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

Tetsu-to-Hagané Vol. 84 (1998), No. 2

  • Thermodynamics on the Formation of Non-metallic Inclusion of Spinel (MgO·Al2O3) in Liquid Steel

    pp. 85-90

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    The non-metallic spinel inclusion, MgO·Al2O3, has a high melting point, and produces an undeformed C-type inclusion in steel products.Therefore, it is very harmful in the production of high grade wire, spring and bearing steels.Thus it is necessary to develop a methodology by which we can predict the occurrence of such inclusion.
    Dolomite crucible was used to increase the magnesium content in liquid iron in the present experiment on thermodynamics of spinel formation.
    Discussed is the thermodynamics of the formation of spinel non-metallic inclusions during co-deoxidation with aluminum and magnesium in the present work, based on the previous work of deoxidation equilibrium with magnesium, calcium and aluminum in liquid iron by the authors in order to evaluate the activities of oxygen and dissolved deoxidizers in the thermodynamic description by the first and second order interaction parameters including the cross product terms.
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  • Accelerative Evaporation Mechanism of Molten Pure Copper and High Carbon Iron by Urea Addition

    pp. 91-96

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    In order to obtain the fundamental information concerning accelerative evaporation of copper dissolved in molten steel with ammoniacal compound, experiments were done by adding urea onto molten pure copper and high carbon iron containing 0.4%Cu at 1150 to 1450°C under an argon atmosphere.
    Evaporated amount of the molten copper increased with the increase of urea amount. At constant urea amount, it increased with increasing temperature at 1150 to 1250°C, while it became constant above 1250°C. Based on the mass balance of evaporated copper to added urea, it was concluded that the molten copper was evaporated as Cu (N3) 2 and this compound decomposed to liberate copper immediately after evaporation.
    In the case of high carbon iron containing 0.4%Cu, iron was preferentially evaporated, so that copper was concentrated in the molten iron. At constant urea amount, evaporated amount of the molten iron increased abruptly with increasing temperature up to 1250°C, while it reached almost constant value at 1250 to 1300°C. After again increasing at 1300 to 1350°C, it remained unchanged above 1350°C. X-ray diffraction analysis showed that the matter entrapped from exhaust gas was composed of graphite, Fe3C and Fe3N. Based on this result and the mass balance, it was concluded that the high carbon iron was evaporated as Fe (CN)3 below 1300°C and Fe (CN)2 above 1350°C.
  • Variation of Inclusion Morphology in Molten Fe-Ni Alloys

    pp. 97-102

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    The morphology changes of the inclusions in molten Fe-42mass % Ni alloy were investigated in the Si-Mn complex deoxidation experiments under CaO-SiO2-Al2O3-MgO slags in order to clarify the mechanism of the morphology control of the inclusions by means of the slag-metal-inclusions reactions.
    An increase in the Al2O3 content of SiO2-MnO inclusions was observed in the process of the deoxidation with time. As the result, SiO2-MnO-Al2O3 ductile inclusions could be produced in Fe-Ni alloy. This was caused by the decomposition of Al2O3 in slag, and the reaction between aluminum in molten alloy and the inclusions. An increasing amount of MgO in the inclusions was also observed simultaneously in that process. That phenomenon caused the formation of MgO-Al2O3 spinel inclusion. It was presumed that transportation of Mg from slags to inclusions would be occurred as the result of the chemical reaction in the same manner as that of aluminum.
  • Lubrication and Friction between Mold and Solidified Shell in Continuous Casting of Steel

    pp. 103-108

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    The laboratory scale experiments have been carried out to evaluate the solid frictional force between the mold and the solidified shell during the continuous casting of steel. The coefficient of solid friction between mild steel and Cu plate increased with increasing the temperature and the measured value was 0.45 at a temperature of 653K. By use of rape seed oil, the coefficient of solid friction decreased from 0.45 to 0.20. The coefficient of solid friction between solid mold flux and Cu plate increased with raising the temperature and the large coefficient of 0.70 was measured at a temperature of 673K. The solid frictional force was independent of the strain rate. The thickness of the liquid mold flux film between the mold and the solidified shell was estimated from the thermal analysis at meniscus zone, which gives the friction force of one third of rape seed oil casting.
  • Effects of Heating and Cooling Rate on Transformation Behaviors in Weld Heat Affected Zone of Low Carbon Steel

