Automatic Temperature Control (ATC) for hot strip mill
24.06.2025 | Salzgitter Flachstahl GmbH
Slabs from continuous casting lines are rolled to form hot strip and wound into coils on Salzgitter Flachstahl’s hot wide strip mill. To do so, the slab is brought to its rolling temperature in one of the pre-heating furnaces and then rolled out to the desired thickness via several roll stands.
The final rolling temperature and subsequent cooling of the strip have a decisive effect on the technical properties of the rolled steel. It’s crucial here to ensure even heat over the entire length of the strip. As a general rule, it can be said that slower cooling makes the steel softer and more flexible, while rapid cooling increases its hardness with a loss of ductility.
Consequently, an automatic temperature-controlled approval system was introduced on the hot wide strip mill over 20 years ago to assess the final temperature. On the basis of a trained neural network, this system assessed the degree to which the final temperature was met, depending on many additional parameters. In this way it was possible to automatically assess the strips produced on the basis of this degree of fulfilment.
The steel grades and hot wide strip mill processes have been refined in the last 20 years with the result that modifications were required to the existing ATC system.
The existing ATC was to be examined as part of a bachelor thesis and suggestions made to refine or redevelop it. Various machine-learning techniques as well as a rules-based method were examined and compared in the process. It transpired that the database required was not sufficient for a machine-learning technique and it was therefore not possible to engage with customer-focused requirements. Therefore, the company opted for a rules-based system.
The data system interface for the development of materials and quality (DEWuQ) was used as the basis for the rules. DEWuQ already has an integrated rules module, meaning that it was only necessary to formulate the corresponding rules.
Based on measurement results, the rules check before and after the cooling process whether the temperature curves are within the limited tolerances specified. These tolerances are defined as a function of grade and thickness specifications for each measurement point and they can be updated by means of a master data table.
To this end, the hot wide strip is divided into segments in which the data points lie within or outside the limited tolerance ranges. If all the data points are within the tolerances, the product is automatically declared as of “good” quality for this criterion. If variances are found, a manual quality assessment will be required. The technical expert will now decide with the support of automatically generated notes whether the variances are acceptable in order to ensure that the desired properties continue to be met.
Overall, the overhaul of the ATC system boosts process reliability with regard to the technical and mechanical properties of the hot strip, and ensures consistently high quality for customers. At the same time, this system will support the work of the quality specialists as a significant part of the strips produced can already be automatically characterized as “good” and only doubtful strips, already labeled with notes, will have to be assessed manually.