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

Author:

Jiang Jinxing12,Zhang Zhongde3,Guo Kai3,Guan Yingping1,Yuan Liangzeng3,Wang Qingfeng3

Affiliation:

1. School of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China

2. Plate Business Department, Nanjing Iron & Steel United Co., Ltd., Nanjing 210035, China

3. Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

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.

Funder

National Key Research and Development Program of China

Key Research and Development Program between Nanjing Iron & Steel United Co., Ltd.

Yanshan University

Publisher

MDPI AG

Subject

General Materials Science,Metals and Alloys

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