103 (2018)© EDP Sciences, 2017https://doi.org/10.1051/metal/2017075

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Effects of electromagnetic stirring on fluid flow and temperaturedistribution in billet continuous casting mould and solidificationstructure of 55SiCrHanghang An1,2, Yanping Bao1,*, Min Wang1, and Lihua Zhao1,2

1 State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China2 School of Metallurgical and Ecological engineering, University of Science and Technology Beijing, Beijing 100083, PR China

* e-mail: b

Received: 12 March 2017 / Accepted: 15 September 2017

Abstract. The optimal combination of current intensity and frequency of mould electromagnetic stirring(M-EMS) in continuous casting billet was a crucial compromise for improving inner quality of cast billet such asreduction in center segregation and porosity of medium-high carbon steel. In the present study, a decoupledthree-dimensional mathematic model of electromagnetic field, fluid flow and heat transfer in continue castingbillet mould with EMS has been developed, and the effects of current intensity and frequency on the system werealso discussed. In addition, the industrial trials were carried out to investigate the magnetic field characteristicsin themould withM-EMS and the influence ofM-EMS on the solidification structure of 55SiCr. According to thecalculations and analysis, the optimal combination range of current intensity and frequency was 300–320A and3–4Hz, respectively. The results showed that inner quality in as-cast billet of 55SiCr has been improvedsignificantly with optimal parameter of 320A and 3Hz. For instance, central equiaxed zone increased from 19%to 33%, the center carbon segregation ratio decreased from 1.13 to 1.05 as well as center porosity has nearlydisappeared.

Keywords: billet continuous casting / M-EMS / numerical simulation / 55SiCr / solidification structure

Mould electromagnetic stirring (M-EMS) has played anincreasingly important role in improving inner quality ofmedium-high carbon steel especially in as-cast billet [1–5].The molten steel is driven to rotate by electromagneticforce (EMF), and M-EMS could promote the dissipationof superheat. Consequently, the morphology of caststructure is improved, and the center segregation andporosity are also reduced. However, inappropriate stirringintensity may increase the deterioration of macrosegregation and center porosity in cast billets duringpractical continuous casting [6–7]. Stirring intensity is afunction of the current intensity and frequency, which aremain operation parameters of M-EMS [8]. So thecombination of the current intensity and frequencyis paramount to desired metallurgical improvement byM-EMS.

A lot of studies have been conducted and mainlyfocused on the following aspects during the past decade. Onone hand, the influence of current intensity and frequencyon the flow field, temperature distribution, solidificationbehavior and inclusion trajectory of molten steel in themould with M-EMS has been analyzed by using numerical

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simulation [9–13]. It had contributed a lot in theunderstanding of basic mechanisms of transport phenome-na under multi-field mutual coupling in the mould withM-EMS. This was supported with many industrial trialsaimed at improvement of the steel quality by optimizingM-EMS according to various steel grades and castingconditions. On the other hand, the influence of thecombination of current intensity and frequency on theequiaxed crystal zone, central shrinkage, subsurface andcentral cracks were investigated for billet and bloomcontinue casting only by a multitude of industrial trials[14–18].

Although those researches provided some usefulinformation for practical production, an explicit 3D flowfield distribution in M-MES was not provided owing to thecomplex external body force terms in the flow equationsand the boundary condition of the electromagnetic field,especially in billet continuous casting. In addition a morebeneficial method seemed to optimize the operation processparameters of M-EMS by combining numerical simulationwith fewer industrial trials in continue casting billet.

In the present study, a three-dimensional mathematicmodel of electromagnetic field, fluid flow and heat transferin continuous casting billet mould with M-EMS has been

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Fig. 1. Schematic sketch of a continue casting billet mould with electromagnetic stirring: (a) geometrical model of mould withM-EMS, (b) position of M-EMS.

Table 1. Main parameters of the electromagnetic stirrer.

Parameters Value

Outside diameter (mm) f900±3Inside diameter (mm) f540±2Total height (mm) 480±2Core materials Cold-rolled silicon steel

sheet 35W300Core height (mm) 282Core width (mm) 73Winding conductor (mm2) 7.35� 10.5Winding symmetry Three-phase winding

symmetric distributionConnection mode One star connectionCoil turns 110Coil height (mm) 388Coil thickness (mm) 148Number of pole pairs 1

Table 2. Geometrical parameters and material propertiesused in simulation.

