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Keywords:

  • inclusion removal;
  • liquid;
  • non-liquid;
  • RH degassing

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Methods
  5. 3 Results and Discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

Inclusion morphology, composition and number at different times of RH vacuum treatment were investigated on API-X70 pipeline steel with ASPEX. It is found that the inclusions are mainly globular CaO–Al2O3–MgO. The inclusion number decreases with time during RH treatment and the removal ratio can reach 72% after 28 min treatment. The initial number of inclusion plays a vital role in final inclusion number, so efforts must be made to eliminate the inclusion before RH treatment to get a highly clean steel. Non-liquid inclusions including C3A (C is CaO and A is Al2O3) and Al2O3 · MgO decrease much more quickly and thoroughly than liquid ones, C12A7. This can be explained by the fact that non-liquid inclusions have a much higher contact angle and interfacial energy but a much lower work of adhesion than liquid ones between inclusion and steel melt.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Methods
  5. 3 Results and Discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

Non-metallic inclusions are mostly formed by deoxidation in the steelmaking process. The presence of non-metallic inclusions gives rise to various problems in the steelmaking process and steel products. Therefore the inclusions should be removed as much as possible before casting. Generally RH is used to de-H, de-N, and de-C, but increasing attention is given to its strong ability to removal inclusions.

Shirabe and Szekely[1] recognized that in essence RH system functions as mixers, rather than purely degassing units, and a key role attributed to these operations was their function to promote inclusion removal. They thought that the dramatic reduction of the total oxygen content after RH was largely attributed to the removal of the inclusions. Soejima et al.[2] briefly examined the inclusion behavior during the RH treatment with emission spectroscopy. They showed that alumina inclusions decreased with time and found it took 8 min and 2–3 min to eliminate alumina inclusions to a steady extent when the initial [O] before Al addition is 182 and 5 ppm, respectively. Miki et al.[3] also found that the RH degasser is capable of lowering Al2O3 content from more than 150 ppm to less than 50 ppm in 12 min. Murai et al.[4] found total oxygen decreased from 80 to 15 ppm in 15–20 min during the RH degassing process. And the total oxygen comes from Al2O3 inclusion. Tanaka et al.[5] found that total oxygen decreased from 22 to 28 ppm at 15 min to 14–22 ppm at 25 min by RH circulation, which was attributed to the inclusion removal. Recently, Katsuaki et al.[6] proposed that RH process time should be elongated up to the maximum duration to keep high productivity in continuous casting to get a steel with fewer inclusions.

All these studies have noticed the significant function of RH to remove inclusions. However, most of them focus on the change of total oxygen,[2-5] rather than on non-metallic inclusion which has a more direct relation with the steelmaking process and steel products. If any, they are limited to Al2O3, other types of inclusion such as calcium aluminates with low melting temperature have never been studied. In addition, the variation of inclusion during the RH vacuum treatment has rarely been investigated so far.

The purpose of this paper is as follows:

  1. To study the transient behavior of inclusion including morphology, composition and number during RH treatment combined with steel and slag composition.
  2. To show light on which kind of inclusion can be removed easily from steel during RH degassing, then adjust steel or slag composition to get the desired inclusion based on slag/steel/inclusion equilibrium.
  3. To determine how much RH circulation time is needed to get the aimed inclusion content matching the required quality.

2 Experimental Methods

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Methods
  5. 3 Results and Discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

The industrial experiment involving two heats was conducted on API-X70 pipeline steel in NISCO steel plant in China. The steelmaking route starts with a blast furnace, followed by powder injection desulfurization, then BOF and a ladle furnace and RH vacuum treatment (RH-MFB), finally a continuous slab caster. The composition adjustment has been completed in LF, and no additives were made during RH vacuum treatment, since it disturbs the high circulation rate of molten steel[6] and increases inclusions.[2] RH-MFB capacity is 150 ton/heat, the flow rate of circulation gas (Ar) is 2000 Nl/min, the circulation rate is 121 ton/min. It needs 3 min to obtain vacuum level of 5 mbar under which the steel is degassed. It should be noted that the RH operating conditions, such as temperature and circulation time, are very steady, and nearly the same for every heat including the two heats in this study.

