Some aspects of non-metallic inclusions during vacuum degassing in ladle treatment- with emphasize on liquid CaO-Al2O3 inclusions

YoungJo Kang

Doctoral Thesis

Department of Materials Science and Engineering Division of Micromodeling

Royal Institute of Technology SE-100 44 Stockholm

Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskloan i Stockholm, framlägges för offentlig granskning för avläggande av Teknologie doktorsexamen, onsdagen den 7 mars 2007, kl. 10.00 i D3, Lindstedsvägen 5, Kungliga Tekniska Högskloan, Stockholm

ISRN KTH/MSE--07/04--SE+MICROMODMETU/AVH ISBN 978-91-7178-571-8

ABSTRACT The present thesis was to study non-metallic inclusions during vacuum degassing in ladle treatment. Emphasize was mostly given to liquid CaO-Al2O3 inclusions. A series of industrial experiments were carried out at Uddeholm Tooling AB, Hagfors, Sweden. To gain an insight into the industrial findings, laboratory investigations were also performed. Large number of steel samples were collected and examined. Liquid calcium aluminate inclusions with low SiO2 and high SiO2 were often found with spinel inclusions before vacuum degassing. Laboratory experiments showed that spinel would react with the dissolved Ca in the liquid steel forming calcium aluminate inclusions. This laboratory results agreed with the industrial observation that spinel phase was quite often found in the center of the calcium aluminate phase.

After vacuum degassing, most of the inclusions were calcium aluminate liquid inclusions. When dissolved Al level was low, 2 types of liquid calcium aluminate inclusions with considerably different SiO2 contents were found to coexist even at the end of the process. In view of the lack of the thermodynamic data for SiO2 activities in the low silica region, thermodynamic measurements were conducted in the CaO-Al2O3-SiO2-MgO system. The experimental results could reasonably explain the coexistence of the two types of the liquid oxide inclusions. While the total number of inclusions decreased during vacuum degassing, the number of bigger inclusions (>11.3 μm) increased generally in used ladles. This finding was in accordance with the previous studies, wherein, ladle glaze was found to be responsible for the supply of bigger inclusions. The behaviors of several types of inclusions in liquid steel were examined using a laser scanning confocal microscope (LSCM). While alumina inclusions tended to impact on each other, agglomerate and grow very quickly, none of the other types of inclusions, such as spinel and calcium aluminate was observed to agglomerate. The results of LSCM study agreed well with the industrial observation. Examination on a huge number of inclusions did not show any indication of impact and physical growth of the inclusions, except the alumina inclusions. The removal of inclusions around open-eye in a gas-stirred ladle was experimentally studied by a cold model with oil and water. Most of the simulated inclusions were brought up to the oil phase by gas-water plume. Inclusion removal into oil layer took place when the inclusions passed through the sphere-bed of the oil layer around the open-eye. A calculation showed that the contribution of metal-gas plume in inclusion removal was much larger than that of buoyancy mechanism. The results of the industrial experiments revealed that the inclusions distribution strongly depended on stirring condition. When a ladle was stirred by both gas and induction, inclusion removal near slag layer was significant. Key words: non-metallic inclusions, ladle refining, vacuum degassing, ladle glaze, spinel, SiO2 activity, agglomeration, cold model, open-eye, inclusion removal

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ACKNOLEDGEMENTS First of all, I would like to express my special thanks to my principal supervisor, Professor Du Sichen, for his excellent guidance and encouragements throughout this work. I also convey my special thanks to my co-supervisor, Professor Kazuki Morita, for his valuable advices and supervision during this work. I am truly grateful to Alf Sandberg, Dr. Mselly Nzotta and Tech. Lic. Karin Steneholm, for fruitful discussions and valuable helps in industrial experiments. Warm kindness of many colleagues in Uddeholm Tooling AB is also appreciated. I also thank to Professor Piotr R. Scheller, for his helpful suggestions and comments as well as supports during my work in Frieberg. I am thankful to Professor Seshadri Seetharaman and Professor Pär Jönsson, for constant support during my study. I owe my gratitude to Fan Li, Liang Yu and Dr. Bahman Sahebakar for their essential contributions and assistances. Financial supports for this work from Uddeholm Tooling AB are gratefully acknowledged. I also thank to Professor Min Dong Joon, for his encouraging advice and comments. Thanks to all my friends in Division of Micromodeling and my colleagues and all faculties in Department of Material Science and Engineering for their friendship and support. I specially thank to all members of KOSAS and Korean friends around me, MY, KB, SS, JY, HS, JS, SW, DH, DY, SY, HN, YN, for numerous helps and kindness, which made me stay in Sweden happily. Finally, I would like send my sincere gratitude to my family in Korea for continuous trust and encouragement in my whole life. Stockholm, January 2007

YoungJo Kang

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SUPPLEMENTS The thesis is based on the following papers: Supplement 1: “Non-metallic inclusions and their distribution in the ladle before and

after vacuum treatment of tool steel” YoungJo Kang, Mselly Nzotta and Du Sichen Steel-Grips, accepted for publication, 2006

Supplement 2: “Mechanism Study on the Formation of Liquid Calcium Aluminate

Inclusions from MgO·Al2O3 spinel” YoungJo Kang, Fan Li, Kazuki Morita and Du Sichen Steel Research International, Volume 77, Nov., 2006, pp. 785-792

Supplement 3: “Activities of SiO2 in some CaO-Al2O3-SiO2(-10%MgO) melts with

low SiO2 contents at 1873K” YoungJo Kang, Du Sichen and Kazuki Morita Sent to ISIJ International for publication

Supplement 4: “Some aspects of physical growth of non-metallic inclusion in ladle

treatment” YoungJo Kang, Bahman Sahebkar, Piotr R. Scheller and Du Sichen METEC InSteelCon 2007, 2007, June, Dusseldorf, Germany

Supplement 5: “Mechanism Study of inclusion removal around open-eye in ladle

treatment” YoungJo Kang, Liang Yu and Du Sichen Ironmaking and Steelmaking, in press, 2007

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Parts of this work have been accepted at the following conferences: 1. “Activity of SiO2 in CaO-Al2O3-SiO2(-10%MgO) melts with low SiO2 content at

1873K” YoungJo Kang, Du Sichen and Kazuki Morita 153rd ISIJ Spring Meeting, 2007, March, Chiba, Japan

2. “Some aspects of non-metallic inclusions in vacuum treatment”

YoungJo Kang and Du Sichen 7th International Conference on Clean Steel, 2007, June, Balatonfured, Hungary

TABLE OF CONTENTS

ABSTRACT i

ACKNOWLEGDEMENTS iii

SUPPLEMENTS v

1 INTRODUCTION 1

2 EXPERIMENTAL 2

2.1 Analysis on industrial samples 2

2.2 Study on the formation of liquid CaO-Al2O3 phase 3

2.3 Measurement of SiO2 activity in CaO-Al2O3 melts with low SiO2 4 content

2.4 In-situ observation on the interactions of non-metallic inclusions 5

2.5 Cold model for the inclusions removal by bubble floatation 6

3 RESULTS 8

3.1 Industrial study 8 3.1.1 Compositions of steel and slag before and after vacuum degassing 8

3.1.2 Types of inclusions before and after vacuum degassing 8

3.1.3 Populations of inclusions before and after vacuum degassing 9

3.1.4 Distribution of inclusions before and after vacuum degassing 10

3.2 Formation of CaO-Al2O3 phase in the presence of MgO·Al2O3 in 12 liquid steel

3.3 Activities of SiO2 in low SiO2 region of the CaO-Al2O3-SiO2(-MgO) 14 system

3.3.1 Activity of SiO2 in the CaO-Al2O3-SiO2 system 14

3.3.2 Activity of SiO2 in the CaO-Al2O3-SiO2-MgO system 15

3.4 Interactions and behavior of various inclusions in liquid steel 17 3.4.1 Attractive interaction of alumina inclusions 17

3.4.2 Behaviors of inclusions other than alumina 18

3.5 Behaviors of inclusions around open eye 20 3.5.1 Phenomena around open-eye and inclusion movements in the bath 20

3.5.2 Attachment of inclusions with different sizes 21

4 Discussion 22

4.1 Formation of liquid inclusions before and after vacuum degassing 22 4.1.1 Formation of liquid CaO-Al2O3 inclusions from spinel 22

4.1.2 Equilibrium between liquid inclusions and liquid steel after vacuum 24 degassing

4.2 Change of inclusions during vacuum degassing 26 4.2.1 Number of bigger inclusions during vacuum degassing 26

4.2.2 Physical growth of various inclusions during vacuum degassing 26

4.3 Removal of inclusions during vacuum degassing 31 4.3.1 Mechanism of inclusion removal around open-eye 31

4.3.2 Contribution of the open-eye in inclusion removal 32

4.3.3 Distribution of inclusions in a ladle 35

5 SUMMARY 37

6 FUTURE WORKS 38

7 REFERENCES 39

1 INTRODUCTION Non-metallic inclusions have been one of the most important concerns in iron and steelmaking for past several decades. It is well known that non-metallic inclusions existing in the final product generally degrade the quality – especially, mechanical properties – of the product. Even though there have been few suggestions to utilize non-metallic oxides,[1, 2] most of efforts have been made focusing on the detrimental effects of the non-metallic inclusions. In order to control the non-metallic inclusions appropriately, sound understanding on the non-metallic inclusions would be extremely necessary. As a matter of fact, various aspects of the non-metallic inclusions have been studied. They range from origin of the inclusions,[3, 4] formation and thermodynamic considerations,[3-8] morphologies[5, 8-11] and interrelationships with steel and slag[11-18], to removal of the inclusions,[19-23] physical properties and effects.[15-18, 24-27] Nonetheless, still many points are left unclear and to be improved. Even in secondary refining, the non-metallic inclusions impose crucial influences on the quality of the product as well as the processes. Recently, great attentions highlight on the secondary refining with increasing demands of cleaner steel. In the sense, ladle treatment plays an important role as not only a transporting process before casting but also as a refining process. During the ladle treatment, the removals of the non-metallic inclusions and impurities such as sulphur, hydrogen are carried out, while temperature and composition of liquid steel are controlled for casting. Since the ladle treatment is often the final step before casting in many cases of the clean steel production, a proper control on the non-metallic inclusions during the ladle treatment may directly affect the quality of the final product. Some experimental works[4-6, 28-30] in Uddeholm Tooling AB, Hagfors, Sweden were carried out for the purpose of better control on the inclusions during the ladle treatment, in last few years. A number of types of the inclusions were observed in steel samples taken from the different steps of the ladle treatment. The formation and developments of the inclusions along the ladle treatment were also studied. Inclusions analyses revealed that ladle glaze is responsible for the formations of some types of the inclusions during the ladle treatment. At the end of the ladle treatment in Uddeholm Tooling AB, the liquid steel experiences vacuum degassing period. After vacuum degassing, the majority of the inclusions in liquid steel were found to be liquid inclusions based on CaO-Al2O3 oxide solution. The liquid calcium aluminate inclusions are frequently observed even in the final product. Since the sizes of these inclusions are rather large, >10μm, the inclusions have harmful impacts on the properties of the end product, causing surface defects or degradation of polishablity.[31] However, the study on the liquid calcium aluminate inclusions during the ladle treatment is very limited. Many aspects of the inclusions during the ladle treatment are still unclear. The aim of the present work is to obtain better understanding on the non-metallic inclusions during the ladle treatment with emphasize on the liquid CaO-Al2O3 inclusions. For this purpose, industrial experiments on the inclusions before and after vacuum degassing in the ladle treatment are carried out. It also consists of a number of laboratory works to get an insight into the formation of the inclusions, the physical growth of them and their removal mechanism.

