EP0719607B1 - Method for regulating the continuous casting between rolls - Google Patents

Abstract

A method is claimed for the regulation of continuous casting between rolls. During casting the separation force (RSF) of the rolls is measured and the position of the bearings of at least one of the rolls is acted on in order to increase or diminish the inter-axial distance between the rolls. In order to maintain this separation force essentially constant a table of force values ( DELTA RSF) is predefined, framing a desired nominal force (RSFo) and position of the bearings are acted on more sharply when the measured force is outside the table of values than when it is within the extreme values in the table.

Description

The present invention relates to continuous casting between cylinders of thin metal products, in particular in steel.

According to this known technique, the product produced, for example a thin steel strip of a few millimeters thick, is obtained by pouring the metal molten in a casting space defined between two cylinders of parallel axes, cooled and driven in counterclockwise rotation. In contact with cold walls cylinders, the metal solidifies and the metal skins solidified, driven by the rotation of the cylinders, join at the neck between the cylinders, to form the said strip, extracted downwards.

The operation of the casting process between cylinders is subject to various constraints relating to both poured product only at the installation of the installation casting.

The casting strip must in particular have a section corresponding, in shape and dimensions, to the desired section.

This implies that the air gap at the neck between the cylinders, i.e. the distance between the two cylinders, or substantially equal to the desired thickness Of the band. In fact, as the tape obtained is conventionally subsequently subjected to rolling, the thickness accuracy is less important than its regularity over the entire length of the strip. So a deviation of a few tenths of a millimeter over the thickness relative to the desired thickness is not detrimental to obtaining a quality finished product, after rolling, while rapid variations thickness along the longitudinal direction of the strip casting may affect the finished product, despite said rolling.

From the point of view of the implementation of the sunk, the main constraint is of course to get a continuous strip, and therefore it is necessary to ensure the strip extraction, and that this strip is sufficiently solidified during its extraction. Over-solidification metal upstream of the neck is not necessarily harmful in the case of metal casting relatively malleable, for example aluminum, but it is unacceptable for harder metals, such as steel, because then such over-solidification leads either the formation of a metal wedge above the neck, preventing extraction, i.e. deterioration of cylinders when excessively passing metal between them solidified.

Conversely, insufficient solidification leads to breakthroughs and rupture of the strip downstream of the neck.

To avoid these two causes of malfunction, it is known to play on the spacing of the cylinders, by bringing together in case of insufficient solidification, or removing them in the event of over-solidification, so that the bottom of the solidification well, between the skins of solidified metal in contact with the walls of the cylinders, be maintained at the level of the neck.

Inevitably this results in variations longitudinal thickness of the product obtained when the solidification conditions vary during casting, for various reasons, especially during startup, during the first turns of the cylinders and their setting operating temperature. These variations are tape quality unacceptable casting.

In addition to the problems indicated above, especially those related to the cylinder runout: the perfect circularity of the cylinders which cannot practical to be obtained, it follows that, for a fixed position of the bearings supporting the cylinders, the spacing between these varies cyclically during their rotation. It will also be noted that, at the initial run-out cylinders, cold, overlap the defects of circularity generated by the original deformations thermal due to overheating and cooling cyclic of the surface of the cylinders at each turn.

We already know various methods of regulation tending to provide a solution to one or more of problems previously mentioned.

Thus, we know, for example by EP-A-138 059 and EP-A - 0194 628, a casting method according to which, for avoid deterioration of the casting cylinders in the event of over-solidification of the cast metal, we act on the spacing cylinders as a function of the spreading force exerted on these by the poured product, this effort being assumed representative of the state of solidification of the metal. But this method leads, as we saw previously, to longitudinal variations in the thickness of the strip obtained.

We also know from the above documents a method by which the speed of cylinders (therefore the casting speed) as a function of variations in spacing or force. This method, based on the fact that, if the speed increases, the time of solidification of molten metal in contact with cylinders is reduced, and therefore the solidification is less (and conversely), however does not allow to react fast enough to avoid problems with over or under-solidification which can appear suddenly. Therefore, it can practically only be used in combination with the previous method of setting the spacing as a function of the spacing force.

