Form of Al–Ti Oxide Produced by Al–Ti Deoxidation Reaction at 1873 and 1473 K
Hiroyuki Matsuura
Lecturer
Department of Advanced Materials Science,
Graduate School of Frontier Sciences,
The University of Tokyo
1. Introduction
Titanium-alloyed steels have been widely used for various products such as automobile
sheets, heavy plates, or stainless steels, because titanium has great advantages not onlyin cost effectiveness but also in improvements of physical and chemical properties ofsteels, such as formability, non-ageing property, prevention of austenite grain growth, or
promotion of fine ferrite microstructure formation. It is well known that titanium forms
various non-metallic inclusions, e.g. oxide, nitride, carbide, or sulfide; especially titaniumoxide exists in several forms such as Ti2O3, Ti3O5, TiO and so on, depending on steel
compositions.
Since titanium has a strong affinity with oxygen, it is normally alloyed after
deoxidation process. Aluminum is one of common strong deoxidizer. Titanium is much
expensive raw material compared to aluminum, and therefore the combined process ofdeoxidation by aluminum and alloying by titanium is popular. The Al–Ti deoxidation
process is one of the common processes for secondary refining. Thermodynamics for Al
deoxidation or Ti deoxidation reaction have been studied well, while the complex
deoxidation by Al and Ti addition has not been comprehensively understood. Ruby-Meyer

et al.1) have estimated the stable oxide phases equilibrated with Fe–Al–Ti melts byemploying the multiphase equilibrium code CEQCSI based on the IRSID slag model2) at
1793 K. The calculated stable phase diagram indicates the formation of Al2O3, Ti2O3,
Al2TiO5 or TiOx–Al2O3 liquid oxide. On the contrary, Jung et al.3) have employed FACT
databases and FactSage software, and calculated the similar phase diagram of oxides for
the Fe–Al–Ti–O system at 1873 K. They have reported the stable region of a Ti3O5 solid
phase which have not existed in the reported diagram by Ruby-Meyer et al.,1) while they
have not found a liquid phase. Later, they have revised the previous phase diagram inwhich stable regions of Al2O3, Ti2O3, Ti3O5 and liquid oxide equilibrating with metal are
shown.4) Kim et al. also reported the phase stability diagram of oxides equilibrating with
Fe–Al–Ti melt at 1873 K. The stable regions for Al2O3, Ti2O3 and Ti3O5 were shown, while
those for other oxides and liquid oxides were not observed.5) As a conclusion, the chemical
stability of Al–Ti–O oxides equilibrating with metal is still open for discussion.
The stability of non-metallic inclusions containing titanium in solid steel would be
more complex due to the formation of not only oxide but also nitride, carbide, sulfide and－ 53 －

