Abstract
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 μm and 43%, respectively, whereas those of the X80 pipeline steel were 2.4 μm 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 ~456 MPa and ~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 ~475 MPa and ~109.8 MPa, respectively.
Highlights
Pipe steels are important materials to transport petroleum and natural gas, and different levels of pipe steels including X52, X60, X70, X80, X100 and X120 have been studied
Due to the poor service environment, high strength and good toughness are required for pipe steels, and the API SPEC 5L standards are used to evaluate the performance of pipe steels
The microstructures of both pipeline steels were of acicular ferrite (AF), bainite ferrite (BF), and fine martensite-austenite (M-A) islands
Summary
Pipe steels are important materials to transport petroleum and natural gas, and different levels of pipe steels including X52, X60, X70, X80, X100 and X120 have been studied. They are divided by different microstructural components. Due to the poor service environment, high strength and good toughness are required for pipe steels, and the API SPEC 5L standards are used to evaluate the performance of pipe steels. Pipeline steel manufacturing has focused on developing pipeline steels with a high grade, large diameter and thick wall [1,2]. The ultimate gauge of hot-rolled X70 and X80 pipe steels with a dominant phase of acicular ferrite could exhibit good comprehensive mechanical properties and act as suitable candidates
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