97 epm paris article avec fig

Proceedings of the International Congress on Electromagnetic Processing Of Materials, 27-29 May 1997, Paris La Défense, Vol.2, p.355-365

IN MOLD DOUBLE STIRRING SYSTEM

IN CONTINUOUS CASTING : EFFECT OF TWO COUNTER ROTATING MAGNETIC FIELDS

S.KUNSTREICH, M.C.NOVE, D.YVES, DANIELI ROTELEC ( FRANCE ) W. COURTHS, E.KORTE, SAARSTAHL A.G. ( GERMANY )

RESUME : Un système à double rotation en lingotière - brasseur et frein - est étudié par simulation sur métal de Wood, par modélisation numérique ainsi que par essais sur site industriel. La diminution espérée de la vitesse de rotation au niveau du ménisque est effectivement obtenue. Mais, conjointement, le frein provoque des instabilités du ménisque par une circulation verticale intense et annule les améliorations obtenues par le brasseur sur la qualité des billettes coulées. ABSTRACT: An in mold double motor system - stirrer plus braker - is investigated by simulation with Wood metal and numerical modeling as well as in plant trials. The intended effect of reduced rotational velocity near meniscus is verified. However, at the same time, the braker induces meniscus instability by strong vertical flow and cancels the improvements obtained by the stirrer in as-cast billet quality. 1. INTRODUCTION

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Electromagnetic stirring (EMS) is largely used in continuous casting of steel, since it improves considerably the quality of the as-cast products. The most frequently used type is rotative stirring in the mold (M-EMS) of billets and blooms. Initially developed to improve the surface and subsurface quality of the products by means of the mechanical rotation of the liquid steel at the very beginning of the solidification, i.e. at the meniscus, it subsequently has proven to produce even better metallurgical results in the internal quality of the as-cast products than the stirring in the secondary cooling zone (S-EMS) below the mold (1). Its ability to improve simultaneously the surface, subsurface and internal quality of the products is the reason of its big industrial success as well as of some remaining controversy and problems. Problems arise if M-EMS is wrongly used at excessive intensity. The tendency to do so comes from the fact that the metallurgical results generally improve with increasing stirring intensity. However, excessive stirring velocity in the mold creates negative effects such as unstable meniscus level (2), powder entrapments (3), lubrication defects (4) or accelerated wear of the submerged nozzle (5). Although some of these drawbacks can be prevented by careful control of meniscus level, of powder feeding and of nozzle immersion depth, in other cases a compromise has to be found between the contradictory requirements: maintain intensive stirring below the meniscus in order to achieve optimal metallurgical results and simultaneously reduce the stirring velocity at meniscus in order to avoid the above described problems. It seems obvious that the most successful method would be the direct control of the meniscus flow by means of an additional inductor positioned on the top of the normal stirrer and connected to an independent electrical power supply. Such "double motor" concept, let’s call it M2-EMS, could maintain high stirring velocity in the lower part of the mold by the stirrer M-EMS which plays the role of the "main motor", whilst the velocity in the meniscus area would be braked down by the braker M-EMB which, as an auxiliary "braking motor", rotates in opposite direction (6). Since this concept implies increased cost for investment and operation, it must provide much better operational results than the existing EMS. We also had in mind some theoretical doubt and remembered some negative results from the early 80’s with counter-rotating mold and submould stirrers (7). Therefore we decided to carefully check the validity of this concept by a four steps program: - 1 - design and manufacture two industrial M2-EMS equipments for 150 sq.mm billets and

262 mm x 336 mm blooms, - 2 - measure the liquid metal flow velocities produced by these equipments in a scale 1 : 1

mold filled with Wood metal, - 3 - develop and calibrate three dimensional computer codes capable to describe the real flow

conditions - 4 - test the equipments under industrial conditions on continuous casting machines.

The present paper describes some of the relevant results of steps 2 to 4.

