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

Tetsu-to-Hagané Vol. 82 (1996), No. 11

  • Numerical Simulation of Change of Temperature and Thermal Stress Generated in MgO-C Bricks Applied to BOF

    pp. 881-886

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    Change of temperature and thermal stress which were generated in MgO-C bricks 1080 mm, 540 mm and 270 mm long was studied under a temperature transition simulated to BOF operation. Thermal and mechanical properties of samples without metal and samples with 3% Al, after heat treating many times at high temperature, were measured at the temperature for data to be used for FEM (Finite Element Method) calculation. (1) Data of thermal expansion coefficient and elastic modulus which were applied for calculation at the start of operation were different from those applied in the middle of operation. (2) In the middle of operation, the temperature regularly changed and large stresses were generated in the fixed inner range from hot face regardless of the brick length. However, the temperature did not change and stress was nearly zero outside the fixed inner range. (3) Large stress which produced a crack was generated at the start of operation and reoperation. (4) A crack of samples with 3% Al could be formed at a closer point to the hot face than that of samples without metal, because the elastic modulus of the former was higher than that of the latter.
  • A Model Design for the Charging Apparatus of a Sintering Machine

    pp. 887-892

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    A model design for the charging apparatus of a sintering machine has been studied to propose a method of adjusting model rules. In this method, dried are similar to the wet original raw mix in size distribution is used for model mix, and Floude number is taken into account in order to decide the operating conditions of the model sintering machine. This method has been checked by comparing size segregation in the bed between original sintering machine and 1/5 practical model, which has resulted in good accordance practically.
  • Development of Mathematical Model of High-Frequency Induction Furnace with Coke Packed-Beds for Scrap Melting

    pp. 893-898

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    The high-frequency induction melting furnace with packed beds of coke has been recently developed from many advantages. However, overall transport phenomena of the furnace is still not clear in spite of its importance for optimizing/designing the furnace. In this study, by summarizing the results obtained in the previous studies, a total mathematical model was proposed for simulating heat transfer and material flow in the furnace. Three experiments at different conditions were also conducted by using low-melting-temperature metal in the laboratory scale furnace with graphite packed beds. The results showed that agreement between observed and predicted data in any experiments is good enough for verification of the model. In addition, the experimental data revealed that the decrease of electric current and the reduction of scrap layer influence significantly productivity and temperature of liquid metal at the outlet, respectively. Finally, the model was applied to an industrial furnace, with both temperature and flow fields of scrap, coke and hot metal simulated for the first time. The major findings due to this analysis were that a thermally-inactive region at the center of the scrap layer and predominant channelling of hot metal near the wall exist.
  • Solid and Liquid Movement Characteristics in Two-dimensional Cold Model for the Dripping Zone of a Blast Furnace

    pp. 899-904

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    Liquid hold-up, liquid discharge distribution and velocity vector of moving particles were measured under the fixed bed and moving bed conditions in a two-dimensional (2D) cold model of 1/10 scale for the dripping zone in a blast furnace, in which stainless steel balls and filtered water were used instead of coke and metal or slag.
    At relatively small liquid flow rates and in a packed bed with scattered voids as in this cold model and a blast furnace, restricted liquid paths were formed in the packed bed, and consequently the liquid distribution became nonuniform. The liquid movement in the moving bed was much different from that in the fixed bed. In the fixed bed, total and static hold-ups of the 2D model agreed with those of a previously reported one-dimensional (1D) cold model, respectively. Each liquid hold-up in the moving bed was remarkably greater than that in the fixed bed. As liquid velocity increased, each liquid hold-up increased. In the moving bed, total and static hold-ups of the 2D model were larger than those of the 1D model, respectively.
  • Reaction Kinetics of Dehydrogenation from Molten Iron under Inert Gas Atmosphere

    pp. 905-910

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    The authors investigate the kinetic aspects of dehydrogenation from molten iron under Ar atmosphere of 1.013×105 Pa at 1580°C. In the experiments, Ar is blown through an alumina lance onto the surface of the inductively stirred molten iron under various conditions of gas flow rate, lance height, and lance diameter. The reaction rate increases with the increase in the Ar flow rate. Simple first order and second order rate equations cannot fit the experimental data. Assuming that the gas side and the liquid side mass transfer processes are the rate determining steps, the authors develop a mixed transport control model. The gas side and the liquid side mass transfer coefficients are deduced from the kinetic data. The reaction rate is not affected by O and S in the explored range of [%O] and [%S]. It is concluded that the reaction is controlled by the gas side and the liquid side mass transfers.
  • Effect of Secondary Cooling on Microstructures in a Rapidly Solidified High Cr Stainless Steel Strip

