WO1989007499A1 - Superheating and microalloying of molten metal by contact with a plasma arc - Google Patents

Superheating and microalloying of molten metal by contact with a plasma arc Download PDF

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Publication number
WO1989007499A1
WO1989007499A1 PCT/AU1989/000057 AU8900057W WO8907499A1 WO 1989007499 A1 WO1989007499 A1 WO 1989007499A1 AU 8900057 W AU8900057 W AU 8900057W WO 8907499 A1 WO8907499 A1 WO 8907499A1
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WIPO (PCT)
Prior art keywords
plasma
stream
arc
molten metal
tundish
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Application number
PCT/AU1989/000057
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French (fr)
Inventor
Louis James Wibberley
Original Assignee
The Broken Hill Proprietary Company Limited
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Application filed by The Broken Hill Proprietary Company Limited filed Critical The Broken Hill Proprietary Company Limited
Publication of WO1989007499A1 publication Critical patent/WO1989007499A1/en
Priority to KR1019890701873A priority Critical patent/KR900700212A/en

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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22DCASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/10Supplying or treating molten metal
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22DCASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/10Supplying or treating molten metal
    • B22D11/11Treating the molten metal
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21CPROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0037Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00 by injecting powdered material
    • C21C7/0043Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00 by injecting powdered material into the falling stream of molten metal

Definitions

  • This invention relates to the treatment of molten metal and has particular but certainly not exclusive application to the treatment of molten steel held in a tundish, for example for feeding a continuous caster.
  • the invention is concerned with superheating molten metal and with making alloy additions to such metal.
  • metal superheat should be about 2-10°C at the submerged entry nozzle (SEN). Lower temperatures can cause SEN blocking and skull formation, whilst higher temperatures increase the cooling load on the mould and require slower casting speeds.
  • Tundish heating achieves closer temperature control than a ladle furnace due to its proximity to the mould, and may lessen the impact of some of the disadvantages of a ladle furnace.
  • Plasma heating by transferred arc to the exposed surface of the metal is disclosed in Tolve et al, 5th International Iron and Steel Congress, Steelmaking Proc. 69 (1986), 689, and is apparently 30-50% efficient. Temperature stratification and contamination by H, N (from atmospheric moisture and N 2 ) and entrainment of slag are envisaged due to high plasma temperatures, arc impaction and an exposed metal surface. Another system relying on plasma heating of the exposed metal surface is disclosed in Kubabara et al, "Development of Tundish Plasma Heater", Nippon Steel Company, presented at ISPC Tokyo 1987. Efficiency is apparently about 80%.
  • the Tolve et al paper also describes a method of alloying by aluminium wire through a hollow stopper rod, and by. cored CaSiBa wire spiralled directly into the liquid metal. Although the paper suggests the use of these techniques for other alloys, the problem of obtaining uniform distribution of high melting point alloys is not discussed.
  • tundish preheating include resistance heating, and submerged or enclosed combustion.
  • Electrical resistance heaters such as Mo ribbon, carbon or MoSi rods, are costly and operate most efficiently by radiative heat transfer directly to the exposed metal. Splash must be avoided. Mo ribbon and carbon must also operate in a protective argon atmosphere. Temperature stratification will also occur. Submerged combustion would cause excessive hydrogen pickup from the combustion gases and is inefficient due to only partial combustion to CO. Combustion in a submerged combustion tube is inefficient and limited by convective heat transfer.
  • the invention provides a method of treating, for example heating, molten metal comprising contacting a moving stream of the metal with plasma-arc.
  • the moving stream is preferably a falling stream of the metal.
  • the molten metal is steel, for example a stream of molten steel being teemed from a ladle into a tundish for a continuous caster.
  • the treatment may advantageously comprise superheating the molten metal by means of the arc plasma.
  • the plasma is preferably a transferred-arc plasma whereby the return electrode is disposed in contact with the molten metal downstream of the contact of the plasma with the stream.
  • Alloying elements may be incorporated into the plasma and thereby added to the stream of molten metal. Incorporation of the alloying elements is preferably achieved by injecting alloying material containing the elements through the centre of a hollow cathode which generates the arc plasma, or by providing the alloying material as a progressively consumed anode with respect to the plasma generating cathode.
  • the invention also provides apparatus for treating molten metal comprising a receptacle for molten metal, means defining a path for a stream of the molten metal being directed to the receptacle, and means for generating and contacting the stream with a plasma-arc.
  • the invention further provides apparatus for treating molten metal comprising means defining an enclosure for a moving stream of the molten metal, and means for generating and contacting the stream with a plasma-arc.
  • Figure 1 is a sectioned isometric and somewhat diagrammatic view of a ladle and tundish assembly fitted with apparatus according to the invention, for superheating molten steel before it is fed to a continuous caster;
  • Figure 2 is a fragmentary cross-section of a modification of the assembly of Figure 1, suitable where headroom at the site is limited;
  • Figure 4 is a graph of tundish operating conditions and the corresponding metal superheat and power requirement profiles for a superheat of about 30°C;
  • Figures 5A-5C are respective fragmentary cross-sections depicting alternative arrangements for incorporating alloying elements in the molten steel.
  • Figures 6A-6C are respective fragmentary cross-sections depicting alternative arrangements for incorporating alloying elements in the molten steel as it enters the submerged entry nozzle from the tundish.
  • the assembly.10 illustrated in Figure 1 includes a ladle 12 for molten steel with a lower nozzle 13 and associated slide gate 14, and a tundish 16 from which the molten metal is fed as a strand through a submerged entry nozzle (SEN) to a continuous caster (not shown).
  • a transferred-arc plasma unit or furnace 20 Fitted between outlet 14 and tundish 16, to the side of the tundish, is a transferred-arc plasma unit or furnace 20 with an electrode 28 for contacting the falling stream of molten metal (depicted for illustrative purposes at 22) with a plasma-arc 23 and thereby heating stream 22 as it falls from outlet 14 into the tundish 16, where it is temporarily retained as a pool feeding nozzle 18.
