art-3A10.1007-2Fs11663-009-9267-6

Transient Behavior of Inclusion Chemistry, Shape,and Structure in Fe-Al-Ti-O Melts: Effect of TitaniumSource and Laboratory Deoxidation Simulation

CONG WANG, NOEL THOMAS NUHFER, and SEETHARAMAN SRIDHAR

In the present study, laboratory-scale deoxidation experiments in a vacuum-induction furnace(VIF) were carried out to elucidate the evolution of inclusions during transient stages after atitanium addition. Three different titanium sources (Fe-70 wt pct Ti, Fe-30 wt pct Ti, andtitanium granules) were employed and the results were compared in terms of the inclusionchemistry, structure, and morphology. It was found that, immediately after titanium additionsto an aluminum-killed melt, titanium-containing inclusions, which are either single-phase ordual-phase particles having a certain amount of titanium and are contrary to melt equilibriumpredictions, were formed. Analysis by transmission electron microscope (TEM) suggested thatthese inclusions could be Al2TiO5. The change in the inclusion chemistry was accompanied by ashift in the inclusion morphology from spherical to irregular. With time, the inclusion chemistryshifted back toward the thermodynamically stable Al2O3, but the change in morphologyremained. The temporary Al2TiO5 inclusions were formed as a result of local high content oftitanium immediately after and at the vicinity of the titanium addition. When comparing thedifferent titanium sources, it was found that they can be ranked as Ti>Fe-70 pct Ti>Fe-30 pct Ti, in terms of the amount of titanium-containing inclusions produced during thetransient stage.

DOI: 10.1007/s11663-009-9267-6� The Minerals, Metals & Materials Society and ASM International 2009

I. INTRODUCTION

NONMETALLIC inclusions in steels form as a resultof the interaction between reactive elements in the steelmelt and its environment (atmosphere, refractory, orslag). In general, the population of inclusions needs tobe minimized, because inclusions can cause problemswith process control. They can also have a deleteriouseffect on the toughness of the final products and on theirductility and weldability.[1,2] This project aims to studythe evolution of oxide inclusions in the ladle during theprocessing of interstitial-free (IF) steels.

Generally, 400 to 700 ppm of dissolved oxygen willremain in steel after the converter process,[3–5] and steelmelts with such a high dissolved oxygen content need tobe deoxidized to prevent blister formation duringcontinuous casting. Deoxidation and adjustment of thefinal steel composition is carried out during ladletreatment.

Aluminum is a common deoxidizer because it isinexpensive compared to other reactive elements and itshigh oxygen affinity results in a fairly low content ofsoluble oxygen. During ladle processing of IF steels,titanium in the form of Fe-Ti alloys is added afterdeoxidation to bind the soluble nitrogen or solublecarbon remaining after the secondary refining processinto TiC or TiN compounds during solidification.Binding of the interstitials is necessary to ensure therequired mechanical properties of IF steel during hotrolling, because interstitial atoms prohibit dislocationrearrangement and reduce the formability of IF steel.Because IF steels are used for structural parts such asdoors in automotive applications, it is vital that form-ability is ensured. While titanium is not added for thepurpose of deoxidation, it can form relatively stableoxides if oxygen is available. Therefore, titanium oxideinclusions can precipitate during solidification becauseof solute rejection and the enrichment of titanium andoxygen in the interdendritic liquid.[6] Furthermore,titanium oxides have good ability to assist ferritenucleation within austenite grains and dramaticallypromote the formation of acicular ferrite microstruc-ture, which is interlocking in nature and deflects thepropagation of cleavage cracks, causing an increase intoughness.[7,8]

Titanium-containing inclusions are, however, gener-ally not wanted, because titanium bound as oxidesdecreases the amount of titanium available to bindinterstitial elements. Furthermore, the presence of tita-nium in the steel melt has been shown to increase the

CONG WANG, Graduate Student, and NOEL THOMASNUHFER, Director of Electron Microscopy and Materials Char-acterization, are with the Center for Iron and Steelmaking Research,Department of Materials Science and Engineering, Carnegie MellonUniversity, Pittsburgh, PA 15253. SEETHARAMAN SRIDHAR,POSCO Professor of Steelmaking, Center for Iron and SteelmakingResearch, Department of Materials Science and Engineering,Carnegie Mellon University, is with the National Energy TechnologyLaboratory, Pittsburgh, PA 15236. Contact e-mail: [email protected]

Manuscript submitted April 24, 2009.Article published online July 21, 2009.

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 40B, DECEMBER 2009—1005

tendency toward clogging of the submerged-entry nozzle(SEN).[9–12] The clogging of the SEN severely decreasescasting productivity and is a major source of surfacedefects on the steel products. It is generally assumed thata clogging deposit forms by the attachment andsubsequent sintering of oxide particles on the innersurface of the SEN refractory. The cause for theincreased clogging has not been resolved, but thepresence of titanium oxide inclusions has been suggestedas a possible cause.[9–12] Because titanium oxide inclu-sions are not predicted thermodynamically from themelt bulk chemistry, it is likely that they could be theresult of transient conditions, such as a local hightitanium concentration near the point of the addition.

The objective of this research is to investigate thetransient ladle reactions by examining the evolution ofthe inclusion composition, morphology, and structureafter the titanium addition, with respect to aluminumdeoxidation. In a previous article by Matsuura et al.,[13]

it was demonstrated that a certain amount of titanium-containing inclusions could be produced during labora-tory deoxidation tests, even when Al2O3 was the stableoxide phase. The purpose of this study is to establish thepossible effects of titanium sources (industrial ferrotita-nium alloys vs pure titanium) and to identify thestructure of the titanium-containing oxide inclusions.

The specific objectives for this study are to do thefollowing:

(1) experimentally establish the transient conditionsunder which titanium-containing inclusions form inaluminum-deoxidized melts;

(2) determine the change in the inclusion structure,chemistry, and morphology that results from thetransient reactions in the cases in which Al2O3 isstable; and

(3) vary the titanium-addition type to compare theinfluence of the various types on the inclusionbehaviors at the transient stage.

II. MATERIALS AND METHODS

In the current study, laboratory-scale deoxidationexperiments were carried out to elucidate the evolutionof inclusions during the transient stage after titaniumaddition. The general methodology was to obtain sam-ples in a melt after an aluminum/titanium addition insidean induction furnace without exposing samples to theoutside atmosphere. In that way, the oxygen content inthe melt was maintained constant during the experiment.Samples were characterized for the following: (1) total

oxygen, (2) soluble aluminum and titanium, and(3) inclusion chemistry and shape. In a limited numberof samples, the inclusion structure was determined.

