Abstract

The effect of different cooling processes on microstructure especially precipitation behavior and properties of high grade pipeline steel was investigated in this paper. The results showed that: By two stage controlled cooling process, ferrite-bainite phase was obtained in X80 pipeline steel with high deformability, the average size of Martensite and Austenitic islands (M/A) was approximately 1μm, the Uniform Elongation (UEl) was 10%. Whereas granular bainite was obtained after conventional laminar cooling process and fine M/A of 1∼2μm size dispersed in grains or along grain boundaries. Compared with conventional pipeline steel, high deformability pipeline steel through two stage controlled cooling process had higher Elongation (El), impact toughness and lower yield ratio, due to the existence of ferrie+bainite phase structure which reduced yield strength (YS) and enhanced deformation performance and fracture toughness. Additionally, the effect of precipitation strengthening of high grade pipeline steel produced by Ultra-fast cooling (UFC) process was superior to that produced by conventional laminar cooling process. A significant amount of nano particles precipitated with the size of 1nm to 5nm were obtained through the UFC process. The application of UFC made the precipitation process avoid the nose temperature of precipitation temperature time (PTT) curve, thus the size of nano particles were smaller and the distribution of precipitation was more dispersive.

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