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

ISIJ International Vol. 56 (2016), No. 11

  • Thermodynamic Properties of the CaO–AlO1.5–CeO1.5 System

    pp. 1893-1901

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    DOI:10.2355/isijinternational.ISIJINT-2016-201

    In this study, the thermodynamic properties of the CaO–AlO1.5–CeO1.5 system have been investigated. The chemical compositions and crystal structures of ternary intermediate compounds in this system, which have not been fully studied before, have been clarified. In addition, the isothermal phase relationships of the CaO–AlO1.5–CeO1.5 system have been investigated at temperatures of 1823 K and 1873 K by using a chemical equilibrating technique. The activities of CaO, AlO1.5, and CeO1.5 were measured by a chemical equilibrating technique by using molten silver as a reference metal. For some measurements of AlO1.5 activity, copper was used as an alternative reference. In addition, the molten iron composition has been calculated using the activities of each component of the CaO–AlO1.5–CeO1.5 system obtained in the present study.
  • Influence of the Interfacial Tension on the Droplet Formation by Bubble Rupture in Sn(Te) and Salt System

    pp. 1902-1909

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    DOI:10.2355/isijinternational.ISIJINT-2016-303

    Using a molten chloride and Sn system, in-situ observation of the bubble rupture and metal droplet formation can be carried out because the molten chloride phase is transparent. The size distribution of droplets with diameters in the range of 0.1 to 1.0 mm is measured using a high resolution camera, and the influence of the interfacial tension is discussed. The following results were obtained: (1) The addition of Te to the Sn decreased the interfacial tension and increased the number of small droplets, while the total surface area and total volume of droplets in the chloride bath decreased; (2) As the gas flow rate increased, the number of droplets in the Sn system formed by Mode B, where the metal emulsion was generated by the disintegration of the metal column, increased and large droplets were observed. In the Sn–Te system, even at the highest gas flow rate, droplets formed by Mode A, where the metal droplets formed by the rupture of the metal film, were observed in addition to those formed by Mode B; (3) In the Sn system, the size of the bubbles produced in Mode B was larger than that for Mode A. In the Sn–Te system the bubble size was generally smaller than that in the Sn system; (4) The total volume of the droplets formed by a bubble increased with increasing bubble size. In particular, the bubble rupture in Mode B generated a greater volume of droplets than in Mode A, even though the bubble size was the same.
  • A Simulation Study of Particles Generated from Pellet Wear Contacts during a Laboratory Test

    pp. 1910-1919

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    DOI:10.2355/isijinternational.ISIJINT-2016-328

    In the blast furnace process, material losses occur due to mechanical wear between charged iron ore pellets and are exhausted in the form of dust in the off-gases. A redesigned tribometer combined with a ventilation chamber was developed to identify the dust emission from the mechanical wear contact of pellets. In order to obtain a better understanding of the measurement results, a coupled drift flux with a unified Eulerian deposition model was adopted to investigate particle dispersion and deposition during tests. Two influential factors, namely the air condition (5–20 L/min) and particle size (1–20 µm) were examined. The predicted results were presented by introducing two parameters, namely the measurable fraction and the deposition fraction. For each air condition, the measurable fraction declines while the deposition fraction rises as particle size grows. The critical size of the particles that becomes airborne and captured at the outlet was identified to be around 20 µm. In addition, a high airflow rate supplied at the inlet was observed to be favorable for improving the measurable fraction. Nevertheless, the results show that nearly 50% of emitted particles (1–20 µm) that failed to be captured during tests. Thus it could be expected that these generated particles would be transported deeply in a blast furnace if they are not efficiently removed from the off-gas. As a consequence, they may influence the quality of the products. Furthermore, the validation of the simulation results against the experimental data was achieved by using the predicted measurable fraction.
  • Influence of Binder Dosage on Granule Structure and Packed Bed Properties in Iron Ore Sintering Process

    pp. 1920-1928

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    DOI:10.2355/isijinternational.ISIJINT-2016-298

