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

ISIJ International Vol. 50 (2010), No. 1

  • Effect of Arc Length on Fluid Flow and Mixing Phenomena in AC Electric Arc Furnaces

    pp. 1-8

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    DOI:10.2355/isijinternational.50.1

    Fluid flow and mixing phenomena in an industrial three phase Electric Arc Furnace has been investigated, as a function of arc length. Velocity and temperature fields as well as mixing times were computed, assuming that liquid steel occupies the whole computational domain and buoyancy forces are responsible for convective flow. It is reported a strong effect of arc length on both fluid flow and mixing time, suggesting a metallurgical practice with long arc operation to improve the melting kinetics of the metallic charge. Thermal stratification has also been confirmed, however the volume of hot steel increases when arc length also increases.
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  • Mathematical Modeling of the Melting Rate of Metallic Particles in the Electric Arc Furnace

    pp. 9-16

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    DOI:10.2355/isijinternational.50.9

    A computational fluid dynamics model coupled to a lagrangian model of melting/solidifying particles has been developed to describe the melting kinetics of metallic particles in an industrial Electric Arc Furnace (EAF), assuming that liquid steel occupies the entire computational domain. The metallic particles represent Direct Reduced Iron (DRI). The use of two previous models, an arc model and a fluid flow model has made possible to evaluate the melting rate of injected DRI in a three phase-EAF, evaluating the influence of the initial particle size, the initial DRI temperature, feeding position, feeding rate, arc length and some of the metallurgical properties of DRI. The frozen shell formed in the early stage of the melting process has also been evaluated in this model.
  • Physical Modeling Study on Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Gas Stirring and Fluid Flow Characteristics in Bath

    pp. 17-25

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    DOI:10.2355/isijinternational.50.17

    The gas stirring and fluid flow characteristics in the bath during the combined side and top blowing AOD refining process of stainless steel were investigated on a water model of a 120 t AOD converter. The geometric similarity ratio of the model to its prototype (including the side tuyeres and the top lances) was 1 : 4. Using the reliably evaluated friction factors of the side tuyeres for the model and its prototype to gas streams, based on the theoretical calculations of the gas flow properties in the tuyeres and the top lances and the modified Froude number, the gas blowing rates utilized for the model were still more reasonably and accurately determined. Thus, sufficiently full kinematic similarity between the model and its prototype was maintained. The influences of the gas flow rates for side and top blowing, the tuyere number and the angle between each tuyere on the characteristics were examined. The results showed that under driving of multiple gas side blowing streams, the liquid in the whole bath was in vigorous agitation and circulatory motion during the blowing process, and there was no evident dead zone in the bath. The gas blowing rate of the main tuyeres possessed a determined role on the gas stirring and liquid flow pattern in the bath. With regard only to the stirring, the larger the gas side blowing rate, the higher the stirring work input, the more vigorous the bath and its liquid surface were. At a given tuyere number and gas side blowing rate, the larger the angle between each tuyere, the more uniform the gas stirring to the bath became. The gas jet from the top lance could change the agitation and liquid flow pattern in the bath caused by the gas side blowing streams, making the liquid turbulence enhance, and the larger the gas top blowing rate, the more obvious the relevant change was. The existing tuyere equipment and arrangement of 7 tuyeres with an angular separation of 18° for the 120 t AOD converter could not provide a perfect stirring and uniform liquid flow pattern in the bath. The agitation power densities of the gas side and top blowing jets to the bath were estimated, respectively.
  • Physical Modeling Study on Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Fluid Mixing Characteristics in Bath

    pp. 26-34

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    DOI:10.2355/isijinternational.50.26

