WO2020061289A1 - Online monitoring and control to eliminate surface defects arising during the production of cast steel strip - Google Patents

Abstract

A method of controlling a continuous caster producing thin cast strip of metal may comprise the steps of obtaining microwave measurements of a first surface of the cast strip with a microwave reflectometer, transforming the microwave measurements of the first surface into SAR images of the first surface, and analyzing the SAR images to determine whether cracks are present in the first surface. For each crack identified, a size of the crack is determined. For cracks equal to or greater than a predetermined threshold size, an angular orientation of the crack is determined. A category of each crack is determined based on crack size and, for cracks equal to or greater than the predetermined threshold, angular orientation. Caster operation may then be adjusted to mitigate the category of detected crack.

Description

Online Monitoring And Control To Eliminate Surface Defects Arising

During The Production Of Cast Steel Strip

Related Applications

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 62/734,258, filed September 20, 2018, and U.S. Provisional Application Serial No.

62/740,081, filed October 2, 2018, which are incorporated by reference.

Background

[0002] In continuous casting of thin steel strip in a twin roll caster, molten metal is introduced between a pair of counter-rotated laterally positioned casting rolls, which are internally cooled, so that metal shells solidify on the moving casting roll surfaces.

[0003] The shell metal shells are brought 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 region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a moveable tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool.

[0004] The thin cast strip passes downwardly through the nip between the casting rolls and then into a transient path across a guide table to a pinch roll stand. After exiting the pinch roll stand, the thin cast strip passes into and through a hot rolling mill where the geometry (e.g., thickness, profile, flatness) of the strip may be modified in a controlled manner.

[0005] Brushes may be provided to clean debris from each of the casting rolls, such as accumulation of metal oxides and other contaminants on the roll surfaces. During casting the surfaces of the casting rolls are continuously cleaned ahead of contacting the molten metal in the casting pool.

[0006] Due to imperfections at the caster, surface defects, particularly surface cracks, may occasionally be produced in the cast steel strip. To check the surface quality of final products during commercial production of cast steel strip, a dye-penetration test is usually performed at the end of each coil.

[0007] The dye-pen test procedure typically involves a full-width sample of each coil being regularly inspected for cracks. The length of the sample is typically 30 cm, but on some occasions longer sample length (~1.5 m) corresponding to one revolution of the casting roll is also evaluated. The samples are stretched by about 3.5% to 6.0% and then scale on strip surface is removed by immersing the samples in an acid tank. After rinsing, dye penetrant is applied and cracks are identified, measured and manually recorded.

However, the dye pen test procedure is applied manually and only to relatively small samples of strips at the end of a coil.

[0008] What is needed is a crack detection apparatus and method that continuously scans the entire surface of a metal strip during the casting process and provides feedback to caster control system in real time. Preferably, both the top strip surface and bottom of strip surface are monitored simultaneously and the caster operators should have access to the crack detection system from operating pulpit to adjust caster operation parameters to eliminate surface cracking in real time.

Summary

[0009] A method of controlling a continuous caster producing thin cast strip of metal may comprise the steps of obtaining microwave measurements of a first surface of the cast strip with a microwave reflectometer, transforming the microwave measurements of the first surface into SAR images of the first surface, and analyzing the SAR images to determine whether cracks are present in the first surface. For each crack identified, a size of the crack is determined. For cracks equal to or greater than a predetermined threshold size, an angular orientation of the crack is determined. A category of each crack is determined based on crack size and, for cracks equal to or greater than the predetermined threshold, angular orientation. Caster operation may then be adjusted to mitigate the category of detected crack.

[0010] The category of crack may be selected from the group comprising

longitudinal cracking, transverse cracking, and micro cracking. The predetermined threshold may be in the range of 2 millimeters to 6 millimeters, for example, 4 millimeters, 3 millimeters or 2 millimeters. Cracks equal to or greater than the threshold are determined to be in the longitudinal cracking or transverse cracking categories, and cracks smaller than the threshold are determined to be in the micro cracking category.

[0011] The step of obtaining microwave measurements of a first surface further comprises obtaining microwave measurements of a second surface. The first surface comprises a bottom surface of the cast strip and wherein the second surface comprises a top surface.

