- CRGO Steel Definition: Cold Rolled Grain Oriented (CRGO) silicon steel is defined as a type of electrical steel with improved magnetic properties due to its specific grain orientation.
- Properties of CRGO Steel: CRGO steel has high magnetic permeability, high resistivity, low losses, and a high stacking factor.
- Cold Rolling Process: CRGO steel is produced through a cold rolling process that aligns grains to enhance magnetic characteristics in the rolling direction, known as Goss texture.
- Transformer Applications: CRGO steel is mainly used in transformer cores because of its high permeability, low losses, and efficient design capabilities.
- Future Scope of CRGO Steel: Research continues to improve CRGO steel by optimizing silicon content and manufacturing processes to achieve better magnetic properties.
Cold rolled grain oriented (CRGO) silicon steel is electrical steel with grains aligned so magnetisation is easiest along the rolling direction. Adding silicon (Si) to iron (Fe) in controlled amounts raises resistivity, cuts eddy-current loss, raises magnetic permeability and lowers magnetostriction. Silicon steel of this kind dates from the late 19th century. The table below shows how certain electrical and magnetic behaviors of iron changes on addition of silicon.
N. P. Goss, the early inventor of the cold rolled grain oriented silicon steel or CRGO steel manufacturing process in 1933 gave the idea in his own words “I have experimental evidence which leads me to believe that there is an apparent relation between the grain size and ductility of a specimen and its magnetic properties. This evidence shows that small, uniform grains and high ductility accompany high permeability”. That process became the basis of modern grain-oriented grades. Based on the orientation of grains there are two types of silicon-steels:
- Grain Oriented Silicon Steel (GO).
- Non-grain Oriented Silicon Steel (NGO).
The rest of this page covers grain-oriented (GO) material, with the focus on cold rolled grain oriented (CRGO) silicon steel and where it is used.
Cold Rolling of Steel
Cold rolling takes silicon-steel strip into a thickness range of about 0.1 mm to 2 mm, thinner than ordinary hot-rolled plate. Transformer CRGO laminations are often 0.23 mm to 0.35 mm. Controlled cold rolling and annealing set the Goss texture (110)[001], the direction of easy magnetization along the rolling direction. Grain-oriented steel is not used in rotating electrical machines where the magnetic field plane changes; non-grain oriented silicon steel is used there instead.
Schematic representation of the (110)[001] rolling texture or Goss texture
Properties of CRGO Steel
CRGO is a soft magnetic material and has the following properties:
- High magnetic permeability.
- Reduced magnetostriction.
- High resistivity.
- High stacking or laminating factor allows compact core designs.
- Low losses.
Grades of CRGO Steel
- The early grades of steel were known as M7(0.7watts /lb at 1.5T/60Hz) and M6(.6watts/lb at 1.5T/60Hz).
- Similarly, M5 M4 and M3 grades were developed in the late sixties.
- Hi-B is a highly oriented grade (originally from Nippon Steel) with lower core loss than conventional CRGO of similar thickness.
Application of CRGO Silicon Steel as Transformer Core
CRGO grade steel mainly finds applications as core material for power transformers and distribution transformers. This can be explained as below
- High magnetic permeability leads to low excitation current for a given flux density.
- Low hysteresis and eddy current losses.
- A high lamination factor supports compact cores and less core steel for a given rating.
- High knee saturation characteristics.
- Very low level of magnetostriction leads to noise reduction.
- The sheet is workable enough for core cutting and winding on a production line.
Future Scope of CRGO Silicon Steel
Nickel-iron, mu-metal and amorphous boron strip are used in some magnetic cores. Amorphous metal near Fe78-B13-Si9 can show lower core loss than CRGO and is used in some distribution transformers. Large power transformer cores still commonly use CRGO because of saturation induction, mechanical handling and cost.
