METHOD AND DEVICE FOR REGULATING A STRAND CASTING SYSTEM

Abstract

A method that includes casting liquid metal in the mould of the strand casting system, extracting a metal strand from the mould using rollers of the strand guide of the strand casting system, determining a measurement variable, which correlates to the fluctuation of the casting level in the mould, cyclically changing the spacing of the opposing rollers of the strand guide in directions opposite to the fluctuations of the casting level to reduce fluctuations of the casting level, and detecting casting level fluctuation frequencies and providing at least one observer, which determines the actual value (ACT) of the roller spacing being used as one of the input variables for the observer in order to compensate a phase shift and/or amplitude of the actual value (ACT) of the roller spacing.

Claims

1. A method for regulating a strand casting plant, wherein the strand casting plant comprises a mold and a strand guide downstream of the mold, wherein liquid metal is poured into the mold, in particular via an inflow unit, which liquid metal solidifies on walls of the mold, so that a metal strand having a solidified strand shell and a still liquid core forms, wherein the metal strand is drawn out of the mold by means of rollers of the strand guide arranged spaced apart, wherein a measured variable is determined, which correlates with the variation of the casting level forming in the mold, this measured variable is processed with incorporation of at least one computing rule and is used to reduce the variations of the casting level, wherein the mutual spacing of opposing rollers of the strand guide is cyclically changed before the complete solidification point (D) to reduce the variations of the casting level, namely by cyclic change of the roller spacing, opposing the variations of the casting level, of opposing rollers of the strand guide, wherein frequencies of the variations of the casting level are detected and at least one observer is provided which, on the basis of these frequencies, determines a compensation value (k) for a target value (SET) of the roller spacing of the rollers, characterized in that the actual value (ACT) of the roller spacing is used as one of the input variables for this observer, in order to compensate for a phase shift and/or amplitude of the actual value (ACT) of the roller spacing.

2. The method as claimed in claim 1, wherein the cyclic changes are in a frequency range up to greater than or equal to 0.6 Hz, preferably up to 5 Hz.

3. The method as claimed in claim 1, wherein multiple roller segments each having one or more rollers are arranged on both sides along the strand guide, wherein at least the inner roller segment located closest to the mold is pivoted normally in relation to the strand guide direction about the axis of rotation of a roller of this roller segment located closest to the mold.

4. The method as claimed in claim 1, wherein frequencies of the variations of the casting level in a frequency range from 0 to 5 Hz are detected and the variations are offset by means of cyclic opposing change of the roller spacing of rollers of the strand guide.

5. The method as claimed in claim 1, wherein frequencies of the variations of the casting level in a first frequency range are detected and the variations are offset by means of cyclic opposing movements of the inflow unit, further frequencies of the variations of the casting level in a second frequency range are detected and the variations are offset by means of cyclic opposing change of the roller spacing of rollers of the strand guide, wherein the second frequency range is greater than the first frequency range, a first observer is provided which determines a first compensation value (k) for a target position of the inflow unit on the basis of frequencies of the first frequency range, and a second observer is provided which determines a second compensation value (k) for a target value (SET) of the roller spacing of the rollers of the strand guide on the basis of frequencies of the second frequency range, wherein the actual value (ACT) of the roller spacing is used as one of the input variables for this second observer.

6. A device for carrying out a method as claimed in claim 1, comprising means for introducing a metal melt into a mold, a strand guide comprising rollers, a measuring unit for measuring variations of the casting level, which is connected to a control unit, wherein an adjustment device connected to the control unit is provided, which is designed to reduce, in particular offset, variations of the casting level by cyclic change, opposing the variations of the casting level, of the roller spacing of opposing rollers of the strand guide, and wherein the control unit comprises at least one observer which is designed in such a way that, based on frequencies of the variations of the casting level, a compensation value (k) for a target value (SET) of the roller spacing of the rollers is determined and the actual value (ACT) of the roller spacing is used as one of the input variables for this observer, in order to compensate for a phase shift and/or amplitude of the actual value (ACT) of the roller spacing.

7. The device as claimed in claim 6, wherein the adjustment device is designed for cyclic changes of the roller spacing in a frequency range up to greater than or equal to 0.6 Hz, preferably up to 5 Hz.

