EP0517629B1 - Method and apparatus for casting semi-finished products - Google Patents

Abstract

The invention relates to a method and a device for the continuous or semi-continuous casting of plates or of billets, in particular in aluminium or in aluminium alloy. The method consists in completely automating the casting, restricting the intervention of the operator, controlling the level of metal in the supply channel, imposing a law on the levels of metal as a function of time, which law is common to all the ingot moulds, after individual laws have been applied to each of the ingot moulds in order to enable them to rejoin this common law, and controlling the descent of the support plate of the false bottoms and the stopping of casting when the programmed length is attained. The device comprises sensors for detecting the heights and levels of metal in the channel and the ingot moulds, an automatic-action system for regulating the levels and controlling various operations: tilting of the furnace, descent of the plate, stopping of casting, and actuators for controlling the flow rate of metal into the ingot moulds. By restricting human intervention, the invention provides greater safety. By permitting more reproducible casting conditions, it affords better cast-product quality. <IMAGE>

Description

1 °) TECHNICAL AREA OF THE INVENTION.

The invention which is the subject of this patent application relates to the continuous or semi-continuous vertical casting of metallic semi-finished products.

• d'un moule de coulée ou lingotière (1) de forme cylindrique ou prismatique droit selon la forme de la section du produit coulé: billette ou plaque. Cette lingotière a un axe de symétrie vertical et est ouverte à ses extrêmités supérieure et inférieure;
• d'un faux-fond (2) obturant l'extrêmité inférieure au démarrage et constituant ainsi le fond d'un moule dont la lingotière constitue les parois;
• d'un système de refroidissement de la lingotière, réalisé par exemple par une lame d'eau (3) frappant la partie supérieure externe de la lingotière et ruisselant le long de ses génératrices, puis le long du produit coulé;
• d'un plateau (4) supportant le faux-fond muni de moyens non représentés permettant sa descente à vitesse réglable et régulière;
• d'un chenal (5) d'alimentation en métal liquide, muni d'une ou plusieurs busettes (6) par où s'écoule le métal dans la lingotière;
• d'un flotteur (7) ayant pour fonctions d'une part de répartir le métal dans toute la section du produit et d'autre part de réguler la hauteur du métal dans la lingotière, la pastille du flotteur éventuellement munie d'un pointeau (8) venant obstruer partiellement la busette quand le niveau a tendance à monter.

The principle of vertical continuous casting is well known: it consists in casting the liquid metal inside a mold formed by the already solidified part of the cast metal. A continuous casting apparatus is constituted in its simplest form (Figure 1):

• a casting mold or ingot mold (1) of cylindrical or prismatic shape, depending on the shape of the section of the cast product: billet or plate. This ingot mold has a vertical axis of symmetry and is open at its upper and lower ends;
• a false bottom (2) closing off the lower end at start-up and thus constituting the bottom of a mold whose mold forms the walls;
• a cooling system for the ingot mold, produced for example by a blade of water (3) striking the upper external part of the ingot mold and flowing along its generatrices, then along the cast product;
• a plate (4) supporting the false bottom provided with means not shown allowing its descent at adjustable and regular speed;
• a liquid metal supply channel (5), provided with one or more nozzles (6) through which the metal flows into the ingot mold;
• of a float (7) having the functions on the one hand of distributing the metal throughout the section of the product and on the other hand of regulating the height of the metal in the ingot mold, the tablet of the float possibly provided with a needle ( 8) partially obstructing the nozzle when the level tends to rise.

At start-up, the plate is in the high position and the false bottom is engaged slightly inside the mold with a clearance such that the liquid metal cannot flow through it. The mold is cooled by the water blade. The liquid metal is poured through the channel (5) and the nozzle (6) into the mold formed by the ingot mold and the false bottom. Solidification begins from the walls of the cooled ingot mold and the false bottom. When a sufficiently solid crust is thus formed, the lowering of the false bottom support plate is controlled and the casting is continued, the limit of the solid and liquid phases called the solidification front having in section the approximate shape represented in FIG. 1 in (9).

• soit de poursuivre la coulée et de l'arrêter lorsque la longueur souhaitée de produit coulé est atteinte: c'est la coulée semi-continue, la seule appliquée aux alliages d'aluminium (appelée cependant le plus souvent "coulée continue")
• soit de poursuivre la coulée jusqu'à vidange complète du four en sectionnant le produit coulé en longueurs successives souhaitées, après l'avoir éventuellement cintré pour le rendre horizontal: c'est la coulée continue au sens strict, appliquée à l'acier.

It is then possible:

• either continue casting and stop it when the desired length of cast product is reached: it is semi-continuous casting, the only one applied to aluminum alloys (however most often called "continuous casting")
• either continue casting until the oven is completely emptied by sectioning the cast product into desired successive lengths, after having possibly bent it to make it horizontal: it is continuous casting in the strict sense, applied to steel.

For productivity reasons, it is usual, at least in the casting of aluminum products, to sink several products at the same time: several ingot molds are arranged on a site, several false bottoms on a support plate and the channel is provided with several nozzles.

The manual start of a casting on such a multiple-flow site has several drawbacks both in terms of product quality and operator safety. The molds do not fill at the same speed; However, since the lowering of the false bottoms can only start when the same required level of metal is reached in all the molds, the operator has to intervene manually to delay the filling of the molds most "in advance" so as to bring them all at the same time to the start level. It is understood that this method which requires great dexterity on the part of the operator does not ensure good reproducibility of the starting conditions and is the source of faults at the feet of the cast products which can lead to the reject of the entire product. In addition, it exposes the operator, in the immediate vicinity of the site, to splashes of liquid metal, to more frequent metal-water explosive reactions under transient conditions of start-up.

2 °) PROBLEM POSED.

The problem posed by the applicant is the complete automation of the semi-continuous casting, making it possible to remove the operator from direct proximity to the site, this operator only intervening to indicate the parameters of the casting: nature of the alloy, shape, size and number of products poured simultaneously and to trigger the casting by a command given to the automation system. This system controlling the casting will therefore include a database containing the conditions as a function of alloys and formats and their possible evolution over time.

3 °) DESCRIPTION OF THE PRIOR ART.

