1 Engineering Research Institute, University of Science and Technology Beijing, Beijing 100083 2 Beijing Laboratory of Modern Traffic Metal Materials and Processing Technology, University of Science and Technology Beijing, Beijing 100083
Cite this article:
ZHAO Zhengzhi, TONG Tingting, ZHAO Aimin, HE Qing, DONG Rui, ZHAO Fuqing. MICROSTRUCTURE, MECHANICAL PROPERTIES AND WORK HARDENING BEHAVIOR OF 1300 MPa GRADE 0.14C-2.72Mn-1.3Si STEEL. Acta Metall Sin, 2014, 50(10): 1153-1162.
Multiphase microstructure which contains ferrite, lath martensite, tempered martensite and a specific proportion of retained austenite with chemical composition of Mn between low Mn and medium Mn (0.14C-2.72Mn-1.3Si, mass fraction, %) belong to C-Si-Mn series was produced using continuous annealing simulator. By means of dilatometric simulation, SEM, TEM, EBSD and XRD, microstructures of the steels in different heat treatments were characterized. The results illustrate that the tested steel sheet gained good comprehensive properties with yield strength of 672 MPa, tensile strength up to 1333 MPa, total elongation A50 of 13% after annealing at 800 ℃, which can be explained by the refined microstructure, appropriate proportion of phases and a specific proportion of retained austenite. This work has deeply analyzed the work hardening behavior, discussed the change of instantaneous work hardening rate n. The multi-stage work hardening behavior was studied by modified C-J analysis, and explored the influence of ( is the volume fraction of martensite, is the equivalent diameter of martensite) and fraction of ferrite on it. The results show that n increases with the rise of true strain and then decreases, but has a different feature in the decrease for the different tested steels; the multi-stage work hardening behavior studied by modified C-J analysis shows 2 or 3 stages because of the different martensite volume fraction. The strain scope of combined action of ferrite and martensite △e is affected by the volume fraction of ferrite: △e is small when the temperature is low, and then △e is large when temperature increases, while the △e maybe small when temperature continue to rise. Above all, the high instantaneous work hardening rate which is helpful for the improvement of strength, plasticity and toughness can be attributed to the proportion, morphology and distribution of ferrite and martensite, which is also the consequence of coordination and combination action of each factor.
Fund: Supported by National Natural Science Foundation of China (No.51271035) and Fundamental Research Funds for the Central Universities (No.FRF-TP-10-001A)
Fig.1 Schematic of annealing process applied to 0.14C-2.72Mn-1.3Si steel
Fig.2 SEM images of tested steel under annealing temperatures of 760 ℃ (a), 780 ℃ (b), 800 ℃ (c) and 820 ℃ (d)
T / ℃
fF /%
fM /%
fRA / %
Cγ / %
760
65.97
28.17
5.86
1.02
780
50.97
44.49
4.54
1.07
800
26.14
68.11
5.75
1.12
820
25.82
70.33
3.85
1.25
Table 1 Volume fraction of each constituent phase and mass fraction of carbon in retained austenite of tested steel under different annealing temperatures
Fig.3 TEM images of tested steel under annealing temperatures of 760 ℃ (a) and 800 ℃ (b)
Fig.4 Mechanical properties of tested steel under different annealing temperatures
Fig.5 Stress-strain curves of tested steel under different annealing temperatures
Fig.6 XRD spectra of tested steel under different annealing temperatures
Fig.7 Volume fraction of retained austenite and mass fraction of carbon in retained austenite in tested steel under different annealing temperatures
Fig.8 TEM images of retained austenite in tested steel after annealing at 800 ℃
Fig.9 Instantaneous work hardening rate n of tested steel under different annealing temperatures
Fig.10 Plots of the work hardening behavior of the tested steel obtained by using the modified C-J analysis (mI, mII and mIII indicate the stress exporents of stages I, II and III, respectively)
Fig.11 EBSD images of quality maps (a, c) and inverse pole figure (IPF) maps (b, d) for tested steel annealed at 760 ℃ (a, b) and 800 ℃ (c, d)
T / ℃
mI
mII
mIII
etr1 / % (mI →mII)
etr2 / % (mII→mIII)
760
2.9
2.0
6.7
0.9
1.8
780
2.6
3.0
8.0
0.8
1.6
800
-
2.8
8.0
-
1.3
820
-
3.3
8.7
-
1.3
Table 2 Stress exponents and strain at the transition points in deformation stages of tested steels by the modified C-J analysis
T / ℃
dF μm
dM μm
(%·μm-1)1/2
nmin
nmax
nmax-nmin
760
4.6
0.23
11.07
0.38
0.58
0.20
780
3.7
0.51
9.34
0.46
0.62
0.16
800
2.2
0.82
9.11
0.47
0.63
0.16
820
1.3
1.42
7.04
0.54
0.64
0.10
Table 3 Equivalent grain size of tested steel after annealing and the extreme values of n
Fig.12 Effects of ferrite volume fraction on strain scope of combined action of ferrite and martensite △e in tested steel (Dotted line indicates the tendency of △e when continue to lower the volume fraction of ferrite)
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