In a finishing mill, the steel strip is first heated so that a phase transformation to austenite takes place. In the subsequent cooling zone, the strip is cooled in a controlled manner to achieve the desired microstructure (phase composition) through what is known as austenite decomposition. As the microstructure is crucial for the mechanical properties of the steel strip, cooling represents a critical process step for the quality and performance of steel products.

As part of a successful collaboration, experts from AIT, TU Wien and voestalpine at the Linz site have implemented a modern control concept for strip temperature in the cooling zone: a non-linear model predictive control (NMPC) system for strip temperature, which enables a significant increase in temperature control accuracy compared to the state of the art. The basis for this is a real-time mathematical model that takes into account both the temperature development and the phase transformations in the steel strip. Using a bespoke algorithm, the controller calculates optimal control variables in real time, such as the fan speeds for cooling, in order to precisely control the temperature of the steel strip along specified setpoints.

Industrial implementation on a production line at voestalpine’s Linz site demonstrates clear improvements in temperature distribution and significantly increased process stability, leading to reduced variation in mechanical properties, a lower scrap rate and more efficient use of resources. Furthermore, the results provide a solid basis for further research into the direct control of microstructure and steel quality.

The system is already in permanent industrial use and has been fully accepted by plant operators – a clear sign of industrial maturity and robustness. The control concept developed is not limited to a single production line. It is fundamentally transferable to different steel grades and other metallurgical heat treatment processes.

The results were also published under the title “Nonlinear model predictive temperature control of a cooling process for steel strips undergoing phase transformations” in the renowned journal “Control Engineering Practice”.