At its Bochum plant for NO grades, thyssenkrupp Steel Europe uses a laser sensor system from nokra to measure the edge waviness of non-grain oriented strip. This optical inline measuring process has replaced the previous practice of manual offline edge-wave measurements. The new system can even handle moving strips, thus enabling the measurement of edge waviness along the entire coil length.
At its NO plant in Bochum, TKSE produces non-grain oriented electrical strip in thicknesses from 0.2 to 1.0 mm. The strip is heat-treated in three continuous annealing furnaces to achieve the physical and magnetic properties required.
According to standard EN 10251, wavy edges have to be measured and documented after the final heat treatment. During the measurements, the strip has to be tension-free.
The offline measurements at the No. 6 annealing furnace used to involve a great manual effort: Specimens had to be cut out at the strip head and tail ends and placed on a flat table. There, the wave lengths were manually measured and the wave heights determined using a wedge, according to the procedure laid out in the applicable standard.
The goal: Making information instantly available
The objective of the “inline waviness measurement” project was to reduce the effort involved in the offline measurements or implement a solution that would completely replace the offline practice. At the same time, the measurements should no longer be restricted to the head and tail areas of the strip. By making measurements at several positions along the coil, it would be possible to obtain additional information about the evolution of edge waves along the entire strip length and use this information as a basis to optimize the cold rolling and slitting processes. Additionally, the customer demanded proof of the system’s measuring capability.
TKSE intended to use the new system not only for stationary measurements, but also for measuring edge waviness along the entire coil length of the running strip – i.e. when the strip is under tension.
The optical wavy-edge measurement system installed at the No. 4 continuous annealing furnace is currently used for stationary measurements. However, nokra’s laser light-section technology is also suitable to measure the waviness of moving strips. This capability had been a key reason for thyssenkrupp to decide in favour of the nokra system.
The project: Inline replaces offline
In January 2022, nokra installed a system of its alpha.fi compact series at the exit of the No. 6 continuous annealing furnace ahead of the cutting-to-length line. During the stationary measurements, the gantry of the system travels along the strip over a length of about 1.5 m. It accommodates two laser sensors, each covering a strip area of 480 mm from the respective edge. However, the gantry is designed to accommodate a third sensor in order to be able to also measure the middle area of the strip, if required.
The system can operate in two measuring modes: The measurements of the stationary strip take over the function of the offline measurements, also with respect to the applicable standards. A novelty is that the system is also able to perform inline measurements of the moving strip under tension.
Measuring mode 1:
For a stationary measurement, the strip is halted so that it will lie still on the integrated measuring table. In this state, the strip is tension-free, as required by the applicable standard. As soon as the strip has stopped, the gantry travels over the about 1.5-meter-long strip section on the measuring table, scanning the height profile and the strip edges. Scanning takes about 15 seconds, the entire measuring process, including braking and restarting of the strip, about 30 seconds. During that time, strip leaving the furnace runs into the accumulator, which had already been part of the line before the measuring system was installed. Usually, one measurement is made at the head, one in the middle and one at the tail end of each coil.
Measuring mode 2:
During the continuous inline measurements, the running strip is guided over two rollers to avoid any contact with the table. The system captures the strip passing the gantry from a defined position. In this case, the measurements are made with the strip under tension.