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Three‐Dimensional Investigations of Inclusionsin Ferroalloys
PER
Yanyan Bi,� Andrey Karasev, and Pär G. Jönsson
As the requirements on material properties increase, there has been a demand on anadditional knowledge on the effect of impurities in the ferroalloys on the properties. Thus,the number, morphology, size, and composition of inclusions in four different ferroalloys(FeTi, FeNb, FeSi, and SiMn) were investigated. This was done in three dimensions (3D) byusing scanning electron microscopy in combination with energy dispersive spectroscopyafter electrolytic extraction of the ferroalloy samples. The non‐metallic and metallicinclusions were successfully analyzed on the surface of film filter. Thereafter, the particlesize distribution was plotted for most of the non‐metallic inclusions. The non‐metallicinclusions were found to be REM oxides in FeTi, FeSi, and SiMn, Al2O3, Ti–Nb–S–O oxides inFeNb and silicon oxides in SiMn. Moreover, the intermetallic inclusions were found to be aTi–Fe phase in FeTi, Ca–Si, and Fe–Si–Ti phases in FeSi and a Mn–Si phase in SiMn. Inaddition, the almost pure single metallic phases were found to be Ti in FeTi, Nb in FeNb, andSi in FeSi.
1. Introduction
Ferroalloys are commonly used in the steel industry to
alloy or deoxidize the steel during the secondary steel-
making process before casting. However, the addition of
ferroalloys may cause a supply of deleterious impurities to
the liquid steel. In some cases, these impurities are clearly
related to the raw material and the way of producing a
ferroalloy and are more or less unavoidable.[1] As the
requirements on material properties increase, the effect of
impurities in ferroalloys on the steelmaking process has
been studied more deeply. Jonsson et al.[2] concluded the
future demands on ferroalloys from the view point of
customers. The first kind of ferroalloys is the high purity
ferroalloys that are used for late additions during ladle
refining. The second is the less refined grades of lower
cost, which should be added earlier in the steelmaking
production line. Thus, in order to meet the forthcoming
demands, ferroalloys should be characterized with respect
to the number, morphology, size, and composition of
inclusions.
Some publications about the inclusions in different
kinds of ferroalloys as well as characterization method are
listed in Table 1. SiO2, Al2O3, and some intermetallic phase
[�] Y. Bi, A. Karasev, P. G. JonssonKTH-Royal Institute of Technology, Brinellvagen 23, Stockholm, SE-10044, SwedenEmail: [email protected]
DOI: 10.1002/srin.201300157
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were observed in FeTi alloys.[1,3–6] Inclusions in FeSi have a
direct link to the Al and Ca contents in the ferroalloys.[3,7–9]
Similarly, the types of inclusions in FeMn depend on the
silicon[10,11] and C[1,14] contents in the ferroalloys. Also,
the inclusions in different grades of FeSi alloys have a
connection to the steel cleanliness. However, inclusions in
FeMn only have a temporary influence on the inclusions in
liquid steel according to laboratory and plant trials.[12,13]
Finally, inclusions in FeMo, FeNb, FeP alloys were also
studied by some researchers.[1,3,14]
However, the above mentioned studies have been
concentrated on two dimensional (2D) investigations,
except for the study by Pande et al.[1,14] They used acids
to dissolve steel and to study the impurities in three
dimensions (3D). However, some impurities might dis-
solve during the acid extraction. Moreover, very few
researchers tried to provide information about the particle
size distribution of inclusions in ferroalloys.[2] Therefore,
the present study is concentrated on three dimensional
investigations of inclusion characteristics (such as com-
position, size, number, and morphology) in the following
four ferroalloys: FeTi, FeNb, FeSi, and SiMn. The
electrolytic extraction (EE) method was used in this study.
