The test results with electron backscattered difraction(EBSD)and scanning electron microscope(SEM) study and mechanical properties examination show that the structure of microalloying X70
X80 and X100 pipeline steels consists of acicular ferrite
granular bainite and a little lower bainite
and with decreasing effective grain size
increasing bainite content and improve structural homogeneity the strength and toughness of high grade pipeline steels increase obviously.