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

ISIJ International Vol. 56 (2016), No. 6

  • Production and Technology of Iron and Steel in Japan during 2015

    pp. 905-916

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    DOI:10.2355/isijinternational.56.905

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    1. Production and Technology of Iron and Steel in Japan during 2014 ISIJ International Vol.55(2015), No.5
    2. Production and Technology of Iron and Steel in Japan during 2012 ISIJ International Vol.53(2013), No.6
    3. Production and Technology of Iron and Steel in Japan during 2013 ISIJ International Vol.54(2014), No.6
  • Thermodynamic Assessment of Liquid Mn–Fe–Si–C–Ca–P System by Unified Interaction Parameter Model

    pp. 917-925

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    DOI:10.2355/isijinternational.ISIJINT-2015-545

    The Unified Interaction Parameter Model was used to evaluate the thermodynamics of Mn–Fe–Si–C–Ca–P system. The calculated results of the activities and activity coefficients of different components as well as the solubility of C and Ca agree well with the experimental data in binary, ternary and quaternary system. These parameters could be applied to calculate the activities of Mn, Si, Fe, C, Ca and P and the equilibrium between Ca and P in carbon saturated Mn–Fe–Si–C–Ca–P melts during the dephosphorization process. They are also useful for understanding the process of ferromanganese and silicomanganese. Using the present UIPM parameters, the calculated results showed that temperature had a weak effect on the activity coefficient of manganese in Mn–C melt. The effect of temperature and Fe content on the carbon solubility in Mn–Fe melts could be expressed as: XCsat=(-229.18/T+0.411)+(-186.54/T+0.014)XFe. Manganese activity monotonically decreased as the silicon content increased and slightly decreased as the temperature increased in the carbon saturated Mn–Si–C system.
  • Effect of Alumina Morphology on the Clustering of Alumina Inclusions in Molten Iron

    pp. 926-935

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    DOI:10.2355/isijinternational.ISIJINT-2015-561

    Clustering of alumina inclusions during liquid processing of steel significantly influences its cleanliness and mechanical properties. We have therefore studied the effect of alumina inclusion morphology on their clustering behavior in molten iron. Alumina inclusions were extracted from iron samples taken at 1 min after Al addition. Dendritic, spherical, plate-like, faceted and clustered alumina inclusions were identified and their clustering degrees were measured. The clustering degree increases in the order of spherical, dendritic, plate-like and faceted inclusions. To explain this, attractive force between two alumina particles with different shape combinations, i.e., sphere-sphere (S-S), sphere-plane (S-P), plane-plane (P-P), was calculated based on the theory of spontaneous cavitation. The attractive force is influenced significantly by particle shape. S-S type has the smallest attractive force and the shortest acting length. P-P type has the largest attractive force and the longest acting length. This explains that spherical inclusions have the lowest clustering degree. The lower clustering degree of plate-like inclusions, compared with faceted inclusions, is due to that molten iron wets plate-like inclusions better.
  • Direct Reduction Behaviors of Titanomagnetite Ore by Carbon Monoxide in Fluidized Bed

    pp. 936-943

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    DOI:10.2355/isijinternational.ISIJINT-2016-040

    On the background of the two stages short process of direct reduction - electric arc furnace melting separation, the gas-solid direct reduction behaviors of South Africa titanomagnetite ore particles by carbon monoxide in fluidized bed have been investigated. The results showed that, due to the lower apparent diffusion activation energy without the gas mass transfer among particles, the CO gas reduction of titanomagnetite in fluidized bed present much higher efficiency than the pellet reduction. Continuous solid solution formed by MgO and MnO in FeO can impede reduction through the barrier effect and increase the required CO reduction potential of FeO, 2FeO·TiO2 and FeO·TiO2 in titanomagnetite. The generated FeO would combine with FeO·TiO2 to form 2FeO·TiO2 in the reduction process and lower the metallization degree after the first reduction step of iron oxides in titanomagnetite. Finally, the relationship between the metallization degree and the equilibrium CO–CO2 gas reduction potential has been analyzed and established, with the impurities effect and the phase transformation taken into consideration.
  • Dynamic Changes in Interfacial Tension between Liquid Fe Alloy and Molten Slag Induced by Chemical Reactions

    pp. 944-952

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    DOI:10.2355/isijinternational.ISIJINT-2015-654

