EP4277756A1

EP4277756A1 - Rolling with minimisation of a drop in the bending force upon entry

Info

Publication number: EP4277756A1
Application number: EP22700615.2A
Authority: EP
Inventors: Andreas Hollaus; Matthias Kurz
Original assignee: Primetals Technologies Germany GmbH
Current assignee: Primetals Technologies Germany GmbH
Priority date: 2021-01-18
Filing date: 2022-01-13
Publication date: 2023-11-22
Also published as: CN116723901A; EP4029618A1; JP2024503871A; US20240075508A1; WO2022152781A1

Abstract

In a roll stand (1), a planar rolling material (8) consisting of metal is rolled. The working roll inserts (5) are pressed apart from one another by a bending system (10). A base set-point value (FBB*) is supplied to a bending controller (14) and is taken into consideration by the bending controller (14) to determine a resultant set-point value (FB*). An actual value (FB) of the bending force is also supplied to the bending controller (14). From this, the bending controller (14) determines a base controlled variable (SB) for the bending system (10) so that, when the bending system (10) is actuated with the base controlled variable (SB), the actual value (FB) is brought as close as possible to the base set-point value (FBB*). From a stabilisation time (t3), which lies after an entry time (t2), the bending controller (14) determines the resultant set-point value (FB*), additionally taking an actual rolling force (F) into consideration. During an entry time period, which starts before the actual entry time (t2) and ends at the latest at the stabilisation time, an additional set-point value (FBZ*) is supplied to the bending controller (14) and is taken into consideration by the bending controller (14) when determining the resultant set-point value (FB*). The actual value (FB) of the bending force is thus greater than the base set-point value (FBB*). Alternatively or additionally, an additional controlled variable (SZ) is added to the base controlled variable (SB), or the controlled variable (SR) supplied to the bending system (10) is limited downwardly by a minimal controlled variable (SM).