This dissolution technique leads to less dissolution of
inclusions in comparison to the acid method.[15] More
specifically, the EE method is recommended for the
analyses of REM oxides, Al2O3, and SiO2 inclusions which
are also observed in this study. After separating the
particles on the surface of film filter, the particle size
distribution was determined for most of the major non-
metallic inclusion types found in the samples.
steel research int. 85 (2014) No. 4 659
Ferroalloya) Method Inclusion Ref.
Composition Shape Size
[mm]
Number Other
FeTi (35) Acid (3D) Si/SiO2,
Al–Ti–O
Irregular 1–50 9–9.5% Titanium oxide is not reduced
completely by the aluminum
during the production.
Pande
et al.[1]SEM
EDS
FeTi (35) SEM Iron oxide
magnesium oxide
Irregular – – – Tiekink
et al.[3]AIA
FeTi (35) SEM Al2O3, TiN,
Al4TiO8
– �20 PSD Inclusions in FeTi35
increased the product
rejection. FeTi70 is cleaner
than FeTi35.
Kaushik
et al.[5]
FeTi (70) EDS Pande
et al.[6]
FeTi (70) Acid (3D) Si/SiO2,
Al–Ti–O
Faceted 1–20 1–1.5% Inclusions in the matrix are
in good agreement with the
extracted inclusions.
Pande
et al.[1]
SEM Al2O3,
Fe–Ti–Al2O3
– – – – Gasik
et al.[4]EDS
FeNb (65) Acid (3D) Not any The microstructure of FeNb
reveals two phases with no
apparent inclusions.
Pande
et al.[1]SEM
EDS
FeSi
(65, 75)
FGA SiO2, Al2O3,
(Al,Ca,Si)xOy,
(Al,Mg)xOy
– – PSD FeSi with lower Al content
results in a low oxygen
content and a small quantity
of Al2O3 inclusions.
Grigorovich
et al.[8]
FeSi (75) QTM FeSi2.5, Si2Ca,
Si2Al2Ca, FeSi2Ti,
Fe4Si8Al6Ca
– 2.8–22.4 PSD Impurities are mostly
intermetallic phases and
oxide inclusions are very rare.
The grade of ferrosilicon has
a direct impact on the
inclusions in the liquid steel.
Wijk and
Brabie[7]SEM
EMP
FeSi (75) SEM – – – – High Si phase with Al, Cu and
Ti, high FeSi phase with V and
Fe. Some phases contain Ce.
Tiekink
et al.[3]AIA
FeSi (75) SEM MgO–Al2O3,
SiO2–Al2O3–MnO–CaO
Spherical 1–3 – Al in FeSi enhace the
formation of spinel inclusion.
However, Ca prevents it.
Mizuno
et al.[9]EDS Irregular
HC FeMn
(75)
Acid (3D) C Powder – 0.3–0.5% There is no inclusion due to
the high C content.
Pande
et al.[1,14]
LC FeMn
(80)
Acid (3D) C, Si/SiO2 Powder – 0.2–0.25% –
HC SEM MnS, SiMn oxide,
Mn oxide
– – – The FeMn alloy contains
distinguishable carbide and
nitride phases.
Tiekink
et al.[3]FeMn (80) AIA
Table 1. Continued
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Ferroalloya) Method Inclusion Ref.
Composition Shape Size
[mm]
Number Other
LC&MC SEM MnO, MnS,
MnO–SiO2–MnS
Dendritic
Rhombic
3–180 PSD The amount of inclusions is
inversely proportional to the
carbon content in FeMn. Low
oxygen FeMn has lower
inclusion mean diameters.
Sjoqvist
et al.[10,12,13]
FeMn (80) PC MIC Spherical
FeMo (70) Acid (3D) Si/SiO2, Al2O3 Spherical 10–50 0.5–0.9% Inclusions in the matrix are
in good agreement with the
extracted inclusions.
