High toughness of large-sized G35 ultra-high-strength steel under incomplete hardening conditions

During the fabrication of large-sized critical components, conventional ultra-high-strength steels (UHSS) generally suffer from a significant deterioration in strength and toughness due to the reduction in cooling rate. In contrast, a newly designed UHSS G35 steel exhibits remarkably enhanced impact toughness without achieving full hardenability. In this study, the evolution of microstructure and the underlying mechanism responsible for toughness improvement in large-sized G35 steel bar were systematically investigated, the microstructure of which primarily consists of a dual-phase structure composed of carbide-free bainite (CFB) and retained austenite (RA). A considerable amount of film-like retained austenite forms between bainitic laths at 5–35 mm radial positions from the surface, which undergoes strain-induced transformation during impact deformation absorbing substantial energy, leading to a significant improvement in impact toughness, reaching 120 J. The high Si/C ratio of G35 steel effectively suppresses carbide precipitation, promotes the formation of carbide-free bainite, and stabilizes carbon-enriched retained austenite. Consequently, G35 steel achieves an excellent combination of strength and toughness under low cooling-rate conditions like air-cooling, exhibiting an ultimate tensile strength of 1395–1450 MPa and an impact energy of 135–145 J, which are significantly superior to those of 30CrMnSiA and 35CrMnSiA steels under identical cooling conditions. This study provides valuable insights into the engineering application of G35 steel for medium- to large-scale load-bearing components in advanced equipment and offers guidance for alloy design and heat-treatment optimization of high-performance ultra-high-strength steels.