Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 84 (1998), No. 7

  • TEM Observation MgO-or CaO-bearing Wustite Solid Solution Reduced to Iron by Hydrogen

    pp. 471-476

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    Wustite pellets containing MgO or CaO was reduced and the morphologies and microstructure of reduced iron phase were observed.
    The reduction rate of wustite pellets increased with increasing CaO or MgO, and the rate accelerating effect of CaO was larger than that of MgO. The oxide particles were uniformly dispersed in the reduced iron. The average particle size of MgO and CaO were 90-120 nm and 10-20 nm, respectively. The particle size distribution of CaO was more narrow than that of MgO. The iron phase reduced from pure wustite was well sintered, whereas sintering was prevented by MgO or CaO particles. Especially the porous iron was formed from wustite containing CaO.
    During the reduction, the concentration of magnesium ion or calcium ion in wustite increased at the reaction interface. The ratio of the vacancy diffusivity to the chemical reaction rate was calculated from the distance between oxide particles in reduced iron.
  • Heating-up Reduction and High Temperature Properties of High Alumina Sintered Ores

    pp. 477-483

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    To investigate the effect of Al2O3 on the reduction and high temperature properties of sintered ores, the heating-up reduction of sintered ores varying the Al2O3 content are carried out. The early softening, the increase of pressure drop, and meltdown temperature were observed at lower temperature with increase of Al2O3 content. From the simultaneous X ray observation of reduction process and EPMA and SEM analysis of quenched samples, a relatively large amount of Wüstite-Gehlenite eutectic phase was observed in the early stage of reduction process at high Al2O3 content sintered ores. Based on these results, it is concluded that Al2O3 rich molten phase was produced at the relatively low temperature with the increase of Al2O3 content.
  • Estimation of Circulation Flow Rate in RH Reactor Using Water Model

    pp. 484-489

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    The effects of the injecting conditions on the circulation flow characteristics in RH reactor were investigated by the measurements of downward flow velocity in down-leg with a laser doppler velocimeter and the flow observation with a video camera in water model experiments. The maximum circulation rate and the critical gas flow rate increased with the number of orifices. It was observed that the circulation flow rate took maximum when the injected bubbles reached to the opposite side of the pipe wall (single injection) or collided with the other (multi-injection). By considering the geographical condition, the critical gas flow rate can be estimated with the empirical equations on the bubble diameter and the penetration length of bubbles. The diameter and the number of orifices affect the circulation characteristics of the reactor, because they change the bubble size and the bubble frequency in the up-leg. The optimum operating condition was estimated using the results of water model experiments, which showed that the optimum condition might be changed in each heat because of the erosion of refractories.
  • Characteristics of Molten Steel Jet Flow in a Continuous Casting Mold with Electromagnetic Brake Technique

    pp. 490-495

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    Numerical calculation and mercury model experiment are performed to analyze jet flow under DC magnetic field and it is found that counter flow generated around the jet flow. Both the results of calculation and theoretical analysis show that the distribution of electric current density is an important factor for generating counter flow. It is also predicted from numerical calculation that the counter flow has a great effect on the molten steel flow in the mold, therefore there is a possibility for the counter flow to have a great effect on slab quality.
  • Effects of Carbon Content and Cooling Rate on Growth Rate of γ-phase during Peritectic Solidification in Iron-carbon System

    pp. 496-501

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    Changes in fractions of δ-ferrite, austenite and liquid phases during peritectic solidification in iron-carbon system are calculated. The peritectic solidification, δ+L→γ, is divided into δ→γ transformation at δ/γ interface and L→γ solidification at γ/L interface. Both the transformation and solidification proceed by (1) carbon diffusion from liquid through austenite into δ-ferrite and (2) precipitation of austenite from δ-ferrite and crystallization of it from liquid due to cooling. Approximately 80% of the austenite formed during the peritectic solidification is the product of δ→γ transformation, which may lead to the generation of tensile stress in the solidification shell because of the difference in density between the two solids. The amount of the δ→γ transformation is largest, when the initial carbon content is 0.17 mass%. However, when the transformation in a well-developed dendrite network is focused on, the carbon content for the largest amount of the transformation decreases to 0.14-0.16 mass%, which corresponds to the carbon content at which surface cracking of continuously cast slabs is reported to be most frequent.
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  • Effects of Overbased Organic Metal Salts on Lubricity in Hot Working

    pp. 502-509

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    A new method to present a lubricating composition which can effectively prevent seizure during hot rolling of stainless steels is proposed: both "the Principles of Hard and Soft Acids and Bases" for improving chemisorption force and "the super fine particles" for decreasing metal/metal contacts. The degree of seizure and coefficient of friction of mineral oil based organic metal salts, lithium grease based graphite, conventional hot-rolling oil are estimated by disc-block type friction test at elevated temperature. Effects of lubricants on roll oxide film are investigated by model hot rolling mill. Chemical reactivity of lubricants for a fresh and oxide surface is investigated by XPS.
    The results obtained are as follows; (1) A lubricating composition included overbased metal sulfonates was effective for preventing seizure by action of a large quantity of calcium carbonate (average particle diameter < 100 Å) in hot metalworking. (2) Calcium sulfonates have the highest lubricity among the alkaline earth salts of the sulfonic acid having the same base number and a higher base number is better. (3) It is made clear that the overbased Calcium sulfonates change FeO into FeO·Ca0 on steels in rolling bite, and prevent roll oxide film from growing up.
  • Effect of TiN Size on Ferrite Nucleation on TiN in Low-C Steel

