Thinner, Harder, Better, Faster: The Fourth-Generation Pair-Cross Mill

Renowned for its ability to roll at high loads and exceptional speeds, the Pair-Cross Mill now features new technological advances.

Primetals Technologies developed the fourth-generation Pair-Cross Mill as its next flagship to meet the needs of modern hot rolling: highly reliable and precise strip-crown control, labor-efficient operation, low maintenance requirements, and advanced automation. The new mill features a simplified design with fully hydraulic cylinders, incorporates built-in sensors for automation, and enables precision control through automatic roll alignment adjustment.

In recent years, demand for rolling thin strip and high-strength steel has increased to meet the automotive industry’s need for lightweight vehicle bodies to improve fuel efficiency and reduce emissions. Additionally, increased efficiency in hybrid and electric vehicles requires higher quality and tighter tolerances in the production of electrical steel. For hot and cold strip processing, this translates to a need for high-load and high-speed rolling - and for precise crown control. Added to these pressures is a decline in the number of skilled manufacturing workers, which calls for increased automation and reduced maintenance requirements for heavy machinery. The Pair-Cross Mill from Primetals Technologies has been designed and refined over the years to respond to these industry-wide changes.

Primetals Technologies introduced the Pair-Cross Mill in 1984 to advance hot rolling by meeting the demands of the period with efficiency and innovation. The mill is comparable to a standard 4-Hi mill but utilizes upper and lower rolls in a “cross” formation. This allows the roll gap at the edges of the strip to be adjusted, which then keeps the gap at the center of the rolls constant. This added precision in controlling the roll gap - and therefore the strip crown - is the key innovation of the Pair-Cross Mill. To meet the growing demand for high-load and high-speed rolling, the work and backup rolls operate in tandem without crossing, which enables the use of flat rolls. This counteracts any issues that may otherwise be caused by a buildup of very high pressure between the work and backup rolls, which can damage the rolls when operating at high speeds. These core benefits are part of the reason why more than 150 Pair-Cross Mill stands are currently in operation, but as demands for tighter tolerances have grown—along with challenges related to maintenance and productivity— Primetals Technologies has continued to advance the mill’s design.

While the original Pair-Cross Mill already comprehensively supported the production of high-quality steel, maintenance concerns and the drive to reduce mechanical complexity have since been factors influencing the refinement of the mill’s design. One example of this evolution is the reduction in the number of parts required for the electromechanical drive units. Beginning with one hundred components - motors, reducers, screws, and nuts - the second and third generations of the drive units reduced this number to a remarkable twenty-five parts. These kinds of improvements lowered the mill’s mechanical complexity, which translated into reduced maintenance effort. For the fourth generation, the design was then completely transformed. Specialists engineered the mill from the ground up; it now employs a direct-drive hydraulic cylinder, further reducing mechanical components.

Rolling a strip straight and evenly is critical to avoid any issues in the rolling mill, especially during thin-strip rolling. Conventional mills inevitably require a gap between the housing and roll chock so that the roll can be pulled out of the housing. In this case, keeping the gap as small as possible and maintaining the roll axis perpendicular to the rolling direction help keep the strip straight. However, this is quite difficult to maintain. In conventional mills, the only option for managing the housing liner is to measure and replace it manually during maintenance downtime. In contrast, the fourth-generation Pair-Cross Mill achieves a “zero gap” effect between the housing and roll chock during rolling, thanks to the Mill Stabilizing Device. The sensor incorporated into the cross cylinder enables automatic correction of the roll position in the rolling direction and compensation for liner wear, which used to be handled manually. Furthermore, because roll-position control is driven directly by the hydraulic cylinder—with no intervening gears, screws, or threaded drives—the system is inherently free of backlash: the small clearance between mating mechanical components that causes a lag in positional response whenever the direction of load changes. Eliminating this dead zone is particularly consequential during strip threading, when the leading edge of the strip enters the roll gap and any delay in positional correction is most likely to cause steering problems.

The fourth-generation Pair-Cross Mill is a proven innovation and was successfully implemented at Nucor Steel Decatur, Alabama, U.S.A. The upgrade began in March 2022 at the F2 stand, with the F1 and F3 stands following over the next eighteen months. Experts at Nucor observed significant improvements in the differential force ratio and roll leveling.

In hot rolling, a difference in rolling force between the work side and the drive side of the mill is one of the principal causes of poor strip tracking. Two mechanisms give rise to this force imbalance. First, the mill housing on each side has a characteristic stiffness—its mill constant—and if these values differ, the two sides deflect by unequal amounts under load, producing an asymmetric roll gap even when both sides are nominally set to the same position. Second, any angular misalignment between the work rolls and backup rolls—known as skewing—generates an axial thrust force at the roll contact; because this force acts unequally on the two sides, it adds a further imbalance to the differential load.

With the introduction of the fourth-generation Pair Cross Mill, the roll alignment with respect to the mill line center is always correctly calibrated, as the Mill Stabilizing Device eliminates the gap between each roll chock and the mill housing. This principle is confirmed by real-world operating data that shows the extent to which the roll skew between work rolls and backup rolls was suppressed, as well as the reduction in differential load (Figure 7).

Ideally, when roll leveling between the work side and the drive side is at zero, the differential force should also be at zero. However, if the roll alignment is unstable and skew occurs between the rolls, the differential load has an offset, and stable operation cannot be achieved. Figure 8 shows the relationship between the leveling amount and the differential force. With the new mill, the differential force approaches zero when leveling is at zero, indicating that perfect roll alignment is maintained.

Nucor Steel’s operators reported stable operations and fewer in-bar interventions in rolling and leveling.

The installation of the fourth-generation Pair-Cross Mill at Nucor Steel Decatur’s hot-strip mill has not only successfully improved operations but also prepared the plant for the future—by anticipating future rolling conditions for specific steel grades. The site’s operators reported stable operations and fewer in-bar interventions in rolling and leveling. Moreover, Nucor Steel Decatur also observed a leap forward in reducing maintenance effort: the plant’s previous mill, a first-generation Pair-Cross Mill, required surveying and liner adjustments at six-month intervals. In contrast, the new fourth-generation mill has demonstrated an average wear of merely 0.4–0.6 millimeters over a thirty- month service period.

By simplifying maintenance procedures, removing the gap between the housing and roll chock, and reducing strip-threading troubles caused by machine-part backlash, the fourth-generation Pair-Cross Mill is the ideal mill stand for modern hot-rolling operations. Additionally, the mill dramatically increases the level of control over the shape of strip cross sections. This is achieved by extending the application range of its functions through proper roll-position and vibration control, which enables high-load and high-speed rolling with precision and quality. For Primetals Technologies, the fourth-generation Pair-Cross Mill is the new standard in hot rolling, meeting the industry’s current demand for high-load, high-speed, and straight rolling. It also leads to higher productivity, lower maintenance expenditures, and a reduction in labor costs. For new plants and existing first-, second-, and third-generation Pair-Cross Mills, the fourth generation is now redefining the very idea of hot rolling.