Inclusion Formation and Interfacial Reactions between FeTi Alloys and Liquid Steel at an Early Stage

Abstract

Titanium is usually added to the liquid steel in the form of ferroalloys with varying Ti concentrations. These titanium sources also contain Al, Ca and O as the main impurities. In the present work, three different titanium sources, namely, pure Ti and two commercially produced Ti alloys i.e. FeTi70 and FeTi35 are studied. The Ti or FeTi was brought in contact with the liquid iron using the suction method, for a predetermined time and quenched. The reaction zone between the liquid Fe and the titanium source was subjected to microstructural investigation. The high Ti concentration region obtained in a pure Ti–Fe reaction couple, the inclusion formation in liquid iron after coming in contact with FeTi70 and the evolution of existing inclusions in FeTi35 after coming in contact with liquid iron have been studied. The present study has helped in understanding the influence of impurities from the Ti source on the dissolution behavior and the inclusion formation. On the basis of this study, it can be concluded that the Ti rich regions formed after the introduction of pure Ti could modify the existing alumina inclusions in liquid steel, the impurities in FeTi70 contribute to the inclusion formation depending upon the availability of O while FeTi35 introduces inclusions to the liquid steel.

1. Introduction

Titanium is added to the ULC (ultra low carbon) steel to bind the interstitial elements like carbon and nitrogen. The titanium requirement is met with the addition of the commercially available ferrotitanium grades like FeTi70 and FeTi35 in an Al deoxidized steel. These FeTi grades differ not only in their Ti contents but also in the nature of impurities present owing to their different processing routes.¹⁾ However, it was shown with previous studies^2,3,4,5) that the introduction of titanium to the Al killed liquid steel results in the formation of Al–Ti–O inclusions. The formation of Al–Ti–O inclusions takes place immediately after the titanium source addition which with time reverts back to pure alumina inclusions accompanied by a change in morphology. This behavior, in turn, affects the castability of steel.⁶⁾ It can be ascertained that the formation of Al–Ti–O inclusions depends upon the local supersaturation of titanium and therefore, the dissolution behavior of the titanium source affects the inclusion formation.

The schematic representation of the dissolution/reaction after the addition of ferroalloy/deoxidizer to the liquid steel is shown in Fig. 1. The dissolution process mainly consists of (1) melting or dissolution depending on the melting temperature with the intermediate formation of a steel shell (stage I), (2) nucleation of inclusions in the vicinity of a deoxidizer depending upon the local supersaturation (stage II) (3) the growth and agglomeration of the inclusion particles in liquid steel (stage III) (4) finally the removal of these inclusions by various mechanisms (stage IV). As the formation of inclusions like oxides is inevitable (stage II), most of the research is directed towards the removal of inclusions from the liquid steel (stage III and IV) to produce superior quality steel. The formation of Al–Ti–O inclusions that can be attributed to stage II, is due to the local supersaturation of Ti in liquid steel which reduces existing alumina inclusions. Pandelaers et al.⁷⁾ studied the dissolution behavior of pure Ti and FeTi70 alloy. The FeTi70 alloy used in his experiments was manufactured on a laboratory scale and devoid of impurities like Al and Ca, which are typically found in a commercial grade alloy. In his study, stage I was mainly concerned (Fig. 1). Wang et al.³⁾ compared the inclusion behavior after the addition of different titanium sources, viz., pure Ti, FeTi70 and FeTi35 to the Al-killed liquid steel on a laboratory scale, simulating the actual steelmaking process. Wang's work can be attributed to the inclusion formation between stage II and stage III. On an industrial scale also, clusters of Al–Ti–O inclusions after the addition of FeTi to Al-killed steel at the end of RH treatment have been reported.⁸⁾ The addition sequence of Al and Ti has been studied in detail⁹⁾ to analyze its influence on inclusion behavior. It was observed that irrespective of the addition time of Al or Ti, the final chemistry of the inclusions was alumina. In our previous study,¹⁾ the investigation suggests that Al in FeTi70 is mostly present in its soluble form while that in FeTi35 is only partly soluble. The Al in FeTi35 was seen in the form of alumina and in the elemental form surrounding the unreduced titanium oxide. In the previous studies,^2,3,4,5,7,9) the formation of Al–Ti–O inclusions was attributed to the local supersaturation of Ti, irrespective of its source. Very little attention, so far, has been paid to the behavior of the impurities in FeTi upon its introduction to the liquid steel. Ferrosilicon containing impurities like Al is known to affect the inclusion formation in stainless steels significantly e.g. an increasing amount of Al in ferrosilicon enhances the spinel formation while it reduces the MnO content of the inclusions.^10,11)

Fig. 1.

Schematic representation of the typical stages during the alloying (and/or deoxidation) practice.

In the present paper, the results of the reaction of liquid iron with commercially obtained FeTi70 and FeTi35 along with pure Ti as a reference source of titanium are shown to elucidate the phenomena occurring between stage I and II i.e. the formation of the reaction zone between the Ti sources and liquid iron including the nucleation of inclusions. To this purpose the liquid metal suction method was used. The aim was to understand the effects of impurities on dissolution as well as the mechanisms of the inclusion formation between the Ti/FeTi and liquid iron.

2. Experimental

2.1. Procedure

The liquid metal suction method^12,13) was used in order to obtain a reaction zone between the iron and the three different titanium sources (Tables 1 and 2) representative for the early stage dissolution.

Table 1. Experimental parameters.

