Degradation of Iron Ore Sinter. Increase in Low-temperature Reduction. due to Hematite-alumina Solid Solution. and Columnar Calcium Ferrite*

Transcription

1 Increase in Low-temperature Reduction due to Hematite-alumina Solid Solution Degradation of Iron Ore Sinter and Columnar Calcium Ferrite* By Iehiro SHIGAKI, * * Mineo SA WADA* * and Norio GENNAI * * * Synopsis Direct observation of lattice images by TEM of magnetite reduced f rom hematite made clear ultramicroscopic partial distortion due to solid solution o f A12O3. Furthermore, the effect o f solid solution of A12O3 with hematite on strain of magnetite, which had reduced grains of the hematite with a mixture gas of CO C02, was estimated by line-broadenings of peaks of X-ray diffraction. It revealed that increasing amounts o f A12O3 increased the strain of magnetite. Fracture strength of the minerals in sintered ore was estimated quantitatively by Vickers indentation, indicating crack propagation-arrest characteristics. Fracture toughness of both glassy silicate and calcium ferrite resulted in values of the same order with a little effect of A12O3. Calcium ferrite, however, had the lowest value for " critical load", indicating crack initiation characteristic among all the minerals tested. An experimental study of the crystallization mechanism of skeletal hematite indicated that magnetite coexisting with liquid silicate decomposed, melted and was oxidized at the falling stage of sintering. I. Introduction Size degradation of iron ore sinter, occurring on low-temperature reduction, causes an increase in the gas flow resistance at the shaft of the blast furnace and in the nonuniformity of gas flow, and exerts an unfavorable influence upon stable operation of blast furnace. In order to solve this problem, a number of metallurgical or mineralogical studies have been done. Many studies have indicated that defects of hematite crystals induced size degradation; Sasaki et al.' observed a lamella of multi-component hematite with transmitted light and concluded that the cause was grain distortion generated by precipitation of impurities as A1203, Ti02 or Mn0 during cooling in sintering process. On the contrary, Inazum.i et al.2~ have pointed out that the cracking might be attributed to the shape of skeletal hematite and to the difference in expansion due to different reaction rates between the hematite and the inclusions. Inoue et al.3~ have pointed out the analytic necessity of dividing two factors; reduction amounts of hematite, 4H, and the fracture strength of a matrix. These studies indicate that secondary skeletal hematite is a major contributor to RIM. This hematite usually contains impurities as solid solution, and includes silicate in its crystal. Furthermore, it generally coexists with columnar calcium ferrite and glassy silicate. Size degradation during reduction of sinter is attributed doubtlessly to volume expansion during reduction at low temperatures from hematite to magnetite. However, there has been no agreed-upon theory about the mechanism from crack initiation to crack propagation, since several factors are mixed each other as solid solution of A1203 with hematite, particular shape of skeletal hematite, and strength of matrixes around hematite which generates stress. The authors indicated in a previous paper4) that the skeletal hematite was easily cracked due to its particular shape, such that the stress arising on reduction is concentrated at the boundary between the magnetite and the inclusion where the radius of curvature is small to crack the inclusion. And the solid solution of A1203 with hematite did not induce lattice distortion and no precipitation of A1203 occurred during cooling in sintering process and reduction to m.agnetite as far as X-ray diffraction could examine. But the problem has not been solved about the effect of solid solution of A1203 with hematite on degradation of sinter during reduction at low temperatures. This report attempts to clarify this subject by direct observation of lattice structures using TEM. Furthermore, several studies5'6~ have indicated recently that the mineral texture composed of skeletal hematite, columnar calcium ferrite, and glassy