Temperature Effect of Hot Rolling Process on Microstructure, Strength and Fracture Toughness of X65 Pipeline Steel

The present work investigates the effect of final temperature of hot rolling process on strength and fracture toughness of API X65 pipeline steel. It has been shown that decreasing the finish rolling temperature from 900 to 800 °C in non-recrystallization region causes a decrease in ferrite grain size by about 16% and change the morphology of ferrite microstructure in non-recrystallization region from polygonal ferrite to quasi-polygonal ferrite. It has been found after measuring the fracture toughness parameter (JIC) for two samples hot rolled in non-recrystallization and dual-phase (α + γ) regions that fracture toughness value of 158 kJ/m2 is reduced by 25% to 118 kJ/m2 due to the probability of proeutectoid ferrite generation in microstructure during rolling in dual-phase region which reduces flexibility and facilitates the crack growth. The results of fatigue crack growth test and da/dN–Δk curve indicate that the fatigue crack growth rate in the dual-phase region of hot-rolled steel is higher than that in the non-recrystallization region due to the inhomogeneity of microstructure and lower fracture toughness.