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

Tetsu-to-Hagané Vol. 89 (2003), No. 6

  • The Model Experiment on Coagulation of Inclusions by Imposing Electoromagnetic Vibration

    pp. 623-628

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    A new process for eliminating inclusions in a molten steel has been proposed. In the process, the electromagnetic vibration force induced by simultaneous imposition of a direct magnetic field and an alternating electric field is imposed on the molten metal to promote collision and coagulation of inclusions. In order to confirm the validity of the proposed process an experimental work was conducted by simulating silicon particles precipitated in an Al-Si alloy as the model of inclusions in a molten steel. It is found from the experiment that the optimum frequency of vibration exists and its reason is discussed from the theoretical viewpoint taking account of particle motion.
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    Readers Who Read This Article Also Read

    1. Twin Morphology Formed on Goss Si-steel Single Crystal Tetsu-to-Hagané Vol.89(2003), No.6
    2. Improvement of Ridging Behaviour of Ferritic Stainless Steel by Spread Rolling Method Tetsu-to-Hagané Vol.89(2003), No.6
    3. The Effect of Nb on High Temperature Properties of Austenitic Steels. Tetsu-to-Hagané Vol.45(1959), No.11
  • Viscosity Change of Molten Oxides by Electric Current Imposition

    pp. 629-636

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    Viscosity measurements of the molten oxides and the mold powders during the electric power supply are conducted by the rotational cylinder method in order to examine the viscosity change by the electric power. It is derived from the measurement that the viscosity increases by AC power supply and that viscosity change is increase or decrease in DC power supply. It is cleared by the MD analysis that growing network is the main mechanism of viscosity change in AC power supply. On the other, it is found that the viscosity change is occurred by electrolysis in DC power suuply. These results indicate the possibility of the viscosity control of the mold powder by the electric power.
  • Characteristics of Flotation, Sedimentation and Entrainment of Particle in Agitated Liquid

    pp. 637-644

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    The characteristics of particle entrainment from free surface or bottom wall into mechanically agitated water have been studied to clarify the inclusion behavior in liquid metal. The critical agitation speed, nc at which a 50% of particle is entrained into bulk water is measured under steady state by using buoyant particles (glass balloon and silica balloon) and sedimental particles (glass bead). It is found that the dependence of nc on the particle diameter, dp, is different between the buoyant and the sedimental particle. For the case of the buoyant particle, nc is independent of dp, whereas for the sedimental particle nc increases with increasing dp, . To account for this difference, two transition times are investigated. One is the entraining time, t1, which is the time needed to reach a steady state after a sudden imposition of agitation with various speeds. The other is the floating/sinking time, t2, which is the time required to reach a steady state after a sudden decrease in agitation speed from a high speed achieving uniform particle dispersion to lower agitation speeds. It is thought that the balance between t1 and t2 should determine the amount of entrained particles in bulk water under steady-state experiments. For the case of the buoyant particle, the dependencies of t1 and t2 on dp are almost same, so that the value of nc does not depend on dp. On the other hand, for the case of the sedimental particle, the dependence of t1 on dp is smaller than that of t2, so that nc increases with increasing dp. By the VTR observation of the particle entrainment from the free surface or bottom wall, particles are found to be entrained mainly by vertical vortices formed in the agitation vessel.
  • Effect of Molten Steel Flow on Initially Solidified Shell in Ultra Low Carbon Steel

    pp. 645-652

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    The surface defects such as a blister and a sliver occur easily on the surface of the ultra low carbon steel sheet. The cause of these defects is bubbles or nonmetallic inclusions entrapped by the fingernail formed by the point's of solidified shell being bent or by the ruggedness part of the solidified shell in the meniscus neighborhood caused by nonuniform growth of solidified shell when molten steel is continuously cast. In this research, the influence of the molten steel flow on the nonuniformity of the solidified shell and on the formation of the fingernail structure in the ultra low carbon steel was examined. The minimum value of the thickness of initially solidified shell increases and nonuniformity decreases by the molten steel flow though nonuniformity of the solidified shell is extremely large in the ultra low carbon steel. Moreover, the size of the fingernail structure decreases as the molten steel flow velocity increases and as the molten steel temperature rises though a big fingernail structure generates in the ultra low carbon steel. In addition, the decrease in the depth of the fingernail structure and in the number of nonmetallic inclusions in continuously cast slabs by the flow which had been given to molten steel in the mold by in-mold electromagnetic stirring were confirmed.
  • Improvement of Ridging Behaviour of Ferritic Stainless Steel by Spread Rolling Method

