EP1647604A2 - Process and device for improving the quality of steel or aluminium sheet during heat treatment in a continuous furnace - Google Patents

Abstract

The procedure, in which steel or aluminium strip (3) passes round upper and lower rollers in a heating or cooling chamber (1), uses a computer (100) to provide real-time calculations of the heat profile of the rollers and the maximum admissible temperature of the metal strip for the most critical rollers. The calculations are effected by reference to at least one integrated physical heat exchange model including conduction exchanges between the strip and rollers, and strip heaters (4) are adjusted accordingly.

Description

The present invention generally relates to a process for improving the production of a vertical heat treatment line for steel or aluminum and / or improving the quality of the products to be treated.

More specifically, the method of the invention relates to vertical lines (or having a vertical chamber) for treating steel or aluminum strips using at least one vertical heating chamber or a vertical cooling chamber, such as the heat treatment lines, in particular continuous annealing lines, or such as coating lines, in particular metal or non-metallic coating lines.

This process aims to maximize the productivity of the line and the quality of the final product by determining the speed, heat transfer and traction of the treated product, optimal steady state and transient avoiding creases on the bandaged.

BACKGROUND OF THE INVENTION

With reference to FIGS. 1 to 4, a general description of the treatment lines of the steel or aluminum strips will be presented.

A vertical chamber for heating a strip processing line made according to the state of the art is constructed according to the principle shown in FIG. 1, on which a heating chamber 1 can be distinguished, transport rollers 2 or referral equipping said chamber, a metal strip 3 passing on said rollers, and heating elements 4. The strip 3 is heated in the chamber 1 mainly by the heating elements 4, which are usually made of electric radiant tubes or to gas combustion.

During its passage in the chamber 1, the band 3 is heated on both sides by the heating elements 4 located on either side of the line of passage, and said band changes line of passage to each return roller . The heating curve of the strip 3 in the chamber 1 is controlled by the indexing of the various heating elements 4 or groups of heating elements operating identically.

A vertical cooling chamber of a strip processing line made according to the state of the art is constructed according to the principle shown in FIG. 2, on which a cooling chamber 1 ', transport rollers 2' are distinguished. or referral equipping said chamber, a metal strip 3 'passing on said rollers, and cooling elements 4'. The strip 3 'is cooled in the chamber 1' mainly by the cooling elements 4 ', which are most often constituted by gas blowing assemblies at a temperature below the temperature of the strip.

During its passage in the chamber 1 ', the band 3' is cooled on both sides by the cooling elements 4 'located on either side of the pass line, and said band changes line of passage at each return roller. The cooling curve of the strip 3 'in the chamber is controlled by the indexing of the different cooling elements 4' or groups of cooling elements operating identically.

PRODUCTIVITY OF THE LINE AND QUALITY OF THE FINAL PRODUCT

The productivity of the line is determined by the capacity of each chamber 1 or 1 'to provide heat transfer of heating or cooling to reach strip temperatures at the exit of rooms that respect given temperature tolerances.

• des tolérances métallurgiques : il faut éviter de surchauffer ou de sous chauffer la bande, afin de respecter un cycle thermique qui détermine les caractéristiques mécaniques finales de la bande,
• des tolérances process qui sont déterminées en fonction des conditions opératoires compte tenu de la configuration de la chambre, de la production, et du produit à traiter. En pratique ces tolérances process sont le plus souvent déterminées par la contrainte de ne pas former des plis sur la bande (communément appelés par les anglo-saxons « heat buckles » dans les chambres de chauffage et « cool buckles » dans les chambres de refroidissement).

Tape temperature tolerances are actually two types of tolerances:

• metallurgical tolerances: it is necessary to avoid overheating or under heating the band, in order to respect a thermal cycle which determines the final mechanical characteristics of the band,
• process tolerances which are determined according to the operating conditions taking into account the configuration of the chamber, the production, and the product to be treated. In practice, these process tolerances are most often determined by the constraint of not forming folds on the strip (commonly referred to as "heat buckles" in heating chambers and "cool buckles" in cooling chambers). .

