[Document Type] SPECIFICATION
[Title of the Invention] DESILICONIZATION AND DEPHOSPHORIZATION
METHOD OF MOLTEN PIG IRON
[Technical Field]
[0001]
The present invention relates to a desiliconization and dephosphorization
method of a molten pig iron, and particularly relates to a desiliconization and
dephosphorization method suitable for the molten pig iron with a high Si concentration
(for example, a Si content: 0.4 mass% or greater).
The Priority is claimed on Japanese Patent Application No. 2011-27338, filed
on February 10,2011, and Japanese Patent Application No. 2011 -41220, filed on
February 28,2011, and the contents of which are incorporated herein by references.
[Background Art]
[0002]
As the usage environment of steel products becomes more severe, the
requirements for a reduction of impurity elements in the steel such as phosphorus and
sulfur become more severe.
In order to meet such requirements, a technique of removing silicon, sulfur,
and phosphorus in pig iron, tapped from a blast furnace, in advance in a process prior
to a decarburization refining; and performing converter blowing to obtain the steel, that
is, a so-called a molten pig iron pretreatment technique has been developed. This
molten pig iron pretreatment was generally performed using a torpedo car in the past.
However, recently, cases where the molten pig iron pretreatment is performed using a
converter have been increased.
[0003]
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•
In particular, recently, demand for a low-phosphorus steel has been increased.
In addition, a reduction in a slag generation amount has also been required due to
problems relating to a rationalization in the production cost of the steel and a slag
treatment.
[0004]
In general, in a dephosphorization reaction of the molten pig iron, in order to
oxidize phosphorus in the molten pig iron and remove phosphorus from the molten pig
iron into a slag, it is necessary that an oxygen source and oxygen ions be applied to
phosphorus in the molten pig iron; and phosphate ions be stably produced in the slag.
In this case, the oxygen ions are supplied to an interface between the molten pig iron
and the slag by a basic oxide such as CaO forming a molten slag.
[0005]
Since the slag contains CaO, it is preferable that slags, produced in processes
other than a dephosphorization process, is recycled by a dephosphorization reaction,
from the viewpoint of reducing the slag generation amount.
[0006]
However, slags produced in a decarburization process or a secondary refining
process or the like (hereinbelow, referred to as "a decarburization slag" and "a
secondary refining slag", respectively) contain high concentrations of SiC^ and AI2O3
other than CaO. Therefore, when these slags are recycled in the dephosphorization
process (a molten pig iron dephosphorization process), there is a problem in that the
slag amount is increased; and the risk of the slopping in which the slag overflows from
a furnace is increased. Among the components, AI2O3 is known as a chemical
component of significantly promoting a slag foaming (for example, refer to Non-Patent
Document 1). In addition, as a countermeasure to slopping, a method of increasing a
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furnace capacity is not preferable from the viewpoint of a facility cost.
[0007]
In addition, the molten pig iron contains impurities other than phosphorus.
In particular, recently, a use of an iron ore having a great Si content has been increased
from the viewpoint of reducing a production cost.
[0008]
From the viewpoint of optimizing the molten pig iron treatment, it is
preferable that silicon and phosphorus contained in molten pig iron be removed in the
same process. As a technique of efficiently preparing a low-phosphorus steel, the
present inventors disclose a technique of removing silicon contained in the molten pig
iron in the same process as a phosphorus removal process (Patent Document 1).
[0009]
When silicon and phosphorus contained in the molten pig iron are removed in
the same process, the slag amount is increased due to SiCh produced by a
desiliconization reaction. Therefore, there is a problem in that slopping is likely to
occur. In order to avoid this problem, the present inventors disclose a technique in
which an oxygen gas, blowing from a top blowing lance, is used as a carrier gas to
blow a burnt lime fine powder to the molten pig iron (Patent Document 2).
[0010]
However, when the molten pig iron having a greater Si content (for example,
0.4 mass% or greater) than that of the related art is treated, the slag amount is increased
due to SiC>2 produced by the desiliconization reaction. Therefore, when the abovedescribed
recycled slags (for example, the decarburization slag or the secondary
refining slag) are used as a source of supplying oxygen ions used for a
dephosphorization and desiliconization reaction, particularly, a problem pertaining to
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slag foaming becomes remarkable.
[0011]
The secondary refining slag described in this specification includes a ladle
slag produced during a secondary refining; and a tundish slag used for preventing the
air oxidation of a molten steel in a tundish during a continuous casting and for
retaining the temperature.
[Prior Art Document]
[Patent Document]
[0012]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. H2-47212
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2002-256320
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. HI 1 -172315
[Non-Patent Document]
[0013]
[Non-Patent Document 1] High Temperature Interfacial Transport
Phenomena in Pyrometallurgical Processes, p. 57, by Ryouji TSUJINO and Eiji AIDA
[Disclosure of the Invention]
[Problem to be Solved by the Invention]
[0014]
An object of the present invention is to solve the above-described problems
and to provide a desiliconization and dephosphorization method of the molten pig iron
in which a recycled slag generated in a molten steel stage is used as a source of
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supplying oxygen ions, which are used for the dephosphorization and desiliconization
reaction of the molten pig iron having a greater Si content (for example, 0.4 mass% or
greater) than that of the related art, to suppress the slag foaming while reducing a total
discharge amount of the slag in the molten steel stage; and as a result, the slopping can
be efficiently prevented without increasing a furnace capacity.
[Means for Solving the Problems]
[0015]
(1) According to an aspect of the present invention, there is provided a
desiliconization and dephosphorization method of the molten pig iron including: an
oxygen gas blowing process of blowing oxygen gas from a top of a furnace toward a
surface of the molten pig iron; a recycled slag addition process of supplying at least
one of a decarburization slag and a secondary refining slag toward the surface of the
molten pig iron as a recycled slag; and a burnt lime fine powder addition process of
supplying a burnt lime fine powder having a maximum particle size of 500 urn or less,
along with the oxygen gas blowing toward the surface of the molten pig iron, in which
the burnt lime fine powder addition process starts at a start time at which a silicon
concentration in the molten pig iron is reduced to 0 mass% or more and 0.15 mass% or
less.
[0016]
(2) In the desiliconization and dephosphorization method of the molten pig
iron according to (1), the silicon concentration in the molten pig iron at the start time
may be 0 mass% or more and 0.08 mass% or less.
[0017]
(3) In the desiliconization and dephosphorization method of the molten pig
iron according to (1) or (2), in the recycled slag addition process, when the silicon
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concentration in the molten pig iron at the time of the start of the recycled slag addition
process is lower than 0.6 mass%, only the secondary refining slag or both of the
decarburization slag and the secondary refining slag may be supplied to the molten pig
iron, and when the silicon concentration in the molten pig iron at the time of the start
of the recycled slag addition process is higher than or equal to 0.6 mass%, only the
decarburization slag may be supplied to the molten pig iron.
[0018]
(4) In the desiliconization and dephosphorization method of the molten pig
iron according to any one of (1) to (3), in the recycled slag addition process, when the
secondary refining slag is supplied toward the surface of the molten pig iron, an
amount of the secondary refining slag per 1 ton of the molten pig iron may be within a
range between 0.1 kg and 16 kg.
[0019]
(5) The desiliconization and dephosphorization method of the molten pig iron
according to any one of (1) to (4) may further include a solid oxygen source addition
process of supplying a solid oxygen source toward the surface of the molten pig iron.
[0020]
(6) The desiliconization and dephosphorization method of the molten pig iron
according to any one of (1) to (5) may further include a massive CaO source addition
process of supplying a massive CaO source, which is derived from an ore having an
average particle size of 5 mm or greater, toward the surface of the molten pig iron, in
which in the massive CaO source addition process, an amount of the massive CaO
source per 1 ton of the molten pig iron may be limited to be less than or equal to 7.5
kg-
[0021]
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i
•
(7) In the desiliconization and dephosphorization method of the molten pig
iron according to any one of (1) to (6), a total amount of MnO in the recycled slag may
be greater than 0 mass% and less than or equal to 25 mass%.
