Processing-Microstructure-Property Relationship in Governing High Strength-High Ductility Combination in Fe-4Mn-4Ni-3Al-0.1C Steel

Breaking trade-off relationship between strength and ductility in steels has been a perpetual topic, since researchers have been pursuing more energy and resource-efficient manner to use steels. In this study, rolling, tempering and inter-critical annealing were innovatively combined to obtain high strength- high ductility combination in Fe-4Mn-4Ni-3Al-0.1C steel. The process referred as multi-step inter-critical annealing (MIA) was designed to accomplish the following objectives: (a) a stretched lamellar-shaped grain structure resulting from rolling at non-recrystallization region of austenite, (b) Mn/C-heterogeneous austenite inherited from carbides formed during tempering. Therein, Mn/C-depleted austenite areas originated from ferrite; meanwhile, Mn/C-enriched austenite areas originated from the carbide dissolution, (c) transformation hardening of Mn/C-depleted austenite area during cooling in the inter-critical annealing, (d) stress relaxation of Mn-enriched austenite area retained as retained austenite at room temperature. The specimen achieved excellent mechanical response through the optimized MIA process (tempering at 400 °C for 3 hours, then inter-critical annealing at 765 °C for 40 minutes). The obtained microstructure was composed of alternating lamellae of bainite and a special annealed martensite on microscale, with the latter consisting of secondary martensite and retained austenite films on nanoscale. The yield strength and total elongation of steel were increased from 858 MPa and 12% in direct inter-critical annealing process to 1011 MPa and 32% in MIA respectively. The modified Crussard-Jaoul analyses were used to evaluate the deformation behavior of the steels. Due to lots of bainite formation, a relatively high initial strain hardening exponent (0.72 m-1) was observed for the DIA steels. While lower initial strain hardening rate (0.32 m-1) and more stages of work hardening because of increasing stable austenite and secondary martensite in the MIA steels ensured the excellent strength–ductility combinations.