Why CRGO is a superior magnetic material

CRGO (cold rolled grain oriented) steel is about 3 % silicon steel that is cold rolled and then annealed so the grains line up with their easy direction of magnetisation (the cube edge) along the rolling direction. This is called the Goss texture. Along this direction its magnetic properties are much better than those of ordinary hot rolled steel, so it is called a superior magnetic material.

Reasons

1. Very high permeability along the rolling direction. Only a small magnetising force is needed, so no-load current is very low.
2. High working flux density: 1.5–1.75 T, against 1.2–1.4 T for hot rolled steel. A smaller core area and fewer turns are needed for the same rating.
3. Very low core loss: about 0.9–1.3 W/kg at 1.5 T, 50 Hz, against roughly 2.5–3.5 W/kg for hot rolled steel. The loop is narrow, and the thin sheets have high resistivity.
4. Thin laminations (0.23–0.35 mm) with a smooth surface and a thin inorganic (carlite/phosphate) coating give a high stacking factor (about 0.95–0.97).
5. No ageing, so the loss stays constant over the transformer's life.
6. Low magnetostriction, so the transformer is quieter.

 B ^         CRGO (rolling dir.)
   |      .------------
   |    /    hot rolled
   |   /  .------------
   |  / /
   | //
   +---------------------> H
 CRGO reaches high B at small H

Conditions for using it well

• The flux must flow along the rolling direction. Across the grain the properties are poor, so corner joints are mitred at 45°. This is why CRGO suits transformers (one-direction flux) better than rotating machines.
• The strips must be annealed again after cutting, and bolt holes avoided, because stress spoils the grain properties.

Because CRGO gives smaller, lighter, cooler and more efficient transformers, it is the standard core material for power and distribution transformers.