Effects of the Ultrasound Treatment on Reaction Rates in the RH Processor Water Model System

Transcription

1 Effects of the Ultrasound Treatment on Reaction Rates in the RH Processor Water Model System Yong tae Kim 1 Kyung woo Yi 1,2 Received: 27 December 2017 / Accepted: 28 June 2018 The Korean Institute of Metals and Materials 2018 Abstract Ruhrstahl Heraeus (RH) processor is widely applied to the refining process to produce steel with very low carbon contents. In this study, to investigate the effect of ultrasound treatment on RH decarburization process, we have developed two kinds of the water models simulated the RH process and study the removal rate of dissolved oxygen. The one is the RH water model of 1/8 size of actual RH degasser simulated the late-stage of the RH process when surface reaction and plume reaction mainly occur. Through this model, it is found that the ultrasound treatment accelerates dissolved oxygen removal reaction and this tendency is maintained even at low concentrations. Also, the results show that there is a difference in the degassing efficiency depending on the frequencies and the positions of the ultrasonic transducer. Also, to simulate the Early-stage Reaction of the process including the inner-site reaction which is difficult to investigate through the RH water model, the other water model has been developed (the RH-ER water model). This model shows that the ultrasound treatment facilitates the earlystage reaction including inner-site reaction, like the RH water model. These results show that the addition of the ultrasound treatment can accelerate decarburization reaction during RH process compared to conventional process. Keywords Metals RH degassing Water model experiment Ultrasonics 1 Introduction Ultra-low carbon steel used for automobile applications requiring high formability can be produced through RH (Ruhrstahl Heraeus) decarburization system. As demands for ultra-low carbon steel increase, the decarburization rates must be increased to decrease processing times and to gain higher yields. One of the methods to enhance the degassing reaction rate is ultrasound treatment. The ultrasound treatment has merits which are environmentally clean, relatively inexpensive [1 4] and easy to install [3, 4]. Due to these advantages, the ultrasound treatment on solidifying metals or alloys has been attempted in many previous studies. According to these studies, the ultrasound treatment can contribute to producing pure materials close to the theoretical density by removing the unnecessary gaseous phase in the melt [1, 2, 5 7] or to refining the grains by the non-linear effects of the ultrasound [8 12]. When the ultrasound is applied to a liquid system, the ultrasound waves propagate from a radiator or a transducer surface into the liquid and result in alternating compression and rarefaction periods in the liquid. If the pressure inside the liquid becomes lower than a specific value during the rarefaction period, the solubility of gaseous species decreases to a super-saturation state. When the sum of the partial pressures of the vapor and the dissolved gas is larger than the sum of the hydrostatic pressure of the liquid, the atmospheric pressure and the capillary pressure caused by surface tension, like Eq. 1 [13], bubbles can be formed in the liquid. * Kyung woo Yi yikw@snu.ac.kr p 0 + ρgh + 2S R p V + p G (1) 1 Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea 2 Center for Iron and Steel Research, Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 08826, Republic of Korea where p 0, ρ, g, h, S, R, p V, p G denote the atmospheric pressure, the liquid density, the gravitational acceleration, the distance from surface to bubble, the surface tension, the bubble radius, the partial pressure of the liquid vapor and the Vol.:( )

2 partial pressure of the dissolved gas, respectively. Since the vapor pressure of the liquid is so small, the internal pressure of the bubble is determined by the partial pressure of the dissolved gas. Bubbles can be formed in the melt because the ultrasound increases p G. This is called the cavitation phenomenon by ultrasound. Generally, these generated bubbles grow by rectified diffusion [13 16], or coalesce with each other [17]. If the bubbles grow large enough by these processes, they can float to the surface by buoyancy force. Through this process, degassing is accelerated [18]. However, it has some limitations that the ultrasound treatment is applied to the steel melt. The process including the steel melt is carried out under very high-temperature over 1500 C. This process environment is so severe that the ultrasonic transducer or horn to transmit ultrasound is corroded [3, 19, 20]. Therefore, in the previously mentioned studies, the ultrasound treatment was almost applied to the metals or alloys whose melting point are low, such as pure aluminium [5, 21], alloys based on aluminum [1, 5 7, 9], alloys based