[0022]
(8) The desiliconization and dephosphorization method of the molten pig iron
according to any one of (1) to (7) may further include a stirring process of blowing a
gas to the molten pig iron and stirring the molten pig iron with a stirring energy density
s of within a range between 1.2 kW/t and 10 kW/t which is defined according to a
following Formula 1.
0.0062x0 xT [ ( H \ ( T \e= ^ x In 1 + ^-5- + 1--^- (5£l)
Wm \ { 1.54J t T
where
e: a bottom blowing stirring energy density (W/t) applied to the molten pig
iron
Qg: an amount of a bottom blowing gas (NL/min; including an evolved gas
from a solid material (for example, a limestone fine powder))
T: a molten pig iron temperature (K) at the time of the start of a bottom
blowing
Tg: a temperature (K) of the bottom blowing gas before entering into a metal
bath
Ho: a blowing depth (distance from a liquid surface of the molten pig iron to a
tip end of a bottom blowing tuyer; (m))
Wm: a weight of the molten pig iron (including a charged scrap; (t))
[0023]
(9) In the desiliconization and dephosphorization method of the molten pig
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iron according to (8), in the stirring process, a limestone fine powder may be supplied
to the molten pig iron along with the gas blowing to the molten pig iron.
[Effects of the Invention]
[0024]
In the desiliconization and dephosphorization method of the molten pig iron
according to (1), a burnt lime fine powder is blown to the surface of the molten pig
iron after the silicon concentration in the molten pig iron is reduced to be lower than or
equal to 0.15 mass%. As a result, even when the silicon concentration in the molten
pig iron is high, the desiliconization and dephosphorization of the molten pig iron can
be efficiently performed in the same process. Furthermore, the desiliconization and
dephosphorization method of the molten pig iron according to (1) includes two
processes of the recycled slag addition process of adding at least one of the
decarburization slag and the secondary refining slag to the molten pig iron; and the
burnt lime fine powder addition process of adding the burnt lime fine powder to the
molten pig iron. Therefore, the recycled slag generated in the molten steel stage is
used to efficiently prevent the slopping without increasing the furnace capacity while
reducing a total discharge amount of the slag in the molten steel stage.
[0025]
When the Si concentration in the molten pig iron is high; and only a topblowing
burnt lime fine powder is used, the cost necessary for supplying a CaO source
is significantly increased. However, when a combination of the recycled slag and the
burnt lime fine powder is used as the CaO source, the cost necessary for supplying the
CaO source can be reduced.
[0026]
In addition, in the desiliconization and dephosphorization method of the
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molten pig iron according to (7), Mn contained in the secondary refining slag and Mn
contained in the decarburization slag are transferred to the molten pig iron; and as a
result, a Mn concentration in the molten pig iron after the desiliconization and
dephosphorization can be improved. Therefore, when steel products containing Mn
according to the production specification are produced, the amount of the usage of an
expensive Mn ore or Mn alloy can be reduced in a post-process.
[Brief Description of the Drawing]
[0027]
FIG 1 is a schematic diagram (vertical cross-sectional view) illustrating a
dephosphorization furnace which is desirably used in an embodiment of the present
invention.
FIG 2 is a diagram illustrating a basic process example when a
desiliconization and dephosphorization method of the molten pig iron according to an
embodiment of the present invention is applied to a steelmaking.
FIG 3 is a diagram illustrating an example of a process chart according to an
embodiment of the present invention.
FIG 4 is a schematic diagram (vertical cross-sectional view) illustrating a
decarburization furnace from which a decarburization slag is discharged.
FIG 5 is a diagram illustrating a relationship between a discharge amount of
the slag and a silicon concentration in the molten pig iron [%Si].
FIG 6 is a diagram illustrating an example of a particle size distribution of a
burnt lime fine powder used in an embodiment of the present invention.
[Embodiments of the Invention]
[0028]
Hereinbelow, preferred embodiments of the present invention will be
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•
described. In particular, an embodiment of the present invention relates to a
desiliconization and dephosphorization method suitable for a molten pig iron with a
high Si concentration (for example, Si content: within a range between 0.45 mass%
and 0.65 mass%). Hereinbelow, % represents mass% unless specified otherwise.
First, massive (an average particle size: 5.0 mm or greater) auxiliary materials
are added from a hopper 8 (a top hopper) to a molten pig iron P in a dephosphorization
furnace 1 (a molten pig iron dephosphorization furnace) illustrated in FIG 1 (an
auxiliary material addition process). In the embodiment, as such auxiliary materials,
recycled slags (post-process slags) such as a decarburization slag and a secondary
refining slag are used, and a solid oxygen source such as iron ore and dolomite can
also be used. As illustrated in FIG. 2, the recycled slags described herein refer to
slags (for example, secondary refining slags such as a decarburization slag, a ladle
slag, and a tundish slag) which are used or produced in processes (for example, a
decarburization blowing, a secondary refining, and a continuous casting) subsequent to
a desiliconization and dephosphorization treatment (corresponding to the
dephosphorization furnace 1). Among such recycled slags, the decarburization slag
to be charged in the hopper 8 is obtained by discharging a slag from a decarburization
furnace; cooling and crushing this slag; removing a base metal from the crushed slag
by a magnetic separation; and crushing the slag to a size of preferably less than 50 mm.
In addition, the secondary refining slag to be charged in the hopper 8 is obtained by
discharging a slag, remaining in a ladle or a tundish after the end of the casting, from
the ladle or the tundish; cooling and crushing this slag; removing a base metal from the
crushed slag; and crushing the slag to a size of preferably less than 50 mm. These
slags are put into the hopper 8 above the furnace and are quantitatively weighed and
added to the dephosphorization furnace 1. However, among the recycled slags, the
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•
secondary refining slag contains a large amount of AI2O3 which causes a slag foaming.
Therefore, when the suppressing of the slag foaming is preferentially necessary, it is
preferable that the decarburization slag be preferentially used. In addition, in the
embodiment, since the addition (amount and timing) of the auxiliary materials is
controlled depending on the kind of the auxiliary materials, multiple hoppers can be
used. In this case, the auxiliary material addition step includes a recycled slag
addition process and, optionally, may further include a solid oxygen source addition
process or a massive CaO source addition process described below. In the
embodiment, a converter type furnace is used as the dephosphorization furnace 1.
[0029]
Next, a nitrogen gas supplied from a nitrogen gas tank 5 is used as a carrier
gas and is blown from the bottom through a bottom blowing tuyer 3 provided in the
bottom of the dephosphorization furnace 1 to blow the limestone fine powder in a
bottom blowing tank 6 (a fine powder bottom blowing tank) (stirring process).
[0030]
In order to secure a stirring energy density necessary for the
dephosphorization reaction, a flow rate of the nitrogen gas and the limestone fine
powder can be appropriately set. In particular, in the embodiment, when a stirring
energy density applied to the molten pig iron P is controlled to be within a range
between 1.2 kW/t and lOkW/t, both the desiliconization and the dephosphorization can
be more efficiently performed even with a molten pig iron having a relatively high
silicon concentration. When the stirring energy density is less than 1.2 kW/t, the
oxidation of silicon in the molten pig iron advances quicker than the oxidation of
phosphorus and thus, the dephosphorization reaction is not likely to occur. On the
other hand, when the stirring energy density is greater than or equal to 1.2 kW/t, the
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•
oxygen activity of a slag S in an interface between the slag S and the molten pig iron P
approaches an oxygen activity determined according to the amount of FeO in the slag.
Therefore, even when the silicon concentration in the molten pig iron P is high, the
dephosphorization reaction is easy to occur.
[0031]
The stirring energy density e is defined by the following (Formula 1).
