Wall thickness monitoring while stretch-reducing tubes

Abstract

A controller (2) and method for controlling a stretch-reducing mill (1) for rolling tubes are presented. The stretch-reducing mill (1) has several roll stands (10) arranged behind one another in a conveying direction (F) of the tubes (R) and at least one outlet-side wall thickness measuring device (20). The controller (2) is set up to receive measurement data from the wall thickness measuring device (20) which identifies one or more outlet-side wall thicknesses (s.sub.r) of a tube (R) exiting from the last roll stand (10) and one or more of the received measurement data wall thickness on the inlet-side (s.sub.l_t), preferably to determine an inlet-side wall thickness profile of the tube (R) before entering the first roll stand (10), and preferably to calculate and control one or more of the roll stands (10), taking into account the determined inlet-side wall thicknesses (s.sub.l_t).

Claims

1. A controller (2) for controlling a stretch-reducing mill (1) for the rolling of tubes (R), the stretch-reducing mill (1) having several roll stands (10) arranged behind one another in a conveying direction (F) of the tubes (R), and at least one outlet-side wall thickness measuring device (20) for measuring outlet-side wall thicknesses (s.sub.r) of tubes (R) exiting from a last of the several roll stands (10), wherein the controller (2) is configured to receive measurement data relating to one or more outlet-side wall thicknesses (s.sub.r) of a tube (R) exiting from the last of the several roll stands (10) from the wall thickness measuring device (20), to determine one or more inlet-side wall thicknesses (s.sub.l_t) of the tube (R) before entering a first of the several roll stands (10) from the received measurement data, and to control one or more of the several roll stands (10) taking into account the determined inlet-side wall thicknesses (s.sub.l_t).

2. The controller (2) according to claim 1, wherein the controller (2) is further configured to determine a deviation from one or more setpoint values from the determined inlet-side wall thicknesses (s.sub.l_t).

3. The controller (2) according to claim 1, wherein the control of the one or more of the several roll stands (10) comprises at least a speed regulation of work rolls thereof.

4. The controller (2) according to claim 1, wherein the controller (2) is further configured to use the determined inlet-side wall thicknesses (s.sub.l_t) for a wall thickness control of one or more subsequent tubes (R).

5. The controller (2) according to claim 1, wherein the controller (2) is further configured to create a forecast of the wall thicknesses of subsequent tubes (R) from the determined inlet-side wall thicknesses (s.sub.l_t), and wherein the creation of the forecast includes an analysis of cyclically recurring and/or statistical regularities.

6. The controller (2) according to claim 1, wherein the controller (2) is further configured to compare the determined inlet-side wall thicknesses (s.sub.l_t) with the measured outlet-side wall thicknesses (s.sub.r).

7. The controller (2) according to claim 1, wherein the controller (2) is further configured to correct the determined wall thicknesses on the inlet-side (s.sub.l_t) by considering a measured inlet-side wall thickness mean value, and wherein the measured inlet-side wall thickness mean value is determined by measuring a weight and/or a length and/or a diameter of one or more incoming tubes (R).

8. The controller (2) according to claim 1, wherein the stretch-reducing mill (1) furthermore has an inlet-side wall thickness measuring device which is set up to measure an inlet-side wall thicknesses (d1) of tubes (R) entering the first roll stand (10), and wherein the controller (2) is further configured to compare one or more measured inlet-side wall thicknesses (s.sub.l) with the determined inlet-side wall thicknesses (s.sub.l_t), and to control one or more of the several roll stands (10) taking into account the comparison result to control and/or to detect a malfunction of the stretch-reducing mill (1).

9. A stretch-reducing mill (1) for the rolling of tubes (R), comprising: several roll stands (10) arranged behind one another in a conveying direction (F) of the tubes (R); at least one outlet-side wall thickness measuring device (20) for measuring outlet-side wall thicknesses (s.sub.r) of tubes (R) exiting from a last of the several roll stands (10); and the controller (2) according to claim 1.

10. A method for controlling a stretch-reducing mill (1) for the rolling of tubes (R), the stretch-reducing mill (1) having several roll stands (10) arranged behind one another in a conveying direction (F) of the tubes (R), and at least one outlet-side wall thickness measuring device (20) for measuring outlet-side wall thicknesses (s.sub.r) of tubes (R) exiting from a last of the several roll stands (10), wherein the method comprises: measuring one or more outlet-side wall thicknesses (s.sub.r) of a tube (R) by the wall thickness measuring device (20); determining, by calculating, one or more inlet-side wall thicknesses (s.sub.l_t), of the tube (R) before entering the first roll stand (10) from the measured outlet-side wall thicknesses (sr); and controlling one or more of the several roll stands (10) taking into account the determined inlet-side wall thicknesses (s.sub.l_t).

