MOULD POWDER COMPOSITION TO ENHANCE CASTING SPEED FOR HIGH CARBON GRADES OF STEEL
INTRODUCTION TO THE FIELD OF INVENTION
The present invention relates to a mould powder having higher melting characteristics, improved mould heat transfer and lubrication to enhance casting speed for high carbon grades of steel. The present invention also relates to modification in oscillation parameters for reduced friction with improved powder consumption for improved lubrication at higher speed.
PRIOR ART AND DRAWBACKS
Rourkela Steel Plant at its Steel melting Shop-I is producing MC-40 and MC-55 grades of steel through the BOF-VAR/VOR-LF-CC (Basic Oxygen Furnace-Vacuum Arc Refining/Vacuum Oxygen Refining- Ladle Furnace-Continuous Casting) route. Casting duration of these grades was high compared to normal grades owing to speed restriction (max 0.75m/min.). The casting speed had been kept low due to fear of sticker type breakouts, which are normal for all high carbon grades. There was no separate mould powder exclusively being used for high carbon grades. Low carbon (LC) powder is normally used for casting of these grades. The powder melting as well as consumption was on the lower side compared to normal grades.
Casting of high carbon grades requires special attention particularly with respect to powder characteristics in terms of physical as well as melting properties. These grades require high mould heat transfer due to low shell growth due to austenitic mode of solidification with high micro segregation, which has lower shell strength. In addition to this, sticking tendency in mould is very high due to lower shrinkage characteristics and lower powder consumption in mould. Mould powder film stability is lower leading to higher

friction, if normal low carbon powder is used. The liquidus temperature of these grades is much lower compared to low carbon grades. In addition, the solidification range is wider, which leads to columnar mode of solidification.
In Japanese Patent Application No. 3-193248 discloses a method of incorporating into mold powder, as accelerators of crystallization, oxides of elements belonging to Group MA and Group IV such as ZrO2, TiO2, Sc2 O3, and Y2 O3. Further, according to this invention a high casting speed is achieved if the viscosity of a molten slag is lowered to 1 poise or less at 1300° C. In the course of being cooled, the molten slag produces crystals which mildly cool the slab surface within a mold. This mild cooling of the slab surface makes the cooling rate of the slab surface uniform.
Japanese Patent Application No. 5-15955 proposed lowering the viscosity of a molten slag and increasing the ratio by weight of T.CaO to SiO2 (T.CaO/SiO2). In this method, T.CaO represents a sum of the amount of CaO contained in the mold powder and the amount of CaO converted from Ca-containing matter assumed to be present in the mold powder in the form of CaF2.
The document EP 0 899 041 A discloses mold powder for continuous steel casting, containing CaO, SiO2 and fluoride as the main components. The ratio by weight of CaO', represented by the following equation, to SiO2 is 0.9-2.8 and the CaF2 content is about 5-60 wt. %. The powder contains Na2O in an amount of 0-25 wt. % and C in an amount of 0-10 wt. %. Also disclosed a method for continuous steel casting which makes use ofthe mold powder.
The document US 6125919 discloses a mold powder for continuous casting which enables manufacture of a slab having excellent surface quality in the casting of steel at a high casting speed. The present invention also

relates to a continuous casting method using the mold powder of this invention.
The document US 6315809 mold powder characterized in that, a weight ratio of CaO to Si02 in the mold powder is within a range of 0.50 to 1.20, the mold powder contains one or two or more species selected from a group consisting of oxides, carbonates, or fluorides of alkali metals, alkaline earth metals, or other metals, and 0.5 to 5 percent by weight of carbon powder, Li20 content is within a range of 1 to 7 percent by weight, F content is within a range of 0.5 to 8.0 percent by weight, crystallization temperature is within a range of 1000 to 1200C0, surface tension at 1300Co is 250 dyne/em or more, and a relationship between viscosity eta (poise) at 1300Co and casting speed V (m/min) satisfies a range represented by an expression 6.0.ltoreq.Eta.V.ltoreq.100.0. By the present invention, there is provided a mold powder which enables stable casting by reducing the likelihood of powder entrapment into the mold without giving rise to surface crack when casting with a thin-slab continuous caster.
Mould Oscillation enables healing of the early fragile solidifying shell. It also helps in consistent supply of mould slag into mould-strand gap for reducing friction. Deficiency of these gives rises to increase in friction in the mould and may lead to breakout in extreme situation. High speed casting of high carbon grades requires higher mould powder consumption to reduce mould friction. Mould oscillation frequency in addition to powder properties plays a vital role in deciding the powder consumption and thereby mould friction.
In view of above, a new mould powder having higher melting characteristics, improved mould heat transfer characteristics and better lubrication characteristics is essential for casting at higher speed. Development of a new powder and optimization of mould oscillation

