阅读说明：本技术 使用单个厚度轮廓仪检测平整度缺陷 (Flatness defect detection using a single thickness profiler ) 是由 J.E.凯弗 R.G.诺宁 于 2019-10-04 设计创作，主要内容包括：在此提供了一种用于控制具有用于生产薄带产品的轧机的设备的系统。该系统包括测厚仪和控制器。所述测厚仪布置在轧机的出口处,以在薄带产品宽度上的多个位置处对薄带产品进行厚度测量。所述控制器耦合至测厚仪并配置为接收厚度测量值,处理厚度测量值以检测与多个控制位置对应的薄带产品厚度波动,并根据厚度波动检测薄带产品中的平整度缺陷。(A system for controlling a plant having a rolling mill for producing a thin strip product is provided. The system comprises a thickness gauge and a controller. The thickness gauge is disposed at the exit of the rolling mill to measure the thickness of the thin strip product at a plurality of locations across the width of the thin strip product. The controller is coupled to the thickness gauge and configured to receive the thickness measurements, process the thickness measurements to detect thin strip product thickness fluctuations corresponding to the plurality of control locations, and detect flatness defects in the thin strip product based on the thickness fluctuations.)

1. A system for controlling a plant having a rolling mill for producing a thin strip product, comprising:

a thickness gauge disposed at an outlet of the rolling mill to perform thickness measurements of the thin strip product at a plurality of locations across the width of the thin strip product; and

a controller coupled to the thickness gauge and configured to receive the thickness measurements, process the thickness measurements to detect a thin strip product thickness fluctuation corresponding to the plurality of control locations, and detect a flatness defect in the thin strip product based on the thickness fluctuation.

2. The system of claim 1, wherein the controller is further configured to determine a phase of the thickness fluctuation at each of the plurality of control locations and detect the flatness defect based on a difference in the phase of the thickness fluctuation.

3. The system of claim 2, wherein the controller is further configured to identify a control location having a leading phase value as indicative of a flatness defect.

4. The system of claim 1, wherein the controller is further configured to determine an amplitude of the thickness fluctuation in a given frequency range at each of the plurality of control locations and detect the flatness defect based on an amplitude magnitude of the thickness fluctuation.

5. The system of claim 4, wherein the controller is further configured to identify control locations having higher amplitude values as an indication of flatness defects.

6. The system of claim 1, wherein the apparatus comprises a twin roll continuous casting apparatus.

7. The system of claim 1, wherein the apparatus comprises a twin roll continuous casting apparatus and the rolling mill comprises a work roll and a plurality of valves and nozzles for providing staged water spray cooling to the work roll, each spray zone of the nozzles comprising one of the plurality of control positions, wherein the controller actuates the valves to differentially cool the work roll in response to a detected flatness defect.

8. The system of claim 1, wherein the apparatus comprises a twin roll continuous casting apparatus and the rolling mill comprises a work roll and a plurality of valves and nozzles for providing staged water spray cooling to the work roll, wherein the controller is further configured to determine a phase of the thickness fluctuation at each of the plurality of locations corresponding to the nozzles and actuate the valves in response to the detected phase of the thickness fluctuation to differentially cool the work roll.

9. A method for controlling an apparatus having a rolling mill for producing a thin strip product, comprising:

measuring the thickness of the thin strip product at a plurality of locations across the width of the thin strip product using a thickness gauge disposed at an outlet of the rolling mill;

receiving a thickness measurement at a controller coupled to a thickness gauge;

processing the thickness measurements to detect thickness fluctuations of the thin strip product corresponding to the plurality of control locations; and is

Flatness defects in the thin strip product are detected from the thickness fluctuations.

10. The method of claim 9, further comprising the steps of:

determining a phase of the thickness fluctuation at each of the plurality of control positions; and is

And detecting flatness defects according to the phase difference of the thickness fluctuation.

11. The method of claim 10, further comprising identifying control locations having leading phase values as an indication of flatness defects.

12. The method of claim 9, further comprising determining an amplitude of the thickness fluctuation in a given frequency range at each of the plurality of control positions and detecting the flatness defect based on the magnitude of the amplitude of the thickness fluctuation.

13. The method of claim 12, wherein the frequency range is 4-7 hertz, and the method further comprises identifying control locations having higher amplitude values as an indication of flatness defects.

