著者

土山 聡宏 荒木 理 高木 節雄

出版者

一般社団法人 日本鉄鋼協会

雑誌

鉄と鋼 (ISSN:00211575)

巻号頁・発行日

vol.106, no.6, pp.382-390, 2020 (Released:2020-05-31)

参考文献数

22

被引用文献数

1 4

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The upper and lower yield points of ferritic steel containing a small amount of carbon were discussed in terms of the critical stress for dislocation emission from a grain boundary, namely, “critical grain boundary shear stress”, on the assumption of the pile-up model. Considering some experimental results such as tensile testing, relaxation testing and nanoindentation testing on grain boundaries, we concluded that both upper and lower yield points could be similarly understood as a phenomenon of dislocation emission from dislocation sources existing at grain boundaries. The difference in stress between upper and lower yield points was explained in terms of the density of mobile dislocations, which determines the extent of stress concentration at grain boundary caused by pile-up of the dislocations. Slow cooling after annealing or aging at low temperature, by which Cottrell atmosphere is formed, leads to a significant decrement of the mobile dislocation density, and this results in an occurrence of the sharp upper yield point because of a reduced number of piled-up dislocations and insufficient stress concentration at grain boundaries.

著者

赤間 大地 土山 聡宏 高木 節雄

出版者

一般社団法人 日本鉄鋼協会

雑誌

鉄と鋼 (ISSN:00211575)

巻号頁・発行日

vol.103, no.5, pp.230-235, 2017 (Released:2017-04-30)

参考文献数

21

被引用文献数

8

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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.

著者

吉武 睦海 土山 聡宏 高木 節雄

出版者

一般社団法人 日本鉄鋼協会

雑誌

鉄と鋼 (ISSN:00211575)

巻号頁・発行日

vol.98, no.6, pp.223-228, 2012 (Released:2012-05-31)

参考文献数

17

被引用文献数

3 22

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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.

著者

的場 理一郎 中田 伸生 二村 裕一 土山 聡宏 高木 節雄

出版者

一般社団法人 日本鉄鋼協会

雑誌

鉄と鋼 (ISSN:00211575)

巻号頁・発行日

vol.93, no.7, pp.513-517, 2007-07-01 (Released:2009-02-13)

参考文献数

9

被引用文献数

17 16

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The “nominal grain size” (average grain size) is generally applied to Hall-Petch relationship to evaluate grain refinement strengthening in polycrystalline materials. However, the steels with wide grain size distribution (duplex-grained structure) may not deform uniformly but yield preferentially from larger grains to finer ones. This phenomenon is called “micro-yielding”. In this study, the effect of duplex-grained structure on the yield stress was investigated by using some IF steels with different grain size distribution. As a result of tensile testing, the yield stress of duplex-grained steels could be conventionally plotted on the Hall-Petch relationship as a function of (nominal grain size)-1/2 in the range from 100 to 10 μm, even though the micro-yielding phenomenon occurred within the coarse grains at a lower stress than the macroscopic yield stress. When the volume fraction of grains with identical size is summed from larger-sized ones, the summated volume fraction (defined as the integrated volume fraction) always reaches 70-80 vol% at the nominal grain size irrespective of the difference in grain size distribution. These results suggest that polycrystalline materials including duplex-grained structure materials cause the macroscopic yielding when the grains of 70-80 vol% are micro-yielded.

著者

吉弘 辰明 土山 聡宏 高木 節雄

出版者

公益社団法人 日本金属学会

雑誌

日本金属学会誌 (ISSN:00214876)

巻号頁・発行日

vol.67, no.7, pp.362-367, 2003 (Released:2008-04-24)

参考文献数

7

被引用文献数

2 2

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Chemical reaction process(Ti+CrB→Cr+TiB) of isolated CrB particle in a Ti powder compact was investigated to find a proper sintering condition for producing TiB/Ti composites by reaction sintering. EPMA analysis revealed that the formation of TiB starts at 1223 K during heated with the rate of 0.17 K/s. The reaction proceeds topochemically with the movement of a reaction band from the particle surface to inside. The unreacted-core model was applied to the reaction of a CrB particle for the analyse the reaction rate of CrB particle. It was found that diffusion of Ti and Cr atoms in a TiB layer controls the whole reaction rate, and then the time of reaction finish at 1223 K was estimated as a function of initial CrB particle radius.