silicate was degraded especially. Therefore, the effect of coexisting minerals on RDI should be considered. Thereupon fracture strength of columnar calcium ferrite was evaluated using indentation microfracture method to study aggravation of RDI. In any case, decrease in an amount of skeletal hematite leads certainly to improvement of RDI. Several mechanisms have been proposed regarding the formation of the skeletal hematite : (a) dissolving of magnetite, which coexists with silicate melt at a high temperature, and crystallizing out abruptly at a temperature range of hematite crystallization in phase diagrams (Inazumi et al.2)), (b) incongruent melting of the calcium ferrite in the heating process (Inoue and Ikeda7~), (c) dissolving of Si02 into the Ca0-Fe203 melt (Matsuno et al.8)), and (d) assimilation of the Ca0-Fe203 melt and silicate melt (Haruna et al.9)). However, no single theory has been established about this. Accordingly this mechanism is studied experimentally to decrease an amount of skeletal hematite. * ** *** Partly published in Tetsu-to-Hagane, 71 (1985), 1180, in Japanese. Manuscript received on June 10, 1985; accepted in the form on January 10, ISIJ Iron & Steel Technology Center, Materials Laboratories, Kobe Steel, Ltd., Wakinohama-cho, Chuo-ku, Kobe 651. KOBELCO Institute Inc., Wakinohama-cho, Chuo-ku, Kobe 651. final (503)

2 ( 504 ) Transactions ISIJ, Vol. 26, 1986 II. Experimental Procedure 1. Lattice Distortion and Strain of Magnetite Crystals Reduced from Hematite Crystals 1. Observation of Crystal Lattice Distortion Recently electron-microscope technique of lattice imaging has been developed to observe the structure of materials directly in the field of mineralogy. This technique was used to study the effect of solid solution of A1203 with hematite on the lattice structure of magnetite crystal reduced from the hematite, using TEM at voltage of 200 kv. Single crystals, necessary for the technique, were prepared as follows. Single crystals of hematite were made by a flux method.10~ Powdered Fe203, 9.9 g, dehydrated borax, 26.5 g, and cupric oxide, 1.0 g, were packed in a platinum crucible and heated up to C in air, and then cooled to 800 C at a cooling rate of 4 C/h. Crystals of hematite were obtained by solving the borax-flux in hot HN03 (1 +4) solution. The obtained single crystals were platelike of 1 N5 mm in diameter and pm thickness. In order to make a solid solution of A1203 with Fe203, the hematite crystals thus obtained were put in a platinum crucible with powder whose weight percent was dehydrated borax 60, Fe20315 and A , heated at C for 20 h in air. Weight percentage of A1203 held in Fe203 was 3.0. The hematite crystals, pure of A1203 containing, were reduced to magnetite at 550 C by a mixture gas (CO/C02=40/60). The reduced specimens adhered to slide glasses were thinned with carborundum and prepared by ion bombardment to make small holes in the center, whose surrounding wall thinner than 0.1 pm could be used for the observation of lattice structure by TEM. 2, Measurement of Crystal Strain Strains were evaluated from line-broadening measurements of X-ray diffraction lines for crystals of magnetite, which were reduced from those of hematite where impurities were held as solid solution. Mixed ratio of impurities was widely changed to reveal clearly their effect on the broadening. Namely, reagent grade chemicals of A1203 or Ti02 were mixed with that of Fe203 at rates of 5 or 10 wt%, and heated at C for 6.5 h in air to be quenched. The same heat treatment was done for a pure reagent grade chemical of hematite. The hematite crystals thus obtained were reduced with a mixture gas (CO/C02=40/60) at 550 C, and presented to the experiment of X-ray diffraction. Methods of diffraction and analysis were the sam.e as those in the previous paper.4) 2. Strength of Minerals Related to Reduction Degradation Degradation characteristic of iron ore sinter should be closely connected with fracture toughness of minerals in sinter against stress. Evans and Charles" revealed that fracture toughness of homogeneous ceramics could be evaluated from length of diagonal of Vickers indent and that of arising crack. Minerals in sinter