Microstructure and strengthening mechanisms of heavy gauge pipeline steel processed by ultra-fast cooling (UFC) versus laminar cooling (LC)

Abstract

ABSTRACT In this study, a novel thermo-mechanical controlled processing (TMCP) schedule, based on ultrafast cooling (UFC), was used to fabricate heavy gauge X80 pipeline steels for solving the low qualification drop-weight-tear test (DWTT) properties. The microstructures of the X80 pipeline steels were all composed of quasi-polygonal, acicular ferrite (AF), and granular bainite (GB) produced by Laminar Cooling (LC) and UFC. The grain sizes were finer and the average length of HAG was higher produced by UFC. The main strengthening mechanisms in LC-process steel were grain strengthening and precipitation strengthening, and dislocation strengthening. However, the main strengthening contribution in UFC-process was related to grain strengthening and dislocation strengthening. The decrease in strength induced by reducing Nb, Mo, and Cr contents in UFC steel was compensated by increasing the contribution of grain strengthening. In conclusion, the low-cost X80 pipeline steels can be produced by UFC without degradation of strength and toughness.