Steel Making K.K. Keshari
[email protected]
What is Steel ? Steel is an alloy of iron (Fe), carbon (C) and alloying elements which are added intentionally to develop certain properties in final product based on end use/requirement.
In engineering, Fe-C alloys are classed into iron, stee steel, l, and and pig pig iro iron n acco accord rdin ing g to the the “ C ” cont conten entt Iron : 0.01 - 0. 0.025% Carbon Steel : 0.025 - 2.06% Carbon Pig Iron : >2.06% Carbon It also contain certain other elements, such as Mn, P, S, Si, Cr, Mo, Ni, Al etc.
What is Steel ? Steel is an alloy of iron (Fe), carbon (C) and alloying elements which are added intentionally to develop certain properties in final product based on end use/requirement.
In engineering, Fe-C alloys are classed into iron, stee steel, l, and and pig pig iro iron n acco accord rdin ing g to the the “ C ” cont conten entt Iron : 0.01 - 0. 0.025% Carbon Steel : 0.025 - 2.06% Carbon Pig Iron : >2.06% Carbon It also contain certain other elements, such as Mn, P, S, Si, Cr, Mo, Ni, Al etc.
Steel Making Process Steelmaking is an oxidising process It refines and oxidises the impurities in hot metal like C, Si, Mn and removes removes S & P to a desired level Fluxes are added to form basic slag to fix S & P Hot metal supplies the necessary heat in Basic Oxygen Furnace (BOF)/Twin Hearth steelmaking & Electrical Power in Electric Arc Furnace(EAF)/ Induction Furnace
Classification of Steel Carbon Steels: Steels: Variations in
properties are obtained by
vary varying ing the the CC- conte content nt Class
C%
Mn %
Low C steel
:
upto 0. 0.25%
0.25 - 0. 0.50
Medium C steel
:
0.25 - 0.60
0.5 0 - 0.90
High C steel
:
0.61 - 2. 2.06
0.30 - 0. 0.90
Alloy Steels: Special purpose steels that contains alloying elements ( such as Cr, Ni, Mo, V, B, W etc.) apart from carbon steels to improve mechanical and other properties - Sta Stainle inless ss Steel Steel (High (High Cr, Cr, Ni, Mn etc) etc) - Non Non-S -Sta tain inle less ss Stee Steell
Development of Steelmaking Crucible process was available till 1855. Acid Bessemer developed in 1855 by Henry Bessemer of Great Britain Basic Bessemer process was developed by Sydney Thomas in 1878.
Development of Steelmaking (cont.) Open- Hearth process developed by Siemens and Pierre Martin in 1856 Electric Arc Furnace steel making started in 1906 at New York and gained momentum after second world war LD process of steelmaking ( BOF) developed in 1950 where pure oxygen is blown from the top of the converter. Induction Furnace steel making started around 1950. Open- Hearth furnace has been
modified
Hearth Furnace ( THF ) in late seventies
to Twin
Raw materials in Steelmaking Hot Metal Scrap / Sponge Iron / DRI Fluxes ¾
Lime / Dolomitic Lime
¾
Raw Dolomite/ Burnt Dolomite
¾
Iron Ore / Mill Scale
¾
Bauxite / Fluorspar
¾
BOF Slag
¾
Oxygen / Air
Typical Hot Metal Analysis for SAIL Plants Plant C %
Si %
Mn %
S%
P%
BSP
4.12
0.55
0.25
0.035
0.17
RSP
3.91
0.91
0.30
0.049
0.21
DSP
4.01
0.84
0.12
0.045
0.22
BSL
4.03
0.60
0.12
0.045
0.18
Process Routes in Steelmaking VAD
BOF RH Ingot Casting
EAF
IF
Primary Vessel
LF
VOD
Secondary Refining
Continuous Casting Casting
Stages of Steelmaking Primary Steelmaking Basic Oxygen Furnace (BOF)
Electric Arc Furnace (EAF)
Induction Furnace (IF)
Twin Hearth Furnace (THF)
Stages of Steelmaking Secondary Refining of Steel Functions of Secondary Refining Precise control of temperature through ELECTRICAL POWER INPUT / CHEMICAL HEATING DE-OXIDATION and adjustment of chemical composition through alloy additions and inert gas stirring Flotation/removal/modification of steel by inert GAS STIRRING Refining under sulphurisation
basic
and
inclusion
reducing
from liquid
SLAG
for
Removal of gases from steel by VACUUM DEGASSING Maintains inert atmosphere during processing
de-
Secondary Refining Units Argon Rinsing Unit (ARU) / LRS Ladle Furnace (LF) Vacuum Arc Degassing & Refining (VADR) Ruhrstahl Heraeus Degasser (R-H / R-H OB) Argon Oxygen Decarburisation (AOD) Vacuum Oxygen De-carburisation (VOD)
