The Principle and Mechanism of Enhancement of Both Strength and Ductility of Martensitic Steels by Carbon
Yonghua RONG,Nailu CHEN()
School of Materials Science and Enigineering, Shanghai Jiao Tong University, Shanghai 200240, China
Cite this article:
Yonghua RONG,Nailu CHEN. The Principle and Mechanism of Enhancement of Both Strength and Ductility of Martensitic Steels by Carbon. Acta Metall Sin, 2017, 53(1): 1-9.
Since quenching-partitioning-tempering (Q-P-T) process was proposed in 2007, our research group have realized the enhancement of both strength and ductility of Q-P-T martensitic steels by increasing the carbon from low content to medium content range. The recent work devoted every effort to extending carbon content to high carbon range. Based on failure of our many trials, a design idea of anti-transformation induced plasticity (anti-TRIP) effect was proposed and the composition and process of high carbon low alloying martensitic steel were designed according to the idea of anti-TRIP effect so that the strength and ductility of high carbon Q-P-T martensitic steel are higher than those of medium carbon Q-P-T martensitic steel, which fulfills the desire of investigators for a century. This paper will mainly expound the background of anti-TRIP effect, the design of composition and process of high carbon Q-P-T martensitc steel as well as its microstructure, the mechanism of high strength and ductility for high carbon Q-P-T martensitic steel, and finally analyze the principle that Q-P-T process makes the enhancement of both strength and ductility by increase of the carbon content.
Fig.1 Comparison of mechanical properties of high carbon quenching-partitioning-tempering (Q-P-T) martensitic steel with other advanced high strength steels (AHSSs)[7] (TRIP—transformation induced plasticity, Q&P—quenching and partitioning, DP—dual phase, M—martensitic steel)
Fig.2 Engineering stress-strain curves of high carbon Q-P-T and Q&T martensitic steels (LN—liquid nitrogen)
Fig.3 Product of strength and elongation and volume fraction of retained austenite increase with rising carbon content in Q-P-T martensitic steels
Fig.4 SEM images of Fe-0.63C-1.52Mn-1.49Si-0.62Cr-0.036Nb high carbon low alloy martensite steel after different heat treatment processes[14](a) as-hot rolled sample(b) as-normalization sample(c) sample A (hot rolled+Q-P-T)(d) sample B (hot rolled+normalization+Q-P-T)
Fig.5 TEM analyses of undeformed Q-P-T sample[14]
(a) bright-field TEM image of sample A(b) dark-field TEM image of retained austenite and inserted SAED pattern of the retained austenite and martensite in sample A(c) bright-field TEM image of sample B(d) dark-field TEM image of retained austenite and inserted SAED pattern of the retained austenite and martensite in sample B(e) dark-field TEM image of sample A and inserted SAED pattern of niobium carbides(f) dark-field TEM image of sample B and inserted SAED pattern of niobium carbides
Fig.6 EBSD analyses of retained austenitein in sample A (a) and sample B (b), and grain size distribution of retained austenite (c)[14]
Fig.7 Variation of average dislocation density with strain[11](a) in martensite in Q-P-T sample or Q&T one (b) in retained austenite
Process
Strain
(εM2)1/2
ρM1
ρM2
ρ?M
(εA2)1/2
ρA1
ρA2
ρ?A
VRA
%
10-3
1014 m-2
1014 m-2
1014 m-2
10-3
1014 m-2
1014 m-2
1014 m-2
%
Q-P-T
0
2.09±0.06
5.94±0.36
4.96±0.30
5.45±0.33
1.57±0.06
13.06±0.88
7.96±0.54
10.51±0.71
28.1
3
1.95±0.04
5.76±0.16
4.80±0.20
5.28±0.18
2.03±0.11
18.51±1.21
0.65±0.73
14.58±0.97
24.5
5
1.85±0.03
4.77±0.12
3.97±0.10
4.37±0.11
2.57±0.11
25.44±1.26
14.16±0.76
19.30±1.01
20.4
8
2.11±0.04
6.34±0.16
5.28±0.14
5.81±0.15
3.24±0.14
38.10±1.28
22.70±0.76
30.40±1.02
15.9
14
2.26±0.05
6.86±0.24
5.72±0.20
6.29±0.22
3.78±0.20
53.38±2.62
31.26±1.56
42.37±2.09
12.8
19
2.46±0.04
8.22±0.17
6.84±0.15
7.53±0.16
10.9
24
2.66±0.04
10.04±0.17
8.36±0.15
9.20±0.16
10.9
28.9
2.92±0.04
10.82±0.20
9.86±0.16
10.84±0.18
8.0
Q&T
0
2.48±0.03
7.94±0.14
7.44±0.12
7.44±0.12
4.3
(LN)
3
2.61±0.06
9.56±0.28
8.12±0.24
8.84±0.26
5
2.87±0.06
11.40±0.33
9.66±0.27
10.53±0.30
8.7
3.25±0.03
14.20±0.15
12.00±0.13
13.10±0.14
Table 1 Microstructure parameters of martensite and retained austenite in Q-P-T and Q&T tensile samples at different strain stages[11]
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