The 10 most-cited articles in 鐵と鋼 : 日本鐡鋼協會々誌, 2018

The reduction of CO2 emission from the ironmaking process is important issue from the view of environmental problems typified by global warming in recent years. Low RAR (reducing agent rate) operation of the blast furnace is one of effective measures for reducing CO2 emission. Injection of HRA (hydrogenous reducing agents) from the tuyere (where is the lower part of blast furnace) is also effective measure. In this study, the influence of HRA injection point on combustion and gasification efficiency of pulverized coal (PC) in the case of simultaneous injection of HRA and PC from double-channel lance was examined by small scale combustion furnace and three-dimensional numerical simulation for improvement permeability in blast furnace. Combustion experimental conditions were in three cases, case1: injected HRA from outer side and PC from inner side of double-channel lance, case2: injected HRA from inner side and PC from outer side of double-channel lance and case3: injected HRA and PC premixed. As a result, the combustion and gasification efficiency was increase in the order of case3,…

られることが分かっている。 (1) σ 0 は温度やひずみ速度，固溶強化などに依存した摩擦力 (Friction stress) ，k は転位強化係数，G は剛性率，b は転位の Burgers vector である。その後，鉄についても，透過型電子 顕微鏡を用いて転位密度を求める研究が行われ，加工した 鉄の降伏応力と転位密度の関係が議論されてきた 2-5) 。著 者らは，これまでに報告されたデータを整理して，鉄の転 位強化量 Δσ が次式で与えられることを示した 6) 。 (2) 転位強化機構については，長範囲での転位間の応力場の影 響を基本とした長範囲応力場説と異なるすべり面上の転位 間の相互作用を基本とした林転位説があり 7) ，永年に亘っ てその妥当性が議論されてきたが，丸川はこれまでの研究 結果を総合的に判断して，後者の説が妥当であるとの結論 に至っている 8) 。ただし，加工した鉄の場合，転位分布は均 一ではなく，転位セルという不均一な組織が形成されるこ とが分かっている 3) 。著者らは，転位密度の高いセル壁と 転位密度の低いコア部を異なる二つの組織と仮定し，複相 組織モデルを適用することによって(2)式の関係を妥当に 説明できることを示した 9) 。その結果によると，10 15 m -2 に 近い転位密度の領域まで(2)式が成立することが予想され るが，2 × 10 14 m -2 を上回る転位密度の領域では，透過型電 子顕微鏡による測定は困難となる。

Time-resolved and in-situ observations using synchrotron radiation X-rays successfully proved that the massive-like transformation, in which the γ phase was produced through the solid – solid transformation and the partition of substitute elements such as Mn and Si at the δ/γ interface could be negligibly small, was selected in the unidirectional solidification of 0.3 mass%C steel at a pulling rate of 50 μm/s. The massive-like transformation produced fine γ grains in the vicinity of the front of δ/γ interface. The coarse γ grains also grew behind the fine γ grains along the temperature gradient. Distance between the δ/γ front and the advancing front of coarse γ grains was as short as 200 μm. Namely, the fine γ grains disappeared within 10 s by the growth of coarsen γ grains along the temperature gradient. In addition, the observation of the δ/γ interface confirmed that a transition from the diffusion-controlled γ growth to the massive-like growth of γ phase occurred at a growth rate of 5 μm/s. Thus, the massive-like transformation is dominantly selected in the carbon steel during…

Effect of crystal structure of surface compound layer on the fatigue strength of nitrided SCM 435 steel was investigated. Specimens in which the crystal structure of the surface compound layer was controlled to γ’-Fe4N phase or ε-Fe2-3N phase were prepared by gas nitriding treatment capable of changing nitriding potential (KN). Axial fatigue test were conducted to the specimens. Even in each test specimen which has different crystal structure of the compound layer, distribution of residual stress and hardness in the nitrogen diffusion layer exhibited almost same values. It is revealed that fatigue strength significantly depends on crystal structure of surface compound layer. Specimens with γ’-Fe4N phase dominant shows substantially high fatigue strength comparing to specimens with ε-Fe2-3N phase dominant. In the fatigue test under stress ratio R=0, the specimen from which the surface compound layer was removed showed the highest strength. The fatigue strength is considered to be governed by the fracture strength of each compound layer. Namely, in the specimen with ε-Fe2-3N phase…

