Theoretical Research And Application Of Ultrahigh-basicity Mold Fluxes For Peritectic Steel

Controlling the heat flux at meniscus in molds is the key aspect to prevent the formation of cracks on peritectic slab surfaces,and the most effective method to control the heat flux is using mold fluxes with a high crystallization tendency.However,conventional mold fluxes for peritectic grades usually result in poor lubrications and cause sticking or even breakout,which makes peritectic slabs casted with relative low speeds.With the development of Steelmaking industry,new situations,such as no-defect slabs,high-speed continuous casting,wide and thick slabs casting,and small rolling-reduction,make it more difficult to balance the heat transfer control and lubrication with conventional mold fluxes,which has seriously restricted the production and development of continuous casting of peritectic steel.Thus,it is critically important to explore new methods to improve the balance between heat transfer control and lubrication at meniscus and uncover its mechanism.Based on the difficulty of conventional mold fluxes to balance heat transfer control and lubrication,the controlling pattern of heat transfer control and lubrication functions of mold fluxes were discussed in wider composition ranges?including basicity?via thermodynamic and dynamic studies.Then,new methods and theories to improve the balance between heat transfer control and lubrication of mold fluxes were found in this study.As the problem that inaccurate results caused by volatilization of fluorides existed in conventional method to measure the crystallization features of mold fluxes,a novel method was established in this study to obtain accurate crystallization features of mold fluxes.To investigate the composition change of remaining liquid slag after crystallization,methods to obtain the remaining liquid flux were explored and the effect of composition change on lubrication ability of liquid flux were also discussed.Based on these studies,some results can be drawn as below.Thermodynamic and dynamic calculations reveal that,the mold fluxes with ultrahigh basicity have lower crystallization temperatures and faster crystallization speeds of cuspidine,and with better lubrication features of remaining liquid fluxes after massive precipitation of crystals.Massive and fast precipitation of cuspidine can weaken the heat transfer near meniscus and decrease the crack ratio on slab surfaces.In addition,the remaining liquid fluxes with low crystallization temperatures and good lubrication capacity can improve the lubrication condition of slabs,which decrease the risk of sticking breakout.The results of this study reveal a new way to improve the balance between heat transfer control and lubrication.Based on these results,further studies on the relationship between flux compositions and its properties,and the mold flux producing process were conducted.After massive using in steel plants,a theoretical system on ultrahigh-basicity mold fluxes for peritectic steel continuous casting was established.During the measurements of crystallization features using hot-thermocouples and DSC,volatilization of fluorides can lead to inaccurate data.A novel method was firstly established in this study to avoid the volatilization,and the whole crystallization process in Pt crucibles can be recorded.The results reveal the crystallization temperature of cuspidine decreases and crystallization ratio increases obviously with an increase of the slag basicity.With a further increase of slag basicity,slag composition no longer in the primary crystal field of cuspidine,and the crystallization temperature of mold fluxes then increased obviously.The increase of F can enhance the critical basicity of the primary crystal field of cuspidine.When the basicity increases from 1.45 to 1.75,the appearance of crystals changes from block to globoid,and with a massive increase of crystallization speed and apparent crystallization ratio.The increase of MgO and Na₂O can decrease the crystallization temperature and ratio.Thus,appropriate MgO and Na₂O contents are required.When slag basicity lower than the critical basicity,break temperatures of fluxes increase at first and then decrease with a constant increasing of basicity.Mold fluxes with higher basicity result to higher level of composition segregation after crystallization,decreasing of liquids temperature of remaining liquid fluxes,and slightly decreasing the crystallization ratio of cuspidine.For CaO-SiO₂-CaF₂based mold fluxes,the lubrication capacities of remaining liquid slags do no decrease,and the melting temperature of remaining fluxes also do not increase after the increase of the basicity.The industrial application of ultrahigh-basicity mold fluxes has been carried out.The developed mold fluxes were adopted in the production of 3.78 million tons of peritectic steels in PanSteel.The developed mold fluxes gave good melting property,appropriate consumptions,and stable heat flux.No stick alarm occurred and the ratio of longitudinal surface cracks decreases from 18.56%to 0.50%.The developed ultrahigh-basicity mold fluxes have been used in ChongSteel from more than 4 years with 1200million tons of peritectic steel production.Which resulting an obvious decreasing of stick alarm,and no stick breakout occurred.After using the developed mold fluxes,the non-defect rate of slab surfaces in ChongSteel increases to 96.5%,which provides high quality slabs for hot charge process in ChongSteel.The ultrahigh-basicity mold fluxes can provide a smooth running;also decrease the crack tendency on slab surfaces in peritectic steel continuous casting,which can improve the balance of heat transfer control and lubrication,and increase the slab quality and casting efficiency.