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

Tetsu-to-Hagané Vol. 83 (1997), No. 1

  • Interfacial Tensions between CaO-SiO2-Al2O3 Slag and Fe-O-S, Fe-Si, Fe-Al, Fe-C, Fe-Cr or Fe-Ni Alloys

    pp. 1-6

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    The interfacial tensions of 35mass%CaO-35mass%SiO2-30mass%Al2O3 slag with Fe-O-S, Fe-Si, Fe-Al, Fe-C, Fe-Cr or Fe-Ni alloys were investigated at 1580°C by the sessile drop method. The interfacial tension for Fe-O-S system decreased with increasing oxygen and/or sulfur contents and their values (σ) with oxygen and sulfur activities (a) were found to follow the relation,
    σ0-σ= 0.3241n(1+116a0+8.5as) ([S] < 1mass%)
    In evaluating the effect of each alloying element on the interfacial tension, the difference in the oxygen content in each alloy was taken in the consideration. The interfacial tension was increased with increasing aluminum and silicon contents, slightly decreased with increasing carbon or nickel contents, and markedly decreased with increasing sulfur and chromium contents in the melt.
  • Estimation of Thermal Diffusivity of Solid Part of Coke Based on Image Data

    pp. 7-11

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    The ratio of thermal diffusivity of solid part to effective thermal diffusivity (αse) was estimated by computer simulation of heat transfer using the image data of microstructure of a sample. Based on the fact that the value αse is characteristic of structure of the sample, the thermal diffusivity of solid part of coke was estimated with the value αse and the effective thermal diffusivity αe measured by LASER flash method.
    The temperature dependency of the estimated thermal diffusivity of solid part of coke was almost the same irrespective of the kind of coke. The estimated thermal diffusivity of solid part of coke was approximately 4.0×10-6 (m2/s) at temperatures below 1000K, and it increased as temperature rises at temperatures higher than 1000K.
    Effect of the accumulative thickness of carbon mesh plane Lc on the estimated thermal diffusivity of solid part of coke was found to be insignificant.
  • Influence of Strength and Reactivity of Formed Coke on Reaction, Heat Transfer and Permeability in Lower Part of Blast Furnace

    pp. 12-17

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    The influence of formed coke properties on the phenomena in the lower part of the blast furnace has been researched with two kinds of model experiment in order to establish the formed coke using technology.
    First, the influence of physical properties of formed coke on the deadman structure was investigated using a semicircular model capable of simulating the heat transfer in the furnace.
    The deadman having low void fraction is generated during using formed coke due to the formed coke properties with high bulk density and low void fraction. This structure results in the increase of metal hold-up, suppresses the gas entering into the deadman and leads to the lack of heat in this area.
    In order to improve the deadman structure for smooth gas entering, it is effective to charge 50mass% formed coke in the peripheral area or to mix 50mass% formed coke with normal coke previously.
    Second, the influence of strength and reactivity of formed coke on the reaction and permeability was investigated using a 90° sector model capable of simulating the reaction and heat transfer in the lower part of the furnace.
    The fine coke generation in the raceway can be suppressed and the permeability is kept stably by use of formed coke having higher strength at room temperature than normal coke.
    High reactive formed coke is burnt rapidly in the raceway, resulting in the acceleration of pulverized coal heating and combustion. The use of high reactive formed coke leads to the improvement of reducing efficiency of the furnace due to the decrease of CO2 gasification temperature and the increase of CO gas amount.
  • Air Leak through Snorkel of RH Degasser and Its Effect on Nitrogen Removal in Molten Steel

    pp. 18-23

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    Experimental and theoretical studies were carried out to understand the effects of air leak through the snorkel of a RH degasser on desorption and absorption of nitrogen in molten steel during vacuum treatment.
    The pressure and gas composition in steel pipes inserted into the refractory of up-and down-legs were examined. Snorkels were sealed with argon gas and change of nitrogen content in steel were compared with conventional treatment without argon gas seal.
    The outer side pressure of the down-leg is higher than the inner side one, whereas the pressure distribution of the up-leg is to the contrary. The main gas composition in the down-leg is nitrogen and that of the up-leg is argon. These mean that the air leak spot exists in the down-leg.
    Desorption and absorption rates of nitrogen are balanced at 20 ppm when snorkels are sealed with argon gas, whereas balance content is 27 ppm for conventional treatment without argon gas seal. The kinetic analysis on nitrogen behavior made clear that the amount of nitrogen gas leak when argon gas was used for sealing snorkels was reduced on the average by one half and standard deviation of that value by one fifth compared with the conventional treatment. The relation between the rate constant for nitrogen removal and nitrogen absorption rate by air leak was shown in order to obtain the aimed final nitrogen content at the fixed treatment time.
  • Mechanism of Fine Bubbles Entrapment beneath the Surface of Continuously Cast Slabs

