Advanced Materials Research
ISSN: 1662-8985, Vol. 803, pp 312-316
doi:10.4028/www.scientific.net/AMR.803.312
© 2013 Trans Tech Publications, Switzerland
Online: 2013-09-10
Research of Fluoride-free Protective Slag Used in low C Steel
Man-tang DING
College of Resources and Environmental Engineering, Pan Zhi Hua University, Sichuan 617000,
China; email: [email protected]
Keywords: fluorine hazards, fluorine-free protection slag, fluorine-free mould slag, Li2O, BaO, B2O3
Abstract: In order to eliminate the hazards of fluoride, the low melting point and boosting
substances of Li2O, BaO, B2O3 were added into the low C peritectic steel mold powder replaced
fluoride. It makes fluorine-free protection slag successfully developed. The crystalline ore phase of
Fluorine-free mould slag is melilite. By adjustment crystallization ratio of melilite in slag film, the
heat flow through the mold can be controlled.
Preface
Continuous casting mold powder is one of the continuous casting three materials, it have a
significant impact on the slab surface quality and process stability. The traditional protective slag
contains large amounts of fluoride. Although it has an important role in the regulation of
crystallization properties, viscosity, surface tension of protective slag, but it is also serious the
hazards to the equipment, the human body and the environment. Fluoride at high temperatures will
been volatile and react with other substances, generated such as HF, NaF, SiF4, AlF3. It could
accelerate the corrosion of the equipment, damage to human organs, pollute water supplies.
The development that sacrificed the environment and non-renewable resources not could be
repeated again in order to achieve the goal of sustainable development task. The development of
fluorine-free protective slag was discussed because fluorine pollution source would be eliminated to
achieve cleaner production and environmental goal of a virtuous circle. Due to peritectic reaction
produced volume shrinkage large to form the crack easily, the low C peritectic steel especially
requires crystallizer slow cooling. It require high performance of fluorine-free protection slag.
Protective slag performance requirements:
- Melting temperature
The melting temperature have influence on the slag film thickness and heat flow that it was
between the solidified shell and mold. The slag film thickness that is at the outlet of the crystallizer
is 1 ~ 4 mm. As the melting temperature is higher, the thermal resistance of the slag film is greater .
In generally, thermal resistance of the slag film was from 40 to 90% of the total thermal resistance[1].
And shell thickness reduced with the rise of fluxes melting temperature. The shell thickness must be
greater than a certain value at the mold outlet, otherwise there will be continuous casting breakout.
Therefore, there is a maximum temperature limit. Of course, it was affected by the casting speed,
steel, section, and other factors. Usually it was required that the melting temperature is lower than
1200 ° C.
- Melting speed
The melting speed was in the 5 ~ 10s. The melting speed not only controls the thickness of the
liquid slag layer, but also controls the speed of the liquid slag flowing into the gap that it is between
the solidified shell and the mold. It could control slag circle formation, prevent from meniscus
solidification, and control lubrication and cooling that it is between the mold and solidified shell. In
order to do these, the liquid slag layer is required in the 8 ~ 15mm.
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Advanced Materials Research Vol. 803
313
- Viscosity
Low viscosity could help slag to absorb inclusion, increase the liquid slag film thickness and
lubrication, heat transfer which they are between the mold and solidified shell. But the viscosity is
too low, it will cause the slag entrapment and increase the depth of oscillation marks. ηv2 = 5 is the
best state.η-liquid slag viscosity, v-pull speed.
- Crystallization temperature
Protective slag′s the crystallization temperature
is different from its solidification
temperature. There is easy to produce glassy in one side of liquid slag film which it contact with the
mold wall because it was suffered from greatly rapid cooling, low temperature, temperature
gradient. There is easy to produce the crystals in the side of the slag film contacting the solidified
shell because the temperature is relatively high. In general the interfacial thermal resistance was
70% of the total thermal resistance. It is to achieve the slow cooling target that cracks
corresponding was reduced. With enhance of the crystallization temperature of the protective slag,
crystallization ratio of protective slag increase than the corresponding, it would result in increase of
thermal resistance and decrease of heat flow.
- Interfacial tension
The interfacial tension of protective slag was directly related with protective slag consumption.
Thus it affected the lubrication between the solidified shell and the mold wall. Low interfacial
tension could cause slag which affected the quality of slab. Therefore, it should be to increase the
interfacial tension appropriately by reduce the content of surface-active substances in protective
slag.
Experimental Methods
On the basis of the low melting point eutectic region of CaO-Al2O3-SiO2 slag system, the right
amount of Na2O, Li2O, B2O3 fluxing substances were added into. The used substances are
chemically pure. First the sample was mixed evenly, followed by heating to 1300 ℃ melted, then
cooled to room temperature, finely pulverized for the next detection. The chemical composition of
sample was shown in Table 1.