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2 EXPERIMENTAL

2.1 Analysis on industrial samples In order to study non-metallic inclusions in liquid steel during ladle treatment, huge numbers of samples were taken. Sampling and analyses were carried out at Uddeholm Tooling AB, Hagfors, Sweden, which is a scrap-based steel mill. Liquid crude steel melted in an electric arc furnace (EAF) from scrap metal is tapped into a ladle. Figure 1 shows the flow of the processes in Uddeholm Tooling AB and the details of the process are described further in supplements. After removing the EAF slag, Al deoxidation, addition of slag former and alloying are carried out with induction stirring. The ladle is then transferred to a degassing station, where the closed chamber is evacuated to 3 mbar. Meanwhile, the ladle is stirred by both gas and induction. Once the target steel composition is met, the liquid metal is sent for casting.

(a) tapping from EAF to ladle (b) ladle treatment (c) vacuum degassing (d) uphill casting

Figure 1 Process at Uddeholm Tooling AB, Hagfors

In most of the experiments, the steel grade ORVAR 2M, whose composition is shown in Table 1, was investigated. The sampling was carried out before and after vacuum degassing (after flotation period). In some heats, a number of samples were taken after deslagging (before deoxidation) and before casting. With the aim of studying the inclusion distribution in a ladle, several sampling positions were used in some heats. The positions are schematically shown in Figure 2. Even the induction stirrer and the porous plugs are shown in the figure. Totally 9 positions (3 positions along the arc of an automatic sampling equipment × 3 depths, viz. 10%, 30% and 50% of the depths from the slag/metal interface) were considered in a ladle. Ordinary sampling position was the position (P2, D1).

induction stirrer

P1

P2P3

D1

3.0m

D2

D3

argon plug

induction stirrer

P1

P2P3

D1

3.0m

D2

D3

argon plug

induction stirrer

P1

P2

P3

2.2m

argon plug

induction stirrer

P1

P2

P3

2.2m

argon plug

Figure 2 Sampling positions in a ladle

By rapid solidification (RS) method, double lollipop samples were obtained. One of them was used for composition analysis, while the other was for inclusion analysis. After sampling, the sample was taken out of the sampler and quenched in water quickly. At each stage of the process, the temperature and dissolved oxygen of the metal bath were measured simultaneously using a Celox® oxygen sensor supplied by Heraeus Electro-Nite.

Table 1 Typical chemical compositions of steel ORVAR 2M

element C Si Mn Cr Mo V mass% 0.39 1.0 0.4 5.3 1.3 0.9

One of the two lollipop steel samples was used for chemical composition analysis. The compositions of the steel were determined by an ARL 9800 XP SIM/SEQ, PANalytical, X-ray fluorescence (XRF) spectrometer and an optical emission spectrometer (OES), QBLF 750. Carbon and sulphur contents were also measured by means of a CS-444 LS, LECO, carbon/sulphur determinator. The other steel sample was used for inclusion analysis. Well-polished samples were examined using a light optical microscope (LOM). According to the Swedish standard SS111116,[32] the inclusion population and the size distribution were evaluated. The compositions of the phases present in the inclusions were analyzed using a JSM-840, JEOL, scanning electron microscope (SEM), equipped with an energy dispersive X-ray analyzer (EDX), INCAEnergy, Oxford

2.2 Study on the formation of liquid CaO-Al2O3 phase The formation of liquid CaO-Al2O3 phase from spinel phase was studied in the laboratory. The experimental setup is schematically shown in Figure 3. A graphite resistance furnace (the Laboratory Furnaces Group 1000) supplied by Thermal Technology Inc., was used. The sample temperature was controlled by Pt-10%Rh/Pt thermocouple in contact with the bottom of the graphite holder. For very fast quenching, a cooling chamber surrounded by water-cooling copper pipe was installed on the top of the furnace. In order to obtain a low oxygen potential in the reaction chamber, Ar gas was passed through a gas cleaning system before it was introduced into the reaction tube. A spinel pellet was produced by sintering pressed powder mixture of Al2O3 and MgO at 1873K for 120 hours. The Al2O3 and MgO powders were all reagent grades and supplied by Sigma-Aldrich. The formation of the spinel phase was confirmed by XRD analysis. The spinel pellet was 11mm in diameter and 5mm in height. Steel samples of ORVAR 2M supplied by Uddeholm Tooling AB were employed in the experiments. Calcium carbide, CaC2, supplied by Akzo Surface Chemistry AB, Sweden (80mass% of CaC2) was used as the Ca source. About 125g of steel pieces and 0.8g of CaC2 were placed in a ZrO2 crucible held in the graphite holder. CaC2 was kept in a hole drilled out in a piece of the steel and covered by a small piece of steel. The spinel pellet was placed on the top of all the steel pieces. In a few experiments, no CaC2 was added. After reaction chamber was closed and sealed tightly, the graphite holder with the sample was inserted into the even temperature zone. The oxygen partial pressure of the outgoing gas appeared as low as 5×10-21atm by the oxygen sensor. The furnace was heated up to the predetermined temperature and kept for a certain period of time. After holding time,

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the samples were quenched by lifting the holder quickly to the water cooled chamber. In a few experiments, furnace cooling was employed to study the effect of cooling rate. From the sample after the reaction, the interface between the steel and spinel was carefully collected. The interfaces were examined by light optical microscope and analyzed by the JSM-840, JEOL, SEM, with an EDX, INCAEnergy, Oxford. The dissolved Ca was analyzed for some of the steel samples using a pulse distribution analysis technique, PDA. Due to the extremely low Ca contents, however, the Ca analyses were subjected to certain level of uncertainties.

Thermocouple

Gas inlet

Alumina radiation shields

Graphite crucible

ZrO2 crucible

Sample

Steel rod

Cooling chamber

Gas outlet

Graphite rod

Lifting deviceSteel rod

Vacuum pumpThermocouple

Gas inlet

Alumina radiation shields

Graphite crucible

ZrO2 crucible

Sample

Steel rod

Cooling chamber

Gas outlet

Graphite rod

Lifting deviceSteel rod

Vacuum pump

Figure 3 Experimental apparatus

2.3 Measurement of SiO2 activity in CaO-Al2O3 melts with low SiO2 content For the consideration of the equilibrium between inclusions and liquid steel, the activity of SiO2 was measured in low SiO2 content region of the CaO-Al2O3-SiO2(-MgO) melts. In the measurement, a reference metal Cu (γO

Si in Cu=0.016[33]) and synthetic oxide solutions were equilibrated in a MoSi2 resistance furnace, SS-1700-200, NEMS. The temperature was controlled with a proportional integral differential (PID) controller equipped with Pt-6%Rh/Pt-30%Rh thermocouple. Copper and oxide solution with various compositions were held in a graphite crucible under controlled oxygen partial pressure at 1873K for 18 hours, which was determined by preliminary experiments. The compositions of the oxide solutions for the measurements are presented in Table 2. The oxygen partial pressure was controlled to be approximately 4.4×10-16atm by the C(s)/CO(g) equilibrium. After the experiments, the metal and the slag were carefully separated from quenched samples for chemical analyses. The content of Si in Cu was analyzed by an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), Seiko Instrument Co. While SiO2 content in slag sample was determined by a gravimetry method, the ICP-AES was also used for determination of the slag compositions other than SiO2.

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Table 2 Charge compositions charge compositions (mass%) sample CaO Al2O3 SiO2 MgO

301 49 40 11 - 302 56 35 9 - 303 48 43 9 - 304 40 51 9 - 305 47 47 6 - 306 55 39 6 - 307 40 54 6 - 308 47 50 3 - 309 40 57 3 -

ternary

310 55 42 3 - 401 41 40 9 10 402 46 35 9 10 403 37 44 9 10 404 42 42 6 10 405 46 38 6 10 406 36 48 6 10 407 41 46 3 10 408 35 52 3 10 409 45 42 3 10

quaternary

410 39 42 9 10 411 47 35 11 7 inclusions 412 48 32 14 6

2.4 In-situ observation on the interactions of non-metallic inclusions The interactions and the behaviors of inclusions in the liquid steel were examined using a confocal scanning laser microscope, CSLM, a 1LM21H, Lasertec. A gold-coated high temperature furnace is equipped with the CSLM, as illustrated in Figure 4. Gold coating and elliptical shape allow that most of reflected beams from a halogen lamp focus on the upper focal point, where a sample is placed. The sample can be heated by radiation. In the CSLM, He-Ne laser is used for the observation. The obtained image can be both watched using a CRT monitor and recorded into a video tape. Steel samples taken before vacuum treatment at Uddeholm Tooling AB were used in the CSLM. In this type of samples, spinel inclusions were often observed. A steel sample was placed in an alumina crucible. To obtain thermal homogeneity in the system, the sample on the sample holder was covered by a platinum lid with opening on the top for the observation. After the sample was placed into the sample holder, the chamber was sealed tightly. The chamber was evacuated to 10-5 bar before argon gas (O2<2ppm) was introduced. The evacuation and argon filling procedure was repeated at least 5 times to ensure a clean atmosphere. In order to study various types of inclusions, accordingly different atmospheres using Ar+H2 mixture were employed. After the system was stabilized at 1873K, the surface of liquid steel was carefully examined. It should be mentioned that the real temperature of the sample was somewhat lower than the recorded value. The difference was estimated to be about 30 K. The image of the surface was examined and recorded by the CRT monitor and the video recorder. Still frames were taken from the recorded movie clip with the time step of 0.5sec, and carefully analyzed. After the experiments in the CSLM, the inclusions observed were identified by SEM, (LEO 430, Leo Electron Microscopy, scanning electron microscope with EDX X-ray analyzer, Röntec-EDX)

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Figure 4 Illustration of high-temperature chamber of a confocal scanning laser microscope

2.5 Cold model for the inclusion removal by bubble floatation Inclusion removal by metal-gas plume under gas blowing was studied by cold model experiments. Water was used to simulate the steel bath, while silicone oils of different viscosities were used to simulate slag layer. Different silicone oils and charcoal powder were employed to simulate the inclusions. 3 types of silicone oils, supplied by BDH Laboratory, Supplies Poole, BH15 1TD, England, with different kinetic viscosities: 50cSt, 100cSt and 200cSt, were used. The corresponding dynamic viscosities in SI units are 0.048, 0.097, 0.194 kgm-1s-1, respectively. Small amounts of oil and water with a volumetric ratio of 1/5 were well mixed by magnetic stirring at 600rpm for 6 hours. To obtain even clearer images, the silicone oil in several cases was colored with an oil-soluble dye. Less than 0.05g of Sudan Blue was added to the oil-water mixture and stirred. Charcoal powder was prepared by crushing pieces of coal supplied by Skogens Kol AB, KILAFORS, Sweden and sorting them into different size groups. Powders were filtered with sieves of 100, 140 and 200mesh successively to get three groups of powder. The image analyses by an optical microscope showed that the average sizes of the charcoal powders in the three groups were 140μm, 87μm and 45μm, respectively. The powder was dried in an oven at 383 K for 12 hours and weighed carefully before being mixed with water in a small glass container. Similar as oil-water system, the charcoal-water mixture was magnetically stirred at 600rpm for 6 hours before usage. For the observation on phenomena around open-eye, a rectangular vessel (l430mm×w210mm×h590mm) with a gas inlet at the center of bottom was used. A silicone oil layer with 25~33mm thickness was laid on the top of water bath filled in the container. The height of water layer was about 180mm. Through the gas inlet nozzle with an inner diameter of 10mm, compressed air was introduced at a predetermined flow rate to produce gas bubbles. Several gas flow rates were adopted. Once the system had been stabilized in about half an hour, the flow pattern in the open-eye area and the oil-water mixing around the open-eye were carefully observed.

