EP0545768A1 - Method and apparatus for the automatic control of continuous annealing furnaces heated by radiant tubes - Google Patents

Abstract

Process for automatic control of a continuous annealing oven with radiation via radiant tubes of the type comprising a number of successive zones for heating to distinct temperatures, employed especially for continuously performing different heat treatments of strips connected to each other and requiring specific annealing operations, in which the position of the strip ends, the speed of forward travel of the strip and the temperature in each of the zones of the oven are determined, characterised in that the change in the temperature of each of the zones of the oven and in the speed of forward travel in the course of time are determined from the measured results and from the characteristics of the strip which has just entered the oven and of the one following it, so that the temperature in the different zones of the oven is always higher than or equal to the values necessary for the heat treatment and so that the speed of the strip is adapted to the necessary period of treatment in each zone as a function of the temperature of the said zone. <IMAGE>

Description

The present invention relates to a process for the automatic control of a continuous annealing furnace with radiation by radiant tubes of the type comprising several heating zones at distinct temperatures, used in particular for carrying out different continuous heat treatments of strips connected together and requiring specific annealing operations.

Currently the continuous annealing method is essential to the detriment of basic annealing. One of the main reasons is the reduction in manufacturing times, from a few days to a few tens of minutes. The current objective is to further improve the productivity of such installations. To achieve this, it is necessary to constantly monitor the speed of travel of the strips as well as the temperature in each of the zones of the oven.

In the prior art, the strips are welded together before passing into the continuous annealing furnace. Strips of different types, different widths, different thicknesses can thus follow one another at the entrance to the oven. These bands may require different heat treatments in order to obtain their own characteristics. In this case it is then necessary to quickly vary the temperature of the different zones of the oven to adapt to the new incoming strip. The thermal inertia of the oven being relatively large, it is impossible to vary this temperature immediately for the heat treatment of the next strip.

For example, the thermal inertia of a radiant tube furnace can be three degrees Celsius per minute; if one wants to go from a 750 ° regime to an 800 ° regime, this results in a variation of 50 °, which requires a transition time of 15 to 20 minutes. If the strip circulates, for example, at 100 meters / minute, this corresponds to a length of 2 km of strip.

In order to solve this problem, intermediate bands are currently introduced between two bands requiring different heat treatments. It is during the passage of these intermediate strips that the oven temperature is caused to vary so as to be at the correct temperature when the next strip arrives. The intermediate strip is then decommissioned and can be reused for similar purposes, except in the case where the strip has been continuously coated after heat treatment.

The temperature control of the various zones of the oven is also managed by the operator, who, depending on the characteristics of the bands present in the oven and arriving from a control panel, sets the temperature set points for each of the oven areas.

This solution has the drawback of requiring the use of intermediate strips which represent a large amount of lost metal, of not using the oven continuously, in particular during the passage of these intermediate strips, and of not optimizing the quantity of energy expended to power the ovens, since an operator indicates the heating instructions for each of the zones of the oven empirically.

To avoid the use of intermediate strips, it is agreed to modify the continuous annealing furnace and to add individually controllable preheating units which inject heating gas directly onto the strip, thus allowing temperature variations in the around 100 ° per second.

This produces a combi steamer using radiant tubes and forced convection by gas jets.

In addition to the cost of installing such an annealing furnace, its operating cost is very high and such a rate of temperature variation applied locally on a part of the strip, in particular on the welded joint between two strips can cause , due to the transformations that the metal undergoes, problems downstream of the continuous annealing, for example during the treatment with the skin-pass.

The object of the invention is to propose a method for automatically operating a continuous annealing furnace with radiation by radiant tubes, making it possible to avoid the use of intermediate bands, without using a device with forced convection.

To this end, the subject of the invention is a process for the automatic control of a continuous annealing furnace with radiation by radiant tubes of the type comprising several successive zones of heating at different temperatures, used in particular for carrying out different heat treatments in continuous of strips connected to each other and requiring specific annealing operations, characterized in that the position of the ends of the strips, the speed of advancement of the strip, the temperature of the strip and the temperature of the oven in each are determined of the zones of the oven, and that the evolution of the temperature of each of the zones of the oven and of the speed of advance over time are determined from the measured results and from the characteristics of the strip having just entered the oven and its next, so that the temperature in the different areas of the oven is always greater than or equal to the values required for the line thermally and that the speed of the strip is adapted to the treatment time required in each zone as a function of the temperature of said zone.

