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

ISIJ International Vol. 35 (1995), No. 12

  • Mean Velocity and Turbulence of Water Flow in a Cylindrical Vessel Agitated by Bottom Air Injection

    pp. 1431-1437

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    DOI:10.2355/isijinternational.35.1431

    Cold model experiments based upon aqueous systems were carried out to study the mean velocity and turbulence intensity in a metallurgical reactor agitated by gas injection. The axial and radial velocity components in a cylindrical bath stirred by bottom air injection through a centered single-hole bottom nozzle were measured using a two-channel laser Doppler velocimeter. The spatial mean kinetic energy for mean flow, i.e., circulating flow, km, v, and that for turbulence fluctuations, kt, v, were determined. It was found that km, v and kt, v depended on 0.60 and 0.72 power of the injected gas flow rate, respectively. The energy supplied by the injected gas into the bath therefore was more consumed to maintain turbulence motions than to do mean flow as the gas flow rate increased. A spatial mean velocity Vm, v=(2km, v)1/2 and a spatial mean turbulence intensity Tuv=(kt, v/km, v)1/2 were newly introduced. The spatial mean turbulence intensity was above unity under the present experimental conditions. These quantities eere correlated as functions of the gas flow rate, bath depth and bath diameter.
  • The Estimation of Convective Heat Transfer Coefficients between a Spherical Particle and Fluid at Lower Reynolds Number

    pp. 1438-1443

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    DOI:10.2355/isijinternational.35.1438

    Convective heat transfer coefficients between a spherical particle and fluid for two flow systems (solid-liquid, -gas) were measured and were calculated with computer simulation at relatively low Reynolds numbers, and the applicabilities of empirical equation, for example, Ranz & Marshall's equation, to the estimations of convective heat transfer coefficients of these systems were investigated.
    An aluminum sphere, 32 mm in diameter, which was kept at about 60°C initially for solid-water system and at 30°C for solid-air system, was dipped into each fluid flow and the relationships between the Nusselt number, Nu, and the Reynolds number, Rep, were obtained. Convective heat transfer coefficients were calculated with computer simulation and were compared with experimental results.
    For solid-water systems, convective heat transfer coefficient was as large as the value estimated by Ranz & Marshall's equation for the range 300<Rep, but for lower Reynolds number region, heat transfer coefficients were slightly larger than the value calculated by Ranz & Marshall's equation. Although Reynolds number measured in solid-air system was small, heat transfer coefficient was as large large as ones calculated with following empirical equation.
    Nu=2.0+0.6Pr1/3Rep1/2+0.43(PrGr)1/4
  • Effect of Mixed-grinding on Reduction Process of Carbonaceous Material and Iron Oxide Composite

    pp. 1444-1451

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    DOI:10.2355/isijinternational.35.1444

    Mixing state and structure of composite significantly influence reduction kinetics of carbonaceous materials/iron oxides mixture. The present study reports the effect of mixed-grinding of the mixture on the reduction of the composite by indirect heating. Mixtures prepared by different combinations of hematite and graphite reagents, iron ores and metallurgical coke were mixed-ground by two types of ball mills. TG-DTA measurements for the composites were carried out under different heating patterns and total pressures. Reaction temperature remarkably decreases by the mixed-grinding operation in any combinations of materials and total pressures. A catalytic effect of iron produced during reaction is observed when using graphite reagents but seems to be inhibited when using metallurgical coke. A simple kinetic analysis suggests that gasification of carbon is promoted through enhancement of the catalytic effect and/or change of rate limiting step from the gasification of carbon to the direct reactions between iron oxides and carbon.
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  • Mechanism of Decarburization in RH Degasser

    pp. 1452-1458

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    DOI:10.2355/isijinternational.35.1452

    Decarburization behavior in RH-Degasser was discussed on the basis of a comparison between simulation and operational result. In the simulation, decarburizations through Ar bubbles, CO bubbles and a free surface were considered. Furthermore, the difference in carbon concentration between the melt in the ladle and that in the vacuum vessel was also considered. As a result of the comparison, it was found that the contribution of the decarburization as CO bubbles to the total amount of decarburization is great. In order to step up the decarburization rate, increasing the circulation flow rate is effective. That is accomplished by reducing the atmospheric pressure in the vacuum vessel, injecting Ar at a deeper position, and increasing the Ar gas flow rate and the snorkel diameter. Increasing oxygen concentration is expected to be effective to promote decarburization.
  • Precipitation Behavior of Fe-Al-O Inclusions under Unidirectional Solidification of Fe-30mass%Ni Alloys Saturated with CaO-Al2O3 Slags

    pp. 1459-1467

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    DOI:10.2355/isijinternational.35.1459

