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

ISIJ International Advance Publication

  • Reduction Behavior and Kinetics of Iron Ore Pellets under H2–N2 Atmosphere

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    DOI:10.2355/isijinternational.ISIJINT-2017-739

    Direct reduction behavior and dynamic characteristics of oxidized pellets under the 75%H2-25%N2 atmosphere at 760, 900, 1000°C are studied in this paper. 500 g oxidized pellets in the size of 10–12.5 mm are reduced by gas with the flow rate of 12 L·min-1 for 1 hour. Weight loss during reduction was recorded and the model of un-reacted core was adopted for dynamic analysis. Morphology of metalized pellets was analyzed through optical microscope and scanning electron microscope. Compressive strength and degradation index were also detected. Results showed that both the reduction degree and reaction rate increased with the increasing temperature. The reduction rate was controlled by chemical reaction with the apparent activation energy of 40.954 kJ·mol-1. The chemical reaction resistance could be effectively reduced through appropriately raising the temperature. However, the effectiveness was weakened with the further increase of temperature. The reaction rate constant at 900°C was 0.025 m·s-1, which was slightly lower compared with reduction kinetics under pure hydrogen. The compressive strength decreased with the increasing temperature, and amounts of holes and cracks were observed at higher temperature. Reduction degradation behavior of pellets under this atmosphere was not obvious.
  • Development of High-Chromium Ferritic Heat-Resistant Steels with High Nitrogen Content

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    DOI:10.2355/isijinternational.ISIJINT-2017-758

    New ferritic heat-resistant steels with high nitrogen content were prototyped and their microstructures and mechanical properties at high temperature were evaluated. The addition of 0.3 mass% N into ferritic steels was achieved without the formation of blowholes by applying pressurized melting methods under an atmosphere of up to 4.0 MPa. The high-nitrogen ferritic heat-resistant steels contained several kinds of nitrides within the lath martensitic structure. V-rich coarse particles were identified as crystallized MN. Fine VN or Cr2N particles were precipitated on the martensitic grain boundaries such as prior-austenite grain boundary, packet boundary, block boundary and lath boundary depending on the V content. The martensitic structure of the high-nitrogen steels contained a hierarchical microstructure including martensitic laths, blocks, packets, and prior-austenitic grains. These martensitic structures satisfied the Kurdjumov–Sachs relationship as with conventional carbon steel. The creep strengths of the prototyped steels were comparable with those of Gr. 91 steel, albeit lower than those of Gr. 92. Additional precipitates other than nitrides are required for further strengthening of the developed steels.
  • Prediction of Void Closure in Metal Forming: One Cylindrical Through-hole

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    DOI:10.2355/isijinternational.ISIJINT-2018-037

    Since voids in a billet or an ingot are detrimental under tensile stress, they are closed by forging or rolling through a combination of collapse and contraction. In plane-strain forging or rolling, a cylindrical void or through-hole was found experimentally as well as analytically to be closed as the effective strain at the center of the void reached a certain value. In the present investigation, this finding was further examined for transverse forging, axial rolling and axial forging, in which the void was elongated or shortened in length. Since strain as well as stress components in length were irrelevant to a description of cross sectional changes, a concept of the planar effective strain and the planar hydrostatic stress was introduced. As a result, void closure on cross section was able to be predicted by the planar effective strain and the normalized planar hydrostatic stress at the center of the cross section, which were obtained from an analysis of non-void model. However, there were two exceptions; one was the case in which the planar hydrostatic stress was positive in sign and the other was the axial forging in which the void never collapsed on cross section.
  • Nanobainite Layer Prepared by Laser Hardening Combined with Isothermal Transformation

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    DOI:10.2355/isijinternational.ISIJINT-2017-591

    Nanobainite layer was prepared by laser hardening combined with isothermal transformation (LHCIT) at 250°C, which between Ms and Md temperature. The microstructures of the nanobainite layer were analyzed by X-ray Diffraction (XRD) and transmission electron microscope (TEM). The nanohardness of the nanobainite layer were measured by the nano-mechanical tester. The results show that the residual stress value of the laser hardening layer is 319.15 ± 21.05 MPa, and the shear band such as stacking fault bundle, nano-twin and ε-martensite can be formed by the stress in austenite during LHCIT. The highest nano-indentation hardness of the nanobainite layer is 8.81 GPa and the average nanohardness of the nanobainite layer is evidently increased from 6.65 GPa to 7.76 GPa. The reduced hardness is bimodal in the laser hardening layer, and it is demonstrate that the high hardness of the nanobainite layer can be obtained by the formation of the shear band.
  • Effect of Microstructure and Precipitates on Mechanical Properties of Cr–Mo–V Alloy Steel with Different Austenitizing Temperatures

