Recently, medium Mn steels (MMnS) have been extensively investigated because of the excellent mechanical combination of strength and ductility achieved at the relatively low alloying cost. Intercritical annealing (IA) is a key process of MMnS to form intercritical austenite that can be retained fully or partially at room temperature, which can trigger transformation-induced plasticity and then improve work hardening during deformation. However, this process leads to a relatively low yield strength because the recovery, recrystallization, grain growth, coarsening, and dissolution of precipitates could occur during IA. In this study, the microstructural evolution and resultant mechanical properties of Cu-V dual alloyed 3Mn steel were examined during two manufacturing processes: hot rolling → warm rolling at 550-650°C → IA at 690°C for 10 min (termed as WR-IA) and hot rolling → aging at 550-650°C for 70 min → IA at 690°C for 10 min (termed as Aging-IA). That is,the two processes differentiate in either the warm rolling or the aging process used as the intermediate process. WR-IA specimens exhibit significantly higher ductility than Aging-IA ones, but they both have the same yield strength. The former is attributed to a large quantity of defects introduced during warm rolling, which promoted austenite reverse transformation during IA and led to a large fraction of retained austenite. The resultant tensile properties include yield strength of 1230-1320 MPa and ductility of 23%-29%, which is superior to those of either V- or Cu-alloyed MMnS published in references. In particular, higher yield strength was achieved because the dual alloying of Cu-V and the two-stage thermomechanical process, that is,warm rolling plus IA, are adopted. The first warm rolling promoted Cu-rich precipitates dispersed for strengthening, and the precipitation of VC during subsequent IA could compensate for the softening caused by IA. Consequently, a high yield strength was achieved. Meanwhile, 25%-30% fraction of austenite was retained, thereby providing transformation-induced plasticity during deformation, leading to high ductility.
Fund: National Natural Science Foundation of China(51831002);Fundamental Research Founds for the Central Universities(06600019;06500151)
Corresponding Authors:
LUO Haiwen, professor, Tel: (010)62332911, E-mail: luohaiwen@ustb.edu.cn; HU Bin, associate professor, Tel: (010)62332911, E-mail: hubin@ustb.edu.cn
Fig.1 Schematic illustration of two manufacturing processes employed in this study (HR, WR, and IA represent hot rolling, warm rolling, and intercritical annealing processes, respectively)
Fig.2 Tensile properties of the 3Mn steel after different manufacturing processes (a) engineering stress-strain curves of WR, WR-IA, and Aging-IA specimens (b) comparison on the tensile properties of the studied steel and Cu or V alloyed medium Mn steels reported[5,6,10,16-23] (TE—total elongation, YS—yield strength)
Specimen
YS / MPa
UTS / MPa
TE / %
WR550
1800 ± 9.2
1870 ± 7.8
2.28 ± 0.21
WR600
1650 ± 11.3
1745 ± 8.7
5.61 ± 0.32
WR650
1455 ± 4.3
1520 ± 3.7
5.43 ± 0.41
WR550-690
1325 ± 13.2
1390 ± 10.3
23.37 ± 0.72
WR600-690
1300 ± 12.7
1365 ± 9.8
24.05 ± 0.47
WR650-690
1230 ± 15.1
1290 ± 12.8
28.92 ± 0.94
HR550-690
1290 ± 10.4
1330 ± 5.6
9.51 ± 0.41
HR600-690
1330 ± 8.8
1370 ± 8.7
10.73 ± 0.38
HR650-690
1265 ± 7.8
1305 ± 5.5
15.75 ± 0.49
Table 1 Summary of mechanical properties of the 3Mn steel manufactured by different processes
Fig.3 Secondary electronic images of the microstructures of WR and WR-IA specimens (TM, α, θ, and γ represent tempered martensite, ferrite, cementite, and austenite, respectively) (a) WR550 (b) WR600 (c) WR650 (d) WR550-690 (e) WR600-690 (f) WR650-690
Fig.4 EBSD band contrast images overlapped with phase distribution on the microstructures of WR-IA and Aging-IA specimens (a) WR550-690 (b) WR600-690 (c) WR650-690 (d) HR550-690 (e) HR600-690 (f) HR650-690
Fig.5 TEM images (a, c, e, g, i, k) and corresponding EDS mappings of elements on the rectangle areas (b, d, f, h, j, l) of the microstructures of WR and WR-IA specimens (The areas marked by dashed circles in Figs.5b, d, f, h, j, and l represent Cu-rich or V-rich precipitates) (a, b) WR550 (c, d) WR600 (e, f) WR650 (g, h) WR550-690 (i, j) WR600-690 (k, l) WR650-690
Specimen
Precipitate
Diameter
nm
Number density
m-3
Volume fraction %
WR550
Cu-rich
8.8 ± 1.9
4.51 × 1023
0.34
WR600
Cu-rich
13.6 ± 2.8
5.85 × 1023
0.77
WR650
Cu-rich
15.7 ± 1.7
3.35 × 1023
0.68
VC
11.9 ± 3.5
2.15 × 1023
0.19
WR550-690
Cu-rich
19.2 ± 2.9
7.89 × 1022
0.29
VC
19.3 ± 6.5
6.64×1022
0.25
WR600-690
Cu-rich
27.1 ± 3.5
1.27 × 1023
0.69
VC
21.1 ± 6.1
5.89 × 1022
0.29
WR650-690
Cu-rich
34.5 ± 3.8
1.44 × 1022
0.31
VC
20.2 ± 3.6
8.57 × 1022
0.37
Table 2 Size, number density, and volume fraction of Cu-rich and VC nanosized precipitates in WR and WR-IA specimens
Fig.6 Calculated dislocation and precipitation streng-thening increments for the studied steel manu-factured by the different processes (a) calculated dislocation strengthening incre-ments by Eq.(3) varied with warm rolling/aging temperatures (b) calculated precipitation strengthening incre-ments of VC and Cu-rich precipitates in WR-IA specimens
Specimen
Before deformation
After deformation
Transformed percentage
WR550
1.3 ± 0.12
None
None
WR600
2.1 ± 0.09
WR650
8.3 ± 0.41
WR550-690
31.4 ± 0.71
5.2 ± 0.46
83.4
WR600-690
22.3 ± 0.56
7.1 ± 0.28
68.1
WR650-690
28.3 ± 0.89
6.4 ± 0.25
77.3
HR550-690
21.1 ± 0.31
7.4 ± 0.36
64.9
HR600-690
15.7 ± 0.43
7.5 ± 0.43
52.2
HR650-690
18.1 ± 0.49
6.8 ± 0.49
62.4
Table 3 Austenite volume fractions in all the specimens before/after the tensile deformations (%)
Fig.7 Size distributions of cementite particles in WR specimens (a) and mechanical stability and C contents (mass fraction) of austenite grains in WR-IA specimens (b) (k—index of austenitic mechanical stability, RA—retained austenite)
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