4. Impact of refractories corrosion on Industrial
processes
FIRE COURSE – Unitecr’2001, October 30th, 2011 Kyoto, Japan
4.1. STEEL MAKING
J. Poirier
CNRS-CEMHTI, University of Orleans
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
FIRE COURSE – Unitecr’2001, October 30th, 2011 Kyoto, Japan
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
Hydrogen
Carbon
Nitrogen
OxygenControl of inclusions
Phosphorus
SulfurControl of inclusions
Non metallic elements
Electromagneticproperties
Deep drawing
Weldability
Weldability
Toughness
Internal soundness
Surface defects
Anisotropy
Fatigue
Bending
Influence of non metallic elements on steel properties
Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories
More and more complex elaboration to eliminate non metallic elements
Vacuum treatment
C content < 15 ppm
is possible !
Desulphuration treatment
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
1. The possibility to keep the chemical composition of the liquid steel for a given process
3 aspects
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
3. The prevention of defects concerning the steel surface
The impact of refractory products on the quality of the metal
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
Magnesia graphiteMagnesia chromeDolomiteHigh alumina, mainly bauxite products Alumina - spinel
Fired and unfired bricks
Unshaped high alumina or High alumina spinel content products
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 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
Steel ladle
Tundish Sprayed magnesia
Plate Al2O3 - C
Al2O3-C Stopper
Ladle Al2O3 - C Shroud
Al2O3 - C andZrO2-C insert
Submerged nozzle
Tundish lining and continuous casting
Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories
Summary of different defect types in steel in relationwith the refractory products
Steel
Spallingof wall
Reactivity Steel
refractory
Al2O3
build up
Materials and assembly of refractories
Corrosion of slag line
Air leakage
Air leakage
Interactions
Mastery of argon injection
PollutionSteel/slag/refractory
Erosion of refractories
Reoxydation
Al2O3 clogging
Thermal transfert
Inclusions and defects
- exogenous inclusions - endogenous inclusions TiN, Al2O3-MgO, MnO-SiO2, Al2O3, SiO2- splitting decohesion (inclusions + gaz)
Steel purity
- Carbon pick up- Sulphide cleanliness- N and H pick up
Longitudinal cracks
Heterogeneity of solidification
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
Sliding gate Stopper Tundish lining Submerged nozzlePart 1. Continous casting
The plates of the sliding gate system
Subjected to severe thermo-mechanical stress
Lead to the cracking of the refractory in use
Cause of air leakage with effects on the cleanliness and the wear
Al2O3 /SiC / C refractory
Sliding gate Stopper Tundish lining Submerged nozzlePart 1. Continous casting
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
Sliding gate Stopper Tundish lining Submerged nozzlePart 1. Continous casting
(a) cracks in a slide gate air leakage
(Pa)
Design of the plates of the sliding gate system
(b) optimised design no crack
In order to reduce cracking and to limit the re oxidation of the steel
Sliding gate Stopper Tundish lining Submerged nozzlePart 1. Continous casting
The stopper
The function : the control of metal flow rate
Al2O3/graphite products
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
Air leakage due to :
an imperfect airtightness of argon injection connection
the permeability of refractory pieces
The stopper may be a
source of reoxidation
The stopper
Injection of argon
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
Etanchéité quenouilles - Mesures à chaud
0
0,5
1
1,5
2
2,5
3
3,5
4
0 20 40 60 80 100 120 140 160 180 200 220
Temps (min.)
D f
uite
(l/m
in.)
