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

ISIJ International Vol. 50 (2010), No. 4

  • Three-dimensional Compressible Flow Simulation of Top-blown Multiple Jets in Converter

    pp. 491-500

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    DOI:10.2355/isijinternational.50.491

    A three-dimensional mathematical model has been developed to simulate the compressible jets flow from the top-blown lance with multi-nozzles in converter to a free surrounding domain. The variations of fluid density and viscosity, high temperature, and Mach number were taken into consideration in this model which was validated against the physical modeling results. More specifically, computations were obtained to compare the widely used realizable k–ε turbulence model against the standard k–ω turbulence model, which shows that the latter one is superior to calculate diverse turbulent conditions within the multiple jets. Moreover, the coalescence pattern of the multiple jets has been illustrated by their Mach number distribution, and each individual jet proceeds in a curve course, bending to the lance center and tending to unite. The effects of the inclination angles on the jets coalescence were also investigated, which indicates that the lower the inclination angle is, the stronger the interfering extent between the multiple jets is. With the help of this model, the dynamic power of the multiple jets to support the cavity formation was demonstrated, and additionally, a mathematical model concerning the effective penetration radius and digressing path of the multiple jets was proposed by taking the inclination angle and the axial distance from the nozzle tips as arguments.
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    1. Effect of Lance Design on Jet Behavior and Spitting Rate in Top Blown Process ISIJ International Vol.41(2001), No.12
    2. Characteristics of Jets from Top-blown Lance in Converter ISIJ International Vol.40(2000), No.1
    3. Numerical Calculation of Circulation Flow Rate in the Degassing Rheinstahl-Heraeus Process ISIJ International Vol.40(2000), No.8
  • Thermodynamic Analysis of Mn-depleted Zone Near Ti Oxide Inclusions for Intragranular Nucleation of Ferrite in Steel

    pp. 501-508

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    DOI:10.2355/isijinternational.50.501

    A series of thermodynamic analyses have been performed to elucidate possible mechanism of Mn-depleted zone (MDZ) development near Ti oxide inclusions as nucleation sites of Intragranular Acicular Ferrite (IGF) transformation in steels, using a computational thermodynamic approach (CALPHAD). It is demonstrated that thermodynamic calculations are able to reproduce experimentally known inclusions evolution in the steels. The MDZ development near the Ti2O3 inclusions is shown to be a result of Mn absorption into the Ti2O3. Moreover, from thermodynamic analysis of phase equilibria in the Mn–Ti complex oxide system, it is proposed that a phase change of inclusion from (Ti3O5) s.s. (pseudobrookite structure solid solution dissolving Mn) to (Ti2O3) s.s. (ilmenite structure solid solution dissolving Mn) accelerate the Mn absorption into the Ti2O3 inclusion in steel. From a series of thermodynamic calculations, optimum thermal heating condition to enhance Mn absorption into Ti2O3 inclusions as well as IGF formation is discussed.
  • Distribution and Coordination State of Titanium Oxides in Microstructures Formed by Spinodal Decomposition in Multicomponent Borosilicate Glass

    pp. 509-514

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    DOI:10.2355/isijinternational.50.509

    Tetrahedrally coordinated titanium oxides implanted into inorganic materials aid in the removal of air-based pollutants by photocatalytic reactions. We investigated the distribution and the coordination state of titanium oxides in a phase-separated glass formed by the spinodal decomposition to obtain porous glass containing tetrahedrally coordinated titanium oxides. Various glass compositions containing titanium oxides were prepared by partially replacing SiO2 with B2O3 and small amounts of TiO2 in multicomponent silicate glass compositions where a spinodal decomposition had been confirmed. Interconnected microstructures that consist of two different phases were observed in heat-treated glass samples using transmission electron microscopy and these microstructures underwent spinodal decomposition. Compositional analyses of these microstructures using energy dispersion spectroscopy revealed that the decomposed phase with higher concentration of SiO2 contains smaller amount of titanium oxides than the other phase. Thermodynamic analysis was conducted to evaluate the distribution of titanium oxides in the glass phases, formed by a phase separation in the multicomponent borosilicate glass, where glass was regarded as a super-cooled liquid phase. In the thermodynamic analysis, the calculated activity of TiO2 in the two decomposed glass phases indicated that the glass phase with higher concentration of SiO2 contains lower concentration of titanium oxides. Porous glasses containing titanium oxide were fabricated from the spinodal-decomposed borosilicate glass by leaching one of the separated phases with an acid solution. Ti K-edge X-ray absorption near edge structure spectra indicated that the porous glass contains tetrahedrally coordinated titanium oxides.
  • Numerical Investigation of the Transient Multiphase Flow in an Ironmaking Blast Furnace

