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Cleanliness of Low Carbon Aluminum-Killed Steelsduring Secondary Refining Processes
Wen Yang, Xinhua Wang, Lifeng Zhang,� Qinglin Shan, and Xuefeng Liu
Efficient secondary refining process is necessary for massive and stable production of low
carbon aluminum-killed (LCAK) steels. Plant trials were performed to investigate the
cleanliness of steels. Characteristics of composition, cleanliness, and inclusions of LCAK
steel during different secondary refining processes, including LF, CAS, RH-LIT, and RH,
were studied and compared. The results showed that CAS, RH-LIT, and RH processes had
better control of low carbon, silicon, and nitrogen than LF process. High cleanliness of
LCAK steels could be achieved by all the mentioned refining processes. It was concluded
that the total oxygen (T.O.) should be <35 ppm to reduce the amount of inclusions and
reach the level of clean steels. The removal rate of inclusions during RH-LIT and RH
processes was much higher than that of LF and CAS. The T.O. content and the amounts of
inclusions during CAS, RH-LIT, and RH could be quickly decreased to a low value within
10min. The results showed that CAS and RH-LIT as well as RH refining processes can
produce LCAK steels that meet the requirements of high efficient, low cost, clean, and
stable production, while LF is more suitable for the heats with poor control of end point of
BOF, and for the process with calcium treatment to control sulfur content and lower the
clogging of Submerged Entry Nozzle during thin slab continuous casting.
1. Introduction
Low carbon aluminum-killed (LCAK) steels are well used
for production of cold rolled sheets used for automobiles,
household appliances, etc. It requires that LCAK steel
should have low carbon, low silicon, and low nitrogen to
obtain good forming property, and the total oxygen (T.O.)
and the size of inclusions of LCAK steels should be strictly
controlled to guarantee high surface quality of the product.
The secondary refining processes of LCAK steels grades
include LT (Ladle Treatment by Gas Stirring), CAS
(Composition Adjustment by Sealed Argon Bubbling),
RH (Ruhrstahl–Heraeus), and LF (Ladle Furnace treat-
ment). Besides improving steel quality, secondary refining
process is the important step to link up steelmaking and
continuous casting. The LT gas stirring treatment was
developed approximately in 1951.[1] This process is simple
and low cost,[2] however, slags are needed to be deoxidized
to prevent the reoxidation of molten steel and prevent the
increase of inclusion amount. Moreover, the molten steel is
easy to be reoxidized by air when the stirring is too strong.
As the tight quality requirement of the steel, LT is hardly
used alone or little used nowadays. The CAS refining proc-
ess was developed in 1975 by Nippon Steel[3] and expanded
gradually at the end of 1970s. Comparing the LT process,
the CAS process utilizes a snorkel, which hinders the
explosion of the molten steel to air and slag and therefore
lowers the reoxidation of the molten steel. Thus, the stir-
ring intensity can be strengthened to promote the removal
of inclusions, leading to the short refining time and rela-
tively high cleanliness of steel and high yield of alloy. RH
refining process was developed by the two companies
of Ruhrstahl and Heraeus, so called RH, in 1959.[4] The
advantage of RH is degassing and removal of inclusions
utilizing high vacuum and circulation flow. Due to the
using of vacuum chamber, the RH process can well avoid
the reoxidation of molten steel from air and slag, and the
stirring of RH can be very strong. Multiple functions of CAS
and RH were expanded in the 1970s and 1980s, such as
CAS-OB, RH-OB, RH-KTB, RH-PB for additional function
W. Yang, Prof. X. Wang, Prof. L. ZhangState Key Laboratory of Advanced Metallurgy and School ofMetallurgical and Ecological Engineering, University of Science andTechnology Beijing, Beijing 100083, ChinaEmail: zhanglifeng@ustb.edu.cnQ. ShanSteelmaking Department of Shougang Jingtang United Iron & SteelCo., Ltd, Tangshan 063200, ChinaX. LiuSchool of Materials Science and Engineering, University of Scienceand Technology Beijing, Beijing 100083, China
DOI: 10.1002/srin.201200213
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of reheating, decarburization, and desulfurization.[5]
Besides, the RH light treatment (RH-LIT) was proposed
in 1975 to reduce the burden of steelmaking furnace.[5]
Different with RH route in which deoxidation is performed
during BOF tapping, in RH-LIT route there is no deoxida-
tion performance during tapping, and the first stage of RH-
LIT refining process is decarburization by the dissolved
oxygen in vaccum condition, following by deoxidation with
aluminum addition, which leads to a lower dissolved oxy-
gen before deoxidation and less alumina generated after
deoxidation. As a result, RH-LIT process allows relatively
high carbon and low dissolved oxygen at the end point of
BOF. LF refining process was developed in 1971,[5,6] the
main characteristic is steel heating by electric arc, thus it
can melt ferroalloy by electric arc heating, adjust compo-
sition, and temperature, perform desulfurization and deox-
idation by a reducing slag, and control the cleanliness and
inclusions in the molten steel. The typical advantages of
the LF refining process include high cleanliness of steel,
modification of inclusions, and stable temperature, thus, it
is widely used nowadays although it was developed later
than other refining processes.