A. Materials

High-purity iron chips (approximately 12 mm indiameter) were employed as the base materials formelting, and high-purity aluminum wires (99.999 pct)were used as deoxidizers. For experiments involving theaddition of pure titanium, reagent-grade pure titaniumgranules (99.9 pct) were selected. In addition to thetitanium granules, two kinds of Fe-Ti alloys, namely,Fe-30 pct Ti and Fe-70 pct Ti by nominal weightpercent, were chosen to simulate exactly the realindustrial practice; their respective compositions,together with that of the aluminum wire and titaniumgranules, are listed in Table I. The Fe-70 pct Ti alloywas produced by melting grounded titanium scraps ortitanium sponges with iron scraps and other sources ofiron in an electric arc furnace. This process is relativelyclean and the resulting alloy contains fewer impurities,due to minimum slag carryover from the furnace.[14]

The Fe-30 pct Ti alloy was produced by the Thermiteprocess, in which rutile (TiO2) and limonite (FeO(OH)ÆnH2O) sand are melted and reduced by using aluminum,which generally produces Al2O3 and the Fe-Al-Tialloy. Existing phases in Fe-70 pct Ti and Fe-30 pct Tialloys were identified using X-ray diffraction (XRD)(Figure 1), which revealed two different phases, namely,alpha titanium and the line compound of FeTi forFe-70 pct Ti, which is in agreement with what isexpected from the reported Fe-Ti phase diagram,[15]

Table I. Chemical Analysis of Starting Materials

Source (Wt pct) Mn Si Cr Al V P Cu Ti O Fe

Fe (chips) — — — — — — — — <0.004 99.996Al (wire) — — — 99.999 — — — — <0.001 —Ti (granules) — — — — — — — 99.9 0.004 —Fe-30 pct Ti 2.002 1.436 0.113 8.217 0.177 0.041 0.129 28.364 0.25 to 0.35 balFe-70 pct Ti 0.11 0.138 0.522 3.481 2.522 0.075 0.175 68.157 0.10 to 0.15 bal

20 30 40 50 60 70 80 900

200

400

600

800

1000

1200

1400

αTi

FeTi

Fe-70%TiFe-30%Ti

FeTi

αTi

FeFe

αTi

Fe

FeTi

Fe2Ti

Fe2TiFe

2Ti

FeTi

Fe

FeTi

αTi

FeTi

αTi FeTi

αTi

αTiαTi

Rel

ativ

e In

ten

sity

2θ

Fig. 1—XRD patterns for Fe-30 pct Ti and Fe-70 pct Ti alloys.

1006—VOLUME 40B, DECEMBER 2009 METALLURGICAL AND MATERIALS TRANSACTIONS B

and alpha titanium, Fe2Ti (laves phase), and FeTi for theFe-30 pct Ti alloy. Typical microstructure and corre-sponding elemental analyses were carried out through ascanning electron microscope–energy-dispersive spec-troscope (SEM-EDS). From Figure 2(a), the Fe-70 pctTi alloy was identified to have Ti2Fe (points 4 and 5)and Ti4Fe (points 1 and 6) in addition to the XRD-detected Fe2Ti (points 2 and 3) and titanium (point 7).The high amount of oxygen in the two ferrotitaniumalloys is most likely in the form of alumina. An exampleis shown in Figure 2(b), in which an Al2O3 inclusion inthe starting Fe-30 pct Ti alloy is shown. Both alloyswere nonhomogeneous in the distribution of theseinclusions and, therefore, a quantitative characterizationof the additions before the experiment could not bemade.

The purpose of the experiment in which relativelypure titanium granule was added was to carry out acomparative experiment with a titanium source that didnot have a high starting total oxygen content. Thiswould provide information on whether the high totaloxygen content (in the form of Al2O3) has an effect onthe deoxidation reactions.

For each experiment, 3 kg of iron chips was meltedinside a high-purity MgO crucible (outer diameter86 mm, inner diameter 71 mm, and height 160 mm)surrounded by an induction coil.

B. Deoxidation Simulation and Experimental Conditions

The preparation of the melt, deoxidation, sampling,and casting were carried out inside a 15-kW vacuum-induction furnace (VIF), to prevent the reoxidation ofthe melt by the surrounding gas atmosphere. Thechamber is connected to an oil diffusion pump and amechanical pump, which can evacuate the chamber toapproximately 1 Pa, which is sufficiently low that thegas-phase mass transport of oxygen to the melt isnegligible during the duration of the experiments. Theexperimental procedure was as follows. First, thechamber was evacuated to reach a low vacuum.Although the degree of vacuum obtained may varyfrom one experiment to the other, the starting oxygenand nitrogen content in the iron melt were generallymaintained in the ranges of 330 to 450 and <19 ppm,respectively. The chamber was then backfilled withargon to protect the melt from being oxidized. Afterthe temperature reached 1873 K, the power supply wasmaintained at a stable level and the iron melt wasmaintained for 20 minutes, to ensure that a stable melttemperature was reached. The melt temperature wasmonitored by a dual-wavelength pyrometer throughthe top window during an experiment and the induc-tion coil output was controlled manually to maintainthe experimental temperature. After waiting for a

Fig. 2—Typical microstructure of (a) Fe-70 pct Ti and (b) Fe-30 pct Ti alloys.


sufficient time, the desired amount of aluminum ortitanium was added to the melt according to differenttime interval requirements. Subsequently, samples ofthe melt were taken into quartz tubes. One tappedsample was acquired by teeming melt into a water-cooled copper mold lined with boron nitride paint. Inthe furnace, a manufactured sampling tool illustratedin Figure 3 was installed; this enables the sampling byattached quartz tubes without exposure to the outsideair atmosphere. A schematic illustration of experimen-tal setup, including both the VIF and the samplingtool, is presented in Figure 3.

In order to obtain a sufficient and consistent set ofsamples of the melt during the transient stage, thesequence of the melting and sampling procedures wascarefully designed. In general, three types of samplingprocedures, schematically illustrated in Figure 4, wereemployed for different purposes. Route (a), which wascarried out by taking one sample 1 minute between thealuminum and titanium addition, ensured the yield andquality of the expected Al2O3 inclusions and had thepotential of identifying unexpected inclusions due toexternal contamination imposed by unexpected elementsfrom crucibles (pre); routes (b) and (c) involved three

Fig. 3—Schematic illustration of experimental setup.

Fig. 4—General melting and sampling procedure illustration: (a) pre, (b) regular, and (c) extended deoxidation.


consecutive samplings immediately after the titaniumaddition at different times, with route (c) having aprolonged 3 minutes for deoxidation to proceed. Basedon the relative differences in sampling times, routes (b)and (c) are named regular and extended, respectively.Each condition that is investigated would therefore becarried out three times (pre, regular, and extended);experimentally, a total of nine samples will be obtained.In this process, some of the samples from the threeexperiments overlapped with respect to time after theaddition and this was used to establish reproducibility.Generally, during industrial practice, titanium wasalways added 1 minute after the aluminum additionand it was retained in the melt for another 10 minutesbefore casting. It should be noted that the first deoxi-dation practice (Al-01) employed simpler sampling andheating procedures, as depicted in Figure 5.