    This paper investigates the influence of binder dosage on the iron ore sintering process. The granule structure and packed bed properties were explored under a wide range of water and hydrated lime addition levels. At the same moisture content, the addition of more hydrated lime improved the granulation efficiency by forming a more cohesive initial layer on the surface of nuclei particles and producing more agglomeration during granulation. The bed voidage-moisture curve could be divided into three regions, and this curve became flatter with higher hydrated lime addition level. A more mechanistic model of bed voidage [ε = ε0 + (1−ε0)exp(−mRn), ε0 = 0.36×(εσ)−0.3209] was proposed which represents the influence of cohesive forces and potential for granule deformation by linking voidage to the spread of granule size distribution (σ) as well as the adhering mass ratio (R). For the tested Asia-Pacific region ore blend, adding hydrated lime improved the bed permeability significantly due to the increase in granule size and bed voidage. However, there is a saturation value of solid binder dosage. The improving action becomes limited at 3 wt% since further increase of hydrated lime addition had no further benefit to bed voidage.
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    Readers Who Read This Article Also Read

    1. A Simulation Study of Particles Generated from Pellet Wear Contacts during a Laboratory Test ISIJ International Vol.56(2016), No.11
    2. Interpretation on Iron Ore Granulation Process Based on Particle-size Analysis ISIJ International Vol.56(2016), No.11
    3. SO2 Absorption in the Sinter Bed during the Sintering Process ISIJ International Vol.55(2015), No.9
  • Parameters Optimization, Sticking Mechanism and Kinetics Analysis of Fine Iron Ore in Fluidized-Reduction Process

    pp. 1929-1937

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    DOI:10.2355/isijinternational.ISIJINT-2016-280

    ORD-S fine iron ore is reduced in fluidized bed, and fine iron ore particles are evaluated through mechanical and dynamic analyses. This study aims to obtain the changing direction of the operating parameters by synthetically analyzing the effect of temperature, velocity of reducing gas, reducing gas atmosphere, size of fine iron ore particle, and coating oxide composition on the fluidized reduction sticking process; draw economic, convenient, and effective operating parameters; and provide theoretical and technical basis for the economical and rational use of the ORD-S fine iron ore. The optimal operating parameters require to reduce ORD-S fine iron ore include 923 K to 1023 K temperatures, particle size range of 0.63 mm to 1.0 mm, linear velocity of 0.6 m/s, hydrogen accounting for 80 vol.% of the total carbon monoxide and hydrogen mixture, and coating MgO accounting for 2 mass% of the total quality of fine iron ore. Under the optimal experimental conditions, the critical radius of solid bridge is 90–105 nm, the apparent activation energy E of the chemical reaction is 22.48 kJ/mol, and the limit link of chemical reaction speed is the internal diffusion speed.
  • Effect of Cr2O3 on the Reduction and Smelting Mechanism of High-Chromium Vanadium-titanium Magnetite Pellets

    pp. 1938-1947

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    DOI:10.2355/isijinternational.ISIJINT-2016-234

    Effect of Cr2O3 on the reduction mechanism of high-chromium vanadium-titanium pellets was firstly studied in the present paper. It is found that Cr2O3 has an obvious effect on the reduction of high chromium vanadium-titanium pellets. With increasing Cr2O3 content from 0.28% to 8.22%, the reduction extents increased slightly and then decreased, and the reduction mechanism was further elucidated by means of XRD and SEM-EDS. Effect of Cr2O3 on the smelting mechanism of high-chromium vanadium-titanium pellets was also studied in the present paper. With increasing Cr2O3 contents from 0.28% to 8.22%, the softening start temperature and the softening temperature gradually increased, and the softening zone increased as a whole. The melting start temperature increased as a whole. The dripping temperature gradually increased, the melting-dripping zone increased quickly to a relatively high value of above 230°C, and the permeability index increased evidently, indicating the deterioration of melting-dripping index. The dripping difficulty increasing with the increase of Cr2O3 contents is in accordance with the observed Cr-multicarbides and Cr-carbides in the XRD pattern and the microscopic examination of the not-dripped and dripped melted iron and slag.
  • Effect of Volume Breakage Due to DI Measurement on Pore Structure in Coke

    pp. 1948-1955

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    DOI:10.2355/isijinternational.ISIJINT-2015-388

    Coke strength is an important property and generally evaluated with Drum Index (DI). It is considered that coke breakage occurs by two types of breakages during DI measurement, surface breakage and volume breakage. In this study, the cause of volume breakage was investigated by image analysis of cross-sectional image of coke before and after the DI measurement. As a result, a new image analysis method was developed. In this image analysis method, pore shape and coke-matrix connectivity were evaluated and new parameters “degree of Irregularity (I)” and “Connectivity Index (CI)” were proposed. I and CI represent pore shape and coke-matrix connectivity, respectively. By using this image analysis method, it was revealed that pore shape and coke-matrix connectivity were improved on adding HPC (High Performance Caking additive). By comparing the cross-sections before and after the DI measurement, it was found that the volume breakage occurred at the portions which have higher porosity, coarse pores, cracks and thin pore wall. Furthermore, it was clarified that the thickness of the pore wall broken by the DI measurement was < 7.6 µm thickness.
  • Analysis of Blast Furnace Hearth Sidewall Erosion and Protective Layer Formation

    pp. 1956-1963

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    DOI:10.2355/isijinternational.ISIJINT-2016-168

