What Is Silicon Steel?
Silicon steel, also called electrical steel or lamination steel, is a soft magnetic iron-silicon alloy with a silicon content of roughly 0.5% to 3.5%. Alloying iron with silicon raises electrical resistivity, lowers hysteresis loss, increases permeability and almost eliminates magnetic ageing, so the material is used wherever electrical energy is converted into magnetic energy and back again. It is supplied as cold rolled strip in coil form or as cut-to-length sheet, and it is produced in two families: grain-oriented (GO) electrical steel and non-oriented (NO) electrical steel.
Because silicon also reduces saturation induction and increases brittleness, mill metallurgy has to balance magnetic performance against workability. Thin strip rolling plus a controlled final anneal is used to grow the Goss texture that gives grain-oriented product its strongly directional magnetic behaviour, while non-oriented strip is processed to keep grain orientation random so that the magnetic properties are acceptable in every direction in the plane of the sheet.
Grades and Standards
Silicon steel is ordered against ASTM, IEC, EN or GB grade systems, and in the metric system the grade name normally encodes both nominal thickness and maximum specific core loss:
ASTM A876 covers flat-rolled grain-oriented silicon steel; the older M-grade series such as M-4 and M-5 still appears on legacy drawings and must be mapped to a current grade before ordering.
ASTM A677 covers fully processed non-oriented electrical steel, while semi-processed and high-frequency grades fall under further ASTM specifications.
IEC 60404-8-4 classifies grain-oriented material and IEC 60404-8-7 covers non-oriented material.
EN 10107 applies to grain-oriented cold reduced sheet and strip; EN 10106 applies to the non-oriented product.
GB/T 2521.1 and GB/T 2521.2 are the Chinese standards for cold rolled non-oriented and grain-oriented grades.
Reading a metric grade name is straightforward: the number before the letter is the nominal thickness in hundredths of a millimetre and the figure after the letter is the maximum core loss multiplied by 100, both measured at 1.5 T and 50 Hz. A 50W470 grade is therefore 0.50 mm thick with a guaranteed maximum loss of 4.70 W/kg, 35W300 is 0.35 mm with 3.00 W/kg, and the grain-oriented designator Q follows the same logic, so 27QG100 is a 0.27 mm oriented grade at 1.00 W/kg. Always confirm the loss reference (magnetisation and frequency) before comparing two quotations, because a grade quoted at 1.0 T cannot be compared with one quoted at 1.5 T.
Key Magnetic and Physical Properties
Four properties decide grade selection: resistivity, saturation induction, magneto-crystalline anisotropy together with magnetostriction, and Curie temperature.
| Property | Typical value | Design consequence |
|---|---|---|
| Silicon content | 0.5-3.5%, grain-oriented 3.0-3.5% | Raises resistivity, lowers saturation induction and ductility |
| Density | about 7.65 g/cm3 at 3% Si | Core weight, stacking factor and clamping loads |
| Electrical resistivity | about 13 micro-ohm-cm at 3% Si against 9.7 for pure iron | Suppresses the eddy current share of core loss |
| Saturation induction | about 1.97 T at 3% Si, 2.03 T for pure iron | Sets the usable flux density and therefore frame size |
| Curie temperature | roughly 740 C at 3% Si, 770 C for pure iron | Upper limit of the magnetic working range |
| Magnetostriction | very low in high-silicon oriented grades | Controls audible noise of transformers and motors |
The trade-off is systematic: every extra tenth of a percent of silicon buys lower loss but costs saturation induction and formability. That is why highly alloyed grades above about 3.5% Si, which show almost zero magnetostriction, are brittle and are usually supplied only as thin strip.
Grain-Oriented vs Non-Oriented Electrical Steel
| Feature | Grain-oriented | Non-oriented |
|---|---|---|
| Crystal texture | Goss texture aligned with the rolling direction | Random grain orientation |
| Easy magnetisation axis | Rolling direction only | Acceptable in all in-plane directions |
| Typical silicon content | 3.0-3.5% | 0.5-3.0% |
| Typical thickness | 0.18-0.35 mm | 0.35-0.65 mm |
| Core loss at 1.5 T, 50 Hz | about 0.9-1.3 W/kg | about 2.5-6.5 W/kg |
| Main applications | Power and distribution transformer cores, wound cores | Motors, generators, relays, pulse transformers, small transformers |
| Handling | Must be cut and stacked with the flux along the rolling direction | Tolerates punching and bending in any direction |
Fabrication, Coating and Applications
Strip carries a mill-applied insulating coating whose class controls what the user may do afterwards. ASTM A976 classifies these coatings, and the coating has to survive slitting, punching, shearing and, where used, a stress-relief anneal. Laminations are stacked with the rolling direction aligned to the flux path, and transformer cores are built with mitred or step-lap joints so that the joint reluctance stays low.
Cutting method influences loss. Mechanical shearing and punching deform the cut edge and increase local loss, while laser cutting produces a narrow heat-affected zone that can be harmful in high-frequency designs because the damaged zone is a larger share of a narrow tooth. A post-cut stress relief anneal at roughly 750-820 C in a controlled atmosphere restores the loss of grain-oriented laminations, but only coatings that are specified as annealing-resistant may be used, because other coating classes burn off or react with the steel.
Typical end uses are power and distribution transformers, current and voltage instrument transformers, wound toroidal cores, industrial motors, alternators, relays, contactors and pulse transformers in power electronics. Selection normally starts from the flux density and frequency of the application, then fixes maximum acceptable core loss, then the thickness that keeps eddy current loss within budget.
Frequently Asked Questions
Q: What is the difference between grain-oriented and non-oriented silicon steel?
Grain-oriented grades have a Goss texture that concentrates magnetic performance along the rolling direction and are used in transformer cores where the flux path is fixed. Non-oriented grades have randomly oriented grains and are used in rotating machines where the flux direction changes continuously.
Q: What does a grade name such as 50W470 mean?
The first figure is the nominal thickness in hundredths of a millimetre, so 50 means 0.50 mm, and the figure after the letter is the maximum specific core loss multiplied by 100, so 470 means 4.70 W/kg at 1.5 T and 50 Hz.
Q: Why is silicon added to electrical steel?
Silicon raises electrical resistivity, which suppresses eddy currents, and it reduces hysteresis loss, increases permeability and suppresses magnetic ageing. The cost is lower saturation induction and reduced ductility above about 3.5% silicon.
Q: Which thickness is used for transformer cores?
Grain-oriented strip from 0.18 mm to 0.35 mm covers most power and distribution transformers. Thinner strip lowers eddy current loss but reduces the stacking factor and raises cost, so 0.23-0.30 mm is the common compromise at 50-60 Hz.
Q: Is silicon steel as corrosion resistant as stainless steel?
No. Silicon steel contains little or no chromium, so it depends on its mill coating plus additional varnish or paint in exposed designs. It is not a substitute for stainless steel in wet or chemically aggressive environments.
Q: What core loss can be expected from non-oriented grades?
Non-oriented grades are named by loss: 35W300 gives 3.00 W/kg, 50W470 gives 4.70 W/kg and 50W600 gives 6.00 W/kg at 1.5 T and 50 Hz. Higher silicon content and thinner strip both reduce these figures.
