Slag Characterisation

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Study On slag Characterization

of EAF High-Cr stainless SteelMaking

ByPranesh Rao K M

12PM07F

Under the guidance ofDr Mohammed Rizwanur Rahman

Dept. of MME;

NITK Surathkal


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Contents

Introduction

Importance of Slag characterization

Experiment

Assumptions

Sample collection

Bulk chemical composition

Petro logical analysis of various phases

Computational thermodynamics


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Introduction

Slags in EAF are product of reaction between

oxides and flux

Oxides may be present in charge or formed

due to reaction of constituent elements

present in melt with oxygen

Slags being light float on steel melt

Hence avoiding oxide inclusion in steel melt


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Introduction

Slag characterization involves following steps:-

Identify the phases at process temperature

calculating the amount of the phases

investigating the influence of the process

parameters on the amount of the phases


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Importance of Slag characterization

A good slag practice is essential for production of ahigh-quality stainless steel

The electrical and material efficiency of the electric arcfurnace (EAF) can considerably be improved by a good

slag practice The metallurgical properties of the slag are strongly

influenced by its high-temperature microstructure

Depending on composition, Slag can be used for

agricultural purposes ,as an additive to cement and asan additive for road construction

Hence characterization of the phases within the EAFslag as well as the determination of the amount ofthese phases is of high importance


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Experiment

In order to study the slag characterization in

EAF high-chromium stainless steelmaking, slag

samples were collected from 14 heats of AISI

304L steel making (18%Cr 10.5% Ni 2%Mn

1%Si 0.03 %C ) two samples per heat

Petrographical and thermodynamic

characterization was performed on the EAFaustenitic steelmaking slags


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Assumptions

The slag system consists Al2O3-CaO-Cr2O3 -FeO-MgO-SiO2-TiO2

Slag is homogeneous and the slag phases are in

equilibrium with each other. The slag temperature was assumed to be 500C

higher than the measured steel temperatures.

All solid compounds were in their stoichiometriccompositions.

Basicity=(%CaO)/(%Sio2)


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Collection of sample


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Collection of slag sample

Slag samplings in the EAF were performed using a

long steel spoon

Spoon content was poured quickly on the cold

concrete floor, freezing the high-temperaturemicrostructure of the slag samples

With each slag sampling, the temperature of the

steel was measured using temperature lances These were dipped into the steel melt by an

automatic sampling system.


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Bulk chemical composition

Fragments of each slag sample were randomlychosen and were crushed to powder andcompacted onto borate plates

The composition of the samples wasdetermined using the X-ray fluorescencespectroscopy

As a results an average value of the chemicalcomposition of the bulk of the EAF slagisdetermined


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The composition of the slag samples and the averaged values

(AISI 304L), in wt%


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Results from Bulk chemical

composition determination

Samples 6A-6B and 11A-11B, have a composition nearto the averaged composition, with a basicity around1.5, Thus represent normal EAF operation

Samples 8A-8B contain high amount of Cr2

O3

compared to average quantity

Samples 10A-10B contained higher amounts of MgOand Al2O3 in comparison to the average value

Based on above sample slag temperature,

Average temperature of A samples: 16750C

Average temperature of B samples: 16860C


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Petrographical analysis

Petrography was initially performed for the slagsamples using light optical microscopy

Thereafter, scanning electron microscopy (SEM) usingbackscattered electrons was done on the samples

SEM instruments were equipped with an energydispersive X-ray spectroscopy (EDS) analyzer to be ableto perform a semi-qualitative compositiondetermination on the phases and particles existing inthe specimens.

X-ray diffraction (XRD) analyses were performed onfine-powdered samples to confirm the existence of theobserved phases


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Slag specimen preparation for

microscopy

Three slag fragments from different parts of

one slag sample were collected and mounted

in a conductive epoxy resin.

Thereafter, the specimens were ground and

polished

Finally, the surfaces of the specimens were

coated with a conductive layer

l i d f


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slag specimen prepared for

microscopy

O ti l i i b ti i


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Optical microscopic observations in

the slag samples

Darker Matrix

Bright large angular particles(>5 m)

Bright small angular particles(


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Angular particles

Samples 8A and 8B with a higher chromium-

oxide content contain both small and large

angular particles.

