The using of Electromagnetic Stirring (EMS) is to strengthen liquid movement steel by means of electromagnetic force induced in liquid core of strand. Concretely, the AC field excited by the stirrer penetrates into the liquid steel of the strand and generates electromagnetic force under the interaction of induced current and local magnet field. The electromagnetic force is the volume force acted on the liquid steel volume element, which can push liquid steel movement. Taking three-phase rotating magnetic field stirrer as an example, the stirrer is acting as motor stator, and the liquid steel is acting as motor rotor, thus working principle of electromagnetic stirring can be understood as that of asynchronous motor with solid rotor.
EMS type refers to flow pattern of liquid steel arising out from electromagnetic stirring.
1．Rotary Stirring: Generated by two-phase or three-phase rotating magnetic field stirrer, the liquid steel rotates at certain speed.
2．Liner Stirring: Generated by traveling-wave magnetic field stirrer, which moves the liquid steel towards one direction.
Whether it is rotary stirring or liner stirring, the flow direction of the liquid steel is always following the magnetic field direction.
3. Rotary stirring generated by two traveling-wave magnetic fields: Two sides of the strand are provided with a pair of traveling-wave magnetic field stirrers to generate traveling-wave magnetic fields in different directions, which linearly moves the liquid steel at two sides towards reverse directions. According to continuity law of hydrodynamics, the liquid steel inside the strand rotates; thereby, two-way traveling-wave magnetic field stirrer also can generate rotary stirring. This rotary stirring is mainly applied to slabs and blooms.
Product Classification Criteria
Currently, most of electromagnetic stirrers are of AC induction type.
1. Phases: Two-phase type or three-phase type; most of them are three-phase type.
2. Magnetic Field Configuration:
Rotating Electromagnetic Stirring (REMS): Similar with stator of normal asynchronous motor, when the stirrer is fed with three-phase or two-phase AC, a rotating magnetic field in the space is excited to rotate at certain speed.
Traveling-wave Electromagnetic Stirring (TEMS): It is derived from stator of normal asynchronous motor. Supposing that one side of the stator is bended along with axial direction to flat, when the stator of motor is fed with three-phase AC to excite rotating magnetic field, after bending, the original rotating magnetic field changes to traveling-wave magnetic field to move the liquids towards one direction at certain speed.
3. Outer looking:
Rotating Magnetic Field Stirrer: The stirrer excited by three-phase power supply is of round ring shape; the stirrer excited by two-phase power supply is of square ring shape.
Traveling-Wave Magnetic Field Stirrer: box type or liner type.
4. Structure of stirrer: Internal MEMS comprises an inductor exciting magnetic field and non-magnetic stainless steel shield in the inner cavity. Except that, all the stirrers comprise inductors to excite magnetic field and non-magnetic stainless steel cases.
Installation position and mode of electromagnetic stirrer: According to its installation positions on metallurgical lengths of casting machine, the following modes are very important to obtain best metallurgical results.
In principle, the magnetic stirrer can be arranged at any positions on metallurgical lengths of casting machine, but basically there are installed in 3 positions:
1. Mold Electromagnetic Stirring (MEMS): The stirrer is arranged on the outside of mold copper tube. Billet and bloom MEMS always use rotating magnetic field stirrer; Slab MEMS uses two-way traveling-wave magnetic field stirrer. All the MEMS use low-frequency power supply excitation.
2. Strand Electromagnetic Stirring (S-EMS) The stirrer is arranged outside the strand. Normal S-EMS adopts one-way traveling-wave magnetic field type, and the stirrer is arranged at one side of inner arc of the strand; some adopt rotating magnetic field type. Slab Strand SEMS adopts two-way traveling-wave magnetic field type, and the stirers are arranged at inner arc and outer arc of the strand.
At present, except for slab, billet and bloom casters rarely use S-EMS.
3. Final Electromagnetic Stirring (F-EMS): It is mainly applicable to billet and bloom continuous casters. The stirrer is arranged outside the strand, and adopts rotating magnetic field stirrer. According to the size of section, it uses mains frequency or low-frequency power supply excitation.
The typical configuration of different stirrers for billet and bloom casters are:
①M+S-EMS: Applicable to medium, high carbon steel (including low alloy steel, spring steel) continuous casting
②M+F-EMS: Applicable to medium, high carbon steel (including low alloy steel, bearing steel) continuous casting
③S+F-EMS: Applicable to medium, high carbon steel large-section bloom continuous casting
④M+S+F-EMS: Applicable to super-high carbon steel billet continuous casting.
At present, M+F-EMS is a most common combination of stirring.
Metallurgical Results of Electromagnetic Stirring
Electromagnetic stirrings at different positions have different metallurgical effects.
|Applicable Steel Type
|Increase equiaxed ratio;
Reduce blowhole and pin-holes on surface and subsurface;
Reduce inclusions on surface and subsurface.
Uniformization of solidification shell;
Improve center porosity;
Improve center segregation;
|Low alloy steel
Medium-high carbon steel
|Increase equiaxed ratio;
Reduce internal crack;
Improve center segregation and porosity;
|Refinement of equiaxed crystal;
Improve center segregation and shrinkage;
Special high carbon steel