Effect of microstructure on the crack propagation behavior of microalloyed 560Â MPa (X80) strip during ultra-fast cooling

A novel thermo-mechanical controlled processing (TMCP) involving ultra-fast cooling (UFC) was used to process microalloyed 560 MPa (X80) strip with critical thickness of 22 mm. The microstructure and mechanical properties including tensile, Charpy v-notch impact toughness, and drop weight tear test (DWTT) properties were studied, with particular focus on the effect of microstructure on fracture and crack propagation behavior during DWTT test. The study underscores that the processed strip comprising of acicular ferrite (AF), bainitic ferrite (BF), together with finely distributed martensite/austenite (M/A) constituent provided excellent combination of strength, toughness and crack arrest property. The yield strength (618 MPa), tensile strength (752 MPa), and low-temperature toughness (upper shelf energy of 302 J, transition temperature of −74 °C) as well as DWTT shear area (100%) met the requirements of API SPEC 5L. The predominantly AF microstructure with small-size M/A constituent improved the low-temperature toughness by increasing the critical fracture stress and increased the ability to hinder crack propagation during Charpy test. The AF and finely distributed M/A constituent effectively deflected the crack propagation during DWTT indicating excellent crack arresting property of pipeline strip. It is the cooling schedule of UFC that was responsible for decreasing the effective grain size and increasing the mechanical properties.