The 10 most-cited articles in 鐵と鋼 : 日本鐡鋼協會々誌, 2017

Several methods on single-peak and multi-peaks line-profile analyses for neutron or X-ray diffraction are critically reviewed. Compared with deconvolutional methods like the classical Williamson-Hall method and the modified Warren-Averbach method combined with the modified Williamson-Hall plot, a recently developed convolutional multiple whole profile (CMWP) fitting method enables to provide the density, arrangement and character of dislocation and crystallite size reasonably. Elasto-plastic deformation behavior in austenite, ferrite, martensite and two-phase steels are studied using such profile analyses, revealing that flow stress is dependent not only on dislocation density but also its arrangement. In particular, in situ neutron diffraction with the CMWP analysis is powerful to monitor microstructural changes during deformation and/or heat treatment. For the evaluation of crystallite or grain size, diffraction, small angle scattering and Bragg edge imaging should be employed taking their applicable length scales into consideration.

Effect of aging treatment at 373 K on Hall-Petch coefficient (ky) was investigated in consideration of the change in friction stress associated with carbide/nitride precipitation in ferritic steels containing 60 ppm carbon or nitrogen (C60 and N60). Tensile tests revealed that the ky was monotonously increased with increasing aging time in both steels, and also, C60 exhibited a larger ky value than that of N60 under the same aging time. As a result of 3DAP analysis and theoretical calculation for grain boundary segregation of carbon and nitrogen, the ky corresponded to the amount of carbon and nitrogen existing at grain boundary. There was no difference in the effect on ky increment between both elements. The larger ky in C60 under the same aging condition was due to the larger amount of segregated carbon compared with nitrogen.

Various methods were employed to measure the austenite volume fraction in a 1.5Mn-1.5Si-0.2C steel. It has been confirmed that the volume fractions determined by transmission electron microscopy, scanning electron microscopy/electron back scatter diffraction, X-ray diffraction and neutron diffraction exhibit a general trend to become larger in this order, although the values obtained by X-ray and neutron diffraction are similar in the present steel because austenite is relatively stable. The austenite volume fractions determined by diffraction methods have been found to be affected by the measuring specimen direction, i.e., texture, even by applying the conventional correcting procedure. To avoid this influence, it is recommended to measure both of volume fraction and texture simultaneously using neutron diffraction. Although synchrotron X-ray shows higher angle resolution, its small incident beam size brings poor statistic reliability. The influence of texture cannot be avoided for transmission Bragg edge measurement, either, which must be overcome to realize 2D or 3D volume…

が盛んに行われている 2-4) 。 線形摩擦接合(Linear Friction Welding：LFW)は，材料同 士を押し付け，線形運動で擦りあわせた際に生じる摩擦熱 を熱源とした接合法である。その歴史は古く，1929 年に初 めての特許が出願された 5) 。この特許の内容は，非常に漠 然としていたものであったが，1969 年に装置に特化した特 許が権利化された 6) 。その後，1980 年代初期に英国溶接研 究所(The Welding Institute：TWI)で LFW 装置が造られて からは，主に航空機材料である Ti，Ni 合金を中心に本接合 法の開発が進められた 7-13) 。このような経緯から，LFW プ ロセスの基礎的な特許で現在有効なものは存在しないとさ れ 14) ，摩擦攪拌接合法(Friction Stir Welding：FSW)等の新 規接合法と比較すると用途拡大に向けて有利な状況であ る。 このプロセスの特徴として，ツールレスな固相接合で あること，極めて高速なプロセスであること，継手の寸法 精度が高いこと 15) ，接合面の制約が少ないことが挙げられ る。さらに，摩擦圧接と同様に 16,17) ，接合に伴って軟化した 材料が接合面外部へと排出され，バリを形成するというこ とが知られている。一方で，この接合法が適用される部材 は主にバルク状のものが多く 18-23) ，薄板への適用はほとん ど見られない。 本研究では中高炭素鋼の新規接合法として LFW に着目 した。しかしながら，従来の LFW では，

Direct simulations of macrosegregation were performed using a numerical model to predict solidification grain structures and macrosegregation based on a three-dimensional cellular automaton finite difference method coupling with flow calculation of shrinkage flow and natural convection. In order to investigate the relationship between solidification structures and macrosegregation, the simulations for a special mold used in model experiments of Sato et al. [Tetsu-to-Hagané, 99(2013), 101], which can form macrosegregation in the central region of the small ingot, were carried out. In the simulations, the bridging of columnar grains formed in the center of ingot during solidification, and then the positive segregation was generated in the region below the bridging. On the other hand, the negative segregation was generated in the region above the bridging. The primary factor of this macrosegregation was the shrinkage flow with the formation of bridging, and the degree of positive and negative segregation was enlarged by the presence of natural convection. As the results, it was…

