Effect of Ferrite/Martensite Phase Size on Tensile Behavior of Dual-Phase Steels with Nano-Precipitation of Vanadium Car
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AL-PHASE (DP) steel, composed of soft ferrite and hard martensite phases, is one type of advanced high strength steels that reveal excellent combination of high strength and good formability.[1–3] Although DP steels with a wide variety of strength levels in the range from 500 to 1200 MPa in ultimate tensile strength have been developed for industrial applications such as automotive products,[3–5] demands for further strengthening without a loss of ductility and formability in DP steels are still increasing for automotive crush performance and fuel efficiency. Mechanical properties of ferrite–martensite DP steels depend on several structural parameters, e.g., the volume fraction,[6–10] the distribution (size, morphology,
ELANGO CHANDIRAN, GORO MIYAMOTO, and TADASHI FURUHARA are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan. Contact e-mail: [email protected]. YU SATO is with the Aishin Seiki Co. Ltd., 2-1 Asahi-machi, Kariya, Aichi 448-8650, Japan. NAOYA KAMIKAWA is with the Department of Mechanical Science and Engineering, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan. Manuscript submitted January 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
connectivity, etc.)[6,9,11–14] and the constituent properties (strength, formability, etc.)[15–26] of the ferrite/martensite phases in the structure. For instance, an increase in the strength of martensite phase is one of the strategies that are often used for strengthening of DP steels, but this often leads to a decrease in the post-uniform elongation.[15,17,20,24] On the contrary, an increase in the strength of ferrite phase by dispersion of nano-precipitates in ferrite phase is a promising strategy to increase the strength of DP steels with maintaining sufficient ductility.[24–26] This type of steels can be termed as ‘‘nano-precipitated DP steels.[26]’’ In previous work, it has been reported[24,25] that DP steels with nano-sized TiC alloy carbides in ferrite phase produced by interphase precipitation have high strength with good formability, compared to conventional DP steels. Our group also reported that dispersion of nano-sized VC particles in ferrite phase significantly improves the strength without a loss of ductility of DP steels.[26] On the other hand, these reports indicate that dramatic improvement of ductility has not been achieved only by using strengthening of ferrite phase by nano-precipitates. Thus, further structural optimization should be required to realize a simultaneous improvement of both strength and ductility in nano-precipitated DP steels.
In the early studies on the ferrite/martensite phase size effect on DP steels, it was often reported[6,9,12,14] that strength is increased by refinement of phase size, while uniform and total elongation are less affected by the phase size. On the other hand, it was also reported[12,14] that the necking is pronounced during tensile test by refinement of ferrite/martensite phases
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