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

ISIJ International Vol. 34 (1994), No. 6

  • Cold Model Experiments for Developing a New Velocimeter Applicable to Molten Metal

    pp. 461-467

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    DOI:10.2355/isijinternational.34.461

    The objective of this study is to develop a new velocimeter utilizing the hydrodynamic drag of a sphere which is applicable to velocity measurement of high temperature molten flow. Although the drag coefficient CD is required for the evaluation of drag, has not been studied for a fluid flow with high turbulence intensity and turbulence scale typical of ironkaming and steelmaking processes. Therefore CD was measured using a sphere placed in a wind tunnel equipped with a turbulence generator and correlated as a function of Reynolds number Re and turbulence intensity Tu. The effect of turbulence scale was found to be negligible.
    A new velocimeter using a sphere was developed and its applicability was examined in cold models such as an air jet of Tu≅2%, a water jet of Tu=30-40%, and a water-air bubbling jet of Tu=40-50%. Agreement between the results obtained by the present velocimeter and those obtained by existing velocimeters, for example, laser Doppler velocimeter, hot wire anemometer and pitot tube was quite reasonable. Velocity measurement in a mercury-air bubbling jet also revealed the possibility of the usage of the present velocimeter for high temperature and high turbulence molten metal flow in metallurgical reactors.
  • Development of Firing Schedule to Prepare Composite Pre-reduced (CPR) Pellets

    pp. 468-475

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    DOI:10.2355/isijinternational.34.468

    The newly developed CPR (composite pre-reduced) pellet, consists of a metallised sponge iron core encased in a partially reduced and sintered iron oxide shell. Such CPR pellets are obtained by firing a composite iron ore-coal char pellet prepared by two stage pelletisation. Iron ore and coal char mixed in desired proportion are pelletised to form core pellet which is further coated with iron ore only during second stage pelletisation. Such CPR pellets being developed mainly to utilise plant waste for preparing a cheap metallised feed for use in blast furnace.
    When composite iron ore-coal pellets are fired in air, CO(g) generated in core reduces not only iron oxide in-situ but also the outer shell partially. The continued firing reoxidised the reduced pellet unwillingly. The present paper gives a basic study of firing time and temperature to study the reduction behaviour of composite pellets followed by microstructural examination of fired and reduced pellets. The firing schedule, thus, developed was tested to prepare bulk quantity of CPR pellets having 28.6% metallic iron, out of total 76.6% Fe in the pellet rendering nearly 48% overall reduction degree together with sufficient strength (120±20 kg) which may enable its use as blast furnace feed.
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  • Combustion Behavior of Pulverized Coal in Tuyere Zone of Blast Furnace and Influence of Injection Lance Arrangement on Combustibility

    pp. 476-483

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    DOI:10.2355/isijinternational.34.476

    Hot model experiments on pulverized coal injection were carried out to understand the combustion mechanism of pulverized coal injected into the blast furnace. The combustion behavior of the pulverized coal was directly observed by a high speed camera, and the temperature distribution of combustion flame was analyzed by an image processor. According to the results, the combustion flame was not uniform across the cross section of the blow pipe and tuyere, and the fluctuation of the flame was observed. Then, it was found that the dispersion of the pulverized coal had a great role on combustibility owing to the rapid consumption of oxygen by the combustion of volatile matter. Moreover, it was confirmed that the arrangement of injection lance influenced on the combustion efficiency, since the dispersion of coal particles was enhanced by the double lance arrangement. On the basis of the above results, the dispersion of coal particles was analyzed by the calculation of the average distance of individual coal particles to easily evaluate the effect of the injection lance on combustibility.
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  • Deoxidation Equilibrium of Hafnium in Liquid Iron, Nickel and Iron-Nickel Alloys

    pp. 484-490

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    DOI:10.2355/isijinternational.34.484