    The blast furnace campaign life is largely determined by the damage to the hearth area. Bricks and skulls in the hearth were sampled and investigated after the blowing-out of one commercial blast furnace. A variety of techniques, such as chemical analysis, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD), were applied in characterizing the erosion and protection mechanism of the hearth sidewall. The upper area of the hearth was heavily eroded. Large amounts of alkali chlorides were identified in the area, which could be the main contribution to the serious erosion. The taphole area was protected by the formation of the skull, which consisted of slag with CaZnSi3O8, Zn2SiO4 and CaMg2Al2O7 phases. The circulation of the hot metal in the hearth is the major cause of brick erosion in the hearth bottom area, which leads to a ring shaped wear. Controlling peripheral hot metal flow across the hearth wall using long taphole and coke with high degradation resistance may slow the carbon brick erosion. Increasing the carbon content in the hot metal can help reducing the dissolution of the carbon bricks and enhance the formation of a graphite layer to protect the hearth lining.
  • Interpretation on Iron Ore Granulation Process Based on Particle-size Analysis

    pp. 1964-1972

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    DOI:10.2355/isijinternational.ISIJINT-2016-096

    The adopted conventional granulation and proposed pre-wetting granulation for six sinter mixtures were reinvestigated and interpreted from the perspective of particle size analysis. To research the experimental data from the two processes, two mass fractal models were adopted and further discussed in the characterization of particle size distribution for these sinter mixtures. It was found that: (1) Compared to conventional granulation, the pre-wetting granulation can significantly change the particle size distributions of the sinter mixtures before and after granulation. (2) A fractal characteristic for the particle sizes of the sinter mixtures was observed in the two granulation processes. The particle size distributions of the mixtures can be simulated by the proposed fractal models. (3) Using such fractal model as a bridge, the granule sizes can be predicted by moisture content and fractal parameters of the researched sinter mixture. However, due to the preferential growth of pre-wetted particles, the predictive models on particle sizes built in conventional granulation did not work well in pre-wetting granulation. (4) A prediction model on green bed permeability was developed with fractal parameters of the granules. With these findings, this work can be meaningful for ore-blending optimization and granulation enhancement.
  • Factors to Determine Inclusion Compositions in Molten Steel during the Secondary Refining Process of Case-Hardening Steel

    pp. 1973-1981

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    DOI:10.2355/isijinternational.ISIJINT-2016-324

    A study was carried out to observe inclusions during the secondary refining process of case-hardening steel to understand the factors to determine inclusion compositions. During the LF refining, inclusions changed from the primary deoxidation product of Al2O3 to MgO·Al2O3 and the CaO–Al2O3 system. At the end of LF refining, the compositions were placed on the tie-line connecting the areas of MgO·Al2O3 and CaO–Al2O3–MgOliq in the CaO–Al2O3–MgO diagram. This change took place by the composition evolution targeting the thermodynamic equilibrium states. After the RH treatment, the inclusion compositions changed mainly to Al2O3 and the CaO–Al2O3 system because MgO·Al2O3 inclusions were removed, while the CaO–Al2O3 system inclusions, the most stable oxide, were remained. This behavior could be described by the interfacial properties between the oxides and the molten steel. Al2O3 inclusions were considered to be newly generated during the RH treatment. It was confirmed that three factors of (1) equilibrium state (2) removal and (3) generation of inclusions dominated to determine the inclusion compositions.
  • A Numerical Study about the Influence of a Bubble Wake Flow on the Removal of Inclusions

    pp. 1982-1988

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    DOI:10.2355/isijinternational.ISIJINT-2016-249