are not so homogeneous as ceramics, but indentation microfracture method was used to evaluate fracture toughness of minerals in sinter, since preliminary experiments showed a range of hardness 6N 7 (GNm-2), and that of fracture toughness 0.7-'1.6 (MNm-3/2), which corresponded to those of ceramics. An assumption was made that in the case of degradation of sinter during reduction, there were at least two steps; viz., (1) crack initiation and (2) crack propagation-arrest. No study has been done on crack initiation by indentation microfracture method, while on crack propagation-arrest Sakamoto et al.12~ showed that prismatic-type calcium ferrite and glassy slag were unstable. In the first place, probability of crack initiation was measured with change of load from 10 to 300 g for 15 s. Definition of " critical load ", P%, by which crack initiation was estimated, was the load when accumulated frequency of crack initiation reached 50 % : a crack should initiate once when an indent is loaded with P50 by two times. Strictly speaking, fracture toughness or stress corresponding to critical load should be estimated, but there is neither equation to estimate the former nor analysis for the latter in the complex elastic/plastic field. P o was, hence, adopted here for estimation of crack initiation as first approximation. In the next place, characteristic of crack propagation-arrest was evaluated from fracture toughness, using Eq. (1) and (2),11) which were derived from median cracks by Vickers indentation of materials as WC/Co (12%), SiC, Si3N4, and so on. Namely, the indentation crack length, C (m), could be related to the impression radius a (m) by: K O/H ~/ a = O. l 5k(C/a)-3/2...(1) H = 0.47P/a2...(2) where, K~ : fracture toughness (N/m3/2) H: hardness (N/m2) P: load (kg) : the constraint factor k : the correction factor. Homogeneous phases only as large as a circle of 5a/2 in radius were necessary for the experiment of crack initiation, while a+c for that of crack propagation-arrest. Since values of a were known to be smaller than 14 pm by a preliminary experiment, minerals larger than 35 pm in sinter made by a testing pot were used to estimate crack initiation characteristic. The measurements were conducted on secondary hematite, magnetite and columnar calcium ferrite crystals, and glassy silicate in order to eliminate the effect of pores. The surfaces of the specimens were polished using diamond paste. Columnar calcium ferrite and glassy silicate of large size were synthesized since specimens as large as 80 pm were necessary for the experiment of crack propagation-arrest to eliminate crack arrest by another mineral phases. Table 1 shows mixing ratio of reagents and composition of synthetic samples. For calcium ferrite, samples after preheating and

3 Transactions ISIJ, Vol. 26, 1986 (505) Table 1. Mixing ratio and composition of synthetic calcium ferrite and glassy silicate. crush were held at C for 30 min in air, cooled to 1200 C at 35 C/ h and taken out of the furnace, while for glassy silicate, samples were held at C for 30 rriin in air and subsequently taken out. Thickness of minerals is important in indentation microfracture method. Thickness of minerals was not measured, but was assumed to be larger than 5a/2 for the experiment of crack initiation and larger than a+c for the experiment of crack propagationarrest. For the experiments of crack initiation, loaded weights for 15 s were g against each 30 points of the minerals, whereas 500 g for 15 s to generate stable median cracks for the experiment of the crack propagation-arrest against each 10 points in measurement of 4 crack lengths developed from angular points of indent. 