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

Tetsu-to-Hagané Vol. 83 (1997), No. 3

  • Analytical Research for the Improvement in Iron Ore Sintering Mechanism with the Design of Pseudo-particles

    pp. 169-174

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    Several pretreatment methods of raw mix have been proposed to improve sintering reaction, such as, melting reaction of fine iron ores with limestone fines, assimilating reaction of coarse iron ores with the melt, and so on. For optimizing these pretreatment processes, it is important to clarify the effect of the structure of sinter mix pseudo-particles made by these processes on the sintering mechanism including the permeability during sintering and the combustion efficiency of coke breeze in sintering bed.
    In this study, analytical research was carried out for evaluating the change in the sintering mechanism with some pretreatment methods of raw mix, i.e. the ordinary granulation [a], the high moisture granulation [b] and the fine part selective granulation method of clayish iron ores ([c] : with limestone, [d] : without limestone). Characteristics of sinter mix pseudo-particles were investigated and sintering pot tests were carried out under these pretreatment methods.
    Permeability of packed bed was improved with increase in 2-5mm even-sized pseudo-particles. For increasing the amount of coke combustion (CO+CO2), it was also effective to concentrate coke breeze at the surface of pseudoparticles. The complete combustion rate of coke (CO2/(CO+CO2)) was improved with limestone concentration at the surface of pseudo-particles as case [d].
    Though sintering speed was improved with these intensified granulating process [b], [c] and [d], the strength of sinter was decreased due to increasing in the cooling rate of sinter in the case of high moisture granulation [b]. By contrast, in case of the selective granulation [c] and [d], the strength of sinter was improved due to the increase in melt formation and the decrease of cooling rate of sinter.
  • Desulfurization of Direct-reduced Iron Produced by Heating Iron-ore/Carbon Composite Pellet with Hydrogen and Hydrogen/Water-vapor Gas Mixture

    pp. 175-180

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    The direct-reduced iron produced from iron-ore/coal composite pellet is generally contaminated with sulfur. In this study, iron-ore/coal composite pellet is partially or completely reduced during heating up to given temperatures, 1173 to 1473K, at a constant rate in nitrogen atmosphere. Subsequently, it is treated with H2-H2O, H2, or CO-CO2 gas mixtures to complete reduction and to perform desulfurization. It is shown that hydrogen treatment is effective to remove the sulfur and water-vapor addition to hydrogen can further enhance the desulfurization kinetically and thermodynamically. The desulfurization rate is very fast and is mainly controlled by chemical reaction on the pore surface of reduced iron. On the other hand, to treat it with a CO-CO2 gas mixture is almost invalid to desulfurization.
  • Influence of Sample Shape on Ion Beam Current in Analysis of Fe-based Alloy by Glow Discharge Mass Spectrometry

    pp. 181-186

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    For the analysis of trace elemental constituents by glow discharge mass spectrometry, ion beam ratios (IBRs), ratios of ion currents of analysed elements to that of matrix element, are observed. In general, IBRs are affected by the shape for pin-shaped samples or the size of mask for disk-shaped samples. These effects were studied systematically for Febased alloys. For disk-shaped samples, IBRs are rather independent of the mask size provided that the diameter is in the range of 10-15mm. For pin-shaped samples, IBRs depend on the sample shape. However, it has been shown that IBRs for arbitrary shapes can be corrected to that of a standard shape using an experimental equation. The equation for correction is a linear combination of r, L and L2 and F, where r and L are diameter and discharge length of pin-shaped samples, respectively, and F is a parameter for the shape of cross section. IBRs depend on r linearly. On the other hand, they are dependent on the second order of L. The fact indicates that there exists a discharge length (Lext) for each element giving the IBR an extremum. LextS are 3-6mm for the most elements. By using sample whose discharge length is close to Lext, it is possible to minimize the effect of discharge length on IBRS. The present correction shows the possibility to reduce the coefficient of variation for samples of arbitrary shapes from 40% to less than 10%.
  • Isolation and Identification of Dross Precipitated in 55mass% Al-Zn Bath

