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(11) | EP 0 453 188 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Plasma tundish heating |
| (57) A method of heating molten material in a tundish employs a plasma torch. The plasma
comprises a mixture of argon and one or more of nitrogen, hydrogen, neon and helium.
A mixture of argon and helium including from 10 to 20% by volume of helium is useful
for heating steel. For an arc of given length, a higher voltage is obtainable than
when pure argon is used as the plasma gas. The torch is preferably engaged by a horizontal
support arm which is pivotally mounted on an extensible support so as to facilitate
location of the torch in a limited space above the tundish. |
[0001] This invention relates to a method and apparatus for heating. In particular, it relates to the heating of molten metal in a tundish by means of a thermal plasma.
[0002] A thermal plasma is a gas of sufficient energy content that a significant fraction of the species present are ionised permitting the conduction of electrical energy. Thermal plasmas are used in for example smelting ores. Energy from a transferred arc torch is typically transferred directly to the ore being smelted, although if desired, a non-transferred arc torch can be used to heat a large volume of gas which in turn is used to heat a feedstock.
[0003] In a conventional metallurgical furnace, the electrode of the plasma torch is typically mounted within a framework on the top of the furnace and passes through a sleeve arrangement in the lid, allowing the length of the electrode on the underside to be varied according to the operating level within the furnace. A gas used to form the plasma is argon.
[0004] It has also been proposed to use a thermal plasma to heat molten metal in a tundish. Tundishes are widely used in the continuous casting of steel to transfer the molten steel from a vessel, for example a ladle, to the casting machines. In practice, the temperature of the molten metal can fall by up to 25°C in the tundish. The effect of this temperature fall can be to produce a variable micro structure in the resulting steel. The grain size may be particularly affected. Where a fine grain size is needed it may not be possible to maintain the metallurgically desirable low level of super heat in the ladle for fine grain structure without employing a means of heating the molten metal in the tundish.
[0005] Heating the molten metal in the tundish by means of a thermal plasma offers two main advantages. First, the ladle tapping temperature can be reduced saving costs on power and electrodes and reducing the interval of time between consecutive batches. Second, it permits the required tundish temperatures to be held throughout casting to improve the quality and the fineness and consistency of grain size.
[0006] In the majority of locations where tundishes are installed, there is limited room above the tundish to accommodate a plasma torch. There is therefore a need to provide a method of heating molten metal in a tundish by means of a thermal plasma, which reduces the amount of room needed for the plasma heating apparatus. The method according to the invention makes this result possible by making possible the achievement of a reduced arc length for a given power and amperage. It is known for an argon stabilised arc that it is not possible to reduce the argon arc length below an optimum without losing power (i.e. without loss of voltage). Although higher amperages can be used to compensate, such higher amperages cause greater cost of current carrying cables or bus bars. In addition, there is a limit to the current a particular plasma torch will carry, and higher currents tend to give increased torch loses and wear. Accordingly, merely increasing the amperage is not a desired solution to the problem of reducing arc length.
[0007] According to the present invention there is provided a method of heating molten material in a tundish by means of a thermal plasma, wherein the plasma is of a gas comprising a mixture of argon and one or more of nitrogen, hydrogen, neon and helium.
[0008] A proportion of nitrogen, neon, helium or hydrogen in the gas from which the plasma is formed increases the voltage for a given arc length and given amperage and hence increases power. Accordingly, a shorter arc length preferably in the range 20 to 45 cm can be employed without loss of power and without increasing the amperage. In general, the order of effectiveness is nitrogen > hydrogen > neon > helium. Nitrogen is however suitable only for use with some steels, hydrogen is generally undesirable for most metallurgical applications, while neon is particularly expensive. Accordingly, helium is usually the additional gas of choice for steel and some non-ferrous metals (e.g. aluminium), and nitrogen for other non-ferrous metals. Preferably, for heating molten metal in a tundish, a gas mixture comprising 50 to 95% by volume of argon and 5 to 50% by volume of helium or nitrogen is employed to form the plasma. More preferably, the mixture comprises 80 to 90% by volume of argon and 10 to 20% by volume of helium or nitrogen.
[0009] In order to facilitate thermal plasma heating of the molten metal in a tundish, the plasma torch preferably comprises a generally, vertical electrode, typically hollow, detachably engaging a generally horizontal support arm which is employed to carry the electrode services, typically comprising the plasma gas, electrical power cable and cooling water for the electrodes.
