Abstract: Continuous cooling phase transformation curves of the DX51D+Z low carbon steel with and without rare earth element Ce were drawn by Formaster-F automatic phase transformation instrument combined with microstructure analysis. The effects of rare earth on the continuous cooling phase transformation points and microstructure of the DX51D+Z steel at different cooling rates were studied. The results show that the addition of trace rare earth Ce (9×10^-6) reduces the Ac₁ transformation point temperature of the DX51D+Z steel by 3 ℃ and increases the Ac₃ transformation point temperature by 9 ℃, increasing the phase transformation interval from α-Fe→γ-Fe during slow heating. When the cooling rate is 0.05-2 ℃/s, the trace rare earth Ce has little effect on the the starting and ending temperature of phase transformation of the tested steel. When the cooling rate is greater than 5 ℃/s, the trace rare earth Ce reduces the phase transformation termination temperature, expands the phase transformation range of ferrite+pearlite and ferrite+bainite, and shifts the static CCT curve to the lower left. Within the cooling rate range of 0.1-20 ℃/s, the grain size grade of the tested steel with rare earth Ce is 0.2-0.4 higher than that without rare earth, and the grain size distribution is more uniform.

Key words: rare earth Ce, DX51D+Z low-carbon steel, CCT curve, microstructure