[0032]
[Expression 2]
0.0062x0 xT i f H \ ( T \]
£= ^ xiln 1 + - ^ - + 1__JL (s£D
Wm \ \ 1.54J { T
[0033]
Herein in this (Formula 1), respective variables are as follows.
e: a bottom blowing stirring energy density (W/t) applied to the molten pig
ironP
Qg: an amount of a bottom blowing gas (NL/min; including an evolved gas
from a solid material (for example, a limestone fine powder))
T: a molten pig iron temperature (K) at the time of the start of a bottom
blowing
Tg: a temperature (K) of the bottom blowing gas before entering into a metal
bath
Ho: a blowing depth (distance from a liquid surface of the molten pig iron to a
tip end of a bottom blowing tuyer; (m))
Wm: a weight of the molten pig iron P (including a charged scrap; (t))
[0034]
Then, an oxygen gas (a high-pressure oxygen) supplied from an oxygen gas
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#
tank 4 is blown to a surface of the molten pig iron P through a top blowing lance 2
inserted into the dephosphorization furnace 1; and the dephosphorization starts (an
oxygen gas blowing process). Furthermore, depending on the state of the slopping,
the oxygen gas blowing from the top blowing lance 2 is used as a carrier gas to blow
the burnt lime fine powder (for example, as illustrated in FIG. 6, a burnt lime classified
into a size of 2 um to 200 um) in a top blowing tank 7 (a burnt lime fine powder top
blowing tank) to the surface of the molten pig iron P (a burnt lime fine powder addition
process).
That is, the high-pressure oxygen is supplied from the oxygen gas tank 4 to the top
blowing lance 2 through a pipe, but the top blowing tank 7 is disposed between the
oxygen gas tank 4 and the top blowing lance 2. Therefore, the burnt lime fine powder
can be blown to the surface of the molten pig iron along with the oxygen gas. As
illustrated in a Gantt chart on the upper section of FIG 3 as an example, plural
processes of the auxiliary material addition process (for example, the recycled slag
addition process), the stirring process, the oxygen gas blowing process, and the burnt
lime fine powder addition process are performed in an overlapping time zone.
In the embodiment, in this way, a burnt lime fine powder source is supplied
from the top blowing lance to an ignition point formed on the molten pig iron surface.
As a result, a molten calcium ferrite (CaOFeO) is formed at the ignition point; the
formed calcium ferrite rapidly reacts with SiCh in the slag to form fine dicalcium
silicate (2CaOSi02); and 2CaOSi02 having a high melting point is grown while
dissolving phosphoric acid and is macroscopically and uniformly dispersed in the slag.
Therefore, since 2CaOSi02 increases a practical solid phase rate of the slag and
suppresses foaming due to the bridge effect, an effect of suppressing the slopping
which causes a production impediment is exhibited.
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[0035]
The suppression of the slopping using this method is particularly effective in a
case where the dephosphorization of a high-Si molten pig iron is performed within a
short period of time, a case where the secondary refining slag containing AI2O3 which
promotes the slag foaming is recycled, or a case where a large amount of converter
slag (the decarburization slag) is recycled.
[0036]
Recently, the recycling of the construction wastes has been progressed and a
most of the construction wastes has a tendency to be used as a construction material.
Therefore, a demand for reducing the out-of-system discharge amount of a slag, which
is used as the construction material, has been increased. In order to reduce the
discharge amount of the slag to outside of steel production system, it is most effective
to recycle CaO in the slag, that is, to recycle the slags produced in post-processes
subsequent to the dephosphorization process. However, due to the reason described
below, it is not easy to reuse the slags, produced in the post-processes, in the
dephosphorization process of molten pig iron.
[0037]
That is, generally, in the dephosphorization reaction of the molten pig iron,
since phosphorus in the molten pig iron is oxidized and removed to a slag, it is
necessary that an oxygen source and oxygen ions are applied to phosphorus in the
molten pig iron; and phosphate ions are stably produced in the slag (in this case, the
oxygen ions are supplied to an interface between the molten pig iron and the slag by a
basic oxide such as CaO forming the molten slag). As the oxygen source, a solid
oxide source such as an iron oxide and an oxygen gas can be used. In (Formula 2) to
(Formula 4), underlined chemical components (P, C, Si) represent chemical
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components in the molten pig iron (in metal); and parenthesized chemical components
((O "), (PO4 "), (SiOa)) represent chemical components in the slag.
2P+50+3(02>2(P04
3") (Formula 2)
C+OCO (Formula 3)
Si+2O(Si02) (Formula 4)
[0038]
An initial phosphorus concentration in the molten pig iron is approximately
0.1%, but an initial carbon concentration in the molten pig iron is higher than or equal
to 4% (40 times the initial phosphorus concentration). Therefore, the decarburization
reaction represented in (Formula 3) unavoidably occurs, thereby generating CO gas.
Due to this CO gas, the slag foaming occurs. When this foaming becomes severe, socalled
the slopping phenomenon in which the slag overflows from the furnace is
caused to occur. When the slopping occurs, there are problems in that the oxygen
supply rate is reduced; an oxygen supply is stopped and the processing is unavoidably
stopped; the processing time is lengthened; and the productivity deteriorates.
[0039]
However, when the slags produced in the post-processes such as the
decarburization slag and the secondary refining slag are recycled as a CaO source in
the dephosphorization process, these slags contains high concentrations of Si02 and
AI2O3 other than CaO which are effective for dephosphorization, thereby increasing
the slag amount. Therefore, the above-described slopping problem becomes
significant. In addition, for example, as described in Non-Patent Document 1, AI2O3
significantly promotes the slag foaming. In particular, when it is necessary that the
operation be finished within a short period of time, it is difficult to reuse the molten pig
iron with a high silicon concentration in the dephosphorization process. The reason is
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that, when the silicon concentration in the molten pig iron is high, the amount of SiC>2
produced in the desiliconization reaction is great and thus the total amount of the slags
produced is increased.
[0040]
To deal with this, the present inventors have thoroughly investigated and have
solved the above-described problems by supplying the burnt lime fine powder to an
ignition point, formed on the molten pig iron, along with the top blowing oxygen at an
i appropriate timing. At the high-temperature ignition point, first, CaOFeO melt is
i
formed; the CaOFeO melt, formed at the ignition point, reacts with SiC>2 in the slag to
rapidly form 2CaOSiC>2 particles; and these 2CaOSiC>2 particles are grown and are
suspended in the slag. The present inventors have clarified that the suspended
2CaOSi02 particles have a high effect of suppressing the above-described slag
foaming. As a result, it was clarified that, even when the high-silicon molten pig iron
is dephosphorized, the recycled slags such as the decarburization slag and the
secondary refining slag can be used.
[0041]
A factor which has the highest effect on the slag amount generated in the
dephosphorization process is the silicon concentration in the molten pig iron. In the
dephosphorization process, since the desiliconization reaction represented in (Formula
4) occurs, the degree of the CaO amount to the SiC>2 amount produced in the
desiliconization reaction, that is, the basicity is reduced. In order to maintain the
dephosphorization ability of the slag, it is necessary that the basicity of the slag is
maintained at approximately 2. When the silicon concentration in the molten pig iron
is high, the CaO amount added to the slag is increased in proportion to the silicon
concentration in the molten pig iron. Furthermore, when the slags produced in the
I
- 16 - |
I i
post-processes subsequent to the dephosphorization process are used as a CaO source,
the slags contain chemical components other than CaO such as SiC»2, AI2O3, MnO, and
FeO. Therefore, the chemical components other than CaO are increased in the
dephosphorization furnace, the slag production amount is greater than that of a case of
using a primary CaO source such as the burnt lime. As a reference, the slag amount
Ws(t) generated in the dephosphorization process is substantially represented in the
following (Formula 5) when only the burnt lime is used as a CaO source.
Ws=[%Si]x60xl000/28/100x[l+(C/S)]xWm' (Formula 5)
In the formula, [%Si] represents the silicon concentration (mass%) of the
molten pig iron; (C/S) represents the target basicity of the slag after the
dephosphorization process (=(%CaO)/(%Si02); (-)); (%CaO) represents the CaO
concentration (mass%) of the slag after the dephosphorization process; (%Si02)
represents the Si02 concentration (mass%) of the slag after the dephosphorization
process; and Wm' represents the molten pig iron amount (other than a charge scrap;
(t))).
