A Novel thermo-mechanical controlled processing for large-thickness microalloyed 560 MPa (X80) pipeline strip under ultra-fast cooling

A novel TMCP involving UFC was used to produce microalloyed 560 MPa (X80) pipeline strip (with critical thickness of 22 mm) at a pilot-plant scale by adopting high temperature coiling process. Re-reddening phenomenon of large-thickness pipeline strip was considered during controlling of coiling temperature. Competitive properties including strength, ductility, and toughness were obtained, which met the requirements of API SPEC 5L specification. Desirable microstructural constituents consisting of acicular ferrite (AF), bainitic ferrite (BF), and degenerate martensite/austenite (M/A) were obtained at high coiling temperature of 500 °C. A high content of nano-sized (Nb, Ti)C precipitates formed because of higher coiling temperature and auto-tempering induced by re-reddening effect. The predominant strengthening mechanisms in the studied steel at coiling temperature of 500 °C are a combination of grain size, dislocations, and nano-sized precipitates, corresponding to dislocation strengthening increment of ~145 MPa and nano-size precipitation strengthening increment of ~108 MPa. The novel process is an environmental friendly approach for processing of large-thickness pipeline strips with lower internal stresses.