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Flatness errors in cold rolling lead to non-uniform distribution of tensile stresses across the strip width that can cause the strip to break. Reliable measurement of strip tension distribution or flatness is a prerequisite for effective control. Siemens has developed the new SI-FLAT contactless flatness measurement device as a measurement and control package for surface finish demanding requirements.
In the last decade, there has been rapid improvement in flatness measurement and control systems in cold rolling mills so that most rolling mills are now unable to be driven without them. These systems are all contact-based radial force measurement systems, using force transducers in a deflector roll.
However, increasing demands on the strip surface, for example, in the production of brightannealed stainless steel, has brought out the weaknesses of contact-based measurement equipment due to strip surface marking resulting from contact with the flatness roll. Interim solutions have been found by coating the rolls with chrome, rubber, or other materials.
CHALLENGE OF STRIP FLATNESS
With these expensive special coatings, scratches on the strip surface can be kept to an acceptable degree. The disadvantage of this solution is very high investment and maintenance costs. Reversing cold mills for bright-annealed stainless steel thus normally require four flatness rolls: Two of them are in operation, one is ready to be changed if the rubber coating of a roll gets damaged because of a strip break, and the fourth roll is in the grinding or re-coating process. Renewing the coating is required approximately every 3 to 6 months, and extra calibration equipment is required to reduce process downtime. From these drawbacks arose the need to develop a contactless flatness measurement system. Development was handled by the Siemens research center in Munich in cooperation with the Cold Rolling Technologies department in Erlangen. Work began in 1998.
PRINCIPLE OF SI-FLAT
The solution embodied in SIFLAT was found in a periodical excitation of the strip and measurement of the excitation amplitudes across the strip width, as a measure of the strip tension distribution. The excitation force is applied by creating a vacuum between the strip and a sensor plate, which is, located approximately 5mm below the...