[Designation of Document] SPECIFICATION
[Title of the Invention] VACUUM DEGASSING TANK AND DEGASSING
METHOD USING THE SAME
[Technical Field]
[0001]
The present invention relates to a vacuum degassing tank having excellent
durability against low-basicity slags, and a degassing method using the vacuum
degassing tank.
[Background Art]
[0002]
In a vacuum degassing tank used for a vacuum degassing apparatus, a
refractory material such as magnesia carbon bricks or magnesia chromia bricks is used.
As the refractory material, a carbon-containing refractory material such as magnesia
carbon bricks has been widely used because of its excellent thermal shock resistance.
[0003]
The carbon-containing refractory material contains carbon which has a weak
point in oxidation resistance at high temperatures and thus the erosion rate thereof is
increased under a strongly acidic atmosphere. Here, adding an antioxidant such as an
Al powder, an Al-Si alloy powder, or an Al-Mg alloy powder to the carbon-containing
refractory material has been performed hitherto (for example, refer to Patent
Documents 1 to 5,10, and 11). This is for the purpose of preventing carbon from
being oxidized by adding a metal powder having a higher affinity to oxygen than
carbon at a high temperature.
[0004]
However, the magnesia carbon brick shows good durability against slags
1
having a slag basicity (CaO/Si02: hereinafter, may be simply referred to as basicity or
"C/S") of about 3 or higher but has deteriorated durability against slags having a lower
basicity. This is because the viscosity of slag having a low basicity is significantly
degraded and thus the slag infiltrates into the bricks and thus the magnesia aggregate is
eroded and causes spalling. Here, a method, in which metal chromium or a
chromium compound being contained in addition to the aluminum alloy powder causes
oxidized Cr203 to generate a MgO-Cr203-based high-melting-point material in the
operation surfaces of the bricks and the reaction layers of the slag and increases the
apparent viscosity of the slag, thereby suppressing the elution of the magnesia
aggregate (refer to Patent Document 6), is suggested. Similarly, a method, in which
calcium zirconate being contained in addition to the aluminum alloy powder causes
CaO dissociated at a high temperature to be integrated with the slag and increases the
viscosity of the slag, thereby suppressing infiltration (refer to Patent Document 7), a
method, in which a rare-earth oxide being contained without the use of chromium in
consideration of the environment causes the rare-earth oxide to react with SiOi in the
slag and increases the melting point of the slag, thereby suppressing infiltration (refer
to Patent Document 8), and the like are suggested. Moreover, a method in which
crushed bricks having MgO as a main component or Al is added to the slag depending
on the basicity of the slag to adjust the slag component to become the initial crystal
zone of MgO, thereby reforming the slag (refer to Patent Document 9), is suggested.
[0005]
In addition, in Patent Document 10, a magnesia carbon brick to which an Al-
Mg alloy is added is described. However, as apparent from a slag basicity (C/S)
under the conditions of facilities described in the text and a corrosion resistance test
being 3, the results of simulating a case where the bricks are used in a converter are
shown.
[0006]
In addition, in Patent Document 11, as a magnesia carbon brick appropriate
for thermal spraying repair, an example in which an Al powder or an Al-Mg alloy is
added so that an Al content is 3 mass% or less while a fixed carbon content is 13
mass% or less is described. However, Background Art and Means for Solving the
Problems of Patent Document 11 specify that this invention is used for a converter.
Therefore, a corrosion resistance evaluation method is performed by using converter
slags for evaluation, and thus the durability against low-basicity slags which is the
object of the present invention is not described.
[0007]
Therefore, according to the related art, although adding a small amount of an
Al-Mg alloy to magnesia carbon bricks used for a vacuum degassing tank for
preventing the oxidation of graphite is known, the durability against low-basicity slags
is not known.