[0012] The caster may be a twin roll caster and the step of adjusting caster operation may further comprise adjusting key caster parameters such as the casting speed the casting roll separation force, the operation of the casting roll brushes, or the steel chemistry in response to the crack characteristic detected. In one example, the caster may comprise a twin roll caster having two counter-rotating casting rolls producing a metal strip wherein the metal strip passes through a first pinch roll stand and a hot rolling mill, and wherein the microwave measurements are obtained between the pinch roll stand and the hot rolling mill. A caster controller maybe coupled to the microwave reflectometer.

[0013] The steps of obtaining microwave measurements and transforming the microwave measurements may be performed by a SAR microwave reflectometer. The SAR microwave reflectometer may comprise a signal source, at least one dual polarized antenna, receiver, and a SAR processor. The steps of transforming the microwave measurements and analyzing the SAR images may be performed by the SAR processor. The antenna may comprise at least one antenna array directed at a first surface of the cast strip and oriented transversely to the direction of travel of the cast strip. [0014] The caster controller adjusts caster operation in response to the category of cracking identified by the microwave reflectometer. For example, the controller may adjust key caster parameters selected from the group consisting of: the casting speed , casting roll separation force, operation of the casting roll brushes or steel chemistry in response to the crack characteristic detected.

[0015] The microwave reflectometer may be located between the pair of casting rolls and the hot mill.

Brief Description of the Drawings

[0016] Figure 1 is an illustration of a thin strip casting plant mill with crack detection according to one aspect of the present invention.

[0017] Figure 2 is an illustration of a twin roll caster.

[0018] Figures 3a and 3b are block diagrams of a SAR microwave reflectometer coupled to a casting plant control according to an aspect of the present invention.

[0019] Figure 4 is an illustration of an antenna array oriented with respect to a cast strip.

[0020] Figure 5 is an illustration of transverse cracking.

[0021] Figure 6 is an illustration of longitudinal cracking.

[0022] Figure 7a is an illustration of micro cracking.

[0023] Figure 7b is an illustration of potential causes of micro cracking.

Detailed Description

[0024] A casting and rolling installation according to one example of the present invention is illustrated in Figures 1 and 2. The installation comprises a twin roll caster denoted generally as 11 which produces a cast steel strip 12 which passes in a transit path 10 across a guide table 13 to a pinch roll stand 14 comprising pinch rolls 14A. After exiting the pinch roll stand 14, the strip passes by two microwave crack detector antenna arrays 44 and into a hot rolling mill 16. The crack detector antenna arrays 44 are located one above the cast metal strip 12 and one below the cast metal strip 12 so as to be able to scan both top and bottom surfaces. The hot rolling mill 16 comprises a pair of reduction rolls 16A and supporting rolls 16B by in which the metal strip is hot rolled to reduce its thickness. The rolled strip passes onto a run-out table 17 on which it may be force cooled by water jets 18 and through a pinch roll stand 20 comprising a pair of pinch rolls 20A, and then to a coder 19.

[0025] Referring to Figure 2, the twin roll caster 11 comprises a main machine frame 21 which supports a pair of parallel casting rolls 22 having casting surfaces 22A. Molten metal is supplied during a casting operation from a ladle (not shown) to a tundish 23, through a refractory shroud 24 to a distributor 25 and thence through a metal delivery nozzle 26 into the nip 27 between the casting rolls 22. Molten metal thus delivered to the nip 27 forms a pool 30 above the nip and this pool is confined at the ends of the rolls by a pair of side closure dams or plates 28 which are applied to the ends of the rolls by a pair of thrusters (not shown) comprising hydraulic cylinder units connected to the side plate holders. The upper surface of pool 30 (generally referred to as the "meniscus" level) may rise above the lower end of the delivery nozzle so that the lower end of the delivery nozzle is immersed within this pool.

[0026] Casting rolls 22 are water cooled so that shells solidify on the moving roll surfaces and are brought together at the nip 27 between them to produce the solidified strip 12 which is delivered downwardly from the nip between the rolls.