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At the same time, still pictures and movie clips were taken using a digital camera, CANON IXUS700 for image analysis. Using the dyed oil-water mixture, the movement of the inclusions and the removal from the water bath were also examined. The silicone oil of 200cSt was used for the oil layer and the inclusions. After 30 min of the gas flow, the prepared oil-water mixture was injected into the water bath. The injection was made at the half height of the water bath and very close to the side wall of the container using a syringe. The total amount of injection of the oil-water mixture was less than 50mL. Again, still pictures and movie clips were taken to capture the flow pattern in the water bath and the movements of the tiny mixture droplets. Semi-quantitative study on the inclusion removal was also carried out. In order to simulate the separation of inclusions in a ladle, a cylindrical container with dimensions ø250mm×h400mm was used. A gas inlet was situated in the center of the bottom. About 8L of water was poured into the container, and the charcoal-water mixture, prepared in advance, was added into the water. Compressed air was introduced from the nozzle at the bottom at a high flow rate to attain the homogeneity of the charcoal particles in the water bath. After stopping the air flow, silicone oil was put on the top of the water bath. To prevent the charcoal powder from attaching to the oil layer before onset, the oil was introduced with special precaution. After the first sampling for the initial particle concentration, a constant air flow started to be introduced. Usually, 6 samples were taken at predetermined blowing times. All the samples were taken at the same position in the water bath using syringes. Each sample was 50mL in volume. Special attention was paid to avoid any entrapment of oil into a syringe. The charcoal powder was separated from water using filter papers, which had been dried for 3 hours and weighed accurately before the filtering. After the filtration, the filters were dried in an oven for 3 hours again and weighed. The weight and the concentration of the charcoal powder in the sample were obtained from the increase in weight of the filter. The details of the experimental conditions are shown in Table 3.

Table 3 Experimental conditions

experiments particle size of inclusion (m)

viscosity of silicone oil (kgm-1s-1)

flow rate (Lmin-1)

1 4.5×10-5 0.097 1.7 2 4.5×10-5 0.097 3.0 3 4.5×10-5 0.048 3.0 4 8.7×10-5 0.048 1.7 5 8.7×10-5 0.048 3.0 6 1.4×10-4 0.097 1.7

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3 RESULTS

3.1 Industrial study

3.1.1 Compositions of steel and slag before and after vacuum degassing The experiments were carried out in 2 different periods of the year, namely Period A and Period B. The typical steel compositions before and after vacuum degassing in the both periods are presented in Table 4. The typical compositions of the ladle slags are also listed in Table 5. It is noted that the total Al contents in the liquid steels in the heats of Period A are considerably higher than those in the heats of Period B before vacuum degassing. Table 4 also shows that Al content decreases after vacuum treatment. This decrease could be attributed to the reduction of the other oxides and the reoxidation at the open-eye region. However, the total Al contents after vacuum treatment in Period A are still higher than those in Period B. Correspondingly, the alumina contents in the slags of Period A are slightly higher than those of Period B both before and after vacuum treatment. The difference in MgO content in the slags of Period A and Period B can also be seen. No substantial difference in the SiO2 content is observed when comparing the slags of the two groups.

Table 4 Chemical compositions of steel samples before and after vacuum mass% C Si Mn Cr V Mo Al Ca

before vacuum 0.38 1.02 0.42 5.15 1.31 0.93 0.070 <0.0006Period A after vacuum 0.39 1.02 0.41 5.16 1.29 0.93 0.029 <0.0006

before vacuum 0.39 1.05 0.39 5.20 1.30 0.92 0.048 <0.0006Period B after vacuum 0.41 1.07 0.39 5.20 1.29 0.92 0.022 <0.0006

Table 5 Chemical compositions of slag samples before and after vacuum mass% CaO MgO Al2O3 SiO2

before vacuum 48 10 24 13 Period A after vacuum 46 12 26 12

before vacuum 47 15 22 13 Period B after vacuum 45 15 23 12

* the compositions are not normalized

3.1.2 Types of inclusions before and after vacuum degassing Totally, six types of non-metallic inclusions are detected in the present steel samples. All these types of inclusions had been also found in the earlier studies on tool steels.[28,

34] To make easier discussion, the same classification and notations of the inclusions are employed. The composition ranges and the appearance of the inclusions before and after vacuum degassing for the both of Period A and B are summarized in Table 6. Inclusions of Type 2 have a spherical shape. They consist of Al2O3, CaO, MgO and SiO2. This Type of inclusions is liquid oxide solution with high SiO2 content. Inclusions of Type 3 are irregular in shape. The composition of these inclusions is within the non-stoichiometric range of MgO·Al2O3 spinel phase. Inclusion of Type 4 is a combination of inclusions of Types 2 and 3, with the spinel phase (Type 3) embedded in the solution (Type 2). The phase of inclusion Type 7 consists mainly of CaO and Al2O3, with some small amounts of MgO and SiO2. The difference between Type 7 and Type 2 is the substantial difference in SiO2 content. Similar as inclusions

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of Type 4, inclusions of Type 6 are also composed of two phases, the MgO·Al2O3 spinel (Type 3) in the center surrounded by a layer containing mainly CaO and Al2O3 with small amounts of MgO and SiO2 (Type 7). Type 8 has also two phases, viz. MgO islands embedded in a solution (Type 7). The SEM micrographs of these inclusions could be found in the previous publications.[5, 6, 28, 34]

Before vacuum degassing, Type 3 and the inclusions with liquid calcium aluminate phase were often detected. Contrary to Period A, considerable numbers of inclusions based on Type 2 were found in Period B. After vacuum degassing, the majority of the inclusions were the inclusions based on the liquid oxide solution with low SiO2. It is noted that Type 2 inclusions were found to coexist with the liquid inclusions having low SiO2 even after vacuum degassing, in spite of their considerable difference in SiO2 content. Type 2 inclusions were never found after vacuum degassing in Period A as shown Table 6. This difference could be due to the difference in aluminum content in the steel as shown in Table 4. It should be also pointed out that Type 8 inclusions were observed only after vacuum degassing in used ladles.

Table 6 Compositions of various types of inclusions and the appearance before and after vacuum degassing composition (mass%) appearance

Period A Period B type* description CaO Al2O3 SiO2 MgO before vacuum

after vacuum

before vacuum

aftervacuum

1 pure MgO 0~4 1~4 0~1 90~97 2 liquid oxide solution with high SiO2 44~48 32~36 11~15 4~7 × × 3 spinel 0~4 68~71 - 26~29 × × 4 coexistence of 2 and 3 × 5 pure Al2O3 6 coexistence of 7 and 3 × × × 7 liquid oxide solution with low SiO2 30~39 45~52 2~6 8~10 × × × × 8 coexistence of 7 and 1 (in old ladle) × ×

* referred after T. Nagendra[34]

3.1.3 Populations of inclusions before and after vacuum degassing The numbers of inclusions in the steel samples were counted using a light optical microscope (LOM). According to the Swedish standard SS111116,[32] the number of inclusions were estimated and classified into 4 different size groups,

• 2.8 μm ~ 5.7 μm • 5.7 μm ~ 11.3 μm • 11.3 μm ~ 22.6 μm • 22.6 μm ~

Based on the counting results, the inclusions are divided into two bigger groups, namely inclusions < 11.3 μm and inclusions > 11.3 μm, for the later discussion. This division is because that the inclusions bigger than 11.3 μm are usually considered more harmful. Figures 5 (a) and (b) show the changes of the populations of inclusions smaller than 11.3 μm and bigger than 11.3 μm in vacuum treatment. The ages of the ladles used are also presented in the figures. The number of inclusions smaller than 11.3 μm is much larger than that of the bigger inclusions. It is clearly seen that the population of

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inclusions smaller than 11.3 μm decreases during vacuum degassing regardless of the ladle age. In contrast to the smaller inclusions, the numbers of inclusions bigger than 11.3 μm generally increase during the vacuum degassing as shown in Figure 5 (b). Only in the case of the new ladle (age 1), the number of inclusions bigger than 11.3 μm decreases during vacuum treatment.

0.0

0.2

0.4

0.6

0.8

1.02.8μm~11.3μm

after vacuumbefore vacuum

10%depthPosition 2

ladle age 1 ladle age 4 ladle age 14 ladle age 16 ladle age 26

num

ber o

f inc

lusi

ons,

mm

-2

Process0.00

0.02

0.04

0.06

0.08 ladle age 16 ladle age 26

11.3μm~

after vacuumbefore vacuum

ladle age 1 ladle age 4 ladle age 14

10%depthPosition 2

num

ber o

f inc

lusi

ons,

mm

-2

Process (a) (b)

Figure 5 Change of the populations (mm-2) of inclusions (a) smaller than 11.3μm and (b) bigger than 11.3μm before and after vacuum treatment

3.1.4 Distribution of inclusions before and after vacuum degassing The inclusions in the samples from different positions in a ladle have been carefully counted to reveal the inclusion distribution. In Period B, an attempt was made to take samples at all nine positions in the ladle both before and after vacuum treatment. The ladle had been used for 17 heats. Unfortunately, two samplings (positions P1 and P2 at D1) were not successful before vacuum treatment. Figures 6 (a) and (b) present the numbers of inclusions before vacuum treatment for inclusions smaller than 11.3 μm and inclusions bigger than 11.3 μm, respectively. Similarly, the numbers of inclusions smaller than 11.3 μm and bigger than 11.3 μm at different positions in the ladle after vacuum treatment are shown in Figures 7 (a) and (b). Note that the sizes of the spheres in these figures are proportional to the numbers of inclusions (number of inclusions per square millimeter). Before vacuum treatment, inclusions in both size groups appear to distribute quite uniformly in the ladle, as shown in Figure 6. On the other hand, after vacuum treatment, the total number of inclusions has decreased. As shown in Figure 7, the distribution of the inclusions is not as uniform as that before vacuum treatment. Irrespective of the size group, the numbers of inclusions at the depth D1 (10% depth from the slag/metal interface) are the smallest in comparison with the other two depths (D2 and D3) at all three positions (P1, P2 and P3). It is also seen in Figure 7 that the numbers of inclusions after vacuum treatment in both size groups show an increasing trend with the depth of the steel bath at positions P1. On the other hand, the numbers of inclusions appear to show higher values at D2 in the case of P3.

10

2.8~

11.3μm

, 18

ladl

e ag

e

0.56

8

0.61

9

0.62

8

0.83

1

0.57

5

0.59

2

0.65

9

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5

posi

tion

depth (%)

1

2

3

11.3μm

~, 1

8 la

dle

age

0.14

2

0.07

4

0.16

9

0.13

7

0.07

1

0.08

7

0.08

2

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5po

sitio

n

depth (%)

1

2

3

2.8~

11.3μm

, 18

ladl

e ag

e

0.06

5

0.14

3

0.24

8

0.06

7

0.16

4

0.19

9

0.09

8

0.11

1

0.06

6

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5po

sitio

n

depth (%)

1

2

3

11.3μm

~, 1

8 la

dle

age

0.02

1

0.06

4

0.11

2

0.01

0

0.08

2

0.02

0

0.07

1

0.03

60.

079

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5

posi

tion

depth (%)

1

2

3

Figu

re 6

The

pop

ulat

ions

(mm

-2) o

f inc

lusi

on (a

) 2.6

~11.

3μm

and

(b) 1

1.3μ

m~

befo

re v

acuu

m tr

eatm

ent a

t diff

eren

t pos

ition

s in

a la

dle,

ladl

e ag

e 18

Figu

re 7

The

pop

ulat

ions

(mm

-2) o

f inc

lusi

on (a

) 2.6

~11.

3μm

and

(b) 1

1.3μ

m~

afte

r va

cuum

trea

tmen

t at d

iffer

ent p

ositi

ons i

n a

ladl

e, la

dle

age

18

(a)

(b)

(a)

(b)

2.8~

11.3μm

, 18

ladl

e ag

e

0.56

8

0.61

9

0.62

8

0.83

1

0.57

5

0.59

2

0.65

9

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5

posi

tion

depth (%)

1

2

3

2.8~

11.3μm

, 18

ladl

e ag

e

0.56

8

0.61

9

0.62

8

0.83

1

0.57

5

0.59

2

0.65

9

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5

posi

tion

depth (%)

1

2

3

11.3μm

~, 1

8 la

dle

age

0.14

2

0.07

4

0.16

9

0.13

7

0.07

1

0.08

7

0.08

2

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5po

sitio

n

depth (%)

1

2

3

11.3μm

~, 1

8 la

dle

age

0.14

2

0.07

4

0.16

9

0.13

7

0.07

1

0.08

7

0.08

2

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5po

sitio

n

depth (%)

1

2

3

2.8~

11.3μm

, 18

ladl

e ag

e

0.06

5

0.14

3

0.24

8

0.06

7

0.16

4

0.19

9

0.09

8

0.11

1

0.06

6

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5po

sitio

n

depth (%)

1

2

3

2.8~

11.3μm

, 18

ladl

e ag

e

0.06

5

0.14

3

0.24

8

0.06

7

0.16

4

0.19

9

0.09

8

0.11

1

0.06

6

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5po

sitio

n

depth (%)

1

2

3

11.3μm

~, 1

8 la

dle

age

0.02

1

0.06

4

0.11

2

0.01

0

0.08

2

0.02

0

0.07

1

0.03

60.