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2. FLOW VELOCITY MEASUREMENTS WITH WOOD METAL 2.1 Experimental Set-up Fig. 1 shows schematically the experimental set-up and the main mechanical, electrical and magnetic data of the equipments. The stirrer M-EMS and the braker M-EMB are arranged around a non magnetic stainless steel tank in exactly the same geometrical position as later on the two continuous casting machines for 150 sq.mm billets and for 262 x 336 mm blooms. The tank is heated by hot water at constant temperature of 90° C and is filled with Wood metal. The stirrer and braker are connected to two independent low frequency power supplies with control of current intensity, frequency and synchronized operation or not. The height of the tanks has been chosen more than 2 times longer than the stirrer/braker arrangement in order to remove possible bottom effects. No metal recirculation between bottom of tank and nozzle is simulated, the nozzle is only submerged into the Wood metal. 2.2 Measurements Magnetic fields produced by the stirrer alone, the braker alone and by the combination of both have been measured inside the tanks ( and later inside the real molds) as function of current and frequency with calibrated Hall sensors. (Those data are used to check the validity of the computer simulations). The movements on the meniscus have been observed and video recorded as function of different current settings of stirrer and braker. The absolute speed of the Wood metal flow inside the tank has been measured with Vives sensors (8), one suitable to measure the horizontal speed components x, y and one to measure the vertical component z. The sensors have been calibrated by immersion into Wood metal contained in a cylindrical tank 300 mm deep and 700 mm in diameter, which rotated under controlled speed. Each speed measurement represents the time average over 120 seconds (20 measurements). The reproducibility was better than + 0,01 m/sec. Measurements have been made from approx. 30 mm below the free surface down to a position below the middle of the stirrer on lines A, B, E, F of the 150 sq.mm billet simulation (Fig.2) and on the lines I, J, K, P, M, N of the 262 x 336 mm bloom simulation (Fig. 3). Parameters were : stirrer alone in different positions with different current settings, stirrer plus braker in same or in opposite direction, with different current settings, synchronized and desynchronized operation, with and without nozzle. 2.3 Results The most relevant results can be summarized as follows (a more complete presentation will be published later (9)) : − Fig. 2 shows the horizontal speed component vx in the point A (x = 0, y = - 25 mm) for

different z positions and at constant stirrer current of 150 A. The curves named 0, 200, 250, 300, 350 refer to the corresponding braker current. In spite of its reduced dimensions with respect to the stirrer, the braker is able to brake down to zero and even to inverse the horizontal rotational speed in a range from the free surface down to approximately the middle between braker and stirrer.

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In the vicinity of zero horizontal speed, the flow conditions become very unstable : a relatively small increase of braker current shifts the speed from zero to largely negative values. The braker does not only brake the surface speed but also reduces the speed inside and below the stirrer by more than a factor of two and a swelling of the free surface in the corners shows strong vertical upright velocities.

− Fig. 3 shows the vertical speed component vz measured in the point P (x = -106 mm, y = -143 mm) for different z positions at constant stirrer current 150 A. The curve 0 (braker

current zero) shows an upward directed speed above the stirrer plane of symmetry, a downward directed speed below this plane and zero on the plane. This is the experimental evidence of two vertical recirculation loops as indicated in Fig. 3, which are induced by the rotative stirring and which were theoretically predicted (10). The upward and downward directed speeds show that both the upper and the lower loop are perfectly symmetrical to the middle of the stirrer (the corresponding downward movement has also been measured in the point A - Fig. 2 - but is not shown here).

− The more the braker brakes the horizontal speed, the stronger becomes the vertical speed in this recirculation loop. Its absolute value is comparable to the horizontal speed. The extension of the upper loop is shifted considerably below the stirrer center by the effect of the braker .

− Comparison of Fig. 2 and 3 shows that the reduction of the horizontal speed by more than 0,1 m/sec in the center of the stirrer reappears in form of vertical speed in and above the stirrer.