Ti source (pure Ti or FeTi)	Contact time (s)	Sample identification
Ti (99.99%)	10	Ti-01
	30	Ti-02
	60	Ti-03
FeTi70	10	Ti70-01
	30	Ti70-02
	60	Ti70-03
FeTi35	10	Ti35-01
	30	Ti35-02
	60	Ti35-03

Table 2. Composition of the Ti sources used for the experiment (wt%).

Material	Ti	S	Mn	Al	Si	Ca	V	T.O.	Fe
Ti	99.99	–	–	–	–	–	–	–	–
FeTi70	69.6	0.006	0.25	2.48	–	0.03	1.83	0.18	Bal
FeTi35	38.5	0.022	0.66	5.05	3.5	0.22	0.42	0.95	Bal
Electrolytic iron	C	S	Mn	N	Si	Ca	Cu	T.O.	Fe
	0.0005–0.0015	0.0001–0.0003	–	0.0005	0.0003	–	0.0001	0.0020–0.0050	Bal

About 85 grams of electrolytic Fe (99.97% Fe) was filled in a magnesia crucible (30 mm inside diameter (ID), 35 mm outside diameter (OD), 50 mm height (H)) which was placed inside a high temperature vertical tube furnace (GERO HTRV 100-250/18, MoSi₂ heating elements) and melted at 1873 K under purified Ar atmosphere. The oxygen content in the off-gas was measured with a solid state ceramic oxygen sensor (Rapidox 2100, Cambridge Sensotec Ltd.). A typical value of the oxygen concentration in purified Ar is about 10^–20 ppm. The temperature profile in the furnace ensures a hot zone, approximately 4 cm in length, in which the temperature variation is within 1°C. Three different Ti sources were cleaned by pickling in 5 vol% NaOH solution for 1 min, followed by washing in de-mineralized water, ultrasonic cleaning by dipping in acetone and drying. Each piece of Ti or FeTi was placed inside a silica tube (8 mm ID, 10 mm OD) which was joined to another tube (6 mm ID, 8 mm OD, 20 mm H) in order to fix these pieces. This is schematically shown in Fig. 2. After about 60 min of temperature homogenization of liquid iron at 1873 K, the silica tube containing Ti or FeTi piece at room temperature was immersed in the magnesia crucible containing liquid iron. A small volume of molten Fe was suctioned into the tube and brought into contact with Ti or FeTi for a predetermined time of 10 s, 30 s and 60 s. The experimental parameters for each test are given in Table 1. The temperature of liquid iron just before the contact was 1873 K. The total oxygen measured in the bulk iron sample after the experiment was 100 to 140 ppm.

Fig. 2.

Schematic representation of the liquid metal suction method.

2.2. Analysis and Characterization

The samples obtained after the experiments were subjected to microstructural investigation by using a high resolution scanning electron microscope (Philips SEM XL-30 FEG), equipped with an EDAX energy dispersive spectrometer (EDS) detector system. The total oxygen content was determined in the bulk Fe samples after the experiments with a Leco combustion analyzer (TC-436DR). The thermodynamic calculations for FeTi35 experiments were carried out using the ‘Equilib' module of FactSage 6.2. The databases used for these calculations were FACT, FSstel and FToxid.

3. Results and Discussion

The early dissolution behavior of three different Ti sources was studied by bringing Ti or FeTi in contact with liquid iron at approximately 2 cm above the bottom of the crucible, inside the temperature zone (temperature variation ±1°C) of the vertical tube furnace. After allowing the Ti or FeTi in contact with liquid iron for the predetermined span, it was found that pure Ti and FeTi35 established a perfect contact with liquid iron and macroscopically, a clear interface was obtained (Fig. 3). However in case of FeTi70, instead of a clear interface, a macroscopically homogenous alloy was obtained suggesting a complete melting of FeTi70 in the liquid Fe. The dissolution mechanisms and the inclusion formation in liquid steel for these systems (liquid Fe – pure Ti, FeTi70, and FeTi35) are discussed in the following sections. In order to understand the various phases formed during the liquid Fe and titanium interaction, the titanium concentration of each phase is given in Table 3 obtained from the Fe–Ti equilibrium phase diagram.¹⁴⁾

Fig. 3.

Fe–Ti reaction couples obtained after introduction of Ti source to the liquid iron.

Table 3. Concentration of Ti in the various phases (wt%).¹⁴⁾

Phase	α-Ti	β-Ti	TiFe	TiFe2
Ti concentration	99.95 to 100	75.3 to 100	45.9 to 48.7	24.6 to 31.8

