Inclusion Formation in AK Steels

Prévia do material em texto

Inclusion Formation and Agglomeration in Aluminum Killed Steels 
 
 
R. Rastogi* and A. W. Cramb 
Department of Materials Science and Engineering 
Carnegie Mellon University 
Pittsburgh, PA 15213 
 
 
Keywords: alumina, continuous casting, clogging, agglomeration, growth, 
inclusions. 
 
 
Abstract 
 
The formation of alumina as a network can occur in two distinct manners: (1) it can 
grow continuously or (2) it can form by agglomeration and sintering. The purpose of this 
paper is to study both types of network to ascertain if there are morphological differences 
between networks formed by these two mechanisms. Clogs from industrial casters were 
analyzed. Alumina networks were grown in levitated droplets and in crucible 
experimentation. Networks were also formed by sintering of sub-micron sized alumina 
particles. Results of this work indicates that alumina networks change as a function of 
time due to sintering and that the clog itself can be a growth site during times of 
extensive reoxidation. Morphologically it is very difficult to tell the difference between 
growth and agglomeration networks due to the effects of sintering. With time at 
steelmaking temperatures, both types of clogs tend to look similar. Thus it is not possible 
to unambiguously determine the manner of clog growth from its morphology. 
 
 
Introduction 
 
Nozzle build-ups that lead to nozzle clogging are a problem as they are a completely 
natural phenomenon when steels are cast containing solid inclusions or liquid inclusions 
which precipitate solid material as they react or cool. Almost all solid inclusions (with the 
exception of titania) are not wet by liquid steel thus they will agglomerate if they are in 
contact with another refractory surface. The history of continuous casting reveals that 
nozzle clogging was one of the first problems to be overcome before continuous casting 
of aluminum killed grades was commercially successful. Table 1 illustrates a number of 
inclusion types that have been found in nozzle build-ups during the continuous casting of 
steel. Note that all deposits are solid. 
 
 
* Present address: Honeywell - Amorphous Metals, 440 Allied Drive, Conway SC 29526. The work 
presented is a part of the Ph.D. Thesis completed at Carnegie Mellon in 2000. 
10472001 IRONMAKING CONFERENCE PROCEEDINGS
Numerous researchers have studied the buildup problem since its increasing 
importance was realized in the late 1960’s. The increasing demands on quality control 
have made the problem more and more important. Yet, for decades it has been treated 
through a mostly qualitative approach. At present there is no universally accepted 
standard method to quantify clogging, though casting shops do have their own methods 
of interpreting casting parameter data to look for clogging. Apart from the obvious 
limitations on the productivity and quality of the casting process, which ultimately 
translates to an increase in costs, the clogging phenomenon may also contribute to 
development of unsafe and potentially dangerous situations in the casting shop. 
 
 
Table 1: Solid Inclusions in SEN Build-up[1] 
 
Inclusion Chemistry Comments 
Al2O3 Always found during the casting of 
aluminum killed steels, can be caused by 
inclusion agglomeration, reaction between 
the steel and the nozzle or air aspiration 
through the nozzle. 
MgO-Al2O3 (spinel) This inclusion was first noted when 
magnesium was injected into liquid steel. 
With the advent of excellent ladle 
metallurgy practices soluble magnesium 
contents in steels are increasing leading to 
the formation of the magnesium aluminate 
spinel phase. 
TiN A precipitation product during casting if 
the equilibrium condition is violated. 
CaO-Al2O3 Found during calcium treatment if 
insufficient calcium is added or there is 
reoxidation. 
CaO-Al2O3-CaS (solid) Typical build-up in higher sulfur 
containing aluminum killed steels where 
the CaS precipitates from the liquid 
calcium aluminate. 
CaO-Al2O3 – MgO.Al2O3(solid) Typical build-up in calcium treated steels 
when soluble magnesium levels in the steel 
are too high. 
 
 
A deposit in the proximity of the nozzle exit ports may cause an uneven flow in the 
mold, resulting in solidification problems and even breakouts. In addition, the disturbed 
flow can also entrain mold slag and chunks of deposit may be broken loose which can 
end up as large inclusion clusters on the shell surface. Clogging can lead to harmful flow 
conditions in the mold much before it is serious enough to be noticed by the operator. 
1048 2001 IRONMAKING CONFERENCE PROCEEDINGS
This is partly because it is not consistently predictable and also because it can occur 
swiftly. Clogging is predominantly experienced during casting of Al-killed steels, 
although other grades also are susceptible to clogging. Titanium alloyed steels contain 
oxide and nitride inclusions in the melt and also exhibit nozzle clogging. 
 
Two questions have been frequently addressed in the literature: 
 
1. What is the source of the depositing particles? 
 2. How and why do these particles deposit to form a clog? 
 
There are many different viewpoints concerning the source of alumina particles that 
are present in the deposit in a nozzle. Some of the popular views are : 
 
I. Reaction between liquid steel and the nozzle refractory: 
 
When the molten steel flows past the refractory, dissolved aluminum in the steel can 
reduce solid or gaseous oxides to form alumina, which is gradually deposited on the 
nozzle wall as the casting proceeds. 
 
