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

ISIJ International Vol. 52 (2012), No. 3

  • Influence of Non-stoichiometric Serpentine in Saprolite Ni-ore on a Softening Behavior of Raw Materials in a Rotary Kiln for Production of Ferro-nickel Alloy

    pp. 333-341

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    DOI:10.2355/isijinternational.52.333

    The investigation of influence of non-stoichiometric serpentine in saprolite Ni-ore on a softening behavior of raw materials has been successfully carried out by SEM-EDS analysis. Serpentine in ore is classified into the low-MgO and high-FeO, high-MgO and low-FeO type. The ratio of that depends on the degree of leaching of MgO during the weathering. The crystal lattice of serpentine is significantly disturbed by the leaching of MgO, and has cation component much lower than stoichiometric composition, which allows replacement of much Mg2+ by Fe3+ to be enhanced during weathering. This leads to the formation of non-stoichiometric low-MgO type serpentine with much Fe2O3. The recrystallization of that continually takes place in accordance with equilibrium. Therefore, it is not until 1300°C that olivine (2(Mg1–xFex)·SiO2) occurs, in that case, low-MgO and high-SiO2-FeO amorphous silicate is simultaneously formed as the residual melt. Further, CaO in flux and Al2O3 in spinel melt into that melt, which allows melting temperature of melt to be still more reduced. In the recrystallization of non-stoichiometric high-MgO type of serpentine, olivine occurs from around 900°C. High-MgO-SiO2 and low-FeO amorphous silicate is simultaneously formed as the residual of recrystallization. Whereas, the contents of higher-MgO and lower-FeO than Ore-A leads to difficulty in melt generation. Thus, melt generation in saprolite Ni-ore occurs as the residual in the recrystallization (i.e., formation of olivine) of non-stoichiometric serpentine, being enhanced by the amount of MgO leached and Fe3+ by which Mg2+ is emplaced in the weathering.
  • Molecular Dynamics Study of the Structural Properties of Calcium Aluminosilicate Slags with Varying Al2O3/SiO2 Ratios

    pp. 342-349

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    DOI:10.2355/isijinternational.52.342

    Molecular dynamics simulation was explored to investigate the change of structure of calcium aluminosilicate slags with varying Al2O3/SiO2 ratios at a fixed CaO content. In practice the results of the study are relevant to the significant changes in slag structure caused by the changes in chemical composition during continuous casting of steels containing high amounts of dissolved aluminum. It was found that Q4 tetrahedral species (tetrahedron with four bridging oxygens) increase while NBOs (non-bridging oxygen) decreases with increasing Al2O3/SiO2 mole ratio, suggesting that a more polymerized network was formed. The concentration of oxygen tricluster increased dramatically up to 24% with increasing Al2O3/SiO2 mole ratio, and the coordination number for Al (CNAl–O) was also observed to increase from 4.02 for sample CAS1 to 4.11 for sample CAS11, suggesting that high coordination number of Al presents in the slag melt with the substitution of [AlO4] for [SiO4]. NBOs prefer to be coordinated with Si and Al tends to be localized in more polymerized environment as network intermediate phases. The degree of Al avoidance was calculated and the Al avoidance principle is applicable in the SiO2 rich regions.
  • Shape Anisotropy of Electromagnetic Materials in Application of Microwave by Micro-sized Numerical Electromagnetic Field Calculation

    pp. 350-354

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    DOI:10.2355/isijinternational.52.350

    As the fundamental characteristics of microwave application, which is expected to be one of the useful technologies in the reduction reaction of iron oxide, electromagnetic anisotropic characteristics of electromagnetic materials in application of microwave are observed by micro-sized numerical high frequency electromagnetic computation. In assuming that the micro-sized unit structure is repeated in 3-dimensional directions and its size is much smaller than the electromagnetic wave length, the micro-sized electromagnetic field calculation using finite element method and jω method is carried out. In considering the bar shape and the cube shape as a micro-structure of electromagnetic material, the bar shape micro-structure has anisotropic characteristics, though cube shape micro-structure has isotropic ones. The generation of depolarization field within the micro-structure causes the difference of electromagnetic characteristics and the anisotropy.
  • Modelling Viscosities of CaO–MgO–Al2O3–SiO2 Molten Slags

    pp. 355-362

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    DOI:10.2355/isijinternational.52.355

