D. STEINER PETROVI^: SUSTAINABLE AND STRATEGIC SOFT-MAGNETIC Fe-Si-Al ALLOYS PRODUCED BY ...
681–686
SUSTAINABLE AND STRATEGIC SOFT-MAGNETIC Fe-Si-Al
ALLOYS PRODUCED BY SECONDARY METALLURGY
TRAJNOSTNE IN STRATE[KE MEHKOMAGNETNE Fe-Si-Al
ZLITINE, IZDELANE S POSTOPKI SEKUNDARNE METALURGIJE
Darja Steiner Petrovi~
Institute of Metals and Technology, Lepi pot 11, Ljubljana, Slovenia
Prejem rokopisa – received: 2023-12-11; sprejem za objavo – accepted for publication: 2023-12-15
doi:10.17222/mit.2023.1072
In Slovenia, Fe-Si-Al alloys for non-oriented, silicon-steel sheets are designed and manufactured in a sustainable manner. Fer-
rous scrap is recycled, and, therefore, CO 2 emissions are greatly reduced. However, sustainable technologies based on secondary
metallurgy have many limitations. Impurities that cannot be effectively removed from the steel melt increase the complexity of
the material’s behavior during processing and use. One of the most contaminating elements is copper (Cu). In this review, the
focus is on phenomena related to the Cu impurity during the specific steps of the metallurgical processing of selected Fe-Si-Al
alloys. The identified challenges concerning the efficiency of some technological phases related to the presence of Cu in
Fe-Si-Al, non-oriented electrical steels might motivate further (inter)disciplinary research, basic or applied. In order to follow
the set goals of the EU and achieve climate neutrality by 2050, silicon electrical steels produced with sustainable circular-econ-
omy approaches must be recognized as a strategic material for the EU.
Keywords: silicon steel, secondary metallurgy, impurities, copper, magnetic losses, EU Green Deal
V Sloveniji so zlitine Fe-Si-Al za neorientirane silicijeve jeklene plo~evine zasnovane in izdelane na trajnosten na~in. V
postopkih sekundarne metalurgije recikliramo jekleni odpad, s ~imer se znatno zmanj{ajo emisije CO 2. Vendar pa imajo
trajnostne tehnologije, ki temeljijo na sekundarni metalurgiji, {tevilne omejitve. Ne~isto~e, ki jih ni mogo~e u~inkovito
odstraniti iz jeklene taline, znatno pove~ajo kompleksnost obna{anja materiala med predelavo v kon~ni proizvod in v njegovi
rabi. Ena najbolj obremenilnih ne~isto~ je baker (Cu). V pregledu je poseben poudarek namenjen pojavom, ki so povezani z
ne~isto~o bakrom in u~inkovitostjo posameznih proizvodnih faz v izdelavi Fe-Si-Al neorientiranih elektroplo~evin.
Identificirani izzivi, povezani s prisotnostjo bakra v zlitinah Fe-Si-Al lahko motivirajo nadaljnje (inter)disciplinarne raziskave,
bodisi temeljne bodisi aplikativne. Da bi sledili zastavljenim ciljem EU in dosegli podnebno nevtralnost EU do leta 2050, bi
morala biti silicijeva elektro jekla, proizvedena s pristopi trajnostnega kro`nega gospodarstva, prepoznana kot strate{ki material.
Klju~ne besede: silicijevo jeklo, sekundarna metalurgija, ne~isto~e, baker, magnetne izgube, Evropski zeleni dogovor
1 INTRODUCTION
The synthesis and processing of metallic materials is
the largest single source of greenhouse-gas emissions in
the world.
1
Metals production accounts for 40 % of all
industrial greenhouse-gas emissions, 10% of global en-
ergy consumption, 3.2 billion tonnes of minerals mined,
and several billion tonnes of by-products every year.
Therefore, metals must become more sustainable.
1
The
European Green Deal is the EU’s new growth strategy,
aiming to transform the EU into a fairer and more pros-
perous society, with a modern, resource-efficient and
competitive economy, with no net emissions of green-
house gases by mid-century.
2
Specific priority topics of
the EU Green Deal have been defined, i.e., Clean En-
ergy, Circular Economy, Efficient Renovations, Sustain-
able Mobility, Sustainable Food, Preserving Biodiversity,
all of which contribute to climate action and zero-pollu-
tion ambitions. However, achieving a climate-neutral and
circular economy requires the full mobilisation of indus-
try. All industrial value chains will have a key role to
play. To meet the EU’s energy and climate targets for
2030, EU countries need to establish integrated national
energy and climate plans that outline how the EU coun-
tries intend to address five areas: energy efficiency,
renewables, greenhouse-gas-emission reductions, inter-
connections, and research and innovation. Therefore, the
whole industry value chains along with the research and
innovation sector will have a key role to play.
