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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 84 (1998), No. 9

  • Reduction of Center Porosity of Round Billet by Electromagnetic Stirring in Horizontal Continuous Casting

    pp. 609-616

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    In order to reduce center porosity of round billet in horizontal continuous casting, experiments of electromagnetic stirring (EMS) were carried out for two kinds of stainless steel containing Nb and Ti, which had a tendency to enhance center porosity. Combined electromagenetic stirrings in mold (M-EMS), in strand (S-EMS) and in final solidification region (F-EMS) were applied to continuous casting of 263 mmφ round billet.
    It has been found that the combined stirrings of M-EMS, S-EMS and F-EMS are most effective to reduce center porosity of the billet. A lack of the stirrings decreases the effect on the reduction of center porosity. S-EMS was proved to have the effect to disperse equiaxed crystals in the molten steel, which were formed in the mold by M-EMS and consequently, enable uniform packing of the equiaxed crystals in the center of billet. F-EMS was proved to be most effective, when it was applied for the liquid core diameter of 40 mm to 80 mm. F-EMS is thought to have the effect to secure the fluidity of mushy zone in the center of billet by the stirring of 15 Hz, but has no influence on solidification structure.
  • Estimation of Mold Powder Consumption in Continuous Casting

    pp. 617-624

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    The inflow phenomena of mold powder between mold and solidified shell are very important for continuous casting opreation and quality of cast products.in this experimenral study, mold powder consumption was measured with a apparatus called 'mold simulator'using several kinds of mold powders. Based on the results from the experiments, the following fidings were obtained. Mold powder consumption, Q, was affected by the powder properties, powder viscosity, η, crystallization temperature, Tcs, and also affected by the mold oscillation, amplitude, A, and frequency, f and casting speed, Vc, and deformation ratio βof sinusoidal curve to non-sinusoidal. As the result, the next equation of mold powder consumption was obatined.
    Q = K·βA0.4/Tcs1.6√ηVccos-1(-1000Vc/2πfA)
    This equation can be used to find the adequate operation conditions when the mold powder properties and the amplitude of mold oscillation are changed.
  • Control of Early Solidification by the Use of High Frequency Electromagnetic Field in the Continuous Casting of Steel

    pp. 625-631

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    In order to improve surface quality of continuously cast steel, alternating magnetic field was applied through the copper mold to the early solidification zone of round billet. The mold has a number of slits and was specifically designed to achieve the efficient penetration of the field as well as high stiffness. A mold size is 0.18 m in internal diameter and is surrounded with a multi-turn induction coil. Casting velocity is 1.2m/min. The capacity of high frequency generator is 300kW and the frequency is 25kHz.
    0.11% carbon steel was cast to investigate the effects of the intensity of magnetic field and the imposing area on the surface quality of billets. Even under the strong magnetic field, the meniscus level was controled precisely by RFLC method (Resonant Frequency Level Control Method) detecting the small shift of resonant frequency of high frequency generator.
    Surface marks on the cast billets had three types by the imposition of electromagnetic field. Regular surface marks caused by the mold oscillation have disappeared by selecting the optimum meniscus level under suitable electric power. By the use of developed two fluids model based on VOF code, dynamics of meniscus shape at the early solidification zone during oscillation were studied theoretically. The model shows that the disappearance of oscillation marks is responsible for the stability of interfacial shape between flux and molten steel by the imposition of electromagnetic field.
  • Changing Process from ζ·Fe-Zn Phase to Al-Fe Intermetallic Compounds in Molten Zn-5mass%Al Alloy Bath

    pp. 632-636

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    As a purpose to investigate the changing process from Fe-Zn intermetallic compounds (IMCs) to Al-Fe IMCs in a molten Zn-5mass%Al alloy, this study has been carried out by immersing the galvanized steel sheet with Γ. Γ'. δ and ζ Fe-Zn phases as a coating layer in a Zn-5mass%Al bath at 450°C.
    ζ·Fe-Zn phase changed to particle products mainly consisted of Zn-containing Al5Fe2 and the melt with less Al concentration than the Zn-5mass%Al bath at an extremely high rate. This change was found to be occurred under the two steps reaction; (1) ζ·Fe-Zn+Al(L)→ Al-Fe-Zn amorphous phase + Zn (L), (2) Al-Fe-Zn amorphous phase →Al5Fe2 + Zn(L). At the first step, the Al-Fe-Zn amorphous phase was formed around the grain boundaries of ζ·Fe-Zn phase in contact with the Zn-5mass%Al melt and then nuclei of Al5Fe2 were formed in this amorphous phase. It is considered that Zn concentration at an area of the amorphous phase surrounded Al5Fe2 is increased with the growth of Al5Fe2. As a result, the area surrounded Al5Fe2 becomes a liquid state and Al5Fe2 changed from ζ·Fe-Zn phase exists as a particle in the Zn-rich melt. The Al-Fe-Zn amorphous phase is considered to be a quasi-equilibrium phase which has a wide range of concentration for Al (1560 at%), Fe (1025 at%) and Zn (1575 at%).
  • Changing Process from Γ·Fe-Zn Phase to Al-Fe Intermetallic Compounds in Molten Zn-5mss%Al Alloy Bath