3.1. Fe-Pure Ti Reaction Couple

3.1.3. Microstructure of the Fe–Ti Interface

Ti and Fe established a perfect contact as shown in Fig. 3. The reaction couples Ti-01, Ti-02 and Ti-03 were obtained after the predetermined times of 10, 30 and 60 s respectively. Titanium oxide inclusions were not observed in the reaction zone of Ti-01, Ti-02 and Ti-03 samples. The result was unexpected. The probable reason is explained at the later part of this section. Figures 4 and 5 indicate that the reaction zone widens with time. The dissolution of Ti into liquid iron (Fig. 4) shows the phases analogous to the Fe–Ti equilibrium phase diagram.¹⁴⁾ The phases β-Ti, TiFe and TiFe₂ are the products of metallurgical reactions i.e. eutectic reaction at 1358 K and eutectoid reaction at 868 K, respectively. The reaction zone between the Fe and Ti was clearly distinguishable for the different reaction times. The composition in the reaction zone near pure Ti consisted of the eutectic mixture of TiFe and β-Ti (Fig. 4(a)). It means that the eutectic alloy (melting point 1358 K) forms having a considerably lower melting point than pure Ti (melting point 1943 K) upon the contact. The reaction zone was extended in the width and pure titanium crystals were seen in the β-Ti and TiFe matrix for the sample Ti-02 and Ti-03 (Figs. 4(b) and 4(c)). This was due to the formation of Ti rich liquid at the interface as the holding time increases, which resulted in the precipitation of pure Ti crystals during cooling. The formation of Ti rich liquid can be attributed to the melting of pure Ti due to the exothermicity of the Fe–Ti dissolution.¹⁵⁾ The overall width of the reaction zone for Ti-03 was comparable to Ti-02 (Fig. 5). However the extent to which different phases exist inside the reaction zone varies for Ti-02 and Ti-03 samples. The measurement of distinct phases in the reaction zone for these samples is shown in Fig. 5. It can be clearly seen from Fig. 5 that the width of the individual phases varies showing the relatively high titanium concentration regions towards the Fe side with holding time.

Fig. 4.

The micrographs of the reaction zone between pure Ti and liquid Fe. The distances in µm are given from the pure Ti towards pure Fe (a) Ti-01 (b) Ti-02 (c) Ti-03.

Fig. 5.

Schematic representation of the extent of the reaction zone and the various phases observed between the liquid Fe and pure Ti (a) Ti-01 (b) Ti-02 (c) Ti-03.

The widths of the reaction zones are 1.3 mm for Ti-01, 4.1 mm for Ti-02 and 4.3 mm for Ti-03 sample as measured in the center of the samples. It can be speculated that the formation of Al–Ti–O inclusions after the addition of pure Ti to the liquid steel can be due to the formation of these reaction zones in which the concentration of Ti is sufficiently high to reduce the existing alumina inclusions in liquid steel. The concentration of dissolved Ti required to reduce the existing alumina inclusions in the liquid steel containing a particular dissolved aluminium content is discussed in the Fe–FeTi35 section.

A large amount of data compiled by Cha et al.¹⁶⁾ for Ti deoxidation experiments from various researchers show that as the Ti content increases above 1 wt%, the oxygen requirement for the oxide formation also increases. In fact, experimental data of the oxygen saturation line corresponding to the Ti concentrations above 10 wt% were not found. In the present experiments, the Ti concentration of the various phases found in the reaction zone was much higher than 10 wt% (Table 3 and Fig. 4). Therefore, the oxygen concentration range (100–140 ppm) in the present experiments was not sufficient for the formation of titanium oxide.

3.2. Fe–FeTi70 Reaction Couple

The FeTi70 contains a higher level of impurities as compared to the pure Ti. The chemical composition of this alloy is given in Table 2 and the microstructure is shown in Fig. 6. FeTi70 is manufactured by alloying titanium sponge and scrap (mainly Ti–6Al–4V) with iron. Therefore, elemental impurities like Al and V are common in this alloy. The microstructure consists of three distinct phases i.e. β-Ti, TiFe and β-Ti+TiFe. These phases are in accordance with the Fe–Ti equilibrium diagram for an alloy containing ～70 wt% Ti. The phase TiFe can dissolve up to 4–8 wt% Al and 2 wt% V.¹⁷⁾

Unlike pure Ti, the interface between Fe and FeTi70 alloy obtained after quenching was not macroscopically visible (Fig. 3). The FeTi70 alloy (melting temperature 1358 K), after coming in contact with liquid iron, melted and subsequently, the mixing took place by convection and diffusion.

Fig. 6.

Distribution of phases in the FeTi70 alloy.

3.2.1. Microstructure of Fe–FeTi70 Reaction Zone

The microstructure of the Fe–FeTi70 reaction zone, due to complete melting of FeTi70, shows the boundary between the Fe+FeTi70 mixture and Fe. All three samples consisted of dark regions distributed in a light grey matrix. The Ti concentration of the dark region in Ti70-01 and Ti70-02 samples (Figs. 7(a) and 7(b)), was intermediate between the β-Ti and TiFe compositions while that of the light grey matrix was less than the Ti concentration of the TiFe₂ phase i.e. it falls between the TiFe₂ and pure Fe. The compositions of each pure phase are listed in Table 3. In the Ti70-3 sample (Fig. 7(c)), the dark areas were TiFe₂ while the composition of the light grey areas was less than 24 wt% Ti. This indicates a shift in the overall composition towards lower Ti concentration from the original eutectic mixture of β-Ti and TiFe phases as observed in the as-received FeTi70. The composition of the dark and light grey areas in the FeTi70+Fe mixture obtained in Ti70-1, Ti70-2 and Ti70-3 samples are shown in Fig. 8. The phase indicated as Fe in Ti70-1 and Ti70-2 samples has typically 2 to 5 wt% Ti and marks the iron rich side.

Fig. 7.

Micrographs of the reaction zone between FeTi70 and liquid Fe for different reaction times (a) Ti70-1 (10 s) (b) Ti70-2 (30 s) (c) Ti70-3 (60 s).

Fig. 8.

Composition of the dark and light grey regions observed in Fe–FeTi70 reaction zone of Ti70-1, Ti70-2 and Ti70-3 samples.

The inclusions of Al–Ti–O type were seen in Ti70-2 and Ti70-3 samples as shown in Figs. 7(b) and 7(c). In case of Ti70-3, in addition to the Al–Ti–O inclusions, Ca–Al–Ti–O inclusions were also observed as shown in Fig. 7(c).

