EP0047218B1 - Process for controlling and regulating the continuous casting of strips between rolls - Google Patents

Abstract

The invention relates to a method for the monitoring and control of operating parameters of a machine for the continuous casting of strips between rolls, including the torque exerted on at least one of the rolls to cause the strip to advance and/or the stress exerted by the strip on at least one of the journals and/or the temperature of the strip as it leaves the rolls and permanently measuring the deviation between the instantaneous value of one of these parameters and the mean value of this parameter over a period of time immediately beforehand. If this deviation exceeds a reference deviation, the casting speed of the machine is reduced until the deviation again becomes less than the reference deviation. The speed of the casting machine is then increased for as long as this deviation remains lower than the reference deviation.

Description

The subject of the present invention is a process for controlling and regulating the operating parameters of a machine for casting strips between rolls, intended to optimize the conditions for obtaining a good quality product and, in particular, to increase the productivity.

Those skilled in the art are familiar with movable mold casting machines of the cylinder type which are used to manufacture, directly, from a molten metal mass, a continuous strip having a width of up to a few meters and a thickness close to the centimeter.

• - d'une part, d'un dispositif d'alimentation en métal liquide comprenant successivement dans le sens d'écoulement du métal: un four de maintien du métal à l'état liquide, une goulotte de circulation équipée d'un système réglant le niveau et le débit du métal, une busette de répartition du métal ayant à son extrémité de sortie une ouverture de section rectangulaire;
• - d'autre part, un dispositif de refroidissement et de laminage comprenant deux cylindres dont les axes sont parallèles et espacés plus ou moins l'un de l'autre suivant l'épaisseur de bande désirée. Ces cylindres sont munis, à chacune de leurs extrémités, de prolongements cylindriques axiaux ou tourillons qui s'engagent par l'intermédiaire de coussinets dans des ouvertures pratiquées dans des traverses support ou empoises, équipées d'un système de serrage et solidaires de deux colonnes verticales qui forment le bâti de la machine. Ces cylindres sont équipés intérieurement d'un réseau de canaux le long desquels circule un fluide de refroidissement et sont en relation avec un moteur qui les anime d'un mouvement de rotation en sens inverse.

These machines mainly consist of:

• - On the one hand, a liquid metal supply device comprising successively in the direction of flow of the metal: an oven for maintaining the metal in the liquid state, a circulation chute equipped with a system regulating the level and flow of metal, a metal distribution nozzle having at its outlet end an opening of rectangular section;
• - On the other hand, a cooling and rolling device comprising two cylinders whose axes are parallel and more or less spaced from each other according to the desired strip thickness. These cylinders are provided, at each of their ends, with axial cylindrical extensions or trunnions which engage by means of bearings in openings made in support crosspieces or chocks, equipped with a clamping system and integral with two columns which form the frame of the machine. These cylinders are internally equipped with a network of channels along which a cooling fluid circulates and are in relation to a motor which drives them in a rotational movement in the opposite direction.

These two devices are placed in relation to each other, so that the outlet section of the nozzle is parallel to the axes of the cylinders and located at a certain distance from the plane passing through these axes which is called the plane of exit.

During the operation of the machine, the metal distributed by the nozzle fills the free space between the cylinders along a circular arc comprised between the plane of the outlet section of the nozzle and the outlet plane of the cylinders.

Under the action of the cylinders, the metal cools, begins to solidify in a place called swamp, due to the presence of a more or less viscous mixture of crystals and liquid, located at a distance from the plane of the section outlet point of the nozzle generally called marsh depth. Then the metal solidifies completely and is drawn towards the exit plane of the cylinders in an increasingly restricted space, where it undergoes a rolling force which gradually brings it to the desired thickness when it escapes by the space between cylinders in the form of a strip which is then taken up by a reel.

This strip is subsequently subjected to various mechanical and / or thermal treatments which lead to products such as thin sheet for example, the mechanical characteristics of which: resistance, elastic limit, elongation, hardness, etc. will be partly depending on the quality of the strip from the casting machine.

