Through-thickness cleavage fracture stress of a Ti-V-N plate steel

Through-thickness cleavage fracture stresses, σ_f*, have been determined for six microstructures of a Ti-V-N plate steel directly by through-thickness four-point bending (4PB) notched specimens. Results showed that σ_f* is higher for ferrite-bainite microstructures than for ferrite-banded pearlite. For an identical finish-rolling temperature (FRT), the plates with a high cooling rate have a higher value of σ_f* than their counterparts with a low cooling rate. Following the same cooling rate, the highest values of σ_f* are obtained for steels finish rolled above A_r3 but below T_NR (nonrecrystallization) and the lowest for steels finish rolled below A_r3, which contain deformed ferrite (DF) with texture components. Cleavage microcracks are observed to initiate at second-phase particle or pearlite-ferrite interface and then to propagate into ferrite matrix. Growing microcracks could be arrested by bainite phase distributed uniformly in ferrite matrix, which contributes to a high value of σ_f*. The low value of σ_f* was attributed to elongated ferrite-deformed ferrite and martensite/austenite (MA) microstructures.