    Based on water modeling of the gas stirring and fluid flow in the bath, the fluid mixing characteristics in the bath during the combined side and top blowing AOD refining process of stainless steel were studied on a water model unit of a 120 t AOD converter. The influences of the angle included between each tuyere, the side tuyere number and the gas flow rates for both side and top blowing on the characteristics were examined. The results illustrated that the combined side and top blowing process possessed a good mixing effectiveness. The gas flow rate of the main tuyeres had a key role on the liquid mixing in the bath. With a physical shielding effect of the gas streams from the subtuyeres on the gas streams of the main tuyeres, increasing suitably the gas flow rate of the subtuyeres could enhance mixing efficiency; and the gas jet of the top lance could prolong the mixing time. For a simple side blowing, at a given tuyere number and gas side blowing rate, an increase in the angular separation between each tuyere could be advantageous for shortening the mixing time. At a given angle between each tuyere and gas side blowing rate, increasing the tuyere number could not necessarily reach definitely a similar result. Moreover, it could make the high temperature zone move towards the sidewall around the tuyere outlets and lower the life of the refractory lining due to reducing the gas flow rate of single tuyere and the horizontal penetration of the gas stream. Relevant to the oxygen top bowing rate of 6600 Nm3/h used in the practice, taking 5 tuyeres with 22.5° or 6 tuyeres with 27° could offer a roughly equivalent and good mixing result. As far as only the mixing in the bath is concerned, for the 120 t AOD converter, the existing 7 tuyeres with 18° would not be a proper equipment and arrangement of tuyere under the blowing operations employed for the practical refining. Using 6 tuyeres with 27° could give a perfect mixing in all the various refining periods. The relationships of the mixing time with the gas blowing rates of main tuyeres and subtuyeres and top lance, the angle between each tuyere, the tuyere number, the agitation power densities, and the modified Froude numbers were determined.
  • Reduction Kinetics of Iron Oxides in Molten Lunar Soil Simulant by Graphite

    pp. 35-43

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    DOI:10.2355/isijinternational.50.35

    In space exploration including the utilization of materials and energy outside the earth, construction of a moon base is one of considerable measures to develop the front line. Due to the huge cost of transportation from the earth, structural materials such as iron should be in-situ produced on the moon. Fortunately, the Apollo 14 mission has reported that the lunar soil contains about 10 mass% of iron oxides. When considering ironmaking from lunar soils by graphite on the moon, understanding the reduction behaviour should come first but has not yet been attempted for iron resources whose concentration of iron oxides is as low as 10 mass%. Thus, the present work has focused on reduction kinetics of iron oxides in molten lunar soil simulant by graphite, and apparent rate constants for reduction reactions have been derived from experiments with systematic variation in reduction conditions for comprehensive discussion about the reaction mechanism. The apparent rate constant derived is 2.0×10−4 cm/s at 1803 K and the corresponding activation energy estimated is about 360 kJ/mol. The use of a graphite rod rotation mechanism enabled the stirring condition to be changed, resulting in a finding that stronger stirring shifts the rate-determining step from reduction of FeO by CO gas to direct reduction of FeO by graphite. On the basis of all these kinetic findings, it is likely that ironmaking from lunar soils by graphite on the moon is possible in that high vacuum and low gravity conditions would probably promote the removal of CO gas film from the graphite surface.
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  • Deoxidation Equilibrium of Titanium in the Iron–Nickel Melts

    pp. 44-52

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    DOI:10.2355/isijinternational.50.44

    The oxygen solutions in the titanium-containing Fe–Ni melts have been thermodynamically analyzed. When the nickel content rises to 40%, the deoxidation ability of titanium decreases but then it sharply rises with increasing nickel concentration in melt. This can be explained by the fact that although the bond strength of titanium in nickel is considerably stronger in comparison with iron (γ°Ti(s)(Fe)=0.0083; γ°Ti(s)(Ni)=0.00019), but the bond strength of oxygen in nickel is appreciably weaker that in iron (γ°O(Fe)=0.0105; γ°O(Ni)=0.357). The oxygen solubility curves pass through a minimum whose position shifts to the higher titanium concentrations with an increase in the nickel content from 0.564% Ti for pure iron to 0.633% Ti for pure nickel. The further addition of titanium causes an increase in the oxygen content in melt. Equilibrium points of oxide phases TiO2, Ti3O5 and Ti2O3 for different alloy compositions at 1873 K were determined. The published results devoted to the oxide phases in iron and nickel deoxidized with titanium are summarized. Deoxidation of Fe–40%Ni melts with titanium was experimentally studied. The experimental and calculated results are in good agreement. The deoxidation abilities of titanium, chromium, manganese, vanadium, silicon, carbon, and aluminum in different Fe–Ni alloys are compared.
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  • Effect of Carbon Structure Crystallinity on Initial Stage of Iron Carburization

    pp. 53-58

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    DOI:10.2355/isijinternational.50.53

