School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China
Cite this article:
Chengwei SHAO, Weijun HUI, Yongjian ZHANG, Xiaoli ZHAO, Yuqing WENG. Microstructure and Mechanical Properties of a Novel Cold Rolled Medium-Mn Steel with Superior Strength and Ductility. Acta Metall Sin, 2019, 55(2): 191-201.
Recently, energy conservation, environmental protection and security are the main factors considered by the automotive manufacturers. Medium-Mn steel with excellent combination of specific strength and ductility have been regarded as the potential candidates for automotive applications. The excellent combination of specific strength and ductility depends on the microstructure under different heat treatment processes of the steels. Therefore, the relationship of the combination of specific strength and ductility and microstructure should be studied in detail. A new alloy system of aluminum-containing medium-Mn steel was developed in this study. The addition of aluminum stabilizes α-ferrite, and facilitates the presence of δ-ferrite during solidification. The addition of Mn and C compensates the effect of aluminum on phase stability and ensures austenite formation. In this investigation, the effects of intercritical annealing temperature on the microstructure and tensile properties of a newly designed cold-rolled aluminum-containing medium-Mn steel (0.2C-5Mn-0.6Si-3Al, mass fraction, %) were investigated by SEM, XRD and uniaxial tensile tests. The tensile results show that an excellent combination of ultimate tensile strength (σb) of 1062 MPa, total elongation (δ) of 58.2% and σb×δ of 61.8 GPa% could be obtained after annealing at 730 ℃. The inverted austenite of the cold-rolled steel coarsenes and gradually changes its morphology from mainly lamellar to mainly equiaxed with increasing intercritical annealing temperature, and a duplex microstructure consisting of multi-scale retained austenite could be obtained at 730 ℃, which possesses suitable mechanical stability and thus presents prolonged transformation-induced plasticity (TRIP) effect during tensile deformation. This kind of sustainable TRIP effect and the cooperative deformation of ferrite are responsible for the superior mechanical properties. The investigation of tensile fracture behavior shows that the nucleation and growth of voids occurred mainly at the interfaces between soft ferrite and hard martensite induced by deformation.
Fig.1 SEM images of cold rolled medium-Mn steel samples as-cold rolled (a) and 700T (b), 730T (c), 750T (d), 770T (e), 800T (f) and 850T (g), showing multiphase microstructure consists of ferrite (F), retained austenite (RA), δ-ferrite (δ-F) and/or martensite (M) (Arrows in Fig.1f show a small amount of martensites)
Fig.2 Distributions and variations of RA grain size for cold-rolled medium-Mn steel samples 700T (a), 730T (b) , 770T (c) intercritically annealed at different temperatures and then low-temperature tempered, and the variation of RA grain size (d)
Fig.3 XRD spectra of cold-rolled medium-Mn steel samples before (a) and after (b) tensile test, measured RA fractions (c) and amount of transformed RA and transformation ratio of RA (d)
Sample
σs / MPa
σb / MPa
δ / %
σb×δ / GPa%
700T
970
1054
25.7
27.1
730T
920
1062
58.2
61.8
750T
865
1140
49.1
56.0
770T
786
1214
40.6
50.9
800T
518
1293
18.3
23.7
850T
746
1379
12.4
17.1
Table 1 Tensile properties of cold-rolled medium-Mn steel samples intercritically annealed at different temperatures and then low-temperature tempered
Fig.4 Dependence of the yiled strength on the grain size (d) of RA of the cold-rolled medium-Mn steel samples intercritically annealed among 700~800 ℃
Fig.5 Engineering stress-engineering strain curves of cold-rolled medium-Mn steel samples 700T, 730T and 770T (a) and their work hardening rate (dσ/dε) curves (b~d)
Fig.6 Plots of the k parameter of samples intercritically annealed at different temperatures and then low temperature tempered
Sample
Mass fraction of Mn / %
Mass fraction of C / %
In RA
In F
In δ-F
In RA
700T
6.34±0.46
5.14±0.53
4.31±0.28
0.772
730T
6.30±0.58
4.89±0.43
4.27±0.35
0.765
750T
6.27±0.50
4.84±0.40
4.27±0.15
0.727
770T
6.25±0.60
4.88±0.38
4.23±0.23
0.701
800T
6.26±0.41
4.68±0.42
4.17±0.15
0.637
850T
6.18±0.29
4.54±0.33
4.11±0.11
0.551
Table 2 EDS measured and XRD calculated concentrations of Mn and C
Fig.7 Longitudinal section SEM images of fractured tensile samples 700T (a), 730T (b), 770T (c) and 850T (d) (The thickness in uniformly strained part of sample is indicated in the case)
Fig.8 SEM images showing the microstructures near the fracture of the cold-rolled medium-Mn steel after tensile deformation for samples 700T (a), 730T (b), 770T (c) and 850T (d) (Circles on the micrographs indicate positions of voids in the F+RA (α’) constituent)
Fig.9 SEM images of the fracture surface of cold-rolled medium-Mn steel samples 700T (a) and 850T (b) after uniaxial tension test
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