Field
[0001] The present invention relates to a desulfurization processing method of molten steel
and a desulfurization agent.
Background
[0002] Recently, high purity steel manufacturing has been highly demanded for enhancing
material characteristics along with adding high value to steel and an increase in
use of steel materials, for example. Particularly, a demand has been highly increased
for ultra low sulfur steel having a low content of sulfur, which is an element that
reduces toughness of steel materials. In melting manufacturing processes of steel
materials, desulfurization processing at a molten pig iron stage and desulfurization
processing at a molten steel stage are available. Steel materials are typically subjected
to only the desulfurization processing at the molten pig iron stage when manufactured
by being melted. In the melting manufacturing processes of ultra low sulfur steel
such as high-grade electromagnetic steel plates and line pipe steel materials, it
is insufficient to perform only the desulfurization processing at the molten pig iron
stage. It is, thus, necessary to perform the desulfurization processing at the molten
steel stage in addition to the desulfurization processing at the molten pig iron stage.
[0003] The desulfurization processing at the molten steel stage is typically performed by
a ladle refining method such as an ASEA-SKF method, a VAD method, or an LF method.
The ladle reining method includes an arc heating means to heat molten steel and a
stirring means to stir molten steel, and further includes a blowing means to blow
powder such as flux or alloy powder to molten steel. In the ladle refining method,
a desulfurization agent is added into a ladle holding molten steel manufactured by
being melted by decarburization refining in a converter, the molten steel and the
desulfurization agent are stirred and mixed with each other or subjected to arc heating,
thereby causing the desulfurization agent to form slag, and a slag-metal reaction
occurs between the slag formed by slag formation of the desulfurization agent and
the molten steel to transfer sulfur in the molten steel to the slag.
[0004] The used desulfurization agent contains CaO (quicklime) as a major component and
Al
2O
3 (alumina), CaF
2 (fluorite), and the like, which are added for the purpose of lowering a melting point
of the desulfurization agent. For achieving effective desulfurization reaction in
the desulfurization processing method by the ladle refining method, it is important
to cause the added desulfurization agent to rapidly form slag and to increase a contact
area between the slag formed by the slag formation of the desulfurization agent and
the metal by increasing stirring strength. The desulfurization agent is typically
added to molten steel in the ladle by being placed on the molten steel. It takes a
long time that the desulfurization agent forms slag in any case where the desulfurization
agent forms slag by arc heating or by being stirred and mixed with the molten steel
after the desulfurization agent is added.
[0005] Patent Literature 1 discloses a method of desulfurizing molten steel in which flux,
a mixture of quicklime, alumina, and fluorite, is added and bubbling processing is
performed such that a slag composition after the desulfurization processing satisfies
that CaO/Al
2O
3 ≥ 1.5 mass% and CaF
2 ≥ 5 mass%. Patent Literature 2 discloses a method in which pre-melt flux (preliminarily
mixed or uniformly dissolved) of CaO-Al
2O
3 or pre-melt flux of CaO-Al
2O
3-CaF
2 is used for promoting the slag formation of the desulfurization agent. As for enhancing
stirring strength of molten steel, Patent Literatures 3, 4, and 5 disclose a method
in which stirring gas mixed with flux is blown, as a means to enhance the stirring
strength without increasing a stirring gas flow rate.
Citation List
Patent Literature
[0006]
Patent Literature 1: Japanese Patent Application Laid-open No. H08-260025
Patent Literature 2: Japanese Patent Application Laid-open No. H09-217110
Patent Literature 3: Japanese Patent Application Laid-open No. S61-91318
Patent Literature 4: Japanese Patent Application Laid-open No. S61-281809
Patent Literature 5: Japanese Patent Application Laid-open No. 2000-234119
Summary
Technical Problem
[0007] The method described in Patent Literature 1, however, has a problem in that, when
a desulfurization agent containing CaF
2 is used, refractory forming the ladle is heavily melted and eroded by CaF
2 in the produced slag, thereby significantly shortening a life-span of the ladle.
The method described in Patent Literature 2 has a problem in that the pre-melt flux
is very expensive, thereby increasing processing cost. The desulfurization agent containing
CaF
2 has the same problem as described above.
[0008] The method described in Patent Literatures 3, 4, and 5, has a limit for a flux blowing
amount with respect to a blowing gas rate (a solid-gas ratio has a limit ranging from
5 to 30 kg/kg). The method, thus, has a limit for increasing stirring strength. When
a stirring gas flow rate is increased, a surface of molten steel in the ladle is heavily
disturbed (waved). As a result, a problem arises in that splashes occur and metal
sticks to a lid of the ladle, or another problem arises in that electrodes and molten
steel are shorted, for example, to cause unstable arc, thereby making it difficult
to perform arc heating.
