著者

Yoshikazu Matsuoka Tatsuya Iwasaki Nobuo Nakada Toshihiro Tsuchiyama Setsuo Takaki

出版者

The Iron and Steel Institute of Japan

雑誌

ISIJ International (ISSN:09151559)

巻号頁・発行日

vol.53, no.7, pp.1224-1230, 2013 (Released:2013-08-20)

参考文献数

14

被引用文献数

139 174

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In order to clarify the grain size dependence of mechanical stability of austenite, deformation-induced martensitic transformation behavior was investigated on uniaxial tensile deformation in a metastable austenitic stainless steel (Fe–16%Cr–10%Ni) with the grain size controlled from 1 to 80 μm. In addition, crystallographic characteristics of deformation-induced martensite were analyzed by means of the EBSD (electron backscattering diffraction) method to discuss the variant selection rule. It was found that mechanical stability of austenite is independent of its grain size, although thermal stability of austenite is remarkably increased by grain refinement. Some special martensite variants tend to be selected in an austenite grain on the deformation-induced martensitic transformation (near single-variant transformation), and this results in the formation of a texture along tensile direction. This suggests that the most advantageous variants are selected in the deformation-induced martensitic transformation to release tensile strain and leads to the grain size independence of mechanical stability of austenite.

著者

Toshihiro Tsuchiyama Yoshikazu Nakamura Hideyuki Hidaka Setsuo Takaki

出版者

The Japan Institute of Metals and Materials

雑誌

MATERIALS TRANSACTIONS (ISSN:13459678)

巻号頁・発行日

vol.45, no.7, pp.2259-2263, 2004 (Released:2005-06-24)

参考文献数

12

被引用文献数

7 10

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Ultrafine grained austenitic structure was obtained in 18Cr-9Ni stainless steel by thermomechanical treatment using reversion from deformation-induced martensite. The superplastic deformation behavior was investigated at 923 K for the steels containing various amounts of retained martensite particles in the initial structure before tensile testing. The retained martensite was effective for suppressing grain growth of austenite and necessary for the superplasticity although it was thermodinamically unstable phase and gradually decreased its volume fraction with superplastic deformation. Therefore, the superplastic elongation was strongly dependent on the initial volume fraction of the retained martensite. The total superplastic elongation was enlarged with increasing the initial amount of martensite, and the maximum elongation of about 270% was obtained when the volume fraction was controlled to be around 10 vol%. However, the increase in elongation was leveled off in the range above 15 vol% martensite. The effect of the retained martensite on the superplasticiy was discussed in connection with the changes in volume fraction of the martensite, austenite grain size and deformation mechanism.

著者

Toshihiro Tsuchiyama Kurato Inoue Katsutoshi Hyodo Daichi Akama Nobuo Nakada Setsuo Takaki Tamotsu Koyano

出版者

The Iron and Steel Institute of Japan

雑誌

ISIJ International (ISSN:09151559)

巻号頁・発行日

vol.59, no.1, pp.161-168, 2019-01-15 (Released:2019-01-17)

参考文献数

37

被引用文献数

9 16

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The microstructure and hardness of martensite in Fe–C and Fe–N alloys with up to 7.5 at% contents of carbon and nitrogen, respectively, were compared. Their difference in hardness was discussed based on four strengthening mechanisms. The martensitic structures of Fe–C and Fe–N alloys with equal contents of carbon and nitrogen, respectively, were nearly identical, except for the amount of retained austenite. Furthermore, Fe–C alloy was considerably harder than Fe–N alloy. This discrepancy gradually increased with carbon and nitrogen contents. The enhanced hardness of Fe–C alloy martensite was attributed to its higher dislocation density and the stronger pinning force of interstitial carbon atoms on dislocations.

著者

Nobuo Nakada Norimitsu Koga Yuki Tanaka Toshihiro Tsuchiyama Setsuo Takaki Masaharu Ueda

出版者

The Iron and Steel Institute of Japan

雑誌

ISIJ International (ISSN:09151559)

巻号頁・発行日

vol.55, no.9, pp.2036-2038, 2015-09-15 (Released:2015-09-29)

参考文献数

17

被引用文献数

13 18

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The strength of pearlitic steel was clearly reduced by annealing, even though cementite stably maintained a lamellar structure. In response, lattice strain of the ferrite phase in pearlite monotonically decreased with increasing annealing time. As a result, a good linear relationship was established between the strength and ferrite lattice strain independent of the interlamellar spacing and morphology of cementite. This suggests that the ferrite/cementite elastic misfit strain contributes to the high strength of pearlitic steel.

著者

Nobuo Nakada Norihide Fukuzawa Toshihiro Tsuchiyama Setsuo Takaki Tamotsu Koyano Takashi Iwamoto Yasuhiro Omori

出版者

The Iron and Steel Institute of Japan

雑誌

ISIJ International (ISSN:09151559)

巻号頁・発行日

vol.53, no.1, pp.139-144, 2013 (Released:2013-01-15)

参考文献数

22

被引用文献数

10 13

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In order to understand the mechanism of isothermal transformation of Fe–N alloy, the isothermal transformation microstructure that forms in a wide temperature range below Ae1 was investigated in Fe–2.6 mass%N hypereutectoid alloy by means of the electron back scatter diffraction method in addition to the conventional microstructural observation methods. High-nitrogen austenite fully decomposed to ferrite and Fe4N over the entire temperature range, and the time-temperature-transformation (TTT) diagram had a C shape with a nose temperature around 700 K. The hardness linearly increased with decreasing transformation temperature because the microstructure became finer, but the morphology of the (ferrite + Fe4N) structure changed discontinuously at around 800 K. From the microstructural and crystallographic analyses, it was concluded that the microstructure formed at higher temperature is a lamellar eutectoid structure, braunite, while the other is an upper bainitic structure containing bainitic ferrite formed through a displacive mechanism and Fe4N formed by concentration and ordering of the nitrogen. Since Fe4N is a counterpart of the cementite in Fe–C alloy, the respective structures are similar to pearlite and upper bainite in carbon steel.

著者

Hirokazu Tsukahara Takuro Masumura Toshihiro Tsuchiyama Setsuo Takaki Koichi Nakashima Kazukuni Hase Shigeru Endo

出版者

一般社団法人 日本鉄鋼協会

雑誌

ISIJ International (ISSN:09151559)

巻号頁・発行日

vol.55, no.1, pp.312-318, 2015-01-15 (Released:2015-02-06)

参考文献数

26

被引用文献数

3

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The range of chemical compositions that can obtain an austenitic single structure was defined for medium-manganese (Mn) carbon (C) steels. Among the potential compositions, Fe-5%Mn-4%Cr-(0.8–1.4)%C (mass%) was selected as the optimized composition range to form a stable austenitic structure. The tensile properties and deformation substructure were investigated in the austenitic steels having this composition. The work hardening behavior of the steels varied depending on the carbon content, which was closely related to the deformation microstructure. In the 0.8%C steel, both a deformation-induced martensitic phase as well as the formation of deformation twins generated a high work hardening until fracture. With an increasing carbon content, which increased the stacking fault energy (SFE), the deformation tended to shift towards dislocation slipping, resulting in a lower work hardening rate. This trend appears similar to conventional twinning-induced plasticity steel where the work hardening behavior is tied to the SFE.