The quality of the final product is therefore highly dependent on the respect of the metallurgical tolerances, but also on the non-formation of folds on the strip (that is to say the respect of the process tolerances). Indeed, the folds can lead to permanent deformations that are not compatible with the end use, or scratches when the folds touch fixed elements located in line, or even breaks in the band.

The metallurgical tolerances are in turn determined reliably by the operator, usually by mechanical tests performed on tape samples taken at the end of the line.

PLISING TRAINING

In heating (Figure 1), there is in the chamber of the heating chamber 1, a temperature difference between the strip 3, which is cold, and the rollers 2, which are hot because placed in a hot environment. When the band 3 passes on the rollers 2, it cools them by contact, in an area that corresponds to its width. This effect is of course more marked for the first rolls.

The temperature distribution along the longitudinal axis of the roll then takes the form of a bowl, as shown in FIG. 3. This inhomogeneous temperature distribution along the longitudinal axis of the roller causes a differential expansion or thermal profile, which follows. the same profile as the temperature profile. The thermal profile is called diabolo.

In cooling (FIG. 2), there exists in the chamber of the cooling chamber 1 'a difference in temperature between the strip 3', which is hot, and the rollers 2 ', which are cold because they are placed in an environment cold. When the band 3 'passes over the rollers 2', it heats them by contact, in an area that corresponds to its width. This effect is of course more marked for the first rolls.

The temperature distribution along the longitudinal axis of the roll then takes the form of a crown, as shown in FIG. 4. This inhomogeneous temperature distribution along the longitudinal axis of the roll causes a differential expansion or thermal profile, which follows the same profile as the temperature profile. The thermal profile is said curved.

The hot profile of the rollers in contact with the strip is the superposition of a machining profile (that is to say cold profile) and a thermal profile (in diabolo in the heating chambers, and in curved in the cooling chambers).

In order to avoid guiding problems, it is absolutely necessary to avoid a "diabolo" hot profile of the roll (ie with a diameter in the median zone of the strip which is smaller than the diameter towards the edges of the roll. the band) but prefer a flat hot profile or with a slight bulge (that is, with a diameter in the median area of the band that is larger than the diameter towards the edges of the band). Indeed, a "diabolo" profile is not self-centering, and when the band is moving towards one of the edges of the roll, it is not biased towards the center, unlike the "curved" profile which is self-centering. This phenomenon has also been widely observed when driving rotating machines by flat belts.

To avoid this phenomenon, the rollers equipping the heating chambers are usually machined with an initial crown, which is sufficient to keep a hot profile with a very slight crown after the thermal profile due to the contact of the strip and the roll, while the rollers equipping the cooling chambers are most generally machined with a flat profile, the very slight final convexity of the hot profile is the thermal profile due to the contact of the strip and the roll.

The passage of a band on a non-cylindrical roll causes differential mechanical stresses depending on the width. When these mechanical stresses, superimposed on the other mechanical stresses (belt tension, dead weight, etc.), exceed the elastic limit (or the buckling limit which may be less than the elastic limit) of the strip at the given temperature, there is has fold formation.

This phenomenon exists in steady state, and much more in transient state. Indeed, when one proceeds to a change of format of the band (thickness and / or width) and / or a change of quality (thermal cycle) of band, said band then passes on rolls which have the profile to hot from the previous band, or from the same band under unstabilized conditions.

For example, in a heating chamber, the risk of formation of folds (commonly called "heat buckle") is all the greater as the profile of the roll under the band moves away from a cylindrical profile and the band is This band then has a lower elastic limit and a lower buckling limit. If we go from a narrow band to a wide band, the initial hot thermal curvature will be too pronounced, and this curvature of the roll too pronounced will therefore lead to a risk of large folds. Similarly, the passage of a thick band to a thin band in a heating chamber presents greater risks than in steady state.

EVOLUTION OF PRODUCTION

The problems of quality or loss of productivity due to the folds are not new, but have an increasingly important criticality for the reasons explained below.