• mesurer le niveau de métal dans le moule,
• fournir un signal représentant ce niveau,
• générer un signal de référence représentatif du niveau de métal désiré,
• comparer le signal représentant le niveau avec le signal de référence,
• générer un signal de sortie pour agir soit sur le débit de métal,soit sur la vitesse de descente du produit coulé.

Patent GB 1449846 (CONCAST) published on 9/15/1976 describes a method for regulating the flow of metal in a continuous casting installation which consists in:

• measure the level of metal in the mold,
• provide a signal representing this level,
• generate a reference signal representative of the desired metal level,
• compare the signal representing the level with the reference signal,
• generate an output signal to act either on the metal flow rate or on the rate of descent of the cast product.

The teaching of this patent does not make it possible to solve two problems: that of the regulation of the continuous casting with several flows and that of the transient conditions of filling of the ingot molds at startup.

U.S. patents 4,498,521 and 4,567,935 (KAISER ALUMINUM & CHEMICAL) respectively describe an automatic casting process and device. The method solves the two problems indicated above since it is a method of regulating the level of metal in a vertical continuous casting with several flows during start-up, making it possible to bring the metal levels in the various ingot molds into the same horizontal plane before the descent of the plateau.

However, this process does not make the entire casting operation completely automatic since it does not support tilting the furnace. Furthermore, as will be explained later, the level laws of the ingot molds individual are substantially parallel, that is to say that they join the common law of the levels at different points. In the present invention, each individual ingot mold has its own law for catching up with the common law, more or less sloping depending on the level delay of the ingot mold considered and adapted so that the common law of levels is joined at the same time.

• détection du niveau de métal liquide,
• comparaison du niveau détecté avec un niveau de référence pour déterminer l'écart,
• variation du débit de métal liquide à l'aide d'un moteur pas-à-pas, les pas du mouvement dudit moteur étant la somme d'un terme proportionnel à l'écart, d'un terme proportionnel à la dérivée de l'écart et d'un terme proportionnel à la dérivée seconde de l'écart.

U.S. Patent 4,660,586 (ALUMINUM COMPANY OF AMERICA) claims a method for regulating a level of liquid metal poured into a container, which comprises the following steps:

• liquid metal level detection,
• comparison of the detected level with a reference level to determine the deviation,
• variation of the flow of liquid metal using a stepping motor, the steps of the movement of said motor being the sum of a term proportional to the difference, of a term proportional to the derivative of the deviation and of a term proportional to the second derivative of the deviation.

The teaching of this patent is entirely general: it can be applied to regulating the level of any liquid in any container and in particular to regulating the level of metal in an ingot mold. However, it also does not automatically take charge of all the successive phases involved in starting and continuing a continuous casting operation.

4 °) STATEMENT OF THE INVENTION.

The invention relates to a method and a device allowing the complete automation of the successive phases involved in the starting and the continuation of a continuous casting operation with several flows of plates or billets of metal alloy and in particular of alloy of aluminum.

• imposer des séquences de remplissage les plus simultanées possible de toutes les lingotières afin que cette phase se déroule dans les mêmes conditions pour tous les écoulements et que le démarrage de la descente du plateau ait lieu à un niveau de métal liquide voisin, prédéterminé et reproductible dans toutes les lingotières;
• éviter cependant d'avoir à fermer complètement l'alimentation en métal liquide dans une lingotière, cette fermeture complète provoquant des replis froids sur les pieds des produits, sources de criques.

Two basic principles guided the conception of the invention:

• impose the most simultaneous filling sequences possible on all ingot molds so that this phase takes place under the same conditions for all flows and that the start of the descent of the tray takes place at a neighboring liquid metal level, predetermined and reproducible in all ingot molds;
• however, avoid having to completely shut off the supply of liquid metal in an ingot mold, this complete shutdown causing cold folds on the feet of the products, sources of cracks.

• 1) Phase préalable de détermination du point de fermeture et de prépositionnement des quenouilles, sortes de pointeaux servant au réglage du débit dans chaque lingotière, dont la description sera donnée plus loin.
• 2) Phase d'alimentation en métal par le four de coulée.
• 3) Phase d'alimentation forcée des lingotières.
• 4) Phase de "rattrapage" individuel du niveau de métal dans chaque lingotière au cours de laquelle on amène les lingotières à un niveau imposé commun.
• 5) Phase de suivi d'une loi de niveau commune à toutes les lingotières.
• 6) Phase de descente du plateau avec suivi d'une loi de niveau en fonction de la longueur coulée.

The method is characterized in that it comprises a preliminary phase and five successive main phases. The operator triggers the first preliminary phase, then the first main phase. The following are linked automatically:

• 1) Preliminary phase of determining the closing point and pre-positioning of the cattails, kinds of needles used for adjusting the flow rate in each ingot mold, the description of which will be given below.
• 2) Metal supply phase by the casting furnace.
• 3) Forced feeding phase of the molds.
• 4) Individual "catch-up" phase of the metal level in each ingot mold during which the ingot molds are brought to a common imposed level.
• 5) Monitoring phase of a level law common to all ingot molds.
• 6) Stage of descent from the plateau followed by a law of level depending on the length of casting.

Each of these phases itself comprises a certain number of successive stages described below:

4.1 Preliminary phase of determining the closing point and prepositioning the cattails.

This phase is launched at the operator's request, before the start of casting.

The system first performs an automatic search sequence for the closing point of each of the cattails. This is a prerequisite for the next step, which is the prepositioning of the stopper rods before casting and for the actual casting steps, where positioning at well-defined openings will also be ordered.

In the general case, the prepositioning of the cattails will be done with zero opening. In a variant of the method, described below in more detail, this prepositioning is done at an opening other than zero, and possibly specific to each of the flows.

4.2 Metal supply phase by the casting furnace.

This phase is launched at the operator's request, and provided that phase 4.1 has been successfully concluded. It is the first of the phases of automatic casting to which the following phases are linked without further operator intervention.

• a) le basculement du four d'alimentation en métal liquide s'il est de type basculant ou l'ouverture de la quenouille obturant le trou de coulée s'il est fixe;
• b) le remplissage du chenal alimentant les lingotières. Dans le cas général, les busettes disposées à la partie inférieure du chenal au dessus de chaque lingotière sont fermées par les quenouilles, de sorte que le métal remplisse le chenal sans s'écouler dans les lingotières. Dans le cas particulier où les busettes ne sont pas fermées par les quenouilles, un barrage est disposé juste en amont de la busette qui est alimentée en premier, toujours pour que le métal remplisse le chenal sans s'écouler dans les lingotières.

