Influence of Quenching and Partitioning Times on Austenite Stability and Tensile Properties of CMnAlSi Quenching and Partitioning Steel

Quenching and partitioning (Q&P) steels have emerged as highly promising contenders for automotive applications due to their exceptional blend of high strength and ductility, achieved through transformation-induced plasticity associated with retained austenite (RA). Hence, ensuring precise tailoring of RA content and morphology is crucial for Q&P steels. The present investigation aims to study the effect of quenching and partitioning times on RA stability, and the mechanical properties of 0.26C-1.87Mn-0.99Al-0.45Si steel subjected to hot rolling followed by direct quenching and partitioning (DQP). This present research introduces a new microstructural-based approach for identifying the optimal quenching and partitioning times to achieve a microstructure comprising of thin films of inter-lath austenite between martensite laths. DQP steels with longer quenching time exhibit blocky RA islands of smaller sizes. On the other hand, with a shorter quenching time, the microstructure contains a larger fraction of RA with higher carbon content, and this RA is predominantly film type along with small-sized blocky RA islands. This results in an excellent combination of mechanical properties (UTS of 1250 +/- 39 MPa and % elongation to failure of 16%). The post-tensile microstructures were also examined using transmission electron microscopy, and film-type RA appears more resistant to strain-induced transformation.