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7/28/2019 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
http://localhost/var/www/apps/conversion/tmp/scratch_6/composition%20of%20splines.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_6/composition%20of%20splines.pptx7/28/2019 Slag Characterisation
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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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Thermodynamic Calculations
Sample 8A
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Thermodynamic Calculations
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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