Investigation on micromechanism of ferrite hardening after pre-straining with different strain rates of dual-phase steel

In this work, the effects of strain rates (10(-4) s(-1)similar to 10(-1) s(-1)) on ferrite hardening of 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 (LAGB) and nano-indentation. The results showed that GNDs and LAGBs in ferrite increased with increasing the strain rate, the average KAM values and nanohardness increased accordingly. Strain localization was observed in the ferrite grains at lower strain rates (10(-4) s(-1)similar to 10(-3) s(-1)), and the considerable strain localization occurred at ferrite/bainite (F/B) interface at higher strain rates. 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. During the tensile deformation after pre-straining, the yield ratio increased while the uniform elongation decreased with strain rates of pre-straining. It was worth noting that the completely ferrite hardening led to the early plastic deformation of tensile tests was interface deformation. This work provides a theoretical basis for the ferrite hardening behavior after pre-straining at different strain rates, and then contributes to the safe service of dual-phase pipeline steel.

Automated summary

The study showed that increasing strain rates in dual-phase steel leads to an increase in GNDs and LAGBs in ferrite, resulting in higher hardness. At lower strain rates, strain localization was observed within ferrite grains, while at higher strain rates, significant strain localization occurred at the ferrite/bainite interface.