Figure 6 is established to show the stability regionsfor various forms of oxide inclusions and how they willchange depending on the variation in soluble aluminumand titanium content levels. Thermodynamic data wereobtained and assessed from the studies of the followingsystems: (1) Fe-Al-O,[16,17] (2) Fe-Ti-O,[18–25] and(3) Fe-Al-Ti-O.[26–31] The ranges of aluminum andtitanium compositions generally encountered in IF steelsare represented by a dashed circle. The star indicates theexperimental conditions investigated in this study; thiswill be discussed later.

The final chemical compositions and their respectiveexperimental purposes, the sampling procedures, andthe operation conditions of all the experiments aresummarized in Table II.

C. Characterization

1. Inclusion morphology and chemistrycharacterizationThe inclusion characterization was completed

through the combination of an SEM (PHILIPS* XL

30) and an EDS (INCA**). For each sample, 50

inclusions were examined and analyzed. The inclu-sion chemistry, inclusion size, and morphology wererecorded for further analysis, to examine the relation-ships among the inclusion size distribution, inclusionshape modification, chemical composition, and time.The definition of the inclusion shapes was expanded inorder to incorporate inclusion types not found in ourprevious studies.[13] The typical morphologies of dif-ferent inclusion categories are shown in Figure 7.Inclusion chemistries are expressed in atomicpercentages.

2. Inclusion and raw materials structurecharacterizationA transmission electron microscope (TEM) with an

energy-dispersive X-ray system was used to identify thespecific chemistries and structures of inclusions. Thethin foils were prepared in the following way. Sampleswere acquired from the iron melts in the quartz tubes.After cutting the samples into relatively thin films(approximately 300 lm) and rinsing them thoroughlyin acetone, methanol, and water, these foils weresubjected to mechanical grinding to achieve a thicknessof ~80 lm, which has proved to be the most appropriatescale for electropolishing. The electropolishing, using ajet polisher process, was performed in an environment ofa 10 wt pct perchloric acid solution, with the balancemethanol, and an operation temperature of approxi-mately 233 K. The working voltage for electropolishingwas set at 20 V. A Tecnai F20 TEM (FEI Company,Hillsboro, OR) was employed to carry out both diffrac-tion patterns and chemical analysis. The XRD methodwas applied, to reveal the existence of certain phasesinside raw materials, in particular, of Fe-Ti alloys, aslisted in Section II–A.

Fig. 5—Melting and sampling procedure illustration of Al-01.

Fig. 6—Calculated oxide phase diagram equilibrated with the Fe-Ti-Al-O system at 1873 K. Inset shows theoretical final soluble alumi-num and titanium content levels of experiments in this study.

*PHILIPS is a trademark of Philips Electronic Instruments Corp.,Mahwah, NJ.

**INCA is a trademark of Oxford Instruments, Abingdon,Oxfordshire, United Kingdom.


Table II. Experiment Conditions and Purposes

Experiment TargetTemperature

(K)SamplingProcedure

Final Chemistry(Wt Pct)

Goal Chemistry(Wt Pct)

Al-01 change in inclusion morphology andchemistry when stable phase is withinAl2O3 region by killing with aluminum

1873 1, 4, and 8 min Al = 0.054 Al = 0.060

AlTi-02 change in inclusion morphology andchemistry when stable phase is withinAl2O3 region by killing withFe-70 pct Ti alloy

1873 (a) Al = 0.037Ti = 0.060

Al = 0.060Ti = 0.060

AlTi-03 (b) Al = 0.059Ti = 0.063

AlTi-04 (c) Al = 0.050Ti = 0.064

AlTi-05 change in inclusion morphology andchemistry when stable phase is withinAl2O3 region by killing withFe-30 pct Ti alloy

1873 (a) Al = 0.064Ti = 0.062

Al = 0.060Ti = 0.060

AlTi-06 (b) Al = 0.060Ti = 0.060

AlTi-07 (c) Al = 0.057Ti = 0.055

AlTi-08 change in inclusion morphology andchemistry when stable phase is withinAl2O3 region by killing with titanium

1873 (a) Al = 0.068Ti = 0.052

Al = 0.060Ti = 0.060

AlTi-09 (b) Al = 0.055Ti = 0.051

AlTi-10 (c) Al = 0.059Ti = 0.061

Fig. 7—Typical inclusion classification: (a) spherical, (b) irregular, (c) polygonal, and (d) dual phase.


3. Chemical analysis for evaluation of total and solubleelements

Care was taken in the handling and preparation of thesamples for chemical analysis. Only the middle of thesample was used for chemical analysis. The sampleswere cleaned by machining off the surface, usuallyreducing the cross section to a 5 9 5-mm square andwith a length of 30 mm. The total oxygen content wasanalyzed by inert gas fusion impulse infrared absorptionspectroscopy and the total nitrogen content was mea-sured by the inert gas fusion thermal conductivitymethod. The total alloying element content levels wereanalyzed through the alkali fusion acid dissolutionmethod. The filtrated residue was finally subjected toinductively coupled plasma–optical emission spectrom-etry, to obtain soluble element content levels.

4. Inclusion density analysisBy adopting a previously described method for

calculating inclusion density,[13] a number density mea-surement was also carried out using an SEM under2500 times, by which 50 9 30-lm images can beobtained. Thus, by designating a step of 50 lm, asuccessive series of images can be obtained withoutleaving out possible inclusion sites. Altogether, 50consecutive pictures were taken for each of the samplesand the inclusions were correspondingly counted for thenumber density calculation.

III. RESULTS

In this section, the experiment, except for Al-01, whichinvolves only the aluminum killing without titaniumadditions, will be named according to the followingnomenclature: AlTi-xx-xx. The middle numbers denotethe experiment number based on Table II; the last twodigits represent the specific sample of the correspondingheat. First, the aluminum killing (Al-01) is presentedand, subsequently, the results from the titanium additionafter the aluminum killing (AlTi-02–10) are discussed.

A. Aluminum Killing without Titanium Additions(Al-01)

In order to establish a baseline for this study, adeoxidation experiment was conducted through alumi-num killing but without any form of titanium addition.This subsection is focused on the effect of a purealuminum killing without any titanium addition on theinclusion behaviors, particularly in terms of the inclusionmorphological evolution, because no essential inclusionchemistry variations are expected. For this experiment,the samples were taken at 1, 4, and 8 minutes after thealuminum addition, as illustrated in Figure 5.

