Device for controlling a stretch-reducing mill

Abstract

The invention relates to a method for controlling a stretch-reducing mill in which tube ends of stretched tubes are optimised by controlling one or more motors of the stretch-reducing mill (1), said method comprising at least one wall thickness measurement on the discharge side and automatic adaptation of the magnitude of the change in the speed of the motors to the tube wall thickness profile, wherein the development over time of the changes in the speed of individual or all the motors is adapted automatically on the basis of the tube wall thickness measurement values.

Description

[0001] The invention relates to a method for controlling a stretch-reducing rolling mill.

[0002] During the stretch reduction process, the tube wall thickness thickens or increases at the tube ends compared to the middle section. This occurs because the longitudinal rolling tension otherwise achieved in the middle section of the tube cannot be achieved at the front or rear end of the tube due to the lack of rolling stands upstream or downstream in the conveying direction. The resulting thickened tube sections exceeding the permissible wall thickness tolerance represent a yield loss and must be cut off.

[0003] Additionally, especially in stretch-reducing rolling mills in seamless tube plants, the mother tubes or hollows used may have upset wall thicknesses at the ends, e.g., due to tool wear in the pre-rolling units. This upset of the mother tubes causes an additional thickening of the finished tube ends.

[0004] Various methods have been devised to reduce end losses. End loss control has achieved practical and widespread significance by dynamically varying the motor or roll speeds as the pipe ends pass through the rolling mill. This increases the speed ratio between the rolling passes closest to the pipe end, thus increasing the rolling tension.

[0005] Such systems have been known for a long time, for example through DE 1 602 181 A or DE 1 962 792 A.

[0006] Numerous embodiments have been proposed, for example, in DE 25 57 707 A1, DE 198 40 864 A1, DE 26 45 497 A1, or DE 30 28 211 A1. However, none of these sources address the problem of the specific setting of the speed change.

[0007] The particular challenge lies, on the one hand, in initiating the aforementioned speed changes in a timely manner, as otherwise they have no effect on the end thickening. On the other hand, the magnitude of the speed change and the transition to the steady-state speeds must be precisely coordinated, as otherwise the sections adjacent to the pipe ends may exhibit unacceptable undershoots of the target wall thickness. The situation is further complicated by the fact that the speed curves of up to 32 drive motors must be adjusted. It is not possible to determine theoretical speed profiles in advance that would achieve the best possible shortening of the thickened ends without further adjustment. However, for the operating crews, manually adjusting the speed profiles is a difficult and time-consuming process.

[0008] This problem has been recognized for a long time and has led to further proposals for possible automation. For example, the aforementioned DE 1 962 792 A1 teaches the use of penetration detection, on the one hand, using sensors upstream of the SRW and, on the other hand, detecting the change in motor speed due to load changes when the pipe enters or exits a rolling pass. This allows for better tracking of the position of the pipe ends and partially automatic adaptation of the control parameters. However, this form of pipe tracking in the SRW is not suitable for rolling mills in which groups of rolling passes are driven by shared motors. Furthermore, the development of modern, frequency-controlled asynchronous motors has meant that speed drops resulting from load changes are minimal and are barely detectable by a pipe end control system.

[0009] Solutions have also been proposed in which additional sensors, such as light barriers or photocells within the SRW, are intended to detect the current position of the front or rear pipe end and thus trigger the use of speed control. However, due to the unavoidable and adverse environmental influences in the SRW, such as spray water, steam, or dust, such systems are not consistently reliable.

[0010] JP H0724614 A describes an automatic adjustment of motor speeds based on pipe measurement data. A method for controlling a stretch-reducing mill is disclosed, wherein pipe ends of stretched pipes are optimized by controlling one or more motors of the stretch-reducing mill, comprising at least one outlet-side wall thickness measurement and an automatic adjustment of the magnitude of a speed change of the motors to the pipe wall thickness profile. However, the operating times and the duration of the effect are not adjusted. Both, however, have a significant influence on the control result. Furthermore, the influence of the incoming pipe is not taken into account. Likewise, a combination of multiple rolling processes to minimize the influence of measurement errors or outliers is not mentioned.