8. The device as claimed in claim 6, wherein multiple roller segments each having one or more rollers are arranged on both sides along the strand guide, wherein at least the inner roller segment located closest to the mold is pivotable normally in relation to the strand guide direction by means of the adjustment device about the axis of rotation of a roller of this roller segment located closest to the mold.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0072] The invention will be explained in greater detail on the basis of an exemplary embodiment. The drawings are exemplary and are to illustrate the concept of the invention, but are in no way to restrict it or even reproduce it exhaustively. In the figures:

[0073] FIG. 1 shows a schematic view of a portion of a strand casting plant according to the invention,

[0074] FIG. 2 shows a schematic view of a strand guide according to the invention,

[0075] FIG. 3 shows the schematic construction of a control unit of the prior art,

[0076] FIG. 4 shows details of the first observer from FIG. 3,

[0077] FIG. 5 schematically shows a monitoring loop according to the invention comprising a first and second observer,

[0078] FIG. 6 shows the time curve of various variables during the regulation of a strand casting plant,

[0079] FIG. 7 shows the time curve of roller spacing and casting level when the roller spacing is not changed,

[0080] FIG. 8 shows the time curve of roller spacing and casting level when the roller spacing ideally keeps the casting level constant,

[0081] FIG. 9 shows the time curve of roller spacing and casting level when the roller spacing shows unusual behavior,

[0082] FIG. 10 shows the time curve of roller spacing and casting level when the unusual behavior of the roller spacing is ideally offset.

EMBODIMENT OF THE INVENTION

[0083] According to FIG. 1, a strand casting plant comprises a mold 1. Liquid metal 3 is poured into the mold 1 via an immersion pipe 2, for example, liquid steel or liquid aluminum. The inflow of the liquid metal 3 into the mold 1 is set by means of an inflow unit 4. A design of the inflow unit 4 as a closure plug is illustrated in FIG. 1. In this case, a position p of the inflow unit 4 corresponds to a stroke position of the closure plug. Alternatively, the inflow unit 4 can be designed as a slide. In this case, the closure position p corresponds to the slide position.

[0084] The liquid metal 3 located in the mold is cooled by means of cooling units (not shown), so that it solidifies on walls la of the mold 1 and thus forms a strand shell. A core 6 is still liquid, however. It only solidifies later. The strand shell 5 and the core 6 together form a metal strand 7. The metal strand 7 is supported and drawn out of the mold 9 by means of a strand guide 8. The strand guide 8 is downstream of the mold 1. It comprises multiple roller segments 8a, which in turn comprise rollers 8b. Only a few are shown of the roller segments 8a and the rollers 8b in FIG. 1. The metal strand 7 is drawn at a draw-off speed v out of the mold 1 by means of the rollers 8b.

[0085] The liquid metal 3 forms a casting level 9 in the mold 1. The casting level 9 is to be kept as constant as possible. Therefore [0086] both in the prior art and also in the present embodiment variant of the inventionthe position p of the inflow unit 4 is tracked to set the inflow of the liquid metal 3 into the mold 1 accordingly. A height h of the casting level 9 is detected by means of a measuring unit 10 (known per se). The height h is supplied to a control unit 11 for the strand casting plant. The control unit 11 determines a manipulated variable S for the inflow unit 4 according to a regulating method, which is explained in greater detail hereafter. The inflow unit 4 is then activated accordingly by the control unit 11. In general, the control unit 11 outputs the manipulated variable S to an adjustment unit 12 for the inflow unit 4. The adjustment unit 12 can be, for example, a hydraulic cylinder unit. Frequencies of the bulging after the mold are detected metrologically and/or determined according to f=v.sub.c/p.sub.Roll*n, wherein v.sub.c corresponds to the draw-off speed of the strand, f corresponds to the bulging frequency, n corresponds to the number of the harmonic frequencies (1, 2, etc.), and p.sub.Roll corresponds to the roller spacings.

