Method for operating a roll stand for stepped rolling

Abstract

A method for operating a roll stand for the stepped rolling of metal strip is described. The metal strip is initially fed into a roll stand on the infeed side, where it is step-rolled. This creates a leading and a trailing section of the metal strip, each with different thicknesses. After the roll stand, the step-rolled metal strip initially passes through an outfeed-side strip accumulator before it is wound into a coil by an upcoiler. The outfeed-side reel is controlled to a constant strip tension. To keep the outfeed-side strip tension sufficiently constant even in the case of rapid changes in the size of the roll gap and the resulting rapid changes in thickness in the rolled metal strip the control of the strip tension with the aid of the upcoiler is supplemented by a position control for a roller unit in the outfeed-side strip accumulator.

Claims

1. A method for operating a roll stand (100) during stepped rolling of a metal strip (200), wherein an outfeed-side strip accumulator (110) with at least one roller unit (114) and an upcoiler (120) are arranged downstream of the roll stand, the method comprising: feeding the metal strip (200) into the roll stand (100) on an infeed side; stepped rolling of the metal strip in the roll stand (100) such that a leading and a trailing section (220, 210) of the metal strip have different thicknesses with a difference in thickness (Ahi); passing the metal strip (200) through the outfeed-side strip accumulator (110); and coiling of the metal strip having the different thicknesses downstream of the strip accumulator (110) into a coil by the upcoiler (120); controlling a position of the roller unit (114) of the outfeed-side strip accumulator (110) to individually predetermined target positions (Psoll v, Psoll n) respectively for the leading and the trailing section of the metal strip (200); and controlling an outfeed-side strip tension of the metal strip (200) with the upcoiler (120) as an actuator to a predetermined outfeed-side target strip tension.

2. The method according to claim 1, wherein the target position (Psoll n with n=i+1) predetermined for the trailing section (210) of the metal strip (200) is calculated from the target position (Psoll v with v=i) predetermined for the leading section (220) of the metal strip as
Psoll n=Psoll v+Δp, wherein a change in target position (Δp) is determined by calculating a change in the outfeed speed (ΔVexi) of the metal strip from the roll stand in accordance with the difference in thickness (Ahi); and integrating the difference in speed (Vex i−Vhi) of the metal strip over time into a resulting change in length (Δsi) of the metal strip (200) on the outfeed side of the roll stand (100); and calculating the change in the target position (Δp) of the roller unit (114) in the strip accumulator to compensate for the change in length (Δsi) of the metal strip during a transition from the leading to the trailing section (220, 210).

3. The method according to claim 2, wherein the necessary change in the target position (Δp) of the roller unit (114) resulting from the difference in thickness (Δhi) is calculated in advance in terms of time in a process model for the stepped rolling, before or while the difference in thickness (Δhi) is realized by a change in a roll gap between work rolls (105) of the roll stand, and wherein the roller unit (114) is adjusted to its new changed target position (Psoll n with n=i+1) and in this respect is pre-controlled before a size of the roll gap is changed by the difference in thickness (Δhi).

4. The method according to claim 1, wherein a drag reel (80) for providing the metal strip is arranged on the infeed side of the roll stand (100), and wherein at least one infeed-side strip accumulator (90) for the metal strip is arranged between the drag reel and the roll stand, wherein the method further comprises the following steps: controlling a rotational speed of the drag reel (80) to a predetermined winding target speed; and controlling an infeed-side strip tension of the metal strip (200) to a predetermined infeed-side target strip tension with the infeed-side strip accumulator (90) as an actuator.

5. The method according to claim 1, wherein the metal strip (200) fed into the roll stand (100) on the infeed side has a constant initial thickness (h0).

6. The method according to claim 1, wherein the metal strip is cold-rolled during the stepped rolling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a rolling mill with a roll stand for the stepped rolling of metal strip;

(2) FIG. 2 shows a detailed view of FIG. 1 to explain the method for operating a roll stand; and

(3) FIG. 3 shows an illustration of the changed target position of the roller unit in the outfeed-side strip accumulator.

DETAILED DESCRIPTION

(4) The invention is described in detail below with reference to the specified figures in the form of exemplary embodiments. In all figures, the same technical elements are designated with the same reference signs.

