Electromagnetic Stirring

ทฤษฎี:

Electromagnetic stirring (EMS) is the process by which a high level of stirring efficiency can be achieved through interaction between the magnetic field from the static induction coil and the electrically conducting metal bath.
From both practical implementation and experimental study the results of EMS can provide huge advantages including reduced defects in the finished product, higher manufacturing yields, rapid start up and quick financial payback from improved efficiency.

In metallurgical processing, effective and reliable stirring of the melt is one of the prerequisites for higher productivity and improved process performance. In over 1200 installations, the steel and aluminum industry have chosen non-contact electromagnetic stirring technology. By electromagnetic stirring (EMS) it is possible to achieve effective stirring by the interaction between the magnetic field from the static induction coil and the electrically conducting metal bath.

Electromagnetic stirrers are mainly used in the ladle furnace and continuous casting to improve the product quality and increase the steel production. Based on vast process experience and accurate simulation tools, ABB are a company that can define the results of implementing EMS. Achievable results depend on customer targets, current process conditions and chosen solutions. ABB conclude the following, based on 130 installations of EMS process on ladle furnaces:

  • 5-15% higher yields of alloys and 5-10°C lower tapping temperature
  • Less synthetic slag addition with an unbroken slag layer
  • Higher heating efficiency and less wear of electrodes
  • Cleaner steel, for example 10% less oxygen and hydrogen content
  • Minimum maintenance costs because of non-moving parts and non-coil contact with the furnace or the melt
  • No impact on existing or new ladle furnace design and functionality
  • Safe and easy stirring operation, typically 1-2 melt rotations/minute, fully variable and reversible
  • Standard stirring units for 20-300 ton capacity ladle furnaces
  • Rapid implementation with one day start-up and pay back within 11 months
  • Turnkey and performance guarantee commitments with financial solutions/package and worldwide service organization.

In the continuous casting process of a steel mill, methods of improving slab quality are important and often ongoing research issues. In addition to modifying the jet flow angle and remaking the submerged nozzle shape, an electromagnetic technique, which is able to control the fluid flow without contact between the liquid steel and a stirrer, has been used as a flow control technique.

One type of electromagnetic technique is electromagnetic stirring (EMS), which generates a fluid flow by the Lorenz force provided by a linear induction motor. According to the setup position and metallurgical aspects, electromagnetic stirring processes can be classified into several types, such as mould (M-EMS), strand (S-EMS), and final (F-EMS).

The EMS applied on the steel caster is basically a magneto-hydraulic process affecting the crystallization processes and solidification of billet steel. The complexity of the entire process is enhanced further by the fact that the temperatures are higher than the casting temperatures of the concast steel. The temperature of the billet gradually decreases as it passes through the caster down to a temperature lying far below the solidus temperature.

From the viewpoint of physics and chemistry, the course of the process is co-determined by a number of relevant material, physical and thermokinetic characteristics of the concasting steel and also the electrical and magnetic quantities. There is also a wide range of construction and function parameters pertaining to the caster and EMS as well as the parameters relating to their mutual arrangement and synchronisation. Numerous works from recent years relate that the exact mathematical modeling of EMS on a caster is still unsolvable.

In the paper of Stransky K. et al the results of a very demanding experimental verification of the effect of electromagnetic stirring (EMS) on the dendritic structure of steel during the concasting of (150 × 150) mm billets was presented. The basic EMS experiment was conducted on a CONCAST billet caster where two individual mixers were working, as in Figure 1.



Figure 1: The positions of the MEMS and SEMS stirrers

The first stirrer, entitled MEMS (Mould Electromagnetic Stirring), is mounted directly on the mould and the second stirrer, entitled SEMS (Strand Electromagnetic Stirring), is mounted at the beginning of the flow directly after the first cooling zones but in the secondary-cooling zone. Nine different variants of concasting were verified during ordinary concasting without EMS, with MEMS mounted on the mould, with SEMS mounted in a secondary cooling zone and using both MEMS and SEMS.

Macroscopic grinding was conducted on the samples taken from cross-sections of individual billets in order to make the dendritic structure visible and evaluate it. The greatest effect of the EMS was experimentally observed during the mixing using both MEMS and SEMS simultaneously. The area of the columnar dendrites oriented perpendicular to the surfaces of the walls has a thickness limited to 1/4 to 1/3 of the billet thickness. In the remaining central part of billets stirred in this way the structure which dominates is the equiaxed dendrite structure. Long-term statistical monitoring of the quality of billets has proven that the application of EMS reduces the occurrence of defects.

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