Description

The present invention is based on an operating method for a roll stand for rolling flat metal stock that has a roll stock head, the roll stand having at least work rolls and back-up rolls, - wherein the work rolls are mounted in work roll chocks and a bending system presses the work roll chocks apart acts on the work roll chocks, - wherein the rolling stock head reaches the roll stand at an actual tapping time, - wherein a basic setpoint value is supplied to a bending controller and the bending controller determines a resulting setpoint taking into account the basic setpoint, - an actual value of the bending force also being fed to the bending controller, - the bending controller using the resulting setpoint and the actual value to determine a basic manipulated variable for the bending system determined, so that when the bending system is controlled with the basic manipulated variable, the actual value is approximated as closely as possible to the resulting target value, - whereby the bending controller uses the resulting target value from a stabilization point in time that is after the piercing point in time, with additional consideration of when rolling the flat actual rolling force occurring on the rolling stock is determined. The present invention is also based on a rolling unit for rolling a flat rolling stock made of metal, which has a rolling stock head, wherein the rolling unit has a rolling stand and a deflection controller, the rolling stand has work rolls and back-up rolls mounted at least in work roll chocks - wherein the rolling stand has a bending system that presses the work roll chocks apart, - wherein the bending controller controls the bending system, - wherein the rolling stand and the bending controller interact with one another during operation of the rolling unit in such a way that they carry out such an operating method. PRIOR ART Rolling stands for rolling a flat rolling stock are often designed as a four-high stand (ie as a rolling stand with work rolls and back-up rolls) or as a six-high stand (ie as a rolling stand with work rolls, back-up rolls and intermediate rolls arranged between the work rolls and the back-up rolls). A metal strip is often rolled in them, sometimes also a heavy plate. A pass schedule calculation is carried out before the rolling of the respective rolling stock. As part of the pass schedule calculation, target values for the individual actuators of the roll stand are determined, with which the actuators are to be operated when rolling the respective rolling stock. The target values include at least the adjustment or the rolling force. They usually also include a target value—hereinafter referred to as the base target value—for the bending force with which the work roll chocks and thus also the work rolls are to be pressed apart. The bending force can be used to influence the contour, profile and flatness of the rolling stock. Other actuators can also be present to influence the contour, profile and flatness, for example a work roll shift or local cooling. In the case of roll stands for stainless steel, strip edges can also be lubricated to influence the profile. However, the other actuators are not relevant within the scope of the present invention. The pass schedule calculation is carried out by a higher-level control device, which is usually referred to as the L2 system in specialist circles. The target values determined as part of the pass schedule calculation are forwarded by the control device to subordinate controllers, which implement real-time control during the rolling of the rolling stock. The entirety of the controllers is usually referred to as the L1 system in specialist circles. The target values are specified before the rolling stock reaches the roll gap between the work rolls of the roll stand, i.e. before the tapping takes place. For example, the target value for the bending force - i.e. the basic target value - is specified to a bending controller. This setpoint is modified by various correction variables during the rolling of the flat rolled stock. One of the correction variables is an additional target value that is determined as a function of the rolling force and – similar to an AGC – is intended to compensate for changes in the roll deflection that occur as a result of a change in the rolling force. However, this additional setpoint is only applied after the instabilities that arise during the tapping process have been corrected again by the controllers of the L1 system. The bending controller therefore determines a basic manipulated variable for the bending system during a piercing period, which begins before the piercing time and ends after the piercing time, based solely on the basic setpoint and the actual value of the bending force and controls the bending system according to the determined basic manipulated variable . The determination is made in such a way that the actual value of the bending force is brought as close as possible to the basic target value at all times. When piercing, the bending force (i.e. its actual value) drops. The bending controller tries to correct this slump as quickly as possible. However, a period of several 100 ms, sometimes up to 500 ms, elapses until the system is completely corrected. On the one hand, the drop in the bending force has a negative effect on the resulting contour and the associated profile, as well as the flatness of the rolling stock. However, this can often be accepted. On the other hand, the dip in the bending force leads to a short-term unstable state, the effects of which on the running of the strip cannot always be foreseen. In particular, it can happen that a hook forms in the rolling stock on the outlet side of the roll stand. In some cases, the hook is so large that it hits the side rails on the exit side of the rolling stand. This can lead to damage to the lateral guides and, in individual cases, even to the rolling stock head getting caught. In this case, the rolling stock head is no longer transported further, while the rolling stand continues to push the rolling stock. As a result, the rolling stock bulges (so-called bulging). This leads at least to an unplanned longer interruption of the operation of the roll stand, sometimes even to considerable damage to the roll stand or downstream equipment. DE 102006059 709 A1 discloses an operating method for a roll stand for rolling flat metal stock, wherein the work rolls of the roll stand are subjected to a bending force during a period when a head of the roll stock has not yet reached the roll stand is at least as great as a balancing force of the upper work roll and the upper back-up roll (and possibly further rolls arranged between the upper work roll and the upper back-up roll). From the point in time at which the head of the rolling stock reaches the roll stand, the bending force is determined according to the technological requirements of the rolling process. The resulting bending force can be greater or less than the minimum force and also greater or less than the balancing force. From JP S57050207 A an operating method for a roll stand for rolling a flat rolled stock made of metal is known, in which the point in time at which a rolled stock head reaches the roll stand is calculated in advance. From this point in time, the work rolls of the roll stand are