Préchauffage Coulée
A argon injection system in the stopper in order to limit air leakage
Graphite compressed
joints
Design to limit air leakage
Time in mn
Preheating of tundish Casting
Lea
kag
e (l
/mm
)
Air tightness of the stopper : measurement of leakage in use ( at high temperature)
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
The tundish lining
Made of magnesia and forsterite (2MgO-SiO2) monolithic
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
The tundish lining
Preheating
Lining after use
The close contact between steel and the refractory lining allows a pollution action ( exchange of oxigen, hydrogen, magnesium, silicium)
In use
Lining with
-a great porosity
- active surface
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
% FeO
Qua
ntity
of o
xyge
n (g
)
0
0,2
0,4
0,6
0,8
1
0 2 4 6 8
Preheating at 1200°C
Preheatingat 180°C
% FeO
Qua
ntity
of o
xyge
n (g
)
0
0,2
0,4
0,6
0,8
1
0 2 4 6 8
Preheating at 1200°C
Preheatingat 180°C
Relationship between oxygen (caught by aluminium) and the FeO content of the tundish refractory (laboratory trials)
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)
Refractory Steel
Lehmann and Al. 2nd Intern. Symp. On advances in refractories for the metallurgy industry, 1996
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
The quantity of spinels is in relation to the magnesia content in the refractory lining
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
Observation of spinel crystals at the interface steel/refractory
laboratory trials
0
5
10
15
20
25
40 60 80 100% MgOdu réfrac
% surfacique de spinelle% spinel
Spalling of the MgO-SiO2 lining can lead to MgO-Al2O3 inclusions in steel
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
00,5
11,5
22,5
33,5
4
0 1 2 3 4Number of casting during a sequence
Hyd
roge
n [
pp
m]
Measurement of the hydrogen content in steel during a sequence of 3 ladles
The tundish lining : hydrogen pick up
Diffusion of water from sray lining occurs and complete expulsion of the moisture cannot be guaranted even when the tundish is well prea-heated
Hydrogen pick up at the beginning of the casting
To limit hydrogen pick up in the steel, it is important to improve the refractory composition and the preheating procedures of the tundish
Sliding gate Tundish lining Submerged nozzlePart 1 Continous casting Stopper
Submerged nozzle materialsAl2O3/graphite products
Alumina deposits in a submerged nozzle
Clogging and unclogging lead to metal contamination by alumina particules or clusters
One of the main problem : alumina clogging for Al killed steels !
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
• 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 factorschoice and assembly of refractory materials
What caused clogging ?
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
Morphology of deposits in submerged nozzles : 3 zones
1 2 3Refractory
A decarburized zone
Alumina particles + vitreous phase
On the hot faceplate like Al2O3 particles
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
PO2 = 10-17 atm PO2 = 10-11 atm
Refractory Steel
Mechanism of condensation
Sliding gate Tundish lining Submerged nozzlePart 1 Continous casting Stopper
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
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
Even if the steel is perfectly clean, the clogging will still occur !
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
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
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
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
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
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
Oxygen pick up and behaviour of submerged nozzle for Al killed steels
Oxidation of liquid steel (Fe-C) and corrosion of refractory by
iron oxydes and/or oxygen
Wear
Oxidation of dissolved Al
Steel oxydation rate
Alumina build up Build up Beyond a certain air leakage, the
quantity of oxygen affect is so large that it doesn’t affect the Al in steel
The steel ther the carbon of the nozzle are oxidized which cause
erosion
Sliding gate Tundish lining Submerged nozzlePart 1 Continous casting Stopper
Oxydation of steel and wear of the submerged nozzle
The oxydation of steel causes the oxydation of the carbon of the
submerged nozzle
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
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 !
