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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 33 (1993), No. 7

  • Recent Trends in the Production and Use of High Strength Stainless Steels

    pp. 711-720

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    DOI:10.2355/isijinternational.33.711

    The strengthening mechanisms of steels have been well studied widely, and are the same of the mechanisms in high strength stainless steel, e.g., grain refinement by the thermomechanical treatment, solid solution strengthening by lattice distortion through the addition of alloying element, transformation strengthening by martensite transformation, work hardening by the formation of strain induced martensite through rolling, strain aging hardening by the tempering or aging of martensite, and precipitation strengthening of intermetallic compounds which are coherent with the matrix. These strengthening mechanisms relate to the thermomechanical treatment of the steel as well as the chemical composition. Various high strength stainless steels are produced by a combination of these mechanisms, and have peculiar mechanical properties depending on the final microstructure.
    Recently, the need for various other properties than high strength, e.g., ductility, toughness and weldability has been recognized. Work-hardened stainless steel, for example, has greater strength than SUS301 and is used for ID blades (inner diameter blade for cutting Si single crystal), etc. The additional strength of the steel results from solid solution hardening and strain aged hardening. Martensite stainless steel and ultra microduplex stainless steel were also developed and are now in use fitting a variety of needs. The former steel has high toughness with more than 1700 N/mm2 tensile strength, and the latter has good strength and ductility but is not softened on welding.
  • Enhancement of Resistance against Oxidation with Carbon Dioxide for Formed Coke and Electrode-grade Graphite and Carbon by Infiltrating Carbon into Pores

    pp. 721-727

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    DOI:10.2355/isijinternational.33.721

    Formed coke, electrode-grade graphite (E.G.) and elctrode-grade carbon (E.C.) were modified by infiltrating carbon within the pores by use of methane cracking. The rate of oxidation with CO2 was lowered for all these samples but the amount of reduction was dependent on pore structures. Decrease in the rate for the electrode-grade carbon was very large compared to other samples. The mechanism of enhancement of resistance against oxidation was explained in terms of the pore structure changes accompanying infiltration and oxidation.
  • Mass Transfer from a Solid Body Immersed in a Cylindrical Bath with Bottom Gas Injection

    pp. 728-734

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    DOI:10.2355/isijinternational.33.728

    As a fundamental study on the melting process of solid bodies immersed in a molten metal bath, cold model experiments were done to reveal the effect of bottom gas injection on the mass transfer coefficient from a solid body. The mass transfer coefficient was measured by means of the electro-chemical method. Nitrogen gas and aqueous H2SO4 and FeSO4 solution were employed, and gas flow rate and inner diameter of nozzle were changed over a wide range. Spheres of three different diameters and a flat plate made of platinum were chosen as representative bodies. Empirical correlations of mass transfer coefficient from these bodies were derived as functions of Reynolds number, Schmidt number and turbulence intensity.
  • Kinetics and Reduction Characteristics of Hematite-noncoking Coal Mixed Pellets under Nitrogen Gas Atmosphere

    pp. 735-739

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    DOI:10.2355/isijinternational.33.735

    Hematite-noncoking coal mixed pellets were reduced isothermally at 900, 950, 1000, 1025 and 1050°C under constant flow rate of nitrogen gas. The surface characteristic of the reduced pellets for different time-temperature schedule were examined by a Scanning Electron Microscope (SEM). Analysis of the data and examination of the SEM microphotographs revealed that the mechanism associated with the reduction changed with increasing temperature and fractional reaction. Activation energy values of the reduction at different levels of fractional reaction were calculated with the help of an Integration Method.
    The effect of Carbon/Hematite molar ratio on the extent of the reduction was also investigated. It was found that at temperatures 900 to 1000°C fractional reaction increased with increasing carbon content up to a critical value of the ratio and then decreased. At temperatures above 1000°C fractional reaction increased linearly with increasing carbon content.
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  • Mixing Phenomena in a Liquid Bath Stirred by Gas Jets through Side and Inclined Nozzles

    pp. 740-747

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    DOI:10.2355/isijinternational.33.740

    The perfect mixing time, tm, of the water bath, during gas injection through bottom (vertical and inclined) and size nozzles has been measured by using electrical conductivity technique. Effects of gas flow rate, bath depth, nozzle angle and location have been examined. The measurements indicate that under the condition of a shallow bath (H/D=0.31) and large gas flow rate (specific power ε>10–1 W/kg) influences of the nozzle angle and nozzle location on the mixing time are significant. On the other hand, effects of those parameters on tm are negligible in a deep bath (H/D=1.25). Change in the tracer concentration with time was predicted by numerical modelling. A satisfactory agreement between the computed and experimental results can be obtained by parameter fitting for the average plume rise velocity and effective viscosity in the mathematical model. It is presumed that turbulent mass transfer of the tracer is dominant in the deep bath, while the convection of liquid is more effective in the homogenization in the shallow bath.
  • Gas Blowthrough of a Liquid Metal Bath by an Upward Directed Submerged Jet

    pp. 748-756

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    DOI:10.2355/isijinternational.33.748

