Study On The Technology And Theory Of Semi-continuous Casting Processing Of Wrought Magnesium Slabs

With the increasing challenges of energy shortage and environmental protection,the massive application of wrought magnesium alloys is imperative.Magnesium sheets and coils are the most popular and staple commercial products,which are manufactured through the advanced hot rolling process,premising the prepared large scale casting slabs with high quality and low cost.However,the control of solidification structures and casting stress are difficult due to the special plane symmetry of heat transfer and solidification in slab fabrication process,especially when the width to thickness ratio is large.As a part of the National Basic research program(973)"The solidification of large-scale magnesium slab with high quality and low cost" and the National Key Technology Support Program "Low-cost and High-efficiency Rolling technique of Magnesium sheets",this work integrate the electromagnetic and ultrasonic fields into the semi-continuous DC casting of magnesium slabs,develop the experimental and simulation work on magnesium slabs with different scales,and investigate the effects of casting speed,cooling rate,melt distribution and external fields on the heat transfer behaviors during the solidification.This work firstly investigate the heat transfer behaviors of magnesium slabs with a section dimension of 130 mm×300 mm by the real-time temperature measurement during DC casting.The results show that the effects of casting speed and secondary cooling water on heat transfer behaviors are much related to the induced heat release change by them and the influence of secondary cooling on primary cooling.With the increasing casting speed,the thickness of mushy zone reduces as well as the thickness diversity of mushy zone throughout the cross section,which is beneficial to obtain a uniform solidification structure and increase the cooling rate and heat flow direction of the melt close to the inner wall of crystallizer at the primary solidifying stage(High temperature zone).It further inhibits the growth of primary dendrites as well as the form of columnar zone.Increasing the secondary cooling rate can achieve similar but a little weaker result.However,it could significantly change the heat release orientation and inhibit the form of columnar zone at the edge area.The results of real-time temperature measurement in LFEC casting process show that the forced convection by low frequency electromagnetic field can remarkably reduce the sump depth,mushy zone thickness and the corresponding diversity of mushy zone throughout the cross section,which could further increase the cooling rate of slabs and finally obtain the uniform and refined solidification structure.However,further increasing the intensity of electromagnetic field can result dendritic coarsening,due to the rising initial cooling rate of the melt at the corner of large face.Strong secondary cooling rate can inhibit the forced convection by electromagnetic field near the inner wall of crystallizer,which could reduce the initial cooling rate at this area and result a dendritic and coarse solidification structure.Local heat insulation at the inner wall of crystallizer have a remarkable influence on the heat transfer and solidification behaviors during LFEC casting process.An all-round heat insulation is better than the partial heat insulation at small face.The results of LFEC DC casting of large-scale magnesium alloy slabs show that a proper casting speed is beneficial to obtain a sound casting with uniform and fine solidification structure.The reasonable casting speed for 300 mmx800 mm slab is about 30 mm/min.Meanwhile,conducting ultrasonic treatment at proper location during the LFEC casting of 350mmx860mm slabs can further refine the solidification structures and prohibit the form of columnar zone and macrosegregation,due to the impact range of ultrasonic field enhanced by the forced convection of electromagnetic field.The observed cracks in large-scale magnesium alloy slabs easily form due to the collective efforts of non-free feeding and casting stress.The cracks induced by non-free feeding easily form near the symmetry plane of the large plane,and the cracks induced by casting stress always occur at the low solidification level of the large plane.The numerical simulation results of LFEC DCcasting of super large-scale 400 mm X 1450 mm magnesium slabs show that the electromagnetic field has remarkable influence on the DC casting process.Enhancing the(current)intensity of electromagnetic field can expand the range of the forced convection at the center of slabs,where the heat transfer is also enhanced.With the increasing casting speed,the sump depth rise significantly,and the solidified shell height reduce notably but the diversity throughout the circumference increase.Increasing the cooling rate of the center position of the large plane can increase the solidified shell height but the diversity throughout the circumference decrease as well as the difference of cooling rate during solidification.Compared to the electromagnetic field,the ultrasonic field has a weak influence on the flow and temperature fields of super large-scale magnesium slabs.Only a slight melt temperature rise can be observed at the underface of the ultrasonic radiator under strong ultrasonic power.Meanwhile,the ultrasonic horn also has weak effects on the electromagnetic distribution in this case.The melt distribution channel has outstanding effects on the temperature and melt flow fields.Decreasing the number of the hole-path close to the center of large plane can reduce the horizontal temperature difference and the sump depth remarkably,which can provide a uniform solidification throughout the cross section.Finally,a semi-continuous casting crystallizer system for super large-scale slabs has been designed based on the investigated heat transfer behaviors during solidification.