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Acta Metall Sin  2019, Vol. 55 Issue (3): 332-340    DOI: 10.11900/0412.1961.2018.00270
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C and Mn Elements Characterization and Stability of Retained Austenite in Different Locations ofQuenching and Partitioning Bainite Steels
Yaqiang TIAN1,Geng TIAN1,Xiaoping ZHENG1,Liansheng CHEN1(),Yong XU1,2,Shihong ZHANG2
1. Key Laboratory of the Ministry of Education for Modern Metallurgy Technology, North China University of Science and Technology, Tangshan 063210, China
2. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

The volume fraction and stability of retained austenite play an important role in the performance of low carbon steels, while the C and Mn elements have a stabilizing effect on the thermal stability and mechanical stability of retained austenite. Therefore, the C and Mn elementals partitioning was promoted by intercritical annealing. As a result, the mechanical properties of the low carbon steels are improved. The microstructure of quenching and partitioning bainitic steels and retained austenite characteristics were studied by means of SEM, TEM and XRD. The partitioning and content of C and Mn elements in retained austenite at different locations were characterized by EPMA, EBSD and nanoindentation. The effect of C and Mn elements on the stability of retained austenite at different locations and phase change law of retained austenite were investigated by combining the tensile stress-strain curves under the treatment of intercritical annealing (partial austenitizing)-quenching and partitioning in the bainitic region process (IQ&PB). In the process of tensile deformation, the transformation induced plasticity (TRIP) effect occurs, the volume of retained austenite decreases, the transformation takes place preferentially in the ferrite grain boundary, and finally occurs between the bainite laths. C and Mn elements have a stabilizing effect on the retained austenite, which make retained austenite is not prone to phase change. The stress at the tensile fracture is concentrated, and the retained austenite is completely transformed into martensite. The volume fraction of retained austenite is 3.12% and 5.03% at 2 mm and 4 mm distances away from the fracture. Film-like retained austenite is more stable than blocky retained austenite, and the retained austenite of the <111>γ crystal orientation is unstable and easily transforms into martensite.

Key words:  quenching and partitioning bainitic steel      retained austenite in different locations      C and Mn elements distribution      stability of retained austenite      TRIP effect     
Received:  25 June 2018     
ZTFLH:  TG142.4  
Fund: National Natural Science Foundation of China(51574107);Natural Science Foundation of Hebei Province(E2017209048)
Corresponding Authors:  Liansheng CHEN     E-mail:  kyckfk@ncst.edu.cn

Cite this article: 

Yaqiang TIAN,Geng TIAN,Xiaoping ZHENG,Liansheng CHEN,Yong XU,Shihong ZHANG. C and Mn Elements Characterization and Stability of Retained Austenite in Different Locations ofQuenching and Partitioning Bainite Steels. Acta Metall Sin, 2019, 55(3): 332-340.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00270     OR     https://www.ams.org.cn/EN/Y2019/V55/I3/332

Fig.1  Schematic of intercritical annealing-quenching and partitioning in the bainitic region (IQ&PB) heat treatment process of sample (Ac3—temperature of all ferrite transformed to austenite, Ac1—start temperature of pearlite transformed to austenite, Bs—initial temperature of bainite transformation, Ms—initial temperature of martensite transformation)
Fig.2  Nanoindentation marker position at 2 mm and 4 mm locations away from the fracture
Fig.3  SEM images of sample after the treatment of IQ&PB process (F—ferrite, B—bainite, M/A—martensite/austenite)
Fig.4  Bright-field (a) and dark-field (b) TEM images of the retained austenite and content changes of C and Mn elements in the retained austenite (c) after the treatment of IQ&PB process (RA—retained austenite. Insets in Fig.4b shows the SAED patterns)
Fig.5  Microstructure (a) and EPMA images of C (b) and Mn (c) elements of sample after the treatment of IQ&PB process (A—area fraction, Aver.—average)
Fig.6  XRD spectrum of sample after the treatment of IQ&PB process
Fig.7  Engineering stress-strain curve (a) and variation in instantaneous work hardening exponents with true strain (b) of sample after the treatment of IQ&PB process
Fig.8  XRD spectra of sample in different locations away from the fracture after the treatment of IQ&PB process
Fig.9  EBSD images of the quenching and partitioning bainitic steels tensile sample in different locations away from fracture (Retained austenite is marked in red)(a) original sample (b) 4 mm away from fracture (c) 2 mm away from fracture (d) fracture
Fig.10  Microstructure (a) and EPMA images of C (b) and Mn (c) elements at a distance of 4 mm away from fracture
Fig.11  Orientation maps of austenite grains at a distance of 4 mm (a) and 2 mm (b) away from the fracture
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