Yielding behavior of triplex medium Mn steel alternated with cooling strategies altering martensite/ferrite interfacial feature

doi:0.1016/j.jmst.2022.04.003

Abstract

Abstract:

In this paper, we report the influence of cooling processes on the yielding behavior of a medium Mn steel (MMS) with triplex microstructure, i.e. austenite (γ), ferrite (α) and as-quenched martensite (α'). After the intercritical annealing (IA) at both 725 ℃ and 750 ℃, the steel was subjected to the two cooling processes, i.e. air cooling (AC) and water quenching (WQ). It exhibits the discontinuous yielding after the AC following the IA at 750 ℃ while the continuous yielding after the WQ. Compared with WQ process, both the dilatometry and the microstructural examinations show that the AC process leads to lower Ms temperature, larger retained austenite (RA) fraction and less martensite, the latter is always companied with geometry necessary dislocations (GNDs) generated near the α/α' interfaces. Considering the complexity of nanosized tri-phases in this steel, the presence of martensite with key features in the resultant specimens was systematically examined by atom probe tomography (APT) on the samples prepared by the specific target lift-out method. The APT results directly revealed the C/Mn co-segregation at the α'/α interfaces in the AC samples but not in WQ samples. The numerical simulation results further suggest that the segregation of C and Mn at the α'/α interfaces may be due to different mechanisms. We conclude that the yielding of triplex MMS is determined by both the quantity of GNDs generated near the α/α' interfaces, which increases with martensite fraction, and the extent of their immobilization resulting from the interfacial segregation of solute atoms when the presence of martensite is sufficient. WQ tends to suppress the discontinuous yielding of MMS since the rapid cooling may promote more martensite formed with the increasing quantity of GNDs and prevent the interfacial segregation of both C and Mn.

Key words: Triplex, medium, Mn, steel, Phase, boundary, Segregation, Discontinuous, yielding

Bin Hu, Xiao Shen, Qinyi Guo, Qinghua Wen, Xin Tu, Cancan Ding, Fanglin Ding, Wenwen Song, Haiwen Luo. Yielding behavior of triplex medium Mn steel alternated with cooling strategies altering martensite/ferrite interfacial feature[J]. J. Mater. Sci. Technol., 2022, 126: 60-70.

Figures/Tables 11

Table 1. Chemical composition of investigated MMS.

Empty Cell	Fe	C	Mn	Al	N
wt.%	Bal.	0.11	4.89	1.96	0.0018
at.%	Bal.	0.50	4.85	3.96	0.007

Fig. 1. Sketch of the thermo-mechanical route employed to produce the air cooled (AC) and water quenched (WQ) specimens.

Table 2. Chemical compositions (in wt.%, converted from at.% data measured by 3D-APT) of different phases (α, α' and γ represent ferrite, martensite and austenite) in 725AC, 725WQ, 750AC and 750WQ, the corresponding Ms temperatures are calculated using Eq. (1).

Specimens	Grains	Ms ( °C)	Alloy elements (wt.%)
Specimens	Grains	Ms ( °C)	Mn	Al	C	Fe
725AC	γ₁	−172.9	13.4	1.2	0.66	84.1
	α'₁	52.3	9.64	1.56	0.55	87.53
	γ₂	−306.6	15.52	1.37	0.81	81.53
	α'₂	109.7	9.41	1.74	0.38	88.06
	α₁	/	3.20	2.32	0.04	94.42
725WQ	α₁	/	2.87	2.48	0.01	94.11
725WQ	α'₁	134.1	9.38	1.70	0.28	87.66
750AC	α₁	/	3.49	1.00	0.009	95.39
750AC	α'₁	163.5	7.37	0.54	0.318	91.59
750WQ	α₁	/	2.84	2.70	0.01	94.34
	α₂	/	3.37	0.89	0.001	95.64
	α'₁	192.1	7.27	0.41	0.24	90.95

Fig. 2. (a, b) Engineering stress-strain curves, in which, (b) is the magnification view of the rectangle area in (a); (c) the HEXRD patterns and (d) volume percentage of retained austenite.

Fig. 3. EBSD band contrast map overlapped by phase distribution images and Kernel Average Misorientation (KAM) maps of BCC and FCC phases showing the microstructure of 725AC (a, a1, a2), 725WQ (b, b1, b2), 750AC (c, c1, c2), 750WQ (d, d1, d2). The grains marked by green in band contrast images represent austenite. The local misorientation of KAM maps is expressed in the rainbow-scale colors.

Fig. 4. (a) TEM bright field image showing the microstructure of 750AC; (b) the selected area electron diffraction (SAED) pattern measured from the white circle in (a).

Fig. 5. Atom probe tomography measurement of 725AC specimen as an example of nanostructure characterization and phase identification: (a) triplex nano-grained phases (α'+α+γ) decorated by different Mn isoconcentration (at.%) surface; (b) 2D contour map of Mn density distribution.

Fig. 6. (a, d) Three-dimensional atom probe tomography maps of manganese in the 725AC (a) and 725WQ (d) specimens; (b, c, e) C/Mn/Al concentration profiles relative to the position of the phase boundaries taken from the selected region of interest (ROI) marked in (a) and (d).

Fig. 7. (a, c) Three-dimensional atom probe tomography maps of manganese in 750AC (a) and 750WQ (c) specimens; (b, d, e) C/Mn/Al concentration profiles relative to the position of the phase and grain boundaries taken from the selected region of interest (ROI) marked in (a) and (c).

Fig. 8. (a) Dilatometric curves of studied steel cooling to ambient temperature by different cooling rates (5 ℃/s, 10 ℃/s, 20 ℃/s, 50 ℃/s, 100 ℃/s) after they are IA at 750 for 10 min; (b, c) EBSD band contrast overlapped by phase distribution images showing the microstructure of dilatometric specimens that were cooling to ambient temperature at the cooling rates of 5 ℃/s (b) and 100 ℃/s (c). The grains marked by green in (b) and (c) represent austenite.

Fig. 9. C (a, c) and Mn (b, d) concentration gradients developed between the ferrite and austenite phases during IA at 725 ℃ (a, b) and 750 ℃ (c, d) for 10 min, and then cooling to 700 ℃, 650 ℃, 550 ℃, 500 ℃ and 450 ℃ at a cooling rate of 20 ℃/s.

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