|Title of Invention||
A CORED WIRE INJECTION PROCESS IN STEEL MELTS
|Abstract||The present invention relates to a cored wire injection process in steel melts. In particular , it relates to the dimension and the injection speed of a cored wire used in steel plants to inject fluxes and alloying additives in molten steel baths.|
|Full Text||FIELD OF INVENTION
The present invention relates to a cored wire injection process in steel melts. In
particular, it relates to the dimension and the injection speed of a cored wire
used in steel plants to inject fluxes and alloying additives in molten steel baths.
The objectives of such additions are either to refine the steel further or to adjust
the composition to meet the chemistry for the final applications of the steel. This
invention is aimed at decreasing the loss of additives during the injection in the
steel bath and thereby reducing the consumption.
Steel making is sentially an oxidation process where the impurities (i.e. the
undesirable demerits) of the molten metal (either pig iron or melted scrap) are
preferentially oxidized to join the slag along with fluxes. Some amount of
oxygen and the inclusions, like alumina formed due to subsequent de-oxidation
process, remain in the steel. These oxygen and inclusions not only create
operational problems during further processing of the steel in continuous casting
and rolling but also are mostly detrimental to the product quality. The major
challenge to the steel plant operators is to reduce their content below a certain
The use of calcium is beneficial in this direction. However, the introduction of it
in liquid steel bath is very difficult due to its low density and low vapour
pressure. The advent of cored wire injection technology and the development of
calcium bearing material like calcium-silicide, calcium-iron etc have enabled the
steel plant operators to introduce the calcium in steel baths.
A large number of steel plants have also started using cored wire with Lead,
Sulphur, Selenium, Tellurium and Bismuth as filing materials. A cored wire is a
continuous steel tube filled with either a calcium bearing material or a ferroalloy
material. 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 very controlled rates into the steel-melts.
The distribution of the amount of calcium injected can be in undesirable
reactions like some amount being vapourised and lost to the atmosphere in
unreacted condition and some amount of calcium reacting with ladle top also
Some amount of calcium will react with the dissolved oxygen and inclusions
present in the steel and join the slag. Some amount of calcium will remain in the
steel as retained calcium. The last mentioned reactions are desirable reactions.
Ideally the injected calcium should be involved in the desirable reactions only.
The yield of calcium can be defined as the rate of amount of retained calcium to
the amount of calcium injected.
The yield of calcium in the cored wire injection process is at the most 30% and
sometimes it becomes as low as 2% depending on grades of steel processed and
the operating conditions.
When the steel plants are desperately looking for cost reduction options, there
exists a need for an improvement in the yield of calcium. An increase of 10% in
the yield of calcium should lead to big savings.
The description for addition of calcium given above holds good for other alloying
SUMMARY OF THE INVENTION
The main object of the present invention therefore, is to increase the yield of
calcium in a cored wire injection process.
It has been observed that the utilization of calcium and other additives is
maximum when the material is released form cored wire very close to the
bottom of the ladle so that the losses through the undesirable reactions
mentioned above can be kept to a minimum. The material in released as and
when the sheath melts completely. The key factors which determine the zone of
release of the material are the speed of injection and the dimensions of the
cored wire keeping the grade of steel processed, treatment temperature and the
material and sheath properties constant.
The main object of the invention is achieved by controlling the zone of release
of the material and thereby the yield of calcium and / or other additives by
changing the dimensions of the cored wire and the speed of injection. The
diameter of the cored wire and the thickness of the mild steel sheath are
varied along with a suitable speed of injection to ensure that the material is
released very close to the bottom.