    pp. 515-523

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    DOI:10.2355/isijinternational.50.515

    Discrete particle simulation (DPS) has been applied to multiphase flow modelling in an ironmaking blast furnace (BF), including burden distribution at the top, gas–solid flow in the BF shaft and raceway, and liquid–solid flow in the hearth. In this work, the approach is further extended to take into account the transient features of gas and particle flow coupled with liquid tapping operation. In the simulation, two types of particles of coke and ore with different physical properties are considered, together with different shapes of the cohesive zone and the shrinkage of size of ore particles in the cohesive zone to present ore reduction. The simulated results show that the flow of both solid and gas phases varies spatially and temporally, particularly in the cohesive zone. Gas flow is strongly affected by the layered structure of ore and coke particles in the cohesive zone. A coke-free zone can form in the hearth, and the boundary profile between the coke-free zone and the coke bed depends on the amount of liquid accumulated in the hearth, gas and solid flow rates in the raceway, and coke consumption in different regions at the interface of liquid and the coke bed. The results show that the complicated transient multiphase-flow in a BF can be captured by the present approach which may be extended to account for heat transfer and chemical reaction in the future.
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    1. Lowering Reduction Temperature of Iron Ore and Carbon Composite by Using Ores with High Combined Water Content ISIJ International Vol.49(2009), No.11
    2. Effects of Coal Inertinite Size on Coke Strength ISIJ International Vol.48(2008), No.5
    3. Effect of Carbon Structure Crystallinity on Initial Stage of Iron Carburization ISIJ International Vol.50(2010), No.1
  • Enhancement of Reactivity of Carbon Iron Ore Composite Using Redox Reaction of Iron Oxide Powder

    pp. 524-530

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    DOI:10.2355/isijinternational.50.524

    Increasing reactivity of burden for the blast furnace can decrease temperature of the thermal reserve zone and reducing agent for producing pig iron. Carbon iron ore composite is considered to be a candidate of high reactive burden. Reactivity of the carbon iron ore composite can be improved by an increase in reactivity of carbon. It is known that Redox reaction of iron enhances the gasification reaction of carbon, therefore carbon supported by iron can accelerate the reaction of the carbon iron ore composite.
    In the present study, the newly designed carbon iron ore composite consisting of biomass char coated with submicron iron oxide powder and iron ore fines was proposed to improve the reduction rate. These submicron iron oxide powders can be generally produced by the fluidized roasting of pickling waste liquor of steel sheet in the steel works. These iron oxide powders with submicron size directly attached to carbon could work as a catalyst to increase the gasification reaction rate of carbon. It was confirmed that addition of submicron iron oxide powder to the carbon iron ore composite with biomass char enhanced its reactivity. On the other hand, the addition of iron oxide powders to coke was not effective. Substitution of small ratio of iron oxide powder for iron ore fines in the carbon iron ore composite with biomass char remarkably improved the reactivity of the composite.
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    Readers Who Read This Article Also Read

    1. Lowering Reduction Temperature of Iron Ore and Carbon Composite by Using Ores with High Combined Water Content ISIJ International Vol.49(2009), No.11
    2. Effects of Coal Inertinite Size on Coke Strength ISIJ International Vol.48(2008), No.5
    3. Effect of Carbon Structure Crystallinity on Initial Stage of Iron Carburization ISIJ International Vol.50(2010), No.1
  • Inversed Phosphorus Segregation in Twin Roll Cast Strips for Improvement of Mechanical Properties and Weathering Resistance

    pp. 531-539

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    DOI:10.2355/isijinternational.50.531

