Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a process for
dephosphorizing molten pig iron. More specifically, it
relates to a process for lowering the phosphorous content
with a small decarburization amount of 1.5 wt.% or less,
by treating molten pig iron at less than 1,450C prior to
charging it into a steel-making furnace.
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DESCRIPTION OF THE PRIOR ART --
It has been known for a long time that phos-
phorus in steel is a harmful impurity since it causes - -
temper brittleness and it lowers the product toughness.
For this reason, dephosphorization of steel has been an
important problem in steel production. It is no exag- -
geration to say that the progress of the steel-making ~ -
process has been directly related to the progress of the
dephosphorization process. However, the steel-making ~ -
process has changed little since the invention of the LD
converter after World War II which provides for a high
dephosphorization rate. It is now the main technique in
the steel-making process.
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However, in recent years the kinds of steel
requiring high touqhness such as steel sheet for low
temperature use and high tension steel have become more
important, and these steels cannot contain phosphorus in
amounts of more than 0.01 to 0.015 wt.%.
It is difficult to produce such low phos-
phorus steel by the normal single slag converter oxida-
tion smelting technique, and thus a double slag converter
oxidation smelting technique has been employed.
According to the condition of the raw ma-
terials, there are instances where pig iron containing
phosphorus in amounts of far more than only 0.1 to 0.2 wt.%
is used, and in such cases the double slag technique has
to be employed if steel conforming to the severe phos-
phorus standard as mentioned above is to be produced, or
even if ordinary steel containing not more than 0.035 wt.%
of phosphorus is to be produced.
However, the double slag technique involves
grave problems due to d~sadvantages such as the drop of
converter productivity caused by long smelting time, the
drop of the iron yield caused by intermediate slagging off,
etc.
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SU~ARY OF THE INVENTION
The primary object of the present invention is
to provide a process for preliminarily dephosphorizing mol-
ten pig iron before charging it into a steel-making furnace.
It is a further object of the present invention to provide
for the production of steel with a low phosphorus content - -
even by the single slag converter oxidation smelting
technique. Thus, the present invention overcomes the
above-mentioned disadvantages of the double slag technique. -
According to the process of the present inven-
tion, (a) dephosphorizing agents are added to the molten
pig iron to adjust the amount of CaO to correspond to -
at least 0.3 times the sum of the amounts of Si02 and P205 -
and to adjust the amount of T.Fe to 15 to 50~ in terms of ~ -
the dephosphorized slag composition after treatment, and,
in addition, (b) iron oxide and/or oxidizing gas corres-
ponding to not more than the amount of oxygen required to
oxidize all the amounts of silicon, manganese, and phos-
phorus and 1.5 wt.% of the carbon contained in the molten
pig iron before treatment are added to the molten pig iron
before it is charged into the steel-making furnace. The
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dephosphorizing agents and oxidizing agents are made to
come into contact with the molten pig iron with stirring
to lower the phosphorus content with a small decarburi- -
zation amount of 1~5 wt.% or less.
5 . - DETAIIi!~D DESCRIPTIO~ OF THE INVE~TION
.
~ In the dephosphorization of molten pig iron,
phosphorus is surmised ~o be absorbed and re ved by
slag as oxide. However, since phosphorus has less af-
finity for oxygen than silicon and manganese in molten
pig iron, sufficient oxidizing power to almost com-
pletely oxidize silicon and mangançse must be provided
- ~ for effective dephosphorization. Such strong oxidizing
power, however, causes concurrent decarburization.
~ ~ In the pretreatment of lten pig iron be-
15 ; fore charging it into the steel-making furnace, it is
; - - desirable to keep the decarburization amount as small as
' possible. In fact the amvunt must be suppressed to 1.5
wt.X or less in light of equipment conditions as well as
the safety of ~he heat source in the following step in
the converterO
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This can be achieved by treating the molten
pig iron at less than 1,450C. The treatment at a temper-
ature of more than 1,450C causes active decarburization;
however it does not allow for the attainment of a high de-
phosphorization with a decarburization amount of 1.5 wt.%
or less. A lower treatment temperature of below 1,450C
allows for a more effective dephosphorization with a
smaller decarburization. For example, treatment at
1,300C allows for the attainment of a high dephosphori- ;
zation with a small decarburization of 0.4 to 0.7 wt.%.
