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

Tetsu-to-Hagané Vol. 86 (2000), No. 8

  • Rate of Reduction of Chromium Ore by Liquid Iron Containing Carbon and Silicon

    pp. 499-506

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    The rate of smelting reduction of top-added chromium ore with fluxes such as SiO2, CaO-SiO2 and Na2B4O7 by liquid iron containing carbon and silicon was measured at 14001600°C under an argon atmosphere. Experiments were carried out to examine effects of initial silicon content, temperature, kind of flux additive and amount of flux addition.
    The rate of reduction increased remarkably with an increase of silicon content in liquid iron. It also increased with the addition of various fluxes. Na2B4O7 was found to have the largest promoting effect on the reduction. When fluxes were added, it was observed that they reacted with chromium ore to form liquid slag. Therefore, it was estimated that the reduction took place mainly at the interface between slag and metal. The initial rate of reduction is not affected by the amount of fluxes addition. However, the chromium yield increased with increased amount of fluxes. The effect of temperature on the rate of reduction was significant. The apparent activation energies were about 217320kJ/mol in the case of formation of liquid slag. The reduction mechanism and the rate-controlling steps were discussed, finally.
  • Effect of Volume Change of Coal during Plastic and Resolidifying Phase on the Internal Gas Pressure in Coke Ovens

    pp. 507-513

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    The coking pressure in coke oven, which is caused by the internal gas pressure in the coal plastic layer, is determined by the gas permeability of the layer. The gas permeability of the plastic layer depends on its density as well as the physical property of the plastic coal itself. The plastic layer is between the coke layer and the coal layer and the effect of the volume change of these outer layers, i.e. contraction and compression, on the density and the internal gas pressure of the plastic layer was studied. Sandwich carbonization test, where different coals were charged in the test coke oven, showed that the internal gas pressure in the plastic layer depends not only on the one kind of coal in plastic phase but also on the other kind of coal in resolidifying phase near the oven walls. The relative volume of coke transformed from the unit volume of coal was measured using X-ray CT scanner and it varied greatly across the coke oven width depending on the kinds of coals. The volume change of coal during plastic and resolidifying phase affects the density and the internal gas pressure of the plastic layer. The relative volume of semicoke and coke transformed from the unit volume of coal near the oven walls is higher for a high coking pressure coal than that for a low coking pressure coal. This leads to the high density of the plastic layer and the generation of dangerously high internal gas pressure in the oven centre.
  • Direct Observation of the Free Surface Behavior of a Molten Metal under the Imposition of an Intermittent Alternating Magnetic Field

    pp. 514-518

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    In continuous casting, the free surface motion of a molten metal plays an important role for determining the surface quality of products. In order to clarify the free surface behavior of a molten metal under the imposition of an intermittent alternating magnetic field, free surface waves of a molten gallium were visualized and recorded by use of a high-speed video camera. It is noticed that the number of capillary waves generated in the radial direction increased with increase of the intermittent frequency and was decreased by inserting a copper shield between a coil and a mold. The number of the waves generated in the azimuthal direction decreased during propagating from the vicinity of a sidewall to the center. Asymmetric gravity waves were excited when the distribution of the magnetic field was non-uniform in the azimuthal direction or the intermittent frequency was adjusted near the resonance frequency.
  • Effect of Differential Tin Coatings on Resistance Seam Weldability of Low Tin-coated Steel for Beverage Cans

    pp. 519-525

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    Under the intense competition among many packages, the welded can such as beverage can, food can and aerosol can occupies a dominant position owing to the rational manufucturing process of resistance seam welding. However, the seam welding process itself is further needed to increase the welding speed to improve the productivity without increasing the steel thickness and also the coating weight. To overcome this subject, the authors investigated the effect of main parameters on weldability and established two-dimensional model for calculation in the former reports. As a result, it was exemplified the importance of the current path length during welding at the interfaces.
    The above result reminds us that the actual welding machine has a pair of different size electrode, therefore the current path length is considered to be different. The differencially coated tinplate sheet was prepared to change the current path length at the interfaces. Incidentally, tin free steel was prepared to change the current path lengths at the interfaces by pre-pressing of welding part. The same model was applied to compare the culculation results of the differentially coated tinplate with that of equally coated one. Finally, the calculation proved the results that the heat balance between two interfaces is very important and the weldability can be improved by the balance of current path lengths by decreasing the tin coating weight of upper side.
  • The Effect of Temperature and Water Vapor on the Initial Stage of High Temperature Oxidation of an Fe-1.5mass% Si Alloy

    pp. 526-533

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    The initial stage of oxidation (up to 600s) in air and air-H2O atmospheres was investigated for Fe and an Fe-1.5mass%Si alloy at 1373 and 1473K. The oxidation kinetics of Fe was parabolic at both 1373 and 1473K and the parabolic rate constants are very similar in both air and air-H2O. For the Fe-1.5Si, at 1373K the oxidation amounts increased rapidly after an incubation period (up to 400s) in air-H2O, while at 1473K oxidation obeyed a linear rate law in both air and air-H2O, because a liquid phase was formed with FeO and Fe2SiO4. The linear rate constants were very similar both in air and in air-H2O. In Pt-marker experiments in air-H2O for Fe-1.5Si it was found that the Pt-marker located between external Fe-oxide and inner FeO+Fe2SiO4 layers at 1373K, while at 1473K the Pt-marker located on the alloy surface. The thickness of each layer was measured as a function of time at 1373K in air-H2O. It was found that after an Si-rich oxide (SiO2+Fe2SiO4) layer at the initial stage of oxidation disappeared, a thick inner FeO+Fe2SiO4 layer formed, accompanied by the formation of Fe3O4inside the outer Fe2O3 scale. Rapid oxidation after 400s proceeded with the growth of an FeO layer in the surface scale. The change of the Si-rich oxide layer to an FeO+Fe2SiO4 mixture is due to penetration of water molecules. A combined process of perforating dissociation and transport of water molecules was suggested to be the cause of the rapid growth of the inner layer.
  • Corrosion Resistance and Protection Mechanism of Hot-dip Zn-Al-Mg Alloy Coated Steel Sheetunder Accelerated Corrosion Environment