On comparing FeTi70 dissolution with pure Ti, it can be observed that the FeTi70 started melting upon the contact due to its low melting temperature (eutectic point) and resulted in the formation of inclusions due to the presence of impurities like Al and Ca. While the dissolution of pure Ti in liquid iron involves partial melting and the mixing mostly depends upon the diffusion of Ti species into the liquid iron. This suggests that the dissolution of FeTi70 is much faster than pure Ti.

The possibilities and the thermodynamic conditions favorable to formation of inclusions as observed in the Ti70- 2 and Ti70-3 samples are discussed below.

3.2.2. Inclusion Formation in the Reaction Zone

The interfacial reactions in the Fe–FeTi70 reaction zone involve mainly liquid iron, dissolved oxygen, dissolved titanium and the impurities from the titanium source.

The inclusion characteristics i.e. size, morphology and chemical composition of the inclusions observed in Ti70-2 and Ti70-3 samples (Fig. 9) are listed in Table 4.

Fig. 9.

SEM micrographs of the inclusions observed in samples: (a) and (b) Ti70-2; (c) and (d) Ti70-3.

Table 4. Inclusion characteristics observed in the Ti70-2 and Ti70-3 samples.

Sample	Inclusion morphology		Size (µm)	Composition (wt%)
				Al	Ti	Ca	O	Fe
Ti70-2	Polygonal		2–3	46.23	11.25		42.52
	Polygonal		2–3	30.49	16.31		24.47	28.74
	Polygonal		1–2	27.64	14.56		20.90	36.89
Ti70-3	Dual phase	spherical core (CaO.Al2O3)	～7	19.78	7.16	28.54	44.52
		Polygonal periphery	～25	24.11	7.29	37.61	30.91
	Dual phase (rounded)	Dark region (CaO.2Al2O3)	～10	31.30	7.23	23.92	37.55
		Gray region			7.32	33.19	54.97	4.52
	Polygonal		2–3	19.82	21.04		29.59	29.54

The standard Gibbs free energies^18,19,20,21) (J/mol) of formation in liquid steel for these inclusions are given below. The standard state of the activities of dissolved elements (indicated by square brackets) was infinitely dilute solution in liquid iron while for the oxides, it was pure solid.

  

$$\begin{array}{c}{\text{Al}}_2{\text{O}}_3(\text{s})+[\text{Ti}]+2[\text{O}]={\text{Al}}_2{\text{O}}_3.{\text{TiO}}_2(\text{s})\\ \mathrm{\Delta }{\text{G}}^{\circ }=-706844+232.25\text{T}\end{array}$$(1)

  

$${\text{Al}}_2{\text{O}}_3(\text{s})+3\text{x}[\text{Ti}]=3{\text{Ti}}_{\text{x}}\text{O}(\text{s})+2[\text{Al}]$$(2)

  

$$\begin{array}{c}2[\text{Al}]+[\text{Ti}]+5[\text{O}]={\text{Al}}_2{\text{TiO}}_5(\text{s})\\ \mathrm{\Delta }{\text{G}}^{\circ }=-1435000+400.5\text{T}\end{array}$$(3)

  

$$\begin{array}{c}[\text{Ca}]+[\text{O}]=\text{CaO}(\text{s})\\ \mathrm{\Delta }{\text{G}}^{\circ }=-629998+144.75\text{T}\end{array}$$(4)

  

$$\begin{array}{c}2[\text{Al}]+3[\text{O}]={\text{Al}}_2{\text{O}}_3(\text{s})\\ \mathrm{\Delta }{\text{G}}^{\circ }=-1225417+393.8\text{T}\end{array}$$(5)

  

$$\begin{array}{c}\text{CaO}(\text{s})+{\text{Al}}_2{\text{O}}_3(\text{s})=\text{CaO}.{\text{Al}}_2{\text{O}}_3(\text{s})\\ \mathrm{\Delta }{\text{G}}^{\circ }=-19246-18\text{T}\end{array}$$(6)

On the basis of the above reactions, the origin of inclusion formation after the introduction of the FeTi70 alloy is discussed. As shown schematically in Fig. 1, when pure Ti or FeTi is added to the melt, a significant supersaturation occurs at the periphery of the dissolving/melting Ti or FeTi and depending upon the oxygen concentration, the formation of oxide inclusions takes place.

The formation of Al–Ti–O type of inclusions, as observed in the present experiments and the earlier work,^2,5) is possible by the Eqs. (1), (2) and (3). However, the possibility of the Al–Ti–O inclusion by Eq. (3) is more likely in the present experimental conditions because of the following reasons: (i) No alumina inclusion was present in the liquid Fe. Alumina inclusions were very rarely observed in the as-received FeTi70 and the few observed were large in size (> 20 µm)¹⁾ while the Al–Ti–O inclusions obtained in the present samples lie in the range of 2–3 µm (ii) all the reaction species i.e. dissolved Al, Ti and O are present at the reaction site in their dissolved state in FeTi70 (iii) the appearance of the inclusions as observed in Figs. 9(a) and 9(b) is single phase.

The oxygen saturation lines, above which there is formation of Al–Ti–O inclusions, are calculated on the basis of Gibbs free energy (Eq. (3)) and the interaction coefficients^20,22,23) data (Table 5) by varying Al and O concentration for a given Ti concentration and the temperature.

