Mechanism of Alumina Adhesion to Continuous Caster Nozzle with Reoxidation of Molten Steel

Transcription

1 , pp Mechanism of Alumina Adhesion to Continuous Caster Nozzle with Reoxidation of Molten Steel Katsuhiro SASAI and Yoshimasa MIZUKAMI 1) Steel Research Laboratories, Nippon Steel Corporation, Shintomi, Futtsu, Chiba-ken Japan. 1) Formerly Nagoya R&D Laboratories, Nippon Steel Corporation. Now at The Iron and Steel Institute of Japan, Otemachi, Chiyoda-ku, Tokyo Japan. (Received on April 18, 2001; accepted in final form on July 9, 2001) Basic experiments were conducted on the adhesion of Al 2 O 3 in molten steel when molten FeO is present in the molten steel. It was found that the adhesion and coalescence of Al 2 O 3 particles are promoted when molten FeO is present in the molten steel, that FeO and FeAl 2 O 4 are observed in the bond between the Al 2 O 3 particles, and that this observed result agrees with the pattern of Al 2 O 3 adhesion to the continuous caster nozzle when the molten steel is reoxidized. Based on these experimental results, a study was made of the mechanism whereby Al 2 O 3 adheres to the continuous caster nozzle when the molten steel is reoxidized. When the oxygen concentration in the molten steel is locally raised by the reoxidation of the molten steel, the adhesion force of the Al 2 O 3 particles due to the liquid bridge force of the molten FeO is far greater than the adhesion force of the Al 2 O 3 particles due to the van der Waals force or the surface tension of the molten steel. The adhesion of Al 2 O 3 to the nozzle interface is thus considered to proceed mainly with the molten FeO serving as binder. Since the molten FeO binder changes into a solid bridge of FeAl 2 O 4 in a short time, the Al 2 O 3 particles adhering to the nozzle interface are considered to integrate to form a network of Al 2 O 3 particles faster than possible with solid-phase sintering. KEY WORDS: continuous casting; nozzle; alumina buildup; adhesion force; reoxidation; liquid bridge. 1. Introduction Many studies have been conducted on the mechanisms of alumina adhesion to continuous caster nozzles, and various alumina adhesion mechanisms have been proposed, based on the reaction between the nozzle and molten steel, 1 3) the interfacial energy between alumina and molten steel, 4) and the interfacial tension gradient near the boundary surface of nozzle. 5) These alumina adhesion mechanisms assume the steady-state casting condition where there is no disturbance, and alumina adhesion to the nozzle in such an unsteady-state that the molten steel is reoxidized is rarely discussed. We have often experienced the rapid clogging of continuous caster nozzle during the reoxidation of molten steel in the ladle, tundish or nozzle, or at the beginning of a cast or the joint between heats in sequence casting where the molten steel is more severely reoxidized than in steadystate portions. Causes of this phenomenon appear not to be made clear enough. The authors 6) already investigated the actual state of alumina buildup to the continuous caster nozzle when the molten steel was reoxidized in the tundish. They found that some of the molten FeO formed in the tundish flows unreduced through the continuous caster nozzle into the mold, that Al 2 O 3 particles, mainly a few to 10 mm in size, are deposited on the continuous caster nozzle and FeAl 2 O 4 is formed in the bond between the Al 2 O 3 particles, and that the molten FeO so readily wets the Al 2 O 3 particles that it easily penetrates between the Al 2 O 3 particles. These results suggest the possibility of Al 2 O 3 particles in the molten steel adhering to the nozzle interface with the molten FeO serving as binder. This possibility could not be fully verified, however. In this study, experiments were conducted on the adhesion of Al 2 O 3 in the molten steel when molten FeO was present in the molten steel. Based on the experimental results, a mechanism was proposed for the adhesion of Al 2 O 3 to the continuous caster nozzle during the reoxidation of the molten steel and was evaluated as to its validity in comparison with conventional Al 2 O 3 adhesion models. 