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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 87 (2001), No. 12

  • Nitride Capacity of the Molten CaO-SiO2-Al2O3 System Containing TiO2 or ZrO2 and Equilibrated with Molten Si

    pp. 727-733

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    Nitride capacity of molten CaO-SiO2-Al2O3 containing TiO2 or ZrO2 was observed. The molten oxides were equilibrated with nitrogen gas of 1013 hPa and with metallic Si to controll acculately oxygen partial pressure in the oxide phase. The experimental procedure was developed by the authors.
    It is estimated that oxygen partial pressure in the molten oxide phase was controlled by the following reaction equation.
    CaO-SiO2-Al2O3 system: Si+ O2 = SiO2
    CaO-SiO2-Al2O3-TiO2 system: TiO2+4Si+O2=TiSi2+2SiO2
    CaO-SiO2-Al2O3-ZrO2 system: ZrO2+4Si+ O2=ZrSi2+2SiO2
    The nitride capacities measured by the procedure are comparatively same as that reported by other investigators.
  • Reduction Behavior of the Mixture of Iron Ore and Carbonaceous Material at High Temperatures

    pp. 734-740

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    Reduction and gasification rates of mixtures of fine iron are and coal char were measured for some types of coal char and ore at temperature from 940°C to 1340°C. Experiments were carried out with an electric furnace by measuring weight losses of samples and gas composition during reaction. The reduction rates were obtained by separating the weight loss of the fine iron ore reduction from that of the coal char gasification. The overall reduction rate of the mixture was controlled by the coal char gasification at 940°C and by the iron ore reduction at 1340°C. The reduction rate of fine iron ore and the gasification rate of coal char were formulated as functions of the particle size, type of raw material, and temperature.
  • Effect of Wettability of Immersion Nozzle on Mold Powder Entrapment around Immersion Nozzle in Continuous Casting Mold

    pp. 741-747

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    Mold powder entrapment in a continuous casting mold is harmful for producing clean steel. As the immersion nozzle is poorly wetted by molten steel in the real processes, the effect of the surface wettability of the nozzle on the descent of mold powder due to pressure difference along the nozzle was investigated in this model study. Concerning a poorly wetted immersion nozzle, mold powder descended most deeply on the surface of the nozzle, while for a highly wetted immersion nozzle the deepest mold powder-molten steel interface appeared at a position away from the nozzle. The poorly wetted immersion nozzle promoted the descent of mold powder significantly compared with the highly wetted immersion nozzle. Accordingly, the mold powder would be more entrapped at the ports of the poorly wetted nozzle. An empirical equation was proposed for the descent of the mold powder.
  • Statistics of Extremes Analysis of Nonmetallic Inclusions Based on 3D Inspection

    pp. 748-755

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    It is well known that the scatter of fatigue strength of high strength steels is caused by nonmetallic inclusions. The lower bound of the scatter of fatigue strength can be predicted by considering the maximum size of nonmetallic inclusions. Thus, it is of practical importance to estimate the maximum size of nonmetallic inclusions by appropriate inclusion rating methods. Most rational and convenient method to predict the maximum size of inclusions is the one based on the statistics of extremes. Therefore, recently the inclusion rating based on the statistics of extremes has been used by many industries, though the rating methods are mostly two-dimensional (2D) optical methods. It is known that the accuracy of the 2D method is lower than the exact 3D method. In addition, when multiple type inclusions having different chemical composition are contained in a material, the statistics of extremes distribution does not necessarily become a single straight line but become a bi-linear line. The objectives of the present study are (1) to clarify the validity of the 2D method and (2) to establish the method to predict the maximum inclusion size when the statistics extremes distribution becomes bilinear. The results obtained show that the 2D method is basically correct as predicted by the computer simulation. When a bilinear distribution is obtained, it is necessary to determine the minimum inspection area Scritfor predicting the maximum size of the larger type inclusions, which become the fatigue fracture origins of components.
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    1. The Origin of MgO Type Inclusion in High Carbon Steel Tetsu-to-Hagané Vol.88(2002), No.1
  • Development of Determination of Trace Oxygen in Steel after Removing Surface Oxide

    pp. 756-761

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    Removing the oxide from sample surface is important in analyzing clean steel by infrared method after fusion under inert gas. Generally electropolishing or chemical polishing methods are used as sample pretreatment. No method without grow-discharge method could be perfectly eliminated surface oxide. It is because of exposing sample to air, re-oxidation caused easily. The oxide is not negligible to analyze trace oxygen in the bulk. In order to control surface oxide, sample was preheated two times at 1050°C in graphite crusible. After sample was preheated at 1050°C, clean surface was obtained. To transfer the sample from graphite crusible to throw position, first oxide caused easily by exposing to air but it controlled. After the first oxide was analyzed at 1050°C, the same procedure to transfer the sample to throw position was done. Second oxide also caused easily. The oxide and bulk oxygen were together analyzed at 2400°C for total oxygen. Both first and second oxides were same volume and character. So we could calculate bulk oxygen to eliminate second oxide from total oxygen. Analytical curve was made from potassium nitrate method. This development method can be a quantitative analysis. The analytical results obtained from the certified materials are low about 1 ppm from certified value, but the precision is satisfactory. It estimated that surface oxide was included in the certified value.
  • Properties of Ultra Low Carbon Steel Produced by Repetitive Shear Deformation of Side Extrusion

    pp. 762-766

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    Ultrafine grained microstructure was produced by repetitive deformation using the side-extrusion method from ultra low carbon steel with an initial grain size of about 150 μm. The repetitive deformations were imposed at a low extrusion speed of 2 mm/min at room temperature. The side-extrusions with lateral pressure were repeated up to 10 passes without rotation. The sample was uniformly deformed by the side-extrusion, and the equivalent strain was 1.15 after a single pass of the side-extrusion. The ultimate tensile strength increased from 200 MPa with an increasing number of side extrusion passes. After 10 passes of side-extrusion, an ultrafine grained steel with a tensile strength of over 1000 MPa and a grain size of 0.5 μm ×0.2 μm was developed.

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