079

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5

posi

tion

depth (%)

1

2

3

11.3μm

~, 1

8 la

dle

age

0.02

1

0.06

4

0.11

2

0.01

0

0.08

2

0.02

0

0.07

1

0.03

60.

079

0 10 20 30 40 50 60 70 80

0.5

11.

52

2.5

33.

5

posi

tion

depth (%)

1

2

3

Figu

re 6

The

pop

ulat

ions

(mm

-2) o

f inc

lusi

on (a

) 2.6

~11.

3μm

and

(b) 1

1.3μ

m~

befo

re v

acuu

m tr

eatm

ent a

t diff

eren

t pos

ition

s in

a la

dle,

ladl

e ag

e 18

Figu

re 7

The

pop

ulat

ions

(mm

-2) o

f inc

lusi

on (a

) 2.6

~11.

3μm

and

(b) 1

1.3μ

m~

afte

r va

cuum

trea

tmen

t at d

iffer

ent p

ositi

ons i

n a

ladl

e, la

dle

age

18

(a)

(b)

(a)

(b)

11

3.2 Formation of CaO-Al2O3 phase in the presence of MgO·Al2O3 in liquid steel

Thirteen experiments were carried out. The experimental conditions along with the phase(s) found at the interfaces are presented in Table 7.

Table 7 Experimental conditions and analysis resultssample temperature (K) time (h) quenching CaC2 addition phase(s) found at the interface

1 1873 2 Yes spinel with low Ca 2 1873 0.5 Yes spinel with low Ca 3 1813 2 Yes spinel with low Ca 4 1873 12 Yes spinel with low Ca 5 1873 1.5 Yes Yes liquid, CaO·2Al2O3

6 1873 1.5 Yes Yes liquid, CaO·2Al2O3

7 1873 1.5 Yes spinel with low Ca 8 1793 1.5 Yes Yes CaO·2Al2O3

9 1843 1.5 Yes Yes CaO·2Al2O3

10 1873 1.5 Yes Yes liquid, CaO·2Al2O3

11 1793 1.5 Yes Yes CaO·2Al2O3

12 1843 1.5 Yes Yes CaO·2Al2O3

13 1873 1.5 Yes spinel with low Ca

In three samples, namely samples 5, 6 and 10, liquid CaO-Al2O3 solution phase was detected at the interface between spinel and liquid steel. In addition to the liquid phase, the compound CaO⋅2Al2O3 was also found in the interface of the 3 samples. It is noted that the composition of the liquid phase vary with position, indicating the existence of concentration gradients in the liquid. The compositions of the liquid phase determined by EDX analysis for these samples are presented in Figure 8. While CaO and Al2O3 are the major components of the liquid phase, small amount of MgO is also found to dissolve in the liquid phase. As an example, Figure 9 presents the mapped images taken from the interface of sample 6. The coexistence of the liquid solution and CaO⋅2Al2O3 is well brought out by the images. The layer of the liquid-CaO⋅2Al2O3 two-phase mixture formed in the interface was found to be several hundreds micron in thickness.

sample 5 sample 6 sample 10

CaO

Al2O3

MgO

mas

s%Ca

Om

ass%Al

2 O3

mass%MgO

80

60

40

2080

60

40

20

80604020

CaO·2Al2O3 MgO·Al2O3

sample 5 sample 6 sample 10

CaO

Al2O3

MgO

mas

s%Ca

Om

ass%Al

2 O3

mass%MgO

80

60

40

2080

60

40

20

80604020

CaO·2Al2O3 MgO·Al2O3

Figure 8 Compositions of formed phases in samples 5, 6 and 10 on the CaO-Al2O3-MgO ternary diagram

12

In four samples 8, 9, 11 and 12, CaO⋅2Al2O3 compound was found to be the only reaction product. In Figure 10, the mapped images taken from the interface of sample 8 are presented. It is clearly seen that a thin solid calcium aluminate layer, about 10μm is formed at the interface between spinel and liquid steel. Notice that the experimental temperatures for these four samples are all lower than 1873 K. As seen in Table 7, no new phase was detected at the interfaces of the rest of the samples of spinel-steel reaction. However, small amount of CaO was found to dissolve in the spinel phase.

Figure 9 Mapping images of the interface of sample 6 by SEM-EDX

Figure 10 Mapping images of the interface of sample 8 by SEM-EDX

To gain knowledge of the level of dissolved Ca in the liquid steel when liquid CaO-Al2O3 solution is formed, the concentrations of dissolved Ca in some of the steel samples were analyzed. The dissolved Ca concentrations in sample 5 and 6 were found to be 0.7 ppm and 0.8 ppm, respectively. The dissolved Ca level is reasonable in view of the formation of the liquid phase. It should be pointed out that the analysis of the dissolved Ca at such low concentration level is associated with considerable uncertainties. Hence, the present result could only be considered as semi-quantitative.

13

3.3 Activities of SiO2 in low SiO2 region of the CaO-Al2O3-SiO2(-MgO) system Using Eqs. [1-3], the activity of SiO2 can be calculated by Si content in Cu and the oxygen partial pressure.

)()()( 22 gOlSisSiO += [1]

65.1049799loglog2

21 +−=

⋅=

Tapa

KSiO

OSi [35] [2]

SiSiSi Xa ⋅= γ [3] where aSi is the activity of Si relative to pure liquid, while aSiO2 is relative to pure soild.

3.3.1 Activity of SiO2 in the CaO-Al2O3-SiO2 system The results of the chemical analysis of the slag samples are presented in Table 8. A comparison of the results with the weighed-in compositions in Table 2 indicates that the slag compositions did not change significantly during the experiments. Table 8 also presents the measured activities of SiO2 in the CaO-Al2O3-SiO2 ternary system.

Table 8 Analyzed compositions of the melts and SiO2 activities in the CaO-Al2O3-SiO3 system analyzed compositions (mass%) sample CaO Al2O3 SiO2

mass%Si in Cu aSiO 2 (×10-3)

301 49 40 11 0.38 1.02 302 55 36 9 0.24 0.64 303 47 44 9 0.34 0.91 304 39 52 9 0.62 1.64 305 48 46 6 0.27 0.72 306 54 40 6 0.07 0.19 307 40 54 6 0.37 0.99 308 46 50 4 0.14 0.38 309 40 56 4 0.18 0.47

ternary system

310 54 42 4 0.04 0.10

Based on the obtained values, iso-SiO2 activity curves were constructed in the iso-thermal section of the CaO-Al2O3-SiO2 system at 1873K. The contour lines are presented in Figure 11. The iso-activities curves estimated by Rein and Chipman[36] are also reproduced in the figure for comparison. The present activity values of SiO2 are higher than the suggestion by the other authors, in spite of the similarity of the shape. Moreover, the SiO2 activity obtained in the present study decreases more drastically with the increase of CaO content in comparison with the results by Rein and Chipman.[36] Note also that significant decrease in the SiO2 activity can be seen, as the composition approaches CaO-saturated line. According to the phase diagram of the CaO-Al2O3-SiO2 system,[37] tricalcium silicate, 3CaO·SiO2 phase exists in equilibrium with liquid solution in CaO-rich side at 1873K. The result of the present study can be verified using the Gibbs energy of decomposition of the tricalcium silicate.

)()(3)(3 22 sSiOsCaOsSiOCaO +=⋅ [4]

350.02.6211log 4 −−=T

K [38] [5]

The SiO2 activity at the coexisting point of CaO and 3CaO·SiO2 is evaluated to be 2.2×10-4, which is in very good accordance with the measured value as presented in Figure 11. This agreement would suggest the reliability of the present measurements.

14

(mas

s% C

aO) (m

ass% SiO80

60 40

20

(mas

s% C

aO) (m

ass% SiO80

60 40

20

(mas

s% C

aO) (m

ass% SiO80

60 40

20

(mas

s% C

aO) (m

ass% SiO

60 40

20

(0.0001)(0.001)

CaO Al2O3(mass% Al2O3)80604020

)

CaO Al2O3(mass% Al2O3)80604020CaO Al2O3(mass% Al2O3)80604020

2

CaO Al2O3(mass% Al2O3)80604020

(mas

s% C

aO) (m

ass% SiO

60 40

20

(mas

s% C

aO) (m

ass% SiO

60 40

20

(mas

s% C

aO) (m

ass% SiO

60 40

20

(mas

s% C

aO) (m

ass% SiO

60 40

20

(0.0001)(0.001)

CaO Al2O3(mass% Al2O3)80604020

)

CaO Al2O3(mass% Al2O3)80604020CaO Al2O3(mass% Al2O3)80604020

2

CaO Al2 3(mass% Al2O3)80604020

0.0010.0005

0.00020.0001

2CaO·SiO2

3CaO·SiO2

(mas

s% C

aO) (m

ass% SiO80

60 40

20

(mas

s% C

aO) (m

ass% SiO80

60 40

20

(mas

s% C

aO) (m

ass% SiO80

60 40

20

(mas

s% C

aO) (m

ass% SiO

60 40

20

(0.0001)(0.001)

CaO Al2O3(mass% Al2O3)80604020

)

CaO Al2O3(mass% Al2O3)80604020CaO Al2O3(mass% Al2O3)80604020

2

CaO Al2O3(mass% Al2O3)80604020

(mas

s% C

aO) (m

ass% SiO

60 40

20

(mas

s% C

aO) (m

ass% SiO

60 40

20

(mas

s% C

aO) (m

ass% SiO

60 40

20

(mas

s% C

aO) (m

ass% SiO

60 40

20

(0.0001)(0.001)

CaO Al2O3(mass% Al2O3)80604020

)

CaO Al2O3(mass% Al2O3)80604020CaO Al2O3(mass% Al2O3)80604020

2

CaO Al2 3(mass% Al2O3)80604020

0.0010.0005

0.00020.0001

2CaO·SiO2

3CaO·SiO2

Figure 11 Iso-aSiO2 lines in the iso-thermal section of the CaO-Al2O3-SiO2 system at 1873K,

solid lines: present work, dotted lines: Rein and Chipman[36]

3.3.2 Activity of SiO2 in the CaO-Al2O3-SiO2-MgO system The measured values of the SiO2 activities in the quaternary system along with the analyzed slag compositions are presented in Table 9. Most of the slag compositions were in the 10mass% MgO section. Two additional slag compositions, which were relevant to the inclusions found in the ladle, were also studied. Similar as the ternary system, no significant change in the slag compositions was noticed after the experiments.

Table 9 Analyzed compositions of the melts and SiO2 activities in the CaO-Al2O3-SiO2-MgO system analyzed compositions (mass%) sample CaO Al2O3 SiO2 MgO mass%Si in Cu aSiO2

(×10-4) 401 42 41 8 9 0.17 3.76 402 46 35 9 10 0.08 2.19 403 38 46 8 8 0.13 3.60 404 43 42 6 9 0.07 1.85 405 46 38 6 10 0.05 1.22 406 36 49 5 10 0.13 3.43 407 41 46 3 10 0.05 1.21 408 36 52 3 9 0.07 1.89 409 45 42 4 9 0.03 0.93

quaternary system

410 40 42 8 10 0.15 4.10 411 47 36 11 6 0.13 3.49 inclusions 412 48 31 15 6 0.23 6.20

A comparison of Tables 8 and 9 shows that the SiO2 activities in the quaternary system are generally lower than that in the ternary system, almost one order of magnitude. Since the content of MgO was only 10mass%, the basic behavior of MgO to decrease the SiO2 activity is very profound. Iso-activity contours of SiO2 in the CaO-Al2O3-SiO2-10mass%MgO system at 1873 K were drawn in the iso-thermal section in Figure 12. The figure shows again that the activity of SiO2 decreases as CaO content increases and Al2O3 content decreases.

