EP0000454B1 - Process for controlling the interstand tension during the hot-rolling of thick products. - Google Patents

Description

The invention relates to the hot rolling of thick, flat or long products in trains fitted with tandem cages and, more specifically, to the control of the tension between successive cages.

The use of rolling mills with tandem cages, that is to say where the product is engaged simultaneously in two or more successive cages, has considerable economic advantages, in particular from the point of view of space and productivity of the installations. However, compared to semi-continuous trains, the main problem posed by this technique is the correct adjustment of the metal flow rates at the outlet of each of the cages. In fact, if an upstream cage delivers too much, the metal accumulates between the cages and the product is subjected to a harmful compression which causes surface defects, such as cracks, and poor geometry of the blank. On the other hand, if an upstream cage does not deliver enough, the next cage risks exerting excessive traction on the product, causing it to be constricted and even, in the extreme, breaking. It is therefore essential to be able to continuously determine the state of the product and, as soon as tension, traction or compression is detected, to act on the settings of the cages so as to bring the product back to Gans optimal rolling conditions.

For hot products which are not very thick, the control of the tension between cages is well resolved by the so-called “looping” method which consists in bending the product in the form of a loop between the cages and in regulating the speeds of the latter so as to maintain constant the length of the loop.

In the case of cold rolling, the tension between cages can be measured directly, by means of a tensiometer and, on the other hand, the control of metal flow rates does not need to be as precise because the tensile strength is high and the band can withstand significant stresses without the necking appear.

These techniques cannot be applied to hot and thick products. Indeed, the closure of the method would require bending of the blank which, in consideration of the thickness of the product, require the development of high powers cover and may damage the surface state by apparent r ⁱ tion of cracks . On the other hand, the critical tensions being extremely low when hot (a tension equal to half the resistance to deformation is enough to cause faults), it would be necessary to be able to measure the traction with extreme precision, which, under the conditions rolling, seems difficult to envisage.

A solution, consisting in using the indications provided by the intensities of the motors of the different cages, has been proposed for thick hot products. Indeed, when there is traction of the blank by a cage, this results in an increase in the intensity of the corresponding motor. while the intensity of the motor of the upstream cage decreases, the effort to be provided being less. But it is quite clear that this method can only provide imprecise indications, more or less usable, with experience, by the personnel responsible for the operation of the installation and that it is not adaptable to automatic operation. of the train.

The present invention proposes to allow the control of the rolling of thick hot products, by indirectly determining and continuously adjusting the tension of the product. The invention uses the basic relationships already known between the speeds of rotation of the working cylinders of consecutive cages and product flow speeds upstream and downstream of each cage, during rolling. These relationships are described, for example, in the review “Tech- nische Mitteilungen AEG-Telefunken •, vol. 66, n ° 6, June 1976, page 257. However, they were not considered to be exploitable by the experts in rolling to carry out the indirect measurement and the adjustment of the tension of the products, which used torque measurements. or the drive current of the cage motors to carry out this regulation. It is also known, for example from English patent n ° 1,163,069, to use the information coming from the measurement of the speed of the rolls and the speed of flow of the product on a stand of the rolling mill to detect the appearance of the skating on this cage, skating which is a harmful parasitic effect, and to modify the speed in order to remove this skating.

• - on mesure en continu la vitesse V, de défilement du produit entre les cages,
• - on mesure en continu la vitesse linéaire V_e de la surface des cylindres de travail d'une des cages,
• - on calcule, à l'aide des mesures précédentes, la valeur du glissement g_o pour cette cage,
• - on agit sur la vitesse de rotation des cylindres de travail d'au moins une des cages de manière à maintenir la valeur du glissement voisine d'une valeur de consigne correspondant à un état de traction faible.

To this end, the subject of the invention is a method of controlling the tension exerted on thick hot-rolled products, engaged between two successive cages comprising working rolls, method according to which:

• - the speed V, of movement of the product between the cages, is continuously measured,
• - the linear speed V _e of the surface of the working rolls of one of the stands is continuously measured,
• - we calculate, using the previous measurements, the slip value g _o for this cage,
• - It acts on the speed of rotation of the working cylinders of at least one of the cages so as to maintain the value of the sliding close to a set value corresponding to a low state of traction.

• - on élabore à l'aide des mesures des vitesses un signal correspondant au glissement g_o par application de la relation g_o = V_s - V_c, V_e
• - on compare en continu ce glissement à la valeur de consigne,
• - on élabore un signal d'erreur,
• - on modifie la vitesse de rotation d'au moins une des cages de manière à annuler le signal d'erreur.

