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

Tetsu-to-Hagané Vol. 82 (1996), No. 12

  • Recent Progress of Iron Ore Agglomeration Technology

    pp. 965-974

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    Sintering technology has evolved steadily in the last decade by upgrading specifications of apparatus around the sinter strand and by the sophisticated process control methods. In order to use much difficult-to-sinter raw material without decreases of productivity and product yield and also deterioration of quality, new types of granulation systems for control of mineral constitution formation, such as HPS (Hybrid Pelletized Sinter) Process, CaO Selective Pregranulation Process and Self-Densificication and High-Melting-Point Liquid-Phase Sintering Process, have been developed and installed successfully. Increased productivity has been achieved through improving product yield and permeability in sintering bed by the development of new types of raw mixture feeder, such as ISF (Intensified Sifting Feeder) etc., and by the technology of homogeneous sintering both horizontally and vertically in the bed, such as SECOS (Sintering Energy Control System) etc. In addition, older and less efficient sintering machines are closed, converging on large -scale and small number machines which in turn renewed process control system based on artificial intelligence and introduced high-efficient and compact environmental systems as replace of old ones. Eventually, the major improvements have been achieved in four items:
    (1) Increase use of cheap raw materials such as limonite are and pellet feed.
    (2) Reduce production costs by productivity increase with high product-yield and by man power saving.
    (3) Uupgrade sinter quality in slag volume and reducibility for acceleration of PCI injection to blast furnace.
    (4) Reduce environmental impact with use of high efficient aparatus.
  • Experimental Study of Deformation Process of a Water Droplet Impinging on Polished and Rough Surfaces Heated to above the Leidenfrost Temperature

    pp. 975-980

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    This paper treats the comparison of the deformation process of a water droplet impinging upon a hot polished (smooth) surface with that upon a hot oxide scaled (rough) surface. The surface whose material is SUS304 stainless steel, has been heated to 773K for both the smooth and rough cases and a water droplet diameter has ranged between 0.3mm to 0.7mm. The rough surface has been made artificially by rapid-quenching the impingement side of the test surface piece, which has been heated to 1273K in a furnace for about three hours, by water spraying. Thereby, the so-called oxide film has been formed on the surface. The droplet deformation process has been measured for the two cases where the droplet is struck on the smooth and rough surfaces, keeping the other experimental conditions constant. The deformation process has been found to be significantly different between the smooth and rough cases. The irregularity of the droplet configuration on the rough surface has been found to occur even in a short time after collision and to be amplified as the droplet deformation proceeds. For various Weber numbers, it has been found that the time needed for a droplet to reach a maximum diameter on surfaces can not be arranged as a function of the Weber number alone. Again, the difference between the smooth and rough cases becomes remarkable with increasing the Weber number.
  • Evaluation of Melting Characteristics of Fine Iron Ores by the Tablet Sintering Test

    pp. 981-986

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    Characteristics of iron ores have influence upon sintering operation and sinter quality. Mineralogical properties of fine iron ores affect significantly the melting behavior in the sintering process. In this study, in order to estimate the macroscopic melting behavior which is directly linked to the coalescing of granules and the formation of pore network in the sintering process, tablets consisted of the mixture of fine iron ores and limestone were prepared. Sintering tests were carried out using the tablets, and the deforming behavior of the tablets was quantified as the deforming temperature. The deforming temperature was strongly affected by gangue minerals in iron ores, mainly because of the rising of peritectic temperature of quaternary calcium ferrite by the dissolution of clay into the calcium ferrite melt. The deforming temperature of iron ore mixture depended on that of each iron ore and its mixing ratio.The influence of MgO sources such as serpentine and Ni slag on the deforming temperature was examined. The means to control the melting behavior in the sintering process by the granule design of raw mixtures was discussed.
  • Basic Characteristics of New Shape Formed Coke in Burden Distribution

