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discovered 02_2012

discovered 02.12 FOCUS WWW.Hzdr.DE is, together with the automobile, engineering and electrical industries one of the largest employers in Germany) energy and raw materials can be saved by an order of magnitude that private households could never match even after making considerable efforts in terms of energy-saving measures. Nevertheless: every effort counts as a step towards achieving energy efficiency and a fair distribution of energy. Looking at old technologies through the eyes of researchers The standards are high for steel plants: nowadays the metal that is produced is not allowed to have even the slightest material defects. This is because one should be able to convert it into increasingly larger and at the same time thinner and lighter parts. Energy-savings can be made at this point, if the steel is perfectly produced during the first casting without having to be melted again. Further, one can save considerable energy – and thereby save costs – if the casting process itself is sped up. However, the molten metal is subject to the laws of thermo-fluid dynamics: faster casting processes can lead to unwanted swirls that in turn increase the rate of rejection. Approximately 90 percent of the world’s steel is manufactured using the continuous casting process, which enables a high casting speed and productivity. It is based on the fact that the hot molten metal is poured at approx. 1,500 degrees into a copper mold, which is open at the bottom and which uses water as a coolant. There the outer part of the molten mass cools off and can be pulled off. At this point the core of the strand is still liquid, but gradually cools off in the air. Today‘s casts take on different forms, depending on their respective requirements. Modern factories for example produce very thin metals, which are then directly processed into forms (i.e. car bodies) in rolling mills. It is easy to see why this kind of solution benefits from dramatic cost and energy savings. What does casting have to do with magnetic fields? Due to the fact that hot molten metals are practically always electrically conductive, one can use magnetic fields as electromagnetic brakes to control the flows of these molten metals. These brakes are based on a law that many of us know from school: Lenz’s law. It is common knowledge that an electric current sets up a magnetic field. According to Faraday‘s law a magnetic field however also produces a current in an electrical conductor, if these move relatively towards each other. Lenz (1804 - 1865) was the first to recognize that the interaction between an induced current and an applied magnetic field produces a force that counteracts the original force. This inductive force is known as the Lorentz Force. It is the workhorse for HZDR experts in this fascinating field of magnetohydrodynamics. Flows of the molten steel usually also affect the properties of the final product. Only when the flow is stable and reproducible, can high-quality steel be produced; if on the other hand there are swirls, problems often arise in terms of the quality of the material or even rejects, meaning that it must be melted again incurring additional costs. The idea behind implementing electromagnetic brakes that have been used now for approx. the last fifteen years is that externally applied magnetic fields control the flow, suppressing unwanted swirls. If the magnetic brakes in the steel factory work well, then the casting speed can be increased and the quality, productivity and efficiency increase proportionately. Bearing in mind that steel strand casting is a 70-year old technology, one would think that the process would have been optimized a long time ago. However, nothing could be further from the truth. There are several reasons for this. On the one hand with today’s conventional methods for taking measurements one cannot look directly at the molten steel and its flow, on the other hand the effect of the electromagnetic brakes have thus far only been conceived and tested on the basis of calculations and integral results. An investigation of the flow in the mold was based exclusively on experiments with water however, but water behaves completely differently from molten metals. Electromagnetic brakes do not brake The experimental work at the LIMMCAST plant from the Helmholtz-Zentrum Dresden-Rossendorf is now for the first time ever able to provide realistic data on flows of molten steel. Barely published, these results had a great response in the scientific community and the steel industry, although they readdress many of the assumptions that for many years were considered to be the basis for using electromagnetic brakes. The experimental results were surprising: in steel casting, electromagnetic fields do not always work solely as brakes, but can also make the flow become unstable under certain conditions and even increase the level of swirls. Hence, it is hardly any wonder that the Rossendorf scientists’ results provoked an overwhelming response at international conferences. The good thing is that industry partners have also expressed a serious interest in the results. Gunter Gerberth, Director of the Institute of Fluid Dynamics at HZDR remembers how, when LIMMCAST was set up some five years ago, there were a few rather critical objections. In the meantime however these voices of dissent have fallen silent. Externally applied magnetic fields control flow.