3 - DEVELOPMENT OF COMPUTER MODELS

The aim of this work is to develop computer models which are capable to predict three dimensional flow distributions of liquid metal created by complex AC magnetic field configurations and which use electromagnetic coupling and free surface conditions. The purpose is to simulate the flow conditions for various sections, casting conditions and EMS configurations in continuous casting. This work is part of a collaboration program with EPM-MADYLAM (11). It uses the code MHD3D (12), which has been developed to couple two computer codes : FLUX-EXPERT (13) which uses finite elements and computes with Maxwell equations the magnetic fields, induced currents and volume forces, CEPHISE (14) which uses finite volumes and computes with Navier-Stokes equations the flow velocity distributions with the turbulent flow model k-ε.

The exact geometry of the bloom experiment has been simulated reproducing the inductors and

the Wood metal volume with free meniscus surface (Fig. 4).The double wall tank made of stainless steel has been considered has a single wall tank with total equivalent thickness. − Fig. 5 shows the computed and measured horizontal speed vy on the line M (x = 81 mm,

y = 0) as function of the vertical z position. The curve with crosses represents the first computation results for stirrer operated at 150 A and braker out of operation. This result is totally wrong, because in reality the rotational speed does not at all stop above and below the stirrer. The turbulent k-ε flow has then be replaced by a non turbulent flow simulated by a 100 times higher viscosity. The new results obtained with stirrer alone (white squares) and with stirrer plus braker (white triangles) match surprisingly well with the experimental data (black squares and triangles), however only inside the stirrer area around z = 600 mm.

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The computed data also describe very well the flow speed for smaller z-values in the area up to

the braker in case the braker is operated (triangles), that means the braking effect covers the effect of the free surface. However, in case of operation without the braker (squares), the flow speed towards the free surface is not correctly predicted by the computations.

This result illustrates well that k-ε model for turbulent flow is not adapted to compute flow distributions with free surface condition. Works with different EMS configurations and with a replacement of the k-ε model are under progress (15).

4 - TESTS ON No.3 BILLET CASTER OF SAARSTAHL A.G.

4.1 Purpose of the tests

SAARSTAHL has installed on the six strands of its 150 sq.mm billet caster N° 3 rotative type S-EMS below the mold at a distance of 1.10 m between top of stirrer and meniscus. The S-EMS is used to improve the internal solidification structure and to reduce center

segregation. Although the metallurgical results are satisfactory, a change from S-EMS to M-EMS could be of interest because of the following reasons : a mold stirrer installed inside the mold assembly is less subject to operational accidents than a strand stirrer, no alteration of secondary cooling and strand supporting rolls occurs, the intensity of S-EMS is limited by the formation of "white bands", a mold stirrer is expected to produce additional surface and subsurface improvements.

Because of these reasons, SAARSTAHL has started again trials with M-EMS. The first experience on this caster has been made in 1985 with a 480 mm long stirrer installed close to the meniscus in a 700 mm high mold. This configuration which is well adapted to open pouring, has produced a strong nozzle wear and an increase of macro-inclusions, even if operated at relatively low power settings (16). In order to meet the requirements of long sequence casting with submerged nozzle and powder, the mold stirrers were replaced by strand stirrers. Because of the newly available M2-EMS with a flat stirrer of only 350 mm height in combination with a braker, new trials have been started in 1995.

4.2 Description of Tests

The tests were started with different current settings in stirrer and braker under open pouring conditions, in order to observe the meniscus movements and to establish appropriate stirring parameters. Based on this observation, the following current settings have been applied later for powder casting with submerged nozzle : − Stirrer: 150 A, 200 A, 250 A (more than 250 A creates very strong meniscus movements) − Stirrer plus braker : Stirrer 200 A, 250 A, Braker 150 A, (more than 150 A on braker

creates heavy turbulences in the meniscus area) Totally 359 heats have been cast in 132 sequences with the M2-EMS equipment installed on one

strand of N° 3 CC. The metallurgical results have been compared with those of the other 5 strands which were stirred with the S-EMS.

