03_Iron and Steel Making

Materials engineering

Iron and steelmaking

Metals: rarely exist in pure state mostly in ores

Ore: Metallic and other compounds,

mostly oxides

Iron ores: 30-70% Fe

Copper ores: 0.1-0.8 % Cu

Molybdenum: 0.01-0.1% Mo

4 basic way to gain the metallic parts from ore:

Reduction by carbon

Electrolytic way

Metallotermical process

Dissociation

Metallic content:

costs

1) Reduction by carbon MeO + C Me + CO

FeO + C [Fe] + {CO}

gasmolten metals

2) Electrolytic way Al2O3 Al23+ + 3O2-

on the cathode: Al3+ + 3e- Al

3) Metallothermical process

TilCl4 + 2Mg [Ti] + 2MgCl

4) Dissociation MeX [Me] + [x]only at high energy level

Iron and steel

Iron and steel making

Blast furnace

Foundry

Steel making

plant

Foundry

Production of molten steel

Purpose: Iron ore Pig Iron

Iron producing processes

ore types: Fe3O4 magnetite ~70% Fe

Fe2O3 hematite ~70% Fe

FeCO3 siderite ~50% Fe

+ tailings: silicates, sand, other non ferrous

MnO, Al2O3, P2O5, etc

Concentration of ore

cost

cost of blast

furnace

cost of concent-

rating

cost of pig iron

Fe % in ore 30 ~ 70 %magnetite

Blast Furnace Plant

Tasks

1) Reduction of the ore

2) Extraction of tailings3) Melting separation of the molten iron from the

molten tailings (spec. weight difference)

Blast Furnace Plant

Dimension of the BF:

Diameter: 4-10 m

Height: 25-30 m

Volume: 300 5000 m3

Charge: Ore + Coke + Limestone

For 1000 t of iron:

2000 t ore +

800 t coke +

500 t limestone +

~ 4000t hot air

Charge moves down (6-8 hours)

- Preheating by gas: coke burns more efficient

Formation of CO

CO reacts with iron ore

- Coke reduces CO2 in the gasC + CO2 2CO

- CO reduces the surface of the iron ore. Indirect reductionFeO + CO Fe + CO2

- Slag producing by limestone.CaCO3 CaO + CO2MgCO3 MgO + CO2

- In the bosh the coke burnsC + O2 CO2 + Heat

- The coke reduces the molten ore. Direct reductionFeO + C Fe + CO

- Molten limestone + other slag components produce eutectic slag

Slag floats over molten iron

Processes in blast furnace

C+O2CO2

Gibbs free energy

Reduction of FeO from 690 C

Processes in blast furnace thermodynamics


Carbon reduces the oxides:

FeO + C Fe + CO

MnO + C Mn + COSiO2 + 2C Si + 2CO

P2O5 + 5C 2P + 5COSO2 + 2C S + 2CO

gasin molten iron

alloying elements

impurities

In BF carbon can reduce S, P, Cr, Mn, Si 70-90%and Ti 10-20%

Sulfur and phosphorous are harmful in pig iron, and they must be removed.


Desulfurization

FeS + CaO FeO + CaS

in molten slag

Dephosphorisation

P2O5 + 5FeO + 5C + 4 CaO CaO4P2O5 + 5Fe + 5CO

in molten iron

in molten slagin molten iron

in molten iron

gas

Result: pig iron

Product of blast furnace

At the bottom of the BF:

Slag on the top

Molten iron on the bottom with~4% C

Near to eutectic composition

Taping at different heights:

different composition different

purpose

C% Mn% Si% S% P%

for casting 3 - 4 < 1 < 4 < 0.1 < 0.1

for steel with Bessemer method 3 - 4 0.4 1 ~ 3 < 0.1 < 0.1

for steel with Thomas method 3 - 4 0.4 1 ~ 2 < 0.1 < 0.1

for steel with Siemens-Martin method

3 - 4 0.4 1 ~ 1 < 0.1 < 0.1

http://www.youtube.com/watch?v=QBLRIEZZEsU


Taping:


Metallurgy

http://www.youtube.com/watch?v=kPH4dJUVOfc

Steel making

Purpose Pig iron steel by fire refining treatments thatdecrease the C content and impurities.

Main steps

1) Charging

2) Oxidation decreasing C content

3) Increasing temp. with decreasing C% the Tmelt increases !

4) Deoxidation decrease FeO and O in molten steel

5) Alloying

6) Casting, solidification

casting of ingots or

continuous casting of bars and billets

Steel making

Processes

Siemens Martin (open hearth furnace)

Bessemer converter process

Thomas converter process

Oxygen converter process (Linz-Donawitz process - LD)

Electric arc steel furnace

Siemens Martin process (1864)

Charging

pig iron+scrap

pig iron + ore

Capacity

10-900 t

6-12 h

Too expensive

carbon

0,3 %/hour

burns out

Bessemer process (1856)

Charging

molten iron

1210-1250C~3% Si

Capacity

5-60 t

15-20 min

First converter method

No external heat

Acidic lining (slag

react.)