[Prior Art Document]
[Patent Document]
[0008]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. S57-166362 (the upper left column of p. 2, Table 1)
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. S58-190868 (the right column of p. 1, Table 1)
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. S63-166751 (the upper right column of p. 2, Table 3)
[Patent Document 4] Japanese Unexamined Patent Application, First
3
Publication No. 2001-139366 (Claim 1, Table 2)
[Patent Document 5] Japanese Unexamined Patent Application, First
Publication No. 2007-182337 (paragraph [0022], Table 2)
[Patent Document 6] Japanese Unexamined Patent Application, First
Publication No. Hl-320262 (Claim 1 in CLAIMS, Table 1)
[Patent Document 7] Japanese Unexamined Patent Application, First
Publication No. 2000-95556 (Claim 1, paragraph [0015])
[Patent Document 8] Japanese Unexamined Patent Application, First
Publication No. 2001-254120 (Claim 1, paragraph [0013])
[Patent Document 9] Japanese Unexamined Patent Application, First
Publication No. 2006-257519 (Claim 1)
[Patent Document 10] Japanese Unexamined Patent Application, First
Publication No. H5-186259 (Paragraph [0016], Table 1)
[Patent Document 11] Japanese Unexamined Patent Application, First
Publication No. 2008-151425 (Table 3)
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0009]
However, it still cannot be said that the durability of the magnesia carbon
brick against the low-basicity slags is sufficient, and for example, in a case where Al-
Si-killed steel, Si-killed steel. Si-added steel, or the like is smelted using a vacuum
degassing tank, low-basicity slag having a basicity of 2 or less is generated, and lining
refractory materials in the vacuum degassing tank are severely consumed during
vacuum degassing.
[0010]
The inventors have diligently researched to solve such problems. As a result,
new knowledge was obtained in a procedure of a detailed examination on the contents
of Al-Mg alloy or graphite in a carbon-containing magnesia refractory material for a
vacuum degassing tank. That is, regarding the Al-Mg alloy which has been used as
an antioxidant according to the related art to prevent oxidation of carbon due to its
higher affinity than carbon, surprisingly, it was found that by causing the amount
thereof to be in a specific range and setting the mass ratio thereof to graphite in a
refractory material to be extremely higher than that according to the related art, a
fiinction of suppressing the erosion of a magnesia aggregate is exhibited and excellent
durability even against low-basicity slags as described above can be exhibited. In
addition, hitherto, it has been known that thermal shock resistance is reduced when the
amount of metals including an Al-Mg alloy is increased (for example, 3 mass% or
higher). However, the inventors found that thermal shock resistance can also be
ensured by setting the amount of graphite with respect to the amount of the Al-Mg
alloy to be in a certain mass ratio range and thus completed the present invention.
[0011]
Therefore, an object of the present invention is to provide a vacuum degassing
tank in which carbon-containing magnesia refractory materials that show excellent
durability against low-basicity slags having a basicity (C/S) of 2 or less are used as
linings.
[0012]
In addition, another object of the present invention is to provide a degassing
method capable of performing degassing while suppressing the wear of a vacuum
degassing tank, in the degassing method of performing secondary refining of steel by
adding a Si source.
[Means for Solving the Problems]
[0013]
The present invention employs the following in order to accomplish the
objects to solve the problems.
(a) According to an aspect of the present invention, there is provided a
vacuum degassing tank which includes an iron shell and a refractory material which
covers an inner portion of the iron shell and performs degassing of molten steel in a
reduced-pressure atmosphere, including: a carbon-containing magnesia refractory
material provided at a part of the refractory material which comes into contact with at
least a molten slag. The carbon-containing magnesia refractory material contains 7
mass% or higher and less than 28 mass% of graphite, 3.5 mass% or higher and 14
mass% or less of an Al-Mg alloy, and magnesia and unavoidable impurities as the
remainder, and a mass ratio obtained by dividing a mass of the Al-Mg alloy by a mass
of the graphite is 0.5 or higher and 2.0 or less.
(b) In the vacuum degassing tank described in (a), a lower limit of the mass
ratio may be 1.0.
[0014]
(c) A vacuum degassing method according to another aspect of the present
invention includes: performing vacuum degassing in which a slag with a low basicity
of CaO/Si02<2 is likely to be generated using the vacuum degassing tank described in
(a)or(b).
(d) In the vacuum degassing method described in (c), the vacuum degassing
may be performed while performing secondary refining of steel by adding Si or a Si
alloy to the vacuum degassing tank.
[0015]
The carbon-containing magnesia refractory material used for the vacuum
degassing tank according to the aspects of the present invention preferably contains 3.5
mass% to 14 mass% of the Al-Mg alloy by inner percentage, and more preferably
contains 3.5 mass% to 10.5 mass% thereof When the amount of the Al-Mg alloy is
less than 3.5 mass%, desired corrosion resistance cannot be ensured. On the other
hand, when higher than 14 mass% thereof is contained, the porosity of the carboncontaining
magnesia refractory material is increased during use and the structure
thereof becomes brittle, also resulting in a degradation of the corrosion resistance.
