The strength and plasticity of (mm) 150 x 150

180 x 220 and 220 x 300 concasting billet of gear steel 20CrMnTiH at 600 ~ 1 300 °C have been studied by Gleeble-1500 thermal simulation machine. Test results showed that the actual strength at fracture S

k

of three size of billet at 650 ~ 950 °C was very low

at 950 ~ 1 200 ℃ the S

k

increased with increasing temperature

then the S

k

decreased at temperature higher than 1 200 °C ； the second brittle zone of steel was at 950 ~ 700 ℃. It is available to prevent crack source evolving into longitudinal crack by increasing the shell thickness of billet out of mold and decreasing holding time at 950 ~ 700 °C zone.