so on, which are normally not found at secondary refining temperatures. The behavior of
non-metallic inclusions in solid state steel is quite important to understand therelationship between microstructure of steel and mechanical properties, and thus tomaximally bring out such properties for best performance. Nevertheless, even thestability of oxides containing aluminum and/or titanium in solid steel at heat treatment
temperatures has not been clarified.
In the present study, firstly the stable region of Al2TiO5 which has not been
experimentally measured among various oxides produced by Al–Ti deoxidation, wasmeasured by chemical equilibration technique at 1873 K. Secondly, the evolution andmodification of oxide inclusions in Fe–Al–Ti steels were investigated and the change ininclusion compositions, morphologies and size with heating time was discussed.6)
2. Experimental
2.1. Equilibrium between Al2TiO5 and Fe–Al–Ti melt
Firstly Fe–Al–Ti metals (Al: 0.0142 to 0.1088 mass%, Ti: 0.0380 to 0.3875 mass%) wereprepared by following procedure. About 150 g of solid electrolytic iron were put in a
porous MgO crucible (OD 38 mm, ID 26 mm and height 123 mm) and then the crucible
was put inside a quartz reaction tube (OD 50 mm, ID 46 mm and length 360 mm). Afterthe atmosphere inside the reaction tube was changed to Ar gas (purity 99.9 %, flow rate
400 cm3/min), the prepared electrolytic iron was heated and melted by an induction
furnace at 1873 K for 1 h. After that Al (purity 99.9 %) and Ti (purity 99.9 %) were addedin sequence with interval of 2 min. Finally the crucible was taken out from the reaction
tube at 10 min after titanium addition, and quenched by water.
Al2TiO5 powder was produced by mixing reagent grade Al2O3 and TiO2 powders inequimolar amounts and sintering about 20 g of the mixture at 1823 K for 24 h inside an
Al2O3 crucible (OD 38 mm, ID 34 mm and height 45 mm). Obtained powder wascompacted in a tablet shape of 18 mm diameter and 1 mm thickness and sintered at 1823
K for 24 h in an Al2O3 crucible to prepare the Al2TiO5 pellet.
Figure 1 shows the schematic diagram of equilibrium experimental apparatus. About
30 g of Fe–Al–Ti sample and 1 g of Al2TiO5 pellet were put in an Al2O3 crucible (OD 38
mm, ID 34 mm and height 45 mm). The crucible was suspended at the top of an Al2O3
reaction tube (OD60 mm, ID 52 mm, length 1000 mm) by winding with tungsten wire.
The reaction tube atmosphere was changed to Ar gas and thus Ar–3%H2 gas wasintroduced by 200 cm3/min. Then, the crucible was moved down to the hot zone of thefurnace. The crucible was quickly pulled up after 3 h and Ar gas was blown for quenching.
One gram of specimen was machined from each Fe–Al–Ti sample and dissolved into
mixed acid (HCl : HNO3 : Water = 1 : 2 : 5) and Al and Ti contents of solution wereanalyzed by ICP–OES (SII SPS7800) after filtration. Total oxygen content was analyzed
by combustion – infrared absorption method (LECO TC600). X-ray diffraction pattern of－ 54 －

oxide pellet before and after experiment was measured by powder XRD equipment(RIGAKU RINT–2500V).
Metal–oxide pellet interfaces of some specimens were observed and analyzed by
SEM–EDS. The specimen was mounted by polyester resin and polished by SiC papers
and diamond suspensions up to 0.25 μm. Contents of Al, Fe, Mg and Ti in inclusions
observed at the polished surface were determined by EDS (JEOL EX–54175JMU).
2.2. Observation of inclusions in metal deoxidized by Al–Ti
Three kinds of Fe–Al–Ti specimens (hereafter, named as 2–A, 2–B, and 2–C) wereprepared. About 200 g of electrolytic iron was put in an MgO crucible (OD 38 mm, ID 26mm, height 123 mm) and the crucible was put in a quartz tube (OD 50 mm, ID 46 mm,
length 360 mm). After the atmosphere of the reaction tube was replaced to Ar gas (purity
99.9 %, flow rate 400 cm3/min), the sample was melted in an induction furnace at 1873 K
for 1 h, and then Al (purity 99.9 %) followed by Ti (purity 99.9 %) were added in sequencewith interval of 2 min to specimens 2–A and 2–C, or only Ti was added to specimen 2–B.
The melt was further kept for 10 min and then the crucible was taken out from thereaction tube and quenched by immersing a crucible into water. The same methods as 2.1.
for analyses of Al, Ti and total O contents were employed.
A piece of sample (25 mm diameter, thickness 10 mm) was machined from each sample
and hanged in the reaction tube of an electric furnace by steel wire. Then, samples were
heated at 1473 K for 1 to 3 hours after the atmosphere was replaced by Ar gas (purity
99.9 %, flow rate 350 cm3/min). After prescribed heating time, the sample was quickly
taken out of the reaction tube and quenched by immersing into water.
The same procedures as 2.1. for sample preparation were applied to observe inclusions
in as-cast and heated samples. Inclusions were characterized through observations by
SEM (JEOL JSM–6060LV) and composition analyses by EDS (JEOL EX–54175JMU).
Compositions of Al, Fe, Mg and Ti of inclusions were determined.
Stainless steel tube
Ar gas inlet

