Introduction to the field of Invention
This invention relates to a process for the manufacture of basic tundish cover powder for
controlling inclusion content in molten low carbon aluminium killed steel.
In another aspect, this invention also relates an improved method for the continuous casting of
low carbon aluminium killed (LCAK) steel.
Prior Art. Drawbacks in the prior art and early attempts in the prior art
In the continuous casting, it is very important to minimize thermal loss of the molten material in
the tundish before casting. This is carried out by using heat insulation material on the surface of
the molten material kept in the tundish.
The most common material, which is used as tundish covering material during continuous casting
is carborised rice husk. This material contains Si02 (>90%) and well suited for the purpose of
thermal insulation of molten steel in tundish so that heat loss is minimized during CaLxicijua=>.
This material has its own limitation and is not suited in the production of clean steel particularly for
low carbon aluminium killed steel requiring deep/extra deep drawing operation, low inclusion
content is required for this purpose.
Attempts have been made to meet these challenging requirements of not only heat insulation but
also minimization of re-oxidation and control of inclusion.
Rice husk, Containing Si02is not suitable for low carbon aluminium killed steel, since it generates
alumina inclusion by re-oxidation of aluminium present in steel, it also increases silicon content of
LCAK steel, which is not desirable.
Current studies
To avoid the formation of undesirable alumina inclusions, our current studies have indicated that
covering compounds, basic in nature is required.
Towards these ends, we have successfully changed the tundish covering material mainly rice
husk to a metallurgically active compound, which minimizes re-oxidation and absorbs non-
metallic oxide inclusion mainly alumina.
Objects of the invention
It is, therefore, a principal object of the invention to propose a method for the manufacturing of
improved covering material for controlling inclusion and re-oxidation of molten steel in the casting
of low carbon aluminium killed steel.
It is another object of this invention to propose such a method, which will produce suitable
covering material, which can be effectively used on the surface of the molten material in a tundish
before the casting of the material.
It is yet another object of this invention to propose such a method, which will produce a covering
material capable of not only providing thermal insulation properties, thereby avoiding o minimizing
heat losses but also minimizing aluminium loss and also almost eliminate silicon pick up.
A still further object of this invention to propose such a process, which will produce suitable
covering mateial for use in a tundish, which will have satisfactory physical properties including
free flowability required for complete coverage of steel; surface in tundish.
Yet another object of this invention to propose an improved casting method for low carbon
aluminium killed steel whereby it is possible to produce clean steel casts, particularly flat products
which require deep/extra deep drawing operation.
These and other objects of this invention will be apparent from the following paragraphs.
Brief statement of this invention.
A method for the manufacturing of basic tundish cover powder for low carbon aluminium killed
steel comprising:
a. Calcining limestone in the temperature range of 1200 - 1250° C to produce
calcined lime having 92% by weight of Calcium Oxide.
b. Packing the product of step (a) in a conventional manner without ingress to
moisture or access to air.
c. Calcining bauxite in the temperature range of 1500 - 1600° C to produce
calcined bauxite having an alumina (Al203) content up to 90.00% by weight.
d. Subjecting the calcined material of step (b) and step (c) to size reduction
independently to a powder.
e. Mixing the thus size reduced calcined lime and calcined bauxite in a
mechanical mixer to obtain a fine blend to the exclusion of air or moisture
and
f. Packing the thus obtained blend to the exclusion of air or moisture.
The calcined lime has at least 92% by weight of calcined Oxide.
The calcined lime has a Si02 content of less than 5% by weight and the preferred Si02 content is
3.0 to 5.0% by weight.
The calcined bauxite has an alumina (Al203) content of at least 85.0% by weight and up to 90%
by weight.
The calcined bauxite has a Si02 not exceeding 6.0% by weight.
The calcined lime and the calcined bauxite are mixed in the weight proportions of 1.0: 0.5 and the
preferred weight proportion of calcined lime to calcined bauxite is in range of 0.9 to 1.0: 0.4 to 0.5
depending upon the chemical compositions of calcined lime and the calcined bauxite.
The different steps are carried out without ingress to air or moisture, more particularly with
respect to the highly hygroscopic lime.
The particle size of the blend is such that almost all the fines pass through 100 mesh screen and
oversize on 200 mesh (ASTM) screen is not more than 10% by weight.
Further according to the invention, the basic powder is to be applied initially after pouring the
steel into tundish. Rich husk is to be added on top of the basic powder layer. Basic powder will
absorbs inclusion and controls re-oxidation whereas rice husk will provide necessary heat
insulation in tundish.
Detailed process of the invention.
The process of the invention has been carefully selected after judicious selection of the raw
material in the first step.
After several laboratory investigations, we have found that the raw material should be basic and
should have high calcined oxide and also high alumina low in iron oxides and should have
permissible loss of ignition.
A judicious selection has made using calcined lime and calcined bauxite.
The sources are limestone and bauxite respectively.
Selection of the raw material
a. Limestone of selected variety is carefully calcined at a temperature of about 1200 -1250° C in
the usual manner and is carefully stored immediately After calcination without access to
moisture/air due to its high hygroscopic nature. The calcined lime should meet the following
characteristics: -
b. b. Similarly, bauxite is calcined at temperature of about 1500 -1600° C to produce calcined
bauxite. The calcined bauxite should meet the following characteristics: -
Sizing of raw materials: The raw materials (calcined lime and calcined bauxite) should be
ground and screened to the size as per requirements. The calcined lime is to be kept inside the
plastic bag immediately after calcination of limestone in order to avoid any moisture pick-up since
lime is highly hygroscopic in nature. The grinding and screening of calcined lime should be done
just before the mixing of the raw materials.
