AU749027B2

AU749027B2 - Vertical continuous casting installation with optimized measurement of the metal level

Info

Publication number: AU749027B2
Application number: AU63407/99A
Authority: AU
Inventors: Etienne Caloz; Jean-Pierre Seppey
Original assignee: Alcan Technology and Management Ltd
Current assignee: 3A Composites International AG
Priority date: 1998-10-23
Filing date: 1999-10-16
Publication date: 2002-06-13
Anticipated expiration: 2019-10-16
Also published as: NO20011978D0; IS5920A; NO20011978L; EP0995523A1; AU6340799A; CA2348613A1; ZA200102812B; EP1133371A1; WO2000024535A1

Description

WO 00/24535 (AMENDED) 1 Vertical Continuous Casting Installation with Optimised Measurement of the Metal Level The invention concerns a vertical continuous casting installation, in particular a vertical continuous casting installation for automatic continuous casting of aluminium alloys, comprising a control unit, several ingot moulds with bases arranged on a lowerable casting table, a transfer trough system for conveying a molten metal from a furnace into the individual ingot moulds, a measurement device for each ingot mould in order to determine the time-dependent level N(t) of the molten metal and a flow control device to control the supply of metal into the individual ingot moulds as a function of the difference between a predetermined nominal set-point curve Nsoll(t) and the measured timedependent level N(t) of the molten metal.

The invention also concerns a process for vertical continuous casting of metals, in particular aluminium alloys, in a casting installation comprising several ingot moulds, in which process the liquid metal is supplied from a furnace by way of a transfer trough system to the individual ingot moulds and by way of a flow control device controlled by a control unit into the ingot moulds which are initially closed during a filling phase by bases arranged on a lowerable casting table, where starting from an initial level Na of the molten metal at which the molten metal level control begins, up to a predetermined start level Ns at which the lowering of the casting table in order to generate the metal continuous casting begins, and during the entire lowering phase, the time-dependent metal level N(t) in each ingot mould is measured with a measurement device and compared with a time-dependent predetermined nominal setpoint curve Nsoll(t), and the metal supply to the individual ingot moulds is controlled by a flow control device according to the time-dependent difference between the Tt ctual and the nominal metal level.

2 In a multi-line continuous casting installation, for faultless operation it is essential in particular to control the starting process, i.e. the optimum control of the metal supply to the individual casting units until the actual casting start which is initiated by the lowering of the casting table.

*o *00 **g *o *oo W;:\i,]ry\MNM INODEL\(,3407-99.doc 3 Typically, the fill level of the ingot moulds for the start of the lowering process is between. 120 and 200 mm. Precise control of the metal level, in particular in the lowering phase, is of decisive importance for faultless operation of a casting installation. Precise control of the metal level in the individual ingot moulds requires a correspondingly precise measurement of the fill depth. Consequently, precise metal level control of a casting installation requires precise and reproducible level measurement over a large measurement range of typically 200 mm. The importance of precise metal level determination is increased greatly, in particular, in the refined control systems of multi-mould continuous casting installations.

To improve the measurement accuracy of the level measurement :e .of a molten metal during the initial phase of ingot mould S: 20 filling, JP-A-08019844 describes a measurement device with two sensors fitted above the ingot mould. During the start phase of casting the molten metal into the ingot moulds, to measure the molten metal level a laser beam or ultrasound sensor is used where the corresponding measurement signal is 25 used to monitor the metal melt level and determine the time for the addition of powder. As soon as the metal melt level reaches the measurement range of the second sensor, the metal level is monitored and the time for starting the lowering process of the ingot mould base determined on the basis of the molten metal level established using a sensor based on an eddy process.

For precise fill level determination inductive or capacitative sensors are suitable. The necessary precision can however only be achieved with inductive sensors within a measurement range of approximately 30 50 mm. Vertical continuous casting installations known from the state of the A art therefore normally use devices in which such sensors are 4 guided by means of precision mechanics in co-operation with a servo or stepped motor so that the measurement range permitted for the required measurement accuracy is not exceeded. Capacitative sensors can be used for large measurement ranges of for example up to 300 mm; however, they are greatly dependent on external measurement conditions so frequent recalibration is required.