Prior to the inclusion characterization, the inclusiondensity along the depth of the crucible was evaluated inorder to identify the inclusion distribution during theexperiment. Therefore, an ingot 10 cm in height and7.1 cm in diameter solidified after deoxidation was slicedinto two equivalent halves, and samples from selectedpositions were cut from the ingot, as indicated in

Figure 8. As shown in Figure 8, eight samples altogether,including six inside and along the center line, one atop,and one on the side, with dimensions of 10 9 10 mm,were sectioned; those samples are subsequently namedafter their characteristic distance from the bottom of theingot. Generally, a vertical distance between thosesamples was maintained at the step of 1.5 cm.The results of the inclusion densities are shown in

Figure 8(a). It can be seen that the distribution of theinclusion densities is rather homogeneous (approximately100/mm2, according to Figures 8(c) and (d)) up to 8.5 cmabove the bottom, with the lowest density (63/mm2)existing in a region around the range from 4.0 to ~7.0 cmabove the bottom. From Figure 8(b), it can be seen,however, that the apparent inclusion densities of the topand topside samples are high (>3000/mm2). Individualinclusions are classified as those that could be distin-guished under an SEM in two-dimensional images. It is,however, likely that this concentrated inclusion distribu-tion actually consists of clusters that have floated to thesurface during the aluminum killing. This is similar towhat has been found by Turkdogan,[32] Kiessling,[33] andTiekink,[12] In particular, Turkdogan pointed out that, inlaboratory experiments with inductively stirred melts(~5-cm deep), most of the oxide inclusions floated out tothe surface of the melt within 5 minutes. Moreover,Turkdogan clarified the stirring effect by claiming thatinclusions could get attached to the surface of the ladlelining, which is equivalent to the MgO crucible in thisstudy. Hence, it is plausible to understand the existence ofhigh-density inclusions concentrated on the top surfaceand side samples. Based on these results, the quartz tubeswere designed to reach positions ranging from 5.0 to~6.5 cmabove the bottom, in order to sample themelt in aregion inwhich the inclusion density is expected to remainrelatively constant, i.e., not be influenced by the clusterson the surface. The clusters at the surface would be eitherattached to the refractory linings or absorbed by a top-covering slag in a ladle and are, therefore, not of interest inthis study of the inclusion evolution inside the iron melt.For each of the samples, 50 inclusions were selected for

characterization. Three major types of inclusions couldbe classified according to their distinctive appearance,viz. irregular, spherical, and polygonal, as defined inFigure 7. Representative inclusions, featured in Figure 9,were chosen from individual inclusion pools. Theirrespective chemistries were also acquired by employingEDS analysis, as described earlier. Figures 9(a), (c), and(e) are typical spherical inclusions; Figures 9(b), (d) and(f) are typical irregular inclusions.The morphological change in the inclusions with time

after the aluminum addition is shown in Figure 10,which combines the trend of the shape variations of thethree samples from the heat Al-01. In this figure,irregular inclusions are represented by pentagrams,spherical classification by filled circles, and polygonalby pentagons. After 1 minute of the aluminum addition,the inclusion is found to be primarily (72 pct) composedof the irregular type, with the rest being complementedby spherical classification. It can be seen from Figure 10that the percentages of the various inclusion morphol-ogies remain relatively constant.


Figure 11 shows the total oxygen content as functionof time; it can be seen that, after 1 minute after thealuminum addition, the total oxygen content droppeddramatically from 447 to 30 ppm and eventually to22 ppm. When considering Figure 8 and the reportedliterature, which suggests that aluminum killing resultsin oxide clusters that float on top, it is likely that this iswhat causes the rapid drop in total oxygen; this is seenin Figure 11, because the sampling is carried out farbelow the surface, where the oxide clusters are notpresent. A similar experiment done by Tiekink et al.[12]

also showed the same variation in the total oxygentendency.

In summary, aluminum killing resulted, as expected,in rapid deoxidation and in the clustering and flotationof the oxide products. The morphology of the inclusionsremaining inside the melt does not change appreciablywith time.

B. Aluminum Killing Followed by Different TitaniumSources Additions

To simulate the conditions in the ladle, during IF steelprocessing, titanium was added after the aluminumkilling. As mentioned earlier, three sources of titaniumwere used, Fe-70 pct Ti, Fe-30 pct Ti, and relativelypure titanium granules. The sampling procedures thatwere employed and the melt chemistry in terms ofaluminum and titanium content levels are described.The AlTi-02–10 was carried out by adding titanium

sources 2 minutes after the aluminum addition; this wasintended to mimic the industrial practice in whichtitanium is generally added after a time period that isexpected to be sufficient to have already deoxidized themelt through aluminum addition. The results from thealuminum deoxidation by Matsuura et al.[13] underthe current experimental conditions suggested that

Fig. 8—Inclusion density measurement along the depth of iron melt by means of ingot sectioning and characterization. (a) Overall illustration ofsample sectioning positions. (b) Inclusion clusters in top sample. Representative inclusion distribution at the position of (c) 5.5 cm above theingot bottom and (d) 1.0 cm above the ingot bottom. Clusters resulting from deoxidation floated up to the surface instead of staying in thecenter of the melt.


deoxidation would be expected to be completed by2 minutes when the titanium is added.

The typically encountered inclusions are shown inFigure 12. First presented (Figure 12(a) through (c)) arethe inclusions acquired in the samples treated by theFe-70 pct Ti alloy. Inclusions of a spherical shapewere common 1 minute after the aluminum addition(Figure 12(a)). However, after 5 minutes of aluminumaddition (or 3 minutes after a titanium addition), theinclusions were found to change in both morphologyand chemistry. Figure 12(b) shows an inclusion identi-fied as dual phase, with the inner part and the outside

shell having different chemistry in terms of oxygen,aluminum, and titanium values. The subsequent image(Figure 12(c)) shows a further change, namely, a single-phase inclusion that has an irregular shape but a pure-Al2O3 composition and is obtained 8 minutes after thealuminum addition. Similarly, typical inclusions forthe experiments treated by the Fe-30 pct Ti alloy(Figures 12(d) through (f)) and titanium granule(Figures 12(g) through (i)) are found to evolve in thesame manner as those treated by the Fe-70 pct Ti alloy.Figure 12 provides examples of individual (but typical)inclusions; the percentages at which different inclusion

Fig. 9—Typical Al2O3 inclusions: (a) spherical inclusion, (b) irregular inclusion 1 min after aluminum addition, (c) spherical inclusion, (d) irregularinclusion 4 min after aluminum addition, and (e) and (f) spherical and irregular titanium inclusions, respectively, 8 min after aluminum addition.

