著者

Toshihiro Tsuchiyama Takayuki Sakamoto Shohei Tanaka Takuro Masumura

出版者

The Iron and Steel Institute of Japan

雑誌

ISIJ International (ISSN:09151559)

巻号頁・発行日

vol.60, no.12, pp.2954-2962, 2020-12-15 (Released:2020-12-16)

参考文献数

13

被引用文献数

7

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Medium manganese steel (Fe-5.0%Mn-1.2%Si-0.10%C alloy) was subjected to interrupted quenching from the austenite single-phase region to a temperature between Ms and Mf followed by intercritical annealing in the ferrite and austenite dual-phase region at 923 K. As a result, a core-shell type second phase, which consisted of a fresh martensite core surrounded by a film-like retained austenite shell, was formed. The mechanism and kinetics of reversion for the interrupted-quenched specimens were analyzed with DICTRA simulation and TEM observation. With regard to the effect of the core-shell type second phase on mechanical properties, it was inferred that the fresh martensite core functioned as a hard second phase and enhanced work hardening by stress partitioning similar to DP steel, while the film-like retained austenite contributed to improved ductility due to the TRIP effect. As the interrupted quenching temperature decreased, the volume fraction of the fresh martensite core decreased, while the stability of the retained austenite shell increased. This showed potential for controlling the strength and ductility balance of medium manganese steel. A possible beneficial effect of the core-shell type second phase on the ductile fracture behavior was also discussed in terms of stress/strain relaxation at the interfaces between hard martensite and ferrite matrix.

著者

Yushi Takenouchi Shuhei Wada Takuro Masumura Toshihiro Tsuchiyama Hiroshi Okano Shusaku Takagi

出版者

The Iron and Steel Institute of Japan

雑誌

ISIJ International (ISSN:09151559)

巻号頁・発行日

vol.62, no.10, pp.2000-2007, 2022-10-15 (Released:2022-10-19)

参考文献数

23

被引用文献数

2

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Stress relaxation tests were conducted in the elastic region of an ultralow carbon martensitic steel (Fe–18%Ni alloy) to quantitatively analyze the effect of mobile dislocations on the low elastic limit of the steel. The elastic limit of the as-quenched material was measured at 255 MPa, although its tensile strength was as high as 720 MPa. The stress relaxation tests, which were performed at 255 MPa, revealed a remarkable stress reduction due to the movement of the mobile dislocations present in the as-quenched material. The total dislocation density barely changed during the test, while the distribution parameter (M-value) decreased significantly, indicating that the mobile dislocations exhibited stable arrangements. The 5% cold rolling before the relaxation tests suppressed the relaxation and simultaneously increased the elastic limit to a maximum, 435 MPa. By estimating the mobile dislocation density by relating the stress reduction in the stress relaxation tests to the distance of the dislocation movement evaluated via transmission electron microscopy (TEM) observations, it was estimated that the mobile dislocation density of the 5%-cold-rolled material was lowered to ~1/10 of that of the as-quenched material.

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.