There have been many studies on the cleanliness of
LCAK steel at different refining processes,[7–23] the T.O.
content in LCAK steels summarized in Table 1.[7–14]
From these studies, it is known that the high cleanliness
of LCAK steel can be achieved by all the refining processes
discussed above. In recent years, the LCAK steel pro-
duction faces many challenges, including the require-
ments of high quality of the steel product and low
production costs, less environment pollution, and low
energy consumption. Thus, high efficient, low cost, and
stable production of high quality steel is necessary for the
sustainable development of steel enterprises. However, it is
lack of comparison between different refining processes of
LCAK steel, leading to considerable divergences on the
choice of refining process for LCAK steel. Accordingly, in
the current paper, characteristics of steel composition,
cleanliness, and inclusions of each refining process includ-
ing LF, CAS, RH, and RH-LIT were studied and compared
to provide a basis for the choice of refining processes.
2. Experimental and Analysis Methodology
Industrial trials for the production of LCAK steel were
performed. The production routes are ‘‘KR (to remove
sulfur from the hot metal)! 300 t BOF!LFþCa treat-
ment/CAS/RH/RH-LIT! continuous casting.’’ The LF
refining with Ca treatment process is for the production
of SPHC steel while the other refining processes are for the
production of DC01 steel. The compositions of SPHC and
DC01 steel are listed in Table 2.
In this work, calcium treatment was assumed to be a
part of LF refining process. In the LF refining route, during
tapping of BOF, a certain amount of aluminum was added
into the molten steel to decrease the dissolved oxygen to
several ppm. Synthetic slag was added into the ladle during
tapping to reduce the FeO and MnO in the slag. During LF
refining, high basicity slag was used and aluminum was
added to improve deoxidation. Steel samples were taken
before LF refining, at the earlier stage, the middle stage, the
end of LF refining, after Ca treatment, and after soft
blowing.
In CAS refining route, during tapping of BOF, a certain
amount of Al and synthetic slag were added into the molten
steel as well. After tapping, slag modifier was added into
the slag to reduce the slag oxidizability. At CAS station, the
snorkel was submerged into the molten steel after the slag
surface was blown open to a bright face, followed by a
strong stirring by argon gas to promote the removal of
inclusions. The temperature was decreased by adding steel
scraps and be raised by adding aluminum and blowing
oxygen. Steel samples were taken before CAS refining,
during process of argon blowing with time interval of
approximately 7 min, and after argon stop.
In the RH refining route, the end point of BOF was
controlled under common practice, deoxidation, and
alloying were performed during tapping of BOF. During
the RH refining, a large argon gas flow rate was employed
to promote the recirculation and removal of inclusions
from the molten steel. While in the RH-LIT route, the
carbon content at the end point of BOF was relatively
higher than the normal RH process, and no deoxidation
or alloying were performed during the tapping of BOF.
Decarburization was firstly performed during RH-LIT
process under a certain vacuum degree. After decarburiza-
tion, aluminum was added for deoxidation. Then, the
generated inclusions were removed by flow transport
under excellent dynamic conditions. Steel samples were
taken after deoxidation. Since the temperature drop was
large during RH process, aluminum addition, and oxygen
blowing were performed to increase the temperature of
the molten steel.
The schematic operation and sampling scheme of
each secondary refining process were shown in Figure 1.
Temperature of the molten steel was measured several
times during the refining process of each heat. The dis-
solved oxygen content was measured by a zirconia sensor
during RH and RH-LIT refining processes and at the begin-
ning of other refining processes.
The contents of C, S, and T.O. of the steel samples were
analyzed using infrared analysis. The concentration of
silicon was analyzed using ICP-AES and the nitrogen con-
tent was analyzed using the thermal conductance method.
Inclusions on the cross-section of each steel sample
were detected using the automated SEM/EDS inclusion
analysis (ASPEX). ASPEX is a computer-controlled scan-
ning electron microscope that is designed for the auto-
mated imaging and elemental analysis of a wide spectrum
of surfaces and particulates. The system provides a fully
integrated SEM and EDX platform for addressing the
microscale visualization needs. It can simultaneously
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Steel works Refining Ladle Tundish Slab Year Ref.
America
Inland No.4 BOF shop RH-OB 60–80 8–40 2003 [7]
Dofasco, Canada EAF, LMF 14 2003 [7]
BOF, LMF 23 2003 [7]
Europe
Dillinger, Germany Tank Degas 10–15 2000 [7]
Sidmar, Belgium 37 2000 [7]
RH 20–56 2002 [8]
Corus strip Products, Port Talbot Works, UK CAS-OB, or RH 10–15 2001 [7]
Teesside Technology Center, Corus, UK 6 2002 [7]
Voest-Alpine Stahl GmbH, Linz, Austria RH-OB 10–36 25 2003 [7]
China
Panzhihua Iron and Steel Co. RH-MFB 20–24 2000 [7]
Ar stirring 15.2 2000 [7]
Ma’anshan Iron and Steel ASEA-SKF 18–24 2000 [7]
BaoSteel CAS 73–100 38–53 14–17 2000 [9]
WISCO, No.2 Works RH (Pressure vessel steel 28–34 24–26 12–19 2000 [7]
WISCO, No.3 Works RH 16 20 15 2002 [7]
Anshan Iron and Steel ANS 35–56 33–35 25 2003 [10]
Lianyuan Iron and Steel LFþCa treatment 30–36 34–42 37–43 2006 [11]
BaoSteel, Meishan works LFþCa treatment 34 50 16 2006 [12]
Qian’an Iron and Steel CAS 19–24 17–20 11–15 2007 [13]
Tangshan Iron and Steel LFþCa treatment 16–39 19–46 12–38 2009 [14]
Table 1. Total oxygen content (ppm) reported during various processing steps for LCAK steel after the year of 2000.
