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

Tetsu-to-Hagané Vol. 66 (1980), No. 12

  • Expansion and Contraction during Hydrogen Reduction of Green and Pre-heated Hematite Compacts Containing Foreign Oxides

    pp. 1593-1602

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    Effects of SiO2 or Al2O3 added to green or pre-heated hematite compacts on the expansion/contraction of the both compacts when reduced in a hydrogen atmosphere were studied by measuring changes in the electrical resistance and thermal expansion/contraction and by examining the relationship of the degree of reduction to the electrical resistance and thermal expansion/contraction. The results obtained are as follows:
    (1) Green compacts are reduced fast in the style of a uniform type where the reduction proceeds simultaneously at the inner parts as well as at the surfaces, while pre-heated compacts are reduced slowly in accordance with the core model where the reduction proceeds gradually from the outer to the inner parts.
    (2) In case of green compacts, the smaller the oxide particle size is, the slower the reduction rate is and also the less the shrinkage due to reduction is.
    (3) Cracking of green and pre-heated compacts containing foreign oxides must be due, among other causes, to the local stresses in compact matrix attributable to the difference in shrinkage among the contained oxides of different sizes and, in some cases, to the so-called phase-separated structure.
    (4) A clear relationship of the electrical resistance and the expansion/contraction is observed over about 95% reduction in case of green compacts except the compact containing γ-alumina and, in case of pre-heated compacts, over about 90% reduction when silica is contained and over about 60% reduction when alumina is contained.
  • Mechanism and Effects of Sintering Method with Previous Preheating Raw Mix

    pp. 1603-1611

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    1. Special sintering method with previously preheating raw mix was investigated by sintering pot test. In this method the upper part of mix bed on the grate was preheated with 300°C hot air before ignition.
    Following results were obtained.
    (1) Degradation of quasi-particles in the mix bed decreased and bed gas permeability increased.
    (2) Coke consumption was reduced without the reduction in production and the deterioration in sinter quality. The potential heat of coke saved was more than the sensible heat of hot air for preheating. It is due to the increase of coke combustion efficiency by preheating mix bed.
    2. Test operation of this method by 460 m2 Dwight Lloyd sintering machine with sinter cooler waste gas (280°C) showed the saving of coke consumption by 4.8 Kg/t-sinter and coke oven gas (ignition gas) consumption by 1.0 Nm3/t-sinter.
  • Melting Property of MgO Containing Sinter

    pp. 1612-1621

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    The solution property of MgO containing materials into the CaO-Fe2O3 system and the melting property of MgO containing sinter during reduction were investigated. The results obtained are summarized as follows:
    (1) The formation of high viscosity silicate and magnetite prevents the slag from being homogeneous and so Cr slag and Ni slag remain unslagged. The formation of dicalcium ferrite prevents periclase from dissolving into the CaO-Fe2O3 system.
    (2) During reduction unslagged MgO makes the shell of solid solution of MgO-FeO with FeO in glassy silicate and its solution is negligibly low.
    (3) For both normal sinter and pellet, all silicate minerals dissolve above about 1 240°C.
    (4) MgO addition in sinter is effective to decrease slag quantity during reduction but the melting property of sinter is not so improved as that of pellet. The proper basicity for sinter and pellet is about 1.4.
  • Effects of Gaseous Sulfide on the Reduction of Iron Oxide Pellets

    pp. 1622-1630

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    Effects of addition of 5% of gaseous sulfide on the rate of reduction of iron oxide pellets were studied in the temperature range of 800°C to 1 000°C with the following results;
    (1) The reduction rate was greatly decreased with the addition of COS and H2S respectively to CO and H2.
    (2) Analysis of the reduction rate based on the unreacted-core model and observation of the fractured surface of partially reduced pellets by using a scanning electron microscope and an electron probe microanalyser demonstrated that the formation of dense FeS shells offered great resistance to the intraparticle diffusion of reducing gas toward the oxide core, thus greatly retarding the reduction of pellets, and that the partial penetration of the sulfide gas into the oxide core was also responsible to the decrease of reduction rate.
    Although the sulfide content of reducing gas in the present work is much higher than that of blast furnace shaft gas, the present results will be helpful for better understanding of the behavior of sulfur in the blast furnace shaft.
  • Rate of Oxidation of Liquid Iron by Pure Oxygen

