Effect of Final Rolling Temperature on the Precipitates and Strengthening‐Toughening Mechanism of V–N Microalloyed X70 Pipeline Steel

In this study, the final rolling temperatures (FRTs) are the key process parameters in the control of precipitates, microstructure, and mechanical properties of V–N microalloyed X70 pipeline steels. As the FRTs decrease from 890 to 780 °C, the yield strength (YS) , −20 °C impact energy and drop‐weight tear test (DWTT) properties first increase to 565 ± 7 MPa, 273 ± 10 J, and 95 ± 2% at 830 °C, and finally decreased to 545 ± 5 MPa, 221 ± 6 J, and 85 ± 3% at 780 °C. The ductile‐brittle transition temperature (DBTT) is −71, −73, −79, and −69 °C, respectively, at FRTS of 890 , 860 , 830, and 780 °C. The diameter of the precipitates was in the range between 5–80 nm under all the FRTs. The optimum microstructure and mechanical properties were obtained at a FRT of 830 °C. The main effect of the FRT on the yield strength was fine grain strengthening, dislocation strengthening and precipitation strengthening in this steel. The significant enhancement in low‐temperature toughness is primarily attributed to transformation strengthening from acicular ferrite (AF) and precipitation hardening from high fraction of 5‐20 nm V(C, N) precipitates. The fine non‐parallel AF with high degree of misorientation could improve the low‐temperature toughness by impeding the dislocation crack propagation.