Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
1
Lecture 9
Electrical Steels
Dr. Javad Mola
Institute of Iron and Steel Technology (IEST)
Tel: 03731 39 2407
E-mail: [email protected]
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
2
Applications of Electrical Steels
Electrical steels account for 97% the soft magnetic material market (6 Mt per year).
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
3
Magnetization Cycle and Hysteresis
The hysteresis loop is represented by the solid red curve; the dashed blue curve indicates the initial magnetization. The area within a loop represents a magnetic energy loss per unit volume of material per magnetization–demagnetization cycle; this energy loss is manifested as heat that is generated within the magnetic specimen and is capable of raising its temperature.
Remanence, Br
Coercive force, Hc
Soft magnet
(easily
magnetized-
demagnetized) Hard magnet
(high resistance to
demagnetization)
Saturation magnetization, Bs
𝐵 = 𝜇𝐻 , 𝐵 = 𝜇𝐻 + 𝜇𝑀
Permeability
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
4
A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in the transformer's primary winding creates a varying magnetic flux in the core and a varying magnetic field impinging on the secondary winding. This varying magnetic field induces a varying electromotive force (emf) or voltage in the secondary winding. Transformers can thus be designed to efficiently change AC voltages from one voltage level to another within power transmission networks.
Transformers
E-core transformer
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
5
Hysteresis losses -When the magnetic field through the core changes, the magnetization of the core material changes by expansion and contraction of the tiny magnetic domains it is composed of, due to movement of the domain walls. This process causes losses, because the domain walls get "snagged" on defects in the crystal structure and then "snap" past them, dissipating energy as heat. This is called hysteresis loss. It can be seen in the graph of the B field versus the H field for the material, which has the form of a closed loop. The amount of energy lost in the material in one cycle of the applied field is proportional to the area inside the hysteresis loop. Since the energy lost in each cycle is constant, hysteresis power losses increase proportionally with frequency.
Transformer Core Losses
Eddy-current losses -If the core is electrically conductive, a changing magnetic field induces circulating loops of current in it, called eddy currents. Eddy current loops flow in planes perpendicular to the magnetic field. The energy of the currents is dissipated as heat in the resistance of the core material (this is used in induction heating). The power loss is proportional to the area of the eddy current loops and inversely proportional to the resistivity of the core material. Eddy current losses can be reduced by making the core out of thin laminations which have an insulating coating. Laminations with surface
insulation coating
Loops of
electrical
current
Monolithic material
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
6
Electrical Resistivity
Red
uce
d e
dd
y cu
rren
ts
Alloying element Fe-X binary alloys, %
Ele
ctri
cal r
esi
stiv
ity
at 2
0 °
C, n
Ω.m
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
7
Electrical Resistivity
Because iron is a relatively good conductor, it cannot be used in rapidly changing magnetic field applications, such as in high frequency transformers, as intense eddy currents would appear. This results in huge energy losses. Use of Fe is therefore limited to direct current (dc) applications. For alternating current (ac) applications, Fe-Si alloys (silicon steels) are used.
Red
uce
d e
dd
y cu
rren
ts
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
8
Saturation Induction
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
9
Saturation Induction and Coercivity
Fe
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
10
Fe-Si Phase Diagram
Fe-Si electrical steels
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
11
Functions of Si in Electrical Steels
Increase of electrical resistivity + Suppression of the loop enabling desirable grain growth by heat
treatment at higher temperatures + Increased solid solution strengthening (smaller deformation near
edges after punching/stamping operations and therefore reduced need for deburring) +
Development of preferred orientation grain structure + Magnetostriction is a phenomenon which causes strain
(deformation) during magnetization. Up to 3%Si: magnetostriction increases (increased noise
emission). – Between 3 and 6.5%Si: magnetostriction decreases and
becomes very low at 6.5%Si. + Reduction of saturation magnetization and Curie temperature – Reduced formability –
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
12
Magnetic (Magnetocrystalline) Anisotropy
111
100
110
easy
magnetization
direction
Observed and calculated B-H curves for differently-oriented single crystals of 3 to 3.5%Si steel
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
13
Grain-Oriented (GO) vs. Non-Oriented (NO)
Grain-oriented (GO): Processed in such a way that optimum magnetic properties
are developed in the rolling direction (coarse-grained sheets with Goss texture)
High permeability and low core losses More expensive than the NO types Used in applications where the direction of magnetic flux is
constant for example in high-efficiency transformers
Non-oriented (NO): Similar magnetic properties in all directions Cheaper than the GO types Used in applications where the direction of magnetic flux is
changing, for example in electrical motors or generators
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
14
Grain-Oriented (GO) vs. Non-Oriented (NO)
Fe-3%Si steel
BmaxGO
BmaxNO
GO steel:
- higher Bs
- lower Hc
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
15
Grain-Oriented (GO) vs. Non-Oriented (NO)
thickness=0.36 mm
Core Loss Magnetic Induction
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
16
Grain-Oriented (GO) Transformer Cores
Note: Preferred orientation in rolled sheets is usually represented in the form of (hkl)[uvw] where hkl denotes the miller indices of the plane parallel to the sheet surface and uvw are
the indices for the direction parallel to the rolling direction.
Fabrication process of GO silicon steels used in the transformer cores
(typically Fe-3 wt%Si steel) aims at developing a (110)[001] texture (also
known as Goss texture or cube-on-edge) so as to minimize the core
loss. This is because the <100> directions are easier to magnetize.
[uvw]
(hkl)
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
17
Core Losses vs. Frequency
Total losses GO 3%Si
0.3 mm
H //[001]
B=1500 mT
Frequency, Hz
Lo
sses,
W /
kg
0 50 100 150 200 250 300
0
6
4
2
12
10
14
Hysteresis
loss
8 Classical
eddy loss
Anomalous
eddy loss
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
18
Low C Steel vs. GO and NO Electrical Steels
Grade Composition µmax ,
relative Resistivity ρ,
µ𝛺cm
Coercive force Hc ,
A/m
Saturation magnetization Bs ,
T
Low C steel Fe-0.05%C 5,000 10 80 2.15
Non-oriented (NO) Fe-Si
Fe-3%Si (50 ppm C,
0.15%Mn) 7,000 60 40 1.97
Grain-oriented (GO)
Fe-Si
Fe-3%Si (30 ppm C,
0.07%Mn) 40,000 47 8 2.00
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
19
Grain Size
High-purity iron at B=1 T (10 kG)
Grain boundaries impede the motion of domain walls, so coarse grains are preferred.
Softer dc magnetic properties
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
20
Grain Size
GO NO
Phys Panomalous
Pclassical
Ptotal = Phys + Pano + Pcla Ptotal = Phys + Pano + Pcla
In ac applications, there may be an optimum grain size range that provides best magnetic performance. The optimum grain size is larger for GO steels.
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
21
Aging of electrical steels increases the core losses because fine carbides oppose the domain wall migration. Low C contents may be achieved by vacuum-degassing in the liquid state and/or later in the solid state by decarburization annealing in H2-containing atmospheres during post-cold rolling annealing.
Carbon Content
Unalloyed iron
B=1 T (10 kG)
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
22
Processing of NO Steels
NO steels constitute nearly 81% of the soft magnetic materials market.
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
23
Processing of GO Steels
GO steels constitute nearly
16% of the soft magnetic
materials market.
Ste
el A
pp
licati
on
/ F
err
ou
s M
ate
rials
II S
S2015
24