2. Experimental Methods Spherical sintered Al 2 O 3 particles with a purity of 99.8% and a size of 0.3 to 0.5 mm, and Fe 2 O 3 reagent were added in specified proportions to 3.0 g of high-purity electrolytic iron, and were thoroughly agitated and mixed. The mixed powder was formed under a pressure of 294 MPa into a cylindrical tablet with a diameter of 10 mm. Figure 1 schematically illustrates the experimental apparatus. An electron beam (EB) melter was used in the experiment. 7) The cylindrical tablet was placed on a water-cooled copper hearth, and the experimental apparatus was evacuated to a high vacuum of 10 4 Pa. The melting current was set at 50 ma, and the electron beam was irradiated on the cylindrical tablet. As soon as the tablet was melted, the ISIJ

2 melting current was reduced to 40 ma, and the melt was held in a button-shaped molten condition for the specified length of time. The melting current was reduced to prevent Al 2 O 3 particles from melting at the top of the button-shaped melt directly exposed to the electron beam. As the Al 2 O 3 particles floated up to the metal surface by the melting of the cylindrical tablet, their behavior was photographed through the peephole in the electron beam melter by video camera. After the experiment, the rapidly quenched metal sample was removed from the electron beam melter, and the Al 2 O 3 particles at the metal surface were observed with a scanning electron microscope (SEM). The average of projected area equivalent diameter of Al 2 O 3 particles was measured from the SEM images with an image analyzer. Some samples were analyzed for chemical composition with an electron-probe microanalyzer (EPMA). 3. Experimental Results The video observation results of the Al 2 O 3 particles during electron beam melting are shown in Figs. 2 and 3. When Fe 2 O 3 was not added (Fig. 2), the way the Al 2 O 3 particles rose to the metal surface as soon as the tablet melted Fig. 1. Schematic of experimental apparatus. was observed. The rise of the Al 2 O 3 particles continued for about 6 s after the start of melting. The Al 2 O 3 particles once floated up to the metal surface remained there and did not appreciably move. On the other hand, when Fe 2 O 3 was added (Fig. 3), the single Al 2 O 3 particles and the small agglomerates of the Al 2 O 3 particles floated up to the surface as soon as the tablet melted and continued to adhere to and coalesce on the metal surface for about 8 s after the start of melting, and formed a large agglomerate of the Al 2 O 3 particles at the center of the metal surface. Figure 4 shows SEM micrographs of rapidly quenched metal surfaces. It is evident that when Fe 2 O 3 is added, the Al 2 O 3 particles adhere and coalesce to form agglomerates, each composed of a few Al 2 O 3 particles. The average size d Al2 O 3 (mm) of the Al 2 O 3 particles was determined from the SEM micrographs. Its change with time is shown in Fig. 5. When Fe 2 O 3 is not added, the average size of the Al 2 O 3 particles changes little at about 0.45 mm, despite increase in the Al 2 O 3 particle concentration from 1 to 3%. This average size is approximately equal to that of the spherical sintered Al 2 O 3 particles used in the experiment. When Fe 2 O 3 is not added, the adhesion and coalescence of the Al 2 O 3 particles progress little. On the other hand, when Fe 2 O 3 is added, the average size of the Al 2 O 3 particles increases for about 4 to 8 s and then remains at a practically constant value. This means that the adhesion and coalescence of the Al 2 O 3 particles proceed in an extremely short time. Increasing the addition of Fe 2 O 3 promotes the adhesion and coalescence of the Al 2 O 3 particles and increases the average size of the Al 2 O 3 particles that reach the metal surface. The observation results of the cross section through the Al 2 O 3 particles that adhered to each other and coalesced are shown in Fig. 6, and the EPMA line analysis results of the bond between the Al 2 O 3 particles are shown in Fig. 7. A bridge composed of FeO (the mole ratio of Fe and O is 1.0 : 1.2 on average according to the EPMA analysis results) and FeAl 2 O 4 (the mole ratio of Fe, Al and O is 1.0 : 1.8 : 4.2 on Fig. 2. Video observation results of Al 2 O 3 particles on the surface of steel melt without the addition of Fe 2 O 3 during electron beam melting. (a) 0 s, (b) 1 s, (c) 2 s, (d) 4 s, (e) 6 s, (f) 8 s ISIJ 1332