By oven temperature, we actually designate either the temperature of the oven atmosphere or the temperature heat exchange on a standard surface, for example 1 m². This oven temperature must be distinguished from a temperature resulting from radiation from the strip or from the walls of the oven.

• la figure 1 représente un schéma d'un dispositif dans la mise en oeuvre du procédé ;
• la figure 2 représente un schéma de la position de trois bandes successives de section décroissante à deux instants donnés ;
• la figure 3 représente le schéma de l'évolution des consignes et de températures d'une zone d'un four en fonction du temps ;
• la figure 4 représente un schéma de la position de trois bandes successives de section croissante à deux instants donnés ; et
• la figure 5 représente le schéma de l'évolution des consignes et de températures d'une zone du four en fonction du temps correspondant à la situation de la figure 4.

The invention will be better understood with the aid of the description which follows, given solely by way of example and made with reference to the appended drawings in which:

• FIG. 1 represents a diagram of a device in the implementation of the method;
• FIG. 2 represents a diagram of the position of three successive bands of decreasing section at two given times;
• FIG. 3 represents the diagram of the evolution of the setpoints and temperatures of an area of a furnace as a function of time;
• FIG. 4 represents a diagram of the position of three successive bands of increasing section at two given times; and
• FIG. 5 represents the diagram of the evolution of the setpoints and temperatures of an area of the oven as a function of the time corresponding to the situation of FIG. 4.

A process for automatic control of the continuous annealing furnace according to the invention manages for each of the zones of the furnace the modification of two types of parameters.

It allows on the one hand to manage product changes, whether it is the geometry of the strip, or the heating cycle necessary for the heat treatment of said strip, on the other hand it allows the adaptation of the temperature of the various zones of the furnace to changes in the speed of the line due to technical requirements or following operator's orders, for example when the annealing furnace is placed before a coating line, which imposes the speed of travel of the strip.

• à déterminer les caractéristiques géométriques de la bande qui entre dans la zone du four, en particulier son épaisseur et sa largeur, le cycle thermique à lui faire subir et les consignes de température à appliquer à la zone du four pour réaliser le cycle thermique, ainsi que les caractéristiques géométriques de la bande suivant celle qui est dans la zone du four et le cycle thermique à lui faire subir ;
• à calculer au moyen d'une loi de commande en fonction des conditions de la bande dans la zone du four, des conditions de la suivante et des contraintes dues aux caractéristiques du four, la durée de transition optimale pour que la zone du four passe de la température pour réaliser le cycle thermique de la bande qui est dans ladite zone à la température pour réaliser le cycle thermique de la bande suivante, et le moment à partir duquel il faut modifier les consignes de température de la zone du four, pour assurer le cycle thermique de la bande suivant celle située dans la zone du four, sans que la température soit inférieure à la température nécessaire au cycle thermique de la bande située dans ladite zone, ainsi que la loi de variation de la température dans ladite zone ;
• à envoyer au dispositif de pilotage du four les consignes de variation de température.

In the case of a change of product, that is to say when there is a succession of two strips of different nature or requiring different heat treatments, or different treatment speeds, the method according to the invention consists for each zone of the individually controllable oven:

• to determine the geometrical characteristics of the strip which enters the zone of the furnace, in particular its thickness and its width, the thermal cycle to be subjected to it and the temperature instructions to be applied to the zone of the furnace to carry out the thermal cycle, thus that the geometrical characteristics of the strip according to that which is in the zone of the furnace and the thermal cycle to be subjected to it;
• to be calculated by means of a control law as a function of the conditions of the strip in the furnace area, the conditions of the next and the constraints due to the characteristics of the furnace, the optimal transition time for the furnace zone to pass from the temperature for carrying out the thermal cycle of the strip which is in said zone at the temperature for carrying out the thermal cycle of the following strip, and the moment from which it is necessary to modify the temperature set points of the zone of the oven, to ensure the thermal cycle of the strip following that located in the zone of the furnace, without the temperature being lower than the temperature necessary for the thermal cycle of the strip situated in said zone, as well as the law of variation of the temperature in said zone;
• send the temperature variation instructions to the oven control device.

At the same time, the method consists in determining the speed of the strip in the oven and the speed at which it must scroll in the case of a speed change, and the moment from which the speed setpoints must be modified is calculated.

