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

Tetsu-to-Hagané Vol. 105 (2019), No. 12

  • Reduction Behavior of Iron Oxide and Influence of Basicity in Initial Melt Formation Zone

    pp. 1099-1107

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    DOI:10.2355/tetsutohagane.TETSU-2019-040

    In order to estimate the influence of initial melt on reduction behavior of iron oxide, heating-up reduction and isothermal reduction were carried out. Samples (FCSA4) were composed of Fe2O3, CaO, SiO2 and Al2O3. Four kinds of samples of different composition (basicity: C/S = CaO/SiO2 = 1.0, 1.5, 1.8, 2.0) were prepared. The reduction rate of FCSA4 with C/S = 1.0, 1.5 was lower than that rate of FCSA4 with C/S = 1.8, 2.0 due to the initial melt formation at the isothermal reduction of 1473 K. The overall reduction rate ka increased with increasing C/S and decreasing of amount of melt. The SEM analysis indicated the influence of melt formation on pore structure of sample. The porosity was estimated by image analysis technique. Defining ka and closing pore ratio εc, the relational equation between ka and εc represents as follow:ka = (–0.36 εc + 18.67) × 10–5In the reduction from 1273 K to 1473 K with 21 vol% CO – 9 vol% H2 – 70 vol% N2 gas mixture, the rates of all samples didn’t decrease. The pore wasn’t closed by the effect of H2 addition. Moreover, the rate of FCSA4M2 with the addition of 0.2 mass% MgO also didn’t decrease. The initial melt formation temperature shifted to higher temperature by the addition MgO.
  • Modeling of Coke Particle Breakage in Blast Furnace Considering Pore Structure by Discrete Element Method

    pp. 1108-1117

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    DOI:10.2355/tetsutohagane.TETSU-2019-060

    Cokes in a blast furnace play an important role as a spacer for keeping gas permeability. Currently blast furnaces with large inner volume, exceeding 5000 m3, become common in Japan. Consequently, use of high strength coke is a crucial issue for modern blast furnaces. However, general methods for evaluating coke strength, for example drum test, are not enough for understanding the breakage behavior of cokes in detail. In order to evaluate the coke breakage behavior in blast furnaces, coke breakage model based on discrete element method (DEM) with cluster particles and bonds was developed.According to the experiments of indirect tensile test, the tensile strength of cokes shows wide distribution because of the randomness of the coke pore arrangement. Then, the DEM simulation model for coke breakage was developed by considering pores with random position. DEM simulations of indirect tensile test condition, with 10 cases of random pore arrangement for each Coke A (small porosity) and Coke B (large porosity), were carried out. The tensile strength obtained from experiments and DEM simulations were compared by using Weibull analysis. The simulation results show an agreement with the experimental results including distribution of coke strength. Finally, the probability distributions of coke breakage obtained from Weibull analysis was applied to a DEM simulation result of material flow in 5000 m3 blast furnace and percentages of coke breakage at deadman region were evaluated for Coke A and B.
  • Gas Permeability Improvement Mechanism at the Lower Part of Blast Furnace by Converter Slag Injection from Tuyere and Quantification of its Effect

    pp. 1118-1125

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    DOI:10.2355/tetsutohagane.TETSU-2019-065

    In order to improve gas permeability around the blast furnace bird’s nest region during high pulverized coal injection rate operation, flux injection technology from the tuyere is surveyed with the purpose of decreasing slag hold-up. In this study, converter slag is focused on as the flux material. The advantages of converter slag as an inductant are its low melting point and its smaller endothermic quantity because it is a high FeO pre-melt slag. Results obtained by experiments and actual blast furnace tests are as follows:(1) Converter slag melted quickly during flight within the raceway and showed excellent assimilation properties with bird’s nest slag.(2) Based on the results of slag dripping experiments, a scheme is established which estimates the improvement of gas permeability using the operational conditions for the blast furnace and the converter slag injection conditions.(3) A decrease in pressure drop at the lower part of the blast furnace was demonstrated by converter slag injection tests at Kobe No.3 blast furnace. The operational results showed fairly good agreement with those estimated by the scheme of the present study.
  • Effect of Driving Torque on the Interfacial Creep for Shrink-fitted Bimetallic Work Roll

    pp. 1126-1134

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    DOI:10.2355/tetsutohagane.TETSU-2019-048

