Study On Structure Characteristics Of Solid Slag Films Of Mold Fluxes For Peritectic Steel Continuous Casting

As an important material used in the continuous casting of steel,mold flux plays significant roles in improving the surface and sub-surface quality of slabs.In the continuous casting of middle carbon crack-sensitive steels,high-basicity mold fluxes are usually used to control the heat flux from steel shell to the mold to avoid the formation of longitudinal cracks on slab surfaces.However,stick or breakout are more frequently occurred when using those high-basicity fluxes.With the development of continuous casting technology,the contradiction between heat transfer control and lubrication of mold fluxes for peritectic steels becomes more obvious.As the lubrication and heat transfer control functions of mold fluxes rely on liquid and solid slag films,analyzing and understanding of the structure evolution of slag films in solidification are with no doubt,of great importance to coordinate the lubrication and heat flux control functions of slag films.In order to understand the structure evolution of solidified slag films,a one-dimensional finite difference model was established to simulate the slag solidification and heat flux acquisition process using a water-cooled copper probe.Based on the frequent non-uniform film structures and poor repeatability of measured heat flux data using conventional large probes,the effect of probe sizes and temperatures of liquid fluxes on the structure of solidified slag films and heat fluxes were investigated.Then,a conventional high-basicity mold flux?with binary basicity R=1.38?,an ultrahigh-basicity mold flux?R=1.74?,a CaO-SiO₂-CaF₂ based low-basicity mold flux?R=0.88?,and CaO-SiO₂-Na₂O based low-fluorine and fluorine-free mold fluxes were selected as the basis of this study.An improved water-cooled probe with a much smaller size was used to obtain solidified films with more representative structures.The structure of films of different mold fluxes solidified in fluxes with various temperatures and probe immersion time was measured and compared.The effect of structures on heat fluxes through slag films was also discussed.Baesd on the calculation results,a conclusion can be drawn that the stability of flux temperature during experiments affects the film structures and heat flux data strongly.Thus,an improved water-cooled probe with a much smaller size was developed to obtain solidified films with more representative structures.Based on the structure analysis,conclusions can be drawn that for high-basicity mold fluxes,the roughness of film surfaces contacted with copper plate develops early,when films are still glass.The formation of this roughness is associated with the porosity at slag-copper interface,and is with no causal relationship with crystallization and devitrification in films;Compared with the convention high-basicity slag films,the roughness of surfaces of ultrahigh-basicity slag films contacted with the copper probe is much larger.The uniform and large surface roughness of the ultrahigh-basicity film controls the growth rate and enhances the uniformity of solid films,which potentially improves the lubrication capacity of mold fluxes.The surface roughness of ultrahigh-basicity film Ra=3⁵ ?m;For conventional high-basicity basicity film,Ra=1.5⁴ ?m.The closed porosity of conventional high-basicity slag films decreases upon solidification,but which remains at a high level during the solidification of ultrahigh-basicity mold fluxes.Which controls the fast growing of solid films of ultrahigh-basicity mold fluxes in some degree.The formation of closed pores in the devitrification layer of conventional basicity and ultrahigh basicity films is with no causal relationship with crystallization.The cuspidine in ultrahigh-basicity films are with large lath-shapes in the outer layer of films,the precipitation of those large lath-shaped cuspidine maintains or increases the closed porosity of films.No obvious differences of thermal conductivities of conventional high-basicity and ultrahigh-basicity mold fluxes were detected.Their measured thermal conductivities are in the range: 3.17³.67 W/m K.Different from high-basiciy slag films,the surface of low-basicity films contacted with the copper probe is smooth and no open pores are detected on this surface,roughness Ra of the surface ranges from 1.08 to 2.67 ?m.A small surface roughness of initial solid film weakens the heat flux control capacity of mold flux at meniscus.The closed porosity of initial solid films lower than 5 vol%,which increases gradually with the increased probe immersion time.For CaO-SiO₂-Na₂O based low-fluorine and fluorine-free films,neither the formation of surface roughness nor the formation of pores in the devitrification layer has relationship with crystallization.The surface roughness Ra of the low-fluorine and fluorine-free films ranges from 2 to 3.5 ?m,the fluctuation of flux temperatures has obvious impact on the roughness,which weakens the uniformity of heat fluxes.The apparent and true densities of low-fluorine and fluorine-free films increase with increased probe immersion time.Compared with the high-basicity solid films in this study,the apparent densities of the initial low-fluorine and fluorine-free films are larger,and with smaller closed porosities.Based on the structure evolution of solidified films in this study,large-scale applications of ultrahigh-basicity mold fluxes were conducted.The results indicate the ultrahigh-basicity mold flux gives better lubrication conditions,lowers the heat flux of molds and increases the surface quality of slabs.