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

Tetsu-to-Hagané Vol. 98 (2012), No. 6

  • Preface to the Special Issue on “Fundamentals and Novel Approaches for New Demands on Work-Hardening Properties of Steels”

    pp. 215-215

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    DOI:10.2355/tetsutohagane.98.215

  • Tensile Properties and Work-hardening Behavior of Fe-3%Al Single Crystal

    pp. 216-222

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    DOI:10.2355/tetsutohagane.98.216

    The tensile deformation behavior of single and poly-crystal Fe-3%Al alloys was investigated at room temperature. The work hardening rate (WHR) of the polycrystalline alloy monotonously decreased with increasing strain and increased with decreasing grain size. The WHR of the single crystal alloy was remarkably lower than that of the polycrystalline alloy, and the single crystal alloy exhibited a peculiar behavior that could be classified into four different types depending on the tensile axes. When the tensile axes were on the line connecting ‹102› and ‹101› in the stereo triangle, the characteristics of deformation of the single crystal alloy were steady and its WHR showed a monotonous decrease similar to that observed in the case of the polycrystalline alloy. For most of the tensile orientations, including ‹001›, the WHR curve changed from concave to convex with a strain increase. The WHR for the ‹101› orientation also showed the maximum and minimum values, i.e., it dropped immediately after reaching the maxima. This behavior was considered ascribable to the transition of the dislocation structure. The deformation for the ‹111› orientation was characterized by the highest yield stress and work softening, which were caused by some intense slip bands with discontinuous stress drops. Lattice rotation with tensile deformation in single crystal alloys was also examined by an electron backscatter diffraction (EBSD) measurement. For determining the rotation direction, the stereo triangle was divided into two areas by a great circle connecting ‹113› and ‹102›.
  • Effect of Carbon and Nitrogen on Work Hardening and Deformation Microstructure in Stable Austenitic Stainless Steels

    pp. 223-228

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    DOI:10.2355/tetsutohagane.98.223

    Stable austenitic stainless steels containing 0.1 % carbon and nitrogen (Fe-18%Cr-12%Ni-0.1%C and Fe-18%Cr-12%Ni-0.1%N alloys) were tensile-tested to clarify the difference between the effects of carbon and nitrogen on the work hardening behavior as well as the deformation microstructure development in austenite. The carbon-added steel exhibited a much larger work hardening rate than the nitrogen-added steel in the high strain region (true strain > 0.25) although the dislocation accumulation was more significant in the nitrogen-added steel. EBSD analysis revealed that deformation twins were more frequently formed in the carbon-added steel, which leads to the TWIP effect. The reason why the nitrogen-added steel showed the less twinning behavior seemed to be mainly related with the short range order (SRO) composed of Cr and N atoms.
  • TWIP Effect and Plastic Instability Condition in an Fe-Mn-C Austenitic Steel

    pp. 229-236

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    DOI:10.2355/tetsutohagane.98.229

    The correlation among deformation twin density, work hardening, and tensile ductility was investigated in an Fe-18Mn-1.2C steel. The twin density was varied by changing tensile deformation temperature from 123 to 523 K. The deformation twin density at a 10% plastic strain decreased with increasing deformation temperature except for the condition (123K) in which martensitic transformation occurred. The work hardening rate at the early deformation stage decreased due to the reduction in the deformation twin density; however, the decrease in work hardening rate did not affect the uniform elongation. The uniform elongation was determined by a plastic instability condition. Thus, the important factor for the uniform elongation is the work hardening rate in the later deformation stage. Additionally, we discussed influences of dynamic strain aging and ε-martensitic transformation which accelerated fracture.
  • Brittle-to-Ductile Transition in Nickel-Free Austenitic Stainless Steels with High Nitrogen

    pp. 237-244

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    DOI:10.2355/tetsutohagane.98.237