14. The method of claim 9, wherein the rolling mill includes a work roll and a plurality of valves and nozzles for providing staged water spray cooling to the work roll, each spray zone of the nozzles including one of the plurality of control positions, wherein the controller actuates the valves to differentially cool the work roll in response to a detected flatness defect.

15. The method of claim 9, wherein the rolling mill includes a work roll and a plurality of valves and nozzles for providing staged water spray cooling to the work roll, wherein the controller is further configured to determine a phase of the thickness fluctuation at each of the plurality of locations corresponding to the nozzles and actuate the valves in response to the detected phase of the thickness fluctuation to differentially cool the work roll.

16. A twin roll continuous casting apparatus for producing a thin strip cast product, comprising:

a pair of counter-rotating casting rolls having a nip therebetween capable of delivering a cast strip downwardly from the nip;

a hot rolling mill comprising work rolls and a plurality of valves and nozzles for providing staged water spray cooling to the work rolls, each spray zone of the nozzles comprising a control position,

a thickness gauge disposed at an outlet of the rolling mill to perform thickness measurements of the thin strip product at a plurality of locations across the width of the thin strip product; and

a controller coupled to the thickness gauge and configured to receive the thickness measurements, process the thickness measurements to detect thin strip product thickness fluctuations corresponding to the plurality of control locations, and detect flatness defects in the thin strip product based on the thickness fluctuations;

wherein the controller is coupled to the plurality of valves and actuates the plurality of valves to differentially cool the work rolls in response to the detected flatness defect.

17. The system of claim 16, wherein the controller is further configured to determine a phase of the thickness fluctuation at each of the plurality of control locations and detect the flatness defect based on a difference in the phase of the thickness fluctuation.

18. The system of claim 17, wherein the controller is further configured to identify a control location having a leading phase value as indicative of a flatness defect.

19. The system of claim 16, wherein the controller is further configured to determine an amplitude of the thickness fluctuation in a given frequency range at each of the plurality of control locations and detect the flatness defect based on the magnitude of the amplitude of the thickness fluctuation.

20. The system of claim 19, wherein the frequency range is 4-7 hertz, and the controller is further configured to identify control locations having higher amplitude values as an indication of flatness defects.

Background

In the continuous casting of thin steel strip, molten metal is cast directly into thin strip by casting rolls. The shape of the thin cast strip is determined by, among other things, the surface of the casting surfaces of the casting rolls.

In a twin roll caster, molten metal is introduced between a pair of counter-rotating laterally disposed casting rolls which are internally cooled so that metal shells solidify on the surfaces of the moving casting rolls and come together at the nip between the casting rolls to produce a thin cast strip product. The term "nip" is used herein to refer to the general area where the casting rolls are closest together. Molten metal may be poured from a ladle through a metal delivery system including a movable tundish and a core nozzle located above the nip to form a casting pool of molten metal supported on the casting surfaces of the casting rolls above the nip and extending along the length of the nip. The casting pool is typically confined between refractory side plates or dams held in sliding engagement with end surfaces of the casting rolls to confine the two ends of the casting pool.

The cast strip passes downwardly through the nip between the casting rolls and then into a transition path, through a guide platform, and to the pinch roll stand. After exiting the pinch roll stand, the cast strip enters and passes through a hot rolling mill where the cast strip may be modified in a controlled manner, typically by reducing the thickness of the cast strip.

Lateral expansion of the cast strip may also occur when the thickness of the cast strip is reduced by axial compression. The direction and amount of expansion is determined by the poisson's ratio of the material and the applied tension. Depending on the tension normally applied during rolling and the geometry of the cast strip, this results in a horizontal spread that is almost entirely in the rolling direction (lengthwise direction). This expansion is called elongation. The percent elongation of a material is proportional to the percent reduction in thickness. If the thickness of the cast strip decreases by different amounts in the width direction of the cast strip (perpendicular to the rolling direction), this will result in the cast strip elongating by different amounts in the length direction.

However, the different extensions are still part of the same sheet metal, and the more elongated portions are constrained by the less elongated portions. This creates stresses in the material which ultimately produce "warpage" in the material as the tension is removed from the metal plate.

Another type of flatness defect is called a bend mark, which leads to short-wave thickness variations in the casting strip in the frequency range of 4-7 Hz. As the bend lines become stronger, the magnitude of the thickness variation increases. As the inconsistent rolling increases, tight buckling from the mill can begin to occur, then folding begins to occur and eventually the strip can tear and break. The term "folding" refers to the phenomenon that the warping on the inlet side of the rolling mill becomes so large as to fold on itself when passing through the work rolls.