Parameters Value

Cross-sectional dimension of billet strand(mm2)

150� 150

Casting speed (m ·min�1) 2.3Tundish superheat (k) 25Grade of steel 55SiCrComputed length of mould (mm) 780The thickness of the copper (mm) 13.6Depth of nozzle (mm) 110Molten steel density (kg ·m�3) 7100Molten steel viscosity (kg ·m�1 · S�1) 5.5� 10�3

Molten steel electric conductivity(V�1 ·m�1)

7.14� 105

Copper mould electric conductivity(V�1 ·m�1)

4.7� 107

Molten steel magnetic conductivity(H ·m�1)

1.257� 10�6

Armature core magnetic conductivity(H ·m�1)

1.257� 10�3

Molten steel thermal conductivity(W ·m�1 ·K�1)

34

Molten steel specific heat (J ·Kg�1 ·K�1) 750Liquidus temperature of molten steel (K) 1471Solidus temperature of molten steel (K) 1386

2 H. An et al.: Metall. Res. Technol. 115, 103 (2018)

developed. Consequently, the transport behavior inM-EMS during billet continuous casting was simulated,and the inner-quality of 55SiCr cast billet was improved.Additionally, the effects of current intensity and frequencyon flow field, electromagnetic field and temperaturedistribution in the mould were studied. The industrialtrials were also carried out to investigate the magnetic fieldcharacteristics and the influence of M-EMS on thesolidification structure of 55SiCr.

1 Model description

1.1 Geometrical model

Figure 1a and b showed a billet strandwithM-EMS and theschematic sketch of M-EMS for billet respectively.

Main parameters of the electromagnetic stirrer areshown in Table 1. The operating conditions and materialproperties [10,12,13] were summarized in Table 2.

Thermo-physical parameters of 55SiCr were obtainedby ProCAST software. The purpose of this work was todetermine the reasonable combination of current intensityand frequency for the implementation of M-EMS inpractical production. In consideration of the situation ofpractical production in billet casting, optimal values ofcurrent intensity and frequency were chosen as one of260A, 280A, 300A, 320A and 3Hz, 3.5 Hz, 4Hz, 5Hz,respectively.

H. An et al.: Metall. Res. Technol. 115, 103 (2018) 3

1.2 Basic assumptions

The following assumptions were employed to simplify themathematical model:
– the molten steel in the mould was assumed to beincompressible Newtonian fluid. The viscosity, thespecific heat, and the thermal conductivity were assumedto be constant over temperature;
–
the additional magnetic field produced by themolten steelflow in themouldwas ignoreddue to a very smallmagneticReynolds number in the mould with M-EMS. Thus thesimulation of electromagnetic field could be decoupledfrom the calculation of flow field and heat transfer;
–
the magnetic field generated by the inductive system ofM-EMS was considered as quasi-steady state and thedisplacement current was ignored. The electromagneticforce was obtained in a timely averaged form;
–
an insulating layer represented the mold powder layerbetween the strand and the mold, so the induced currentwas enclosed within the strand;
–
the influence of Joule heating generated by the inducedcurrent was ignored, and solidification in the mould werenot also taken into account.
1.3 Electromagnetic field model

In order to carry out a coupled simulation of theelectromagnetic field and flow field in the billet mould withM-EMS, the magnetic field distribution of M-EMS in themouldwas calculatedfirst, and then electromagnetic force asa momentum source was loaded into the momentumequation of themoltenmetalflow.ANSYSMaxwell softwarewas used in the calculation of the magnetic field and theelectromagnetic force. The electromagnetic field distribu-tions in the mould, including the magnetic flux density andthe induced current density were obtained by solvingMaxwell’s. In the coupled calculation of electromagneticfield and flow field, the time-average induced electromag-netic force is used, details can be found elsewhere [19]. In thecoupled calculation of electromagnetic field and flow field,the time-average induced electromagnetic force can besolved in equations from Wang et al. [19] research.

1.4 Fluid flow model equations

The flow field was simulated by ANSYS-Fluent software,and the molten metal flow in the mould with EMS wassolved by treating the Lorentz force discussed above as asource term to the momentum equations as follows, whichwere used to describe the flow field of molten steel in themould and expressed in equations (1) and (2).