For fill level and distance measurements, in principle laser optical, ultrasound and microwave processes are known.

Laser optical processes can only be used within limits for fill level measurement of highly reflective measurement products. Such processes are therefore suitable mainly for 15 level measurement of metal alloys, for example aluminium alloys, during the filling phase of the ingot moulds and at the start of the lowering phase. During the lowering phase, a few minutes after the start of the lowering phase, a highly reflective oxide layer forms at least in aluminium alloys which severely hinders or renders impossible the use of laser optical processes for level measurement.

Ultrasound and microwave processes according to the radar principle have a large measurement range and allow contactless level measurement but do not however have the S" necessary measurement accuracy, at least not for the lowering phase of the continuous casting process. Ultrasound fill level measurement processes are also highly temperature-dependent and microwave fill level measurement processes are influenced by the measurement environment.

4a The above discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of any of the claims.

It would be desirable to provide a vertical continuous casting installation of the type described initially with a precise, reliable and low cost fill level measurement and a process of the type specified initially in which metal level measurement can be performed in a simple manner with high precision.

According to the present invention, there is provided a vertical continuous casting installation, in particular a vertical continuous casting installation for automatic continuous casting of aluminium alloys, including a control unit, several ingot moulds with bases arranged on a lowerable casting table, a transfer trough 15 system for conveying a molten metal from a furnace into the individual ingot moulds, a measurement device for each ingot mould in order to determine the time-dependent level N(t) of the molten metal and a flow control device to control the supply of metal to the individual ingot moulds as a function of the difference between a predetermined non-linear nominal set-point curve Nsol(t) and the 20 measured time-dependent level N(t) of the molten metal, wherein the measurement device consists of two measurement systems working physically differently, each with one sensor, where the first measurement system is used to measure the time-dependent molten metal level N(t) during a first filling phase from an initial time ta determined by the control unit until reaching a time tw determined by the control unit, and the second measurement system is used to measure the time-dependent molten metal level N(t) during the subsequent filling and lowering phase, the sensors of each measurement device are fixed at a predetermined and set distance from the ingot mould, the first measurement system has a measurement accuracy of at least 2 mm in a measurement range of at least 200 mm, and the second measurement system has a measurement accuracy of at least 0.1 mm in a measurement range of at least 20 mm.

The present invention also provides a process for vertical continuous casting of metals, in particular aluminium alloys, in a casting installation including several 4b ingot moulds, in which process the liquid metal is supplied from a furnace by way of a transfer trough system to the individual ingot moulds and by way of a flow control device controlled by a control unit into the ingot moulds which are initially closed during a filling phase by bases arranged on a lowerable casting table, where starting from an initial level (Na) of the molten metal at which a molten metal level control begins, up to a predetermined start level (Ns) at which the lowering of the casting table to create the metal continuous casting begins, and during the entire lowering phase, the time-dependent metal level N(t) in each ingot mould is measured with a measurement device and compared with a time- 0io dependent predetermined non-linear nominal value Nsoi(t), and the metal supply to the individual ingot moulds is controlled by a flow control device according to the time-dependent difference between the actual and the nominal metal level, wherein measurement of the time-dependent metal level N(t) is performed by a measurement device consisting of two measurement systems working physically 5 differently, where starting from an initial time ta determined by the control unit until reaching a time tw determined by the control unit, a first measurement system with a first sensor is used which has an accuracy of at least 2 mm in a measurement range of at least 200 mm, and for further measurement of the time-dependent metal level N(t) during the subsequent filling and lowering phase a second 20 measurement system with a second sensor is used which has an accuracy of at least 0.1 mm in a measurement range of at least 20 mm, and the sensors of the two measurement systems assume a fixed position in relation to the ingot mould which is constant throughout the entire continuous casting process.