Various control means have been developed to control the shape of the work rolls of a hot rolling mill to reduce flatness defects. For example, work roll bending cylinders are provided to affect a symmetrical change in the center region of the roll gap profile of the work roll relative to the regions adjacent the edges. The curvature of the rolls enables correction of the symmetrical shape defects common to the central zone and the two edges of the strip. In addition, the pressure cylinder can affect an asymmetric change in the roll gap profile on one side relative to the other. The force cylinders of the rolls are capable of deflecting or tilting the roll gap profile to correct shape defects in the strip that occur asymmetrically on either side of the strip, where the draw on one side is tighter than the average draw stress on the strip and the draw on the other side is looser than the average draw stress on the strip.

Another method of controlling the shape of the work rolls, and thus the elongation of the cast strip passing between the work rolls, is to locally segment the work rolls. For example, see U.S. patent 7,181,822, which is incorporated herein by reference. By controlling the localized cooling of the work surfaces of the work rolls, the profile of the upper and lower work rolls can be controlled by thermal expansion or contraction of the work rolls to reduce shape defects and localized buckling. Specifically, control of localized cooling may be achieved by increasing the amount of time the pulse width modulation valves are open, which effectively increases the relative amount of coolant that is injected through the nozzles onto the work roll surfaces in one or more regions of the observed cast strip shape buckle region, causing the work roll diameters of one or both of the work rolls in that region to shrink, thereby increasing the roll gap profile and effectively reducing the amount of elongation in that region. Conversely, by effectively reducing the relative amount of coolant sprayed by the nozzles onto the work surface of the work roll, the work roll diameter in this region may be caused to expand, thereby reducing the roll gap profile and effectively increasing the amount of elongation. Alternatively or additionally, the cooling of the work surface of the work rolls may be controlled internally in the region of the work rolls by locally controlling the temperature or the amount of water circulating through the work rolls in the vicinity of the work surface, thereby achieving control of the local cooling. While such controls are known, such controls are typically manually operated and have no real-time feedback as to the presence of flatness defects.

It has been found that attempts to measure the flatness of the strip directly downstream of the hot rolling mill are unsatisfactory for achieving practical control of the hot rolling mill. The high temperature of the cast strip at the outlet of the hot rolling mill makes it difficult to measure the strip flatness by direct contact.

For example, attempts have been made to provide a method of detecting flatness by measuring the tension difference across the width of the strip. Typically, a physical device (often referred to as a "profiler" roll) is placed in line with the sheet material. As part of this process, the sheet should have some deflection (or wrap angle) around the rolls and be under tension. The device typically measures the tension difference across the width of the roll by measurement of displacement or force. The low tension areas are indicative of locations where warpage is present. However, the devices used to measure the tension difference across the width of the strip tend to be very expensive. Furthermore, they do not generally last for a long time in a hot rolling environment.

Non-contact optical methods have been used to measure flatness. Some measuring devices use optical or radiation detection methods to detect the height of the warp in the strip in a stereoscopic manner. However, the optical device is based on the warpage being visible. This non-contact flatness measurement results in a local flatness measurement because only a portion of the strip exhibits a measured flatness defect at any given time. When a material is in an extended state, it is elastically deformed. This may tend to hide the warpage, thereby preventing optical inspection until the flatness defect becomes very large.

There is a need for a system and method for determining the flatness of a cast strip metal product that is robust enough to withstand the hot rolling environment and can detect characteristics that may cause flatness defects when tension is released, even if the flatness defects of the metal sheet are not optically detectable when the metal sheet exits the hot rolling mill. Such measurements can then be used to automate certain aspects of the rolling process to produce a product free of flatness defects.

Disclosure of Invention

A system for controlling a plant having a rolling mill for producing a thin strip product is provided herein. The system comprises a thickness gauge and a controller. The thickness gauge is disposed at the exit of the rolling mill to measure the thickness of the thin strip product at a plurality of locations across the width of the thin strip product. The controller is coupled to the thickness gauge and configured to receive the thickness measurements, process the thickness measurements to detect thin strip product thickness fluctuations corresponding to the plurality of control locations, and detect flatness defects in the thin strip product based on the thickness fluctuations.