    The authors investigated the change in the interfacial tension with time for various combinations of molten slag and liquid Fe to elucidate the mechanism of the change in interfacial tension between liquid Fe alloy and molten slag over time accompanying reduction/oxidation reactions. The behavior of the change in the interfacial tension over time can be explained by the adsorption of oxygen at the interface and the diffusion of oxygen from the interface into the bulk of the liquid Fe and molten slag. In addition to that, we found that the interfacial tension decreases slowly and greatly from its initial value to a minimum point and then increases slowly to the final equilibrium state when molten silicate slag with low viscosity is brought into contact with liquid Fe without Al content and some of its SiO2 decomposes and dissolves into the liquid Fe. From these results, we suggest that the detachment of oxygen adsorbed at the interface into the liquid Fe is very slow and may be the rate-limiting step.
  • Reduction Roasting of Tin-Bearing Iron Concentrates Using Pyrite

    pp. 953-959

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    DOI:10.2355/isijinternational.ISIJINT-2015-717

    A novel method of reduction roasting of tin-bearing iron concentrates using pyrite (FeS2) is proposed, in which hematite in tin-bearing iron concentrates is reduced to magnetite (Fe3O4) and cassiterite (SnO2) is transformed to volatile tin sulphide (SnS). In a certain range, tin removal and iron reduction rates both increase with roasting temperature, pyrite amounts and residence time. When pyrite amounts exceed 5.00 mass%, some of the produced magnetite will be sulfurated to FeS and the evaporation of SnS (g) will also be obstructed. Tin content of the concentrate is decreased to 0.07 mass% and hematite is reduced to magnetite completely under the conditions of N2 flow rate of 5.00×10-6 m3/s, roasting temperature of 1473.0 K, residence time of 60 min, pyrite addition amounts of 5.00 mass% and concentrates particle size of 0.075 mm. The roasting products can meet the standard of BF ironmaking, which requires tin content in iron ores less than 0.08 mass%, and the tin removed can be recycled through smoke dust collection.
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    2. Prediction of Bainite Start Temperature in Alloy Steels with Different Grain Sizes ISIJ International Vol.54(2014), No.4
    3. Phase Transformations during Low Temperature Nitrided Inconel 718 Superalloy ISIJ International Vol.56(2016), No.6
  • Evaluation of Dust Generation during Mechanical Wear of Iron Ore Pellets

    pp. 960-966

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    DOI:10.2355/isijinternational.ISIJINT-2015-680

    Iron ore pellets undergo mechanical wear during handling, transportation and use in a blast furnace. This results in a loss of valuable raw materials and causes environmental problems in form of dust in off-gases from a blast furnace. Thus, this study is focused on the investigation of the mechanical wear of iron ore pellets and the dust formation. The characteristics of industrial pellets (such as size, weight, density and hardness) have been investigated. Moreover, the influence of pellet characteristics on the wear mechanism (sliding/abrasion and impact/collisions wear) and the characteristics of generated dust have been investigated. It was observed that the size of pellets can influence the wear rate under the given experimental conditions. The pellets with larger size (13.5<deq<15.0 mm) showed 10–20% higher wear rate as compared to small sized pellets (9.5<deq<12.50 mm). SEM studies of the dust generated during wear experiments inferred that larger contribution of impact/collisions in wear of pellets is the reason for the higher wear rate of large size pellets. Further, a relationship between the critical diameter of dust particles, which can be removed with off-gases from the blast furnace, and the velocity of off-gases in top part of blast furnace was developed.
  • Phase Equilibria in the System “FeO”–CaO–SiO2–Al2O3–MgO with CaO/SiO2 1.3

    pp. 967-976

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    DOI:10.2355/isijinternational.ISIJINT-2016-099

