Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF DECARBURISATION AND DEPHOSPHORISATION OF A MOLTEN
METAL
Field of the invention
[0001] The present invention relates to a treatment
of molten metals, in particular for the production of Ultra
Low Carbon (ULC) and Super Ultra Low Carbon (SULC) molten
steels, including the steps of decarburisation,
dephosphorisation and chemical heating.
[0002] The present invention is also related to a
lance required to perform said treatment in a degasser
device.
Background of the invention
[0003] In the production of steel, the step of
decarburisation, meaning reduction of carbon-content of the
molten metal in a degasser device is known. This step is
performed by blowing oxygen on the molten metal in the
degasser device, said oxygen being in gaseous or solid
oxide form. Because of the temperature drop caused by the
decarburisation reaction, a reheating of the molten metal
is performed.
[0004] In particular for the production of ULC
steels as of SULC steels, such a combined process of
decarburisation and chemical heating in the degasser by
oxygen supply has been described in EP-A-0347884.
[0005] In US-A-4198229, a dephosphorisation
technique is described whereby a flux composition and a
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halide of alkali earth metal such as calcium fluoride is
added to the molten metal bath.
[0006] The dephosphorisation, as described in the
latter document, has to be performed in the secondary
metallurgy device, after the steel has passed through a
converter device and prior to decarburisation in a degasser
device. However, this method has the following drawbacks .
- a loss of time due to this additional step in the process
route,
- as a consequence, the higher temperature losses,
- the necessity to have an amount of dissolved oxygen
available in the steel at the secondary metallurgy
device.
[0007] W096/16190 describes a process for blowing a
pulsating stream of oxygen or an oxygen containing gas onto
the surface of a molten metal bath, e.g. a molten steel in
a degasser device, as a means of decarburisation. A fuel
gas can be added to the oxygen stream, as well as a supply
of solid oxide particles. This document also describes the
lance used for the supply of said substances. Called the
Mesid type 1St generation, this lance has an annular
channel for oxygen, and inside this channel, a movable
circular channel for addition of solid oxides, further a
Laval section for accelerating the oxygen, containing
nozzles for adding fuel, and an annular cooling sleeve on
the outside.
[0008] In this type of lance, the oxygen is used as
the transport gas for blowing solid oxides on the--molten
steel, which makes it difficult to regulate both oxygen and
solids flows independently from each other.
[0009] Another drawback of this type of lance is the
fact that the refractory of the degasser device cannot be
heated from ambient temperature using said lance. An
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additional heating system must be provided to heat up the
refractory, prior to the use of this type of lance. The
closed structure of the lance mouth also causes high noise
levels originating from this type of lance.
(0010] Finally, the necessity of having a movable
channel for powder blowing complicates the design of this
type of lance.
[0011] EP-A-0879896 describes an apparatus and a
method for decarburisation of molten metal, said apparatus
being a degasser device containing a number of fixed lance
nozzles in the side of the degasser device, each having an
inner tube through which gaseous oxygen is blown at
supersonic speeds and an outer tube through which a cooling
gas is blown. This installation provides however no
possibility to add oxygen in the form of solid oxides.
Aims of the invention
[0012] The main aim of the present invention is to
improve the state of the art methods of producing ULC and
SULC steels
[0013] In particular, the present invention aims to
provide a method to produce ULC and SULC steels, generating
a time gain in the method.
(0014] A further aim of the present invention is to
provide an improved lance used to blow oxygen and/or solids
on a molten metal in a degasser device.
Summary of the present invention
[0015] The present invention is related to a method
of treatment of a molten metal in a degasser device,
comprising the steps of decarburisation and
dephosphorisation, characterised in that said
decarburisation and said dephosphorisation are performed
simultaneously in said degasser device.
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[0016] In a preferred embodiment of the method
according to the present invention, said method comprises
the steps of
- blowing an amount of oxygen on the molten metal in said
degasser device, said amount depending on the need for
oxygen to perform said decarburisation,
- simultaneously with said decarburisation, blowing a
first powder, consisting of a lime based flux, on said
molten metal, to decrease the phosphorus content of said
metal,
- simultaneously with said dephosphorisation and said
decarburisation, blowing a further amount of oxygen on
said molten metal, said further amount depending on the
need for oxygen to perform said dephosphorisation.
[0017] According to a preferred embodiment of the
method according to the present invention, said metal is
steel and said method is performed in an RH-vessel to
produce Ultra Low Carbon and Super Ultra Low Carbon steels.