 
II. Deposition of indigenous alumina inclusions or deoxidation products: 
 
This hypothesis is the most popularly stated theory in the literature. Many authors 
have reported that the inclusions deposited at the nozzle orifice did not form in situ but 
were formed elsewhere and deposited at the orifice. Snow and Shea [2] realized as early 
as 1949 that the depositing particles are similar to the deoxidation products found in steel. 
 
III. Precipitation of alumina on the refractory surface: 
 
This viewpoint suggests that the growth of the alumina deposit is by the precipitation 
of the dissolved aluminum and oxygen. These precipitating species are suggested to have 
three possible origins: 
 
1. Diffusion of air through the refractory: Many studies conclude that diffusion of air 
through the refractory pores (or a leakage through the gas injection ports and from in 
between the refractory joints) would cause oxidation of the dissolved aluminum in the 
steel. Alumina would be formed as a product and subsequently grow as a deposit on 
the refractory wall. 
 
 
2. Oxygen provided by the refractory materials: Poirier et al. {72}[3] proposed that 
the oxygen from the refractory is transported to the refractory / steel interface as CO 
which oxidizes aluminum. They concluded that the deposition takes place by in situ 
nucleation of alumina from the steel/refractory reaction and thus, even if the steel 
were perfectly clean, clogging would still occur. 
 
10492001 IRONMAKING CONFERENCE PROCEEDINGS
 
3. Temperature drop precipitation at the nozzle: Since there is a heat loss from the 
nozzle to the atmosphere, the temperature of the steel drops when it passes through 
the nozzle. This drop in temperature supersaturates the steel with respect to aluminum 
and oxygen, resulting in the precipitation of alumina. In addition due to the lower 
local temperature, steel can solidify in the inter-particulate spaces and reinforce the 
clog [4]. 
 
 
All of the events listed above may possibly contribute to clogging, although they are 
given varying significance by different authors in the literature. 
 
Making steel as clean as possible is considered to bethe first step towards avoiding 
clogging. However, it is unlikely that steel cleanliness improvements will completely 
eliminate nozzle clogging. Dawson [5] calculated that for typical casting conditions, 
nozzle blockage could occur if as little as one in every 1500 nonmetallic inclusions were 
deposited on the nozzle wall. Given the lack of a complete understanding of the clogging 
phenomenon, most plant solutions are merely changes and modifications in plant 
practices which were found to be useful in avoiding clogging. These changes and 
modifications have evolved and developed into some basic rules over the years. For 
example, one expects clogging to be less severe (for a given steel grade) if the melt is 
comparatively cleaner in terms of oxide inclusions. Current practices are continually 
modified to achieve optimum results. Many times a modification in the practice works 
well even though the reason that it works may not be entirely explained. 
 
Figure 1 is an external view of a clogged SEN used in the casting of an aluminum 
killed carbon steel. White bulk alumina deposit can be seen in the ports. In case of 
aluminum killed steel, the matrix of oxide deposit was identified in 1971 by Singh [6] as 
a 3-dimensional network of small alumina particles sintered to each other. In all build-
ups a 3-dimensional network is seen. 
 
The formation of alumina as a network can occur in two ways: (1) it can grow 
continuously or (2) it can form by agglomeration and sintering. The purpose of this paper 
is to study both types of network to ascertain if there are morphological differences 
between networks formed by these two mechanisms. Networks from numerous clogs 
(provided by different steel companies) were documented. Alumina networks were 
formed experimentally by reaction and also by sintering. A comparison between the 
morphologies was conducted. 
 
 
 
 
 
 
 
 
1050 2001 IRONMAKING CONFERENCE PROCEEDINGS
 
 
Figure 1: External view of a clogged nozzle. 
 
 
Nozzle Clogging Observation 
 
Numerous actual clogs were observed in detail. The clog samples were provided by
the various steel companies supporting this work. These clogs came from different
locations e.g. from tundish well nozzle, from the SEN including the length from the slide
gate to the exit ports. Also, different steel grades were represented by these samples. All
of the clogs were analyzed using several different techniques including SEM observation
and chemistry determination by XRD. Some of the salient features found in these
samples are presented below.
Clog Deposit Structure in Alumina Graphite Shrouds 
 
The following description is of deposits found in clogged SENs where the clogging 
occurred in the port area during the casting of various aluminum killed steels. 
 
Macrostructure of the deposit 
The bulk of the deposit is composed of oxide particles (usually alumina) of micron 
size. The white powdery alumina deposit has a porosity of 80% or greater. The voids may 
be empty or filled with steel. In case of absence of the steel, the deposit is very friable 
and care has to be taken while handling to avoid any damage while preparing the sample. 
Deposit at the exit
ports 
10512001 IRONMAKING CONFERENCE PROCEEDINGS
 
Figure 2 shows a schematic of the deposit thickness in two perpendicular sections 
along the length of the SEN. We can see that the bulk deposit is not of uniform thickness 
along the length and it is also different in different sections at same height. A slide-gate 
system shows the most prominent dependence on the flow. The deposit is preferentially 
present at the sites where the flow eddies are present. 
 
 
 
 
Figure 2: Cross-sectional schematics show a spatial difference in the shape 
of the deposit. On the left is a section along the port exit. Figure on the right is a 
section at 90° from the ports (of a two port SEN). 
 
