Influence of Steel Grade on Oxidation Rate of Molten Steel in Tundish

Transcription

1 , pp Influence of Steel Grade on Oxidation Rate of Molten Steel in Tundish Katsuhiro SASAI and Akihiro MATSUZAWA Oita R & D Laboratories, Nippon Steel Corporation, 1 Oaza-Nishinosu, Oita, Oita-ken, Japan. sasai.katsuhiro@nsc.co.jp (Received on October 6, 2011; accepted on December 22, 2011) The influence of steel grade on the oxidation rate of molten steel in tundish was studied by conducting oxidation experiments on the Ti and Ti Al deoxidized molten steel and comparing the obtained oxidation rates with that of the Al deoxidized molten steel as measured in a previous report. In the still state, the oxidation rate of the Ti deoxidized molten steel is faster than those of the Ti Al deoxidized molten steel and the Al deoxidized molten steel, showing dependence on the steel grade. This means that in the still state, while the oxidation rates of the Ti Al and Al deoxidized molten steel are controlled by the mass transfer of oxygen in the oxide film, the oxidation rate of the Ti deoxidized molten steel is controlled by the mass transfer of O 2 gas in the gas phase because the surface is not covered with oxide films. In addition, in the stirred state, the oxidation rates of the Ti and Ti Al deoxidized molten steel become faster than that of the Al deoxidized molten steel in the region where the O 2 gas partial pressure exceeds 10 kpa. This dependence on the steel grade can be explained by the mechanism of accelerating the mass transfer in the gas phase due to active iron evaporation in the Ti and Ti Al deoxidized molten steel, in which the surface disturbance is larger than that in the Al deoxidized molten steel. KEY WORDS: continuous casting; tundish; reoxidation; oxidation rate; steel grade dependence; Ti deoxidized molten steel; Ti Al deoxidized molten steel. 1. Introduction The cleanliness of molten steel in the ladle has been improved dramatically with the technological advances of secondary refining processes. However, the contamination of molten steel in the tundish has surfaced, and the prevention of this contamination has become an extremely important issue when producing super-clean steel. The entrapment of ladle slag, reaction with tundish cover powder, and the wear of refractories, etc., are cited as important factors responsible for the contamination of molten steel in the tundish. Further, the secondary oxidation of molten steel by air has a particularly significant effect and is considered the principal cause of the contamination of molten steel in the tundish. In an effort to clarify the oxidation behavior of molten steel by air, the authors have determined the oxidation rate of: the Al deoxidized molten steel both in the still state and the stirred state, 1) the Al deoxidized molten steel covered with tundish cover powder, the Al deoxidized molten steel that has been poured, 2) and the Al deoxidized molten steel in an incompletely sealed tundish. 3) In addition, the authors have presented reoxidation models under these conditions by applying kinetic analyses, while also determining the quantities of the reoxidation of molten steel in the tundish caused by various factors using such models. 4) However, the oxidation behavior of molten steel as caused by air may vary depending on the steel grade, other than the stirring state of molten steel and the mass transfer conditions in the gas phase mentioned in previous reports, 1 3) thus the oxidation behavior cannot be elucidated sufficiently by studies concerning only Al deoxidized molten steel. Since the authors experienced that the cleanliness of ultra low-carbon molten steel was lower than that of low-carbon Al deoxidized molten steel in tundish through the commercial operation, it is useful for engineering to elucidate the influence of steel grades on the oxidation rate of molten steel from such a point of view. Furthermore, only a few studies 5,6) have been reported in which the oxidation rate of molten steel is measured under relatively low oxygen partial pressure, such as in air, by changing the type of deoxidizing element used; therefore, the mechanism and conditions under which the oxidation rate of molten steel exhibits steel grade dependence have not yet been made thoroughly clear. Thus in this study, attention has been paid to Ti as an important deoxidizing element of ultra low-carbon steel, and the oxidation rate of the Ti deoxidized molten steel and that of the Ti Al deoxidized molten steel were measured both in the still state (the portion excluding a tundish inlet where molten steel is stirred relatively weekly.) and the stirred state (the portion at a tundish inlet where molten steel is strongly stirred.). These oxidation rates of molten steel have been compared with that of the Al deoxidized molten steel that were obtained from studies previously reported, 1,2) and the steel grade dependence of the oxidation rate of molten steel has been examined by conducting kinetic analyses ISIJ