Table 5. Interaction coefficients.

i	j	$e_i^j$	Reference
O	Al	–0.83	22)
	Ti	–0.6	20)
	O	–0.2	20)
Al	Al	0.045	20)
	Ti	0.004	23)
	O	–1.4	22)
Ti	Al	0.0037	23)
	Ti	0.013	20)
	O	–1.8	20)

Considering that the initial temperature of the liquid iron was 1873 K and the local temperature was likely to decrease upon contact with FeTi70, the saturation lines are calculated for two temperatures. In the Ti70-2 and Ti70-3 samples, the titanium concentration in the vicinity of Al–Ti–O inclusions (marked by circular regions in Figs. 7(b) and 7(c)) was around 3 to 5 wt% while the Al concentration was less than 1 wt% (～2000–7000 ppm) as measured by the EDS. The total oxygen concentration of the iron after the experiments was 100 to 140 ppm determined by the Leco combustion technique. The dissolved O in the liquid iron was assumed to be the same as that of total oxygen. The melting point of Fe containing 3 to 5 wt% Ti is above 1773 K. On combining the compositional data with temperature, it further shows that favorable thermodynamic conditions for the formation of Al–Ti–O inclusions exist for the oxygen concentration between 100 to 140 ppm and Al concentration between 2000 to 7000 ppm as marked by a rectangular region in Fig. 10. This rectangular region lies above 2 wt% and below 4 wt% Ti line for both 1873 K and 1773 K temperatures. This is in good agreement with the EDS measurement whereby Ti concentration was found to be 3 to 5 wt%.

Fig. 10.

Oxygen saturation lines drawn for varying Al, O and Ti concentration for 1773 K and 1873 K. The compositional region in which the inclusions were observed in the present experiments for Ti-02 and Ti-03 samples is indicated by a rectangle. Al and Ti concentrations were measured by EDS while the O concentration was assumed to be 100–140 ppm as measured by the LECO combustion technique.

Also calcium aluminate inclusions containing up to 7–8 wt% titanium (Ca–Al–Ti–O) were observed in the present experiments (Figs. 9(c) and 9(d) and Table 4). These complex oxides can be formed by the reduction of alumina or Al–Ti–O inclusions by Ca. The concentration of Ca in FeTi70 was 50–300 ppm.¹⁾ Approximately 0.8 g of FeTi70 (starting weight) was melted after coming in contact with 4 g of liquid Fe (the quantity suctioned into the silica tube) during the experiment. This results in a total calcium concentration of 7 to 40 ppm in the resulting Fe–FeTi70 mixture. The thermodynamic calculations for the reactions (Eqs. (4) to (6)) shows that for Ca to be present in the elemental form, so that it can reduce alumina inclusion or Al–Ti–O inclusion, the activity of oxygen in liquid iron has to be extremely small. The presence of high oxygen in liquid iron (100–140 ppm) and about 1800 ppm of total oxygen in FeTi70, suggests that the existence of Ca in oxide is more favorable than the elemental form. Therefore, it seems that the calcium aluminate inclusions may have already existed in the FeTi70.

The formation of Ca–Al–Ti–O inclusions by the reaction between dissolved Ti and already existing Ca–Al–O inclusions seems to be the most likely possibility based on the observations and the thermodynamic calculations. However, the formation mechanism of Ca–Al–Ti–O inclusion requires more attention and needs to be investigated further. It is, so far, clear that the impurities from FeTi70 contributes towards the inclusion formation.

3.3. Fe–FeTi35 Reaction Couple

FeTi35 was the most impure source of Ti introduced to the liquid steel. The chemical composition of the alloy is given in Table 2. This alloy was quite fragile and porous in nature. The variation in the chemical composition of this grade in the various samples taken within the same batch was found to be very large.¹⁾ The presence of alumina inclusions and its size distribution in the as-received FeTi35 (FeTi35-0) are shown in Fig. 11.

Fig. 11.

FeTi35: (a) Distribution of phases and inclusions in the FeTi35 alloy (Matrix A – Al-4 to 10 wt%; Si-3 to 5 wt%; Ti-34 to 42 wt%; Fe-29 to 45 wt%; O-0 to 10 wt%) (b) Inclusion size distribution.

The inhomogeneous composition (Fig. 11(a)) can be attributed to the processing route of FeTi35 which is the reaction product of aluminothermic reduction of ilmenite. Therefore, the resultant product contains Ti, Al, alumina and unreduced Ti–O_x inclusions. The total oxygen content of FeTi35 as measured with the LECO combustion technique was found to be very high (4000 ppm to 1 wt%)¹⁾ and can be largely attributed to the presence of alumina inclusions. The alumina inclusion size distribution is shown in Fig. 11(b). The compositional inhomogeneity and the presence of inclusions, makes FeTi35 quite distinct from the previous two Ti sources i.e. pure Ti and FeTi70. As per the Fe–Ti equilibrium diagram, the approximate melting temperature of the FeTi35 (containing 38 wt% Ti) is found to be 1623 K which lies between the melting temperature of pure Ti (1943 K) and FeTi70 (1358 K). FeTi70 was found to be completely molten while pure Ti was partially melted. The micrographs of the FeTi35 samples on introduction to liquid iron, namely, Ti35-1, Ti35-2 and Ti35-3 are shown in Fig. 12.

Fig. 12.

Micrographs of the FeTi35 and liquid iron interface (a) Ti35-1 (10 s) (b) Ti35-2 (30 s) (c) Ti35-3 (60 s).

