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

ISIJ International Vol. 35 (1995), No. 7

  • Preparation of High Ash Indian Coals for Carbonization

    pp. 819-825

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    DOI:10.2355/isijinternational.35.819

    The quality parameters of coke produced from a particular coal blend can be significantly influenced by the coal preparation technique adopted. In Indian coking industry, generally 8-10 types of coals are used as raw material base for carbonization and they differ widely in terms of their grindability characteristics, caking/coking properties, dulatation characteristics, petrographic analysis etc. The Indian coals contain very low percentage of vitrinite (<50%), higher content of middlings and rejects (>1.6 Sp. gr. fraction) and higher ash content (18-23%). Conventional blend crushing of these coals together for carbonization leads to concentration of inerts and middlings/rejects in the coarser size fraction (+6 and 6-3 mm) of the charge leading to heterogeneity in the properties of different size fractions. Difference thermal transformation to plastic state and shrinkage characteristics in the semi-coke state between coarser and finer fractions generates internal stresses in the coke matrix leading to poor strength characteristics.
    With the use of increased percentage of low ash imported coal in the blend (15-40%), the SAIL plants are trying to contain the coal blend ash to a level of about 17%. The imported coals generally have low mineral matter content, high HGI values and increased proportion of fines (60-65% below 3 mm), These circumstances have necessitated development of new coal preparation schemes for carbonization viz. groupwise crushing, blend crushing with prescreening of fines and groupwise crushing with prescreening of fines. The paper discusses in detail the above new coal preparation techniques along with the methodology to determine the degree of homogeneity of the coal blend.
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  • Rate of SiO2 Inclusion Removal from Molten Cu to Slag under Mechanical Stirring Condition

    pp. 826-831

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    DOI:10.2355/isijinternational.35.826

    A kinetic study has been made on SiO2 inclusion removal from molten Cu to slag (Li2O-SiO2-Al2O3) under static and mechanical stirring conditions. From optical microscopic observation (×400), it is found that SiO2 inclusions in the metal have spherical shape. The inclusion radius is less than 8.25 μm under the present experimental conditions. The oxygen content which originates from SiO2 inclusions in the metal phase agrees with the total oxygen ocntent reasonably well. Hence, the change in the total oxygen concentration of the metal, [mass%O]T, corresponds to that in the amount of inclusions in the metal. A rate constant of inclusion removal, kO, is obtained from the log[mass%O]T-time relation. In the explored range of the rotation speed of the stirrer, the change in the mechanical stirring condition does not affect the rate of inclusion removal from the metal phase. However, the rate of includion removal under the static condition is lower than that under the mechanical stirring condition. The rate constant, kO, increases with increasing initial oxygen concentration, [mass%O]T0.
  • Rate of SiO2 Inclusion Removal from Molten Cu to Slag under Gas Injecion Stirring Condition

    pp. 832-837

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    DOI:10.2355/isijinternational.35.832

    Kinetic studies have been made on SiO2 inclusion removal from molen Cu to Li2O-SiO2-Al2O3 slag under Ar gas injection stirring condition. The effect of gas injection on the inclusion removal rate has been investigated. Experiments were done at 1523 K. The gas flow rate of injected Ar was in the range of 3×10-6-6×10-5 m3/s under the conditions of 1523 K and 1.013×105 Pa. The crucible diameters were 30, 40, 60, and 75 mm. During each experimental run, the frequency of bubble formation was measured by means of a pressure pulse technique.
    The bubble size ranged from 7 to 18 mm. From the [mass%O]T-time relation, the rate constant of inclusion removal, kO, was obtained. It is shown that kO increases with increasing Ar gas flow rate, Vg. The rate of inclusion removal increases with increasing gas bubble-metal interfacial area. A mathematical model is developed to explain the experimental result. In the model, the inclusion particles are assumed to adhere to the gas bubble-metal interface during the bubble formation and ascent periods. The apparent mass transfer coefficient, km, for inclusion removal obtained experimentally, is independent of the gas flow rate and the crucible diameter. Thus, it is considered that the inclusion particles are removed from the melt mainly through adhesion to the gas bubble-metal interface.
  • Treatment of Steel with Alkaline-earth Elements

    pp. 838-844

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    DOI:10.2355/isijinternational.35.838

