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

増村 拓朗 谷口 大河 浦中 祥平 平嶋 一誠 土山 聡宏 丸山 直紀 白幡 浩幸 植森 龍治

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.11, pp.835-843, 2020 (Released:2020-10-31)

参考文献数

22

被引用文献数

1 7

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Electrical resistivity of low-carbon martensitic steels was measured to estimate the carbon concentration in solid solution. Since electrical resistivity is influenced not only by solute carbon but also by substitutional elements, lattice defects and second phase, the effects of these factors need to be subtracted from total electrical resistivity, in order to obtain the accurate solute carbon concentration by this method. As a result, the effects of dislocations and grain boundaries were much smaller than that of solute elements, being only 1 - 2% of the measured electrical resistivity, in martensitic steel. On the other hand, substitutional elements and retained austenite were found to be significantly effective. By subtracting these effects from the measured value, the change in electrical resistivity due to solute carbon, Δρsol.C, could be formulated as a function of the carbon concentration in solid solution of martensite, Csol, as follows:Δρsol.C [mΩmm] = 0.25 × Csol [mass%]The estimated solute carbon concentration was confirmed to correspond to the directly measured value by atom probe tomography.

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

土山 聡宏

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.100, no.9, pp.R20-R21, 2014 (Released:2014-08-31)

参考文献数

2

著者

土山 聡宏

出版者

日本鉄鋼協会

雑誌

ふぇらむ = Bulletin of the Iron and Steel Institute of Japan : (一社)日本鉄鋼協会会報 (ISSN:1341688X)

巻号頁・発行日

vol.26, no.8, pp.497-501, 2021

著者

横井 龍雄 首藤 洋志 池田 賢一 中田 伸生 土山 聡宏 大村 孝仁 峯 洋二 高島 和希

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.102, no.5, pp.244-252, 2016 (Released:2016-04-30)

参考文献数

38

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Dual Phase (DP) steel is used in automotive body parts for weight saving and crashworthiness, however there is an issue of DP steel in low stretch flange ability evaluated by hole expanding tests. In order to improve stretch flange ability of DP steel, it is important to estimate the damage of punching quantitatively and to clarify the change of microstructure before and after punching because the hole expansion ratio is decided in the ductility remained after pre-strain equivalent to punching. Therefore we tried to measure the damage of punching by unique techniques of Electron Backscatter Diffraction (EBSD), nano-indentation and micro-tensile testing and to observe fracture surface by Scanning Transmission Electron Microscope (STEM). Average EBSD-Kernel Average Misorientation (KAM) value and pre-strain damage have strong correlation, thus average KAM value can become the index of the damage. The nanohardness and tensile strength using micrometer-sized specimens increased with increasing average KAM value in the ferritic phase as approaching the punching edge. A shear type fracture occurred without necking in the specimen cut out in the area of the edge. The ultrafine-grained ferritic microstructure was observed in the sample cut out in the same area with STEM. It seems that the ductility loss of the punched DP steel was probably attributed to localized strain into the ultrafine-grained ferritic microstructure.

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。