Microstructure and properties of pipeline steel with acicular ferrite

The X70 grade and X80 grade pipeline steel strip with acicular ferrite microstructure have been researched and developed. The properties of the steel with acicular ferrite were studied by using tensile test, Charpy impact test, CTOD test and DWTT test, and compared to that of the steel with ferrite and pearlite. The microstructure of acicular ferrite was analyzed by using optical metallographic microscopy, scanning electron microscopy and transmission electron microscopy. It shows that the acicular ferrite pipeline steel has high strength, high toughness, low ductile-brittle transition temperature and high fracture-arrest toughness, the excellent properties are benefit from the uniform microstructure, clean steel, and the low carbon acicular ferrite in which consist of interaction of very fine precipitated particles and high-density dislocation.