3 Fig. 3. Video observation results of Al 2 O 3 paricles on the surface of steel melt with the addition of Fe 2 O 3 during electron beam melting. (a) 0 s, (b) 1 s, (c) 2 s, (d) 4 s, (e) 6 s, (f) 8 s. Fig. 4. SEM micrographs of rapidly quenched metal surfaces. (a) 0mass%Fe 2 O 3 1mass%Al 2 O 3, 10 s, (b) 0.2mass%Fe 2 O 3 1mass%Al 2 O 3, 10 s, (c) 0.3mass%Fe 2 O 3 1mass%Al 2 O 3, 10 s, (d) 0.4mass%Fe 2 O 3 1mass%Al 2 O 3, 10 s. average according to the EPMA analysis results) is formed in the bond between the Al 2 O 3 particles. This agrees with the pattern of the Al 2 O 3 adhesion to the continuous caster nozzle during reoxidation of the molten steel. 6) The added FeO is considered to have served as binder to help the Al 2 O 3 particles to adhere and coalesce and to have then reacted with the Al 2 O 3 particles to form FeAl 2 O Discussion One possible mechanism for the adhesion of Al 2 O 3 particles in the molten steel to the continuous caster nozzle is considered here. The Al 2 O 3 particles are brought to the nozzle interface by the vortexes formed in the turbulent boundary layer, adhere to the nozzle interface to minimize the interfacial energy with the molten steel, and sinter at high temperatures to form a network. 4) The mechanism of the Al 2 O 3 particles in the molten steel adhering to the Al 2 O 3 particles at the nozzle interface is essentially the same as the mechanism of the Al 2 O 3 particles in the molten steel agglomerating and coalescing. Concerning this agglomeration and coalescence, several studies 8,9) point out that when the ISIJ

4 Fig. 5. Change with time in size of Al 2 O 3 particles at metal surface. Fig. 7. EPMA line analysis results of bond between Al 2 O 3 particles. Fig. 8. Model of Al 2 O 3 particle adhesion with molten FeO as binder. Fig. 6. Observation results of cross section through adhered and coalesced Al 2 O 3 particles. Al 2 O 3 particles difficult to wet the molten steel come into contact, a void is formed around each contact point and the Al 2 O 3 particles attract each other due to the interfacial tension (surface tension) between the void and the molten steel, and that this adhesion force is greater than the buoyant force and flow resistance that act on the Al 2 O 3 particles. Although the adhesion and buildup of Al 2 O 3 particles in the molten steel on the continuous caster nozzle may be explained differently in terms of energy or dynamics above mentioned, it may be basically considered that the Al 2 O 3 particles difficult to wet the molten steel adhere to the nozzle interface by the action of interfacial tension and integrate in a short time by solid-phase sintering at high temperatures. As already described, however, the investigation of actual nozzle buildups 6) and Al 2 O 3 particle adhesion model experiments point to the possibility that the Al 2 O 3 particles in the molten steel may adhere to and build up on the nozzle interface with FeO serving as binder. This is different from the conventional Al 2 O 3 adhesion mechanisms. The mechanism whereby the Al 2 O 3 particles in the molten steel adhere to and build up on the nozzle interface, together with the molten FeO formed by reoxidation of the molten steel in the tundish and other parts of the continuous caster, is studied here in two separate processes : one process in which the Al 2 O 3 particles in the molten steel adhere to the nozzle interface and the other process in which the Al 2 O 3 particles integrate at the nozzle interface Process of Al 2 O 3 Particles Adhering at Nozzle Interface Adhesion Force Due to Liquid Bridge Force The adhesion force due to the liquid bridge force formed by the molten FeO between the Al 2 O 3 particles is evaluated on the two isospherical Al 2 O 3 particles shown in Fig. 8. The geometrical conditions yield the following equation: R 2 2 2R 1 R 2 2R 1 r cosq Al2 O 3 FeO 0...(1) where R 1 is the radius (m) of curvature of the liquid bridge; R 2 is the radius (m) of the neck of the liquid bridge; q Al2 O 3 FeO is the contact angle ( ) between Al 2 O 3 and molten FeO; and r is the radius (m) of the Al 2 O 3 particles. The Laplace equation is given by 2001 ISIJ 1334