It is therefore important to try to maintain good quality from the start to the end of the tape casting. For this, the machine must be operated under the conditions most favorable for obtaining such a result, even when it is used at its maximum speed.

Good quality presupposes the absence of faults such as cracks, cracks or sagging of metal at the outlet of the cylinders. However, several causes of the appearance of defects are known on the strips produced from the casting machines concerned here. These are, in general, variations of certain operating factors such as the temperature of the metal supplying the machine, the flow of cooling water from the cylinders, the surface condition of the cylinders linked to the lubrication conditions, the composition of the metal. poured, the level of the metal in the chute, etc ...

Any variation of one of these factors can, depending on its magnitude, disrupt the operation of the machine, that is to say cause at certain points of the strip an instability of the swamp which can reach the exit plane of the cylinders. This local instability of the marsh causes the appearance of more or less significant defects on the strip, sometimes requiring its scrapping.

The causes of faults are also linked to the casting speed. Indeed, it has been found that beyond certain speeds, the machine's operating stability becomes more critical and more sensitive to certain hazards, and, in particular, to variations in the operating factors listed above, which is reflected by an increased frequency of appearance of faults.

Among these causes of faults, it is necessary to distinguish those which are due to factors of which one can easily detect a variation such as: temperature, metal level, water flow. In these cases, it is easy to carry out an automatic control with alarm allowing the operator of the machine to quickly remedy this variation. But, when it comes to factors such as the composition of the cast metal or the surface condition of the cylinders, it seems quite difficult, if not impossible, to be able to continuously detect their variation. Then, the operator will not be able to react until the appearance of the fault, which will lead to reject a part of the strip.

Whether the variation is detectable or not, this requires the operator to be permanently available. But, the manufacturing staff tion, due to external constraints, can not always react instantaneously or have their eyes riveted on the strip, and this sometimes results in the appearance of a major defect such as a sag which then forces the machine to stop. This is the reason why casting machines are often used at speeds much lower than their possibility, so as to avoid the appearance of these faults due to possible variations in the operating factors.

It is to avoid these drawbacks that the applicant has studied and developed a process for controlling and regulating the operating parameters of the casting machine intended to optimize the conditions for obtaining a good quality product and, in particular , to increase productivity.

Certainly, there is already known from US-A-3,869,891 a process for optimizing the speed of a metal strip moving between the rolls of a rolling mill. It consists, in order to obtain a strip of constant thickness, first of all to predetermine and fix a maximum value and a minimum value of a rolling mill variable, such as the pressure of the rolls or the tension on the belt when entering or leaving the machine, then after a start-up period, maintaining the speed at a preset maximum value so that the running variable remains between the preset maximum and minimum values.

It should be noted that the values of the running variable and the speed are predetermined as a function of the strip to be laminated and that for a given strip, the maximum speed of the rolling mill is fixed in advance and cannot therefore be exceeded as well as the also shows Figure 4.

The applicant has designed a control and regulation process in which the values of the speed and of the walking variables are not predetermined but where only a fixed deviation value to be respected and this reference deviation are fixed only for the walking variables is maintained by varying the speed. This difference corresponds to the difference between an average of values of the walking variable during a previous moment and the instantaneous value so that the values of the walking variables change over time without ever being dependent on a predetermined limit value; Since the walking variables are constantly changing, the speed will change in the same way without ever being limited by a predetermined value, and will therefore lead to maximum productivity.

This process in which we consider at least one of the parameters belonging to the group consisting of the torque exerted on at least one of the cylinders to advance the strip, the force exerted by the strip on at least one of the pins , the temperature of the strip at the outlet of the cylinders, is characterized in that the difference between the instantaneous value of the parameter considered and the average value of this same parameter is continuously measured during an immediately preceding period of time; when this difference becomes negative for the torque and the force, and positive for the temperature, and greater in absolute value than a reference difference, less than 10% of the average value, the casting speed is reduced until the deviation becomes again lower than the reference deviation, then, the casting speed is increased as long as the deviation between the instantaneous value of the parameter and the average value of this parameter during an immediately preceding period of time does not exceed the reference deviation.