    The inclusions composition, size, morphology and distribution in an Fe-30mass%Ni alloy were studied as a function of the initial solute contents, which were controlled by the CaO-Al2O3 slags, and of the solidification parameters. The Al2O3-rich Al2O3-CaO inclusions were observed at the initial contents of Al=300-400 and O<6, while the FeO and the FeO-rich FeO-Al2O3 inclusions were observed at the initial contents of Al=2-6 and O=20-130 on mass ppm base. The FeO-rich FeO-Al2O3 inclusions were located in later stage of solidification in the intercellular or the interdendritic region. And most of the Al2O3-rich Al2O3-CaO inclusions were located in later stage of solidification, but some distributed in the intercellular or the interdendritic region. The dependence of the composition and the size of these inclusions on the cooling rate was not observed. The precipitation of FeO and Al2O3 was discussed by using the solute segregation models.
  • Precipitation Behavior of Al-Ti-O-N Inclusions in Unidirectionally Solidified Fe-30mass%Ni Alloy

    pp. 1468-1476

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    DOI:10.2355/isijinternational.35.1468

    The composition, size, morphology and distribution of the Al-Ti-O-N inclusions in the unidirectionally solidified Fe-30mass%Ni alloy were studied under the different solidification conditions as a function of solute contents which were controlled by using the CaO-Al2O3-TiOx-TiN slags. The TiOx-Al2O3 inclusions were observed in the initial contents of Ti=20-110 and N=60-130, while the TiN inclusions were observed in the initial contents of Ti=700-1100 and N=90-150 on mass ppm base. The Ti content and the size of the TiOx-AL2O3 inclusions increased with an increase in the cooling rate. The TiOx-rich TiOx-Al2O3 and the TiN inclusions were located in last stage of solidification in the interdendritic and the intercellular region. The precipitation of TiN and TiOx-Al2O3 inclusions was discussed by using the segregation models.
  • Effect of the Composition of Oxide on the Reaction between Oxide and Sulfur during Solidification of Steels

    pp. 1477-1482

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    DOI:10.2355/isijinternational.35.1477

    In order to elucidate the reaction between oxides and sulfur in molten steel during solidification, the effect of composition of oxides in continuously cast steels has been observed and analyzed. The results obtained are as follows.
    CaO-SiO2-Al2O3 and CaO-SiO2 oxides are observed in low Al content steel (Al=0.005 mass%) and CaO-Al2O3 oxides are observed in high Al content steel (Al=0.031 mass%). CaO-SiO2-Al2O3 and CaO-Al2O3 oxides have higher S content than CaO-SiO2 oxides. High S content in oxides was obtained with high liquid fraction and high sulfide capacity of oxides at the solidification temperature of the steel.
    Because desulfurization by oxides which have high sulphide capacity and high liquid fraction occurs during solidification, S segregated in the center of slab is absorbed by the oxides.
  • Effects of Cooling Rate on Growth Behavior of Austenite Phase during Peritectic Reaction in Iron-Carbon Binary Alloy

    pp. 1483-1488

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    DOI:10.2355/isijinternational.35.1483

    A model experiment of peritectic reaction during cooling of an iron-carbon binary alloy is performed by using a solid-liquid diffusion couple method, and the effects of cooling rate on the growth behavior of austenite phase is investigated. A numerical simulation of the growth process of austenite phase during the peritectic reaction also is performed by using the method previously proposed by the Present authors. Both the experimental and simulated results show that austenite phase formed at δ-ferrite/liquid interface grows into both δ-ferrite phase and liquid phase during cooling and that its growth rate increases with increasing cooling rate.
  • Lateral Roll Force in Asymmetrical Rolling and Its Influence on Rolling Stability

    pp. 1489-1493

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    DOI:10.2355/isijinternational.35.1489