    pp. 637-642

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    Following the previous report, it has been investigated the changing process from Fe-Zn intermetallic compounds (IMCs) to Al-Fe IMCs in a molten Zn-5mass%Al alloy at 450°C.
    Γ·Fe-Zn phase changed directly to Zn-containing Al5Fe2 at an extremely high rate.Γ phase had a strong orientation of {110} plane parallel to the steel surface, but Zn-containing Al5Fe2 had not a significant orientation. And a special orientation relationship between Γ phase and Al5Fe2 could not be found. Combining these results with the previous results on the changing process from ζ·Fe-Zn phase (two steps reaction; ζ·Fe-Zn phase→Al-Fe-Zn amorphous phase→Al5Fe2), it is considered that whether Fe-Zn IMCs change to Al5Fe2 by the direct reaction or the two steps reaction depends on the Fe atomic concentration of these IMCs. After the whole of Γ·Fe-Zn layer changed to Zn-containing Al5Fe2, Al5Fe2 layer peeled off the steel substrate. This is considered to be due to that a Zn-rich phase which had been formed like a chain in Zn-containing Al5Fe2 layer changed to voids by the outer diffusion of Zn in this phase. The Zn-rich phase is understood to be formed by the concentration of Zn discharged from Γ·Fe-Zn phase during changing to Zn-containing Al5Fe2.
  • Precipitation and Growth of γ' and γ″ Phases in a 41.5Ni-40Fe-16Cr-2.9Nb-1.8Ti-0.2Al Alloy

    pp. 643-649

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    A 41.5Ni-40Fe-16Cr-2.9Nb-1.8Ti-0.2A1 alloy is a superalloy strengthened by the precipitation of γ' and γ″ phases. The knowledge of the nucleation and growth behaviors of the γ' and γ″ precipitates is very important for a appropriate usage of the alloy. We investigated the precipitation characteristics of this alloy by micro-Vickers hardness test. X-ray diffraction and transmission electron microscopy. The hardness of the alloy aged in various conditions closely related to the size of the γ' and γ″ precipitates. With increasing aging time, the lattice parameter of the alloy matrix decreases and reaches a constant value at each aging temperature after a certain aging time. The volume fractions of γ' and γ″ phases precipitated in the alloy at 993K are 7.2% and 13.3%, respectively. The growth kinetics of the γ' and γ″ precipitates followed the predictions of LSW theory of diffusion controlled growth at all aging temperatures. The activation energy values for the growth of γ' and γ″ precipitates are estimated to be 278kJ/mol and 431kJ/mol, respectively. Some γ″ precipitates contain γ' precipitates inside their bodies or are in contact with the γ' precipitates. It is considered that some γ' precipitates of this alloy act as preferential nucleation sites for the γ″ precipitates.
  • A Mathematical Model for Nb(C, N) Precipitation Kinetics in Austenite Controlled Both by Chemical and Interfacial Energy

    pp. 650-657

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    A mathematical model for predicting precipitation kinetics of Nb(C, N) in austenite has been developed on the basis of the classical nucleation theory and the diffusion controlled growth theory. The local equilibrium condition is assumed to maintain at the interface in which the contribution of the interfacial energy due to the curvature of precipitates is taken into account. With this modification, the changes in diameter and its distribution of precipitates can be evaluated as well as the fraction precipitated in the overall reaction including the Ostwald ripening. Calculated diameters and fractions of Nb(C, N) in both extra low carbon and HSLA steels are in good agreement with those observed. The present model predicts the existence of stasis in the change in average diameter of precipitates prior to the Ostwald ripening. This stasis can be understood as a result of the change in the diameter distribution of precipitates due to the change of the principal driving force for the movement of the interfaces.
  • Influence of Aging at Hot Bands on Recrystallization Temperature of Cold Rolled Sheets in Nb-bearing Ferritic Stainless Steel

    pp. 658-663

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    The influence of aging at hot bands on the recrystallization of cold rolled sheets in a 15mass% Cr-0.46mass% Nb ferritic stainless steel was investigated. The results obtained are as follows:
    (1) The recrystallization temperature of cold rolled sheets which were aged at the temperature range from 700°C to 750°C for 2 hours or long times in hot bands was lower than that of no aging.
    After aging of hot bands, the content of Nb as precipitates was about 0.45%, which was nearly equal to the total content of Nb. The main precipitates was detected as Fe2Nb-Laves phase. It is considered that Fe2Nb-Laves phase, which grows coarsely during hot band aging, is contributed to lowering the recrystallization temperature of cold rolled sheets.
    (2) In the case of no aging in hot bands, a large amount of solute Nb existed in hot bands before cold rolling. Fine Fe2Nb-Laves phase precipitating at approximately 700°C might mainly suppress the recrystallization of cold-rolled sheets.
  • Precipitation Behaviors of γ', ε and η Phases in Fe-Ni-Co-Nb Based Superalloys

