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

Tetsu-to-Hagané Vol. 81 (1995), No. 5

  • Influence of Potassium Absorption on the Change of Crystallite Size of Coke with Heating Temperature

    pp. 513-517

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    Effects of potassium absorption on relation between graphite crystallite size of coke and heat-treatment temperature have been examined. The results are summarized as follows :
    1) A crystallite size of coke, Lc, decreases when the heat-treated coke absorbed potassium. However a general tendency that the Lc increased with increasing temperature is maintained. On the other hand, another crystallite size, La, is not affected by the potassium absorption.
    On the basis of the above observation, it is estimated that the remarkable deterioration of the coke is due to formation of an intercalation compound in the crystal.
    2) The Lc decreases with increasing potassium content and then it reaches a limiting size when the potassium content reaches about 6 mass%. The limiting size depends on the heat-treatment temperature.
    The decrease in the Lc in a low drum index coke was greater than that in an ordinary coke, but it was impossible to explain as due to the difference of the limiting concentration.
    x

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  • Rapid Solidification Analysis of 18Cr-8Ni Stainless Steel Considering Dendrite Growth Conditions

    pp. 518-522

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    The initial stage of rapid solidification of 18Cr-8Ni stainless steel was numerically analyzed using the relationship between tip temperature and growth rate of a free dendrite. The generation rate of latent heat of fusion was evaluated from the growth rate of the interface which was given as a function of tip temperature of a dendrite. A phase selection rule that the phase with larger growth rate under a given tip temperature could grow was assumed in the analysis.
    The calculated length of cellular structure of metastable γ phase was in good agreement with experimental data, when the reported relationship between cooling rate and initial undercooling at the onset of solidification was assumed in the calculations.
  • Process Analysis of Non-Contact Continuous Casting of Materials Using Cold Crucible

    pp. 523-528

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    An axi-symmetrical process model for the cold crucible continuous casting of a conductive material is developed based on analyses of electromagnetic and temperature fields coupled with force balance around the free boundary of melt. The electromagnetic field around the system is predicted by means of a wire model using the vector potential method. A characteristic temperature field in the charge which is electromagnetically repelled by the crucible is given by a finite difference solution of the heat balance equation taking transitional phase change into consideration. The validity of the proposed theoretical model has been confirmed by experimental measurements of the electromagnetic field around the cold crucible and temperature field in the charge. Numerical predictions show that keeping a molten charge without contact of a surrounding crucible is possible when the position and shape of solidification front is properly controlled by a regulated water cooling of the surface in the lower part of the charge. A laboratory experiment has been conducted to support this predicted fact. Theoretical operational criteria as to casting speed, cooling position and its rate which allow a stable and non-contacting melting of the charge are shown.
  • Pinhole and Inclusion Defects Formed at the Subsurface in Ultra Low Carbon Steel

    pp. 529-534

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    Excellent surface characteristics are necessary for the production of ultra low carbon steel sheet which is used for the automobile. Crack formation is not a serious problem in continuously cast ultra low carbon steel because of good ductility at high temperatures. However, pinhole defects in slabs and sliver defects in cold sheets are often observed, and they are the most harmful defects in producing ultra low carbon steel.
    In this paper, the reason why pinhole defects are more likely to occur in ultra low carbon steel than other carbon steels is discussed based on the investigations of gas composition in the pinhole defects and the relationship between pinhole defects and solidification structure. Furthermore, the origin of sliver defects with inclusions is discussed based on investigations of inclusions found inside of the pinhole in the slabs and near the sliver defects in the cold rolled sheets.
  • Prevention of Cracks in Gas Injection Type Immersion Nozzle

