High strain capacity X60 linepipe steels with superior strain aging resistance

Linepipe steels used for pipeline construction in arctic and seismically active environments are designed to withstand significant plastic strain in the longitudinal direction to ensure mechanical integrity. High uniform elongation and low yield-to-tensile strength ratio (YR) are part of the overall linepipe specification to ensure adequate strain capacity in these applications. These properties can be achieved in commercial-grade linepipe steels, e.g., X60, with a dual phase (DP) microstructure. However, upon short-duration exposures at relatively low temperatures during the application of fusion bonded epoxy (FBE) coating, a significant degradation in uniform elongation and YR is observed, a phenomenon commonly referred to as strain aging. This paper describes the mechanistic understanding of strain aging in dual phase X60 grade linepipe steels and the development of commercial-grade steels resistant to strain aging through modifications to manufacturing processes and steel chemistry.