    pp. 987-992

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    Basic characteristics in burden distribution and charging pattern of new shape formed coke developed in order to improve the properties of small size and low void fraction that pillow-type formed coke has were conducted by 1/3 scale charging model and mathematical model of blast furnace. Basic characteristics, those are, inclination angle, coke-collapse and trajectory of new shape formed coke are almost the same as those of conventional coke. In the case of wall cha.rging of new shape formed coke, until 60% of total charged coke, new shape formed coke is able to be charged without it's rolling to the center. It is possible to apply RABIT model7) to new shape formed coke charging without it's major modification. In the case of new shape formed coke wall charging, the fluctuation in furnace is supposed to be smaller than that in the case of pillow-type formed coke wall charging. Moreover, it's center charging is supposed to be applied to actual blast furnace. More accurate estimation of in-furnace phenomena by mathematical model considering coke reactivit y is a subject to be worked out in future.
  • Effect of the Structures of Injection Facilities on Combustion Characteristics in the Pulverized Coal Injection into a Blast Furnace

    pp. 993-998

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    Concerning a large amount of pulverized coal injection into a blast furnace, the effect of the structures of injection facilities on the combustion of pulverized coal in a raceway cavity has been studied theoretically and experimentally.
    The mixing of pulverized coal with oxygen is important to promote the combustion of pulverized coal in the raceway cavity. The effect of the mixing of pulverized coal with oxygen on the subsequent combustion in the raceway cavity was estimated theoretically. With respect to the structure of injection parts, the effects of the spouting velocity and angle of oxygen jet on the pulverized coal combustion was estimated experimentally using the empty combustion furnace. As the results, the larger spouting velocity of oxygen jet and larger cross angle of the pulverized coal and oxygen injections were concluded to be preferable for the promotion of the combustion of pulverized coal. Furthermore, the effect of increasing pulverized coal flow rate and the steam injection on the combustion of pulverized coal was estimated.
    Based on above discussions, the injection burner and tuyere of the actual size was made and equipped on the commercial blast furnace for availability test. As the results, the performance of the developed pulverized coal-oxygen burner was confirmed for commercial use from the points of combustibility, durability, abilities of the operation, maintenance and security.
  • Mechanism of Internal Cracking in Continuous Casting

    pp. 999-1004

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    To clarify the mechanism of internal cracking in continuous casting, a series of tensile tests of cylindrical ingot with liquid core and internal cracking tests by intentional misalignment in continuous caster were carried out. By analysing the above experimental results with strain accumulation model, it wad found that internal cracking occurs and grows in the range where the total amount of strain given between ZST and ZDT exceeds the critical strain. Severe longer internal cracks which often generates in continuously cast strand is estimated to be caused with the development of solidification. To prevent internal cracking, not only suppression of each incremental strain, but the determination of casting condition and roll-lavout to suppress total amount of accumulated strain smaller, considering the range between ZST and ZDT is significant.
  • Control of Initial Solidification of Steel by Application of High Frequency Magnetic Field

    pp. 1005-1010

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    This research work was conducted to find the possibility of controlling the initial solidification in the continuous casting processes with the triple point by appling high frequency electromagnetic field. Some continuous casting processes such as the horizontal continuous casting have the triple point defined as the intersection of a refractory, a mold and molten steel. Since the triple point has a role of initiating solidification shell, proper initiation of solidification at the point is required. The imposition of high frequency electromagnetic field on the triple point was investigated to achieve the appropriate solidification control.
    First of all, numerical calculations were done to estimate the magnetic pressure and the joule heat. Then, bench scale experiments and plant tests with the horizontal continuous caster in Keihin Works were carried out. An induction coil was installed around the triple point and driven at 3kHz in both cases. For the bench scale experiments, 250kg of plain carbon steel was cast into a round billet with 65mm in diameter, and for the plant tests 50t of high carbon steel was cast into billets with 330mm in diameter. The cast surface appearance and subsurface solidification structure were examined. Without electromagnetic field, the triple point defects such as rough surface and irregular subsurface solidification structure appeared on the cast. On the other hand, almost no such defects were observed.
    Better bonding between the shell tip and the molten metal near the triple point was supposed to be promoted by the electromagnetic field.
  • Creep Damage Evaluation on 2.25Cr-1Mo Heat Exchange Tubes by Intra-granular Distortion Analysis