  • Furnace 20 consists of a refractory lined cylinder 26 forming an enclosure about a plasma chamber 27 about 600mm across.
  • Cylinder 26 may be 12mm stainless steel and the refractory lining a 125mm thick high alumina castable. Cooling may be provided but is usually unnecessary.
  • the cylinder 26 is of sufficient length for the minimum free length (shroud-path height) of stream 22 to be about 250mm to reduce the tendency for stream breakup at low metal flowrates.
  • a sliding cover (not shown) on plate furnace 20 forms a gastight seal with the housing 15 of ladle slide gate 14.
  • the cover is necessary to prevent contamination of the argon atmosphere in the furnace, and to protect the ladle slide gate mechanism from radiant heat from the arc and hot plasma gases.
  • the bottom rim of cylinder 26 is similarly gas-tight sealed with the tundish housing and with a baffle 17 in the tundish.
  • Each gas-tight seal may include a suitable gasket.
  • sealing of the bottom of the furnace 20 could be eliminated by immersing the lower end of the furnace into the metal pool in the tundish.
  • Figure 2 illustrates modified structure, suitable where headroom is limited, in which the electrode is incorporated into a lid 20' for the tundish, but is still directed at the metal stream.
  • Cathode 28 is a graphite rod, for example of 100mm diameter and 1200 mm long.
  • the cathode is conveniently hollow, e.g. a 10mm axial bore, to allow entry of argon plasma gas and, if required, an ignition device.
  • Argon flow should be kept to the minimum to control arc plasma shape. Excessive argon flow may cause undesirable turbulence in the main body of the tundish, and also increase toxic gas or fume emissions (0_, NO , Pb etc from the steel or impurities in the argon) .
  • the cathodes may be cooled, e.g. by water or compressed air if desired.
  • Electrode 28 is moveable pneumatically or electromechanically (e.g. using a rack and pinion mechanism not detailed) along a support mast 19 to three positions - fully retracted for electrode replacement or tundish exchange, standby position with the electrode advanced just inside the furnace wall, and ignition position and heating positions 0-200mm back from the stream.
  • the electrode is therefore slideable through a sealed port 21 in the wall 20a of furnace 20.
  • positive location of the electrode support mast 19 with respect to the port 21 is provided by a mounting bracket 19a for the mast secured to the outside of furnace 20.
  • the electrode is directed downward (e.g. 10-40°) at an acute ange to the direction of flow of stream 22, to help deflect steel splash and to allow arc contact with the bath during periods of unstable stream conditions.
  • Initial arc length can be estimated from the electrode length (determined by mast position at electrode entry point) and the estimated slide gate position, or, by advancing the electrode until the arc voltage drops to below 30V (assumed zero arc length) and then retracting 150mm.
  • Electrode 28 and the furnace 20 Electrical insulation betwen the electrode 28 and the furnace 20 is provided by the refractory lining of the furnace chamber.
  • a clearance at port 21 of 24-40mm is provided to prevent arcing between the electrode and hot interior refractory surfaces near the po ⁇ t.
  • Additional gas sealing is provided by 25mm thick flanges of kaowool insulation board.
  • External insulation and protection of the electrode may be provided by a vented enclosure or grill (not shown) : this would also prevent operator exposure to arc light in the event of an electrode breaking outside the heater.
  • Quick connect electrical and water supplies are provided to enable removal of the heater during tundish changes, typically after 5-8 heats, or for grade changes. This allows any necessary patching of refractories and servicing of electrodes (resetting or replacement), and replacement of the sealing gaskets.
  • the return current path is provided by an in situ immersed anode 36 of J-configuration slung on the wall of the tundish 16 at each end of the tundish.
  • Each anode 36 is a mild steel plate sandwiched between the existing tundish refractories 35 and an insulating board 38. The lower end of the anode is bent up at 37 to reduce erosion and the tip encased in castable 39.
  • Alternative return electrode arrangements include insitu electrodes in the wall of the tundish, a separate arcing return electrode, a dipped pipe protected by skulling, a mould return (i.e through the continuous cast mould lubricant and strand - mould contacts, with earth straps if necessary to protect load cell structures from stray currents), a mould return with a separate return roll or slide to share the current load, and an in situ ladle return electrode seated in the floor of the ladle.
  • Heat is transferred to the metal stream by direct arc impingement and radiation, this ratio varying mainly with arc length.
  • furnace 20 about 60-70% of the energy is thought to be transferred by direct impingement, with only 30-40% transferred by direct radiation from the plasma and multiple reflection from the refractories to the stream and the metal pool in the tundish.
  • the electrodes may be powered by an AC or a DC system but a DC system is presently favoured as it offers improved control, longer electrode life, and higher efficiency over that for an AC system.
  • Figure 3 is a schematic diagram for the power supply.
  • the preferred method of power control is by varying the current.
  • the arc voltage should remain relatively constant and independent of current, at around 120-150V for argon support gas.
  • the flat voltage characteristic is due to the increase in arc area and temperature (i.e reduced arc resistance) with increased current. Therefore, stream temperature increase should be approximately linear to the furnace current. Operation and current control would be determined by a combination of slide gate opening, ladle and tundish load cells, and tundish thermocouples.
  • the ladle 12 is lowered over the tundish and nozzle 13 located through the furnace cover plate.
  • the slide gates are opened to commence teeming and the electrode 28 advanced on its mast 19 tothe standby position. If temperature measurements indicate low steel temperature, the electrode is slowly advanced toward the stream 22 up to the estimated stream centre line until ingnition occurs.
  • the starting power is set at, e.g. about 150KW, with an open circuit voltage limit of about 200V. Current control may alternately be used for starting.
