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Products and solutions, 2000-03-20, 01:00 AM

Steel segment: narrow tolerances and high strength – water-quenched and tempered wide strip from Thyssen Krupp Stahl AG

At Hanover Fair 2000, Thyssen Krupp Stahl AG (TKS) will be exhibiting a telescopic jib set weighing several tonnes for use in mobile cranes. This heavyweight exhibit represents a new product with which TKS has carved out a new niche for itself in the market: high-strength water-quenched and tempered wide strip with thickness tolerances of less than 0.40 mm over the entire length of the strip. Strips of this type with thicknesses between four and twelve millimeters are used in the construction of mobile crane jibs which are subjected to extreme loads. The minimum tensile yield strengths of the steels are between 690 and 1,100 Newtons per square millimeter. TKS sells them under the product names N-A-XTRA and XABO.

Plates with these levels of strength are manufactured by adding alloying elements such as carbon, chromium or molybdenum and by applying a special heat treatment process, water-quenching and tempering. In this process, the plates are initially heated in an annealing furnace until they adopt an austenitic structure, then cooled with pressurized water so quickly that the structure is converted into a hard bainite/martensitic structure. Finally, the plates are tempered in order to relieve the stresses in the structure and thus attain a high level of toughness. XABO 1100 is the latest material developed using this process with a minimum tensile yield strength of 1,100 Newtons per square millimeter. This steel makes it possible to achieve a weight saving of about ten percent in jib production versus the XABO 960 grade which has already been introduced.

When it comes to rolling the plates, it is important to maintain the narrowest possible thickness tolerances. The customer benefits from lower processing costs because less time and effort is required to straighten the components and there is less superfluous weight to be moved. In turn, this increases the efficiency of the mobile cranes. ThyssenKrupp Stahl AG is the only plate manufacturer able to produce water-quenched and tempered plates with thickness tolerances of less than 0.40 millimeters. This is because whereas water-quenched and tempered plates are normally rolled on four-high trains, TKS uses an advanced hot strip mill instead. The hot strip is subsequently cut to length on a high-performance line. The feedstock continuously runs through several roll stands on the hot strip mill. This mill operates with significantly greater precision than a four-high stand which is designed for high roll forces and torques and in which the individual plates are pushed backwards and forwards by two working rolls with a reversing feature. Thickness deviations in production with the four-high train are about twice as great as those with the TKS method. Halving the tolerances reduces the inherent weight of the jib construction by about 5 percent.

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