Transmission electron microscopy of martensite/austenite islands in pipeline steel X70

The fine structure of martensite/austenite (M/A) islands in pipeline steel X70 was investigated using transmission electron microscopy. It was shown that the microstructure of the steel undergoing thermo-mechanical controlled processing, comprises a majority of irregular blocky ferrite and acicular ferritic constituents and a small amount of so-called M/A islands which in turn consisted of retained austenite and martensite plates different in size and orientation. Microtwins and midrib are usually formed within a martensite plate, which is an indication of the occurrence of high-carbon martensite. It was in addition found that the amount of the M/A islands decreased slightly with increasing the cooling rate after the hot deformation, and the morphology of the M/A islands changed from thick, irregular long strip to thin, short rods. The holding temperature after the fast cooling had no evident effect on the amount of the M/A islands, though lowering the holding temperature could yield a finer and more dispersive M/A islands. (c) 2006 Published by Elsevier B.V.