Continuity equation:

∂ðruiÞ∂xi

¼ 0: ð1Þ

Navier–Stokes equation:

∂ðruiujÞ∂xj

¼ � ∂p∂xi

þ ∂∂xj

meff

∂ui

∂xjþ ∂uj

∂xi

� �� þ Fi; ð2Þ

where r, m, P and g were the density, fluid velocity,pressure and gravitational acceleration, respectively.

Standard k-emodel were shown in equations (3) and (4).

∂ðruikÞ∂xi

¼ ∂∂xi

meff

sk

∂k∂xi

� �þG� re; ð3Þ

∂ðruieÞ∂xi

¼ ∂∂xi

meff

sk

∂e∂xi

� �þ C1

eK

G� C2e2

K; ð4Þ

where meff, k and e were the effective viscosity, turbulentkinetic energy and its dissipation respectively; sk and se arePrandtl number corresponding to k and e and they wereconstant.

The effective viscositywas given in equations (5) and (6).

meff ¼ mt þ m; ð5Þ

mt ¼ rCmk2

e; ð6Þ

where mt was the diffusive coefficient of turbulent kineticenergy, m was viscosity, and Cm was a constant.

1.5 Energy equation

The heat transfer conservation equation was expressed as:

∂∂t

ðrHÞ þ ∂∂xj

ðrmjHÞ ¼ ∂∂xj

lþ Cpmt

st

� �∂H∂xj

� �; ð7Þ

where lwas the thermal conductivity, stwas the turbulentPrandtl number, and st=1.0 was assumed.

Calculation formula of liquid fraction (fL) wasexpressed as:

fL ¼0 T � Ts:

1� TL � T

TL � Ts; Ts < T < TL;

1; T≥TL;

:

8><>: ð8Þ

1.6 Boundary conditions and solution method

An air cylinder around the total geometry was used forcapturing most of the magnetic induction lines in thesurrounding air. No magnetic flux through the externalsurface and magnetic insulation was used as a boundarycondition.

The flow field was simulated by ANSYS-Fluentsoftware. The inlet velocity was computed according tothe mass conservation between the inlet and outlet basedon the casting speed. The values of turbulent kinetic energyand the turbulent dissipation rate at the SEN inlet werebased on the semi-empirical expression. The free surfacewas specified as a zero-shear condition. The velocity of thestrand wall was specified as casting speed. The outletguarantees the mass conservation. All walls were set to anon-slip boundary condition, and standard “wall functions”near the wall were used.

Fig. 2. Comparison of the calculated magnetic flux density withmeasured data at the centerline of mould (I=270A, f=3Hz) (i.e.with copper pipe) along axial direction.


The inlet temperature was set as the sum of the liquidustemperature and the superheat. The free surface and theSEN refractory wall were treated as adiabatic. A fixedtemperature equal to the liquidus was imposed along all themould walls.

The numerical modeling was performed by three steps.In the first step, ANSYS-Maxwell software was used in thecalculation of the 3D distribution of the electromagneticfield in the billet strand by solving Maxwell’s equationswith finite element method, which can obtain the magneticfield distribution and electromagnetic force field. Thendata files on electromagnetic field were extracted from thepost-processor of ANSYS-Maxwell software. In the secondstep, data files on electromagnetic field in the first step wereloaded on the corresponding node as an external body forceterm in the momentum equation by the MHD module ofthe ANSYS-Fluent software. In the last step, the flow fieldand temperature field in the mould were computed bysolving the turbulent Navier-Stokes equation and energyequation with the finite volume method.

The computational domain was meshed by eight-nodehexahedral elements, except for the atmospheric regionwhere four node tetrahedral elements were used. In thewhole domain, there were about 1 000 000 element grids.

Fig. 3. The distribution contour of magnetic field intensity in theentire mould.

2 Results and discussion

2.1 Experimental validation for electromagnetic field

In order to ensure the accuracy of the mathematicalmodels, the calculated results from the magnetic inductionwere compared with those measured data in a real billetmould by using the digital teslameter and the comparisonwas displayed in Figure 2. As shown in Figure 2, thecalculated results were in good agreement with themeasured data. It indicated that the developed mathemat-ic model was reasonable for M-EMS. Although somedifferences existed between the calculated and measureddata, these deviations could be ignored owing to magneticfield leakage and measured or computed error.