    The inclusion removal mechanism due to a bubble wake flow was studied using a water model and a three dimensional numerical model. In the experiments, a high-speed camera was used to record the bubble movement and the inclusion behavior connected to the flow. In the numerical model, the bubble induced fluid dynamics in the liquid was simulated using the volume of fluid (VOF) method. Also, the individual particle motion was tracked by the discrete phase model (DPM). A two-way coupling approach was used to model the interaction between the continuous phase and the discrete particles. The calculated results were compared with the experimental observations in the air-water-particle system. Calculations were also extended to an argon-steel-inclusion steelmaking system. The predictions show that the removal percent per bubble is increased with an increased bubble size. However, the inclusion removal percent per unit bubble volume can be improved by decreasing the bubble size. Also, the inclusion rising zone was found to be 1.625 and 5 times of the bubble size in width and height, respectively. It is also shown that the bigger inclusions are more easily removed compared to the smaller ones.
  • Three-dimensional Observation of Nonmetallic Inclusion Clusters in Solid Metal by X-ray Micro-CT

    pp. 1989-1995

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    DOI:10.2355/isijinternational.ISIJINT-2016-218

    In this study, we developed a method of observing three-dimensional (3D) inclusion clusters in metal. The theory of X-ray micro-computed tomography (CT) is generally introduced. The criteria on determining the sample size and energy of X-ray beam is set according to the characteristics of the beamline BL20XU available at Spring-8, which is the largest synchrotron radiation facility. The Al–TiB2 and Al–SiC system were measured by the X-ray micro-CT available in beamline BL20XU at Spring-8. The determination of the sample size the energy of X-ray beam are discussed on basis of the X-ray linear attenuation coefficient (LAC) and the transmission rate of Al–TiB2 and Al–SiC system. The limitation of the charge coupled device (CCD) camera, such as spatial resolution and observing field, are taken into consideration as well. Phase retrieval method is applied to reduce the noises of tomographic images for Al–SiC system due to its poor contrast. The 3D particle clusters of TiB2 and SiC are reconstructed by self-developed program. The fractal dimension of TiB2 and SiC clusters are calculated around 2.7. The feasibility of observing alumina cluster in steel is discussed on basis of the latest capability of beamline BL20XU at Spring-8.
  • Precipitation Behavior of Primary Precipitates in Ti-microalloyed H13 Tool Steel

    pp. 1996-2005

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    DOI:10.2355/isijinternational.ISIJINT-2016-199

    The characteristics and thermal stability of the primary precipitates in H13 ingot modified with 0.034 wt% Ti were studied. A large number of (Ti,V) carbonitrides were observed, including the homogeneous Ti-rich ones and the inhomogeneous Ti-V-rich ones. The atomic ratios of Ti/V in Ti-rich precipitates range from 3.36 to 9.53, while those in Ti-V-rich precipitates range from 0 to 7.60. A certain number of sulfide and Mo-Cr-rich carbides also exist in the ingot. Ti-rich carbonitride is stable at 1150°C and 1250°C. Ti-V-rich precipitates with low content of Ti are unstable and start to decompose after holding a short time at high temperature. No Mo-Cr-rich carbide is left after 3 h at 1150°C or even 0.5 h at 1250°C. These precipitates are generated during solidification and the generating process could be well speculated by Thermo-Calc. During solidification, Ti-N-rich carbonitride precipitates first and the composition changes little until the generation of Ti-V-rich precipitates. With the development of solidification, the contents of Ti and N in newly generated Ti-V-rich precipitates decrease, while that of V and C increase. Sulfide and Mo-Cr-rich carbides precipitate at the end of solidification.
  • Operation Optimization of Slab Reheating Process Based on Differential Evolution

    pp. 2006-2015

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    DOI:10.2355/isijinternational.ISIJINT-2016-211

    In the slab reheating process, the temperature distribution of walking beam furnace is critical for the temperature of slab, as well as the subsequent hot rolling. Besides, this process is dynamic, nonlinear, and time varying due to coupling with complex physical and chemical reactions. In this paper, we focus on an operation optimization problem of tuning furnace temperature to optimize the slab reheating process, which considers the reheating quality and production cost. For this purpose, we develop a novel operation optimization method. Specifically, first, from the view of mechanism, a heat transmit model based on the heat-exchange of slab and furnace, an energy consumption function, and especially, an oxidation loss function are established. Therefore, incorporating the mechanism models, we build an optimization model to minimize temperature deviation of slab, energy consumption, and oxidation loss to describe our problem. Then, based on the features of the problem, we propose a modified differential evolution algorithm, which includes a space contraction scheme, a new self-adaptive parameter strategy, and a new mutation operator. Finally, to evaluate the performance of our method, extensive numerical experiments are implemented by comparing it with other well-known evolutionary algorithms based on practical data. The experimental results demonstrate the effectiveness of proposed operation optimization method on solving our problem.
  • Boiling Heat Transfer during Impingement of Two or Three Pipe Laminar Jets onto Moving Steel Sheet

    pp. 2016-2021

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    DOI:10.2355/isijinternational.ISIJINT-2016-295