3. Crystallization Mechanism of the Skeletal Hematite Several mechanisms have been proposed as previously mentioned regarding the formation of the skeletal hematite, containing impurities as A1203. Experiments were conducted here on these mechanisms to verify the formation of skeletal hematite and to study what was the surest in an actual sintering process. Table 2 shows mixing ratio of reagents before making samples. The shape of the secondary hematite following incongruent melting of the quaternary calcium ferrite was examined in the following way: sample G in a platinum envelope was heated in air at a rate of 13 C/s, simulating a heating pattern in a commercial process, to the planned temperatures of 1250, and C, held for 2 min at each temperature, respectively, and then quenched by dropping into water. In the next stage, we examined the shape of the secondary hematite by dissolving glassy silicate, or Si02, into the calcium ferrite melt. Homogeneous calcium ferrite was made from sample H by repetition of heating at C, quenching and crushing, whereas glassy silicate was synthesized from sample I at 1400 C. For each 1 g of the calcium ferrite, the glassy silicate thus obtained and Si02 of reagent grade were pressed by a die to a diameter of 10.5 mm. Each sample of calcium ferrite + glassy silicate and calcium ferrite + Si02, where calcium ferrite was set on the underside, was heated in the same pattern as in the experiment with sample G. Finally, sample J was heated at C for 10 min in air, cooled at 2 C/min to the planned temperatures of and C, quenched and then prepared for microscopic observation. III. Table 2. Mixing ratio of reagents before making samples. Results and Discussion 1. Effect of A1203 on Lattice Distortion and Strain of Magnetite 1. Crystal Lattice Distortion Photograph 1 shows a lattice image of magnetite crystal reduced from pure Fe203, whose electron diffraction pattern indicates the plane as (211). The edge adjacent to the hole in the middle part of the photograph is not clear due probably to contamination of hydro carbide. The left side of the photograph shows the lattice image of magnetite, whose contrast varies with the periodicity of the lattice. Photograph 2 was enlarged partly from Photo. 1. The specimen does not show distortion especially even after the reduction and thinning. The space group of pure magnetite is Fd3m, whose sites of atoms are shown in a book.13~ It is possible to calculate sites of atoms on any plane by multiplication of matrices. Figure 1 shows arrangement of 02- ions, thus obtained, on (211) plane. Lattice images by TEM do not correspond generally to the atomic structures of crystals. But mean widths of white spot arrays in Photo. 2 were 4.9 and 3.0 A by measuring lengths of each ten arrays, which correspond respectively to interplanar spacings of (111) and (220) obtained theoretically as shown in Fig. 1. Consequently, white spots correspond exactly to the arrangement of sites occupied by oxygen atoms in the structure of Fe304; there is one-to-one cor-

4 (506) Transactions ISIJ, Vol. 26, 1986 Fig. 1. Theoretical disposition of 02_ ions on (211) plane of pure magnetite crystal. Photo. 1. Lattice image of magnetite reduced from pure hematite and its electron diffraction pattern of (211) plane. Photo. 2. High-resolution lattice from pure hematite. image of magnetite reduced respondence between lattice image contrast and structure. Photograph 3 shows a lattice image of magnetite crystals reduced from solid solution of A1203 with Fe203. The parts where lattice distortion existed or the leaf specimen was curved appear dark in the photograph due to defocus from Bragg's condition. Mean interplanar spacings were from 2.7 to 3.0 A for (220), while from 4.7 to 5.1 A for (111), indicating elastic distortions in the crystals. Photograph 4 is a high-resolution lattice image of a different part from Photo. 3. The right figure in this photograph was treated with the naked eye. Lower parts from line a to line f in Photo. 4 agreed well to the disposition of 02- ions in Fig. 1. Lattice distortions were detected in the upper part from line a to line h and the lower part from line i to line m. The previous measurement of lattice constant4~ indicated that A1203 originally held in hematite maintains a state of solid solution statistically even after reduction to magnetite, and the experiment Photo. 3. Lattice image of magnetite reduced from Fe203 -A1203 solid solution. mentioned above showed that lattice strains of magnetite were increased by the solid solution. Moreover, direct observation using TEM, however, revealed existence of the lattice distortion by solid solution of A1203 as mentioned above. Furthermore, ultramicroscopic precipitation of A1203 or generation of a different substance may occur, though the area was too small to use the electron diffraction technique. 2. Strain of Magnetite Lattice distortion was found in magnetite reduced from hematite solid solution with A1203. The distortion thus observed might induce strain and hence strains were evaluated by line-broadening of difrac-