2.2 Distribution characteristic of magnetic inductionand electromagnetic force

Figure 3 presented the distribution contour of thecalculated magnetic field intensity with current intensityand frequency (320A/3Hz) in the entire mould. As shownin Figure 3, the magnetic induction was confined mainly inthe region surrounded by the inductive coil. Magnetic fieldintensity generated byM-EMS was unevenly distributed inthe entiremould, whose distribution along an axial directionwas big at both ends, but small in themiddle, andmaximumvalue appeared to be around the center of stirrer.

The azimuthal EMF distribution with different currentintensity and frequency were shown in Figure 4. As shownin Figure 4, the magnitude of the azimuthal EMF at theedge of the billet was much higher than that at the center ofthe mould. The current intensity as well as frequency hadan opposite relation with the azimuthal EMF. It wasobviously observed that the azimuthal EMF increased with

current intensity and decreased with frequency along theradius of the billet. The maximum electromagnetic forceappeared at the edge of the billet strand, and the maximumazimuthal EMF along the radius was 2.25 kN ·m�3 with afrequency of 3Hz and 2.25 kN ·m�3 at a current intensity of320A, respectively.

2.3 Characteristics of fluid flow and temperature

In order to analyze the effect of M-EMS on flow field andtemperature distribution, simulated calculations wereperformed both with and without M-EMS. The mainresults were presented in Figures 5–9.

As shown in Figures 5a and 6a, when MES was notapplied, the molten steel from SEN flowed into the mould,and most of the superheated molten steel directly floweddownward, and only a small quantity flowed to the freesurface of the mould to form a pair of recirculation zones.

Fig. 4. Distributions of azimuthal EMF along the radius at the center of stirrer: (a) different current intensity (f=3Hz), (b) differentfrequency (I=320A).

Fig. 5. Velocity field at meridional planes of mould: (a) withoutEMS, (b) with EMS (I=320A, f=3Hz).

Fig. 6. Three-dimensional streamline in mould: (a) withoutEMS, (b) with EMS (I=320A, f=3Hz).


The recirculating direction of the molten steel was from thecenter of the mould flowing to the mould wall, and thedistribution of the streamlines was a radial pattern as aresult of molten steel flow downward at the central regionand upward near the edge. There was no rotary motion onhorizontal plane (at the center of stirrer) in Figure 7a. Itwas also observed clearly that the distribution of thestreamlines was a radial pattern as a result of molten steelflow downward at the central region and upward near theedge. This flow pattern did not decrease the superheat andbenefit the enlargement of the equiaxed crystal zone.WhenM-EMS was applied, an obvious change was found in the

flow field of molten steel in the mould. The molten steelcoming from the SEN formed two pairs of recirculationzones in the mould. Most of the molten steel floweddownward from a pair of recirculation zones in the lowerregion of the mould as well as the upward molten steel froma pair of recirculation zones in the upper region of themould. The recirculating direction of molten steel was fromthe mould wall flowing to the center of the mould, as shownin Figures 5b and 6b. The molten steel in the mould had arotating velocity distribution by the action of the rotatingEMF, and the whole rotary action of molten steel appeared,in Figure 7b.

Fig. 7. Distribution of velocity field along the radial direction at the center of stirrer: (a) without EMS, (b) with EMS (I=320A,f=3Hz).

Fig. 8. Distributions of temperature field at meridional planes ofmould: (a) without EMS, (b) with EMS (I=320A, f=3Hz).

Fig. 9. Temperature distribution of cross-section billet at the outle


Without M-EMS, the superheat of the molten steel inthe mould disappeared only slowly, the core temperature ofthe billet was high and the high temperature zone in thebillet core extended to the bottom of themould, as shown inFigure 8a, and the temperature fell from the billet core tothe side of the solidified shell sharply. With M-EMS, it wasobviously observed that the super-heated molten steelchanged from a vertical downward movement to ahorizontal rotation, so that the axial temperature declinedquickly and the radial temperature rose. Most of thesuperheated molten steel remained in the upper region ofthe mould, as shown in Figure 8b. Figure 9 revealedtemperature distribution of cross-section billet at the outletof mould. As shown in Figure 9, in comparison, the regionwith fL=1 became much smaller, and the region withfL=0.7∼0.9 becamemuch larger afterM-EMSwas applied.The core temperature of the billet reduced dramatically,whereas the temperature at the solidification front of thesolidified shell increased. Consequently an increase intemperature gradient was beneficial for heat transfer.

