The role of silicon in silicon steel

Silicon's Good Effects

1) As [Si] increases, the resistivity of steel increases, thereby reducing the eddy current loss;
2) Reduce the anisotropy of the crystal, making magnetization easier and reducing magnetic resistance, thereby reducing hysteresis loss;
3) Promote the coarsening of ferrite grains, reduce the grain interface, reduce coercive force, and increase magnetic permeability;
4) Promote the graphitization of carbon in steel and improve the harm of carbon to magnetism;
5) A strong deoxidizing element that can remove oxygen that is detrimental to magnetism. (1) Si
Si is conducive to magnetization, worsens brittleness, and rusts (easily oxidized). Hot rolling ≤ 4.6%, cold rolling ≤ 3.5%.
However, due to vacuum and progress in processing technology, steel types with Si>6.5% have appeared.

(2)C
Extremely harmful element.
↓B, ↑P (expand γ)
Cold rolled steel [C]＜0.005%; hot rolled steel [C]＜0.012～0.015%.
During smelting, [C] is <0.06%, and during continuous decarburization and annealing, [C] is <0.005%.

(3)S
There are both pros and cons
S<0.003%:
↑Hot brittleness;
↓Pn (sulfide inclusions, uneven structure, hindering the growth of F grains during annealing)
However, MnS is beneficial to single-oriented cold-rolled silicon steel sheets (coarse grains are obtained during secondary crystallization, and favorable grain orientation can be obtained by heat treatment after cold rolling). (4)P
Harmful elements.
Like Si, ↓γ promotes grain growth, ↑ρ, ↓P, and ↑electromagnetic properties.
However, it is cold brittle and difficult to cold work. Cold rolled materials should be removed as harmful elements, [P]<0.015%.
For hot-rolled steel with low [Si] for motors, insufficient brittleness is the main contradiction, but for the sake of electromagnetic performance, the control of P can be relaxed.

(5) Al
Similar to Si:
↑ρ, ↓P;
Promote grain growth, graphitization (C concentration, little impact on electromagnetic properties), deoxidation, etc.;
Improve electromagnetic properties, can replace part of Si (because Si does not make steel brittle); fix N, reduce magnetic aging (non-oriented steel 0.15 ~ 0.65% Al)

(6)Mn
Limit elements.
Expanding γ, α + γ appears, which causes phase transformation stress and the appearance of A during annealing, which is detrimental to magnetism.
Mn can ensure the beneficial effect of S, and the generated MnS will precipitate along the grain boundaries to hinder the growth of primary grains, which is conducive to the development of secondary recrystallization and the acquisition of grain orientation.
Mn/S=8～10.

(7) Gas
Oxygen: Oxide + dissolves in α, which is very harmful to electromagnetic properties: ↑P; ↓μ; ↓B
N is the same as O: ↑coercive force; ↓μ; magnetic failure (O is dissolved in α for this reason, as time goes by, P↑; B↓).
When 0.005%N→0.002%, P↓15%
But: Oriented steel, an appropriate amount of AlN can improve the orientation texture and obtain excellent magnetic properties. Hi-B: 0.0062～0.0092%N, 0.022～0..032%Al.
H: ↑P; H brittle; swelling (bubble defects during rolling). Generally, vacuum treatment is required.