During this phase, the following steps take place:

• a) the tilting of the liquid metal supply furnace if it is of the tilting type or the opening of the stopper closing the tap hole if it is fixed;
• b) filling the channel supplying the molds. In the general case, the nozzles arranged at the lower part of the channel above each ingot mold are closed by the cattails, so that the metal fills the channel without flowing into the ingot molds. In the particular case where the nozzles are not closed by the stopper rods, a dam is placed just upstream of the nozzle which is supplied first, always so that the metal fills the channel without flowing into the ingot molds.

This phase ends when the level of liquid metal in the channel has reached a predetermined value.

4.3 Forced feeding phase.

This phase begins when the level of liquid metal in the channel has reached a predetermined value or threshold level and ends, for each ingot mold when the level detector which will be discussed below detects a certain height dh of metal on the false bottom. . The end of this phase, and therefore its duration, varies with the ingot molds.

• c) à cet instant T₀ où le niveau seuil de métal liquide dans le chenal est atteint, positionnement des quenouilles alimentant les lingotières à une valeur d'ouverture dite d'ouverture initiale et déclenchement de l'origine des temps d'une loi de variation de niveau de métal N en fonction du temps, qui prendra effet un peu plus tard, et qui sera commune à toutes les lingotières : $\text{N = f(T)}$
  
  .

Obtaining the threshold level in the channel triggers the following steps:

• c) at this instant T₀ when the threshold level of liquid metal in the channel is reached, positioning of the cattails supplying the ingot molds at an opening value called initial opening and triggering of the origin of the times of a law of variation of metal level N as a function of time, which will take effect a little later, and which will be common to all ingot molds: $\text{N = f (T)}$
  
  .

This step can be modified by the following variant: at time T₀ when the level of liquid metal in the channel reaches a predetermined value, opening of the cattails feeding the molds to an open position strong enough to have a high metal flow avoiding freezing, triggering of the origin of the times of the future law of variation of level of metal common to all the molds, then partial reclosing of the cattails to the so-called initial opening position.

.In the particular case where the previous phase took place with open cattails, and where the metal was contained by a dam, the sequence of the step is as follows: opening of the dam located just upstream of the first nozzle and triggering of the origin of time T₀ of the future law of variation of the metal level $\text{N = f (T)}$

.

• d) maintien d'un niveau constant dans le chenal à l'aide de moyens de détection de ce niveau et d'un système de régulation agissant sur le basculement ou l'ouverture de la quenouille du four de maintien;
• e) lorsque l'un quelconque des détecteurs de niveau de métal liquide installé au dessus de chaque lingotière détecte une hauteur de métal déterminée au-dessus du faux-fond dh, la phase d'alimentation forcée fait place à la phase de rattrapage pour cette lingotière alors qu'elle se poursuit pour les autres où cette hauteur dh n'a pas encore été détectée;
• f) si la hauteur dh n'est pas atteinte, dans chaque lingotière, au bout d'une période de temporisation déterminée à partir du temps T₀, ouverture de la quenouille par incréments successifs à partir de l'ouverture initiale

In this particular case, the previous variant can also be used. The pre-positioning of the cattails is then carried out in the over-opening position, and a partial reclosing to the so-called initial opening position takes place shortly after the opening of the dam.

• d) maintaining a constant level in the channel using means for detecting this level and a regulation system acting on the tilting or opening of the stopper distaff;
• e) when any of the liquid metal level detectors installed above each ingot mold detects a determined metal height above the false bottom dh, the forced feed phase gives way to the catch-up phase for this ingot mold while it continues for the others where this height dh has not yet been detected;
• f) if the height dh is not reached, in each ingot mold, after a delay period determined from time T₀, opening of the stopper rod in successive increments from the initial opening

4.4) "Catch-up" phase.

.This phase is specific to each ingot mold: it begins at different times, but ends at the same time T₁ and at the same level N₁ located on the curve $\text{N = f (T)}$

.

• g)Lorsque la phase d'alimentation forcée a pris fin pour une lingotière, mise en route d'une régulation de niveau dans cette lingotière, selon une loi de niveau croissante en fonction du temps pour amener ce niveau à une valeur N₁ déterminée à l'avance, la régulation agissant sur la position de la quenouille de la lingotière correspondante et cette loi de niveau étant linéaire entre le point de détection et le point N₁,T₁;
• h) au fur et à mesure que la hauteur de métal dh est détecté dans les autres lingotières, mise en route de la régulation de niveau dans ces lingotières, selon une loi de niveau croissante en fonction du temps propre à chaque lingotière pour amener ce niveau à la même valeur N₁ et au même temps T₁ que ceux des autres lingotières, la régulation agissant toujours sur la position de la quenouille d'alimentation des lingotières, cette loi de niveau étant toujours linéaire entre le point de détection et le point N₁, T₁;

La phase de "rattrapage" prend fin au temps T₁. Il n'est pas tenu compte,pour ce changement de phase , d'un retard éventuel d'une lingotière, car ce retard pourra se combler dans la phase suivante.This second phase includes the following stages:

• g) When the forced feeding phase has ended for an ingot mold, initiation of level regulation in this ingot mold, according to a law of increasing level as a function of time to bring this level to a value N₁ determined at l 'advance, the regulation acting on the position of the stopper rod of the corresponding ingot mold and this level law being linear between the detection point and the point N₁, T₁;
• h) as the height of metal dh is detected in the other ingot molds, initiation of the level regulation in these ingot molds, according to a law of increasing level as a function of the time proper to each ingot mold to bring this level at the same value N₁ and at the same time T₁ as those of the other ingot molds, the regulation always acting on the position of the feed stopper of the ingot molds, this level law being always linear between the detection point and the point N₁, T₁ ;

The "catch-up" phase ends at time T₁. No account is taken of a possible delay of an ingot mold for this phase change, since this delay may be filled in the following phase.

4.5) Monitoring phase of a common level law.