    The bimetallic work rolls are widely used in the roughing stands of hot rolling stand mills. The rolls are classified into two types; one is a single-solid type, and the other is a shrink-fitted construction type consisting of a sleeve and a shaft. Regarding a shrink-fitted construction type, the interfacial creep sometimes appears between the shaft and the shrink-fitted sleeve. This interfacial creep can be regarded as the relative displacement between the sleeve and the shaft, which often causes the roll damage. In this paper, the FEM simulation is performed to clarify the effect of driving torque on the interfacial creep by considering the driving motor torque. It is found that the relative displacement in the interfacial creep is accelerated by the presence of the motor torque significantly. With increasing the shrink fitting ratio, the relative displacement in the interfacial creep decreases. The effects of motor drive torque on the stress, the displacement and average displacement along the interface were also discussed by varying the motor torque.
  • Effects of Solid-Solute V on the Phosphatability of Hot-Rolled Steel Sheets

    pp. 1135-1142

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    DOI:10.2355/tetsutohagane.TETSU-2019-029

    The phosphatability of hot-rolled steel sheets has become increasingly important with application of higher strength and thinner steel sheets to automotive parts. Although V is an alloying element which is often added to high strength hot-rolled steel sheets, the effect of V on phosphatability was still unclear. This study investigated the phosphatability of V-added hot-rolled steel sheets by using V-free, 0.20% V and 0.47% V steel sheets as test specimens. After phosphate treatment, phosphate crystals covered the whole surface of the V-free and 0.20% V steel sheets, but there were no phosphate crystals on the surface of the 0.47% V steel sheets. In order to clarify the mechanism, potentiostatic electrolysis in the phosphate treatment solution was carried out. Phosphate crystals were found on the surface of the V-free steel sheet after both cathodic and anodic polarization. In contrast, no phosphate crystals were found on the surface of the 0.47% V steel sheet after anodic polarization, but similarly to the V-free steel sheet, phosphate crystals had formed after cathodic polarization. A surface analysis by XPS revealed that V oxides had precipitated on the surface of the 0.47% V steel sheet after anodic polarization. Evolution of hydrogen ions in the oxidation reaction of V inhibits the formation of phosphate crystals. Thus, oxidation of V on the steel surface in the phosphate treatment solution was identified as the factor responsible for deterioration of the phosphatability of V-added hot-rolled steel sheets.
  • Determination of Facet Plane and Cleavage Fracture Plane of the Top Dross Formed in a Molten Zinc

    pp. 1143-1152

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    DOI:10.2355/tetsutohagane.TETSU-2019-067

    In a molten zinc bath of a continuous galvanizing line, top dross particles crystallize as Fe2Al5 intermetallic compound containing Zn. These particles easily adhere to the steel sheets causing surface defects. Therefore, controlling the top dross particles is a key issue. The present study focused on the determination of facet plane of top dross using three-dimensional analysis of morphology of the top dross by simultaneous exploitation of the serial sectioning process and electron back scattering diffraction (EBSD). Furthermore, the crystallographic plane of the cleavage fracture surface of the top dross was determined by EBSD, after cleavage crack was introduced by Vickers hardness indentation. The following results were obtained: (1) The facet planes of the top dross consist of two planes of (001), four planes of {110} and eight planes of {111}. In addition, the top dross particles grow fastest in the [001] direction. Consequently, the top dross particle is concluded to have the polyhedron structure with 14 facet planes. (2) The cleavage fracture surface of the main crack in the top dross is (100) plane.
  • Effects of Excess C on New Grain Formation and Static Recrystallization Behavior at Shear Bands in Cold-Rolled Ultra-Low Carbon Steel Sheets

    pp. 1153-1162

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    DOI:10.2355/tetsutohagane.TETSU-2019-019

    To investigate the effect of excess trace amounts of solute carbon on recrystallization of cold-rolled steel sheets, the recrystallization behavior of Ti-bearing ultra-low carbon steels was studied. Recrystallization was accelerated by the presence of solute carbon, and microstructure observation confirmed that recrystallized grains were generated at ferrite grain boundaries or deformation bands. In steel containing a trace amount of solute carbon, observation of many recrystallized grains generated from ferrite grain boundaries indicated that the existence of fine grains before cold-rolling of steel containing solute carbon is one cause for accelerated recrystallization.Detailed observation of recrystallized grains revealed that a large number of new recrystallized grains were generated in deformation bands in the steel containing a trace amount of solute carbon, whereas such grains were not generated in large numbers around grain boundaries in the interstitial free steel. Moreover, a fine structure was formed in the deformation bands in the steel containing a trace amount of solute carbon. These results indicated that the grain boundary migration of recrystallized grains was accelerated by high stored energy and high-angle boundaries in deformation bands.
  • Effect of Carbon and Nitrogen on Md30 in Metastable Austenitic Stainless Steel

    pp. 1163-1172

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    DOI:10.2355/tetsutohagane.TETSU-2019-062

