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

Tetsu-to-Hagané Vol. 88 (2002), No. 11

  • High Strain Rate Deformation Behaviour of Steels for Auto-Body

    pp. 703-714

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    It has been one of the major interests in automobile industries to improve crashworthiness of auto-bodies as well as the fuel consumption. Crash energy absorption of a component depends not only on the geometry but also the mechanical properties of the materials used. Buckling of thin wall tube structures, which is a typical example of the deformation during crash, is studied theoretically to clarify the effect of strain rate dependence of flow stress. Geometrical and material factors that control the energy absorbing property of components are reviewed. Flow stress at a strain rate up to approximately 103/s can be estimated by using thermal activation models for dislocation motion. Theoretical background and problems remained for application of the models are briefly reviewed. These methods are of use for machine design coupled with materials selection and/or development when high-speed deformation like automobile collision is taken into consideration.
  • Mass Transfer from a Single Stationary Bubble in Highly Turbulent Flow

    pp. 715-722

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    A single CO2 bubble was held in a downward highly turbulent flow field realized in a tapered pipe. The dissolution process of the bubble was recorded with a CCD camera and the images were processed on a personal computer to determine the mass transfer coefficient for the bubble. An empirical equation was proposed for the mass transfer coefficient by referring to the previously obtained experimental findings. The equation is expressed as a function of the size of the bubble and the turbulence intensity in addition to the well-known Schmidt number and Reynolds number. The measured values of the mass transfer coefficient were approximated by the equation within a scatter of ±60%. Such estimation accuracy is acceptable in this kind of measurement.
  • Prediction of the Formation of Eutectic Nb(C, N) in Large Ingot of High-Cr Ferritic Steel

    pp. 723-729

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    Nb addition is widely used for strengthening of low-alloy steel. In case of large ingots, however, where the cooling rate during solidification is low, eutectic Nb-carbide can easily coarsen. Nb addition can substantially reduce the ductility and toughness of the material by the influence of such large eutectic carbide. As the effect of Nb on the strengthening is advantageous in high Cr steel, 12% Cr rotor forgings generally contain about 0.05wt% Nb in order to improve the high temperature strength. In case of 12% Cr steel, Nb forms Nb carbo-nitride (Nb(C, N)), because 12% Cr steel usually contains N of several hundred ppm. This research shows a guideline of chemical composition to avoid the formation of Nb(C, N) for a large 12% Cr steel ingot manufactured by conventional method. Formation condition of eutectic Nb(C, N) in 12% Cr steel and low-alloy steel were compared. It is confirmed that Nb(C, N) is formed less easily in 12% Cr steel than in low-alloy steel. The experimental results were arranged such that the amount of eutectic Nb(C, N) was expressed in an experimental formula as a function of contents of Nb, C and N, and the cooling rate. The experimental formula was verified the investigation of a large 12% Cr steel ingot weighing 68 t, and its validity was confirmed.
  • Investigation on Chemical Structure of Inorganic Matter in Coal by 27Al MQMAS and 1H→27Al CP/MQMAS NMR

    pp. 730-735

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    High resolution two-dimensional 27Al multiple-quantum magic-angle-spinning (MQMAS) NMR being capable of averaging the second-order quadrupolar interaction was applied in order to characterize the mineral matter in dry coals that have been spotlighted recently in new approach to the coal use. Octahedral aluminum compounds between kaolin and other mineral phases could be clearly distinguished by 27Al MQMAS method. According as the total carbon percent increases, octahedral aluminum species tend to vary from kaolin. The 1H→27Al CP/MQMAS NMR that is a combination of cross polarization and MQMAS was also employed to evaluate the connectivity between aluminum atom and hydrogen atom. As a result, the octahedral compounds containing the linkage of Al-O-H such as kaolin and excluding the Al-O-H bond were separated by this technique. MQMAS and CP/MQMAS is very effective in characterization of inorganic matter including quadrupolar nuclei, which is particularly well suited to the analysis of clay minerals that are noncrystalline materials and present in coals although XRD is difficult to analyze the amorphous compounds.
  • Development of Monitoring System of Dehydration of Iron Ore