    pp. 911-916

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    The rapid solidification process, which omits the hot rolling stage, was employed for the production of a thin strip made of materials which typically have poor workability. The process required the solidified strip to be cooled carefully so that embrittlement or recrystalization did not occur: 30Cr-14Ni stainless steel, solidified to a thin strip by a twin roll caster, was subjected to another cooling (secondary cooling) before coiling. The transition of strip temperatures during the cooling was estimated both by measurement and by calculation. From the experimentally observed relationship between the heat treatment and embrittlement, the optimum condition for the secondary cooling was obtained. Experiments showed that the strip became embrittled when it was subjected to 930 K for 5 min. Therefore the cast strip was passed through the secondary cooling zone of water jets. The coiling temperature was kept below 700 K and no embrittlement was observed. As a result, a strip was delivered as an overlay welding material on a semi-commercial basis.
  • Alloying Degree Analysis Method for Galvannealed Steel Sheets

    pp. 917-922

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    One of the advantages of galvannealed (GA) steel sheet over electroplated one is that the thick coating is realized in lower cost. The increase of the alloying degree, the iron concentration in GA coating is favorable for coating adhesion and corrosion resistance after painting, whereas accelerates powdering in the press forming process. Therefore, it is important to control the alloying degree in the production of GA steel sheet.
    The accuracy σd of the alloying degree analytical methcd applicable to on-line analysis is reported as 0.6 mass% using the analytical curve, i.e., the relationship between the alloying degree and the X-ray diffraction (XRD) intensity ratio of (633) plane of Γ alloy phase in the coating to the background. However, the accuracy is not good enough and little is known about the influence of GA steel sheet production condition, e.g., temperature and Al concentration in galvanizing bath, oxidizing degree and temperature of steel sheet before galvanizing and the galvanizing time on the alloying degree analysis. Moreover, few reports deal with the comparison of correlation between the alloying degree and several XRD intensity ratios.
    We found out that the XRD intensity ratio most correlated was Γ(633)/ζ(160) and that the analytical curve using Γ(633)/ζ(160) intensity ratio shifted according to the production condition due to the change of the thickness ratio Γ/ζ. Therefore, we introduced the standard analytical curve that predicts the alloying degree from the corrected XRD intensity ratio; corrected according to the production condition. By using this curve and the corrected XRD intensity ratio Γ(633)/ζ(160) for the unknowns, the alloying degree was analyzed with the accuracy ad of 0.46 mass%.
  • Simple and Rapid Determination of Trace Manganese in Steel by Differential Pulse Cathodic Stripping Voltammetry

    pp. 923-928

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    Manganese at the μg/g level in steel is determined by differential pulse cathodic stripping voltammetry in a strongly acidic sample solution without separating iron matrix. The method is based on the accumulation and subsequent stripping of hydrated manganese (IV) oxide on a platinum disk electrode. Optimum experimental conditions include the use of a 2 ml aliquot of the 0.23 M nitric-0.18 M sulfuric acid mixture electrolyte containing 2 mg of a sample, and pre-electrolysis at +1.45 V vs. SCE followed by a negative-going scan at a rate of 200 mV/s to +0.65 V vs. SCE.
    The peak height of manganese decreased with increasing the amount of iron (III) in the electrolyte and increased linearly with pre-electrolysis time until the electrode surface became saturated. The determination range could broaden by using manganese-free acids and longer pre-electrolysis times; thus the calibration graphs were linear between 1.8×10-8 and 1×10-4 M of manganese (II), within 600 s pre-electrolysis, and passed through the origin. Possible interferences are evaluated.
    The determination of 2.8 to 4280 μg/g of manganese in steels was achieved with relative standard deviations of less than 6%(n=10) within 15 min. The detection limit is 0.3 μg/g for a pre-electrolysis time of 600 s. The proposed method is more suitable than the commonly used JIS methods (G 1213 and G 1257) for the determination of trace manganese in steel from the viewpoints of simplicity, less experimental skill, rapidity and high sensitivity as well as excellent precision.
  • Effect of Roll Cooling Water Property on Corrosive Wear of Tungsten Carbide Rolls

    pp. 929-934

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    Tungsten Carbide Cemented Rolls are widely used in hot-steel-products rolling because of good wear resistance. At the early period, small diameter ring rolls with tungsten carbide mounted on steel parts were used in finishing cantilever blocks of wire rod mills. Recently, medium diameter ring rolls and solid rolls with tungsten carbide have been used in modern intermediate stands of rod mills and stretch reducing stands of tube mills. Moreover, they have started to be applied to finishing stands in stainless section mills because of good galling resistance. But the tungsten carbide rolls is very expensive compared with the conventional cast iron rolls. To apply tungsten carbide rolls instead of cast iron rolls economically, it is necessary to get about five to ten times roll life of cast rolls. In this report, the corrosive characteristic of tungsten carbide alloy in high temperature water, which is one of main reasons for roll wear, is investigated. It was found that the optimization of the pH and calcium concentration of cooling water is effective to suppress corrosion of rolls. This effect was assured in the real production of three roll type stretch reducing stands of the tube mill in Kashima Steel Works.
  • Non-contact Evaluation of Mechanical Properties of Electroplated Wear Resistant Ni-P Layer from the Velocity Dispersion of Laser SAW