On the other hand, when the decarburization slag, the secondary refining slag,
and the like are used as the recycled slags, the slag amount Ws(t) generated in the
dephosphorization process is represented in the following (Formula 6).
Ws=[%Si]x60x 1000/28/1 OOx [1+100(C/S)x(^/100+^/(%CaO)LD+^3/(%CaO)
SR)]xWm' (Formula 6)
In the formula, t,\ represents the ratios (-) of the burnt lime fine powder in the
total CaO amounts, £2 represents the ratios (-) of the decarburization slag in the total
CaO amounts, and £3 represents the ratios (-) of the CaO amount in the secondary
refining slag, respectively; (%CaO)LD represents the CaO concentration (mass%) in the
decarburization slag; and (%CaO)sR represents the CaO concentration (mass%) in the
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secondary refining slag.
In addition, generally, [%X] represents the mass percentage of the chemical
component X in metal (in this case, the molten pig iron); and (%X) represents the mass
percentage of the chemical component X in the slag.
[0042]
In this case, the chemical compositions of the decarburization slag and the
secondary refining slag are, for example, as shown in Table 1. In the secondary
refining slag, the chemical composition (the amounts of chemical components such as
CaO and AI2O3) greatly varies. In addition, since an Al deoxidation is performed on
the molten steel during the secondary refining, the AI2O3 concentration is high and the
CaO concentration is relatively low in the secondary refining slag. Therefore, when
the only the secondary refining slag is reused as a CaO source for dephosphorization,
the slag amount is 2 times to 3 times that of the case of using only the burnt lime fine
powder as a CaO source; the above-described slopping becomes severe; and the real
operation is difficult to perform.
On the other hand, when the burnt lime fine powder is used as a part
(preferably within a range between 25% and 90% and more preferably within a range
between 25% and 80% (for example, approximately 30%)) of a CaO source necessary
for controlling the slag basicity; this burnt lime fine powder is blown to the ignition
point at a high-temperature of approximately 2000°C along with a top-blowing oxygen
gas. As a result, the liquid phase rate of the slag is reduced and thus, the slag foaming
can be efficiently suppressed.
In this case, first, the calcium ferrite is produced from the burnt lime fine
powder at the ignition point; and this calcium ferrite is transferred into the slag and is
changed into solid dicalcium silicate particles while rapidly dissolving Si02 and
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•
phosphoric acid in the slag. Furthermore, the dicalcium silicate particles are grown
while fixing a liquid slag between the dicalcium silicate particles, thereby forming a
granular slag. As a result, the effect of substantially reducing the liquid phase rate of
the slag can be obtained.
[0043]
TABLE 1
(HASS%)
I I CaO I SiQ2 I MnO 1 HgO I P2O5 1 AI2O3 I T-Fe I
DECARBSUGZATI0H 4 ( U 11"6 5' 5 5"6 2'1 31 17-8
R E F S I L A G 31- 2 t 1 2 8- 7 7- 9 °8 180 14.2
[0044]
By the way, in Patent Document 2, the present inventors discloses a method of
suppressing the foaming by reducing the liquid phase rate of a slag in which the
basicity of the slag is controlled to 1.2 or more and less than 2.5 by using a CaO source
having a particle size of less than 5 mm. However, in this method, since the burnt
lime, of which the basicity in the slag can be easily controlled, is used in an amount of
15.5 kg per 1 ton of the molten pig iron, there is a problem in that the cost is greatly
increased. In particular, in order to crush the burnt lime, a large amount of energy is
necessary. When the whole amount of 15.5 kg/t of the burnt lime has an average
particle size of less than 5 mm (in particular, the burnt lime fine powder), the cost is
significantly increased. Conversely, when a massive burnt lime having an average
particle size of 5 mm or greater is used in the dephosphorization process, there is a
case where the melting rate of the massive burnt lime in the slag may deteriorate; and
- 19 -
•
the dephosphorization efficiency may deteriorate.
When the slags produced in the post-processes are used in the
dephosphorization process, the slag volume is significantly increased; and AI2O3
contained in the recycled slags promotes the foaming. Therefore, when the recycled
slags are used in the method disclosed in Patent Document 2, the cost is significantly
increased due to an increase in the amount of the burnt lime fine powder used; and a
considerable amount of liquid phase remains in the slag, which promotes the foaming.
When the recycled slags are used, in order to increase the dephosphorization rate,
generally, a large amount of the massive burnt lime is simultaneously added to increase
the basicity.
On the other hand, in the embodiment, the burnt lime fine powder is supplied
to the high-temperature ignition point at an appropriate timing. As a result, while
suppressing the amount of burnt lime fine powder used to an appropriate amount, the
dephosphorization promoting effect and the foaming suppressing effect, caused by the
burnt lime fine powder, can be sufficiently secured. That is, as described above, a
calcium ferrite liquid phase is instantaneously formed, and then 2CaOSi02 is formed.
Since 2CaOSiC>2 takes in a slag liquid phase therebetween while dissolving
phosphoric acid; and are grown into the massive fine powder having a particle size of
within 10mm and several tens mm as a whole, the foaming is suppressed, that is, the
desorption property of gas bubbles in the slag is significantly improved.
Furthermore, the recycled slags having a low melting point are supplied
toward the surface of the molten pig iron (optionally, molten pig iron containing scrap)
at the initial stage (start) of the dephosphorization process. Therefore, as compared a
case where only the massive burnt lime is supplied, an initial-stage melt, which is
effective for simultaneously performing the desiliconization and dephosphorization,
- 20 -
can be rapidly formed. In addition, since the recycled slags can rapidly supply CaO
in the slag on the molten pig iron, the cost can be reduced without impairing the
efficiency of the simultaneous desiliconization and dephosphorization process.
Furthermore, owing to the supply of the recycled slags and the supply of the burnt lime
fine powder at an appropriate timing, CaO can be efficiently used for
dephosphorization. Therefore, the slag discharge amount in the entire steelmaking
processes can be reduced; and a dephosphorization slag having almost no free lime,
which can be directly used as a construction material, can be obtained.
[0045]
The present inventors determined that, if the desiliconization and
dephosphorization were simultaneously performed as in the case of the embodiment,
the silicon concentration in the molten pig iron would be reduced to almost zero;
gradually, the decarburization reaction would be preferentially performed; and the
generation rate of the CO gas, generated in the reaction of (Formula 3) would be
increased, which would cause a problem of the slag foaming. Therefore, the present
inventors found that it is most preferable time that the burnt lime fine powder is added
after the silicon concentration is sufficiently reduced. Specifically, as illustrated in
FIG 3, the process of supplying the burnt lime fine powder as a CaO source along with
the oxygen gas blowing to the surface of the molten pig iron starts after the time (start
time) at which the silicon concentration in the molten pig iron is reduced to 0 mass%
or more and 0.15 mass% or less. In order to suppress the amount of the burnt lime
fine powder used to the minimum, it is preferable that this start time is the time at
which the silicon concentration in the molten pig iron is reduced to 0 mass% or more
and 0.08 mass% or less. It is preferable that the silicon concentration in the molten
pig iron at the start time be higher than or equal to 0 mass%. However, in
- 21 -
consideration of the generation rate of CO gas by the decarburization reaction, the
silicon concentration in the molten pig iron at the start time may be higher than or
equal to 0.001 mass% or higher than or equal to 0.01 mass%. In this way, the
desiliconization reaction rate is reduced according to (Formula 4); and the
dephosphorization rate is improved according to (Formula 2) at a timing at which the
decarburization reaction rate is increased according to (Formula 3). As a result, a
time zone in which the foaming causes a problem is significantly reduced; and the
desiliconization and dephosphorization can be efficiently performed while suppressing
the foaming.
As described above, by controlling the timing at which the burnt lime fine
powder is supplied, the amount of an expensive burnt lime fine powder used can be
reduced and the large amounts of the decarburization slag and the secondary refining
slag can be used.