    pp. 109-114

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    Effects of welding thermal cycle pattern and microstructure of a low carbon base steel on transformation behavior of heat affected zone(HAZ)were investigated in order to know the relationship between microstructure and the deterioration in toughness of HAZ of high heat input welding.
    Thermal dilatation and microstructural observations showed that the reverse transformation from ferrite to austenite and from austenite to δ-ferrite occurred displacively at rapid heating.Ac1 temperature was decreased by heating rate of 200°C/s in TMCP steel, but it was not decreased in normalized steel.This meaned that reaustenitization was accelerated in non-equilibrium microstructure.Austenite and δ-ferrite transformed displacively were characterized by inner stress and substructure with defects, which promote the formation of grain boundary ferrite (GBF) and acicular ferrite(AF) in cooling.Slow cooling in stable austenite region above 900°C caused the relaxation of inner stress and the restoration of substructure, which were followed by the retardation of the formation of GBF and AF.GBF related to the precipitation of pearlite bordering on GBF.It was already reported by authors that the lowering toughness of HAZ was attributed by the precipitation of degenerate pearlite.
    These results indicate that it is important to simulate thermal cycle accurately in order to evaluate toughness of HAZ of high heat input welding.
  • Fatigue Strength Properties of Bainitic Microalloyed Steels

    pp. 115-120

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    Relation between fatigue strength and static strength, and the relation between fatigue crack initiation and microstructure in bainitic microalloyed steels were investigated, using various bainitic steels changed in heat treatment conditions. Results obtained are as follows : (1) Lowering austenitizing temperature raises the tensile strength (σB), 0.2% yield strength (σy) and fatigue limit (σw) of the air-cooled bainitic steels by reason of the refinement of packet size. An increase in cooling rate raises σB, σy and σw of the air-cooled bainitic steels by reason of lowering bainite transformation temperature. (2) The fatigue limit ratios (σwB) of the air-cooled bainitic steels are almost the same as those of the tempered martensitic steels and the tempered bainitic steels except the bainitic steels austenitized at a higher temperature (1603K). (3) Under fatigue conditions, linear surface reliefs made of slip bands are found to be generated mainly along bainitic ferrite lath in the air-cooled bainitic steels, and a fatigue crack initiates along a linear surface relief. (4) Coarsening of prior austenite grain size over about 100μm reduces the fatigue limit of the air-cooled bainitic steels. This is because the longer bainitic ferrite laths in coarser prior austenite grains are regarded as small fatigue cracks in the bainitic steels with fine prior austenite grains.
  • Effect of Loading Rate on Deformation and Fracture Properties of Notched Steels

    pp. 121-126

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    The effects of loading rate on deformation and fracture properties of structural steels have been investigated using notched round bar steel specimens. FEM analysis was carried out to evaluate stress and strain condition around notch region. Because of high speed straining, plastic work leads to temperature increase in the vicinity of notch tip. Strain rate dependence of tensile strength was evaluated equally as temperature by strain rate-temperature parameter (R parameter) considering temperature increase in the notch region. It is proved that ductile-brittle transition temperature did not rise with dynamic loading in the case of the steels with good deformability or the specimen with low stress concentration factor. This is because that large amount of plastic deformation leads to high temperature increase around the notch under dynamic loading, which eliminates the effect of strain rate for brittle fracture.
  • Thermo-mechanical Treatment for Improvement of Superplasticity of SUS304

    pp. 127-130

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    A martensitic transformation which was induced by large plastic deformation on austenitic stainless steel SUS304(similar to AISI304)is reversely transformed into primary austenite phase with grain size of about 1μm by annealing.One more such thermo-mechanical treatment is applied for SUS304 with fine microstructures, grain sizes are measured and superplastic behaviors are examined.The grain sizes are extremely hyper-fined below 500nm, and the maximum ductility over 400 pct and the strain rate sensitivity index(m)of about 0.45 are obtained at high strain rate of 1.8×10-3s-1.The changes of microstructure during deformation are observed and discussed under the tensile conditions exhibiting the maximum elongation by using a transmission electron microscope.The same quantities of dislocation are seen at 200 pct and 300 pct deformation.The superplastic behaviors of SUS304 are accommodated to dynamic recrystallization.
  • Relationship between Charpy Impact Toughness and Microstructure in Heat Affected Zone of High Heat Input Low Carbon Steel Weldment