    pp. 653-658

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    A rolling method to increase the width of materials would have advantages in multi-width strip production, giving high yield efficiency and productivity. In addition, textures of strips could be chnged by the traverse strain. It is, however, still difficult to produce transverse metal flow in thin strips. The authors suggested a new method for spread rolling of thin strips by multi-pass grooved rolling. In this study, the method has been applied to a Cr 11% ferritic stainless steel strip and its effect on the ridging behaviour has been investigated. The thin 1.0 mm thick and 70 mm wide steel strips are widened up to approximately 72 mm by 35%-reduction. The textures of the spread rolled steel contain relatively shaper γ-fiber band and weaker α-fiber than the flat rolled strips, so that the planer anisotropy of the spread-rolled strips are smaller than that of the flat-rolled strip. The texture is generally weakened by subsequent recrystallization annealing. Applying EBSP analysis on ND-plane of the spread rolled strips, it is found that the grain clusters of certain orientations aligning along the rolling direction, which causes severe ridging, are scattered in the strips by spread rolling and the allocation of <001>//ND grains is remarkably different from those of the stock and flat rolled strips. It is notable that the improvement of the ridging behaviour on the ferritic steel is brought by the spread rollling.
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    Readers Who Read This Article Also Read

    1. Twin Morphology Formed on Goss Si-steel Single Crystal Tetsu-to-Hagané Vol.89(2003), No.6
    2. The Model Experiment on Coagulation of Inclusions by Imposing Electoromagnetic Vibration Tetsu-to-Hagané Vol.89(2003), No.6
    3. The Effect of Nb on High Temperature Properties of Austenitic Steels. Tetsu-to-Hagané Vol.45(1959), No.11
  • Influence of Water Vapor on Surface Hot-shortness of CuSn Containing Steel

    pp. 659-665

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    This paper describes the influence of water vapor on the surface hot-shortness of 0.3%Cu-0.05%Sn containing steel, mentioning the comparison of the hot-shortness in water vapor containing atmosphere to that in the air. Surface hot-shortness was assessed by measuring the number of surface cracks occurring in the hot-deformed specimens after 1250°C heating in x%H2O-1%O2-bal.N2 (x=0, 10, 20, 30) atmosphere. The microstructure at the scale/steel interface was closely observed by optical microscopy and SEM. Structural analysis of scale was also made by X-ray diffraction and EPMA.
    The number of surface cracks increased with an increase in the water vapor content, and the width and depth of surface cracks increased as well. The hot shortness observed in specimens subjected to 20-30% water vapor occurred more severely than that for the specimens heated in the air, although the amount of scale for the former specimens is less than that for the latter.
    Discussion about the severe hot-shortness in water vapor containing atmosphere was given in terms of the flatness of scale/steel interface and Cu (Sn) enriched liquid alloys precipitated on Fe2SiO4 at the interface.
  • Texture and Microstructure of Ultra-low Carbon IF Steel Strip Processed by Conshearing

    pp. 666-672

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    The Equal Channel Angular Extrusion (ECAE) was successfully used as an intense straining process to obtain ultra-fine grained metals. However, it has a problem in applicability to long, thin coiled strips. The authors proposed an alternative continuous shear deformation process, i.e., Conshearing. In this study, 1 mm thick ultra low carbon interstitial-free (IF) steel strip is subjected to the conshearing process at room temperature up to four passes. The deformation characteristics, the changes in microstructure, texture and mechanical properties have been investigated. The shear strain is introduced uniformly through the thickness except around surfaces, and increases with the number of passes. The tensile strength increases from 282 to 442 MPa by four passes. The processed materials shows relatively higher elongation (26%), even after 4 passes. The shear texture with {110} <001> and {112}<111> components is formed in the sheared region.
  • Effect of Silicon Content on Tempered Hardness, High Temperature Strength and Toughness of Hot Working Tool Steels

    pp. 673-679

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    In order to prevent heat cracks of tool steels in hot working, steels of high toughness and superior high-temperature strength has been developed. Typical hot-working tool steels, such as JIS SKD6, 61 and 62, contain about 1 mass% silicon. In this study, the influence of the silicon content was investigated on the 4 mass%Cr-MMo-V-Ni tool steel in the following points;
    1) secondary hardening after tempering
    2) high-temperature strength and room-temperature toughness with various cooling rates during quenching
    The secondary hardening peak of the tool steel decreases with the decrease in its silicon content, due to the cementite precipitation during tempering. Both its high-temperature strength and room-temperature toughness are increased by decreasing its silicon content.
    The extraction residuals analysis, the TEM observation and the EDX analysis are performed on the quenched and quenched-tempered specimens. The effects of decreasing the silicon content of the tool steel are summarized as follows;
    A) The undissolved carbides (mainly MC type) are decreased at the quenching temperature, while these precipitates are increased after tempering.
    B) The grain boundary carbides (M23C6) are decreased after tempering.
    C) In the bainite structure, the silicon content does not affect the toughness, due to the precipitation of M2C type carbides. The high-temperature strength, however, is still dependent on the silicon content.
  • Improvement of Hot Wear Characteristic of High Speed Tool Steel Roll by Increase in Cr and Mo Contents

