BACKGROUND

Steel coils are side trimmed with rotary knives as part of the manufacturing process: to remove edge defects, to minimise edge defects during subsequent processing, and to produce the ordered coil width. Cut edge quality is affected by a number of variables such as knife gap, knife sharpness, steel coil grade and thickness. Figure 1 shows a coil with good edge quality. Occasionally, poor edges as illustrated in Figures 2 and 3 can be produced, resulting in downgrading, re-trimming or scrap coils, resulting in considerable cost, particularly if the effects are not quickly noticed and remedied. Some edge defects such as burrs can frequently damage rollers and cause surface-quality problems. In traditional trimming lines, up to three or four coils could be affected before the problem was noticed and corrections attempted, but of course unless the technical relationships between edge cut quality and cause are well understood these corrections may not be adequate. 

EDGE TECH

To address these issues Unilux, a company that specialises in industry vision technology, has developed the Edge Tech remote inspection system using a combination of lighting, cameras and powerful software that allows detailed inspection of the trimmed edge on both edges of the strip immediately after the trimming operation is performed. Strip edge quality, rotary side trimmer knife wear and knife cracks can be seen in real time on a display, corrected, and then checked to ensure that the problems have been eliminated. Edge Tech has applications in standard coil processing lines, pickling lines and galvanising lines.

Because edge quality and knife wear can be seen in real time the necessary plant adjustments can be made long before the coil is complete, eliminating the need for additional processes like re-trim, downgrading or complete coil rejection. Figure 4 shows a coil with a burr detected as it is just beginning.

In order for these benefits to be realised, operators must be In order for these benefits to be realised, operators must be empowered with the following information while trimming:

1. Immediate knowledge of a defect (see Figure 4)

2. Ability to identify the defect

3. The proper corrective action to address the problem

4. Immediate feedback that the corrective action resolved the problem

5. A record of the problem so that further corrective actions can be assessed to ensure the issue does not happen again

EDGE TECH SYSTEM FEATURES

1. Immediate viewing of the coil edge right after knife trim operation on both edges of the strip

2. Optimal cut-to-break ratio defined for benchmarking

3. Coil edge quality captured in real time

4. Examination of the entire circumference of trim knives as represented by the quality of trim

5. Allows for knife setting adjustments

6. Inspect edge detail in slow motion with pause, play and back-up capabilities

7. 8x magnification of the strip edge

8. On-screen measurement of anomalies for problem assessment

9. High definition image capture and display

10. On-demand image storage

11. Machine learning features provide faster response to defects identified as being critical

BENEFITS

Data from Edge Tech installations is indicating the following benefits:

1. Inspect proactively and document production data

2. The short time scale to identify problems, make adjustments and verify the success of corrections is possible after less than 100m of coil at full production speed

3. Higher yield and productivity resulting from less on-site downgrading. Re-trimming and scrap plus better inspection has also reduced the number of coils rejected by customers

4. The return on investment can be measured in numbers of coils instead of months. Because there is more confidence in coil edge quality there is less need to inspect coils further down the process line and to schedule jobs more efficiently

5. The large amount of data produced is an ideal source of information for all sorts of process improvements, using cause and effect analysis and statistical process control

6. Improved operator safety by limiting interaction with the line to only when necessary

7. Retrofittable to existing lines

The Edge Tech system is mounted within 1m downstream of the knives, as shown in Figures 5a and 5b. Images of the edges provided immediately after the cut not only enable operators to assess the condition of the edge, but also of the knife and its effect on the coil trim. By knowing the knife wear, operators can either change knives early or extend trimming use, instead of just using a designated trim distance for knife changes. This becomes even more critical with new knives designed to work with the growing number of high strength steels.

At any time during the processing of each coil, operators can look at a monitor in the safety of the control room or pulpit to see if trimming is within tolerance and see if the knife requires adjustment. A visual display of the knife’s cut-to-break ratio can alert them when the ratio is out of range, enabling them to make an adjustment or just fine-tune the settings for the best results.The inspection system then allows them to verify immediately that they’ve made the correct adjustment. Before Edge Tech and it's knife circumference monitoring capabilities, there would be one to three more coils requiring secondary action.

DEFECT IDENTIFICATION

Figure 6 shows some typical screen displays with defects highlighted. Figure 7 shows a knife crack, how it appears on the Edge Tech monitor and where it can be seen in a processed, rewound coil. Detecting the knife crack as soon as possible allows the operator to schedule knife replacement to minimise coil re-trimming.

