STEEL MAKING TAPPING TEMPERATURE AND DEOXIDATION PRACTICE


Type of Steel According To Deoxidation


De-oxidation means removal of oxygen from liquid steel through the various deoxidation agents like aluminum, silicon, Manganese, Titanium, lime etc. From which aluminum and silicon is the best de-oxidiser for steel.

Deoxidation is nessary because, present of oxyegn create blow hole and pinhole or fracture during sheet metal work that is in hot rolling mill. But in some case deoxidation is not necessary or partially.

So that it is divided in to four categories according to deoxidation practice.

1. Killed steel ------ ( a ) aluminum killed
                                 ( b ) silicon killed
2. Semi killed
3. Rimmed steel
4. Capped steel

Killed steel :

Duringsolidification no gas evolution takes place in killed steel. All steel containing more than 0.3% C are killed. In this steel the heat is worked in such a way that by the time C level drops to the required percentage. The refining should be over. Then the heat is blocked by Fe-Si, high Si pig iron or Si-Mn to be blocking drop in O2 reaction.

Semi killed Steel :

It is practically deoxidized steel such that only a smaller amount of gas is evolved durring solidification C contain is the range 0.15 to 0.30%. The required deoxidation is carried out in the furnace by Fe-Mn, Fe-Si of steel in a mould. Generally the top layer solidified when the gas evolution occurs. This type of steel is used to prepare structural shape, plates etc.

Rimmed Steel :

It require lot of gas evolutions during solidification. It contains enough dissolved O2 which is possible in low C steel that is when C is less than 0.15% no oxidation is carried out in side the furnace. For this steel some deoxidation may be done in the laddle or mould itself, by adding Fe-Mn or Am as deoxidizer.

In this ingot the primary and secondary blow hole is known as rim which is the characteristics of Rimmed steel. It has smooth surface which is clean due to brick gas evolution during solidification. This ingots are cast in narrow end up (NEV) mould.


Capped steel :

A variety rimming steel where gas wevolutio is much less brick than inusual rimming steel. Steel is cast in battle shaped NEU moulds. Where the constructed top facilities mechanical capping of the ingot there C percentage is about 0.15%.

DEOXIDATION PRACTICE :


After refining of steel bath can be deoxidized either inside the furnace which called furnace deoxidation or in time of tapping which is known as ladle deoxidation.

●  The deoxidation operation decrease the oxidizing potential of the bath. If the oxidizing refining slag is present in contact with the metal the deoxidation may result in reversion of the refining reactions.

●  In an acid steelmaking process silica and manganese oxide,being stable oxides,but in a basic process P2O5 from the slag is very readily reduced back to the metal bath if the oxygen potential drop.

●  In a basic process however the refining slag containing P2O5 should be removed as far as possible before deoxidizing the bath inside the furnace.In a basic process the deoxidation can alternatively be carried out in the ladle.At times a majority of the refining slag is flushed off in a basic process and the deoxidation may be carried out partly in the furnace and major part in the ladle.

●  Depending upon the extension of deoxidation there are three different types of steel ingots are teemed i.e. killed or fully deoxidized, semi killed or balanced and the rimming steel. If the carbon content is less than 0.15% the liquid steel containing enough dissolved oxygen and then rimmed variety can be produced.

●  The rimming steel are not deoxidized because dissolved oxygen is essential for rimming action during solidification of steel in ingot moulds. If the carbon is more than 0.16%, the level of dissolved oxygen rapidly fall down with increasing carbon content. The low carbon steel may be partially deoxidized to produce semi killed or balanced variety of steel ingots.


TAPPING TEMPERATURE :


The tapping temperature of any refined steel depends on mount of superheat in the steel melt and on several factor that are

1. Chemical composition of the melt.
2. Amount of addition like recarburization, deoxidation and alloying addition to be made in ladle.
3. Holding time of the melt in the ladle before teeming starts.
4. Teeming practice as perferapre.


●  Basically steel melt in the ladle should be at the required teeming temperature, which is at least 30 - 70℃ above it's liquidious temperature. This temperature drop during the entire teeming period is worked out by computing the temperature drop during holding period and the entire teeming period with the total addition in the ladle.

●  At the higher side of the tapping temperature is conditioned by the process of refining and the refractory used in the furnace. Similarly the temperature loss due to the cold alloying addition for the usual variety of addition under idealised adiabatic condition.

●  So only low alloying steels can be satisfactorily made by ladle addition alone. If extra superheat is not available even for small addition preheated addition can be considered as a way out.

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