On the Variants of Thermal Process in Developing Strong and Ductile Medium Mn Steel

Abstract

In this contribution, we investigate the influence of three heat treatments, including intercritical annealing (IA), quenching and partitioning (Q&P), and combination of IA and Q&P (IA-Q&P), on microstructure and mechanical properties of the medium Mn steel. The steel treated by the IA process has the largest volume fraction of austenite, which is responsible for the longest elongation and largest energy absorption with the operation of both transformation-induced plasticity (TRIP) effect and twinning-induced plasticity (TWIP) effects. The medium Mn steel produced by IA-Q&P treatment has a mixture microstructure with retained austenite, ferrite, and lath martensite. The existence of martensite in the IA-Q&P steel makes the higher yield strength than that of the IA steel. The better uniform elongation of the IA-Q&P steel than that of the Q&P steel is ascribed to the larger volume fraction of retained austenite and higher mechanical stability of austenite. The Q&P steel has the highest yield stress due to its martensite matrix. It is expected that the tensile properties of medium Mn steel can be tuned by different thermal processes (IA, Q&P, and IA-Q&P) to facilitate its broad automotive applications.

Strong and ductile medium-Mn steel achieved via controlled martensite tempering

“…The chemical composition of medium Mn steel employed for the present investigation is Fe-10Mn-0.2C-2Al-0.1 V (in wt-%). The alloying of 10pct Mn content is to secure proper hardenability of steel so that sufficient amount of austenite is preserved after quenching down to room temperature [18][19][20]. The present medium Mn steel is prepared with a vacuum induction furnace and cast into ingots.…”

“…The martensite in Q&P steels is generated during the quenching process down to a temperature below M s temperature and is tempered at an elevated temperature to allow carbon partitioning into austenite [7,9,17]. This Q&P process can be further simplified by selecting the quenching temperature as the roomtemperature, leading to the development of roomtemperature quenching and partitioning (RT Q&P) steel [18][19][20][21]. The RT Q&P process can effectively separate the quenching and partitioning processes, allowing the partitioning process to be done in appropriate time without instantaneous after the quenching process [18].…”