Optimisation of the rolling process (pass sheduling) to avoid roll spalling and surface defects

This project has focused on rolling conditions that cause roll spalling or surface defects on the strip or plate. Roll spalling is a type of fatigue failure. Fatigue crack initiation during rolling has been modelled for cyclic loading using a non-linear isotropic/kinematic hardening law. The results have been used together with selected fatigue initiation model for analysis of spalling failure and prediction of roll lifetime. Several sets of data from production have been used for the final tuning of the roll spalling model. The contact pressure distribution between BUR and WR along the axial direction has been studied in three different mills. A modification of the BUR taper profile for two of the mills reduced the peak pressure with about 30-40 %. Surface defects have been observed and analysed in a cold rolling mill. Shelling of the strip is the main cause of severe defects, repeated marks on the strip. Variation of the rolling parameters has no or very small effect on the surface structure unless the temperature reaches 90 °C during the last pass. A modified pass schedule can decrease the total rolling time of almost 25 %. In a plate mill, when rolling thin plate, the head and tail ends are so cold that excessive compressive loads generate marks in the work rolls. A model indicated that the load in a 'static' analysis is of the same order of magnitude as the dynamic load. An industrial solution was made according to this result.