[0027] in some embodiments, a method of continuous scanning for cracks includes microwave reflectometry. A microwave reflectometer typically comprises a signal source generating a microwave frequency signal, an antenna, and a receiver. Referring to Figures 3a and 3b, advantageous examples of a microwave reflectometer comprise one or more Synthetic Aperture Radar (SAR) microwave reflectometers 40a, 40b, which are capable of detecting the length of cracks and determining an angular orientation of detected cracks. SAR microwave reflectometers 40a, 40b comprise a microwave signal source 42, one or more antennas, which may comprise one or more antenna arrays 44, a receiver 46, and SAR processor 48 to convert reflected and received microwave signals into SAR images of the cast strip. The SAR reflectometer may be coupled to a controller 50 for the twin roll caster. The SAR microwave reflectometer 40b includes two antenna arrays 44, one to measure a top surface of the cast strip and one to measure a bottom surface. Alternatively, two distinct SAR microwave reflectometers 40a may be used to measure the top and bottom surfaces of a cast strip.

[0028] In one example, the SAR microwave reflectometer 40a, 40b is adapted to detect cracks in cast steel strip 12 produced by a twin roller casting process as the steel is being cast. Referring to Figure 4, a one-dimensional array of antennas 44 is oriented with its lengthwise axis perpendicular to the direction of travel of the cast strip 12. The array of antennas 44 may be located to measure the cast strip 12 before it enters the hot rolling mill. For example, a first SAR microwave reflectometer antenna array 44 may be located to image the bottom surface of the cast strip, and a second SAR microwave reflectometer antenna array 44 may be located to image the top surface of the cast strip before entering the hot rolling mill 16. Because cast steel strip 12 exiting the nip is very hot, on the order of 1400° C, the antennas 44 should be protected against the heat. Alternatively, the antenna array may be located to measure the cast strip 12 after the hot rolling mill 16. The antenna array 44 may be dimensioned to be able to image the entire width of the cast strip 12 as it travels past the antenna array 44.

[0029] In one example, the antenna array comprises four antennas are arranged in a linear array. Additional or fewer antennas may be used. To maximize the chance of detecting a crack, the polarization of the wave should be orthogonal to the lengthwise angular orientation of the crack. Because the angular orientation of cracks in cast metal strips may occur in various directions, one or more circularly polarized electromagnetic waves may be transmitted and received so that all directions are scanned for cracks. In one example, both left and right hand circularly polarized waves may be transmitted and received during scanning. Cross-polarized linearly polarized antennas may also be used. [0030] In some embodiments, measurements are made continuously as the strip steel passes by the one-dimensional array, providing a two-dimensional set of

measurements in another embodiment, measurements are made periodically. In a preferred embodiment, the two-dimensional set of measurements is processed into SAR image data. The SAR processor 48 transforms the measured received signals from the frequency domain to the spatial domain. The output of the SAR processor 48 comprises complex values with x, y coordinates corresponding to a location on the measured surface, and a z coordinate corresponding to a height of the surface. Converting the reflected electromagnetic wave complex values to SAR image data improves the signal to noise ratio of the plurality of the electromagnetic wave complex values, and improves the resolution of the surface measurements. In one example, the SAR algorithm is an Omega-k algorithm. Application of the Omega-k algorithm to SAR image processing is well known to persons of skill in the art.

[0031] A surface defect may be detected and its location identified from magnitude information derived from at least one of the SAR images. The size of the defect may also be determined from magnitude information derived from at least one of the SAR images.

Determining a size and location of a surface defect may be improved by multiplying the magnitude information derived from the SAR images from the two different receive polarizations.

[0032] The SAR images from the two different receive polarizations may also be used to determine an angular orientation of the defect. This may be accomplished by determining a difference between phase information derived from one SAR image relative to the other SAR image. The difference between the phase information derived from the two SAR images may be determined by subtracting phase information derived from one SAR image from the phase information derived from the other SAR image.

[0033] For each detected crack, a length may be estimated from the magnitude information. The magnitude information represents the intensity of the reflected signal from the sample. In the absence of a crack, the magnitude will be uniform. The presence of a crack will cause a localized increase in measured intensity, and is detectable in the magnitude information. The larger the crack, typically the higher the increase in magnitude at the locations corresponding to the crack. Accordingly, the size of a crack or other surface defect may be determined from magnitude information.

