Rolling so-called jumbo beams that are up to 140 mm thick and weigh up to 1,377 kg per meter requires the use of extremely large beam blanks as feedstock. The past practice of rolling from slabs is less reliable and requires many passes in the breakdown stand, resulting in temperature losses, limited productivity, and unfavorable yield. In recent decades, however, the possibility of continuously casting ultra large and heavy beam blanks has become a reality.
The use of beam blanks for rolling large and heavy beams has brought about a change in the rolling technology used in the breakdown stand, meaning fewer passes, stable product quality, and increased yield. At the same time, it was also possible to reduce the size of the breakdown mill and the drive power (compared with the use of slabs).
The rolling mill equipment needs to be more rigid to accommodate much higher loads. Reversing tandem mill stands require larger roll diameters, higher horizontal and vertical roll separating forces, as well as more drive power. Since jumbo beam mills typically also produce the standard range of beams and other products such as sheet piles, the CCS®-type mill stand (Compact Cartridge Stand) is designed to accommodate different roll sets with varying diameters and barrel lengths. This feature allows the use of existing and new rolls for modernization projects like that at Nucor Yamato Steel, or the use of a range of roll diameters that suit the different products within the product mix.
Despite the use of heavy beam blanks, the rolled stock can only be reduced to a limited extent in the rolling mill. In addition, since the yield strength decreases as the thickness increases, the beams must be thermally treated. To achieve the required mechanical properties, such as high yield strength, improved toughness (at low temperatures) and good weldability, in-line thermal treatment is essential, since normalizing rolling routes are unable to achieve these mechanical properties for the finished products.
Thermo-mechanical beam rolling, selective flange cooling as well as quenching and self-tempering (QST) processes for beams were developed and introduced back in the 1990s. Thermo-mechanical rolling is used for thinner beams, QST for thick-flange beams, while selective flange cooling is used for both applications. Hence, the economical production of beams with increased tensile properties, excellent toughness, improved weldability and low operating costs and the reduced cost for alloying elements in steelmaking is now possible.
SMS group’s design of such in-line treatment facilities is distinguished by its flexibility for use with the entire product range recipes for different beams and grades, generated using a so-called BeamCool® process model, however without the need to change equipment for varying beam sizes. Due to the thickness of the flanges and web, it is not possible to cut the jumbo beams to length in cold conditions or straighten them by roller straightening. Indeed, despite the fact that SMS group built the world’s largest CRS® straightening machine for the heavy section mill at Masteel in China, the jumbos still need to be gag pressed.
Hot sawing is required for cut-to-length operations. The hot saws are distinguished by large saw blade diameters and high drive power.
In the last five years, SMS group has built a heavy section mill for jumbo beams in China, and another is under construction in North Africa. In addition, two heavy section mills were revamped in the USA and Korea. This underlines the increasing demand for these products.