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

Tetsu-to-Hagané Vol. 85 (1999), No. 10

  • Effects of Limestone Size on Pore Structure of Sintercake of Iron Ore

    pp. 711-716

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    It is well-known that size of limestone affects productivity and reduction degradation index (RDI) in iron are sintering, and that the change in productivity is due to change in permeability of sinter bed and the change in RDI is due to change in mineralogy of sinter. This work has measured structure and permeability of sintercake by an X-ray CT scanner and pressure proves inserted into sinter bed, respectively, and dis-cussed how their changes contribute to the productivity and RDI. As a result, with increasing in size of limestone, the diameter of pores in sintercake increases because of increase in the volume of free melt. The shape of sinter becomes spherical in the same manner. The change in sintercake structure contributes to the increase in bed-permeability during sintering by about 20%. The change in the shape of pore in sinter reinforces tumbling strength, possibly resulting in improvement of RDI.
  • Coke Disintegration Behavior in the Raceway of the Blast Furnace at Pulverized Coal Injection

    pp. 717-723

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    To clarify the effect of pulverized coal injection on coke disintegration behavior in the raceway, hot model experiments were carried out and changes in reaction mechanisms of coke in the raceway were examined by a mathematical model. Furthermore, a coke disintegration model had been newly developed to analyze the effects of the pulverized coal injection and the coal combustibility on the coke disintegration rate quantitatively.
    As the result of the analysis using this model, pulverized coal injection had a great effect on increasing the coke disintegration rate due to the shrinkage of the oxidation zone existing in front of the tuyere that had a burn-out effect of the degraded layer on coke particles generated by the solution-loss reaction in the depths of the raceway. However, the disintegration rate could be decreased by improvement of the combustibility of the pulverized coal. It was assumed that increase in consumption of carbon dioxide by the pulverized coal resulted in suppress the formation of the degraded layer on the coke particles.
    Based on above mentioned results, the injection lances at Fukuyama No.5 blast furnace were changed from the conventional single lance to the eccentric double lance with high combustibility, then the permeability at the lower part of the blast furnace was considerably improved, and it was suggested that the improvement of the lance structure could be an effective way to restrain the generation of coke fines from theraceway.
  • Determination of Trace Amounts of Arsenic, Bismuth and Antimony in Iron andSteel by Continuous Hydride Generation and ICP-AES or ICP-MS

    pp. 724-727

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    Trace of hydride-forming elements, As, Bi and Sb in iron and steel have been simultaneously determinend by continuous hydride generation and inductively coupled plasma atomic emission spectrometry or inductively coupled plasma mass spectrometry. The effects of masking reagents of matrix elements and acid concentration on the hydride generating conditions are investigated. A standard addition method is necessary for the determination of Bi, to compensate for the interference from alloy element, Ni. Detection limits of As, Bi and Sb are 0.5, 0.8, 0.5 ng/ml by ICP-AES and 0.03, 0.02, 0.03 ng/ml by ICP-MS, respectively, improved a sensitivity of a factor of 1 to 2 orders of magnitude from the conventional solution nebulization method.
  • Electrodeposition Behavior and Wear Resistance of Co-Ni Alloys from Sulfamate Baths for Continuous Steel Casting Mold

    pp. 728-734

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    Electrodeposition behavior of Co-Ni alloys, which has been applied in the coating of continuous steel casting mold, was studied by measuring the polarization curves using a Cu substrate in sulfamate baths of various compositions at 55°C under galvanostatic conditions. The wear resistance of electroplated Co-Ni film was also investigated by sliding wear test of ring-on-plate type at 300°C. The results obtained were as follows:
    (1) In a wide range of bath composition, Co-Ni alloy deposition shows a feature of anomalous codeposition in which the less noble Co is electrodeposited in preference to Ni. The Ni deposition is greately polarized when compared to the Co deposition from their single baths.Therefore, Co is deposited in preference to Ni, though Ni deposition from the alloy plating bath is more largely depolarized than that from its single bath.
    (2) The wear resistance of Co-Ni coating is improved with increasing Co content more than 50% in the deposits. The higher Co content in the deposit is, the thicker oxide film mainly composed of CoO and Co3O4 is formed on the surface of Co-Ni coating, which improves sliding property to prevent adhesive wear between Co-Ni coating and press ring.
  • Field Surveys of Hydrogen Attack in 0.5Mo Steel Equipment and Subsequently Observed Metallurgical Effect