[0011] DE-A1-38 19 571 describes a method for controlling wall thickness during pipe stretch reduction. Changing the speeds in such a way that the temporal progression of the speed changes of individual or all motors is automatically adjusted based on the pipe wall thickness measurements is not provided for.

[0012] DE-A1-10 2015 118 065 describes a method for adjusting the speed of continuous pipe rolling mills. Changing the speed in such a way that the temporal progression of the speed changes of individual or all motors is automatically adjusted based on the pipe wall thickness measurements is not provided for.

[0013] A disadvantage of the current technology is that in practical operation, rolling mill operators usually have to make corrective adjustments to the CEC, or at least at the beginning of a rolling campaign. Adjustments may also have to be made during a rolling campaign, for example, due to tool wear.

[0014] It is the object of the invention to provide a method for controlling a stretch-reducing rolling mill in which a yield loss due to thickened pipe ends is reduced.

[0015] This object is achieved according to the invention for a method mentioned above with the features of claim 1. By adjusting the rotational speeds during the passage of a pipe, a particularly precise influence on the resulting course of the wall thickness in the region of the pipe ends can be achieved.

[0016] In automatic operation, a CEC independently monitors and evaluates the achieved wall thickness results at the pipe ends and readjusts the strength and timing of the speed change at the pipe ends for the following pipes.

[0017] Further advantages of the invention include reducing the workload for rolling mill operators. Optimal CEC settings are found more quickly and are better maintained throughout a rolling campaign.

[0018] According to the invention, the temporal progression of the rotational speeds is characterized by the start time of the rotational speed change and the end time of the rotational speed change. It is provided that the temporal progression is characterized by a start time or end time and a rate of change.

[0019] For more precise optimization, it may be provided that the evaluation of the wall thickness profile is carried out on at least three sections of the wall thickness profile.

[0020] It may be generally advantageous to provide for the evaluation of the wall thickness progression to be carried out from several target variables.

[0021] In a particularly preferred embodiment of the invention, it can be provided that the method is combined with a wall thickness control system for automatically controlling the wall thicknesses outside the thickened ends.

[0022] According to the invention, the wall thickness curves at the ends are examined for cyclic patterns, whereby such patterns are taken into account in the control of the motors.

[0023] In a possible further development of the invention, a measurement of an incoming shell wall thickness profile can be performed, with the magnitude and temporal progression of the speed changes of the tube end control being adapted to the shell wall thickness measurements. In this way, intervention is made very early in the forming process of a shell into a tube as the final product with the aim of improving the tube end diameter.

[0024] In a preferred further development, the wall thickness curves at the ends of the shells can be examined for cyclic patterns and such patterns can be taken into account.

[0025] Furthermore, it can preferably be provided that the method is combined with a wall thickness control system for automatically controlling the wall thicknesses outside the thickened ends.

[0026] A further measure improving the invention is automatic puncture detection.

[0027] A further measure improving the invention consists in taking into account the actual wall thickness profiles at the ends of the incoming mother pipes.

[0028] A further measure improving the invention consists in specifying nominal or ideal shapes of the pipe ends of each dimension.

[0029] A further measure improving the invention consists in the use of pattern recognition algorithms to evaluate the wall thickness profile of each pipe end.

[0030] A further measure improving the invention consists in a simulation to calculate in advance the effect of a change in setting.

[0031] A further measure improving the invention consists in an iteration of the CEC setting over several loops to find a stable optimum

[0032] A preferred embodiment of the invention is described below and explained in more detail with reference to the accompanying drawings. Fig. 1 shows a schematic representation of a stretch-reducing mill with its control system.

[0033] A stretch-reducing mill comprises several rolls in rolling stands 1, which are driven by variable-speed motors. Stretch-reducing of a rolled stock 2 is achieved by controlling the motors at different speeds, so that the rolled stock is subjected to tensile stress between the rolls.

[0034] The motors are supplied with electrical power via a programmable logic controller (PLC) 3. The PLC 3 queries and / or calculates the motor speeds during the rolling process.

