At the hot strip mill of Salzgitter Flachstahl, slabs from the continuous casting plants are rolled into hot strip and wound into coils. For this process, the slab is heated to rolling temperature in one of the heating furnaces and then rolled to the desired thickness through several rolling stands.
The thickness profile
During the rolling process, the material acquires a thickness profile depending on the shape of the rolls and their positioning through bending/swiveling, as well as the temperature. This profile describes the variation in thickness across the strip width. A broad distinction is made between flat, hollow, and convex profiles, although hybrid forms can also occur.
Depending on the type and severity of the profile features, different behaviors result, for example, regarding oil redistribution or strip flow in downstream processing equipment. A more convex profile is particularly advantageous in cold rolling, while a flatter profile is preferred for oil redistribution.
Depending on the application, different profile requirements arise, which are described by various parameters. The Cx value (e.g., C40 or C100) describes how much the strip thickness at a defined distance from the strip edge deviates from the thickness in the center of the strip. Connecting the two measurement points creates a profile line. Two further characteristic profile values can be read from this line: crowning and wedge.
Crowning is the difference between the normalized zero point and the vertical intersection with this connecting line. Wedge is the absolute difference between the two Cx values to the left and right of the strip (see figure).
Additionally, so-called ridges can occur, which describe an increase in thickness at the strip edges. If the ridge is too pronounced, edge waviness can occur during cold rolling.
The data system for the development of materials and quality (DEWuQ)
With the introduction of “DEWuQ,” the data system for material development and quality, it became possible for the first time to conduct a plant-wide evaluation of production quality across all production lines. DEWuQ has an internal set of rules that evaluate products according to predefined criteria and block them from manual review if these criteria are violated.
With increasing system capabilities on the one hand, and further new requirements on the other (e.g., bead analysis), the task arose to also integrate profile analysis into DEWuQ.
Based on thickness measurement data, the implemented rules check after hot rolling whether the specifications regarding crowning, wedge, and bead have been met in relation to the customer-specific requirements.
For the evaluation, the selected measurement data are first normalized to zero, as described above, so that the profile data is set to (0, 0) at the strip center. Subsequently, crowning and wedge are calculated based on C40 or C100 points, depending on the profile identifier. After removing the wedge, the normalized thickness profile is checked for ridges.
Overall, this change in profile assessment increases process reliability regarding the profile properties of the hot-rolled strip and ensures consistently high quality for customers. At the same time, it supports the work of the quality assessor, as a significant portion of the produced strips can now be characterized automatically, meaning that only strips with deviations from the quality assessment require manual evaluation.