 
1052 2001 IRONMAKING CONFERENCE PROCEEDINGS
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SEN
Bulk Deposit
 
Figure 3: Schematic and examples showing a difference in the deposit above 
and below the meniscus level. 
 
In general a bulk deposit is present heavily in the areas submerged inside the melt 
pool in the mold. The straight part of the SEN is essentially clean. In case of tundish well 
nozzles the deposit is heavy near the entry of metal from the tundish into the nozzle. 
Figure 3 shows a schematic of the deposit structure along the length of the nozzle. This 
schematic shows the difference in the shape of the clog at these two different positions. In 
all the clogged nozzle samples, received as whole pieces, a distinct relation of the deposit 
to the meniscus was seen. Due to presence of steel up to the meniscus on the outside of 
the nozzle the steel flow inside is expected to be different from that in the upper region of 
the SEN. It can be thought that the presence of steel in form of a pool at the bottom of the 
SEN will cause recirculation patterns in the steel which will accelerate the inclusion 
deposition many-fold. This reasoning can be used to explain the more than noticeable 
difference in the deposit below and above the meniscus levels. Looking at two examples 
of SEN designs in Table 2 it can be seen that the SEN bore is probably never full of 
liquid steel during casting. 
 
Table 2: Cross-sectional opening areas at different location in two nozzles. 
 
Sample Cross-sectional Area at different locations in the 
SEN, cm2 
 Entry Bore Ports (combined) 
1 53 53 135 
5 26 45 53 
 
10532001 IRONMAKING CONFERENCE PROCEEDINGS
Clog Deposit Structure in Tundish Well Nozzles 
 
Figure 4 shows the cross-sections of two different tundish well samples. We can see 
the curvature in the deposit profile which makes the effect of flow obvious. 
 
Figure 5 shows the photographs of horizontal cross-sections of the deposit in a 
tundish well nozzle. The sample represents a typical clog with a network of alumina 
particles connected in 3-dimensions that formed the bulk of the clog. Figure 6 shows the 
SEM micrographs of a polished section of this deposit. The only difference compared to 
SEN clogged nozzle samples was the presence of greater amount of steel throughout the 
clog, although some pockets of alumina without steelwere present. 
 
No variation of steel chemistry is detected along the thickness of the bulk clog as 
indicated by the WDX analysis. Table 3 is an example of WDX measurement done on a 
tundish well nozzle clog sample: 
 
 
 
Figure 4: Longitudinal section of tundish well nozzle specimens. 
 
Free steel 
stream 
Deposit 
1054 2001 IRONMAKING CONFERENCE PROCEEDINGS
 
 
Figure 5: Photographs of a tundish nozzle well used for a re-phosphorized 
low carbon steel. The horizontal cross-sections are at two different positions 
along the length. A thick deposit of alumina in a steel matrix is seen. Light 
colored part in the middle indicates the steel which solidified after the casting 
was stopped. 
 
 
 
Figure 6: Micrographs of the above tundish nozzle well after metallographic 
polishing. Image on the left shows the boundary between inner clog and the free 
steel stream. 
 
Table 3: The concentrations of silicon and carbon in the steel present in the 
clog. 
Location Si (%) C (%) 
Near the nozzle wall .04 .05 
Middle of deposit .04 .03 
Bulk Steel .05 .04 
10552001 IRONMAKING CONFERENCE PROCEEDINGS
 
The high magnification micrographs of the particles reveal growth striations and/or 
faceting on all the particles, which can give us some idea of the conditions in which they 
formed. In the above two specimens from the tundish well nozzle, the deposit was 
penetrated by fully dense steel. It is possible that the steel penetrated into the clog 
material after the casting was stopped since these specimens were in a location above the 
slide gate. But it is also possible that the steel was present at the time the casting was 
going on. This is because, unlike in the case of an SEN where the pressure of the flowing 
steel may even be below atmospheric level, in case of a well nozzle there is a sufficient 
pressure head from the steel present in the tundish. 
Alumina particles 
The refractory particles forming the deposit are in the size range of a few of microns. 
Figure 7 shows typical morphologies of inclusions found in the deposit. In this case, all 
of these particles have come from the same clogged nozzle. It should be noted that the 
shape and size of these alumina particles is very similar to the deoxidation products found 
in steel. 
 
 These micrographs were obtained by collecting some of the friable alumina deposit 
on an SEM sample holder (a carbon embedded polymer sticker on top of an aluminum 
stub). Micrographs in which the steel has been etched away to reveal the clog show 
exactly the same structure with alumina particles forming a network. There is no clearly 
apparent difference between the alumina which didn’t contain any steel in the voids 
against the alumina which did have steel. Although it appears that the particles without 
any steel tend to exhibit heavy faceting and the presence of clearly visible striations. This 
observation may be related to a difference (made by the absence or presence of steel) in 
the way alumina particles change their shape when held at high temperatures. 
 
1056 2001 IRONMAKING CONFERENCE PROCEEDINGS
 
 
 
 
 
Figure 7: High resolution SEM micrographs of the deposit particles. In all 
the above photographs the particles are alumina found in the clog material from 
different SENs. 
 
