Rolling & Efficiency of Cluster Mill
Cluster mills are used for rolling of either blooms or slabs. Cluster rolling is similar to the traditional break down rolling sequence with a series of two, three, or four two-high mills arranged sequentially, with the direction of theoretical force reversed at each pass. This type of arrangement eliminates the undesirable axial thrust associated with an arrangement of two adjacent two-high mills, simplifying the arrangement of rolls, stands, and drives and resulting in improved efficiency.
Generally, a four-high reversing mill is used instead of the three-high cluster mill. Utilizing a four-high reversing mill requires fewer passes and less raw material wastage, resulting in improved efficiency and cost savings for the steel maker.
Cluster mills may be constructed that have the housing for all of the rolls in a single cast frame, allowing for a greater stiffness of the system and greater breaking forces than a four-high reversing mill. The individual rolls of the cluster mill may be connected to their drives through either universal joints or multiple gears, depending on the size of the mill. The configuration of the series of rolls in the cluster mill facilitates a faster rolling speed than is possible with a reversing mill.
The finished product of a cluster mill is most often of a lower quality and smaller size than that possible with a four-high reversing mill. This is due to the fact that it is impossible to guarantee a completely level product throughout the mill passes. As a result, excess energy is often required to decrease the size of the product and straighten it.
In addition to its visual differences in the product, cluster mills also require more passes than their four-high reversing mill counterparts. This is because the cluster mill does not have the same large reduction capabilities in each pass as a reversing mill.
Another factor affecting the quality of the finished product is the angle of rolls. The use of an angle less than 0 degrees (or greater than 90 degrees) will cause the rolls to lead to each other instead of pushing against each other. This leads to a decrease in the amount of reduction possible in the pass, leading to an increased amount of passes required to achieve the desired size of the product.
Cluster mill technology has been around for some time and can provide many benefits for certain rolling operations. Advantages include improved efficiency and cost savings as well as improved quality of the finished product. The main issues/drawbacks when it comes to the cluster mill are extra passes and higher energy requirements to compensate for the decreased product quality and increased size discrepancies between passes.