• i) à partir du temps T₁, la régulation impose au niveau de métal dans l'ensemble des lingotières la loi de niveau commune $\text{N = f(T)}$
  
  . Cette loi peut présenter un changement de pente en T₂,N₂ ou même plusieurs.

This phase has only one step:

• i) from time T₁, the regulation imposes on the metal level in all the molds the common level law $\text{N = f (T)}$
  
  . This law can present a change of slope in T₂, N₂ or even several.

The third phase ends in T₃, N₃ (or in T _n , N _n if there are several changes in slope in the law of the levels) where the descent of the false-bottom support plateau begins.

4.6) Descent phase of the platform.

• j) après contrôle des niveaux atteints et du temps écoulé, mise en route de la descente du plateau supportant les faux-fonds au temps T₃, le niveau théorique étant N₃, application d'une loi de vitesse de descente prédéterminée et régulation du niveau N dans chacune des lingotières selon la loi $\text{N = f(L)}$
  
  , L étant la longueur de produit coulé.

This phase includes the following stages:

• j) after checking the levels reached and the time elapsed, starting the descent of the platform supporting the false bottoms at time T₃, the theoretical level being N₃, application of a predetermined rate of descent speed and regulation of the level N in each of the molds according to the law $\text{N = f (L)}$
  
  , L being the length of product poured.

• k) lorsque la longueur programmée de renvoi du four (inférieure à la longueur de produit coulé puisque le métal contenu dans le chenal permet encore de couler une certaine longueur) est atteinte, basculement arrière du four ou fermeture de la quenouille du four pour arrêter l'alimentation en métal, poursuite de la coulée grâce au métal contenu dans le chenal jusqu'à une baisse déterminée du niveau de métal dans les lingotières, relevage et inclinaison de la portion de chenal située au-dessus des lingotières pour évacuer, par l'ouverture ainsi créée le métal restant encore dans le chenal et arrêt de la descente du plateau.

A variant whose detail and interest will be explained later may consist in starting the descent of the plateau as soon as, within a small time interval before T₃, the levels are all in a small interval around N₃ .

• k) when the programmed length of return from the oven (less than the length of product poured since the metal contained in the channel still allows a certain length to flow) is reached, tilting the oven back or closing the stopper of the oven to stop the '' metal supply, continuation of the casting thanks to the metal contained in the channel until a determined drop in the level of metal in the molds, lifting and tilting of the channel portion located above the molds to evacuate, by the opening thus created the metal still remaining in the channel and stop the descent of the plateau.

• régulation à action proportionnelle. Le déplacement de la quenouille est alors simplement proportionnel à l'écart entre le niveau de métal détecté et sa valeur de consigne;
• régulation à action proportionnelle et intégrale.Le déplacement de la quenouille est la somme de deux termes: l'un proportionnel à l'écart entre le niveau de métal détecté et sa valeur de consigne et le deuxième proportionnel à l'intégrale de l'écart en fonction du temps;
• régulation à action proportionnelle, dérivée et intégrale. Le déplacement de la quenouille est la somme de trois termes: le premier proportionnel à l'écart, le deuxième proportionnel à la dérivée de l'écart en fonction du temps, le troisième proportionnel à l'intégrale de l'écart en fonction du temps.

The choice of the principle of regulation controlling the level of metal in the ingot mold also constitutes one of the means of the invention. We know that there are several principles:

• proportional control. The displacement of the stopper rod is then simply proportional to the difference between the metal level detected and its set value;
• proportional and integral action regulation. The displacement of the stopper is the sum of two terms: one proportional to the difference between the detected metal level and its set value and the second proportional to the integral of the difference as a function of time;
• proportional, derivative and integral control. The displacement of the stopper rod is the sum of three terms: the first proportional to the deviation, the second proportional to the derivative of the deviation as a function of time, the third proportional to the integral of the deviation as a function of time .

The Applicant has found that the most suitable system for the application under consideration is the second, with proportional and integral action. It is fast enough and sensitive and in particular avoids "pumping" and instabilities.

• a) un moyen d'alimentation en métal liquide à partir d'un four de maintien, par exemple un système de basculement ou de quenouille,
• b) un chenal alimentant les lingotières de coulée continue à l'aide d'orifices calibrés munis de moyens d'obturation totale ou partielle,
• c) des moyens de maintien d'un niveau constant dans le chenal comprenant un système de détection de ce niveau et un système de régulation agissant sur le basculement ou l'ouverture de la quenouille du four de maintien
• d) des moyens d'obturation totale ou partielle des orifices calibrés de chaque lingotière, comprenant chacun un pointeau appelé quenouille, et un actionneur de ce pointeau,capable de chercher automatiquement le point de fermeture de l'orifice,
• e) des détecteurs de niveau de métal liquide installés au dessus de chaque lingotière,
• f) un système de régulation asservissant le niveau de métal dans chaque lingotière d'abord à une loi de croissance déterminée propre à cette lingotière puis à une loi de croissance commune à toutes les lingotières, par commande de l' actionneur agissant sur la section de l'orifice d'alimentation de cette lingotière,
• g) un système détectant qu'un niveau déterminé N₃ est atteint dans l'ensemble des lingotières et commandant la descente du plateau supportant les faux-fonds obturant le fond des lingotières au démarrage selon une loi de vitesse prédéterminée,
• h) un système détectant que la longueur programmée de renvoi du four est atteinte et commandant l'arrêt de l'alimentation du chenal à partir du four de coulée, l'arrêt de la descente du plateau, et le relevage et l'inclinaison de la portion de chenal située au-dessus des lingotières .

The corresponding device includes:

• a) a means for supplying liquid metal from a holding furnace, for example a tilting or stopper system,
• b) a channel supplying the continuous casting ingot molds using calibrated orifices provided with total or partial sealing means,
• c) means for maintaining a constant level in the channel comprising a level detection system and a regulation system acting on the tilting or opening of the stopper distaff
• d) means for total or partial sealing of the calibrated orifices of each ingot mold, each comprising a needle called a stopper, and an actuator of this needle, capable of automatically seeking the point of closure of the orifice,
• e) liquid metal level detectors installed above each ingot mold,
• f) a regulation system controlling the level of metal in each ingot mold firstly with a determined growth law specific to this ingot mold then with a growth law common to all the ingot molds, by command of the actuator acting on the section of the feed orifice of this ingot mold,
• g) a system detecting that a determined level N₃ is reached in all of the ingot molds and controlling the descent of the plate supporting the false bottoms closing the bottom of the ingot molds at start-up according to a predetermined speed law,
• h) a system detecting that the programmed length of the furnace return has been reached and controlling the stopping of the channel supply from the casting furnace, the stopping of the descent of the tray, and the lifting and tilting of the portion of channel located above the molds.

