Interpretation of dynamic tensile behavior by austenite stability in ferrite-austenite duplex lightweight steels.

Phenomena occurring in duplex lightweight steels under dynamic loading are hardly investigated, although its understanding is essentially needed in applications of automotive steels. In this study, quasi-static and dynamic tensile properties of duplex lightweight steels were investigated by focusing on how TRIP and TWIP mechanisms were varied under the quasi-static and dynamic loading conditions. As the annealing temperature increased, the grain size and volume fraction of austenite increased, thereby gradually decreasing austenite stability. The strain-hardening rate curves displayed a multiple-stage strain-hardening behavior, which was closely related with deformation mechanisms. Under the dynamic loading, the temperature rise due to adiabatic heating raised the austenite stability, which resulted in the reduction in the TRIP amount. Though the 950 °C-annealed specimen having the lowest austenite stability showed the very low ductility and strength under the quasi-static loading, it exhibited the tensile elongation up to 54% as well as high strain-hardening rate and tensile strength (1038 MPa) due to appropriate austenite stability under dynamic loading. Since dynamic properties of the present duplex lightweight steels show the excellent strength-ductility combination as well as continuously high strain hardening, they can be sufficiently applied to automotive steel sheets demanded for stronger vehicle bodies and safety enhancement.

Dramatic improvement of strain hardening and ductility to 95% in highly-deformable high-strength duplex lightweight steels.

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.