Accordingly, in the vertical continuous casting installation, \V y\MMH NODEL\63407-99.doc the measurement device consists of two measurement systems working physically differently, each with one sensor, where the first measurement system is used to measure the timedependent molten metal level N(t) during a first filling phase from an initial time ta determined by the control unit until reaching a time tw determined by the control unit, and the second measurement system is used to measure the timedependent molten metal level N(t) during the subsequent filling and lowering phase, the sensors of each measurement device are fixed at a predetermined and set distance from 0 the ingot mould, the first measurement system has a measurement accuracy of at least 2 mm in a measurement range of at least 200 mm, and the second measurement system has a measurement accuracy of at least 0.1 mm in a measurement range of at least 20 mm.

The solution according to the invention is based on the idea o" that at the start of the continuous casting process, i.e.

during the first filling phase of the ingot mould initially closed by the base, during the remaining filling phase and during the lowering process of the casting table, various 20 level measurement devices can be used which take optimum account of the specific requirements during the various principal phases.

S The invention is also based on the knowledge that a large measurement range of approximately 200 mm is required only during the first filling phase of the ingot mould initially closed by the base, and in the subsequent filling and lowering phase of the casting table a smaller measurement range of for example 15 20 mm is sufficient. Also during the starting phase a lower measurement accuracy is required than in the subsequent filling and lowering phase as the fill level changes very quickly in the first filling phase.

During the subsequent filling and lowering phase in contrast a very high measurement accuracy is required.

AIN IlS I IODEL 1,3407-99 do, 5a The term measurement range means a measurement value range in which the values can lie in the entire range between a maximum and a minimum measurement value, where the difference between the maximum and minimum value corresponds in amount to the measurement range. For example the measurement values in a measurement range of 200 mm lie in a value range between 0 and 200 mm.

A measurement device is preferred in which the first measurement system is based on an optical, capacitative, ultrasound or microwave process, and the second measurement system on an inductive, capacitative or optical process.

6 Particular preference is given to a measurement device in which the first measurement system is based on an optical or ultrasound or microwave process and the second measurement system on an inductive or capacitative process.

The first filling phase of the ingot mould closed by the base is usually performed at the maximum possible speed so that the metal level rises very quickly at the start of the ingot mould filling. As a result a turbulent flow is formed in the ingot mould at the start of the filling phase so that at the start of the filling process there is no flat melt surface so the reflection properties of the melt surface are substantially less than those of a flat surface of the same metal. For this reason, and because of the oxide skin not yet formed in this process phase, the first filling phase allows measurement by means of laser optical processes. For the subsequent casting phase i.e. during the remaining filling phase and during the lowering process of the casting table, use of a laser optical level measurement process is not suitable for all alloys because of the high reflection of the substantially flat molten metal surface.

According to the invention the first measuring system can concern measuring recorders or sensors which are based on one of the fill level measurement process described below: a) ultrasound process b) optical process c) microwave process on the radar principle d) capacitative process.

A fill level measurement with ultrasound is based either on measurement of the run time of a sound pulse or on measurement of sound absorption. Measurement of the run time of an ultrasound pulse is preferred i.e. the distance of the melt surface is calculated from the run time between the signal being sent and received. The run time process normally works on the echo principle i.e. an electric pulse is converted into an ultrasound pulse for example by a 7 piezoelectric oscillator arranged on the base or the lower area of the ingot mould, where this pulse is emitted into the melt and partly reflected by the melt-air boundary layer, where the reflected ultrasound pulse (echo) meets a similar piezoelectric oscillator in which the echo is converted back into an electric pulse. The fill level is calculated from the run time of the sound pulse and the speed of sound. The fill level can be measured using the same echo principle if the ultrasound emitter and receiver are arranged in the air chamber above the melt surface. For ultrasound fill level measurement the temperature of the measurement media must be taken into account as the speed of sound is temperature-dependent.