[0087] The roller spacings, which correspond to the strand thickness d shown, can be intentionally adapted by means of pivot axis 23 and/or adjustment device 24. This can take place, as shown here in FIG. 1, in that in the first segment at least one roller segment 8a comprises a fixed outer frame, for example, the roller segment 8a located on the left directly below the mold 1 here. The opposing roller segment 8a, and/or the inner frame supporting it, is pivotable around a pivot axis 23, which extends normally in relation to the plane of the drawing. The pivot axis 23 can coincide with a rotational axis of a roller 8b, with the rotational axis of the upper roller 8b here, but could also be provided at another point, of course. Due to the pivoting, the roller spacing changes in the lower roller pair of the uppermost roller segment 8a in FIG. 1, while the roller spacing of the upper roller pair remains the same. This is not disadvantageous because the change of the roller spacing due to the method according to the invention is generally only in the range of a few tenths of millimeters up to 2 mm.

[0088] Possible guide rollers, which are directly connected to the mold and would be arranged above the uppermost roller segment 8a shown here, are not shown in FIG. 1. These guide rollers are generally not adjustable in relation to one another and normally in relation to the strand guide direction, however.

[0089] Alternatively to the pivoting, the left uppermost roller segment 8a, i.e., for example, its outer frame, could be fixed and the right upper roller segment 8a, i.e., for example, its inner frame, could be displaced in parallel normally to the strand guide direction toward the left roller segment 8a and away from it. The roller spacing of all roller pairs thus changes by the same absolute value in each case. This could also be carried out using one or more hydraulic cylinders (distributed along the strand width and/or along the strand guide direction).

[0090] In FIG. 2, only one strand guide 8 is shown, which can replace the strand guide 8 in FIG. 1 or also supplement it-after the uppermost segment. In FIG. 2, in each of the three illustrated segments, each roller segment 8a has three rollers 8b on each side. However, there could also be only two or more than three rollers 8b per roller segment 8a. In continuation of FIG. 1, the fixed strand shell 5 and the liquid core 6 of the strand are illustrated here up to the complete solidification point D. Accordingly, adjustment devices 24 are also provided in all segments 8a up to the complete solidification point D. The adjustment devices 24 can adjust each of the roller segments 8a by pivoting or by parallel displacement, as already explained in FIG. 1. In this example, the inner roller segment 8a of the first (uppermost) segment is adjusted by pivoting around the pivot axis 23, and the inner roller segment 8a of the second segment is adjusted by parallel displacement by means of two adjustment devices 24. The connection of the adjustment devices 24 to the control unit 11 is not shown here.

[0091] The control unit 11 implementssee FIG. 3inter alia, a casting level regulator 13. The height h of the casting level 9 is supplied to the casting level regulator 13. Furthermore, a target value h* for the height h of the casting level 9 is supplied to the casting level regulator 13. Furthermore, further signals are supplied to the casting level regulator 13. The further signals can be, for example, the width and the thickness of the cast metal strand 7 (or more generally the cross section of the metal strand 7), the draw-off speed v (or its target value), and others. The casting level regulator 13 then determines on the basis of the deviation of the height h of the casting level 9 from the target value h* in particular a preliminary target position p* for the inflow unit 4. The casting level regulator 13 can use the further signals for its parameterization and/or for determining a pilot control signal pV.

[0092] The control unit 11 furthermore implements a first observer 14. The height h of the casting level 9 and its target value h*, the further signals and a final target position p* for the inflow unit 4 are supplied to the first observer 14. The first observer 14 determines a first compensation value k. The first compensation value k is added to the preliminary target position p* and the final target position p* is thus determined. The manipulated variable S, using which the inflow unit 4 is activated, is then determined on the basis of the deviation of the actual setting p from the final target position p *. In general, the control unit 11 implements a lower-order position regulator (not shown) for this purpose.

[0093] For the sake of good order, it is to be emphasized once again that the first and second observers 14, 25 are not persons, but rather function blocks implemented in the control unit 11.

[0094] The difference between the preliminary target position p* and the final target position p* corresponds to the first compensation value k determined by the first observer 14. Since the first compensation value k is determined by the first observer 14 and it is therefore known to the first observer 14, alternatively to the final target position p*, the preliminary target position p* can also be supplied to the first observer 14. Because of the circumstance that the first compensation value k is known to the first observer 14, the first observer 14 can thus readily determine the final target position p* from the preliminary target position p *. A tapping point 15, at which the (preliminary or final) target position p*, p* is tapped can thus be located before or after a node point 16 as needed, at which the first compensation value k is added to the preliminary target position p *. The tapping point 15 is to be located before a node point 16, however, at which the pilot control signal pV is added on.