    pp. 1631-1639

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    The rate of oxidation of liquid iron by oxygen gas has been studied using a volume constant technique.
    The process of oxidation consists of two stages of very rapid oxidation in the early stage and relatively slow oxidation in the second stage after the formation of iron oxide covering the surface of melt. The rate of the dissolution of oxygen into the melt was studied under the presence of three phases of gas, liquid oxide, and liquid iron to make clear the mechanism of oxidation.
    From the experimental results obtained, it was considered that the rate-etermining step under the co-existence of three phases was the transport of solute gas through the stagnant layer in the oxide phase at the gasoxide interface. This mechanism is supported by the following experimental facts: a) The rate of oxidation is proportional to the square root of oxygen pressure in the gas phase. b) The reaction rate decreases with decreasing the stirring of the melt. c) The rate is not influenced by the amount of oxygen and sulphur in liquid iron.
    The apprarent activation energy of the reaction is low, giving a value of 5 kcal/mol.
  • Fracture Behavior of Some Structural Steels under Hydrostatic Tension and Their Fracture-Mode-Negative Pressure-Temperature Diagrams

    pp. 1650-1658

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    The fracture behavior of some structural steels under the hydrostatic tensile stress state has been studied by means of the Bridgman-type specimens. The fracture behavior of these steels had strong dependence on the hydrostatic tensile stress state. It was shown that the higher the hydrostatic tension and the lower the testing temperature, the lower the fracture strain was. A transition from ductile to cleavage appeared on the specimens subjected to the larger hydrostatic tension.
    The ductile-brittle transition pressure (negative pressure) DBTP is suggested to exist from the analogy of ductile-brittle transition temperature DBTT.
    The three dimensional "Fracture Behavior Diagrams" in which hydrostatic tension is taken into consideration have been presented in order to discuss the fracture behavior of these steels in general.
  • Dynamic Recrystallization of Austenite in 18-8 Stainless Steel and 18 Ni Maraging Steel and Its Related Phenomena

    pp. 1659-1668

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    The dynamic recrystallization behavior of austenite in an 18-8 stainless steel and an 18 Ni maraging steel was studied mainly by microstructural observations of the specimens which were water-quenched immediately after the tensile deformation to various strains at temperatures ranging from 800 to 1 200°C and strain rates of 10-310-1s-1. The changes in austenite grain size and hardness at room temperature due to the dynamic recrystallization were also studied.
    The deformation conditions for the occurrence of dynamic recrystallization in both steels were made clear as a function of deformation temperature (T), strain rate (ε) and strain (ε). Dynamic recrystallization takes place under the deformation conditions with Z (Zener-Hollomon parameter) less than the critical value of Zc. Zc is increased with an increase in strain or with a decrease in initial austenite grain size. The addition of Cr seems to have the retarding effect on the occurrence of dynamic recrystallization of austenite. The dynamic recrystallized grain size (D-) is determined only by the deformation condition Z, and does not depend on the strain and the initial grain size. Present result indicates that the dynamic recrystallization is effective for refining austenite grain size. The room temperature hardness of dynamically recrystallized austenite in 18-8 stainless steel and the martensite formed from the dynamically re-crystallized austenite in 18 Ni maraging steel is higher than that of conventionally heat-treated specimens. However, such an increment of hardness due to dynamic recrystallization is not so large.
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  • Effect of Sea Water on Fatigue Crack Growth Rate in High Yield Strength Steels

    pp. 1669-1678

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    Fatigue crack growth rates were obtained in air at 16.7 Hz, (da/dN)air, and those in sea water at 0.167 Hz, (da/dN)cor, for HT80, HY140, HY180, HY240, HP9-4-20, PH13-8Mo, and 18Ni(200) maraging steels. Stress ratios, R, of 0.10 and 0.50 were used. The ratio of (da/dN)cor/(da/dN)air, r0.167Hz, were used as the degree of the effect of sea water on fatigue crack growth. The following results are obtained:
    (1) The value of r0.167Hz was lower in HY140 tempered at 615°C and HP9-4-20 steels.
    (2) The maximum value of r0.167Hz was obtained at ΔK = 50 to 100 kg/mm3/2 for R = 0.10 in all steels used. At ΔK<2040 kg/mm3/2 and ΔK>250 kg/mm3/2, no effect of sea water on fatigue crack growth rate was obtained.
    (3) The value of r0.167Hz depended on Kmax and ΔK at lower and depended on ΔK at higher crack growth rate. The maximum value of r0.167Hz depended on Kmax.
    (4) A positive interrelation was obtained between r0.167Hz and area fraction of brittle fracture in 18Ni (200) maraging steel.
  • The Effect of Sub-critical Quenching Temperature on Fatigue Properties of Press-fitted Axles

