Bubble Size Calculation and Telescoping Problems in Hot Strip Coiling

A hot strip mill rolls steel slabs to produce steel strips varying in thickness from 1.2 mm to 12.7 mm and in width from 550 mm to 1600 mm. The output from the hot strip mill is in the form of coils and hence the coiling process is vital as it determines the final form of the product. The formation of a coil begins when the strip exiting the pinch rolls is bent over the mandrel by the coiler rolls. These coiler rolls, known as wrapper roll No. 1 (or bending roll), wrapper roll No.2 (or guiding roll) and wrapper roll No.3, play an important role in the formation of the first few wraps. It is observed in practice that a bubble is formed on the very first wrap during coiling and the bubble subsequently diminishes and disappears after more wraps are coiled. Telescoping is one of the major coil shape problems observed in hot strip mills and it causes loss to the mill directly and indirectly. Though the primary eye telescoping is attributed to camber or wedge of the strip and delay in shortstroke guiding, it is the bubble formation that initiates secondary eye telescoping. This paper explains the formation of the bubble and describes a model which takes into account the centrifugal force and springback at the head end of the strip to estimate the bubble size. Once the bubble size is determined, the Blazevic's formula can be used to calculate the number of wraps needed to fill the bubble. The analysis of bubble formation and determination of its size are major steps towards understanding and remedying telescoping problems.