Application Summary
Continuous casting process lines used for manufacturing steel, all around the world, contain components that undergo extremely high pressures and temperatures, and they often run for weeks without stopping. When the molten steel is ready to leave the furnace, it goes into a water-cooled mold and forms a slab. The slab comprises a solid outside layer that surrounds a molten core. The metal then cools further as it passes through several water spray stations.

Typically, several pairs of rolls arranged in segments on a 65-foot radius contains the slab while it solidifies. During this time, each pair of rolls must maintain a gap tolerance of 0.040 inches. As this takes place, a dedicated data acquisition system continuously monitors the processing equipment for certain variables such as temperature, cast speed, and mold behavior. After the slab completely solidifies, it is cut into lengths according to the customer’s order. The next step takes it to a hot strip mill. Here, the slab passes through a furnace where it is reheated to a uniform temperature before it runs through a processing line composed of a series of rolling stands. The hot slab is descaled and run through a roughing mill to further reduce its thickness. The slab gradually reforms into a long bar, runs through a series of finishing stands, becomes a sheet of steel, and finally reaches a thickness specified by the customer, typically less than 1/8-inch. The steel sheet is then coiled at a high rate of speed and either shipped to the customer or sent to a cold-rolling mill for further processing.

Each step in the process is monitored to ensure that all the mechanized equipment remains within operating tolerances, which guarantees that a quality product is formed in the hot-mill finishing stands and remains so as it runs through the cold mill where high-speed rotating rollers form the strip. One common but serious problem that often shows up in a roller stand is a vibration that causes it to lose its ability to maintain critical thickness accuracy of the steel coil.

Potential Solution
Most mills employ one or more of a variety of permanent, distributed data acquisition systems that continuously monitor temperature, vibration, force, displacement, and speed. In addition to the permanent data system, a portable system is used to process vibration and speed information from accelerometers and tachometers located on the rolling stands when certain problems crop up. They measure roll speed and vibrations at each reducing stage that might be caused by faulty rollers and roller bearings. The data acquisition system measures and analyzes these vibrations along the process line to help pinpoint the source of the problem. Resident software then defines the source of any abnormality in the process line, such as worn rolls, faulty bearings, and interstand tensiometer problems.

IOtech’s Solution
Ken Ives, an independent consultant from Lebanon, Ohio, visits numerous steel and tin mills every year, measuring anomalies that can eventually cause serious quality and maintenance problems. Although he occasionally uses the onsite data equipment, usually he finds it limited and prefers to use his own IOtech ZonicBook for measuring vibration and speed from his customer’s accelerometers and tachometers. Ives finds the ZonicBook convenient to carry on trips to different facilities and easy to set up with his laptop computer anywhere in the plant.

“Vibrations at one stage on the line can upset the roller calibration not only at that stage, but also at another stage downstream from it,” says Ives. “I place capacitive type accelerometers next to the screws located on top of the mill that can measure very low frequencies.” Vibration and tachometer measurements are synchronized and correlated using an FFT algorithm with a waterfall presentation. “This maps out a graph of events that defines the rolling coil of steel. I record this high-speed information for two to three minutes, then I use special software to process the data and highlight the critical frequency information for the bearings and the mesh frequency of the gear teeth,” continues Ives. The ZonicBook also takes information from the thickness gages at the end of the mill. The system tracks the tachometer and thickness perturbation data from each rolling stand in a specific order. After Ives analyzes the accelerometer signals he can identify the rolling element on the stand that produced the thickness variations.

“Some of the data we gather with the ZonicBook are quite revealing,” says Ives. “For example, we had a problem with one line in a tin mill that repeatedly yielded substandard surfaces. We discovered that the spindle gears were generating a 161-Hz forcing function that produced 5th octave chatter.” The geared couplings contain as much as 0.020-in. of slop. Ives had the geared spindles replaced with universal joints at that location and eliminated the chatter and increased the tin mill’s quality. By comparison, the universal joint slop is typically only 0.002-in., a 10 times improvement.

“The eZ-Analyst software from IOtech is easy to use and presents the data in a number of formats that is convenient and useful,” says Ives. “In addition, I use third-party software to insert the cursers and calculate critical frequencies. I examine the waterfall presentation and use a color format to highlight the vibration intensity.”

Conclusion
Ken Ives, an independent consultant, uses the IOtech ZonicBook to measure vibrations and analyze problems on cold- and hot-strip rolling mills as well as continuous casters. He finds the ZonicBook fast and easy to set up and use. The software also provides him with a variety of data presentations that help him pinpoint the source of the problem. The FFT algorithm and waterfall presentation combined with eZ-Analyst software are extremely helpful to him in diagnosing faults.

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