Gas outlet
Silicone rubber

Alumina shieldTungsten wire
Pt30%Rh-Pt6%Rh
thermocouple

MoSi2 heater
Al2O3 crucible
Al2TiO5 pellet

Alumina tube

Fe-Al-Ti alloy

Alumina tube
Alumina supporter
Alumina shield
Silicone rubber
Gas inlet

Figure 1

Schematic diagram of apparatus for equilibrium experiment.
－ 55 －

3. Results and Discussion
3.1. Equilibrium between Al2TiO5 and Fe–Al–Ti melt
Table 1 shows the compositions of specimens after equilibrium experiments. Oxide
pellets after experiments could be easily separated from metals after all experiments.
However, it was expected from the position and shape of samples that oxide pellet andmetal contacted well each other at 1873 K. In addition, since Al2O3 crucibles were used,molten metal was doubly saturated by Al2TiO5 and Al2O3. The shape of oxide pellet was
the same as the green pellet and the formation of liquid oxide phase was not confirmed byvisual observation.
The dissolution reactions of Al2O3 and Al2TiO5 into molten iron are expressed as
Reactions (1) and (2). Interaction parameters applied for the calculation of Gibbs freeenergy changes from present results are shown in Table 2.7–9)
Al2O3 (s) = 2 Al (mass%) + 3 O (mass%)(1)

Al2TiO5 (s) = 2 Al (mass%) + Ti (mass%) + 5 O (mass%)(2)

Table 1Exp.
No.

Results of equilibrium experiments.

Compositions of metal

Equilibrium constant∆G º of dissolution

(ppm)

(-)

(J/mol)Sol. Al

Sol. Ti

T. O

Al2O3

Al2TiO5

Al2O3

Al2TiO5

1–1

16.3

342.4

50.6

2.98×10−13

2.42×10−19449000667000

1–2

0.74

1.5

249

6.5×10−14

5.6×10−21

470000730000

2.4×10−21

490000740000

1–3

0.94

5.1

145

2.3×10−14

1–4

0.66

1.2

203

3.0×10−14

1.4×10−21

490000750000

1–5

0.67

1.3

272

6.8×10−14

6.0×10−21

470000730000

1–6

2.2

10.2

82.6

2.5×10−14

1.7×10−21

490000740000

1–7

105.8

2051

60.2

1.31×10−11

6.70×10−17

390000580000

1–8

48.2

878.7

46.7

1.77×10−12

2.86×10−18

421000629000

1–9

17.4

286.7

54.6

4.30×10−13

3.43×10−19443000662000

1–10

19.9

359.5

38.6

1.98×10−13

9.85×10−20

45500068200047.3

4.37×10−12

1.29×10−17

407000606000

1–11

84.3

1836

Table 2

Interaction parameters.

eij (jÆ)

Al

Ti

O

Al

0.04307)0.00409)−1.987)Ti

0.00379)

0.0498)−1.038)O−1.177)−0.3408)−0.177)－ 56 －

Gibbs free energy changes of Reactions (1) and (2) calculated from experimental results
are shown in Table 1. Average values of Gibbs free energy changes of the dissolution
reaction of Al2O3 or Al2TiO5 into molten iron at 1873 K were determined as follows.
Al2O3 (s) = 2 Al (mass%) + 3 O (mass%)∆G º = 450000 J/mol(1873 K)

Al2TiO5 (s) = 2 Al (mass%) + Ti (mass%) + 5 O (mass%)∆G º = 680000 J/mol(1873 K)

Gibbs free energy change for Al2O3 dissolution reaction obtained from present study isalmost the same as that reported by Itoh et al.,7) 450600 J/mol.
The Al–Ti oxide stability diagram equilibrated with Fe–Al–Ti melt at 1873 K was
calculated by using obtained Gibbs free energy changes for the dissolution reactions of
Al2O3 and Al2TiO5 into molten iron, and reported equilibrium constants for the
dissolution reactions of Ti2O3 and Ti3O5 expressed as Eqs. (3) and (4).8) Figure 2 shows thecalculated stability diagram with present experimental results. In the present study,
oxygen content of metal may be overestimated since total oxygen content in metal
samples was analyzed by combustion method. Indeed, oxygen content of metal is larger
than estimated by calculation.
Ti2O3 (s) = 2 Ti (mass%) + 3 O (mass%)log K Ti2O3 = −9.986(1873 K)

(3)

