Effect of final cooling temperature to microstructure and property relationship of V-N microalloyed X70 pipeline steel

Herein, the final cooling temperatures (FCTs) are the key process parameters to achieve high strength and toughness combination in V-N microalloyed X70 pipeline steels. This research shows that the steels primarily consisted of polygonal ferrite (PF) and a small quantity of pearlite (P) phase at the FCT of 630 °C, while the PF and a small fraction of granular bainite (GB) and acicular ferrite (AF) were obtained at 580 °C. As the FCTs decrease from 580 °C to 450 °C, the proportion of AF and GB gradually increases. The AF was composed of fine non-parallel ferrite platelet structures with higher dislocations densities compared with other phases. The nanoscale precipitates were divided into two types under all the FCTs, large-scale Ti(C, N) precipitates of diameter 20–50 nm that precipitated at high temperatures and small V(C, N) precipitates of diameter 5–20 nm at low temperatures, which significantly contributed toward strength-toughness. As the FCTs decrease from 630 °C to 450 °C, the yield strength −20 °C impact energy and drop-weight tear test properties first increased to 565 ± 7 MPa, 505 ± 9 J and 99 ± 1% at 520 °C, and finally decreased to 520 ± 6 MPa, 256 ± 9 J and 82 ± 2% at 450 °C. The optimum microstructure and mechanical properties were obtained at the FCT of 520 °C. The fine-grain strengthening and precipitation strengthening played a crucial role in affecting the thermo-mechanical properties in this steel, such that fine-grain strengthening of 37.5%, and precipitation strengthening of 32.3% at the FCT of 520 °C. The transformation strengthening from AF and precipitation hardening from high fraction of 5–20 nm V(C, N) precipitates played a crucial role in improving the low-temperature toughness and strength.