Abstract
Three kinds of X70 steels with the same chemical composition and different microstructures fabricated by varying processes were compared in aspect of the microstructures and mechanical properties using SEM, TEM and tensile testing machine. The experimental results showed that the steel 1 with acicular ferritic microstructure fabricated by thermal-mechanical processing with online accelerated cooling (TMCP) exhibited an excellent combination of strength and toughness, but provided high yield ratio of 0.85, low uniform elongation of 8.3% as well as low strain hardening exponent of 0.09, indicating poor deformability. In contrast to above steel 1, the steel 2 and steel 3 having ferrite-bainite dual-phase microstructure respectively fabricated by TMCP and intercritical annealing exhibited the improved deformability in terms of the low yield ratio of 0.69 and 0.68, high uniform elongation of 12.8% and 11.8%, and strain hardening exponent of 0.157 and 0.155. It is argued that the optimum properties combination of strength, toughness and deformability can best be achieved by obtaining a ferrite-bainite dual-phase microstructure. This kind of ferrite-bainite dual-phase pipeline steel is appropriate to transmitting oil and natural gas in seismic zone and permafrost.
Highlights
Pipeline steels are usually used to transport crude oil or natural gas over a long distance
By adjusting the parameters of thermal-mechanical processing with online accelerated cooling (TMCP), a dual-phase microstructure of polygonal ferrite (PF) plus bainite colonies at ferrite grain boundaries can be obtained in steel 2 (Figure 1c,d)
By adjusting the parameters of TMCP, a dual-phase microstructure of polygonal ferrite plus bainite colonies at ferrite grain boundaries could be obtained in steel
Summary
Pipeline steels are usually used to transport crude oil or natural gas over a long distance. In this condition, high strength and toughness are usually required. For X70 pipeline steel, an acicular ferritic microstructure is an optimal microstructure because of its excellent combination of strength and toughness. Dual-phase X70 steel can been fabricated by intercritical annealing with acicular ferrite X70 pipeline steel These three kinds of X70 steels with the same chemical composition fabricated by different processes were compared in aspect of the microstructures and mechanical properties in the present work. The results will be beneficial to the practical application of ferrite-bainite dual-phase pipeline steels with high deformability
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