Intercritical tempering enables nanoscale austenite/ε-martensite formation in low-C medium-Mn steel: A pathway to control mechanical properties

In a 0.05C-5.4Mn-0.2Si-0.8Cr-wt% steel intercritically tempered at 700 °C for 50 min a new hexagonal closed-packed phase (ε-martensite) appears on former reversed austenite by an air-cooled process. The tensile strength of the tested steel increased from ~ 840 MPa after intercritical tempering at 650 °C for 50 min to ~ 1002 MPa with the tempering temperature increased to 700 °C, and the product of strength and elongation was improved from 19.9 GPa % to 21.6 GPa %; moreover, the impact energy at − 40 °C was decreased from 143 J to 68 J, and the yield ratio was reduced from 0.84 to 0.50. This result was attributed to the weakening of TRIP effect in existence of ε-martensite. The partitioning of alloying elements at different intercritical tempering conditions and the calculation of SFE (stacking faults energy) indicated that ε-martensite was produced in the ~ 50-100 °C temperature range while cooling in air after tempering at 700 °C for 50 min ε-Martensite affected the stability of the reversed austenite and the fracture mode. The fracture mode was changed with the increased tempering temperature. The results reveal that a multiphase low-C medium-Mn steel can be produced via intercritical tempering accompanied by a unique phase transformation process. A high-temperature and long-time tempering condition will lead to a thermally-induced ε-martensite transformation due to lower SFE possibly associated with more homogenized (or less amount of) alloying elements in austenite, and affects the mechanical behaviors of medium Mn steel.