Stages of Steelmaking Steel Casting Ingot Casting
Ingot Molten steel
Casting pit
Slab/Bloom Soaking pit
Blooming/ Slabing mill
Scarfing
Rolling
Steel Casting Continuous Casting
Slab/Bloom/Billet Molten steel
Continuous caster
Rolling
Why Continuous Casting? Ingot Molten steel
Casting pit
Soaking pit
Slab/Bloom Blooming/
Scarfing
Rolling
Slabing mill
Slab/Bloom/Billet Molten steel
Continuous caster Reduction in processing stages Increase in yield & productivity
Rolling
Continuous Casting A method of pouring steel directly from a ladle through a tundish into water cooled molds, shaped to form billets, blooms or slabs. Castings are continuously withdrawn whilst teeming of the metal is proceeding. A faster method of making steel than traditional methods which allows molten steel from the ladle to be cast directly into slabs, blooms or billets. Eliminates the need to pour liquid steel into ingots and can accept liquid steel on a continuous basis.
Tundish - a Metallurgical Reactor
Distribution of liquid steel to strands with thermal, chemical & dynamic uniformity Separation of non-metallic inclusions Maximise sequence length Helps in smooth ladle changeover without interrupting the casting speed Prevents inclusions and slag from entering into mold
Continuous Casting Mould
Continuous casting molds are made of copper alloys and are internally cooled with water Mould determines the strand shape Solid shell formation having sufficient strength to contain ferrostatic liquid head at entry into the secondary cooling zone Mold water transfers heat from the solidifying shell The active cooling length of molds ranges from 600 to 900 mm To decrease the friction between mould and strand, a lubrication medium is added to the mould
Cast Steel Structure Chill zone near the strand surface Consists of fine equiaxed crystals Columnar zone Dendrites extend inwards from the chill zone Equiaxed zone at central region Chill zone
Randomly oriented equiaxed crystals
Continuous Cast Shapes Billet : (75 – 200) x (75 – 200) mm
Bloom : (250 – 450) x (250 – 450) mm
Slab : (150 – 250) x (800 – 1800) mm
Thin Slab : (50 – 125) x (800 – 1800) mm
Rounds : (150 – 500) mm dia
Abnormalities in Continuous Casting Break Out Nozzle chocking Freezing Continuous Cast Defects Surface cracks / sub-surface cracks Internal cracks Blow holes, Pin holes, Central looseness Remedial Measures Control of superheat Proper designing of secondary cooling Steel chemistry Casting speed
BOF Steelmaking
[Si] + O2 = SiO2 [C] + ½ O2 = CO, [C] + O2 = CO2 2 [P] + 5/2O2 = P2O5 [Mn] + 1/2O2 = MnO [Fe] + 1/2O2 = FeO, FeO + [C] = Fe + CO
Sequence of operation in BOF Lime / dolomite converter bottom
addition at
Scrap charging Hot metal charging Oxygen blowing Addition of during blow
fluxes
in
batches
After blowing oxygen lance is lifted and converter tilted for sample and temperature recording Tapping in ladle Addition of de-oxidiser in ladle during tapping
Functions of BOF Removal of impurities like C, Si, Mn, P and to a lesser degree Sulphur with the help of oxygen. Oxidation products lead to acidic slag except carbon. To achieve desired end point condition in shortest possible time without any hindrance and delays. Aim slag basicity ~ 3.0 with addition of lime for removal of Phosphorous and sulphur. Remove the impurities through slag.
Factors affecting output in BOF Input materials i.e. Hot metal, Scrap, fluxes, and oxygen of right quality Blowing practice of right kind Proper planning & Maintenance of various facilities
Blowing Practice Supply of oxygen at desired flow rate and pressure Oxygen is injected in converter via oxygen lance Specific Oxygen blow rate vary form 3.0 to 4.0 Nm 3/tmin. Oxygen blow rate depends on design of lance, Converter capacity Oxygen purity should be 99. 5% Oxygen is supplied at high pressure at about 12-14 bar.