    pp. 187-192

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    Based on studies of the electrochemical stability of the intermetallic compound (Al-Fe-Si-Zn) in a 55mass% Al-Zn bath, a method of isolating and identifying the composition of the dross was established.
    The dross was extracted by controlled potential electrolysis at-1.4V vs. SCE in a 10mass% NaOH solution. A small amount of residual Zn was removed by dissolving with concentrated HNO3. The extracted dross was dissolved in HCl and the amounts of Al, Fe, Si and Zn were determined by ICP-AES. The concentration and chemical composition of the dross in rapid-cooled and air-cooled samples were almost equivalent.
    The method of controlled potential electrolysis in an NaOH solution was also found to be applicable to the removal of the upper layer of 55mass% Al-Zn plate for electron microscope observation of the alloyed layer (Al-Fe-Zn-Si intermetallic compounds).
  • Relative Sensitivity Factors for Fe-based and Ni-based Alloys in Glow Discharge Mass Spectrometry

    pp. 193-198

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    Relative sensitivity factors (RSFs) in the analysis of glow discharge mass spectrometry were determined for Fe-based and Ni-based alloys. From 12 Fe-based alloys and 15 Ni-based alloys, pin-shaped and disk-shaped samples were prepared. RSFs of 42 elements in Fe-based alloys and 35 elements in Ni-based alloys were determined using both pin-shaped and disk-shaped samples. The effect of the matrix on RSFs is not recognized. The values of RSFs agree quite well with previous work. However, a systematic deviation of the values of RSFs has been recognized between pin-shaped and disk-shaped samples. This deviation is expressed well by a linear relation. RSFs for disk-shaped samples are obtainable from those for pin-shaped samples using the linear relation and vice versa.
  • Effect of Intermediate Layer of Diffusion Barrier on Tensile Strength of Brazed Joint of Stainless Steel Coated with Ni-Cr-B Alloy Filler

    pp. 199-204

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    An aluminum oxide (Al oxide) intermediate layer was applied as an diffusion barrier to the brazing of a stainless steel using a filler of evaporated nickel-chromium-boron alloy (Ni-Cr-B) film. When the diffusion barrier was not applied, the tensile strength of the brazed joint of a Ni-Cr-B filler coated steel was increased as the filler thickness increased from 7μm to 10μm. On the other hand, a Ni-Cr-B filler of less than 7μm thickness was almost ineffective in increasing the tensile strength of joints without the diffusion barrier. It is presumed that Ni and B in the filler metal diffused into the substrate during the thermal cycle for brazing, and this raised the liquidus temperature of the filler metal to a temperature above the brazing temperature. An intermediate layer of Al oxide was effective in reducing the diffusion of Ni and B in the filler into the substrate. Hence it increased the tensile strength of the brazed joint. The brazed joint of the steel plated with Ni-Cr-B/Al oxide double layers showed sufficient tensile strength when used in an automobile metal honeycomb catalyser.
  • Phase Transformation of Al2O3 Scales Formed on Al-deposited Fe-Cr-Al Alloy Foil

    pp. 205-210

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    The phase transformation of Al2O3 formed on an Al-deposited Fe-Cr-Al alloy foil and an Fe-20Cr-5Al alloy foil of the similar composition, and their oxidation kinetics have been studied in air at 1073-1373K using XRD, SEM and SIMS.
    It was found that the deposition of Al promotes the growth of metastable aluminas. The transformation of metastable Al2O3 to α-Al2O3 formed on the Al-deposited foil is slower than that on the alloy foil. The values of kp for the growth of α-Al2O3 on the Al-deposited foil is smaller than that on the alloy foil in the temperature range 1273 to 1373K. These are attributable to the purity of the Al2O3 initially formed on Al-deposited foil.
    The phase transformation of Al2O3 is reflected by the scale morphology. The nucleation of θ-Al2O3 occurs on the surface of granular γ-Al2O3. θ-Al2O3 having a shape of whisker grows on the oxide surface. The morphology of whiskers is more round, after θ-Al2O3 transforms to α-Al2O3. We obtained the TTT diagrams for the phase transformation of Al2O3 scales formed on both foils in air in the temperature range 1073 to 1373K.
  • Effect of Retained Martensite on High Temperature Deformation of SUS304 with a Fine Grain Size