2 2. Experimental Methods 2.1. Oxidation Experiments of Molten Steel in Still State An atmosphere-controllable Tammann furnace was used for the oxidation experiments of molten steel in a still state. Electrolytic iron in the amount of 500 g was melted in an alumina crucible with an inside diameter of 40 mm and a height of 150 mm in an Ar atmosphere. After having maintained the temperature of molten steel at C, Ti was added to a target concentration of 0.15 mass% for the Ti deoxidized molten steel (Fe Ti melt), and Ti and Al were added to the target concentrations of 0.15 mass% and 0.1 mass%, respectively, for the Ti Al deoxidized molten steel (Fe Ti Al melt). In some experiments, S was added in an amount of 0.1 mass% to both the Ti and Ti Al deoxidized molten steel. An Ar O 2 gas mixture was also introduced through the bottom of the Tammann furnace at a flow rate of Ncm 3 min 1 for 2 min to adjust the gas phase to the specified partial pressure of O 2 gas. Then, the flow rate of the Ar O 2 gas mixture was reduced to Ncm 3 min 1, and the oxidation experiment of molten steel was started. Samples of molten steel were taken at specified time intervals using transparent quartz tubes with an inside diameter of 6 mm and were analyzed regarding the Ti and Al concentrations. As experimental conditions, the partial pressure of O 2 gas in the gas phase was changed over the range of 5 to 20 kpa Oxidation Experiments of Molten Steel in Stirred State For the oxidation experiments of molten steel in a stirred state, a high-frequency vacuum induction melting furnace (50 kw, 2.7 khz) was used. Electrolytic iron in the amount of 80 kg was melted in a magnesia crucible with an inside diameter of 220 mm and a height of 400 mm in an Ar atmosphere under a total pressure of 40 kpa. The temperature and composition of molten steel were the same as those of the oxidation experiments conducted using the Tammann furnace. Ar gas and O 2 gas were then introduced into the vacuum induction melting furnace. The partial pressure of O 2 gas in the gas phase was adjusted to the desired value, 4 to 23 kpa, at a total pressure of 101 kpa, and the oxidation experiment of molten steel was started. During the experiment, molten steel samples were taken at specified time intervals. These samples were analyzed regarding the Ti and Al concentrations Rapid Oxide Quenching Experiments Because the collection of the oxide films was difficult in the oxidation experiments mentioned above, the rapid oxide quenching experiments were carried out in order to examine the oxide films that formed on the surface of molten steel during oxidation. 25 g of electrolytic iron was melted in an alumina crucible with an inside diameter of 11 mm and a height of 100 mm in an Ar atmosphere using the Tammann furnace. To facilitate the observation of oxide films, 0.2 mass% of Ti was added to the Ti deoxidized molten steel, and 0.2 mass% of Ti and Al, respectively, was added to the Ti Al deoxidized molten steel. Using a similar method to the oxidation experiment with the Tammann furnace, the partial pressure of O 2 gas in the gas phase was adjusted to 20 kpa, and the oxidation experiment of molten steel was started. The experiments were conducted for 5 min, and then the power supply was turned off and the crucible containing the molten steel was rapidly cooled. The sample of molten steel was vertically divided into two, and the oxide films that formed on the surface of molten steel were observed using a scanning electron microscopy (SEM). The characteristic portions of the oxide films were analyzed quantitatively by an electron probe microanalyzer (EPMA). 3. Experimental Results 3.1. Observation of Molten Steel Surface during Oxidation Experiments During the oxidation experiments, the surface of molten steel was observed visually. In the oxidation experiments of the Ti deoxidized molten steel, both in the still state and the stirred state, a large number of the oxides that formed on the surface of molten steel moved toward the walls of the crucible while changing shape, so that the surface of molten steel was not covered with oxide films. Especially in the stirred state, it was observed that as the partial pressure of O 2 gas increased, the oxides moved very fast, and a large amount of iron vapor was generated so that the surface of molten steel appeared dim in reddish-brown. On the other hand, in the oxidation experiments of the Ti Al deoxidized molten steel, coverage by oxide films on the molten steel surface varied depending on the degree of stirring. In the still state, the surface of molten steel as a whole was covered with oxide films, whereas in the stirred state, the surface was not covered with formed oxides because the surface of molten steel was in an actively flowing state. Furthermore, when the partial pressure of O 2 gas was high, it was observed that a large volume of iron vapor was generated from the surface of molten steel. Judging from the visual observation, the surface of the Ti Al deoxidized molten steel and that of the Ti deoxidized molten steel in the stirred state were almost in the same condition Examination of Oxides on Surface of Molten Steel Formed during Oxidation SEM micrographs of the surface oxide films on the molten steel obtained in the rapid quenching experiments are shown in Fig. 1, while the EPMA quantitative analysis results of the surface oxide films are shown in Fig. 2. The chemical compositions of the surface oxide films, the liquid fractions f L of the surface oxide films at C as calculated by FactSage software, 7) along with the generally estimated phases of the oxide films are shown in Table 1. For the Ti deoxidized molten steel, a relatively dense oxide film (a) including a few pores with a thickness of 10 to 20 μm partially formed on the surface of molten steel and had the compositions of: 86 mass%tio 2-14 mass%feo (from the results of EPMA analysis (Ti) = 52 mass%, (Fe) = 11 mass%, (O) = 37 mass%) and 50 mass%tio 2-50 mass%feo (from the results of EPMA analysis (Ti) = 30 mass%, (Fe) = 39 mass%, (O) = 31 mass%). The former liquid fraction was 61% (approximately a liquidus temperature of C), while the latter was 100% (approximately a liquidus temperature of C) at the experimental temperature of 2012 ISIJ 832

