Abstract
Microstructure of high strength pipeline steels containing different Nb contents was investigated using optical microscopy and transmission electron microscopy. Second phase particles were analyzed by extraction replica method. The results show that microstructure of the low Nb steel comprises granule bainite and the low bainite with a few of martensite-austenite (M-A) constituents. However, microstructure is consisted of acicular ferrite with M-A constituents in high Nb content steel. Moreover, M-A constituent is consisted of twinning martensite, lath martensite and retained austenite. In the low Nb steel precipitates are a large of square TiN particles. Second particles of high Nb content are mainly the large size duplex type (Nb,Ti)C and small NbC precipitates.
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