Z-High® Mills were developed as a compromise between the 4-High mill and the 20-high. The work roll on the 4-High is often too big, resulting in roll flattening, while the work roll of the 20-high can sometimes be too small, resulting in skidding. The work roll of the Z-High® Mill is just right.
For example, a typical 50” (1,250 mm) 4-High might have an 18” (457 mm) work roll, while a similar 20-high mill might have a 2.5” (64 mm) work roll. The comparable Z-high® Mill would have a work roll diameter of 5.5” (140 mm).
Sendzimir’s Z-High® Mill is often described as a side-supported 6-High mill, and this is easily the best way to view its advantages.
The small-diameter work roll, compared with a comparable 4-High mill, permits a higher reduction per pass for very hard materials and/or thin strip as well as the elimination of intermediate annealing.
Like its sibling, the 20-High cluster mill, Sendzimir’s Z-High® Mill benefits from high gauge accuracy because of the effectively stiffer mill modulus (compared with the 4-High). It also benefits from other Z-mill advantages, such as lightweight, chockless work rolls that can be replaced in under a minute, that can be ground with a crown unique to the strip shape about to be rolled, or that can be refreshed on the last pass to ensure the best possible surface finish.
The Z-High® Mill achieves excellent shape control because it has a lateral adjust design similar to that of the Sendzimir 20-High cluster mill and bending/counter-bending. This results in less edge cracking and edge drop-off compared with a similar 4-High design.
It’s slightly larger work roll, however, gives it ample rolling torque transmission for wide strip.
The Z-High® Mill has a longer back-up roll life than the comparable 4-High mill because its smaller work roll is associated with less roll separating force and therefore less wear and tear.
The following image shows a cross-sectional and longitudinal view of a typical Z-High configuration.
The following image shows a detail of the work roll area.
One distinct difference between the Z-High® Mill and Sendzimir’s 20-High cluster mill is the various installation configurations possible with the Z-High.
The Z-High® Mill can be designed as a retrofit to an existing 4-High housing. This design eliminates the cost of a new housing; the back-up rolls, pinion stand, and main drives can often be reused. The result is a much higher quality mill at a fraction of the cost.
The retrofit design can also be configured to allow for an interchangeable 4-High/Z-High combination. The 4-High can be used for temper pass rolling or very heavy gauge breakdown rolling. Remove the 4-High mill work rolls, insert the Z-High cartridge, and you have a mill stand capable of rolling much harder materials down to thinner gauges with better gauge accuracy. This retrofit design is also applicable to the classic 6-High design.
Finally, many customers have preferred a completely new Z-High® Mill with no relation to the 4-High. Such an installation provides many of the benefits of both the 4-High and Sendzimir’s 20-High cluster mill but with a smaller mill housing, back-up rolls, and bearings, which means less of a capital investment than either of the alternatives.
Sendzimir’s Z-High® Mill is particularly suitable for “hot” situations such as the entry section of a hot stainless anneal and pickle line.
The size of the work roll is small enough to allow high specific pressure yet large enough to give a sizeable draft reduction on heavy-gauge materials. Reductions in stainless steel of up to 35% per stand have been documented.
The lightweight, chockless work roll can be exchanged in under a minute, thus ensuring virtually nonstop performance in a continuous production situation.
Avesta Sheffield’s Nyby Bruks works in Sweden, J&L Specialty Metals in Midland, Pennsylvania, and Ugine Isbergues, in Isbergues, France, have each installed Sendzimir Z-High® Mills in their A&P lines.
In the future, Sendzimir’s Z-High® Mill promises to figure largely in the continuous casting of stainless steel. It is the only design capable of reducing continuously cast stainless steel strip to cold-roll gauges in a single pass. Our in-house paper on the Process-Line Z-High describes the details.
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About one-quarter as many Z-High® Mills have been installed worldwide as Sendzimir 20-High Cluster Mills. This is not surprising, given that the cluster mill was invented in the 1930s and the Z-High in the 1970s.
As can be seen in the following illustration, most Z-High® Mills are fairly narrow. Irregardless, it is a Z-High® Mill that is rolling the widest stainless steel strip in the world!