    The deoxidation equilibrium of hafnium in liquid iron, nickel and iron-nickel binary alloys has been studied at the temperature ranges from 1873 to 1973 K using hafnia crucibles.
    The effect of temperature on the equlibrium constant of the deoxidation reaction in liquid iron with hafnium was found to be:
    log KHf(Fe)(=aHf·aO2)=-35840/T+11.39
    while the deoxidation product in iron was expressed as follows:
    logK'Hf(Fe) (=[%Hf] [%O]2)=logKHf (Fe)+5.85(2[%Hf]+11.1 [%O])    (0.01-0.1 mass% Hf)
    The temperature dependence of the equilibrium constant of the deoxidation in liquid nickel was determined as the expression.
    log KHf(Ni)=aHf·aO2= –33800/T+8.40
    in the meantime, the deoxidation product in nickel was given by:
    log K'Hf (Ni) (=[%Hf] [%O]2)=log KHf (Ni)+9.75 (2 [%Hf]+11.1 [%O])     (0.002-0.1 mass% Hf)
    The deoxidation of iron-nickel binary with hafnium was measured over whole concentration ranges of the binary system at temperatures of 1873 and 1923 K. The activity coefficients of oxygen and hafnium in the binary based on pure liquid iron were given by the following expression:
    log fO(Fe)Ni+ 1/2 log fHf(Fe)Ni=0.005 [%Ni]
    below 60 mass% of nickel, while the activity coefficients on the nickel basis were expressed by:
    log fO(Ni)Fe+ 1/2 log fHf(Ni)Fe= –0.025 [%Fe]
    below 10 mass% of iron.
  • Evaporation Rate of Zinc in Liquid Iron

    pp. 491-497

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    DOI:10.2355/isijinternational.34.491

    An investigation of evaporation rate of Zn from liquid iron has been undertaken at 1873 K by impinging Ar onto the surface of inductively melted iron containing Zn. The evaporation rate of Zn was found to be first order with respect to Zn content in the metal. No measurable effects of the Ar flowrate and carbon or sulfur content in the metal on the rate were observed with Ar flowrate above 5l/min, but the rate was strongly affected by stirring condition of the melt. It was, therefore, concluded that the evaporation rate of Zn was controlled by the liquid phase mass transfer under the condition. The mass transfer coefficient in the liquid phase was estimated to be 0.032 cm/s at 1873 K. The removal rate of Zn or other tramp elements in the conventional practical operations or vacuum treatment was also discussed based on the present result.
  • Effect of Length of Vertical Section on Inclusion Removal in Vertical Bending-type Continuous Casting Machine

    pp. 498-506

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    DOI:10.2355/isijinternational.34.498

    Steel sheet defects caused by inclusions are a serious quality problem. The vartical bending-type continuous casting machine is said to be more effective than the bending-type continuous casting machine for removal of inclusions, but the most effective length of the vertical section for inclusion removal has not been studied. In this paper steel sheet defects are investigated, and flow of molten steel in the mold is evaluated using the k-ε turbulence model. The most effective vertical section length for inclusion removal was found to be 2.5 to 3.0 m, irrespective of casting conditions and inclusion size.
  • Description of Transfer and Deposition during PLD of Thin Ceramic Films

    pp. 507-515

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    DOI:10.2355/isijinternational.34.507

    Thin films of ceramic materials (Al2O3, ZrO2) for technical applications are deposited at different laser parameters (wavelength, fluence, mode of operation) and processing variables (processing gas pressure and composition, rf bias, distance target-substrate). The material transfer is studied by high-speed photography and emission spectroscopy as a function of laser parameters and processing variables. Time-resolved (0.01-10 μs after the beginning of the laser pulse) measurements of the geometry, dynamics, velocity of the vapour/plasma front (max. v=60000 m/s), composition, ionization state and electron-temperature (40000-140000 K) are obtained. The morphology, structure and composition of the films are investigated by SEM, XRD and EDX. The deposited films show a broad variety of different structures which correspond to sputtered films on heated substrates. The results are discussed in view of applications. The overall view of experimental results allows the description of the material transfer which is related to the properties of the films deposited.
  • Continuous Cooling Transformations in Martensitic Stainless Steels

    pp. 516-521

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    DOI:10.2355/isijinternational.34.516