    pp. 131-136

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    Although it has been known that toughness of heat affected zone (HAZ) in low carbon steels lowers with increasing heat input, high heat input welding has been demanded from economical reason. Toughness of HAZ has been often investigated using synthetic HAZ, but thermal cycle in high heat input welding has not been well known. The thermal cycle in tandem submerged arc butt welding in a low carbon steel was measured using thermocouples specially designed. Cooling rate of HAZ markedly changed in around A1 transformation temperature in high heat input welding, that is high cooling rate above the temperature and low cooling rate below the temperature. Toughness of synthetic HAZ of high heat input welding with high cooling rate above 700°C and low cooling rate below 700°C deteriorated compared with that of synthetic HAZ cooled at a constant low cooling rate. This result was related with degenerated pearlite precipitation in synthetic HAZ of high heat input welding.
    Charpy impact absorbed energy of actual HAZ scatters in a wide temperature range of transition zone. This has disturbed to understand toughness of HAZ. The scattering of absorbed energy occurred in a narrow temperature range in synthetic HAZ and in a wider one by Izod impact test. It appears that the scattering of absorbed energy in actual HAZ is caused by strength heterogeneity of actual joints because of no strength heterogeneity in synthetic HAZ and larger strength heterogeneity by Izod impact test compared with Charpy impact test.
  • The Estimation of Bainite Transformation Temperatures in Steels by the Empirical Formulas

    pp. 137-141

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    The empirical formula showing the relation of the BS temperature to the chemical composition of steels was obtained from published isothermal transformation diagrams. The bainite transformation temperatures on the continuous cooling at the critical cooling rates corresponding to the beginning of the pearlite region and to the appearance of the fully pearlitic stage transformation were obtained from published CCT diagrams, and these temperatures (B1 and B2) were correlated to the chemical composition of steels for the wide range of the steel composition.
  • Microstructure and Grain Size Dependence of Creep and Creep-fatigue Properties of Low Carbon Medium Nitrogen Type 316 Steel

    pp. 142-147

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    The effect of grain size on the creep and creep-fatigue behavior of low carbon medium nitrogen type 316(316FR) steel has been investigated at 550°C, compared with conventional type 316(SUS316) steel.The microstructure of ruptured specimens has been examined with electron microscope.The creep rupture strength of both steels decreased with increasing grain size.The rupture strength of 316FR was higher than that of SUS316 at any grain size.The rupture ductility of 316FR, where very small Laves precipitated on the grain boundary during creep, slightly decreased with increasing grain size, while that of SUS316, where carbide precipitated on the grain boundary, did not change with grain size.The rupture elongation of 316FR at largest grain size, however, was much higher than that of SUS316.This difference in rupture ductility is attributed to the grain boundary carbide developed during creep.The creep-fatigue life of both steels decreased with increasing grain size.Because 316FR showed longer life and smaller dependence on grain size than SUS316, 316FR had much longer life than SUS316 at large grain size.The fracture mode of 316FR changed from transgranular to intergranular with increasing grain size, while that of SUS316 was intergranular at any grain size.Although 316FR showed intergranular fracture at very large grain size, it had much longer life than SUS316.This means that the crack propagation along grain boundary of 316FR is more difficult than SUS316.Such difference in crack propagation behavior may be depends on the grain boundary precipitation, that is, Laves or carbide.
  • Development of Car Production in East Asian Countries by Japanese Manufacturers and Its Influence on the Demand for Steel

    pp. 148-153

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    We evaluated the managerial environment of East Asian countries for car production at present and in future by Japanese manufactures and made clear the relation between the total capital and the managerial environment. Then the increase of steel demand corresponding the car production in this region was estimated. In this research, we used the Analytical Hierarchy Process (AHP) in taking into account qualitative factors too. As a result, the steel demand for cars in 2000 in this region was estimated 3, 210 thousand ton per year, which increases by 74.1% in comparison with the value in 1994.
  • Seebeck Effect of Fe-Al-Si Alloy and Low Temperature Thermoelectric Properties

    pp. 154-158

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    The thermoelectric conversion offers unique possibility of a gigantic electric power generation utilizing low-temperature heat sources below 600K. Such sources are solar heat, terrestrial heat and exhaust waste heat from the central-station steam-electric plant. They are thermodynamically low grade but infinite and gratis. The immense production of electricity by this principle is made possible exclusively by means of the thermoelectric junction between the iron-based alloys as the conversion materials because these components of the generator can be manufactured on an efficient mass production basis.
    Among several possible thermoelectric junctions basically consisted of iron, the Fe13%Al12%Si (p) -Fe12%Al (n) alloy couple has been found to exhibit a relatively high value of thermoelectric power, 71μV/K. The merit of this thermoelectric power junction with respect to the thermoelectric conversion has also been described by measuring other thermoelectric properties of these elements.

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