Role of Direct Quenching and Partitioning Processes in Improving Austenite Stability and Stretch Flangeability in a Low C Steel

The current research work introduces a novel processing technique involving a combination of hot rolling and a direct quench and partitioning treatments to produce an ultra-high strength, low-carbon and lean-composition steel with superior mechanical properties and enhanced stretch flangeability. The methodology involves the introduction of a secondary partitioning step after a one-step direct quenching and partitioning (DQP) process. A detailed investigations on microstructures, tensile properties and stretch flangeability (using hole expansion testing) were carried out. The martensite-austenite two phase microstructure resulted in a remarkably improved product of strength and elongation (PSE, 24 GPa.%), the hole expansion ratio of 45% and a total elongation of 21.7%. It is shown that the stability of retained austenite, rather than its volume fraction, has a significant impact on the strain hardening rate, and therefore influences strength, ductility and stretch flangeability. The results indicate that tailoring retained austenite stability is essential for optimizing the mechanical performance and stretch flangeability of quenched and partitioned steels. Introducing secondary partitioning into the Q&P process provides a feasibility to achieve a large fraction of total retained austenite, (predominantly film-type, along with small-sized blocky retained austenite islands in the microstructure), which results in high-strength Q&P steels with excellent global and local formability.