  • the electrode After ignition of the arc, the electrode is retracted to give an estimated arc length of, say, about 150mm. Power is increased to give the required temperature rise in the steel. Monitoring and control is continued through the duration of ladle run out.
  • Figure 4 shows typical tundish operating conditions (curves C and D) , and the corresponding power profile (curve A) required to maintain an ideal metal superheat (curve B) of about 30°C.
  • a modified assembly (not illustrated) , plural electrodes are provided, equi-angularly spaced about the metal stream.
  • the furnace may be conveniently annular and there may be, for example, three electrodes offset at different vertical heights relative to the direction of metal flow.
  • Such a configuration may be advantageous by maximising the area of the stream under arc impact and to reduce refractory stress from arc radiation.
  • Alloy additions vary from 0.2kg/t for microalloys such as Nb to lOkg/t for Mn.
  • the vaporisation power required is highest for metallic molybdenum, about 1.5MW.
  • complete vaporisation in the plasma is only required for extremely rapid dissolution, such as required for the strand additions discussed below. Melting and some degree of atomisation should be sufficient to ensure rapid dissolution of most alloys.
  • the alloying mechanism is envisaged to be as follows:
  • Figure 5A shows a hollow cathode, e.g a hollow carbon cathode, for which the simplest arrangement would involve injecting the alloys through the centre of the electrode. If wire feeding is used, as is illustrated, the wire 40 would be maintained at electrode potential. As an arc will be present at all times (from the electrode carbon), good control and stability is ensured.
  • the wire spool should be maintained at an elevated potential (about 50V), although this could be avoided by maintaining the wire at earth potential and forming the arc directly between the cathode and the wire: however, this would require a more complex electrode involving an insulated central wire guide.
  • Figure 5B depicts a plasma powder torch 42.
  • specific metal vaporisation rates range from 50-100g/MJ, with efficiencies of 70-85%.
  • the main loss of efficiency is due to the high argon flows required for non-transferred arc plasmas and water cooling of the torch.
  • this torch has a lower efficiency, it provides a modular/self contained device and does not require a return electrode.
  • the main limitation with this device is the shorter residence time of the solids in the plasma, which would probably restrict its use to either fine wires or powders.
  • FIG. 5C The use of a consumable wire electrode 44 is shown in Figure 5C.
  • Heavy wires may be added by making the wire the anode, and feeding directly into the metal pool in the tundish. As relatively large droplets are formed by wire melting, this technique may not be suitable for alloys with high melting points (e.g. ferromolybdenum) or low solubility (e.g. lead).
  • the main engineering problem with this technique is the requirement to have the wire feeder and spool at about 50V above ground potential.
  • the presently preferred option is to use the hollow carbon cathode or the consumable wire-electrode method. These enable higher injection rates and the use of larger wires than possible with non-transferred arc torches, as the residence time of particles in the transferred arc is about three orders of magnitude greater than that in a powder torch (l-5s versus * lms), and with wire feeding, direct arc impingement gives much higher heat fluxes.
  • the ability to produce metal fog from alloys, including the high melting point elements, may allow individual strand alloying at the submerged entry nozzle 18. This operation would be particularly beneficial for the production of small tonnage, value-added grades, and leaded steels, where direct strand injection would obviate the environmental precautions required for refurbishing tundishes and eliminate the problems of lead segregation to the floor of the tundish.
  • Figure 6A depicts a submerged arc injector 50.
  • This simple arrangement uses a consumable wire electrode 52.
  • the alloy wire 52 is melted by a transferred arc with the liquid steel in the recess beneath the injector.
  • a steeply drooping power supply would be used to maintain a low current arc with the carbon wire guide during low wire feed rates (i.e current decreases rapidly with increasing voltage) .
  • the main problem with this arrangement is the need to maintain the wire and spool at about 20-50V above ground.
  • a plasma torch injector is shown in Figure 6B'.
  • a small (about lOOkW) plasma torch 54 is located in the top of a large bore refractory injection lance 56.
  • the alloy material is introduced into the hot plasma gases as either fine wire or powders.
  • the molten, partially vaporised material then travels down the injector under gravity in the plasma gases.
  • the power input would be varied to maintain the inside of the probe at about 1600°C to prevent skulling - internally and at the outlet.
  • a higher argon flow is required, and the method is not suited for rapidly melting large diameter wires.
  • Figure 6C shows an arc-ultrasonic atomising injector.
  • This arrangement involves arc melting of a rapidly vibrating alloy wire 58. The purpose is to utilise the rapid heating of a transferred arc, with the droplet size controlled by ultrasonic atomisation. Droplets are torn from the liquid film formed by arc heating, rather than dripping from the wire under gravity. Radio frequency heating could also be used.
  • strand injection according to the invention would overcome the problems of lead addition in particular by providing extremely fine lead droplets and an even dispersion in the steel entering the strand, and so avoiding the formation of lead stringers in the end product.
  • the sealed environment at the point of addition also minimises environmental problems, a special concern with lead.
  • the most suitable device for strand leading may be a piezoelectric or magnetostrictive atomiser which can produce a fine, narrow droplet size distribution.
  • the simplest method is to provide liquid lead to the atomiser.
  • the arc-ultrasonic atomiser of Figure 6C could be used.
  • Powdered lead or lead compounds may be introduced either by direct addition or by plasma torch. The advantage of the plasma torch is that larger sized powders may be used and improved dispersion would be obtained. Powders may include lead concentrates, sulphides, carbonates or metallic lead.
  • the method is capable of readily providing a temperature increase of up to 20°C, even 30°, in the metal entering the tundish. It permits accurate addition of refractory and toxic alloys by wire, shot or powder feeding, yet contamination of the teeming stream is prevented.
  • the equipment is non-intrusive to the already busy tundish area, may be removed and serviced with normal tundish changes, and is non capital-intensive and economic to operate.