    The impingement of pipe laminar jets is commonly used in run-out-table cooling in hot rolling mills. In this process, a moving hot steel sheet is cooled by pipe laminar array jets. When the spacing between two neighbor jets is small in the sheet width direction, flow interaction of cooling water on the sheet is inevitable, resulting in complex heat transfer phenomena. In the present study, the boiling heat transfer during the impingement of two or three pipe laminar jets onto a moving steel sheet was studied by laboratory-scale experiments. The test coolant was water at room temperature. Water jets were produced from 5-mm-diameter pipe nozzles at a mean velocity of 0.8 m/s. The nozzle spacing between two jet centers was 8, 12, or 16 mm. A 0.3-mm-thick stainless steel sheet with a moving velocity of 1.5 m/s was used as the test substrate. The temperature of steel ranged from 300 to 500°C. The flow was observed by flash photography, and the heat transfer characteristics were studied by an infrared thermography technique. It was found that high heat flux regions were formed near the jet impact points on the moving solid. Flow interaction occurred between two jets, where the heat removal rate was relatively small compared to that in the jet impact regions. The effects of the nozzle spacing, number of nozzles, and temperature of the solid on the boiling heat transfer characteristics were studied in detail from an industrial viewpoint.
  • Evaluation of Solidification Cracking Susceptibility for Austenitic Stainless Steel during Laser Trans-Varestraint Test Using Two-dimensional Temperature Measurement

    pp. 2022-2028

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    DOI:10.2355/isijinternational.ISIJINT-2016-302

    Laser beam welding at high speeds enhances solidification cracking susceptibility, and brittleness temperature range must be measured under a high cooling rate to quantitatively evaluate the influence of welding speed upon the susceptibility. This work investigated the influence of welding speed upon the solidification cracking susceptibility of type 310S stainless steel using the laser Trans-Varestraint test. A multi-sensor camera based on two-color thermometry measured two-dimensional temperature distribution around the molten pool to obtain the temperature range along different solidification crack directions, and the applicability and accuracy of the two-dimensional temperature distribution technique for measuring temperature range of cracks was investigated. The brittleness temperature range was measured precisely with this method; it was nearly constant around 102°C at welding speeds from 0.2 to 2.0 m/min. The results demonstrated the effect of the welding speed from 0.2 to 2.0 m/min upon the solidification cracking susceptibility was very small.
  • Evaluation of Epoxy Coating for Ballast Tanks under Thermal Cycling by Electrochemical Impedance Spectroscopy

    pp. 2029-2036

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    DOI:10.2355/isijinternational.ISIJINT-2016-228

    The degradation of epoxy coating for ballast tanks was examined by electrochemical impedance spectroscopy (EIS). EIS spectra were measured in LiCl solutions using approximately 200-µm-thick epoxy-coated steel in the temperature range 30–70°C. To clarify the effect of water activity (aw), three different LiCl concentrations were used: 0.01 M (aw = 1), 5 M (aw = 0.7), and 10 M (aw = 0.3). The EIS spectra in 0.01 M LiCl solution revealed capacitive behavior over the whole 1 kHz–1 mHz frequency range at 30°C, whereas distinct resistive behavior arose in the low-frequency region above 40°C. Increasing temperature and water activity slightly increased the epoxy film capacitance (Cf) and drastically decreased the film resistance (Rf). The long-term monitoring of the coating degradation by EIS was also performed under thermal cycling in the 0.01 M LiCl solution, from which it was found that Rf decreased and Cf increased gradually with increasing cycle number. Observation by scanning electron microscopy after 250 thermal cycles revealed that several defects (voids and cracks) that could act as water absorption sites formed inside the coating and at the coating/steel interface. The epoxy coating was removed from the steel substrate and a trace amount of Fe3O4 was confirmed to have formed over the whole surface of the steel substrate. EIS is extremely useful for monitoring the degradation of thick polymer coatings under thermal cycling.
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  • Combined Multi-scale Analyses on Strain/Damage/Microstructure in Steel: Example of Damage Evolution Associated with ε-martensitic Transformation

    pp. 2037-2046

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    DOI:10.2355/isijinternational.ISIJINT-2016-272

