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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 82 (1996), No. 8

  • Gas Velocity Distribution for a Confined-type Gas Atomization Nozzle

    pp. 635-640

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    Gas flow characteristics for a confined-type gas atomization nozzle have been studied to design properly the fine powder atomization nozzle through increasing the gas jet speed in the pulverizing region. Before flowing downward vertically, discrete gas jets spouted from the atomization nozzle converge around the geometrical focus, where the vertical gas velocity in the center portion reaches a saturation. The gas velocity distributions on the horizontal plane are influenced by apex angle and protrusion length. When the apex angle is less than 50°, the effect of protrusion length on the velocity distribution is relatively small. Whereas, in case of larger apex angle than 65°, the protrusion gives interference of suppressing the gas flow under the condition that sufficient suction pressure is formed at the melt orifice. The gas velocity distribution around the geometrical focus is not affected by changing the impinging gas pressure at the same mass flow rate. In order to increase the gas velocity in the pulverizing region, adjacent arrangement of gas jets to the melt delivery tube tip is important because of much reduced energy loss.
  • Coke Degradation Mechanism and Suppression Measures during High-rate Pulverized Coal Injection

    pp. 641-646

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    The coke degradation mechanism was considered in order to suppress the fine coke increase during high-rate pulverized coal injection. The following conclusions were derived as a result of model experiment.
    (1) The reasons why the fine coke increases at the tuyere level during high-rate pulverized coal injection are the degraded coke surface existence because of the lack of gasification by O2 and the increase of gasification by CO2 in the raceway.
    (2) The use of high reactive coke is effective in order to suppress the coke degradation.
    (3) The permeability in the lower part of the blast furnace is controlled by the fine coke amount at the bosh level during high-rate pulverized coal injection. The decrease of load on the coke bed is effective in order to suppress the coke degradation at the bosh level.
    (4) In order to apply the model experimental results to the actual blast furnace, it is necessary to consider the effect of coke strength at room temperature and the decrease of coke surface strength after gasification during the use of high reactive coke.
  • Development of Measuring Method and a Fixed Quantity for Wetted Area between Liquid and Particles in a Packed Bed for Ironmaking

    pp. 647-652

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    Evaluation of wetted area between liquid and particles in a packed bed for ironmaking is quite important for an analysis of transport phenomena, however, measurement of wetted area in the metallurgical packed bed process has never been reported in the literature. Therefore, we have studied a novel method for the measurement of the wetted area under the same wettability conditions as an ironmaking process. In this experiment the contact angle between liquid and particles in the packed bed was controlled to be from 103° to 121°, and a sheet of regularly arranged, hemispherical convexity was lined in the inner wall of the packed bed for eliminating wall effect. Liquid with dye was uniformly supplied to the packed bed, then the particles were dried after being reached steady state, to estimate the dyed area on the surface of all the particles. Wetted area was measured at various conditions of contact angle and liquid flow rate, and then a new empirical formula for the wetted area was related to dimensionless groups of Reynolds number, Froud number, Weber number and non dimensional surface tension.
  • Reduction Behavior of Sinter with Medium and Low Temperature Carbonized Coke in Mixed Bed

    pp. 653-658

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    Modification of present coke oven and development of different type of coking process are investigated by many institutions. In such a trend, much attentions are paid to lower the production cost by decreasing the carbonization temperature.
    In this study, Medium Temperature Carbonized Coke (MTCC) or Low Temperature Carbonized Coke (LTCC) and sinter in mixed bed were served to heating-up reduction experiments. Experimental conditions were as follows: heatingup rate was 10°C/min from 200°C to 1450°C and gas composition was Ar-25.5%C0-4.5%CO2.
    Coke carbonized at lower temperature has generated larger amount of H2 and had much effects on reduction. So H2 from LTCC had larger effects on reduction both directly and indirectly than MTCC.
    In the temperature range from 600°C to 900°C, gasification reaction of high reactive carbon from carbon deposition reaction below 600°C increased the reduction potential of gas and promoted the reduction reaction.
    In temperature range from 900°C to 1250°C, not only the reduction by generated H2 directly, but also gasification and reduction were promoted simultaneously by mutual activation phenomenon between gasification and reduction.
  • The Rate of Hot Metal Desulfurization by Na2O-CaO Flux