  • Strand injection uses external heating involving arc or radio frequency means to produce finely divided alloy particles capable of rapid dissolution and uniform mixing into the steel entering the SEN. Further dispersion of the alloying material is obtained by ultrasonic vibrations or in the case of lead injection, high pressure liquid lead from an external dispenser.

Abstract

A method of treating high melting point metals in the form of a moving stream with a plasma arc is disclosed. The method has particular application to the superheating and/or microalloying of molten steel at the tertiary stage i.e., during teeming from the ladle (12) into a tundish (16), prior to its entry into a continuous casting machine. The transferred plasma arc (23) is struck between the cathode (28) [and microalloy (40, 42, 44)] and the teeming stream (22), thus superheating the molten steel. If added, the microalloy is dispersed as a fog, by melting, partial vapourisation and condensation and drawn down by the teeming stream (22) into the bath of molten steel in the tundish (16), where it is alloyed by the intense mixing that occurs. Microalloying with a transferred plasma arc and microalloy (52, 56, 58) submerged below the surface of the turbulent molten steel is also disclosed as an alternative. Apparatus for carrying out the method consists of a tundish (16), a ladle nozzle (13) which defines the teeming stream (22), a cathode (28) mounted adjacent to the teeming stream (22)/bath and associated electrical circuitry, as shown in Figure 3. A protective refractory hood (50, 56) for the cathode is required for a submerged plasma arc. A return electrode is positioned in the tundish, (so as to dip into the molten steel), downstream from the cathode.

Description

S.UPERHEATINGANDMICROALLOYINGOFMOLTEN METALBYCONTACTWITHAPLASMAARC.
FIELD OF THE INVENTION
This invention relates to the treatment of molten metal and has particular but certainly not exclusive application to the treatment of molten steel held in a tundish, for example for feeding a continuous caster. In particular preferred aspects, the invention is concerned with superheating molten metal and with making alloy additions to such metal.
BACKGROUND ART
With continuous casting, efforts by steelmakers to ensure accurate temperature, composition and inclusion shape control are easily negated by the long standing time in the ladle. Problems include ladle temperature stratification, aluminium fade, decreased recovery of alloys, -and inclusion formation due to interaction with refractories and slag.
An additional requirement of continuous casting is the need for higher tap temperatures, up to 70°C above that required for ingot steel, made necessary to ensure sufficient thermal reserve for ladle treatment, losses to the tundish and possible longer standing times. The higher tap temperatures cause a marked increase in converter refractory wear and also increase the flux requirements for phosphorus control.
A further difficulty with continuous casting is the initial tundish temperature dip, and temperature sag during the cast. Ideally, metal superheat should be about 2-10°C at the submerged entry nozzle (SEN). Lower temperatures can cause SEN blocking and skull formation, whilst higher temperatures increase the cooling load on the mould and require slower casting speeds.
Although part of the temperature problem can be offset by use of a ladle furnace, this equipment is capital intensive, creates large shifts in the base power requirements of the plant (30-40MW), greatly reduces ladle life, and does not overcome the problem of temperature sag during casting.
The most satisfactory solution to these problems to date has been to provide facilities for heating (5-20°C temperature increase) and tertiary alloying at the tundish, the intermediate receptacle to which the steel is delivered f om the ladle, and from which it passes to the continuous caster via the SEN. Tundish heating achieves closer temperature control than a ladle furnace due to its proximity to the mould, and may lessen the impact of some of the disadvantages of a ladle furnace.
Two methods of in-cast tundish heating have been cited in recent literature - by radiofrequency (RF) heating and by transferred plasma-arc to the top of the tundish.
RF heating, described in Mabuchi et al, 5th International Iron and Steel Congress, Steelmaking Proc. 69 (1986), 737, is capital intensive, requiring a non-suscepting stainless steel tundish or tundish modifications. Heating efficiency ranges from 50-85% depending on configuration.
Plasma heating by transferred arc to the exposed surface of the metal is disclosed in Tolve et al, 5th International Iron and Steel Congress, Steelmaking Proc. 69 (1986), 689, and is apparently 30-50% efficient. Temperature stratification and contamination by H, N (from atmospheric moisture and N2) and entrainment of slag are envisaged due to high plasma temperatures, arc impaction and an exposed metal surface. Another system relying on plasma heating of the exposed metal surface is disclosed in Kubabara et al, "Development of Tundish Plasma Heater", Nippon Steel Company, presented at ISPC Tokyo 1987. Efficiency is apparently about 80%. Although contamination is prevented by the use of a hood over the plasma torch, to provide steel temperature homogeneity, and to prevent superheating of the tundish slag layer and rapid refractory wear, this system has been used only in small shallow tundishs, apparently less than 15 tonne, and inert gas stirring of the metal below the zone of arc impingment is recommended by the manufacturers.
The Tolve et al paper also describes a method of alloying by aluminium wire through a hollow stopper rod, and by. cored CaSiBa wire spiralled directly into the liquid metal. Although the paper suggests the use of these techniques for other alloys, the problem of obtaining uniform distribution of high melting point alloys is not discussed.
Other possible approaches to tundish preheating include resistance heating, and submerged or enclosed combustion. Electrical resistance heaters, such as Mo ribbon, carbon or MoSi rods, are costly and operate most efficiently by radiative heat transfer directly to the exposed metal. Splash must be avoided. Mo ribbon and carbon must also operate in a protective argon atmosphere. Temperature stratification will also occur. Submerged combustion would cause excessive hydrogen pickup from the combustion gases and is inefficient due to only partial combustion to CO. Combustion in a submerged combustion tube is inefficient and limited by convective heat transfer.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a treatment of molten metal which may be utilised for improved heating and/or tertiary alloying of molten steel as or before it is fed to a continuous caster.
The invention provides a method of treating, for example heating, molten metal comprising contacting a moving stream of the metal with plasma-arc.
