Influence of deformation Substructure on the strain compatibility of ferrite and bainite in a commercial pipeline steel

The strain instability due to hardening of the soft phase is a key factor in premature failure of pipeline steels after work hardening. In this work, we reveal the hardening mechanism of ferrite is actually the strengthening effect of deformation substructures on the flow stress, where the strengthening contribution by the low angle grain boundaries (LAGBs) is the main stress enhancement way. It is able to subdivide the ferrite into many sub-grain structures through dynamic evolution between geometrically necessary dislocations (GNDs), LAGBs and high angle grain boundaries (HAGBs) during work hardening. These subgrains have independent orientations and can be further subdivided, thus causing the Hall-Petch strengthening effect. Based on the hardening mechanism of ferrite, the representative volume element model with different levels of work hardening is developed. As the level of work hardening increases, the strain compatibility is gradually reduced between ferrite and bainite. The strain localization factor is increased from 0.023 to 0.061, the plastic deformation difference between the two phases is increased from 6.71% to 10.93%, and strain partitioning coefficient is reduced from 7.11 to 3.07. This work will contribute to better understand the substructure strengthening of dual-phase materials and provide a new idea for the protection of work-hardened metals.