11. The method according to claim 10, wherein a deviation from one or more setpoint values is determined from the determined inlet-side wall thicknesses (s.sub.l_t).

12. The method according to claim 10, wherein the determined inlet-side wall thicknesses (s.sub.l_t) are used for a wall thickness control of one or more subsequent tubes (R).

13. The method according to claim 10, wherein a forecast of the wall thicknesses of subsequent tubes (R) is created from the determined inlet-side wall thicknesses (s.sub.l_t), which includes an analysis of cyclically recurring and/or statistical regularities.

14. The method according to claim 10, wherein the determined inlet-side wall thicknesses (s.sub.l_t) are compared with the measured outlet-side wall thicknesses (s.sub.r).

15. The method according to claim 10, wherein the determined inlet-side wall thicknesses (s.sub.l_t) are corrected by a measured inlet-side wall thickness mean value, and wherein the measured inlet-side wall thickness mean value is determined by measuring a weight and/or a length and/or a diameter of one or more incoming tubes (R).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic view of a stretch-reducing mill with a wall thickness measuring device on the outlet-side.

(2) FIG. 2 is a diagram showing an exemplary tube wall thickness as a function of the position in the axial direction of the tube, for explaining an iterative method for calculating an inlet-side tube wall profile.

DETAILED DESCRIPTION

(3) Preferred exemplary embodiments are described below with reference to FIG. 1.

(4) FIG. 1 shows a schematic view of a stretch-reducing mill 1. The stretch-reducing mill 1 has several roll stands 10, here ten as an example. The roll stands 10 are preferably individually controllable. In particular, the speeds of the work rolls (not shown in FIG. 1) of the roll stands 10 can be set individually.

(5) The roll stands 10 are controlled via a controller 2, preferably via a computer. If necessary, the controller 2 takes over the control of further components of the stretch-reducing mill 1. It should be pointed out that the term “controller” includes both centralized and decentralized structures for controlling the stretch-reducing mill 1. The controller 2 therefore does not have to be at the “location” of the stretch-reducing mill 1 or part of it. In addition, control tasks, data processing steps, etc. can be distributed to different computing devices, which then fall under the term “controller” as a whole. Furthermore, the communication of the controller 2 with the components to be controlled can take place both physically via cable and wirelessly.

(6) To roll a tube R, it passes in a conveying direction F through the stretch-reducing mill 1. Before entering the stretch-reducing mill 1, the tube R has an inlet-side wall thickness Si, which can fluctuate along the longitudinal extension of the tube R, i.e., locally. When it exits from the last roll stand 10, the tube R has a changed wall thickness s.sub.r.

(7) The outlet-side wall thickness s.sub.r is measured by means of a wall thickness measuring device 20, which can be, for example, a radiometric measuring device. The measurement is preferably carried out without contact, such as by measuring the attenuation of the radiation emitted by a radiator, for example a cesium radiator. However, the outlet-side wall thickness s.sub.r of the tube R can also be measured in other ways. This also includes in particular indirect measurement methods in which the wall thickness is not measured directly, but one or more other physical parameters are measured from which the wall thickness can be calculated.

(8) In addition to the wall thickness measurement on the outlet-side, other parameters can optionally be measured, such as the inlet-side and/or outlet-side speed of the tube R, which is measured by means of speed measuring devices 21 and 22, the inlet-side and/or outlet-side weight of the tube R, etc. Such technical means are omitted in FIG. 1 for the sake of clarity.

(9) The measured values from the wall thickness measuring device 20 and any further measuring stations are transmitted to the controller 2. The measurements can essentially be carried out continuously or discretely, i.e., in cycles.

(10) The measured outlet-side tube wall thickness s.sub.r (more generally: the tube wall thickness profile measured on the outlet-side) is used by the controller 2 in order to theoretically derive the inlet-side tube wall thicknesses s.sub.l_t (more generally: the tube wall thickness profile on the inlet-side). Such a back-calculation can take place, for example, by means of a material flow simulation, described in detail below.

