Steel Making
Lecture 28 & 29
Emergence of Steel
In the beginning, there was iron...
Then came cast iron...
...and finally steel
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General Issues about Steel
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•
Steel has been called the world’s most useful and inexpensive metal.
•
In recent decades, steel manufacturers have been forced to take a much
closer look at what their processes have been doing to the fragile
environment around them.
What is Steel used for - I
The structure of the Pyramid of the Louvre, canned food,
oil platforms, catalytic converters, paper clips, mounts
for electronic chips...are all made of steel.
1.
In the construction of bridges or buildings...
2.
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In the automobile sector...
What is Steel used for-II
3.
For everyday uses:
cans,pots,containers,etc...
5.
In communications...
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4.
6.
At the heart of food preservation...
In Pipelines
Desirable properties
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Low cost and availability
Hot and cold formable
Weldablility
Suitable machinability
Hard, tough and wear resistant
Corrosion resistant
Heat resistant and resistance to deformation at high temperatures.
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Primary steel making
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Pig iron usually contains 3~4% of carbon, 2-4% of silicon, 1-2% of
manganese and 1-1.2% of phosphorous which makes it very brittle and
not useful directly as a material except for limited applications
Primary Steel making is about refining pig iron to reduce these
impurity amounts
Typical MS composition
Carbon
Silicon
Manganese
Sulphur
Phosphorus
0.16-0.18%
0.40% max
0.70-0.90%
0.040% Max
0.040% Max
Overview of steel making

To make steel, carbon content needs to be decreased to desired level and
concentrations of impurities be reduced much.

Steelmaking processes are essentially the removal of the excess carbon,
manganese, phosphorus, sulphur and silicon by chemical reaction, principally
oxidation, under a slag whereby the excesses are transformed to slag.

The primary reaction is the removal of carbon to the specified level by
oxidation.
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Overall refinement scheme of B/F iron
 Reduction of iron and other metallics from ore
Resulting pig iron containing high carbon and other impurities
 Oxidation of C, Si, Mn by exposing to oxygen & flux
Resulting steel contains high oxygen levels
 S,P removed in slag by fluxing
Linked to deoxidation
 Oxygen and other dissolved gases must be removed
Vacuum gassing and other methods
 Recarburization and Ladle additions for fine chemistry control
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How is Steel made-I
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Steel Production-Path
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Steel products
Semi-finished products(continuous cast steel)
Finished products (rolled steel)
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How is Steel made-II
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How is Steel made-III
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Steel Making Plant
Electric Arc Furnace
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Ladle Furnace
Continuous Casting Machine
Refining reactions
•In steelmaking, the presence of an element or compound in a particular
phase is denoted by brackets except for the gaseous phase.
•Concentration in the metallic bath use brackets [ ]
•Concentration in the slag use parentheses ()
•(FeO) + [C] = CO + [Fe] means Iron oxide dissolved in the slag reacts
with carbon dissolved in the steel bath to form gaseous carbon
monoxide (which escapes into the atmosphere) and iron dissolved in the
bath .
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Refining Oxidation reactions
1. [Fe]
+
[O] = (FeO)
2. [C]
+
[O] = CO
3. [Si]
+
2 [O] = (SiO2)
4. [Mn]
+
[O] = (MnO)
5. 2[P]
+
5[O] = (P2O5)
• All reactions are exothermic.
•C is removed as gas and except carbon, all other impurities are removed as
oxides .
•Iron oxidation is unavoidable and its oxidation must be controlled to avoid
loss in productivity.
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Slag
❑ Slag is a generic term and in steelmaking ,it is mostly a solution of oxides and sulphides
in the molten state and the multi-crystalline phases in the solid state.
❑ Slag is a separate phase because
1. It is lighter than molten steel
2. It is immiscible in steel
Slag plays an important role in steelmaking. It is said, “make a slag and slag makes steel”
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Acid and Basic steels
❑ The impurities removed, depend on whether an acid (siliceous) or basic (limey) slag is
used.
❑ An acid slag necessitates the use of an acid furnace lining (silica) and a basic slag
needs a basic lining (magnesia or dolomite) with lime in the charge.
❑ In acid process, Si, Mn, and carbon only are removed by oxidation. Consequently, the
raw materials must not contain P and S in amounts exceeding those permissible in the
finished steel.
❑ In basic process, Si, Mn, carbon, P and to some extent S can be removed from the
charge but normally the raw materials contain low Si and high P contents.
Steel making processes
•Bessemer process
•LD process
•Electric Arc process
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Bessemer furnace
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The Bessemer process
Converter and its lining
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Small furnaces (converter) (~20 tons) consisting a steel casting
Lined with siliceous and quartzite rock (acidic material)in acid process
Lined with burnt dolomite mixed with tar in basic process
It can be rotated from a vertical position to a horizontal position in order to
facilitate charging and pouring.
Sequence in operation
➢Charge in horizontal position
➢Air blowing in vertical position
➢Tapping in upside down
➢Deoxidation and recarburization in ladle
1.Charge
Charge in Acid process
Molten Pig Iron
Composition:
C=3-4 %
Si=1.2-2%
Mn=.75-1 %
P,S=.04 %
Charge in Basic process
Molten Pig Iron
Composition:
C=3-4 %
Si=1%
Mn=1 %
P=1.5%
S=.1% max
Lime
Air Blow in acid and basic process
Elements removal in case of Acid process
Elements removal in case of basic process
Pouring off
Slag and hot metal is poured off in different ladle.
Slag produced in basic process are SiO2 . CaS, P2 O5, MnO, FeO
Recurburization and deoxidisers
➢Coke ( carbon source) ,ferrosillicon ( Si), ferromanganese (Mn) ,Al are added
FeO + C= Fe +CO
FeO +Mn=Fe +MnO
Comparison of the acid and Basic Bessemer process
Advantages of Bessemer process
Disadvantages of Bessemer process
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LD Process:

The name LD stands for Linz and Donawitz, these were the two places in Austria
where the process was born.

