EP0200981A2 - Process for the automatic control of the starting operation of a continuous metal casting machine - Google Patents

Abstract

At the latest before the level measuring device responds, the degree of opening of the outflow (5) is brought close to the normal pouring degree and after receiving a clear signal from the level measuring device, or when a first predetermined level is reached, the extraction machine is switched on and the withdrawal speed for determining the Degree of opening of the outflow consulted. For this purpose, the speed of the rollers is measured by means of a tachometer (18), the signal given is standardized, then multiplied by the signal determining the normal pouring degree of opening and the degree of opening of the outflow is controlled with the signal thus obtained. After the start-up phase of the fume cupboard or after reaching a second predetermined fill level, which is above the first fill level above, the I-regulation of the degree of opening of the outflow alone and after reaching a third predetermined fill level, which roughly corresponds to the normal desired fill level, is on passed an IP regulation.

Description

The invention relates to a method for automatically controlling the start-up operation of a metal continuous casting installation. As is known, such a system essentially consists of a water-cooled mold which is supplied with melt from a vessel provided with a controllable bottom outflow opening via a dip tube, and a take-off machine for the solidifying strand. The mold is provided with a fill level measuring device, by means of which the opening of the floor discharge or in free-wheel operation the speed of rotation of the take-off rollers is controlled during operation. Before the start of casting, the lower mold opening is closed with a cold strand. When casting on, the molten steel is welded to the start-up head, which usually contains refrigerated scrap. The water-cooled mold removes so much heat from the molten metal that a solidified shell forms. As soon as the fill level in the mold has reached a predetermined level, the extraction machine is switched on and the cold strand is continuously drawn off with the solidifying strand. The further continuous casting is then subjected to a conventional mold level control.

Automatic pouring is problematic since the fill level measuring device is arranged in the area of the desired height of the bath level and only gives usable measured values when the bath level approaches its desired height. In addition, it must be taken into account that strong bath movements and splashes occur during casting, which falsify any level measurements or make their evaluation more difficult. Automation of the casting process is known, for example, from DE 32 21 708. The process described therein is characterized in that the bottom outlet opening is automatically opened intermittently for a sip filling of the continuous casting mold during the period from the start of the melt supply to filling the cavities in the area of the start-up head and / or until the initially strong bath movements in the continuous casting mold have subsided.

It is known from BE 704.306 to continuously fill the mold in a first phase and to close the outflow opening again when the bath level is detected by the level measuring device. The extractor is then started and the opening of the floor outlet is regulated by a P controller.

However, such processes fail if you want to shed small cross sections. Here the volume to be filled by the steel is very small; the thermal inertia of the steel mold system is drastically reduced. In addition, it must be borne in mind that the plugs that close the floor outlet opening are moved from the open to the closed position by the commercially available actuators in about 0.8 seconds, i.e. if the level measuring device gives the first values, even a half-open discharge could hardly be closed early.

When filling in sips, especially with very cold batches (approx. 20 ° C above liquidus), there is a great risk that the heat input per stopper stroke will be too low with the small stopper strokes and therefore the outflow or the immersion tube will often be smeared Pouring on after changing the immersion tube increases this risk, since the immersion tubes are not preheated. It was even observed that in the case of qualities that are very difficult to cast, the steel froze in the dip tube during a stroke pause and the dip tube detached from the vessel on the next stroke. As a remedy, attempts were made to extend the stopper strokes in order to supply the spout and the immersion tube with as much heat as possible per stroke. However, this is only possible to a limited extent, because an extension of the opening time automatically Increasing the stopper stroke entailed the risk of entering the measuring range with the stopper very wide open, which can lead to steel overflow. In addition, the undefined stopper position when reaching the transition position is disadvantageous for setting the transition from the sprue to the control.

In the case of batches with a very high Al content (without CaSi), these problems are exacerbated. This is aggravated by the fact that each time the stopper is closed, alumina builds up on the spout so that the stopper zero position shifts upwards. An uneven build-up of this clay often results in a plug that does not close tightly (so-called plug runner). Here, the greatest possible energy supply must be taken into account to prevent smearing.

The object of the invention is to propose a method for automatically controlling the start-up mode, which overcomes the disadvantages mentioned above and ensures problem-free casting of small cross sections.

This object is achieved by the invention specified in claim 1. Advantageous further developments are given in the subclaims.

The advantages achieved by the invention are essentially a safe cast-on, the regulating and control unit being able to be constructed from cheap and proven electronic parts. In addition, there is the possibility of casting steel whose temperature is just above liquidus.