    Iron carburization is one of the most important reactions in iron making process. If the rate and efficiency of carburization reaction are increased, energy consumption of the process will be reduced to a large extent. The purpose of this study is to clarify the effect of carbon structure on initial iron carburization behavior for suggestion of effective carbon utilization in iron making process. Samples of different carbon structures were prepared by heat treatment of CH4 decomposed carbon at various temperatures. Carbon structures of prepared carbon samples were analyzed by a Raman spectroscopy. For experiments, high-purity iron samples directly-contacted with carbon were prepared. A laser microscope combined with an infrared image furnace was used for heating of the sample and “in-situ” observation of carburization phenomena. Carbon concentration profile in iron sample was analyzed by a wavelength dispersion type electron probe micro analyzer. The results of this work showed that primary Fe–C liquid formation temperature became lower from 1439 to 1431 K with use of lower crystallization carbon. It was indicated that utilization of the lower crystallization degree of carbon structure causes formation of primary Fe–C liquid at the lower temperature.
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  • Magnetic Separation of Phosphorus Enriched Phase from Multiphase Dephosphorization Slag

    pp. 59-64

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    DOI:10.2355/isijinternational.50.59

    The authors have found that phosphorus exhibits remarkable segregation in exhausted actual hot-metal pretreatment slag (dephosphorization slag), and it exists as a 3CaO·P2O5–2CaO·SiO2 solid solution with the FeO–CaO–SiO2 matrix. Since their magnetic properties are significantly different, it is possible to separate them with the aid of a superconducting strong magnetic field. To investigate the effects of magnetic field strength, the particle size of the slag, etc., an experiment on magnetic separation has been carried out using simulated dephosphorization slag (18.1FetO–45.9CaO–20.3SiO2–6.6P2O5–2.5MnO–5.5MgO in mass%) and a superconducting magnet with 0.5–2.5 T. In a stronger magnetic field, the quality of the recovered slag becomes better due to less contamination of the FetO matrix phase, while its quantity worsens and the amount of recovered slag declines. However, the quantity of the recovered slag can be improved by repeating the procedure. In the present experiment, about 65% of the phosphorus enriched phase can be recovered with less than 10% of FetO matrix phase contamination at conditions of 0.5 T, a particle size of less than 35 μm, and a water/slag ratio of 32 with a single procedure. The P2O5 content in the recovered slag is close to that in the phosphorus-enriched phase in the initial slag, and the FeO content decreases markedly with magnetic separation.
  • Recycling Effects of Residual Slag after Magnetic Separation for Phosphorus Recovery from Hot Metal Dephosphorization Slag

    pp. 65-70

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    DOI:10.2355/isijinternational.50.65

    In a previous study, the authors found that phosphorus exhibits remarkable segregations in industrial hot metal pretreatment slag where it exists as a Ca3P2O8–Ca2SiO4 solid solution together with a FeO–CaO–SiO2–MnO–MgO matrix. Since the magnetic property of each phase is significantly different, it is possible to separate the phases with the aid of a superconducting strong magnetic field. By applying a strong magnetic field of 0.5 to 2.5 T to the crushed slag, more than 60% of the concentrated phosphorus phase in the slag was recovered. If most of the phosphorus can be removed from the slag, the residual slag will comprise FeO–CaO–SiO2–MnO–MgO with less P2O5, and thus may be recycled to iron- and steel-making processes, such as sintering, hot-metal desiliconization, and hot-metal dephosphorization processes. In the present work, the recycling effect of subjecting the residual slag to the dephosphorization process was simulated based on a mass balance calculation. A significant reduction in total slag generation and CaO input was demonstrated by the mathematical model considering phosphorus recovery and recycling of residual slag as a dephosphorization agent. Using the waste input–output model, it was shown that phosphorus recovery from dephosphorization slag and the recycling of residual slag in a hot-metal dephosphorization process have potentially great environmental and economic benefits.
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  • Enhancement of Iron Melting Rate under the Co-existence of Graphite and Wüstite

    pp. 71-80

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    DOI:10.2355/isijinternational.50.71