[0034] Cracks have been detected in a range from 0.8mm to 10mm or larger in size. In cast steel strips, a crack length of 4mm or more is indicative of transverse or longitudinal cracking. An angular orientation must be determined for cracks having a length 4mm or greater before they can properly be identified as transverse or longitudinal. Because cracks having lengths of less than 4mm is indicative of micro-cracking, angular orientation need not be ascertained. Accordingly, 4mm may be selected as a threshold as to whether angular orientation needs to be determined. While 4mm may be an appropriate threshold in some applications, the threshold may be selected from a range of 2mm to 6mm, or a different range, depending on the technology, materials, and processes at issue.

Alternatively, angular orientation may be determined for all detected cracks. The invention is not limited for use in twin roll steel casting plants, and may be adapted to other metal producing plants. In such cases, the type size and orientation of may be indicative of other forms of cracking.

[0035] The SAR image data from the two different receive polarizations may also be used to determine an angular orientation of the defect. This may be accomplished by determining a difference between phase information derived from one SAR image relative to the other SAR image at the location of the detected crack. The difference between the phase information of the two images may be determined by subtracting phase information derived from the complex values of one SAR image from the phase information derived from the complex values of the other SAR image.

[0036] The size and angular orientation of cracks may provide valuable information to control operation of the casting rolls 22. For example, in cast steel strips, transverse cracking is cracking in the direction perpendicular to the rolling direction, and having a length greater than 4mm. An example of transverse cracking 62 is illustrated in Figure 5. Transverse cracking 62 is often due to excess amount of pool turbulence, and/or chattering in the mold which disrupt the solidification process at the meniscus. [0037] One form of chatter defect which may cause transverse cracking, called "low frequency chatter”, is produced at low casting speeds due to premature solidification of the metal high up on the casting rolls so as to produce a weak shell which subsequently deforms as it is drawn further into the casting pool. Another form of chatter defect, called "high frequency chatter”, occurs at higher casting speeds when the shell starts forming further down the casting roll so that there is liquid above the forming shell. This liquid, which feeds the meniscus region, cannot keep up with the moving roll surface, resulting in slippage between the liquid and the roll in the upper part of the casting pool, thus giving rise to high speed chatter defects appearing as transverse deformation bands across the strip. Chatter may be eliminated by adjusting casting parameters such as casting roll separation force.

[0038] Referring to Figure 6, longitudinal cracking 64 is cracking in the direction parallel to the rolling direction, and having a length greater than 4mm. Longitudinal cracking has several potential causes, including uneven shell solidification due to casting roll surface defects, inadequate brushing of the casting roll (allowing too little or too much oxide accumulation on the casting rolls), excess N, S, H, etc. in the liquid steel chemistry, and too much mushy (molten metal with a very high solid fraction) between shells leaving the casting roll nip which eventually solidifies and leaves shrinkage voids, creating stress concentration at the centerline of strip and which can initiate cracks that propagate to the surface.

[0039] An additional form of cracking is micro cracking or "crocodile skin" cracking (also known as a "croco-defect”). An example of micro cracking is provided in Figure 7a. Micro-cracks have no predominant direction, and normally have a length 78 of less than 4mm.

[0040] The‘croco defect' manifests as depressions on the surface of as-cast strip formed during solidification. Referring to Figure 7b, a first shell 72 and a second shell 74 are combined at the nip 27 between the casting rolls 22. Some areas 76 between the shells may solidify later than other areas. The defect is generated during d to g solid state phase transformation which is associated with 0.3% volume shrinkage and the stresses induced from this transformation generate croco depressions on the strip. These surface depressions lead to uneven heat transfer and shell formation as illustrated in Figure 7b. Cracks are mostly formed in the 'valleys' where the shell is thinner due to lower heat transfer and higher stress concentration. This defect can be eliminated by optimizing liquid steel chemistry and by optimizing roll cleaning.

[0041] Thus the detection and identification of each of the above forms of cracking may provide information to a casting plant operator or casting plant control system to improve the quality of the cast strips being produced. This requires that the length and angular orientation of cracking is detected and provided to operators and/or control systems.