The silicon concentration in the molten pig iron described herein can be
obtained from an actual operating data or a simulation under various operating
conditions such as the silicon concentration in the initial-stage molten pig iron.
[0046]
In the embodiment, as a CaO source, a primary CaO sources such as the burnt
lime (including CaO as a major component), a slaked lime (including Ca(OH)2 as a
major component), and the limestone (including CaC03 as a major component); and
the recycled slags such as the decarburization slag and the secondary refining slag are
used. In addition, the burnt lime fine powder which is supplied along with the
oxygen gas is fine powders having a maximum particle size of 500 urn or less and a
raw material derived from an ore (that is, a raw material which is obtained after a
thermal decomposition and has the chemical composition composed of CaO and
- 22 -
impurities such as a gangue). Examples of the particle size distribution measurement
results of the burnt lime fine powders used in Examples and Comparative Examples
described below (a volume frequency distribution and a cumulative distribution thereof
measured by a laser diffraction particle size distribution analyzer) are illustrated in FIG.
6. In this example, the particle size is in a range of 2 umto200 um. As the particle
size becomes greater, the contact efficiency between chemical components relating to
the dephosphorization deteriorates. Therefore, it is necessary that the maximum
particles size of the burnt lime fine powder be less than or equal to 500 urn. In
addition, when the maximum particle size of the burnt lime fine powder is excessively
great (for example, greater than 500 um), a pipe portion is extremely worn, which is
not preferable. On the other hand, the lower limit of the maximum particle size of the
burnt lime fine powder is not particularly limited. By setting the lower limit not to be
excessively low, the crushing cost and the scattering loss, which may be caused by the
burnt lime fine powder desorbing from the blowing gas, can be reduced. Therefore,
the maximum particle size of the burnt lime fine powder may be greater than or equal
to 1 um. In order to prevent the generation of unnecessary gases and the energy loss
in the slag, the burnt lime fine powder is used (including CaO as a major component
(for example, 90 mass% or greater)) as a CaO source which is supplied along with the
oxygen gas.
In addition, the average particle size of the limestone fine powder used for the
bottom blowing is preferably 32 um to 75 um. This limestone fine powder is mainly
used for stirring although also functioning as a CaO source.
Furthermore, in order to finely adjust the basicity, a massive CaO source (for
example, the primary CaO sources) derived from an ore may be provided from the
hopper 8. However, unless the fine adjustment of the basicity is necessary, it is
- 23 -
II% s
I
preferable that the massive CaO source be not used as much as possible in order to
increase the recycling rate of the recycled slags and to increase the usage efficiency of
the CaO sources. Specifically, the amount of the massive CaO source having an
average particle size of 5 mm or greater per 1 ton of the molten pig iron is preferably
limited to be less than or equal to 7.5 kg, more preferably limited to be less than or
equal to 7.0 kg or less than or equal to 5.0 kg, still more preferably limited to be less
than or equal to 1 kg, and most preferably limited to be 0 kg. In addition, when the
massive CaO source is supplied into the dephosphorization furnace, the massive CaO
source addition process may start at the same time as that of the recycled slag addition
process.
In addition, in consideration of controlling the basicity of the slag and
suppressing the foaming, it is preferable that the amount of the burnt lime fine powder
(in the case of not adding the massive burnt lime) or the total amount of the burnt lime
fine powder and the massive CaO source be limited to be less than or equal to 3.2 kg
per 1 kg of Si in the molten pig iron.
Furthermore, when the silicon concentration in the molten pig iron at the time
of the start of the recycled slag addition process is lower than 0.6 mass%, it is
preferable that only the secondary refining slag or both of the decarburization slag and
the secondary refining slag be used; and it is more preferable that both of the
decarburization slag and the secondary refining slag be supplied to the molten pig iron.
On the other hand, when the silicon concentration in the molten pig iron at the
time of the start of the recycled slag addition process is higher than or equal to 0.6
mass%, it is preferable that only the decarburization slag be supplied to the molten pig
iron.
By using the recycled slags as described above, the recycled slags can be used
- 24 -
with a high efficiency and a balance while suppressing the slag foaming to the
minimum.
The initial-stage silicon concentration (before blowing the oxygen gas) in the
molten pig iron of the dephosphorization furnace which is applied to the embodiment,
is not particularly limited. For example, the initial-stage silicon concentration may be
higher than or equal to 0.3 mass%, higher than or equal to 0.4 mass%, or higher than or
equal to 0.5 mass%.
[0047]
In the embodiment, as described above, the recycled slags are reused.
As a result, the slag discharge amount in the entire steelmaking processes can
be reduced; and the desiliconization and the dephosphorization can be simultaneously
performed while suppressing the slag foaming. Furthermore, in the embodiment, as a
modification example, when a steel product containing Mn is produced as described
below, the cost required for controlling the Mn amount can be suppressed by using a
slag containing MnO as a recycled slag. In modification examples described below (a
first modification example and a second modification example), since a partial
configuration is the same as that of the embodiment, the description of such a
configuration will not be repeated.
As a steel product containing Mn, a demand for a steel product such as a high
tensile strength steel sheet has been increased. In such a steel product, the Mn
amount contained in the steel product is regulated according to the product
specification. When adjusting the Mn content in the steel, only Mn contained in the
molten pig iron as a major component is used, and the Mn amount is insufficient.
Therefore, a final adjustment of adding a Mn alloy is performed during the period from
a primary adjustment of adding a Mn ore in the steel during the decarburization
- 25 -
blowing (the decarburization furnace) or from the finish of the decarburization blowing
(when the molten steel is tapped from the decarburization furnace to a steel ladle) to
the finish of the secondary refining.
In addition, as disclosed in Patent Document 3, a Mn ore is also added to the
molten pig iron in the molten pig iron pretreatment. In this case, when steel products
containing Mn according to the production specification are produced, a large amount
ofaMn ore or Mn alloy is required and the cost is increased. On the other hand, in
the desiliconization and dephosphorization process, when a slag containing MnO is
used instead of a Mn ore, the cost is effectively reduced.
[0048]
FIG 2 is a diagram illustrating a basic process example when the
desiliconization and dephosphorization method of the molten pig iron according to the
embodiment is applied to the steelmaking. As illustrated in FIG 2, the
desiliconization and dephosphorization process of the molten pig iron (corresponding
to the embodiment) is performed in the dephosphorization furnace; the decarburization
blowing is performed in the decarburization furnace; the secondary refining is
performed; the molten steel is prepared from the molten pig iron; and the molten steel
is continuously casted to produce the steel. When steel products containing Mn are
produced, the MnO amount is likely to be greatest in a slag (that is, the secondary
refining slag) discharged in the secondary refining or the continuous casting among the
slags discharged in the above-described processes. Therefore, in the desiliconization
and dephosphorization process of the molten pig iron, it is preferable that the
secondary refining slag containing Mn be returned and added to the molten pig iron of
the dephosphorization furnace (the first modification example). In addition,
particularly when a Mn ore is added to the decarburization furnace, it is preferable that
- 26 -
the decarburization slag containing Mn as well as the secondary refining slag be
returned and added to the molten pig iron of the dephosphorization furnace (the second
modification example).
[0049]
In this way, in the first modification example of the embodiment, the
secondary refining slag (at least one of a ladle slag and a tundish slag) is returned and
put into the dephosphorization furnace 1. When the steel products containing Mn are
produced, the secondary refining slag contains Mn. However, as described above, the
secondary refining slag contains a large amount of AI2O3. Therefore, when the
secondary refining slag is put into the dephosphorization furnace, the slag foaming is
significantly promoted and thus the slopping phenomenon in which the slag and the
molten pig iron overflows from the furnace is caused to occur. In this way, due to the
operational problems, the technique of putting the secondary refining slag into the
dephosphorization slag has not yet been put into a practice.
[0050]
As described above, in the first modification example of the embodiment, the
secondary refining slag is added to the molten pig iron; and the burnt lime fine powder
is blown to the surface of the molten pig iron along with the top-blowing oxygen gas.