[0009] The invention is made in view of above problems, and aims to provide a desulfurization
processing method of molten steel and a desulfurization agent that can efficiently
perform desulfurization processing without using CaF
2 and pre-melt flux.
Solution to Problem
[0010] To solve the problem and achieve the object, a desulfurization processing method
of molten steel according to the present invention is a method including: adding a
desulfurization agent containing quicklime into a ladle holding the molten steel;
and stirring the molten steel in the ladle to reduce a sulfur concentration in the
molten steel, wherein the desulfurization agent contains quicklime, where a sum of
volumes of pores having a pore diameter ranging from 0.5 µm to 10 µm (inclusive of
0.5 µm and 10 µm) in the quicklime is equal to or larger than 0.1 mL/g.
[0011] Moreover, in the desulfurization processing method of molten steel according to the
present invention, the quicklime contains particles, where 90% or more of the particles
contained in the quicklime has a particle diameter ranging from 1 mm to 30 mm (inclusive
of 1 mm and 30 mm) .
[0012] Moreover, a desulfurization agent according to the present invention includes quicklime,
where a sum of volumes of pores having a pore diameter ranging from 0.5 µm to 10 µm
(inclusive of 0.5 µm and 10 µm) in the quicklime is equal to or larger than 0.1 mL/g,
wherein the quicklime contains particles, where 90% or more of the particles contained
in the quicklime has a particle diameter ranging from 1 mm to 30 mm (inclusive of
1 mm and 30 mm).
[0013] Moreover, in the desulfurization processing method of molten steel according to the
present invention, the stirring is performed so as to satisfy a stirring power density
represented by the following formula (1). In the present specification, "Nm
3" means a volume of gas at an atmospheric pressure of 101325 Pa and a temperature
of 273.15 K under standard conditions.
[0014] Moreover, in the desulfurization processing method of molten steel according to the
present invention, an amount of aluminum supplied into the molten steel within 10
minutes after start of the desulfurization processing, the desulfurization processing
starting after the molten steel is tapped from a converter, satisfies the following
formula (2).
[0015] Moreover, in the desulfurization processing method of molten steel according to the
present invention, argon gas is blown into the ladle such that an oxygen concentration
in the ladle is equal to or smaller than 15%. Advantageous Effects of Invention
[0016] The desulfurization processing method of molten steel and the desulfurization agent
according to the invention can efficiently perform desulfurization processing without
using CaF
2 and pre-melt flux.
Brief Description of Drawings
[0017]
FIG. 1 is a schematic diagram of a side surface of an LF facility used when the invention
is implemented.
FIG. 2 is a diagram illustrating slagging ratios in invention examples and comparative
examples.
Description of Embodiment
[0018] The inventors of the invention have earnestly studied for solving the problems described
above by focusing attention on a particle size and a pore diameter of caustic lime
and molten steel components. More specifically, the inventors of the invention have
performed various experiments and researches for the purpose of causing flux added
as a desulfurization agent to rapidly form slag to achieve efficient desulfurization
processing without using CaF
2 as a part of the desulfurization agent when low sulfur steel having a sulfur concentration
equal to or smaller than 0.0030 mass% is manufactured by being melted by desulfurization
processing by a ladle refining method using a desulfurization agent having a CaO containing
material as a major constituent material.
[0019] As a result, the inventors of the invention have found that the temperature of molten
steel when flux is added, a sol. Al concentration, and the particle size and the pore
diameter of caustic lime are important in order to promote flux added as a desulfurization
agent to form slag. The temperature of molten steel is determined by the temperature
of molten steel when the molten steel is tapped from a converter. As the temperature
of the molten steel when the molten steel is tapped is increased, the refractory of
the converter is increasingly melted and eroded, thereby causing processing cost to
be increased. An immoderate increase in tapping temperature is, thus, inadvisable.
[0020] The inventors of the invention have found that the desulfurization processing can
be performed highly efficiently using a powder desulfurization agent containing quicklime
as a major component and the quicklime satisfies that a sum of volumes of pores having
a pore diameter ranging from 0.5 to 10 µm in all of the pores included in the quicklime
is equal to or larger than 0.1 mL/g. The inventors of the invention, thus, have conceived
the invention. The pore diameter distribution of quicklime was measured by the following
method.
[0021] As pretreatment, quicklime was dried at a constant temperature of 120 °C for 4 hours.
Using Micromerities autopore IV 9520, a pore diameter distribution of dried quicklime
having a pore diameter ranging from approximately 0.0036 to 200 µm was obtained by
a mercury intrusion method, and a cumulative pore volume curve was calculated. In
addition, a sum of volumes of pores having a pore diameter ranging from 0.5 to 10
µm was obtained from the calculated cumulative pore volume curve.