    We studied damage evolution behavior associated with ε-martensite in a Fe-28Mn alloy. Visible factors of damage evolution associated with ε-martensite are considered to be strain distribution, microstructure, micro-void and crack. Combinatorial use of replica digital image correlation, electron backscattering diffraction, and electron channeling contrast imaging enables to clarify the distributions of strain, microstructure and damage. Through quantitative damage analysis, damage evolution behavior was classified into three regimes: (i) incubation regime, (ii) nucleation regime, and (iii) growth regime. In the incubation regime, an interaction of ε/ε-martensite plates and impingement of ε-martensite plates on grain boundaries caused plastic strain localization owing to plastic accommodation. In the nucleation regime, accumulation of the plastic strain on the boundaries caused microvoid formation. The damage propagated along with the boundaries through coalescence with other micro-voids, but the propagation was arrested by crack blunting at non-transformed austenite. In the growth regime, the arrested damage grew again when a further plastic strain was provided sufficiently to initiate ε-martensite near the damage.
  • Combined Effect of Ausforming and Warm Tempforming on the Strength and Toughness of An Ultra-High Strength Steel

    pp. 2047-2056

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    DOI:10.2355/isijinternational.ISIJINT-2016-275

    A 0.4C-2Cr-1Mo-2Ni steel (in mass%) was austenitized at 1123 K, followed by ausforming (AF) using multi-pass caliber rolling with a rolling reduction of 74%. The AF samples were subsequently tempered at 773 K and deformed by multi-pass caliber rolling (i.e. warm tempforming, WTF) with a rolling reduction of 46%. Their microstructures and mechanical properties were investigated and compared to those of the quenched and tempered samples (non-AF samples) which were subjected to the sequent WTF with rolling reductions of 46% and 74%. The WTF with rolling reductions ranging 46 to 74% resulted in the strengthening and toughening of the AF and non-AF samples through the evolution of anisotropic and ultrafine grain structures with strong <110>//rolling direction (RD) fiber textures. The AF sample demonstrated the faster kinetics of the microstructural changes, i.e. refinement in the transverse grain size and development of highly elongated grains than the non-AF sample. As a result, the AF sample exhibited a better combination of ultra-high strength and toughness than the non-AF sample, when compared at the rolling reduction of 46%. The combined effect of AF and WTF was especially pronounced in the enhancement of toughness resulting from delamination.
  • Application of the Quenching and Partitioning (Q&P) Process to D6AC Steel

    pp. 2057-2061

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    DOI:10.2355/isijinternational.ISIJINT-2016-257

    Quenching and partitioning (Q&P) was successfully applied to a medium carbon and low alloy martensitic D6AC steel. Fully austenitized samples were quenched to temperatures below the Ms temperature (317°C), ranging from 240 to 275°C, and partitioned at the quench temperature for 300 s. While the as quenched sample exhibits a fully martensitic microstructure, all the Q&P processed samples have a retained austenite fraction up to about 6%. These samples were tensile tested at room temperature and their mechanical properties were compared to those obtained from different heat treated D6AC samples. Regardless of the partitioning temperature (equivalent to the quench temperature), the retained austenite in the Q&P processed samples transformed to martensite during the tensile testing, providing additional ductility above 10% of total elongation for the high strength material with a tensile strength around 1500 MPa.
  • Precipitation Behavior of Copper Sulfides in Fe–Si–Cu–S Ferritic Steel

    pp. 2062-2067

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    DOI:10.2355/isijinternational.ISIJINT-2016-056

    Precipitation behavior of copper sulfides in Fe-3.2%Si-0.4%Cu-0.02%S (mass%) ferritic steel was studied by X-ray diffraction (XRD), microstructure examination by transmission electron microscope (TEM) and scanning electron microscope (SEM). From the TEM results, it was found that FeS and cubic Cu2-xS, precipitated in as-cast specimens. XRD peaks due to the cubic Cu2-xS almost disappeared at 1173 K. XRD peaks due to the hexagonal Cu2-xS were identified in specimens annealed above 1373 K. Assuming that the hexagonal Cu2-xS is metastable in steel, which precipitates during cooling after dissolution of the stable cubic Cu2-xS, it can be suggested that the cubic Cu2-xS was dissolved at least 1173 K. Results of thermodynamic calculations are consistent with this suggestion, which indicates that the dissolution temperature of the cubic Cu2-xS is lower than those of other sulfides such as MnS and FeS in the ferritic steel.
  • Quantification of Large Deformation with Punching in Dual Phase Steel and Change of its Microstructure –Part I: Proposal of the Quantification Technique of the Punching Damage of the Dual Phase Steel

    pp. 2068-2076

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    DOI:10.2355/isijinternational.ISIJINT-2016-312

