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Acta Metall Sin  2018, Vol. 54 Issue (6): 859-867    DOI: 10.11900/0412.1961.2017.00315
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Effects of Intercritical Annealing Temperature on the Tensile Behavior of Cold Rolled 7Mn Steel and the Constitutive Modeling
Feng YANG1, Haiwen LUO2, Han DONG3()
1 Central Iron and Steel Research Institute, Beijing 100081, China
2 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
3 School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China
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Abstract  

Medium Mn steel is composed of sub-micron grained ferrite and austenite, the unstable austenite may transform to martensite during plastic straining. Although the mechanical properties of medium Mn steel could be easily tested by tensile test, it is quite difficult to directly measure the influences of different constituent phases on the tensile and work hardening behavior. Thus, at the present work, EBSD, TEM, XRD and a constitutive model based on dislocation density have been used to study the effects of intercritical annealing (IA) temperature on the tensile properties and work hardening behavior of a newly designed medium Mn steel, Fe-7%Mn-0.3%C-2%Al (mass fraction). Experimental results showed that with the increase of IA temperature, the mechanic stability of reverted austenite decreased gradually and the kinetics of strain induced martensite rose rapidly. The stability of the reverted austenite was moderate when intercritically annealed at 700 ℃, this led to the best plasticity and the optimal mechanical properties. Simulated results exhibited that the mechanic stability of austenite has a decisive influence on the tensile behavior of the material. The austenite stability will be too high if the IA temperature is lower, and this will lead to the lower work hardening rate and uniform elongation; when the IA temperature is moderate, the stability of austenite will be optimum, consequently strain-induced martensite would be progressively produced during straining and result in the higher work hardening rate and prolonged uniform elongation; the stability of austenite will be too lower if the IA temperature is higher, thus larger volume fraction of strain-induced martensite would be formed in a short period, and this would result in the higher tensile strength but the inferior uniform elongation.

Key words:  medium Mn steel      austenite stability      TRIP effect     
Received:  25 July 2017     
ZTFLH:  TG335.5  
Fund: Supported by National Natural Science Foundation of China (No.U1460203)

Cite this article: 

Feng YANG, Haiwen LUO, Han DONG. Effects of Intercritical Annealing Temperature on the Tensile Behavior of Cold Rolled 7Mn Steel and the Constitutive Modeling. Acta Metall Sin, 2018, 54(6): 859-867.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00315     OR     https://www.ams.org.cn/EN/Y2018/V54/I6/859

Fig.1  EBSD phase maps of S680 (a), S700 (b) and S720 (c) (The white phase is ferrite and the gray phase is austenite, the black lines are high-angle grain boundaries with misorientation angles of over 15°, the gray lines are low-angle grain boundaries with misorientation angles of 2°~15°)
Fig.2  TEM images of S680 (a), S700 (b) and S720 (c) (γ denotes austenite grains and the rest are ferrite grains)
Sample Size / μm α β n
Ferrite Austenite
S680 0.91 0.42 4.0 5.5 7.0
S700 0.99 0.60 4.0 6.0 7.0
S720 0.73 0.62 4.7 7.0 3.5
Table 1  The average grain sizes of ferrite and austenite and parameters for calculating martensite volume fraction (VM)
Fig.3  VM of S680~S720 after deformed to various strains[16] and the corresponding fitted curves
Fig.4  True stress-true strain curves of cold-rolled 7Mn steel after annealed at different temperatures
Fig.5  TEM images of deformed microstructures in S700 after the strain of 0.095 (a) and 0.35 (b) and the dark-field image of austenite in Fig.5b (c) (Insets show the SAED patterns of the circles in Figs.5a and b. SF—stacking fault, γ—austenite, M—strain induced martensite)
Phase G GPa b
nm
α M K
MPaμm1/2
ρ0 / m-2 k1 k2 dc
μm
S680 S700 S720 S680 S700 S720 S680 S700 S720
Ferrite 81.6 0.25 0.38 2.95 120 9×1013 3×1013 1×1013 0.004+
0.03VM
0.004+
0.03VM
0.0035+
0.03VM
1.3 1.3 1.5 1.6
Austenite 72.0 0.25 0.35 2.95 420 9×1013 3×1013 1×1013 0.045+
0.01VM
0.05+
0.01VM
0.05+
0.02VM
0.8 0.6 0.6 1.6
Martensite 81.6 0.25 0.38 2.95 - 1×1015 1×1015 1×1015 0.04 0.04 0.05 1.0 1.0 1.0 0.3
Table 2  Parameters used in the constitutive model calculations for cold rolled 7Mn steel at room temperature
Fig.6  Measured[16] and calculated dislocation densities (ρ) of austenite
Fig.7  Measured and calculated true stress-true strain curves (a) and corresponding curves of work hardening rate (WHR) (b)
Fig.8  The calculated true stress-true strain curves (a, c, e) and curves of work hardening rate of the composite and each constituent phase (b, d, f) in S680 (a, b), S700 (c, d) and S720 (e, f)
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