Microstructural evolution and mechanical properties of a micro-alloyed low-density ?-TRIP steel

Ultrahigh tensile strength over 1200 MPa has not been reported for low-density delta-transformation-induced-plasticity (8-TRIP) steels. In addition, the trade-off between strength and ductility is still inevitable for those steels. This study fills the gap by controlling morphology and volume fraction of 8-ferrite. A micro-alloyed low-density 8-TRIP steel with a chemical composition of Fe-3Al-0.84Cr-0.94Mn-0.31Mo-1.18Ni-0.31Si-0.25V-0.3C (weight percentage, wt.%) was prepared, and the high Al addition resulted in the reduced density of ~5% with 7.49 g cm-3 in density. By applying varying heat treatment parameters, three steels with different microstruc-tures, i.e. steels A, B and C were prepared. Microstructural morphologies of the three steels are significantly different. Steel A has a medium volume fraction of 56% for dendritic 8-ferrite with moderate average thickness of 3 pm while steel B contains the highest volume fraction of dendritic 8-ferrite (61%) with the smallest thickness of 1 pm. In contrast, steel C has the lowest volume fraction of dendritic 8-ferrite (32%) with the largest thickness of 4 pm. In addition, the volume fractions of lamellar RA are 14%, 8% and 19% for steels A, B and C, respectively. Steel A has an ultrahigh tensile strength of 1270 MPa, while a good ductility of 17% is retained. However, steel B shows the lower ductility of 14% in spite of the higher tensile strength of 1220 MPa. The thickness and volume fraction of 8-ferrite band influence not only the TRIP effect but also the initiation and propagation of cracks. The main reason for steel B with the lower ductility is associated with its smallest thickness and the highest volume fraction of 8-ferrite among the three steels. In contrast, the utmost strength-ductility product of steel C is attributed to its moderate thickness and the lowest volume fraction of 8-ferrite bands, which can alleviate the local stress concentration at the ferrite/martensite boundaries and leads to the improvement of ductility.

Automated summary

This study addresses the trade-off between strength and ductility in low-density delta-transformation-induced-plasticity steels by controlling the morphology and volume fraction of 8-ferrite. It is found that the thickness and volume fraction of 8-ferrite bands significantly influence the TRIP effect and the initiation and propagation of cracks.