Sticking and scratching problems after batch annealing, including coil compression stress effects

General The completion date of the project was postponed by one year, due to delays in the development of the industrial stress measurement system by BFI at EKO-Stahl. Objective of the project Batch annealing remains a significant workshop in cold plants. Two of its major problems are sticking and scratches, both defects of surface aspect unacceptable for automotive industry. The global objective of the present project was to enlarge the scope of previous studies on sticking, generally done on sheet samples, both with other parameters (atmosphere, steel composition), and by changing size of samples (laboratory tests on narrow coils, industrial tests). The change in size for sticking experiments allows to validate previous results on sheet sample and to experimentally quantify the effect of parameters inaccessible on sheet samples such as coiling tension, thermal stresses, flatness, thickness and thickness profile (global and localized). Another objective was to investigate and quantify scratching phenomenon, particularly as a counterpart of sticking. This can only be made using coils and is easier at laboratory scale (narrow coils). We also aimed at defining connection between force adherence inside coil and sticking or scratching occurrence. These relationships had to be established with the help of a model assessing coiling stresses and thermal stresses, validated by a stress measurement at the inner wrap level of industrial coils while coiling. All the laboratory tests had to be validated on industrial trials and statistical studies made at EKO Stahl, which encounters sticking and even more scratching problems, so the conclusions get more foundations. Comparison of initially planned activities and work accomplished The main deviations from the initial plan of activities come from unsuccessful development of the industrial coiling stress measurement system by BFI. With the industrial coiling stress developed by BFI and implemented at EKO-Stahl, we expected to study the links between 3 things: defect apparition, coiling stresses and parameters influencing the coiling stresses (namely the coiling tension, the thickness profile and the flatness). Without any reliable measurement, this was impossible, and no longer was useful the comparison with existing models for coiling stresses and thermal stresses.