In order to investigate the behavior of inclusion during the RH treatment, steel samples are collected at times of 0, 6, 10, 16, 22, and 28 min at the same position. And slag samples are collected at 0 and 28 min. Zero minute was the time at which RH begins to be vacuumed.

Steel samples are automatically prepared by System Laboratory with Rubin 530 which is an electronically controlled grinding and polishing device. To get a much more reliable experiment result, a much larger scan area is adopted in this study. The scan area is around 100 mm2, and thousands of particles are detected. The onerous work is done by ASPEX PSEM EXPLORER with Automated Feature Analysis system, which can record the inclusion feature quickly and automatically, including chemistry, morphology, number and so on. Inclusions less than 1 µm are not detected in this study.

The acid-soluble [Al]s and [Ca] contents of steel are analyzed by ICP-AES method. Total oxygen content in the steel sample is determined by fusion and infrared absorption method. Composition of slag is analyzed by an X-ray fluorescence spectrometer.

3 Results and Discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Methods
  5. 3 Results and Discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

3.1 Chemical Compositions of Steel and Slag

The chemical compositions of steel and slag of the two heats are given in Table Table 1 and Table 2, respectively. [N] decreases 0.0013% for both heats during RH vacuum treatment, indicating that RH has a strong ability to remove nitrogen. Besides, T.O is reduced 0.0008% and 0.0016% to 0.0016% and 0.0013% after RH treatment, thus it can be concluded that RH also has a strong ability to remove inclusions because T.O is mainly in the form of inclusion when [Al]s is between 0.027% and 0.048% in this study. [Al]s, [Ca], and [Mg] in the steel decrease during the RH vacuum treatment. [Al]s decreases 0.0150 and 0.0095% and [Ca] decreases 0.0009 and 0.0008% for Heat 1 and 2. [Mg] decreases 0.0003% for both heats. The loss of [Al]s could be attributed to the reduction of FeO in slag, and the reactions can be written as Equation (1), which could introduce Al2O3 inclusions in steel. And [Ca] and [Mg] are mainly lost in the form of vapor due to high vapor pressure of calcium and magnesium and the high vacuum degree during RH vacuum treatment as well as the flotation of Ca and Mg-containing inclusions. Slag composition nearly keeps constant except FeO and MgO. FeO decreases due to the reduction of FeO by [Al]s for both heats but decreases much more in Heat 1, nearly double the amount of Heat 2. This is the reason why [Al]s in Heat 1 decreases much more than Heat 2. MgO in slag increases, this can be caused by severe erosion of refractory and the absorption of Mg-containing inclusions into slag.

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Table 1. Steel composition and temperature before and after RH treatment, mass%
HeatSample[C][Si][Mn][Al]s[Ca][Mg]T.O[N]T [°C]
1Before RH0.0540.241.560.04200.00280.00090.00290.00401640
1After RH0.0540.241.530.02700.00190.00060.00130.00271585
2Before RH0.0640.251.480.04800.00210.00070.00240.00331641
2After RH0.0670.251.500.03900.00130.00040.00160.00201590
Table 2. Slag composition before and after RH treatment, mass%
HeatSampleCaOSiO2Al2O3MgOMnOFeO
1Before RH63.7912.0414.292.420.151.28
 After RH63.0211.9314.913.840.160.64
2Before RH66.198.2017.312.510.130.82
 After RH67.478.1915.313.370.170.51

3.2 Evolution of Non-Metallic Inclusions

3.2.1 Morphology and Composition of Inclusions

Figure 1 shows the typical inclusion morphology and composition of both heats during RH vacuum treatment. As can be seen, the inclusions are globular and the composition is mainly CaO–Al2O3–MgO throughout the RH treatment.