Thus, the method according to the invention consists first of all in considering only certain operating parameters such as the torque exerted on at least one of the cylinders to advance the strip, the force exerted by the strip on one at least of the journals of each cylinder, the temperature of the strip at the outlet of the cylinders. Control and regulation can be carried out on one or two or on the three parameters at the same time.

The use of these parameters instead of the market factors listed above comes from the fact that it has been found that these parameters somehow integrate the fluctuations of these different factors as well as other measurable factors and that their value varies so depending on these factors. Thus, when these factors vary in such a way that the marsh becomes locally unstable, the torque and the force decrease continuously over time while, on the contrary, the temperature increases. We thus found an original way to detect faults. It suffices to measure the value of these parameters and monitor their evolution.

The measurement means are conventional: for the torque, it is possible for example to use the intensity of the electric current which feeds the engine for controlling the rotation of the cylinders or to use the indications of a strain gauge placed on one of the pins ; for the force exerted by the strip, one can place, for example, a strain gauge between the chocks of the cylinders and the clamping screws or measure the hydraulic pressure of the clamping circuit; for temperature, one can take for example a thermoelectric probe or an optical pyrometer mounted so as to be able to scan the entire width of the strip and to make it possible to determine an average value of the temperature.

However, the realization of the invention can be done using any means of measurement whatsoever.

The measurements thus made are sent to a computer such as a computer, minicomputer, microprocessor or programmable controller capable of calculating averages. In fact, the invention is characterized in that the difference between the instantaneous value of the parameter and the average value of this same parameter is measured for an immediately preceding period of time. It is therefore necessary that the device which receives the information on the values of the parameters can memorize them over a given period of time At and calculate the average. At this time, it must also be able to measure the difference between the value of the parameter at time t and the average value ne of this same parameter during the period At which has just passed, then to compare it to a reference difference, and, according to the value of this difference, to output or not a signal intended to modify the speed of casting of the machine.

Any deviation will not be taken into account by the calculator. Indeed, it has been found that with regard to the torque and the force, the fluctuations liable to cause the appearance of defects such as cracks, cracks, drips, etc. are accompanied by a reduction in the value of these parameters; the computer therefore only records, in this case, the negative deviations between the instantaneous value and the average value; on the other hand, for the temperature, it is the elevation which announces the fault, and, in this case, the computer will only retain the positive deviations.

In all three cases, the comparison is made between the absolute value of this deviation and a reference deviation which has been determined beforehand and entered into the computer memory.

Any deviation value less than the reference deviation will result in the absence of a signal at the computer output; on the contrary, any higher value will result in a signal to reduce the casting speed. This reduction can be done gradually or in fixed-term steps and continues until the difference becomes less than the reference difference. During this entire phase, the average value retained by the computer is that which corresponds to the time period At immediately preceding that when the deviation has become greater than the reference deviation.

This reference deviation is, in general, less than 10% of the average value of the parameter considered, and this in order to have a suitable response time.

The time period At is preferably less than ten minutes to best follow the fluctuations of the parameter.

In the case where the speed reduction is done in successive stages, the reduction corresponding to each stage is less than 15% of the speed value at the instant immediately prior. Each level lasts less than five minutes.

After a greater or lesser reduction in speed, the deviation becomes smaller than the reference deviation. At this moment, the computer sends a speed increase signal whose value is less than 10% of the speed value the previous instant and continues to compare the difference between the instantaneous value of the parameter and the average value of the parameter for an immediately preceding period of time less than five minutes. This increase can also be done gradually or in successive stages. In the latter case, the increase in speed at each level is less than 10% of the speed value at an instant immediately prior, the duration of the level being less than five minutes. The increase continues as long as the deviation for one of the three parameters does not exceed the reference deviation.