subjected to a bending force by means of a bending system. JP S59061512 A discloses an operating method for a roll stand for rolling a flat rolled stock made of metal, in which a loop lifter arranged upstream of the roll stand detects whether the rolled stock is subjected to tension on the inlet side of the roll stand. As a result, the tapping and the tapping are recorded. During tapping, bending forces acting on the work rolls of the roll stand are adjusted in such a way that the thickness of the rolling stock decreases towards its lateral edges. During tapping, the bending forces are adjusted in a similar manner as soon as a drop in the load on the looper and thus the discharge of the flat rolled stock from the upstream rolling stand is detected. DE 4331261 A1 discloses an operating method for a roll stand for rolling flat metal stock, in which the work rolls can be subjected to positive and negative bending forces of different magnitudes by means of a bending system. Summary of the invention The object of the present invention is to create possibilities by means of which the unstable state can be avoided as far as possible. The object is achieved by an operating method with the features of claim 1. Advantageous configurations are the subject of the dependent claims 2 to 7. According to the invention, an operating method of the type mentioned is configured in that during a tapping period that begins before the actual tapping time and ends after the actual tapping time, the bending controller in addition to the base setpoint an additional target value is supplied so that the bending controller determines the resulting target value during the piercing period, taking into account not only the basic target value but also the additional target value and the actual value of the bending force is therefore greater than the basic target value immediately before the actual piercing time, and/or - by adding an additional manipulated variable to the basic manipulated variable, a resulting manipulated variable is determined, which is supplied to the bending system and the bending system is thereby controlled in such a way that the resulting manipulated variable is greater than the basic manipulated variable, and/or - an A The selection element is supplied with the basic manipulated variable and a minimum manipulated variable and the selection element supplies the bending system with the maximum of the basic manipulated variable and the minimum manipulated variable. If the actual value of the bending force immediately before the tapping time is greater than the basic target value, the bending force begins to drop at a higher level. This reduces the likelihood and magnitude of potential hook formation. If the resulting manipulated variable is greater than 0, the hydraulic valve, by means of which hydraulic fluid is supplied to the bending system, is at least partially open at the time of tapping. It therefore does not have to be opened at the time of tapping. The piercing of the rolling stock therefore leads to a smaller drop in the bending force. This also reduces the probability and the extent of a possible formation of a hook. A resulting manipulated variable greater than 0 can be correct set size must be guaranteed. For example, the additional manipulated variable can be set to 110% of the maximum possible value. Even if the bending controller determines the minimum possible value as the basic manipulated variable, the hydraulic valve is opened by at least -100% +110% = 10%. As an alternative to specifying the additional manipulated variable, the minimum manipulated variable can also be supplied directly. In this case, if the hydraulic valve is already open due to the basic manipulated variable and the minimum manipulated variable is smaller than the basic manipulated variable, the hydraulic valve remains open without changing the open position. However, regardless of the value of the basic manipulated variable, the hydraulic valve is always opened at least to the extent specified by the minimum manipulated variable. By suitably selecting the minimum manipulated variable (namely greater than 0), it can also be ensured in this case that the hydraulic valve is at least partially open at the time of tapping. It is possible that the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable are switched abruptly to their maximum value at the start of the tapping period. It is also possible for the additional setpoint value and/or the additional manipulated variable and/or the minimum manipulated variable to be abruptly reduced to zero at the end of the tapping period. Preferably, however, the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable are increased strictly monotonically from 0 to their maximum value with a finite increase from the beginning of the tapping period and/or strictly monotonically from their maximum value to zero at the end of the tapping period with a finite increase lowered. This results in smoother transitions, which place less stress on the bending controller, the hydraulic valve and the bending system in particular and also lead to a more stable transition, especially when the additional setpoint value and/or the additional manipulated variable and/or the minimum manipulated variable is reduced to 0. Possibilities for raising and lowering with a finite slope are well known and familiar to those skilled in the art. For example, ramping can take place or a binary switching process can be smoothed out by appropriate filtering. As a rule, an expected tapping time is determined by tracking the path of the rolling stock. In this case, the start of the tapping period is preferably a predetermined early period of time before the expected tapping time. The predetermined early period of time is dimensioned, for example, in such a way that the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable reach their maximum value at a time whose distance from the expected tapping time is at least as great as an error tolerance between the actual and the expected tapping time. The person skilled in the art can readily estimate the error tolerance based on the inaccuracies in tracking the path of the rolling stock head, which are known to him. The predetermined period of time is typically in the range between 0.5 s and 2.0 s, in particular between 0.8 s and 1.5 s, for example around 1.0 s. It is possible that the end of the tapping period is a predetermined late period of time after the expected tapping time. However, the end of the tapping period is preferably a predetermined late period of time after the actual tapping time. The actual piercing time can be detected without further ado, for example on the basis of an abrupt increase in the actual rolling force or the rolling torque actually applied by the drives of the work rolls. In both cases, the predetermined late period of time is dimensioned in such a way that the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable up to their maximum value maintained at a time whose distance from the expected or actual tapping time has a predetermined value. From this point in time, the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable can be reduced to 0. The specified value – i.e. the period of time during which the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable are still kept at their