    pp. 535-540

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    The mechanism by which cracks are occurred in the immersion nozzle with silicaless inner porous refractory developed to stabilize the injection of argon gas into molten steel, and the prevention of nozzle cracking have been investigated. The results obtained are as follows.
    (1) Models for estimating the crack formation pressure and hot argon gas pressure for the immersion nozzle were proposed. Using those models, a theory for preventing nozzle cracking according to casting conditions was worked out.
    (2) With the immersion nozzle with silicaless inner porous refractory, the hot argon gas pressure did not drop. Therefore, when the initial argon gas pressure is substantially high, a longitudinal crack tends to occur easily in the powder line which deteriorates in strength due to corrosion in the latter half of casting operation.
    (3) By increasing the immersion nozzle wall thickness to improve its strength and controlling the hot argon gas pressure below the crack formation pressure, it is possible to prevent cracks in the immersion nozzle with silicaless inner porous refractory.
    x

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  • Strength Evaluation Method for Roll Neck of Rolling Mills

    pp. 541-546

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    There was no technical data on stress concentration of deform cross section such as a roll neck shaft, that has a plane on both sides. Accordingly, we had conventionally estimated maximum stress by using a stress concentration factor data of the inscribed shaft in a plane. However, there was a fear to overestimate stress concentration by this method. Therefore, we have done stress analysis of the roll neck by the three-dimensional F. E. M. (finite element method) and stress measurement by the strain gauges on the roll neck model.
    In our research, we have developed an experimental equation for estimating a stress concentration factor. As a result, it was cleared up that the stress concentration factor, calculated by the conventional strength evaluation method, was excessive. And, it could be possible to design a rolling mill drives which have better strength balance.
  • Effect of Strain Induced by Hearth Rolls and Tension on Ductility of Strip Processed on Continuous Annealing and Processing Line

    pp. 547-552

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    Unlike the steel strip annealed in the laboratory, the strip processed on continuous annealing and processing line (C.A.P.L.) contains considerable dislocation. Such dislocation, or plastic strain, is considered to deteriorate the ductility of continuously annealed material.
    Since the strip near the soaking furnace is softened, the effect of strip tension is considerably large. Therefore, assuming that C.A.P.L. also functions as a tension leveler, the relationship is obtained between the amount of plastic strain, which is imparted to the strip, and the strip tension with respect to roll diameters. As a result, the strip is stretched each time it passes between rolls. The experiment shows that the amount of strain imparted to the strip is proportional to the degree of deterioration in the total elongation.
    In order to improve the mechanical properties of continuously annealed material, it is effective to reduce strip tension and increase roll diameters. In particular, if the strip tension : σt is maintained below the certain value obtainable by the next equation, σt≤Dσe2/2hE, where D : roll diameter, σe : yield stress, h : thickness of strip and E : modulus of elasticity, there exists a condition which practically enables no-strain annealing where strain does not accumulate even when the strip passes between rolls.
  • Effects of Surface Finishing of Stainless Steel Sheets on Formation of Plasma Polymerized Thin Film

    pp. 553-558

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    The relation between surface finishing of stainless steel sheets and formation of plasma polymerized thin film of Perfluoromethylcyclohexane (PFMCH) was investigated by methods of EPMA and A.C. impedance analysis.
    By observation of EPMA, homogenious and thin film formed by plasma polymerization on the surface of SUS304BA was obtained, but on the surface of SUS3042D, thinner film was formed at the boundary of crystalline than on the surface of crystalline. A.C. impedance analysis showed that the value of A.C. impedance of SUS304BA was higher than that of SUS3042D, and it is assumed that in the case of SUS3042D, the thickness of the film formed by plasma polymerization on the surface of crystalline was about ten times that of the boundary of crystalline.
  • Effect of S in Steel on Hydraulic-descaling-ability in Si-added Hot-rolled Steel Sheets