[0042] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. For example, the methods described herein may be performed with less or more steps/acts or the steps/acts may be performed in differing orders. Additionally, the steps/acts described herein may be repeated or performed in parallel with one another or in parallel with different instances of the same or similar steps/acts. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

What is claimed is:

1. A method of controlling a continuous caster producing thin cast strip of metal, comprising:

obtaining microwave measurements of a first surface of the cast strip with a microwave reflectometer;

transforming the microwave measurements of the first surface into SAR images of the first surface;

analyzing the SAR images to determine whether cracks are present in the first surface;

for each crack identified, determining a size of the crack;

for cracks equal to or greater than a predetermined threshold size, determining an angular orientation of the crack;

determining a category of each crack based on crack size and, for cracks equal to or greater than the predetermined threshold, angular orientation of the crack; and

adjusting caster operation to mitigate the category of detected crack.

2. The method of claim 1, wherein the category of crack is selected from the group comprising longitudinal cracking, transverse cracking, and micro cracking.

3. The method of claim 2, wherein the predetermined threshold is 4 millimeters.

4. The method of claim 3, wherein cracks equal to or greater than the threshold are determined to be in the longitudinal cracking or transverse cracking categories, and cracks smaller than the threshold are determined to be in the micro cracking category.

5. The method of claim 1, wherein the step of obtaining microwave measurements of a first surface further comprises obtaining microwave measurements of a second surface.

6. The method of claim 5, wherein the first surface comprises a bottom surface of the cast strip and wherein the second surface comprises a top surface.

7. The method of claim 1, wherein the caster is a twin roll caster and the step of adjusting caster operation further comprises adjusting key caster parameters selected from the group consisting of: casting speed, casting roll separation force, operation of the casting roll brushes, and steel chemistry in response to the crack characteristic detected.

8. The method of claim 1, wherein the caster comprises a twin roll caster having two counter-rotating casting rolls producing a metal strip wherein the metal strip passes through a first pinch roll stand and a hot rolling mill, and wherein the microwave measurements are obtained between the pinch roll stand and the hot rolling mill.

9. The method of claim 1, wherein the steps of obtaining microwave measurements and transforming the microwave measurements are performed by a SAR microwave

reflectometer.

10. The method of claim 9, wherein the SAR microwave reflectometer comprises a signal source, at least one dual polarized antenna, a receiver, and a SAR processor.

11. The method of claim 10, wherein the steps of transforming the microwave

measurements and analyzing the SAR images is performed by the SAR processor.

12. The method of claim 1, wherein the predetermined threshold is 3 millimeters.

13. The method of claim 1, wherein the predetermined threshold is 2 millimeters.

14. A twin roll caster for producing a cast strip, comprising:

a pair of parallel casting rolls arranged to provide nip between the casting rolls, the casting rolls being cooled to solidify molten metal which exits from the nip as a cast strip;

a hot rolling mill for reducing the cast strip thickness;

at least one microwave reflectometer including at least one antenna array, the at least one antenna array directed at a first surface of the cast strip and oriented transversely to the direction of travel of the cast strip, the microwave reflectometer being configured to:

identify cracks in the cast strip for each crack identified, determine a size of the crack;

for cracks equal to or greater than a predetermined threshold size, determine an angular orientation of the crack;

determine a category of each crack based on crack size and, for cracks equal to or greater than the predetermined threshold, angular orientation of the crack; and

a caster controller coupled to the microwave reflectometer, the caster controller adjusting caster operation in response to the category of crack

determined by the microwave reflectometer.

15. The twin roll caster of claim 14, wherein the microwave reflectometer is located between the pair of casting rolls and the hot mill.

16. The twin roll caster of claim 14, wherein the microwave reflectometer is further configured to:

obtain microwave measurements of the first surface of the cast strip;

transform the microwave measurements of the first surface into SAR images of the first surface; and

analyze the SAR images to determine whether cracks are present in the first surface.

17. The twin roll caster of claim 14, wherein the microwave reflectometer further comprises a second antenna array directed at a second surface of the cast strip and oriented transversely to the direction of travel of the cast strip.

18. The twin roll caster of claim 14, wherein the caster controller adjusts caster parameters selected from the group consisting of: casting speed, casting roll separation force, operation of the casting roll brushes and steel chemistry in response to the crack characteristic detected.