Effect Of Superheat On The Internal Quality Of Continuous-casting Billet For Bearing Steel

Bearing steel is mainly used to produce rolling elements for rolling bearing.Due to its high carbon content,bearing steel trends to solidify over a wide mushy zone at the solidification front,which leads to an increase of permeability in the molten steel and enrichment of segregated elements during remaining liquid with progress of solidification,and it generally constitutes major defects such as segregation which limits the application range and service life of bearing steel.Therefore,to improve the internal quality by improving the production process is an important task for the production of bearing steel.The reasonable superheat can improve the equiaxed zone ratio of continuous casting billet,which was beneficial to suppress segregation and porosity.In order to ensure the continuous casting process and ensure the quality of the product,the superheat of molten steel must be controlled accurately.The quality of bearing steel was investigated at different superheats(20 ℃,25 ℃and 35 ℃)by experimental observations to investigate the internal quality of the samples at different superheats.On the basis of obtaining solidification structure,equiaxed zone ratio was measured.It is found that equiaxed zone ratio of GCrl5 billets at different superheats decreases with the increase of superheat.For comparison of the internal quality of GCrl5 billets more precisely,the same area(30mmx30mm)called central equiaxed grain zone in equiaxed grain zone of the GCrl5 billets was chosen in consideration of different equiaxed zone ratios at different superheats.The internal quality of bearing steel continuous casting billet at different superheat was compared by using the segregation ratio of central equiaxed grain zone,the area of large dark point and secondary dendrite arm spacing(SDAS).In addition,the factors affecting the quality of central equiaxed grain zone are analyzed.It is found that the segregation ratio,the area of large dark point and the secondary dendrite arm spacing were the minimum values at 25 ℃ superheat.All these results indicated that compared to the GCrl5 billets at 20℃ and 35℃ superheats,the quality of central equiaxed grain zone of the billet at 25 ℃ superheat is improved,which indicates 25 ℃ superheat is the optimal superheat.Morever,using the ANSYS simulation software,the temperature field of the solidification process of GCr15 billets at different superheats can be obtained.Based on the temperature field,the center solidification time tMcentre,later solidification time tMlater and earlier solidification time tMearlier can be solved.It is found that central equiaxed grain zone SDAS of GCr15 billets is decreased with the decrease of later solidification time tMlater rather than the center solidification time tMcentre.That is to say,there is a certain error in characterizing the central equiaxed zone SDAS with the center solidification time tMcentre.Due to the presence of macro segregation and convection,when the temperature of the molten steel drops below the liquidus,there are no second branches in the liquid steel.This period of time is included in the earlier solidification time of tMearlier,which leads to invalidation of center solidification time tMcentre.Based on the analysis of GCr15 continuous casting billets,the thermal simulation experiment was carried out under the laboratory condition to study the influence of superheat and other solidification conditions on the quality of the billets.It can provide some references for the selection of superheat during casting process and the optimization of continuous casting process.A benchmark sample is set up during the experiment.Its cooling rate shall be referenced to GCr15 continuous casting billet.On this basis,the cooling rate at different stages and superheat were changed to study the influence of solidification conditions on the solidification structure.It was found that,during the superheat elimination stage and the earlier solidification stage,the cooling rate increases,the solidification macrostructure changes from cellular crystalline to columnar crystal.However,increase the cooling rate of later solidification stage,the solidification macrostructure basically kept the morphology of the benchmark sample,mainly with cellular crystal.When the heating temperature of each sample is the same,increasing the coolong rate of the earlier solidification stage and the later solidification stage can increase the compactness of the solidification macrostructure,especially increasing the coolong rate of the earlier solidification stage.Taking into account the difference between the thermal simulation test and the actual casting process,it is further proved that for the GCr15 continuous casting billet sample analysis:using earlier solidification time tMearlier to characterize its internal quality has a certain failure.When the cooling rate of each sample is the same during the solidification process,the quality of the sample can be improved by lowering the heating temperature.When the samples were heated at the same temperature,it can be found that the different solidification structure can be obtained by changing the cooling rate of different solidification stages.According to the results of thermal simulation experiment,it can extend to the actual casting process.The results show that the quality of continuous casting billet can be improved by increasing the cooling rate at different solidification stage,although molten steel is not the best pouring temperature.