    To evaluate the potential benefits of strontium treatment of steel compared to calcium, steel-clad wires of calcium-silicon, calcium and strontium were fabricated. Lengths of these wires were injected to 35 kg heats of AISI 1045 steel. Samples for total calcium or strontium, oxygen, sulphur, and aluminum were taken as frequently as possible during and after the injection. The recoveries of the alkaline-earth elements in steel for the three wires were 8.3, 4.6 and 0.41%, respectively. There was virtually no inclusion modification with the strontium wire compared to the calcium-containing wires. It was found that the absorption rate of the alkaline-earth elements increased with the sulphur and oxygen contents of the steel. A mathematical model for this enhanced dissolution rate was developed which identifies the fundamental limitations of the alkaline-earth elements for steel treatment.
  • Activity of Silica in the Slag of an Electric Arc Furnace Using Direct Reduced Iron for Steelmaking

    pp. 845-850

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    DOI:10.2355/isijinternational.35.845

    Test charges consisting of DRI and scrap were melted in a 70t UHP electric arc furnace. Slag samples were collected and used to estimate the activity coefficient and activity of silica in the slag and to investigate the influences of basicity, temperature and optical basicity on both of them. A relationship between the activity coefficient and temperature was obtained which shows that the relative partial molar enthalpy of solution of SiO2 in the slag is 70.0 kJ/mol. A simple linear equation has been obtained for the variation of the activity of silica with its concentration in the slag. Metal samples containing from 0.01 to 0.03 wt% of silicon show that the mean activity coefficient of silicon dissolved in the bath is 0.991.
  • Phase Equilibria and Thermodynamics of FeO·Cr2O3-MgO·Cr2O3-MgO·Al2O3 Spinel Structure Solid Solution Saturated with (Cr, Al)2O3

    pp. 851-858

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    DOI:10.2355/isijinternational.35.851

    Thermodynamic properties of chromium ore are imortant to control the new process for crude stainless steel production in a converter, in particular, pre-reduction of the ore in the process. In the present work, phase equilibria and the activities of iron chromite in FeO·Cr2O3-MgO·Cr2O3-MgO·Al2O3 spinel structure solid solution saturated with (Cr, Al)2O3 have been determined at 1573 K, by the equilibrium of iron dissolved in liquid silver held in the spinel structure solid solution crucible wih CO-CO2 gas mixture.
    No measurable solubility of FeO·Cr2O3-MgO·Cr2O3-MgO·Al2O3 solid solution in (Cr, Al)2O3 phase was detected, while small solubility of (Cr, Al)2O3 in the spinel structure solid solution phase was found. Tie-lines between (Cr, Al)2O3 and the spinel structure phases were determined.
    The activity of FeO·Cr2O3 in FeO·Cr2O3-MgO·Cr2O3-MgO·Al2O3 spinel structure solid solution saturated with (Cr, Al)2O3 showed negative deviation from ideality. It was observed that the spinel structure solid solution showed regular solution behavior, and α-functions between each constituent in the FeO·Cr2O3-MgO·Cr2O3-MgO·Al2O3 system were determined as follows;
    αFeO·Cr2O3 - MgO·Al2O3 = –92000 (J),
    αMgO·Cr2O3 - MgO·Al2O3 = – 30000 (J).
  • Cold Model Experiments on the Application of Gas Lift Pump to the Transportation of Molten Metal

    pp. 859-865

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    DOI:10.2355/isijinternational.35.859

