Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 88 (2002), No. 9

  • Catalytic Effect of Iron on the Carbon Gasification Reaction and Its Rate Constants in Iron-Carbon Composite Pellet

    pp. 479-486

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    The rate of the gasification reaction of coal char- and graphite-electrolytic iron composite pellets were gravimetrically measured in CO-CO2 mixtures from 850 to 1000°C. The gasification curves of high accuracy were obtained by a combination of weight loss measurements and carbon analysis by combustion method for reacted pellets. By applying the integrated first-order reaction rate equation to the curves, the apparent gasification rate, k', was estimated. Based on the relation between effectiveness factor and Thiele modulus for the Langmuir-Hinshelwood type catalytic reaction in a spherical catalyst, the straight line relation between (1/k')2 versus R2 was extrapolated to zero R2 to obtain the intrinsic gasification rate. By comparing the intrinsic gasification rates for carbon-electrolytic iron, the catalytic effect of electrolytic iron on the gasification is to increase the rates by the factors of 1.54.9 and 14.3 times respectively. The gasification rate parameters of the rate equation, k=(k0PCO2)/(1+k1PCO+k2PCO2), were evaluated from the two series experiments in the following conditions. Series A: PCO+PCO2=101325 Pa, PCO2=(0.10.2)×101325 Pa. Series B: PCO/PCO2 =4, PCO +PCO2+PN2 =101325 Pa, PCO2=(0.10.2)×101325 Pa. The catalytic effect of the electrolytic iron was in good agreement with that of the iron reduced from a hematite ore, and the results obtained in this study are applicable to the reduction of the iron ore-coal char composite pellet.
  • Development of Cast Copper Cooling Staves and Its Application to Commercial Blast Furnace

    pp. 487-492

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    Cast copper cooling staves have been newly developed to prolong blast furnace campaign life. Mathematical analysis by using the finite element method was carried out to investigate the thermal and mechanical characteristics of both the conventional cast iron and cast copper cooling staves. These calculation results showed that temperature of cast copper cooling staves was lower than that of cast iron cooling staves due to the higher thermal conductivity and the temperature distribution of cast copper cooling staves became uniform. As a result, thermal stress of cast copper cooling staves, caused by temperature difference, was much lower than that of the cast iron type.
    According to approval examination of test staves at actual blast furnace, heat flux through the copper staves was estimated the same level as the cast iron staves by the help of formation of slag accretion layer on the surface of the staves. It became clear that there was no difference between rolled and cast copper staves in practical use although thermal conductivity of cast copper was slightly lower than that of rolled copper. Therefore, it was favorable to use cast copper staves from the viewpoints of the easiness of manufacturing and economical reason.
    In the case of high rate injection of pulverized coal, peripheral gas flow tends to be intensified at the lower part of blast furnace. It was considered that cast copper staves were particularly effective for long life campaign in such a high heat load condition.
  • The Growth of MnS Precipitates in Fe-Si Alloys

    pp. 493-499

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    "In-situ" observation of the growth of MnS precipitates in Fe-Si alloyswas made on cooling using a confocal scanning laser microscope. The MnS precipitates in δ phase grew fast, but those in γ phase grew slowly. Two calculation models for the growth of MnS precipitates were compared each other. One is "diffusion controlled" model, and the other is "Ostwald ripening" model. In this case, MnS precipitates appeared in solid iron supersaturated with Mn and S, so the growth of MnS precipitates would be represented by "diffusion controlled" model. The "diffusion controlled" model was combined with the diffusion and redistribution model of solute elements during the phase transformation in Fe-Si alloy. If the coefficients of volume diffusion were used for calculation, the size of MnS precipitates by calculation model was much smaller than that by "in-situ" observation. It would be because of the effects of the short circuit diffusion, especially the surface diffusion. So the calculation model was arranged to consider both the surface diffusion and the volume diffusion. The calculation results became agree with the observation results. The size of MnS precipitates was measured in surface area and bulk area, and that in surface area was larger than in bulk area. This fact suggests that the surface diffusion is effective.
  • Development of Detection System by Optical Method for Slag Entrained to Teeming Stream in Continuous Casting Process