    pp. 736-740

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    A new monitoring system for water in iron ore has been established. Ore sample in nitrogen gas flow was heated with an electric furnace, and generated water was lead continuously to the Fourier-transform infrared spectrometer (FT-IR) with nitrogen gas carrier. This monitoring system has a time-resolved of 12 sec minimum on the condition with invented gas measuring cell and gas lines. Using this system, different profiles of water generated from several drop-shaped iron ore samples could be obtained. It was found that they should have two peaks of water generated from goethite [α-FeO(OH)] and Kaolin at the different range of temperature.
    Furthermore, the calibration curve with copper sulfate penta hydrate of a standard material showed good linearity. The analytical results of water in ore samples were in good agreement with their values analyzed by JIS method.
    The present system will be applied to characterize the mechanism of pore formation on sinter reaction of iron ore.
  • Determination of Polycyclic Aromatic Hydrocarbons in Coal Combustion Gas Using High Performance Liquid Chromatography

    pp. 741-746

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    This study describes a sampling and analysis procedure for polycyclic aromatic hydrocarbons(PAH) at high temperatures in exhaust gas. Particulate matter sampling was used in conjunction with gas phase sampling. Particulates were collected on quartz fiber filter heated at same temperature as exhaust gas. Vaporous PAH not retained by the filter were cooled at 55°C and trapped from the gas phase on Tenax-GC polymer beads of 10g. The sample volume was about 1m3. Tenax-GC has demonstrated high collection efficiency for benzo(a)pyrene(B(a)P)generated at 375°C under a stream of nitrogen. PAH were extracted with n-pentane for 4h by continuous PAH extractor developed on this study. It demonstrated 99% extraction efficiency for B(a)P spiked on the adsorbent and it was more effective than Soxhlet extraction. The extracts were concentrated to 1ml of n-pentane in a Kuderna-Danish evaporator. Qualitative and quantitative analysis of the extracts were performed by high performance liquid chromatograph (HPLC) with ultraviolet/fluorescence detection. Eight PAH (3, 4, 5, 6-dibenzocarbazole, phenanthrene, anthracene, fluoranthene, pyrene, 2-methylanthracene, benz(a)anthracene, benzo(a)pyrene) were determined in coal combustion gas on reducing NOx procedures. It was demonstrated that the tendency to reduce NOx levels leads to an increase in the PAH present. Moreover total concentration of four PAH (phenanthrene, fluorancene, pyrene, benzo(a)pyrene) in this study is satisfactory agreement with those measured in the emissions of coal-fired power stations in the literature.
  • Mathematical Modeling for the Simultaneous Prediction of Rolling Force and Microstructure Evolution in the Tandem Hot Rolling of Fine Grain Steel Sheets and Plates

    pp. 747-754

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    A new mathematical model to predict rolling force and microstructure evolution in tandem hot strip rolling of fine grain steel sheets and plates has been proposed. This model is composed by combining Orowan's theory for plastic deformation, FDM analysis temperature and incremental modeling for the evolution of microstructure. This model enables us to predict rolling force, rolling torque and microstructure simultaneously from finishing train to the run out table with quite short computing time, although flow stress is directly calculated by dislocation density and the residual dislocation of austenite is reflected to phase transformation.
    Proposed model has been used to characterize tandem hot rolling of fine grain steel sheets and plates. In order to manufacture fine grain steel sheets and plates, heavy reduction tandem hot rolling under low temperature is needed. As rolling force and rolling torque in finishing stands are higher than conventional rolling schedules, it is strongly requested to know whether rolling can be done within the capacity of rolling mill. The proposed model is helpful to design the best schedule to roll fine grain steel sheet and plate considering rolling reduction, rolling speed and rolling temperature.
    Also, there is good relationship between ferrite grain size of the hot rolled steel sheets and plates and residual dislocation density of austenite phase at the onset of phase transformation. Then, regressive equation to describe relation between them is newly proposed. This equation could be applicable to other rolling sequence such as bar rolling, because it is given by general analytical scheme for the evolution of microstructure.
  • Fracture Behavior of Oxide Inclusions during Rolling and Drawing

    pp. 755-762

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    The present study is concerned with the fracture behavior of oxide inclusions during hot rolling and cold drawing of steel rod and wire. Change in the number and size of alumina, zirconia, zircon and silica inclusions during rolling and drawing was investigated. These oxide inclusions were extracted from the steel by the nitric acid dissolution method. The results obtained are as follows:
    (1) All the oxide inclusions (alumina, zirconia, zircon and silica) are fractured during hot rolling.
    (2) Alumina inclusions are hardly fractured, although zirconia, zircon and silica are fractured during cold drawing.
    (3) The magnitude of fracture of oxide inclusions is affected by the compressive strength of oxides.
    (4) Furthermore, the magnitude of fracture of oxide inclusions can be predicted from Young's modulus and mean atomic volume of the oxides.
  • Influence of B in Steel on the Morphology of Selectively Formed Mn-oxides on the Steel Sheet Surface during Recrystallization Annealing