著者

高木 節雄

出版者

公益社団法人 日本金属学会

雑誌

日本金属学会誌 (ISSN:00214876)

巻号頁・発行日

vol.83, no.4, pp.107-118, 2019-04-01 (Released:2019-03-25)

参考文献数

37

被引用文献数

8 15

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Yielding and deformation behavior was overviewed for polycrystalline ferritic steels with various grain sizes and carbon contents. In the well-annealed ferritic steels, yielding is governed by the mechanism of grain refinement strengthening (GRS) and yield stress follows the Hall-Petch relation as to grain size. Conversely, yielding of cold rolled ferritic steels is governed by the mechanism of dislocation strengthening (DS) and yield stress follows the Bailey-Hirsch relation as to dislocation density. Under the same percentage of deformation, dislocation introduction is promoted with decreasing ferrite grain size and this results in higher yield stress (or flow stress) in specimens with smaller grain size. Yielding elongation appears due to the plastic instability that is realized during the change of strengthening mechanism from GRS to DS. Therefore, the extent of yielding elongation is determined in the relation between yield stress and work hardening behavior of matrix. Temper rolling plays a role to make the strengthening mechanism change from GRS to DS. When the extent of temper rolling is so small as about 1%, yield stress of temper rolled specimens is lowered below that of the as-annealed specimen. This is the reason why yield stress of the temper rolled specimen is lowered below that of as-annealed specimen. In the relation between GRS and DS, there is not additional but competitive relationship.

著者

荒木 理 藤井 浩平 赤間 大地 土山 聡宏 高木 節雄 大村 孝仁 高橋 淳

出版者

一般社団法人 日本鉄鋼協会

雑誌

鉄と鋼 (ISSN:00211575)

巻号頁・発行日

vol.103, no.8, pp.491-497, 2017 (Released:2017-07-31)

参考文献数

28

被引用文献数

15

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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.

著者

高木 節雄 土山 聡宏 中田 伸生 中島 孝一

出版者

九州大学

雑誌

基盤研究(B)

巻号頁・発行日

2006

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粒子分散強化は金属材料の基本的な強化機構の一つであり、鉄鋼材料の場合、セメンタイトを代表とする炭化物が一般的に強化分散粒子として使用されている。一方、近年ではナノテクノロジーによる鉄鋼材料の高機能化研究が盛んに行われており、数nm~数十nm の非常に微細な分散粒子(ナノ分散粒子)を利用して鉄鋼材料の高強度化を図ろうとする試みがなされ、その一つとしてナノCu 粒子が注目されている。ただし、ナノCu 粒子分散鋼の優れた機械的性質は、単に分散粒子のサイズが微細であることだけでなく、「分散Cu 粒子自体が鉄基地に比べて十分軟質である」というCu 粒子の特徴によってもたらされている事実も示唆されている。今後、大きな降伏強度と加工硬化率を有し高強度・高延性を兼ね備えた材料を得るためには、炭化物とCu 粒子を同時に最適な状態で分散させ、それぞれの特長を融合させてやること(ハイブリッド化)が有効であると考えられる。そのような鉄鋼材料、「ハイブリッド鋼」の有効性を証明することを本研究の最大の目的とし研究を遂行した。その結果、様々なハイブリッド鋼(フェライト型ハイブリッド鋼、マルテンサイト型ハイブリッド鋼、パーライト型ハイブリッド鋼など)の創製に成功し、炭化物とCu 粒子の複合析出により鋼の強度-延性バランスが大幅に改善することが明らかとなった。さらに、炭化物とCu 粒子それぞれの分散状態を制御することにより鋼の降伏強度と加工硬化率を独立して任意にコントロールできる可能性が示唆された。

著者

飛鷹 秀幸 木村 勇次 高木 節雄

出版者

社団法人日本鉄鋼協会

雑誌

鐵と鋼 : 日本鐡鋼協會々誌 (ISSN:00211575)

巻号頁・発行日

vol.85, no.1, pp.52-58, 1999-01-01

被引用文献数

6

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Mechanical milling using high energy planetary ball mill was applied to Fe-C alloy powders with (ferrite+cementite) two-phase structures to give an ultimate large strain into the powders. Dissolution behavior of cementite during mechanical milling was investigated in relation to ultra grain refining of ferrite matrix, and dissolution capacity of cementite was discussed in terms of carbon content in the powders. Ultra grain refining of ferrite matrix to about 10nm results in full dissolution of cementite in the powders with carbon up to 2 mass% C. Most of carbon, which has been rejected from decomposed cementite, is suggested to segregate at grain boundary to form amorphous layer. Thus, it was proposed that the dissolution limit of cementite depends on both volume fraction of the grain boundary amorphous layer and carbon concentration therein. For example, as the maximum carbon content of the grain boundary amorphous layer was to be about 4.2 mass% C, the dissolution limit of cementite was estimated at 30 vol% from the mass valance for carbon content in the case ferrite grains were refined to around 10nm. This volume fraction of cementite is just correspondent to that in Fe-2mass%C alloy.