    A structurally-based viscosity model (using a few optimized parameters) is proposed to represent viscosity as functions of both temperature and composition for the CaO–MgO–Al2O3–SiO2. The model represents the slag structure through the different types of oxygen ions formed in the melt. Approximate methods for calculating the concentrations of these different types of oxygen ions are proposed and are then used to describe the effect of melt structure on viscosity. The model provides a good description of the viscosity behavior varied with composition and temperature within the CaO–MgO–Al2O3–SiO2 system. This includes pure systems: Al2O3 and SiO2; binary systems: CaO–SiO2, MgO–SiO2, Al2O3–SiO2 and CaO–Al2O3; ternary systems: CaO–MgO–SiO2, CaO–Al2O3–SiO2 and MgO–Al2O3–SiO2; quaternary system: CaO–MgO–Al2O3–SiO2. The different roles of CaO and MgO on viscosity are also discussed; these tend to differ in melts with or without Al2O3. In the absence of Al2O3, CaO reduces the viscosity more than MgO. In contrast, when Al2O3 is present, the Ca2+ ions take priority over Mg2+ ions in the charge-compensation of Al3+ ions which leads to the formation of more stable Ca–AlO45– tetrahedral structures, which, in turn, results in an increase in viscosity. However, when there is enough basic oxide (CaO or MgO) present to generate non-bridging oxygens, CaO reduces the viscosity more effectively than MgO.
  • Carbothermal Reduction and Nitridation of Ilmenite Concentrates

    pp. 363-368

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    DOI:10.2355/isijinternational.52.363

    Carbothermal reduction and nitridation of ilmenite concentrates of different grades and synthetic rutile were studied in isothermal experiments in hydrogen – nitrogen gas atmosphere and pure nitrogen in a tube reactor. Concentrations of CO and CO2 in the off gas were measured online using infrared gas analyser. The reaction products were analysed by X-ray diffraction. Iron oxides in ilmenite concentrates were reduced to metallic iron, and titanium oxides were converted to titanium oxycarbonitride. Reduction of ilmenite concentrates and synthetic rutile in hydrogen containing gas was significantly faster than in pure nitrogen. The rate of conversion of titania to titanium oxycarbonitride in the 50 vol% H2-50 vol% N2 gas mixture decreased with increasing grade of ilmenite concentrate. In nitrogen, reduction of synthetic rutile was faster than reduction of ilmenite concentrates.
  • Influence of Additives on Cokemaking from a Semi-soft Coking Coal during Microwave Heating

    pp. 369-377

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    DOI:10.2355/isijinternational.52.369

    Coke was produced from a Waterberg semi-soft coking coal using microwave heating and selected microwave susceptors. Waterberg semi-soft coking coal is poorly susceptible to microwave heating, especially below 500°C, and therefore requires microwave susceptors. Susceptors were selected from ferroalloy fines and their respective ores.
    Various batch experiments were performed on compressed discs utilizing a resonant microwave cavity at a constant 915 MHz frequency to heat a batch of about 5 to 7 kg of the semi-soft coking coal (sscc) to obtain coke. Materials were characterized using Inductive Coupled Plasma (ICP) analysis and coke strength tests.
    Dielectric property results showed that chrome and manganese ores, as well as their respective high carbon ferrochrome and ferromanganese alloys, are suitable microwave susceptors to enable rapid coke formation during microwave heating. Coke formation was completed within 2 to 3 hours up to 1100°C compared to 21 hours for a commercial plant since microwave heating reduces the “cold centre” in a coke oven. Obtained coke strengths were slightly lower than for a commercial coke but still of a high quality. It was also shown that the admixture of chrome ore resulted in its partial reduction which will be advantageous to the ferrochrome industry since this method allows for recycling of fines without additional pelletisation. The results also showed that microwave energy has the potential to be employed during commercial coke formation, either on its own or as a hybrid technology.
  • Optimizing the Mass Ratio of Two Organic Active Fractions in Modified Humic Acid (MHA) Binders for Iron Ore Pelletizing

    pp. 378-384

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    DOI:10.2355/isijinternational.52.378