on Al Si [10 12] and alloys based on magnesium [8], etc. [3, 21]. If this limitation is not solved, the ultrasound treatment cannot be used repeatedly to the high-temperature melt. Therefore, some studies attempted to introduce the ultrasound indirectly to the steel melt. Liu et al. have shown that the ultrasound treatment had effects on the grain refining and the improvement of the mechanical properties when the ultrasound treatment was applied to solidifying 1Cr18Ni9Ti stainless steel [22] melt and T10 steel melt [23]. To prevent the transducer from corroding, they attached the transducer to the side of the mold including melt whose temperature is over 1600 C. Komarov et al. [3] have introduced various ultrasonic modules including a cooling system to endure high temperature. They have mentioned that these modules can be directly immersed in the high-temperature melt and have listed the results that some researchers investigated the effects of the ultrasound treatment with these modules on grain refining of various metals and alloys including steel. Nowacki [24] developed a transducer including the cooling system for an investigation into effects of the ultrasound treatment on grain refining of low carbon steel, too. As other methods to introduce ultrasound directly to steel melt, some studies have focused on radiators that are connected to the transducer and can avoid the corrosion at high temperature. Abramov [21] have introduced radiators made of various materials whose melting temperature is so high, such as Ta, Nb, Fe, Ti, Mo, W, in order to investigate grain refining of not only alloys based on the aluminium and magnesium but also various steels, such as ferritic, austenitic and carbide steels and high boron steels. Recently, new radiators, such as Ti-alloy [1, 6 9], Mo Al 2 O 3 ZrO 2 ceramic metal [25, 26] or Si 3 N 4 [27] are developed. Furthermore, there is an example using a radiator made of the same material as the steel melt used in an experiment [20]. However, these trials are still not perfect. Generally, though the ultrasonic modules have the cooling system, it is difficult to endure high temperature for a long time. Also, it is hard for the compound radiators to produce. The other limitation is the ultrasound attenuation in the steel melt. The steel melt has relatively high density and viscosity. Due to these material properties, the effects of the cavitation is limited, compared to air or the other light metal alloy [28]. As a result, it is difficult that the introduced ultrasound affects the entire melt of large-scale. In fact, some researchers [4, 25, 26] used low carbon steel melt of only under 1 kg for the investigation of the effects of the ultrasound treatment on degassing. Even, the ultrasound treatment is applied to the small size melt of only several kilograms in the experiments with low melting temperature metal and alloy [1, 6, 7, 10 12]. Though there are some grain refining results for hundreds of kilograms of the steel melt [24, 29], this limitation is critical of steelmaking that mass production is required. Therefore, there are a few attempts to apply the ultrasound treatment to the large-scale steel melt. It is one of the solutions that the power of the introduced ultrasound is increased, in order to overcome this limitation. Most of the studies show that as the power increases, the positive effects of the ultrasound treatment are also enhanced. However, the relations between power, frequency, material, and experimental conditions, etc. aren t clear, so further studies about them are needed. Despite these limitations, the ultrasound treatment is attractive method applicable to degassing process of the steel melt. In this study, we investigate the effects of ultrasound treatment on the RH process. However, it is very difficult to observe the melt behavior and the degassing phenomenon in the RH degasser. Therefore, water model [30 34] have been proposed as a good alternative for simulating steel melt flow or evaluating the degassing efficiency. In this study, we develop two kinds of water models to investigate the effects of ultrasound treatment on the RH process. One is an RH water model of 1/8 size of the actual RH degasser equipped with ultrasonic devices. This model can show the effects of the ultrasound treatment during the latestage of the RH process when the carbon concentration is low. Meanwhile, the carbon concentration is high during the early-stage of the RH process. Since it is difficult that this model simulates the reaction during the early-stage of the RH process, the other model is required. Thus, we develop the new water model to simulate the Early-stage Reaction of the RH process (RH-ER water model). Using this model, we investigate the effect of the ultrasound treatment on the