    pp. 735-742

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    Most equipment made of 0.5Mo steel has been operating successfully for a long term in the petroleum and petrochemical industries. However, unusual incidents of hydrogen attack below the Nelson chart imply that metallurgical variations such as microstructure and concentration of impurity elements have an important role in hydrogen attack resistance. The deletion of the Nelson curve is a major concern to the oil refining industry, which now has to assess the integrity of aged, existing equipment made of 0.5Mo steel.
    The sample of 156 heat was investigated by ultrasonic inspection and metallurgical examination. All damaged samples taken from retired vessels and piping after long term service possessed an unusual microstructure of pro-eutectoid ferrite and pearlite with massive M23C6 carbides. On the other hand, all components with standard pro-eutectoid ferrite and acicular bainitic microstructures with Fe3C were not damaged below the critical curve of Nelson chart 3rd edition.
    Based on these surveys of the hydrogen attack incidents, a critical operating limit of temperature and hydrogen partial pressure was determined which accounts for variations in microstructure and the type of precipitated carbide.
  • A Method of Predicting Hydrogen Attack in 0.5Mo Steel Equipment

    pp. 743-750

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    Authors investigated the conditions for hydrogen attack initiation in base metal and weld heat affected zones (HAZ) by comparing operating conditions and material properties on the basis of field surveys. Operating conditions are expressed in terms of hydrogen attack parameter Pw, derived from Shewmon's equation for determining the expansion rate of steel corresponding to methane bubble formation.
    The threshold value of Pw for hydrogen attack initiation varies proportionally to massive M23C6 carbide ratio in base metal, and Pw=-7.8 is obtained as the threshold value for normalized and tempered (NT) base metal showing proeutectoid ferrite and acicular bainite with Fe3C.On the other hand, the resistance of HAZ is correlated to hardness at HAZ. If the hardness is lowered below Hv 230 by post weld heat treatment, HAZ can be considered resistant as NT base metal is.
    Authors defined the severity of hydrogen partial pressure and temperature as HAT (Hydrogen Attack Tendency), by formulating based on Shewmon's equation. Then, a nomogram named a HAT chart is developed to express the length of incubation period by comparing HAT and the threshold value of Pw. The resistance of 0.5Mo equipment to hydrogen attack can be characterized nondestructively by testing hardness at HAZ and identifying carbide morphology of base metal. And then, the remaining life of 0.5Mo equipment can be predicted by the HAT chart.
  • Formation Mechanism of {111} Recrystallization Texture in Ferritic Steels

    pp. 751-757

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    Recrystallized ND//(111) texture is obtained from cold rolled ND//(111) matrix even in single crystals where no grain boundary exists. These ND//(111) recrystallized grains are analyzed to emerge by (112) rotation from cold rolled ND//(111) matrix. This rotation is related to{110} (111) slip systems operated during cold rolling. Recrystallized orientations from cold rolled {111}(110) single crystal are 18 degrees rotated around these (112) axes and deviate from ND//(111) by 6 degrees. Since this deviation is not recognizable in polycrystalline material owing to its symmetry, ND//(111) recrystallized texture in cold rolled commercial ferritic steels such as IF steels or electrical steels, may also be explained by the same (112) rotation, not by conventional ND//(111) rotation.
  • Transition from Ductile Crack Growth to Brittle Fracture of Heavily Deformed Steel Plate

    pp. 758-764

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    The tensile and bend tests were carried out using cracked specimens to clarify the transition behavior from ductile crack growth to brittle fracture. The brittle fracture initiation which occurred after the large plastic deformation was evaluated by comparing such fracture mechanical parameters as a stress intensity factor K and a CTOD δ obtained using the initial crack length with those calculated using the total length include ductile crack extension at the brittle fracture. The latter parameters were denoted as a K mod and a δmod. Although the values of K, δ, and δmod at the brittle fracture depended on the loading mode and/or the crack length, the Kmod gave almost constant value.
    In order to clarify the significance of Kmod, that dependence of K and δ on specimen geometry, in which the stress distribution ahead of crack was equal to that at the brittle fracture initiation under large deformation, was investigated using FEM analysis. Although the values of δ depended on specimen geometry, the values of K were almost constant. It was considered that Kmod was afected by plastic constraint theoretically. Kmod, however, was hardly affected by plastic constraint when the ductile crack grew enough to form a strain distribution peculiar to materials ahead of the crack. Kmod could be used as the criterion for the transition from ductile crack growth to brittle fracture.
  • Strain Rate-temperature Dependency of Impact Tensile Properties and Ductile Fracture Behavior in Ductile Cast Iron