0 1 2 3 4 5 6 7 8 9-10

0

10

20

30

40

50

60

70

80

Irregular Spherical Polygonal

Per

cen

tag

e o

f d

iffe

ren

t ty

pes

of

incl

usi

on

Time, minute

Fig. 10—Inclusion morphological evolution in terms of percentagesof different types of inclusions of Al-01 samples. Pentagrams are forirregular classification; filled circles are for spherical-type and penta-gons for polygonal inclusions.

-2 -1 0 1 2 3 4 5 6 7 8 90

100

200

300

400

500447 ppm

To

tal o

xyg

en c

on

ten

t, p

pm

Time, min

Al a

dd

itio

n

Fig. 11—Total oxygen variation in Al-01 samples. It is clearly seenthat, after 1 min after the aluminum addition, the oxygen contentdropped dramatically from 447 to 30 ppm and eventually to 22 ppm.


morphologies are represented are discussed later. Theinclusions shown in Figures 12(b), (e), and (h) areconsidered transient ones that are present immediatelyafter the titanium addition. Because they are notthermodynamically stable, their population decreasesat extended times.

In addition to the SEM-EDS examination, a TEMcharacterization was carried out to reveal the structureof titanium-containing inclusions, especially those com-ing from the samples obtained immediately after thetitanium addition, which should include inclusions fromthe transient stage. A typical inclusion, which isacquired 5 minutes after the aluminum addition fromsample AlTi-03-03, is shown in Figure 13. Through theEDS inspection and diffraction pattern, the inclusion isconfirmed to be Al2TiO5, which has an orthorhombicstructure with the zone axis of ½1�11�: As can be seen fromthe bright-field image (Figure 13(a)), the size of thisinclusion is approximately 0.8 lm and the shape isapproximately spherical, which matches well with theSEM-detected inclusion sizes and classifications. Thestructure information was confirmed through a com-parison of the acquired diffraction pattern and thesimulation database,[34] which gives the angle betweenplanes ð�112Þ and ð110Þto be 121 deg, while the measuredangle is 119 deg.

In order to illustrate the tendency of the inclusionchemistry to change in titanium content, Figure 14 is

presented, which combines three experiments for eachseries in a sequential way, to compare the percentages oftitanium-containing inclusions in each sample. In thisfigure, the red triangles are for the three samples fromthe first heat, the cyan triangles represent those of thesecond heat, and blue ones are for the extendeddeoxidation samples.For the Fe-70 pct Ti alloy-added ones (Figure 14(a)),

initially there are only Al2O3 inclusions, but the per-centage of titanium-containing inclusions almost keepsincreasing for the first two heats. The total number oftitanium-containing inclusions peak at a value of 44 pctat approximately 5 minutes for the third sampleacquired from the second heat. With time, the tita-nium-containing inclusion population decreases andfinally approaches zero at 9 minutes after titaniumaddition. It can be seen that the Fe-30 pct Ti alloy-treated deoxidation practice (Figure 14(b)) results innearly the same inclusion compositional and morpho-logical evolution as that seen in the Fe-70 pct Tialloy-added ones. The percentage of titanium-bearinginclusions undergoes a surge from 0 to 26, but reverts toalmost 0 after 8 to 9 minutes. For the case in whichtitanium granules were added (Figure 14(c)), it can beseen that the percentage of titanium-containing inclu-sions increases very quickly after the addition oftitanium granules and that the peak value of thetitanium-containing inclusions is higher than for the

Fig. 12—Typical inclusions found in aluminum killing followed by titanium additions experiments: (a) through (c) are spherical, dual, and irreg-ular inclusions, respectively, encountered in Fe-70 pct Ti alloy-treated experiments; (d) through (f) are spherical, dual, and irregular inclusions,respectively, encountered in Fe-30 pct Ti alloy-treated experiments; (g) through (i) are spherical, dual, and irregular inclusions, respectively,encountered in titanium granule treated experiments. Sampling events are adopted at 1, 5, and 8 min after aluminum addition.


two alloys. The peak also appears to be somewhatflatter. The classification of titanium-containing inclu-sions includes the following three cases: (1) aluminum-titanium oxides, (2) dual-phase particles that containboth alumina and titanium-containing parts, and(3) pure titanium oxides. It should be noted that veryfew pure titanium oxide inclusions were encountered.

Figure 15 shows the percentages of different types ofinclusions from parallel experiments, in sequential orderfor all of the deoxidation experiments. By connectingthe respective percentages according to the differentdeoxidation and sampling procedures, three types ofinclusions are found to follow different evolutionpathways. A general tendency exists when consideringall three of the deoxidation experiments treated withtitanium sources, that is, the irregular portion increasesafter the addition of titanium, and the total percentagereaches nearly 100 when the deoxidation process isexecuted in the extended procedures. The sphericalpercentage, on the other hand, evolves in an almostcomplementary manner with respect to the irregularportion. For the samples taken from the extendeddeoxidation, almost all of the inclusions are no longerspherical. Last, the fraction of polygonal inclusions isalways kept at a very low level. In addition, anotherevident point is that, for the sequentially overlappingsamples, there are no major differences in the inclusiontypes. This suggests that the experiments are wellcontrolled in the sense that the values are reproducedin the time regions in which experiments overlap withone another. When comparing the chemistry evolutionfrom Figure 14 with the morphological evolution inFigure 15, it appears that the inclusion chemistryundergoes a temporary modification in which titaniumis incorporated in the oxides; this is accompanied by achange in the inclusion morphology. The chemistry

eventually reverts back to Al2O3, but the inclusionmorphology does not.The changing tendency of the inclusion density

against deoxidation time is presented in Figure 16. Itcan be seen that the inclusion density remains atrelatively low values and does not change appreciablythroughout the deoxidation process. The density valuesfrom the samples taken from molten melts are generallyconsistent with those acquired from the sliced solidifiedingot, as illustrated in Figure 8.The variation in total oxygen content with time is

shown in Figure 17, to depict the tendency of oxygeninside the melt with respect to the deoxidation time. Thetrends in Figure 17 are nearly identical to those foundfor the total oxygen in Figure 11. The total oxygencontent after the aluminum killing is primarily in theform of Al2O3 oxides that cluster and float to thesurface. Hence, deoxidation is completed beforethe titanium or Fe-Ti is added.