3 reaction rate when the carbon concentration is relatively high. 2 Physical Modeling Procedures 2.1 The Similarity of the RH Water Model to the RH Processor and the Feature of the RH Water Model For the RH water model to be valid, flow behavior in this model should be similar to the RH processor. When inertia and viscosity mainly govern the flow behavior, it depends on the kinematic viscosity (viscosity/density). Because the effects of the natural convection on the flow behavior in the RH processor are small and the kinematic viscosity of the water is similar to that of the molten steel, the RH water model is valid for evaluating the flow velocity [32, 33]. To investigate the flow and the degassing behavior during the RH process, the RH water model that is made of transparent acryl is used, as shown in Fig. 1a. The diameters of the ladle, the vacuum vessel and the snorkels are 300, 500 and 100 mm, respectively, The length of each snorkel is 210 mm. Also, the up-snorkel has four nozzle-inlets whose diameters are 4 mm for circulation gas (Fig. 1b). The first step in the RH process is to combine the ladle filled with steel melt and the vacuum vessel. After then, a vacuum pump reduces the pressure in the vessel to raise the melt from the ladle to the vessel and the circulation gas is blown through the nozzles connected to the up-snorkel to circulate the steel melt. The RH water model is similar to the RH processor except that the steel melt and Ar gas are replaced by the water and N 2 gas, respectively. The pressure in the vessel is maintained by a manometer installed between a vacuum pump and the vessel and the quantity of N 2 gas is controlled by an MFC installed between the up-snorkel and a gas-bombe, respectively. Because a DO-sensor measuring the dissolved oxygen concentration may be affected by ultrasound, it is installed in the ladle. An ultrasonic transducer with a protective cap whose diameters are 90 mm is installed above one of the snorkels, as shown in Fig. 2. Frequencies of the transducers used in this modeling are 20 khz, 28 khz and 40 khz, and the power is 60 W. The circulation gas, N 2, is injected at a rate of 15L/ min. To evaluate the degassing efficiency of the RH water model, its reaction order should be identical to that of the RH processor. The decarburization reaction is two-step reaction, which is combined with the chemical reaction and the physical process, as seen in Eqs. 2 and 3. [C] + [O] [CO] (2) Fig. 1 a The schematic configuration of the RH water model apparatus with ultrasonic devices: 1 ultrasonic transducer, 2 ultrasound generator, 3 manometer, 4 pressure controller, 5vacuum pump, 6 MFC, 7 nitrogen gas bombe, 8 DO-sensor; b the shape of circulation gas inlet of up-snorkel [CO] CO (g) (3) Among the two steps, the first step is very fast due to high temperature. On the other hand, in the second step, the dissolved gas phase must overcome the activation barrier of gas/melt interface for bubble formation. So, the second step is the rate-determining step of this decarburization reaction. Thus, decarburization reactions in the RH processor can be considered as the apparent first-order reactions like Eq. 3. In the RH processor, there are three degassing reaction sites at the melt surface, at the circulation gas surface and

4 Plume reaction K P = A P ρ melt h c ([ ] wt% C 100 [ wt% C ] ) P (5) Fig. 2 The positions of the ultrasonic transducers (dash line) in the RH water model Fig. 3 Degassing reaction sites in a the RH decarburization system and b the RH water model system [18] in the melt, as shown in Fig. 3a [33, 35]. The reactions at each site are called the surface reaction, the plume reaction and the inner-site reaction, respectively. The previous study [35] has proposed the rate constants at each site as follows. Surface reaction K S = A s ρ melt h c ([ ] wt% C 100 [ wt% C ] ) S (4) Inner-site reaction dc dt = K I [wt%c][wt%o] where K, A, ρ melt, h c denote the reaction rate, the reaction area, the density of the melt and the mass transfer coefficient of the dissolved carbon, respectively. Equations 4 and 5 show that decarburization rates at the surface and plume depend on transport rate of carbon to the reaction sites. Since the surface and the plume reactions are assumed to occur immediately when the carbon and oxygen meet in the melt, the reaction rate is proportional to the only carbon concentration at the reaction sites [35]. Also, if the oxygen concentration is sufficiently high in the system as compared to the carbon, the change in the oxygen concentration is relatively small during the process. Therefore, [wt%o] in Eq. 6 can be considered as a constant. If so, the inner-site reaction is also assumed to the apparent first-order reaction. Harashima et al. [36] have reported that the decarburization reaction is regarded as the apparent first-order reaction when the carbon concentration is less than 200 ppm. The water contains the dissolved oxygen. When nitrogen is blown into the water and the ambient circumstances change to the nitrogen atmosphere, dissolved oxygen is released