Composition, Structure and Properties of Cold-Rolled Non-Oriented Electrical Steel
Dec 14, 2023
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Composition of Cold-Rolled Non-Oriented Electrical Steel
Cold-rolled non-oriented electrical steel is an ultra-low carbon silicon soft magnetic alloy used in the magnetic circuits of motors, generators and small and medium power transformers. The commercial grades used in rotating machine duty are designated by their specific loss, for example 50W600, 50W800 and 50W1300 on a 0.50 mm nominal thickness, where the three digits give the loss in W/kg at a polarisation of 1.5 T and a frequency of 50 Hz. Across such a grade family the deliberately varied alloying elements are silicon and aluminium; the residual elements are kept at low levels so that the magnetic properties remain stable.
| Element | Typical range in ultra-low carbon non-oriented steel | Effect on magnetic behaviour |
|---|---|---|
| Silicon | about 1.5 % to 3.0 % | raises resistivity and lowers total loss; lowers saturation polarisation and increases brittleness in cold working |
| Aluminium | about 0.2 % to 0.6 % | acts like silicon on resistivity, loss and anisotropy, with a weaker effect on strength and hardness |
| Carbon | kept at very low level, typically below about 0.005 % | residual carbon precipitates on ageing and degrades loss, so the carbon level is held as low as practical |
| Manganese | about 0.2 % to 0.5 % | combines with sulphur and affects grain growth and texture during processing |
| Sulphur | normally below about 0.01 % | forms sulphide particles that control grain size, which in turn controls loss and permeability |
| Phosphorus | usually a residual, generally below about 0.05 % | improves resistivity and punchability but embrittles the sheet at high level |
Because silicon raises resistivity in proportion to its content, it reduces the eddy current component of total loss, which is the dominant part of loss at the higher frequencies used by inverter driven machines. The same addition lowers the saturation polarisation, so above roughly 3.0 % silicon the loss gain is offset by a loss of magnetic induction and by cold working difficulty. Aluminium behaves in a similar way and is the second lever available to the metallurgist, but its influence on grain size, resistivity, anisotropy and mechanical hardness is not as strong as that of silicon.
Structure and Texture Control
Two structural features decide the final properties: grain size and crystallographic texture. A larger grain reduces the grain boundary area and therefore the hysteresis loss, but an excessively coarse grain degrades the high frequency behaviour and the mechanical properties. Texture matters because the easy magnetisation direction of the body-centred cubic lattice is the cube edge, so a favourable texture of the {100} type aligned with the sheet plane improves induction, while the conventional non-oriented product is deliberately kept as close to random as the process allows so that properties are the same in every in-plane direction.
The process route that delivers this structure is a controlled chain: hot rolling, normalising, single or double cold reduction with intermediate annealing, final recrystallisation annealing and the application of a surface insulation coating. Precipitation of fine aluminium nitride and manganese sulphide is used during processing to restrict grain growth at the wrong stage, then dissolved or coarsened at the final anneal so that recrystallisation can proceed to the intended grain size. Coating thickness and coating type determine interlaminar insulation resistance, which is what limits eddy current loss in a laminated stack.
Magnetic and Mechanical Requirements
Grade designations follow the national and international series, with IEC 60404-8-4 and ASTM A677/A677M applied internationally and GB/T 2521.2 for the Chinese designation system. Specific loss is measured on an Epstein frame to IEC 60404-2 or on a single sheet tester to IEC 60404-3, and the guarantee must state the polarisation, the frequency and the test method. Performance grades used in high speed and miniaturised machines are increasingly specified by loss at 400 Hz and above rather than only at 50 Hz, together with a minimum magnetic induction at a stated field strength. Mechanical requirements cover yield strength, elongation, hardness and punchability, because the steel has to be stamped into stator and rotor laminations with a burr height small enough to avoid excessive interlaminar short circuits.
Effects of Alloying and Processing on Loss
Total loss is conventionally separated into hysteresis, eddy current and anomalous components. Silicon and aluminium attack the eddy current term through resistivity. Grain size and texture attack the hysteresis term. Thickness reduction attacks the eddy current term again, which is why thin gauge products are used for high frequency rotating machines even though they cost more per kilogram. Stress relief annealing after stamping attacks both terms by removing the dislocations introduced by shearing, and it is standard practice for high efficiency motor cores where the added process cost is repaid by the loss reduction.
The practical consequence for a buyer is that two sheets bearing the same grade designation can differ in high frequency loss, induction and punchability, because the designation fixes only the 50 Hz loss limit and the thickness. A specification for a motor or an inverter driven application should state the frequency of interest, the minimum induction and the mechanical properties, and it should require a test certificate measured by the named method.
Frequently Asked Questions
Q: What does the number in a grade such as 50W800 mean?
A: The 50 states the nominal thickness of 0.50 mm, W indicates non-oriented electrical steel and 800 states a specific loss of 8.00 W/kg measured at 1.5 T and 50 Hz.
Q: Why is silicon added to non-oriented electrical steel?
A: It raises electrical resistivity, which lowers eddy current loss, and it reduces magnetic anisotropy; the addition is limited to about 3.0 % because higher silicon content lowers saturation polarisation and makes cold working difficult.
Q: Why is carbon kept so low in electrical steel?
A: Residual carbon precipitates during service and increases loss through magnetic ageing, so ultra-low carbon practice keeps the level near or below about 0.005 % and the mill gauges the result on the finished product.
Q: Which standards specify non-oriented electrical steel and its measurement?
A: IEC 60404-8-4 and ASTM A677/A677M for the material, GB/T 2521.2 for the Chinese grade series, and IEC 60404-2 or IEC 60404-3 for the loss measurement method.
Q: Is a coarser grain always better for low loss?
A: Not always; a coarser grain lowers hysteresis loss but can raise the high frequency loss and reduce mechanical strength, so the target grain size is chosen against the frequency band of the application.
Q: Why does punching affect magnetic properties?
A: Shearing introduces dislocations and residual stress near the cut edge, which raises hysteresis loss locally; stress relief annealing after stamping restores a large part of that loss.