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 IA process has largest volume fraction of austenite, which is responsible for the longest elongation and largest energy absoprtion with 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 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.
Highlights
High-performance steels are desirable to develop lightweight structural components with high safety coefficient in various areas, such as automotive and aerospace
The average prior austenite grain size is about 7.7 ± 2.7 μm, which is much larger than that of the intercritical annealing (IA)-quenching and partitioning (Q&P) steel due to the grain growth at the higher annealing temperature
The austenite volume fraction of the IA, IA and Q&P (IA-Q&P), and Q&P steels obtained from the electron backscatter diffraction (EBSD) measurements is 47.2, 30.3, and 23.1%, respectively
Summary
High-performance steels are desirable to develop lightweight structural components with high safety coefficient in various areas, such as automotive and aerospace. The medium Mn steel is processed by intercritical annealing (IA) to achieve a dual-phase microstructure consisting of ferrite and retained austenite (γR) (Lee and De Cooman, 2014; Wang H. et al, 2018; Chandan et al, 2019). The IA temperature and annealing time are crucial to the quantity and stability of the retained austenite (Lee and De Cooman, 2013; Lee and Han, 2014; Han et al, 2015). The amount of retained austenite usually reaches a peak value and decreases with increasing IA temperature (Gibbs et al, 2011; Ding et al, 2020). It is reported that the Mn content in the retained austenite decreases continuously with the ascent of IA temperature, which leads to lower mechanical stability of retained austenite (Lee et al, 2011b).
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