Continuous Cooling Transformation Curves and Microstructures of Copper Bearing Low-Carbon Si-Mn Steel

The dilatometric curves of the copper bearing low-carbon Si-Mn steel at different cooling rates (1-150 ℃·s-1) were determined by Gleeble-3800 thermal mechanical simulator. And the continuous cooling transformation (CCT) curves of the steel were obtained. The effects of the cooling rates on the microstructure evolution and microhardness were also analyzed with the microstructure observation and hardness measurement. The results show that at the cooling rate between 1-5 ℃·s-1, the microstructure mainly consisted of ferrite and pearlite. The martensite appeared at the cooling rate of 10 ℃·s-1 and the content increased with the increase of cooling rate, meanwhile the pearlite degenerated and decreased and the content of ferrite also decreased, among which the acicular ferrite increased and polygonal ferrite decreased then disappeared. The acicular ferrite disappeared and the microstructure consisted of martensite and retained austenite when the cooling rate was higher than 120 ℃·s-1. The microhardness of the steel increased with the cooling rate increase.