A Study on the Transient Inclusion Evolution during Reoxidation of a Fe Al Ti O Melt

Transcription

1 , pp A Study on the Transient Inclusion Evolution during Reoxidation of a Fe Al Ti O Melt Cong WANG, 1) Neerav VERMA, 1) Youjong KWON, 1) Wouter TIEKINK, 2) Naoki KIKUCHI 3) and Seetharaman SRIDHAR 1,4) 1) Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA USA. sridhars@andrew.cmu.edu 2) Tata Steel Research, Development & Technology, P.O. Box 10,000, 1970 CA IJmuiden, The Netherlands. 3) Steel Research Laboratory, Steelmaking Research Department, JFE Steel Corporation, 1 Kokan-cho, Fukuyama, Hiroshima Japan. 4) National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, PA USA. (Received on August 19, 2010; accepted on December 3, 2010) The effect of a simulated reoxidizing environment on the chemical and morphological evolution of nonmetallic oxide inclusions was studied. Additions of 545 ppm and 274 ppm of soluble oxygen were introduced to an Al killed melt containing approximately 600 ppm of Ti and 600 ppm of Al. It was found that inclusion chemistry evolved from Al 2 O 3, Al 2 TiO 5 and eventually to Ti 3 O 5 for the higher oxygen addition case and to Al Ti complex oxides for the lower oxygen addition one. Morphologically, it was observed that irregular inclusions gradually were replaced by spherical ones during the reoxidation process. These changes are discussed through the coupling of thermodynamic prediction and experimental conditions, and considerations on the local variations of O and metallic element activities. KEY WORDS: inclusions; reoxidation; transient stage. 1. Introduction One of the challenging issues for the manufacturing of clean steels, especially of interstitial-free (IF) steels, is the problem of reoxidation associated with the formation of exogenous inclusions caused by unwanted variations in the oxygen level due to reactions between the melt and its environment, i.e. refractory, reducible slags and the gas environment. 1 3) Industrially, Fe Ti alloys are used as sources for Ti addition. Due to the high solubility of oxygen in Ti and the processing routes through which the Fe Ti alloys are manufactured, oxygen can potentially enter the melt locally during alloy additions as well resulting in a high Ti and O content. IF steels, due to their high formability, non-ageing property and cost effectiveness, have been widely used since the last three decades for automotive sheet applications. Recent trends in sheet production have been towards the production of very high purity steels with C and N contents less than 30 ppm, total O content less than 20 ppm and elimination of visible surface defects. 4) Ladle processing of IF steels involves de-oxidation by Al followed by Ti addition for binding the interstitial elements C and N. The de-oxidation products are often found to contain Ti 5) and this is not predicted by equilibrium conditions. The Ti-containing oxide inclusions have been suggested to aggravate clogging of the submerged entry nozzle which decreases continuous casting productivity and is a source of surface defects on the steel products. 6 8) Systematic effort has been applied to simulate the transient conditions after the Ti addition under laboratory conditions, and the resulting inclusions have been identified, 9 11) and the effect of Ti sources, the influence of Ti/Al ratio and the effect of gradual increase of Ti addition, on inclusion evolution behaviors, primarily in terms of composition and morphology, have been described elsewhere. 9 11) It was found that the addition of Ti to Al-killed melts produce transient changes in chemistry resulting in a significant amount of Ti-containing inclusions that eventually with time revert back to the thermodynamic stable alumina. The transient reactions however caused a change in morphology that caused the population of spherical inclusions to diminish while the number of irregular ones increased. Reoxidation is likely to cause unexpected lowering of soluble Al and depending on the local Ti content, this could result in inclusions other than Al 2 O 3 to form. The reoxidation process is therefore likely to involve both transient and permanent changes to the inclusion population. In this study the reoxidizing conditions were simulated by increasing the available O content through the addition of Fe 2 O 3 powders in the melt, a certain period after the Ti addition. The results are compared to those obtained under normal conditions without reoxidation, in terms of inclusion characteristics, with the purpose of identifying possible reactions that might occur during industrial practice. 375

