Steel MAking: Lecture BOS and EAF

27/09/2010 Materials Engineering NEDUET Karachi 2
Basic Oxygen Steelmaking Converter Plant Layout

BOS process
Basic Oxygen Steelmaking Converter
• The Basic Oxygen process is the major method of making steel.
• Modern furnaces, or converters, will take a charge of up to 350 tonnes at a time and
convert it into steel in little more than 15 minutes
• The molten iron accounts for
about 80% of the total
charge, the rest consisting of
steel scrap.

A water-cooled oxygen lance is
lowered into the converter, and
high-purity oxygen is blown on
to the metal at very high
pressure.
The oxygen forms a chemical
bond with the unwanted
elements and removes them as a
slag which floats on top of the
liquid steel.
These oxidation reactions – the
chemical boil – produce heat, and
the temperature of the metal is
controlled by the quality of the
scrap and iron ore coolant added.

Functions of the Basic Oxygen
• The main functions of the Basic Oxygen Furnace (BOF)
are to decarburize and remove phosphorus from the
hot metal, and to optimize the steel temperature so
that any further treatments prior to casting can be
performed with minimal reheating or cooling of the
steel

Critical Operations of BOS
• According to the changes in metal characteristics
during refining, which of these operations are the
most critical? (choose three answers)
 C removal
 Mn removal
 Si removal
 P removal
 S removal
 Temperature adjustment

Critical Operations of BOS
• According to the changes in metal characteristics during refining, which of these
operations are the most critical? (choose three answers)
 C removal
 Mn removal
 Si removal
 P removal
 S removal
 Temperature adjustment
• Yes. A large amount of carbon has to be extracted ( about 45 kg
per ton of steel) and a narrow bracket of final is required, e.g.
0.05/- 0.01 %.
• No. In general Mn oxidation proceeds without problem. There is
no strict constraint on final Mn in the BOF. Mn alloying is
performed in the ladle.
• No. Si oxidation is very easy and takes place during the first 1/3
of the blow. Si alloying is performed in the ladle.
• Yes. P removal can in practice be made only in the converter (or
during hot metal pretreatment in Japan). All P entering the steel
ladle (in particular from slag carryover) will remain in the steel.
• No. Hot metal desulfurisation is a common practice, and ladle
steel desulfurisation is performed for ultra low S contents, e.g. S
less than 0.0010 %.
• Yes. Final temperature has to be reached within a narrow
bracket (e.g. / -10% 0 C) to allow a harmonious ladle metallurgy
process. All temperature adjustments in the ladle (through
chemical reaction- aluminothermy- or use of a ladle furnace
when available) are very costly in time and for metal quality.

The raw materials charged into the converter are:
• Liquid 'hot metal' from the blast furnace after specific pre-treatments, i.e.
desulfurization or dephosphorization
• Other iron-containing additions, essentially scrap and ore, calculated so as to
adjust the thermal balance and obtain the required steel temperature
• The additions necessary to form a slag of appropriate composition, involving
mainly lime (CaO) and dolomitic lime (CaO-MgO), usually in the form of 20 to
40 mm lumps
• Pure oxygen injected either through a multi-hole lance or through bottom tuyeres
After a completed blowing operation the materials produced are:
• Liquid steel
• Exhaust gas rich in CO (about 80-90%), recovered through the closed or
suppressed combustion hood, and is often used in the burners of the reheating
furnaces
• Slag, poured out of the vessel after the steel
• Both gas and slag are valuable by-products, provided they are properly recovered
and stored.
Inputs, Additions and Outputs

Chemical Reactions
• The hot metal charge is refined by rapid oxidation reactions on contact
with the injected oxygen, under conditions far removed from
thermodynamic equilibrium with the other elements present:
• C + ½ O2 → CO
• CO is partially oxidized into CO2 above the melt (post-combustion). These
gaseous reaction products are evacuated through the exhaust hood. The
ratio CO2/(CO+CO2) is called post combustion ratio (PCR).
• Other oxidation reactions occurring during refining include:
• Si + O2 → SiO2
• 2P + (5/2)O2 → P2O5
• Mn + ½O2 → MnO
• Fe + ½O2 → FeO
• 2Fe + (3/2)O2 → Fe2O3
• These oxides combine with the other oxides charged (lime, dolomitic lime)
to form a liquid slag which floats on the surface of the metal bath

Slag/Metal Reactions
Slag/metal reactions concern:
Si removal
• This reaction is very fast and the total amount of silicon is
transferred to the slag in the first one third of the blow
P and S removal
• These reactions require a very accurate control of slag
formation and of the final slag composition in order to
guarantee the low contents sought for the liquid steel.
• Of the two, dephosphorization is the most crucial and is
analyzed here. To be successful and cost efficient, the
steelmaker has to consider both thermodynamic and
kinetic aspects.