 
 
 
 
 
 
Figure 8: Example of calcium aluminate clog particles from an SEN. In this 
case the particles have heavy faceting compared to alumina particles. 
 
10572001 IRONMAKING CONFERENCE PROCEEDINGS
 
 
Figure 9: Example of calcium aluminate clog particles in a tundish well 
nozzle. In this case there is no steel in the clog. The calcium aluminate phase is 
fully dense. 
Figure 8 and Figure 9 show some examples of particles found in calcium-aluminate 
clogs. Here we can see that the particle structure is quite different from alumina clogs. 
The high resolution micrographs of the deposit material from different nozzles show the 
following trend for a calcium aluminate based clog: 
 
1. In the case of a pure alumina deposit, the particles are usually distinct. They 
invariably have growth striations (although facets are also seen in some cases) and 
are connected to each other with what looks like a sintered neck growth. 
 
 
2. In the case of complex deposits, such as calcium-aluminate or calcium-titanate or 
some such combination, the particles have heavy faceting. Particles cannot be seen 
distinctly, rather they appear to be growing on top of each other. 
 
 
3. The calcium aluminate is composed of two distinct phases finely mixed together. 
 
Chemistry of the deposit 
 
To determine the different constituents of the deposit material two different methods 
were utilized. X-ray diffraction (XRD) analysis provides a more accurate determination 
of the chemical compounds present in the clog, while the Energy Dispersive 
Spectroscopy (EDS), which works in conjunction with the SEM, provides a fast and 
fairly accurate way of determining the content of the elemental species. Also available in 
conjunction with the SEM is the facility of Wavelength Dispersive Spectroscopy (WDS); 
which is a more accurate method of quantifying the elemental species. 
 
X-ray diffraction was carried out on only a few samples. In all other cases the oxide 
deposit chemistry was analyzed using EDS or WDS. While EDS can detect amounts of 
an element down to a fraction of a percent, it does not give a very accurate measure of the 
amount present. However, most clogs showed Al and O in the elemental analysis and can 
1058 2001 IRONMAKING CONFERENCE PROCEEDINGS
be assumed to be Al2O3. In the case where calcium is present as shown by the EDS, we 
can only say that it is some form of calcium aluminate. XRD is a tedious method and is 
hindered by special treatment of the specimen; relatively large amounts of oxide bulk 
deposit are needed with little or no steel present. 
X-ray diffraction analysis 
The deposit was removed from the inside surface of the nozzle for XRD analysis. 
The clog material was removed and crushed to prepare an adequate amount of powder 
sample for the analysis. The deposit was found to be composed of pure α-alumina in the 
case of a clogged nozzle used to cast plain carbon steel. In case of a specimen used to 
cast calcium treated carbon steel the deposit was found to be composed of calcium-
aluminates, the major phases being CaO.2Al2O3 and 3CaO.5Al2O3. Both these phases are 
rich in alumina (and thus are solid at steelmaking temperatures) compared to the low 
melting calcium aluminate phase, CaO.Al2O3. 
 
While XRD provides a highly accurate method of determining the compounds 
present in a sample it is a bulk determination method in that a big chunk of the clog 
sample has to be taken out and crushed to obtain a powder sample to be used in the XRD 
equipment. In this method the local characteristics of the deposit are lost. As seen in the 
SEM observation of the calcium-aluminate clogs, the two different phase of CaO.xAl2O3 
were intermeshed finely at a micron or sub-micron scale, such that they were not 
distinguishable in the SEM. As has been reported in the literature frequently, the clogs 
from the rephosphorized grades did not show any phosphorus. The clog structure was 
indistinguishable from the typical alumina clog. It should be noted that rephosphorized 
steel grades are notorious for clogging. 
EDS & WDS measurements 
Though the measurement of amount of Si present in metal is not strictly quantitative 
by EDS, we can observe a difference in the Si levels present in the top and bottom 
portions of the SENs. An interesting observation which supports the theory of reaction 
between the refractory and steel is that the droplets of steel entrapped in this matrix of 
oxides/metalshow a gradient in the concentrations of Si as one goes from the nozzle wall 
to the inside of the bulk deposit. Concentrations of Si as high as 10 wt.% have been 
observed in the deposits of the nozzles which were used to cast carbon steels having very 
low silicon in the product. In LCAK and TiSULC steels the Si level in typically below 
0.03 %wt. This may be due to an accelerated rate of the following reaction as frequently 
proposed in literature: 
SiO2 + Al = Al2O3 + Si Equation 1 
Although this observation may indicate that there is a reaction going on between the 
constituents in the steel and the refractory material of the nozzle, this localized high Si 
presence among other elements may be coming from the glaze present on the unused 
SENs. 
 
10592001 IRONMAKING CONFERENCE PROCEEDINGS
SENs are ceramic composites, and are sensitive to thermal shock. Therefore, they are 
preheated with an open flame through the bore at the mill prior to use. Since SENs also 
contain graphite, which will oxidize during preheating, the purpose of application of 
glaze is to protect the SEN from oxidation. All SEN manufacturers apply glaze to 
nozzles. Glazes for all applications use a suspension of glass frit along with additives. 
Three commercially used glass formers are Al2O3, SiO2 and B2O3. Therefore, any glaze 
would generally contain one or more of these components. It should be noted that the 
reaction layer on the SEN above the melt meniscus level may be a result of the preheating 
of the SEN in air prior to installation on tundish for casting. 
 
