Cold Rolled Ordinary Grain-Oriented Silicon Steel: Grades and Processing
Oct 10, 2025
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What Ordinary Grain-Oriented Silicon Steel Is
Ordinary grain-oriented silicon steel, often abbreviated CGO for conventional grain oriented, is cold rolled silicon steel in which a strong Goss texture has been developed so that the easy magnetisation direction of the crystals lies along the rolling direction. Silicon content is around 2.9-3.3 %, which raises resistivity and cuts eddy current loss without making the strip impossible to roll. The texture is produced by a long sequence of hot rolling, cold rolling with intermediate annealing, decarburising and a high temperature secondary recrystallisation anneal that leaves grains many millimetres long and aligned with the strip.
Because the texture is directional, the material is anisotropic by design: loss measured along the rolling direction is much lower than loss measured across it. That is exactly what a transformer core wants, since flux travels along the limbs and yokes in one direction.
Grades, Thickness and Loss Levels
| Property | Typical value | Note |
|---|---|---|
| Silicon content | 2.9-3.3 % | Gives the resistivity needed for low eddy loss |
| Standard thickness | 0.23, 0.27, 0.30, 0.35 mm | Thinner gauges for lower loss and noise |
| Conventional grade loss | about 1.10-1.30 W/kg at 1.7 T, 50 Hz | Typical values for ordinary CGO grades |
| High permeability grade loss | about 0.85-1.05 W/kg at 1.7 T, 50 Hz | Higher induction and lower loss |
| Domain refined grade loss | down to about 0.70 W/kg at 1.7 T, 50 Hz | Extra surface treatment divides the domains |
European designations follow EN 10107, where the number carries the maximum specific total loss at 1.7 T and 50 Hz multiplied by 100 and the thickness is given in hundredths of a millimetre, so an M120-30 grade is a 0.30 mm strip guaranteed at 1.20 W/kg. Suffixes distinguish conventional, high permeability and domain refined material. IEC 60404-8-7 is the parallel international specification, while ASTM A876/A876M covers flat rolled, grain-oriented silicon iron in the fully processed condition and quotes grade limits for core loss and induction at defined test conditions. Loss is verified on Epstein specimens to IEC 60404-2 or by single sheet tester to IEC 60404-3.
Surface Coating and Stress Engineering
The coating on grain-oriented strip does more than insulate. During the final high temperature anneal, magnesium oxide applied to the surface reacts with the steel to form a thin forsterite glass film; a phosphate based layer and a final insulation coating are then applied on top. This layered system puts the strip under controlled tensile stress, which narrows the magnetic domains and lowers loss, and it must survive shearing, annealing in the customer's works and the varnish cure. Coating thickness, adhesion and resistance are therefore part of the specification, not cosmetic details.
Shearing, Building and Stress Relief
Every mechanical operation damages the texture locally: shearing creates a stressed zone a few millimetres wide, punching a slightly larger one, and bending at a corner disrupts the domain structure. Practical rules are to keep burr height low, avoid re-cutting offcuts smaller than the stressed edge, keep laminations flat, and use a stress relief anneal where the design allows it. Clamping pressure and joint design then decide how much of the material's advantage survives into the finished core.
Applications
Ordinary grain-oriented steel serves distribution transformers, small and medium power transformers, instrument transformers and reactors. Where very low no-load loss or very low noise is contractual, high permeability or domain refined grades replace the conventional material, and amorphous strip competes at the lowest loss end of distribution transformer design.
Frequently Asked Questions
Q: What is the difference between ordinary and high permeability grain-oriented steel?
A: Both are textured along the rolling direction, but high permeability grades reach lower loss and higher induction through tighter texture control, quoted typically at 0.85-1.05 W/kg at 1.7 T and 50 Hz against about 1.10-1.30 W/kg for conventional grades.
Q: Why is grain-oriented steel only good in one direction?
A: The Goss texture aligns the easy magnetisation axis with the rolling direction, so loss and permeability are much better along that axis than across it.
Q: What does M120-30 mean in EN 10107?
A: A 0.30 mm grain-oriented strip whose maximum specific total loss is 1.20 W/kg measured at 1.7 T and 50 Hz.
Q: Can ordinary grain-oriented steel be welded?
A: Welding destroys the local texture and creates a short circuit between laminations, so cores are normally clamped or bolted rather than welded along the flux path.
Q: Is stress relief annealing needed for cut cores?
A: For wound and cut cores it usually is, because it removes winding and cutting stress, but the anneal temperature and atmosphere must be compatible with the insulation coating.
Q: How does thickness affect noise?
A: Magnetostriction makes the core vibrate; thinner strip and better clamping both reduce the resulting sound level, which is why low noise transformers often specify 0.23 mm or 0.27 mm material.