    The aim of this present paper was to systematically optimize and determine the mass ratio of two organic active fractions (fulvic acid and humic acid) in a type of novel organic binder (named Modified Humic Acid (MHA)) developed in China for iron ore pelletizing. The experimental data demonstrated that the mass ratio of Na-HA to Na-FA had an obvious influence on the performance of MHA binders. The drop number of green pellets grew to reach a plateau of 11.7 times when the mass ratio of Na-HA to Na-FA was increased to 7:3. Obviously, the crushing strength of dried and fired pellets was increased with the increase of mass ratio of Na-HA to Na-FA. Especially, dried or preheated pellets remarkably increased their crushing strength after the mass ratio of 6:4. From these results, the optimum mass ratio Na-HA to Na-FA in MHA binders for iron ore palletizing was concluded to be 7:3. The pelletizing performance of MHA binders could be undoubtedly improved by optimizing the mass ratio of Na-HA to Na-FA. The explanation for improving the pelletizing performance of MHA binders by optimizing the mass ratio of two fractions lies in that the influencing factors (acidic groups and viscosity) for deciding the pelletizing performance of MHA binders can be optimized and balanced by changing mass ratio of Na-HA to Na-FA. The reason for that mechanical strength of fired pellets was influenced by mass ratio of active fractions in MHA binders, was considered to be attributed to the thermal stability difference of active fractions.
  • Recycling Waste Polymers in EAF Steelmaking: Influence of Polymer Composition on Carbon/Slag Interactions

    pp. 385-393

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    DOI:10.2355/isijinternational.52.385

    Interactions between blends of metallurgical coke and polymers with EAF slag (30.5% FeO) at 1550°C have been investigated using a sessile drop arrangement to determine the influence the polymer and its chemical composition on carbon/slag interactions. Two polymers, namely polyethylene terephthalate (PET) and polyurethane (PU), were used in this study. The CO emissions during carbon/slag interactions for PET/Coke and PU/Coke blends were lower as compared to corresponding emissions from metallurgical coke. An improvement in slag foaming, as determined by the dynamic changes in the volume of the slag droplet, was observed when PET/Coke and PU/Coke blends were used compared to the coke alone. Relatively greater number of gas bubbles was found to be entrapped in the slag droplet along with reduced iron droplets. Higher level of H2O formation was also seen in the case of PET/Coke and PU/Coke blends as a result of FeO reduction by H2. These results indicate that volatiles (H2 and CH4) released from the polymer/coke blends can influence the interactions between carbon and slag (in addition to reduction reactions by solid carbon). This study shows that a variety of waste polymers can be utilised as a carbon resource in EAF steelmaking processes.
  • Mathematical Modeling of Refining of Stainless Steel in Smelting Reduction Converter Using Chromium Ore

    pp. 394-401

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    DOI:10.2355/isijinternational.52.394

    For predicting and controlling the process of chromium ore smelting reduction in a converter preferably, the coupling dynamic model is established for the first time based on the kinetic models of chromium ore dissolution and interfacial reducing reaction between the slag and the metal. It can decrease the refining time considerably and improve the production efficiency for producing 12 wt% Cr stainless steel crude melts in a converter when the initial hot metal contains the definite chrome metal at 1560°C. Under the conditions of the mathematical model, the effect of chromium ore size on the process of producing stainless steel crude melts by chromium ore smelting reduction in a converter is great when the particle size of chromium ore increases to a certain value. In such condition, the dissolution of chromium ore in the slag is the rate controlling step at a certain stage of refining and the rate of chromium ore smelting reduction is controlled jointly by the dissolution and the reduction of chromium ore. The effect of temperature on the process of chromium ore smelting reduction in a converter is significant. The mean reduction rate of chromium ore increases from 0.052 wt%·min–1 to 0.175 wt%·min–1 with the increasing of temperature from 1540°C to 1580°C.
  • Modeling and Validation of an Electric Arc Furnace: Part 1, Heat and Mass Transfer

    pp. 402-412

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    DOI:10.2355/isijinternational.52.402

