Study On Mechanism And Control Method Of Spheroidization For GCr15 Bearing Steel During Warm Ring Rolling

GCr15 bearing steel is the most widely used bearing material.In the bearing forming process,whether it is cold ring rolling or hot ring rolling,spheroidizing annealing is an indispensable process for the subsequent cutting of the material.However,the conventional spheroidizing annealing process usually requires an isothermal temperature of more than 10 hours,which is extremely wasteful of energy.In this paper,a new type of warm forming technology is proposed.By controlling the pearlite divorced eutectoid transformation and deformation parameters during the warm rolling process,it is possible to obtain a fine and fine round carbide structure with good uniformity in a short time.The forming effect is a new energy-efficient process.This paper will systematically study the feasibility,spheroidization mechanism,control methods and industrial specific practices of this new warm forming technology.In this study,the multi-pass deformation and single-pass deformation and the warm compression experiments with divorced eutectoid transformation and eutectoid transformation are used to realize the feasibility of tissue spheroidization in the process of deformation and transformation during warm ring rolling.The analysis of the properties revealed that the morphology,warm deformation parameters and pearlite transformation of the original pearlite group had an important effect on the spheroidization effect of the carbide.Subsequently,the spheroidization model of pearlite transformation process under small stress and large deformation conditions was established by Jeziorny method and pearlite diffusion nucleation phase transition theory respectively,and the mechanism of tissue spheroidization during warm deformation was explored.It is found that in the process of warm deformation of bearing steel,there is competition for pearlite divorced eutectoid transformation and eutectoid transformation,and competition for phase spheroidization and deformation spheroidization.It is believed that in this new type of warm forming technology,the spheroidization essence is to control the proportion of divorced eutectoid transformation to pearlite transformation by warm deformation,and at the same time to mechanically break the layered carbide structure to achieve rapid spheroidization.In order to specifically study the interaction mechanism between the deformation and transformation of the warm forming spheroidization process and control it,the deformation experiment before the transformation and phase transformation of pearlite divorce eutectoid transformation was designed.Through the deformation texture,ferrite grain size,carbide precipitation position,subgrain distribution,pole figure,residual stress distribution,stress-strain relationship,etc.,the deformation evolution and the structural evolution and dynamic recrystallization of the thermoforming process are controlled.The ferrite recovery status was analyzed.It indicates that the deformation can promote the transformation,and the transformation can improve the warm forming effect.The precise regulation of deformation and transformation is the necessary means to realize the short-flow warm ring rolling forming process.Finally,an attempt was made to industrial warm ring rolling on the hot ring rolling production line of Chengdu Tianma Railway Bearing Co.,Ltd.The microstructures of different regions of different warm rolling schemes such as pre-rolling,temperature rise,two-phase rolling,pre-hot rolling,etc.were analyzed.According to the results,a two-phase and two-deformation warm ring rolling process was designed.It is found that by precisely controlling the deformation and transformation parameters during the ring rolling process,the spherical cementite structure with extremely high roundness and few irregular carbides can be obtained within 130 min.The martensite grain size after quenching and tempering also meets the standard.