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

ISIJ International Vol. 31 (1991), No. 3

  • Microwave Application in Carbothermic Reduction of Iron Ores

    pp. 241-245

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    DOI:10.2355/isijinternational.31.241

    Reduction of hematite fines and magnetite concentrates containing carbon using microwave energy was investigated. The reduction rate was found to be very rapid under all conditions employed. The nature of the ore mineral and flux (lime and limestone) did not influence the reduction but the nature of carbon had an effect, with charcoal being superior to coke.
    Microwave reduction is a highly non-isothermal process and bearing this in mind, the effect of temperature on reduction was exponential. Difference between the reduction of identical samples with microwave and conventional heating (1000°C) was remarkable–in favour of microwave heating.
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  • The Flow Field in Air-Water Vertical Bubbling Jets in a Cylindrical Vessel

    pp. 246-253

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    DOI:10.2355/isijinternational.31.246

    Bubble frequency and gas holdup in bubbling jets were measured by means of an electro-resistivity probe. The axial, radial and tangential velocity and fluctuation components of water in the bath were measured with a laser Doppler velocimeter. On the basis of the experimental results, the flow field in an air-water vertical bubbling jet in a cylindrical vessel was divided into 4 regions, i.e., the momentum, transition, buoyancy and surface regions with respect to the axial distance from the nozzle exit. The profiles of gas holdup, axial velocity and root mean square value of axial velocity fluctuation in each region were discussed in detail. Also, the flow rate, momentum, and kinetic energy of rising water were determined, being necessary to refine previous theoretical models predicting the fluid behavior induced by bubbles.
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  • Development of a New Mold Oscillation Mode for High-speed Continuous Casting of Steel Slabs

    pp. 254-261

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    DOI:10.2355/isijinternational.31.254

    High-speed continuous casting is a prerequisite for the successful operation of hot direct rolling (HDR). However, as casting speed increases, breakout caused by solidified shell sticking tends to occur. It is therefore necessary to reduce the frictional force between the mold and the solidified shell in order to prevent shell sticking in high-speed casting.
    The lubrication mechanism in the mold was analyzed, and the results led to the establishment of a new modl oscillation mode which reduces frictional force. This offers an oscillation movement where the time of upward motion of the mold is longer than that of downward motion in a single oscillating cycle. The effect of this new mode on the reduction of the frictional force was ascertained by casting tests using an experimental continuous caster.
    On the basis of fundamental studies the new mold oscillation mode has been utilized in the No. 5 slab caster at NKK Fukuyama Works. Plant tests confirmed that the new mode is more effective in reducing frictional force than the sinusoidal mode. A high-speed casting of 3.0 m/min has been successfully achieved with this new mold oscillation mode. No difference in the surface quality of slabs could be observed between casting with the sinusoidal mold oscillation and this new mode.
  • Warp Control in Strip Processing Plant

    pp. 262-267

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    DOI:10.2355/isijinternational.31.262

    Strip is bent many times by turn rolls in the continuous annealing and surface treatment lines. Roll diameter of the turn rolls is usually made as small as possible due to limitation of space and cost. Therefore, plastic bending occurs and at the same time, the strip thus treated develops longitudinal and transverse warps (curvatures). It is proposed in this paper to refer to these two features as "curl" and "gutter", respectively.
    Curl is restrained by line tension, but gutter (crossbow) appears down-stream of the turn roll. Gutter causes some line problem, especially in the plating line where it causes nonuniformity of coating thickness across the strip width.
    This report provides theoretical analysis and basic information about gutter and how to control it in order to unify coating thickness in electrogalvanizing and hot dip galvanizing line. In the former line, gutter elimination just before the vertical plating cells, in the latter line, configuration of pot rolls are the main technical points to be discussed.
  • Hot-ductility Recovery by Manganese Sulphide Precipitation in Low Manganese Mild Steel

    pp. 268-277

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    DOI:10.2355/isijinternational.31.268

    The hot-ductility of low Mn mild steel has been investigated as a function of temperature, annealing time, and cooling rate after soaking treatment at varied levels of Mn and S contents. A drastic reduction of hot-ductility was brought about by soaking treatment at 1250°C, followed by a slow cooling at a rate less than approximately 30°C/s, although its ductility was excellent upon heating to the test temperature directly from room temperature. A further deterioration was made by an increased S content. This hot-ductility loss was attributed to S segregation to austenite grain boundaries. An extremely fast cooling rate after soaking, greater than 100°C/s for example, resulted in an improved hot-ductility. The binding energy between S atom and austenite grain boundary has been determined to be 198.4 kJ/mol from the S content dependence of hot-ductility. The hot-ductility of the embrittled steel improved by annealing at a test temperature. This was accompanied by MnS precipitation. A pre-deformation and a reduced S content accelerated the hot-ductility recovery. The apparent activation energy for the hot-ductility recovery has been determined to be 461.2-491.3 kJ/mol. This high activation energy has been discussed in terms of the diffusion of Mn and the binding energy between S and austenite grain boundary. The presence of free O retarded the recovery of the hot-ductility, rendering the reduction of S content useless.
  • Comparison of Dynamic Recrystallization and Conventional Controlled Rolling Schedules by Laboratory Simulation

    pp. 278-288

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    DOI:10.2355/isijinternational.31.278

