EP0391822B1 - Method and apparatus for manufacturing thin metal products by continuous casting - Google Patents

Abstract

The invention relates to a method and an apparatus for obtaining thin metal products by continuous casting. The device comprises an ingot mould 2 and pinch rolls 6, 6' intended to cause, by reduction of the thickness of the said product, the closure of the solidification pool. The separation force F exerted on the pinch rolls by the product is measured, and to the core of the still molten product is applied, by means of inductors 8, 8' housed in the pinch rolls or upstream of the latter, a variable magnet field slaved by slaving means 9 to the said separation force so that the latter does not exceed, on a long- term basis, an upper limit value given in advance which represents the stress which can be tolerated momentarily by the pinch rolls without damage. <IMAGE>

Description

The invention relates to obtaining thin metallic products directly by continuous casting. It relates more particularly to obtaining thin steel slabs, that is to say flat products whose final thickness rarely exceeds 100-120 mm, but is most often around 20-50 mm.

Continuous casting installations for steel slabs having these small thicknesses are distinguished from continuous casting installations for slabs of standard thickness (of the order of 150-200 mm), in particular by the presence, downstream of the ingot mold in the direction of extraction of the product, from at least one pair of rollers, called pinch rollers. These have the role of bringing, without actual rolling, the product to its final thickness after it leaves the ingot mold, by only bringing the large faces of the cast product together. The ingot mold, the design of which derives from that of conventional continuous casting ingot molds, does not in fact make it possible to directly obtain the desired small thicknesses. The spacing of the pinch rollers is fixed, in principle, throughout the casting and equal to the thickness desired for the product. They exert their grip on the product so as to bring about the early closure of the liquid well. On leaving the rolls, the product is therefore normally fully solidified throughout its section. In any case, it no longer has a completely liquid heart.

To achieve a maximum thickness reduction, it is advantageous to place the pinch rollers immediately at the outlet of the mold. However, this location is not without drawbacks. In particular, it risks leading to premature withdrawal of the product poured from the cooled wall of the mold in the final part thereof, which can cause a breakthrough. It is therefore possible to design installations in which the pinch rollers are sufficiently distant from the outlet of the ingot mold, by a distance of the order of 1 m for example.

A problem to be solved in the operation of such a machine is the adjustment of the depth of the liquid well (also called "metallurgical height"). As we have seen, pinch rollers are designed to shape a product whose core is still liquid. If the liquid well closes upstream of the pinch rollers, these must therefore act on a product which is entirely solid or pasty state. To bring the product to the desired thickness, they will have to undergo a severe spacing effort on the part of the product, which in fact amounts to real rolling on a solid product. Such an effort, if it is too great, or if it is repeated too frequently and for too long, can cause serious damage to the rollers themselves and to their holding members. On the other hand, a machine capable of withstanding such large spacing forces would be hardly compatible in size with the rest of the casting installation, because of the great rigidity of the frame which would be necessary. In any event, its cost would be much higher than that of a normal installation.

The object of the invention is to propose a method for rapid adjustment of the depth of the liquid well, making it possible to reduce the force undergone by the pinch rollers to its nominal value as soon as an abnormal increase in this force is observed.

• on mesure la force d'écartement exercée en permanence sur les rouleaux pinceurs par le produit ;
• et, dans une zone située en amont des rouleaux pinceurs, ou entre ceux-ci, on applique au coeur du produit encore liquide un champ magnétique variable, tout en asservissant l'action dudit champ magnétique à la valeur de la force d'écartement mesurée, de manière que celle-ci ne dépasse pas durablement une valeur limite supérieure donnée à l'avance représentative de l'effort tolérable momentanément sans dommages par les rouleaux pinceurs.

To this end, the subject of the invention is a process for the continuous casting of thin metallic products, in particular steel, in an installation comprising an ingot mold followed, in the direction of extraction of the cast product, by pinch rollers, intended to cause , by reducing the thickness of said product during casting, closing the solidification well, characterized in that:

• the spreading force permanently exerted on the pinch rollers by the product is measured;
• and, in a zone located upstream of the pinch rollers, or between them, a variable magnetic field is applied to the core of the still liquid product, while controlling the action of said magnetic field to the value of the spacing force measured , so that it does not durably exceed an upper limit value given in advance representative of the force tolerable momentarily without damage by the pinch rollers.