[0035] The PLC 3 is connected to sensors 5, 6 via a network 4 in the form of a fieldbus system, so that measured values ​​flow directly into the PLC. Sensors 5 are, in this case, exemplary position sensors, for example, in the form of light barriers. Sensors 6 determine additional measured values ​​for monitoring the rolling process, in particular the diameter, wall thickness, and temperature of the rolled material.

[0036] The PLC 3 can also communicate with a process control computer 7a of a process control level via a non-real-time capable network 7.

[0037] A method according to the invention for controlling a stretch-reducing mill can be carried out on a plug-in reducing mill described above as an example. In this process, tube ends of stretched tubes are optimized by controlling one or more motors of the stretch-reducing mill.

[0038] At least one wall thickness measurement is performed on the outlet side by sensors 6, and the magnitude of a speed change of the motors is automatically adjusted to the measured pipe wall thickness profile. According to the invention, the temporal progression of the speed changes of individual or all motors is also automatically adjusted based on the pipe wall thickness measurements.

[0039] The temporal progression of the speeds is characterized by the start time of the speed change and the end point of the speed change. In particular, the temporal progression is also characterized by a rate of change of the speeds.

[0040] An assessment of the wall thickness profile is carried out on at least three sections of the wall thickness profile.

[0041] In addition, the wall thickness progression is evaluated based on several target variables.

[0042] The process for controlling the pipe end thickness is combined with a wall thickness control system for automatically controlling the wall thicknesses outside the thickened ends.

[0043] The measured values ​​from the sensors 6 are analyzed by means of programs, whereby the wall thickness curves at the ends are examined for cyclic patterns and such patterns are taken into account when controlling the motors.

[0044] In addition to the measurement of the wall thicknesses of the (partially) stretched pipes, a measurement of an incoming shell wall thickness profile is carried out, whereby the sizes and the temporal courses of the speed changes of the pipe end control are adapted to the shell wall thickness measured values.

[0045] The wall thickness curves at the ends of the shells are examined for cyclic patterns and such patterns are taken into account.

[0046] Overall, the process is combined with a wall thickness control system for automatic control of the wall thicknesses outside the thickened ends. List of reference symbols

[0047] 1Rolling stands with rolls and motors 2Rolling stock 3PLC = Programmable Logic Controller 4Bus system, fieldbus 5Sensors, position sensors 6Sensors for diameter, wall thickness, temperature, etc. 7Network at process control level 7aProcess control computer

Claims

1. Method of controlling a stretch-reducing rolling mill, wherein tube ends of stretched tubes are optimised by control of one or more motors of the stretch-reducing rolling mill (1), comprising at least one wall thickness measurement at the outlet side and an automatic adaptation of the amount of a change in rotational speed of the motors to the tube wall thickness profile, wherein the time plot of the rotational speed changes of individual or all motors is also automatically adapted on the basis of the tube wall thickness measurement values, wherein the time plot of the rotational speeds is characterised by the start instant of the rotational speed change and the end instant of the rotational speed change, wherein the time plot is characterised by start instant or end instant and a rate of change and wherein that the wall thickness plots at the ends are investigated with respect to cyclic patterns and such patterns are taken into consideration in the control of the motors.

2. Method according to the preceding claim, characterised in that the evaluation of the wall thickness plot is carried out at at least three sections of the wall thickness profile.

3. Method according to one of the preceding claims, characterised in that the evaluation of the wall thickness plot is carried out from a plurality of target magnitudes.

4. Method according to any one of the preceding claims, characterised in that the method is combined with a wall thickness checking system for automatic control of the wall thicknesses outside the thickened ends.

5. Method according to any one of the preceding claims, characterised in that a measurement of an entering wall thickness profile of a tube blank is carried out, wherein the amounts and time plots of the rotational speed changes of the tube end control are adapted to tube-blank wall thickness measurement values.

6. Method according to claim 5, characterised in that the wall thickness plots at tube blank ends are investigated with respect to cyclic patterns and such patterns are taken into consideration.

7. Method according to one of claims 5 and 6, characterised in that the method is combined with a wall thickness checking system for automatic control of the wall thicknesses outside the thickened ends.

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