For fill level measurement with microwave using the radar principle, the microwave transmitter and receiver and an antenna are fitted above the ingot mould. The surface of the molten metal partly reflects the usually frequency-modulated electromagnetic waves or pulses emitted by the microwave transmitter. The distance between antenna and melt surface is measured on the radar principle. In a preferred embodiment of this process, a microwave signal of constant amplitude is emitted and after reflection received again and mixed with part of the transmission signal: the frequency of the mixer output signal is proportional to the run time and hence a measure of the distance between the transmitter and melt surface.

Optical fill level measurement processes can be interferometric distance measurement, a laser run time process or a triangulation process.

For interferometric distance measurement, the distance of the reflective melt surface from a sensor is measured. As a measurement signal either the phase angle difference between the reflected and non-reflected modulated laser beam is analysed or the displacement of the reflector surface (melt surface) is measured with a counter laser interferometer. In 8 interferometric distance measurement suitably a monochromatic laser beam is split on a semi-transmissive mirror into a measurement and a reference beam. The two beams are each reflected by a reflector, one fixed and one moving. The reflected beams are overlaid on the semitransmissive mirror causing interference stripes which lie transverse to the receiver and are analysed by this. A distance change of the melt surface of X/4 (I wavelength of laser beam) causes a maximum change in light intensity so that the change in melt level arises from the number of registered maxima or minima and the wavelength.

In laser run time processes, the fill level measurement can be carried out by a direct run time measurement of a light pulse or by phase measurement. In phase measurement the transmission signal is suitably modulated onto a carrier signal, for example in the MHz range, where measurement of the phase shift takes place after demodulation in the receiver. Direct run time measurement takes place on the radar principle where the smallest time difference is measured in the nano- to pico-second range.

In the triangulation process the fill level measurement is traced back to an angle measurement. A tightly focussed light beam of a laser hits the melt surface at an acute angle and is reflected from this. Depending on the fill level the reflected light beam hits a certain point on the receiver i.e. for example a position detector. The position detector can for example consist of a CCD line (chargecoupled device) which consists of a high number of lightsensitive elements (pixels) arranged in a line. With a CCD line, this position can be detected and converted into the fill level by the angle or path difference. The place of breakdown of the reflected laser beam on the position detector moves as a function of the distance of the melt surface from the sensor.

9 In capacitative fill level measurement, the capacitance is measured as a function of the melt level. The capacitance changes, for example due to the degree of cover or the distance between two given surfaces. The capacitance however also changes on a change in dielectric constant of air) due to the introduction of molten metal. A change in capacitance is shown, for example, by the change in a capacitative resistor.

According to the invention the second measurement system concerns measurement recorders or sensors which are based on one of the fill level measurement processes described below: e) capacitative f) inductive g) optical.

Inductive sensors are preferably based on measurement of the change in inductive resistance XL with: XL OL, where L N 2 L A

S

and N number of windings S path length of magnetic flux lines A area crossed by magnetic flux lines L permeability of material.

According to the invention the sensors are arranged at a predetermined and fixed distance from the ingot mould, i.e.

the two measurement systems have no device for height adjustment of the sensors.

Preferably also, measurement systems are used which have no mechanically moving parts, and in particular no mechanical precision parts. Measurement systems which work contactless in relation to the molten metal are also preferred.

A measurement device for each casting unit (ingot mould) is preferred in which the first measurement system is based on 10 an optical process, in particular a triangulation process, and the second measurement system on an inductive process.

Particularly preferred is a first measurement system with a measurement range of up to 200 mm, where an accuracy of 1 mm is achieved over the entire measurement range.

The measurement accuracy of the first measurement system is typically between 0.1 mm and 2 mm, preferably between 0.1 mm and 1 mm.

The measurement range of the second measurement system is typically 20 to 50 mm where the measurement accuracy is typically between 0.01 mm and 0.1 mm, preferably between 0.01 mm and 0.08 mm.

