Effect of Retained Austenite Stabilized via Quench and Partitioning on the Strain Hardening of Martensitic Steels

A novel heat-treating process, quench and partitioning (Q&P), has been proposed as a fundamentally new way to produce martensitic microstructures containing retained austenite. The two-step process hypothesizes carbon enrichment of the austenite by decarburization of the martensite. Significant amounts of retained austenite have been measured in the final microstructure, although evidence for transition carbide formation in the martensite also exists. The mechanical properties obtained via Q&P are reported for a CMnAlSiP steel after intercritical annealing for A50 specimens. Tensile strength/total elongation combinations, ranging from 800 MPa/>25 pct to 900 MPa/20 pct to 1000 MPa/10 pct, indicate that Q&P is a viable way to produce high strength steel grades with good ductility. The instantaneous strain hardening of Q&P steels shows a significant dependence on the partitioning conditions applied. Lower partitioning temperature (PT) leads to continuously decreasing instantaneous n-values with strain, similar to the strain hardening behavior observed for dual-phase (DP) steels, whereas higher PTs for the same partitioning time increase the strain hardening significantly. After an initial increase, the observed n-values remain high up to considerable amounts of strain, resulting in similar strain hardening behavior observed for austempered transformation-induced plasticity (TRIP) grades. Assessment of the mechanical stability of the retained austenite indicates that the TRIP effect is effectively contributing to the increased strain hardening as function of strain.