Study on continuous cooling transformation and strength–toughness of X70 pipeline steel with high deformability

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...