Modeling the dependence of strain-hardening exponent on ferrite content and grain size for dual-phase riser steel

Abstract

The increasing demand for deep-sea oil and gas exploration requires riser steels with superior strain hardening performance. However, the quantitative relationship between microstructural parameters (ferrite content and grain size) and the strain-hardening exponent ( n -value) in dual-phase riser steels remains unclear. In this study, a two-dimensional representative volume element model is developed based on the actual metallographic structure of X70 riser steel. Finite-element tensile simulations are systematically performed to investigate the microscopic deformation mechanisms and mechanical response of the dual-phase steel. The results indicate that plastic strain is mainly concentrated in the ferrite phase, while deformation incompatibility between the ferrite and bainite phases is identified as the primary cause of localized damage. A quantitative predictive model for the n -value is established: n = 0.097 f c − 0.0023 g s + 0.015 f c g s + 0.0767, where f c is the ferrite content and g s is the grain size grade. The model is validated using simulation data from X70 riser steel, demonstrating good applicability and reliability. This study provides a quantitative reference for microstructural optimization of ferrite–bainite dual-phase riser steels within the investigated composition and processing range.