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Submerged nozzles are straight through types with bottom diameter 40 x 80 mm ; The nozzle

wear has been analyzed after each end of sequence casting by measuring the bottom wall thickness and the annular erosion at the meniscus level. The nozzle from the test strand is compared with the nozzles from the other strands.

Internal solidification structure of the as-cast billets has been analyzed by sulfur prints from longitudinal and transverse cut samples and quoted as per an internal quality index ranging from 1 to 6. This index depends on diameter of the equiaxed zone, existence of mini-ingots and V-type segregation, length of the dendrites in the transition zone. Big diameter of equiaxed zone, absence of mini-ingots and of V-type segregation and short dendrites are considered to reflect good quality. Since these data depend not only on EMS, but also and even very strongly on other parameters like steel grade, superheat etc., the internal quality is analyzed in terms of difference between the M2-EMS strand and the S-EMS strands.

4.3 Results − Fig. 6 shows the wear of the submerged nozzle of the test strand as function of the stirring

parameters and of the sequence length (1-5 casts). The wear itself is quoted as ‘’same, slightly more, more, much more ‘’with respect to the strands with S-EMS. Except one case after 5 heats, no additional nozzle wear can be detected with the stirrer alone in operation at 150 A. At 200 A or more, increased and partially strong additional wear appears frequently. Therefore this setting has been stopped after a few sequences. The use of the braker, except two cases of slightly more wear, permits to stop the nozzle erosion and shows the same nozzle wear as the reference strands with S-EMS.

− Fig. 7 shows the internal billet quality of the M2-EMS test strand as function of the stirring

parameters. The quality itself is quoted in terms of ‘’same, better, worse, much worse’’ as compared to the S-EMS stirred strands. Operation of the stirrer alone at 150 A gives clearly worse results, operation at 200 A or more better results than the S-EMS. Simultaneous operation of stirrer and braker gives worse results than the S-EMS.

− Fig. 8 resumes the results for the different stirring configurations and adds the obvious

information that the soundness of the subsurface area (i.e. freeness from slags and pinholes) is better in all cases of mold stirring than in strand stirring.

5 - DISCUSSION

Observation of the meniscus for both, Wood metal and steel, shows the same qualitative

phenomena : the braker is an efficient tool to control the horizontal rotational speed at meniscus induced by the stirrer. It can brake down to zero and even inverse this speed. However, in the vicinity of zero horizontal speed, the flow conditions become very unstable and strong vertical flow velocities create meniscus fluctuations. This is in contradiction with results from Mercury experiments, which reported a braking down of the speed at meniscus in any direction together with a meniscus flat and devoid of disturbance (17). We conclude that mercury trials are not suitable to simulate continuous casting of steel.

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The braker clearly stops the problem of submerged nozzle erosion induced by the stirrer. This

can easily be understood from the liquid flow velocities in Fig. 2, if one admits that the erosion effect mainly comes from rotative horizontal flow velocity.

Unfortunately, the braker also completely cancels the superiority of the mold stirrer over the strand stirrer in terms of internal billet quality. This superiority is well established if M-EMS is compared with S-EMS (1). It is explained by a thermal, not by a mechanical effect : the mold stirrer dissipates much better the superheat than the strand stirrer. Considering the flow velocities of Fig. 2 and 3, we can imagine the following explanation : a) The braker stops the horizontal flow velocities in the upper part of the mold. Therefore

heat exchange, which mainly exists in the upper part of the mold (18) will be reduced. b) The braker increases the speed of the vertical recirculation loop and extends the upper loop

into a region below the stirrer. Therefore the lower loop will be situated further downwards and its upraising vertical flow in the centre will no more brake the hot liquid steel coming from the nozzle. Furthermore, the strong and long upper loop will enhance the hot liquid steel going below the mold. There will be less superheated liquid inside the mold and the dissipation of superheat by the M-EMS will be decreased.

c) The braker strongly reduces the rotative flow velocity inside and below the stirrer and therefore impairs the residual effect of the mold stirrer below the mold.