from the water. The removal reaction of the dissolved oxygen in the RH water model is expressed in Eq. 7. [ ] O2 O2 (g) (7) This reaction is the apparent first-order reaction similar to the decarburization reaction (Eq. 3) in the RH processor. When the oxygen transports to the reaction sites the surface and the plume, the removal reaction of the dissolved oxygen occurs, as seen in Fig. 3b. So, the reaction mechanisms in the RH water model are similar to the RH processor. The RH process can divide into two stages according to the dominant reactions, as seen in Fig. 4. This figure is numerical analysis result obtained from a comprehensive numerical Fortran program, RH-3D that can calculate the flow [37 39] and the carbon concentration during the RH process [35, 39]. This shows the change in the contribution of each reaction to the overall decarburization reaction in the RH processor over the processing time. As seen in this figure, when the carbon concentration is relatively high during the early-stage of the process, the inner-site reaction is predominant, and then its contribution decreases sharply over time. On the other hand, when the carbon concentration is low during the late-stage of the process, the surface and the plume reactions are dominant but the inner-site reaction hardly occurs [40, 41]. (6)

5 Fig. 4 The contribution ratio of each reaction over the processing time Some studies have investigated this relationship through numerical analysis [35] or comparative analysis used actual plant data and numerical results [40, 41]. However, since it is difficult to decrease the internal pressure of the RH water model to very low level like the RH processor, a surface and a plume reaction only occur [33], as seen in Fig. 3b. Thus, the RH water model simulates the late-stage of the RH process. 2.2 The Feature of the RH Water Model for the Early Stage Reaction (RH ER Water Model) It is practically difficult to investigate the early-stage of the RH process including the inner-site reactions through the RH water model. Because the inner-site reaction takes place in the super-saturated state by a pressure drop, the significantly low pressure in a vessel is required. Since the density of water is far smaller than that of steel melt, it is difficult to use the RH water model under the low pressure. Therefore, numerical models [35, 42] are used more frequently than water models, to simulate the entire process including the inner-site reactions. Thus, a new water model is suggested to simulate the earlystage of the RH process including the inner-site reaction and to evaluate the degassing efficiency. The new RH water model for the Early-stage Reaction (RH- ER water model) is shown in Fig. 5. For the new model to endure enough low pressure, the wall of this model is made of much thicker and stronger materials than the RH water model. The positions of the ultrasonic devices and the pressure control device are similar to the RH water model. To keep a measuring point constant and avoid interruption of ultrasound, the distance between a DO-sensor and a water surface is same as that of the RH water model. Fig. 5 The schematic configuration of the RH-ER water model apparatus: (1) ultrasonic transducer, (2) ultrasound generator, (3) manometer, (4) pressure controller, (5) vacuum pump, (6) DO-sensor 3 Results and Discussions 3.1 The Evaluating of Degassing Efficiency in the RH Water Model To compare with the removal tendency of the dissolved oxygen, experiments with various degassing treatments in the RH water model are performed. All experiments are performed under the pressure of about mmhg in the vessel, the 28 khz transducer is used for the ultrasound treatment (UST) and the quantity of blown N 2 gas for circulation (Cir) is 15 L/min. The temperatures of the water used in the experiments are C, and the starting points are set when the dissolved oxygen is a saturated state at the beginning temperature of each experiment. The reuslt about these experiments is shown in Fig. 6. When

6 Fig. 6 The dissolved oxygen removal behaviors comparison with each treatment in the RH water model: UST, Cir mean the ultrasound treatment with 28 khz ultrasonic transducer and circulation with 15 L/min of the blown N 2 gas, under pressure of about 735 mmhg in the vessel, respectively the Y-axis is set to log-scale, a graph shape of the degassing tendency over processing time is like straight. This means that this reaction is the first-order reaction. This result shows that the addition of the UST in the RH water model facilitates degassing compared to the conventional process. In the absence of the circulation, however, it seems that there is little change in the dissolved oxygen concentration. This is because the degassing reaction by UST may mainly occur in a vessel near the transducer and little in the ladle with a DO sensor. Kang et al. [28] have reported the attenuation of the ultrasound using