The BASIC OXYGEN STEEL-making (BOS) Process

The BASIC OXYGEN STEEL (BOS) - making Process
This can be described via the following steps:
The process occurs in a furnace which is a specially designed steel vessel that may be rotated through 360 in a vertical plane.
The vessels are lined with heat resistant (refractory) materials eg.dolomite (MgCO₃/ CaCO₃) and magnesite (entirely MgCO₃). These last for up to 2 weeks before they have to be replaced.

1) Scrap steel is placed in the vessel followed by molten iron directly from a blast furnace.
2) the vessel is stood upright and a water-cooled lance is lowered from above and oxygen is blown onto the surface at high speed. Carbon in the iron is oxidised to CO and CO₂:

C(S) + O₂(g) > CO₂(g)
CO₂(g) + C(S) > 2CO(g)
2CO(g) + O₂(g) > 2CO₂(g)
3) After a few minutes the oxygen is stopped. A flux of calcium fluoride (fluorspar, CaF₂) and "burnt lime" (CaO) is then added.
Burnt lime is calcium oxide formed by heating calcium carbonate until it decomposes:

CaCO₃(s) > CaO(s) + CO₂(g)
This is introduced as a method of removing unwanted Si and P, since:

4P + 5O₂ > P₄O₁₀
6CaO + P₄O₁₀ > 2Ca₃(PO₄)₂

Si + O₂ > SiO₂
CaO + SiO₂ > CaSiO₃
this produces a slag.
Manganese is oxidised but remains in the iron, sulfur dissolves in the slag and is removed.
4) the lance is lowered again for another blow of oxygen, the vessel contents are analysed to see whether step 3 should be repeated.
If all is OK then:
5) the vessel is tilted and the steel is tapped off below the slag level
6) the slag is removed by tilting the vessel in the opposite direction.
The whole cycle takes about 45 minutes.

Types of Steel

In general, steels can be divided into 2 groups:

Carbon Steels
small % of C (0.03 to 1.5%),
residual amounts of other metals
comprises 90% of all steels produced

At high temperatures, 907-1400 C, Fe is cubic close-packed (ABCABC) and can hold 1 C for every 12 Fe atoms.
This form of steel is called 'austenite'.

With excess carbon, an interstitial steel is produced called 'cementite' which has a precise composition of Fe₃C.

'austenite' is stable down to 690 C where it converts to:
a body centred form called 'ferrite' which has less carbon and also 'cementite'.

It is possible to retain the 'austenite' structure below 690 C by adding Mn.

Alloy Steels
contain C and controlled amount of other metals; eg Mn, Cr, Ni, Mo.

C with increasing C content, hardness and tensile strength
increases, but ductility and welding properties decrease
Mn increases strength and hardness, but less effective than C
good for high energy impact, battering - rifle barrels etc.
Cr improves corrosion and abrasion resistance
Ni increases toughness, tensile strength, hardness and corrosion resistance
Mo increases hardness and tensile strength especially at high temperatures

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Created and maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.

Created July 2000. Links checked and/or last modified 20th October 2005.
URL http://wwwchem.uwimona.edu.jm/courses/BOS.html