15

0.00010.0002

0.0003 0.0004

(mas

s% C

aO) (m

ass% SiO

2 )

90CaO10MgO

90Al2O310MgO(mass% Al2O3)

80

80

60

60

4020

20(m

ass%

CaO

) (mass%

SiO2 )

90CaO10MgO

90Al2O310MgO(mass% Al2O3)

80

80

60

60

4020

20

0.00010.0002

0.0003 0.0004

(mas

s% C

aO) (m

ass% SiO

2 )

90CaO10MgO

90Al2O310MgO(mass% Al2O3)

80

80

60

60

4020

20(m

ass%

CaO

) (mass%

SiO2 )

90CaO10MgO

90Al2O310MgO(mass% Al2O3)

80

80

60

60

4020

20

Figure 12 Iso-aSiO2 lines in the iso-thermal section of the CaO-Al2O3-SiO2-10%MgO system at 1873K

16

3.4 Interactions and behavior of various inclusions in liquid steel

3.4.1 Attractive interaction of alumina inclusions A great number of inclusions were seen under the CSLM when Ar gas was employed. Post-analysis revealed that most of the inclusions were pure alumina. The high oxygen level (>2ppm) in Ar gas along with the high Al content (≈0.07mass%) in steel samples would lead to the formation of a big number of Al2O3 inclusions. It was found that the alumina inclusions collided very frequently, and the collisions always resulted in agglomeration. A series of images in Figure 13 demonstrate the process of the agglomeration of the alumina inclusions. It is clearly seen that several groups of inclusions collide and agglomerate within seconds. This observation is in accordance with the other researchers.[9, 10] It was also observed that the alumina inclusions often attracted each other. The attraction sometimes functioned even between long distance. Figure 14 shows the trajectories of alumina inclusions on the liquid steel surface. Four inclusions were traced along the frames. All the inclusions were alumina inclusions.

AA AAB

AB

A

(a) 0.0sec. (b) 1.0sec. (c) 2.0sec.

AB

C

AB

C

A

BC

A

BC

A

B

C

A

B

C

(d) 3.0sec. (e) 4.0sec . (f) 5.0sec.

Figure 13 Images from the confocal scanning laser microscope, showing the agglomeration of alumina inclusions

Figure 14 Trajectories of a group of the alumina inclusions, showing the self-attraction

The collision between inclusions A and B and consequently their agglomeration took place in about 3 seconds. Each frame was taken every 0.5sec. The moment when inclusion A floated up was regarded as t = 0.0. The initial distance between inclusions

17

A and B was 135μm. From the next moment, the inclusion B moves towards the inclusion A, while the inclusions A and other inclusions move in almost the same manner, which may be caused by surface movement. The approaching of inclusions A and B towards each other would be strong evidence that the attractive interaction is applied between the two inclusions. Immediately before the 2 inclusions start agglomerating, the distance between them is estimated as about 28μm. However, it is also seen that inclusions C and D maintain the inter-particle distance less than 100μm for about 3 seconds.

3.4.2 Behaviors of inclusions other than alumina When Ar+1%H2 gas mixture was used, the population of the inclusions observed under CSLM was lower than that in case of Ar gas. According to SEM analysis carried out after the experiments, the inclusions found in the samples were mostly spinel inclusions. The spinel inclusions had usually sharp edges (solid), which was a useful indication for identification of this type of inclusions during observation under CSLM. It was also found that most of the spinel inclusions were smaller than 10μm and the size of the inclusions hardly increased with time. No collisions as well as attractions were observed. The typical example of the movements of the spinel inclusions on the surface are shown in Figure 15. When inclusion A was traveling towards left-upwards along the surface flow, another inclusion (marked as B) floated up to the surface (see Figure 15 (b)). At the moment that inclusion B floated up, the distance between the 2 inclusions was about 25μm. Although this distance was shorter than the pre-agglomeration distance between the 2 alumina inclusions discussed earlier, the two inclusions kept the distance constant for 3 seconds without attracting or getting closer.

AAA

B25μm

AB

25μm

(a) 0.0sec (b) 1.0sec

AB

AB

AB

AB

(c) 2.0sec (d) 3.0sec

Figure 15 Images from the confocal scanning laser microscope, showing a pair of spinel inclusions staying apart from each other

The use of Ar+2%H2 gas mixture resulted in even lower oxygen partial pressure. In this case, the majority of the observed inclusions were another kind of small solid inclusions. The SEM analysis revealed that the compositions of the inclusions were

18

very similar to CaO·2Al2O3. Most of the inclusions were in the size range between 10~20μm. No agglomeration and consequently no growth in size were seen for this type of inclusions. This aspect is exemplified by the images in Figure 16. Figure 16 (b) shows one calcium aluminate inclusion (inclusion B) just floated up to the surface next to another (inclusion A). The distance between these two inclusions was less than 40μm. At the next moment, the inter-particle distance slightly increased. For more than 3 seconds, the inter-particle distance remained almost constant.

AA AB AB

(a) 0.0sec. (b) 1.0sec.

AB ABAB AB

(c) 2.0sec. (d) 3.0sec.

Figure 16 Images from the confocal scanning laser microscope, showing a pair of CaO·2Al2O3 inclusions staying apart from each other

It should be mentioned that in both cases of the solid calcium aluminate and the spinel inclusions, a huge number of inclusions were examined. Many of them experienced very short inter-particle distance. On the other hand, no attractive interaction among the inclusions was ever observed. It is reasonable to conclude that the solid calcium aluminate and the spinel inclusions rarely collide and agglomerate.

19

3.5 Behaviors of inclusions around open eye

3.5.1 Phenomena around open-eye and inclusion movements in the bath An open-eye would form when a critical gas flow rate was reached. Once an open-eye was formed, the oil layer exhibited a sphere-bed structure composed of many oil droplets, each of which was coated with a thin water film. This observation is in accordance with the previous study carried out by the present laboratory.[30] The movement of inclusions in water bath stirred with gas injection was examined by introducing imitating inclusions into the bath. In case of oil-water mixture, the size of the inclusions (tiny oil droplets) varied from 0.1mm to 5mm. However, attention was only given to the small droplets (less than 1mm), which could be relevant to the simulation of inclusions. The big inclusions floated up very quickly owing to the buoyancy force. The small inclusions once injected, moved towards the center of the water bath due to the momentum. In the vicinity of bubble-water plume, the tiny oil droplets started to follow the plume. Figures 17 (a)-(e) illustrate the movements of a number of oil droplets with time.

(a) t= 1.5sec. (b) t= 2.1sec. (c) t= 2.7sec.

(d) t= 3.6sec. (e) t= 4.2sec.

Figure 17 Movement of a group of fine oil droplets with time

20

When the inclusions reached the open-eye along the stream line of the flow, most of the inclusions entered into the openings between the rotating oil droplets in the sphere-bed of the oil layer. During the passage through the sphere-bed of the oil layer, a part of inclusions attached to the surface of oil droplets and captured, while the rest just passed through. The sphere-bed of the oil layer behaved like a filter to the inclusions passing through. Visual observations showed that the bigger inclusions had bigger chance to be trapped by the filter of the sphere-bed of the oil droplet and to be removed. Many small inclusions which were brought back to the water started approaching the plume again and moving up following the stream lines. This circulation was repeated 2~3 times before the majority of inclusions disappeared. Some smaller inclusions could still be observed to stay in the water bath even after more than 60 sec.

3.5.2 Attachment of inclusions with different sizes To study the efficiency of the sphere-bed of the oil layer in capturing the inclusions, the concentration of the inclusions in the water bath was studied. Charcoal particles were employed as inclusions in this series of experiments. The changes in the concentrations of inclusions with time are plotted in Figure 18 for a number of experimental conditions. The normalized concentration, viz. the ratio of the instant concentration over the initial concentration (the first sample) is used in Figure 18. Irrespective of the experimental conditions, the concentration decreases dramatically in the earlier stage. The rate of the inclusion removal decreases with gas blowing time as indicated by the slope. It is seen that the removal rate depends very much on the experimental conditions, especially on the size of the inclusions. Bigger particle size leads to much faster removal. Figure 18 also shows that the higher gas flow rate is associated with faster inclusion removal.

0 1000 2000 3000 4000 5000 60000.0

0.2

0.4

0.6

0.8

1.0

45μm, 100cts, 1.7Lmin-1

45μm, 100cts, 3.0Lmin-1

45μm, 50cts, 3.0Lmin-1

87μm, 50cts, 1.7Lmin-1

87μm, 50cts, 3.0Lmin-1

140μm, 100cts, 1.7Lmin-1

C /

C0

Time (s)

Figure 18 Normalized concentrations (C/C0) of charcoal particles as a function of time

21

4 Discussion

4.1 Formation of liquid inclusions before and after vacuum degassing

4.1.1 Formation of liquid CaO-Al2O3 inclusions from spinel The existence of spinel inclusions before vacuum degassing have been well explained in previous works.[4, 6, 28] At the same time, large numbers of inclusions with liquid oxide soultion were also observed. After vacuum degassing, inclusions of Type 6 are very often seen, as shown in Table 6. Figure 19 shows the SEM pictures of a typical example of Type 6 inclusions. The coexistence of the liquid calcium aluminate and the spinel phases are well brought out by the mappings of the elements.

50μm50μm

Figure 19 Mapping images of a calcium aluminate inclusion bearing a spinel island

The SiO2 content in the liquid phase of the inclusions exemplified in Figure 19 is usually less than 5 mass%. This content is much lower than the SiO2 concentration in the synthetic ladle slag. It is less likely that the liquid calcium aluminate inclusions are generated by the slag entrapment, since the compositions are very different. This type of inclusions could be either formed by the spinel-slag reaction or spinel-steel reaction or both. As presented in Table 7, no liquid and CaO⋅Al2O3 were detected in samples 1~4, which were not quenched. The sample 4 was even kept at 1873 K for 12 hours. Hence, only the results of quenched samples were used in the discussion. All the experiments with CaC2 addition carried out at 1873K revealed the formation of liquid phase and CaO⋅Al2O3 compound in the quenched samples. The formation of CaO-Al2O3 liquid phase can be explained by the exchange reaction between Ca and Mg.[6, 28] Thermodynamically, the spinel phase is expected to be in equilibrium with the liquid phase during the reaction. The composition of initially formed liquid phase would be at the 3-phase (liquid, spinel and CaO·2Al2O3) equilibrium point as shown Figure 8. In order to gain an insight of the reaction mechanism, the required Ca activity for the formation of the liquid phase is evaluated on the basis of the following reaction,

MgCaOOAlMgOCasOAlMgO +++=+⋅ 3232 2)(2 [6] The activity values of the oxide components in the initial liquid phase could be calculated using the KTH model, Thermoslag.[39] The dissolved Mg in the steel is

22

very difficult to determine. The total Mg content in the deoxidized steel is below 2 ppm.[28] It is reasonable to expect that the dissolved Mg is considerably lower than this level. For convenience, 2 ppm is used for the present discussion. The calcium content required to initially form the liquid phase estimated to be 0.14 ppm. In order to confirm this results, similar calculations are carried out using the activity values reported by Hino et al.[40] for the CaO-Al2O3-MgO system. As seen in Table 10, the required calcium content for the formation of the initial phase is about 5×10-5 ppm.

