Characteristics of Fine Oxide Particles Produced by Ti-M Complex Deoxidation

ISIJ International, Vol. 48 (2008), No. 11, pp. 1507–15161507©2008ISIJstead of an Fe–C alloy which is preferable to be used. How-ever, there are two reasons why it was not to be used. One is that; oxide particles obtained on a film filter are analyzed by SEM. If an Fe–C alloy is used, oxide particles on a film fil-ter are difficult to analyze due to the presence of other residual particles and/or precipitates whose amount is in-creased with an increase in carbon content. It was con-firmed that amount of such residual particles and/or precip-itates was small in the case of an Fe–CϽ0.05mass%C alloy. Another is that; the yield of Mg in metal is significantly high due to the fact that the solubility of Mg in liquid Ni is considerably higher than that in liquid Fe. This leads to the excellent reproducibility in the measurement of particle size distribution, because the characteristics of deoxidation particles such as size, number, composition and dispersion are reproducible in an Fe–10mass%Ni alloy.2.Experimental2.1.ProcedureThe melting of an Fe–10%Ni alloy was carried out in an induction furnace (100kHz) under an Ar atmosphere. In order to eliminate the fluid flow due to induction current, a graphite susceptor (10mm of wall thickness) was installed between induction coil and crucible. A charge (70g) con-taining high-purity electrolytic iron (99.99%) and globular nickel (99.97%) was melted at 1600°C and held for 30min at this temperature for homogenization of metal composi-tion. High purity Al2O3and MgO crucibles were used for0.20% Ti experiments (Exp. Nos. 3 to 5) and all other de-oxidation experiments, respectively. The Fe–50%Ti, Ni–12%Mg and Fe–10%Zr alloys were melted previously in an arc furnace under an Ar atmosphere and used as deoxidants in all experiments.The melt with an initial oxygen content in the range from 0.010 to 0.014% was deoxidized with Ti (0.025 and 0.20%), Mg (0.1%) or Zr (0.08 and 0.20%) in Exp. Nos. 1to 13. After stirring for 20s using an Al2O3rod, the meltwas held for 1, 3, 5 or 10min at 1600°C, followed by rapid quenching in water. Reoxidation during quenching was pre-vented in such a way that the flow rate of Ar gas from the bottom was first increased and immediately after a crucible containing a sample was taken out from the furnace, He gas was blown onto melt surface, followed by quenching in water. In Exp. Nos. 14 to 19 the melt was first deoxidizedwith 0.025% Ti, stirred for 20s using an Al2O3rod andheld for 40s, followed by addition of Mg or Zr. After stir-ring for 20s, the melt was held for 1min at 1600°C and quenched in water immediately. In Exp. No. 20 the appro-priate amount of Ti (0.025%) and Mg (0.1%) was added in the melt at once. In Exp. Nos. 21 and 22 the melt was first deoxidized with 0.06% Ti, stirred for 20s and held for 30min, followed by addition of Mg (0.1%) or Zr (0.05%). Then, the melt was stirred for 20s and held for 1min at 1600°C, followed by rapid quenching in water.2.2.Observation of Spatial ParticlesFor the analysis of particle size, number and composi-tion, a metal sample was dissolved by using a potentiostatic electrolytic extraction (Ϫ150mV, 45 to 55mA and 350 coulomb). The 10% AA (10v/v%acetylacetone–1w/v% tetramethylammonium chloride–methanol) and 2% TEA (2v/v%triethanol amine–1w/v%tetramethylammonium chloride–methanol) were used as electrolyte for the Ti, Zr and Ti/Zr and the Mg and Ti/Mg deoxidation experiments, respectively. The weight of dissolved metal was from 0.07 to 0.10g. After electrolytic extraction the solution with par-ticles was filtrated by using a polycarbonate film filter with an open pore size of 0.05m m. The particles and clusters ona surface of film filter were observed by using the scanningelectron microscope (SEM) at a magnification of 1000, 2000, 5000 and 10000. The spatial size, dV, of each parti-cle on a photomicrograph was obtained by using a semi-au-tomatic image analyzer. The method for the calculation of number of particles per unit volume, NV, is described else-where.15)The composition of particles was determined by using the electron probe microanalysis (EPMA). Qualitative analysis of particle composition was obtained for particles larger than 0.05m m. Estimation of particle composition in percentage was made for particles