Steelmakers have more and more formats (thickness, width) or tape qualities (thermal cycle) to process. The number of transitions increases (it is not uncommon to change thickness, bandwidth and / or band quality every two to five reels processed).

The formats of the bands evolve towards widths which are bigger and bigger. Plates 2000 mm wide are now common, whereas they rarely exceeded 1600 mm a few years ago. Moreover, the improvement of the final mechanical properties of the steels makes it possible to reduce the thickness, leading to a decrease in weight. Overall the width / thickness ratio therefore increases substantially, resulting in greater sensitivity to the folds.

The appearance of modern low carbon shades, in particular the generalization of steels without interstitials, requires for these stamping steels an annealing to higher temperature, that is to say a band with very low elastic limit and very low limit buckling. The resulting drop in mechanical strength at high temperature further accentuates the risk of wrinkles.

The appearance of fast cooling chambers directly from the annealing temperature accentuates the thermal profiles of the rollers generated by the band (very cold environment, and hot band).

The increase in productivity imposes an increase in speed, implying a need for more pointed guidance to avoid lateral offsets.

STATE OF THE ART

Various solutions have been implemented in the past, to improve the productivity of the lines and the quality of the products to be treated, as recalled hereinafter by way of example.

Variations in web tension have been tried by empirically or pseudo-empirically lowering the tensile strength for certain formats or tape qualities. This method often leads to a reduction in speed being associated with this decrease in traction in order to mitigate the risk of tape offset which increases when traction decreases, which implies a loss of productivity. Finally, given the lack of modeling, this method is not completely reliable, and risks of offset and / or folds remain, which results in quality and / or productivity losses.

Room pleat sensors have sometimes been used to control web speed (speed reduction). This technique, however, has the disadvantage of having no predictive action, and involves a loss of production.

We have also sought a production scheduling limiting the transitions and therefore the risks folds, and consequently the quality and / or productivity losses. However, this process is very restrictive because it involves a planning of the products to be processed, which imposes a stock upstream of the line, and thus lengthens the response time to a specific format and / or cycle request. a customer.

Transition (ie, non-commercialized or decommissioned) coils have also been used during format transitions and / or band quality at the risk of wrinkles, which avoids wrinkles on the strips marketed or otherwise. decommissioned, with consequences for production losses (tape breakage for example) and / or quality, but this implies a loss of productivity of the line.

A modification of the roll itself has been proposed (see for example JP-A-04-06733), but this technique is very expensive and difficult to implement at high speed.

Interposition of thermal screens (fixed or movable) between the edges of the roll and the radiant tubes of the heating chamber (see for example JP-A-06-228659) has also been tested, possibly supplemented by a curtain of atmosphere gas (see JP-A-02-282431) or heating elements (see JP-A-63-038532). This technique certainly allows to act on the thermal roller, but requires the implementation of complex equipment and relatively expensive investment and maintenance. Indeed, screens alone are not enough, and remain in any case passive actuators.

Dynamic screen methods have also been used, ie the reduction of the folds formed in the heating zone of the continuous treatment lines of the strips by action on the thermal crown of the transport rollers, this modification of the thermal state of rollers being carried out directly by modulating the heating of the radiant tubes located in the vicinity of these rollers. For example, reference may be made to EP-A-1 229 138. This method acts on the thermal crown only according to empirical criteria determined by the overall temperature tolerances (ie without dissociation of the metallurgical tolerances and process tolerances), and does not drive the web pull, and therefore does not optimize the productivity of the line. In addition, on most existing heating chambers, the radiant tubes are grouped together for heating control in vertical control zones, so that a control of the only radiant tubes located in the vicinity of the rollers imposes heavy modifications and therefore no economic.

• ils ne calculent pas en temps réel les échanges thermiques entre la bande et chaque rouleau, et donc n'intègrent pas l'effet de la conduction lié à la vitesse de la ligne, et
• ils ne dissocient pas les tolérances de température process et les tolérances de températures métallurgiques.

