Analysis of the migration behavior of non-metallic inclusion groups during continuous casting of bearing steel

 　　The bearing steel is used in a harsh environment with various alternating loads, which can easily cause fatigue cracks on the surface of the shaft sleeve or rolling element. Researchers have long revealed that fatigue life is closely related to the type and size of non-metallic inclusions. In aluminum deoxidized steel, Al2O3 type inclusions are present in 2 μ When it is below m, it is prone to calcification and degeneration, and the size is very small, greatly reducing the harm; But 5 μ Al2O3 type inclusions above m often have incomplete calcification and denaturation, forming point like D-type inclusions or composite with dissolved magnesium in steel to form Al2O3 · MgO · CaO type inclusions. The composite coating thickness can reach tens to tens of micrometers, seriously affecting fatigue life; Some literature studies have also shown that semi encapsulated Al2O3 · MgO · CaO inclusions are accompanied by clustered Al2O3 or composite Al2O3 · CaO inclusions, with equivalent diameters ranging from tens to hundreds of micrometers, indicating the phenomenon of aggregation between inclusions.

　　By reviewing the work of researchers, it can be found that after the refining and soft blowing processes are completed, the size of inclusions is basically controlled between 2-5 μ m. Even so, it is still impossible to avoid inclusions of tens to tens of micrometers remaining in the casting billet. The experiment by Japanese scholar Ohno T et al. revealed that the number of inclusions gradually increases during the pouring process from ladle to tundish and then to crystallizer, and the probability of large inclusions appearing increases; Inclusion aggregation bands appeared at the subcutaneous position of 20-50 mm in the casting billet, with inclusions ranging from tens to hundreds of micrometers. In the exploration of the source of inclusions in casting billets, a series of researchers such as Javurek M, Pfeiler C, and ZHANG L F have established their own inclusion trapping calculation models within the Euler Lagrange model framework. The research results show that small inclusions near the surface are more easily trapped, while large inclusions ranging from tens to hundreds of micrometers are distributed near the surface, within a range of 20-50 mm from the surface of the casting billets. In the research process of these scholars, it is necessary to assume the initial particle size and quantity of inclusions, without considering the impact of changes in the number of inclusions caused by the aggregation and growth of inclusions during the pouring process, and without considering the impact of electromagnetic stirring on the aggregation and trapping of inclusions. In the research field of electromagnetic stirring, Fujisaki K, Natarajan T T and others focus on the research of stirring mode and magnetic field intensity distribution, while Satou K and others focus on the research of the influence of electromagnetic stirring on the flow field and liquid level fluctuation, and do not involve the aggregation, growth and deployment of inclusions. Based on these research conclusions, in the control of inclusions in the casting billet (especially larger inclusions), the main measures are to prevent foreign large inclusions (such as flocculent particles at the nozzle) and slag entrapment caused by liquid level fluctuations. However, in practice, it has been found that the surface of the billet is prone to 10-30% μ The inclusions of m, most of which are spherical or hemispherical in shape, show signs of aggregation of inclusions in the flow field of the crystallizer in terms of composition and quantity. This phenomenon is also an undeniable influencing factor on the source of larger inclusions.