LAUSR

Strain‐controlled fracture is investigated in 20CrNi2Mo steel using the Hall–Petch relationship, electron backscatter diffraction (EBSD), and other techniques. An interesting result is that the ductility of the tested steel increases with a coarsening of prior austenite grains. This finding is in contrast to conventional understanding. Meanwhile, the strain after necking is approximately 2/3 of the total strain, and the value of UC (crack propagation energy per unit volume) reaches 95% of UK (static toughness). The ductility of 20CrNi2Mo steel also depends on crack propagation. Furthermore, the Hall–Petch relationships between ductility and multilevel microstructure demonstrate that lath is the effective control unit of the plasticity of the tested steel. Lath also plays a key role in crack propagation that results from the rotating, bending, and shearing of lath, as confirmed by fracture analysis and EBSD.