    Carburization and melting processes of solid Fe under the co-existence of graphite and wüstite has been investigated based on a confocal laser scanning microscope “in-situ” observation of the movement of the interface between Fe–C melt and solid Fe at 1523 K. In the experiments, strong stirring flow at the surface of the Fe–C melt is observed. This flow is found to be Marangoni flow and is introduced by the surface tension gradient due to the difference of oxygen concentration between the Fe–C melt interface with graphite and solid iron. The velocity of Marangoni flow is measured from the moving velocities of bubbles at the molten Fe–C surface. The melting rate of Fe is analyzed by computational fluid flow simulations of the Fe–C melt. Based on these results, it is confirmed that the carburization and melting rate of solid Fe can be enhanced by allowing Fe–C melt to come into contact with wüstite and graphite simultaneously.
  • Charcoal Behaviour by Its Injection into the Modern Blast Furnace

    pp. 81-88

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    DOI:10.2355/isijinternational.50.81

    Nowadays the use of charcoal in metallurgy is intimately linked to small blast furnaces in Brazil. Due to the challenge for CO2 mitigation, interest for charcoal use as a renewable energy source is rising. In the scope of European efforts to mitigate carbon dioxide emissions in the steel industry in the post-Kyoto period, the use of charcoal in cokemaking and ironmaking has been investigated. This paper presents results of an experimental study on charcoal behaviour under the blast furnace simulating conditions performed at the Department of Ferrous Metallurgy, RWTH Aachen University and at the National Centre for Metallurgical Investigations, Madrid. Conditions in the raceway and in the furnace shaft were simulated using thermo-analytical, laboratory and pilot facilities. Charcoal samples were produced in two furnaces for pyrolysis from different wood types at various carbonisation conditions. Furthermore technological and ecological assessment of blast furnace process when injecting different types of charcoal was performed using a mathematical model. All the experiments and calculations were also performed with reference mineral coals for injection. Conversion efficiency of all the tested charcoals is better or comparable with coals. Change in coke rate, furnace productivity and further operation parameters when replacing pulverised coal with charcoal depends on charcoal ash content and composition.
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  • Effect of Bottom Bubbling Conditions on Surface Reaction Rate in Oxygen–Water System

    pp. 89-94

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    DOI:10.2355/isijinternational.50.89

    In secondary steelmaking, the enhancement of the reaction rate in the low carbon period during the decarburization of steel is considered the most effective method to produce ultralow carbon steel. In a previous study, it was revealed that the surface reaction is dominant during the final stage of the actual refining process. In order to improve the surface reaction rate, it is necessary to enlarge the reaction region, which is usually achieved by increasing the plume eye area. In this study, water model experiments were carried out to estimate the influence of bottom stirring conditions on the gas-liquid reaction rate; for this purpose, the deoxidation rate during the bottom bubbling process was measured. Five types of nozzle configurations were used to study the effect of the plume eye area on the reaction rate at various gas flow rates. The results reveal that the surface reaction rate is influenced by the gas flow rate and the plume eye area. An empirical correlation was developed for the reaction rate and the plume eye area. This correlation was applied to estimate the gas–liquid reaction rate at the bath surface.
  • Laboratory Study on Evolution Mechanisms of Non-metallic Inclusions in High Strength Alloyed Steel Refined by High Basicity Slag

    pp. 95-104

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    DOI:10.2355/isijinternational.50.95