    A series of “FeO”-containing slags are present in iron blast furnace. Phase equilibria play an important role in optimum operation of blast furnace which involves complex chemical reactions. Phase equilibria studies have been carried out in the system “FeO”–CaO–SiO2–Al2O3–MgO in equilibrium with metallic iron. The technique used includes high temperature equilibration followed by quenching and electron probe X-ray microanalysis. Pseudo-ternary sections of the phase diagrams are presented in the form of (CaO+SiO2)–Al2O3–MgO at fixed CaO/SiO2 ratio of 1.3 and “FeO” concentrations of 5 and 10 wt%. This is the first systematic study in the system “FeO”–CaO–SiO2–Al2O3–MgO important to the blast furnace operations. It was found that melilite, Ca2SiO4, merwinite, spinel and (Mg,Fe2+)O are the primary phase fields in the composition range investigated. Liquidus temperatures increase in the melilite and spinel primary phase fields and decrease in the merwinite and Ca2SiO4 primary phase fields with increasing Al2O3 concentration. The liquidus temperature is not sensitive with Al2O3 in the spinel primary phase field. Effects of MgO and “FeO” on liquidus temperature are also discussed. Extensive solid solutions can be formed in this system that have significant influence on the liquidus temperatures and the data will be used for optimisation of the thermodynamic models.
  • Parameter Estimation by Inverse Solution Methodology Using Genetic Algorithms for Real Time Temperature Prediction Model of Ladle Furnace

    pp. 977-985

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    DOI:10.2355/isijinternational.ISIJINT-2015-712

    In the process of developing mechanistic dynamic models which faithfully represent characteristics of a process, accurate estimation of parameters is a very crucial step. Inverse solution methodology combined with evolutionary optimization algorithms has been proved to be a very potential technique for offline parameter estimation. Advanced industrial automation systems capable of generating and storing enormous volumes of sensory data have indeed fostered the usage of this approach. In the present work, inverse methodology combined with Genetic Algorithms has been successfully employed for estimating parameter of a dynamic model aimed to predict liquid steel temperature in Ladle Furnace. The parameter evaluated in this study was heat transfer coefficient of ladle refractory walls. The optimal value evaluated was obtained as 10.62 W/m2.K.
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    3. Analysis of Melt Flow in Metallurgical Vessels of Steelmaking Processes ISIJ International Vol.53(2013), No.4
  • Effective Desiliconization Method with Swirling Flow of Hot Metal at Blast Furnace Casthouse

    pp. 986-994

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    DOI:10.2355/isijinternational.ISIJINT-2016-023

    Hot metal desiliconization is carried out by adding iron oxide at a tilting runner in a blast furnace casthouse to improve BOF operation. Aiming at effective silicon removal by enhancing mixing of desiliconizing agent and hot metal, desiliconization experiments at an actual blast furnace casthouse was performed with the tilting runner that was specially designed to generate swirling flow of hot metal.
    1. Amount of silicon removed from the hot metal through the entire desiliconization operation was larger under the swirling flow condition of hot metal compared with conventional non-swirling one.
    2. Hot metal samples taken at the exit of the tilting runner showed that change in silicon content of hot metal in the tilting runner was larger with swirling condition.
    3. Hot metal samples were also taken from the torpedo car, and desiliconization behavior in a transient basin part of the torpedo car was evaluated. Calculated silicon content in the basin part was lower with swirling condition.
    4. A reaction analysis was made assuming that present system is a semi-batch reactor. Reaction rate constant of desiliconization through the swirling part of the tilting runner and the basin part of the torpedo car was 2.2 times higher with swirling condition. This result qualitatively agrees with the results of previous 5 ton hot metal experiments and reflects enhanced mixing of desiliconizing agent and hot metal by swirling flow in an industrial-scale operation.
  • Characteristics and Generating Mechanism of Large Precipitates in Nb–Ti-microalloyed H13 Tool Steel

    pp. 995-1002

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    DOI:10.2355/isijinternational.ISIJINT-2015-569