[0095] The first observer 14 comprises a determination block 17. The height h of the casting level 9, the further signals, and the final target position p* are supplied to the determination block 17. The determination block 17 comprises a model of the strand casting plant. By means of the model, the determination block 17 determines on the basis of the further signals and the final target position p* an expected height (i.e., computed with model support) for the casting level 9. On the basis of the expected height, the determination block 17 then determines an expected variation value dh (i.e., computed with model support) for the height h of the casting level 9, i.e., the short-term variation. For example, the determination block 17 can perform averaging of the height h of the casting level 9 and subtract the resulting mean value from the expected height. The determined variation value h thus reflects the expected variation of the height h of the casting level 9. On the basis of the variation value h, the determination block 17 then determines the first compensation value k.

[0096] The procedure previously explained in conjunction with FIG. 3 corresponds to the procedure of the prior art. It is also used in this embodiment variant of the present invention. The first observer 14 having the determination block 17 is illustrated once again in FIG. 4. In the scope of the present invention, the determination block 17 is merely one of multiple components of the first observer 14 in accordance with the illustration in FIG. 4, however. Thus, for example, the first observer 14 additionally comprises a first analysis element 18. The variation value h is supplied to the first analysis element 18. The first analysis element 18 determines the frequency components of the variation value h therefrom. In addition, a second analysis element 19 is preferably also provided. A secondary signal Z is supplied to the second analysis element 19. The second analysis element 19 determines the frequency components of the secondary signal Z therefrom.

[0097] The secondary signal Z can be a withdrawal force F, using which the metal strand 7 is withdrawn from the mold 1 by the rollers 8b of the strand guide 8. The withdrawal force F is oriented parallel to the draw-off speed v. Alternatively, it can be the draw-off speed v itself. These two alternatives are preferred. However, it is also possible to use, for example, a force signal E, which is applied to (at least) one of the roller segments 8a of the strand guide 8, as the secondary signal Z. The direction to which the force signal F is related is orthogonal to the draw-off speed v. The secondary signal Z can again alternatively be a local strand thickness d, which is measured by means of a measuring unit 21 (see FIG. 1) in the strand guide 8. The first analysis element 18 supplies the frequency components determined thereby to a selection element 22. If provided, this also applies in a similar manner to the second analysis element 19. The selection element 22 determines, in conjunction with the draw-off speed V1 the associated wavelengths which correspond to the frequency components of the variation value h and possibly also of the secondary signal Z. The draw-off speed v is supplied for this purpose to the first observer 14 and to the selection element 22 within the first observer 14. The selection element 22 determines the wavelengths at which the associated frequency component of the variation value h and possibly also the associated frequency component of the secondary signal Z is greater than a threshold value S1, S2. The respective threshold value S1, S2 can be defined individually for the frequency components of the variation value h, on the one hand, and the frequency components of the secondary signal Z, on the other hand. These wavelengths are preselected by the selection element 22. Within ranges, which are each coherent per se, of preselected wavelengths of the variation value h, the selection element 22 then determines the wavelengths i (i=1, 2, 3, . . . ), at which the respective frequency component of the variation value h assumes a maximum. The number of wavelengths i is not restricted. The selection element 22 (finally) selects these wavelengths i. The selection element 22 supplies the selected wavelengths i to the determination block 17. The determination block 17 carries out a filtering of the height h of the casting level 9 and the final target position p* for the wavelengths i selected by the selection element 22. The determination block 17 determines the first compensation value k solely on the basis of the filtered height h of the casting level 9 and the filtered final target position p*. The determination block 17 leaves the other frequency components of the height h of the casting level 9 and the final target position p* unconsidered in the scope of the determination of the first compensation value k. Furthermore, predetermined wavelength ranges can be specified to the selection element 22. In this case, the predetermined wavelength ranges represent an additional selection criterion. In particular, wavelengths at which the associated frequency component of the variation value h and possibly also the associated frequency component of the secondary signal Z are above the respective threshold value S1, S2 are only selected if they are additionally within one of the predetermined wavelength ranges. Otherwise, they are not selected even if the associated frequency component of the variation value h and possibly also the associated frequency component of the secondary signal Z is greater than the respective threshold value S1, S2.