    pp. 1679-1688

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    In order to apply a sub-critical quenching method for the improvement of fatigue strength of press-fitted parts of car axles, the effect of sub-critical quenching temperature (500700°C) on the microstructure, hardness, residual stress distribution and fatigue strength was investigated using 12 mmφ notched specimens (0.35%C steel) and press-fitted specimens (0.4%C steel) of 50 mmφ.
    (1) The maximum surface hardness, about 1.2 times higher than the normalized state, was obtained by 650°C sub-critical quenched specimens. The cause is attributable to the solution hardening effect of carbon into ferrite. And it can be considered that the hardness increase in the surface would have a beneficial effect to improve the fatigue limit for the crack initiation, σw1, of notched or press-fitted parts.
    (2) The maximum value of the compressive residual stresses in the surface was generated by 650°C sub-critical quenching, but depths of the compressive residual stress zone were almost independent on the sub-critical quenching temperatures.
    (3) The maximum value of the fatigue limit for failure, σw2, was obtained by 650°C sub-critical quenching. Moreover, a linear relationship between σw2 and the values of the compressive residual stress in the surface was clarified by the sub-critical quenched specimens and the normalized ones.
    (4) It is concluded from the above results that the optimum sub-critical quenching temperature is about 650°C.
  • Properties of Quenched and Tempered 9 % Ni Steel Pipes for LNG Service

    pp. 1689-1695

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    The development of a 9% nickel steel pipe suitable to the transpotation of the liquefied natural gas has been studied to both laboratory and mill scale.
    (1) As-rolled 9% nickel steel plates were formed to the pipes by UOE process. The pipes were welded by using the 9% nickel steel wire with SAW or MIG welding equipment, and quenched and tempered by continuous heating pipe QT installation. As a result the developed pipes had good qualities and properties in both pipe body and weld portion, and satisfied A333 Gr. 8 in the Standard of ASTM.
    (2) The microstructures in the weld portion of the 9% nickel steel pipes produced by pipe-QT process were consisted of the tempered martensite and partial retained austenite structures as same as in the pipe body, and the maximum hardness in the weld portion was only Hv. 264.
    The toughess of both pipe body and weld portion, and the macrostructure of welded joint were of good properties.
    (3) The optimum condition for the rapid heating cycle in a short time is to quench at 790°C and temper at 620°C for 4 min.
  • The Effects of Hydrogen on the Plastic Behavior of Iron

    pp. 1696-1703

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  • Research Activities on Ferrous Metallurgy for 34 Years in the Author's Laboratory

    pp. 1704-1717

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  • Inter-element Effects in Photoelectric Emission Spectrometric Analysis of Steel

    pp. 1718-1726

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  • 良い英文を書くために

    pp. 1727-1728

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  • 抄録

    pp. 1766-1772

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  • Effect of Composition of Slag on Heat Transfer Characteristics in Electroslag Remelting Process

    pp. 1940-1949

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    Temperature distribution in slag bath and heat transfer rate to cooling water at various interfaces were investigated on 80 mmφ ESR furnace. Effective thermal conductivity of slag crust was also measured. CaO-Al2O3-CaF2 slags were used in these experiments. Effects of heat transfer characteristics on melting rate and the shape of metal bath are discussed.
    The results obtained are as follows:
    (1) Overall heat transfer coefficient of the interface region between slag bath and cooling water in ESR and effective thermal conductivity of CaO-Al2O3 are 0.014 cal/cm2·s·deg and 2.2×10-3 cal/cm·s·deg, respectively, and they both increase with the increase in CaF2 content.
    (2) In case of oxide-base slag, temperature in slag bath gradually decreases in radical directions, but in fluoride-base one, it doesn't change so much to the vicinity of slag crust.
    (3) The temperature gradient in slag crust of CaO-Al2O3 slag is very steep. However, it becomes gentle and the temperature gap at the slag crust-mold wall interface increases with the increase in CaF2 content.
    (4) The shape of metal bath differs according to slag composition. Its depth depends on the temperature of slag bath. On the other hand, the height of the cylindrical part of metal bath is related to the heat transfer rate across the slag bath-metal bath interface.

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