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

Tetsu-to-Hagané Vol. 107 (2021), No. 3

  • Influence of Ore Assimilation and Pore Formation during Sintering on Reduction Behavior of Sintered Ores

    pp. 185-193

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    DOI:10.2355/tetsutohagane.TETSU-2020-087

    Sintered ores with superior high-temperature reducibility can enhance indirect reduction at the shaft and permeability at the cohesive zone through improving the softening-melting behavior, resulting in stable operation of the blast furnace at low coke rates. As the depletion of high-grade ore deposits limits control of the chemical composition of sintered ores, small pores less than 10 µm in diameter in the microstructure of sintered ores were focused on to increase the high-temperature reducibility. Sintering conditions for increasing small pores in constant raw material conditions were examined. Furthermore, considering the heterogeneous structure of the sintered ores, reduction behaviors of relict ores and assimilated structures were estimated individually. The contribution of each structure to small pore formation during sintering and their influence on high-temperature reducibility was discussed. Sintering with a sharp temperature profile led to many small pores in the sintered ores by increasing the amount of relict ores with small pores even in constant raw material conditions. Both, for relict ores and assimilated structures, low-temperature reducibility was determined by the total porosity including large pores, whereas the Al2O3 content in gangue minerals, the <10 µm pore volume fraction, and the amount of gangue mineral influenced high-temperature reducibility. Assimilated structures involving granular hematite contained many small pores, compared with other types of assimilated structures. Results of plant trials for two different methods to increase small pores, in relict ores and in assimilated structures, revealed their potential for improving the high-temperature reducibility of sintered ores without controlling the chemical composition.
  • Effect of Large Amount of Co-injected Gaseous Reducing Agent on Combustibility of Pulverized Coal Analyzed with Non-Contact Measurement

    pp. 194-201

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    DOI:10.2355/tetsutohagane.TETSU-2020-107

    Nowadays, reduction of amounts of CO2 from ironmaking process is important from the aspect of prevention of the global warming. COURSE50 is Japanese national project which aims to reduce the amount of CO2 emission from ironmaking process by 30% by 2050. In COURSE50, we try to reduce iron oxide with H2 to decrease amounts of carbon use, by injecting large amount of gas containing H2 and pulverized coal (PC) from tuyere. In that case, PC combustibility can be different from that in general blast furnace condition, due to high co-injected reducing gas ratio. Though a large number of researches about PC combustion around tuyere of blast furnace has been carried out, the effect of large amounts of co-injected reducing gas on PC combustibility was hardly investigated. To evaluate that, we conducted experiments with two experimental furnaces equipped with various non-contact measurement apparatus and found that;1) The larger amounts of co-injected reducing gas were, the faster O2 and CO2 consumption, and CO and H2 generation in the raceway.2) The amounts of co-injected reducing gas should be optimized for higher PC combustibility.3) Co-injected reducing gas activated PC combustion by raising PC temperature, and that resulted in acceleration of PC.4) Trade-off relationship between rapid heating effect and O2 consumption of co-injected reducing gas could determine the optimum amounts of reducing gas.Consequently, we elucidated how we could co-inject reducing gas with PC as reducing agents without deteriorating PC combustibility.
  • Solidification Behavior of Ti-6Al-4V Alloy

    pp. 202-209

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    DOI:10.2355/tetsutohagane.TETSU-2020-083

    It is important to understand the solidification behavior of titanium alloys for optimizing the casting conditions. In this study, to evaluate the solidification behavior of the Ti-6Al-4V alloy, an experiment was conducted using a lab-scale electron beam furnace. After melting the surface layer of the ingot through electron beam heating, the surface layer was allowed to solidify. Based on the measurement results of the cooling curve of the surface of the ingots, it was observed that the solid was subject to undercooling during its formation. The cooling rate of the ingot could be predicted through numerical simulation, for the melting and solidification of the ingot. The primary and the secondary dendrite arm spacing were examined with respect to the cooling rate. The concentrations of Al and V in the dendritic region were analyzed using electron probe microanalysis (EPMA). It is clarified that the Al is segregated into the dendrite core during solidification, and that V is segregated into the interdendritic region.
  • Accuracy Improvement of the XRD-Rietveld Method for the Quantification of Crystalline Phases in Iron Sintered Ores Through the Correction of Micro-absorption Effects

    pp. 210-218

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    DOI:10.2355/tetsutohagane.TETSU-2020-098