We also know a casting process according to which we act on the position of the bearings of the cylinders to take into account the circularity of the surface of the cylinders by measuring these circularity defects and by correcting the position of the bearings accordingly depending on the angle of rotation of the cylinders. This method does not allow, as will be understood easily resolve problems related to the condition of solidification of the cast metal.

• de couler sans risque de rupture de la bande ou de percées,
• d'éviter l'endommagement des cylindres,
• d'éviter ce qui est appelé "zones brillantes" sur les cylindres, qui sont le signe de fortes concentrations d'effort d'écartement, et qui reflètent une modification localisée de l'état de surface (rugosité) des cylindres, préjudiciable à la régularité ultérieure de la solidification de la première peau solidifiée,
• et surtout, d'obtenir une bande de métal d'épaisseur la plus constante possible sur toute sa longueur, et l'obtention de cette épaisseur régulière le plus rapidement possible après le début de la coulée.

The present invention aims to jointly solve the problems mentioned above, and aims in particular to allow:

• to flow without risk of band breakage or breakthroughs,
• to prevent damage to the cylinders,
• avoid what are called "shiny areas" on the cylinders, which are the sign of high concentrations of spreading force, and which reflect a localized modification of the surface state (roughness) of the cylinders, detrimental to the subsequent regularity of solidification of the first solidified skin,
• and above all, to obtain a metal strip of thickness as constant as possible over its entire length, and obtaining this regular thickness as quickly as possible after the start of casting.

With these objectives in view, the object of the invention is a regulation process for continuous casting between cylinders, according to which, during casting, we measure the spacing force of the cylinders, and we act on the position of the bearings of at least one of the cylinders for increase or decrease the distance between the said cylinders, characterized in that, in order to maintain said effort substantially constant, we predefine a range of force values framing a desired nominal force, and we act on the position of the bearings more sharply when the value of the measured effort is outside the said range only when included in the said fork.

Thus, in accordance with the invention, account is taken the size of the gap between the spreading effort measured and the nominal effort desired to act on the position of the cylinder bearings: as long as the force stays within the predetermined range, that is, it deviates relatively little from the nominal force value, the reaction of moving the bearings of the cylinders to compensate for this variation in effort will moderate, or even zero, while if the effort comes out of the said fork, the reaction will be more intense.

According to a particular arrangement of the invention, the position of the bearings being regulated to a setpoint position, said setpoint position is fixed by a reference position value d _r , determined by providing an initial setpoint value d _o of position of the bearings a variable corrective value Δd as a function of the difference between the measured spacing force and the nominal force, said corrective value being greater when the measured force is outside of said range than when it is included in the said range.

Preferably, the modulation of the intensity of the corrective action, in response to a difference between the set value of the spreading force and its actual measured value, is carried out by providing the signal E representative of this difference with a correction , defined by a function f , such that it reduces the intensity of this signal when the force measured is within the predefined range, and this is the signal thus corrected E '= f (E) which is then used in the regulation loop to generate the corrective value Δd which is added to the initial position value d _o of the position of the bearings to form the reference position value d _r , which in turn is used as the instruction in a loop standard type of regulation for regulating the position of bearings.

As the speed of movement of the bearings is conventionally, in such a regulatory loop, in proportion with the difference between the effective position of bearings and the set position, it follows that the action on the position of the bearings is all the more long live that said reference position value is far from the value of the actual position measurement. And as the said correction has the effect of displacing the setpoint position beyond the setpoint position initial, and in the direction leading to increasing the gap between setpoint position and actual position of steps, and all the more so as the force measured is distant from the nominal effort, it follows that the reactivity of the bearing position regulation is increased when the force measured leaves said range.

In other words, said correction leads to generating an artificial reference position value, which defines a setpoint position offset from the initial set position in driving direction conventionally to compensate for a variation in effort spacing, i.e. in the direction of a spacing cylinders in response to an increase in said force spacing and vice versa. And as this value of reference position, used as a setpoint for the bearing position regulation, is then far from the value of the actual position measurement of levels, this regulation will react more strongly, to move the bearings, only if the set position was remained the initial setpoint position.

According to a particular embodiment, the signal corrected E 'is increasing according to the difference between the measured spacing force and the nominal force. In this case, the greater the difference between the measured effort and the nominal force is large, the more lively the reaction will be. Preferably then, the corrected signal E 'increases more quickly when the force measured is outside the said fork that when included in said fork. It follows then that not only the reactivity increases with the said difference between measured effort and nominal effort, but it grows all the more quickly that the gap is large.

