Relationship Between Retained Austenite Stability and Cryogenic Impact Toughness in 0.12C-3.0Mn Low Carbon Medium Manganese Steel
Long HUANG,Xiangtao DENG(),Jia LIU,Zhaodong WANG
State Key Laboratory of Rolling Technology and Automation, Northeastern University, Shenyang 110819, China
Cite this article:
Long HUANG,Xiangtao DENG,Jia LIU,Zhaodong WANG. Relationship Between Retained Austenite Stability and Cryogenic Impact Toughness in 0.12C-3.0Mn Low Carbon Medium Manganese Steel. Acta Metall Sin, 2017, 53(3): 316-324.
Low carbon and low alloy steels require good combination of strength and ductility to ensure safety and stability of structures, and the low temperature toughness has become more significant to low carbon low alloyed high performance steel recently. Retained austenite plays a great role in a multiphase system to improve the toughness of steel as a result of the deformation induced transformation of retained austenite when the steel deformed. In this work, the characterization of multiphase microstructure including retained austenite, tempered martensite and intercritical ferrite which obtained by a three-step intercritical heat treatment in a low carbon medium manganese steel were studied, and the low-temperature impact toughness evolution from -40~-196 ℃ during the process were analyzed. The results showed that C and Mn distributed unevenly after intercritical quenching and were benefit to martensite inverse transformation to austenite, and the enriched C and Mn elements can improve the stability of reverted austenite during the tempering process. The impact energy of the steel is 200 J at -80 ℃ during the processes at intercritical quenching temperature 720 ℃ and tempering temperature 640 ℃, and the energy of impact crack formation and propagation at different temperature were also analyzed.
Fund: Supported by National Natural Science Foundation of China (Nos.51234002, 51504064 and 51474064) and National Basic Research Program of China (No.2016YFB0300601)
Fig.1 Schematic of three-step intercritical heat treatment of the 0.12C-3.0Mn steel
Fig.2 Determination of critical points of 0.12C-3.0Mn steel after hot rolled and intercritical quenched by dilatometric method
Fig.3 SEM images for 0.12C-3.0Mn steel after different stage heat treatments (a) austenitizing quenching (b) intercritical quenching (c~f) critical quenching at 620 ℃, 640 ℃, 660 ℃ and 680 ℃, respectively
Fig.4 EBSD images of retained austenite in samples of CQ620 (a), CQ640 (b), CQ660 (c), CQ680 (d) and CQ680 grain boundary (e)
Fig.5 XRD spectra of retained austenite in samples
Fig.6 Bright-field (a) and dark-field (b) TEM images of retained austenite in sample CQ680
Fig.7 Images of distributions of C (a) and Mn (b) elements in sample CQ680
Sample
σs / MPa
σb / MPa
A / %
Ag / %
CQ620
505
640
30.4
16
CQ640
480
625
34.8
22
CQ660
455
625
36.0
24
CQ680
440
710
40.4
24
Table 1 Mechanical properties of samples after heat treatment at different temperatures
Sample
Volume fraction
Impact energy / J
of RA / %
-40 ℃
-80 ℃
-100 ℃
-150 ℃
-196 ℃
CQ620
7
257
181
123
42
3
CQ640
10
278
210
120
47
4
CQ660
13
262
100
88
36
8
CQ680
15
113
76
63
11
8
Table 2 Volume fraction of retained austenite (RA) and impact energy of CQ620, CQ640, CQ660 and CQ680
Fig.8 Fracture morphologies of impact samples of CQ640 (a, d), CQ660 (b, e), CQ680 (c, f) at -40 ℃ (a~c) and -80 ℃ (d~f)
Fig.9 Impact energy curves of samples at -40~-196 ℃ (a) and load and absorbed energy vs displacement curves (b~d) of samples at -40 ℃ (b), -80 ℃ (c) and CQ660 at -40 ℃ and -80 ℃ (d) (Points 4~7 are ending points of impact curves)
Fig.10 EBSD images of impact sample CQ660(a) 10 mm distance from the impact fracture at -40 ℃(b) near the impact fracture at -40 ℃(c) impact fracture at -80 ℃
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