4. impact of refractories corrosion on industrial processes fire course – unitecr2001, october...

$: 4. Impact of refractories corrosion on Industrial processes FIRE COURSE – Unitecr2001, October 30th, 2011 Kyoto, Japan 4.1. STEEL MAKING J. Poirier CNRS-CEMHTI,$
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


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


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


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



2. The achievement of the required metal cleanliness : the amount and the nature of non metallic inclusions


3. The prevention of defects concerning the steel surface



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



Secondary metallurgy : for degassing devices

RH/OB

Magnesia-chrome and alumina unshaped products

(containing or not spinel MgO-Al2O3)



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


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


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


(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


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


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)


The tundish lining

Made of magnesia and forsterite (2MgO-SiO2) monolithic


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


% 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


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


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 !


• 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 ?


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



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 !


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


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


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


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


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 !

Exemple of a catastrophic wear

In extreme situation, the permeability of the refractory system becomes very important and the submerged nozzle is damaged


Erosion of submerged nozzle / effect of the Al2O3-C refractory

Materialwith silica

Pure materialwithout silica

High erosion

no erosion


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



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


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 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


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


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


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


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


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


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)


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


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


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


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


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


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


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


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


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


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


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


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


The dissolved oxygen content is directly converted to a oxygen partial pressure


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


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


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


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 !


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


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


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


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


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 % )


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


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


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


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


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

4. impact of refractories corrosion on industrial processes fire course – unitecr2001, october...

Documents

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