Fundamental Research On The Mold Slags Used For High-Al Steels

Mold slag is a synthetic slag added to the mold during the continuous casting of steel, and its property significantly determines the stability of the continuous casting process of steel and surface quality of the slab. Slag is fed onto the top of the mold to provide thermal insulation; the molten slag protects the steel meniscus from reoxidation and absorbs inclusions from the steel into the molten slag pool; liquid slag ?ows into between solidified shell and a mold, the liquid slag film helps lubrication between the water cooled mold and the solidified shell surface of the steel to prevent sticking against the mold surface, which occasionally leads to breakout, and solid slag film controls heat transfer to achieve uniform shell.During the continuous casting of high-Al steel, [Al] is likely to react with （SiO₂） in the mold slag, which results in increase of w（Al₂O₃）/w（SiO₂） ratio and the Al₂O₃ content, respectively. The changes of mold slag composition would lead to the property changes, which influence the smooth operation of the continuous casting process. So, how to design a suitable mold slag is a key issue for the continuous casting of high-Al steel.Based on the characteristics of reaction between the [Al] and （SiO₂）, that the reaction between the [Al] and （SiO₂） sourced from normal mold slag is inevitable, and this reaction would reach equilibrium in a comparatively short time, two ideas about how to design a suitable mold slag for high-Al steel were proposed: （i） CaO-SiO₂ mold slag with low basicity, this slag would have a suitable property after the reaction between the [Al] and （SiO₂） reached equilibrium; （ii） CaO-Al₂O₃ mold slag with low SiO₂ content and high Al₂O₃ content, this slag would avoid the reaction between the [Al] and （SiO₂）.Hemisphere point temperature was used to study the melting characteristics of mold slag. The results showed that, for CaO-SiO₂ mold slag, the melting points would increase with the increase of w（Al₂O₃）/w（SiO₂）; B₂O₃ and Li₂O could decrease the melting point, especially B₂O₃ exhibited the potent capability to decrease the melting point; for CaO-Al₂O₃ mold slag, the melting point would decrease with the increase of B₂O₃, Li₂O, Na₂O, F-, BaO, SrO, and among these slags, Li₂O had a more evidence influence than others.Rotation viscosity was used to study the rheological property of mold slag. Low basicity CaO-SiO₂ slag would be transformed into CaO-Al₂O₃-SiO₂ mold slag during casting of high-Al steel; the molten slag viscosity would change with the increase of w（Al₂O₃）/w（SiO₂）, Al₂O₃ presented amphoteric behavior; the viscosity of slag containing both Li₂O and B₂O₃ is suitable and ?uctuates only moderately with the increase in w（Al₂O₃）/w（SiO₂）. For CaO-Al₂O₃ series slag, B₂O₃, Li₂O Na₂O and F- could decrease the viscosity, while the viscosity would decrease and then increase with the increase of MgO. With the increase of BaO and SrO, the viscosity of CaO-Al₂O₃ mold slag would increase. Based on the Riboud model, a suitable viscosity mathematical model for the mold slag with high Al₂O₃ content was established.Heat flux simulator of mold slag film was used to study the heat transfer behavior of slag film. For the CaO-SiO₂ slag, the heat flux through the slag film would decrease with the increase of w（Al₂O₃）/w（SiO₂）; heat flux increases with the increase of Li₂O and/or B₂O₃ content in the mold slag. For the CaO-Al₂O₃ slag, B₂O₃ could decrease the crystallization rate, decrease the thickness of slag film, and increase the heat flux; Li₂O could increase the crystallization rate obviously, decrease the thickness of slag film, and decrease the heat flux; Na₂O could decrease the crystallization rate, decrease the thickness of slag film, and increase the heat flux; F- could increase the crystallization rate, decrease the thickness of slag film, and increase the heat flux; MgO and SrO could decrease the crystallization rate, decrease the thickness of slag film, and increase the heat flux appreciably; using the BaO to replace the CaO, the slag film thickness and crystallization rate would decrease, and the change tendency of heat flux was smoothly.The influence of the slag chemical composition on the incubation time of mold slag was studied by constructing Temperature Time Transformation （TTT） diagram, the influence on the critical cooling rate was obtained by constructing of Continuous Cooling Transformation （CCT） using SHTT, in addition, SHTT was used to studied the kinetics of crystallization and crystallization activation energy of CaO-Al₂O₃ slag. For the CaO-SiO₂ slag, Li₂O have little effect on the incubation time, and B₂O₃ could prolong the incubation time obviously. For the CaO-Al₂O₃ slag, B₂O₃, Na₂O, MgO, BaO and SrO would prolong the incubation time; Li₂O would shorten the incubation time, at the same time, increase the rate of crystallization of the slag; F- had little effect on the incubation time, but could increase the rate of crystallization of the slag. B₂O₃ and Na₂O could decrease the crystal growth rate, increase the crystallization activation energy; F- and Li₂O could increase the crystal growth rate, decrease the crystallization activation energy; MgO could increase the crystal growth rate and the crystallization activation energy. Through the practical casting application of high-Al content non-manganese steel in the Taiyuan Iron&Steel（Group） Co.,Ltd., and expected results was obtained.