    Laboratory study was carried out to discuss evolution mechanisms of non-metallic inclusions in high strength alloyed steel refined by high basicity slag, aiming at formation of lower melting temperature inclusions to improve anti-fatigue properties of steel.
    It is found that steel/slag reaction time had great effect on inclusion types, compositions and shapes. With reaction time extended from 30 to 180 min, solid MgO–Al2O3 and MgO-based inclusions were finally changed into CaO–MgO–Al2O3 system inclusions lower melting temperature (<1773 K). While shapes of inclusions varied in the route blocky/angular→near spherical→spherical.
    Al2O3/MgO·Al2O3/MgO and MgO/MgO·Al2O3/CaO·2Al2O3 stability diagram were obtained by thermodynamic calculation. The results indicated that MgO and MgO·Al2O3 inclusion would be formed at early stage of steel-slag reaction because activity of Mg is much larger than that of Ca in steel. However, with increase of Ca activity, solid MgO·Al2O3 and MgO inclusions would be inevitably and gradually transferred into complex liquid inclusions even dissolved [Ca] is as low as 0.0002%. Thus, SEM-EDS mappings of CaO–MgO–Al2O3 system inclusions are characterized by high melting temperature solid MgO·Al2O3 or MgO-based inclusion cores surrounded by lower melting temperature CaO–Al2O3 outer surface layers, which would be softer during hot rolling and therefore be helpful to improve anti-fatigue properties of steel.
    Model was established to elucidate change mechanisms of inclusions. Transferring kinetics of inclusions was discussed qualitatively to analyze velocity controlled steps. It is found that diffusion of Mg and Ca in solid inclusion core and the formed CaO–Al2O3 outer surface layer would be probably the limited step during evolution of inclusions. However, further work should be done to discuss evolution kinetics of inclusions quantitatively.
  • Performance Assessment of CO2 Treated Fluxed Iron Oxide Pellets in Basic Oxygen Steel Making Process

    pp. 105-114

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    DOI:10.2355/isijinternational.50.105

    Lump lime and iron ore are generally used in BOF as flux and cooling material respectively. Owing to high melting point, poor dissolution property, fines generation and hygroscopic nature of lump lime, delay in process and operational complexities are generally encountered. On the other hand, iron ore charging creates slag foaming. In order to alleviate the above problems and to utilize waste materials, fluxed iron oxide pellets containing waste iron oxides (~70%) and lime fines (~30%) were prepared and subsequently strengthened with CO2 gas treatment. This carbonated fluxed pellets exhibited very good CCS (30 kg/pellet) and DSN (150), suitable for cold handling. The performance of this pellet has been assessed in a laboratory scale bottom blowing converter for its specific use in BOF. The refining characteristic of a liquid bath, prepared with pig iron chips was studied using a laboratory scale bottom blowing converter with three flux conditions namely fluxed pellet plus additional lime, iron ore lump plus lump lime and only lump lime. It has been observed that use of these pellets improves decarburization and dephosphorization, increases the metallic yield, decreases oxygen consumption and reduces foaming. Overall, the developed pellet shows very good application potential in basic oxygen steel making.
  • Hydrogen Desorption Behavior of Pure Iron and Inconel 625 during Elastic and Plastic Deformation

    pp. 115-123

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    DOI:10.2355/isijinternational.50.115

