Transcript Part 2

FIRE COURSE – Unitecr’2001, October 30th, 2011 Kyoto, Japan
4. Impact of refractories
corrosion on Industrial
processes
4.1. STEEL MAKING
J. Poirier
CNRS-CEMHTI, University of Orleans
FIRE COURSE – Unitecr’2001, October 30th, 2011 Kyoto, Japan
4. 1 STEEL MAKING - CONTENTS OF THE PRESENTATION
• Introduction
•Part I (4.1.1) : Flow control and interactions of refractories
and steel during continuous casting
o Protection between ladle and tundish
o Tundish lining
o Submerged nozzles
•Part II (4.1.2) : Corrosion, cleanliness and steel quality
o Reactions between refractories, steel and slag
o Metallurgical consequences
Control of oxide cleanliness, Steel desulphuration, Ca treatments of
inclusions, Elaboration of ULC steels
• Conclusion
INTRODUCTION
Surface micrograph showing fine particles at
grain boundaries
Steel-maker’s challenge
To propose steel grades with :
• narrower composition ranges
• lower guaranteed contents of residuals
• controlled inclusion size distributions
To obtain
reproducible service
properties
TRIP 800
Introduction
Steel challenge Cleanliness /chemistry
Non metallic elements
Impact of refractories
Two main keys to the production of quality steel products
Chemistry and inclusion control
These results can only be reached by a strict control of process
In particular, steel cleanliness and purity requirements make
the selection of refractory products more and more important
Introduction
Steel challenge Cleanliness /chemistry
Non metallic elements
Impact of refractories
Influence of non metallic elements on steel properties
Non metallic elements
Internal soundness
Hydrogen
Electromagnetic
properties
Carbon
Deep drawing
Nitrogen
Surface defects
Toughness
Oxygen
Control of inclusions
Weldability
Phosphorus
Weldability
Sulfur
Control of inclusions
Fatigue
Anisotropy
Bending
Introduction
Steel challenge
Cleanliness /chemistry
Non metallic elements
Impact of refractories
More and more complex elaboration to eliminate
non metallic elements
Vacuum treatment
Desulphuration treatment
 C content < 15 ppm
is possible !
S content~ a few ppm 
Element
P
C
S
N
H
O
ppm
10
5
5
10
<1
5
Lower limits of residual elements in steel making elaboration
Introduction
Steel challenge
Cleanliness /chemistry
Non metallic elements
Impact of refractories
The impact of refractory products on the quality of the metal
3 aspects
1. The possibility to keep the chemical composition of
the liquid steel for a given process
Introduction
Steel challenge
Cleanliness /chemistry
Non metallic elements
Impact of refractories
The impact of refractory products on the quality of the metal
2. The achievement of the required metal cleanliness :
the amount and the nature of non metallic inclusions
Introduction
Steel challenge
Cleanliness /chemistry
Non metallic elements
Impact of refractories
The impact of refractory products on the quality of the metal
3. The prevention of defects concerning the steel surface
Introduction
Steel challenge
Cleanliness /chemistry
Non metallic elements
Impact of refractories
Main classes of refractories in relation
with the quality and metal cleanliness
 Secondary metallurgy : for steel ladle
Fired and unfired bricks
Unshaped high alumina
or High alumina spinel
content products
Introduction
Steel challenge
Magnesia graphite
Magnesia chrome
Dolomite
High alumina, mainly bauxite products
Alumina - spinel
Cleanliness /chemistry
Non metallic elements
Impact of refractories
Main classes of refractories in relation
with the quality and metal cleanliness
 Secondary metallurgy : for degassing devices
RH/OB
Magnesia-chrome and alumina unshaped products
(containing or not spinel MgO-Al2O3)
Introduction
Steel challenge
Cleanliness /chemistry
Non metallic elements
Impact of refractories
Main classes of refractories in relation
with the quality and metal cleanliness
 Tundish lining and continuous casting
Steel ladle
Al2O3-C  Stopper
Plate 
Tundish 
Al2O3 - C
Ladle  Al2O3 - C
Shroud
Sprayed magnesia
Submerged
nozzle
Introduction
Steel challenge

Al2O3 - C and
ZrO2-C insert
Cleanliness /chemistry
Non metallic elements
Impact of refractories
Summary of different defect types in steel in relation
with the refractory products
Steel
Interactions
Steel purity
- Carbon pick up
- Sulphide cleanliness
- N and H pick up
Inclusions and defects
- exogenous inclusions
- endogenous inclusions
TiN, Al2O3-MgO, MnOSiO2, Al2O3, SiO2
- splitting decohesion
(inclusions + gaz)
Longitudinal cracks
Heterogeneity of
solidification
Pollution
Steel/slag/refractory
Materials and assembly