[0022] The pore diameter was calculated using Washburn's equation represented in the following
formula (3). In formula (3), P is the pressure, D is the pore diameter, σ is the surface
tension (= 480 dynes/cm) of mercury, θ is the contact angle (= 140 degrees) between
mercury and the specimen.
[0023] Molten pig iron tapped from a blast furnace is received by a hot metal transfer vessel
such as a hot metal ladle or a torpedo car, and transferred to a converter in which
decarburization refining is performed as the next process. Typically, during the transportation,
hot metal pretreatment such as desulfurization processing and dephosphorization processing
are performed on molten pig iron. The invention is the technique to manufacture low
sulfur steel. The desulfurization processing is, thus, performed. Even when the dephosphorization
processing is not required in accordance with the compositional standard of the low
sulfur steel, the dephosphorization processing is performed to prevent rephosphorization
from converter slag in desulfurization processing after tapping from the converter.
[0024] The decarburization refining is performed on the molten pig iron on which the desulfurization
processing and the dephosphorization processing are performed, and resulting molten
steel is tapped to the ladle. In the decarburization refining in the converter, a
little amount of quicklime (CaO) and a little amount of dolomite (MgCO
3-CaCO
3), or calcined dolomite (MgO-CaO) is used as flux, and the flux forms slag in the
converter (hereinafter described as the "converter slag"), because the desulfurization
processing and the dephosphorization processing are already performed on the molten
pig iron. The converter slag has a role to promote dephosphorization reaction of the
molten pig iron. The dephosphorization processing is, however, already performed on
the molten pig iron. The main role of converter slag is, thus, prevention of occurrence
of iron splashes in blowing refining and melting and erosion of the lining refractory
of the converter.
[0025] At the last stage of tapping, the converter slag is mixed into the molten steel and
flows into the ladle. Slag flow-out prevention measures, which are typically taken,
are performed to prevent the flow out of the converter slag. It is, however, difficult
to perfectly prevent the converter slag from being flowed out even when the slag flow-out
prevention measures are taken. Some amount of converter slag is mixed into the molten
steel in the ladle and flows out from the converter. After tapping, the converter
slag that is mixed into the molten steel and flows into the ladle may be removed from
the ladle. The converter slag may, however, not be removed because SiO
2 component in the converter slag contributes to the slag formation of a CaO containing
material added later as the desulfurization agent.
[0026] In order to form CaO-MgO-Al
2O
3-SiO
2 desulfurization slag having a certain composition in the ladle, a CaO containing
material, a MgO containing material, an Al
2O
3 containing material, and a SiO
2 containing material are added into the ladle as flux. As described above, MgO has
a lower desulfurization ability than that of CaO, the MgO containing material may
not be added. Metallic Al is added into the ladle for deoxidation of the molten steel
and reduction of the slag (reduction of Fe oxides and Mn oxides in the slag).
[0027] Those materials may be added into a facility in later process that performs desulfurization
processing by any of an ASEA-SKF method, a VAD method, and an LF method. From a point
of view of promoting the slag formation of CaO, those materials are preferably added
into the ladle at tapping from the converter to the ladle or just after the tapping.
It is preferable for quicklime added just after the tapping that a sum of volumes
of pores having a pore diameter ranging from 0.5 to 10 µm in all of the pores included
in the quicklime is equal to or larger than 0.1 mL/g, and the quicklime contains particles
90% or more of which have a particle diameter ranging from 1 to 30 mm.
[0028] Respective additive amounts of the CaO containing material, the MgO containing material,
metallic Al, the Al
2O
3 containing material, and the SiO
2 containing material are determined, by considering a mass and a component composition
of converter slag flowed into the ladle, such that the composition of the slag produced
in the ladle after the slag formation of the added flux, i.e., the slag formed from
the flux and the converter slag, satisfies that the SiO
2 content is in a range from 5 to 15 mass% and a value of [(mass% CaO) + (mass% MgO)]/(mass%
Al
2O
3) is in a range from 1.5 to 3.0, and, preferably, the value of [(mass% CaO) + (mass%
MgO)]/(mass% Al
2O
3) is in a range from 1.8 to 2.5.
[0029] In this case, the respective additive amounts are more preferably determined such
that a value of (mass% MgO)/(mass% CaO) of the produced slag is equal to or smaller
than 0.10. Those materials are added into the ladle by the determined additive amounts.
All of the additive amount of metallic Al does not become Al
2O
3. Some amount of metallic Al is dissolved and remains in the molten steel. A ratio
of Al
2O
3 in slag to Al dissolved in molten steel is obtained preliminarily by an experiment.
