J. Mater. Sci. Technol. ›› 2020, Vol. 49: 70-80.DOI: 10.1016/j.jmst.2020.01.051

• Research Article • Previous Articles     Next Articles

Kinetic transitions and Mn partitioning during austenite growth from a mixture of partitioned cementite and ferrite: Role of heating rate

Geng Liua, Zongbiao Daia, Zhigang Yanga, Chi Zhanga, Jun Lib, Hao Chena,*()   

  1. a Key Laboratory for Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China
    b Research Institute of Baoshan Iron and Steel Co., Ltd, Shanghai, 201900, China
  • Received:2019-11-26 Revised:2020-01-08 Accepted:2020-01-11 Published:2020-07-15 Online:2020-07-17
  • Contact: Hao Chen

Abstract:

Austenite formation from a ferrite-cementite mixture is a crucial step during the processing of advanced high strength steels (AHSS). The ferrite-cementite mixture is usually inhomogeneous in both structure and composition, which makes the mechanism of austenite formation very complex. In this contribution, austenite formation upon continuous heating from a designed spheroidized cementite structure in a model Fe-C-Mn alloy was investigated with an emphasis on the role of heating rate in kinetic transitions and element partitioning during austenite formation. Based on partition/non-partition local equilibrium (PLE/NPLE) assumption, austenite growth was found alternately contribute by PLE, NPLE and PLE controlled interfaces migration during slow-heating, while NPLE mode predominately controlled the austenitization by a synchronous dissolution of ferrite and cementite upon fast-heating. It was both experimentally and theoretically found that there is a long-distance diffusion of Mn within austenite of the slow-heated sample, while a sharp Mn gradient was retained within austenite of the fast-heated sample. Such a strong heterogeneous distribution of Mn within austenite cause a large difference in driving force for ferrite or martensite formation during subsequent cooling process, which could lead to various final microstructures. The current study indicates that fast-heating could lead to unique microstructures which could hardly be obtained via the conventional annealing process.

Key words: Cementite, Austenite, Kinetics, Elements partitioning, Fast heating