Electromagnetic Braking Improves Steel Quality In .

CONTINUOUSCASElectromagneticbraking improvessteel qualityin continuouscastingTINGVarious arrangements of the staticmagnetic fields have been tested, andsome major improvements have beenachieved through this work. However, thetests are costly and time-consuming;furthermore, the ambient conditions posea problem. In view of this, ABB IndustrialSystems has focused its attention ondeveloping theoretical models for computing the flow behaviour with differentfield coil arrangements [1, 2].With the help of a developed turbulence model for the steady-state condition in the strand, three different EMBRconfigurations were studied 3 :IConventional EMBR, in which twoElectromagnetic ‘braking’ of the hot metal flow in the mold of a continu-magnetic fields are placed and actous casting machine improves the quality of the cast steel by reducinglocally across the strand width.the penetration of non-metallic inclusions. These are especially likely toII EMBR Ruler, in which one magneticoccur during high-speed casting. Numerical studies carried out at ABBfield covers the entire strand width.Industrial Systems provide an insight into the influence electromagneticIII FC Mold (Flow Control Mold); here, thebraking exerts on the behaviour of the metal flow in the mold.entire width of the strand is coveredby two parallel magnetic fields, withCthe nozzle for the molten steel lyingontinuous casting was developedachieved over the entire strand width 2 .as the answer to industry’s demand forBy reducing the risk of non-metallic inclu-The mathematical model was verified byan improved steel quality and higher pro-sions, the EMBR considerably improvescomparing the computed fluid flow withduction rates. Instead of discrete ingots,the quality of the cast steel.results from full-scale measurements incontinuous casting produces, as itsAlthough ‘electromagnetic brake’ hasname suggests, a continuous slab forbecome the generic name for systemsrolling into sheet metal or sectional steel,of this kind, their function is describedetc 1 .betterbytheterm‘electromagneticbetween them.different installations.Conclusions drawnContinuous casting is, however, aflow controller’, this name also helpingfrom the numerical studiescomplex process in which harmful non-to explain the apparent paradox in theThe most important results of the studiesmetallic inclusions, such as slag or gas,statement that the ‘casting speed iscan be summarized as follows:can easily become entrapped in theincreased by the electromagnetic brake’. molten metal. The risk of such inclusionsThe EMBR ensures a uniform velocity forfield have a considerable influence onincreases with the casting speed, sincethe molten steel over the entire cross-the molten steel flow in the strand.the jet of molten steel penetrates deepsection of the strand; hence, the castingElectromagnetic braking is most effec-into the mold, pulling mold powder andspeed can be increased without anytive at improving the flow field whenother impurities down with it. The pres-degradation of the steel slab quality.the jet of steel is directed towards theence of such impurities in the solidifiedThe argon gas and the static magneticzone covered by the magnetic field. metal seriously impairs the quality of thesteel.Non-metallicinclusionspenetratingdeep into the center of the slab are re-To eliminate this problem, ABB devel-duced by the magnetic field configur-oped and patented the electromagneticAnders Lehmanation acting on the full strand widthbrake (EMBR). The EMBR uses a staticGöte Tallbäckas compared with the configurationmagnetic field to control the flow of hotÅke Rullgårdwith two magnetic fields acting locally.metal in the mold, allowing a uniformABB Industrial Systemscasting speed and temperature to be4ABBReview1/1996 The temperature at the meniscus israised by 5–15 C when electromag-

CONTINUOUSCASTINGarrangement and the strength of themagnetic field. The tendency for low-frequency, highamplitude oscillation to occur at themeniscus is suppressed very effectively by the action of the static magneticfield in the mold. The risk of moldpowder being pulled downwards withthe flow is reduced, since such pulldown is the result of high accelerationand a high average velocity of the flowat the meniscus. Further areas of application for electromagneticbrakingincontinuouscasting may be expected in thecontext of smaller strand sizes: thereduced penetration depth of thesteel jet and the higher sub-meniscustemperature have a positive effect onthe quality of the steel. With the helpof the EMBR it should therefore beContinuous slab casting in the SSAB steelworks in Luleå,northern Sweden1possible in the future to use significantly higher casting speeds withoutthe quality being degraded. This hasspecial advantages for the casting ofnetic braking is used. When castingWith the FC Mold configuration, inwide slabs at a low speed it is possiblewhich the nozzle lies between the twothat the power will have to be reducedparallel magnetic fields, limited ‘brak-with the EMBR Ruler, since the steeling’ of the jet occurs in the direction offlow at the narrow faces of the slabsthe narrow faces, resulting in a smallercould become stagnant if the brakingreduction in the average velocity at theeffect is too strong. Under certainmeniscus. This configuration has thecircumstances, this might reverse theadvantage that the steel flow from thedesired effect, ie, the temperaturenarrow faces towards the nozzle canbecomes too low and the molten steelbe maintained.at the meniscus freezes. EMBR23456The depth of the jet’s penetration isThe action of a static magnetic field inreduced with all three configurationsthe mold will often cause a strongcompared with ‘unbraked’ casting.reduction in the average velocity ofHowever, the optimum flow and steelthe steel directly below the meniscus.quality are largely dependent on theFlow in a continuously cast strand without EMBR (left)and with EMBR (right)Sthin slabs.72Strand width1Without EMBR1 Deep penetrationof non-metallic inclusions2 Mold powder layer3 Disturbed meniscus4 VorticesWith EMBR5 Calmer, hotter meniscus6 Braked area7 Reduced penetration depthof non-metallic inclusionsSABBReview1/19965