• La figure 1 illustre, ainsi que nous l'avons indiqué plus haut le principe de la coulée continue de demi-produits telle qu'elle est pratiquée dans l'art antérieur.
• La figure 2 et sa variante 2 bis représentent le système de régulation de niveau du métal utilisé dans l'invention.
• La figure 3 représente l'organigramme des étapes successives d'une opération de coulée continue selon l'invention.
• La figure 4 représente, en fonction du temps, et en trait plein, un exemple de loi de niveau imposée dans les lingotières et les niveaux réels atteints dans les différentes lingotières, au nombre de trois dans l'exemple considéré.
• Les figures 5, 6 et 7 représentent la position des niveaux de métal au moment de la mise en route de la descente du plateau par rapport au niveau de consigne et les trois variantes de conditions à remplir pour déclencher cette mise en route.
• La figure 8 représente une forme préférée de réalisation d'un capteur capacitif de niveau.
• La figure 9 représente le schéma du pont de mesure utilisé pour la mesure du niveau.
• Sur la figure 2, on peut voir que le système de régulation du niveau de métal comprend:
  • un capteur de niveau (10) qui délivre un signal proportionnel au niveau du métal dans la lingotière,
  • un régulateur (11) qui compare ce signal à une valeur de consigne représentant le niveau souhaité,
  • un actionneur (12) qui peut déplacer verticalement la quenouille (13) en fonction de l'écart détecté par le régulateur,
  • une quenouille (13) dont la partie inférieure, généralement tronconique, obture plus ou moins, en fonction de sa position, la section supérieure de la busette (14), agissant ainsi sur la débit de métal liquide arrivant par le chenal (15).

The detailed description of the process and the device will be better understood using the figures:

• Figure 1 illustrates, as we indicated above the principle of continuous casting of semi-finished products as practiced in the prior art.
• Figure 2 and its variant 2 bis represent the metal level control system used in the invention.
• FIG. 3 represents the flow diagram of the successive stages of a continuous casting operation according to the invention.
• FIG. 4 represents, as a function of time, and in solid lines, an example of a level law imposed in the ingot molds and the actual levels reached in the various ingot molds, three in number in the example considered.
• Figures 5, 6 and 7 show the position of the metal levels at the start of the descent of the plate relative to the set level and the three variants of conditions to be met to trigger this start.
• FIG. 8 represents a preferred embodiment of a capacitive level sensor.
• Figure 9 shows the diagram of the measurement bridge used for level measurement.
• In Figure 2, we can see that the metal level regulation system includes:
  • a level sensor (10) which delivers a signal proportional to the level of the metal in the mold,
  • a regulator (11) which compares this signal with a set value representing the desired level,
  • an actuator (12) which can move the stopper rod (13) vertically as a function of the distance detected by the regulator,
  • a stopper (13) whose lower part, generally frustoconical, more or less closes, depending on its position, the upper section of the nozzle (14), thus acting on the flow of liquid metal arriving through the channel (15).

• prépositionnement des quenouilles à une ouverture non nulle (position dite d'ouverture initiale, ou, le cas échéant, de surouverture)
• fermeture d'un barrage situé juste en amont de la première busette;

   Remplissage du chenal jusqu'à un niveau seuil;
   Ouverture du barrage et lancement de l'origine des temps de la loi de niveau commune;
   Poursuite du remplissage comme dans le cas général.The stopper rod (13), instead of closing the upper section of the nozzle, can, as a variant, close its lower section. Such an arrangement is shown on the figure annex 2bis.
In this case, it is not possible to fill the channel with the nozzles closed, because then the metal freezes between the nozzle and the stopper. The procedure used is therefore the variant described above:
Before casting:

• prepositioning the cattails at a non-zero opening (so-called initial opening position, or, if necessary, over-opening)
• closure of a dam located just upstream of the first nozzle;

Fill the channel to a threshold level;
Opening of the dam and launching of the origin of the times of the common level law;
Continuation of filling as in the general case.

In the flow diagram of FIG. 3, the successive stages of the process (traditionally outside the prior phase) have been represented in a traditional manner by rectangles and the stages presenting an alternative function of the embodiment (Y for yes) or no (N for no) of an external condition by diamonds. The text inside the rectangle indicates the operation in question; the one inside the diamond formulates the external condition.

• basculement arrière du four ou fermeture de sa quenouille de coulée pour arrêter l'alimentation en métal liquide,
• poursuite de la coulée avec le métal contenu dans le chenal jusqu'à une baisse déterminée du niveau de métal dans les lingotières,
• relevage et inclinaison de la portion de chenal située au-dessus des lingotières et arrêt de la descente du plateau.

