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Acta Metall Sin  2026, Vol. 62 Issue (3): 477-488    DOI: 10.11900/0412.1961.2025.00219
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Effect of Mn Pre-Partitioning on Bainite Transformation During Medium-Temperature Continuous Cooling of Medium Mn Steel
ZHENG Qinyuan1,2, LIU Peng1, LU Yi1,2, ZHU Hailong1,2, ZHENG Chengwu1,2(), LUAN Yikun1, LI Dianzhong1,2()
1.Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Cite this article: 

ZHENG Qinyuan, LIU Peng, LU Yi, ZHU Hailong, ZHENG Chengwu, LUAN Yikun, LI Dianzhong. Effect of Mn Pre-Partitioning on Bainite Transformation During Medium-Temperature Continuous Cooling of Medium Mn Steel. Acta Metall Sin, 2026, 62(3): 477-488.

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Abstract  

In the development of third-generation advanced high-strength steels, achieving a balance between strength and ductility while minimizing alloying and production costs is critical. Among the promising candidates, medium Mn steels (MMnS) have the desired design flexibility for achieving a certain amount of metastable austenite, thereby exhibiting an enhanced transformation-induced plasticity (TRIP) effect. Owing to the weakened alloying effect in low-Mn content MMnS, more efforts should be devoted to enhancing the stability of austenite during intercritical annealing. This study explores the possibility of developing high-strength, high-ductile MMnS via continuous cooling from medium temperatures. A 0.2C-3Mn-1.5Si (mass fraction, %) MMnS was selected to analyze the influence of Mn pre-partitioning on bainite transformation and the volume fraction of retained austenite in low-Mn content MMnS using various characterization methods, including SEM and EBSD, as well as mechanical property testing methods. The results indicate that the carbide-free bainite transformation occurring during the medium-temperature continuous cooling enables the acquisition of retained austenite in MMnS. Mn-rich austenite lamellae can be initially produced via Mn pre-partition during intercritical annealing. Subsequently, bainite transformation is restricted to occur within the undercooled austenite lamellae in the medium-temperature continuous cooling process, resulting in a refined multiphase microstructure comprising film-like retained austenite, bainitic ferrite, and intercritical ferrite. The volume fraction of retained austenite in low-Mn content MMnS substantially increases because of the enrichments of Mn and C from Mn pre-partition and bainite transformation, respectively, thereby enhancing the strength and ductility of MMnS.

Key words:  medium Mn steel      carbide-free bainite      Mn partition      retained austenite      TRIP effect     
Received:  02 August 2025     
ZTFLH:  TG142  
Fund: National Natural Science Foundation of China(52321001);National Natural Science Foundation of China(52071322)
Corresponding Authors:  ZHENG Chengwu, professor, Tel: (024)23971973, E-mail: cwzheng@imr.ac.cn;
LI Dianzhong, professor, Tel: (024)23971281, E-mail: dzli@imr.ac.cn

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2025.00219     OR     https://www.ams.org.cn/EN/Y2026/V62/I3/477

Fig.1  Schematic of the continuous cooling process from medium temperature in medium Mn steel (Ae3—the highest temperature at which ferrite and austenite phases can coexist in equilibrium, Ae1—the lowest temperature at which ferrite, cementite, and austenite phases can coexist in equilibrium, C˙—cooling rate, Ts—start cooling temperature. PA—prior austeniting annealing, PI—prior intercritical annealing, CB—continuous cooling with bainite transformation)
SampleProcessPre-annealing processCB
Ts / oCC˙ / (oC·s-1)
PA-CB400-0.05PAAnnealing at 850 oC for 10 min4000.05
PI-CB370-0.05PIAnnealing at 780 oC for 10 min3700.05
PI-CB430-0.054300.05
PI-CB400-0.054000.05
PI-CB400-0.14000.1
PI-CB400-0.24000.2
Table 1  Samples of the 0.2C-3Mn-1.5Si medium Mn steel (MMnS) heat-treated with various pre-annealing and continuous cooling processes
Fig.2  Dilatation curve (a), SEM image (b), EBSD-band contrast (BC) map overlapped with the phase map of austenite (c), and Mn distribution map (d) of the 0.2C-3Mn-1.5Si MMnS austenitizing pre-annealed at 850 oC followed by continuous cooling from a medium temperature of 400 oC (ΔL—change in sample length, αB—bainitic ferrite, αM—martensite, γ—retained austenite. Inset in Fig.2a shows a magnified view of the dilatation infection due to bainite transformation during continuous cooling. The red arrows in Fig.2b point to the flak-like retained austenite that forms during the bainite transformation)
Fig.3  SEM images (a1-c1) and EBSD-BC maps overlapped with the phase maps of austenite (a2-c2) of the austenitizing pre-annealed 0.2C-3Mn-1.5Si MMnS during the medium-temperature continuous cooling process when cooling to temperatures of 395 oC (a1, a2), 390 oC (b1, b2), and 370 oC (c1, c2)
Fig.4  Dilatation curve of the intercritically pre-annealed 0.2C-3Mn-1.5Si MMnS during medium-temperature continuous cooling (a); SEM images (b1, b2), EBSD-BC maps overlapped with the phase maps of austenite (c1, c2) and Mn distribution maps (d1, d2) after intercritically pre-annealing (b1-d1) and after medium-temperature continuous cooling (b2-d2) (Inset in Fig.4a shows a magnified view of the dilatation infection due to bainite transformation during continuous cooling)
Fig.5  SEM images (a1-c1) and EBSD-BC maps overlapped with the phase maps of austenite (a2-c2) of the intercritically pre-annealed 0.2C-3Mn-1.5Si MMnS during the medium-temperature continuous cooling process when cooling to temperatures of 395 oC (a1, a2), 390 oC (b1, b2), and 370 oC (c1, c2)
Fig.6  Engineering stress-strain curves (a) and strain-hardening rate (SHR) curves alongside true stress-strain curves (b) of the 0.2C-3Mn-1.5Si MMnS pre-annealed at different temperatures followed by continuous cooling processing from the medium temperature of 400 oC
Fig.7  Dilatation curves (a), XRD patterns (b), and retained austenite (RA) volume fractions (c) of the intercritically pre-annealed 0.2C-3Mn-1.5Si MMnS continuously cooled from different Ts at a cooling rate of 0.05 oC/s (The black arrow in Fig.7a marks the dilatation inflection resulted from the martensite transformation)
Fig.8  Engineering stress-strain curves (a) and mechanical properties (b) of the intercritically pre-annealed 0.2C-3Mn-1.5Si MMnS continu-ously cooled from different Ts at a cooling rate of 0.05 oC/s (YS—yield strength, UTS—ultimate tensile strength, TE—total elongation)
Fig.9  Dilatation curves (a), XRD patterns (b) and RA volume fractions (c) of the intercritically pre-annealed 0.2C-3Mn-1.5Si MMnS continuously cooled from 400 oC at different cooling rates (The black arrow in Fig.9a marks the dilatation inflection resulted from the martensite transfor-mation)
Fig.10  Schematics of carbide-free bainite transformations during continuous cooling processing from a medium temperature in MMnS
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