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

Tetsu-to-Hagané Vol. 89 (2003), No. 7

  • Evaluation of Gas-Liquid Mass Transfer and Interfacial Area in Bubbling Jet

    Takehiko KUMAGAI, Manabu IGUCHI, Tadatoshi NAKATANI

    pp. 735-741

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    Abstract

    Experimental investigation was carried out on the gas-liquid mass flux and interfacial area in a bottom blown bubbling jet. The mass flux was measured by two methods in a relatively low gas flow rate regime. One is based on a change in the bubble size and the other is based on a change in the pH in the bath. When the gas flow rate exceeded a certain critical value, the mass transfer coefficient was satisfactorily apporoximated by an previously proposed empirical equation for a clean liquid, regardless of the degree of contamination. An empirical equation was proposed for the total interfacial area in a bubbling jet as a function of the physical properties of fluids and process parameters.

  • Effects of CaF2, MgO and SiO2 Addition on Sulfide Capacities of the CaO-Al2O3 Slag

    Mitsuhiko OHTA, Takatomi KUBO, Kazuki MORITA

    pp. 742-749

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    Abstract

    The sulfide capacities of the CaO-Al2O3, CaO-Al2O3-CaF2, CaO-Al2O3-SiO2-CaF2 and CaO-Al2O3-SiO2-MgO slags were determined at 1873K to estimate sulfur distribution ratios between these slags and molten steel in the secondary refining process.
    In prior to the sulfide capacity measurement, equilibrium constant for the reaction of sulfur dissolution into molten copper was investigated at 1673-1873K.
    Substitution of MgO for CaF2 was found to be effective for desulfurization when CaO content of the slags was high, but the sulfide capacity considerably decreased when SiO2 was calculated with added slightly in the slags. In addition, the relationship between the sulfur distribution ratio and oxygen content in steel was calculated with the sulfide capacity.

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  • Visualization of Boron in Molybdenum by α-rays Track Etching Method and Tritium Autoradiography

    Hideo SAITO, Fumio MORITO

    pp. 750-757

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    Abstract

    Molybdenum alloys addited with <0.02 ppm B to 160 ppm B were analyzed by α-rays track etching (ATE) method irradiated by thermal neutron for 12 hours using atomic reactor of Rikkyo University and Japan atomic reactor of JRR-4. It was found that boron was segregated along grain boundaries and in the matrix. We analyzed boron distribution in the vicinity of the triple junctions at grain boundaries and in the matrix by the statistical frequency of α-rays tracks. Also we studied tritium autoradiography by cathodic charging method. Visualization of boron distribution was confirmed along the grain boundary which seemed to be effective trapping sites of hydrogen.

  • Experimental and Theoretical Investigation on Tilting in Bar and Rod Rolling UsingDiamond-Square Pass and Square-Oval Pass

    Motoo ASAKAWA, Hideyuki MIYAZAWA, Makoto TOI, Yuki KATAYAMA, Jun YANAGIMOTO

    pp. 758-764

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    Abstract

    Metals under bar and rod rolling often exhibit tilting, due to smaller width height ratio of bar and wire rod under rolling. Design of calibers and rolling conditions to reduce tilting of stock is of industrial importance, as tilting may result in wrinkling and overfilling of rolled product, or miss rolling of bar and wire rod. Characteristics of tilting under bar rolling are investigated experimentally and analytically, aiming to clarify threshold for the occurrence of overturning and restoration resulting from tilting. Tilting of bar under rolling can be classified into two modes. Mode 1 tilting is resulting from steady-state twisting deformation in roll bite. This mode takes place even though position of entrance cross-section is constrained strongly by guide or previous stand, and this mode can be directly analyzed by steady-state FE analysis. Mode 2 tilting takes place in the bar before rolling, when constraint from guide or previous stand is relatively small. This mode can be evaluated using normalized moment acting to entrance of bar obtained by FE analysis. Steady-state FE analysis of bar under rolling is successfully applied to predict tilting of bar in Square-Diamond Pass and Square-Oval Pass. Predicted results agree well with the experimental measurements for tilting of mode 1 as well as mode 2. It can be concluded that FE analysis is helpful to the design of calibers and rolling conditions for bar rolling and rod rolling.