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    DOI:10.2355/isijinternational.ISIJINT-2017-531

    The mechanical properties of as-quenched and tempered steels are affected by austenitizing temperature. The present work has investigated the effect of austenitizing temperature on martensitic microstructure, carbide precipitates and mechanical properties of 30NiCrMoV12 alloy steel for the axle of high-speed train. The martensitic microstructure and carbide precipitates were studied using OM, FE-SEM, TEM, EBSD and EDS. Thermodynamic calculation of equilibrium precipitation were carried out by Thermo-Calc software. The results showed that the prior austenite grains, martensitic packets, blocks and laths were coarsening with increasing austenitizing temperature. Besides, with increasing austenitizing temperature, after tempering the amount of large size carbides precipitated at martensitic lath boundaries decreased while the amount of small size carbides precipitated in matrix increased. Meanwhile, phase transformation from M23C6 to M7C3 during tempering was enhanced with increasing austenitizing temperature. Coarse grains and wide martensitic laths were beneficial to reducing the amount of strip-like M23C6 carbides precipitated at martensitic lath boundaries due to the reduction of boundary area and thereby obtaining more fine precipitates in matrix. The strength and impact toughness could be improved to a certain extent by refining carbides in tempered steel with higher austenitizing temperature. However, the degree of favorable influence on impact toughness resulting from refining carbides was lower than the negative effect from coarse martensitic structures. Therefore, the toughness is deteriorated and the strength is improved with increasing austenitizing temperature.
  • Research on the Bimetallic Composite Roll Produced by an Improved Electroslag Cladding Method: Mathematical Simulation of the Power Supply Circuits

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    DOI:10.2355/isijinternational.ISIJINT-2017-703

    Bimetallic composite roll has been given more and more attentions because of its superiority in playing the advantages performance of the internal and external materials at the same time. In the present study, a 2D quasi-steady state mathematical model of the electroslag cladding technology for producing bimetallic composite roll was developed by the Fluent software with the UDS and UDF function. Characteristics of the electromagnetic field, flow field and temperature field of the composite roll system have been numerically simulated and the laboratory scale experiments with the different power supply circuits were also developed to provide a verification of the mathematical models. The results indicate that: simulation results of the temperature distribution in the composite roll were well verified by the corresponding experiments. With the using of current supplying mold (CSM®), the improved conductive circuit is more beneficial to improve the distributions of current density and Joule heat in the slag pool and keep the high temperature zone away from the roll core surface than the conventional conductive circuit. On one hand, it makes the roll core be no longer as a pole of the electroslag process and the temperature adjustment of the slag pool become more flexible. On the other hand, it leads to a partial micro-melting of the roll core surface which is beneficial to form a metallurgical bonding and effectively avoid the mechanical mixing of the liquid metal between the roll core and composite layer, so, it can improve the comprehensive performance of bimetallic composite roll.
  • Recovery of Zinc from Zn–Al–Fe Melt by Super-gravity Separation

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    DOI:10.2355/isijinternational.ISIJINT-2017-561

    The removal of iron-containing dross particles and recovery of zinc from galvanizing dross by super-gravity separation was investigated using a model Zn–Fe–Al alloy. After super-gravity separation, the high purity molten zinc went through the filter, while the residue mainly consisting of dross particles was intercepted by the filter and separated from the molten zinc. The effects of gravity coefficient and separating temperature on zinc recovery and iron removal were investigated. The preliminary results show the super-gravity separation is a promising method of recovering zinc from galvanizing dross.
  • Effect of Annealing Temperature on the Selective Oxidation and Reactive Wetting of a 0.1C-6Mn-2Si Advanced High Strength Steel During Continuous Galvanizing Heat Treatments

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    DOI:10.2355/isijinternational.ISIJINT-2017-688