• - Fig. 1 eine schematische Darstellung einer Stranggiessanlage,
• - Fig. 2 ein Blockschaltbild einer erfindungsgemässen Steuerung des Anfahrbetriebes,
• - Fig. 3 eine Variante einer erfindungsgemässen Steuerung und
• - Fig. 4 ein Zeit-Stopfenstellung Diagramm beim Angiessen.

The invention is explained in more detail below with reference to drawings. They show

• 1 shows a schematic representation of a continuous casting installation,
• 2 shows a block diagram of an inventive control of the starting operation,
• 3 shows a variant of a control according to the invention and
• - Fig. 4 is a time-plug position diagram during casting.

As can be seen from FIG. 1, the upper end of the water-cooled continuous casting mold 1 is supplied with melt 3 via a dip tube 2. The melt 3 is located in an intermediate container 4, the outflow opening 5 of which can be closed with a stopper 6. The position of the stopper 6 is determined by a hydraulic actuator 17. The position of the actuator is influenced by the positioner 7. The lower side of the mold is closed with the start head 8, on which cooling scrap 11 is located; the cold strand 9 connected to it is clamped in the take-off rollers 10. At the upper end of the mold 1 there is a radiation source 12, usually Co-60, which extends over a height H. The radiation originating from the source 12 is recorded by a sensor 13. After appropriate amplification and processing (see reference number 14), the received signal is forwarded to the control and control unit 15. The bath level is deduced from the radiation attenuation. In the further description, the fill level is given as a percentage of the measuring range. For example, the target level is 70%, i.e. the target level is reached when an output signal of 70 percent of the maximum value is present.

The regulating and control unit 15, which is shown in somewhat more detail in FIG. 2, receives the fill level values from the amplifier 14 and the respective position of the plug 6 from the actuator 17 and acts on the positioner 7 and on the controller 16 of the extraction machine. The regulating and control unit 15 is also informed of the respective take-off speed of the (cold) strand via a tachometer 18 attached to a take-off roller.

2 shows details of the regulating and control unit. The position of the various switches before the start of casting is shown. Using known electronic measures, the switches change the position only once, i.e. If the metal mirror falls during casting, the switches cannot fall back into their start-up operating position.

Capacitor 38 maintains the instant stopper zero position. A voltage is tapped at the potentiometer 20, which experience has shown to lead to the normal stopper position during casting, while the desired bath height in the mold is set via the potentiometer 30. The limit indicator 31 acts on the switching devices 24, 26 and 29, and on the control unit 16 as soon as the metal has reached predetermined fill levels in the mold. In the multiplier module 25, the stopper position set via the potentiometer 20 (the zero position of which is taken into account by means of the capacitor 38 and the amplifier 19) is multiplied by a value proportional to the withdrawal speed of the line. The circuit further comprises an I controller 27 and a P controller 28, the controllers being used not only to control the start-up operation but also to regulate the normal casting operation.

The operation when starting is as follows: the switch 23 is closed and at the same time the switch 22 is opened. The zero position correction of the stopper is thus recorded in the capacitor and the opening setpoint determined on the potentiometer is passed directly to the positioner 7 after a zero correction by the adder 21. As stated above, this opening setpoint advantageously corresponds to the middle stopper position during casting. In any case, the bottom outlet opening should neither be opened too far, because otherwise a steel overflow will be preprogrammed, nor too small, because then the spout or the plug will smear.

As soon as the casting level measuring device indicates a 20% fill level, the casting machine is started. At the same time, the switch unit 24 is switched over; the stopper opening setpoint is thus additionally changed by means of the multiplier module 25 as a function of the current casting speed. It goes without saying that the signal emitted by the tachometer is standardized for this purpose, ie the multiplier value is between 0 (casting machine is stationary) and 1 (target speed has been reached). This ensures that the stopper is first moved briefly in the closing direction (Giessge speed is low) and then opened in proportion to the run-up curve of the fume cupboard. If there is a breakdown in the fume cupboard, the outflow opening is closed as quickly as possible. The multiplier value can be linear or, for example, logarithmically linear in a first speed range and ultimately exponential depending on the casting speed. Especially in the case of very small cross-sections (less than 100mm), routine tests are required to carefully try out which mathematical (partial) dependencies of the multiplier value on the casting speed optimally prevent breakthrough or overflow during casting.

When a 50% fill level is reached, the above position control is replaced by a level control, and specifically the I branch 27 of the control is switched on by switching the switch unit 26. It goes without saying that up to this point in time the I-controller 27 had been adjusted to the position of the stopper in the separate storage described above (see dashed arrow and reference number 17), since otherwise it would have reached its saturation limit at this point and when the switch unit was switched over 26 would cause a brutal tearing up of the plug.