The first diamond (21) at the top of the diagram represents the tilting order of the oven. As long as it is not given (N), the program loops on itself.
If it is given (O), we go to the next stage: Tipping the oven (22). The liquid metal flows into the channel and, with the ingot feed rods closed, the level of metal rises in this channel. The level detector placed in the channel constantly compares the actual level with a setpoint level (23). If this level is not reached (N), the tilting of the oven continues. As soon as it is reached, the next step is triggered: (24)
The ingot feed rods all open in a position such that the flow of liquid metal is sufficient to avoid untimely freezing. At this time, the origin of the times of the law of variation of metal level in the ingot mold is triggered, common to all the ingot molds. Such a law is shown in Figure 4 and will be discussed below. Then, after a very short predetermined time, the stopper rods all partially close at a fixed initial opening. It is also possible, as indicated above, to go directly to the initial opening without prior over-opening.
From this stage, the successive operations take place independently for each mold. The diagram represents the continuation of the stages only for a single ingot mold but it is the same for each of the others.
The diamond (25) represents the comparison of the height of metal above the false bottom detected in the ingot mold with a preset value dh, 3 mm for example. If this height dh is not reached within a determined period of time, (26), a law of opening the stopper rod in increments as a function of time (27) is then launched so as to accelerate the filling of the molds until at the setpoint. If, in any one of the ingot molds, despite the successive increments of opening the stopper rod, the target height is not reached after a determined period (28), then there has been an incident: the casting is then stopped (29).
In the normal case (25, O), the liquid metal has reached the target height dh, but not at the same instant in each ingot mold. It involves bringing all the molds at the same time, to the same level corresponding to the law of levels as a function of time. To this end, the program calculates for each individual ingot mold a particular law of catch-up starting from the detection point and allowing the metal levels in each ingot mold to be connected to the law of the levels, at point A (T₁, N₁) in Figure 4, for example (rectangle 30).
From this point A in FIG. 4, the level law as a function of time is common to all the ingot molds and is represented by this same FIG. 4 (31).
In (32), the level sensors of the molds compare the metal level detected with a setpoint level N₃ assigned for the descent of the plate supporting the false bottoms. If this level is reached, the descent of the platform is controlled (33).
The next step is to compare the length of product poured with the programmed length (34). If this length is reached, the casting is stopped (35). The stopping of the casting includes the following successive operations not represented on the flowchart:

• rear tilting of the oven or closing of its casting stopper to stop the supply of liquid metal,
• continuation of the casting with the metal contained in the channel until a determined drop in the level of metal in the ingot molds,
• lifting and tilting the portion of channel located above the molds and stopping the descent of the platform.

FIG. 4 represents, as a function of time, the level of metal identified with respect to the mold, for a continuous casting installation, comprising, for reasons of simplification, three flows a, b, c only. But the description applies in the same way to an installation comprising a higher number of flows.
For clarity, we have shown, next to the vertical axis of the levels, a half-ingot mold in section (40), the false bottoms in the initial position (41, a, b, c) of the three ingot molds and a metal level at a given time (42). The law of levels imposed, which depends on the alloy and the format, is represented by the line A, B, C, D in solid lines. The height dh is the minimum height detected in step (25) of FIG. 2. It can be seen that this height dh is successively reached by the metal of the molds c, a and b. The program then calculates three laws of linear levels of different slopes (represented by dashes), according to the "delay" of the molds and the position of the false bottom at the start and on which the individual regulation of each of the molds will be calibrated, so that the metal levels are found identical at point A. Then, from A to B, then from B to C, the individual regulations of each ingot mold impose on all the levels, with the fluctuations inherent in any regulation, the common law provided up to level N₃ which controls the descent of the plateau. The levels actually observed are represented by dotted lines.

• T > T₃
• pour toute lingotière n, ${\text{N}}_{\text{n}}{\text{= <figure-callout id="3" label="N" filenames="imgf0001.png" state="{{state}}">N</figure-callout>}}_{\text{3}}$
  
  .

The descent is thus triggered by the simultaneous achievement of two conditions:

• T> T₃
• for any ingot mold n, ${\text{NOT}}_{\text{not}}{\text{= <figure-callout id="3" label="N" filenames="imgf0001.png" state="{{state}}">N</figure-callout>}}_{\text{3}}$
  
  .

In practice, and due to the fluctuations inherent in any regulation, the second condition can lead to completely stopping the feeding on the most advanced mold for a relatively long time. This is contrary to the second principle mentioned above and can lead to defects at the bottom of the cast product.

This situation is shown diagrammatically in FIG. 5, representing in a system of time-level axes the law of the levels in solid bold line and in dotted line the evolution of the levels observed in all of the molds limited in this example to three: a, b and c. The ingot mold reached level N₃ at time T _a , the ingot mold b at time T _b and the ingot mold c at time T _c . The descent of the plateau does not begin therefore that at time T _c . The ingot mold a remains unpowered for a fairly long period T _c - T _a , the ingot mold b slightly less. This expectation is, in any case, detrimental to the quality of the products.

Also, have we imagined two variants shown diagrammatically in FIGS. 6 and 7.

These two figures are identical to FIG. 5. They represent the same law of the levels and the same evolution of the levels observed in the three ingot molds a, b, c.

• T > T₃
• pour toute lingotière n, N_n dans une fourchette admissible autour de N₃

Dans la variante de la figure 7, la comparaison des niveaux N_b et N_c avec N₃ est faite non pas seulement à partir de l'instant T₃ mais à partir d'un intervalle de temps prédéterminé dT avant T₃. Dès lors que, à partir du temps T₃ - dT,l'ensemble des niveaux dans les trois lingotières sont à l'intérieur de la fourchette de niveau définie, la descente est déclenchée.
Les conditions pour le début de la descente deviennent:

• ${\text{T > T}}_{\text{3}}\text{- dT}$
  
• pour toute lingotière n, N_n dans une fourchette admissible autour de N₃

Le fonctionnement de l'invention qui vient d'être décrite implique:

• des mesures de niveau précises et fiables,
• des moyens de positionnement précis de chacune des quenouilles relativement à chacune des busettes.

In the variant of FIG. 6, at time T₃, only the ingot mold aa has reached the level N₃; its power has been cut off since time T _a . The levels reached at time T₃ in the two other ingot molds are then compared with the setpoint level N₃. If these levels N _b and N _c are within a predetermined admissible range, symmetrical or not around N₃, then the descent is triggered. This variant has the advantage of considerably limiting the period during which the ingot mold, the most advanced, ceases to be supplied.
The conditions for the start of the descent become:

• T> T₃
• for any ingot mold n, N _n in an admissible range around N₃

In the variant of FIG. 7, the comparison of the levels N _b and N _c with N₃ is made not only from time T₃ but from a predetermined time interval dT before T₃. As soon as, from time T₃ - dT, all of the levels in the three ingot molds are within the defined level range, the descent is triggered.
The conditions for the start of the descent become:

• ${\text{T> T}}_{\text{3}}\text{- dT}$
  
• for any ingot mold n, N _n in an admissible range around N₃

The operation of the invention which has just been described implies:

• precise and reliable level measurements,
• means for precise positioning of each of the stopper rods relative to each of the nozzles.

• un capteur capacitif de niveau du métal,
• un actionneur intégrant un dispositif de détection du point de fermeture de la busette.

