Abstract
To obtain high strength and excellent deformability for ferrite/bainite dual-phase (F/B DP) pipeline steel for gas pipelines based on strain-based design, the volume fractions of ferrite and bainite should be considered first. In this work, abstract representative volume elements (RVE) of finite element models (FEMs) of mesostructure for F/B DP pipeline steel with volume fractions of bainite between 30% and 58% were established, and the effects of volume fraction of bainite on the tensile properties and deformation compatibility were studied. Results show that the stress and strain in the mesostructure were primarily distributed in the bainite and ferrite, respectively, and strain concentration occurs at the ferrite/bainite interface. With increasing volume fractions of bainite, the strain localization factor (SLF) and strain ratio of ferrite with bainite ( ε F / ε B ) decrease, which can improve the deformation compatibility of the F/B DP pipeline steel. However, the stress ratio of bainite with ferrite ( σ B / σ F ) and the contributions of bainite to stress and strain sequentially increase, and, as a result, the strength increases and the ductility decreases. Therefore, a balance of strength and deformability can be obtained when the optimal volume fraction of bainite is in the range of 40% to 48%.
Highlights
The strain-based design has been adopted in the pipeline for passing through the geological hazardous areas, such as oceans, swamps, landslides, frozen soils, and earthquakes [1,2,3,4,5,6]
The deformation resistance of the pipeline pipe becomes an important parameter to prevent deformation collapse caused by geological and ocean current movement [7, 8]. e deformation resistance of the pipeline pipe is related to the pipe diameter, wall thickness, and longitudinal strainhardening exponent of the pipeline steel [6, 8]. erefore, it is necessary for the pipeline steel to exhibit high strain-hardening ability, i.e., excellent deformability. e strain-hardening ability and deformability of the pipeline steel are generally characterized by stress ratio, yield ratio, strain hardening exponent, and uniform elongation [1, 9], which have been specified in some pipeline steel specifications such as DNV 2000 [6, 9] and Q/SY GJX 118-2012 [10]
The simulated results show that the yield strength linearly increases with an increasing volume fraction of bainite, while, for the tensile strength, the increment of tensile strength when the bainite fraction increases from 30% to 40% is higher than the increment when the bainite fraction increases from 40% to 58%
Summary
The strain-based design has been adopted in the pipeline for passing through the geological hazardous areas, such as oceans, swamps, landslides, frozen soils, and earthquakes [1,2,3,4,5,6]. Erefore, it is necessary for the pipeline steel to exhibit high strain-hardening ability, i.e., excellent deformability. For the F/B DP pipeline steel, the balance of strength and ductility strongly depends on the volume fraction, distribution, morphology, and mechanical properties of each phase in the ferrite and bainite steel [1, 9, 11, 12]. Research has been more focused on the effects of the volume fraction of bainite on strength, yield ratio, uniform elongation, and strain hardening exponent. E deformation compatibility between the ferrite and bainite during deformation is an important factor affecting the strain and stress concentration and crack initiation at the F/B interface. Bainite must accommodate high deformation characteristics of ferrite because of the stress concentration at the F/B interface, where the crack first initiates. e deformation compatibility between the ferrite and bainite during deformation is an important factor affecting the strain and stress concentration and crack initiation at the F/B interface. erefore, it is important to consider the deformation compatibility between the ferrite and bainite, which can help to explain the deformation mechanism of the DP steel
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