Phase and Microstructure Evolution of a Low-Alloyed Steel During Intercritical Annealing and Quenching

Abstract

Strong, yet ductile steels can be manufactured by applying optimized thermal treatments during production. Such treatments can be performed on low-alloyed steels and their mechanical properties can be tuned within relatively wide limits. Although dual-phase steels only contain low concentrations of alloying elements due to optimized thermal processing they can reach ultimate strengths of 1000 MPa or elongations to fracture of up to 30 pct. During their production ferrite (α) partially transforms into austenite (γ) and then during quenching further into martensite (α′). So far, many assumptions had to be made about these transformations since they were difficult to observe at elevated temperatures and during quenching. In this study, we combine heating and quenching inside an SEM with EBSD measurements to track the evolution of the microstructure at the surface of a small steel sample throughout a simulated production path. The orientation relationships of austenite and martensite formation are investigated by EBSD mapping of the parent and product phases. The microstructure is analyzed before and after quenching and the orientation relationships associated with the γ–α transformation as well as the γ–α′ transformation are compared. For the first time, the orientation relationships (OR) between ferrite, austenite, and martensite are directly compared within the same location of a sample. The results show that the Kurdjumov–Sachs (KS) and Nishiyama–Wassermann(NW) ORs only approximately describe the γ–α and γ–α′ transitions. The experiment reveals the role of KS/NW boundaries in the intercritical regime and microscopically observes the consequences of the γ–α′ transition. It was found that the formation of martensite causes highly deformed ferrite in its vicinity which most likely affects the mechanical properties of the dual-phase steel. It is expected that this type of experiment will help in better understanding microstructural mechanisms during heat treatments and eventually will contribute in the development of steels with tailored microstructures.