    pp. 500-506

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    In a continuous casting process, it has been desired to detect slag entrained to a teeming stream from a ladle to a tundish under sealed condition in order to protect molten steel from reoxidization. "Detection system for slag entrained to teeming stream" which enables to detect slag in the teeming stream effectively by an optical method is developed.
    The principle of the slag detection is based on the difference in emissivity and a stream diameter between slag and steel. Both the emissivity and the stream diameter of slag are larger than those of steel.
    This system was tested in continuous casting processes. The mean value of the optical power from slag is about 1.54 times of that from molten steel only. Its mean value from molten steel mixed with slag is about 1.2 times of that from molten steel only. The timing of alarm outputs from the system were compared with the timing of visual detection for 35 times. 97% of alarm output timing appeared in 5 secs before visual detection. And successful results were also obtained from all of the sealing pour tests without visual detection.
    Developed detection system for slag entrained to teeming stream by the optical method which has the features of simple probe structure, easy operation and high accurate slag detection, is expected to contribute to stable operation and casting of good quality products in continuous casting machines.
  • Development of High Temperature Infrared Spectrophotometer System and Its Application for Analysis of Thermal Decomposition Behavior of Coals

    pp. 507-512

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    Thermal decomposition behaviors of coals were measured by high temperature infrared spectrophotometer (high temperature IR) system. It is important to know the thermal decomposition process of coals in order to understand their chemical properties during coking reaction. Two coals were analyzed by this system. The coal samples were heated from 50°C up to 600°C in a N2 ambience, and IR spectra were measured in transmission mode every 10°C. The changes of IR spectra as sample heating were well obtained. Especially, high temperature region over 400°C, a peak intensity based on aliphatic C-H bond was decreased. While a peak intensity based on aromatic C-H bond was stable below 600°C. At least, two types hydroxyl groups were obtained in coal samples. One type is dehydrate below 400°C, another type is stable in high temperature region over 400°C. Additionally, dehydrate behavior of Kaolin which is mineral matter in coal was well obtained. Dehydrate temperature of Kaolin is used for good monitor of sample temperature. The high temperature IR system can measure of functional groups during high temperature region, and useful for analysis of thermal decomposition behavior of coals.
  • Effect of Electrolysis Factors on the Formation of Poly(2-vinylpyridine) Coatings Electropolymerized on Zn

    pp. 513-519

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    The effects of electrolysis factors on the formation of poly(2-vinylpyridine) coatings electropolymerized on Zn substrate have been investigated by cyclic voltammetry technique. The amount of poly(2-vinylpyridine) coatings electropolymerized by cyclic voltammetry was significantly dependent on the pH of the bath and the maximum amount of coating was obtained at pH 5. This result suggested that the specific amount of proton in the bath should play an important role in the electropolymerization of 2-vinylpyridine. The morphology of the coatings was evidently affected by the hydrogen evolution. Under the electrolysis conditions where the hydrogen evolution rate was increased, the coatings contained many pits and streaks caused by vigorous hydrogen evolution. The poly(2-vinylpyridine) coatings produced by cyclic voltammetry were fine in the bath of pH 5 at 20 to 30°C. On the other hand, the current density for polymerization by chronoamperometry was abruptly decreased in the initial stage of two-dimensional coating formation, then it was subsequently decreased with an increase in the thickness of sound coating. The anode and cathode current density of Zn plate in 3% NaCl solution was decreased by poly(2-vinylpyridine) coating formation on it.
  • Effect of Trace Amounts of S on the Steam Oxidation Behavior for Mod.9Cr-1Mo Steels