    Md30 is defined as the temperature at which 50 vol.% of α’-martensite is formed at a true tensile strain of 0.3 in metastable austenitic steels. It has been generally believed that the effect of carbon content on Md30 was estimated to be identical to that of nitrogen as shown by Nohara’s equation. However, we found in this study that Md30 in carbon-added steel is lower than that in nitrogen-added steel, which indicates that the effect of carbon content on the mechanical stability of austenite is more significant than that of nitrogen. In addition, the relationship between Md30 and carbon and nitrogen content is not linear. The effect of carbon and nitrogen content on Md30 is higher at lower carbon and nitrogen content region (<0.1%). As this effect was not considered in the previous study, the austenite-stabilizing effects of both the elements were underestimated. Therefore, in this study, new equations are proposed to accurately estimate Md30 of a Fe-Cr-Ni alloy system. As a result, modified Md30 equation is suggested as below:Md30(K) = 800 – 333 C eq – 10.3Si – 12.5Mn – 10.5Cr – 24.0Ni – 5.6MoCarbon equivalent, Ceq is a function of carbon and nitrogen concentrations and temperature.Ceq = C + aNa = 0.931 – 0.000281exp (0.0219T)Above equations show that the difference in austenite-stabilizing effects of carbon and nitrogen increases with rising temperature, owing to the difference in stacking fault energy between carbon-added and nitrogen-added steels.
  • 1011-cycle Gigacycle Fatigue Properties of High-strength Steel

    pp. 1173-1178

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    DOI:10.2355/tetsutohagane.TETSU-2019-071

    Fatigue tests were conducted up to 1011 cycles on high-strength steel to clarify a fatigue limit. The fatigue limit of the high-strength steel was not confirmed by gigacycle fatigue tests up to 1010 cycles, while our previous study suggested that the fatigue limit was probably confirmed by those up to 1011 cycles. However, the 1011 cycles fatigue testing was challenging since it took 2 months even by using ultrasonic fatigue testing at 20 kHz. In this study, 3 specimens were tested beyond 1010 cycles. Although a test on a specimen was terminated at around 5 x 1010 cycles, 2 specimens reached 1011 cycles without failure. In other word, no specimen failed above 1010 cycles. These results demonstrated the fatigue limit on high-strength steel in a gigacycle region. The fractured specimens below 1010 cycles revealed internal fractures originating from oxide-type inclusions. When the specimens failed in long life regions, clear ODAs (Optically Dark Areas) were observed on the fracture surfaces at around the internal fracture origin, while the ODAs were obscure in case of failure in short life regions. The runout specimens up to 1011 cycles were forcibly fatigue-fractured at higher stress amplitudes in the short life regions. As the result, the ODA was observed on the forcibly fatigue-fractured surface. This meant that small internal cracks existed in the runout specimens since the ODA was a trace of small internal crack growth. Namely, non-propagating cracks were the mechanism of the appearance of the fatigue limit.
  • Evaluation Method for Small Fatigue Crack Growth Life of Low Carbon Steel with Fine Grain HAZ Microstructure Simulated with Heat Treatment

    pp. 1179-1188

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    DOI:10.2355/tetsutohagane.TETSU-2019-072

    Small fatigue crack growth tests utilizing DIC technique are carried out for low carbon steel with two different heat treatment to establish the small fatigue crack growth evaluation method. As a result, small fatigue crack growth rate (FCGR) looked larger than that of large crack by evaluating with usual stress intensity factor range. The small fatigue crack opening stress, successfully measured by using DIC technique, were decreasing with loading stress was increasing. In addition, by using effective cyclic J integral range calculated with measured crack opening stress, even small fatigue crack growth rates were almost agreed with that of large crack data. Due to this evaluation, microstructural resistance, appeared in the region of crack length is 0.2 mm or less, is successfully visualized. Finally, estimated small fatigue crack growth life by using Paris’s law of large crack, effective cyclic J integral range and conventional approximation formula of crack opening stress was almost agreed with corresponding experimental data.

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