2
Becoming
the world’s first climate-neutral continent by 2050 is a
once-in-a-lifetime opportunity to modernise the EU’s
economy and society towards a sustainable future.
The purpose of the article is to present the role of the
soft-magnetic material silicon steel and give an overview
of the state of the key challenges related to the material
produced via circular-economy routes. Since silicon
steels are designed and manufactured in a sustainable
manner also in Slovenia, a special focus is dedicated to
the phenomena related to Cu impurity during the specific
steps of the metallurgical processing of selected Fe-Si-Al
alloys.
Materiali in tehnologije / Materials and technology 57 (2023) 6, 681–686 681
UDK 669.14.018.583 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(6)681(2023)
*Corresponding author's e-mail:
darja.steiner@imt.si (Darja Steiner Petrovi~)

2 SOFT-MAGNETIC SILICON STEELS
Soft-ferromagnetic materials are alloys that magnet-
ise and demagnetise with low hysteresis losses.
3,4
A mag-
netic material is considered "soft" when its coercive field
strength is of the order of or lower than the earth’s mag-
netic field (about 40 A/m). A soft-magnetic material can
be employed as an efficient flux multiplier in a large va-
riety of devices, including transformers, generators, and
motors, to be used in the generation, distribution, and
conversion of electrical energy, and a wide array of appa-
ratus, from household appliances to scientific equip-
ment.
5
Silicon steels are the most important soft-mag-
netic material occupying most of the soft-magnetic mar-
ket. Almost 97 % of soft-magnetic materials produced
today are silicon electrical steels made of Fe-Si or
Fe-Si-Al alloys. Over 12 million tons are produced annu-
ally, around 80 % of which are non-oriented (NO)
grades, the remainder being grain-oriented (GO) silicon
steels. In electrical machines NO grades are used almost
exclusively.
6,7
The development of new classes of
high-specific-power electrical machines and drives is
constantly required.
8
In Slovenia, non-oriented electrical steels are pro-
duced by SIJ Acroni d.o.o., Jesenice.
9
Their SIWATT
brand includes non-oriented electrical steel that is essen-
tial for the economical production, conversion, distribu-
tion and use of electricity. Various grades of Fe-Si-Al
electrical steels under SIWATT are characterised by
magnetising ability, high magnetic permeability and low
losses and are built into the magnetic cores of electric
motors and generators.
For the optimal design of the soft-magnetic material
ex-ante material’s characterization is necessary, includ-
ing a definition of the chemical composition of the steel
along with extensive thermodynamic and microstructure
modellings, and magnetic-loss prediction. Ex-post chem-
ical, mechanical, metallographic and magnetic character-
ization analyses are dedicated to the optimization of the
material’s properties. These generally include analyses
of the steel’s chemical composition and purity, mechani-
cal tests, microstructure analysis and grain-size measure-
ments, the texture characterization and the magnetic-loss
measurements. Many of their properties derive from an
interplay of processing, microstructure, and chemistry,
on scales that reach from manufacturing dimensions.
This turns research on magnetic materials not only a
multi-physics and multi-scale problem, but also requires
close collaboration between characterization, processing,
and theory.
3
3 THE METALURGY OF SILICON STEELS
PRODUCED USING CIRCULAR-ECONOMY
ROUTES
The accumulation of impurities in the recycling of
steel impacts the quality of secondary steel. Understand-
ing impurity levels is crucial in the context of the prolif-
eration of circular-economy policies, expected high recy-
cling rates, and the growth of scrap consumption.
10
The
complexity of the materials’ behaviour, and conse-
quently, the metallurgical processing of silicon steels for
magnetic applications increases with the complexity of
steels’ chemical composition. Si being a ferrite stabilizer
increases the ferrite phase fraction of Fe.
11
The addition
of Si to iron brings notable changes in the physical, me-
chanical and magnetic properties of the Fe. The most no-
table effect regards the electrical resistivity. Alloying
with Al instead of Si leads to similar physical and struc-
tural effects, while not causing material embrittlement.
However, Al is very reactive and prone to oxide forma-
tion, but it brings an increase in magnetostriction.