The size and morphology of alumina inclusions were not changed much in the Ti35-1 sample as compared to the as-received FeTi35-0 sample (Figs. 11(a) and 12(a)). In the Ti35-2 sample, the microstructure of FeTi35 becomes quite homogenous in appearance with a distribution of Ti–O_x and the alumina inclusions (Fig. 12(b)). In Ti35-3 sample, Ti rich regions (β-Ti) containing from 72 to almost 100% Ti was seen in the coagulated microstructure. It shows that there was partial melting inside the FeTi35, as outlined in Fig. 12(c). One shared phenomenon which was observed for all samples was the development of an oxide layer (mainly Al and Ti oxide) between FeTi35 and the liquid iron which was grown in thickness with time. Due to the formation of a complex oxide layer between the liquid Fe and FeTi35, only a very low level of Ti (up to 2 wt%) was observed in the iron side within a distance of 200 µm from the oxide layer.

The development of the oxide layer between the FeTi35 and liquid iron and the inclusion evolution in the FeTi35 side are discussed in the following sections.

3.3.1. Oxide Layer Growth between FeTi35 and Liquid Fe

The oxide layer consisting of Fe–Al–Ti–Si–O was grown as a function of time from 10 to 60 s. The oxide layer thickness vs time is plotted and shown in Fig. 13. The oxide layer micrographs are shown in Fig. 14. The thin oxide layer between Fe and FeTi35 with a thickness of ～6 μm was found in Ti35-1 sample. This oxide layer consisted of loosely packed alumina and Al–Ti–O inclusions as can be seen in Fig. 14(a). In Ti35-2 sample, the oxide layer was uniform and about 35 µm thick. The composition of oxide layer in Ti35-2 sample consisted of Al–Ti–O embedded in a continuous Ti–O_X layer. The variation in thickness of the oxide layer formed in Ti35-3 sample was large as can be seen from Fig. 12 and the thickness was about 85 µm on average. The relationship between the oxide layer thickness and the contact time looks parabolic with its axis of symmetry lying on the y-axis suggesting accelerated growth of the oxide layer with increasing contact time. This is in contrast to the generally observed parabolic behavior with the axis of symmetry lying on the x-axis indicating that the oxidation rate decreases with time due to the increasing oxide layer thickness which itself acts as a diffusion barrier. However, considering (1) the limited data (only three points) and (2) a large variation in the thickness of the oxide layer in Ti35-3 sample (contact time = 60 s), a linear relationship between the oxide layer thickness and contact time was assumed indicating that the oxide layer growth is reaction rate controlled rather than diffusion controlled.

Fig. 13.

Oxide layer thickness vs time of contact for FeTi35-1, Ti35-2 and Ti35-3 samples.

Fig. 14.

Micrographs of the oxide layer (a) Ti35-1 (b) Ti35-2 (c) Ti35-3.

Practically, the oxidation process is complex and in these experiments, an alloy (FeTi35) was involved. A mechanism of the oxide layer formation on the basis of observations and the compositional data, is proposed:

(i) In Ti35-1 sample (contact time = 10 s), when liquid iron comes in contact with FeTi35, Al and Ti from FeTi35 combines with the dissolved oxygen from the liquid iron to form small alumina and Al–Ti–O inclusions (≤2 µm) as can be seen in the Fig. 14(a). The formation of a thin layer consisting of small inclusions between liquid Fe and FeTi35 prevents significant diffusion of Ti in liquid iron. Additionally, the possibility of the attachment of a few inclusions from FeTi35 to the interface also exist owing to (a) the presence of a large amount of inclusions in FeTi35 and (b) careful observation of the interface after 10 s (Fig. 14(a)) shows loosely adherent inclusions with crystalline morphology.

  

$$\begin{array}{c}\underset{\_}{\text{Al}}\mathrm{\hspace{0.17em} }\text{and}\mathrm{\hspace{0.17em} }\underset{\_}{\text{Ti}}\mathrm{\hspace{0.17em} }(\text{FeTi35})+\underset{\_}{\text{O}}\mathrm{\hspace{0.17em} }(\text{liquid}\mathrm{\hspace{0.17em} }\text{iron})={\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}+\text{Al}-\text{Ti}-\text{O}\\ \text{Al}\mathrm{\hspace{0.17em} }\text{and}\mathrm{\hspace{0.17em} }\text{Ti}\mathrm{\hspace{0.17em} }\text{oxides}\mathrm{\hspace{0.17em} }(\text{FeTi35})={\text{TiO}}_{\text{x}}+\text{Al}-\text{Ti}-\text{O}\mathrm{\hspace{0.17em} }(\text{at}\mathrm{\hspace{0.17em} }\text{interface})\end{array}$$

(ii) After 30 s (Ti35-2), further oxidation takes place on the FeTi35 side forming a continuous Ti–O_X layer with Al–Ti–O inclusions embedded in it at some locations (Fig. 14(b)). Titanium, for which the concentration was significantly higher than the other elements in FeTi35, oxidized forming a continuous Ti–O_X layer. The oxide layer formed after 10 s was very thin and mainly consisted of a loosely adherent particles. Therefore, there was a contribution of dissolved oxygen from liquid iron. The possibility of adhering the already existing inclusions in FeTi35 to the oxide layer also exist. The growth of the oxide layer can be attributed to   

$$\begin{array}{c}\underset{\_}{\text{Ti}}\hspace{0.17em}(\text{FeTi}35)+\underset{\_}{\text{O}}\hspace{0.17em}(\text{liquid}\mathrm{\hspace{0.17em} }\text{iron})\hspace{0.17em}={\text{TiO}}_{\text{X}}\\ \text{Al}\mathrm{\hspace{0.17em} }\text{and}\mathrm{\hspace{0.17em} }\text{Ti}\mathrm{\hspace{0.17em} }\text{oxides}\mathrm{\hspace{0.17em} }(\text{FeTi}35)={\text{TiO}}_{\text{X}}+\text{Al}-\text{Ti}-\text{O}\mathrm{\hspace{0.17em} }(\text{at}\mathrm{\hspace{0.17em} }\text{interface})\end{array}$$

