EP0998360B1 - Method for starting continuous metal casting operation - Google Patents

Abstract

The process is characterized in that, before starting casting, an initialization position is determined for the stopper rod control actuator when the stopper rod is resting on its seat under its own weight and after having filled the tundish equipped with this stopper rod, the actuator is operated to place it in a controlled superclosed position and, to start casting, the actuator is driven according to an imposed movement versus time law, time (t1) for start of casting being determined from this law and the control actuator continues to be driven in opening direction to enable the metal to flow into the ingot mould.

Description

The present invention relates to continuous casting metals, including steel, and more particularly a method of starting the casting, from the situation where the casting plant is ready to receive the molten metal contained in a transport container such as a steelworks pocket.

Such an installation conventionally comprises a distributor equipped with a pouring nozzle and a mold. The distributor also includes means shutter, called distaff, to shut off the nozzle and adjust the flow rate of molten metal during casting.

Before the start of casting, a dummy is put in place in the installation, this mannequin has a dummy head which is inserted into the mold for shut it off temporarily at the start of casting, and stopper rod is placed in the closed position.

To start the casting, the metal of the ladle is spilled into the distributor.

Then we open the stopper so that the metal fills the mold by flowing through the nozzle. When the metal reaches a predetermined level in ingot mold, we drag the dummy down, to start the extraction of the cast product, at least partially solidified in contact with cooled walls of the mold.

A problem which arises is to define the moment when we are going to start the extraction, taking into account in particular the level required in ingot mold and time required for the product is sufficiently solidified before start the extraction. However, the arrival of metal at the level required in the mold depends on the metal flow in the nozzle and therefore in particular of the open position of the distaff.

Also, to automate the startup, it is already known to use a level detector to detect the arrival of the metal at the prescribed level in the mold, and to control the start of extraction by this detector.

Furthermore, it is also known to use a level detector placed on the mold to regulate the flow rate or the extraction speed during casting, so as to maintain a substantially metal level constant in the mold during the entire casting.

But these detectors can only be placed top of the mold. In addition, they have not conventionally a small detection distance and are positioned so that you can measure the level variations at near the setpoint level during casting. From this done, they only detect metal in an ingot mold when it approaches the setpoint level. So during almost the entire filling time of the mold, you can't control the metal level. Otherwise, when the detector can finally detect the presence of metal, and therefore order the start of the extraction, it takes a while before getting in speed, and the metal level may exceed largely the setpoint level. This can be in part avoided by simultaneously ordering the closure of the stopper rod to reduce the feed rate. But the reaction time of the stopper rod, the positioning is ensured by a jack, cannot be reduced enough to completely avoid the problem supra. In addition, the inertia of the metal flow and that of the means for controlling the stopper rod level fluctuations that can last for a some time before the regulation and therefore the level stabilize and the flow becomes regular.

Another problem that arises is to arrive at determine the instant of the actual start of the casting, that is to say the instant when the metal contained in the distributor begins to flow when ordering the opening of the distaff. This problem is also linked to that of being able to control the rise in level in the ingot mold, this level not being detectable during much of the filling, as we have seen previously. The only way to control this climb level is therefore to act on the metal flow flowing from the distributor, which depends on the position exact distaff.

For example, document DE 3421344 describes a casting start process whereby for the purpose in particular to avoid parasitic solidifications at starting, and starting from a situation where the distaff is leaning on his seat, we proceed to a first maximum opening, then closing phase partial, and finally holding in the open position with possibly oscillations, to regulate the flow.

But determining the position of the distaff is conventionally carried out by a device located on the control means of the distaff and not on the distaff itself. he it follows that the indication given by these measuring means is not exactly representative of the position of the distaff itself, this being due in particular to the games inevitable means of mechanical connection of the distaff to its control means. It follows that not only there is a time lag between the command to open the stopper rod and the start of the actual opening, and therefore the start of flow of the metal, but also, subsequently, the position indication distaff is not an exact reflection of its position effective, which determines the metal flow rate. Now, this is only by knowing the precise moment of the start of casting, and flow rate, which can be determined precisely the level in the mold during the filling.