    A novel instrumentation system has been developed which can measure the fraction of time that a gas jet penetrates from its point of injection to the surface of a liquid metal bath. Such penetration of the bath by injected gas is also termed "blowthrough". Using this new instrument the conditions required for blowthrough have been studied using a liquid tin model of a bath smelting vessel. The experimental results confirm a criterion previously published by Kato et al. for the onset of blowthrough provided the gas injection rate is low. It is shown that the limit of applicability of the Kato criterion corresponds to the onset of compressibility effects in the injected gas. A modification to the Kato criterion is developed which extends its range of applicability into the compressible gas flow regime.
    Experimental results confirm that the modified Kato criterion correctly predicts the influence of injection nozzle diameter, gas density and gas sonic velocity on the gas flow required to cause blowthrough at a given bath depth.
  • Model of Mixed Control for Carbon and Silicon in a Steel Converter

    pp. 757-763

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    DOI:10.2355/isijinternational.33.757

    The removal of carbon in a steel converter has been studied under Gas-Liquid reaction considerations; diffusion and reaction parameters were determined and control regimes tested. Improvement of the prediction of C results in the end of the blowing process can be obtained considering the variation of factors during the process. This is also obtained by introducing the Si-removal in the simulation obtaining the C and Si evolution. Application to control is discussed in relation with the use of a slag reaction model.
  • Mathematical Modelling of Heat Transfer Phenomena in Continuous Casting of Steel

    pp. 764-774

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    DOI:10.2355/isijinternational.33.764

    A steady-state three-dimensional heat flow model based on the concept of artificial effective thermal conductivity has been developed. On the basis of available literature information, boundary conditions to the governing heat flow equation have been applied, and the equation was solved via the control-volume based finite difference procedure. The model is sufficiently general and can be applied to various geometrical shapes of relevance to continuous casting of steel. Sensitivity of the predicted results to various numerical approximation including grid configurations, as well as to other modelling parameters such as axial conduction, mushy zone modelling procedure, choice of value of Keff have been extensively studied. It has been shown that assumptions and numerical procedures influence the computed results significantly. Finally, numerical predictions have been compared with three sets of experimental measurements reported in literature on shell thickness in industrial casters. In contrast to some earlier claims, these indicated only poor to moderate agreement between model predictions and experimental results.
  • Effects of Second Phase Morphology on Retained Austenite Morphology and Tensile Properties in a TRIP-aided Dual-phase Steel Sheet

    pp. 775-782

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    DOI:10.2355/isijinternational.33.775

    The relationship between second phase morphology and retained austenite morphology and the influences of these two kinds of morphology on tensile properties of a 0.17C-1.41Si-2.00Mn (mass%) TRIP-aided dual-phase steel have been investigated in a temperature range between 20 and 400°C.
    A large amount of fine retained austenite was obtained when the second phase morphology was "a network structure" or "an isolated fine and acicular one." The retained austenite particles were nearly isolated in the ferrite matrix away from bainite islands and were moderately stable. On the other hand, "an isolated coarse structure" of second phase resulted in a small amount of more stable retained austenite film along bainite lath boundary.
    The influence of second phase morphology on the flow curve significantly differed from that of a conventional ferrite-martensite dual-phase steel. Isolated retained austenite particles lowered the flow stress, and resultantly reduced the effects of second phase morphology (i.e., network effect or fine grain size effect) on flow stress. However, the isolated retained austenite particles enhanced effectively the ductility, particularly at 50-100°C, due to the moderate strain induced transformation. On the other hand, retained austenite films along bainite lath boundary scarcely influenced on tensile properties of the steel. These results were discussed on the basis of a continum theory.
  • Effect of Initial Orientation on the Recrystallization Behavior of Solidified Columnar Crystals in a 19% Cr Ferritic Stainless Steel

    pp. 783-792

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    DOI:10.2355/isijinternational.33.783

