Abstract: The pearlite (P) and ferrite (F) morphology evolution and its influence on the hardness in the interface, after stress relief annealing, on substrate side of composite plate joint between 304 steel and Q245R steel by explosive welding were investigated using scanning electron microscope (SEM) and nano indentation. The results show that the diameter of recrystallized equiaxed grain was 1 μm in fine grain zone, the size of ferrite subgrain among equiaxed grains formed in the original pearlite position was less than 100 nm. The subgrain boundary consisted of discontinuous spheroidized cementite and continuously distributed carbon atom. Spheroidization of cementite and precipitation of ferrite subgrain occurred in broken pearlites in fiber zone, and some unbroken lamellar pearlites age softened. In distorted grain zone, spheroidization of cementite and precipitation of ferrite subgrain occurred in a few broken pearlites, but other unbroken lamellar pearlites age hardened. At bonded interface, the hardness of ferrite and pearlite changed depending on the softening or hardening mechanisms in different regions.