Ti3O5 (s) = 3 Ti (mass%) + 5 O (mass%)log K Ti3O5 = −16.50(1873 K)

(4)

Figure 3 shows the XRD patterns of an oxide pellet before and after an experiment.
Only peaks corresponding to Al2TiO5 phase were observed in the case of oxide pellet
before experiment, while both peaks corresponding to Al2TiO5 and Al2O3 phases wereobserved from the pellet after experiment. Therefore, the oxide pellet after experiment is
the mixture of Al2TiO5 and Al2O3.
Compositions of oxides at the interface between metal and oxide pellet in somespecimens were measured. Figure 4 shows the typical interface of metal–oxide pellet.
Light and dark gray regions were observed inside oxide pellet. Measured compositions ofeach phase were shown as the Fe–Al–Ti ternary system in the figure, in which light grayregion is TiOx phase containing approximately 10 mol%Al while dark gray region was
Al2O3 phase containing about 3 mol%Ti. Figure 5 shows the measured compositions ofoxide phases observed at the metal–oxide pellet interfaces as the Al–Ti binary system.
Initial Al2TiO5 phase (Ti content as the Al–Ti binary system is 33.3 mol%) was notobserved at the metal–oxide pellet interface, while Al2O3 phase containing TiOx and TiOx
phase containing Al2O3 were observed. These different oxide phases were also confirmed
from SEM images.
－ 57 －

0

10

Ti2O3

0.001

-1

10

Ti3O5

[mass%Ti]-210

Al2TiO5
Al2O3

-3

10

-4

10

0.05

0.02
-4

[mass%O]
0.002 0.001 0.0005

0.01 0.005
-3

10

-2

10

-1

10

10

[mass%Al]
Figure 2

Compositions of metals after equilibrium on the calculated stable oxide phase

diagram for the Fe–Al–Ti–O system equilibrated with molten Fe–Al–Ti at 1873 K.

Al2TiO5

Intensity (a. u.)

Al2O3

Oxide pellet after Exp. 1-11

Prepared oxide pellet

10

Figure 3

20

30

40
50
2θ (deg)

60

70

80

XRD patterns of oxide pellets before and after equilibrium experiment.

－ 58 －

10 μm(mol%)
Al 10.5
Ti 88.9
Fe 0.6

Resin

Oxide

Steel

Figure 4(mol%)Al 96.2
Ti 2.7
Fe 1.1

SEM image at the interface between metal and oxide phases,
and compositions analyzed by EDS. (Exp. 1–11)

Exp. 1-8

Exp. 1-11

Al2TiO5
0

20

40

60

80

100

Ti content of oxide phase onto the Al-Ti binary system (mol%)Figure 5

Compositions of oxide phase analyzed by EDS.

Oxide inclusions dispersed in some metals after experiments were analyzed to measure
the compositions of oxide phases which could be equilibrated with metal sufficiently
without any effect from gas phase. Figure 6 shows the example of measured inclusions
compositions onto the Al–Ti–Fe ternary system. Inclusion compositions were plotted on
the line from the Fe corner because Fe contents of inclusions were overestimated due tothe X-ray signal from Fe matrix during EDS analysis of fine inclusions, and thus precise
analysis of Fe content of inclusions was hard. Therefore, measured inclusion compositions
were converted to the Al–Ti binary system by removing Fe content for the sake of
convenience. Figure 7 shows the inclusion compositions converted to the Al–Ti binary
system. Similar to the case of oxides at the metal – oxide pellet interfaces, Al2TiO5 oxide
was not observed but Al2O3 phase containing TiOx and TiOx phase containing Al2O3 were
found.
As a result, compositions of inclusions could not be estimated from the stability－ 59 －

diagram for the Al–Ti oxides equilibrated with the Fe–Al–Ti melts at 1873 K shown in Fig.
2. It was concluded that the further investigations are required to clarify the stablephases for the Al2O3–TiOx oxide system.

0 100
Exp. No.