5 Transactions ISIJ, Vol. 26, 1986 (507) Photo. 4. High-resolution lattice image of magnetite reduced from Fe203-A1203 solid solution. Fig. 2. Effect of A1203 on line-broadenings of magnetite reduced from Fe203-A1203 solid solution at 550 C. tion peaks.41 Figure 2 shows the effect of A1203 on line-broadening of magnetite. Figure 3 shows the effect of Ti02 as well on strain of magnetite analyzed in the same way, since Ti02 is considered to increase degradation during reduction. Figure 4 shows the strain of magnetite calculated from the slope of lines in Figs. 2 and 3. Lattice strain of magnetite increases with increasing of impurities, whose effect was larger for A1203 than Ti02. Relation between size degradation during reduction and increasing magnitude of strains of magnetite Fig. 3. Effect of Ti02 on line-broadenings of magnetite reduced from Fe203-FeO. Ti02 solid solution at 550 C. due to solid solution of A1203 was considered as follows. In comparison with data of actual fracture strength a~ with a calculated value of ideal fracture strength r, for materials as glass, SiC, Si3N4, iron whisker or high carbon music wire, the ratio of ac/ th is from 1/10 to 1/100 and those of brittle materials take smaller values.14~ Furthermore, theoretical calculation revealed the relation Qth E/10.15) Assuming that the strains are produced by a constant elastic stress Q, Hooke's law can be applied using

6 (508) Transactions ISIJ, Vol. 26, 1986 Fig. 5. Probability of crack initiation with change of load. Table 3. " Critical load " and hardness of minerals. Fig. 4. Effect of impurities held in of reduced magnetite. hematite on the strain Young's modulus F so that; Consequently, materials should be broken when values of s reach 1/100 to 1/10. Assuming that cr(/ i h = l 100 for magnetite crystals, the values of s in this study took figures down one place in comparison with 1/100. Obtained value of about 1 N 2 x 10-3, however, showed mean value of strain, hence larger values should exist partially. Fracture is normally generated from the weakest part of materials, so it is expected that magnetite crystals with larger mean value of E have higher frequency of breaking. The authors have shown in the previous paper4) that solid solution of A1203 with hematite did not induce strains and no precipitation of A1203 occurred during cooling in sintering process and reduction to magnetite. However, impurities held in hematite as solid solution influence strains of reduced magnetite. 2. Fracture Toughness o f Minerals Degradation characteristics of sinter during reduction is influenced not only by impurities held in hematite as solid solution but also by the strength of matrix suffering stress. First, crack initiation was analyzed. Figure 5 shows probability of crack initiation against load. Values of critical load P o and hardness H are summarized in Table 3. Columnar calcium ferrite had the lowest value of P o. On the contrary, magnetite and glassy silicate had larger values. Hematite has an intermediate value due to very large value for hardness. Next, crack propagation-arrest characteristics of glassy silicate and calcium ferrite were analyzed. Equation (1) was obtained by using materials with values of O.9'-l6.0 (MNm-3~2) for K~, l (GNm-2) for H and for Poisson's ratio v. And k-value was calculated from data at larger C/a values than 3. In this experiment, values of glassy silicate and calcium ferrite were 6'- 7 (GNm~2) for H and O.7'-l.6 (MNm32) for K, and 2.6'-3.8 for C/a. Values of v for both glassy silicate and calcium ferrite were not measured, but available data were for various kinds of glasses.'4~ Many brittle materials have values of v from 0.2 to 0.3. Hence, assuming that v of calcium ferrite is close to this range of value, it is assumed to adopt Eq. (1) for both glassy silicate