IV. DISCUSSION

From the results, the following trends can be identi-fied: (1) Al2O3 inclusions were, as expected, producedafter aluminum additions to the melts, resulting in theconsumption of most of the soluble oxygen and flotationof the large clusters; (2) immediately after titaniumadditions to an aluminum deoxidized melt, titanium-containing inclusions were formed, contrary to meltequilibrium predictions; (3) this is accompanied by ashift in the inclusion morphology from spherical toirregular; and (4) with time, the inclusion chemistry wasfound to shift back toward the thermodynamicallystable Al2O3, but the change in morphology remained.The transient titanium-containing inclusions are either

Fig. 13—TEM identification of a transient stage Al2TiO5 inclusion 5 min after aluminum addition from Fe-70 pct Ti alloy-treated experiment:(a) bright-field image and EDS result show the size is approximately 0.8 lm and stoichiometry is Al2TiO5 and (b) zone axis of the incident beamis along the direction of ½1�11�.


duplex ones consisting of an Al2O3 part and a part withtitanium+aluminum oxide (Figures 12(b), (e), and (h))or single-phase oxides that contain aluminum andtitanium (Figure 13). The latter would include the phaseAl2TiO5, which was identified through the TEM. Asdiscussed earlier, the Al2TiO5 phase, with its composi-tion listed in Figure 13(a), has an orthorhombic struc-ture. Figure 18 shows a comparison of the TEM-EDSidentified inclusion composition and the correspondingSEM-EDS acquired inclusion compositions. It can beseen that the only difference in the ideal stoichiometry ofAl2TiO5 (gray square) and the TEM identified one(black hexagonal) is the difference in oxygen content.The oxygen discrepancy could be due to the inaccuracycaused by the EDS technique. On the other hand, theAl/Ti ratio of that inclusion is very close to 2/1,confirming the fact that aluminum and titanium sitesare intact and the inclusion could be indexed as Al2TiO5.

The formation of the transient inclusions, which arenot stable, according to the bulk melt chemistry, can beformed if the titanium concentration was temporarily

and locally high immediately after the addition. InFigure 6, this corresponds to temporarily shifting thecomposition toward the left into the Al2TiO5-rich, oreven Ti3O5-rich, region and, as a result, the transienttitanium inclusions are produced. Ruby-Meyer et al.[28]

have suggested the existence of a viscous (or liquid)phase field instead of the Al2TiO5 phase. It is possiblethat transient inclusions could exist in this region but,among the inclusions characterized in this study, mosttransient titanium-containing inclusions were irregularin shape and did not resemble other liquid inclusions,such as those resulting from Ca-treated aluminum-killedmelts.When titanium is added to an aluminum-killed melt,

the following five reactions are possible at locations atwhich the soluble titanium content is high. Reactions [1],[2], and [3] require that soluble oxygen is available,whereas [4] and [5] are metallothermic reductionreactions.

Oþ xTi! TixO ½1�

-1 4 50 1 2 3 6 7 8 9

0

10

20

30

40

(a)

Fe-70%TiP

erce

nta

ge

of

Ti-

bea

rin

g In

clu

sio

ns

Time, min

Pre

Regular

Extended

Al a

dd

itio

n

Ti a

dd

itio

n

-1 40 1 2 3 5 6 7 8 9-5

0

5

10

15

20

25

30

Pre

Regular

Extended

(b)

Fe-30%Ti

Per

cen

tag

e o

f T

i-b

eari

ng

incl

usi

on

s

Time, min

Al a

dd

itio

n

Ti a

dd

itio

n

-1 0 1 2 3 4 5 6 7 8 9-10

0

10

20

30

40

50

60

70

PreRegularExtended

(c)

Pure Ti treated

Per

cen

tag

e o

f T

i-b

eari

ng

incl

usi

on

s

Time, min

Al a

dd

itio

n

Ti a

dd

itio

n

Fig. 14—Percentage of titanium-containing inclusions: (a) Fe-70 pct Ti alloy-treated experiments; (b) Fe-30 pct Ti alloy-treated experiments, and(c) titanium-granule-treated experiments. Each experiment employs three sampling events.


2Alþ 5Oþ Ti! Al2TiO5ðsÞ ½2�

Al2O3ðsÞ þ 2Oþ Ti! Al2TiO5ðsÞ ½3�

5Al2O3ðsÞ þ 3Ti! 3Al2TiO5ðsÞ þ 4Al ½4�

Al2O3ðsÞ þ 3xTi! 3TixOðsÞ þ 2Al ½5�

A precipitate growth model from the literature[35] wasused to estimate the soluble oxygen required to precip-itate titanium-containing inclusions of the sizes observedin this study. It is assumed that the local titanium contentis high enough that its supply does not control thegrowth of the inclusion. This seems reasonable, becausethe titanium content locally would need to be high toallow for a metastable condition of titanium oxidestability in the first place. It is instead assumed that theoxygen supply in the already aluminum-killed meltcontrols the growth rate. In this model, the precipitation

process is approached by assuming equivalent spheresthat confine the flux and have identical radii of re definedby the separation between inclusions. By applying themass conservation requirement and proper boundaryconditions, the following equation results:

c0 ¼r2ðtÞc002Dt

þ c0 ½6�

where r(t) is the actual inclusion size, t is the requiredtime of precipitates of corresponding sizes, c0 is theconcentration at re, c¢ is the equilibrium composition ofthe solute in solution, and c¢¢ is the concentration perunit volume of solute in the precipitate. It is known thatthe oxygen diffusion coefficient in liquid iron is1.12 9 10�8m2s�1.[36] Therefore, corresponding c0 val-ues for the formation of either Al2TiO5 or Ti3O5, whichare the two possible titanium-containing products thatare be predicted through the stable phase diagram(Figure 6), could be calculated.In this calculation, r(t) is the typical inclusion size of

each type of inclusion 60 seconds after the titanium

-1 0 1 2 3 4 5 6 7 8 9-10

0102030405060708090

100110

(a)

(c)

Fe-70%Ti

Irregular, Extended Spherical, Extended Polygonal, Extended Irregular, Regular Spherical, Regular Polygonal, Regular Irregular, Pre Spherical, Pre Polygonal, Pre

Per

cen

tag

e o

f d

iffe

ren

t ty

pes

of

incl

usi

on

Time, min

Al a

dd

itio

n

Ti a

dd

itio

n

-1 0 1 2 3 4 5 6 7 8 9-10

0102030405060708090

100110

Fe-30%Ti


(b)

Al a

dd

itio

n

Ti a

dd

itio

n

Time, min

Per

cen

tag

e o

f d

iffe

ren

t ty

pes

of

incl

usi

on

-1 0 1 2 3 4 5 6 7 8 9-10

0102030405060708090

100110

Pure Ti treated


Per

cen

tag

e o

f d

iffe

ren

t ty

pes

of

incl

usi

on

Al a

dd

itio

n

Ti a

dd

itio

n

Time, min

Fig. 15—Percentage of different types of inclusions: (a) Fe-70 pct Ti alloy-treated experiments, (b) Fe-30 pct Ti alloy-treated experiments, and(c) titanium-granule-treated experiments. Each experiment employs three sampling events.