The combination of an optical sensor with an inductive sensor allows the provision of a compact and efficient measurement device which firstly has no expensive, sensitive and complex mechanical sensor guidance devices and secondly, because of the sensor properties adapted in measurement range and precision to the individual casting phases, allows an efficient level measurement of the molten metal with sufficiently high precision. The continuous casting installation according to the invention is particularly suitable for a control algorithm for introduction of metal into the individual ingot moulds in which non-linear nominal set-point curves are used to control the molten metal level.

By avoiding mechanical sensor guidance devices, for level measurement considerably less space is required with regard to the vertical dimensions, so the casting system can be constructed more compactly.

To solve the task according to the invention with regard to the process, the measurement of the time-dependent metal level N(t) is performed by a measurement device consisting of two measurement systems working physically differently, 11 where a first measurement system with a first sensor is used starting from an initial level until a predetermined melt level is reached, and a second measurement system with a second sensor is used for further measurement of the timedependent metal level N(t) during the subsequent filling and lowering phase, and the sensors of the measurement systems assume a fixed position in relation to the ingot mould which is constant throughout the entire continuous casting process.

The fill phase begins on introduction of the liquid metal onto the base and ends at the start of lowering of the base i.e. when the start level N s is reached at time t s The metal level control normally only begins when a certain metal level Na is reached at time ta in the ingot mould initially closed by the base. The first fill phase indicates the period from the start of introduction of liquid metal into the ingot mould until time tw at which the system switches to the second measurement system for determining the metal level, where tw indicates the time at which the metal level in the ingot mould initially closed by the base reaches a predetermined height N w The time between tw and t s describes the second or further fill phase, or the first subsequent fill phase. The lowering phase begins on reaching the start level N s at time t s and lasts until the end or interruption of the continuous casting process.

The flow control device is controlled as a function of the difference of the measured metal level N(t) from the nominal set-point curve Nsoll(t) using a control unit where the flow control device determines the quantity of molten metal flowing into the ingot mould. The control unit, for example, determines the initial time ta of ingot mould filling, time tw for the change of measurement system, start time t s of the lowering process, the filling of the ingot mould or the quantity of the metal to be introduced into the ingot mould per time unit during the filling phase and during the lowering process, the lowering rate of the casting table and 12 the control of the measurement system responsible for level measurement The control unit is also however sometimes used for monitoring and control of further process parameters e.g. coolant water supply, CO 2 supply, supply of grain refining agents, EMC power supply, and for example automatically initiates the lowering process of the casting table.

In a preferred embodiment of the process according to the invention, the metal supply in the individual ingot moulds is controlled only until start time t s directly by the timedependent difference between the actual and nominal values of the metal level. Thereafter, direct control of the metal supply after the start of the lowering process takes place as a function of the ingot length i.e. as a function of the vertical casting table position, and based on the difference between the actual and nominal value of the metal level. The control of the metal supply after the start time t s takes place by the ingot length-dependent difference between the actual and nominal value of the metal level. As the lowering process normally proceeds at constant rate, the ingot length increases linearly with time so that the metal supply even during the lowering process is controlled according to the time-dependent difference between the actual and nominal values of the metal level.

I

Preferably time ta of the start of level control is determined by measurement of the metal level, in particular by level measurement by means of a laser optical process.

The triggering of level measurement using the second measurement system, i.e. the time of change of measurement systems, can take place either by a control unit on the basis of the metal level measured, or according to a preferred embodiment, in particular when using a second measurement system with an inductive measurement process, be triggered directly by the melt level in that the measurement system change takes place at a time at which the melt 13 enters, for example, a cavity formed by an inductively working measurement coil.

The process according to the invention is suitable for casting installations with just one ingot mould; in particular however it is suitable for vertical continuous casting installations with several ingot moulds.

According to the invention the sensors of the two measuring systems of a casting unit assume a fixed position in relation to the ingot mould, constant throughout the entire continuous casting process, i.e. measurement is carried out without any mechanical guidance device in the form of a height adjustment of the sensors, for example. Preferably, level measurement takes place during the first fill phase contactless in relation to the molten metal.