5 Fig. 9. Model of Al 2 O 3 particle adhesion due to surface tension of molten steel. DP FeO Fe s FeO Fe (R 1 1 R 1 2 )...(2) where DP FeO Fe is the pressure difference (Pa) between the molten FeO and the molten steel; and s FeO Fe is the interfacial tension (N/m) between the molten FeO and the molten steel. Eliminating R 1 in Eq. (1) and rearranging Eq. (1) by using Eq. (2) yield Eq. (3). DP FeO Fe R 2 2 3s FeO Fe R 2 2s FeO Fe r cosq Al2 O 3 FeO 0...(3) When Eq. (3) is solved for R 2, Eq. (4) is obtained. R 2 { 3s FeO Fe (9s 2 FeO Fe 8s FeO Fe DP FeO Fe r cosq Al2 O 3 FeO )0.5 }/(2DP FeO Fe )...(4) The adhesion force F A,L (N) between the two isospherical Al 2 O 3 particles due to the liquid bridge is calculated by the Fisher equation 10) as follows : F A,L p R 22 DP FeO Fe 2p R 2 s FeO Fe...(5) The adhesion force between the two isospherical Al 2 O 3 particles due to the liquid bridge force of the molten FeO can be calculated by substituting the value of R 2 obtained by Eq. (4) into Eq. (5) Adhesion Force Due to Surface Tension When the Al 2 O 3 particles difficult to wet the molten steel contact each other, a void is formed between the Al 2 O 3 particles as shown in Fig. 9. The adhesion force F A,S (N) created between the two isospherical Al 2 O 3 particles is expressed as the sum of the pressure difference DP Fe (Pa) between the void and the molten steel and the surface tension s Fe (N/m) of the molten steel and is given by F A,S p R 42 DP Fe 2p R 4 s Fe...(6) where R 4 is the radius (m) of the neck of the void. Equation (7) is derived from the geometrical conditions of Fig. 9. R 2 4 2R 3 R 4 2R 3 r cosq Al2 O 3 Fe 0...(7) where R 3 is the radius (m) of curvature of the void; and q Al2 O 3 Fe is the contact angle ( ) between Al 2 O 3 and the molten steel. Equation (8) holds true according to the Laplace relation. DP Fe s Fe (R 1 3 R 1 4 )...(8) Eliminating R 3 from Eqs. (7) and (8) and rearranging them yields Eq. (9) concerning R 4. DP Fe R 4 2 3s Fe R 4 2s Fe r cosq Al2 O 3 Fe 0...(9) R 4 is calculated by Eq. (9) as follows: R 4 { 3s Fe (9s 2 Fe 8s Fe DP Fe r cosq Al2 O 3 Fe )0.5 }/(2DP Fe )...(10) The adhesion force between the two isospherical Al 2 O 3 particles due to the surface tension of the molten steel can be thus calculated by solving Eq. (10) for R 4 and substituting this value of R 4 into Eq. (6) Adhesion Force Due to van der Waals Force The interaction that operates when two macroscopic objects, such as a particle and a particle or a particle and a wall, approach each other consists of the repulsion force that results from the overlapping of the diffusive electrical double layers and dispersion force (van der Waals force). Since there is no need to consider the diffusive electrical double layer for nonmetallic inclusions in the molten steel, the adhesion force F A,V (N) due to the van der Waals force only acts on the two isospherical Al 2 O 3 particles as approximated by 11) FA,V H r/(12a2 )...(11) where H is the Hamaker constant (J); and a is the surface distance between the Al 2 O 3 particles and meets the condition a r. Taniguchi et al. 12) report that the Hamaker constant between the Al 2 O 3 particles through the medium of molten steel is J. As can be seen from Eq. (11), the van der Waals force greatly changes with the surface distance between two particles. For the particles to stably exist in the position where the potential energy of the particles is minimum, the surface distance between the particles must be usually mm or less. It is thus possible to estimate the adhesion force due to the van der Waals force acting between the two isospherical Al 2 O 3 particles in the molten steel by using Eq. (11) Estimation of Interfacial Tension between Molten FeO and Molten Steel Assuming that the molten steel and the molten FeO are in a completely wetted condition, the interfacial tension between the molten FeO and the molten steel can be estimated by the following Antonov equation 13) : s FeO Fe s Fe s FeO...