Effect of final helical electromagnetic stirring on crack formation in large bloom

Internal crack is one of the common defects for special steel bloom. Electromagnetic stirring is the common technical means to improve the uniformity of microstructure and composition of the bloom. The helical electromagnetic stirrer is not only simple in structure, convenient in installation, but also has obvious stirring effect. To investigate the effect of final helical electromagnetic stirring (F-HEMS) on the liquid steel flow, heat transfer and crack formation in large bloom, experiments and numerical simulations were carried out. It is found that the liquid steel in the liquid core is spirally flowing along the casting direction under the action of electromagnetic force, and the range is concentrated in the height range of the electromagnetic stirrer. The temperature gradient between zero ductility temperature and liquid impenetrable temperature (TZD-TLI) in continuous casting bloom is increased by 31.56%-42.27% and the TZD-TLI zone width is decreased by 6.6 mm with F-HEMS. Under the influence of F-HEMS, the liquid fraction in the mushy zone decreases. The distributions of temperature, Von-Mises stress and elastic equivalent strain in continuous casting bloom are all stratified. After the application of F-HEMS, Von-Mises stress and elastic equivalent strain in TZD-TLI zone of continuous casting bloom decrease by 0.11 MPa on average and 0.02 percent point on average, respectively. Based on numerical simulations and experiments, a cracking index formula is proposed to predict the internal cracking risk. It is found that the crack index when using F-HEMS is 0.158-0.280 smaller than when not using F-HEMS, which proves that F-HEMS can reduce the probability of crack formation in continuous casting bloom. However, the formation of internal cracks is affected by many factors, and the cracking prediction of large bloom in continuous casting process still needs to improve the simulation method and be proved by a large number of experimental data. F-HEMS affects liquid steel flow and heat transfer in continuous casting large bloom, reduces the probability of internal crack formation. The research results provide a theoretical basis for industrial production.