Title: Study on continuous cooling transformation and strength-toughness of X70 pipeline steel with high deformability

Authors: Bin Wang; Cui Zhou; Jia-xiang Zhu; Xiao-yu Tian

Addresses: School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China ' School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China ' School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China ' School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China

Abstract: Microstructure evolution and mechanical properties of X70 pipeline steel with high deformability was studied in this paper, phase transformation of this steel was investigated by continuous cooling transformation (CCT) method on Formastor-FII automatic transformation instrument. It was concluded that the phase transformation of the steel mainly occurred at 800∼500ºC with Ac1 = 730ºC and Ac3 = 890ºC. Furthermore, as the cooling rate increased, the phase transformation temperature decreased and the hardness of the steel increased. When the cooling rates was 5~20ºC/s, the microstructure was composed of quasi-polygonal ferrite and bainite, the content of bainite increased as cooling rates increased; when the cooling rates were lower than 2ºC/s, bainite disappeared, the microstructure was polygonal ferrite and pearlite. The stress-strain curve of X70 pipeline steel with high deformability exhibited round-house type and the steel had excellent strength-toughness and uniform elongation(>10%), low yield ratio (<0.8), high strain hardening exponent, the brittle-ductile transition temperature was between −100∼−120ºC.

Keywords: X70 pipeline steel; high deformability; CCT; continuous cooling transformation; microstructure; strength; toughness; deformation; phase transformation; steel hardness; ferrite; bainite; pearlite; stress; strain.

DOI: 10.1504/IJMMP.2014.066561

International Journal of Microstructure and Materials Properties, 2014 Vol.9 No.2, pp.136 - 146

Published online: 30 Apr 2015 *

Full-text access for editors Full-text access for subscribers Purchase this article Comment on this article