Introduction to steel making processes

Steel Making Processes;
Historical Development

Steel classifications
What is steel?
Steel is an alloy of basically iron & carbon
ranging from 0.025-2%. Other alloying elements
are occasionally added to improve different
properties in the ultimate steel.
The composition of steel broadly divides it into,
i) Plain carbon steel
ii) Alloy steels

The plain carbon steels are of three major types:
i) Low carbon or mild steels upto 0.25%C
ii) Medium carbon steels 0.25-0.65%C
iii) High carbon steels 0.65-1.70%C
Alloy steels are broadly divided into three different
types:
i) Low alloy steels total alloy content < ~5%
ii) Medium alloy steels ‘’ 5-10%
iii) High alloy steels ‘’ >10%

Steelmaking is a process of selective oxidation of
impurities i.e. reverse of iron making. During
steelmaking impurities like C, Si, Mn, P are
oxidized to their respective oxides and
eliminated either as gas or as liquid slag. Other
impurity ‘S’ can be minimized in reducing
condition by a slag-metal interface reaction.

Bessemer Converter Process
The Bessemer Converter Process was first developed in
1856. the original process was suitable for only low
‘P’ iron as it used acidic refractory lining. The basic
bessemer process, suitable for high ‘P’ hot metal was
developed later by Thomas Gilchrist and is known as
Thomas process.
The Bessemer converter process has now been
completely replaced by basic oxygen converters.

The converter & operational
practice
The bessemer converter is a pear shaped vessel with a
detachable bottom. It is mounted on a trunnion and
can be rotated through 360⁰. After charging the hot
metal blowing is started through the bottom tuyeres.
At the end of the blow, the converter is again tilted
upside down and the metal and slag are tapped out.
The Bessemer converter process is completely
autogeneous, i.e. no external heat supply is needed.
The energy requirement for steelmaking is provided
by the exothermic chemical reaction of the bath.

Introduction to steel making processes

Silicon oxidation occurs first, then Mn and ‘C’
oxidation peaks only after ‘Si’ and ‘Mn’.
Carbon oxidation is indicated by appearance
of the long flame over the mouth of the
converter. It drops after the completion of C-O
reaction.
‘P’ oxidation reaction continues for another 3-4
mins. This period is known as after-blow
period.

Composition of suitable hot metal
Element% Acidic process Basic process
C 3.5-4.0 3-3.6
Si 2.0-2.5 0.6-1.0
S 0.04max. 0.08-0.10
P 0.04max. 1.8-2.5
Mn 0.75-1.0 1.0-2.5

Decline of the Bessemer Process
In spite of the basic simplicity, the Bessemer steel
suffered from several limitations:
• A large part of the heat generated through
exothermic reactions is lost in the form of sensible
heat of the N2 gas. So, the scrap melting ability of
this process is very limited.
• The ‘N’ content in the Bessemer steel is high-of the
order of 0.012%. This is not desired with extremely
low level (50-60ppm) required in most commercial
steels.

• The need to change the bottom section
frequently was a severe problem.
• The Bessemer process could not refine high
‘Si’ medium ‘P’ iron in one single stage. In
Indian condition, a duplex or triplex process
became necessary to treat this type of hot
metal.

Open hearth furnace steelmaking
The process was originally developed by
Siemens in Germany and Martin in France in
late nineteenth century. The ability of OH
furnaces to melt both light & heavy scraps to
produce liquid steel of any ‘C’ content and any
desired chemical compositions had saved this
process till eighties.

Construction of OH Furnace
It is basically a reverberatory furnace in which
hot metal and molten steel scraps are refined
in a shallow basic lined hearth. The fuel
burners heat up the bath through the
intermediate slag layer. The hot exhaust gases
are conducted through a set of regenerators.
The sensible heat of hot gases preheat the
refractory checker works. The regenerators
preheat the air and fuel.

Operation of the basic OH furnace
There is usually a wide choice of raw materials in basic OH
furnace. In Indian plants , usually steel scrap + hot metal mix
constitutes the basic charge materials. The ‘Si’ content of the
hot metal was maintained as low as possible usually ≤ 1.0%,
to ensure optimum basicity of the slag. The ‘P’ content ranged
from 0.22-0.30%. The formation of basic oxidising slag
required addition of iron ore + limestone.
In integrated steel plants the usual practice was to charge steel
scrap, lime/limestone and iron ore first. The charge was
heated upto a state of incipient fusion. Then hot metal was
charged.