To investigate the characteristics of dislocation evolution in ferritic and austenitic stainless steels under tensile deformation, neutron diffraction line-profile analysis was carried out.The austenitic steel exhibited higher work hardening than the ferritic steel.The difference in the work hardening ability between the two steels was explained with the dislocation density estimated by the line-profile analysis.The higher dislocation density of the austenitic steel would originate from its lower stacking fault energy.Dislocation arrangement parameters indicated that the strength of interaction between dislocations in the austenitic steel was stronger than that in the ferritic steel.This would mainly originate from the difference in dislocation substructures; while dislocation tangle, which can be prompted by the cross slip, was expected in the ferritic steels, highly dense dislocation walls induced by planar glide of dislocations as well as the tangle were expected in the austenitic steel.It was confirmed that the stronger interaction between dislocations in the austenitic steel…

The effects of hydrogen charge on the formation of vacancy-type defects in low alloy martensitic steel were investigated by using positron lifetime spectroscopy. During slow strain rate tensile tests (SSRT), dislocation and vacancy were introduced and increased with increasing tensile strains. Vacancy clustering was significantly promoted by hydrogen charge, although there was no remarkable difference in the dislocation density and mono-vacancy equivalent vacancy density between hydrogen-charged and uncharged samples under the same tensile strains. Increase in hydrogen concentration in the steel promoted vacancy clustering. Preferential conditions for the vacancy clustering showed a good agreement with the conditions for significant elongation loss by SSRT, implying the vacancy clustering has an big effect on hydrogen embrittlement. In the case of constant load tests under applied stress in elastic region, no increase in vacancy and dislocation density was observed. This implies plastic deformation is necessary for the formation of vacancy-type defects even under hydrogen charging.

We constructed an automatic in situ fatigue observation system to monitor small fatigue crack behavior at the microstructural level. Applying a digital image correlation (DIC) technique in combination enabled us to continuously and automatically track and record microscopic deformation behavior. To verify the effectiveness of this system, we applied it to small fatigue crack evaluation in heat-treated low-carbon steel. The results demonstrated the feasibility, using our developed system, of automatically tracking and recording microstructurally small fatigue crack initiation and early growth behavior. By utilizing DIC analysis, we also succeeded in visualizing microscopic deformations such as inhomogeneous strain concentration by microstructures causing fatigue cracks and small fatigue crack opening-closing behavior. Although early stage fatigue crack growth rate accelerated compared to long crack data, it was consistent with long crack data by using ΔKeff calculated from crack open stress measured by DIC.

In Modified Williamson-Hall / Warren-Averbach (MWH/WA) method, the estimation of parameter α is of vital importance for obtaining other parameters accurately. Almost all researchers use the procedure for determining parameter α proposed by Ungár et al. (route 1), which includes the rough approximation. On the other hand, Takebayashi et al. suggested an improved procedure (route 2) which could give more precise results. By comparing above two routes, it is found that route 1 can result in overestimated (about twice) α values and significantly impacts further analysis in MWH/WA method due to ignored terms. Since the difference of α value affects the crystallite size D and parameter φ, the application of route 2 is recommended in MWH/WA method.

The effects of the crosshead speed, hydrogen content and temperature on fracture strength and fracture surface morphology were investigated using a tempered martensitic steel containing 1.67 mass % of Si (H-Si) and one containing 0.21 mass% of Si (L-Si). When L-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from quasi-cleavage (QC) to intergranular-like (IG-like) to intergranular (IG) at room temperature. In contrast, when H-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from QC to IG-like at room temperature. This transition in the fracture surface morphology can be explained by the magnitude relationship between intergranular and transgranular strengths under hydrogen charging. At a temperature of –196 °C, hydrogen did not lower the fracture strength nor did it change the fracture surface morphology. Hence, hydrogen embrittlement at room temperature was presumably caused by hydrogen accumulation and lattice defect formation during stress application as well as by hydrogen trapped…