    pp. 680-685

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    The influence of alloying elements on microstructure and wear resistance of High Speed tool Steel roll (HSS roll) was studied in order to improve wear resistance and surface deterioration resistance of HSS roll. Although the volume of eutectic M7C3 carbide increased with the increase in C and Cr contents, wear resistance was not improved. The increase in Cr and Mo contents in good balance improved the wear resistance of HSS Roll remarkably, as eutectic carbide of M7C3 type became tough and the damage on the worn surface was prevented.
    The destructive prevention mechanism of carbide was discussed from change of the alloy concentration and the lattice constant of carbide by increasing in Cr and Mo contents.
  • Twin Morphology Formed on Goss Si-steel Single Crystal

    pp. 686-690

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    The twin morphology of Goss single crystal of silicon steel was investigated using electron back scattering diffraction (EBSD) method of FESEM, and was compared with that of W-SEM.
    The orientation analysis parallel to ND, RD and TD showed a very narrow band of about 3 μm width perpendicular to the rolling direction. The twin formed between the boundary of the Goss single crystal matrix and the very narrow band. This boundary showed step-like morphologies of about 70-200 nm. The angle between the two <110> orientations of the (211) twin was 70.5 deg.
    The origin of twin due to the alternate formation of twin boundary was also observed. It is suspected that this permitted to do the formation energy of twin as low as possible
  • Precipitation of Z-phase and Precipitation Sequence during Creep Deformation of Mod.9Cr-1Mo Steel

    pp. 691-698

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    Changes in type, size and number density of precipitates during creep exposure have been investigated in Mod.9Cr-1Mo steel, and precipitation behaviour of Z-phase and its influence on another precipitates and creep strength has been discussed. Three type precipitates of M23C6 carbide, NbX and VX carbonitrides were detected in the as tempered condition. Precipitation of Laves phase was observed after short-term creep exposure at 600°C and that of Z-phase was found in the long-term region after several thousands hours of creep exposure at both 600 and 650°C. Mean diameter of MX carbonitride was about 1/3 to 1/7 of that of M23C6 carbide, and those phases were roughly separated by size. With precipitation and rapid coarsening of Z-phase, decrease in number density of fine MX carbonitride and disappearance of NbX was observed. Penetration of nitrogen from atmosphere was not observed during creep deformation. A lot of fine MX carbonitride at the vicinity of prior austenite grain boundary caused by coarse primary MX should promote precipitation of Z-phase. Composition of metallic elements in NbX, VX and Z-phase was limited in small range and unchangeable independent of temperature and exposure time. Nucleation and rapid coarsening of Z-phase after long-term creep exposure results in decrease in creep strength as a result of decrease in precipitation strengthening effect of fine MX carbonitride.
  • Creep Damage Estimation based on the Softening Behavior of 10Cr-1Mo-1W-VNbN Steel

    pp. 699-704

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    Microstructural observations were carried out on 10Cr-1Mo-1W-VNbN steels artificially deteriorated by ageing, creep and creep interrupted tests under various conditions. Martensitic lath structure was collapsed during high temperature exposure regardless of stress. The quantitative analysis of lath structure was performed by measuring lath width. The mean lath width increased with time and the increasing trend corresponded to both the creep strain and softening. The theoretical formulation of the softening was designed based on the kinetics of the dislocation structure. The formula gave quantitative correlation among the creep stress, temperature, time and hardness. The creep damage at any given condition was estimated by this formula, and the estimated result was in good agreement with the experimental value of the hardness at any creep time fraction t/tr.
  • The Influence of Chemical Composition and Temper Heat-treatment on Strength and Impact Toughness in Ni-Cr-Mo-V Steels