The measurement systems comprise continuous and discontinuous action level detection systems. Consequently, the process according to the invention can be performed by a continuous and/or discontinuous level measurement. In level measurement with the second measurement system, a continuous metal level measurement is preferred. Equally preferably, measurement takes place with the first measurement system at discrete times, in particular with 3 to 10 measurement values, whereas the level measurement with the second measurement system is performed continuously.

In casting installations with several ingot moulds, lowering of the casting table with the bases suitably begins as soon as the start level is reached in one ingot mould.

Preferably, the metal level in the transfer trough system from the start of the filling phase of the bases and the ingot moulds up to and including the stationary casting phase (lowering phase) is kept at a constant level.

14 Further advantageous embodiments of the process according to the invention are described in the sub-claims.

The continuous casting installation according to the invention and the process according to the invention are suitable for casting all continuously castable metals, but preferably for continuous casting of aluminium, magnesium and copper alloys. The continuous casting installation according to the invention and the process according to the invention are however suitable in particular for continuous casting of aluminium alloys.

Further advantages, features and details of the invention arise from the examples shown in Figures 1 and 2 and the description of figures.

Figure 1 shows diagrammatically a simplified cross-section through part of an ingot mould with the base retracted.

Figure 2 shows diagrammatically a nominal set-point curve of the temporal development of the metal level in another ingot mould.

The vertical continuous casting system shown in Figure 1 contains an ingot mould 10 with a base 14 arranged on a lowerable casting table 16, a lift/lower device 11 for the casting table which is driven by a motor 12, where the motor is controlled by way of a control unit 34, a metal level measurement device consisting of two measurement systems 22, 26, a transfer trough system 20 for conveying molten metal 18 from a furnace (not shown) to the ingot mould 10, where a flow control device 30 controlled by the control unit 34 determines the quantity of molten metal introduced into the ingot mould. The control unit 34 determines amongst others the starting point ta of ingot mould filling, the start time t s of the lowering process, the filling of the ingot mould or the quantity of the metal 18 to be introduced per time 15 unit into the ingot mould 10 during the fill phase and during the lowering process, and the lowering rate of the casting table 16, where the control unit 34 works as a function of the metal level measurement N(t) and a predetermined nominal set-point curve Nsoll(t The flow control device 30 shown as an example in Figure 1 substantially consists of an inlet opening 33 in the transfer trough system 20 which can be closed by a vertically moving stopper 32. The stopper 32 can be brought into the closed position by lowering into the inlet opening 33, or by lifting this the opening cross-section and hence the supply of molten metal 18 into the ingot mould 10, can be enlarged accordingly. The stopper 32 has a stopper rod which is guided by a holder device and driven by a motor 31, where the motor is controlled by the control unit 34.

Before the start of casting, during a testing phase all settings in the casting system are checked. When all starting conditions are fulfilled, by tipping the furnace containing the liquid metal, the transfer trough system is filled to a preset metal level. As soon as a sensor for example an inductive measurement value emitter indicates a preset fill level in the transfer trough system 20, the inlet opening 33 of the transfer trough system 20 is cleared by raising the stopper 32 of the flow control device 30 and the filling of the base 14 and the ingot mould 10 with liquid metal 18 begins. The metal level N(t) in the base 14 or the ingot mould 10 is determined, e.g. PID-controlled, by way of a measurement device containing two measurement systems 22, 26.

The ingot mould 10 shown in Figure 1 is shown in closed stage i.e. the base lies against the ingot mould 10, where the lowering process has not yet begun. The filling phase is however almost concluded as the ingot mould 10 is already filled to close to the second sensor 28 with liquid metal 16 The first sensor 24 is at a greater distance from the base 14 than the second sensor 28. This ensures that the first sensor 24, based on a laser optical process, cannot come into contact with the melt 18. The second sensor 28, based on an inductive measurement process, however requires at least partial direct contact with the melt 18.

Sensors 24 and 28 are connected at a fixed distance with the respective remainders of the measurement systems 22 and 26.