The type of the Al-Mg alloy is not particularly limited, and a general type added to
magnesia carbon bricks as an antioxidant may be used. Appropriately, an alloy is
composed of a composition of Al^Mgi? may be used. In addition, an Al-Mg alloy
having a particle diameter of 40 |j.m to 200 |.xm may be used.
[0016]
The graphite contained in the carbon-containing magnesia refractory material
used for the vacuum degassing tank according to the above aspects may be graphite
generally used in magnesia carbon bricks. For example, flake graphite, earthy
graphite, artificial graphite, expanded graphite, and the like are given. Appropriately,
natural flake graphite having well-developed crystals may be used. The amount of
graphite may be 7 mass% or higher by inner percentage, preferably 7 mass% to 28
mass%, and more preferably 7 mass% to 14 mass%. When the amount of the
graphite does not reach 7 mass%, the role of carbon to function as so-called carboncontaining
magnesia refractory material cannot be sufficiently achieved. When the
amount of the graphite exceeds 28 mass%, mold forming becomes difficult, and the
filling ability of the refractory material cannot be ensured. When the amount of the
graphite is 14 mass% or less, the filling ability is fiirther enhanced.
[0017]
In addition, in the aspects, the mass ratio of the Al-Mg alloy to the graphite
(Al-Mg alloy/graphite) is 0.5 or higher, preferably 1.0 or higher, and more preferably
in a range of 1.0 to 2.0. In the magnesia carbon bricks according to the related art,
graphite which is less likely to be wet by molten slags remains on the operation surface
and thus moistening by the molten slags can be prevented. However, during
degassing at a reduced pressure by a vacuum degassing apparatus such as RH
(Ruhrstahl-Heraus) or DH (Dortmunt-Horde), the chemical reaction as shown in the
following expression (1) proceeds between the MgO aggregate and C of the magnesia
carbon bricks and thus there is a concern that the bricks may be vaporized.
MgO(s)+C(s)-^Mg(g)t+CO(g)t.. .(1)
[0018]
Here, in the aspects, by the Al-Mg alloy being contained in the abovedescribed
range and setting the mass ratio of the Al-Mg alloy to the graphite (Al-Mg
alloy/graphite) to be 0.5 or higher, the partial pressure of Mg in the carbon-containing
magnesia refractory material is increased to suppress the reaction of the expression (1),
thereby preventing the erosion of the MgO aggregate. Regarding the upper limit of
the mass ratio, when the amount of the Al-Mg alloy added is increased too much,
cracks are generated in the refractory material due to excessive sintering, and thus the
mass ratio of Al-Mg alloy/graphite is preferably 2.0 or less. When the carboncontaining
magnesia refractory material in the aspects is used as the lining refractory
material in the vacuum degassing tank, even during secondary refining of steel in
which slag with a low basicity of CaO/Si02<2 is generated by adding Si or a Si alloy
to smelt Al-Si-killed steel. Si-killed steel. Si-added steel, or the like, the wear of the
vacuum degassing tank is suppressed as much as possible, and thus the degassing can
be appropriately performed.
[0019]
The magnesia contained in the carbon-containing magnesia refractory
material used for the vacuum degassing tank of the aspects may be magnesia generally
used as the aggregate of the refractory material, and for example, sintered magnesia
such as natural magnesia or the like made by firing natural magnesite, electrofused
magnesia made by melting a magnesia raw material in an electric furnace and recrystallizing
the resultant, and the like may be used. As the magnesia aggregate,
those pulverized into sizes of about 3 to 5 mm or smaller to adjust their granularity are
generally used, but there is no particular limitation. Furthermore, in addition to the
graphite and the Al-Mg alloy, a binder resin described later or additives that can be
blended in in a range that does not depart from the object of the present invention may
be added, and excluding the blended components, the amount of the magnesia may be
set to the blending amount of the magnesia as the remainder of the refractory raw
materials, and preferably 56 mass% or higher thereof, by inner percentage, may be
contained.
