Fundamentals of Stainless steel melting In EAF-AOD route

ByAmartya Talukdar

E-mail: amartya.talukdar@gmail.com

Stainless steel demand is growing

continuously but prices are stagnant

To be successful it is necessary to

• Optimise Quality

• Maximise Productivity

• Reduce Operating cost

Presentation Road map

CARBON-CHROMIUM-TEMPERATURE

RELATIONSHIPS

Slag Characteristics in Stainless steel

making

Kinetics of gas adsorption/ de-sorption in

Stainless steel-w. r. t nitrogen

Genesis & types of inclusions & steel

cleanliness

• 1960: Laboratory investigations on CARBON-

CHROMIUM-TEMPERATURE RELATIONSHIPS

Isothermal chemical reaction process of stainless

steel making

• ARGON was introduced in the REACTION ZONE to

control TEMPERATURE

• REDUCED CARBON level can be achieved

WITHOUT ANY MAJOR LOSS OF Cr

• RESULTS COULD NOT BE REPEATED IN EAF –

HENCE AOD

Preamble

Equilibrium of Chromium and Carbon with variation in Temperature

• Line 1: Equilibrium C-Content is decrasing at higher temperature.

• Line 2 : Equilibrium C-Content is incrasing at higher Cr-content.old standard

Process

Triplex

Duplex

Prime Reactions in Stainless steel making

When oxygen is injected into the liquid metal reactions for Chromium & carbon

oxidation

• 4[Cr] + 3{O}= 2(Cr2O3) not preferred

• [C]+{O}={CO}

• 3CO+Cr =Cr2O3+3C

A major loss of valuable chromium to the slag in the form of oxides is not

acceptable

Thus preferential oxidation of Carbon is desired over Cr oxidation

reaction

(Cr2O3)+3[C] =2[Cr ]+3{CO} preferred

[C]+{O}={CO} preferred

ConCept of oxide ellingham / oxygen potential diagram

A reaction proceeds forward spontaneously when ∆Go the

free energy of the reaction is negative and those are

reactions are more favorable compared to other where ∆Go is

more negative

-∆Go = -∆ Ho +T ∆So

If this equation is plotted - ∆Go as y and T as x- axis for any (in

y= mx+C format) particular reaction we get Ellingham Diagram

Where ∆ Ho formulate intercept & ∆So decide the slope

Oxide Ellingham / oxygen Potential Diagram

Control of Temperature

• Ellingham diagram : At Higher temperature C

oxidation is favorable than Cr oxidation

• The critical temperature for kinetic reasons

1680-1700oC

• Not feasible and detrimental to vessel lining

• Oxygen lancing on low temperature bath: lots

of Cr oxidised & lands in slag as Cr2O3

• Late O2 injection & High Oxygen lancing

rate is preferred in EAF

Reaction involved

3CO+2Cr = Cr2O3+3C

aCr2O3 X aC3

Equilibrium constant K = --------------------- where aCr2 & aCr2O3 =1

aCr2 X pCO

3

aC is directly proportional to CO partial pressure i.e. lower equilibrium C level at reduced p CO

ai - activities of reaction constituents

At reduced CO partial pressure pCO : 2C+O2= 2CO will preferentially proceed over Cr ► Cr2O3 reaction

At reduced CO partial pressure pCO high Cr should be in equilibrium low carbon in melt

Explanation with Sivert’s law

Oxygen potential diagram with conditions like reduced partial pressure & C, Cr concentrations etc. imposed on it for

C CO► &

Cr ► Cr2O3

reactions-120

-60

-70

-90

-110

-80

-100

pco= 0.01 atm.

pco= 1.0 atm.

1200 1600 200018001400

RT

ln

Po

2

Temperature oC

Carbon – Chromium equilibrium in AOD/VOD

Decarburisation is effected

high Temperature

Reduced pCO

• Cr2O3 level in slag can vary from 5-

20 %

• High Cr oxide in slag make the slag

stiff viscous & non workable

SLAG CHARACTERISTICS

Viscosity of Cr2O3 slags at 1550oC

High Cr oxide in slag make the slag stiff , viscous & non workable

Area of Low viscosity

Cr2O3 scale

CaO scale

Role of Slag basicity on Cr2O3 Reduction

Cr2O3

Cr2O3

Cr2O3Cr2O3

Cr2O3Cr2O3

SiO2 SiO2SiO2

CaO

Typical SiO2 –Cr2O3 Tetrahedral Slag Structure

Process of breaking tetrahedron by CaO start at basicity 1.05

Resultant Slag• Viscous Stiff slag in Acid regime

• Until CaO breaks structure to individual Cr2O3 - CaO

units Cr2O3 can not be released for reduction to Cr

• High Basicity : more slag volume and more Cr2O3

entrapment

• Thus Slag basicity must be optimum Between 1.3

to 1.8

• This is also true for AOD slags under reduction

Typical Evolution of Cr2O3 In slag During EAF

0

2

4

6

8

10

12

14

16

EndMelting

End oxyblow

StartReduction

FinishReduction

Tap ladle

Cr2O3 in Slag min

Cr2O3 in Slag max

Preferred Slag Basicity

After Melt 2.0

Melting & Oxy Injection 1.9

Reduction start 1.8

Reduction finish 1.7

Tapping 1.5

EAF SLAG CHARACTERISTICS

ISO-Activity contours of CrOx for CaO-SiO2- CrOx system at 1873 K with Po2=6.95x10 -11