[0098] As already previously mentioned, the second observer 25 comprises identical components as the first observer 14, analyzes frequencies of the bulging after the mold 1, and specifies a second compensation value k for the adjustment device 24, namely the compensation value for the target value SET of the roller spacing. This target value SET is a static target value which generally corresponds to the desired strand thickness. A monitoring loop is shown in FIG. 5, which comprises a first and a second observer 14, 25. The first observer 14 specifies a first compensation value k for the inflow unit 4 of the mold 1, whereby the casting level 9 in the mold 1 is regulated. Stated in simplified terms, the first observer 14 and the inflow unit 4 of the mold 1 together represent a standard system for regulating the casting level 9 of the mold 1, which is used for the compensation of frequencies in the first frequency range and thus represents a controller 27 for frequencies of the first frequency range. The second observer 25, which is connected to the adjustment device 24, represents a controller 26 for frequencies of the second frequency range and specifies a second compensation value k.

[0099] This second compensation value k is fed to the regulator 28 for roller adjustment, which calculates a manipulated signal 29 for the roller spacing from a target value SET and an actual value ACT and passes this manipulated signal 29 to the adjustment device 24. In addition, the actual value ACT is then also passed to the second observer 25, which takes this into account in the calculation of the second compensation value k.

[0100] Instead of the first observer 14, which controls and/or regulates the inflow unit 4 of the mold 1, another regulating method could be provided.

[0101] Only a single regulating method could also be provided, which only controls and/or regulates the adjustment device 24 of the rollers 8b, while the inflow unit 4 of the mold 1 is not used at all for adjusting out the variations of the casting level. This single regulating method could be the second observer 25. In this case, the second observer 25 would generally cover a greater frequency range than in the case of two regulating methods. This frequency range could then cover, for example, the frequencies from 0 to 0.6 Hz, 0 to 1 Hz, 0 to 2 Hz, 0 to 3 Hz, 0 to 4 Hz, or 0 to 5 Hz.

[0102] FIG. 6 shows an example of a suppression of cyclic oscillations. The time t is plotted along the horizontal axis. The position of the inflow unit 4, inscribed with Pos (4), is illustrated along the vertical axis in the first (uppermost) illustration, in the second figure the height of the casting level 9 in the mold 1, inscribed with M_L, and in the third figure the steel flow in the strand, inscribed with St_Fl. For better comprehension, the regulation Comp is still deactivated at the point in time t=0 and is then switched on, which is illustrated in the last figure with the states 0 for the deactivated regulation and 1 for the activated regulation. It is well recognizable in the first three illustrations that the position of the inflow unit 4 cyclically changes, and also the height of the casting level 9 and as a result also the steel flow out of the mold 1. The cyclic variations of the casting level M_L are reduced with the activation of the regulation, by changing the position Pos (4) of the inflow unit 4 here. In the method according to the invention, additionally or alternatively to changing the position Pos (4) of the inflow unit 4, one would cyclically change the mutual spacing of the rollers 8b in the uppermost segment accordingly to reduce the variations of the casting level.

[0103] FIGS. 7 to 10 each contain two illustrations: the upper illustration shows the time curve of the casting level 9, in which case the casting level 9 ideally follows the horizontal central line. In the lower illustration, the dotted line shows the time curve of the actual value ACT of the roller spacing, the dashed line shows the time curve of the roller spacing EST calculated in advance by the model of the observer, and the solid line shows the time curve of the target value SET of the roller spacing corrected with the second compensation value k. The target value SET of the roller spacing substantially corresponds to the desired strand thickness d. The second compensation value k is added to said target value and the resultant signal can then be used as a manipulated signal 29 for the roller spacing. The target value SET of the roller spacing is therefore a static value which the, generally periodically, varying second compensation value k decreases and increases, hence generally likewise periodically. The signal that arises as a result of the second compensation value k being added to the static target value SET is thus as it were the final target value.

