Control Of Inclusion Size And Shape Distribution In Bearing Steel By Applied Electric Field

The continuity of the matrix will be damaged by non-metallic inclusions in the bearing steel structure,which will have a great impact on the quality.Therefore,in steelmaking production,how to effectively control inclusions is an important issue to improve the mechanical properties of steel,it is very valuable for the purification of molten steel to further optimization of the process on the basis of the original purification process.This thesis introduces a new type of inclusion removal process,by applying an electric field to the molten steel to control the movement of inclusions,to achieve the refinement and control of inclusions.GCr15 bearing steel was used in the experiment.The effects of DC current and pulse current on the distribution of inclusions were compared by comparative experiments.Through the in-depth discussion of the theory of pulse current controlling the evolution and migration of inclusions,the mechanism of pulse current is clarified,which provides a theoretical basis for the application of pulse current treatment technology in ultra-clean smelting.Through the combination of high temperature electrowetting experiment and theoretical calculation,the results show that with the increase of electric field strength,the interface free energy between molten steel and Al₂O₃ inclusion decreases gradually.When the electric field strength increases from 0 to 20 V（threshold）,the wetting angle decreases from 132°to 94°.When the voltage continues to increase,the contact angle between the two phases does not change,the inclusion nucleation and growth process will be affected.When a direct current was applied to the molten steel,the percentage of inclusions in the upper and lower parts was 82.7%when the direct current was 0.8A,which increased by 13.4%compared to 69.3%in the unenergized steel sample,and the percentage of inclusions in the middle part was reduced from 30.7%to 17.3%,while the percentage of inclusions in the upper and lower parts was 86.5%when the direct current was 1.5A,which increased by 3.8%compared to 17.3%in the unenergized steel sample.The number of inclusions in the steel sample increased by 3.8%,and the number of inclusions in the middle part decreased from 17.3%to 13.5%.The matrix of the steel sample is cleaner than the unenergized treatment.Scanning electron microscope energy spectrum analysis shows that the use of direct current treatment increases the number of small and medium particle inclusions in the steel,and direct current is effective in controlling not only the larger Al₂O₃ inclusions but also the distribution of the smaller Al₂O₃-Mg O and Al₂O₃ inclusions.When the pulse current was applied to the molten steel,the number of inclusions in the upper and lower parts increased by 87.4%and the number of inclusions in the middle part decreased from 30.7%to 12.6%when the pulse current was 0.8A compared to the unenergized steel sample,while the number of inclusions in the upper and lower parts increased by 88.1%and the number of inclusions in the middle part increased by18.8%and decreased to 11.9%when the pulse current was 1.5A compared to the unenergized steel sample.18.8%,and the number of inclusions in the middle part was reduced to 11.9%.In order to reduce the interfacial free energy,the alumina inclusions have a tendency to reduce their own area and gradually tend to spherical shape.Meanwhile,increasing the pulse current size,Al₂O₃ inclusions accumulate more in the lower part of the steel sample,indicating that with the increase of current intensity,Al₂O₃ inclusions are subjected to greater migration force and more obvious migration,which in turn improves the morphology of inclusions and helps to improve the tissue properties of the steel sample.The comparison between pulsed current and direct current shows that the number of inclusions in the middle part of the steel samples treated by pulsed current decreases more and the number of inclusions in the upper and lower parts increases more,thus it can be found that the pulsed current drives the inclusions to migrate better.