The area of the oven is called each part of the oven which can be individually controlled, that is to say having its own adjustment means making it possible to modify the temperature conditions in this zone without modifying the conditions in the neighboring zones.

In the case where there are only speed changes, the method acquires the value of the target speed to be reached and the instant when the speed must be reached, determines the transient parameters in speed and in temperature to be applied so that the change of speed does not affect the heat treatment of the belts.

The transient parameters in speed and temperature are determined taking into account any transients due to changes in the thermal cycle in progress or planned.

In order to adapt the temperature of the oven to the strip which arrives at the inlet of said oven, it is necessary that the maximum time is equal to the time of passage of the product to which this transition is applied and, on the other hand, the instructions relating to a transition are determined with an anticipation equivalent to the dead time and the establishment time between the initial state and the final state.

Taking into account the heat treatment necessary for the strip following that being treated, and the speed of advance of the line, the method according to the invention provides for the calculation of the oven temperature set points which will be applied to the different zones of the oven. during the transition and in particular the duration and the slope of the temperature ramps.

Two cases are possible during a temperature change to treat two successive coils or bands. Either the temperature is caused to be increased, for example due to the increase in the thickness of the strip, or on the contrary it is caused to be decreased.

When the temperature in one zone of the oven has to be reduced, for the next strip, the date of the start of variation of the setting instructions is determined at the passage of the solder between the two strips at the exit of said zone of the oven. From this date, the temperature is lowered according to a parameterized variation law referring to a class of products and based on portions of linear variation with variable duration and slope, up to the processing temperature of the second strip. Indeed, the heat treatment temperature of a metal can be higher than the target temperature without damage to it, as long as one remains within the limits provided for by metallurgical criteria, while a lower temperature does not meet the conditions for a good course of treatment.

Information relating to the specific characteristics of each band is provided by a management computer system and transmitted directly to the appropriate computers which generate instructions to be applied to the equipment.

In addition, the automats as well as the various measurement sensors, distributed along the line in particular in each zone of the oven, provide information on the progress of the strip, which makes it possible to monitor each product while along the line from the input pads to the output pads.

As can be seen in Figure 2, representing the scrolling of two successive strips of decreasing thickness, one thicker denoted 11 and the other less thick denoted 12 at three given instants, one denoted I, initial instant, instant A corresponding to the exit from the zone and instant B corresponding to the end of the transition.

Figure 3 shows the evolution of the temperature setpoint of an oven zone whose output is marked by line 13 (Figure 1) from time I between temperatures T1 and T2 with T1 higher at T2 taking into account an additional anticipation corresponding to the dead time due to the inertia of the furnace. It can be seen that the ramp of negative slope begins at the passage of the connection point between the strips 11 and 12 at the instant I corresponding to a distance Dm before the end of the zone of the oven whose temperature is sought to be controlled. The progressive drop in temperature takes place during the passage of the strip 12 requiring the lowest temperature T2 over a distance De after the exit from said zone.

In these figures, the time interval LA corresponds to the dead time due to the inertia of the oven and the time interval AB to the time of establishment of the new oven temperature. The distances Dm and De correspond respectively to these time intervals.

However, according to the configuration and the average load of the installation, it is possible to consider that the instant of triggering anticipated compared to the exit from the zone can be identical to a synchronized triggering on the entry into said zone.

The opposite situation is described in FIGS. 4 and 5 since it involves the passage from a strip of thin thickness denoted 11 to a strip of greater thickness denoted 12. The strip 11 requires heat treatment at temperature T1 and the strip 12 a heat treatment at the temperature T2 with T1 lower than T2. FIG. 4 represents the position of the two respective bands at three separate instants, the initial instant A, the intermediate instant B and the final instant F corresponding to the end of the transition.

As can be seen in FIG. 5, the temperature transition takes place at time A, that is to say when the strip 12 is at a distance De + Dm from the entrance to the zone. The intermediate instant B represents the end of the dead time while the weld is at the distance De from the entry of the zone. The final instant F is such that during the passage of the weld between the strip 11 and the strip 12, taking into account an additional anticipation to take into account the delay due to the dead time, at the entry of the zone of the oven, the oven has already acquired the temperature T2 necessary for the processing of the strip 12. The variation of the temperature of the zone is therefore effected by the application of temperature setpoints increasing continuously from time A of temperature T1 to temperature T2.