their numerical analysis. They made a 2D symmetry model which is R50 mm H180 mm and simulated velocity distribution in various melts. When the ultrasound with 200 W and 20 khz was introduced to the water, the maximum distance affected by the ultrasound was 140 mm from the transducer. Compare to this model, the RH water model is larger and the power of the used ultrasound is lower. Thus, although the degassing may occur in the vessel, it is difficult to expect to degas by UST in the entire system without circulation, as seen in Fig. 6. This means that there is the difference of the degassing mechanisms depending on the presence or absence of the circulation gas. Generally, cavitation bubbles grow the specific way so-called rectified diffusion during the degassing by the ultrasound processing [14 16]. For the growth by rectified diffusion, the bubbles pulsate in an area influenced by ultrasound during a few cycles. However, the cavitation bubbles cannot grow by the rectified diffusion in the RH process because the circulation gas pushes out the cavitation bubbles out of this area. Furthermore, the plume or bubbles Fig. 7 The changes in dissolved oxygen measured in RH water model when ultrasonic transducers are installed above the down-snorkel disengaged from the plume can merge with the small cavitation bubbles failed to float. It is the reason that the presence of the circulation gas is more important than the UST in RH process. The degassing efficiency can be quantified by determining the rate constant of a degassing reaction. In the case of the first-order reaction, the concentration change over the time can be expressed as Eq. 8 [32, 33]. dc dt = KC ln C = Kt + Q K = ln ( ) [DO] t [DO] 0 Δt where C, t, K, [DO] i denote dissolved gas concentration, processing time, rate constant and dissolved oxygen concentration at the time i, respectively. By separating variables and integrating Eq. 8 for the concentration and the time, Eq. 9 is obtained and K is represented as Eq. 10. In fact, K is proportional to a slope of the graph in the Fig. 6. The unit of the rate constant of the first-order reaction is the reciprocal of the time, (min 1 ). 3.2 The Effects of Ultrasound Frequencies and Transducer Positions on the Rate Constants in the RH Water Model When the ultrasonic transducer is installed above the downsnorkel, the change in dissolved oxygen concentration measured in the RH water model is shown in Fig. 7. In the index of each experiment, the first two-digit number, the (8) (9) (10)

7 next one character and the last two-digit number denote the applied ultrasound frequency, the transducer position and the quantity of blown circulation gas, respectively. For example, 40D15 means that 40 khz of ultrasound is applied, an ultrasonic transducer is located above the down-snorkel and 15 L/min of circulation gas is blown. Also, for the sake of convenience, we will call the experiment 0 khz when the experiment is perforemed that the transducer is installed and not operated. The measured values fluctuate in experiments with the UST as seen in Fig. 7. These phenomena grow stronger when the applied frequency is lower and processing time is shorter. These are the common features of all experiments. These may be supposed to be due to complex factors such as generation of bubbles or flow change by ultrasound and the further studies about these phenomena may be needed in the future. After fitted by the regression analysis due to these fluctuations, the results can be quantified by calculating the rate constant using Eq. 10. R-square values of fitting curves are over except the results of 20 khz experiments that have The rate constants of each experiment are presented in Fig. 8. There are two significant features in Fig. 8. The first is that as the frequency is higher, the removal rate constant of the dissolved oxygen is larger. These results are related to the size of cavitation bubbles by the ultrasound. Because rarefaction is short in the higher frequency, the size of the bubbles is relatively smaller [18]. Since the bubbles whose radius is small have high internal pressure, as shown in Eq. 1, it is not easy for cavitation bubbles to disappear even in the compression region of the ultrasound. Thus, the higher Fig. 8 The rate constants according to the positions and frequencies in RH water model: the results of 20 khz represent the dashed line since the fluctuation of their degassing behavior is too severe, as seen Fig. 7 the frequency, the greater the number of not eliminated bubbles. As previous mentioned, when the ultrasound is applied to the water and there is flow circulation by the plume, the cavitation can escape from the influence of the ultrasound by the plume within a few cycles. After then they are raised by ascending flow. As the bubbles rise, the hydrostatic pressure around the bubbles decreases. Also, they can coalesce with each other bubbles, or they can merge into the