maximum value – is determined by the design and dimensioning of the bending system. The value is usually in the range between 0.1 s and 1.0 s, in particular between 0.2 s and 0.6 s, for example 0.3 s or 0.4 s the additional manipulated variable and/or the minimum manipulated variable before rolling the flat rolled stock as a function of properties of the rolled stock and/or as a function of an expected rolling force. As a result, the corresponding maximum value can be optimally matched to the specific roll pass to be carried out. Alternatively or additionally, it is possible for the maximum value of the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable to be determined in such a way that the resulting manipulated variable assumes its maximum possible value immediately before the actual tapping time. This procedure can be useful in particular in the case of the front stands of a multi-stand finishing train or in the case of a roll stand for rolling heavy plate (plate mill). The additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable are preferably determined in such a way that a drop in the actual value of the bending force at the actual piercing time, which would occur without the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable, is compensated for at least 50%. So if the basic setpoint has the value X and without the additional setpoint and/or the additional manipulated variable and/or the minimum manipulated variable there would be a dip to the value Y when tapping, the additional setpoint and/or the additional The manipulated variable and/or the minimum manipulated variable is preferably determined in such a way that the bending force collapses to a maximum of (X+Y)/2, preferably even only to a value that is greater than (X+Y)/2. It is particularly preferred if the bending force collapses at most to the basic target value, ie to the value X. The object is also achieved by a rolling unit having the features of claim 8. According to the invention, in a rolling unit of the type mentioned at the outset, the roll stand and the deflection controller interact with one another during operation of the rolling unit in such a way that they carry out an operating method according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS The properties, features and advantages of this invention described above and the manner in which these are achieved will become clearer and more clearly understandable in connection with the following description of the exemplary embodiments, which is detailed in connection with the drawings be explained. 1 shows a rolling stand from the side before rolling a rolling stock, FIG. 2 shows the rolling stock from FIG. 1 from the side at the point of tapping, i.e. at the start of rolling of the rolling stock, FIG 4 shows part of the rolling stand of FIGS. 1 to 3 from the front, FIG. 5 shows part of a control structure for the rolling stand of FIG. 1 to 4, FIG 1 to 4 according to the prior art, FIG. 7 a part of a control structure for the rolling stand of FIGS. 1 to 4 according to a first embodiment of the present invention, 8 shows a timing diagram for an operating method for the roll stand in FIGS. 1 to 4 according to the first embodiment of the present invention, FIG. 9 shows part of a control structure for the roll stand in FIGS. 1 to 4 according to a second embodiment of the present invention 10 shows a timing diagram for an operating method for the rolling stand in FIGS. 1 to 4 according to the second embodiment of the present invention, and FIG. 11 shows part of a control structure for the rolling stand in FIGS. 1 to 4 according to a third embodiment of the present invention Invention. DESCRIPTION OF THE EMBODIMENTS According to FIGS. 1 to 4, a roll stand 1 has work rolls 2 and back-up rolls 3 . In the context of the present invention, this represents the minimum configuration of the roll stand 1. In addition, the roll stand 1 could also have intermediate rolls. In this case, the intermediate rolls would be arranged between the work rolls 2 and the back-up rolls 3 . As shown in FIG. 4, the work rolls 2 have bearing journals 4 with which the work rolls 2 are mounted in work roll chocks 5 . In an analogous manner, the back-up rolls 3 have bearing journals 6 with which the back-up rolls 3 are mounted in back-up roll chocks 7 . A rolling force F is applied to the back-up roll chocks 7 and thus also to the back-up rolls 3 as a result of the rolling of a rolling stock 8 . The rolling force F is transmitted to the work rolls 2 via the back-up rolls 3 . This is well known to those skilled in the art. The rolling stock 8 itself consists of metal, for example steel or aluminum. It is a flat rolling stock, for example a strip or a heavy plate. It has a rolling stock head 9 . The rolling stock head 9 is that area of the rolling stock 8 which is first rolled in the roll stand 1 . Correspondingly, a transport direction of the rolling stock 8 is denoted by x in FIGS. The roll stand 1 also has a bending system 10 . The bending system 10 generally consists of at least two hydraulic cylinder units 11, 12 which act on the work roll chocks 5 on the drive side and on the operator side and thereby press the work roll chocks 5 apart. The bending system 10 is used to adjust the contour, profile and flatness of the rolling stock 8. In some cases, several hydraulic cylinder units 11, 12 act on the work roll chocks 5. In this case, there are correspondingly more hydraulic cylinder units 11, 12. According to FIG. 5, the roll stand 1 is controlled by a control structure. The control structure usually includes a control device 13 and in any case a bending controller 14. The control device 13 is a higher-level control device that acts as an L2 system, ie determines their target values as part of a pass schedule calculation for subordinate controllers. In FIG. 5, only a single controller is shown in this regard, namely a bending controller 14. In practice, of course, there are also other controllers. In the context of the present invention, however, only the bending controller 14 is important. Therefore, only the bending controller 14 is shown and only the bending controller 14 is explained below. The pass schedule calculation is carried out for the rolling stock 8 even before the rolling stock 8 is rolled in the roll stand 1 (see FIG. 1). As part of the calculation of the pass schedule, the control device 13 determines target values for the adjustment of the roll stand 1, if necessary the roll shift and others. In particular, the control device 13 determines a base setpoint value FBB* of the bending force as part of the pass schedule calculation for rolling the rolling stock 8 in the roll stand 1 . The basic setpoint FBB* can be a single, singular value that is constant over time. Alternatively, separate base target values can be set for different sections of the strip to be rolled. values FBB* are determined. In this case, the basic setpoint FBB* varies over time. The base reference value FBB* is supplied to the bending controller 14 from a point in time t1 (see FIG. 6). The point in time t1 is referred to below as the default point in time t1. At the default time t1, the rolling stock head 9 has not yet reached the roll stand 1 (see FIG. 1). The basic setpoint value FBB* is generally supplied by the control device 13. In principle, however, the