    pp. 935-940

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    We developed a new laser surface acoustic wave (SAW) system and applied this to estimate the mechanical properties of the wear-resistant Ni-P layer electroplated on a stainless steel. The velocity dispersions of Rayleigh wave of the asplated and heat-treated Ni-P layer were obtained by the one point time domain signal processing. The Ni-P layers with excellent wear resistance produced by the heated treatment higher than 725K were found to show higher Rayleigh velocities than that of the substrate steel, while the Ni-P layer with poor wear resistance showed lower velocities. Young's moduli of the Ni-P layer, estimated so as the computed velocity dispersion agreed with the measured one, increased with the increase of wear resistance.
  • Influences of Vanadium and Thermo-mechanical Treatment on Microstructures of Medium Carbon Microalloyed Steels

    pp. 941-946

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    The austenite recrystallization behaviors during the hot-deformation and the characteristics of the ferrite-pearlite structures transformed during continuous cooling from the hot-deformed austenite were investigated for the medium carbon steels containing 0.4% carbon with 0 to 0.3% vanadium. The results are as follows: (1)The vanadium additions inhibit the recrystallization of hot-deformed austenite and elevate the austenite recrystallization temperatures in the medium carbon steels, more effectively than those in the low carbon steels. The austenite recrystallization-stop temperature of 0.4% carbon steel added 0.3% vanadium is over 200K higher than that of 0.4% carbon steel. This is considered to be due to the increase of the strain induced precipitation of VC in deformed austenite, through the increase of the super saturation of VC with the increase in the carbon content. (2)The ferrite-pearlite structures of 0.4%C-0.3%V steels hot-deformed at non-recrystallization temperature (below 1223K) are refined remarkably, through the increase of the ferrite nucleation site consisted of the flattend austenite grain boundary and the annealing twin formed in the austenite grain. (3)0.4%C-0.3%V steels hot-rolled at non-recrystallization temperature on the practical mill have superior toughness in spite of high tensile strength over 900N/mm2 because of the microstructure refinement through the non-recrystallization austenite deformation.
  • Improvement of Delayed Fracture Strength of High Strength Steels by Intergranular Ferrite Precipitation

    pp. 947-952

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    A new method for improving delayed fracture strength by the precipitation of ferrite along prior austenite grain boundaries has been demonstrated with high strength steels of 1300 MPa in tensile strength. Intergranular fracture which is characteristic to the delayed fracture of tempered martensitic steels is suppressed and is changed to quasi-cleavage by the intergranular ferrite precipitation. Hydrogen thermal desorption analysis has revealed that the amount of diffusive hydrogen increases under loading. The mean hydrogen occlusion rate rather than the hydrogen absorption under no loading or the amounts of hydrogen at the time of failure corresponds to the delayed fracture strength levels. The mechanism of the effect of intergranular ferrite on the hydrogen embrittlement based on the accumulation of lattice defects has been discussed.
  • Macrostructure Control and Magnetic Property of Rare-earth Magnet Pr-Fe-B Alloy Ingot

    pp. 953-958

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    In order to produce the high quality rare-earth magnet, macrostructure control of Pr-Fe-B alloy ingot was carried out. Production method of this allov consists of melting, casting, hot rolling and heat treatment. The columnar structure of ingot is necessary to obtain the high magnetic properties. The columnar structure was produced by two kinds of method. One is thin plate casting and the other is directional solidification. By the thin plate casting sound and columnar structure ingot was produced. This alloy is cooled rapidly from the liquidus temperature to the peritectic temperoture and then cooled slowly below the peritectic temperature because this alloy is brittle. By this method the columnar structure was produced and the crack of this thin plate was protected.
    In order to decrease the production cost of ingot, 400kg large size ingot was produced by the directional solidification using the hot top heating. The temperature gradient in ingot was so increased by the hot top heating that the large part of ingot showed the columnar structure. The magnetic properties such as (BH) max and iHc were improved by the hot top heating.
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    1. Improvement of Delayed Fracture Strength of High Strength Steels by Intergranular Ferrite Precipitation Tetsu-to-Hagané Vol.82(1996), No.11
  • Temperatures of Metallic and Ar Atoms in Ar Arc Plasma on Melting of Metal

    pp. 959-964

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    In order to examine the effect of metallic vapor on temperature of Ar plasma, graphite, pure iron and Fe-5mass%Mn alloy were melted in Ar arc plasma as an anode material. The high speed and high sensitive multi-channel monochromator was used for measurement of intensity of spectral lines. Based on these intensities, distribution of temperatures of metallic and Ar atoms were calculated. Measured temperature of metallic atom was different from that of Ar atom, and local thermal equilibrium was not established.
    The temperatures of Ar atom in the center of arc column were about 9000K, 11000K and 9000K for graphite, pure iron and Fe-5mass%Mn alloy, respectively. This is due to the different plasma current density and electric conductivity of plasma. The details of configuration of plasma column and plasma current characteristic were discussed.

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