According to another embodiment, the signal corrected is zero when the value of the measured effort is included in the said range, and increasing in function of the difference between the measured separation force and nominal force when the value of the measured force is outside the said range. In this case, as long as the force measured remains within the said range, the bearing position regulation acts normally to keep these at the initial set position, this which amounts to tolerating variations in effort without seek to compensate for them by a displacement of the bearings, as long as they stay in the said range. On the other hand, as soon as the measured effort goes out of this range, the action on the position of the bearings will be all the more vivid as the force measured moves away from the limits of the range.

According to another particular provision, the correction is reduced after a start-up time predetermined. So we add to the modulation, explained above, of the intensity of the corrective action in function of the measured effort, additional modulation depending on the casting phase. This modulation allows to further increase the reactivity of regulation during the start-up period, so as to obtain the most quickly as possible a stable diet, and reduce this reactivity once this substantially stable regime is obtained, so as to avoid while a peak of effort of very short-term, occurring after the start-up period, lead to a significant variation in the spacing of cylinders, as would be the case during the so-called start-up period. Note that this second modulation applies regardless of whether the effort measured either in the said range or out of it.

Similarly, and with a substantially equivalent, the range of effort may be relatively narrow during the start-up period, and then be enlarged.

• à assurer une très forte réactivité de la régulation pendant la phase de démarrage, pour compenser au mieux les variations brusques des paramètres de coulée survenant lors de la mise en régime de l'installation et dues à la mise en vitesse des cylindres, à leur mise en température et à leurs déformations consécutives, en privilégiant alors l'aspect continuité de la coulée, quitte à tolérer des variations d'entrefer,
• et à réduire ensuite cette réactivité pour privilégier la constance de l'épaisseur du produit coulé, et en tolérant plus facilement d'éventuels pics d'effort sans agir (ou avec une action modérée) sur la position des paliers.

The last two provisions above are aimed at:

• to ensure a very high reactivity of the regulation during the start-up phase, to best compensate for the sudden variations in the pouring parameters occurring when the system is brought into operation and due to the speeding up of the cylinders, their setting in temperature and in their consecutive deformations, while favoring the continuity aspect of the casting, even if it means tolerating variations in the air gap,
• and then to reduce this reactivity in order to favor the consistency of the thickness of the cast product, and by more easily tolerating possible peaks of effort without acting (or with a moderate action) on the position of the bearings.

Other characteristics and advantages will appear in the description which will be given by way of example of a continuous casting process between cylinders of thin steel strips.

• la figure 1 est une vue frontale schématique d'un dispositif de coulée entre cylindre de type connu en soi,
• la figure 2 est un schéma de la boucle de régulation utilisée conformément à l'invention pour réguler l'effort d'écartement des cylindres,
• la figure 3 est une représentation de la courbe de correction de l'effort d'écartement mesuré, utilisé dans la boucle de régulation de la figure 2,
• les figures 4 et 5 sont des représentations graphiques montrant l'évolution en fonction du temps, au début de la coulée, de la vitesse d'extraction, de l'angle de rotation d'un point de la surface d'un cylindre, de la position des paliers du cylindre mobile, et de l'effort d'écartement des cylindres exercé par le produit coulé ;
• les figures 6 et 7 illustrent deux variantes de la correction d'effort E' = f(E).

Reference is made to the appended drawings in which:

• FIG. 1 is a schematic front view of a device for casting between cylinders of a type known per se,
• FIG. 2 is a diagram of the regulation loop used in accordance with the invention to regulate the force of spacing of the cylinders,
• FIG. 3 is a representation of the correction curve of the measured spacing force, used in the regulation loop of FIG. 2,
• Figures 4 and 5 are graphical representations showing the evolution as a function of time, at the start of casting, the extraction speed, the angle of rotation of a point on the surface of a cylinder, the position of the bearings of the movable cylinder, and of the force of spacing of the cylinders exerted by the cast product;
• Figures 6 and 7 illustrate two variants of the force correction E '= f (E) .