    Dual Phase (DP) steel is used in automotive body parts for weight saving and crashworthiness, however there is an issue of DP steel in low stretch flange ability evaluated by hole expanding tests. In order to improve stretch flange ability of DP steel, it is important to estimate the damage of punching quantitatively and to clarify the change of microstructure before and after punching because the hole expansion ratio is decided in the ductility remained after pre-strain equivalent to punching. Therefore we tried to measure the damage of punching by unique techniques of Electron Backscatter Diffraction (EBSD), nano-indentation and micro-tensile testing and to observe fracture surface by Scanning Transmission Electron Microscope (STEM). Average EBSD-Kernel Average Misorientation (KAM) value and pre-strain damage have strong correlation, thus average KAM value can become the index of the damage. The nanohardness and tensile strength using micrometer-sized specimens increased with increasing average KAM value in the ferritic phase as approaching the punching edge. A shear type fracture occurred without necking in the specimen cut out in the area of the edge. The ultrafine-grained ferritic microstructure was observed in the sample cut out in the same area with STEM. It seems that the ductility loss of the punched DP steel was probably attributed to localized strain into the ultrafine-grained ferritic microstructure.
  • Quantification of Large Deformation with Punching in Dual Phase Steel and Change of its Microstructure –Part II: Local Strain Mapping of Dual Phase Steel by a Combination Technique of Electron Backscatter Diffraction and Digital Image Correlation Methods

    pp. 2077-2083

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    DOI:10.2355/isijinternational.ISIJINT-2016-310

    To evaluate heterogeneous strain distribution developed by pre-deformations in dual phase (DP) steel accurately, a combinational technique of Electron Backscatter Diffraction (EBSD) and Digital Image Correlation (DIC) methods was newly introduced in this study. A good correlation is established between kernel average misorientation calculated by EBSD and local equivalent strain measured by DIC in ferrite matrix of DP steels regardless of the difference in deformation process, which means that an EBSD orientation map can be easily converted into an applicative strain map by employing the individual correlation formula. This new technique reveals that very large strain region is locally formed within dozens of micrometer from the punched edge in a punched DP steel. On the other hand, hard martensite grains dispersed in DP steel remarkably promote the heterogeneity of strain distribution in ferrite matrix. As a result, the large strain region is also developed in the form of bands in a cold-rolled DP steel by only 60% thickness reduction at least, as if it is affected by the distribution and morphology of martensite grains. In addition, the local strain mapping demonstrates that the equivalent strain of the large strain band in cold-rolled material is comparable to that of the heavily deformed edge in punched one. The very large strain band in ferrite matrix is characterized by ultrafine grained structure, which leads to the possibility for the losing ductility in ferrite matrix and the martensite cracking.
  • Quantification of Large Deformation with Punching in Dual Phase Steel and Change of its Microstructure –Part III: Micro-tensile Behavior of Pre-strained Dual-phase Steel

    pp. 2084-2092

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    DOI:10.2355/isijinternational.ISIJINT-2016-318

    The deformation behavior of inhomogeneous microstructures developed by pre-straining was studied by micro-tensile testing to elucidate the cause of low hole expandability of ferrite–martensite dual-phase (DP) steels. Slip bands developed in the ferritic phase, when the DP steel was cold-rolled (CR) at a reduction of 60% in thickness; in the 88% CR microstructure, ultrafine ferrite grains with a strong texture were locally observed. While the nanohardness increased with increasing pre-strain in the ferritic phase, it was constant in the martensitic phase. Tensile tests using micrometer-sized specimens with ferritic and martensitic phases revealed that the ultrafine-grained ferritic microstructure exhibited high yield strength but low ductility when compared to the slip band ferritic microstructure. While a shear type fracture occurred without necking in the former, the latter exhibited a chisel-edge type failure. In the absence of ultra grain refinement by pre-straining, the inhibition of slip transfer by the interphase boundary was a major contributor to the strengthening in the DP steel. The ductility loss of the severely deformed DP steel was attributed to localized strain in the ultrafine-grained ferritic microstructure.
  • Influence of Gas Injection Pipe on CO2 Decomposition by CaCl2–CaO Molten Salt and ZrO2 Solid Electrolysis

    pp. 2093-2099

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    DOI:10.2355/isijinternational.ISIJINT-2016-179