    The hydrogen desorption behavior of pure iron with a body-centered-cubic (BCC) lattice and Inconel 625 with a face-centered-cubic (FCC) lattice was examined during tensile deformation using a quadrupole mass spectrometer in a vacuum chamber integrated with a tensile testing machine. Hydrogen and water desorption was continuously detected simultaneously under the application of a tensile load and strain to the specimens. Hydrogen desorption promoted by tensile deformation can be found by deducting both fragment hydrogen dissociated from H2O and H2 desorbed under unloading from the total amount of hydrogen desorbed from hydrogen-charged specimens during tensile deformation. Hydrogen desorption from hydrogen-charged specimens was detected under various strain rates of 4.2×10−5/s, 4.2×10−4/s and 4.2×10−3/s. Hydrogen desorption rarely increased under elastic deformation. In contrast, it increased rapidly at the proof stress when plastic deformation began, reached its maximum, and then decreased gradually with increasing applied strain for both pure iron and Inconel 625. This desorption behavior is closely related to hydrogen dragging by moving dislocations. The amount of desorbed hydrogen promoted by tensile deformation was measured by thermal desorption analysis (TDA). The TDA results showed that the amount of desorbed hydrogen differed at each strain rate. The largest amount of desorbed hydrogen promoted by tensile deformation was 16% of the initial hydrogen content in pure iron with a high hydrogen diffusion rate when the specimen was deformed at a strain rate of 4.2×10−4/s. In contrast, that of Inconel 625 with a low hydrogen diffusion rate was 9% of the initial hydrogen content when the alloy was deformed at a strain rate of 4.2×10−6/s. This difference in the amount of desorbed hydrogen transported by dislocations depends on the balance between the hydrogen diffusion rate and mobile dislocation velocity.
  • Application of Water Based-TiO2 Nano-fluid for Cooling of Hot Steel Plate

    pp. 124-127

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    DOI:10.2355/isijinternational.50.124

    A nano-fluid was prepared by dispersing 0.1 wt% TiO2 nano particles in water along with a small amount of surfactant. The performance of the nano-fluid vis-a-vis water as coolant was studied on hot steel plates by measuring the cooling curves and by appropriate metallographic investigations. Significant enhancement in cooling rate was observed for the nano-fluid which could be due to an enhancement of convective heat transfer by jet impingement, coupled with lower surface tension of the nano-fluid, as compared to water.
  • The Influence of Microstructures on the Notched Tensile Fracture of Ti–6Al–6V–2Sn Welds at Elevated Temperatures

    pp. 128-132

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    DOI:10.2355/isijinternational.50.128

    Notched tensile tests of Ti–6Al–6V–2Sn welds, which were subjected to distinct post-weld heat treatments (PWHTs), were carried out in air at 150, 300 and 450°C. The results were compared with those of mill-annealed base metal (MB) specimens tested at same temperatures. The notched tensile strength (NTS) of the MB specimens decreased significantly with increasing temperature. Moreover, the welds with high hardness showed notch brittleness at room temperature but had a higher NTS than the MB specimen. The presence of grain boundary α layer promoted grain boundary shear, therefore, reduced its notch brittleness at room temperature for the weld with the PWHT at 704°C (the W-704 specimen). A lack of deformation compatibility at the interfaces between the α and β phases caused interfacial separations in the MB specimen as well as void formation in the W-704 specimen, leading to the low NTS of these specimens at elevated temperatures.
  • Bacterial Attachment and Initiation of Biofilms on the Surface of Copper Containing Stainless Steel

    pp. 133-138

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    DOI:10.2355/isijinternational.50.133

    Type 304 stainless steel, copper containing stainless steel, and oxygen free copper were subjected to antibacterial tests and short term exposure experiments in a laboratory. Antibacterial tests showed that the copper containing stainless steel, as well as oxygen free copper, was antibacterial, yet the antibacterial activity of the copper containing stainless steel was lower than that of the oxygen free copper. In short term exposure experiments, the copper containing stainless steel, as well as type 304 stainless steel, didn't sterilize planktonic bacterial cells, while the oxygen free copper reduced the number of alive planktonic bacterial cells. The copper containing stainless steel did not protect itself from bacterial adhesion, but sterilized about 75% of sessile bacterial cells and reduced formation of biofilms on its surface. Such experimental results indicate that the copper containing stainless steel is effective against biofilm related impacts.
  • The Role of New Ferrite on Retained Austenite Stabilization in Al-TRIP Steels

    pp. 139-146

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    DOI:10.2355/isijinternational.50.139