    The effects of peak annealing temperature and annealing time on the selective oxidation and reactive wetting of a prototype medium-Mn Fe-0.1C-6Mn-2Si third generation advanced high strength steel were investigated. Annealing heat treatments were carried out in a N2-5 vol% H2 243 K (-30°C) dew point process atmosphere at 963 K (690°C) and 1073 K (800°C) for 120 s and 600 s. TEM observations of the sample cross-sections revealed internal oxidation of the subsurface grains and grain boundaries. EELS results showed that the internal oxide network had a multi-layered structure with SiO2 at the oxide core and MnSiO3 as the surrounding shell; however, MnO was the only species detected at the surface of all samples. The effect of annealing temperature on the surface structure development and its impact on reactive wetting of the substrates annealed for 120 s at both peak annealing temperatures by a Zn-0.20 wt% Al (dissolved) galvanizing bath was also determined. In contrast to the 1073 K steel, the 963 K substrate showed superior reactive wetting, owing to the much thinner, finer and wider spacing of the MnO nodules on the pre-immersion surface. TEM+EELS analysis of the coated steels showed that infiltration of the bath alloy and partial reduction of MnO resulted in lift-off of the surface oxides and partial formation of Fe2Al5ZnX interfacial layer, indicating that reactive wetting had occurred for the 963 K × 120 s substrate.
  • Effect of Side-blowing Arrangement on Flow Field and Vanadium Extraction Rate in Converter Steelmaking Process

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    DOI:10.2355/isijinternational.ISIJINT-2017-463

    To improve the vanadium extraction rate, the iron ore powder was used to be injected into the molten bath by side-blowing. Both simulation numerical and water experiment model for analyzing the behaviors of molten bath flow field had been developed to investigate the mixing time, velocity profile and dead-zone volume of molten steel under various injection arrangements and gas flow rates. The result showed that the mixing time would reduce, with increasing the distance between side-blowing nozzle and molten bathe surface. Moreover, although the kinetic energy would be removed by the mutual influenced between bottom-blow bubbles and the side-blowing jet at a certain content, the stirring effect of molten bath is still improved by the side-blowing jet. Based on the result of simulation, an injection arrangement was used in the 150 t vanadium extraction converter. Based on the results, the vanadium content of semi-steel and T. Fe in the slag with side-blowing arrangement is 0.033% and 34.1%, respectively. And the vanadium content of semi-steel and T. Fe in the slag with side-blowing arrangement is 0.044% and 32.1%, respectively.
  • Effect of Zr, Al Addition on Characteristics of MnS and Formation of Intragranular Ferrite in Non-Quenched and Tempered Steel

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    DOI:10.2355/isijinternational.ISIJINT-2017-617

    In this paper, the characteristics of inclusions, microstructures and mechanical properties in non-quenched and tempered steel with Zr and Al addition were investigated to compare the effect of ZrO2 and Al2O3 particles on the precipitation behavior of MnS and the formation of intragranular ferrite. The results show that oxides, sulfides and oxy-sulfides in Zr addition steel are all finer than those in Al addition steel. As a result, the volume fraction of intragranular polygonal ferrite (IPF) induced by tiny MnS + V(C, N) particles in Zr addition steel has increased to 14.9%, resulting in higher transverse plasticity. Low lattice mismatch between ZrO2 and MnS inclusions plays an important role in refinement of MnS inclusions. Besides, the single ZrO2 particle, which is not wrapped by MnS inclusions, would induce formation of IPF further due to the small mismatch with α-Fe.
  • In Situ Temperature Measurement of Sinter Beds at High Spatial and Time Resolution

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    DOI:10.2355/isijinternational.ISIJINT-2017-636

    In the sintering process during iron making, the sintering reaction proceeds in a packed bed along with the combustion of coke particles. Although detailed temperature information is necessary to improve the process, it is difficult to measure the temperature distribution inside the packed bed with high spatial and time resolution. We performed in situ temperature measurement inside the sinter bed at high spatial and time resolution, i.e., 2 mm and 10 s, respectively, during sintering. A sheathed thermocouple was scanned at optimized scan speed along the inside of the thin-wall alumina tube which was held perpendicular inside the sinter bed. The information on the temperature variation during sintering showed a clear correlation between the quality of the sinter and the sinter heat pattern for each layer. Further analysis also showed that the flame front speed is proportional to the O2 consumption in the sinter bed. The temperature measurement technique enabled an unprecedented detailed discussion with the temperature distribution inside the sinter bed during sintering. This technique will not only help to improve the sintering process but also provide beneficial information on the chemical reactions occurring inside packed beds.
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  • Dissolution Mechanism of Carbon Brick into Molten Iron

