Steel and aluminum go through several processing steps before they are ready to be formed into finished products. Hot rolling is widely considered the most critical step to make uniform sheet metal of specific thickness, depending on the end use. Hot rolling of steel occurs at very high temperatures, above the recrystallization temperature of the material, producing malleable sheets that can be formed in a variety of shapes. The process produces sheets that can be extremely long, which are then rolled into coils. While these coils are tested to ensure they meet certain specifications, the process can be challenging, as explained in the thefabricator.com article, Troubleshooting sheet metal defects:
“Aside from the normal tolerances of the reheating furnace and the natural cooling that occurs during rolling, obtaining identical properties from coil to coil is made more challenging because processing speeds are as high as 30 miles per hour…each 10- to 20-ton master coil can have different properties based on how it was rolled. These properties are determined from a tensile test…Tests are taken from the coil head end or tail end because that is where samples are easily obtained. Mill practice dictates whether this sample is taken from the quarter-width or center-width position, and it will likely be in the rolling direction (grain direction) of the coil.
Process control is supposed to ensure that the coil end test is representative of the entire mile-long coil across the width, but the coil ends are subject to the greatest likelihood of variation in temperature, cooling rates, and thickness spikes. The edges of the coil width are also where variation can occur. The edges cool differently than the body of the coil.
If the coil ends are not within the ordered specification requirements for tensile or thickness properties, then a lap or two is removed, and the coil is retested. Once satisfactory properties are obtained, the coil is weighed and shipped. Remember that if you are ordering based on chemistry only, these coil ends have the same composition as the body of the coil, so it can be shipped to you in spite of potentially having tensile properties outside of what you typically receive.
Furthermore, if you are ordering mild steel to ASTM specifications, only chemistry is required to be satisfied.”
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The hot strip steel mill is universally considered one of the harshest environments for process instrumentation and sensors but as the material speed and temperature increases, so does the importance of the measurement values. To ensure product uniformity, hot rolling mills need reliable, repeatable process control data to identify operating issues; off-gauge and off-tolerance product results in the downgrading or even scrapping of coils. However, when hot material exits the rolling mill, the vertical position and orientation of the strip is not sufficiently defined to allow consistent, accurate profile measurement.
Stereoscopic x-ray profile gauges have the ability to determine both the cross-thickness profile and the physical position of the strip in space. By using a narrow x-ray fan beam to measure the strip, the system can record the height of each point of the strip in the time domain, and the flatness, or shape, of the strip can be calculated.
- Maintaining Dimensional Quality in the Hot Strip Mill
- High Speed Archiving of Thickness and Profile Data in the Hot Strip Mill
- The Benefits of ‘Hot’ Coating Weight Measurements to Coating Control