Temperature Control in the Hot Rolling Mill
Aluminum sheet and plate products are used for a wide range of applications, including can stock, brazing, automotive and aerospace. These industries demand exacting tolerances and precise mechanical properties, particularly for new, technically challenging high-strength alloys. As a result, the modern aluminum hot rolling mill demands previously unobtainable levels of temperature measurement accuracy for the control of rolling mill bite, pressure, speed, and coolant. To meet this need, Williamson offers two multi-wavelength infrared technologies able to provide the unprecedented accuracy this industry now demands for temperature readings throughout the hot rolling process.
Current Measurement Technologies and Limitations
Some plants use and thermocouple probes to get a temperature measurement throughout the process and make adjustments based on these contact measurement points. Thermocouples - while a contact measurement, are never really accurate and repeatable - the temperature output can change depending on how hard they are pressed onto the metal, they often read lower than the true temperature as they are influenced by air temperature, and need to be sharpened and checked against a calibration device often to be trusted (which pretty much never happens).
Pyrometers are non-contact temperature sensors that measure the infrared energy that is emitted from a target. Aluminum is a notoriously difficult material to measure because it is a non-greybody material, which means that it has a low and varying emissivity that varies at different wavelengths. For these type of non-greybody materials a multi-wavelength pyrometer is the recommended option (see point 1 in this blog post for more information). However, the aluminum undergoes such a dramatic change during the roughing/reversing mill process (from ingot, to slab, to strip) that traditional multi-wavelength pyrometers often need to have an adjustment to the reading so that they can be more accurate in their reading. The needed adjustments are different for different alloys, and also vary by the thickness (pass number) of the aluminum strip.
While the multi-wavelength technology is highly repeatable and accurate under very specific process conditions, any time these conditions vary (and they do), there will be a measurement error. As a result, many plants will have very detailed matrices, models, or recipe systems that will adjust the offset for each pyrometer based on individual alloy, strip thickness, and position (if the algorithms worked for all conditions, they wouldn't need the offsets!). These model and/or recipe systems are extremely complicated and difficult to manage if anything in the process changes or if the pyrometer is not functioning properly, which makes them difficult to rely upon.
A New Approach - MWx
With the increasing demand for tighter temperature tolerances from the automotive and aerospace industries, the existing pyrometer technologies were not cutting it and we set out to develop a new approach