Summary of clog observations 
A variety of microstructures were observed in the different samples. Different kinds 
of structures were found. Figure 10 shows an alumina clog from a rephosphorized carbon 
steel. The individual pure alumina particles can be seen along with the joints between the 
particles. We can see striations and facets on the individual particles. These structures 
were thought to be a result of the way a clog grows. Figure 11 shows a dendritic alumina 
structure found in a clog. The dendrite itself is a monolithic structure and its arms are 
connected to the other parts of the clog material. 
 
Non-alumina clogs show an even greater and more complex variety of structures. 
Calcium aluminate clogs typically have a dense structure and it is not apparent that these 
clogs are composed of distinct particles. The structures seen in Figures 12 and 13 appear 
to be monoliths with Figure 14 showing only some hint of individual particle structure. 
 
A simple particle by particle deposition and sintering is not adequate to explain the 
observed clog structures. 
 
 
Figure 10: SEM images of alumina from the bulk clog of a rephosphorized 
carbon steel. 
1060 2001 IRONMAKING CONFERENCE PROCEEDINGS
 
Figure 11: SEM micrograph of the bulk alumina clog from carbon steel. 
Notice the size of the dendrite of alumina. 
 
 
 
Figure 12: High magnification SEM micrograph of the calcium-aluminate 
structures found in the deposit from a Ca-treated carbon steel. 
10612001 IRONMAKING CONFERENCE PROCEEDINGS
 
 
Figure 13: High magnification SEM micrograph of the clog showing 
dendritic growth of calcium-aluminate from a Ca-treated carbon steel. 
 
 
Figure 14: SEM image of the clog composed of complex oxide from a 
stainless steel. 
 
1062 2001 IRONMAKING CONFERENCE PROCEEDINGS
Alumina Networks by Growth 
 
Levitation melting and deoxidation 
 
Experiments were done with levitation melting and deoxidation. The major 
advantages of levitation deoxidation experiments were: 
 
1. Absence of any contact of the melt with a refractory surface. 
2. Ability to quench the samples rapidly. 
 
In one experiment a piece of 2g pure iron was levitation melted in an argon 
atmosphere. The piece was melted quickly and aluminum was added by touching an 
aluminum wire to the top surface of the molten iron droplet. It was visually noted that the 
surface of the iron droplet was covered with an oxide layer immediately after adding 
aluminum. Right after deoxidation the power was cut-off allowing the droplet to fall into 
the copper mold placed below the coil. Figure 15 shows an example of the levitation 
deoxidized sample. In this case the droplet was cooled while levitated by adjusting the 
power and the solidification time is estimated to be less than one minute. The egg-shaped 
solid iron thus obtained was cut longitudinally and polished metallographically, followed 
by etching. In the micrograph we can see the dark shaded alumina deoxidation products 
present near the top surface of the droplet. A closer look reveals that these massive 
clusters are composed of a three-dimensionally connected alumina network (Figure 16). 
 
 
 
 
 
 
 
 
10632001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
b) 
Figure 15: A levitation deoxidation sample after polishing and etching: a) 
three big rafts on the top end of droplet are seen. The small dark spots around 
the periphery are artifacts from polishing and etching. 
 
1064 2001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
b) 
Figure 16: Close-up of alumina clusters seen in Figure 14. The three 
dimensional network type structure of the alumina is seen. 
 
10652001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
b) 
Figure 17: The structure of alumina obtained after electrolytic extraction 
from levitation melting and deoxidation of pure iron. 
 
1066 2001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
b) 
Figure 18: Close-up structure of the alumina deoxidation product in Figure 
17. 
Figure 17 shows examples of the three-dimensional structure of the alumina. These 
micrographs were obtained after electrolytic extraction of the alumina from the iron 
droplet. We can see dendritic structure (a) and a comparatively denser structure (b). 
Figure 18 is a still closer look at these alumina products with the continuity of the 
structure clearly apparent. We cannot see distinct inclusions of alumina with sintering 
necks connecting them together. Rather, it appears to be a continuous growth of alumina 
on itself. 
10672001 IRONMAKING CONFERENCE PROCEEDINGS
Crucible deoxidation with aluminum 
Pure iron (electrolytic iron from Fisher Scientific) was melted in crucibles. The 
sample weight varied from 50g to 300g. In all experiments about 1 wt. % aluminum was 
added to make sure that enough aluminum was available for consuming the dissolved 
oxygen and further to go into solution. 
 
The following steps take place on the addition of the aluminum: 
 
• Aluminum, initially at room temperature, contacts the hot melt and melts. 
• The aluminum reacts with the dissolved oxygen to form alumina along with a 
release of exothermic reaction heat. 
• The growth of alumina takes place along the surface of the melt and into the 
melt at a rapid rate. 
• After the initial burst of growth the concentration of oxygen in the vicinity of 
already formed alumina falls and the growth stops. 
 