(12) At C, the surface tension s FeO of the molten FeO is 0.57 N/m, 13) and the surface tension s Fe of the molten steel changes with the oxygen concentration in the molten steel and is given by 13) s Fe ln(1 111 [O])...(13) When the oxygen concentration in the molten steel is in equilibrium with the molten FeO ([O] 0.21 mass%, 14) s Fe 0.89 N/m), s FeO Fe is 0.32 N/m. This value practically agrees with the interfacial tension of 0.3 N/m measured by Ogino et al. 14) between the molten FeO and the molten steel. When the oxygen concentration in the molten steel is in equilibrium with FeAl 2 O 4 ([O] mass%, 15) s Fe 1.25 N/m), s FeO Fe is 0.68 N/m and is approximately equal to the interfacial tension of 0.7 N/m of an FeO CaO Al 2 O 3 slag when the oxygen concentration in the molten steel is the same. 14) Since the interfacial tension between the molten steel and slag essentially depends on the oxygen ISIJ

6 concentration in the molten steel and the difference in interfacial tension between different slag compositions is very small, 14) the calculated value of s FeO Fe is judged to be practically valid when the oxygen concentration in the molten steel is in equilibrium with FeAl 2 O 4. The interfacial tension between the molten FeO and the molten steel as estimated by Eqs. (12) and (13) is thus used below Study of Adhesion Force between Al 2 O 3 Particles The relationship between the interfacial tension between the molten FeO and the molten steel and the adhesion force between the Al 2 O 3 particles due to the liquid bridge force of the molten FeO is shown in Fig. 10. The contact angle between the molten FeO and the Al 2 O 3 particles was set at 0 from the measurement value of the previous report. 6) The molten steel level in the tundish was set at 1 m, and the pressure difference between the molten FeO and the molten steel was estimated at Pa. When the continuous caster nozzle deposits were investigated in the previous report, 6) FeAl 2 O 4 was observed in the deposited metal. This finding suggests that the oxygen concentration in the molten steel flowing through the continuous caster nozzle locally or temporarily rose to the oxygen concentration of mass% in equilibrium with FeAl 2 O 4 due to the reoxidation of the molten steel. In this case, the interfacial tension between the molten FeO and the molten steel is 0.68 N/m, and adhesion forces of and N act between two 1 mm radius isospherical Al 2 O 3 particles and between two 10 mm radius isospherical Al 2 O 3 particles, respectively, due to the liquid bridge force of the molten FeO according to Eq. (5). Figure 11 shows the relationship between the surface distance between the Al 2 O 3 particles and the adhesion force between the Al 2 O 3 particles due to the van der Waals force. When the surface distance between the Al 2 O 3 particles is mm, 16) adhesion forces of and N due to the van der Waals force act between the 1 mm radius isospherical Al 2 O 3 particles and between the 10 mm radius isospherical Al 2 O 3 particles, respectively, according to Eq. (11). These adhesion forces due to the van der Waals force are about to times as large as the adhesion force due to the liquid bridge force of the molten FeO, and are very small as adhesion forces acting between the Al 2 O 3 particles. Figure 12 shows the relationship between the contact angle between the molten steel and the Al 2 O 3 particles and the adhesion force between the Al 2 O 3 particles due to the surface tension of the molten steel. The surface tension of the molten steel was set at 1.79 N/m, and the pressure difference between the void and the molten steel was estimated at Pa by setting the molten steel level in the tundish at 1 m. When the oxygen concentration in the molten steel is reduced by aluminum deoxidation, the contact angle between the Al 2 O 3 particles and the molten steel is ) and adhesion forces of N and N due to the surface tension of the molten steel act between the 1 mm radius isospherical Al 2 O 3 particles and be- Fig. 11. Relationship between surface distance between Al 2 O 3 particles and adhesion force of Al 2 O 3 particles due to van der Waals force. Fig. 10. Relationship between interfacial tension between molten FeO and molten steel and adhesion force of Al 2 O 3 particles due to liquid bridge force of molten FeO. Fig. 12. Relationship between contact angle between molten steel and Al 2 O 3 particles and adhesion force of Al 2 O 3 particles due to surface tension of molten steel ISIJ 1336