2 2. Experimental Methods Fig. 1. General melting and sampling procedure illustration. Table 1. Chemical analysis of each sample. The experimental set-up is similar to that described elsewhere. 9) The general methodology is to deoxidize the iron melt, which contains 450 ppm of O, through the addition of the appropriate amount of Al, and to achieve different stable phases, by controlling the amount of Ti addition 2 min after Al deoxidation at a constant temperature of K. The chemistry of the melt can be characterized by a target of 600 ppm of soluble Al and soluble Ti, respectively, to achieve a Ti/Al of 1. This is similar to the melt chemistry of IF-steel melts after de-oxidation and Ti addition. 9) Immediately after Ti treatment, typical samples were taken out from the melt and the resulting inclusions were characterized in terms of composition, structure as well as morphology via energy dispersive spectroscopy (EDS) equipped scanning electronic microscopy (SEM). For each sample of interest, a total amount of 50 inclusions were randomly picked for characterization. Figure 1 is used to illustrate the uniqueness of these sets of experiments, with a simulated reoxidation environment being applied after Ti treatment. Two sets of experiments were designed to achieve final stable phases of Al 2 TiO 5 by having 40 ppm soluble Al and 600 ppm soluble Ti (named as 1st set) and of Al 2 O 3 by having 300 ppm soluble Al and 600 ppm soluble Ti (2nd set), respectively, and the required amounts of Fe 2 O 3 were estimated according to the thermodynamic and mass balance calculations. Essentially, these two sets of experiments follow the same procedure. The first part of the experiment to simulate Ti addition to Alkilled melts, in the absence of re-oxidation, was carried out in the same manner as what was done in previous studies. 9) The end of the transient stage after Ti addition was found to be confined within 6 min and thus the first sample obtained 6 min after Ti addition (8 min after Al deoxidation), is expected to be close to the end stage of the transient reactions that might be occurring after Ti is added. At this point inclusions are expected to be largely Al 2 O 3 and the morphology mostly irregular. 9,10) Two minutes after the first sampling event, appreciable amounts (approximately 5.4 and 2.7 g, equivalent to 545 and 274 ppm of soluble O, respectively) of reagent grade Fe 2 O 3 powders, which are primarily intended to serve as the external O source to provide a reoxidizing environment, were added to cause an increase in the melt oxygen content and simulate a reoxidation occurrence. A remark should be made about the behaviour of the Fe 2 O 3 when introduced in low oxygen steel. In the first sample after introduction of the Fe 2 O 3, no inclusions were detected that showed partial reduction of the Fe 2 O 3 substrate or FeO Al 2 O 3 or FeO TiO x type inclusions. The reduction and dissolution of Fe 2 O 3 particles and the melt was thus assumed to be fast and thus the Fe 2 O 3 was assumed to function as a source of dissolved oxygen. After the external O input, three other samples were obtained consecutively at 11, 14 and 18 min to investigate how and to what extent the inclusions could evolve under the assigned reoxidizing environment. The final chemistry of each sample, in terms of respective total and soluble element, is listed in Table 1. It should be noted that for the oxygen analysis, only the total value, which includes both the dissolved part and the part associated with aluminum to form alumina, is measured because insertion of an oxygen probe is not possible for the current furnace configuration. However, previous studies 9,10,12) have shown flotation removes the majority of alumina and thus the variation in total oxygen in the melt is an indication of the soluble oxygen. 3. Results Typical inclusions found during the reoxidation experiment of the 1st set are listed in Fig. 2. As demonstrated from Figs. 2(a) to 2(c), which were acquired right before the reoxidation occurred and the thermodynamic stable phase is Al 2 O 3, inclusion morphology is irregular in nature, and inclusion chemistry is dominated by either more or less pure Al 2 O 3 or Al-rich inclusions with small amounts of Ti content, which are believed to be the solid solution of Ti 3 O 5 into the Al 2 O 3 matrix. This result is representative of the end stage of transient reactions and is consistent with previous studies. 