Ultrahigh strength and low yield ratio of niobium-microalloyed 900 MPa pipeline steel with nano/ultrafine bainitic lath

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2010-06, Vol.527 (16), p.3886-3892
• Main Authors: Guo, Aimin, Misra, R.D.K., Xu, Jinqiao, Guo, Bin, Jansto, S.G.
• Format: Article
• Language: English
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; Elasticity. Plasticity; Exact sciences and technology; Linepipe; Low yield ratio; Materials science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Microalloyed steel; Nb-microalloyed steel; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Precipitation; Ultrahigh strength
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2010.02.067

An ultra-low carbon niobium-microalloyed steel with yield strength of ∼900 MPa has been processed on a pilot-plant scale. The microstructure of the steel is primarily characterized by lower bainite and acicular ferrite, with small fraction of lath-martensite and martensite–austenite (MA) constituents. Bainite is present as fine domains. A combination of niobium and titanium precipitates was observed at the grain boundaries and in the interior of the grains and includes irregular (∼40–150 nm of (Nb, Ti)(C, N)) and fine cuboidal/spherical particles of NbC (∼30–50 nm). It was observed that accelerated cooling inhibited the precipitation of Nb and Ti carbides. The Charpy impact toughness at −20 °C was 200 J and tensile elongation was 15% with the yield ratio of less than 0.84. The good matching of high strength and low yield ratio was realized by two-stage thermo-mechanical rolling combined with fast cooling.