    pp. 763-770

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    Influence of B on the selective surface oxidation behavior of Mn during recrystallization annealing was investigated by glow discharged spectroscopy, secondary electron microscopy and auger electron spectroscopy, on the 0.4mass% Mn added ultra low carbon steel.
    It is well known that both Mn and B are segregated on the surface due to the selective surface oxidation during the annealing in a reducing atmosphere. When the amount of B in the steel was 11ppm, a B-Mn compound oxide is considered to be in molten phase, because the melting point of the compound oxide is lowered than the annealing temperature. As a result, the compound oxide was in globular shape in annealing at 850°C, due to the effect of surface tension.
    In case of B free steel, Mn was found on the grain-boundary as Mn oxide, because of preferable diffusion through grain-boundary after recrystallization annealing. In case of B added steel, B segregated precedingly before recrystallizing, both on the grain-boundary and in the grain. Mn segregated after recrystallizing, forming a B-Mn compound oxide in molten phase. Because the compound oxide is considered to coagulate and move on the steel surface during annealing, the oxide was eventually distributed both on the grain-boundary and in the grain.
    The morphology of a B-Mn compound oxide changed diversely according to the amount of the B in steel. It is considered that the melting point and the chemical composition of the compund oxide influence the morphology of the selective surface oxide of the annealed B added steel.
  • Relationship between Microstructure and Hydrogen Absorption Behavior in a V-bearing High Strength Steel

    pp. 771-778

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    Hydrogen thermal desorption analysis and TEM observation were performed to understand the relationship between microstructure and hydrogen absorption behavior in a V-bearing steel (0.4C-1.0Cr-0.7Mo-0.35V; mass%). The hydrogen absorption behavior was changed by controlling (V, X)C precipitation with change of tempering temperature from 300 to 700°C. Hydrogen was absorbed into the specimen by cathodic charging at a constant condition, and its content was examined by thermal desorption analysis. Energy-filtering and high-resolution TEM methods were employed to observe the size and distribution of (V, X)C precipitates. When the specimen was tempered at around 600°C, the absorbed hydrogen content was markedly increased; 3 times higher than that of the as-quenched specimen. TEM observation showed that this significant hydrogen absorption was attributed to nano-scale (V, X)C coherent precipitates. When the specimen was tempered at 700°C, the absorbed hydrogen content was decreased but was still as high as that of the as-quenched specimen. This was due to spherical (V, X)C incoherent precipitates with about 20nm in diameter. It was estimated from hydrogen desorption profiles that activation energy of hydrogen evolution from trapping sites for the fine coherent precipitates was similar to that for dislocations and lower than that for the coarse incoherent precipitates.
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  • Effects of the Prior Deformation on Dispersion of Carbide Precipitates during Isothermal Transformation in 12%Cr-0.1%C Steel

    pp. 779-785

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    Effect of prior deformation on the microstructure formed through isothermal transformation was investigated for the purpose of controlling the dispersion of carbide particles in 12%Cr-0.1%C steel. Prior deformation promoted isothermal transformation and softening of the steels. In particular, the isothermal transformation above the nose temperature of TTT diagram (1000K) was found to be very effective for shortening the treating time and obtaining a soft material which has equiaxed ferritic structure with homogeneously dispersed carbide particles. However, the behavior of microstructural evolution significantly depends on the prior austenite grain size as well as transformation temperature. In the case of transformation of coarse-grained austenite (>40μm), cellular eutectoid structure is formed along austenite grain boundaries and this results in the hardening of the material. It was also confirmed that the carbon is enriched into the untransformed austenite during isothermal transformation especially in the coarse-grained materials, and this leads to the formation of the cellular eutectoid structure. In this paper, two ways were proposed for suppressing the formation of cellular eutectoid structure and obtaining homogeneous carbide dispersion: One is the refining of austenite grains by the deformation at recrystallization temperature (1213K) and the other is the promotion of intragranular nucleation of ferrite by the deformation at unrecrystallization temperature (1133K).
  • 1010-cycle Fatigue Properties for SUP7 Spring Steels Tempered at 430 and 500°C