The casting installation, shown only partially in Figure 1, includes so conventional, and known per se, two cylinders 1, 2, of axes parallel, spaced apart from each other by a distance corresponding to the desired thickness of the casting strip. The two cylinders 1, 2 are rotated in direction otherwise, at the same speed. They are carried by bearings 3, 4, schematically represented, of two supports 5, 6 mounted on a chassis 7. The support 5, and therefore the axis of the corresponding cylinder 1, is fixed relative to the chassis 7. The other support 6 is movable in translation on the chassis 7. Its position is adjustable and determined by thrust cylinders 9 acting so as to bring together or move the supports away from each other. Means of measurement of the spacing force of the cylinders, such as load cells 8, are arranged between the fixed support 5 and the chassis 7. Sensors 10 make it possible to measure the position of the mobile support 6, and therefore the variations in position relative to a setpoint position predetermined according to the desired thickness of the bandaged.

During casting, the molten metal is spilled between the cylinders, and begins to solidify at contact of their cooled walls by forming skins solidified which are driven by the rotation of cylinders and meet substantially at the neck 11 between the cylinders to form the solidified strip pulled down. In doing so, the metal exerts on the cylinders an RSF spacing force, measured by the weigh 8, this effort being variable in particular depending the degree of solidification of the metal.

To regulate this effort, and guarantee continuity of the casting, one acts on the thrust cylinders 9. Thus, for example, to reduce the RSF spacing effort, we acts on the cylinders 9 in the direction leading to a spacing of the cylinders and, conversely, for increase the effort, we act on the cylinders in the direction an approximation of the cylinders.

This action is performed automatically by a regulation which, according to the invention, makes it possible to obtain a substantially constant spreading effort, very quickly after the start of casting, as well as a thickness of the band obtained also substantially constant.

The schematic diagram of the regulating loop for the spacing force of the cylinders is illustrated in FIG. 2. In this regulating loop, the difference E between the value of the spacing force RSF, measured by the load cells 8, and the effort setpoint value RSF ₀ is calculated by the calculation unit 20. This difference E is entered into a correction device 22 which determines a corrected value E ′ function of E, according to a relationship which will be described in more detail later. The value E 'is introduced into an adjustable gain amplifier 24 which converts E' to a speed v, proportional to E, which is itself integrated into the integrator 26 to provide a corrective value Δd.

The corrective value Δd is introduced into an adder 28 which also receives an initial position setpoint value d ₀ and a runout compensation value Cfr, and develops a position reference value d _r .

The position reference value d _r , which serves as a reference in the regulation of the position of the bearings, is introduced into a comparator 30 which also receives the measured value d _m of the position of the bearings, measured by the sensors 10, and develops a signal E _p representing the difference between the actual position of the bearings and the setpoint position. This signal is introduced into a conventional regulation loop 32 (PID) which provides a signal i _sv to a servovalve 34 for controlling the thrust cylinders 9. The actuation of the thrust cylinders acts on the course of the casting (symbolized by the "process" box 36) during which the value of the RSF spacing force is measured.

It will be noted that the cycle time of the position regulating loop of the thrust cylinders 9 (loop shown diagrammatically by the dotted frame 36) is, for example, 2.10 ^-3 seconds while the overall cycle time (dotted frame 38) is for example 10.10 ^-3 seconds.

The correction f provided by the correction device 22 is shown graphically in FIG. 3, in which numerical values of E and E ′, expressed in tonnes, have been indicated only by way of example.

In this example, the nominal value RSF ₀ of the spreading force is 6 T (6 tonnes or approximately 6000 daN), and the range of forces ΔRSF is 4 T. As long as the measured value of the force spacing is between 4 and 8 T, the correction of the difference E is expressed by E '= 0.3 E ; when the spreading force drops below 4 T or above 8 T, the correction becomes E '= E - 1.4 T .

It can be seen that, according to this example, and with reference to the diagram in FIG. 2, the corrective value Δd generated from the value E ', will increase continuously as a function of the difference between the spacing force measured RSF and the nominal force RSF ₀ , but moreover increases more strongly as soon as the spacing force leaves the range ΔRSF. Consequently, the reactivity of the position regulation of the bearings is somewhat reduced as long as the spacing force measured remains within the said range, and increased or beyond.