    pp. 520-525

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    The oxidation behavior of six heats of Modified 9Cr-1Mo steel has been investigated in steam at four temperatures between 873 and 1023K for up to 3.6 Ms, comparing with that of 9Cr-0.5Mo-1.8W-V-Nb steel and 11Cr-0.4Mo-2W-Cu-V-Nb steel. The difference of weight gain due to steam oxidation among the six heats of Modified 9Cr-Mo steel is not large at a low temperature of 873K, but it becomes more significant with increasing test temperature. The heats containing higher sulfur exhibit smaller weight gain than those containing lower sulfur and also than other steels examined. It is concluded that a good resistance to steam oxidation of Modified 9Cr-1Mo steel is closely correlated with higher sulfur content.
  • Change in Hardness and Substructure during Creep of Mod.9Cr-1Mo Steel

    pp. 526-533

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    In order to investigate the structural degradation process during creep, interrupted creep tests were conducted of a Mod.9Cr-1 Mo steel in the range of stress and temperature from 71 to 167 MPa and 873 to 923K. The change of hardness and tempered martensitic lath width were measured in the grip and gauge parts of interrupted specimens. The lath structure was thermally stable in static conditions, but was not stable during creep, and the structural evolution was enhanced by creep strain. The relation between the change in lath width and strain was described in the form, ΔW=α(WS-W0)·ε, where ε is the strain, W0 is the initial lath width, WS is the final lath width depending solely on stress, and α is the constant of the magnitude of 0.67μm/strain. The change in Vickers hardness was expressed by a one-valued function of creep life consumption ratio. A model describing the degradation process of tempered martensitic structures was proposed on the assumption that dislocations were eliminated by the migration of lath boundaries. The model could explain the relation between change in hardness and creep life consumption ratio satisfactorily. The influence of precipitates on subboundaries was discussed in connection with the abnormal subgrain growth appearing in low stress regime.
  • Alloy Designing of High Strength Bainite Steels for Hot Forging

    pp. 534-538

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    Effects of alloying elements on microstructure and mechanical properties in conventional microalloyed bainite forging steels were examined. Regression equations of carbon equivalent Ceq and Bs temperature for reheated and continuously cooled bainite steel were obtained. Coefficients of Ceq equation and Bs equation were well correlated. When steels were strengthened to the same extent by alloying elements, microstructures became fine especially by alloying of Cr, Mn, and Mo. Tempering at 473K improved the impact value. It is assumed that termpering at 473K promoted precipitation of supersaturated carbon in bainitic ferrite. Tempering at 573K decreased retained austenite and improved the 0.2% proof stress.
  • Effect of Co on Creep Deformation and Precipitation Behavior of High-Cr Ferritic Steels

    pp. 539-546

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    The creep behaviour of alpha iron with Co was investigated at temperature of 600°C under stress of 30 to 40 MPa. The minimum creep rate for model alloyed with Co was two orders of magnitude lower than that of alpha iron. The changes of strengthening factor in alpha iron with Co are consistent with the content of Co. The creep behaviour of martensitic 10% Cr steels with Co was also investigated at temperature of 600°C under stress of 250 to 350 MPa. In contrast to alpha iron with Co, the changes of strengthening factor in martensitic steels with Co show no monotonic, but the curve has a maximum, at around 3% Co, as a content of Co, associated with the solid solution strengthening of Mo and W. An excess of Co content like 5% Co acccelerates the Laves phase precipitation reaction in a way similar to increasing the Mo and W content, suggesting that Co increases the supersaturation of Mo and W. This leads to the appearance of the curve with a maximum.
  • Effect of Al Content on r-value of Boron-bearing Al-killed Steel Sheet