Tab 1 The chemical composition of sample(%)
sample
CaO
SiO2
Al2O3
1#
31.5
30.2
4.6
2#
31.5
30.2
3#
31.5
4#
MgO
Li2O
Na2O
2.5
2.0
4.6
2.5
30.2
4.6
31.5
30.2
5#
31.5
6#
31.5
B 2O 3
TiO2
BaO
MnO
S
10
<0.5
<0.1
3.0
11
<0.5
<0.1
2.5
2.0
11
8
<0.5
<0.1
4.6
2.5
2.0
12
10
<0.5
<0.1
30.2
4.6
2.5
2.0
12
10
1
<0.5
<0.1
30.2
4.6
2.5
2
10
10
1
<0.5
<0.1
8
Experimental results and analysis
The temperature performance parameters of protective slag were shown in Table 2.
Tab 2
The temperature performance parameters of protective slag (℃)
sample
melting temperature
crystallization temperature
1#
2#
3#
4#
5#
6#
1237.2
1207.4
1216.5
1201.1
1198.4
1170.8
1207.3
1209.0
1199.5
1198.6
1197.3
1159.3
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Frontiers of Chemical Engineering, Metallurgical Engineering and Materials II
It is known from Table 2 that: First，an amount of 2.0% that Li2O was added in is relatively
suitable. Second, melting temperature and crystallization temperature of 3# and 4# protective slag
significantly decreased than 1# and 2# slag. The melting temperature decreased by 20.7 °C, 6.3 °C,
the crystalline temperature decreased by 7.8 °C, 10.4 °C when 3 # contrasted on 1 # or 4#
contrasted on 2#. As 3# sample compared to 4#, the melting temperature and crystallization
temperature respectively reduce the 15.4 °C, 0.9 °C. Because B2O3 was added in, the melting
temperature and crystallization temperature were greatly reduced. It helped to ensure the thickness
of the liquid slag layer and the slag film thickness, and to ensure the lubrication and cooling
between the solidified shell and the mold. Third, when 2% of TiO2 was added in the protective slag,
it can effectively prevent sticking Breakout because the melting temperature and crystallization
temperature of the protective slag decreased than 4# slag. Fourth, after adding 8% of BaO, it fully
meet slow cooling and to prevent cracks requirements of crack sensitive steels, the low C peritectic
steel etc. , because the melting temperature and the crystallization temperature is substantially
reduced.
Thermal analysis curve of the heating and cooling process of 6# slag were shown in Figure 1.
Flow /(ml/min)
DSC /(mW/mg)
↓ exo
250
0.6
TG /%
[1.1] 20110924-al2o3-nolid-shanghai-15k-n2-1300-400-protective slag.dsu
purge2
protective
100
Flow /(ml/min)
DSC /(mW/mg)
↓ [1.3]
exo
250
TG /%
[1.3] 20110924-al2o3-nolid-shanghai-15k-n2-1300-400-protective slag.dsu
purge2
protective
86.5
0.5
98
Complex Peak:
Area:
118.8 J/g
Peak*: 1170.8 °C
Onset: 1134.5 °C
End: 1233.7 °C
96
0.15
86.0
Complex Peak:
Area:
-72.09 J/g
Peak*: 1159.3 °C
Onset: 1093.5 °C
End: 1209.0 °C
200
0.4
85.5
200
0.10
[1.1]
0.3
94
150
0.2
85.0
[1.3]
0.05
150
84.5
0.00
92
100
100
0.1
84.0
-0.05
90
83.5
0.0
50
50
88
[1.1]
[1.1]
-0.10
[1.3]
83.0
-0.1
86
-0.15
0
[1.1]
-0.2
200
Fig. 1
400
600
Temperature /°C
800
1000
0
1200
500
600
700
800
900
Temperature /°C
1000
1100
1200
TG-DSC thermal analysis curves of 6# slag heating and cooling process.
Note: There is not melt processing before thermal analysis.
It could meet requirements of melting temperature and crystallization properties of
non-fluorine slag used in the low C peritectic steel, because it made the melting and crystallization
temperature of the slag continued to drop when BaO was added in the protection slag contained of
Li2O, B2O3, TiO2.
Discussion and analysis
- Impact of Li2O
Li2O has fluxing action that it can reduce the melting temperature and crystallization
temperature of protective slag. It can promote to form liquid slag layer and slag film, to raise the
speed that the liquid slag flowed into the gap between the solidified shell and mold to strengthen the
role of lubrication and heat transfer.
It was said in the literature [2] that an amount of Li2O added in wax > 2.0%, the crystallization
temperature will improve. In the range of <2%, an increase of Li2O can reduce the crystallization
properties of the non-reactive protective slag. In the range of 2% to 5%, an increase of Li2O can
promote slag crystals, shorten the time of crystal precipitation, improve crystallization temperature,
generate slag circle and slag strip excessive.
Li2O content rise 1% for every, the melting point average was lower 36 °C. Li2O can reduce the
melting temperature of the protective slag because Li2O belong to the outer body of the network,
the Li﹢ charge is small , has a large radius and the smaller the role between it and O2－.
