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AUTHORS: Zhu Miaoyong, Cheng Nailiang and Yang HongliangNorth-eastern University, Shanghai Meishan Co, Ltd,Baosteel Group and ABB (China) Ltd
The main demands on ladle metallurgy are goodcontrol of steel cleanliness, temperature, chemical
analysis, alloying yields and degassing. Various refiningmethods have been developed to meet these demands,such as RH, ASEA-SKF, VOD and CAS-OB; and there are different ways to agitate and move liquid steel in the ladle, such as vacuum lifting, gas stirring andelectromagnetic (induction) stirring.
First introduced in 1965, LF-EMS has been installed in100 plants and has achieved great success both in theproduction of high quality steel, in acting as a bufferbetween steel plant and caster and enabling a reductionin furnace tapping temperature to be achieved.
In this paper the principles, components andcharacteristics of the LF-EMS system are described. Themetallurgical performance of LF-EMS is discussed andcompared with argon gas stirring. Finally, theimplementation of LF-EMS in a steel plant with a thin slabcasting machine will be discussed.
LF-EMS SYSTEMThe main components of the LF-EMS system are anelectromagnetic coil, frequency converter, transformerand a water station for the cooling of theelectromagnetic coil (see Figure 1). A low-frequencytwo-phase current generated by the converter is fed intothe coil to generate the electromagnetic field. Themagnetic field is strengthened by an iron core locatedbehind the coil which is made of tubes that are cooledby low-conductivity, de-ionised water. The ladle shell infront of the coil must be made of non-magnetic stainlesssteel so that the low-frequency magnetic field canpenetrate into the melt. The coil and iron core areplaced in a stainless steel box and are protected fromthe heat in the ladle by a layer of refractory material.The coil box should be placed as close as possible to theladle in order to achieve the highest possible magneticfield in the melt.
Electromagnetic stirring during ladle refining (LF-EMS) provides excellent control of deoxidation, ladlecomposition and temperature through its efficient and controllable stirring action. Use of LF-EMS inconjunction with gas stirring for desulphurisation optimises the advantages of both processes.
Electromagnetic stirring in ladle refining processes
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MAGNETIC FORCE AND FLUID DYNAMICSThe low-frequency magnetic field induces current in themelt. A Lorenz force is then formed as a result of the co-existence of the two fields:
[1]where: is the induced current, is the magnetic field
and is the Lorenz force.
r Fig.1 Schematic view of the LF-EMS system
r Fig.2 Calculated Lorenz force in the ladle
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distributed steel velocity in the ladle and there is nosignificant difference between the velocities at thebottom and surface of the melt, as in the gas-stirred ladle.Figure 4 shows the distribution of turbulent energy in theladle. As with the Lorenz force the turbulent energy ismore homogeneously distributed than in the gas-stirredladle (lance or porous plug).
METALLURGICAL PERFORMANCEDeoxidation and steel cleanliness LF-EMS generatescontrolled stirring in the melt and can keep the slag unbroken to minimise oxygen ingress and oxidation ofaluminium and other elements. The homogeneouslydistributed stirring energy is favourable for the formation
of larger inclusions and for theirtransport to the steel-slag and steel-refractory interfaces where they arecaptured. As a result the total oxygencontent is about 20–30% lower thanthat of gas-stirred steel.
Figure 5 shows the effect of the stirring power on the oxygenremoval rate constant for differentprocesses. Excessively strong stirring isdetrimental since the upwardcirculation of steel into the slag layer
may expose an open surface or ‘eye’ to reoxidation, andthe lining may be seriously eroded. It can be seen that amaximum deoxidising effect of LF-EMS (ASEA-SKF) isobtained by optimising the stirring power.
Figure 6 describes the deoxidising process. When adeoxidant such as aluminium is added to steel it reactsrapidly with the dissolved oxygen via the nucleation ofsmall alumina inclusions. This lowers the dissolved oxygencontent to an equilibrium value determined by both thedeoxidant and the temperature. The total oxygen contentdoes not decrease as fast as the dissolved oxygen contentas the oxide inclusions need time to be removed from thesteel by absorption into the slag.
These oxides increase in size mainly from collisionscaused by velocity gradients. The separation of the oxidesfrom the steel to the slag has been found to be enhancedby a buoyancy force which is initiated by densitydifferences between the oxide and the steel, and by smalleddies in the turbulent flow. The rate of separation ofinclusions can be increased with the help of stirring, whichcauses inclusions to collide, as well as lifting the inclusionsup to the top slag, where they can be assimilated.
LF-EMS can provide uniformly distributed turbulentenergy (small eddies) in the bulk melt, which is beneficialto the collision of small inclusions, thereby increasingtheir size. An optimised flow speed on the melt surfaceensures that the inclusions are easily absorbed by the slag
Figure 2 shows the calculated Lorenz force in the ladle. Itis concentrated in the left side close to the coil and itsdirection may be upwards (as shown in the figure) ordownwards by controlling the current phase difference.Figure 3 compares the calculated flow in an induction-stirred and gas-stirred ladle.
It can be seen that LF-EMS gives a more homogeneously
r Fig.3 Calculated steel flow in (a) induction- and (b) gas-stirred ladles
r Fig.4 Distribution of turbulent energy in the ladle
r Fig.5 The effect of stirring power on the oxygenremoval rate constant
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layer. The stronger the deoxidising elements used, themore important becomes the issue of avoiding exposureto the furnace atmosphere to minimise reoxidation andaid removal of inclusions.