    pp. 534-541

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    Corrosion behavior of hot-dip Zn-6%Al-03%Mg alloy coated steel sheets in cyclic corrosion test (CCT) has been investigated.
    The corrosion resistance was improved with increasing Mg content in the coating layer, and the highest corrosion resistance was observed at 3% Mg. In Zn-6%Al-3%Mg alloy coated steel sheet, the formations of zinc carbonate hydroxide and zinc oxide were suppressed for longer duration compared with Zn-0.2%Al and Zn-4.5%Al-0.1%Mg alloy coated steel sheets. As a result, zinc chloride hydroxide existed stable on the surface of the coating layer. From the polarization behaviors in 5% NaCl aqueous solution after CCT, it was found that the corrosion current density of Zn-6%Al-3%Mg alloy coated steel sheet was much smaller than those of Zn-0.2%Al and Zn-4.5%Al-0.1%Mg alloy coated steel sheets. As zinc carbonate hydroxide and zinc oxide had poor adhesion to the coating layer and had porous structures, these corrosion products were considered to have little protective action for the coating layer. Therefore, it was concluded that Mg suppressed the formation of such nonprotective corrosion products, resulting in the remarkable improvement of corrosion resistance.
  • Effect of Initial Microstructure on Long Term Creep Strength of a Low Alloy Ferritic Steel

    pp. 542-549

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    The effect of initial microstructure, martensite, tempered martensite, bainite and ferrite+pearlite, on long term creep strength property up to about 80000h at 848K has been investigated on a 0.5Cr-0.5Mo low alloy ferritic steel. No significant effect of tempering after quenching on the long term creep strength has been observed. However, strong effect of transformation process during cooling from austenite has been observed. Creep strength of bainite microstructure is higher than those of martensitic microstructure, independent of tempering after quenching. In the low stress condition in which applied stress is lower than proof stress, creep strength of full annealed microstructure which consists of ferrite and pearlite has been found to be higher than the others. The smallest creep rate of a full annealed microstructure in the long term creep region should be caused by both very low dislocation density of the initial microstructure and small amount of dislocations generated under the low stress condition.
  • Heterogeneous Recovery and Precipitation of Z Phase during Long Term Creep Deformation of Modified 9Cr-1Mo Steel

    pp. 550-557

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    Degradation of 9Cr-1Mo-V-Nb steel during long term creep deformation is caused by a preferential recovery at the vicinity of prior austenite grain boundary (PAGB). Changes in microstructure, especially on precipitates have been investigated in order to understand a mechanism of such heterogeneous recovery. Heterogeneous recovery has been quantitatively demonstrated by the distribution of subgrain size. Three types of the precipitates, M23C6, V(C, N) and Nb (C, N), have been found in the as tempered condition. Precipitation of Laves phase and Z phase have been found in the creep ruptured specimen. Laves phase is found in all the creep ruptured specimen. On the other hand, Z phase is found in the specimens creep ruptured after more than 10000 h. Complex nitride phase of Z phase is rich in Cr, Nb and V of the form Cr(Nb, V)N. Precipitation of Z phase takes place in the long term region and coarsening rate of it is much faster than that of MX, such as V(C, N) and Nb(C, N). It has been concluded that a precipitation of Z phase in the long term region is one of the reason of preferential recovery at the vicinity of PAGB in 9Cr-1Mo-V-Nb steel.
  • Effect of Solute Copper on Hardness and Ductile-to-Brittle Transition in α-Iron

    pp. 558-562

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    Effects of solute Cu on strengthening and ductile-to-brittle transition (DBT) of α-iron have studied for Fe-(02)mass%Cu alloys. The Fe-Cu alloys exhibit massive ferritic transformation on water-quenching from solution treatment temperature. With increasing Cu content, the massive ferrite grains were refined and the hardness was raised. Effect of the solid solution hardening is predominant to the strengthening, but the refinement of massive ferrite grains does not affect so much. Charpy impact tests showed that Cu addition substantially lowers DBT temperature of α-iron. The embrittleness at low temperature was caused by the onset of cleavage cracks at ferrite grain boundaries where deformation twins impinged. Copper in solution suppresses the formation of deformation twins, and this results in the shift of DBT temperature to lower side.
  • Impact Properties of Low Alloy TRIP Steels

    pp. 563-569

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    The impact properties of high-strength low alloy TRIP steels such as "TRIP-aided dual-phase steel" and "TRIP-aided bainitic steel" associated with the transformation-induced plasticity (TRIP) of retained austenite were investigated for some applications to the automotive impact members. The TRIP steels possessed far higher impact absorbed value and lower ductile-brittle transition temperature than the conventional ferrite-martensite and ferrite-pearlite steels. The stress relaxation resulting from the strain-induced transformation of retained austenite islands or films improved the impact properties in terms of suppressing void and/or crack initiation and these propagation. The best impact properties were completed in the steel composing of uniform fine bainitic lath structure and a large amount of stable interlath retained austenite films.

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