1-1

20

80

co
Ti

ent
(m
ol%
)

1-7

Fe

60

60

1-8

)
ol%
(m
nt
nte

con
t

40

1-9

40

1-10

Al2TiO5

80

20

1-11
Al2TiO5
100
0

20

40

60

0
100

80

Al content (mol%)Figure 6

Projection of compositions of

0
20
40
60
80
100
Ti content of inclusions onto the Al-Ti binary system (%)

Figure 7oxide inclusions on the Al–Ti–Fe ternary

Compositions of oxide inclusions

analyzed by EDS.

system. (Exp. 1–10)

3.2. Observation of inclusions in metal deoxidized by Al–Ti
Compositions of prepared specimens are summarized in Table 3. According to thestability diagram shown in Fig. 2, compositions of specimens 2–A, 2–B and 2–C arelocated in the Al2O3 stable region, the Ti3O5 stable region, and at the boundary between
Al2O3, Al2TiO5, Ti2O3 and Ti3O5 stable regions, respectively. Mg content in all inclusions of
all specimens was lower than 1 mass%, and therefore the observed inclusions wereconsidered as the Al2O3–FeO–TiOx system.

Table 3Compositions of specimens.

Exp.

Compositions of metal (mass%)

No.

Sol. Al

Sol. Ti

T. O

2–A

0.0341

0.04440.0060

2–B

–

0.028

0.0007

2–C

0.01620.29

0.0035－ 60 －

(1) Specimen 2–A (Al: 0.0341 mass%, Ti: 0.0444 mass%)
Typical SEM images of inclusions in as-cast, and heated samples are shown in Fig. 8.
The shape of inclusions in an as-cast sample was mainly spherical and it did not change
clearly by heating at 1473 K.
Compositions of inclusions are represented on the Al–Fe–Ti ternary system as shown in
Fig. 9. Most of inclusions observed in an as-cast sample had the composition of almost
pure Al2O3 and Ti content in inclusions was less than 4 mol%. Although Fe content wasobserved up to 25 mol%, Fe content of smaller inclusions tend to increase due to the X-ray
signal from Fe matrix. Therefore, the inclusion composition was concluded as almost pureAl2O3 containing small content of TiOx.
The inclusion compositions changed from Al2O3 to Al–Fe–O, or Al–Ti–Fe–O system byheating at 1473 K, which were not observed in an as-cast sample. After 60 min, three
types of inclusions were observed, namely Al2O3, Al–Fe–O, and Al–Ti–Fe–O inclusions.
Most Al–Fe–O inclusions contained from 30 to 40 mol% Fe as the Al–Fe binary system.
The composition of Al–Ti–Fe–O inclusions as the Al–Fe–Ti ternary system was in therange between 35 and 75 mol%Fe, and 5 and 20 mol%Ti.
After heating for 180 min at 1473 K, Al2O3 inclusion was no longer observed and only
Al–Fe–O or Al–Fe–Ti–O inclusions were observed. Composition of Al–Fe–O inclusions
dispersed widely and the Fe content as the Al–Fe binary system was between 30 and 75
mol%. The Al–Ti–Fe–O inclusions contained from 10 to 20 mol%Al, from 60 to 80 mol%Fe,
and from 10 to 20 mol%Ti as the Al–Fe–Ti ternary system. Number of inclusions withlarge Fe content increased with increasing heating time. From these results, Al2O3
inclusions changed to Al–Fe–Ti–O inclusions by heating at 1473 K.
Figure 10 shows the size distribution of inclusions. The size distribution of inclusions
was not affected clearly by heating although composition changed significantly.

(a)Figure 8

5 μm

(b)

2 μm

(c)

2 μm

SEM images of typical inclusion observed in Specimen 2–A; (a) as cast, (b) after
60 min heating at 1473 K, and (c) after 180 min heating at 1473 K.

－ 61 －

60

60

40

80

80

20

100
0

20

40

60

Al content (mol%)

20

100
0

0
100

80

20

40

80

0
100

(b)

40

80
100
0

60

Al content (mol%)(a)60

Figure 9

40

20

20

40

60

Al content (mol%)

0
100

80

(c)

Composition of inclusions in Specimen 2–A; (a) as cast, (b) after 60 min heating
at 1473 K, and (c) after 180 min heating at 1473 K.