Pande
et al.[1]
SEM CaO–SiO2–Al2O3,
SiO2–Al2O3
– – – – Gasik
et al.[4]EDS
FeP (30) Acid (3D) (Fe,P,Mn,Ti)O Angular 10–80 0.3–0.4% The distribution of Ca, Mn,
and Ti is inhomogeneous and
the best addition sequence is
obtained.
Pande
et al.[1,6]SEM
EDS
FeP (35) SEM – – – – High P phase with Si and low
P phase with Mn, Ti, and Cu.
Tiekink
et al.[3]AIA
HC, high carbon; LC, low carbon; AIA, automated inclusions analysis; PSD, particle size distribution; FGA, fractional gas analysis; QTM,Image Analyzer Qantimet; EMP, electron microprobe.a)(), the content of alloying element.
Table 1. Inclusions in different ferroalloys.
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2. Experimental
The investigations of inclusions in this study were
carried out by using the following commercially
obtained ferroalloys: FeTi, FeNb, FeSi, and SiMn. The
typical chemical compositions of these ferroalloys are
listed in Table 2. The specimens were cut from a
ferroalloy lump to small pieces (15� 10� 3mm3) before
EE.
In this study, EE was applied for 3D investigations of
inclusion characteristics in the ferroalloys. The EE of
ferroalloys was carried out using a 10% AA (10 v/v%
acetylacetone – 1w/v% tetramethylammonium chloride –
methanol) electrolyte. The current density was set to 30–
40mA cm�2 during the EE. The weight of the dissolved
Ferroalloys C N S P Fe Si
FeTi 0.18 0.23 0.008 0.012 22.426 0.21
FeNb 0.15 – 0.05 0.06 28.296 2.48
FeSi 0.089 – 0.006 0.023 22.386 75.975
SiMn 0.053 – 0.0059 0.046 8.771 29.219
Table 2. Composition of different ferroalloys (wt%).
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ferroalloys during EE was 0.08–0.14 g. After extraction, the
solution containing inclusions was filtrated through a
polycarbonate (PC) membrane film filter with an open
pore size of 1 or 3mm. The 3mmfilmfilter was only used to
analyze the large size (>3mm) inclusions in FeTi. The
extracted inclusions were investigated in 3D on a surface
of film filters by using scanning electron microscopy
(SEM). The size of the inclusions dV, is the diameter of
a spherical inclusion. The equivalent size of a non-
spherical inclusions and clusters was calculated from
Equation 1:
dV ¼ Lmax þWmax
2ðfor non� spherical inclusions and clustersÞ
ð1Þ
Al Ca Mn Ti Cr Ni Nb
3.14 – 0.05 71.56 – – –
1 – – 0.82 – – 66.23
1.166 0.224 0.032 0.083 0.0058 0.0039 –
– – 61.471 0.2 0.03 0.03 –
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where Lmax and Wmax are the maximum length and widthof the inclusion, respectively.
The number of inclusions per unit volume (NV) was
calculated as follows:
NV ¼ n � Af
Aobs� rmW dis
ð2Þ
where n is the number of inclusions in the appropriate size
interval, Af is the area of the film filter (1396mm2), Aobs is
the total observed area, rm is the density of the ferroalloy
matrix and Wdis is the dissolved weight of the ferroalloy
during extraction.
The composition of inclusions was determined by
energy dispersive spectroscopy (EDS). The total ob-
served area of film filter for different samples was
varied from 0.22 to 2.23mm2. Here, it should be
pointed out that there might some undissolved ferro-
alloy matrix pieces on the surface of the film filter
after filtration. However, these can easily be distin-
guished from the inclusions based on composition and
morphology.
Type Type A Type B
Typical photo
Size range (mm) 6–25 1–8
Composition Ti–Fe Ti–Fe
Percentage (%) 9 75
Table 3. Classification of inclusions in FeTi alloys.
