Effect of finish rolling temperature on microstructure and mechanical properties of X80 pipeline steel by on-line quenching

A Ti-V microalloyed X80 seamless pipeline steel was prepared using on-line quenching and off-line tempering technology. The influence of finish rolling temperatures (FRTs-950 degrees C and 900 degrees C) and tempering temperatures (600 degrees C-700 degrees C) on microstructure and mechanical properties was investigated. Fully recrystallized austenite was formed at 950 degrees C FRT, but partially at 900 degrees C FRT. The quenched microstructure consisted of lath bainite, granular bainite, cementite and TiC precipitates. With the decrease in FRT, more granular bainite and strain -induced TiC precipitates were obtained in the quenched microstructure. The original quenched structure significantly impacted the subsequently tempered microstructure. The tempered microstructure was composed of polygonal ferrite, tempered bainitic laths, cementite and (Ti, V)C precipitates. As FRT decreased, the tempered microstructure exhibited more polygonal ferrite, smaller grain size and better precipitation characteristics of (Ti, V)C precipitates, simultaneously improving the strength and impact toughness. An ideal combination of the yield strength (-632 MPa), tensile strength (-707 MPa), impact value (-249 J at-20 degrees C) and shear-fracture area (-97% at-20 degrees C) was achieved at 900 degrees C FRT and 680 degrees C tempering temperature, which satisfied the API requirements of X80 pipe. Therefore, 900 degrees C FRT and 680 degrees C tempering temperature were suitable process parameters for on-line quenching technology to produce X80 seamless pipeline steel.