Fraction of inclusion number (%)

50

As castAfter 60 min
After 180 min

40

30

20

10

0

0-2

2-3

3-4

4-5

5-6

6-7

7-8

8-9

9-10

10-

Inclusion size (μm)

Figure 10

Size distribution of inclusions in Specimen 2–A; (a) as cast, (b) after 60 min
heating at 1473 K, and (c) after 180 min heating at 1473 K.

(2) Specimen 2–B (Ti: 0.028 mass%)
Figure 11 shows the typical inclusion shapes in Specimen 2–B. Inclusions in an as-castsample had very irregular shape, while mainly spherical shape inclusions were observedin samples after heating at 1473 K. The size of inclusions became smaller with heatingtime as explained in detail below.
Figure 12 shows the compositions of inclusions observed in Specimen 2–B. In an as-castsample, TiOx inclusions with small Fe content were mainly detected. On the contrary,
Ti–Fe–O inclusions were observed in samples heated at 1473 K. In the sample heated at－ 62 －

1473 K for 60 min, Fe content as the Fe–Ti binary system varied from 50 to 100 mol% andthe average composition was 75 mol%Fe–25 mol%Ti. Composition of inclusions in asample heated for 180 min clearly showed the change in composition from TiOx to 75
mol%Fe–25 mol%Ti as the Fe–Ti binary system.
Figure 13 shows the size distribution of inclusions. Inclusion size clearly became
smaller with heating samples at 1473 K. In an as-cast sample, inclusion size dispersed
widely from less than 1 to over 10 μm, while many fine inclusions were found in samples
heated at 1473 K. More than half of observed inclusions in the heated sample for 180 min
were in the size range between 2 and 3 μm.
From compositional and size change of inclusions during heating at 1473 K, it isconsidered that the dissolution of TiOx inclusions into the metallic phase and thenucleation of smaller Fe–Ti–O inclusions occurred simultaneously, because TiOx is nolonger stable in Fe–0.028 mass% Ti at 1473 K but Fe–Ti–O oxide is.

(a)

(b)

10 μm

Figure 11

(c)

5 μm

2 μm

SEM images of typical inclusion observed in Specimen 2–B; (a) as cast, (b)after 60 min heating at 1473 K, and (c) after 180 min heating at 1473 K.

60
80
100
0

60

40

80

20

20

40

60

100
0

0
100

80

Fe content (mol%)40

60

Fe content (mol%)

(a)

80

0
100

(b)

40

80

Figure 12

20

20

60

100
0

40

20

20

40

60

Fe content (mol%)

80

0
100

(c)

Composition of inclusions in Specimen 2–B; (a) as cast, (b) after 60 min
heating at 1473 K, and (c) after 180 min heating at 1473 K.
－ 63 －

Fraction of inclusion number (%)

50

As castAfter 60 min
After 180 min

40

30

20

10

0

0-2

2-3

3-4

4-5

5-6

6-7

7-8

8-9

9-10 10-

Inclusion size (μm)
Figure 13

Size distribution of inclusions in Specimen 2–B; (a) as cast, (b) after 60 min
heating at 1473 K, and (c) after 180 min heating at 1473 K.

(3) Specimen 2–C (Al: 0.0162 mass%, Ti: 0.29 mass%)
Typical inclusions in as-cast and heated samples of Specimen 2–C are shown in Fig. 14.
The shape of inclusions in an as-cast sample were similar to those of Specimen 2–B,
namely very irregular shape. Although the shape of inclusions did not clearly change byheating at 1473 K, many inclusions in the sample heated for 180 min at 1473 K had twophases in an inclusion as shown in Fig. 14(c)Figure 15 shows the compositions of inclusions in Specimen 2–C on the Al–Fe–Ti
ternary system. Though Al2O3 inclusions were mostly observed, various types ofinclusions such as TiOx with small FeO content, or FeO with small Al2O3 and TiOx
contents were also found. This is because the metal composition is at the boundary
between Al2O3, Al2TiO5, Ti2O3 and Ti3O5 stable regions at 1873 K as shown in Fig. 2.
Therefore, the formed inclusions have not been equilibrated comprehensively with metal
before solidification.
As explained above, the compositions of inclusions changed to two compositions, one is
Al–Fe–O inclusions containing approximately 40 mol% Fe as the Al–Fe binary system
and also with small Ti content, the other is Al–Ti–Fe–O inclusions which averagecomposition is 20 mol%Al–20 mol%Ti–60 mol%Fe as the Al–Ti–Fe ternary system.
Though some Al2O3 inclusions were also observed, the fraction of Al2O3 inclusions
decreased by heating. The changing behavior of inclusion compositions with heating was
similar to that observed in the case of Specimen 2–A. However, many observed inclusions
in Specimen 2–C after heating had two phases as shown in Fig. 14, which was different－ 64 －

