Simultaneous Increase of Both Strength and Ductility of Medium Mn Transformation-Induced Plasticity Steel by Vanadium Al
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ngth and ductility are key mechanical properties of steels for structural applications in automotive industry.[1,2] To increase the strength without sacrificing ductility, the transformation-induced plasticity (TRIP) effect is frequently employed, leading to the development of TRIP-assisted multi-phase steels[3,4] and medium Mn TRIP steels.[5–11] The TRIP effect that is resulted from martensitic transformation could provide medium Mn steels high ultimate tensile strength and good ductility.[6,9,10] Nevertheless, medium Mn steel has a low yield strength due to the soft ferrite matrix.[12] Different strategies have been proposed to increase the yield strength of medium Mn steel, such as the introduction of hard martensite by quenching and partitioning (Q&P) process[13–15] and the precipitation
B.B. HE and M.X. HUANG are with the Shenzhen Institute of Research and Innovation, The University of Hong Kong, Shenzhen, 518000, China and also with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China. Contact e-mail: [email protected] Manuscript submitted October 13, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
strengthening by microalloying.[16–18] Nevertheless, for the microalloying strengthening, the formation of carbide can decrease the C content of retained austenite grains and therefore deteriorate their mechanical stability.[19] However, it is noted that the size of austenite grains also affects their mechanical stability.[20–22] The microalloying such as titanium (Ti) and vanadium (V) can refine the austenite grain size and therefore improve their mechanical stability. Therefore, it is still not clear whether or not microalloying will deteriorate the mechanical stability of austenite grains in medium Mn steel. The present work is to investigate the effect of V-alloying on the mechanical stability of austenite grains in medium Mn steel. It is found that the V-alloying can lead to proper mechanical stability of austenite grains and consequently increases both strength and ductility of medium Mn steel simultaneously. Two medium Mn steel grades with the chemical composition of Fe-10 pct Mn-0.45 pct C-1 pct Al and Fe-10 pct Mn-0.45 pct C-1 pct Al-0.7 pct V are employed for the present investigation. These two steels are termed as V-free steel and V-alloyed steel hereafter. Both steels are cast and forged into billet followed by hot rolling to strips with a final thickness of 4 mm. The tensile samples with a gage length of 25 mm are wire cut along the rolling direction of the strips. The tensile samples are annealed at 1000 C for 1 hour, followed by water quenching and deep cryogenic treatment in liquid nitrogen for 10 minutes. Then the tensile samples are tempered at 620 C for 5 hours. The above heat treatment is termed as quenching, tempering, and partitioning (Q-T&P) process.[23] The tensile tests are performed at a strain rate of 5 9 10 4 s 1 under room temperature. The scanning electron microscopy (SEM) observation and electron backscattered diffraction (EBSD) measurement are performed in
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