[0011] The invention also provides apparatus for heating molten metal in a tundish including a plasma torch comprising a generally vertical electrode detachably engaging a generally horizontal support arm carrying necessary services to the electrode, the support arm being pivotally mounted on an extensible support.
[0012] If desired, particularly in an elongate tundish, more than one plasma torch may be employed to heat the molten metal therein.
[0013] The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram illustrating a general arrangement of a ladle and a tundish fitted with a plasma torch adapted to be operated in accordance with the invention;
Figure 2A is a schematic side view of a plasma torch for use in the method according to the invention and Figure 2B is a plan view thereof.
Figure 3 is a schematic perspective drawing illustrating a method according to the invention employing two plasma torches and showing the system for supplying services to the torches.
[0014] Referring to Figure 1 of the drawings, a ladle 2 mounted on a turret (not shown) has a tapping outlet 4 which terminates beneath the level of molten metal in a tundish 6. The tundish is provided with a plasma radiation shroud 8 through which extends the end of a transferred arc plasma torch 10. Means (not shown) may be provided for adjusting the height of the tip of the torch 10 above the level of molten metal 12 in the tundish 6. The torch 10 typically comprises a hollow cathodic electrode made of copper. The electrode is preferably formed with passages (not shown) in its wall for the circulation of a liquid coolant typically water. It is also provided with an electrical power supply by means not shown in Figure 1. The gas from which the plasma is formed is supplied to the hollow interior of the torch 10. In accordance with the invention the plasma gas comprises a mixture of argon and one or more of nitrogen, helium, hydrogen and neon. For heating steel, the gas mixture preferably comprises argon and helium.
[0015] If a cathodic plasma torch is employed, the tundish is provided with an anode return which may be a plate of steel built into an appropriate dummy wall in the tundish. Alternatively, a dip graphite electrode may be used for this purpose.
[0016] In operation of the apparatus shown in Figure 1, the tundish 6 is filled from the ladle 2 via the submerged outlet 4. Stopper or starter cones (not shown) can be used to prevent mould feeding from the tundish until the operating level of steel in the tundish has been reached. This is usually at a depth of 400 to 800 mm in a 1000 mm deep tundish. Heating can be effected by striking an arc between the tip of the torch 10 and the molten metal in the tundish 6. The power to the individual torch may be varied by control of arc current using a thyristor rectifier unit (not shown) with a smoothing reactance. Then, the arc length is set and the out voltage finds its own value from the particular system. The given arc length for a given voltage and amperage will vary according to the particular characteristics of the apparatus used, but for any given apparatus in a particular environment, the arc length for a given voltage and amperage may be reduced by including a greater proportion of the additional gas (one or more of helium, nitrogen, neon and hydrogen) in the gas mixture with argon. If the apparatus gives an arc length of some 300 mm for a given voltage and amperage when argon is the sole plasma gas, then by using a gas mixture in accordance with the invention containing up to 50% by volume of helium, the arc length may be reduced by up to 40% for a given voltage (or the voltage similarly increased for a given arc length). For example, with an arc length of 300mm it is possible to increase the voltage from 120V to 150 to 180V by using as the plasma an argon-helium gas mixture containing from 10 to 20% by volume of helium.
[0017] It is generally not necessary to use the method according to the invention to heat the molten metal in the tundish continuously. For example, in the first ladle in a sequence, the steel will tend to be cool and the method according to the invention may be used at this stage because of the low steel temperature and high filling rate. Full depth in the tundish is typically reached generally in 5 to 10 minutes. While the ladle supplies steel within a metallurgically acceptable temperature window, the method according to the invention need not be used to apply extra heating. As it cools at the end of the ladle, the method according to the invention is applied to prevent such cooling resulting in a lowering of the temperature. The temperature can typically be controlled within + or - 5°C.
[0018] The plasma torch 10 typically has a power up to 1.25 MW and operate with a current in the range of 1000 to 8000 amps DC and a voltage of 100 to 200 volts DC. It may typically be supplied with the plasma gas at pressure (say 3 atmospheres) at a rate of 25 to 50 l per minute per 1000 A. The length of the cathodic electrode may typically be in the range 1000 to 2000 mm, and it may typically have a diameter in the order 50 to 100 mm.
[0019] A deionised water coolant may be employed typically at elevated pressure (for example in the range 10 to 15 bar) with a coolant flow rate of 50 to 200 l per minute.