5
When
circular-economy approaches are involved, i.e., second-
ary-metallurgy routes, the steel’s cleanliness becomes an
important factor for silicon steels that are to be further
processed into grain-oriented or non-oriented electrical
steel sheets and coils. In the electric-arc-furnace (EAF)
steelmaking process, the main raw material is ferrous
scrap, and an inspection of its quality is the first crucial
step in producing clean steel. Ferrous scrap is the most
recycled material in the world, and through its utiliza-
D. STEINER PETROVI^: SUSTAINABLE AND STRATEGIC SOFT-MAGNETIC Fe-Si-Al ALLOYS PRODUCED BY ...
682 Materiali in tehnologije / Materials and technology 57 (2023) 6, 681–686
Figure 2: Rotor magnetic cores for electric motors
9
Figure 1: Coils of soft-magnetic Fe-Si-Al steel (SIWATT brand of SIJ
Acroni d.o.o., Jesenice, Slovenia)
9

tion, CO
2
emissions can be significantly reduced. Ac-
cording to the statistical data of The World Steel Associ-
ation,
12
the imports and exports of scrap rose globally to
>200 million metric tonnes in total, and that value is still
increasing. However, efforts must be focused on the
overall chemical composition of the steel. Since different
qualities of scrap steel are used, the contents of impurity
elements Cu, Sn, Sb, As, Ti, etc. can vary, and the con-
trol of the residual elements must be appropriate.
13,14
It is
well known that the Cu content constantly increases in
secondary steels. The first cross-national comparison of
impurity accumulation in recycled steel
10
has shown that
the content of Cu impurity was higher in Western Europe
and Japan than in Ukraine, Vietnam, and China. The
main sources of copper contamination are automotive
scrap, waste electric and electronic equipment (WEEE)
and demolition waste. Cu is mixed with cut metal scrap
during shredding and is difficult to separate, unless done
manually. This scrap is not fully separated from copper
parts in electric motors and electrical wires. Another Cu
source is scrap with a high Cu content, like rebars and
stainless steel. Because information on the type of con-
sumed scrap in all investigated countries was not avail-
able, the variation of the Cu content between countries
was considered only in terms of the separation practices
during recycling.
10
4 CHALLENGES CONCERNING COPPER
IMPURITY
Impurities that cannot be effectively removed from
the steel melt increase the complexity of the material’s
behavior. One of the most contaminating elements in
secondary steels is copper (Cu).
4.1 Decarburization
The decarburization of industrial cold-rolled sheets
made of Fe-Si-Al steels produced by secondary metal-
lurgy is performed by annealing in a wet gas mixture at
temperatures around 850 °C. The principal reaction that
controls the decarburization
7,15,16
is equation (1):
[C]
Fe
+H
2
O(g) = CO(g) + H
2
(g) (1)
The exact technological parameters depend mainly
on the chemical composition of the steel and are carried
out according to prescribed technological methods. The
decarburization is determined by the chemical reactions
between the gas mixture and the carbon at the steel’s sur-
face. The temperature of the annealing, the composition
of the gas mixture and the chemical composition of the
steel are the influencing parameters that determine the
kinetics and the mechanism of decarburization. The
decarburization process of steel consists of several steps,
among which is the diffusion of carbon to the steel sur-
face among the most influential ones. One of our previ-
ous studies shows that the carbon surface segregation in
the Fe-1.8Si-0.5Al alloys was hindered by the presence
of larger amounts of copper (e.g, up to 0.6 w/% Cu) in
the steel.
Carbon diffusion within the matrix of silicon steel is
considered the rate-limiting step in the early stages of
decarburization.
17
Elucidating the effectiveness of this
metallurgical process in complex alloy systems such as
secondary Fe-Si-Al steel is of utmost importance.
4.2 Magnetic properties
The core loss of the non-oriented electrical steel is
defined as the dissipation of electrical energy in the form
of heat during magnetization by an alternating current. In
spite of the fact that the core-loss prediction in laminated
magnetic circuits has generated relentless scientific re-
search efforts, the almost forty-year-old Statistical The-
ory of Losses is still prevailing in this domain.
18
Since
modern electrical energy conversion exposes laminated
magnetic circuits to higher frequencies and larger excita-
tion fields, novel models for core-loss prediction are pro-
D. STEINER PETROVI^: SUSTAINABLE AND STRATEGIC SOFT-MAGNETIC Fe-Si-Al ALLOYS PRODUCED BY ...
Materiali in tehnologije / Materials and technology 57 (2023) 6, 681–686 683
Figure 3: Carbon surface segregation in Fe-1.8 w/%Si-0.5 w/%Al alloys with different contents of Cu (0.3 w/%Cu and 0.6 w/%Cu, respectively)
after annealing in vacuum

posed.
18–22
Numerous parameters, including the chemical
composition and the production technology, influence
the magnetic properties of the electrical steel.
23
Increased
core loss could also result from the increased cumulative
(total) value of the impurity elements in the steel. Impu-
rities can have a synergy effect on the deterioration of
the magnetic properties. Their elevated contents mean a
larger number of inclusions and precipitates.