The controller may be further configured to determine a phase of the thickness fluctuation at each of the plurality of control positions, and detect the flatness defect according to a phase difference of the thickness fluctuation. The controller may also be configured to identify a control location having a leading phase value as an indication of a flatness defect.

The controller may be further configured to determine an amplitude of the thickness fluctuation within a given frequency range at each of the plurality of control positions, and detect the flatness defect according to an amplitude magnitude of the thickness fluctuation. The frequency range may be 4-7 hz. The controller may also be configured to identify control locations having higher amplitude values as indicative of flatness defects.

The apparatus may include a twin roll continuous casting apparatus and the rolling mill may have work rolls and a plurality of valves and nozzles for providing staged water spray cooling to the work rolls, each spray zone of the nozzles including a control position, wherein the controller actuates the valves to differentially cool the work rolls in response to a detected flatness defect. The controller may be further configured to determine a phase of the thickness fluctuation at each of the plurality of locations corresponding to the nozzles, and actuate the valve to differentially cool the work roll in response to the detected phase of the thickness fluctuation.

According to another aspect of the invention, a method for controlling an apparatus having a rolling mill for producing a thin strip product comprises: performing thin strip product thickness measurements at a plurality of locations across the width of the thin strip product using a thickness gauge disposed at an exit of the rolling mill, receiving the thickness measurements at a controller coupled to the thickness gauge, processing the thickness measurements to detect thin strip product thickness fluctuations corresponding to the plurality of control locations; and detecting flatness defects in the thin strip product based on the thickness fluctuations.

The method may further comprise the steps of: a thickness fluctuation phase is determined at each of the plurality of control positions, and flatness defects are detected from a difference in the thickness fluctuation phases. In one example, the method includes identifying a control location having a leading phase value as an indication of a flatness defect.

The method may further include determining an amplitude of the thickness fluctuation in a given frequency range at each of the plurality of control positions, and detecting the flatness defect based on the magnitude of the amplitude of the thickness fluctuation. Control positions with higher amplitude values can be used to identify flatness defects.

The rolling mill may include a work roll and a plurality of valves and nozzles for providing staged water spray cooling to the work roll, each spray zone of the nozzles including one of the plurality of control positions, and the controller may actuate the valves in response to a detected flatness defect to differentially cool the work roll.

Drawings

Operation of an exemplary twin roll casting apparatus of the present invention is described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic of a thin strip casting plant having a hot rolling mill capable of controlling the shape of cast strip in accordance with an aspect of the present invention;

FIG. 2 is an enlarged cross-sectional side view of the continuous caster of the thin strip casting plant of FIG. 1;

FIG. 3 is a partial side view of the hot rolling mill of the thin strip casting plant of FIG. 1 showing the arrangement of the partial cooling apparatus;

FIG. 4 is a partial plan view illustrating a cooling pattern of a partial cooling apparatus of the hot rolling mill of the thin strip casting plant of FIG. 1;

FIG. 5 is a partial plan view illustrating a cooling pattern of a partial cooling apparatus of the hot rolling mill of the thin strip casting plant of FIG. 1;

FIG. 6 is a block diagram of a control system of another aspect of the present invention;

FIG. 7 is a schematic illustration of in-phase thickness fluctuations;

FIG. 8 is a schematic illustration of heterogeneous thickness fluctuations;

FIG. 9 is a flow chart of a method of another aspect of the present invention;

fig. 10 shows a histogram of the bend intensity distribution (frequency ═ number of coils) divided between coils marked with bend scales and coils not marked with bend scales when detected downstream;

fig. 11 is a flow chart of another method of another aspect of the present invention.

Detailed Description

The exemplary casting and rolling facility shown in fig. 1 and 2 includes a twin roll caster, generally indicated by reference numeral 11, which produces a thin cast strip 12, which thin cast strip 12 enters a transition path, passes through a guide platform 13, and reaches pinch roll stand 14. After exiting the pinch roll stand 14, the thin cast strip 12 enters and passes through a hot rolling mill 15, the hot rolling mill 15 consisting of back-up rolls 16 and upper and lower work rolls 16A and 16B at the location of which the thickness of the strip is reduced. Upon exiting the mill 15, the strip 12 reaches an output platform 17 where the strip 12 may be forcibly cooled by a water spray 18, and the strip 12 then passes through a pinch roll stand 20 comprising a pair of pinch rolls 20A and to a strip coiler 19. The exit thickness gauge 90 measures the thickness of the cast strip after it exits the rolling mill 15 and provides a signal indicative of the measurement to a controller 92.