Transition calculation models have also been proposed to limit the temperature difference between the two tape formats during a transition, so that the temperature of the hottest product is compatible with predefined temperature tolerance guidelines. These models, however, have two major drawbacks:

• they do not calculate in real time the thermal exchanges between the band and each roll, and therefore do not integrate the effect of the conduction related to the speed of the line, and
• they do not dissociate process temperature tolerances and metallurgical temperature tolerances.

The production is therefore not optimized, in particular because the temperature tolerances are not maximum in some cases. Finally, anti-wrinkle strategies are generally global and applied to all transitions, which limits the productivity of the line.

Moreover, any combination of the above solutions has no major appeal.

To complete the state of the art, it is also worth mentioning some other documents that illustrate different approaches proposed to reduce wrinkles and / or obtain a stable guidance of the band on the rollers.

JP-A-08 013 042 thus describes a method of reducing wrinkles for a steel strip circulating in a heating chamber, wherein the heating elements are piloted to adjust the temperature of the heating chamber and the temperature of the heating chamber. the tape, based on the experience of previous operations. It should be noted that no real-time calculation of the hot roll profile and the web temperature is expected. The line speed is only determined empirically from acceptable temperature ranges, so no reference to an integrated physical heat exchange model is provided.

JP-A-07 278 682 describes a process analogous to the previous one, in which a gaseous atmosphere is piloted to prevent the formation of folds by modifying the instructions of a direct fire preheating, with the determination of a range of temperatures. eligible for the band. There is therefore no concern in this process of the roll profiles.

JP-A-02 030 721 describes a method for obtaining a stable guidance of the strip on the rollers (the question of the folds is therefore not directly concerned), based on the temperature differences between the edges and the center of the rolls, which deviations are kept below a predetermined threshold by changing the line speed. We therefore try to predict by calculation the distribution of the temperatures in the rollers according to the measurements of thickness, width, temperature, and speed of the band. However, it is not suggested to calculate in real time the hot roll profile, and no reference to an integrated physical heat exchange model is planned.

JP-A-62 089 821 discloses a method for preventing both side slip of the web and ply formation, of estimating the curvature of the rolls and modifying the line speed so that the curvature remains in the web. a predetermined range. It is therefore a purely empirical approach with reference to graphs, consisting of empirically selecting band heating profiles in the heating chamber and then interpolating. It should be noted that the estimation of the curvature of the rollers in question has nothing to do with a real-time profile calculation.

JP-A-60 021 335 discloses a fold detector by a sensor which transmits an alert message in the event of a detected bend, which message triggers a readjustment of the parameters of temperature, line speed, and band voltage. .

JP-A-08 060 255 discloses a method of detecting and predicting fold formation by repetitive calculations of web temperature, roll temperature, and roll profile. It should be noted that no reference to an integrated physical heat exchange model is planned.

Finally, JP-A-04 056 733 discloses a roll structure with an inner heating sleeve for a more uniform distribution of temperature.

OBJECT OF THE INVENTION

The aim of the invention is to optimize the line speed and / or the strip tension and / or the temperature tolerances. process economically to maximize line production and the quality of the final product.

GENERAL DEFINITION OF THE INVENTION

The technical problem mentioned above is solved according to the invention by means of a process for improving the production of a vertical heat treatment line for steel or aluminum and / or for improving the quality of the products to be produced. process by reducing the folds formed in a heating or cooling chamber for a metal strip passing on transport and / or return rollers equipping said chamber, with a determination of acceptable strip temperature tolerances for non-forming folds on the rollers, in stable or transient state, said method being characterized by a real-time calculation of the hot profile of the most critical roll or rolls for the formation of folds, and a real-time calculation of the maximum allowable strip temperature for the hot profile or profiles calculated for the at least one most critical roll, said calculations being made by reference to at least one mod an integrated physical heat exchange shield including at least the conductive exchange between the strip and the rolls of the heating or cooling chamber according to claim 1.

