WO2021090336A1 - Innovative cored wire with two or more steel strips cladded together - Google Patents

Abstract

This invention relates to 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. The main object of this invention is to improve the efficiency of cored wire. This is by utilising the natural insulation formed while multiple steel strips are cladded together to manufacture the cored wire tube containing the filler material. The cored wire consists of the filler material covered with two or more steel strips cladded together and the air gaps between steel strips act as an insulation. The width of steel strips may vary between 15 mm and 120 mm and thickness of the steel strips may vary between 0.10 mm and 1.20 mm.

Description

INNOVATIVE CORED WIRE WITH TWO OR MORE STEEL STRIPS

CLADDED TOGETHER

FIELD OF INVENTION: This invention relates to increase in the efficiency due to cladding of one or more steel strips over another which acts as natural insulation due to air gap in between and thereby melting of filler material is delayed.

The cored wire consists of filler material covered with one steel strip and cladded with one or more steel strips over the first strip.

BACKGROUND INFORMATION:

Steel making is essentially an oxidation process where the impurities of molten metal are preferentially oxidized to join the slag along with fluxes. The major challenge to steel makers is to reduce oxygen and inclusions, which are formed due to subsequent de-oxidation as they may remain intact after casting process & are thus detrimental to the product quality.

The use of calcium is beneficial but its introduction in liquid steel bath is very difficult due to its low density and high vapour pressure. The advent of cored wire injection technology and development of calcium bearing material like calcium- silicide, calcium-iron, pure calcium cored wire etc. have enabled steel plant operators to introduce calcium efficiently in steel bath. A cored wire is a continuous steel tube filled with either a calcium bearing material or a ferroalloy material. The steel tube is usually made of a single strip. In general thickness of steel strip used in industry is 0.3-1.2mm for cored wire.

The efficiency of calcium treatment of steel melt is measured by the yield (or recovery) of calcium after cored wire injection. This yield of calcium can be defined as the ratio of calcium retained in steel 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. The low yield of calcium is due to its high vapour pressure and low boiling point (1439°C). As this temperature is lower than the working temperature of liquid steel, calcium vaporizes as soon as it is released in the liquid steel.

Thus, the calcium yield and its consistency are of paramount importance to the steelmakers for producing high quality steel with least cost. The present invention aims at improving the calcium yield and its consistency and at the same time reduce the cost of cored wire to enable steelmakers with an efficient and cost effective calcium treatment process.

SUMMARY OF THE INVENTION:

The main object of this invention is to improve the efficiency of cored wire. This is by utilising the natural insulation formed while multiple steel strips are cladded together to manufacture the cored wire tube containing the filler material. In other words, in the present invention, the cored wire consists of the filler material covered with two or more steel strips cladded together and the air gaps between steel strips act as an insulation. The width of steel strips may vary between 15 mm and 120 mm and thickness of the steel strips may vary between 0.10 mm and 1.20 mm.

It is well established that the utilization of calcium and other additives is maximum when the material is released form the cored wire very close to the bottom of the ladle. It ensures reduced losses through the undesirable reactions like early vaporisation, due to insufficient depth of penetration in the ladle and hence loss to the atmosphere in unreacted condition, and also loss of filler material through reaction with ladle top slag are kept at a minimum. The zone of release of the material in this invention is close to the bottom of the ladle due to cladding of steel strips. There will be air gaps between steel strips which will act as natural insulation and effectively reduce the conductive heat transfer and thus delay the melting of filler material. This enables delayed melting of the cored wire and dispersion of the filler material at a higher depth inside the liquid steel bath. DETAILED DESCRIPTION OF THE INVENTION:

Calcium addition in liquid steel is done at the end of secondary treatment process i.e. either in ladle furnace or in vacuum degassing unit. The major functions of cored wire are modification of inclusion morphology, deoxidation & desulphurization. Cored wire is used for calcium treatment and for chemistry adjustment to suit the requirement of the next processing unit i.e. casting unit. Calcium treatment has of late become an essential feature of liquid steel processing. Cored wire is injected into the ladle through wire feeder which is capable of feeding cored wire at a very controlled rate into the steel-melts. The efficiency of calcium treatment of steel melt is measured by the yield (or recovery) of calcium after cored wire injection. This yield of calcium can be defined as the ratio of calcium retained in steel to the amount of calcium injected. It is well established that the yield of calcium and other additives is maximum when the material is released from the cored wire very close to the bottom of the ladle.