Referring to FIG. 2, the twin roll caster 11 comprises a main frame 21 which supports a pair of laterally positioned casting rolls 22, the casting rolls 22 having casting surfaces 22A and forming a nip 27 therebetween. During casting, molten metal is supplied from a ladle (not shown) to a tundish 23, through a refractory shroud 24 to a removable tundish 25 (also referred to as a distributor vessel or transition piece), and then through a metal delivery nozzle 26 (also referred to as a core nozzle) between the casting rolls 22 above the nip 27. Molten steel is introduced from tundish 23 into removable tundish 25 through the outlet of shroud 24. The tundish 23 is provided with a slide gate valve (not shown) to selectively open and close the outlet 24 and effectively control the flow of molten metal from the tundish 23 to the caster. Molten metal flows from removable tundish 25 through the outlet and optionally to and through core nozzle 26.

The molten metal delivered to the casting rolls 22 thereby forms a casting pool 30 above the nip 27 supported by the casting roll surfaces 22A. The casting pool is confined at the ends of the rolls by a pair of side dams or plates 28, which side dams or plates 28 are applied to the ends of the rolls by a pair of thrusters (not shown) comprising hydraulic cylinder units connected to the side dams. The upper surface of the casting pool 30 (often referred to as the "meniscus" level) is generally elevated above the lower ends of the delivery nozzles 26 so that the lower ends of the delivery nozzles 26 are submerged in the casting pool.

The casting rolls 22 are internally water cooled by coolant supplies (not shown) and driven in opposite rotational directions by drive means (not shown) so that shells solidify on the moving casting roll surfaces and are brought together at the nip 27 to produce a thin cast strip 12, which thin cast strip 12 is delivered downwardly from the nip between the casting rolls.

Referring to figure 1, below a twin roll caster 11, cast strip 12 passes within a sealed enclosure 10 to a guide platform 13, which guide platform 13 guides the strip to a pinch roll stand 14, through which pinch roll stand 14 the strip exits sealed enclosure 10. The enclosure 10 may not be completely sealed but is adapted to control the atmosphere within the enclosure and to control the contact of oxygen with the cast strip within the enclosure. After exiting the sealed enclosure 10, the strip may pass through an additional sealed enclosure (not shown) after the pinch roll stand 14.

Thin cast strip 12 is delivered from pinch roll stand 14 to hot rolling mill 15, which includes upper work rolls 16A and lower rolls 16B. Referring to FIGS. 3, 4 and 5, a header 70A is disposed adjacent the upper work roll 16A and supplies coolant to three rows of nozzles 71A and 72A. The row of nozzles 71A closest to the strip contains 24 nozzles capable of delivering coolant from the header 70A at a pressure of 100 psi, for example at a flow rate of 470 gallons per minute. The nozzles 71A are not individually adjusted during the casting process, but cool the upper work rolls 16A throughout the casting process. Of the remaining two rows of nozzles 72A, one row of 12 nozzles can deliver coolant at 100 psi, for example at 235 gallons per minute; another row of nozzles, consisting of 13 nozzles staggered from the previous row of nozzles, can deliver coolant from the header 70A at 100 psi, for example at 400 gallons per minute. The nozzles 72A in the two rows are spaced so that the sprays from the nozzles do not interfere with one another so as not to reduce the cooling efficiency of the sprays. Coolant spray 75 from nozzle 71A and coolant spray 76 from nozzle 72A may be manually controlled by upper manifold valve 73A or by flow meter 73A preset by an operator to a desired flow rate.

Further, the sprays 76 emitted from the nozzles 72A may be individually controlled by individual control valves 74A. The individual control valves 74A may be actuated by a controller 92 (fig. 1, 6) or manually adjusted. The individual control valve 74A may be a pulse width modulated valve and the controller may adjust the duty cycle of the pulses. It should be appreciated that depending on the particular embodiment of the hot rolling mill, a single control valve 74A may control more than one nozzle 72A if zoned cooling is desired. Typically, however, a separate control valve 74A is provided for each nozzle 72A to provide greater flexibility and effectiveness in the operation of the hot rolling mill to control the shape of the work rolls 16A and, thus, the shape of the cast strip. Typically, the nozzles 72A may be arranged at about 50 mm intervals. The sprays emitted from the nozzles 72A are arranged so that the sprays substantially overlap between regions across the working surface 77A of the work roll 16A. In this way, the controllable nozzle 72A is able to respond to and effectively control any shape defects throughout the strip material 12. In particular, control valve 74A may be controlled to increase or decrease the roll gap profile to reduce or eliminate the differential elongation. A sliding brush bar 81 is also provided to expel the coolant from the sprays 75 and 76 from the nozzles 71A and 72A after the coolant impacts the working surface 77A, thereby preventing contact of the coolant with the strip 12, which could cause defects due to localized cooling.