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Steel C Si� Mn P� S� Alt N�
SPHC 0.05 0.03 0.2 0.015 0.012 0.015–0.05 0.006
DC01 0.03 0.03 0.2 0.015 0.012 0.015–0.05 0.006
Table 2. Composition of SPHC and DC01 steel (%).
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Steel C Si� Mn P� S� Alt N�
SPHC 0.05 0.03 0.2 0.015 0.012 0.015–0.05 0.006
DC01 0.03 0.03 0.2 0.015 0.012 0.015–0.05 0.006
Table 2. Composition of SPHC and DC01 steel (%).
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Figure 1. Key operation and sampling scheme of each refining process (Sam., sampling; meas., measuring; add., adding).
Steel C Si� Mn P� S� Alt N�
SPHC 0.05 0.03 0.2 0.015 0.012 0.015–0.05 0.006
DC01 0.03 0.03 0.2 0.015 0.012 0.015–0.05 0.006
Table 2. Composition of SPHC and DC01 steel (%).
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detect the size, shape, area, number, location, and elemen-
tal composition of inclusions in steel samples. The working
magnification was set at �225 and the minimum detect-
able inclusion was �1.0 mm in the current study.
A statistical parameter, area fraction, is defined as the
following formula to characterize the amount of non-
metallic inclusions.
AF ¼ Ainclusion
Atotal(1)
where, AF is the area fraction of inclusions, ppm; Ainclusion
is the total area of detected inclusions, mm2; and Atotal is the
sample detection area, mm2.
Since an inclusion with irregular shape especially a
cluster might be cut into two or more particles on one
cross section, AF is more accurate than the number density
to describe the amount of the irregular shape inclusions
since the area contains the information of both number
and size.
In the current study, in order to reveal the three-dimen-
sional morphology of inclusions in the steel produced by
CAS and RH refining processes, inclusions were extracted
from steel samples.[24,25] After the surface of the steel
sample being polished, about 3 g of each steel sample
was dissolved in hydrochloric acid solution (1:1 in volume)
at 708C. After the iron matrix was completely dissolved,
deionized water was added to dilute the solution. Then, the
inclusions were filtered out using a polycarbonate filter
paper propelled by a vacuum pump. The pore size of the
filter was 0.2 mm in order to capture most of the inclusions.
As alumina-based inclusions are insoluble to HCl acid,
they will remain on the filter paper. The extracted
inclusions were then observed using SEM after depositing
carbon powder over the filter paper under vacuum
conditions.
3. Results and Discussions
The content of carbon, silicon, sulfur, T.O., nitrogen, and
inclusions in steel samples taken during steel refining
processes were evaluated.
3.1. Carbon Content
Figure 2 shows the carbon content in steel during
secondary refining processes. Since the carbon content
of SPHC is a little higher than that of DC01, the carbon
content in steel during LF refining process is higher than
other processes. This figure shows that the carbon content
during LF, CAS, and RH was slightly changed during
refining. While at the initial stage of RH-LIT process
the carbon content decreased significantly to <0.01%
due to the decarburization, after the addition of carburant,
the carbon content increased to the target value. Since
decarburization can be performed in RH-LIT process
route, the carbon content at the end point of BOF can
be controlled in a higher value and wider range than the
normal RH process, leading to an easier control to
reach the target value at the end point of BOF. While in
other refining routes, since no decarburization was per-
formed and might occur carbon pick-up during refining, it
is required that the carbon content in steel should be
controlled to a low value and in a narrow range at the
end point of BOF, which increases the difficulty of BOF
control and increases the damage possibility of the
BOF refractory.
3.2. Silicon Content
To obtain good forming property of LCAK sheets, [Si] in
LCAK steel should be <0.03%. The variation of [Si] in each
refining process as well as the comparison of [Si] content
and [Si] pick-up after different refining processes are
shown in Figure 3, indicating that the order from high
to low [Si] content by different refining processes is:
LF>CAS, RH>RH-LIT, while the order from large to small
silicon pick-up during refining is: LF>CAS, RH, and RH-
LIT.
The reaction inducing the increasing of silicon is
3ðSiO2Þ þ 4½Al� ¼ 2ðAl2O3Þ þ 3½Si� (1)
The larger [Si] pick-up in LF refining process is due to the
active reaction between slag and molten steel. During the
LF refining process, the activity of SiO2 in slag is improved
by decreasing the oxidizability of the slag. Besides, the
stirring of steel caused by argon blowing provides a good
dynamic condition to the reaction.
