| The increases in strain rate generally enhances the difference in strain partitioning between ferrite and bainite of dual-phase steel,which may induce the strain localization,then affect the strain hardening behavior.Therefore,the micro-strain formed by plastic deformation of different strain rates of pipeline steel in service environment is simulated,and the strain hardening response of pre-strain to the microstructure has been studied.Furthermore,controlling the strain rate of pipeline steel and determining a reasonable deformation range is of great significance for the safety evaluation of pipeline steel in complex geological environments.In this work,the effects of strain rates(10-4 s-1?10-1 s-1)on ferrite hardening of X70 dual-phase steel were investigated through pre-straining tests.The strain localization was analyzed by the evolution of Kernel average misorientation(KAM).The micromechanism of ferrite hardening was elaborated by the evolution analysis of geometrically necessary dislocations(GNDs),low angle grain boundaries(LAGBs)and nano-indentation.The strain hardening behavior of dual-phase steel were studied through finite element simulation analysis and dislocation evolution analysis.The results showed that GNDs and LAGBs in ferrite increased with increasing the strain rate,the average KAM values and nanohardness increased accordingly,and the strain hardening also increases.The plastic deformation and strain localization occurred in ferrite at different strain rates,which expanded from the interior to the ferrite/bainite(F/B)interface with increasing the strain rates.At a lower strain rate(10-4 s-1?10-3 s-1),the inner of ferrite grains acted as a source of dislocations and slip bands during plastic deformation,and strain localization was observed in the ferrite grains.Then,severe dislocation tangles were formed in the inner of ferrite,which led to considerable strain localization,while the degree of ferrite hardening was not sufficient for interface deformation.Due to the nearly saturated GNDs and LAGBs density at the strain rate of 10-2 s-1,the completely ferrite hardening occurred and its nanohardness was close to dual-phase interface.Then,the dislocation cells at F/B interface led to the highly-saturated dislocation density of ferrite,resulting in severe strain localization.Therefore,the stress concentration and strain localization were enhanced with increasing the strain rate,and then the F/B interface deformation was induced to coordinate plastic deformation.During the tensile deformation after pre-tensioning,the stress-strain curves of tensile tests showed continuous yield behavior at the strain rates ranging from 10-4s-1 to 10-3 s-1,while the yield behavior was not significant at higher strain rates.The strain hardening of tensile deformation after pre-tensioning at lower strain rates(10-4s-1?10-3 s-1)showed two stages,while it showed one stage at higher strain rates(10-2 s-1?10-1 s-1).And the strain hardening ability and uniform elongation of dual-phase steel decreased with the increase of strain rate,while the yield ratio increased,which showing obvious strain rate strengthening characteristics.This work provides a theoretical basis for the variable speed destructive behavior of pipeline steel in engineering applications. |
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