According to the invention, preferably:

• a capacitive metal level sensor,
• an actuator incorporating a device for detecting the point of closure of the nozzle.

The principle of the capacitive sensor is as follows:
A flat capacitor is produced, one of the reinforcements of which is a metal disc and the other of which is the upper surface of the liquid metal. We know that the capacity of a plane capacitor C is equal to the product of the surface of the armature by the dielectric constant of the medium separating the armatures divided by the distance between the armatures. The measurement of the capacitance of the capacitor is an indirect measurement of the distance between the two armatures and therefore of the level of the metal.

In practice, we operate schematically as follows, as shown in FIGS. 8 and 9.

FIG. 8 shows the sensor itself, in place above the upper surface of the liquid metal in an ingot mold.

FIG. 9 represents the diagram of the bridge for measuring the capacitance of the capacitor thus formed.

In FIG. 8, the level of liquid metal, represented by the reference (50) constitutes one of the reinforcements of the capacitor. The second frame (51) is part of the sensor itself. A third armature (52), disposed at a fixed distance and above the armature (51), constitutes, with the latter a second capacitor whose capacity will serve as a reference in the measurement bridge which will be discussed below. . For example, the distance e between the liquid metal level and the armature (51) is 18 mm at equilibrium, as is the distance between the two armatures (51) and (52).

The capacitance of the capacitor C _x formed by the plates (50) and (51) is constantly compared with the reference capacitance C _r of the capacitor formed by the plates (51) and (52) using a measuring bridge the diagram of which is shown in FIG. 9. The bridge comprises 4 branches linked together by 4 vertices (53), (54), (55), (56). The branches (53) - (54) and (54) - (55) are supplied with sinusoidal alternating current by means of two identical transformers (57) and (58) whose primaries are connected in series to a current source high frequency (80 kHz, for example). In branch (55) - (56), place the capacitor C _x and in branch (56) - (53), the reference capacitor C _r . The top (55) corresponding to the armature constituted by the liquid metal of C _x is connected to the ground. This is easily achieved by means of the metal plate supporting the false bottom on which the cast product rests. The opposite vertices (54) and (56) are interconnected via a transformer (59) to a current detector (60). When the distance between the reinforcements (50) and (51) of C _x is equal to a reference value, of 18 mm for example, the capacities of C _x and C _r are equal, the bridge is balanced and no current passes through the detector (60). When this distance decreases or increases, the bridge is unbalanced and the detector is crossed by a current. An electronic system then sends an order to a servomotor which raises or lowers the sensor so as to bring it back to a distance from the liquid metal equal to the reference value of 18 mm for example. The copying of the successive movements of the sensor makes it possible to generate at every instant a signal corresponding to the level of liquid metal in the ingot mold.

Such a device is placed above the liquid metal in each ingot mold and makes it possible to measure and regulate the level of the metal.

The actuator is essentially characterized by its device for detecting the point of closure of the nozzle. According to the invention, the actuator preferably consists mainly of an electric motor-reducer assembly with precise control of the position of the rod. The rod is hollow and has inside an axis capable of sliding a few mm. This axis is kept released by a spring type device. It is to this axis that the distaff is fixed.

When the rod moves in the "out" direction, if the stopper rod encounters an obstacle on its path, the spring is pressed, the axis slides in the rod and comes to actuate a limit switch.

Thanks to this detection device, the point of closure of the nozzles can be determined by an automatic procedure. This point corresponds to the minimum exit position of the rod allowing the limit switch to be actuated, corrected for the stroke traveled by the axis in order to depress the spring and actuate the limit switch.

The opening position of the stopper rod is then determined from this reference point.

5 °) EXAMPLE.

The device described above was mounted on a continuous casting site intended for the simultaneous casting of 5 plates of format 1360 mm by 610 mm in aluminum alloy 5052 (Standard Aluminum Association). The molds are arranged parallel to each other and transversely to the axis of the feed channel.
Their height is 115 mm. The set value for dh is 3 mm above the double curvature false bottom. The setpoint for N₃ is 48 mm below the upper level of the mold. The descent speed is 42 mm / minute. These parameters have been introduced into the automation system.
A first order launched the preliminary step of finding closure points. Depending on the flows, these points were found to be located between 18 and 29 mm from the actuator stroke which is 100 mm (0 mm = rod fully extended). At the end of this step, the actuators placed the cattails at their closing point.
A second order launched the actual casting. At the end of the channel filling step, obtaining the threshold level triggered the opening of the cattails 7 mm above the closing point.
The first ingot mold to reach the height dh reached it 18 seconds after the opening of the cattails, the last 25 seconds after this opening.
All the ingot molds have reached level N₁ at the setpoint time T₁ = 40 s, following specific setpoint laws.
The level N₃ was reached by all of the ingot molds 85 seconds after the opening of the cattails, at which time the support platform for the false bottoms began its descent.

Claims (10)

1. Process for the starting-up and continuation of automatic casting of plates or billets of a metal alloy, and in particular of an aluminium alloy, with a plurality of outflows on a continuous casting unit, characterised in that it comprises a preliminary phase triggered by a first command and a succession of five main phases triggered by a second command, which perform the following stages:

   1) During the preliminary phase:
   Automatic searching by their respective stopper rods (13) for the points of closure of the spouts (14) supplying the ingot moulds from the runner (15) located above the ingot moulds, and pre-positioning of the stopper rods (13),

   2) During the metal supply phase by the casting furnace.

   a) tilting (22) or opening of the stopper rod sealing the casting hole of the furnace supplying molten metal,

   b) filling of the runner (15) supplying the ingot moulds, the metal being prevented from running out into the ingot moulds by one or more suitable devices,

   3) During the phase of forced supply.

   c) at the moment T₀ when the level of molten metal in the runner (15) reaches a predetermined value, opening of the device or devices preventing the metal from running out into the ingot moulds, positioning of the stopper rods (13) at an aperture value known as the initial aperture, and release (24) of the origin of the times of a law of variation of metal level N as a function of time, which will take effect in the further stages and which will be common to all ingot moulds: $\text{N = f(T)}$

   

   ,

   d) maintenance (23) of a constant level in the runner (15) by means for detecting this level and by a control system acting on the tilt or the aperture of the stopper rod of the holding furnace;

   e) each time the molten metal sensor (10) installed above each ingot mould detects a specified height of metal dh above the bottom block of a particular ingot mould, this ingot mould passes to the stage g described below,

   f) if the height dh has not been reached in each ingot mould after a specified dwell time starting from the time T₀, the stopper rod opens by gradual increments starting from the initial aperture.