0 10 20 30 40 50 600.00
0.01
0.02
0.03
0.04
0.05
0.06
Car
bon
cont
ent (
%)
Time of refining (min)
LF RH-LIT CAS RH
Carburant additionin RH-LIT
Figure 2. Variation of carbon content during refining processes.
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3.3. Desulfurization
The casting speed of the thin slab continuous casting is
higher than that of the traditional continuous casting proc-
ess. Low sulfur content is required to avoid slab breakout
during casting. Besides, calcium treatment is usually used
to modify inclusions from solid to liquid to prevent the
submerged entry nozzle from being clogged, and low sulfur
content is needed to improve the efficiency and stability of
the calcium treatment. Thus, the desulfurization during
different refining processes is discussed as well.
The [S] content in steel and the efficiency of desulfur-
ization of each secondary refining process are shown
in Figure 4, which indicates that the LF refining process
has the best desulfurization efficiency and with the lowest
[S] content in steel after refining.
0 10 20 30 40 50
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035 LF CAS RH-LIT RH
Time of refining (min)
[Si]
in s
teel
(%)
Deoxidation in RH-LIT
LF CAS RH-LIT RH0.000
0.005
0.010
0.015
0.020
0.025
0.030
[Si]
and
[Si]
pick
-up
afte
r ref
inin
g (%
)
[Si] after refining [Si] pick-up
(a) (b)
Figure 3. Variation of [Si] during refining processes (a), and comparisons of [Si] content and [Si] pick-up after different refiningprocesses (b).
0 10 20 30 40 50 600.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014 LF RH-LIT CAS RH
[S] i
n st
eel (
%)
Time of refining (min) LF CAS RH-LIT RH0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
[S] c
onte
nt a
fter r
efin
ing
(%)
(a) (b)
LF CAS RH-LIT RH0
10
20
30
40
50
60
70
Effic
ienc
y of
des
ulfu
rizat
ion
(%)
(c)
Figure 4. Variation of [S] content during refining processes (a), [S] content in steel after secondary refining (b), and efficiency ofdesulfurization (c) of each refining process.
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The desulfurization reaction is expressed by[26]
3ðCaOÞ þ 3½S� þ 2½Al� ¼ 3ðCaSÞ þ ðAl2O3Þ (2)
According to Equation 2, in order to increase the efficiency
of desulfurization, the activities of CaO and [Al] should be
higher. Since the steel grade decides the [Al] content in
steel, the desulfurization mainly depends on the activity of
CaO in slag. Because a high basicity slag was used in LF
refining process in the current study, the activity of CaO
was high, leading to the highest desulfurization efficiency.
On the other hand, LF process is designed to desulfurize
because the heating method is more suitable for the use of
high basicity slag.
3.4. Temperature Control
Larger temperature drop rate requires more temperature
compensation to ensure castability of the molten steel,
leading to more cost and possible worse steel cleanliness.
Thus, small temperature drop rate is usually respected.
In the current study, the temperature drop rate is
defined as the average drop rate in the range from the
beginning of refining to the first temperature rising oper-
ation except for the LF refining process, in which the
parameter is defined as average temperature drop in the
refining time except the time period of slag addition, elec-
trical arc heating, and soft blowing. Figure 5 presents the
variation of temperature of molten steel during refining
processes and the average temperature drop rate of each
refining process. The arrows in Figure 5a show the first
temperature rising operations during refining processes. It
is indicated that the order of temperature drop from large
to small during different refining processes is: RH>RH-
LIT>LF and CAS. The large temperature drop rate during
RH and RH-LIT refining which is owing to the strong argon
blowing and the larger circulation rate of the molten steel
will increase the burden of BOF for higher temperature
requirement.
The heating methods of the refining processes are
different from each other. The attribute of LF refining
process is electric arc heating of steel, thus the temperature
control is much easier and more accurate, which is good
for discharging the burden for the furnace, slagging during
the refining, and keeping suitable temperature of the melt
to caster, but it may induce carbon pick-up and nitrogen
pick-up. While the temperature of the molten steel during
CAS or RH refining is raised by aluminum addition and
oxygen blowing, which increases the T.O. and the amount
of inclusions in steel.
3.5. Total Oxygen and Nitrogen
In most refining processes except RH-LIT, deoxidation
were performed by adding aluminum during tapping of
BOF, thus the oxygen content at the end point of
BOF is assumed to be the dissolved oxygen before
deoxidation. Figure 6 shows the contents of [C] and [O]
before deoxidation in CAS, RH, and RH-LIT process, and at
0 10 20 30 40 501560
1580
1600
1620
1640
1660
1680 LF CAS RH-LIT RH
Tem
pera
ture
of m
olte
n st
eel (
)
Time of refining (min)
slag
LF CAS RH-LIT RH0
1
2
3
4
5
6
7
Ave
rage
tem
p. d
rop
rate
(°C
/min
)
(a) (b)
Figure 5. Variation of temperature (a) and the average temperature drop rate (b) of each refining process.