Few bright large angular particles(>5 m)

were observed in samples 6A-6B and 11A-11B

Where as small angular particles were

observed in all samples

O ti l i i i L


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Optical microscopic image - Large

angular particles.


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Bright elongated particles

In 6A, 8A-8B,and 10A samples containing high

Cr2O3 some elongated particles are also observed


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Composition of Angular particles

The chemical composition of the angular particles wereanalyzed by EDS

The result suggests that these particles are mainlycomposed of Cr ,Mg ,Al ,O

The composition and angular shape of these particlescorrespond to the spinel phase The

General formulation of the spinel is A2+B23+O4, A and B

are divalent and trivalent cations, respectively

Spinels crystallize in octahedral habits, which is theexplanation behind the angularity of the cross sectionof these particles
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Ternary oxide system diagram of splines

It was assumed that the spinels are constituted of Cr2O3, MgO and Al2O3Bold dots in the diagrams represent the compositions of the angular

particles

Spinel phase area shaded in gray in the diagram

Examination of the ternary diagram reveals that the spinel phase area is

a solid solution of MgOCr2O3 and MgOAl2O3

O ti l i f M t lli


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Optical microscopy of Metallic

spherical particles

These spherical droplets were most likely splashed from the meltbath into the slag layer during the melting process

The droplets were classified into three different groups

depending on their sizes:

i) smaller than 5m,ii) between 5 and 20m

iii) larger than 20m

The slag contains a large

numbers of small droplets.

But number of the

medium-size droplets was less

than that for large-size

droplets


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Composition of spherical particles

Chemical composition of the droplets for different size classes

was analysed using SEM-EDS technique

Ni and Cr Wt% in spherical particles is less than that in liuid steel

It can be seen that, on average, large droplets contain more Niand Cr than smaller droplets.

Large droplets with a Ni content up to 46 wt% were observed.

Cr and Fe have higher affinity to O compared to Ni due to large

surface area available in case of large particles Fe and Cr get

Oxidized leaving behind Ni

Optical microscopic image of Typical


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Optical microscopic image of Typical

dendrite pattern.

We can observe a typical dendrite-shaped phase in the slag

which is brighter than the matrix

It is believed that this kind of microstructure is evolved

during the solidification period and that it does not exist at

rocess tem eratures


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Composition of dendrite

SEM-EDS analyses showed that the composition

of this phase is very similar to spinel particles.

They are mainly composed of Cr, Mg, Al, O.

This similarity implies that these dendrites are

secondary spinels which have formed during

cooling.

Size and the shape of the dendrites have beenfound to be dependent on the cooling rate of

samples


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COMPUTATIONAL THERMODYNAMICS

The calculations were carried out using Thermo-

Calc software package.

The software was used to predict the equilibrium

phases within the slag at the processtemperatures using the global slag composition

Influence of the parameters such as the slags

composition, temperature and basicity on theequilibrium phase assemblage within the slag

were studied


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Thermodynamic calculations results - Phases

theoretically predicted in the slag samples.

MgCr2O4 spinel is assumed to be a stoichiometric compound though it is asolid solution

These results are in good agreement with microscopic analyses such aslarge amount of spinels in 8A-8B samples, absence of spiels in 10B sample

Th d i C l l ti


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Thermodynamic Calculations

Sample 6A


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Sample 8A


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Sample 10B


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Effect of the process parameters on the

amount of spinel particles in the slag


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Effect of the process parameters on the

amount of spinel particles in the slag


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Conclusion

Qualitative and quantitative study ofStudy Onslag Characterization of EAF High-Cr stainlessSteel Making is conducted

Thermodynamically predicted phases wereverified

The non convergence of result observed is due tothe assumptions made

Amount of solid particles (spinels) in slag whichdetermine physical properties of slag is directlyrelated to amount of Cr2O3 in slag


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Reference

A Study of EAF High-Chromium Stainless Steelmaking SlagsCharacteristics and Foamability by Saman Mostafaee -

Department of Materials Science and Engineering -RoyalInstitute of Technology

Fundementals of steelmaking by E T turkdogan


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Slag Characterisation

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