FIELD OF INVENTION
The present invention relates to an aluminium wire injection method in steel
melts. In particular, the invention relates to an improved aluminium wire
injection process in steel melts in which the dimension and the injection speed of
an useable aluminium wire is designed in correspondence with the capacity of
the laddie including the height of the liquid column.
BACKGROUND OF THE INVENTION
Steel making is essentially an oxidation process where the impurities (i.e. the
undesirable elements) of the molten metal (either pig iron or melted scrap) are
preferentially oxidized to join the slag along with fluxes. Some amount of oxygen
thus remains in the steel. This oxygen not only creates operational problems
during further processing of the steel in continuous casting but also forms
inclusions which are detrimental to the product quality. The de-oxidants like
aluminium is added to the steel to reduce the oxygen content below a certain
level.
All such additions are either to deoxidize the steel further or to adjust the
composition to meet the chemistry for the final applications of the steel.
However, the introduction of aluminium in liquid steel bath is very difficult due to
its low density and low melting point. The advent of aluminium wire injection
technology has enabled the steel plant operators to introduce the aluminium in
steel baths more efficiently.

An aluminium wire is a continuous solid cylinder of almost pure aluminium,
produced by rolling of cast aluminium. This wire is fed in the liquid steel bath
contained in a ladle with the help of a wire feeder. This appears to be the most
suitable means to introduce a particular element into the melt while attaining a
high degree of homogenization and ensuring its metallurgical effectiveness.
There exists equipment today that is capable of feeding wire at a very controlled
rates into the steel-melts.
An inappropriate distribution of the amount of aluminium injected can result in
undesirable reactions for example, some amount may be vapourised and lost to
the atmosphere in unreacted condition and some amount of aluminium may
unfavourably react with the ladle top slag and atmospheric oxygen will be also
lost. Some amount of aluminium will react with the dissolved oxygen present in
the steel the remaining amount of aluminium are analysed as retained
aluminium. The last two phonomena are however desirable.
Ideally the injected aluminium should be involved in the desirable reactions only.
The utilization of aluminium can be defined as the ratio of the sum of retained
aluminium in the steel bath and the aluminium consumed for bringing down the
oxygen level of the steel bath to the total amount of aluminium injected. The
utilization of aluminium in the aluminium wire injection process should ideally be
100%.
When the steel plants are desperately looking for cost reduction options, there
exists a need for an improvement in the yield of aluminium. An increase of 10%
in the yield of aluminium should lead to big savings.

OBJECTS OF THE INVENTION
Accordingly an object of the present invention is to propose an improved
aluminium wire injection process in steel melts which decrease the loss of
aluminium during the injection in the steel bath and thereby reduce the
consumption of aluminium wire.
This object of the present invention is achieved by ensuring that the aluminium
wire melts at an appropriate place in the ladle by selecting a suitable speed of
injection and appropriate dimensions of the wire.
It has been observed that the utilization of aluminium is the maximum if it melts
very close to the bottom of the ladle so that the losses through the undesirable
reactions mentioned hereinabove can be kept at the minimum. The key factors
which determine the point of release of the material are the speed of injection
and the dimensions of the cored wire while, keeping the bath superheat and the
liquid column height constant. The bath superheat is the difference between the
treatment temperature and the liquidus in steel bath.
The diameter of the aluminium wire is selected along with a suitable speed of
injection to ensure that the wire melts very close to the bottom of the ladle.
The diameter of wire, for example for a140 ton ladle having 3 meter liquid
column height can be selected from 12.5 mm to 18mm. The exact combination
of the diameter and the speed will depend on the bath superheat and the liquid
column height in the ladle.

The present invention thus provides an aluminium wire injection process for
deoxidation and alloying in liquid steel bath, in which the bath temperature and
the chemistry of liquid steel is adjusted in a secondary treatment unit according
to the requirements. The process is characterized in that the aluminium is caused
to melt close to the bottom of the ladle by injecting a prefabricated aluminium
wire of appropriate dimensions at a predetermined speed depending on the bath
superheat and liquid column height.
v.-
The invention will now be described with the help of the accompanying
drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows in schematic form the application of aluminium wire in steel
bath.
Figure 2(a) - a graphical representation of the traveled distance before melting
of a 12.5 mm aluminium wire when injected in steel bath at different bath
superheats.
Figure 2(b) - a graphical comparison showing variation of traveled distance
before melting of aluminium wires of different diameters (0) when injected in
steel bath at a bath superheat of 100°C.
Figure 2(c) - shows useful speed range to achieve a traveled distance of around
3 meter before melting of the aluminium wires of figure 2(b) when injected in
steel bath at different bath superheats.