[0020]
The carbon-containing magnesia refractory material used for the vacuum
degassing tank of the aspects can be obtained by, like the well-known magnesia carbon
bricks according to the related art, blending a phenolic resin or the like as a binder with
the magnesia, the graphite, and the Al-Mg alloy, kneading the refractory raw materials,
molding the resultant using a press machine or the like, and then drying the resultant.
In addition, additives such as other metal powders or metal compound powders may
also be blended in as refractory raw materials in a range that is not out of the object of
the present invention.
In addition, when the carbon-containing magnesia refractory material is lined
in the vacuum degassing tank, the carbon-containing magnesia refractory material may
be formed at least on a part that comes into contact with the molten slag, and it may
also be formed on the entire lining surface as the lining.
[Advantage of the Invention]
[0021]
According to the aspects of the present invention, a carbon-containing
magnesia refractory material which shows excellent durability even during degassing
in which low-basicity slag with a C/S mass ratio of 2 or less is generated, while having
thermal shock resistance which inheres in the carbon-containing magnesia refractory
material, can be obtained. In addition, by using the carbon-containing magnesia
refractory material according to the aspects of the present invention for the lining
refractory material in the vacuum degassing tank, degassing can be appropriately
performed when Al-Si-killed steel. Si-killed steel. Si-added steel, or the like is smelted.
[Brief Description of the Drawing]
[0022]
FIG. IA is a graph showing the relationship between content of graphite and
wear depth in a rotary erosion test.
FIG. IB is a graph showing the relationship between content of graphite and
wear index in the rotary erosion test.
FIG. 2A is a graph showing the relationship between addition amount of metal
and wear depth in the rotary erosion test.
FIG. 2B is a graph showing the relationship between addition amount of metal
and erosion index in the rotary erosion test.
FIG. 3 A is a graph showing the relationship between addition amount of metal
10
and mass reduction ratio in an oxidation resistance evaluation test, and shows a case
where the added metal is metal Al.
FIG. 3B is a graph showing the relationship between addition amount of metal
and mass reduction ratio in the oxidation resistance evaluation test, and shows a case
where the added metal is an Al-Mg alloy.
FIG. 4 is photographs showing the cut cross-section of a refractory material
after oxidation firing in the oxidation resistance evaluation test.
FIG. 5 is a graph showing the relationship between mass ratio of Al-
Mg/graphite and wear depth in an erosion test using a vacuum melting furnace.
FIG. 6 is a longitudinal sectional view of a vacuum degassing tank according
to an embodiment of the present invention.
[Best Mode for Carrying Out the Invention]
[0023]
A vacuum degassing tank and a degassing method using the same according
to an embodiment of the present invention will be described below with reference to
the drawings.
FIG. 6 illustrates a vacuum degassing tank 1 of this embodiment. The
vacuum degassing tank 1 is a furnace in which degassing of molten steel is performed
using a reduced-pressure atmosphere and is configured by coaxially assembling an
upper tank 2 and a lower tank 3. The upper tank 2 includes a cylindrical iron shell 21
and a refractory material 22 which covers the inner peripheral surface thereof The
upper end of the upper tank 2 is covered with a top cover 23. In addition, the side
face of the upper tank 2 is provided with an alloy injection port 24 and an exhaust port
25.
The lower tank 3 includes an iron shell 31 having substantially the same
11
diameter as that of the iron shell 21 of the upper tank 2 and a refractory material 32
which covers the inner peripheral surface thereof The lower end of the lower tank 3
is provided with two circulation tubes 33 in the vertical direction. Moreover, two
dipping tubes 4 are mounted to extend from the lower ends of the circulation tubes 33,
and the dipping tubes 4 are dipped into the molten steel in a ladle 5.
[0024]
During an operation, air in the vacuum degassing tank 1 is discharged from
the exhaust port 25 (arrow Al of FIG. 6) to form a reduced-pressure state, and the
molten steel in the ladle 5 is suctioned up into the vacuum degassing tank 1. In
addition, Ar is blowed from a gas blow-in port formed on one side of the dipping tube
4 to cause the molten steel in the vacuum degassing tank I to flow in and scatter
(arrow BI of FIG. 6). In this manner, degassing of the molten steel is performed in
the vacuum degassing tank 1, and the degassed molten steel is returned into the ladle 5
from the other dipping tube 4 (arrow B2 of FIG. 6).