    The following paper presents an approach to the mathematical modeling of heat and mass transfer processes in a 3–phase, 80 MVA AC, electric arc furnace (EAF) and represents a continuation of our work on modeling the electric and hydraulic EAF processes. This paper represents part 1 of the complete model and addresses issues on modeling the mass, temperature and energy processes in the EAF, while part 2 of the paper focuses solely on the issues related to the thermo-chemical relations and reactions in the EAF. As is generally known, the chemical, thermal and mass processes in an EAF are related to each other and cannot be studied completely separately; therefore, the work presented in part 1 and part 2 is related to each other accordingly and should be considered as a whole. The presented sub-models were obtained in accordance with different mathematical and thermo-dynamic laws, with the parameters fitted both experimentally, using the measured operational data of an EAF during different periods of the melting process, and theoretically, using the conclusions of different studies involved in EAF modeling. In conjunction with the already presented electrical and hydraulic models of the EAF, the heat-, mass- and energy-transfer models proposed in this work represent a complete EAF model, which can be further used for the initial aims of our study, i.e., optimization of the energy consumption and development of the operator-training simulator. The presented results show high levels of similarity with both the measured operational data and the theoretical data available in different EAF studies, from which we can conclude that the presented EAF model is developed in accordance with both fundamental laws of thermodynamics and the practical aspects regarding EAF operation.
  • Modeling and Validation of an Electric Arc Furnace: Part 2, Thermo-chemistry

    pp. 413-423

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    DOI:10.2355/isijinternational.52.413

    The following paper presents an approach to the mathematical modeling of thermo-chemical reactions and relations in a 3–phase, 80 MVA AC, electric arc furnace (EAF) and represents a continuation of our work on modeling the electric and hydraulic processes of an EAF. This paper is part 2 of the complete EAF model and addresses the issues relating to chemical reactions and the corresponding chemical energy in the EAF, which are not included in part 1 of the paper, which is focused on mass, temperature and energy-exchange modeling. Part 2 and part 1 papers are related to each other accordingly and should be considered as a whole. The developed and presented sub-models are obtained according to mathematical and thermo-chemical laws, with the parameters fitting both experimentally, using the measured operational data of an EAF during different periods of the melting process, and theoretically, using the conclusions of different studies involved in EAF modeling. Part 2, part 1 and the already published electrical and hydraulic models of the EAF represent a complete EAF model, which can further be used for the initial aims of our project, i.e., optimization of the energy consumption and the development of an operator-training simulator. Like with part 1, the obtained results show high levels of similarity with both the operational measurements and theoretical data available in different studies, from which we can conclude that the presented EAF model is developed in accordance with both the fundamental laws of thermodynamics and the practical aspects relating to EAF operation.
  • Improvement of Hardenability and Static Mechanical Properties of P20 + 0.5Ni Mold Steel through Additions of Vanadium and Boron

    pp. 424-433

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    DOI:10.2355/isijinternational.52.424

    Constant demand to optimize production of plastic components puts pressure on mold steels manufacturers to supply pre-hardened blocks with larger cross-sections. As the size of mold blocks increases, it becomes more difficult to maintain the same microstructure throughout their volume. Thus, the size of mold steel blocks is limited by the cooling rate reachable at their center. This situation has created the need for the development of new alloys with improved hardenability. Additions of vanadium up to 0.35 wt-% have been made to P20 + 0.5Ni mold steel. The addition of vanadium significantly increased hardenability, allowing the production of pre-hardened blocks with a cross section 88% larger than the same alloy without vanadium. Such additions of vanadium also increased the impact toughness by 120%, raised the yield strength by 17% and improved hardness after tempering by 24%. The effect of boron on the hardenability of the P20 + 0.5Ni steel was also investigated. It resulted in a significant increase in hardenability and, in theory, could permit the production of pre-hardened blocks with a cross-section 400% larger than the original material. Finally, our results show that 0.35 wt-% of Ni can be substituted by 0.15 wt-% of vanadium in a P20 + 0.5Ni steel and still maintain the same hardenability.
  • Effects of Second Phase Particle Dispersion on Kinetics of Isothermal Peritectic Transformation in Fe–C Alloy

    pp. 434-440

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    DOI:10.2355/isijinternational.52.434

