Effect of rolling processing on the microstructure and M/A islands in a X70 grade pipeline steel

The effects of cooling rate, deformation temperature and reduction on the microstructure and martensite/austenite (M/A) islands in a Nb-V-Ti micro-alloyed steel were studied by Gleeble thermal-mechanical simulation tests and microstructural observations. The results show that an increase in cooling rate and a decrease in deformation temperature can refine the mierostructure, reduce polygonal ferrite and enhance acicular ferrite (AF) in the steel. Under different controlled rolling processes, there is a certain amount of M/A island structure in the matrix, while the effect of deformation temperature on the M/A is small. Appropriately increasing the cooling rate and reduction can reduce the volume fraction of islands and make them be refined and dispersed, so the proper combination of AF and M/A can be obtained by controlling deformation parameters to improve the properties of the steel.