<p>In view of the volume instability of steel slag aggregate leading to the quality problem of expansion damage in asphalt road construction, the 4.75–9.5 mm steel slag particles were treated by autoclaved carbonation technology, and the effects of the carbonation system (temperature and time) on the autoclaved pulverization rate, <i>f</i>-CaO content, and the relationship between them for the carbonated steel slag were investigated. In addition, the microstructure of the carbonated steel slag was analyzed by X-ray diffractometer (XRD), scanning electron microscope and energy dispersive spectrometer (SEM-EDS), metallographic microscope and X-ray fluorescence imaging spectrometer (XRF). The experimental results indicate that, under the initial CO₂ pressure of 1.0 MPa, increasing the carbonation temperature leads to the increase in the crystal plane spacing of Ca(OH)₂ that was generated by the hydration of minerals in steel slag, and promotes the transformation of carbonated CaCO₃ from the orthorhombic system to the hexagonal system, resulting in the increase of the crystal planes spacing of them, meantime, accelerates the decomposition of RO phases and also the outward migration of Ca²⁺, Fe²⁺, and Mn²⁺ ions to cover and coat on the Si⁴⁺, Al³⁺ ions, and impels the formation of hydroxides such as Fe(OH)₃ and the formation of carbonates such as Ca(Mg)CO₃, FeCO₃ and MnCO₃. Carbonation at the temperature of 90 °C for 3 h can reach the center of 4.75–9.5 mm steel slag particles. Meanwhile, the increase of temperature can promote the mineral reaction in steel slag, resulting in the fuzzy interface between mineral phases, increase of burrs, dispersion, crossover, reduction of grain size, and rearrangement of mineral particles.</p>