Abstract: Based on the hot-rolling production line， a novel processing of ultra-fast cooling(UFC)was developed to produce X65 pipeline strip. Microstructure of the experimental steel after the new processing was studied and the strengthening mechanism was further discussed. The results showed that microstructure of the UFC processed X65 pipeline strip primarily consists of fine acicular ferrite(AF)， quasi-polygonal ferrite(QPF)， martensite/austenite(M/A)and degenerate pearlite(DP). The effective grain size is 2.93μm and the proportion of the high-angle boundary in the microstructure is 31.5%. Small block ferrite with the size ranging from 200 to 1000nm is observed in the substructure. A large fraction of nano-sized Nb(C，N)particles(＜10 nm) are observed in the ferrite matrix. Moreover， mechanical properties of the experimental steel met to the API SPEC 5L standard. The main strengthening mechanism for the UFC processed X65 pipeline strip is composed by grain refining strengthening， solid solution strengthening， dislocation strengthening and precipitation strengthening， where the strengthening contributed by nano-sized precipitations is 96.1MPa.

Key words: hot strip rolling, ultra-fast cooling, X65 pipeline steel, acicular ferrite, strengthening mechanism