[0025]
[Rotary Erosion Test]
Using a MgO material made of magnesia clinker as an aggregate, which had a
particle diameter of 1 to 5 mm and a purity of 98% or higher, and a magnesia fine
powder having a particle diameter of less than 1 mm, flake graphite having a particle
diameter of 100 to 400 mm and a purity of 97% or higher, an Al-Mg alloy powder (a
composition of AlnMgi?, a purity of 99.0% or higher) having a particle diameter of 40
to 200 mm, a metal Al powder (a composition of Al (metal Al), a purity of 99.5% or
higher) having a particle diameter of 10 to 100 mm, and a phenolic resin, test
refractory raw materials of TestNos. 1 to 12 shown in Table 1 were prepared.
Moreover, a test refractory raw material (Test No. 16) which includes 10 mass% of
- 12
graphite, 4 mass% of an Al-Mg alloy, 2 mass% of a phenolic resin, and 82.30 mass%
of a MgO material was prepared. In addition, after kneading each test refractory raw
material with an Omni-mixer, a standard brick (size: 65 mmxll4 mmx230 mm) was
molded using a press machine. In addition, the standard brick was dried and heated at
200°C, and was cut into sizes of an upper bottom of 41 mm, a lower bottom of 67 mm,
a height of 48.5 mm, and a length of 114 mm, thereby obtaining refractory materials
for a rotary test.
[0026]
In addition, the obtained test refractory materials were lined as linings in a
rotary drum-type erosion test apparatus (not illustrated), test slags having a slag
composition shown in Table 2 was put therein and rotated for 8 hours while being
heated at 1700°C to perform the rotary erosion test, and the height dimension (residual
dimension) of each test refractory material as the linings was measured. In addition,
the test slags were replaced with a new one every 20 minutes in the rotary drum-type
erosion test apparatus. In addition, a wear index indicates an index which uses the
wear depth of the refractory material of Test No. 4 as 100, and wear becomes a lower
degree as the number decreases.
13
[0027]
[Table 1]
Test
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Test refractory raw material (unit: mass%)
MgO
Remainder
80.36
82.30
Phenolic
resin
2
2
2
Graphite
3
5
10
7
7
7
7
7
7
7
7
7
7
9
8
10
Added metal
Al-M
2
2.5
2.5
0
1.75
3.5
7
0
0
0
0
10.5
14
9
8
4
S
Al-
Mg/C
0.67
0.5
0.25
0
0.25
0.5
1
0
0
0
0
1.5
2
1
1
0.4
Al
0
0
0
0
0
0
0
10.5
1.75
3.5
7
0
0
0
0
0
Rotary erosion test
Residual
dimension
(mm)
36.98
37.98
39.04
36.11
38.36
40.09
40.84
32.93
38.69
39.01
38.73
39.9
---
32.15
Wear
depth
(mm)
11.52
10.52
9.46
12.39
10.14
8.41
7.66
15.57
9.81
9.49
9.77
8.6
-
--
16.35
Wear
index
93
85
76
100
82
68
62
126
79
77
79
69
-
--
132
Oxidation
resistance test
Mass
reduction ratio
(%)
--
-
7.68
6.44
2.07
1.15
-
6.42
4.72
1.70
--
--
-
Vacuum
erosion test
Wear depth
(mm)
-
-
-
9.76
8.69
7.29
5.60
---
-
5.66
5.71
5.49
5.62
-
14
[0028]
[Table 2]
Test slag (unit: mass%) CaO
27.75
Si02
27.75
MnO
25
T-Fe
2
AI2O3
12.5
MgO
5
Basicity (C/S)
1
[0029]
As a result of the rotary erosion test as described above, first, as shown in
FIGS. 1A and IB, from the results of Test Nos. 1 to 3 and 5 in which the amounts of
the Al-Mg alloy added were substantially the same, it was confirmed that erosion was
suppressed as the amount of graphite was increased, but the degree of wear was not
changed much even when the amount thereof exceeded 7 mass%.