Performance in SAIL Parameters
BSP THF
Numbers
DSP
BOF
4
3
Capacity, t
250
Production, t Heat weight, t
BOF
BSL (SMS-II) BOF
3
2
120
120
300
2.368
1.600
1.67
2.188
247
114.4
118.0
281.1
1-07
1-07
1-07
Duration, Hr-Min.
5-18
Hot Metal, kg/t
944
1050.7
1061
1007
Scrap, kg/t
158.9
97.9
77.7
122.4
Burnt Lime,kg/t
-
91.7
50.8
Burnt Dolo ,kg/t
13.1
-
-
26.8
Raw Dolo, kg/t
26.5
3.8
29.8
-
63.3
Electric Arc Furnace
Input Scrap + Ferro-alloy / Sponge Iron / DRI Fluxes ¾ Calcined Lime ¾ Iron Ore ¾ Spar Electrical Power Oxygen De-oxidiser & Ferro-alloy
Charge Preparation Charging
De-oxidation
Melting
Tapping
Slag Formation
Power Consumption
Slag off
Tap to tap time
Induction Furnace
Components of a Typical Induction Heating System AC power supply Frequency (low frequencies of 5 to 30kHz are
effective for thicker materials & higher frequencies of 100 to 400kHz are effective for smaller parts Induction coil Work piece (material to be heated or treated).
Principle
In induction heating, heat is actually "induced" within the part itself by circulating electrical currents. Heat is transferred to the scrap via electromagnetic waves, the part never comes into direct contact with the coil Coil does not get hot
Twin Hearth Furnace
Twin Hearth Furnace Twin-bath furnace is a modified version of Open hearth furnace. It has two bath, A and B, with a common roof connected by a passage through which gases can move. Each of the bath has a tap hole, a higher hole in the rear wall for skimming off the slag during melting Carbon monoxide is evolved when oxygen is blown into the bath during melting & refining While the metal in bath A is being blown with oxygen, bath B is charged, and heated up with the flame from bath A, regenerating part of its heat. The fuel for bath B is mainly the CO in the fumes passing through the passage from bath A
Oxygen Content of Deoxidised Steel
Killed steel deoxidised with Al : 2-4 ppm Semi killed Steel de-oxidised with - Si/Mn
: 50-100 ppm
- Si/Mn/Al: 25-40 ppm - Si/Mn/Ca: 15-20 ppm
Rimming steel deoxidised with Mn : 250-350 ppm
Mixing phenomenon in a gas stirred ladle
Pressure
Upward movement of gas in the melt
Temperature Transfer of kinetic energy of gas to melt Volume Expansion of Argon gas
Stirring intensity is mainly controlled by gas flow rate
Functions of Ladle Slag Protects liquid steel from atmosphere - Re-oxidation - N2 pickup Absorbs inclusion Prevents heat loss due to radiation De-sulphurisation of liquid steel Reduces level fluctuation for smooth arcing
Function of Steam Ejector Vacuum is a volume of space which is substansively empty of matter,…“ [wikipedia] Steam Inlet Connected to Vacuum Chamber
Jet Nozzle Mixing Region Diffuser
Spray water nozzle
Condenser
Principal of Vacuum Degassing Carbon Removal Reaction for C & O removal is given by, [C] + [O] = CO [C] + 1/2O2 = CO [C] + (FeO) = CO + Fe Hydrogen Removal 2[H] = H2 [H] = k √p H2 Nitrogen Removal 2[N] = N2 [N] = k √p N2
Argon Rinsing Unit
Function Heating
ARS X
Homogenisation Inclusion Flotation Degassing
X
De-sulphurisation
X
Ladle Furnace
Function
LF
Heating Homogenisation Inclusion Flotation Degassing De-sulphurisation
X
VADR
Function Heating Homogenisation Inclusion Flotation Degassing De-sulphurisation
VAD
R-H (Late 1950s) / R-H OB Function
R-H
De-carburisation
X
Heating
X
R-H OB
Homogenisation Inclusion Flotation Degassing De-sulphurisation
X
X
AOD Function
AOD
De-carburisation Heating
X
Homogenisation Inclusion Flotation Degassing De-sulphurisation
X