  • Properties of Warm-rolled Steel Plates with Ultrafine-grained Ferrite and Cementite Structures

    Akio OHMORI, Shiro TORIZUKA, Kotobu NAGAI

    pp. 765-772

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    Abstract

    A novel rolling process characterized by multi-pass bi-axial reduction at warm temperatures without a fairly large reduction per pass was pro-posed to obtain ultrafine-grained ferrite structures for steel plates. The process was applied to 0.15%C-0.3%Si-1.5%Mn steel. Steel plates with a thickness of 18 mm, a width of 75 mm and a length of 1100 mm were obtained. The ultrafine-grained structures composed of ferrite and cementite were obtained in most part of the bi-axially rolled plates, while the coarse deformed structures composed of elongated ferrite grains or subgrains remained in many part of the uni-axially rolled ones. The bi-axial reduction likely enhanced the formation of high angle boundaries. The average sizes of the newly evolved ferrite grains were approximately 0.6 μm and 1.2 μm at the rolling temperature of 773K and 873K respectively. The evolution of the ultrafine-grained structure through the process increased the yield strength from 330 to 780 MPa and decreased the fracture appearance transition temperature (vTrs) below 77K. Severe warm rolling at the lower temperature, however, caused the development of a deformation texture and enhanced {100} and {111} ferrite grain colony bands causing "separations" on the fracture surfaces of the Charpy impact specimens with eventual deterioration of the Carpy impact value. The bi-axial reduction was effective for controlling the microscale texture and suppressing the separation.

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  • Spontaneous Reverse Transformation due to Adiabatic Deformation Heating in 0.3%C-9%Ni Steel

    Tomoyuki YOKOTA, Masaru MIYAKE, Yasuhiro SODANI, Masakazu NIIKURA

    pp. 773-780

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    Abstract

    A new concept for ultra refinement processing in austenite grain size; Spontaneous Reverse Transformation (SRT) due to adiabatic deformation heating has been proposed. The occurrence of SRT was verified using a 1000 ton laboratory mill. 0.3wt%C-9wt%Ni steels with configuration of 50×50×300mm were heated to 550°C which is lower than cementite dissolution temperature. The microstructure was tempered martensite before rolling. Reheated steels were heavily rolled with a total reduction of 60%, 70% and 90% under a rolling speed of 50 m/min. Measured plate surface temperature rose up to 675°C which was higher than Ae3 temperature in the case of 90% rolling reduction. SRT occurred only in the mid thickness region for 70% rolling reduction, while all through the thickness for 90% rolling reduction. The microstructure of the region in which SRT occurred consisted of a fresh and fine martensite with high dislocation density and cementite was no longer observed in this region. The size of grains surrounded by high angle boundaries was roughly estimated as around 0.5 μm, which was considered to be either prior γ grain size or martensite packet size. Deformation analysis incorporated with thermal analysis revealed that SRT occurrence behavior was well explained by temperature distribution along the plate thickness caused by adiabatic deformation heating and roll chill.

  • Evolution of Ultrafine-grained Ferrite Structure through Multi-pass Warm Caliber-rolling

    Akio OHMORI, Shiro TORIZUKA, Kotobu NAGAI, Naoshi KOSEKI, Yasuo KOGO

    pp. 781-788

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    Abstract

    Ultrafine-grained steel bars with a cross section of 18 mm square were fabricated through a multi-pass warm caliber-rolling for a 0.15%C-0.3%Si-1.5%Mn steel. The average ferrite grain sizes of 0.43 μm, 0.70μm and 1.2μm were obtained in the isothermal rolling processes at 773K, 823K and 873K, respectively. Despite of inhomogeneous strain distribution by the caliber-rolling, finally, large plastic strain more than 2 or 3 could be introduced into the whole region of the bar samples. Strain accumulation due to the multi-pass warm deformations was confirmed by comparing microstructural evolution through the multi-pass deformations with that through single-pass deformations. The hardness and the grain size of the ultrafine-grained structures formed through severe warm deformations depend on Zener-Hollomon parameter. These indicate that the formation process of the ultrafine-grained structures is "continuous recrystallization" or "in-situ recrystallization". The similarity to single-pass deformations reveals the multi-pass warm rolling to be an effective method to obtain ultrafine-grained ferrite structures.

