Heterogeneous Microstructure of Low-Carbon Microalloyed Steel and Mechanical Properties

The microstructure plays a major role in the performance of metallic materials, which can be tailored through the composition and/or processing technique. In this study, a heterogeneous microstructure was developed for low-carbon microalloyed API X65 steel, the most commonly used pipeline steel for oil and gas transportation, using a heat treatment process. The heat treatment process involved intercritical heating of the steel followed by high-temperature isotheral cooling, allowing for phase transformation, as well as alloying element partitioning. The heat treatment transformed the banded ferrite–pearlite microstructure of rolled steel to a quasi-polygonal ferrite microstructure, with the sporadic presence of austenite at the grain boundaries. The quasi-polygonal ferrite was distributed in a heterogeneous form with a fine-grain shell surrounding the coarse-grained core. The heterogeneity in the microstructure, despite being single phase, led to a significant improvement in the tensile yield strength, ultimate tensile strength, ductility and toughness of the steel, with a marginal reduction in microhardness values.