    The mass fraction of each crystalline phase in inorganic materials can be investigated using the Rietveld refinement of the X-ray diffraction (XRD) patterns. For quantitative analysis, differences in the values of the linear absorption coefficient, μ, among the crystalline phases must be considered when certain X-ray sources are used, because such differences often affect their mass fractions. Herein, we evaluate the effects of the differences between the Cu and Co X-rays on the mass fractions of the crystalline phases in iron sintered ores using the XRD-Rietveld method by performing two types of XRD measurements. Type 1 samples modeled materials with two different particle size combinations of α-Fe2O3 and ZnO. Type 2 samples used powder mixtures to simulate iron sintered ores composed of α-Fe2O3, and synthesized SFCA and SFCA-I in various mass fractions. Moreover, a correction method was developed using the Taylor-Matulis (TM) correction that considers the μ of each phase and the average particle diameter of each crystalline phase determined by scanning electron microscopy with energy dispersive spectroscopy. For type 1 samples, results that were in good agreement with the initially-charged mass fractions could be obtained using the TM correction, even in the presence of significant differences in R between α-Fe2O3 and ZnO. The results for type 2 samples confirmed that quantitatively accurate mass fractions could be obtained using the TM correction with an accuracy of approximately ±3 mass% for Cu and Co sources, whereas the error was greater than ±3 mass% for Cu source when the TM correction was not applied.
  • Application of an Inverse Heat Conduction Solution with Laplace Transform Technique for a Steel Cooling Processes

    pp. 219-228

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    DOI:10.2355/tetsutohagane.TETSU-2020-095

    When we evaluate complicated cooling heat transfer characteristics of high-temperature steel with water, a solution of the inverse heat conduction problem (IHCP) is essential to specify the surface heat flux and surface temperature change with time from the measurement of the inside solid temperatures. In the present study, the analytical solution of the IHCP with the Laplace transform technique proposed by Monde is applied to steel cooling processes. This technique deals uniform and constant thermal properties of the solid, and the measured temperature change at two depths is approximated with half-power polynomials of time. The steel cooling processes show a very high cooling rate and a large temperature drop after quenching. Therefore, the temperature dependence of thermophysical properties could not be negligible, and much better accuracy of the approximate functions is also required for the complicated measured temperature history. We applied modifications to the analysis, such as a simple and explicit treatment of the temperature-depending properties and superimposing approximate functions. The accuracy of the modified technique was assessed for quenching temperature histories in a steel specimen. From the sensibility of approximation parameters constructing the superimposed functions, the optimum parameters were specified for the measured temperature of quenching experiments. The explicit treatment of the thermal properties ensures accurate estimation results for a higher cooling rate, in which the product of the temperature gradient near the surface and the temperature gradient of the thermal conductivity is small.
  • Effect of Reaction Product of Epichlorohydrin and Imidazole on the Electrodeposition Behavior of Zn-Ni Alloy from Alkaline Zincate Solution

    pp. 229-236

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    DOI:10.2355/tetsutohagane.TETSU-2020-108

    Electrodeposition of Zn-Ni alloy was performed on a Cu electrode at a current density of 10–5000 A·m−2 and a charge of 5 × 104 C·m−2 in an unagitated zincate solution at 293 K containing the reaction product of epichlorohydrin and imidazole as brightener. The effect of brightener on the deposition behavior of Zn-Ni alloy was investigated. The transfer current density at which the deposition behavior shifts from the normal type to anomalous was 50 to 100 A·m−2 in brightener-free solution, while it became 10 to 20 A·m−2 with an addition of brightener, indicating that the brightener greatly decreased the transfer current density. The transfer current density corresponded to the current density at which the potential of total polarization curve significantly shifted from the more noble region than the equilibrium potential of Zn to less noble region. With an addition of brightener, the decrease in transfer current density is attributed to suppression of hydrogen evolution, and the current efficiency for alloy deposition at high current density region decreased due to suppression of both Zn and Ni deposition. The Ni content of deposited films decreased with brightener, indicating that Ni deposition was more suppressed with brightener. With increasing current density, the crystals of films deposited from the solution containing brightener became smooth and showed the significant brightness. The oxidation reaction of films deposited from the solution containing brightener is suppressed, as a result, the corrosion potential shifted to noble direction.
  • Hydrogen Effect on Microstructural Alteration before Fatigue Fracture on Austenitic Stainless Steels

    pp. 237-246

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    DOI:10.2355/tetsutohagane.TETSU-2020-104

