Metallurgical design of high strength austenitic Fe-C-Mn steels with excellent formability (Metaldesign) : final report

Five TWIP steel grades (Fe-Mn-C-N and Fe-Mn-Al-Si-C) were selected, adopting stacking fault energy (SFE) and austenite phase stability as the main metallurgical characteristics for alloy design. The selected TWIP grades were characterised in terms of SFE, mechanical properties, microstructure, recrystallisation, welding and forming ability. The performance analysis revealed that only TWIP2 and TWIP3 clearly behave as expected from TWIP steels, with the typical hardening stage associated with profuse deformation twinning. The variants with lower Mn and C content (TWIP1, TWIP4, TWIP5) require a further set-up of steel chemistry to better balance the SFE and avoid the formation of second phases (ferrite or deformation-induced martensitic phases). The results achieved in the project allowed to define the suitable industrial manufacturing route to produce the TWIP steel. However two main issues resulted with an importance stronger than expected in the technical annex: hydrogen embrittlement susceptibility and strip surface decarburization. The first issue has suggested to delay the industrial heat scheduling to evaluate the risk of hydrogen embrittlement for TWIP steels and the need to include a vacuum degassing facility for industrial TWIP steel manufacturing. The decarburization issue revealed that during the annealing process a proper atmosphere control is necessary to avoid within the decarburised layer the formation of deformation induced martensitic phases (α'+ε).The activity results revealed that the investigated TWIP steels have excellent tensile properties together with good welding and galvanizing ability that make them extremely attractive for automotive applications both for structural reinforcement (body in white) and for energy absorption.