Research on Hot Stamping-Carbon Partition-Intercritical Annealing Process of Medium Manganese Steel

In order to improve the plasticity of hot stamping parts, this paper combines the heat treatment process with the plastic forming of sheet metal, and creatively proposes a new process of hot stamping-carbon partitioning-intercritical annealing. The mechanical properties and microstructure are characterized under the newly proposed process, the quenching-partition (QP) process, and the intercritical annealing (IA) process, respectively. The new process firstly undergoes incomplete austenitizing treatment at 610 °C, then carries out distribution treatment while stamping at 300 °C, and finally conducts annealing treatment in critical zone at 680 °C in two-phase zone. The results show that a multi-phase refined microstructure composed of lath martensite, retained austenite, fresh martensite, and carbides are obtained by the new process. Most of the retained austenite is shaped in the thin film due to martensitic shear, in which carbon and manganese elements diffuse from martensite to austenite by heat treatment, thus stabilizing the retained austenite. Retained austenite with a volume fraction of 33.7% is obtained in the new process. The retained austenite with higher content and better stability is completely consumed during the stretching process, which gives full play to discontinuous TRIP effects, thus delivering the elongation of 36.8% and the product of strength and elongation (PSE) reached as high as 43.6 GPa%.