Deformation Temperature Path Effects on Martensitic Hierarchy and Strength–Ductility in Automotive Ti–Zr Microalloyed Steel

This study examines how deformation temperature paths affect martensitic hierarchy and the strength–ductility balance of an experimental low‐carbon Ti–Zr microalloyed ultrahigh‐strength steel. Isothermal three‐pass compression at 950, 1000, or 1050 °C (routes A–C) was compared with decreasing‐temperature paths of 1050–1025–1000 °C and 1050–1000–950 °C (routes D and E). After water quenching, the microstructures were characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM), together with microtensile testing and hardness measurements. The 1050 °C isothermal route coarsened prior‐austenite grains and martensitic packets/blocks, whereas decreasing‐temperature deformation suppressed coarsening and enhanced martensitic subdivision. Route D produced the most homogeneous hierarchical structure, the highest high‐angle‐boundary fraction, and coordinated kernel average misorientation (KAM) and geometrically necessary dislocation (GND) distributions, yielding 915.3 MPa yield strength, 1043.0 MPa ultimate tensile strength, and a Vickers hardness of 315.4 HV. Route E produced the finest hierarchy and greater dislocation‐associated stored energy, with 766.4 MPa yield strength, 928.9 MPa tensile strength, and 25.2% elongation. These results show that the deformation temperature path balances boundary‐network homogenization against stored‐energy retention, while Ti/Zr‐rich carbide precipitates provide auxiliary strengthening.