Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
14th Generation PC Mill Promoting
DX (Digital Transformation)for Saving Maintenance Works

P r i m e t a l s Te c h n o l o g i e s J a p a n , L t d .

In the hot and cold strip rolling process in steel manufacturing, there has been
greater-than-ever demand for thin strip and high strength steel rolling (demand for high-loadrolling) along with high productivity (demand for high-speed rolling), to meet the recent need forlight-weight vehicle bodies for better fuel efficiency and tightened emissions control regulations in
the automobile industry. Meanwhile, due to improved efficiency in motors for hybrid vehicles andelectric vehicles, a small strip crown is desirable for the electrical steel sheets used in the ironcores of motors (demand for greater control over the cross-section shape of the strip).
Furthermore, in order to address the imminent decrease in the size of the working population,
demand for labor saving and automation in facility maintenance has also been increasing. Amongthe many technologies developed to realize these needs, the key for hot rolling equipment is a hotrolling mill with high ability to control the cross-section shape of the strip. Primetals Technologies
has been developing a hot rolling mill called the Pair Cross Mill (PC Mill) that has the capability
to address current needs. The PC Mill offers greater competitive advantages over high-loadrolling or high-speed rolling, since it can utilize a flat roll for the work roll and back up roll. On
the other hand, it also has a disadvantage in that the large number of parts makes maintenancedifficult. The 4th generation PC Mill developed as the next flagship hot rolling mill has fewer parts
requiring maintenance due to its full hydraulic cylinder control and is capable of automatic rollposition adjustment in the rolling direction, which used to be handled manually. This has beenachieved through the utilization of a sensor embedded in the cylinder. This report will introduce
the features and other relevant information of the 4th generation PC Mill, as well as actual orders
from Nippon Steel Corporation in Setouchi Works (Hirohata Area) and a U.S. company forinstallation in hot rolling mills.

|1. Thinner, harder and faster
The 4Hi mill shown in Figure 1 has many applications in hot rolling mills and the PC Mill
has the same 4Hi mill structure. As you can see in Figure 1, when the strip is rolled, the work roll isdeformed by the rolling load, which creates a strip crown. Rolling out a hard material causes a highrolling load, which generates a large strip crown. Figure 2 illustrates the principles of the PC Mill.
The PC Mill can adjust the roll gap at the end of the strip by crossing the upper and lower rolls
across the strip without changing the roll gap in the center of the strip, which allows control of thestrip crown. On the other hand, the high pressure on the contact surface between the work roll andback-up roll and high-speed rotation can damage the rolls. One of the features of the PC Mill is tolet the work roll and back-up roll work in tandem without crossing them where flat rolls can beused, which is a greater competitive advantage over high-load rolling and high-speed rolling. Since
launch, these features have been highly evaluated, with more than 150 stands of the PC Mill in
actual use around the world.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
2Figure 1

4Hi mill

Figure 2

Principles of PC Mill|2. Further challenges of high-load rolling and high-speed rolling
Under the conditions of high-load rolling and high-speed rolling, the mill vibration becomessignificant and depending on the extent thereof, the rolling load is restricted. In order to solve thisissue, the Mill Stabilizing Device (MSD) was developed as a vibration damping device for thehydraulic cylinder models, and has been in use since the 3rd generation PC Mill. It has been
confirmed on actual equipment that the use of the MSD reduces the mill vibration as shown inFigure 3 and improves the restriction of the rolling load as shown in Figure 4.

Figure 3

Reduction of mill vibration by MSD

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
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Figure 4

High load rolling by MSD|3. History to the development of the 4th generation PC MillAs described above, while the PC Mill offers advantages, it also had a disadvantage in thatmaintenance work is difficult due to the large number of parts. Therefore, the history of the PC Millupgrade is both a path to simplification as well as performance enhancement. Figure 5 shows thehistory of the PC Mill simplification. If we say the number of parts required for the mechanicalelectromotive drive unit of the 1st generation PC Mill (i.e., motors, reducers, screws, nuts, etc.) was100, the 2nd generation PC Mill was reduced to 40 and the 3rd generation PC Mill was furtherreduced to 25. In the 4th generation, by completely removing the mechanical electromotive drive
unit and driving the mill directly by the hydraulic cylinder, we have achieved a significant
reduction in the number of parts.

Figure 5

History of PC Mill simplification|4. Contribution to labor saving and DX promotion
To roll a strip straight and even is very important in order to avoid problems occurring inside
the rolling mill, which is of particular importance in thin strip rolling. Conventional mills as shownon the left-hand side of Figure 6 inevitably require a gap between the housing and roll chock sothat the roll can be pulled out of the housing. It is considered that it is very effective to keep the gap
as small as possible and to maintain the roll axis perpendicular to the rolling direction in order tothe strip in straight, which, however, is quite difficult to manage. In terms of conventional mills,
the only option for managing the housing liner is to measure and replace it manually at the time ofmaintenance shutdowns. On the other hand, while the 4th generation PC Mill as shown on theright-hand side of Figure 6 has achieved a “zero gap” between the housing and roll chock during
rolling thanks to the MSD, the sensor incorporated in the cross cylinder enables the automatic
correction of the roll position in the rolling direction and liner wear compensation, which used to behandled manually. Furthermore, since the roll position control is driven directly by the hydrauliccylinder without interposing any mechanical parts such as gears or screws, the structure not only
allows a low parts count, but also prevents machine backlash. Consequently, strip threading
troubles attributable to machine backlash are also reduced.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
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Figure 6 Labor saving and automation in liner management|5. Further development
We have scheduled installation of the 4th generation PC Mill into hot rolling mills at Nippon
Steel Corporation in Setouchi Works (Hirohata Area), as well as at a U.S. company. Thisinstallation aims to achieve better productivity by simplifying maintenance procedures, removing
the gap between the housing and roll chock and reducing troubles in strip threading caused bybacklashes of machine parts, while enhancing the control over the cross-section shape of strips withthe extended range of application of the PC Mill functions through the proper administration of theroll position and the vibration control effect.
In the future, we will continue to extend the range of application of the 4th generation PC
Mill to cover newly-constructed hot rolling mills and provide solutions to meet the demand forhigh-load rolling, high-speed rolling, straight rolling, higher productivity, less maintenance andlabor saving.