    Effects of dispersion of insolvable particles on peritectic transformation kinetics in Fe–C binary alloy system have been investigated by means of a model experiment based on a diffusion couple method. During isothermal holding of the diffusion couple of δ and liquid phase samples, the peritectic transformation proceeds by migrations of ferrite(δ)/austenite(γ) interface and liquid(L)/γ interface. It was observed that the existence of insolvable particle, ZrO2, in δ phase retarded the migration of δ/γ interface and also the migration of L/γ interface. The retarding effect by the ZrO2 particles becomes strong as volume fraction of particles increases and/or particle radius decreases, which is qualitatively coincident with the effect of particles on grain growth (Zener pinning). These findings were verified by multi-phase field simulations.
  • Influence of Melting Conditions on Graphite Morphology in Ni–C Alloy and Grain Number of Matrix

    pp. 441-446

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    DOI:10.2355/isijinternational.52.441

    The influence of the melting conditions on the graphite morphology and the grain number of the matrix in a Ni–C alloy was studied. Two groups of parent alloys were prepared using high purity materials, one group containing the spheroidizing element of Ce, Mg or Ca, while in the other group, S was added as an anti-spheroidizing element.
    The spheroidizing element-added sample of 0.5 g was melted and held for 15 minutes at 1673 K in an Ar atmosphere and then cooled. At the high cooling rate of 1000 K/min, an irregular graphite appeared. When an Ar+3%H2 atmosphere and a 10-min holding time at 1673 K were used, the formation of the spheroidal graphite was confirmed at the cooling rate of 1000 K/min in both groups.
    The atmosphere and the holding time were then independently changed at the cooling rate of 1000 K/min. It was found that the 3%H2 did not significantly affect the spheroidal graphite formation. Moreover, the holding times of 1 min and 20 min also did not significantly affect the spheroidal graphite area fraction in the Ni–C alloy, while they affected the graphite morphology of the samples containing the spheroidizing elements like Mg.
    The grain number of the matrix around the graphite phase was also investigated in this study based on EBSD observations. The results showed that the matrix around the massive graphite morphology is polycrystalline, but is mainly single crystalline around the spheroidal graphite. The mechanism of the difference in the crystal structure of the matrix in the two different graphite morphologies was also discussed.
  • Numerical Investigation of Electro-magnetic Flow Control Phenomenon in a Tundish

    pp. 447-456

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    DOI:10.2355/isijinternational.52.447

    The liquid steel flow phenomenon in a tundish is important to improve the steel quality. It is governed by the design of the tundish and its flow control devices. Pouring chamber is the most popular flow control device used inside the tundish. Flow control using electro-magnetic forces is the latest development in this area. The present work investigates the electromagnetic flow control phenomenon in the tundish. A 3-D Magnetohydrodynamics (MHD) model was used to study the flow behavior governed by electromagnetic forces in the tundish. The innovative concept of using electromagnetic dam as a flow modifier for tundish was proposed in current work. Computational Fluid Dynamics (CFD) simulations were performed for different combinations of electromagnetic dams and results were compared with pouring chamber. The advantage of electromagnetic flow control over other mechanical devices was established through CFD simulations.
  • Quantitative Analysis of Inclusions in Low Carbon Free Cutting Steel Using Small-angle X-ray and Neutron Scattering

    pp. 457-463

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    DOI:10.2355/isijinternational.52.457