larger than 1.0m m due to the difficulty in accurate quantitative analysis of composi-tion for particles below 1.0m m.2.3.Chemical AnalysisIn order to determine soluble and insoluble contents of elements ([sol.M] and [insol.M]), a metal sample was dis-solved by using a potentiostatic electrolytic extraction method (Ϫ150mV, 45 to 55mA and 1500 coulomb). The weight of dissolved metal was from 0.3 to 0.5g. The elec-trolyte solution was 10% AA for Ti, Zr and Ti/Zr deoxida-tion experiments and 2% TEA with Ba (0.1 to 0.2w/v% Ba) for Mg and Ti/Mg deoxidation experiments. After elec-trolytic extraction the solution was filtrated by using a poly-tetrafluoroethylene (PTFE) film filter with an open pore size of 0.1m m. The solution and residue on a film filter after filtration were used for analysis of soluble and insolu-ble M contents, respectively, by using inductively coupled plasma (ICP) emission spectrometry.3.Results and DiscussionThe experimental conditions for the sample preparation, chemical composition of metal and particle characteristics are given in Table 1.3.1.Effe ct of Comple x De oxidation on Total ParticleSize DistributionTotal particle size distributions obtained in Ti, Mg and Ti/Mg deoxidations are shown in top, middle and bottom diagrams in Fig. 1, respectively. In Ti/Mg complex deoxi-dation, Ti (0.025%, the amount of initial addition) is added and 1min later Mg (0.1%) is added, followed by quenching after 1, 3 or 5min. This method denotes Ti/1min/Mg deox-idation, hereinafter.The size distribution in Ti deoxidation at 1min corre-sponds to the primary and secondary deoxidation particles (primary and secondary particles, hereinafter). The curve at 5min corresponds to mostly secondary particles except for large particles. These are the particles crystallized and/or precipitated during cooling and solidification since dis-solved oxygen content is relatively high in Ti deoxidation.Most of primary particles after deoxidation floats out and/or coagulates.In Mg deoxidation at 1min, a bimodal curve is observed.The curves at large and small size particles correspond to primary and secondary particles, respectively. When the melt containing initial oxygen level of about 100ppm is de-oxidized with Mg, dissolved oxygen content does not im-mediately approach the oxygen level determined by the Mg/MgO equilibrium. For this reason secondary particles are produced during rapid cooling and solidification. MgO particles formed at 1min are easy to be in cluster and float out, because of high contact angle (qϭ125°) with respect 1508©2008ISIJto liquid Fe on MgO substrate and low density of MgO (3.65g/cm3). As a result, the number of primary particles decreases with increasing holding time and the particle size distribution at 3min has a single peak, although it consistsof primary and secondary particles. Furthermore, dissolved Mg vaporizes rapidly with holding time at 1600°C and ini-tial MgO particles change to MgO–FeO particles which will be explained in Sec. 3.2. As shown in the middle dia-gram, the number of particles in Mg deoxidation experi-ment at 5min decreases drastically. This is due to the fact that most of primary MgO particles coagulate and float out.The number of secondary particles after 5min is also low because the content of dissolved oxygen is almost equal to the oxygen level determined by the Mg/MgO equilibrium.In Ti/1min/Mg deoxidation at 1min, a bimodal curve is observed, as shown in the lower diagram. The primary par-ticles, which are correlated with the peak on the right-hand side for large size particles, are produced by the deoxida-tion of soluble oxygen with Mg and Ti and by the reduction of primary TiO X particles with Mg. The secondary parti-cles, which are correlated with the peak on the left-hand side for small size particles, are produced by the reaction of dissolved oxygen with Mg and Ti during rapid cooling and solidification. Large size primary particles and clusters float out with holding time. It is considered that the curve at 5min corresponds to mostly secondary particles except for large particles.Total particle size distributions obtained at Zr and Ti/1min/Zr deoxidations are shown in the upper and lower diagrams of Fig. 2as a function of holding time at 1600°C.As given in Table 1, total N V values decrease with holding time in both deoxidations. A bimodal curve