Si + O2 SiO2

from the air

Bessemer process (1856)

During the blow C, Si, Mn % decreases.

%

C

blowing time

15 min.

C 1700C

1250C

4%3%

1%

Si

Mn

O

N

Charging

molten iron

1210-1250C~2% P

No external heat

4 P +5 O2 2 P2O5

from the air

Thomas converter process (1878)

Similar to Bessemer, but basic lining for slag reaction.

Oxygen-converter process (LD)

Charging

molten iron

~3% C

~0.5% Mn

~1% Si

~0.1% P, S

Capacity

15-400 t

No external heat

To avoid overheating when blowing iron ore or scrap are changed.Limestone is changed for desulfurization & dephosphorization.

Variants of Oxygen-converter process

OLP process

oxygen limestone powder

oxygen & CaO powder is blown through the lance

AOD process

argon oxygen decarbonizingoxygen & argon is blown through the lance

Mixed gas system decreased partial pressure of oxygen C% decreases

up to 0.002% C e.g. for stainless steels

Electric Arc Steel Furnace

Charging

scrap + solid pig iron

~3% C

~0.5% Mn

~1% Si

~0.1% P, S

Capacity

5-200 t

For high grade steels

T > 2500C intensive reaction, N2 dissociation

Charging scrap

http://www.youtube.com/watch?v=nolpiat6Sk0


during work

http://www.youtube.com/watch?v=G6Uxh-xtU-g


Electrode in the furnace

http://www.youtube.com/watch?v=3gg9_zTlg4M


Steel making - oxidization

Purpose decrease C% and oxidize the impurities (S, P)

In open hearth and electric arc furnace

In air or oxygene blowing converters

C + FeO Fe + CO

from scrapor iron ore

turbulence in the charge

2C + O2 2CO

from blowing air

The dissolved oxygen contentincreases

Hamiltons law

At the given temperature [C][O]=constant

O%

C%0.01

p=1bar

0.1 1

0.0001

0.001

0.1

0.01

p=1mbar

Stainless steels oxidization

requires vacuum

C < 0.02%O < 0.01%

The law of distribution and mass action

At a given temperature the ratio of the amount of a given compoundIn the molten iron and in the molten slag is constant.

() =

The law of distribution

The law of mass action

Determines the direction of the reaction

mA + nB pABv1

v2

v1=k1(CAB)p V2=k2(CA)

m(CB)n

At equilibrium v1=v2

Effect of nonmetallic elements S, P, O, N

Effect of sulfur

S does not dissolve, forms FeS eutectic with iron.

T

FeS %

~80% 100%

1000

1600

0%

Crystallization at the grain

boundaries.

Cold and hot brittleness

To reduce the effect: desulfurization

2) Increase S content1) Alloy with Mn

FeS + Mn MnS + Fe generally S < 0.035%

MnS is formable at high temperature

grain

FeS at grain boundaries


Effect of sulfur - desulfurization

To achieve low S%

increase L : increase the temperature.

increase CaO content in the slag

increase CaS content in the slag

increase FeO content in the slag

=

=

=

=

increases with temperature

The slag must be changed


Effect of nitrogen

nitride compounds precipitation

and/or the solidification of nitrogen in

interstitial solid solution.

Increases strength decreases toughness.

Ageing


Effect of phosphorous

Keep P content under 0.035%

(0.001%)

T

15%

1000

1.2%

RmRp02Z Rp02

P [%]

Rm

Z

TTTKV

Impact energy

TTKV

P%


Effect of phosphorous - dephosphorization

To achieve low P%

decrease the temperature.

increase CaO content in the slag

increase phosphate content in the slag

increase FeO content in the slag

=[4 25] [] 5

2 5[]4

The slag must be changed

2 P + 5 FeO + 4 CaO (CaO4 P2O5) + Fe

in molten iron in molten slag Dissolves only in slag

in molten iron

[] =[4 25] []

5

5[]4


Effect of oxygen

In form of O or FeO.

TTTKV

Impact energy

Brittel-to-ductile transition temperature

Work done till fracture

TTKV

O%

strain

after long service period

after forming

initial state

stress

Ageing

To reduce the effect: deoxidation

Methods:

Settling

Diffusional deox.

Synthetic slag

Ladle metallurgy


Effect of oxigene

RmZ

O [%]

Rm

Z

Deoxidation: settling

Deoxidizing elements are loaded into the molten steel.

General reaction:

FeO + Me MeO + Fe

The amount of deoxidizing elements are limited by their

disadvantageous effect on the properties:

Mn < 1% causes grain coarsening & brittleness

Si < 0.5 % it decreases the toughness.