DATA PROCESSING

The Level 2 interface to the mill collects data pertinent to the coils under inspection and they become part of the records used for documenting quality for customers, making this indexed information available for internal review. As operators make adjustments to trimming knives, the Edge Tech System can automatically capture images and permanently store all the information in data packets related to the coil. Process engineers can also reference the data when developing solutions to specific problems.

With the automatic image collection and storage capability, image-storage capacity is limited only by the storage space provided by the mill or coil processing facility. Further, IT administrators can set up internal systems to store those images and other production data in folders that will make them more accessible to personnel who need the information. The net result of the Level 2 computer interface tie-in is that Edge Tech now becomes an integral part of a coil management system instead of an add-on. This automated data capture can become critical for pinpointing actions taken because of metallurgical differences in each coil.

ACCOUNTING FOR METALLURGICAL VARIATION

Even though a number of coils being processed consecutively may have the same composition specification, a number of factors in the rolling process can change their properties in highly significant ways. Changes in strength or temperature can change metallurgical properties which, in turn, can affect how the trim knives interact with the coil. While the differences in metallurgy may be imperceptible to plant operators, their effects on trim quality can be picked up by the Edge Tech cameras, and operators can make the necessary adjustments.

The enhanced view of coil edges allows for more involvement from plant metallurgists to isolate and monitor the effects of mill operations on coil quality. This inproduction perspective is then used to set protocols for critical mechanical settings and, as a secondary level of response, the effects of changes in upstream processes such as coil heat treatment.

Figures 8 and 9 illustrate the differences in cut/fracture surface and edge profile of hard and soft steel sheets. It should be noticed that the harder steel has less knife penetration but more tearing and end distortion.

The ratio of these values is called the Cut-To-Break ratio and is based on the material strength (MPa) and thickness:

• 1021/22 stainless steel (and many of the higher strength low alloy steels): typically 10% cut, 90% break (10:90), but some may require as little as 5% cut with 95% fracture

• 1005/06 mild steel = 25% cut, 75% break (25:75)

• 1001/03 aluminium = 50% cut, 50% break (50:50)

• Some mills may also choose to set a minimum and maximum for cut-to-break if they run a consistent hardness, eg: 20:80 minimum to 25:75 maximum

Edge Tech allows better control of this ratio by numerical algorithms versus ‘trial and error’.

The operator is able to determine that the proper cut-tobreak ratio is being met, and should any variation outside of an acceptable range occur, the operator is alerted in real time and immediate action can be taken.

Incorrect depth of the cut can lead to burrs at the bottom of the strip which in turn can damage the surface of work rolls, resulting in surface defects on future coils.

THE EFFECTS OF KNIFE VARIABLES

Knife manufacturers have long-established guidelines for the identification of common defects that can occur. Mills without edge inspection typically replace knives based on time in use or linear product runs such as every ‘X’ number of hours or in ‘n’ x 100km based on a ‘guess’ that it is time to make a change.

In-line inspection systems have established a new standard for a detailed view of the strip edge while in production which means new ways to increase the life of knives and better control of preventative maintenance of knife changes. With visual feedback from Edge Tech, they can make an educated decision and extend the knife life by up to 1.5 times or more, but have the ability to replace them sooner if they start to deteriorate.

The effect of variables that can impact knife life, such as different grades of steel, upstream variables, or metallurgical properties can be monitored directly. A real-time visual indication of these variables can be used to determine and document the ideal knife setting based on the historical data for each grade of steel. A mill could also use this information to build a history of knife quality, such as performance and life from different manufacturers.

Lastly, a mill can use the documented data to optimise run time based on the history of each type of knife on different materials. Teams can identify at what point in the run time would an adjustment in knife settings extend life or determine the true effects of adjustments commonly made at the end of the knife life to finish a run, such as increasing lap or downward pressure causing damage to work rolls.

CONCLUSIONS

Inspection systems eliminate trial-and-error. Operators respond to a quality concern by correcting the issue immediately, before any further damage is done. In this example, technology empowers the operator to no longer guess if corrective action will solve the issue but to know it is correct, and document their experience for repeatability.

Further consistency is achieved via the ability to establish ideal settings for repeatability of each coil and job. Operators follow a set process, react immediately based on established protocol and confirm results in real time. Mills can now build a library of observed cause and preferred reaction, which can lead to further enhancements such as the automation of knife settings by the mill based on real-world scenarios as experienced by mill personnel on mill equipment.

Mike Simonis is President, Unilux USA, Saddle Brook,
New Jersey, USA

CONTACT: unilux@unilux.com