Effect of Nb-Ti Microalloying Balance on Microstructural Evolution and Mechanical Properties of Complex Phase Steel

Three low-carbon complex-phase steels with different Nb–Ti microalloying balances were investigated to clarify the relationships between microalloying composition, precipitation behavior, microstructural evolution, and mechanical properties. The steels underwent intercritical annealing at 790 °C, followed by isothermal holding at 460 °C and air cooling. Their microstructures and mechanical properties were characterized using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and room-temperature tensile testing. Thermo-Calc, TC-PRISMA, and a strengthening-component model were further employed to analyze precipitation behavior and the contributions of different strengthening mechanisms. The results show that the Nb–Ti microalloying balance is associated with distinct precipitation characteristics and subsequent microstructural evolution. Among the three compositions investigated, Steel 2, containing 0.035 wt.% Nb and 0.025 wt.% Ti, exhibited well-developed (Nb,Ti)C precipitation, refined grains, and a relatively high bainite fraction, resulting in the most favorable strength–ductility balance. Its estimated Orowan strengthening contribution reached 246 MPa. In comparison, Steel 1 exhibited more limited precipitation, whereas Steel 3 showed more pronounced precipitate coarsening. Although Steel 3 achieved the highest tensile strength, its relatively high martensite fraction contributed to its increased strength, while its lower elongation and mixed ductile–quasi-cleavage fracture morphology reflected a less favorable strength–ductility synergy. These findings demonstrate the close association between Nb–Ti microalloying balance, precipitation behavior, multiscale microstructural evolution, and mechanical properties, providing insights into the role of Nb–Ti microalloying in the development of complex-phase steels.