It will be noted that the expressions of E 'indicated above are to be considered relatively, since the value E 'is then multiplied by the gain of amplifier 24, and integrated over a cycle time, to give the correction Δd.

It should also be noted that an equivalent effect in this concerning the calculation of Δd could be obtained by directly entering the difference E into the amplifier 24 and by varying the gain thereof as a function of E, i.e. by increasing the gain when the effort gap is out of range, relative to gain when said effort is within range.

However, as we will see next, the gain can also be set according to the time elapsed from start of casting. It would then follow that the gain should be set according to two parameters, the time and the spacing effort, which can in practice complicate the implementation of regulation.

The variation of E 'as a function of E could also be defined differently, for example E 'being zero or substantially zero as long as the spreading force is included in the said range, and increasing in function of E outside of it, as shown in dotted lines in figure 3.

In the latter case, the reference position d _r would then be corrected only when the spacing force would come out of said range, and any variation of force remaining in said range would not cause any displacement of the bearings of the cylinders.

Preferably, the correction made to the reference position of the bearings is reduced after a predetermined start-up time, which can be easily achieved by decreasing the gain, and therefore the value Δd.

In addition, the width of the fork can be increased. These two measures ensure a very high reactivity of the regulation when starting the casting, but not to cause displacement substantial cylinder bearings when peaks of effort occur after the said start-up period.

• le tracé 40 représente la vitesse des cylindres,
• le tracé 50 représente la position angulaire d'un cylindre, l'intervalle entre deux pics de cette courbe correspondant à un tour de cylindre,
• le tracé 60 représente les variations de la force d'écartement RSF, mesurée en tonnes ( échelle graduée de gauche du graphique),
• le tracé 70 représente les variations de la position des paliers, mesurées en mm ( échelle graduée de droite).

To illustrate the results obtained thanks to the invention, FIG. 4 shows the evolution as a function of time, from the start of the casting, of four parameters:

• line 40 represents the speed of the cylinders,
• the plot 50 represents the angular position of a cylinder, the interval between two peaks of this curve corresponding to one revolution of the cylinder,
• plot 60 represents the variations in the spreading force RSF, measured in tonnes (graduated scale on the left of the graph),
• the plot 70 represents the variations of the position of the bearings, measured in mm (graduated scale on the right).

These lines correspond to a casting carried out according to the process according to the invention, by fixing nominal force at 6 tonnes and a fork width ΔRSF of 2 tonnes for approximately 35 seconds, enlarged to 4 tons then.

We note that, after a significant peak of effort 61 at start-up, the effort still varies significantly during the first turns of the cylinders, with some excursions outside the range 5 - 7 tonnes. Correlatively, we see on plot 70, during this same period, the significant variations corresponding to displacements of the bearings of the movable cylinder to compensate the so-called effort variations. We note however that after the first turn of the cylinders, the spreading effort stays in the said range.

When the range is widened to 4 - 8 T, after the start-up period, the effort variations remain weak, and moreover, the bearings of the cylinders do not practically move more, which is explained by the fact that the spreading force is maintained in the center of said fork, and that its variations, lessened by the correction indicated above, do not produce practically no effect on the position regulation of bearings.

It can therefore be seen that the implementation of the method according to the invention allows to quickly obtain, and subsequently maintain a spacing effort, as well as a spacing of the axes of the cylinders, substantially constant.

The corresponding records represented Figure 5, in the case where the nominal force has been fixed at starting at 15 tonnes and a fork width of 4 tonnes, show that the spreading force stabilizes also, as well as the position of the bearings, but this stabilization in this case requires a longer time, this which reveals the advantage of setting a value at startup the lowest possible nominal force with a width also low range, as in the case of figure 4.

Note that, in addition to the regulation described above, the method according to the invention incorporates a runout regulation, to take account of faults circularity of the cylinders and compensate them so as not to no cyclic variations in strip thickness casting.

For this, we determine the circularity deviations cylinders by measuring variations in force spacing as a function of the angle of rotation of the cylinders, this measurement being made during the first cylinder turns when starting the casting, and, then, we modify the said reference value of position of the bearings as a function of the angle of rotation, to compensate for the said circularity deviations.