of refractories
Corrosion of slag line
Spalling
of wall
Erosion of
refractories
Reoxydation
Air leakage
Reactivity
Steel
refractory
Mastery of argon
injection
Al2O3
build up
Al2O3 clogging
Thermal transfert
Air leakage
PART 1. (4.1.1) FLOW CONTROL
INTERACTIONS OF REFRACTORIES AND STEEL
DURING CONTINUOUS CASTING
- Sliding gate system
-Protection between ladle and tundish
- Tundish lining
- Submerged nozzles
Sliding gate system
consists of a mechanical assembly containing the refractory plates
The basic function : the control of metal flow rate
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
The plates of the sliding gate system
Subjected to severe thermo-mechanical stress
 Lead to the cracking of the refractory in use
Al2O3 /SiC / C refractory
Cause of air leakage with effects on the cleanliness and the wear
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Effect of the plate cracks on the nitrogen pick up
Shape of plates
2 points of
blockage
3 points of
blockage
Length of cracks
 121 mm
76 mm 
N pick up
 1.96 ppm
0.58 ppm
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Design of the plates of the sliding gate system
(Pa)
(a) cracks in a slide gate
 air leakage
(b) optimised design
 no crack
In order to reduce cracking and to limit the re oxidation of the steel
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
The stopper
Al2O3/graphite products
The function : the control of metal flow rate
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
The stopper
Injection of
argon
The stopper may be a
source of reoxidation
Air leakage due to :
an imperfect airtightness of
argon injection connection
the permeability of refractory
pieces
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
A argon injection system in the stopper in order
to limit air leakage
Graphite
compressed
joints
Air tightness
of quenouilles
the stopper
: measurement of
Etanchéité
- Mesures à chaud
leakage in use ( at high temperature)
4
3,5
Design to limit air
leakage
D f uit e ( l/min.)
3
Preheating
Préchauffage
of
tundish
2,5
Coulée
Casting
2
1,5
1
0,5
0
0
20
40
60
80
100
120
140
160
180
200
220
Temps (min.)
Time in mn
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
The tundish lining
Made of magnesia and forsterite (2MgO-SiO2) monolithic
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
The tundish lining
The close contact between steel and the refractory lining allows a
pollution action ( exchange of oxigen, hydrogen, magnesium, silicium)
Preheating
Lining with
-a great porosity
- active surface
Lining after use
In use
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Reduction of silica and iron oxydes present in refractories with
oxygen pick up in steel
3 (SiO2)refract. + 4 [Al]steel  3 [Si]steel + 2(Al2O3)
3 (FeO)refract. + 2 [Al]steel  3 [Fe]steel + 2(Al2O3)
Steel
1
Relationship between oxygen (caught by
aluminium) and the FeO content of the
tundish refractory (laboratory trials)
Preheating
at 180°C
0,8
Quantity of oxygen (g)
Refractory
0,6
0,4
Preheating
at 1200°C
0,2
0
0
2
4
6
8
% FeO
Lehmann and Al. 2nd Intern. Symp. On advances in refractories for the metallurgy industry, 1996
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Transfer of magnesium and formation of MgO-Al2O3 spinels
Plant trials as well as the laboratory experiments demonstrate also a
chemical transformation of the forsterite into the MgO-Al2O3 spinel
3(2MgO-SiO2) refr. + 4 [Al]steel  2(MgO-Al2O3)refr. + 4 (MgO)refr. +3 [Si]steel
% surfacique
de spinelle
%
spinel
25
20
15
10
5
0
40
Observation of spinel crystals
at the interface steel/refractory
laboratory trials
60
% MgO
80
100
du réfrac
The quantity of spinels is in relation to the
magnesia content in the refractory lining
Spalling of the MgO-SiO2 lining can lead to MgO-Al2O3 inclusions in steel
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
The tundish lining : hydrogen pick up
Hydrogen [ppm]
Diffusion of water from sray lining occurs and complete expulsion of the
moisture cannot be guaranted even when the tundish is well prea-heated
4
3,5
3
2,5
2
1,5
1
0,5
0
Hydrogen pick up at the
beginning of the casting
0
1
2
3
4
Number of casting during a sequence
Measurement of the hydrogen content in steel during a sequence of 3 ladles
To limit hydrogen pick up in the steel, it is important to improve the
refractory composition and the preheating procedures of the tundish
Part 1 Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Submerged nozzle materials
Al2O3/graphite products
One of the main problem : alumina
clogging for Al killed steels !