    Type 304 stainless steel is typical austenitic one where fatigue life is deteriorated in hydrogen environments. There are few studies for fatigue crack initiation process compared to those of fatigue crack propagation. In this study, the authors experimentally investigated influence of hydrogen on dislocation structures and phase distribution before the fatigue crack initiation. A solution heat treated Type 304 plate was used as a sample, and round-bar fatigue specimens with a notch were machined. Fatigue tests were performed for the hydrogen-charged or -free specimens in fully reversed loading conditions at room temperature. The test was terminated before the crack initiation. Then, the dislocation structures and the phase distribution underneath the notch root were analyzed by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). For the hydrogen-charged specimen, planar dislocations were observed in the TEM images. With increase in fatigue cycles, Area of stacking faults (SFs) increased and ε martensite (εM) appeared on the (111)γ planes. α’ martensite (α’M) were observed at crossover sites of the εM phases on different (111)γ planes. In EBSD analysis, the α’M was often observed in a plate form parallel to the (111)γ planes. For the hydrogen-free specimens, on the other hand, dislocations cell structures and massive α’ M were observed in the TEM images. Neither SFs nor εM were formed. Thus, hydrogen increases dislocation planarity and changes martensitic transformation from γ to α’M, or γ to α’M through ε, resulting in different α’ M morphologies.
  • Three-dimensional EBSD Analysis and TEM Observation for Interface Microstructure during Reverse Phase Transformation in Low Carbon Steels

    pp. 247-256

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    DOI:10.2355/tetsutohagane.TETSU-2020-105

    For the development of advanced steels, phase transformation from ferrite(α) to austenite(γ) is essentially important to control the austenite phase in the heating process. Formation of austenite during the initial stage of α→γ transformation from the recrystallized ferrite in low carbon steel has been studied from the view-point of the orientation relationships and the interphase boundary structure. At high temperature, the in situ electron backscattering diffraction (EBSD) analysis of austenite grain growth during the α→γ transformation indicates that the different migration behaviors according to different α/γ interfaces, derive from the interfacial coherency with the specific orientation relationships. The orientation and microstructure of the interface between ferrite and austenite have been investigated using the 3D crystal orientation analysis and transmission electron microscopy (TEM) observations. When the crystal orientation relationship between ferrite and austenite grain are close to the Kurdjumov–Sachs relationship, the grain boundary normal itself is also close to the {111}α and {011}γ, respectively. The microstructure of these interfacial planes is revealed to be flat using 3D-EBSD and TEM analysis. These coherent planes are strongly connected to the formation of the austenite phase on heating and also affect the slow migration of the grain-growth process.
  • Effects of Microstructural Anisotropy on the Dwell Fatigue Life of Ti-6Al-4V bar

    pp. 257-267

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    DOI:10.2355/tetsutohagane.TETSU-2020-109

    Cyclic fatigue, dwell fatigue and crack growth properties were evaluated in the axial direction (L) and transversal direction (T) of Ti-6Al-4V forged round bar. In the SN curve where the stress is normalized by 0.2% proof stress, the cyclic fatigue life in the L/T direction is almost the same, whereas the dwell fatigue life in the T direction is as short as 1/5. In dwell fatigue, ductile fracture occurred when the maximum stress was higher than 95% of 0.2% proof stress. At stresses below 870 MPa, the inelastic strain range and the strain increase rate in the T direction gradually decreased with decreasing stress, and the fracture mode transitioned to that with fatigue crack growth. The gradual change must have been caused by the mixture of anisotropic microtexture regions. At stresses below 825 MPa, the fracture mode transitioned rapidly in the L direction, where the soft oriented microtexture regions were dominant. In the low ΔK region (≤15MPam), the crack growth rate in the axial direction was about twice that in the radial direction of the bar. The shorter dwell fatigue life in the T direction under stress conditions showing fatigue crack growth was explained by the significantly earlier crack initiation compared to that in cyclic fatigue and the faster crack growth along the microtexture in the axial direction of the bar.

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