3 Fig. 1. SEM micrographs of surface oxide films on the molten steel obtained in rapid quenching experiments. (a) Comparatively dense oxide film of the Ti deoxidized molten steel, (b) Coarse oxide film of the Ti Al deoxidized molten steel, (c) Dense oxide film of the Ti Al deoxidized molten steel. Fig. 2. EPMA quantitative analysis results of surface oxide films on the molten steel. (a) Comparatively dense oxide film of the Ti deoxidized molten steel, (b) Coarse oxide film of the Ti Al deoxidized molten steel, (c) Dense oxide film of the Ti Al deoxidized molten steel ISIJ

4 1 600 C. In addition, taking into account the fact that a few pores regarded as an effect of the solid phase were observed, it is considered that the liquid phase-based oxide film including a little solid phase formed on the surface of the Ti deoxidized molten steel. For the Ti Al deoxidized molten steel, the mixture of a coarse oxide film (b) with many pores and a very dense oxide film (c) covered the entire surface of the molten steel, with film thickness ranging from 40 to 80 μm. The oxide film (b) was composed of 55 mass%tio 2-39 mass%al 2O 3-6 mass%feo (from the results of EPMA analysis (Ti) = 33 mass%, (Al) = 21 mass%, (Fe) = 5 mass% and (O) = 41 mass%) and 60 mass%tio 2-19 mass%al 2O 3-21 mass%feo (from the results of EPMA analysis (Ti) = 36 mass%, (Al) = 10 mass%, (Fe) = 16 mass% and (O) = 38 mass%). The former liquid fraction was 41% (approximately a liquidus temperature of C), while the latter was 96% (approximately a liquidus temperature of C) at C. Detailed observation of the oxide film (b) shows that it consists of a solid portion of granular oxides (mainly at the gasoxide interface) and a liquid portion of relatively dense oxides with few pores (mainly at the oxide-metal interface). Therefore, judging from the composition and the morphology of the oxide film, it is presumed that under the experimental temperature of C, the oxide film (b) was in a combined state of solid and liquid phases. On the other hand, a very dense oxide (c) was composed of 30 mass%tio 2-11 mass%al 2O 3-59 mass%feo (from the results of EPMA analysis (Ti) = 18 mass%, (Al) = 6 mass%, (Fe) = 45 mass% and (O) = 31 mass%) and partly composed of 54 mass%tio 2-8 mass%al 2O 3-38 mass%feo (from the results of EPMA analysis (Ti) = 33 mass%, (Al) = 4 mass%, (Fe) = 30 mass% and (O) = 33 mass%). These liquid fractions were 100% at C both (the former liquidus temperature was about C, while the latter was about C). Furthermore, taking into consideration that the oxide film was very dense, it is thought that the oxide film (c) was in the liquid phase under the experimental temperature. The results mentioned above show that, in the Ti Al deoxidized molten steel, the oxide film (b) in a combined state of solid and liquid phases in which the compositions are TiO 2-high Al 2O 3 ( 19 mass%)-low FeO (<38 mass%) and the oxide film (c) in the liquid phase in which the compositions are TiO 2-low Al 2O 3 (<19 mass%)-high FeO ( 38 mass%), coexist on the surface of molten steel Oxidation Rates of Ti and Ti Al Deoxidized Molten Steel In the experiments, the oxidation rate of the Ti deoxidized molten steel is obtained as a change in the Ti concentration over time, while the oxidation rate of the Ti Al deoxidized molten steel is obtained as a change in both the Ti and Al concentrations over time; thus, these oxidation rates cannot be compared directly. Therefore, in this paper, the oxidation rates of the Ti and Ti Al deoxidized molten steel are examined using the amount of O absorbed [O] Abs, which is obtained by regarding the amount of O which reacts with Al and Ti in the molten steel to form the oxides as the total amount of O absorbed, as an index. Here, taking into consideration that the reaction of Al and Ti with O absorbed from the gas phase proceeds fast and the increase in the dissolved O concentration is negligible small because Al and Ti are relatively strong deoxidizing elements, it is judged that the above supposition would be appropriate. Fig. 3. Change in the amounts [O] Abs of O absorbed over time during the oxidation of the Ti deoxidized molten steel in the still state. Table 1. Chemical compositions of the surface oxide films, the liquid fractions f L of the surface oxide films at C as calculated by FactSage software, 7) along with the generally estimated phases of the oxide films. Steel type Oxide film compositions/mass% TiO 2 Al 2O 3 FeO f L/% Estimated phases Fe Ti melt (a) Liquid-based phase, including a little solid Fe Ti Al melt (b) (c) Combined state of solid and liquid phases Liquid phase Fig. 4. Change in the amounts [O] Abs of O absorbed over time during the oxidation of the Ti deoxidized molten steel in the stirred state ISIJ 834

5 Al in the molten steel reacts with the absorbed O to form Al 2O 3. However, the valence of Ti changes in the range between TiO and TiO 2, and thus the amount of O absorbed cannot be calculated precisely from the change in the Ti concentration. Therefore, quantitative analyses were made using EPMA on the oxides formed in the molten steel during the oxidation of the Ti and Ti Al deoxidized molten steel, and, from the analytical results, the molar ratio of the Ti and O contained in the oxides was calculated. For both the Ti and Ti Al deoxidized molten steel, the average composition of the Ti oxide was expressed in TiO The changes in the amounts of O absorbed over time during the oxidation of the Ti and Ti Al deoxidized molten steel according to the stirring conditions are shown in Figs. 3 to 6. Here, W Fe is the weight of molten steel, and P O2 is the O 2 gas partial pressure in the gas phase. O absorption of the Ti and Ti Al deoxidized molten steel was calculated from the Al and Ti concentration decreases, according to the Fig. 5. Change in the amounts [O] Abs of O absorbed over time during the oxidation of the Ti Al deoxidized molten