    Application of a gas lift pump to a molten metal system was evaluated by determining the effects of liquid properties on the pump's transportation characteristics using a water bath, a glycerol solution bath, and a Wood's metal bath. The flow in the gas lift pipe was in a "slug flow" regime when the pump was working. The critical gas flow rate, which is defined as the lowest gas flow rate required to transport the liquid, decreased with increase in the submergence and with decrease in pipe diameter in all systems. The density of the liquid strongly affected the lifted liquid volume whereas the viscosity had little influence on it. The empirical equation to predict the transportation of molten metal was derived using the slug flow model based on the corrected Nicklin's equation. The model was able to reproduce the critical gas flow retes obtained by the experiment.
  • Macrosegregation Behavior in Continuously Cast High Carbon Steel Blooms and Billets at the Final Stage of Solidification in Combination Stirring

    pp. 866-875

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    DOI:10.2355/isijinternational.35.866

    The influence of various Electro-Magnetic Stirring (EMS) modes on reducing macrosegregation has been investigated by conducting a series of plant tests using a continuous bloom caster and a billet caster of POSCO ad well as the relevant laboratory experiment. The emphasis was put on the reduction mechanism of macrosegregation due to a combination stirring including a final EMS (FEMS) in continuous casting high carbon steel blooms and billets. The variation of mushy zone shapes and crater end positions depending on EMS modes was also examined through a Pb addition test providing a close relationship between them and macrosegragation behavior.
    The optimum stirring pool thicknesses at final EMS (FEMS) have been determined as a result for each carbon steel tested providing a valuable production line data for plant operation and their relation with carbon content are also discussed in view of the theoretical calculation from a solidification model and a relative permeability measurement using a Tamman furnace. Finally, the reduction mechanism of macrosegregation has been clarified as due to the enhanced solidification rate, finely distributed segregation spots in the whole centre region, and narrowed width of mushy zone during the final stage of solidification under a combination stirring mode.
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  • The Influence of Lattice Defects on the Solidification Process of Al-Cu Alloys

    pp. 876-885

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    DOI:10.2355/isijinternational.35.876

    An experimantal study of the Al-Cu and Al-Sn systems has been performed in order to determine the fraction of solid formed after quenching in a DTA-furnace. The fraction of solid formed after quenching has been calculated using the presently accepted theory for the solidification process and is compared with the experimental results. It is concluded that the latent heat is not constant and the value found in literature is only true for very slow solidification rates. This deviation is assumed to be due to lattice defects formed during the solidification process. The defects formed are assumed to be mono-vacancies. The quantity of mono-vacancies required to give the fraction of solid found experimentally is calculated. It is shown that the free energy of the solid increased due to the increase of the vacancy concentration during solidification. The change in the free energy decreases the melting point and increases the partition coefficient between solid and liquid. The homogenisation time required to anneal the supersaturated vacancies formed, and the critical cooling rate to form this amount are calculated. It is concluded that the presently accepted solidification law for prediction of the fraction formed solid must be modified by introducing the formation of lattice defects during the solidification process.
  • Prevention of Red Scale Formation during Hot Rolling of Steels

    pp. 886-891

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    DOI:10.2355/isijinternational.35.886

    Red scale defects usually observed in high Si hot rolled strip were reproduced in a laboratory 3 stand tendem mill. The effects of hot rolling and descaling conditions on the strip surface color and scale structure were examined.
    Irrespective of Si content in steel, the hot rolled strip surface became red when the scale thickness before rolling was above 20 μm and the rolling temperature was below 900°C. It was found that surface part of the scale (mainly FeO) was broken to powder by hot rolling at the temperature below 900°C. The red scale of Fe2O3 was formed by the oxidation of powdered scale during cooling. Thick scale formed during slab soaking was completely removed by hydraulic descaling before rolling in low Si steel, whereas that was not removed in high Si steel. This remained scale caused the red scale defects after rolling and cooling. The application of obtained results to the hot strip mill production of red scaleless strip was discussed.
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  • Ellipsometric Study of the Initial Layer Formation of the Preferential Zn Deposition in Zn-Ni Electroplating

    pp. 892-899

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    DOI:10.2355/isijinternational.35.892

