Achieving excellent cryogenic strength-ductility synergy of ultra-low carbon austenite stainless steel by cryogenic rolling and two-step annealing

Abstract

This study proposes a novel processing method combining cryogenic rolling (cryo-rolling) and a two-step annealing process to further enhance the heterogeneity of 316L austenitic stainless steel. Deformation-induced martensite (DIM) was formed after cryo-rolling due to the reduction in stacking fault energy (SFE), accompanied by microstructural refinement and dislocation accumulation. After the first annealing at 700 °C, most of the DIM reverted to austenite with fine grains (FG, 1∼5 μm), while the stored energy was significantly reduced. In the second annealing at 750 °C, recrystallization became the dominant mechanism of microstructural evolution, resulting in a microstructure consisting of ultrafine grains (UFG, <1 μm), fine grains (FG), and coarse grains (CG, >5μm). After the two-step annealing process, an excellent combination of mechanical properties was achieved, including a yield strength (YS) of 1057 MPa, an ultimate tensile strength (UTS) of 1510 MPa, and a total elongation (EL) of 62.5%. The high YS primarily arises from UFG and dislocation strengthening. The enhancement of heterogeneity facilitated the interaction between UFG, FG, and CG, significantly improving the strain-hardening ability, which can primarily be attributed to the heterogeneous deformation-induced (HDI) effect in the early deformation stage. The transformation-induced plasticity (TRIP) effect was identified as the main mechanism in the later deformation stage.