Clogging and unclogging lead
to metal contamination by
alumina particules or clusters
Alumina deposits
in a submerged nozzle
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
What caused clogging ?
• Hydrodynamic factors :
metal flow velocities, turbulence zones associated with dead
zones, shape of submerged nozzles
• Metallurgical factors:
steel grades, cleanliness and deoxidation
• Thermal factors:
steel temperature, heterogeneous bath, insufficient
preaheating of nozzles
• Interactions Al2O3-C refractories / steel and refractory
factors
choice and assembly of refractory materials
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Morphology of deposits in submerged nozzles : 3 zones
A decarburized
zone
Refractory
1
2
3
On the hot face
plate like Al2O3 particles
Alumina particles + vitreous phase
Interactions Al2O3-C refractary/steel : deposit build up mechanism
 Dissolution of the carbon of the Al2O3-C refractory into the steel
 Build up of a first layer of deposit by volatilization and oxidation
reactions
Refractory
PO2 = 10-17 atm
Steel
PO2 = 10-11 atm
Mechanism of condensation
Part 1 Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Interactions Al2O3-C refractary/steel : deposit build up mechanism
 Dissolution of the carbon of the Al2O3-C refractory into the steel
 Build up of a first layer of deposit by volatilization and oxidation
reactions
 Alumina formation through oxidation of aluminium by
Carbon monoxide
CO (ref) [C]Fe + [O]Fe
CO(g) forms in the refractory
Aluminium oxidation
2[Al]Fe + [O]Fe Al2O3
Deposit formation
Even if the steel is perfectly clean, the clogging will still occur !
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Interactions Al2O3-C refractary/steel : deposit build up mechanism
Consequences
The alumina deposit increases with the content of oxide phases in the
Al2O3-C refractories (silica, alkalines) that are likely to be reduced by carbon
 Alumina clogging does not occur
with high carbon content steel
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Oxygen pick up and permeability of refractory products
Oxygen plays a fundamental role in the build up of deposits
in submerged nozzles
• oxydation of dissolved Al in steel
• condensation of the Na,K, Si, SiO gaz compounds into a
oxyde vitreous phase
Many sources of reoxydation
• permeability of the refractory products
• reduction of oxides by C ( SiO2, K2O, Na2O, B2O3)
• imperfect assembly seal of the refractory parts
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Prevention of alumina build up in submerged nozzles
The alumina build up is caused by a gaseous transfert of oxygen
The permeability of the refractory and the air tightness
of the assembly play an important part
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Oxygen pick up and behaviour of submerged nozzle for
Al killed steels
Build up
Alumina build up
Beyond a certain air leakage, the
quantity of oxygen affect is so large
that it doesn’t affect the Al in steel
Steel oxydation rate
Oxidation of
dissolved Al
Oxidation of liquid steel (Fe-C)
and corrosion of refractory by
iron oxydes and/or oxygen
Wear
The steel ther the carbon of the
nozzle are oxidized which cause
erosion
Part 1 Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Oxydation of steel and wear of the submerged nozzle
The oxydation of steel causes the
oxydation of the carbon of the
submerged nozzle
We observe a significant erosion by
disintegration of the bonding phase.
The alumina particles are thus
drawn into the metal
 This is a new source of
contamination by alumina of
refractory origin !
