A set of top and bottom rolls (Pinch Rolls) help flatten the coil before it enters the finishing mill. Pinch rolls provide sufficient reverse bending action to flatten the coil which is bent due to coiling at coil box. Pinch rolls (refer Figure 2) are manufactured from high alloy steel material to resist the harsh conditions observed in Hot Strip Mill (HSM), which is a mixture of stresses generated on the surface of rolls due to hot rolling and corrosion inducing conditions such as steam. The roll material is supposed to be capable of resisting degradation of roll surface, which could happen in terms of any physical deformation (material flow) or hardness drop in a band (locally) or across the length of roll surface. However, such roll surface degradations are observed during inspection (refer Figure 1).
The surface quality of pinch rolls (Dia. - 400 mm, Barrel length – 1700 mm) is extremely critical in ensuring good quality cold rolled product. Any material, hot rolled through pinch rolls having degraded roll surface will exhibit defects that are visible at cold rolling stage only. The flow of material from HSM to Cold rolling mill generally takes anywhere from 10 to 30 days. Thus, poor surface quality of pinch roll poses a threat of quality and monetary loss on the huge tonnage produced through HSM within those 10 to 30 days. Hence, it is very important to have an inspection system (frequent, the better) for pinch rolls.
It is not possible to access the pinch rolls and inspect its surface quality during running condition of line & during short downtime (short downtimes are like 2-4 hours for 3-4 times in a month). Presence of high amount of steam around the pinch rolls make it difficult to have a visual appearance of the rolls during running condition. Presently, it’s possible to inspect the pinch roll surface quality, manually, only during long shutdown. Long shutdowns are 16-24 hours once in 21 days.
Solution is being sought for online (real time) inspection system of pinch roll surface quality. It would be beneficial if hardness measurement feature could be included in the inspection system.