To study the effects of the deformation degree and cooling rate on the microstructure and phase transformation temperature for the B1500HS steel,the samples were heated at 900 ℃ for 5 min,compressed by 10,20,30 and 40％ at the strain rate of 0.1 s-1,and then cooled down at the rates of 50,40,25,20 and 15 ℃/s by the thermo-mechanical simulator,respectively.The start and finish temperatures of the phase transformation were determined by the tangent method,and the volume fraction of the phase transformation was ascertained by the level principle according to the dilatometric curves.The volume fraction of the retained austenite was determined by X-ray diffraction.The results show that the volume fraction of the bainite rises with an increase in the deformation degree as the cooling rate is lower than the critical rate.At the same cooling rate,the phase transformation temperature rises with an increase in the deformation degree,and the sizes of both the martensite and bainite phases reduce due to the austenite grain refinement induced by the deformation.The volume fraction of the retained austenite reduces as the deformation degree increases.The critical cooling rate of the undeformed samples is approximately 25 ℃/s and the critical cooling rate rises as the deformation degree increases.