    pp. 1011-1016

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    Creep damages of 2.25Cr-1Mo steel heat exchange tubes used for 80, 000, 190, 000 and 221, 000 hours in fossil power plants were quantitatively evaluated in terms of intra-granular distortion by TEM. A method for calculating the average intra-granular distortion of 2.25Cr-1Mo steel developed in our previous study was applied. Average intra-granular distortion used for 221, 000 hours was calculated as 0.058 deg./μm and estimated 2.4% creep strain. Relation between creep strain and creep life consumption ratio indicated more than 70% of creep life was consumed at service condition. Heat exchange tube used for 80, 000 hours was calculated 20% of creep life was consumed. Among the three specimens, creep life consumption ratio was linearly increased with operation period. Near the leak portion used for 221, 000 hours high creep strain rate was anticipated from the dislocation structure and non-uniform distribution of intragranular distortion in the tube. It was concluded intra-granular distortion (IGD) analysis was applicable for the evaluation of 2.25Cr-1Mo heat exchange tube employed under and over designed period.
  • Phase Analysis of Ca-containing Inclusions in Ca-treated Steels

    pp. 1017-1022

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    The determination and phase control of Ca-containing inclusions are important for the successful production of Ca-treated steels such as hydrogen induced cracking resisted steels. In this work, the quantitative isolation of calcium aluminate (mCaO·nAl2O3), CaS and CaO in Ca-treated steels has been investigated. All operations of isolation procedure from matrix dissolution to filtration were carried out in a glovebox with an Ar atmosphere. The atmosphere was controlled to a relative humidity of 25% to prevent H2O from attacking the inclusions. Using this protection, 10% bromine-methanol method was capable of the quantitative isolation of mCaO·nAl2O3, whose reactivity with H2O was found to be increased with increasing the ratio of m/n. Constant current electrolysis in 10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte (AA electrolysis method) was capable of the quantitative isolation of mCaO·nAl2O3 and CaS. However, CaO was not isolated. On the basis of these results, the method for evaluating the amounts of the Ca-containing inclusions has been developed. The amount of mCaO·nAl2O3 was evaluated by converting the analytical values of metal elements in the residue isolated by 10% bromine-methanol method into oxide forms. The amount of CaS was calculated from the difference between the amounts of Ca isolated by AA electrolysis method and 10% bromine-methanol method. The amount of CaO was calculated from the difference between the amounts of total oxygen and oxygen as mCaO·nAl2O3.
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  • Plasma Arc Seam Welding Process with Preheating for Welded Stainless Steel Pipe

    pp. 1023-1028

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    To improve the productivity of welded stainless steel pipe, new plasma arc welding with preheating has been developed. The critical welding rate of plasma seam welding is restricted by the amount of molten metal that is generated in welding, and the separate bead that breaks out by the small amount of molten metal in high speed welding must be prevented. The preheating by high frequency current is efficient to increase this molten metal, and optimum preheating temperature for high speed welding is about 300°C. This new welding process has twice productivity as high as conventional plasma arc welding, and enables to manufacture the pipe of 3mm in thickness at a rate of 2.5m/min. At the preheating temperature over 300°C, critical welding rate is decreased caused by the lack of plasma heat transmission to base metal with the surface oxide layer generated in preheating.
  • Effect of Carbon and Phosphorus Contents on the Graphitization of Cementite in High Carbon Sheet Steels

    pp. 1029-1034

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    In the steel with lower Mn and lower P, carbon occasionaly precipitates as graphite instead of cementite under certain annealing conditions. Then, the elongation became to be independent of amount of C contents because of the decrease in cementite and increase in ferrite grain diameter. In this study, in order to produce graphitized sheet steel with good ductility in short annealing period, the following investigation was carried out.
    0.15%Si-0.15%Mn-0.004%S cold rolled sheet steels, with C ranging from 0.05 to 0.65%, with P ranging from 0.001 to 0.05% were provided. These steels were annealed at the temperature ranging from 550 to 700°C for the time ranging from 0.25 to 30h. The microscopic observation was conducted at every annealing condition.
    Under the condition with higher annealing temperature and longer annealing period, a remarkable stimulation of graphitization was brought in the lower P steel with various C, because of the acceleration of C diffusion. Particularly, in the case of steels with low C, the longer annealing period and the extremely lower P contents is necessary for graphitization. This result was thought to be due to the segregation of P, inhibiting C diffusion, on the graphite nucleation site.
  • Effect of Partial Solution Treatment on Martensitic Transformation of 12%Cr Steels

