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
It is done to reduce the thickness of the steel in the range of 0.1 mm to 2 mm which cannot be achieved with hot rolling. During this process, under carefully controlled conditions optimum magnetic characteristics are achieved in the direction of rolling. This direction is also known as Goss texture (110) which is the direction of easy magnetization in the rolling direction. This can be shown in the figure below. The grain-oriented steel is not used in rotating electrical machines in which the magnetic field is in the plane of sheets but the angle between the magnetic field and rolling direction keeps changing. For this purpose nongrain oriented silicon steel is used.
Schematic representation of the (110) 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
- 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
While there have been alternatives to the CRGO grades of steel such as nickel-iron, mu-metal, amorphous boron strip, superglass etc still CRGO steel is the superior choice in the transformer industry. Alloys like the amorphous metal Fe78-B13-Si9 has shown to have much lesser core losses when used as the core of distribution transformer as compared to the CRGO grade steel. An optimum constituent of silicon addition in steel can change the texture so as to achieve desirable magnetic properties when manufactured under controlled conditions.