Randomization of Ferrite/austenite Orientation Relationship and Resultant Hardness Increment by Nitrogen Addition in Vanadium-microalloyed Low Carbon Steels Strengthened by Interphase Precipitation


Interphase precipitation of nano-sized alloy carbides is recently used to strengthen low carbon steels for its excellent contributions to strength and formability. The effects of nitrogen addition on the hardness of vanadium-microalloyed low carbon steels were investigated by considering both the dispersion of inter- phase precipitation and the ferrite/austenite crystallography. Three-dimensional atom probe analysis reveals that interphase precipitation of vanadium carbide is hardly affected by increasing the nitrogen content, although the nanohardness of ferrite is slightly increased. Another important factor determining the overall hardness of ferrite is found to be the ferrite/austenite crystallography. At lower transformation temperature, nitrogen addition reduces the amount of Widmanstatten ferrite and bainite, which are formed in absence of interphase precipitation. Instead, relatively harder allotriomorphic and idiomorphic grain boundary ferrite without Kurdjumov-Sachs orientation relationship against austenite are formed extensively.


phase transformation; interphase precipitation; crystallography; atom probe tomography; nanoindentation.


Yongjie Zhang (1), Kunio Shinbo (1), Takahito Ohmura (2), Takuya Suzuki (2), Kaneaki Tsuzaki (3), Goro Miyamoto (1), Tadashi Furuhara (1)


1) Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi, 980-8577 Japan.
2) National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047 Japan.
3) Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395 Japan.

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ISIJ International, Vol. 58, No. 3, 2018, pp. 542–550.