From Calcium Powder Injection to Calcium-Silicon Cored Wire Injection.

Powder injection developed in the 1970’s, but had frequent problems due to slopping and excessive reactivity, hydrogen pickup and unreliable operations due to lance plugging.

Use of calcium silicon cored wire quickly displaced powder injection due to numerous operational and cost advantages.

Studies indicate that in powder injection the powder particle loses momentum rapidly after leaving the gas jet. As a result the calcium reaction takes place in an area of the ladle with highest vertical velocity and the particles may actually be entrapped in the transport gas. These issues with powder injection, and the relative ease of use with calcium silicon cored wire quickly resulted in numerous studies and a good understanding of the use of calcium silicon cored wire became widespread.

What is different in thin slab casting is the frequent necessity for the addition of calcium without silicon due to metallurgical requirements for galvanized and drawing quality flat rolled steel. Thin slab casting’s requirements resulted in a need for a new process – one which can quickly and consistently add pure calcium (without silicon or carbon) in a cost effective manner.

Adding Ca-Fe

At the inception of the thin slab casting era, the predominant pure calcium cored wire product was a mixture of calcium particles and iron powder.

Iron or steel powder is added to calcium powder to reduce the reactivity of calcium by acting as a heat sink of inert particles; these wires were able to work but had several distinct disadvantages.

Due to the large differences in shape, size and density between calcium and iron powders, it is extremely difficult to provide a consistent product which will achieve reproducible results.

The calcium particles are frequently carried out of the ladle by gas bubbles prior to reacting with the steel resulting in low recovery and high reactivity.

The necessity of cladding and using large amounts of inert filler material (typically 70% of the fill is iron powder) results in relatively high production and treatment costs.

Finally, due to calcium’s natural propensity for oxidation and hydration, particularly as a powder with a large surface to volume ratio, shelf life becomes an issue and a contributor to erratic performance.

Adding Pure Calcium

These requirements – cost, speed and consistency are not unique to thin slab casting but they become more important due to difficulties in adding pure calcium to molten steel.

Challenges

• Despite its beneficial effects in steel, the properties of calcium make its addition a difficult proposition.
• It melts at very low temperatures.
• It has a boiling point below steelmaking temperatures.
• Its density is well below that of steel, it has a low solubility in iron and a relatively high vapor pressure.
• calcium’s natural propensity for oxidation and hydration, particularly as a powder with a large surface to volume ratio, causes large degrees of oxydation and hydration. As such, shelf life becomes an issue and a contributor to erratic performance.

The addition of silicon to calcium (or its presence in steel) serves to increase calcium’s solubility and lower its vapor pressure - thus the relatively easy application of CaSi wire to steel where silicon content is not an issue - but this fact is not the case in the low silicon steels typically seen in thin slab casters.

Manufacturers of calcium metal and cored wires (along with the respective steel plants) have addressed this new opportunity and results show significant improvements in technology along with substantial decreases in treatment cost.