    The characteristics of precipitates larger than 1 µm in Nb–Ti microalloyed H13 tool steel were studied. Four types of large phases exist in the as-cast ingot according to the compositional characteristics, that is, (Ti,Nb,V)(C,N), V-rich carbide, Mo–Cr-rich carbide and sulfide. (Ti,Nb,V)(C,N) could be further classified as the Ti-rich one, the Nb-rich one and the Ti/Nb transforming one. V-rich carbide normally has a strip shape, while Mo–Cr-rich carbide presents a eutectic appearance. The compositional characteristics of large precipitates have little relation with the sample positions. V-rich and Mo–Cr-rich carbide have the tendency to dissolve at 1000°C and 1100°C, while the (Ti,Nb,V)(C,N) is more stable and there are no apparent changes on the morphology after holding 6 h at 1000°C and 3 h at 1100°C. These precipitates are generated during solidification. The precipitating process of (Ti,Nb,V)(C,N) could be well speculated through Thermo-Calc software. Ti-rich carbonitride precipitates first. Nb-rich carbonitride appears later, singly or on the Ti-rich carbonitride. V-rich carbide precipitates at the end of solidification. Eutectics consisting of iron matrix and Mo–Cr-rich carbide will be generated when the solutes in liquid steel reach the eutectic point.
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    3. Phase Equilibria in the System “FeO”–CaO–SiO2–Al2O3–MgO with CaO/SiO2 1.3 ISIJ International Vol.56(2016), No.6
  • Multi-scale Simulation for the Columnar to Equiaxed Transition in the Weld Pool

    pp. 1003-1012

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    DOI:10.2355/isijinternational.ISIJINT-2015-738

    The heat and mass transfer behavior and the morphological evolution of the solidification structures in the weld pool were simulated by the multi-scale model which combines the finite element (FE) and the cellular automata (CA) methods to investigate the columnar to equiaxed transition (CET) process during welding. The grain structure evolution within the entire weld and the competitive dendrite growth at different locations were studied to better understand the CET process. The results indicate that with the decrease of the distance to weld center, the undercooled zone width and the maximum undercooling increase. In this case, more equiaxed dendrites form in the undercooled melt, and the distance between the equiaxed dendrites and the columnar front also becomes lager. There is more space for the equiaxed dendrites to grow up and block the columnar dendrites. Therefore, the equiaxed dendrites become more competitive, and the CET tendency increases.
  • Interaction between Alumina Inclusions in Molten Steel Due to Cavity Bridge Force

    pp. 1013-1022

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    DOI:10.2355/isijinternational.ISIJINT-2016-038

    The formation and extinction behavior of cavity bridges has been experimentally evaluated by allowing the cylindrical particles simulating inclusions to approach and separate off in mercury and in molten steel. An interaction model of spherical alumina particles close to the actual inclusion shape due to cavity bridge force has been developed on the basis of the experimental results. Using this interaction model, the processes of agglomeration and separation of alumina inclusions in molten steel have been analyzed, and the agglomeration force due to cavity bridge force has been discussed in comparison with different agglomeration forces that are derived from the van der Waals force in molten steel and the capillary force on the surface of molten steel. When two isospherical alumina inclusions are approaching each other in molten steel, a large agglomeration force of 1.54d·σFe (d: the diameter of alumina inclusions, σFe: the surface tension of molten steel.) is generated by the cavity bridge formation from the interparticle surface distance of 0.07d, and then the agglomeration force also gradually increases to reach the maximum value of 1.88d·σFe in complete contact state. Conversely, when two isospherical alumina inclusions in molten steel are separated from the contact state, a large agglomeration force of 0.92d·σFe and above is maintained until the cavity bridge extinction in the interparticle surface distance of 0.12d, whereas the agglomeration force gradually decrease from 1.88d·σFe. In addition, it is assumed that alumina inclusions in aluminum deoxidized molten steel principally agglomerate and coalesce on the basis of the agglomeration force derived from very strong cavity bridge force in comparison with the van der Waals force in molten steel and the capillary force on the surface of molten steel, and coarse alumina clusters are thus formed in molten steel.
  • Selection of Sulfide Morphology in SUS303 during Solidification

    pp. 1023-1030

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    DOI:10.2355/isijinternational.ISIJINT-2016-050