Figure 3 - shows the percentage utilization of the aluminium wire according to
prior art speed of injection and according to the speed of injection of the present
invention.
Figure 4 - shows the average overall % utilization of the aluminium wire in
respect of prior art speed of injection and the speed of injection as modified
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The process of injection aluminium wire has been illustrated schematically in
Figure 1.
After the steel is made in the primary steel-making vessel, the liquid steel is
carried in a ladle to the secondary treatment unit. The main purpose of the
secondary treatment unit is to further refine the steel and adjust the bath
temperature and chemistry to suit the demand of the next processing unit i.e.
casting unit. The presence of dissolved oxygen in the liquid steel poses problem
to the smooth operation of casting and also deteriorate the product quality. The
deoxidation of the steel, thus, becomes essential to control the dissolved oxygen
level. The liquid steel is treated with aluminium and / or other deoxidants in the
secondary processing units.
In the present invention a mathematical model has been developed to predict
distance traveled by an aluminium wire before its complete melting. Based on
the model results and the experimental results described hereinafter in the
'experimental work' section of the text, it will be clear that the most common

aluminium wires of 12.5 mm diameter is not suitable for superheats in excess of,
say 80°C, in a 140 ton capacity ladle with around 3 meter liquid column height.
The better wires for such applications should have 16 or 18 mm diameter and
the injection speed should be in the range of 60 m/min. to 80 m / min.

The parameters of the wire which effect the distance traveled are described
below. The expression 'distance traveled' categorically means the distance
traveled by the wire before the aluminium melts' is an indicator of dispersion of
the molten aluminium in the ladle.
The melting of wire depends on the amount of heat transferred from the bath to
the wire which in turn depends on the heat transfer coefficient only when the
superheat and the wire diameter are constant. The heat transfer coefficient is
directly proportional to the wire speed. Thus, the speed of injection decides the
melting behaviour when all other parameters are constant; for example higher
speed results in a lower melting time.
A general methodology of wire injection as depicted in Figure 2(a) shows the
variation of distance traveled for a typical wire specification at different
superheats. It is observed that the distance traveled by the wire does not
monotonically increase with the increase in speed; rather it passes through a
maximum and beyond a critical it decreases drastically.
Figure 1 shows a payoff wire (1) from which a feeder (2) is receiving the wire
and being injected in a steel bath (5) via a guide (4), the guide being connected
to the feeder (2) by means of a flexible hose (3).

As it is described hereinabove, the melting time decrease with the increase in
speed. However the decrease in the melting time on account of this factor is not
necessarily accompanied by a decrease in the distance traveled. On the contrary,
as evident from the Figure 2(a), the distance traveled, initially increases with
speed (up to line AA') and reaches a maximum at a speed at the intersection
with line AA' and then decrease after line AA'. The position of this intersection
point changes according to the bath superheat.
The change in the distance traveled by the wire with the increase in the speed of
injection is dependent on the relative dominance of the two competing factors.
The increase in speed clearly implies that if the melting time were to remain
unchanged, the distance traveled would be more. However, since the heat
transfer coefficient also increases with the speed, the melting time decreases.
Clearly, whether the injected wire will move deeper or not would be dictated by
whether the decrease in melting time is significantly higher or not. The melting
behaviour of the wire is characterized by a combined factor for example, the
formation of frozen layer(s) of the bath steel formed on the aluminium wire
when immersed, the melting of this frozen layer(s) and the melting of the
original aluminium wire. The melting time of the frozen layer may be extended
by the secondary growth of this layer. In the region where speed is lower than
the value indicated by the line AA', the distance traveled increases with the
speed. After this point ('transition point'), the secondary growth ceases and the
presence of only primary frozen layer drastically decreases the melting time and
hence the traveled distance. Such a phenomenon suggests that depending on
the prevailing conditions in a steel shop, an increase in speed may not
necessarily help the wire to travel nearer to the bottom of the ladle before
complete melting.