0.00 0.01 0.02 0.03 0.04 0.05 0.060
400
800
1200
1600
2000 CAS RH RH-LIT at end
point of BOF RH-LIT after
decarburization C-O equilibrium
)mpp( tnetnoc ]
O[
[C] content (%)
1225
645
Figure 6. Contents of [C] and [O] of steel in different refiningprocesses.
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the end point of BOF in RH-LIT process is shown in the
figure as well. The dissolved oxygen content increased with
the decreasing of carbon at the end of BOF, and smaller
carbon content induced more deviation of dissolved oxy-
gen from its equilibrium value, which is calculated at the
pressure of 1 atm. In RH-LIT refining route, the [O] content
before deoxidation was smallest,�215 ppm. In RH refining
route, the value was 645 ppm higher than that of RH-LIT.
The [O] content before deoxidation was highest in CAS
route, �1225 ppm higher than that in RH-LIT. The high
oxygen content at the end point of BOF might be due to the
excessive oxidation of melt by oxygen blowing. Lower
carbon content induces higher oxygen content and easier
excessive oxidation. Thus, it is harmful to pursue too low
carbon content at the end point of BOF.
Figure 7 shows the variation of T.O. and N contents
during each secondary refining process. At the earlier stage
of refining processes of CAS and RH, the content of
T.O. decreased significantly, with a decreasing speed of
�4 ppm min�1, as shown in Figure 8a. During LF refining
0 10 20 30 40 50-9
-6
-3
0
3
6
Heating of RH-LIT
LF CAS RH-LIT RH
Time of refining (min)
)nim/
mpp( H
R/SA
C/FL fo td/.O.Td-
Heating of RH
-50
-40
-10
0
10
20
30 -dT.O./dt of R
H-LIT (ppm
/min)
LF CAS RH-LIT RH0
10
20
30
40
50
60
T.O
. afte
r sec
onda
ry re
finin
g (p
pm)
(a) (b)
Figure 8. The decreasing rate of T.O., �dT.O./dt of each refining process (a) and T.O. contents (b) after secondary refining.
0 10 20 30 40 500
20
40
60
80
100
LF T.O. N
T.O
. and
N in
ste
el (p
pm)
Time of refining (min)
Ca treatment
0 10 20 30 40 500
10
20
30
40
50
60
70
T.O
. and
N in
ste
el (p
pm)
Time of refining (min)
CAS T.O. N
Argon blowing
Al addition
(a) (b)
(c) (d)
16 20 24 28 32 36 400
30
60
90
120
180
200
220
Time of refining (min)
RH-LIT T.O. N
T.O
. in
stee
l (pp
m)
Heating by Aladdition and O
2 blowing
0
4
8
12
16
20
N in steel (ppm
)0 10 20 30 40 50
0
20
40
60
80
T.O
. and
N in
ste
el (p
pm)
Time of refining (min)
RH T.O. N
Heating by Al addition and O2 blowing
Figure 7. Variation of T.O. and N during refining processes, a) LF, b) CAS, c) RH-LIT, and d) RH.
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process, the T.O. decreased gradually, while during CAS
and RH refining, after a fast decreasing at the first 8–9 min,
the T.O. content kept almost constant. During RH-LIT and
RH refining processes, the heating of steel by the addition
of aluminum and oxygen blowing caused a sharp increase
of T.O. At the end of each refining process, the T.O.
decreased to a low value – 33 ppm by LF, 34 ppm by
CAS, 45 ppm by RH-LIT, and 28 ppm by RH, as shown
in Figure 8b, indicating that high cleanliness of steel can
be achieved by all the refining processes discussed.
However, T.O. during RH-LIT was relatively higher than
that of other refining processes due to the shorter net time
for the removal of inclusions.
Figure 9 shows the nitrogen content and nitrogen pick-
up for each refining process. It is indicated that the order
of nitrogen content from high to low is: LF>CAS and
RH>RH-LIT, and the nitrogen pick-up was largest during
LF refining process, which was as high as 12 ppm and
mainly occurred during Ca treatment, as shown in
Figure 7a, which might be attributed to the air absorption
caused by the boiling of molten steel on the near-surface of
the molten pool at time of Ca–Fe wire injection that was
induced by the high steam pressure of calcium.
3.6. Inclusions in Steel during Refining
Inclusions in LCAK steel after deoxidation are mainly
alumina-based. Large alumina-based inclusions have a
detrimental effect on the castability of the molten
steel[27–29] and the surface quality of the steel sheet[30–32].
Currently, large alumina-based inclusions are either modi-
fied into low melting ones by calcium treatment,[9,33,34] or
removed by flow transport,[35,36] or become defects of the
steel product.[26]
Some aspects of inclusion characteristics, including
composition, morphology, amount, size, and remove rate
in each refining process, were analyzed.