    pp. 24-29

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    Origin of fine bubbles beneath the surface of continuously cast slab has been verified. In addition, the influence of fluid flow condition in mold on entrapment of the bubbles beneath meniscus has been investigated through a simulation experiment with water models. The results are summarized as follows.
    (1) The gases compositions in the bubbles were N2, Ar, H2 and CH4, and the origin of each composition was air leakage, Ar gas injected at slide gate, diffusive hydrogen in molten steel, and coal-tar pitch soaked in refractory of slide gate respectively.
    (2) Air leakage at the slide gate increased with passage of casting time, and the volume of leakage was estimated to be 100550Ncm3/s.
    (3) The size of bubble contained N2 gas was smaller than that contained Ar gas and found in mainly center of slab width direction.
    (4) The distribution of bubbles existence in slab width direction depended on bubble size and molten steel flow beneath meniscus.
    (5) An application of the airtight slide gate reduced the number of bubbles beneath the slab surface in the middle of width direction.
  • Separation of Inclusions in Liquid Metal Using Fixed Alternating Magnetic Field

    pp. 30-35

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    The principle on separating inclusions in a molten metal by using a fixed alternating magnetic field has been confirmed through experiments with molten aluminium. The separating efficiency of inclusions is studied through experimental and analytical ways under different operating conditions. The mathematical model to predict the separating efficiency is developed and the efficiencies predicted by the model are compared with the experimental ones.
    A feasibility study on the application of an electromagnetic inclusion separation in a molten steel is performed. An operating non-dimensional parameter, Y ≡(ρfdp2Be2LtS) (μfμeWdt2) which characterizes an electromagnetic inclusion separation process is newly proposed.
  • Deformation Behavior of Continuously Cast Strand during Soft Reduction

    pp. 36-41

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    Three dimensional deformation behavior of continuously cast strand during soft reduction, which is considered to be an important factor when applying optimum reduction rate in soft reduction technology for reducing center segregation, was experimentally and theoretically analyzed and following results were obtained.
    (1) Continuously cast strand deforms during soft reduction not only in the thickness direction but also in the width and casting directions. The longitudinal elongation and width spreading are proportional to the reduction amount.
    (2) Theoretical model to simulate deformation behavior during soft reduction has been developed.
    (3) Reduction efficiency defined as the ratio of reduction area for unsolidified region to the total reduction area is expressed as a function of the ratio of unsolidified width to the total strand width and decreases with the decreases in unsolidified width and aspect ratio of the strand.
  • Application of Laser Ablation ICP-AES to Steel Analysis

    pp. 42-47

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    A new system of the laser ablation-ICP (Inductively Coupled Plasma) AES (Atomic Emission Spectrometry ) was developed for rapid analysis of steels. A lamp pumped Nd; YAG laser or a laser diode pumped solid-state laser is used for ablating samples with the Q-switched mode of high pulse repetition rate. The optical system for the laser radiation is adjusted to satisfy a criterion of laser energy density. The optics is also adjusted to scan the laser beam in the whole area of a square repeatedly. The composition of particles generated by the system is almost the same as that of the solid matrix, and their sizes are<0.1μm and near uniform. These features of particles bring high sensitivity and precision to this system.
    The developed laser-ICP system shows some analytical merits superior to the spark source AES. The analytical performances of this system are excellent precision in the trace level concentration, good linearity of analytical curve, little or no sample preparation, the ability of the analysis of hot samples (1000°C), accurate control of analytical point. On carbon determination in steel, the influence of metallurgical history which causes the serious error in the spark source AES, is not appeared in the developed system.
    The developed laser-ICP system has been applied to two analytical systems in steelmaking works. One is a full automatic analyzing system of steel product samples and the other is a rapid analyzer of defects in steel sheets. Both of the systems are working smoothly.
  • Determination of Trace Amount of Carbon in Steels by Atomic Emission Spectrometry

    pp. 48-53

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    In the present study, analytical conditions for higher precision analysis of trace amount of carbon by atomic emission spectrometry were considered.
    The results obtained are summarized as follows:
    (1)(1)CI 165.8nm as a carbon spectral line and FeI 287.2nm as an internal standard spectral line, (2)spark area (time-resolving technique) as photometric area, (3)preburn:1500, analysis:1500, reverse:1000 as pulse number of discharge were most appropriate conditions.
    (2)The analytical value of carbon increased 0.2ppm with 1°C rise in sample temperature before discharge. Therefore, to analyze carbon in low carbon steel accurately, the sample temperature before discharge should be adjusted to room temperature (26 27°C).
    (3)Carbon intensity increased when the time intervals between analyses were prolonged. It could be depressed by cleaning discharge before analysis.
    (4)By conditions showed in (1), sample temperature control and cleaning discharge before analysis, analytical repeatabilities in standard deviation at carbon concentration of 6 50ppm were 0.51.3ppm.
  • Development of Cold Rolling Technology for High Thickness Accuracy and Excellent Tensile Property Sheet