steel in the still state. stoichiometric relation, by assuming that, as described previously, all O absorbed into molten steel reacts with Ti and Al to form TiO and Al 2O 3, respectively. Under any conditions, O absorption almost increases linearly over time, and the rate of increase in O absorption becomes faster as the O 2 gas partial pressure in the gas phase increases. In addition, when O absorption of the Ti and Ti Al deoxidized molten steel in the stirred state is 0.08 mass% or more, the amount of O absorbed deviates slightly from the line increasing in proportion to time (in a dotted line). This is caused by the increase of the dissolved O concentration in equilibrium with deoxidizing elements in the molten steel because Al and Ti concentrations in the molten steel decrease considerably by a large amount of O absorbed. In the region where Al and Ti concentrations are relatively high and the increase of the dissolved O concentration is negligible, i.e., in the region where the change in O absorption over time maintains linearity, the oxidation rate of molten steel is evaluated as discussed below. Figure 7 shows the relation between the oxidation rate of molten steel in the still state, d[o] Abs/dt, and the partial pressure of O 2 gas in the gas phase, while Fig. 8 shows the relation between the oxidation rate of molten steel in the stirred state and the partial pressure of O 2 gas in the gas phase. In both Figs. 7 and 8, the previously reported 1) oxidation rates of the Al deoxidized molten steel in the still state and the stirred state are also shown. In the still state (Fig. 7), the oxidation rate of the Ti deoxidized molten steel increases in proportion to the first power of the O 2 gas partial pressure in the gas phase, whereas the oxidation rate of the Ti Al deoxidized molten steel is slower than that of the Ti deoxidized molten steel; similar to the Al deoxidized molten steel, that of the Ti Al deoxidized molten steel increases in proportion to the 0.5th power of the O 2 gas partial pressure in the gas phase. In the stirred state (Fig. 8), similar to the Al deoxidized molten steel, the oxidation rates of both the Ti and Ti Al deoxidized molten steel increase in proportion to the first power of the O 2 gas partial pressure in the gas phase Fig. 6. Change in the amounts [O] Abs of O absorbed over time during the oxidation of the Ti Al deoxidized molten steel in the stirred state. Fig. 7. Relation between the oxidation rate d[o] Abs/dt of molten steel in the still state and the partial pressure P O2 of O 2 gas in the gas phase ISIJ

6 Fig. 8. Relation between the oxidation rate d[o] Abs/dt of molten steel in the stirred state and the partial pressure P O2 of O 2 gas in the gas phase. chemical reaction with O would be extremely fast. 9) Moreover, taking into account the fact that O absorption increases linearly over time, the chemical reaction f) can be excluded from the rate-determining step in the Ti and Ti Al deoxidized molten steel. It has been reported that for cases in which the gas absorption rate of molten steel is controlled by a chemical reaction step such as gas absorption and desorption at the gas-metal interface, S in the molten steel reduces the reaction rate. 10) This study examined the influence of S on the oxidation rate of molten steel by adding 0.1 mass% of S in the Ti and Ti Al deoxidized molten steel both in the still state and the stirred state. As shown in Figs. 7 and 8, neither the Ti nor the Ti Al deoxidized molten steel showed any reduction in the oxidation rate caused by S in the molten steel. The fact that there is no influence of S in the molten steel supports the assumption that mass transfer is a ratedetermining step, and it seems that, as described previously, the interfacial chemical reaction processes of b), d), and f) would not become the rate-determining step in the Ti and Ti Al deoxidized molten steel as the previously reported 1) Al deoxidized molten steel. when the O 2 gas partial pressure is 10 kpa or less. However, when the O 2 gas partial pressure exceeds 10 kpa, the oxidation rates do not increase in proportion to the first power of the O 2 gas partial pressure, and they become faster than that of the Al deoxidized molten steel. 4. Discussions 4.1. Effect of Chemical Reactions on Oxidation Rates of Ti and Ti Al Deoxidized Molten Steel The oxidation reaction of the Ti and Ti Al deoxidized molten steel can be considered, similar to the case of the previously reported 1) Al deoxidized molten steel, to consist of the elementary steps of a) to g) below: a) Mass transfer of O 2 gas in the gas phase b) Chemical reaction at the interface between the gas phase and the oxide film c) Mass transfer of oxygen in the oxide film d) Chemical reaction at the interface between the oxide film and the molten steel e) Mass transfer of O in the molten steel f) Chemical reaction between O, Al, and Ti in the molten steel g) Mass transfer of Al and Ti in the molten steel Of the elementary steps shown above, the influences of the chemical reaction processes of b), d), and f) on the oxidation rate of molten steel are discussed below. In general, it is thought that the chemical reaction proceeds quickly at a high temperature. The oxidation rate of molten steel when the O 2 gas partial pressure was relatively as low as the air was measured in this study and was lower than the oxidation rate of molten steel using pure oxygen gas, 8) which was reported to be controlled by the chemical reaction at the interface between the gas phase and the oxide film. Taking these factors into account, there is the small possibility of either interfacial chemical reaction b) or d) becoming the rate-determining step. In addition, since Al and Ti are relatively strong deoxidizing elements, it is expected that the 4.2. Rate-determining Mechanism in Oxidation Reaction of Ti Deoxidized Molten Steel From the considerations in 4.1 above, the oxidation rate of the Ti deoxidized molten steel would be controlled by one of the mass transfer steps a), c), e), or g). The rate-determining step in the oxidation reaction of the Ti deoxidized molten steel is examined below using the experimental results and the reaction rate models. For cases in which the oxidation rate of the Ti deoxidized molten steel is controlled by g) the mass transfer of Ti in the molten steel, the oxidation rate is expressed by Eq.