The additive amount of metallic Al is set on the basis of the ratio. No CaF
2 is added.
[0030] In the invention, "the composition of the slag in the ladle after the desulfurization
processing is adjusted to the composition that does not substantially contain CaF
2" means that the slag composition of after the desulfurization processing is adjusted
without using a fluorine compound such as CaF
2 as a slag formation accelerator of CaO, and even when fluorine that is unavoidably
mixed into the used CaO containing material and Al
2O
3 containing material, for example, and brought into the ladle is present in the slag
after the desulfurization processing, the slag in the ladle is defined that the slag
substantially does not contain CaF
2.
[0031] As for the CaO containing material to be added, quicklime (CaO), limestone (CaCO
3), slaked lime (Ca(OH)
2), dolomite (MgCO
3-CaCO
3), and calcined dolomite (MgO-CaO) are used, for example. As for the MgO containing
material to be added, magnesia clinker (MgO), dolomite (MgCO
3-CaCO
3), and calcined dolomite (MgO-CaO) are used, for example.
[0032] It is preferable for a particle size of caustic lime that an average particle diameter
of the caustic lime is in a range from 1 to 30 mm from a point of view of a reaction
efficiency and an addition yield. From a point of view of reducing an amount sucked
in an exhaust system, an amount of fine powder is preferably small. An amount of caustic
lime having an average particle diameter equal to or larger than 30 mm is, thus, preferably
small. The measuring method of the average particle diameter is as follows. One kilogram
of a desulfurization agent was collected. The collected desulfurization agent was
sieved into nine classes, i.e., equal to or smaller than 500 µm, 500 µm to 1 mm, 1
to 5 mm, 5 to 10 mm, 10 to 15 mm, 15 to 20 mm, 20 to 25 mm, 25 to 30 mm, and equal
to or larger than 30 mm. The average particle diameter was obtained by calculating
the weight ratio represented in the following formula (4).
[0033] As for the Al
2O
3 containing material, aluminum dross (contains 20 to 70 mass% metallic Al and main
component of the balance is Al
2O
3), bauxite (Al
2O
3·2H
2O), and calcined alumina (Al
2O
3) are used, for example. Aluminum dross can be used as alternative of metallic Al.
As for the SiO
2 containing material, silica sand (SiO
2) and wollastonite (CaO-SiO
2) are used, for example. When the mass of the converter slag flowed in the ladle is
large, the SiO
2 containing material may not be required to be added. The MgO containing material
may not be required to be added when the slag composition satisfies that a value of
[(mass% CaO) + (mass% MgO)] / (mass% Al
2O
3) is in a range from 1.5 to 3.0, preferably in a range 1.8 to 2.5 without addition
of the MgO containing material.
[0034] The ladle holding the molten steel is transferred to the facility that performs the
desulfurization processing by any of the ASEA-SKF method, the VAD method, and the
LF method, and the desulfurization processing is performed on the molten steel by
the facility. In the invention, the desulfurization processing is performed by an
LF facility as an example. FIG. 1 is a schematic diagram of a side view of the LF
facility used when the invention is implemented. FIG. 1 illustrates an LF facility
1, a ladle 2, an elevating lid 3, arc heating electrodes 4, submerged lances 5 and
6, bottom-blowing porous bricks 7 and 8, molten steel 9, slag 10, a row material supply
chute 11, and an Ar gas introduction pipe 12.
[0035] In the LF facility 1, the ladle 2 that contains the molten steel 9 and is mounted
on a traveling carriage (not illustrated) is disposed at a certain position just under
the lid 3. The lid 3 is moved downward to be tightly in contact with the upper end
of the ladle 2. While the contact is kept, Ar gas is supplied from the Ar gas introduction
pipe 12, resulting in a space surrounded by the ladle 2 and the lid 3 becoming Ar
gas atmosphere. Ar gas is preferably blown from piping provided on the periphery of
the furnace lid such that an oxygen concentration in the ladle 2 is equal to or smaller
than 15%. The reduction of the oxygen concentration in the ladle 2 makes it possible
to reduce an amount of Al lost by reaction with oxygen in the air in the LF processing.
The flow rate of Ar gas blown from the ladle 2 preferably satisfies that a value of
πL
2/4Q is in a range from 50 to 150 (m/min) and more preferably 70 to 100 (m/min). L
is the diameter (m) of the ladle and Q is the flow rate of Ar gas (Nm
3/min). When the flow rate of Ar gas is small, the oxygen concentration is not sufficiently
reduced. In contrast, when the flow rate of Ar gas is too large, the molten steel
temperature is caused to be reduced.
