Research Of Morphology Evolution On Solidification Structure Of Bearing Steel Under Electromagnetic Stirring

Bearing steel has high hardness, wear resistance, contact fatigue strength and elastic limit, also has good impact toughness, fracture toughness, antirust properties and cold/hot working properties. The world recognized that bearing steel level is the symbol to measure the level of a national metallurgy. The carbon segregation in casting billet and other defects can cause that the local strength or hardness of bearing is lower than that with normal structure, which resulting in the decrease of fatigue strength, and fatigue failure of parts prematurely in use. Therefore, the improvement of solidification structure quality is an important task for the production of bearing steel.Electromagnetic stirring (EMS) is an effective method to reduce or eliminate the segregation and porosity, accelerate the center overheating dissipation, increase the rate of equiaxial crystal, and reduce the harmful elements in steel, etc. However, the EMS is sensitive to the stirring position, and can’t change the installation position easily. Meanwhile, the casting billet is easy to appear white band in-strand electromagnetic stirring (S-EMS).The research based on the method of pouring the billet of100×100mm bearing steel and bringing to the technology of EMS in the proceeding of solidification in different opportunities, then sample analysis with the solidification structure of bearing steel casting billet. Through the research on dendrite show, cooling speed, permeability and carbon segregation detection, discuss the effect on the solidification structure of bearing steel with EMS.On this experiment condition, the following conclusions were obtained:(1) From surface area to center, solidification structure of bearing steel divides fine grain area, columnar crystal area and equiaxial crystal area, surface fine grain area ratio changed little generally and around13.5%in different stirring opportunities; In-mold electromagnetic stirring (M-EMS) can effectively keep columnar crystal from developing, promote the formation of small equiaxial crystal, columnar crystal area ratio from69.11%without EMS to31.79%with EMS, meanwhile, equiaxial crystal area from17.75%without EMS to52.12%with EMS.(2) Width of primary dendrite has no apparent coarsening and is about145μm, along with the advancement of solidification from surface to center, it has little influence with or without EMS. M-EMS can effectively reduce the primary dendrite arm spacing, primary dendrite arm spacing suddenly become35%more distance and then quickly reduced when S-EMS is used.(3) Apparent coarsening appears in the process of the secondary dendrite arms are solidifying, the later joined of EMS, the greater coarsening rate (Kc). Coarsening rate from2.52of no EMS gradually reduced to1.76of M-EMS.(4) The higher of the cooling speed (ε), the local solidification time (τ) is shorter, second dendrite arm spacing is smaller, meanwhile, the permeability (Kp) is smaller, carbon segregation index (C偏) is lower.(5) The parameters of M-EMS is350A/12Hz, electromagnetic force is536.99N·m-3, it Can effectively reduce the central carbon segregation index when liquid core’s width of casting billet is37mm. It will be go against improvement of center segregation when liquid core’s width is small.(6) When stirring intensity is350A, S-EMS makes a white belt where first appear positive segregation and after is negative segregation. Stirring intensity decreased to300A, the white belt is basic eliminated.