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  • Formation Process of White Etching Area under Rolling Contact in Bearing Steel

    Hisashi HARADA, Noriyasu OGUMA, Atsushi YAMAMOTO, Harushige TSUBAKINO

    pp. 789-795

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    Abstract

    The formation process of White Etching Area (WEA) under rolling contact was investigated to characterize the WEA formation mechanism using radial type rolling fatigue test machine. The examination was carried out under rapid acceleration and deceleration of rotating speed. From observation of test pieces by Scanning Ion Microscope (SIM) and Transmission Electron Microscope (TEM), it was found that acicular structures were formed before forming WEA. The length and width of acicular structures were about 20 μm and about 2 μm respectively. Acicular structures were formed separately from each other and fine granular structures formed from martensite. It was considered that acicular structures were formed by locally microstructural change of martensite due to rolling contact fatigue. The distance from surface to acicular structures were concerned with the location of maximum value of shear stress under rolling contact. An angle between rolling contact surface and acicular structures wasn't consistent with the analytical shear stress angle. It suggested that the stress to govern the forming the acicular structure was not only shear stress under rolling contact but also secondary dynamic stress.

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  • Ageing Effect on Creep-fatigue Properties of Super-clean 9% CrMoV Steel for Steam Turbine Rotors of Combined Cycle Power Plants

    Li-Bin NIU, Masato OKADO, Tsukasa AZUMA, Yoshihiro SAKAI, Mitsuyuki KOBAYASHI, Hiroshi TAKAKU

    pp. 796-802

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    Abstract

    Focusing on the heating degradation of material properties, which is one of the main factors controlling the lives of steam turbine rotors of power plants, the ageing effect on the creep-fatigue properties of a newly developed super-clean 9% CrMoV steel was investigated in this work, comparing with that of the present 1% CrMoV steel for high-pressure steam turbine rotors. Creep-fatigue tests for the as-received steels and their aged materials were mainly conducted at 566°C in air.
    The creep-fatigue strengths of the two as-received steels show almost the same values. Although the decreasing of creep-fatigue strengths of the aged 1% CrMoV steel is extremely remarkable due to heating degradation, that of the aged super-clean 9% CrMoV steel is very small. By observing the fracture surfaces and the longitudinal sections of the creep-fatigue ruptured specimens, and basing on the microstructural changes, the effects of ageing treatments on the creep-fatigue strengths are discussed. From the viewpoint of creep-fatigue strength, the super-clean 9% CrMoV steel will be a very promising rotor material for the ageing corresponded to the long-term service.

  • Effect of Aluminum and Silicon on Magnetic Properties and Microstructures of Fe-Cr-Ni-C-Al and Fe-Cr-Ni-C-Si Alloys

    Shin-ichiro YOKOYAMA, Tsutomu INUI, Yoritoshi MINAMINO, Nobuhiro TSUJI

    pp. 803-810

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    Abstract

    The magnetic properties and microstructures of Fe-17.5mass%Cr-2.0mass%Ni-0.5mass%C-a mass%Al (a = 0.04-2.38) and Fe-17.5mass%Cr-2.0mass%Ni-0.5mass%C-a mass%Si (a = 0.18-1.94) alloys were investigated in order to develop a magnetic material exhibiting high electric resistivity and softmagnetism when annealed at low temperatures, and paramagnetism when dissolution treated at high temperature. The addition of Al or Si was effective in increasing the electric resistivity, and improving the softmagnetism of the alloys step-annealed at 1053K and 923K. The improvement of the softmagnetism was discussed from the increase in the transformation temperature and variation of the microstructures. In the lower Al or Si content (≤0.7% Al or 1.6% Si), the alloys consisted of (α+ γ +M23C6) phases at 1053K, the first annealing temperature. The mole fraction of the γ phase decreased with increasing Al or Si content because of the increase in the Acl temperature. This change caused the decrease in the dislocation density due to the γ→α' transformation, and resulted in the improvement of the softmagnetism. On the other hand, most of the alloys were transformed to the paramagnetic γ phase by dissolution treatment at 1473K. However, the higher Al content alloys (≥1.91% Al) consisted of (γ + α) phases, and exhibited higher relative permeability than the others. This phenomenon was also interpreted from the increase in the transformation temperature with increasing Al content.

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  • © 2003 The Iron and Steel Institute of Japan

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