9,10) Previous studies 9,10) have investigated the time dependent reactions that take place in induction stirred melts that lead to the formation of various non-metallic inclusions not predicted by thermodynamics. It was revealed that when Ti is added to Al, a transient process is identified whereby the Ti content in the inclusions temporarily exceeds that predicted by thermodynamics. It was found that if Ti is at amounts corresponding to Ti/Al ratios of 1/4 or less, then the transient inclusions are primarily Ti dissolved at small amounts inside Al 2 O 3. However, if it is added in higher amounts, second phase Ti-inclusions, including the complex Al Ti O inclusions and a permanent change in morphology occur, promoting irregularly shaped inclusions. The inclusion characteristics that would typically prevail at the end of the stirring after Ti-addition would be represented by Figs. 2(a) 2(c) and the end of the dashed lines in Fig. 4 Fig. 6. Inclusions are largely modified both in shape and in composition after the introduction of Fe 2 O 3 powers into the melt. One minute after the addition of external O source, as evidenced from Figs. 2(d) to 2(f), inclusions chemistries evolve from the Al 2 O 3 -dominating state to one where the Ti content ranges from 3.63 to at%. Based on the Al/Ti ratio, the inclusion shown in Fig. 2(d) might be a solid solu- 376

3 Fig. 2. Typical inclusions encountered in the 1st set of experiment: (a) (c) are obtained 8 min after Al deoxidation; (d) (f) are obtained 11 min after Al deoxidation (1 min after reoxidation); (g) (i) are obtained 14 min after Al deoxidation (4 min after reoxidation) and (j) (l) are obtained 18 min after Al deoxidation (8 min after reoxidation). Fig. 3. Typical inclusions encountered in the 2nd set experiment: (a) (c) are obtained 8 min after Al deoxidation; (d) (f) are obtained 11 min after Al deoxidation (1 min after reoxidation); (g) (i) are obtained 14 min after Al deoxidation (4 min after reoxidation) and (j) (l) are obtained 18 min after Al deoxidation (8 min after reoxidation). tion of Al2O3 into the Ti3O5 matrix and the inclusion shown in Fig. 2(e) might be Al2TiO5. The inclusion in Fig. 2(f) might be a solid solution of Ti3O5 into Al2O3. Distinguishably, inclusion shape is altered from irregular-shaped ones toward spherical ones. As the reoxidation process proceeds further, inclusions can be seen to have more Ti content at the cost of Al (Figs. 2(g) to 2(i) and Figs. 2(j) to 2(l)). More importantly, in terms of inclusion morphology, inclusions are primarily clearly-defined spheres. This is a significant difference from the situation when re-oxidation did not occur when irregular shaped inclusions were predominant. It should be noted that in terms of inclusion stoichiometry, these inclusions could be considered as the solid solution of Al2O3 into respective Ti3O5 matrixes. Similarly, typical inclusions found in the 2nd set of experiment (with Al2O3 being the aimed stable phase by having 300 ppm soluble Al and 600 ppm of soluble Ti) are displayed in Fig. 3. Before the external oxygen input, inclusions are dominated by irregular Al2O3 (Figs. 3(a) 3(c)). As with the 1st set experiment, inclusions are substantially changed both morphologically and compositionally after 1 min of oxygen addition, with more spherical and Ti-containing inclusions produced (Figs. 3(d) 3(f)). With time, the majority of the inclusions are spherical shape and contain small amount of Ti contents, which is believed to form through solid solution of Ti3O5 into the Al2O3 matrix (Figs. 3(g) 3(l)). Information regarding the morphological evolution of both sets of experiments is exhibited in Fig. 4. The two curves essentially follow the same trend. As can be seen clearly, inclusions are typically irregular after 6 min of Ti treatment, when the transient stage is approaching the end. However, an approximately equal spherical-irregular split is Fig. 4. Percentages of different types of inclusions with respect to deoxidation time. Dashed curve schematically shows the evolution trend of the percentage of irregular inclusions in Ref. 9). Fig. 5. Percentage of Ti-containing inclusions with respect to deoxidation time. Dashed curve schematically shows the evolution trend of the percentage of Ti-containing inclusions in Ref. 9). observed 1 min after Fe2O3 treatment. With time, spherical inclusions are taking the majority at the compensation of irregular types. By using the previously adopted approach of characterizing inclusion chemistry, Fig. 5 was constructed to show the change in percentage of Ti-containing inclusions (only in377

