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

Tetsu-to-Hagané Vol. 97 (2011), No. 3

  • Influence of Product Material Property on Tool Wear in Shearing Process

    pp. 99-107

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    DOI:10.2355/tetsutohagane.97.99

    In shearing process, tool wear is important to provide constant quality of product. The influence of flow stress of product material on tool wear is investigated. Piercing process experiment is carried out 50,000 strokes under the different conditions of sheared material and the clearance between punch and die. The punch edge radius and the amount of tool side wear are measured to evaluate the variation of tool wear. Additionally, the shape of sheared surface is measured to investigate the influence of tool wear on the product shape. Furthermore finite element analysis of shearing process is carried out under the same condition to confirm the influential factor on tool wear. In the experimental result, punch and die edge radius similarly increase with increasing of stroke number. Tool side wear causes increasing of clearance. However, these tendencies are different for sheared material. The shape of sheared surface is affected by tool edge wear than tool side wear. From analytical result, tool wear is predicted qualitatively by using friction energy, which is calculated from relative slip velocity between tool and material and surface pressure. These values are affected by material flow stress. Friction energy increases with increasing of flow stress and it causes increasing of tool wear.
  • Numerical Analysis for Corrosion near Shear Cut Edge of Galvanic Steel Sheet

    pp. 108-116

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    DOI:10.2355/tetsutohagane.97.108

    We have developed a new mathematical model for galvanic corrosion with ions movement, reactions and electroneutrality. In this model, ion density distribution is corrected by solving Poisson's equation to satisfy electroneutrality. Major ion species and reactions are considered in NaCl solution, MgCl2 solution and artificial sea water on a Fe/Zn galvanic couple.
    Distribution of corrosion products in NaCl solution and MgCl2 solution obtained by numerical analysis agreed well with measured by FTIR method qualitatively.
    Corrosion progresses at shear cut edge of galvanic steel sheet in NaCl solution, MgCl2 solution and artificial sea water were simulated and discussed. In addition, galvanic corrosion under a paint film near the shear cut edge was computed. The numerical analysis result indicated that concentration of Cl under the paint film was increased by more than factor of 10.
  • Effect of Cr Contents on Corrosion Resistance of FeAl in HCl Solution

    pp. 117-122

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    DOI:10.2355/tetsutohagane.97.117

    Addition of Cr in intermetallic compound FeAl improves its corrosion resistance protecting grain corrosion. At the same time it has been clarified that the addition of Cr in FeAl has the possibility to improve the corrosion rate to almost zero. The relationship between corrosion resistance of FeAl(Cr) and Cr contents has been proved.
    The phenomenon of corrosion rate of FeAl(Cr) that was fast with little Cr addition but slow with much Cr addition was observed. Corrosion rate of FeAl(Cr) which is added until 8 at% Cr is faster than that of FeAl or same. It is getting late at added 10 at% Cr and then addition of 12 at% Cr makes it almost zero. The corrosion of not only FeAl but FeAl(Cr) starts from grain corrosion and it has become obvious there are much Fe at grain boundary of FeAl and FeAl(Cr).
    Anodic polarization curves for the specimens in 0.1 mol/L HCl solution showed that addition of Cr increased corrosion resistance by forming protective film easily.
  • Effect of Chemical Compositions of Case Hardening Steels for Distribution of Carbon and Cementite during Vacuum Carburizing

    pp. 123-129

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    DOI:10.2355/tetsutohagane.97.123

    To develop the model to predict microstructures of case hardening steels during vacuum carburizing, the effects of chemical compositions such as Cr and Si on carburizing behaviors were investigated by applying three steels based on Fe–1.1Cr–0.8Mn–0.25Si–0.2C(SCr420H). Distributions of Carbon(C) near the carburizing surfaces and grain boundary cementite (θ) were analyzed by GD-OES and image analysis of microstructures respectively. It was revealed that the C distributions and the volume fractions of θ ware affected by the chemical compositions i.e. they increased with the Cr amount and decreased with Si amount. The analyzed results by XRD and FE-EPMA showed the C content in the retained austenite (γ) near the surface was around 1.4–1.6 mass% which was higher than the equilibrium condition.
    By the comparison of experimental and calculation results which was obtained from the commercial multi-elemental diffusion solver, DICTRA, it was confirmed that the C in the γ was supersaturated during vacuum carburizing and the surface condition was estimated as equal to the C activity of the meta-equilibrium boarder of γ/(γ+graphite). Additionally the result of θ distribution suggests volume fraction of θ can not be predicted by equilibrium calculations but a kinetic model for θ growth is required.
  • Calculation of Microstructure in Vacuum-carburizing Incorporating Kinetics Modeling of Grain-Boundary Cementite

    pp. 130-135

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    DOI:10.2355/tetsutohagane.97.130

    In vacuum carburizing of steels, a short-time carburizing is usually followed by a diffusion period to eliminate the film-like cementite (θ) grown on austenite (γ) grain boundary surface. Due to a radical reaction with decomposed hydro-carbon gas, the surface C content in γ was found higher than the solubility of θ. It was recognized as the metastable equilibrium of supersaturated γ with graphite until the θ covered all the surfaces. Based on this finding, a calculation model predicting not only C concentration profiles but also quantitatively the grain-boundary θ has been developed in this work.
    The existing model estimates the amount of θ with the equilibrium fractions for local C contents in a framework of finite difference method (FDM). Even with the equilibrium surface condition of γ plus θ with graphite, it overrates the amount of θ observed after several minutes of carburizing. In the developed model, a parabolic law was assumed for the thickening of θ, and the rate controlling process was considered Si diffusion rejected from θ under the isoactivity condition of γ/θ interface and supersaturated γ. The rate coefficient (αSi) has been validated using multicomponent diffusion simulation for moving velocity of the γ/θ interface. A 1-D FDM program calculates an increment of θ for all gridpoints using the updated diffusivities and local equilibrium by a coupled CALPHAD software. Predictions of carbon profile and volume fraction of cementite represent the experimental analysis much better than the existing models especially for short-time carburizing.
  • Influence of Microstructure on Ductile Damage Behavior of Dual Phase Steel

    pp. 136-142

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    DOI:10.2355/tetsutohagane.97.136