from the case of Specimen 2–A.
Figure 16 shows the size distribution of inclusions. The effect of heating on the change
of inclusion size distribution was not clearly observed, which was similar to Specimen
2–A.

(a)

(b)

5 μm

Figure 14

(c)

5 μm

5 μm

SEM images of typical inclusion observed in Specimen 2–C; (a) as cast, (b)after 60 min heating at 1473 K, and (c) after 180 min heating at 1473 K.

0 100

0 100
20

20

80

40

40

60

60

40

60

100
0

0
100

80

40

80

20

20

60

60

40

80
100
0

80

20

20

(a)

Al content (mol%)

40

60

80

0
100

Al content (mol%)

(b)

0 100
20

80

40

60

60

40

80
100
0

20

20

40

60

Al content (mol%)Figure 15

80

0
100

(c)

Composition of inclusions in Specimen 2–C; (a) as cast, (b) after 60 min
heating at 1473 K, and (c) after 180 min heating at 1473 K.

－ 65 －

Fraction of inclusion number (%)

50

As castAfter 60 min
After 180 min

40

30

20

10

0

0-2

2-3

3-4

4-5

5-6

6-7

7-8

8-9

9-10

10-

Inclusion size (μm)

Figure 16

Size distribution of inclusions in Specimen 2–C; (a) as cast, (b) after 60 min
heating at 1473 K, and (c) after 180 min heating at 1473 K.

(4) Mechanisms of Size and Compositional Change of Inclusions in Solid State Steels
Effect of heating at 1473 K on the size distribution and composition of inclusion wasstudied for three different specimens. Regarding the effect on the size distribution, thefraction of smaller inclusions increased in the case of Specimen 2–B, while the size
distribution did not change obviously in Specimens 2–A and 2–C. Therefore, thenucleation and dissolution of inclusions expressed as Eqs. (5) and (6) occurred
simultaneously and thus the fraction of finer inclusions increased by heating in Specimen
2–B,
TiOx (s) → Ti + x O(5)Fe + Ti + (x+1) O → FeO–TiOx (s)(6)

On the other hand, the dissolution of inclusions did not occur and therefore the size
distribution did not change obviously in the case of Specimens 2–A and 2–C. Although thereason of this difference is not clear at this moment, one possibility is the kinetic effect,
namely the dissolution of Al2O3 might be much smaller than that of TiOx.
In the present study, neither the valence of Ti nor the mineralogical phase of inclusions
was measured, and the chemical stability of oxide phase in solid steel has not beenreported. Therefore, the stability of titanium oxide was evaluated by assuming thecondition that Gibbs free energy change for reaction (7) can be applied for solid iron.
5 Ti2O3 (s) = 3 Ti3O5 (s) + Ti (mass%)－ 66 －∆G ° = 189500 − 93.6T J/mol(7)