This variable magnetic field can be mobile and therefore exert a stirring action on the liquid core of the cast product, stirring which, as is known, promotes the evacuation of heat, or on the contrary be stationary and exert an inductive heating action on the cast product.

The invention also relates to a continuous casting device for implementing this process, characterized in that it comprises means for measuring the spacing force exerted by the product poured on the pinch rollers, at least one inductor generating a variable magnetic field in the product poured upstream of the pinch rollers or between them, and means d slaving of said magnetic field to the value of the spreading force exerted by the product on the pinch rollers.

The inductor (s) can be located between the ingot mold and the pinch rollers, or housed in the pinch rollers, as described for example in Luxembourg patent n ° 67,753 filed on 07.06.1973 and the content of which is incorporated by reference herein description.

These inductors can impart stirring movements to the liquid core of the cast product, or exert a heating action on the cast product.

As will be understood, it is through the stirring action exerted on the liquid core to facilitate the evacuation of the overheating or on the contrary by the reheating action exerted on the product that adjusts the depth of the liquid well.

The mixing of the liquid core of the product during casting is today very widespread in continuous casting of steel products of conventional formats: blooms, billets and slabs with a thickness of approximately 200 mm. Its usual aims are to obtain solidification structures favorable to the properties of use of the final product, such as a high proportion of solidified section in equiaxial mode, and a reduction in segregation. However, stirring also plays a role in the removal of calories from the metal: this is all the more rapid as the setting in motion of the liquid metal is more intense.

All other things being equal, an increase in the stirring action of the liquid core therefore tends to accelerate the solidification of the product and to raise the closing point of the liquid well. Conversely, a decrease in the stirring action tends to delay the solidification of the product and bring down the closing point of the liquid well.

In continuous casting of products of large cross-section, this effect of stirring on the cooling of the product is only of relatively secondary importance, since the cooling is mainly ensured, below the mold, by means of a cooling fluid sprayed onto the surface of the product. On the other hand, no attempt is made to adjust with great precision the point of closure of the liquid well, which is generally located several meters below the ingot mold. On the other hand, in the casting of thin slabs, the smaller thickness of the liquid core makes it very sensitive to all the phenomena which can accelerate its solidification, and in particular to forced convection movements such as those induced by a mobile magnetic field. By establishing such movements permanently within the liquid core and by modulating their intensity, there is therefore a possibility of adjusting the depth of the solidification well without intervening in the operating parameters of the machine. We have seen that, in the case of the continuous casting of thin slabs with pinch rollers distant from the mold, such an adjustment was of very great importance. This possibility can be added to or substituted for other means of cooling the product acting from its surface, such as the projection of a cooling fluid. It has the advantage of acting directly on the liquid core and of providing a short reaction time.

The intensity of the stirring is modulated as a function of the measurement of the force exerted by the product on the pinch rollers, the spacing of which, in normal operation, is kept constant and equal to the thickness desired for the product. The pinch rollers are designed to undergo a force, the value of which, which is measured by sensors continuously or at close time intervals, corresponds to the closing of the liquid well. If F becomes greater than its normal value, it is the indication that the pinch rollers undergo an effort on the part of a product whose core is no longer in the liquid state. Such a deviation from normal operation of the machine may be due to factors such as a voluntary reduction or not in the casting speed, a drop in the temperature of the liquid metal feeding the ingot mold, etc. The start and the end of the casting, during which the temperature of the liquid metal and the speed of movement of the product can vary in significant proportions, also constitute critical periods. Indeed, it is then difficult to adjust the operating parameters of the machine so as to permanently locate the closing point of the liquid well at the desirable level. The possibility to influence the speed of solidification of the metal by means of the intensity of stirring of the liquid core provides the operator with an additional means of action for controlling the operation of the machine.

