Effect of Nb Content on the Microstructure and Impact Toughness of High-Strength Pipeline Steel

Abstract

In this study, X80 pipeline steel is prepared with different Nb contents through the thermo-mechanically controlled rolling process. The effects of using two different Nb contents on the impact toughness and microstructure of the pipeline steel are examined using various experimental techniques. The results show that with the increase in Nb content, the transformation temperature Ar3 decreases, and the nucleation and growth of bainitic ferrite with lath features (LB) are promoted, while those of granular bainite (GB) are inhibited. In addition, the stability of the austenite phase increases with the increase in Nb content. Therefore, the volume fractions of LB and martensite–austenite (M/A) constituents increase, while the proportion of high-angle grain boundaries (HAGBs) decreases. The impact energy of pipeline steel at −45 °C is closely related to the Nb content. Specifically, the impact energy decreases from 217 J at 0.05 wt.% Nb to 88 J at 0.08 wt.% Nb. The cracks are preferentially formed near the M/A constituents, and the HAGBs significantly inhibit the crack propagation. The steel with 0.05 wt.% Nb has a lower content of M/A constituents and a higher proportion of HAGBs than the one with 0.08 wt.% Nb. In addition, as the Nb content increases, the crack initiation energy and the crack propagation energy decrease. Thus, the 0.05 wt.% Nb steel has a higher low-temperature impact energy.

Effect of aging on the structure and mechanical properties of austenite stainless steel N50 alloyed with nitrogen, niobium and vanadium

“…These precipitates act as stress concentrators in the material and reduce the impact strength. Various studies have shown that large precipitates of Nb can have a negative impact on toughness, mainly [5,6,24,25]. These precipitates formed in the structure before aging.…”

Heat Treatment Process, Microstructure, and Mechanical Properties of Spring Steel with Ultra-High Strength and Toughness

“…Considering the beneficial effect of large-angle grain boundaries on impact toughness, an appropriate amount of V is added in the composition design to make the experimental steel exhibit more stable impact toughness under the same conditions as much as possible. Jiang et al [24] conducted a study on the influence of different Nb contents on the microstructure evolution and impact behavior of M/A, and found that an increase in Nb content leads to an increase in the proportion of M/A components, and a decrease in the crack initiation energy. Further analysis revealed that an increase in the proportion of M/A components has a certain adverse effect on impact toughness.…”

Study on Fracture Behavior and Toughening Mechanisms of Ultra-High-Strength Pipeline Steel

“…The toughening mechanism below the X100 level has been well studied. There, a good match of strength and toughness was achieved [8][9][10][11][12][13][14]. The contradiction between strengthening and toughening becomes more prominent when the steel grade reaches X100 [7,[15][16][17][18][19].…”