Iron and steelmaking

Department of Materials Science and Engineering

Materials Engineering

Prof. Péter János SZABÓ

Department of Materials Science and Engineering

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 making

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Blast furnace

Foundry

Steel making

plant

Foundry

Production of molten steel

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Iron producing processes

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Purpose: Iron ore → Pig Iron

ore types: Fe3O4 magnetite ~70% Fe

Fe2O3 hematite ~70% Fe

FeCO3 siderite ~50% Fe

+ tailings: silicates, sand, other non ferrousMnO, 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

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Blast Furnace Plant

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Dimension of the BF:

Diameter: 4-10 m

Height: 25-30 m

Volume: 300 – 5000 m3

Charge: Ore + Coke + Limestone

Tasks

1) Reduction of the ore

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

molten tailings (spec. weight difference)

For 1000 t of iron:

2000 t ore +

800 t coke +

500 t limestone +

~ 4000t hot air

Processes in blast furnace

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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 + CO2

MgCO3 → 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

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C+O2→CO2

• Gibbs free energy

• Reduction of FeO from

690 °C

Processes in blast furnace- thermodynamics

Processes in blast furnace

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Carbon reduces the oxides:

FeO + C → Fe + CO

MnO + C → Mn + CO

SiO2 + 2C → Si + 2CO

P2O5 + 5C → 2P + 5CO

SO2 + 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.

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Processes in blast furnace

Desulfurization

FeS + CaO → FeO + CaS

in molten slag

Dephosphorization

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

in molten iron

in molten slagin molten iron

in molten iron

gas

Result: pig iron

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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

Product of blast furnace

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

Taping:

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Product of blast furnace

Metallurgy

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

Steel making

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Purpose Pig iron → steel by fire – refining treatments that

decrease 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

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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)

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Charging

pig iron+scrap

pig iron + ore

Capacity

10-900 t

6-12 h

Too expensive

carbon

0,3 %/hour

burns out

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Siemens Martin process (1864)

Bessemer process (1856)

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Charging

molten iron

1210-1250ºC

~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

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Bessemer process (1856)

During the blow C, Si, Mn % decreases.

%

C

blowing time

15 min.

ºC 1700ºC

1250ºC

4%3%

1%

Si

Mn

O

N

Thomas converter process (1878)

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Charging

molten iron

1210-1250ºC

~2% P

No external heat

4 P +5 O2 → 2 P2O5

from the air

Similar to Bessemer, but basic lining for slag reaction.

Oxygen-converter process (LD)

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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

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OLP process

oxygen – limestone – powder

oxygen & CaO powder is blown through the lance

AOD process

argon – oxygen – decarbonizing

oxygen & 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

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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 > 2500ºC intensive reaction, N2 dissociation

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Electric Arc Steel Furnace

Charging scrap

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

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during work

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

Electric Arc Steel Furnace

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Electric Arc Steel Furnace

Electrode in the furnace

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

Steel making - oxidization

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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

Hamilton’s law

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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 massaction

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

𝐿(𝑇) =𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔

𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛

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)

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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

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Effect of sulfur - desulfurization

To achieve low S%

• increase L : increase the temperature.

• increase CaO content in the slag

• decrease CaS content in the slag

• decrease FeO content in the slag

𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛 =𝐶𝑎𝑆𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔 ∙ 𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔

𝐾 ∙ 𝐶𝑎𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔∙ 𝐿

𝐾 =𝐶𝑎𝑆𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔 ∙ 𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛

𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛 ∙ 𝐶𝑎𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔=

𝐶𝑎𝑆𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔 ∙ 𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔

𝐹𝑒𝑆𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛 ∙ 𝐶𝑎𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔∙ 𝐿

𝐿 =𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑠𝑙𝑎𝑔

𝐹𝑒𝑂𝑖𝑛 𝑡ℎ𝑒 𝑖𝑟𝑜𝑛increases with temperature

The slag must be changed

Effect of nonmetallic elements (S, P, O, N)

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Effect of nonmetallic elements (S, P, O, N)

Effect of nitrogen

nitride compounds precipitation

and/or the solidification of nitrogen in

interstitial solid solution.