 
Further growth takes place after the diffusion of aluminum and oxygen to different 
heterogeneous solid surface locations; as for example the crucible walls and bottom. 
Presence of adequate amounts of dissolved Al and O is required for the growth of 
alumina. 
 
Addition of aluminum 
 
In one of the deoxidation experiments pure iron was melted under vacuum. The bell-
jar was evacuated and flushed twice with argon before the final evacuation. The vacuum 
pump was kept operating while melting and deoxidation was completed. As seen in 
Figure 19 the aluminum contacts with the melt surface and immediately reacts with the 
dissolved O to form alumina. The figures shows addition and reaction of aluminum with 
dissolved O to form alumina which rapidly covers the surface of the melt. Theframes 
have 30 to 90 milliseconds elapsed between them (with the exception of the last frame). 
The total time elapsed from frame 1 to frame 7 is 350 milliseconds. Note the movement 
of the alumina rod from the center of the first image to the right in the last image. The 
growing alumina pushed this thin alumina rod which was kept in the melt to study 
alumina growth on a refractory surface. The whole surface of the melt is covered with 
alumina in a less than 100 milliseconds. The growth velocity of surface alumina can be 
estimated to be in excess of 0.66 m/s. Note that this growth velocity cannot be attributed 
to the bulk convection flow in the crucible melt. Firstly, the melt appears relatively quiet 
because the induction coil couples with the graphite susceptor, thus shielding the iron 
melt from inducing currents and flow. Secondly, as mentioned, the alumina grows 
outwards in all directions from the point of impact on the melt surface. 
 
1068 2001 IRONMAKING CONFERENCE PROCEEDINGS
 
 
 
 
Figure 19: Video frames showing the events taking place on the addition of 
aluminum to the steel melt. 
10692001 IRONMAKING CONFERENCE PROCEEDINGS
Post-mortem analysis of deoxidation samples can show us the morphology of these 
deoxidation products. Figure 20 shows a schematic of alumina growth observed in the 
deoxidized ingots. Figure 21 shows the surface of an iron ingot after deoxidation with 
aluminum. The alumina appears to have formed as distinct grains a few microns across. 
The alumina sheet is non-porous with every grain attached all around the periphery 
connected to other grains around it. This morphology of surface alumina is observed in 
all deoxidation samples. The image on the right shows the surface alumina (seen as flat 
surface facing left) with the attached bulk alumina, seen as a 3-D network of alumina 
rafts. The alumina appears to be formed as distinct grains a few microns across. 
 
 
Figure 20: A schematic of deoxidation product growth on the surface of the 
melt and on crucible surfaces. 
From the video recordings of aluminum deoxidation we can see that alumina on the 
surface forms in a burst of less than 100 milliseconds. A curious event is observed in one 
experiment when thin alumina rods are actually pushed away from the point where 
aluminum was added, by outward growing alumina layer. These thin rods of alumina 
were placed in the melt before aluminum addition to study growth on rods. Also worth 
noting is the fact that alumina growth is seen taking place starting from the middle of the 
Aluminum
Growth on crucible
surfaces
Alumina growth on melt
surface
1070 2001 IRONMAKING CONFERENCE PROCEEDINGS
melt surface, around the location of addition of aluminum, outwards towards the crucible 
sides. The aluminum added has solids already present that provide nucleation sites. It is 
also observed that aluminum has a layer of alumina, as the aluminum wire being added 
tends to retain its shape at temperatures well above the melting point of aluminum. 
 
a) 
b) 
 
Figure 21: The surface of iron ingot after deoxidation with aluminum. 
 
10712001 IRONMAKING CONFERENCE PROCEEDINGS
 
Figure 22: A polished cross-section of the ingot showing inclusions growing 
into the bulk of the ingot from the surface. 
 
 
 
Figure 23: An etched image near the top surface of aluminum deoxidized 
iron ingot. 
 
1072 2001 IRONMAKING CONFERENCE PROCEEDINGS
We can observe the inclusions using different methods. Figure 22 shows a polished 
cross-section of the ingot with alumina inclusions growing into the bulk of the ingot from 
the surface. Figure 23 is from a similar sample. In this case, the sample was etched after 
polishing the longitudinal section of the ingot. The gray patches all over the bulk of the 
sample are facets cut in iron grains due to etching. The dark areas touching the curved 
ingot surface are the alumina deoxidation inclusions. These methods of observing 
inclusions are often ignored in the literature. The usual practice is to look for inclusions 
in the bulk of the ingot. As can be seen, a major part of the actual picture is missed in 
such metallographic analysis. We do not notice the presence of surface alumina. In fact 
the surface of the experimental samples are usually cleaned by removing the surface layer 
mechanically. It is treated as an undesirable and dirty part of the ingot and the importance 
of surface is not realized. 
 