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Figure 1. Typical inclusion morphology and composition during RH treatment. a) 0 min, b) 6 min, c) 10 min, d) 16 min, e) 22 min, f) 28 min.

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CaO–MgO–Al2O3 ternary diagrams are applied to demonstrate the composition change of inclusions in the present study, since the main components of inclusion are the three oxides CaO, MgO, and Al2O3, of which the sum is not less than 85% for all samples. The inclusion compositions of all samples for two heats are shown in Figure Fig. 2 and Fig. 3, respectively. In these two figures, each open circle represents an individual inclusion, and the filled square represents the average composition of all inclusions detected. The line is calculated by Thermo-Calc within which is the liquid region below 1873 K. The inclusions in this region are fully liquid and classified as C12A7 type inclusions, while those outside this region are non-liquid ones including fully solid or solid + liquid complex inclusions and can be divided into two types based on the composition. One is C3A type inclusions of which the CaO content is over 50%, the other is Al2O3 · MgO type inclusions of which the CaO content is less than 50%. From the two figures, it can be found that inclusions at different times mainly are C12A7, C3A and Al2O3 · MgO in both heats. The average compositions of inclusions are locating in the liquid region for all samples, but changes slightly with time. Al2O3 increases while CaO and MgO decreases, which are caused by the loss of [Ca], [Mg] and the reaction expressed by Equation (1).

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Figure 2. Change of composition distribution of inclusions with time during RH vacuum treatment in Heat 1. a) 0 min, b) 6 min, c) 10 min, d) 16 min, e) 22 min, and f) 28 min.

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image

Figure 3. Change of composition distribution of inclusions with time during RH vacuum treatment in Heat 2. a) 0 min, b) 6 min, c) 10 min, d) 16 min, e) 22 min, and (f) 28 min.

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3.2.2 Number of Inclusions

Figure 4 shows the change of inclusion number with time during RH treatment. It can be observed that the total number of inclusion decreases with time. And that is the same for both fully liquid and non-liquid inclusions. And the removal ratio of inclusion for Heat 1 and 2 can reach up to 63% and 72% after 28 min treatment. As for the decrease of the inclusion, Miki et al.[3] thought inclusion coagulation due to collision are mainly responsible for the decrease in inclusion population for the small inclusions less than 5 µm, and inclusion flotation by attachment to argon bubbles decreases mainly the number of large inclusions. They are removed to the slag layer above the ladle or the top free surface of the RH degasser. It can also be observed that liquid inclusions are far more than non-liquid ones. It means that the C12A7 type inclusions are much more than the total of C3A and Al2O3 · MgO type inclusions.

image

Figure 4. Change of inclusion number during RH treatment. a) Heat 1, b) Heat 2.

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Figure 5 compares the variation of inclusion number with time for two heats. In Y axis, t is time (min), Nt and N0 are inclusion number at t and the start of the RH treatment. As seen in Figure 5, the inclusion number decreases but the decreasing rate slows down with time. In addition, the trends of Nt/N0 for both heats are almost the same. The result indicates that under the same operating conditions, the initial number of inclusion plays a vital role in the final inclusion number after RH treatment, the more the initial inclusions, and the more the final inclusions. So efforts must be made to control the inclusion number before RH treatment to get a highly clean steel in a short time. Also it can determine how much time is needed to get the aimed inclusion content based on Figure 5. Since inclusion is closely related to the defects of steel,[2] and different steel grade has a different requirement on inclusion content. It made possible to select the optimum operating condition matching for the required quality, such as RH circulation time.

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Figure 5. Comparison of inclusion number with time.

