Research of Strain Aging in Pipeline Steel with a Ferrite/Martensite Dual-Phase Microstructure

By applying ferrite-martensite microstructural control, high-deformability linepipes of grade X70 have been developed. The steel with an acicular ferrite microstructure was intercritically heat-treated at 820 degrees C for 10min, followed by rapidly water cooling, in order to get a ferrite/martensite dual-phase microstructure. After aging at 200-250 degrees C for 5-15min, the microstructure and mechanical properties of the ferrite-martensite microstructural steel were studied. The pipe with a ferrite/martensite dual-phase microstructure exhibited a superior strain aging resistance aging at 200-250 degrees C for 5-15min. The yield ratios were below 0.77 and the pipe still exhibited continuous yielding behavior, meanwhile tensile strength, uniform elongation and impact toughness did not experience significant change. By intercritical annealing, carbides and carbonitride containing Nb and Ti precipitated out of the ferrite phase further, at the same time, solute carbon atoms could adequately diffused from the ferrite to the uniformly distributed austenite derived from the acicular ferrite initial microstructures, leading to the reduction of the amount of free carbon and nitrogen to pin the mobile dislocations in the ferrite phase, therefore, strain aging effects were not pronounced in the temperature and time range investigated.