A simultaneously improved strength and ductility on carbide free bainite steel via novel ausrolling and twinning process based on SFE controlling

In this paper, we developed a strategy to circumvent the strength-ductility trade-off in high-strength carbide-free nanobainite steel. Two successive cycles of deformation were designed in the corresponding stacking fault energy ranges of supercooled austenite and retained austenite, which are respectively dominated by dislocation slip and twinning mechanism. The results indicate that the ausrolling and twinning process yielded a high density of dislocations and nanotwins in steel. A high density of dislocations in bainite ferrite trapped more carbon atoms, reducing the carbon content in retained austenite and the corresponding stability. Therefore, martensitic transformation of retained austenite was accelerated, during the tensile process, resulting in an excellent work hardening capacity and consequently ultrahigh strength in the resultant material. Meanwhile, multistage gamma (face- centered cubic)-* e (hexagonal close-packed)-* alpha' (body-centered cubic) martensitic transformation and interaction of nanotwins with dislocations provide superior ductility by alleviating shear strain accumulation and retarding crack initiation. The excellent strength (2.4 GPa) and ductility (15.7%) of the ausrolling and twinning steel indicates that the proposed strategy has potential applications in the development of high-strength, high- ductility materials.

Automated summary

This paper presents a strategy to overcome the strength-ductility trade-off in high-strength carbide-free nanobainite steel. By designing two successive cycles of deformation within specific stacking fault energy ranges, the study successfully achieved a high density of dislocations and nanotwins in the steel, resulting in excellent strength and ductility. The proposed strategy has potential applications in the development of high-strength, high-ductility materials.