    The microstructure of inclusions in low carbon free cutting steel without lead addition was investigated using small-angle X-ray scattering (SAXS) coupled with small-angle neutron scattering (SANS). The two-dimensional (2D) SAXS pattern shows clear scattering due to inclusions composed of large elongated particles aligned along the rolling direction, and small isotropic particles. From a comparison of the simulated and experimental 2D SAXS patterns, the shapes of the inclusions are regarded as ellipsoid for the larger inclusions and spherical for the smaller inclusions. The length of the minor axis in the large inclusion is 6.9 μm, while the diameter of the small inclusion is 0.50 μm. The aspect ratio of the large inclusion is estimated to be 3.8 in the lower q region, and is reduced slightly to 3.5 in the higher q region from the azimuthal plots. The results of an alloy contrast variation (ACV) analysis using both the SAXS and SANS data indicate that the chemical composition of the inclusions is almost NaCl-type manganese sulfide, and that the amount of iron sulfide is low. The volume fractions are 1.4% for the large inclusions and 0.2% for the small inclusions. This is consistent with the area fraction estimated using an optical microscope, and indicates that nearly all of the sulfur in the steel sample forms the manganese sulfide inclusions.
  • Effect of Process Parameters on Microstructures and Mechanical Properties of a Nano/ultrafine Grained Low Carbon Steel Produced by Martensite Treatment Using Plane Strain Compression

    pp. 464-470

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    DOI:10.2355/isijinternational.52.464

    In this work, the martensite treatment consisting of cold deformation by plane strain compression and subsequent annealing was used for producing the nano/ultrafine grained structure in a low carbon steel. The equivalent strain was varied from 0.1 to 2, while the annealing process was carried out in the temperature range of 400–600°C for 0–180 min. The microstructural evolution and mechanical properties of the as-deformed and annealed specimens were investigated. The results showed that in the as-deformed specimens, increase in strain intensified the volume fraction of the martensite cell blocks and consequently the strength. Fully equiaxed nano/ultrafine grained ferrite was developed from the martensite cell blocks during the annealing at warm temperature around 500°C for sufficient time lengths. It was concluded that the final multi-phased microstructure composed of ultrafine ferrite grains, block-tempered martensite, and fine cementite precipitates was responsible for the obtained superior mechanical properties.
  • Effect of Silicon and Aluminium on Austenite Static Recrystallization Kinetics in High-strength TRIP-aided Steels

    pp. 471-476

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    DOI:10.2355/isijinternational.52.471

    High-strength transformation-induced-plasticity steels contain high concentrations of silicon or aluminium or a combination of the two. The effects of up to 1.5 wt.% Si and up to 1.9 wt.% Al on the static recrystallization kinetics of austenite in 0.2C–2.0Mn–0.6Cr steel have been investigated and the activation energies of deformation (Qdef) and static recrystallization (Qsrx) have been determined. Addition of aluminium increases Qdef and Qsrx more strongly than addition of silicon. Aluminium alloying also retards static recrystallization more strongly than silicon alloying. In both cases, the effects of silicon and aluminium are non-linear saturating towards the highest concentrations studied.
  • Effect of Welding Parameters on Microstructure and Mechanical Properties of Friction Stir Welded Plain Carbon Steel

    pp. 477-482

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    DOI:10.2355/isijinternational.52.477

    In the present investigation, friction stir welding was carried out for plain carbon steel under variable rotational and traversing speed of tool keeping all other welding parameters same. Microstructural characterization was carried out for welded samples along with determination of microhardness distribution and evaluation of mechanical properties. Weld nugget microstructure principally consisted of ferrite-pearlite, however ferrite grain size and pearlite area fraction were varied depending on welding parameters. Substantial grain growth was found in heat affected zone. Sub-size tensile specimens exhibited improvement in strength; whereas, standard tensile samples showed lowering in strength with respect to parent alloy. Ductility of subsize and standard specimens showed smaller value in comparison to parent alloy and dependent on local microstructural characteristics.
  • Influence of Welding Speed on Microstructures and Properties of Ultra-high Strength Steel Sheets in Laser Welding

    pp. 483-487

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    DOI:10.2355/isijinternational.52.483