Once the initial burst of deoxidation product growth near the addition site ceases 
because of local depletion of dissolved species, further growth can take place at other 
heterogeneous sites once the species have diffused there. One such site is the surface of 
the refractory crucible. Figure 24 shows the alumina growth on the side of the alumina 
crucible. The white circles and curved lines are the growth of alumina along the edges of 
the bubbles found on the ingot. The small globules are iron droplets stuck to the crucible 
wall. Figure 25 shows the close-up SEM images showing growth of alumina on the 
crucible surfaces on deoxidation with aluminum. The image on the right shows the close-
up of alumina growth near the edges on the bubbles on ingot surface (also seen in 
Figure 24 as circles formed by white dots). Figure 26 shows further close-up images. 
It can be clearly seen that the alumina particles are growing from the alumina grains of 
the crucible. The continuity of alumina between the crucible grains and the growing 
particles confirms that there is actual growth of particles taking place as opposed to 
sintering of particles already formed elsewhere. Also the particles are seen in different 
stage of growth. 
 
 
Figure 24: Alumina deoxidation product growth on the side of the alumina 
crucible. 
10732001 IRONMAKING CONFERENCE PROCEEDINGS
 
a) 
b) 
 
Figure 25: SEM images showing growth of alumina on the crucible surfaces 
on deoxidation with aluminum. 
 
1074 2001 IRONMAKING CONFERENCE PROCEEDINGS
 
a) b) 
 
c) d) 
Figure 26: Close-up of alumina deoxidation product growth on the side of 
the alumina crucible. 
Crucible Deoxidation with Titanium 
Experiments were done with addition of alternate deoxidizers to the iron melts. From 
the observations made on these samples we can compare the products to those observed 
in aluminum deoxidation. It is seen that the general characteristic of the presence of 
deoxidation products on the top surface of the ingot is similar. 
 
Figure 27 shows the surface of an iron ingot deoxidized with titanium. In this case 
we see surface titanium oxide which appears to be similar to the alumina, but the grain 
size is larger and shows a greater variation from grain-to-grain, unlike alumina which has 
a very restricted distribution. Here we can see dimensions of grains exceeding 20 µm, 
while other grains are only a few microns in size. This and other differences from 
alumina can be attributed to the difference in the way the two deoxidation products grow 
and interact with liquid iron. 
 
10752001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
 
b) 
 
Figure 27: The surface of the iron ingot deoxidized with titanium. 
 
1076 2001 IRONMAKING CONFERENCE PROCEEDINGS
 
 
Figure 28: Polished section of titanium deoxidized iron. Very small amount 
of titanium-oxide in the bulk is noticed. 
Figure 28 shows the polished section of an area near the top of the ingot. The top 
part of the ingot is seen with the surface curvature against crucible side suggesting a 
wetting interface. When compared to aluminum deoxidation, a lack of titanium-oxide 
deoxidation product in the bulk is noticed. Figure 29 shows the titanium oxide inclusions 
on the side surface of the ingot. Again, compared to alumina deoxidation productformation this case may differ in the wetting properties of titanium oxide with iron. This 
difference may account for the lack of Ti-oxides growing into the bulk of the ingot, and 
in the difference in the shape of the small individual particles. 
These growth experiments have shown that networks of alumina can be formed by 
growth. The networks precipitate on pre-existing heterogeneous nuclei. The formation 
time is very short and cannot be due to a sintering type of model as no evidence of sinter 
necks are seen. The growth appears to form a 3 dimensional continuous network. This 
network can exhibit many of the morphological features seen in industrial clogs. 
 
 
10772001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
b) 
 
Figure 29:Titanium oxide inclusions on the side surface of the ingot 
deoxidized with titanium. 
 
 
 
1078 2001 IRONMAKING CONFERENCE PROCEEDINGS
Sintering of Alumina Particles 
 
Sintering of alumina particles results in coarsening and bonding of particles in the 
form of a structure which, in appearance, is very similar to a clog. Figure 30 and Figure 
31 show examples of the result of sintering alumina particles. Figure 30 shows particles 
obtained (from Alfa Aesar, pure gamma-alumina) with a catalogued cluster size of 3 
microns. As can be seen from the figure the actual individual particles are much smaller 
in size. After sintering in air at 1600 °C for 4 hrs the resulting structure is very similar to 
that of a clog deposit. Although the particle size is around 2 microns, as seen in a clog, 
the particle packing is denser. That is, there are lesser voids in between the particles. 
Figure 31 illustrates a different example of alumina particles before and after sintering in 
argon for 1hr at 1600 °C. In this case also the smaller particles have disappeared and the 
resulting particles have smooth edges. Note that in this case perhaps the sinter duration 
was much less, which may not have been sufficient for a structure more similar to a clog 
to develop. Also, because of the bigger initial particle size the kinetics of modification 
could have been slow. 
 
Sintering of these same alumina particles (and clog particles) while immersed in 
steel in the absence of oxygen is described in the next section. The results are similar as 
in the case of sintering in air or argon atmosphere. This raises the question of whether 
what we see now is what was actually present during the growth of a clog. Figure 32a 
shows actual clog particles from a re-phosphorized grade tundish well nozzle. Figure 32b 
shows the resulting particles after sintering for 1hr at 1600 °C in argon. The result shows 
that the appearance is almost the same though the particle packing has become denser as 
compared to the initial particle network. Experiments of sintering in air have shown that 
iron-aluminates can result from iron and alumina particles in presence of oxygen. 
 