Reasons for decline of OH process
• OH steelmaking is a very slow process. It cannot
match the productivity of modern BOF where the tap
to tap time is of the order of 40-60mins.
• The dependence of external fuel supply is a serious
constraint of the process.
• Construction and maintenance of the roof and
substructure of the OH furnace is more difficult than
the overall maintenance of a basic oxygen converter.

Top-blown Basic Oxygen Converter
Process
The easy availability of oxygen gas in the post Second World War
period facilitated research on oxygen lancing through the
throat of the converter. As a result the top-blown basic
oxygen converter process, popularly known as LD process was
developed. LD stands for Linz and Donawitz towns in Austria,
where the developmental work was carried out. In course of
time the process also came to be known as Basic Oxygen
Furnace (BOF) process.
Because of its flexibility, it can refine hot metal of varying
compositions to produce low ‘C’, high ‘C’ and low alloy steels.

A basic oxygen converter is a pear-shaped vessel with a
concentrically positioned oxygen lance. The steel
shell is suitably lined with basic refractories. O2
(99.9%) is blown through a water cooled lance fitted
with a copper nozzle. The position of the lance w.r.to
the bath and the flow rate of O2 are automatically
controlled. The capacity of a modern converter
ranges from 100T-400T.

BOF Steelmaking practice
1. Scrap Charging
• Scrap Iron and Steel are tipped into the Furnace. The Iron and
Steel comes from old or scraped cars, bridges, buildings, etc.
Also used is Iron or Steel that when manufactured into a
product was not of good enough quality to be used for its
intended purpose.
2. Molten Iron Charging
• Molten Iron, which comes straight from the Blast Furnace is
then tipped into the Furnace. The Furnace is now ready for
the blow. The hot metal to scrap ratio ranges70:30 to 100:1.

3. The Blow
• The Gas Offtake Hood is lowered onto the Furnace. The water
cooled Oxygen Lance is then lowered. This carries the hot
Oxygen to the surface of the hot metal, increasing the
temperature in the Furnace and melting all of the metal. The
Oxygen combines with the impurities to form oxides in the
form of gases and slag.
4. Sampling
• During the Blow the temperature of the Furnace is monitored,
and at regular intervals samples of the molten metal are taken
to be analysed. When the Steel is of the right composition,
then the Steel workers can move onto the next stage.

5. Pouring
• When the Steel is of the right composition the Gas Offtake
Hood and the Oxygen Lance are removed. The molten Steel is
then poured out the Top-hole by turning the Furnace to one
side. The Steel is then cast into ingots, or processed by
continuous casting.
6. Slagging
• When all of the Steel has been poured out, the Furnace is
turned upside down, in the opposite direction to that when
pouring, and the Slag is removed.

Oxygen jet characteristics
• In BOF process, O2 is blown at a pressure of 8-10atm. through
a convergent –divergent nozzle. The O2 jet is supersonic and
has a speed of 1.5-2.2 times the speed of sound. A supersonic
jet is characterized by a supersonic core in which the jet
velocity is higher than the speed of sound.
• As the jet travels away from the nozzle, it is retarded by the
converter atmosphere so that the supersonic core shrinks
radially and the axial velocity gradually decreases until at
some distance away from the nozzle, the jet becomes fully
subsonic. This point marks the end of supersonic core.

• The jet ultimately impinges on the liquid metal surface to
form a cavity. The impingement of the jet and the dissipation
of the jet momentum causes circulation of the liquid bath in
the upward direction at the vessel central axis. The intensity
of the jet-bath interaction is expressed in terms of ‘Jet Force
No.’ (JFN) and is defined as,
JFN = Gas pressure x Nozzle dia./ Lance height

• At low JFN, dimpling with a slight surface depression
is observed.
• At medium to high JFN, splashing with a shallow
depression.
• At very high JFN, penetrating mode of cavity with
reduction in splashing.
Only the last two types of behavior are encountered in
BOF. Metal droplets are formed on the lip of the
cavity and get ejected in both modes.