    In the present paper, twin roll strip casting was applied to fabricating the strips of low carbon steels containing different P contents. P was spontaneously segregated near strip surfaces by the deformation in mushy zone during strip casting. Uniform surface layers enriched in P can be formed after subsequent processing of cold rolling and annealing, which can be used as surface coatings for the improvement of both weathering resistance and mechanical properties including toughness and elongation. In terms of plasticity, the elongation of conventional strips started drastically decreasing with P content beyond 0.15% by weight. By contrast, the elongation of the cast strips containing 0.26% P remained at the level of about 30%, with the tolerated ability of P being increased by about 0.1% by weight by twin roll strip casting. The ductile–brittle transition temperatures (DBTT) for the cast strips after cold rolling and annealing were measured to be lower than those for the conventional strips after hot rolling, cold rolling and annealing by 10–15°C. For 120 cycles of corrosion exposure, the weight gains of the 0.15 and 0.26P cast strips were reduced by more than 30% as compared with that on the conventional strip containing 0.08% P. It has been demonstrated that phosphate enriched layers could be formed beneath the corrosion scales of the 0.15 and 0.26P cast strips after cold rolling and annealing, which reduced the corrosion rate and improved the corrosion resistance.
  • Assessment of Hot Ductility with Various Thermal Histories as an Alternative Method of in situ Solidification

    pp. 540-545

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    DOI:10.2355/isijinternational.50.540

    Much work has been devoted to improving the hot ductility of steels to make them less prone to transverse cracks during the unbending stage of continuous casting. As a more positive method to avoid cracks, developing a fine grain size within the surface and sub-surface region of a slab through a double phase transformation was proposed. The in situ solidification method is usually adopted for assessing the effect of a double phase transformation, resulting from intensive cooling and reheating, on hot ductility. There exist, however, some weak points to simulating the continuous casting process by an in situ solidification. In the present work, two kinds of thermal history were adopted during the tensile test to assess the effect of a double phase transformation more precisely, instead of in situ solidification, for simulating the unbending operation. Eliminating the perpetual experimental problems of an in situ solidification tensile test, a solidified slab was used throughout the present work.
  • Influence of Thermomechanical Processing on the Austenite–Pearlite Transformation in High Carbon Vanadium Microalloyed Steels

    pp. 546-555

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    DOI:10.2355/isijinternational.50.546

    Multipass torsion tests were carried on with several V-microalloyed high carbon steels, using different deformation sequences in order to modify the austenite state prior to transformation. Both recrystallized and deformed austenite microstructures were studied. After deformation, different cooling rates were applied. The results show that accumulating strain in the austenite before transformation seems to slightly increase the interlamellar spacing for a given cooling rate, this increase being related to the pearlite transformation taking place at higher temperatures because of the increase in the austenite grain boundary area per unit volume (SV). On the other hand, the retained strain significantly contributes to a refinement of the “ferrite units”, this effect being more significant as vanadium and nitrogen contents rise. A relationship between the mean “ferrite unit” size with SV and cooling rate was determined. Similarly, empirical expressions to predict strength as a function of vanadium microalloying addition, SV and cooling rate were derived.
  • Electroless Nickel Coating Kinetics on TMT Rebar Surface and Coating Characterisation

    pp. 556-560

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    DOI:10.2355/isijinternational.50.556

    The corrosion resistance Ni–P–Fe alloy coating was obtained on rebar surface by an electroless process using glycolic acid as a complexing agent. During dipping of the iron scale free rebar in electroless solution, Fe first dissolves and the surface is activated. Subsequently, Ni, P and Fe are co-deposited by an autocatalytic process. The coating was characterised using SEM, EDS and XRD techniques. The weight percentage of Ni, P and Fe in the coating showed a relationship with the coating time. The Tafel and salt spray tests were conducted to find out corrosion resistance performance of coated samples. Electrochemical behavior of the coated rebar in simulated concrete environment is influenced by the pH of the concrete pore solution and the P content in the coating. Corrosion potential and the corrosion rate of the coatings increased with the increase in P content in the coating and pH of the pore solution whereas the resistance against chloride attack increased with increase in P content in the coating. Coated rebars showed reduction in bond strength in the range of 8 to 14% compared to the bare rebars. The maximum (~14%) drop in bond strength was observed for longer coating time. This can be attributed to the smoother coating surface. The coating obtained under longer coating time showed higher amount of P which contributed to maximum surface smoothness. However, this bond strength was much above the necessary strength requirement according to Indian standard specification.
  • Coating Degradation in Hot Press Forming

    pp. 561-568

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    DOI:10.2355/isijinternational.50.561

