GB2219232A

GB2219232A - Molten metal pouring apparatus

Info

Publication number: GB2219232A
Application number: GB8813108A
Authority: GB
Inventors: Douglas Colin Lillicrap
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1988-06-03
Filing date: 1988-06-03
Publication date: 1989-12-06
Anticipated expiration: 2008-06-03
Also published as: GB8813108D0; GB2219232B

Abstract

Molten metal pouring apparatus comprises a tiltable vessel (1) having a discharge nozzle (3) above the normal level of molten metal (6) in the vessel (1), there being an electrical induction coil (4) about the nozzle (3) to control discharge therefrom when the vessel (1) is tilted. The apparatus is constructed in to maintain a substantially constant discharge stream trajectory with electromagnetically controlled flow rate regulation. <IMAGE>

Description

MOLTEN METAL POURING APPARATUS This invention relates to molten metal pouring apparatus.

In the pouring of molten metal from a vessel it is known for the dispensing vessel to be tilted so that metal runs over a pouring spout or lip at the top of the vessel or through a nozzle or other opening in the side of the vessel. In some applications the dispensing vessel is tilted about an axis which passes through or close to the pouring spout or nozzle. This technique is widely used for pouring molten metal from a furnace into a transfer ladle or from a ladle into the pouring basin of a casting mould.

However, such simple pouring practices cannot be readily adapted to produce a rapid response metal dispensing system to meet the requirements of an automated high throughput casting line.

In GB-A-777213 there is disclosed an apparatus in which electromagnetic forces are used to control or prevent the discharge of molten metal through an opening in the wall of a dispensing vessel. In this apparatus the vessel is provided with an opening below the level of the molten metal, and an induction coil is placed around the vessel in the vicinity of the opening. When an alternating electric current is applied to the coil, the electromagnetic forces acting on the molten metal move the metal away from the opening thus preventing flow of metal through the opening until the coil is de-energised. Thus, metal flow can be controlled by de-energising the coil over a succession of timed periods.

In DE-A-1037789 there is disclosed an apparatus similar to that described above, but in which discharge of molten metal from the vessel is by way of a nozzle projecting from the vessel wall, the induction coil being located about the nozzle which is preferably formed of ceramic material.

According to this invention there is provided molten metal pouring apparatus comprising a vessel to contain the molten metal, the vessel having a discharge nozzle projecting therefrom at a position above the normal level of molten metal in the vessel when in an upright position, the vessel being tiltable thereby to cause molten metal to flow out of the vessel through the nozzle; an electrical induction coil located about the nozzle; and means to supply an alternating electric current to the coil whereby the coil provides an electromagnetic field which induces electric currents in any molten metal in the nozzle when the vessel is tilted, interaction between the electromagnetic field provided by the coil and the currents induced in the molten metal in the nozzle providing a force which serves to control the flow of the molten metal through the nozzle.

Preferably the nozzle wall thickness is the minimum necessary to give the nozzle mechanical strength, and the clearance between the induction coil and the nozzle is just sufficient to allow the induction coil to fit over the nozzle. This provides efficient coupling of the electromagnetic forces from the induction coil with the metal stream flowing through the nozzle. The electromagnetic forces are directed radially towards the axis of the nozzle so that a magnetic over-pressure is created in the nozzle. This magnetic over-pressure causes the metal stream in the nozzle to slow down thus providing a means of smoothly regulating the flow rate simply by varying the current to the induction coil. Thus, the flow can be smoothly varied from a maximum value to some minimum value or even to zero if specially designed nozzles are used.

This invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a diagrammatic vertical sectional view through an apparatus aCcording to the invention; Figure 2 is a diagrammatic vertical sectional view through a modified form of the apparatus of.

Figure 1; and Figure 3 is a graph illustrating operation of apparatus according to the invention.

Figure 1 shows a dispensing vessel 1 having refractory material lined walls 2 into one of which is set a discharge nozzle 3. The nozzle 3 protrudes beyond the outside of the wall 2 and an electric induction coil (4) is closely fitted around the protruding portion. The nozzle 3 is positioned close to the open upper end of the vessel so that when the vessel 1 is in an upright position the level of molten metal 6 in the vessel 1 below the nozzle 3.