    pp. 664-671

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    An Fe-38 Ni-13 Co-4.7Nb-1.5 Ti-0.4 Si alloy (alloy 909) and an Fe-29.5 Ni-19.7 Co-4.0 Nb-1.3 Ti-0.6 A1-0.4 Si alloy (alloy 929) are the latest low thermal expansion superalloys. This study was carried out to elucidate the relation between the age-hardening and the precipitation be-haviors of γ', ε and η phases in these alloys by micro Vickers hardness test and transmission electron microscopy.
    The results obtained in this study are as follows;
    (1) In aging at 893K hardening proceeds gradually with aging time and reaches to the maximum hardness; the hardness after aging for 720 ks in alloys 909 and 929 are HV400 and HV460, respectively. The rate of hardening increases with increaseing aging temperature. After reaching to the maximum hardness the hardness decreases gradually because of the coarsening of γ' phase and the precipitation of ε and η phases. The precipitation and growth rates of these phases in alloy 929 are lower than those in alloy 909.
    (2) The precipitation in alloy 909 follows in a sequence of supersaturated γ phase →γ' phase →Widmanstätten ε phase →cellular η phase. That in alloy 929 follows in a sequence of supersaturated γ phase →γ' phase →Widmansätten (ε+ η) phases.
    (3) The main constituent elements of the ε and η phases in these alloys are Ni, Co, Nb, Ti, and Fe and Ni, Nb, Co, Fe, and Si, respectively. The ε phase contains more Co and less Ni contents than the η phase. There is no distinct difference in the composition between the Wid-manstätten and cellular η phases.
  • Effect of Particle Size of Secondary Carbide on Fatigue Life in Cold Die Steel

    pp. 672-677

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    It has been known that mechanical properties of die steels are affected by carbides, especially primary carbides of which shape is coarse and rod-like.It appears that relatively large and spherical secondary carbides which precipitates in austenite, also influence mechanical properties. The effect ofthe particle size (diameter) of secondary carbide on the fatigue life of JIS SKD11 die steels was studied by tension-compression fatigue tests with stress amplitude between 1.1 and 1.8 GPa and frequency of 0.17 Hz. The diameter of secondary carbides was changed by soaking time at 1100°C under the constant width of primary carbide. The fatigue life decreased with an increase in the diameter of secondary carbide. The effect of diameter of secondary carbide on fatigue life depended on stress amplitude in fatigue tests. It became larger in low stress amplitude. This shows that fatigue life is mainly influenced by primary carbide in high stress amplitude and by both primary and secondary carbides in low stress amplitude. The relationship between fatigue life, N, at stress amplitude of 1.1 GPa and the diameter, d/μm, of secondary carbide was shown by the following equation: log N=4.04-0.48d. The fraction of carbides with void, which was observed after fatigue tests, increased in steels with large secondary carbide. Voids were initiated at all of secondary carbides more than 3 μm in diameter and no of carbides less than 1μm. Tensile strength also decreased with an increase in the diameter of secondary carbide. Voids were initiated at interfaces between secondary carbides and matrix in plastic deformation of tensile test. Tensile stress for void initiation was inversely proportional to the square root of the diameter of secondary carbide.
  • Improvement of High Cycle Fatigue Strength of P/M Ti-6Al-2Sn-4Zr-2Mo/TiB Particulate Composites by the Modification of Matrix Microstructure

    pp. 678-684

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    The present authors have previously shown that in the Ti-6Al-2Sn-4Zr-2Mo/10TiB particulate composites the fatigue fracture originated neither from the interior of particulate nor from the particulate/matrix interface, but rather from the matrix area of the composites. This finding strongly suggested that the high cycle fatigue strength is related with the matrix microstructure. Therefore, the present study aimed at improving the high cycle fatigue strength by the modification of matrix microstructure.
    The Ti-6Al-2Sn-4Zr-2Mo/10TiB particulate composites were produced by the blended elemental P/M method using extra low chlorine titanium powder smaller than 45 μm in diameter. The smooth axial fatigue tests were carried out at an R ratio of 0.1 in the load-controlled mode.
    The matrix microstructure was highly dependent on the cooling rate after the sintering treatment. The faster cooling rate of 10K/sec produced the well-defined colony microstructure. The massive colony microstructure was obtained by the cooling rate slower than 0.3K/sec. The modification of the matrix microstructure from a colony into a fine acicular α microstructure resulted in the increase in high cycle fatigue strength. For example, fatigue strength at 107 cycles was increased from 490 MPa to 550 MPa. The three main contributions to the fatigue strength were suggested as the contribution from the increase in Young's modulus (18%), tensile strength (39%) and microstructural modifi-cation (43%), respectively.
  • Comments on "Formation and Falling Mechanism of Black Oxide Layers in High Speed Steel Rolls during Hot Strip Milling"

    pp. 685-687

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