8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 1/11

SUBMITTED BY: ARPIT

08108047

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 2/11

The purpose of the Basic Oxygen Steelmaking (BOS) process is to refinethe hot metal produced in the blast furnace into raw liquid steel, which

may be subsequently refined in the secondary steelmaking shop.The main functions of the Basic Oxygen Furnace (BOF) are to

decarburize and remove phosphorus from the hot metal, and to optimizethe steel temperature so that any further treatments prior to casting can

be performed with minimal reheating or cooling of the steel.The exothermic oxidation reactions that occur during BOS generate a lot

of heat energy - more than is necessary to attain the target steeltemperature. This extra heat is used to melt scrap and/or iron ore

additions.Typical compositions and temperatures of the hot metal charge and

tapped steel are as follows:

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 3/11

•Relevance and relation of carbon removal with oxygentransport/diffusion to gas metal interface

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 4/11

The reaction of oxidation of carbon practically does nottake place at the slag metal interface because of thedifficulty in nucleating gas bubbles there.

The reaction takes place at the gas metal interface sinceit eliminates the necessity of nucleation of gas bubble.

The decarburisation rate (dc/dt) is controlled by the

rates of diffusion of either carbon or oxygen to the gasmetal interface.

In the bottom blown bessemer converters a continuousseries of fresh air bubbles pass through the melt. It

results in increasing the total area of gas-metalinterface enormously.

The turbulence caused by the bottom blown operationfavours the mass transport.

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 5/11

•Mechanism of oxygen transport from the gas phase tothe gas metal interface

•Kinetics of carbon oxygen reaction

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 6/11

Pneumatic Processes: In pneumatic processes oxygen is readily present at the

gas metal interface.

The rate of reaction is affected by diffusion of carbonto the interface.

The stirring of bath caused in these processes help tohasten carbon transport and thus decarburisation rate

upto 10% per hour are obtainable in these processes

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 7/11

 A bessemer convertor

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 8/11

Hearth processes: Oxygen from the furnace atmosphere has to diffuse

across the slag and the metal layers to reach the ore-metal interface.

The physical solution of oxygen in slag is negligible. Itmust dissolve in slag in ionic form.

The mechanism of oxygen transport from the gas

phase is described below: 

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 9/11

 At the gas slag interface oxygen dissolves as:

½{O2 } + 2e- = (O2-)

 And the iron in the slag gets oxidised as:

  2(Fe2+) = 2(Fe3+) + 2e-

  so the overall reaction is:

2(Fe

2+

) + ½(O2 ) = 2(Fe3+

) + (O2-

)

 Due to thermal diffusion these migrate from gas-slagto slag-metal interface and a reverse reaction as:

 2(Fe3+) + (O2-)= 2(Fe2+) + [O2] takes place.

The ferrous ions again migrate to the gas-slag interfaceand complete the complete the cycle which repeatsitself. The oxygen dissolved in the metal diffuses to thepore-metal interface and combines with carbon.

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 10/11

The process of diffusion of oxygen can be hastened by eliminating the diffusion path across the slag layer by adding lumps of ferric oxide which, being heavier than

the slag but lighter than the metal settles down at theslag metal interface.It supplies oxygen as :

<Fe2O3> = 2(FeO)+[O]

 Which is an endothermic reaction.It is known as ‘oreingof slag’. 

In absence of oreing when oxygen solely comes fromfurnace atmosphere, under the usual conditions of open

hearth practice, a maximum dc/dt of 0.12-0.18% C/hr isattainable.

If oreing is carried out to its maximum extent the ratecan increase upto o.6 % C/hr .The limit is due to the

inadequate heat transfer across the slag layer since the

8/4/2019 8047 Arpit Arora - Steel Making

http://slidepdf.com/reader/full/8047-arpit-arora-steel-making 11/11

2/3<Fe2O3> + 2[C] = 4/3[Fe] + 2{CO} ; H = +65Kcal

Is endothermic in nature

If low pressure oxygen is sprayed on the slag surface theabove endothermic reaction is replaced by 

[C]+[O]={CO} ; H=-89Kcal With a net heat gain and hence dC/dt upto 3% C/hr have

been obtained. Higher dC/dt values can be obtained inhearth processes by oxygen lancing of the bath during

refining period. This is the principle of the modified hearth processes of 

steel making.