4 last two samples carry high Al content with minimal error ranges. Complimentarily, the average Ti content is only about 3 at%, as indicated by the open squares in Fig. 6(b). It is largely because the inclusions are composed of inclusions formed through the solid solution of Ti 3 O 5 into the Al 2 O 3 matrix as displayed in Fig. 3. Fig. 6. (a) Average Al content of inclusions with respect to deoxidation time. Dashed curve schematically shows the evolution trend of the average Al content percentage of Alcontaining inclusions in Ref. 9). The horizontally dashed line represents the Al content in Al 2 TiO 5 under theoretical stoichiometry. (b) Average Ti content with respect to deoxidation time. clusions with 1 at% Ti contents are counted as Ti-containing inclusions) with time. The results represent 50 inclusions obtained over an area of 0.15 mm 2. Again, the results of both sets of experiments share good similarity. Initially, the percentage of Ti-containing inclusions is low ( 15%), but this value keeps increasing after the external O input occurred. Around 60% inclusions are Ti-containing, 1 min after the addition of Fe 2 O 3, and almost all of the inclusions are Ti-containing thereafter. In addition, since each individual inclusion contains certain amounts of Al, but not necessarily Ti, inclusion chemistry is alternatively expressed via the average Al content representation versus deoxidation time (Fig. 6(a)) by mathematically averaging the Al atomic percentage of the 50 inclusions from each sample. While the previous Fig. 5 results showed the number of inclusions containing Ti regardless of Ti content, Fig. 6 is an indication of how the Al or Ti content changes with time for a typical inclusion. For both cases, 8 min after Al killing, inclusions are more or less entirely composed of Al 2 O 3, as demonstrated earlier in Ref. 9), and the average Al content is around 31 at%, fairly consistent with both inclusion stoichiometry and the corresponding values revealed in the previous investigations. For the 1st set of experiment, immediately following the addition of the reoxidizing agent, the average Al content drops first to nearly 20 at%, then to 5 at% and eventually to a negligible 2 at%. It is noteworthy that the error bar after 1 min of O input is quite significant, and this is the reflection of the coexistence of both Al 2 O 3 inclusions and complex Al Ti O inclusions with higher and/or lower Al content (Fig. 2 and Figs. 3(d) and 3(e)). The very low value of average Al content of the last two samples is an indication that the inclusion population has changed from Al 2 O 3 to Ti-containing oxides. For the 2nd set of experiment, however, the error range for the first sample after oxygen addition is still large, implying the same situation as demonstrated in the 1st set of experiment. It is noteworthy to point out that the 4. Discussion Through the morphological and compositional comparison of inclusions before and after the simulated reoxidation treatment, the following findings were noted: (1) The inclusion chemistry evolved from Al 2 O 3, Al 2 TiO 5 and eventually to Ti 3 O 5 for the higher oxygen addition case and to Al Ti oxides (mostly through the solid solution of Ti 3 O 5 into the Al 2 O 3 matrix) for the lower oxygen addition one. Consequently, percentages of Ti-containing inclusions keep increasing with time for both cases. (2) Inclusion morphology reverses the initially irregularly shaped-dominating scenario to the situation where spherical inclusions dominate. (3) For the high oxygen addition case the Ti content in the inclusions continuously increased but in the low oxygen addition case the Ti content increased initially but dropped to a plateau. The compositional change could be explained through either/both of the following two scenarios: ( i) A non-equilibrium reaction between the dissolved oxygen and the dissolved Ti. (ii) A successive lowering of Al, until Ti-containing inclusions become stable. In Fig. 2 and Fig. 3, it can be seen that the Ti content increases already at the first sampling, 1 