    To gain further insight into the nonequilibrium transformations produced by non-isothermal cycles in martensitic stainless steels, the study experimentally determined the CCT diagrams of four Cr13 and CrMoV14 type steels, which were cooled from two different austenizing temperatures. The results show that the changes in the chemical composition of the austenite, which are caused by the carbide dissolution and precipitation processes, have a considerable effect ont the phase transformations that occur during continuous cooling.
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  • Effect of Pre-cold-working on Diffusional Reversion of Deformation Induced Martensite in Metastable Austenitic Stainless Steel

    pp. 522-527

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    DOI:10.2355/isijinternational.34.522

    Metastable austenitic stainless steels undergo deformation induced transformation to bcc martensitic structure during cold-working. The martensite induced reverts to austenite at a relatively low temperature and this leads to the formation of ultra fine austenite grains of less than 1 μm in diameter. In this paper, the effect of pre-cold-working on the morphology of reversed austenite was investigated by means of transmission electron microscopy and tensile test. The alloy used is an Fe-18.08%Cr-8.65%Ni alloy. Since this alloy has metastable austenitic structure at room temperature, it almost transforms to lath-martensite by 50% cold-rolling. Further cold-rolling above 50% deforms transformed martensite itself, and results in the formation of dislocation-cell structure instead of lath-martensitic structure.
    The diffusional reversion of deformation induced martensite takes place at around 900 K and precold-working to lath-martensite not only promotes the reversion but also gives a large effect on the microstructure of reversed austenite: Reversed arstenite is characterized, in a specimen with 50% pre-cold-working, by the stratum structure of austenite laths and blocks, which looks like a lath-martensitic structure, while in a specimen with heavy pre-cold-working, by the structure of fine equiaxed grains. On the discussion of grain boundary strengthening in the former case, 0.2% proof stress depends not on the lath size but on the block size of reversed austenite.
  • In situ Solidified Hot Tensile Test and Hot Ductility of Some Plain Carbon Steels and Microalloyed Steels

    pp. 528-535

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    DOI:10.2355/isijinternational.34.528

    The metallurgical structure and hot ductility of continuous casting steels mainly depends on temperature history after solidification. The usual hot ductility tests do not reproduce the actual temperature path of continuous casting steels during current production processes because the deformation is achieved on tensile specimens reheated from room temperature. A laboratory hot tensile test on in situ solidified sample has been achieved to investigate the thermomechanical properties of steels directly after solidification, without a cooling step down to room temperature. The basic principle of this test consists in melting a sample of the studied steel to obtain a notched tensile specimen. The tensile test is then achieved at the required temperature after a rate controlled cooling. In comparison with other tests, the notched specimens offer a great interest in producing very depressive hydrostatic stress, significant triaxiality and so, favouring microvoid growth and coalescence. The late step being quickly reached when the specimen notched zone has a poor ductility.
    Experiments in the range of 700 to 1100°C are achieved on a set of C-Mn steels and Nb-V microalloyed steels to establish influence of the temperature path on hot ductility. The results show some significant differences on the ductility curves between tests performed on in situ solidified specimens. It is found that the differences are mainly related to segregation distribution, austenite grain size, ferrite formation kinetics and dynamic precipitation of niobium/vanadium carbonitrides.
  • Some Aspects of Tempered Martensite Embrittlement in 3Cr-Mo-V Steel

    pp. 536-540

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    DOI:10.2355/isijinternational.34.536

    The paper deals with the tempered martensite embrittlement phenomenon in the low-carbon low alloy 3Cr-Mo-V steel tempered in the temperature range of 523-773 K.
    To determine the tempered martensite embrittlement the Charpy-V test at the temperature 223 K was performed. The methods of light microscopy, transmission electron microscopy (TEM) scanning electron microscopy (SEM) and hardness testing (HRC) were applied.
    In temperature dependance of CVN the minimum at 623 K was found to exist. The mechanism of transgranular cleavage corresponding to a transgranular mode of the tempered martensite embrittlement was found out during the failure. In the microstructure of the quenched states the retained austenite around the martensitic laths as well as the fine carbide particles inside the laths were observed. This can be considered as a cause of the transgranular mode of the tempered martensite embrittlement. The difference in grain size (0.1–0.8 mm) affects the tempered matensite embrittlement of steel only weakly.

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