Effect of thermo-mechanical control process on microstructure of high strength X100 pipeline steel

The transformation of undercooling austenite in a X100 pipeline steel was investigated by means of thermal simulation test. Based on the obtained results, a thermo-mechanical control process (TMCP) by which a mixed microstructure mainly consisted of granular bainite and lath bainite can be obtained has been proposed and effects of finish rolling deformation amount, cooling rate and final cooling temperature on microstructure have been analyzed. The results show that the microstructure of tested steel gradually refines and the volume fraction of lath bainite decreases with the increase of deformation amount. With the increase of cooling rate and the decrease of final cooling temperature, microstructure refines and volume fraction of lath bainite increases obviously. High volume fraction of lath bainite in microstructure is good for high strength. However, excessively high volume fraction of lath bainite and M/A islands with the shape of acicular have harmful influences on toughness.