    The thermo-mechanically induced microstructure changes occurring in a type 1 aluminized coating on hot press forming (HPF) steel were studied in detail. The formation of intermetallic phases at the soaking temperature prior to die quenching revealed that the coating matrix consists mainly of FeAl2 intermetallic phase by the time the press forming carried out. Kirkendall void formation was observed to take place. The thermal oxidation of aluminized coating during HPF was found to be limited, with the coating acting as an effective barrier for the oxygen during heating. The deterioration caused by the high temperature plastic deformation was shown to lead to coating cracking without loss of adhesion. The steel surface was oxidized where it was exposed between the coating segments.
  • Improvement of Aluminum Erosion Behavior and Corrosion Resistance of AISI H13 Tool Steel by Oxidation Treatment

    pp. 569-573

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    DOI:10.2355/isijinternational.50.569

    In this work, H13 tool steel was prepared with different oxidation treatments to investigate the effects of oxidation temperature and soaking time. In order to improve erosion and corrosion resistance, this study tested with oxidation temperatures of 560°C, 580°C, and 600°C, and oxidation soaking times of 1 h, 2 h, and 3 h, respectively. Experimental results showed that the oxidation treatment at 600°C and for 3 h is the optimum process. The average thickness of the oxide layer was 9.2 μm. It showed that the oxide layer (Fe3O4) can protect and improve the aluminum erosion of H13 steel. The specimens that underwent the oxidative procedure were proven to effectively reduce the ratio of Al–Fe–Si compounds during erosion tests of A380 alloy solution. In addition, the results showed that the oxide layer can enhance polarization resistance, and quickly generate a passivation layer to increase the ability of corrosion resistance.
  • Characterization of the Bake-hardening Behavior of Transformation Induced Plasticity and Dual-phase Steels Using Advanced Analytical Techniques

    pp. 574-582

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    DOI:10.2355/isijinternational.50.574

    The bake-hardening (BH) behavior of TRansformation Induced Plasticity (TRIP) and Dual-Phase (DP) steels after intercritical annealing (IA) has been studied using transmission electron microscopy, X-ray diffraction and three dimensional atom probe tomography. It was found for the DP steel that carbon can segregate to dislocations in the ferrite plastic deformation zones where there is a high dislocation density around the “as-quenched” martensite. The carbon pinning of these dislocations, in turn, increases the yield strength after aging. It was shown that bake-hardening also leads to rearrangement of carbon in the martensite leading to the formation of rod-like low temperature carbides in the DP steel. Segregation of carbon to microtwins in retained austenite of the TRIP steel was also evident. These factors, in combination with the dislocation rearrangement in ferrite through the formation of cells and microbands in the TRIP steel after pre-straining, lead to the different bake-hardening responses of the two steels.
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  • General Method of Phase Transformation Modeling in Advanced High Strength Steels

    pp. 583-590

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    DOI:10.2355/isijinternational.50.583

    In the present paper, a new modeling approach is proposed for the austenite to ferrite and bainite transformation kinetics in transformation induced plasticity (TRIP) and complex phase (CP) steels. Based on experimental data obtained by dilatometry during continuous cooling, Rios' method has been successfully applied assuming additivity to calculate the parameters for the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, i.e. exponent n and the rate parameter k. Limitations of the Rios' method have been identified when k is a function of both temperature T and fraction transformed X. For these cases that are in particular relevant for the bainite transformation, a new modeling method has been developed to investigate the exact relationship of k with T and X. The new method has been used to describe the transformation kinetics in a TRIP and a CP steel. Good agreement has been obtained between the calculated and measured transformation data. The proposed new modeling method provides a general modeling approach that promises to be useful in predicting the complex phase transformation kinetics during industrial processing of advanced high strength steels (AHSS).
  • Grain Boundary Migration in Fe–3mass%Si Alloy Bicrystals under a Magnetic Field

    pp. 591-595

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    DOI:10.2355/isijinternational.50.591