The determination of the circularity deviations can be made by a computer which extracts from the curve variations in the measured spacing force, the cyclic variations, significant of circularity defects, and develops a corrective value CFr which is added to the value initial setpoint d ₀ and correction Δd to form the position reference value d _r .

The drawings in FIGS. 6 and 7 represent two variants of the correction f which can be used by the correction device 22.

In the variant represented in FIG. 6, the range ΔRSF is no longer centered on the nominal value RSF ₀ , as in the case of FIG. 3, but shifted to the right, that is to say in the direction of increasing forces . With such a correction, the reactivity of the bearing position regulation is reduced, as indicated above, only when the distance force measured RSF is greater than the set point RSF ₀ . On the other hand, if the force measured is less than the setpoint, the regulation acts normally, that is to say more strongly, which avoids an excessively abrupt reduction in the force, and therefore avoids reaching a value d excessively low effort. This is particularly useful when the RSF ₀ setpoint is itself low, for example of the order of 2 tonnes.

In the variant shown in Figure 7, the correction applied when the spreading force remains in the vicinity of the setpoint is similar to that shown FIG. 3, that is to say providing a reduction in the reactivity of the regulation as long as the effort measured RSF remains within the predefined ΔRSF range. On the other hand, a maximum value E'Max is imposed on the corrected value E ', when the force measured exceeds a certain threshold (defined by Es in Figure 7). So while retaining a strong reactivity of the regulation when the effort measured comes out of the ΔRSF range, a gap is avoided excessive cylinders in response to a peak of effort very high but very brief, and therefore we provide a more cylinders in their normal position as soon as the peak effort has passed.

Obviously, these last two variants of correction may be combined.

Claims (11)

1. Control process for twin-roll continuous casting, in which, during casting, the roll separation force (RSF) is measured and the position of the bearings of at least one of the rolls is varied in order to increase or decrease the centre-to-centre spacing of the said rolls, characterized in that, with a view to keeping the said force substantially constant, a band (ΔRSF) of force values bracketing a desired nominal force (RSF₀) is predetermined and the position of the bearings is varied more sharply when the value of the measured force lies outside the said band than when it lies within the said band.
2. Process according to Claim 1, characterized in that, the position of the bearings having been regulated to a set position, the said set position is fixed by a reference position value (d_r) which is determined by applying a correction (Δd), which can be varied as a function of the difference between the measured separation force (RSF) and the nominal force (RSF₀), to an initial set value (d₀) for the position of the bearings, the said correction (Δd) being greater when the value of the measured force lies outside the said band than when it lies within the said band.
3. Process according to Claim 2, characterized in that the correction (Δd) is computed on the basis of a corrected signal (E') obtained by making a correction defined by a function (f) to the difference (E) between the measured separation force (RSF) and the nominal force (RSF₀).
4. Process according to Claim 3, characterized in that the corrected signal (E') increases as a function of the difference between the measured separation force (RSF) and the nominal force (RSF₀).
5. Process according to Claim 4, characterized in that the corrected signal (E') increases more rapidly when the value of the measured force (RSF) lies outside the said band (ΔRSF) than when it lies within the said band.
6. Process according to Claim 3, characterized in that the corrected signal (E') is zero when the value of the measured force (RSF) lies within the said band (ΔRSF) and increases as a function of the difference between the measured separation force and the nominal force when the value of the measured force lies outside the said band.
7. Process according to either of Claims 5 and 6, characterized in that the said band (ΔRSF) is shifted with respect to the nominal force (RSF₀) in the direction of increasing force.
8. Process according to one of Claims 5 to 7, characterized in that a maximum value (E'_max) is assigned to the corrected value (E') when the value of the measured force (RSF) exceeds a predefined threshold (E_t).
9. Process according to one of Claims 2 to 8, characterized in that the said correction (Δd) is reduced after a predetermined start-up period.
10. Process according to one of Claims 2 to 9, characterized in that the said force band (ΔRSF) is widened after a predetermined start-up period.
11. Process according to one of Claims 2 to 10, characterized in that circularity deviations of the rolls are determined by measuring the variations in the separation force (RSF) as a function of the angle of rotation of the rolls, this measurement being made during the first revolutions of the rolls during the start-up of casting and, subsequently, the said reference value (d_r) for the position of the bearings is modified as a function of the angle of rotation in order to compensate for the said circularity deviations.

Images