    Roll system forces and their relationships with each other during asymmetrical rolling have been studied for deciding the direction and magnitude of the lateral roll forces that could affect the rolling stability, rigidity and loading conditions of a rolling mill. Results obtained from experiment is in agreement with theoretical analysis which indicates that kinematical asymmetry in strip rolling creates lateral roll forces and magnitude of the forces is dependent on the angle of force inclination affected by roll speed mismatch ratio i, pass reduction ε and applid strip tensions T. It has been found that, for optimizing mill design and operation, front tension (or tension difference) in asymmetrical rolling should not exceed the level which is required to preserve rolling stability. The appropriate value of i should not be higher than 1.28 to avoid bearing overloading whilst still retain roll force reduction characteristics of cross shear rolling.
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  • Coating Adhesion and Interface Structure of Galvannealed Steel

    pp. 1494-1501

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    DOI:10.2355/isijinternational.35.1494

    The effect of Fe content of coating and Al content in galvanizing bath on the coating adhesion of galvannealed ultra-low carbon IF (Interstitial Free) steel sheet was investigated using a lap-shear method. The exposed steel surface after removal of the galvannealed coating with hydrochloric acid solution was examined so as to discuss the relation between the geometrical shape of the coating/steel interface and the coating adhesion.
    Galvannealed coating exhibits the minimum adhesion strength in the region of 8-10 mass% Fe, which was thought to relate to the extinction of η-phase. Further increment of Fe content raised the coating adhesion, and high Al addition to galvanizing bath also raised the minimum adhesion strength of the coating around 9 mass%Fe.
    The steel surface exposed with the acid was even and smooth for the low Al content coatings with 8-9 mass% Fe, while that was uneven or rugged for the coatings with relatively high coating adhesion. The further increment of Fe content in the coating developed Γ-phase to form ledges with the pitch of about 100 nm at the coating/steel interface, and the increase of Al in galvanizing bath resulted in uneven alloy formation, particularly discontinuous formation of Γ-phase to cause uneven erosion of the steel surface and form the ruggedness with the pitch comparable with α-Fe grain size at the coating/steel interface. The ruggedness with the pitch comparable with α-Fe grain size was thought to result from the dependency of Fe-Zn interaction on the microstructure of the base steel. It was assumed that the coating adhesion is governed by the ruggedness of the coating/steel interface and formation of Γ-phase contributes to increasing the adhesion strength.
    x

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  • The Growth of Cellular δ Phase in a 69Ni-15Cr-8Fe-6Nb Alloy

    pp. 1502-1508

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    DOI:10.2355/isijinternational.35.1502

    The growth behavior of cellular δ phase in Ni-base alloy, modified Inconel X-750 type alloy (X-750M), was investigated mainly using optical and transmission electron microscopy.
    The values of n in the Johnson-Mehl equation for the growth of the cellular δ phase in X-750M which was aged in various conditions after a solution heat treatment were estimated to be 1.69-1.84.
    The values of n in the case of X-750M which was pre-aged at 943 K for 253 ks after a solution heat treatment followed by aging in various conditions were estimated to be 1.44-1.67. The obrtained value of n suggested that the growth of the cellular δ phase in X-750M after the pre-aging treatment obeyed a volume diffusion rate control with zero nucleation rate. Transformation curves of γ+γ'' phases to the cellular δ+γ phases in the specimens aged after the pre-aging treatment were consistent with a theoretical one. The apparent activation energy for the growth of the cellular δ phase in X-750M aged after the pre-aging treatment was estimated to be 232 kJ/mol, which is nearly equal to that for diffusion of solute atoms in the alloy.
    x

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  • Thermo-mechanical Treatment of a High Nb-High V Bearing Microalloyed Steel

    pp. 1509-1517

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    DOI:10.2355/isijinternational.35.1509

    The response of a Nb-V microalloyed steel to thermal-mechanical treatment is examined. Constant strain rate, isothermal compression tests of axially symmetrical samples, to a strain of unity, are used to determine the metal's response at high temperatures. Four-stage, constant true strain rate compression of the samples is used to simulate partially the process of strip rolling. The loading is interrupted at various strains for ever shorter intervals. The resulting true stress-true strain curves are used to examine the steel's response to interrupted loading. The ability of the microalloying components to retard to dynamic restoration processes, after an interruption, is observed. Microstructural observations indicate that the most important process parameter is the total strain.
  • Mixing Enthalpy Measurements of Liquid Fe-Ti Alloys by Levitation Alloying Calorimetry and Calculation of the Thermodynamic Properties of Mixing

    pp. 1518-1522

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    DOI:10.2355/isijinternational.35.1518

