Improvement of rolled strip surface and geometry by advanced automatic diagnosis of surface defects and optimisation of friction in the roll gap

In this multi-partner, multi-national project the main objectives were to determine ways of improving rolled strip surface quality and geometry. These objectives were pursued by mathematical modelling, laboratory experiments and by full-scale trials in hot strip mills. Preliminary FE modelling was carried out to show how the evolution and formation of defects could be studied through different passes in a hot strip mill, but further work required characterisation of friction at the roll/stock interface. A technique for determining mean friction coefficients using hot plane strain compression testing was developed, but aspects of the available test machine meant the applicability of the results was limited. A programme of tests was completed using an experimental hot friction test rig with High Chrome Iron and High Speed Steel discs in both the un-oxidised and oxidised condition. A variable friction relationship has been derived and used in a model of the first pass of a finishing mill. A comprehensive advanced concept for an automatic on-line diagnosis system has been developed for hot strip. This integrates the results of surface inspection and relevant processing conditions, and feeds back its findings to the process control. The development of a new system for surface inspection, combining both shape measurement and surface inspection, allows the assessment of local strains to be included in the potential causes of defects such as surface or edge cracks. The prototype for this system has been successfully tested in a hot strip mill. Laboratory trials were carried out to investigate primary and secondary scale growth, and to determine the effects of descaling and work roll lubrication on the hot rolled strip surface. A recently installed Parsytec surface inspection system in a hot strip mill was used to investigate the effect of finishing temperature and inter-stand cooling in reducing tertiary scale defects. This has proved that a modern surface inspection system can be used to evaluate the effect of the settings of a surface cooling system. A literature study on tertiary scale was conducted and the findings of the study have also been proven by mill trials. Strip chemistry has been found to have a big influence on the appearance of tertiary scale defects, and ways to inhibit tertiary scale have been found that don't adversely affect the rollability of certain products. An automatic system has been installed in the finishing stands of a hot strip mill to measure changes in back-up roll bearing oil film thickness as a consequence of force and speed changes. Models have been developed and implemented at each stand to allow initial comparisons with the results obtained from the automatic oil film tests. The comparisons show quite good agreement when the test data are consistent but improvements are still required in the data capture system and data pre-processing software before the new gap compensation can be allowed to shadow the existing compensation.