Table 10 Compositions and activity values of components of each liquid and Ca activity for their formation activity composition

(mole fraction) Thermoslag[39] Hino et al.[40]

CaO 0.37 aCaO 0.035 0.020 Al2O3 0.54 aAl2O3 0.404 0.050 MgO 0.08 aMgO 0.122 0.050

initial liquid

aCa (ppm) 0.141 5×10-4

CaO 0.51 aCaO 0.144 - Al2O3 0.31 aAl2O3 0.087 - SiO2 0.06 aSiO2 3.5×10-4 - MgO 0.12 aMgO 0.259 -

liquid inclusion

aCa (ppm) 0.242 -

Although the calcium activities to form the liquid solution predicated by the two models[39, 40] do not agree with each other, both models predict extremely low Ca activities for the formation of the initial liquid. As the dissolved Ca in samples 5 and 6 were found to be 0.7 ppm and 0.8 ppm, these levels of calcium contents would be thermodynamically sufficient to form the liquid phase. However, it must be pointed out that the Ca analyses might be subjected to considerable uncertainties. The amounts of Ca additions in the form of CaC2 were much high with respect to the analyzed Ca contents in the samples. For the inclusion shown in Figure 19, a calculation can be made using the composition of the liquid phase. A typical composition of the liquid phase found around the spinel phase is presented in Table 10 along with the evaluated oxide activities using Thermoslag.[39] As shown in the table, the formation of this liquid phase by reaction [6] would require a calcium activity of 2×10-5 (0.2 ppm). This would need very low concentration of dissolved Ca. However, the above consideration is only from thermodynamic view point. The formation of the liquid oxide layer around spinel phase would need certain amount of Ca. A Ca source is needed to pursue reaction [6]. Once the liquid phase is formed, another equilibrium must be considered. The other side of the product layer is in contact with liquid steel. Since dissolved Ca is supplied from liquid steel to the reaction site, the composition of the initial liquid phase would move to CaO-rich side. In fact, the liquid phase with high CaO content were frequently found in the vicinity of the interface in contact with steel in samples 5, 6 and 10. On the other side, the equilibrium between the spinel and the initial liquid phase should be maintained by calcium supply. Dissolved Ca, therefore, will diffuse through the liquid layer to the interface of the spinel and newly forming liquid phase. The formation of CaO·2Al2O3 phase between the spinel and the liquid phase would be caused by the local depletion of Ca.

It is seen in Table 7 that the reaction temperature and addition of Ca have great impact. The reaction between spinel inclusions and steel can be controlled by temperature.

23

For example, lower process temperature would lead to the formation of CaO⋅2Al2O3 on the spinel surface, while higher temperature would result in a product layer of CaO⋅2Al2O3 and liquid mixture. Moreover, the chemical compositions of steel and slag impose a great influence on the chemical characteristics of the liquid CaO-Al2O3 phase. The present work as well as previous studies[4-6, 28, 34 ] have often found inclusions having MgO in the centre surrounded by the oxide solution (Type 8) with other inclusions based on the oxide solution. The source for Type 8 inclusions has evidently been identified to be the ladle glaze.[4, 5, 34] The appearance of Type 8 inclusions only after vacuum degassing suggests that the entrapment of the ladle glaze takes place during vacuum degassing. This aspect is well explained by the increase of the number of inclusions > 11.3 μm during vacuum degassing, as shown in Figure 5 (b).

4.1.2 Equilibrium between liquid inclusions and liquid steel after vacuum degassing A comparison of the types of inclusions found before and after vacuum treatment in Period A and Period B reveals a considerable difference. As shown in Table 6, inclusions of Type 2 and Type 4 were not detected in the samples taken before vacuum treatment in Period A; but were found in the samples of Period B. While two types of inclusions (Types 7 and 8) were detected after vacuum treatment in Period A, 4 types of inclusions were found at the same stage in Period B. The results of Period A agree generally with the previous investigations.[28, 34] It should be mentioned that a clear difference in SiO2 content has been observed between Type 2 and Type 7. A further comparison of their compositions with the slag composition suggests that none of these two types are from the ladle slag. The coexistence of the two types of inclusions and the clear composition gap between them with respect to SiO2 would strongly suggest that these two types of inclusions are from different sources. In order to get better understanding of the difference between Period A and Period B, a thermodynamic consideration is necessary. An examination of the steel compositions in Table 4 reveals that the Al contents in Period A and Period B are considerably different. In view of the fact that the metal is deoxidized by aluminum and the system is far from homogeneity and thermodynamic equilibrium before vacuum treatment, the following reaction is considered.

( ) ( ) SisOAlAlsSiO 3243 322 +=+ [7]

43

32

72

32loglogAlSiO

SiOAl

aaaa

K⋅

⋅= [8]

For the first approximation, the dissolve Al is considered to be the same as the analyzed total Al listed in Table 4. This assumption is justified by the fact that the difference between the total Al and total O contents is much larger than the total O content (Hence, most of Al would be in the dissolved form). The total oxygen level has been reported as the order of 10ppm.[5, 6, 34] The dissolved Si content appears to be nearly constant regardless of the periods as shown in Table 4. Since rises to a power of 4 in eq. [8], while rises only to 2, it is expected that the content of aluminum in the steel would have strong impact on the inclusion chemistry. As revealed by eq. [8], the low Al content in Period B would possibly lead to high SiO

Ala

32OAla

2

24

activity in the inclusions. Hence, the inclusions of Type 2, which have higher SiO2 content, may exist in the steel even after vacuum treatment in Period B. For better understanding on the coexistences of the two types of liquid inclusions with different SiO2 contents at the end of the ladle treatment, the equilibria of the two types of inclusions with liquid steel are necessary to be discussed. For the purpose, a thermodynamic consideration using the activity of SiO2 in the inclusions is helpful. The activity of SiO2 in the equilibrium with the liquid steel is obtained using the following reaction.

OSisSiO 2)(2 += [9]

59.1130414loglog2

2

9 +−=⋅

=Ta

aaK

SiO

OSi [10]

[ ]ifa ii %= [ ]jefj

jii %log ∑= [11]

where aSi and aO are the activities of silicon and oxygen in liquid steel relative to a dilute solution; aSiO2 is the activities of SiO2 with pure solid as the standard state; [%i] and fi are the concentration of element i and its activity coefficient on a weight percent basis with dilute solution as its reference state; and ei

j is the first order interaction parameter of j on i. After vacuum degassing, most of the inclusions were calcium aluminate liquid inclusions.[6, 34] On the basis of the steel composition in Table 4, the activities of dissolved Si and O after vacuum degassing were evaluated to be 1.8 and 4.2×10-5, respectively. Correspondingly, the equilibrium activity of SiO2 after vacuum degassing could be evaluated as 1.4×10-4 using Eq. [10]. The composition range of the inclusions of Type 7 indicates that the activity of SiO2 in the inclusions is positioned between 0.0001~0.0002 in the iso-SiO2 activity contour shown in Figure 12. This is in very good accordance with the value 1.4×10-4, which is evaluated from aSi and aO in the steel. This agreement strongly suggests that the inclusions are in equilibrium or at least very close to the equilibrium with the steel after vacuum degassing. In the case of the liquid inclusions having high SiO2 content found after vacuum degassing, the experimentally determined SiO2 activities in Table 9 could be used directly for the discussion. While the two values (the last two rows) are somewhat higher than the aSiO2 in the inclusions with low SiO2 content, they could be considered to be quite similar. As mentioned in the previous section, the main reason for the coexistence of the two types of inclusions could be the difference in their CaO contents. The inclusions having higher SiO2 content have considerably higher CaO content. The difference between the measured activities in the last two rows in Table 9 and the value calculated from aSi and aO in the steel could be due to either or both of the following two factors. (i) The EDX analyses to determine the compositions of the inclusions were possibly associated with uncertainties. As shown in the last two rows in Table 9, slight change in the compositions would result considerable difference in the aSiO2. (ii) A final equilibrium between the inclusion with steel has not been reached. Nevertheless, the present results would suggest that the SiO2 activities of the 2 types of inclusions are rather close to each other. Both types of inclusions are very close to the equilibrium with the steel after vacuum treatment.

25

4.2 Change of inclusions during vacuum degassing

4.2.1 Number of bigger inclusions during vacuum degassing As seen in Figure 5 (b), the numbers of inclusions bigger than 11.3 μm show an increasing trend for all heats except for the heat using a fresh ladle. Although the numbers of inclusions > 11.3 μm are not very high and therefore associated with experimental uncertainties, the consistent trend shown in Figure 5 (b) still indicates that the number of inclusions >11.3 μm is not reduced by vacuum treatment. Similar results have been reported by Steneholm et al[41]. Generally, the inclusions bigger than 11.3 μm are more detrimental. Moreover, bigger inclusions are easier to float up and to be removed than the smaller ones taking into account of buoyancy force and drag force (by liquid metal) as shown Figure 18. Nonetheless, the bigger inclusions are increasing through vacuum degassing. Following this reasoning, it is expected that new inclusions in this size range are supplied during vacuum treatment. Previous studies have found that ladle glaze is one of the major sources supplying non-metallic inclusions.[4, 5, 29] Ladle glaze is formed during the casting process. The top slag adheres to the ladle lining, while it goes down following the steel melt during the draining of the ladle. The adhered slag will penetrate into the pores of the ladle wall. The studies have also shown that inclusions of Types 6-8 could all partially be generated by ladle glaze.[4, 29]

The structure of the glaze in the ladle for making tool steels has been systematically studied by Beskow et al.[4] Four layers are found in the wall of a magnesite-lined ladle, namely the original carbon bearing magnesium oxide, a decarburized layer, a slag infiltrated layer and the outer slag layer. Ladle glaze is mostly composed of the latter two layers. The outer layer of the glaze would be removed after the filling of the ladle in the next heat, while the slag-infiltrated layer would not be easily removed because of the MgO matrix. A strong stirring of the ladle by either induction or argon or both would flush off pieces of the slag-infiltrated layer resulting in inclusions of Type 7 and Type 8. It is reasonable to expect that the inclusions generated in this manner would be relatively bigger in view of the interfacial energy constraint and unevenness of the wall. It is worthwhile to mention that a considerable fraction of the inclusions bigger than 11.3 μm are Type 8. It is also noted that inclusions of Type 8 are never found in the ladle of age 1 by both the present authors and the previous work.[5, 29] The trend shown in Figure 5 (b) could be well explained by the contribution of ladle glaze. The present finding that vacuum treatment does not reduce the number of bigger inclusions in a used ladle is in conformity with the results of the previous studies[5, 6, 34,

41]. In a fresh ladle (age 1), no ladle glaze is attached on the walls. Because of the absence of inclusion supplied by the glaze, the numbers of inclusions in both size groups decrease as seen in Figures 5 (a) and (b). Hence, the results of ladle age 1 show another evidence of the effect of glaze.

4.2.2 Physical growth of various inclusions during vacuum degassing Bigger non-metallic inclusions would have better probability to be removed to the slag phase by buoyancy force. On the other hand, inclusions having bigger sizes are

26

more harmful when they remain in the final product. The previous section showed that the number of inclusions > 11.3 μm increased during vacuum treatment. The main reason of the increase of the bigger inclusions is expected to be related to ladle glaze. Nonetheless, the relationship between the number of inclusions and inclusions growth during vacuum degassing still needs to be understood. A good knowledge of physical growth of the inclusions due to agglomeration is essential to optimize the ladle processes. The CSLM results show evidently that alumina inclusions strongly attract each other and agglomerate readily. Consequently, the alumina inclusions grow fast by agglomeration. The growth of the alumina inclusions by self-agglomeration is well brought by Figure 20. The sizes of the alumina inclusions are in the order of 10μm in the initial state (Figure 20, (a)). On the other hand, many inclusions bigger than 50μm can be seen after only 60 sec. (Figure 20, (b)) As a matter of fact, the agglomeration behavior of the alumina inclusions has been well reported by many researchers.[9, 42, 43] Yin et al.[9] have also observed long-range attraction between alumina inclusions. These authors have also estimated the attraction force using the mass of particles and acceleration. It would be interesting to make similar calculation to see whether the attraction force due to the mass is a dominating factor in the attraction and agglomeration of the inclusions.

(a) before, t = 0sec. (b) after t = 60sec.