Making steel: EAF process

In the Electric Arc Furnace (EAF), recycled steel scrap is melted and
converted into high quality steel by using high-power electric arcs. The main
task of most modern EAFs is to convert the solid raw materials to liquid crude
steel as fast as possible and then refine further in subsequent secondary
steelmaking processes. Nevertheless, if time is available, almost any
metallurgical operation may be performed during flat bath operation period
(after melting), which is usually performed as a pre-treatment to the secondary
steelmaking operations.
This module introduces the equipment, raw materials and processes used to
produce steel in the EAF, finishing with a simulation, which allows you to put all
this in practice and melt your own cast

Electrode Breakage
• Electrode breakage occurs occasionally in the EAF, predominantly during the melt-down
operation. This should be avoided due to the high costs associated with a breakage
Electrode breakage is usually a consequence of mechanical overload on the electrode from
the surrounding scrap:
• Scrap is caving in from the side as the electrode penetrates into the scrap pile inside the
furnace. If heavy pieces hit directly on the electrode side breakage may occur.
• The electrodes are moving downward without detecting that a non-conducting material
is present at the tip of the electrode. As the electrode continues to push downwards,
breakage may occur.
• Animation:
• http://www.steeluniversity.org/content/html/eng/default.asp?catid=25&pageid=2081271938

Slag Foaming
• Foaming slag is used to increase the thermal efficiency of the
furnace during the refining period, when the side walls are fully
exposed to the arc radiation. A foaming slag will rise and cover the
electric arcs, thus permitting the use of a high tap setting without
increasing the thermal load on the furnace walls. In addition, an
electrical arc covered by a foaming slag will have a higher efficiency
in transferring the energy into the steel phase.
• Slag foaming is obtained by injecting oxygen into the liquid steel,
where mainly iron is oxidized according to the reaction:
• O2 + 2 Fe = 2 (FeO)
• Carbon powder is then injected simultaneously into the slag phase
where iron oxide is reduced.
• (FeO) + C = Fe + CO (g)
• The resulting CO gas is a critical component in order to obtain a
foaming slag.

Slag Foamability
• Slag foamability is as critical a parameter as CO gas generation in order to obtain
foam. Foamability is controlled by the slag phase physical properties viscosity,
surface tension and density. These properties vary with slag composition.
• Due to the nature of the EAF – with large variations in temperature and
composition in different locations of the furnace – the slag is likely to be partly
solid at some positions during some stages of the operation. A slag that is not fully
liquid, but contains some solid material such as undissolved lime, also influences
the foamability, since there is a change in the apparent viscosity:
• where:
• η = apparent viscosity of the solid-containing melt
• η0 = viscosity of the solid-free melt
• f = volume fraction of solid particles in the melt
• a and n are constants.

Slag Reduction
• Although the oxidation of alloys is prevented as far as possible, furnace
operations will result in some valuable elements such as Cr being present
in the slag phase at the end of the operation. For economic reasons, these
need to be recovered by the addition of reductants such as FeSi onto the
slag which melt and react with the slag oxides, according to:

Deslagging
• A slag door is positioned in the side wall at the back of the
furnace. This opening is used by the operator for visual
inspection of the inside of the furnace, injection of oxygen
and carbon using consumable lances, and for deslagging.
• During slag foaming, slag is continuously pouring out
through the slag door and a limited deslagging is achieved.
• A more deliberate deslagging is made whenever a new slag
is needed in order to perform the next metallurgical
operation. This is always the case when a shift in oxygen
potential is needed, e.g. shifting from highly oxidizing
operating conditions to reducing conditions.

Tapping
• Tapping of the furnace is initiated by the operator when the
processing in the furnace is finalized and the target temperature
has been reached. Tapping should be performed as fast as possible
in order to save time.
• There are two common furnace designs that have different tapping
configurations.
1. Eccentrically-Bottom Tapping (EBT) furnaces have a taphole
positioned off-center in the base of the furnace. Such a
configuration enables slag-free tapping. In these cases a "hot heel"
(small amount of remaining metal and slag) is retained in the
furnace between the heats.
2. Spout furnaces are used for some steel grades. Tapping via a spout
causes the slag to be carried over to the ladle, where it is
thoroughly mixed with the steel. In these cases all the metal is
poured out, without any hot heel remaining in the furnace.

Steel MAking: Lecture BOS and EAF

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