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Figure 6 shows the variation of the inclusion number of different types with time in each heat. For both heats, both liquid and non-liquid inclusions decrease with time, but non-liquid inclusions decrease faster than liquid ones at the beginning of RH treatment. And the removal ratios, which can be calculated by Equation (2), are different, it only needs 6 min for non-liquid inclusions to get a removal ratio of 60%, while it costs 28 min to get the same removal ratio for liquid ones. And the removal ratio is 60% for liquid inclusions and 80% for non-liquid ones after the RH treatment. These results indicate that non-liquid inclusions can be removed more easily and thoroughly during the RH treatment.

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Figure 6. Change of inclusion number of two types with time. a) Heat 1, b) Heat 2.

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Murai et al.[4] found the removal of total oxygen content (T.O) during RH treatment is in the form of Equation (3), of which C is T.O and K0 is the removal rate of T.O. Similar to their result, the removal of inclusion during RH is also in such form. But in this study, C is Nt/N0 and the K0 is the inclusion removal rate. The calculation result is shown in Figure 7, the ln(Nt/N0) is the mean value of two heats, and the absolute value of the slope is the inclusion removal rate K0. It can be observed that ln(Nt/N0) is proportional to time and decreases steadily throughout the time for liquid inclusions, but for non-liquid inclusions, it decreases quickly at 6 min, then slows down. The difference could be explained as follows: for non-liquid inclusions, they are abundant in the first 6 min, and they are easy to collide into bigger ones, so they are removed faster. After 6 min, most of the inclusions have been removed and the number of remaining inclusions is so small that the collision and coagulation hardly happen even though a big stirring energy exists, so the remaining inclusions can only be removed in the form of small mono-particles, rather than big coagulated ones. But the removal rate of smaller inclusions particles is slower than that of bigger ones,[4] so the removal rate slows down after 6 min. As for liquid inclusions, however, the inclusions are abundant throughout the time, they can collide and coagulate into bigger ones and get removed, so the removal rate does not change. Furthermore, it could be concluded that in the first 6 min both non-liquid and liquid inclusions are removed in coagulated forms, and the non-liquid inclusions are removed faster than liquid ones, which are 0.150/min and 0.035/min. After 6 min, non-liquid inclusions are removed as mono-particles and liquid inclusions are still removed in coagulated forms, non-liquid inclusions are smaller than liquid ones, but non-liquid inclusions are decreased in almost the same speed as liquid ones, both are 0.035/min. All this could prove that non-liquid inclusions are removed more easily than liquid ones

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Figure 7. Comparison of removal rate for liquid and non-liquid inclusions with time.

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As mentioned above, liquid inclusions are C12A7 type, while non-liquid inclusions can be divided into C3A and Al2O3 · MgO type inclusions. Figure 8 shows the removal behavior of C3A and Al2O3 · MgO type inclusions. It can be observed that the removal ratio of C3A type inclusions are nearly the same in both heats, and that's also the case for Al2O3 · MgO type inclusions. Just like the removal behavior of total non-liquid inclusions in Figure 7, both C3A and Al2O3 · MgO type inclusions decrease quickly in the first 6 min, then slow down. And they are removed easily than C12A7 type inclusions. These results obtained from Figure Figs. 7 and 8 indicate that there must be some property that can determine the similarity of removal behavior of either liquid (C12A7) or non-liquid inclusions (C3A and Al2O3 · MgO) in both heats, and might explain why non-liquid inclusions can be removed more easily than liquid ones.

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Figure 8. Comparison of removal ratio for C3A a) and Al2O3 · MgO b) with time.

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The contact angle, work of adhesion and interfacial energy between steel melt and inclusion are employed to explain the different removal behaviors of liquid and non-liquid inclusions.

Figure 9 shows a sketch of the contact angle. Figure 9a shows the case when either inclusion or steel melt is solid (Case 1), while Figure 9b shows the case when both inclusion and steel are liquids (Case 2). In the figure, θC is the contact angle, α is the visible contact angle. γSI, γSG and γIG are interfacial energy of steel-inclusion, steel-gas and inclusion-gas, respectively. The γSI in Figure 9a and 9b can be calculated by Equation Eq. (4) and Eq. (5), respectively. The work of adhesion, which is the work (Wad) required separating the inclusion form steel melt, is represented by Equation (6).