3.6.1. Composition and Morphology
Figure 10 shows the composition of inclusions at the end
of each refining process, detected by ASPEX. The curve in
the figure represents the boundary of liquid inclusion
region. In LF route, after calcium treatment the oxide
inclusions transferred to CaO–MgO–Al2O3 (Figure 10a),
and many of them were located in the liquid region, which
is good for preventing nozzle clogging and improving the
castability of the steel. However, Figure 10b represents that
after calcium treatment a vast amount of CaS generated,
most of them were precipitaed on the oxides, this would
decrease the modification effect of alumina inclusions.
Thus, lower sulfur content is required to improve the
inclusion modification. At the end of CAS, RH-LIT, and
RH refining processes, since there was no calcium treat-
ment, inclusions were solid alumina, which would induce
nozzle clogging during casting. Thus, on the point of view
of the castability of the molten steel, LF refining (including
calcium treatment) is better than CAS, RH-LIT, and RH.
The transformation of inclusions during LF refining
process was owing to the reaction of calcium treatment.
High basicity reductive slag was used during LF refining,
the reaction between the slag and the molten steel changed
the composition of inclusions, especially after calcium
addition. Inclusions were modified to liquid ones mainly
according to the following reaction.
7ðAl2O3Þ þ 12½Ca� þ 12½O� ¼ ð12CaO� 7Al2O3Þ (3)
As mentioned above, oxide inclusions after LF refining was
mainly MgO–CaO–Al2O3 that could be partially dissolved
by hydrochloric acid, while after CAS and RH refining
inclusions were mainly alumina that would not be dis-
solved by hydrochloric acid. Accordingly, the morphology
of inclusions after LF refining was directly observed by
SEM, and inclusions after CAS and RH refining were firstly
extracted using HCl acid and then observed by SEM to
reveal the three-dimensional morphology of alumina
inclusions.
Figure 11 shows the morphology of inclusions at the
end of each refining process. Since inclusions after RH
refining were the same as those after RH-LIT refining,
inclusions only after RH-LIT refining are shown here.
After LF refining, inclusions were transformed to spherical,
and most of them were small. CaS was precipitated on the
gap part of the spheres shown in Figure 11a. However, after
CAS and RH-LIT refining, there were still many irregular
inclusions, and large clusters and aggregates were
observed, which were very detrimental to the subsequent
casting process. Besides clusters and aggregates, many
small spherical and polyhedral particles were observed
as well in the steel samples after CAS and RH refining.
3.6.2. Amount and Removal Rate
The amount of inclusions is characterized by their area
fraction (AF). Figure 12 shows the variation of AF of
inclusions during each secondary refining process.
LF CAS RH-LIT RH0
5
10
15
20
25
30
35
40N
and
N p
ick-
up a
fter
refin
ing
(ppm
) N after refining N pick-up
Figure 9. N content and N pick-up during each refiningprocess.
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Figure 10. Composition of inclusions at the end of each refining process: a,b) LF, c) CAS, d) RH-LIT, e) RH.
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During LF refining process, before calcium treatment, the
AF of inclusions decreased, while after calcium treatment,
the amount of inclusions increased sharply, which was
caused by the massive generation of new inclusions especi-
ally sulfide (as shown in Figure 10b) and slight reoxidation
of molten steel (as shown in Figure 7a) during calcium
treatment. During other refining processes, the variation of
AF was consistent with the tendency of T.O. At the early
stage of refining process (�10 min), the AF of inclusions
decreased significantly, and then decreased slowly. During
RH-LIT and RH refining processes, the heating of molten
steel by adding aluminum and blowing oxygen extremely
increased the AF of inclusions due to the massive gener-
ation of new alumina inclusions.
The relationship between the AF of alumina-based
inclusions and the T.O. is shown in Figure 13. It is clearly
indicated that the AF of alumina-based inclusions
increased with the increasing of T.O. in steel, and it
increased slowly when the T.O. was<35 ppm but increased
exponentially after the T.O. was over 35 ppm. Thus, the
T.O. in steel should be controlled to<35 ppm to reduce the
amount of inclusions. As discussed by Zhang,[22,37] the T.O.
only represents the inclusions smaller than 50 mm. How to
control large inclusions that are mainly exogenous ones
Figure 11. Morphology of inclusions after refining processes a) LF, b) CAS, and c) RH-LIT.
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from slag and lining refractory materials is another import-
ant task of steelmakers.
The AF of inclusions at the end of each refining process
is shown in Figure 14, where LF(�Ca) means before
calcium treatment and LF(þCa) means after calcium treat-
ment. The AF of inclusions was smallest by refining of
RH-LIT or RH, �37 ppm, around 52 ppm at the end of
CAS refining and before calcium treatment in LF refining
process. However, after calcium treatment, the AF was
sharply increased to �232 ppm owing to the massive
generation of CaS and slight reoxidation of the steel during
the practice of calcium treatment.
The minimum AF of inclusions characterizes the
removal ability of inclusions from the molten steel, while
10 20 30 40 50 600
50
100
150
200
250
300
Time of refining (min)
AF
of in
clus
ions
(ppm
)
Ca treatment
LF
0 10 20 30 40 500
50
100
150
200
250
300
AF
of in
clus
ions
(ppm
)
Time of refining (min)
CAS
Argon blowing
16 20 24 28 32 36 40 44
0
200
400
600
800
1000
AF
of in
clus
ions
(ppm
)
Time of refining (min)
Heating by adding Aland blowing O2
RH-LIT
0 10 20 30 40 50
0
100
200
300
400
500
600
700
AF
of in
clus
ions
(ppm
)
Time of refining (min)
RH
Heating by adding Al and blowing O2
Figure 12. AF of inclusions during each secondary refining process.