basic setpoint value FBB* can also be supplied to the bending controller 14 in some other way. An actual value FB of the bending force is also supplied to the bending controller 14 . Possibilities for detecting or determining the actual value FB are generally known to those skilled in the art. For example, to determine the bending force FB, working pressures pP, pT in the working spaces of the hydraulic cylinder units 11, 12 can be mathematically combined with one another in connection with the effective working surfaces. The bending controller 14 controls the bending system 10. In particular, the bending controller 14 uses a resulting setpoint value FB* and the actual value FB to determine a basic manipulated variable SB for the bending system 10. The basic manipulated variable SB is determined in such a way that the actual value FB corresponds to the resulting setpoint value FB*. is approximated as closely as possible if the bending system 10 is controlled with the basic manipulated variable SB. The resulting setpoint value FB* is determined by the bending controller 14 using at least the basic setpoint value FBB*. The resulting setpoint FB* can temporarily be identical to the base setpoint FBB*. At least temporarily, however, other variables are also included in the resulting setpoint value FB*. This will become apparent. Using the basic manipulated variable SB, the bending controller 14 determines a resulting manipulated variable SR. The resulting manipulated variable SR can temporarily be identical to the basic manipulated variable SB. In normal operation, ie during the stable rolling of the flat rolling stock 8, the deflection controller 14 gives the resulting control variable SR to the bending system 10 and thereby controls the bending system 10. The bending controller 14 determines as a basic manipulated variable SB and also as a resulting manipulated variable SR, in particular, an opening state for hydraulic valves 15, 16, by means of which work spaces of the hydraulic cylinder units 11, 12 with a high working pressure pP (pump pressure) and a low working pressure pT (tank pressure). The hydraulic valves 15, 16 are usually continuously adjustable valves, ie proportional valves or servo valves. Due to the specification of the basic setpoint FBB*, the bending controller 14 thus initially determines a relatively large basic manipulated variable SB from the preset time t1, possibly even a maximum possible value MAX of the basic manipulated variable SB (and thus also the resulting manipulated variable SR). However, it reduces the basic manipulated variable SB back to 0 or almost to zero as soon as the actual value FB of the bending force is as close as possible to the basic setpoint value FBB*. In addition, it is pointed out in this connection that within the scope of the present invention, positive values of the basic manipulated variable SB correspond to an increase in the bending force (up to a technically maximum possible value), negative values to a reduction in the bending force. At a point in time t2, the rolling stock head 9 reaches the rolling stand 1 (see FIG. 2). The point in time t2 is referred to below as the actual tapping point in time t2. The actual piercing time t2 can easily be detected, for example by recognizing a significant increase in the rolling force F or a rolling torque of a drive of the work rolls 2. During piercing, the bending force FB drops significantly. Drops of 50% and more are quite possible. Within a relatively short time, the bending controller 14 opens the hydraulic valves 15, 16 by specifying a corresponding basic manipulated variable SB and thereby sets the bending force back to its resulting desired value FB*. The time to The creation of the bending force is usually well below 1 s, for example around 500 ms. After the actual tapping time t2, the rolling stock 8 is rolled (see FIG. 3). Immediately after the tapping time t2, however, the roll stand 1 is in a comparatively unstable state, which is corrected again by the various controllers assigned to the roll stand 1 (including the bending controller 14). A stable state is reached again at a stabilization point in time t3. The time interval between the stabilization point in time t3 and the tapping point in time t2 is determined by the design and dimensioning of the roll stand. As a rule, it is in the range of 1 s and less, for example 500 ms or even less. A correction value δFB* is determined by a determination unit 17 from the stabilization time t3 onwards. The correction value δFB* is applied to the basic setpoint FBB*. From the stabilization point in time t3, the resulting setpoint FB* is the sum of the basic setpoint FBB* and the correction value δFB*. The correction value δFB* is determined in the determination unit 17 as a function of the (actual) rolling force F. The determination unit 17 thus implements a so-called DPC (=“bending AGC”). If necessary, further correction variables can also be supplied to the bending controller 14 from the stabilization time t3, for example from a flatness control or from a profile feedback control. A correction based on thermal factors is also possible. However, at least the compensation of influences caused by the rolling force is given. At a point in time t4, a rolling stock foot 18 (see FIGS. 1 to 3) of the rolling stock 8 leaves the roll stand 1. Analogous to the actual tapping time t2, the actual tapping time t4 can also be recorded without further ado, in particular re by recognizing a clear drop in the rolling force F or a rolling torque of a drive of the work rolls 2. The time t4 is referred to below as the tapping time. As a rule, the application of the correction value δFB* to the basic setpoint FBB* is frozen shortly before tapping time t4, ie the correction value δFB* last determined is retained. In the context of the present invention, however, this is of secondary importance. The core of the prior art procedure explained above is retained, but modified and supplemented according to the invention. A possible modification and addition is explained in more detail below in connection with FIGS. 7 and 8, another possible modification and addition in connection with FIGS. 9 and 10 and another modification and addition in connection with FIG 7 and 8, the bending controller 14 is supplied with an additional setpoint value FBZ* during a piercing period—in addition to the base setpoint value FBB*. The additional setpoint value FBZ* can be fed to the bending controller 14 by the control device 13 . However, it can also be specified in some other way, for example by an operator (not shown). The tapping period begins at a start time t5 and ends at an end time t6. The start time t5 is before the actual tapping time t2. The end time t6 is after the actual tapping time t2. It is usually before the stabilization point in time t3. It can also coincide with the stabilization point in time t3. At least as a rule, the end time t6 should not be after the stabilization time t3. Because from the stabilization point in time t3, the sense and purpose of the regulation of the rolling stand 1 is no longer to ensure a stable start of rolling. Rather, it is now the sense and purpose of the regulations of the rolling stand 1 to roll the rolling stock 8 to its target properties, in particular to its target thickness and thickness Target