Controlled cooling adjacent the lower work roll 16B is achieved by supplying coolant from header 70B to three rows of nozzles 71B and 72B. The row of nozzles 71B closest to the strip contains 24 nozzles capable of delivering coolant from the header 70B at a pressure of 100 psi, for example at a flow rate of 470 gallons per minute. The nozzles 71B are not individually adjusted during the casting campaign, but rather provide coolant to cool the lower work rolls 16B throughout the casting campaign. Of the remaining two rows of nozzles 72B, one row of 12 nozzles can deliver coolant at 100 psi, for example at 235 gallons per minute; another row of nozzles, consisting of 13 nozzles staggered from the previous row of nozzles, can deliver coolant from the header 70B at 100 psi, for example at 400 gallons per minute. Here, the nozzles 72B in the two rows are also spaced so that the sprays from the nozzles do not interfere with each other, so as not to reduce the cooling efficiency of the sprays. Coolant spray 75 from nozzle 71B and coolant spray 76 from nozzle 72B may be manually controlled by lower manifold valve 73B.

Further, spray 76 from nozzle 72B may be individually controlled by individual control valves 74B. The individual control valves 74B may be actuated by the controller 92 or manually adjusted. The separate control valve 74B may be a pulse width modulated valve and the controller may adjust the duty cycle of the pulses. It should be appreciated that depending on the particular embodiment of the hot rolling mill, a single control valve 74B may control more than one nozzle 72B if zoned cooling is desired. However, a separate control valve 74B is typically provided for each nozzle 72B to provide greater flexibility and effectiveness in controlling the strip shape during operation of the hot rolling mill. The nozzles 72B may be arranged at a pitch of about 50 mm. The nozzles 72B may be arranged such that the spray emitted from the nozzles substantially overlaps between regions across the working surface 77B of the work roll 16B. In this way, the controllable nozzle 72B is able to respond to and control the shape of the working surface of the work roll 16B at any location, and thus respond to and control shape defects at any location in the strip 12. In particular, control valve 74B may be controlled to increase or decrease the roll gap profile to reduce or eliminate the differential elongation.

In the use of rotating equipment, periodic variations or other factors that fluctuate at a periodic frequency are often introduced into the material. Cast metal strip produced by rolls often has some degree of periodic thickness variation. This variation is typically minimized as much as possible, but unless it exceeds customer requirements, it is not considered a defect. For example, in casting metal strip, the exit profile thickness gauge 90 may be used to detect periodic fluctuations in thickness.

For example, the metal strip may have a thickness that tapers from the center of the strip to the edges. For example, the thickness of the center may fluctuate between 1450 microns and 1470 microns, and the thickness of the edge may fluctuate between 1410 microns and 1430 microns. As long as the peaks and valleys of these undulations are aligned across the width of the metal strip and the lines defining the peaks and the lines defining the valleys are perpendicular to the rolling direction of the metal strip, the metal strip does not undergo differential stretching and is expected to have fewer or no flatness defects. Such fluctuations may be referred to as in-phase fluctuations. Fig. 7 shows the in-phase fluctuation. Fig. 7 is not drawn to scale and the fluctuations are exaggerated in order to make the effect more noticeable. In FIG. 7, the x-axis represents the length direction of the cast strip, the y-axis represents the width direction of the cast strip, and the z-axis represents the thickness direction of the cast strip. Line 94 represents thickness measurements taken at intervals across the width of the cast strip. The peaks 94A and valleys 94B of the measured values are approximately in a straight line across the width of the web.