The moving stream is preferably a falling stream of the metal. In one advantageous application, the molten metal is steel, for example a stream of molten steel being teemed from a ladle into a tundish for a continuous caster.
The treatment may advantageously comprise superheating the molten metal by means of the arc plasma. The plasma is preferably a transferred-arc plasma whereby the return electrode is disposed in contact with the molten metal downstream of the contact of the plasma with the stream.
Alloying elements may be incorporated into the plasma and thereby added to the stream of molten metal. Incorporation of the alloying elements is preferably achieved by injecting alloying material containing the elements through the centre of a hollow cathode which generates the arc plasma, or by providing the alloying material as a progressively consumed anode with respect to the plasma generating cathode. The invention also provides apparatus for treating molten metal comprising a receptacle for molten metal, means defining a path for a stream of the molten metal being directed to the receptacle, and means for generating and contacting the stream with a plasma-arc.
The invention further provides apparatus for treating molten metal comprising means defining an enclosure for a moving stream of the molten metal, and means for generating and contacting the stream with a plasma-arc.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 is a sectioned isometric and somewhat diagrammatic view of a ladle and tundish assembly fitted with apparatus according to the invention, for superheating molten steel before it is fed to a continuous caster;
Figure 2 is a fragmentary cross-section of a modification of the assembly of Figure 1, suitable where headroom at the site is limited;
Figure 3 is a simple block diagram of the power supply for the plasma-arc heating unit in Figure 1;
Figure 4 is a graph of tundish operating conditions and the corresponding metal superheat and power requirement profiles for a superheat of about 30°C;
Figures 5A-5C are respective fragmentary cross-sections depicting alternative arrangements for incorporating alloying elements in the molten steel; and
Figures 6A-6C are respective fragmentary cross-sections depicting alternative arrangements for incorporating alloying elements in the molten steel as it enters the submerged entry nozzle from the tundish.
BEST MODES OF CARRYING OUT THE INVENTION
The assembly.10 illustrated in Figure 1 includes a ladle 12 for molten steel with a lower nozzle 13 and associated slide gate 14, and a tundish 16 from which the molten metal is fed as a strand through a submerged entry nozzle (SEN) to a continuous caster (not shown). Fitted between outlet 14 and tundish 16, to the side of the tundish, is a transferred-arc plasma unit or furnace 20 with an electrode 28 for contacting the falling stream of molten metal (depicted for illustrative purposes at 22) with a plasma-arc 23 and thereby heating stream 22 as it falls from outlet 14 into the tundish 16, where it is temporarily retained as a pool feeding nozzle 18.
Furnace 20 consists of a refractory lined cylinder 26 forming an enclosure about a plasma chamber 27 about 600mm across. Cylinder 26 may be 12mm stainless steel and the refractory lining a 125mm thick high alumina castable. Cooling may be provided but is usually unnecessary. The cylinder 26 is of sufficient length for the minimum free length (shroud-path height) of stream 22 to be about 250mm to reduce the tendency for stream breakup at low metal flowrates.
A sliding cover (not shown) on plate furnace 20 forms a gastight seal with the housing 15 of ladle slide gate 14. The cover is necessary to prevent contamination of the argon atmosphere in the furnace, and to protect the ladle slide gate mechanism from radiant heat from the arc and hot plasma gases. The bottom rim of cylinder 26 is similarly gas-tight sealed with the tundish housing and with a baffle 17 in the tundish. Each gas-tight seal may include a suitable gasket. In an alternate arrangement, sealing of the bottom of the furnace 20 could be eliminated by immersing the lower end of the furnace into the metal pool in the tundish.
Figure 2 illustrates modified structure, suitable where headroom is limited, in which the electrode is incorporated into a lid 20' for the tundish, but is still directed at the metal stream.
Cathode 28 is a graphite rod, for example of 100mm diameter and 1200 mm long. The cathode is conveniently hollow, e.g. a 10mm axial bore, to allow entry of argon plasma gas and, if required, an ignition device. Argon flow should be kept to the minimum to control arc plasma shape. Excessive argon flow may cause undesirable turbulence in the main body of the tundish, and also increase toxic gas or fume emissions (0_, NO , Pb etc from the steel or impurities in the argon) . The cathodes may be cooled, e.g. by water or compressed air if desired.
Electrode 28 is moveable pneumatically or electromechanically (e.g. using a rack and pinion mechanism not detailed) along a support mast 19 to three positions - fully retracted for electrode replacement or tundish exchange, standby position with the electrode advanced just inside the furnace wall, and ignition position and heating positions 0-200mm back from the stream. The electrode is therefore slideable through a sealed port 21 in the wall 20a of furnace 20. To prevent damage and to facilitate electrode entry into the furnace chamber, positive location of the electrode support mast 19 with respect to the port 21 is provided by a mounting bracket 19a for the mast secured to the outside of furnace 20.
The electrode is directed downward (e.g. 10-40°) at an acute ange to the direction of flow of stream 22, to help deflect steel splash and to allow arc contact with the bath during periods of unstable stream conditions.
Initial arc length can be estimated from the electrode length (determined by mast position at electrode entry point) and the estimated slide gate position, or, by advancing the electrode until the arc voltage drops to below 30V (assumed zero arc length) and then retracting 150mm.
As the arc length will vary with slide gate position and electrode wear (about lOOmm/h at maximum output) periodic resetting of the electrode position may be necessary.
Electrical insulation betwen the electrode 28 and the furnace 20 is provided by the refractory lining of the furnace chamber. A clearance at port 21 of 24-40mm is provided to prevent arcing between the electrode and hot interior refractory surfaces near the poτt. Additional gas sealing is provided by 25mm thick flanges of kaowool insulation board. External insulation and protection of the electrode may be provided by a vented enclosure or grill (not shown) : this would also prevent operator exposure to arc light in the event of an electrode breaking outside the heater.