Exemple of a catastrophic wear
In extreme situation, the permeability of the refractory system becomes very important and the submerged nozzle is damaged
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
Erosion of submerged nozzle / effect of the Al2O3-C refractory
Materialwith silica
Pure materialwithout silica
High erosion
no erosion
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
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
IFSInterstitial free steel
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
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
Prevention of alumina build up in submerged nozzles
1. Refractory solutions
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
• 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
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
Sliding gate Tundish lining Submerged nozzlePart 1. Continous casting Stopper
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 non-dissolved 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
Spalling
Pollution of the slag Pollution of the steel
Slag lineMgO-C
WallAl2O3
Deposit of slag at the end of the previous casting
Reactive
slag
Direct transfert Ref steel
Dissociation and dissolution
Corrosion by slag :Dissolution and erosion of refractory
Steel ladle
Metallurgical consequences
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Exemple : basic oxygen furnace (BOF) slag SiO2 TiO2 Al2O3 FeO MnO MgO CaO P2O5 LOI 1000°C
wt % 12.8 0.7 1.4 18.4 2.9 5.2 52.4 2.3 0.3
Study of phase assemblage with temperature- mineralogical path- microstructural changes
Slag / MgO-C microstructure
Some considerations about the slag chemistry and mineralogy
The slag behavior is very important in determining the steel quality
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Thermodynamic prediction
Decrease of th
e temp
erature
Basic oxygen furnace (BOF) slag
Fe(s)MgO
CaO
Ca3Ti2O7
SLAG
T(C)
wei
gh
t %
0900 1100 1300 1500 1700 19000
10
20
30
40
50
60
70
80
90
100
Ca3SiO5
Ca2SiO4
MnOCa2Fe2O5
Ca3MgAl4O10
• 1650°C : Slag + CaO(s)
• Calcium silicates
Ca3SiO5 (C3S)
Ca2Si04 (C2S) + CaO
• Calcium ferrite
Ca2Fe2O5
• MgO
• Minor phases
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Small dendritic crystals 20-80 µm
Heterogeneous crystals.50-150 µm
Homogeneous crystals180-250 µm
Introduction
Conclusion
Industrial cooling~ 24 -48h
1600°C
Rapid cooling~ 3-5s
10°C/h
Slow cooling~ 72h
Effect of thermal conditions on the kineticsof cristallisation
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 Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
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
16 heats : small crack in the lining 24 heats : great evolution of the defect
Observation of steel ladle lining degradations in service
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
SlagPrecipitation
zone Refractory
Steel ladle
Impregnationzone
Several zones of attack with
different textures
Identification of the reactional mechanisms
Slag
penetration
Chemical
dissolution
Structural
spalling
Slag
penetration
Chemical
dissolution
Structural
spalling
Part2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
0
10
20
30
40
50
60
70
80
90
-2 0 2 4 6 8 10
Oxi
de
con
ten
t (w
t %
)
Init
ial
inte
rfa
ce
Distance (mm)
Precipitation zone
Corundum Mullite
SiO2
CaO
Al2O3
RefractoryImpregnation
Hexa-aluminate
of lime
Mullite Mineralphases
Profil of composition of
liquid phase
Slag Precipitation zone RefractoryImpregnation
Slag
Evolution of the liquid composition at high temperature (1600°C)
Part2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Interactions Steel /slag /refractory
Dissolution
Volatilisation
Carbo reductionOxido reduction
Formation of new compounds
Reactions which contribute to degrading the steel quality
Dissolution and precipitation Dissociation
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
The gradient of composition is the driving force of the corrosion process
CArefractory
Slag RefractoryBoundary layer
CAslag
Initial interface
2 elementary steps : a thermochemical reaction at the solid/liquid interface and
a diffusion of species
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Chemical exchanges are controlled by a boundary layer
at the liquid/refractory interface
Study of dissolution in laboratory
Slag
[MgO] = f(t)
Steel
4
9
14
19
24
0 50 100 150
Time ( mn)
MgO
% in
sla
g
slag CaO-SiO2 with SiO2/CaO = 0.9
Saturation solubility of MgO
T = 1630°C
Dissolution of MgO in MgO-C refractory for different times by CaO-SiO2 slag
MgO
Slag/MgO interface
500 m
Slag
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
Slag Refractory
CBrefractory
CBslag
Initial interface
CArefractory
CBAB2/B
CBAB/AB2 CA
AB/AB2
CAAB2/B
CAslag
Boundary layer
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
Corundumlayer
CA2 layer
CA6 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
CA2 : CaO-2Al2O3
CA6 : CaO-6Al2O3
Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact
Dissociation, volatilization
Overview of the brickwork of a vacuum degasser (RH/OB)
Vacuum = 10-3 atm
Example : chromium volatilization of the magnesite-chrome lining in RH/OB vacuum degazer
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
Ex. SiO2 + Al => Al2O3 + Si
Standard reference: activity = 1
Oxido-reduction
The reduction of oxides by the desoxidation metals occurs in the steel
This table indicates the oxides which are reduced by desoxidation metals
Part 2 DissolutionVolatilization Carbo-reduct.New compounds Metallurgical impactOxydo-reduction.
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 Carbo-reduct.New compounds Metallurgical impactOxydo-reduction.