(1). 1) d[o] Abs/dt=1.854M O A k M,Ti [Ti]/(M Ti V)... (1) Where [O] Abs is the amount of O absorbed by the molten steel (mass%); t is time (s); Mo is the atomic weight of O (g mol 1 ); M Ti is the atomic weight of Ti (g mol 1 ); A is the surface area of molten steel (cm 2 ); K M,Ti is the mass transfer coefficient of Ti in the molten steel (cm s 1 ); [Ti] is the mass concentration of Ti in the molten steel (mass%); and V is the volume of molten steel (cm 3 ). If the oxidation rate of molten steel is controlled by step g), then d[o] Abs/dt would be in proportion to [Ti] from Eq.(1). In contrast, in the Ti deoxidized molten steel, though the Ti concentration in the molten steel decreases with the lapse of time, O absorption increases linearly over time, irrespective of whether or not the molten steel is stirred, and the Ti concentration dependence of the oxidation rate is not recognized. In addition, the oxidation rates for cases in which they are controlled by step g) are calculated using Eq. (1) and are shown in Figs. 7 and 8. As can be seen from these figures, the oxidation rate of the Ti deoxidized molten steel is lower than the calculated value. Here, it was assumed that the Ti concentration in the molten steel was 0.15 mass%. For all mass transfer coefficients in the molten steel, the value ( cm s 1 in a still state, 0.09 cm s 1 in a stirred state 12) ) obtained as follows was used: the N 2 absorption experiment was conducted in an atmosphere of 100% N 2, and the mixed-control rate model of the chemical reaction at the gas-metal interface (for the 2012 ISIJ 836

7 chemical reaction rate constant, the value reported by K. Harashima et al. was used 11) ) and the mass transfer of N in the molten steel were applied to the results. 12) From these results, it is expected that step g) would not become the ratedetermining step in the oxidation reaction of the Ti deoxidized molten steel. Next, if the oxidation rate of molten steel is controlled by e) the mass transfer of O in the molten steel, the oxidation rate can be expressed by Eq. (2). 1) d[o] Abs/dt=A k M,O [O] (F-M)/V... (2) Where k M,O is the mass transfer coefficient of O in the molten steel (cm s 1 ) and [O] (F M) is the O concentration in the molten steel at the interface between the oxide film and the molten steel (mass%). Since, in the Ti and Ti Al deoxidized molten steel, the oxide films containing 6 59 mass% of FeO were formed on the surface of molten steel, it is regarded that the O concentration at the interface between the oxide film and the molten steel is in equilibrium with FeO, and thus [O] (F M) at a temperature of C would be 0.23 mass%. 1) Therefore, if the oxidation rate of molten steel is controlled by step e), [O] Abs should change linearly with t, and d[o] Abs/dt should be constant regardless of the O 2 gas partial pressure. However, the oxidation rate of the Ti deoxidized molten steel, both in the still state and the stirred state, actually changes with the partial pressure of O 2 gas. Furthermore, as can be seen from Figs. 7 and 8, the oxidation rate of the Ti deoxidized molten steel is slower than that calculated by Eq. (2) for cases in which step e) is a ratedetermining step. It is thus thought that step e) is excluded from the rate-determining steps concerning the oxidation reaction of the Ti deoxidized molten steel. Also, if the oxidation rate of molten steel is controlled by c) the mass transfer of oxygen in the oxide film, Eq. (3) holds. 1) d[o] Abs/dt=100M O A k F P 0.5 O2 /(ρ V)... (3) Where k F is the rate constant (mol Pa 0.5 cm 2 s 1 ); P O2 is the partial pressure of O 2 gas in the gas phase (Pa); and ρ is the density of molten steel (g cm 3 ). From Eq. (3), if [O] Abs increases linearly with t and if d[o] Abs/dt is in proportion to P O2 0.5, the oxidation rate of molten steel is controlled by step c). However, no such O 2 gas partial pressure dependence of P O2 0.5 on d[o] Abs/dt in the oxidation process of the Ti deoxidized molten steel, both in the still state and the stirred state, can be seen. In addition, in the oxidation process of the Ti deoxidized molten steel, regardless of whether or not the molten steel is stirred, the surface of molten steel is not covered with the formed liquid phase-based oxides and most of O 2 gas is directly absorbed from the gas phase into the molten steel, thus it is hard to assume that the oxide films constitute resistance to O absorption. Therefore, it is unlikely that the oxidation rate of the Ti deoxidized molten steel is controlled by step c). Finally, if the rate-determining step in the oxidation reaction of molten steel is a) the mass transfer of O 2 gas in the gas phase, the oxidation rate is given by Eq. (4). 