    The sulfide morphology in SUS303 was investigated and the mechanism of monotectic sulfide formation was evaluated. The sulfide of metastable monotectic type was observed at the surface region of ingot with the higher solidification cooling rate and stable eutectic type was observed at the inner region. The formation of monotectic sulfide was promoted by Ca addition and the formation of eutectic type sulfide was promoted by Al addition with the lower oxgen content. It is thought that the nucleation behavior greatly affect the monotectic sulfide formation because of the lower interfacial energy with liquid oxide inclusion. However, solute distribution, undercooling and the decrease of S activity by adding Ca or increasing O content may also influence on the eutectic/monotectic morphology selection.
  • Effect of Additional Boron Amount on Surface Roughness after Lathe Turning in h-BN Dispersed Type 304 Stainless Steels

    pp. 1031-1037

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    DOI:10.2355/isijinternational.ISIJINT-2015-684

    The authors have reported that the machinability (lathe turning, drilling and sawing) of SUS304 (Type 304) austenitic stainless steel is improved by precipitating hexagonal boron nitride (h-BN). In this study, we investigated the surface roughness after lathe turning in the h-BN precipitated SUS304 steels, where the nitrogen content was fixed at 0.2% and the boron content was changed as 0, 0.03, 0.05, and 0.1% (mass%). Surface roughness increased as the boron content increased, especially at a lower cutting speed of 22 m/min. However, SUS304 steel with boron content of up to 0.05 mass% had similar or smaller surface roughness compared with a commercial SUS304 steel, and the SUS304 steel containing 0.1 mass% boron had smaller surface roughness compared with a SUS303 (Type 303) sulfur-added free-cutting steel. Precipitation of h-BN increased the size of the build-up edge and increased the surface roughness. On the other hand, solute nitrogen increased the Vickers hardness, leading to decreased separation size of the build-up edge and decreased surfaced roughness.
  • Modelling of Static Recrystallization Behavior of High Manganese Austenitic Steels with Different Alloying Contents

    pp. 1038-1047

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    DOI:10.2355/isijinternational.ISIJINT-2015-733

    During hot rolling, austenite recrystallization determines the grain size evolution and the extent of strain accumulation, and therefore, it can be used to control the microstructure and improve the mechanical properties of the final product. However, at the moment, experimental data and models describing the recrystallization kinetics of high-Mn steels are scarce, and they do not take into account the effect of the different C and Mn alloying contents usually present in these steels. The aim of this work is to provide a quantitative model for the determination of the static recrystallization kinetics and recrystallized grain size that is valid for a wide range of high-Mn steel compositions. In order to do this, softening data determined in previous works for steels with different Mn (20 to 30%), Al (0 to 1.5%) and C (0.2 to 1%) levels were considered. In addition, new tests were carried out to determine the effect of deformation conditions on the static softening kinetics and the recrystallized grain size. The static recrystallization kinetics of the high-Mn steels follows Avrami’s law, with n Avrami exponents which are temperature dependent and lower than those determined for low C steels. A dependence of the t0.5 (time for 50% fractional softening) on the carbon content has been observed and it was incorporated into an equation for the calculation of this parameter. An expression that is valid for predicting the recrystallized grain size as a function of deformation conditions is also proposed.
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    2. Corrosion Behaviour of Alloy 800H in Low Density Superheated Steam ISIJ International Vol.56(2016), No.6
    3. Multi-scale Simulation for the Columnar to Equiaxed Transition in the Weld Pool ISIJ International Vol.56(2016), No.6
  • Effect of Initial Ni Plating on the Structure and Hardness of Electrodeposited Ni–W Alloys with and without Annealing

    pp. 1048-1056

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    DOI:10.2355/isijinternational.ISIJINT-2016-110