Mechanism of refining
• During refining, controlled oxidation of the impurities takes
place once O2 is blown at supersonic speed. The interaction
of the O2 jet with the bath produces crater on the surface,
from the outer lips of which a large number of tiny metal
droplets get splashed. These droplets reside for a short time
in the slag above the bath.
• So, the existance of metal-slag- gas emulsion within the vessel
during the entire blowing period is an integral part of the BOF
steelmaking. This is the reason why slag-metal
(dephosphorization) and gas-metal (decarburization) reaction
proceed so rapidly in BOF steelmaking.

Bath conditions at various stages of
blow
0 min. 5 min. 7 min. 20 min.
GAS-SLAG-
METAL
EMULSION

Reactions in BOF
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 5 10 15 20 25
C,Si,Mn,wt%
Blow time, min
C%
Si%
Mn%
P%

• When the metal bath reacts with O2 jet
oxidation of iron occurs,
2[Fe] + {O2} = 2(FeO)
4(FeO) +{O2} = 2(Fe2O3)
An iron oxide rich slag forms in the early stage of
blowing due to oxidation of iron and addition
of iron ore/mill scale in the charge.

• The (FeO) reacts with the impurity elements in the metal &
slag.
(FeO) + [Si] = [Fe] + (SiO2)
(SiO2) + 2(FeO) = (2FeO.SiO2)
[Mn] + (FeO) = (MnO) + [Fe]
2(MnO) + (SiO2) = (2MnO.SiO2)
A (FeO) rich slag quickly dissolves lime, and the following
reactions proceed,
(2MnO.SiO2) + 2(CaO) = 2(CaO.SiO2)

Carbon removal
• The most important reaction in BOF is oxidation of
carbon. The rate decarburization is initially low, it
increases to a peak value in the middle of the blow
and then decreases again.
• In the initial period the (FeO) in slag may raise to 14-
16%. During peak period it decreases to 7-9%,
because during this period more O2 is consumed
than supplied. When the ‘C’ content of the bath
drops to <0.2% the decarburization kinetics drops
and (FeO) rises again.

Variation of rate of decarburization
during the blow
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 20 40 60 80 100 120
Carbonoxidationrate(%/min)
% of Blow time
C oxidation rate

Mn removal
• [Mn] from the bath is lost in the slag as (2MnO.SiO2),
so there is reduction in Mn% in the bath in the initial
period. As the slag basicity increases due to lime
dissolution, (MnO) is gradually released & is reduced
by carbon,
(MnO) + [C] = [Mn] + {CO}
So, the [Mn] content in the bath increases again. As the
intensity of C-O reaction decreases towards the end
of the blow, Mn is reoxidised from the bath. This
accounts the characteristic “Mn hump” in the
reaction diagram.

Phosphorus removal
• The early formation of a basic slag enables
dephosphorization to proceed simultaneously
with decarburization. The reactions are,
2[P] + 5 (FeO) = (P2O5) + 5 [Fe]
(P2O5) + 4 (CaO) = (4CaO.P2O5)

Sulphur removal
• Although the oxidizing slag in BOF is not suitable for desulphurization,
some ‘S’ removal may occur due to highly basic character of the slag and
high temperature (1680-1700⁰C).
The slag-metal desulphurization reactions are,
(FeS) + (MnO) = (MnS) + (FeO)
(FeS) + (MgO) = (MgS) + (FeO)
(FeS) + (CaO) = (CaS) + (FeO)
Part of the ‘S’ may be removed in the initial stage of the blow through the
reaction with Mn,
[Mn] + [S] = (MnS)

Production of High Carbon Steels in
BOF
• High carbon steels like rail steels(0.65- 0.74%C, 0.6-
1.0%Mn, 0.27-0.3%Si), Ball bearing steels (1.0%C,
1.2%Cr) etc. are manufactured in BOF converter by
“catch carbon technique”.
• In this technique, dephosphorization is accelerated
and completed before decarburization. Extra lime
and fluorspar are charged and the lance is raised to a
higher position for maintaining a soft blow condition
till ‘P’ removal is completed.

• Thereafter, decarburization is continued by a harder
blow till the bath carbon content drops to the
desired level.
• Alternatively, blowing may be continued to complete
both dephosphorization and decarburization.
Required amount of carburizer (petroleum coke
/graphite) is then added to the low carbon steel bath
to raise the ‘C’ content to the desired level. However,
this method involves risk of increasing the inclusion
& nitrogen % in steel, picked up from the carburizer.

Introduction to steel making processes

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Introduction to steel making processes