Edition 01/2019

Through a comprehensive investment programme, ArcelorMittal Bremen is pursuing the goal of improving the productivity and quality of the continuous casting plant. This also required investments in a better cooling system. The following rule applies: the more effective the cooling, the faster the continuous casting plant can produce. The liquid steel from the steelworks constitutes the beginning. The material flows into a distributor and through a casting tube into the mould. This is a shaping construction made of copper. The copper walls of the mould are water-cooled and give the steel its later rectangular shape. In this area, however, the solidified outer skin is still as sensitive as a raw egg and must be further cooled into so-called slabs before the continuous strand is cut. ArcelorMittal Bremen has a 16 metre long track available for this purpose.
In Bremen, finely atomised steam with compressed air is used for cooling. Before the nozzles wet the four outer surfaces of the continuous casting process with the water mist, the water previously cleaned in filter systems is swirled around a chamber with the compressed air - comparable to the carburettor of an internal combustion engine. The result is finely atomised droplets that allow high heat absorption and bring the steel from liquid 1,600 to solid 800 degrees Celsius in a short time. “The water enters the mixing chamber at 12 bar and is mixed there with a good 5 bar of air,” explains Bernd Grosse from the steel manufacturer‘s engineering department.
The challenge in cooling the steel is to reduce the temperature of the continuous casting after the mould, but only in a frame that allows the steel to pass through a groove from the vertical to the horizontal. “We have to create a shell that is so soft that the steel passes through the curve without leaking,” says Grosse, describing the process. If pure water were used here, the control spectrum would be very limited - a real disadvantage on the 16 metre long cooling section, as poorly controlled cooling processes have an unfavourable effect on the solidification process, increasing the risk of cracks and blowholes. Process engineers have therefore developed sophisticated cooling formulations for the respective products. Slabs from Bremen are up to 220 millimetres thick in various steel grades and measure between 950 and 2,670 millimetres wide. Further processing takes place in Bremen in the hot rolling mill.

This brief insight into the process shows the importance of compressed air in continuous casting. The metallurgical network measures 20 kilometres and is fed from the compressed air centre at a pressure of around 5.5 bar(g). The frequency-controlled screw compressors of AERZEN type VMT4W with their motor power of 545 kW deliver a two-stage peak volume flow of 4,000 m³/h, i.e. one third of a turbo machine. The control range in daily operation is between 2,000 and 4,000 m³/h at a maximum pressure of 8.5 bar(g). Both systems have been precisely adapted by AERZEN.
The two-stage design of two oil-free screw compressors generates a pressure of 4.5 bar (abs) in the first compressor stage. The air then has a temperature of about 250 degrees and must be cooled down to below 60 degrees before entering the second stage. AERZEN has installed an extremely effective water cooler between the two stages, which is also supplied with water from the river Weser. Depending on the weather and season, this corresponds to about 25 to 30 degrees Celsius. This is thus only about ten Kelvin above the temperature of the river. After the second stage, cooling down to about 35 degrees takes place again. At this stage, the compressed air is still 100 percent saturated with water. But the steel mill needs dry air with a pressure dew point of +3 degrees Celsius.

In view of the fact that AERZEN received the order for a complete package, the dehumidification was part of the development and engineering work, as well as the subsequent installation and commissioning. ArcelorMittal Bremen opted for screw compressor technology from AERZEN in the early planning stage of the modernisation work. “From experience we feel safe with AERZEN,” emphasises Technician Sven Ress. Against this background, the turbo machines and the two-stage compressors in the compressed air centre are vibration-monitored. “Thanks to the online analysis of the frequencies that occur, we can identify bearing damage at an early stage and plan repairs accordingly,” explains his colleague Volker Merrath. In order to save as much time as possible for this work and also for regular maintenance, AERZEN has installed the screw compressors without acoustic hood in the acoustically insulated compressed air centre.