Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 91 (2005), No. 8

  • Basic Characteristic of Swirl Motion in Tapered Bath Agitated by Gas Injection

    pp. 623-629

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    A tapered bath agitated by bottom gas injection showed two types of swirl motions; shallow-water wave type and deep-water wave type. The occurrence conditions of the swirl motions were experimentally determined. Empirical equations were proposed for the period and amplitude of the swirl motions of the deep-water wave type. The numerical results for the period and amplitude were in good agreement with the values calculated from the empirical equations even for much larger scale vessels than that used for experiments. This fact suggests that the empirical equations are applicable to real scale baths.
  • Effects of Inclusion and ODA Sizes on Gigacycle Fatigue Properties of High-strength Steels

    pp. 630-638

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    Gigacycle fatigue tests up to 109 cycles were conducted for SCM440 and SUJ2 steels. Five types of each steel were prepared with different melting processes. The gigacycle fatigue properties were divided into two groups: One group, consisting of SCM440 and double-melted SUJ2 steels, revealed higher fatigue strengths than the other group consisting of single-melted SUJ2 steels. The origins of fish-eye fracture were mostly an small Al2O3 in the high fatigue strength group and entirely a large (Cr, Fe)3C in the low fatigue strength group. ODA sizes normalized with inclusion sizes at the fracture origin showed dependency on the inclusion sizes. The normalized ODA sizes increased according to the inclusion size decrease when the inclusion sizes were below about 10 μm, although the normalized ODA sizes were constant when above that size. Relating to this dependency, fatigue limits at 109 cycles were independent of the inclusion sizes when the inclusion sizes were enough small. The inclusion sizes, below which fatigue limit lost dependency on the inclusion sizes, were expected to exist between 10 and 20 μm. On the other hands, fatigue limits depended on inclusion sizes when the inclusions were large enough to exceed that size, as well as on ODA sizes. The ODA and large inclusions were similar to small cracks in which ΔKth depends on the crack size to the 1/3th power. This meant that the fatigue limits depended on the ODA and large inclusion sizes to the-1/6th power.
  • Wear Reduction of Carbide Tools Observed in Cutting Ca-added Steels for Machine Structural Use

    pp. 639-646

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    The effect of Ca addition for improving machinability in carbide tool machining operation has been widely known. It has been proposed that oxide inclusions, which would work as "belag" on the tool surface during machining operation, has been responsible for this effect. In recent years, when Ca-treated steels containing higher sulfur are machined, MnS has been observed to form on the rake face, and this "inclusion layer, " instead of the oxide belag, would serve favorably for machinability.
    In this paper, we have investigated the influence of several inclusions and the addition of Ca and/or Mg on rake face wear of carbide tools. The rake face wear of Ca-added steel was appreciably reduced due to the MnS inclusion layer formed on the rake face. However, the concurrent addition of Ca and Mg to the steel did not improve the rake face wear. Combined with a TEM observation in a cross section of MnS layer formed on the rake face, these results are understood by the different nature of oxide inclusions in terms of abrasive wear.
  • Superplastic Anisotropic Behavior in Magnesium Alloy of AZ91D Grain Refined by Isothermal-rolling

    pp. 647-654

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    Thin magnesium alloy of AZ91D sheets worked by isothermal-rolling and then annealed around the temperature between 0% and 100% recrystallization have been prepared, and tensed at right angles to rolling direction at elevated temperatures and at several strain-rates. Both the total elongation and strain-rate sensitivity m have been obtained and discussed.
    The m values was found to be 0.5 in range of 1.0 × 10-4s-1 to 1.0 × 10-3s-1. The activation energy required for superplastic deformation was calculated to be about 87 kJ ·mol-1, which was almost same of the activation energy for self-diffusional grain boundary coefficient in Mg.
    Although the high strain-rate sensitivity and the activation energy corresponding to self-diffusional grain boundary were obtained, contrary to expectations, the values of total elongation didn't exceed over 100% at all testing temperatures and at all strain-rates.
    Tensed at right angles to rolling direction, gap and offset presupposed during tensile testing were not accommodated by recrystallized anisotropic fine particles. Therefore, a large number of cavities caused, and consequently the total elongations decreased.
  • Dependence of Magnetic Properties on Dislocation Density in Commercial Steels Deformed in Tension