Type Ti Fe Al Si Ni
Type A 45–76 21–49 0–1.5 0–1.3 1–5
Average 61 36 0.9 0.2 2
Type B 45–55 43–47 1–4 0–0.2 1–2
Average 48 45 1.7 0.05 1
Type C 89–97 0–4 0.5–3 – –
Average 92 1 1 – –
Type D – 0–2 0.6–2.0 6–8 –
Average – 0.6 1.4 7 –
Table 4. Composition of different type of inclusion in FeTi alloys (w
662 steel research int. 85 (2014) No. 4
3. Results
3.1. Inclusions in FeTi
The typical photos, size ranges, composition, and per-
centage of the inclusions observed after EE of FeTi alloy are
shown in Table 3. A more detailed composition informa-
tion is listed in Table 4. There are two types of intermetallic
phases in ferrotitanium, a faceted Type A and a flower-like
Type B. The size range of the Type A inclusions is much
larger than that for Type B inclusions. However, the
percentage of Type B inclusions is eight times larger than
the Type A inclusions. The Ti, Fe contents, and the ratios
are different in these two types of intermetallic phases. The
element N is present in both types of intermetallic phases,
as can be seen from the elemental mapping in Figure 1.
However, the light element, such as C and N cannot be
analyzed in an accurate manner by EDS. The average
value of N in these two types of inclusions is 0.008 and
0.1mass%, respectively. Therefore, it is not possible to say
that the titanium carbides and nitrides, which has high
melting points, exist or not in this case. The Type C
Type C Type D
3–15 1–21
Ti–Fe–Al–O REM-Si–Cr–Al–O
10 6
Ce La Pr Nd O N
– – – – 0–1 0–0.5
– – – – 0.7 0.008
– – – – 0–3 0–0.6
– – – – 1.7 0.1
– – – – 3–10 0–1.7
– – – – 5 0.3
27–35 28–32 2–3 6–7 17–22 –
30 30 3 7 20 –
t%).
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Figure 1. Elemental mappings of different inclusion types in FeTi alloys.
0 2 4 6 8 10 12 14 160
1000
2000
3000
4000
Nv
(mm
-3)
dv
(µm)
Figure 2. Particle size distribution of Type D inclusions in FeTialloys.
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inclusions are almost pure Ti, with small amounts of O, Al,
and Fe. There is also some N in this type of inclusions, as
can be seen fromFigure 1. However, the distribution of N is
not homogenous. Also, the Type D inclusions mostly
consist of REM (Ce, La, Pr, Nd) oxides. These oxides easily
form clusters, which may cause nozzle clogging.[16–18]
According to the literature review, no REM oxides have
been observed in FeTi. The existence of the REM oxides
might be due to the raw material. The particle size
distribution of this type of inclusion is shown in Figure 2.
According to the results reported by Pande et al.,[1,6] the
quantity of inclusions in a FeTi35 alloy ismuch higher than
that in a FeTi70 alloy. Moreover, the size range of
inclusions in a FeTi35 alloy is wider because of the
different manufacturing routes for these alloys. This is in
good agreement with the result reported by Kaushik
et al.,[5] who showed that the sliver index in an FeTi33 alloy
is higher than in an FeTi70 alloy. In addition, Al2O3 and
SiO2 were observed in a FeTi70 alloy.[1,5] However, REM
oxides were detected instead of Al2O3 and SiO2 in the
FeTi70 alloy in this study. The reason for this might be that
the REMmetal has a higher affinity to oxygen compared to
aluminum and silicon.
According to the Fe–Ti–Al phase diagram,[19] the
melting points of Type A, Type B, and Type C inclusions
are about 1200, 1300, 16008C, respectively. The melting
points are lower or approximately equal to a steel
temperature of 16008C. Therefore, it is assumed that these
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intermetallic phases will dissolve after the addition to the
liquid steel.