As mentioned above, in dephosphorization the
phosphorus is absorbed and removed by the slag as oxide.
In this case, the slag basicity represented by CaO/(SiO2 +
P205) and the oxidizing power of the slag represented by
T.Fe are important factors. That is, the higher the slag -
basicity and T.Fe, the higher the dephosphorization which
can be attained. ----
However, if the oxidizing power is provided
mainly by an oxidizing gas as in the conventional steel- - -
20 making process, the temperature rises greatly, thus not ~-
allowing for the treatment at less than 1,450C, and
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decarburization is cauaed. This does not provide for
a high T.Fe of 15X or re (due to a high carbon content).
In the case of the present invention, if the
oxidizing power is provided mainly by iron oxide, the
treatment te_perature can be held at less than 1,450C, -
- keeping the decarburization amount small, and the T.Fe of
the slag can be obtained at 15% to 50%.
Further re, the low temperature treatment is - -
advantageous for dephosphorization also because the ther-
~ _~dynamics allows for dephosphorization at a low basicity
which would be inconceivable in the steel-ma~ing process.
If the treatment temperature is as low as 1~300C, the
slag T.Fe of 40 to 50% allows the dephosphorization of 40
~ ~ to 60X even with a basicity of 0.3~ However, if the
lS ~ basicity becomes smaller than 0.3, effective dephosphori- `-
-~ zation cannot be made, however high the T.Fe may be. Of
course, the higher the basicity of the slag, the re --
effective the dephosphorization is. If the T.Fe is 15X
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~ ~ or more in this treatment process at less than 1,450C,
a~ a~ 3-20 n~ a basicity of ~ allows for sufficient dephosphorization,
and a baPicity higher than it is not required. From the
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- above, it can be said that the dephosphorized slag in
the range from 0.3 to 3 in basicity and in the range
from 15 to 50a in T.Fe is the most suitable for this
`process.
For dephosphorization of molten pig iron with
such dephosphorizing slag, it is necessary, as mentioned
above, to provide sufficient oxygen to oxidize all the
amounts of silicon and manganese with stronger affinity
for oxygen than phosphorus so as to oxidize silicon,
manganese and phosphorus and less than about 1.5 wt.~ of the
carbon. If oxygen or any other oxidizing gas is used as
the only oxygen source, the temperature rise by the - ~-
treatment is great. Therefore, unless the temperature
of the molten pig iron before treatment is as low as
about 1,200C, it is difficult to keep the tempertaure
of the molten pig iron at less than 1,450C during treat-
ment. On the contrary, if the oxygen is provided by iron
oxide such as iron ores or scale, the temperature drop
;; by the treatment is great. Therefore, unlesss the temper-
ature of the molten pig iron is more than about 1,450C,
it is difficult to keep the temperature of the molten pig
`~ iron at more~than 1,250C after treatment. For dephos-
~ - phorization of molten pig iron which does not meet the
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above temperature conditions, iron oxide, such as iron
ores or scale, and an oxidizing gas, such as oxygen,
must be used together as the oxygen source.
The dephosphorizing agents for producing de-
phosphorizing slag must comprise CaO and iron oxide as
mentioned above.
When sufficient dephosphorization can be
obtained at the relatively low basicity as in the process
of the present invention, LD converter slag can in part --
be used as the CaO and iron oxide source. In addition,
if limestone is used as the CaO source, it is changed
into CaO at the treatment temperature, and quick lime
itself can of course be used. From the above, the de-
phosphorizing agents can be (1) LD converter slag and iron
oxide such as iron ores or scale, (2) limestone and iron
oxide such as iron ores or scale, and (3) quick lime and
iron oxide such as iron ores or scale. When these de-
phosphorizing agents and oxidizing agents are used for
dephosphorization, the slagging of these additions is
very important. If the mixture poses difficulty in slagging,
the addition of Fluorspar and/or soda ash promotes slagging.
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When these are used for dephosphorization, the
stirring of the ~olten pig iron and dephosphorizing agents
or dephosphorizing slag is important for sufficient re-
action and for shortening of the treatment time. As a
means of stirring, an impeller is the most effective, but
bubbling by using an inert gas such as nitrogen, argon,
etc., or air can be used, too, for stirring.