Quick connect electrical and water supplies are provided to enable removal of the heater during tundish changes, typically after 5-8 heats, or for grade changes. This allows any necessary patching of refractories and servicing of electrodes (resetting or replacement), and replacement of the sealing gaskets.
The return current path is provided by an in situ immersed anode 36 of J-configuration slung on the wall of the tundish 16 at each end of the tundish. Each anode 36 is a mild steel plate sandwiched between the existing tundish refractories 35 and an insulating board 38. The lower end of the anode is bent up at 37 to reduce erosion and the tip encased in castable 39. Alternative return electrode arrangements, not illustrated, include insitu electrodes in the wall of the tundish, a separate arcing return electrode, a dipped pipe protected by skulling, a mould return (i.e through the continuous cast mould lubricant and strand - mould contacts, with earth straps if necessary to protect load cell structures from stray currents), a mould return with a separate return roll or slide to share the current load, and an in situ ladle return electrode seated in the floor of the ladle.
Heat is transferred to the metal stream by direct arc impingement and radiation, this ratio varying mainly with arc length. In furnace 20, about 60-70% of the energy is thought to be transferred by direct impingement, with only 30-40% transferred by direct radiation from the plasma and multiple reflection from the refractories to the stream and the metal pool in the tundish.
The electrodes may be powered by an AC or a DC system but a DC system is presently favoured as it offers improved control, longer electrode life, and higher efficiency over that for an AC system. Figure 3 is a schematic diagram for the power supply.
The preferred method of power control is by varying the current. The arc voltage should remain relatively constant and independent of current, at around 120-150V for argon support gas. The flat voltage characteristic is due to the increase in arc area and temperature (i.e reduced arc resistance) with increased current. Therefore, stream temperature increase should be approximately linear to the furnace current. Operation and current control would be determined by a combination of slide gate opening, ladle and tundish load cells, and tundish thermocouples.
A typical operating sequence for the assembly 10 of Figure 1 will now be described. The ladle 12 is lowered over the tundish and nozzle 13 located through the furnace cover plate. The slide gates are opened to commence teeming and the electrode 28 advanced on its mast 19 tothe standby position. If temperature measurements indicate low steel temperature, the electrode is slowly advanced toward the stream 22 up to the estimated stream centre line until ingnition occurs. The starting power is set at, e.g. about 150KW, with an open circuit voltage limit of about 200V. Current control may alternately be used for starting.
After ignition of the arc, the electrode is retracted to give an estimated arc length of, say, about 150mm. Power is increased to give the required temperature rise in the steel. Monitoring and control is continued through the duration of ladle run out.
Figure 4 shows typical tundish operating conditions (curves C and D) , and the corresponding power profile (curve A) required to maintain an ideal metal superheat (curve B) of about 30°C.
In a modified assembly (not illustrated) , plural electrodes are provided, equi-angularly spaced about the metal stream. The furnace may be conveniently annular and there may be, for example, three electrodes offset at different vertical heights relative to the direction of metal flow. Such a configuration may be advantageous by maximising the area of the stream under arc impact and to reduce refractory stress from arc radiation.
The absence of air and slag, and the high plasma temperatures and intense mixing in the tundish make the stream furnace 20 an ideal location for microalloying e.g with Al, B, Cr, Mn, Mo, Nb, Ni, Pb, Ti, and V. Accurate carbon trimming could also be achieved under these conditions.
In this context, the plasma-arc heating so far described can:-
- provide rapid melting of large alloy material, e.g sheathed wires, therefore preventing the formation of a skull around the wire,
- offset the chilling effect of adding cold alloys, and
- create a metal fog from the alloy - which is important for slow dissolving lead or for high melting point additions such as ferroniobium (Tm 1530°C), ferrovanadium (Tm 1770°C) and ferromolybdenum (Tm 1900°C).
Alloy additions vary from 0.2kg/t for microalloys such as Nb to lOkg/t for Mn. The vaporisation power required is highest for metallic molybdenum, about 1.5MW. However, complete vaporisation in the plasma is only required for extremely rapid dissolution, such as required for the strand additions discussed below. Melting and some degree of atomisation should be sufficient to ensure rapid dissolution of most alloys.
The alloying mechanism is envisaged to be as follows:
(a.) Melting and partial vaporisation by plasma heating.
(b.) Condensation of vapours and coalescence to form submicron alloy fog.
(c.) Entrainment of droplets, fog, vapour and any larger particles by the teem steam.
(d.) Rapid dissolution and dispersion by the intense mixing in the tundish pouring box.
(e.) Float or sink-out of any remaining large particles (floats Ti; sinks Ni, Cr, Mo, Pb) followed by slower dissolution. Three alternative arrangements for introducing alloying materials are depicted in Figures 5A to 5C. Figure 5A shows a hollow cathode, e.g a hollow carbon cathode, for which the simplest arrangement would involve injecting the alloys through the centre of the electrode. If wire feeding is used, as is illustrated, the wire 40 would be maintained at electrode potential. As an arc will be present at all times (from the electrode carbon), good control and stability is ensured. The wire spool should be maintained at an elevated potential (about 50V), although this could be avoided by maintaining the wire at earth potential and forming the arc directly between the cathode and the wire: however, this would require a more complex electrode involving an insulated central wire guide.
Figure 5B depicts a plasma powder torch 42. For non-transferred arc plasmas, specific metal vaporisation rates range from 50-100g/MJ, with efficiencies of 70-85%. The main loss of efficiency is due to the high argon flows required for non-transferred arc plasmas and water cooling of the torch. Although this torch has a lower efficiency, it provides a modular/self contained device and does not require a return electrode. The main limitation with this device is the shorter residence time of the solids in the plasma, which would probably restrict its use to either fine wires or powders.
