A dual-precipitation strategy for tailoring the strengths of ferrite and martensite in dual-phase steels

Conventional strengthening strategies for ferrite–martensite dual-phase (DP) steels generally focus on strengthening individual constituent phases, often increasing the strength mismatch between ferrite and martensite and thereby limiting mechanical performance. Here, we propose a dual-precipitation strategy that enables simultaneous precipitation strengthening of both ferrite and martensite through controlled phase transformation followed by tempering. This approach introduces two distinct precipitate populations: interphase precipitates in ferrite and tempering-induced precipitates in martensite. Precipitation behaviour in Ti microalloyed steels, with and without Mo addition, was systematically investigated across varying ferrite–martensite fractions using transmission electron microscopy (TEM), small-angle neutron scattering (SANS), and atom probe tomography (APT). Increasing ferrite fraction progressively reduces precipitation sensitivity during tempering, producing coarser precipitates with lower number density. Mo addition primarily promotes early-stage precipitate nucleation rather than substantially enhancing coarsening resistance, resulting in higher precipitate densities and enhanced strengthening. Precipitate chemistry exhibits pronounced size dependence, with smaller precipitates being increasingly metal-rich, while Mo promotes carbon vacancy incorporation within B1-type precipitates. Strengthening estimation indicates that even ultrafine precipitates strengthen the material predominantly through the Orowan bypassing mechanism. These findings demonstrate that dual-precipitation strategy provides a new microstructural design strategy for tailoring phase-specific strengthening and strength mismatch, offering opportunities to optimise the balance between strength and formability in advanced DP steels.