Other exemple of oxydo-reduction
Oxydo reduction SiO2 dense layer Coefficients of diffusion
Oxydation SiCRéduction FeO
ΔG0 (T) :
3SiC + 2FeO 2 FeSi +SiO2 + 3C
∂aO2 / ∂V
100μm SiC SiO2 Slag
SiO2SiCSlag
CaO, MgO
K2O, Na2O
Mechanisms Driving force Key parameters
FeSi
Part 2 DissolutionVolatilization Carbo-reduct.New compounds Metallurgical impactOxydo-reduction.
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
100 m
Disappearanceof fused SiO2 aggregates
Microstructure of Al2O3-C refractory
used in continuous casting
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 Carbo-reduction New compounds Metallurgical impactOxydo-reduct
Formation of new compoundsExemple : Al2O3-MgO in situ spinel castables
- Multicomponent and heterogeneous ceramic- Microscopic observations at room temperature
Al2O3-MgO castable corroded by a lime rich slag in a steel ladle
Sla
g
Impact pad
Imp
regn
atio
n z
one
Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impactOxydo-reduct
Corrosion of MgO-Al2O3 castable by a lime rich slag
with the matrix : spinels (Mg,Fe,Mn)O(Fe2Al2)O3
spinels
Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impactOxydo-reduct
Interaction between slag and matrix
0
0,2
0,4
0,6
0,8
1
0 0,2 0,4 0,6 0,8 1
MgO(s)FeO(s)MnO(s)SiO2(s)CaO(s)Al2O3(s)Fe2O3(s)MgAl2O4(sp)FeAl2O4(sp)spinelSlagAl8O12(sp)MnAl2O4(sp)
com
posi
tion
an
d ra
te o
f sl
ag
an
d s
pin
el (
wt.
%)
<A>
slag
Al2O
3(slag)
CaO(slag)
MnO(slag)MgO(slag)
FeO(slag)Fe
2O
3(slag)
spinel
MgAl2O
4(sp)
Al8O
12(sp)
MnAl2O
4(sp)
FeAl2O
4(sp)<A>
Com
pos
itio
n a
nd
rat
e of
sla
g an
d s
pin
el (
wt%
)
(Mg,Fe,Mn)O(Fe2Al2)O3
SEM observation
Glassy phase
P = 1 at.
T= 1600°C
Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impactOxydo-reduct
Interaction between slag and matrix
Weight% of FeO, Al2O3, MgO and MnO in the liquide state
0
0,2
0,4
0,6
0,8
1
0 0,2 0,4 0,6 0,8 1
rate
of
oxi
de
s in
sla
g p
has
e (
wt.
%)
<A>
MgO
FeO
Al2O
3
MnO
<A>
Rat
e of
oxi
des
in s
lag
ph
ase
(wt
%)
P = 1 at.
T= 1600°C
Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impactOxydo-reduct
Combination of the refractory and a non-dissolved element in steel
Formation of MnSiO3 crystals at the interface clay refractory / steel
Reoxydation of the steel with the formation of solid
inclusions + glass
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
Quickly drawn into steel
Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impactOxydo-reduct
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
Inclusions of oxydes
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
PART II. (4.1.2) Corrosion, cleanliness and steel quality
INTERACTIONS OF REFRACTORIES AND STEEL DURING THE PROCESS OF SECONDARY METALLURGY
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 Ca treatment Desulfurization Carbon pick upO2 content
The dissolved oxygen content is directly converted to a oxygen partial pressure
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
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
1600°C
PO2 = 10-15at
PO2 < 10-15atRefractories
Al2O3
MgOCaOTiO2
PO2 > 10-15atRefractories
Cr2O3
SiO2
2 zones
Index of oxygen potential (in Kcal/mol O2)
Influence of the refractory material on the oxygen contents
Al Killed steel at 1600°C
Ar atmosphere
50 Kg induction furnace
and 3t ladle furnace
The refractory material has a significant
influence on the oxygen content of steel
Metal/Slag / Refractory reactions : spalling of Al2O3 refractory lining and cleanliness of Si killed steels (steel cords)
Liquid silicates
Liquid silicates+ MgO.Al2O3
%
MgO
(sl
ag)
% Al2O3 (slag)
Corrosion of slag line
MgO
Spallingof walls
Al2O3
Precipitation of MgO-Al2O3 oxydes
Hard inclusions
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 complexwith 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
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
Metallurgical consequences : efficiency of Ca treatments of steel
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
Before Ca treatment After Ca treatment
MnS sulphur
Alumina
SilicoaluminatesAl2O3/SiO2/MnO
Al2O3
CaO
CaS
Globular calcic inclusion
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 Ca treatment Desulfurization Carbon pick upO2 content
Formation of spinel inclusions in Al killed steels created by reaction of the dolomitic lining with calcium addition in excess.