1) d[o] Abs/dt=200M O A k G P O2 /(ρ V R T)... (4) Where k G is the mass transfer coefficient of O 2 gas in the gas phase (cm s 1 ); R is the gas constant (Pa cm 3 mol 1 K 1 ); and T is the absolute temperature (K). As can be seen from Eq. (4), for cases in which the oxidation rate is controlled by step a), there is a linear relation between [O] Abs and t, and d[o] Abs/dt is in proportion to P O2. The oxidation rate of the Ti deoxidized molten steel in the still state increases in proportion to the first power of the O 2 gas partial pressure and completely satisfies the conditions applicable when it is controlled by step a) from Fig. 7. On the other hand, as shown in Fig. 8, the oxidation rate of the Ti deoxidized molten steel in the stirred state increases in proportion to the first power of the O 2 gas partial pressure and agrees with the oxidation rate of the Al deoxidized molten steel that is controlled by the mass transfer of a) in the region where the O 2 gas partial pressure is low, while when the O 2 gas partial pressure exceeds 10 kpa, it deviates from the line that is proportional to P O2. However, even in the region where the O 2 gas partial pressure exceeds 10 kpa, the oxidation rate of the Ti deoxidized molten steel is affected by the O 2 gas partial pressure and is shifted to the oxidation rate faster than that of the Al deoxidized molten steel, which is proportional to P O2. Thus, it is difficult to conclude that the rate-determining step in the oxidation reaction of the Ti deoxidized molten steel changes when the O 2 gas partial pressure exceeds 10 kpa. Furthermore, taking into account the fact that neither of the above mentioned steps c), e) and g) become the ratedetermining step, it is considered that the oxidation rate of the Ti deoxidized molten steel is controlled by step a), or the mass transfer of O 2 gas in the gas phase, irrespective of whether or not the molten steel is stirred Rate-determining Mechanism in Oxidation Reaction of Ti Al Deoxidized Molten Steel The mass transfer of steps a), c), e) and g) can be considered as the rate-determining steps in the oxidation reaction of the Ti Al deoxidized molten steel. Using the experimental results and the reaction rate models, the mass transfer step that controls the oxidation rate of the Ti Al deoxidized molten steel is studied as discussed below. If the oxidation rate of molten steel is controlled by g) the mass transfer of Al and Ti in the molten steel, the oxidation rate is expressed by Eq. (5). d[o] Abs/dt=1.5M O A k M,Al [Al]/(M Al V) M O A k M,Ti [Ti]/(M Ti V)... (5) Where k M,Al is the mass transfer coefficient of Al in the molten steel (cm s 1 ); [Al] is the mass concentration of Al in the molten steel (mass%); and M Al is the atomic weight of Al (g mol 1 ). From Eq. (5), if the oxidation rate is controlled by step g), d[o] Abs/dt should be affected by [Ti] and [Al]. However, in the Ti Al deoxidized molten steel, though the Ti or Al concentration in the molten steel decreases with the lapse of time, O absorption increases linearly over time regardless of whether or not the molten steel is stirred, and no dependence of the oxidation rate on the Ti or Al concentration can be seen. The oxidation rates that are controlled by step g) are calculated using Eq. (5) and are shown in Figs. 7 and 8. As can be seen from these figures, the oxidation rate of the Ti Al deoxidized molten steel is lower than the calculated value. Here, it was assumed that the Al concentration in the molten steel was 0.1 mass% and that the Ti concentration ISIJ

8 was 0.15 mass%. It is thus thought that the oxidation rate of the Ti Al deoxidized molten steel is controlled by a step other than step g). When the oxidation rate is controlled by e) the mass transfer of O in the molten steel, it does not depend on the O 2 gas partial pressure and remains constant. The oxidation rate of the Ti Al deoxidized molten steel changes depending on the O 2 gas partial pressure, both in the still state and the stirred state. Furthermore, as evident from Figs. 7 and 8, the oxidation rate of the Ti Al deoxidized molten steel is slower than that calculated by Eq. (2) or is controlled by step e). Therefore, it is judged that the oxidation rate of the Ti Al deoxidized molten steel is not controlled by step e) either. In the still state, the entire surface of the Ti Al deoxidized molten steel is covered with the relatively thick oxide film consisting of the solid-liquid coexisting phase of TiO 2-high Al 2O 3-low FeO and the liquid phase of TiO 2-low Al 2O 3-high FeO, and O 2 gas is absorbed into the molten steel through this oxide film. The oxidation rate of the Ti Al deoxidized molten steel in the still state is in proportion to the 0.5th power of the O 2 gas partial pressure, and this satisfies the condition applicable when it is controlled by c) the mass transfer of oxygen in the oxide film. On the other hand, in the stirred state, since the oxides consisting of the solid-liquid coexisting phases and the liquid phase formed in the oxidation process of the Ti Al deoxidized molten steel move rapidly toward the wall of the crucible, the surface of molten steel would not be covered with oxide film, and thus O 2 gas is absorbed not through the oxide film but directly into the molten steel. The oxidation rate of the Ti Al deoxidized molten steel in the stirred state is in proportion to the first power of the O 2 gas partial pressure in the region where the O 2 gas partial pressure is low, and this meets the condition applicable when the oxidation rate is controlled by a) the mass transfer of O 2 gas in the gas phase. When the O 2 gas partial pressure exceeds 10 kpa, the oxidation rate of the Ti Al deoxidized molten steel deviates from the line showing that the oxidation rate is in proportion to the first power of the O 2 gas partial pressure. However, taking into account that the oxidation rate shifts in the direction in which the oxidation rate becomes faster and that the oxidation rate is still affected by the O 2 gas partial pressure, the oxidation rate of the Ti Al deoxidized