    The initial layer formation of Zn, Ni, and Zn-Ni deposition on Au electrode in sulfate bath has been investigated by three-parameter ellipsometry. The formation of deposited layer has been monitored by the in-situ optical method in the thickness range of nm with the complex refractive index of the layer.
    The underpotential deposition (UPD) of Zn is observed from the change of ellipsometric parameters and the Zn deposition is found to start in the UPD potential region more positive than 0.5 V than the redox potential of Zn2+/Zn. The Ni deposition is strongly inhibited by the presence of Zn2+ ion. It is suggested that the thin layer of Zn initially formed in the UPD potential region inhibits the Ni deposition. The inhibition by the thin surface layer of Zn, which is preferentially formed, is thought to occur also on the growing Zn-Ni electrodeposited alloy. It is, therefore, speculated that the preferential deposition of Zn in the Zn-Ni codeposition process is caused by the thin Zn layer on the deposited Zn-Ni alloy.
  • Effect of Addition of β-Phase Stabilizing Elements (Nb, Mo and V) on Plastic Behaviour of Ti3Al Single Crystals with the D019 Structure

    pp. 900-907

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    DOI:10.2355/isijinternational.35.900

    Effect of the addition of Nb, Mo and V which are thought to stabilize the bcc structure on plastic behaviour of Ti3Al based single crystals was investigated in the range from room temperature to 900°C. {1010}<1210> prism, (0001)<1210> basal and {1121}<1126> pyramidal slip systems were operative at all temperatures depending on crystal orientation. Activation of the prism slip was most favourable among the three slip systems, while the pyramidal slip was observed only at limited orientations near the [0001] corner because of high CRSS. The CRSS for the prism and basal slips decreased monotonously as the temperature rose, while that for the pyramidal slip increased rapidly with rising temperature and reached a peak. Addition of V and Mo led to violation of Schmid's law for the prism slip, while the CRSS in binary Ti3Al and ternary Ti3Al-Nb crystals exhibited no orientation dependence. Anomalous strengthening and peak temperature in the CRSS-temperature curves for the pyramidal slip were very sensitive to the added elements in Ti3Al. The addition of V, Mo and Nb caused more effectively anomalous strengthening in this order.
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  • Prediction of the Onset of Static Recrystallization after Hot Deformation

    pp. 908-913

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    DOI:10.2355/isijinternational.35.908

    In order to determine the kinetics of recrystallization in plain carbon steels, double-hit compression tests have been performed using a computerized Gleeble machine. Based on the nucleation mechanism of subgrain coalescence and the measured softening data, a kinetic model was developed to predict the start times for the static recrystallization occuring after hot deformation. Both the model calculations and experimental observations indicate that the onset of recrystallization is decelerated with increasing reheating temperatue but speeded up as the deformation temperature is increased. The former case can be attributed to the larger initial austenite grain size produced at higher reheating temperatures, which in turn results in larger subgrains and reduces the rate of coalescence. The latter case is due to the higher boundary diffusivity and mobility attained at higher deformation temperatures, which accelerates the process of subgrain coalescence. The accelerating effects of strain and strain rate can also be similarly explained by means of the higher dislocation density and finer subgrains generated during the higher strain and strain rate deformation. The good agreement obtained between model predictions and experimental results finally supports the view point that the coalescence of subgrains could be the rate controlling mechanism for the nucleation process of static recrystallization in austenite.
  • Effect of Mn and Ti Precipitates on the Hot Ductility of Low Carbon and Ultra Low Carbon Steels

    pp. 914-919

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    DOI:10.2355/isijinternational.35.914