Experiments involving holding of alumina particles or actual clog at high 
temperature have shown that the shape and size of the alumina particles can be modified. 
This fact points to a possibility that the observed particles in the clog, in shape and size, 
may not be the original ones which were present in the clog while the casting was going. 
This is especially important if the clog actually forms as a different phase and then after 
some duration turns into the clog that we observe post mortem. Sintering in air has shown 
that the alumina particles can undergo a chemical change to form an iron-aluminate when 
held in iron at steelmaking temperatures under high oxygen atmosphere. This would 
mean that the iron-aluminate in the clog as found by us, and as has been reported in the 
literature may not have been a part of the clog. Perhaps it formed after the casting was 
stopped and the nozzle was removed, exposing it to the atmospheric air while still hot. 
Comparison of the sintered alumina particles with the clog particles shows that the clog 
particles do not have such defined necks. 
 
 
 
 
10792001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
b) 
Figure 30: Alumina particles (synthetic alumina from Alfa Aesar) before 
and after sintering at 1600 °°°°C in air for 4hrs. 
 
1080 2001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
b) 
Figure 31: Alumina particles from Alcoa before, and after sintering at 1600 
°°°°C in argon for 1hr. 
 
10812001 IRONMAKING CONFERENCE PROCEEDINGS
a) 
b) 
Figure 32: Actual clog particles (from re-phosphorized steel) (a) before and, 
(b) after sintering in argon at 1600 °°°°C for 1hr. Some densification of the alumina 
network can be seen. 
 
 
 
 
 
 
1082 2001 IRONMAKING CONFERENCE PROCEEDINGS
In brief, the following observations were made in these experiments: 
 
• Sintering experiments indicated that the alumina particles can be modified by holding 
at steelmaking temperatures. Resulting alumina appears to be similar, in shape and 
size of particles, to a clog alumina. 
• Sintering kinetics are too slow for sintering of alumina particles of 2-5 µm in size to 
account for the morphologies observed in deoxidation experiments of short duration. 
Thus, growth as well as agglomeration can form alumina networks that are 
morphologically similar to clogs. 
• The clog particles that we observe may change (physically or chemically) after 
casting. This was shown by conversion of alumina to iron-aluminates. 
 
Discussion
 
Alumina networks that can be formed by agglomeration and sintering or by growth 
have similar morphologies. 
 
This work indicates that the classical view of clogging as sintering of individual 
particles may not be sufficient to explain the morphology of all alumina clogs. This is not 
to say that in very dirty steels that the predominant mode of clogging is not 
agglomeration. However, from post mortem examination of clogs one cannot decide the 
mechanism of clog formation unambiguously. Thus there are times when the clog can be 
agglomeration and sintering, growth or a combination of both. 
 
This work indicates that in very clean steels where particle capture mechanisms are 
not the major method of clogging, it is possible to grow clogs which will resemble the 
clogs commonly seen during the casting of fairly dirty steels. The major difference will 
be in the time it takes for the clog to grow to a significant size. In this mechanism the 
SEN itself or the clog acts as a heterogeneous nucleation site for alumina growth. 
 
Often in the literature temperature drop supersaturation is quoted as a reason for 
clogging. However, if one calculates the potential growth rate in a turbulent flow 
scenario, the growth rate in aluminum killed steels due to temperature decrease against 
the SEN is of the order of 0.05 cm/hour and cannot explain most observed clogging. If 
there is transient supersaturation due to local reoxidation because of air aspiration into the 
nozzle, this rate can rise to close 1cm per hour or higher. This work thus indicates that in 
times of transient supersaturation due to excessive reoxidation or reaction between the 
refractory and the steel, the clog itself will grow. This could explain the rapid clog 
growth rates that are seen during extremely transient operations when there is air 
aspiration into the SEN. 
 
Clogs are dynamic due to fast sintering kinetics at steelmaking temperatures. Thus 
the clogs that are viewed after casting are a result of the time and temperature history of 
the process and are not simply a picture of the accumulation of the inclusions present in 
the steel. 
 
10832001 IRONMAKING CONFERENCE PROCEEDINGS
Conclusion 
 
Alumina networks that can be formed by agglomeration and sintering or by growth 
mechanisms will have similar morphologies due to the effects of sintering on the 
observedmorphologies. 
 
 
Acknowledgement 
 
The authors would like to thank the member companies of the CISR for their support 
during the progress of this work. 
 
 
References 
 
(1) R. Maddalena, R. Rastogi, B. El Dasher and A. W. Cramb: Nozzle Deposits in 
Titanium Treated Stainless Steels, Trans ISS, 2000, Iron and Steelmaker, December 
2000 
(2) Snow,R.B., Shea,J.A., Marbaker,E.E., Benedict,V., Mechanism of erosion of nozzles 
in open-hearth ladles, Journal of American Ceramic Society, 1949, 32, p187-194. 
(3) Poirier,B.T., Guiban,M.A. , Provost,G., Mechanisms and countermeasures of alumina 
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(4) Farrel,J.W., Hilty,D.C., Steel flow through nozzles: Influence of deoxidizers, Electric 
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Steel Making Conference Proceedings, 1990,p15-31. 
(6) Singh, S.N., Morphology of alumina deposits from aluminum-killed steel frozen in a 
constricted nozzle, Metallurgical Transactions, Vol.2, 1971, p3248. 
 
 
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