Composition of inclusions obtained by an too large addition of SiCa to steel in a dolomite ladle
Initial composition of liquid inclusions
Final composition of inclusions
55%MgO-35%CaO- 10%Al2O3
Transformation path
Solid at casting temperature
Participate in nozzle clogging
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
Metallurgical consequences : desulphurization
Obtained by metal – slag stirring in secondary metallurgy
Porus blocs in a steel ladle
Reaction of desulphurization : CaO + S = CaS + O
liquid slag close to lime saturation
Low oxygen content in steel
Requirements
For aluminum killed steels the final sulphur contents is less than 10 and even 5 ppm !
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
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 Ca treatment Desulfurization Carbon pick upO2 content
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 Ca treatment Desulfurization Carbon pick upO2 content
Lime saturation indexes smaller than 1 correspond to liquid slag
Effect of degree of lime saturation of the slag on desulphurisation and refractory wear
Desulphurization index
Refractory wear
Consequences : advanced desulphurization can only be reached reliably and reproducibly in ladles with a basic lining
Best S conditions
Bannenberg and Al. 6 Int. Iron and Steel congress, 1990, Nagoya
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
Industrial applications: S vacuum treatment in basic ladles
Sur saturation
in CaO
Slag line Refractory wear /
S treatment [MgO]%
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 Ca treatment Desulfurization Carbon pick upO2 content
C Mn P S N Si Al Ti
3 150 7 7 3 7 20 60
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
Typical chemical composition of a Ti-containing IF steel for drawing applications (concentration in 10-3 % )
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
Relationship between carbon pick up and iron content in slag for a ultra low carbon steel (killed Aluminium)
Mechanism of carbon transfert from MgO-C refractory to IF steel
Carbon pick up strongly varies with the composition of the slag and the importance of argon stirring
0
2
4
6
8
10
12
14
16
0 2 4 6
[Fe] (%) in slag
Car
bon
pic
k u
p (
pp
m)
in
stee
l (
aft
er k
ille
d w
ith
Al) Slag lineSteel
ladle
ULC steel
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
0
2
4
6
8
10
12
14
16
0 2 4 6
[Fe] (%) in slag
Car
bon
pic
k u
p (
pp
m)
in
stee
l (
aft
er k
ille
d w
ith
Al)
Relationship between carbon pick up and iron content in slag for a ultra low carbon steel (killed Aluminium)
Mechanism of carbon transfert from MgO-C refractory to steel
Carbon pick up rises sharply when the slag is strongly deoxidized and contains less than 2% of iron oxide
+ 10 ppm ΔC
ULC steel
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
02468
1012141618
0 1 2 3 4
Car
bon
pic
k u
p af
iter
deo
xida
tion
(p
pm)
Mean wear rate of MgO-C slag line (mm/heat)
Evolution of the carbon pick up of ULC steel
Strong correlation between carbon pick up of ULC steels and MgO-C refractory wear rate of the ladle slag line
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 Ca treatment Desulfurization Carbon pick upO2 content
At the interface , condensation of Mg(g) Mg(g) + FeO MgO + Fe
Oxido reduction and vaporisation of magnesium
Mg
0.2 mm
C
MgO
0
2
4
6
8
10
12
14
16
0 2 4 6
[Fe] (%) in slag
Car
bon
pick
up
(ppm
) in
stee
l ( a
fter
kill
ed w
ith A
l)
Mechanism of carbon transfert from MgO-C refractory to steel
Formation of a dense MgO layerwith a positive effect on the corrosion
Presence of iron oxydes in slag
Limitation of carbon pick up
Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick upO2 content
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