    pp. 765-770

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    Impact tensile tests of as-cast and ferritic annealed ductile cast irons are carried out at strain rates up to 103/s using a servo-hydraulic high speed testing machine. The fractography of the tensile specimens is carefully carried out with a scanning electron microscope. The experimental results demonstrate that the 0.2% proof stress and ultimate tensile strength of the materials used increase with increasing strain rate and lowering temperature. For the as-cast material with higher pearlite content in the matrix, the strain rate sensitivities of the strengths at room temperature are found to be higher than those of ferritic material. According to the relationships between the tensile properties and strain rate-temperature parameter, R-value, it is considered that for the deformation mechanisms of ductile cast irons at strain rates up to 103/s, the thermally activated process is predominantly. Also, the constitutive equations of the impact tensile strengths for these irons are made based on the strain rate-temperature parameter R. It is believed that the proposed equations can be appropriate for engineering applications. While both the work-hardening exponent and the tensile ductility increase by decreasing the R-value. However, the tensile ductility decreases by decreasing the R-value less than 4500K, because of the over-strengthening by deformation constraining occurs: It is supported by observations of fracture surfaces in the tensile specimens that micro-dimple fracture surface area induced from debondings at a large number of small inclusions near the eutectic cell boundaries increases with decreasing R-value.
  • Effect of Strengthening Mechanisms on Limit Stress for Slip Bands Formation under Cyclic Deformation of Hot-rolled Sheet Steel

    pp. 771-777

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    The purpose of this study is to clarify the effect of strengthening mechanisms on fatigue limit for slip bands formation(σws).Various kinds of hot rolled steels were prepared by adding various alloying elements (C, Si and Ti) separately to ferrite plus pearlite steel and ferrite plus martensite steel. Fatigue tests were performed under load controlled condition. Test specimens were fatigue-tested until they failed or reached 107th cycle. After the tests, slip bands were observed by SEM. The ratio of ferrite grains forming slip bands to all the ferrite grains (SBFR) was calculated by using test specimens subjected to the stresses just above σws. As a result, the followings have been clarified: (1) Strengthening of ferrite by addition of solid solution or precipitation elements markedly increased the ratio of σws to tensile strength. This result is quite the same as for fatigue limit for fracture, which was already found in the previous studies. (2) The plastic strain amplitude for slip bands formation tends to decrease as σwsincreases. On theother hand, SBFR increased linearly with plastic strain amplitude. Therefore in creasing in σws is concluded to promote localization of slip bands. (3) And also the calculation of microscopic plastic strain limit for slip bands formation strongly suggested that the strain limit is a kind of material constant of ferrite phase.
  • Manufacturing Process and Mechanical Properties of Fine TiB Dispersed Ti-6A1-4V Alloy Composites Obtained by Reaction Sintering

    pp. 778-783

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    Fine TiB dispersed Ti-6Al-4V alloy composites, which expect high hardness, high strength, high wear resistance and superplasticity, were developed using a powder metallurgy method. Boride powder and Ti-6Al-4V prealloyed powder were mechanically blended in a high energy ball mill.The powder obtained was pressed into dies and consolidated by reaction sintering. The dispersed boride particles produced reactive TiB in the matrix during sintering. In following hot isostatic press (HIP), samples showed a very fine microstructure of TiB dispersing homogeneously. At room temperature, hardness, compressive strength and wear resistance had the tendency to increase as blended boride content increases. Especially, the composite obtained by blending MoB powder showed small size needle-like TiB dispersion and fine near β phase matrix, and in blending MoB of 15 vol% showed a good wear resistance. In compressive and tensile test at 1173K, the TiB dispersed composite exhibits m values of no less than 0.38 and large elongation, and resultantly superplasticity was confirmed.

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