(11) The calculated or simulated inlet-side tube wall thickness profile is now used by the controller 2 as an input variable for the local wall thickness control of subsequent tubes R or tube sections. In the simplest case, for example, the back-calculated tube wall thickness profile or a mean value from several calculated tube wall thickness profiles is assumed as an approximation for the actual tube wall thickness profile. With this theoretical profile as a basis, the dynamic speed correction of the roll stands 10 takes place.

(12) It has been found in practice that deviations in the tube wall thickness can have regular properties, in particular cyclic or periodic. For example, similar wall thickness deviations can occur at equivalent points in successive tubes R, or wall thickness profiles that are repeated after a certain number of tubes R. The first case occurs, for example, when there are problems with the heating of the starting material, which is reflected in the incoming wall thickness s.sub.1 of the tube R. The second case occurs, for example, when individual tools in the preliminary units, such as hole plugs or rolling bars, are worn out or are more worn out than others. Since such tools are often used in circulation in the preliminary units, worn tools and the wall thickness deviations caused by them appear with a certain periodicity, which corresponds to the number of tools used in circulation. For these reasons, the controller 2 is preferably set up to examine the inlet-side tube wall profiles calculated for such characteristics—for example characteristics which are common to all tubes R or which recur periodically—and to predict the wall thickness profile of subsequent tubes R therefrom. In this way, the accuracy of the calculated wall thickness profiles can be improved.

(13) Based on this preferred exemplary embodiment, it can be useful if the controller 2 only begins to apply correction speeds for the roll stands 10 when these regularities are recognized with a certain degree of certainty in the tube wall thickness profile and/or the number of tools in circulation in the preliminary units.

(14) A further improvement in the forecast quality can be achieved if the calculated inlet-side tube wall thickness profile is corrected by a measured mean value of the inlet-side wall thickness. Such a wall thickness mean value can be determined, for example, by measuring the weight, the length, and the diameter of the incoming tube R, which is technically comparatively simple and inexpensive.

(15) According to a further exemplary embodiment, the controller 2 can carry out a comparison between the calculated inlet-side tube wall thicknesses s.sub.l_t and the measured inlet-side tube wall thicknesses s.sub.l. Such a comparison can be used, on the one hand, to perform a plausibility check of the back-calculation or forecast and, on the other hand, to perform an adaptive correction of the control behavior of the controller 2 in order to further optimize the rolling result. In this way, for example, the usage periods and/or amounts of the changes in the rotational speed of the roll stands 10 made for a wall thickness deviation can be automatically adjusted.

(16) The proposed technical solution, which is based on a back-calculation of the inlet-side tube wall thicknesses s.sub.l_t from the outlet-side tube wall thicknesses s.sub.r, offers a compromise between investment and operating costs of the stretch-reducing mill and an improvement in the rolling accuracy, which is aimed for by taking into account the inlet-side wall thickness fluctuations. The theoretically determined inlet-side wall thicknesses s.sub.l_t do not have to exactly match the actual inlet-side wall thicknesses s.sub.l. It is sufficient if their application leads to an improvement in the rolling quality.

(17) The back-calculation described is also useful if technical means are available for measuring the tube wall thickness on the inlet-side. On the one hand, in this case, malfunction situations can be overcome in the event of failure of the inlet-side wall thickness measurement without having to interrupt or stop the operation of the system. On the other hand, an adaptation of the control model and/or an error detection can be carried out by comparing between the back-calculated tube wall thicknesses and those measured on the inlet-side.

(18) In the following, a method for calculating an inlet-side tube wall profile is presented with reference to FIG. 2. For this purpose, the controller is configured to assign an incoming tube wall thickness to the measured tube wall thickness on the outlet-side. The boundary conditions are the speed setting of the roll stands 10 of the stretch-reducing mill 1, since these determine the rolling tension and stretching. The tube R on the inlet-side is hereinafter referred to as a “shell” for differentiation.

(19) Calculation methods are known which can be used to calculate the speeds of a stretch-reducing mill in order to roll a desired tube wall thickness from a specified inlet-side shell wall thickness, see for example: H. Biller, “The reduction of tubes, theory and application” in “Production of tubes”, Düsseldorf 1975, pages 48-63.

(20) The calculation methods are reversible, i.e., with a given tube wall thickness and known speeds, the associated shell wall thickness can be calculated on the inlet-side. In simplified terms, an inverse function is carried out, which calculates the shell wall thickness on the inlet-side from the roll speeds and the tube wall thickness:
s.sub.l=F.sup.−1(n.sub.i,s.sub.r)  (1)

(21) Here, s.sub.l is the inlet-side wall thickness of the shell, n.sub.i is the roller speed of the i-th roll stand 10 and s.sub.r is the outlet-side wall thickness of the tube R.

