By Gleeble3500 thermomechanical simulator to determine the dilation curves at continuous cooling conditions

the critical transformation points of steel NM400 are Ac

1

=719. 4 ℃ and Ac

3

=847. 8 ℃，and combined with the metallographic method

the continuous cooling transformation (CCT) curves of super cooling austenite of tested steel are plotted using Origin software. The results show that with the increase of cooling rate

the microhardness of steel increases gradually

and the phase transformation gradually changes from ferrite and pearlite to bainite and martensite

and the ferrite content decreases gradually ； when the cooling rate is less than 1 ℃/s

the pearlite content increases gradually

and the grain size gradually decreases; when the cooling rate is greater than 5 ℃/s

the ferrite and pearlite phase transition disappears

and with the increase of the cooling rate the bainite phase decreases and marte

nsitic phase is gradually increasing.