When oxidizing gas is supplied to the molten
pig iron, if the dephosphorizing slag layer is thick, it
10 can be supplied through an immersion pipe, but if the de- --
phosphorized slag layer is relatively thin, it may be
blown into molten pig iron through the slag by using a
water-cooled lance.
This dephosphorizing treatment of molten pig -
iron can be performed in a ladle used for charging molten
- pig iron into a converter, and this method is advantageous
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for skimming of slag after treatment and for charging into
the converter. The treatment of molten pig iron in a tor-
pedo car can be performed effectively, too.
Embodiments of the present invention will be
given below.
Embodiment I
With 45 tons (t) of molten pig iron stored in
a ladle, 30kg/t of converter slag and 40kg/t of scale were
added to the molten pig iron as dephosphorizing agents,
and further 35kg/t of scale was added as an oxidizing agent.
While 7.2Nm3/t of oxidizing gas was blown into the molten
pig iron by using an immersion pipe, the mixture was
stirred by an impeller at 100 rpm. The results as shown in
Table 1 were obtained.
Table 1
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Temperature of molten pig iron 1,320C
before treatment
Temp~rature of molten pig iron 1,275C
after treatment
_
Components of molten pig iron
before treatment (% by wt.) C Si Mn P S
4.06 0.82 0.50 0.112 0.027
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Components of molten pig iron
after treatment (~ by wt.) C Si Mn P S
3.53 trace trace 0.023 0.027
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Components of dephos- CaO/(SiO2+P205) = 0.37
phorized slag T.Fe = 41.1
Treatment time 35 min.
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As shown above, the treatment in the ranqe
from 1,275 to 1,320 allowed for a high dephosphorization
amount of 79% with a small decarburization amount of
0.53 wt.%.
Embodiment II
With 45t of molten pig iron stored in a
ladle, 30kg/t of quick lime and-25kg/t of iron ore as
dephosphorizing agents and lOkg/t of Fluorspar as a
slagging promoter were added to the molten pig iron and --
further 20kg/t of iron ore was added as an oxidizing
agent. While 10.2Nm3/t of oxygen was blown through an
immersion pipe, N2 bubbling was performed for stirring
by using another immersion pipe. The results as shown - -
in Table 2 were obtained.
- Table 2
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Temperature of molten pig iron 1,350C
before treatment
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Temperature of molten pig iron 1,370C
after~treatment
Components of molten pig iron
~- before treatment (% by wt.) C Si Mn P S
4.07 0.64 0.59 0.113 0.012
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Components of molten pig iron
after treatment (% by wt.) C Si Mn P S
3.54 trace 0.13 0.015 0.013
~Components of dephos- CaO/(SiO2+P205) = 1.79 - -
phorized slag T.Fe = 15.8
Treatment time 40 min.
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As shown above, the treatment in the range ~
from 1,350 to 1,370C allowed for a high dephosphori~ r
zation amount of 87X with a s~all decarburization amount --
of 0.53 wt.~
S Embodiment III
With 41.5t of molten pig iron stored in a
lad~e, 30kg/t of limestone and 32kg/t of iron ore were
added to the molten pig iron as dephosphorizing agents,
~- and further 20kg/t of iron ore was added as an oxidizing
agent. And ll.5Nm3/t of oxygen gas was blown into the
- molten pig iron by using a water-cooled lance, the ~ix-
ture was stirred by an impeller at 100rpm. The results
as shown in Table 3 were obtained
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Table 3
~ Temperature of molten pig iron1,370C -
before treatment
Temp~rature of molten pig iron1,345C
. after treatment
A' ~ CDmponents of molten pig iron 5~ `
before treat~ent (% by wt.) C ~ Mn P S F
4.18 0.~2 0.48 0.121 0.032 f
oomponents of molten pig iron S~'
. after treatment (X by wt.) C S Mn P S
~ 3.56 trace 0.07 0.021 0.030
Components of dephos- CaO/(SiO2+P2Os) ~ 0.92
: ~ phorized slaq _ T.Fe = 36.2X
Treatment time 37 min.
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As shown above, the treatment in the range from
1,370 to 1,345C allowed for a high dephosphorization amount
~- ~ of 82.6X with a small decarburization rate of 0.62 wt.%~ ~
t