Accordingly, the whole rotary action of molten steelappeared and the superheat of the molten steel in themould was reduced. It was in favor for breaking offcolumnar crystal and enlarging the area of equiaxed crystalzone. Consequently, the internal quality of the billet wasimproved.

t of mould: (a) without EMS, (b) with EMS (I=320A, f=3Hz).

Fig. 10. Flow field at meridional planes of mould with different current intensities (f=3Hz): (a) streamlinemap, (b) velocity contour.

Fig. 11. Velocity distribution along radial direction at the centerof stirrer with different current intensities (f=3Hz).


2.4 Effect of the current intensity and frequency onflow field

The important part of this study was to determine thereasonable combination of current intensity and frequencyregarding the implementation of M-EMS in practicalproduction. Therefore, it was essential to investigate theeffect of current intensity and frequency on the flow field.

Figure 10 displayed streamline map and velocitycontour at the meridional plane of the mould with thefrequency of 3Hz and current intensity of 260, 280, 300 and320A, respectively. As shown in Figure 10, the flowpatterns of the molten steel exhibited various distributionsat different current intensities. There were the upperrecirculation zones in the upper region of mould and therotational flow zone in the lower region of mould with allthe current intensity of 260, 280, 300 and 320A inFigure 10a. A pair of rounded recirculation zones did notarise in the lower region of the mould with the currentintensity of 260A. As the current intensity increased to300A and 320A, it was obviously observed that a pair ofrounded recirculation zones in the lower region of themould in Figure 10b. Moreover, the impacting depth of jetflow decreased, upper circulating region shortened, lowerrotational flow strengthened, flow zone enlarged andmoved up as the increase of the current intensity inFigure 10. It was beneficial to decrease the superheat andenlarge the equiaxed crystal zone.

Velocity distribution along the radial directions at thecenter of stirrer with the frequency of 3Hz and differentcurrent intensities were shown in Figure 11. As shown inFigure 11, velocity increased first and then decreased fromthe edge to the center of the billet, finally dropped to nearlyzero at the center of the billet. The changing tendency ofazimuthal velocity along the radial directions was consis-tent with that of azimuthal EMF from Figure 4. Besides itwas observed that the maximum azimuthal velocity alongthe radius of the mould increased from 0.053m · s�1 with acurrent intensity of 260A to 0.076m · s�1 with a currentintensity of 320A, and it appeared at the location

corresponding to the EMS core. The maximum azimuthalvelocity along the radial direction appeared at the edge ofthe billet.

Figure 12 demonstrated streamline map and velocitycontour at meridional planes of the mould with the currentintensity of 300A and the frequency of 3, 3.5, 4 and 5Hzrespectively. It was observed from Figure 12 that therecirculating flow of molten steel in the upper regions ofmould strengthened, the recirculating velocity increased,the center of the vortex of the circulating region graduallymoved up with the decrease of frequency, especially in thecase of 3Hz, and a pair of recirculation zones in the lowerregion of the mould appeared when the frequency exceeded3Hz, the recirculation zone got shorter and absolutelyvanished with the frequency of 5Hz.

Velocity distribution along the radial directions at thecenter of the stirrer along the radial directions at the centerof stirrer with the current intensity of 320A and different

Fig. 12. Flow field at meridional planes of mould with different frequency (I=320A) : (a) streamline map, (b) velocity contour.

Fig. 13. Velocity distribution along radial direction at the centerof stirrer with different frequency (I=320A).

Fig. 14. Sampling schematic for acid etch.


frequency were shown in Figure 13. The changing tendencyof azimuthal velocity with increasing frequency wasconsistent with that of EMF. As shown in Figure 13b,the maximum azimuthal velocity along the radius of themould decreased from 0.067m · s�1 with a frequency of 3Hzto 0.038m · s�1 with a frequency of 5Hz, and it appeared atthe location corresponding to the EMS core. Themaximumazimuthal velocity along the radial direction appeared atthe edge of the billet.

According to the simulation and analysis above, thesignificant swirl flow with M-EMS prevented the super-heated jet moving downward and weakened the invasiondepth of the jet, and thus the location of the hot zone in themould moved up evidently. It could be concluded that theoptimal combination range of the current intensity andfrequency was 300–320A and 3–4Hz. Moreover, it wasevident that the effect of decreasing frequency on the flowfield of molten steel was more sensitive than that of the

increasing current intensity. What’s more, flow field inmould has great variation with small change of currentintensity compared with bloom or round bloom [10–13].

