Investigation on micromechanism involved in ferrite hardening after prestraining of dual-phase steel

The evolution of low angle grain boundaries (LAGBs) and Taylor factor (TF) was investigated by EBSD analysis to reveal the micromechanism of ferrite hardening for dual-phase steel during prestraining from 1% to 5%. The results showed plastic deformation was dominated by soft ferrite phases during prestraining from 1% to 3%, and subsequently, transformed to interface deformation during prestraining from 3% to 5%. Due to the evolution of LAGBs, dislocation substructures were generated during prestraining. The fraction of soft grains within the range of TF similar to 2-3 gradually decreased after the hardening, indicating the slip system was more difficult to be activated within ferrite. The hardening of ferrite was attributed to the formation of stable dislocation subgrain structures and the hardening of grain orientation as well. When the prestraining degree went above 3%, the ferrite phases can be completely hardened. After ferrite hardening, the strain hardening ability of the dual-phase steel was reduced, the yield strength was increased and the uniform elongation was decreased at the same time. It was noteworthy that the stress drop (up to 10 MPa) occurred on the stress-strain curve after the completion of strain hardening, and the plastic deformation was initiated directly at interface, exhibiting a mixed mode of failure involving both cleavage brittle and dimple ductile fracture. This work is aiming to provide detailed insights and experimental evidence for ferrite hardening behaviour after prestraining, which is expected to make great contribution for better service protection of the pipeline using dual-phase steel.

Automated summary

The evolution of low angle grain boundaries and Taylor factor in dual-phase steel during prestraining was investigated, showing a transition from plastic deformation dominated by soft ferrite phases to interface deformation. The hardening of ferrite was attributed to the formation of stable dislocation subgrain structures, leading to reduced strain hardening ability, increased yield strength, and decreased uniform elongation. The stress drop observed after strain hardening completion initiated plastic deformation directly at the interface, resulting in a mixed mode of failure involving both cleavage brittle and dimple ductile fracture. This study provides insights and experimental evidence for ferrite hardening behavior after prestraining in dual-phase steel.