of particle size distribution is observed in Zr deoxidation at 1min. This is due to the coexistence of primary and secondary particles.The primary ZrO 2and ZrO 2–FeO particles are produced after Zr deoxidation. However, dissolved oxygen content is higher than that determined by the Zr/ZrO 2equilibrium. As1509©2008ISIJTable 1.Experimental conditions, composition of metal phase and characteristics of particles.Fig.1.Total particles size distributions in Ti, Mg and Ti/Mg de-oxidation experiments.a result, the secondary particles are produced by crystalliza-tion and/or precipitation of ZrO 2and ZrO 2–FeO particles during cooling and solidification. The modal values for pri-mary particles move to lower particle size and a single peak curve is obtained with holding time due to the particle flotation and coagulation. It is considered that the curves at 3 and 5min correspond to only primary particles, since the contribution of secondary particles is small due to low solu-ble oxygen content at 3min after Zr addition.In Ti/1min/Zr deoxidation, particle size distribution at 1min shows single peak curve which is different from those obtained in Mg, Zr and Ti/1min/Mg deoxidations at 1min.The content of soluble oxygen in Ti/1min/Zr deoxidation at 1min after Zr addition is lower in comparison with that for Zr deoxidation experiment. Thus, it is considered that the contribution of secondary particles is smaller. On the other hand, the number of primary particles in Ti/1min/Zr deoxi-dation is higher and the mean particle size is smaller than that in Zr deoxidation experiments. As a result, two peaks for primary and secondary particles are combined to one peak. The number and size of particles in Ti/1min/Zr deox-idation decrease gradually with increasing the holding time compared with those in Zr deoxidation experiments.Variation of particle number per unit volume, N V , with holding time at 1600°C is shown in the upper diagram of Fig. 3for Ti, Mg and Ti/1min/Mg deoxidations which correspond to the results shown in Fig. 1. The results for the simultaneous addition of Ti and Mg (Ti ϩMg) and those for Ti addition, followed by Mg addition after 30min (Ti/30min/Mg) which will be discussed in Sec. 3.3. are in-cluded in the upper diagram. The N V values for Mg and Ti/1min/Mg deoxidation at 1min are nearly the same, but those for Mg deoxidation decrease faster than those for Ti/1min/Mg deoxidation with holding time. It can be seen that the N V value for Mg deoxidation within 5min of hold-ing time is about half of those for Ti/1min/Mg deoxidation.The N V values for Zr and Ti/1min/Zr deoxidation which are shown in Fig. 2 are plotted against holding time in the lower diagram of Fig. 3. The results for Ti addition, fol-lowed by Zr addition after 30min (Ti/30min/Zr) which will be discussed in Sec. 3.3. are included. The N V values for Zr deoxidation decrease faster than those for Ti/1min/Zr deox-idation and within 5min of holding time are about half of those for Ti/1min/Zr deoxidation.The N V values for particles below 0.5m m in Mg and Zr deoxidations tend to decrease with holding time. This can be explained by the decrease of soluble oxygen after deoxi-dation with an increase of holding time in these experi-ments. It is of interest to note that the N V values for parti-cles below 0.5m m in Ti/1min/Mg and Ti/1min/Zr deoxida-tions decrease slightly during holding time from 1 to 5min.The fraction of the small size particles to total N V values in-creases due to the flotation of large particles with increas-ing holding time.3.2.Variation of Size Distribution and Composition of Particles with Holding Time at 1600°CThe effects of Ti addition and holding time at 1600°C on composition and size distribution of particles are studied in M and Ti/M (M ϭMg and Zr) deoxidation experiments in an Fe–10%Ni alloy. The qualitative composition analysis was made for the particles above 0.05m m on film filter. The par-ticle compositions which are discussed in Sec. 3.4 were de-termined for the particles in the range of d V Ն1.0m m.It should be pointed out that the composition of particles precipitated during rapid quenching corresponds to non-equilibrium phases containing FeO. This is due to the rela-tively high content of supersaturated oxygen in the melt.The content of FeO in particles depends also on amount of deoxidant addition, deoxidation