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

ISIJ International Vol. 42 (2002), No. 5

  • Observation of Inclusion in Aluminum Deoxidized Iron

    pp. 459-466

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    DOI:10.2355/isijinternational.42.459

    Aluminum-deoxidized iron at 1 873 K was solidified at 3 different cooling speed; (1) the ultra-rapid cooling of iron using twin rollers, (2) the quenching of iron into copper mold, and (3) the quenching of the iron-bearing crucible in a water bath; the most rapid cooling rate achieved with (1), which was probably about 105K/s, followed (2) and (3). Dendritic, maple-like, polygonal, network-like, coral-like and spherical inclusions were observed in the samples. The dendritic, maple-like and polygonal inclusions varied in size from several tens to a few μm and were classified as primary inclusions since their sizes were independent of cooling speed. However, the network-like and coral-like inclusions (in the sample cooled ultra-rapidly and quenched into copper mold), and spherical inclusions were classified as secondary inclusions since they decreased in sizewith increased cooling speed. A few large spherical inclusions, which would be primary inclusions, were also present.
    The analysis of the electronic diffraction of the inclusions established that the α, γ, δ-alumina were presentas secondary inclusions. An amorphous silica spherical inclusion was also observed.
  • Thermodynamic Analysis on Metastable Alumina Formation in Aluminum Deoxidized Iron Based on Ostwald's Step Rule and Classical Homogeneous Nucleation Theories

    pp. 467-473

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    DOI:10.2355/isijinternational.42.467

    A mechanism for the formation of unstable alumina (δ, γ, and κ-alumina) in Al-deoxidized iron has been investigated on the basis of Ostwald's Step Rule and two homogeneous nucleation theories. The two homogeneous nucleation theories include a simplified theory in widespred use and the other is a modified theory in which the Gibbs free energy change of the parent iron phase is taken into consideration.
    Comparison of the chemical potential of various aluminas from the standpoint of the Ostwald's Step Rule showed that the unstable alumina could form from the liquid iron alloy supersaturated with oxygen. Moreover, the rule revealed that there is a possibility of liquid alumina formation. The two homogeneous nucleation theories also proved the possibility of the formation of unstable and liquid alumina. The latter nucle-ation theory indicated the easier nucleation of liquid and unstable aluminas than that for α-alumina from the liquid iron containing oxygen contents in excess of the critical value of nucleation at 1 873 K. In contrast, nucleation was rare for oxygen contents less than the critical point at 1 873 K. However, unstable alumina can be formed during cooling and solidification since the oxygen contents of the critical point decreases with temperature decrease. When iron solidifies whilst in the supercooled condition, the nucleation rates of unstable and liquid alumina can be accelerated as a result of the oxygen enrichment at the solidifying front. The authors propose a mechanism of the network-like or coral like inclusion formation by taking into consideration of the liquid alumina formation.
  • Thermodynamics of the Al2O3–SiO2–TiOX Oxide System at 1 873 K

    pp. 474-481

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    DOI:10.2355/isijinternational.42.474

    Isothermal phase relations for the Al2O3–SiO2–TiOX system have been investigated under strongly reducing conditions, achieved by equilibrium between graphite and CO gas, PO2=4.56×10-16 atm at 1873 K, in equilibria with a saturating oxide tablet.
    Activities of Al2O3 and SiO2 were measured by the Knudsen effusion method. Activities of SiO2 were also measured by the chemical equilibrium between the oxide and the Fe–Si–C alloy. Activity of TiOX was also estimated from the composition of the Si–Al–Ti alloy in equilibrium with the oxide assuming that the alloy conformed to a regular solution. In addition, steel composition equilibrated with the oxide of arbitrary composition has been calculated by the activities of each component of the oxide system obtained in the present study.
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  • Gaseous Reduction Behavior of Powdered Iron Ore Sinter and Analysis on the Basis of Rist Model for Fixed Bed

    pp. 482-488

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    DOI:10.2355/isijinternational.42.482

    For the analysis of a blast furnace process, it is necessary to investigate the reduction behavior of iron ore sinter, which is the main source of an iron oxide charged into the blast furnace. It is impossible to accurately analyze the reduction behavior of the iron ore sinter without considering the existence of a quaternary calcium ferrite (CF), which has another reduction equilibrium different from hematite. From this point of view, the gaseous reduction behavior of powdered iron ore sinter has been investigated mainly at 1 173 K and the analysis has been conducted on the basis of a model derived by modifying Rist model for a fixed bed in consideration of CF. The hydrogen reduction behavior is found to be in reasonable agreement with the reduction curve calculated from the derived model at temperatures ranging from 1 073 to 1 273 K. Subsequently, in order to confirm the validity of the derived model, the hydrogen reduction behavior of a synthesized CF, hematite, and a mixture of the CF and hematite was investigated. The trend was observed that the reduction behavior of hematite and the mixture of the CF and hematite is almost the same and that the reduction behavior of the single CF sample has a delay for the others, in reasonably accordance with the relation shipderived from the model. Moreover, the reduction behavior can be almost explained by the derived model regardless of the mixture ratio of H 2 and CO in the ingoing gas under the present experimental conditions; the reduction progress has a delay by the difference of the reduction equilibrium by replacing H2 with CO(–CO2).
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  • Stability of Cuspidine (3CaO · 2SiO2 · CaF2 ) and Phase Relations in the CaO–SiO2 –CaF2 System

    pp. 489-497

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    DOI:10.2355/isijinternational.42.489

