5th International High Manganese Steel Conference - HMNS 2022

Quenching and partitioning (Q&P) process has shown a great potential to achieve enhanced tensile properties in advanced high strength steel (AHSS). It is primarily attributed to the retained austenite in martensitic matrix that exhibits TRIP effect. In order to obtain austenite in the final microstructure, the cold-rolled steel is austenitized and then quenched to the temperature between Ms and Mf, followed by a partitioning process which allows the diffusion of carbon supersaturated in martensite into adjacent austenite. Consequently, the carbon enriched austenite can be retained upon final cooling to room temperature. According to the earlier works, the first quenching temperature after the austenitization has a significant influence on the mechanical properties because it determines the fraction of constituent phases. However, it is often difficult to control the first quenching temperature precisely in the manufacturing process, leading to the deviation in the mechanical properties. To overcome this difficulties, a room temperature Q&P process has been paid attention, in which the chemical composition of the steel is adjusted that the interval between Ms and Mf temperature includes the room temperature. However, high Mn content that is necessary to control Ms and Mf temperature possibly causes degradation of galvanizability and weldability. In the present study, we investigated the room temperature Q&P process starting from the chemically heterogeneous microstructure. Without increasing Mn content, highly heterogeneous Mn distribution in the initial microstructure successfully produced the Q&P microstructure consisting of martensite and retained austenite even subjected the room temperature Q&P process. Detailed microstructure evolution and corresponding mechanical properties will be discussed with respect to the influence of chemically heterogeneous initial microstructure.