    pp. 510-515

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    The ferrite nucleation model based on a classical nucleation theory for a rectangular TiN was derived to clarify its nucleation potency. The calculation revealed that a ferrite nucleates most easily on the facets of TiN rather than on its edges, corners, and austenite grain boundary and also indicated that the minimum size of TiN exists being influenced by the chemical driving force. These theoretical findings were confirmed experimentally for the TiN precipitate in low C-steel. The minimum size of TiN which has the potency of a ferrite nucleation was experimentally determined for various chemical driving force conditions by measuring the maximum size distribution of TiN which existed in the not transformed austenite at transformation temperature. These values showed a good agreement with the calculated ones.
  • Effect of Nitrogen Addition on Localized Corrosion Behavior in SUS316L Stainless Steels

    pp. 516-521

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    The effect of nitrogen addition up to 0.556 mass% on corrosion behavior in SUS316L type austenitic stainless steels was examined, where a particular emphasis was put on chemical analysis of an anolyte by the macro-cell method. It is found that the pitting potential becomes higher with an increase in nitrogen content. The critical pitting temperature is elevated by nitrogen addition and well summarized by using the pitting index of Cr+3Mo+26N (mass %). From the analysis of the anolyte cell solution by the galvanostatic electrolysis with macro-cell instrumentation, NO3- is detected only in nitrogen bearing steels when the potential is kept at a higher level, while NH4+ is detected in nitrogen free steels and in nitrogen bearing steels when the potential is lowered. The potential-pH equilibrium diagram for nitrogen-water system shows that NH4+ is stabilized at lower potentials and NO3- is done at higher potentials. It is believed that NO3- acts as a powerful inhibitor against CΓ attack to the passive film while NH4+ raises pH in a corrosion pit so as to suppress the localized corrosion.
  • An Anomaly in Young's Modulus of High-manganese Austenitic Steel in Cold-worked States

    pp. 522-525

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    High manganese austenitic steels show a marked anomaly in the temperature dependence of elastic constants: an abrupt drop near the Néel temperature TN and successive lowering below TN Young's moduli in annealed and cold-rolled states of 18Mn-4Cr steels were measured from 150K to 350K by the resonance method of free-free transverse vibration. The abrupt drop of Young's modulus of this steel near TN was extremely large in the annealed state, amounting to the ratio of about 20%. Such a step-wise change in Young's modulus disappeared with increasing the reduction of area in cold rolling and recovered gradually by subsequent annealing. This effect of cold rolling seems to be attributed to the predominance of internal stresses of dislocations over the generation of magnetostriction in the antiferromagnetic range.
  • Short-term Creep Behavior Analysis of a Modified 9Cr-1Mo Steel

    pp. 526-533

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    The Ω and a conventional methods were applied to analyze the short-term creep behavior of a Mod.9Cr-1Mo steel. The stress and temperatuer ranges investigated are from 107.9 to 196.1 MP and from 873 to 923K, respectively. Creep life, tr was related to the time to minimum creep (tm), the minimum creep rate (εm) and the imaginary initial creep rate (εo ) astm =0.22×tr1.03, ε m Z·tr KMG and εoY·tr= Ko, where the magnitudes of Z, KMG, Y, and Ko are 0.801, 0.332, 0.884 and 0.216, respectively. The constitutive equations for the minimum and imaginary initial creep rates are expressed as εm = Am·σonm·exp (-Qm/RT) and εο= Ao·σono exp(-Qo/RT), where Am and Ao are structure factors, σo is the initial stress, nm and no are the stress exponents, and Nm, NoQm and Qο are the activation energies for the minimum creep rate and imaginary initial creep rate, respectively. The magnitudes of Am, Ao, NQm and Qo are 9.2 × 103/s, 1.08 × 108/s, 12.8 ±0.2, 13.0±0.2, 666 ± 5 kJ/mol and 751 ±5kJ/mol, respectively.
  • Evaluation of Material Recycling System with Logit Model

    pp. 534-539

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    Recycling of various materials has been urged worldwide for environmental reasons. Promotion of recycling is expected to reduce environmental pollution caused by material production. However, recycling of materials can lead to increase in tramp element concentration in the material, which often causes difficulties in manufacturing and other processes. Therefore, it is necessary to evaluate every aspect of recycling in order to achieve both reduction in environmental burden and efficient use of the material.
    In this study, we assume that the historic change of the amount of material stock can be approximated by logistic equation and made long term simulation of tramp element accumulation, CO2 emission, etc. in recycling system of steel. By using this method, we evaluated various recycling systems and showed that combination of several policies can achieve significant reduction of CO2 emission while maintaining quality of the material.

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