• LPBF 316L steel annealed at 1200 °C for 20 h achieves an outstanding synergy of enhanced strength and exceptional ductility, with ultimate tensile strength of 613 MPa and total elongation of 73.8%.
• Controlled high-temperature annealing significantly increases the proportion of coincidence site lattice (CSL) grain boundaries, improving microstructural stability and mechanical performance.
• Quasi-in-situ tensile testing reveals that deformation twinning reorients crystals, promoting further slip and plastic deformation, which underpins the enhanced plasticity.
• The synergistic activation of slip and twinning mechanisms is the key to achieving superior ductility in heat-treated LPBF 316L steel, offering a pathway for optimizing additively manufactured stainless steel components.