3 Effect of M-EMS on solidification structure

In order to investigate the effect of M-EMS on thesolidification structure of 55SiCr, industrial trials werecarried out on the billet caster at HANGSTEEL LTD. Thecombination of current intensity and frequency wasevaluated comprehensively by macrostructure, macrosegregation ratio and secondary dendrite arm spacing(SDAS) of samples.

3.1 Effect of M-EMS on macrostructure

Figure 14 displayed the sampling schematic for acid etch inas-cast billet, and the solidification macro-defects of thesamples were revealed by using hydrochloric acid erosionmethod.

Fig. 15. Comparison between the cross-section morphologies of as-cast billet under different stirring technological parameters:(a) 250A, 4.5Hz, (b) 300A, 3Hz, (c) 300A, 3.5Hz, (d) 320A, 3.5Hz, (e) 320A, 3Hz.

Table 3. Result of the cross-section morphologies ofas-cast billet under different stirring technological para-meters.

Current/frequency

Cornercrack

Centerporosity

Center equiaxedcrystal ratio (%)

300A/3Hz 0.5 0.5 29320A/3Hz 0 0 33300A/3.5Hz 0.5 0.5 28320A/3.5Hz 0 0 30250A/4.5Hz 0.5 1.5 18

Fig. 16. Sampling schematic for wet chemical analysis of carboncontent.


Figure 15 gave the comparison between the cross-section morphologies of typical defects for 55SiCr withdifferent stirring parameters. The statistical results ofaverage defect grade were presented in Table 3. As shownin Figure 15 and Table 3, compared with the originalstirring parameter with 250A and 4.5Hz, the billet centralsoundness has been improved remarkably under optimizedstirring parameter. As an example, defects such as cornercracks and oversized center porosity disappeared in as-castbillet. The grades of center porosity decreased from 1.5 to 0and, in addition, the center equiaxed crystal ratio increasedfrom 18% to 33% under the stirring parameter of 320A and3Hz especially.

3.2 Effect of M-EMS on macro segregation

The carbon segregation ratios on the 55SiCr billet weremeasured quantitatively by the chemical analysis ofdrillings. Figure 16 showed the detailed locations of drillingsamples for chemical analysis of carbon content at the

Fig. 17. Carbon segregation ratio in cross-section of billet under different stirring technological parameters: (a) the inner-outer arccenterline, (b) diagonal direction.

Fig. 18. Analysis schematic for SDAS: (a) sampling schematic, (b) measuring schematic.


cross-section of as-cast billet, where the drilling sampleswere obtained using a 4-mm-diameter drill up to a depth of8mm.

To quantitatively describe the macro segregation underthe different stirring parameters, carbon segregation ratioalong the inner-outer arc centerline and diagonal directionin cross-section of billet under different stirring parameterswere shown in Figure 17a and b respectively. It wasobserved that the distributions of carbon along the inner-outer arc centerline and diagonal direction in cross-sectionof billet under different stirring parameters were in thesame trend. Compared with the case of the original stirringparameter with 250A and 4.5Hz, macro segregation hasbeen improved significantly in cross-section of billet underoptimized stirring parameter. The maximum carbonsegregation ratio at the as-cast billet center decreasedfrom 1.24 to 1.04 and solute element distribution becamemore homogenized under the stirring parameter with 320Aand 3Hz especially.

3.3 Effect of M-EMS on SDAS

Smaller the SDAS at the center was, the finer the equiaxedstructure was, and the lower the extent of segregation wasdue to the reduced permeability of the mushy zone [20]. Formeasurement of SDAS metallographic samples were cutfrom the billet section at an interval 15mm from thesurface to the center of billets. Samples were collected fromall identical locations of billet cross-section to study thevariation of SDAS across all faces. Samples were polished,etched and examined in the image analyzer. On an average,30 measurements have been recorded for each location andaverage SDAS was reported. The sampling schematic forSDAS in as-cast billet is shown in Figure 18.

As shown in Figure 19, dendritic microstructure such asthe chill, columnar, columnar-equiaxed and equiaxed zoneswere observed in the macrostructure of the billet fromsurface to center with electromagnetic stirring of 320A and3Hz. Non-oriented fine equiaxed structure was present in

Fig. 19. Dendritic microstructure from surface to center in cross-section of billet under the stirring parameters of 320A and 3Hz:(a) 7.5mm, (b) 22.5mm, (c) 37.5mm, (d) 52.5mm, (e) 67.5mm.

