Strip can be traded by length or by weight. The end customer is mainly interested in price per area of strip, as long as the strip thickness is within the tolerance limits. When strip is traded by length, it is obvious that the strip producer can save material cost by rolling closer to the low tolerance limit.
But also when strip is traded by weight, a producer can benefit from producing thin material. By being able to provide more square meters per ton of strip, he will give his customer more value for the money and thus be more competitive.
In both cases it is important to know the thickness of the thinnest part of the strip, with the best accuracy achievable. In the production process, the thinnest part of the strip should then be controlled to be just above the lower side of the tolerance limit.
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. Closer thickness tolerances are made possible by this new gauge that measures as accurately in production as in the laboratory.
The repeatability of the gauge is 0.3 µm and the total inaccuracy in the mill is 0.2 %. MTG measures the thickness 150 mm from the strip edge. This position of the measuring area has been chosen to be well away from the outer part of the edge zone. Yet, it will give a true and representative value of the thinnest part of the strip, independent of uncertainities in the strip profile.
Example
Let us assume rolling of a strip 1200 mm wide and 1.000 mm thick. Further, assume that low side tolerance is 0.975 mm and that the profile could be described according to figure 1. (The percentage relation indicates the probability for a certain profile.)
Let us consider two measurement alternatives:
- With an X-ray gauge in the center. We assume that total gauge accuracy is ±4 µm. Thickness is controlled towards a value of 1.000 mm at the center.
- With Millmate Thickness Gauge, that has a total gauge accuracy of ±2 µm. MTG measures 150 mm from the edge. Thickness is controlled towards a value of 0.985 mm at this measuring position.
If we use the above assumptions as the basis for a calculation of achieved thickness distributions in the two cases, we will get a result according to figure 2. With Millmate Thickness Gauge the mean strip thickness is 0.5 % thinner and still even less material is out of specification. As the mean thickness is thinner, the result will be more square meters per ton.
From the example we can also see that the spread in thickness is smaller in the case using MTG, leading to a more consistent product.