plume [17]. As a result, the size of the bubbles becomes larger and the bubbles can float easily by larger buoyancy forces. Thus, as the frequency increases, there are many cavitation bubbles, so that the degassing rate increases. The second is that the rate constants are better when the transducer is located above the down-snorkel than above the up-snorkel, regardless of frequencies. The direct cause of these phenomena is the reduction of a surface area by the transducer. As seen in Fig. 4, the surface is one of the main reaction sites during the late-stage of the RH process. The ultrasonic transducer with a protective cap occupies 9% of the water surface in the entire vacuum vessel, which reduces the surface area, so that the overall reaction rate decreases. In fact, The rate constant of the degassing experiment without installing an ultrasonic transducer is about min 1 and it is larger than the rate constants of experiments when the transducer is installed and not operated (0 khz), as seen in Fig. 8. The surface above the up-snorkel is that the area is relatively more extensive, as schematically shown in Fig. 3, and concentration is higher [42] than those above the down-snorkel, due to the ascending plume. This means that the surface above the up-snorkel is the most active area of surface reaction. Since the transducer covers this area, the rate constants of 00D15 and 00U15 decrease to and min 1, respectively. Despite the surface reduction, the rate constants increase when the UST is applied. The rate constants of all cases when the transducer is installed and operated are larger than that when the transducer is not installed, except when the transducer is located above the up-snorkel and the ultrasound of 20 khz is applied. To investigate the rate constant change by the UST, the degree of improvement is calculated, as expressed in Eq. 11. ( ) KUST K 0kHz Degree of improvement (%) = 100 (11) K 0kHz where K UST, K 0kHz denote the rate constants of the experiment with the UST and 0 khz, respectively. Table 1 represents the degree of improvement in degassing efficiency. According to this table, the degassing efficiencies of experiments with the UST increase by 5% 15% compared to results of the 0 khz experiments. Also, as the frequency increased, the degree of improvement increased.

8 Table 1 The improvement in degassing efficiency according to the ultrasound frequencies and positions of the ultrasonic transducers 20 khz (%) 28 khz (%) 40 khz (%) Above down-snorkel Above up-snorkel While the dissolved oxygen concentration decreases by 1 ppm, the changes in the rate constants are shown in Fig. 9 and listed in Table 2. These results show that the improvement in degassing efficiency is similar to the entire process, and the degree of improvement in the interval rate constants with the UST can increase the degassing efficiency by 5% 15% like Table 1. This tendency is maintained in the low concentration condition. The degree of improvement is relative difference between with UST and without UST. It is larger when the transducer is located above the up-snorkel than above the down-snorkel. If only these results are only considered, it seems that the appropriate positions of the transducer are above the up-snorkel. When the transducer is located above the up-snorkel, the ultrasound breaks the plume. The pieces of the plume can merge with cavitation bubbles. This is why the degree of improvement is higher. However, as the rate constant is still smaller due to the reasons mentioned previously, the position of the transducer may be more appropriate to be above the down-snorkel. There are two main ways to increase process efficiency. One is to reach the target concentration quickly (line 1 in Fig. 7) and the other is to maintain a high reaction rate even at low concentrations (line 2 in Fig. 7). Figure 8 and Table 1 show that the RH process with UST can be rapidly reduced to the target concentration of the dissolved oxygen. On the other hand, Fig. 9 and Table 2 show that Table 2 The improvement in degassing efficiency during reduction of 1 ppm according to the ultrasound frequencies and positions of the ultrasonic transducers 28 khz (%) 40 khz (%) Above down-snorkel 6ppm to 5ppm ppm to 4ppm ppm to 3ppm Above up-snorkel 6ppm to 5ppm ppm to 4ppm ppm to 3ppm the improvement in the reaction rate due to the UST is effective even when the concentration is low. 3.3 The Evaluating of Degassing Efficiency in the RH ER Water Model To investigate the dissolved oxygen reduction behavior under low pressure, we perform the experiments using the RH-ER model. Under the internal pressure of 150 mmhg, the one is that the ultrasound of 40 khz is introduced and the other is that the transducer is installed but not operated. As seen in Fig. 10, the dissolved oxygen concentration decreases slowly during the initial 30 s and then sharply in both experiments. The initial 30 s is the time it takes for the air inside the vessel to be released to the outside, and then the inner-site reaction by the super-saturation occurs. Fig. 9 The interval rate constants between each 1 ppm in the RH water model Fig. 10 The dissolved oxygen removal behaviors depending on the VaT and UST in the RH-ER water model: there are only data during 420 s in the experiment with UST. This is attributable to the severe environment inside the vessel such as the low pressure, which is only 1/5 of the atmospheric pressure, and humid atmosphere, causing the transducer to be short-circuited and destroyed

