- 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.
The addition of silicon(Si) in iron(Fe) in right proportions with the help of certain manufacturing process significantly improves the magnetic and electrical properties of iron. By the end of 19th century, it was discovered that the addition of silicon to iron significantly improves the resistivity of iron and so silicon steel or what we know today as electrical steel was developed. It not only brought down the eddy current losses in steel, but significant improvement in magnetic permeability and reduction in magnetostriction was observed. 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”. This idea led to a revolution in the steel industry leading to the production of high-grade steels. Based on the orientation of grains there are two types of silicon-steels:
- Grain Oriented Silicon Steel (GO).
- Non-grain Oriented Silicon Steel (GNO).
In the coming sections, we will discuss the GO steel. Specifically, we will discuss cold rolled grain oriented (CRGO) silicon steel and its applications.
Cold Rolling of Steel
Cold rolling is used to reduce steel thickness to a range of 0.1 mm to 2 mm, which hot rolling cannot achieve. This process, conducted under carefully controlled conditions, optimizes magnetic characteristics in the rolling direction, known as Goss texture (110)[001], the direction of easy magnetization. Grain-oriented steel is not used in rotating electrical machines where the magnetic field plane changes; instead, non-grain oriented silicon steel is used for those applications.
Schematic representation of the (110)[001] rolling texture or Goss texture
Properties of CRGO Steel
It 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.
- A new material called Hi-B has a remarkable degree of orientation and is 2 – 3 grade better than conventional CRGO steel products.
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 currents and lower inductions.
- Low hysteresis and eddy current losses.
- Excellent lamination factor leads to better and compact designs and hence low material required.
- High knee saturation characteristics.
- Very low level of magnetostriction leads to noise reduction.
- Enhances ease of winding and improves productivity.
Future Scope of CRGO Silicon Steel
Although alternatives like nickel-iron, mu-metal, and amorphous boron strip exist, CRGO steel remains the top choice in the transformer industry. Alloys like the amorphous metal Fe78-B13-Si9 have shown lower core losses compared to CRGO steel. Optimizing the silicon content in steel and manufacturing it under controlled conditions can further enhance its magnetic properties.