Combination of Slag CharaCteriStiCS & melt CompoSition

• Optimum Slag basicity

• Optimum Silicon level in melt

Rel

ativ

e C

r 2O

3 %

in

Sla

g

Rel

ativ

e C

r 2O

3 %

in

Sla

g

Combination of Slag Characteristics & Melt composition

Rel

ativ

e C

r 2O

3 %

in

Sla

g

• Preferable Transfer Carbon to AOD should be ~1.25 -

1.50%

• Decarburisation in AOD is more efficient compared to

EAF

• Thus optimum Transfer/ Tap Carbon preferred

Basicity

• B = ( CaO+MgO ) / SiO2

• Start with Si from hot metal : B = 5 • Reduction: B = 1,4• Desulfurisation: B = 2,0• Combined Red. + Desulf. : B = 2,2

Cr oxidation reaction and Cr protection during melting in EAF

• Cr to Cr2O3 oxidation & its Reduction to Cr is highly

complicated process

• Dissolved Silicon and carbon should be high to

protect Cr and to secure sound reduction of

Cr2O3

• At least 0.2% Silicon should be maintained melting to

tapping to protect Cr from being oxidised

• Substantial improvement in Cr recovery may obtained

when Silicon level in melt is improved from 0.1 to 0.40 %

• Cr2O3 solubility in slag varies with Cr in metal

• At 14-20% Cr in metal Cr2O3 solubility in slag

is~9-10% at 1690oC• Industrial slag shows much higher solubility

this is primarily due to solvent effect of Al2O3

• Quantity or Percent of Cr2O3 in slag can be estimated through some approximate - rule

(Cr) Slag / [Cr] Metal = 0.3(%FeO) Slag

Cr2O3 in solubility slag and Cr2O3 partition

Rel

ativ

e C

r 2O

3 %

in

S

lag

• Preferable Al2O3 level in slag ~6 - 8%

• Al2O3 in slag increases Cr2O3 activity

• Al2O3 in slag increases slag fluidity and kinetic conditions for Cr

transfer rate at 1500-1600oC

• Cr recovery rate is slow and improves with increase in Al2O3 %

in slag

• Al3+ replaces the Cr3+ ion in Cr2O3 spinel based particles

Combination of Slag Characteristics & Melt composition

Control points to restrict Cr oxidation & enhance Cr recovery

• High Temperature

• Chemistry of melt

• Slag composition

• Extent of dilution of CO

In AOD extent /degree of chromium oxidation depends of blowing procedures

AOD fundamentals

Fundamentals converted into AOD Actions

• High input metal temperature

• Optimum C & Silicon content in AOD input metal

• Moderate to low slag volume in transfer

• Deep Blow from top lance

• Increased oxygen lancing rate at initial decarburisation period

• Quick reduction of temperature after blow finish

• Optimum basicity of slag during reduction

Gas Phase

Diatomic

Liquid/Gas Inter-phase Liquid Phase

N2

[N][N]

N2

[N]

Schematic representation for nitrogen adsorption

Sulphur & Oxygen put hindrance at this surface

Nitrogen at the end of the AOD varies between 150-500 PPM in metal depending on blow gas regimes

[N] = k* √ pN2

[N]

[N]

[N]

[N]

Ar/N2

Ar/N2

Ar/N2

Ar/N2

Ar

{Ar}

Depth o f B

ath

Schematic representation of nitrogen removal by argon rinsing

Increase in nitrogen content of stainless steel

Use of High Nitrogen containing inputs

CO-Boil in AOD reduces the nitrogen level

Oxygen purity of blowing should be reduced

After Sulphur & Oxygen removal during

reduction step Nitrogen should be blown

Inclusions in 304 type stainless steel

Change in constituent percentage in Inclusions with temperature during steel making

Al2O3 & TiO2 content

increase as the process

proceeds from AOD to Mold

Al2O3 & TiO2 form during

deoxidation

MgO content : constant or

decrease due to dilution

Inclusions nucleate

heterogeneously on surface

of AOD or slag particles

Inclusions in 304 type Ti treated stainless steel

Inclusions in AOD• Inclusions in Mold

Inclusion compositions In 304 type Stainless on CaO- Al2O3-SiO2 – 10 MgO quasi-ternary

Normally the nucleation site for

Inclusion formation in AOD –

suspended AOD slag droplets

Mainly four types• Spinel MgO-Al2O3

• Perovskite CaO-TiO2

• Pure Rutile TiO2

• Spinel in MnO-Silicate

matrixReduction in Aluminium content of metal

Inclusions in 304 type stainless steel

• Inclusions are scattered in

boundary of Sphene (CaO-SiO2-

TiO2) and Perovskite (CaO-TiO2)

region

• Inclusion Crystallization

temperature Highly sensitive to

basicity Perovskite- high melting point

Sphene- lower melting point

Reduction in AOD Basicity to avoid formation ofhigh temperature phases

Inclusions in AOD • Inclusions in Mold

Inclusion compositions In 304 type Stainless on CaO- TiO2-SiO2–10

MgO quasi-ternary

Perovskite

Sphene

Rutile