Japanese companies still use the most HSS rolls, but several North American steelmakers have successfully installed the longer-lasting rolls in their mills. Even with such variations, the performance of the different suppliers' rolls is about the same, Zeik says. The operators at U.S. Steel Fairfield use HSS rolls in Stands 1-3 for 90 percent of their production. By early next year, the plant plans to use HSS rolls in the reversing-roughing mill, too. Fairfield reduced roll changes from 2,400 to 1,600 a month when it switched from high-chrome to HSS rolls, Zeik says.
Mills recoup their costs only if they significantly extend their rolling campaigns.
At Fairfield, complete six-stand roll changes take 15 minutes, compared to less than 10 minutes at some mills. Changing machines automatically install and eject rolls from stands, but an overhead crane must maneuver rolls to and from the mill during a roll change. The fewer roll changes with HSS thus added 23 hours of potential uptime per month to the hot-strip mill at Fairfield. Now it can roll more slabs than Fairfield's casters can produce. A surface crack of a high-chrome roll can branch beneath the roll surface, creating small but unstable roll portions that can break off during rolling.