    pp. 786-792

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    Giga-cycle fatigue properties were investigated for SUP7 spring steels tempered at 430 and 500°C. Tensile strength levels were 1423 and 1730MPa. Rotating bending, electromagnetic, high-speed servohydraulic and ultrasonic testing machines were used for 30 or 100Hz, 120 Hz, 600Hz and 20kHz tests, respectively. 1010-cycles fatigue properties were obtained in 3 years under 100Hz rotating bending and in 7days under 20kHz uniaxial loading, using 6-and 3-mm-diameter hourglass specimens. 108-cycles fatigue properties were also obtained under 120 and 600 Hz uniaxial loading, using 6- and 3-mm-diameter hourglass specimens. Almost all specimens fish-eye-fractured at internal inclusions of Al2O3. Optically dark areas (ODAs) were formed around inclusions even at 20kHz. The data obtained was analyzed according to the predicted fatigue limit and control volume of specimen for fish-eye fracture proposed by Murakami and others. S-N curves for 430°C tempered steel under 100Hz rotating bending, and 600Hz and 20kHz uniaxial loading were coincident each other, because the control volume was almost the same of 34 and 33mm3. S-N curves under 120Hz unixial loading with the large control volume of 227mm3 shifted to the lower strength level. The inclusion size was nearly proportional to the control volume. Modified S-N curves, that is σaw' versus Nf curves, were independent on the test speed, loading type, specimen configuration and strength level. Here, σw' is the fatigue limite for fish-eye fracture predicted by Murakami and others. These results give us two findings. One is that fish-eye fracture is not the simple hydrogen embrittlement process. Another is that 20kHz ultrasonic machine is effective for accelerated testing method of giga-cycle fatigue.
  • Optimization of Cr Content in High Cr Heat Resistant Steels on the Basis of Long Term Creep Strength

    pp. 793-800

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    There have been many investigations to improve creep strengths of 9-12% Cr ferritic steels, aiming to increase the efficiency of power generation and to reduce CO2 emission to the global environment. We have tried to develop a heat resistant ferritic steel for a steam turbine rotor which would be tolerable even in the ultra super critical condition (e.g., steam temperatures of 620 to 650°C). In the beginning of alloy design, the Cr content was increased to strengthen the average chemical bond between the atoms in steel, which resulted in high creep strength in the test for a short term under the condition of large applied stresses. However, it was found from the creep test for a longer term under the condition of 650°C, 157MPa that a 10% Cr steel was much superior in creep strength to an 11.5% Cr steel, contrary to the results obtained from the creep test for a short term. Thus a series of creep tests was performed with six ferritic steels varying in Cr content in the range of 8.5 to 11.5%. As a result, it was found that an excess amount of Cr had a detrimental effect on the long-term creep strength, because it acted in some ways to promote the recovery and the softening of the martensitic microstructure in the steels. By this effect, a 9% Cr steel exhibited the highest creep strength among the six steels under the very long-term creep condition of 650°C, 98MPa.
    Thus the developing of advanced ferritic steels necessitates the optimization of the Cr content.
  • Plastic Anisotropy in an Electrodeposited Pure Iron

    pp. 801-807

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    Deformation behavior of electrodeposited plates of pure iron was studied. The plate consisting of fine cdamnar grains with (III) fiber texture was found to show Rankford (γ) value as large as 7.6 at 293K. When test temperature was lowered to 77k, the r-value was decreased. The r-value was also decreased when specimens were annealed to enlarge grain size. From observations of surface believes and substructures of deformed specimens, it is speculated that such an extromoly high r-value is mainly caused by grain boundary sliding.
  • Effect of Mo on Strain Aging Behavior in Ultra Low Carbon Steel Sheets

    pp. 808-813

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    In the autobody fabrication, both delayed aging properties at room temperature to suppress stretcher-strain and high bake hardenability are required for bake hardening steel sheets. In relation to the requirement, the effect of Mo on aging behavior has been investigated for ultra low carbon steels. Obtained results are as follows. Increase of yield point elongation during aging at 40°C after skinpass rolling is retarded by the addition of 0.025 mass% Mo. The effect of Mo for reducing aging deterioration is enfeebled as aging temperature rises, then bake hardening treatment at 170°C for 20min provides high hardenability. Thus, it is concluded that the addition of Mo is effective to compromise delayed aging properties and high bake hardenability. The effect of reducing aging deterioration is considered to be caused by short-range atomic interaction.

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