Understanding variability in mechanical properties of hot rolled microalloyed pipeline steels: Process-structure-property relationship

We analyze here the basic philosophy of alloy design and process-structure-property paradigm in pipeline steels. Microstructure of pipeline steels of yield strength in the range of 65 ksi (450 MPa) to 120 ksi (840 MPa) was characterized and the key factors that influenced the microstructure and mechanical properties of pipeline steels were examined. Based on the experimental results and analysis, it is underscored that optimizing C and Mn contents is important to eliminate undesired microstructure and obtain high toughness in pipeline steels. Adequate amount of ferrite is necessary to ensure high deformability of high grade pipeline steels. In ultrahigh strength pipeline steels, bainite obtained through rational alloy design (Mo, B), and suitable processing route (especially high cooling rate and low end cooling temperature) is beneficial in obtaining high strength-toughness combination.