As a result, 2CaOSi02 is rapidly formed at the hot spot (an ignition point) on the
surface of the molten pig iron; and the liquid phase rate of the slag is reduced.
Therefore, since the fluidity of the slag deteriorates and the slopping phenomenon is
difficult to occur, the secondary refining slag can be added in the desiliconization and
dephosphorization process. Meanwhile, the MnO concentration in the slag liquid
phase is increased due to a reduction in the liquid phase rate of the slag. Accordingly,
while reducing a distribution ratio of MnO in the slag to Mn in the molten pig iron
- 27 -
(that is, (%MnO)/[%Mn])5 the Mn concentration in the molten pig iron can be
improved by transferring Mn in the secondary refining slag to the molten pig iron; or
the transfer of Mn in the molten pig iron to the secondary refining slag by the oxidation
can be suppressed. Since the CaO amount in the secondary refining slag greatly
varies, there is a case where the effect as a substitute of a dephosphorization agent may
be lower than that of the burnt lime.
[0051]
The amount of the secondary refining slag added to the molten pig iron is
preferably within a range between 0.1 kg/t and 16 kg/t. When the amount of the
secondary refining slag added is excessively small, the effect of improving the Mn
concentration in the molten pig iron cannot be sufficiently obtained as compared to the
related art. Conversely, when the amount of the secondary refining slag added is
excessive, the slag amount in the desiliconization and dephosphorization process is
increased and thus the slopping phenomenon is likely to occur. In this case, in order
to suppress the slopping phenomenon, a large amount of the burnt lime fine powder is
necessary.
[0052]
In the above-described first modification example, only the secondary refining
slag is added to the dephosphorization furnace. However, in the secondary
modification example, in addition to the secondary refining slag, the decarburization
slag, produced in the decarburization blowing process of the molten pig iron after the
desiliconization and dephosphorization process described below and illustrated in FIG.
4, is also added to the dephosphorization furnace. Since this decarburization slag also
contains MnO, like the first modification example, the Mn concentration in the molten
pig iron can be improved by transferring Mn in the slag to the molten pig iron; or the
- 28 -
•
transfer of Mn in the molten pig iron to the secondary refining slag by the oxidation
can be suppressed. Since the other configurations of the second modification
example are the same as that of the first modification example except that the
decarburization slag is used as a recycled slag, the description thereof will not be
repeated.
[0053]
Hereinbelow, the reason that the putting of the secondary refining slag into the
dephosphorization furnace is effective in the above-described modification examples
(the first modification example and the second modification example) will be
described based on the flow in the flowchart illustrated in FIG 2. FIG 1 is a diagram
illustrating the desiliconization and dephosphorization process in the
dephosphorization furnace, and FIG 4 is a diagram illustrating the decarburization
blowing in the decarburization furnace.
[0054]
A massive auxiliary material containing at least the secondary refining slag is
added from the hopper 8, which is provided above the dephosphorization furnace 1, to
the molten pig iron P. In addition, optionally, a bottom-blowing flux (for example,
the above-described limestone fine powder) is blown from the bottom blowing tuyer 3,
which is provided in the bottom of the dephosphorization furnace 1, to the molten pig
iron P along with the gases such as a fuel gas, an oxygen gas, and an inert gas; and the
molten pig iron P is stirred. Then, the oxygen gas is blown from the top blowing
lance 2 to the surface of the molten pig iron P; and a CaO source functions as the
desiliconization and dephosphorization agent. As a result, the desiliconization and
dephosphorization are simultaneously performed. In the dephosphorization furnace
1, Si and P in the molten pig iron are transferred to the slag and thus, the Si
- 29 -
$
concentration and the P concentration in the molten pig iron are reduced.
[0055]
Furthermore, the molten pig iron in which the Mn concentration is improved
as compared to the related art is transported to a decarburization furnace 10 illustrated
in FIG 4, followed by the decarburization blowing. A hopper 15 is provided above
the decarburization furnace 10. A massive auxiliary material (for example, the burnt
lime or a Mn ore) is added from the hopper 15 to a molten pig iron PLD- In the abovedescribed
modification examples, as this auxiliary material, a necessary amount of Mn
ore is used in order to increase the Mn concentration in the molten pig iron PLD to be in
the production specification range. However, since the Mn concentration in the
initial-stage molten pig iron PLD of the decarburization furnace 10 is increased as
compared to the related art, the necessary amount of Mn ore is significantly reduced as
compared to the related art. Accordingly, the amount of an expensive Mn ore or Mn
alloy, which is used during the decarburization blowing or the secondary refining, is
reduced as compared to the related art and thus, the cost can be reduced. After the
auxiliary material is added to the molten pig iron PLD, in the decarburization furnace
10, a gas in a bottom blowing gas tank 13 is blown from a bottom blowing tuyer 14,
which is provided in the bottom of the decarburization furnace 10, to the molten pig
iron PLD- Next, an oxygen gas in an oxygen gas tank 12 is blown from a top blowing
lance 11 to the molten pig iron PLD, thereby performing the decarburization blowing.
The molten steel produced by the decarburization blowing is transported to
the secondary refining process to adjust chemical components in the molten steel
during the secondary refining. The molten steel after the secondary refining is
transported to a continuous casting machine, and a steel such as a slab is produced
from the molten steel. In FIG 2, the recycled slags are added to the
- 30 -
dephosphorization furnace from the same line. However, the recycled slags can also
be added to the dephosphorization furnace from a different line from the
dephosphorization furnace which is applied to the above-described embodiment and
the modification examples thereof.
[0056]
Unlike the embodiment, it can be also considered that the secondary refining
slag is added to the decarburization furnace 10 instead of the dephosphorization
furnace 1. However, in the decarburization furnace, the C concentration in the molten
pig iron is lower than that of the dephosphorization furnace; and the concentration of
an oxidizing atmosphere is high. Therefore, the reduction reaction from MnO to Mn
is not likely to occur and a Mn yield is low. In addition, in order to recycle Mn in the
recycled slags, it is necessary that the recycled slags contain Mn (that is, MnO). That
is, during the recycling of Mn, the MnO concentration in the recycled slags is
necessarily higher than 0%, preferably higher than or equal to 6.0%, and more
preferably higher than or equal to 10%. The upper limit of the MnO concentration in
the recycled slags is not particularly limited, but, for example, may be 25% in
consideration of the CaO amount in the slags.
[0057]
The above-described embodiment and the modification examples thereof
include the oxygen gas blowing process of blowing oxygen gas from a top of a furnace
toward a surface of the molten pig iron; the recycled slag addition process of supplying
at least one of a decarburization slag and a secondary refining slag toward the surface
of the molten pig iron as a recycled slag; and the burnt lime fine powder addition
process of supplying the burnt lime fine powder having a maximum particle size of
500 urn or less along with the oxygen gas blowing to the surface of the molten pig
- 31 -
•
iron. Among these processes, the burnt lime fine powder addition process starts at a
start time at which a silicon concentration in the molten pig iron is reduced to 0 mass%
or more and 0.15 mass% or less. Therefore, in the above-described embodiment and
the modification examples thereof, the desiliconization and the dephosphorization can
be simultaneously performed with high efficiency while reducing a total discharge
amount of the slag in the entire steelmaking processes.
In addition, in the above-described modification examples, the recycled slags
containing Mn are used. Therefore, even when the steel products containing Mn
according to the production specification are produced, the amount of a Mn ore or Mn
alloy used can be reduced as compared to the related art and thus, an increase in cost
can be suppressed.
[0058]
In the related art, when the silicon concentration in the molten pig iron is
higher than 0.3%, the total amount of slags, generated during the dephosphorization
and desiliconization process, the decarburization blowing process, and the secondary
refining process as illustrated in FIG 5, is approximately greater than 100 kg/t per 1
ton of the processed molten pig iron. However, by adopting the embodiment and the
modification examples, even when the silicon concentration in the molten pig iron is
0.5%, the total amount of slags can be suppressed to be lower than 100 kg/t. The slag
discharge amount illustrated in FIG 5 is the total amount of slags, which are not
recycled in the desiliconization and dephosphorization process, among a slag (a
dephosphorization slag) generated in the desiliconization and dephosphorization
process, the decarburization slag, and the secondary refining slag. In addition, the
each numerical values of legends in FIG 5 represent the above-described £1, £,2, and
^3 (Formula 6) in order from left to right, respectively.