In the present work, the innovative design of the cored wire coupled with the prescribed injection speed ensures that the material is released close to the bottom of the ladle. The cladding of steel strips creates one or more air gaps, depending on number of steel strips cladded, between them. These air gaps between steel strips act as natural insulation and effectively reduce the conductive heat transfer and thus delay the heating up of the wire. This enables delayed melting of the cored wire and dispersion of the filler material at a higher depth inside the liquid steel bath.

In addition, in the current invention different widths of steel strips have been planned for inner and outer layers; inner layer(s) being always of lower width. Similarly, use of different thickness of steel strip is also being suggested. The width of inner steel strip(s) has been planned such a way that it covers the filler material to a maximum extent without covering the area of filler material which is lying just below the locking zone of the outer most strip. By leaving a gap for locking area which is covered by the locking of 3 folds of outer strip thereby enabling near uniform cross section of the coil without getting overlapped on the inner strip. This modification, for instance in one such design where two steel strips are cladded, allows over 10% reduction in steel consumption in comparison to conventional steel strip used in cored wire. For this study, the ratio of thickness between the inner and outer steel strips is equal to one. The reduction in steel strip consumption reduces the cost of the cored wire.

1. Industrial Trials:

Industrial trials successfully conducted in low carbon grade and in medium carbon grade. The recovery obtained by using the innovative cladded steel strip of cored wire is shown in figure-1.

Figure 1 describes recovery inlow carbon (LC) and medium carbon (MC) grades of steel for the innovative cored wire over conventional cored wire.

Ladle size is in between 150-250MT. In both grades there is over 10% improvement in recovery than conventional product.

Therefore, recovery is more in case of new innovative product than conventional product.

Due to cladding of steel strips there is natural insulation provided in between which effectively reduce the conductive heat transfer and thus delay the melting of filler material thereby enabling the deeper penetration of the cored wire in the molten metal bath before its own melting and dispersion. This innovative cored wire has increased the recovery over 10% at customer end which has given significant savings in calcium treatment cost and also helped them to reduce the consumption of cored wire by 10% which reduced the transportation cost thereby reducing carbon footprints.

SPECIFICATION:

> Finished Cored wire diameter: 4 mm - 25 mm,

> Steel strip thickness:0.10 mm - 1.20 mm

> Steel strip width: 15 mm - 120mm

> Number of strips - 2 to 4

Claims

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 a cored wire consists of filler material covered with one steel strip and cladded with one or more steel strips over the first strip.

3. The process as claimed in claim 2, wherein the dimensions of said cored wire are 20-50 mm inner strip width and 0.1-1 mm inner strip thickness.

4. The process as claimed in claim 2, wherein the dimensions of said cored wire are 30-100 mm outer strip width and 0.1-1 mm outer strip thickness.

5. The process as claimed in claim 2, wherein due to the change in number of steel strips used for producing cored wire there is reduction of steel consumption during manufacturing by 20%.

6. The process as claimed in claim 2, wherein due to the change in number of steel strips used for producing cored wire there is improvement in yield.

7. The process as claimed in claim 5 and claim 6, wherein due to reduction of steel consumption during manufacturing of cored wire by 20% and improvement in yield at customer end will help to minimize carbon emission and contribute towards our efforts in achieving sustainability goals.

8. The process as claimed in claim 2, wherein due to the change in number of steel strips there is natural insulation in between steel strips.