    The brittle-to-ductile transition (BDT) behaviour in nickel-free austenitic stainless steel with high nitrogen was investigated. Fall-weight impact tests revealed that Fe-25mass%Cr-1.1mass%N austenitic steel exhibits a sharp BDT behaviour in spite of an fcc alloy. The aspects of plastic deformation after the impact tests indicate that the BDT observed in this austenitic steel is induced by poor ductility at low temperatures as is the same as that in ferritic steels. In order to measure the activation energy for the BDT, the strain rate dependence of the BDT temperature was examined by using four-point bending tests. The weak dependence of the BDT temperature on the strain rate was observed. The Arrhenius plot of the BDT temperature against the strain rate elucidated that the activation energy for the BDT of Fe-25mass%Cr-1.1mass%N is much higher than that of low carbon ferritic steels. The origins of such distinct BDT behaviour and its large value of the activation energy in this high-nitrogen steel are discussed in terms of the reduction of dislocation mobility at low temperatures due to the interaction between glide dislocations and nitrogen solute atoms.
  • Effect of Solution Carbon and Retained Austenite Film on Development of Deformation Structure of Low Carbon Lath Martensite

    pp. 245-252

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    DOI:10.2355/tetsutohagane.98.245

    The development of deformation structure in the low carbon lath martensite steel was clarified using transmission electron microscopy observation with Kikuchi pattern analysis. The retained austenite films on the martensite lath boundaries transform to high carbon martensite films by light deformation, and the high carbon martensite films protect martensite lath boundaries from deformation. The specific deformation structures of low carbon lath martensite, such as kinked laths, irregularly bent lamellas and lamellar dislocation cells are formed by the retained austenite films. The tempered lath martensite structure without retained austenite films easily disappears by light deformation. We also clarified the relationship between the mechanical properties, such as work-hardening ratio, and the development of the the deformation structure.
  • Effect of Solute Carbon Content on Microstructures of Cold-rolled Ferritic Steel
    -The Same Area Analyses by Using TEM and SEM-EBSD-

    pp. 253-261

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    DOI:10.2355/tetsutohagane.98.253

    Deformation microstructures developed in cold-rolled ultra low carbon (ULC) steel as well as those in low carbon (LC) steel have been investigated by using TEM and SEM-EBSD techniques. Particular attention has been paid to the effect of solute carbon on the development of those microstructures. Dislocation structures characteristic to the preferred orientations such as γ-fiber (ND//‹111›) and α-fiber (RD//‹011›) have been revealed by the same area observation employing the above two techniques.
    TEM images of dislocation cell boundaries observed in ULC are sharper than those in LC structures. Images of dislocation line segments were separately distinguished in cell structures in ULC, while in LC they were indistinguishable because of high density of dislocations. This indicates that dislocation density increases with increasing the amount of solute carbon, which was confirmed also by XRD measurement. In grains of ND//‹111›, fine microbands and/or shear bands (SBs) were developed while in RD//‹011› grains such remarkable inhomogeneous microstructures were not observed, which suggests that work-hardening in ND//‹111› grains is more prominent than that in the other preferred orientations. In {111}‹211› grains of LC steel, the same kinds of shear bands as observed in Fe-Si steels were formed as the most characteristic microstructure, where elongated fine-grained structures with the orientation scattering of 35° between the {111}‹211› and {110}‹001› Goss orientation were found.
  • Neutron Diffraction Study on Anisotropy of Strain Age Hardening in Ferritic Steel

    pp. 262-266

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    DOI:10.2355/tetsutohagane.98.262

    The work-hardening characteristics of anisotropic tensile deformations and the corresponding residual strain changes of pre-strained ferritic steels without and with aging treatment were investigated by using angle dispersive neutron diffraction and electron backscatter diffraction pattern analysis. The plastic deformation along the pre-strained direction leads to evident work-hardening at the beginning stage, showing discontinuous yielding behavior. Comparably, the plastic deformation perpendicular to the pre-strained direction shows continuously yielding. The tensile and compressive residual strains were found in the ‹200› and ‹110› grains along the pre-strained direction, respectively. It is also found that the difference in various oriented grains after strain aging become more evident along the pre-strained direction but smaller perpendicular to the pre-strained direction, revealing a higher work hardening capability in the former case than in the latter case.
  • Effect of Strain Path Change and Strain Aging on Anisotropic Work-Hardening Behavior in Ferritic Steel

    pp. 267-274

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    DOI:10.2355/tetsutohagane.98.267

