Chemical Composition Control of Silicon Steel Sheet for GO and NO Grades
Mar 20, 2023
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Why Chemical Composition Controls Magnetic Performance
Silicon steel, also called electrical steel, is the standard soft magnetic material for transformers, motors and generators. Core loss, permeability and magnetising current are largely fixed at the steelmaking stage, because they depend on the chemical composition of the melt and on the subsequent rolling and annealing route. Two product families come from the same alloy system: grain-oriented (GO) sheet for transformer cores and non-oriented (NO) sheet for motors and generators.
Silicon Content: Resistivity and Eddy-Current Loss
Silicon is the defining alloying element. It raises electrical resistivity, which suppresses eddy-current loss, but too much silicon embrittles the strip, reduces cold workability and lowers saturation induction. Practical ranges are summarised below.
| Type | Typical Si range | Typical grades | Main use |
|---|---|---|---|
| Non-oriented (NO) | 0.5-3.5 % | 50W470, 35W300 | Motors and generators; isotropic magnetic properties |
| Grain-oriented (GO) | 2.9-3.5 % | 30Q130 and similar | Transformer cores; low loss along the rolling direction |
Carbon: Ultra-Low Levels in Both Families
Residual carbon is the most damaging impurity in electrical steel. In GO material carbon must fall to about 0.003-0.005 % or lower after decarburisation annealing, because carbon held in solution prevents the secondary recrystallisation that develops a sharp Goss texture. In NO material the limit is less severe but still tight, typically not more than 0.005 %, since carbon precipitates in service and causes magnetic ageing - a slow rise in core loss over the life of the machine. Vacuum degassing, careful deoxidation practice and a wet-hydrogen decarburisation anneal are the usual control measures.
Manganese, Phosphorus and Sulfur Limits
Manganese: typically 0.1-0.5 % depending on grade. It improves hot workability and counteracts sulfur brittleness, but excess manganese raises loss through solid-solution effects.
Phosphorus: increases resistivity and hardness and improves punching quality in thin NO sheet, yet it impairs cold workability and promotes segregation, so it is normally held below about 0.05-0.1 %.
Sulfur: a harmful impurity that forms MnS inclusions which pin domain walls and hinder grain growth. Final sulfur is normally 0.005 % or less.
Aluminium, Nitrogen and Steel Purification
In GO production, fine MnS and AlN precipitates are deliberately retained as inhibitors. They restrict normal grain growth during primary recrystallisation so that Goss-oriented grains can grow abnormally during the final high-temperature anneal. Once the texture is established the inhibitors must be dissolved and the steel cleaned. Purification combines vacuum degassing (RH or VD type), calcium treatment or ladle metallurgy to modify inclusion shape, continuous casting with electromagnetic stirring for homogeneous solidification, and a final high-temperature hydrogen anneal.
Correct composition control gives stable low core loss together with low hysteresis loss, because clean steel allows easy domain-wall movement. Composition windows and the resulting magnetic properties are specified in GB/T 2521, the IEC 60404-8 series, ASTM A876 for GO grades and ASTM A677 for NO grades. An order should always state the grade designation, the guaranteed core loss at a defined flux density and frequency such as 1.7 T at 50 Hz for GO grades, and the density basis used for the loss calculation.
Frequently Asked Questions
Q: Why is silicon added to electrical steel?
Silicon raises electrical resistivity and therefore reduces eddy-current loss; the addition is limited by brittleness and by falling saturation induction.
Q: What is the maximum carbon content of grain-oriented silicon steel?
After decarburisation annealing the carbon content is normally held at 0.003-0.005 % or lower so that the Goss texture can develop.
Q: What causes magnetic ageing in non-oriented sheet?
Carbon left in solution precipitates during service and raises core loss gradually; keeping carbon at or below about 0.005 % prevents it.
Q: Why is sulfur harmful in electrical steel?
Sulfur forms MnS inclusions that pin domain walls and restrict grain growth, which increases hysteresis loss.
Q: What is the role of aluminium and nitrogen in GO steel?
They form fine AlN precipitates that act as grain-growth inhibitors during primary recrystallisation, and are then removed during the purification anneal.
Q: Which standards define silicon steel grades and core loss?
GB/T 2521, the IEC 60404-8 series, ASTM A876 for grain-oriented grades and ASTM A677 for non-oriented grades.