Thus, the casting machine will always operate at the maximum speed compatible with the absence of faults.

• - il assure une production maximum,
• - il évite tous les défauts de solidification apparents ou non, pouvant survenir en marche continue et ceux consécutifs aux arrêts de la machine,
• - il diminue la main d'oeuvre nécessaire à la surveillance de la machine,
• - il fait travailler la machine dans de meilleures conditions car elle ne s'arrête plus,
• - il fournit par l'intermédiaire du calculateur, un enregistrement des paramètres de coulée qui est une sorte de fiche signalétique de la bande de coulée.

We can say that the process of the invention optimizes the conditions for obtaining a good quality product, because, all at the same time:

• - it ensures maximum production,
• - it avoids all solidification defects, apparent or not, which may occur in continuous operation and those consecutive to machine stoppages,
• - it reduces the manpower necessary for monitoring the machine,
• - it makes the machine work in better conditions because it never stops,
• - it provides, via the computer, a recording of the casting parameters which is a sort of data sheet for the casting strip.

The invention will be better understood using the single figure representing a type of machine and on which we can see a feed nozzle (1) through which the liquid metal is admitted between the two cylinders (2 and 2 ') , whose pins (3 and 4) are held by the chocks (5 and 6) integral with the column (7). After cooling and rolling, the metal leaves the casting in the form of a strip (8) which is wound in (9).

The cylinders are rotated in opposite directions by means of the motor (10).

We detect in (11) on the motor supply circuit, the intensity which allows the torque to be evaluated.

On the hydraulic circuits (12 and 12 ') arranged in the columns at (7 and 7 _' ), two manometers (13 and 13 ') are placed making it possible to evaluate the rolling effect, while an optical pyrometer scans in (14) the width of the strip to determine the temperature. These three measurements are transmitted to the computer (15) which controls at (16) the speed of the motor by modifying the voltage of the supply current.

The application of the invention is illustrated by the following example: A machine of the SCAL Jumbo 3C type is used as well as a PER-KIN ELMER 1620 minicomputer. The cast alloy is 1050, the composition of which is described in "Standard for Aluminum Mill Products ”published by the Aluminum Association. The thickness of the strip is 10mm.

The torque is measured through the intensity of the machine's cylinder drive motor (direct current motor).

The rolling forces are measured by means of two manometers measuring the pressure of the hydraulic circuit on each of the two columns.

The temperature of the strip, at the outlet of the cylinders, is measured by an optical pyrometer, which scans the entire surface of the strip.

• - on fait varier la vitesse linéaire des cylindres toutes les deux minutes, d'une valeur de 0,01 m/mn;
• - les valeurs de l'intensité des pressions hydrauliques et de la température sont enregistrées par le calculateur toutes les secondes;
• - la détection des défauts se fait de la façon suivante: à chaque seconde, l'intensité du moteur d'entraînement des cylindres ainsi que les valeurs des pressions hydrauliques dans les colonnes et la température de la bande sont comparées aux valeurs moyennes calculées à partir des 120 mesures précédant l'instant considéré. Si les écarts entre les valeurs instantanées et les valeurs moyennes calculées respectivement de l'intensité de la pression sont négatifs et inférieurs en valeur absolue à un écart correspondant à 2% des valeurs moyennes, la coulée est jugée stable et la procédure d'augmentation de la vitesse décrite plus haut est poursuivie. De même, si l'écart entre la température mesurée à cet instant et la température moyenne est positif et ne dépasse pas 5°C, la coulée est jugée stable. Si au moins un des écarts sur l'intensité et la pression est négatif et dépasse en valeur absolue 2% de la valeur moyenne du paramètre correspondant et/ou si l'écart sur la température est positif et dépasse 5°C, la coulée est jugée instable.