This problem is particularly troublesome in the case continuous casting between cylinders because the determination of the exact moment of the start of the extraction is essential in this technique. It is therefore necessary to know precisely the exact moment of start of flow and filling rate, all the more more than in this technique, the filling time between the start of the flow and the start of the extraction is very short.

The invention aims to solve the different aforementioned problems, and in particular to allow the precise determination of the start of casting time effective, and the flow rate during the filling the mold.

With these objectives in view, the object of the invention is a method of starting a casting operation continuous metal, in a casting installation comprising a distributor which includes a drain closable by a stopper rod coming to rest on a distaff seat, mechanical connection means between the stopper rod and a control cylinder displacement of the distaff, and an ingot mold receiving the metal flowing through said orifice.

a) la quenouille est posée sur son siège sous le seul effet de son propre poids, le vérin de commande étant inactif et dans une position d'initialisation définie par la position de la quenouille,

b) on détermine la dite position d'initialisation du vérin de commande,

c) on actionne le vérin de commande dans le sens de la fermeture pour pousser la quenouille sur son siège,

d) on remplit le répartiteur avec le métal liquide,

e) on actionne le vérin de commande pour l'amener dans une position de surfermeture contrôlée définie par une distance prédéterminée de la position du vérin de commande par rapport à la position d'initialisation,

   et, pour démarrer la coulée :

f) on actionne le vérin de commande dans le sens de l'ouverture selon une loi prédéfinie imposée de déplacement du vérin de commande en fonction du temps, l'instant de début de coulée étant déterminé à partir de cette loi, en calculant avec cette loi le temps mis pour que le vérin de commande passe de la position de surfermeture à la position d'initialisation,

g) et on continue à actionner le vérin de commande dans le sens de l'ouverture, pour laisser le métal s'écouler dans la lingotière.

According to the invention, this method is characterized in that, before starting the casting:

a) the stopper rod is placed on its seat under the sole effect of its own weight, the control cylinder being inactive and in an initialization position defined by the position of the stopper rod,

b) the said initialization position of the control jack is determined,

c) the control cylinder is actuated in the closing direction to push the stopper rod on its seat,

d) the distributor is filled with liquid metal,

e) the control cylinder is actuated to bring it into a controlled over-closing position defined by a predetermined distance from the position of the control cylinder relative to the initialization position,

and, to start casting:

f) the control cylinder is actuated in the opening direction according to a predefined law imposed on the displacement of the control cylinder as a function of time, the instant of start of casting being determined from this law, by calculating with this law the time taken for the control cylinder to pass from the over-closing position to the initialization position,

g) and the actuator continues to be actuated in the opening direction, to allow the metal to flow into the ingot mold.

As will be better understood later, the method according to the invention makes it possible to determine precisely when the molten metal will be able to start to flow between the stopper rod and its seat.

This moment is therefore just when the distaff leaves his seat.

Theoretically, it would suffice to maintain the distaff precisely in this position, and start moving it up to move it away from his seat, the precise moment of the start of the trip defining the start time of casting.

In practice, this is impossible. In fact, would it be because of the inevitable games existing in the mechanical connection means between stopper rod and cylinder control, and by the thrust exerted on the stopper rod by the liquid metal contained in the distributor, it is clear that even if the control cylinder is held in fixed position, in said initialization position where the stopper rod is just placed on its seat, the position exact distaff will vary when filling the distributor by molten metal, particularly following the catching up with mechanical play, or the phenomena of expansion.

It follows that the tightness of the stopper rod on his seat would be broken, and metal flows molten could happen unexpectedly even before the dispatcher is filled.

To avoid this, conventionally according to the technique operator, the operator actuates the control cylinder displacement of the distaff, before the start of the filling the distributor so as to press strongly the distaff on his seat. Therefore, it is practically impossible for the operator to know exactly when the distaff will end up in watertight limit position on his seat, when he goes operate the control cylinder in the other direction, since there is not an exact match between the position of the control cylinder and the position of the distaff.

The principle of the invention is in fact to restore artificially this correspondence, starting from the idea that if there is not an exact match of respective positions of the stopper rod and the cylinder command when the latter is actuated in one direction then in the other, however, we restore this correspondence if we only consider displacement in a sense, that of opening.

