It is important to understand master coil shape problems and how to properly use the slitting line to improve or eliminate the problem. The most common problems in heavy gauge slitting are coil set and cross bow.
“Coil Set” means the strip exhibits a curvature in the direction of its length. It is caused when the fibers on one surface of the strip have been stretched longer than the fibers on the other surface. The strip then curves toward the side having the shorter fibers. The difference in fiber length on opposite surfaces of the strip is most often caused by winding the coil to a very tight radius, or from bending the strip over a passline roll that is too small.
“Cross Bow” results from attempting to correct coil set. In this case, the strip exhibits curvature across its width.
The first attempt at correcting shape in a heavy gauge slitting line should be at the peeler blade and hold-down roll. With the hold-down roll applying pressure against the outside diameter of the coil, the peeler blade is positioned and extended so that the first wrap of the coil can be “pryed” open. The peeler blade can impart a permanent reverse bend in the material. A series of these reverse bends will result in the lead end of the strip being flat enough for threading into the line.
However, as the coil is processed and its diameter gets smaller, the coil set becomes even more severe. An additional piece of equipment is the only way to correct coil set along the coil’s entire length.
Shape correction in either a leveler or flattener is the result of bending the strip back and forth over a configuration of work rolls. The goal is to achieve a dominant number of fibers all the same length and this typically happens by elongating shorter fibers.
A leveler is typically designed with a large number of small diameter work rolls where a flattener employs larger rolls. The goal is to work the fibers enough to exceed the yield point of the steel. It will have some “spring back”.
Material run through a flattener, with its larger rolls, will have greater internal residual stresses since not all the fibers will have exceeded their yield points. This means that machining or heat treating later may cause the strip to go out of flat again.
The smaller diameter rolls and close centerline dimensions from work roll to work roll allow cold working of the strip beyond the yield point for nearly all the fibers. A leveler also typically includes precision flights of back-up rollers, a very rigid frame and a drive to each roll making it more expensive than a flattener.
The key to success is working with your coil processing equipment supplier to assess your needs and develop an equipment and operating solution that meet both processing and budget goals.