Grain-oriented silicon steel (CGO/HGO/HiB) is specified by lamination thickness, silicon content and guaranteed core loss at 1.5 T/50 Hz, with non-oriented (NGO) grades picked on the same loss class plus magnetic permeability at the operating flux density. Buyer-side catalogues list transformer-grade coils at US$700–20,000 per piece with a 1-piece MOQ, signalling the wide price spread that comes from grade, gauge and lamination finish rather than volume [S1].
The selection problem is not "which brand" but which combination of thickness, Si% and domain-refined treatment will hit a target no-load loss budget inside a given stator or core window. Transformer and motor designers who skip the loss-per-kg math end up overpaying at the coil stage or burning it back as operating cost over the asset life.
Grain-Oriented vs Non-Oriented: Crystallography Drives the Trade
Grain-oriented electrical steel (GOES, grades typically designated M2–M6 in legacy US practice and 30Q120–30G130 in GB/T 2521 notation) is rolled so the Goss texture {110}<001> aligns the easy magnetisation axis with the rolling direction, giving core loss in the 0.8–1.2 W/kg band at 1.5 T/50 Hz for 0.30 mm CGO material.
Non-oriented silicon steel (NGO, e.g. 50W470–35W270 family) is used where flux rotates during a cycle — stator and rotor laminations in induction motors, small transformers and reactors. Selection is governed by core loss at 1.0 T or 1.5 T and by magnetic polarisation at 5000 A/m, both tabulated in the mill test certificate. NGO cannot match GOES in the rolling direction, but it is isotropic in the plane, which is why motor designers who mistakenly specify GOES in a rotating-flux path pay for a directional premium they never use.
Silicon Content and Lamination Thickness: The Two Levers You Move First
Silicon raises resistivity, narrows the hysteresis loop and suppresses magnetostriction, but it also embrittles the strip and limits cold reduction. Above ~3.5% Si the alloy becomes too brittle for conventional cold rolling, which is why amorphous and 6.5% Si ultra-thin strip are sold as specialty products rather than commodity coils.
Thickness is the second hard lever. Halving lamination gauge roughly quarters the eddy-current component of core loss, which is why the trend from 0.35 mm to 0.27 mm to 0.23 mm tracks the efficiency classes of distribution transformers (the 0.18–0.20 mm HiB band exists for high-frequency transformers and traction). The penalty is stacking factor: a 0.23 mm lamination stacks to ~94–96% theoretical density, while 0.35 mm reaches 97–98%, so core cross-section grows for the same net iron area when you downgauge.
Core Loss Class, Magnetostriction and Noise: Hidden Specification Lines
Core loss is the line item that gets a number on the datasheet (P1.5/50 in W/kg for transformer grades, P1.0/50 or P1.5/50 for NGO), but magnetostriction is the spec that makes a transformer quiet. GOES magnetostriction at 1.5 T peaks around 1–3 × 10⁻⁶ for stress-relief-annealed (SRA) material and can rise tenfold if the core is clamped too hard, which is why "domain-refined + SRA + tension coating" appears as a bundle on HiB quotes rather than three line items. [S1]
For motors, NGO selection adds thermal class and weldability: the same 50W470 coil that drops 4.7 W/kg at 1.5 T/50 Hz will not survive the welding currents of an automatic stator line as well as a 50W600 with tighter composition control, so the welding and annealing route inside a motor plant often overrides the loss class on paper. Buyers who only chase the lowest W/kg number frequently re-specify within a quarter because their stamping dies chip on the higher-Si coil.
Coating, Insulation and Stacking Factor
Mill-applied insulating coatings (C-2, C-3, C-4, C-5 in AISI convention, or the equivalent inorganic/organic/inorganic-organic hybrid layers per IEC 60404-1-12) serve two purposes: interlaminar resistance to block eddy currents across the stack, and stress relief to keep magnetostriction low. For transformer cores the coating must survive a 780–840 °C stress-relief anneal; for motors it must resist die lubricant and welding spatter. Mixing a transformer-grade coating into a motor line is one of the most common spec errors, because the coating chemistry that anneals clean in a transformer oven either burns off early in a motor anneal or pollutes the stator bore. [S2]
Stacking factor is the quiet number that changes every other dimension: 0.23 mm HiB with forsterite coating typically stacks to 94–95%, while 0.35 mm CGO with C-5 insulation reaches 97–98%. Core designers therefore specify the net iron area first and back-solve lamination count and stack height from the density budget. A specifier who locks in thickness before solving the area budget ends up over-rating the core or under-utilising the winding window.
Selection Criteria Compared: CGO vs HGO/HiB vs NGO
On a four-criterion cut the choice usually shakes out as: (1) flux path — unidirectional transformer core → GOES, rotating flux machine → NGO; (2) target no-load loss — <0.8 W/kg at 1.5 T/50 Hz forces HiB with domain refinement, 0.8–1.2 W/kg is the CGO band, 2.0–4.7 W/kg covers the standard NGO families; (3) operating frequency — 50/60 Hz favours 0.23–0.35 mm gauges, 400 Hz and above pushes to 0.10–0.18 mm or to amorphous ribbon; (4) total cost of ownership — HiB commands a 15–30% price premium over CGO at the same thickness, recovered over the transformer service life in iron-loss electricity [S1][S2].
Material families cross-cut by application: GOES covers power and distribution transformers, audio output transformers, and high-efficiency reactor cores; NGO covers induction and synchronous motor laminations, small transformer cores, and ballasts; amorphous and 6.5% Si strip cover distribution-transformer retrofits and high-frequency chokes where core loss dominates the operating budget. The wrong family in the wrong application is the most expensive error — a HiB coil in a rotating-flux motor delivers no loss benefit and pays the directional premium for nothing. For related decision logic on the broader alloy family, the alloy steel vs aluminum specifier cut is a useful parallel read on how grade, density and modulus trade against cost in adjacent material families.
Sourcing Levers: Mill Audit, Test Certificate and Slitting Tolerance
Procurement signals that matter at the quotation stage include: mill test certificate per heat with actual Si%, thickness profile across the coil, and core-loss verification at 1.5 T/50 Hz (Epstein frame per IEC 60404-2 or single-sheet tester per IEC 60404-3). Slitting tolerance on transformer strip is typically ±0.02 mm on width and a burr limit below 25 µm; strip width below 50 mm and tolerances tighter than ±0.01 mm are specialty slitter territory and move the price band [S2].
A specifier watching both signals can usually lock the right grade at a 3–6 month price valley rather than chasing spot. For complementary reading on the broader electrical-steel ecosystem, the silicon steel encyclopedia entry consolidates metallurgy and grade maps, and the alloy steel reference is the right landing page when the application drifts away from electrical grades and into structural/HSLA territory.
For component-level specifications, see silicon carbide.