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Exemple of a catastrophic wear
In extreme situation, the permeability of the refractory system
becomes very important and the submerged nozzle is damaged
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Erosion of submerged nozzle / effect of the Al2O3-C refractory
no
erosion
High
erosion
Pure material
without silica
Material
with silica
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Effect of steel grades on the behavior of the submerged nozzles
Steel grades
Clogging
Corrosion
decarburising
Mechanisms
Al killed
High
None
Moderate
Decarburation, oxidation of
aluminium , sticking of Al2O3
IFS
Erratic
Weak
High
Formation of Al2TiO5
Clogging/unclogging
Steel with SiCa
treatment
None
High
Moderate
Dissolution of alumina
aggregates and formation of
a low melting phase
High
Manganese
None
High
Moderate
Corrosion of alumina
aggregates with formation
of MnAl2O3
High
Phosphorus
None
High
Moderate
Corrosion of alumina
aggregates with formation
of aluminate of phosphate
High
carbon
Weak
None
Weak
Sticking of Al2O3 or
Fe2+ (Fe3+,Al 3+) 2 O 4
Interstitial free steel
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Prevention of alumina build up in submerged nozzles
1. Refractory solutions
•
improve the purity of Al2O3-C refractories with as little
silica and impurities as possible
•
reduce the permeability of the products
•
use internal layers to limit the clogging
o
Not permeable to gaseous exchange
o
Chemically inert with steel
o
Thermal shock resistant
o
Mechanically resistant to steel flow
A submerged nozzle with a carbon
free liner
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
Prevention of alumina build up in submerged nozzles
2. Process and metallurgical solutions
To ensure perfect steel
cleanliness in the tundish
To avoid steel reoxidation between the sliding
gate of the steel ladle and the mould
Part 1. Continous casting
Sliding gate
Stopper
Tundish lining
Submerged nozzle
PART II. (4.1.2) Corrosion, cleanliness and steel quality
INTERACTIONS OF REFRACTORIES AND STEEL DURING
THE PROCESS OF SECONDARY METALLURGY
I.1. Reactions between refractories, steel and slag
o Dissolution
o Dissociation/volatilization
o Oxydo-reduction / carbo reduction
o Formation of new compounds
o Combination of the refractory and a nondissolved element in steel
I.2. Metallurgical consequences
o Inclusionnary cleanliness
o Efficiency of Ca treatments of steel
o desulfurization
o Carbon pick up
Steel cord
Defects on the surface
The refractory- slag – steel system in secondary metallurgy
Corrosion by slag :Dissolution
and erosion of refractory
Steel
ladle
Slag line
MgO-C
Reactive
Wall
Al2O3
Direct
transfert
Ref steel
Dissociation and
dissolution
slag
Spalling
Deposit of slag at the end
of the previous casting
Pollution of the slag
Pollution of the steel
 Metallurgical consequences
Part 2 Dissolution
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Some considerations about the slag chemistry and mineralogy
The slag behavior is very important in determining the steel quality
 Study of phase assemblage with temperature
- mineralogical path
- microstructural changes
Exemple : basic oxygen furnace (BOF) slag
wt %
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
P2O5
LOI 1000°C
12.8
0.7
1.4
18.4
2.9
5.2
52.4
2.3
0.3
Slag / MgO-C microstructure
Part 2 Dissolution
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Basic oxygen furnace (BOF) slag
 Thermodynamic prediction
100
• 1650°C : Slag + CaO(s)
SLAG
80
• Calcium silicates
Ca3SiO5 (C3S)
Ca2Si04 (C2S) + CaO
• Calcium ferrite
Ca2Fe2O5
• MgO
weight %
70
60
50
Ca2SiO4
40
Ca3SiO5
30
20
Ca2Fe2O5
10 MnO
MgO
Ca3MgAl4O1
Fe(s)
Ca3Ti2O7
0
0
0900
• Minor phases
CaO
1100
1300
1500
1700
Decrease of the temperature
90
1900
T(C)
Part 2 Dissolution
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Effect of thermal conditions on the kinetics
of cristallisation
1600°C
10°C/h
Rapid cooling
~ 3-5s
Small dendritic crystals
20-80 µm
Industrial cooling
~ 24 -48h
Heterogeneous crystals.
