Charge Materials

The LD vessel charge consist of molten metal, cold pig iron, steel scrap, calcined lime and dolomite, iron ore, fluorspar and pure oxygen of 99% purity.

Hot Metal :

● LD process is often used to make soft steels for flat products with general requirements for extra low sulphur specification necessitated a constant reduction in sulphur contents of the final steel product. The continuous casting also need that the sulphur should be lower below 0.02% to produce transverse crack free product.
hot metal terpedo - ld process raw materials
hot metal terpedo

● In special steel plates the normal sulphur specification is 0.01% and there is demand for extra low sulphur steels with as low as 0.001% coupled with very low carbon
Ex : The pipe Line tube steels.

● The oxidising condition of LD process are not conductive to desulphurisation. The external desulphurisation of molten iron in transfer ladles is adopted that is HMDS(hot metal de-sulphurisation) in which calcium carbide and magnesium base mixture are injected separately or together to obtain sulphur content below 0.001%.

● The silicon content of such iron must be as low as possible preferably below 0.65% but usallus below 1.2%. Every extra bit of silicon required extra lime to make slag of the required basicity, and then salg volume goes up with silicon content.

● A certain manganese content results in enough manganese in the bath prevent the over oxidation of bath. A high manganese in slag also tends to retard dephosphorisation of the bath. Manganese content in the range of 0.5 to 1.1% are tolerated but 0.7% is considered better for steelmaking.

● And at last carbon levels in iron up to 3.5 to 4.5% is better for steel making. The temperature of the hot metal at the time of charging is usually around 1250 to 1350℃. The propertion of hot metal in the charge varies in the range of 75 to 90% that is 10 to 25% scrap depending upon the availability.
Cold Pig Iron :
● It may be necessary to include cold pig iron in the charge to consume the excess of pig iron produce in the plant. It can be included along with the scrap. Granulated cold pig iron however tends to settle down at the bottom and it doesn't come up until toward the end if refining.

● It causes reaction to take place while tapping the vessel that may even necessitate a small reblow. Cold pig iron tends to prolong the blow by disturbing the usual blow characteristic of the scrap plus hot metal charged and hence is neither preferred by operator nor recommend.

Fluxes :

● The reason to preferred lime as a flux instead of limestone is straightway available for fluxing and formation of slag. If limestone is added as a flux, lime will available only after calcination that is that the slag formation will be delayed.
ld process fluxes
ld converter fluxes

● Efficiency and economy of the process very much depends on the formation of thin, oxidising and basic slag as early as possible during the blow. Use of silica and fluorspar as a flux is bring lime quickly in solution. But in plants use dolomite to help early formation of slag, which may be useful in safeguarding the lining of the vessels but it adds to one more item to be handled on the shop.

● Line reactivity is an important parameters to judge its quality as a first approximation in term of particles size. A lumpy line is slower to dissolve in the slag than the finer powdered lime. Conversely fine powder tends to fly off above the slag. A small granular from of lime is thus best in used.

● Lime consumption varies around 2 to 5% of the weight of the metal charge. The amount to fluorspar, silica added as flux very little amount on the local condition which, do not exceed more the few kg/ton of steel.

Scrap or Iron Ore :

● These are used as a coolant to best utilize the excess heat energy available during refining. LD process can take up to 25% of the metal scrape. Some case it used to hasten the formation of the early slag. Iron ore can act as a coolant but is not preffered because the available of excess thermal energy can be econimicalle used in melting maximum load of circulating scrap in an steel plant.

● If scrap is not available iron ore is added as coolant to the tune up to 100kg per tonne of steel. In its cooling effect 1% ore in the charge is nearly equivalent to 3.5% steel scrap.

Oxygen :

● During the decarburization of the bath form 4.5% to nearly 0.12% C. The nitrogen level remains the same for a given purity of gas. But below 0.12% C the nitrogen level increases, if oxygen purity is below than 97%, Where as it decreases if purity is above 99%.

● Hence oxygen of 99.5% purity is always desirable to keep nitrogen level finished steel below 0.0016%. It finally gives nitrogen content below 0.003% in the steel ingots.

● The consumption of oxyegn per tonne of steel made, varies with proportion of scrap and ore added as coolents, and also with single and multi hole Lance desings.

References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty

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