    Ultra-high strength steels (UHSS) are welded using an Nd:YAG laser welding system. The influence of welding speed on joint appearance, microstructure, hardness and tensile properties are investigated. The results indicate that the size of the fusion zone decreases with the increasing of the welding speed and partial penetration occurs if the welding speed exceeds 100 mm/s. The microstructure of the fusion zone and its approaching heat affected zone (HAZ) is lath martensite leading to the high strength and hardness. There exists a tempered zone (softening area) in the HAZ approaching to the base metal, where the microstructure is tempered maretensite and ferrite. When changing the welding speed from 70 to 130 mm/s, the width of the tempered zone decreases from 100.4 to 48.2 μm and the degree of softening slightly decreases with the minimum hardness ranging from 310 to 336 HV. The highest tensile strength and the maximum elongation at fracture are both obtained at the welding speed of 90 mm/s, the specimen fails in a ductile fracture mode.
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  • Arc Behavior in Non-uniform AC Magnetic Field

    pp. 488-492

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    DOI:10.2355/isijinternational.52.488

    The authors developed an oscillating arc driven by an alternating magnetic field perpendicular to a transferred arc. In this work, the arc behavior was investigated under a non-uniform magnetic field. To prevent heat flux from decreasing near the edge of the arc oscillation, a specially designed non-uniform magnetic field was imposed to the arc. Theoretical consideration of the arc deflection under the non-uniform magnetic field was conducted, where the upper field was in inverse direction to the lower one. The magnetic flux density distribution in each side was assumed to be symmetric with respect to point z=zo. The heat flux decrease was minimized by adjusting the position of zo. Experimental results confirmed the theoretical predictions.
  • Influence of Electrode Force on Weld Expulsion in Resistance Spot Welding of Dual Phase Steel with Initial Gap Using Simulation and Experimental Method

    pp. 493-498

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    DOI:10.2355/isijinternational.52.493

    By increasing the electrode force, it would be an alternative way to avoid weld expulsion in resistance spot welding (RSW) of dual phase (DP) steel with initial gap. To investigate the influence of the electrode force on weld expulsion, a weld expulsion estimation method based on a finite element model were used to analyze the contact state, electrical field, temperature distribution and weld nugget formation process. The simulation results were validated by the experiments. It was shown that a great electrode force could be helpful to inhibit weld expulsion by improve the contact state between sheets. For 0.8 mm DP600 steel with 1.6 mm gap spacing, the minimum electrode force without expulsion is about 2.5 kN. For DP600 steels with different thickness from 0.8 to 1.8 mm, minimum electrode forces have been determined to inhibit the weld expulsion with different gap spacing to improve the weld quality of RSW.
  • Effects of Post-oxidizing Treatment on Melting Loss and Corrosion Resistance of Gas Nitrided AISI H13 Tool Steel

    pp. 499-504

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    DOI:10.2355/isijinternational.52.499

    This study conducted gas nitriding on AISI H13 steels, followed by oxidization treatments at various temperatures (560°C, 580°C, 600°C) and soaking times (1 h, 2 h, 3 h), to investigate the melting loss and corrosion resistance of A380 aluminum alloy. The experimental results indicated that the samples of oxynitriding (post-oxidizing treatment after nitrided) at 580°C for 3 h, 600°C for 2 h and 600°C for 3 h have better ability to resist melting loss due to their thicker oxidized layers. The weight loss rate also showed that they are near zero after a 4 h erosion test. The increasing weight loss rate is relative to the appearance of the FexAlySiz compound. In addition, the difference in corrosion current is closely related to the corrosion rate. A smaller corrosion current will lead to a smaller corrosion rate. As a result, increasing the temperature and soaking time of oxidization is advantageous to the corrosion resistance, and a small variation in corrosion current will result in a significant change to the polarization resistance of corrosion. Consequently, the oxide layer can enhance the polarization resistance, and quickly generate a passivation layer that increases the corrosion resistance ability.
  • Interaction of Boron with Copper and Its Influence on Matrix of Spheroidal Graphite Cast Iron

    pp. 505-509

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    DOI:10.2355/isijinternational.52.505