50-150 µm
Slow cooling
~ 72h
Homogeneous crystals
180-250 µm
Size of crystals differs significantly depending on the cooling time:
a slow cooling promotes the growth of crystals
M. Gauthieu, J. Poirier, F Bodenan, G Franchescini, Wascon 2009
Par 2 Dissolution
Introduction
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical
impact
Conclusion
An industrial example of interaction refractory/ slag
corrosion of MgO-C in steel ladles
Wear of the slag line
Dissolution/corrosion of MgO-C
Part 2 Dissolution
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Correlations between metal cleanliness, corrosion mechanisms of
MgO-C in steel ladle and critical slag parameters
Steel types
Important wear
mechanism of MgO-C
Critical slag
parameters
Al deoxidized steels
Dissolution of magnesia
in CaO-Al2O3 slag
[CaO]/[Al2O3]
Initial MgO
Si deoxidized steels
Dissolution of magnesia
in CaO-SiO2-Al2O3 slag
[SiO2]/[CaO]
[Al2O3]
Slag T°C
Ultra low [C] steels
Oxidation of carbon by
the slag iron oxide
[FeO]
Part2 Dissolution
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Example : case of deoxidation with Al
Influence of the [CaO]/[Al2O3] ratio on the MgO saturation of
CaO-Al2O3 slags at 1600°C and on the corrosion of MgO-C slag line
the variation of [CaO]/[Al2O3] has an important effect on wear
In the same time, the solubility of magnesia in the slag increases strongly
P Blumenfeld and Al. Effect of service conditions on wear mechanisms of steel ladle refractories Unitecr’97 New Orleans
Part2 Dissolution
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
An industrial example of interaction refractory/ steel
spalling of bauxite walls
Observation of steel ladle lining degradations in service
16 heats : small crack in the lining
Part 2 Dissolution
24 heats : great evolution of the defect
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Identification of the reactional mechanisms
Steel ladle
Slag
Chemical
Structural
penetration
dissolution
spalling
Several zones of attack with
different textures
Slag
Part2 Dissolution
Precipitation
zone
Impregnation
zone
Refractory
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Evolution of the liquid composition at high temperature (1600°C)
Slag
Precipitation zone
Impregnation
Refractory
90
Slag
Precipitation zone
Hexaaluminate
of lime
70
Corundum
Impregnation
Refractory
Mullite
Mullite
50
40
30
Mineral
phases
SiO2
60
Initial interface
Oxide content (wt %)
80
Al2O3
Distance (mm)
CaO
Profil of
composition of
liquid phase
20
10
0
-2
Part2 Dissolution
0
2
4
6
8
10
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Reactions which contribute to degrading the steel quality
Dissolution
Volatilisation
Dissolution and
precipitation
Interactions
Steel /slag /refractory
Oxido reduction
Dissociation
Formation of new
compounds
Carbo reduction
Combination of the refractory and a
non-dissolved element in steel
Part 2 Dissolution
Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Direct dissolution
Chemical exchanges are controlled by a boundary layer
at the liquid/refractory interface
The gradient of composition is the driving force of the corrosion process
2 elementary steps : a thermochemical reaction at the solid/liquid interface and
a diffusion of species
Slag
Boundary
layer
Refractory
CArefractory
CAslag
Initial interface
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Study of dissolution in laboratory
Dissolution of MgO in MgO-C refractory for
different times by CaO-SiO2 slag
[MgO] = f(t)
Slag
Steel
Slag
MgO
MgO % in slag
24
Saturation solubility of MgO
19
T = 1630°C
14
slag CaO-SiO2 with
SiO2/CaO = 0.9
9
4
0
500 m
50
100
150
Time ( mn)
Slag/MgO interface
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Dissolution with precipitation of new compounds
Heterogeneous mechanism with the precipitation of new phases
 Decrease of the wear rate
Initial
interface
CBrefractory
CAslag
CAAB2/B
CBslag
CBAB/AB2
CAAB/AB2
CBAB2/B
CArefractory
Slag
Boundary
layer
Refractory
F. Qafssaoui, J. Poirier, J.P. Ildefonse, P. Hubert :Influence of liquid phase on corrosion behaviour of andalusite-based refractories. Refractories
Applications Transactions, 1 (2005) , 2-8
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Transition between the different monomineral
layers : in bauxite and andalusite refractories
CA2 layer
CA2 : CaO-2Al2O3
CA6 : CaO-6Al2O3
CA6 layer
Corundum
layer
200 m
Bauxite brick
100 m
Andalusite brick
Corrosion of high alumina refractories by Al2O3-CaO slag, T=1600°C
Dissolution – precipitation processes inside a liquid phase
A slow precicipation from the a liquid phase
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Dissociation, volatilization
Example : chromium volatilization of the magnesite-chrome lining
in RH/OB vacuum degazer
Vacuum = 10-3 atm
Overview of the brickwork of a vacuum degasser (RH/OB)
D. Brachet, F. Masse, J. Poirier, G. Provost : Refractories behaviour in the Sollac Dunkirk RH/OB steel degasser, Journal of the Canadian Ceramic
Society, 58 (1989), 61-66
Part 2 Dissolution Volatilization
Oxydo-reduct. Carbo-reduct. New compounds Metallurgical impact
Chrome pick up in steel
20 and 100 ppm of ΔCr in steel in correlation with oxygen blowing
Part 2 Dissolution Volatilization
Oxydo-reduct. Carbo-reduct. New compounds Metallurgical impact
Oxido-reduction
The reduction of oxides by the desoxidation metals occurs in the steel
Ex. SiO2 + Al => Al2O3 + Si
This table indicates the oxides which are reduced by desoxidation metals
Standard reference:
activity = 1
Part 2 DissolutionVolatilization Oxydo-reduction. Carbo-reduct. New compounds Metallurgical impact
Example of oxido-reduction reaction
Submerged nozzle in fused silica
The fracture of the tube occurs after one hour.
