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 aluminum infrared temperature sensor 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
Thermocouples
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).
Multi-Wavelength Pyrometers
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 to making accurate and repeatable temperature measurements for the aluminum hot rolling industry. We wanted to eliminate the need for these complex recipes systems and see if we could develop a dynamic algorithm that would automatically adjust the temperature reading to adjust for different alloys, thicknesses and surface character. This dynamic technology completely eliminates any need to define and make adjustments for alloy, pass number or thickness that is so often associated with the reversing mill application and gives the user a real-time accurate temperature measurement.
Over the past 2 years, we have been developing and refining this technology to the point where we have improved the accuracy and repeatability of the pyrometer across the most popular alloys for all passes, without making any adjustments to the sensor parameter settings.
Reversing Mill Sample Data
With its Dynamic ESP Technology, no adjustments to the MWx were required to achieve the following results. These results were obtained using the Reversing Mill algorithm and using the same default parameter settings across all alloys and for all passes. Results for 1000, 4000, and 8000 series alloys are pending.
So Now What?
The MWx technology has a repeatable and accurate temperature reading without any adjustment, so what? The truth is that this level of accuracy is not necessary or integral to the finished product for all different types of alloys and products. There are certain operating windows that the aluminum slab has to hit in order to make good product. For example, the tolerances/temperature window of can-stock (used in the beverage and packing industry) are much looser than those for the automotive and aerospace industry. From what we understand it is around a 30°C window for can-stock alloys and a 15°C window for automotive and aerospace alloys. So as long as the temperature measurement technology is accurate to within those ranges and quality product is still being produced everything should be fine – right? Well, yes, everything would be fine if companies were producing quality product. However, the lack of real-time accurate temperature measurement means that the rolling process is not optimized and there are many areas for efficiency gains. For example, real-time temperature knowledge allows you to better control the speed of the rollers which results in quicker throughput while maintaining the same level of quality. And in the aluminum hot rolling business, efficiency, throughput and quality are of the utmost importance.
Speaking of quality, the final quality of the product is dependent on the temperature history of the aluminum throughout the rolling process. Many automotive and aerospace companies demand the highest quality from their suppliers and if they are quick to change suppliers if the product quality is not up to par. With the demand for automotive and aerospace aluminum expected to jump in the upcoming years, there is a great opportunity to be able to supply those industries. So while the existing temperature measurement technologies might be working fine and there is a detailed plan and defined process conditions, it might be worth taking a look at this new technology to see if it can help improve process efficiency and quality.
For more detailed information on the MWx and the specific measurement points along the aluminum hot rolling mill click the link below to download our comprehensive application guide.
