The SiFe / silicon steel are purified, refined, melted and rolled into sheets / coils for further processing. Careful and precise control is needed at every stage if high quality, consistent magnetic properties are needed, and that's where cores from reliable manufacturers are only used in high quality transformers to ensure the materials are meeting specifications and consistent. If you get them from unknown source, then good luck with the outcome!
The silicon steel is rolled at controlled high temperature in high gauge coils, and further rolled at cold temperature to the ultimate desired gauge and further annealed. The silicon steel are usually cold reduced and further processed to develop their grain orientation if needed.
The silicon steel (for transformers use) are produced to meet magnetic property specifications rather than specific chemical composition. Magnetic properties are of utmost importance for transformer use, although it is highly dependent on the chemical composition and processing control.
Transformer cores is made up of a few ingredients:
- silicon: primary alloy, to increase volume resistivity, to reduce eddy current, help in attaining the desired grain structure of steel, reduced hysterisis component for lower core loss, avoid phase change in the orientation processing
- aluminum: ~<1% added for metallurgical effects, and affect the grain structure
- manganese: ~<1%, added for metallurgical effects, and affect the grain structure
- others: other impurities such as carbon, sulfur, nitrogen
What we are generally interested are High Silicon Steel (~M15 to M47) and Grain-Oriented Silicon Steel (~M2 - M10), for power and audio transformers use. The silicon steel are normally supplied in specific gauges, and for transformers, are usually 0.64mm, 0.47mm, 0.35mm, 0.27mm, 0.23mm and 0.18mm.
Oriented and non-oriented silicon steel
Oriented - crystal structure having magnetic properties that are better in a given direction
Non-oriented - not oriented intentionally oriented but will still have very minor orientation magnetic properties due to process methods.
Fully controlled processing are required to produce grain oriented silicon steel. After hot rolling, cold reduction with intervened annealing will be needed. The steel grains/crystals are elongated and oriented during the rolling. Further final mill annealing will be needed for secondary re-crystallization to grow the crystals to a final desired size. The end results is that the silicon steel will be most easily magnetized in the grain direction/orientation (parallel to the direction of rolling).
Advantages of grain oriented silicon steel
* increase permeability at high flux densities with reduced core loss (M15-47 >> M2-M6)
* higher efficiency / energy-efficient / less heat
* cost reduction
* size / weight reduction
* reduced ampere-turns ratio
Thickness of the silicon steel lamination influences the core loss under mains (AC) and audio applications due to the eddy current component of the core loss. Therefore, for mains (AC) and audio use, we need to limit the maximum thickness to <=0.64mm and below. Higher lamination thickness will increase the exitation current and increased eddy current + skin effects.
Skin effects (caused by eddy currents within each lamination) causes crowding of magnetic flux out of the middle thickness section of the lamination. The eddy current produces counter magnetomotive force that caused this effect, where only a portion of the lamination cross section will be effective in carrying the flux - a great loss and may cause early saturation due to such effects.
Although so, not all applications requires the thinner lamination. All are case by case bassis and sometimes using thinner than needed is actually wasteful and brings no benefits.
J&K Audio Design