Journal
MATERIALS CHARACTERIZATION
Volume 136, Issue -, Pages 20-28Publisher
ELSEVIER SCIENCE INC
DOI: 10.1016/j.matchar.2017.11.058
Keywords
Medium Mn steel; Austenite stability; Strength and toughness; TRIP effect; Work hardening
Categories
Funding
- National Natural Science Foundation of China [51604072]
- National High-tech R&D Program (863 Program) [2015AA03A501]
- Fundamental Research Funds for the Central Universities [N150704003]
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Medium-Mn steel was innovatively designed in terms of alloying elements for medium to heavy steel plates to obtain combination of high strength, good ductility, and excellent low temperature toughness. Martensite was the dominant microstructure in the directly quenched steel plate because of high hardenability obtained by the addition of 5.6 wt% Mn. Three intercritical annealing processes were adopted to ensure transformation-induced-plasticity (TRIP) effect through optimization of the volume fraction, morphology, and C and Mn-enriched reversed austenite. On annealing, the dislocation density of martensite plate was decreased because of recovery and recrystallization, and metastable austenite film nucleated at the interface. Both thermal and mechanical stability of austenite decreased with increase in annealing temperature. The high yield strength of 840 MPa, good elongation after fracture of 24.3%, and excellent toughness at - 60 degrees C of 130.3 J was obtained at inter-mediate annealing temperature of 650 degrees C, and the volume fraction of reversed austenite at room temperature and - 80 degrees C was 22% and 17%, respectively. The reversed austenite exhibited different stability with increased tensile strain, leading to improved ductility and delayed necking. Moreover, the crack initiation energy and crack propagation energy were increased via dynamic stress partitioning and relaxation together with the transformation of metastable austenite. Thus, significant degree of TRIP effect induced by moderate stability of austenite played an important role in governing mechanical properties.
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