[0018] Said oxygen for decarburisation and/or
dephosphorisation may be supplied in gaseous form or in the
form of a second powder containing solid oxides, such as a
powder containing substantially Fe203.
[0019] Said lime based flux is a powder containing
calcium oxide (Ca0). It may for example consist of 70% Ca0
and 30% calcium fluoride (CaF2).
[0020] The amount of said lime based flux lies
between 1 and 4 kg per ton metal. The rate at which said
lime based flux is blown on said molten metal is at least
50 kg/minute, and preferably 100 kg/minute.
[0021] The present invention is equally related to a
multifunctional lance used for the treatment of molten
metal, in particular of steel in a degasser device, said
lance comprising a central channel, coaxially surrounded by
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a channel which is cooled by a fluid, said lance being
characterised in that .
- said central channel is fixed to the inside wall of said
cooling channel by way of at least two ribs
- a channel is placed eccentrically with respect to said
central channel.
[0022] According to one embodiment of the the lance
according to the present invention, an axial opening is
present in one of said ribs, said eccentric channel being
inserted in said axial opening.
[0023] According to another embodiment, said
eccentric channel is inserted in the space between two of
said ribs.
[0024] Said eccentric channel may be a powder-supply
channel, said powder supply channel being connected to a
first supply system for a powder consisting of a lime based
flux and to a second supply system for a powder containing
solid oxides and to a transport gas supply system. Said
eccentric channel may be fixed, or it may be movable in
axial direction.
[0025] According to a preferred embodiment, said
cooling channel is equipped with a ring at its mouth
section, said ring having an inner diameter ranging from
0.8 X D to 1.6 X D, where D is the outer diameter of the
central channel. Preferably, said ring has an inner
diameter ranging from 0.9 X D to 1.2 X D.
[0026] According to a preferred embodiment, said
ring has an opening through which the stream of powder
coming from the eccentric channel may flow to the molten
metal.
[0027] According to a preferred embodiment, the
mouth section of the central channel is placed at a
distance (a) from the mouth section of said lance, so that
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(a) is maximum 3 times D', wherein D' is the inner diameter
of said mouth section of said central channel.
[0028] According to a preferred embodiment, an
actuator is used to move said lance in a vertical
direction.
[0029] The lance according to the invention may
further comprise an ignition device.
[0030] According to a preferred embodiment,
regulating devices are present to regulate the flow of the
various substances to the lance according to the present
invention.
Brief description of the drawings
[0031] Figure 1 situates the invention within the
different typical process stages at the steel works.
[0032] Figure 2 describes industrial test results of
the classical route and of the route following the
invention.
[0033] Figure 3 describes the process using a top
blowing lance according to the invention.
[0034] Figures 4a and 4b describe the lance
according to a first embodiment of the invention. Figure
4a represents the front view, figure 4b represents the
section view along A-A'.
[0035] Figure 5 represents a section view of a
second preferred embodiment of the lance according to the
present invention.
Detailed description of the invention
[0036] Figure 1 places the invention amongst the
stages of the metallurgical process, and compares the
result to the process of the prior art, for the production
of ULC and SULC steels.
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[0037] After the treatment of the pig iron in the
converter, the steel may follow the classical routes (100,
200) or the invention route 300.
[0038] The classical route is subdivided in two
routes having 2 or 3 treatment stages between the converter
and the continuous casting device. In the first classical
route 100 (without dephosphorisation), the steel is going
directly to the degasser device. Directly after the
decarburisation and chemical heating, the SULC or ULC steel
undergoes a deoxidisation an and alloying treatment in the
degasser or in the secondary metallurgy device. In the
second classical route 200 the steel is first
dephosphorised in the secondary metallurgy device and then
treated in the same way as the previous case.
[0039] According to the invention route 300, the
steel goes to the degasser device for decarburisation, and
it is simultaneously dephosphorised by means of a powder
containing lime based fluxes which is blown together with
or independent of oxygen on the surface of the steel into
the degasser device. Directly after the
decarburisation/dephosphorisation, the SULC or ULC steel
undergoes an oxidisation and alloying treatment in the
degasser or in the secondary metallurgy device.
The combined decarburisation/dephosphorisation in the
method according to the invention avoids a time loss and
temperature loss. Moreover, no oxygen supply or high
dissolved oxygen content before degassing is necessary in
the secondary metallurgy device.
[0040] According to the invention route, the molten
steel enters the degassing device immediately after the
converter stage.
