Beyond traditional steel grades, demand has been growing steadily for AHSS (advanced high-strength steels), in particular for use in the automotive industry. Harder and thinner flat-steel products contribute significantly to weight reduction of vehicles and consequently to improved fuel economy. Steel producers are therefore faced with the challenge of producing dual-phase or TRIP (transformation induced plasticity) steels for the automotive industry and silicon steels for electrical appliances, which frequently exceeds the capability of installed rolling facilities.
Established 4-high and 6-high cold-rolling mills (typically with driven work rolls) have known limits in their reduction capability. This is mainly due to excessive work-roll flattening that takes place with high rolling forces. This widely acknowledged fact has led in the past to the introduction of cluster mill types (for example, 20-high or Z-high) with much smaller work-roll diameters to be able to produce stainless steel and special steel grades.
Introduction of the Flex-HI mill solution
As new mill installations require substantial investments, steel producers prefer to continuously upgrade existing equipment to maintain and strengthen their competitive position. In order to meet current and future requirements of mill operators, Primetals Technologies has developed a unique solution that significantly extends the cold-rolling capabilities of existing 4-high mills with only minor modifications. This new mill technology, called Flex-HI, enables the operation of individual stands either in 4-high mode with large work rolls or in 6-high mode with small work rolls (X-HI mode). This is achieved with the use of quickly interchangeable cassettes. Flex-HI thus makes it possible to maintain high productivity for soft to medium-strength materials in the 4-high mode, and to extend the product mix with new generations of AHSS or silicon steels in the X-HI configuration (see Figures 1 and 2). This solution thus represents the innovative combination of two proven technologies of Primetals Technologies.
Flex-HI mill technology can be ideally installed in existing 4-high reversing mills and in tandem cold-rolling mills. In 5-stand tandem cold-rolling mills, for example, the intermediate mill stands (Nos. 2, 3 and 4) can be outfitted with a Flex-HI system. Reduction capability in the X-HI mode, especially in mill stands 3 and 4, is significantly greater than in the 4-high mode since the incoming strip into these mill stands is already work-hardened and thin.
Comparison of 4-high and X-HI roll configurations. A rolling mode change is carried out by exchanging the 4-high and X-HI roll cassettes within a typical roll-change time
The FLEX-HI MILL
The X-HI mill insert consists of driven intermediate rolls, small work rolls and lateral support rolls on both sides of the work roll. The lateral support rolls are mounted on movable arms, which are connected to the intermediate roll chocks. This simple cassette system makes it possible to change from the 4-high rolling mode to the X-HI mode and back without increasing work-roll change time. This elegant solution allows the highly flexible production of a wide range of steel grades. The back-up rolls remain unchanged and are used in both rolling modes.
Compared to traditional 4-high and 6-high technology, the work-roll diameters of the X-HI mill insert are much smaller, which results in significantly reduced rolling forces and rolling torques. Especially for the production of hard and thin products, the small work-roll diameter permits large thickness reductions without costly intermediate annealing treatment.
Figure 3 illustrates an example of achievable product thickness after cold rolling for an incoming hot-rolled dual-phase 980 steel grade with a strip thickness of 2.0 mm and width of 1,270 mm produced on an Arvedi ESP line. For a 5-stand tandem mill with a classical work-roll diameter of approximately 635 mm and with all five stands working at the operating force limit (roughly 22 MN roll separating force), the achievable product thickness is approximately 0.95 mm. A simple mode change in mill stands 3 and 4 from 4-high to X-HI with a work-roll diameter of 180 mm allows the product thickness to be reduced to 0.5 mm at significantly lower rolling-force levels (9–12 MN roll-separating force).
Cooling and lubrication is provided by a water/oil emulsion. In the patented system of Primetals Technologies, the nozzles are mounted in the side support arms close to the work rolls. This ensures reliable roll-gap lubrication and effective roll cooling to achieve rolling speeds in X-HI mode of up to 500 m/min.
Meeting market challenges
The Flex-HI solution from Primetals Technologies allows an existing cold-rolling mill to be operated in both the established 4-high configuration, and, after a roll-cassette change, in a 6-high cluster-mill configuration to extend the product mix to include new generations of high-strength steels. The described solution offers steel producers an attractive and cost-effective option to meet the increasingly demanding market challenges. The first engineering order for the supply of Flex-HI roll cassettes was received in June 2015 from a Midwestern U.S. steel producer.