Silica was reduced by desoxidation elements (Al,Mn,Ca) presents in liquid steel
Part 2 DissolutionVolatilization Oxydo-reduction. Carbo-reduct. New compounds Metallurgical impact
Other exemple of oxydo-reduction
Mechanisms
Driving force
Oxydo reduction
∂aO2 / ∂V
Slag
Key parameters
SiO2 dense layer
Coefficients of diffusion
SiO2
SiC
CaO, MgO
K2O, Na2O
FeSi
ΔG0 (T) :
3SiC + 2FeO  2 FeSi +SiO2 + 3C
Slag
SiO100μm
2
SiC
Oxydation SiC
Réduction FeO
Part 2 DissolutionVolatilization Oxydo-reduction. Carbo-reduct. New compounds Metallurgical impact
Carbo reduction
At high temperature, carbo reduction reactions occur in the oxide-carbon
refractories
Ex. SiO2 + C  SiO (gas) + CO (gas) at 1550°C
SiO2 + C  Si (gas) + 2 CO (gas) at 1550°C
Disappearance
of fused SiO2 aggregates
Microstructure of Al2O3-C refractory
used in continuous casting
100 m
C. Taffin, J. Poirier :The behaviour of metal additives in MgO-C and Al2O3-C refractories. Interceram International, 43 (1994), 356-358
Part 2 Dissolution Volatilization Oxydo-reduct Carbo-reduction New compounds Metallurgical impact
Formation of new compounds
Exemple : Al2O3-MgO in situ spinel castables
Impregnation zone
Slag
Impact
pad
- Multicomponent and heterogeneous
ceramic
- Microscopic observations at room
temperature
Al2O3-MgO castable corroded by a lime rich slag in a steel ladle
Part 2 Dissolution Volatilization Oxydo-reduct Carbo-reduct
New compounds
Metallurgical impact
Corrosion of MgO-Al2O3 castable by a lime rich slag
spinels
with the matrix : spinels
(Mg,Fe,Mn)O(Fe2Al2)O3
Part 2 Dissolution Volatilization Oxydo-reduct Carbo-reduct
New compounds
Metallurgical impact
Interaction between slag and matrix
(Mg,Fe,Mn)O(Fe2Al2)O3
Glassy
phase
SEM observation
and rate of slag and spinel (wt%)
Composition
composition and rate of slag and spinel (wt. %)
1
slag
P = 1 at.
0,8
T= 1600°C
Al O (slag)
2
0,6
3
0,4
spinel
CaO(slag)
MgAl O (sp)
2
4
Al O (sp)
8
0,2
MnO(slag)
Fe O (slag)
2 3
FeO(slag)
MgO(slag)
0
0
0,2
Part 2 Dissolution Volatilization Oxydo-reduct Carbo-reduct
12
0,4
<A>
<A>
0,6
New compounds
0,8
MnAl O (sp)
2 4
1
FeAl O (sp)
2
4
Metallurgical impact
Interaction between slag and matrix
1
FeO
P = 1 at.
