Two quenching methods were applied in the experiments carried out on Gleeble 1500 thermomechanical simulator: (1) spurting water; (2) making the samples drop into water, and the cooling rate distributed un-uniformly in the samples quenched by the former and uniformly by the latter. The critical cooling rate of preventing ferrite precipitation was determined, and the microstructure observation position in which the microstructure could be frozen to room temperature in the samples quenched by spurting water was also determined and using this method the Ad3 was determined to be about 830℃ for tested materials. Using second quenching method, single-pass deformation experiments at different temperatures were carried out, and the microstructure analysis indicates that, as the deformation temperature decreased, the ferrite morphology changed from Widmanstätten and allotriomorphs to equiaxed at about 800℃. Combining the dilation-temperature curves measured during cooling process after deformation at different temperatures, the Ad3 was determined to be also about 830℃ for tested materials.