Ferrite (α) recrystallization behavior during annealing is focused on because the recrystallized grains affect the distribution of martensite in dual-phase structure. The driving force of recrystallization increased as the increase of dislocation density (ρ). Not only cold-working but also martensitic transformation may induce a high ρ. Thus, as-quenched martensite is a promising microstructure in which α recrystallization occurs without cold-working before annealing. In this study, the characteristics of martensite required for the α recrystallization in low-carbon steel were clarified using neutron diffraction with line profile analysis.

The 0.1C–2.0Mn (mass%) steel was chosen. The 30-mm-thick steel plates were hot-rolled at the finishing temperature of 900 °C into the 3.0-mm-thick sheets, and subsequently followed by water quenching below 100 °C to develop a martensitic structure (AQ). The as-quenched martensitic steel sheets were cold-rolled with the reduction of 40% (40CR) and 67% (67CR) in the thickness, respectively. Those steel sheets were heated to 650 °C at the heating rate of 0.5 °C/s, and subsequently followed by water quenching to room temperature. The dislocation structure such as ρ and dislocation distribution was characterized using a diffraction profile analysis where the convolutional multiple whole profile (CMWP) method was performed for the initial microstructures.

No recrystallized α grains were detected in the annealed martensite of the AQ, while recrystallization was almost completed in both the cold-rolled sheets at 650 °C. Each ρ in the three martensitic steels was higher than that in the cold-rolled ferritic–pearlitic steel in which recrystallization occurred. Only the AQ shows the martensite structure consisted of randomly distributed dislocations, which was represented by a dimensionless parameter M* (=R_e*ρ^0.5, where R_e* is the effective cut-off radius of a dislocation). In contrast, the strong interactions between dislocations were confirmed in the 40CR and 67CR. Therefore, the interactions between dislocations are responsible for the recrystallization behavior and its progress.