(22) In the simplest case, the inverse function is solved by trial and error, i.e., by specifying a shell wall thickness on a trial basis. A series of speeds is calculated from this in order to obtain the tube wall thickness from this assumed shell wall thickness. If the determined speeds deviate from the target speeds, the shell wall thickness is changed until the desired speed range is reached or at least the deviations between the specified values and the calculated speed range are less than an error threshold.

(23) However, this relatively simple method is not readily suitable for calculating the local wall thickness profile. This is in particular due to the fact that the speeds of the rolls in the stretch-reducing mill 1 are generally not constant over time, but are subject to variations over time in order to obtain a targeted influence on the stretching of the stretch-reducing mill 1, for example to control the thickness of the end of the tube or the local control of the wall thickness. Therefore, the above-mentioned inversion of the speed calculation cannot necessarily be carried out immediately without possibly receiving large errors in the calculated shell wall thickness.

(24) The calculation of a shell wall thickness profile from the measured wall thickness profile of a tube R can, however, be improved by an iterative procedure and simple metrological measures. According to one embodiment, in addition to the outlet-side wall thickness measuring device, two speed measuring devices 21, 22—preferably operating without contact—are set up on the inlet-side of the stretch-reducing mill 1 and on the outlet-side immediately in front of and behind the stretch-reducing mill 1, see FIG. 1. Such measuring devices are comparatively inexpensive and easy to maintain and service. The measured values of both speed measuring devices 21, 22 are fed to a measured data memory of the controller 2 in a synchronized manner. The purpose of the speed measuring devices 21, 22 is to accurately record the material flow through the stretch-reducing mill 1.

(25) The outlet-side wall thickness measuring device 20 in turn provides the controller 2 with a sectional wall thickness profile of the tube R, in which a mean wall thickness s.sub.j, which is assumed to be constant in the section j, is specified for small sections j of length l.sub.j see FIG. 2.

(26) According to the proposed method, the constancy of the roll speeds over time is now examined first. For each roll stand 10, the mean roll speed during the rolling of a tube R is determined:

(27) $\begin{array}{cc}{\stackrel{\_}{n}}_i^{\mathrm{ges}}=\frac{1}{t_{\mathrm{ges}}}{\int }_0^{t_{\mathrm{ges}}}\left(n_i\left(t\right)\right)\mathrm{dt}& \left(2\right)\end{array}$

(28) Here, n.sub.i.sup.ges denotes the mean roll speed of the i-th roll stand 10 during rolling, n.sub.i(t) denotes the roll speed of the i-th roll stand 10 at time t, and t.sub.ges denotes the total rolling time for the tube R.

(29) The current roll speeds needed to form the mean value are taken by the controller 2, for example from the data memory of the drive motor control.

(30) As the next step, the system examines the greatest deviations of the roll speeds of a roll stand 10 from the respective mean value:

(31) $\begin{array}{cc}{\Delta }_i=\left[\frac{\max (n_i\left(t\right){|}_0^{t_{\mathrm{ges}}}-\min \left(n_i\left(t\right)\right){|}_0^{t_{\mathrm{ges}}}}{\stackrel{\_}{n_i^{\mathrm{ges}}}}\right]& \left(3\right)\end{array}$

(32) If this value is less than a lower tolerance limit, for example 1%, it is assumed that the roll speeds were sufficiently constant during rolling, and the back-calculation of the shell wall thickness s.sub.lj can now be started for each tube section length l.sub.j and wall thickness s.sub.rj:
s.sub.lj=F.sup.−1(n.sub.i.sup.ges,s.sub.rj)  (4)

(33) If the roll speeds had an invalid range of changes, the speed curve is gradually divided into time segments until the current speeds deviate only slightly from the mean speed in the sub-ranges, i.e., the criterion suggested above is satisfied, if possible, according to equation (3).

(34) To do that, the procedure goes as follows: A mean tube wall thickness for the entire tube R is formed from the tube wall thickness profile on the outlet-side, e.g., according to the following approach:

(35) $\begin{array}{cc}\stackrel{\_}{s_r^{\mathrm{ges}}}=\frac{\underset{j}{.Math.}\left(s_{\mathrm{rj}}{l}_j\right)}{l_r^{\mathrm{ges}}}& \left(5\right)\end{array}$

(36) Here, l.sub.j denotes the length of the measured tube section j with the wall thickness s.sub.rj and l.sub.r.sup.ges denotes the total length of the tube.