[0036] When the CaO containing material, the MgO containing material, metallic Al, the Al
2O
3 containing material, and the SiO
2 containing material are not preliminarily added into the ladle 2, and when the additive
amounts of those materials are insufficient, the flux containing those materials and
metallic Al are supplied into the ladle 2 via the row material supply chute 11. Metallic
Al is preferably added within 10 minutes after start of the desulfurization processing
such that the following formula (5) is satisfied. It is, thus, preferable for promoting
the desulfurization processing that metallic Al is added in accordance with the Al
concentration after tapping from the converter to increase the Al concentration in
molten steel.
[0037] Then, the electrodes 4 are energized to generate arc, if necessary, to heat the molten
steel 9 and to cause the added flux to form slag. Thereafter, the submerged lance
5 or 6 is immersed into the molten steel 9 and then Ar gas serving as a stirring gas
is blown into the molten steel 9 from at least one of the submerged lances 5 and 6,
or the bottom-blowing porous bricks 7 and 8 to stir the molten steel 9. As a result
of stirring the molten steel 9, the flux is mixed with the molten steel 9, thereby
causing the flux to form slag. As a result, slag 10 is produced.
[0038] The produced slag 10 is stirred and mixed with the molten steel 9 as a result of
stirring of the molten steel 9. As a result, a slag-metal reaction occurs between
the molten steel 9 and the slag 10 and, thus, a desulfurization reaction occurs in
which sulfur in the molten steel 9 transfers into the slag. From a point of view of
promoting the desulfurization reaction, as described above, one or more kinds of Ca
alloy powder, metallic Mg powder, and Mg alloy powder are preferably blown into the
molten steel 9 together with Ar gas from the submerged lances 5 and 6, or, at least
one period in the desulfurization processing, blowing of the stirring gas from the
submerged lances 5 and 6 and blowing of the stirring gas from the bottom-blowing porous
bricks 7 and 8 are preferably performed simultaneously.
[0039] As for the Ca alloy powder, Ca-Si alloy powder and Ca-Al alloy powder are used, for
example. As for the Mg alloy powder, Mg-Al-Zn alloy powder and Mg-Si-Fe alloy powder
are used, for example. The particle diameters of those metallic powder are not limited
to specific diameters as long as those metallic powder can be added by being blown.
From a point of view of keeping a reaction interfacial area, the maximum particle
diameter is preferably equal to or smaller than 1 mm. When the sulfur concentration
in the molten steel 9 becomes equal to or smaller than 0.0010 mass%, blowing Ar gas
into the molten steel 9 stops to end the desulfurization processing. When the temperature
of the molten steel 9 is lower than a target temperature at the end of the desulfurization
processing, arc heating is performed. When the composition of the molten steel 9 is
not in a target range, an alloy iron and metals for composition adjustment are supplied
via the row material supply chute 11. After the completion of the desulfurization
processing, degassing refining is performed by an RH vacuum degassing apparatus, for
example. Thereafter, a strip cast slab is casted by a continuous casting machine.
[0040] As described above, in the invention, the slag composition after the desulfurization
processing is adjusted such that the SiO
2 content is in a range from 5 to 15 mass% in the desulfurization processing of the
molten steel 9 by the ladle refining method using the desulfurization agent containing
the CaO containing material as the major constituent material. SiO
2, thus, functions as the slagging accelerator to promote the slag formation of CaO.
In addition, the slag composition after the desulfurization processing is adjusted
such that a value of [(mass% CaO) + (mass% MgO)]/(mass% Al
2O
3) is in a range from 1.5 to 3.0, thereby causing the slag 10 to have high desulfurization
ability. As a result, the desulfurization processing can be efficiently performed
on the molten steel 9 without using CaF
2 as a part of the desulfurization agent and pre-melt flux as the desulfurization agent.
The above description is an example where the invention is implemented using the LF
facility. The invention can also be applied to an ASEA-SKF facility and a VAD facility
according to the manner as described above.
[First example]
[0041] Molten pig iron tapped from a blast furnace was subjected to the desiliconization
processing, the desulfurization processing, and the dephosphorization processing.
The processed molten pig iron was, then, charged into a converter to be subjected
to the decarburization refining. As a result, obtained was approximately 250 tons
of molten steel having a carbon concentration ranging from 0.05 to 0.09 mass%, a sulfur
concentration ranging from 0.0041 to 0.0043 mass%, and a phosphorous concentration
ranging from 0.004 to 0.010 mass%. After tapping, the converter slag flowed in a ladle
was not discharged. Metallic Al, quicklime, lightly calcined dolomite, and aluminum
dross were added into the ladle, and the ladle was transferred to the LF facility
illustrated in FIG. 1. The desulfurization processing was performed for approximately
30 minutes in such a manner that the molten steel was stirred by blowing Ar gas at
2000 NL/min from the submerged lances into the molten steel while the arc heating
was performed by the electrodes immersed in the slag so as to achieve a target sulfur
concentration equal to or smaller than 0.0024%.