    Ni–W alloys were electrodeposited onto steel sheets with an initial Ni plating. The electrodeposition was conducted in unagitated sulfate solution containing citric acid at pH 5 and 60°C under coulostatic (9.0 × 105 C·m-2) and galvanostatic (100–5000 A·m-2) conditions. The effect of the initial Ni plating on the structure and hardness of the deposited Ni–W alloys was investigated before and after annealing. At all current densities and in the presence and absence of the Ni plating, Ni4W and NiW were precipitated in the annealed deposits. Without initial Ni plating, the Ni in the deposits diffused into the steel substrate during annealing, increasing the W content near the steel substrate. Consequently, many large precipitates were found in that vicinity. With initial Ni plating, the Ni in the plating diffused into the Ni–W deposits during annealing, and the W content of the deposits decreased around the plated Ni. The precipitates in the Ni–W deposits with initial Ni plating were finer than those without initial Ni plating. Before annealing, the W content in the deposits was lower in the initially Ni-plated sheet than in the unplated sheet. During annealing, Ni diffused from the plated Ni to the Ni–W deposits, apparently repressing the formation of large Ni4W and NiW precipitates. The fine, uniformly deposited Ni4W and NiW precipitates increased the hardness of the deposited Ni–W alloys after annealing.
  • Scaling Physical Model Study on Dynamic Accumulation of Bottom Dross in a Molten Zinc Pot

    pp. 1057-1066

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    DOI:10.2355/isijinternational.ISIJINT-2015-645

    A theoretical framework of similarity and its related experimental methods concerning scaling physical model are presented to investigate dynamic behavior of bottom dross in a zinc pot during the continuous hot-dip galvanizing process. Based on the similarity criteria of Froude number, turbulent Reynolds number, Shields number and particle Reynolds number, an analytical framework is developed to describe similarity of dross dispersion, transport and deposition in a stirred molten zinc tank. A complete set of experimental methods are suggested correspondingly. A 1/5 reduced scale model is adopted to study steady flow field, dynamic accumulation rules and stable shape of bottom dross. NaCl aqueous solutions and black particles of acrylonitrile butadiene styrene are employed as model fluid and bottom dross, respectively. The results show that two impact flows are the key factors in the formation of dross deposition morphology in the bottom of zinc bath. The first impact flow strikes bottom drosses located in the floor of tank and near the front wall and results in three impact craters for symmetric distribution. The other impact flow hits bottom drosses deposited blow the sink roll. A pair of pits with curved edges and a ridge between them are formed. The dispersed bottom drosses under impact are carried by the main circular flow and heaped up toward the back wall similar to a mobile dune. The greater the difference between current shape and steady state morphology of bottom dross, the more dross particles are scattered.
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    3. Selection of Sulfide Morphology in SUS303 during Solidification ISIJ International Vol.56(2016), No.6
  • Corrosion Behaviour of Alloy 800H in Low Density Superheated Steam

    pp. 1067-1075

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    DOI:10.2355/isijinternational.ISIJINT-2015-651

    The integrity of materials is of great concern in the construction of nuclear reactors. During the design of Gen IV supercritical water reactors, materials characterization under simulated conditions must be carried out to aid proper material selection. In this study, the long term corrosion resistance of Alloy 800H under low pressure superheated steam (SHS) at a temperature of 800°C and 0.1 MPa was tested for up to 3000 hours. The results showed that all six samples experienced weight gain in the first 2000 hours while both weight gain and weight loss were found after 3000 hours. Visual inspection and SEM surface analysis suggest the likelihood of oxide exfoliation at a later stage of the testing in SHS. Additionally, chromia formed on all samples after 1000, 2000 and 3000 hours as well as spinel and hematite.
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    1. Phase Transformations during Low Temperature Nitrided Inconel 718 Superalloy ISIJ International Vol.56(2016), No.6
    2. Multi-scale Simulation for the Columnar to Equiaxed Transition in the Weld Pool ISIJ International Vol.56(2016), No.6
    3. Reduction Roasting of Tin-Bearing Iron Concentrates Using Pyrite ISIJ International Vol.56(2016), No.6
  • Phase Transformations during Low Temperature Nitrided Inconel 718 Superalloy