The problem of early melting may result in higher losses through undesirable
reactions. The possibilities of increasing distance traveled by the wire in such
situations by modifying wire dimensions have been assessed in this section. Now
if the wire diameter is increased, the total heat requirement for melting of the
wire increase as there is more wire mass to be melted and as a result the
traveled distance is increased.
To find out the suitable dimensions of the wire for certain critical applications,
the study was carried out for three wire diameters (12.5, 16 and 18mm) and the
results have been plotted in Figure 2(b). It is evident from the results depicted in
the accompanying drawings that if the wire diameter is increased, the traveled
distance prior to the transition point increases though the transition point (the
speed where the secondary growth of the frozen layer ceases) remains the same
and also the effect of speed on the traveled distance after the transition point is
very low.
Thus, it may be concluded from the above findings that the common practice of
injecting aluminium wire in a liquid steel bath with a superheat of 100°C in a
ladle, of say 140 ton liquid steel capacity with 3 meter liquid column height, at a
speed of 150 m/min is not effective and the speed of injection should be brought
down to the range of 60 to 80 m/min to maximise the utilization of the wire. The
potential increase in the injection duration should be compensated by increasing
the diameter of the wire beyond 12.5 mm preferably to 16 to 18mm. The useful
speed range for a liquid column height of 3 m has been furnished in Fig. 2(c).
The speeds available at a particular bath superheat are the speeds in between
the upper and lower curves shown in this figure.

EXPERIMENTAL WORK
Trials have been conducted in a steel plant in 52 low carbon aluminium killed
steel heats; results of which have been shown in Figures 3 and 4. The aluminium
wire used was the conventional wire of 12.5 mm diameter and the injection was
done at a steel bath having superheat in excess of 80°C. The samples have been
collected at the end of 2 minutes inert gas stirring following the aluminium wire
injection. The reduction of injection spged. from 150 m/min to 70 m/min has
shown an improvement in the utilization of aluminium as shown in Figure 3. The
average overall improvement in the utilization of aluminium in these 52 heats
was around 10% as shown in figure 4. This increase in utilization is an indicator
of potential decrease in the aluminium wire consumption.

WE CLAIM
1. An improved wire injection process for introducing a deoxidant for
example, aluminium in a secondary liquid steel bath to control the
dissolved oxygen level in the liquid steel including adjustment in the
bath temperature, the liquid steel being produced in a primary steel
making vessel and carried in a ladle to the secondary liquid bath,
wherein a prefabricated aluminium wire is caused to move at a
predetermined speed for injection in the secondary steel bath such
that the aluminium melts close to the bottom of the bath, and wherein,
the dimension such as diameter of the aluminium wire including the
moving speed of the wire being selected in correspondence with the
superheat of the steel bath and the liquid column height, characterized
in that said predetermined speed of injection is 70 m/min, in that the
diameter of said aluminium wire is more that 12.5 mm, and in that the
bath superheat is in excess of 80°C in a 140 ton ladle with a 3 m liquid
column height.
2. The process as claimed in claim 1, wherein the diameter of said
aluminium wire is 16 mm and the speed of injection is 60-80 m/min.
3. The process as claimed in claim 1, wherein the diameter of said
aluminium wire is 18 mm and the speed of injection is 60-80 m/min.
4. The process as claimed in claim 1, wherein the melting behaviour of
the 'injected' aluminium wire in the secondary bath is selected based
on the time taken to form a frozen layer of the liquid steel on the
aluminium wire when immersed, the melting time of the formed frozen
layer, and the melting time of the original aluminium wire.

5. An improved wire injection process for introducing a deoxidant for
example aluminium in a secondary liquid steel bath, as substantially
described herein and illustrated with reference to the accompanying
drawings.

The invention relates to an improved wire injection process for introducing a
deoxidant for example, aluminium in a secondary liquid steel bath to control
the dissolved oxygen level in the liquid steel including adjustment in the bath
temperature, the liquid steel being produced in a primary steel making vessel
and carried in a ladle to the secondary liquid bath, wherein a prefabricated
aluminium wire is caused to move at a predetermined speed for injection in
the secondary steel bath such that the aluminium melts close to the bottom
of the bath, and wherein, the dimension such as diameter of the aluminium
wire including the moving speed of the wire being selected in correspondence
with the superheat of the steel bath and the liquid column height,
characterized in that said predetermined speed of injection is 70 m/min, in
that the diameter of said aluminium wire is more that 12.5 mm, and in that
the bath superheat is in excess of 80°C in a 140 ton ladle with a 3 m liquid
column height.