    pp. 659-664

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    Study was made on the effect of Na2O content and phosphorus content of hot metal on the desulfurization rate by Na2O-CaO mixed flux at 1623K under argon gas atmosphere.
    In the case of high phosphorus content of hot metal, the optimum Na2O content was observed. Na2O may act not only as a desulfurization material but also as a oxidizer at slag-metal interface by decomposition reaction. The optimum Na2O content appeared as a compromise between oxygen activity and sulfide capacity. Higher degree of desulfurization was obtained when hot metal of higher phosphorus content was treated. This result was ascribed to lowered oxygen activity at slag-metal interface caused by reduction of Na2O activity and suppression of Na2O decomposition by P2O5, formed by dephosphorization reaction. The highest degree of desulfurization could be achieved by Na2O-SiO, flux, and this result support the mechanism described above.
    Through the experiment, large amount of unknown sulfur was observed and the rate of unknown sulfur increased with increase in Na2O content in flux. To reveal this reaction mechanism, coupled reaction model calculation was performed. Although no evidence of gas formation at slag-metal interface could be obtained directly from this analysis, thermo-dynamic considerations suggested the possibility of SO2 gas formation reaction in slag bulk phase.
  • Pressure Distribution around the Melt Delivery Tube for a Confined-type Gas Atomization Nozzle

    pp. 665-670

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    Gas flow characteristics in the gas atomization have been studied to design atomization nozzles for fine powder production preventing blockage. A conical jet flow is formed for the confined-type atomization nozzle that has a melt delivery tube protruding into center portion and discharges discrete gas jets toward the geometrical focus. Converging around the geometrical focus, the gas jet flows vertically downward with making vortices and reverse gas flow beneath the melt orifice. The stability of atomization is determined by the pressure at the melt orifice and vortices' size, which vary with protrusion length of melt delivery tube. Molten metal fed out of the melt delivery tube spreads into the conical gas flow region, and is pulverized near the geometrical focus. When the protrusion of the melt delivery tube is insufficient, horizontal flow of molten metal along the end of melt delivery tube is formed, resulting in the blockage of the nozzle. In case the atomization nozzle has larger apex angle, upward gas flow along the outer surface of the tube is observed in spite of suction pressure at melt orifice, leading to an unstable gas flow due to the adhesion and growth of molten metal droplets. The suction pressure at melt orifice can be explained in terms of Bernoulli's equation.
  • Effect of Method of Ca Treatment on Composition and Shape of Non-metallic Inclusions

    pp. 671-676

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    A major objective of calcium treatment in steelmaking is to improve product performance through inclusion modification. In order to establish inclusion modification, shape and composition of inclusions were monitored using SEM during and after addition of calcium wire into 2kg heats of steel under the various conditions of Ca consumption, Ca addition pattern and flux composition. From experimental studies of calcium treatment, the mechanism of modification of calcium aluminate and formation of CaS and MgO in inclusions was discussed from the thermodynamic point of view. The results obtained are summarized as follows.
    (1) Calcium addition pattern affected composition variation of inclusions, although it had no effects on relation between [Ca] and (%CaO) in inclusions. Apparent equilibrium constant log (Kcao) was approximated to be -6.4 by thermodynamic calculation.
    (2) CaS in inclusions were observed under the conditions of high Ca consumption, high [S] and low (SiO2) in flux.
    (3) [Mg], increased by reduction of MgO in flux or crucible after Ca addition, reacted with calcium aluminate and formed MgO in inclusions. Critical condition for formation of MgO was represented with relation between [Ca] and [Mg].
    x

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  • Surface Brightness of Stainless Steel Strip by Axially Ground Rolls in Cold Rolling

    pp. 677-682

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    Recently, to improve productivity of stainless steel strip, development of cold rolling technology in tandem mill is required.
    Previous research works, however, show that a surface brightness of strip decreases due to increase of micro-defects such as oil pits caused by high speed rolling with large diameter work rolls. In this paper, to obtain high brightness surface in cold rolling, a new method for decreasing micro-defects by "Axially ground rolls", which are finished by grinding to axial direction instead of conventional grinding to circumferential direction, is proposed.
    Main results are as follows:
    (1) The surface brightness of stainless steel strip is significantly improved by rolling with axially ground rolls.
    (2) This new rolling method has an effect to decrease the oil pits and the imprint of roll asperity on strip surface due to relative slip between roll and strip.
    (3) Although a rolling friction is increased by this method, an occurrence of seizure is restrained.
  • Influences of Cold Rolling and Annealing on Orientations of Recrystallization Nuclei and Grain Growth in Invar Alloy

    pp. 683-688

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    Fe-36Ni(Invar) steel sheet is used for shadow masks in color cathode ray tubes. For using shadow masks, it is necessary to obtain a low yield strength, high r- value and excellint etching property after annealing. So, nuclei orientations of recrystallization, and grain growth of invar alloy were studied by transmission electron microscopy (TEM). The results are as follows:
    (1)In case of cold rolling with light cold rolling reduction and annealing, nuclei orientations of recrystallization are mainly (011) and (011) nuclei orientations have matrices of (011) orientation.
    (2)In case of cold rolling with heavy cold rolling reduction and annealing, nuclei orientations of recrystallization are mainly (011) and (011). Both orientations have matrices of (011) orientation.
    (3)The grains of (011) orientation in sample with heavy cold rolling reduction are increased with progressing annealing.
  • Friction and Wear Properties of C/C and Ceramics Partircle Dispersed C/C Composites in a Zinc Plating Bath