    pp. 1076-1082

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    DOI:10.2355/isijinternational.ISIJINT-2015-697

    The effects of the salt bath nitriding parameters on the microstructure, microhardness and corrosion behavior of Inconel 718 superalloy at temperature ranging from 425–500°C were investigated by X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and corrosion test. Experimental results indicated that the microstructure and phase constituents of the surface nitridization are highly process dependent. When Inconel 718 superalloy was subjected to salt bath nitriding, the predominant phases of the nitrided layer were identified as expanded austenite (S phase), austenite and CrN. The thickness of the nitrided layer increased with the time of nitridization. Meanwhile, salt bath nitriding improved the surface hardness dramastically. The maximum value of hardness measured from the treated surface was 2100 HV0.1 after 16 h at 500°C, which is about 5 times as hard as the untreated material (420 HV0.05). Proper low temperature nitriding can improve the erosion corrosion resistance. The sample that was nitrided for 4 h at 475°C had the best corrosion resistance.
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    1. Corrosion Behaviour of Alloy 800H in Low Density Superheated Steam ISIJ International Vol.56(2016), No.6
    2. Multi-scale Simulation for the Columnar to Equiaxed Transition in the Weld Pool ISIJ International Vol.56(2016), No.6
    3. Reduction Roasting of Tin-Bearing Iron Concentrates Using Pyrite ISIJ International Vol.56(2016), No.6
  • Hydrogen Embrittlement of Ultrafine-grained Austenitic Stainless Steels Processed by High-pressure Torsion at Moderate Temperature

    pp. 1083-1090

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    DOI:10.2355/isijinternational.ISIJINT-2015-664

    The effect of hydrogen on the plasticity of ultrafine-grained austenite was studied on type 304, 316L, and 310S stainless steels processed by high-pressure torsion at moderate temperature. Like austenitic steels with ordinary grain sizes, the hydrogen-induced ductility loss for the ultrafine grains became more pronounced with decreasing stability of the austenitic phase. For all the steels, the uniform elongation was limited by strengthening due to the ultra grain refinement; the ultrafine-grained 310S stable austenitic steel further exhibited a small local elongation due to a lack of martensitic transformation. The ductility loss due to hydrogenation for 316L steel with an intermediate austenite stability was retained to a moderate level. The ultrafine-grained 304 metastable austenitic steel exhibited a serious ductility loss induced by hydrogen showing localised shear deformation. This suggests that the dynamic martensite transformation plays a crucial role in the hydrogen embrittlement of ultrafine-grained metastable austenitic steel.
  • Dependence of Elevated Temperature Intergranular Cracking on Grain Size and Bulk Sulfur Content in TP347H Austenitic Stainless Steels

    pp. 1091-1096

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    DOI:10.2355/isijinternational.ISIJINT-2015-721

    Regardless of the bulk sulfur content, heat–resistant TP347H austenitic stainless steels of the smaller grain size show a ductile fracture after rupture test, while the steels of the larger grain size show a typical intergranular cracking. The intergranular cracking is mainly due to the sulfur segregated at grain boundaries, and it is not inhibited even by the extremely low bulk sulfur content of 7 ppm. In steels showing the intergranular cracking, the time to failure is rather longer in the steel of the higher bulk sulfur content. This is due to MnS precipitates incoherently formed within the matrix, the interface of which acts as a strong sink of free sulfur segregating to grain boundaries. The fracture mode and the time to failure are determined by the combination of bulk sulfur content and grain size influencing the creep resistance and the equilibrium grain boundary segregation concentration of sulfur.
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    pp. 1097-1102