The speed reduction and increase program by the computer is as follows:

• - the linear speed of the cylinders is varied every two minutes, with a value of 0.01 m / min;
• - the values of the intensity of the hydraulic pressures and of the temperature are recorded by the computer every second;
• - fault detection is carried out as follows: every second, the intensity of the cylinder drive motor as well as the values of the hydraulic pressures in the columns and the temperature of the strip are compared with the average values calculated from of the 120 measurements preceding the instant considered. If the differences between the instantaneous values and the calculated average values of the pressure intensity respectively are negative and less in absolute value than a difference corresponding to 2% of the average values, the casting is deemed to be stable and the procedure for increasing the speed described above is continued. Likewise, if the difference between the temperature measured at this instant and the average temperature is positive and does not exceed 5 ° C, the casting is deemed to be stable. If at least one of the differences in intensity and pressure is negative and exceeds in absolute value 2% of the average value of the corresponding parameter and / or if the difference in temperature is positive and exceeds 5 ° C, the flow is deemed unstable.

In this case, the computer immediately gives an order to slow down the linear speed of the cylinders to a value of 0.05 m / min. If after 10 seconds all the differences between the instantaneous values of the cylinder engine intensity, the two pressures and the belt temperature and the average values of these parameters have not become smaller than the reference deviations, the computer gives a second order to slow down the cylinders by 0.05 m / min. This procedure is repeated until each of the deviations is less than the reference deviation. The casting is then again judged stable and the computer repeats the procedure for increasing the speed described above.

The maximum instantaneous speed measured under these conditions is 1.25 m / min, which corresponds to a productivity of approximately 2.02 tonnes per hour and per meter of bandwidth produced. Beyond this, solidification defects appear. On the other hand, maintaining continuous casting at this speed is very difficult because it requires perfect consistency of the temperature of the liquid metal in the pouring spout, the lubrication of the cylinders, etc. The process described above however made it possible to maintain an average speed of approximately 1.22 m / min in continuous operation, ie an average productivity of 1.98 tonnes per hour and per meter of width, and this without any defect in solidification on the strip .

The method according to the invention finds its application in the continuous casting of metals between cylinders, in all cases where it is desired to optimize the conditions for obtaining a quality product, and, in particular, to increase productivity.

Claims (9)

1. A method for the monitoring and control of the operating parameters of a machine for the continuous casting of strips between rolls in which at least one of the parameters from the group constituted by the torque exerted on at least one of the rolls to cause the strip to advance, the stress exerted by the strip on at least one of the journals and the temperature of the strip upon leaving the rolls is considered, characterised in that the deviation between the instantaneous value of the parameter under condisera- tion and the mean value of the same parameter is permanently measured over a period of time immediately beforehand; if this deviation becomes negative for the torique and stress, and positive for the temperature and higher in absolute value than a reference deviation which is less than 10% of the mean value of the parameter, the speed of casting is reduced until the deviation again becomes lower than the reference deviation; and the casting speed is then increased for as long as the deviation between the instantaneous value of the parameter and the mean value of this parameter over a period of time immediately beforehand does not exceed the reference deviation.

2. A method according to claim 1, characterised in that the mean value of the parameter is established over the period of time immediately prior to that when the deviation becomes higher than the reference deviation.

3. A method according to claim 1, characterised in that the period of time immediately beforehand is less than 10 minutes.

4. A method according to claim 1, characterised in that the speed is reduced in successive stages.

5. A method according to claim 1, characterised in that the casting speed is reduced in stages and is reduced at each stage by a quantity of less than 15% of the speed value at the moment immediately beforehand.

6. A method according to claim 1, characterised in that the casting speed is reduced in stages and in that each stage in the reduction of the speed has a duration of less than 5 minutes.

7. A method according to claim 1, characterised in that the speed is increased in successive stages.

8. A method according to claim 1, characterised in that the speed of casting is increased in stages and is increased at each stage by a quantity which is less than 10% of the speed value at a moment immediately beforehand.

9. A method according to claim 1, characterised in that the casting speed is increased in stages and in that each stage in the increase in speed has a duration of less than 5 minutes.

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