min after oxygen input and this could be reflective of the non-equilibrium Tioxides that form as the dissolved O encounters dissolved Ti. If the Al content is sufficiently high, these Ti-oxides would be metallothermically converted to alumina but from Fig. 2 and Fig. 3 there is no indication that this is occurring which might be due to that the Al content is lowered rapidly and there is a successive evolution of the thermodynamically stable phase. Initially, when the first sample is taken out from the melt, the final soluble Al and Ti contents are 390 ppm and 580 ppm, respectively, which dictates that the final stable phase should be Al 2 O 3. However, since it is also the end stage of the transient reactions as defined before, it is natural to have a low percentage of Ti-containing inclusions, as can be seen in Figs. 2(a) 2(c) and Figs. 3(a) 3(c). For the morphological change, spheres can form in a few different ways such as: 1) precipitate spheres at high oxygen vs. metallic element ratios suggested by Steinmetz 29) and 2) liquedification (being viscous). Figure 7(a) is used to illustrate the stable phases particularly for the higher oxygen addition case since the lower oxygen addition case theoretically does not involve stable phase changes. Relevant thermodynamic data 13 16) regarding major reactions, and concerning interaction coefficients 17,18) of different species, are listed in Tables 2(a) and 2(b). After Al killing and Ti treatment, the stable phase is predicted to be Al 2 O 3, which is governed by the soluble Al and Ti concentrations, as indicated by the black square in Fig. 378

5 Fig. 7. (a) (b) Table 2. (a) Calculated equilibrium phase diagram for the Fe Al Ti O system at K in terms of soluble Al and Ti contents. The area enclosed by short dots schematically represents the potential liquid phase proposed by Ruby-Meyer et al. 26) and Jung et al. 27) Squares are matching compositions of each sample of the higher oxygen addition case from Table 1. Circles represent the melt compositions of the lower oxygen addition case. (b) Illustration of O and Al variations. Pentagrams represent available Al and O contents when reaching the Al 2 O 3 /Al 2 TiO 5 boundary, and available Al and O contents when reaching the Ti 3 O 5 /Al 2 TiO 5 boundary, respectively. The circle on the solid line denotes the equilibrium oxygen content when soluble Al content equals 300 ppm. (a) Reactions in the Fe Al Ti O system with standard free energy changes; (b) interaction coefficients. 7(a). Initially, with the addition of reoxidizer, it would be expected that the dissolved Al rapidly reacts and as a result, the overall thermodynamic condition is altered largely due to the lowering of the soluble Al but the overall Ti content does not change. It is calculated that for the general case, after Al deoxidation, 5.8 ppm of soluble O would be retained as indicated by the thermodynamic equilibrium requirement, and this value is going to be constant throughout the following Ti treatment. However, when the soluble Al content is continuously lowered as a result of the O addition (as indicated from the change of the squares from point (1) to (2) and to point (3) in Fig. 7(a)), the theoretical stable phase will be changed when the chemistry is such that the boundary of Al 2 TiO 5 and Al 2 O 3 is reached. Based on a mass balance of consumed Al (from 390 to 80 ppm) and O, this will occur at an oxygen level of 32 ppm, as shown in Fig. 7(b), which presents the relationship between soluble Al and O, by the pentagram and the connecting dashed lines. Equilibrium between O and Al is thus not established. The possible de-oxidation reactions now become (ignoring the variations in Al/Ti ratio that might exist): 2Al 5O Ti Al TiO 2 5 Al O Ti 2O Al TiO (1)...(2) However, given the fact that the O activity is still relatively high and pre-existing Al 2 O 3 could have been floated to the top part of the melt, the reaction described by Eq. (1) is likely to be the most prevalent reaction. From Fig. 7(b) it can be seen that upon reaching the Al deoxidation equilibrium, the Al content is 390 ppm while the O content is 5.1 ppm, as computed from thermodynamics. After adding Fe 2 O 3 as the external O source, O content is elevated to 545 ppm, enabling the formation of Ti-containing inclusions possible. According to the mass balance of O and Al, 32 ppm of O is available if the phase boundary of Al 2 O 3 /Al 2 TiO 5 is reached. Similarly, 