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In this study, the experimental procedure involved hot rolling (1100°C–850°C) of six novel lightweight steels from different groups: ferritic, ferrite-based and austenite-based, followed by a heat treatment schedule to achieve the desired mechanical properties and compatibility for an industrial continuous annealing line. The heat treatment consisted of soaking at various temperatures between 600 and 1000 °C for 60 s, followed by cooling to the austempering temperature (460 °C) with a holding time of either 3 or 120 s. Microstructural and tensile test characterisation was carried out. Microstructural observation revealed a constituent of a single ferrite, a complex matrix containing austenite, martensite and carbides. Depending on the heat treatments applied, a wide range of mechanical properties was obtained. Generally, in ferritic steels, increasing the annealing temperature does not affect tensile properties; however, in duplex steels, it increases ductility at the expense of tensile strength. For instance, in steel (0.25C-5.7Mn-3.9Al-0.09Nb wt.%), the yield strength (YS) and ultimate tensile strength (UTS) decreased by approximately 300 MPa, while total elongation (TEl) increased from 6.7% to 21.8% as the annealing temperature was raised from 600°C to 1000°C. Isothermal holding at 460°C for an extended period improved strength but reduced ductility in ferritic and austenite-based steels. In ferrite-based steels, mechanical properties largely depend on an annealing temperature, which is attributed to microstructural evolution. Fractography of the tensile specimens revealed various fracture morphologies (brittle, mixed brittle-ductile, and ductile modes) depending on the alloys’ composition and phase constituents. The results indicate that controlled thermomechanical processing is essential for balancing the high-strength requirements of Al-containing lightweight steels.
Acknowledgement: The authors would like to acknowledge Pro-quality program for support in starting scientific activities in new research topics as part of the Excellence Initiative – Research University program, grant no. 32/014/SDU/10-22-002.