V

Ti < 0.1 % they decreases the toughness.

Al


Effect of deoxidizing element on the dissolved oxygen

O [%]

Si

Me [%]

0.01

CV

Mn

Ti

0.1 1

0.0001

0.001

0.01

0.1

Al

Zr


Rimmed steel Deoxidizing with Mn only

susceptible on ageing

Semi-killed steel Deoxidizing with Mn + Al

for continuous casting

Killed steel Deoxidizing with Mn + Si

lower TTKV than rimmed steel

Dead killed steel Deoxidizing with Mn + Si + Al/V/Ti/Zr

best quality from the point of brittleness

Deoxidation: diffusional and synthetic slag meth.

Diffusional method

Deoxidizing element loaded on the top of the molten

slag. Diffusion of O to slag.

Diffusional synthetic slag method

The molten steel is poured on the top of prepared

FeO-free slag.molten steel

FeO free slag

Deoxidation: ladle metallurgy

Ladle metallurgy

Powder injection

deoxidizing, desulfurizing, and dephosphorising

powder with Ar gas are blown into the molten

steel.

molten steel

This technology with the converter method is the most up-to-date steel

making process

- Inclusions are lifting to the slag.

- Almost isotropic

Vacuum handling

A process for deoxidation and degasification

The effect of vacuum on steel

1) Decreasing the partial pressure of the gas above the

molten steel

2) Decrease the content of oxygen.

3) Increase the vaporization rate of low melting point

metals (Zn, Pn, Sn, As)

4) Separates the compounds by dissociation.

Fe4N

CrN

FeO

AlN

TiN

SiO2

Al2O3

~10-6 bar ~10-9 bar 10-12-15 bar

Practically impossible

Vacuum handling

Two type of process

Molten steel

Vacuum chamber

Steel stream

ladle

Degasificated

steel

Va

cu

um

Vacuum ladle degassing Vacuum stream degassing

Effect of dissolved gases on steel

CO - in the rimmed steel produces gas bubbles

O2 - produces gas inclusions and oxide and silicate

inclusions

N2 - increase the ability for aging and nitride inclusions

H2 - flocking H2 bubbles cracking


flocking H2 bubbles

reason

[H]

Temp

A3 A4 Tmelting

[H]

Tmelting

A4

A3


H2 solution

let the gas atoms depart by diffusion

slow cooling after casting (several days)

forging by very soft deformation to make cohesion between the surfaces of the cracks

+ for N make stable nitrides my mircoalloying elements

Al, V, Ti AlN, VN

Alloying, casting

Alloying

Can only take place after a perfect deoxidation, otherwise

alloying elements would burn.

Casting

Two types: casting of ingots continuous casting

Casting of ingots

simple

High productivity

More homogeneous

slow

Casting of ingots

Solidification process for ingots

- Shrinking effect

- Crystallisation, grain-arrangement, mircostructure

- Segregation

Casting of ingots

Shrinking effect

the top 12-15% of the total weight of killed

steel ingot must be cut off (rimmed steel only

3-5%)

Casting of ingots

Crystallisation, grain-arrangement, mircostructure

R

N N number of crystal nuclei

T

R rate of crystall growing

Supercooling under theequilibrium

Casting of ingots

Segregation normal segregation

During the solidification the liquid phase becomes enriched

with alloying elements and impurities.

T

Concentration of the

liquid phase

B [%]

R rate of crystall growing

P%

S%

C%

cross section of the ingot

The difference can be300% for S500-600% for P

Casting of ingots

Segregation inverse segregation

Because shrinkage the alloying elements

and impurities can move inwards between

dendrites.

The impuritiy concentration is higher

between the dendrites arm.

Concentration %

dendrite

liquid

phase

Casting of ingots

Microstricture and segragation in ingot

http://www.substech.com/

Casting of ingots

Continuous casting

https://www.youtube.com/watch?v=d-72gc6I-_E

Continuous casting

Steel refining methods

All of these methods have a remelting and solidification period to:

- Decrease the dissolved gas content and the amount of inclusions

- Produce a homogeneous fine grained crystal structure

- Produce a homogeneous distribution of alloying elements

Used for

- tool steels

- high alloy steels


Vacuum arc remelting process

Removal of dissolved gases, such as hydrogen, nitrogen and CO;

Reduction of undesired trace elements with high vapor pressure;

Improvement of oxide cleanliness; Achievement of directional

solidification of the ingot from

bottom to top, thus avoiding

macro-segregation and reducing

micro-segregation.


Vacuum induction remelting process

Removal of undesired trace elements with high vapor pressures

Removal of dissolved gases (hydrogen and nitrogen)


Electroslag remelting process

http://www.substech.com/

Similar technology:

electron beam

remelting process

03_Iron and Steel Making

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Transcript of 03_Iron and Steel Making