| High-end bearing steel has extremely harsh requirements on comprehensive mechanical properties,such as super high strength,hardness,wear resistance,and contact fatigue life,which put forward very high requirements for each production process.To improve the mechanical properties of bearing steel,a large number of insoluble elements are added to the steel to enhance the solid solution strengthening and precipitation strengthening effect.However,the addition of such elements leads to the nucleation and precipitation of heterogeneous microstructure due to concentration gradient,which seriously damages the machinability of the bearing steel.Therefore,the corresponding heat treatment is usually carried out to modify the microstructure at different stages:Firstly,the as-cast bearing steel should be homogenized to eliminate the interdendritic carbide and element segregation for the subsequent heat deformation treatment;Secondly,the network carbide precipitation along the austenite grain boundary can be inhibited by controlled rolling and cooling,thus preventing intergranular cracking during service.Eventually,the hotrolled bearing steel is treated by the spheroidizing annealing to promote the cementite transformation from lamellar to spherical particles to reduce the strength and improve fracture toughness.The cold-forming properties of bearing steel are significantly improved.As a common treatment method in the bearing steel production process,the heat treatment process has been widely studied in the past 100 years.However,there are two limitations of traditional heat treatment methods.Firstly,the heat treatment methods applied to the homogenization of bearing steel require higher temperatures and a longer time to eliminate heterogeneous microstructure.Secondly,no effective technology is yet available to eliminate the network carbides after hot rolling.Based on the above two considerations,it is urgent to develop an efficient and green treatment method suitable for the whole process of high-end bearing steel production,which can eliminate the heterogeneous microstructure.This study uses the pulsed electric current to treat the as-cast and hot-rolled bearing steel differently to control the heterogeneous microstructure.By eliminating eutectic carbides,network carbides,and controlling the spheroidization of lamellar cementite,the microstructure uniformity of the bearing steel is finally improved.The mechanism of pulsed electric current regulating multi-type,multi-scale and multi-configuration heterostructures are studied by microstructure characterization,composition analysis,and numerical calculation.The research work is composed of three parts:Firstly,the continuous casting billet of GCr15 bearing steel was used as the object of study to explore the correlation between the pulsed electric current treatment and the rapid homogenization of the as-cast organization of the material.By analyzing the variation of elemental segregation coefficient under the pulsed electric current and establishing the mathematical model for the eutectic carbide,the regulation mechanism of interdendritic microstructure uniformity was investigated.Secondly,taking the hot-rolled bar of GCr15 bearing steel as the research object,the dissolution behavior of network carbide under the pulse current was explored,and the difference in the diffusion rate of atoms in the grain and the grain boundary was calculated to clarify the dissolution mechanism of network carbides enhanced by pulsed electric current.Thirdly,the dissolution and precipitation behaviors of lamellar cementite under pulsed current were explored in the hot-rolled bar of GCr15 bearing steel.By analyzing the micro defect and calculating the thermodynamics under pulsed electric current,the essence of pulsed current treatment promoting the uniform distribution of granular carbides in a beneficial size range is revealed.The main research results are listed as follows:(1)The mechanism of microstructure homogenization of as-cast bearing steel by the pulsed electric current was studied.The eutectic carbides and element segregation in the interdendritic of the as-cast bearing steel can be eliminated by the pulsed electric current treatment at 1100℃ for 30min.Compared with the industrial homogenization process(1220℃-300min),it can effectively avoid grain coarsening by reducing the processing temperature and shortening the processing time.The non-uniform distribution of current density and the change of electrical free energy of the system were the main driving forces of carbide dissolution.The electric migration can significantly improve the diffusion coefficient of solute atoms and shorten the homogenization process.(2)The elimination mechanism of the network carbide in hot-rolled bearing steel by the pulsed electric current was investigated.The network carbides distributed along the prior austenite crystal boundaries can be eliminated rapidly(30 min)at a lower temperature(740℃)by applying the pulsed electric current.The thermodynamic calculations demonstrate that the network carbides on grain boundary were preferentially dissolved during the pulsed current treatment,and the size differences between the carbides were eliminated.Kinetic analysis showed that the high-density current region on the surface of network carbide and the strong electron wind force significantly improved its dissolution kinetics.The difference between the diffusion coefficients of C atoms in the grain and the grain boundary under the pulse current was 132 times that under the thermal field.The intergranular diffusion played an important role in the dissolution process of the network carbide.(3)The dissolution and precipitation behaviors of lamellar cementite under the pulsed electric current were investigated.The cementite was rapidly transformed from lamellar to granular under the pulsed electric current.For the second stage of the spheroidizing annealing,it was found that the cementite was not coarsened under the pulsed electric current.Structural characterization found that the number density of carbide particles in the material increased significantly,and the increase of nucleation particles inhibited the growth of the original cementite particles.Compared with the traditional industrial spheroidizing annealing process,the carbide particles in the samples treated by pulsed electric current were more uniform and finer.Comparing the tensile properties of the samples in different states,it was found that the plasticity of the materials treated with external field assistance was the best,and the fracture morphology characterization further confirmed the correlation between the differences in the microstructure and the mechanical properties. |
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