Microstructure and Strengthening Mechanisms in a Newly Designed High Strength Low-Density Duplex Steel

Abstract

A new lightweight steel of Fe-18.17 pct Mn-10.51 pct Al-1.04 pct C-5.88 pct Ni was homogenized and finish rolled at 1223 K (950 °C) followed by rapid quenching which resulted in 80 pct austenite, 20 pct elongated and globular B2 precipitates with high dislocation density (PD1). Annealing the quenched alloy at 1203 K (930 °C) produces an additional 4 pct platelets of B2 (PD1-A). Further aging of PD1-A at 823 K (550 °C) precipitates 5.9 pct k-pearlite at austenite grain boundary (PD1-B). PD1 and PD1-A report yield strength of 1138 MPa and 1015 MPa, respectively. K-pearlite provides additional strengthening of 167 MPa in PD1-B. Calculated yield strength shows that the strengthening is dominated by solid solution, but followed by dislocations in PD1, second phase strengthening in PD1-A and PD1-B. Significant work-hardening mainly by B2, existing dislocations, and grain boundary and or in combination with k-pearlite results in high tensile strength of 1296 MPa in PD1, 1254 MPa in PD1-A, and 1430 MPa in PD1-B. The tensile properties of the low-density steel are superior to a few automotive grade dual-phase and complex-phase steels and comparable to that of press-hardened or martensitic steels. The work-hardening behaviors of all three steels follow Ludwigson flow of easy glide and steady-state work hardening. Work-hardening rate increases with increasing volume fraction but decreasing size of precipitates, however, limited ductility reduces activities of dislocation and avoids dynamic recovery in PD1-B. Coarse size and low volume fraction of B2 precipitate result in a low work-hardening rate in PD1. The higher volume fraction and uniform distribution of B2 precipitate in PD1-A lead to a maximum amount of work hardening. Non-uniform distribution of lamellar k-pearlite restricts plastic elongation of 9 pct in PD1-B. © The Minerals, Metals & Materials Society and ASM International 2024.