An example of implementation of the method will now be described. By means of mathematical modeling and tests on the machine, it is determined for given casting conditions (product format, casting speed, temperature of the liquid metal feeding the ingot mold, etc.) which operating parameters of the mixing of the liquid core in principle allows the closing of the liquid well to intervene at the most favorable level, that is to say at the level of the pinch rollers. Furthermore, we set for F two threshold values Fo and F1. Fo is a value below which the force is considered abnormally low and indicates a risk of non-closing of the liquid well. F1 is a value beyond which it is considered that the force undergone by the rollers is too great and must not be constantly supported. If this value F1 is reached, this means that the solidification well closes too early. Consequently, the operator gradually decreases the intensity of the electromagnetic stirring so as to reduce the movements of the metal to slow down its solidification. After each incremental decrease, the force exerted on the rollers is measured. If it remains greater than F1, a further decrease in the intensity of the mixing is carried out. If it is between Fo and F1, the new intensity of stirring is maintained until, possibly, a return to normal of the pouring conditions makes this stirring intensity insufficient and moves the closing point of the liquid well below princely rolls. Such a displacement must result in an effort less than Fo, and it is then necessary to increase the intensity of stirring to bring the effort into the interval [Fo, F1].

Furthermore, it is possible to set a threshold value F2 greater than F1, beyond which it is considered that the force tending to spread the pinch rollers must imperatively not be supported for more than a few moments. The crossing of this threshold F2 must then lead not only to a significant and abrupt reduction in the intensity of stirring, but also to a separation of the rollers in stages. After each level, the force supported by the rollers is measured. If it remains greater than F2, the spacing of the rollers is increased. When it becomes less than F2, the rollers are then brought together. Then, if the force becomes greater than F2, the rollers are again moved aside. If, on the contrary, after bringing the rollers closer together, the force remains below F2, the approximation is continued, and so on until the rollers have returned to their nominal spacing, with a force lower than F2. From there, to reduce the force to a value between Fo and F1, we only play on the intensity of the mixing.

To take account of the inertia of the magnetic field action, it is also possible to set a warning value F3 less than F1 such that, when increasing, the spreading force reaches F3, the modification of the action of the magnetic field. We can thus anticipate crossing the threshold F1 and reduce the risk of damage to the machine. Symmetrically, one can set a warning value F4 greater than Fo such that, when decreasing, the spreading force reaches F4, the magnetic field is modified. This further limits the risk of defects in the product, which would be due to too late closing of the liquid well.

The organs which control the intensity of the stirring and the spacing of the pinch rollers are controlled by the operator or the data processing unit which manages the operation of the casting machine, according to the information provided by the necessary measuring instruments: measurements of the force on the rollers, the distance between the rollers, the electrical parameters of the electromagnetic stirrers, the casting speed, etc.

The electromagnetic stirrers with a sliding field that can be used can belong to the various known types of stirrers, imparting vertical translational movements to the metal or perpendicular to the direction of extraction, according to the large faces of the product or rotational movements. Advantageously, but not exclusively, they are of the type known as "brewing rollers", described for example in patent FR 2 187 468 in the name of the applicant. They are then included in rollers which also serve to support the product (without compressing it) between the lower part of the mold and the pinch rollers. Plan brewers, arranged opposite the large sides of the product in this same area of the machine can also be used.

The single appended figure shows schematically an example of installation for continuous casting of thin slabs according to the invention, seen in section and in profile. The poured product 1 leaves the ingot mold 2, supplied with liquid metal by the nozzle 3, in the partially solidified state: the core 4 is in the liquid state, and it is enclosed by a peripheral layer 5 in the pasty state or fully solidified, the thickness of which increases as the product progresses through the machine, solidifying. The installation includes a pair of pinch rollers 6, 6 ′ in free rotation or controlled in the direction indicated by the arrows, placed at a certain distance from the mold, outside of its immediate vicinity. These rollers reduce the thickness of the product to the desired value by bringing together the layers of metal which have started to solidify on the large faces of the mold, so as to close the liquid well. They are provided with means 7.7 ′ for adjusting their spacing, and for maintaining it at a determined value. The installation also comprises means of electromagnetic stirring of the liquid core 4. They can be constituted, as shown in the figure, by a pair of polyphase planar stirrers 8, 8 ′ arranged opposite the large faces of the product, between the bottom of the ingot mold and the pinch rollers and generating a sliding magnetic field. Sensors measure the rolling forces F and F ′ exerted by the product respectively on the pinch rollers 6 and 6 ′, and transmit the results of these measurements to an information processing unit 9. This unit controls, according to the information transmitted by the sensors, the operating parameters of the brewers which determine the intensity of the movements inside the liquid core, and this permanently. On the other hand, this unit also controls the spacing of the pinch rollers in cases where the rolling force exerted on one of them by the product exceeds the threshold F2 previously defined.