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    DOI:10.2355/isijinternational.ISIJINT-2015-719

    A universal equation for understanding various phenomena observed on the engineering stress-strain curves of polycrystals, which includes the Hall-Petch relation concepts, is derived. The derivation of the equation begins from the simple condition that the given constant strain rate is the sum of the elastic strain rate and the plastic strain rate. The physical meanings for the various phenomena on engineering stress-strain curves are analyzed from the viewpoint of the competition between two strain rates.
  • Application of Fertilizer Made of Steelmaking Slag in the Recovery of Paddy Fields Damaged by the Tsunami of 2011

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    DOI:10.2355/isijinternational.ISIJINT-2015-640

    The great earthquake of 2011 triggered a tsunami that damaged large areas of paddy fields in northeastern Japan. In an effort to address the salt damage, supplementation of Ca-containing materials to exchange Na adsorbed on soil surface has been recommended. In addition, Si has also been shown to enhance paddy growth. Steelmaking slag, which contains a water-soluble solid solution phase of 2CaO·SiO2, can supply Ca and Si for soil remediation. In this study, the dissolution behaviors of nutrient elements from fertilizer made of steelmaking slag were investigated using the column test. In addition, crop cultivation experiments were also conducted using tsunami-damaged paddy fields. In column test, Ca content in soil solution increased by the application of fertilizer made of slag, but the Na content did not change significantly. These trends were also observed in the pore water of the actual paddy in crop cultivation experiments. In addition, the incremental trend of silicate content in the pore water by the application of fertilizer made of slag was more apparent than that in the column test. Paddy growth was enhanced and the yield of brown rice was increased by the application of fertilizer made of slag. In conclusion, the fertilizer made of steelmaking slag has the following three effects: (1) mitigating the damage caused by the Na ion through the supplementation of Ca, (2) enhancing the mineralization of soil N by increasing the pH, and (3) accelerating photosynthesis by the supplementation of silicate.
  • Silico-ferrite of Calcium and Aluminum Characterization by Crystal Morphology in Iron Ore Sinter Microstructure

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    DOI:10.2355/isijinternational.ISIJINT-2016-044

    Correct identification of mineral phases in iron ore sinters is key for further research, in which individual sinter properties are derived directly from mineral phases. Nowadays widespread image analysis and techniques based on the visual quantification may not exactly evaluate mineral type, what has an unfavorable effect on the overall phase analysis and all functions derived from it. Some specifics were demonstrated by microscopic examination of sinter samples and pointing out differences in calcium ferrite identification. It was found the dendritic crystallization may cause such crystal shape of low-Fe SFCA, that it ultimately looks like plates of high-Fe SFCA-I. Aggregates of SFCA-I plates crystallized adherent are misidentifiable as prismatic SFCA crystals. The pathway for SFCA-I crystals covered with SFCA was also discussed.
  • Rapid Borate Fusion Pre-treatment for ICP-AES Determination of Tungsten in High-speed Steel

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    DOI:10.2355/isijinternational.ISIJINT-2015-648

    This paper suggests a rapid decomposition and dissolution method for stable solution preparation in inductively coupled plasma atomic emission spectrometry when tungsten in a high-speed steel is quantified. The steel sample was decomposed with a mixture of hydrofluoric and nitric acid, and then the resulting solution was fused with lithium tetraborate. The vitrified borate was able to dissolve in nitric acid containing tartaric acid to prepare the final sample solutions. This pre-treatment method has a superior feature: borate could effectively prevent tungsten hydrolysis in the prepared solution. The standard for quantification could be prepared in a similar procedure using metal standard solutions. The present method enabled more rapid and accurate determination of tungsten in certified reference materials of high-speed steel compared to the conventional dissolution procedure.

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