Study on preparation and mechanism of high-strength-toughness niobium-containing duplex stainless steel: Synergistic annealing-induced TRIP and TWIP effects

To develop Lean duplex stainless steels (LDSS) with excellent strength-ductility balance and cost-effectiveness, a novel Mn-N-saving niobium-containing LDSS (Fe-0.03C-21Cr-5Mn-0.3 N-0.14Nb) was designed. The microstructural evolution and mechanical properties during annealing after heavy cold rolling were systematically investigated. Optimized annealing yielded an ultimate tensile strength (UTS) of ∼1 GPa and total elongation (TE) of ∼70 %. Mechanistic analysis revealed that this exceptional performance originates from unique phase transformation and recrystallization behaviors. Above the austenite transformation temperature, the martensite→ austenite reverse transformation was dominated by a shear mechanism, accompanied by abnormal thermal expansion along the rolling direction (RD). During annealing, recrystallization exhibited significant phase-dependent differences: Ferrite with high stacking fault energy (SFE) completed recrystallization at 800 °C, while austenite recrystallization was delayed until 850 °C due to suppressed recovery by low SFE. Strain hardening analysis indicated that shear-formed lath austenite contained high-density dislocations, enhancing stability but inhibiting transformation-induced plasticity (TRIP). As annealing temperature increased, austenite underwent recovery and recrystallization, reducing dislocation density and reactivating TRIP, thereby improving work hardening. Crucially, annealing at 950 °C elevated austenite SFE to a critical level, triggering synergistic TRIP and twinning-induced plasticity (TWIP) effects, ultimately imparting outstanding mechanical properties. This study provides new experimental insights and theoretical guidance for designing high-performance lean DSS.