and calcium ferrite. Photographs 5 and 6 show crack initiation and propagation-arrest of glassy silicate and calcium ferrite, respectively, by Vickers indentation, indicating that crack initiates at four corners of indent. Both photographs corresponding to the experiments on crack propagation-arrest appear to point out clearly the growth of median cracks. Furthermore, Fig. 6 obtained with change of load shows a proportion between P and C3~2, which signifies the growth of median cracks. In the second place, fracture toughness calculated by Eq. (1) is shown in Figs. 7 and 8. Figure 7 reveals that the value of fracture toughness of calcium ferrite (m Ca0 Si02-n CaO.3(Fe, A1)203) is smaller for a composition of rn/n ratio of 0.49 than for that of 0.31; increase in A1203 content decreases fracture toughness. Figure 8 shows the effect of A1203 on fracture toughness of glassy silicate, indicating independence of A1203 content. The fracture toughness of glassy silicate and columnar calcium ferrite resulted in values of the same order. 3. Difference in Degree of Degradation among Minerals Indentation microfracture method revealed that columnar calcium ferrite and glassy silicate were weak

7 Transactions IsIJ, Vol. 26, 1986 (509) Photo. 5. Crack initiation and propagation-arrest of care after Vickers indentation. glassy sili- Fig. 6. Relation between load and length of crack. Fig. 7. Fracture toughness n[ca(fe, A1)6015] of calcium ferrite: in[casi03]- Photo. 6. Crack initiation and propagation-arrest calcium ferrite after Vickers indentation. of columnar on crack propagation;, arrest characteristic, and the former was particularly weak on crack initiation characteristic. Sized particles after the RDI test were studied to examine propriety of indentation microfracture method. Figure 9 shows amounts of magnetite and calcium ferrite contained in various sized particles after the RDI test on sinter produced in a sintering pot. X- ray internal standard method and chemical analysis17a indicated that weight proportions of minerals in the sinter tested were 50 % for hematite, 10 % for magnetite, 30 % for calcium ferrite, 5 % for glassy silicate and 5 % for dicalcium. ferrite. Furthermore, long holding time of 4.2 min above C during sintering resulted in comparatively large amounts of skeletal hematite and columnar calcium ferrite. In comparison with these amounts, Fig. 9 reveals that increased amount of magnetite and calcium ferrite were contained in smaller-sized particles, which in turn indicates that parts composed of hematite and calcium ferrite were mainly cracked by volume expansion during reduction from hematite to magnetite. Hematite grains in smaller sized particles were increased due to large amount of skeletal hematite not only by larger Fig. 8. Effect of A1203 content on fracture toughness of glassy silicate. Fig. 9. Mineral content of classified samples after RDI test. strains but also by larger reduction amounts of JH because of existence around pores. Judging from the fact that there is no difference in crack propaga-

8 ( 510 ) Transactions ISIJ, Vol. 26, 1986 tion-arrest characteristic but great difference in crack initiation characteristic, plenty of calcium ferrite in degraded smaller-sized particles may be attributed to a low value of "critical load." 4. Formation of Skeletal Hematite Grains The authors have come to the conclusion that skeletal hematite grains, solid solution with A1203, induce strains of reduced magnetite grains to increase low-temperature reduction degradation. Besides, RDI increases for the reason that columnar calcium ferrite and glassy silicate generally coexist with skeletal hematite grains. It is, hence, important to clarify the mechanism to suppress formation of skeletal hematite. Photograph 7 shows the secondary hematite generated by the incongruent melting of calcium ferrite. The shape of the hematite is granular and no skeletal hematite can be observed. On the other hand, Photo. 