Abstract
Ferrite + austenite duplex lightweight steels have been actively developed by adding low-density Al for overcoming a limitation of stiffness deterioration by a traditional approach to obtain a weight reduction. Multiple-stage deformation mechanism in lightweight steels, i.e., simultaneous formation of deformation-induced martensite and deformation twin and additional plasticity by twinning, has been nominated as an attractive strategy, but shows a steady flow behavior with early plastic instability. Here, we present a newly designed Fe-0.3C-9Mn-5Al steel in order to obtain an optimal level of stability of austenite and a resultant outstanding combination of tensile strength and ductility, e.g., 874 MPa and 72%, together with sufficiently high strain hardening. These enhanced properties are attributed to the decreased austenite stability by controlling the austenite size and alloying partitioning due to variation in austenite fraction inside duplex microstructures. The present work gives a promise for structural applications requiring both reduced specific weight and remarkable deformability.
Highlights
Effective control of global warming and greenhouse gas reduction have become key issues in automotive industries[1,2,3,4,5]
Since it is sensitively working for the strain hardening effect even when small amounts of alloying elements are varied, a remarkable improvement of strain hardening without plastic instability as well as high yield strength can be achieved by further optimization of austenite size and fraction
Since the active TRansformation Induced Plasticity (TRIP) is required for the high strain hardening rate, the present alloy design mainly focuses on the achievement of higher strain hardening effect by decreasing the austenite stability, while the excellent ductility is maintained
Summary
Effective control of global warming and greenhouse gas reduction have become key issues in automotive industries[1,2,3,4,5]. Key ideas of (TRIP + TWIP) mechanisms are based on the distribution of stored energy by TWIP as well as TRIP, and on the maximization of delayed necking induced by additional plasticity due to TWIP They contributes the ductility enhancement, but the TRIP rate is too low, while the TWIP does not greatly contribute the strength enhancement, thereby showing a steady flow behavior in the strain range of 30~77%. The austenite stability is indirectly decreased by the increased austenite fraction due to increased Mn and decreased Al contents, instead of the direct decrease in austenite stability due to the decreased C content Since it is sensitively working for the strain hardening effect even when small amounts of alloying elements are varied, a remarkable improvement of strain hardening without plastic instability as well as high yield strength can be achieved by further optimization of austenite size and fraction
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