Evaluating Air Knife Adjustments with the Aid of a Coating Weight Model

1. Evaluating Air Knife Adjustments with the Aid of a Coating Weight Model R. Wilhelm Objective Control Ltd. D. Maas Pro-Tec Coating Co. A. Hueve Pro-Tec Coating Co.

2. Background: Who are we? • Pro-Tec is a USS-Kobe Steel Joint venture. • Primarily involved in automotive Galvanized and Galvannealed sheet product • Objective Control is a control system design and consulting company.

3. What motivated these tests? • Desire to improve wiping efficiency • Achieve lower coating weights • Increase throughput on low coating weights • Improve uniformity over wide range of products

4. Planned Procedure • Reference Test • Verify present operating characteristics of knives in current use (FOEN 1.5 mm bowtie gap set horizontal). • Compare coating characteristics with those predicted by the model currently used for coating weight control. • Comparison Tests • Determine the operating characteristics of similar knives with 1.7mm flat gap set horizontal. • Determine the characteristics of the 1.7 mm knives set at –1.5° angle.

5. What we learned • Widening the gap appears to have had the opposite of the intended effect. • Narrowing the gap appears to have yielded the desired results. • Direct comparisons of raw data are difficult and can be misleading. • A coating weight model can partially compensate for unmeasured influences and clarify comparisons. • Further testing is required to corroborate our results and conclusions.

6. Experimental Method • Modified factorial experiment • 4 values of line speed covering full range of operation • 3 values of knife distance covering full range of operation • 4 values of pressure for each speed, covering full range of operation • Short dwell time at each point • Coating weight data synchronized to compensate for knife-to-gauge distance.

7. Planning the test with the coating weight model • Coordinates (speed, distance, pressure) chosen to cover full range of operation. • Black dots in the figure represent test coordinates selected using existing control model. • Surfaces represent coating characteristics at different line speeds.

8. Why use a coating weight model? • Assists in planning the experiments. • Selection of appropriate test coordinates to cover the range of operation. • Permits prediction of wiping efficiency at any operating point from relatively few experimental data points. • Can help to identify and compensate for the effects of extraneous variables.

9. Reference Test Results Compared with Predictions • Measured coat weights fell somewhat above those predicted by the existing model, developed 2 years earlier.

10. Compensating for Unmodeled Variables • The model can be normalized to compensate for the effects of unmodeled variables. • The normalized 1998 model is a very accurate predictor of the reference test results

11. Scatter diagram illustrates agreement between model and all data collected. Bounding lines are +/- 5% Outliers are mostly due to misalignment of delayed measurements during transients. New Reference Model(1.5 mm bowtie gap) • Horizontal clustering is due to short term fluctuations in weight, often caused by strip shape or flutter between knives.

12. 1.7 mm gap:Raw Comparison • Data collected with 1.7mm gap compared with characteristics of 1.5mm gap. • Measured weights were lower where the knives have least impact, but higher where the knives have greatest impact.

13. 1.7 mm gap:Normalized Comparison • Normalizing the two models at the point of least knife impact leads to a different conclusion.

14. Causes of Variation • Changes in coating weight between tests could be explained entirely by changes in stripping efficiency due to the changes in knife parameters. • Changes in coating weight from test to test could also be influenced by changes in either withdrawal flux or stripping efficiency, induced by unmeasured variables, such as strip temperature, surface roughness, bath chemistry, etc. • Changes in coating weight during a single test could also be influenced by changes in unmeasured parameters. • Changes in coating weight either during one test or between tests could be due in part to measurement errors induced by strip temperature or other effects.

15. Observations • During the current series of tests strip temperature was known to vary considerably, both between tests and during individual tests. • During one of the tests, the sequence in which the variables were changed was reversed. The raw data from this test deviated most significantly from the expected behavior. • Based on experience and prior testing of the measurement system, significant measurement errors due to temperature on galvanize product are unlikely.

16. Further Observations • Our normalization procedure is designed to compensate for biases caused by a change in withdrawal flux. • Stripping theory indicates that a consistent change in slope in the coating characteristic is a better indicator of a change in stripping efficiency than the absolute weight value. • Comparisons between the 1998 model and reference model for the same knives clearly illustrate the ability of the model to compensate for changes due to exogenous variables.

17. Raw data show coating weight to be substantially higher except where knives have their greatest impact. Changes of this magnitude in the region of least knife impact are unlikely to be due to this small change in angle. Raw data at –1.5°, 1.7mm compared to horizontal 1.7mm characteristics

18. The corrected comparison indicates much less difference. We believe this comparison to be tainted by the fact that one test was run in reverse order. Normalized data for -1.5° angle, 1.7mm compared to horizontal 1.7mm characteristics

19. The surprises and uncertainties surrounding the initial results prompted us to plan and conduct a second series of tests, this time with a 1.35mm gap. Once again, the raw data show a significant deviation at the points of least knife impact. Round 2: 1.35 mm gap

20. Normalized data from 1.35mm gap compared to 1.5mm reference model • Now the normalized characteristics indicate a decrease in weight throughout the operating range.

21. Predicted Throughput Advantage • At 6 psi & 0.5 in, the model suggests that a 1.7mm gap requires the line to run 35 fpm slower than a 1.5mm gap to produce 40gsm. • The 1.35mm gap should permit an increase of 20 fpm.

22. Conclusions • We emphasize that our conclusions are tentative. There are many reasons to be cautious about interpretation of our results: • Flaws in experimental procedure • One test sequence reversed • Non-steady-state temperature conditions • No data for withdrawal flux without air wipe • Small number of experiments • Raw data difficult to interpret • We have not taken operational actions based on these results.

23. Conclusions (2) • One must be very careful in interpreting raw differences in coating weight. • We believe it is much more reliable to examine differences in rate (partial derivatives). • The model-adjusted method has been demonstrated to be effective when comparing data from the same knives at different times. • The model-adjusted results for our small changes in knife angle also yield more consistent results.

24. Conclusions (3) • We intend to perform further testing using improved and more efficient methods. • These will be made easier by the installation of a new coating weight control system, which is currently in progress. • Additional tests are planned on a pilot galvanizing testbed where strip temperature, bath temperature, and surface roughness can be carefully controlled. • Once corroborated, we plan to take advantage of the improved performance in production.