Microstructural evolution and mechanical behavior of phase reversion-induced bimodal austenitic steels

Abstract

The present study aimed at achieving excellent mechanical properties in austenitic stainless steel by the design of bimodal (BM) microstructure fabricated through cold rolling and reverse annealing. The evolution of the BM microstructure during reverse annealing was investigated in detail. Moreover, the strengthening mechanism and the work-hardening behavior of BM steels were also analyzed. It was found that when the annealing temperature was below 634 °C, the reversion of strain-induced martensite (SIM; approximately 80%) into austenite (α' → γ) was mainly controlled by the diffusion mechanism. However, when the annealing temperature exceeded 634 °C, the reversion of SIM was gradually controlled by the shearing mechanism due to an increase in the driving force for shearing reversion. When annealing was carried out at 800 °C for 1–1000 s, the BM microstructure was successfully obtained because of the differences in nucleation rate and stored energy between the reversed austenite zone (approximately 80%) and the deformed austenite zone (approximately 20%) during recrystallization. The yield strengths (YS; 658–775 MPa) and the tensile strength (TS; 1157–1183 MPa) of the as-prepared BM steels were significantly improved as compared to those of the solid-solution-treated (coarse-grained; CG) steel (YS = 332 MPa and TS = 1022 MPa). Furthermore, the effects of twinning-induced plasticity (TWIP) and multi-stage transformation-induced plasticity (TRIP) (caused by the BM microstructure) contributed to strong work-hardening ability and high ductility. The uniform elongation and the total elongation of the as-obtained BM steels were measured as ~45% and ~55.5%, respectively.