[0041] In said degasser device, a combined
decarburisation/dephosphorisation is to take place. For
both reactions, a sufficient amount of dissolved oxygen
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must be present in the molten steel bath. Oxygen can be
added by way of the lance of the invention, in the form of
gaseous oxygen or in the form of solid oxides. The oxygen
and/or solid oxides that have to be blown into the degasser
are calculated based upon the initial oxygen content and
the aimed oxygen content after decarburisation. The latter
depends on the desired amount of chemical reheating.
[0042] If the initial oxygen content is too low,
because the initial carbon content is too high to reach the
necessary ultra low carbon level, forced decarburisation
can be applied by blowing gaseous oxygen or solid oxides in
the early stage of the decarburisation reaction by the aid
of the lance according to the invention.
(0043] According to the invention, a powder
containing lime based fluxes is then blown immediately
after or simultaneously with said oxygen blowing, in order
to remove the phosphorus.
(0044] Phosphorus is removed by the following
reaction:
(Ca0) + 2 [P] + 5 [O] H (CaO.P205) slag Eq (1)
The reaction of Eq. (1) proceeds the more to the right as
the oxygen content in the molten steel and the lime content
of the fluxes are higher.
[0045] A necessary precondition is the presence of a
sufficient quantity of dissolved oxygen in the molten steel
bath when starting the dephosphorisation. According to
equation 1, the higher the oxygen activity in the steel,
the more efficient the dephosphorisation. In order to have
sufficient dephosphorisation, a sufficient oxygen content
during decarburisation is needed.
[0046] If the initial oxygen activity is sufficient
to fulfil said requirement, only the powder containing the
lime based fluxes is added.
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[0047] If not enough oxygen is available, oxygen can
be added as gaseous oxygen or as a powder containing solid
oxides (e.g. iron oxides) by the aid of the lance of the
invention before or during the addition of the lime based
flux. The choice between gaseous oxygen and solid oxides
depends only on the aimed temperature at the end of the
treatment.
[0048] If the temperature would be too high, solid
oxides (e. g. iron oxides) co-injected with the lime
containing fluxes by the same said lance will provide the
necessary oxygen in order to promote the dephosphorisation
reaction without repeating.
[0049] In case the temperature is too low for the
continuous casting operation, chemical repeating is
realised by blowing an excess amount of oxygen and adding
aluminium through the alloying gutter 15 during the
decarburisation/dephosphorisation. However, said amount of
oxygen and aluminium is added preferably after the
decarburisation, during the deoxidisation of the steel.
[0050] The amount of lime based fluxes, added to
perform the dephosphorisation is 1 to 4 kg/to steel,
preferably 2 to 3 kg/to steel, and is determined by the
initial phosphorus content in the liquid steel, and the
aimed phosphorus level after degassing. The lime based flux
amounts must be blown at a flow rate which is high enough
to avoid time loss due to the powder blowing, at a minimum
rate of 50 kg/minute, and preferably at a rate of
100kg/minute.
[0051] The slag for dephosphorisation is mainly
formed by the injected powder, so dephosphorisation
increases as a function of the mass and of the fluidity of
the powder blown.
[0052] After the treatment, the slag must have
enough phosphorus capacity and must be fluid enough to
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prevent P pickup in the steel. Preferably, an excess of 20%
lime is added to the process slag or the process slag is
skimmed and replaced by a new slag, formed by addition of
lime or lime containing fluxes.
[0053] By prolonging the treatment time and by
blowing a higher amount of lime based flux, higher
phosphorus removal will be obtained if needed.
[0054] The reaction will be speeded up when a good
contact between the lime containing fluxes and metal is
obtained. This is realised by blowing the finely dispersed
powders on the melt surface in the degasser device, said
powder being mixed with the liquid steel by the turbulence
of the steel due to the vacuum action. The powder blowing
lance is at a height preferably lower than 5 meters above
the steel level in the degasser device.
[0055] Figure 2 shows industrial results of the
classical route (normal represented by the cloud of black
points), i.e. degassing without dephosphorisation and of
the route following the invention (10 white circles around
their regression line), i.e. with dephosphorisation during
decarburisation. The graph compares P-levels after tapping
from the converter (X-axis) to P-levels taken before the
continuous casting device (Y-axis). By the method
according to the invention, more than 20o phosphorus
removal is realised.
[0056] Figure 3 describes the process using a lance
according to the invention. The degassing device 1 is
placed above the ladle 2 containing the liquid steel 3. The
up-leg snorkel 4 and the down-leg snorkel 5 are penetrating
the liquid steel through the primary slag 6. The vacuum
action pumps the liquid steel into the degassed chamber 7.
[0057] The oxygen 8 and the powders 9, e.g. a lime
based flux, are blown through the top-lance 10 to the top
of the steel-slag surface 11 within the degassing chamber.
