Correlation between microstructure and fracture mechanism of 960 MPa Grade ultra-high toughness steel

This study investigated the correlation between microstructure and fracture mechanisms in a 960 MPa grade ultra-high toughness high strength low alloy steel produced through controlled rolling and off-line heat treatment. The steel features a low-carbon composition (≤0.1 wt%) and a fine-grained(≤10 μm) bainitic microstructure. Through microstructural characterization and fractographic analysis, extensive quantitative analysis of grain, packet, dimple, and cleavage facet sizes was conducted. Fracture surfaces were examined to identify crack initiation sites. The findings indicate that grain refinement significantly enhances toughness. In fine-grained conditions, bainitic packets that approximate the size of the grains modulate crack propagation pathways. The near-oriented bainite across adjacent grains forms structures comparable to abnormally large grains, crucial for crack nucleation. Moreover, martensite/austenite (M/A) islands, while crucial to the initiation of cracks, require the synergistic interaction with grain boundaries and inclusions to significantly influence fracture initiation. This investigation elucidates the complex interplay between microstructural refinement and fracture mechanisms. •Achieved 960 MPa yield strength grade steel and 250 J toughness at −40 °C.•Grain refinement enhances toughness, but comparable bainitic packet size limits crack propagation pathways.•M/A islands are predominant nucleation sites, but require combined factors for fracture initiation.•Critical micro-mechanism is cleavage crack crossing grain boundaries.