It has been found that if the peaks 94A and valleys 94B of the thickness measured at different points along the width of the strip do not generally follow a substantially straight line perpendicular to the longitudinal direction, the metal strip is stretched to a different degree. Fig. 8 shows the case where the peaks 94A and valleys 94B of the thickness measurement do not follow straight lines across the width of the web. This fluctuation may be referred to as out-of-phase fluctuation. The degree of flatness defects can be inferred from the detected different degrees of elongation. For example, if a 1430 micron peak at the edge is detected before a 1470 micron peak at the center in the direction of strip travel, the amount of elongation of the metal strip is greater at its edge than at its center. When tension is removed from the strip, the elongation may warp, resulting in flatness defects. The magnitude of the elongation distance is indicative of different degrees of elongation and unevenness. In this way, flatness detection can be accomplished using a single thickness gauge without the need to directly measure the actual flatness of the metal strip.

Since the peaks and valleys fluctuate periodically at a frequency of about 4-5 hz, the relative phase of the fluctuations at different measuring points across the width of the metal strip indicates whether the peaks and valleys are aligned and perpendicular to the rolling direction. When the undulations are in phase, the peaks and valleys are aligned and do not characterize the elongation difference or flatness defects. When the fluctuations are out of phase, the elongation difference and flatness defects are characterized.

Referring to fig. 9, a method 100 for detecting flatness defects is provided. In step 102, thin strip thickness measurements are made at a plurality of spaced points across the width of the strip. In one example, the width of the cast strip is two meters. The thickness gauge 90 can make 400 measurements on a two meter wide strip. This results in a measurement interval of 5 mm. Different widths of the cast strip may be measured and larger or smaller measurement intervals may be used, which may result in different numbers of measurements. The measurements may be taken at 0.02 second intervals. Shorter or longer time intervals may be used.

In step 104, the controller 92 receives the thickness measurement and processes in step 106 to detect fluctuations. In one example, the thickness measurements are converted to a two-dimensional image in which the thickness of the cast strip may be displayed in color. In step 110, the operator of the casting apparatus may observe the thickness fluctuations to determine the relative course of the thickness fluctuations in the strip 108 and detect flatness defects based on the phase difference. If the undulations occur substantially linearly across the width of the cast strip, no corrective action is required. However, if the undulations appear to bend from the middle to the edges of the cast strip, the undulations in the bending are indicative that some portions of the cast strip may have experienced excessive stretching. In step 112, the operator may adjust the staged cooling (or heating) of the casting rolls to reduce the differential elongation. For example, the operator may manually adjust the control valves 74A, 74B to increase the coolant flow to the work rolls in the regions of the strip having a wave ahead of the central portion of the strip to reduce the elongation.

In another example, the measurements may be analyzed to detect differences in elongation (steps 108, 110), and the valves 74A, 74B may be automatically controlled to reduce or eliminate the differences in elongation (step 112). An illustrative example is provided herein in fig. 1 and 6, but the present invention is not limited to this example. In one example, in step 104, thickness measurements may be received and stored for processing, such as in a multi-dimensional array. One dimension of the array may be a measured distance across the width of the cast strip (e.g., each measured position spaced 5 millimeters across the width). Another dimension may be measurement time (e.g., 0.02 second time interval).

If the measurements are to be used to control the work roll shape, the resolution of the measurements may be higher than the resolution used to control the work roll. In this case, the measurements taken across the width of the cast strip may be averaged for a particular work roll control. In the example of an adjustable work roll described above, the nozzles 72A, 72B may be spaced 50 millimeters apart, thereby creating control positions that are 50 millimeters wide, while measurements may be taken at 5 millimeter intervals. Thus, the measurements for the ten measurement locations may be averaged as one average measurement per nozzle.

In another example of step 108, ten measurements corresponding to the center of the width of the cast strip are averaged for each measurement taken at 0.02 second intervals. The frequency and phase of the thickness fluctuation can then be determined over time for the averaged measurements. For example, a vector of resulting averaged measurements may be subjected to a Fast Fourier Transform (FFT) analysis along a time axis to identify the frequency of thickness fluctuations.

This analysis may be performed for each work roll control position. In the above example of a segmented cooled work roll, the spray area of each nozzle 72A, 72B includes a control area. Since the spacing of the nozzles was 50 mm and the measurements were carried out at 5 mm intervals, the average value was calculated in sections in units of 10 measurements over the width of the cast strip. Each segment represents a 50 mm wide position corresponding to a nozzle and provides an average thickness for that position. In one example, for a strip 1.68 meters wide, 33 segments (negligible edges) are obtained when measured at every 0.02 second interval. The first maximum within one sampling period (determined from the results of the FFT analysis) is identified to determine the phase of the undulations for each 50 mm segment across the width of the ribbon. The sampling period may be, for example, five seconds.

