Process Design and Microstructure Control of Medium Manganese Steel with Continuous Yield and High Strength Yield Ratio
ZHANG Guangying1, LI Yan2,3, HUANG Liying4, DING Wei1()
1 School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China 2 School of Rare Earth Industry, Inner Mongolia University of Science and Technology, Baotou 014010, China 3 Key Laboratory of Integrated Exploitation of Bayan-Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Baotou 014010, China 4 Department of Mechanical and Electrical Engineering, Hebei Vocational University of Technology and Engineering, Xingtai 054000, China
Cite this article:
ZHANG Guangying, LI Yan, HUANG Liying, DING Wei. Process Design and Microstructure Control of Medium Manganese Steel with Continuous Yield and High Strength Yield Ratio. Acta Metall Sin, 2024, 60(4): 443-452.
Researches have focused on the development of lightweight and high-performance steel materials to ensure automobile safety. Medium manganese steel is a potential candidate owing to its excellent mechanical properties and low production cost. However, the problem of plastic instability (Lüders strain, Portevin-Le Chatelier effect) is one of the main factors restricting the development of medium manganese steel. Therefore, resolving the plastic instability of medium manganese steel is a prerequisite for its development and hence to ensure the benefits of its mechanical qualities. Many studies have found that the stability of retained austenite is directly related to the plastic instability of medium manganese steel. In this work, cold rolled low-carbon medium manganese steel is selected, and multi-stable retained austenite is obtained by designing pretreatment and critical annealing process. The phase transformation of the retained austenite with different stability in each stage of the tensile process and its influence on the mechanical properties are studied. The results show that the microstructure containing pearlite + ferrite + martensite is obtained after pretreatment of medium manganese steel. After annealing, pearlite transformed into filmy retained austenite and ferrite phase; while martensite transformed into blocky retained austenite. Mn content in the filmy retained austenite is higher than that in the blocky retained austenite, making the filmy retained austenite more stable than the blocky one. The blocky retained austenite has poor stability, and phase transformation occurs at the initial stage of plastic deformation, eliminating Lüders strain. In contrast, the filmy retained austenite has high stability, and phase transformation occurs in the middle and late deformation, contributing toward its high strength and plasticity. The specimens containing double-stable retained austenite not only maintain the tensile strength (> 1000 MPa) and high fracture elongations (> 30%), but also have the characteristics of continuous yield and high strength yield ratio.
Fig.1 Schematic of heat treatment process (A1—start temperature of pearlite transformed to austenite, A3—temperature of all ferrite transformed to austenite, A—austenite, F—ferrite, P—pearlite, M—martensite, RA—retained austenite, RA+—Mn-rich RA, RA-—Mn-poor RA)
Fig.2 SEM images of A730-3 (a, b) and PA730-5 (c, d) samples before (a, c) and after (b, d) annealing, and schematics of austenite transformation of PA730-5 sample (CEME—cementite) (e)
Fig.3 RA grain size distributions of A730-3 (a) and PA730-5 (b) samples, and average grain sizes of each phase in the microstructure (c)
Fig.4 TEM images and SAED patterns (insets) (a, d), and corresponding EDS element mappings (b, e) and EDS point concentrations (c, f) of Mn element of blocky (a-c) and film (d-f) RA in PA730-5 sample after annealing (Red dots in Figs.4a and d show EDS scanning points)
Fig.5 Schematic of sample location (a) and corresponding SEM images (b-d) away from fracture successively in PA730-5 sample
Fig.6 XRD spectra of A730-3 and PA730-5 samples before (a) and after (b) tensile fracture
Fig.7 Engineering stress-strain curves of A730-3 and PA730-5 samples (PLC—Portevin-Le Chatelier)
Sample
YS
MPa
UTS
MPa
TE
%
UTS / YS
UTS × TE
GPa·%
A730-3
922
1041
43
1.12
44.76
PA730-5
652
1025
34
1.57
34.85
Table 1 Mechanical properties of A730-3 and PA730-5 samples after annealing
Fig.8 Work hardening index curves of A730-3 and PA730-5 samples
Fig.9 TEM image of PA730-5 sample after fracture (a) and schematic of motion of Lüders band (b) (VL—Lüders band moving rate, VC—tensile rate)
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