Owing to less rolling passes and non-control cooling rate device after finishing rolling

the two kinds of structure-pearlite + ferrite and bainite + martensite in Φ70 ~ 80 mm round bar of steel 40CrMo (/% : 0. 40 ~0. 41C

0. 97 〜1.05Cr

0. 17Mo) produced by continuous rolling mill strain easily occur. By using Gleeble-2000 thermal simulation machine the continuous cooling transformation (CCT) curves of two kinds of original structure specimens of heat batch Nol and No2 are measured

and the structure and micro-region analysis of finished products by heating at 1 150 ℃ for 5 min then cooling with 10 °C/s to 900 丁 for 10 s and deforming 30% with strain rate 20 s

-1

and cooling with 0. 2 ~ 10 ℃/s to 200 °C are studied. Results show that the martensite structure occurs as the finished products is cooling with large cooling rate after rolling ； with cooling speed

0. 1 ~ 1 ℃ /s for round bar after rolling the steel 40CrMo can get normal structure i. e. bainite and/or pearlite structure.