[0042] Table 1 illustrates the sulfur concentrations (chemical analysis values) before and
after the desulfurization processing and the desulfurization ratios in respective
desulfurization tests. The remarks column in Table 1 illustrates "invention examples",
which are the tests according to the invention, and "comparative examples", which
are the tests other than those according to the invention. The desulfurization ratio
is the value of a ratio of a difference in sulfur concentration in the molten steel
before and after the desulfurization processing to a sulfur concentration in the molten
steel before the desulfurization processing and the value is expressed by percentage.
The desulfurization evaluation "Good" means that the sulfur concentration in the molten
steel after the desulfurization processing was equal to or smaller than 0.0024% while
the desulfurization evaluation "Poor" means that the sulfur concentration in the molten
steel after the desulfurization processing exceeded 0.0024%.
Table 1
Test No. |
Sum of volumes of pores 0.5-10 µm |
Average particle diameter |
[S] Component change |
Desulfurization evaluation |
Remarks |
Before processing [S] |
After processing [S] |
Desulfurization ratio |
mL/g |
mm |
Mass [%] |
Mass [%] |
[%] |
- |
1 |
0.03 |
10 |
0.0042 |
0.0028 |
33.3 |
Poor |
Comparative example |
2 |
0.06 |
10 |
0.0043 |
0.0027 |
37.2 |
Poor |
Comparative example |
3 |
0.09 |
10 |
0.0041 |
0.0025 |
39.0 |
Poor |
Comparative example |
4 |
0.15 |
10 |
0.0042 |
0.0021 |
50.0 |
Good |
Example |
5 |
0.2 |
10 |
0.0042 |
0.0019 |
54.8 |
Good |
Example |
6 |
0.2 |
0.5 |
0.0044 |
0.0022 |
50.0 |
Good |
Example |
7 |
0.2 |
0.8 |
0.0041 |
0.0020 |
51.2 |
Good |
Example |
8 |
0.2 |
1.5 |
0.0043 |
0.0018 |
58.1 |
Good |
Example |
9 |
0.2 |
3 |
0.0043 |
0.0016 |
62.8 |
Good |
Example |
10 |
0.2 |
5 |
0.0042 |
0.0016 |
61.9 |
Good |
Example |
11 |
0.2 |
15 |
0.0041 |
0.0015 |
63.4 |
Good |
Example |
12 |
0.2 |
25 |
0.0043 |
0.0014 |
67.4 |
Good |
Example |
13 |
0.2 |
28 |
0.0042 |
0.0016 |
61.9 |
Good |
Example |
14 |
0.2 |
32 |
0.0043 |
0.0017 |
60.5 |
Good |
Example |
15 |
0.2 |
40 |
0.0042 |
0.0019 |
54.8 |
Good |
Example |
[0043] Table 1 also illustrates the test levels and the results. In the comparative examples
(test numbers 1 to 3), in which the sum of volumes of pores having a pore diameter
ranging from 0.5 to 10 µm is inadequate, the desulfurization ratios were lower than
those in the invention examples (test numbers 4 to 15). In the invention examples
having test levels in each of which the average particle diameter of quicklime is
in a range from 1 to 30 mm, slag formation was promoted and the desulfurization ratio
of molten steel were higher.
[Second example]
[0044] Molten pig iron tapped from a blast furnace was subjected to the desiliconization
processing, the desulfurization processing, and the dephosphorization processing.
The processed molten pig iron was, then, charged into a converter to be subjected
to the decarburization refining. As a result, obtained was approximately 250 tons
of molten steel having a carbon concentration ranging from 0.05 to 0.09 mass%, a sulfur
concentration ranging from 0.0041 to 0.0043 mass%, and a phosphorous concentration
ranging from 0.004 to 0.010 mass%. After tapping, the converter slag flowed in a ladle
was not discharged. Metallic Al, quicklime, lightly calcined dolomite, and aluminum
dross were added into the ladle, and the ladle was transferred to the LF facility
illustrated in FIG. 1. The desulfurization processing was performed for approximately
30 minutes in such a manner that the molten steel was stirred by blowing Ar gas at
500 to 2000 NL/min from the submerged lances into the molten steel while the arc heating
was performed by the electrodes immersed in the slag so as to achieve a target sulfur
concentration equal to or smaller than 0.0024%.
[0045] Table 2 illustrates the sulfur concentrations (chemical analysis values) before and
after the desulfurization processing and the desulfurization ratios in respective
desulfurization tests. The desulfurization evaluation "Good" means that the sulfur
concentration in the molten steel after the desulfurization processing was equal to
or smaller than 0.0024%.