7,24–26
An investigation of four different grades of fully-fin-
ished, non-oriented, electrical steels manufactured of
secondary Fe-Si-Al alloys, with the silicon content from
1.0 to 2.5 w/% has shown that the slope of the total mag-
netic losses curve with increasing frequency is deter-
mined by the rate of magnetic domains mobility. The
higher is the density of magnetic domains, the smaller is
their mobility. The complexity of the magnetic domains
and their surroundings is decreasing with increasing con-
tent of alloying elements. The rate of magnetic domains
mobility is affected by the number of non-metallic inclu-
sions and precipitates, which hinder the movement of
magnetic domains and eddy currents, which produce
losses and limit the mobility of domain walls.
27
4.3 Magnetic aging
A necessary condition for obtaining high-quality
electrical steel is a minimal amount of magnetic ag-
ing.
28,29
Therefore, the electrical steel must be free of car-
bon and contain only small quantities of nitrogen. A low
carbon content is important because the carbon that is in
the form of  -carbide at temperatures above 150 °C is
eliminated within the crystal grains, and with the nitrides
acting as a barrier to the movement of Bloch walls, there
is a substantial deterioration in the electromagnetic prop-
erties. This process is called magnetic aging. In terms of
the magnetic aging of non-oriented electrical steel of dif-
ferent qualities in reference to Si content, at temperatures
of 225 °C and 300 °C, the most significant impact comes
from the carbon content in the steel. To prevent magnetic
aging associated with the precipitation of carbides, pre-
dominantly Fe
3
C and  -carbide Fe
2.4
C, the decarburi-
zation of the cold-rolled steel sheets is a very important
processing step.
7
Various iron carbides precipitate and
degrade the magnetic properties by interfering with mag-
netic domain-wall motion. The slow precipitation of car-
bides during service can cause a substantial increase in
the core losses. A study on the magnetic aging
30
of
Fe-Si-Al alloys produced using secondary metallurgy
has shown that the extent of the magnetic aging was af-
fected by the copper content. The results indicate that the
proportion of magnetic aging decreases with increasing
copper content in samples.
30
In general, the mobility and
rotation of the magnetic domains are affected by the
non-metallic inclusions and precipitates, which consti-
tute barriers to the movement of the magnetic domains.
To clearly determine the role of copper precipitates
on the magnetic behavior, final magnetic properties and
magnetic aging of non-oriented electrical steels, further
studies are needed.
5 POTENTIAL RESEARCH TOPICS ON THE
SUSTAINABILITY OF METALS
Recently, Raabe has proposed several novel direc-
tions for a potential basic research on the direct
D. STEINER PETROVI^: SUSTAINABLE AND STRATEGIC SOFT-MAGNETIC Fe-Si-Al ALLOYS PRODUCED BY ...
684 Materiali in tehnologije / Materials and technology 57 (2023) 6, 681–686
Figure 5: Epstein tester for measurements of magnetic properties of
soft-magnetic silicon steels
9
Figure 4: Instrument for measurements of magnetic properties of
soft-magnetic silicon steels
9
– MPG 200D Brockhaus Messtechnik

sustainability of metals,
1
some of them being: (i) effi-
ciency of metallurgical processes, (ii) recycling of
well-sorted scrap, (iii) the nanoscrap recycling from
complex modern products (microelectronics and cata-
lysts), (iv) a deep understanding of all impurity and con-
taminant effects in the entire spectrum of alloys, scrap,
reductants, by-products, waste products and minerals, (v)
multi-element and high-quality recycling, (vi) material-
and recycling-oriented product design, etc. Based on
this, metallurgical sustainability can be expected to break
down into a large spectrum of different disciplines and
branches.
1
6 SUMMARY
Ferrous scrap is the most recycled material in the
world, and through its utilization CO
2
emissions can be
greatly reduced. In order to follow the set goals of the
EU and achieve climate neutrality by 2050, silicon elec-
trical steels produced with sustainable circular-economy
approaches must be recognized as a strategic material for
the EU. Therefore, the soft-magnetic silicon steels are
strategic not only in the economic, but also in the social
sense. Sustainable technologies based on the secondary
metallurgy that recycles steel scrap have many limita-
tions. Impurities that cannot be effectively removed from
the steel melt increase the complexity of the material and
its behavior. One of the most contaminating element in
secondary Fe-Si-Al alloy is copper (Cu). The identified
challenges concerning the unexplained mechanisms and
efficiency of some metallurgical processes related to the
presence of Cu in Fe-Si-Al steels (e.g., decarburization,
magnetic domains mobility, magnetic aging, etc.) may
motivate further (inter)disciplinary research, basic or ap-
plied.
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