Dilatation curves of C-Mn-Si series low carbon （/% : 0.11C

1.15Si

1.85Mn

0.032A1

0.003Ti

0.002 4N） and medium carbon （/% ： 0.35C

1.1 ISi

1.82Mn

0.041 Al

0.002Ti

0.004 2N） bainite steel continuous cooling with cooling rate 0.5 ~ 30 ℃/s have been measured by using thermal dilation method and metallographic examination with the Gleeble-1500 hot simulation machine to define the phase transformation points

and combined with microstructure observation

the continuous cooling transformation （ CCT） curves of two steels are respectively plotted by Origin software. Results show that with cooling rate ≤ 1 °C/s

the structure of 0. 11% C steel is ferrite + bainite + martensite

as cooling rate ≥ 2 °C

its structure is bainite + martensite

and with cooling rate ≥ 0.5 °C/s the 0.35% C steel has bainite 4- martensite structure ； with increasing carbon content

the transformation temperature of bainite and martensite all decreases.