0 20 40 60 80 1000
100
200
300
400
AF
of in
clus
ions
(ppm
)
T.O. content in steel (ppm)
Figure 13. Relationship between the AF of inclusions and theT.O. of the steel.
LF(- Ca) LF(+ Ca) CAS RH-LIT RH0
20
40
60200
220
240
AF
of in
clus
ions
(ppm
)
Figure 14. AF of inclusions at the end of each secondaryrefining.
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the removal efficiency of inclusions is characterized by
average removal rate of the AF at the fast decreasing stage
of AF, defined as Equation 4.
hIR ¼½AF�in � ½AF�low
DtIR(4)
where, hIR is the removal rate of inclusions in AF, ppm/
min; [AF]in is the initial AF of inclusions, ppm; [AF]low is the
AF of inclusions at the end of fast decreasing stage shown
in Figure 12, ppm; DtIR is the time of the fast decreasing
stage, min. The schematic diagram is shown in Figure 15.
The calculated removal rate of inclusions during each
refining process is shown in Figure 16, indicating that the
removal rate of inclusions in AF in RH-LIT and RH was far
larger than that in LF and CAS owing to the quick circu-
lation flow of molten steel. The very low removal rate
during LF refining was due to the relatively small bottom
argon blowing rate. Another reason is that the initial AF of
inclusions considered in the calculation (at 14 min) might
not be included in the actual initial fast decreasing stage
that is generally during the initial 10 min of the refining
process.
3.6.3. Size Distribution
Figure 17 shows the average size and size distribution of
inclusions during refining processes. The number density
(ND) of inclusions is defined as the number of inclusions
per unit detection area. The tendency of average size of the
inclusions was similar to the AF of inclusions. During LF
refining, before calcium treatment, the size of inclusions
decreased gradually because of the removal of >2 mm
inclusions. Immediately after calcium treatment, the aver-
age size decreased to only 1.53 mm owing to the massive
generation of 1–2 mm inclusions by the calcium treatment.
As the time increasing during soft blowing, inclusions
aggregated and their average size increased. During CAS
refining, during argon blowing process, the average size
decreased, while after argon blowing was off, the average
size increased firstly due to the aggregation of small
inclusions and then decreased due to the floating removal
of large inclusions. During RH-LIT refining, at the early
stage of steel heating, many large alumina generated which
led to a large average size. At the later stage of heating,
it quickly deceased due to the fast removal of large
inclusions. During RH refining, at the early stage, it
decreased significantly due to the removal of inclusions,
and increased obviously during the heating stage for the
same reason as that during RH-LIT.
The average size of inclusions at the end of each refining
process is shown in Figure 18, indicating that the average
size of inclusions after LF refining was smallest due to the
modification by slag–steel reaction and calcium treatment.
The actual size of inclusion in CAS or RH refining may be
larger than the measured one because of the dominant
irregular and clustered alumina inclusions.
3.7. Total Refining Time
The secondary refining process is a connection link
between the preceding steelmaking and the following
casting, thus besides functions mentioned previously,
the secondary refining process is also a regulation of the
production tempo. To improve the production efficiency
and reduce manufacturing costs, high speed continuous
casting is expected, which requires shorter secondary
refining time.
The refining time in the current work is defined as the
time from start to the end of treatment, e.g., for LF and CAS
refining it is the time from the start of to the end of argon
blowing, and for RH-LIT and RH refining it is the time from
the start to the end of vacuum. The refining time of each
process is shown in Figure 19, indicating that the refining
time of LF was longest and that of CAS was shortest. Here,
the LF refining process included calcium treatment. RH-
LIT process contained a decarburization stage and a steel
heating process, leading to a relatively long refining time.
Although there was no decarburization stage during RH
process, however, there were two steel heating processes,
LF CAS RH-LIT RH0
10
20
30
40
50
60
70
Rem
oval
rate
of i
nclu
sion
sin
AF
(ppm
/min
)
Figure 16. Removal rate of inclusions during each refiningprocess.
Figure 15. Schematic of removal rate of inclusions.
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Figure 17. Average size and size distribution of inclusions during each refining process.
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which induced much longer time than expected.
Therefore, the temperature rising operation should be
shortened as far as possible to lower the total refining time.
This requires a compact and reasonable plants layout
and more scientific management of the manufacturing
process.
3.8. Material Consumption
Low cost manufacturing is necessary and urgent nowadays
for the environmentally friendly and sustainable develop-
ment of steel industries. The key of low cost manufacturing
is to produce high quality products with the least consump-
tion of resource raw materials. In the current study, material
consumption of each refining process was analyzed.
