EP0545768B1 - 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 method of automatic operation of a continuous annealing furnace at radiation by radiant tubes of the type comprising several heating zones at separate temperatures used in particular to carry out heat treatments different continuous bands connected between them and requiring specific annealing operations.

Currently the continuous annealing method is essential at the expense 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 objective current is to further improve the productivity of such facilities. To achieve this it is necessary constantly monitor the speed of the bands as well as the temperature in each of the areas of the oven.

In the state of the art, the strips are welded together before going into the oven continuous annealing. Bands of different nature, different width, different thickness, so can follow one another at the entrance of the oven. These bands can require different heat treatments so 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 band. 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 an oven with radiant tubes can be three degrees Celsius per minute; if you want to go from a 750 ° regime to a 800 ° speed, this results in a 50 ° variation, which requires a transition time of 15 to 20 minutes. If the tape runs, 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 being introduced between two bands requiring different heat treatments. It is when passing these intermediate bands that the oven temperature will vary so as to be at the right temperature when the tape arrives next. The intermediate belt is then downgraded and can be reused for similar purposes, except in the case where the strip has been continuously coated after treatment thermal.

The temperature control of the different zones of the oven is on the other hand managed by the operator, who depending on the characteristics of the bands present in the oven and arriving rule from an array of controls the temperature setpoints for each of the oven areas.

This solution has the disadvantage of requiring the use of intermediate bands which represent a large amount of metal lost, not to use the oven at all times, especially when passage of these intermediate bands, and not optimize the amount of energy spent to power the ovens, since an operator indicates the instructions for heats each of the oven zones empirically.

To avoid the use of intermediate bands, it was agreed to modify the continuous annealing furnace and to add controllable preheating units to it individually injecting heating gas directly on the tape, allowing variations temperature in the oven of around 100 ° per second.

A mixed radiation oven is then obtained by radiant tubes and forced convection by gas jets (see also FR-A-2 406 667 and EP-A-0 181 830).

Besides the cost of installing such an oven annealing, its operating cost is very high and such temperature variation rate applied locally on part of the strip, especially on the welded joint between two bands can cause, due to transformations that the metal undergoes, problems downstream of the annealing continuous, for example during skin pass treatment.

The object of the invention is to propose a method automatic operation of a continuous annealing furnace at radiation by radiant tubes to avoid the use of intermediate bands, without use a device with forced convection.

To this end, the subject of the invention is a method automatic operation of a continuous annealing furnace at radiation by radiant tubes of the type comprising several successive heating zones at temperatures separate, used in particular to perform treatments continuous thermal different from connected strips between them and requiring annealing operations specific. This process comprises the steps indicated in claim 1.

By oven temperature, we actually designate either the temperature of the oven atmosphere or the temperature of the heat exchange on a standard surface, for example 1 m ² . 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 an oven 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.

An automatic oven control process continuous annealing according to the invention manages for each furnace areas the modification of two types of settings.

On the one hand, it allows you to manage changes in products, whether it's the geometry of the strip, or of the heating cycle necessary for the heat treatment of said strip, on the other hand it allows the adaptation of the temperature of different areas of the oven to changes line speed due to imperatives technical or following operator's orders, by example when the annealing furnace is placed before a coating line, which imposes the running speed Of the band.

• 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 event of a change of speed, and we calculate the moment from which it takes modify the speed setpoints.

Each part of the oven which can be controlled individually, i.e. having own adjustment means making it possible to modify temperature conditions in this area without modify the conditions in neighboring areas.

In the event that there are only changes in speed, the process acquires the value of speed target to be reached and the instant the speed should be reached, determines the transient parameters in speed and temperature to apply for the change speed does not affect the heat treatment of bands.

Transient parameters in speed and temperature are determined taking into account possible transients due to changes in thermal cycle in progress or planned.

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

Taking into account the necessary heat treatment to the strip following that being processed, and line speed, the following process the invention provides for the calculation of temperature setpoints of the oven 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 when changing temperature for processing two successive coils or strips. Either the temperature is brought to be increased, made for example of increasing the thickness of the band, or on the contrary it is brought to be diminished.