    Hot ductility and precipitation behavior of titanium-bearing and titanium-free low carbon and ultra low carbon steels have been investigated by an elevated temperature tensile test and quantitative analysis of Ti, Mn and S precipitates. In the steel without titanium, the main precipitate was MnS in austenite. The amount of Mn precipitates increases as temperature is lowered until a saturation level. The dectile brittle transition temperrature can be evaluated by the Mn/S ratio. Ductility can be improved by holding at 1100°C or cooling-reheating process. On the other hand, in the titanium-bearing steel, the amount of Mn precipitates was very small due to Ti precipitated in austenite. The ductile-brittle transition temperature corresponds to the amount of Ti4C2S2 precipitated, and the ductility cannot be improved by holding at 1100°C or cooling-reheating process.
  • Effect of Testing Atmosphere on Low Cycle Fatigue of Hot Work Tool Steel at Elevated Temperature

    pp. 920-926

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    DOI:10.2355/isijinternational.35.920

    Effects of atmosphere and strain amplitude on low cycle fatigue behavior of AlSl H11 hot work tool steel were investigated. The fatigue life in vacuum was twice as long as that in air. From the result of the fatigue test in vacuum, it became clear that slip bands on the specimen surface were formed during fatigue and they grew up with increasing number of cycles. The initial fatigue cracks were generated at the intersection of slip bands and were vertical to loading direction. The fatigue cracks mainly initiated at internal inclusions when tested in vacuum, whereas superficial cracks were observed on the same fracture surface when applied higher strain amplitude. In contrast, nucleation sites of fatigue cracks when tested in air were specimen surface irrespective of the magnitude of strain amplitude. The extension of low cycle fatigue life in vacuum was mainly due to the decreasing of growth rate of superficial and internal cracks to the critical crack length.
  • Effects of Alloying Elements on Deformation Mode in Ti-V Based β Titanium Alloy System

    pp. 927-936

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    DOI:10.2355/isijinternational.35.927

    We have discussed the rule by which the predominant cold deformation modes of metastable β phase are governed depending on their chemical compositions through the examinations of the effects of Sn, Al, and Zr on β-quenched and slightly cold rolled microstructures in a Ti-V based alloy system. It seems in Ti-V binary alloys, that orthorhombic martensitic transformation temperatures Ms and Md are depressed far below room temperature by the athermal ω phase at over about 15%V. Tin and aluminum intrinsically lower these temperatures like vanadium. On the other hand, tin and aluminum simultaneously suppress the athermal ω phase of Ti-16V to first rise Md and even Ms up to above room temperature, respectively. The deformation mode of β phase consequently depends on both the Md temperature and the degree of suppression of the athermal ω phase formation. Alloys having a Md above room temperature undergo stress-induced martensitic transformation. As for alloys having a Md below room temperature, alloys where the athermal ω phase is sufficiently suppressed undergo slip, whereas alloys where it is not so done {332}<113> twinning. Since aluminum strongly suppresses the athermal ω formation, increased Al additions change the deformation mode of Ti-16V from {332}<113> twinning to stress-induced martensitic transformation via a quenched α'' region or that of Ti-16V-4Sn from stree-induced martensitic transformation to slip. On the other hand, since tin does not suppress it so much as aluminum, increased Sn additions first change the deformation mode of Ti-16V from {332}<113> twinning to stress-induced martensitic transformation but subsequently revive {332}<113> twinning again before slip. The deformation mode of Ti-V-Al-Sn alloys can be interpreted by superposing the effects of V, Al, and Sn. Zirconium also depresses martensitic transformation temperatures and Ti-14V-6Zr undergoes stress-induced martensitic transformation. However, Ti-16V based Zr-added alloys undergo {332}<113> twinning in the wide range of Zr content because the athermal ω phase formation is rarely suppressed by zirconium. This interpretation has solved the discrepancy of the transition of deformation modes of β titanium alloys.

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