By tilting the vessel 1 as shown the metal 6 can be made to rise above the nozzle 3 so that gravity causes the metal 6 to run out through the nozzle 3.

The vessel 1 is steadily tilted so that a depth of metal P is maintained above the discharge end 11 of the nozzle. To minimise the overall height of the vessel, it is convenient to close off part of the open end of the vessel 1 with a wall 5 as shown, so that the metal depth P above the nozzle can be maintained even when the vessel 1 is tilted through 900. This arrangement also allows all the metal 6 to be drained off through the nozzle.

In the absence of an electromagnetic field, i.e. when the coil 4 is de-energised, the metal stream issues from the nozzle with a velocity V0 given by

where g is the acceleration due to gravity. The stream leaving the nozzle contracts slightly having a minimum area known as the vena-contracta which is determined by the discharge coefficient for the nozzle. Thus, the mass flow depends on the square root of the metal head P, the diameter of the discharge end 11 of the nozzle and the discharge coefficient of the nozzle.

On leaving the nozzle 3 the metal stream 7 falls in a parabola, and is shown falling into a pouring basin 8 of a casting mould 9. The tilt axis 10 of the vessel 1 is located with respect to the discharge end 11 of the nozzle 3 and the top of the pouring basin 8 such that as the vessel 1 is tilted the discharge end 11 of the nozzle 3 rotates about the axis 10 on an arc of radius R. If the axis 10 is located a height H vertically above the centre of the pouring basin 8 such that

then the stream 7 will fall into the centre of the pouring basin 8 for all angles of tilt.

When a high frequency electric current is passed through the coil 4 the magnetic over-pressure created in the nozzle 3 causes the stream within the nozzle to slow down. The ratio of the velocities with and without the magnetic field are given by:

where B is the magnetic induction at the surface of the metal stream, p is the magnetic permeability of the metal, p is its density, g is acceleration due to gravity, V is the velocity with the field applied and V0 the velocity without the field.

This equation is only valid for B2/2ppgP < 1.

The induction B is a maximum on the mid-plane of the coil 4 and diminishes as the ends of the coil are approached, tending to zero sufficiently far from the coil.

The magnetic over-pressure acting on the stream of metal has a similar distribution with a maximum on the mid-plane of the coil 4. Therefore, the stream slows down to a minimum as it flows towards this mid-plane and then accelerates under the action of both gravity and the imposed magnetic pressure gradient as it flows away from the mid-plane. In fact, sufficiently far from the coil 4, the stream will accelerate to the velocity it would have in absence of the magnetic field which is determined by the depth P of metal. Consequently, even when the magnetic field is applied, the stream of metal will continue to fall into the centre of the pouring basin 8 for all angles of tilt of the vessel.

If the velocity of the stream at the mid-plane of the coil is reduced by the magnetic field, then the mass flow through the nozzle is reduced.

However, as the stream accelerates away from the mid-plane, continuity demands that the diameter of the stream reduces. Thus, at the point where the stream has accelerated to the value V0 again, the stream must contract to a diameter given by:

where d is the diameter of the stream with the field applied and do the diameter of the stream without the field, when both are measured at a point below the coil, at a distance R from the pivot axis 10, where the velocity is given by VO = 2gP.

Clearly, for the stream to contract it must separate from the walls of the nozzle 3 in the region of maximum field strength. Provided the coil 4 is short, this will not seriously affect the trajectory of the stream which will continue to fall into the pouring basin 8 even though its diameter, and therefore the mass flow rate, have been reduced.

This ability to maintain a substantially constant stream trajectory while electromagnetically regulating the flow rate is a particular advantage of the apparatus of this invention.

There are two types of nozzles, the subjects of British Patent Application No. 8809693.8 and Application No. 8811015 which can be used to control the flow of metal.