The variation in the diameter of wire for a 140 ton ladle having 3 meter bath
depth is from 13 mm to 18 mm and the variation of sheath thickness is from 0.4
mm to 0.8 mm. The exact combination of the diameter, sheath thickness and
the speed depends on the grade of steel processed and the treatment
Thus the present invention provides a cored wire injection process for
introducing fluxes and alloying additives in liquid steel bath, comprising the steps
of adjusting the bath temperature and chemistry of the liquid steel in a
secondary treatment unit according to requirements; and releasing said additives
from said cored wire, while controlling the zone of release of said additives,
thereby controlling yield of the additives by changing the dimensions of said
cored wire and speed of injection to suit the grade of steel processed and the
The invention will now be described with the help of the accompanying
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows in schematic form the use of cored wire in steel bath.
Figure 2(a) shows the travelled distance before melting of 13 mm wire with 0.4
mm sheath thickness.
Figure 2(b) shows variation of travelled distance with different wire
Figure 3 shows an improvement in the yield of material;
Figure 4 shows the reduction in consumption of material.
13 mm diameter with 0.4 mm sheath thickness is not suitable for steel grades
having high liquidus temperature and / or high treatment temperature in 140 ton
capacity ladle with around 3 meter bath depth. The best wire for such
applications should have 18 mm diameter and 0.8 mm sheath thickness and the
injection speed should be around 110 m/min.
The parameters of the wire which effect the distance travelled are discussed
below. The distance travelled is the distance travelled by the wire before the
material is set free into the melt and is an indicator of the point of release of the
material in the ladle.
The melting of wire and subsequent release of the material 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 wire diameter are
fixed. The heat transfer coefficient is directly proportional to the wire speed.
Thus, the speed of injection decides the melting behavior when all other
parameters are constant; for example higher speed results in a lower melting
Figure 2(a) shows the variation of distance travelled for a typical wire
specification. It is observed that the distance travelled by the wire does not
monotonically increase with the increase in speed; rather it passes through a
maximum and beyond a critical speed it decreases again. As it is already
discussed the melting time decreases 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 travelled. On the contrary, as
evident from the Figure 2(a), the distance travelled, initially increases with speed
(up to line AA) and reaches a maximum at a certain speed (speed at the
intersection with line AA") and then decreases (after line AA'). The position of
this intersection point changes with the bath temperature.
The change in the distance travelled by the wire with 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 travelled 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. In the region
of speed lower than the value indicated by the line AA', the first factor dominates
and thus, the distance travelled increases with the speed. After this point, the
dominance of the second factor prevails and so as the speed increases the
distance travelled decreases in this region. It 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 release of the
The second rise in the curves of distance travelled after AA' is not of practical
interest because of the unrealistic speed and / or very high treatment
temperatures. However, this phenomena occurs as there is a minimum time
required for the casing to heat up to its melting point to initiate melting. The
wire, travelling at a very high speed, travels to a higher distance by this time and
thus, the distance travelled by the wire increases at a very high speed.
The problem of early release of material may result in higher evaporation loss as
well as loss of unreacted material by the reaction with the top slag. The
possibilities of increasing distance travelled 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
increases as there is more wire mass to be melted and as a result the release of
the material is delayed. Similarly, if the casing thickness is increased, the heat
requirement for its melting increases which again results into the delayed melting
of the wire.
To find out the suitable dimensions of the wire for certain critical applications,
the study was carried out for three wire diameters (13,16 and 18 mm) and three
casing thicknesses (0.4, 0.6 and 0.8 mm) and the results have been plotted in
Figure 2(b). The process parameters for a typical low carbon heat (liquidus of
bath as 1525°C and bath temperature at the time of injection as 1630°C are
considered for this figure. Three curves for 0.4 mm casing thickness, if
compared, clearly shows the consistent increase in distance travelled when the
diameter is increased from 13 mm (dashed line 'c') to 16 mm (dashed line 'b')
and then to 18 mm (dashed line 'a'- Similarly observation can be made when
the three curves for 0.6 mm casing thickness of different wire diameter (solid
lines 'a', 'b' and 'c' are compared.
