High-Temperature Flow Stress and Recrystallization Characteristics of Al-Bearing Microalloyed TWIP Steels

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EXTENSIVE efforts have been directed in recent years to improve the fuel efficiency and crash worthiness of automobiles through the development of advanced light-weight, high-strength steel sheets with excellent ductility. Notable among the new developments are the austenitic steels with 16 to 25 pct Mn that exhibit enhanced ductility and energy absorption ability through twinning-induced plasticity (TWIP).[1–3] The yield strength of austenitic TWIP steels with 16 to 25 pct Mn is relatively low, below 400 MPa, and therefore new efforts are necessary to improve it. One obvious method is to refine the grain size that can result in a significant increase of yield strength or alternatively, prestraining of a sheet followed by recovery and partial recrystallization of that sheet.[3] It is also well known for decades that precipitation strengthening by microalloying can be used to increase the yield strength of ferritic steels, but it has not been utilized to the same extent in the case of TWIP steels. Only a couple of papers concern the improvement of the performance of high-Mn TWIP steels by increasing their yield strengths using microalloying.[3–5] Chateau et al.[4]

MAHESH CHANDRA SOMANI, Senior Specialist Researcher, DAVID A. PORTER, Professor and Head, and L. PENTTI KARJALAINEN, Emeritus Professor, are with the Faculty of Technology, Materials Engineering, Centre for Advanced Steels Research, University of Oulu, PO Box 4200, 90014 Oulu, Finland. Contact e-mail: mahesh.somani@oulu.fi ATEF S. HAMADA, Associate Professor, is with the Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez University, Box 43721, Suez, Egypt. Manuscript submitted February 16, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A

achieved an increase of 140 MPa owing to the precipitation of semicoherent VC precipitates below 30 nm in size in an Fe-22Mn-0.6C steel (hereinafter concentrations in wt pct, unless mentioned otherwise) with 0.21 at. pct V. Scott et al.[5] investigated precipitation strengthening in three cold-rolled and annealed TWIP steels and observed that V is the most efficient strengthening element and an increase of yield strength by about 200 MPa is realized by alloying with 0.35 pct V and even a yield strength of 900 MPa could be achieved by alloying with 1 pct V. The potentials of Ti and Nb alloying remained less pronounced. Precipitation strengthening by Nb and V in microalloyed TWIP steels has also been targeted in a recent project funded by the Research Fund for Coal and Steel (RFCS).[6] While the influences of chemical composition on the microstructures and mechanical properties of TWIP steels at room temperature have been extensively investigated and published in the literature, the information on the constitutive flow and recrystallization behaviors of these steels at high temperatures is scarcer. Hamada et al.[7–9] have investigated high-temperature flow stress, dynamic recrystallization (DRX) and static recrystallization (SRX) behaviors of Fe-24Mn-0.10C and Fe-26Mn-3.4Al-

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