The first type, illustrated in Figure 1, consists of a main bore 12 of radius RB and an exit bore 13 of radius RE such that RB is greater than RE. The frequency of the current supplied to the coil is sufficiently high to satisfy the condition

For maximum efficiency, the mid-plane of the coil 4 is positioned across the upstream end of the exit bore 13 where it joins the main bore 12. The stream velocity is therefore a minimum at the entrance to the exit bore 13 and increases downstream. Under these conditions the stream would be expected to separate from the nozzle walls at the corner where the exit bore 13 meets the main bore 12. With this nozzle configuration the flow rate can be varied from typically 110% to 30% of the rate for zero magnetic field, although the stream cannot be shut off entirely with this nozzle.

The tilting vessel, equipped with an electromagnetic flow control nozzle as described above can be used to dispense metal in a number of ways. The dispensing vessel, often a ladle or furnace weighing many tons, would be slowly tilted to maintain an approximately constant metal depth P above the nozzle 3, without causing large disturbances to the surface of the melt. In the simplest application, a metal level sensor would be used to monitor the depth P and thus provide a signal to a suitable control circuit to regulate the current supplied to the coil 4. This control circuit could be set up to deliver a constant flow rate or a pre-programmed variation in flow. Otherwise, the current supplied to the coil 4 can be controlled by a signal from a sensor monitoring the metal level or weight of metal in the receiving vessel.For example, the receiving vessel might be a tundish in which the metal level must be held constant.

Otherwise, the receiving vessel might be a continuous casting mould which is supplied directly from sequential ladles, each equipped with an electromagnetic flow control nozzle. In each case a closed control loop can be set up to provide a fast acting flow control system to maintain the required metal level or flow rate. The electromagnetic flow control nozzle would compensate for unavoidable variation in the metal depth P, and, within certain limits, for erosion or partial blockage of the nozzle.

The dispensing vessel can otherwise be equipped with the second type of nozzle as illustrated in Figure 2, with which it is possible to stop the metal flow completely. This is achieved by placing a centre member 14 in the nozzle 3 leaving a gap 15 between the member 14 and the nozzle wall through which the molten metal 6 flows.

The top of the centre member 14 is on the mid-plane AA of the coil 4, so that the maximum magnetic pressure B2/2p is applied to the metal just above the centre member 14. For the full over-pressure on the stream to be developed, the frequency of the current supplied to the coil 4 must be sufficiently high to satisfy the condition:

where Rc is the radius of the top of the centre member 14.

When the magnetic induction B is such that B2/2ppgP < 1, the nozzle of Figure 2 behaves in a similar manner to a nozzle without a centre member, that is the magnetic over-pressure causes the stream to slow down thus reducing the mass flow. However, when B2/2ppgP slightly exceeds 1, the electromagnetic forces are large enough to lift the metal away from the wall of the nozzle 3. The induction B at the surface of the metal decreases as the metal is displaced away from the coil. Hence the metal is displaced until the magnetic pressure balances the static pressure. When the induction B is large enough to displace the metal onto the top 16 of the centre member 14 the flow through the nozzle is cut off.Under these conditions of no flow, the electromagnetic forces, with a contribution from the surface tension forces, balance the static pressure pgP due to the depth of metal P.

This arrangement allows the nozzle 3 to deliver a single stream of molten metal as the flow rate is throttled from maximum down to a very small value, when the stream breaks up into discrete droplets.

For practical metal dispensing applications the nozzle allows a smooth regulation of the flow rate while maintaining a single stream down to the minimum value required, when it can be turned off sharply by rapidly increasing the current supplied to the coil 4.

In a working embodiment of apparatus as shown in Figure 2, the main bore of the nozzle was 30 mm diameter and the circular top 16 of the centre member 14 had a diameter of 22mm which was off-set by 2mm with respect to the axis of the main bore. The vessel was pivoted about an axis so that the nozzle discharge end 11 described an arc of radius 100mm about the tilt axis. A receiving vessel was positioned 33.3mm below the tilt axis. The coil 4 consisted of a single turn of water cooled copper placed around the nozzle 3 so that the mid-plane AA of the coil 4 coincided with the top 16 of the centre member 14.