In work hardened metals, it is known that the strength of metals is heightened with increasing dislocation density. In the case of cold worked iron, dislocation is introduced heterogeneously and forms dislocation cell structure. Yield stress increases with an increase of volume fraction of cell wall where dislocation density ρ is much higher than the matrix. Applying the composite model to cold worked iron with dislocation cell structure, it was theoretically explained that linear relationship is realized between √ρ and the increment of strength Δσ, so called Bailey-Hirsch relation. On the other hand, the Bailey-Hirsch equation; Δσ[GPa]=18√ρ/109 was constructed from the experimental data which were already reported for cold worked iron. In previous papers, dislocation density was measured directly by transmission electron microscopy. As a result, it was confirmed that experimental data agree well with the result which was theoretically introduced based on the composite model and that the above Bailey-Hirsch equation can be applied for cold worked iron in which dislocation density is…

Factors causing hydrogen embrittlement of cold-drawn pearlitic steel fractured in plastic/elastic region have been investigated from the viewpoint of lattice defects. Tensile tests were conducted for hydrogen-charged specimens containing 1.5 and 4.0 ppm hydrogen under various crosshead speeds. The amount of tracer hydrogen which corresponds to the amount of lattice defects in the specimens subjected to plastic strain or elastic stress was measured using a thermal desorption analysis. As a result, specimens containing 1.5 ppm hydrogen fractured in the plastic region under all experimental conditions in the present study. In contrast, specimens containing 4.0 ppm hydrogen fractured in the elastic region at crosshead speed of 0.01 mm·min–1 or less and fractured in the plastic region at 0.1 mm·min–1 or more. Subjecting plastic strain in the presence of hydrogen increased the amount of lattice defects corresponding to vacancies. In contrast, the presence of hydrogen had no effects on the formation of lattice defects under subjecting elastic stress. The amount of lattice defects in the…

1． 背景：高加工硬化能の起源としての 動的ひずみ時効1882 年に Hadfield 鋼が開発されて以来，Fe-Mn-C オース テナイト鋼の高加工硬化能の起源は鉄鋼科学における歴史 的研究課題の一つである 1-7) 。Fig.1(a)に一例として，Fe-18Mn-1.2Cオーステナイト鋼(本論文における化学組成表 記はすべて mass%)の応力ひずみ応答を示す。広い温度範 囲で，高い加工硬化能および延性-強度バランスを維持 するところに特徴がある。この高加工硬化能に関する研 究は，近年の Fe-Mn-C 基双晶誘起塑性(TWIP：TWinning-Induced Plasticity)鋼の開発により大きく加速した 8,9) 。著し い加工硬化発現のキーファクターは双晶変形と動的ひずみ 時効(DSA：Dynamic Strain Aging)である。積層欠陥エネ ルギー(SFE)が極めて小さい場合(20 mJ m -2 周辺)は， 双晶変形が最重要因子である 10-12) 。双晶変形と加工硬化の 関係については，その理論 2,13,14)

The dislocation strengthening was estimated by applying the dislocation theory for a Fe-18%Ni alloy which has a lath martensitic structure. The yield stress of highly dislocated metals is dependent on both the friction stress and the dislocation strengthening. Regarding the coefficient of dislocation strengthening, it is governed by the shear modulus of metals. Ni addition plays a role in increasing the friction stress but decreases the shear modulus. This means that the coefficient of dislocation strengthening is smaller in the Fe-18%Ni alloy than pure iron. It was confirmed that the yield stress, which was experimentally obtained in Fe-18%Ni alloy, is reasonably explained by the mechanism of dislocation strengthening, taking the effects of Ni into consideration. On the other hand, in the case of lath martensite with a dislocation density of 2×1015 /m2, it was also found that the effect of Ni addition does not appear on the yield stress because the increment of solid solution strengthening is cancelled out by the decrement of dislocation strengthening.

Effect of thermo-solutal convection on the macrosegregation in continuously cast slabs was investigated by numerical simulation using a finite-volume scheme. The thermo-solutal convection and solidification shrinkage induced flow was considered in the numerical model. The thermos-solutal flow induced the vortex flow during the solidification. The solute-rich liquid was washed out from the mushy zone by the vortex flow and the V-shaped segregation pattern was formed. The solidification shrinkage induced flow from top side to bottom side was dominant at the last stage of the solidification. However, the segregation pattern induced by this flow could not be calculated by well accuracy because of the grid size and convergence error.