    pp. 54-59

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    In order to meet customer's growing demand for thickness accuracy of cold rolled strip, it becomes more important to minimize the thickness deviation in the transverse direction as well as in the longitudinal direction. Through our research work, it was revealed that strip edge profile can be controlled by changing a roll crown and the control range is large when a reduction in thickness is large. A Pair Cross (PC) mill has been introduced in stands 1-3 of the new 5-stand tandem cold strip mill at Kashima Steel Works of Sumitomo Metal Industries, Ltd. Using the PC rolling technology and a newly developed automatic gage control (AGC) system, a production system for a high thickness-uniformity sheet has been developed.
    Excellent tensile property sheet is also required recently by automobile companies. By rolling ultra-low carbon steel with high reduction excellent tensile property has been obtained.
  • Load Prediction Model of 17%Cr-7%Ni Stainless Steel in Cold Rolling

    pp. 60-65

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    The load prediction model of 17%Cr-7%Ni stainless steel in cold rolling has been developed. In this model, the flow stress equations included deformation-induced martensitic transformation and the mathematical model of rolling load based on the approximate theory of Bland & Ford are considered. The volume fraction of martensite by ferrite scope was modified by X-ray diffraction method. The flow stress were investigated from the tension tests of sheets prestrained by rolling with temperature range from 20°C to 200°C, and the flow stress equations were established from parameters of the volume fraction of martensite, material temperature, equivalent strain and strain rates. The rolling load, the volume fraction of martensite and the strip temperature calculated by this load prediction model were consistent with the experimental results. By using this load prediction model, it is made clear that the rolling load and the volume fraction of martensite depends on strip temperature and rolling speed greatly.
  • Influence of Carbon and Nitrogen on Intergranular Corrosion of an Fe-18Cr Alloy

    pp. 66-71

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    In order to study the difference between the effects of carbon and nitrogen on intergranular corrosion of ferritic stainless steels, Fe-Cr alloys were chosen as a model system. Five different Fe-18Cr alloys with carbon concentrations of 0.001 to 0.013 mass percent and nitrogen concentrations of 0.002 to 0.013 mass percent were utilized to observe the susceptibilities to intergranular corrosion and the growth rates of (Fe, Cr) 7C3 carbide and Cr2N nitride particles precipitated on grain boundaries. The intergranular corrosion of the alloys, which were homogenized at 1273K for 600s and then annealed at 873K for 100 to 1000s, in chrolide solutions is more pronounced at high carbon concentrations than at high nitrogen concentrations. The difference in the susceptibilities to intergranular corrosion is due to greater thickness of chromium depleted zones along grain boundaries in the high carbon alloys than in the high nitrogen alloys, resulting from a greater growth rate for the carbide than for the nitride.
  • Effects of Grain Boundary Precipitates on Long-Term Creep Rupture Properties of SUS347H Steel

    pp. 72-77

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    Heat-to-heat variation in long-term creep rupture strength and ductility for a SUS347H austenitic stainless steel has been investigated on the basis of microstructure observations. The long-term creep rupture tests up to about 105h were carried out at 600, 650, 700 and 750°C. The microstructural changes during creep were observed for three different heats of the steel after interrupting creep tests at various testing times. M23C6 and NbC carbides precipitate during the creep tests. The major controlling factor for the creep rupture strength and the ductility was found to be the formation and the propagation of grain boundary cracks, resulting from grain boundary sliding. The size distribution of M23C6 particles precipitated along grain boundaries strongly depends on the content of boron in the heats with 4 to 27 ppm boron. There is an optimum boron content of 12 ppm to minimize the propagation of grain boundary cracks. It is concluded that the heat-to-heat variation is caused by the difference in the boron content which affects the size distribution of M23C6 precipitates and hence the formation and the propagation of grain boundary cracks.
  • Life Cycle Assessment for Spray Coatings Applied to the Heating Tubes of PFBC Boiler

    pp. 78-83

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    LCA (Life cycle assessment) is a systematic process used to calculate and evaluate the environmental impacts of products. Because boiler components are now exposed a more severe erosion/corrosion environment by improving efficiency of thermal power plant, it is expected that the use of thermal spray coatings will increase.
    The LCA method was attempted to apply to various thermal spray coatings for PFBC (Pressurized Fluidized Bed Combustion) and evaluate the eco-friendly coatings. The result was gained that all the alumina coatings have good characteristics. In fact the Al2O3-40%ZrO2 coating by APS has the lowest environmental impact and the best erosion resistance, it may be considered the most effective coating.

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