Static fracture toughness of fail-safe steel

A 1800 MPa class steel with ultrafine elongated grain structures was fabricated by multi-pass caliber rolling and a static three-point bending test was conducted. The strength–toughness balance of the developed steel was remarkably improved compared with conventionally quenched and tempered steel with a martensitic structure. The static fracture toughness of the developed steel was about 40 times higher than that of the conventional steel. The microstructural design that improves the toughness in ultrahigh-strength steels is clarified.