X80 pipeline steel,which is mainly composed of ferrite/bainite,is an important structural steel for pipeline transportation.The plastic deformation of X80 pipeline steel at different strain rates caused by geological and human factors deteriorates its strength.Microstructural transformation and strain localization during deformation are the fundamental factors that deteriorate the mechanical proper-ties of steel.Therefore,in this study,the strain partitioning behavior and microstructure evolution mecha-nism of ferrite and bainite in X80 pipeline steel at different strain rates(10-4 s-1 to 10-1 s-1)under 5%defor-mation were revealed using representative volume element models and electron backscatter diffraction technology.The results show that when the strain rate is low(10-4 s-1 to 10-3 s-1),ferrite has sufficient time to complete the evolution of geometrically necessary dislocations(GNDs)to low-angle grain boundaries(LAGBs)and the transformation of LAGBs to high-angle grain boundaries(HAGBs).Ferrite can release strain distortion energy,which can weaken the strain localization behavior of X80 steel.As the strain rate increases,the strain response time decreases,hindering the transition from LAGBs to HAGBs.This re-sults in the accumulation of high-density GNDs and LAGBs in ferrite,thereby intensifying strain localiza-tion.Additionally,when the strain rate is high(10-2 s-1 to 10-1 s-1),the strain partitioning coefficient be-tween ferrite and bainite could be reduced,thereby producing the strain gradient in the vicinity of the inter-face and resulting in GNDs accumulation and back stress formation.Furthermore,ferrite and bainite could show compressive and tensile stresses,respectively,thus limiting the strain coordination between the two phases significantly,increasing the stress concentration near the interface,and reducing the strain hardening ability.The strain partitioning behavior between ferrite and bainite was further revealed to better understand the plastic deformation of X80 pipeline steel.
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