For this, according to the invention, we therefore define a position of the control cylinder, precisely measurable and therefore reproducible, called overclosure position controlled, as well as a cylinder displacement law of control in the opening direction, i.e. corresponding to a displacement of the stopper rod towards the high.

The controlled overclosure position is defined by a preset distance considered from the position of the control cylinder when it just causes the detachment of the stopper rod from its seat, i.e. the initialization position.

Note that this initialization position of the control cylinder is not operator defined nor by any action on the said cylinder, but that it results solely from the forces of gravity acting on the installation and in particular on the stopper rod. It is so only the contact of the distaff on his seat, under the effect of its weight, which defines the position control cylinder initialization. We will therefore note that when determining the position initialization, it is the distaff which fixes the position of the control cylinder, while casting, it is obviously the control cylinder which fixes the position of the stopper rod.

The law controlling the displacement of the jack in function of time is defined experimentally, in depending on the characteristics of the casting installation and process, so as to establish a good relationship defined between displacement of the control cylinder and displacement of the distaff, as soon as, moving towards at the top, the distaff leaves contact with its seat. By cons, before this moment, there is no such well defined relationship but only a definition of the position of the control cylinder as a function of time, without the actual position of the stopper rod being connected to that of the cylinder.

This law will therefore allow, at the beginning of the stage f), to ensure a displacement of the jack, without there being proportional displacement of the stopper rod, this displacement of the jack corresponding in a way to a relaxation of the constraints generated by the support effort distaff on his seat.

Then, as soon as these constraints are removed, that is to say from the moment there is a break in the contact between the stopper rod and its seat, the displacement of the jack leads to a displacement of the stopper rod, and therefore to the flow of the metal contained in the distributor, the flow rate of the molten metal being then regulated by acting on the cylinder, and depending on the position of said cylinder.

The foregoing explanation is intended to explain the principle of the invention, and is therefore quite theoretical. In practice, it is obvious that the beginning real flow does not exactly match the instant at which contact between the distaff and its seat is broken, especially because the geometry contacting surfaces is not ideal and that come into play the physical characteristics of molten metal (fluidity, surface tension, ... etc). This is why the cylinder displacement law is determined experimentally, one of the aims of the invention basically being able to provide a reproducibility of the conditions for the start of casting of a casting to another.

According to a particular provision of the invention, during step c), push the distaff on its seat until the pushing force exerted by the means order value reaches a predetermined value.

Alternatively, push the distaff on its seat until control means reach a predetermined position.

Whatever the case, the thrust exerted on the distaff before the introduction of molten metal into the dispatcher must be sufficient to guarantee a perfect sealing of the stopper rod on its seat, without risk that this seal is disturbed during filling the distributor. However, this position of overclosure will be located beyond, in the direction of the closing, from the controlled over-closing position.

According to another additional provision, when start of the casting and after the passage of the means of command in the initialization position, we continue automatic opening of the stopper rod according to a law imposed opening, to a position called filling. This filling position is maintained during the filling of the mold. This provision actually allows for filling the ingot mold under controlled flow, so that the rise liquid metal in the mold is done the most calmly possible and that level regulation, classically known, can come into play without creating jolts when the level of metal in the ingot mold arrives near nominal level. In particular, this avoids any risk of metal overflowing out of the mold. This also ensures a smooth transition between the start-up phase, i.e. until the level in an ingot mold is substantially at nominal level, and the start of extraction.

According to yet another preferential provision, before the level of metal in the ingot mold reaches a nominal predetermined level of casting, put into service level regulation which regulates level as soon as the metal level comes close to the nominal level. Level regulation, well known in continuous casting facilities, is therefore put in service long before the metal level can be detected by the sensor, conventionally used in regulation systems. The regulation is however saturated so as to avoid that it tends to provoke an additional opening of the distaff (which would normally be the case since the metal level is then well below the normal level). On the other hand, the control circuits being already in service before that the sensor detects the metal cast in the mold, the regulation intervenes without delay as soon as the level of metal is detected. As a result, the reaction, caused by said regulation when the cast metal comes close to the nominal level, is less lively and does not generate a sudden displacement of the stopper rod or sudden variation in the extraction speed.