In this second case of temperature evolution, the time interval AB corresponds to the dead time and the time interval BF to the establishment time. It can be seen that the corresponding dead time distances Dm and establishment time De are distributed differently since in this case, it is necessary that at the final instant F, the new higher set temperature is obtained.

The method according to the invention here provides for determining from the speed of advancement of the strip and the characteristics of the oven and the class of the product, the date of the start of the temperature ramp as well as the value of its slope.

During the entire course of the heat treatment, in accordance with the process according to the invention, the values of the main characteristic elements of the strip and of the installation are recorded periodically. heat treatment and in particular the speed of the strip, the condition of the line, the temperature of the strip and the temperature of the oven in each of the zones, to refine the temperature and speed setpoints applied to the installation.

When the strip continues to advance in the furnace and the next strip arrives in the considered zone of the furnace, the method calculates the law of variation of the temperature to be applied for the transition between the next strip and the strip still after and possibly restores causes the previously calculated transition between the strip leaving the oven zone and the next strip, in particular when the latter must cause a drop in temperature.

This has the effect of avoiding lowering the temperature too much if it has to be raised immediately afterwards.

When the line must change speed, for example due to an increase in speed requested by the coating installation following the annealing installation on the line, the process acquires the value of the target speed to be reached and the instant where the target speed must be reached.

Due to the line speed change, it will be necessary to modify the oven temperature setpoints so that the thermal cycle of the strip is respected with the new speed.

For this, the computer determines the transient in speed and in temperature to be applied so that the speed change does not affect the heat treatment of the strips, that is to say the start time of the temperature and speed changes. and the laws of speed and temperature modifications.

These parameters are determined taking into account any transitions due to changes in the thermal cycle in progress or planned, this to avoid by example a drop in temperature dictated by the change in line speed followed by a drop in temperature dictated by the change in thermal cycle.

In certain use cases, it may happen that the speed must be reduced significantly before being immediately increased again to return to its initial value. In such circumstances, provision is made not to allow the temperature of the oven to drop and to keep the temperature present so as not to consume excessive energy to raise the temperature to this same temperature level subsequently.

In the case where the speed change is undergone, that is to say detected after the fact, after detecting the speed change, the method makes it possible to trigger a transient on the temperature to adapt to the new speed.

In Figure 1, we can see the diagram of a device for implementing the method according to the invention comprising a continuous annealing furnace 1 consisting of six distinct zones 1A to 1F through which passes a strip 2 shown in progress treatment, followed by a strip 3 welded to the strip 2 according to a weld zone 4. This weld zone 4 comprises a hole 5 opening out on each side of the strip and allowing the location thereof, at particular points of the line, during its running, by means of sensors 6. On the rest of the line, the position as well as the speed of the strip are determined by measuring sensors 22 installed on the different rollers 23 of the line and which, taking into account the speed of travel of the strip and the radius of the rollers which drive it, integrate the abscissa of the strip. These sensors are connected to the information processing unit 7.

Strip temperature sensors 8 are placed in each of the different zones of the oven. The information collected by these sensors 8 is transmitted to the information processing unit 7. Information and program storage means 9 are also provided and connected to the information processing unit 7. This unit 7 controls devices 10 for adjusting the temperature of each of the zones of the oven, as well as a device for adjusting the speed of advance of the strip, not shown in FIG. 1. A device 21 for viewing the information collected at the level of the oven and at the level of the strips is also provided to inform the operator and allow him to control the installation in certain particular circumstances.

Thus the use of such a device for the implementation of the process allows a heat treatment of strips of different nature without the addition of intermediate strips and without unnecessary consumption of energy for heating the oven, during transient conditions, in the limit where the variations in the product class parameters are sufficiently small, for example by assuming that variations in thickness do not exceed 20%.