Preferably, the method comprises optimizing the heat profile (s) calculated in real time from the most critical roller (s) by acting on the heating or cooling elements of the strip equipping the heating or cooling chamber.

Advantageously, the method comprises calculating and applying in real time the maximum allowable strip speed and / or the maximum allowable strip tension for the hot profile or profiles calculated in real time from the most critical roll or rolls. .

The invention also relates to a device for implementing an improvement method having at least one of the abovementioned characteristics, said device being remarkable in that it comprises a process calculator using at least one integrated physical model of heat exchange including at least the conductive exchanges between the strip and the rolls of the heating or cooling chamber, said calculator being arranged to determine acceptable strip temperature tolerances for non-wrinkling on the rolls, and for calculate in real time the hot profile of the most critical roll or rolls for the formation of folds and the maximum permissible temperature for the hot profile or profiles calculated for the at least one most critical roll, with reference to said at least one model integrated physics of heat exchange.

Preferably, the process computer is also arranged to calculate in real time the allowable speed and / or web tension for the one or more hot profiles calculated in real time from the most critical roll or rolls.

Other characteristics and advantages of the invention will emerge more clearly in the light of the following description of a particular embodiment, with reference to FIGS. 5 and 6, in which the same references have been kept for the homologous members. than those used in Figures 1 to 4.

DETAILED DESCRIPTION OF THE MEANS FOR IMPLEMENTING THE INVENTION

According to the invention, and contrary to the solutions of the state of the art, only the metallurgical temperature tolerance constraints are indicated as set by the operator for a given band, and a mathematical model optimizes the line speed, process temperature tolerances and / or tensile tape to avoid wrinkles and ensure maximum productivity based on the real-time calculation of steady and transient roll profiles. The mathematical model integrates in real time the effect of conduction between the band and the rollers.

An example of implementation of the invention in a heating chamber (FIG. 5) is constituted by a process computer 100 which mainly models the heat exchanges between the strip 3 and the heating elements 4, the heat exchanges between the strip 3 and the rollers 2, and the heat exchange between the chamber 1 and the band 3, in order to calculate the real-time thermal profile of each roll or critical roll for forming folds. The computer 100 is thus able to take and / or indicate the actions necessary to prevent the formation of folds.

An exemplary implementation of the invention in a cooling chamber (FIG. 6) is constituted by a process computer 100 which mainly models the heat exchanges between the strip 3 'and the cooling elements 4', the heat exchanges between the band 3 'and the rollers 2', and the heat exchanges between the chamber 1 'and the band 3', in order to calculate the real-time thermal profile of each roll or each critical roll for the formation of folds. The computer 100 is thus able to take and / or indicate the actions necessary to prevent the formation of folds.

The implementation in one or more heating chambers 1 can naturally be combined with the implementation in one or more cooling chambers 1 '.

The integrated physical heat exchange model used by the process computer 100 concerns at least the conductive exchanges between the 3 or 3 'band. and the rollers 2 or 2 'of the heating or cooling chamber. However, it may also concern other heat exchanges, in particular radiation exchanges (between the strip and the heating or cooling elements, or between the strip and the heating or cooling chamber), or convection exchanges ( between the strip and the gaseous atmosphere prevailing in the heating or cooling chamber).

When it has been determined which roll or rolls 2 or 2 'are the most critical for the formation of folds, the integrated physical model serves as a basic reference for heat exchanges between the 3 or 3' strip and the rollers 2 or 2 ', in particular the rollers which are upstream of the most critical roller or rollers, because the latter obviously have a predominant influence.

• i. maximise la vitesse de bande en régime stable (c'est-à-dire pour une même bande et un même cycle thermique) pour respecter cette tolérance de température process (ainsi que les tolérances de température métallurgiques) ;
• ii. maximise la ou les vitesses des bande en régime transitoire (c'est-à-dire lors d'un changement de cycle thermique et/ou d'un changement de format) pour respecter cette tolérance de température process (ainsi que les tolérances de température métallurgiques) ;
• iii. agit au besoin sur la traction de bande pour respecter les impératifs de guidage et de non formation de plis compte tenu des profils de rouleaux calculés ; et
• iv. agit au besoin sur les organes de chauffage ou de refroidissement de bande pour optimiser le profil de rouleau afin d'augmenter la tolérance de température process.