28 ppm of O is available if the phase boundary of Al 2 TiO 5 /Ti 3 O 5 is reached. Both of these values are higher than their corresponding equilibrium ones which are 19 ppm in Al 2 O 3 and 22 ppm in Al 2 TiO 5 (see black circles in Fig. 7(b)), respectively. It can be seen that after 1 min of reoxidizer addition, inclusions begin to contain more Ti, (Figs. 2(d) 2(f) and Figs. 3(d) 3(f)), which indicates that at least partially the melt is converted to a condition favorable for the formation of Al 2 TiO 5, as exemplified via Fig. 2(e) and Fig. 3(d). Moreover, inclusions with very low Ti content could also be identified in these samples (Fig. 2(d) and Figs. 3(e) 3(l)). For the higher oxygen addition case, if the situation predicted by Eq. (1) is going to be true, contents of soluble Al and Ti are going to be successively reduced until the dashed line hits the boundary of Al 2 TiO 5 and Ti 3 O 5 (from point (3) to point (4)). The formation of Ti 3 O 5 could be possible because of: 1) controversies on the selection of reliable thermodynamic data, especially of the Fe Al Ti O system; 2) inherent inaccuracy due to the minor requirement and difference of the desired Al amount for these two stable regions. In other words, the reoxidizing effect by the introduction of Fe 2 O 3 could be a progressive process. Initially, soluble Al content is decreased to an extent at which the formation of Al 2 TiO 5 is favored. Since the added O source amount could be larger, even though not that appreciable, than the precisely required amount for the stabilization of a pure Al 2 TiO 5 state, the realization of a Ti 3 O 5 stable regime could be highly possible because the remaining soluble Al content is far below, as exhibited by the squares in Fig. 7(a). A similar discussion could be applied to the formation of Ti-containing inclusions in the lower oxygen addition case (2nd set of experiment). For the lower oxygen addition case, the stable phase does not change even though appreciable amount of reoxidizer has been added. As shown by 379

6 the pentagram in Fig. 7(a), the stable phase is still in the Al 2 O 3 region (Al 390 ppm and Ti 640 ppm, also as shown in Table 1) and is far away from either the Al 2 TiO 5 and/or the liquid region boundary. Due to this reason, the formation of Ti 3 O 5 and Al 2 TiO 5 inclusions is unlikely, which can also be evidenced in Figs. 3(a) 3(c). From Table 1 it can be seen that after the reoxidizer was added soluble Al and Ti contents drop simultaneously with the increase of total oxygen, which indicates the effect of simulated reoxidation process (sample 2). With increased time, however, soluble Al and Ti contents remain essentially unaltered, but the total oxygen content keeps decreasing. A possible reason could be ascribed to the inclusion flotation that replaces the oxygen from the melt. It should be noted that, by considering the thermodynamic prediction in Fig. 7(a), sample 3 and 4 (indicated by circles in Fig. 7(a)) are essentially falling into the Al 2 O 3 stable region (Table 1), though being fairly close to the Al 2 O 3 /Al 2 TiO 5 phase boundary, implying the formation of inclusions with less Ti content (Figs. 3(d) 3(l)) is possible because of the local super-saturation hypothesis as discussed in the preceding papers. 9,10) The Ti content in the inclusions prevails and this could be due to that Ti is present as dissolved oxides within Al 2 O 3. A detailed discussion on the formation thermodynamics of this kind of inclusions can be found in Ref. 11). It is also likely that there is a wide spectrum of stoichiometries in terms of Al, Ti and O contents, with many of them being believed to be the solid solution of Ti 3 O 5 into Al 2 O 3 matrixes, or vice versa. Ti oxides belong to the complex Magneli phase family, and they share great similarities in stoichiometries and structures, though they essentially don t belong to the same space group. TiO x gets dissolved in the Al 2 O 3 matrix in the form of either Ti 2, Ti 3 or Ti 4, as have been reported by many researchers 19 24) and demonstrated by the TEM results in Refs. 9) 11). Thus, it might be practical to extend the existing knowledge of the Al 2 O 3 TiO 2 and Al 2 O 3 Ti 2 O 3 systems to the Al 2 O 3 Ti 3 O 5. In addition, according to available thermodynamic data, Ti 3 O 5 phase is predicted to be the stable phase under the current experimental conditions. It could be pertinent, to a large extent, to