Other features and advantages will emerge of the description which will be made of a method of start of continuous steel casting in accordance with the invention.

• la figure 1 est une vue schématique d'une installation de coulée continue d'acier en lingotière,
• la figure 2 est un graphique montrant la position mesurée du vérin de commande de la quenouille en fonction du temps.

Reference is made to the appended drawings in which:

• FIG. 1 is a schematic view of an installation for the continuous casting of steel in an ingot mold,
• Figure 2 is a graph showing the measured position of the stopper control cylinder as a function of time.

The continuous casting installation, shown in Figure 1, during casting, includes a distributor 1 containing molten steel 2, provided with an orifice drain 3 fitted with a nozzle 4. The drain port 3 can be closed by a stopper 5 coming to rest on its seat 6. The displacements of the distaff are carried out by a control cylinder 7, connected to the stopper 5 by mechanical connection means such as that an articulated lever 8 pivoting in a bearing 9.

The installation also includes, so known per se, an ingot mold 20 whose walls are energetically cooled to cool and solidify the molten metal poured into the mold by the nozzle 4. Under normal casting conditions, the metal at least partially solidified in the form for example of a slab 21, is extracted from the mold down, at extraction roller means 22 driven in rotation by motors not shown.

The control cylinder 7 is equipped with a position 10, which continuously measures the exact position of the cylinder rod. The installation furthermore comprises a regulation system 11, schematically represented in the drawing, and to which is also connected with a level 12 detector allowing detect and measure the level 23 of the metal in the mold.

The regulation system 11 is also connected to a solenoid valve 13, or equivalent control means, to control the movements of the jack 7, as well as motors of the extraction rollers 22 to adjust their speed.

All these means are conventionally known in the existing casting plants.

Note, however, that by control cylinder, must obviously understand not only a cylinder conventional comprising a rod movable in translation in a cylinder body, such as the cylinder 7 shown in FIG. 1, but also any other actuator capable of ensuring the same displacement function of the stopper rod.

The graph in FIG. 2 shows, by way of example of the method according to the invention, the variations in the position d of the jack 7 as a function of time t, from an instant situated before the start-up process, until the arrival in normal casting regime.

Plot 31 corresponds to the position initialization "O" of the control cylinder, that is to say the measured position of the cylinder rod when the distaff 5 rests under the effect of its own weight on its seat 6. The jack is then not subjected to any pressure by the solenoid valve 13, the position of the rod then being solely determined by the position of the stopper 5. In the example shown, we will understand easily that the weight of the distaff and that of the cylinder rod exert efforts on the lever 8 directed downward, and that, as a result, the games inevitable at all joints are located upwards for joints 51 and 61 of the distaff and cylinder rod on the lever as well as for the pivot joint 9 of the lever on installation.

Starting from this position, the jack is then controlled so that its rod moves by a value dl, suitable for compensating for the various clearances indicated above and for strongly pressing the stopper rod on its seat. This position is maintained throughout the filling of the metal distributor, as shown by the trace 32. Instead of defining at this stage a cylinder position d ₁ , it is also possible to define a force or a pressure for supplying the cylinder , as already mentioned.

When the distributor is filled, the actuator is then controlled to move its rod into the so-called controlled over-closing position (line 33 in Figure 2). This position is defined by a distance d ₂ from the initialization position. This distance can for example be a predetermined percentage, for example 3%, of the total stroke of the jack. In practice, this distance will be determined experimentally so as not to be too great, but nevertheless sufficient for the stopper rod to remain firmly pressed on its seat and for no play to appear at the level of the various joints.

This position will be considered later as a starting point for actuating the cylinder under the law imposed opening already mentioned.

This law is represented in FIG. 2 by the trace 34. As represented, this law, fixing the displacement of the cylinder rod as a function of time, is linear. However, this is not mandatory, the curve representative of the said law being able some deviate from a straight line, depending on the kinematics own connecting means between the cylinder and the distaff, and also flow conditions of filling of the mold, as will be seen by after.