    Primary crystallization field of cuspidine (3CaO · 2SiO2 · CaF2 ) has been experimentally determined in the 2CaO · SiO2–SiO2–CaF2 system by quenching method and differential thermal analysis (DTA). Hermetically closed platinum capsules were used as a sample container to prevent fluorine loss in the form of HF and SiF4 by reaction of CaF2 with moisture or SiO2 for quenching experiments. Combining the present study with the partial phase diagrams previously reported, the whole phase diagram of the CaO–SiO2–CaF2 system has been developed. Cuspidine equilibrates with (2CaO · SiO2)2CaF2, CaO · SiO2, 3CaO · 2SiO2, 2CaO · SiO2 and CaF2 in solid state. Cuspidine coexists with liquid phases at temperatures between 1 387 K and 1 680 K.
  • Measurement of Thermal Diffusivity of Steels at Elevated Temperature by a Laser Flash Method

    pp. 498-503

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    DOI:10.2355/isijinternational.42.498

    In order to obtain thermal diffusivity of steels at elevated temperature with sufficient reliability using a laser flash method, an accurate value of specimen thickness is essentially required. Accordingly, a linear thermal expansion coefficient of steels was systematically measured in the temperature range from room temperature to 1 676 K. Since the decrease in specimen thickness was detected after the measurement of thermal diffusivity under vacuum at high temperature close to solidus, such a factor was quantitatively estimated from the thickness values of quenched specimens. Combining these two results, the thermal diffusivity values were successfully determined for ultra low carbon, low carbon, medium carbon, 1.2% silicon, 9% nickel and 13 % chromium steels in the temperature range from room temperature to 1676 K using the laser flash method.
  • Indicators of the Internal State of the Blast Furnace Hearth

    pp. 504-511

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    DOI:10.2355/isijinternational.42.504

    The hearth is a crucial region of the blast furnace, since the life of its refractory may be decisive for the campaign length of the furnace. Excessive growth of skull on the hearth wall and bottom, in turn, reduces the inner volume of the hearth, causes drainage and other problems that limit productivity, and has a negative effect on hot metal temperature and chemistry. A set of indicators that reflect the internal state of the hearth has been developed. The motivation for the indicators is outlined and their application to hearth state detection is illustrated with several examples from the operation of two Finnish blast furnaces.
  • Influence of Cooling Rate on the Hot Cracking Formation of Nickel Rich Alloys

    pp. 512-519

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    DOI:10.2355/isijinternational.42.512

    Hot cracking formation and its mechanism in invar alloys (Fe–36mass%Ni) during continuous casting was investigated. The invar alloy is very sensitive to hot cracking due to its low transition temperature of brittle to ductile fracture even though its solidification interval is narrow. At the cooling rate of 10°C/min the transition temperature of brittle to ductile fracture is about 113°C below the solidus temperature. An increased cooling rate in invar alloy increases transition temperature of brittle to ductile fracture. It is increased to about 43°C below the solidus temperature when the cooling rate is increased to 100°C/min. A mechanism of hot cracking formation in invar alloys has been proposed. Hot cracks in invar alloys with a fast cooling rate are formed between the primary dendrites due to the equiaxed solidification structure. However, at slow cooling rate, hot cracks are formed between the grain boundaries due to the columnar structure.
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  • A Three Dimensional Modified Cellular Automaton Model for the Prediction of Solidification Microstructures

    pp. 520-526

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    DOI:10.2355/isijinternational.42.520

    A three dimensional modified cellular automaton model (3-D MCA) was developed in order to simulate the evolution of microstructures in solidification of alloys. Different from the classical cellular automata in which only the temperature field was calculated, this model included the solute redistribution both in liquid and solid during solidification. The relationship between the growth velocity of a dendrite tip and the local undercooling, which consists of thermal, constitutional and curvature undercooling terms, was calculated according to the KGT (Kurz–Giovanola–Trivedi) and LKT (Lipton–Kurz–Trivedi) models. The finite volume method, which was coupled with the cellular automaton model, was used to calculate the temperature and solute fields in the calculation domain. The present 3-D MCA model was applied to predict the microstructures, such as the free dendritic growth from an undercooled melt, the competitive dendritic growth in practical casting solidification. Some of the simulated results were compared with those obtained experimentally.
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  • Damage Mechanisms in Salt Bath Nitro-carburised and Plasma Nitrided Hot Forging Dies of H11 Tool Steel

    pp. 527-533

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    DOI:10.2355/isijinternational.42.527