When the temperature in an area of the oven should be decreased, for the next band, the start date of variation of the adjustment setpoints is determined at passage of the weld between the two strips at the outlet from said oven area. From this date, the temperature is lowered according to a law of variation parameterized referring to a product class and based on portions of linear variation with duration and variable slope, up to treatment temperature of the second band. Indeed, the processing temperature thermal of a metal can be higher than the target temperature without damage to it, as long as we stay within the limits provided by the criteria metallurgical, while a lower temperature does not does not meet the conditions for the smooth running of the treatment.

Characteristic information specific to each band are provided by a system IT management and transmitted directly to suitable computers which generate instructions for apply to equipment.

In addition, the machines and the various measurement sensors, distributed along the line in particular in each zone of the oven, provide information on the progress of the tape, which allows to make follow-up of each product 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 thickness decreasing, one thicker denoted 11 and the other thinner noted 12 to three times given, one noted I, initial instant, instant A corresponding to the output of the zone and the instant B corresponding to the end of the transition.

Figure 3 shows the evolution of the temperature setpoint of an oven zone whose outlet is marked by line 13 (figure 1) from the moment I between temperatures T1 and T2 with higher T1 at T2 taking into account an additional anticipation corresponding to the dead time due to the inertia of the furnace. We finds that the ramp of negative slope begins on passing from the connection point between bands 11 and 12 to the instant I corresponding to a distance Dm before the end the area of the oven whose temperature is sought to be controlled. The gradual drop in temperature takes place during the passage of the strip 12 requiring the most low temperature T2 over a distance From after the exit of said area.

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

However, depending on configuration and load average of the installation, it is possible to consider than the anticipated trigger time relative to the exit from the zone can be identical to a trip synchronized with entry into said area.

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

As can be seen in Figure 5, the transition of temperature takes place at time A, that is to say when the strip 12 is at a distance De + Dm from the entrance to the area. Intermediate B represents the end of the dead time while the weld is at distance From the entrance to the area. The final moment F is such that during the passage of the weld between the band 11 and band 12, taking into account an anticipation additional to take into account the delay due at idle time, at the entrance to the oven area, the oven has already acquired the temperature T2 necessary for the treatment of strip 12. The variation of the temperature of the zone is therefore carried out by applying temperature setpoints increasing continuously from the moment A from 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. We can see 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 moment F, the new higher set temperature, obtained.

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

Throughout the heat treatment, in accordance with the process according to the invention, periodically reads the values of the main characteristic elements of the tape and the installation heat treatment and in particular the speed of the tape, the condition of the line, the temperature of the band and oven temperature in each zone, to refine the temperature and speed setpoints applied to the installation.

When the strip continues to advance in the oven and the next band arrives in the area under consideration of the oven, the process calculates the law of variation of the temperature to be applied for the transition between the strip next and the tape again afterwards and eventually hands over involved the previously calculated transition between the strip leaving the oven area and the next strip, especially when this must cause a drop in temperature.

This has the effect of avoiding lowering the temperature if it must be raised immediately after.

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

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

For this, the computer determines the transient in speed and temperature to be applied so that shifting does not affect processing thermal bands, i.e. the start time of the changes in temperature and speed and the laws of changes in speed and temperature.

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

In some use cases, it can happen that the speed must be reduced significantly before being immediately increased again for return to its initial value. In such circumstances, it is planned not to let the temperature drop from the oven and keep the temperature present so as not to consume excessive energy to raise the temperature to the same temperature level thereafter.

In case the gear change is suffered, that is to say detected after the fact, the process allows after having detected the gear change triggering 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 following method the invention comprising a continuous annealing furnace 1 consisting of six distinct zones 1A to 1F across from which passes a strip 2 shown during treatment, followed by a strip 3 welded to the strip 2 along a weld zone 4. This weld zone 4 includes a hole 5 opening on each side of the strip and allowing the location thereof, at points individuals of the line, during its scrolling, at by means of sensors 6. On the rest of the line, the position as well as the speed of the tape are determined by measurement sensors 22 installed on the different rollers 23 of the line and which, given the tape speed and roll radius which drive it, integrate the abscissa of the band. These sensors are connected to the processing unit information 7.

Band 8 temperature sensors are placed in each of the different areas of the oven. The information collected by these sensors 8 is transmitted to the information processing unit 7. Des means of storing information and programs 9 are also provided and connected to the processing unit information 7. This unit 7 controls devices 10 for adjusting the temperature of each zone of the oven, as well as a speed control device advancement of the strip not shown in Figure 1. An information display device 21 collected at the furnace level and at the eastern strips also intended to inform the operator and allow him to order the installation under certain circumstances particular.

