Mechanism of crack propagation in 1800 MPa class ultrahigh-strength steel by ultrafine-grained structure (Development of fracture control from microstructure design)

Abstract

A 1800 MPa class steel bar with an ultrafine elongated grain (UFEG) structure was fabricated by multipass caliber rolling at 500°C. The static three-point bending test was conducted in a temperature range from 100°C to -196°C. The behaviors of crack propagation on the developed steel were studied on the basis of the microstructural features. The conventionally quenched and tempered steel with a martensitic structure showed a catastrophic fracture behavior which fractured with a peak bending load in a temperature range from 23°C to -196°C. The crack propagated directly across the center portion of the test bar. On the other hand, the developed steel exhibited a noncatastrophic fracture behavior with evidence of stepwise load increases beyond the first load drop. The microcrack occurred, from near the notch root, with normal to the loading direction (LD) or with an angle of 45° to the LD, and the crack propagated with many zigzag cracks branching from the zigzag crack along the longitudinal direction of the test bar. The occurrence of such brunching cracks corresponded to the spatial distribution of {100} cleavage planes and boundaries of the elongated grains. Namely, the microstructural damage is not localized but rather is widely distributed over very large dimensions. In the bending load - displacement curve, many load drops appeared, and the bending load did not decrease with an increase in displacement due to two effects: the stress shielding effect associated with the interference of multiple cracks and the effect of the improved plastic deformation associated with grain refinement and texture.