In another example of step 110, the phase shift difference may be measured by starting at the center and proceeding towards the two edges. A zero phase may be assigned to a central measurement segment and the phase of another measurement segment may be determined relative to the center. For example, the leading phase is positive and the lagging phase is negative with respect to the center. Phase may be represented by phase shift angle (multiplied by frequency) or time delay or advance. In the present example, for a 1.68 meter wide ribbon, there should be 33 average measurements indicating the phase shift compared to the waveform at the center of the ribbon. The measurements can be normalized by averaging all the phase shift measurements and then subtracting the average from each measurement (whereby the average of all measurements becomes zero).

In another example of step 112, the relative phase of the undulations at different segments can be used to control a casting or rolling operation, such as the staged cooling of the work rolls. For each 50 mm segment across the width of the strip, the resulting vector should have a value that characterizes the phase shift compared to the "average" thickness fluctuation phase. Zero phase shift characterization is free of flatness defects and therefore this is the goal. Each value in the resulting vector may be multiplied by a gain constant and integrated with respect to time. The resulting integrated value can be used as an offset of the associated segmented cooling spray. For a positive phase value, the control valves 74A, 74B corresponding to the differentially extended regions should be opened in proportion to the magnitude of the phase value to increase the flow rate of the cooling water. Increased cooling at this location causes the diameter of the work roll to shrink, thereby reducing elongation. For a negative phase value, the control valves 74A, 74B corresponding to the non-elongated portions should be closed in proportion to the magnitude of the phase value to reduce the flow rate of the cooling water. As the location heats up, the diameter of the work roll portion expands, thereby increasing elongation.

The measurement and control adjustments may be made repeatedly to bring the phase difference close to zero, a value that characterizes the absence of flatness defects. At this point, the measurement continues, but no further control is required to adjust the work roll diameter until a flatness defect is detected.

To control bending, a quadratic curve fit may be performed on the resulting measurements. The quadratic term characterizes the symmetry unevenness of the strip. The target value for this term is zero. The quadratic term may be multiplied by a gain value and integrated with respect to time and then used as the bend offset.

Referring to FIG. 11, another method of detecting flatness defects 120 includes determining the amplitude of a periodic fluctuation. An increase in the amplitude of the wave in a portion of the strip width indicates a bend. For example, referring to FIG. 10, bend mark defects were not identified in the most frequently occurring ribbon with amplitudes in the 0-20 micron range. However, bend mark defects are found in the case where the undulations frequently include an amplitude in the range of 25-60 microns.

Thickness measurements are taken at a plurality of spaced points across the width of the strip as before in step 122, and the thickness measurements are received at the controller 92 in step 124. In one example, a one-dimensional data array is generated for each measurement point. The one-dimensional array includes a variation over time of measurement data obtained from a given sensor at a point in width. In one example, 500 measurements representing 20 seconds of data are stored in the one-dimensional array. The measurement time interval may be set in step 126. In one example, it ranges from 0.02 to 0.04 seconds. In step 126, the data is processed to detect fluctuations. In one example, the data is filtered to remove variations other than the periodically fluctuating 4Hz to 7Hz frequencies. The filter may comprise an Infinite Impulse Response (IIR) bandpass filter, which may employ a third order butterworth filter having a passband of 3.75 hertz to 7.7 hertz.

In step 128, a Blackman window is applied before Fourier transforming to obtain an amplitude spectrum. Since the non-bend mark frequencies have been filtered out, the remaining amplitudes are summed for each point across the strip width to give the total average thickness variation for each point. The minimum of this new one-dimensional array (representing all points across the width at one point in time) can be subtracted from all the data points in the array to remove the consistent thickness variation caused by sources other than the bend mark. The resulting data is a measure of the amplitude of the fluctuation over a 20 second sampling period for each point on the width. The maximum value for each time step within the web can then be averaged over the entire web to give the bend mark strength of the web. In step 130, flatness defects are identified by identifying a fluctuation amplitude (e.g., in the range of 25-60 microns). This can be used to adjust the mill operation in step 132.

The above-described phase and amplitude methods may be used in combination. The bend marks appear as periodically fluctuating local thickness "bends". The phase detection techniques disclosed above may help identify bends or screen out false positives with lower amplitude fluctuations.

The present invention is not limited to controlling segmented cooling nozzles. The detection of flatness defects can also be used for the bending of the work rolls and the control of the work roll force cylinders.