    pp. 559-564

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    Effect of S on red scale defect, which is often observed as abnormal red scale stripes within normal black scale matrix especially in hot-rolled Si-added steel sheets, has been studied by means of laboratory hot rolling test. It was found that the area of red scale defect on the sheet surface decreases with increasing the S content. This can be explained by the formation of the eutectic compound of FeO/FeS with the low melting point of 940°C between steel substrate and upper eutectic compound of FeO/Fe2SiO4 with the melting point of 1173°C, i.e., the descaling ability of the primary scale is largely improved by the existence of the liquid phase in the process. The cause of formation of only FeS without MnS at the scale/steel interface in spite of significantly high amount of 1.5%Mn, is also discussed on the basis of chemical equilibrium diagrams calculated by the thermodynamic database "MALT2".
  • Microstructure and Tempering Behavior of Rapidly Solidified Layer of High C-high-Alloy Steels Formed by Laser Surface Melting

    pp. 565-570

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    Microstructure and tempering behavior of rapidly solidified layer formed by laser surface melting has been investigated in high C-high alloy tool steels SKH51 and SKD11, and medium C-medium alloy tool steel SKD62.
    In SKH51 and SKD11 which contain a large amount of primary carbides even after conventional quenching, solution of alloying elements is promoted by rapid solidification. This results in formation of supersaturated martensite in SKH51 and meta-stable austenite in SKD11.
    Subsequent heat treatment at 550°C of rapidly solidified layer leads to extremely high secondary hardening to HV 1100 in SKH51, probably because of the increase of the amount of carbides precipitation.
    Heat treatment of rapidly solidified SKD11 at 600°C also leads to hardening from HV500 to HV750 because of decomposition of meta-stable austenite to ferrite+carbide phase and martensite.
    In SKD62 in which carbides are almost dissolved in matrix even in conventional quenching, substructure and tempering behavior of rapidly solidified layer is similar to conventionally quenched one.
  • Role of Vanadium in Creep Rupture Strength of 9Cr-1Mo-V-Nb-N Steel

    pp. 571-576

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    The effect of V addition on the mechanical properties of 9Cr-1Mo-V-Nb-N steel, especially creep rupture strength (CRS), has been examined and the factors that control the CRS have been discussed.
    Though the specified V addition is 0.18 to 0.25 mass% in ASME SA387 Grade 91 steel, the increase in V addition from 0.15 to 0.32 mass% resulted in the decrease in high temperature proof strength (PS) and CRS. The δ ferrite is not observed even in the 0.32 mass% V containing steel. The δ ferrite is not the cause of decrease of CRS. The change of CRS should be explained by precipitation strengthening ; not only by the inter-precipitate distance but also by the size of precipitate and the coherency strain around precipitate.
    The increase in V addition causes the sparse precipitation of VN. This could be the reason of the decrease in CRS by increased addition of V. The increased V addition also causes the increase in size of VN and the decrease in coherency strain around VN. Supposing the local climb model for creep deformation, these are also the factors that affect the CRS : their effects are estimated to be almost same in magnitude and to cancel each other.
  • Contraction of Cold Rolled Austenitic Stainless Steels during Annealing at Low Temperature

    pp. 577-582

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    Austenitic stainless steel has good corrosion resistance because of its high chromium content. Moreover, it can be hardened significantly by cold-working, though it is not hardened by heat treatment. Cold rolled austenitic stainless steel is, therefore, widely applied in spring parts, transportation equipment, architectural materials and so on. However, this cold rolled austenitic stainless steel has the peculiar behavior during annealing at low temperature which is not observed in annealed stainless steel. (1)It contracts significantly parallel to the rolling direction during annealing without load. (2)Creep occurs at low stress below the proof strength. To study the mechanism of contraction, the effect of stability of austenite, hardness and martensite induced plasticity on contraction were investigated, and 30% reduced austenitic stainless steel was found to contract by 317×10-4 during annealing. Contraction is considered to be connected with ε-martensite and stress relaxation.
  • Mezzo Scopic Analysis of Cleavage Fracture Toughness in Low Carbon Steels and Low Alloy Steels