frequency utilising on-line mould friction measurement was carried out to increase casting speed for high carbon grades.
BACKGROUND OF INVENTION
Mould powders play an important role in the stability of the continuous casting process of steel at all casting speeds. The main functions of mould slag (i.e. molten powder) are to provide sufficient lubrication and to control the heat transfer in horizontal direction between the developing steel shell and the copper mould.
The physical properties of mould powders are directly related to the chemical and mineralogical composition i.e. the raw material choice of a mould powder. Thus, the work will concentrate on the translation of process data from the caster to physical properties and to the chemical and mineralogical requirements.
Mold powder is a synthetic slag which is continuously fed onto the liquid pool surface during casting. The powder melts and flows down between the mold walls and the strand shell. Choosing the right mold powder is a critical choice to ensure a good enough surface quality of the cast material. The chosen powder primarily influences oscillation mark depth and mold powder consumption.
The function of casting powders is to: act as a lubricant between strand and mold; improve heat transfer from strand to mold; provide thermal insulation of the top surface of the molten pool; protect liquid steel against reoxidation; and absorb inclusions that rise to the metal surface.
Mold powder consumption depends not only on the chosen type of mold powder but also on the oscillation settings and casting speed. Since the

molten mold powder is pumped by the oscillating movement of the mold into the mold/strand gap the oscillation settings have an essential influence on the mold powder consumption.
Mould powder properties for high carbon steel:
SMS-I of Rourkela Steel Plant produces high carbon grades of steel through the BOF-VAR/VOR-LF-CC route. Casting speed of these grades is restricted to maximum of 0.75 m/min. The typical chemistries of the two grades are shown below.

The mould powder for low carbon is generally being used for casting these high carbon grades. The specification of the powder is given in TABLE-I & II.


Table-ll: Physical Properties of Mould Powder for low carbon grade
Basicity CaO/SiO2 0.86
Viscosity [Pa*sec] IRSID (ex. 0.12at1300oC
RIBOUD)
Melting characteristics Softening point 1040oC
Melting point 1085°C
Flow point 1125oC
Characterisation of existing powder
Mould Oscillation enables healing of the early fragile solidifying shell. It also helps in consistent supply of mould slag into mould-strand gap for reducing friction. Deficiency of these gives rise to increase in friction in the mould and leads to surface defects and breakout in extreme situation. Mould friction during casting was measured through tri-axial accelerometer-based system. The friction behaviour during casting of HC grades with existing powder and oscillation frequency of 100xcasting speed can be seen in Fig. 1 at casting speed range of 0.7 - 0.8 m/min.
In view of above, a new mould powder having higher melting characteristics, improved mould heat transfer characteristics and better lubrication characteristics is essential for casting at higher speed. Development of a new powder and optimization of mould oscillation frequency utilizing on-line mould friction measurement was carried out to increase casting speed for high carbon grades.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a new chemistry of mould powder having higher melting characteristics, improved mould heat