(37) With this mean tube wall thickness, a mean shell wall thickness is now formed in the first approach with the help of the above-mentioned inverse function and the mean roll speeds:
s.sub.l.sup.ges=F.sup.−1(n.sub.i.sup.ges,s.sub.r.sup.ges)  (6)

(38) This mean shell wall thickness may be erroneous but is used as a starting value for further refinement of the solution. Tube R and shell are each subdivided into the same number of equivalent partial volumes. The measured values of the wall thickness measurement on the outlet-side can be used directly for tube R:

(39) $\begin{array}{cc}{V}_k=\underset{j=m}{\overset{n}{.Math.}}\left(\pi \left(D_r-s_{\mathrm{rj}}\right)s_{\mathrm{rj}}{l}_j\right)& \left(7\right)\end{array}$

(40) Here, V.sub.k denotes the k-th partial volume between the measured value sections m to n and D.sub.r denotes the tube diameter.

(41) For the shell, the associated partial volume is formed with the help of the shell wall thickness which is determined approximately in the first step:

(42) $\begin{array}{cc}{V}_k=\underset{j=m}{\overset{n}{.Math.}}\left(\pi \left(D_r-s_{\mathrm{rj}}\right)s_{\mathrm{rj}}{l}_j\right)& \left(8\right)\end{array}$

(43) Here, x.sub.n, x.sub.m denote the length coordinates on the shell and D.sub.l denotes the shell diameter.

(44) By the speed measuring device 21 on the inlet-side one can now determine the point in time t.sub.m at which the hollow section or the partial volume V.sub.k between the longitudinal coordinates x.sub.n-x.sub.m has entered the stretch-reducing mill 1. In addition, with the aid of the measured values of the outlet-side wall thickness measuring device 20 and the outlet-side speed measuring device 22, it is possible to determine the point in time t.sub.n at which the partial volume V.sub.k has left the stretch-reducing mill. Since the law of constant volume applies to the forming of metallic materials, a tube section on the outlet-side can be clearly assigned to an inlet-side shell section by considering the volumes. In the next step, the mean wall thickness of the tube R in the partial volume is determined:

(45) $\begin{array}{cc}\stackrel{\_}{d_r^k}=\frac{\underset{j=m}{\overset{n}{.Math.}}\left(d_{\mathrm{rj}}{l}_j\right)}{\underset{j=m}{\overset{n}{.Math.}}{l}_j}& \left(9\right)\end{array}$

(46) Then, for each roll stand 10, its mean speed is determined during the passage of the partial volume V.sub.k:

(47) $\begin{array}{cc}{\stackrel{\_}{n}}_i^k=\frac{1}{\left(t_n-t_m\right)}{\int }_{t_m}^{t_n}\left(n_i\left(t\right)\right)\mathrm{dt}& \left(10\right)\end{array}$

(48) For each of the sections k, it is now checked whether the speeds within this section were sufficiently constant. The procedure is analogous to the error criterion from equation (3), but now only for the speed deviations in the time segment t.sub.m to t.sub.n. The time segments in which the deviations from the respective mean speeds are too high are further subdivided according to the method shown in relations (7) to (10). This subdivision can be repeated until no further improvement in results can be expected. Finally, each tube wall thickness measured on the outlet-side is assigned to a set of associated mean roll speeds, which either represent the actual speed curve at the time the tube section is rolled in accordance with the error criterion (3) within a permissible fluctuation range, or a deviation of which from the actual speed curve cannot be reduced any further:
s.sub.rj.fwdarw.n.sub.i.sup.k  (11)

(49) With this relation, a shell wall thickness profile is now calculated using the inverse function:
s.sub.ij=F.sup.−1(n.sub.i.sup.k,s.sub.rj)  (12)

(50) The method presented is only an example and can be modified insofar as it enables one or more of the roll stands 10 to be controlled, taking into account the determined inlet-side wall thicknesses.

(51) As far as applicable, all of the individual features set out in the exemplary embodiments can be combined with one another and/or exchanged.

LIST OF REFERENCE SYMBOLS

(52) 1 Stretch-reducing mill 2 Controller 10 Roll stand 20 Wall thickness measuring device 21 Inlet-side speed measuring device 22 Outlet-side speed measuring device R Tube F Conveying direction s.sub.l Wall thickness of the tube on the inlet-side s.sub.l_t Theoretically determined wall thickness of the tube on the inlet-side s.sub.r Wall thickness of the tube on the outlet-side