Knowing therefore the cylinder displacement law, the distance d ₂ , and the instant t _{0 at} which the cylinder displacement is controlled in accordance with this law, the precise start time t ₁ will be determined by calculation as being the instant t ₀ + Δt, Δt being the time taken for the cylinder rod to move by the distance d ₂ . At this time, the displacement of the jack will have released the pressure exerted by the stopper rod on its seat during the controlled over-closing phase, and will also have caught up with all the play in the joints, in the opposite direction to what had occurred during pressurizing the jack before filling the distributor. The assembly of the actuator, the lever, and the stopper is then found to be substantially in the same situation as during initialization since the forces exerted are practically the same as those exerted during initialization, the only difference being that c 'is then the jack which pulls the stopper upwards, whereas during initialization, it was the stopper which held the jack.

From time t1, the flow orifice therefore opens gradually, the progression of the opening being controlled by the displacement of the jack, which displacement continues to take place under the control of the law imposed, until a point defined by a distance d ₃ , this distance d ₃ being determined so as to correspond to a given opening of the stopper rod. It may be different from the maximum opening provided for the normal casting regime, which will only occur after the start of extraction. From time t ₁ , the metal contained in the distributor therefore begins to flow into the ingot mold at a rate determined by the opening of the stopper rod, that is to say gradually increasing until the jack reaches position d ₃ , then stabilizes at an imposed value while the mold continues to be filled (trace 35).

During the filling time of the mold, while the extraction has not yet started, the flow of metal can therefore be different from the nominal flow under which metal will flow from the distributor into the ingot mold after starting the extraction rollers. Only when the metal level in the mold comes close to the level detector 12 that the level regulation, of a type known per se, takes the relay to control cylinder 7, as well as possibly the speed of the extractor rollers 22, to adapt the flow at extraction speed, so as to keep a substantially constant level of metal in the mold, as is well known.

The invention is not limited to the method of startup which has just been described only as example. In particular, the method according to the invention can advantageously be implemented in continuous casting systems between cylinders.

Also, instead of using sensor 10 directly placed on the cylinder and measuring the position of its stem, position measurements can be performed by any other means of measurement capable of determining precisely the position of the control means of the distaff.

Claims (5)

1. Method of starting up a continuous metal casting operation in a casting plant comprising a tundish (1) which has a draining orifice (3) that can be closed off by a stopper rod (5) bearing on a stopper rod seat (6), mechanical linkage means (8) between the stopper rod (5) and a cylinder (7) for controlling the displacement of the stopper rod, and a mould (20) which receives the metal (2) flowing out through the said orifice,
   characterized in that, before the casting run:

   a) the stopper rod (5) is placed on its seat (6) merely under the effect of its own weight, the control cylinder (7) being inactive and in an initialization position (31) defined by the position of the stopper rod;

   b) the said initialization position of the control cylinder is determined;

   c) the control cylinder is actuated in the direction of closing, in order to push the stopper rod onto its seat;

   d) the tundish is filled with the liquid metal;

   e) the control cylinder is actuated in order to bring it into a controlled overclosed position (33) defined by a predetermined distance (d2) of the position of the control cylinder with respect to the initialization position;

   and, in order to start the casting run:

   f) the control cylinder is actuated in the direction of opening according to an imposed law (34) of displacement of the control cylinder as a function of time, the instant (t1) of the start of casting being determined from this law, by calculating with this law the time taken for the control cylinder to pass from the controlled overclosed position (33) to the initialization position; and

   g) the control cylinder continues to be actuated in the opening direction, in order to let the metal flow out into the mould.
2. Method according to Claim 1, characterized in that, during step c), the stopper rod is pushed onto its seat until the thrust force exerted by the control cylinder reaches a predetermined value.
3. Method according to Claim 1, characterized in that, during step c), the stopper rod is pushed onto its seat until the control cylinder reaches a predetermined position (32).
4. Method according to any one of Claims 1 to 3, characterized in that, during the casting start-up and after the control cylinder has passed into the intialisation position, the stopper rod continues to be opened until a filling position (35) lying beneath a fully open position, this filling position being maintained while the mould is being filled.
5. Method according to Claim 4, characterized in that, before the metal level (23) in the mould reaches a predetermined nominal casting level, a level-regulating procedure, which ensures level regulation as soon as the metal level comes close to the nominal level, is implemented.

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