Study On The Control Of Carbides In Bearing Steel GCr15 Based On Electroslag Remelting Process

Carbide has a significant effect on the properties of bearing materials.The formation of large primary carbide in GCr15 bearing steel is mainly due to the segregation of elements during solidification of molten steel.With the increase of the demand for high quality bearing steel,it is very important to strictly control the segregation of carbides during solidification.Controlling of carbides for high quality bearing steel in China has obvious gap with the advanced technologies in other developed countries.The studies about the control of carbides are very few,especially about the control of primary carbides.In this thesis,GCr15 bearing steel produced by electroslag remelting and rolling process was chosen as research object.This work focused on the control of segregation of elements and carbides in the electroslag remelting and rolling process,in order to provide guidance and theoretical basis for the development and production of high quality bearing steel.The main research contents and conclusions are as follows:Numerical simulation was carried out using MeltFlow software to study the influence of cooling intensity on the temperature distribution of molten pool and solidified ingot,the liquid fraction distribution of molten pool and solidification structure.The results show that the temperature of molten pool decreases with the increase of cooling intensity,the depth of molten pool becomes shallower,the dendrite spacing decreases and the segregation of C decreases.The formation of precipitates and their composition during solidification in GCr15 bearing steel were calculated by Thermo-Calc software.The carbides in GCr15 bearing steel were analyzed by non-aqueous electrolysis method,SEM-EDS and XRD.The results show that the carbides in GCr15 bearing steel are mainly Fe-rich M3C carbides,and a small amount of Cr-rich M7C3 and M3C2 carbides.The precipitation of primary carbide M3C from liquid steel according to eutectic reaction occurs when the solid fraction of liquid steel exceeds 0.902.Due to the segregation of elements,the existing temperature range of M3C carbides containing a small amount of carbon and chromium in bearing steel is enlarged.The effect of secondary cooling intensity of electroslag remelting on element segregation,microstructure,cementite and carbide in GCr15 bearing steel was investigated.The results show that the element segregation and amount of primary carbides could be lower by ESR process.The type of carbides in bearing steel is not changed,namely M3C,M3C2 and M7C3.The morphology of primary carbides changes from angular shape to lump-like shape.After ESR process,the dimension of primary carbide and the mass fraction of Cr in primary carbide decrease.With the increase of secondary cooling intensity of ESR process,the microstructure of ESR ingot become much finer and uniform,the size of primary carbides decreases.Suppressing the formation of cementite at grain boundaries and primary carbide during electroslag remelting of GCr15 steel is beneficial to inhibit the formation of large secondary carbides after annealing.There is a decreasing trend in the maximum orientation deviation angles between columnar grains and ingot axial with the increase of secondary cooling intensity,which favors the refinement of microstructure.The effect of rolling parameters on the microstructure and carbides in GCrl5 bearing steel was studied.The results show that with the decrease of rolling temperature,the increase of cooling rate and pass reduction,the degree of network carbides in GCr15 bearing steel decreases.The rolling parameters for reducing the precipitation and segregation of carbides in bearing steel were proposed in this study.Compared with the rolling of continuous casting billet,the segregation of carbides in bearing steel is reduced after rolling of ESR ingot,the amount of large carbides decreases,pearlite structure and network carbides become finer.After rolling,the pearlite structure and network carbides are finer in the ESR ingot produced by ESR using a larger secondary cooing intensity.