In addition to or instead of the brewers located between the bottom of the mold and the pinch rollers, it is also possible to use brewers included in the mold. However, since they must act on a liquid core of a relatively large volume and since they are located far enough from the bottom of the liquid well, these Brewers in an ingot mold may be insufficient to ensure alone a sufficiently sudden variation in the speed of solidification of the liquid core.

Another variant consists in housing the inductors in the pinch rollers themselves, made tubular for this purpose. They are then particularly well suited to an acceleration of solidification in the case where the liquid well closes below the pinch rollers. They can also stir the liquid metal only in the latter case, or be permanently active like the devices described above. They can be used alone or in combination with other agitation devices for the liquid heart.

Finally, it is possible to replace some or all of the moving field inductors ensuring the mixing of the liquid core by variable but stationary field inductors, the role of which is to provide a permanent or intermittent flow of heat to the liquid heart. This heat flow can be modulated so as to keep the liquid well closed at the pinch rollers. It is increased when the liquid well closes upstream of the nip rollers, and decreased when the liquid well closes below the nip rollers. Such a device is particularly advantageous to implant inside the pinch rollers, since it can then act in the same area where its effect must be felt as a priority.

Claims (10)

1. Process for the continuous casting of thin metal products, particularly steel products, in an installation comprising an ingot mould (2) followed, in the direction of withdrawal of the cast product, by squeezing rolls (6,6′) intended to cause, by a reduction of the thickness of the said product, the closing of the solidification pool, characterized in that:

   - the separation force exerted on the squeezing rolls by the product is measured;

   - and, in a zone located upstream of the squeezing rolls or between the latter, a variable magnetic field is applied to the still molten core of the product, matching the action of the said magnetic field to the value of the separation force measured so that the latter does not exceed, over a long period, an upper limit value given in advance and representing the force that the squeezing rolls can tolerate temporarily without damage.
2. Process according to Claim 1, characterized in that the magnetic field is a stationary variable magnetic field, having an inductive heating effect on the product.
3. Process according to Claim 1, characterized in that the magnetic field is a variable magnetic field which is mobile in translation, having a stirring effect on the molten core of the product.
4. Process according to Claims 1 to 3, characterized in that the action of the said magnetic field is matched to the value of the rolling force measured so that the latter is maintained between two values which are fixed in advance: an upper limit value F1, representing the separation force which is not to be exceeded over a long period in order not to damage the rolls, and a lower limit value Fo, representing the presence downstream of the rolls of a core of the product which is still molten.
5. Process according to Claim 4, characterized in that, if the separation force exceeds a fixed value F2 which is greater than or equal to the said limit value F1, the separation of the rolls is increased by increments so as to bring the separation force back to a value which is less than F1, and the separation of the rolls is then brought back, in stages, to its nominal value, by ensuring that the separation force does not exceed the value F2.
6. Process according to Claim 4, characterized in that, in order to take into account the inertia of the action of the magnetic field, when the separation force, when increasing, reaches a fixed warning value F3 which is less than F1, the action of the magnetic field is modified.
7. Process according to Claim 4, characterized in that in order to take into account the inertia of the action of the magnetic field, when the separation force, when decreasing, reaches a fixed warning value F4 which is greater than Fo, the action of the magnetic field is modified.
8. Device for the continuous casting of thin metal products, particularly steel products, of the type comprising an ingot mould (2) followed, in the direction of withdrawal of the cast product, by squeezing rolls intended to cause, by reduction of the thickness of the product, the closing of the solidification pool, characterized in that it also comprises:

   - means for measuring the separation force exerted by the cast product on the squeezing rolls (6, 6′),

   - inductors, (8,8′) housed in at least one of the squeezing rolls or upstream of the latter and which can generate a variable magnetic field in the part of the cast product which is still molten, located upstream of the squeezing rolls (6,6′) or between these,

   - and means (9) for matching the said inductors to the separation force exerted by the product on the squeezing rolls in order to prevent this force exceeding over a long period an upper limit value given in advance and representing the force that the squeezing rolls can tolerate temporarily without damage.
9. Device according to Claim 8, characterized in that the inductor is a multiphase sliding-field inductor.
10. Device according to Claim 8, characterized in that the inductor is a single-phase induction coil.

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