Abstract: The static CCT curve of the HG05 as-quenched-tempered seamless steel tube with high strength and toughness is measured by the Gleeble 3500 thermal simulation machine, and then the critical quenching-cooling rate of the steel is determined.The outer layer, the middle layer and the inner layer of the hot-formed HG05 seamless steel pipe are sampled and analyzed.All the said layers are of the bainite and ferrite structures, and the ferrite structure changes greatly.The size of ferrite in the middle layer is the largest, and the average size is 10 μm;and the inner ferrite volume fraction is the highest as about 12.4 vt%.The outer, the middle and the inner layers of the hot-formed steel tube are quenched with the thermal simulation method, and the critical quenching-cooling rate of the as-quenched-tempered steel, and the incompletely quenched microstructure is obtained.Since the quenched-tempered microstructure is homogeneous, there is little austenitic nucleation core, and after being heated with the thermal simulation, the austenite grain size is large, the critical quenching-cooling rate of austenite is 20 ℃/s, whereas the critical quenching-cooling rate of the hotformed microstructure is higher than that of the austenite.