Microstructure-Mechanical Properties Relationships in Quenching and Partitioning (Q&P) Processed Advanced High Strength Steels (AHSS)

DoctorThe use of advanced high-strength steels (AHSS) has increased significantly in recent years, particularly in the automotive industry where the need for mass avoidance is paramount in efforts to increase fuel economy without sacrificing passenger safety. The quenching and partitioning (Q&P) processing of AHSS has been shown to result in a clear improvement of their strength and ductility combinations. The selection of the quench temperature during Q&P processing makes it possible to obtain a wide range of mechanical properties for the final martensite-austenite microstructures. Although various Q&P steels have been investigated in earlier studies, fundamental questions regarding the mechanisms operating during the partitioning stage have remained. The present study provides a direct atomic-scale evidence for the partitioning of both interstitial carbon and substitutional Mn and Si, during the Q&P processing of medium Mn steel by means of 3-dimensional atom probe tomography. The experimental results were compared to results of a numerical simulation of the kinetics of carbon, Si and Mn partitioning during Q&P processing assuming an immobile martensite-austenite phase boundary. The present work also analyzed the application of Q&P processing to medium Mn steels and press hardening steels (PHS) to obtain a new type of ultra high-strength steel with a martensite+austenite microstructure. The selection of the quench temperature makes it possible to vary the ultimate tensile strength within a range of 500 MPa. The processing leads to low carbon lath martensite matrix with a controlled volume fraction of retained austenite. A physically-based model describing the relationship between the microstructure and the mechanical properties of Q&P processed steels was developed