(iii) With longer holding times (60 s), the oxide layer was difficult to distinguish between different oxides within the reaction layer (Fig. 14(c)). The resultant oxide layer seems to be the solid solution of mainly Al and Ti oxides. The elemental mapping of this (Ti35-3 sample) oxide layer is shown in Fig. 15. It can be seen that the strong oxide forming elements like Al and Ti with some Si in FeTi35 were oxidized resulting in the formation of a complex oxide.   

$$\begin{array}{c}{\text{Al}}_2{\text{O}}_3+{\text{TiO}}_{\text{x}}+\text{Al}-\text{Ti}-\text{O}+\text{Fe}\mathrm{\hspace{0.17em} }\text{and}\mathrm{\hspace{0.17em} }\text{Si}(\text{FeTi}35)\\ =\text{Al}-\text{Ti}-\text{Si}-\text{Fe}-\text{O}\end{array}$$

Fig. 15.

Elemental mapping of the oxide layer in Ti35-3 sample.

On the basis of the the elemental mapping of the oxide layer (Fig. 15), it can be deduced that the oxide layer as observed in the micrographs (Fig. 14) has grown more towards the FeTi35 side as the presence of the Fe in the bulk oxide layer was minimal. It means that the contribution of oxygen and/or oxides from FeTi35 side was substantially higher than that of the liquid Fe side. The possible sources for oxygen and or oxides are dissolved oxygen (100–140 ppm) in liquid iron, the air pockets in FeTi35 because of its porous nature and the oxides/inclusions in FeTi35 (total oxygen 4000 to 10000 ppm). In the beginning, the availability of oxygen can be attributed to the dissolved oxygen from liquid iron side to a larger extent and; air infiltration due to the porous nature of FeTi35 to a smaller extent up to 30 s. After 30 s, the growth of the oxide layer on the FeTi35 side can be attributed to the air infiltration to a larger extent and to a smaller extent, to the adherence of the existing inclusions in FeTi35. The inclusion evolution in the bulk of the FeTi35 is further discussed in detail in the following section.

3.3.2. Inclusion Evolution

The micrographs of the inclusion evolution for Ti35-1, Ti35-2 and Ti35-3 samples are shown in Fig. 16. The inclusion size distribution, average size and the area fraction of these inclusions as measured in FeTi35 side of these samples is shown in Figs. 17(a) and 17(b).

Fig. 16.

Microstructural evolution and the Al based inclusions inside FeTi35 (compositional data was measured in the vicinity of inclusions as indicated by a circular ring).

Fig. 17.

Evolution of Al based inclusions as a function of time in Ti35-1, Ti35-2 and Ti35-3 samples (a) Inclusion size distributions (b) Average inclusion size and area fraction.

The inclusion size distribution for these samples indicates that the number of large sized inclusions decreases as the holding time increases i.e. the number of large sized inclusions (60–90 µm) have decreased from Ti35-1 to Ti35-2 sample while the inclusions less than 10 µm have increased in number. In Ti35-3 sample the inclusions in the size range 50 to 90 µm had disappeared (Fig. 17(a)). The decrease in the large sized inclusions has affected the other inclusion characteristics e.g. it can be observed that the number, area fraction and the size of the inclusions continuously decrease from Ti35-1 to Ti35-3 sample (Figs. 17(a) and 17(b)). Another possibility for observing less inclusions in Ti35-3 sample is that the distribution of the alumina inclusions was inhomogenous throughout the microstructure in the as-received FeTi35. As in these experiments, only a limited contact surface was available as shown in Fig. 12, incidently, an area of Ti35-3 with a low number of inclusions could have been contacted.

The decrease in large sized alumina inclusions with increase in holding time can be attributed to the reduction of alumina inclusions by the high Ti concentrations surrounding them. The unreduced Ti–O_x by Al and; alumina inclusions by Ti were reduced depending upon the local composition, specifically, the Ti/Al ratio (Ti and Al in wt%).

The reduction of oxides is explained on the basis of the thermodynamic calculations (FactSage). The local composition i.e. Al and Ti concentration surrounding the alumina inclusions (Fig. 16) was measured by EDS. The alumina reduction reactions by Ti are shown below by Eqs. (7) and (8).   

$$\text{x}/3\mathrm{\hspace{0.17em} }{\text{Al}}_2{\text{O}}_3(\text{s})+\underset{\_}{\text{Ti}}={\text{TiO}}_{\text{x}}(\text{s})+2\text{x}/3\mathrm{\hspace{0.17em} }\underset{\_}{\text{Al}}$$(7)

  

$${\text{Al}}_2{\text{O}}_3(\text{s})+\text{m}\mathrm{\hspace{0.17em} }\underset{\_}{\text{Ti}}={\text{Al}}_{2-\text{n}}{\text{Ti}}_{\text{m}}{\text{O}}_3(\text{s})+\text{n}\underset{\_}{\text{Al}}$$(8)