    High strength steel, which often contains a few tens of ppm B, has been used in the automobile industry due to weight reduction for energy savings. If this steel scrap is recycled for producing high strength spheroidal graphite iron castings, the alloying element, B, changes the matrix from pearlite to ferrite. In this research, the influence of B on the matrix of the high strength spheroidal graphite iron castings and the means of counteracting its bad effects were investigated.
    The result of the study on the ferritization effect of B on the matrix showed that the ferrite volume fraction increased with the addition of B. Meanwhile, the smoothness of the spheroidal graphite surface was damaged.
    In order to investigate the counteracting measures to the ferritization effect of B, N or Ti was added to the parent alloy to form BN or TiB2. It was found that the addition of N or Ti reduced the ferrite volume fraction slightly, and the effect of Ti was more obvious.
    Some tests were carried out to discuss the interaction of Cu with B. The existence of a thin Cu film on the spheroidal graphite surface was observed in the B-free Cu-added sample; nevertheless, it could not be confirmed in the B-added sample. When Cu was replaced by Mn, there was no difference in the ferrite fraction with the B addition. Therefore, it could be confirmed that the ferritization effect of B exists only in the presence of Cu in spheroidal graphite cast iron.
  • Variant Selection of Low Carbon High Alloy Steel in an Austenite Grain during Martensite Transformation

    pp. 510-515

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    DOI:10.2355/isijinternational.52.510

    In this study, the development of a lath martensite structure in low carbon high alloy steel was observed in situ using high-temperature laser scanning confocal microscopy. The crystallography of the martensite structure was analyzed using electron backscatter diffraction patterns. It was observed that martensite transformation starts from the prior austenite grain boundary. Then, the variant (Σ1) of another packet belonging to the same bain correspondence was observed in the early stage of martensite transformation. Another block in the same packet was observed in the next stage of martensite transformation. Finally, transformation occurred among the neighbors of the transformed martensite block.
  • High-resolution Observation of Steel Using X-ray Tomography Technique

    pp. 516-521

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    DOI:10.2355/isijinternational.52.516

    A common, serviceable form of steel has been observed by employing synchrotron X-ray microtomography technique. Spatial resolution has been optimized using a test object with varying experimental conditions. Reasonably high resolution, which is close to the theoretical upper limit for the projection-type X-ray tomography, has been achieved for a reasonably large specimen. Its application has made it possible to clearly observe a fatigue crack and its opening behavior in steel, and to demonstrate some quantitative mechanical evaluations.
  • Effect of Texture Components on the Lankford Parameters in Ferritic Stainless Steel Sheets

    pp. 522-529

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    DOI:10.2355/isijinternational.52.522

    Two ferritic stainless steel (FSS) sheets were produced to display different recrystallization textures. The in-plane variation of the Lankford parameter R(α) was calculated from the sheet textures and compared to the R-values determined experimentally. The textures of the FSS sheets were further decomposed into a number of individual texture components to clarify the effect of the main texture constituents on R(α). The in-plane variation of R(α) is mainly governed by the main texture component, while minor texture components generally reduce the overall level of R(α). A desirable high normal anisotropy R and small in-plane variations of R(α) are obtained for texture components with an Euler angle Φ lying in the range 50° ≤ Φ ≤ 60°, which comprise the γ-fiber orientations close to {111}//ND.
  • Influence of Low Temperature Heat Treatment on Iron Loss Behaviors of 6.5 wt% Grain-oriented Silicon Steels

    pp. 530-534

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    DOI:10.2355/isijinternational.52.530

    In this study, the influence of low temperature heat treatment on iron loss behaviors has been investigated in 6.5 wt% grain-oriented silicon steels fabricated by a SiO2 textile method. Heat-treatment under hydrogen atmosphere was carried out for 1 hr in the temperature range of 500 to 800°C. As a result, the minimum iron loss was obtained in a sample annealed at 600°C. Transmission electron microscopy analysis showed that B2 and D03 antiphase domains existed in all heat-treated samples but D03 size was maximized after 600°C heat-treatment. In addition, magnetic domains cut through these antiphase boundaries. Therefore, the low iron loss of the sample annealed at 600°C should be ascribed to the reduction in the area of the antiphase boundaries which acted as pinning centers in the magnetic domain walls.
  • Data Arrangement and Consideration of Evaluation Standard for Silico-Ferrite of Calcium and Alminum (SFCA) Phase in Sintering Process

    pp. 535-537

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    DOI:10.2355/isijinternational.52.535

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