    Microstructure and mechanical properties of two high Al, low-Si TRIP steels with different Cr and Mo contents were studied using continuous galvanizing line (CGL) laboratory simulation. Combined use of specific etching methods, optical and electron microscopy observations and EBSD characterization led to verify the epitaxial growth of ferrite during cooling at a moderate rate from the intercritical annealing to the isothermal holding temperature. The amounts of “new” ferrite formed during cooling and retained austenite obtained after processing are much higher in the steel with lower content of hardenability-promoting elements. Measured tensile properties and mechanical behavior of the steel strongly depend on the amount of new ferrite and retained austenite. It is found that the formation of new epitaxial ferrite from intercritical austenite can effectively contribute to the chemical and particle size stabilization of untransformed austenite as well as to obtain the desired TRIP effect under processing conditions highly compatible with industrial practice, i.e. cooling rates near 15°C/s and isothermal holding times at 460°C shorter than 60 s.
  • Pseudoelastic Behavior of Fe–Al Polycrystals

    pp. 147-151

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    DOI:10.2355/isijinternational.50.147

    Pseudoelastic behavior of Fe–Al polycrystals at room temperature was examined focusing on Al concentration, the crystallographic texture and the grain size. Perfect pseudoelasticity derived from the reversible motion of 1/4 ‹111› superpartial dislocations dragging the anti-phase boundaries (APB) took place in Fe–25.0at%Al polycrystals at a total strain of 1.0%. The amount of strain recovery in Fe–Al polycrystals showed a maximum at 25.0 at% Al and the deviation from the concentration led to a decrease in strain recovery. The backward stress due to APB and dislocation configuration in Fe–Al alloys was closely related to the dependence of the pseudoelasticity on Al concentration. The pseudoelastic behavior of Fe–25.0at%Al polycrystals depended strongly on the loading axis, which could be accounted for in terms of the Taylor factor. Moreover, recovery strain of Fe–23.0Al polycrystals increased with increasing average grain size suggesting that the grain boundaries suppressed the forward and backward motion of the superpartials and were harmful for the pseudoelasticity.
  • Delamination Toughening of Ultrafine Grain Structure Steels Processed through Tempforming at Elevated Temperatures

    pp. 152-161

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    DOI:10.2355/isijinternational.50.152

    The deformation of tempered martensitic structures, namely tempforming treatments, were applied to a 0.6C–2Si–1Cr steel at 500, 600 and 700°C using multi-pass caliber-rolling with an accumulated area reduction of 80%. The tensile and Charpy impact properties were investigated to make clear the relation between the microstructure and the delamination behavior of the tempformed (TF) samples. The tempforming treatments resulted in the evolution of ultrafine grain structures with strong ‹110›//rolling direction (RD) fiber deformation textures and fine spheroidized cementite particles distributions. In contrast to the ductile-to-brittle transition of the conventional quenched and tempered (QT) samples, the TF samples exhibited inverse temperature dependences of the impact toughness due to the delaminations, where the cracks branched in the longitudinal direction (//RD) of the impact test bars. As a result, high strength with excellent toughness was achieved in the TF samples. A yield strength of 1364 MPa and a V-notch Charpy absorbed energy of 125 J were obtained at room temperature in the sample that was tempformed at 500°C. The delamination was shown to occur due to the microstructural anisotropy of the TF samples, and the dominating factors controlling the delamination toughening were the transverse grain size, the grain shape and the ‹110›//RD fiber deformation texture. The discussion also indicated that the ultra refinement of the transverse grain structure was the key to enhancing both the yield strength and the toughness of the TF steel while lowering the ductile-to-brittle transition temperature.
  • Formability of Al–Nb Bearing Ultra High-strength TRIP-aided Sheet Steels with Bainitic Ferrite and/or Martensite Matrix

    pp. 162-168

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    DOI:10.2355/isijinternational.50.162