It was also assumed that the elemental species like Al and Ti were in the dissolved state because of the partial melting inside the FeTi35 as can be seen from Fig. 12(c). The Ti/Al ratio (concentration in wt%) as measured by EDS in the region as indicated in Fig. 16 was found to lie in the range of 3 to 5 in Ti35-1 sample and 7 to 15 in Ti35-2 and Ti35-3 samples. This local compositional data was used for the FactSage calculations. The FactSage calculations were carried out for a 100 g of iron containing varying concentrations of Al (0.5 to 2 wt%) and Ti (0.2 to 25 wt%) with 0.05 wt% alumina at two different temperatures i.e. 1873 K and 1773 K. The calculation results show that with higher titanium concentration, especially when the Ti/Al (in wt%) ratio is greater than 10, the alumina starts diminishing and the Ti₂O₃ and ilmenite starts forming as shown in Fig. 18. The pure Al₂O₃ was found to continuously decrease as the ratio of Ti/Al approaches a value of 10 at 1873 K and 12 at 1773 K while the Al content was increased. These calculation results are found to be in very good agreement with Matsuura et al.²⁾ and the present observations as indicated in Figs. 16 and 17. The inclusions in the Ti35-3 were comparatively small in size and a few in numbers. The Ti–O_x inclusions were found to be extremely small in size (less than 5 µm) while Al–Ti–O inclusions were found to be relatively large in size in the Fe–Ti matrix of Ti35-3 sample.

Fig. 18.

Dependence of the stability of pure Al₂O₃ on Ti/Al ratio at 1873 and 1773 K.

It is clear from the above discussion that the FeTi35 addition introduces alumina and/or Al–Ti–O inclusions to the liquid steel.

4. Summary and Conclusions

Generally, the formation of Al–Ti–O inclusions is attributed to the local supesaturation of Ti immediately after the addition. However, the present study shows that the impurities and the inclusions present in the Ti source alters their dissolution behavior and subsequently, the inclusion formation.

The present work is summarized in the context of actual steelmaking processing as follows:

(i) Pure Ti: The high titanium concentration regions after the reaction between pure titanium and liquid iron suggests that the titanium dissolution process is a relatively slow process owing to its high melting point. In the actual steelmaking process where the titanium addition is made in the Al deoxidised liquid bath, soluble aluminium typically lies in the range of the 300–600 ppm which ensures very low dissolved oxygen content (～4–6 ppm). This in turn, makes the formation of pure titanium oxide inclusions difficult. However, it can be speculated that if the dissolving Ti encounters an alumina inclusion, the alumina inclusion can be reduced to form Al–Ti–O because the Ti/Al (in wt%) ratio in liquid steel surrounding the alumina inclusion can easily be above 10. The micrographs of the Fe-pure Ti reaction zone reveals the regions of very high Ti concentration in the vicinity of dissolving pure Ti. The modification/reduction of alumina inclusions in liquid steel to Al–Ti–O inclusions as explained by the previous researchers can be attributed to these regions of high titanium concentration.

(ii) FeTi70: Apparently, FeTi70 meets two requirements to reduce the local supersaturation in order to prevent the Al–Ti–O formation i.e. its low melting point makes it a faster melting alloy and as compared to pure Ti, it is a more dilute source of titanium. However, FeTi70 contains elemental impurities like Al and Ca. Until now, the behavior of these impurities was unclear. The formation of Al–Ti–O inclusions after FeTi70 addition gives two possibilities (a) the reduction of alumina inclusions by dissolved Ti and or (b) the formation of Al–Ti–O inclusions by combining the dissolved Ti, Al and O as observed in the present experiments. FeTi70, being a low melting eutectic alloy, was found to be dissolved much faster than pure Ti. Therefore, after the addition of FeTi70, the type of inclusion formation is controlled by the availability of dissolved oxygen. The source for oxygen can be either reoxidation or the FeTi70 alloy itself. Nonetheless, in the present experiments, it is clear that the impurities from FeTi70 contribute towards the inclusion formation like Al–Ti–O and Ca–Al–Ti–O inclusions.

(iii) FeTi35: The oxide layer at the interface between FeTi35 and iron grows in thickness with increase in the contact time. The growth of oxide layer can be attributed to the local oxidation of the elements like Al and Ti as well as the attachment of inclusions from FeTi35 to the interface. It has prevented the significant mixing of the FeTi35 alloy in liquid iron. However, the evolution of the alumina inclusions inside the FeTi35 microstructure was observed. The size of pure alumina inclusions were found to have been decreased with time. At 1873 K, if the Ti/Al (in wt%) ratio exceeds a value of 10, the alumina inclusions can be reduced by Ti as shown with the thermodynamic calculations. The high Ti concentration at the edges surrounding the alumina inclusions was found to reduce them to Al–Ti–O inclusions. In actual steelmaking, it is quite unlikely that the FeTi35 would take such a long time to dissolve as the other much more dominating mechanisms like eddy diffusion and convection will enhance the mixing process. However, the short time for dissolution means less time for the modification of existing alumina inclusions. Therefore, it is certain that the FeTi35 addition to the liquid steel introduces alumina and Al–Ti–O inclusions. This has also been observed and reported in our earlier study at the industrial scale.²⁴⁾

If FeTi35 is produced in the same manner as FeTi70 i.e. by alloying, the advantage of lower local supersaturation can be taken due to Ti dilution and faster melting as compared to the FeTi35 produced by aluminothermic reduction. It will also prevent the introduction of alumina/Al–Ti–O inclusions maximizing the overall Ti yield/recovery.

Acknowledgements

This work was performed with the financial support of ArcelorMittal Industry Gent (Sidmar) and the IWT (project no. 070277).

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