8 shows the results of dissolving Si02 and glassy silicate into the melt of' the CaO-Fe203 system. Si02 has already started assimilation at C; this generates glassy silicate, and hematite at the boundary of the calcium ferrite and the glassy silicate. The shape of the hematite is, however, granular. Furthermore, we think that the mechanism of assimilation of glassy silicate with the melt of the CaO- Fe203 system is identical to Si02 dissolution. Granular hematite generates at the boundary at 1250 C, while some skeletal hematite generates locally due to the great assimilation speed of silicate at C. The authors concluded that this mechanism of skeletal hematite generation was very unlikely to occur during actual sintering, for the reason that there were great differences between their textures. Finally we examined the mechanism proposed by Inazumi et al.2~ Photograph 9(A) shows that the texture at C which was cooled slowly from 1450 C is composed of glassy silicate and euhedral crystals of magnetite. The texture at C comprises the skeletal hematite, including magnetite and glassy silicate. The included magnetite, however, loses its euhedral shape, which indicates that hematite is oxidized through decomposition and melting of magnetite. The authors, in another paper,5) have proposed increasing the size of limestone particles as a concrete Photo. 7. Secondary hematite of calcium ferrite. formed by incongruent melting Photo. 9. Photo. 8. Secondary hematite by calcium ferrite melt. dissolution of Si02 or glassy silicate into Skeletal hematite by decomposition melting of magnetite. and

9 Transactions ISI1, Vol. 26, 1986 (511) measure to decrease the amount of skeletal hematite by way of suppression of formation of magnetite. Iv. Conclusions Effects of A1203 on size degradation during reduction of iron ore sinter concerning lattice distortion and strength of minerals were studied using TEM and indentation microfracture method. Results obtainedd were as follows. (1) Direct observation by TEM of lattice of m.agnetite crystal reduced from hematite made clear partial distortion due to solid solution of A1203. (2) X-ray analysis indicated that solid solution of A1203 with hematite increased the magnitude of lattice strains of reduced magnetite, which may be related to size degradation during reduction. (3) Fracture toughness of both glassy silicate and calcium ferrite resulted in values of the same order with a little effect of A1203 content. Calcium ferrite, however, had the lowest value for critical load, which may be the cause of its poor degradation characteristics. (4) Experimental study of the crystallization mechanism of the skeletal hematite indicated that magnetite coexisting with liquid silicate decomposed, melted and was oxidized at the falling stage of sintering. Acknowledgements The authors would like to thank Mr. T. Yamada of Kobe University for his preparation of the specimens by ion thinning. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) REFERENCES M. Sasaki, T. Enokido, S. Kondo and T. Nakazawa: Tetsu-to-Hagane, 59 (1973), T. Inazumi, K. Shinada and M. Kawabe: Tetsu-to-Hagane, 68 (1982), K. Inoue, H. Hayashi and K. Yoshioka: The 54th Committee (Ironmaking), Japan Soc. Promotion of Science (JSPS), Rep. No (Feb., 1983). I. Shigaki, M. Sawada, M. Maekawa and K. Narita: Trans. ISIJ, 22 (1982), 838. I. Shigaki, M. Sawada, K. Yoshioka and T. Takahashi : Tetsu-to-Hagane, 71 (1985), T. Takada, H. Soma, T. Irida, E. Kamishima, H. Kimura and T. Isoyama: Tetsu-to-Hagane, 70 (1985), S86. K. Inoue and T. Ikeda: Tetsu-to-Hagane, 68 (1982), F. Matsuno : 7 etsu-to-hagane, 64 (1978), J. Haruna, S. Suzuki, M. Takasaki and K. Sato : Tetsu-to- Hagane, 67 (1981), A Tasaki and S. Iida : J. PI ys. Soc. Japan, 18 (1963), A. G. Evans and E. A. Charles: J. Amer. Ceram. Soc., 59 (1976), 371. N. Sakamoto, H. Fukuyo, Y. Iwata and T. Miyashita : Tetsu-to-Hagane, 70 (1984), 512. International Tables for X-ray Crystallography, I, ed. by N.F.M. Henry and L. Lonsdale, The Kynoch Press, Birmingham, (1965), 340. A. Kamei and T. Yokobori : Ceramics Japan, 11 (1976), 119. T. Yokobori : Zairyo kyodogaku (Strength and fracture of solids), Iwanami, Tokyo, (1984), 9. Kagakubinran (Kisohen), ed. by Chemical Society of Japan, Maruzen, Tokyo, (1975), 564. I. Shigaki, M. Sawada, K. Yoshioka and T. Takahashi : The 54th Committee (Ironmaking), Japan Soc. Promotion of Science (JSPS), Rep. No (Nov., 1984).