(5) FIG. 1 shows a rolling mill for the stepped rolling of metal strip 200. This mill includes a roll stand 100 with work rolls 105, which span a roll gap. The size of the roll gap is variable in stepped rolling for introducing differences in thickness Δh into the metal strip. As a consequence, a leading section 220 of the metal strip then has a different thickness than a trailing section 210 of the metal strip 200. The difference in thickness is Δhi; see FIG. 2. Thereby, the running parameter i with i=1 . . . I denotes a respective section of the metal strip.

(6) In accordance with FIG. 1, the metal strip with a preferably constant initial thickness h.sub.0 is provided by a drag reel 80 on the infeed side of the roll stand 100. It then optionally passes through an infeed-side strip accumulator 90 before it enters the roll gap between the work rolls 105 of the roll stand and is rolled there. On the outfeed side, the metal strip 200 that is then rolled initially passes through an outfeed-side strip accumulator 110 before being wound into a coil by means of an outfeed-side upcoiler.

(7) Due to changes in the size of the roll gap during rolling, the metal strip wound into the coil has different thicknesses in different sections.

(8) The method for operating the roll stand 100 is described in detail below with reference to the figures. The method applies in particular to the cold rolling of metal strip; however, application of the method to hot strip is not excluded.

(9) The method is aimed in particular at keeping the strip tension on the outfeed side of the roll stand 100 constant even in the case of rapid changes in the thickness of the metal strip, such as those that occur during stepped rolling. For this purpose, the upcoiler 120 is used as an actuator for a strip tension control system, with which the outfeed-side strip tension is controlled to a predetermined outfeed-side target strip tension.

(10) At a constant cross-section of the metal strip, the strip tension is proportional to a tensile force acting on the metal strip in the longitudinal direction of the metal strip. The control to the constant target strip tension can be realized, for example, by changing the target torque of the upcoiler in a manner dependent on time according to the continuously increasing radius of the coil during winding. In this respect, the claimed control of a constant target strip tension on the outfeed side can be realized by a suitable control of the torque of the upcoiler, wherein the target torque to be provided must then be changed as a function of the coil radius, which changes over time. The specified control to the constant strip tension corresponds to a control to an average value of the speed of the metal strip at the exit of the roll stand.

(11) In order to also be able to control rapid changes in the roll gap and resulting rapid changes in the thickness of the metal strip with regard to a desired constant strip tension, as described above, a rapid position control for the roller unit 114 of the outfeed-side strip accumulator 110 is superimposed on the specified control of the strip tension by means of the upcoiler. Such position control provides that, for each section i of the metal strip with a different or individual thickness, as the case may be, the roller unit in the outfeed-side strip accumulator is controlled to an individually predetermined target position. The necessity for setting such individual target positions for the sections of the metal strip of different thicknesses is explained in more detail below, in particular with reference to FIG. 2.

(12) It is assumed that the metal strip 200 enters the roll gap between the work rolls 105 of the roll stand 100 with an initial thickness h.sub.0. In the roll gap, the metal strip 200 initially undergoes a thickness reduction by a difference in thickness Δhi, as shown in FIG. 2. This thickness reduction Ahi of the metal strip on the outfeed side of the roll stand 100 leads to a change in the outfeed speed Vex i of the metal strip from the roll stand by an amount ΔVex i in the leading section 220 of the metal strip, in particular compared to the circumferential speed VU of the work rolls 105 of the roll stand 100, which is simplistically assumed to be constant.

(13) The change in the output speed ΔVex i depends on a large number of parameters, as they are not exhaustively listed in the following physical function.

(14) ΔVi=f (ho, Δhi, η: coefficient of friction in the roll gap, kf: flow curve of the material to be rolled, roughness, diameter and circumferential speed VU of the work rolls, degree of lubrication in the roll gap, temperature, roughness of the metal strip, tensile force FZug i on metal strip at the exit of the roll stand before the outfeed-side strip accumulator, tensile force Fzug Hi on the metal strip at the exit of the outfeed-side strip accumulator, etc).