    pp. 547-552

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    It has been demonstrated that boron had a beneficial effect on the mechanical properties of continuously annealed Al-killed steel sheets since boron improved the grain growthability by substituting larger BN for AlN. It has been reported that deep drawability of boron-bearing steels could be improved either by optimizing the B/N ratio between 1.0 and 1.5 in wt% or by reducing Al content. However, the effect of aluminum content on r-value in the boron-bearing steel sheets has not been well discussed yet. In this study, the effect of aluminum content onr-value and recrystallization texture formation was investigated for laboratory-melt boron-bearing low carbon Al-killed steel sheets.
    The r-value of continuously annealed boron-bearing Al-killed steel increased with decreasing aluminum content under the same grain size. In the steels with lower aluminum (Al=-0.005%) content, the carbide coarsening in hot-rolled sheet was more significant as compared to those with higher aluminum (Al=0.055%) content. The recrystallization temperature was lowered with decreasing aluminum content, and the development of <111>//ND components was accelerated while that of <110>//ND components was retarded. Since the increase in aluminum content accelerates AlN precipitation in hot-rolled sheets, boron might remain as in solution and segregate to grain boundaries. On the other hand, the reduction in aluminum content causes preferential precipitation of BN in hot-rolled sheets. The improvement of r-value by reducing aluminum content in B-bearing low carbon steels was caused by the development of <111>//ND components which was reduced byboron in solution.
  • Fretting Fatigue Characteristics of New Biomedical β-type Titanium Alloy in Air and Simulated Body Environment

    pp. 553-560

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    In general, fretting fatigue may occur in the artificial hip joint and bone cement as a result of the cyclic loading and fretting between these parts. Fretting decreases dynamic durability and produces wear debris, etc. These can cause various effects on the human body and therefore, it is necessary to know the fretting fatigue characteristics of biomaterials in vivo in order to avoid these effects. In this study, plain fatigue and fretting fatigue tests were carried out in air and in vivo environment, that is, in Ringer's solution on Ti-29Nb-13Ta-4.6Zr, which has been developed recently for biomedical materials, and also on conventional biomedical material, Ti-15Mo-5Zr-3Al.
    In both low cycle fatigue and high cycle fatigue life regions, fretting fatigue strength of Ti-29Nb-13Ta-4.6Zr and Ti-15Mo-5Zr-3Al subjected to various heat treatments decreases remarkably as compared with their plain fatigue strength. In this case, the decreasing ratio of fatigue life by fretting increases with increasing the crack growth area, which is caused by the tangential force at the contact plane of pad. In fretting fatigue in air, degree of damage by fretting, which is indicated by Pf/Ff where Pf and Ff are the plain fatigue limit and fretting fatigue limit, respectively, increases with increasing elastic modulus and hardness. In fretting fatigue in vivo environment, passive film on specimen surface is destroyed by fretting action even in Ti-29Nb-13Ta-4.6Zr, which has excellent corrosion resistance, and, as a result, corrosion pits that lead to decreasing fretting fatigue strength especially in high cycle fatigue life region, are formed on its surface.
  • Analysis of Strain Distribution during Tensile Deformation of Sheet Steel and Determination of Uniform Elongation Using a Constitutive Equation

    pp. 561-566

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    Uniform strain distribution in a tensile specimen during an initial stage of tensile deformation of sheet steel breaks down into the localization of strain after the maximum load point corresponding to uniform elongation. These deformation behaviors were analyzed by a simplified constitutive equation derived from a thermally activated process of dislocation movement. The average deformation behavior measured at fixed gage length is co-related with the local deformation behaviors by means of the equality of load applied on a tensile specimen. Relation between local strain and local strain rate derived from this co-relation exhibits an U-shaped curve which goes through a point of average strain and strain rate, which composes a straight line at fixed strain rate on this strain and strain rate diagram. The U-shaped curve becomes tangent to the straight line expressing average strain and strain rate relation at a strain corresponding to the uniform elongation. Development of strain fluctuation imposed at a local point was analyzed on this diagram. At smaller average strain which corresponds to the left half of the U-shaped curve, the local strain higher than the average exhibits local strain rate lower than the average, which results in a convergence of strain fluctuation and thus uniform deformation. On the other hand, at higher average strain, the local strain higher than the average exhibits local strain rate higher than the average, which results in a divergence of strain fluctuation and the development of localized deformation. Uniform elongation is defined as the strain at the boundary of convergence and divergence of local strain fluctuation.
  • Wear Characteristics of Surface Oxidation Treated New Biomedical β-type Titanium Alloy in Simulated Body Environment