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Figure 9. A sketch of the contact angle. a) Case 1, b) Case 2.

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Since it's impossible and unnecessary to study the removal behavior of every single particle of thousands of inclusions, average compositions of C12A7, C3A and Al2O3 · MgO in this study are applied. Due to the similar inclusion behavior of both heats and nearly consistent inclusion composition in each heat with time during RH treatment, the first sample at RH 0 min in Heat 1 is selected, and the composition is shown in Table 3. As can be seen, the C3A type inclusions have a higher CaO content, and C12A7 type inclusions have a medium CaO content, while Al2O3 · MgO type inclusions have higher Al2O3 and MgO.

Table 3. Average composition of inclusions at RH 0min in heat 1
ParticleOxideAl2O3MgOCaO
LiquidC12A751841
Non-LiquidAl2O3 · MgO64297
C3A34263

The surface energy γSG is 1844 cm−2 calculated by Equation (7).[7] The [O] and [S] used in this calculation is 0.00026% and 0.0012%. The [O] is calculated by Equation (8) assuming the activity of Al2O3, the activity coefficient of [O] and [Al]s are unity and [Al]s at RH 0 min in Heat 1 is 0.042%[8]

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Table 4 shows the interfacial energy between inclusion and gas (γIG), contact angle (θC), the work of adhesion (Wad), the interfacial energy (γSI) between inclusion and steel melt at 1873 K. 54.3° and 670 erg cm−2 are chosen as θC and γIG of C12A7 since its average composition is very close to the data reported by Bretonnet et al.[9] The visible contact angle α for C12A7 is needed to calculate γSI since it's liquid at 1873 K, and the data is 35.7°[9] in their study. The γSI and Wad for C12A7 are calculated by Equation Eq. (5) and Eq. (6). Shinozaki et al.[10] have measured the contact angle of Al2O3 · MgO, they found it nearly did not change with the increase of MgO for molten steel with small oxygen content. The measured contact angle of Al2O3 · MgO with 20% MgO is 134.1°. So 134.1° is applied here to represent the contact angle of Al2O3 · MgO in which MgO is 29% in this study. The γSI and Wad for Al2O3 · MgO are calculated by Equation Eq. (4) and Eq. (6). Because no θC and γIG are available for C3A to date, only Al2O3 · MgO is compared with C12A7 to elucidate the different behaviors between liquid and non-liquid inclusions.

Table 4. Interfacial energy between inclusion and gas (γIG), contact angle (θC), work of adhesion (Wad), the interfacial energy (γSI) between inclusion and steel melt at 1873 K
ParticleOxideγIG/erg cm−2θC/degWad/erg cm−2γSI/erg cm−2
SolidAl2O3750[12]144.0[13]3522242
Al2O3 · MgO512[10]134.1[10]5611795
MgO710[14]125.0[15]7861768
LiquidC12A7670[9]54.3[9]11571357

As can be seen in Table 4, the θC of Al2O3 · MgO is 134.1°, much larger than C12A7, 54.3°. Besides, Al2O3 · MgO has a higher γSI than liquid one C12A7, which means the Al2O3 · MgO inclusions are more likely to collide and detach from steel melt. Both behaviors will reduce the total interfacial energy of the inclusion-steel system, which is the product of interfacial energy multiplying the contact area, through reducing the contact area, and these processes are spontaneous. Furthermore, Al2O3 · MgO has a much lower Wad, which indicates it will be much easier to be removed from the steel bulk since it needs less energy to detach form the steel. Table 4 also lists relevant date for Al2O3 and MgO. It can be inferred that for a given particle size, Al2O3 can be removed faster than Al2O3 · MgO, MgO, and C12A7, since it has the largest γSI and lowest Wad. And it's supported by other authors'[2, 3, 5] experiment result, they all found that Al2O3 can be removed quickly from the steel melt. Furthermore, Yin et al.[11] confirmed that the collision and agglomeration of liquid CaO–Al2O3 particles are more difficult compared with Al2O3 at inert gas/molten steel interface due to the different interfacial parameters. Also it can be deduced form Table 4 that MgO can be removed faster than C12A7, because the γSI of MgO is larger than C12A7 but the Wad of MgO is smaller than C12A7.

