Abstract: The effects of the time of applying rotating electromagnetic stirring on the solidification microstructure of GCr15 bearing steels were investigated via graphite mold casting. The results showed that imposing rotating electromagnetic stirring at different periods during the solidification process of the molten steel significantly influences the solidification microstructure of the bearing steel ingots. The sooner rotating electromagnetic stirring is imposed， the larger magnetic Taylor number (the characteristic nondimensional parameter to describe the forced convection by the magnetic field) is. Accordingly， the electromagnetic force generates remarkably in the molten steel， which indicates that the fluid flow is strong enough for the columnarequiaxed transition (CET). The equiaxed crystal ratio of the ingot increased to 52% when the rotating electromagnetic stirring is imposed during the period of 0~05min after the accomplishment of pouring. Furthermore， the fluid flow generated by rotating electromagnetic stirring promotes the breaking， melting and drifting away of the dendrites in the solidification front. The fragments of the dendrites proliferated and were brought to the centre of the molten pool which became the heterogeneous nucleus under the strong fluid convection. The constitutional supercooling degree increases with the decrease of temperature gradient which consequently in favor of the promotion of CET.

Key words: bearing steels, electromagnetic force, solidification microstructure, supercooling degree, magnetic Taylor number