addition. The sizes (r(t)) of the Al2TiO5 phase arecalculated by taking the average sizes of the titanium-containing inclusions, including both single- and

dual-phase ones. When encountering dual-phase parti-cles, only the titanium-containing parts are measuredand taken into account. It should be noted that puretitanium oxides are seldom encountered and theiraverage size is taken as equivalent to that of Al2TiO5

phase. The c¢ values are obtained through thermody-namic equilibrium considerations; the c¢¢ values arerealized through the conversion of the oxygen contentfrom the atomic percentage to the appropriate unit(parts per million). Based on the results in Table III, itcan be argued that the estimated values for c0, requiredfor a diffusion-controlled growth of titanium-containinginclusions of the observed sizes are not sufficiently highthat reactions with dissolved oxygen (Eqs. [1] through[3]) can be excluded.It is difficult to establish what the source of the soluble

oxygen is that causes Reactions [1], [2], and [3]. Theresults from Figures 11 and 17 and the observedinclusion-cluster flotation suggest that the aluminumkilling is very rapid, similar to that suggested in theliterature.[32] If this is the case, the equilibrium solubleoxygen concentration would then be approximately3 ppm, which is low compared to the estimations inTable III. It is, however, possible that oxygen is supplied

-1 0 1 2 3 4 5 6 7 8 90

20

40

60

80

100

120

140

160

180

200

PreRegularExtended

Nu

mb

er d

ensi

ty, m

m-2

Time, min

Fe-70%Ti

Al a

dd

itio

n

Ti a

dd

itio

n

(a)

-1 0 1 2 3 4 5 6 7 8 90

20

40

60

80

100

120

140

160

180

200

PreRegularExtended

Nu

mb

er d

ensi

ty, m

m-2

Al a

dd

itio

n

Fe-30%Ti

Ti a

dd

itio

n

Time, min

(b)

-1 0 1 62 3 4 5 7 8 90

20

40

60

80

100

120

140

160

180

200PreRegularExtended

Nu

mb

er d

ensi

ty, m

m-2

Al a

dd

itio

n

Ti a

dd

itio

n

Time, min

Pure Ti treated

(c)

Fig. 16—Inclusion density as a function of deoxidation time: (a) Fe-70 pct Ti alloy-treated experiments, (b) Fe-30 pct Ti alloy-treated experi-ments, and (c) titanium-granule-treated experiments. Each experiment employs three sampling events.

-2 -1 0 1 2 3 4 5 6 7 80

50

100

150

200

250

300

350

400

450

500

Ti a

dd

itio

n

Fe-70%Ti, Pre Fe-30%Ti, Pre Ti granule, Pre Fe-70%Ti, Regular Fe-30%Ti, Regular Ti granule, Regular Fe-70%Ti, Extended Fe-30%Ti, Extended Ti graule, Extended

To

tal o

xyg

en c

on

ten

t, p

pm

Time, min

447 ppmStarting oxygen content

Al a

dd

itio

n

Fig. 17—Variation in total oxygen content with respect to deoxida-tion time for Fe-70 pct Ti alloy-added deoxidation practice.


from other sources, i.e., the following: (1) argon gasatmosphere, (2) MgO crucible, (3) local nonhomogene-ity in oxygen content, and (4) soluble oxygen in thetitanium source. In the case of the argon gas atmo-sphere, the mass-transfer coefficient of oxygen throughthe liquid-phase boundary layer near the gas/metalinterface is so low that only a negligible amount of bulkoxygen could be changed during the experiment.[37]

With regard to the MgO crucible, there was nomagnesium oxide found during the characterization ofany of the samples and, thus, while the crucible cannotbe excluded as an oxygen source, there is no evidencethat it is the source. Whether the local nonhomogeneityin the melt concentration existed cannot be verified.From Table I, it can be seen that the two ferrotitaniumalloys, indeed, contain high oxygen content. Unfortu-nately, the amount of soluble oxygen could not bequantified due to the nonhomogeneity of the material,and a significant amount of the oxygen is most likelybound as alumina. In the case of the titanium granule(AlTi-07-10), however, the oxygen in the starting mate-rial is negligible and oxygen supply from the titaniumsource is not likely. Thus, based on the estimations inTable III, it seems that Reactions [1] through [5] are allpossible. However, as mentioned earlier, the quantity ofpure titanium oxides was very limited and many ofthe inclusions consisted of duplex inclusions, with an

Al2O3- and a titanium-rich part. It was also shown earlier,in Figure 16, that the inclusion density did not increaseafter the titanium addition. Hence, it seems unlikely thatthe reaction described by [1] is occurring. Also, thechances of forming Al2TiO5 phase via Reaction [2] arequestionable, because the possibility of gathering all ofthe required numbers of dissolved molecules of alumi-num, titanium, and oxygen is somewhat unlikely. It wasalso shown earlier, in Figure 16, that the inclusion densitydid not increase after the titanium addition, whichindicates that, if Reactions [1] and [2] did occur, theywould occur on the surface of existing alumina inclusions.It is interesting to note that, in the dual-phase inclusions,the sizes of the Al2O3 part were generally smaller thanthose of the original aluminum-killed oxide products.This scenario can be envisioned as the direct reduction ofAl2O3 by the dissolved titanium, which, in turn, supportsthe reactions listed in [4] and [5].The transient nature of the titanium oxides is such

that they are not thermodynamically stable and, thus,because the titanium/aluminum ratio is recovered afterthe titanium addition, the following reactions occur:

3Al2TiO5ðsÞ þ 4Al! 5Al2O3ðsÞ þ 3Ti ½7�

3TixOðsÞ þ 2Al! Al2O3ðsÞ þ 3xTi ½8�

The aforementioned equations describing the reactionmechanisms could be well explained through the changein the average titanium percentage among all thetitanium-containing inclusions within a population atvarious times. Figure 19 presents the measured variationin the mean titanium content by arithmetically averag-ing all of the titanium atomic percentages of thetitanium-containing inclusions from the samples of theFe-70 pct Ti alloy-treated practice. In the two othercases, the Fe-30 pct Ti and the titanium granules, theresults were similar. From this figure, it can be clearlyseen that, after the titanium addition, the average value

0.00 0.25 0.50 0.75 1.00

0.00

0.25

0.50

0.75

1.000.00

0.25

0.50

0.75

1.00

Al c

onte

nt(m

ole

frac

tion) Ti content (m

olefraction)

O content (mole fraction)

Al2TiO5

Fig. 18—Illustration of Al2TiO5 inclusion identified under the TEMwithin the context of ternary inclusion chemistry representation dia-gram of the sample AlTi-03-03. The actual composition of Al2TiO5

phase identified under the TEM-EDS is represented by the blackhexagonal; the ideal stoichiometry is represented by the gray square.The others are individual chemistry obtained with an SEM-EDS.