    pp. 567-574

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    Wear resistance is one of the important characteristics for biomaterials. An excessive friction wear is one of the main causes of looseness in artificial hip joints because the progress of wear depresses the mobility and load capacity of the artificial hip joints. While, titanium and its alloys are well known to have excellent biocompatibility among the metallic materials. However, wear resistance of titanium and its alloys is considerably lower than that of Co-Cr alloy and SUS316L stainless steel, which are conventional biomaterials. In this study, therefore, the oxidation treatment was carried out to improve wear resistance of Ti-29Nb-13Ta-4.6Zr, a new β-type titanium alloy, and Ti-6Al-4V ELI, a typical conventional titanium alloy, for biomedical applications. The following results were obtained.
    In oxidized Ti-29Nb-13Ta-4.6Zr, the α phase precipitates below oxide layer on the specimen surface. The highest hardness is obtained in the vicinity of boundary between oxide layer and oxygen solution layer in oxidized Ti-29Nb-13Ta-4.6Zr. When the oxidation treatment is conducted with Ti-29Nb-13Ta-4.6Zr, oxygen solute in the alloy is greater than that in Ti-6Al-4V ELI. Therefore, the oxide layer of Ti-29Nb-13Ta-4.6Zr shows no exfoliation. For a oxidation treatment temperature of 773K, weight loss of Ti-29Nb-13Ta-4.6Zr decreases with increasing treatment time. However, weight loss of Ti-6Al-4V ELI increases in this case. For a oxidation treatment temperature of 1073K, weight loss decreases in both Ti-29Nb-13Ta-4.6Zr and Ti-6Al-4V ELI with increasing treatment time. From the above results, it can be concluded that oxidation treatment is the effective way to improve wear resistance of Ti-29Nb-13Ta-4.6Zr.
  • Analysis of Stainless Steels by Direct Current Helium Glow Discharge Mass Spectrometry

    pp. 575-579

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    The analysis of stainless steel by means of the direct current helium glow discharge mass spectrometry (GDMS) has been studied. Disk samples were specular grinding with alumina paste (1 μm) and preliminary exposed to glow discharge for 7.2 ks in the discharge cell. Ion beam intensities of discharge gas ionic species were measured as a function of discharged voltage (500-2000 V). The generation of excessive diatomic molecule ion (He2+) was not found at a range of 1250-2000 V. Optimum glow discharge conditions were examined and we adjusted the discharge voltage and current to 1000 V and 3 mA, respectively. The relative sensitivity factors (RSFX, Fe; Fe=1) for fifteen analytes obtained using the JSS CRMs, NIST and IARM SRMs were almost less than 1. The analytical results of a practical sample, stainless steel, showed good agreement with results obtained by X-ray fluorescence spectrometry based on the fundamental parameter method and by the chemical analysis.
  • Development of Recycling Process for Stainless Steel Pickling Acids

    pp. 580-585

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    The waste pickling solution used with stainless steel, which consists of mixed fluoric acid and nitric acid, is treated by pH adjustment, and the larger part of the neutralized sludge product is dewatered and disposed of in landfills. Thus, with recent pressure on landfill sites and stricter controls on the fluorine and nitrogen concentrations in waste water, the development of a recycling technology for this waste pickling solution has become an urgent task.
    In the present research, useful metal components and fluorine were separated and the separated fluorine solution was recycled as a pickling solution on both beaker and bench scale levels with the aim of developing a 100% recycling process for stainless steel pickling solutions.
    As a result, a multi-step recycling process was developed at the bench scale level. The features of the process are as follows.
    (1) Fluorine is separated from the neutral sludge by neutralizing the waste pickling solution to pH 10 or higher with alkali.
    (2) The neutral sludge from which fluorine has been separated is washed and dried, and useful metals such as Ni, Cr, Fe, etc. are then recovered using smelting reduction technology.
    (3) On the other hand, the acids and alkali are separated by electrodialysis.
    The regenerated fluoro-nitric acid is concentrated by again applying the electrodialysis technique, and is recycled and used as a pickling solution for stainless steel.

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