transfer and lubrication to enhance casting speed for high carbon grades of steel.
It is another object of the invention to reduce the melting point of the mould powder to enhance powder melting rate for higher powder consumption.
It is another object of the invention to increase powder viscosity to improve slag film stability between strand and mould at higher casting speed to avoid sticker formation.
It is another object of the invention to reduce the basicity of the mould powder to achieve higher heat flux.
It is another object of the invention to propose modification of casting parameters for improved lubrication at higher speed for high Carbon grades.
It is further object of the invention to propose modification in oscillation parameters for reduced friction with improved powder consumption for improved lubrication at higher speed.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1: Frictional values of four sensors with variation of casting speed from 0.8 to a.7m/min.
Fig. 2(a): Mould strand frictional value at 0.9 m/min
Fig. 2(b): Mould friction values at 1.0 m/min
Fig. 3: Lower frictions at low frequency of oscillation

DETAILED DESCRIPTION OF THE INVENTION
General principle of preparation of mould powder is based on Si02-CaO-Al203 system of the low melting point, low viscosity zone-based, and with the amount of Na20, CaF2 and other parameter adjustments. And compared with ordinary continuous casting fluxes, high-speed continuous casting fluxes require a lower viscosity and lower melt temperature. Common mold powder for general use Na20, CaF2 and so help protect the financial agent to reduce the viscosity and melting temperature of slag, but to meet the requirements of high-speed continuous casting, with increasing Na20, CaF2 is not desirable. In addition, high levels of fluoride intake can also cause serious erosion of the invasion. Adding high-speed continuous casting mold powder slag CaO can significanlly reduce the viscosity. At the same time, protect the basicity increase, but also accelerated the dissolution flux, the absorption rate of oxide inclusions. However, with the temperature, the viscosity of alkaline flux varies greatly. Near the liquidus, the crystaliizaiion of the flux capacity enhancemen,, there has been crystal precipitaiion, serious damage to the glass of the slag. Alkaline flux increases, ehe crystal precipitaiion temperature will lead to increased tendency to crystaliize increases. The use of such flux, the frictional resistance will increase the mold, increasing the probability of occurrence of breakout. Lower the melting temperature of base material and the protection of slag basicity, reducing the amount of solvent added, the inhibition of flux tendency to crystaliize, to prevent leakage of steel is very effective.
Therefore, high-speed mold powder in the added amount of CaO should be controlled in a more appropriate range, can effectively reduce the viscosity of flux. High-speed continuous casting mold in a certain amount of Li20 and MgO, also reduce the viscosity of fluxes and softening temperature. Early in this way can reduce crystal precipitation temperature, the expansion of the liquid slag lubrication zone, and reduce the alkalinity of slag to reduce fluoride content in the slag. And BaO, 8203 for reducing the viscosity and softening

temperature of the slag, inhibition of crystal precipitation, increasing the consumption of flux with a certain role. Casting machine casting speed, mold powder viscosity and slip rate of the negative mold and other factors, a great influence on the mold friction.
In the casting speed, viscosity and increases the negative value of slip rate, mold friction increases. Meanwhile, the high viscosity of the flux residue consumption will reduce the amount of slag flow decreases, so that the slag film thinning and uneven, the results will increase the frictional resistance of mold. Initial solidification solidified shell by additional external force, can cause defects in billets, in severe cases may lead to breakout. Reduce the viscosity of mold powder, softening temperature and crystallization temperature of slag increased consumption, reduce mold friction is very effective.
As a high speed continuous casting, casting speed, casting speed changed greatly, making a significant reduction in the consumption of flux. Flux should be good at high casting speed and casting speed change can be maintained under a large enough advantage of slag consumption. Therefore, the protection required for high-speed continuous casting of molten slag should have a faster speed, to protect the slag layer thickness sufficient to meet the filling into the mold and solidified shell membrane between the slag consumption needs. However, the melting rate of mold powder is not as fast as possible, melting too fast is bound to affect their insulation performance, easy to cool the skin and cause skin to form slag and more depth of oscillation marks and other defects. When the slag layer thickness of 10 mm to 30 mm, the flux is generally believed that the melting rate more appropriate. In general, the melting rate by the flux of carbon to adjust. Carbon type, dosage, method of particle size and distribution of carbon flux in the melting rate and melt structure greatly. Adding the higher the carbon content of carbon materials, the greater the dispersion, the role of melting