The two measurement systems 22 and 26 are also firmly connected together mechanically i.e. normally the two measurement systems together form one mechanical unit.

The distance of the sensors 24, 28 from the ingot mould is constant throughout the entire continuous casting process, i.e. the distance of the sensors 24, 28 from the metal surface changes constantly, in particular during the ingot mould filling phase. Consequently, the distance of sensors 24, 28 from the molten metal surfac, or the base 14, is greatest at the start of the filling phase, whereas this distance diminishes continuously or discontinuously during the filling phase and remains substantially constant after reaching the start level Ns i.e. at the start of and during the lowering process.

The embodiment shown in the drawing relates to continuous casting with a conventional ingot mould. The vertical continuous casting installation according to the invention however also includes other casting processes, for example casting in an electromagnetic alternating field (EMC) i.e.

using an electromagnetic ingot mould.

Figure 2 shows as an example a nominal set-point curve Nsoll(t) for the process according to the invention. As soon as the metal in a ingot mould 10 has reached a predetermined starting level Na at starting time ta, the metal level control begins on the basis of the set-point curve Nsoll(t) 17 and the measured metal level N(t) until the metal level has reached the starting level Ns at start time t s in the ingot mould 10 closed by the base 14, whereupon lowering of the casting cable to generate the metal continuous casting begins.

The nominal set-point curve Nsoll(t) shown in Figure 2 is polygonal and suitable for example for discontinuous control of the metal level. In an area close to the starting level Na, the nominal set-point curve Nsoll(t) has a gradient greater than the mean gradient (N s Na) (t s ta). In contrast the nominal set-point curve Nsoll(t) in an area lying close to the starting area Ns has a gradient less than the mean gradient.

At time tw nominal value Nsoll(tw) has height Nw. At time tw a switch occurs from the first measurement system 22 to the second measurement system 26. In the second measurement system 26, based on an inductive measurement process, and shown in Figure 1, time tw is determined by the entry of the melt into a cavity formed by an inductively working measurement coil. The metal height N(t) above the base 14 during the first fill phase, i.e. until the fill height as reached the value Nw, is determined with the first measurement system 22 which has a large measurement range.

After time tw the metal height is determined with the second measurement system 26, the measurement range of which is less than that of the first measurement system 22 but which has a high measurement accuracy. The high measurement accuracy is essential in particular after time tw as thereafter the nominal set-point curve Nsoll(t) runs preferably flatter than the mean gradient and hence the metal supply to the individual casting units in a continuous casting system with several ingot moulds 10 can be controlled optimally until the actual casting start t s which is initiated by lowering of the casting table 16.

18 The start level Ns, i.e. the height of the surface of the liquid metal 18 above the base 14 at start time ts, typically lies between 100 and 200 mm, and in particular between 120 and 190 mm. The start level N s starting from an initial time ta is typically reached in a time of 20 to s or preferably within 25 to 45 s.

Claims

1. Vertical continuous casting installation, in particular a vertical continuous casting installation for automatic continuous casting of aluminium alloys, including a control unit, several ingot moulds with bases arranged on a lowerable casting table, a transfer trough system for conveying a molten metal from a furnace into the individual ingot moulds, a measurement device for each ingot mould in order to determine the time-dependent level N(t) of the molten metal and a flow control device to control the supply of metal to the individual ingot moulds as a function of io the difference between a predetermined non-linear nominal set-point curve Nsi 1 (t) and the measured time-dependent level N(t) of the molten metal, wherein the measurement device consists of two measurement systems working physically differently, each with one sensor, where the first measurement system is used to measure the time-dependent molten metal level N(t) during a first filling phase o *o* from an initial time ta determined by the control unit until reaching a time tw determined by the control unit, and the second measurement system is used to measure the time-dependent molten metal level N(t) during the subsequent filling and lowering phase, the sensors of each measurement device are fixed at a predetermined and set distance from the ingot mould, the first measurement system has a measurement accuracy of at least 2 mm in a measurement range of at least 200 mm, and the second measurement system has a measurement accuracy of at least 0.1 mm in a measurement range of at least 20 mm.