The use of a consumable wire electrode 44 is shown in Figure 5C. Heavy wires may be added by making the wire the anode, and feeding directly into the metal pool in the tundish. As relatively large droplets are formed by wire melting, this technique may not be suitable for alloys with high melting points (e.g. ferromolybdenum) or low solubility (e.g. lead). The main engineering problem with this technique is the requirement to have the wire feeder and spool at about 50V above ground potential.
From the available information, the presently preferred option is to use the hollow carbon cathode or the consumable wire-electrode method. These enable higher injection rates and the use of larger wires than possible with non-transferred arc torches, as the residence time of particles in the transferred arc is about three orders of magnitude greater than that in a powder torch (l-5s versus *lms), and with wire feeding, direct arc impingement gives much higher heat fluxes.
The ability to produce metal fog from alloys, including the high melting point elements, may allow individual strand alloying at the submerged entry nozzle 18. This operation would be particularly beneficial for the production of small tonnage, value-added grades, and leaded steels, where direct strand injection would obviate the environmental precautions required for refurbishing tundishes and eliminate the problems of lead segregation to the floor of the tundish.
Successful strand alloying, especially for the more refractory elements, requires introducing the alloy in a finely divided form, say "lOOμm. This requires atomisation of the liquid alloy, or the formation of metal fog by the vaporisation-condensation mechanism. Because of the short residence time in the injector, both require high heat transfer rates.
It is proposed to introduce the molten alloying material as a fog or mist via a vertical lance located above the strand outgate from tundish 16 at nozzle 18, using a "strand injector". Three possible arrangements are shown in Figures 6A-6C, using both transferred arc and plasma torch heating.
Figure 6A depicts a submerged arc injector 50. This simple arrangement uses a consumable wire electrode 52. The alloy wire 52 is melted by a transferred arc with the liquid steel in the recess beneath the injector. A steeply drooping power supply would be used to maintain a low current arc with the carbon wire guide during low wire feed rates (i.e current decreases rapidly with increasing voltage) . The main problem with this arrangement is the need to maintain the wire and spool at about 20-50V above ground.
A plasma torch injector is shown in Figure 6B'. In this arrangement, a small (about lOOkW) plasma torch 54 is located in the top of a large bore refractory injection lance 56. The alloy material is introduced into the hot plasma gases as either fine wire or powders. The molten, partially vaporised material then travels down the injector under gravity in the plasma gases. The power input would be varied to maintain the inside of the probe at about 1600°C to prevent skulling - internally and at the outlet. A higher argon flow is required, and the method is not suited for rapidly melting large diameter wires.
Figure 6C shows an arc-ultrasonic atomising injector. This arrangement involves arc melting of a rapidly vibrating alloy wire 58. The purpose is to utilise the rapid heating of a transferred arc, with the droplet size controlled by ultrasonic atomisation. Droplets are torn from the liquid film formed by arc heating, rather than dripping from the wire under gravity. Radio frequency heating could also be used.
It is expected that strand injection according to the invention would overcome the problems of lead addition in particular by providing extremely fine lead droplets and an even dispersion in the steel entering the strand, and so avoiding the formation of lead stringers in the end product. The sealed environment at the point of addition also minimises environmental problems, a special concern with lead.
The most suitable device for strand leading may be a piezoelectric or magnetostrictive atomiser which can produce a fine, narrow droplet size distribution. The simplest method is to provide liquid lead to the atomiser. However, if steel sheathed lead wire is to be used, the arc-ultrasonic atomiser of Figure 6C could be used. Powdered lead or lead compounds may be introduced either by direct addition or by plasma torch. The advantage of the plasma torch is that larger sized powders may be used and improved dispersion would be obtained. Powders may include lead concentrates, sulphides, carbonates or metallic lead.
By way of conclusion, it is proposed to summarize the principal advantages of the inventive method as applies to the treatment of molten steel as it is fed from a ladle to a tundish of a continuous caster. Foremost, the method is capable of readily providing a temperature increase of up to 20°C, even 30°, in the metal entering the tundish. It permits accurate addition of refractory and toxic alloys by wire, shot or powder feeding, yet contamination of the teeming stream is prevented. The equipment is non-intrusive to the already busy tundish area, may be removed and serviced with normal tundish changes, and is non capital-intensive and economic to operate.
Strand injection uses external heating involving arc or radio frequency means to produce finely divided alloy particles capable of rapid dissolution and uniform mixing into the steel entering the SEN. Further dispersion of the alloying material is obtained by ultrasonic vibrations or in the case of lead injection, high pressure liquid lead from an external dispenser.

Claims

1. A method of treating molten metal comprising contacting a moving stream of the metal with a plasma-arc.
2. A method according to claim 1 comprising heating said molten metal stream with the plasma-arc.
3. A method according to claim 1 or 2 wherein said moving stream is a falling stream of the metal.
4. A method according to claim 1, 2 or 3 wherein said molten metal is steel.
5. A method according to claim 1 for superheating molten steel in a tundish, comprising plasma-arc heating the stream of molten steel as it is teemed from a ladle into the tundish.
6. A method according to any preceding claim wherein said plasma is a transferred-arc plasma whereby a return electrode is disposed in contact with the molten metal downstream of the contact of the plasma with the stream.
7. A method according to any preceding claim wherein said plasma-arc is directed into contact with said stream at an acute angle to the direction of flow of said steel.
8. A method according to any preceding claim further comprising adding alloying elements to the plasma-arc and thereby to the stream of molten metal.
9. A method according to claim 8 wherein said alloying elements are added by injecting alloying material containing said elements through the centre of a hollow cathode which generates the plasma-arc.
10. A method according to claim 8 wherein said alloying elements are added by providing alloying material containing said elements as a progressively consumed electrode.
11. Apparatus for treating molten metal comprising a receptacle for molten metal, means defining a path for a stream of the molten metal being directed to said receptacle and means for generating and contacting said falling stream with a plasma-arc.
12. Apparatus according to claim 11 further comprising a ladle from which said molten metal may be teemed as said stream, and wherein said receptacle is a tundish for temporary retention of said metal.