    pp. 1035-1040

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    When high chromium martensitic steels are subjected to partial solution treatment in (austenite+M23C6 carbide) two-phase region, not only the coarsening of austenite grains is effecitively suppressed by insoluble-carbide particles, but also martensitic structure, which has formed during cooling, is refined through the treatment. In this study, the mechanism of refining of martensitic structure was discussed in Fe-12%Cr-C ternary alloys by investigating the relation between Ms temperature and solution treatment conditions, and the effect of insoluble-carbide particles on the substructures such as martensite-lath or -block. The Ms temperature of steels with full solution treatment simply depends on the content of C and Cr, but that of steels with partial solution treatment is dependent on the effective chemical composition of matrix, which is evaluated by taking the amount of C and Cr in insoluble-carbide away from their total content. Refining of martensite-block structure within grains is not caused directly by insoluble-carbide particles, but done indirectly through the refining of austenite grain size. While, on the martensite-lath structure, insoluble-carbide particles supply nucleation sites for laths, and work also as obstacles for growing laths. As a result, martensite-laths in steels with partial solution treament are divided finely by insoluble carbide particles, and the arrangement of laths is tend to be confused.
  • Creep Deformation Mechanism of a High Nitrogen 9Cr-2Co Steel

    pp. 1041-1046

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    The role of nitrogen, which was forcibly introduced up to 0.2mass% to a 9Cr-2Co steel, was investigated in a quasisteady state creep in the range of stress and temperature from 873 to 1000K and 70 to 100MPa. The analysis showed that the stress exponent of quasi-steady state creep rate was about 5 irrespective of nitrogen concentration. Mechanical differential tests conducted during the quasi-steady state creep revealed that there was no instantaneous plastic strain on an abrupt stress change regardless of soluble nitrogen concentration from 240 to 410ppm and the effective stress exponent of average dislocation velocity was nearly equal to unity. The dislocation density determined from the differential tests proved to be raised with the 3.5 to 4th power of applied stress. This larger stress exponent was reduced to 1.8 to 2.6 when an elementary addition rule was taken into account between an internal stress component from precipitates and a dislocation-dislocation interaction. The reduced stress exponent whose origin was ascribable to the dislocation-dislocation interaction was compatible with the stress exponent of single phase materials. These results suggest that the dislocations drag I-S (Interstitial-Substitutional) atmospheres behind them although a large fraction(about 90%) of internal stress in flow stress and their stress exponents of the quasi-steady state creep are both favorable to the metal type behavior of creep.
  • Excitation of Waves on a Molten Metal by an Intermittent Alternating Magnetic Field

    pp. 1047-1052

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    A new process for wave generation on the free surface of a molten metal is proposed where an intermittent alternating magnetic field with the stepwise change of amplitude is adopted to induce the time variation of magnetic pressure generating a wave motion.
    Based on a linear small-amplitude wave theory, a mathematical model has been developed, which describes the wave motion as a function of operating variables such as intermittent frequency, amplitude of magnetic pressure and vessel size. The function is indicated as combination of a free vibration determined by a vessel size and a forced vibration, in which the amplitude of the latter is in proportion to the magnetic pressure difference at the maximum and the minimum magnetic fields.
    To confirm the validity of the mathematical model, an experiment with liquid gallium was performed. It was found from FFT analysis of the wave motion measured by a laser level sensor that the frequencies of the wave motion are composed of an intermittent frequency of the magnetic pressure and the intrinsic frequencies of a vessel. A resonance phenomenon was observed when the intermittent frequency coincided with the intrinsic frequency. Except the resonance case, the amplitude of the wave motion was in proportion to the amplitude of the magnetic pressure difference. These observed results are well predicted by the mathematical model.

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