The apparatus was tested using aluminium with the vessel 1 being tilted to maintain a depth of metal of 150mm above the nozzle discharge end 11.

The mass flow m was measured for various values of induction B. These values were non-dimensionalised by dividing by mO, the mass flow for zero magnetic field. This ratio squared (m/mO)2 plotted against B2/2ppgP is shown in Figure 3.

This plot was very nearly a straight line of slope -1 as would be expected from equation 1. As expected the flow is cut off for values of B2/2ppgP slightly greater than 1.

Fluctuation in the depth of metal P causes variations in the trajectory of the metal stream 7 from the nozzle 3. In the present example a + 20mm variation in P caused a + 7mm variation in the position at which the stream 7 entered the receiving vessel. Most receiving vessels are large enough to accommodate fluctuations of this size.

Claims

1. Molten metal pouring apparatus comprising a vessel to contain the molten metal, the vessel having a discharge nozzle projecting therefrom at a position above the normal level of molten metal in the vessel when in an upright position, the vessel being tiltable thereby to cause molten metal to flow out of the vessel through the nozzle; an electrical induction coil located about the nozzle; and means to supply an alternating electric current to the coil whereby the coil provides an electromagnetic field which induces electric currents in any molten metal in the nozzle when the vessel is tilted, interaction between the electromagnetic field provided by the coil and the currents induced in the molten metal in the nozzle providing a force which serves to control the flow of the molten metal through the nozzle.

2. Apparatus as claimed in Claim 1, in which the depth P of molten metal above the discharge end of the nozzle when the vessel is tilted, the radius R described by the discharge end of the nozzle about the tilt axis when the vessel is tilted, and the height H of the discharge end of the nozzle above the receiving vessel into which the molten metal is being poured, satisfy the relationship

3. Apparatus as claimed in Claim 1 or Claim 2, in which a signal from a sensor monitoring the level of the molten metal in the vessel is used to control the tilt of the vessel to maintain a substantially constant level and also to control the current supplied to the coil to maintain a substantially constant molten metal flow rate through the nozzle.

4. Apparatus as claimed in any preceding claim, in which a signal from a sensor in the receiving vessel is used to control the supply of current to the coil.

5. Molten metal pouring apparatus substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the drawings.

5. Apparatus as claimed in any preceding claim in which the supply of current to the coil is varied so as to deliver a specified mass of metal into the receiving vessel.

6. Molten metal pouring apparatus substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the drawings.

Amendments to the claims have been filed as follows 1. Molten metal pouring apparatus comprising a vessel to contain the molten metal, the vessel having a discharge nozzle projecting therefrom at a position above the normal level of molten metal in the vessel when in an upright position, the vessel being tiltable thereby to cause molten metal to flow out of the vessel through the nozzle the depth P of molten metal above the discharge end of the nozzle when the vessel is tilted, the radius R described by the discharge end of the nozzle about the tilt axis when the vessel is tilted, and the height H of the discharge end of the nozzle above the receiving vessel into which the molten metal is being poured, satisfying the relationship R = H; an electrical induction coil located about the nozzle; and means to supply an alternating electric current to the coil whereby the coil provides an electromagnetic field which induces electric currents in any molten metal in the nozzle when the vessel is tilted, interaction between the electromagnetic field provided by the coil and the currents induced in the molten metal in the nozzle providing a force which serves to control the flow of the molten metal through the nozzle.

2. Apparatus as claimed in Claim 1, in which a signal from a sensor monitoring the level of the molten metal in the vessel is used to control the tilt of the vessel to maintain a substantially constant level and also to control the current supplied to the coil to maintain a substantially constant molten metal flow rate through the nozzle.

3. Apparatus as claimed in Claim 1 or Claim 2, in which a signal from a sensor in the receiving vessel is used to control the supply of current to the coil.

4. Apparatus as claimed in any preceding claim, in which the supply of current to the coil is varied so as to deliver a specified mass of metal into the receiving vessel.