- 32 -
[0059]
Furthermore, the dephosphorization slag generated in the embodiment and the
modification examples thereof has almost no free lime. Therefore, even when a
steam aging process or an air aging process over a long period of several months is not
performed as in the related art, the water immersion expansion rate, which is measured
with a measurement method defined in JIS A5015, is extremely low at 0.5% or lower.
In addition, in a case where a dephosphorization slag is finely crushed and immersed in
pure water when the ratio of the water to the slagis 5:1, in the dephosphorization slag
of the related art having a high free lime concentration, the pH is approximately 12.5;
however, in the dephosphorization slag generated in the embodiment and the
modification examples thereof, the pH is reduced to approximately 11. Accordingly,
the dephosphorization slag generated in the embodiment and the modification
examples thereof can be safely used as a construction material.
[Examples]
[0060]
In all the Examples 1 to 14 and the Comparative Examples 1 to 3 described
below, the following processes were performed. A converter type furnace was
charged with the predetermined amounts of the molten pig iron and the scrap. Next, a
nitrogen gas and the calcium carbonate fine powder (the limestone fine powder) were
blown from the bottom of the furnace; and the oxygen gas was blown from a main
lance while adding an auxiliary material ("Initial-Stage Top-Addition Flux" in Table 3)
from a hopper provided above the furnace, thereby performing a dephosphorization
process. The deterioration in the basicity of the slag, caused by the desiliconization,
is cancelled out with the addition of a CaO source to adjust the basicity of the slag in
the dephosphorization process. The decarburization slag and the secondary refining
- 33 -
slag having the chemical compositions (slag compositions) shown in Table 1 were
used.
In the Examples 1 to 14, as CaO sources to be added from the above, the
recycled slags such as the secondary refining slag and the decarburization slag were
supplied into the furnace; and the burnt lime fine powder was supplied to an ignition
point along with the top-blowing oxygen gas.
The calcium carbonate, which was bio wen from the bottom of the furnace into
the molten pig iron, generated the CO gas with the reaction according to (Formula 7)
and contributed to the stirring (C represents C in the molten pig iron).
CaC03+C=CaO+2CO (Formula 7)
[0061]
In the Examples 1 to 9 shown in Tables 2 to 4, at least one of the secondary
refining slag and the decarburization slag was used as the recycled slag; and a C
amount [%C], an Si amount [%Si], and a P amount [%P] in the molten pig iron before
and after the dephosphorization process were measured. In these examples, the
silicon concentration could be reduced to substantially 0% and the phosphorus
concentration could be sufficiently reduced to approximately 0.1% to 0.035%.
In the Example 4 (the Si concentration in the molten pig iron was
approximately 0.15%), by appropriately controlling the supply start time of the burnt
lime fine powder, a large amount of phosphorus could be removed from the molten pig
iron to the slag as compared to the Comparative Example 2 in which the amounts of
the recycled slags and the burnt lime fine powder used were at the same level.
Likewise, in the Examples 1 to 3 (the Si concentration in the molten pig iron was
approximately 0.08%), by more appropriately controlling the supply start time of the
burnt lime fine powder, a large amount of phosphorus could be removed from the
- 34 -
!
•
molten pig iron to the slag as compared to the Examples 4 to 6 in which the amounts of
the recycled slags and the burnt lime fine powder used were at the same level.
Furthermore, when the silicon concentration in the molten pig iron (an initialstage
molten pig iron) at the time of the start of adding the recycled slags was higher
than or equal to 0.6%, in the Example 3, only the decarburization slag was used; and
thus, a large amount of phosphorus could be removed from the molten pig iron to the
slag as compared to the Example 7 in which both the secondary refining slag and the
decarburization slag were used.
Furthermore, in the Examples 1 to 3, by reducing the amount of the massive
burnt lime, a large amount of the recycled slags could be efficiently used at the same
level of the target basicity as compared to the Example 8.
In addition, in the Examples 1 to 3, since a more sufficient stirring energy
density was applied to the molten pig iron, the Examples 1 to 3 could be desirably
applied to the molten pig iron containing a large amount of Si. In addition, a large
amount of phosphorus could also be removed from the molten pig iron to the slag as
compared to the Example 9.
[0062]
On the other hand, in the Comparative Example 1, as in the case of the related
art, the massive burnt lime having an average particle size of approximately 20 mm
were used in combination with the decarburization slag and the secondary refining slag
without using the burnt lime fine powder, but the slopping was severe. Therefore, in
order to promote the settlement of the slag, it was necessary that the dephosphorization
process be stopped for 5 minutes and thus, the productivity significantly deteriorated.
In addition, the slag overflew from the furnace and an iron loss was increased.
Furthermore, when the dephosphorization process was stopped, that is, when the
- 35 -
supply of the oxygen gas was stopped, FeO in the slag was reduced by the carbon in
the molten pig iron. Therefore, the oxidizing power in the furnace deteriorated and
the dephosphorization ability (the dephosphorization amount) deteriorated. When the
oxygen amount is increased in order to compensate for the deterioration in the
dephosphorization ability, the decarburization reaction advances more than necessary
and thus, the melting point of the molten pig iron is increased. As a result, the
amount of a base metal attached to the furnace may be increased; or a base metal may
be attached to a ladle after being tapped from the furnace to the ladle.
In addition, in the Comparative Example 2, since the supply of a relatively
expensive burnt lime fine powder started at a timing at which the dephosphorization
and the desiliconization competed against each other, the burnt lime fine powder could
not be sufficiently used for a high-efficiency dephosphorization.
[0063]
- 36 -
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- 39 -
[0066]
As shown in Tables 5 to 7, in the Examples 10 to 14 and the Comparative
Example 3, in addition to the above-described [%C], [%Si], and [%P], the Mn amount
[%Mn] in the molten pig iron before and after the dephosphorization process was
measured. From the Mn amount [%Mn] after the dephosphorization process, the
necessary amount of a Mn ore put into the decarburization furnace (in terms of Mn in
the molten pig iron of the decarburization furnace, "Mn Ore in Decarburization
Furnace" in Table 7) was calculated. Generally, the amount of an Mn ore put into the
decarburization furnace was determined from an empirical value of a reduction rate of
an Mn ore (that is, an increase amount of [%Mn] due to an Mn ore), a final target
[%Mn], and [%Mn] before the decarburization process (that is, after the
dephosphorization process). In the Examples 10 to 14 and the Comparative Example
3, at least one of the secondary refining slag and the decarburization slag was used as
the recycled slag.
In the Examples 10 to 14, the silicon concentration could be reduced to
substantially 0% and the phosphorus concentration could be sufficiently reduced to
approximately 0.1% to 0.015%.
In the Examples 11 to 14 in which the secondary refining slag (the ladle slag)
having a sufficiently high MnO concentration was added to the dephosphorization
furnace, the Mn concentration after the dephosphorization process was higher and the
necessary amount of an Mn ore put into the decarburization furnace was reduced as
compared to the Example 10 in which the secondary refining slag was not used. In
the Example 13, in addition to the secondary refining slag, the decarburization slag
was also put thereinto.
In the Comparative Example 3, the secondary refining slag (the ladle slag)
- 40 -
was put into the dephosphorization slag, but the burnt lime fine powder was not
supplied thereto. Therefore, in the Comparative Example 3, the high-efficiency
dephosphorization could not be used at the ignition point; and the P amount removed
from the molten pig iron to the slag was reduced as compared to the Examples 10 to
14. In addition, in this case, in order to prevent the slopping, it was necessary that the
amount of the secondary refining slag put be limited as a slight amount; and the
efficiency of reusing Mn in the secondary refining slag could not be sufficiently
obtained.