[0104] FIG. 7 shows the time curve of roller spacing and casting level 9 when the roller spacing is not changed. The casting level 9 changes its height periodically if the actual value ACT of the roller spacing, the roller spacing EST calculated in advance, and the final target value of the roller spacing remain constant, i.e. in particular no second compensation value k is added to the static target value SET. Therefore, the adjustment device 24 here does not change the roller adjustment.

[0105] FIG. 8 shows the time curve of roller spacing and casting level 9 when the roller spacing ideally keeps the casting level 9 constant. For this purpose, the second compensation value k added to the target value SET of the roller spacing has to be changed with the same frequency as the unregulated casting level 9 (FIG. 7) and generally with a corresponding phase shift with respect to the casting level 9, thus resulting in a common curve of the roller spacing EST calculated in advance and the actual value ACT of the roller spacing, which common curve has the same frequency as the target value SET plus the second compensation value k, but is only phase-shifted with respect to the target value SET plus the second compensation value k. The actual roller adjustment thus corresponds to the roller spacing EST calculated in advance.

[0106] FIG. 9 shows the time curve of roller spacing and casting level when the actual roller spacing shows unusual behavior. A periodic variation of the casting level 9 arises despite regulation on the basis of the target value SET plus the second compensation value k. In other words, everything is done the same as before in FIG. 8, but the result is different because the rollers 8b behave unexpectedly. Therefore, FIG. 9 reveals a difference in both phase and amplitude between the actual value ACT of the roller spacing and the roller spacing EST calculated in advance.

[0107] FIG. 10 shows the time curve of roller spacing and casting level when the unusual behavior of the roller spacing from FIG. 9 is ideally offset. As a result of the feedback of the actual value ACT to the second observer 25, the latter can adapt the second compensation value k in such a way that this unusual behavior is also offset. It is evident that for this purpose the phase of the target value SET plus the second compensation value k has to be shifted relative to FIG. 9 in order that the casting level 9 is ideally offset again.

[0108] Typical strand thicknesses d during thin slab casting are around 100 mm, and typical casting speeds are between 2 and 6 m/min. The constant roller division over relatively long portions of the strand guide in the transport direction is typically in the range of around 200 mm. Casting speed and roller division then yield the frequencies of the fundamental wave and of the harmonic waves of the oscillations of the casting level which are to be offset by the method according to the invention and the device according to the invention.

LIST OF REFERENCE SIGNS

[0109] 1 mold

[0110] 1a walls of the mold

[0111] 2 immersion pipe

[0112] 3 liquid metal

[0113] 4 inflow unit

[0114] 5 strand shell

[0115] 6 core

[0116] 7 metal strand

[0117] 8 strand guide

[0118] 8a roller segments

[0119] 8b rollers

[0120] 9 casting level

[0121] 10 measuring unit

[0122] 11 control unit

[0123] 12 adjustment unit

[0124] 13 casting level regulator

[0125] 14 first observer

[0126] 15 tapping point

[0127] 16, 16 node points

[0128] 17 determination block

[0129] 18, 19 analysis elements

[0130] 20 temperature sensor

[0131] 21 measuring unit

[0132] 22 selection element

[0133] 23 pivot axis

[0134] 24 adjustment device

[0135] 25 second observer

[0136] 26 controller for frequencies of the second frequency range

[0137] 27 controller for frequencies of the first frequency range

[0138] 28 regulator for roller adjustment

[0139] 29 manipulated signal for roller spacing

[0140] ACT actual value of the roller spacing

[0141] D complete solidification point

[0142] d strand thickness

[0143] EST roller spacing calculated in advance

[0144] F withdrawal force

[0145] F force signal

[0146] h height of the casting level

[0147] h* target value for the height of the casting level

[0148] k first compensation value

[0149] k second compensation value

[0150] p position of the inflow unit

[0151] p*, p* target positions

[0152] pv pilot control signal

[0153] S manipulated variable for the inflow unit 4

[0154] SET target value of the roller spacing

[0155] S1, S2 threshold values

[0156] T temperature

[0157] V draw-off speed

[0158] Z secondary signal

[0159] h variation value