Effect of austenitic state on the multi-phase control in X90 pipeline steel with high Nb content

This work focuses on the effects of different austenization temperatures on the continuous cooling transformation kinetics and the ultimate microstructures characteristics of 620 MPa grade (X90) pipeline steel. The results indicated that the austenitic state has significant influence on the diffusive transformation with the cooling rate of 1 or 15 degrees C/s, while slight influence with high cooling rate (30 degrees C/s). The transformation of ferrite in multi-phase steel could be significantly promoted by the refinement of austenite grains or the increase of deformation at austenitic non-recrystallization region. While the intermediate and low temperature transformation products, such as lath bainite or martensite, could be obtained from coarsen or undeformed austenite. By controlling the prior austenitic state and the cooling process, multi-phase (including QPF, BF, AF and M/A) X90 industry trails were carried out, and the results of mechanical tested indicated that the rolled plate has excellent mechanical properties. (C) 2015 The Authors. Published by Elsevier td.