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    DOI:10.2355/isijinternational.ISIJINT-2017-659

    In order to investigate the dissolution mechanism of carbon brick into molten iron, cylindrical specimens were immersed into molten iron to carry out the experiments. The dissolution reaction of carbon was considered as the dominant reaction through thermodynamic analysis, the result of SEM revealed the hole diameter decrease from the reaction interface to the center position of carbon brick. The quantitative relationship between element content and erosion was obtained through the experimental results, the characteristic parameters were selected to compare the influence degree of element content on the erosion. The calculation model of mass transfer coefficient was established, the dissolution reaction of the sample is controlled by interfacial reaction and mass transfer of carbon when the phosphorus content up to 0.2% in molten iron. The adsorption of sulfur on the iron-carbon interface covers part of the effective surface, the degree of adsorption on the interface depends on the proportion of sites covered by sulfur.
  • Void Nucleation, Growth, and Coalescence Observed by Synchrotron Radiation X-ray Laminography during Tensile Deformation of Fe–0.02 mass% N Alloy

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    DOI:10.2355/isijinternational.ISIJINT-2017-609

    In this study, the effect of the state of N atoms in a Fe–N alloy, namely, whether N exists as a dissolved atom or as a nitride on void nucleation, growth, and coalescence during tensile testing was examined and the factors dominating local elongation was discussed. Two types of Fe–0.02 mass% N alloy specimens were used; one was heated to the ferritic phase region before water-quenching, while the other was subjected to aging.The local elongation of the water-quenched specimen was lower than that of the aged specimen. Secondary-ion mass spectrometry and electron backscattering diffraction analyses revealed that N atoms were segregated around grain boundaries similarly in both the water-quenched and the aged specimens, while N atoms precipitated as Fe4N in the aged specimen. Void nucleation, growth, and coalescence were observed using synchrotron radiation X-ray laminography. In the water-quenched specimen, voids suddenly grew and coalesced before fracture, while in the aged specimen sudden growth and coalescence were suppressed. Nano-indentation hardness measurements showed that the difference in hardness between the regions around the grain boundaries and grain centers was smaller in the aged specimen compared to that in the water-quenched specimen. This result indicated a lower plastic strain gradient around the grain boundaries in the aged specimen. This lower plastic strain gradient in the aged specimen was caused by precipitation of N atoms as Fe4N. From these results, it was concluded that the main factor permitting the greater local elongation of the aged specimen was the lower plastic strain gradient around the grain boundaries.
  • Effect of Main Gas Composition on Flow Field Characteristics of Supersonic Coherent Jets with CO2 and O2 Mixed Injection (COMI) at Steelmaking Temperature

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    DOI:10.2355/isijinternational.ISIJINT-2017-580

    As an efficient oxygen supplying technology, coherent jets are widely applied in electric arc furnace (EAF) steelmaking processes to strengthen chemical energy input, speed up smelting rhythm and promote the uniformity of molten bath temperature and compositions. Recently, the supersonic coherent jets with CO2 and O2 mixed injection (COMI) was proposed and through industrial experiments, it can be found that the supersonic coherent jets with COMI showed remarkable advantages in reducing the dust production during EAF steelmaking. In this study, based on the eddy dissipation concept (EDC) model with the detailed chemical kinetic mechanisms (GRI-Mech 3.0), a computational fluid dynamics (CFD) model of supersonic coherent jets with COMI was built. Compared with one-step combustion reaction, GRI-Mech 3.0 consists of 325 elementary reactions with 53 components and can predict more accurate results. The numerical simulation results were validated by the combustion experiment data. The jet behavior and the fluid flow characteristics of supersonic coherent jets with COMI at steelmaking temperature 1700 K was studied and the results show that the chemical effect of CO2 significantly weakens the shrouding combustion reactions of CH4 and the relative importance of the chemical effect of CO2 increases with CO2 concentration increasing. The potential core length of supersonic coherent jets decreases with the volume fraction of CO2 increasing. Moreover, it also can be found that the potential core length of supersonic coherent jets can be prolonged with higher ambient temperature.

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