In light of the above, induction stirring offers the bestpossibilities for clean steel production and it has beenestimated that induction stirring will result annually in0.2% fewer rejects for internal steel quality as comparedto gas stirring.
Alloying and analysis control Because of the ability ofLF-EMS to produce a controlled ‘eye’ and efficient bottomstirring energy, the melting and homogenisation of alloysis rapid. Figure 7 shows the calculated mixing time in gas-stirred and induction-stirred ladles. It can be clearly seenthat induction stirring gives much better mixing than gasstirring. LF-EMS is an excellent tool for production ofhighly alloyed steels because of the high active arc powerfor melting of alloys and the rapid mixing time because ofinduction stirring. Figure 8 shows an example of alloyadditions at Ellwood Uddeholm Steel Company in theproduction of a 45t AISI H-13 tool steel heat. 80kg of
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FeCr/tonne steel was added into the EMS ladleand homogeneity was achieved less than 15minutes after the start of the FeCr addition.Note the low superheat after the FeCr addition.
The operational experiences from a largenumber of installations with induction and gasstirring show that the rate of oxidation ofdeoxidising elements is generally lower forinduction stirring than for gas stirring (seeFigure 9). Differences in aluminium oxidationrates between gas stirring and inductionstirring of 2–5ppm/min have been reported.This means that the corresponding amount ofaluminium added can be saved. Also, thisdifference can increase if there is contact
between the steel and the furnace atmosphere duringtreatment caused by leakages through a roof that doesnot seal perfectly against the ladle rim.
For low carbon grades, excessive carbon pick-up fromelectrodes during arc heating can be detrimental. Figure10 shows the results of the carbon yield in induction- andargon gas-stirred ladles, indicating that gas-stirred heatshave a carbon yield considerably over 100% and ofgreater variability compared to induction-stirred ones.
r Fig.6 The principle of the deoxidising process
r Fig.7 The calculated mixing time of (a) gas-stirred and (b) induction ladles
r Fig.8 Alloying and temperature control in an induction-stirred ladle
a
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This relates to the longer arc length and greater exposureto air during gas stirring. By implication electrodeconsumption is lower with LF-EMS.
Desulphurisation Good desulphurisation requires goodmixing of the slag and melt. Experience shows that themost complete desulphurisation is obtained with acombination of induction stirring, gas stirring and archeating working in a complementary manner. Gas stirringis used when there is a need for an intensified interactionbetween slag and steel to maximise the desulphurisationreactions. This is followed by a period of smooth inductionstirring to let the CaS inclusions float out of the steel.
PLANT OPERATIONSTemperature control Efficient and controllablereheating cycles are important in order to minimisesecondary effects like lining wear and unexpectedcomposition changes of the melt, as well as minimiseenergy use and heating times. The arc length and slagthickness are both important factors in energy transferto the melt; the longer the arc the thicker the slag layerneeds to be. Due to the relatively calm slag layer, LF-EMS can use a short arc to heat the melt, which ensures
a higher electrical efficiency of the arc energy transfer tothe bath. A 50–70% higher efficiency has beenmeasured, which is of great importance to short,efficient reheating cycles.
Induction stirring makes it possible to operate withshorter arcs (lower voltage) without unacceptable carbonpickup. Heating is normally made with a cos fi of 0.7–0.71with induction stirring, and with a cos fi of 0.78–0.81using porous plugs. This advantage makes it possible torun on a higher power with induction stirring compared togas stirring for the same refractory wear index. A higherpower will give a shorter heating time, which will minimisethe risk of dissolution of atomic nitrogen from the arczones when heating Al-killed steel. From our ABB LFinstallations we have learned that running on high powerand waiting gives a lower nitrogen level and a better ladle
life than running continuously on a lowerpower.
Buffer between steelplant andcaster Shorter heating times mean fewerdelays and hence potential for greateroutput. When combined with argon gasstirring, LF-EMS is also beneficial, becauseit opens the porous plug due to its goodtemperature homogenisation.
THIN SLAB CASTINGThin slab casting is increasingly becoming a viablealternative to conventional slab casting. Increasedemphasis, however, has to be placed on temperaturecontrol and homogeneity, steel cleanness (low oxygencontent) and low nitrogen content to guarantee productquality. LF-EMS is ideally suited to this process because ofits excellent control of these parameters. This has beenproved in steel plants operating with both LF-EMS in theladle furnace and thin slab casting.
CONCLUSIONSLF-EMS has excellent performance with respect todeoxidation, temperature control, chemistry analysiscontrol, alloy yield and the production of clean steel. Inorder to improve the desulphurisation performance LF-EMS can be complemented by argon gas stirring. LF-EMSis also an important tool for reliable and economic thinslab casting. MS
Zhu Miaoyong, North-eastern University, Shengyang, PR China, Cheng Nailiang, Shanghai Meishan Co Ltd,Baosteel Group, Nanjing, P. R. China and YangHongliang, ABB (China) Ltd, Beijing, PR China
CONTACT: [email protected]
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r Fig.9 Aluminium loss during stirring
r Fig.10 Carbon yield over total ladle furnace treatment
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