    Grain boundary migration under a magnetic field was studied using Fe–3mass%Si alloy bicrystals with ‹001›/Σ5 (φ=36°) and ‹001›/random (φ=45°) tilt boundaries at temperatures between 1223 K and 1323 K. A capillarity method (Sun and Bauer method) was used to observe grain boundary migration. A 6 T magnetic field was applied parallel to the [100] direction, which is one of easy magnetization directions, in a grain whose area increases with occurring grain boundary migration. It was found that the application of a magnetic field observably increased the velocity of grain boundary migration for both bicrystals. The abrupt change in the velocity of grain boundary migration was observed at a critical capillary driving force irrespective of whether the magnetic field was applied. However, this critical driving force was decreased by application of a magnetic field. It was suggested that the magnetic field act as a driving force for grain boundary migration rather than increase its mobility.
  • Microstructural Evolution and the Effect on Mechanical Properties of S30432 Heat-resistant Steel during Aging at 650°C

    pp. 596-600

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    DOI:10.2355/isijinternational.50.596

    In this paper, the mechanical properties and microstructural evolution of S30432 heat-resistant steel during aging at 650°C were investigated, and the effect of microstructural evolution on the yield strength, tensile strength, hardness and impact toughness was discussed. The results show that, the change of strength and hardness can be divided into three stages. In the first stage (before 500 h), the precipitation of fine ε-Cu plays a main role in the significant increase of the strength and hardness. In the second stage (500–5000 h), the coarsening of ε-Cu is the key factor to decrease the strength and hardness. After 5000 h of aging, there is no obvious change in the strength and hardness. Similarly, the change of impact toughness during aging of S30432 steel at 650°C can also be divided into three stages. The sharp decrease of impact toughness in the first stage results from the precipitation of M23C6 and ε-Cu particles. At stage II, the impact toughness keeps on declining as a result of gradual coarsening of M23C6, ε-Cu and Nb(C,N). Finally, M23C6, ε-Cu and Nb(C, N) are relatively stable, so that the impact toughness tends to be stable gradually.
  • Influence of Reheating Temperature and Vanadium Content on Transformation Behavior and Mechanical Properties of Medium Carbon Forging Steels

    pp. 601-606

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    DOI:10.2355/isijinternational.50.601

    The influence of vanadium and nitrogen on microstructure and mechanical properties of medium-carbon steels has been studied by means of metallography and mechanical testing. Vanadium addition to the low nitrogen steel suppresses the formation of ferrite–pearlite following the low reheating temperatures and microstructure consists of bainitic sheaves. Increasing nitrogen at the same vanadium level promotes the acicular ferrite formation. For high reheating temperatures, dominantly acicular ferrite structure in both the low nitrogen and the high nitrogen vanadium steels is obtained. The results suggest that vanadium in solid solution promotes the formation of bainite. The effect of nitrogen is related to the precipitation of VN particles in austenite with high potency for intragranular nucleation of acicular ferrite and to the precipitation of V(C, N) particles in ferrite with high potency for precipitation strengthening. Addition of both vanadium and nitrogen considerably increases the strength level, while CVN20 impact energy increases on changing the microstructure from bainitic ferrite to the fine ferrite–pearlite and acicular ferrite.
  • Effect of Tempering on Mechanical Properties of V-added AISI 4335 Steel

    pp. 607-612

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    DOI:10.2355/isijinternational.50.607