3.2. Inclusions in FeNb
The typical inclusions in a FeNb alloy are shown in Table 5
and the detailed composition information is listed in
Table 6. As can be seen, Type A inclusions aremostlymade
steel research int. 85 (2014) No. 4 663
Type Type A (I) Type A (II) Type B Type C
Typical photo
Size range (mm) 2–12 5–27 1–14 2–21
Composition Al–O Al–O Ti–Nb–S–O Nb–Ti–O
Percentage (%) 20 4 17 59
Table 5. Classification of inclusions in FeNb alloys.
Type Fe Nb Al Ti Si O S
Type A 0–0.9 0–2 42–54 – – 45–56 –
Average 0.1 0.4 50 – – 49 –
Type B – 10–26 0–0.7 28–53 0–1.5 8–23 21–32
Average – 15 0.3 44 0.5 12 26
Type C 0–4 80–94 0–2 0–3 0–0.4 2–15 –
Average 0.7 91 0.5 1.2 0.2 7 –
Table 6. Composition of different type of inclusion in FeNb alloys (wt%).
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up of pure aluminum oxides. Also, inclusions are present
as single inclusions (Type A (I)) as well as clusters (Type A
(II)). The presence of Al2O3 is linked to the aluminothermic
reduction of FeNb. It is well known that Al2O3 inclusions
can significantly affect the mechanical and fatigue
properties and also cause nozzle clogging problems during
casting.[20] Type B inclusions are Ti–Nb–S–O inclusions.
The sulfur content in a FeNb alloy is 0.05mass% (Table 2).
This is the highest value among the ferroalloys that have
Figure 3. Elemental mapping of Type B inclusions in FeNb alloys.
664 steel research int. 85 (2014) No. 4
been analyzed in this study. According to the elemental
mapping of this type of inclusion in Figure 3, the
distribution of Ti, Nb, S, and O is homogenous, which
means that only one phase exists in this type of inclusions.
However, up to now, only the Ti–Nb system[21] and the
Ti–S system[22] were studied. Therefore, the physical and
chemical characteristic of the Ti–Nb–S–O system is not
known. However, nomatter if they will dissolve or not after
the addition to the liquid steel, they will definitely be
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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 300
1000
2000
3000
4000
5000
6000
7000
Type A (I) Type A (II) Type B
Nv
(mm
-3)
dv
(µm)
Figure 4. Particle size distribution of Type A and Type B inclusionsin FeTi alloys.
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deleterious to the cleanliness of the steel due to the
presence of S and O. The particle size distribution of
these two type inclusions is shown in Figure 4. The Type C
inclusion is almost a pure Nb phase. The melting
point of the Type C phase is 24008C[23] if only the Nb
and Ti are considered. In addition, the Type C inclusions
Type Type A Type B
Typical photo
Size range (mm) 2–20 5–9
Composition REM-Si–Fe–Ti–O Ca–Si–Al–N
Percentage (%) 36 4
Table 7. Classification of inclusions in FeSi alloys.
Type Si Fe Ti Al Ca
Type A 3–10 0–8 0–5 0–1 –
Average 6 2 2 0.4 –
Type B 35–49 0–3 – 2–6 40–57
Average 44 0.6 – 3 45
Type C 30–35 34–41 24–30 1–3 –
Average 33 36 27 2 –
Type D 80–94 0–2 – 0–2 –
Average 89 0.4 – 0.3 –
Table 8. Composition of different type of inclusion in FeSi alloys (w
� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
represent the majority (59%) of the inclusions in a FeNb
alloy. This is followed by the Type A (I) and Type B
inclusion. Finally, a Type A (II) inclusion is present, which
is an Al2O3 cluster. The size range of the Type A (II) and
Type C inclusions is wider than that for Type A (I) and
Type B inclusions.
No inclusions could be observed in a FeNb alloy in
Pande’s study after dissolution when using strong acids.[1]
However, the characteristics of inclusions in FeNb, such
as the number, morphology, size, and composition can
accurately be investigated on the surface of film filter after
EE. This implies that the EE method is more suitable than
the acids dissolution method for extraction of inclusions
present in FeNb.