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Due to the blowing pressure, the blown oxygen and the lime
based fluxes are penetrating the liquid steel surface. They
are then entrained into the ladle by the internal steel
movement, through the down-leg snorkel and mixed with the
liquid steel.
[0058] The CO + C02 produced by the reduction of the
carbon in solution and other gases are evacuated through
the exhaust gas duct 12. The alloying gutter 15 is used
for adding substances during reheating, or after the
combined decarburisation/dephosphorisation process, for
alloying in the degasser device (see figure 1).
[0059] Figures 4a and 4b show the lance according to
a preferred embodiment of the present invention. This
lance offers the opportunity to inject powder together or
without extra oxygen supply, and to regulate the flow of
powder indepently from the flow of oxygen gas. The powder
may consist of solid oxides for decarburisation, or of a
lime based flux for dephoshphorisation, or of a mixture of
both. This means that all the oxidic and lime based flux
additions can be supplied at the same time, which leads to
the possibility of using the whole decarburisation time for
supplying powder independently of oxygen blowing and
treatment time.
[0060] The lance, as shown in figures 4a and 4b
comprises a central channel 13 with a Laval type section 33
at the end. To the central channel are connected . a
supply system for oxygen gas 51, a supply system 52 for
fuel, e.g. natural gas, and a supply system 53 for a
protection gas, e.g. argon. This protection gas is sent
through the central channel, when the oxygen flow is
stopped, to prevent said central channel from possible
damage. The central channel 13 is surrounded by an annular
cooling channel 21, which is connected to the supply system
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of coolant 54, for example water. Between the outer wall
of the central channel 13 and the inner wall 24 of the
cooling channel 21 is another annular channel 19, which is
equally connected to said oxygen supply system 51.
[0061] The central channel 13 is fixed to the inner
wall 24 of the cooling channel 21 by way of at least two
ribs 14. A channel 31 is placed eccentrically with respect
to the central channel 13. In the embodiment shown in
figure 4b, this eccentric channel 31 is inserted in the
space between two of said ribs 14. This channel 31 is
connected to a supply system 55 for powder consisting of a
lime based flux and to a supply system 56 for powder
containing solid oxides. Said channel 31 is also connected
to a supply system for an inert transporting gas 57, which
may be the same gas, e.g. argon, as said protection gas.
In another embodiment of the invention shown in figure 5,
one of the ribs has an axial opening 17, into which the
channel 31 is inserted.
[0062] The channel 31 is preferably fixed.
According to another embodiment of the invention, the
channel 31 may however be made movable along its axis so
that it can be extended until its mouth section is flush
with the mouth section 25 of the lance.
[0063] There is a distance (a) between the mouth
section 18 of the central channel 13 and the mouth section
25 of the lance. The distance (a) is preferably smaller
than three times the inner diameter D' of the mouth of the
central channel 13. At the mouth section 25 of the lance,
a ring 26 is placed, having an inner diameter d ranging
from 0.8 X D to 1.6 X D, and preferably from 0.9 X D to 1.2
X D, where D is the outer diameter of the central channel
13. Said ring 26 has an opening 27 through which the
stream of powder coming from the channel 31 may flow to the
molten metal. In case said channel 31 is movable, said
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opening 27 is large enough to accommodate said channel 31
in its extended position.
[0064] An actuator 61 is present, by which means the
lance can be moved vertically along its axis, so that its
mouth section 25 is placed closer or farther away from the
molten metal in the degasser device.
[0065] In the preferred embodiment shown in figures
4a and 4b, an ignition device 32 is present, in the form of
a channel, connected to the fuel supply system 52, to the
oxygen supply system 51 and to the protection gas supply
system 53. This ignition device is used for igniting the
flame when the lance is used as a burner, i.e. when oxygen
and fuel are sent through the central channel 13, for
example to heat the refractory of the degasser device,
prior to decarburisation/dephosphorisation.
[0066] Regulating devices are present to regulate
the flow of the various substances which are supplied to
the lance, namely for the regulation of the flow of gaseous
oxygen 71, coolant 76, fuel 73, powder consisting of a lime
based flux 74 and powder containing solid oxides 77,
transporting gas 75 and protection gas 78.
[0067] Compared to the state of the art, the lance
according to the present invention allows independent
regulation of the addition of oxygen gas and powders. The
lance may comprise an ignition device 32 allowing the
heating of the refractory of the degasser device from
ambient temperature. The powder channel 31 may be fixed
and the open structure at the mouth section allows a noise
reduction compared to existing types of lances.