molten steel is thought to be controlled by step a), even in the region where the O 2 gas partial pressure exceeds 10 kpa. The above results show that, in the oxidation process of the Ti Al deoxidized molten steel, the coverage of the oxide film on the surface of molten steel varies depending on whether or not the molten steel is stirred. It is revealed that the oxidation rate of the Ti Al deoxidized molten steel in the still state is controlled by c) the mass transfer of oxygen in the oxide film, while that in the stirred state is controlled by a) the mass transfer of O 2 gas in the gas phase Influence of Steel Grades on Oxidation Rate of Molten Steel It is clear from Fig. 7 that the oxidation rate of the Ti deoxidized molten steel in the still state is faster than those of the Al and Ti Al deoxidized molten steel and that it exhibits dependence on the steel grade. In the oxidation process of the Al and Ti Al deoxidized molten steel, the entire surface of molten steel was covered with oxide films consisting of solid and liquid phases (in the case of the Al deoxidized molten steel, the solid phase of FeAl 2O 4 and the liquid phase of FeO 1) ), and these oxidation rates were controlled by the mass transfer of oxygen in the oxide films. On the other hand, in the oxidation process of the Ti deoxidized molten steel, the formed liquid phase-based oxides did not cover the surface of molten steel, and thus there was no resistance of the oxide film to the mass transfer of O 2 gas and, therefore, the oxidation rate now became controlled by the mass transfer of O 2 gas in the gas phase. In other words, the dependence of the oxidation rate of molten steel in the still state on the steel grade can be explained by the change of the rate-determining step due to the coverage of oxide films. In the stirred state, it was observed that iron vapor was actively generated from the surfaces of the Ti and Ti Al deoxidized molten steel in a high O 2 gas partial pressure region and, as shown in Fig. 8, the oxidation rates of the Ti and Ti Al deoxidized molten steel became faster than that of the Al deoxidized molten steel in which no significant iron evaporation was observed. As the oxidation rates of the Al, Ti, and Ti Al deoxidized molten steel in the stirred state can be considered to be controlled by the mass transfer of O 2 gas in the gas phase, the dependence of the oxidation rate on the steel grade in the stirred state cannot be explained by the difference of the rate-determining step. Furthermore, since the surface of molten steel was not covered with the oxides and O 2 gas was directly absorbed from the gas phase into the molten steel through the entire surface, which was the maximum reaction interface, regardless of the O 2 gas partial pressure in oxidation process of the Al, Ti and Ti Al deoxidized molten steel in the stirred state, it is hard to assume that these oxidation rates of molten steel exhibited the steel grade dependence due to the reaction interface increase in the region where the O 2 gas partial pressure exceed 10 kpa. Therefore, as a possible reason for why the oxidation rates of the Ti and Ti Al deoxidized molten steel become faster than that of the Al deoxidized molten steel, the mechanism is suggested that the significant iron evaporation in high O 2 gas partial pressure stirs the gas phase and enhances the mass transfer of O 2 gas in the gas phase. This mechanism suggests that: 1 when the influence of iron evaporation in the Al, Ti, and Ti Al deoxidized molten steel is eliminated and when the mass transfer conditions in the gas phase are adjusted, these oxidation rates decrease and can be matched: and 2 when iron evaporation in the Al deoxidized molten steel is accelerated to the contrary, the oxidation rate of molten steel increases and it is possible to make the oxidation rate of the Al deoxidized molten steel agree with those of the Ti and Ti Al deoxidized molten steel. The validity of the above mentioned mechanism is verified below by confirming 1 and 2 by means of experiments. For 1, an oxidation experiment of molten steel was conducted after uniformly covering their surfaces with 300 to 500 g of alumina reagent in order to eliminate the influence of iron evaporation on the gas phase. 2) Figure 9 shows the relation between the amounts W Al2 O 3 of alumina added to cover the surface and the oxidation rate of molten steel. The solid line represents the oxidation rate of molten steel calculated from the model to estimate the mass transfer coeffi ISIJ 838

9 Fig. 9. Relation between the amounts W Al2 O 3 of alumina added to cover the surface and the oxidation rate d[o] Abs/dt of molten steel. cient in the alumina layer. 2) The oxidation rates of the Al, Ti, and Ti Al deoxidized molten steel almost coincide with those obtained based on the assumption that the oxidation rate is controlled by mass transfer in the alumina layer and, moreover, there is no difference in the oxidation rate of molten steel between each steel grade. In other words, the oxidation rate of molten steel would not become dependent on the steel grade as expected in 1 when the influence of iron evaporation is eliminated and the mass transfer rates of O 2 gas in the gas phase are identical. While iron vapor flows against the O 2 gas flow from the molten steel surface toward the gas phase, it captures a part of O 2 gas as FeO to lower the O 2 gas concentration in the gas phase. If Ar gas can be injected dispersing into the molten steel, flows from the surface of molten steel toward the gas phase and reduces the O 2 gas partial pressure in the gas phase in the same way as iron vapor, it may be possible to simulate the effect of iron evaporation on the gas phase by injecting Ar