According to a preferred embodiment of the invention:

• - using the velocity measurements, a signal corresponding to the slip g _{o is developed} by applying the relation g _o = V _s - V _c , V _e
• - this slip is continuously compared with the set value,
• - an error signal is produced,
• - the rotation speed of at least one of the cages is modified so as to cancel the signal error.

According to a particularly advantageous characteristic of the method according to the invention, the setpoint value chosen is equal to the value of the slip in the absence of traction increased by 2 to 5%.

According to a particularly advantageous embodiment of the invention, the speed of the product is measured by the so-called “correlation” method, known per se, which will be recalled below.

The values of the rotational speeds of the rolls corresponding, for a product, to rolling under low tension, can be used as preset values for a subsequent blank of similar characteristics.

V_s et V_e étant respectivement les vitesses de sortie et d'entrée du. produit et V_c la vitesse linéaire du cylindre.As will be understood, the method according to the invention is based on an indirect determination of the traction between cages, using the measurement of the slip. By sliding is meant the difference between the speed of the product and the peripheral speed of the cylinder, reduced to the speed of the cylinder. We can thus define a backward sliding, if we refer to the speed of the product at the entry of the cage, and a forward sliding, if we consider the speed of exit of the product. Let, for a cage, g the sliding forward and g 'the sliding back, we will therefore have:

V _s and V _e being respectively the exit and entry velocities of the. product and V _c the linear speed of the cylinder.

In the absence of traction, for example when the product is engaged in a single cage, there exists, for the latter, a unique value of the slip which, the friction being independent of the speed in hot rolling, depends only on the sole geometrical conditions (radii of the cylinders, thicknesses of entry and exit ...). This slip can be calculated, quite easily, for flat products from geometrical considerations, for other thick products we can only determine an experimental value which we know is independent of the rolling speed. On the other hand, when the product is subjected to a tension, traction or compression, in particular when, engaged in a cage, it enters the next one whose adjustment is incorrect, the slip varies. The rotational speeds of the cylinders are generally controlled, they therefore remain constant despite the tension; on the other hand the speed of the product undergoes a modification: it increases in the event of traction, decreases if there is compression.

The applicant's work enabled him to establish that it was possible to use the relationships already known by themselves between the variations of the front slip for one cage (or of the rear slip for the next) with the value of the tension exerted on the product by the downstream cage to indirectly measure the tension exerted on the product. It is therefore possible, by observing fluctuations in this slip, calculated from simultaneous measurements of product and cylinder speeds, to determine the corresponding changes in tension. In order to obtain low traction conditions, it is then possible to act on the speed of one of the stands to bring it back to a value close to the "ideal" value corresponding to rolling without tension.

The invention will in any case be well understood by referring to the description which follows, given by way of example with reference to the attached drawing board in which the single figure shows two consecutive cages of a continuous roughing train.

We have considered here the case of a flat product, but it is obvious that this example could be immediately transposed to any long hot-rolled product by replacing the notion of thickness by that of section.

Le produit 1 n'étant pas encore en prise dans la cage3, g_o est la valeur du glissement sans traction. Lorsque l'ébauche 1 entre dans la cage 3, si cette dernière est réglée correctement, elle en ressort avec une épaisseur diminuée et une vitesse supérieure à celle d'entrée. Par contre, si la vitesse de la cage 3 est trop faible ou trop élevée, cette dernière ne peut assurer le débit correct du métal entrant, ce qui se traduit respectivement par une compression ou une traction du produit. Dans les deux cas la vitesse de sortie de la cage 2 du produit 1 est modifiée, elle devient respectivement inférieure ou supérieure à la vitesse V, imposée par les seules conditions géométriques. Il en résulte une variation du glissement avant g qui devient respectivement inférieur ou supérieur à g_µ, V restant inchangée.In the figure, product 1 during rolling is represented at the moment when, engaged in a cage, referenced 2, it will enter the next cage 3. For reasons of simplicity, the two cages have been shown diagrammatically by their working rolls only. At the entry of the cage 2 the blank has a speed V _e , it emerges therefrom, after reduction of thickness, with the speed Vg. The cylinders having a linear speed V _c , the forward sliding of the cage 2 is given by the formula:

The product 1 not yet being engaged in the cage 3, g _o is the value of the slip without traction. When the blank 1 enters the cage 3, if the latter is adjusted correctly, it emerges with a reduced thickness and a speed greater than that of entry. On the other hand, if the speed of the cage 3 is too low or too high, the latter cannot ensure the correct flow of the incoming metal, which results respectively in compression or traction of the product. In both cases the output speed of the cage 2 of the product 1 is modified, it becomes respectively lower or higher than the speed V, imposed by the only geometric conditions. This results in a variation of the slip before g which becomes respectively less than or greater than g _µ , V remaining unchanged.

