An In-Depth Study of Mechanical Performance of API 5L Steels Through Micromechanical Modelling and Comprehensive Parametric Study of Microstructure Characteristics

The global utilization of steel from the American Petroleum Institute (API) 5L family is widespread in the construction of pipelines for crude oil and natural gas transportation due to its cost-effectiveness, high strength, and low alloy content. However, like other alloys, API 5L steels are susceptible to ductile failure and softening when subjected to stresses beyond their yield point, leading to inelastic deformation through void nucleation, growth, and coalescence. This study employs a microstructural-based approach using three-dimensional representative volume element (3D RVE) modelling to investigate the effects of void and microstructure characteristics on the yield stress of API 5L steels. The 3D RVE effectively predicts the mechanical behaviour of these steels under quasi-static strain conditions. Additionally, a dislocation density-based mathematical model is used to evaluate the impact of void characteristics during inelastic deformation. Response Surface Methodology (RSM) is applied to analyze the effects and interactions of microstructural parameters and void characteristics on the mechanical properties of API 5L steels. The optimization using RSM predicts an optimal yield stress ( \({\sigma }_{\text{y}}\) ${\sigma }_{\text{y}}$ ) of approximately 521.4 MPa, achievable with a void size of 0.5 μm, void volume fraction of 0.5%, ferrite grain size of 2 μm, bainite grain size of 5 μm, and bainite volume fraction of 50%. The anisotropic flow stress curves predicted by the 3D RVE model align well with experimental findings, validating the model's effectiveness. The study highlights those microstructural parameters, particularly ferrite grain size (FGS) and bainite volume fraction (BVF), have a significant influence on the mechanical properties and yield stress of API 5L steels, offering valuable insights for optimizing steel pipeline materials through controlled microstructural design.