    Microstructure, tensile properties and stretch-flangeability of 980–1470 MPa grade Al or Al–Nb bearing TRIP-aided cold-rolled sheet steels with bainitic ferrite and/or martensite matrix microstructure (TBF steels) were investigated for automotive applications such as impact member reinforcements, sheet flames and so on. In addition, these properties were related with the microstructure and the retained austenite characteristics. Complex additions of 0.5% Al and 0.05% Nb into a base steel with chemical composition of 0.2% C, 1.5% Si and 1.5% Mn (in mass%) significantly enhanced the total elongation and stretch-flangeability, especially when austempered at temperatures below martensite-start temperature. The excellent stretch-flangeability was primarily associated with (i) refined prior austenitic grain by NbC precipitates and (ii) uniform fine mixed matrix microstructure of bainitic ferrite and martensite, as well as (iii) TRIP effect of metastable retained austenite.
  • Removal of Fe3O4 Film Using Atmospheric Transferred Plasma at Low Temperature

    pp. 169-173

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    DOI:10.2355/isijinternational.50.169

    Ar transferred thermal plasma was applied onto Fe3O4 film covering an Fe rod using a hybrid plasma furnace composed of both non-transferred and transferred thermal plasma. A tungsten rod was used as an electrode. The flame from the transferred plasma spread over the surface of the Fe3O4 film. The transferred plasma direct current was set to each of the values (0, 2, 3, 4, and 5 A) after the non-transferred plasma made contact with the Fe3O4 film. The removal mass of the Fe3O4 film directly increased as the transferred plasma current increased. The oxygen potential of the output gas decreased when the plasma was irradiated. The temperature on the surface was 1073 (K) during the 5 A plasma irradiation (this temperature is under the melting point), even though a trace of melted Fe3O4 was observed after the plasma treatment.
  • Effects of PO2 at Flux State on the Fluorine Dissolution from Synthetic Steelmaking Slag in Aqueous Solution

    pp. 174-180

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    DOI:10.2355/isijinternational.50.174

    This paper studies effect of oxygen partial pressure at molten flux state on the fluorine dissolution from synthetic steelmaking slag solidified from the flux. The PO2's were set as 6.6×10−8, 8.1×10−5, 2.9×10−3, 1.5×10−2, 1.1×10−1, or 6.6×103 Pa, by changing the CO/CO2 ratio of environmental flowing gas. The Ca : Al : Fe atomic ratio of major cations of the slag (oxide) is 23 : 9 : 13, and minor constituents are Mg (~3 at%) and F (~0.3 at%). The phases and their distribution is characterized using SEM, EDS (point and mapping), Rietveld refinement of XRD. Increase of PO2 reduces fluorine dissolution, and the amounts of dissolution are higher than 1 mg/L when processed at PO2<10−2 Pa, but are about or less than 0.5 mg/L when processed at PO2>10−2 Pa. These results indicate that the low PO2 processed slag has large fraction of low density Mayenite (Ca12Al14O33) with most of fluorine in its sparse crystal structure, and liberates large amount of fluorine. On the contrary, the high PO2 processed slag is mainly composed of high density Brownmillerite (Ca2AlxFe2−xO5), and the fluorine exists as CaF2 particles not to liberate fluorine easily. It is concluded that the fluorine dissolution is strongly governed by the amount of phases, the density and empty space of each crystal structure to accommodate fluorine, and their distribution.
  • Carbon Recycling for Reduction of Carbon Dioxide Emission from Iron-making Process

    pp. 181-185

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    DOI:10.2355/isijinternational.50.181

    A new energy transformation concept based on carbon recycling, called the Active Carbon Recycling Energy System, ACRES, is discussed. In this system, hydrocarbons are regenerated from carbon dioxide by using a heat source that does not emit carbon dioxide, allowing the hydrocarbons to be re-used cyclically as energy carrier media. The thermodynamic feasibility of ACRES is compared with that of a hydrogen energy system. Carbon monoxide has a higher energy density than hydrogen and is highly compatible with conventional chemical, steel, and high-temperature manufacturing processes. Thus, it is a suitable carbon medium for ACRES. A high-temperature gas type nuclear reactor is a good power source for ACRES. The combination of the nuclear reactor and ACRES with carbon monoxide is expected to form the basis of a new iron-making process that has low carbon dioxide emissions.
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