Table 2
Test No. |
Stirring power density |
Slagging ratio 5 min. after start of LF processing |
[S] Component change |
Desulfurization evaluation |
Remarks |
Before processing [S] |
After processing [S] |
Desulfurization ratio |
[W/t] |
[%] |
Mass [%] |
Mass [%] |
[%] |
- |
- |
16 |
10 |
35 |
0.0042 |
0.0019 |
54.8 |
Good |
Example |
17 |
15 |
50 |
0.0042 |
0.0018 |
57.1 |
Good |
Example |
18 |
27 |
57 |
0.0041 |
0.0018 |
56.1 |
Good |
Example |
19 |
35 |
68 |
0.0042 |
0.0018 |
57.1 |
Good |
Example |
20 |
45 |
85 |
0.0042 |
0.0017 |
59.5 |
Good |
Example |
21 |
55 |
94 |
0.0042 |
0.0017 |
59.5 |
Good |
Example |
22 |
74 |
100 |
0.0042 |
0.0017 |
59.5 |
Good |
Example |
23 |
100 |
100 |
0.0043 |
0.0015 |
65.1 |
Good |
Example |
24 |
135 |
100 |
0.0041 |
0.0014 |
65.9 |
Good |
Example |
25 |
158 |
100 |
0.0042 |
0.0013 |
69.0 |
Good |
Example |
26 |
167 |
100 |
0.0042 |
0.0012 |
71.4 |
Good |
Example |
27 |
185 |
100 |
0.0044 |
0.0011 |
75.0 |
Good |
Example |
[0046] Table 2 also illustrates the test levels and the results. It was found that with
an increase in stirring power, the slagging ratio after 5 minutes from start of the
LF processing and the desulfurization ratio were increased. It was found that high
slagging ratio and desulfurization ratio were obtained because the stirring power
density satisfies the following formula (6).
[Third example]
[0047] FIG. 2 is a diagram illustrating the slagging ratios of the invention examples and
the comparative examples. The invention examples used quicklime satisfying that the
quicklime has a particle diameter equal to or smaller than 20 mm and the sum of the
volumes of pores having a pore diameter ranging from 0.5 to 10 µm is 0.2 mL/g. The
comparative examples used quicklime satisfying that the quicklime has a particle diameter
equal to or smaller than 20 mm and the sum of the volumes of pores having a pore diameter
ranging from 0.5 to 10 µm is 0.03 mL/g. As illustrated in FIG. 2, it was found that
slagging was more promoted in the invention examples than that in the comparative
examples even at the identical stirring power density (135 W/t).
[Fourth example]
[0048] Molten pig iron tapped from a blast furnace was subjected to the desiliconization
processing, the desulfurization processing, and the dephosphorization processing.
The processed molten pig iron was, then, charged into a converter to be subjected
to the decarburization refining. As a result, obtained was approximately 250 tons
of molten steel having a carbon concentration ranging from 0.05 to 0.09 mass%, a sulfur
concentration ranging from 0.0041 to 0.0044 mass%, and a phosphorous concentration
ranging from 0.004 to 0.010 mass%. After tapping, the converter slag flowed in a ladle
was not discharged. Metallic Al, quicklime, lightly calcined dolomite, and aluminum
dross were added into the ladle, and the ladle was transferred to the LF facility
illustrated in FIG. 1. The LF processing used quicklime satisfying that the quicklime
has a particle diameter equal to or smaller than 20 mm and the sum of the volumes
of pores having a pore diameter ranging from 0.5 to 10 µm is 0.2 mL/g.
[0049] Table 3 illustrates the sulfur concentrations (chemical analysis values) before and
after the desulfurization processing and the desulfurization ratios in respective
desulfurization tests. [sol.Al]
1 is the upper limit value (mass%) of an Al concentration standard of a steel grade
to be manufactured by being melted and [sol.Al]
2 is the Al concentration (mass%) in the molten steel after tapping from the converter.
The desulfurization evaluation "Good" means that the sulfur concentration in the molten
steel after the desulfurization processing was equal to or smaller than 0.0024%.