2. Vertical continuous casting installation according to claim 1, wherein the first measurement system is based on an optical, capacitative, ultrasound or microwave process, and the second measurement system is based on an inductive, capacitative or optical process.

3. Vertical continuous casting installation according to claim 1 or 2, wherein the first measurement system is based on an optical or ultrasound or microwave process, and the second measurement system on an inductive or capacitative process. li y\hI M I i NO DE L\03407-99.doc

4. Vertical continuous casting installation according to any one of claims 1 to 3, wherein the first measurement system is based on an optical process, in particular a triangulation process, and a second measurement system on an inductive process.

Vertical continuous casting installation according to any one of claims 1 to 4, wherein the first measurement system of each measurement device works contactless in relation to the molten metal. lo

6. Process for vertical continuous casting of metals, in particular aluminium alloys, in a casting installation including several ingot moulds, in which process the liquid metal is supplied from a furnace by way of a transfer trough system to the individual ingot moulds and by way of a flow control device controlled by a control unit into the ingot moulds which are initially closed during a filling phase by S 15 bases arranged on a lowerable casting table, where starting from an initial level of the molten metal at which a molten metal level control begins, up to a predetermined start level (Ns) at which the lowering of the casting table to create the metal continuous casting begins, and during the entire lowering phase, the time-dependent metal level N(t) in each ingot mould is measured with a 20 measurement device and compared with a time-dependent predetermined non-linear nominal value and the metal supply to the individual ingot moulds is controlled by a flow control device according to the time-dependent difference between the actual and the nominal metal level, wherein measurement of the time-dependent metal level N(t) is performed by a measurement device consisting of two measurement systems working physically differently, where starting from an initial time ta determined by the control unit until reaching a time tw determined by the control unit, a first measurement system with a first sensor is used which has an accuracy of at least 2 mm in a measurement range of at least 200 mm, and for further measurement of the time-dependent metal level N(t) 3o during the subsequent filling and lowering phase a second measurement system with a second sensor is used which has an accuracy of at least 0.1 mm in a measurement range of at least 20 mm, and the sensors of the two measurement systems assume a fixed position in relation to the ingot mould which is constant ,throughout the entire continuous casting process. \V A :\Ii y\M M H NO DEL\0W407-99.do

7. Process according to claim 6, wherein the first measurement system is based on an optical, capacitative, ultrasound or microwave process and the second measurement system on an inductive, capacitative or optical process.

8. Process according to claim 6 or 7, wherein the first measurement system is based on an optical process, in particular a triangulation process, and the second measurement system on an inductive process.

9. Process according to any one of claims 6 to 8, wherein the predetermined melt level (Nw) at which the measurement system changes is determined by the vertical position of the lower opening of a cylindrical cavity formed by an inductively working measurement coil, where the longitudinal axis of the measurement coil lies substantially perpendicular to the surface of the molten metal.

10. Process according to any one of claims 6 to 9, wherein the metal level N(t) S-is measured with the first measurement system at discrete times and the metal level N(t) is measured with the second measurement system continuously.

11. Process according to any one of claims 6 to 10, wherein the metal is supplied to the ingot moulds on the basis of the corresponding difference between the nominal set-point curve Nsol 1 and the metal level measurement value N(t)under PID control.

12. Cast metal formed by the process of claim 6.

13. A vertical continuous casting installation, substantially as herein described with reference to the accompanying drawings. \V:\iatl N I I NODEL\63407.99.doc

14. A process for vertical continuous casting of metals, substantially as herein described with reference to the accompanying drawings. DATED: 28 September 2001 PHILLIPS ORMONDE FITZPATRICK Patent Attorneys for: ALUSUISSE TECHNOLOGY MANAGEMENT LTD. ego* 0 0 W\~u NJI I NOlDEL\63407.99.d.,