DESC:
FIELD OF INVENTION

? The present invention relates to a cored wire injection process in steel bath with an improvement in the yield / recovery of calcium.
? In particular, it relates to the dimension of the cored Wire and its injection speed in liquid steel bath to inject additives like fluxes, mainly calcium bearing material and alloying additives in molten steel bath.

BACKGROUND INFORMATION
? Steel making is essentially an oxidation process where the impurities of the molten metal 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. The major challenge to steel makers is to reduce oxygen and inclusions, which are formed due to subsequent de-oxidation as they remain intact after casting process & are detrimental to the product quality.
? Calcium usage is beneficial in this direction. However, the introduction of calcium in steel is a challenge to steel makers because of its low melting point, low boiling point and high vapour pressure. Old techniques for calcium injection in secondary metallurgy are manual method, Bullet shooting which resulted in low calcium recovery. New technique for calcium injection in secondary metallurgy is Cored wire which is a continuous steel tube filled with either a calcium bearing material (generally calcium silicide or pure calcium) or a ferroalloy material.

? Calcium cored wire injected into the ladle through wire feeder. It is capable of feeding wire at very controlled rates into the steel-melts. This is the most suitable method to introduce a particular element into the melt while attaining a high degree of homogenization and ensuring its metallurgical effectiveness.
? Calcium injected is lost 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 slag
? Desirable reactions of calcium are calcium reacting with the dissolved oxygen and inclusions present in the steel which join the slag. Some amount of calcium will remain in the steel as retained calcium. Ideally the injected calcium should be involved in the desirable reactions only.
? Calcium enhances the cleanliness and smoothness in continuous casting process. It also takes care of inclusion morphology control and modification. Due to injection of calcium cored wire there is effective deoxidation and desulphurization which is beneficial to the steel melt
? The yield of calcium is defined as the ratio 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 40% 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 lead to big savings in steel making process.

SUMMARY OF THE INVENTION
? The main object of the present invention therefore, is to increase the yield of calcium by altering the dimensions of cored wire during the injection in the steel bath and thereby to reduce the consumption.
? It has been observed that the utilization of calcium and other additives is maximum when material is released from cored wire very close to the bottom of the ladle so that the losses through the undesirable reactions mentioned above can be kept at a minimum. The material is 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, grade of steel processed, treatment temperature and ladle height.
? The main object of the present invention is achieved by ensuring that the additives from the cored wire are released at an appropriate place in the ladle and selecting a suitable speed of injection and appropriate 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 are as follows.

1. Varied diameter of the cored wire along with optimized addition feed rate are selected
2. Along with the above a suitable speed of injection is also selected to ensure that the material is released very close to the bottom of the ladle.
3. The variation in the diameter of wire can be selected from 13 mm to 16 mm
? The exact combination of the diameter and the speed will depend on the grade of steel processed, treatment temperature and the ladle size or liquid column height in the ladle thereby controlling yield of the cored wire

DETAILED DESCRIPTION OF THE INVENTION

? Calcium addition is done in secondary treatment unit i.e mainly in ladle furnace or vaccum degassing unit. The main purpose of the secondary treatment unit is to further refine the steel, adjust the bath temperature and chemistry to suit the demand of the next processing unit i.e. casting unit. The presence of dissolved oxygen and inclusions in the liquid steel poses problem to the smooth operation of casting and also deteriorate the product quality. The calcium treatment of the steel, thus, becomes essential to control the dissolved oxygen level as well as the shape and characteristics of the inclusions. The liquid steel is treated with the calcium and / or other additives bearing cored wires in the secondary processing units.

? The present invention shows that the variety of steel grades a steel shop produces requires varying specification of the cored wire to exploit maximum benefit from it. From the earlier embodiment US7682418B2 it has been already established that, if the additives are released at the maximum possible depth of the bath (i.e. close to the ladle bottom), the maximum benefit can be obtained. To protect the filler material these patents (US7682418B2) describes the experimentation on different thickness of outer sheath material.
? Here in the current embodiment the higher diameter cored wire along with optimized addition feed rate has been experimented.

? 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 additive 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 depend on the amount of heat transferred from the bath to the wire which in turn depend on the heat transfer coefficient only when the superheat and wire diameter are constant. 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; an optimum speed should be chosen based on high calcium vapour pressure and also the ladle size.

Fig.1: Recovery of 16mm cored wire(HD wire) vs 13mm cored wire (MD wire)

HD- Higher diameter
MD- Medium diameter
? 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.
? The ladle size is in between 150-250MT and here medium carbon grades are treated with 16mm CaSi cored wire which gave 10% more recovery than 13mm CaSi cored wire where medium carbon grades are treated
? Therefore recovery is more in case of higher diameter cored wire than medium diameter cored wire.
? Speed of injection should be optimized which may help the wire to travel nearer to the bottom of the ladle before release of the material. In this case 150m/min speed was used for both 16mm cored wire and 13mm cored wire
? However the higher diameter wire(16mm) has more recovery than medium diameter wire (13mm). By usage of 16mm diameter wire steel consumption during manufacturing has been decreased by 200kg/MT when compared to 13mm diameter wire
SPECIFICATION
? Diameter of the cored wire: >13-17.5 mm
? Thickness of steel strip: 0.3-1mm
? Locking: Either seamless or folded single or double lock

EXPERIMENTAL WORK

? Trials have been conducted in a steel plants of which results have been shown [above.] in Figure1. The wires used are the conventional calcium-silicide material bearing wire of 13 mm diameter with sheath thickness of 0.4 mm and 16 mm diameter wire with sheath thickness of 0.4mm was used. and the injection was done at a steel bath temperature of 1590o C in medium carbon grades. Superheat of liquidus + 500C is maintained in this plant, accordingly injection temperature varies for different carbon composition. There is improvement in the yield by changing from 13mm to 16mm which is evident from the Figure 1

,CLAIMS:WE CLAIM:

1. A cored wire injection process for introducing fluxes and alloying additives in liquid steel bath after adjusting bath temperature and the chemistry of liquid steel in a secondary treatment unit according to requirements: characterized in that said additives are released close to the bottom of the ladle by injecting at a predetermined speed a prefabricated cored wire of appropriate dimensions, depending on the grade of liquid steel, treatment temperature and ladle size / liquid column height.
2. The process as claimed in claim 1, wherein dimension of said cored wire are preferably more than 12 mm in diameter and optimum speed is used to suit steel grades of high liquidus temperature and / or treatment temperature in a 160 – 250MT ladle with, 3 - 4.5m liquid column height respectively.
3. The process as claimed in claim 2, wherein the dimensions of said cored wire are 13 mm in diameter and 0.4 mm in sheath thickness
4. The process as claimed in claim 2, wherein the dimensions of said cored wire are 16 mm in diameter and 0.4 mm in sheath thickness
5. The process as claimed in claim 2, the speed of injection used during wire addition of both 16mm and 13mm cored wire was 150m/min
6. The process as claimed in claim 2, wherein due to the change in dimension from 13mm diameter to 16mm diameter cored wire there is reduction of steel consumption during manufacturing by 200kg/MT
7. The process as claimed in claim 2, wherein due to the change in dimension from 13mm diameter to 16mm diameter cored wire there is improvement in yield
8. The process as claimed in claim 1, wherein said additive is a calcium bearing material.
9. The process as claimed in claim 7, wherein said calcium bearing material comprises of calcium-silicide.