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

ISIJ International Vol. 34 (1994), No. 8

  • The γ→α Transformation in Low Carbon Irons

    pp. 615-630

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    DOI:10.2355/isijinternational.34.615

    The influence of cooling rate on transformation temperatures in low carbon irons is reviewed. Plateaux are obtained in plots of transformation temperature vs.cooling rate. Comparison with later published TTT diagrams show that these plateaux correspond with the nose of 'c' curves on TTT diagrams. This leads to a proposed TTT diagram for iron. Transformations in Fe-Ni, Fe-Cr and Fe-C alloys, and low carbon irons are reviewed. The morphology and kinetics of the individual transformations, namely equi-axed ferrite, massive ferrite, bainitic ferrite, lath (massive) martensite and twinned martensite are then discussed. A recent theoretical treatment of the equi-axed ferrite transformation is summarised. This theory indicates that the kinetics of the equi-axed ferrite transformation is controlled by growth. Correlation is made between microstructures obtained in binary alloys and those obtained in Fe-Nb-C alloys and commercial HSLA steels.
  • Effect of Serpentine on Mechanical Strength and Reducibility of Iron Ore Briquettes

    pp. 631-640

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    DOI:10.2355/isijinternational.34.631

    In the present work, the effect of serpentine addition on mechanical strength and reducibility of iron ores briquettes is investigated. The iron ore used was mixed with the serpentine and a chemical reagent of CaO to adjust the CaO/SiO2 ratio in 0.3 and 1.2. The briquettes were fired at temperatures of 1523, 1548, 1573 and 1598 K. These briquettes were reduced under an atmosphere at 60%Ns/40%CO and 60%Ns/20%CO/20%CO2, at temperature of 1173 and 823 K, respectively.
    From the experimental results, it is concluded that serpentine influences more positively on mechanical strength of briquettes when CaO/SiO2=1.2, at all firing temperatures. It was also observed that serpentine influence on reducibility is not very significant when briquettes are reduced at 1173 K, but basic briquettes have better reducibility than acid ones at this temperature. At a temperature 823 K, there is not a significant reducibility defference between basic and acid briquettes.
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  • Combustion Behavior of Pulverized Coal in a Raceway Cavity of Blast Furnace and Its Application to a Large Amount Injection

    pp. 641-648

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    DOI:10.2355/isijinternational.34.641

    For a large amount of pulverized coal injection into a blast furnace, the combustion of pulverized coal in a raceway cavity has been studied theoretically and experimentally.
    The theoretical formula, which can estimate the combustion efficiency of pulverized coal in a raceway cavity, were derived. The effects of the injecting and operation conditions on the limit of pulverized coal combustion in a raceway cavity were evaluated.
    The effect of mixing of pulverized coal with oxygen on the combustion rate of pulverized coal was studied in a combustion test using both empty and coke packed furnace. From the results, the validity of the derived theoretical equation was verified, and is was also confirmed that the making sure of a raceway depth and the adoption of a injection technique which promotes the mixing of pulverized coal with oxygen are important to promote the combustion of pulverized coal.
    Based on these discussions, the upper limit of pulverized coal injection rate in the hot blast furnace operation was estimated. To inject a large amount of pulverized coal more than the actually accomplished amount, it is necessary to increase the oxygen concentration in blast and adopt a injection technique which promote the mixing of pulverized coal with oxygen.
  • Decarburization Reaction of Molten Iron of Low Carbon Concentration with Solid

    pp. 649-656

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    DOI:10.2355/isijinternational.34.649

    A kinetic study has been made on decarburization of molten iron of low carbon and oxygen concentrations with solid oxides. Electrolytic iron was melted in a MgO, Al2O3 or CaO crucible. The mass of iron and the initial carbon concentration were varied between 137 and 627 g and between 30 and 480 ppm. The initial oxygen concentration was below 50 ppm, and oxygen for the decarburization was supplied by the crucible oxide. Experiments were made with blowing argon gas (1000 Ncm3/min) onto the melt surface at 1580°C.
    The rate of decarburization decreases in the order of MgO, Al2O3 and CaO and is almost independent of the stirring condition of the bath. For the carbon concentration >100 ppm, the rate of decarburization does not change with the initial carbon concentration. From these results, it is considered that the rate is controlled by the dissociation reaction of the oxide. For the carbon concentration <100 ppm. The rate decreases with decreasing carbon concentration. In this case, the rate is presumed to be controlled by the dissociation reaction of oxide, formation reaction of CO at crucible-melt interface and CO mass transfer. A mathematical model is proposed to explain the rate of decarburization.
  • Decarburization Reaction of Molten Iron of Low Carbon Concentration with Vacuum Suction Degassing Method

    pp. 657-662

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    DOI:10.2355/isijinternational.34.657

    A kinetic study has been made on decarburization reaction of molten iron of low carbon and oxygen concentrations with solid oxides. A new degassing method, namely Vacuum Suction Degassing (VSD) method, was applied to decarburize molten iron to an ultra-low carbon concentration. A porous oxide tube, the inside of which was evacuated, was immersed into molten iron. The initial carbon concentration was varied between 30 and 110 ppm. The initial oxygen concentration was below 50 ppm. The experiments were performed under Ar atmosphere (1.01×105 Pa). The experimental temperature was 1853 K.
    The VSD method can greatly increase the rate of decarburization of molten iron and the carbon concentration decreases to a very low value of a few ppm. The rate of decarburization with an Al2O3-SiO2 tube is higher than that with an Al2O3 tube. Increase in gas permeability of a porous tube enhances the decarburization reaction. The apparent rate constant of decarburization at the porous tube-molten iron interface with evacuation in the porous tube is about 10 times larger than that without evacuation.
  • Formation of H2S in the Bath Smelting Process

    pp. 663-669

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    DOI:10.2355/isijinternational.34.663