Al O
2
T= 1600°C
Rate of oxides in slag phase
rate of oxides in slag phase (wt.%)
(wt%)
0,8
0,6
3
MgO
MnO
0,4
0,2
0
0
0,2
0,4
<A>
<A>
0,6
0,8
1
Weight% of FeO, Al2O3, MgO and MnO in the liquide state
Part 2 Dissolution Volatilization Oxydo-reduct Carbo-reduct
New compounds
Metallurgical impact
Combination of the refractory and a non-dissolved element in steel
Far exemple, consider the reduction of the silica of the
refractory by the dissolved manganese in steel
2 Mn + SiO2  2 MnO + Si
MnO + SiO2  MnSiO3
Reoxydation of the steel
with the formation of solid
inclusions + glass
Quickly drawn
into steel
Formation of MnSiO3 crystals at the interface clay refractory / steel
Part 2 Dissolution Volatilization Oxydo-reduct Carbo-reduct
New compounds
Metallurgical impact
PART II. (4.1.2) Corrosion, cleanliness and steel quality
INTERACTIONS OF REFRACTORIES AND STEEL DURING
THE PROCESS OF SECONDARY METALLURGY
I.1. Reactions between refractories, steel and slag
o Dissolution
o Dissociation/volatilization
o Oxydo-reduction
o Carbo reduction
o Formation of new compounds
I.2. Metallurgical consequences
o Inclusionnary cleanliness
o Efficiency of Ca treatments of steel
o desulfurization
o Carbon pick up
Part 2
Metallurgical impact cleanliness O2 content
Inclusions of oxydes
Ca treatment Desulfurization Carbon pick up
Metallurgical consequences : inclusionnary cleanliness
Oxide cleanliness is measured by the total mass of
oxide inclusions formed in the liquid steel
Aluminum or silicon additions are used to transform
soluble oxygen into alumina (or silica)
Total dissolved oxygen contents :
Less than 20 ppm for Al killed steels
 lower than 5 ppm for specialty steels
Inclusions of alumina
Structural steel
Part 2 Metallurgical impact cleanliness
O2 content
Ca treatment Desulfurization Carbon pick up
The dissolved oxygen content is directly converted
to a oxygen partial pressure
Part 2
Metallurgical impact cleanliness
O2 content
Ca treatment Desulfurization Carbon pick up
What consequences does this low oxygen partial pressure have
for the selection of refractories ?
To limit the possibility of oxygen pick up, the refractory ’s oxygen potential
must be lower than that of the steel
PO2 > 10-15at
Refractories
Cr2O3
SiO2
2 zones
PO2 = 10-15at
PO2 < 10-15at
Refractories
Al2O3
MgO
CaO
TiO2
1600°C
Influence of the refractory material on the oxygen contents
Ar atmosphere
50 Kg induction furnace
and 3t ladle furnace
The refractory material
has a significant
influence on the oxygen
content of steel
Al Killed steel
at 1600°C
Index of oxygen potential (in Kcal/mol O2)
Metal/Slag / Refractory reactions : spalling of Al2O3 refractory
lining and cleanliness of Si killed steels (steel cords)
Corrosion of
slag line
% MgO (slag)
MgO
Liquid silicates
+ MgO.Al2O3
Spalling
of walls
Al2O3
Precipitation of
MgO-Al2O3 oxydes
 Hard inclusions
Liquid silicates
% Al2O3 (slag)
Oxide cleanliness can be affected by exogenous inclusions
from corrosion or erosion of refractories
Case of deoxidation with Si
Influence of CaO-SiO2-Al2O3 slag composition on the corrosion of
MgO-C with a temperature between 1600 and 1650°C
The situation
is complex
with 3 cases
1.
Solid in suspension in Al2O3 poor slags slow corrosion
2.
Solids precipitated which MgO saturated in contact with the
refractory slow corrosion
3.
Totally liquid slag  rapid corrosion
Metallurgical consequences : efficiency of Ca treatments of steel
Purpose
improving the castability of aluminum killed steels by transforming the
alumina deoxidation inclusions into liquid lime aluminate inclusions
Advantage
These liquid inclusions do not stick to the nozzle refractories
Before Ca treatment
After Ca treatment
Alumina
MnS sulphur
Silicoaluminates
Al2O3/SiO2/MnO
CaS
Al2O3
CaO
Globular calcic
inclusion
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Impact refractories in the efficiency of Ca treatments of steel
Ca has a high affinity for oxygen
Possibility to reduce some constituents of the refractories
SiO2, Cr2O3, Al2O3, …..
Improvement in the efficency of a calcium tretment
when high alumina ladle refractories are replaced by
dolomite or magnesia refractories
Even with the use of basic refractories, possibility to a
transfer of magnesia towards the inclusions
Part 2
Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Composition of inclusions obtained by an too large
addition of SiCa to steel in a dolomite ladle
Transformation
path
Initial composition
of liquid inclusions
Final composition of
inclusions
55%MgO-35%CaO- 10%Al2O3
Solid at casting
temperature
Participate in nozzle
clogging
Formation of spinel inclusions in Al killed steels created by reaction of the
dolomitic lining with calcium addition in excess.
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Metallurgical consequences : desulphurization
Obtained by metal – slag
stirring in secondary metallurgy
Reaction of desulphurization
Requirements
Porus blocs in
a steel ladle
: CaO + S = CaS + O
liquid slag close
to lime saturation
Low oxygen
content in steel
For aluminum killed steels the final sulphur contents is less
than 10 and even 5 ppm !