Table 3
Test No. |
[sol.Al]1 |
[sol.Al]2 |
Left side of Formula (5) |
Right side of Formula (5) |
WAl |
[sol.Al]3 |
RH processing time |
[S] Component change |
Desulfurization evaluation |
Remarks |
Before processing [S] |
After processing [S] |
Desulfurization ratic |
% |
% |
kg/t |
kg/t |
kg/t |
% |
min |
Mass [%] |
Mass [%] |
[%] |
- |
28 |
0.050 |
0.023 |
0.77 |
1.27 |
1 |
0.039 |
30 |
0.0042 |
0.0011 |
73.8 |
Good |
Example |
29 |
0.050 |
0.025 |
0.75 |
1.25 |
1.1 |
0.043 |
29 |
0.0043 |
0.0011 |
74.4 |
Good |
Example |
30 |
0.050 |
0.026 |
0.74 |
1.24 |
1.2 |
0.047 |
31 |
0.0041 |
0.0010 |
75.6 |
Good |
Example |
31 |
0.050 |
0.024 |
0.76 |
1.26 |
1.3 |
0.051 |
36 |
0.0042 |
0.0009 |
78.6 |
Good |
Example |
32 |
0.050 |
0.025 |
0.75 |
1.25 |
1.4 |
0.055 |
39 |
0.0042 |
0.0008 |
81.0 |
Good |
Example |
33 |
0.050 |
0.024 |
0.76 |
1.26 |
1.5 |
0.059 |
42 |
0.0044 |
0.0008 |
81.8 |
Good |
Example |
[0050] As illustrated in Table 3, in the test levels in which the amount of Al supplied
within 10 minutes after start of the LF processing is in the range represented by
formula (5), the value of [sol.Al]
3 at the end of the LF processing was within the standard and the desulfurization ratio
was higher. In the test levels in which the amount of Al supplied within 10 minutes
after start of the LF processing was larger than the range represented by formula
(5), the value of [sol.Al]
3 at the end of the LF processing exceeded the upper limit value of the standard. Dealuminization
was, thus, necessary in the next process RH and the processing time in the RH was,
thus, extended.
[Fifth example]
[0051] Molten pig iron tapped from a blast furnace was subjected to the desiliconization
processing, the desulfurization processing, and the dephosphorization processing.
The processed molten pig iron was, then, charged into a converter to be subjected
to the decarburization refining. As a result, obtained was approximately 250 tons
of molten steel having a carbon concentration ranging from 0.05 to 0.09 mass%, a sulfur
concentration ranging from 0.0041 to 0.0044 mass%, and a phosphorous concentration
ranging from 0.004 to 0.010 mass%. After tapping, the converter slag flowed in a ladle
was not discharged. Metallic Al, quicklime, lightly calcined dolomite, and aluminum
dross were added into the ladle, and the ladle was transferred to the LF facility
illustrated in FIG. 1. The LF processing used quicklime satisfying that the quicklime
has a particle diameter equal to or smaller than 20 mm and the sum of the volumes
of pores having a pore diameter ranging from 0.5 to 10 µm is 0.2 mL/g. Metallic Al
was added so as to satisfy formula (5) within 10 minutes after start of the LF processing.
[0052] Table 4 illustrates the sulfur concentrations (chemical analysis values) before and
after the desulfurization processing and the desulfurization ratios in respective
desulfurization tests. The desulfurization evaluation "Good" means that the sulfur
concentration in molten steel after the desulfurization processing was equal to or
smaller than 0.0024%.
Table 4
Test No. |
Oxygen concentration |
Al loss in processing (Air entrainment) |
[S] Component change |
Desulfurization evaluation |
Remarks |
Before Processing [S] |
After Processing [S] |
Desulfurization ratio |
- |
% |
kg/t |
Mass [%] |
Mass [%] |
[%] |
- |
34 |
20 |
0.50 |
0.0042 |
0.0014 |
66.7 |
Good |
Example |
35 |
18 |
0.47 |
0.0043 |
0.0014 |
67.4 |
Good |
Example |
36 |
16 |
0.45 |
0.0041 |
0.0013 |
68.3 |
Good |
Example |
37 |
14 |
0.37 |
0.0042 |
0.0012 |
71.4 |
Good |
Example |
38 |
12 |
0.32 |
0.0042 |
0.0011 |
73.8 |
Good |
Example |
39 |
10 |
0.30 |
0.0044 |
0.0010 |
77.3 |
Good |
Example |
[0053] As illustrated in Table 4, in the test levels (test numbers 37 to 39) in which the
oxygen concentration in ladle is equal to or smaller than 15%, it was found that the
Al loss in processing was reduced. The Al loss in processing (air entrainment) was
obtained by using the following formula (7).
Industrial Applicability
[0054] The invention can provide the desulfurization processing method of molten steel and
the desulfurization agent that can efficiently perform the desulfurization processing
without using CaF
2 and pre-melt flux. Reference Signs List
[0055]
- 1
- LF facility
- 2
- ladle
- 3
- lid
- 4
- electrode
- 5, 6
- submerged lance
- 7, 8
- bottom-blowing porous brick
- 9
- molten steel
- 10
- slag
- 11
- row material supply chute
- 12
- Ar gas introduction pipe