9 Fig. 11 The reaction rate according to the concentration of the dissolved oxygen in the RH-ER water model As seen in Fig. 10, regardless of the UST, the dissolved oxygen concentration decreases sharply and then there is little change in concentration after reaching a certain concentration. This is consistent with the results in Fig. 4, in which the inner-site reaction only contributes to the early-stage of RH process and rarely contributes to the late-stage. Depending on the presence or absence of the UST, the difference of the final concentration is about 1 ppm. Besides, the time to reach the final concentration is also short. Figure 11 shows the removal rate of the dissolved oxygen with respect to the concentration in experiments with and without the UST using Eq. 12. Since the time to reach target pressure, 150 mmhg, is 40 s, the initial concentration in Eq. 12 is the value after 40 s from the beginning of the pressure drop. Reaction rate = dc dt where C, t, [DO] i denote the concentration, the processing time and the dissolved oxygen concentration at the time i, respectively. In all concentration region, the reaction rate of the experiment with the UST is larger than that of the experiment without the UST. These results show that the UST during the early-stage including the inner-site reaction can improve the degassing efficiency, too. 4 Conclusions = [DO] t [DO] 0 Δt (12) In this study, two kinds of the water models are used to investigate the effects of the ultrasound treatment (UST) on the degassing behavior during the RH process. To quantitatively evaluate the efficiency, reaction rate constants and reaction rates are calculated and compared. From these models, the degassing efficiency in the process with the UST is better than those without the UST. The RH water model is simulated the late-stage of the RH decarburization process in which the surface and the plum reactions mainly occur. From the results of this modeling, it is found that the UST is effective in improving the degassing efficiency, which depends on ultrasound frequencies and the transducer positions. The higher the ultrasound frequency, the larger the rate constants. Though the rate constants when the transducer is located above the down-snorkel are larger than that when above the up-snorkel, the degree of improvements of the UST are larger when transducers are located above the up-snorkel than when the transducers are located above the down-snorkel. This is why negative effects of installation of the transducer are greater than the positive effects obtained by the UST. Also, this tendency is seen not only in the overall reaction but also in every reaction interval. These results show that by adding the UST in the RH water model, not only the target concentration can be reached quickly but also a lower concentration can be reached during the same processing time. The other model devised in this study, the RH-ER water model (the RH water model for the Early-stage Reaction), is simulated the early-stage of the RH process including the inner-site reaction. This model also shows that the UST is useful in facilitating the reactions related to the early-stage of the process. The degassing rates become faster and the final concentration is lower when the experiment with the UST is performed. The experimental results of the two models show that the UST facilitates the degassing reactions during all stages of the process. Acknowledgements The authors would like to express their gratitude to the Center for Iron and Steel Research at Research Institute of Advanced Materials (RIAM) in Seoul National University for their financial support of this work. References 1. H. Xu, T.T. Meek, Q. Han, Effects of ultrasonic field and vacuum on degassing of molten aluminum alloy. Mater. Lett. 61(4 5), (2007) 2. Q. Han, Ultrasonic processing of materials. Metall. Mater. Trans. B 46(4), (2015) 3. S.V. Komarov, M. Kuwavara, O.V. Abramov, High power ultrasonics in pyrometallurgy: current status and recent development. ISIJ Int. 45(12), (2005) 4. M. Xu, Q. Liu, D. Ma, G. Wu, B. Hu, L. Ma, Effects of ultrasound on the degassing of molten steel in the RH refining process. Steel Res. Int. 85(5), (2014) 5. G.I. Eskin, Cavitation mechanism of ultrasonic melt degassing. Ultrason. Sonochem. 2(2), S137 S141 (1995)

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