    Studies have been made to compare the effect of salt bath nitro-carburising and plasma nitriding treatments, on the working life of hot forging dies made of H11 grade of tool steel. Working life of the surface modified dies, that are used for the production of bearing races by high speed closed die forging, was evaluated in the real plant operation. Salt bath nitro-carburised tools are found to give an average life of 12 000 races with a very wide variation between 1 000 to 35 000 races. In sharp contrast, plasma nitrided tools showed a consistent two to four fold higher life than the salt bath nitro-carburised tools. The surface of worn out dies was examined by scanning electron microscopy (SEM) to understand the nature of damage processes involved in the dies subjected to two different surface treatments. Fatigue cracks, resulting from cyclic thermal and mechanical loading during operation, are the dominant modes of damage in the case of salt bath nitro-carburised as well as plasma nitrided dies. The severity of die damage by mechanical fatigue was more in the salt bath nitro-carburised dies. Abrasive wear due to micro cutting was observed to result in significant damage of the salt bath nitro-carburised die. Die damage due to micro cutting was not observed in the plasma nitrided dies. However, signatures of chipping and plastic deformation were noticed at few locations. The results obtained on service life and wear mechanisms after the two different surface treatments are examined in the light of the characteristics of the modified surfaces.
  • Mechanical Properties of Native Rust Layer Formed on a Low Alloy Steel Exposed in Marine Atmosphere

    pp. 534-539

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    DOI:10.2355/isijinternational.42.534

    Low alloy steel panels, exposed in marine atmosphere for 4 years, formed a compacted rust layer which could suppress the penetration of corrosive electrolyte and lower the corrosion rate. But in practical applications, the protective reliability is limited by the mechanical properties of the rust layer under the action of various loads. This paper presented the first attempt to evaluate the variation of mechanical properties of the protective rust layer with the exposure term by means of micro-indentation testing. Based on the analysis of energy release during cracking, the fracture toughness and the adhesion strength of the rust layer were evaluated. Results showed that the fracture toughness and adhesion strength of the rust layer were improved with the prolongation of the exposure term, the adhesion strength of the rust layer was higher than the fracture toughness of the rust layer itself.
  • Effects of Colloidal Silica Addition on the Self-healing Function of Chromate Coatings

    pp. 540-544

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    DOI:10.2355/isijinternational.42.540

    The corrosion prevention mechanism of the chromated metal materials is considered to be attributable to the self-healing function of the chromate coatings. In this study, the corrosion behaviors of dry-in-place type chromate coated galvanized steel specimens have been studied using a scanning vibrating electrode technique. By this method, the effects of colloidal silica addition in the chromate coating on the self-healing function have been examined. The results confirmed that the chromate coatings prevent the corrosion of metal under coating or at coating defects by their self-healing function. The self-healing function of the chromate coatings is due largely to the formation of a Cr compound layer from the dissolved Cr(VI) ions in the electrolyte. The addition of colloidal silica greatly enhances the self-healing function and corrosion protection of the dry-in-place type chromate coating.
  • Solubility of Titanium Carbosulfide in Austenite

    pp. 545-550

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    DOI:10.2355/isijinternational.42.545

    A titanium carbosulfide solubility product of log [Ti][C]0.5[S]0.5 = -14 646/T15.51 and a Gibbs free energy of formation for titanium carbosulfide of Δ°GTi4C2S2 = -1 121.9+0.196 T kJ/mol were calculated from dissolution temperature experiments using a series of titanium-modified secondary hardening ultra-high strength steels.
    The presence of other precipitates such as TiN, TiC and TiS could be neglected for this class of steels. The effect of the alloying additions on the activity of carbon and hence on the dissolution temperature of titanium carbosulfide was also found to not be significant for these steels. The experimentally determined solubility product and Gibbs free energy of formation are compared with previously published data for titanium carbosulfide.
  • Bauschinger Effect in α–γ Dual Phase Alloys Studied by In Situ Neutron Diffraction

    pp. 551-557

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    DOI:10.2355/isijinternational.42.551

    In situ neutron diffraction experiments during tension-compression deformation were performed on five Fe–Cr–Ni alloys with the volume fraction of ferrite (α) ranging from 0.0 to 100 %. Tensile deformation was applied in a step by step manner up to a strain of 1.3–2.0 % followed by compressive deformation, and neutron diffraction spectra were recorded during temporary stops of a deformation machine with fixed crosshead. (111) reflection of austenite and (110) of ferrite, respectively, were measured simultaneously by using a position sensitive detector. Elastic lattice strains in both constituent phases were evaluated from measured diffraction spectra as a function of external load. Based on these experimental results, heterogeneous deformation behavior in the α–γ dual phase alloys is discussed considering the Bauschinger effect. It is concluded that large compressive residual lattice strains detected in the γ phase after tensile pre-strain-ing, causes the large Bauschinger effect in α–γ dual phase alloys.
  • Effect of Slag Components on Reducibility and Melt Formation of Iron Ore Sinter

    pp. 558-560

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    DOI:10.2355/isijinternational.42.558

  • The Existence of a Ternary Phase in the Al2O3–CaO–FeO System

    pp. 561-563

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    DOI:10.2355/isijinternational.42.561

  • Evaluation of Dislocation Density from the Flow Curves of Hot Deformed Austenite

    pp. 564-566

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    DOI:10.2355/isijinternational.42.564

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