    pp. 705-710

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    To meet the requirements for optimum utilization of power station sites and economic realization of coal fired thermal units, tandem compound 60 Hz 1000 MW large thermal units were developed. One of the most critical items was development of a large-size high strength generator rotor forging with comparable toughness to the conventional rotor forging. Based on the investigation results of existing rotor forging, chemistry optimization was conducted within the specification of conventional turbine generator and low pressure rotor forging by using laboratory heat materials and 0.28%C-0.25%Mn-4%Ni-1.75%Cr-0.4%Mo-0.12%V high purity steel was selected for the candidate material. Double tempering heat treatment of 550°C and 580°C was also developed for increasing the yielding ratio of 0.02% yielding stress to tensile strength to meet the requirements of mechanical properties. According to those laboratory studies, one trial rotor forging with the same diameter as production ones was successfully produced.
  • Significance of Size and Type of Inclusions on Giga-cycle Fatigue in High-strength Steels

    pp. 711-717

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    Fatigue tests were carried out up to 108 cycles for a series of 1800 MPa-class spring steels in order to investigate the relation between inclusions and fatigue properties. The fatigue tests were carried out both for billets and bars rolled from the billets and, in case of billets, the tests were both for RD and TD directions.
    Al2O3rich inclusions were undeformable and not lengthened in rolling. On the other hand, SiO2 rich inclusions were divided into small pieces and the effective sizes for the axial direction were little reduced in rolling although the inclusions were deformable. In turn the difference of fatigue strength between billets and bars was not so large in most cases. The TD direction fatigue tests showed fish-eye fractures with the origins of large and lengthened MnS inclusions and the fatigue strengths were about a half of those in the RD direction.
    The fatigue data plotted on a modified S-N diagram, in which stress amplitude were normalized by fatigue limits estimated by Murakami's equation, were divide into two groups. One consisted of large and lengthened MnS inclusions and large artificial notched defects. The other consisted of small defects such as Al2O3 and TiN inclusions and matrix cracks. The result showed that the effects of the lengthened MnS inclusions were almost equal to artificial notches and apparently different from relatively small defects. Moreover, the small defect group was also divided into Al2O3 inclusion group and others, suggesting the effect of properties of defects on the fatigue strengths.
  • Effects of Retained Austenite on Tension-Compression Fatigue Properties of Cold Work Tool Steel

    pp. 718-725

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    Effects of retained austenite (γR) on tension-Compression fatigue properties of cold work tool steel have been investigated using modified SKD11 (Mod.SKD11), which contains 0.8C-8Cr-2Mo-0.5V. Tension-Compression fatigue specimens were tempered at 180°C, 470°C and 540°C after quenching from 1030°C. These conditions vary the amount and stability of γR with equal hardness of 60HRC. The amount of γR in these specimens tempered below 500°C was 15 vol%. The γR stability to stress induced transformation in specimen tempered at 470°C can be considered to increase than that of 180°C, because C content in γR for the specimen tempered at 470°C is higher than that of the specimen tempered at 180°C. Above 500°C, the γR in specimens was decomposed and secondary hardening occurred. Fatigue tests were done under fully reversed stress amplitude of 1000 MPa. Each result was plotted on Weibull probability sheet. Fatigue lives of the specimens tempered at 470°C were longest, and decreased in the sequence 180°C→540°C. Fisheyes were observed on the fracture surface of each specimen. The diameter of fisheye for the specimens tempered at 180°C and 470°C was larger than that for the specimen tempered at 540°C. It seems that γR contributes to the increase of the resistance for crack propagation. As a result of measurement of γR volume fraction after fatigue test, it is found that γR in specimen tempered at 180°C is unstable and the volume fraction of γR decreased approximately 5%, while volume fraction of γR in specimen tempered at 470°C was unchanged. Thus, it is considered that the amount and stability of γR contribute to the increase of fatigue lives.
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    Readers Who Read This Article Also Read

    1. Improvement of Strength and Toughness of SKD11 Type Cold Work Tool Steel Tetsu-to-Hagané Vol.84(1998), No.3
  • Low-and High-cycle Fatigue Properties of Ultrafine-grained Low Carbon Steels

    pp. 726-733

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    It has been well known that fatigue properties of steels are improved by grain refinement. In the STX-21 project, we have been developing low carbon steels with sub-micron meter grain sizes and reported that their tensile strength is 800 MPa and DBTT is around 77K.
    In this report, low-and high-cycle fatigue properties of ultrafine-grained steels with different grain diameters of 0.7 and 0.9 μm and different carbon contents of 0.05 and 0.15 mass% were investigated. The grain refinement improved high-cycle fatigue properties, whereas it did not affect low-cycle fatigue properties. Fatigue limit showed the similar Hall-Petch type relationship for several studies carried out hitherto.

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