Carbide precipitation and microstructure refinement of Cr-Co-Mo-Ni bearing steel during hot deformation

YUAN Xiao-hong(袁晓虹)^{1, 2}, ZHENG Shan-ju(郑善举)³, YANG Mao-sheng(杨卯生)², ZHAO Kun-yu(赵昆渝)¹

(1. Faculty of Material Science and Engineering, Kunming University of Science and Technology,
Kunming 650093, China;
2. Central Iron and Steel Research Institute, Beijing 100081, China;
3. New Metallurgy Hi-tech Group Co., Ltd., China Iron and Steel Research Institute Group, Beijing 100081, China)

Abstract:The dynamic recrystallization and carbides precipitation of the Cr-Co-Mo-Ni bearing steel were investigated by hot compression tests performed at temperatures ranging from 850 °C to 1080 °C with strain rate of 1-20 s^-1. The activation energy (Q) for the tested steel is calculated to be around 682.99 kJ/mol at a deformation strain of 0.6. Microstructural analysis by SEM shows that the dynamic recrystallization (DRX) behavior is dependent sensitively on the deformation strain, temperature and strain rate, while an exponential relationship between DRX grain size and Z parameter is obtained from the computational formula. Moreover, the M₆C-type carbides (<1 μm) act as the main prohibitor of grain coarsening, and the polynomial regression relationship between them is worked out. With electron backscatter diffraction (EBSD) observation, DRX is the main nucleation mechanism responsible for the formation of new grains during hot compression. In conclusion, the interaction between DRX affected by hot deformation parameters and carbides precipitation determines the ultimate grain size refinement.

Key words:high-alloy steel; Z parameter; grain size refinement; carbides; dynamic recrystallization (DRX) mechanism