    It has been well known that anisotropy in yield stress and the work-hardening rate is induced by pre-strain and aging. However, such an origin has not been adequately understood. In the present study, stress-strain curves in different directions were investigated after 2% pre-straining and post-heat treatment at 150°C in ferritic steel. When the applied strain path was changed to the orthogonal direction of the pre-straining path, the re-yield stress was lowered and the work-hardening rate in the low plastic strain was increased. The heat treatment following 2% pre-straining caused an increase of the re-yield stress in the parallel direction to the pre-strain and caused no change on the re-yield stress in the orthogonal direction. The work-hardening rate was increased in both directions after the heating. Electron back scatter diffraction pattern (EBSP) analysis was also conducted to measure the kernel average misorientation (KAM) value, which corresponded to the density of the geometrically necessary dislocation, on each [hkl]-oriented family grain for the pre-strained and the post-heated materials. The EBSP results indicated that heterogeneous work-hardening behavior among the [hkl]-oriented family grains could strongly effect the anisotropy induced by strain path change and aging.
  • Measurement and Material Modeling of Work Hardening Behavior of Cold Rolled IF Steel Sheet Using Multiaxial Tube Expansion Test

    pp. 275-282

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    DOI:10.2355/tetsutohagane.98.275

    Deformation behavior of cold rolled IF steel sheet (SPCE) under biaxial tension has been investigated for large plastic strain range over 0.15. The test material was bent and TIG welded to form a tubular specimen with an inner diameter of 44.6 mm and wall thickness of 0.8mm. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens using a servo-controlled tension-internal pressure testing machine developed by one of the authors [T. Kuwabara, K. Yoshida, K. Narihara, S. Takahashi, Anisotropic plastic deformation of extruded aluminum alloy tube under axial forces and internal pressure, Int. J. Plasticity 21, 101-117 (2005)]. Moreover, biaxial tensile tests using a cruciform specimen were also performed to precisely measure the deformation behavior for a small strain range following initial yielding. True stress-true plastic strain curves, contours of plastic work in stress space and the directions of plastic strain rates were precisely measured and compared with those calculated using selected yield functions. Plastic deformation behavior up to a work equivalent plastic strain of 0.19 was successfully measured and the test material exhibited differential hardening. The Yld2000-2d yield function with an exponent of five most closely predicts the contours of plastic work and the directions of plastic strain rates.
  • Numerical Homogenization Methods Based on Heterogeneous Microstructure in Multi-Constituent Steels

    pp. 283-289

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    DOI:10.2355/tetsutohagane.98.283

    The purpose of this study is to present the applicability of homogenization methods based on heterogeneous microstructure in multi-constituent steels. At first, basic equations of a standard elastoplastic constitutive model and the homogenization methods have been discussed to describe nonlinear stress-strain curves of metallic materials. Here the homogenization methods have been classified into three types with the theoretical differences in concept. Numerical demonstrations for Dual-Phase steel have been performed to express the potential of these homogenization methods. The results of numerical demonstrations have indicated that a class of the numerical homogenization approaches is especially useful in both a quantitative evaluation of macroscopic material response and an investigation of microscopic mechanism in consideration of morphology of microstructure.
  • Comparative Study of Microscopic Morphology and Mechanical Behavior between Experiments and Numerical Analyses for Ferrite-Pearlite Dual Component Steel

    pp. 290-295

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    DOI:10.2355/tetsutohagane.98.290

    Relationship between microscopic morphology and mechanical behaviors has been examined with numerical analyses and experimental tests for Ferrite-Pearlite dual component steels characterized with different volume fractions of Pearlite microstructure. In the numerical analyses, three types of microscopic morphology have been modeled with finite elements and the deformation state of the finite element model has been computed by means of finite element analysis based on mathematical homogenization method. For experimental tests, specimens of some volume fraction of Pearlite have been made under adjusting strengthening factors except the volume fraction of Pearlite as much as possible. By comparing these numerical and experimental results, the effect of the microscopic topology to macroscopic mechanical behavior has been discussed in this study.
  • Deformation Behavior in High-Strength Dual-Phase Steel Sheets during Bend Forming

    pp. 296-302

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    DOI:10.2355/tetsutohagane.98.296