The speeds of the cylinder and of the product are measured continuously, the first for example using a tachometric dynamo, for the second any sufficiently precise means can obviously be used, but the applicant preferably recommends the process and the device described in French Patent No. 2,161,757. This method consists in calculating the speed by detection and correlation of two similar random signals, shifted in time and linked to the moving product. Two optical detectors 4 and 5, for example photoelectric receivers, are placed between the cages, above of the product, and are located at a well-defined distance from each other with respect to the direction of travel of the rolled product. The signals emitted by the detectors 4 and 5 are sent to a time-correlated correlator: the latter, not forming part of the invention, has not been shown here and, for more details on this apparatus, reference may be made to the aforementioned patent. To determine the initial value of the delay, the signals supplied by the passage in front of the detectors of the head of the product are used, therefore the average speed of movement of the head of the product. This method thus makes it possible to know, at all times, the real speed of the blank with great precision.

The signals corresponding to the speeds of the cylinders and of the product are sent to a device making it possible to calculate the value of the slip. This value is continuously compared with a setpoint value equal, in the example considered, to the value of the slip without traction increased by 3%. When the actual slip value becomes lower or higher than the set value, the comparator generates an error signal. The latter is transmitted to the control computer of the rolling line which immediately triggers an action on the speed of the stands, so as to reduce the error signal to zero. This modification can be carried out on cage 2, on cage 3 or on both simultaneously. Thus, in particular, when a traction of the cage 3 on the product 1 is detected, the speed of the cage 2 can be increased until the error signal is canceled.

Generally one laminates rather long series of sketches of similar characteristics; the values of the rotational speeds determined during the rolling of the first blank using the method according to the invention can be used as preset values before the engagement of the following ones.

In the example which has just been described, only the sliding before of the upstream cage has been considered, but it is understood that the sliding behind the downstream cage could also have been used.

The method according to the invention can very well be implemented on a manually controlled rolling mill, the information collected then being used directly and the corrective actions carried out by the personnel responsible for conducting operations. However, its application to partially or fully automated trains is of even greater interest: the error signal then becomes one of the parameters controlling the speed regulation of the cages. When the fully automated rolling is carried out using models, it is even possible to take into account slip fluctuations caused not by variations in tension, but linked in particular to the change in the thickness of the entry or of the rolling force, in order to avoid unnecessary or even erroneous corrections.

Currently, in hot trains for rolling thick products, they are rarely engaged in more than two cages at a time. However, the application of the method according to the invention is not limited to single tandem cages, but can easily be generalized to three and then to any number of cages where the product is taken simultaneously. A first approximation, generally sufficient, consists in considering the cages as independent two by two. Thus, in the case of three cages, when the last has been incorrectly preset, it appears at the engagement of the product a variation in the slip between the first two cages and a value different from the reference between the last two. A satisfactory result is then obtained by considering the last two cages as isolated from the first and by only modifying the speed of rotation of the third to bring the slip back to the set value. However, when models are available, it can be taken into account more rigorously that the deviation from the setpoint, when engaging in the third cage, is partly due to the tension which is established between the first two and which, compared to the same isolated cages, causes a change in the tension-slip relationship.

In all cases, the method according to the invention, making it possible to detect traction or compression as soon as it appears and to take immediate corrective action, appreciably reduces the number of blanks rejected because of their bad geometry and presents , therefore, a certain economic interest.

Claims (4)

1. A method of monitoring the interstand tension exerted on thick products which are hot- rolled, engaged between two successive stands comprising working rolls, characterised in that

- the speed of passage V_s of the product between the stands is measured continuously,

- the linear speed V_e of the surface of the working rolls of one stand is measured continuously,

- the slip value go for this stand is calculated from the previous measurements,

- the speed of rotation of the working rolls of at least one of the two stands is acted upon so as to maintain the slip value close to a reference value corresponding to a state of weak traction.

2. A method as claimed in claim 1, characterised in that :

- a signal corresponding to the slip go is calculated using the speed measurements by applying the relationship

- this slip is continuously compared with the reference value,

- an error signal is calculated,

- the speed of rotation of at least one of the stands is modified in order to cancel the error signal.

3. A method as claimed in one of claims 1 and 2, characterised in that the reference value is equal to the slip value in the absence of tension increased by 2 to 5 %.

4. A method as claimed in one of claims 1 to 3, characterised in that the speed of passage of the product is measured by detection and correlation of two identical random signals offset in time and linked to the product, the correlation being carried out by a delayed serv6 correlator, the initial value of the delay corresponding to the mean speed of passage of the head of the product in front of the detectors.

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