    pp. 655-661

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    Dependences of two magnetic parameters, coercive field Hc and coefficient c, on dislocation density were investigated in tensile deformed Fe polycrystal and SFVQ-1A commercial steel. The dislocation density was measured by using transmission electron microscope (TEM). Magnetic parameters were analyzed from hysteresis loops. Coercive field Hc and coefficient c were proportional to ρ1/2 and ρ, respectively. These parameters have simple dependences on dislocation density, and would be useful for nondestructive evaluation of dislocation structure in commercial steel.
  • Effect of Mn on Strain Aging Behavior of Ultra Low Carbon Steel Sheets

    pp. 662-669

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    Aging behavior of low Carbon steel depends on chemical compositions. Mn were reported to reduce BH (Bake Hardening) value at low carbon mild steel, but its mechanism are not cleared. In this paper, strain aging behavior of ultra low carbon steel with Mn were examined by measuring Yield Point, Yield Point Elongation and BH.
    The following results are obtained:
    (1) Aging rate is reduced by 1.0% Mn addition. BH value is also reduced by Mn addition, which means that Mn does not improve the relationship between aging rate and BH value.
    (2) The BH values of 300 min aging of the samples with Mn show higher values than the BH with 20 min, and show almost same values of the samples without Mn. It seems that no stable phase is formed by Mn addition.
    (3) Atomic level interaction between Mn and Carbon is assumed to exist and its value is smaller than that of Mo-C. The trapping ratio of C by Mn at 25°C is smaller than that of Mo, and the trapping ratio of C by Mn at 170°C is larger than that of Mo, which cause the BH deterioration at Mn added steel.
  • Effects of Silicon, Molybdenum, and Nitrogen on Elevated Temperature Properties of Low Carbon Austenitic Stainless Steels

    pp. 670-675

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    To improve the high temperature mechanical properties of low carbon medium nitrogen type 316 stainless steels (316FR) the effect of Si addition on high temperature properties was investigated in relation to Mo and N which are the strengthening elements of 316FR. Tensile and creep rupture tests at 550°C and aging tests at 550, 600, and 650°C were conducted. The microstructures of ruptured specimens and aged ones were examined with an electron microscope and extracted residues analysis. The addition of 3 mass% Si, 2% Mo, and 0.08% N increased both tensile strength and elongation. Creep rupture strength was slightly increased by the addition of Si with or without Mo and N, while the addition of Mo or N remarkably increased rupture strength. Although rupture elongation increased by the addition of Si to a base alloy and a Mo containing one, it did not changed with Si in case of a Mo and N containing alloy. The addition of Si to base alloy caused precipitation of G phase on grain boundaries, while in case of a Mo containing alloys Si remarkably accelerated precipitation of G phase on grain boundaries and Laves phase in matrix.
  • Dynamic and Static Restoration Processes in the α Region of Commercially Pure Titanium, and Effects of Oxygen and Iron on These Processes

    pp. 676-684

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    Hot deformation behavior in the α region of commercially pure titanium (C.P. titanium) was investigated together with the effects of oxygen and iron on dynamic and static restoration processes. Hot compression tests were performed using a hot working simulator of THERMECMASTER-Z, and stress-strain (S-S) curves and dynamically restored microstructures were obtained under various hot deformation conditions covering the temperature from 873 to 1073K and strain rate from 1 × 10-3s-1 to 1 s-1. Static restoration behavior was investigated through variation of softening ratio with holding time obtained by intermittent two-steps hot deformation. Three types of S-S curves were obtained and classified by the deformation temperature and strain rate. Dynamic restoration process in the α region of C.P. titanium was confirmed to be dynamic recovery type from both of stress-strain curves and observation of dynamically restored microstructure. Both activation energy values in dynamic and static restoration processes were almost same as approximately 185 kJ/mol, which was very close to the activation energy for self-diffusion of titanium in α titanium. The effect of oxygen or iron with a respective content of 0.3% on flow stress and static restoration kinetics was relatively small, although the temperature-strain rate region for occurrence of dynamic recovery was slightly expanded by addition of these elements. These results appeared to be brought about by relatively high diffusivity of these elements in α titanium.

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