CONCLUSION Control of steel flow in the mold by double motor system M2-EMS (stirrer M-EMS plus braker M-EMB) has been investigated by means of Wood metal model and numerical simulation as well as plant trials in billet caster. The following preliminary conclusions can be drawn : 1. Horizontal and vertical flow velocity of Wood metal in a scale 1:1 model of 150 sq.mm

billets and 262 x 336 mm blooms shows that the intended effect of the braker to reduce the rotational flow velocity near meniscus is obtained. A theoretically predicted vertical flow is evidenced. Braking down the horizontal flow at meniscus enhances the vertical flow which causes a turbulent meniscus in the product corners. The intended effect of a quiet meniscus which has been reported from Mercury experiments is not obtained.

2. Numerical flow modeling with k-ε model is not satisfactory ; but a non turbulent flow

simulation appears promising - with further development required. 3. Continuous Casting plant trials with 150 sq.mm billets confirm the braker effect to stop

horizontal flow velocity at meniscus and to create turbulence by strong vertical flow. The positive effect of the braker on wear of the submerged nozzle entails a concomitant deterioration in internal billet quality ; consequently, use of the braker cannot be recommended.

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REFERENCES 1 - ROTELEC leaflet : "Electromagnetic Stirring for continuous casting of billets and

blooms" : Figure 10. 2 - ROTELEC : "Know-How Book" : Chapter 1-4 3 - Tsuneaki Nagamichi : "Flow phenomena in continuous casting mold of small section

round billet" - CAMP - ISIJ-Vol 2 (1989) - 1254 4 - Hirotaka Miki et al. : "Flow characteristics of molten flux in round billet continuous

casting mold" 112th ISIJ Meeting - Lecture n° 295 - Oct. 1986 - Nagoya University 5 - J.M Bastian : "Casting conditions of large rounds through continuous caster of Gandrange

using mold EMS". The Institute of Metals - London - Continuous Casting'85 - Paper 24. 6 - Beitelman, J.A. Mulcahy : "Flow control on the meniscus of continuous casting mold with

an auxiliary A.C magnetic field"- International Symposium on Electromagnetic Processing of Materials, Nagoya -1994 - ISIJ - pp 235 - 241

7 - ROTELEC : "Data base of customer results" 8 - R. Ricou, Ch. Vives: "Local velocity and mass transfer measurements in molten metals

using an incorporated magnet probe" - International Journal of Heat and Mass Transfer 1982.- Vol. 25, n° 10 - pp 1579-88,

9 - ROTELEC - under preparation 10 - K.H. Spitzer, M. Dubke and K. Schwerdtfeger : "Rotational Electromagnetic Stirring in

Continuous Casting of Round Strands", Met. Trans., Vol. 17B, March 1986, pp 119-131. 11 - EPM/MADYLAM - Internal report - Ref. 95/0487 - March 1995 12 - C. Trophime : "Modélisation numérique du couplage Magnétohydrodynamique (MHD)

fort....."- PhD Thesis INPG - (EPM MADYLAM) -1995 13 - DT2i/EPM- France 14 - Ch. Raffourt : "Modélisation numérique de la thermohydraulique d'une coulée

semi-continue de plaques d'aluminium" Ph. D Thesis - INPG - 1991 15 - C. Strohm - Ph. D Thesis under preparation in EPM/MADYLAM 16 - R. Jauch, W. Courths et al. : "Electromagnetic stirring at ARBED SAARSTAHL's bloom

and billet casters" - ROTELEC congress - Cannes - 1984 17 - F.C Chang and J.R Hull, L. Beitelman : "Simulation of fluid flow induced by opposing

AC magnetic fields in a Continuous Casting mold" - 13th PTD conference - Proceedings 1995-79

18 - M. Wolf : "Mold heat Transfer and Lubrification Control" - Process Technology Conference Proceedings - Vol 13 (1995) - pp 99-117

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97 epm paris article avec fig

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