Thus the use of such a device for the implementation of the process allows heat treatment of bands of different nature without the addition of bands intermediaries and no unnecessary energy consumption for heating the oven, during transient conditions, within the limit where the variations of the parameters of product classes are sufficiently low, by example by admitting that variations in thickness do not not exceeding 20%.

Claims (10)

1. A method of automatically operating a continuous annealing furnace irradiated by radiant tubes and having significant thermal inertia of the type comprising several successive heating zones which are at distinct temperatures and are individually controllable, and a device for controlling the furnace, the furnace being used, in particular, to carry out different continuous thermal treatments on interconnected strips requiring specific annealing operations, in which:

   the position of the ends of the strips, the strip advancing speed and the temperature in each of the zones of the furnace are determined,

   for each zone of the furnace, the geometric characteristics of the strip entering the zone of the furnace are determined, particularly its thickness and its width, the thermal cycle to which it is to be subjected and the temperature setpoints to be applied to the zone in order to implement the thermal cycle, and the geometric characteristics of the strip following the strip which is in the zone of the furnace and the thermal cycle to which it is to be subjected are also determined, characterised in that:

   for each zone of the furnace, a control function based on the conditions of the strip in the zone of the furnace, the conditions of the next strip and the constraints due to the characteristics of the furnace is used to calculate

   the optimum transition period required for the zone of the furnace to change from the temperature for implementing the thermal cycle for the strip which is in the zone to the temperature for implementing the thermal cycle for the next strip, and

   the moment from which the temperature setpoints of the zone of the furnace must be modified to ensure the thermal cycle for the strip following the strip which is located in the zone of the furnace, without the temperature falling below the temperature required for the thermal cycle for the strip located in the zone; and

   the temperature variation setpoints are sent to the furnace control device.
2. A method of automatically operating a continuous annealing furnace according to claim 1, characterised in that the temperatures in the different zones of the furnace are controlled by applying, during the transition phases, furnace temperature setpoints, the triggering time, duration and gradient of which are calculated from the measured parameters of the characteristics of the strips and from other parameters originating from the different product classes.
3. A method of automatically operating continuous annealing furnacc according to any one of claims 1 and 2, characterised 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 setpoints 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 the furnace.
4. A method of automatically operating a continuous annealing furnace according to any one of claims 1 to 3, characterised in that

   in parallel, the speed of the strip in the furnace and the speed at which it must travel in the event of a change in speed are determined, and the moment from which the speed setpoints must be modified is calculated.
5. A method of automatically operating a continuous annealing furnace according to claim 4, in which the strip advancing speed must be modified, characterised in that:

   the value of the target speed to be reached and the instant at which the speed must be reached are obtained;

   the parameters of the speed and temperature transients to be applied so that the change in speed does not affect the thermal treatment of the strips are determined.
6. A method of automatically operating a continuous annealing furnace according to claim 4 or 5,
   characterised in that the parameters of the speed and temperature transients are determined, taking account of any transients due to changes in the thermal cycle in progress or the intended thermal cycle.
7. A method of automatically operating a continuous annealing furnace according to any one of claims 1 to 6, characterised in that the transition conditions are periodically recalculated to take account of any change in the operating point on the product in progress in relation to the forecasts.
8. A method of automatically operating a continuous annealing furnace according to any one of claims 1 to 7, characterised in that decreasing temperature ramps are applied in order to adjust the temperature of a zone of the furnace from a first value to a second, lower value when the start of the strip requiring the second value for the zone of the furnace arrives at a planned distance (Dm) upstream ot the exit from the zone of the furnace.
9. A method of automatically operating a continuous annealing furnace according to any one of claims 1 to 7, characterised in that increasing temperature ramps are applied in advance in order to adjust the temperature of a zone of the furnace from a first value to a second, higher value while the strip preceding the strip requiring the second temperature value is present, such that the second temperature is reached when the strip enters the zone of the furnace.
10. A method of automatically operating a continuous annealing furnace according to any one of the preceding claims, characterised in that the variations in temperature in the zones of the furnace are limited when three strips in succession requiring high, low and then high temperatures respectively are noted during the passage of the strip requiring a low temperature treatment.

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