▪ Increases strength decreases

toughness.

▪ Ageing

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Effect of nonmetallic elements (S, P, O, N)

Effect of phosphorous

Keep P content under 0.035%

(0.001%)

T

15%

1000

1.2%

αγ

Rm

Rp02

Z Rp02

P [%]

Rm

ZT

TTKV

Impact energy

TTKV

P% ↑

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Effect of nonmetallic elements (S, P, O, N)

Effect of phosphorous - dephosphorization

To achieve low P%

• decrease the temperature.

• increase CaO content in the slag

• decrease phosphate content in the slag

• increase FeO content in the slag

𝐾 =[𝐶𝑎𝑂4 · 𝑃2𝑂5] ∙ [𝐹𝑒] 5

𝑃 2 𝐹𝑒𝑂 5[𝐶𝑎𝑂]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

[𝑃] =[𝐶𝑎𝑂4 · 𝑃2𝑂5] ∙ [𝐹𝑒]

5

𝐾 𝐹𝑒𝑂 5[𝐶𝑎𝑂]4

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Effect of nonmetallic elements (S, P, O, N)

Effect of oxygen

In form of O or FeO.

TTTKV

Impact energy

Brittle-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

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Effect of nonmetallic elements (S, P, O, N)

Effect of oxygen

Rm

Z

O [%]

Rm

Z

Deoxidation: settling

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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

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Deoxidation: settling

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

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Deoxidation: settling

Rimmed steel Deoxidizing with Mn only

susceptible to 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 method

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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

slag

molten steel

Deoxidation: ladle metallurgy

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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

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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

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Vacuum handling

Two type of process

Molten

steel

Vacuum chamber

Steel stream

Ladle

Degasificated

steel

Vacuum

Vacuum ladle degassing Vacuum stream degassing

Effect of dissolved gases on steel

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CO - in the rimmed steel produces gas bubbles

O2 - produces gas inclusions and oxide and silicate

inclusions

N2 - increase the ability to aging and nitride inclusions

H2 - flocking – H2 bubbles → cracking

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Effect of dissolved gases on steel

flocking – H2 bubbles

reason:

[H]

Temp

A3 A4Tmelting

[H]

Tmelting

A4

A3

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Effect of dissolved gases on steel

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 by mircoalloying elements

Al, V, Ti → AlN, VN …

Alloying, casting

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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

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• simple

• High productivity

• More homogeneous

• slow

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Casting of ingots

Solidification process for ingots

- Shrinking effect

- Crystallisation, grain-arrangement, mircostructure

- Segregation

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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%)

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Casting of ingotsCrystallisation, grain-arrangement, mircostructure

RN N – number of crystal

nuclei

ΔT

R – rate of crystall growing

Supercooling –under theequilibrium

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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

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Casting of ingots

Segregation – inverse segregation

Because shrinkage the alloying elements

and impurities can move inwards between

dendrites.

The impurity concentration is higher

between the dendrites’ arm.

Concentration %

dendrite

liquidphase

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Casting of ingots

Microstricture and segragation in ingot

http://www.substech.com/

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Casting of ingots

Continuous casting

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Continuous casting

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

Steel refining methods

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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

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Steel refining methods

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.

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Steel refining methods

Vacuum induction remelting process

▪ Removal of undesired trace elements with high vapor pressures

▪ Removal of dissolved gases (hydrogen and nitrogen)

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Steel refining methods

Electroslag remelting process

http://www.substech.com/

Similar technology:

electron beam

remelting process

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THANK YOU FOR YOUR ATTENTION!Prof. Péter János Szabó – szpj@eik.bme.hu

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