Sheet steel is an essential material for our modern lives. With its strength and flexibility, steel is the ideal material for industrial and consumer applications, from the buildings we live in, to the cars we drive, to the containers that hold and transport our food. It can be recycled over and over again, reducing landfill waste and saving energy.
With the ever-growing concern for efficiency and sustainability, steel producers strive to provide quality strip in a manner that maximizes mill yield and minimizes scrap material while meeting stringent customer specifications for thickness, width, hardness, and strength. An excellent way to accomplish this is by processing the material through a cold rolling mill. Cold rolling is a metal forming process in which a sheet of metal is pressed through a pair of rolls to reduce thickness, increase strength and improve surface finish.
The capital investment in a rolling mill is substantial. In order to achieve the maximum ROI and shortest payback time, the mill needs to produce high quality sheet at the fastest possible speeds, 24 hours a day, seven days a week. The high speed production achieved by modern cold rolling mills requires reliable, robust thickness sensors that can keep up in real time; accurate measurements are needed within milliseconds to optimize mill throughput and efficiency. The wrong sensor choice will result in inferior or out-of-spec product quality, lost production time, wasted raw materials, and inefficient plant operations. An idle mill costs money both in terms of labor and lost product revenue, but production volume is only half the picture; customers must be satisfied with the quality of the material produced.
To achieve high quality steel strip, Automatic Gauge Control (AGC) is an essential component of modern cold rolling mills. Some of the key AGC input parameters are speed and tension. Conservation of mass dictates that the mass per unit time entering the mill must equal the mass per unit time exiting the mill, so as the material is rolled thinner, the speeds must increase. If the drive motors are off, even by a few centimeters per minute, the strip may break or cobble, with catastrophic results. There is a delicate balance between the reduction caused by the mill force, and the reduction caused by drawing (extruding) the material through the gap.
Thickness gauges are primarily used to look for dramatic deviations in thickness. Other than that, their feedback is used to check the predictions of the mass flow models. Small changes are corrected by slight adjustments in speed and tension, as the motor power can be controlled very quickly. Larger changes are compensated by the hydraulic cylinders in the mill stand, which can react in a matter of milliseconds.
There are several choices in thickness gauge technology, including direct contact or optical laser, but only x-ray gauges provide the speed and accuracy required to operate a cold rolling mill. X-ray based sensor systems provide extremely accurate high speed/low noise measurements, permitting producers to realize material savings and quality improvements.
Read this X-Ray Strip Thickness Gauge Specification Sheet to learn about other advantages of an x-ray-based thickness sensor, including:
- Compliance with the IEC 61336 “Thickness Measurement Systems Utilizing Ionizing Radiation – Definitions and Test Methods” guidelines
- Immediate digitization of the analog signal so the data can be processed without time consuming ADC/DAC conversions. The reduction of a few milliseconds of process delay time can assure the AGC has time to fully correct any strip thickness deviations.
- Data archiving can be configured to accept and record data from other sensors within the mill. Any data point that is available to the mill computer through an Ethernet connection can be collected by the system to allow for comprehensive data analysis, thus permitting the pairing of thickness measurement output to mill tensions and speeds in such a way that complies with the IEC 61336 testing standards.