The hot rolling mill is used to heat up steel products to a temperature where they can be rolled out or shaped into different products like strip, plate, rods, bars, wires, or rails. Once the steel is formed into its desired shape, it is cooled so that the mechanical properties of the steel are locked into place. Needless to say, temperature control is critical throughout the hot rolling mill both in the heating and cooling zones. How to accurately measure steel temperature in the cooling zone can be a challenge, but here are a few tips to get the best results.
How to Measure Wire in the Cooling Zone:
After the wire has been shaped, it is coiled and laid on a cooling conveyor to be air-cooled. The wire is generally thin enough so that it can cool relatively quickly with just air and fans. Temperature still needs to be measured at this area of the mill to make sure that the product is cooled in time before it is packaged to be sent off to a customer.
Temperature measurement can be made after the wire has been bundled up either by a contact thermocouple or a non-contact infrared pyrometer. This measurement is usually used to make sure that the wire bundle is cool enough to be handled by other operators and machines. While this is an important safety check, this measurement doesn’t allow you to change the operating process (increase the fans) if it is still too hot.
For plants that are interested in optimizing their cooling process, it is better to take a temperature reading before the product is packaged, so that they can adjust the process controls in real time. One of the challenges with wire is that it can have a very small diameter; and for some pyrometer technologies, you would need to ensure consistent and accurate alignment in order to assure a good temperature reading.
Dual-wavelength pyrometer technology automatically compensates for misalignment as it is designed to measure the hottest temperature it sees in its field of view. Single-wavelength pyrometers require a full field of view to be able to give an accurate temperature reading.
How to Measure in the Strip and Plate Cooling Zones:
The cooling process for strip and plate mills is different than at the wire mill. For these products, water is the main form of cooling the steel. The cooling rate is a critical factor that affects the physical properties of the steel – especially for high-tensile strength and dual-phase steel alloys. Accurately monitoring the temperature throughout the cooling process ensures quality product and an efficient process. There are two main areas of measurement interest – the laminar quench and the coiler.
Laminar Quench: Steel goes through a “water shower” at the laminar quench, where it is rapidly cooled in order to achieve the right mechanical properties. Dumping a whole lot of water onto the steel introduces a whole bunch of problems for traditional infrared pyrometers. Mainly, the pyrometer has to look through standing water and steam at a low temperature in order to get an accurate temperature reading. So what many plants do is move their pyrometers far away from this quench area and use air knives to blow off excess water to get a repeatable temperature reading. While the temperature reading might be more repeatable, it is not measuring at the location of interest near the quench area where you can have real-time process control adjustments. Williamson’s specialty-wavelength pyrometer (SP-WA-21) is specifically designed at a wavelength that can view through 3-5mm of water and steam with little to no interference. Measuring in the quench zone allows for better process control and can reduce overall energy costs from overcooling, all while assuring the desired quenched product properties.
Coiler: Temperature control at the coiler is another critical area to measure as it is often the last part of the HRM where you can confirm the steel has cooled enough to achieve the right physical properties. Like the laminar quench, there are a number of interferences to deal with – steam, water, scale, dirty-optics and low temperatures. Dual-wavelength pyrometers are the ideal choice at the coiler as they are able to best compensate for steam, scale, and can measure low-temperatures. Accurately monitoring the temperature at the coiler has the benefit of preventing over cooling and reduces the associated energy costs.
How to Measure in the Rod and Bar Cooling Zones:
Rod and bar mills can have a combination of both air and water cooling. Often times after rods/bars/tubes have been formed into their desired shape, a walking beam conveyor is used to cool the newly formed products. Like other steel products, cooling the steel at the correct rate is critical to achieving the desired mechanical properties.
Similarly, there are a number of challenges associated with temperature measurement in the cooling zone. These include – scale, emissivity variation, misalignment, steam, and low temperatures. Single-wavelength pyrometers cannot compensate for scale, emissivity variation and misalignment. Two-color pyrometers cannot view through steam, cannot measure low temperatures, and are only OK at compensating for scale. Dual-wavelength pyrometers are ideal for this type of measurement because they can minimize and eliminate all of the above listed interferences and temperature measurement challenges.
With accurate temperature measurement at the cooling zone, operators can adjust the line speed to optimize the process while still maintaining quality product.
Temperature control in the cooling zones of the HRM plays a critical role in determining the final physical properties of the steel product. Accurate temperature measurement is crucial to optimizing the steel making process. Selecting the right wavelength technology will give you the best results to optimize your hot rolling mill. The cooling zones are just one area of the mill where the process can be optimized. To learn more about how to optimize other areas of the steel making process through proper steel infrared temperature sensor selection, download our white paper, Understanding Infrared Pyrometers for Demanding Steel Mill Applications. If you have any questions about which pyrometer technology is most appropriate for your application, please don’t hesitate to reach out to our team with any questions.