In either of the two cases above, the process computer 100 determines the process temperature tolerance (maximum band temperature) for non-formation of folds taking into account the profile of each calculated roll and:

• i. maximizes the steady-state bandwidth (ie for the same band and thermal cycle) to meet this process temperature tolerance (as well as metallurgical temperature tolerances);
• ii. maximizes the transient band speed (s) (that is, during a thermal cycle change and / or format change) to meet this process temperature tolerance (as well as temperature tolerances) metallurgical);
• iii. it acts on the web tension if necessary to respect the requirements of guidance and non-formation of folds considering the calculated roll profiles; and
• iv. works on tape heating or cooling elements as needed to optimize the roll profile to increase the process temperature tolerance.

• gain de productivité de la ligne, par application instantanée de la vitesse maximum compatible avec la non formation de plis ;
• gain de qualité et de productivité par garantie de non formation de plis (avec les conséquences associées de production de second choix, de ralentissement de ligne, ou de casse de bande) ;
• gain de flexibilité par une possibilité de passage de transition de format et ou de qualité de bande non réalisable avec les solutions traditionnelles.

The invention provides very important advantages, which are recalled below:

• productivity gain of the line, by instantaneous application of the maximum speed compatible with the non-formation of folds;
• quality and productivity gain through a non-wrinkle guarantee (with the associated consequences of production of second choice, line slowing, or tape breaking);
• flexibility gain through a possibility of passage of format transition and or band quality not achievable with traditional solutions.

Claims (6)

Process for improving the production of a vertical heat treatment line for steel or aluminum and / or for improving the quality of the products to be treated by reducing the folds formed in a chamber (1; 1 ' ) for heating or cooling for a metal strip (3; 3 ') passing on transport and / or return rollers equipping said chamber, with a determination of acceptable strip temperature tolerances for non-wrinkling on the rolls in stable or transient conditions, characterized by a real-time calculation of the hot profile of the most critical roller (s) (2; 2 ') for ply formation, and a real-time calculation of the maximum permissible strip temperature for the hot profile or profiles calculated for the at least one most critical roll, said calculations being made by reference to at least one integrated physical heat exchange model including at least the exchange conductive between the strip (3; 3 ') and the rollers (2; 2 ') of the heating or cooling chamber (1; 1'). A method according to claim 1, characterized by optimizing the hot calculated profile (s) in real time of the most critical roll or rollers (2; 2 ') by acting on the heating or cooling elements (4; 4'). strip of equipping the chamber (1; 1 ') for heating or cooling. A method according to claim 1 or claim 2, characterized by calculating and applying in real time the maximum allowable web speed for the one or more hot profiles calculated in real time from the one or more rolls (2; more critical. A method according to claim 1 or claim 2, characterized by the calculation and application in real time of the maximum allowable tensile strength for the one or more hot profiles calculated in real time from the most critical roller (s) (2; 2 '). Device for implementing an improvement method according to one of claims 1 to 4, characterized in that it comprises a process computer (100) using at least one integrated physical heat exchange model including all the minus the conductive exchanges between the strip (3; 3 ') and the rollers (2; 2') of the heating or cooling chamber (1; 1 '), said calculator being arranged to determine acceptable strip temperature tolerances for non-forming folds on the rollers, and for real-time calculation of the hot profile of the most critical roll or rolls (2; 2 ') for the formation of folds and the maximum permissible temperature for the hot runner (s) calculated for said one or more most critical rolls, with reference to said at least one integrated physical heat exchange model. Device according to claim 5, characterized in that the process computer (100) is also arranged to calculate in real time the speed and / or the strip tension admissible for the hot profile or profiles calculated in real time of the roll or rolls (2; 2 ') the most critical.

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