profile or its target contour. A specification of the additional setpoint value FBZ* going beyond the stabilization point in time t3 would be disadvantageous for this. The additional setpoint FBZ* is switched to the basic setpoint FBB*. The supply of the additional desired value FBZ* to the bending controller 14 causes the bending controller 14 to determine the sum of the basic desired value FBB* and the additional desired value FBZ* as the resulting desired value FB*. The basic manipulated variable SB is thus determined in such a way that the actual value FB of the bending force is as close as possible to this sum. Due to the modified target value (FBB*+FBZ* instead of FBB*), the actual value FB of the bending force immediately before piercing time t2 is greater than the basic target value FBB*. As part of the embodiment according to FIGS. 9 and 10, an additional manipulated variable SZ is switched onto the basic manipulated variable SB during the tapping period. The sum of the basic manipulated variable SB and the additional manipulated variable SZ is thus supplied to the hydraulic valves 15, 16 as the resultant manipulated variable SR. As a result, the resulting manipulated variable SR is greater than the basic manipulated variable SB immediately before the actual tapping time t2. The additional manipulated variable SZ can be fed to the bending controller 14 by the control device 13 . However, it can also be specified in some other way, for example by an operator (not shown). In the embodiment according to FIG. 9, the basic manipulated variable SB and the additional manipulated variable SZ are added on the output side of the bending controller 14 . In the embodiment according to FIG. 11, on the other hand, the basic manipulated variable SB and a minimum manipulated variable SM are supplied to a selection element 19 on the output side of the bending controller 14 . The selection element 19 selects the larger of the manipulated variables SB, SM supplied to it and supplies the selected manipulated variable to the bending system 10 as the resulting manipulated variable SR. In this embodiment, it is not necessary to specify the additional setpoint value FBZ*, since the bending controller 14 can cause the resulting manipulated variable SR is greater than the minimum manipulated variable SM. However, the bending controller 14 cannot cause the resulting manipulated variable SR to be smaller than the minimum manipulated variable SM. The minimum manipulated variable thus defines a minimum activation state of the bending system 10. As a rule, it is sufficient to either take the procedure according to FIGS. 7 and 8 or the procedure according to FIGS. 9 and 10. In principle, however, it is also possible to combine the two procedures. For example, the additional manipulated variable SZ can primarily be applied, so that the actual value FB of the bending force is increased. In this case, the additional setpoint value FBZ* can be tracked accordingly at the same time, so that the bending controller 14 does not counteract the increase in the bending force due to the deviation of the actual value FB of the bending force from the basic setpoint value FBB*. However, even without tracking the additional setpoint FBZ*, the resulting manipulated variable SR can be forced to be positive. All that is required for this is to select the additional manipulated variable SZ sufficiently large. The configuration according to FIG. 11 generally does not have to be combined with one of the configurations of FIGS. 7 to 10. Various advantageous configurations of the present invention can also be seen in particular from FIGS. 8 and 10, and also from FIGS. 7 and 9 in individual cases. As a result, the same also applies to the configuration of FIG. 11. These configurations are not necessary for the realization of the basic principle of the present invention, but offer additional advantages. The configurations are explained individually in more detail below. They can be implemented independently of one another, but can also be combined with one another as required. Furthermore, the configurations are explained below without exception in connection with FIG. 8 and, in part, FIG. 7, ie for the case in which the additional setpoint value FBZ* is specified. However, the advantageous configurations can also be implemented in a completely analogous manner if the additional manipulated variable SZ or the minimum manipulated variable SM can be specified. A possible embodiment relates to the way in which the additional setpoint value FBZ* is specified from the start time t5. In particular, the additional setpoint value FBZ* is preferably raised strictly monotonically from the start time t5 and with a finite gradient from 0 to a maximum value FBZ0*. The period during which this increase takes place can be in the range of several 100 ms. The lifting should be completed before the actual tapping time t2. Appropriate grading techniques are well known to those skilled in the art. A further possible embodiment relates to the manner in which the additional setpoint value FBZ* is lowered after the actual tapping time t2. In particular, the additional setpoint value FBZ* is reduced from its maximum value FBZ0* to 0, preferably in a strictly monotonous manner and with a finite increase. The period of time during which this reduction takes place can in particular also be in the range of several 100 ms. Corresponding procedures for gradual lowering are well known to those skilled in the art. However, the value 0 must be reached by the stabilization time t3 at the latest. A further possible configuration relates to the definition of the starting point in time t5. In particular, an expected piercing time t7 can be determined as part of a path tracking of the rolled stock head 9 (the implementation of a path tracking is generally known to those skilled in the art). Accordingly, it is easily possible to determine the starting point in time t5 in such a way that it is a predetermined early period of time T1 before the expected tapping point in time t7. The actual tapping time t2 can be before or after the expected tapping time t7. However, the time deviation is at most as large as a pre- knew error tolerance δt. The actual tapping time t2 is therefore in the interval [t7-δt;t7+δt]. The predetermined early period of time T1 can in particular be dimensioned in such a way that the additional setpoint value FBZ* has already definitely reached its maximum value FBZ0* at the actual tapping time t2. This configuration makes it possible, in particular, to ensure that the actual value FB of the bending force is already adjusted as far as possible to the sum of the basic setpoint value FBB* and the additional setpoint value FBZ*. Alternatively, the predetermined early period of time t1 can also be dimensioned in such a way that the additional setpoint value FBZ* has definitely not yet reached its maximum value FBZ0* at the actual piercing time t2. This refinement makes it possible in particular to ensure that the resulting manipulated variable SR has a positive value at the actual tapping time t2. The predetermined period of time T1 is typically in the range between 0.5 s and 2.0 s, in particular between 0.8 s and 1.5 s, for example around 1.0 s The early time span T1 can also be combined with a special way of determining