    A seamless tube mill rolling process was simulated with the aim of optimizing the ferrite grain refinement by decreasing the finish rolling temperature during the stretch reducing mill (SRM) stage. Tests were performed on one Nb-V and two Ti-V micralloyed steels. For the Ti-V steels, maximum grain refinement is achieved when the SRM exit temperature is just above the Ar3; at a cooling rate of 3.5°C/s during transformation, ferrite ASTM grain size numbers of #13 and higher are produced. When the reductions are applied below the no-recrystallization temperature or Tnr, the strains are accumulated at first as a result of the absence of static recrystallization during the short interpass times. At large strains, dynamic recrystallization is initiated, due to the absence of strain-induced precipitation during the brief unloading intervals. Under these conditions, the rolling loads are lower than when a conventional controlled rolling schedule is employed. In the latter case, the interpass times are sufficiently long for the occurrence of appreciable strain-induced precipitation, which is responsible for the prevention of dynamic recrystallization, and the consequent accumulation of strain and pancaking of the austenite. For the Nb-V steels, when the SRM cooling curve passes through the nose of the CCP (continuous-cooling-precipitation) PS curve, austenite pancaking takes place. By contrast, when the cooling curve passes above or below the CCP curve, the dynamic recrystallization of austenite is initiated, leading to ferrite grain sizes which are generally finer than those produced by the pancaking process.
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  • Microstructural Characterization of Twin-roll Cast Gamma Titanium Aluminide Sheets

    pp. 289-297

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    DOI:10.2355/isijinternational.31.289

    As an effective route for near-net-shaping of sheets of poor workability materials, twin-roll casting is applied to production of gamma titanium aluminide sheets. An experimental process has been established for shaping of flat 100-mm wide sheets with a uniform thickness in a range of 1 to 2 mm. Changes in parameters controlling the rate of solidification in twin-roll casting provide a fairly wide variety of microstructures in a stoichiometric gamma titanium aluminide alloy. The microstructures are characterized with special attention to their through-the-thickness variations for understanding of the path of solidification and solid state transformation in direct sheet casting. In the subsurface layer of cast sheets a primary alpha phase forms from liquid and the alpha dendrites grow along the heat flow to form a sharp preferred orientation with the ‹0001›α direction parallel to the sheet normal. Subsequently the alpha grains transform into an extensively refined (γ+α2) structure and develop a transformation texture of ‹111›γ fiber parallel to the sheet normal with an orientation relationship of {0001}α//{111}γ. In the intermediate layer gamma single phase increases in relative fraction and evolves a growth texture of [001]γ parallel to the sheet normal. In the dendrite and interdendritic gamma interfaces, gamma grains accommodate thermal stresses generated during cooling. Toward the central zone the growth directions of dendrite deviate from the sheet normal with increasing depth below the surface as a reflection of meniscus profile. In the central zone shrinkage cavities are formed in the absence of liquid metal on solidification. Based on these findings metallurgical guides for further improvements in casting procedure and for controlling of cast structure and discussed.
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  • Determination of Dynamic Elastic-Plastic Fracture Toughness by a Drop-weight Impact Testing Machine

    pp. 298-303

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    DOI:10.2355/isijinternational.31.298

    A drop-weight type tensile testing machine has been designed and built. The testing machine elementally consists of a drop weight, a loading rod, and a stopper. A test is performed by dropping the weight on the stopper. The use of the stopper which has an oil damper within results in dampening the initial shock waves, and tensile speed as high as a quarter of falling speed of drop weight is obtained. Using this stopper, the measured data can be similarly analyzed as those in a static test.
    Load-line displacement is estimated from the crack opening displacement on the front surface of the specimen. The estimated load-line displacement was confirmed to be in good agreement with the load-line displacement measured directly in a static test. Furthermore, the onset of crack propagation can be detected by an ultrasonic method. Using these methods, dynamic elastic-plastic fracture toughness JId of a nodular cast iron was determined at the stress intensity rate KI of less than 1.9×105 MNm–3/2 s–1. The specimens used were 25 mm-thick CT specimens. In the upper-shelf fracture toughness region, JId of nodular cast iron increases with increasing KI.
  • Effect of Cobalt Addition on Transformation Behavior and Drawability of Hypereutectoid Steel Wire

    pp. 304-311

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    DOI:10.2355/isijinternational.31.304

    The grain boundary cementite and Widmanstätten cementite consisted in a hypereutectoid steel have been considered to cause the brittle fracture. In this study, the effect of cobalt addition on the transformation behavior of the hypereutectoid steel has been examined in order to control the grain boundary cementite precipitation. It was found that in the case of hypereutectoid steels containing carbon content less than 1.3 wt%, the grain boundary cementite precipitation could be suppressed with the cobalt addition and patenting treatment.
    By suppression of grain boundary cementite, the hypereutectoid steel showed good drawability comparable with that of eutectoid steel and the increase of tensile strength comparing with eutectoid steel. The analysis using Embury-Fisher relation showed that the increase of workhardening rate was due to the refinement of lamellar spacing, which was confirmed by microstructural observation. Furthermore, it was found that refinement of lamellar spacing was attributed to increased carbon content in the case of fully pearlitic microstructure. The effect of cobalt addition on the transformation behavior was also discussed from CCT curves.
  • Effect of Porosity on the Swelling Behaviour of Iron Ore Pellets and Briquettes

    pp. 312-314

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    DOI:10.2355/isijinternational.31.312

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