7 tween the 10 mm radius isospherical Al 2 O 3 particles, respectively. These adhesion forces are about 1.73 to 1.83 times as large as the adhesion force due to the liquid bridge force of the molten FeO, and are considered to be mainly responsible for the adhesion of Al 2 O 3 when the oxygen concentration in the molten steel is reduced by aluminum deoxidation. When the oxygen concentration in the molten steel rises locally or temporarily to the oxygen concentration in equilibrium with FeAl 2 O 4, the contact angle between the molten steel and Al 2 O 3 becomes smaller than 90 17) and the adhesion force due to the surface tension of the molten steel acts little. These results suggest that when the oxygen concentration in the molten steel is raised locally or temporarily by reoxidation of the molten steel, the adhesion force of the Al 2 O 3 particles due to the liquid bridge force of the molten FeO is much greater than the adhesion force of the Al 2 O 3 particles due to the van der Waals force or the surface tension of the molten steel, and that the adhesion of Al 2 O 3 to the nozzle interface proceeds with the liquid bridge force of the molten FeO Process of Al 2 O 3 Particle Integrating at Nozzle Interface Sintering Rate between Al 2 O 3 Particles The sintering rate of two isospherical Al 2 O 3 particles by the volume diffusion mechanism is given by 18) x 5 /r 2 a s Al2 O 3 d 3 D V t/(k T)...(14) where x is the radius (m) of the Al 2 O 3 particle neck; a is a constant that varies with the geometrical shape of the Al 2 O 3 particle neck; s Al2 O 3 is the surface tension of Al 2 O 3 and is 0.75 N/m; 17) d 3 is the void volume in the Al 2 O 3 particle and is m 3 ; 19) D V is the volume self-diffusion coefficient and is m 2 /s; 19) t is the time (s); k is the Boltzmann constant; and T is the absolute temperature (K). Ooi et al. 20) investigated the formation behavior of Al 2 O 3 clusters, and found that the sintering of Al 2 O 3 particles mainly proceeds by such a volume diffusion mechanism that the neck surface is a vacancy source and the grain boundary is a vacancy sink and that the sintering rate is expressed by Eq. (14) when the value of a is 80. The change with time in the neck radius between the two isospherical Al 2 O 3 particles with solid-phase sintering can be thus estimated by using Eq. (14) Rate of Solid Bridge Formation between Al 2 O 3 Particles When Al 2 O 3 particles adhere to each other due to the liquid bridge force of the molten FeO, FeO reacts with Al 2 O 3 particles to form FeAl 2 O 4. Given the fact that the melting point of FeAl 2 O 4 is C, the liquid bridge of FeO changes to a solid bridge of FeAl 2 O 4 to integrate the Al 2 O 3 particles when the molten steel temperature is about C. In the solid bridge formation model shown in Fig. 13, the neck radius x (m) of the solid bridge is given by Eq. (15) according to the geometrical relationship if the thickness of the FeAl 2 O 4 layer is denoted by x (m). x (x 2 2x r) (15) The formation rate of FeAl 2 O 4 by the additive reaction of Fig. 13. Model of formation of solid bridge. Al 2 O 3 and FeO is estimated to be controlled by mass transfer in the FeAl 2 O 4 layer, and the parabolic law of Eq. (16) holds true for the thickness of the FeAl 2 O 4 layer in a plane reaction. x 2 k D t...