    pp. 583-588

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    The cleavage fracture toughness of steels was analyzed on the basis of the statistical local criterion approach. Statistical stress criterion at the crack tip region suggests that the cleavage fracture toughness can be described as a function of the yield stress and the cleavage fracture stress of the materials. In the present work, the formulation of the cleavage fracture toughness was examined in order to analyze mezzo-scopically the mechanical and metallurgical factors. Good correlation between the cleavage fracture toughness, and the yield stress and the cleavage fractue stress which evaluated by round bar specimens was obtained for 47 kinds of low carbon steels and low alloy steels. Based on the correlation, the fracture toughness can be predicted from the tensile test properties.
  • Effect of B on Textures of Fe-36%Ni Alloy

    pp. 589-594

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    Effect of B addition (B≤0.012%) on the cold rolling and recrystallization textures of Fe-36%Ni alloy has been studied.The {001} <110> preferred orientation, which is increased with increasing B contents, developes in the surface layer. The surface rolling texture penetrates from the surface to a depth of less than about 20μm. The pure-metal type rolling texture of {4 4 11} <11 11 8> which is found in the inner layer, irrespectively of B contents. The surface recrystallization texture of B containing alloys is almost random. The inner recrystallization texture showes a gradual change with increasing B contents from the {100} <001> orientation to the {4 4 11} <11 11 8> orientation of rolling texture which is retained after completion of recrystallization.
  • Effect of a Small Amount of Carbon and Carbon + Lanthanum on the Creep Characteristics of a Fe-20Cr-5Al Alloy

    pp. 595-600

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    The present paper reports the effect of a small amount of carbon (50-300 ppm) and carbon plus lanthanum (0.04-0.15mass%) on the high temperature creep characteristics of a Fe-20Cr-5-Al ferritic stainless steel. Testing was conducted over the stress range from 3.9 to 6.9 MPa at 1173 K. It was found that creep life increases with increase in caron concentration. The beneficial effect of carbon, however, decreases with increase in applied stress and carbon concentration especially when the carbon concentration exceeds 150 ppm. For steels with 0.1 mass % lanthanum, creep life increases with increase in carbon up to 300 ppm. The average creep rate of the steels with 150 ppm carbon is inversely proportional to the lanthanum concentration. This suggests that dislocations drag their solute atmosphere of lanthanum. For the steels with 0.1 mass % lanthanum and various amounts of carbon, the average creep rate is inversely proportional to the carbon concentration whereas that for the steels without lanthanum is not proportional to the reciprocal of carbon concentration. The effect of this type of carbon was interpreted by the I-S interaction with which the carbon increases the apparent size misfit of lanthanum atom.
  • Ductile Fracture Bchavior in CTOD Test of Multi-pass Weld Heat Affected Zone of Quenched and Tempered High Strength Steels

    pp. 601-606

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    CTOD property of 780MPa class high strength steel weld HAZ was investigated by means of HAZ thermal cycle simulation. CTOD property improves as hardenability of steel increases as well as charpy impact property does in case the CTOD specimen fractures in brittle manner.
    As the hardenability of steel increases, fracture mode changes from brittle to ductile one. However, steel with high hardenability shows low CTOD value, less than 0.2mm, even with ductile fracture mode. CTOD value with ductile fracture mode (δm) decreases with increasing hardness of steel.
    Decrease in δm due to increasing hardness is attributed both to deterioration of CTOD value at ductile fracture initiation (δ1) and ductile crack growth resistance (K).
    δm value can be estimated from strain hardening exponent, ductile fracture properties (δ1, K) and initial ligament size of a CTOD specimen.
  • Introduction to Reaction Kinetics for Material Processing (II)

    pp. N214-N224

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  • Analytical Atomic Spectrometries Utilizing in Iron and Steel Industry

    pp. N225-N232

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  • Isothermal Hot Working of Intermetallics for Structural Use

    pp. N233-N239

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