Transformation-induced plasticity (TRIP) 590 steel, with a chemical composition of 0.16C-0.25Si-1.46Mn-1.5A1 (wt%), was treated with an austenitizing pretreatment followed by conventional TRIP heat treatments. Retained austenite (RA) is an important phase in TRIP steel. However, the physical characteristics of nanoscale RA grains cannot be systematically characterized by conventional electron backscatter diffraction and transmission electron microscopy analyses. In contrast, transmission Kikuchi diffraction (TKD) analysis can be employed to systematically characterize the grain size, morphology, and distribution of RA grains. The TKD analysis of TRIP590 steel samples subjected to austenitizing pretreatment revealed a large proportion of lamellar RA grains with a grain size range of 80 nm to 3 mu m, and a more homogeneous distribution of RA grains was obtained around the ferrite grains than samples formed by conventional heat treatments alone. In addition, the volume fraction of the RA phase and its carbon content were greater than those obtained in samples formed by conventional heat treatments alone. Moreover, the austenitizing pretreatment significantly increases the ultimate tensile strength and elongation of the TRIP steel samples, and the product of the ultimate tensile strength and elongation increased by an average of 23%. The pretreated steel samples exhibit excellent mechanical properties, where the product of the strength and elongation reached 26448 MPa%, with a yield strength of 556 MPa, tensile strength of 696 MPa, and an elongation of 38%.