    The effect of the initial orientation of each grain on the recrystallization behavior at 973 K after 70% cold-rolling of solidified columnar crystals in a 19% Cr ferritic stainless steel have been investigated. It was demonstrated that the recrystallization kinetics, the recrystallizaztion microstructure and the orientation of recrystallized grains in each columnar grain strongly depend on the initial orientation. The (001)[100] initially-oriented grains which showed the cold-rolled structure composed of fine elongated deformation bands readily recrystallized and formed fine recrystallized grains. On the other hand, the (001)[110] initially-oriented grains which showed a smooth-etching deformation structure were difficult to recrystallize and formed coarse recrystallized grains after long-time annealing. Because of this difference in the recrystallization behavior among columnar grains, the fully recrystallized structure of the columnar crystal specimen was extremely inhomogeneous in grain size. The fully recrystallized structure was also characterized by the "cube" texture (i.e., (001)[100] texture) which has been scarcely reported in polycrystalline bcc metals and alloys. Most of the recrystallized grains with cube orientation appeared in the (001)[100] initially-oriented grains. These features are discussed relative to the inhomogeneous cold-rolled microstructures of columnar crystals.
  • Creep Properties of α+α2 Type Ti-Al-Sn-Zr-Nb-Si Alloys Designed with the Aid of Thermodynamics

    pp. 793-798

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    DOI:10.2355/isijinternational.33.793

    Effects of microstructural and compositional factors on creep and room temperature tensile properties of α+α2 titanium alloys were examined by using five Ti-Al-Sn-Zr-Nb-Si alloys designed by thermodynamic calculation to have the volume fraction of α2 precipitate (Vα2) of 0.1 or 0.2.
    Increase in the creep strength due to additions of Nb and Si was confirmed in β treated alloys. In the relationship between the creep strength and the amount of primary α phase (Vαp), the peak was observed for the alloys GT-88, 89, and 90, while the creep strength of the GT-91 decreased rapidly with increasing Vαp, and that of the GT-79 was not changed so much with the change in Vαp. The observed dependence of Vαp on the creep strength seemed to result from variations of the Vα2 and the grain size due to the heat treatment. In a room temperature tensile test, remarkable improvements in elongation were obtained by the α+β solution treatment in alloys having Vα2 of about 0.1. From the present results, the Vα2 of about 0.1 at 823 K combined with the α+β solution treatment is proposed as the best design condition for α+α2 type high temperature titanium alloys.
  • Influence of Composition and Thermal History on the Dynamic Recrystallisation and Subsequent Hot Ductility of Mild Steels

    pp. 799-806

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    DOI:10.2355/isijinternational.33.799

    Torsion tests at strain rates 1×10–3, 1.6×10–1 and 1 s–1 and at temperatures between 850 and 1100°C were carried out (1) on Al and Al-Ti mild steels after heating directly to the testing temperature, (2) on Al steel soaking for 30 min at 1200°C, then cooling the specimen to the testing temperature and (3) after soaking, cooling to 700°C and then heating to the testing temperature. Both steels showed a clear trough in ductility between 850 and 1050°C, the ductility being lowest for the directly heated Al treated steel and the soaked, cooled below A1 and heated again, highest for the 1200°C soaked steel and intermediate for the directly heated Al-Ti steel. The lowest ductilities are associated with the inhibiting effect of AlN and TiN exerted on grain boundary migration and preventing the completion of dynamic recrystallisation, as shown also by their effect of increasing the peak strain and making the reaching of equilibrium dynamic recrystallised grain size difficult, which would markedly improve ductility. Soaking at 1200°C, by dissolving and/or coarsening the AlN precipitates has the marked effect of increasing ductility. The present results can be explained in terms of the drag exerted by the dispersed particles making dynamic recrystallisation and grain growth difficult.
  • Influence of Weld Thermal Cycle on Properties of Flash Butt Welded Mn-Cr-Mo Dual Phase Steel

    pp. 807-815

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    DOI:10.2355/isijinternational.33.807

    Flash butt welding of 3.7 mm thick Mn-Cr-Mo dual phase steel was carried out at different final jaw distance (FJD), where besides normal machine cooling (NMC), various instantaneous post weld cooling such as forced air cooling (FAC) and water spray cooling (WAC) were used. The energy input during welding was kept practically constant. The aim of the investigation was limited to study the influence of variation in weld thermal cycle, resulting from the change in FJD and post weld cooling, on the properties of the weldment. It was observed that during NMC the variation in FJD influences the weld thermal cycle and properties, but the application of forced cooling reduces the influence of FJD on them. The increase of cooling rate was found to reduce the softening of HAZ, in presence of reduction in tempering of martensite at a distance of about 6.0 mm from the weld centre. During slow cooling, especially under NMC, the variation in FJD was found to reduce relatively the elongation of the weldment without affecting its UTS significantly where, the weldment was generally found to fail from HAZ, 6-7 mm away from weld centre. However, in case of fast cooling, especially under WSC, the variation in FJD was found to have an insignificant effect on the tensile properties of the weldment and the weldments were generally observed to be fractured from the base material.

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