To estimate the effect of casing thickness alone, the set of three curves for 13
mm wire diameter with three different casing thickness viz. 0.4 mm (dashed line
'c', 0.6 mm (solid line 'c') and 0.8 mm (solid line 'd') are compared. It is evident
from this figure that effect of casing thickness is more prominent than that of
wire diameter. For example, while the increase in diameter from 13 mm (dashed
line 'c') to 18 mm (dashed line 'a' keeping the casing thickness fixed at 0.4 mm
has a negligible effect on the distance travelled (at the injection speed of 200
m/min), the increase in casing thickness from 0.4 mm (dashed line 'c' to 0.8
mm (line 'd' for the wire diameter of 13 mm increases the distance travelled by
around 0.8 m (at the injection speed of 200 m/min).
However, from practical point of view casing thickness can not be increased too
high. Also there is a limitation on the injection speed; injection speed usually
can not be lowered below 110 m/min. Considering the above practical aspects,
there should be a judicial choice of wire diameter, casing thickness and the
speed of injection to enable the wire melting near the bottom of the ladle. For
example, Figure 2(b) suggests that the 13 mm wire with 0.8 mm casing is more
suitable than the 13 mm wire with 0.4 mm casing in case of high superheat
melts as the former reaches closer to the ladle bottom before releasing the
material. However, the speed of injection required ( wire (13 mm diameter with 0.8 casing) to reach the bottom of the ladle is
somewhat impractical from the operational point of view. The more workable
solution, in such cases, would be to increase the wire diameter too along with
the increase in casing thickness. Curve 'e' presents such solution; the 18 mm
diameter wire with 6.8 mm casing thickness, in this case, can reach the bottom
of the ladle at a reasonable injection speed of 110 m/min.
Trials have been conducted in a steel plant result of which has been shown
above. The wire used was the conventional calcium-iron material bearing wire of
13 mm diameter with 0.4 mm sheath thickness and the injection was done at a
steel bath temperature of 1630° C when the liquidus of bath was 1525° C. The
reduction of injection speed (V) from 240 m/min to 150 m/min has shown an
improvement in the yield of calcium as shown in Figure 3.
The next phase of trial was conducted using 16 mm calcium iron material
bearing wire having 0.4 mm sheath thickness. The further improvement in the
yield is evident from the Figure 3. The reduction in material consumption to
achieve the same level of treatment efficiency is shown in Figure 4.
1. A cored wire injection process for introducing fluxes and alloying additives in
liquid steel bath after adjusting bath temperature around 1630° C and the
chemistry of liquid steel in a secondary treatment unit according to requirements,
the ladle f said treatment unit having a 3 m liquid column height, characterized in
that, said additives are released very close to the bottom of the ladle at a depth
of approximately 3 m by injecting, at a predetermined speed of around 110
m/min, a prefabricated cored wire of appropriate dimensions for maximum
utilization of said additives, said dimensions being 18 mm in diameter and 0.8
mm in sheath thickness, these dimensions of said prefabricated cored wire and
this predetermined speed of injection being determined depending on the grade
of liquid steel, liquidus/treatment temperature, ladle size/liquid column height and
properties of cored wire material.
2. The process as claimed in claim 1, wherein said ladle is a 140 ton ladle.
3. The process as claimed in claim 1, wherein said additive is a ferro-alloy
4. The process as claimed in claim 1, wherein said additive is a calcium bearing
5. The process as claimed in claim 4, wherein said calcium bearing material
6. The process as claimed in claim 4, wherein said calcium bearing material
The present invention relates to a cored wire injection process in steel melts. In
particular , it relates to the dimension and the injection speed of a cored wire used in
steel plants to inject fluxes and alloying additives in molten steel baths.
|Indian Patent Application Number||57/KOL/2004|
|PG Journal Number||36/2011|
|Date of Filing||11-Feb-2004|
|Name of Patentee||TATA STEEL LIMITED|
|Applicant Address||RESEARCH AND DEVELOPMENT DIVISION, JAMSHEDPUR|
|PCT International Classification Number||C21C 7/00|
|PCT International Application Number||N/A|
|PCT International Filing date|