    The individual deformation behavior of ferrite and martensite was investigated microscopically by using nanoindentation technique to clarify bendability of dual-phase steel sheets. The cold-rolled steel sheets with composition of Fe-0.16mass%C-1.0mass%Si-1.5mass%Mn were annealed at 775°C for 600 s, water-quenched and tempered at 350°C for 300 s. V-shape bending tests with bending angle of 90° were performed for annealed specimens with the thickness of 1.0 mm. Nanoindentation experiments were performed with several peak loads to measure the distribution of the hardness in sub-micron areas, using a cube corner type indenter. The microstructure of the specimen obtained by the heat treatment was ferrite-martensite dual-phase. Ferrite was the dominant microstructure with a volume fraction of approximately 65 %. The nanohardness for ferrite and martensite was 3.2 GPa and 5.9 GPa in the annealed specimen, respectively. Both ferrite and martensite were hardened in the specimen with a bending radius of 1.0 mm. Especially, ferrite was hardened locally in the specimen. The nanohardness for ferrite in the markedly hardened area was 4.0 GPa and it was 3.6 GPa in another area. A distinct plastic flow such as a shear band was observed near the markedly hardened area. Voids nucleated on the ferrite side in the area. It was concluded that the development of shear bands during bending deformation with a small radius caused the local and remarkable work-hardening and that a larger partition of strain into ferrite than martensite domain resulted in the nucleation of micro voids in ferrite.
  • Visualization of Plastic Strain Distribution in a Dual-Phase Steel Using High-Precision Grid-Markers

    pp. 303-310

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    DOI:10.2355/tetsutohagane.98.303

    The aspect of inhomogeneous deformation in a dual- phase steel has been quantitatively visualized by a new method using high-precision grid-marker which was lithographed on the specimen surface by electron beam. This method enables imaging of plastic strain distribution in wide areas of millimeter scale with nano-scale spatial resolution, which makes it possible to obtain general view of the detailed plastic strain distribution in deformed multi-constituent steels with fine complex microstructures.
    A small plate specimen with a microstructure containing 50%ferrite and 50%martensite was deformed in tension at room temperature, and the grid-marker of the same area was observed at the average equivalent strains of 7.2% and 13.7%. Strain mapping indicated the abundant formation of characteristic bands with strains much higher than the average values. The bands were inclined by 45 degrees to the tensile axis, and they developed with increasing strain particularly in the ferrite areas adjacent to the interface between ferrite and martensite. It was also found that martensite areas actually deformed plastically and their deformation contributed to one fifth of the total plastic strain of the specimen.
  • Quantitative Analysis of Tensile Deformation Behavior by In-Situ Neutron Diffraction for Ferrite-Martensite Type Dual-Phase Steels

    pp. 311-319

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    DOI:10.2355/tetsutohagane.98.311

    The yielding and work-hardening behavior of ferrite- martensite type dual-phase (DP) alloys were clearly analyzed using the in-situ neutron diffraction technique. We successfully established a new method to estimate the stress and strain partitioning between ferrite and martensite phase during loading. Although these phases exhibit the same lattice structure with similar lattice parameters, their lattice strains on (110), (200) and (211) are obviously different from each other under an applied stress. The misfit strains between those phases were clearly accompanied with the phase-scaled internal stress (phase stress). Thus, the martensite phase yielded by higher applied stress than macro-yield stress, which resulted in high work-hardening rate of the DP steel. We also demonstrated that ferrite phase fraction influenced work-hardening behavior.
  • Effect of Martensite Fraction on Tensile Properties of Dual-Phase Steels

    pp. 320-327

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    DOI:10.2355/tetsutohagane.98.320

    Effects of martensite fraction on tensile properties and inhomogeneous deformation of microstructures in dual-phase steels have been investigated by TEM, SEM-EBSD and strain mapping using high-precision grid markers drawn by electron beam lithography. As is well known, when volume fraction of martensite is increased from 25% to 75%, yield and tensile strengths markedly increase while elongation decreases. It was found by EBSD that the average value of kernel average misorientation in ferrite increase with martensite fraction. Strain mapping using the marker method showed that both equivalent plastic strains of ferrite and martensite increase with martensite fraction, where it is to be noted that the strain increase of martensite is larger than that of ferrite, suggesting that the stress redistribution between ferrite and martensite is enhanced with increasing the martensite fraction.
  • Understanding of Stress Redistribution Due to the Internal Stress of Dislocation Pile-Up in Pearlite Steel

    pp. 328-330

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    DOI:10.2355/tetsutohagane.98.328

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