Fig. 20. Variation of the measured values of average SDAS fromsurface to center in cross-section of billet under different stirringtechnological parameters.


the chill zone owing to characteristic of rapid solidificationof liquid steel at the meniscus in Figure 19a. Rapidcoarsening of columnar dendrite occurred because ofreduction of heat extracted from the surface inFigure 19b, where primary dendrites arose. Both primaryand secondary dendrites co-existed in columnar-equiaxedzones and fine secondary dendrites were found inFigure 19c. A fully equiaxed structure was observed inthe center of the cross-section of billet and secondarydendrites became coarse in Figure 19e.

Figure 20 showed a variety of SDAS from surface tocenter in cross-section of billet under different stirringparameters. SDAS increased from a distance of 5mm to adistance of about 37.5mm beneath the surface and then itdecreased. Electromagnetic stirring could break offcolumnar dendrite and made equiaxed dendrite becomefine after remelting, and furthermore electromagneticstirring could increase the cooling rate which reduces thelocal solidification time. According to the study of Wanget al. [8], stirring intensity was proportional to SDAS.However, stirring intensity was in proportion to currentintensity and was inversely proportional to frequencybeyond the optimal frequency. A value of SDAS decreasedfrom 118mm under the stirring parameters with 250A and4.5Hz to 108mm with 320A and 3Hz in the centralequiaxed zone. Fine equiaxed dendrite in central equiaxedzones was in favor of reduction of center segregation andcenter porosity, which were consistent with the reflection ofthat in Figures 15 and 17.

4 Conclusions

In the present study, a three-dimensional mathematicmodel of electromagnetic field, fluid flow and heat transferin billet continue casting mould with EMS was developed,and the effects of the current intensity and frequency offlow field and temperature distribution have been discussedby numerical simulation. Moreover, industrial trials werecarried out to investigate the magnetic field characteristicsand the influence of M-EMS on the solidification structureof 55SiCr. The main conclusions are as follows:


–
there was good consistence between the predictedmagnetic flux density by electromagnetic field modelsand the measured data. Magnetic field intensitydistribution along an axial direction was big on bothends, but small in the middle. The rotating azimuthalEMF along the radius of the billet is parallel to the edge ofthe billet, which increased with the current intensity anddecreased with frequency when the frequency was morethan the optimal frequency value, and the maximumoccurred at the edge of the billet;
–
the effect of the current intensity on flow field was nearlyopposite compared with frequency, and the effect ofdecreasing frequency on the flow field of molten steel wasmore sensitive than that of the increasing currentintensity. The azimuthal velocity of flow field, alongthe radial directions in the mould, increased first andthen decreases from the edge to the center of the billet,finally velocity dropped to nearly zero at the center of thebillet;
–
the significant swirl flow with M-EMS prevented thesuperheated jet moving downward and weakened theinvasion depth of the jet, and the location of the hot zonein the mould moved up evidently.What’s more, flow fieldin the mould had great variation with small change ofcurrent intensity compared with bloom or round bloom;
–
according to the calculations and analysis, the optimalcombination range of current intensity and frequencywas 300–320A and 3–4Hz respectively. Industrialapplication revealed that the inner quality of 55SiCrwas improved obviously as a result of M-EMS with theoptimal stirring parameter of 320A and 3Hz. As anexample, a central equiaxed zone increased from 19% to33%, the minimum SDAS with 108mm was obtained inthe central equiaxed zone. In addition center segregationratio decreased from 1.13 to 1.05 and center porosity hasnearly disappeared.
The authors would like to acknowledge the financial supportprovided by the Natural Science Foundation of China(No. 51274029.) and State Key Laboratory of Advanced Metal-lurgy Foundation (No. 41614014). The authors are also thankfulto HANGSTEEL for the support on the field test.

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Cite this article as: Hanghang An, Yanping Bao, Min Wang, Lihua Zhao, Effects of electromagnetic stirring on fluid flow andtemperature distribution in billet continuous casting mould and solidification structure of 55SiCr, Metall. Res. Technol. 115, 103(2018)

effects of electromagnetic stirring on fluid flow and ... · parameters of m-ems by combining...

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