    pp. 211-214

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    The high temperature deformation properties of two stainless steels, austenite single phase and austenite/martensite dual phases have been studied. These two materials were not conventionally produced but reversely transformed into austenite phase from martensite phase which was induced by large plastic deformation on austenitic stainless steel SUS304 (similar to AISI304) below the temperature of Md point.
    Tensile tests on both alloy sheets with initial grainsize refined below 1μm by such above processing have been conducted at different temperatures from 923K to 1023K at strain rates ranging from 3.0 × 10-5to 1.0 × 10-2 s-1. These both materials exhibit a maximum elongation exceeding 200 pct with very low flow stress. The dual phase material shows more elongation and less flow stress in comparison with the single phase material, and the lower testing temperatures or the higher strain rates, the larger this tendancy becomes clearly. This is the reason why a slight martensite phase which is included in the dual phase material prevents the grain size of austenite phase from coasening during large deformation.
  • Effect of P and C Segregation and Grain Size on Bending Strength of the Carburized and Induction Hardened Steel

    pp. 215-220

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    In order to increase the bending strength of automotive differential gears, a new heat treatment, where induction hardening is carried out after carburizing, was studied. The bending test specimens of a modified JIS SCM822 grade were gas-carburized, followed by quenching, induction hardening and low-temperature-tempering.
    Induction hardening after carburizing prevented intergranular fracture of the case. The results of Auger electron spectroscopy showed that induction heating decreased the phosphorus and carbon segregation at the austenite grain boundaries, which is the cause of the case intergranular embrittlement of carburized steels. The calculation of the diffusion distance indicated that the duration of the induction heating was too short for phosphorus and carbon to resegregate fully to the austenite grain boundaries which were reproduced during the heating.
    Such reduction in segregation and the austenite grain refinement in the case of the carburized and induction hardened steel increased the bending strength by the amount of 36 per cent.
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    1. Estimation of Delayed Fracture Property of Steels Tetsu-to-Hagané Vol.79(1993), No.2
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  • Influence of Ti and N on Strength and Toughness of 780MPa Class Steels and Their Simulated HAZ Toughness

    pp. 221-226

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    Influence of Ti and N on strength and toughness of base plate and HAZ in 780MPa class steels with ultra-heavy thickness of over 150mm was examined using a thermal-cycle simulator.
    (1) Relationship between Ti content and mechanical properties of the base plate under as-quenched condition varies depending on N content. While, toughness of medium-N steels (N = 40ppm) is improved by adding Ti due to microstructural change, toughness of low-N steels (N = 20ppm) deteriorates with increasing Ti content due to precipitation hardening by TiC.
    (2) Influence of Ti and N on simulated HAZ toughness is more significant for double cycle condition than that for single cycle. Ti addition gives remarkable improvement in simulated HAZ toughness of the medium-N steels for double cycle condition. On the other hand, simulated HAZ toughness of the low-N steels deteriorates with increasing Ti content. The governing factors of the simulated HAZ toughness in double-cycle condition are the same as those of the toughness of base plate.
    (3) It is concluded that the low-N steel without Ti addition and medium-N steels with Ti addition of 0.005 0.01% attain both high simulated HAZ toughness and high strength and toughness of base plate.
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    Readers Who Read This Article Also Read

    1. Analysis of Precipitates and Precipitation Behavior in Nb-Ti Bearing Steels Tetsu-to-Hagané Vol.82(1996), No.2
    2. Rate of Dephosphorization of Liquid Iron by Solid Lime Tetsu-to-Hagané Vol.58(1972), No.14
    3. Strengthening Mechanisms of Ti and Nb Bearing Steel Plates Produced by Thermomechanical Control Process Tetsu-to-Hagané Vol.82(1996), No.6
  • Influences of Microstructures of Fatigue Strength of Medium Carbon Microalloyed Steels

    pp. 227-232

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    Influences of the microstructures on the fatigue properties of the medium carbon microalloyed steels were investigated under the rotating bending fatigue tests. Results obtained are as follows: (1) Under fatigue conditions surface relief made of slip bands are found to be generated largely on ferrite in the medium carbon ferrite pearlite steels. The fatigue crack initiates along the surface relief and propagates along the ferrite/pearlite boundary in the early stage of the fatigue crack propagation. (2) Increase in vanadium contents raises the slip band initiation stress and the fatigue strength through strengthening ferrite. Increase in carbon or manganese contents raises the fatigue strength through increasing pearlite volume fraction and refining ferrite grain size. Increase in vanadium contents are more effective to improve the fatigue limit ratio. (3) Coarsening of prior austenite grain size reduces the fatigue limit. This is because the ferrite precipitated along austenite grain boundary are regarded as small fatigue cracks in medium carbon ferrite pearlite steels with fine prior austenite grain size.

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