rate, the stronger, so the role of carbon black is much stronger than graphite. Greater dispersion of carbon black, the lower ignition temperature,at a lower temperature can take advantage of the role of spacer material particles.
Common mold powder slag layer is generally divided into powder layer, transition layer and the slag layer 3 layer, while the High Speed continuous casting mold slag structure with 4 layers, namely, numerous slag layer, sintering layer, semi-molten layer and the slag layer. Semi-molten layer should have sufficient thickness, so that one can increase the flux of the thermal insulating properties, on the other hand to ensure the continued provision of slag to the slag layer drops.
When the High Speed or relatively large changes in casting speed, the slag layer thickness should be able to maintain adequate to ensure the continuous flow and to prevent flow of solid residue slag particles depends on the melting rate of carbon residue in the mixed material. Between carbon and the base material because of high interfacial tension, so difficult to be slag carbon absorption, infiltration and assimilation. For the general type of carbon materials and carbon graphite-based ink, the former dispersion of small, high ignition point, the lower the excess surface free energy, which the slag at high temperatures can act as a good framework; the latter a larger dispersion, low ignition point can be at low temperatures from the skeleton. Made of carbon composite with the melting rate increases with the change of carbon content in slag to change very gently. Melting in the heat flux, the melting point will occur more or less the phenomenon. This may make the slag powder residue off in time and space, the average composition of slag particles, thereby resulting in local changes in slag significant traits that cause defects in the slab surface.

Formulation of New powder for High-C steel casting
A mould powder has been formulated to enhance caster speed with improved mould lubrication for high carbon grades. The melting point of mould powder has been reduced to enhance powder melting rate for higher powder consumption. The powder viscosity has been increased to improve slag film stability between strand and mould at higher casting speed to avoid sticker formation. Basicity of the mould powder has been reduced to achieve higher heat flux. The chemistry and physical properties of the powder is given in TABLE-III & IV respectively.
Table- III: Chemical Analysis of formulated mould powder
% %
SiO2 34.58 K2O 0.30
CaO 25.82 F 10.41
MgO 5.18
AI2O3 5.33 C-free 7.31
TiO2 0.25 CO2 5.48
Fe2O3 1.52 Ctot 8.80
MnO2 0.11
B2O3 0.0 LOI 15.28
Na2O 6.78 H2O total 0.12
The physical properties of the modified mould powder are shown in Table- IV. Table-IV : Physical Properties of SYNTHERM AGS 712/H1
Basicity CaO/SiO2 0.75
Viscosity [Pa*sec] IRSID (ex. 0.14at1300°C
RIBDUD)
Melting characteristics Softening point 1045°C
Melting point 1048°C
Flow point 1090oC

Results of high speed trial with new mould powder and modified oscillation frequency
High speed trial of high carbon grade slabs were carried out using new mould powder and modified oscillation frequency. The mould friction and powder consumpiion were measured. The casting speed was kept in the range of 0.9 - 1.0 m/min. The superheat of all the heats was kept to <40 deg. C and secondary cooling was kept at 1.0 L/kg.
The friction index measured using new mould powder is shown in Fig. 2(a) &2(b).
Comparison of friction data of earlier and new powders cJearlyshowdd that mould friction index was lower and consistent Fig. 2(a) & (b) compared to earlier powder (Fig. 1) indicating improved lubrication behaviour with new mould powder even at higher casting speed (1.0m/min.). This had been corroborated by improved mould powder consumpiion (0.45 kg/m2) with new mould powder compared to existing mould powder (0.35 kg/m2).
In addition to the formulation of new mould powder, mould oscillation frequency (cycle/min.) has been reduced from 100xcasting speed to 90xcasting speed. On-line friction measurement data showed reduced mould friction with lower oscillation frequency (Fig. 3).
Usefulness of the Innovation:
(A)Technical/Technological: The innovative technique of development of mould powder and modification of oscillation parameters through on¬line friction measurement could be used for increasing casting speed