The ground material should be sieved. The size of the material is as follows (ASTM E-11 -70)
Grain size Residue
+ 200 mesh (53 n) <=10%
+ 100 mesh (110 n) <=1.0%
Mixing of raw materials: The basic powder is to be prepared by simple mechanical mixing of
powdered calcined lime with powdered calcined bauxite. The type of mixture to be used for
mixing is either muller or sigma type mixture. The mixing time is 60 minutes. Proper care should
be taken to avoid any moisture pick-up.
Packing of basic tundish powder: The powder is to be kept in a double lined with a inner
plastic layer to avoid any moisture pick-up till its practical Application. The shelf life of this packed
material is 6 months after the date of manufacture. The material should not be used after its shelf
life is over.
Experimental verification
] We have subjected the novel cover material to practical trials and have found very satisfactory
; results.
In a trial run, we have used the material at the rate of 0.25-0.3 kg/ton steel cast. As an example,
approximately, 150 kg is required to be added in a sequence of 4 heats of a heat size 150 ton.
The entire amount is added at the start of the sequence and immediately, the tundish is to be
covered by the rice husk. Basic powder should not be added further during casting of heats in the
sequence. Rice husk to be added as and when it is required to avoid any red appearance at the
top layer.
Results on Plant trial
41 heats in 14 sequence (approximately, 6000 ton of LCAK steel) were cast with this basic
powder. The heat size was 150 ton. The tundish capacity where trial conducted was 32 ton.
Inclusion level in final product
The improvement of inclusion level with basic powder was 50% compared to heats where only
rice husk was added. The inclusion volume fraction of the final product was reduced to 0.16
(average) with basic tundish powder from a level of 0.31 (average) with rice husk.
Aluminium loss and Si-pick-up in tundlsh
Table 1 shows the aluminium loss and Si-pick-up in tundish sample during the study period.
Basic powder shows lower aluminium loss and silicon pick-up in tundish (which is very important
for LCAK grades) compared to rice husk.
Advantages
Temperature loss and other operational aspect: Temperature loss in tundish during casting
with double layer tundish cover powder (basic powder + rice husk) did not show any significant
difference vis-a-vis normal practice. In addition to this, the powder showed excellent flowability
and there was no jam formation. The developed powder did not pose any problem in stopper
movement during casting. During the trials, no instances of aborted heat or partially returned
heats due to temperature loss in tundish were observed.
Chemical composition of basic tundish cover powder:
CaO - Al203 - Si02 based system selected because of the following:
• High alumina absorption capability.
• Less corrosive to refractory.
• Lower melting temperature.
WE CLAIM
1. A process for the manufacturing of basic tundish cover powder for low
carbon aluminium killed steel comprises:
a. Calcining limestone in the temperature range of 1200-1250° C to
produce calcined lime having 92% by weight of Calcium Oxide.
b. Packing the product of step (a) in a conventional manner without
ingress to moisture or access to air.
c. Calcining bauxite in the temperature range of 1500 - 1600° C to
produce calcined bauxite having an alumina (Al203) content up to 90.05
by weight.
d. Subjecting the calcined material of step (b) and step (c) to size
reduction independently to a powder.
e. Mixing the thus size reduced calcined lime and calcined bauxite in a
mechanical mixer to obtain a fine blend to the exclusion of air or
moisture and
f. Packing the thus obtained blend to the exclusion of air or moisture.
2. A process as claimed in claim 1 wherein the Calcium Oxide content is not
above 94% by weight.
3. A process as claimed in claim 2 wherein the calcined lime has a Si02 content
of less than 5% by weight.
4. A process as claimed in claim 3 wherein the preferred Si02 content is 3.0 to
5.0% by weight.
5. A process as claimed in claim 1 wherein the Al203 content is at least 85.0%
oy weight.
6. A Process as claimed in any of claims 1 to 6 wherein the calcined bauxite has
a Si02 not exceeding 6.0% by weight.
7. A process as claimed in any of claims 1 to 5 wherein the calcined lime and
the clacined bauxite are mixed in the weight proportion of 1.0 : 1.5.
8. A process as claimed in claim 7 wherein the preferred weight proportion of
the calcined lime and the clacined bauxite is 1.0 : 0.5.
9. A process as claimed in any of claims 1 to 8 wherein the different step are
carried out without ingress to air moisture, more particularly wit respect to
the highly hygroscopic lime.
10. A process as claimed in claim 7 wherein the particle size of the blend is such
that almost all the fines pass through 100 mesh screen and oversize on 200
mesh screen is not more than 10% by weight.
11. A process for the manufacture of improved tundish cover material for use in
the casting of steel to control steel re-oxidation and absorption of inclusions
substantially as herein described.

There is provided a method for the manufacturing of basic tundish cover powder for low carbon
aluminium killed steel comprising:
(a) Calcining limestone in the temperature range of 1200 - 1250° C to produce calcined lime
lowing 92% by weight of Calcium Oxide.
(b) Packing the product of step (a) in a conventional manner without ingress to moisture or
access to air.
(c) Callcining bauxite in the temperature range of 1500 - 1600° C to produce calcined bauxite
having an alumina (Al2O3) content up to 90.05 by weight.
(d) Subjecting the calcined material of step (b) and step (c) to size reduction independently to a
powder.
(e) Mixing the thus size reduced calcined lime and calcined bauxite in a mechanical mixer to
obtain a fine blend to the exclusion of air or moisture and
(f) Packing the thus obtained blend to the exclusion of air or moisture.