the additional setpoint value FBZ* (or its maximum value FBZ0*). In particular, the early period of time T1 can be determined in such a way that at the actual piercing time t2 “the bending force FB has already been adjusted as far as possible to the sum of the basic setpoint value FBB* and the additional setpoint value FBZ*”. At the same time, the additional target value FBZ* (or its maximum value FBZ0*) can be determined in such a way that the actual value FB of the bending force cannot even reach the sum of the basic target value FBB* and the additional target value FBZ* (for this reason the above formulation placed in quotation marks). The result of this procedure is that the resulting manipulated variable SR is forced to go to a positive value—often even to the maximum value MAX—and remains there because the actually desired result (FB=FBB*+FBZ*) cannot be achieved. A further possible embodiment relates to the specification of the end time t6, subject to the condition that the end time t6 is not after the stabilization time t3. Because, as already mentioned, the actual tapping time t2 can be recorded without further ado or can be determined on the basis of recorded measured variables. Accordingly, it is possible without any problems to determine the end time t6 in such a way that it is a predetermined late time span T2 after the actual tapping time t2. The predetermined late period of time T2 is preferably dimensioned in such a way that the additional setpoint value FBZ* maintains its maximum value FBZ0* up to a point in time whose distance from the actual piercing point in time t2 has a predetermined value. In particular, this value can be in the range between 0.1 s and 1.0 s. For example, it can be between 0.2 s and 0.6 s. A value between 0.3 s and 0.4 s is particularly preferred. After this latter point in time, the additional setpoint value FBZ* is lowered—possibly abruptly, preferably gradually—from its maximum value FBZ0* to 0. Reaching the value 0 corresponds to this End time t6. Since the time period during which the additional setpoint value FBZ* is lowered is also known, the end time t6 can be determined without further ado based on the actual tapping time t2. Alternatively, it is possible to determine the predetermined late period of time T2 starting from the expected tapping time t7. In this case, the determinations are not based on the actual tapping time t2, but based on the expected tapping time t7. A further possible embodiment relates to the manner in which the additional setpoint value FBZ* (or its maximum value FBZ0*) is determined—for example by the control device 13 . In particular, properties of the rolling stock 8 can be utilized. The properties are a on the other hand, actual values or expected values of the rolling stock 8, which the rolling stock 8 has or presumably has before rolling in the roll stand 1. Examples of such variables are the width, the thickness, the temperature and the chemical composition and possibly also the pretreatment of the rolling stock 8. The properties are, on the other hand, target variables which the rolling stock 8 has after rolling in the roll stand should have 1. Examples of such variables are the width and the thickness of the rolling stock 8. Furthermore, mechanical properties of the roll stand 1 are known, for example the modulus of elasticity of the stand, the diameter of the work rolls 2, the diameter of the back-up rolls 3 and others. Finally, for example by the control device 13, expected values for operating parameters of the roll stand 1 for the rolling of the rolling stock 8 are determined as part of the pass schedule calculation, in particular an expected value FE for the rolling force F. The additional setpoint value FBZ* or whose maximum value FBZ0* is determined as a function of the properties of the rolling stock 8 and/or the expected value FE of the rolling force F. If necessary, the mechanical properties of the roll stand 1 can also be taken into account. The specific determination can be made using a formula or a table, for example. The formula or the table can be stored in the control device 13, for example. A further possible embodiment also relates to the way in which the additional setpoint value FBZ* or its maximum value FBZ0* is determined. In particular, the additional setpoint value FBZ* can be determined in such a way that the resulting manipulated variable SR assumes its maximum possible value immediately before the actual tapping time t2. This determination of the additional setpoint value FBZ* means that the hydraulic valves 15, 16 are fully open at the actual tapping time t2 and the entire working pressure pP of the hydraulic system (including accumulators) thereby stabilizes the tapping. This procedure can be used in particular in a heavy plate mill and in the front roll stands of a multi-stand finishing train (for a metal strip). In principle, however, this procedure can also be used for the rear roll stands of a multi-stand finishing train. A final possible embodiment also relates to the way in which the additional setpoint value FBZ* or its maximum value FBZ0* is determined. In particular, the additional target value FBZ* can be determined in such a way that a drop in the actual value FB of the bending force at the actual piercing time t2, which would occur without the additional target value FBZ* being supplied to the bending controller 14, is compensated for by at least 50%. In many cases it will be sufficient if the hydraulic valves 15, 16 are not fully open but only slightly open. For such cases in particular, configurations are useful in which the minimum manipulated variable SM is specified and the minimum manipulated variable SM has a relatively low value, for example a value between 8% and 20% of the maximum possible modulation of the hydraulic valves 15, 16. However, a specification of a larger minimum manipulated variable SM for other cases should not be ruled out. The present invention has many advantages. In particular, the tapping is significantly stabilized. Furthermore, there is a reduction in the period of time that elapses from the piercing time t2 until the actual value FB of the bending force again reaches the basic setpoint value FBB*. Finally, the threading process and the rolling process are stabilized as such. Although the invention has been illustrated and described in more detail by the preferred exemplary embodiment, the invention is not restricted by the disclosed examples and other variants can be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention . LIST OF REFERENCE NUMERALS 1 roll stand 2 work rolls 3 back-up rolls 4, 6 bearing journals 5 work roll chocks 7 back-up roll chocks 8 rolling stock 9 rolling stock head 10 bending system 11, 12 hydraulic cylinder units 13 control device 14 bending controller 15, 16 hydraulic valves 17 determination unit 18 rolling stock foot 19 selection element F rolling force FE expected value FB* resulting setpoint FBB* base setpoint FBZ* additional setpoint FBZ0* maximum value FB actual value of the bending force MAX maximum possible value pP, pT working pressures SB basic manipulated variable SM minimum manipulated variable SR resulting manipulated variable SZ additional manipulated variable t1 to t7 times T1, T2 time periods x transport direction δFB* correction value δt error tolerance