(16) where k D is the rate constant (m 2 /s). Minowa et al. 21) measured the amount of FeAl 2 O 4 formed in tablets composed of an FeO Al 2 O 3 mixed powder in the temperature region of 950 to C and obtained the rate constant for the formation of FeAl 2 O 4 by assuming that the value follows the Jander equation. When this rate constant is converted into that in the parabolic rate equation or Eq. (16) and extrapolated to C, the value of k D obtained is m 2 /s. The change with time in the neck radius of the solid bridge can be thus calculated by obtaining the thickness of FeAl 2 O 4 layer from Eq. (16) and substituting the obtained value into Eq. (15) Force Imposed on Al 2 O 3 Particles by Molten Steel The buoyant force F B (N) acting on the Al 2 O 3 particle can be evaluated by Eq. (17). F B 4p r 3 (r Fe r Al2 O 3 ) g/3...(17) where r Fe is the density of the molten steel and is kg/m 3 ; r Al2 O 3 is the density of the Al 2 O 3 particle and is kg/m 3 ; and g is the gravitational acceleration (m/s 2 ). The drag force F D (N) to which the Al 2 O 3 particles are subjected by their relative motion with the molten steel is given by F D C D r Fe v 2 S/2...(18) where C D is the drag coefficient; v is the flow velocity of the molten steel (m/s); and S is the projected area (m 2 ) of the Al 2 O 3 particle in the flow direction of the molten steel and is p r 2 for the spherical Al 2 O 3 particles. If the diameter of the Al 2 O 3 particles is a few tens of micrometers, the Reynolds number Re P ( 2r v/n) of the Al 2 O 3 particles calculated from the flow velocity of the molten steel through the continuous caster nozzle is less than The drag coefficient can be estimated by using Eq. (19) because the values calculated by Eq. (19) agree well with the experimental values in this Re P range. 16) C D 24( Re 2/3 P )/Re P...(19) The kinematic viscosity n of the molten steel was put at ISIJ

8 Fig. 14. Change with time in radius of neck formed between Al 2 O 3 particles by solid bridge and solid-phase sintering. Fig. 15. Change with time in shear stress acting on neck between Al 2 O 3 particles m 2 /s. The shear stress t (N/m 2 ) acting on the neck between the Al 2 O 3 particles is given by Eq. (20), using the drag force. t F D /(p x 2 )...(20) Study of Integration Rate of Al 2 O 3 Particles The radius of the neck formed between two isospherical Al 2 O 3 particles by solid bridging and solid-phase sintering was calculated. Its change with time is shown in Fig. 14. The radius of the neck between the Al 2 O 3 particles increases with elapse of time in both mechanisms of solid bridging and solid-phase sintering. The radius of the neck formed by solid bridging is approximately 24 to 45 times larger than the radius of the neck formed by solid-phase sintering. This means that the growth rate of the neck formed between the Al 2 O 3 particles by solid bridging is very high. When the buoyant force acting on Al 2 O 3 particles ranging from 1 to 10 mm in radius is calculated by Eq. (17), it is to N. If the flow velocity of the molten steel through the continuous caster nozzle is set at 2 m/s, the drag force acting on the Al 2 O 3 particles in the radius range of 1 to 10 mm is calculated by Eq. (18) to be to N, and is far larger than the buoyant force. This suggests that the force required to break the bond of Al 2 O 3 particles adhering to the nozzle interface is mainly the drag force due to the flow of the molten steel. When the flow velocity of the molten steel through the continuous caster nozzle is 2 m/s, the change with time in the shear stress acting on the neck between the Al 2 O 3 particles due to the drag force is calculated and shown in Fig. 15. In both mechanisms of solid bridging and solid-phase sintering, the shear stress acting on the neck between the Al 2 O 3 particles due to the drag force decreases with elapse of time and increases as the Al 2 O 3 particles increase in size. If the shear strength of the neck formed between the Al 2 O 3 particles by solid bridging or solid-phase sintering can be approximated by the shear strength 22) of sintered Al 2 O 3, then it is Pa. If the Al 2 O 3 particles range from 1 to 10 mm in size, the time during which the shear stress acting on the neck falls below the shear strength is about 0.02 to 0.37 s for solid-phase sintering and is very short at about to s for solid bridging. This indicates that since the growth rate of the neck between the