(15) The change in output speed ΔVex i is typically predicted by a process model based on initialized values for the individual parameters. The initialized values are often only estimates or empirical values because the values of many of the parameters are not exactly known or reproducible. During the execution of the rolling process, it is therefore recommended to observe the individual parameters with the aid of an observer, that is, to record them metrologically and to compare their observed or measured values with the values currently used in the process model. In the event of a detected deviation of individual parameter values, it is then recommended to adjust the process model in order to continuously improve it.

(16) The total speed Vex i of the metal strip at the outfeed of the roll stand in the transport direction of the metal strip in front of the outfeed-side strip accumulator 110 is calculated as follows:
Vex i=VU+ΔVex i

(17) To be distinguished from this is the speed of the metal strip VHi at the outfeed of the outfeed-side strip accumulator 110 or the infeed of the upcoiler 120.

(18) By integrating the difference in speed Vex i-VHi over time, the change in length Δs i of the metal strip on the outfeed side of the roll stand resulting from the original change in thickness Δhi can be calculated in accordance with the following formula:
Δs i=∫(Vex i−VHi)dt

(19) Such change in length Δs i must be compensated for in the outfeed-side strip accumulator 110 so that the change in length has no negative influence on the constancy of the strip tension on the outfeed side of the roll stand. For this purpose, the present disclosure provides for calculating from this calculated change in length Δsi a required change in the target position Δpi of the roller unit in the outfeed-side strip accumulator 110 in such a manner that the specified change in length Δs in the strip accumulator is preferably fully compensated for. FIG. 3 shows an example of the change in the target position Δpi of the roller unit. The specified change in the target position Δpi corresponds, as an example, in rough approximation to half of the change in length Δsi of the metal strip, wherein, however, angular relationships and the length sections of the metal strip guided in the outfeed-side strip accumulator must also be taken into account. The change in the target position Δp i of the roller unit is carried out within the scope of a position control or position regulation, which is superimposed on the traditional strip tension control of the metal strip on the outfeed side of the roll stand.

(20) The roller unit only has a much lower mass and inertia compared to the upcoiler, in particular if the latter carries the metal strip wound into a coil. Therefore, the roller unit or strip accumulator, as the case may be, can advantageously react much faster to rapid changes in the length of the metal strip, such as those that occur in particular during stepped rolling, than the much more sluggish upcoiler. Changes in length are proportional to changes in strip tension in accordance with the following proportionality relationship:

(21) $\Delta F_{\mathrm{Zug}}i=\Delta \mathrm{si}.Math.\left(\frac{E.Math.A}{L}\right)$
with ΔF.sub.Zug Change in the strip tension Δs Change in length of the metal strip due to a change in thickness of the metal strip E E-modulus of the metal strip A Cross-sectional area of the metal strip L Stretched length of the metal strip.

(22) While the outfeed-side upcoiler 120, due to its inertia, winds the metal strip 200 essentially at a constant circumferential speed VHi, which corresponds to an average outfeed speed of the metal strip from the roll stand 100 even with changed thicknesses, the position-controlled strip accumulator compensates for the fluctuations of the outfeed-side strip speed Vex i around the specified average value VHi shown in FIG. 2. In this manner, the object to keep the outfeed-side strip tension constant even in the case of highly dynamic changes in the roll gap and thus in the thickness of the outfeed-side strip, is ensured.

LIST OF REFERENCE SIGNS

(23) 80 Drag reel on the infeed side 90 Infeed-side strip accumulator 100 Rolling mill 105 Work rolls of the roll stand 110 Outfeed-side strip accumulator 114 Roller unit 120 Upcoiler 200 Metal strip 210 Trailing metal strip section 220 Leading metal strip section Δhi Difference in thickness between the leading and trailing sections of the metal strip Δsi Change in the target position of the roller unit of the strip accumulator Δvi Change in the outfeed speed of the metal strip from the roll stand due to the difference in thickness Δh P.sub.Soll v Target position of the leading section of the metal strip P.sub.Soll n Target position of the trailing section of the metal strip I or v (Leading) section of the metal strip i+1 or n (Lagging) section of the metal strip Vo Speed of the metal strip at the entrance of the roll stand Vex i Speed of the metal strip at the outfeed of the roll stand in front of the strip accumulator on the outfeed side VHi Speed of the metal strip behind the outfeed-side strip accumulator at the infeed of the upcoiler. VU Circumferential speed of the work rolls