[0030]
Next, regarding the amount of the Al-Mg alloy powder added, as shown in
FIGS. 2 A and 2B, from the results of Test Nos. 4 to 7 and 12 (the Al-Mg alloy powder
was added) in which the amount of graphite added was constant and Test Nos. 8 to 11
(the metal Al powder was added), when the amount of the Al-Mg alloy powder was 3.5
mass% or higher (that is, the mass ratio thereof to graphite (Al-Mg alloy/graphite) was
0.5 or higher), better results than the metal Al powder were shown, and the best results
were shown particularly in a case where the mass ratio was 1. Moreover, according
to the other test, it was confirmed that in a case where the amount of graphite was 7
mass%, until the amount of the Al-Mg alloy powder added was 14 mass%, good results
were shown as described above.
[0031]
[Oxidation Resistance Evaluation Test]
The test refractory raw materials of Test Nos. 4 to 7 and 9 to 11 were kneaded,
molded, and dried as in the rotary erosion test, and thereafter cut into columnar shapes
having a size of (pSOxheight 50 mm, thereby obtaining refractory materials for an
15
oxidation resistance test. The obtained test refractory materials were put in an electric
furnace (not illustrated) in a state of being buried in coke breeze, were heated to
1000°C at a temperature increase rate of 5°C/min, and were subjected to pre-firing for
10 hours in a reducing atmosphere. In addition, the inside of the electric furnace was
set to an air atmosphere after measuring the mass of each test refractory material,
which were then fiirther subjected to oxidation firing at 1400°C for 4 hours.
[0032]
After finishing the oxidation firing, the mass of each test refractory material
was measured, and the mass reduction ratio thereof after the oxidation firing was
obtained. The results are shown in Table 1 and FIGS. 3A and 3B. From the results,
it was seen that when the addition amount of metal was 3.5 mass% or higher, the test
refractory materials to which the Al-Mg alloy powder was added had a small mass
reduction ratio and excellent oxidation resistance. In addition, each test refractory
material after finishing the oxidation firing was cut into round pieces, and photographs
of the transverse cross-sections thereof are shown in (a) to (g) of FIG. 4. From the
photographs, it was also seen that (c) and (d) to which 3.5 mass% or higher of the Al-
Mg alloy powder was added had excellent oxidation resistance.
[0033]
[Erosion Test using Vacuum Melting Furnace]
Test refractory raw materials of Test Nos. 4 to 7 and 12 as prepared for the
rotary erosion test; a test refractory raw material made of 7 mass% of graphite, 14
mass% of the Al-Mg alloy, 2 mass% of the phenolic resin, and the MgO material as the
remainder (Test No. 13); a test refractory raw material made of 9 mass% of graphite, 9
mass% of the Al-Mg alloy, 2 mass% of the phenolic resin, and the MgO material as the
remainder (Test No. 14); and a test refractory raw material made of 8 mass% of
16
graphite, 8 mass% of the Al-Mg alloy, 2 mass% of the phenolic resin, and 80.36
mass% of the MgO material (Test No. 15) were prepared, and kneaded, molded, and
dried as in the rotary erosion test. Thereafter, they were cut into sizes of an upper
bottom of 46 mm, a lower bottom of 70 mm, a height of 30 mm, and a length of 230
mm, thereby obtaining samples for the vacuum melting furnace erosion test. The
obtained test refractory materials were lined as linings in a 50 kg vacuum melting
furnace (not illustrated), test slags having a slag composition shown in Table 2
described above were put therein and degassing was simulated while heating the test
slags at 1650°C. After reducing the pressure to 1.3 kPa (10 Torr) and holding said
pressure for 3 hours, the height dimension (residual dimension) of each test refractory
material as the lining was measured, thereby obtaining the wear depth.
[0034]
Even under a reduced pressure with the low-basicity slags as described above,
from the results of Test Nos. 4 to 7, 12, and 13 in which the amount of graphite added
was constant, as shown in Table 1, it was determined that the wear depth was reduced
when the amount of the Al-Mg alloy powder is 3.5 mass% or higher and thus good
results were shown. As can be seen from FIG. 5 showing the results as a graph, it was
confirmed that when the mass ratio of the Al-Mg alloy to graphite (Al-Mg
alloy/graphite) was 0.5 or higher, the wear amount (wear depth) was reduced and
furthermore, in a case where the mass ratio was 1 to 2, good results were shown.
[0035]
From the test results, the gist of this embodiment is described as follows.