power, order of deoxidant addition and holding time at 1600°C.The size distributions of particles above 0.05m m ob-tained in Ti deoxidation experiments are shown in Fig. 4in which the results for the initial addition of 0.20 or 0.025%Ti are shown. In 0.20% Ti deoxidation at 1, 3 and 10min of holding time, the particles are mostly TiO X –FeO and con-sist of 79–99% TiO X , 1–15%FeO and 0–7% Al 2O 3. In the 0.025% Ti deoxidation at 1 and 5min of holding time,the particles are FeO–TiO X (78–95% FeO and 5–22%TiO X ). High content of FeO in these particles is due to the high content of soluble oxygen after the deoxidation with1510©2008ISIJFig.2.Total particles size distributions in Zr and Ti/Zr deoxida-tion experiments.Fig.3.Variation of number for small (d V Յ0.5m m) and large(d V Ͼ0.5m m) oxide particles with holding time at 1600°C in M (M ϭMg or Zr) and Ti/M deoxidaton exper-iments.small addition of 0.025% Ti.The curves at 1min shown in the upper diagram consist of primary and secondary particles for large and small par-ticles, respectively. Particle size distributions have single peak because the number of primary particles is consider-ably smaller in comparison with that of secondary particles.The curve at 3min for 0.20% Ti shown in the middle dia-gram has a bimodal curve due to the decrease of secondary particle number as a result of decrease in soluble oxygen content at 3min. The curves at 5 and 10min for 0.025 and 0.20% Ti, respectively, shown in the lower diagram corre-spond to secondary particles except for large size particles.Most of large primary particles float out or coagulate with holding time and high dissolved oxygen content determined by the Ti/TiO X equilibrium promotes the formation of sec-ondary particles. It should be pointed out that pure FeO particles are observed in 0.025% Ti deoxidation in some amount, but these particles are not shown in the figure.In Mg deoxidation pure MgO and MgO–FeO particles are observed and size distributions of particles above 0.05m m are shown in Fig. 5. A bimodal curve is obtained at 1min, as shown in the top diagram, and high and low modal values correspond to primary and secondary parti-cles, respectively. The presence of secondary MgO particles suggests that supersaturated oxygen exists even in the pres-ence of dissolved Mg and MgO particles. This high oxygen level exists only in the initial period when Mg is added into the melt containing high initial soluble oxygen such as 100ppm, because after Mg addition soluble oxygen content is not immediately lowered to the value determined by the Mg/MgO equilibrium.The number of secondary particles decreases with in-creasing the holding time at 1600°C since soluble oxygen content approaches the equilibrium value. The number of primary particles also decreases due to the coagulation and flotation of MgO particles, as discussed in the Sec. 3.4. As a result, the curves at 3 and 5min shown in the middle and lower diagrams have single peak. Furthermore, soluble Mgcontent decreases rapidly with holding due to the evapora-tion of Mg from the melt. Consequently, the particle com-position changes from pure MgO to MgO–FeO. As shown in the middle diagram, particle compositions are mostly the MgO–FeO due to small content of soluble Mg. Moreover,the MgO content in these particles decreases from 55–91 to 17–48% with decreasing the particle size from 1–10 to 0.8–2m m, whereas the FeO content increases from 9–45 to 52–83%. The number of pure MgO and MgO–FeO parti-cles at 5min decreases drastically because these particles are easy to be in clusters and float out, as discussed in the Sec. 3.4. The particle composition approaches MgO since a melt tends to be saturated with a MgO crucible.In Ti/1min/Mg deoxidation, MgO and MgO–FeO, MgO–TiO X –FeO and TiO X –FeO particles are obtained and size distributions of these particles are shown in Fig. 6as a function of holding time. When 0.025% Ti is added to the melt containing about 100ppm dissolved oxygen, the pri-mary particles of TiO X –FeO are produced. The reduction of TiO X –FeO particles with Mg occurs after the addition of 0.1% Mg and then MgO–TiO X –FeO particles are produced.Due to insufficient amount of dissolved Mg remained in the melt, all TiO X –FeO particles cannot be reduced to MgO–TiO X and/or MgO. In addition to the aforementioned reduc-tion, added