Material consumption mainly includes the consump-
tion of: alloys, slags, argon gas, oxygen gas, power, and so
on. For the current study, the steel grade was similar and
the consumption of Mn–Fe for alloying was almost the
same. The slags added during the tapping of BOF were
nearly the identical except that the slag modifier added
after tapping in RH-LIT route was less. Thus, the materials
with large consumption mainly includes aluminum metal
for deoxidation and alloying such as Al–Fe, Al gravels, other
alloy such as Ca–Fe, slags added during refining process
such as synthetic slag and fluorite, argon gas for stirring,
and materials for temperature rising such as electrical
power during LF refining and aluminum and oxygen
during RH-LIT and RH refining.
Table 3 shows the consumption of the main auxiliary
materials for each refining process, the power for vacuum
pumps in RH-LIT and RH was not considered. It indicates
that the material consumption during LF refining process
was much larger than those during other processes.
The comparison on the aspects analyzed above among
the four secondary refining processes is summarized
in Table 4, indicating that CAS and RH-LIT as well as
RH can well meet the requirements of high efficient, low
cost, and stable production of LCAK steels, while LF is
LF CAS RH-LIT RH0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0A
vera
ge s
ize
of in
clus
ions
(μ)
Figure 18. Average size of inclusions at the end of each refiningprocess.
LF CAS RH-LIT RH0
10
20
30
40
50
60
70
Tota
l ref
inin
g tim
e (m
in)
Figure 19. Refining time of each secondary refining process.
Materials LF CAS RH-LIT RH
Al–Fe (kg t�1-steel) 3.45 6.94 0 5.55
Al grain (kg t�1-steel) 1.29 0.30 1.11 0.70
Ca–Fe (kg t�1-steel) 3.82 0 0 0
Synthetic slag (kg t�1-steel) 8.77 0 0 0
Fluorite (kg t�1-steel) 4.62 0 0 0
Argon (Nm3 t�1-steel) 0.091 0.088 0.408 0.387
Electricity for heating (kWh t�1-steel) 23.37 0 0 0
Al grain for heating (kg t�1-steel) 0 0 1.34 0.87
Oxygen for heating (m3 t�1-steel) 0 0 1.08 0.91
Table 3. Main materials consumption of each secondary refining process.
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more suitable for the case with a low level of refining
technology, or for the case with poor control of end points
of BOF steelmaking, or for thin slab continuous casting
process.
4. Conclusions
Characteristics of the composition, cleanliness, and
inclusions of LCAK steels by different secondary refining
processes were studied based on the plant trials, and then a
comprehensive comparison was performed. The following
conclusions were obtained:
(1) The processes of CAS, RH-LIT, and RH could keep low
values of carbon, silicon, and nitrogen which are
required for the production of LCAK steels, while
higher pick-up of silicon and nitrogen were found
for LF refining process.
(2) High cleanliness of LCAK steels could be achieved by
all the mentioned refining processes. The T.O., e.g., was
33 ppm for LF, 34 ppm for CAS, 45 ppm for RH-LIT, and
28 ppm for RH. The T.O. should be <35 ppm to reduce
the amount of inclusions.
(3) The amount of inclusions characterized by the AF after
each refining was: 52 ppm before calcium treatment
and 232 ppm after calcium treatment for LF process,
52 ppm for CAS, 37 ppm for both RH-LIT and RH proc-
esses. The higher amount of inclusions after calcium
treatment was due to the massive generation of CaS
and slight reoxidation during calcium treatment.
(4) The removal rate of inclusions during RH-LIT and RH
was much higher than during LF and CAS processes.
The T.O. content and the amount of inclusions during
CAS, RH-LIT, and RH could be quickly decreased to a
low value within 10 min refining.
(5) According to the comprehensive comparison, it is
known that CAS and RH-LIT as well as RH can well
meet the requirements of high efficient, low cost, and
Aspects Items LF CAS RH-LIT RH
Composition and
temperature control
Resist [C] fluctuation at the end of BOF D D * D
Resist temperature fluctuation at the end of BOF * D D D
Decarburization � � * D
Desulfurization * D � �
Avoid Si pick-up � * * *
Temperature drop * * � �
Temperature compensation * D D D
Cleanliness [O] before deoxidation � � * D
T.O. after refining * * D *
[N] after refining � D * D
Avoid [N] pick-up D * * *
Inclusions Modification * � � �
Amount � D * *
Average size * D D D
Removal rate � D * *
Castability of steel * D D D
Efficiency and cost Refining time � * D D
Production cost � D * D
Construction cost D * � �
Note: *, Good; D, Fair; �, Poor.
Table 4. Comparison of different refining processes.
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stable production of LCAK steels, while LF is more
suitable for the case with low level of refining technol-
ogy, or the case with poor control of end points of BOF,
or for thin slab continuous casting process.
Acknowledgments
The authors are grateful for support from the National
Science Foundation China (grant no. 51274034), the
High Quality Steel Consortium at University of Science
and Technology Beijing (China), and Shougang Jingtang
United Iron & Steel Co. (China) for industrial trials.
Received: August 21, 2012;
Published online: December 5, 2012
Keywords: cleanliness; inclusion; LCAK steel; secondary
refining
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