Claims

Claims 1. Operating method for a rolling stand (1) for rolling a flat rolling stock (8) made of metal, which has a rolling stock head (9), - wherein the rolling stand (1) has at least work rolls (2) and back-up rolls (3), - wherein the work rolls (2) are mounted in work roll chocks (5) and a bending system (10) which presses the work roll chocks (5) apart acts on the work roll chocks (5), - the rolling stock head (9) forming the roll stand (1) into one - actual piercing time (t2) is reached, - a basic target value (FBB*) being supplied to a bending controller (14) and the bending controller (14) determining a resulting target value (FB*) taking into account the basic target value (FBB*), - wherein an actual value (FB) of the bending force is also supplied to the bending controller (14), - the bending controller (14) using the resulting setpoint value (FB*) and the actual value (FB) to generate a basic manipulated variable (SB) for the bending system (10) determined so that when driving u ng of the bending system (10) with the basic manipulated variable (SB), the actual value (FB) is brought as close as possible to the resulting target value (FB*), - with the bending controller (14) measuring the resulting target value (FB*) from a stabilization point in time (t3 ), which is after the tapping time (t2), also taking into account an actual rolling force (F) occurring during the rolling of the flat rolling stock (8), characterized in that during a tapping period that is before the actual tapping time ( t2) begins and ends after the actual piercing time (t2), - the bending controller (14) is supplied with an additional setpoint (FBZ*) in addition to the basic setpoint (FBB*), so that the bending controller (14) receives the resulting the setpoint (FB*) taking into account not only the basic sis target value (FBB*), but also the additional target value (FBZ*) and thus the actual value (FB) of the bending force is greater than the basic target value (FBB*) immediately before the actual piercing time (t2), and/or - by switching an additional manipulated variable (SZ) on the basic manipulated variable (SB), a resulting manipulated variable (SR) is determined, which is fed to the bending system (10) and the bending system (10) is thereby controlled in such a way that the resulting manipulated variable (SR) is greater than the basic manipulated variable (SB) and/or - the basic manipulated variable (SB) and a minimum manipulated variable (SM) are fed to a selection element (19) and the selection element (19) supplies the bending system (10) with the maximum of the basic manipulated variable (SB) and minimum manipulated variable (SM).

2. Operating method according to Claim 1, d a d u r c h g e n n z e i c h n e t that the additional setpoint (FBZ*) and/or the additional manipulated variable (SZ) and/or the minimum manipulated variable (SM) from the start (t5) of the tapping period with a finite increase in a strictly monotonous manner from 0 to a maximum value (FBZ0*) are raised and/or are lowered from their maximum value (FBZ0*) to zero at the end (t6) of the tapping period with a finite increase in a strictly monotonous manner.

3. Operating method according to Claim 1 or 2, d a d u r c h g e n n z e i c h n e t that an expected tapping time (t7) is determined by means of path tracking for the rolling stock (8) and that the start (t5) of the tapping period is a predetermined early period of time (T1) before expected tapping time (t7).

4. Operating method according to claim 1, 2 or 3, characterized in that the end (t6) of the tapping period is a predetermined late period of time (T2) after the actual tapping time (t2).

5. Operating method according to one of the above claims, d a d u r c h g e n n z e i c h n e t that a maximum value of the additional setpoint (FBZ*) and/or the additional manipulated variable (SZ) and/or the minimum manipulated variable (SM) before the rolling stock (8) is rolled in the roll stand (1) in Depending on properties of the rolling stock (8) and/or depending on an expected rolling force (FE).

6. Operating method according to one of the above claims, since the maximum value of the additional setpoint (FBZ*) and/or the additional manipulated variable (SZ) and/or the minimum manipulated variable (SM) is determined in such a way that the resulting manipulated variable (SR) is at the actual tapping time (t2) assumes its maximum possible value (MAX).

7. Operating method according to one of the above claims, d a d a d u r c h g e n n z e i c h n e t that the additional setpoint (FBZ*) and/or the additional manipulated variable (SZ) and/or the minimum manipulated variable (SM) are determined in such a way that the actual value (FB) of the bending force collapses at the actual piercing time (t2), which would occur without the additional setpoint (FBZ*) and/or the additional manipulated variable (SZ) and/or the minimum manipulated variable (SM), is compensated by at least 50%.

8. Rolling unit for rolling a flat rolling stock (8) made of metal, which has a rolling stock head (9), - the rolling unit having a roll stand (1) and a bending controller (14), - the roll stand (1) has work rolls (2) and backup rolls (3) mounted at least in work roll chocks (5), - the roll stand (1) having a bending system (10) that presses the work roll chocks (5) apart, - the bending controller (14) controlling the bending system (10) drives, characterized in that the roll stand (1) and the bending controller (14) interact with one another during operation of the rolling unit in such a way that they carry out an operating method according to one of the above claims.