Mg reacts with dissolved oxygen, whereby leading to the formation of MgO and/or MgO–FeO parti-cles. As shown in the top diagram, primary MgO–TiO X –FeO particles are observed at 1min. It was found that the MgO content in these particles decreases from 31–94 to 1–6% with decreasing the particle size from 5–10 to 1–2m m, whereas the FeO content increases from 1–20 to 22–90%. The TiO X content in these particles varies be-tween 6 to 78%. Based on these results it is suggested that the MgO–TiO X –FeO particles below 2m m are formed when the content of soluble Mg decreases significantly due to evaporation and aforementioned reactions. Secondary and some number of primary TiO X –FeO particles are ex-pected to be present from the bimodal curve of TiO X –FeO1511©2008ISIJFig.4.Particle size distributions in Ti deoxidation experimentsas a function of holding time at 1600°C.Fig.5.Particle size distributions in Mg deoxidation experimentsas a function of holding time at 1600°C.particles. It is not certain whether MgO–FeO particles are primary or secondary in the present study. However, these particles seem to be secondary judging from the particle size.Large size primary particles of MgO–TiO X –FeO and TiO X –FeO float out by coagulation with holding time and their modal values move to the left-hand side. The contribu-tion of secondary particles to the size distribution of these particles increases due to the decrease in soluble Mg con-tent through vaporization and to the increase in soluble oxygen content, as shown in the bottom diagram. The num-ber of primary and secondary particles of TiO X –FeO de-creases with increasing the holding time and the size distri-butions at 3 and 5min show single peak curve.The N V values for the particles above 0.05m m in Ti, Mg and Ti/1min/Mg deoxidations which are shown in Figs. 4to 6, respectively, are summarized in Fig. 7in which the fraction of respective particles to total particles is indicated by a bar diagram. In Ti (0.025 and 0.20%) deoxidations,the N V values of TiO X –FeO particles decrease slowly with holding time. This is due to small number of primary parti-cles. Furthermore, the number of secondary particles is high and changes only slightly with holding time due to high soluble oxygen content in Ti deoxidation. The FeO contents in TiO X –FeO particles at 0.025 and 0.20% Ti de-oxidations are 78–95 and 1–15%, respectively.In Mg deoxidation the N V values decrease rapidly with holding time. As mentioned above, the decrease in the N V values is explained by the competition of the flotation de-gree of large particles and that of the crystallization and/or precipitation of secondary particles. The particle composi-tion changes from pure MgO to MgO–FeO during holding time due to the decrease of soluble Mg content.In Ti/1min/Mg deoxidation, the N V values for MgO–FeO particles remain almost constant during holding time. The N V values for TiO X –FeO and MgO–TiO X –FeO particles de-crease continuously with holding time. The reason for the decrease in N V values for these particles may be explained by the aforementioned competition.The size distributions of pure ZrO 2and ZrO 2–FeO parti-cles in Zr deoxidation are shown in Fig. 8. Bimodal distri-butions are observed at 1min for ZrO 2and ZrO 2–FeO parti-cles, thus suggesting that primary and secondary particles are present. When Zr is added to the melt containing about 100ppm of initial oxygen, the non-equilibrium phase of (30–93%) ZrO 2–(7–70%)FeO particles and the equilibrium phase of pure ZrO 2particles are produced. It is considered1512©2008ISIJFig.6.Size distributions of particles in Ti/Mg deoxidation ex-periments as a function of holding time at 1600°C.Fig.7.Variation of number of oxide particles with holding timeat 1600°C in Ti, Mg and Ti/Mg experiments.Fig.8.Size distributions of particles in Zr (0.08%) experimentsas a function of holding time at 1600°C.that dissolved oxygen content is supersaturated and higher than that determined by the Zr/ZrO 2equilibrium even in the presence of dissolved Zr. This leads to the formation of sec-ondary ZrO 2–FeO and ZrO 2particles during cooling and solidification. The number of primary ZrO 2and ZrO 2–FeO particles above 1m m decreases significantly with increasing the holding time due to the coagulation of these particles,as discussed in the Sec. 3.4, and flotation