An investigation of the conditions leading to strip adhesion in industrial steel coils

[摘要] During the production of steel strip, a significant amount of work hardening takes placewhen the steel is rolled into thin strips, which are stored in a coiled form. These steelcoils are batch annealed in order to reduce the hardness and restore formability, beforefurther processing takes place. The development of diffusion welds between spirals ofsteel coils, during batch annealing, is of particular interest because it prevents the coilsfrom being unwound for further use. This problem is often referred to as strip adhesion orstickering.In order for strip adhesion to develop, it is essential for some coil spirals to be in contact,while the inter-diffusion between spirals takes place. Furthermore, high temperatures alsoaid in the diffusion process. It is therefore useful to study the temperature and resultingthermal stress distributions in the coil, during batch annealing. The temperaturedistribution allows for the calculation of thermal stress, which is the driving force forestablishing contact between spirals, and in addition to this, the temperature distributionalso provides some clues regarding the likelihood of inter-diffusion.In this study, models of temperature and stress are presented. A two-dimensional finitedifference model for temperature is presented and confirmed by an analytical solution ofthe same problem. Analysis of a three-dimensional temperature model in the third chaptershows that, as far as heat transfer is concerned, a cylindrical coil can be wellapproximated by a solid cylinder with a concentric hole. All the temperature modelingwas done for the interior of a coil. Further discussion in later chapters shows that acylindrical coil can also be treated as a solid cylinder for thermal stress modeling.A long cylinder stress model in the fifth chapter provides some useful insight, as far asstrip adhesion is concerned, even though it is a one-dimensional model that does notconsider the effect of axial heat transfer. The radial compressive stress during the coolingstage of the batch annealing process was identified as a paramount ingredient to stripadhesion. The thermal stress calculations are later extended to include a linear cooling temperatureramp and these results are arguably the most important findings of this study. Accordingto these results, when the a cylinder is cooled in such a way that the outer edge lagsbehind the cooling of the inner edge, by a few hours, the compressive radial stress isgreatly reduced. Consequently, the contact pressure between spirals at the most criticalstage of batch annealing (where strip adhesion occurs) is decreased and the chance ofstrip adhesion developing is reduced.

[发布机构] University of the Free State