13. Apparatus according to claim 11 or 12 wherein said path defining means comprises an enclosure for said stream, mounted to said receptacle.
14. Apparatus according to claim 11 or 12 wherein said path defining means comprises lid means for the receptacle having an opening for said stream.
15. Apparatus according to any one of claims 11 to 14 wherein said plasma-arc generating means comprises a first electrode for directing said plasma-arc into contact with said stream as a transferred-arc plasma, and a second electrode disposed in contact with said molten metal in said tundish.
16. Apparatus according to claim 15 wherein said first electrode is arranged to direct said plasma-arc into contact with said falling stream at an acute angle to the direction of flow of the stream.
17. Apparatus according to claim 15 or 16 wherein said first electrode is an elongate electrode moveable to and from an operative position with respect to said falling stream.
18.. Apparatus according to any one of claims 15 to 17 wherein said first electrode is a cylindrical carbon electrode having an axial bore for admission of plasma gas
19. Apparatus according to any one of claims 11 to 18 further comprising means to add alloying elements to the plasma-arc and thereby to the stream of molten metal.
20. Apparatus according to claim 19 wherein said adding means comprises a progressively consumed electrode of alloying material containing said alloying elements.
21. Apparatus for treating molten metal comprising means defining an enclosure for a moving stream of the molten metal, and means for generating and contacting said stream with a plasma-arc.
22. Apparatus according to claim 11 wherein said plasma-arc generating means is arranged to generate a plasma-arc effective to heat said molten metal.
23. A method of adding an alloying element to molten steel, comprising entraining particles of the alloying element in an arc plasma and contacting the plasma with the molten steel.
24. A method according to claim 23 wherein the plasma contacts themolten steel at the top of a strand of the steel being directed into a continuous caster.
PCT/AU1989/000057 1988-02-09 1989-02-09 Superheating and microalloying of molten metal by contact with a plasma arc WO1989007499A1 (en)

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GB2240499A (en) * 1990-02-05 1991-08-07 Labate Michael D Method for producing clean steel for continuous casting
FR2666819A1 (en) * 1990-09-19 1992-03-20 Inst Aluminievoi Magnievoi METHOD AND DEVICE FOR MANUFACTURING A COMPOSITE MATERIAL FROM A BASE METAL.
WO1995029022A1 (en) * 1994-04-26 1995-11-02 Mannesmann Ag Method and device for heating a metal melt
WO2001092585A2 (en) * 2000-06-01 2001-12-06 L'air Liquide Societe Anonyme Pour L'etude Et, L'exploitation Des Procedes Georges Claude Method and apparatus for treatment of metal cascades in flame and gas
WO2003008143A1 (en) * 2001-07-18 2003-01-30 Netanya Plasmatec Ltd. Riser(s) size reduction and/or metal quality improving in gravity casting of shaped products by moving electric arc
US7243701B2 (en) 2000-12-12 2007-07-17 Netanya Plasmatec Ltd. Treating molten metals by moving electric arc
US9156053B2 (en) 2011-10-27 2015-10-13 Graco Minnesota Inc. Melter
US9174231B2 (en) 2011-10-27 2015-11-03 Graco Minnesota Inc. Sprayer fluid supply with collapsible liner
US9796492B2 (en) 2015-03-12 2017-10-24 Graco Minnesota Inc. Manual check valve for priming a collapsible fluid liner for a sprayer
JP2018034180A (en) * 2016-08-31 2018-03-08 新日鐵住金株式会社 In-tundish molten steel heating method and tundish plasma heating device
JP2020011288A (en) * 2018-07-20 2020-01-23 日本製鉄株式会社 Apparatus and method for in-tundish alloy addition
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Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2240499B (en) * 1990-02-05 1993-08-11 Labate Michael D Method for producing clean steel
GB2240499A (en) * 1990-02-05 1991-08-07 Labate Michael D Method for producing clean steel for continuous casting
FR2666819A1 (en) * 1990-09-19 1992-03-20 Inst Aluminievoi Magnievoi METHOD AND DEVICE FOR MANUFACTURING A COMPOSITE MATERIAL FROM A BASE METAL.
WO1995029022A1 (en) * 1994-04-26 1995-11-02 Mannesmann Ag Method and device for heating a metal melt
WO2001092585A2 (en) * 2000-06-01 2001-12-06 L'air Liquide Societe Anonyme Pour L'etude Et, L'exploitation Des Procedes Georges Claude Method and apparatus for treatment of metal cascades in flame and gas
WO2001092585A3 (en) * 2000-06-01 2002-05-30 Air Liquide Method and apparatus for treatment of metal cascades in flame and gas
US7243701B2 (en) 2000-12-12 2007-07-17 Netanya Plasmatec Ltd. Treating molten metals by moving electric arc
WO2003008143A1 (en) * 2001-07-18 2003-01-30 Netanya Plasmatec Ltd. Riser(s) size reduction and/or metal quality improving in gravity casting of shaped products by moving electric arc
US9156053B2 (en) 2011-10-27 2015-10-13 Graco Minnesota Inc. Melter
US9174231B2 (en) 2011-10-27 2015-11-03 Graco Minnesota Inc. Sprayer fluid supply with collapsible liner
US9796492B2 (en) 2015-03-12 2017-10-24 Graco Minnesota Inc. Manual check valve for priming a collapsible fluid liner for a sprayer
US10315787B2 (en) 2015-03-12 2019-06-11 Graco Minnesota Inc. Manual check valve for priming a collapsible fluid liner for a sprayer
JP2018034180A (en) * 2016-08-31 2018-03-08 新日鐵住金株式会社 In-tundish molten steel heating method and tundish plasma heating device
JP2020011288A (en) * 2018-07-20 2020-01-23 日本製鉄株式会社 Apparatus and method for in-tundish alloy addition
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