The significance of multi-step partitioning: Processing-structure-property relationship in governing high strength-high ductility combination in medium-manganese steels

Intercritical annealing, flash process and tempering were innovatively combined to obtain high strength-high ductility combination in 0.12C–4.89Mn-1.57Al steel. The process referred as multi-step partitioning (MSP) was designed to accomplish the following objectives: (a) enrichment of austenite with Mn to enhance the stability of retained austenite, (b) transformation hardening during quenching in the flash process and (c) stress relaxation and carbon enrichment of retained austenite. The tensile strength of steel increased from ∼667 MPa in intercritically annealed steel to ∼986 MPa in flash processed steel. The product of strength and elongation of flash steel and tempered steel were 23.2 GPa• % and 24.9 GPa• %, respectively and higher than the intercritically annealed steel (21.3 GPa• %). The high ductility, especially the uniform elongation of flash steel (16.2%) and tempered steel (19.4%) is attributed to ∼15–19% by volume of Mn-rich stable retained austenite and efficient TRIP (transformation induced plasticity) effect. Thermodynamic calculations enabled us to understand the partitioning behavior of alloying elements in MSP. C, Mn and Al reverse partitioning during the flash process led to increased stability of retained austenite. The unique distribution of chemical constituents contributed to two types of martensitic transformation during the flash process: (a) austenite → α′-martensite transformation dominated at high temperature and contributed to the formation of stacking faults and ε-martensite transformation and (b) austenite → ε-martensite → α′-martensite phase transformation dominated at lower temperature. The stability of retained austenite and interaction with stress concentration contributed to highly efficient TRIP effect in flash processed and tempered steel. The experiment findings were consistent with the diffusion-controlled transformation simulation analysis.