Research And Development On Mould Flux For Thin Slab Continuous Casting

Since the 1980s,thin slab continuous casting and rolling technology has been rapidly developed due to its advantages of compact,efficiency,energy-saving and high-value-added products such as thin gauge and high strength.Continuous casting mould flux is an important functional material that determines the stable production of continuous casting and the slab quality.The desirable mould flux for thin slab continuous casting should meet the needs including fast slag formation,sufficient consumption;suitable thickness and uniform distribution of slag film,and sufficient slag layer thickness.Additionally,the mould flux should be able to dissolve and absorb inclusions,and to control the heat transfer.The good effect on thermal insulation and reducing formation of meniscus slag ring is also needed.Therefore,the mould flux is vital for the smooth production of different steel grades under different continuous casting process conditions and for achieving high quality slab.The main work and conclusions of this paper are summarized as follows:(1)The influence of the composition on the main physical and chemical indicators of mould flux was studied.The results show that the addition of Li2CO3,NaF,Na2CO3,artificial cryolite and borax to the mould flux has certain influence on the melting characteristics,viscosity characteristics,fluidity(consumption)and adsorption of Al2O3 inclusions under the experimental conditions.The effect of the composition on tension is not obvious.Both Li2CO3 and NaF are benefical for reducing melting temperature,accelerating melting,reducing viscosity,improving flow and adsorption.In the consideration of environmental concerns,it is necessary to use low-fluorine or non-fluorine slag instead to control the NaF content in slag.The Na2O produced by the decomposition of Na2CO3 is network restrictor,which can provide non-bridge oxygen atoms,destroy the structure of the silicon oxygen network.Consequently,the melting temperature of slag decreases with increasing melting rate.The viscosity of slag decreases while the fluidity(consumption)and the adsorption inclusion capacity are improved.However,with excessive Na2O,nepheline(Na2O·Al2O3·2SiO2)with high melting point would be precipited,which affects the performance of the mould flux,therefore,the amount of Na2CO3 addition must be limited.The main components of artificial cryolite are Na2O,F,Al2O3 and SiO2.NaF is the network modifier,Al2O3 is the network intermediate,and SiO2 is the network generator.The combination of multiple components determines the performance of mould flux.The main components of borax are Na2O and B2O3.Na2O and B2O3 are network modifiers.B2O3 has a low melting point and can react with CaO,MgO,Al2O3 and Na2O to form a variety of low melting point eutectic,which causes the network structure to disintegrate and effectively decreases the melting temperature of mould flux.With excessive borax added,Na2O in borax reacts with other substances to precipitate high-melting phase nepheline,which can affect the properties of mould flux.The order of influence of the added components on the consumption is Li2CO3>NaF>Na2CO3>artificial cryolite>borax.The order of the ability of the added components to adsorb Al2O3 is:Li2CO3>NaF>Na2CO3>artificial cryolite>borax.(2)After non-dimensional normalization of components and properties data,the viscosity and melting temperature of mould flux can be predicted by the ANFIS prediction model after.The average relative error of viscosity and melting temperature was 4.323%and 0.3442%respectively.Compared with other predictive models,the adaptive neuro-fuzzy inference system is more precise,therefore,it is feasible to predict the performance of the protective slag.The system model has a good expansion function and can be used to establish other properties of the slag,such as solidification temperature,crystallization temperature,crystallization rate,interface characteristics,adsorption characteristics,and the thermal insulation characteristics,etc.On the basis of the adaptive neuro-fuzzy inference system,it is possible to to design and optimize the performance of mould flux,so that we can reduce the test work in the design of composition,shorten the development cycle,and reduce the development cost.(3)The analysis of slag film structure of thin slab continuous casting shows that the slag film is mainly composed of glass phase and crystalline phase.The slag film of low carbon steel contains cuspidine(3CaO·2SiO2·CaF2)and akermanite(2CaO·MgO·2SiO2).The slag film of medium carbon steel contains cuspidine(3CaO·2SiO2·CaF2),an akermanite(2CaO·MgO.2SiO2),and nepheline(Na2O·Al2O3·2SiO2).The mould flux for low carbon steel should have good thermal conductivity and low crystallization rate.Adjusting the crystallization rate of solid slag film can improve heat transfer property of mold and reduce surface defects of slab.It is preferable to reduce crystallization temperature to increase thickness of liquid slag film,so that the t consumption of the protective slag can be increased while the lubricating performance is improved and the friction is reduced.The medium carbon steel has strong crack sensitivity and adhesion sensitivity.In order to avoid cracking,slow shell cooling rate in mould and low casting speed is needed.In consideration of lubrication,the mould slag is preferable to have high basictity,high solidification temperature,high crystallization temperature,high crystallization rate,and low viscosity.(4)According to the technical characteristics of thin slab continuous casting,the design of mould flux should focus on slag consumption and the formation of uniform slag film.Based on the analysis of the steel grade,process conditions,slab quality and defect causes of thin slab continuous casting,the composition and properties of the slag were determined,and the development principle of thin slab continuous casting slag was proposed.For low carbon steel with w[C]=0.01-0.08%,in the consideration of lubrication and slag consumption,the mould flux should have low solidification shrinkage rate and small crack sensitivity to prevent steel breakout by sticking.The basicity of mould flux is 0.8-1.0 with the melting temperature of 1050?,melting speed of 25-30s,and the viscosity is 0.04 Pa·s,with the bulk density of 0.4-0.6 g/cm3.The content of Al2O3 in slag is within 5%.For medium carbon steel(peritectic steel)with w[C]=0.09-0.53%,in the consideration of heat transfer control,the larger solidification shrinkage rate and stronger crack sensitivity of mould flux is needed to prevent cracks.The properties of mould flux can be summarized as following:The basicity is 1.1-1.15,thickness of liquid slag layer is 10-12mm,melting temperature is 1080?,melting speed is 30s,viscosity is 0.14Pa·s,and bulk density is 0.6-0.8g/cm3.The industrial practice shows that both the two types of mould flux have good spreadability with little slag agglomeration at the surface,and the slag ring is small and thin,which can ensure sufficient slag consumption.The longitudinal crack of the slab is reduced,and the surface quality failure rate is reduced.Additionally,the number of breakout predictions is reduced with good practical feedback.