    The electrochemical decomposition of carbon dioxide to form carbon and oxygen gas in CaCl2–CaO molten salt was studied. A water model experiment was carried out to study the influence of the tip shape of the pipe, the pipe diameter and the wettability of the gas injection pipe on the bubble shape in the molten salt. Bubbles were formed with both a horizontal tip and an oblique tip when the wettability of the gas injection pipe was good. The specific surface area of the bubbles when using the oblique tip was smaller than when using the horizontal tip. On the other hand, slug flows appeared when wettability was poor. In a hot model experiment, the current density was measured, and it was found that the CO2 gas concentration decreased. Precipitated carbon was detected from the sample after the experiment. With the same pipe, the decrease of the CO2 gas concentration when using the oblique tip was more remarkable than when using the horizontal tip. It is likely that the form of the CO2 gas in the molten salt was bubble-shaped.
  • A Field Experiment of Sulfide Reduction in Silty Sediment Using Steel-making Slag

    pp. 2100-2106

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    DOI:10.2355/isijinternational.ISIJINT-2015-622

    Hydrogen sulfide is often generated through sulfate reduction under anaerobic conditions in enclosed coastal seas. It is highly toxic, depletes oxygen and forms blue tides. To evaluate the sulfide reduction effect of steel-making slag, we conducted field experiments in Fukuyama inner harbor, where people have suffered from odors caused by gasses including hydrogen sulfide generated from the sediments. We placed steel-making slag on the sediments, and monitored the quality of interstitial water in the sediments and the overlying water. Hydrogen sulfide gas was also collected and measured. Dissolved sulfides in the interstitial water of the steel-making slag construction area were suppressed to below 5 mg/L (as sulfur), while levels ranged from 100 to 350 mg/L in control plots; this reduction lasted for about 2 years. It was assumed that Fe ions eluted from steel-making slag may have reacted with the sulfide. Species number and individual numbers of macrobenthos increased in the steel-making slag construction area. The results imply that capping deteriorated sediments with steel-making slag can effectively improve water and sediment quality of coastal areas.
  • Electrical-resistivity Measurements of Liquid Fe–C Alloys Using the Four-terminal Method

    pp. 2107-2109

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    DOI:10.2355/isijinternational.ISIJINT-2016-143

    Electrical-resistivity measurements of liquid Fe–C alloys were attempted using a new probe developed on the basis of the four-terminal method. The four graphite electrodes (except their ends) were covered with alumina tubing and were combined together with an outer alumina tube such that the sample diameter was determined by the outer tubing. The as-prepared probe was dipped into the liquid samples for measuring their resistivity. With the aim to confirm the reliability of the technique, the probes were firstly used to measure the electrical resistivity of liquid Ga and Sn samples. Subsequently, electrical-resistivity measurements were conducted for Fe-4.31mass%C and Fe-4.57%C samples at ca. 1585 K; these measurements yielded values of 1.48×10−6 and 1.58×10−6 Ω·m, respectively. A comparison with the reported data suggests that the electrical resistivity of liquid iron progressively increased with the carbon concentration up to 3 mass%, beyond which it showed a drastic increase. This resistivity change might have occured because of the structure of the liquid Fe–C alloy.
  • Separation of Arsenic and Lead by Electrolysis for Total Reflection X-Ray Fluorescence Analysis

    pp. 2110-2112

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    DOI:10.2355/isijinternational.ISIJINT-2016-017

    In total reflection X-ray fluorescence (TXRF) analysis of trace amounts of arsenic in the presence of lead in environmental water, spectrum interference between the As Kα line (10.54 keV) and Pb Lα line (10.55 keV) is an important issue. In this note, a method was reported for the separation of these elements by electrolysis. Lead was not detected from the electrolyzed solution after the electrolysis of a solution containing 0.1 mg/L of lead. When a solution containing 0.1 mg/L of arsenic (III) and 0.1 mg/L of lead and a solution containing 0.1 mg/L of arsenic (V) and 0.1 mg/L of lead were electrolyzed, lead species were more easily removed than the arsenic species from the solutions, and almost all of the arsenic species remained in the electrolyzed solutions. The utility of electrolysis for decreasing the concentration of lead in solution samples makes it feasible as a method for accurate determination of arsenic concentration by TXRF analysis.
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