Al 2 O 3 particles is much higher for solid bridging than for solidphase sintering, the Al 2 O 3 particles adhering to the nozzle interface with the molten FeO as binder integrate in a short time and form a network of Al 2 O 3 particles without being broken by the flow of the molten steel through the continuous caster nozzle. When the molten steel is deoxidized by aluminum, the oxygen concentration in the molten steel, locally raised by reoxidation, gradually declines with the elapse of time, and the molten steel becomes more difficult to wet the Al 2 O 3 particles and tends to be ejected from between the Al 2 O 3 particles. As a result, the solid bridge of FeAl 2 O 4 no longer comes into contact with the molten steel and becomes more difficult to reduce, probably allowing the Al 2 O 3 particles to maintain their network. 5. Conclusions Basic experiments were conducted on the adhesion of Al 2 O 3 in the molten steel when molten FeO was present in molten steel. Based on the experimental results, a study was made of the mechanism of the Al 2 O 3 adhesion to the continuous caster nozzle when the molten steel was reoxidized. The following conclusions were drawn : (1) In the Al 2 O 3 adhesion experiments, the adhesion and coalescence of the Al 2 O 3 particles themselves were promoted when molten FeO was present in the molten steel. In the bond between the Al 2 O 3 particles, FeO and FeAl 2 O 4 were observed. This agreed with the pattern of Al 2 O 3 adhesion to the continuous caster nozzle observed when the molten steel was reoxidized. (2) When the oxygen concentration in the molten steel is raised locally or temporarily by the reoxidation of the molten steel, the adhesion force of the Al 2 O 3 particles due to the liquid bridge force of the molten FeO is much greater than the adhesion force of the Al 2 O 3 particles due to the van der Waals force or the surface tension of molten steel. This 2001 ISIJ 1338

9 suggests that the adhesion of Al 2 O 3 to the nozzle interface mainly proceeds with the molten FeO working as binder. (3) Since the growth rate of the neck formed between the Al 2 O 3 particles is very fast with solid bridging compared with solid-phase sintering, the Al 2 O 3 particles adhering to the nozzle interface with FeO as the binder integrate in a short time and form a network of the Al 2 O 3 particles without being broken by the flow of the molten steel through the continuous caster nozzle. REFERENCES 1) T. Kaneko, T. Ohno and S. Mizoguchi: Tetsu-to-Hagané, 66 (1980), S868. 2) K. Sasai and Y. Mizukami: ISIJ Int., 34 (1994), ) K. Sasai and Y. Mizukami: ISIJ Int., 35 (1995), 26. 4) S. N. Singh: Metall. Trans., 5 (1974), ) K. Mukai, R. Tsujino, I. Sawada, M. Zeze and S. Mizoguchi: Tetsuto-Hagané, 85 (1999), ) K.Sasai and Y. Mizukami: Refractories (Taikabutsu), 53 (2001), 56. 7) Y. Nuri and K. Umezawa: Tetsu-to-Hagané, 75 (1989), ) K. Knüppel, K. Brotzmann and N. W. Förster: Stahl Eisen, 85 (1965), ) V. I. Baptizmanskii, N. Bakhman and Yu. V. Dmitriev: Izv. V.U.Z., Chernaya Metall., 3 (1969), ) R. A. Fisher: J. Agric. Sci., 16 (1926), ) S. Taniguchi and A. Kikuchi: Tetsu-to-Hagané, 78 (1992), ) S. Taniguchi, A. Kikuchi, T. Ise, T. Wada and N. Takase: Recent Trends in Ultra-clean Steel Research, Ultra-clean Steel Research Committee, Division of High-Temperature Processes, ISIJ, Tokyo, (1999), ) A. W. Cramb and I. Jimbo: Proc. W. O. Philbrook Memorial Symp., ISS-AIME, Warrendale, PA, (1988), ) K. Ogino, S. Hara, T. Miwa and S. Kimoto: Trans. Iron Steel Inst. Jpn., 24 (1984), ) A. McLean and R. G. Ward: J. Iron Steel Inst., 204 (1966), 8. 16) Editorial Committee on Fundamentals of Powder Engineering: Fundamentals of Powder Engineering, Nikkan Kogyo Shimbun, Tokyo, (1992), 119, ) K. Nogi and K. Ogino: Can. Metall. Q., 22 (1983), ) W. D. Kingery and M. Berg: J. Appl. Phys., 26 (1955), ) T. B. Braun, J. F. Elliott and M. C. Flemings: Metall. Trans. B, 10B (1979), ) H. Ooi, M. Sekine and G. Kasai: Tetsu-to-Hagané, 59 (1973), ) S. Minowa, M. Yamada and M. Kato: Tetsu-to-Hagané, 51 (1965), ) Japan Institute of Metals:Metal Data Book, Third Ed., Maruzen, Tokyo, (1993), ISIJ