gas into the molten steel. In order to confirm 2, the oxidation experiments of the Al deoxidized molten steel was conducted under the O 2 gas partial pressure of 20 kpa while injecting Ar gas of to Ncm 3 min 1, as it seems impossible to directly change the rate of iron evaporation in the Al deoxidized molten steel. As for the Ar gas injection nozzle, eight alumina insulating tubes with an inside diameter of 1 mm were attached radially at regular intervals near the tip (the sealed side) of the alumina protecting tube with an inside diameter of 16 mm, so that Ar gas bubbles could be injected dispersing into the molten steel. Figure 10 shows the relation between the Ar gas flow rate Q G,I and the oxidation rate of molten steel. As the Ar gas flow rate increases, the oxidation rate of molten steel increases. When approximately to mol min 1 of Ar gas is injected into the Al deoxidized molten steel, the oxidation rate of the Al deoxidized molten steel can be made identical to those of the Ti and Ti Al deoxidized molten steel. On the other hand, it has been reported that the iron evaporation rate increases as the O 2 gas partial pressure rises and can reach mol cm 2 h 1 at maximum. 13) When this value is applied to iron evaporation in the crucible with Fig. 10. an inside diameter of 22 cm, as used for this study, the maximum iron evaporation rate in the oxidizing atmosphere would be mol min 1. The gas flow rate ( mol min 1 ) required to make the oxidation rate of the Al deoxidized molten steel almost identical to those of the Ti and Ti Al deoxidized molten steel is from 32 to 44% of the maximum iron evaporation rate. This iron evaporation rate is possible if an oxidizing atmosphere is used. Therefore, it is presumed that even in the Al deoxidized molten steel, as the iron evaporation is accelerated, the oxidation rate will increase and reach near those of the Ti and Ti Al deoxidized molten steel. From the above results, the oxidation rate dependence of molten steel in the stirred state on the steel grade can be explained by the acceleration of the mass transfer in the gas phase due to iron evaporation. It is not clear why the iron evaporation rate varies depending on the steel grade. However, from the observations of the molten steel surface, it seems that when the molten steel to which Ti is added (the Ti and Ti Al deoxidized molten steel) is oxidized, and when the oxides containing titanium are heterogeneously formed on the surface of molten steel in the stirred state, the disturbance on the surface of molten steel is promoted, and, as a result, iron evaporation becomes active. 5. Conclusions Relation between the Ar gas flow rate Q G,I and the oxidation rate d[o] Abs/dt of the molten steel. As basic research to quantify the oxidation rates of molten steel for various steel grades in the tundish, the influence of steel grade on the oxidation rate of molten steel was studied by conducting oxidation experiments on the Ti and Ti Al deoxidized molten steel and comparing the obtained oxidation rates with that of the Al deoxidized molten steel. As a result, the following conclusions were drawn: (1) The oxidation rate of the Ti deoxidized molten steel by air is controlled by the mass transfer of O 2 gas in the gas phase both in the still state and the stirred state. (2) Since the resistance of the oxide film to mass trans ISIJ

10 fer varies depending on whether or not the molten steel is stirred, the oxidation rate of the Ti Al deoxidized molten steel by air is controlled by the mass transfer of oxygen in the oxide film in the still state, while it is controlled by the mass transfer of O 2 gas in the gas phase in the stirred state. (3) In the still state, the oxidation rate of the Ti deoxidized molten steel is faster than those of the Ti Al deoxidized molten steel and the Al deoxidized molten steel, showing dependence on the steel grade. This can be explained by the change of the rate-determining step due to the resistance of the oxide films, as while the surfaces of the Ti Al deoxidized molten steel and the Al deoxidized molten steel are covered with oxide films, the surface of the Ti deoxidized molten steel is not covered with oxide films and contacts directly with the gas phase. (4) The oxidation rates of the Ti deoxidized molten steel, Ti Al deoxidized molten steel and Al deoxidized molten steel in the stirred state are almost identical in the region where the O 2 gas partial pressure is low; however, when the O 2 gas partial pressure exceeds 10 kpa, the oxidation rates of the Ti and Ti Al deoxidized molten steel become faster than that of the Al deoxidized molten steel. This dependence on the steel grade can be explained by the mechanism of accelerating the mass transfer in the gas phase due to active iron evaporation in the Ti and Ti Al deoxidized molten steel, in which the surface disturbance is larger than that in the Al deoxidized molten steel. REFERENCES 1) K. Sasai and Y. Mizukami: ISIJ Int., 36 (1996), ) K. Sasai and Y. Mizukami: ISIJ Int., 38 (1998), ) K. Sasai and Y. Mizukami: ISIJ Int., 51 (2011), ) K. Sasai and Y. Mizukami: ISIJ Int., 40 (2000), 40. 5) T. Emi and R. D. Pehlke: Metall. Trans., 6B (1975), 95. 6) H. Ooi and M. Morishita: Kawasaki Steel Giho, 2 (1970), 14. 7) FactSage 6.1 Thermochemical Software. Thermofact and GTT- Technologies, (2009). 8) T. Emi, W. M. Boorstein and R. D. Pehlke: Metall. Trans., 5 (1974), ) Y. Miyashita: Tetsu-to-Hagané, 52 (1966), ) T. Choh and M. Inouye: Tetsu-to-Hagané, 54 (1968), ) K. Harashima, S. Mizoguchi, M. Matsuo and A. Kiyose: ISIJ Int., 32 (1992), ) S. Mukawa, Y. Mizukami and Y. Ueshima: Tetsu-to-Hagané, 84 (1998), ) E. T. Turkdogan, P. Grieveson and L. S. Darken: J. Phys. Chem., 67 (1963), ISIJ 840