    The structure–property relations of V-added AISI 4335 steel forgings tempered at different temperatures have been examined to delineate the effect of tempering on their ductility and toughness characteristics. The experimental work involved characterization of the microstructures and determination of hardness, tensile properties, impact toughness and plane strain fracture toughness of a series of hardened and tempered samples. The tempering treatments were carried out at eight different temperatures in the range of 633–833 K. The obtained results have shown that hardness and strength decrease in a monotonic fashion with tempering temperature while impact toughness and ductility increase with increasing tempering temperature with an inflection around 753 K. Analyses of the results highlight that the variations of impact toughness and ductility with tempering temperature possess the potential to directly reveal the embrittlement temperature range. It has been also found here that the embrittlement temperature range can be delineated from the variation in the rate of change of fracture toughness with tempering temperature. These observations have been discussed considering the energy absorbed by the initiation and growth of cracks during fracture of a specimen by tensile, impact and fracture toughness tests.
  • Elastic–Plastic and Inelastic Characteristics of High Strength Steel Sheets under Biaxial Loading and Unloading

    pp. 613-619

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    DOI:10.2355/isijinternational.50.613

    Biaxial tensile tests followed by biaxial unloading and reloading are carried out for BH340 and DP590 steel alloys. The contours of plastic work and the directions of plastic strain rates were measured at different levels of plastic work in the first and second quadrants of stress space. The applicability of conventional anisotropic yield functions, Hill's quadratic function and the Yld2000-2d function to the accurate prediction of the plastic deformation behavior of these steel alloys is discussed using the measured data. The measured work contours and directions of plastic strain rates were in good agreement with those calculated using the Yld2000-2d yield function with an exponent of four. The initial and subsequent elastic moduli after prestraining and the instantaneous tangent moduli during subsequent unloading after an equivalent plastic prestrain of ε0p=0.02 are measured from the biaxial loading, unloading and reloading experiments. The moduli of elasticity at reloading were lower by 9 to 17% than those at initial loading. The amount of strain recovery along the rolling direction (RD) is more than that along the transverse direction (TD) for uniaxial unloading, as well as for biaxial unloading. An exponential decay model is proposed that provides good reproduction of the unloading stress–strain relations, (σ/σu)–Δε/(σu/E2), of both materials under different stress ratios.
  • Yielding Behavior of Nb Micro-alloyed C–Mn–Si TRIP Steel Studied by In-situ Synchrotron X-ray Diffraction

    pp. 620-629

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    DOI:10.2355/isijinternational.50.620

    The stress and strain partitioning between the different micro-structural constituents, and the initiation of the martensitic transformation during the yielding of multi-phase transformation-induced plasticity steel were studied by in-situ synchrotron X-ray diffraction experiments under tensile tests. Position, intensity and width of retained austenite and ferrite diffraction peaks were used to determine lattice strain and phase fractions. At low tensile stress, small elastic lattice deformations were observed. Whereas the Poisson strains in the ferrite were found to be reduced at the macroscopic yield stress, the strain in the austenite increased. The results clearly show that at the elasto–plastic transition the transformation of some of the retained austenite is stress induced.
  • Serial Batch Elution of Electric Arc Furnace Oxidizing Slag Discharged from Normal Steelmaking Process into Fresh Water

    pp. 630-638

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    DOI:10.2355/isijinternational.50.630

    The serial batch leaching test of an electronic arc furnace oxidizing slag was performed on the basis of JIS K 0058-1 to investigate the safety of the slag and to clarify the elution mechanism. The slag which was discharged at the time of refining of normal steel was used for this experiment. The slag was dissolved in water whose initial pH was 6.0. After that, the slag was naturally dried. This operation was performed 12 times successively.
    The dissolution behaviors of Ca and Mg were expressed by the parabolic law. That of Si was expressed by the linear law. The dissolutions of them from the slag were controlled by the diffusion through pore and the surface layer through the surface layer where compositions of FeO and Al2O3 were high. Generally, the dissolve concentrations decreased with the increase in the number of the elution times. The environmentally regulated substances were not detected or less than the levels of the environmental quality standards for soil. Ba, Mn, Mo, K, Na, Sr and W were detected as minor element except for the environmentally regulated elements. Generally, the detected environmentally regulated substances and the other minor elements decreased with the number of the elution times. During the elution of the slag, the pH of the aqueous solution steeply increased initially, and it decreased afterwards. The pH decreased with the increase in the number of the elution times.
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