ascribe the Ti content, if in low amount, from solid solution of Ti 3 O 5 into the Al 2 O 3 maxtrix, and vice versa. Further work involving TEM analysis is required to confirm this. The inclusion evolution should however also be discussed in light of the area enclosed by short dots in Fig. 7(a) which would result in liquid (viscous) inclusions. The existence of this region has been suggested in several papers 8,25 28) but there has to date not been an experimental thermodynamic study carried out which irrefutably prove its existence. If one considers this region rather than the Al 2 TiO 5 reaction (2) would become: 5. Concluding Remarks Current study addresses the effect of the simulated reoxidizing environment on inclusion evolution behaviors. It has been shown that the introduction of a proper amount of rexal yo zti Al O Ti x y z...(3) It implies that the inclusions formed might not have fixed stoichiometries as predicted through thermodynamic calculations and demonstrated through phase diagrams. When examining the inclusion morphologies that prevail at the end stages for both cases (Figs. 2(j) 2(l) and Figs. 3(j) 3(l)), it appears that spherical shaped ones dominate. For the higher oxygen addition case, spherical shapes could be an indication that reactions such as (3) are the ones responsible for the Ti-containing inclusion formation rather than reactions (1) and (2). This could be indicative of the existence of a liquid phase region in which spherical shape would easily form as is the case in Ca treated Al killed grades. 28) If this was the case, the liquid region would extend beyond the Ti 3 O 5 /Al 2 TiO 5 boundary in order not to precipitate Ti 3 O 5 (or any other solid Ti-oxides) which would disrupt the spherical shape. As for the lower oxygen addition case, the melt chemistry is unlikely to enter the liquid region. However, according to the discussion presented by Steinmetz et al., 29) spherical oxide inclusions are favored to form under conditions of a high ratio of oxygen activity to metal activity. In this case, the soluble oxygen content, which can be regarded as the oxygen activity, although keeps decreasing as reoxidation goes on, might be still high since the true equilibrium is still far ahead. Under this circumstance, it could be possible that new spherical precipitations form due to high O/Al ratio. Moreover, since the final chemistry of the higher oxygen addition case is fairly close the liquid region; it is also plausible to speculate that the viscous condition could also be favorable for the formation of spherical inclusions for this case, but not necessarily for the lower oxygen addition case. During the casting of Ti-containing IF steels it is generally noted that IF steels show a higher clogging tendency than similar steels without Ti. For the industrial practice it is important to know whether for instance a bulk property of a heat like the total oxygen value is related to the clogging tendency in IF steels or that for instance the Ti/Al ratio in the steel, as described in this work, is of more importance. Several ladle treatment practices for the production of IF steels were developed world wide. In some cases the time between the Al and Ti addition is seen as an important factor. The reoxidation power of the ladle slag can be reduced by lowering the FeO and MnO content in the slag, which will create less reoxidation after Al is added to the steel, 8,30) and the post-circulation time, which is the time after Al was added to deoxidize the steel, can be optimised to attain a low amount of inclusions. In literature it is discussed 6) that Ti-containing inclusions will create easier deposits than pure alumina particles. The studies, described in this work, clearly show that Ti-containing inclusions will form in Ti-containing IF steels when a fierce reoxidizer, like Fe 2 O 3, is added to the Al and Ticontaining steel melt. A while after the Fe 2 O 3 addition, alumina content in the inclusions increases again. From the above a huge reoxidation during casting is considered as a risk for clogging. During a huge reoxidation inevitably the total oxygen will, at least locally, increase. Because the Ticontaining inclusions and the total oxygen separately give rise to a higher clogging tendency, reoxidation should be prevented at all times late in the steel making process, e.g. during teeming and casting. 380

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