From approximately 70X of the fill level, which, as noted above, corresponds to the desired fill level, the P-channel 28 of the control is then switched on by closing the switch unit 29. Now the stopper position is subjected to a classic I-P control, in which the actual value output by the amplifier 14 is compared with a setpoint determined by the potentiometer 30 and the stopper position is changed in the event of a deviation. Experience has shown that pulling in the P branch prematurely leads to violent fluctuations in the metal quantity supply and an increased risk of steel overflow. Under no circumstances may the P controller be switched on before the 1 controller.

Instead of bringing the opening degree of the outflow close to the normal pouring opening degree immediately as described above, it can be advantageous to immediately supply a large amount of heat at the start by first taking a large sip of metal into the Let the mold run in and then reduce the degree of opening of the outflow close to the normal degree of pouring. For this purpose, two are used instead of one potentiometer 20 (see reference numerals 120 and 220 on FIG. 3, which shows the upper part, with changes, of the block diagram according to FIG. 2), the potentiometer 220 being the one corresponding to the large sip of metal Degree of opening and with the potentiometer 120 as described in association with Fig. 2, the middle stopper position Wc is set during casting. Further components shown in FIG. 3 correspond to those of FIG. 2, the reference number being increased by one hundred for the same parts.

Before the start of casting, switch 221 is closed; switch 222 is thus open. When casting on, the switch 123 is closed (time to - see FIG. 4) and the stopper 6 moves from the zero position Wo to the maximum value WM specified by the potentiometer. After approximately 1 second (time t1), switch 123 is opened (switch 122 is closed) and the stopper moves back into its new zero position Wol, which is caused by hardened metal. Approximately simultaneously with the switch 123, the switches 221 and 222 are switched over, so that now the setpoint set in the potentiometer 120 controls the position of the stopper as soon as the switch 123 (time t3) is closed again. The waiting time t2 to t3 depends on the type of distributor, the degree of overheating of the melt, the steel grade, etc. and is between 0 and approx. 15 seconds. Even if a pause is not required, it has proven advantageous to briefly close the bottom outflow opening completely in order to be able to take into account the instantaneous zero position of the plug Wo1 in the subsequent process step by means of the capacitor 118. From time t3, the procedure is as stated above, i.e. the plug moves into a position Wc, which depends on the voltage tapped at the potentiometer 120 and the charge on the capacitor 138. At time t5, approx. 20% of the fill level is reached, the extraction machine is started; at 30% of the fill level, the I controller takes over control, etc.

It is important to move the closure member into a position during the casting in which the pouring opening is still closed when the measuring device delivers the first useful values and then to change the degree of opening depending on the pouring speed. The fastest possible run-up of the strand to the desired speed is desirable and has proven to be advantageous in the control method according to the invention.

Although the method was explained in more detail by means of analog circuits, there is of course also the possibility to carry out the automatic control and regulation in digital technology, although here it is important to ensure that the cycle times are sufficiently short.

Claims (8)

1. A method for automatically controlling the start-up operation of a continuous metal casting installation which essentially consists of a water-cooled mold (1), which is supplied with melt (3) by a vessel (4) provided with a controllable bottom outflow opening, and a take-off machine for the solidifying material There is a strand, the mold being provided with a device (12, 13) for measuring the fill level, characterized in that at least shortly before the fill level measuring device responds, the opening degree of the outflow (5) is brought close to the normal pouring opening degree, and that after receiving a clear, usable signal from the level measuring device, or when reaching a first predetermined level, the extraction machine is switched on and the speed of the rollers is used to determine the degree of opening of the outflow. 2. The method according to claim 1, characterized in that from the start of pouring until receipt of a clear, usable signal from the fill level measuring device, or up to a first predetermined fill level, the degree of opening of the outflow is close to the normal degree of pouring. 3. The method according to claim 1, characterized in that a larger sip of metal is allowed to run into the mold at the start of casting, the degree of opening of the outflow being greater than the normal degree of opening during casting. 4. The method according to claim 3, characterized in that said degree of opening of the outflow is at least twice as large as the normal degree of opening during casting. 5. The method according to claims 3 or 4, characterized in that the bottom outlet opening is then closed at least briefly before the degree of opening is brought close to the normal degree of pouring. 6. The method according to claim 1, characterized in that the speed of the take-off rollers is measured by means of a tachometer (18), the signal emitted is normalized, then multiplied by the signal determining the normal degree of pouring opening and by the signal thus obtained the degree of opening of the outflow controls. 7. The method according to claim 1, characterized in that after the start-up phase of the extraction machine or after reaching a second predetermined fill level, which is above the first fill level, the sole I control of the degree of opening of the outflow. 8. The method according to claim 7, characterized in that after reaching a third predetermined fill level, which corresponds approximately to the normal desired fill level, a transition to an I-P control of the degree of opening is made.

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