    pp. 689-694

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    Friction and wear properties of C/C and ceramic particle dispersed C/C composites sliding against sialon ceramics were investigated in a zinc plating bath.
    The results are summarized as follows:
    (1) SiC particle dispersed C/C has the highest friction coefficient and the lowest wear resistance. BN particle dispersed C/C has lower friction coefficient but lower wear resistance than C/C composite. Y2O3 particle dispersed C/C has the lowest friction coefficient and the highest wear resistance.
    (2) Dispersion of some ceramic particles into C/C composite was useful in improving its friction and wear properties, although some dispersions had the oppsite effect.
    (3) Dispersion of ceramic particles with higher hardness than dross but lower hardness than sialon ceramics into C/C composite was found to improve both the fricition and wear properties of the composite.
  • Effect of Undissolved Cementite and Grain Size on Bending Strength of the Carburized and Induction Hardened Steel

    pp. 695-700

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    A new heat treatment, where induction hardening is carried out after carburizing, was studied to increase the bending strength of automotive differential gears. The modified JIS SCM822 grade was prepared and was cold-forged to form the differential gears, which were plasma-carburized, induction-hardened and low-temperature-tempered. The bending test was carried out using an automotive differential unit.
    In the hardened case, massive hyper-eutectoid cementite, which was produced during the carburizing and remaind during the induction heating, was observed along the austenite grain boundaries. Although the austenite grain was refined by induction heating, bending strength decreased with an increase in the amount of undissolved cementite. At the fracture origin, an intergranular fracture containing undissolved cementite on its surface was observed.
    These results indicate that the increase of the massive undissolved cementite decreased the strength between the austenite grain boundaries, and then diminished the contribution of grain size refinement to the increase of bending strength.
    The formation of hyper-eutectoid cementite during carburizing should be minimized, since this cementite cannot be fully dissolved by induction heating.
  • The Effect of Alloying Elements on the Mechanical Properties of the Fe-36% Ni Alloys

    pp. 701-706

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    The effect of single addition (C, B<0.041%, others<2.0%) of various minor alloying elements such as C, Si, Mn, Ni, Cr, Co, Nb, Ti, V, Zr, and B in the Fe-36%Ni alloy on the mechanical properties, especially 0.2%proof stress and Young's modulus, was investigated. The results obtained are as follows:
    (1) Cr is the only element which decreases the 0.2% proof stress in the Fe-36%Ni alloys.
    (2) There is a good relation between the magnitude of the effects of alloying elements on the proof stress and the radius of atoms of alloying elements.
    (3) The addition of C, Si, Ni, Co decreases the Young's modulus, while the addition Mn, Cr, Nb, Ti, Zr, B increases it. The effect of such alloying elements except B on the Young's modulus can be classified by the period and group in the periodic table.
    (4) From the view of the number of valence electrons and the texture, the origin of the effect of alloying elements on the Young's modulus has been discussed.
  • Ductile-Brittle Transition Evaluation of Laser Welded Steel Metal by Means of Small Specimen Impact Test

    pp. 707-712

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    Small specimen impact tests using miniaturized specimens with 1.5, 1.0 and 0.7mm square and 20mm length were carried out to evaluate the toughness of a laser welded metal which has been difficult to estimate accurately by the standard full or half size specimens due to a fracture path deviation from weld metal to base metal. Three kinds of steel welded metals with a quite same chemical composition were examined: submerged arc welded metal (Type S), laser welded metal on the submerged arc welded metal (Type L), and the submerged arc welded metal subjected to a thermal cycle with water quenching after welding (Type T).
    A reliable ductile-brittle transition temperature (DBTT) and fracture appearance transition temperature (FATT) based on the data partitioning method were obtained by using the miniaturized specimens with keeping correlations to those of the standard size specimen. The DBTTs and FATTs given by each size specimens showed a simple linear correlation, though lower DBTT and FATT were given by smaller specimens. The laser welded metal composed of an extremely fine acicular ferrite structure was estimated to have such an excellent toughness as 168K DBTT in conversion to the standard size specimen. The small specimen impact tests using miniaturized specimens were established as a useful method to estimate toughness in a small and narrow region of welded steels.
  • Effect of Thermal Ageing on the Complex Creep Deformation Behaviour of Carbon Steel and Its Inherent Creep Strength

    pp. 713-718

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    Complex creep deformation behaviour which reveals several peaks and local minima in creep strain rate has been investigated on the carbon steel taken from tube for boilers and heat exchangers (JIS STB 410) and discussed from a view point of "Inherent Creep Strength". Complex creep deformation phenomena observed on the un-aged steel has been disappeared by ageing prior to creep test. Common creep strain dependence of creep strain rate has been observed for both steels with and without prior ageing, and explained by both an inherent creep strength and an increase in stress during constant load creep testing. It has been concluded that complex creep deformation behaviour is caused by decrease in creep strength due to microstructural change and advent of inherent creep strength.

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