Table III. Calculation of Required Concentrationsfor Formation of Different Titanium-Containing Inclusions

OxideFormed r(t) (m) t (s) c¢ (ppm) c¢¢ (ppm) c0 (ppm)

Al2TiO5 0.68 9 10�6 60 15.15 4.43 9 105 15.30Ti3O5 0.68 9 10�6 60 10.34 3.68 9 105 10.47

-1 0 1 62 3 4 5 7 8 9

0

2

4

6

8

10

12

14

16

18

20

22

24

Ave

rag

e T

i co

nte

nt

of

Ti-

con

tain

ing

incl

usi

on

s

Fe-70%Ti Alloy

Al a

dd

itio

n

Ti a

dd

itio

n

Pre

Regular

Extended

Time, min

Fig. 19—Average titanium content of titanium-containing inclusionsfrom samples treated by Fe-70 pct Ti alloy as a function of deoxida-tion time. Error bars are added to indicate the standard deviations.


begins to increase until it reaches a maximum region,which matches well with occasion of the appearance ofthe peak values illustrated in Figure 14(a), suggestingthat the formation of rich titanium-containing inclu-sions is, indeed, taking place. After this plateau region,the average titanium content decreases, although not inan appreciable way, indicating that the reduction oftitanium based on Reaction [7] or [8] is achieved.

If Reactions [1] through [5] would first occur, fol-lowed by Reactions [7] and [8], the inclusions could thenundergo a change such as that shown schematically inFigure 20.

A comparison among three titanium-treated deoxida-tion series suggests that the percentage of the titanium-containing inclusion peak percentage varies dependingon the titanium source. The peak value of the Fe-70 pctTi alloy-treated experiment is 44 pct; this value for theFe-30 pct Ti alloy-treated one is 26 pct, while that forpure titanium is as high as 58 pct. These peak valuesappear at 5, 6, and 6 minutes, respectively. Therefore,titanium sources can be ranked as Ti>Fe-70 pctTi>Fe-30 pct Ti, in terms of the amount of titanium-containing inclusions produced during the transientstage.

These phenomena could be ascribed to the following.First, compared with the Fe-70 pct Ti alloy, theFe-30 pct Ti alloy has a relatively high melting point,which might extend the time for melting and the fulldispersion of the alloy into the liquid melt. Therefore,the reaction between the inclusion and the ‘‘dissolved’’Fe-30 pct Ti alloy might proceed in a slower manner.Second, the pre-existing Al2O3 particles that were foundin the alloy and that possibly existed in the Fe-70 pct Tialloy (Figure 2(b)) would likely be retained in the pinsamples after deoxidation. Therefore, titanium-granule-added heats would more profusely produce titanium-containing inclusions inside the melts. However, due tothe high melting point, titanium granules might bemelted in a tardy manner, which constitutes the reasonfor the later appearance of the peak titanium-containingpercentage value, as demonstrated in Figure 14 (c).

As far as the inclusion morphology is concerned, itwas demonstrated that, after the aluminum addition butbefore the titanium addition, inclusion shapes can beclassified into three categories: spherical, irregular, andpolygonal types, as defined in Figure 7. For all of thedeoxidation experiments, the inclusions are dominatedby the spherical and irregular classifications, with asmall number of polygonal ones 1 minute after thealuminum addition (Figures 13 and 15).

The inclusion morphology is primarily determined bythe degree of supersaturation, which is determined by

the activities of the soluble oxygen and reactive metalelement.[38–40] Immediately after aluminum addition, thesupersaturation is relative high since both solublealuminum and oxygen are at their highest levels.Therefore, the inclusion growth will be independent onthe crystal-steel interface, which results in the unstablebut isometric growth and, thus, the formation ofspherically shaped inclusions.[39] When the degree ofsupersaturation is gradually lowered, the unstablegrowth nature generally remained, but the crystalgrowth will follow along the supersaturation gradient,so the facets would be developed. However, furtherdeoxidation would occur at a supersaturation at whichthe aluminum content remains nearly the same asbefore, but the soluble oxygen content is very low.Under these conditions, the formation of irregularinclusions would be favored because the conditions forfacets forming would be violated. From Figure 10, itcan be seen that there are both spherical and nonspheri-cal inclusions and the populations of these inclusionmorphologies change relatively little with time.When titanium is added 2 minutes after the aluminum

addition, the morphology does change (Figure 15) andirregular inclusions become dominant as transient tita-nium oxide(s) are formed (Figure 12). When Al2O3

eventually reforms, the irregularly shaped inclusionscontinue to dominate. It appears, therefore, that themetallothermic reaction leads to irregular inclusions.

V. CONCLUSIONS

In the present study, laboratory-scale deoxidationexperiments were carried out to clarify the evolution ofinclusions during the transient stages after the titaniumaddition. Iron melts with controlled amounts of alumi-num, titanium, and oxygen additions were made in aninduction furnace. Sampling of the melt was carried outas a function of the time after the aluminum andtitanium additions. Three different titanium sourceswere employed and the respective results were comparedin terms of the inclusion chemistry, structure, andmorphology when the thermodynamically stable inclu-sion was predicted to be Al2O3. The following conclu-sions were drawn from this study.

1. Aluminum killing without titanium addition wascarried out as a control experiment. Immediatelyafter the aluminum killing, the total oxygen contentof the melt decreased to 30 ppm within 1 minute andremained at this level for the remainder of theexperiment. It was revealed that inclusions were

Fig. 20—Schematic illustration of the entire reaction process.


distributed uniformly inside the ingot, except for thetop part, which was more populated with Al2O3

inclusion clusters that most likely formed during thealuminum-killing process. The morphology anddensity of the inclusions inside the melt in the uni-form region was found not to change appreciablywith time after the aluminum killing was completedbeyond 4 minutes.

2. The titanium addition was made 2 minutes after thealuminum addition. The melt chemistry after thetitanium addition was such that Al2O3 was the onlystable inclusion. It was found for all three cases that,immediately after the titanium addition to the alu-minum deoxidized melt, titanium-containing inclu-sions, contrary to the melt equilibrium predictions,were formed and this was accompanied by a shift inthe inclusion morphology from spherical to irregular.With time, the inclusion chemistry shifted backtoward the thermodynamically stable Al2O3, but thechange in morphology remained.

3. Though TEM characterization, it is confirmed thattransient stage inclusions have the structure ofAl2TiO5. It is suggested that the temporary titaniumoxide inclusions were formed as a result of the localhigh content level of titanium immediately after andat the vicinity of the titanium addition. It was dem-onstrated that these titanium sources can be rankedas Ti>Fe-70 pct Ti>Fe-30 pct Ti, in terms of theamount of titanium-containing inclusions producedduring the transient stage.

ACKNOWLEDGMENTS

Financial support from the Center for Iron andSteelmaking Research, Carnegie Mellon University, isgratefully acknowledged. The fruitful discussions withJ. Lehmann, W. Tiekink, and P. Kaushik are greatlyappreciated.

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