of large size parti-cles and clusters. The content of dissolved oxygen ap-proaches the equilibrium value with holding time and the number of secondary particles also decreases. Therefore,the curves for ZrO 2and ZrO 2–FeO particles at 3 and 5min have single peak.The particle size distributions in Ti/1min/Zr deoxida-tion whose particle compositions are pure ZrO 2, ZrO 2–(1–13%)FeO, ZrO 2–(3–12%)TiO X and (12–85%)ZrO 2–(11–46%)TiO X –(1–36%)FeO are shown in Fig. 9. The curves for these particles at 1min shown in the top diagram indicate single peak. This is different from the results of bi-modal curves obtained in Zr deoxidation at 1min shown in the top diagram of Fig. 8. When Zr is added to the melt containing dissolved oxygen content determined by the Ti/TiO X equilibrium, dissolved oxygen content easily ap-proaches the value determined by the Zr/ZrO 2equilibrium.Thus, the number of precipitated secondary particles is small. This is different from the formation of secondary particles in Zr deoxidation in which non-equilibrium phase affects and controls dissolved oxygen content. It is con-firmed that FeO content decreases and becomes negligibly small with further holding time.The N V values in Zr (0.08 and 0.20%) and Ti/1min/Zr deoxidations whose size distributions of respective particles are shown in Figs. 8 and 9, respectively, are summarized in Fig. 10by a bar diagram. The fraction of respective parti-cles to total particles is indicated as a function of holding time. In Zr (0.08 and 0.20%) deoxidation shown in the upper diagram, the N V values for ZrO 2–FeO and those for ZrO 2decrease with holding time. The fraction of ZrO 2par-ticles to total particles at 0.20% Zr is significantly higher than that at 0.08% Zr and is dependent on the content of dissolved Zr.The N V values for ZrO 2and ZrO 2–FeO particles in Ti/1min/Zr deoxidation tend to increase with holding time,whereas those for ZrO 2–TiO X –FeO particles tend to de-crease with holding time, particularly, from 3 to 5min. No TiO X –FeO particles are observed in Ti/1min/Zr deoxida-tion, but the TiO X –FeO particles are observed in Ti/1min/Mg deoxidation, as shown in Fig. 7. This is due to the fact that dissolved Zr reduces TiO X –FeO particles completely to ZrO 2–TiO X –FeO and ZrO 2–TiO X particles.3.3.Effect of Deoxidation Method on Size Distribution and Composition of ParticlesThree different deoxidation methods have been used to study the effect of deoxidation method on size distribution and composition of particles for Ti and Mg deoxidation. Ti-tanium (0.025%) and Mg (0.1%) were added simultane-ously, followed by quenching after 1min. This method de-notes Ti ϩMg and the results are shown in the top diagram of Fig. 11. Titanium (0.025%) was added and 1min later Mg (0.1%) was added, followed by quenching after 1min.This method denotes Ti/1min/Mg, as already mentioned.The results are shown in the middle diagram. Titanium (0.06%) was added and 30min later Mg (0.1%) was added,followed by quenching after 1min. This method denotes Ti/30min/Mg. The results are shown in the bottom dia-gram.In Ti ϩMg deoxidation method, MgO, MgO–FeO,MgO–TiO X , MgO–TiO X –FeO and TiO X –FeO particles are observed and the size distributions of these particles are shown in the upper diagram of Fig. 11. The compositions of most particles are MgO–TiO X and MgO–TiO X –FeO. Size distributions of these particles show single peak curve, al-though primary and secondary particles are considered to be present. The primary particles above 1m m consist of MgO–(8–51%)TiO X and (46–65%)MgO–(33–52%)TiO X –(1–5%)FeO. Pure MgO and MgO–FeO particles show a bi-modal curve consisting of primary and secondary particles.It was determined that the content of FeO in MgO–FeO par-ticles is also below 5%. The size distribution of TiO X –FeO1513©2008ISIJFig.9.Size distributions of particles in Ti/Zr experiments as afunction of holding time at 1600°C.Fig.10.Variation of number of oxide particles with holding timeat 1600°C in Zr and Ti/Zr experiments.particles shows single peak curve with low modal value and consists of mostly secondary particles. The content of FeO in TiO X –FeO particles changes from 50 to 70%.In Ti/1min/Mg method, most particles are MgO–TiO X –FeO and produced by primary deoxidation, as shown in the middle diagram. In Ti ϩMg deoxidation method, the pri-mary TiO X –FeO particles are not precipitated as is precipi-tated in Ti/1min/Mg deoxidation method. As a result, the number of primary MgO–TiO X and MgO–TiO X –FeO parti-cles in Ti ϩMg deoxidation is considerably smaller com-pared with Ti/1min/Mg experiment. On the other hand, the secondary MgO–TiO X –FeO particles in Ti ϩMg deoxida-tion are produced in a large amount as a result of high solu-ble oxygen content.The size distributions of MgO, MgO–FeO, MgO–TiO X ,MgO–TiO X –FeO and TiO X –FeO particles in Ti/30min/Mg deoxidation method are shown in the bottom diagram of Fig. 11. Most of primary particles in 0.025 and 0.20% Ti deoxidations after 5 or 10min float out and thus most of particles are secondary ones, as shown in the bottom dia-gram of Fig. 4. The number of primary particles in 0.06%Ti deoxidation after 30min is significantly small and solu-ble oxygen content is close to that determined by the Ti/TiO X equilibrium. Under these conditions the following reactions occur after Ti addition, followed by Mg addition after 30min: i) Mg deoxidation, ii) reduction of suspended TiO X particles with Mg, and iii) crystallization and/or pre-cipitation of MgO–TiO X and MgO–TiO X –FeO particles.The curves of particle size distribution for MgO and MgO–FeO and those for MgO–TiO X and MgO–TiO X –FeO show an irregular bimodal curve consisting of one peak and one shoulder. The shoulder part corresponds to the primary particles which are produced by Mg deoxidation and the reduction of suspended TiO X particles with Mg. The com-plex primary particles above 1m m consist of MgO–(2–48%)TiO X and (44–87%)MgO–(11–53%)TiO X –(1–13%)FeO. The peaks correspond to the crystallization and/or precipitation of secondary particles. The TiO X –FeO particle size distribution shows single peak curve with low modal value and consists of only secondary particles.The size distributions of ZrO 2and ZrO 2–FeO, and ZrO 2–TiO X –FeO particles obtained by Ti/1min/Zr and Ti/30min/Zr deoxidation methods are shown in the upper and lower diagrams of Fig. 12, respectively. The results in the top dia-gram for Ti/1min/Zr deoxidation are already shown in the top diagram of Fig. 9 and discussed in the Sec. 3.2. In the Ti/30min/Zr experiment, the following three reactions occur after Ti addition, followed by Zr addition after 30min: i) Zr deoxidation, ii) reduction of suspended TiO X particles with Zr, and iii) crystallization and/or precipitation of ZrO 2–TiO X –FeO particles. The curve of ZrO 2–TiO X and ZrO 2–TiO X –FeO particles corresponds to mostly the sec-ondary particles produced by the crystallization and precip-itation of ZrO 2–TiO X –FeO particles. The reduction of TiO X particles with Zr seems to be small due to a small amount of suspended TiO X particles. The complex particles above 1m m consist of ZrO 2–(2–34%)TiO X and (23–91%)ZrO 2–(5–43%)TiO X –(1–31%)FeO.In Ti/1min/Zr deoxidation method the amount of pri-mary ZrO 2–TiO X –FeO particles is significantly larger than that in Ti/30min/Zr deoxidation method. This is due to the fact that many primary TiO X particles are suspended in the melt and are reduced by Zr very effectively in the former deoxidation method.The N V values for the particles with different composi-tions are plotted against the deoxidation method in the upper and lower diagrams of Fig. 13for Mg and Ti/Mg,and Zr and Ti/Zr deoxidations, respectively. It can be seen that the N V values are nearly the same among different de-oxidation methods since initial oxygen content is almost the same and the degree of the particle flotation is small at 1min holding time except for Ti/30min/M method. How-ever, the method of deoxidation strongly affects the compo-sition of particles. It is seen that the N V values for pure M O y and M O y –FeO particles (M ϭMg and Zr) decrease in the order of M ϾTi/30min/M ϾTi ϩM ϾTi/1min/M . The N V val-ues for M O y –TiO X –FeO particles increase in the order of Ti/30min/M ϽTi ϩM ՅTi/1min/M . These results indicate that a large amount of M O y –TiO X –FeO particles is obtained in Ti/1 min/M deoxidation method compared with other de-oxidation methods since the reduction of TiO X particles1514©2008ISIJFig.11.Size distribution of oxide particles for different deoxida-tion methods in Ti/Mg experiments.Fig.12.Size distribution of oxide particles for different deoxida-tion methods in Ti/Zr experiments.。