Microstructure and Strengthening/Toughening Mechanisms of Heavy Gauge Pipeline Steel Processed by Ultrafast Cooling

Heavy gauge pipeline steels experience a low qualification in drop-weight-tear test properties because of the low cooling capability of conventional thermomechanical controlled processing. To solve this problem, a new-generation thermomechanical-controlled processing technology based on ultrafast cooling was applied to prepare heavy gauge pipeline steels. The microstructure, strengthening and toughening mechanisms of 25.4 mm X70 and 22 mm X80 pipeline steels that were processed by ultrafast cooling were studied. The microstructures of the 25.4 mm X70 and 22 mm X80 pipeline steels consisted of bainitic ferrite, M-A island and acicular ferrite with a large fraction above 85%. The grain size and high-angle grain boundary fraction of X70 pipeline steel were 2.7 mu m and 43%, respectively, whereas those of the X80 pipeline steel were 2.4 mu m and 45%, respectively. The strengthening and toughening mechanisms were studied for the ultrafast cooling method. The main strengthening mechanism for 25.4 mm X70 pipeline steel was solution and grain-refining strengthening and precipitation strengthening with contributions of similar to 456 MPa and similar to 90.5 MPa, respectively. In the 22 mm X80 pipeline steel, the main strengthening mechanism was the solution and grain-refining strengthening, and dislocation strengthening with contributions of similar to 475 MPa and similar to 109.8 MPa, respectively.