Basic Research On The Application Of Final Helical Electromagnetic Stirring Technology Of Special Steel Continuous Casting Large Bloom

With the strong demand for large-size and high-quality special steel round blooms in major projects and the advanced equipment manufacturing industry,the massive production of large round blooms is faced with the problem of frequent defects,and electromagnetic stirring is a common method used to improve internal defects.To develop the basic research on the application of final helical electromagnetic stirring（F-HEMS）technology,a multi-physical mathematical coupling model of helical electromagnetic stirring at the solidified end of special steel round bloom was established,and its accuracy was verified by industrial experiments.The effect of F-HEMS is related to the installation position of the electromagnetic stirrer and appropriate matching between electromagnetic stirring parameters and continuous casting process parameters.To determine an optimal installation position for F-HEMS,its effects were evaluated at different liquid fractions（42%,36%,31%and 26%）while considering solidified shell formation in large round blooms.The numerical simulation results show that the thicker the solidified shell,the smaller the magnetic induction and electromagnetic force inside the bloom.The installation of F-HEMS in the position where the liquid fraction of the large round bloom is about 36%can not only avoid the stirring intensity caused by the high liquid fraction,but also avoid the low flow rate of the liquid steel caused by the insufficient electromagnetic stirring range.Industrial tests demonstrate that installing F-HEMS at a distance of 9.3m from the mold meniscus can mitigate internal shrinkage and cracking to some extent.To determine the reasonable parameters of F-HEMS,a numerical simulation was carried out.The results show that the increasing current frequency initially increases the X-Velocity value before decreasing,maximum value occurs at 7Hz.Liquid steel flow range and velocity increase with current intensity,but the X-Velocity gradient is only 0.00012m·s^-1 between 600A and 700A.Therefore,（600A,7Hz）is selected as reasonable parameters for F-HEMS.The random sampling of multiple sets of industrial test results demonstrates that center shrinkage grade significantly reduces after applying F-HEMS while the center crack range decreases on average by 26mm.The effects of F-HEMS on the stress and strain distribution and crack formation in large round billet were studied.The results show that the temperature and thermal stress distributions at the center of the large round bloom are centrosymmetric.The radial section of the bloom exhibits a centralized and segmented temperature gradient,with greater absolute values resulting in higher thermal stresses.Upon applying F-HEMS,which is a reduction in thermal stress within the electromagnetic stirring range.An internal cracking formula based on thermal stress is presented,showing that F-HEMS results in a smaller internal cracking index compared to without it,thereby reducing internal cracks in large round blooms.The industry test results show that F-HEMS has a positive effect on the internal crack control of large round blooms,the internal crack range of large round blooms is reduced by approximately 2.3%compared to without it.