The vacuum degassing tank 1 of this embodiment illustrated in FIG. 6
includes the iron shells 21 and 31 and the refractory materials 22 and 32 that cover the
inner portions of the iron shells 21 and 31, and performs degassing of molten steel in a
17
reduced-pressure atmosphere. In addition, on the entire refractory materials 22 and
32 or at least parts thereof which come into contact with the melted slag, the carboncontaining
magnesia refractory material is formed as the lining. Moreover, the
carbon-containing magnesia refractory material contains 7 mass% or higher and less
than 28mass% of the graphite, 3.5 mass% or higher and 14 mass% or less of the Al-Mg
alloy, the mass ratio obtained by dividing the mass of the Al-Mg alloy by the mass of
the graphite is 0.5 or higher and 2.0 or less, and the remainder thereof is made of
magnesia and unavoidable impurities. In addition, it is more preferable that the lower
limit of the mass ratio is 1.0.
In addition, the unavoidable impurities in the present invention contain a
binder such as a phenolic resin which is added in a small amount during the
manufacturing process. A portion of the binder is vaporized during the drying
process but the remainder thereof remains in the refractory materials. In the present
invention, not only the unavoidable impurities contained in the raw materials but also
the remainder of the binder is defined as the unavoidable impurities.
In addition, the content of the magnesia may be 58 mass% or higher and 89.5
mass% or less. The reason is that in a case where the amount of graphite does not
exceed 28 mass% as the upper limit and the content of the Al-Mg alloy is 14 mass%,
the mass ratio obtained by dividing the mass of the Al-Mg alloy by the mass of the
graphite has a value slightly higher than 0.5, and the amount of magnesia as the
remainder is 58 mass% or higher. In addition, in a case where the amount of graphite
is 7 mass% and the amount of Al-Mg alloy is 3.5 mass%, the mass ratio obtained by
dividing the mass of the Al-Mg alloy by the mass of the graphite has a value of 0.5,
and from this, the amount of the magnesia of the remainder is less than 89.5 mass%.
[0036]
18
In the vacuum degassing method of this embodiment, the vacuum degassing
in which slag with a low basicity of CaO/Si02<2 is likely to be generated is performed
by using the vacuum degassing tank 1 having the above configuration. In addition,
the vacuum degassing may also be performed while performing secondary refining of
steel by adding Si or a Si alloy to the vacuum degassing tank 1.
[Industrial Applicability]
[0037]
As described above, it was confirmed that since the carbon-containing
magnesia refractory material of this embodiment contains 3.5 to 14 mass% of the Al-
Mg alloy powder and the mass ratio thereof to the graphite is 0.5 or higher, excellent
durability against the low-basicity slags is shown and excellent durability is shown
even under a reduced pressure in the degassing. This overcomes the disadvantages of
the carbon-containing magnesia refractory material according to the related art and
thus significantly contributes to an increase in the life span of the refractory materials
under the operational conditions of degassing in the steel industry.
[Description of Reference Signs]
[0038]
1: VACUUM DEGASSING TANK
21,31: IRON SHELL
22, 32: REFRACTORY MATERIAL

j^^^f^THKU 9367-13!
[Designation of Document] CLAIMS
[Claim 1]
A vacuum degassing tank which includes an iron shell and a refi-actory
material which covers an inner portion of the iron shell and performs a degassing of a
molten steel under a reduced-pressure atmosphere, the vacuum degassing tank
comprising:
a carbon-containing magnesia refractory material provided at least at a contact
part of the refractory material which comes into contact with a molten slag,
wherein the carbon-containing magnesia refractory material contains 7 mass%
or higher and less than 28 mass% of a graphite, 3.5 mass% or higher and 14 mass% or
less of an Al-Mg alloy, and a magnesia and unavoidable impurities as the remainder,
and
a mass ratio obtained by dividing a mass of the Al-Mg alloy by a mass of the
graphite is 0.5 or higher and 2.0 or less.
[Claim 2]
The vacuum degassing tank according to claim 1,
wherein a lower limit of the mass ratio is 1.0.
[Claim 3]
A vacuum degassing method comprising: performing vacuum degassing in
which a slag with a low basicity of CaO/Si02<2 is likely to be generated using the
vacuum degassing tank according to claim 1 or 2.
[Claim 4]
The vacuum degassing method according to claim 3, wherein the vacuum degassing is performed while performing a secondary
refining of a steel by adding a Si or a Si alloy to the vacuum degassing tank.