    High tensile strength steel is used to save weight of the automotive body for reduction of CO2 emissions. Because of its high tensile strength, the deformation of high tensile strength steel is difficult. Many kinds of high strength steel are developed to achieve a good balance between high strength and formability. Dual phase steel is one of these steels. It has ferrite and martensite phase as a soft phase and hard phase respectively. In Dual Phase steel, ferrite phase has formability and martensite phase has strength part. However, influences of their volume fraction, chemical composition and heat treatment conditions on material properties are not clear. Therefore, in this study, mechanical properties and micro void behavior on ductile fracture of dual phase steel are investigated. The specimens with difference chemical composition and heat treatment condition are provided. Basic material test is carried out to obtain these mechanical properties. Additionally, the change of void volume fraction during forming is measured and void nucleation site is observed by using laser microscope and image analysis. In the result, dual phase steel containing large volume fraction of ferrite phase shows high uniform elongation and that of martensite phase shows high local elongation. Furthermore, the gradient of void volume fraction of the steel which shows high equivalent fracture strain is lower than that of the steel which shows low equivalent fracture strain.
  • Delayed Fracture Using CSRT Method and Hydrogen Trapping Characteristics of V-bearing High Strength Steel

    pp. 143-151

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    DOI:10.2355/tetsutohagane.97.143

    The delayed fracture characteristics of V-bearing steel were evaluated using the CSRT method, and the hydrogen trapping and de-trapping behavior was studied with the specimens hydrogen-charged and then held in air for up to 2.5 months.
    The CSRT tests were carried out at two test sites and nearly the same delayed fracture characteristics were obtained using the V-bearing steel. The fracture appearance changed from quasi-cleavage to inter-granular fracture with an increase of hydrogen content. The boundary hydrogen content of the fracture appearance change is around 4 mass ppm.
    The hydrogen charged in V-bearing steel was composed of diffusible one, which comes out in 24 h when held in air of 30°C and two kinds of (weakly and strongly) trapped ones. The strongly trapped hydrogen remained in specimen after 2.5 months held in air. By analyzing the thermal desorption profiles with Gaussian function, the peak temperatures of these hydrogen were 100°C, 167°C and 198°C, which corresponds to diffusible, weakly and strongly trapped hydrogen.
    The specimens hydrogen-charged more than 4 mass ppm were fractured in inter-granular mode. After held in air and the hydrogen content became less than 4 mass ppm the fracture mode changed to quasi-cleavage. Re-charging the hydrogen more than 4 mass ppm, the fracture mode became inter-granular again.
  • Evaluation of Dislocation Accumulation and Work Hardening in Ferritic Steels Containing Plastically Deformable Soft Particles

    pp. 152-158

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    DOI:10.2355/tetsutohagane.97.152

    The work hardening behavior and the change in dislocation density during tensile deformation were investigated in ferritic steels containing hard carbide/oxide particles or soft Cu particles with different volume fraction and particle diameter. The magnitude of work hardening were significantly different between both types of steels. In the hard particle dispersion steels, the results of dislocation density measurements agreed well with the theory estimating GN dislocation density proposed by Ashby. On the other hand, in soft Cu particle dispersion steels, the increase in dislocation density during tensile deformation was relatively small compared with the hard particle dispersion steels, resulting in the smaller work hardening rate. This is due to the plastic deformation of Cu particle itself leading to the partial accommodation of the shear strain given in the existence area of particle. A quantitative evaluation of accumulation of GN dislocations was attempted in the soft Cu particle dispersion steels by modifying Ashby's theory using ‘particle plastic accommodation parameter’. With this modification, it was found the work hardening behavior in the early stage of tensile deformation of ferritic steels could be systematically explained by the accumulation of GN dislocations regardless of the stiffness of dispersion particles.
  • Effect of Humic Substances on Iron Elusion in the Method of Restoration of Seaweed Beds with Steelmaking Slag

    pp. 159-164

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    DOI:10.2355/tetsutohagane.97.159

    Barren grounds in coastal area are serious problems in Japan and throughout the world. Although several factors have been proposed to account for barren grounds, we have especially focused on lack of dissolved iron for restoring seaweed beds. It has been developed a method that the mixture of steelmaking slag and humus materials, such as composts, were supplied in seawater. A concentration of dissolved iron can be increased by using the method, since complexes, iron-humates, are produced from iron in steelmaking slag and humic substances in compost. In this study, we evaluated the effect of humic substances in this method for increasing dissolved iron concentration. A laboratorial iron elution test by using actual seawaters was attempted. Three kinds of samples for iron elution, only steelmaking slag, only humus materials, and a mixture of steelmaking slag and humus materials, were prepared. The change of iron concentration in each small tank had been monitored. We found that iron elution rate in the case of the mixture of steelmaking slag and compost was faster than that in the case of only steelmaking slag. Furthermore, it was expected that the characteristic of the structure of humic substances were related to increase iron elution from steelmaking slag. The mixture of steelmaking slag and humus materials was more effective not only for increasing iron concentration in seawater and but also for extending the life time of Fe elution.

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