for high carbon grades in slab casters of India and abroad for high speed casting of high carbon grades.
(B)Economcc:The development of powder and modification of oscillation frequency led to improvement in productivity of the caster in terms enhancement of casting speed for high carbon grades.
(C) Impact in industry:The innovative technique of developing grade specific mould powder and oscillation frequency has the capability to increase casting speed of any grade of steel resulting in improvement of caster productivity.
Different embodiments of the invention are possible to achieve the best method of performance and to obtain the product as stated above to meet the object of the invention. It will be understood that skilled persons with many modifications, variations and adaptations may carry out the invention into practice without departing from its spirit or exceeding the scope of claims in describing the invention for the purpose of illustration.

WE CLAIM:
1. A mould powder composition to enhance casting speed for high.carbon grades of steel containing 34.58% wt of Si02, 25.82 % wt of CaO, 5.18% wt of MgO,5.33 % wt of Al2 O3,0.25 % wt of Ti02, 1.52% wt of Fe2O3, 0.11 % wt of MnO2, 6.78 % wt of Na2O,0.30 % wt of K20, 10.41% wt of F, 7.31% wt of C-free,5.48 % wt of CO2, 8.80% wt of C total,15.28 % wt of LOI and 0.12% wt of water.
2. A mould powder composition to enhance casting speed for high carbon grades of steel as claimed in claim 1, wherein the mould powder is having higher melting characteristics, improved mould heat transfer and lubrication to enhance casting speed for high carbon grades of steel.
3. A mould powder composition to enhance casting speed for high carbon grades of steel as claimed in claim 1, wherein the ratio of CaO to SiO2 is 0.75.

4. A mould powder composition to enhance casting speed for high carbon-grades of steel as claimed in claim 1, wherein the viscostty of the composition is0.14Pa*sec at 1300°C.
5. A mould powder composition to enhance casting speed for high carbon grades of steel as claimed in claim 1, wherein the softening and the melting point of the composition is 1045°C and 1048°C respectively.
6. A mould powder composition to enhance casting speed for high carbon grades of steel as claimed in claim 1, wherein the flow point of the composition is 1090°C.
7. A method for continuously casting steel by use of mould powder composition to enhance casting speed for high carbon grades of steel containing 34.58%

wt of SiO2, 25.82 % wt of CaO, 5.18% wt of MgO.5.33 % wt of AI2O3,0.25 % wt of TiO2, 1.52% wt of Fe2O3, 0.11 % wt of MnO2l 6.78 % wt of Na2O,0.30 % wt of K2O 10.41% wt of F, 7.31% wt of C-free,5.48 % wt of CO2, 8.80% wt of C total,15.28 % wt of LOI and 0.12% wt of water.
8. A method for continuously casting steel by use of mould powder composition to enhance casting speed for high carbon grades of steel as claimed in claim 7, wherein the ratio of CaO to SiO2 is 0.75.
9. A method for continuousyy casting steel by use of mould powder composition to enhance casting speed for high carbon grades of steel as claimed in claim 7, wherein the viscostty of the composition is 0.14 Pa*sec at 1300° C.
10.A method for continuously casting steel by use of mould powder composition to enhance casting speed for high carbon grades of steel as claimed in claim 7, wherein the softening and the melting point of the composition is 1045°C and 1048°C respectively.
11. A method for continuously casting steel by use of mould powder composition to enhance casting speed for high carbon grades of steel as claimed in claim 7, wherein the flow point of the composition is 1090oC.
12.A mould powder composition and the method for continuously casting steel by use of mould powder composition to enhance casting speed for high carbon grades of steel substantially as herein described.

ABSTRACT

The present invention relates to a mould powder having higher melting
characteristics, improved mould heat transfer and lubrication to enhance
casting speed for high carbon grades of steel. The present invention also
relates to modification in oscillation parameters for reduced friction with
improved powder consumption for improved lubrication at higher speed.