Better mechanical properties of SAF2507 duplex stainless steel formed by cold rolling and normalizing

The SAF2507 duplex stainless steel with the heterogeneous structure is produced through cold rolling (ε = 90%) and subsequent 1-min normalizing at 1273 K. Compared with the corresponding initial samples, the grain sizes of ferrite and austenite phases in the 90%-1273 sample are refined by 90%, which led to an increase in the number of grain or phase boundaries between the two phases. The yield strength (YS) of the 90%-1273 sample increased to 715 MPa from 541 MPa, while the uniform elongation of 24.8% is slightly smaller than that of the coarse-grained sample. The higher YS is mainly contributed by the strengthening of grain refinement and dislocation strengthening by incorporating geometrically necessary dislocations (GNDs). During tensile deformation, the accumulation of GNDs in the heterostructured sample continuously generates hetero-deformation-induced (HDI) stress. The HDI stress plays a positive role in the 90%-1273 sample showing an ultimate tensile strength of 939 MPa. Additionally, the amount of twin boundaries in the 90%-1273 sample rose from 4.5% to 35% during tensile deformation, which indicated that the twinning induced plasticity effect was helpful in retaining the ductility of the 90%-1273 sample. This work provides strategies for optimizing the mechanical properties of duplex stainless steels and contributes to understanding the essence of a superior combination of strength and ductility of heterostructured materials.