3.3. Inclusions in FeSi
Typical inclusions and the information of them in FeSi are
shown in Table 7 and 8. Type A inclusions are made up by
REM oxides with some amounts of Si, Fe, and Ti. Ce oxides
have also been observed by other researchers in FeSi
alloys.[3] The particle size distribution of this type inclusion
is shown in Figure 5. Type B inclusions are CaSi
intermetallic inclusions that contain small amounts of
Type C Type D
2–10 1–26
i–O Fe–Si–Ti–Al–O Si–O
20 40
Ni Mn Ce La Pr Nd O
– 0–1 26–40 21–33 1–3 3–7 15–26
– 0.3 34 27 2 4 20
2–5 – – – – – 4–9
2 – – – – – 5
– 0–2 – – – – 0–4
– 0.6 – – – – 1
– – – – – – 5–20
– – – – – – 10
t%).
steel research int. 85 (2014) No. 4 665
0 2 4 6 8 10 12 14 16 18 20 220
1000
2000
3000
4000
5000
6000
7000N
v (m
m-3
)
dv
(µm)
Figure 5. Particle size distribution of Type A inclusions in FeSialloys.
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Al and Ni. The melting point of the CaSi intermetallic
phase is about 13008C[24] if only the Ca and Si contents are
considered. Type C inclusions represent a Fe–Si–Ti–Al
intermetallic phase. The phase diagram of Fe–Si–Ti–Al
system calculated by the Therm-Calc software is shown in
Figure 6. As can be seen, the melting point of Type C
inclusions is about 14608C. The Type B and Type C
Figure 6. Phase diagram of Fe–Si–Ti–Al system.
666 steel research int. 85 (2014) No. 4
intermetallic compounds were also detected by Wijk and
Brabie.[7] Also, the Type D inclusions contain almost only
pure Si, but with small amounts of O and Al. The melting
point of the pure Si phase is about 14008C.[19] Overall, the
Type D and Type A inclusions are the main inclusion
present in an FeSi alloy. Moreover, the size range of them
are wider than that for Type B and Type C inclusions.
It is reported that Al in FeSi enhances the formation of
MgO · Al2O3 spinel inclusions, and that Ca in FeSi has an
effect to prevent it.[9] However, Ca may react with
aluminum and form calcium aluminates that lead to
nozzle clogging.[25] Therefore, the Al and Ca contents in
FeSi alloys should be accurately determined before being
added to the steel. It should be mentioned that the acid
extraction is not suitable to use for an FeSi alloy due to that
Si is not directly soluble in acids.[1] Therefore, the EE
method is more appropriate.
3.4. Inclusions in SiMn
The characteristics of inclusions in SiMn are shown in
Table 9 and a detailed composition information is listed in
Table 10. As can be seen, Type A inclusions contain REM
oxides with some amounts of Si,Mn, andMg, Al.Moreover,
Type B inclusions are almost pure Al2O3 inclusions. Type C
inclusions are mostly silicon oxide with some amounts of
Ca and Mg. Type D inclusions are also silicon oxide, but
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Type Si Mn Fe Mg Al Ca Ce La Pr Nd O
Type A 3–10 0–4 – 0–2 0–1 – 20–40 10–30 1–2 3–6 20–34
Average 8 2 – 1 1 – 32 22 2 4 28
Type B – – – – 42–56 – – – – – 43–60
Average – – – – 50 – – – – – 48
Type C 30–47 – – 0–2 0–1 6–10 – – – – 47–59
Average 37 – – 1 0.3 8 – – – – 53
Type D 36–57 0–3 – – – – – – – – 40–63
Average 45 1 – – – – – – – – 54
Type E 24–38 50–68 0–8 – – – – – – – 6–10
Average 30 59 4 – – – – – – – 7
Table 10. Composition of different type of inclusion in SiMn alloys (wt%).