    Mixing enthalpies, ΔHm, of liquid iron-titanium alloys were measured in dependence of the composition by levitation alloying calorimetry. The measurements were carried out at T=1950 K up to xTi=0.42 in the iron rich concentration range and at T=2112 K up to xFe=0.31 in the titanium rich concentration range. The mixing enthalpy was found to be exothermic. The temperature dependence of ΔHm could be described with help of the regular associate model. On the basis of an adapted function for the mixing enthalpy calculations of the excess heat capacity, the free energy of mixing, the partial free energies of mixing and the activities of iron and titanium are performed. The comparison of the calculated activities with corresponding experimental data from the literature shows a good accordance.
  • Softening and Flow Stress Behaviour of Nb Microalloyed Steels during Hot Rolling Simulation

    pp. 1523-1531

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    DOI:10.2355/isijinternational.35.1523

    The roles of softening and precipitation were investigated by means of hot torsion experiments under conditions simulating either plate or sheet rolling. Six microalloyed steels containing Nb were studied. During the first few finishing passes in the sheet rolling simulations, the mean flow stress (MFS) increased as the interpass time was decreased. Due to strain accumulation, the rate of static recrystallization (SRX) increased significantly after each pass. By taking both strain accumulation and grain refinement into account, it is shown that SRX plays a marked role under sheet rolling conditions, even at temperatures below the no-recrystallization temperature for plate rolling conditions. The accumulated or retained strain reaches the critical value required to initiate dynamic recrystallization only at the lowest entry and rolling temperatures and shortest interpass times. The kinetics of the strain-induced precipitation of NbCN under continuous cooling conditions, taking partial SRX into account, indicate that precipitation begins after 2 to 5 passes when 3 s interpass times are employed, thus reducing further softening. But when 1 s interpass times are used, most of the passes take place before copious precipitation, so that static and post-dynamic (i.e. metadynamic) softening may continue to take place. As a result, the MFS level decreases as the interpass time is shortened during the final passes. The extent of grain refinement was similar in both the sheet rolling and plate rolling simulations. The ferrite grain size is shown to depend on MFS of the final pass, and is independent of the chemical composition of the microalloyed steel.
  • Dynamic Strain Aging and the Wire Drawing of Low Carbon Steel Rods

    pp. 1532-1540

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    DOI:10.2355/isijinternational.35.1532

    The dynamic strain aging behaviour of low carbon steel wire rod was examined at room temperature to 450°C using tensile testing at strain rates of 10-4 to 10-1 s-1 The effects of temperature and strain rate on the yield stress, flow stress, UTS, fracture stress, and fracture strain were investigated in detail. In agreement with previous studies, work hardening peaks, minima in ductility, and negative strain rate dependences of the flow stress were observed between 100 and 400°C, the positions of which depended on the strain rate. A model for dynamic strain aging is employed to predict whether or not it will occur at the strain rates and temperatures involved in commercial wire drawing. For a steel containing 32 ppm N, a temperature higher than about 315°C must be attained for dynamic strain aging to occur; this is higher than the temperatures usually encountered in drawing. However, the model also predicts that if the N content is increased to 115 ppm, the minimum temperature for dynamic strain aging decreases to about 250°C, which can be attained if the die and capstan cooling are not adequate. The negative rate dependence of the flow stress attributable to dynamic strain aging is considered to promote flow localization and, therefore, to be a possible cause of wire breaks during drawing.
  • Recent Advances in the Theory of Free Dendritic Growth

    pp. 1541-1550

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    DOI:10.2355/isijinternational.35.1541

    A review of the recent advances in the theory of free dendritic growth is given. We present the basic ingredients for the currently accepted theory and summarize the main results for two and three dimensional growth. Some thousand references over the past five years have been analyzed and a selection of some two-hundred of them are classified by an index of keywords.

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