This process is also called as Basic Oxygen Process.

LD process is a refining process which is carried out in a LD vessel(or LD
converter/BOF).
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LD Converter
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Construction:

LD converter is a welded construction of non-ageing steel plates (8mm), the height
of the vessel varies from 7-10m.

L.D converter has a basic lining of magnesite bricks (permanent lining) and Dolomite
bricks (working lining).

Oxygen lance (8-10m & 20-25cm dia) is made of concentric steel tubes and the tip
of the lance is made with Copper.
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Why basic lining?
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The refining of the impurities specially P and S require the slag to be basic in
nature.
If the slag is not basic, these elements cannot be kept in the slag and revert
back into the melt
Thus most modern steel refining operations utilize a basic process, i.e create a
basic slag, by adding fluxes appropriately
If the refractory lining of the furnace is not basic as well then the slag and
lining will react with each other, causing unacceptable lining wear.
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Steps involved in LD process:
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1.
Charging
2.
Blowing
3.
Sampling
4.
Tapping
5.
Slag off
1. Charging:
i) Scrap:
Home scrap generated in the plant is charged. It acts as a coolant & utilizes the excess heat
energy generated during refining. LD process can take upto 25% of the metal charge as scrap.
Any more scarp, if charged, cannot be melted and heated to the desired temperature because
of insufficient heat generation
i)
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ii) Hot Metal (75-90%):
A minimum amount of scrap has to be charged. Otherwise the temperature
will get uncontrollably high.
The analysis of iron required to use in LD process as follows:
C
4.10 - 4.30%
Si
0.50 – 0.85%
Mn
0.50 – 0.80%
S
0.02 – 0.03%
P
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0.10 – 0.25%
iii) Fluxes:
❑ Lime
(95+%CaO) and dolomite (58%CaO, 39%MgO) are the two primary fluxes.
iv) Coolants:
❑ Limestone,
scrap, iron ore, and sponge iron are all potential coolants that can be
added to a heat that has been overblown and is excessively hot.
v) Oxygen:
❑ 99.5%
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of pure oxygen is used as refining agent.
2.Blowing:
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After charging, the vessel is rotated to vertical position,
lance is lowered to blowing position and O2 is turned
on.
Oxygen blows at a pressure of 150 psi . which increases
temperature (16000C) ( at hot spot around 25000C)and
burns off impurities.
The blow continues for about 18 minutes.
Oxygen consumption: 50-60 Nm3/t of steel.
Exothermic heat is generated by the oxidation reactions
during blowing
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Emulsion results from vigorous
evolution of CO
This accelerates the refining
process greatly, as the surface
area of the metal exposed is
increased many folds
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3.Sampling:
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Slag and metal samples are taken out for analysis.

Temperature of the bath is measured by immersion of thermocouple.
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4.Tapping:
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If the analysis & tapping temperature are in the required range, then the molten steel is
tapped in the laddle.
Deoxidizers and alloying additions are made in the laddle.
Tap-to-tap time is 40 – 50 min.
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De-oxidation of Converter Steel
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De-oxidation is the final process in which dissolved oxygen in the steel is removed.
The de-oxidizers i.e. Al, Fe-Si and Fe-Mn are added to the steel, which combines with
dissolved oxygen and forms their oxides.
FeO + Al
FeO + Fe-Si
FeO + Fe-Mn
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Fe + Al2O3
Fe + SiO2
Fe + MnO
5.Slag off:

After tapping steel into the ladle, and turning the vessel upside down and tapping the
remaining slag into the "slag pot“.
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Chemical Reactions:
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1.
[Fe]
+
[O] = (FeO)
2.
[C]
+
[O] = CO
3.
[Si]
+
2 [O] = (SiO2)
4.
[Mn]
+
[O] = (MnO)
5.
2[P]
+
5[O] = (P2O5)
6.
[FeS/MnS]
+
(CaO) = (CaS) + (FeO/MnO)
Sequence of elimination of impurities
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Advantages of LD converter

L-D process is about ten times faster than the open hearth process.

It is this use of pure oxygen instead of air that improves upon the Bessemer process,
as the nitrogen (a particularly undesirable element) and other gases in air do not
react with the charge and also decrease efficiency of furnace. This eliminates the
harmful effects of nitrogen

It produces steel with low S & P content from raw materials of ordinary quality.

It does not use an external source of heat or fuel.
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Disadvantages of LD converter

The major disadvantage of L-D process is that the charge must include a
considerable quantity of molten pig iron, thus limiting the amount of scrap that can
be used.

Steel wastage due to splashes by oxygen lancing is more.

Insufficient depth of penetration of O2, leads to thermal gradient in the bath.
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