Figure 20 Images from the confocal scanning laser microscope, showing the growth of alumina inclusions

With some approximations, the attraction force is calculated using the product of the equivalent mass and acceleration. In Table 11, the ranges of calculated mass, volume, velocity and acceleration are summarized for the alumina inclusions that have been found to approach each other and agglomerate. The attractive force that results in approaching and agglomeration is in the order of 10-14~10-16N. This range agrees well with the previous study.[9]

Table 11 Estimated force range applied to the agglomerating alumina inclusions averaged diameter (m) volume (m3) mass (kg) acceleration (ms-2) force (N)

10-6~2×10-6 8×10-16~2×10-15 10-11~5×10-11 2×10-4~10-5 10-14~10-16

As seen in Figure 14, inclusions C and D did not approach towards each other, while inclusions A and B were self-attracting and agglomerating. The inter-particle distance between C and D (about 85μm) is considerably shorter than the initial distance between A and B (about 135µm). The image analysis showed that the sizes of C and D inclusions (16.5μm, 15.5μm) are somewhat smaller than those of A and B inclusions (21.0μm, 18.0μm). Theoretically, the gravitation force is proportional to

27

28

the masses of the moving objects and the reciprocal of square of the distance between them,

221

RMMFgravtation⋅

∝ [12]

The ratio of the attractive force between A and B over that between C and D is estimated to be 3.9. The big difference in the mass attractive force is in accordance with the difference in attraction between the two pairs of inclusions. It is reasonable to believe that the mass attractive force is one of the dominating factors in the attraction of the inclusions. Contrary to the alumina inclusions, no attraction or agglomeration was observed for spinel and CaO·2Al2O3 particles. Figure 21 presents the inter-particle distances as functions of time for different types of inclusions. The values marked for each line stand for the geometric average mass of the pair of inclusions observed. The great difference in the behavior of agglomeration between alumina inclusions and the spinel as well as CaO·2Al2O3 inclusions is well brought out by the figure.

0 1 2 3 4 50

20

40

60

80

100

120

140

160

12μm

alumina spinel CaO.2Al2O3

4.8μm3.8μm

4.5μm

8.5μm

7.5μm 10μm

16μm

20μm

inte

r-pa

rticl

e di

stan

ce (μ

m)

time (sec.)

Figure 21 Inter-particle distance of various types of inclusions as functions of time

To examine whether the attractive force is the only major force leading to the collision and agglomeration of the inclusions, the gravitation forces for different pairs of alumina, spinel and CaO·2Al2O3 inclusions using eq. [12] are evaluated on the basis of the CSLM observation. While spinel inclusions exhibit lower attractive forces due to their smaller sizes, the attractive forces between the CaO·2Al2O3 inclusions and between alumina inclusions lay at the same level. Nonetheless, CaO·2Al2O3 inclusions remain separated without attracting, despite shorter initial inter-particle distances. The comparable mass attractive forces between CaO·2Al2O3 inclusions and between alumina inclusions and their different behaviors in attraction suggest that the mass attractive force is not the only dominating force for the attraction and agglomeration. Though the capillary force also has been suggested to be responsible for the attractive interaction of inclusions, it can be applied only to liquid metal surface or slag/metal interface. Therefore, a comparison of the results of CSLM with the industrial data is absolutely essential to have a better grip of this aspect. A large number of steel

samples were analyzed under a light optical microscope. The samples were taken during ladle treatment in Uddeholm Tooling AB. The types and morphologies of inclusions were examined. In Table 12, the analysis results are summarized.

Table 12 Amount of different types of inclusions examined and their attachment and agglomeration stages

after deslagging

during deoxidation[28]

beforevacuum

after vacuum total

attachment agglomeration

analyzed area (mm2) 6525 ~2500 19873 22927 51825 inclusions observed 4934 ~2060 16315 5956 29265

oxide solution, high SiO2 (Type 2, 4) 65% ~60% 19% 11% No No

spinel (Type 3) 35% ~40% 33% - No No alumina (Type 5) - a few - - Yes Yes

oxide solution, low SiO2 (Type 6, 7, 8) - - 48% 89% No No

It was found that the steel samples taken during deoxidation at Uddeholm Tooling AB contained alumina clusters[28]. The existence of Al2O3 clusters in the steel sample is in very good agreement with the CSLM observation. As shown in Table 12, however, no indication of attachment or agglomeration of the inclusions except alumina is observed. At least some attachments of the spinel inclusions would have been found in the industrial samples, if the inclusions had had a tendency to attract and agglomerate in the ladle. The examination of almost 8000 spinel inclusions would provide a very strong evidence for the absence of the agglomeration of spinel particles. This industrial finding is also evidently supported by the CSLM results. Inclusions of Type 2, 4, 6, 7 and 8 in Table 6 have all a liquid oxide phase in contact with liquid steel. Unfortunately, only a few liquid inclusions were detected under CSLM. Since no pair of liquid inclusions close to each other was seen under CSLM, it is difficult to make conclusion based on the CSLM results. On the other hand, no evidence for the attachment of these types of inclusions was found in the examined more than 20000 inclusions in the industrial samples. It should be pointed out that two liquid spherical droplets would still be spherical when merging together, unless they were caught during the merging process. This fact would raise some difficulties for the conclusion regarding the attachment of the inclusions based on liquid calcium aluminate. Nevertheless, the absence of the sign of merging in the observation of more than 20000 inclusions would in some way support the argument that these types of inclusions might not attract each other. In this connection, it is worthwhile to mention the results reported by Yin and his coworkers.[10] They reported that inclusions based on liquid calcium aluminate did not attract with each other. The liquid inclusions have been found to maintain separated in spite of probable contact. Figure 22 is reproduced from their publication[10] showing the images of three liquid calcium aluminate inclusions at different moments taken by a confocal scanning laser microscope. It is seen that at the moments 10 sec. and 13 sec., two droplets are almost touching each other. However, instead merging together, they depart from each other. The conformity of the industrial data and the in situ observation by CSLM indicates that the growth of alumina inclusions needs consideration in ladle treatment. Compare

29

to the alumina inclusions, the growth by agglomeration or attachment is not likely to take place for the other types of inclusions, including the liquid calcium aluminate inclusion.

Figure 22 Images reproduced from H. Yin and coworkers [10], showing the liquid calcium aluminate inclusions departing from each other without agglomeration

30

31

4.3 Removal of inclusions during vacuum degassing

4.3.1 Mechanism of inclusion removal around open-eye The findings of the water model may throw some lights on understanding the phenomena around the open-eye in the ladle and inclusion removal during vacuum degassing. For the separation of the inclusions from the steel bath to the slag, the direct contact between the inclusions and the slag is necessary. Since it was reported that the major part of the metal coming in contact with the slag is brought up by the metal-gas plume,[30] most of the inclusions would be also brought into contact with slag by the argon-metal plume. Another necessary condition for the removal of inclusions from the metal phase to the slag phase is the energy criterion. The energy state of the inclusion-slag contact must be lower than the state of inclusion-metal contact for the inclusion removal to take place. The observation that oil-water mixture inclusions pass sphere-bed without attachment despite almost zero interfacial tension suggests again the importance of considering the energy barrier. Direct contact between the inclusion and slag is only a necessary condition but not the sufficient condition for inclusion removal. In fact, the sphere-bed of the oil layer acts as a filter. It captures a certain fraction of the tiny oil droplets or charcoal particles when they pass through the channels between the bigger oil droplets that form the sphere-bed. Based on the present observation, the mechanism of inclusion removal from steel to the slag around the open-eye in an argon stirred ladle could be suggested as following:

1. An open-eye is formed when a critical gas flow rate is achieved. 2. A sphere-bed slag layer composed of slag droplets, each covered by a thin

metal film, is formed around the open-eye. 3. Small inclusions follow the stream lines of the flow before they are brought

into contact with slag by the liquid metal. 4. Around the open-eye, metal along with the inclusions brought up by the plume

has good contact with the slag; and a large fraction of the metal along with the inclusions would pass through the sphere-bed of the slag droplets.

5. The sphere-bed of the slag droplets acts as a filter, which captures a fraction of inclusions and allows the rest of them to pass through.

6. The capture of an inclusion by the slag droplet in the sphere-bed requires energy fluctuation – An energy barrier must be overcome before the breaking of the metal film between the inclusion and the slag droplet.

7. The bigger inclusions may rebound and come back in new contact with the slag due to buoyancy force, while the smaller inclusions would follow the stream coming back to the metal bath. These inclusions need to wait for the new turn to be brought into the stream of the upward flow.

8. Outside the open-eye, metal along with inclusions can also be brought into contact with slag by shear flow.[30] This occasion can only occur when the inclusions are very close to the top slag.

This mechanism is schematically illustrated in Figure 23.

32

Figure 23 Schematic illustration of mechanism of inclusion removal to slag around open eye

4.3.2 Contribution of the open-eye in inclusion removal The present results show that the slag around open-eye acts as a filter when inclusions coming with the metal pass through it. The change of the number of inclusions in a control volume in which metal is in contact with slag can be described as,

tPV

VCC

Cell

metalPlume

o

Δ⋅⋅−= −&

ln [13]

where C(m-3) and C0 (m-3) are the instant and initial concentrations of inclusions, P is the probability of the capturing of inclusions by the slag droplets in the sphere-bed,

metalPlumeV −& (m3s-1) is the volume of metal passing through the sphere-bed per second

around the plume and VCell (m3) is the control volume under consideration. Since the sphere-bed of the slag around the open-eye acts as a filter, the probability of the capturing of inclusions passing through the bed would be proportional to the area of the cross section of the inclusions, and therefore proportional to the square of rinclusion, (rinclusion is the radius of the inclusions). The average size of the slag droplets in the sphere-bed is a function of the interfacial tension between slag and metal, the density of the slag, the viscosity of the slag and the velocity of the metal flow. Bigger droplet size would lead to bigger gaps between the droplets and thereby easier passage of the inclusions. Hence, P is expected to be inversely proportional to the area of the cross section of the slag droplet,

2

2

droplet

inclusion

RrP ∝ [14]

where Rdroplet is the average radius of the slag droplets.

It would be interesting to compare the contribution of the metal-gas plume in inclusion removal with that of buoyancy. To make this comparison, a ladle furnace of 65 ton capacity with an argon flow of 160 Lmin-1 is considered. The open-eye is about 0.3 m in diameter.[44] The data used in the calculation are summarized in Table 13.

Table 13 Physical parameters of a ladle used in the present calculation viscosity of liquid steel (kgm-1s-1) 0.007 density of liquid steel (kgm-3) 7000 density of inclusion (kgm-3) 3500 diameter (m) 2.2 depth (m) 3.0 flow rate (Lmin-1) 160 open-eye size (m) 0.3 surface velocity of liquid steel at the slag-metal interface (ms-1) 0.2 vertical velocity of liquid steel in the metal-gas plume (ms-1) 0.5

Outside the open-eye region, two forces exert on an inclusion rising in the liquid metal, viz. buoyancy force and drag force. Since the drag force is a function of the relative velocity of the inclusion with respect to the liquid metal, a terminal velocity of the inclusion will be reached. This terminal velocity, v∞ can be calculated by,

( )Fe

Feinclusioninclusion grv

μρρ −

=∞

2

92 [15]

where ρinclusion and ρFe are the densities of the inclusion and liquid metal respectively; and μFe is the viscosity of the metal. The calculated terminal velocities of inclusions of different sizes are presented in Table 14.