   4) During the catching-up phase (30).

   g) When the height dh has been detected in an ingot mould, adjustment of the level in this ingot mould according to a level law which increases as a function of time and is liner between the point of detection and the point (N₁, T₁) to bring this level to a value N₁ determined in advance, this adjustment acting on the position of the stopper rod (13) of the corresponding ingot mould,

   h) as the height of metal dh is detected in the other ingot moulds, setting in motion of level control in these ingot moulds according to a level law which increases as a function of the time for each ingot mould, to bring this level to the same value N₁ and to the same T₁ as those of the other ingot moulds, this control still acting on the position of the stopper rod (13) for supply of ingot moulds,

   5) During the phase of following a common level law (31)

   i) starting from the time T₁, automatic control of the metal level in all the ingot moulds by the common level law $\text{N = f(T)}$

   

   , this law being capable of having a change of gradient at T₂, N₂, or even a plurality of changes of gradient.

   6) During the phase of lowering of the table (33).

   j) at the time T₃, and after monitoring of the levels relative to the theoretical level N₃, setting in motion of lowering of the plate supporting the bottom blocks, application of a predetermined law of speed of lowering, and adjustment of the level N in each of the ingot moulds according to a new law $\text{N = f(L) = f (length cast)}$

   

   .

   k) when the length of the product cast reaches a programmed value, tilting back of the furnace or closure of the stopper rod of the furnace to stop the supply of metal, continuation of casting with the metal contained in the runner (15) until the metal in the ingot moulds is lowered to a predetermined level, raising and tilting of the portion of the runner (15) located above the ingot moulds, and halting of the descent of the table.
2. Process for starting-up and casting according to claim 1, characterised in that:

   - the stopper rods (13) seal the spouts (14) at the level of their upper section,

   - the device used to prevent the metal from running out into the ingot moulds during the stage of filling of the runner consists of stopper rods (13) located at the point of closure during the preliminary stage.
3. Process for starting-up and casting according to claim 1, characterised in that:

   - the stopper rods (13) seal the spouts (14) at the level of their lower section,

   - the device used to prevent the metal from running out into the ingot moulds during the stage of filling of the runner consists of a dam located just upstream of the first nozzle,

   - the stopper rods (13) are already pre-positioned at the value known as the initial aperture during the preliminary stage.
4. Process for starting-up and casting according to one of claims 1 to 3, characterised in that, during stage c, when the level of molten metal in the runner (15) reaches a predetermined value, the stopper rods (13) are opened to or held in, if already open, a position of rather wide aperture so as to ensure a high flux of metal to avoid setting, a common law of variation of the metal level, $\text{N = f(T)}$

   

   , is set for all the ingot moulds, and then the stopper rods (13) are partially reclosed to the so-called initial aperture position.
5. Process according to one of claims 1 to 4, characterised in that lowering of the panel begins as soon as the two following conditions are met:

   - T > T₃

   - for any ingot mould n, N_n is in an acceptable bracket about N₃.
6. Process according to one of claims 1 to 4, characterised in that lowering of the table begins as soon as the two following conditions are met:

   - ${\text{T > T}}_{\text{3}}\text{- dT}$

   

   , dT being a predetermined time interval

   - for any ingot mould n, N_n is in an acceptable bracket about N₃.
7. Process according to one of claims 1 to 6, characterised in that the level adjustments take place by proportional and integral action.
8. Apparatus for starting-up and continuing automatic casting of plates or billets of a metal alloy, and in particular of an aluminium alloy, with a plurality of outflows on a continuous casting unit, this device comprising:

   a) means for supplying molten metal from a holding furnace, a tilting system or stopper rod for example,

   b) a runner (15) for supplying the continuous casting ingot moulds by means of spouts (14) and stopper rods (13),

   c) means for maintaining a constant level in the runner (15), comprising a system for detecting this level and a control system acting on the tilting or position of the stopper rod of the holding furnace,

   d) means for total or partial sealing of the calibrated apertures of each ingot mould, each comprising a pin known as a stopper rod (13) and an actuator (12) for this pin, capable of finding automatically the closing point of the aperture,

   e) level sensors (10) for the molten metal level, which are installed above each ingot mould,

   f) a control system (11) governing the metal level in each ingot mould according to a specified law of increase particular to this ingot mould by control of the actuator (12) acting on the section of the supply aperture of the ingot mould,

   g) a system for detecting that a specified level N₃ has been reached in all the ingot moulds and controlling the descent of the table supporting the bottom blocks sealing the bottom of the ingot moulds at starting-up, according to a predetermined law of speed,

   h) a system for detecting that the programmed length of the cast product has been reached and controlling halting of the descent of the table, complete closure of the means for sealing the supply apertures of the ingot moulds, and halting of the supply of the casting runner from the casting furnace.
9. Apparatus according to claim 8, characterised in that the actuator (12) of the stopper rod is an electric back-geared motor with automatic position control, comprising:

   - a hollow rod,

   - a shaft inside said rod, capable of sliding several millimetres, to the end of which the stopper rod is fixed and which is held extended by a spring-type device,

   - a stroke limit actuated by the sliding shaft when the latter is pushed back inside the rod against the action of the spring.
10. Apparatus according to one of claims 8 or 9, characterised in that the system (10) for detecting the molten metal level in the ingot moulds is a capacitive level probe formed of:

    - a plane capacitor, whose first electrode is the upper surface (50) of the metal in the ingot mould and the second a plate (51) parallel to this surface and at a certain distance therefrom,

    - a measuring bridge (Fig. 9) for comparing the capacitance (Cx) of this plane capacitor to a fixed reference capacitance (Cr),

    - a servomotor automatically controlled at the measuring bridge and capable of moving the second electrode (51) upwards or downwards to keep the capacitance between the second electrode (51) and the surface of the metal (50) constant,

    - a system for generating a signal proportional to the displacement of the second electrode (51).

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