20-High-Mills

Sendzimir 20-high cluster mills commonly carry the prefix ZR, which stands for “Zimna,” the Polish word for “cold,” and “reversing.” This prefix was used the first time to describe Silesia’s first reduction mill. The numbers, and sometimes letters, following this prefix describe the mill’s geometry, the relative size of its work roll, and the widest strip that the mill can roll.

The number immediately following the “ZR” indicates the mill “section” size comprising a combination of the backing bearings, the work rolls, and so on. For example, a ZR 23 has backing bearings with a diameter of 8.858 inches (225 mm) and work rolls with a diameter of 1.578 inches (40 mm). A ZR 32, on the other hand, has backing bearings of 1.875 inches (48mm) in diameter and work rolls of 0.250 inches (6.5mm) in diameter. The significance of the difference is that a ZR 23 can roll ferrous and nonferrous materials down to a minimum gauge of 0.002 inches (50 microns), while a ZR 32 can roll ultrathin nonferrous foil down to 0.0001 inches (2.5 microns), which is 20 times thinner!

Being able to change work rolls quickly and without damaging the strip or the equipment is important. Sendzimir therefore developed a modification to the sprayboards that uses hydraulic cylinders to raise or lower individual sprays, thus pushing the strip out of the way of the work roll to be changed. This modification facilitates work roll change when the strip is in the mill by making the rolls more accessible.

Cluster mills first came into existence in the 1930s. They are characterized by small-diameter work rolls that are supported by a cluster of rolls as shown in Figure 1 (which shows the location of eight backing assembly shafts: “A” through “H” containing eccentric saddles and bearings). More than 600 cluster mills are now operating around the world. Of these, approximately 400 are of the Sendzimir type, in which, with few exceptions, the housing is monoblock with very short columns and rigid construction, resulting in a high mill modulus (stiffness).

In 1995–98, Krupp VDM, Emitec GmbH—the world’s largest manufacturer of metal catalytic converters—the Fraunhofer Institute for Applied Materials Research, and Wuppertal University discovered a new material – Aluchrom YHf – whose cousin, Aluchrom 7Al YHf, promises to meet the stringent new air pollution standards that will soon be imposed on automobiles in the United States and Europe. Patented by Krupp VDM, the alloy contains chromium, iron, and rare earth elements with as much as 7 percent aluminum by weight. The composition can heat up faster than any other material used in catalytic converters because of its relatively high thermal resistivity and its ability to be rolled to as thin as 0.001 inch (0.025 mm) without jeopardizing the material’s operating life. Moreover, the extreme thinness maximizes surface area, and thus catalytic efficiency, without increasing the weight or volume of the unit or its resistance to air flow. Variations on this alloy are being produced by others, but Aluchrom apparently has the greatest life.

ZR & ZS Mills

Nisshin Steel Co. placed the world’s first Sendzimir tandem mill into operation at its Shunan works in 1969. Located in Nanyo, Japan, the plant occupies a site of approximately one-third square mile. Occupying about 18% of the 315-ft (96 meters) total installation length are four Sendzimir stands: one ZR 22N-50 and three ZR 21B-50. Once up and running, the mill’s maximum finished monthly production capacity totaled 18,300 net tons — 35% 400 series; and 65% 300 series stainless steel.

By John W. Turley

THE problem of control of strip flatness in cold rolling has challenged the industry for a long time. The problem can be separated into two parts: measurement of flatness; and adjustment of roll gap profile to correct any deviation from the target detected by the measuring device. The flatness measurement problem has largely been solved, and several flatness measuring devices (shapemeters) are now available. These devices all have their limitations, eg, high cost, some are suitable for low tensions only, and frequent discrepancies may be found between their indicated flatness values and actual flatness (as measured subsequently on the strip). However, in general, for a given application, it is possible to obtain a shapemeter that will perform satisfactorily