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Sulfur partition coefficient at equilibrium between liquid
slag of the CaO-Al2O3-SiO2-MgO system and steel
a (Al) = 0.03
1625°C
+ 10% Al2O3 in slag
Final S
2 or 3
To obtain reproducible results in industrial conditions, it
is necessary to control well the slag composition
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Effect of alumina and dolomite refractories on desulphurisation
Consequences : advanced desulphurization can only be
reached reliably and reproducibly in ladles with a basic lining
Alumina
Alumina
Dolomite
Richter and Wolf Plannenzustellung beim TN-Verfahren Document VDEh 1985
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Effect of degree of lime saturation of the slag on desulphurisation
and refractory wear
Consequences : advanced desulphurization can only be
reached reliably and reproducibly in ladles with a basic lining
Desulphurization
index
Best S conditions
Refractory
wear
Lime saturation indexes smaller than 1 correspond to liquid slag
Bannenberg and Al. 6 Int. Iron and Steel congress, 1990, Nagoya
Part 2
Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Industrial applications: S vacuum treatment in basic ladles
Slag line
Refractory wear /
S treatment
 [MgO]%
Sur
saturation
in CaO
Desulphurization index
Is = [CaO]/[CaO]s at the end of the treatment
Correlation between :
- the optimal desulfuration rate
- the slag composition
- the corrosion of the magnesia refractories
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Metallurgical consequences : carbon pick up of ULC steel
Ultra-low carbon steel, such as intertitial free steel are elaborated
by metal-gas reaction under vacuum in oxidizing conditions
C
Mn
P
S
N
Si
Al
Ti
3
150
7
7
3
7
20
60
Typical chemical composition of a Ti-containing IF steel
for drawing applications (concentration in 10-3 % )
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Mechanism of carbon transfert from MgO-C refractory to IF steel
ULC steel
Carbon pick up (ppm) in
steel ( after killed with Al)
Carbon pick up strongly varies with the composition of
the slag and the importance of argon stirring
Steel
ladle
16
14
Slag line
12
10
8
6
4
2
0
0
2
4
6
[Fe] (%) in slag
Relationship between carbon pick up and iron content in slag for a
ultra low carbon steel (killed Aluminium)
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Mechanism of carbon transfert from MgO-C refractory to steel
ULC steel
Carbon pick up (ppm) in
steel ( after killed with Al)
Carbon pick up rises sharply when the slag is strongly
deoxidized and contains less than 2% of iron oxide
16
14
12
+ 10 ppm
ΔC
10
8
6
4
2
0
0
2
4
6
[Fe] (%) in slag
Relationship between carbon pick up and iron content in slag for a ultra
low carbon steel (killed Aluminium)
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Evolution of the carbon pick up of ULC steel
afiter deoxidation (ppm)
Carbon pick up
Strong correlation between carbon pick up of ULC steels
and MgO-C refractory wear rate of the ladle slag line
18
16
14
12
10
8
6
4
2
0
0
1
2
3
Mean wear rate of MgO-C slag line (mm/heat)
4
 The wear of MgO-C slag line by the deoxidized slag plays
an important role in the transfert of carbon to steel
Part 2 Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
Mechanism of carbon transfert from MgO-C refractory to steel
Oxido reduction and vaporisation
of magnesium
C
At the interface , condensation of Mg(g)
Mg(g) + FeO  MgO + Fe
Mg
Carbon pick up (ppm) in
steel ( after killed with Al)
MgO
0.2
mm
Presence of iron
oxydes in slag
16
14
12
Formation of a dense MgO layer
with a positive effect on the corrosion
10
8
6
4
2
Limitation of carbon pick up
0
0
2
4
6
[Fe] (%) in slag
Part 2
Metallurgical impact cleanliness O2 content
Ca treatment Desulfurization Carbon pick up
CONCLUSION
The refractory products are strategic for the
production of steel
They have a direct role on the quality of elaborated grades
 chemical composition of the liquid steel
 cleanliness : the amount and the nature of non metallic
inclusions
 The prevention of defects concerning the steel surface
Prospects
The future evolutions of the refractory products
should be made by taking into account the
interactions : steel quality / refractory reactivity
In conjunction with metallurgists efforts to elaborate
clean steels, this improvement combines simultaneous
-control of refractory composition
-Porosity
-Permeability
-And reactivity
Thank you for
your attention