    With the direct utilization of coal to produce hot metal, impurities in the coal, such as sulfur, enter into the process. The amount of sulfur in each phase (metal, slag and gas) depends on the operating conditions. Trials at American Iron Steel Institute (AISI) Direct Steelmaking and Japan (DIOS) indicate that much of the sulfur enters the gas phase and the thermodynamics indicate that H2S will be the predominant specie. The sulfur can enter the gas phase by a reaction of the smelting gas with the slag or during the devolatilization and combustion of the coal. The rate of formation of H2S by the reaction of an Ar-H2-H2O gas with the slag was measured. It was found that the initial rate is dependent on PHO and the sulfur content of the slag. Under present experimental conditions the rate is controlled by liquid phase mass transport. The diffusion coefficient of the sulfur in liquid slag, assuming mass transfer control, is estimated to be 6.65×10-7 cm2/s at 1723 K which is in agreement with published values. The rate of H2O formation was also measured when gas was bubbled through the slag. A rate expression was derived for desulfurization with a foamy slag. The sulfur content of the slag as a function of time was calculated from the derived rate expressions for different circulation rates of slag from bulk to foam. The sulfur content of the bath smelting gas was estimated based on experimental results. Some coal samples were devolatilized and partially combusted with simulated bath smelting gas. The experimental results indicate that a large portion of the sulfur enters tha gas phase during devolatilization and combustion of the coal.
  • Improvement in Descaling of Hot Strip by Hydrochloric Acid

    pp. 670-678

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    DOI:10.2355/isijinternational.34.670

    Mechanism of descaling process by hydrochloric acid was observed and a mathematical model was established, which indicated correct direction of improvement in pickling method, such as installing predescaler prior to acid tanks.
    Various factors dominating descaling time were also investigated by experiments. By combining all the facts and theories obtained, a sumulation program was developed which made possible the prediction of acid concentration distribution along the acid tanks. By utilizing the simulator optimization of pickling facilities specification became possible.
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  • The Influence of Large Boride Particles on Microstructural Evolution in AISI 304 Steel

    pp. 679-688

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    DOI:10.2355/isijinternational.34.679

    Microstructural evolution in AISI 304 stainless steel and in a similar steel containing 1% boron during hot torsion was studied by means of quantitative optical microscopy and transmission electron microscopy. Particular attention was focused on the influence of the large volume fraction of hard boride particles on the dynamic recrystallization paramenters and the characteristics of the dislocation substructure.
    Dynamic recrystallization in the austenitic matrix was accelerated significantly in the presence of boride particles. This was accounted for by the superposition of both local strain accumulations around the curved particle interfaces and higher overall strain energy stored in the austenite. Hot workability, however, remained extremely poor probably because of the high strain rate used in this study. Changes of the dislocation substructure in the austenitic matrix during hot deformation were found to be similar in both steels and were interpreted according to a model that has been so far used predominantly for cold deformation. The study of the substructure of the boride particles showed that they remained almost undeformed during the deformation process.
  • Static Recrystallization of Hot Deformed Austenite and Induced Precipitation Kinetics in Vanadium Microalloyed Steels

    pp. 689-696

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    DOI:10.2355/isijinternational.34.689

    Using torsional tests and applying the back extrapolation method the kinetics of the static recrystallization of hot deformed austenite has been studied in three microalloyed steels with different vanadium contents and this has been modelled at temperatures above and below the static recrystallization critical temperature (SRCT). The SRCT has been found to be a function of the strain, the austenite grain size and naturally the microalloy contents, in this case N and V. A study has also been made of the kinetics of the precipitation induced by the strain and the precipitation-time-temperature (PTT) diagrams have been determined for the three steels. The no-recrystallization temperature (Tnr) has also been determined and the conclusion was reached that the difference between the Tnr and SRCT is mainly due to the austenite grain size, which is smaller in the determination of the Tnr due to the successive recrystallizations which occur before precipitation takes place.
  • Temperature Dependence of the Dissociation Width of Dislocations in a Commercial 304L Stainless Steel

    pp. 697-703

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    DOI:10.2355/isijinternational.34.697

    The dissociation width of dislocations in a commercial AISI 304L stainless steel was measured as a function of temperature. Two types of experiments were carried out. In one, specimens were deformed at room temperature, followed by annealing at various temperatures, and in the other, specimens were deformed directly at various temperatures. In both experiments the width of near-edge dislocations remained unchanged upto 300-400°C, then decreasing gradually. On the other hand, the width of near-screw dislocations showed the maximum at around 400°C, when specimens were deformed directly at high temperatures: When specimens were deformed at room temperature and then annealed at high temperatures, the maximum was less evident. The Suzuki segregation was invoked to explain behaviour of the dissociated dislocations in a commercial AISI 304L stainless steel.
    It is proposed that some typical features of dynamic strain aging observed at around 400°C such as enhanced work hardening and planar slip could be caused in part and in conjunction with other more effective mechanisms than those caused by the slightly enhanced dissociation width of near-screw dislocations in the same temperature range.
  • Comments on the Role of Yttria in Gettering Sulfur in the “Y2O3 Dispersion Effect on Al2O3 Protective Coating Examined on the Basis of Five Models”

    pp. 704-706

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    DOI:10.2355/isijinternational.34.704

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