1 Hebei Key Laboratory of Modern Metallurgy Technology, Hebei United University, Tangshan 063009
2 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016
Cite this article:
Liansheng CHEN, Jianyang ZHANG, Yaqiang TIAN, Jinying SONG, Yong XU, Shihong ZHANG. EFFECT OF Mn PRE-PARTITIONING ON C PARTITIONING AND RETAINED AUSTENITE OF Q&P STEELS. Acta Metall Sin, 2015, 51(5): 527-536.
The chemical compositions of C and Mn have a strong influence on the stability of the metastable retained austenite at room temperature. In the intercritical annealing process, Mn element improves the stability of the austenite by partitioning from ferrite to austenite and the enrichment of Mn in austenite can also impact on the diffusion of C element from martensite to retained austenite in partitioning process. Based on C partitioning, Mn partitioning can further improve the product of strength and elongation, and has no negative effect on weldability of the low carbon high strength steel. Thus, it can effectively solve the contradiction between mechanical property and weldability of low carbon high strength steel in traditional quenching-partitioning (Q&P) process. In this case, it is of great significance to study the Mn pre-partitioning mechanism and its influence on C partitioning and retained austenite of the low carbon high strength steel. Therefore, one low alloy C-Si-Mn steel was studied in the present work. The Mn pre-partitioning behavior and its effect on C partitioning and the stability of the retained austenite were investigated by means of intercritical heating-quenching (IQ) process, Q&P and intercritical heatingaustenitizing-quenching-partitioning (I&Q&P) process. The results showed that during the process of phase transformation in the intercritical reheating, C and Mn elements constantly diffused from ferrite to austenite. When this process ended, C and Mn elements enriched in austenite. While Mn element in microstructure at room temperature was still enrichment and C element enriched regularly between the martensite laths in the I&Q&P treated steel. With the increase of C partitioning time in both Q&P and I&Q&P process, the tensile strength of steel was decreased constantly, while the elongation showed an increasing fristly and then decreasing trend. The product of strength and elongation of the steel treated by I&Q&P process reached 23478 MPa·% with the C partitioning time of 90 s. The more austenite in martensite phase would be obtained after the first quenching with the Mn pre-partitioning. It was important to prompt more C diffusing into austenite during C partitioning process to stabilize more retained austenite at room temperature of the steel after the second quenching. With the same experimental conditions, the retained austenite of the combined effects of C and Mn partitioning during I&Q&P process would be increased 2.4% than the effect of C partitioning during Q&P process.
Fund: National Natural Science Foundation of China (Nos.51254004 and 51304186), Natural Science Foundation of Hebei Province (No.E2014209191) and Science Foundation of Department of Education of Hebei Province (No.YQ2013003)
Fig.1 Schematics of different heat treatments applied to experimental steels (Ac3—temperature of all ferrite transformed to austenite, Ac1—start temperature of pearlite transformed to austenite, Ms—initial temperature of martensite transformation, Mf—final temperature of martensite transformation)
Fig.2 Microstructure of hot-rolled experimental steel (F—ferrite, P—pearlite)
Fig.3 Microstructure (a) and EPMA analysis of C (b) and Mn (c) in experimental steel after IQ process (M—martensite)
Fig.4 Content changes of C and Mn along the scanning line in Fig.3a in experimental steel after IQ process
Fig.5 Microstructure (a) and EPMA analysis of C (b) and Mn (c) in experimental steel treated by I&Q&P process with C partitioning time of 90 s (RA—retained austenite, A—Mn-rich area, B—Mn-poor area)
Fig.6 SEM images of experimental steel after Q&P process with C partitioning times of 30 s (a), 90 s (b) and 180 s (c)
Fig.7 SEM images of experimental steel after I&Q&P process with C partitioning times of 30 s (a), 90 s (b) and 180 s (c)
Fig.8 TEM images and SAED patterns of experimental steels after Q&P (a) and I&Q&P (b) processes with C partitioning time of 90 s
Fig.9 XRD spectra of experimental steels after Q&P and I&Q&P processes with different C partitioning times
Process
t / s
mRA / %
Rm / MPa
A / %
Rm·A / MPa·%
Q&P
30
8.4
1360
14.6
19856
90
11.2
1326
16.7
22144
180
9.0
1305
15.2
19836
I&Q&P
30
11.0
1332
16.3
21712
90
13.6
1319
17.8
23478
180
10.4
1298
16.0
20768
Table 1 Mechanical properties and volume fraction of retained austenite in experimental steel treated by Q&P and I&Q&P processes
Fig.10 Mass fraction of carbon in retained austenite in experimental steels after Q&P and I&Q&P processes with different partitioning times
Fig.11 Relationship between volume fraction of retained austenite and mass fraction of Mn in austenite (L0—theoretical volume fraction of RA after Q&P, P1—volume fraction of RA after Q&P, P2—volume fraction of RA after I&Q&P)
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