Effect Of Mineralogical Structure Of Flux Film On Heat Transfer And Slab Longitudinal Cracks For Hypo-peritectic Steel

Recently, with casting speeds increasing, some slab quality defects, which including surface longitudinal cracks, inclusions, and breakout, have been reported. This is especially true in the case of hypo-peritectic steel, which tends to undergo volume shrinkage, making it even easier for longitudinal cracks or breakouts to occur during continuous casting. Therefore, knowledge regarding the properties of mold flux is becoming increasingly essential. Current studies on heat transfer of flux film are mostly limited to the thickness and crystallization ratio of slag film. However, researches of mineralogical structures of flux film are relatively few, and the characteristics of film structure of hypo-peritectic steel are not clear. Based on this, we analyze the effects of film structure on mold heat transfer and strand quality from the perspective of mineralogical structure of slag film.The results show that the thickness of spot hypoperitectic slag film is nonuniform and the flux film has a layered structure. The glass phase distributes in the mold side and the crystalline phase distributes in the strand side. The main phases of flux films are melilite, wollastonite and cuspidine. The crystallization ratio of normal slag film is higher,which is more than 90%, and the crystalline phases of flux film are melilite, cuspidine and wollastonite, while the crystallization ratio of accidental slag film is lower by65~70%. The mineral compositions are the same with the normal slag film, but the content of each component is significantly different. The melilite significantly decrease,and the cuspidine and wollastonite obviously increase. The crystal development degree of accidental slag film is poor, and the glass phase of flux film relatively increase, which is detrimental to control the heat transfer.The experimental flux films were mainly prepared using spot mold flux to analyze the mineralogical structures. It turned out that the crystallization ratio and mineral compositions of flux film are similar to the spot one, but the layered structure is not obvious. Three types of crystalline mineral appear in the flux films. The melilite crystals tend to be interlaced and acinose, whereas cuspidine crystals appear more fine granular and spearhead-shaped, wollastonite crystals are more prone to be fibrous shaped or thin strips. Compared to the spot flux films, the particle size of minerals of experimental flux film is relatively small and the crystal development is incompleted. It is found that the surface roughness, crystallization ratio and thermal-conduction resistance of flux film are the important factors to mold heat transfer. The crystallization ratio of flux film of hypo-pertiectic steel is as high as 90% with a certain amount of cuspidine precipitation,which can effectively reduce the heat transfer between the mold and strand, and prevent longitudinal cracks occur. To reduce the heat transfer and avoid the occurrence of strand longitudinal cracks, the crystallization ratio and surface roughness should be increased,and a moderate amount of cuspidine is suggested for the casting process of hypoperitectic steel.