30000
40000
50000
60000
-3)
Type Type A Type B Type C Type D Type E
Typical photo
Size range (mm) 1–26 2–5 6–12 1–8 3–16
Composition REM-Si–Mn–O Al–O Si–Ca–Mg–O Si–Mn–O Mn–Si–Fe–O
Percentage (%) 56 2 6 8 28
Table 9. Classification of inclusions in SiMn alloys.
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with small Mn contents. Type E inclusions are Mn–Si
intermetallic phases with small Fe contents. The melting
point of a Type E inclusion is 12508C, if only Mn and Si are
considered.[26] Overall, REM oxides are the main type of
inclusions found in SiMn alloys. This is followed by Type E
intermetallic phases. The particle size distribution of
Type A inclusions is shown in Figure 7.
0 2 4 6 8 10 12 14 16 18 20 22 24 260
10000
20000Nv
(mm
dv
(µm)
Figure 7. Particle size distribution of Type A inclusions in SiMnalloys.
4. Discussion
4.1. Soluble Impurities
According to the future demands on the ferroalloys from
the stand point of the customers in the steel industry, it is
imperative that more comprehensive investigations of
the dissolved element contents in the ferroalloys will be
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needed.[2] The ferroalloy quality depends on the specificuse. However, in general the oxygen, carbon, sulfur,
phosphorous, and nitrogen contents are important to
know. The total oxygen content does not only reflect the
total amount of oxides inclusions, but it also represents the
mean diameter of the inclusions in ferroalloys.[10] It is
reported[10] that a similar trend was observed between the
C content and the amount and mean diameter of
inclusions, just in an inverse proportionality. Also, sulfur
and phosphor is known to have a deleterious effect on the
mechanical properties of steels.
Strong deoxidizers like Al, Ca, Mg, Ti, and Ce have a
large effect on the inclusion formation in the steel. These
elements may react with the oxides that exist in the liquid
steel after the addition of a ferroalloy to form complex
inclusions. For example, the TiO2 inclusions change to
Al–Ti–O complex inclusions after the addition of Al to the
steel.[27] Another problem is that large size inclusions may
be formed through the growth of a new layer on the surface
of the existing inclusions. For example, large size REM
oxides with hollow holes were observed after the addition
of REM alloys to the liquid steel.[28] A very small amount of
Al in FeSi enables the formation of MgO · Al2O3 spinel
inclusion in molten steel. Also, calcium in the FeSi alloy
may form Ca-aluminates that lead to severe clogging
problems during casting. Furthermore, Ti in FeSi will
prevent the effective grain growth due to the precipitation
of TiO2 at the grain boundaries. Ti will also react with
C and N and form hard carbon-nitrides that will
negatively affect the material properties of the final steel
product.
Trace elements such as Pb, Sn, Sb, Zn, and Bi should
also be investigated more in-depth. The use of ferroalloys
with lower contents of these tracing elements reduce the
bad quality steel from 30.8 to 4%.[4] Therefore, the exact
quantity of these non-metal impurities, strong deoxidizers
and trace elements in the ferroalloy should be known
before its introduction to the liquid steel. This is especially
important if additions are needed during the final stage of
the secondary refining, since there is only a short time to
promote inclusion separation by optimized stirring.
4.2. Insoluble Impurities
The dissolution kinetics of ferroalloys in the steelmaking
process was studied.[29] The dissolution time for 10–30mm
size ferroalloys is in a range from 2 to 160 s for FeSi,
SiMn, and FeMn at 1873K. However, the dissolution of the
insoluble impurities in the ferroalloys is not mentioned.
4.2.1. Non-Metallic InclusionsThe inclusions in ferroalloys may undergo some physical
or chemical change after the addition to the liquid steel.