Table 14 Comparison of the contributions of the metal-gas plume and of buoyancy in inclusion removal diameter of inclusion (m) 1.0×10-5 2.0×10-5 5.0×10-5 1.0×10-4

weight of inclusion (kg) 1.8×10-12 1.5×10-11 2.3×10-10 1.8×10-9

buoyancy force (N) -3.7×10-11 -2.9×10-10 -4.6×10-9 -3.7×10-8

terminal velocity of inclusion (ms-1) -2.8×10-5 -1.1×10-4 -6.9×10-4 -2.8×10-3

traveling time (s) 5 5 5 5 m ximum thickness (m) a 1.3×10-4 5.3×10-4 3.3×10-3 1.3×10-2

V．

Buoyancy-metal (m3s-1) 1.0×10-4 4.2×10-4 2.6×10-3 1.0×10-2

F Buoyancy-inclusion (s-1) 9.1×10-6 3.6×10-5 2.3×10-4 9.1×10-4

F uoyancy-inclusionB (min ) -1 5.5×10-4 2.2×10-3 1.4×10-2 5.5×10-2

V．

Plume-metal (m s ) 3 -1 3.5×10-2 3.5×10-2 3.5×10-2 3.5×10-2

F Plume-inclusion (s-1) 3.1×10-3 3.1×10-3 3.1×10-3 3.1×10-3

F Buoyancy-inclusion (min-1) 0.186 0.186 0.186 0.186 * A negative sign stands for the upward direction.

The maximum horizontal traveling time of an inclusion in the vicinity of the slag-metal interface outside the open-eye region could be evaluated by the horizontal velocity of the metal at the interface and the radius of the ladle. In an argon-stirred ladle, the velocity of the metal at the slag-metal interface could be higher than 0.3ms-

1.[44] A lower value, 0.2ms-1 is used to estimate the maximum horizontal traveling time of an inclusion. It is about 5 seconds. On the basis of the terminal velocity of an inclusion and the maximum horizontal traveling time, the maximum thickness of the metal layer, wherein, the inclusion would have the possibility to meet the slag, can be evaluated. The estimated maximum thicknesses for different sizes of inclusions are listed in Table 14. The volume of metal brought into contact with slag per second by horizontal flow outside the open-eye area could also be estimated using the horizontal velocity of the metal at the slag-metal interface and the maximum layer thickness. This volume is denoted as (mmetalBuoyancyV −

& 3s-1). The calculated values of for different sizes of inclusions are also included in Table 14. The maximum number of inclusions that have the possibility to meet slag per second outside the open-eye region can be expressed by,

metalBuoyancyV −&

33

metalBuoyancyinclusionBuoyancy VCN −− ⋅= & (s-1) [16] As a first approximation, the volume concentration of inclusions, C, could be assumed to be uniform in the ladle. Hence, the maximum fraction of inclusions brought into contact with slag per second by buoyancy force could be estimated by

Total

metalBuoyancy

Total

metalBuoyancy

Total

inclusionBuoyancyinclusionBuoyancy V

VVC

VCVC

NF −−−

− =⋅

⋅=

⋅=

&& (s-1) [17]

where VTotal stands for the total volume of the liquid metal. The FBuoyancy-inclusion values evaluated for different sizes of inclusions are also given in Table 14. The inclusions in the metal-gas plume are mostly brought upwards by the plume, while the contribution of buoyancy force in the plume is negligible. It is reasonable to assume that the metal along with the inclusions brought to the open-eye in the metal-gas plume has good contact with the slag around the open-eye when coming back to the metal bulk. Hence, the fraction of inclusions brought into contact with the slag per second by the metal-slag plume could be estimated by,

Total

metalPlumearea

Total

metalPlume

Total

inclusionPlumeinclusionPlume V

vAVC

VCVC

NF −−−

−

⋅=

⋅⋅

=⋅

=&

(s-1) [18]

where NPlume-inclusion is the number of inclusions brought into contact with the slag per second by the metal-gas plume, Aarea is the area of the cross section of the plume and vPlume-metal is the velocity of the metal in the plume at the level of the slag-metal interface. The evaluated FPlume-inclusion values are also included in Table 14. As seen in Table 14, the fractions of inclusions brought into contact with slag per unit time by buoyancy force are very low. In the case of small inclusions having size of 10μm, the fraction of inclusions having the possibility to reach the slag per minute is as small as 5×10-4. Even for the inclusions of 20μm the FBuoyancy-inclusion is very low. On the other hand, the big inclusions, > 50μm would have much higher fraction to reach the top slag by buoyancy force. A comparison of the values of FBuoyancy-inclusion and FPlume-inclusion shows that, irrespective of the size of the inclusions, the fraction of inclusions brought into contact with the slag by the metal-gas plume is much larger than the fraction due to buoyancy. This is especially true for smaller inclusions. The ratio of FPlume-inclusion over FBuoyancy-inclusion for inclusions of 20μm is as high as 80 and for inclusions of 10μm, 340. Although a number of assumptions are made to make the comparison, Table 14 shows evidently that the metal-gas plume in a gas-stirred ladle plays much more important role in bringing the inclusions into contact with the slag phase in comparison with the buoyant mechanism. As a matter of fact, whether the inclusions are dragged up by the plume or brought up by attaching on the bubbles would not change the above discussion, as they can only be removed by the metal-slag contact. If the above discussion is basically correct, the removal of inclusions in an argon-stirred ladle would be mostly by the contribution of metal-gas plume. Consequently, relationship [14] should hold. On the basis of the experimental data in Figure 18 and

the values of Cell

metalPlume

VV −&

, the probabilities can be evaluated. The control volume could

be regarded as that of water volume, approximately 8L; and can be estimated as the volume of water brought up to the open-eye by the plume per second.

metalPlumeV −&

34

35

Using the previous approximation, the values of metalPlumeV −& are estimated to be

0.00031 and 0.00035 m3s-1 for the flow rates 1.7Lmin-1 and 3.1Lmin-1, respectively.

The plot of P against 2

2

droplet

inclusion

Rr is presented in Figure 24. It is seen that the regression

shows reasonably good linearity. It is a further verification of the finding that the contribution of the metal-gas plume plays the major role in the inclusion removal. It should be pointed out that the regression does not go through the origin. This could plausibly be attributed to the separation of inclusions due to buoyancy force.

0 10 20 30 40 50 600.00

0.01

0.02

0.03

0.04

prob

abili

ty

r2/R2 X 106

Figure 24 Dependence of P on 2

2

droplet

inclusion

Rr

4.3.3 Distribution of inclusions in a ladle As mentioned in the process description, the liquid bath is subjected to very strong induction stirring during deoxidation. As a consequence, inclusions would be uniformly distributed in the melt before vacuum, as shown in Figure 6. After the vacuum treatment and the flotation period (about 10 min), the situation would be quite different. As shown in Figure 5, the total number of inclusions decreases substantially after vacuum treatment. This decrease would have certain effects on the inclusion distribution. The flotation period, wherein the metal bath is weakly stirred by one-direction induction would also have impact on the inclusion distribution. Since one direction stirring is applied, the system would be close to steady state after a few minutes. Hence, a fixed flow pattern would have been established when the samples are being taken. In this period, the stream lines at position P3 (see Figure 2) would direct upward in the upper half of the bath, while a downward flow would be expected at position P1. The velocity of the liquid steel at the slag/metal interface would not be high. The relatively low velocity at the interface would favor the removal of inclusions, especially the bigger inclusions in the region close to the interface. It is seen in Figure 7 that the numbers of inclusions at depth D1 from the slag/metal interface have the lowest values for both size groups regardless of the horizontal position. The difference of the number of inclusions between 10% depth (D1) and the other two depths (D2 and D3) are more profound in the case of inclusions bigger than 11.3 μm.

It implies that bigger inclusions are more easily removed to the slag, in accordance with Figure 18. It is well known that drag force plays a very important role in the movement of inclusions in a liquid bath. As shown in Figure 17, the inclusions were found to follow the stream lines of the flow. It would be interesting to examine the distribution of inclusions at D2 and D3. As discussed above, the liquid metal flows upward at P3 and downward at P1. Accordingly, the numbers of inclusions increase from depth D3 to depth D2 at P3 (upward flow) due to accumulation and decrease from depth D3 to depth D2 at P1 (downward flow) for both size groups. This reasoning could not be applied to depth D1, since a large fraction of inclusions would be removed to the slag at this level as previously discussed. As reported earlier, it is difficult to further reduce the number of inclusions beyond a certain level.[41] While the supply of new inclusions (usually bigger than 10 μm) by the glaze is greatly responsible for this difficulty, the poor efficiency of the inclusion removal stands for another one. To improve the inclusion removal an optimization of the flow in the bath would be also essential. The following factors could be taken into account in the optimization.

1. Inclusions are mostly removed by the slag. An increase of the rate of metal-slag contact would increase the probability of the capturing of inclusions by slag.

2. Higher momentum of the liquid steel at the ladle wall would enhance the erosion and therefore increase the number of inclusions, especially the bigger ones. The velocity of the liquid steel at the ladle walls should be modest.

3. An optimized velocity at the slag/metal interface is necessary. Too low velocity would reduce the rate of metal-slag contact, while too high velocity would reduce the probability of the capturing of the inclusions by slag.

4. The stream lines are also required to be optimized to bring the inclusion to the upper part of the ladle.

36

5 SUMMARY Non-metallic inclusions during vacuum degassing in ladle treatment were studied with emphasize on liquid calcium aluminate inclusions in the present study. A large number of steel samples were taken in the ladle process. Inclusions in these samples were carefully analyzed. Laboratory experiments were also carried out to gain a better understanding of the behaviors of the inclusions in ladle treatment. On the basis of both industrial study and laboratory experiments, the formation of the inclusions involving CaO-Al2O3 phase and their equilibrium with liquid steel were discussed. The physical growth by agglomeration and the removal of the inclusions around the open-eye region were also investigated. Before vacuum degassing, spinel inclusions and the liquid calcium aluminate inclusions were often observed. The results of laboratory experiments showed that the reaction between the spinel and dissolved Ca in the liquid steel was a very important formation mechanism of the liquid calcium aluminate inclusions having low SiO2. On the other hand, after vacuum degassing, liquid calcium aluminate inclusions with low SiO2 and high SiO2 contents were found to coexist, in spite of considerable difference in SiO2 content. To understand this coexistence, the activities of SiO2 in CaO-Al2O3-SiO2-MgO liquid solutions having low SiO2 contents were measured using slag-Cu equilibrating technique. The SiO2 activity values of the 2 types of inclusions were quite similar, which was in accordance with the industrial observation. The number of bigger inclusions >11.3 μm increased during vacuum degassing. The increase of the bigger inclusions could be explained by the entrapment of ladle glaze during vacuum degassing. Using a laser confocal scanning microscope, the behaviors of inclusions in liquid steel were examined. Alumina inclusions had strong attractive interaction and grew rapidly by agglomeration. Contrary to the alumina inclusions, no interaction and agglomeration were observed in the case of spinel and CaO·2Al2O3 inclusions. This finding agreed well with the industrial observation, that no indication of agglomeration of these types of inclusions was found in a huge number of steel samples. Cold model experiments were carried out to study the phenomena around the open-eye in a gas-stirred ladle. It was found that most inclusions followed gas-water plume up to the open-eye and passed the sphere-bed. A fraction of inclusions were trapped by the sphere-bed, while the rest followed the water back to the water bulk. The open-eye area appeared to be very effective in inclusion removal. Industrial experiments showed that the distribution of inclusions after vacuum degassing was strongly affected by stirring conditions. The inclusions, especially the bigger ones, were readily removed near slag layer during the vacuum treatment. Evidence for the movement of the inclusions with the stream lines of the flow was observed. The results of the present work provide both experimental and theoretical base for further study on the improvement of inclusion removal and inclusion control.

37

6 FUTURE WORKS • It was found that liquid CaO-Al2O3 inclusions might be formed by the reaction

between spinel inclusions and dissolved Ca in liquid steel. Although dissolved Ca level to form liquid CaO-Al2O3 phase would be very low, a more systematic study on the Ca supply would be interesting. The activity data of other components in the liquid inclusions than SiO2 would be essential to study the equilibrium between the inclusions and liquid steel. Other mechanisms of formation of the liquid calcium aluminate inclusions are also needed to be considered.

• The inclusion removal still needs to be quantitatively developed. It was also found

that strong stirring in a ladle would improve slag/metal contact and the efficiency of inclusion removal. However, the erosion of the ladle wall would be significant at high stirring condition. Therefore, an optimization of stirring condition in a ladle should be further considered.

• Better knowledge of the properties of liquid calcium aluminate inclusions will be

helpful to improve inclusion control.

38

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