COINTINUOUSCASIITINGIII3Configurations of the electromagnetic brake (EMBR)I Conventional EMBR: two magnetic fields are placed and act locally across the strand widthII EMBR Ruler: one magnetic field covers the entire strand widthIII FC Mold: two parallel magnetic fields cover the entire strand width; nozzle opening between fieldsConventional EMBR –the molten steel flow in the strand asperature just below the meniscus rose bylocal magnetic fieldswell as to reduce the number of non-5 to 10 C. However, there is a risk of theIn the first-generation EMBR two mag-metallic inclusions. Generally, the resultsflow stagnating when the full brakingnetic fields were placed and act locallyof measurements have shown that thispower is applied. What is more, whenacross the strand width 3 . It was devel-configuration reduced the penetrationcasting narrow slabs a single, main flowoped to suppress deep penetration ofdepth by up to 50 percent, while the tem-channel caused by the zero magneticBasic EMBR configuration: calculated flow field at cross-sections A, B and C for different flux densities Bof the local magnetic fieldsABCross-section in middle of strandCross-section 100 mm from nozzletowards narrow face of strandC Cross-section20 mm below meniscusabcCBCAa6ABBBbReviewStrand size 245 1600 mmCasting speed 1.6 m/minSubmerged nozzle depth 190 mmNozzle outlet angle –45 Specific meniscus power 75 kW/m2Argon gas flow 10 l/minSuperheat temperature 10 CB 0TB 0.16 TB 0.32 T1/1996CABcA4

CONTINUOUSCASTINGfield in the middle of the strand sometimes causes an increase in the nonCmetallic inclusions entrapped in the so-Clidified shell. This underscores the importance of carrying out simulations todetermine the optimum configuration forEMBRs.The calculated three-dimensional flowis shown in 4 , the induced currents andLorentz forces in 5 .EMBR Ruler –Bone magnetic field coversAaBAbthe entire strand widthThe second-generation EMBR, known asthe EMBR Ruler 3 , uses one magneticfield which acts on the entire strandwidth. The first EMBR of this type was in-Induced currents (a) and Lorentz forces (b) in cross-sectionsA, B and C with local magnetic fields.The casting data is the same as in Fig. 4, however, with a magnetic fluxdensity of 0.32 TEMBR Ruler: absolute velocity maps in the cross-section through the middle of the strand.The velocity in the black regions is 0.4 – 1.0 m/s.Data valid for all maps:Submerged nozzle depth 150 mm,Submerged nozzle depth 250 mm,Strand size 225 1300 mmnozzle outlet angle 0 nozzle outlet angle –30 Casting speed 1.5 m/mina EMBR off, argon gas flow 0e EMBR off, argon gas flow 0Specific meniscus power 75 kW/m2b EMBR off, argon gas flow 10 l/minf EMBR off, argon gas flow 10 l/minSuperheat temperature 20 Cc EMBR on, argon gas flow 0g EMBR on, argon gas flow 0Max. flux density 430 mmd EMBR on, argon gas flow 10 l/minh EMBR on, argon gas flow 10 l/minbelow meniscus 0.3 Tae1300bcdfghABBReview1/1996567

CONTINUOUSCASTING2500abc7EMBR Ruler: tracking of 200-µm particles in the middle of the strand for a magnetic flux densityof 0 T (a), 0.15 T (b) and 0.30 T (c)Strand size 250 2500 mmCasting speed 0.9 m/minSubmerged nozzle depth 225 mmNozzle outlet angle –20 Specific meniscus power 150 kW/m2Argon gas flow 5 l/minSuperheat temperature 15 Cstalled in 1991 at the Sollac casting plantalmost horizontal, the fluid flow will tendthe nozzle. As a rule, the EMBR Ruler in-in Dunkirk, France, and at Hoogovens into float above the magnetic field region.creases the temperature at the meniscusIJmuiden, Holland.Increasing the depth of the nozzle andby 5 to 15 C.The impact of the amount of argonpointing the nozzle further downwardsCalculations and field measurementsgas and the depth of the submergedcauses the molten steel to penetrate thehave shown that a static magnetic field innozzle on the absolute velocity is shownmagnetic field region direct, resulting inthe mold can be used to control thein 6 .effective braking of the nozzle jet. If theflow at the meniscus. If there is noIf the submerged nozzle is located atEMBR is positioned too high, however,EMBR, the flow is normally determineda shallow depth and the outlet flow isargon gas bubbles may collect close toby the slab size, casting speed, amount8Temperature maps for a 2500 mm wide strand, with EMBR Ruler; the magnetic fluxdensities are 0 T (a), 0.15 T (b) and 0.30 T (c).The superheat temperature in the black region is 6–8 C (casting data, see Fig. 7).ACross-section in middle of strandCCCross-section 20 mm below meniscusCC2500a8ABBAReview1/1996bAcA