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Lecture 9

Electrical Steels

Dr. Javad Mola

Institute of Iron and Steel Technology (IEST)

Tel: 03731 39 2407

E-mail: mola@iest.tu-freiberg.de

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Applications of Electrical Steels

Electrical steels account for 97% the soft magnetic material market (6 Mt per year).

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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

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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

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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

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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

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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

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Saturation Induction

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Saturation Induction and Coercivity

Fe

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Fe-Si Phase Diagram

Fe-Si electrical steels

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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 –

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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

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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

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Grain-Oriented (GO) vs. Non-Oriented (NO)

Fe-3%Si steel

BmaxGO

BmaxNO

GO steel:

- higher Bs

- lower Hc

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Grain-Oriented (GO) vs. Non-Oriented (NO)

thickness=0.36 mm

Core Loss Magnetic Induction

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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)

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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

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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

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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

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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.

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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)

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Processing of NO Steels

NO steels constitute nearly 81% of the soft magnetic materials market.

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Processing of GO Steels

GO steels constitute nearly

16% of the soft magnetic

materials market.

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