Strip rolling is not merely a matter of giving the strip a certain thickness. There are other important quality aspects on rolled strip, such as flatness, surface quality and metallurgical qualities.
Yet, thickness is the key parameter in rolling and the primary target is to keep the thickness as constant and precise as possible.
Hitting the target thickness is not easy. Both users and producers have significant costs for non-conforming strip thickness. One of the main reasons for this is erroneous thickness measurement, since existing gauging technologies are sensitive to other physical parameters than thickness only. High energy radiation technologies, based on X-ray or Isotope, are strongly influenced by the alloy composition, coolant on strip, and temperature of the air in the measuring gap. Despite sophisticated compensation functions in these systems, measurement accuracy is still relying on certain conditions outside the control of the gauge itself. The alloy compositions must be constant and the mill environment around the gauge must be clean and stable.
The Millmate Thickness Gauge (MTG) from ABB is based on the patented Pulsed Eddy Current technology. It is a non-contact gauge without the drawbacks of common systems based on X-ray, isotope or contact measurement. It is insensitive to alloy grade and will measure true strip thickness even in the harshest environmental conditions. The system does not require any information about the rolled alloy. Neither are material specific calibration routines needed. Closer thickness tolerances are made possible by this new gauge that measures as accurately in production as in the laboratory.
Example:
Copper mills usually roll a large variety of alloys. From a gauging point of view, some alloys are more difficult than others. Let us consider Bronze alloys, normally containing large amounts of Tin. A change in the tin content of 1% will change the thickness value with 3% in an X-ray system.
Let us assume rolling of a 1.000 mm thick bronze strip with a tin content of 6.2%. The non-compensated X-ray measurement will be 18.6% to high or 1.186 mm. An X-ray system with calibration based on calibration plates or standard composition for the alloy is not at all sufficient in this case, since the differences between different charges of the same alloy are far to big.
Assume therefore that the rolling mill uses an X-ray system that has a calibration based on information about actual charge composition. Still there are inhomogenities in the charge causing changes in the tin content during rolling of the coil. These changes are experienced to be within ±0.4%, but occasionally larger deviations can occur. The tin content variations cause an error in the compensated measuring value of ±12 µm. Since this inaccuracy is to high to be acceptable a contact gauge has been purchased, which is used for calibration of the X-ray gauge.
The contact gauge is fundamentally material independent, but has a temperature drift that is typically in the range of 5 µm. It is also influenced by bent strip edges and dirt build-up on the measuring tips. The performance of the thickness measurement is now normally within ±5 µm, but occasionally errors in the range of 15 µm can occur. The gauging system consists of two different gauges. It is obvious that the investment cost as well as the cost for maintenance has "doubled". The mill down-time has increased, since there are two separate systems needed to control the mill.
Today this rolling mill has a new alternative for the thickness gauging function. A Millmate Thickness Gauge from ABB will measure this strip with an accuracy of ± 2µm. The purchase cost for a MTG system is half of an X-ray/Contact gauge combination. The cost for maintenance and calibration will be considerably lower. An investment in the new MTG system will increase the thickness target hit rate and will soon pay for itself.