Depending on the melting point and the thermodynamic
stability of the specific inclusions at the steelmaking
temperature and composition, the inclusions will dissolve
668 steel research int. 85 (2014) No. 4
or stay solid. If the inclusions dissolve, a local high oxygen
or sulfur content may lead to a formation of new
inclusions. If the inclusions stay solid, the first possibility
is that strong deoxidants that dissolved in the liquid steel
may react with them under the formation of complex
inclusion. The second possibility is that these solid
inclusions may act as nucleation sites if the sizes are
small. The third possibility is that these solid inclusions
collide with each other and form clusters, which have a
bad effect on the casting process as well as on the final
product.
The non-metallic inclusions observed in this study are
the REM oxides in FeTi, FeSi, and SiMn, Al2O3 and Ti–Nb–
S–O in FeNb, silicon oxides in SiMn. The REM oxides and
Al2O3 inclusions probably remain solid and form clusters.
These two types of clusters are known as responsible for
the nozzle clogging and to cause decreased mechanical
properties in the final product. Also, silicon oxides might
dissolve in steel and thereafter form new inclusions.
Furthermore, they might react with strong deoxidizers to
form complex or large size inclusions. Overall, it is seems
that further studies of the Ti–Nb–S–O inclusions are
needed in the future.
4.2.2. Metallic InclusionsThe intermetallic phases observed in this study are a Ti–Fe
phase in FeTi alloy, Ca–Si, and Fe–Si–Ti phases in FeSi
alloy and a Mn–Si phase in SiMn alloy. Furthermore, the
mostly pure single metallic phase is pure Ti in FeTi, Nb in
FeNb, and Si in FeSi. According to the phase diagram of
these metallic phases, the majority of the melting points of
them are lower than the steelmaking temperature 16008C,with the exception of the almost pure Nb phase in FeNb.
Therefore, the majority of the metallic phases are assumed
to dissolve after the addition to steel. The effect of an
insolublemetallic phase that themelting points lower than
16008C on the cleanliness of the steel is that they can be
treated as soluble elements. Although the size range of the
metallic inclusions in ferroalloys in this study is large and
the proportion of the number percentage is tremendous,
no further investigation is conducted in this study.
However, the metallic inclusions (the almost pure Nb
phase, 24008C), which have a melting point that is higher
than the steelmaking temperature should be studied
more in depth in the future. In addition, the solubility of
them in steel for steelmaking conditions should also be
considered.
5. Conclusions
The inclusion contents in the following commercially
obtained ferroalloys were studied: FeTi, FeNb, FeSi, and
SiMn. The characteristics (such as morphology, size,
composition, and number) of inclusions were investigated
in 3D after EE. The influence of the soluble and insoluble
� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.steel-research.de
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PA
inclusions can be beneficial or detrimental depending on
the specific demands on the steel. The following con-
clusions were obtained.
PER
1.� 2
The inclusions in ferroalloys (FeTi, FeNb, FeSi, and
SiMn) can be successfully analyzed after a separation of
the inclusions from the matrix using the EE method.
2.
The non-metallic inclusion types observed in this studyare REMoxides in FeTi, FeSi, and SiMn alloys, Al2O3 and
Ti–Nb–S–O in FeNb alloys and silicon oxides in SiMn
alloys.
3.
The following intermetallic inclusions can be found: aTi–Fe phase in FeTi alloys, Ca–Si and Fe–Si–Ti phases
in FeSi alloys and a Mn–Si phase in SiMn alloys.
Furthermore, the mostly pure single metallic phases
are Ti in FeTi alloys, Nb in FeNb alloys, and Si in FeSi
alloys.
4.
The effect of the impurities in ferroalloy on steelmakingprocess depends on the specific use. The characteristic
of the soluble and insoluble impurities should be
known before its addition to the liquid steel.
Acknowledgements
The China Scholarship Council (CSC) is acknowledged for
financial support to this study.
Received: April 23, 2013;
Published online: August 29, 2013
Keywords: ferroalloy; non-metallic and metallic
inclusions; electrolytic extraction; three-dimensional
investigations
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