Advanced Materials Research Vol. 803
315
it can offer a non-bridging oxygen atoms in the protective slag structure, so that the ratio of O / Si is
increased. It has a larger role in the destruction on the mesh structure of the slag ,and becomes it
to be simple. Consequently, it reduces the slag melting point.
- Impact of B2O3
B2O3 often used to a binder phase of slag and lining knotted compound, its melting point was
only 450 °C. It is easy to form a low-melting phase with other substances in the protective slag,
lower the melting temperature of the protective slag. With the amount of B2O3 added in increases,
the melting point is decreased gradually. In the range of 4% to 10%, an amount of B2O3 was
increase of 1% for each, the melting temperature would lower 8 ℃ averagely[3].
On the one hand B2O3 belongs to the network to form the body, increase the slag viscosity; on
the other hand the B2O3 can significantly reduce the melting temperature of the slag, make the
degree of superheat of the slag to increase, thereby it reduce the viscosity of the liquid slag. It was
introduced in data [3] that the viscosity and transition temperature of the liquid slag have the trend
of downward when B2O3 is from 4% to 8%. The heat transfer characteristics time and heat flux
density of slag film increased. Slag film thickness and the ratio of crystallization decreased. The
crystalline incubation time and full crystallization time increased significantly. Of course, since its
crystalline rate is slower, the slag rim (or circles of slag) was produced less. But B2O3> 8%, this
change process trended to slow.
In summary, It was best that B2O3 should be controlled in the range of 8 to 10%.
- Impact of TiO2
After TiO2 was added into, the transition temperature and crystallization ratio of protection slag
could be greatly reduced. TiO2 that is acidic substances is conducive to the solidified shell
lubrication. It is able to achieve requirements of the fluorinated Fluxes. When R was over 1.2, the
crystallization temperature will be dramatically improved, the slag film can not guarantee
lubrication of slab solidified shell if TiO2 was added into.
In this experiment that R was 1.0, the adversely affect that 1% of TiO2was added into will be
effectively controlled.
- Impact of BaO
With BaO content increased, the melting temperature decreases more, crystallization
temperature decrease relatively small. The crystalline mineral phase of non-fluorine protective slag
is melilite. Its composition is solid solution of aluminum melilite (Ca2Al2SiO7), akermanite
(Ca2MgSi2O7), and sodium melilite (NaCaAlSi2O7). Compared with the fluorine-containing type
protective slag, cuspidine was not separated out during condensation, but melilite that its melting
point and thermal conductivity slightly are above than cuspidine can be separated out. The amount
of heat flowing through the mold can be controlled by adjusting the proportion of the crystallization
of the melilite in slag film. Thereby, the problems that the heat transfer and lubricating between the
mold and the solidified shell were not controlled in non-fluorine protective slag solidification
process could been good solved[4]. BaO will form a low melting point compounds with Li2O, MgO
and other together is the reason why BaO drop melting temperature of slag[5-7].
Although the effect of BaO that it cut down mold fluxes melting temperature and
crystallization temperature was lower than Li2O. But as mentioned, in the slag Li2O> 3%, Li2O
causes the crystallization temperature to rise, the rate of crystallization increases, and was not
conducive to high-speed casting.
When R is constant, with BaO increasing, slag viscosity decreased, it can meet the
requirements of low melting temperature and the low-crystalline, low viscosity of protective slag
with high-speed casting.
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Frontiers of Chemical Engineering, Metallurgical Engineering and Materials II
-Impact of Na2O
The melting temperature of Na2O is lower. It belong to restriction body of the chain structure. It
could been analyzed from two aspects. First, O2-can increase the ratio of oxygen to silicon in slag
protection. Second, Na+ and the corner of the Silicon-oxygen tetrahedron was bonding to prevent
network chains of the Silicon-oxygen tetrahedral that was formed or disconnected. Thereby it could
reduced the melting temperature and viscosity of the protective slag. But adding excessive, there is
nepheline separated out. It would reduced viscosity, so that it was unfavorable for lubrication.
In addition, it was showed from Table 1, Table 2 and the above-mentioned analysis that the
relationship that the fluxing substance influenced on base-slag system was not a simple addition.
The mechanism of common action among them need to been studied.
Conclusions
- Fluorine-free protective slag used in low C peritectic steel continue casting mold was successfully
developed when the appropriate amount of strong fluxing material of Na2O, Li2O, B2O3 etc. were
added into based slag of R = 1.04.
-The crystalline mineral phase of non-fluorine protective slag is melilite. There is not cuspidine
precipitated. The heat flow through the mold can be controlled by adjusting the proportion of the
crystallization of melilite in the slag film. Thus, the problems that the heat transfer and lubrication
between the mold and solidified shell can not been control because there will not be precipitated
cuspidine in non-fluorine slag condensation process could be solved.
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