Claims (11)

Method for automatically operating a continuous annealing furnace with radiation by radiant tubes of the type comprising several successive zones of heating at distinct temperatures, used in particular for carrying out different continuous heat treatments of strips connected to each other and requiring annealing operations specific, in which the position of the ends of the strips is determined, the speed of advancement of the strip, the temperature in each of the zones of the furnace, characterized in that the evolution of the temperature of each of the zones of the oven and the speed of advance over time from the measured results and the characteristics of the strip just entering the oven and its next, so that the temperature in the different areas of the oven is always higher or equal to the values necessary for heat treatment and that the speed of the strip is adapted to the milking time ment necessary in each zone depending on the temperature of said zone. Process for the automatic control of a continuous annealing furnace according to claim 1, characterized in that for each zone of the furnace: the geometrical characteristics of the strip which enters the zone of the furnace, in particular its thickness and its width, the thermal cycle to be subjected to it and the temperature set points to be applied to the zone of the furnace to carry out the thermal cycle, are determined, as well as the geometrical characteristics of the strip according to that which is in the zone of the furnace and the thermal cycle to be subjected to it; - one calculates by means of a control law according to the conditions of the strip in the zone of the furnace, the conditions of the following strip and the constraints due to the characteristics of the furnace, the optimal transition time for the furnace zone to go from temperature to carry out the thermal cycle of the strip which is in said zone at the temperature to carry out the thermal cycle of the following strip, and the time from which it is necessary to modify the temperature setpoints of the furnace zone, to ensure the thermal cycle of the strip following that situated in the furnace zone, without the temperature being lower than the temperature necessary for the thermal cycle of the strip situated in said zone, as well as the law of variation of the temperature in said zone; - the temperature variation instructions are sent to the oven control device; and - At the same time, the method consists in determining the speed of the strip in the oven and the speed at which it must run in the event of a speed change, and the time from which the speed setpoints must be modified is calculated. Process for the automatic control of a continuous annealing furnace according to Claims 1 and 2, characterized in that the maximum time for a temperature transition is that of the treatment of the product to which this transition is applied and in that the instructions relating to a transition are determined with an anticipation equivalent to the temperature establishment time plus a dead time due to the inertia of the zone of said furnace. Process for the automatic control of a continuous annealing furnace according to Claims 1 and 2, characterized in that the temperature of the various zones of the furnace is controlled by applying, during the transition phases, temperature setpoints for the furnace for which the trigger date, duration and slope from the measured parameters of the characteristics of bands, and other parameters from different product classes. Process for automatic control of a continuous annealing furnace according to Claims 1 and 2, characterized in that the transition conditions are periodically recalculated to take into account any change in the operating point on the product in progress compared to the forecasts . Process for automatic control of a continuous annealing furnace according to Claims 1 to 3, characterized in that decreasing temperature ramps are applied in order to adapt the temperature of an oven zone from a first value to a second lower value when the start of the strip requiring the second value of said zone of the furnace, arrives at a predicted distance (Dm) before the exit of said zone of the furnace. However, depending on the configuration and the average load of the installation, it is possible to consider that the instant of tripping anticipated in relation to the exit from the zone can be identical to a synchronized trigger on the entry of said zone. Process for automatic control of a continuous annealing furnace according to Claims 1 to 3, characterized in that increasing temperature ramps are applied in advance in order to adapt the temperature of an area of the furnace to a first value to a second higher value during the presence of the strip preceding that requiring the second temperature value so that said temperature is reached when said strip enters said zone of the furnace. Method for automatically operating a continuous annealing furnace according to any one of the preceding claims, characterized in that the temperature variations of the zones of the furnace are limited when notes a succession of three strips respectively requiring high, low and then high temperatures during the passage of the strip requiring treatment at low temperature. Process for the automatic control of a continuous annealing furnace according to Claims 1 and 2, in which the speed of advance of the strip must be modified, characterized in that: - the value of the target speed to be reached and the instant when the speed must be reached are acquired; - the parameters of the speed and temperature transients to be applied are determined so that the speed change does not affect the heat treatment of the strips. Process for the automatic control of a continuous annealing furnace according to claim 9, characterized in that the parameters of the speed and temperature transients are determined taking into account any transients due to changes in the thermal cycle in progress or planned. Device for implementing the method according to any one of the preceding claims, characterized in that it comprises, on the one hand, detectors (6) of holes (5) placed along the path traveled by the strip and, on the other hand, measurement sensors (22) installed on rollers (23) which drive the strip, integrate the movement of the latter and thus carry out monitoring throughout the line, means (8) for measuring and regulation of the temperature in each of the zones of the oven, means for knowing the characteristics of the strip present in the oven and its next, means (9) for storing information and programs, processing means (7) information acquired, and display means (21) of the main operating parameters of the furnace.

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