As the activity of titanium relative to that in Fe–1 mass%Ti calculated at equilibrium
was 0.0148, the titanium oxide in Specimen 2–B is considered to be Ti2O3. Furthermore,
Fe content in inclusions may be overestimated because of the X-ray signal from Fe matrix
during inclusion analysis by EDS as previously explained. From above reasons, the actual
average compositions of inclusions in Specimen 2–B would be approximately 30
mol%Ti–70 mol%Fe. When Ti2O3 was assumed to be formed as Ti oxide in the sample, theestimated composition of the Fe–Ti–O inclusion corresponds to 4FeO·Ti2O3.
The change of inclusion compositions and the final compositions after heating for 180
min at 1473 K were the same for Specimens 2–A and 2–C, namely Al–Fe–O or
Al–Ti–Fe–O inclusions. Inclusions firstly changed from Al2O3 to Al–Fe–O, and secondly
from Al–Fe–O to Al–Ti–Fe–O by heating. Assuming the formation of Ti2O3 as a stabletitanium oxide and the overestimation of Fe composition as well, the Al–Fe–O (averagecomposition: 60 mol%Al–40 mol%Fe as the Al–Fe binary system) and the Al–Ti–Fe–O(average composition: 15 mol%Al–15 mol%Ti–70 mol%Fe as the Al–Ti–Fe ternary
system) inclusions correspond to Al2O3·FeO and Al2O3·Ti2O3·4FeO, respectively. However,since these oxide forms were considered based on aforementioned assumptions and nomineralogical phase for this oxide has been reported, further study is required to reveal
the form of oxides.
Homogeneous oxide phase in each inclusion was observed in Specimen 2–A, while
many inclusions having two phases in one oxide particle were observed in Specimen 2–C.
This difference in inclusion morphology between Specimens 2–A and 2–C is not clearly
understood and the further study must be conducted.
Since the clear change of size distribution of inclusions by heating was not observed inSpecimens 2–A and 2–C, it is considered the dissolution of Al2O3 inclusions into Fe matrix
did not occur as described previously. Therefore, the change of Al2O3 inclusion to Al–Fe–O
inclusion, and further change of Al–Fe–O inclusion to Al–Ti–Fe–O inclusion are
expressed as Eqs. (8) and (9), respectively.
Al2O3 (s) + Fe + O → Al2O3·FeO (s)or
4/3 Al2O3 (s) + Fe → Al2O3·FeO (s) + 2/3 Al(8)

Al2O3·FeO (s) + 3 Fe + 2 Ti + 6 O → Al2O3·Ti2O3·4FeO (s)or
5/2 Al2O3·FeO (s) + 3/2 Fe + 2 Ti → Al2O3·Ti2O3·4FeO (s) + 3 Al

(9)The oxygen source would be the dissolved oxygen in Fe matrix or the partial dissolutionof Al2O3, depending on the steel composition. Further physicochemical properties must be
clarified for understanding the behavior of inclusions more in detail, such as the chemical－ 67 －

stability diagram for the Fe–Al–Ti–O system in solid steel, or the activities of Al, Ti andO.
4. Conclusions
Equilibrium experiments between molten Fe–Al–Ti metals and Al2TiO5 pellets wereconducted to determine the important oxide stability diagram for estimation of the phaseof oxides produced by Al–Ti deoxidation. However, Al2TiO5 phase was not observed at theinterface between metal–oxide after equilibrium, but Al2O3 phase containing TiOx and
TiOx phase containing Al2O3 were identified. Observation and analyses of non-metallic
inclusions in metals after experiments found similar compositions. Therefore, it was
concluded that inclusion compositions cannot be estimated from the oxide phase stability
diagram determined by the present study and thus further study for the stable oxide
phase of the Al2O3–TiOx system is required.
Behavior of non-metallic inclusions in Al–Ti deoxidized steels at 1473 K was studied
with three different steels. In the case of Fe–0.0341mass%Al–0.0444mass%Ti and
Fe–0.0162mass%Al–0.29mass%Ti,

different

inclusions

were

observedinas-castspecimens, while Al–Fe–O and Al–Ti–Fe–O inclusions were observed in both samples
after heating at 1473 K and those compositions between two specimens were the same.
The size distribution of inclusions did not clearly change by heating. In the case of
Fe–0.028mass%Ti, Ti–O inclusions with small content of Fe were observed in an as-castsample and composition of inclusions changed to Ti–Fe–O after heating at 1473 K, andthe fraction of fine and coarse inclusions increased and decreased, respectively. Though
the stable phase of oxide equilibrated with Fe–Al–Ti steel at 1473 K has not been clarified,equilibrium phases in solid steel at 1473 K may be different from those at steelmaking
temperature and thus the inclusion size distribution, morphology, or composition wouldchange inside solid steel by heat treatment.
Acknowledgment
This research was supported by JFE 21st Century Foundation for FY 2010. The author
expresses his great gratitude to their support.
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