[0020] Referring now to Figures 2A and 2B, there is shown a plasma torch apparatus suitable for use with a tundish in a situation in which access above the tundish is limited. The torch includes a hollow tubular vertically disposed electrode body 20 engaged in a plasma head 22. Typically the body 20 may engage the head 22 by means of a push fit, the body 20 being retained in the head 20 by means of a jubilee or other clip 24. By this means, quick electrode changes are possible, with all the electrode services connections being broken and re-made merely by loosening the retaining clip 24, removing the old electrode 20, inserting the replacement and tightening the clip.
[0021] The electrode head 24 forms part of a pivotally mounted rigid hollow arm 26 which carries the services to the torch. (These services are not shown in Figure 2A and 2B.) The services comprise plasma gas, cooling water flowing to and from the electrode 20, and electricity. The services are connected to the electrode head 22 typically by means of flexible tubing and conductors. Such flexible tubing and conductors exit the opposite end of the arm 26 and may be connected to the necessary sources of supply (not shown in Figures 2A and 2B) with sufficient slack to enable the electrode to be lowered and raised, and the arm rotated to and from the working site. Alternatively, the services may be taken through the arm 26 using metal or other relatively inflexible pipework and suitably insulated electrical bus bars for connecting manifold at the pivoted end of the arm remote from the electrode head. At its other end from the head 22, the arm 26 is pivoted to an extensible support pillar 28. The arrangement is such that the electrode body 20 may be swung into and out of position over a tundish (not shown in Figures 2A and 2B), while the height of the support pillar 28 may be adjusted in accordance with the desired arc length between the tip of the electrode 20 and the tundish.
[0022] The tip of the electrode 20 may be fabricated from thoriated tungsten. The tip may be attached to the body 20 (which is typically made of copper) by means of soldering using a silver solder.
[0023] Figure 3 of the drawings illustrates the services supplied to an apparatus for use in performing the method according to the invention where two separate plasma torches are used to heat the metal in the tundish. Referring to Figure 3 there is shown schematically tundish 40 provided with torches 42 and 44 at either end. The torches respectively have service manifolds 46 and 48. The manifolds 46 and 48 receive a DC electricity supply via means comprising high voltage switch gear 50, a transformer 52, and a thyristor rectifier 54 having a smoothing reactance. The electrical power is supplied from the rectifier 54 to the torches 42 and 44 via the manifolds 46 and 48 with current return being provided by anodes 56 and 58 associated with the tundish 40. The rectifier 54 will also typically be associated with a control box 60 (which if desired may be located in a control room (not shown)) which may be used to adjust the rectifier appropriately.
[0024] The manifolds 46 and 48 are also associated with respective high frequency start units 62 and 64 which are operable to enable an arc to be struck between the respective torch and the molten metal in the tundish 40. Each torch manifold 46 and 48 also receives a flow of cold water via conduit 70 and 72 respectively and returns the cooling water via conduit 74 and 76.
1. A method of heating molten material in a tundish by means of a thermal plasma, wherein
the plasma is of a gas comprising a mixture of argon and one or more of nitrogen,
hydrogen, neon and helium.
2. A method according to Claim 1, wherein the molten material comprises a steel and the
gas comprises a mixture of argon and helium.
3. A method according to Claim 2, wherein the gas comprises a mixture of 50 to 95% by
volume of argon, and 5 to 50% by volume of helium.
4. A method according to Claim 3, wherein the mixture comprises 80 to 90% by volume of
argon and 10 to 20% by volume of helium.
5. A method according to Claim 1, wherein the gas comprises a mixture of argon and nitrogen.
6. A method according to Claim 5, wherein the mixture comprises 80 to 90% by volume of
argon and 10 to 20% by volume of nitrogen.
7. A method according to any one of the preceding claims, wherein the plasma is created
by operation of a transferred arc plasma torch.
8. A method according to Claim 7, wherein the plasma torch comprises a generally vertical
electrode detachably engaging a generally horizontal support arm carrying necessary
services to the torch, the support arm being pivotally mounted on an extensible support.
9. A method as claimed in claim 7 or claim 8, in which the arc length is from 25 to 45
cm.
10. Apparatus for heating molten metal in a tundish comprising a plasma torch comprising
a generally vertical electrode detachably engaging a generally horizontal support
arm carrying necessary services to the torch, the support arm being pivotally mounted
on an extensible support.
11. Apparatus according to Claim 10, in which the plasma torch is of the transferred arc
kind.
12. Apparatus according to Claim 10 or Claim 11, in which the electrode is hollow and
cathodic.