[0067]
The target basicity in Tables 3 and 6 was calculated according to (CaO)/(Si02)
from the CaO amount and the SiC>2 amount in the all the auxiliary materials (for
example, CaCC>3, the burnt lime fine powder, the massive burnt lime, the
decarburization slag, the secondary slag, and iron ore)added to the dephosphorization
furnace, and the Si amount in the molten pig iron. That is, the target basicity is the
value obtained by dividing the total CaO amount in the auxiliary materials (that is, the
amount in terms of CaO) by the sum (that is, the amount in terms of Si02) of the total
Si02 amount in the auxiliary materials and the total Si02 amount generated by the
oxidation of Si in the molten pig iron. In addition, the process time in Tables 4 and 7
represents the supply time for which the supply of the top-blowing oxygen gas to the
dephosphorization furnace was continued; and does not include the downtime for
which the supply of the top-blowing oxygen gas was stopped.
[0068]
- 41 -
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- 42 -
[0069]
2E
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ec
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iW?3_ o w esi ^- <- r^
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u - Q - ^ C M T - e M C M ( M C M ! = >
= £= >-
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CQ I . I
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_, . a
_ _ — 3 r e > o o 0 i n i o g
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- 43 - !
[0070]
[Table 7]
I!
UJ
I—
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s I
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LU-—«t «^ T- CO CO T— <£> Oi
g § 5 » w ** ^ ui eo" e\i
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_ _ _ _ _ | _ . |
o c a s - - - - - - - T— T - —- —- T - - - —»—»--
Be"—"fe
t—
,—, n M C J i o e o o n i n » i - o } r -
n ^-,o>—-o>—-eft-—-ococo—-a>—-
g a f t o o o o o o o o o c > oo
§ —' o o o o o o o o o o o 'o
t> __
HH 3 £, o o o d ° ' d ° ' c> ° ' o' ° ' o
PQ §
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gi S« W
u j S ' E o l IJF c*j c«i co •*-
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3 3 3 ^s 3 3 S S P< SfB >e S
UJ LU LU u UJ "-1
- 44 -
[0071]
As described above, by recycling Mn contained in the secondary refining slag
and the decarburization slag, the Mn concentration in the molten pig iron after the
desiliconization and dephosphorization process can be increased as compared to the
related art. Therefore, even when steel products containing Mn according to the
product specification are produced, the amount of an expensive Mn ore or Mn alloy
used can be reduced.
[0072]
Hereinabove, the preferred examples of the present invention have been
described. However, the present invention is not limited these examples. Additions,
omissions, substitutions, and other modifications can be made for the configurations
within a range not departing from the concepts of the present invention. The present
invention is only limited to the scope of the accompanying claims without being
limited to the above descriptions.
[Industrial Applicability]
[0073]
It is possible to provide a desiliconization and dephosphorization method of a
molten pig iron in which a recycled slag generated in a molten steel stage is used to
suppress a slag foaming while reducing a total discharge amount of the slag in the
molten steel stage; and as a result, the slopping can be efficiently prevented without
increasing a furnace capacity.
[Brief Description of the Reference Symbols]
[0074]
1: DEPHOSPHORIZATION FURNACE (MOLTEN PIG IRON
DEPHOSPHORIZATION FURNACE)
- 45 -
€>
2: TOP BLOWING LANCE
3: BOTTOM BLOWING TUYER
4: OXYGEN GAS TANK
5: NITROGEN GAS TANK
6: BOTTOM BLOWING TANK (FINE POWDER BOTTOM BLOWING
TANK)
7: TOP BLOWING TANK (BURNT LIME FINE POWDER TOP
BLOWING TANK)
8: HOPPER (TOP HOPPER)
10: DECARBURIZATION FURNACE
11: TOP BLOWING LANCE
12: OXYGEN GAS TANK
13: BOTTOM BLOWING GAS TANK
14: BOTTOM BLOWING TUYER
15: HOPPER
P: MOLTEN PIG IRON
S: SLAG
PLD: MOLTEN PIG IRON
SLD: SLAG
- 46 -

[Designation of Document] CLAIMS
[Claim 1]
A desiliconization and dephosphorization method of a molten pig iron, the
method comprising:
an oxygen gas blowing process of blowing an oxygen gas from a top of a
furnace toward a surface of the molten pig iron;
a recycled slag addition process of supplying at least one of a decarburization
slag and a secondary refining slag toward the surface of the molten pig iron as a
recycled slag; and
a burnt lime fine powder addition process of supplying a burnt lime fine
powder having a maximum particle size of 500 urn or less, along with the oxygen gas
blowing toward the surface of the molten pig iron,
wherein the burnt lime fine powder addition process starts at a start time at
which a silicon concentration in the molten pig iron is reduced to 0 mass% or more and
0.15 mass%orless.
[Claim 2]
The desiliconization and dephosphorization method of the molten pig iron
according to Claim 1,
wherein the silicon concentration in the molten pig iron at the start time is 0
mass% or more and 0.08 mass% or less.
[Claim 3]
The desiliconization and dephosphorization method of the molten pig iron
according to Claim 1,
- 47 -
wherein in the recycled slag addition process, when the silicon concentration
in the molten pig iron at the time of the start of the recycled slag addition process is
lower than 0.6 mass%, only the secondary refining slag or both of the decarburization
slag and the secondary refining slag are supplied to the molten pig iron, and
when the silicon concentration in the molten pig iron at a time of a start of the
recycled slag addition process is higher than or equal to 0.6 mass%, only the
decarburization slag is supplied to the molten pig iron.
[Claim 4]
The desiliconization and dephosphorization method of the molten pig iron
according to Claim 1,
wherein in the recycled slag addition process, when the secondary refining
slag is supplied toward the surface of the molten pig iron, an amount of the secondary
refining slag per 1 ton of the molten pig iron is within a range between 0.1 kg and 16
kg-
[Claim 5]
The desiliconization and dephosphorization method of the molten pig iron
according to Claim 1, the method further comprising
a solid oxygen source addition process of supplying a solid oxygen source
toward the surface of the molten pig iron.
[Claim 6]
The desiliconization and dephosphorization method of the molten pig iron
according to Claim 1, the method further comprising
- 48 -
•
a massive CaO source addition process of supplying a massive CaO source,
which is derived from an ore having an average particle size of 5 mm or greater,
toward the surface of the molten pig iron,
wherein in the massive CaO source addition process, an amount of the
massive CaO source per 1 ton of the molten pig iron is limited to be less than or equal
to 7.5 kg.
[Claim 7]
The desiliconization and dephosphorization method of the molten pig iron
according to Claim 1,
wherein a total amount of a MnO in the recycled slag is greater than 0 mass%
and less than or equal to 25 mass%.
[Claim 8]
The desiliconization and dephosphorization method of the molten pig iron
according to Claim 1, the method further comprising
a stirring process of blowing a gas to the molten pig iron and stirring the
molten pig iron with a stirring energy density s of within a range between 1.2 kW/t and
10 kW/t which is defined as a following Formula 1.
0.0062x0 xT if Hn W T„ VI
Wm \ \ 1.54J [ T)where
s: a bottom blowing stirring energy density (W/t) applied to the molten pig
iron !
Qg: an amount of a bottom blowing gas (NL/min; including an evolved gas •
- 49 -
from a solid material (for example, a limestone fine powder))
T: a molten pig iron temperature (K) at the time of the start of a bottom
blowing
Tg: a temperature (K) of the bottom blowing gas before entering into a metal
bath
Ho: a blowing depth (distance from a liquid surface of the molten pig iron to a
tip end of a bottom blowing tuyer; (m))
Wm: a weight of the molten pig iron (including a charged scrap; (t)) 1
[Claim 9]
The desiliconization and dephosphorization method of the molten pig iron
according to Claim 8,
wherein in the stirring process, a limestone fine powder is supplied to the
molten pig iron along with the gas blowing to the molten pig iron.