Concurrent enhancement of ductility and toughness in an ultrahigh strength lean alloy steel treated by bainite-based quenching-partitioning-tempering process

A low-carbon Mn-Si-Cr-Mo alloyed steel was treated by two different heat treatment routes, namely bainite-based quenching plus tempering (BQ-T) and bainite-based quenching- partitioning- tempering (BQ-P-T). The strength, ductility and toughness were enhanced concurrently after BQ-P-T treatment (i.e., ultimate tensile strength: 1416 MPa, the PSE: ~ 25.5 GPa%, the CVN impact energy at 20 °C and − 40 °C: ~ 95 J cm−2 and ~ 45 J cm−2, respectively). These enhanced mechanical properties were attributed to the refined ductile bainite/martensite and the filmy retained austenite multiphase microstructure. The microstructural characterization were carried out by conducting scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, transmission electron microscopy and dilatometry. The carbon partitioning between bainite/martensite and austenite can not only stabilize the austenite films but also hinder the coalescence of the bainitic plates and promote the formation of ultrafine bainitic plates. Besides, both the carbon partitioning and the enhanced tempering of martensite/bainite in the BQ-P-T condition contribute to generating the ductile bainite/martensite. Finally, the microstructural factors in controlling the strength, ductility as well as impact toughness were investigated through the analyses of the fracture surface morphology and the retained austenite evolution beneath the fracture surface.