4 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Methods
  5. 3 Results and Discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

Inclusion morphology, composition and number coupled with steel and slag composition during the RH vacuum treatment are investigated on API-X70 pipeline steel with ASPEX. Based on the experiment results and analysis, the following conclusions are obtained:

  1. RH has a strong ability to remove inclusions besides degassing. [N] decreases 13 ppm, and T.O decreases 8 and 16 ppm after RH vacuum treatment.
  2. The inclusions are mainly globular CaO–Al2O3–MgO, and can be classified into liquid and non-liquid inclusions. The average composition changes slightly with the increase of Al2O3 and decrease of CaO and MgO with time because of the loss of [Ca] and [Mg] and the formation of Al2O3 due to reduction of FeO in slag by [Al]s.
  3. The inclusions decrease with time during RH treatment, and the removal ratio can reach 72% after 28 min treatment. This is caused by inclusion coagulation and removal to the slag layer above the ladle or the top free surface of the RH degasser.
  4. The initial number of inclusion plays a vital role in the final inclusion number, the more the inclusions at the beginning of RH, and the more the inclusions after RH. So efforts must be made to eliminate the inclusion before RH treatment to get a highly clean steel.
  5. Non-liquid inclusions including C3A and Al2O3 · MgO decrease much more quickly and thoroughly than liquid ones, C12A7. This can be explained by the fact that non-liquid inclusions have a much higher contact angle and interfacial energy but a much lower work of adhesion than liquid ones.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Methods
  5. 3 Results and Discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

The authors gratefully express their appreciation to the National Natural Science Foundation of China (No. 51204014) for sponsoring this work. Sincere gratitude and appreciation should be expressed to NISCO who gives us such a great platform to do this study.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Methods
  5. 3 Results and Discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References
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    K. Shirabe, J. Szekely, Trans. Iron Steel Inst. Jpn. 1983, 23, 465.
  • 2
    T. Soejima, J. Kobayashi, H. Matsumoto, 114th ISIJ meeting, October 1987, S970.
  • 3
    Y. Miki, Y. Shimada, B. G. Thomas, Iron Steelmaker 1997, 24, 31.
  • 4
    T. Murai, H. Matsuno, E. Sakurai, Tetsu-to-Hagane 1998, 84, 13.
  • 5
    H. Tanaka, S. Kobayashi, A. Ishizaka, Development of commercially “pure iron” with extra low inclusions at NKK Keihin Works. 83 rd Steelmaking Conference 2000, 91.
  • 6
    M. Katsuaki, T. Tomomichi, K. Kyoichi, The 4th International Congress on the Science and Technology of Steelmaking, ISIJ, Gifu, Japan 2008, p. 457.
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    N. Keiji, Tetsu-to-Hagane 1994, 80, 383.
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    G. Sigworth, J. F. Elliott, Metal Sci. 1974, 8, 298.
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    N. Shinozaki, N. Echida, K. Mukai, Tetsu-to-Hagane 1994, 80, 748.
  • 11
    H. Yin, H. Shibata, T. Emi, ISIJ Int. 1997, 37, 946.
  • 12
    K. Ogino, K. Nogi, Y. Koshida, Tetsu-to-Hagane 1973, 59, 1380.
  • 13
    B. V. Tsarevski, S. I. Popel, Tchern. Met. DRSS 1960, 15, 8.
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    S. K. Rhee, J. Am. Ceramic Soc. 1975, 58, 441.
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    K. Nogi, K. Ogino, Can. Metall. Quart. 1983, 22, 19.